Page 28 - Plastics Engineering

General Properties of Plastics 11

Rubber particles

Tkrmoplastic matrix
I polypmpylenc~
Fig. 1.1 npical structure of olefinic TF'R

The matrix is usually polypropylene and it is this which melts during processing
to permit shaping of the material. The rubber filler particles then contribute the
flexibility and resilience to the material. The other type of TPR is the polyamide
and the properties of all five types are summarised in Table 1.4.

1.3.6 Polymer Alloys
The development of new polymer alloys has caused a lot of excitement in recent
years but in fact the concept has been around for a long time. Indeed one of
the major commercial successes of today, ABS, is in fact an alloy of acryloni-
trile, butadiene and styrene. The principle of alloying plastics is similar to that
of alloying metals - to achieve in one material the advantages possessed by
several others. The recent increased interest and activity in the field of polymer
alloys has occurred as a result of several new factors. One is the development
of more sophisticated techniques for combining plastics which were previously
considered to be incompatible. Another is the keen competition for a share of
new market areas such as automobile bumpers, body panels etc. These appli-
cations call for combinations of properties not previously available in a single
plastic and it has been found that it is less expensive to combine existing
plastics than to develop a new monomer on which to base the new plastic.
In designing an alloy, polymer chemists choose candidate resins according
to the properties, cost, and/or processing characteristics required in the end
product. Next, compatibility of the constituents is studied, tested, and either
optimised or accommodated.
Certain polymers have come to be considered standard building blocks
of the polyblends. For example, impact strength may be improved by using
polycarbonate, ABS and polyurethanes. Heat resistance is improved by using
polyphenylene oxide, polysulphone, PVC, polyester (PET and PBT) and acrylic.
Barrier properties are improved by using plastics such as ethylene vinyl alchol
(EVA). Some modern plastic alloys and their main characteristics are given in
Table 1.2.