Polyethylene

General Information

Polyethylene is produced in many companies in Jurong Island, including Exxonmobil and Shell Chemicals. Polyethylene is the most popular plastic in the world, and it is heavily used in consumer products (over 60 million tons worldwide every year)

Polyethylene is a polymer consisting of long chains of the monomer ethylene. A molecule of polyethylene is a long chain of carbon atoms with two atoms attached to each carbon atom. Below are the molecular formulae of polyethylene in two forms.

Fig. 3 Fig. 4

Production of polyethylene

There are two common types of polyethylene: high density polyethylene (HDPE) and low density polyethylene (LDPE). Both types are commercially produced by polymerization of ethane (C2H4).

Production of LDPE by addition polymerization requires temperature ranging from 100-300oC and very high pressure of 1500-3000 atmospheres. Oxygen or an organic peroxide such as dibutyl peroxide, benzoyl peroxide or diethyl peroxide is needed as initiator. An initiator is a substance which is added in small quantities and is decomposed by light or heat to produce a free radical (R.). A free radical is formed when a covalent bond is broken and a bonding electron is left on each part of the broken molecule. Since the O-O covalent bond is weak, free radicals are easily formed from oxygen or peroxides. Benzene or chlorobenze is used as the solvent. Water or other liquids may be added to dissipate the heat of reaction as the polymerization reaction is highly exothermic.

HDPE can also be produced by addition polymerization. It requires temperature above 300oC and 1 atmosphere pressure (101.3kPa). Aluminium-based metal oxide catalyst is needed. Moreover, the ethene (ethylene) monomer is fed with a paraffin or cycloparaffin diluent (diluting agent). After polymerization the polymer (polythene) is recovered by cooling or by solvent evaporation.

Besides addition polymerization, HDPE can also be produced by coordination polymerization. Temperature 50-75oC and slight pressure are required. A coordination catalyst is prepared as a colloidal suspension by reacting an aluminium alkyl and titanium chloride (TiCl4) in a solvent such as heptane (C7H16). When the polymerization is completed, the catalyst is destroyed by adding water or alcohol to the reaction mixture.

Applications of polyethylene

Polyethylene is one of the world’s most widely utilized thermoplastics. The most important application is its use in piping system. Today we have many kinds of pipes, such as polyethylene pipes, HDPE pipes, polyethylene slotted pipes, polyethylene smooth pipes and pipes for pressure systems. They are widely used in industry. The applications include steel pipe coating, pipeline, polyethylene cables, sewage pipes, cable jacketing, pipelines corrosion protection and anti-corrosion coating tapes.

Now let us focus on a few uses and look at why they are suitable for these applications. Polyethylene pipes are made to transport oil. The advantage is that it is a flexible, tough and lightweight piping product which is suitable for oil transporting. Polyethylene pipes can accept repetitive pressure surges preventing pipe bursting, sliplining, and plow and plant. After the success of its use in oil field, it is used in transporting natural gas. This coilable, corrosion-free piping material could be fusion joined in the field to ensure a “leak-free” method of transporting natural gas to home and businesses.

Besides, high density polyethylene is used to distribute and store potable water. This material neither tuberculates nor supports bacterial growth. As such, HDPE pipes have excellent chemical resistance and are suitable for even harsh environments. It is also important to note polyethylene is dielectric (i.e. non-conducting), which means it is not subject to corrosion and maintains its flow capability over time. Another advantage is that HDPE pipe offers excellent fusion integrity, preventing leakage and enabling the development of one continuous pipeline system.