Materials for engineering, 3rd Edition - (Malestrom)

Organic polymeric materials 179
Screw fitments
Self-tapping screws are the most commonly used form of mechanical fastener
for polymers. Thermoplastics can employ thread-forming screws, where elastic
relaxation processes ensure a tight fit. Thermosets are too brittle for this
technique and they tend to crack in use, so recourse has to be made to threadcutting
screws for these materials.
5.6 Polymer degradation
Plastics are frequently preferred to metals for use in structural applications
because of their resistance to corrosion. They are often regarded as more
corrosion resistant than metals, but this view is an oversimplification of the
situation and there are in fact a number of ways in which polymeric materials
may degrade over a period of time. These are as follows:
1. Oxidative degradation
2. Radiation degradation
3. Mechanical degradation
4. Microbiological degradation and
5. Chemical attack.
We will consider these in turn.
5.6.1 Oxidative degradation
Oxidative degradation is an autocatalytic process of attack on the hydrogen
atoms, to form hydroperoxides. The stability of polymers is thus inversely
proportional to the number of hydrogen atoms with the carbon atoms present
in the polymer chain. The degradation is catalyzed by heavy metals such as
copper.
The degradative reaction can be inhibited by the presence of hydrogendonating
compounds such as hindered phenols or by peroxide decomposers.
Mixtures of different types of stabilizers appear to be synergistic in their
effect. Natural rubbers and other elastomers can be protected against
attack by ozone by the addition of microcrystalline wax. More permanent
stabilization is obtained by the addition of derivatives of the phenol p-
phenylenediamene.
5.6.2 Radiation degradation
Engineering plastics are commonly used out of doors and it is now recognized
that sunlight with a wavelength less than 290 nm is responsible for the
photo-oxidation of polymer surfaces. Long wavelengths have insufficient