Polyethylene is a thermoplastic that contains chains of the monomer called ethylene. Polyethylene is a lightweight, durable thermoplastic with variable crystalline structure. It is one of the most widely produced plastics in the world. They are used in applications ranging for films, tubes, plastic parts, laminates, etc. in several markets (packaging, automotive, electrical, etc.). Polyethylene chemical formula is (C2H4)n.
Polyethylene belongs to polyolefin family of polymers and is classified by its density and branching. Different temperatures, pressures and catalysts during the polymerization reaction can have drastic effects on the branching of polyethylene molecules. Branching is the formation of side chains off the basic polymer backbone. These side chains are formend when the hydrogen-carbon bond is broken during the polymerization reaction. The most common types of polyethlene are:
- Branched versions
- Low-density polyethylene (LDPE)
- Linear low-density polyethylene (LLDPE)
- Linear versions
- High-density polyethylene (HDPE)
- Ultra-high-molecular-weight ployethylene (UHMWPE)
- Cross-linked polyethylene (PEX or XLPE)
Low-density polyethylene (LDPE)
Low-density polyethylene is a semi-rigid and translucent polymer. They have great flexibility, impact toughness and stress cracking resistance. LDPE will have a density ranging from 0.910-0.925 g/cm3. LDPE is polymerized under conditions of high temperature and high pressure. Because LDPE is processed under extreme conditions, the molecular structure is mostly amorphous. Because of the amorphous structure, the branches are very high in quantity and length.
High-density polyethylene (HDPE)
High density polyethylene is a cost-effective thermoplastic with linear structure and no or low degree of branching. they are the most rigid among the other common types of polyethylene. They have a density ranging from 0.935-0.960 g/cm3. They are produced when the polymerization process is done at low temperature and low pressure. Because of this limited breaching, HDPE is more crystalline, leading to the increased density. HDPE can be processed in three different ways: slurry particle reactor, gas phase and metallocene catalyst. HDPE is the plastic that is used in making fuel tanks because of its low permeabilityu and superb chemical resistance.