Polypropylene - PP

Excerpt from the book "Plast og Miljø" (Plastics and the Environment) by Lars Borch Pedersen, Teknisk Forlag

3.1.2 Polypropylene - PP

Propylene is a colourless and flammable gas. Polypropylene (PP) exists as three isomers (substances with the same gross formula but with different structural formulas). It is possible to achieve two rigid, semi-crystalline structures, depending on the position of the methyl group (-CH3) and a soft, elastic (amorphous) structure.

Only the semi-crystalline PP (degree of crystallinity 60-70%) is applied commercially, with the methylene groups positioned on the same side. PP is formed at a low pressure and temperature, in the same way as polyethylene (HDPE), by using a Ziegler-Natta catalyst.

Additives may be included during the actual PP production immediately after polymerisation, but they may also be added in connection with the final processing (compounding). Antioxidants are always used as PP oxidises easily. Processing of PP is mainly done by extrusion, rotational or injection moulding or by thermoforming.

For extraction and production of the raw material (granules), the energy consumption is approx. 80.0 MJ/kg PP.

Effects on the environment and health

The production processes are continuous processes in closed systems with no immediate exposure to organic solvents. The processing of thermoplastics is based on granules or powder. Consequently, there is a risk of dust exposure when handling the PP granules or powder.

In connection with the cleaning of machinery, contact with and emissions of solvents or propylene may occur.

Propylene

The toxicity of propylene is very low. At high concentrations (over 65,000 ppm) slight intoxication and reduced powers of concentration occur. Higher concentrations have stronger effects. Propylene is classified as a highly flammable substance.

All forms of heating renders it possible to split off volatile compounds. Production and processing temperatures are adapted to avoid thermal breakdown. Heat-oxidation breakdown will impair the mechanical properties of the plastic.

At irregular operation, e.g., overheating or the presence of oxygen during the production or processing, various oxidation products may develop, e.g., formaldehyde, acetaldehyde, acrolein, and various acids and ketones, such as formic acid and acetic acid, with the resulting environmental and health effects.

Overheating of PP means temperatures over 220°C to 280°C. The recommended processing temperature for PP ranges between 100°C and 120°C, but processing is possible at temperatures up to 310°C. The risk of overheating and subsequent formation of oxidation products is minimal.

At normal processing of polyolefins, only small amounts of the above substances may develop, since small molecules are always released (one can smell the processing of plastics). The risk of developing chemical substances in amounts exceeding the hygienic threshold values is minimal.

Material characteristics

Polypropylene is a semi-crystalline thermoplastic, the glass transition temperature Tg is approx.

-10°C-5°C, and the melting temperature Tm is approx. 165°C. Maximum application temperature in air and water is Tml ˜ 80°C-120°C and Tmv ˜ 90°C-100°C, respectively. The density of PP is 0.900-0.910 g/cm3 (homopolymer). The modulus of elasticity is 1.05-2.10 GPa (homopolymer) for PP and 3.50-7.00 GPa (ASTM) for 40% glass-fibre reinforced PP.

1: Satisfactory, 2: test required, 3: Unsatisfactory, S: Risk of stress-cracking degradation).

PP is resistant to boiling water. PP is attacked by oxidising acids at sustained temperatures over 90° C. Generally, polypropylene has good electrical insulation properties. PP has good mechanical properties in the form of great hardness and rigidity, it is easy to process and an inexpensive material in terms of price.

The application of PP is somewhat limited by the fact that it is a fire-promoting material and that non-stabilised polypropylene is broken down by UV light. PP becomes brittle at temperatures below -20° C, see also Appendix B. Other types of plastic with similar properties include polyethylene (HDPE).

Copolymerisation

Polypropylene is widely applied in copolymers, one example being copolymerisation with polyethylene, which means that the lower application temperature for PP is lowered.

As a thermoplastic elastomer, PP is copolymerised with EPM rubber (PP and polyethylene cross-linked by means of peroxide), and it is noted for having high ozone and weather stability in bright colours (carbon black not added), see also section 3.11.

Typical fields of application

PP is used for containers, packaging, coating, films, pipes, cable insulation, furniture, household appliances, medical utensils, toys, etc.

Disposal

Perfect combustion of PP without additives leads to formation of CO2 and water.

PP is fire-promoting and has a so-called drip effect, meaning that burning material may drip and thereby lead to fire propagation. Fire-impregnated PP may therefore release potentially hazardous substances or compounds at combustion, see section 2.5.

Compared with combustion of conventional fuel (Dansk elproduktion 1992), a reduced amount of CO2, corresponding to a reduction of 890 g CO2/kg of PP, is released at combustion, see Appendices C and D.

Production, application and disposal of polypropylene (PP) do not involve major environmental hazards.

Category 1

The substances that are added, formed or converted during the production, application or disposal phase must not occasion special cleaning measures and protection requirements, or seriously affect health or the environment.

The plastics have a low consumption of energy at the production of the raw material and during processing, and they have a high energy content at combustion.

5.1 Category 1

Polypropylene - PP

The energy consumption at extraction and production of PP is 80 MJ/kg, which is low.

Production and processing of PP cause no material hazards to health or the environment. Disposal of PP through combustion at complete oxidation only causes emissions of CO2 and water.

The calorific value of PP is 43.5 MJ/kg, indicating a high energy content.