Soft and malleable or hard and tough, polyethylene is an incredibly flexible and very popular polyolefin – in fact, it’s the most commonly produced plastic in the world. Although low-density polyethylene was produced and patented by Imperial Chemical Industries in the late 1930s, it was Karl Ziegler and Giulio Natta who developed the revolutionary catalyst process that made the commercial production of polyethylene viable in the 1950s. Ziegler and Natta were jointly awarded the Nobel Prize in chemistry in 1963 for their contributions to the field of polymers, and the Ziegler-Natta catalyst is named after them. Since the early days, polyethylene has become invaluable in all sectors of the plastics market. Scientists continue to develop polyethylene with new properties and structural characteristics.
Types and Properties
There are four main types of polyethylene on the market today, and are classified according to their density and branching. All are non-toxic and recyclable. High-density Linear polyethylene (HDPE) has a simple molecular structure: a saturated chain of hydrocarbons. It is strong, dense, stiff and crystalline. Low-density polyethylene (LDPE) is slightly more complex: there are chains of polyethylene that branch off the carbon atoms instead of a single hydrogen atom. It is flexible, ductile and tends to stretch under strain. First developed in the late 1960’s, linear low-density polyethylene (LLDPE) is customizable and easier to produce. Ultra-high molecular weight polyethylene is dense and possesses a tensile strength that exceeds that of steel.
To make HDPE, ethylene gas is subjected to low pressures and temperatures together with Ziegler-Natta or Phillips catalysts. LDPE is produced with high temperatures and pressures together with oxide initiators to produce a much more flexible material. LLPE involves the copolymerization of ethylene with 1-hexene, 1-butane, 1-octane and catalysts. This process is customizable and conserves energy. When linear polyethylene is produced in extremely high molecular weight units of up to 6,000,000, it is classified as UHMWP.
Polyethylene’s first main use was for electrical insulation. Soon enough, its versatility and application areas expanded rapidly. Today, you’ll find polyethylene in cling foil, packaging, bottles and all sorts of plastic bags. It forms or reinforces pipes, tubes and fuel tanks. Harder types of polyethylene are used for containers and lids, covers, bearings, filters, gaskets, gear wheels, reinforcements and bullet-proof vests. Polyethylene is often used to injection mold in its melted state, producing an extruded, cooled and ready-formed product for end-use. Blow molding and rotation molding also benefit from polyethylene’s flexibility and reliability.