Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
energy to break covalent bonds in organic compounds, but ultraviolet
wavelengths will selectively excite electrons in the polymer chain. This
increases the vibrational and rotational energy of the covalent bonds, leading
to degradation by bond cleavage. Fortunately, the intensity of light in the
290–320 nm wavelength region is estimated to account for no more than
0.5% of the radiant energy of the sunlight at noon in southern regions. If this
were not the case, few plastics would be of use outside.
Typical types of degradation include yellowing, chalking, surface
embrittlement, loss of tensile or impact strength, and cracking. The process
usually proceeds from the surface layers and the weakened surface can act as
a site for crack nucleation. Such cracks may then propagate into the undegraded
material beneath, causing failure.
No photochemical changes occur when the light is dissipated harmlessly
as heat, and pigments such as carbon black are commonly added to absorb
ultraviolet radiation and then re-emit it as thermal energy. Hydroxybenzophenones
and benzotriazoles are also widely used as ultraviolet
absorbers.
Aliphatic polymers such as PE are degraded by gamma radiation, although
aromatic polymers, such as PS, are relatively resistant to high-energy radiation.
Utensils made from PP may be embrittled by radiation damage if they are
subject to sterilization treatment by doses of gamma radiation of 2.5 Mrad.
Equipment should be designed to last 40 years when subjected to a dose of
20 Mrad.
5.6.3 Mechanical degradation
Mechanical degradation may occur when stress is imposed on a polymer
through machining, stretching or ultrasonics. Bond cleavage may occur,
forming macroradicals which can add oxygen and produce compounds which
will undergo degradative reactions.
5.6.4 Microbiological degradation
Most widely used plastic materials are inert in the presence of microbes and
this stability is important in many applications. Only short-term performance
is required in certain situations before the material is discarded, for example
in fast food packaging. It is considered an advantage if the discarded plastic
degrades when exposed to microbes and it is a challenge to polymer scientists
to develop plastics that possess the requisite properties for their anticipated
service life, but which are eventually capable of degrading in a safe manner.
In 1987, 6 × 10 9 kg of plastics were used for short-term packaging applications
and today this figure exceeds 60 × 10 9 kg.
We may define biodegradable plastics as those whose physical integrity is