Practical Guide to
Polypropylene
Practical Guide to
Polypropylene
By
Devesh Tripathi
Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK
Rapra Technology Limited
Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK
© 2002, Rapra Technology Limited ISBN: 1-85957-282-0
All rights reserved. Except as permitted under current legislation no part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means—electronic, mechanical, photocopying, recording or otherwise— without prior permission from the copyright holder.
1 Introduction... 1 1.1 Background ... 1 1.2 Major Advantages ... 2 1.3 Major Disadvantages... 3 1.4 Competitive Materials... 3 1.5 Applications ... 5
1.6 Market Share and Consumption Trend ... 6
1.7 Major Suppliers... 7 1.8 Material Price... 8 2 Basic Types of PP ... 9 2.1 Homopolymer ... 9 2.2 Copolymer... 9 2.2.1 Random Copolymer ... 10 2.2.2 Block Copolymer ... 10 2.3 Elastomer-Modified Polypropylene ... 11 2.4 Controlled Rheology ... 11 2.5 Metallocene Polymers... 12
2.6 Syndiotactic and Atactic PP ... 13
2.7 Filled Grades of PP ... 13
2.7.1 Talc Filled PP... 14
2.7.2 Calcium Carbonate Filled PP ... 14
2.7.3 Glass Fibre Reinforced PP ... 14
2.7.4 Mica Reinforced PP ... 15
2.8 Additives for PP ... 15
2.9 Identification of PP Type ... 16
3 Structure ... 19
3.1 Molecular Weight ... 19
3.2 Molecular Weight Distribution ... 20
3.3 Crystallinity... 20 3.4 Orientation ... 22 3.5 Isotacticity... 22 4 Properties... 24 4.1 Density ... 24 4.2 Thermal Properties... 24
4.2.1 Glass Transition Temperature and Melting Point ... 24
4.2.2 Maximum Continuous Use Temperature ... 27
4.2.3 Heat Deflection Temperatures and Softening Points ... 28
4.2.4 Brittle Temperature ... 29
4.2.5 Specific Heat ... 30
4.2.6 Thermal Conductivity ... 31
4.3.1.1 The Effect of Test Speed ...33
4.3.1.2 The Effect of Temperature...33
4.3.1.3 Time-temperature Superposition...34
4.3.2 Impact Strength ...34
4.3.2.1 Falling Dart Impact Test...35
4.3.2.2 Notched Impact Strength... 35
4.3.2.3 Tensile-impact Strength...36
4.3.3 Creep ...36
4.3.4 Fatigue...39
4.3.5 Dynamic Fatigue ...39
4.3.6 Mechanical Properties of Filled Grades ...40
4.3.7 Biaxial Orientation ...43 4.4 Electrical Properties ...44 4.5 Optical Properties...46 4.5.1 Transparency ...46 4.5.2 Gloss...47 4.5.3 Haze...47 4.6 Surface Properties ...47
4.6.1 Hardness and Scratch Resistance ...47
4.6.2 Abrasion Resistance ...48
4.6.3 Friction ...49
4.7 Acoustic Properties ...49
4.8 Biological Behaviour...50
4.8.1 Assessment Under Food and Water Legislation ...50
4.8.2 Resistance to Microorganisms...50
4.8.3 Physiological Compatibility...51
4.9 Additives ...51
4.9.1 Antistatic Agents ...51
4.9.2 Electromagnetic Interference/Radio Frequency Interference Shielding ...52
4.9.3 Slip and Antiblocking Agents ...53
4.9.4 Metal Deactivators and Acid Scavengers...53
4.9.5 Blowing Agents...53 4.9.6 Nucleating Agents ...54 4.9.7 Antifogging Agents ...54 4.9.8 Biocides...54 4.9.9 Flame Retardants...55 4.10 Performance in Service ...56
4.10.1 Thermal or Heat Stability...56
4.10.2 Stability to Light and Ultraviolet Rays...57
4.10.3 Chemical Resistance ...59
4.10.5 Sterilisation ... 61
4.10.5.1 Autoclave and Ethylene Oxide Sterilisation... 61
4.10.5.2 Radiation Sterilisation ... 62
5 Design ... 65
5.1 Product Design... 65
5.1.1 Design for Rigidity and Toughness... 65
5.1.2 Weld Lines ... 66
5.1.3 Shrinkage and Dimensional Stability... 66
5.1.4 Sinks and Voids ... 67
5.1.5 Design for Assembly... 68
5.1.6 Integral Hinges... 68
5.1.7 Design to Avoid Failure and Durability... 69
5.1.8 Design Safety Factors ... 69
5.2 Mould Design... 70
5.2.1 Flow Length ... 70
5.2.2 Feed Systems... 71
5.2.3 Venting... 72
5.2.4 Mould Cooling ... 72
5.2.5 Taper and Ejection ... 73
5.2.6 Surface Finish ... 73
5.2.7 Filled Grades ... 74
6 Processing of PP ... 75
6.1 Rheology ... 76
6.1.1 Melt Flow Rate... 76
6.1.2 Viscosity Versus Shear Rate ... 76
6.2 Injection Moulding... 80
6.3 Extrusion ... 82
6.3.1 Fibre and Filament ... 82
6.3.2 Film Extrusion... 82
6.3.2.1 Cast Film... 82
6.3.2.2 Blown Film... 83
6.3.2.3 Biaxially Oriented Film... 83
6.3.3 Coextrusion ... 83
6.3.4 Stretched Tapes ... 83
6.3.5 Sheet Extrusion ... 83
6.3.6 Pipes and Tubes ... 84
6.4 Blow and Stretch Blow Moulding ... 85
6.5 Thermoforming and Vacuum Forming ... 86
6.6 Calendering ... 87
6.7 Rotational Moulding ... 87
7.1.1.1 Heated Tool Welding... 90
7.1.1.2 Hot Gas Welding ...90
7.1.1.3 Friction and Vibration Welding ...90
7.1.1.4 Ultrasonic Welding ... 91
7.1.1.5 Radio Frequency Welding ... 91
7.1.1.6 Other Welding Techniques ...91
7.1.2 Solvent Bonding...91
7.1.3 Adhesive Gluing...92
7.1.4 Sealability...92
7.2 Assembly and Fabrication...92
7.2.1 Machining...92
7.2.2 Snap-fit Joints...92
7.2.3 Mechanical Fastening...93
7.3 Decorating ...93
7.3.1 Printability and Paintability...93
7.3.2 Metallising and Electroplating ...94
7.3.3 Appliques ...94
8 Causes of Failure ...95
9 Product Development Issues ...97
9.1 Material Selection ...97
9.2 Design ...97
9.3 Processing and Post Assembly...98
9.4 Performance in Service ...98
References ...101
1 Introduction
1.1 BackgroundPolypropylene (PP) was first produced by G. Natta, following the work of K. Ziegler, by the polymerisation of propylene monomer in 1954 (Figure 1). The macromolecule of PP contains 10,000 to 20,000 monomer units. The steric arrangement of the methyl groups attached to every second carbon atom in the chain may vary (see Figure 2). If all the methyl groups are on the same side of the winding spiral chain molecule, the product is referred to as isotactic PP. A PP structure where pendant methylene groups are attached to the polymer backbone chain in an alternating manner is known as syndiotactic PP. The structure where pendant groups are located in a random manner on the polymer backbone is the atactic form.
CH2 = CH CH3
Figure 1 Propylene monomer
CH2 CH CH2 CH3 CH CH2 CH3 CH CH3 CH2 CH CH3 n CH2 CH CH3 CH2 CH CH3 CH2 CH CH3 CH2 CH CH3 n syndiotactic polypropylene CH2 CH CH3 CH2 CH CH3 CH2 CH CH3 CH2 CH CH3 n atactic polypropylene isotactic polypropylene
Only isotactic PP has the requisite properties required for a useful plastic material. Stereospecific or Ziegler-Natta catalysts are used to polymerise PP in this form. All the applications of PP described in this book are for isotactic PP, although brief mention is made of the main applications and properties of syndiotactic and atactic PP.
The pendant methylene group in PP is replaced by a chlorine atom in polyvinyl chloride (PVC), by a benzene ring in polystyrene (PS) and by a hydrogen atom in polyethylene (PE). The pendant group significantly affects the properties of the polymer, and consequently the properties of PP are very different from other commodity plastics such as PE, PVC and PS (Section 4).
In 1957, PP was commercially produced by Montecatini as Moplen. Recently, metallocenes have attracted widespread attention as the new generation of olefin polymerisation catalysts. Metallocene catalysts provide enhanced control over the molecular make up of PP, and grades with extremely high isotacticity and narrow molecular weight distribution (MWD) are possible. Properties of metallocene-polymerised PP are further discussed in Section 2.5.
1.2 Major Advantages
PP is very popular as a high-volume commodity plastic. However, it is referred to as a low-cost engineering plastic. Higher stiffness at lower density and resistance to higher temperatures when not subjected to mechanical stress (particularly in comparison to high and low density PE (HDPE and LDPE)) are the key properties. In addition to this, PP offers good fatigue resistance, good chemical resistance, good environmental stress cracking resistance, good detergent resistance, good hardness (5 on the comparative ranking utilised in Table 4) and contact transparency and ease of machining, together with good processibility by injection moulding and extrusion. These advantages of PP are further elaborated in later sections.
Table 1 Comparison of unmodified PP with other materials: Advantages [1]
Property PP LDPE HDPE HIPS PVC ABS
Flexural modulus (GPa) 1.5 0.3 1.3 2.1 3.0 2.7
Tensile strength (MPa) 33 10 32 42 51 47
Specific density 0.905 0.92 0.96 1.08 1.4 1.05
Specific modulus (GPa) 1.66 0.33 1.35 1.94 2.14 2.57
HDT at 0.45 MPa. (°C) 105 50 75 85 70 98
Maximum continuous use
temperature (°C) 100 50 55 50 50 70
Surface hardness RR90 SD48 SD68 RM30 RR110 RR100
Cost (£/tonne) 660 730 660 875 905 1550
Modulus per unit cost
(MPa/£) 2.27 0.41 1.97 2.4 3.31 1.74
ABS = acrylonitrile butadiene styrene HIPS = high impact polystyrene RR = Rockwell R
RM = Rockwell M SD = Shore Durometer
The properties of unmodified PP are compared with other competitive thermoplastics in Table 1. It can be seen from the table that PP offers advantages over most of its competitive materials on the basis of specific modulus (modulus to density ratio), heat deflection temperature (HDT), maximum continuous use temperature or modulus to cost ratio. Environmental and food legislation may further tip the balance in favour of PP.
1.3 Major Disadvantages
The major disadvantages of unmodified PP compared with other competitive thermoplastics are evident from Table 2. It can be seen that PP has significantly higher mould shrinkage, higher thermal expansion and lower impact strength, particularly at sub-ambient temperatures, than HIPS, PVC and ABS. However, PP has lower mould shrinkage and thermal expansion coefficient than HDPE and LDPE. Poor UV resistance and poor oxidative resistance in the presence of certain metals such as copper are other disadvantages of PP. As any semi-crystalline material, PP also suffers from high creep under sustained load in comparison to an amorphous plastic such as ABS or PVC. Other disadvantages of PP are difficult solvent and adhesive bonding, poor flammability, warpage, limited transparency, poor wear properties, unsuitability for frictional applications and poor resistance to gamma radiation. (Further discussion of the properties of PP may be found in Section 4). However, most of these disadvantages could be overcome, either completely or to a certain degree, by proper selection of material, sensible design and good processing. The processing of PP by thermoforming and blow moulding is difficult. Vacuum forming of PP is also difficult.
Table 2 Comparison of unmodified PP with other materials: Disadvantages [1]
Property PP LDPE HDPE HIPS PVC ABS
Mould shrinkage (%) 1.9 3.0 3.0 0.5 0.4 0.6
Thermal expansion (x10-5) 10 20 12 7 6 8
Notched Izod impact
strength (kJ/m) at 23 °C 0.07 >1.06 0.15 0.1 0.08 0.2
PP is not hazardous to health, however, it can release volatile organic compounds (VOCs) into the surrounding air during high-temperature processing. Workers at the processing plant can be subjected to these VOCs through inhalation or skin contact. Good ventilation using exhaust fans can minimise the exposure. Residual monomer and catalysts present in the resin can increase the toxicity.
1.4 Competitive Materials
PP is most frequently compared with PE but other competitive materials are polystyrene and its derivatives, cellulose acetate (CA), cellulose acetate butyrate (CAB) and PVC. PP is used to replace engineering plastics, such as polyethylene terephthalate (PET), polyamide (PA), polycarbonate (PC) and ABS, etc., in kitchen appliances and domestic appliances. In non-plastics, PP faces competition from glass and metal.
Major competitive materials for PP and their crude advantages/disadvantages over PP are given in Table 3. This table is for broad comparison only. In many cases, polymers are filled or modified to improve properties or to reduce cost which makes the distinction between the properties of two polymers for a particular application quite blurred. Consequently, choice of a particular material for a given application will require a careful study of the product requirements, material properties and other commercial, environmental and legislative issues.
Table 3 Comparative advantages/disadvantages of other thermoplastics to PP
Polymer Advantages Disadvantages
LDPE Higher impact resistance
Lower brittle temperature
Lower strength and stiffness Lower surface hardness
Lower heat distortion temperature
HDPE Lower cost
Higher impact resistance Lower brittle temperature
Lower strength and stiffness Lower surface hardness
Lower heat distortion temperature
HIPS Lower shrinkage and warpage
Better gloss Better rigidity
Lower chemical resistance Higher cost
Environmental stress cracking
PVC Better clarity
Better processing window Better weather resistance
Worse environmental acceptance Lower solvent stress crack resistance
Lower heat deflection temperature
PET Higher clarity
Better oxygen barrier Better impact properties
Worse water barrier properties Unsuitable for hot fill and sterilisation
Higher price
ABS Better stiffness
Better gloss Better processibility
Higher cost and weight Lower solvent resistance Lower heat resistance
PA 6, 66 Higher toughness
Better feel
Better hydrocarbon resistance
Higher water absorption Higher cost and density
PC Better transparency
Higher toughness and modulus Higher continuous use
temperature
Higher cost and density Notch sensitive
Lower fatigue resistance
CA Better transparency
Better impact strength at lower temperatures
Higher modulus
Lower solvent resistance Greater moisture absorption Higher cost
CAB Better transparency
Better gloss
Lower solvent resistance Greater moisture absorption Higher cost
A typical material selection involves many properties which are not easily quantifiable in numerical terms (such as weathering, warpage, surface finish, ease of machining, etc.) or which may have very obscure units (such as transparency, fatigue, wear,
bonding, detergent resistance, etc.). These properties for PP are compared with other competitive materials on a judgemental value basis on a scale of zero to nine in Table 4.
Table 4 Comparative ranking of different plastics on a scale of 0 to 9 where 0 represents unfavourable property while 9 represents favourable property [2]
Property PP
homo-polymer
PP
co-polymer LDPE HDPE ABS PVC HIPS
Bonding 5 5 5 5 8 9 9 Brittle temperature 1 2 7 7 5 7 3 Detergent resistance 8 7 4 4 5 9 7 Dimensional stability 4 4 5 5 9 5 7 Fatigue index 9 9 7 8 2 6 3 Flammability 1 1 1 1 2 8 2 Friction 5 5 1 6 1 2 1 Gamma radiation 2 2 5 4 6 7 8 Hydrolytic stability 8 8 9 9 8 7 8 Shrinkage 3 2 1 1 5 7 6 Surface finish 8 8 7 8 8 4 8 Toughness at room temperature 4 6 9 6 7 5 6 Toughness at –40 °C 3 4 8 7 7 3 6 Transparency 5 5 5 5 0 7 5 Weathering 3 3 2 3 3 7 3 Warpage 5 4 5 5 8 8 8 Water absorption 9 8 9 9 4 7 6 Wear 5 5 4 5 2 3 1 Extrusion 8 8 9 9 8 7 9 Injection moulding 8 8 9 8 8 3 8 Machining 8 8 5 8 9 5 4 Vacuum forming 3 2 3 4 8 9 9 1.5 Applications
The main applications of PP in different market sectors are given in Table 5. Some of the critical requirements for these applications are explained in Table 6.
Table 5 Typical applications of PP Sector Typical applications
Household goods
Buckets, bowls, bottle crates, toys, bottle caps, bottles, food processor housing, video cassettes, luggage
Automotive industry
Radiator expansion tanks, brake fluid reservoirs fittings, steering wheel covers, wheel arch liner, bumpers, bumper covers, side strips, spoilers, mudguards, battery cases, tool boxes
Fibres
Artificial sport surfaces, monofilaments for rope and cordage, stretched tapes, woven carpet backing, packaging sacks and tarpaulins, staple fibres, coarse fibres, filament yarns, fine fibres
Table 5 (cont.) Typical applications of PP
Domestic appliances
Dishwasher parts such as top frame, basement, tubs, extruded gaskets, water duct, water softener compartment, etc.
Washing machine parts such as detergent dispenser, door frames, inlet and outlet pipes, bellows, feet and wheel, housings and ducts, etc.
Refrigerator parts such as boxes, containers, drawers, ducts, inlet and outlet pipes etc.
Microwave oven cabinet, irons and coffee maker body parts Packaging
Margarine and ice-cream tubs, films, compartmentalised meal trays, thin-walled packaging for, e.g., disposable food trays, dessert cups and confectionery boxes, strapping tapes, blister packaging Pipes and
fittings
Solid rods, punching plates, hot wire reservoirs, tower packings for distillation columns, domestic wastewater pipes, pressure pipes, heat exchangers, corrugated pipes, small diameter tubing, e.g., biro cartridges, drinking straws
Furniture Stackable chairs
Table 6 Critical requirements for applications where PP is one of the best choice of material
Application Critical requirements
Chairs Good rigidity, good toughness, colourability, mouldability in
complex shapes
Car bumper High impact strength at low temperatures, excellent weathering,
high rigidity Hair dryers, irons and
kitchen appliances
Rigidity, brilliant surface gloss, good heat ageing resistance, antistatic properties, high HDT, mar resistance
Disposable food packaging
Rigidity, transparency (if required), heat sterilisable, no taste, good flow and fast cycling, low cost
Syringes, tubes, cartridges
Transparency, sterilisable and unbreakability (toughness), good flow length
Video cassette boxes Fatigue strength, high flexibility, warpage
Pipes and fittings Low frictional loss, good chemical resistance, high continuous use temperature, low noise
Luggage Impact strength, warpage
1.6 Market Share and Consumption Trend
Over the last four decades, PP has established itself as one of the major commodity plastics. PP is now the third largest consumed plastic material after PE and polyvinyl chloride. The consumption of PP in comparison to other plastics is shown in Figure 3 [3]. Demand for PP has grown consistently, managing an impressive growth even during recessions. Western European PP consumption in 1995 was estimated at about 5 million tonnes against a production capacity of about 6 million tonnes. Approximately 55% of PP is used in extrusion and the rest in injection moulding [3]. Sixty percent of
the PP consumed is homopolymer, 20% block copolymer, with the rest either compounded or random copolymer grades. It is estimated that the growth of PP in the coming decade will be around 6%, the strongest growth pattern for the bulk polymers.
Figure 3 Consumption of PP in comparison to other major plastics in the UK [3]
1.7 Major Suppliers
The major manufacturers of PP and their trade names are given in Table 7.
Table 7 Major PP manufacturers
Manufacturer Trade name
Atofina Appryl Basell Novolen
Borealis Borstar PP
BP Acclear, Accpro, Acctuf
Dow Polypropylene Homopolymer, Impact Copolymer
DSM Stamylan P, Vestolen P
Exxon Exxon Mobil PP
Repsol
1.8 Material Price
The price of PP is compared with that of other competitive thermoplastics in Table 8. It can be seen from the table that commonly used engineering plastics, e.g., acetals, PC, PET and PA are more costly than PP. The different PEs are similar in price to PP, whilst styrenics and PVC are generally more costly. Since the prices of different materials depend on the grade, the quantity purchased, the supplier, etc., these prices should be taken for guidance only. The prices of different grades of PP are compared in Table 9.
Table 8 Comparison of indicative prices of different raw materials
Polymer Price (US cents/lb)
PP homopolymer 30 PP copolymer 35 LDPE 50 HDPE 35 ABS 70 PS 45 HIPS 50 PVC 30 PA 6 125 Acetal 100 PC 140 PET 100
Source of data: Plastics News, March 11, 2002, 21
Table 9 Indicative prices of different types of PP
PP type Price (US cents/lb)
Homopolymer, injection 34 Extrusion grades Fibre 33 Film 35 Profile 39 Sheet 36 Random copolymer Blow moulding 39 Film 38 Injection 37
2 Basic Types of PP
PP, a semi-crystalline thermoplastic, is made in its homopolymer form by polymerising propylene monomer using stereospecific Ziegler-Natta catalysts. The catalyst system is termed stereospecific because it controls the position of the side (methyl) group in each propylene unit in the polymeric chain. A typical catalyst system may be prepared by combining titanium trichloride with tributyl aluminium or its variants. Most commercial PP is isotactic.
The physical properties and processing characteristics of PP are mainly determined by the molecular weight (average number of propylene units in a chain), the molecular weight distribution (variation in average length of chains) and the type and amount of copolymerising monomer. The selection of the right grade of PP for a specific application involves
• choosing between homopolymer and copolymer, • choosing a reactor or controlled rheology grade,
• defining the melt flow rate required and the appropriate additive system.
However, with changes in manufacturing technology, operating conditions and catalyst systems, the traditional differences between the properties of homopolymers and copolymers have blurred [4-6]. Hence, an open mind is necessary to select a proper grade for a particular application.
2.1 Homopolymer
Homopolymer PP is made by polymerising propylene in the presence of a stereospecific catalyst. Homopolymers are more rigid and have better resistance to high temperatures than copolymers but their impact strength at temperatures below zero is limited (Section 4.3.2).
Typical applications for homopolymer polypropylene include windshield washer tanks, shrouds for fans and steering columns, housings for domestic appliances such as hair dryers, sterilisers, irons, coffee makers, toasters, etc., extrusion of fibres and filaments for carpet backing, upholstery fabrics, clothing, geotextiles, disposable diapers, medical fabric and automotive interior fabrics.
2.2 Copolymer
The properties of PP depend on the type and amount of comonomer. There are two basic types: random copolymer and heterophasic or block copolymer. The random polymers contain 1.5% to 6% by weight of ethylene or higher alkenes (such as butene-1) in random distribution and in a single chemical phase. The essential difference between a random and a block copolymer is that the block copolymer contains
comonomer in the form of a dispersed rubber phase [7]. The structure of random and block copolymerised PP is shown schematically in Figure 4.
—P—P—P—P—E—P—P—P—E—P—P—P—P—P—E—P—P—P—P—P—P—E—P—P— random copolymer
—P—P—E—E—P—P—P—P—E—E—E—P—P—P—P—P—E—E—E—E—E—P—P—P— block copolymer
Figure 4 Structure of random and block copolymerised PP molecules. P and E represent propylene and ethylene monomer units, respectively
Copolymerised PP gives a softer feel to film and fibre products compared to homopolymers. However, PP copolymers are more expensive than the homopolymers (see Table 8). Typical applications of copolymerised PP are battery cases, bumper filler supports, interior trim, glove boxes, package trays and window mouldings, video cassette boxes, office chairs, disposable containers, boxes and appliance housings.
2.2.1 Random Copolymer
The random copolymer of PP contains chains with a small number (~1.5–6%) of ethylene or higher olefin units (such as butene or hexane), dispersed randomly among the propylene units. The presence of ethylene in the polymer chain reduces the tendency to crystallise and results in improved impact strength, a softer feel, a wider range of heat sealability, resistance to creasing and improved clarity. Some of the inherent rigidity of the homopolymer is sacrificed by copolymerisation. Due to the lower crystallinity, random copolymers have a lower melting point and specific gravity than the homopolymer. This combination makes copolymers attractive for injection-moulded houseware, thermoforming, stretch blow mouldings and films [4]. Random copolymer grade can be used to replace PVC, PS and PET in food packaging and stationary applications.
2.2.2 Block Copolymer
PP homopolymer is copolymerised with ethylene. In block copolymers, the ethylene content is much higher than the random copolymers. The copolymerised part of the material is rubbery and forms a separate dispersed phase within the PP matrix. As a result, block copolymerised PP is much tougher than homopolymerised PP and can withstand higher impact even at low temperatures but at the expense of transparency and softening point. The main applications of the block copolymerised PP are similar to those of elastomer-modified PP but where the impact property requirement is not that critical.
2.3 Elastomer-Modified Polypropylene
Extremely high toughness at low temperatures can be achieved by modifying PP with elastomers, mainly ethylene propylene rubber (EPR), ethylene propylene diene rubber (EPDM) or plastomers. Plastomers are very low density (<0.88 g/cm3) copolymers of ethylene and an olefin produced using metallocene technology. Plastomers can have narrow molecular weight distribution and more long chain branching than EPR and EPDM.
Modification of PP with elastomers causes loss of hardness and stiffness. If elastomer-modified PP is considered for food-related applications, relevant national and international regulations should be checked for compliance. Shrinkage of elastomer-modified grades is lower than the copolymer grades due to reduced crystallinity and increased free volume. Further, elastomer-modified grades allow good paint adhesion since the rubber phase provides sites for etching or surface treatment. For outdoor applications, black-coloured or light-stabilised materials are required. Low melt flow rate grades are used for extrusion and blow moulding where there are higher impact requirements than can be met with PP homopolymers. However, higher melt flow rate grades are more suitable for injection moulding, once again where better impact strength is justified in terms of application suitability. Elastomer blends are commonly used in the automotive industry for bumpers, bumper covers, protective side strips, spoilers, steering wheel covers, mudguards for tractors and lorries, and other parts which are likely to encounter high impact stresses.
2.4 Controlled Rheology
The polymerisation techniques for PP lead to a wide range of molecular weight. The molecular weight distribution can be controlled by splitting the PP chains using hydrogen peroxide into smaller units in the post-reaction stage. This reduces molecular weight and narrows its distribution and, consequently, increases melt flow rate. Most of such controlled rheology (CR) grades have melt flow rate (MFR) values higher than 20 g/10 min at 230 °C at 2.16 kg load. It can be as high as 120–150 g/10 min or more. Moulding cycles for the CR grades can be up to 15% faster, and warpage and shrinkage is reduced because of reduced orientation of polymer chains in the flow direction and the reduction in injection pressure due to easy flow of the material. Reactor grades of PP have a broad molecular weight distribution (Mw to Mn ratio of 5–12), but CR grades
offer a substantially lower ratio (~3–5). However, the breakdown of polymeric chains might lead to formation of low molecular weight polymers or oligomers that can cause odour problems (organoleptic problems) in PP (Section 4.8.1). The other problem with the CR grades is the reduction in impact strength due to the reduction in molecular weight. The impact properties of the CR grades should be carefully monitored, particularly at low temperatures.
CR grades are available both as homopolymers and copolymers. Copolymer-based CR grades for injection moulding flow well and are highly resistant to warpage and internal stresses. These grades find application in thin-walled packaging for food and
pharmaceuticals, video cassettes, automotive parts, machine housing parts, suitcases, crates and freezer containers and other warpage prone parts. However, the arrival of metallocene-catalysed PP (which offers advantages such as better organoleptic properties and narrow molecular distribution) is set to challenge the use of CR grades in traditional applications.
2.5 Metallocene Polymers
Metallocenes are a new generation of olefin polymerisation catalyst. They have attracted widespread attention because of their high activity and versatile performance with different monomers. The principal obstacles to their use in PP production have been that their melting point and molecular weight are too low. These problems are now solved with newly designed stereo-specific zircocenes making isotactic and syndiotactic PP of high molecular weight and varying stereoactivity. Metallocene catalysts provide enhanced control over the molecular make up of PP [8]. Reactor grades with extremely high isotacticity (~1% atacticity in comparison to a minimum 3–4% atacticity of conventionally polymerised PP) and narrow molecular weight distribution are possible. The narrow molecular weight distribution results in lower shear sensitivity of the PP resin and provides low melt elasticity and elongation viscosity in extrusion (Section 6.1.2).
Metallocene-polymerised copolymers offer the same mechanical properties as the conventional Ziegler-Natta catalysed polymers, similar deflection temperature under load but with lower melting point (147–158 °C). Metallocene-catalysed PP significantly improves the property window of conventionally polymerised PP. Significant improvements in modulus and hot tack strength are observed while water vapour transmission rate, haze and heat seal initiation temperature is reduced [9]. High melt flow properties without the use of organic peroxides (as required by CR grades) means that the metallocene polymers offer superior organoleptic properties. The properties of metallocene-polymerised PP are compared with Ziegler-Natta homopolymer and copolymer PP in Table 10 [10]. It can be seen that the mechanical properties (tensile modulus and tensile yield strength) of metallocene-catalysed PP are similar to that of homopolymer PP while the optical properties (gloss and haze) are similar to random polymer. This unique combination of mechanical and optical properties is associated with ease of flow resulting from a higher MFR value and narrower molecular distribution. Because of the lower molecular weight distribution, the metallocene-based PP offers low warpage and is particularly suitable for thin-walled packaging products for dairy products such as yoghurts and cheese. Other targeted markets for metallocene polymers are medical products such as petri dishes and syringe bodies.
Europe’s first commercial metallocene-catalysed PP was launched by Targor, the joint venture between BASF and Hoechst (now Basell), with the trade name of Metocene. Exxon Mobil (Achieve) and BP have also produced grades of metallocene-catalysed PP.
Table 10 Comparison of the properties of metallocene-polymerised PP with PP homopolymer and copolymer manufactured using Ziegler-Natta catalyst [10]
Property Metallocene PP Ziegler-Natta homopolymer PP Ziegler-Natta random copolymer PP MFR (230 °C/2.16 kg) 60 48 48
Tensile yield strength (MPa) 35 35 29
Tensile modulus (GPa) 1.7 1.55 1.15
Charpy impact strength at 23
°C (kJ/m2) 90 103 180
Haze (%) 7 60 7
Specular gloss at 20° (%) 77 57 65
2.6 Syndiotactic and Atactic PP
Syndiotactic PP is available from, e.g., Fina Oils and Chemicals, and Mitsui Toatsu Chemicals, polymerised using metallocene catalysts. It is claimed that the syndiotactic structure provides better impact strength, greater flexibility, lower haze, lower heat deflection temperature and lower residual monomer content. However, the full properties of these polymers are still to be evaluated and it remains to be seen whether syndiotactic PP can offer properties which are unique enough to market it as superior to isotactic PP and which can provide justification for the higher cost of material [8, 11, 12].
Atactic PP is an amorphous material and has little strength. The main application of atactic PP is in coatings in conjunction with bitumen or asphalt.
2.7 Filled Grades of PP
While most of the PP produced is used without mineral filler, the use of such materials is more common in PP than with PE. PE has very low modulus and stiffness. Consequently, the improvement in mechanical properties achieved by addition of fillers is not significant. By choosing the appropriate filler, PP type and compounding technology, it is possible to design products with properties approaching those of some engineering polymers. For these reasons, fillers are used not only to reduce the polymer content and cost but also to enhance its performance. As a result, a significant number of filled and reinforced PP grades has been developed and are successfully used in different applications. The improved stiffness and heat deformation resistance has led to the use of such compounds for the manufacture of heater housings, car mounting components and several domestic appliances. The main fillers and reinforcements for PP are discussed in this section. Their impact on its mechanical properties is discussed in Section 4.3.6.
It is reported that products made from PP have no effect on the biosphere after landfill disposal. However, mineral fillers may remain on the disposal site for a very long time or build up in incinerators.
2.7.1 Talc Filled PP
Control of the average particle size, the particle size distribution, the purity and the aspect ratio of the filler is necessary to achieve consistent product quality in talc filled PP. In some grades of talc filled PP, water absorption may be an important factor. This will affect the surface appearance of the moulded product and the adhesion of the resin to the filler.
Grades filled with 10% to 40% talc by weight have been produced. Both homopolymer and copolymer grades of PP are used. Talc filled grades offer higher stiffness, better surface aesthetics, lower coefficient of thermal expansion, lower shrinkage, and improved scratch and mar resistance than non-filled grades. Heat deflection temperature and mould shrinkage are also improved by the addition of talc. Flexural modulus increases dramatically with added talc at the expense of tensile strength. In some cases, impact modifiers are added to maintain the impact strength, but at the expense of stiffness. Filled copolymer grades offer higher yield elongation at the expense of tensile yield strength.
The main applications for talc filled PP grades are in car heater casing, motor housing, dryer drums, textile bobbins, industrial and agriculture plant components. Talc filled PP sheet is used as an alternative to carton board.
2.7.2 Calcium Carbonate Filled PP
Calcium carbonate is also commonly used as a filler for PP. In comparison to the talc filled grades, the calcium carbonate filled grades are claimed to have higher impact strength, brighter colour, higher thermal stability and improved fatigue strength, but lower stiffness and tensile strength. Calcium carbonate is added to PP at the same loading as talc, from 10–40% by weight. However, in a highly filled system, non-uniformity of mechanical properties can result from poor dispersion during the compounding process.
The main applications of calcium carbonate filled PP are in instrument panels, motor vehicle grills, heater boxes and garden furniture.
2.7.3 Glass Fibre Reinforced PP
Glass fibres are used to confer enhanced strength and rigidity. These fibres are usually coated with silanes, lubricants, film formers and, sometimes, antioxidants and antistatic agents. These coatings provide better fibre-matrix adhesion, consequently enhancing the mechanical properties of the product. These coatings are also intended to reduce
breakage of the glass fibre during manufacturing and processing. Substantial improvements in tensile strength and modulus are only realised after a coupling reaction takes place between organofunctional silanes on the glass fibre and reactive groups introduced into the PP molecule. There are many commercial glass fibre grades that impart enhanced performance in PP.
The higher aspect ratio of glass fibre imparts higher reinforcing efficiency than talc, calcium carbonate or mica. Glass fibre reinforced PP has been successfully used to replace engineering thermoplastics in various applications. It has replaced PC, ABS, polyesters and PA in hand-held tools, automotive grill opening reinforcing panels and pump housings. Glass fibre reinforced grades are used for car and truck fan shrouds, car rear light housing, radiation expansion tank, grills, headlamp housing, furniture frames and washing machine components.
2.7.4 Mica Reinforced PP
Mica is a generic term encompassing a family of minerals, mainly hydrated potassium aluminosilicates. Due to its high aspect ratio (about 50–100) mica gives higher flexural modulus than talc or calcium carbonate at the same loading. More significant improvement in tensile strength is obtained upon the use of appropriate coupling agents. Due to overall mechanical property profile and high temperature resistance, mica reinforced PP is used in several automotive applications, e.g., crash pad retainers, battery and fan shrouds. As mica is dark in colour, it is not suitable for light-coloured articles.
2.8 Additives for PP
Many other additives can be added to PP to provide or improve different functionality. Commonly used functional additives are given in Table 11 and further discussed in Section 4. However, it should be noted that the improvement in a certain property (or properties) on addition of additives is generally at the expense of some other useful properties. Hence, any change in material should be considered thoroughly to understand its full impact on the product quality, specification and suitability for the intended application.
The presence of additives in PP can significantly increase the toxicity of the resin. These substances can migrate into food or water through plastic packaging or to the body through medical devices. The handling of the additives might require special handling instructions and they can produce toxic degradation products during processing. Detailed information about the toxicity and hazard of special additive or material may be obtained from the Material Safety Data Sheets from the manufacturer. Some ingredients known to cause health and safety problems used in PP are blowing agents, peroxides, fillers (such as glass fibre), pigments (particularly lead- and cadmium-based pigments) and flame retardants.
As a rule of thumb, if any additive is added to the formulation of the PP, it should be tested for its likely impact on food and medical applications. Resin should conform to the regulations for health and safety.
Table 11 Commonly used functional additives for PP Additive Functionality
Antistatic agents To reduce accumulation of dust and associated possible fire hazard
Slipping agents To decrease the friction between the film and the machinery during processing
Antiblocking agents To avoid films sticking together
Metal deactivators To reduce degradation due to the presence of metals
Blowing agents To reduce density
Nucleating agents To improve transparency and clarity Antifogging agents To prevent condensation forming
Biocides To control the growth of micro-organisms and bacteria
Flame retardants To reduce flammability of the material or to suppress smoke
Antioxidants To prevent thermal oxidative degradation during processing and
service
Lubricants To lower melt viscosity and prevent sticking to metal surface
UV stabilisers To protect against harmful UV radiation
Light stabiliser To provide stability against visible light
2.9 Identification of PP Type
Identification of a plastic component may be required for various reasons, e.g., the identification of the material of a competitive product or defective products returned from the field. The simplest technique to identify PP is by burning a small specimen. PP burns with a blue flame with a yellow tip and smells of burning candle when the flame is extinguished. PP floats on water and can be easily cut providing smooth surfaces. PP is soluble in hot toluene. Most of the above observations for identification of PP are similar to those of PE. Hence, further tests are invariably required for confirmation of polymer type. The results from flame testing are further complicated by the presence of comonomers, fillers and additives such as flame retardants, blowing agents, lubricants and stabilisers. Hence, chemical and thermal analysis is required for positive identification of the polymer. Infrared (IR) spectroscopy is the most widely used technique for the positive identification of PP. Typical IR spectra (transmittance (T) plotted against wavenumber) for different types of PP are shown in Figure 5. IR spectroscopy can provide limited information about the fillers as well. Differential scanning calorimeter (DSC) thermograms may be required to confirm the presence of ethylene comonomer in the case of copolymerised PP or to measure the degree of crystallinity in the PP artefacts (Section 3.3).
Further information about the fillers can be obtained from thermogravimetric analysis (TGA) and X-ray fluoroscence spectroscopy (XRF). In TGA, the weight loss and
derivative weight loss of the polymer are measured as a function of temperature while XRF provides the elemental analysis of the polymer compound.
Figure 5a Typical IR spectrum for homopolymer PP [13]
3 Structure
Similar to PE, PP is a linear hydrocarbon polymer containing little or no unsaturation. It is, therefore, not surprising that PP and PE have many similarities in their properties, particularly in their swelling and solution behaviour and in their electrical properties. In spite of many similarities, the presence of a methyl group attached to alternate carbon atoms in the chain backbone does alter the properties of the polymer in a number of ways. For example, it causes slight stiffening of the polymer chain and interferes with the molecular symmetry. The first effect leads to an increase in the crystalline melting point whereas the interference with molecular symmetry would tend to depress it. However, the increase in the melting point due to the presence of pendent group is much higher than the corresponding reduction due to decrease in molecular symmetry. The melting point of PP is approximately 50 °C higher than that of PE. The melting point of HDPE ranges from 120–130 °C. The crystalline melting point of PP ranges from 160–170 °C. Further, due to the presence of pendant methyl groups, PP generally has higher tensile, flexural and compressive strength and higher modulii than PE. The methyl side groups can also influence some aspects of chemical behaviour. For example, the tertiary carbon atom provides a site for oxidation so that PP is less stable than PE to the influence of oxygen. Thermal oxidation (Section 4.10.1) and high-energy radiation (Section 4.10.5) lead to chain scission rather than crosslinking.
The detailed discussion of the structure-property relationship is a very complex issue and is not within the scope of this book. Further details can be found in many textbooks [e.g., 14]. However, many aspects of structure such as molecular weight, molecular weight distribution, crystallinity, etc., significantly influence the properties of PP and, hence, are briefly discussed here.
3.1 Molecular Weight
The molecular weight of PP is normally estimated from the simple measurement of viscosity. Intrinsic viscosity and limiting viscosity numbers can be established by solution techniques. Melt flow rate is more commonly used to measure the viscosity and is defined as the weight of the polymer which can be extruded through a defined orifice in a given time at a defined temperature and pressure. Melt flow rate is inversely related to molecular weight. Easy flowing grades are generally less tough than those of higher molecular weight and stiffer flow.
More sophisticated techniques such as gel permeation chromatography are used for measuring the molecular weight (Section 3.2).
The influence of molecular weight on the bulk properties of PP is often opposite to that experienced with most other well-known polymers. Although an increase in molecular weight leads to an increase in melt viscosity and impact strength, in accord with most other polymers, it also leads to a lower yield strength, lower hardness, lower stiffness and softening point. This effect is generally believed to be due to the fact that a high
molecular weight polymer does not crystallise as easily as lower molecular weight material and it is the differences in the degree of crystallinity which affects the bulk properties. It may also be mentioned that an increase in molecular weight leads to a reduction in brittle point.
3.2 Molecular Weight Distribution
The distribution of molecular weight in a polymer is a measure of the degree of variation in length of molecular chains since not all the chains grow to the same length during polymerisation. Molecular weight distribution is expressed in a number of ways. Polydispersity is the ratio of weight-average molecular weight (Mw) and
number-average molecular weight (Mn) and can be determined by fractionation techniques, such
as gel permeation chromatography, or by interpreting rheological data. A typical gel permeation chromatography curve for PP homopolymer is shown in Figure 6. Published data on PP indicate that molecular weight is in the range Mn = 38,000–60,000 and MW=
220,000–700,000, with values of MW/Mn from about 5.6–11.9 [14]. The controlled
rheology grades have significantly lower MW/Mn ratio (3–5). The molecular weight
distribution influences the processibility of the resin (Section 6).
Figure 6 A typical gel permeation chromatography curve for PP showing molecular weight distribution
3.3 Crystallinity
The molecular chains in PP are linear so they are able to pack together in an ordered crystal structure. Since chains may be entangled or otherwise imperfect (e.g., branching), the structure is not completely regular. Hence, PP is best described as a semi-crystalline polymer.
The degree of crystallinity and crystal structure in a polymer depends on its thermal history. A rapid quenching gives a tough clear product since it suppresses the formation
of crystals, while annealing or slow cooling of the product leads to a rather brittle and hazy product. Increased crystallinity increases hardness, modulus, strength, abrasion and wear resistance, creep resistance, barrier properties, shrinkage and density. Low crystallinity offers the advantages of good processibility, better transparency, economical melt processing and good thermoforming capability. Depending on the processing conditions, 60%–70% crystallinity in the finished product could be achieved.
(a)
(b)
Figure 7 Typical differential scanning calorimeter thermograms for PP showing the effect of cooling rate on the formation of crystalline structure (a) Annealed
Crystallinity in the final moulded artefact could be measured using differential scanning calorimetry (Figure 7). In differential scanning calorimetry, the energy absorbed or produced is measured by monitoring the difference in energy input into the substance and into a reference material as a function of temperature. It can further provide information about melting, crystallisation and glass transition temperature. It can be seen from Figure 7 that the heat taken by the product to melt the crystals depends on the cooling rate of the sample. Quenching suppresses the formation of cystallites, reflected by the lower heat required for melting of crystals (19.87 cal/g compared to 21.87 cal/g). The morphological structure in an injection-moulded article can be quite complex, with graduated layers of different crystallinity. The details of the crystal structure depend on the shape of the article and the conditions under which it is moulded. Thicker sections in a moulding or extrusion may vary in crystallinity, with the rapidly cooled surface having a tough skin while the slower cooling interior has larger spherulites and is relatively brittle. Consequently, moulding shrinkage, internal stresses, dimensional stability and warpage depend on the crystalline structure (Section 5.1.3). PP is often referred to as warpolene because of the warpage problems associated with the processing of the material.
The size of spherulites in PP may vary from 1 to 50 microns and can be seen using an optical microscope under a cross polariser. 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 from the melt. This results in less scattering of light. This technique is used in injection moulding to improve clarity and rigidity, and to reduce set-up time. Further details are given in Section 4.9.6.
3.4 Orientation
PP may be oriented either in the melt phase or by stretching when it is solid. In both processes, the polymer chains are aligned in the perfect direction usually along the line of flow or stretch. Deliberately introduced orientation in fibres or oriented films can lead to dramatic changes in molecular and crystalline arrangements. As a result, major variation in the properties of the article can be expected. Orientation produced by stretching increases tensile strength and reduces elongation in the direction of stretch. Biaxial orientation of PP film improves clarity. Further effects of biaxial orientation on the mechanical properties of PP are explained in Section 4.3.7.
3.5 Isotacticity
Isotacticity is the measure of the percentage of side methyl groups aligned on one particular side of the polymer chain. The isotacticity of commercially produced grades is measured in terms of isotactic index, the percentage of the polymer insoluble in n-heptane. The isotacticity index for most commercially available grades of PP varies from 85% to 95%. It is understood that within the range of commercial polymers, the greater the amount of isotactic material, the greater the crystallinity, and hence the greater the softening point, stiffness, tensile strength, modulus and hardness [14].
4 Properties
The mechanical and thermal properties of PP are dependent on the isotacticity, the molecular 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. These grades 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 lower than the homopolymer grades (Table 12). On the other hand, elastomer-modified, filled or 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. The mechanical, electrical or chemical properties of plastics cannot be considered without knowing the temperature at which the values are derived. The thermal properties of a polymer 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 of detergents.
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
mobile. A polymer above its glass transition temperature acts a tough ductile material while below it, the material is hard and glassy. On cooling, the glass transition temperature is sometimes known as the freeze temperature. Glass transition temperature is measured using a dynamic mechanical thermal analyser (DMTA) or differential scanning 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 20°C depending on the frequency/heating rate. • Crystalline melting point between 160–170 °C depending on the grade and the
frequency/heating rate.
• Recrystallisation temperature on slow cooling of the melt between 115 °C and 135 °C.
A typical temperature curve for shear modulus and mechanical loss factor, measured using 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 loss factor 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. The secondary 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 lower melting points in comparison to homopolymerised PP.
Recrystallisation temperature is quite important for injection moulding. Since the recrystallisation temperature of PP is between 115–135 °C, most of the crystallisation occurs during the cooling of the artefact in the mould. Since the recommended mould
temperature is in the region of 20–60 °C, this allows the possibility to restrict warpage and improve dimensional stability during processing (Section 5.1.3). In addition, PP continues to crystallise after processing at a rate varying with moulding conditions and storage 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 be used 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 the elevated 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 use temperature than PP. However, polycarbonate has a higher maximum continuous use temperature in comparison to PP.
Table 13 Maximum continuous use temperature of different plastics [1]
Polymer °C PP 100 HDPE 55 LDPE 50 PVC 50 ABS 70 HIPS 50 PA 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 of thumb, 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 would be 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 grade and its heat stabilisation system and should be carefully noted from the relevant trade literature. However, the functionality of the polymer for high temperature application might be quite limited in the presence of stresses.
4.2.3 Heat Deflection Temperatures and Softening Points
Heat deflection temperature is defined as the temperature at which a standard test bar deflects 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 are compared 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 more expensive 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 90–95 °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 maximum temperature 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 88–95 50–60 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 between those 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-moulded specimens are relatively stress free.
The data obtained by these tests cannot be used to predict the behaviour of plastic materials 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 of stresses, maximum continuous use temperature (Section 4.2.2) can provide a suitable
criterion for the selection of material. In addition, if load bearing properties are required from 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 been plotted 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 from the 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 sharply when 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 transition temperature. 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 closely related to the second-order glass transition temperature (Section 4.2.1). Brittle temperature 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 PP
PP grade Brittle Temperature
Homopolymer 5 to 15 °C
Random copolymer –10 to 15 °C
Due to the comparatively higher brittle temperature of PP, its use in low-temperature environments should be carefully considered and compared with other available thermoplastics. Low-temperature brittleness of some common thermoplastics is compared in Table 4. It can be seen that LDPE, HDPE, ABS and PVC offer lower brittle temperature. Impact strength at lower temperatures, e.g., –40 °C, should also be considered as a useful criterion for material selection for use under arctic conditions.
4.2.5 Specific Heat
Many features of the processing behaviour of PP may be predicted by consideration of thermal properties. The specific heat of PP is lower than that of PE but higher than that of PS. Therefore, the plasticising capacity of an injection moulding machine using PP is lower than when PS is used but generally higher than with HDPE. The plasticising capacity is defined as the amount of the material which can be melted and plasticised in the barrel in a given time in a given injection moulding machine.
Specific heat is a function of temperature below melting temperature. However, a significant rise in specific heat is observed near the melting point due to the partial crystalline nature of the polymer. However, the specific heat of the polymer melt is virtually independent of temperature. The specific heat, or more precisely the enthalpy, of the material controls the cooling of the artefact in the mould and predominantly the design of the cooling channels in the mould. The heat requirement for cooling of a PP artefact can be calculated from a graph such as that illustrated in Figure 10. To achieve faster cycles, mould cooling requirements should be considered from the beginning. The cooling system should balance the heat flow from the part to ensure uniform part cooling and minimise residual stresses, differential shrinkage and warpage. Other thermal properties of PP are given in Table 16.
Table 16 Thermal properties of PP
Property Value
Specific heat (J/g °C) at 23 °C 1.68
Specific heat (J/g °C) at 100 °C 2.10
Thermal conductivity at 20 °C (W/m K) 0.22
Linear coefficient of thermal expansion (/°C) 20–60 °C 10 x 10-5
60–100 °C 15 x 10-5
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, gives protection 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 is rather limited (owing to cost factors). PP has been used for food packaging of refrigerated foodstuffs due to its suitability for food applications rather than its suitability 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 1–2% in moulding, 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 thermal expansion on shrinkage, warpage and dimensional tolerances is discussed in Section 5.1.3.
4.3 Mechanical Properties
The mechanical properties of PP depend on several factors and are strongly influenced by the molecular weight. General observations suggest that an increase in molecular weight, keeping all other structural parameters similar, leads to a reduction in tensile strength, 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-known plastics.
The properties of some PP grades with different melt flow indices and structure are compared 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 melt flow 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 the cross 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 similar to 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 same value of melt flow rate or molecular weight. The difference in their tensile stress/strain curves is highlighted in Figure 12.
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 lower than 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 candidate for 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 of limited use and should only be used for pre-selection of material. In reality, plastic components 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 in nature. 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 of different grades of PP as a function of temperature is shown in Figure 13. PP homopolymers are slightly stiffer than copolymers at room temperature. However, the difference between the two types is diminished as the temperature rises. The flexural modulus of elastomer-modified PP is significantly lower than the homopolymer or copolymer 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.
