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Polypropylene (PP plastic) is a semi-crystalline thermoplastic polymer obtained by the polymerisation of propylene. It is one of the polyolefin group of plastics (obtained by the polymerisation of olefins). It is one of the most widely used plastics. It can be used as a plastic or as a fibre, which is why it has several applications: textiles, medical devices or automotive components. It is a polymer very similar to polyethylene, although there are some differences between polypropylene and polyethylene due to their chemical compositions.
Characteristics of PP plastic
- Propylene has good mechanical and chemical resistance.
- It is a water resistant plastic.
- It has a low density (0.895-0.92g/cm³), which makes it light, and its melting point is from 160ºC, which makes it ideal for working with it at high temperatures.
- Great versatility, making it compatible with most processing techniques (giving it a wide range of uses).
- It is a partially crystalline material.
- Being rigid, it is a material that is difficult to deform even on impact. At low temperatures, it is a material that becomes more brittle.
Types of polypropylene
There are mainly two types of polypropylene, which are also divided into other grades:
- PP Homopolymer: this is created by polymerising pure propylene. There are three types:
- Isotactic PP: has high crystallinity, mechanical strength and toughness.
- Atactic PP: polymerises leaving the side methyls especially in disarray. It is a sticky material so it sticks well to surfaces.
- Syndiotactic PP: has low crystallinity, is more elastic than isotactic but less tough.
- Copolymer PP: achieved when 5-30% ethylene is added in the polymerisation process. It has more impact resistance than homopolymer. There are 2 types:
- Random copolymer: propylene and ethylene are put into the same reactor at the same time, thus creating polymer chains in which the two take turns randomly.
- Block copolymer: first the propylene is polymerised in one reactor and then the ethylene is added in another once the PP has been formed. Its resistance is very high, which is why it is also called impact/shock PP.
PP plastic recycling
Polypropylene is recyclable and its identification number is 5. PP plastic waste can come in the form of industrial scrap or in the form of post-consumer waste. It is 99% carbon and hydrogen so it is not chemically polluting. As it has energy almost as much as fossil fuels, many PP wastes are used as an alternative fuel to produce electricity and heat. The most commonly used methods for recycling PP are extrusion, injection moulding and mechanical recycling.