Plastics 191
The process used to produce more than 80% of commercial PVC polymers is
suspension polymerisation. An aqueous suspension of vinyl chloride monomer
is agitated vigorously in a pressurised vessel together with colloids (detergents)
to hold monomers in suspension, and buffers to control pH. The resulting PVC
particles are roughly spherical and range from 50–250m in diameter.
Commercial PVC is essentially an amorphous material, although a small
amount of crystallinity is present (about 5% as measured by X-ray diffraction
methods) and is attributed to the fact that that the bulky chlorine atoms do not
align and pack readily. Despite this low percentage, crystallinity greatly influ-
ences the properties of PVC in solution and solid phases.
PVC is thermoplastic, so it softens on warming. The presence of the chlor-
ine atoms in the structure increases the attraction between chains due to CCl
dipolar interactions, imparting great hardness and stiffness in the polymer.
The glass transition temperature of commercial grade PVC is approximately
80–84°C and the melting point around 212°C. Maximum service temperature
for commercial compounds is between 65°C and 80°C. The polarity intro-
duced by the presence of chlorine means that PVC is soluble in polar solvents;
examples are tetrahydrofuran and cyclohexanone. The high concentration of
chlorine in the polymer (56.8% by weight) imparts flame-retardant proper-
ties, a property which has been utilised in electrical insulation cables and
housings.
During polymerisation by the condensation technique, a reactive chemical
group on one monomer reacts randomly with another group on a second
monomer molecule, with the formation of a small molecule as the monomers
join together to form a chain. The small molecule is usually water or alcohol that
splits off during polymerisation. Since the reaction is only dependent on
monomermolecules coming in contact with others, a significant increase in
average molecular weight will occur with reaction time. Polymers formed by
condensationpolymerisation include polyesters, nylons and formaldehyde
polymers.
Rearrangement polymerisation may be considered to follow a reaction type
intermediate between those of addition and condensation. In common with
addition polymerisation, there is no splitting off of small molecules, but the
kinetics are otherwise similar to condensation processes. The preparation of
polyurethane polymers occurs by rearrangement polymerisation. Polyurethane
polymers are products of a polyol, based either on a polyester or polyether, with
several alcohol groups (OH), a di-or poly-isocyanate with several cyanate
groups (NCO) and a chain extender. The chain extender reacts with
the polyol’s alcohol groups, initiating an imbalance in negative and positive
charges throughout the molecule, which after reaction with the isocyanate,
results in the formation of a urethane group (NHCOO) as shown in
Reaction Scheme 3.