Polypropylene Practical Guide

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Practical Guide to Polypropylene

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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.

Polypropylene Practical Guide

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