test methods of rubber materials and products · summary chapter “rubber raw material testing”...

TEST METHODS OF RUBBER MATERIALS AND

PRODUCTS

MATADOR RUBBER S.R.O.

Summary Chapter “Rubber Raw Material Testing” describes chemical analyses of raw materials used in the rubber and tyre-making industry. This part is dedicated to a principal explanation of basic determinants in raw material analysis. Such analyses are used primarily to determine basic chemical and physical-chemical constants that are directly related to purity of the used raw materials. The second part describes chemical analyses of vulcanizates. More complex procedures are used and instrumental analytical methods are applied in chemical analyses of vulcanizates. This part explains principles of instrumental analyses used in rubber-making practice. Chapter “Rubber Compound and Vulcanizate Testing” is related to the chapter on “Rubber Raw Material Testing” and provides a comprehensive description of the system of testing rubber compounds, materials and vulcanizates, starting from sampling and testing vessels up to evaluation of test results according to specific standards. The chapter is divided into four parts as follows: • Rubber Compound Testing

(determining viscosity, scorching, vulcanization characteristics) • Testing Rheologic Properties of Compounds

(rheologic properties of elastomer systems, liquid classification, factors affecting polymer viscosity, rheometry)

• Vulcanizate Testing (determining stress-strain properties, hardness, rebound resilience, tear strength, resistance to abrasion, aging test, dynamic tests, adhesion tests)

• Dynamical-Mechanical-Thermal Analysis of Vulcanizates Chapter “Laboratory Tyre Testing” discusses measuring and testing tyres in laboratory conditions. Laboratory tests are divided according to two criteria – into specific categories of vehicles (passenger, utility, agricultural and special vehicles) depending on their intended use and into dynamic and static tests depending on the condition of the tyre to be tested. The chapter describes preparation of tyre casings for the tests, simple and more complex measurements, speed tests, endurance tests and special dynamic measurements of tyre casings for passenger, utility and agricultural vehicles based on international, regional and national testing methodology. Chapter “Tyre Testing in Real Conditions” deals with a classification of tests in terms of vehicle’s behaviour on the road, in terms of the tested roadway surface, specialties and the character of the assessment process and with measurement of properties. Tyre preparation before tests is an important part. The subsequent section describes life testing methods and special tests and their classification into subjective and objective tests.

©Matador Rubber s.r.o 2007 2

Table of contents Summary ................................................................................................................... 2

Table of contents ....................................................................................................... 3

TESTING OF RUBBER RAW MATERIALS, MIXTURES AND VULCANIZATES ..................................................................................................... 5

1. Testing of rubber raw materials ........................................................................... 5

2. Chemical analysis of vulcanizates........................................................................ 8 2.1 Determination of extractive parts ............................................................................ 9 2.2 Determination of sulphur........................................................................................ 10 2.3 Determination of polymers ..................................................................................... 11

2.3.1 Instrumental methods of determination............................................................................12 2.4 Determination of carbon black............................................................................... 17

2.4.1 Carbon black identification ..............................................................................................17 2.5 Determination of inorganic components, fillers (analysis of ashes) ................... 18 2.6. Identification of accelerators and antidegradants .............................................. 19 2.7 Identification of reinforcing materials.................................................................. 20

LABORATORY TESTING OF TYRES................................................................. 22

1 Introduction..................................................................................................... 22 1.1 Preparation of tyres for laboratory tests ............................................................... 22

2 Passenger car tyres ......................................................................................... 23 2.1 Static measurements......................................................................................... 23

Cross-ply tyres ......................................................................................................... 25

Radial tyres.............................................................................................................. 25 2.2 High speed tests................................................................................................. 38 2.3 Fatigue tests for passenger car tyres ............................................................... 43 2.4 Dynamic measurements of passenger car tyres ............................................. 44

2.4.1 Measurement of tyre rolling resistance ............................................................................44 2.4.2 Force method....................................................................................................................45 2.4.3 Torque method .................................................................................................................46 2.4.4 Measurement of dynamic directional characteristics .......................................................47 2.4.5 Measurements on high speed uniformity test ...................................................................48 2.4.6 Measurement of temperature distribution ........................................................................48 2.4.7 Measurement of tyre deformations...................................................................................49

3 Tyres for utility vehicles.................................................................................. 50 3.1 Static measurements......................................................................................... 50 3.2 Speed tests ......................................................................................................... 53 3.3 Fatigue tests....................................................................................................... 54 3.4 Special dynamic measurements....................................................................... 56

4 Agricultural and special tyres......................................................................... 58


TESTING OF THE TYRES UNDER REAL CONDITIONS............................... 61

1. Wearing tests (working lifetime / tyre mileage) ................................................. 62

2. Special tests ......................................................................................................... 62 2.1 Subjective tests ......................................................................................................... 62 2.2 Objective tests .......................................................................................................... 65

TESTING OF RUBBER COMPOUNDS AND VULCANIZATES...................... 68

1. Introduction......................................................................................................... 68

2. Testing of Rubber Compounds....................................................................... 68 2.1 Determination of Viscosity and Scorch (DIN 53 523, ASTM D 1646, ISO 289) 68 2.2 Determination of vulcanization properties (DIN 53 529, ASTM D 2084) ... 71

3. Testing of compounds rheological properties ................................................ 76 3.1 Rheological properties of electrometric systems............................................ 76 3.2 Classification of liquids .................................................................................... 77 3.3 Factor influencing viscosity of polymers ........................................................ 79 3.4 Measurement of rheometric properties of substances – rheometry............. 81

4. Testing of vulcanizates.................................................................................... 87 4.1 Assessment of physical-mechanical properties .............................................. 87

4.1.1 Assessment of tensile properties (ISO 37, ASTM D 412) ..........................................87 4.1.2 Hardness assessment (ISO 7619, DIN 53 505, ASTM D 2240) ......................................93 4.1.3 Assessment of reflection elasticity (ISO 4662, DIN 53 512)......................................95 4.1.4 Assessment of structural strength (ISO 34-1, ASTM D 624) .....................................96 4.1.5 Abrasive resistance assessment (ISO 4649, DIN 53 516)...........................................97

4.2 Ageing tests........................................................................................................ 98 4.2.1 Assessment of accelerated thermal ageing in air (ISO 188, ASTM D 865, DIN 53 508) 99 4.2.2 Assessment of accelerated thermal ageing in oxygen (ASTM D 572) .....................100 4.2.3 Assessment of resistance against ozone influence (ISO 1431-1)..............................100 4.2.4 Assessment of liquids influence (ISO 1817, DIN 53 521, ASTM D 471)................102 4.2.5 Assessment of permanent tensile deformation (DIN 53 518) ...................................104 4.2.6 Assessment of permanent compressive deformation (ISO 815) ...............................105 4.2.7 Assessment of the brittle temperature limit (ISO 812, ASTM D 746)......................106

4.3 Dynamical tests ..................................................................................................... 106 4.3.1 Assessment of resistance against cracks creation and growth by crimping (ISO 132, ISO 133)..................................................................................................................................107 4.3.2 Assessment of fatigue resistance and temperature increase on the DPGi apparatus (PN) 108

4.4 Adhesion tests.................................................................................................. 108 4.4.1 Assessment of rubber adhesion with metal (ISO 814)..............................................109 4.4.2 Assessment of adhesion between layers during separation.......................................109 4.4.3 Assessment of rubber static adhesion with fabric cord (H-test) (ISO4647)..............109 4.4.4 Assessment of adhesion after dynamical deformation by Henley method................110

5. Dynamical-mechanical-thermal analysis (DMTA) ......................................... 111


TESTING OF RUBBER RAW MATERIALS, MIXTURES AND VULCANIZATES

1. Testing of rubber raw materials

Rubber raw materials’ checking includes essential chemical-analytic and physical-mechanic measuring, the results of which are mostly physical parameters. On the base of these physical parameters it is possible to assume the raw material purity. The parameters are such as: determination of the melting point, moisture volume, dry mass volume, percentage of ash or volatile substances, determination of liquid samples viscosity, specification of effective substance volume, specification of heavy metals (of rubber toxicants), refractive index, acid value, and others. From the methods used for entry raw material quality control it is required to be quick, exact and their results should be in correlation with material purity. Melting point [°C] – it is temperature at which the sample change of consistency takes place; it is from the solid state into a liquid state (the liquid phase and the solid phase are in dynamic balance). It is one of the compound purity criteria. The temperature is dropping down at the moment when small dirt content is present. With impure substances the transmission is not so sharp; therefore there is a temperature melting point interval expressed along with it. Determination of mixture melting point is based on an assumption that the mixture of two different non-isomorphic substances is melting at considerably lower temperature than respective pure compounds. Therefore if two substances have the same melting point and they are same then their melting point will also be the same. If they are different their melting point will be lower. Melting point is determined at accelerators, antidegradants and others. Dropping point – it is temperature at which the substance, while being heated by certain rate, will be softened up to such a point that originated drop will drop down by its weight. It is similar to determine dropping point by a circle and a ball, where generated drop will drop down by gravity force effect of the steel ball. Dropping point is determined at resins, colophony and asphalt. Volatile substances – they are compounds that are being released from a sample during temperature increasing (the temperature of 105°C is mostly used at determination of compounds). In most cases it is water or volatile organic compounds expressed to the original sample backfill in %. It is determined at carbon black, silica, organic chemicals (accelerators, antidegradants) and others. Dry mass – it is the rest of the sample after defining of volatile substances or, if appropriate humidity. It is stated in % to the original sample backfill. It is determined at liquid samples of cements and separating agents. Ash – it is composed of organic, at a certain temperature non-volatile and further indecomposable compounds, which are parts of the sample. Due to decomposition of some inorganic compounds (e.g. carbonates, sulphides) the ash volume does not have to define inorganic parts share in a very exact way. It is stated in % to the original sample backfill and it is necessary to indicate temperature to the result


(usual execution of ash volume determination is at 550°C or, if appropriate 950°C). It is defined at carbon blacks, organic chemicals, fillers, auxiliary rubber aditives. Refractive index (refraction) – it is ratio between speed of propagation of light in vacuum and in the sample. Speed of propagation of light reduction is caused by induction of dipole moment in environment molecules by electro-magnetic field around a light beam. It depends on the exposure length (light dispersion – the base of light decomposition into a spectrum) as well as on relative dielectric constant factor εr, therefore it is indicated for sodium discharge lamp and defined temperature (subsequently being marked as nD – D is marking of a characteristic lines doublet in sodium emission spectrum). Specific refraction r is

here: n – is absolute refractive index

atio between compound refraction index at defined temperature and constant wave

composition of specific refraction and compound M mol

ol refraction of a certain substance does not depend on temperature and pressure,

efractive index is characteristic magnitude that defines purity of rubber raw

iscosity – it is a measure of inner liquid friction. Newton’s equation determines

he measure of dynamic viscosity is Pa.s; kinematic viscosity is a ratio of dynamic

ρ×

+

−=

12n1nr

2

2

W ρ – is density of material Rlength and its density: Mol refraction Rm is weight:

ρnMrRm ×

+=×=

22

Mn −12

Mat the same time it is only slightly influenced by a change of consistency. Mol refraction of organic compounds can be figured out as a sum of atom and bonding refractions that are tabulated. Comparison of experimental and calculated refraction values is used for confirmation of compound structure. Rmaterials and solvents. Vviscosity as a constant of proportionality η between tangential stress τ and speed gradient (dυ/dy).

dy×η=τ

dυ

Tviscosity and liquid density with the measure m2.s-1. In practice, different relative viscosity measures are still often used, they define viscosity value of a sample in the ratio toward standard fluid; such as water (Engler’s °E, Redwood’s °R and Seybold’s degrees °S).


With oils a viscosity – density constant is used. Higher value of viscosity – density constant shows higher portion of aromates in oil. It can be figured out in according to the relation:

718,0))8,0log((log1,0)276,0(

VHK 5015 +η××−−ρ=

Where: ρ15 is density at 15 °C in g/cm3

η50 is cinematic viscosity at 50 °C in mm2/sec.

Viscosity is an important constant, similar to other physical constants because it is closely related to purity of substances and thus also related to their quality. It is determined at liquid softeners and oils. Acid number – it is amount of alkali hydroxide necessary for neutralisation in the sample of present compounds. It is defined in mg KOH/g of the sample. It is determined by volumetric methods at samples which contain free acids such as resins and some softeners. Active substance determination – ordinary industrial chemicals are mostly not used as pure ones, but they form a mixture with various additives which make processing easier, in some cases they may improve hygiene of work (dust nuisance). Due to appropriate quantization it is necessary to know the content of substance active of the industrial chemical. For determination of substance active different analysing methods are used, beginning with volumetric analysis (redox, neutralisation, coagulating or complex-forming titrations) up to modern instrumental methods (GC, HPLC). Carbon Blacks testing – it covers essential determinations, accessible in each laboratory. They are such as determination of pH of carbon black (boiling slurry and sonic slurry), determination of volatile substances, ash content, sieve residue analysis, oil absorption number (dibutylphthalate absorption), iodine adsorption number, tint strength, transmittance of toluene extract, powder density, individual pellet hardness, dust content. Interaction degree of rubber with carbon black depends on three main factors:

• Contact surface size of carbon black and rubber – co called extensive factor (BET, CTAB adsorption, iodine adsorption number)

• Structure of carbon black, so called geometric factor (oil adsorption number and modified oil adsorption number – compressed oil adsorption number)

• Surface activity, so called intensive factor (content of volatile substances, pH…)

Carbon blacks testing also covers the following methods:

• determination of iodine adsorption number (iodine number indicates milligrams´ number of iodine, that is being adsorbed on the surface of 1g of carbon black, by number iodine number is equal at usual retort carbon black as their specific surface determined by nitrogenous adsorption),

• determination of cetyltrimethylamonium bromide adsorption – CTAB (adsorption of large molecules, which cannot enter microspores, it gives


information about effective outer surface that is accessible for rubber molecules),

• determination of specific measuring surface – BET, STSA (it is based on nitrogen adsorption at low temperature, when monomolecular layer is being formed. Nitrogen has small molecules, it is chemically inert/non-reactive and its adsorption does not depend on chemical character of carbon particulate surface),

• determination of oil absorption number – dibutylphthalate adsorption (dibutylphthalate is added to carbon black and torsion moment is being read. At dibutylphthalate saturation by carbon black the torsion moment is suddenly increased or the resistance against kneading),

• volatile substance content – carbon black sample is being heated up at certain temperature and the content is determined from difference between the weight before and after heating,

• pH carbon black – carbon black are dispersed in distilled water (boiling slurry and sonic slurry) and pH of such dispersion in water is determined.

PRI (Plasticity Retention Index) – this mechanical testing substitutes more time demanding and more expensive analysing determination of heavy metals in rubbers. It is based on the matter of fact that some heavy metals present in the rubber sample at increased temperatures cause destruction of rubber polymer string (polymer molecular weight is dropping down as well as its physical-mechanical properties – viscosity, plasticity). Raw rubber sample is recalendered and divided into two parts. One of them is thermally loaded with 140°C for 30minutes. Plasticity value (plasticity deformation measure) will be determined for both samples. PRI is figured out as follows:

%100PP

PRI0

30 ×= Where: P30 is plasticity after thermal loading P0 is plasticity of a sample that was not thermally loaded If a sample does not contain increased amount of heavy metals, PRI value is low.

2. Chemical analysis of vulcanizates

From the point of chemical-analysing view vulcanizate forms a very complex matrix with analytes content from trace parts up to main components, because rubber mixture prescriptions are usually formed of 10 or more components (even the sole components are mostly not chemical individuals and last but not least, many compounds are created, or original ones are disintegrated at mixture vulcanisation). Analyses themselves follow considerably time demanding sample preparation, or pre-concentration of analyte trace parts. In the next text vulcanizate analysis is described only in a short form, not all analysing methods are given there because of great range of used analysing methods. The emphasis is laid mainly on using of modern analysis methods.


Rubber mixture analysis covers the following procedures: 1. extraction of extractive parts from the mixture (extract may contain oils,

softeners, fragments of accelerators, antideradants, waxes, processing ingredients and free sulphur),

2. Sulphur content determination. 3. identification of polymers´ type and their qualification, 4. determination of carbon black content, 5. identification and qualification of inorganic mixture´ components, 6. identification of accelerators and antidegradants 7. identification of reinforcing materials

2.1 Determination of extractive parts Vulcanizate extraction by appropriate extracting agent serves for sample preparation before further analyses as well as for determination of extractive substances´ content. It is possible to use several types of mostly organic solvents for extraction (extracting agents) depending on purpose of the analysis. Selection of appropriate solvents is given in the following table (in according to references of DIN 53 553): Table 1: Selection of appropriate solvents for extraction

Extracting agent for

Polymer type raw rubber, not vulcanized mixtures

vulcanizates

Natural rubber (NR), synthetic polyisoprene (IR) acetone

acetone chloroform/trichloromethane1)

Styrene butadiene rubber (SBR) Styrene butadiene rubber (set by oil) cis- butadiene rubber (BR) Ethylene-propylene-diene rubber (EPDM)

ETA2)acetone chloroform/trichloromethane1)

Polyisobutylene rubber (IIR) Chlorinated polyisobutylene rubber (CIIR) Brominated polyisobutylene rubber (BIIR)

butanone (MEK) acetone

butanone acetone

Chloroprene rubber (CR) Chlorosulphonated polyethylene (CSM) Vynilidenefluoride-hexafluorpropylene interpolymer (FPM)

2-propanol (isopropanol) methanol

methanol

Nitrile rubber(NBR) 2-propanol (isopropanol) methanol

2-propanol (isopropanol) methanol

1) – if bituminous substances are present, extraction is carried out by chloroform. Chloroform extraction lasts approximately half of the time spent at extraction by acetone (it is allowed to use mixture of both solvents).

2) – azeotropic mixture created by 68 % volume of ethanol and 32 % volume of toluene, the boiling point is at 76.7 °C. In the case of ethanol containing water 70% of ethanol and 30% of toluene are formed, water will be distilled of and medium fraction will be used. Front fraction contains water (tertiary mixture: water, ethanol, toluene with the boiling point of 74.4 °C).


The most used acetone extract. Out of the sample acetone extracts resins, free sulphur, in acetone soluble softeners and antioxidants, procesing rubber aditives, mineral oils, waxes, organic accelerators and their reactive products and fat acids. It partially extracts bituminous substances, highly molecular hydrocarbons and soaps. Content of free sulphur, wax and mineral oils may be determined of acetone extract. Chloroform extracts bituminous substances and we can consider it as indicator of mixture components presence. Chloroform extract involves also other components including small rubber amounts. It is applied on the sample that was extracted by acetone in advance (after determination of acetone extract). In according to analysis it is also possible to extract by other softeners, e.g. ethanol dilution of potassium hydroxide (KOH), total extract (extraction ETA) and others. Extraction takes place in extraction equipments (e.g. extraction device according to Soxhlet or Twisselman). Exact sample backfill is being extracted for a rated period by appropriate solvent. Evaluation may be executed in two ways. The first one reclines upon weighing of extracted sample after perfect removal of solvent rest from the sample (vulcanizates have the property to swell up in organic solvents) – it can be done by drying, the best way to do that is in vacuum dryer (differential weighing). The second way is based on weighing of the distillation bulb with the extract after evaporation of extracting agent (direct extract weighing). T he first method, providing that solvent quantity will be removed out of the sample, provides more correct results. In the second case there might be some losses due to volatility of some components (e.g. amines formed by decomposition of some accelerators). Obtained extract is used for further determinations (content of non-saponifiable share in acetone extract, content of paraffinic hydrocarbons or content of mineral oil). Content of non-saponifiable share is determined in the rest from acetone extract. 1 M dilution of KOH is added to the rest from acetone extract and it is being heated under reverse cooler. Hydrolysable components are submitted to hydrolysis and then they are extracted by diethyl ether. The share of acetone extract that has not been hydrolyzed will be evaluated. The content of paraffinic hydrocarbons is determined after previous testing. Absolute ethanol is added to the sample, it is cooled down to -5°C and precipitated paraffin is filtered and weighed. 2.2 Determination of sulphur In practice, as cross-linking (vulcanizing) agent is mostly used by elementary sulphur. After cross-linking (vulcanization) sulphur in vulcanizates appears in different forms depending on used accelerating system and vulcanization degree. Apart from that also some rubber raw materials contain sulphur in their molecule. Vulcanization is a reaction of sulphur with unsaturated bonds of rubber. At vulcanization sulphur reacts while various types of sulphide cross bonds are formed between rubber bonds but also modifying of bonds occurs at the same time (combined sulphur). However, part of sulphur stays in unreacted form and we define it as free sulphur. Total sulphur is the most often defined, it presents overall


amount of sulphur in rubber (added sulphur, sulphur issued from raw materials). Determination of sulphur is carried out by several methods. The principle for all of them is oxidation of sulphur atoms onto sulphur dioxide or sulphates. Nowadays determination of sulphur is executed by the means of elementary analyzer. Elementary analyzers allow determination of not only sulphur but also content of nitrogen, carbon, hydrogen and oxygen. The sample is being burnt in the oxygen atmosphere at the temperature higher than 1200°C; burnt gases are cleared and lead into detector. The most used detectors are either thermal conductivity detector or infra-red detector. Sample weigh between hundredths of milligrams up to approximately 1g, detection limits are in dependence on sample type and detector type from 0.005mg. The whole analysis is lasting for 15minutes. It is usual to use methods of volumetric or gravimetric analyses in operational laboratories. The most often used is the method of sample burning, which contains sulphur in according to Schöniger. The sample is packed into ash less paper that does not contain sulphur compounds and it is burnt in oxygen atmosphere in Erlenmeyer’s flask. Burnt products are absorbed in dilution of hydrogen peroxide. Generated sulphur dioxide is absorbed in dilution and at the same time it is oxidized while sulphates are being formed. Sulphates may be assessed by gravimetric analysis, by precipitation with soluble barium salt or by volumetric methods. Final determination of sulphates is possible to carry out also by the use of automatic titrator. Determination of free sulphur is executed in the sample of butanone extract or in the sample of vulcanizate by boiling with sodium sulphite. In the first case free sulphur is extracted into butanone. After adding of bromine it is oxidized while sulphates are being formed and they are determinated by volumetric method and by precipitation with BaCl2. With the second method sulphite reacts while boiling with elementary sulphur when tiosulphate is being formed and it is assessed by titration. 2.3 Determination of polymers Determination of polymer type and amount belongs to essential determinations. There are various methods; the most used are, however, instrumental methods of chemical analysis. At the same time determination of polymers ranks among the most complex analyses. Procedures based on volumetric methods: Determination of polyisoprene rubber amount (NR) is based on sample oxidation by chrome-sulphuric mixture. Polyisoprene is oxidized while acetic acid is being formed. Formed acetic acid is determined after distillation alkali metrically by dilution of NaOH. Other present polymers influence the determination.


2.3.1 Instrumental methods of determination Thermogravimetric analysis Thermogravimetry is a method based on monitoring of sample volume changes in depending on its temperature. By its help it is possible to make more complex sample classification, not only to do evaluation on the presence of certain polymers. The sample is exposed to controlled thermal program and continuously its mass is being monitored:

– Up to approximate temperature of 150 °C volatile parts are released (humidity, volatile organic components...)

– Above the mentioned temperature volatile softeners start to be releasing (e.g. oils). In according to their character (paraffinic, aromatic…) the temperature of maximum change rate is shifted to lower or higher values. In many cases this action overlaps the beginning of polymer matrix pyrolysis in vacuum (decreasing boiling temperature of volatile softeners, such as oils).

– At the temperature of 370°C polyisoprene rubber starts to pyrolyze, it can also be used for its identification. Rubber butadiene types are being decomposed at temperatures higher than 420°C due to their lower non-saturating character. Complete decomposition of organic substances takes place up to the temperature of approximately 500 °C.

– at the temperature of 750 °C decomposition of present calcium carbonate takes place and the results are calcium oxide and carbon oxide:

23 COCaOCaCO +→

– Carbon black determination is based on their oxidation by oxygen (change of atmosphere into oxidizing). Some organic substances (e.g. compounds containing heteroatom or aromates) produce pyrolyze carbon particles at pyrolysis, therefore the value may be assessed faulty positive. It is possible to determine analyses in such a way that separated burning of pyrolyze carbon particles and filler carbon particles will take place.

– Sample weight at the end of determination shows the amount of ashes.


100.0 200.0 300.0 400.0 500.0 600.0 700.0 800.0 900.0Temperature /°C

0.00

20.00

40.00

60.00

80.00

100.00

TG /%

-6.00

-5.00

-4.00

-3.00

-2.00

-1.00

0.00

1.00

DTG /(%/min)

0.0

50.0

100.0

150.0

200.0

Flow /(ml/min)

TG

DTG

nitrogen air

Residual Mass: 10 .88 % (988.8 °C)

-11.27 %

-19.76 %

-37.85 %

-1.64 %

-18.60 % 382 .8 °C

455 .7 °C

679.5 °C

887.5 °C

Figure No. 1: Example of TG record of a vulcanizate sample. Weight reduction 11.27 % is related to evaporation of volatile softeners, 19.76 % approximate loss of

polyisoprene, 37.85 % loss of butadiene rubbers, mass of carbon black after the change of atmosphere into oxidizing one (nitrogen – air)18.60 % and amount of

ashes 10,88 %.

Temperature [°C]

Figure No. 2: Comparison between TG record of the same sample in vacuum and

the record in nitrogen atmosphere. The blue curved line means measuring executed in vacuum and the red curved line means measuring executed in N2 (significantly

better separation of volatile softeners from the beginning of sample polymer matrix decomposition).

Many decomposition actions run at approximately same temperatures. Due to that fact the beginning and the end of these actions are difficult to be noticeable; therefore the first derivation of TG curve is used. We get so called DTG curve and its minimum (if appropriate maximum) presents the greatest rate of weight change and this temperature is usually evaluated as characteristic temperature. This temperature is, however, influenced by geometric sample configuration and location


of temperature sensor; therefore it is comparable only with the results measured on the same device. Thermo-gravimetry nowadays allows determination of kinetic measuring and on the base of their results it allows determination of sample lifetime from the point of thermal loading view in exploitation. Kinetic measuring is carried out at various sample heating rates. They can be measuring at low heating rates, e.g. at 2°C/min, 5°C/min, 10°C/min. Product lifetime is determined in according of sample thermal decomposition degree and is figured out at selected temperature. For example it will be calculated how much time is needed for certain decomposition degree at selected temperature. Thus determined lifetime is important at products which are exposed to high temperatures. There are several kinetic models of sample decomposition. For polymers it is possible to apply the model of free kinetics (Model Free Kinetics). Due to variability of polymer decomposition mechanisms also other models of kinetics are used (e.g. reactions of 0-th order or n-th order).

Activation energy

Figure No. 3: Kinetic measuring by the means of TG, executed at different heating

rates. There is a possibility of thermo-gravimetric analyzer connection the most often with infra-red or mass spectrometer that allows to identify pyrolyze gases and on their basis to identify components in the sample. Sometimes there is thermogravimeter used to be connected with gas chromatograph. This technique is indicated as „EGA – Evolved Gas Analysis“. TG measuring and spectra monitoring are executed at the same time and the software permits its simple and fast processing. So called Gram-Schmidt diagram is evaluated, it shows dependence between overall absorption (concentration) of forming pyrolyze gases and the time or temperature.

TG recordmg

4

6

8

10

°C200 400

Conversion curves% kJmol^-1

300

50 200

100

0

50 %200 400 °C

alpha(5%)alpha(3%)

alpha(2%) alpha(1%)

h 1.5e+006

1.0e+006

5.0e+005

0.0e+000 -20 20 0 40 °C60


Figure No. 4: 3D graph – wavenumber [cm-1], temperature [°C], absorbance, of TG/FTIR analysis.

Infrared spectrometry Infrared spectrometry is one of the most used instrumental methods, appropriate for identification of polymers in vulcanizates. It is necessary to extract the sample before analysis in order to remove all softeners which inhibit correct identification of polymers. It is done by applicable solvent. Extracted and dried sample is then pyrolyzed in inert atmosphere. Pyrolytic product is caught and applied on KBr tablet and infrared spectrum is measured. The measured spectrum is compared with spectra of known polymers or with spectra listed in the library. Semi quantity analysis is executed with the sample after dissolving in o-dichlorobenzene and after removal of carbon black by the means of column chromatography. The mentioned method is indirect that means it is necessary to make calibration curve with known polymer content.


891.93

800 10001200140016001800 Wavenumber cm-1

0.000

0.005

0.010

0.015

0.020

0.025

Absorbance Units

Figure No. 5: Infrared spectrum of unknown sample measured in connection with TG/FTIR

Pyrolytic gas chromatography Extracted sample is pyrolyzed and a part of pyrolyze gases is conducted into the gas chromatograph. There is separation of individual components and identification can be performed within the column. The most applicable detection method is the use of mass detector that allows identification of components on the base of their mass spectrum, apart from identification by the means of elution properties.

Figure No. 6: Chromatographic record of pyrolyze gas-liquid chromatography with mass detector. The mentioned combination enables identification and determination of different types of rubber and organic compounds in the vulcanizate. Nuclear magnetic resonance: Nuclear magnetic resonance is used in a similar way as infrared spectrometry – for identification and determination of polymers. Measuring of the following spectra is mostly used: 1H spectra, in some cases 13C spectra. NMR allows performance of measuring even in a solid state; eventually it allows utilization of imaging method of selected atoms.


2.4 Determination of carbon black Determination of carbon black can be executed in different ways, e.g. decomposition of organic matrix by the means of nitric acid and then it can be pyrolysis of the sample in inert atmosphere or by TG. Decomposition by nitric acid is applicable only on unsaturated rubbers. Nitric acid oxidizes rubber matrix. The sample is subsequently filtered dried and burned in a muffle furnace. The difference in weight before and after burning is equal to carbon black content within the sample. Pyrolysis is based on a similar principle. The sample is pyrolyzed in inert atmosphere while carbon black are being formed. These are subsequently burned in oxidizing atmosphere and the content of carbon black is found out from the difference of weight. The method is not applicable for the samples that produce pyrolyze carbon black. As the most applicable method is TG at which it is possible to eliminate creation of pyrolyze carbon black.

2.4.1 Carbon black identification It is possible to use several methods for identification of unknown carbon black. However, many of them have their limitations. For identification of carbon black the following methods might be used:

– determination of iodine adsorption (iodine number indicates milligrams number of iodine, that is being adsorbed on the surface of 1g of carbon black, by number iodine number is equal at usual retort carbon black as their specific surface determined by nitrogenous adsorption),

– determination of cetyltrimethylamonium bromide adsorption – CTAB (adsorption of large molecules, which cannot enter microspores, it gives information about effective outer surface that is accessible for rubber molecules),

– determination of specific measuring surface – BET, STSA (it is based on nitrogen adsorption at low temperature, when monomolecular layer is being formed. Nitrogen has small molecules, it is chemically inert/non-reactive and its adsorption does not depend on chemical character of carbon particulate surface),

– distribution determination of parts size by the help of transmission electron microscope – it provides the best results regarding identification of used carbon black in vulcanizates.

Carbon black are isolated from the sample by pyrolysis in inert atmosphere. Greater amount of silicon dioxide within the sample is removed by hydrofluoric acid; otherwise it would interfere in the case of BET determination.


2.5 Determination of inorganic components, fillers (analysis of ashes) By the term fillers are indicated those inorganic materials, apart from sulphur and carbon black, which were added into the rubber mixture. Out of inorganic components especially the content of heavy metals is determined (of rubber toxicants), the content of zinc oxide (activator of vulcanization), if applicable the content of cobalt (adhesive mixtures), the content of magnesium oxide, antimony trioxide, lead oxide (special heatproof mixtures, particularly conveyer belts) and the content of titanium dioxide. Inorganic components determination is based on mineralization of the sample. The sample can be mineralized by different techniques, by dry or wet method, in a closed or open system. The most reproducible and the most correct results are reached at wet, closed mineralization. Mineralization is the most crucial operation from the point of errors done at analysis and the analysis duration. At mineralization in an open system there are relevant losses of volatile metals (As, Sb); with the presence of halogens there are also losses of zinc and other metals (creation of volatile halogenides of metals). Due to this fact there are preferred closed mineralizing systems at which - with the effect of high temperature, pressure and decomposition agents (mineral acid HNO3, H2SO4 and hydrogen peroxide) – almost complete decomposition of organic matrix takes place. The heating is provided by microwave radiation. Vulcanizate samples in routine analysis, provided that they do not contain halogens, are mineralized by dry way (by burning in fire pot in the muffle furnace at 550°C). Other determinations are performed out of such gained ashes. The ashes are dissolved by diluted hydrochloric acid and subsequently the content of silicon dioxide is determined. Determination of metals is performed by atom absorption spectrometry (AAS), by voltamperometric method or spectrophotometer method of UV/VIS region. Higher metal concentrations can be possibly determined by volumetric methods, e.g. content of zinc oxide. Atomic absorption spectrometry is based on radiation absorption measuring by free atoms of the sample at its atomization. Free atoms in gas state are in according to Kirchhoff´s law able to absorb radiation of those wavelengths which they emit themselves (this phenomenon is noticeable on the Sun as so called Frauenhofer´s lines). If an atom is exposed to radiation activity of applicable wavelength there absorption of light quantum takes place and atom getting to adequate excited state. Sufficient concentration of free atoms is reached by temperature increase. Atomization is possibly performed in different flame types or by electrothermal method, the most often in graphite cuvette. The determination limit in the flame for most of metals is approximately up to 0.1mg/l, at electrothermal atomization the determination limit is hundred up to thousand times lower. It is caused by significant “dilution” of the sample by combustion gases of the flame. Mainly metals are determinated by the AAS method. By AAS it is also possible to determine some non-metals, whose analytic absorption lines lie under 190nm (it is necessary to remove air from optical path – radiation absorption by oxygen).


2.6. Identification of accelerators and antidegradants Commonly used antidegradants and accelerators used in rubber industry are organic compounds. It is possible to isolate them from vulcanizate by extraction with applicable extracting agent. Extract contains oils, softeners, fragments of accelerators, antidegradants, processing admixtures and free sulphur. Identification of accelerators can by preformed by one of the separating analytic techniques such as gas chromatography, chromatography on a thin layer or high performance liquid chromatography in connection with appropriate detector. Connection of separating technique and detector is a very effective one, detector allows identification of unknown sample on the base of measuring of its spectrum (mass, infrared, NMR, such as combination of GC/MS, GC/FTIR, GC/NMR or HPLC/MS, HPLC/FTIR, HPLC/NMR). Most of accelerators are decomposed by vulcanization; therefore it is convenient to identify them on the basis of the presence of reactive products (e.g. out of N-cyclohexyl-2-benzthiazolesulphenamide (CBS) cyclohexylamine is created, that is being identified).

S

NS N

H

S

NSH NH2+

Figure No. 7: Decomposition of CBS accelerating agent during vulcanization

Similar to the above mentioned facts it is possible to identify molecules of antidegradants, or processing admixtures such as some vulcanization inhibitors, plasticizing agents and others.

Figure No. 8: Chromatogram of vulcanizate extract

Figure No. 9: Mass spectrum of antidegradant 6PPD, identified in the previous figure

Among the use of separating techniques for analysis of rubbers there is HPLC (HPLC – High Performance Liquid Chromatography). By the method of gel permeation chromatography it is possible to determine distribution of rubber molecular weights, or their average molecular weight.


Figure No. 10: Determination of polyisoprene rubber molecular weights by the

method HPLC 2.7 Identification of reinforcing materials Tyre is a composite product. Apart from rubber it is composed of different reinforcing material types. The most used ones are textile reinforcing materials and steel reinforcing materials (they are used in the form of cord or fabrics). The most used method applied for identification of reinforcing materials is infrared spectrometry. The sample is dissolved in applicable solvent and then it is analyzed by appropriate technique (e.g. KBr tablet). It is possible to identify reinforcing material on the base of its melting point. Melting point is detected by measuring of DSC record.

Figure No. 11 DSC record of unknown reinforcing material (PAD 6.6)


Figure No. 12 DSC record of unknown reinforcing material (PAD 6)

DSC (differential scanning calorimetry) belongs to thermal analysis methods. It is based on the principle of energy measuring (of electric input), that is being consumed for maintaining of the same sample temperature and standard temperature (e.g. carborundum). Enthalpy of sample variables is measured by this method. The method DSC is in the greatest extent applied at the study of phase variables, primarily of polymer materials. It allows evaluation of the sample thermal capacity, glass transition temperature and various phase variables. By the means of this method it is possible to detect “thermal history” of the sample. On the basis of the chemical analyses results it is possible to “reconstruct” original vulcanizate recipe. It is feasible to use analytic techniques at problem solutions of new products development, optimizing of properties and others. Due to the wide range of the mentioned issue there were not all methods listed within the text neither circles of analyses, also there were not detailed analytic method principles listed there.


LABORATORY TESTING OF TYRES

1 Introduction Laboratory tyre tests are carried out in laboratories under exactly defined and regulated conditions. The advantage of laboratory tests lies in their high degree of repeatability and reproducibility, which is given with the ability of exact regulation of testing conditions and parameters. Depending on the character of the Testing we include under these parameters velocity, radial load, inflating pressure, camber angle, angle of directional deviation, temperature of the environment, relative humidity of the environment, as well as the duration period of individual test steps. Laboratory tyre tests can be classified according to several viewpoints. One method of classification is the division according to the use into individual vehicle categories:

• tyre tests for passenger cars, • tyre tests for utility vehicles, • tyre tests for agricultural vehicles and forest tractors, multi-puropse vehicles,

earthmoving machines and special vehicles , • tyre tests of solid tyres and solid wheels for industrial carts, manipulators,

barrows, etc.

From the viewpoint of the character we divide tyre tests dependent on the condition of the tested tyre in following manner:

• static tests and measurements – during these tests no rolling and rotation of the tyre occurs and the properties are being measured in static condition

• dynamic tests – the principle of these tests is the simulation of tyre rolling on a testing device equipped with a drum, usually in a shape of a cylinder with a defined surface, for the purpose of tyre life determination,

• special dynamic measurements – their aim is the determination of various properties of a rotating tyre, which influence the behaviour of the vehicle during operation.

1.1 Preparation of tyres for laboratory tests The procedure of tyre preparation for laboratory tests is usually described in national standards and regulations. Prior to the fitting, tyres, tubes and flaps undergo a visual inspection for the purpose of the visible defects detection. Prior to the tests the tyres can be exposed to non-destruction analyses and tests (röntgen, ultrasound, holography, interferometry, uniformity test). Tested are tyres, which have been produced at least 120 hours prior to the test start. For the purpose of operativeness, this time can be reduced for production quality control up to 24 hours. During this period tyres are being conditioned in a room at a temperature from 5 to 30°C, during last 12 hours at the temperature of the test room. Only after that it is allowed to mount the tyre on a rim (respectively together with the tube and flap) and to proceed according to the relevant methodology of the test. The tyre is


being mounted ot the test rim in such a way, that no damage can occur to the tyre, tube or flap. In order that the bead fits well on rim shoulder, the inflation pressure can be doubled, but maximally up to 1 200 kPa (for passenger car tyres maximally up to 600 kPa). Thenafter the inflation pressure shall be adapted to a value required by the methodology and checked for any pressure drop. With a pressure drop exceeding 20 kPa, the airtightness must be checked by means of immersing into water, with the use of a water-detergent mixture or by means of a special agent for leakage detection. In case of need, the tyre can be additionally stabilized with a running-in at a drum test device. This run in is usually carried out at a speed of 60 to 120 km/h, at a load corresponding from 0,8 up to 1-multiple load of the maximum permitted tyre load and at an inflation pressure corresponding to this load. The stabilization by means of a run in , which can last for 1 to 6 hours, serves for the securing the stability of specific properties and to eliminate mechanical stresses, which act in the tyre after its cure.

2 Passenger car tyres Passenger car tyres include a group of tyres, which is destined predominantly for car categories M1, N1, O1 and O2, i.e. cars for passenger transport up to a weight of 3,5 t ( up to 9 seats including the driver, respectively), vehicles up to a load of 3,5 t designated for load transport (mainly pick-up-type vehicles and light trucks) and their braked or unbraked trailer vehicles with a total weight up to 3,5 t. This group can include also a part of tyres designed for vehicles of the M1G category, i.e. free time vehicles (SAV – Sport Activity Vehicles, SUV – Sport Utility Vehicles) [1]. Laboratory tyre tests of this kind consist of static measurements, dynamic tests and special dynamic measurements. Laboratory tyre tests, in contrary to the testing under exploatation, are carried out with the use of reinforced disc wheels, consisting of a reinforced disc and reinforced rim. These disc wheels are manufactured from steels according to STN EN 10020 [2] by turning, in contrary to pressed or cast disc wheels used on cars. These reinforced disc wheels have very tight production dimensional tolerances and are designed for loads exceeding several times the permissible loads of tyres formed together by casing, valve (or tube and flap in case of tube type tyres) and inflating medium. 2.1 Static measurements As has been already mentioned before, during static measurements, the properties and chracteristics of casings and tyres are determined in a non-rotating state. Following measurements predominantly belong to the static measurements:

• measurement of tyre weight, • measurement of tyre crown thickness, • measurement of tread hardness, • measurement of basic tyre outer dimensions, • measurement of static radial strength, • measurement and analysis of tyre footprint,


• measurement of specific pressure distribution in the tyre footprint, • measurement of mandrel puncture strength in the tyre crown, • determination of tyre resistance to water pressure destruction, • measurement of bead resistance to rim skid of a tubeless tyre, • measurement of the tread groove depth and the height of tread wear

indicators, • measurements of fit forces in the tyre bead.

Measurement of tyre weight has its significance during the monitoring of production processes, as well as during analysis of a competing product, due to the environmentally based effort for tyre weight reduction and thereby also the burden reduction of environment due to waste and possible higher demands on consumption of motor vehicles. In principle there are two possibilities of tyre weight measurement – either the weight of individual pieces is determined, or procedure is executed by measuring series of tyres containing more pieces. Passenger car tyres are weighed on platform balances with an accuracy of 20 to 50 g. Pendant balances equipped with an electronical dynamometer can also be used.

Measurement of tyre crown thickness is being used by some producers for serial production quality control of new, as well as renewed (retreaded) tyres. The measuring device consists of a dimension gauge and a supporting base on which the tyre is positioned in such a way, that the supporting base lies in the middle of the tyre crown in the inner rubber. Then the dimension gauge is being pressed onto the middle of the tyre crown from outside and the tyre crown thickness is measured.

Tread hardness is under European conditions determined by a Shore durometer in Shore units. Its significance lies in the comparison of different tyres, tread mixtures, as well as quality control in tyre retreading by so called warm method, i.e. by application and subsequent cure of the new material. Tread hardness is measured in the centre of tread pattern. The hardness value is being read 3 seconds after the durometer application on the tread pattern. The hardness value is dependent on tyre tread temperature and therefore the tyre must be conditioned at room temperature prior to the measurement. Tread hardness is measured either on three locations, on both edges and in the middle of tyre crown, or on six locations, regularly distributed on the tyre circumference, nearest possible to the middle of the tyre crown [3].

Measurement of basic outer tyre dimensions is realised on a tyre composed of a tubeless casing, rim and valve, or a tube casing, flap and rim, respectively. The measurement of basic outer tyre dimensions, which consist of total width and outer diameter, influences significantly the final values of inflating pressure and width of used rim. The test procedure is following. The tubeless tyre, or the tyre with tube and flap is being inflated from 300 to 350 kPa. Thenafter the inflating pressure is being adjusted to value according to Chart 1. The inflated tyre is being conditioned at the laboratory temperature (25±5)°C for at least 24 hours. Afterwards the inflating pressure is being adjusted again to value according to table 1. The total


width is measured by sliding dimensional gauge on six locations, more or less uniformly distributed on the tyre circumference. At measurement of total tyre width , the protrusions caused by tyre descriptions, decorations, or eventual protecting strip above the bead are also considered. At measurement of tyre profile width, the smallest linear distance between outer sidewall edges of an inflated tyre is being determined, whereby the protrusions caused by tyre descriptions, decorations, or eventual protecting strip above the bead are not being considered. As the resulting value, the biggest of all six measurement values is considere, less frequently their arithmetical mean value. The circumference of a tyre is measured by a steel strip in the middle of the tyre crown, to the nearest 1 mm. The outer diameter of the tyre is calculated dividing the tyre circumference value by π (conventionally 3,1416) [4].

Table 1. Assignment of inflation pressure for the measurement of tyre dimensions according to EEC directive Nr. 30.

Cross-ply tyres Speed Index Symbol Conventional number of

carcass plies (PR) L, M, N P, Q, R, S T, U, H, V 4 170 kPa 200 kPa — 6 210 kPa 240 kPa 260 kPa 8 250 kPa 280 kPa 300 kPa Bias-belted tyres Standard tyres: 170 kPa Radial tyres

Standard Reinforced Spare tyres for temporary use (type T)

180 kPa 230 kPa 420 kPa Total tyre width is evaluated depending on tyre design and profile ratio. Tyres with so-called normal profile (profile number 82, which is not mentioned on the sidewall in the tyre size designation) have their profile width values listed together with the measuring rim width in a chart. For tyres of metric series with designated profile ratio, the profile width is calculated according to following relation:

)AA(KSS 11 −+= , (1)

where S – calculated profile width, mm,

S1 – nominal width designated on the tyre sidewall, mm,

K – which value is 0,4 in case of standard tyres ; in case of non-standard (asymmetrical) fit of tyres on rim, as for example various types of emergency tyres (so-called run-flat tyres) it has the value 0,6,

A – measuring rim width expressed in mm,


A1 – width of so-called theoretical rim, i.e. such a rim, on which a tyre, inflated to prescribed pressure, has its width equal to nominal width. The width of theoretical rim is calculated as xSA 11 = , where x is so-called x-factor equal to 0,70 in case of tyres with profile ratio equal or higher than 50 and 0,85 in case of tyres with profile ratio from 20 to 45 [6].

The total width of tyre, as measured with a sliding gauge by the a.m. method can be smaller than the calculated profile width. This value of calculated profile width can not be surpassed by as much as 4 % in case of tyres with radial and as much as 6 % in case of cross-ply and bias-belted design. Apart from that, if the tyre is equipped with special protecting ribs or bead strips, these values can be surpassed, increased by these tolerances, by further 8 mm. For tyres, identified from the viewpoint of tyre mount on the rim, with the symbol „A“, the total tyre width in the lower sidewall part can equal maximally the nominal rim width, on which the tyre has to be mounted according to the manufacturer, increased by 20 mm. The evaluation method of the outer tyre diameter is simiilar as with the total tyre width. The limit values of outer diameter Dmin and Dmax can be calculated from equations:

a.H2dDmin += (2) bHdD .2max +=

These equations take into account the geometrical arrangement of the tyre, where the total diameter is given by the sum of rim diameter and tyre profile height. The tyre profile )(5,0 dDH −= height H is given as in case of tyres with normal profile, where the values D and d are for each dimension published in relevant regulations [4]. For tyres with a profile ratio 80 and less, the profile height is calculated with the help of profile ratio from the nominal tyre width as

, where S01,0..1 aRSH = 1 is the nominal tyre width and Ra is the nominal profile ratio. The values of factors „a“ and „b“, dependent on tyre design, are listed in the table 2:

Chart 2 Factor values „a“ and „b“, needed for the calculation of limit values for the outer tyre diameter

Tyre design Factor a Factor b Radial 1,04 Bias-belted 1,08 Cross-ply

0,97 1,08

On M+S tyres, the limit value Dmax is being increased by further 1 % for the reason of higher tread blocks .

The principle of the method of static radial strength determination is the deformation measurement of inflated tyre, dependent on the tyre radial load


magnitude [7]. The test is carried out on a test device consisting of a supproting part with a a shaft, bearing the tested inflated tyre. Another substantial part is the measuring table with a supporting flat plate, parallel with the tyre rotation axis. During the test, radial load and radial deformation are recorded. The inflating pressure usually corresponds to the maximum tyre load, but can be selected due to the purpose of measurement. Prior to the tyre load itself, the slack circumference of the tyre is measured with a steel strip. This serves for the calculation of static radius. Afterwards the tyre is pressed onto the measuring table with a velocity of 0,8 to 2,5 mm.s-1. The tyre deformation is recorded beginning from the moment of the tyre contact with the measuring table up to the radial load corresponding 150% of the maximum tyre load. It is measured at least on four different locations of the tyre, evenly distributed on the tyre circumference. The inflation pressure is not adjusted during the test. Arithmetical mean value is calculated from the measured dependencies of radial deformation at load corresponding to 75, 100 a 125 % of maximum permissible tyre load. Static radial strength of the tyre Cr in kN.mm-1 is calculated according to the equation:

75125

75125

rr

rrr SS

FFC−−

= (3)

where: Fr125 – radial load corresponding to 125 % of maximum tyre load in kN, Fr75 – radial load corresponding to 75 % of maximum tyre load in kN, Sr125 – arithmetical mean of radial tyre deformation values at radial load corresponding to 125 % of the maximum tyre load in mm, Sr75 – arithmetical mean of radial tyre deformation values at radial load corresponding to 75 % of the maximum tyre load in mm. Statictyre diameter rs is calculated as follows:

rs Slr −=π20 (4)

where: l0 – slack tyre circumference, mm,

Sr – arithmetical mean of radial tyre deformation values at radial load corresponding to maximum permissible tyre load, mm.

Static radial strength of the tyre is a very important property, because it deals with deformation properties of the tyre , which happen under real exploatation conditions of the tyre as a part of vehicle chassis assembly. Apart from static radial strength, static lateral, circumferential and torsional strength can be measured, by which detailed information about the tyre strength in all directions can be obtained.

Static tyre footprint is evaluated from the footprint, prepared on the same load equipment, where the static radial strength is being measured. The tyre, consisting


of a casing mounted on prescribed rim, eventually together with tube, is inflated to the prescribed pressure and is pressed radially on hard paper (drawing paper) on the measuring table. The tread surface of the tyre, which comes into contact with the paper and which forms the tyre footprint is coated with stamp ink, or eventually with ink, prior to the pressing. Duration of the contact depends on the given test method. The tyre can be unloaded immediately after achievment of the maximum set load, or can be maintained under this load under specified period of time (30 to 120 seconds) and is unloaded thenafter. The tyre footprint is scanned after drying and the footprint is evaluated, as well as solidity ratio, i.e. the ratio of contact surface to total footprint surface.

Distribution of contact pressure in the tyre footprint (so-called nominal pressure) is not uniform and sometimes it can be seen from the ink imprint of the tyre as surfaces with various intensity of colouring. Exact quantitative evaluation of such imprint is practically not possible and therefore other methods of contact pressure measurement in the tyre footprint have been developped. Modern methods allow the measurement of nominal pressure distribution in the tyre footprint continuously on the whole footprint by electronical or optical manner. By electronic method, the tyre is being pressed on special plate consisting of contact pressure sensosrs, which record the pressure on the entire footprint. This method is favourable, because it requires no material for consumption. An advantage lies also in the possibility of contact pressure monitoring on individual measuring spots in dependene from time. Disadvantage lies in the high sensor plate price and bigger pitch of scanning spots like with optical methods. Optical methods are based on the use of special pressure-sensitive foil, which is inserted between the fixed support and the tyre. There are two types of pressure-sensitive foils, so-called double-sheet type and single-sheet type. Double-sheet type is composed of a so-called A-foil and C-foil. A-foil is composed from a PET-carrier with microgranulated colour producing material. C-foil is also composed from a PET-carrier coated with a developing material. The rough surfaces of both foils are applied on each other before the measurement and inserted into the location of pressure measurement. When pressure is applied, microgranules are disturbed and the colour producing material reacts with the developing material under colour formation. Microgranules are developped in such a way, that they react in dependence on pressure level and the colour thickness corresponds to this pressure level. After exposure to the defined load, the foils are evaluated with help of reading instruments, or with help of scanner equipped with a special software, which enables a complete analysis of the footprint. The scanned image is being converted into a colour projection, in which individual intervals of contact pressure have assigned certain colour projection (Picture 1). The disadvantage of this method is, that the foil records only the maximal applied pressure, evaluated after its unloading and therefore the monitoring of contact pressure changes in course of time is not possible, as well as online values recording.


Picture 1 Distribution of contact pressure in tyre footprint

Tyre strength is verified by mandrel puncture in the tyre crown or by a water pressure test. During the mandrel puncture test a rounded mandrel with a prescribed shape and size is being indented into the centre of casing crown of an inflated and fixed tyre. The purpose of the test is the assesment of strength of used reinforcing materials. The device used for this measurement must fulfill following conditions: − to be capable of applying a mandrel indentation force, which is at least 300 % of

the load corresponding to maximum premissible tyre load, − to allow the repeating of the test at least on five locations of the tyre

circumference, − during the fastening of the tyre in the testing device, the tyre rotational axis

must be perpendicular to the mandrel movement direction, − must be equipped with a mandrel of cylindrical shape, manufactured from steel

with a radius of 19,05 mm, terminated with a spherical cap with radius identical to the cylinder radius, the cap surface must be polished,

− the length and stroke of the mandrel must be at least 120 % of tyre profile height,

− indentation velocity of the mandrel must be at least 0,85 mm.s-1, − must allow the recording of the development course of the loading force and

deformation until the moment of tyre puncture.

Prior to the test, according to the regulation of FMVSS Nr. 109 , the tyre is being inflated to a value set in the table 3 [8].


Table 3 Tyre inflating pressure for tyre strength test and bead resistance to rim skid according to the Regulation FMVSS Nr.109 in dependence from the maximum permissible tyre inflation pressure

Maximum permissible tyre inflating pressure in PSI

Maximum permissible tyre inflating pressure in kPa

Maximum permissible tyre inflating pressure in kPa (1)

32 36 40 60 240

280

300

340

350 290 330 350 390

Inflating pressure in kPa

165 193 221 359 180

220

180

220

180 230 270 230 270

(1) Only for CT casings (tyres with reversed bead) Test is being carried out at a surrounding temperature of (25±5) °C. The tyre is being fastened on the measuring device shaft and measuring locations on the sidewall are being numbered. The mandrel must be located closest possible to the tyre alignment plane so that the test location is in the middle of the tread pattern (Picture 2). The measurement is carried out at least in five locations roughly evenly distributed on the tyre circumference. The criterion for the test end is either the tyre puncture, the reaching of required value of deformation work (puncture energy), or contact of inner tyre rubber with the rim. The result of the test is puncture energy, calculated as mean value of individual measurements. For individual measurements it is calculated as follows:

310.2.PFW = (5)

where: W – puncture energy under mandrel tests, in J, F – force, N,

P – mandrel intrusion, mm. Minimum puncture energy for passenger car tyres measured by this method is tabelated in the Regulation FMVSS Nr. 109 in dependence from the tyre width and in case of cross-ply tyres also dependent on used reinforcing material. The values of minimum puncture energy for radial tyres are listed in Table 4, and for cross-ply tyres in Tables 5 and 6:


Table 4 Minimum puncture energies of radial tyres for passenger cars, J Maximum permissible tyre inflating pressure in PSI units

in kPa units

in kPa units (1)Nominal width

32 36 40 24 28 30 34 35 290 330 350 390 Less than 160 220 33 44 22 44 22 44 22 220 441 220 441 160 mm or 294 44 58 29 58 29 58 29 294 588 294 588

(1) only for CT -tyres (tyres with reversed bead)

Table 5 Minimum puncture energies for passenger car tyres with cross-ply and 6 inch profile width, J

Maximum permissible inflating pressure Cord material 32 PSI 36 PSI 40 PSI 240

kPa 280 kPa

300 kPa

340 kPa

Viscose (rayon) 186 291 373 186 373 186 373

Polyamide (nylon) or polyester

294 441 588 294 588 294 588

Table 6 Minimum puncture energies of passenger car tyres with cross-ply design and with a profile width less than 6 inches, J

Maximum permissible inflating pressure Cord material 32 PSI 36 PSI 40 PSI 240

kPa 280 kPa

300 kPa

340 kPa

Viscose (rayon) 113 212 283 113 283 113 283

Polyamide (nylon) or polyester

220 331 441 220 441 220 441


Picture 2 Test arrangement of tyre carcass strength and mandrel puncture

measurement

The second test for tyre strength determination is the test by water pressure. This method lies in the strength determination of a tyre fitted on a rim, where it is stressed by the pressure of water filled into the tyre. At the test tyres are fitted on special reinforced disc wheels or segments with fitting surface in the shape of a rim. In case of need a tube can be used also for a tubeless tyre. Tyre is fastened on a stand, vacuum up to 10 kPa is being produced inside of it and thenafter water is being filled inside. In case of tubeless tyres, the air can be expulsed through a syringe needle equipped with a valve under gradual filling of the tyre with water. Pressure in the tyre is built-up permanently, until the tyre breakdown. During the test the relation between inner tyre volume and volume of the filling medium can be recorded. The result of the test is the character, location and the extent of tyre breakdown, pressure, and eventually also the inner tyre volume in the moment of breakdown. A part of the result can be the inner tyre volume under specified pressure, which is obtained from assessed relation volume-pressure.

Among the measurements of the tubeless tyres safety determination belongs also determination of bead resistance to rim skid. The proceeding of the test is set in the Regulation FMVSS Nr. 109 [8]. The tyre bead resistance to rim skid is reaction against acting lateral force and is characterized by a limit force composed of friction, pressure and adhesion influences, which indicates the lowest loading force at which a bead skid (slip) on th eseating surface of rim occurs with a following air leakage from the tyre. Tyre bead resistance ro rim skid due to excessive lateral forces is induced in laboratory by gradual pressing of the bead with a loading force, acting by means of a shaped segment. During the test, the size of the loading force


leading to bead skid from rim of the test disc wheel is being measured. The dvice for bead rim skid must fulfill these conditions: • it must be capable of exerting a force , which stresses the tyre at least to 300 %

of the load corresponding to maximum tyre load, • it must fasten the tyre in such a manner , that its rotational axis and segment

travel direction are paralel, • it must allow the repetition of the test at least on four different locations on the

tyre circumference.

The principle of the device is given in the Picture 3. The shaped segment serving for pressing of the sidewall is manufactured from aluminium, with a determined surface roughness. The tyres are being tested under inflation according to table 3.

Figure 3 The arrangement of the bead rim skid testing device

(dimensions in mm) Segment travel velocity is 0,85 mm.s-1. The test is carried out at a surrounding temperature of (25±5) °C. The tyre is mounted without tube and without the use of assembly greases or emulsions on a clean test rim. Prior to the test the beads of the tyre are washed with water and dried. In case of need the rim can be cleaned in the same way. Tyre is inflated to a pressure of cca 300 kPa in order to obtain the proper seating of both tyre beads. The inflating pressure is afterwards adjusted to the value listed in Table 3. Tyre is mounted on a jig in horizontal position and the position of shaped segment is adjusted according to the distance A, listed in the Table 7. With the help of a loading device the loading force is increased until the bead skid from the rim and abrupt air leakage (Picture 4). The test is repeated at least on four locations roughly uniformly distributed on the tyre circumference under simultaneous recording of force size at which the bead skid occurs. The relation between loading force and deformation can be also recorded during the measurement.


Table 7 Distance A of the pressing segment from the vertical tyre axis Dis[mm

tance A dependent on tyres ] Code of the

nominal rim diameter Standard For temporary

use, type T 19 330 305 18 318 290 17 305 269 16 292 251 15 279 239 14 267 226 13 254 213 12 241 11 229 10 216 320 216 340 229 345 235 365 248 370 254 390 279 415 292

Picture 4 Detail of a shaped segment acting on the tyre sidewall during test of a tubeless tyre bead resistance against rim skid

For the tread evaluation of new and retreaded tyres, as well as tyre wear resistance and for the evaluation of the tyre wear, serves the measurement of read groove depth and of the tread wear indicators height. For the measurement of read


groove depth and of the tread wear indicators height depth gauges are used, with the support base surface of at least 450 mm2 and scale increments 0,1 mm, eventually even more precise. Prior to the measurement all impurities, which could influence the measurement, are removed from the tyre tread. Some regulations and standards require measurement of read groove depth, resp. of the tread wear indicators on a tyre inflated at least to 50 % of the maximum permissible inflating value, others do not require any specific conditions during measurement (EEC Regulations Nr. 30, Directive 92/23/EEC and FMVSS Nr. 109) [4, 5, 8]. The tip of the depth gauge must be perpendicular to the tread surface during measurement. The depth of tread grooves is determined at least on six locations uniformly distributed round the tyre circumference. It is measured in the proximity of tread wear indicators in the main tread grooves. Main tread grooves are in the sense of the definition of EEC Regulation Nr.30 the wide grooves located in the central part of tyre tread, which comprise inside tread wear indicators. In case of tyre measurement under irregular wear of the tread surface, the measurement is carried out in the location with smallest tread groove depth. The height of wear indicators is determined in such a manner, that the tread groove depth in immediate proximity of wear indicators at its toe is measured first and afterwards the tread groove depth direct on the indicator is being measured. The difference of these values provides the wear indicator height. The wear indicator height in some regulations is given in a following way: − − ( )mm for passenger car tyres according to EEC Regulation Nr. 30 and

Directive 92/23/EHS,

60,000,060,1 +

−

− 1,60 mm for passenger car tyres according to regulation FMVSS Nr. 109. The result of the measurement is the arithmetical mean of individual tread groove depth and tread indicators height measurements on new and retreaded tyres. At tread groove depth of tyres in operation, the result is the smallest value of tread groove depth. This divverence in evaluation is caused by the fact, that the legislation in individual countries prescribes the minimum residual tyre tread groove depth of vehicles in the public traffic and this is connected with minimum safety requirements on vehicle handling, especially on wet surface. With the measurement of seating forces in the tyre bead the radial stiffness of the tyre bead is determined. Radial stiffness of the bead is a parameter, which is in significant relation with the safety and fitting capabilities of tubeless tyres on the rim of a disc wheel. Low radial stiffness can bear as a consequence bead skid from the rim at application of lateral forces on the tyre during various driving manoeuvres in transversal inclination. High radial stiffness has in contrary as a consequence a difficult tyre fit on rims equipped with safety humps , where a damaging of the bead wires and consequent tyre burst can occur during the overrun of the safety hump. Therefore an optimum radial bead stiffness of the tyre must be secured, which lies between so-called safety limit and limit of fitting capability.


The test is carried out on a device equipped with jaws, consisting of individual segments, which are expanded by means of hydraulic cylinders. Each segment pair, located opposite to each other has a dynamometer, which is measuring the segment force acting on thetyre bead and vice-versa (figure 5). Apart of that, the device must be equipped with a segment position sensor compared to zero position, corresponding to nominal rim diameter. The test is normalized in the methodology of German Rubber Industry association WdK 116 [9]. The tyre can be tested at least after 72 hours after its production. Prior to the measurement the tyre must be stored and conditioned at least fo 24 hours under room temperature (23±5) °C in vertical position. Jaws, dependent on tyre bead parameter, are mounted on the testing machine. Nominal jaws diameter Dnom, corresponding to the so-called zero position is listed in the Table 8. This position is checked with the help of so-called calibrating rings, having precise shape and inner diameter and are laid on jaws in the same manner like the tyre bead. The streading velocity of the jaws is set to 6 mm.min-1. Talc is applied on both tyre beads. Tyre is laid with its serial side (side comprising DOT, or serial number) on jaws, which are in their initial position completely drawn back to the minimum diameter, allowing tyre bead fitting. After resetting of dynamometers, the jaws are spread to a diameter Dnom+0,8 mm, securing tyre bead prestressing before measurement, after which the jaws travel back into their initial position. The value of radial bead stiffness is determined by repeated spreading of the jaws at tyre bead deformation values of Dnom-0,29 mm for the safety limit and Dnom+0,38 mm for the fitting limit. Afterwards the bead on the opposite tyre side is measured in the same manner. So far a repeated measurement must be carried out on the same tyre, this can be done after 48 or 72 hours respectively, due to the tyre relaxation.

Table 8 Nominal jaw diameter serving as zero position for the measuring of seating forces in the tyre bead, according to the Standard WdK 116

Nominal rim diameter code Nominal jaws diameter Dnom

[mm] 10 251,87 12 302,67 13 328,07 14 353,47 15 378,87 16 404,27 17 435,22 18 460,62 19 486,02 20 511,42 21 536,82 22 562,22


Picture 5 Testing machine for measuring of seating forces in the tyre bead. View of

the jaws, consisting of individual segments.

Airtightness of tubeless tyres is evaluated by determination o fair pressure drop by two methods: Method A – measuring of airtightness by tyre immersion into water, Method B – direct measuring o fair losses by means of a manometer.

Prior to the test the tyre and rim undergo a visual check. Tyre beads must not be damaged. Testing rim must be clean, not injured and must have a relevant surface ptotection treatment. Tyre beads and rim shoulders are painted with fitting grease and the tyre is fitted in way, that no tyre bead as well as no tubeless valve damage can occur. Under method A the tyre is inflated to a value corrsponding to maximum permissible load. Afterwards i tis immersed into water. After immersion no bubbles can escape into water through the tyre walls , bead area or the valve. So far as air is leaking through the valve, spring insert, or the entire valve are exchanged for new and test is repeated. During test under method B the temperature in the test room is kept during the conditioning as well as during the measuring itself in a span of (25±5) °C. Prior to the measuring the tyre id conditioned for 24 hours under above mentioned inflating pressure. Afterwards the inflating pressure is adjusted again to this value and tyre is stored for a period of 28 days. After this period the tyre air pressure loss is measured by a manometer of precision class 0,6 maximum. The value of pressure drop shall not be smaller than 5kPa. In case, that the initial and final temperature in the test room differ from each other, a calculation of pressure loss ∆p in kPa to a reference temperature 25 °C is to be carried out according to following equation:


( ) ⎟⎟⎠

⎞⎜⎜⎝

⎛++

−++

+=∆273100

273100.273

2

2

1

1

tp

tptp ref (6)

where: tref – reference temperature 25°C, t1 – air temperature in the tyre at the beginning of the test, °C, t2 – air temperature in the tyre at test end, °C, p1 – air pressure in the tyre at the beginning of the test , kPa, p2 – air pressure in the tyre at test end, kPa. 2.2 High speed tests By means of dynamic tests the durability characteristics of tyres are checked by the rolling on outer or inner surface of steel testing drum under fulfilment of prescribed conditions of speed, radial load, inflating, temperature of the environment, camber, directional deviation, acceleration, braking and duration period of individual steps. Duration of dynamic tests is set either by prescribed time limit, or the discontinuity of tyre structure and integrity. Speed tests of passenger car tyres are focused on checking of tyre durability under high speeds, close to, or exceeding the speed rating of the tyre. Their duration is short in comparison with fatigue tests. Typical duration of high speed tests is cca 60 to 150 minutes, dependent on test type speed rating, and tyre design. Typical tyre damage at speed tests is the tearing of rubber from the tread, tearing of tread blocks and separation of reinforcing strips (breakers). At high speed tests, a radial tyre load of cca from 68 % to 85 % of maximum permissible tyre load is used. This level of load allows the use of steel disc wheels, or eventually disc wheels manufactured from light alloys, instead of special reinforced ones. Single-position or double-position testing machines are used for high speed tests. The main part of this machine is a steel drum with a smooth surface. Drums with a diameter of 2 000 mm ± 1 % or 1 707 mm ± 1 % are used for passenger car tyres testing. These are prescribed in individual international and national standards and regulations. In the past, drums with diameters of 1 592 mm, or 1 512 mm respectively, have been used. The drum is fitted in a supporting frame, which must secure the needed stiffness of the whole machine. The drum is driven by an electric motor with the help of driving belts. Further, the machine is equipped with one or two test positions, consisting of a rotary head, on which the tested tyre is fitted. Rotary head is seated on bearings with low friction. Usually it has a disc brake for simplifying of disc wheel fit and for its braking after finishing of the test. Testing position is also equipped with a system of end position sensors, securing the withdrawing of the tyre at separation in the tyre casing, or at decreasing of the dynamic radius of spinning tyre due to inflating pressure drop (Picture 6). Testing position can be equipped with a mechanical device for camber adjustment, or directional deviation adjustment. The tyre is pressed preumatically or hydraulically on the test machine drum. At present, the use of mechanical loading with the help of


weights or endless screw is abandoned. The testing machine can be also equipped with a system of regulation, measuring, or eventually monitoring of test parameters like drum speed, radial load, temperature of the environment, dynamic tyre radius, camber angle, directional deviation angle, inflating pressure or tyre temperature. For high speed tests, higher requirements are put on regulation precision and test parameter measuring, than for fatigue tests. Basic and most widely used high speed test for passenger car tyres is test in accordance with EEC Regulation Nr. 30 [4]. By this test the basic safety characteristics of tyres are verified. The principle of this speed test, as well as of all speed tests is gradual drum speed increase aunder a constant radial load in prescribed time steps until the prescribed time duration is reached, or a tyre destruction takes place. New or retreaded tyre is fit on a prescribed testing rim. The tyre is inflated to relevant pressure listed in Table 9:

Table 9 Tyre inflating pressure in kPa for tests according to methodology EEC Nr. 30

Tyres with cross-ply design (bias – ply)

Tyres with radial design

Conventional ply

Bias – belted tyres

Speed category rating

4 6 8 Normal Reinfor

ced Normal L, M, N 230 270 300 240 280 — P, Q, R, 280 300 330 260 300 260 T, U, H 280 320 350 280 320 280 V 300 340 370 300 340 — W 320 360 — Y 320 360 — Tyres for temporary use ,type T: 420

The tyre is conditioned to the room temperature (25±5)°C at least for 3 hours. Afterwards the tyre pressure is adjusted again to above mentioned value. Tyre is mounted on test shaft and pressed to the outer smooth surface of the testing drum having a diameter of 1 707 mm ± 1 % or 2 000 mm ± 1 %. The tyre is exposed to radial load which equals to a certain multiplier of load corresponding to tyre load index according to Table 10:


Table 10 Multipliers for the maximum tyre load for the determination of radial load according to EEC Regulation Nr.30

Speed category symbol Multiplier of the maximum tyre load for determination of radial load

≤ H 0,8 V 0,728 W, Y 0,68

The pressure in the tyre is not adjusted during the test and a constant radial load is maintained. The temperature in the test room during the test is kept in a span of (25±5) °C or at a higher temperature, if the manufacturer agrees. During the test, following conditions are continuously fulfilled: 1. Tyres of speed category 120 km/h (Symbol L) up to 270 km/h (Symbol W):

Time duration of a continuous transition from zero speed to the initial speed is 10 minutes. The tyre is then spinning at this speed for another 10 minutes. Afterwards, the speed is increased by 10 km/h in steps after every 10 minutes, until the maximum test speed is not reached. This speed step lasts for 20 minutes and then the tyre is stopped.

2. Tyres of speed category 300 km/h (Symbol Y): Time duration of a continuous transition from zero speed to the initial speed is 10 minutes. The tyre is then spinning at this speed for another 20 minutes. Afterwards, the speed is increased by 10 km/h in steps after every 10 minutes, until the maximum test speed is not reached. This speed step lasts for 10 minutes and then the tyre is stopped.

Initial test speed is a speed corresponding to the speed category of the tyre, reduced by 40 km/h in case of a test drum with a diameter of 1 707 mm ± 1 % or by 30 km/h in case of a test drum with a diameter of 2 000 mm ± 1 %. Maximum test speed is a speed corresponding to the speed category of the tyre, reduced by 10 km/h in case of a test drum with a diameter of 1 707 mm ± 1 % or corresponding to the tyre speed category in case of a test drum diameter of 2 000 m ± 1 %. With tests of tyres suitable for speed above 300 km/h, one additional test is carried out according to EEC Regulation Nr. 30. A radial load, corresponding to 80 % of maximum load assigned to maximum speed set by the tyre manufacturer is used. The starting time from zero to maximum speed as set by the manufacturer is 10 minutes. Then the tyre is being tested at this testing speed for 5 minutes. The test is carried out continuously. Tyre submitted to these tests shall not show any tread, ply or cord separations, as well as rubber tearings or cord breakage.


Figure 6 Tyre during a high speed test

Other speed tests of passenger car tyres are a test modification of EEC Nr.30 methodology, or have other proceeding. Because the tyre test according to EEC Regulation Nr.30 serves only to verify the basic safety of new tyres, different modifications of method and testing conditions are used, under which the test runs continuously until dhe tyre destruction. Test load can be used for the evaluation of basic tyre safety, which corresponds to the following higher load index than the tested tyre.

Speed tests of this kind are usually designed in such a manner, that the tyre must withstand the entire speed step, corresponding to tyre speed category without any damages. Another type of speed tests are high speed tests under zero-free tyre camber. This camber is usually 4° and corresponds to the angle between the dividing plane of the tyre and test drum. Because the typical tyre injury at high speed is the edge separation of reinforcing strips, or the tearing of tread blocks, a test under camber causes increased tyre stress on the side turned towards the test drum. Apart from high speed tests, during which the testing speeds are close to the tyre speed category, some regulations, like FMVSS Nr.109, FMVSS Nr.139 and Gulf Standard 53/1986 contain speed tests at a speed independent from the tyre speed category [8, 10, 14]. In some territories of the world the tyres are not marked by the


operation description consisting of speed category symbol in combination with load index, but only by maximum tyre load. Test proceedings according to methodology FMVSS DOT 109 and Gulf Standard Nr. 53/1986 are following. First, the tyre is run-in for 2 hours on a drum with a diameter of 1 707 mm at a speed of 80 km/h and radial load, representing 88 % of maximum tyre load designated on the tyre sidewall . Tyre is then cooled down at a surrounding temperature of (38 ± 3) °C and the inflating pressure is adjusted again to the appropriate value specified in Table 11:

Table 11 Inflating pressure for high speed test according to regulations FMVSS Nr. 109 and Gulf Standard Nr. 53/1986

Maximum tyre inflating pressure in PSI units in kPa units in kPa units(1)

32 36 40 60 240 280 300 340 290 330 350 390 Tyre inflating pressure for the test in kPa

207 234 262 400 220 260 220 260 270 310 270 330

(1) Only for CT tyres (tyres with reversed bead)

The test is carried out under a free increase of inflating pressure at a speed of 120 km/h for 30 minutes, 128 km/h also for 30 minutes and 136 km/h during another 30 minutes. Immediately after the tyre test, its inflating pressure is measured. Tyre is left to cool down for 1 hour. Then the air is deflated out of the tyre, tyre is dismantled from the test rim and is checked if it corresponds to the set requirements. The Standard FMVSS Nr. 139 [14] has been introduced in 2003, valid for new radial tyres for light vehicles (vehicles with a total weight of 10 000 pounds/4 536 kgs and less). The proceeding is similar as with standards FMVSS Nr. 109 and Gulf Standard Nr. 53/1986 with the difference, that the test is carried out under harder conditions. Inflating pressure is selected according to Table 12.


Table 12 Inflating pressure for high speed test according to Regulation FMVSS Nr. 139

Tyre type Inflating pressure[ kPa ]

P-metric: Standard 220 Reinforced 260

LT: Load Range C 320 Load Range D 410 Load Range E 500

CT: Standard 270 Reinforced 310

Run-in and subsequent cooling of the tyre is carried out in the same way as in previous case, under a radial load corresponding to 85 % of the maximum tyre load designated on the sidewall. Test is run continuously and without any interruption for 90 minutes and consists of three subsequent steps with a duration of 30 minutes at speeds of 140, 150 and 160 km/h. Radial load during test is 85 % of the maximum tyre load, the inflation is left freely increasing. Immediately after the tyre test, its inflating pressure is measured. The tyre is left to cool down for 1 hour. Then the air is deflated out of the tyre, the tyre is dismantled from the test rim and checked whether it corresponds to the set requirements. 2.3 Fatigue tests for passenger car tyres

Tyre fatigue tests are dynamic tests serving to establish the tyre durability. The duration of these tests is much longer than with speed tests. Typical time span of fatigue tests for passenger car tyres is 80 to 250 hours. Fatigue tests are usually carried out at a constant speed. Other test parameters, like radial load and inflation can be constant during the tests, but can also be modified. Test conditions are chosen according to the fact, on which tyre part the test is focused. No universal fatigue test exists. Fatigue tests are usually focused on tyre beads and carcass area, mainly reinforcing strips. At bead fatigue tests a lower speed and higher load is used. These test conditions provide lower tehermal and mechanical stress of the tyre in the crown area and a higher load in tyre bead area. During bead fatigue tests a speed of 60 km/h is normally used, radial load is chosen dependent on test methodology and tyre aspect ratio from 1,5 to double maximum permitted tyre load. Inflating pressure depends on tyre methodology, but usually it is 250 kPa. Fatigue tests are generally executed at temperatures (25±5)°C, but temperatures of test rooms (38±3)°C are used more and more often.


At carcass fatigue tests a speed of 80 to 120 km/h i sused and a radial load representing 1,2 to 1,6-times of the maximum premissible tyre load. Tyre is usually over-inflated in this test. These test conditions secure tyre injuries typical for this kind of test: tread separations, separations of reinforcing strips (breakers), tearing of tread blocks, cracks between tread blocks in tread grooves, or broken cords, respectively. After the execution of speed, as well as fatigue tests, the tyres are analyzed and reasons of tyre injury are searched for. For the reason of analysis the tyre is usually cut in radial direction in two locations in such a manner, that the damage of the tyre can be evaluated from the collected section. Optical microscopy can be also used in this operation.

2.4 Dynamic measurements of passenger car tyres Under laboratory dynamic measurements we understand such tests, during which the tyre is rolling over the surface of a drum or running flat steel belt on test machine. During the rolling of the tyre, different tyre properties and characteristics are being measured, which are manifested in the exploatation of the tyre, i.e. during its use on motor vehicle or trailer. These properties influence actively the behaviour of the chassis assembly and, consequently, of the entire vehicle. Measurement of rolling resistance, dynamic directional characteristics, measuring of tyre properties at high speed uniformity test, measurement of the tyre noise and measurement of temperature distribution on tyre surface belong among important dynamic measurements. An important attribute of dynamic measurements is, that no damage occurs to the tyre and the tyre can be later submitted to further measurements or tests.

2.4.1 Measurement of tyre rolling resistance Under rolling resistance we understand an energy loss, or labour consumed for a specified unit of distance during the rolling of the tyre straight forward in a position perpendicular to the outer drum surface under stable conditions. During the rolling resistance measurement, it is inevitable to measure small forces under presence of far bigger forces, which requires the use of equipment with appropriate accuracy. The unit of rolling resistance Fr in the SI system og units is Nm.m-1, which equals to the unit of trailing force, N. For practical reasons the rolling resistance factor Cr is used for the presentation of rolling resistance, calculated as the ratio of rolling resistance in N and radial tyre load in N. Tyre rolling resistance is measured with the help of following methods, and with every method the measured results are transferred to rolling resistance force on the dividing line tyre/drum:

• Force method, • Torque method, • Performance method, • Decceleration method.


Rolling resistance is established on test machines having a cylinder drum with a diameter from 1 500 to 3 000 mm. The surface of the drum is made of smooth steel, which has to be kept clean. A rough surface with a specified roughness can also be used. The measurement of rolling resistance requires accuracy of used rims, zero camber and directional deviation angle, radial load and drum speed control and measurement of tyre inflating pressure. The rolling resistance is assessed according to various methodologies of tyre and car manufacturers, even when the International Standard ISO 8767 [11] is worldwide available. The passenger car tyre rolling resistance is usually measured at three different speeds, 50, 80 a 120 km/h, where the arithmetical mean from the rolling resistance coefficient values at these speeds is taken as a result. But the rolling resistance can be measured at higher speeds as well. Afact is valid, that with increasung speed the rolling resistance increases, and thereby the rolling resistance coefficient(factor) as well. To provide a repeatability of measurements, the tyre must be run in on the measuring drum at least for 1 hour at a speed of at least 80 km.h-1 ,under a load 80 % of maximum tyre load and with inflation pressure corresponding to this load. After the run in the tyre is allowed to cool down for at least 12 hours at temperature of surrounding area. Prior to the test, the inflated tyre is conditioned in the test room temperature at least for 3 hours. Before the measurement start, the inflating pressure is adjusted to appropriate value, set by the test methodology. Then the tyre is being pressed on the drum with radial load, usually representing 80 % of the maximum tyre load. Afterwards the tyre is rolling at a constant speed until a stable rolling resistance value is achieved, which occurs at least after 20 minutes in the first step and at least 15 minutes in following speed steps. At the end of each speed step the rolling resistance value is measured by means of variables, depending on the kind of employed measurement method. In the following, two most utilized methods are described, the force and torque method.

2.4.2 Force method

During force method, the reaction force at the tyre shaft is measured, from which passive losses caused by shaft friction and aerodynamic tyre resistanse are subtracted. These passive losses are measured as the force on tyre shaft during tyre rolling, at a load close to 50 N, i.e. practically withour rolling resistance. The rolling resistance is then calculated as:

[ ])/(1)( RrFFF Lptr +−= (7) where: Fr – rolling resistance, N, Ft – force measured at tyre shaft, N, Fp – unwanted losses in N, measured at a radial load close to 50 N, rL – dynamic tyre radius in m – distance of tyre center line from the outer drum surface, R – test drum radius, m.


2.4.3 Torque method During the torque method, the driving torque M of the test drum with tyre at prescribed radial load is measured, as well as passive torque Mp under tyre radial load cca 50 N. The tyre rolling resistance is then calculated from the following equation:

R

MMF p

r

−= (8)

where: Fr – rolling resistance, N, M – measured torque value, in N.m, Mp – measured value of passive torque in N.m under radial load close to 50 N, R – test drum diameter, m. Rolling resistance coefficient Cr according to both methods, is calculated as:

FFC r

r = (9)

where: Fr – rolling resistance, N, F – radial load, N. So far the measurement is carried out at a temperature different from 25°C (but inside of the span between 20 to 30°C), it is inevitable to correct the values of tyre rolling resistance to the temperature of 25°C with the help of following equation:

[ ])25(125 −+= tKFF rr (10) where: Fr – measured value of rolling resistance at surrounding temperature t , in N,

t – surrounding temperature in °C, K – value equal to 0,01 for passenger car tyres. Results obtained from drums of various diameters differ from each other. Following correction can be used at their comparison:

0102 . rr FKF ≈

( )( )T

T

rRrRRRK

++

=1

221 / (11)

where: R1 – diameter of drum 1 , m, R2 – diameter of drum 2 , m, rT – nominal tyre radius , m, Fr01 – rolling resistance value, measured on drum 1, N, Fr02 – rolling resistance value, re-calculated for drum 2 , N.


2.4.4 Measurement of dynamic directional characteristics During the measurement of dynamic directional chracteristics, the lateral force and reversing torque of a tyre rolling at constant speed on the surface of test drum are established. From dependence of lateral force, or reversing force on the angle of directional deviation , the directional stiffness, or reversing torque stiffness, is established. Dynamic directional characteristics have big expressive value in relation to tyre behaviour on vehicle steering axles, where they determine the tyre behaviour in relation to transfer of lateral forces under given steering wheel wind and relating angle of directional deviation of the wheels. The lateral force and reversing torque are measured on so-called multiple-component heads ( two-component, five-component, six-component). These are special hubs, on which disc wheels are mounted comprising inside a set of tensometers or piezoelectric sensors, after an amplification on measuring amplifier, capable of measurement of individual force and torque components acting on tyre:

• Radial force, • Lateral force, • Circumferential force, • Reversing torque, • Pitch torque • Braking or eventually deriving torque.

During the assesment, regulated inflation is used, i.e. the inflating pressure is during the measurement maintained at a constant, preset value. The tyre is pressed on test drum and is kept rolling for a certain period of time at preset speed and radial load, zero camber angle and zero directional deviation, which is needed for its thermal stabilization. This time is at least 30 minutes. Directional deviation angle is then changed to preset value and a preset rate of directional deviation angle change the transition from given value into opposite side takes place, under simultaneous registering the dependence of lateral force size and reversing torque on the directional deviation angle. At a certain value of directional deviation angle, the tyre is not capable of bearing the lateral forces any more and tyre slipping on drum surface takes place, which is demonstrated by the decrease of absolute value of lateral force, as well as reversing torque together with increase of absolute value of directional deviation. Forces and torques which originate during tyre rolling are influenced by many factors: surface type and roughness, speed, temperature, water on the surface, dynamic properties of condition changes in tyre operation, driving and braking tyre torques, etc. The properties of forces and torques are in their substance independent on speed, as long the tyre is rolling without extensive slipping in the contact area with the surface. Dry surface test have shown, that speed influence is low in a wide extent of directional deviation angle. More methods can be used for the test. One of


four variable inputs (radial load, inflating pressure, directional deviation angle, camber) changes usually gradually or in a jump, with others remaining constant.

2.4.5 Measurements on high speed uniformity test High speed uniformity test is carried out additionally to common tyre uniformity test on testing machines, which are in their design similar to testing machines for high speed tests. The essential part of the machine is a drum, fitted and driven in a base frame, having a diameter of 1 500 to 3 000 mm, smooth surface and a tyre loading system. The frame has the property that it is sufficiently rigid and has own high frequences, causing that a resonance of the system is eliminated during high speed measurement of a rotating tyre. During high speed uniformity test similar properties are being measured, than on a common uniformity test. But as a consequence of high rolling speeds, centrifugal forces are acting on the tyre and the tyre behaviour is similar to behaviour under real conditions on the vehicle.

2.4.6 Measurement of temperature distribution Rotation of a loaded tyre and its continual deformation cause as their consequence heat generation, manifested by temperature increase of the tyre closed in the tyre and by tyre temperature increase. This tempereture rises continuously, until it reaches a balanced level under stabilized test conditions, where the heat formation rate is equal to heat passage from the tyre into environment. Temperature distribution in the tyre, like temperature values themselves depend on radial load, camber, speed of rotation and temperature of the environment. In the location where a separation or tyre integrity breach takes place, a local temperature increase occurs. The measurement of tyre temperature can therefore be during destruction tests utilized as an effective instrument for the disclosure of beginning tyre injuries. Continuous temperature measurement inside the tyre is very complicated, as resistance or voltage temperature sensors have to be built into the tyre body and such sensors act as foreign element in the casing and influence its behaviour. Contact-free optical measurement of surface temperature of the tyre is therefore utilized in the practice. Cameras, sensitive to infrared area of radiation are used here, capable of temperature measurement of rotating bodies. Afterwards there is no difficulty to process a signal of such camera in a computer, display the tyre surface and to assign a specific display colouring to each temperature interval (figure 7). The measurement of rotating tyre temperature has its significance apart from product quality control also in the verification of dynamic mathematical tyre model.


Figure 7 Record from tyre surface temperature measurement in a dynamic

condition

2.4.7 Measurement of tyre deformations Measurement of tyre deformation in a dynamic condition is also necessary for the purpose of research and development. This can be determined by optical methods based on triangulation principle, where the object displacements in a dynamic mode are being measured. Tyre is rolling on a rotating drum, loaded with a chosen load and inflated to a required inflating pressure. The testing machine generates an impulse for measurement start in dependence from the position of the tested tyre, because for the identification of individual measuring locations in a dynamic mode it is necessary, that their position remains always the same. For the identification of measuring locations it is also necessary to prepare a black-and-white pattern on the surface of measured object, in our case the tyre sidewall. After scanning of the measured object in various modes, the system finds corresponding measuring locations, based on black-and-white structure and calculates the displacements and deformations in regard to the reference mode, a static mode in this case. But in reality, the reference shots are executed at a low speed (10 km/h), for the securing of measurement synchronization by means of output signal from the test device. Other shots are carried out at given speeds due to needs in dependence from speed category of the tested tyre. The tyre sidewall is scanned. Displacements in all three orthogonal directions and deformation at a giben rolling speed are obtained as output parameters (figure 8). On the base of these information’s it is possible to compare indirectly sidewall stiffnesses in dynamic condition among individual optional solutions of the designed product, as well as to compare tyres manufactured by various manufacturers.


Figure 8 Record from measurement of displacements on the tyre sidewall in radial

(left) and axial (right) direction

3 Tyres for utility vehicles Tyres for utility vehicles comprise a group of tyres, designed mainly for vehicles of categories M2 and M3 (buses), vehicles of category N2 and N3 ( vehicles designed for load transportation) and vehicles of category O3 and O4 (braked trailers with a total weight more than 3,5 t) [1]. The character of the tests and measurements is the same as with tests of tyres designed for passenger cars. There is a certain difference with dynamic tests, where a bigger emphasis is laid on fatigue tests of the carcass and tyre bead, and a smaller emphasis on speed tests. This is corresponding with the philosophy, that the maximum designed speed of this vehicle category, given also by the obligatory use of speed restrictors, set from 80 do 100 km/h, dependent on vehicle category. Because the principle of tyre tests for truck tyres is similar to the test principle of passenger car tyres, only the differences to these methodologies will be mentined in the following. 3.1 Static measurements Tyres for utility vehicles and their trailers are submitted to following tests: − Tyre weight measurement, − Measurement of tyre crown thickness, − Measurement of tread hardness, − measurement of basic tyre outer dimensions, − measurement of static radial strength, − measurement and analysis of tyre footprint, − measurement of specific pressure distribution in the tyre footprint, − measurement of mandrel puncture strength in the tyre crown, − determination of tyre resistance to water pressure destruction, − measurement of bead resistance to rim skid of a tubeless tyre,


− measurement of the tread groove depth and the height of tread wear indicators, − measurements of tyre surface electric resistance . Weight, crown thickness and tyre trad hardness are measured in the same way as with passenger car tyres. The measurement of outside tyre dimensions is equivalent, with the difference, that as inflating presssure value for measurement, the inflating pressure corresponding to maximum tyre load is used, which is set by the tyre manufacturer, eventually by charts in poblications of tyre producers, associations. During assesment of tyre dimensions according to EEC Regulation Nr.54 [12] the same procceding i sused, as with evaluation of passenger car tyres in accordance with EEC Regulation Nr.30. The difference is in the coefficients (factors) „a“ and „b“, listed in the Table 13: Table 13 Coefficient values „a“ and „b“, needed for the calculation of outer tyre diameter limiting values

Design a b Radial nominal profile width less or equal to 305 mm

1,04

Radial nominal profile width bigger than 305 mm

1,02

Cross-ply (diagonal)nominal profile width less or equal to 305 mm

1,08

Cross-ply (diagonal)nominal profile width bigger than 305 mm

0,97

1,04

For M+S tyres , the limiting value Dmax is increased by further 1 %.

Tests of static radial, tyre footprint and contact pressure distribution in the tyre footprint are equivalent to tests for passenger car tyres. The test of tyre carcass strength and resistance strength to mandrel puncture in the tyre crown in accordance with the Regulation FMVSS Nr. 119 has a big importance for utility vehicle tyres [13]. By this test, the compliance with conditions for tyre strength marking Load Range is verified, which is important mainly for non-european countries, where the load index is not used as a part of the tyre description. The tyre is inflated to a pressure corresponding to tyre load in a twin-assembly, prior to the test. If the tyre description on the sidewall contains only the maximum load for a single assembly, the inflating pressure corresponding to maximum load in a single assembly is used. Mandrel with the same geometry, as described in the chapter 2.1 is used for the test, with a diameter depending on tested tyre. The mandrel diameter dependent on tyre size is listed in the Table 14:


Table 14 Mandrel diameters for utility car tyres and their trailers

Tyre type for Mandrel diameter [mm]

Light utility vehicles 19,05 Tyres for rims with a nominal tyre rim diameter 12 and less 19,05

Tyres different from those listed above: Tubeless:

With nominal rim diameter 17,5 and less 19,05 With nominal rim diamater bigger than 17,5:

− Load Range F or less 31,75 − Load Range higher than F 38,10

Tube type: − Load Range F or less 31,75 − Load Range higher than F 38,10

Minimum puncture energies corresponding to Load Range designation according to Regulation FMVSS Nr. 119 are listed in Table 15.

Table 15 Minimum puncture energies in J for utility car tyres in accordance with Load Range designation according to Regulation FMVSS Nr. 119

Tyre characteristics, due to nominal rim diameter code 12 and 17,5 viac ako 17,5

Load Range all tubeless T.type tubeless T.type tubeless A 68 226 — — — — B 136 294 — — — — C 203 362 769 576 — — D 271 514 893 735 — — E 339 576 1 413 972 — — F 407 644 1 786 1 413 — — G — 712 — — 2 283 1 695 H — 769 — — 2 600 2 091 J — — — — 2 826 2 204 L — — — — 3 052 — M — — — — 3 221 — N — — — — 3 391 —

Note: For the assesment of tyres with viscose cord, puncture energies with a size 60 % from listed values are used.

Test proceeding and its evaluation are identical with the proceeding in case of passenger car tyres. Measurement of tyre surface electric resistance has big importance for vehicles designed for transportation of flammable and explosive substances. It is very


important, that rubber, which has properties of insulators under normal conditions, is capable of transferring the accumulating electric charge from the motor vehicle or the trailer, preventing an electric discharge accompanied by a spark. Such a spark could cause a fire or explosion of the transported material. Surface electric resistance is measured on a clean and dry tyre. An ohmmeter for assesment of insulating resistances equipped with special electrodes and a range of 1010 Ω minimum i sused for the measurement. During measurement the compliance with the measuring conditions is important, especially relative humidity of surrounding air, which can influence to a big degree the electric resistance measurement, and which can not exceed the value 55 %. Electric surface resistance is assessed on five locations by means of special ring electrodes between the tread centre and a locationon the sidewall, where tyre contacts the rim. Other method is the surface electric resistance measurement directly on the vehicle. For this method, a conductive plate, insulated from the earth, is positioned under the wheel, and electric resistance between this plate end the wheel rim is measured. . None of the measured values of surface electric resistance can exceed the value of 1.106 Ω. 3.2 Speed tests Speed tests for utility vehicle tyres have an importance mainly for tyres for light utility vehicles, as for example pickups, van type vehicles, lorries, pick-up, SUV – Sport Utility Vehicles, SAV – Sport Activity Vehicles. They have smaller importance for tyres designed for trucks, buses and their trailers, where high speed tests are usually not executed. Nevertheless, test methodologies are provided, used by some tyre manufacturers and test laboratories. Stress-mechanical deformation and thermal stress occur during high speed tests, mainly in the tyre crown. Typical reasons for test termination of such test are the reinforcing strip and tread separation, or tearing of tread blocks of rubber from thetyre tread. A high speed example for light utility vehicle tyres is test methodology according to EEC Regulation Nr. 54, valid for tyres with load index 121 and less with speed category 160 km/h and higher [12]. New tyre is fitted on a test rim. For a tyre designed for a tube use, a new tube is utilized, or a combination of tube, valve and flap. The tyre is inflated to a pressure corresponding to the maximum tyre load, or to the inflating pressure corresponding to the maximum tyre load in a single assembly. The inflated tyre is conditioned at the temperature of test room (25±5) °C for at least 3 hours, then the tyre inflation is adjusted to the above-mentioned pressure. Afterwards, the tyre is fitted on a test shaft and is pressed on the smooth outside surface of the driven test drum, whereby its width is at least so big as the tread width of the tested tyre. Radial load is chosen according to drum diameter in following way:


− 90 % of the load corresponding to load index in single-assembly in case of drum with a diameter 1 707 mm ± 1 %,

− 92 % of the load corresponding to load index in single-assembly in case of drum with a diameter 2 000 mm ± 1 %.

Initial test speed is a speed lower by 20 km/h than the speed category of the tested tyre. Transition period from stopped condition to the initial test speed is 10 minutes. Duration of first test step is 10 minutes. Test speed in the second test step is lower by 10 km/h than the speed category of the tested tyre. Duration of second test step is 10 minutes. Last test speed corresponds to the speed category of the tyre. Duration of last test step is 30 minutes. Total test duration is 60 minutes. Inflation of the tyre can not be adjusted during the test and the test load is maintained at a constant level in the course of each test step. This speed test is carried out without interruption. After the test no tread, ply or cord separations can appear on the tyre, as well as any rubber tear or cord break. 3.3 Fatigue tests The goal of fatigue tests is the same as with passenger car tyres, i.e. the establishment of tyre durability in its various parts. Usual duration time of fatigue tests for utility vehicle tyres is from 50 to 250 hours. Fatigue tests are usually executed at a constant speed. Basic fatigue test is a test, accepting the requirements of EEC Regulation Nr. 54 [12]. The tyres are tested at a inflating pressure corresponding to PSI symbol, or at an inflating pressure marked on the tyre for maximum tyre load in a single-assembly. Prior to the test, tyres are conditioned at the room temperature (25±5) °C, at which they are also tested. The inflating pressure is not adjusted during the test and is left to increase gradually. This test is the fundamental safety test in european territory and serves as the base for the tyre homologization. Test conditions for the test are listed in the Table 16. Tyres for special use are tested at a speed corresponding to 85 % of the prescribed speed for equivalent normal tyres. Tyres with load index 122 or higher and with a speed category symbol higher than M are not produced for the use in road traffic so far. For this reason they can not be awarded with a homologation and such test is not carried out.


Table 16 Test proceeding according to EEC Regulation Nr. 54

Drum speed Load as percentage of load corresponding to maximum tyre load in a single-assembly Load

Index

Symbol of tyre speed category

Radial tyres [km/h]

Cross-ply tyres [km/h]

7 h 16 h 24 h

F 32 32 G 40 32 J 48 40 K 56 48 L 64 —

122 or more

M 72 — F 32 32 G 40 40 J 48 48 K 56 56

66 % 84 % 101 %

L 64 56 70 % 88 % 106 % 4 h 6 h M 80 64 75 % 97 % 114 % N 88 — 75 % 97 % 114 %

121 or less

P 96 — 75 % 97 % 114 %

A detailed test, but for non-european territories, originates from the Iegulation FMVSS Nr. 119 [13]. The method of the test is here similar, but due to the fact, that in these territories the tyres are designated neither with speed category, nor the load index, the method of the test depends on the Load Range Index. Test conditions are listed in the Table 17. Prescribed test temperature is 35 C.

Table 17 Test method according to Regulation FMVSS Nr. 119

Load as percentage of load corresponding to maximum tyre load in a single-assembly Load

Range Drum speed [km/h]

7 h 16 h 24 h A, B, C, D 80 75(1) 97(2) 114 E 64 70 88 106 F 64 66 84 101 G 56 66 84 101 H, J, L, N 48 66 84 101 (1) 4 hours for tyres submitted to high speed requirements (2) 6 hours for tyres submitted to high speed requirements

Both tests serve mainly for certification purposes, or, partly, for quality control. Due to tyre type the test lasts only 34, or 47 hours. Bead fatigue tests and carcass fatigue tests serve for real assesment of tyre durability.


Bead fatigue tests are executed at lower speeds and a higher load. This combination of testing conditions provides the stress of the tyre in the bead area. The inflation in these tests is regulated and maintained at a value of 75 % to 100 % of the inflating pressure corresponding to maximum permissible load in a single-assembly. The speed in bead fatigue tests varies in dependence on the methodology between 20 and 60 km/h. To prevent any injury in the tread crown area, having the biggest thickness of rubber in the shoulder, tyres must be adjusted prior to the bead fatigue tests. Rubber behaves as a thermal insulator and transfers very poorly the heat originated by repeated tyre deformation. During bead test of an unadjusted tyre, heat accumulation would take place in tyre shoulder due to the unstable process, leading to thermal degradation of the material in the shoulder and to separations in the tyre crown area, which is not the goal of bead fatigue tests. Cutting-off the tread and subsequent roughening of this surface reduces the thermal resistance of the tyre, which is proportional to the material thickness. In this way, the dissipation of produced heat is better and no injury takes place in the tyre shoulder. The radial load in this type of tests usually varies between 1,3- and do 2,4-times of the maximum tyre load in a single-assembly. Radial load during test is either constant, or is gradually increased in individual load steps. Prior to the test the tyre is conditioned at least for 3 hours at the test room temperature, which is (25±5) °C, or (38±3) °C.

Carcass fatigue test are focused on tyre durability in the area of tread and tyre reinforcing strips. Tyre does not require any special preparation before the test. The tyre conditioning prior to the test takes at least 3 hours at the temperature of the test room. Inflating pressure is regulated during the test, its value usually corresponds to the value for maximum tyre load in a single-assembly, or is given in a table from tyre manufacturers, associations (e.g. European Tyre and Rim Technical Organisation). Speed during tests usually varies between 40 and 80 km/h, load from 100 % of maximum permissible load and higher. Load is constant during the test, or is eventually increased in steps. Some tyre manufacturers carry out carcass fatigue tests also with a non-zero directional deviation angle at a constant lateral force. Tyre fatigue tests for light utility vehicles are similar as tyre tests for passenger cars. Test speeds usually vary from 30 km/h for bead tests, and from 40 km/h for carcass fatigue tests.

3.4 Special dynamic measurements Special dynamic tyre measurements for utlility vehicles are similar to tyre measurements for passenger cars. These measurements consist of assesment of rolling resistance, dynamic directional characteristics, tyre noise development and temperature distribution on tyre surface. Each laboratory carries out these tests according to own methodologies The only normalized methodology in this area is the measurement of rolling resistance, which is normalized in the form of International Standard ISO 9948


[15]. But this standard is used in different variations, because not all parameters of the test are described precisely. The assesment is realized on drum test machines with a drum parameter of 1 700 to 3 000 mm at a test room temperature of 20 to 30 °C under stabilized regulated conditions, at zero camber angle and zero directional deviation. A very precise test device must be used for the measurement, because small forces and torques are measured in the presence of bigger forces ( of a higher order). Physical measurement principle is the same, as with the assesment of tyre rolling resistance for passenger cars (see Chapter 2.4). Following methods are used for the establishment of rolling resistance:

• force, • torque, • performance, • decceleration method.

Tyre is rolling on a smooth steel drum during the measurement. The use of a drum with a rough surface is also allowed, with a roughness of 180 µm. The surface temperature of the test drum at the start of the test must be roughly the same as surrounding temperature. During the establishment of rolling resistance, the pressure is left to increase freely, i.e. the tyre valve is closed during the measurement. The inflating pressure at the beginning of the test corresponds to the inflating pressure corresponding to maximum permissible tyre load in a single-assembly. Priorr to the measurement, the tyre must be run in on the test machine with a drum diameter of at least 1 700 mm, a run in on the vehicle is eventually also permitted. The conditions for tyre run in the Standard ISO 9948 are not precisely described. The tyre run in normally lasts 6 hours at a speed of 60 km/h, a radial load corresponding to 80 % of maximum permitted tyre load in a single-assembly and an inflating pressure coreresponding to maximum permitted tyre load in a single-assembly. The speed and duration of individual steps in the Standard ISO 9948 is not clearly prescribed. One of possible examples for rolling resistance measuring method is given in the Table 18: Table 18 Example of tyre rolling resistance measuring method for utility vehicles

Step Duration [min]

Drum speed [km/h]

1 150 502 45 603 45 804 45 90

The recording of individual measured variables and their processing is the same as with measurement of tyre rolling resistance for passenger cars. The difference lies in the application of rolling resistance temperature correction to 25 °C, which is calculated due to following equation:


[ ])25(125 −+= tKFF rr (12)

where: Fr – measured rolling resistance value at surrounding temperature t, in N

t – surrounding temperature, °C

K – value equal to 0,006 for tyre with load index 122 and higher for trucks, buses and their trailers

0,01 for tyres with load index 121 and less for trucks, buses and their trailers.

An eventual correction of the rolling resistance value to other drum diameter is calculated in the same way as during measurement of tyre rolling resistance for passenger cars.

4 Agricultural and special tyres This group comprises tyres used on tractors, suspended machines and trailer vehicles in agriculture, forestry, mining works in quarries, mines, on earthmovinch machines, military vehicles, etc. Tests of such tyres are executed with the help of similar test methodologies, like for utility vehicle tyres. The test choice depends on the planned use of respective tyres, with an emphasis on critical areas. These are e.g. the requirement for low contact pressure in the tyre footprint in agriculture, resistance to punctures during the use in mining industry, etc. The only internationally recognized regulation, but also valid only for tyres used in the area of agriculture and forestry, is the EEC Regulation Nr. 106 [16]. From the point of view of fundamental safety, two tests are important according to this regulation, the establishment of tyre resistance against destruction from water pressure and dynamic test by load and speed. During the test by ware pressure, the tyre strength and its resistance to increased inflating pressure is verified. New tyre is mounted on a special reinforced rim or on a test device equipped with solid circular discs with a seating surface for the tested tyres, allowing the adjustment of their distance corresponding to rim width. Rims, or these special discs used for the test, must withstand without any deformation the highest possible value of pressure attainable during this test. In case of disc use, tyre beads are thoroughly centered and the outside distance between the beads is adjusted to the value corresponding with rim width in dependence from the tested tyre dimension. The tyre is gradually filled with water in such a way, that the air is pressed out. Then the device is started and the water pressure inside the tyre is increased to rech gradually a pressure value, which is 2.5-times higher than the maximum inflating pressure, as set by the tyre manufacturer. Under any


circumstances, the pressure can not be, according to EEC Regulation Nr. 106, lower than 600 kPa, or higher than 1 000 kPa. Afterwards, the constant value of this pressure is maintained for at least 10 minutes. After expiration, the pressure is gradually reduced to zero value, the tyre is dried and inspected. As long as the pressure inside the tyre exceeds the pressure of the environment, nobody is permitted to stay in the test room, which must be securely closed. During the test, the tyre must withstand the test pressure and no damage of beads, used reinforcing materials and separastions of individual tyre parts can take place. Dynamic test under load and speed is carried out on a test device with a drum diameter of 1 707 mm and its width corresponding at least the tread of the tested tyre. The test according to this method is realized only in case of tyres with speed category 65 km/h (Symbol D). Tyre preparation and test method is the same as for tyre tests for utility vehicles according to EEC Regulation Nr. 54 (see Chapter 3.3). Test conditions are given in the Table 19. The test room temperature is maintained in a span of 20 to 30 °C.

Table 19 Program of test under load and speed according to EEC Regulation Nr. 106

Drum speed [km/h] Step Step duration

[h]

Load as load percentage corresponding to maximum tyre load

1 7 66 %2 16 84 %20 3 24 101 %


INDEX OF USED SPECIALIZED LITERATURE, REFERRED TO IN THE TEXT

[1] Consolidated Resolution on the Construction of Vehicles (R. E. 3),

Economic Commission for Europe, Working Party on the Construction of Vehicles

[2] STN EN 10020 Definície a triedy ocelí [3] ISO 7619 Rubber, vulcanized or thermoplastic – Determination of

indentation hardness – Part 1: Durometer method (Shore hardness) [4] Agreement Concerning the Adoption of Uniform Technical Prescriptions for

Wheeled Vehicles, Equipment and Parts Which Can Be Fitted and/or Be Used on Wheeled Vehicles and the Conditions for Reciprocal Recognition of Approvals Granted on the Basis of These Prescriptions, Addendum 29: Regulation No. 30 Uniform Provisions Concerning the Approval of Pneumatic Tyres for Motor Vehicles and Their Trailers

[5] Council Regulation 92/23/EEC from 31.March, 1992 about Tyres of Motorized Vehicles and their Trailers and their Fitting

[6] European Tyre and Rim Technical Organisation (E.T.R.T.O.): Engineering Design Information

[7] STN 63 1511 Tyre Testing, Determination of Static Radial Stiffness and of Static Radius

[8] § 571.109 Federal Motor Vehicle Safety Standard No. 109: New pneumatic tires

[9] WdK 116 Personenkraftwagenreifen, Messung der Wulstkennung [10] Gulf Standard No. 53/1986 (Saudi Standard No. 448/1986) Passenger Car

Tyres, Part 3: Methods of Tests [11] ISO 8767 Passenger car tyres – Methods of measuring rolling resistance [12] Agreement Concerning the Adoption of Uniform Technical Prescriptions for

Wheeled Vehicles, Equipment and Parts Which Can Be Fitted and/or Be Used on Wheeled Vehicles and the Conditions for Reciprocal Recognition of Approvals Granted on the Basis of These Prescriptions, Addendum 53: Regulation No. 54 Uniform Provisions Concerning the Approval of Pneumatic Tyres for Commercial Vehicles and Their Trailers

[13] § 571.119 Federal Motor Vehicle Safety Standard No. 119: New pneumatic tires for vehicles other than passenger cars

[14] § 571.139 Federal Motor Vehicle Safety Standard No. 139: New pneumatic radial tires for light vehicles

[15] ISO 9948 Truck and bus tyres – Methods of measuring rolling resistance [16] Agreement Concerning the Adoption of Uniform Technical Prescriptions for

Wheeled Vehicles, Equipment and Parts Which Can Be Fitted and/or Be Used on Wheeled Vehicles and the Conditions for Reciprocal Recognition of Approvals Granted on the Basis of These Prescriptions, Addendum 105: Regulation No. 106 Uniform Provisions Concerning the Approval of Pneumatic Tyres for Agricultural Vehicles and Their Trailers


TESTING OF THE TYRES UNDER REAL CONDITIONS It is testing on real vehicle, on real road, by real driver. Test of the tyres under real conditions are performed on standard vehicles in normal road traffic as well as on specially built testing roads. We call them as vehicle tests of tyre. Advantage of these tests is in maximum approach to real conditions in operation eventually to customer’s needs. The tests are performed under two kinds of the vehicle behaviour on the road:

- standard - extreme

Different conditions, mainly weather conditions, have also other significant influence that must be taken into consideration and respected. For instance, air temperature, roadway temperature, wind force and direction, roadway humidity, driver skill, state of his mind and many other factors influence of which is impossible to eliminate. The most significant condition of testing occurring on the roadways is the kind of test surface:

- dry roadway - wet roadway - packed snow - ice (plain, clean)

These conditions can be used in natural exterior or can be arranged artificially, to some extent. The tyres are tested on vehicles, which use given type of tyre and their assembly is not significantly modified in any way. Vehicle tests of the tyres are divided according to testing methodologies onto: - wearing tests (working lifetime / tyre mileage), - tests on proving grounds, are called also as special tests. Under group of special tests these are divided according to character onto: - subjective, - objective.

Preparation of the tyres Tyres dedicated for the tests are fit on assembly machine to the wheel rims prescribed by testing vehicle producer for given vehicle type and tyre dimension, afterwards these are pumped up to pressure prescribed by vehicle producer and balanced on the wheel balancing machine. The tyres prepared in this way are mounted onto the testing vehicle.


1. Wearing tests (working lifetime / tyre mileage) Working lifetime is a distance, which a vehicle can pass to wear the tread pattern to given limit. Initial slot depth is measured before the first drive. The vehicle is afterwards driven on testing road circuit and slot depth is repeatedly measured on all tyre crowns on the vehicle after periodically repeated track. Driving and measurements of the slot depth continues until slot depth of the tire tread pattern of any tyre achieves minimum remaining depth (based on the public road traffic law). The test is finished after final measurement of the slots depth. Depth of each slot is expressed by the smallest value from the depth measurements around the whole circumference of appropriate slot. Each tyre is represented by minimum value of data achieved by this means from individual tyre slots. The tyre is measured at the same time in each slot on six areas around the whole circumference. All the slot depth measurements are recorded into program in PC that processes the data, shows graphics and can additionally calculate expected tyre lifetime up to remaining slot depth of tyre in advance of the test completion. Result of the test is normally supported with attached photographic documentation from the tyre before test eventually after test as well as subjective evaluation of the tested tyres properties.

2. Special tests

2.1 Subjective tests For this reason we appointed drivers who are able to manoeuvre with vehicle under extreme situations on the limit of the tyres possibilities and at the same time they are able to review very sensitively behaviour of the tyres from subjective point of view. The tyres are evaluated from view of driving properties, noise emitted by rolling of tyres on the road and driving comfort, i.e. extent of pleasantness during vehicle handling with given tyres. Driving properties: − zero position - during straight drive on flat track covered with asphalt or concrete

surface by steady balance speed in two scopes: 40 - 60 km/h and 110 - 130 km/h. Driver performs during drive an oscillating motion of the steering wheel from small deviation up to the moment when the vehicle recognizable reacts to the driving direction change. Afterwards the driver makes the steering wheel deviation (approx. 20°) under constant speed and leaves the steering wheel released. We evaluate how big the tyre effort to return steering back into direct position is. During manoeuvring we evaluate sensitiveness of the steering around


the basic position of the steering wheel and tire self-aligning torque of the steering.

− reaction respond – during this test the driver evaluates by ca 90° turning of the steering wheel how big delay of the tyres on the front and rear axle is. The test is performed under different speed according to attainable technical conditions given by maximum vehicle speed and the tyres speed rating (60, 100, 140 km/h).

− sensitiveness to a rut – this test is performed during normal road traffic. Stretch of a road with a ruts without big uphill gradient and in straight direction must be selected for this test. Passing through the road ruts is in longitudinal and crossway (crosswise) direction in the closed angle. Acceleration and deceleration is done during drive in tracks or crossover. During this test we evaluate the vehicle behaviour, its tendency to copy the tracks and intensity of necessary steering corrections.

− changed load – it is tested under constant speed on the circular track with big side acceleration of ca 0,7g at the lowest possible gear driving position engaged. Steady-state circular-course driving is followed by sudden deceleration without changed steer angle. We monitor the change of the path curvature radius caused by changed speed, steering mechanism system flexibility rate and consequential change of the wheels loading. During this test the tyres should not cause oversized path curvature.

− transient characteristic – it is based on straight vehicle movement and monitoring is oriented to ability of the vehicle to change direction and side force increase after very fast steering angle and steering wheel holding.

− marginal range - it is tested under fast steering direction changes, most commonly in slalom road, eventually during obstacle avoidance manoeuvre. We evaluate the scope of such steering wheel deviations still allowing vehicle control and tyres cannot increase anymore the side force under additional steering wheel deviation. We evaluate also way of the tyres transmission from status that still allows reacting on the direction change to the sideslip status (sudden or gradual transition into skidding).

Noise Subjective noise test is performed on following types of surfaces under dry conditions:

- smooth road surface, - cobblestone pavement, - repaired road, - anti-skid road surface (similar to the highway surface).

We evaluate the noise intensity generated by tyres and rate of the noise unpleasantness, i.e. level of the personnel intrusion by noise coming from tyres. Following indicators and effects are evaluated during tests on these surface types: basic noise tone, created interactions with other noises, noise in curve, drumming


noise, howling, roaring, noise during acceleration and deceleration or braking. Evaluation is considered in relation to low, middle and height frequencies in the whole possible scope of speeds during vehicle acceleration, constant speed and deceleration breaking.

Comfort Driving is done under low speed in scope from 30 km/h to 50 km/h and also under higher speeds:

- smooth asphalt surface, - smooth asphalt surface with repairs on the road, - concrete surface with joints and sporadic unevenness (road surface bumps), - undulated road - reversed reaction to over-springing (absorption) of the tyres.

Vehicle passes also through single obstacles, e.g. repairs on the road, channel covers, plates placed on the tract in cross or sidelong direction. We evaluate passage hardness trough the obstacle and absorption, speed and size of vibrations transferred by tyres into the vehicle structure, seats and steering wheel. Under low speed we evaluate vulnerability of tyres to create vibrations.

We evaluate each of the tests set according to 10-point scale (Table 1). Verbal evaluation of the driver containing observation of tested tyres is also important.

Table 1: Scale of the point evaluation and meaning of individual steps

1 2 3 4 5 6 7 8 9 10 unsatisfactory insufficient satisfacto

rygood very

goodexcellent

from customer’s point of view corrective measures are necessary

properties of tyres are acceptable, but despite this fact improvements are necessary

properties of tyres are acceptable, no special improvements are necessary

unsatisfied customer -evaluation is not acceptable

satisfied customer - evaluation is acceptable

Scale of the point resolution: 1 - 6 point evaluation – non-acceptable 7 - 10 point evaluation – acceptable Difference of one point may be divided in case of necessity because

of more-accurate defining of the difference in given property in this way:

Differences in point evaluation of compared properties to selected comparison (reference) product:

0,25 point - characterizes difference that is normally insensible but under concentration onto some given property it is noticeable by experienced driver.

- corresponding products, very small resolution of compared property


0,5 point - characterizes remarkable difference by experienced driver without concentration onto evaluated property.

small, but remarkable resolution of compared properties, that must be taken into consideration

0,75 point - characterizes clear remarkable difference in evaluation of the products´ properties mainly in scope of the acceptability limit evaluation

very clear resolution of compared properties 1,0 point - characterizes difference easily remarkable by evaluating person

also without concentration onto evaluated property. - easy resolution of compared properties

2.2 Objective tests These are the tests oriented to concrete properties measured by measurement apparatuses placed inside or outside of the vehicle. As the conditions for testing of tyres cannot be totally stabilized the tests are performed by means of comparison procedure. It means that before measurement of the tested tyres properties we have to measure properties of so called reference tyres under assumption that their properties fulfil specific demands on their operation and quality. These tyres are mostly produced by significant foreign producers. − Braking/stopping distance – largely determines safety of tyres. We measure

distance passed over by vehicle during full braking effect of the vehicle operating brake. Driver will move the vehicle on the straight direct track on selected speed based on possibilities that can be achieved in testing area, e.g. 100 km/h. He keeps the vehicle on this speed until approaching specified place dedicated for starting of the braking manoeuvre. Driver will operate the vehicle braking system in learnt way to achieve increase of the brake fluid pressure in shortest possible time and he will finish the braking manoeuvre under jam on the brake pedal until total stoppage of the vehicle. Measurement equipment located on vehicle will record the distance moved, speed, deceleration and time from the jam on the brake pedal. Such testing of tyres is performed on dry, wet, snow-covered and icy surface, eventually on continual frozen ice area. During the test it is important to have chassis of the vehicle and its braking system in good technical conditions, but also settled climatic conditions are important.

− Vehicle handling/steering control – it is the most complex test from all

objective tests, because present influence of all tyres properties onto vehicle behaviour will show up. Driving takes place on closed circuit in length from 400 to 700 m, where the straight track parts are changed with curves of different radius. Driver’s task is to pass the circuit with vehicle in shortest possible time without mistake. Vehicle completes the circuit more times whilst the time of each lap is measured and these values are statistically processed. The test can be performed also on dry, wet, snow-covered and icy surface, eventually on


continual frozen ice area. Also during this test it is important to have chassis of the vehicle in good technical conditions and settled climatic conditions.

− Slalom test – it is manoeuvre consisting of the road part in determined length

with marked obstacles by means of plastic road cones that must be by-passed in slalom way. Goal of this test is monitoring of the tyres under extreme side loading, discovering of the side toughness influence and adhesion to manoeuvre passing speed. Measured item is the passing time and vehicle cross acceleration. During this test it is necessary to pass the manoeuvre in shortest possible time and that put high demands on driver’s technique and routine practices. Measured values are statistically processed.

− Passing manoeuvre – the test represents by-passing of suddenly created

obstacle by lateral breakaway from normal traffic lane into contra-flow-lane and repeated return into normal lane. By its character it reminds bypassing living animal suddenly running onto the road thus it is sometimes called "reindeer test". Goal of this test is determining the vehicle steering control during usage of individual types of tyres under limiting situations and high speeds. Demands on maintaining the vehicle stability are high. The tests are performed on track with shape determined by ISO 3888 suggestion. The test requires high manoeuvring abilities of the driver and separate regulation for order and number of tested tyres that are mutually compared. It is time demanding test and lasts many days depending on amount of the tyres alternatives. During test it is also necessary to pass the manoeuvre in shortest possible time.

− Trailing throttle / free coasting – by means of this test we compare size of the

tire rolling resistance share to total vehicle passive resistance. Principle of this test consists in moving of the vehicle by speed of 45 km/h and afterwards the gearing mechanism is put into neutral position. We measure a distance passed by vehicle before total stoppage. It is important to perform the test under lowest possible wind speed and changes.

− Noise measurement on vehicle – is the test result of which provides very

important information about tested tyres – noise level created by tyres during rolling on the road in interaction with the whole vehicle they are installed on. The noise is measured during passing of the vehicle in specified distance from microphone of the measurement sound-level-meter. This is an important value because lower noise of the vehicle tyres is an important factor for protection of favourable living environment.

− Effective rolled perimeter – this is the only test when it is not necessary to

compare results with reference tyres. The test is performed under constant speed of 60 km/h on approximate distance 100 meters. Except passed distance we record also amount of the tyre rotations. Distance representing one rotation is searched value. This value provides information about suitability of the tested


tyres usage on vehicle normally using different tyres (e.g. other - similar dimension) from view of correctness of speedometer data. Effective rolled perimeter for one measurement is:

nso = [ m ; m , - ]

where o – effective rolled perimeter of the wheel, s – actual passed distance on measured track,

n – number of the wheel rotations on measured track.


TESTING OF RUBBER COMPOUNDS AND VULCANIZATES

1. Introduction The Chapter „Testing of rubber compounds and vulcanizates“ continues where the Chapter „Rubber raw material testing“ stops, and it provides a detailed description of procedures applied in testing of rubber compounds, materials and vulcanizates, from preparation of samples and testing or speciments to evaluation of results in accordance with respective standards.

The Chapter is divided into four sections

• Testing of Rubber Compounds • Testing of Rheological Properties of the Compounds • Testing of Vulcanizates • Dynamic Mechanical Thermal Analysis of Vulcanizates

each describing more in detail particular testing procedures and methods based on national and international standards and/or technical literature focused on this subject.

2. Testing of Rubber Compounds Testing of rubber compounds includes two basic testing methods – Determination of Viscosity and Scorch, and Determination of Vulcanization Characteristics (as described below). 2.1 Determination of Viscosity and Scorch (DIN 53 523, ASTM D 1646, ISO 289)

Rubber is typically characterized by its viscosity (older literature uses incorrect expression „plasticity“; which is a property negatively correlated to viscosity) dependent on molecular weight and its distribution. Viscosity determines rubber processability, i.e. its mechanical processing ability. The most commonly used method is Mooney viscosity determination. Viscosity of rubber and/or rubber compounds dependents on temperature. Determination of viscosity is based on torque measurement of shearing disc embedded in specimen, enclosed in heated chamber of the measuring instrument. Tests are carried out on Mooney rotating disc viscometer. Torque is measured on axis of the shearing disc. Two different-size rotors are used and rotation speed is set


to 2.00 rpm (revolutions per minute). Large rotor diameter is 38.10 mm, while the small one is 30.48 mm; both discs are 5.54 mm wide. Mooney viscosity is normally measured at temperature 100 °C using large rotor. Small rotor is used only for rubbers or compounds having their viscosity above the instrument upper limit (usually 200 ML). Test is carried out as follows [1]: specimen is put in the instrument heated chamber, which is then closed. Due to high dependency of viscosity on temperature, specimen is given approximately one minute to preheat to required temperature (measurement at isothermal conditions). After the first minute is elapsed, rotor starts – torque measurement is initiated. Rotor torque is usually read in the fourth minute and subsequently calculated to viscosity. When the measurement is completed, rotor stops. Mooney viscosity curve is shown on Figure 1.

Fig. 1 Mooney viscosity determination Mooney viscosity is given in Mooney units, adding a used rotor code (example: result value may be reported as 50ML(1+4)100°C, which means that the measured viscosity was 50 Mooney units, „ML“ indicates that a large standard rotor was used (L-large, S-small), The “1” represents the preheat time before the rotor starts to turn. The “4” indicates the running time of actual rotation of the rotor before the final Mooney viscosity measurement is made at 100°C. Viscosity values measured with the use of large rotor are not comparable to the small rotor results, however, they may be recalculated using the following (approximate correlation calculation) formula: ML=1,8*MS-1,0. In addition to Mooney viscosity, the measurement may also indicate other compound characteristics, such as Delta Mooney, scorching and stress relaxation. Delta Mooney (rubber processability characteristics) is the difference between two Mooney viscosity values measured in two time intervals at preset temperature. Delta Mooney (∆M) is defined as between the viscosities measured in minute 15 and in minute 1.5 from the rotor start. Stress relaxation test may determine elastic and structural-viscosity properties of rubber. Relaxation is determined as following: after viscosity measurement has been completed, residual torque (viscosity) is


measured on switched-off rotor in second 30 and 60. Mooney stress relaxation (in %) is calculated according to formula:

[%]100.)41(ML

MMR 30

30 += (1)

[%]100.)41(ML

MMR 60

60 += , (2)

where: M30 – value of viscosity in Mooney units 30 seconds from rotor switch-off, M60 – value of viscosity in Mooney units 60 seconds after rotor is started.

One of useful tests is a scorch time and scorch speed determination (t5, t35). Scorch time is the time elapsed to the point, when scorching starts at determined temperature (as a criteria we use increase of ML or MS viscosity by 5 units from the minimum value; this value is very important for determination of time during which the product safely withstands process temperature without scorching, before it is to be vulcanized in press). Scorch time curve is shown on Figure 2.

Fig. 2 Scorch time curve

Scorch time can be calculated from the following formula:

[min]ttt 535 −=∆ (3) Scorch time is determined at 120°C or 140°C (any other used temperature must be recorded in test protocol). Among commonly used testing instruments we may include viscometer MV 2000E made by Alpha Technologies:


2.2 Determination of vulcanization properties (DIN 53 529, ASTM D 2084)

Vulcanization process is objectively and representatively shown on vulcanization curves. The curves allow determination of optimum production-related technical conditions and exact compounds control. In addition, they serve as a realistic basic data for the vulcanization mechanism research. The curves can be determined using classical methods, or by use of special instruments, so called vulcameters. The classical method (consecutive vulcanization) is based on preparation of larger number of test specimens vulcanized over different-length time intervals and evaluation of net density, or modulus. The described procedure is a very labor- and time-consuming. As the vulcanizate quality demands are still higher, the requirement is to provide more accurate assessment of vulcanization by use of recorded and evaluated vulcanization curves. To this purpose, special instruments (vulcameters) have been recently constructed and retrofitted. These instruments can automatically record the whole vulcanization curve for each single specimen; this curve represents the dynamic shear modulus of cyclically stressed test specimen during vulcanization. Oscillating disc vulcameters are most commonly used (e.g. Monsanto, Göttfert etc.). Vulcanization curves records include considerably higher number of points with lower scattering, than can be obtained using the consecutive vulcanization methodology. Hence, it allows not only determination of technologically important data, but also sufficiently accurate velocity constants of individual vulcanization stages and their respective activation energies.


By heating the rubber compound to higher temperature a chemical reaction (vulcanization) between netting agent and rubber carbon hydride is induced, while concentration of vulcanizing (networking) agent falls almost to zero. Rubber compounds are vulcanized at certain pressure, however, the pressure has no impact on the vulcanization speed (it prevents from creation of pores due to development of gaseous substances, particularly water; thence, the used pressure is higher than saturated vapor tension at the given vulcanization temperature (e.g. at the saturated vapor tension at 154°C is 0,42 MPa)). Vulcanization is a key process of rubber-production technology, because of its highest energy- and time demands. Vulcanization speed is one of the main productivity-determining factors. Hence, it is very important to have as many information on vulcanization as possible. Vulcanization speed follows the same laws as the speed of other chemical reactions. Vulcanization process is characterized by the following stages: • Induction period, • Networking reaction, • Changes in developed net. The vulcanization speed is determined only by the second stage, and the total vulcanization time is a sum of times of the stage one and stage two. The vulcanization induction period is characterized by slow chemical reaction between vulcanizing agent, rubber and other compound constituents. However, it is an important vulcanization stage, as it allows safe processing of the compound and its perfect spreading inside the mould cavity. The vulcanization induction period is not the scorch time equivalent (STN 62 1415), because it is assessed as the time from which the second vulcanization stage (linearized according to specific mathematic formula (rate law)) is extrapolated. The networking reaction itself is characterized by creation of network bindings (diamond work). Shape of dependence of its speed on time is determined by the vulcanization kinetic order. This stage usually has exponential behavior, and thence the vulcanization is commonly evaluated as the first order reaction. The last stage chemical reactions depend on the rubber type, vulcanization system and temperature. In an ideal case, the result is a constant vulcanization degree value (some real vulcanizing systems actually behave like this). Some compounds show the drop/decrease of vulcanization degree (this relates to splitting of polysulphidic cross-bindings, and sometimes to splitting of rubber chain), called reversal/retrogression. Opposite to this, an additional networking may occur (caused by networking reactions induced e.g. by influence of hyperoxides) [2]. The above described chemical reactions are graphically expressed by kinetic curve (rubber-producing industry uses the term „vulcanization curve“). From the curve behavior, the rubber compound vulcanization characteristics are determined (such as t10, t90, Mmax, Mmin (Fig. 3)).


• t10 is the time required for torque increase by 10 % of difference between the minimum and the maximum vulcanization curve torque.

• t90 is the time representing 90 % of the difference. t90 is the basis for assessment of so-called vulcanization optimum.

• Mmax and Mmin are maximum and minimum torque values characterizing the compound „stiffness“. The vulcanization velocity constant can be calculated from the vulcanization curve; vulcanization curves measured at different temperatures serve for calculation of the vulcanization activating energy.

Torque

Fig. 3 Determination of vulcanization characteristics

Compound initial torque Mo – is the torque, when the rubber compound is practically cold (several seconds after inserting the specimen in the measuring chamber with biconical disc). It is given in N.m.

Compound minimal torque Mmin – is the lowest torque on the vulcanization curve, equal to viscosity of the compound heated to vulcanization temperature; this torque value characterizes the compound stiffness. It is given in N.m.

Vulcanizate maximum torque Mmax – is the highest torque on the vulcanization curve, equal to the value of the vulcanized compound shear modulus at the given temperature; this torque value characterizes the vulcanizate stiffness at the end of vulcanization process. It is given in N.m.

t102

t 50 t 90 tr (98)

M min M o

M 50

M ma

M x 98

M 90

M´ max (60´) m

[ N.]

] [min. time


If the vulcanization curve constantly climbs up (so-called „walking modulus“), the torque value Mmax reached in the certain time is determined (e.g. in minute 30, 60, 120) depending on the measurement temperature. This value is then indicated as [M´max.(30´,60´, 120´)].

Vulcanizate torque interval ∆M – is the torque given in N.m, representing the difference between the maximum Mmax and the minimum Mmin torque.

]Nm[MMM minmax −=∆ (4)

Compound scorch time t02 – is the time, when the torque exceeds the value Mmin by 0,1 N.m at the rotor oscillation amplitude 1°, and by 0,2 N.m at the rotor oscillation amplitude 3° or 5°. In other words, it is the time, when the torque exceeds the Mmin value (at the given temperature) by 1 or 2 units (of the range 100).

Optimum time of vulcanization t90 – this may be considered the time required for reaching 90 % of the maximum achievable torque or network density, at the given temperature [8.5].

[min]M.9,0M)MM.(9,0MM minminmaxmin90 ∆+=−+= (5)

Time in minutes equal to the value M90 is called the optimum time of vulcanization t90. The above described method may be used for determination of vulcanizing time, equal to any torque change (e.g. by 50%). The time representing M50 is called the half-time of the vulcanization t50.

Approximate time of vulcanization t´90 – it is being determined for compounds

characterized by the „walking modulus“. It is equal to maximum change of torque (usually by 90%) in the given time (e.g. in minute 60).

[min])M´)60(M.(9,0M´M minmaxmin90 −+= (6)

Time equal to the M´90 value will be an approximate vulcanization time (t´90). This time may, however, significantly vary from the actual vulcanization time, required for reaching of the complex optimum properties of the given compound.

Reversal period tr (98) – is the time in minutes, in which the maximum torque drops by 2 %; this time is equal to the value M98 in the sloped-down section of the curve.

[min])MM.(98,0MM minmaxmin98 −+= (7)

Time equal to the value M98 on the vulcanization curve is called the reversal period - indicated as tr (98).


Reversal velocity coefficient Rr [min.-1] - is defined by the formula:

][min))98((

100 1

max

−

−=

ttR

rr , (8)

where tmax – in which the value Mmax is reached.

Vulcanization velocity/speed coefficient Rv [min.-1] - is the parameter equal to average slope of the vulcanization curve; it is defined by the formula:

][min)tt(

100R 1

0290r

−

−= , (9)

where: t90 – optimum vulcanization time,

t02 – scorch time.

Approximate velocity constant k´ [min.-1] - is defined by the formula:

][min)tt(

3,2´k 1

0290

−

−= (10)

This is an auxiliary value that serves for estimate of non-isothermal behavior of the vulcanization, if the measurement is done at several temperatures. Vulcanization characteristics are measured on instruments called „vulcameters“(older instruments, offering graphical record as the only output, should be called „vulcanographs“) or „rheometers“. Older instruments have a biconical rotor installed in the heated chamber (Fig. 12.4); newer ones are rotorless. The rotorless instruments use smaller specimens, thus partly eliminating result errors caused by the specimen heat transfer kinetics (the most commonly known instrument is MDR 2000 (Moving Die Rheometer) made by Alpha Technologies, former Monsanto).


Fig. 4 Measuring chamber in vulcameter: 1 – upper die, 2 – lower die, 3 – upper heating plate, 4 – bottom heating plate, 5 – disc, 6 – disc seal, 7 – heating elements,

8 – calibrated thermal sensors, 9 – piston rod

The rotor-free instrument functions as following: the specimen is placed into the heated chamber, which has the bottom part oscillating at the given frequency and amplitude. The oscillations are transferred through the specimen to the mould upper part, where is the torque sensor. The torque that increases as the vulcanization proceeds, is recorded.

3. Testing of compounds rheological properties 3.1 Rheological properties of electrometric systems Rheology can be in general defined as the science on deformation and flow of mass. In terms of the rubber compounds, the only relevant states of matter are liquid, and solid (resp. their transition phase, which is characteristic for macromolecular substances). Rheology is oriented on the three basic areas:

• determination of type and behavior of the flow • rheological modeling, allowing to obtain material functions of the particular

substance (liquid) at the chosen flow conditions • experimental determination of rheological properties, which is the most

important part in terms of practical application [22]. The objective of rheology is a detailed description (by use of mathematical model) of non-ideal behavior of polymeric liquids. Viscosity is the basic flow property. It is a function of temperature and shear rate, as well as molecular characteristics of


polymers (molecular weight, molecular weight distribution, monomeric unit structure), additives concentration (e.g. filling rate), and other factors (pressure....). Rheology, as mentioned earlier, provides information on flow and deformation. For an ideal liquid, which is deformed by acting of external forces, if this deformation is a function of time (e.g. the substance is flowing), we may derive the following formula, also called the Newton law:

(11) •

= γησ . where σ is the tension; η is the viscosity, and γ˙ is the shear rate. On the other hand, for ideal solid substance, which is being deformed by acting of external forces independently of time, the so-called Hook law applies:

(12) γσ .G= where σ is the tension; G is the modulus (shear), and γ is the deformation. At present, rheology of polymeric melts focuses mainly on studying of correlation between elastic and plastic part of the deformation. Unlike elasticity, caused by orientation of macromolecule segments during flow, the basic factors influencing the polymeric melt rheology are molecular weight, and distribution of molecular weight. 3.2 Classification of liquids Liquids may be in general divided into the following groups [22] (Fig. 5):

• viscous fluids – Newtonian and non-Newtonian (pseudoplastic and dilatant) • fluids with time-dependant deformation speed factor (thixotropic and

rheopectic) • fluids with plastic speed factor (Bingham) • fluids with elastic speed factor (elastoviscous and viscoelastic)


Shear rate γ&

Shea

r stre

ss σ

Flow curves 1

4

2

3

Vis

cosi

ty η

Shear rate γ&

1

3

2

4

Viscosity curves

– Pseudoplastic fluid 2

– Bingham fluid 4– Dilatant fluid 3

– Newton fluid 1

Fig. 5 Flow curves of individual types of fluids (gives the relationship between the tension and the shear rate) – Newtonian behavior is characterized by linear flow

curve, other kinds of liquids (pseudoplastic, dilatant, bingham) are non-Newtonian Newtonian liquids are purely viscous liquids, and their rheological behavior is described by the formula:

(13) •

= γησ .Z

where ηz is the apparent viscosity, which in general is a function of shear rate. If this parameter is a constant, then η is called dynamic viscosity and the formula (13) represent the Newton Viscosity Law (i.e. it is identical to the formula (11)). Liquids with such rheological behavior are called Newtonian, while all other liquids are called non-Newtonian.. Melts of polymers are usually non-Newtonian pseudoplastic liquids, with their viscosity decreasing, as the shear rate increases [22]. Pseudoplastic, i.e. structural viscous liquids have three characteristic areas, where viscosity changes, or doesn’t change with the increasing shear rate:

- First Newtonian area – is a linear area, where viscosity decrease is not observed yet; it is represented by the linear value η0, shear rate is low, not causing disentanglement of any polymeric chain/coil.

- Non-Newtonian area – where viscosity decrease occurs, due to orientation of macromolecular string segments.

- Second Newtonian area – η00 is a constant value that doesn’t change anymore with the increasing shear rate; it has Newtonian behavior, practically all segments are oriented (this applies at high shear rates).

Decrease of viscosity of polymeric melts is monitored over the wide range of shear deformation rates; at high shear rate values, the viscosity may be several orders lower than at low shear rates. This Newtonian behavior of polymeric liquids is very important for practical processing of elastomers; viscosity decrease makes


processing of molten polymer considerably easier, and considerably reduces energy consumption [22]. Flow behavior can be mathematically described by so-called flow curve (Fig. 5). It is a functional dependency between the shear rate γ˙ and the shear tension σ (sometimes indicated as „τ“). The easiest and most-widely used mathematical description (so-called rheological model) of purely viscous liquids is the exponential law:

(14) n

K•

= γσ . This model contains two parameters, where K is the coexistence coefficient, and n is an index, characterizing rate of variation from the Newtonian behavior. For Newtonian liquids, n = 1, for pseudoplastic liquids n < 1 (in contrast to dilatant liquids, where n > 1). If we modified the formula (14) as following,

(15) •••

− == γηγγσ ... 1Z

nK the result would be a general formula describing rheological behavior of a purely viscous liquid (13). The result of the formula (15) is, that by applying the exponential law, we can express the apparent viscosity ηz as a function of the shear rate γ˙ by use of the formula:

(16) 1

.−•

=n

Z K γη The advantage of the exponential model is its simplicity – it contains only two parameters, which can be easily experimentally determined. However, a certain disadvantage of this model is that for constant values of the parameters K and n, it approximates experimentally obtained values σ and γ˙ only in relatively narrow range of the shear rate values γ˙ (interval of one, to two orders) [22]. 3.3 Factor influencing viscosity of polymers Several factors have influence on the polymeric melts viscosity. The most significant factors influencing the melt viscosity are temperature, pressure/tension, molecular characteristics, volume of added filler, and structure of the polymeric string and addition of auxiliary processing additives. In general, the influence of these factors may be characterized by increase, resp. decrease of the viscosity curve, where

• temperature, fluxes and processing additives are acting towards decreasing of viscosity, and

• pressure, molecular weight, structural branching, and adding of filler are viscosity increasing factors [22].


Temperature has the most significant impact on flow properties/behaviors of the polymeric melts. Melt viscosity is considerably changes with the changed of process temperature. As the temperature increases, the viscosity decreases, however, different types of polymers respond differently to similar temperature changes. For the temperatures considerably exceeding the glass transition temperature Tg, the viscosity may be described as a function of the temperature, by use of the modified Arrhenius relation (17): η = B . e Ea/RT (17) where constant B, characterizes polymer and its molecular weight, and Ea is the activation energy of the polymeric melt flow. For amorphous polymers, with the temperature exceeding the glass transition temperature (Tg) by less than 100°C, the Arrhenius relation can not describe the viscosity temperature dependency correctly. In such temperature interval, the Williams – Landel – Ferry formula (18) is used for description of thermal dependency:

( )( )gg

gg

g TTcTTc−+

−⋅=

2

1logηη (18)

where Tg is the reference temperature, ηg is the viscosity at the glass transition temperature, the constants c1g and c2g are independent from temperature and their universal values are: c1g = 17,44 a c2g = 51,6. In addition to temperature, the melt viscosity is significantly influenced also by solid particles of filler, dispersed in dispersion phase (polymeric matrix); with increasing amount of the filler, the melt viscosity increases too. The influence of fillers on the Newtonian liquid viscosity can be described by the Einstein formula: η = η1 . (1 + kE . Φ2) (19) where η is the suspension viscosity, η1 is the dispersing medium viscosity, Φ2 is the volume fraction of dispersed filler , and kE is the Einstein coefficient (for ball particles kE = 2) [22]. The Einstein formula can be applied only to systems with low filler concentration; viscosity is neither a function of the filler chemical nature, nor size of its particles. Relative melt viscosity is better expressed by use of the Mooney formula:

m

kE

ΦΦ

−

Φ⋅=⎟⎟

⎠

⎞⎜⎜⎝

⎛

2

2

1 1ln

ηη (20)

where Φm is the defined ratio of actual, and apparent volume of the filler in dispersion.


Critical molecular weight of polymer Me is the weight exceeding the value, above which the melt viscosity is influenced by angulations of macromolecular string segments. The melt viscosity below the critical molecular weight Me is an non-linear function of the molecular weight Mw, however, if molecular weight exceeds the critical molecular weight Me, the viscosity in the area of low shear rate values is an exponential function of the molecular weight, and described by the formula: η = K. Mw α (21) where K is the constant and a function of temperature; it is also dependant on molecular structure, and the exponent α reaches values from the interval (3,4 – 3,5) [22]. The molecular weight distribution range defines the shear rate value, from which the melt non-Newtonian behavior prevails. Polymers with the wide molecular weight distribution range are characterized by non-Newtonian behavior of the melt flow at lower shear rates, compared to polymers with the narrow molecular weight distribution range. Polymers characterized by wide molecular weight distribution range are in general easier processable, because we can achieve the same viscosity at considerably lower shear rate [22]. 3.4 Measurement of rheometric properties of substances – rheometry Rheometry is an extensive branch of science, studying rheological properties of substances. In general, it includes rheometric methods, methodology and instruments on which the measurements are performed – rheometers. Rheometric methods may be divided into two basic groups, depending on obtained state values, which may be either absolute, or just informative:

• absolute rheometric methods • commercial rheometric methods

Absolute methods for measuring of rheological properties of rubber compounds require exact knowledge of the rheometer function elements geometry, as the basic rheological values (i.e. shear stress σ and shear rate γ˙) are not measured directly, but indirectly derived from measurable values, such as force, torque, discharge speed, etc. Principal-based division of rubber compounds rheological properties measuring instruments is as following [22]:

• rotating rheometers • oscillating rheometers • capillary rheometers • flow rheometers


Rotating rheometers are based on measurement of medium resistance against the special-shaped object rotating at circular orbit speed ω. The medium resistance against the rotation is measured by torque Mt, which is equal to shear stress. The angular rate ω is an independent variable, equal to shear rate. Based on the determined shear fields values, we can plot the flow curve σ = f (γ), that can be described by the exponential rheological model (formula 16). Depending on the type of the function values geometry, rotating rheometers may be divided into the following types: plate – plate, plate – cone, cylinder – cylinder. Plate-plate type rheometers are suitable for the Mooney viscosity measurements; the shear rate changes along the plate diameter. The flow curve can be plotted from the shear stress (formula 22) and the shear rate (formula 23), both - based on directly measured values and exact geometry of rheometer function elements - equaling:

σ = 2Mt / π R3 ; (22), (23) ( ) hR /.ωγ =•

where h is the slot height, and R is the plate diameter. Typical instruments are viscosimeter Mooney (mentioned in the Chapter 2.1), and vulcameter Monsanto (also previously mentioned in the Chapter 2.2). The following picture shows the viscosimeter chamber:

Fig. 6 Mooney viscosimeter chamber (Alpha Technologies)

On the plate – cone type rheometers, the shear rate is constant along the cone base (resp. fixed plate) diameter; this type of rheometer is suitable for normal stress measuring too. In order to plot the flow curve, the shear rate and the shear stress values are used (similar to previously described type plate – plate); the following formula is applied:

σ = 3Mt / 2π R3 ; (24), (25) αωγ /=•

where α is the conical angle between the plate and the cone [22]. Oscillating (rotorless) rheometers are working on the principle described in the Chapter 2.2. Among these instruments, commonly used for the flow properties measurement, we may include rheometers MDR 2000 (mentioned above) and RPA 2000).


The difference between the two instruments is, that while neither frequency, nor amplitude oscillation can be changed during measurement on the instrument MDR (and the new processing technologies require better knowledge of compounds rheological properties), this is possible on the instrument RPA 2000 (Rubber Process Analyzer). In addition to changing of amplitude (from ± 0,02 to ± 90°), and frequency (from 0,033 Hz to 33 Hz), there is also the possibility to change temperature (from 40°C to 200°C). RPA 2000 allows measuring of viscoelastic properties of raw, vulcanized, and overvulcanized compounds. Figure 7 shows the instrument testing chamber.

Fig. 7 Testing chamber of rheometer RPA 2000 Capillary rheometers principle is based on measurement of the melt flow, by a pressure applied through the precisely dimensioned capillary. The values measured directly in the capillary rheometry are:


• volume flow rate Q˙ - is determined from the volume of melt, which flows through the capillary over the defined time; this value may be considered equal to the shear rate γ˙

• pressure drop in capillary (∆ p/L) – defines the difference between the capillary input and output pressures, and is relative to the unit length; the pressure drop in capillary may be considered a value equal to the shear stress.

The measured values Q and (∆ p/L) may be used for determination of shear rate and stress values, based on the melt flow curve may be plotted; its rheological properties can be characterized by the exponential rheological model [22]. For analysis of capillary ratios, we assume that the melt flow in the capillary is:

• isothermal – no viscosity change due to temperature gradient is considered • stable – pressure gradient is considered to be constant • the melt is considered to be incompressible, i.e. Q = const.

Based on the capillary shape, the capillary rheometers are divided as following:

• circle cross-section capillary rheometers • rectangular cross-section capillary rheometers

Based on the melt flow analysis performed in the circle cross-section capillary, the flow curve may be plotted from the following relations between the shear rate and the shear stress:

( ) 3.3

RQm

πγ ⋅+−=•

; LRP

2.∆

=σ (26, 27)

where R is the capillary diameter, L is its length, m is the index of deviation from the Newtonian flow (here m= 1/n), Q is the melt volume. The displacement (volume of extruded melt) is determined as follows (28):

⎟⎠⎞

⎜⎝⎛

++

=31.. 0

3

mmvRQ π (28)

where v0 is the velocity in the capillary axis. Based on the melt flow analysis performed in the circle cross-section capillary, the flow curve may be plotted from similar relations between the shear rate and the shear stress:

( ) 2.24

HTQm ⋅+−=

•

γ ; L

PHw 2

.∆=σ (29, 30)

where H is the rectangular die opening/hole height, T is the rectangular die opening width, and L is the capillary length.


The displacement (volume of extruded melt) is determined based on the formula (31):

η.12.. 3

LPHTQ ∆

−= (31)

where η is the viscosity. The above-mentioned relations defined in the capillary rheometry may only be used for interpretation of the measurement results when the measurement errors are eliminated. Influence of some measurement errors may be eliminated simply if we use capillary rheometer of an appropriate construction. However, some can not be eliminated, and thence the obtained results need to be corrected [22]. Major sources of errors in the capillary rheometry are:

• end effects at the capillary input (unsteady flow), and output (shrinkage, or growing after the nozzle)

• outgoing flow kinetic energy • slip at the capillary walls • capillary elastic deformation • heat losses (i.e. transformation of pressure energy into thermal energy) etc.

Among the most commonly used correction methods, we may include Bagley correction and Rabinowitsch correction. Bagley correction corrects the flow unsteadiness at the capillary input (i.e. it is the capillary length-related correction); this method must be used with relatively short capillaries, shorted than L/R <50, where the pressure drop at the capillary input doesn’t reach constant values. The capillary shall be extended by so-called fictious length, so as to keep the pressure drop constant over the whole capillary length. In capillary rheometry, the indirectly determined values of shear stress and shear rate have their apparent meaning, i.e. they are equal to the melt with the Newtonian behavior. Since polymeric melts show considerable deviation from the Newtonian behavior, a correction must be applied on calculated apparent shear values, in order to achieve better characteristics of the polymeric melt pseudoplasticity. For this purpose, the Rabinowitsch correction is used, which based on the exponential model corrects apparent values of the shear stress and rate [22]. Capillary rheometers are among the most commonly instruments used in the capillary rheometry. For characterization of rubber compounds, the suitable capillary rheometers are laboratory spiral devices (offered on market by several companies, e.g. Thermo Haake, or Brabender).


Example of such instrument can be seen on the following picture:

This laboratory extruder may be equipped with different-type extruding heads – it may be either capillary head with circular cross-section, as well as the capillary head with rectangular cross-section, or some different-extruding heads (e.g. extruding head „Garvey“ – see following picture

or the circular cross-section head (next picture)


and various diameter dies (next picture)

4. Testing of vulcanizates 4.1 Assessment of physical-mechanical properties Basic physical-mechanical properties of vulcanizates consist of density, tensile properties (strength, elongation and modulus), hardness, rebound resilience, structural strength.

4.1.1 Assessment of tensile properties (ISO 37, ASTM D 412) Tensile tests give an orientation view about rubber material properties. Besides this they are used also for inspection of the technological processes smoothness, e.g. mixing quality of rubber mixture. They were used also for optimum curing assessment before implementation of rheometers into rubber practice etc. Dependency of loading on prolongation is characteristic for each rubber mixture. Graphic expression of this dependency is so called tensile curve. It is possible to discover maximum loading and prolongation from this curve but also the loading necessary for some prolongation – modulus, serving very often as evaluation measure of rubber elastic properties. This modulus is not identical with Young’s elastic modulus that indicates the stress needed for doubling of original length.

Modulus stated in rubber practice for specified prolongation expresses internal stiffness (STN ISO 37) presents following definitions [3]:

• tensile stress S – the stress causing elongation of the original testing piece, • elongation E – elongation incurred by tensile stress acting on the testing

piece, • tensile strength TS – maximum tensile stress recorded during elongation of

the testing piece till breaking moment,


• breaking tensile strength TSb – the tensile stress recorded in the moment of the testing piece breaking,

• breaking elongation Eb – tensile deformation of the testing piece operation length in breaking moment,

• elongation at specified stress Es – tensile deformation of the testing piece operation length during achievement of tensile stress specified value,

• stress at specified elongation Se – the tensile stress causing specified prolongation of the testing piece operation length,

• strength at yield limit Sy – the tensile stress in the first point on stress-deformation curve where additional deformation increase runs without any stress increase. This point is defined by the curve inflexion point or it is the curve maximum point,

• elongation at yield limit Ey – the tensile deformation in the first point on stress-deformation curve; next deformation increase is not followed with the stress increase,

• length of the testing piece operation part in the shape of double-sided blades – beginning distance between marked points determining operating part in narrowed part of testing piece in shape of double-sided blade, used for measurement of elongation.

Explanation of individual expressions is shown on Fig. 8.

Monitoring of graphic record for dependency of stress on elongation can discover that this dependency is linear only at negligible loadings (keeps track of Hook’s elasticity law), but afterwards it adheres the curve more or less concave and at the end it sharply rises, rubber is solidified, and increase is rising up to final sample destruction.

The strength as well as modulus is stated in conversion to original sample cross-section. This method is used in spite of the fact that cross-section change during sample tensile deformation is very significant. The main reason is based on the fact that measurement of instant sample cross-section in breaking moment is not simple. Difference between tensile strength calculated onto the original and instant cross-section is significantly higher e.g. strength 13 MPa calculated for original cross-section under elongation 1000 % after re-calculation onto instantaneous cross-section in breaking moment is 145 MPa [2].


Fig. 8: Graphic presentation of tensile curves

Sufficient strength does not guarantee high rubber quality in general. There is no relation between tensile strength and resistance to freeze, effervescing, gas permeability, unclear relation is to wear resistance, ambiguous to dynamic properties. The main chemical character of rubber determines the rubber mark. This in some extent influences physical-mechanical properties and also type of cross-links. The cross-links may have different chemical disposition and different length. From this reason also strength of cross-links is different. Their strength goes up in this order:

C – Sx – C (x > 2) C – S2 – C C – S – C C – N – C C – O – C C – C


Also thermal resistance of appropriate vulcanizates and their resistance to permanent deformation goes up in the same order. But in the same order the strength goes down for vulcanizates having cross-links created by the above mentioned structures. So there is a paradox: the stiffer cross-links are created during curing the lower mechanical strength of vulcanizate. This phenomenon is caused by the fact that cross-links are not arranged regularly in real vulcanizate. In some areas the concentration of cross-links is significantly higher than average one, but in other areas it is significantly lower. If this network is deformed then the stress on shorter chains is considerably higher because of their lower expansion. Under some deformation breaking of the most loaded cross-links (the shortest one) occurs, if the cross-links are approximately as stiff as cross-links of the main chain of the rubber hydrocarbon. Thereby, stress in adjacent cross-links is immediately increased and avalanche cracking of additional cross-links occurs together with following destruction of testing piece or product. In case of some less stiff cross-links present in loaded vulcanizate in comparison with the main chain of the rubber hydrocarbon, continual cracking of less stiff cross-links happens already under lower stress than in the first case. Thus created macro-radicals have sufficient time to create new cross-links in micro-regions with lower stress. Thereby „sliding of cross-links“occurs and this results to more equal stress distribution to bigger amount of network chains. Thus the testing piece or product is more deformable (has higher ductility) and avalanche cracking of cross-links occurs only at higher total stress, it means under higher total strength [2]. Rubber tensile properties are greatly influenced by proportioning of fillers besides used rubber. All fillers change the main physical properties of rubber and mostly hardness, tensile strength, modulus, abrasive resistance and resistance to effervescing. For filling mixtures the most common criterion is value of the stress under 300 % elongation (modulus 300 % or M300). This value is not linear function of the filler concentration in the whole scope, but it has a parabolic shape. It is changing lightly with small concentrations, but is sharply mounting up with higher concentration. There is great difference in influence of individual fillers to modulus. Practice did show that under specified filler concentration satisfactory linear dependency is valid between value M300 and value of the oil adsorption, it means that the main parameter of fillers (mostly carbon blacks) influencing size of modulus is their structure [2]. Firmness of rubber composed of natural dry rubber and other rubber gelling under tensile deformation is only lightly influenced by fillers. Active fillers increase it only slightly under optimum feeding, without non-active change and slight reductions under higher concentrations. The fillers have tremendous influence on tensile strength of non- gelling rubbers (e.g. SBR). Strength of unfilled rubber is approximately 2 MPa and all fillers increase it. The biggest influence has the most active fillers under optimum feeding (strength up to 28 MPa). Activity of fillers is measured mainly by value of their specific surface. Tensile strength initially rises with increasing concentration of fillers, but lowers again after achievement of some maximum value.


Tensile tests are done on shredders by suitable loading scope. Different types of testing pieces are used for test (see: obrázky\IMG_6998.jpg). The most frequently used testing pieces have shape of double-sided blade, but sometimes also testing pieces in shape of rings are used. The main criterion for selection of testing pieces in shape of rings or blades is:

• tensile strength: testing pieces in shape of double-sided blades are more suitable

for statement of tensile strength. Rings afford lower (sometimes much lower) values of tensile strength than double-sided blades

• elongation at brake: rings offers approximately same values as double-sided blades under provision that: − elongation of rings is calculated in percentages from their initial internal

perimeter − double-sided blades are cut-off in upright orientation direction, if marked.

Double-sided blades are used always when monitoring of orientation influence is required. The rings are not suitable as testing pieces for this test. • Elongation under specified stress and stress under specified prolongation:

preferably testing pieces in shape of double-sided blades are recommended in type 1 and 2, serving preferably for values specification. The rings and double-sided blades affords approximately same values provided that: - elongation of rings is calculated in percentages from their initial internal perimeter - double-sided blades are used for determination of average values from pieces cut-off in longitudinal and upright orientation direction, if marked.

• small testing pieces can afford different and mostly higher values of tensile strength and elongation at brake when compared with big testing pieces.

The testing pieces are prepared by cutting with suitable cutting knife (see: nect figure) from cured plate or ring.

Thickness of testing pieces in shape of double-sided blade is 2 mm ± 0,2 mm, thickness of rings is 4 mm. Some rubber companies have their own testing pieces for tensile test. Dimensions of individual cutting knives (dies) in shape of double-


sided blades in accordance with STN ISO 37 are stated in tab. 1 according to [3] and their shape is shown on Fig. 9: Tab. 1: Dimensions of cutting knives (dies) in shape of double-sided blades for assessment of tensile properties

Type of testing piece Dimensions of cutting knives (dies) Type 1 Type 2 Type 3 Type 4 Total length (minimum) (A) [mm] 115 75 50 35

Width of blades (B) [mm] 25,0±1,0 12,5±1,0 8,5±0,5 6,0±0,5

Length of narrowed part (C)[mm] 33,0±2,0 25,0±1,0 16,0±1,0 12,0±0,5

Width of narrowed part (D) [mm] 4,00,06 +

− 4,0±0,1 4,0±0,1 2,0±0,1

External transition radius (E)[mm] 14,0±1,0 8,0±0,5 7,5±0,5 3,0±0,1

Internal transition radius (F) [mm] 25,0±2,0 12,5±1,0 10,0±1,0 3,0±0,1

Length of operating part [mm] 25,0±0,5 20,0±0,5 10,0±0,5 10,0±0,5

Fig. 9: Cutting knife (die) in shape of double-sided blade (Explanation of symbols in tab. 1)

Testing pieces in shape of standard type A rings must have internal diameter of 44,6 mm ± 0,2 mm. Axial thickness diameter and radial width diameter must be of 4,0 mm ± 0,2 mm. Testing pieces in shape of standard type B rings must have internal diameter of 8,0 mm ± 0,1 mm. Axial thickness diameter and radial width diameter must be of 1,0 mm ± 0,1 mm. Testing is performed under specified deformation speed and constant temperature. The loading speed for testing pieces type 1 and 2 is 500 mm.min-1, and value of deformation speed for testing pieces type 3 and 4 has magnitude of 200 mm.min-1, this value for testing piece in ring shape type A is 500 mm.min-1 and for type B it is 100 mm.min-1 [3].


Stress value is scanned by means of suitable sensor, prolongation is most often measured by contact or optical extensometer eventually video-extensometer (Fig. 10). Presently, the shredders are normally equipped with computer unit serving for data collection and evaluation of measured data.

Fig. 10: Detail of the testing piece in shape of double-sided blade fixed in jaws of

shredder. There is shown also fixing of extensometer jaws for assessment of elongation.

In some cases, tensile test can discover value of equilibrium modulus, tensile elasticity and relative static hysteresis, eventually relative static relaxation. Tensile equilibrium modulus is stated on testing piece in shape of double-sided blade or ring in such way that testing piece is stretched with speed 500 mm.min-1 up to achieved required prolongation of the testing piece operating part. In this moment the stretching is stopped and falling stress is recorded; it will stabilize on value characterizing specified material. Tensile elasticity is ratio of returned work to work delivered during fluent stretching and releasing of testing piece, relative static hysteresis is defined as ratio of difference between delivered work and returned work to delivered work during its fluent stretching and releasing. Tensile elasticity and relative static hysteresis is calculated from areas rate under tensile curve.

4.1.2 Hardness assessment (ISO 7619, DIN 53 505, ASTM D 2240)

Hardness measurement is based on the depth measurement of a spike penetration with defined dimensions into material. Theoretically valid fact is that rubber resistance against penetration is dependent at small deformations on the elasticity modulus value, testing piece dimensions and size of impressed piece. For roller intender the impressing value is directly proportional to loading and indirectly proportional to its diameter.


The penetration depth is specified by this formula:

dP.Kh = , (32)

where: K – constant factor including elasticity modulus, P – compressive loading, d – roller intender diameter. Different structures of hardness testers are used for rubber hardness testing (e.g. electronic hardness tester ZWICK Shore A: next figure).

Their principle is based on measured depth of measurement piece penetration into sample. Differences between individual hardness testers are mainly in shape and dimensions of testing spike, loading magnitude and impact character and also results presentation method. The most widely used method of the hardness measurement is Shore A. This one is suitable for rubber hardness measurement in interval from 35 to 80 Shore A. Testing tip of the Shore A hardness tester is produced from hardened steel with diameter 1,25 mm. Its tip has shape of cut cone with peak angle 35° and diameter of the cut tip is 0,79 mm. Spring tension is specified by formula [4]:

AH.75550F += , (33) where: F – compressive force in milinewtons, HA – hardness value, measured on hardness tester of A type. Hardness tester Shore D is used for measurement of small testing pieces; it differs from Shore A hardness tester by different compressive force and spike geometry.


Compressive spring tension of the Shore D hardness tester is specified by this formula:

DH*445F = (34) Depth of tested piece must be at least 6 mm. To achieve this thickness, the testing piece can be produced from thinner layers, but because of possible imperfect contact between layers the results on such connected pieces may not correlate with results achieved on pieces produced from one piece. The testing piece must be sufficiently big to allow measurement in distance bigger than 12 mm from its border. Because of strong visco-elastic (during hardness measurement viscous) rubber properties, the hardness measurement must be performed in strictly specified time after applied hardness tester to testing piece. The standard STN 62 1431 specifies this time onto 15 s ± 1 s [12.5]. Rubber hardness is increased with addition of any fillers and it is linear up to high concentrations. Influence of fillers is characterized by filler constant depending on filler type and it is related to the value of oil adsorption.

4.1.3 Assessment of reflection elasticity (ISO 4662, DIN 53 512) Assessment of reflection elasticity is one of the best known methods to determine the rubber ability to absorb mechanical energy during impact. This measurement is based on the height measurement of the metallic pendulum bounce from testing piece. Two methods are based on the above mentioned principle: Schobe and Lüpke method. Reflection elasticity value depends on testing piece thickness, Schobe reflection elasticity is increasing with thickness of testing piece up to 12 mm, Lüpke method has optimum thickness value of 13 mm up to 16 mm. Temperature and testing piece surface have significant influence on reflection elasticity value. Schobe pendulum is created by stiff hammer, ended with hemispherical bumper with sphere diameter of 15,0 mm ± 0,05 mm, and weight of 0,25 kg - 0,1 kg. Pendulum impact speed is 2,00 m.s-1 + 0,6 m.s-1. Deformation energy density must be 427 kJ.m-3 for testing pieces with thickness of 12,5 mm. Testing unit for assessment of Schobe reflection elasticity (ZWICK company) can be seen on:


Testing piece should have diameter 29 mm ± 0,5 mm and thickness 12,5 mm ± 0,5 mm. If testing pieces are prepared from products having lower thickness, it is possible to place them at each other. If testing pieces are prepared by means of pressing or cutting with rotation knife, thus they should not contain bracing material. Procedure for measurement of reflection elasticity is this: the first testing piece will be fixed to apparatus and pendulum will fall six times on sample surface but the first three strokes are not taken into account. Next three strokes are taken into median calculation. The same process is done with the second testing piece. Result of this test is stated as arithmetic average value from medians of two testing pieces [5].

4.1.4 Assessment of structural strength (ISO 34-1, ASTM D 624) This test belongs to structural tests together with assessment of abrasion resistance. Assessment principle consists on tensile loading of suitable testing piece. Selection of suitable testing piece is very important because mostly under influence of big rubber deformation ability and high value of structural strength of wrongly selected shape and size of testing piece the value of breaking force expresses tensile strength instead of structural strength (force needed for breaking the vulcanizate structure). Following types of testing pieces are used (Fig. 11):

• crescent (without notch), • graves (with notch or without notch), • trousers.

The tests are performed on shredders similarly as in case of tensile properties assessment; deformation speed is 500 mm.min-1. Result is expressed as force needed for breaking of the sample with specified thickness in kN.m-1 [6].


c.

b. a.

Fig. 11: Shapes of testing pieces for structural strength assessment (a- crescent,

b- graves, c- trousers)

4.1.5 Abrasive resistance assessment (ISO 4649, DIN 53 516) The abrasive resistance assessment is important mostly for products that are frictionally loaded during their exploitation (e.g. tire treads/protectors, covering layers of conveyor belts). Product life can be expected in accordance with the abrasive resistance stage. Until now approximately 30 different laboratory apparatuses types were developed for the abrasive resistance assessment. But there is no criterion existing for examination of the specific apparatus applicability. Absolute conformity between laboratory results of abrasiveness and vehicle tests practically does not exist. The method procedure (Bussen-Schlobach method) is based on fact, that testing piece in shape of roller with diameter 16 mm and minimum height of 6 mm is rubbed on roller having its surface covered with abrasive agent. Standard abrasive-coated papers with alumina (Al2O3) grains of specified size are used as abrasive agent. Standard mixture on such surface should have the weight loss of 180 – 220 mg after completed path of 40 m and pressure of 10 N. Testing piece is reground before testing, then its weight is measured and it is rubbed on rotating roller covered with the abrasive agent. The weight loss is discovered after completed path of 40 m and relative volume loss ∆V is calculated (for non-rotating testing piece):

]mm[m

200.VV 3

g

t=∆ , (35)

where: Vt - volume loss of testing sample in mm3,

mg - weight loss of standard rubber in mg.


Abrasive resistance index ARI for identical testing piece (rotating or non-rotating) is calculated on the basis of this formula [7]:

[%]100.VV

ARIt

s= , (36)

where: Vs - volume loss of standard rubber in mm3, Vt - weight loss of testing rubber in mm3.

The equipment used for testing is shown on Fig. 12:

Fig. 12: Structure of the apparatus for abrasive resistance assessment

4.2 Ageing tests Under the influence of different outside factors different changes, in particular chemical occurs in rubber and these are resulting to reduced product life. These changes can be totally characterized as ageing. Main factors causing reduction of material life are aerial oxygen, ozone, increased temperature and electromagnetic radiation, in particular from near ultraviolet and visible spectrum part. The above mentioned factors have mutual influence during product exploitation. Determination of these influences under natural conditions is very time consuming and achieved results are not be reproducible. From this reason the tests of accelerated ageing were developed with modified testing conditions in such way that only one of the above mentioned factors prevails (e.g. ozone ageing, oxygen influence is negligible under given conditions) or the above mentioned factors can be combined (accelerated thermal ageing in oxygen or in the air).


The most ageing resistant rubbers are those not containing double linkages in chain (saturated rubbers). Majority of products are produced from non-saturated rubber types. The most significant influence on rubber has ozone present in the air in concentrations from 0,01 ppm up to 0,06 ppm, that is created on the Earth’s surface under ultraviolet radiation effect eventually the electric discharges in atmosphere. Ozone received its name under characteristic odor (from Greek word „ozein“, which means „bad smelling“, and it is toxic). Ozone reacts with non-saturated carbons of rubber hydrocarbon in exposed and deformed rubber, molozonit is created and this is changed into isoozonit in next stage. Consequently cracks are created across the stress direction. Accrued crack exposes un-attacked rubber and originally small cracks are rapidly increasing. Aerial oxygen causes so called oxidation ageing of the rubber. Under normal temperature this ageing will occur only after very long period, but increased temperature greatly accelerates oxidation ageing. The less resistant rubbers are those with great concentration of double linkages (oxygen attacks the most reactive area in the rubber chain) and that is in case of non-saturated rubbers in alpha-position to double linkage (on carbon atom adjacent to non-saturated carbon). Hydro-peroxides are created, and those are dissolved under increased temperature or catalytic effect (light, rubber toxicants) onto free radicals (mainly type RO2). Radicals afterwards react with rubber hydrocarbon and thus splitting or screening of the rubber chains occurs. In polyisoprenes predominates splitting of chains, reversely oxidation screening in homo-polymers and butadiene co-polymers. Sulfur vulcanizates lacking ozone are subject to changes that are directly proportional to increasing concentration of polysulphidic cross-links (anaerobic ageing). Disulphidic and monosulphidic cross-links are created from polysulphidic cross-links and additional new cross-links are created from released sulphur. Rubber module is growing and also other physical-mechanical properties of rubber are changed (ductility and dynamic loading resistance are decreased) [2].

4.2.1 Assessment of accelerated thermal ageing in air (ISO 188, ASTM D 865, DIN 53 508) Method is based on comparison of physical-mechanical properties of testing pieces exposed in non-deformed status to impact of the hot circulating air during defined time in thermostat (method A) or in test-tubes (method B) and testing pieces, which were not exposed to increased temperature influence. Resistance against accelerated thermal ageing in air will be determined on a set of physical-mechanical properties important during practical usage of the product. Unless such set is stated, it is recommended to state following properties: tensile strength, module during specific prolongation, ductility and hardness. Selection of testing pieces is chosen on the basis of selected monitored physical-mechanical properties and in accordance with appropriate testing standard. Testing equipment for method A utilizes a thermostat with forced air circulation to achieve complete exchange of air in the thermostat


operating area at least 3 and maximum 10 times per hour. Method B utilizes test-tubes inserted in heated metal block, eventually liquid baths with the same air exchange as used in method A. Testing pieces must not fill the thermostat chamber volume for more than 10 % from its volume. Distance in between testing pieces in thermostat must not be smaller than 10 mm and distance of the testing piece from the chamber wall less than 50 mm for method A and for method B these values are minimum 5 mm and 10 mm. Temperature selection and time of increased temperature influence is chosen in accordance with the test goal and rubber character (increased temperature effect should not cause visible damage on testing pieces. One from following temperatures is used for testing: 70°C, 100°C, 125°C, 150°C, 175°C, 200°C, 225°C, 250°C, 300°C and 350°C. The test duration is 24, 72, 168, 240 hours or multiples of 168 hours. It is not recommended to perform common tests of different rubber types because of sulphur migration, antidegradants, peroxide eventually softeners. In marginal conditions it is allowed to place into thermostat chamber at the same time testing pieces composed of the same type rubbers with approximately same sulphur content and one accelerant type, with content of one antidegradant type, with content of approximately same softeners amount of equal type. Changed values of the physical-mechanical properties (symbol S) (except hardness) is calculated on the basis of formula (37) (symbols A1, A0 express specific physical-mechanical property (e.g. strength, ductility etc.):

[%]100.A

AAS

0

01 −= (37)

Changed hardness is calculated in this way:

01 HHH −=∆ , (38) where: H1 – hardness value after ageing,

H0 – hardness value before ageing;

4.2.2 Assessment of accelerated thermal ageing in oxygen (ASTM D 572) Principle and also procedure of the method is equal with the previous method, difference is only in usage of different testing equipment that consists of the oxygen chamber and thermostat. After removal of the air from chamber the pressure of oxygen inside of the chamber will be adjusted to 2,1 MPa and temperature to 70°C [9].

4.2.3 Assessment of resistance against ozone influence (ISO 1431-1) The testing pieces are exposed to static tension stress in closed chamber at constant temperature in atmosphere containing specified ozone concentration. The testing pieces are periodically inspected if there are no cracks on their surface. Three alternatives of evaluating procedures are described as to select the ozone concentrations and exposition time:


• assessment of attendance or absence of cracks and in case of necessity to decide about cracking degree after exposition during specified time at selected deformation,

• assessment of time, when the first cracks will show at any selected deformation value,

• assessment of threshold deformation at any chosen exposition time; Threshold deformation is the highest tensile stress, when the rubber can be exposed to air with specified ozone concentration, at specified temperature, in specified exposition time without creating cracks on it. Limiting threshold deformation is tensile deformation under which the time needed for formation of ozone cracks grows very quickly and can become practically unlimited. The testing pieces have the strip shape (width minimum 10 mm, thickness 2 mm, length minimum 40 mm, see: next figure) or double-sided blade (width 5 mm, length 50 mm), their surface must not be damaged (ground, cut etc.).

The test is performed under one of these ozone concentrations: 25 pphm, 50 pphm, 100 pphm, 200 pphm. Preferably the tests run at 40°C, but also other temperatures can be used, e.g. 23°C. Normally the tests are performed at one value or more prolongation values (tensile deformation): 5%, 10%, 15%, 20%, 25%, 30%, 40%, 50%, 60%, 80%. The testing pieces are deformed during method A by tension to 20 % ductility and then inspected after 72 hours of exposition in testing chamber, if there are no ozone cracks shown on their surface. Also other values of deformation and exposition times can be used. Result of the test is statement on attendance or absence of cracks, eventually degree of cracking is stated (description of cracks). Under method B the testing pieces are deformed onto one or more prolongation values (ozone spectrum)


and occurrence of ozone cracks is inspected after 2, 4, 8, 16, 24, 48, 72 and 96 hours, eventually also in other suitable intervals after the lapse of this time. The time when crack occurs during each deformation is recorded. Under method C the testing pieces are deformed onto four deformation values and surface of testing pieces is inspected in intervals used under method B to enable to determine threshold deformation. The result of this test is stated in range with threshold deformation specifying the highest prolongation value without observed cracks creation and the lowest prolongation value with cracks creation discovered [10]. Next evaluation method for resistance against ozone is test assessment of rubber resistance against ozone by threshold deformation method on testing pieces in trapezium shape (STN 62 1529). The testing piece of trapezium shape (see figure above) is fasten into fixture in such way that bottom part of the piece in contrary to the top one is not deformed. The range of deformation values is chosen from six possible ranges. The piece fastened in this manner is continuously deformed along its height from zero deformation up to maximum. The test duration (24, 8 and 4 hours) is selected in accordance with used ozone concentration (50, 100 and 200 pphm of ozone). The threshold deformation is calculated as ratio of distance of the first created crack from non-deformed edge and maximum deformation value to the height of the whole testing piece. It is indicated in %. The testing piece from vulcanized rubber is shown on Fig. 13:

Fig. 13: Testing piece for assessment of resistance against ozone by threshold

deformation method

4.2.4 Assessment of liquids influence (ISO 1817, DIN 53 521, ASTM D 471) Characteristic property of rubber materials is their very low resistance against solvents, in particular organic ones. Non-vulcanized rubber is well dissolved in organic solvents, but this property is getting lost by rubber vulcanization (cross-linking). Vulcanizate is only swelling in solvents. Solvent extracts without cross-linking to low molecular fractions (used during chemical-analytical analysis of vulcanizates to assessment of the extract by chosen solvent extraction). The swelling is stopped at some concentration ratio of rubber and solvent. Achieved equilibrium depends mostly on the rubber cross-linking degree. Thus it is possible to determine this vulcanization degree by experimental assessment of swelling


degree (except rubber cross-linking degree also active fillers influence the swelling, as they reduce constitutional degree of swelling). Besides these two above stated forms of liquids influence (extraction of ingredients from rubber and liquid absorption), the solvent may chemically react with the rubber. Absorption effect is usually bigger than extraction effect, which normally leads to increased volume of testing piece. Curve course dependency of volume change on time results in the fact that volume of testing piece grows quickly at the beginning, than the volume change grows slower and in linear way. Liquid absorption as well as extraction of soluble rubber components (e.g. softeners) has influence on physical-mechanical properties of vulcanizate (change of volume, dimensions, weight, tensile properties, hardness etc.). During assessment of resistance against liquids it is very important to select correct testing temperature and air access amount, as increased temperatures speed up rubber oxidation, evaporation or destruction of testing liquid. As the speed of liquid penetration into rubber depends on several factors, it is not possible to select only one time interval for test (it is recommended to evaluate the liquid influence after some intervals to achieve time change curve of physical-mechanical properties, preferably chosen testing time leads to steady state). Assessment of the volume and weight change is done on three testing pieces with volumes from 1 cm3 up to 3 cm3 and thickness of 2 mm. Testing pieces are fully immersed into tank with testing liquid and they must neither touch each other nor the tank walls. Liquid volume must be 15 times bigger than volume of testing pieces. Percentage change of volume or weight is calculated according to these formulas:

[%]100.)mmm(

)mmm()mmm(V

521

521543100 +−

+−−+−=∆ (39)

[%]100.m

mmm

1

13100

−=∆ , (40)

where m1 – beginning weight of testing piece on air,

m2 – beginning apparent weight of the piece (plus tag, if used) in water,

m3 – weight of testing piece on air after liquid influence,

m4 – apparent weight of the piece (plus tag, if used) in water after liquid influence,

m5 – apparent weight of the tag in water; Appropriate measurement system is used for assessment of changed dimensions (length, width, thickness) and the best one is optical unit without contact. The testing piece must be in rectangular shape with length of 50 mm, width of 25 mm and thickness of 2 mm. Test result is percentage change of testing piece surface:


[%]100.1l.ll.l

Aba

BA100 ⎥

⎦

⎤⎢⎣

⎡−=∆ , (41)

where lA a lB – lengths of diagonals after liquid influence, la a lb – lengths of diagonals before liquid influence.

Physical properties changes can be stated immediately after testing liquid influence or after it removal or drying. Evaluated is change in tensile properties and hardness, but in specific cases also other physical-mechanical properties. Testing piece for assessment of tensile properties should have shape of ring or double-sided blade according to standard ISO 37, testing piece for assessment of hardness should have thickness of 2 mm and other dimensions at least 8 mm (ISO 48). Test procedure, selection of solvent, liquid effective time and testing temperature is identical with the procedure used for assessment of changed dimensions. Tests without liquid drying must be done within 3 minutes after taking test piece out of testing liquid. Assessment of physical properties change after drying should be done on testing pieces dried at temperature 40°C and pressure of 20 kPa, dried onto constant weight (weight change after 30 minutes should not overrun 1 mg). Results expression is the same as during assessment of accelerated thermal ageing. The test of liquid influence is done only on one surface in case of relatively thin flat materials that are exhibited to liquid influence on one side. A special fitting is used for this test and that allows contact of testing piece with liquid only from one side. Changed weight per unit area is evaluated (g.m-2) [11].

4.2.5 Assessment of permanent tensile deformation (DIN 53 518) Goal of this test is to assess the permanent tensile deformation value of the vulcanizate that characterizes viscous share of rubber viscous-elastic behavior. It serves for evaluation of appropriate usage of rubber products as tire-tubes, membranes, gloves. Value of permanent tensile deformation as well as compressive deformation depends on vulcanizate structure. The higher ratio of polysulphidic cross-links contained in vulcanizate, the higher values of permanent deformations are achieved. This phenomenon is related to the strength of cross-links, the less firm is cross-link the easier cracking occurs and during this also mass flow happens. After some time new cross-links are created and those provide fixation of the piece deformed shape. Basis of the test are in assessment of remaining length on operating part of testing piece loaded under constant deformation conditions, testing temperature during testing period. The testing pieces are used according to standard STN ISO 37 type 1, shape of double-sided blade. After fixing the testing piece into fixture it is deformed to one from prescribed deformation values (25 %, 50 %, 100 %, 200 %, 300 %). Selected value should not be higher than 1/3 of the tested rubber ductility. For normal assessment it is suggested to use value of the constant deformation


100%. After that the sample is inserted together with fixture (see next figure) into hot-air oven preheated onto specified temperature (23°C, 70°C, 85°C, 100°C, 125°C, 150°C) and is left there during specified period (24 hours, 72 hours, 168 hours). After testing piece removal the distance between marks is measured and permanent deformation value is calculated on the basis of this formula (42):

[%]100.llll

TD0s

01

−−

= , (42)

where: l0 – distance between reference marks on testing piece before test in mm, ls – distance between reference marks of stretched testing piece in mm, l1 – distance between reference marks on testing piece after recovery in mm.

4.2.6 Assessment of permanent compressive deformation (ISO 815) Testing procedure is similar to the assessment of permanent tensile deformation, but difference is in shape of the testing piece. The testing pieces in shape of roller type A and B can be used. The testing piece type A is a roller with diameter of 29 mm and thickness of 12,5 mm. The testing piece type B is a roller with diameter of 13 mm and thickness of 6,3 mm. Deformation magnitude is selected in accordance with the rubber hardness from 10% to 25%. Calculation is similar to the previous case (12.14). Also fixture shown on Fig. 14 can be used for the test.


Fig. 14: Fixture for assessment of the permanent compressive deformation

4.2.7 Assessment of the brittle temperature limit (ISO 812, ASTM D 746) Basis of this method are in assessment of temperature, when the testing piece remains intact. On the basis of this value an approximate temperature limit for usability of this product under low temperatures is discovered. Testing pieces with width of 6mm, length of 25mm and thickness of 2mm are used for this test (operating part of double-sided blade for tensile test). Testing piece is fixed into testing equipment that creates fixing unit with movable hammer having moving speed (2,0 ± 0,2) m.s-1 and cooling chamber with mixer and temperature sensor. The hammer falls down onto the sample after its fixing and cooling (by mixture of denatured alcohol with solid carbon oxide). Damage of the sample after this impact is detected. The temperature regarded as brittle limit is that one, when the sample still remains without cracking [14].

4.3 Dynamical tests

All the above mentioned tests are static tests, it means that sample is relatively standstill during the test. But many rubber products are dynamically loaded during exploitation (e.g. automotive tire – during movement, material destruction occurs very often under dynamic loading and that is in spite of the fact that deformations are not higher than critical – the reason is in fatigue of material). From this reason appropriate usage of rubber mixture in automotive tire is determined by knowledge


of its dynamical properties. The tests are suitable in particular for dynamically loaded mixtures e.g. sidewalls of the automotive tire.

4.3.1 Assessment of resistance against cracks creation and growth by crimping (ISO 132, ISO 133)

This test is based on repeated bending of the testing piece by constant frequency when the surface change is monitored (crack created in narrowed sample part) or crack dimensions artificially created before test (width of initial crack is 2 mm) in dependence on number of bends. In the first case a graph is designed where damage degree is stated on y axis and corresponding number of bends is put on x axis. The points will show a curve and amount of bends needed for each damage degree can be deducted. The test set-up is shown on Fig. 15. Growth of cracks is evaluated in similar way as creation of cracks, only axis y is used for plotting the crack size. Amount of bends needed for following growth of cracks is deducted from achieved dependency [15]:

• from l0 to l0 + 2 mm,

• l0 + 2 mm to l0 + 6 mm.

• l0 + 6 mm to l0 + 10mm.

1- Bottom fixing (movable) jaw 2 - Testing piece 3 - Top fixing jaw 4 - Deformed testing piece

Fig. 15: Sample deformation during assessment of resistance against cracks creation and growth by crimping


4.3.2 Assessment of fatigue resistance and temperature increase on the DPGi apparatus (PN)

Testing piece in shape of roller with diameter 20 mm and length of 100 mm is fixed into rotation jaws of the DPGi apparatus and required bending angle is adjusted by movement of the fixing unit. Bending magnitude is controlled by means of a radius gauge. The testing piece rotates with frequency 15 Hz. During assessment of fatigue resistance, the testing piece is kept in rotation up to its destruction and consecutive reading of performed revolutions. Material warming is determined by measurement of sample internal temperature after completion of some amount of revolutions (most often after 10 000, 20 000 and 30 000 revolutions) by means of appropriate sensor [16]. The test principle is clear from Fig. 16.

Počítadlo ohybov - Counter of bends Skúšobné teliesko - Testing piece Dotakový vypínač - Contact switch Oblúková dráha - Arch path

Fig. 16: Assessment of DPGi

4.4 Adhesion tests

Tire is a composite material, which means that it is composed from different structural materials like rubber mixtures themselves, steel cord and textile-cord carcass stiffening materials. From safety drive point of view and life of the pneumatic tire it is extremely important to assure their mutual adhesiveness in the highest possible level to avoid separation of different layers of the stiffening materials. High adhesion level between so different materials as metal (eventually textile) and rubber is based on condition that chemical bonds will be created on the phases interface. The evaluation methods for the stiffening materials adhesion may be static or dynamic and have crucial importance for so called adhesion mixtures (amount of the adhesion mixtures in the pneumatic tire is low in comparison with other products). In principle, all the adhesion evaluation methods measure the force needed for separation of the stiffening material from the rubber under specified conditions. Difference can be in samples preparation, setting of the experiment, measurement conditions.


4.4.1 Assessment of rubber adhesion with metal (ISO 814)

Three methods for assessment of rubber-metal adhesion are described here. The first method (one board method) is based on usage of one metallic plate pre-cured with sample of rubber mixture. This pre-cured sample is inserted into shredder and the force needed for „detachment“is measured. The second method is based on assessment of the force needed for detachment separation of metallic rollers connected with pre-cured rubber mixture in between. The third method is similar to the second one, except the fact that the metallic rollers are not plane parallel, but operating areas create the cone. Test results are stated as adhesion values expressed in kN/m for the first case, in MPa for the second case and in kN for the third case [17].

4.4.2 Assessment of adhesion between layers during separation This standard describes assessment of adhesion between layers rubber-rubber,

rubber-textile, rubber-filling textile and filling textile-filling textile. The force needed for detachment separation of individual layers by shredder is recorded. The sample is prepared by pre-curing of individual layers in vulcanization form. Basis for calculation of average value is 80 % of the middle part on graphic record force-prolongation. The result is stated in kN/m [18]. Samples separation plane must be on a level with tension force impact, failing which can cause different results as shown on Fig. 17:

Rovina separácie - Separation level Správne – Correct; nesprávne - Incorrect

Fig. 17: Assessment of adhesion between layers during separation

4.4.3 Assessment of rubber static adhesion with fabric cord (H-test) (ISO4647) This method is based on assessment of adhesion between rubber and fabric cord on the basis of measured force needed for tearing out the cord from molded rubber block. The testing piece has shape of „H“ letter, where the name „H-test“ comes from. The test principle is clear from Fig. 18:


Smer sily - Force direction Kord - Cord

Fig. 18: The testing piece for assessment of rubber-cord adhesion (H-test) Test result is stated as adhesion value expressed in N, stated in shredder

under specified conditions (speed of the traverse movement, temperature) [19].

4.4.4 Assessment of adhesion after dynamical deformation by Henley method A cord sample is cured into rubber roller (see next figure), that is loaded with pressure in the middle part (Fig. 19). Deformation is adjustable in extent from 35 % to 55 %, deformation frequency is 7,5 Hz. The testing piece is dynamically stressed under increased temperature (80 °C).

After finished deformation the roller is cut into three parts taking care to leave the cord intact. The cord is cut near that dynamically stressed part. In this way three testing pieces are achieved; two of them are non-deformed (A) and one is dynamically stressed (B). The adhesion is determined on shredder by means of special fixing jaws. The test result is given in average adhesion value of dynamically non-stressed and stressed part of the sample and adhesion loss calculated by usage of this formula (43), [20]:

[ ]%100.A

BAlossadhesion −= (43)


Fig. 19: Testing piece and its dynamic/impulse stress during Henley test 5. Dynamical-mechanical-thermal analysis (DMTA) Dynamical-mechanical analysis can be simply described as application of the oscillation force on sample and analysis of the material response to this force (Fig. 20). Properties are calculated from this and also flow tendency (called viscosity) from the phase shift and stiffness (module) from sample recovery [21].

Fig. 20: Operation principle of DMA – DMA will add oscillation force causing mostly sinusoidal loading applied on the sample, which creates sinusoidal

deformation. Measurement of material response to loading will discover the phase shift between applied force and real deformation behavior in tested material and values of module, viscosity and absorption can be calculated. (applied stress =

aplikované zaťaženie (napätie); material response = materiálová odozva, phase lag = fázové oneskorenie; amplitude = amplitúda)

Applied force is called stress (σ). When material is put under load, deformation (γ) will be expressed. These data are traditionally achieved from mechanical tensile


tests under constant temperature and result is shown as stress-deformation curve on Fig. 21. Slope of the straight line indicates relation of stress to deformation and extent of material stiffness, so called module. Module is dependent on temperature and applied stress [21].

Fig. 21: Stress-deformation curve gives the force into connection with deformation. Ratio of stress (loading) to deformation is module (E) as the extent of the material

stiffness or its resistance to deformation. Young’s modulus – slope of the initial linear part of the stress-deformation curve is normally used as indicator of the

material behavior. Fundamental measured values from DMA analysis are force and changed dimensions of tested material. These values are used for calculation of modules; the module type depends on type of dynamical loading (i.e. shearing, tensile, bending and pressure stress etc.) and viscosity value that characterizes creep material resistance. One of DMA advantages is that we can achieve module every time and application of sinusoidal curve gives us analysis during the whole length of temperature, frequency, deformation and time scope. Module measured by means of DMA is not exactly the same as Young’s modulus of classical stress-deformation curve (Fig. 22). The Young’s modulus is slope of stress-deformation curve in its initial linear area. In case of DMA, the complex modulus E*, elastic modulus E´ and imaginary (loss) modulus E´´ are calculated from material response of the sinusoidal behavior. These different modules allow us better characterize the material as we can better explore the material ability to return back or stored (retained - elastic) energy (E´) to its energy loss ability – plastic component (E´´), and ratio of these effects (tan delta), which is also called absorption [21].


Fig. 22: Mutual relation between modules E*, E´, E´´, tan delta and η*

Materials also show some types of creep behavior and it is valid also for those materials we think of as solid and rigid. Such materials have final viscosity very high and generally we can state that „if we wait sufficiently long time thus everything flows“. These times are sometimes so long that it is not possible to measure them, but creep tendency can be calculated. This example illustrates the fact that the question of rheology is not “if the subjects creep”, but “how long will they creep” [21]. One example for utilization of DMA to explore the material properties is development of graphic dependency of the elastic modulus on temperature (in case we scan the sample under constant heating or cooling speed), where we can notice modulus change under low temperatures. This transition is marked as glass transition (Tg) and it is also called alpha-transition (Tα). Tg or Tα can be awarded to partial chain motions. Except of the glass transition it is possible to scan also so called beta-transition (Tβ) which is accredited to other changes in molecular motions. β-transition is often connected with side chains or motions of groups and may be in relation with polymer stiffness [21]. Idealized DMA temperature analysis is shown on Fig. 23.

Fig. 23: Idealized DMA temperature analysis (melting = mäknutie; Rubbery

Plateau = kaučukové plató; glass transition = sklený prechod; local motions = lokálne pohyby; bend and stretch = ohyb a pretiahnutie; side groups = bočné

skupiny; gradual main chain = čiastočný hlavný reťazec; large scale chain = dlhý stupňovitý reťazec; chain slippage = preklzovanie reťazca)


As we can utilize DMA for fast mapping of material modulus in dependence on temperature, similarly we can use DMA also for fast examination of the shearing speed effect or frequency on viscosity. Frequency materials behavior can give us information about molecular structure. The intersection point between E´ and η* curves or between E´ and E´´ can be related to molecular weight and molecular weight distribution. By means of frequency analysis under low frequencies we can achieve so called zero-shear plateau (Fig. 24). In this area the frequency changes do not end into viscosity changes as the deformation speed is too small to evoke responses in sample. Similar effect - endless shear plateau – occurs during very high frequencies. This viscosity plateau at zero shear can be in direct relation to molecular weight above critical molecular weight according to this formula:

η = k (Mw3,4) (44)

where k is specific material constant factor [21].

Fig. 24: Zero shear plateau – one of the main utilizations of frequency data for

molecular weight estimate. The zero shear plateau can be used for calculation of polymer molecular weight by means of the above stated formula in case of known

material constant factor k and molecular weight (MW) is higher than critical value (Mc).


By the way, many types of DMA offered on the market allow creep and relaxing testing. Creep (flowing) is one of the most fundamental tests of material behavior and it is directly applicable for the final product properties. Creep-relaxing testing is also very strong analytic tool. These experiments allow to state material response to the constant loading and its behavior after removal of this loading. Creep-relaxing testing allows to achieve inside view - how the material will respond when loaded with constant load [21]. We should remember that creep is not a dynamical test – the constant loading is applied during creep step and it is removed for next relaxation step (Fig. 25).

Fig. 25: Creep – relaxing testing – Creep-relaxing experiments allow stating such balanced properties as modulus Ee and viscosity ηe. These values allow forecasting

of material behavior under imitative conditions of real usage in practice. (irrecoverable creep is lost forever = nenávratné tečenie je úplne stratené; rate of

strain = rýchlosť deformácie; retardation time is a measure of recovery = retardačný čas je mierou zotavenia)

The units that allow dynamical-mechanical testing, so called DMA (or DMTA) apparatuses are offered by many companies on the market. Equipment Qualimeter – Eplexor 500N supplied by Gabo company is shown on the next figure:


Different types of jaws can be used for individual testing types, some of them are shown on following figures: pressure jaws

dual cantilever type jaws

three point bending jaws


tension jaws


LIST OF USED SPECIALIZED LITERATURE

[1] ISO289 Rubbers and rubber mixtures, Assessment of viscosity and curing

on viscosity-meter Mooney [2] Ducháček V.: Rubber raw materials and their processing, VŠCHT Praha

1999 [3] ISO 37 Rubber or thermoplastic elastomers, assessment of tensile properties [4] ISO 7619 Rubber, plastics and ebonite, assessment of hardness by inserting

of the hardness tester point (Shore hardness) [5] ISO 4662 Rubber, assessment of rubber transition flexibility [6] ISO 34-1 Rubber, assessment of structure strength [7] ISO 4649 Rubber, assessment of abrasion resistance on apparatus

with rotating drum [8] ISO 188 Rubber, assessment method of accelerated thermal ageing in the air [9] ASTM D 582 Rubber, assessment method of accelerated thermal ageing in

oxygen [10] ISO 1431/1 Rubber or thermoplastic elastomers, cracking resistance under

ozone influence, part 1: test under static deformation [11] ISO 1817 Rubber, assessment of liquids influence [12] DIN 53518Rubber testing, assessment of permanent tensile deformation [13] ISO 815 Rubber or thermoplastic elastomers, assessment of permanent

compressive deformation under laboratory, increased or decreased temperatures

[14] ISO 812 Rubber, assessment method of the brittle temperature limit [15] ISO 132, ISO 133 Rubber, assessment method of resistance against cracks

creation and growth by crimping [16] PN Assessment of fatigue resistance and temperature increase on the DPGi

apparatus [17] ISO 814 Rubber, assessment of rubber adhesion with metal [18] Rubber, assessment method of adhesion between layers during separation [19] ISO 4647 Rubber, assessment of rubber static adhesion with fabric cord (H-

test) [20] Rubber testing, assessment of rubber adhesiveness with cord after dynamical

deformation by method Henley [21] Kevin P. Menard: Dynamic Mechanical analysis – A Practical Introduction [22] Production technology of rubber products – part rheology (lecture notes TrU

- FPT in Púchov)


test methods of rubber materials and products · summary chapter “rubber raw material testing”...

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