Polyethylene (PE) is a thermoplastic polymer composed of repeating ethylene (C₂H₄) units and is one of the most widely produced plastics in the world. Its molecular structure consists of long hydrocarbon chains with varying degrees of branching, which strongly influence density, crystallinity, and mechanical properties. Low-density polyethylene (LDPE) contains significant chain branching and exhibits lower crystallinity, resulting in greater flexibility and toughness. High-density polyethylene (HDPE), by contrast, has a more linear chain structure, higher crystallinity, and correspondingly greater stiffness, strength, and chemical resistance. These variations arise from differences in polymerization methods and catalyst systems, which control chain architecture during manufacture.
The thermal and mechanical behavior of polyethylene is governed by its semi-crystalline structure. Amorphous regions provide ductility and impact resistance, while crystalline regions contribute to strength and dimensional stability. Polyethylene generally exhibits a relatively low glass transition temperature and a melting temperature that depends on its degree of crystallinity and molecular weight. It also offers excellent resistance to moisture and many chemicals due to its nonpolar hydrocarbon backbone, though it is susceptible to ultraviolet degradation without stabilizing additives. These characteristics make PE suitable for applications requiring toughness, corrosion resistance, and electrical insulation.
Polyethylene is processed using common thermoplastic manufacturing methods such as extrusion, blow molding, and injection molding, enabling its use in films, containers, piping, and molded components. A related material, crosslinked polyethylene (PEX), is produced by introducing covalent crosslinks between chains, improving high-temperature performance, creep resistance, and long-term dimensional stability. While PEX is derived from polyethylene, the presence of crosslinking alters its behavior from that of a conventional thermoplastic, and it is therefore often treated as a distinct material category in engineering applications, particularly in fluid handling and piping systems.