Polystyrene (PS)
0.7-1.6 usd/kgPolystyrene (PS) is a cheap plastic used for moulded packaging and disposables, as well as a rigid foam, expanded PS (EPS). Both have their own unique set of qualities but may still be useful for the same applications, but for different reasons. For example, food containers – one thin and transparent, and the other an insulating and protective foam. While the monomer styrene is extremely hazardous and a known carcinogen, it is considered non-hazardous once polymerised. However, there may be instances where the monomer is not fully polymerised and can potentially leach out, such as into lipophilic food.
As a moulded plastic it has excellent transparency and resistance to water. It is predominately used in packaging and medical containers, as well as brightly coloured tableware and disposables. PS is brittle and will breakdown relatively quickly when exposed to sunlight, becoming yellow. Where transparency and durability are required, there are many alternative options available depending on the requirements, such as polycarbonate (PC), polymethyl methacrylate (PMMA), acrylic and polyethylene terephthalate (PET), polyester. Plastic free options include soda lime glass and borosilicate glass.
Expanded polystyrene (EPS) is a lightweight and rigid foam that provides excellent impact absorption and insulating properties. These qualities are utilised for packaging, safety and as a construction material. It provides an important role as insulation, saving energy throughout its life. However, it is quite voluminous and this presents some challenges with transportation and recycling. Only a very small percentage is PS (typically 2-5%), which means it may be considered too expensive or impractical to collect and recycle.
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Expanded polystyrene (EPS) is an ultralight rigid foam used for moulded packaging, insulation and impact protection. Packaging applications range from food containers (cooler boxes for medicine and frozen foods) to impact protection for domestic appliances. It is lightweight, cheap and insulating, which make it popular for single use food packaging. The problem with this is it is often not recycled. This means it typically ends up in refuse where it takes up a lot of space and breaks down into microplastics. Paperfoam is a compostable and biodegradable starch-based plastic packaging material that provides a more sustainable option for many applications.
As an insulation material it is used for acoustics, thermal management and energy saving in buildings (walls, floors and ceilings), domestic appliances (fridge and freezer), and automotive applications. It is used alone, or as part of a sandwich structure such as with aluminium skins. Over its lifetime, it will likely save many times more energy and CO2 than was used in its production.
It is moulded into safety items, such as crash helmets and child car seats. Tensile strength and compressive strength increase inline with density. The surface is prone to denting and scratching, and so is usually protected with a plastic skin, such as moulded polypropylene (PP) or acrylonitrile butadiene styrene (ABS). It is also used for things like surfboard blanks, boat hulls, sculpture and model making. It is very easy to cut and shape. Once skinned with composite, such as glass fibre reinforced epoxy or polyester resin (UPR), it produces a very strong and lightweight structure.
As with polyurethane resin (PUR) foam and other plastic foams, EPS requires a blowing agent. EPS is produced from small granules of PS mixed with blowing agent. Manufacturers used to use chlorofluorocarbons (CFC) as the blowing agent, but this has since been banned and replaced with hydrofluorocarbons (HCFC). This is better, but still has significant negative effects on the ozone. Safer alternatives include pentane and CO2.
The beads are pre-expanded before moulding. When mature, they are placed into the mould cavity and fully expanded with steam and pressure. The process is carried out at pressures much lower than normal injection moulding, so the mould tools are relatively less expensive. The granules expand to fill the cavity and formed the consolidated part.
Examples include Styropor, Styrodur and Neopor (BASF), and Styrofoam (Dupont).