Page 43 - Plastics Engineering

26 General Properties of Plastics

to be aware of this failure mode because it is a common error, amongst those
accustomed to dealing with metals, to assume that if the material is capable of
withstanding the applied (static) load in the short term then there need be no
further worries about it. This is not the case with plastics where it is necessary
to use long-term design data, particularly because some plastics which are tough
at short times tend to become embrittled at long times.
Fatigue. Plastics are susceptible to brittle crack growth fractures as a result
of cyclic stresses, in much the same way as metals are. In addition, because
of their high damping and low thermal conductivity, plastics are also prone to
thermal softening if the cyclic stress or cyclic rate is high. The plastics with
the best fatigue resistance are polypropylene, ethylene-propylene copolymer
and PVDF. The fatigue failure of plastics is described in detail in Chapter 2.
Toughness. By toughness we mean the resistance to fracture. Some plastics
are inherently very tough whereas others are inherently brittle. However, the
picture is not that simple because those which are nominally tough may become
embrittled due to processing conditions, chemical attack, prolonged exposure
to constant stress, etc. Where toughness is required in a particular application it
is very important therefore to check carefully the service conditions in relation
to the above type of factors. At mom temperature the toughest unreinforced
plastics include nylon 66, LDPE, LLDPE, EVA and polyurethane structural
foam. At sub-zero temperatures it is necessary to consider plastics such as ABS,
polycarbonate and EVA. The whole subject of toughness will be considered
more fully in Chapter 2.

1.4.2 Degradation
Physical or Chemical Attack. Although one of the major features which might
prompt a designer to consider using plastics is corrosion resistance, nevertheless
plastics are susceptible to chemical attack and degradation. As with metals, it is
often difficult to predict the performance of a plastic in an unusual environment
so it is essential to check material specifications and where possible carry out
proving trials. Clearly, in the space available here it is not possible to give
precise details on the suitability of every plastic in every possible environment.
Therefore the following sections give an indication of the general causes of
polymer degradation to alert the designer to a possible problem.
The degradation of a plastic occurs due to a breakdown of its chemical
structure. It should be recognised that this breakdown is not necessarily caused
by concentrated acids or solvents. It can occur due to apparently innocuous
mediums such as water (hydrolysis), or oxygen (oxidation). Degradation of
plastics is also caused by heat, stress and radiation. During moulding the mat-
erial is subjected to the first two of these and so it is necessary to incorporate
stabilisers and antioxidants into the plastic to maintain the properties of the
material. These additives also help to delay subsequent degradation for an
acceptably long time.