Polypropylene (PP)

Polypropylene (PP) is a linear hydrocarbon polymer, expressed as CnH2n. PP, like polyethylene (see HDPE, L/LLDPE) and polybutene (PB), is a polyolefin or saturated polymer. Polypropylene is one of those most versatile polymers available with applications, both as a plastic and as a fibre, in virtually all of the plastics end-use markets. Looking to buy polypropylene?

Contents

1 Development
2 Properties
3 Chemical Structure
4 Manufacture
    4.1 Processing
5 Processing Characteristics
    5.1 Important Shrinkage Factors
6 Grades Available
    6.1 Physical Properties
    6.2 Resistance to Chemicals
7 Applications
  7.1 Breakdown of Injection Moulding
Applications in the UK

8 Grade Selection
 8.1 Advantages
 8.2 Disadvantages
 8.3 Applications
9 Examples of Use
  9.1 Flexible Packaging
  9.2 Rigid Packaging
  9.3 Automotive
  9.4 Consumer Products
      9.4.1 Fibre     
      9.4.2 Industrial
      9.4.3 BOPP Film
10 Find a Supplier of Polypropylene or Another Material

1 Development

Following the work by Ziegler in Germany, the process for producing "stereoregular" polymers was perfected by Professor Giulio Nattain in Italy.  Natta produced the first polypropylene resin in Spain in 1954. Natta utilised catalysts developed for the polyethylene industry and applied the technology to propylene gas.

These new polymers with their ability to crystallise soon became popular and polypropylene is now a very successful product in many areas.  Commercial production began in 1957 and polypropylene usage has displayed strong growth from this date. The versatility of the polymer (the ability to adapt to a wide range of fabrication methods and applications) has sustained growth rates enabling PP to challenge the market share of a host of alternative materials in plethora of applications including... 

Oriented polypropylene (OPP) has seen considerable growth, having replaced cellophane in virtually all applications.

2 Properties

The properties of Polypropylene include...

• Semi-rigid
• Translucent
• Good chemical resistance
• Tough
• Good fatigue resistance
• Integral hinge property
• Good heat resistance

PP does not present stress-cracking problems and offers excellent electrical and chemical resistance at higher temperatures. While the properties of PP are similar to those of Polyethylene, there are specific differences. These include a lower density, higher softening point (PP doesn't melt below 160oC, Polyethylene, a more common plastic, will anneal at around 100oC) and higher rigidity and hardness. Additives are applied to all commercially produced polypropylene resins to protect the polymer during processing and to enhance end-use performance.

3 Chemical Structure 

Linear hydrocarbon polymer, little or no unsaturation. Similar to polyethylene in many properties, especially solution and electrical. However, the presence of the methyl group attached to every alternate backbone chain carbon atom can alter the properties in a number of ways:  

(i)    it can cause a slight stiffening of the chain - increasing the crystalline melting point (Tm);  

(ii)    it can interfere with the molecular symmetry - depressing crystallinity and hence Tm.

Isotactic


Syndiotactic

Atactic

In the case of very regular polymers, ISOTACTIC form, the net result is a melting point some 30oC higher than that for HDPE.

The methyl group also has a chemical effect, i.e. tertiary carbon atom provides a site for oxidation - PP less stable than PE, also peroxides and radiation lead to chain scission rather than cross-linking.

Manufacturers quote "isotactic index" - amount insoluble in hot solvent - which indicates (?) the percentage isotactic polymer in the grade. N.B. usually 90-95% isotactic. Glass transition around 0oC presents problem of embrittlement and this can be altered by copolymerisation (with ethylene  block copolymers) and/or blending with rubber, e.g EPM. Polypropylene does not crystallise so readily as polyethylene, it tends to supercool, and this becomes worse at higher molecular weights.
 

4 Manufacture

Production of polypropylene takes place by slurry, solution or gas phase process, in which the propylene monomer is subjected to heat and pressure in the presence of a catalyst system. Polymerisation is achieved at relatively low temperature and pressure and the product yielded is translucent, but readily coloured. Differences in catalyst and production conditions can be used to alter the properties of the plastic.

Propylene is obtained, along with ethylene, by cracking naphtha (crude oil light distillate). Ethylene, propylene, and higher alkenes are separated by low temperature fractional distillation. From being a byproduct of ethylene in the early 1950's, propylene is now an important material in its own right.

Polypropylene is a major tonnage polymer with a growth rate higher than the norm for such thermoplastics. This growth rate is partially because of the versatility of polypropylene, and hence the wide range of application areas as indicated above. However, in the UK we have always used more polypropylene than in other European countries, e.g. West Germany. This is particularly the case in injection mouldings, which elsewhere might well be made from high density polyethylene.

5 Processing Characteristics

Polypropylene is a relatively easy materials to injection mould in spite of its semicrystalline nature. The absence of any real need for high molecular weight, from the mechanical properties view point, leads to low melt viscosity (easy flow). The pseudoplastic nature of polypropylene enhances this effect at high shear rates (fast filling rates).

Typically melt temperatures for injection moulding are between 200 and 250oC. Though they can be as high as 280, or even 300oC, for short periods of time. With Flame Retardant grades it recommended that 220oC is not exceeded.  Mould filling rates are generally on the high side, to ensure good surface finish and strong mouldings  free of weld lines and flow fronts. Adequate venting of the mould is essential to prevent burn marks.

The melt flow index gives a rough guide to melt flow behaviour, but as a result of the pseudoplastic nature of polypropylene (which is strongly dependant on the molecular weight distribution) this should not be taken too literally. Spiral flow mould data is sometimes provided by manufacturers as a practical means of assessing the flow behaviour, but this is no substitute for the fundamental melt rheological and thermal data which is now becoming more widely available.

Moulding shrinkage of polypropylene is typically around 1%,  but prediction of the actual value is difficult due to the strong influence of moulding conditions. Important factors which affect the shrinkage of polypropylene mouldings include: 

As a result of the ease of flow of polypropylene materials they are often chosen for large area mouldings where it is desirable to operate with minimum clamp forces. However for many current polypropylene mouldings, especially where good surface finish is required, it is common practice to use melt temperatures of 250oC combined with high packing pressures (typically about 80 MPa).
 

6 Grades Available

Three types of polypropylene are currently available. Each suits particular specifications and costing (although there is often some overlap).

Homopolymers - A General Purpose Grade that can be used in a variety of different applications.

Block copolymers - incorporating 5-15% ethylene, have much improved impact resistance extending to temperatures below -20oC. Their toughness can be further enhanced by the addition of impact modifiers, traditionally elastomers in a blending process.

Random copolymers - incorporate co-monomer units arranged randomly (as distinct from discrete blocks) along the polypropylene long chain molecule. Such polymers typically containing 1-7% ethylene are selected where a lower melting point, more flexibility and enhanced clarity are advantageous.

Different PP grades are available dependent on the application and chosen processing method.

 7 Applications

Polypropylene can be processed by virtually all thermoplastic-processing methods. Most typically PP Products are manufactured by: Extrusion Blow Moulding, Injection Moulding, and General Purpose Extrusion. Expanded Polypropylene (EPP) may be moulded in a specialist process.

8 Grade Selection

The choice of grade for any application is based on consideration of any, or all, of the following points:

Homopolymer: stronger, stiffer - higher HDT

Copolymer: better impact, more transparent

MFI: ease of flow vs. toughness.

Special grades

• talc filled 10  40% talc increases hardness, and HDT, but at the expense of toughness.
• glass reinforced 30% glass fibre increases strength, stiffness, and HDT, but drastically reduces the impact.

8.1 Advantages

Good chemical resistance. Good fatigue resistance. Better temperature resistance than HDPE. Lower density than HDPE.

8.2 Disadvantages

Oxidative degradation is accelerated by contact with certain materials, e.g. copper. High mould shrinkage and thermal expansion. High creep. Poor U.V. resistance.

8.3 Applications

Buckets, bowls, crates, toys, medical components, washing machine drums, battery cases, bottle caps. Elastomer modified for bumpers, etc. Talc filled for additional stiffness at elevated temperatures - jug kettles, etc. OPP films for packaging (e.g. crisps, biscuits, etc.). Fibres for carpets, sports clothing.

9 Examples of Use

9.1 Flexible Packaging

PP is one of the leading materials used for film extrusion and has in recent years benefited versus cellophane, metals and paper on account of its superior puncture resistance, low sealing threshold and competitive price. PP Film is available either as Cast Film or bi-axially orientated PP (BOPP). The film market may be divided in to three main sectors:

• Food and Confectioneries
• Tobacco
• Clothing

The food and confectioneries sector is the largest of the film markets with usage ranging from confectioneries to crisps and biscuits. Tobacco products represent a significant market for PP (second largest after food and confectioneries). Rigid packaging subdivides into a multitude of packaging applications from caps and closures to pallets and crates.

9.2 Rigid Packaging

Reusable and collapsible/stackable crates are a great application for PP, providing ease to transport (both full and empty) and allow simple, safe and efficient storage of products and are ideal for Just-in-Time (JIT) storage solutions. As a consequence, supermarkets are beginning to revert to use and similar products are finding application in the automotive supply chain.

Caps and Closures manufactured of PP have benefited from growth in the PET bottle market, particularly for mineral water containment and that of edible oil.

PP is blow moulded to produce bottles for the packaging of a range of products including condiments, detergent and toiletries markets, PP thin-walled containers (e.g. yoghurt pots) are also common. PP competes with PS in this field, offering a cheaper material option (processing costs can, however, offset this benefit). PP is semi-crystalline product and consequently has a narrower processing window than PS and tends to display higher shrinkage. Modern thermoforming machinery is capable (with two sets of tools) of processing either PP or PS, consequently the future infiltration of PP is very much dependent on price fluctuations.

9.3 Automotive

In the automotive sector PP is utilised as a monomaterial solution for automotive interiors. The monomaterial dashboard is becoming increasingly achievable, PP film cushioning, film skins, and powder slush moulding and even blow moulded parts with integral PP textile covers are emerging.

Bumpers, cladding, and exterior trim are also available manufactured from polypropylene. Polypropylene developed for such applications provides low coefficient of linear thermal expansion and specific gravity, high chemical resistance and good weatherability, processability and impact/stiffness balance. Improvements with colour-at-the-press and pre-coloured PP have also reduced or eliminated the need for painting in some applications.

9.4 Consumer Products

Products classified in this sector are Housewares, Furniture, Appliances, Luggage, Toys, Battery Cases and other "durable" items for home, garden or leisure use. Injection Moulding dominates the the conversion process used for these products.

9.41 Fibre

PP Fibre is utilised in a host of applications including tape, strapping, bulk continuous filament, staple fibres, spunbound, and continuous filament.

9.42 Industrial

PP is used to manufacture a range of Sheet, Pipe, Compounding and Returnable Transport Packaging (RTP). With the exception of RTP where Injection Moulding is used, extrusion dominates the conversion process used for these products. Some PP is utilised by the construction sector, most notable domestic drainage pipes.

9.43 BOPP Film - Heat Seal

Customers often experience inconsistent heat seal temperature and heat seal strength with newly developed BOPP film. This is often because of an inconsistent distribution of the heat seal layer thickness at high line speed and extra thin skin gauge. BP has successfully achieved very thin and consistent skin gauge BOPP film products, delivering high clarity (haze <1%), as well as a consistent low SIT heat seal film.

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