Polyethylene

Polyethylene (PE) is the most widely used plastic in the world. Its main use is in the packaging industry – for film, carrier bags, bags, sacks, containers, bottles and more. There are many different types of polyethylene, and most of them have the chemical formula (C₂H₄)_n.

Polyethylene at Elplast

At our own production site in Asenovgrad, Bulgaria, we process polyethylene on 10 extruders with a capacity of 200 tonnes per month. From low-density polyethylene (LDPE) we produce LDPE packaging – film, tubing, semi-tubing, sacks and bags from 25 to 200 µm thick. From linear low-density polyethylene (LLDPE) we produce stretch film, and from recycled polyethylene – packaging from recycled material. We also extrude polyethylene into extruded plastic nets.

Physical properties

Polyethylene is a thermoplastic polymer made of long hydrocarbon chains. Depending on molecular weight and crystallinity, it may show a distinct melting point, which varies considerably between types. For the most common grades of low-density polyethylene (LDPE) the melting point is between 105 and 110 °C; for high-density polyethylene (HDPE) it is between 120 and 130 °C.

Chemical properties

Most grades of low-, medium- and high-density polyethylene (LDPE, MDPE and HDPE) have excellent chemical resistance – they do not react with strong acids or bases and resist mild oxidising agents. Polyethylene burns slowly with a blue flame with a yellow tip and smells of paraffin. When removed from the heat source it keeps burning and forms drips of melt. At room temperature polyethylene does not dissolve; at elevated temperatures it can be dissolved in aromatic hydrocarbons such as toluene or xylene, or in chlorinated solvents such as trichloroethane and trichlorobenzene.

Types of polyethylene

Polyethylene is classified by density and chain branching. Its mechanical properties depend mainly on the branching of the molecular chain, the crystal structure and the molecular weight. The most widely used types are HDPE, LLDPE and LDPE.

• Ultra-high-molecular-weight polyethylene (UHMWPE) – molecular weight usually between 3 and 6 million, which makes it an extremely hard and tough material. Used for engineering machine parts, bearings, gears, protective edges, body armour and medical implants.
• High-density polyethylene (HDPE) – density of 0.941 g/cm? or more. It has little chain branching and high tensile strength. Used in packaging – carrier bags, sacks, bags, canisters, plastic pipes, toys and more.
• Medium-density polyethylene (MDPE) – density from 0.926 to 0.940 g/cm?. It has high impact strength and good overall mechanical properties and is more resistant to stress cracking than HDPE. Used mainly for gas pipes and fittings, heavy-duty bags and covers, bottle caps and shrink film.
• Linear low-density polyethylene (LLDPE) – a linear polymer with many short branches, made by copolymerising ethylene with short-chain alpha-olefins. Density from 0.915 to 0.925 g/cm?. It has higher tensile strength than LDPE and outperforms it in impact, tear and puncture resistance. Used for thin films, bags, cable ducts, buckets, pipes and more, and most widely for stretch film thanks to its excellent mechanical properties and high elongation.
• Low-density polyethylene (LDPE) – density from 0.910 to 0.940 g/cm?. It has many short and long branches, lower crystallinity, lower tensile strength and higher ductility than the other types. Used for films as well as rigid containers, and most often for polyethylene film packaging.