Ethylene is a rather stable molecule that polymerizes only upon contact with cata­lysts. The conversion is highly exothermic, that is the process releases a lot of heat. Coordination polymerization is the most pervasive technology, which means that metal chlorides or metal oxides are used. The most common catalysts consist of titanium(III) chloride, the so-called Ziegler-Natta catalysts. Ethylene can be pro­duced through radical polymerization, but this route has only limited utility and typically requires high-pressure apparatus.

15.1.2.2. CLASSIFICATION

PE is classified into several different categories based mostly on its density and branching. Its mechanical properties depend significantly on variables such as the extent and type of branching, the crystal structure and the molecular weight. With regard to sold volumes, the most important polyethylene grades are HDPE, LLDPE and LDPE.

• Ultra-high-molecular-weight polyethylene (UHMWPE)

• High-density polyethylene (HDPE)

• Linear low-density polyethylene (LLDPE)

• Low-density polyethylene (LDPE)

(a) Ultra-high-molecular-weight polyethylene (UHMWPE)

These include can and bottle handling machine parts, moving parts on weaving machines, bearings, gears, artificial joints, edge protection on ice rinks and butch­ers’ chopping boards. It competes with aramid in bulletproof vests, under the trade names Spectra and Dyneema, and is commonly used for the construction of articular portions of implants used for hip and knee replacements.

(b) High-density polyethylene (HDPE)

HDPE is used in products and packaging such as milk jugs, detergent bottles, butter tubs, garbage containers and water pipes. One third of all toys are manufac­tured from HDPE. In 2007 the global HDPE consumption reached a volume of more than 30 million tons.

(c) Linear low-density polyethylene (LLDPE)

Lower thickness may be used compared to LDPE. Cable covering, toys, lids, buckets, containers and pipe. While other applications are available, LLDPE is used predominantly in film applications due to its toughness, flexibility and rela­tive transparency. Product examples range from agricultural films, saran wrap, and bubble wrap, to multilayer and composite films.

(d) Low-density polyethylene (LDPE)

LDPE is created by free radical polymerization. The high degree of branching with long chains gives molten LDPE unique and desirable flow properties. LDPE is used for both rigid containers and plastic film applications such as plastic bags and film wrap.