Polyethylenes General : A family of closely related commodity thermoplastics that are manufactured in large volumes in a very wide range of grades. Traditionally differentiated by their density (a good measure of degree of crystallinity), from a scientist's point of view the better distinction is the degree of chain branching. The first polyethylene, later called LDPE, was and is made by a high pressure process using a free radical initiator/catalyst and is a polymer with a high degree of chain branching. Subsequently low pressure processes using Ziegler-Natta or related catalysts were developed which produce a much more linear molecule and generally higher crystallinities. These were and are called high density polyethylene (HDPE); this technology is now used to make a family of (chemically) closely related PE's all of which are linear - ULDPE (ultra low density ..), LLDPE (linear low ..), MDPE (medium ..), HMWPE (high molecular weight ..) and UHMWPE (ultra high molecular weight ..). On the other hand, PE copolymers (e.g. with vinyl acetate or acrylates) are normally branched polymers.
General Description : A semi-crystalline (typically around 50%), whitish, semi-opaque commodity thermoplastic that is soft, flexible and tough - even at low temperatures - with outstanding electrical properties but poor temperature resistance. It also has very good chemical resistance but is prone to environmental stress cracking; it has poor UV resistance (unless modified) and poor barrier properties, except to water.
Applications include containers, chemically resistant linings, films (which are transparent if thin) for packaging etc. and electrical/electronic applications including cable insulation and cores in UHF cables.