TABLE OF CONTENTS Albeit having a simple chemical formula, (CH 2 CH 2) n , polyethylene is a broad family with versatile properties that depend on which of the three main polymerization processes is used:
Free radical vinyl polymerization, the oldest process, leads to branched low density polyethylene (LDPE). Macromolecules have numerous short branches, which reduce the melting point, tensile strength and crystallinity. Polymers are relatively flexible because of the high volume of the branched molecule and the low crystallinity.
Ziegler-Natta polymerization leads to linear unbranched polyethylene, the so-called high density polyethylene (HDPE), which is denser, tougher and more crystalline. By copolymerization with other alkenes it is possible to obtain linear low density polyethylene (LLDPE) with better mechanical properties than LDPE. Blends of LLDPE and LDPE are used to combine the good final mechanical properties of LLDPE and the strength of LDPE in the molten state.
Metallocene catalysis polymerization is the most recent technique, growing fast to produce a consistent, uniform distribution of molecular weight resulting in enhanced toughness, impact and puncture strengths, better cold behaviour and optical properties. These advantages allow the downgauging or enhancement of performances for the same weight of polymer. Metallocene catalysis allows the production of all densities, from ultra-low density to ultra-high molecular weight polyethylenes (UHMWPE).
In addition to their structural diversity, polyethylenes can be crosslinked.
Polyethylenes can be classified versus density and molecular weight:
ultra-low and very low density, ULDPE and VLDPE
low density, LDPE and LLDPE
medium density, MDPE
