Page 44 - Plastics Engineering

General Properties of Plastics 27

As regards the general behaviour of polymers, it is widely recognised that
crystalline plastics offer better environmental resistance than amorphous plas-
tics. This is as a direct result of the different structural morphology of these
two classes of material (see Appendix A). Therefore engineering plastics which
are also crystalline e.g. Nylon 66 are at an immediate advantage because they
can offer an attractive combination of load-bearing capability and an inherent
chemical resistance. In this respect the anival of crystalline plastics such as
PEEK and polyphenylene sulfide (PPS) has set new standards in environmental
resistance, albeit at a price. At room temperature there is no known solvent for
PPS, and PEEK is only attacked by 98% sulphuric acid.
Weathering. This generally occurs as a result of the combined effect of
water absorption and exposure to ultra-violet radiation (u-v). Absorption of
water can have a plasticizing action on plastics which increases flexibility but
ultimately (on elimination of the water) results in embrittlement, while u-v
causes breakdown of the bonds in the polymer chain. The result is general
deterioration of physical properties. A loss of colour or clarity (or both) may
also occur. Absorption of water reduces dimensional stability of moulded arti-
cles. Most thermoplastics, in particular cellulose derivatives, are affected, and
also polyethylene, PVC, and nylons.
Oxidation. This is caused by contact with oxidising acids, exposure to u-v,
prolonged application of excessive heat, or exposure to weathering. It results
in a deterioration of mechanical properties (embrittlement and possibly stress
cracking), increase in power factor, and loss of clarity. It affects most thermo-
plastics to varying degrees, in particular polyolefins, PVC, nylons, and cellulose
derivatives.
Environmental Stress Cracking (ESC). In some plastics, brittle cracking
occurs when the material is in contact with certain substances whilst under
stress. The stress may be externally applied in which case one would be
prompted to take precautions. However, internal or residual stresses introduced
during processing are probably the more common cause of ESC. Most organic
liquids promote ESC in plastics but in some cases the problem can be caused
by a liquid which one would not regard as an aggressive chemical. The classic
example of ESC is the brittle cracking of polyethylene washing-up bowls due
to the residual stresses at the moulding gate (see injection moulding, Chapter 4)
coupled with contact with the aqueous solution of washing-up liquid. Although
direct attack on the chemical structure of the plastic is not involved in ESC
the problem can be alleviated by controlling structural factors. For example,
the resistance of polyethylene is very dependent on density, crystallinity, melt
flow index (MFI) and molecular weight. As well as polyethylene, other plastics
which are prone to ESC are ABS and polystyrene.
The mechanism of ESC is considered to be related to penetration of the
promoting substance at surface defects which modifies the surface energy and
promotes fracture.