Polyethylene or polythene is the most common plastic in use today. It is a polymer, primarily used for packaging. As of 2017, over 100 million tonnes of polyethylene resins are being produced annually, accounting for 34% of the total plastics market.
Many kinds of polyethylene are known, with most having the chemical formula ₙ. PE is usually a mixture of similar polymers of ethylene, with various values of n. It can be low-density or high-density: low-density polyethylene is extruded using high pressure and high temperature, while high-density polyethylene is extruded using low pressure and low temperature. Polyethylene is usually thermoplastic, but it can be modified to become thermosetting instead, for example, in cross-linked polyethylene.