Question

How is polyvinyl chloride prepared? Give its uses and properties.

Answer 1

PREPARATION:

poly vinyl chloride is prepared by polymerisation of monomer vinyl chloride

– About 80 percent of polymerisation includes suspension polymerisation.

-12% emulsion polymerisation and 8% bulk polymerisation.

Process :  VCM and water are introduced into the reactor and a polymerization initiator, along with other additives. The reaction vessel is pressure tight to contain the VCM. The contents of the reaction vessel are continually mixed to maintain the suspension and ensure a uniform particle size of the PVC resin. The reaction is exothermic, and thus requires cooling. As the volume is reduced during the reaction (PVC is denser than VCM), water is continuously added to the mixture to maintain the suspension.

The polymerization of VCM is started by compounds called initiators that are mixed into the droplets. These compounds break down to start the radical chain reaction. Typical initators include dioctanoyl peroxide and diacetyl peroxydicarbonate, both of which have fragile O-O bonds. Some initiators start the reaction rapidly but decay quickly and other initiators have the opposite effect. A combination of two different initiators is often used to give a uniform rate of polymerization. After the polymer has grown by about 10x, the short polymer precipitates inside the droplet of VCM, and polymerization continues with the precipitated, solvent-swollen particles.

Once the reaction has run its course, the resulting PVC slurry is degassed and stripped to remove excess VCM, which is recycled. The polymer is then passed though a centrifuge to remove water. The slurry is further dried in a hot air bed, and the resulting powder sieved before storage or pelletization. Normally, the resulting PVC has a VCM content of less than 1 part per million.

USES:

1. It is used for sewerage pipes and other pipe applications where cost or vulnerability to corrosion limit the use of metal.                                                                                                                     2.With the addition of impact modifiers and stabilizers, it has become a popular material for window and door frames.                                                                                                                                 3.By adding plasticizers, it can become flexible enough to be used in cabling applications as a wire insulator.



[NOTE: Vinyl Chloride Monomer (VCM) is the key material from which PVC is made.VCM is a gas with a molecular weight of 62.5 and boiling point of -13.9°C, and hence has a high vapour pressure at ambient temperature]