polypropylene practical guide
TRANSCRIPT
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4 Properties
The mechanical and thermal properties of PP are dependent on the isotacticity, themolecular weight and its distribution, crystallinity, and the type and the amount of
comonomer. Additionally, PP is, like other thermoplastics, a viscoelastic material.
Consequently its mechanical properties are strongly dependent on time, temperature
and stress. The properties of 7 commercial materials (all made by the same
manufacturer and subjected to the same test methods) are compared in Table 12. Thesegrades are of approximately the same isotactic content but differ in molecular weight
(indicated by the change in melt flow rate) and in being either homopolymers, random,block copolymer or controlled rheology grades.
4.1 Density
The typical density of PP is 0.9 g/cm3
and it is the lightest among the widely used
thermoplastics (see Table 1). Therefore, it offers the advantage of being able to
manufacture more items for a given weight of the polymer. Polymethylpentene (TPX),
a commercially available semi-crystalline transparent thermoplastic, has a lower density
(0.83 g/cm3) than PP. Unlike PE, where changes in the degree of crystallinity result in
quite large variations in density, the density of PP changes little over the whole range of
homopolymer and copolymers. The density of the random polymers is marginally lowerthan the homopolymer grades (Table 12). On the other hand, elastomer-modified, filledor reinforced grades might have significantly higher density depending on their
formulation. For example, a 40% talc-filled grade has a density of 1.2 g/cm3.
4.2 Thermal Properties
Unlike metals, plastics are extremely sensitive to changes in temperature. Themechanical, electrical or chemical properties of plastics cannot be considered without
knowing the temperature at which the values are derived. The thermal properties of apolymer typically determine its low- and high-temperature applications, impact
properties and processing characteristics. Typical low-temperature applications for PP
are in refrigerator parts and food packaging for refrigerated shelves. The applications
where high-temperature properties of PP are of particular interest include sterilisation,particularly steam sterilisation, microwave oven proof containers and parts for
dishwashers and washing machine which are subjected to hot water in the presence ofdetergents.
4.2.1 Glass Transition Temperature and Melting Point
The mechanical properties of PP at a particular temperature are dependent on the glass
transition temperature. At very low temperature, the macromolecules are largely
immobile. As the polymer is heated, the restricted macromolecular zones become
progressively more mobile. At the transition temperatures, the material changes from a
glassy hard state to a soft tough state because certain molecular segments become more
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mobile. A polymer above its glass transition temperature acts a tough ductile materialwhile below it, the material is hard and glassy. On cooling, the glass transition
temperature is sometimes known as the freeze temperature. Glass transition temperatureis measured using a dynamic mechanical thermal analyser (DMTA) or differentialscanning calorimeter (DSC). PP has the following transition temperatures:
Second-order glass transition temperature at 10 C (predicted). The actual value
may be observed between 0 to 20C depending on the frequency/heating rate.
Crystalline melting point between 160170 C depending on the grade and the
frequency/heating rate.
Recrystallisation temperature on slow cooling of the melt between 115 C and135 C.
A typical temperature curve for shear modulus and mechanical loss factor, measuredusing torsion pendulum, for different grades of PP is shown in Figure 8. It can be seen
from the figure that a copolymerised grades of PP has two peaks in the mechanical lossfactor curve while PP homopolymer has only one peak. The first peak, above 0 C,
denotes the glass transition temperature, similar to that of homopolymer PP. Thesecondary transition peak at 45 C is due to the presence of comonomer which
provides some mobility to polymer chains above 45 C, thereby, giving enhanced
impact properties.
Figure 8 A typical DMTA trace of PP showing different transition temperatures
PP copolymers, due to lower crystallinity, and metallocene-catalysed PP have lowermelting points in comparison to homopolymerised PP.
Recrystallisation temperature is quite important for injection moulding. Since the
recrystallisation temperature of PP is between 115135 C, most of the crystallisationoccurs during the cooling of the artefact in the mould. Since the recommended mould
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temperature is in the region of 2060 C, this allows the possibility to restrict warpageand improve dimensional stability during processing (Section 5.1.3). In addition, PP
continues to crystallise after processing at a rate varying with moulding conditions andstorage or treatment temperature.
Brittle temperature is very closely related to the glass transition temperature and
determines the minimum temperature at which a semi-crystalline polymer could beused without significant loss of its impact properties.
4.2.2 Maximum Continuous Use Temperature
Maximum continuous use temperatures are based upon the Underwriters Laboratories(UL) rating for long-term (100,000 hours) continuous use, and specifically on theelevated temperature which causes the ambient temperature tensile strength of the
material to fall to half its unexposed initial value following exposure to that elevated
temperature for 100,000 hours. The tests provide a continuous use temperature for a
plastic in the absence of stresses. The maximum use temperature of PP is compared
with other thermoplastics in Table 13. It can be seen that other commodity plastics and
some other engineering plastics have a significantly lower maximum continuous usetemperature than PP. However, polycarbonate has a higher maximum continuous use
temperature in comparison to PP.
Table 13 Maximum continuous usetemperature of different plastics [1]
Polymer CPP 100
HDPE 55
LDPE 50
PVC 50
ABS 70
HIPS 50PA 6 80
PA 66 80
PC 115
Occasionally it is required that the service life of the component is predicted at a
temperature above its maximum continuous use temperature or vice-versa. As a rule ofthumb, a 10 C increase in temperature is equivalent to a decade increase in time. Since
the maximum continuous use temperature of PP is 100,000 hours at 100 C, this wouldbe equivalent to 10,000 hours at 110 C or 1,000,000 hours at 90 C. Hence, certain
grades of PP may be theoretically suitable for a very short-term use at 140 C.
However, the maximum use temperature of a polymer depends on the specific gradeand its heat stabilisation system and should be carefully noted from the relevant trade
literature. However, the functionality of the polymer for high temperature applicationmight be quite limited in the presence of stresses.
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4.2.3 Heat Deflection Temperatures and Softening Points
Heat deflection temperature is defined as the temperature at which a standard test bardeflects by a standard amount under a standard load. Generally loads of 0.45 and 1.80
MPa are used. The values of heat deflection temperature of various plastics arecompared at different loads in Table 14. It can be seen from the table that the heat
deflection temperature of PP is higher than the PE but, it is outranked by moreexpensive engineering thermoplastics.
The Vicat softening temperature is the temperature at which a flat-ended needle of 1
mm2
circular cross-section area will penetrate a thermoplastic specimen to a depth of 1
mm under a specified load using a selected uniform rate of temperature rise. The Vicat
softening point of PP lies between 9095 C and is considerably higher than the PEs.Above the Vicat softening point, the material becomes progressively softer and the
crystalline melting point of PP homopolymer is about 165 C, depending on the grade.The practical application of the Vicat softening point data is limited to quality control
and material characterisation. However, it is taken as a rough estimate of the maximumtemperature for ejection of the artefact from the injection moulding machine.
Table 14 Thermal behaviour of other thermoplastics in comparison to PP [1]
Polymer
Heat deflection
temperature at 0.45 MPa(C)
Heat deflection
temperature at 1.8 MPa(C)
PP 8895 5060
HDPE 75 46
LDPE 50 35
PVC 70 67
ABS 98 85
HIPS 85 75
PA 6 200 80
PA 66 200 100
PC 143 137
Heat deflection temperature is a single point measurement and does not indicate long-
term heat resistance of plastic material. However, it may be used to distinguish betweenthose materials that are able to sustain light loads at high temperatures. The heat
deflection temperature of a specimen is affected by the presence of residual stresses.Warpage of the specimen due to stress relaxation may lead to erroneous results. In
addition, injection-moulded specimens tend to give a lower heat deflection temperature
than compression-moulded specimens. This is because compression-mouldedspecimens are relatively stress free.
The data obtained by these tests cannot be used to predict the behaviour of plasticmaterials at elevated temperature nor can it be used in designing a part or selecting and
specifying material. If an article is subjected to high temperature in the absence ofstresses, maximum continuous use temperature (Section 4.2.2) can provide a suitable
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criterion for the selection of material. In addition, if load bearing properties are requiredfrom the component at high temperatures, the modulus of the plastic as a function of
temperature (Section 4.3.1.2) could provide data for the design calculations.
The flexural modulus of different plastics as a function of temperature have beenplotted in Figure 9. The remarkable difference between different polymers can be
explained on the basis of amorphous and semi-crystalline polymers. It can be seen fromthe figure that the amorphous polymers, such as PVC and ABS, maintain their strength
quite well up to their maximum use temperature. However, their strength falls sharplywhen they reach their glass transition temperature. On the other hand, semi-crystalline
polymers, such as PE and PP, slowly lose their strength above the glass transitiontemperature. However, the residual strength of a semi-crystalline material may be
higher than the amorphous material at a higher temperature, and an amorphous polymer
may be stronger at the lower temperature.
Figure 9 Flexural modulus of different plastics as a function of temperature [2]
4.2.4 Brittle Temperature
At low temperatures, all plastics tend to become rigid and brittle. This happens mainly
because the mobility of polymer chains is greatly reduced. Brittle temperature is closelyrelated to the second-order glass transition temperature (Section 4.2.1). Brittletemperature is defined as the temperature at which 50% of the specimens tested exhibit
brittle failure under specified impact conditions. The brittle temperature of different
grades of PP are given in Table 15.
Table 15 Brittle temperature of different types of PPPP grade Brittle TemperatureHomopolymer 5 to 15 C
Random copolymer 10 to 15 C
Block copolymer 40 to 10 C
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Figure 10 Specific heat of PP as a function of temperature
4.2.6 Thermal Conductivity
The lower thermal conductivity of PP and other plastics compared to metals, givesprotection against external temperature changes and so PP could be used for insulation
applications. However, the use of PP, unless foamed, as a primary insulating material israther limited (owing to cost factors). PP has been used for food packaging of
refrigerated foodstuffs due to its suitability for food applications rather than itssuitability as an insulating material. Lower thermal conductivity limits the production
cycles and can result in cooling strains in thick sections, which may lead to warpage of
the article. Similar to other plastic materials, the conductivity of the PP is a function of
density and foamed PP has lower conductivity than the unfoamed PP.
4.2.7 Thermal Expansion
The coefficient of thermal expansion is defined as the fractional change in length or
volume of a material for a unit change in temperature. The coefficient of thermal
expansion of plastics is considerably higher than metals, up to 6 to 10 times as high.
This difference in the coefficient of thermal expansion can lead to internal stresses and
stress concentrations. Consequently, premature failure may occur. Thermal expansion
in PP gives significant volume changes on melting. It thus shrinks by 12% inmoulding, this must be allowed for when designing the tool. Mould shrinkage and
thermal expansion values for PP are compared with other thermoplastics in Table 2.The use of filler lowers the coefficient of thermal expansion considerably and brings the
value closer to that of metals and ceramics (Section 4.3.6). The effect of thermalexpansion on shrinkage, warpage and dimensional tolerances is discussed in Section
5.1.3.
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4.3 Mechanical Properties
The mechanical properties of PP depend on several factors and are strongly influencedby the molecular weight. General observations suggest that an increase in molecular
weight, keeping all other structural parameters similar, leads to a reduction in tensilestrength, stiffness, hardness, brittle point but an increase in impact strength. This effect
of molecular weight on the properties of PP is contrary to most other well-knownplastics.
The properties of some PP grades with different melt flow indices and structure arecompared in Table 12. It can be observed that an increase in mechanical properties is
not necessarily reflected in a trend predicted only on the basis of molecular weight, and
other structural parameters, particularly crystallinity, play a very important role. Hence,the prediction of the mechanical properties on the basis of molecular weight or meltflow rate should be treated with caution. Appropriate data for the properties of the
material should always be consulted.
4.3.1 Short-term Mechanical Properties
A tensile test reveals that tensile force increases with increasing elongation, up to the
yield point (Figure 11). After this, force initially decreases, i.e., the material can be
further stretched with a smaller force. This is accompanied by a marked necking of thecross section of the test specimen. When this necking down has progressed along the
entire length of the specimen, force increases again until elongation at break is reached.The second increase in deformation resistance is due to partial orientation of the
macromolecules which strengthens the material. This typical behaviour of PP is similarto other ductile plastics. It can be seen from Table 12 that the mechanical properties of
random and block copolymer grades are lower than the homopolymers for the samevalue of melt flow rate or molecular weight. The difference in their tensile stress/strain
curves is highlighted in Figure 12.
Figure 11 A schematic tensile stress/strain curve for PP
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Figure 12 Tensile stress/strain curves for different types of PP
It can be seen from Table 1 that the flexural modulus and tensile strength of PP is lowerthan most of the plastic materials except LDPE and HDPE. However, PP offers an
advantageously high flexural modulus to cost ratio which makes it an ideal candidatefor replacement material to many engineering plastics on the cost reduction basis.
The short-term stress/strain data of different grades of PP (and for other plastics) is oflimited use and should only be used for pre-selection of material. In reality, plasticcomponents are seldom designed and subjected to such high levels of strain as applied
in short-term tensile tests. In addition, most of the cases of product failure are brittle innature. Consequently, the long-term creep and fatigue properties of PP, discussed in
Sections 4.3.3 to 4.3.5, are more important for structural applications.
4.3.1.1 The Effect of Test Speed
Like other viscoelastic thermoplastics, the mechanical properties of PP depend on the
speed of the test. For instance, raising the speed of the test decreases the observed
flexibility and increases the observed brittleness.
4.3.1.2 The Effect of Temperature
The stiffness of PP is a function of temperature. The variation of flexural modulus ofdifferent grades of PP as a function of temperature is shown in Figure 13. PP
homopolymers are slightly stiffer than copolymers at room temperature. However, thedifference between the two types is diminished as the temperature rises. The flexural
modulus of elastomer-modified PP is significantly lower than the homopolymer orcopolymer PP, and its service temperature is around 90 C, much lower than that of
homopolymer PP. PP becomes more ductile as the usage temperature increases, shown
by an increase in elongation at break and decrease in ultimate tensile strength and yield
stress.
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Figure 13 Flexural modulus of different grades of PP as afunction of temperature [2]
4.3.1.3 Time-temperature Superposition
PP is a viscoelastic material, and, consequently, its mechanical properties are strongly
dependent on time, temperature, the level and type of applied stress and the testingspeed. The apparent stiffness or elastic modulus of all plastics reduces with time underload due to the processes of stress relaxation and creep. Similarly, the modulus reduces
with increasing temperature. In other words, the effect of time and temperature on the
mechanical properties is interchangeable. The theory behind this behaviour of
polymeric materials was given by Williams, Landel and Ferry. The detailed description
of this theory can be found in standard textbooks [e.g., 14]. However, at this point, it
will suffice to say that the effect of time during service could be simulated in the short-term using high temperature. This superposition of time and temperature could be used
in practice to predict the durability of the products.
4.3.2 Impact Strength
The second-order transition temperature of PP homopolymer is 10 C. This explainsthe drop in its impact strength at temperatures around 0 C. The impact strengths of
different grades of PP at different temperatures are given in Table 12. Several methodsare used for measuring the impact strength of PP. However, none of the methods
satisfactorily predict performance under conditions of end use. In the Izod or Charpy
test, a notch is incorporated in the sample to concentrate stress; this normally leads to
brittle failure. Impact strength is reduced as the notch gets sharper. Consequently, sharp
corners in load-bearing sections must be avoided in the design of the article, as a
general rule for all the plastics.
The impact strength of an article depends on the inherent molecular structure of the
grade used and the morphology arising from the processing conditions. Changes in the
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geometry of an item can have a major effect on its toughness rating. Impact strengthincreases with the molecular weight but more markedly with comonomer content. The
most important way of improving the impact strength of PP is by incorporating arubbery phase, as in heterophasic copolymers. Toughness increases rapidly with higherrubber content, and its transition from ductile to brittle failure occurs at lower
temperatures.
One of the major reasons for the failure of PP artefacts is the brittle failure. This is
mainly caused by the incorrect selection the PP grade, particularly the use of PPhomopolymer in place of copolymer or use of wrong material at the moulding floor.
Infrared microscopy and gel permeation chromatography can quickly identify thesource of the problem.
4.3.2.1 Falling Dart Impact Test
The falling weight or dart drop test method simulates actual day-to-day abuse and can
be carried out either on standard laboratory specimens or on the articles themselves.
Failure may occur in various ways ranging from brittle to ductile failure (Figure 14).
Particular care must be taken to avoid the brittle failure by proper selection of grade. Attemperatures below 20 C, elastomer-modified PP is more impact resistant than PP
copolymer and homopolymer.
Ductile Bructile Brittle
Figure 14 Example of ductile, bructile or brittle material failureReproduced with permission from Shell, Shell, website: www.basell.com
4.3.2.2 Notched Impact Strength
PP copolymer has higher impact strength than homopolymers even at low temperature
(Figure 15). Higher molar mass provides better impact and notched impact strength
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above 0 C. Elastomer-modified PP shows high notched impact strength even attemperatures below 0 C.
Figure 15 Typical notched impact strength of PP as a function of temperature
4.3.2.3 Tensile-impact Strength
Impact strength tests permit no differentiation between specimens undergoing the testwithout failure. In this respect, the tensile-impact strength test is superior. Other test
variables such as notch sensitivity, loss factor and specimen thickness are eliminated inthe tensile-impact strength test. In addition, tensile-impact strength tests can be used for
very thin specimens. The tensile-impact strength test consists of a specimen-in-head
type of set up. In this case, the specimen is mounted in the pendulum and attains fullkinetic energy at the point of impact. One end of the specimen is mounted in the
pendulum and the other end be gripped by a crashing member, which travels with the
pendulum until the instant of impact. The energy to break by impact in tension isdetermined by kinetic energy extracted from the pendulum in the process of breaking
the specimen. The superior impact properties of elastomer-modified PP are, once again,observed.
4.3.3 Creep
PP is a viscoelastic material and, like all other thermoplastics, it exhibits creep (or cold
flow). Creep is the deformation or total strain which occurs after a stress has beenapplied. Its extent depends on the magnitude and nature of stress, the temperature and
time for which the stress is applied. Over a period of time, PP undergoes deformation,
even at room temperature and under relatively low stress. After removal of stress, a
moulding more or less regains its original shape, depending on the time under stress and
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magnitude of stress. Recoverable deformation is known as elastic deformation andpermanent deformation as plastic deformation.
Typical creep curves plot deformation or creep against time on logarithmic scale for a
range of loads or stresses. This basic creep data could be used to plot isochronousstress/strain curves, isometric stress curves or creep modulus as a function of time. In
an isochronous graph, stress is plotted against strain at a constant series of timeintervals (Figure 16). In isometric curves, stress or strain is plotted as a function of time
for a series of constant strains or stresses. Creep modulus curves are the time-dependentvalue of modulus (Figure 17). As the properties of polymers are a function of
temperature, these curves can be produced at different temperatures. This type of data isavailable from the raw material suppliers in most of the cases. However, sometime the
creep data for the conditions which the component might observe in service are not
available, hence the data is extrapolated to the required conditions. Care should be
exercised in extrapolating the data to higher temperatures or longer durations outside
the experimental creep data range.
Copolymer type and melt flow rate also influence the creep behaviour. Copolymer
grades of PP have substantially lower creep modulus than the homopolymer grades. PP
has a similar modulus to high density PE, but its resistance to creep is much better and,
at a equivalent time under similar load, the creep modulus of PP is more than that of
high density PE. However, the creep resistance of amorphous plastics is much betterthan the semi-crystalline plastics such as PP and PE. Creep resistance of PP could befurther improved by addition of fillers or reinforcements. The creep behaviour of
moulded artefacts is affected by the residual stress or orientation effect in the mouldedarticle.
Figure 16a Isochronous stress/strain curves of PP at 23 C
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Figure 16b Isochronous stress/strain curves of PP at 80 C
Figure 17 Tensile creep modulus of PP as a function of time under stress(T1 = 23 C, T2 = 65 C and T3 = 110 C)
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PP shows different responses to different stresses or combination of stresses. For PP, itis reported that, up to strains of 0.8%, stress is proportional to strain measured during
the creep tests. Up to this level of deformation, stress/strain behaviour under bothcompressive and flexural stress can be approximately calculated from the tensile creeptests.
4.3.4 Fatigue
An alternative case to creep, where the deformation of the material is measured as a
function of time at a constant stress, is fatigue (or stress relaxation). In this case, the
material is subjected to constant strain, the relaxation in the stress in the component is
measured as a function of time. This scenario occurs in press fits, springs, interferencefits, screws and washers, etc., which during service undergo stress relaxation. A typicalstress relaxation curve for PP is shown in Figure 18. A significant relaxation in the
tensile modulus occurs over the 10 year period depicted in this graph (logarithmicscale), the value dropping from around 1,000 N/mm
2to around 350 N/mm
2.
Figure 18 Tensile stress relaxation modulus for PP homopolymer at 23 C
4.3.5 Dynamic Fatigue
Materials subjected to cyclic loads or stresses fail at a point far below the ultimate
strength measured in short-term mechanical tests. The cyclic loads may be caused by
periodic or intermittent loading in on-off situations. It is well known that the amorphousplastics are more susceptible to fatigue than semi-crystalline plastics such as PP. But itshould be noted that the semi-crystalline materials also suffer from dynamic fatigue and
the stress level decreases significantly as the number of cycles increases, though semi-crystalline materials do not undergo the ductile to brittle transition of amorphous
materials. The stress levels for cycles to failure for different plastics are compared in
Figure 19. This figure clearly demonstrates that the amorphous plastics (PC and ABS)
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are seriously prone to fatigue related problems. However, most semi-crystalline plasticsshow a similar slope in the fatigue strength curve. A notable exception is acetal resin
which shows a transition. However, it should be noted that PP is not a very stiff plastic,hence the safe stress level for PP under fatigue conditions is very low.
Figure 19 Stress levels for cycles to failure for different plastics
Fatigue data is usually published in the form of Wohler curves where stress or strainamplitude is plotted against the number of cycles to failure on a logarithmic scale.
Dynamic fatigue is a complex issue. However, the following common observations can
be made:
Fatigue strength decreases with increasing temperature.
Fatigue strength is sensitive to stress concentration such as notches or sharp
corners.
Fatigue strength depends on the stress frequency. The effect of fatigue at low
frequencies is much more severe. In other words, at low frequencies the failurecould occur earlier than that predicted using high frequency tests.
4.3.6 Mechanical Properties of Filled Grades
The properties of filled or reinforced grades of PP are heavily influenced by the type
and amount of the filler. For example, the density of a heavily filled grade can be up to50% higher than the unfilled material. In the next paragraphs, the effect of fillers or
reinforcing agents on the properties of PP is explained. The typical properties of filled
grades of PP are given in Table 17.
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It can be seen from the table that the mechanical properties of filled grades of PP aresubstantially modified by the presence of filler. Depending on the type and content of
filler or reinforcement, PP may even show a brittle failure at low applied strains at lowtesting rates. The improvement/reduction in the tensile strength of the filled grades ismarginal (with the exception of 30% glass fibre reinforced PP with coupling agent) due
to stress concentration effects. However, the modulus is significantly improved onaddition of fillers and reinforcements, particularly for glass fibre reinforced grades with
a suitable coupling agent.
The impact properties of the glass fibre reinforced grades are reduced. However,
reinforced copolymer grades provide good low-temperature impact properties but at theexpense of rigidity. The notched impact strength of the glass fibre reinforced grades is
better due to blunting of the crack propagation mechanism.
Reinforced grades of PP have a distinctly higher surface hardness than the non-reinforced grades; hardness varies according to the type of reinforcing material and its
proportion by weight. The reinforced materials can be used for sliding elements but thewear of other materials in contact may be very high.
The crystalline melting temperature and glass transition temperature of reinforced PP is
not substantially different to those of the unreinforced grades. However, substantial
changes in HDT values are observed. Reinforced PP grades have reduced specific heatvalues since the reinforcing materials have considerably lower values than the basepolymer.
The coefficient of linear expansion, to a large degree, is dependent on the orientation
and distribution of the reinforcing material. In general, it is lower for a reinforcedmaterial than for an unreinforced material. Shrinkage of the filled or reinforced grades
of PP is dependent on the aspect ratio of the filler. Differential shrinkage, the differencebetween longitudinal and transverse shrinkage, is relatively low for talc or calcium
carbonate filled grades in comparison to glass fibre or mica reinforced grades due to thehigher aspect ratio of glass fibre and mica reinforcements. The effect of shrinkage due
to molecular orientation in the partially crystalline matrix is encountered when
reinforcing material is incorporated. As a result, with an increase in the proportion of
spherical filler (e.g., talc or calcium carbonate), increasingly isotropic shrinkage occurs.Consequently, injection-moulded articles made from reinforced PP containing such a
filler have less tendency to distort than the parts moulded from non-reinforced PP andcan be manufactured with closer tolerances. On the other hand, during injection
moulding, glass fibres are generally oriented in the flow direction. The glass fibres
oriented in the melt bring about a very large reduction in shrinkage in the direction oforientation while a smaller reduction takes place at right angles to the direction oforientation. This difference between the shrinkage in two directions can give rise to
distortion problems.
Filled and reinforced grades of PP have a substantially higher creep modulus thanunfilled grades. This is due to the higher initial modulus of the filled or reinforced
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grades and the reduced amount of the polymer in the material which is able to creep.The glass fibre reinforced grades offer higher creep modulus, particularly when used
with proper coupling agents. The general discussion about the fatigue and creep ofunmodified PP discussed in Sections 4.3.24.3.5 is also valid here.
Choice of compounding method is very important to limit the effect of fibre length
distribution on the mechanical properties of glass fibre reinforced PP. Since glass fibresare damaged under high shear compounding conditions and during injection moulding,
knowledge of screw design and moulding conditions is essential to control the fibreattrition and reproducibility of product performance.
The weld lines in the glass fibre reinforced components are particularly weak in
comparison to the rest of the moulded articles since the reinforcing fibres are orientedperpendicular to the direction of flow. Thus the weld strength in reinforced artefact is
similar to the unreinforced material. This is one of the major causes of weakness inreinforced articles and proper care is required in designing the gate.
4.3.7 Biaxial Orientation
The second increase in deformation resistance of PP is exploited by uniaxially
stretching of monofilament and polymer tapes, and uniaxial or biaxial stretching offilm. The various microstructural changes occurring during stretching of PP are shown
in Figure 20. The difference between the original length (or width) of a monofilament,
tape or film and its length after stretching is known as the stretch ratio. After stretching,
the material has considerably higher tensile strength and lower elongation at break inthe stretch direction. By using suitable stretching rates and temperatures below the
crystalline melting temperature, optimum stretch ratios and hence very high degrees oforientation can be obtained. Depending on the stretch ratio, tensile strength values
several times higher than that of the unstretched material can be attained. Elongation atbreak is greatly reduced. Typical figures illustrating these effects are given in Table 18.
Table 18 Comparison of cast, monoaxially oriented and biaxiallyoriented PP films [16]
Property Cast filmMonoaxially oriented
Biaxiallyoriented
Tensile strengthmachine direction (MPa) 39 55 180
Tensile strengthtransverse direction (MPa) 22 280 152
Elongation at breakmachine direction (%) 425 300 80
Elongation at breaktransverse direction (%) 300 40 65
Gloss (ASTM D523) 7580 >80 >80
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Figure 20 Schematic representation of structural changes during stretching of PP
4.4 Electrical Properties
PP is an excellent electrical insulator, as can be expected from a non-polar hydrocarbon.The electrical properties of PP are very similar to those of PE and are compared in
Table 19.
Table 19 Electrical properties of PP and PE [1]Property PP LDPE HDPE
Volume resistivity ( cm) 10
17
10
16
10
17
Dielectric strength (MV/m) 28 27 22
Dielectric constant at 1 kHz 2.28 2.3 2.3
Dissipation factor at 1 kHz 0.0001 0.0003 0.0005
The following terms are commonly used to describe the electrical properties of amaterial:
Dielectric strength is a measure of dielectric breakdown resistance of a material
under an applied voltage. The dielectric voltage just before breakdown is dividedby the specimen thickness to give the value in kV/mm or MV/m. However,
thickness should be specified since the results depend on the thickness.
Volume resistivity is the electrical resistance when an electrical potential is applied
between the opposite faces of a unit cube of material. It is usually measured in
cm.
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Surface resistivity is a measure of the ability to resist the surface current. It is the
resistance when a direct voltage is applied between two surfaces mounted
electrodes of unit width and unit spacing. The value is expressed in ohms.
Arc resistance is the measure of the time (in seconds) required to make an
insulating surface conductive under a high voltage, low current arc. When an
electric current is allowed to travel across the surface of an insulator the surfacewill become damaged over time and become more conductive.
Dielectric constant, or relative permittivity, is defined as the ratio of the electric
flux density produced in a material to the value in free space produced by the same
electric field. It is a ratio, and thus dimensionless.
Power and dissipation factors are the measures of the fraction of energy absorbed
per cycle by the dielectric from the field. These terms arise by considering thedelay between the changes in the field and the change in polarisation which in turn
leads to a current in a dielectric material leading the voltage across it. The angle of
the lead is known as the phase angle (). The power factor is defined as Cos and
dissipation factor is Tan(90-). The loss factor is the product of dielectric constant
and dissipation factor.
Typical electrical properties of a few grades of PP are given in Table 12 and are afunction of temperature and frequency. In general, PP shows outstandingly high
resistivity, low dielectric constant and negligible power factor, all substantiallyunaffected by temperature, frequency and humidity over the usual range of service
conditions. The electrical properties of PP are independent of melt flow rate. However,
certain additives and fillers can have an adverse effect on the electrical properties. The
electrical properties of the PP are unaffected by prolonged immersion in water. Further,
low values of dielectric constant can also be achieved using PP in the form of structural
foam. The power factor is critically dependent on the amount of catalyst residues in the
polymer.
Typical electrical application of PP is in insulating power cable, particularly for
telephone wires. The other functional requirements for this application are high impact
strengths at low temperature and heat stabilisation for use in contact with copper.
However, with the increasing use of optical fibres, the use of PP in this application is
limited.
The dissipation factor of PP is low and is hardly affected by temperature and frequency.
The low dissipation factor rules out the use of high frequency heating and welding ofPP. Hence, special techniques are required for welding of PP, discussed in Section 7.1.
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4.5 Optical Properties
PP granules are naturally white and translucent. However, the final appearance of thematerial can be very different and ranges from hard, fairly rigid, brightly coloured,
glossy, flexible or transparent film to high tenacity fibre. Mouldings made from thenatural coloured homopolymer are semi-transparent, depending on the thickness and
other processing and material parameters.
The optical properties of a material are defined in terms of refractive index, clarity or
transparency, haze and gloss. The refractive index of PP is 1.49. The remaining catalyst
residue in the resin may affect the opacity of the PP resin and produce yellowness.
Different catalyst systems may have different effects on the transparency and
yellowness of the resin. Hence, the optical properties of equivalent grades of PP may bedifferent.
4.5.1 Transparency
Transparency may be defined as the state permitting perception of objects through or
beyond the specimen. It is often assessed as that fraction of the normally incident lighttransmitted with a deviation of less than 0.1 degree from the primary beam direction. A
material with good transparency will have high transmittance and low haze.Transmittance is the ratio of transmitted light to incident light and is complementary to
reflectance.
Uncoloured PP is translucent in thick sections. In thin sections or films, it can betransparent or opaque depending on the grade and the processing conditions.
Homopolymers can be converted into transparent film with good optical properties. The
light scattering due to formation of crystal structure should be minimised. Because of
their phase structure, copolymers which have high impact strength do not in general
yield transparent films. However, single phase random copolymers which suppress the
formation of the crystal due to their irregular structure usually have better clarity thanthe homopolymers. Furthermore, it is important that the refractive index is constant
throughout the sample in the line of direction between the object in view and the eye.The presence of interfaces between regions of different refractive index will cause
scattering of the light rays. A high melt flow rate material should be used. Biorientationof PP film improves transparency since layering of the crystalline structures reduces the
variations in refractive index across the thickness of the film, and this in turn reduces
the amount of light scattering.
Transparency of PP articles can be improved by using moulds or dies that provide avery good surface finish. Further improvements can be made by choosing theprocessing conditions which restrict the formation of spherulites, e.g., rapid cooling,
low melt and mould temperature. However, low mould temperature will reduce thesurface gloss of the moulding. Nucleating agents and clarifying agents which suppress
the formation of spherulites can further improve the transparency of PP artefacts.
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Transparency is improved by contact with liquids to the point where the liquid levelinside a container can be seen from outside (contact transparency).
4.5.2 Gloss
Gloss is defined as the relative luminous reflectance factor of a specimen at the specular
angle. This method has been developed to correlate the visual observations of surface
shininess made at different angles. A highly polished black glass is assigned a specular
gloss value of 100. Three basic angles of incidence are used for measuring gloss: 20,
60 and 85. The gloss increases as the angle of incidence increases. The gloss is a
function of the reflectance and the surface finish of the material which, in turn, depends
on the finish of the mould.
Gloss is an extremely important factor where replacement of ABS with PP articles is
considered on cost grounds.
4.5.3 Haze
Sometimes a polymer may have a cloudy or milky appearance, generally known as
haze. It is often measured quantitatively as the amount of light deviating by more than
2.5 degrees from the transmitted beam direction. Haze is often the result of surfaceimperfections. Recent developments in sheet manufacturing machinery with two
cooling lines, which polish both sides of PP sheet, have resulted in low haze and highgloss sheets.
4.6 Surface Properties
4.6.1 Hardness and Scratch Resistance
Although PP can be scratched with a metal point, its hardness is sufficient to resist therule of thumb which often distinguishes between the polyolefins. LDPE is easily marredby a thumbnail, HDPE is scratched in this way with difficulty but PP is marked little, if
at all.
Hardness is defined as the resistance of a material to deformation, particularlypermanent deformation, indentation or scratching. Hardness is purely a relative term
and should not be confused with wear and abrasion resistance of plastics. For example,polystyrene has a high hardness but a poor abrasion resistance. Many tests have been
devised to measure hardness. However, Rockwell and Durometer hardness tests arecommonly used. The Rockwell hardness test measures the net increase in depthimpression as the load on an indentor is increased from a fixed minor load to a major
load and then returned to a minor load. The hardness numbers derived are just numberswithout units. Rockwell hardness numbers in increasing order of hardness are R, L, M,
E and K scales. The higher the number in each scale, the harder is the material. TheDurometer hardness test is based on the penetration of a specified indentor forced into
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the material under specified conditions. Two types of durometers are commonly used:Type A for softer materials and Type D for relatively harder materials.
The hardness values of different PP grades are compared in Table 12. Furthermore, the
indentation hardness of PP decreases with the temperature (Figure 21). The hardness ofPP depends on its crystallinity. With decreasing molar mass, crystallinity decreases and
so does the hardness.
Figure 21 Ball indentation hardness of PP as a function of temperature (C)
4.6.2 Abrasion Resistance
Resistance to abrasion is defined as the ability of a material to withstand mechanical
action that tends to progressively remove material from its surface. Abrasion resistanceof polymeric materials is a complex subject. The resistance to abrasion is closelyrelated to other factors such as hardness, resiliency and the type and amount of added
fillers and additives. Resistance to abrasion depends on factors such as test conditions,type of abradant and development and dissipation of heat during the test cycle. This all
makes abrasion a difficult mechanical property to define as well as to measureadequately.
A materials ability to resist abrasion is most often measured by its loss in weight when
abraded with an abrader. The Taber abrader is widely used to measure abrasion
resistance. In the Taber abrasion test, the test specimen is placed on a revolving
turntable with a suitable abrading wheel under a set certain dead weight. The weight
loss after a large number of revolutions (at least 5000 revolutions) is measured. Forsofter materials, less abrasive wheels with a smaller load on the wheels may be used.
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Wear may be high when reinforced parts are in contact with unreinforced parts. Thissituation could lead to excessive wear of unreinforced parts. Use of lubricated
reinforced grades may reduce wear when in contact with the unreinforced component.
China clay additives may be used to improve scratch and mar resistance of PP. Scratchresistance is quite important for car body panels. Recently, development of high-
crystallinity copolymer grades has occurred in response to the automotive sectorrequirement for low-cost, low-weight materials for interior trim that do not show
evidence of scratching [17]. These grades are around 15% lighter than ABS, withhigher stress crack resistance and the same toughness. Highly crystalline PP
copolymers have similar scratch resistance to talc filled PP but, because they do notcontain white filler, they do not show the evidence of scratch. However, disadvantages
of the high-crystallinity copolymers are 30% less stiffness in comparison to talc filled
counterpart and higher gloss, which is not preferred by the automotive manufacturers,
greater shrinkage and less acoustic damping.
4.6.3 Friction
PP has a friction coefficient between 0.25 to 0.45, higher than the friction coefficientfor typical bearing materials (from 0.10 to 0.25). However, the friction performance of
PP can be improved by addition of silicone oil and/or polytetrafluoroethylene (PTFE)additives. PTFE-modified grades of PP with very low dynamic and static friction
coefficients are available from resin compounders. The poor mechanical properties ofPP, particularly low shear yield strength, also contribute to its near zero use in sliding
applications. One reported sliding application of PP is in a conveyor belt guide bar for
the beverage industry.
Friction characteristics are very important for fibre to fibre interaction in spin
technology. Internal lubricants can be used to reduce the friction which can be quiteimportant in slide applications such as medical syringes.
4.7 Acoustic Properties
PP offers excellent acoustic damping properties with considerable rigidity at normal
service temperatures. Articles made from PS, ABS or HDPE have their own resonance
and tend to rattle. On the other hand, components made from PP are more or less
acoustically dead because acoustic vibrations are heavily damped in this plastic. TheDMTA or torsional pendulum curves (Figure 8) provide primary data about the acoustic
properties of the material. A good loss factor with sufficient rigidity is required toachieve damping of vibrations. It is primarily the components own vibrations which
are heavily damped, e.g., eigen tones or natural vibration frequencies of the housing.However, PP does not have good air-borne sound absorption because of its rigidity. To
achieve effective sound insulation, additional measures are required such as suitablecladding or spring mounting of the noise source.
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4.8 Biological Behaviour
4.8.1 Assessment Under Food and Water Legislation
PP is generally accepted as a non-toxic and non-carcinogenic material. PPhomopolymers and copolymers are used in many food contact applications ranging
from simple beverage closures to retortable pouch applications. The main requirements
for contact with food are that the article must not impart odour or taste to the food and
should be suitable for the intended application. The main reason for assessment of PP
for contact with food or potable water comes from the use of additives in material
formulation. Additives, monomers, catalyst residues, polymer degradation products,
etc., can migrate to any food in contact if the concentration of these substances is lower
in the food than in the plastic. The migration of these species is a function of time andtemperature. The rate of migration of chemicals or additives is inversely proportional to
the molecular weight of the PP. The migration of these species could produce toxicityor the formation of undesirable flavours or odours, commonly known as organoleptic
problems.
The application of PP in contact with food and water is covered by the relevantstandards/regulations by different authorities in different countries. Health assessment
of plastics under food legislation varies from one country to another. In USA, clearance
from the Food and Drug Authority (FDA) is required while relevant Europeandirectives form the basis of suitability of resin for food contact applications. Normally,the migration/extraction of resins and additives is measured for contact with different
food simulants, e.g., distilled water, vegetable oil or acetic acid. Although similar
principles apply, it is advisable to check in each case with the raw material
manufacturers. Many grades are available which have the material composition meeting
the requirements of the various regulatory authorities.
Provided the approved grades are used and compliance with relevant regulations is
checked, there should not be any problem in using PP for food and water contactapplications. However, the finished part must also meet the requirements of the relevant
regulations. The degradation of material during processing, use of mould release agents,
etc., can make the final product non compliant.
4.8.2 Resistance to Microorganisms
PP is not a nutrient medium for microorganisms and is therefore not attacked by them.
It cannot be penetrated by microorganisms provided the wall or film thickness is at least
0.1 m. In thinner walls, small pores may be introduced during manufacture. Lowmolecular weight additives, such as plasticisers, lubricants, stablisers and antioxidants,may migrate to the surface of plastic components and encourage the growth of
microorganisms. The detrimental effects can be readily seen through the loss ofproperties, change in aesthetic quality, loss of optical transmission and increase in
brittleness. Preservatives, also known as fungicides or biocides, are added to plastic
materials to prevent the growth of microorganisms.
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effect persists for a considerable time and articles made from this plastic can be washedin water without any marked decrease in antistatic activity. Grades with low
crystallinity (e.g., random copolymers) allow faster migration of antistatic agents tosurface, thereby, achieving the optimum performance quickly.
The incorporation of an antistatic agent has little or no effect on the mechanical,
chemical or thermal properties of PP and no effect at all on the processing conditions.However, the presence of antistatic agents may affect the transparency and printability
and may make PP non compliant to FDA regulations, although FDA approved antistaticagents are available. With the inherent hygroscopic nature of antistatic agents, it is
advisable to pre-dry the granules before processing.
Antistatic agents can be ionic or non ionic in nature. Glyceryl monostearate, a non ionicantistatic agent, is commonly used in injection moulding of PP up to a loading of 1% or
more. The exact amount of the antistatic agent added depends on other propertyrequirements such as transparency, FDA approval, printability and its compatibility to
the polymer. Artefacts can be coated with suitable antistatic agents also. To a lesserdegree, lubricants can also act as an antistatic agent and vice versa by reducing the
friction. Typical applications where antistatic properties are required are household
items, housings for electrical and electronic appliances, parts which undergo friction or
sliding, etc.
4.9.2 Electromagnetic Interference/Radio Frequency InterferenceShielding
There might be few occasions when a reduction in the dielectric properties of PP might
be desirable, e.g., antistatic properties. However, for certain applications, the reductionin surface resistivity achieved by addition of antistatic agents may not be sufficient. The
two cases where greater reduction might be required are
to achieve electrostatic dissipation (ESD) where a PP component is in contact with
semi-conductor devices or is used in a hazardous environment, or
to shield the component against electromagnetic interference (EMI) or radio
frequency interference (RFI). Many electronic and electrical devices such as
computers, mobile phones, etc., emit signals which may interfere withcommunications. Regulations now require that the plastic enclosures used to house
these devices should eliminate or attenuate these signals.
The high dielectric strength, low dielectric constant and dissipation factor of PP make ita bad choice of material for EMI/RFI shielding. In these cases, carbon black can be
added to provide increased electrical conductivity. This might be sufficient in manycases. However, secondary shielding methods, such as metal deposition using
metallising or electroplating, or adding metal fillers or nickel coated graphite fibresmight be required to achieve sufficient protection from EMI/RFI.
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4.9.3 Slip and Antiblocking Agents
During the use of PP films and laminates, the line speeds and reliability of thepackaging machine are very important. To decrease the friction of the film on the
processing and conversion equipment, a lubricating agent known as a slip agent isadded. Slip agents migrate to the surface and reduce the coefficient of friction. Once on
the surface, slip agents are able to lubricate the machine allowing for the faster linespeeds. Primary amides, secondary amides and ethylenebisamides are commonly used
as the slip agents. Reduction in the coefficient of friction and thermal stability of theslip agents are important issues. Slip agents should not interfere with corona treatment
or printing and it should not have a deleterious effect on the clarity of clear films. Slipagents should meet FDA or equivalent approval for the recommended application.
Another important property in the manufacture of films is blocking. Blocking is a term
describing the polymer film sticking to itself as a result of storage in roll form.Antiblocking agents are added to overcome this problem. Typical antiblocking agents
are silica, talc or diatomaceous earth. These materials are inorganic, are not misciblewith the polymer and migrate to the surface. They create microscopic roughness on an
otherwise smooth film surface. Addition of just the right amount and good dispersion
are the critical factors.
4.9.4 Metal Deactivators and Acid Scavengers
As with most polyolefins and polydiene, the presence of metals has a strong adverse
effect on PP and most antioxidants are relatively ineffective. Copper and cuprous alloysproduce the greatest effects, but other metals such as iron and nickel also accelerate
degradation. Metal deactivators are widely used to deactivate metal residues present inthe formulation due to catalyst residues, impurities in additives, direct contact with
copper in wire and cable applications and metal inserts. Metal deactivators work bychelation of the metal. Metal deactivators are organic molecules containing hetero-
atoms or functional groups such as hydroxyl or carboxyl groups. Good results may beachieved by the use of 1% of a 50:50 blend of phenol alkane and dilauryl
thiodiproprionate instead of the 0.10.2% of antioxidants commonly used in PP. Acid
scavengers (or antacids) are used in PP to neutralise acidic catalyst residues. Calcium or
zinc stearate are commonly used, and also function as internal lubricant.
Inserts should, therefore, be made of light metal or be nickel or chromium plated. No
adverse effect with brass screw fittings has been detected at temperatures below 100 C.
4.9.5 Blowing Agents
Through the addition of suitable chemical blowing agents to PP, fine-celled foams with
apparent density down to 0.6 g/cm3
can be produced by conventional extrusion or
injection moulding. Injection-moulded foamed articles produce a very fine-celled
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structure creating a smooth surface finish. The blowing agent concentrates are generallyadded in amounts of 1% to 3%. High melt flow rate grades of PP are preferred.
Extruded, foamed PP film and tape can be stretched and thermoformed for trays for
meat applications, cups for drinks, tape yarns for carpet backing. Within the specifiedloading limits for blowing agent, the approval of PP for food applications is not
compromised.
4.9.6 Nucleating Agents
The use of nucleating agents can further modify the crystallinity and crystal structure of
PP by providing numerous sites for growth of small spherulites during cooling of themelt. This results in less scattering of light. This technique is used in injection mouldingto improve clarity and rigidity [18]. As seen in polarised light photomicrographs, PP
containing nucleating agents has crystallites of much finer and more uniform size
relative to unclarified PP. As the crystallites in the clarifier containing grades are
generally smaller than the wavelength of visible light, scattering is reduced and clarity
is greatly improved.
As nucleating agents are generally organic in nature, they melt during normal
processing and thus become completely and evenly distributed in the resin.Furthermore, many organic compounds and metal salts can act as nucleating agents,
including pigments and residual monomer. The use of dimethyl benzylidene sorbitol,
dibenzylidene sorbitol and sodium di 4-t-butylphenol phosphate has been reported.
These additives are also used in resins to provide reduced manufacturing cost throughincreased productivity and reduced set up cost.
4.9.7 Antifogging Agents
Fogging occurs when water droplets formed from exposure of the moisture in foods tolow storage temperatures condense on the inside surfaces of packaging films. Use ofpolyglycerol ester or a sorbitan ester of a fatty acid has been reported for antifogging
applications in refrigerator packaging. Slip agents can also, to a certain degree, act as
antifogging agents.
4.9.8 Biocides
Biocides in PP are not as common as in plasticised PVC. In traditional PP applications,
the need for a biocide to control the growth of microorganisms is virtually non-existent.Some trash cans and other waste disposal containers make use of these components to
inhibit bacterial growth. The improvement is both aesthetic and sanitary. The product isable to resist the formation of unpleasant and unsightly organisms.
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4.9.9 Flame Retardants
PP is basically flammable and ignites at a temperature of about 600 C, although itsburning rate is slow. PP ignites in contact with flame and burns with a faintly luminous
flame. It continues to burn when the ignition source is removed and melts with burningdrips. The spontaneous burning temperature of PP is 360 C and the temperature at
which ignition is induced from an external source is 345 C. Combustion of unfilled PPproduces no environmentally relevant pollutants. The burning produces very little soot,
unlike PS or styrene-acrylonitrile copolymer (SAN), and no char, unlike oxygen-containing polymers such as polyphenylene ether (PPE) or PC. Polymers with superior
fire resistance are thermosetting resins, fluorinated plastics and other plastics containingsulphur such as polyether sulphone and polyphenylene sulphide.
The flammability of the polymers is commonly measured using Underwriters
Laboratories, Inc., (UL) specifications. The UL 94 standard covers classification of amaterials tendency to ignite in the presence of a flame and to continue to burn after the
ignition source is removed. There are three distinct flame tests in UL 94 which are mostoften applied to a PP product: horizontal burn (94HB), vertical burn (94VB) and 125
mm vertical burn (94-5VA, 94-5VB). Recognition under 94HB does not imply any self-
extinguishing character of the resin. However, the 94VB specification measures the
self-extinguishing character of burning and V-0 is the most stringent specification.
Flame retardants are added to reduce the flammability of PP. PP is one of the mostdifficult plastics to make flame retardant. A high level of flame retardant is required to
achieve necessary protection against fire required by the application, a level which will
impair the mechanical performance of the material. The unmodified PP grades
generally have a 94HB rating up to 0.8 mm thickness. Suitably flame retarded grades
with V-0 rating at 3.2 mm thickness are available from compounders. Flame retardants
can reduce the processibility and interfere with the function of certain hindered aminesas light stabilisers. Flame retardant grades of PP are generally not suitable for use in
food contact applications. The integral hinge properties of flame retarded PP are greatlyreduced.
Flame retardants include halogens (bromine and chlorine), aluminium trihydrate,
magnesium hydroxide, phosphates and antimony oxide. Each of these flame retardantshas its own strengths and weaknesses when flame retarding PP products as well as on
its effect on the stability of the resin. Brominated compounds commonly used for flameretardation of PP are decabromodiphenyl oxide and octabromodiphenyl oxide.
Antimony oxide is generally added to halogenated flame retardants for a synergistic
effect. The halogenated flame retardants act as free radical scavengers. Halogenatedflame retardants have many problems. They are known to interfere with hindered aminelight stabilisers due to the production of halogenated acids which could react with
hindered amines, to cause the corrosion of the processing machinery, to produce toxic
decomposition products such as brominated dioxins and furans. Magnesium and
aluminium hydroxides dissociate in the presence of heat to form water and metal
oxides. Water dilutes the combustion gases and takes away the heat from burning
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plastics. Aluminium oxide or magnesium oxide form an insulating char layer on theburning plastic. Use of magnesium hydroxide requires careful processing conditions
since it dissociates at the processing temperature of PP.
The limiting oxygen index (LOI) test is used to measure the minimum concentration ofoxygen necessary for candle-like burning for 3 minutes or more. A higher LOI indicates
that more oxygen is needed to support combustion. The presence of flame retardantsincreases the LOI. Unmodified PP has a LOI of 17. The key fire properties of some
plastics are compared with PP in Table 20. Suitable flame retarded grades of PP canhave a LOI of 28.
Table 20 Fire performance of different plasticsPolymer Flammability Limiting oxygen index
PE HB 17
PP HB 17
PS HB 18
UPVC V-0 45
CPVC V-0 50
ABS HB 19
PC V2 25
PPO/PS HB 20
PA HB 22
4.10 Performance in Service
4.10.1 Thermal or Heat Stability
Owing to the high susceptibility to oxidation due to the pendant methyl group,
unstabilised PP can begin to decompose at high temperatures. Unstabilised PP oxidises
in the presence of air and the rate of oxidation increases with increased temperature.
Oxidation leads to embrittlement, surface cracking, discolouration and loss ofmechanical properties and clarity. High temperatures are encountered during melt
processing or service. This degradation process is accelerated by contact with certainmetals. All commercial grades of PP are incorporated with stabilisers which give
protection against oxidation during processing. Stabilisers also provide protectionduring normal service conditions. It is, therefore, essential to determine the likely end-
use conditions before the choice of the grade and stabilisation system is made.
The mechanism of thermal oxidation in PP is through the formation of free radicals
which react with environmental oxygen to produce peroxides. It could also occur due toradiation, light or the presence of metal residues. Primary antioxidants inhibit theoxidation reaction by combining with free radicals. Hindered phenolics (e.g., butylated
hydroxytoluene (BHT)) are commonly used as primary antioxidants. BHT is FDAapproved for many applications but suffers from high-temperature volatility.
Furthermore, phenolics suffer from oxidation in the presence of metal residues left from
catalysts and produce a yellow colour. Different residues or different amounts of
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residue can produce different extents of yellowness. High molecular weight phenolicsare used for high-temperature processing or high-temperature service conditions.
Secondary antioxidants, also called peroxide decomposers, inhibit oxidation of PP by
decomposing hydroperoxides. Phosphites and thioesters are commonly used assecondary antioxidants. Secondary antioxidants are usually combined with primary
antioxidants to produce a synergistic effect on oxidation. With proper selection of twoantioxidants, it is possible to achieve protection against oxidation which is greater than
the sum of protection given by the two antioxidants when working separately.
Stabilisation with antioxidants may render PP unsuitable for food contact application as
they may directly migrate into the food products, hydrolysing and imparting odour or
taste to the food. Careful selection of a suitable stabiliser system is necessary for foodcontact applications. In addition, heat stabilisers could adversely affect the working of
light stabilisers.
In applications involving no undue mechanical stresses, PP articles will withstand 100
C for a long period of time, depending on the stabiliser systems. Consequently, the
heat and thermal stability of PP is closely related to its maximum continuous usetemperature (Section 4.2.2). Short-term exposure to 140 C is also possible. It has been
observed that a properly heat stabilised and properly processed material can undergo up
to five processing cycles without noticeable reduction in molecular weight or the levelof antioxidant content.
4.10.2 Stability to Light and Ultraviolet Rays
Most plastics are affected by ultraviolet (UV) light in the presence of air. PP is no
exception and, when unstabilised, it very rapidly becomes brittle when exposed to
sunlight. Degradation is accompanied by marked deterioration in mechanical properties.
Mouldings generally lose gloss after short exposure; the surface and the material
immediately beneath suffers the most. The appearance of chalky powder at the surfaceof the heavily degraded PP article has also been reported. The amount of the
degradation depends on the duration of the exposure and whether the article is usedbehind glass. However, the screening effect of the glass may not be sufficient to prevent
degradation if the stabilisation system is inadequate for the application. Like mostplastics, PP exhibits little or no change under short-term exposure to radiation in the
visible light range. Prolonged exposure to direct sunlight can, however, cause a
deterioration in properties due mainly to UV radiation.
UV absorbers are added to provide protection against harmful UV radiation. The UVabsorbers absorb UV light and release the excess energy as heat. Commonly used UVabsorbers for PP are derivatives of benzophenone, benzotriazoles and esters. The use of
hindered amines has also been reported.
The efficiency of the UV absorbers to protect the part depends on the part thickness.
Stabilisation in thick parts is more effective than the thin parts, films or sheets.
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Carbon black in a suitable concentration can make the article sufficiently protected for10 years continuous outdoor exposure in temperate climates. The use of carbon black is
clearly restricted in fibres and films. In addition, the heat stability of the carbon blackmodified grades may be poor in comparison to unmodified grades.
Hindered amine light stabilisers function as free radical scavengers and can double as
thermal antioxidants. They are, though costly, effective at very low concentrations.Hindered amines can interact with other additives (e.g., phenolic antioxidants, titanium
dioxide) in the PP to produce yellowishness. Halogenated flame retardants can reactwith hindered amine light stabilisers and render them ineffective.
The protection of PP against UV radiation and heat is a very complex issue. The idea of
this section is not to go in depth into formulation issues which are best left for resinmanufacturers/formulator or stabiliser suppliers. However, from the above discussion, it
can be appreciated that:
The heat stabiliser and light stabiliser can interact with each other as well as other
additives present in the PP formulation. The mechanical properties of the modified
PP will depend on the amount of stabilisers.
The addition of stabilisers may make material unsuitable for food applications by
leaching and migration.
The reduction in the molecular weight of the PP on exposure to harmful UV radiation
can be easily seen using gel permeation chromatography (Figure 22). It can be seen
from the figure that the reduction in molecular weight is more severe at the part surface.
Molecular weight distribution
Figure 22 Gel permeation chromatogram showing the degradation of polymercaused by UV exposure
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Light stabilised grades have very good light, UV radiation and weathering resistance inboth natural and colour formulations. These grades are supplied with different degrees
of stabilisation to match the various application requirements. It should be noted thatmany additives and pigments can seriously impair the effectiveness of the stabilisersystem. Hence a balance of stabilisation system, pigments and homogeneous
distribution of additives is necessary to achieve optimum resistance to light, UVradiation and weathering. Light stabilised grades are generally unsuitable for food
contact applications. However, formulations can be developed based on requirements.Compatibility of the light stabilisers is very important since they are added in
significant quantities. Blooming and the migration of low molecular weight light
stabilisers are important issues.
The most accurate method to test the suitability of the material is the use of material in
its intended environment for a long period of time. Due to the long-term nature of
outdoor weathering, accelerated testing using weatherometers is common. Different
light sources are used, such as xenon arc lamp, carbon arc lamp and fluorescent sun
lamp. Filtered xenon most accurately reproduces the spectral energy distribution ofsunlight. However, the results from accelerated testing may be different from the long-
term outdoor testing.
HDPE offers inherently better oxidation and UV resistance in comparison to PP. Whilst
these properties may be greatly improved in PP by the use of additives, these mayincrease the cost of PP compounds to beyond that which is considered economicallyattractive. It is for this reason that HDPE has retained a substantial part of the crate
market. Nevertheless, PP blow mouldings have been commercially used in horticulturalsprayers and motor car parts.
4.10.3 Chemical Resistance
As a non polar, high molecular weight paraffinic hydrocarbon, PP has outstanding
chemical resistance, the best of all thermoplastics to organic chemicals. Indeed, there isno solvent for PP at room temperature, although it may swell in some cases. Since PP
contains only hydrogen and carbon and does not contain polar atoms, non polar
molecules (such as hydrocarbons and chlorinated solvents) are more easily absorbed by
PP than polar molecules (such as soaps, wetting agents and alcohols), causing swelling,softening or surface crazing.
PP is extremely resistant to inorganic environments. It is not affected by aqueous
solutions of inorganic salts, nor by most mineral acids and bases, even when
concentrated. It is, however, susceptible to attack by oxidising agents such aschlorosulphuric acid, pure fuming nitric and sulphuric acids, and the halogens. It must
be remembered that stabilisers and other additives can be attacked by aggressive
chemicals to which PP is resistant. This might affect the stability and properties of thematerial.
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Generally speaking, higher temperatures can considerably impair resistance dependingon the chemical environment. Up to 60 C, PP is resistant to many solvents but
aromatic and halogenated hydrocarbons, certain fats, oils and waxes cause swelling. Attemperatures up to 30 C, the effect is only slight.
The higher the degree of crystallinity in a PP material, the greater its chemical
resistance. Consequently, homopolymers of PP have more chemical resistance than therandom copolymer.
Environmental stress cracking (ESC) is the surface initiated brittle fracture of a polymer
under stress in contact with a medium in the absence of which fracture does not occur
under the same conditions of stress. Combinations of external and/or internal stresses
may be involved, and the sensitising medium may be gaseous, liquid or solid. A stressraiser or notch, an external and/or internal residual stress and a stress cracking medium
must be present for ESC to occur. For example, PE products prematurely fail in thepresence of detergents and other active environments. PP is widely used for packaging,
fluid containment and transportation. However, PP is virtually free from environmentalstress cracking observed in other polymers and attempts in the laboratory to identify a
pure ESC agent for PP have failed.
Many plastics are inclined to environmental stress cracking or embrittlement on
prolonged contact with boiling detergent solutions. The PP components specially madefor washing machines do not exhibit these disadvantages. A reflux test involving 1000hours in boiling detergent solution is used to measure water absorption, embrittling and
change of the dimensions. It has been reported that suitable grades show 0.5% higher
water absorption than the normal grades when soaked in detergent solution.
Furthermore, no embrittlement is observed and the yield stress, ultimate tensile
strength, dimensions, surface hardness, rigidity and toughness of PP are not changed.
4.10.4 Permeability
4.10.4.1 Permeability of Water and Liquids
PP is virtually impermeable to water and water-based products. It does not, therefore,
swell when immersed in water. The water vapour permeability of PP film is compared
with other plastics in Table 21. Changes in relative humidity have no effect on the
properties of the material. The very slight water uptake can be determined when there isa change of temperature in a hot damp atmosphere. It is due entirely to surface
adsorption. Fillers, reinforcements and additives may slightly increase the uptake of
water. It is advisable to dry the material immediately before processing or to degas itduring plasticisation. This particularly applies to carbon black pigmented PP. However,there is appreciable adsorption and permeation with certain organic solvents (especially
if non-polar). The degree of permeation increases with temperature.
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Table 21 Water vapour permeability of various plastics relative to thepermeability of oriented PP
Polymer Water vapour (g/m
2
24 h)Oriented PP film 1.0Cast PP film 2.0
HDPE 1.0
PVDC 0.10
LDPE 5.0
Rigid PVC 10.0
Plasticised PVC 3070
EVA 12.0
PA 11 20
PA 6 100Oriented PS 40
Cellulose acetate 400
Cellophane P 1000
4.10.4.2 Permeability of Gases
The permeation rate of PP with different gases is compared to other polymers in Table
22. It can be seen that the resistance to permeability of gases improves with orientation.In addition, PP offers good resistant to permeability of gases which is comparable to
HDPE but significantly better than LDPE.
In PP packaging for solvent containing substances or strong smelling products,
migration of solvent must be expected and, as a result, loss of weight in the contents
during prolonged storage. Most of the flexible packaging materials require stiffness,good printability, high gloss and good barrier properties. Oriented PP is coextruded
with polyvinylidene fluoride (PVDF) to achieve suitable product requirements whereexcellent barrier properties are required.
4.10.5 Sterilisation
4.10.5.1 Autoclave and Ethylene Oxide Sterilisation
Devices may be sterilised by one of three main techniques: autoclave, ethylene oxide or
radiation treatment. Steam autoclaving is generally carried out at temperatures of 120135 C. Being resistant to high temperatures and water, PP is one of the obvious
choices. It is entirely unaffected by the autoclaving as long as the treatment temperature
is kept below its softening temperature. Random polymers have lower softeningtemperature than the homopolymer and block copolymers and, hence, are not preferred
for steam autoclaving. Use of ethylene oxide sterilisation is now on the decline due to
the mutagenic nature of the material. PP is generally unaffected by ethylene oxide
treatment.
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Table 22 Permeability to gases of various plasticsPolymer N2 O2 H2O CO2 SO2
Oriented PP film 7.0 35 140 175 280Cast PP film 18 90 360 450 720
HDPE 15 75 300 375 600
Polyvinylidenechloride (PVDC)
0.06 0.3 1.2 1.5 2.4
LDPE 50 250 1000 1250 2000
Rigid PVC 1 5 20 25 40
Plasticised PVC 5200 251000 1004000 1255000 2008000
Ethylene vinylacetate copolymer
80 400 1600 2000 3200
PA 11 3.0 15 60 75 120PA 6 0.5 2.5 10 12.5 20
Oriented PS 20 100 400 500 800
Cellulose acetate 25 125 500 625 1000
Cellophane P 10 50 200 250 400
Medical sterilisation at temperatures above 100 C demands two opposing requirementsfrom the polymer. The polymer should have high crystallinity to provide good tensile
strength retention and heat resistance while low crystallinity is required to improve the
transparency. PE plastomers based on metallocene technology have been blended with
PP to achieve the optimum properties for syringe applications. The random copolymershave lower resistance to autoclave sterilisation due to their lower heat distortion
temperature. Parts which are significantly stressed can deform during autoclave
treatment. The stresses in a syringe may arise from pressure, vacuum, weight of the
liquid or due to pressure exerted on syringe.
4.10.5.2 Radiation Sterilisation
Radiation sterilisation is most damaging to PP. When PP is subjected to high energyionising radiation, deterioration in its physical properties and changes in its molecular
structure occur detectable by infrared (IR) spectroscopy. PP that will be radiation
sterilised requires unique stabilisation packages. Since radiation sterilisation causes
increased oxidation of the polymer, additive levels are also increased. Sterilisation of
disposable articles is generally carried out with a radiation dose of about 25 kJ/kg or 2.5Mrad. Although toughness suffers to some extent and shade changes are possible,
articles properly made from PP still retain their serviceability. Suitable trials should beconducted to properly assess the effect of radiation. Thick-walled parts are less affected
than thin-walled components. PP copolymers have markedly better resistance to highenergy radiation than homopolymers, due to lower crystallinity and greater free volume.Flex to failure test is used for evaluating the radiation tolerance of PP. Main areas of
application include syringes, needle shields, surgical trays and blow mouldedcontainers. El Paso Products Co., Nested Chemicals International and Exxon Chemicals
have reported development of suitable grades of gamma radiation sterilisable PP to
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satisfy the requirements for blow moulded containers and injection moulded articles[19, 20].
PE has considerable better radiation resistance than PP due to the absence of the tertiary
carbon atom which initiates degradation of the material and loss of the mechanicalproperties.
Irradiated PP has been used as a plant wrapping which gradually degrades in the soil.
These wrappings are suitable for both manual and machine planting of forests and
fields.
PP can encounter radiation from various sources when used in the nuclear industry.
Gamma radiation, used for medical sterilisation, is far more penetrating than beta,electron radiation or alpha radiation. The radiation dosage level harmful for PP is quite
low. Hence, use of PP in the nuclear industry where high radiation occurs is not
possible. Suitable resins for use in nuclear industry include phenolics, polystyrene,
polyester, particular epoxies, silicone, etc.