compounding

Introduction

Polypropylene (PP) sold for commercial consumption has some types of additive. We will define'unmodified'PP as representing only those additives without which PP could not be viably processed incommercial extruders, moulding machines and the like. Accordingly, we will define'modified'PP as thatwhich has additives designed to provide special environmental, processing or physical properties.

Typical additives to unmodified PP include antacids such as calcium stearate., calcium pelargonate, zincoxide or hydrocalcite. These compounds are required to neutralize catalyst residues that could otherwiseform acids detrimental to the converter's equipment. The second basic additive to PP is antioxidant. This isrequired, as a minimum to protect the polymer from chain scission during processing and ageing. Typicalprocess stabilizer antioxidants are hindered phenols.

Most additional additives and modifiers such as clarifiers, flame retardants etc. are targeted to affect acertain class of properties. Further, fillers and reinforcements are incorporated to impart "Value Addition"to PP. High value added PP products can be made by "Modifying PP with elastomers such as EPM/EPDM.

Compounding is defined as the process of incorporating additives, modifiers into PP for achievinguniformity on a scale appropriate to the quality of the articles subsequently made from the compound. It isalso known as hot or melt blending.

2.0 Value added PP products

2.1 Low level additives2.1-1 Environmental Property Enhancement

In addition to process stabilizers, additional heat stabilizers are used to improve the long term heat ageingof PP. Several antioxidants will afford improvements, with the choice dependant on the environmentrequires. An important parameter is the continuous use temperature. Typical PP homopolymer withmoderate stabilization has a continuous use temperature of 105 deg. c. Special stabilization can increasethat temperature to 125 deg. C. Thioethers can act as long term heat stabilizers.

The UV resistance of unmodified PP is poor. However, stabilization can improve its performancesignificantly. Typically, hindered amines-can be used in combination with other appropriate additives todeliver service life or more than 5 years outdoors in both pigmented and non-pigmented products. This canbe done without affecting the appearance of a component. In addition, if a black appearance is acceptable,special grades of carbon black at levels above 2% can provide a service life of more than 20 years.

In addition to the stabilized applications listed above, there are many specialized uses of PP that require lowlevels of stabilizers. Among the applications for these stabilizers are the protection of PP components inradiation sterilizing environments, the prevention of colour development in various environments and thestabilization of wire-coating resin against the effects of copper. Each of these unique applications requiresspecific additive packages.

2.1-2 Processing Property Enhancements

In addition to the need for application heat stabilizers, some manufacturing processes require exceptionalmelt stability. For example, if products will be reprocessed repeatedly, if high levels of scrap are used or ifextruded sheet subsequently will be exposed to high temperatures, as in a thermoforming application, thegoal would be to minimize degradation in the extrusion process.. In these cases, additional melt stabilizersare required. Phosphite-based products are used for this purpose.

In film resins, slip agents and antiblocks are added to improve the processing and performancecharacteristics of the film. Slip agents are typically amides, with the particular selection depending on thecharacteristics being sought. Typical antiblocks are utilized to prevent the film from sticking to itself, while

3

slip agents are utilized to prevent the film from sticking to other surfaces, such as the converting equipment.Also used in film for processing and application performance are antistats. These products act to minimizestatic build-up as the film passes over the downstream equipment, as well as to minimize the dust pick upinherent in packaged products. For the latter applications, antistats are sometimes incorporated into PP usedin sheets, bottles and moulded articles. Monoglycerides and/or diglycerides are typical PP antistats.

Injection moulding products may utilize additives to adjust processing performance in the form of lubricantsand/or mould release additives. These additives may interfere with decorating.

2.1-3 Physical Property Enhancements

As crystallinity is responsible for many of the characteristics of PP, he ability to control it allows one toinfluence the physical properties of the resin. Nucleators are employed that provide sites for the initiation ofcrystals. The choice of nucleator will determine which properties are affected most significantly. The targetproperties are usually stiffness and heat deflection temperature, contact clarity and/or see-through clarity.Nucleators may also affect processing performance owing to their effect on the rate of polymercrystallization.

2.2. High Level additives (Fillers and Reinforcements for PP)

With the exception of the carbon black at more than 2% level, all the additives mentioned in Section 2.1 areutilized at very low levels. Typically, the total add level of these types of additives will remain below 2%.The properties that are most significantly affected by these type of additives are processing andenvironmental characteristics. TO achieve significant differences in properties, fillers and reinforcementsare used at much higher levels. Fillers and reinforcements are generally distinguished by the resultingdifferences in properties that they provide to PP.

Fillers are defined as additives in solid form that differ from polymer matrix with respect to theircomposition and structure. They are generally inorganic in nature and less frequently organic and are usedat levels of 10% and above. Different types of fillers can be classified as under:

Inert or Extender Fillers: These fillers occupy space in the polymer increase bulk density and lower cost andthereby extend the polymer.

Active Fillers: These produce specific improvements in physical and/or mechanical properties.

Reinforcements: These are specific type of active fillers. They increase tensile strength and flexuralmodulus of PP.

PP has the capability of accepting large amounts of mineral fillers (upto 70% by weight are marketed asmasterbatches). These include various forms of cellulose, hydrated oxides, clays, glass, metal powder,carbon fibre, wollastonite, asbestos, talc calcium carbonate, mica and combinations of these. Each hascertain characteristics that it imparts to a PP compound and will be discussed in detail elsewhere.

One of the original aims of introducing fillers was to decrease the cost of polymer by use of inexpensivefillers. However, it soon became apparent that improved properties were possible. This allowed use of highcust fillers, i.e. higher-purity, better colour and special surface treatments to improve adhesion at thedispersed phase interface. The most important fillers for PP are calcium carbonate, talc and mica. Glassfibre is used to reinforce PP.

Therefore, the aim today in compounding fillers and reinforcements in PP is selective modification ofproperties for specified end-use or in other words "Tailor-Making" of properties.

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2.2.-1 Selection criteria for fillers and reinforcements in PP

when incorporating fillers and reinforcements into PP, a number of factors have to be considered. These aresummarized below:

Particle shape, mean particle size and particle size distribution of the fillersDispersability and adhesion (linkage) with the PP matrix.Abrasive action of the filler on the processing machines.

Properties of the filler/reinforcement such as specific gravity, intrinsic strength, inorganic impurities.

Problems associated with dust when handling fine powders.

Cost of the filler/reinforcement

Reinforcement is justified only when a distinct improvement of properties/cost reduction compared tounreinforced PP is found or when a specific combination of properties is not achievable by other means.

Some general rules for selecting fillers and reinforcements in PP are summarized below:

Property Improvement Sought Choice of Filler/ReinforcementImproved surface finish Calcium carbonate, talc, glass beadsImproved tensile strength and flexural modulus Glass fibres, wollastonite, carbon fibresIncreased conductivity Carbon powder, A1 flakes, Ni-coated mica,

stainless steel fibre

Properties of filled and reinforced plastics

The main difference between inactive and active or reinforcing fillers is their influence on physical andmechanical properties. Modulus of elasticity and stiffness are increased to some extent by all fillers, eventhe spherical types such as CaCO3 and glass spheres. On the other hand, tensile strength can only beimproved by fibre reinforcement. Also the temperature of deflection under load(HDT) can not be increasedto the same extent as by fibre reinforcement. Fillers in platelet form, such as talc or mica, produced amarked improvement in these properties.

The use of extender fillers can result in the following changes in the properties of thermoplastics

Increase in densityIncrease in modulus of elasticity, as well as in compressive and flexural strength (stiffening)Lower shrinkageIncrease in hardness and improvement in surface qualityIncrease in HDT

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- Less temperature dependence of mechanical and physical properties- Cost reduction

Reinforcing fillers produce the following improvements in thermoplastics:

Increase in Tensile strength at break and compressive, shear and flexural strengthIncrease in modulus of elasticity and stiffness of the composite materialIncrease in HDT and decreasing temperature dependence of mechanical propertiesLower shrinkage

Two discrete phases are always present in reinforced plastics. The discontinuous filler phases should exhibithigher tensile strength and higher modulus of elasticity than the polymer matrix, whereas the continuouspolymer phase should possess higher elongation at break than the fibre. For this reason, fibres are suitableas reinforcing agents.

When the fibre reinforced material is subjected to a tensile load, local tensile stresses are transferred to thepolymer/fibre interface by shear forces and distributed over the fibre surface. For this purpose, the fibremust adhere well to the polymer and possess a specific length, since otherwise it slips out of the matrixmaterial. The higher the modulus of elasticity of the matrix polymer, the smaller can be the minimumlength of the fibre. Adhesion can be considerably improved by coupling mechanism between the filler andthe plastic.

2.2-2 Calcium Carbonate Filled PP

Calcium Carbonate (CaCO3) can be classified asMineral ground or naturalPrecipitated or syntheticNaturally occuring calcium carbonate is found as chalk, limestone, marble and is the preferred variety forfiller incorporation into PP.

A typical composition of filler grade calcium carbonate is shown below

CaCo3 : 98.5-99.5%MgCO3 : Upto 0.5%Fe2O3 : Upto 0.2%

Other impurities include silica, alumina and aluminium silicate, depending on location, source of the ore.

Typical mineral properties areDensity : 2.70 g/ccMoh;s hardness : 3Degree of whiteness 85-95%Oil absorption : 9-21g/100 g. powderSpecific surface : 1-15 m2/gmarea

6

Loadings of calcium carbonate in PP typically run from 10 to 50%, although concentrations as high a 80%have been produced. The filler is available in a variety of particle sizes and size distributions can be coatedor uncoated. Generally speaking, large particle size, greater than 5 um CaCO3 are less expansive, but theyreduce the impact strength of the PP compound. Smaller particle sizes (less than 1 um) cost more and aremore difficult to compound, but provide superior impact strength and improved surface appearance. CaCO3is usually selected as a filler when a moderate increase in stiffness is desired. minimal sacrifice in impactstrength can be tolerated.

Other effects of the mineral filler are to increase the density of the PP compound, reduce shrinkage, whichcan be helpful in terms of part distortion and the ability to mould in tools designed for other polymers. Attypical levels 10-50%, the viscosity of the compound is not significantly affected by the CaCO3.

The main secondary additive employed in CaCO3 formulations is a stearate. The stearate acts as aprocessing aid, helping to disperse the finer-particle size CaCO3. It also helps to prevent the absorption ofstabilizers into the filler. Finally, as an added benefit, it acts to cushion the system, resulting in improvedimpact. (Figure 1.)

Table (1) illustrates several properties available based on CaCO3 formulations. Applications of this productrange are in furniture, flush tanks, fan regulator covers, textile bobbins, refrigerator parts. It is aninexpensive substitute for polystyrene and ABS co-polymer. IN PP films, incorporation of CaCO3improves drawing properties. In PP tapes, use of CaCO3, upto 5-6% greatly reduces tendency of PP tofibrillate.

2.2-3 Talc Filled PP

Talc, chemically is hydrated magnesium silicate and can be represented at 3 MgO.4SiO2-H2O. Naturallyoccuring talc can exist in various forms like fibrou,s lamellar, needle, modular etc.

A typical composition of talc is as under:

SiO2 : 40-62%Al2O3 : 0.2-11%Fe2O3 : 0.1-0.5%FeO : 0.1-6.0%CaO : 0.3-1.0%MgO : 30-33%H2O : 16-17%

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Figure 1. (a) Effect of filler level on falling weight impact strength of PPat 23 deg. C.

(b) Effect of filler level on notched izod impact strength PP at 23deg. C.

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Table -1

Typical Product Properties of CaCO3, Filled-PP

Homopolymer standardCaCO3

Copolymer standardCaCO3

AlternateCaCO3

Property

Unifilled 20% 40% Unfilled 20% 40%

20% 40%

Melt flow rate, conditionL,g/10 min.

4 3 2 4 3 2 3 2

Density, g/cc 0.903 1.05 1.22 0.899 1.03 1.2 1.04 1.22Tensile strength (yield) MPa. 36 32 26 28 25 21 22 19Flexural modulus(1% secant),MPa

1656 2311 2725 1311 1794 2242 1414 1725

Rockwell R hardness 99 98 97 82 86 87 82 82Heat deflection temperature(deg. C)

97 117 120 85 86 94 83 83

Notched izod (23C), J/m 42 48 42 133 69 42 NBFa NBFaa No break, flex.

Table 2

Typical product properties of talc filled - PP

Homopolymer CopolymerPropertyUnfilled 20% 40% Unf

illed

20% 40%

Melt flow rate, condition L,g/10 min. 4 3.5 2.8 4 3.5 2.8Density, g/cc 0.903 1.05 1.22 0.89

91.04 1.22

Tensile strength (yield) MPa. 35 34 31 27 28 26Flexural modulus(1% secant), MPa 1656 2484 3278 752 2208 2898Rockwell R hardness 99 98 95 82 87 85Heat deflection temperature (deg. C) 88 110 118 77 105 114Notched izod (23C), J/m 42 32 21 133 53.38 32

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Typical Mineral properties areDensity : 2.90 g/ccOil absorption 28-51 g/100 g. powderMoh's Hardness: 1

Talc is generally selected to achieve higher stiffness than is possible with CaCO3, although at the cost ofreduced impact strength and greater sensitivity to moisture. It helps to decrease shrinkage and warpage andincreases thermal conductivity of the PP compound. Additional effects are higher HDT's and higher tensilestrength as compared to CaCO3 formulations.

Table (2) summarizes typical physical properties of talc filled PP.

In addition to structural and thermal property improvements, talc contributes to better dimensional stability,enhanced thermoforming opacity and depending on talc grade, white colour. The purity of talc isparticularly important to the thermal properties. The presence of metal ions can catalyze the degradation ofPP, which will decrease the long-term thermal stability of the composite. Small amounts of stabilizerpackages are often used in combination with talc, as in under- the hood applications. Surface treated talc donot affect the thermal properties of PP adversely.

Talc-filled masterbatches are availabale at loading upto 75%. Dark talc is often used for exterior andinterior automotive engineering applications where colour is not so critical Although the reinforcementproperties of dark talc are lower than those of white-talc grades, the dark talc provides acceptableperformance and costs less.. Often compounds for such applications contain both talc and CaCO3. Whitetalc are used in PP garden furniture and domestic appliances such as washers, dryers etc.

PP is the largest volume plastic usage for talc. Talc provides the structural strength (stiffness) and hightemperature resistance needed for automotive and appliances.

2.2-4 Mica Filled PP

Mica as a filler is not quite as popular as CaCO3 or talc, but it does offer some unique characteristics.

Mica occurs as complex structures of potassium and/or aluminium sillicates namely,

Typical mineral properties are:Density : 2.80 g/ccMoh's hardness: 4Oil absorption :48-500 g/100 g powder

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Table 3 a

TYPICAL PRODUCT PROPERTIES OF MICA FILLED -PP

Property Unfilled 40% 40% Coupled 50% 50% CoupledMelt flow rate,condition L,g/10 min.

4 2 2 1.5 2

Density, g/cc 0.903 1.23 1.23 1.36 1.36Tensile strength(yield) MPa.

35 43 46 45 50

Flexural modulus(1%secant), MPa

1656 5796 6555 7245 8211

Rockwell R hardness 99 88 82 85 887Heat deflectiontemperature (66 psi),deg.C

88 136 138 138 138

Heat deflectiontemperature (264 psi),deg.C

96 111 114 118 118

Table 3 b

Mechanical Properties of Mica & Glass - PP

Property Unfilled 20% 40%Tensile strength, MPa 34 42 43Flexural modulus, MPa 1311 6417 7176Izod impact, J/mNotched at 220 deg. CUnnotched at 22 deg. C

24No Break

42501

35235

heat deflection temperaturedeg. C at 264 psi

58 125 108

Mould shrinkage, % lengthwise 2.0 0.3 0.8

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Following the trends that have been established, mica is the next step up in the stiffness/HDT ladder.Likewise, it results in further decreased impact. Mica is found in a platelet form rather than in the moreparticular form of CaCO3. As the fillers pass from the particulate form through the platelet form on theirway to a fibrous-type form, the changes in physical properties correspond. From another view point, as theaspect ratio, or length-to-diameter (or thickness) ratio of the filler increases, strength tends to increase andimpact to decrease.

Mica offers outstanding stiffness as well as increased HDT. These properties are further enhanced by theaddition of a coupling agent. Table (3a) summarizes data on some mica-filled PP.

Mica-PP composites can be as stiff as steel sheet, but weight only 45% as such. Mica filled PP products areconsidered inexpensive substitute for glass fibre filled PP (Table 3b). An interesting use of mica is in PP-PEcoopolymer foam for loud speakers and musical instruments due to excellent acoustic properties of micamineral. The higher speed of sound in mica allows for a more compact speaker cone.

2.2-5 Glass -Fobre Reinforced PP

Glass fibre reinforced PP is a high growth segment of the market. These products have high tensile strengthand HDT. Applications include those areas requiring the chemical resistance of PP with the strength ofengineering resins. A drawback of these materials had been their tendency to distort in the final product.Recent advances have resulted in easy-flow grades that significantly reduce this tendency to warp. Otherdevelopments include the production of higher impact grades. These improvements have opened newapplications in the appliance and industrial markets.

Typical fibre lengths of glass are1/8 to 3/16 inch, although longer fibres are available for specialtyapplications. The standard glass diameter for PP applications is 13 microns. The factors influencingproperties are the base resin, the starting glass-fibre geometry, the compounding and processing techniques,the presence or absence and type of a coupling agent. Table (4) lists typical properties of glass filled PP.Table (5) illustrates the effect of polymer type and melt flow rate of physical properties using standard 3/16inch starting fibre length and a coupling agent. As can be seen the property balances available comparefavourably with many other engineering materials.

Table (6) demonstrates the effect of the type of coupling agent on PP. As can be seen, the coupling agentprovides a considerable increase in tensile strength. In addition, other properties, such as creep resistanceare also improved. The mechanism of the coupling agent is to form a bond between the sizing agent on theglass fibre and the specially treated PP resin.

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Table 4

Typical Product Properties of Glass filled-PP

Property Units Base PPHomopolymer

20% glassfilled PP

30% glasssfilled PP

40% glassfilled PP

Density g/cc 0.90 1.06-1.08 1.15-1.17 1.19-1.21Tensilestrength atyield

MPa 35 35.5 36.0 37.0

Tensilestrength atbreak

MPa 23 32.5 35.0 36.0

Elongationat break

% 60 30 20 30

Flexuralstrength

MPa 330 440 450 450

HDT 66 psi deg.C. 75 90 97 112

Table 5

Typical Product Properties of Glass Reinforced, Coupled Material

Homopolymer CopolymerProperty20% 40% 20% 40% 20% 40%

Melt flowrate,condition L,g/10 min.

3 2 18 12 1.8 1.5

Density,g/cc

1.04 1.22 1.04 1.22 1.05 1.14

TensileStrength(yield) MPa

83 103 77 99 61 86

Flexuralmodulus(1% secant),MPa

4209 6831 4071 7590 3450 4830

HeatDeflectiontemperature(66 psi )deg. C.

141 144 144 145 141 143

Heatdeflectiontemperature

234 136 134 137 141 143

NotchedIzod (23deg. C) J/m

85 112 75 96 149 192

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Table- 6

Effect Coupling Agent on a 30% Glass-Reinforced PP

Property No. of Coupling Agent Coupling Agent A Coupling Agent BMelt flow rate,condition L, g/10 min.

15 12 12

tensile Strength(yield)MPa

71 88 100

Yield Elongation (%) 1.8 2.1 2..9Flexural modulus(1%secant), MPa

6141 6279 6348

Heat deflectiontemperature (264 psi)deg. C.

143 146 146

Notched Izod (23deg.C) J/m

69 85 96

Applications of glass filled PP are in fan blades, head lamp housing, chemical process equipment, washingmachine tanks etc.

Table (7) shows a comparison of properties of filled PP homopolymer

2.2.6 Other fillers and Reinforcements

Many other fillers and reinforcements also can be used with PP, including wood flour, ground corn stalksand other cellulose containing substances. The cellulosic based products provide low-cost opportunities forachieving high stiffness, but their applications is limited owing to their tendency to char and generate waterat processing temperatures.

For the achievement of conductivity and/or static dissipation properties, metal powders, silver coated glassspheres, metal wires and conductive carbons have been used. TO reduce the coefficient of friction and toimprove wear characteristics. TeflonR and Silicone are used. These systems are highly specialized anddesigned for particular applications.

2.2-6 Colour Systems

The production of precoloured PP can be accomplished through the use of previously selected pigmentsystems that have been distributed uniformly in the polymer. For successful colour matching, the viscosityof the resin, the ease of pigment dispersion, the compounding equipment used and the selection ofdispersing aids must be considered. In order to ensure adequate dispersion, pigment systems are oftenpredispersed in a masterbatch. When utilizing a masterbatch, the carrier must be compatible with PP matrix.

In addition, the possible nucleating effect of certain pigments, heat stability of pigments which could affectprocessing and physical properties of PP respectively must also be considered.

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2.3 Flame Retardant PP

Materials used as flame retardants can be broadly classified as inorganic fillers and organic compounds.However, for making polypropylene flame retardant most of the conventional inorganic fillers cannot beused. This is because of high processing temperatures of polypropylene. At times, to achieve desired levelof flame retardance substantial quantities of inorganic fillers are required to be used. This affects theproperties of end product drastically. Hence, polypropylene is made flame retardant by using organic flameretardants.

Table 7

Comparison of Properties of filled PP homopolymer

Property Units Base PPHomopolymer

20%CaCO3filled

20% talcfilled

20%glassfilled

20% glassreinforced

MFI g/10min.

4 3 3.5 3.5 3.5

Density g/cc 0.9 1.06-1.08 1.06-1.08 1.06-1.08 1.06-1.08MouldShrinkage

% 2.1-2.3 1.6-1.8 1.5-1.7 0.8-1.0 0.8-1.0

Tensilestrength atbreak

MPa 23 18 25 32.5 80.0

Elongationat break

% 60 70 50 30 -

Flexuralstrength

MPa 33 35 47 44 61

deg.C 75 75 99 90 141

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Table 8

Flame retardant polypropylene- homopolymer chlorinated system

Property Units PP-FRDensity gm/cc 1.15-1.17Mould shrinkage % 1.5-1.1.6Mechanical propertiesTensile strength at yield

kg/cm2 340

Tensile strength at break kg/cm2 270Elongation % 20Flexural strength kg.cm2 450

Izod ImpactNotched kg.cm/cm2 2.0

20.0Unnotched kg.cm/cm2

Thermal PropertiesHeat Distortion temp. at 18.5kgs.load

deg. C 60

Flammability UL 94 V-0

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There are three broad categories of alme retardant polypropyleneA) Chlorine based systemsB) Bromine based systemsC) Non-Halogenated systems

There are a number of flame retardants available all over the world in each of the above system. The rightchoice among them depends on the end application and the cost. Another important inorganic filler isantimony trioxide which acts as a synergist with halogen i.e. mainly chlorine and bromine containing flameretardants. The halogen liberated from these during burning reacts with antimony trioxide to form volatileantimony halides and oxyhalides. These enter the gaseous part of the flame and help quench reactionoccuring, thereby neutralizing free radicals. The antimony trioxide also reacts directly with the polymer togive water which cools and dilutes the flame.

Manufacture of flame retardant polypropylene

In order to achieve uniform flame retardancy in the end product, compounding of polypropylene with allflame retardant additives becomes extremely important. Some applications also require many otheradditives and fillers to be incorporated in polypropylene along with flame retardants. This demandssophisticated compounding equipment and quality control.

To manufacture high quality flame retardant polypropylene sophisticated machinery and technology arerequired. Investment in machinery is very high even for medium sized manufacturing unit. Theseoverheads, together with the high rate of power consumption per unit weight of material manufactured,substantially add to the cost of the finished product. However, this increase can be compensated by addingfillers, which increases the complexity of compounds. Higher safety standards required by applicationsnormally justify higher cost of flame retardant materials.

UL Classification

Following is the basics of Underwriters Laboratory specifications:

This is a flammability test devised by Underwriters Laboratories for plastic materials used in electricaldevices and appliances. In this test, specimens are exposed for two successive 10 seconds ignitions from3/4" burner flame. They are classified according to the time it takes the flame to extinguish and the lengthof time any"after glow" persists.

Rating are as follows:

94 V-O

Extinguishment time 0.5 secondsAfter glow time 0.30 seconds

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94 V-1Extinguishment time 6-25 secondsAfter glow timeNo. flaming drips 0-60 seconds

94 V-2

Extinguishment time 0.25 secondsAfter glow timeFlaming drips permitted 0-62 seconds

Chlorine based systems:

This is the most economic way to make Flame Retardant Polypropylene although recently highly advancedand expensive systems are also available suitable not only for polypropylene but also for nylon, PBT etc.

Table 8 shows properties of flame retardant polypropylene made out of polypropylene homopolymer.Similarly Table 9 shows properties of flame retardant polypropylene copolymer. These are only typicalexamples of data sheet properties. However, enough scope exists to further modify the properties as per thedemands of applications.

It is also necessary to understand the limitations of this most economic system. The thermal stability andheat resistance of flame retardant poly propylene is greatly reduced. Flame retardants also plasticisepolypropylene is mouldable at lower temperature. This reduces the heat distortion temperature and sets alimit to the applications which need higher performance temperature. Chlorine based systems findapplication in small injection moulded parts and is not recommended for any of the extrusion applications asit poses greater degree of damage to screw, barrel, dies etc.

Bromine based system:

This is the best way of making propylene flame retardant. The end product has better thermal stability andhence can be processed by injection moulding and extrusion. Heat deflection temperature is also notadversely affected.

Table 10 shows typical properties of polypropylene homopolymer based compounds.

Since this system is versatile and can be used in wider applications, it is extensively studies from the pointof view of smoke and toxicity hazards.

Polypropylene compounds having required properties can be tailor-made for give application byincorporating various mineral fillers and glass fibres.

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Table 9

Flame retardant polypropylene- copolymer chlorinated system

Property Units PPCDensity gm/cc 1.15-1.17Mould shrinkage % 1.5-1.6Mechanical propertiesTensile strength at yieldTensile strength at break

kg/cm2kg/cm2

300250

Elongation % 40Flexural strength kg. cm2 400Izod ImpactNotchedUnnotched

kg.cm/cm2kg.cm/cm2

5.040.0

Thermal PropertiesHeat Distortion temp. at 18.5 kgs.load

deg. C. 55

Flammability UL 94

Table 10

Flame retardant polypropylene- homopolymer brominated system

Property Units PPFRV-1Unfilled PPH

Density gm/cc 1.24Mould shrinkage % 1.8-2.0Mechanical propertiesTensile strength at yieldTensile strength at break

kg/cm2kg/cm2

330275

longation % 40Flexural strength kg. cm2 460Izod ImpactNotchedUnnotched

kg.cm/cm2kg.cm/cm2

3.033.0


deg. C. 96.0

Flammability 1mm UL 94V-1

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Non-halogen based systems

Smoke density and toxicity regulations limit the use of halogen based flame retardant polypropylene.Acidic halogen based gases evolved during burning also damage expensive electronic circuitary. Thedevelopment of non-halogenated systems took place considering these aspects. However, it is the mostexpensive system today for injection moulding applications. It is widely used for extrusion applicationsmainly polyethylene based cables.

Table 11 gives properties of polypropylene homopolymer based product. Similarly compounds based onco-polymer fillers and other additives can be made.

2.4 Elastomer Modified PP products

One of the most common reasons to utilize a rubber/elastomer "modifier" in PP is to improve its lowtemperature impact resistance. Originally, the frist impact PP products were formulated from blends of PPhomopolymer with butyl rubber. Subsequently, it was discovered that ethylene-propylene (EPM) rubberoffered greater toughening power and was easier to disperse and compound into PP. The industry thenevolved towards fine tuning of the EPM materials with the closely related ethylene-propylene-diameterpolymers (EPDM)..

When EPM/EPDM rubbers are thoroughly blended with either PP homopolymer or copolymer to an extentof 10-40% by weight, a new family of thermoplastic materials called rubber or toughened PP is produced.They have flexural modulus of elasticity within 600-1200 MPa, together with an exceptional impactresistance down to - 50 deg. C according to the grade and percentage of EPDM used. Such a combinationhas been the main reason for the success of these materials in the automobile industry.

The three properties, elastic modulus in flexural mode, impact strength and brittle/tough transitiontemperature represent the most important indicators for end-use behaviour of elastomer modified PPtechnical articles.

A single rubber particle acts as a centre of absorption and elastic redistribution in the surrounding area of theshock wave caused in the material when a local impact is applied. The impact energy is, thereforedistributed across a large volume of material, decreasing local intensity when the finely dispersedelastomeric particles are numerous. Table (12) lists some properties of EPDM modified PP vs EPDMcontent.

Modification by Other Components

Elastomer modified PP grades require greater stiffness at elevated temperatures, better HDT and shapestability. Filler can be added to improve these properties. Talc is generally used in 15 to 40% by weight offinished products. EPDM-PP Talc compositions permit a wide range of possible proportions andcorresponding broad performance area for the commercial products.

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Table 11

Non-halogenated Flame retardant polypropylene-homopolymer

Property Units Unfilled PPHDensity gm/cc 1.04Mould shrinkage % 1.8-2.0Mechanical propertiesTensile strength at yieldTensile strength at break

kg/cm2kg/cm2

315270

longation % 38Flexural strength kg. cm2 430Izod ImpactNotchedUnnotched

kg.cm/cm2kg.cm/cm2

2.331.0


deg. C. 87.0

Flammability 1mm UL 94V-0

Table 12Properties of Elastomer Modified Polypropylene:

Property TestMethod

Units Blend I Blend II Blend III BlendIV

Blend V

Density D 1505 gm/cc 0.9-1.0 0.9-1.0 0.9-1.0 0.9-1.0 0.9-1.0Tensile strength at yield D 638 kg/cm2 45 100 150 200 220Elongation D 638 %

kg/cm25001000

500 500 350 100

Flexural Modulus D790 % 7500 7500 10000 15000Izod Impact-Notched-Unnotched

D256D256

kg. cm/cmkg.cm/cm

N.BN.B

N.BN.B

N.BN.B

60NB

30NB

Shore Hardness D2240 Shore 85A 90A 70D 77D 85DApplications O riungs,

hoses, matsand mud flaps,bellows

Automobiletrims, toys,refrigeratorliners,windowprofiles

Bumpers Dashboards,internal panesl

I II III IV V

PP: 50 60 70 80 90EPDM: 50 40 30 20 10

Applications:

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In the recent past, the most important uses of PP-EPDM blends has been in the automobile industry.Comparative lightness, low cost and a wide range of mechanical properties are the main factors responsiblefor the continuous growth of these materials since 1975. Their main use in car bumpers using either PP-EPDM flexible shapes mounted on a rigid steel framework or fabricated self-supporting finished articlessuch as complete car front masks.

Suitable blends contain from 10 to 35% by weight of EPDM, according to flexibility and imapctrequirements. EPDM-PP talc ternary blends find their biggest outlet as interment panel dash boards inmany models of low to medium cost cars. For this application stiffness and shape stability, coupled with animpact resistance high enough to avoid splintering failures or dangerous sharp fragments in medium speedcrashes provide these blends with an ideal application.

According to an estimate 90% of the OO-EPDM blends used are for the automotive industry atpresent. Asummary of filled and reinforced-PP products in the automobile industry is given in Table 13.

3.0 Compounding Machinery

Compounding of PP with additiives, fillers and reinforcements can be achieved using various types ofmachines depending on cost and quality of mixing desired. A few of these are discussed in the section.

3.1 Compounding low add levels

PP is generally an easily compounded material and the compounding of low-add-level products usuallyrequires nothing more than a dry solids mixer for preblending and a single-screw extruder for pelletizing.Extrusion temperatures will depend primarily on the viscosity or melt flow rate of the PP homopolymer orcopolymer component., with melt temperatures typically falling between 190 deg. c. and 2245 deg. c.Depending on the equipment, PP may give lower output rates on extruders than other polymers as a result ofboth its low density and its rheological properties.

At high melt flow rates, pelletizing may present some problems owing to the low melt strength. Withspecialized equipment, melt flow rates as high as 1000 have been pelletized. Both hot-cut and cold-cutpellletizing systems are suitable for PP. The choice usually depends on the equipment available as well asdownstream processing considerations. In some subsequent extrusion and injection moulding processes, theshape and size of the pellets will influence the products processing behaviour. For example, the flat, thinwafer configuration characteristic of the hot cut has been reported to cause slipping problem in the extrusionof thin film.

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Table 13

Plastic Components in Maruti Esteem

Part Name Material Wt. in gms. No. of parts Wt. per car(grams.)

Instrument Panel Filled PP 4000 1 4000Glove box Filled PP 350 1 350Glove box lid Filled PP 200 1 200Garnish cowlventilator

Filled PP 210 2 420

Oil filer housingassembly

Filled PP 650 1 650

Fan blade 80 W Filled PP 180 1 180Fan Blade 120 W Filled PP 236 1 236AC blower Filled PP 160 1 160Air filter assembly Filled PP 800 1 800Trim rear pillar Filled PP 150 2 300Air ventilatorassembly

Filled PP 1275 1 1275

AC case and covercooling

Filled PP 550 1 550

Boot component Filled PP 500 2 1000Bumper (Frontupper)

PP modified 780 1 780

Bumper (Frontlower)


Bumper (Rearupper)


Bumper (Rearlower)


When particularly good dispersion is required, a better option for incorporating these additives is to producean additive concentrate. To obtain the necessary dispersion in the final product, the concentrate is producedutilizing a fluxed melt mixer such as a Banbury prior to extrusion. The extruded concentrate may then beblended with the polymer matrix during a final compounding stage or may be added during the finalmoulding or extrusion step. Colour concentrates are a well known example, but lubricants. UV/lightstabilizers may also be incorporated in this manner.

3.2 High Add Levels3.3 3.2.1 Single screw extrusion

The addition of higher-add-levels of fillers and reinforcements may be done on a single screw or a twin-screw extruder. The choice is determined by the type of filler/reinforcement. In general, the single screwuse is limited to low-aspect-ratio additives such as Ca CO3 and talc.

The production of CaCO3 filled and talc filled materials may be accomplished using only a dry-solid mixerand a two-stage vented extruder. An Auger-type hopper stuffer assists feeding. As the filler level increases,melt fluxing mixers, such as Farrel Continuous Mixer (Figures 2) may be used to optimize mixing anddistribution. The Farrel Continuous Mixer is a counter rotating, non-intermeshing twin-rotor mixer. It canbe divided into 3 zones: (a) Feed zone (b) Mixing zone (c) Discharge zone. The feed zone of the mixer isdesigned to opimize ingestion and delivery of feed m material (s) into the mixing zone. Upon entering the

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mixing zone, the feed material is melted and then continuously fluxed, pumped, backmixed and it exitsthrough the discharge zone. The maximum achievable loading will depend, to some extent, on the meltviscosity of the polymer, but primarily on the type of filler. For example, specialized Ca CO3 concentrateshave been produced with filler levels upto 80%, while it is difficult to reach a 50% loading with mica.

At high filler loadings, intensive mixing may not be necessary, as long as the filler is uniformly distributed.Over-mixing can cause separation. In general, both batch (Banbury) and continuous(FCM type) mixers aresuitable.

metered feeding is necessary for all high filler-level systems. Fillers such as CaCO3 are liable to bridge inthe hopper. In addition, there is a potential for separation and build-up of the filler on vertical metalsurfaces. The critical design features of the feeders are the agitator and the feed screw and their relationshipwith each other. The agitator should sweep the hopper walls as closely as possible to prevent filler build-up.The hopper should be equipped with a level control feeder.

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SECTIONAL VIEW OF TYPICAL FCM IN OPERATION

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The compoudning extruder should have relatively shallow flights. A compression ratio of 2.5:1 is preferredand no additional mixing devices are generally required.

Mineral fillers tend to be hygroscopic. Therefore, a vacuum vent is utilized during the extrusion step.When a cold-cut pelletizing system is used, the emerging strands from the die face is taken into water bathand taken out quickly, taking care to ensure integrity of the strands. A blower assists water removal fromthe strands prior to chopping.

3.2-2 Reciprocating Extruders

In these types of machines, both rotational and reciprocating action of the screw occur simultaneously andare continuous. These machines are specifically designed to enhance mixing performance and are ususallyreferred to as Ko-Kneaders.

A schematic sketch of a Ko-Kneader is shown in Figure (3). The Kneader is a single screw extruder withinterrupted screw flights and stationary pins or teeth in the barrel. The screw flights can be continuous inthe feed section and vent section to improve forward conveying. In this section, no pins are located in thebarrel. The mixing sectionof the screw is typically double-fighted with a small helix angle of 13 degree.The interruptions in the screw flights leave flight segments that look like vanes. There are three rows ofpins or teetha along the length of the barrel. The barrel has a clamshell design, which facilitates cleaning ofthe barrel. Modern kneaders have a typical L/D ratio of about 11.

PP can be compounded with CaCO3, talc and glassfibres on Ko-Kneaders,

3.2-3 Twin screw extruders

The compounding of fillers, reinforcements, flame retardants (solids/liquids) can be achieved with a twin-screw extruder. Twin-screw extruders are designed to allow flexibility in the method of componentintruduction, in addition to the ability to change screw configuration. The extruder comes configures withmultiple feed ports, allowing the polymer components to be added as specified by the compounder. Forexample, the resin is generally added at the beginning of the extruder to ensure melting, mixing anduniformity of melt temperature, while fillers/reinforcing agents or liquid additives are added furtherdownstream.

Twin-screw extruders make use of starve feeding systems as opposed to flood feeding system utilized withsingle-screw extruders. Side feeders for filler addition are also employed. Venting, pelletizing anddownstream handling requirements are all similar for systems described in section 3.2-1.

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Feed alternatives (depending on formulation)

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3.2-3.1 Flame Retardant PP

A schematic sketch of a typical twin-screw set up for introducing liquid flame retardant into PP is shown inFigure (4). The extruder set up requires two feeding ports, one for PP and one injection port for additivelocated downstream. The first section of the extruder creates a melt pool before the liquid is injected. Theviscosities of the polymer melt and liquid are matched near the injection port and then the mass is allowed tohomogenize in the latter part of the extruder. The factors which govern the quality of the final product arethe temperature profile in the extruder, screw design and extruder geometry.

3.2-3.2 Filled PP

Filled PP grades can be compounded with twin screw extruders. Dispersion requirements for talc andCaCO3 vary according to the final product application. The primary objectives are uniform incorporationand mixing. A schematic diagram of mixing elements which make up a configurations shown in Figure (5).

Fillers are added downstream t a melt to isolate their abrasive effects. A melt seal is formed upsteam wherelarge pitch screws are used. The downstream section is configured for low shear mixing for lowperformance applications. Applications requiring high degree of dispersion are compounded using a highshear intensive mixing section.

3.2-3.3. Glass Fibre Filled PP

Compounding technique for incorporating glass fibre into PP requires isolation of high and low shearsections along the screw length. Chopped fibres or continuous rovings can be used. The design of thescrew determines the final firbre length in PP. PP in powder/pellet form is metered into the feed zone withstabilizers and lubricants. The additives can be preblended or metered individually. A relatively high shearplasticizing and mixing section is used to generate a homogeneous melt. Fibres are introduced into the meltvia second feed opening located downstream (figure 6.) Metering equipment is required for choppedstrands. Continuous rovings are continuously unwound from the core, drawn in by the turning screw. Oneinside the barrel, the continuous rovings are chopped to length by the kneading elements locateddownstream of the feed opening. A less intensive mixing section is needed for chopped fibre, since thefibres have only to be wetted out and dispersed. Degassing and die extrusion follows the compounding step.

Table (14) shows the effect of compounding technique on physical properties of glass fibre filled PPproduct. It is, therefore, important to select the correct compounding machine to obtain reinforced productswith desired mechanical properties.

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Table - 14Effects of Compounding Technique on 20% glass reinforced-PP

Property Standard twin screw Intensive twin screw Fluxed melt mixerMelt flow rate,condition L.g/10 min.

4 5 9

Tensile strength (yield)MPa

79 70 49

Yield elongation, % 2.8 3.1 5.1Flexural modulus (1%secant), MPa

4071 3726 26.22

Heat deflection temp.(264 psi), deg. C.

134 134 104

Fibre length (average),um

550 550 300

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Twin screw-extruder for introducing liquid FR into PP

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Twin screw Extruder

Continuous Twin Screw

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Typical configuration for glass fibre incorporation into PP

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Schematic view of a Banbury Type Internal Mixer

compounding

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