Polystyrene (PS) is a thermoplastic polymer made by the polymerisation of styrene, a key chemical used in the manufacture of various plastics. It is one of the most widely used plastics in industry. It was first produced industrially in 1930 in Germany.
It is a versatile plastic used to manufacture a wide variety of consumer products. Because it is a hard, solid plastic, it is often used in products that require transparency, such as food packaging and laboratory equipment. When this material is combined with various dyes, additives and other plastics, polystyrene is used to make household appliances, electronics, automotive parts, toys and more.
Types of polystyrene
There are four main types of polystyrene:
- Crystal PS or GPPS: is the product of the polymerisation of pure styrene. It is hard but brittle and is transparent. At 100°C and above, it is easily moulded and can be formed into many shapes.
- High-impact PS or HIPS: rubber has been added to make it more resistant. It can withstand more violent impacts without breaking. It is an opaque material.
- Expanded PS or EPS: it is 5% polystyrene and 95% of a gas that reduces the density of the material through bubbles. It is lightweight and resistant to moisture. It is widely used to make food packaging, and is also used as an insulating material in the building industry.
- Extruded PS: Similar to expanded PS. It is a rigid foam obtained by gas injection when glass polystyrene is melted. It is denser and more impermeable than EPS. It is used as a thermal insulator or for the protection of fragile products.
Polystyrene production processes
The most commonly used process for polystyrene is mass radical polymerisation. The process for crystal polystyrene would be as follows:
- Conditioning of raw materials: as it does not rely on catalysts, no purification of the raw material is carried out as it can accept impurities.
- Reaction: Styrene polymerises suddenly and faster the higher the temperature rises.
- De-volatilisation: unconverted styrene and ethylbenzene are separated from polystyrene.
- Purification of the recycle: the elements separated in the previous step have many impurities. In some plants they are purified and in others a part is purified.
- Granulation: the molten polystyrene comes out of a row of holes forming threads that are cooled in a water bath, dried and cut into small cylinders (pellets).
- Shipping: PS is shipped to be sold in bulk or packed on pallets.
Depending on the material to be manufactured, the manufacturing method can be injection moulding, blow moulding, extrusion or thermoforming.
PS plastic recycling
Many polystyrene products are not recycled due to lack of investment in compactors and logistics systems. Polystyrene foam has a low density and is therefore not an economical collection, but if compacted it would become a high value material for recycled pallet producers. Recycled polystyrene can be used to manufacture new EPS parts (packaging, sheets...), for soil improvement (once the EPS is crushed it is mixed with soil), it is incorporated in other building materials (light bricks, mortars...), EPS pellets are produced (pens, coat hangers...) and used as filling material (stuffed animals, cushions...).
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