Common Polymers and Additives used for Processing - Introduction to Polymeric Composites with Rice Hulls - page 45

being the phthalates. It is insoluble in alcohol but slightly soluble in tetrahydrofuran.
PVC is used widely in construction work because it is more effective and usable than
traditional materials such as copper, iron or wood in pipes or profiled applications.
PVC is also used in applications such as electrical cable insulation, upholstery, coating
textiles, inflatable products, window frames, and conduits.
4.3.1 Production
PVC is produced by polymerisation (joining of basic units) of the monomer: vinyl
chloride. About 80% of production involves suspension polymerisation. Emulsion
polymerisation accounts for ≈12% and bulk polymerisation accounts for the
remaining 8%. The monomer is introduced into a pressure-tight reactor with a
polymerisation initiator, along with other additives. The contents of the reaction vessel
are mixed continuously to maintain a suspension to ensure a uniform particle size
of the resulting PVC resin. The reaction is exothermic so needs cooling and, because
the volume is reduced during the reaction, water is added continuously to maintain
the suspension. Polymerisation is started by compounds called 'initiators', which
are mixed into the droplets. These compounds break down to start the radical chain
reaction. Some initiators start the reaction rapidly but slow down, whereas others
have the opposite effect. A combination of both is often used to achieve a uniform
rate of polymerisation. After the polymer has grown by about tenfold, the short
polymer precipitates inside the droplets of the monomer, and polymerisation continues
with the precipitated solvent-swollen particles. Once the reaction has finished, the
resulting PVC slurry is degassed and stripped to remove excess monomers, which
can be recycled. The slurry is dried further in a hot air bed and the resulting powder
is sieved before storage or pelletisation.
4.3.2 Additives for Polymers
The product resulting from polymerisation of vinyl chloride monomers is unmodified-
PVC. Before this PVC resin can be processed into finished products, it is necessary
to convert it into a usable compound by the incorporation of additives such as heat
stabilisers, UV stabilisers, lubricants, plasticisers, impact modifiers, thermal modifiers,
fillers, flame retardants, biocides, blowing agents, and pigments. All types of monomers
after polymerisation, with the resins in the raw state, will need additives to make them
into workable polymers. All these additives are not needed as a whole but only some
are needed; this factor is dependent upon the final grade to be produced to suit end
applications. Some of the important additives and properties are presented below.