Plastics 193
For example, raw PVC polymer is a brittle, inflexible material with rather
limited commercial possibilities. Attempts to process PVC using heat and
pressure result in severe degradation of the polymer. Compounding PVC
involves incorporating sufficient additives with the raw polymer to produce a
homogeneous mixture suitable for processing and with the required final
properties for the lowest possible price. Table 2 shows that PVC can be used
to produce rigid guttering, plastic grass, soft toys, electric cable insulation,
photograph pockets and shoe soles simply by varying the amount of plasti-
ciser in the formulations between 16% and 50% by weight.
During processing, PVC is milled together with liquid plasticiser, so that the
latter physically attaches itself to the surfaces of the polymer particles, separat-
ing them from each other and allowing them to flow over each other, increasing
flexibility on a macro-scale. Of the one million tonnes of plasticisers used annu-
ally in Europe, approximately 90% comprise phthalate esters; the largest single
product used as a general purpose plasticiser since the 1950s is di(2-ethylhexyl)
phthalate (DEHP). The effects of DEHP on health, particularly that of children,
has been of concern since the 1980s, and toys and accessories containing DEHP
intended for children younger than three years are not allowed to be sold in
Europe.
Polymer additives or modifiers may be generally grouped into those which
alter physical properties, those which change chemical properties and those
which improve the appearance of the finished plastic product. Additives, which
alter physical properties either during processing or in the final plastic object
include fillers, plasticisers, lubricants and flow promoters, impact modifiers and
foaming agents. Additives, which change chemical properties during process-
ing and in the final plastic product include anti-ageing additives. Additives,
which improve appearance include colourants in the form of pigments and dyes.
Table 3 summarises the types and function of the additives that are usually
incorporated into polymers. There is some overlap between the properties
and actions of many polymer additives. For example, plasticisers may also
act as impact modifiers, lubricants and stabilizers. Conversely, a commercial
Table 2Typical applications for PVC formulations
plasticised with di(2-ethylhexyl) phthalate (DEHP)
DEHP (% by weight)* Application
16.7 Vinyl flooring
23.1 Upholstery cover
28.6 Document folder
33.3 Garden water hose
37.5 Electrical cable sheath
44.4 Shoe sole
50.0 Rubber (wellington) boots
*Based on PVC and DEHP.