Polyethylene is a thermoplastic
, meaning it becomes pliable when heated and rigid when cooled. This particular polymer is synthesized from long chains of ethylene monomers that are composed of carbon and hydrogen and form double covalent bonds in a number of structural configurations. Because it is available in many forms, polyethylene is the most commonly used plastic
. Annual production rates for this plastic alone are approximately eighty million metric tons. Such a large number is necessary to accommodate the incredible diversity of objects made from polyethylene. It is found in bottles, packaging sheets, shower curtains, pipes, wire insulation, plastic bags and containers, bulletproof vests, artificial knees and hip replacement components, flooring, storage and more. Resistant to water, acids, alkalis, solvents and chemical corrosion, polyethylene is popular in automotive, packaging, transport, medical, pharmaceutical, food and chemical processing, entertainment and electronics industries, to name a few. While this plastic is non-biodegradable, it is recyclable; it can be melted, remolded and rehardened numerous times. Additionally, researchers have been developing new techniques aimed at synthesizing polyethylene from components found in sugarcane in order to create bioplastics.
While bioplastics continue to be studied, most polyethylene is synthesized from ethylene, though several different methods for this are possible. Radical polymerization, anionic addition polymerization, ion coordination polymerization and cationic polymerization are all used in the production of PE through heat and pressurization. As each process results in a slightly different type of polyethylene, the plastics are classified based on density and branching or crystallization. The three main categories are high-density polyethylene, which has a melting point in the range of 120 to 130°C, low-density polyethylene, which melts at around 115°Celsius and finally medium-density polyethylene. In total there are ten different divisions including ultra high density and very low density, but the main three will often suffice. These polymer resins are formed into plastic rods
through injection molding, casting and extrusion. While stock forms such as these may sometimes be used as the finished product, most will undergo additional operations. Secondary processes employed on plastic materials such as these include drilling, welding, fastening, turning, milling and precision machining. It is important to consider both the manufacturing processes and finish product when selecting a polymer.