POLYTHENE or POLYETHYLENE

Polythene or polyethylene is very useful and one of the most common polymers today. Polythene or polyethylene is prepared from pure ethylene (pure ethylene obtained from petroleum) by polymerization process either by high pressure method or by low pressure method. Polythene or polyethylene which is manufactured by high pressure method possesses a low density, hence this type of polythene or polyethylene is known as high pressure polythene or low density polythene. Polythene or polyethylene which is manufactured by low pressure method possesses a high density, hence this type of polythene or polyethylene is known as low pressure polythene or high density polythene.

High Pressure Method

When polythene or polyethylene is synthesized by high pressure method a free radical yielding catalyst such as oxygen or peroxide is required.

Ethylene with highly pure quality is polymerized at a pressure of 1000 – 1500 atm and at temperature of 250 ⁰C in the presence of a free radical yielding catalyst. From the reactor, the molten liquid passes to a separator. In the separator the unreacted ethylene is removed and recycled. So, now in the separator molten liquid is water white polythene or polyethylene. This can be extruded, chilled and solidified, chopped and stored. By adding fillers and colouring matter to the molten polymer, moulding powder can be prepared.

Since polythene or polyethylene prepared by high pressure method possesses a low density, so this type of polythene or polyethylene are highly branched, consists of several short and long branches, as shown in the following structure----

High pressure polythene or low-density polythene is extremely poor conductor of electricity, chemically inert, tough, flexible over a wide range of temperature. This type of polythene or polyethylene is not attacked by acids, alkalies or oil.

High pressure polythene or low density polythene is used for the fabrication of king toys, making pipes, making containers, wires and cable insulation, in films, in packings, articles of domestic use, making bottles.

Low Pressure Method

When polythene or polyethylene is synthesized by low pressure method a metal derived catalyst is required. Some examples of metal derived catalyst are titanium tetrachloride in a hexane solution of aluminium triethyl, chromium oxide on a silica alumina support etc.

Ethylene with highly pure quality and cyclohexane solvent along with a suitable metal derived catalyst are fed to a reactor. Here cyclohexane solvent serves for several purpose such as----

Cyclohexane solvent acts as a medium for dissipating the heat of reaction.

Cyclohexane solvent controls the rate of consumption of ethylene.

Cyclohexane solvent protects the growing polymer from chain breaks.

In the reactor under 35 atm pressure and at a temperature of 60 – 200 ⁰C ethylene is polymerized. From the reactor, the molten liquid passes to a flash drum. In the drum excess cyclohexane and unreacted ethylene are removed. The polymer produced due to polymerization of ethylene is then centrifuged and filtered. The polymer which is ash free thus obtained, is precipitated from the solution in a stripper to get a slurry. By floatation, the slurry is separated from water. It is then dried and finished.

Since polythene or polyethylene prepared by low pressure method possesses a high density, so this type of polythene or polyethylene molecules contains linear unbranched chains. These chains can be close packed in the solid state. Due to this close packing, this type of polythene or polyethylene has high density.

Low pressure polythene or high density polythene is more stronger, chemically more inert, more stiffer and harder than low density polythene.

High density polythene possesses high softening temperature and greater tensile strength.

Low pressure polythene or high density polythene is used for making toys, bottles, containers etc. This polythene is also used in the injection moulding of house wares.