Polyethylene (PE) is a cheap and versatile plastic used in many everyday food contact applications, as well as industrially demanding situations. It is odourless and non-toxic. Examples range from packaging films, milk bottles and cutting boards, to corrugated pipe and chemical tanks. It is known for its toughness, puncture and tear strength, low density (it will float on water and this helps with sorting in recycling) and ease of processing. It is not durable to UV (although this can be countered with additives), but has very good resistance to chemicals and corrosion.

It is similar to polypropylene (PP) in many respects and both plastics are consumed in very large quantities. PE collectively accounts for around 25% of global plastic production. This helps to keep the price low, and increases the chances of recycling, because there are so many products, especially packaging bottles, made of these relatively simple polymers.

PE has some unique properties, that make it useful in niche applications. For example, its slippery surface (low coefficient of friction) is utilised on the underside of skis and snowboards, and ultra high molecular weight polyethylene (UHMWPE) has exceptional strength to weight that is used in safety critical applications ranging from bullet proof armour to high strength rope.

Homopolymer PE (>965 kg/m3) is not so common. Instead, PE tends to be copolymerised and this reduces its density to high (HDPE, 941-965 kg/m3), medium (MDPE, 926-947 kg/m3), low (LDPE, 910-925 kg/m3), linear low (LLDPE, 910-940 kg/m3) and ultra low (ULDPE, <910 kg/m3). While strength and stiffness increase with density, toughness and stress crack resistance are reduced. Environmental stress cracking is a kind of premature failure (cracking) as a result of specific temperature and stress conditions in the presence of certain chemicals. The main processes involved in the production of the PE are naphtha cracking for ethylene, high-pressure polymerisation for low-density polyethylene (LDPE), solution polymerisation for linear low-density polyethylene (LLDPE) and suspension polymerisation for high-density polyethylene (HDPE). Cross-linked PE (PEX, XPE or XLPE), used in place of polyvinyl chloride (PVC) pipe for example, has higher stress crack resistance and impact properties than regular thermoplastic PE. The cross-links are formed once the plastic is formed, such as with the addition of peroxide or silane. The cross-links are permanent and mean that the material cannot be melt processed or welded, which makes it harder to repair. Pipe is typically based on HDPE and co-extruded cable based on LDPE. It is used for things like domestic water heating systems, whereby the cross-linking yields a very flexible pipe that is highly resistant to cracking and bursting even after prolonged use with exposure to heat chemicals. The flexibility and toughness means long pipes can run continuously without joins, which allows for quicker and cheaper installation.