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ORGANIC TANNING IN THE PRODUCTION OF LEATHER FOR AUTOMOBILE UPHOLSTERY

U. Sammarco

Leather Consultant

1. Introduction The term wet-white (WW) is used to define leather that is pre-tanned using organic substances, which can be easily shaved. After shaving the wet-white is tanned using synthetic and vegetable tannins, normally treated with synthetic polymers and with various auxiliaries of a multiple chemical nature. Only organic substances are applied, in this manner we obtain finished leathers which are devoid of chrome, zirconium, titanium and aluminium, which feature a shrinkage temperature of approximately 80°C. The resulting leather is defined as Metal-free (MF)1,2

Naturally, this process must not involve the use of substances which may be considered harmful to the health of the end user of the leather article, nor compounds which may generate such in the leather through secondary reactions. Here we refer, for example, to tri and pentachlorophenol, to dyes which may form one or more carcinogenic aromatic amines, formaldehyde or oxazolidine, to the urea-formaldehyde, dicyandamide and melamine resins which can, through secondary reaction, generate chrome 6, antimony, arsenic, cadmium, mercury, lead, tin, nickel etc. In recent years there has been a growing interest in wet-white for a number of reasons. In order to avoid confusion and misunderstanding, we must immediately clarify that, at the present state of the art, this technology cannot replace the classic chrome tanning in the production of the vast majority of articles. It is effectively impossible to conceive the achievement of this objective, at least in the near future, as a result of the quality limits of the chrome-free articles in comparison to those tanned using chrome. Despite this, the use of articles devoid of chrome and other heavy metals is spreading with an interesting trend in certain specific sectors of leather utilisation, and more specifically in that of automobile upholstery. The reasons for this demand are various, and some of them directly concern the automotive industry. Effectively, in 2005 a regulation will come into force in the European Union which lays down that 95% of abandoned vehicles must be recycled. Obviously, leather devoid of heavy metals is more easily degradable that that which contains chrome. Moreover, the presence of chrome in the leather can, in certain conditions, following phenomena of oxidation, cause the formation of chromium hexavalent3,4,5,6. Cr(VI) is a carcinogenic ion, even at very low concentrations. In addition to this, the waste from the processing of leathers tanned using chrome, such as shaving, fluffing and trimming, pose problems in incineration, so that they have to be disposed of in authorised landfills. Finally, leathers tanned using organic substances, subjected to tests of accelerated ageing in conditions of high temperature and low relative humidity, feature a resistance to shrinkage and a dimensional stability superior to the chrome leathers. As a result of the elevated shrinkage, the chrome tanned leathers become deformed, while the leather tanned using organic substances remains practically unaltered. For this reason it is used to line automobile dashboards. However, we must bear in mind that if the MF leather is subjected to ageing tests conducted simultaneously at high temperatures and high relative humidity, it is completely destroyed7.

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TABLE I

THE ADVANTAGES OF M-F LEATHER OVER CHROME-TANNED LEATHER

- More easily degradable - Non-pollutant processing waste (shaving, trimming, etc.) - No risk of presence of Cr(VI) in the finished leather

- Greater resistance to shrinkage (greater dimensional

stability) in conditions of high temperature and low humidity

FIG. I

INFLUENCE OF CLIMATIC CYCLES

1 a

1 b

1 c

Test on chrome-tanned leather at low humidity and high temperature

Test on wet-white leather at low humidity and high temperature

Test on wet-white leather at high humidity and high temperature

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2. Performance and requirements for leather for automobile upholstery Car manufacturers demand high performance from all the components of the vehicles, and the leather used is no exception. Leather destined to be used in vehicle upholstery is subjected to more stringent tests than those for any other type of article, especially in the case in which it is sold directly by the tannery to the car manufacturer. In such cases these are defined as primary components. The requirements demanded of the so-called after-market articles, which are marketed by wholesalers supplying the car manufacturers, are considerably less stringent. The physical properties required of leather for automobile upholstery have assumed fundamental importance; car owners, moreover, demand that the internal decor does not deteriorate over the entire duration of the life of the vehicle. The leather used for automobile upholstery is during use subjected for a long period to extremely drastic climatic conditions. For example, it can be exposed to extremely elevated variations of temperature. When the vehicle is parked in the sun during the summer period, the interior can reach a temperature of approximately 100°C. Similarly, in some countries, the internal temperature of a vehicle parked outside at night can fall to levels of less than -10°C. The concomitance of customers’ expectations and the severity of the conditions of use demand that most of the leather used for car upholstery is heavily pigmented, so as to achieve a surface which is resistant to the most extreme stress. Aniline and velvet leathers are rarely used, only for specific requirements or where a particularly elegant look is called for. Many car manufacturers have their own analytic methods, and demand particular specifications, so that there is an extremely high number of tests available8. This lack of standardisation creates serious difficulties for the tanning companies. Precisely because of these difficulties, the tanneries tend to produce leather for just one, or at most two, automobile manufacturers. Even though the individual car manufacturers can specify their own methods, there are inevitably similarities between the tests utilised. For the metal-free leathers, the most important properties are those shown in Table II. We shall deal with these and other parameters, the requisite values and the specified methods of analysis, in a more exhaustive and specific manner in the section in which we assess the properties of the leathers produced using the technological system proposed in this report.

TABLE II

PRINCIPAL PROPERTIES TESTED ON THE METALFREE LEATHERS FOR AUTOMOBILE UPHOLSTERY

- TENSILE STRENGTH (tear strength) - FOGGING

- COLOUR FASTNESS TO RUBBING - COLOUR FASTNESS TO LIGHT - COLOUR FASTNESS TO THE SIMULTANEOUS

ACTION OF HEAT AND HUMIDITY - RESISTANCE TO SHRINKAGE - RESISTANCE TO PERSPIRATION - RESISTANCE TO ABRASION - FLEX RESISTANCE - SMELL - COLD CRACK RESISTANCE - FINISH ADHESION

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- FLAMMABILITY - RESISTANCE TO AGEING IN VARIOUS CONDITIONS - EMISSIONS

The mechanical properties are influenced by the manner in which the chemical process is performed. The tensile strength, for example, can be influenced by the wet operations, while obviously the flex resistance is closely linked to the type of finish performed. An adequate formulation of finishing can increase the resistance to abrasion and the colour fastness to dry cleaning or the colour fastness on rubbing. The effect of light and of ultra-violet rays can cause variations in the colour of the exposed leather. This phenomenon can be attributed to the use of products with limited colour fastness to light. Leathers that are finished with the appropriate mixtures of pigments, binders and auxiliary solids rarely give rise to complaints from customers. Lightly coated leathers, which have a natural and elegant appearance, may present this type of problem. In this case the colour fastness to light of the finished product is determined by the crust material. The situation can be partially improved by using an appropriate system of tanning and using dyes, fat liquors and re-tanning agents that are colour fast to light. For leather to be used for vehicle upholstery, these methods would not give results satisfactory for the current demands of the automobile industry. The effects of heat and humidity within the automobile can have a particularly negative effect on the leather. The problem of the resistance to shrinkage is important above all if the leather is to be used to cover the dashboards, which may be subjected to temperatures of well over 100°C. Depending on the tanning process used, this may give rise to a more or less obvious hardening of the leather. Particularly high temperatures may cause variations in colour as a result of the yellowing of the individual components of the finishing mixture, or of the migration of dyes which have not been adequately fixed in the leather. The fogging, as is well known, is caused by the condensation of volatile substances on the surface of the windscreen, thus reducing the visibility of the driver. The fogging value is determined by weighing the mass of the condensate (gravimetric method) or by evaluating the degree of opacity of a glass surface (reflectometric method). Sector literature clearly indicates which substances more than others negatively influence the fogging value9. As regards this aspect, the agents that are prevalently responsible are the fat liquors and the finishing products, followed in descending order by the emulsifiers, the degreasing agents and the ammonium salts. Among the stuffing agents, the sulfited fish oils are those which perform best, especially when the fogging is calculated using the gravimetric method. The emissions are made up of volatile or moderately volatile organic substances which are formed as a result of the increase of temperature inside the automobile. Naturally, the formation of these substances gives rise to an unpleasant small. This indicates a possible contamination of the environment with substances which may be damaging to health. The volatile substances are generally made up of acetone, ethanol, pyridine, butoxyethanol and N-methylpyrrolidinone contained in the polyurethanes, while the moderately volatile substances are represented by organic compounds such as fatty acids, alcohols and glycols10. The former are contained predominantly in the finishing products, while the latter are present in the stuffing agents. The emissions are measured through head space gas chromatography.

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3. The production of wet white: practical observations Among the possible pre-tanning agents, glutaraldehyde (GDA) has to date proved to be the moat efficient substance in the pre-tanning of wet white. Many other tanning agents have been experimented, with inferior results. The vegetable and synthetic tannins proved to be less efficacious in terms of the overall quality of the wet white obtained. Among the other aldehydes, glyoxal revealed considerable limitations and formaldehyde, despite yielding reasonable results, cannot be utilised on account of toxicological problems. The isocyantes and the epoxide resins, which yield covalent bonds similar to the aldehydes, are used very rarely 11. The essential points for the production of a good quality wet white are fundamentally the following: The preliminary beamhouse operations, the soaking and the liming are performed using the same procedures as in the processing of wet blue. The deliming must be completely penetrating so that the glutaraldehyde can easily penetrate the entire cross-section of leather. Deliming agents based on ammonium salts enhance the yellowing of the leather treated with glutaraldehyde, so that especially in the case of leather dyed in pastel shades, their use should be contained within acceptable levels. On the other hand the use of deliming agents completely devoid of ammonium makes an in-depth deliming extremely difficult. As is well known, the blends of bating agents contain a certain quantity of ammonium salts. Consequently, when deliming and bating are complete, the leather must be thoroughly rinsed to completely remove any traces of ammonium salts. The pickeling exercises a decisive influence on the penetration of the glutaraldehyde in the cross-section of the leather. The pH value must be less than 3 in the entire cross-section, so that the distribution of the pre-tanning agent is uniform.

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Pre-tanning with glutaraldehyde plays a decisive role in the production of leather devoid of heavy metals and aluminium salts. The parameters which influence this are laid down in table III.

TABLE III

FACTORS INFLUENCING PRE-TANNING

- the concentration of the pre-tanning agent - the pH at the end of pre-tanning - the type of glutaraldehyde utilised - the duration of the process - the procedures for basifying - the nature of the synthetic tannins utilised

3.1 The concentration of the pre-tanning agent With a supply of 0.5% of GDA per 100% of active substance the leathers feature a Tc of approximately 72°C, sufficient to ensure a correct operation of shaving. With the increase of the amount of GDA a diminishing of the tensile strength is observed together with a greater yellowing, with the consequent undesirable effects in the processing of leathers to be dyed in pale colours.

Fig 2 Pre-tanning with glutaraldehyde

Deliming with ammonium salts

Deliming without ammonium salts

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With the increase of the concentration of the GDA in pre-tanning, the result is a greater yellowing, clearly undesirable in the processing of leathers which are to be dyed in pastel shades.

3.2 pH level at the end of pre-tanning The reactivity of the GDA grows with the increase of the pH, and consequently the Tc of the leather treated with the same becomes higher, until it reaches a maximum value at pH=4. Strange to say, above this level, an increase in the quantity of GDA fixed to the leather is not matched by a corresponding increase in the Tc. With the reduction of the pH beneath 4, there is a corresponding decrease of the Tc of the leather. In practice, therefore, the pre-tanning begins at a pH of less than 3 to enable the aldehyde to penetrate efficaciously in cross-section; subsequently the pH is gradually raised to a value of 4.0 – 4.2 . Higher levels of pH have the sole effect of intensifying the yellowing of the leather without further increasing the hydrothermal stability. 3.3 The utilisation of modified GDA The modified GDA is less reactive than the product in itself. Thanks to its lesser astringency, it enables a more uniform penetration in cross-section. The advantages deriving from this concern the achievement of a finer grain and a paler colour of the wet white. 3.4 The duration of the pre-tanning This depends on the thickness of the leather. Obviously the time required for the complete penetration of the pre-tanning agent in the cross-section is greater for a leather of full thickness than for a leather of split pelt. Generally, for the purpose of achieving the perfect penetration of the GDA and the optimal tanning effect, after basifying the skins should remain in the pre-tanning bath overnight, on automatic.

Fig. 3 Differences in the colour of leather tanned with increasing quantities of glutaraldehyde

% of GDA

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3.5 The basifying Generally, the basifying begins 90 minutes after the addition of the GDA. Then the pH is gradually raised, avoiding overly abrupt shifts of pH, so that the aldehyde can react with the leather slowly without overcharging the grain. This inconvenience is counterbalanced with an easy penetration, which leads to a coarse grain and in extreme cases also renders it fragile. In general the bath is basified initially using salts with a bland alcaline reaction, such as sodium formate and subsequently, very slowly, with sodium bicarbonate. Alternatively, an auxiliary synthetic tannin with neutralising effect can be used, the action of which is milder than that of the traditional basifying agents. 3.6 The influence of synthetic tannins in pre-tanning These contribute to the improvement of the pressing and shaving of the wet white. They help to make the leather less sticky, so that it doesn’t stick to the cylinder of the shaving machine, making the shaving operation simpler. It is advisable to utilise synthetic tannins which are colour fast to light, with a composition based on dihydroxydiphenylsulphone. Originally, there was a tendency in pre-tanning to utilise reduced quantities of synthetic tannins, sufficient to ensure an easier pressing and shaving; at present instead the trend is towards a heavier pre-tanning. In this type of pre-tanning, in which between 3 to 5% of synthetic tannin is used, the thickness of the leather is easier to control, in that there is less fluctuation between the shaving thickness of the wet white and that of the finished leather. In this case the shaving does not damage the papillary layer and the leather is not subjected to an excessive and irregular swelling in the course of the main tanning that follows. On the basis of these observations, we record the most rational formulation for the production of wet white.

TABLE IV

CONTROL OF THE FUNDAMENTAL PARAMETERS IN PRE-TANNING

- Conventional soaking and liming - Utilisation of deliming agents with low ammonium content - Completely penetrated deliming - End pickeling pH lower than 3.0 - Utilisation of modified GDA - Gradual raising of pH - End tanning pH of around 4.0 - Utilisation of a synthetic tannin colour fast to light (from 3 to 5% ) - Overnight permanence in bath - Final rinsing with sodium bisulphite

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3.7 Pre-tanning for the production of wet-white on split pelt cattle hide for automobile upholstery

% referred to pelt weight The leathers are soaked and limed in the conventional manner, fleshed, split, and thoroughly delimed, bated, and rinsed extremely well to eliminate all traces of ammonium salts 40 % H2O 20°C 6.0 % NaCl rotate 10 min. d = 7.0°Bé + 0.5 % HCOOH 85% (1:5) rotate 30 min. + 1.2 % H2SO4 96% (1:10) rotate 90 min. pH = 2.7-2.8 cross-sect.:uniform to V.B.C. + 3.0 % G.D.A. mod., 25%, (1:3) rotate 90 min. + 1.0 % Stuffing stable on electrol. (1:4) rotate 20 min. + 1.5 % Synth. tannin neutralis. polv. 3.5 % Synth. tannin of substitution rotate 60 min. + 1.0 % HCOONa polv. rotate 20 min. Bring the temperature up to 30°C 0.5 % NaHCO3 ( 1:10) rotate 120 min. pH = 3,9-4,0 + 0.1 % TCMTB 30%(1:3) rotate 40 min. Night on automatic, rotate 5 min, every 30 min, next morning rotate 10 min., pH = 3,9-4,0, Drain 150% H2O 20°C 0,5% NaHSO3 (1:5) rotate 20 min. Unload, press, shave mm 0.9-1.0- Tc: 75°C The treatment with bisulphite serves to eliminate the excess of glutaraldehyde from the grain, which otherwise can cause a reduced elasticity. 4. Retanning In effect, this represents the true principal tanning, in the course of which there is an increase in the Tc of the leathers of approximately 70°C at 80-82°C. In general, various types of tanning substances are used simultaneously: vegetable tannins, synthetic tannins and polymers of varied chemical nature. The formulation depends on the type of article required. The use of the GDA in this phase too brings with it a series of advantages: it enables the reduction of the quantity of

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chemical products normally used and it improves the penetration of the tanning products and the fat liquors applied in the subsequent phases. The result is a softer leather with a greater constancy of production. The leathers have a better reaction to the wet frame drying, which is not always conducted in optimal conditions of temperature and humidity. The joint utilisation of vegetable tannins makes it advisable to start the re-tanning in a concentrated bath, at a low temperature combined with dispersing synthetic tannins in order to facilitate the penetration of the natural tannins. After the re-tanning operation, conducted at a low number of revolutions and for a sufficiently long time to ensure the perfect penetration of the re-tanning agents, the bath is diluted at a temperature of 40°C and fixing is induced by means of a slight acidification. Before the principal stuffing operation, a decisive acidification is performed. Effectively, the leather treated with the above-mentioned products features a very low isoelectric point and therefore the anionicity of the support has to be lowered in order to achieve a satisfactory fixing of the dyes (eventual topping) and of the fat liquors. The use of excessive quantities of natural tannins in general provokes a pronounced reduction of the mechanical-physical strength.12

TABLE V

MECHANICAL PARAMETERS RELATED TO THE USE OF DIFFERENT QUANTITIES OF NATURAL TANNIN ( TARA)

PARAMETERS

10%TANNIN

15%TANNIN

25%TANNIN

Tensile strength 135 N

115 N

95 N

Tensile elongation 52%

38%

35%

Resistance to ripping ( tear growth

resistance)

29 N

18 N

15 N

Bursting strength (breaking load)

23 Kg

21 Kg

9 Kg

The type of natural tannin used has a great influence on the quality of the merchandise and on the mechanical-physical strength of the leathers manufactured. In this regard, the leathers produced were compared, all the other processing conditions being the same, using on each occasion the following natural tannins: mimosa, tara, myrabolams, sumach. The leathers treated with tara revealed the best results in terms of both softness and body and surface handle and finally also in terms of the intensity of colour with the same amount of the same dye. As regards the Tc, the differences are negligible, as are the levels of colour fastness to light. In comparison to the other tannins used, the tara also offers superior values in terms of tensile strength, tear resistance and resistance of the grain to bursting, elongation and breaking etc. Finally, the tara, like other natural tannins, contributes to the reduction of the eventual presence of formaldehyde in the leather. The joint utilisation of acrylic polymer re-tanning and stuffing agents, in view of their elevated capacity for fixing to the leather, contribute together with the sulfited fish oils to achieve good performance as regards fogging.

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4.1 Organic tanning for the production of full grain metal-free leather Shaving mm. 0.9-1.0, 5 referred to shaved weight RINSING: 200 % Water 30°C 0.3 % Formic acid 30 min. PRE-TREATMENT 100 % Water 35°C NEUTRALISATION 1.5 % GDA, 25% 0.5 % Sulfited fish oil, 80% 60 min. 2.0 % Synth. tannin neutralis. 10 min. 1.5 % Sodium formate 20 min. 0.4 % Sodium bicarbonate 40 min., pH = 4,7-4,9 Drain, rinse PRE-STUFFING 50 % Water 30°C RE-TANNING 4.0 % Synthetic oil, 70% 30 min. DYING 5.0 % Acrylic polymer, fat liq., re-tan., 20 min. 6.0 % Synth. tannin, dihydroxydiphenylsulphone 7.0 % Tara 30 min. 8.0 % Synth. tannin, dihydroxydiphenylsulphone 7.0 % Tara x % Dye 180 min., cross-sect. penetrated, + 80 % Water 45°C 4.0 % Natural sulfited oil ,80% 90 min. 0.7 % Formic acid 20 min. 1.0 % Formic acid 30 min., pH = 3.7-4.0 RINSING 200 % Water 45°C 10 min. Drain STUFFING 100 % Water 50°C 10.0 % Sulfited fish oil, 80% 90 min. 1.0 % Formic acid 30 min. 0.8 % Formic acid 30 min., pH = 3.2-3.3

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wet nail, stake, dry mill, wet nail, finish. 5. Evaluation of the article manufactured

For the sake of brevity, we will not linger on the finishing technique which could eventually be the subject of a further discussion. Many mechanical-physical properties, such as adhesion, flexing, rubbing and resistance to ageing under the effects of light and heat, and the emissions, largely depend on the techniques and products used in finishing. In the case in question, this consists of the application of 25-30 g/sq.ft of a base-coat, sprayed in two stages and 4-5 g/sq.ft of a top-lacquer. The former is based on polyurethane dispersions, filler, pigment and anti-tack-agent, the latter on polyurethane dispersions which produce a very hard film, matting agents bonded with polyurethanes and matting agents without binders. The addition of pigment in the top too greatly increases the resistance of the finish to ageing, in the sense that no changes in colour are recorded under the action of light and heat. Conclusions With this type of technology we have obtained a metal-free article for automobile upholstery with excellent generic and mechanical-chemical-physical qualities. The tables below record an example of the tests on the characteristics of an article of this type. The tables illustrate, alongside the characteristic, the values demanded by the automobile manufacturers for each parameter, the value recorded and the related analysis method.

TABLE. VI

MECHANICAL, AND CHEMICAL-PHYSICAL CHARACTERISTICS OF THE

METAL-FREE LEATHER MANUFACTURED

PARAMETER REQUIRED VALUE METHOD RECORDED

VALUE

Thickness 1.1-1.4 mm DIN 53326 1.2 OK

Bulk density 0.6-0.8 g/m3 DIN 53327 0.7 OK

Mass per unit of area 700-900g/m2 DIN 53326 990 OK

Fat content 7/12% DIN EN ISO 4048 10.6 OK

Ph value >=3.5 DIN EN ISO 4045 4.1 OK

Chrome oxide < 0.1 DIN 53309 < 0.1 OK

Tensile strength >=130 N DIN 53328 Stab 2 167 OK

Elongation at break 35-60% DIN 53328 Stab 2 52 OK

Stitch tear resistance >=60 N DIN 53331 66 OK

Resistance to ripping >=-25 N DIN 53329/A 29 OK

Flexibility grain side

<=1.6 N

PV 3903 OK

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flesh side <=1.0 N

Watertightness 1ml of distilled water

OK OK

Colour fastness to crocking with alkaline solution Evaluation

>=4,0 s.g. DIN EN 20105 A03

4.5 OK

Flexometer In normal conditions In tropical conds:70°C,70%ur.,168 h Cold flexibility:-10°C After 3 periods in light

>=100000

>=1000 >=30000 >=10000

DIN EN 13334 No break

“ “ “

OK

Water vapour permeability >=2.0 mg/cm2h DIN 53333 2.2 OK

Resistance to heat 144h at 100°C 4h at 120°C

>=4 s.g. >=4 s.g.

PV 3941 5 5

OK

Colour fastness to light after 3 periods Evaluation

>=4 s.g.

DIN 75202 4 OK

Adhesion of finish dry wet after 10000 flexes after 3 periods at xenotest

>=4.0 N >=1.2 N >=3.5 N >=3.5 N

11.7 5.3 NC NC

OK

Resistance to rubbing dry/500 cycles wet/75 cycles to alkaline perspiration/50 cycles to gasoline/10 cycles to soap solution/50 cycles to alcohol-heptane/5 cycles

5 s.g.

>=4 s.g. >=4 s.g. >=4 s.g. >=4 s.g. >=4 s.g.

DIN 53339 5 4 4

4.5 4 4

OK

Abrasion-Tabertest cs-.10 10N >=300 cycles DIN 53109 No break OK

PCP Total absence Absent OK

Shrinkage 96 h, 70°C,70% u.r. 168 h,120°C

<=5% <=5%

PV 1200 5 5

OK

Oil repellence >=4 s.g. PV 3922 4 OK

Flammability <=100mm/min. TL 1010 D <=100mm/min

OK

Emissions <=100 micro gC/g PV 3341 <=100micr. gC/g

OK

Fogging <=5 mg PV 3015 4.5 OK

Smell <=3 PV 3900 2 OK

Formaldehyde <=10 mg/Kg PV 3925 3 mg/kg OK

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Bibliography (1) Wolf, G., Igl, G.. Breth, M., Scandinavian Tanners Meeting, May 1999 (2) Sammarco, U., Simoncini, A., CPMC, 70, 5, 205, 1994 (3) Sammarco, U., CPMC, 74, 83, 1998 (4) Graf, D., JALCA, 96, 5, 2001 (5) Häuber, G., Germann, H.P., Leder und Hätemarkt, 9, 5, 1999 (6) Candar, V., Palma, J.-J., Zorluoglu, Y., Reetz, I., XXVI IULTCS Congress Proceedings,

2001 (7) Schwaiger, W., Franken, M., Heinzelmann, F., XXVI IULTCS Congress Proceedings, 2001 (8) Rowley, G., Leather International, 24, May 2003 (9) Kaussen, M., JALCA, 93, 16, 1998 (10) Breitsamer, M., Goetz, O., XXVII IULTCS Congress Proceedings, 2003 (11) Wolf, g., Breth, M., Carle, J. Igl, G., JALCA, 96, 111, 2001 (12) Sammarco, U., CPMC,70, 5, 205, 1999