Page 170 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

The City & Guilds Textbook: Plumbing Book 1

Degradation of plastics

The use of plastics is becoming common in the plumbing industry. Everything
from hot and cold water services to central heating and drainage can now be
installed in some form of plastic material. Problems, however, can occur with
plastics under certain conditions. Degradation of plastics can occur from a
variety of causes such as:
l heat
l light
l oxygen
l ultraviolet (UV) degradation.

Heat (thermal degradation)
One of the limiting factors when using plastics in high temperature applications
is their tendency to not only soften but also to thermally degrade. In some
instances, thermal degradation can occur at temperatures much lower than
those at which mechanical failure is likely to occur.
All plastics experience some form of degradation during their life. The chemical
reactions that occur with thermal degradation lead to both physical and optical
changes, such as:
l reduced ductility and embrittlement
l chalking
l colour changes
l cracking.
KEY POINT Light (photodegradation)
Photodegradation takes This occurs due to the action of light, whether from natural sunlight or electrical
place in direct light, fluorescent lighting, and generally causes a yellowing of the plastic material. It is
even electric light,
whether heat is present usually more pronounced on light-coloured plastics but can affect all colours.
or not. UV degradation
takes place in daylight, Oxygen (oxidative degradation)
whether the Sun is This is decomposition of the plastic due to the presence of oxygen, which
present or not. Its effects alters the plastic’s properties. Colour change is often the first sign of oxidative
occur even on cloudy degradation, coupled with a change in flow, mechanical and electrical properties
days and as such it is of the plastic, even if the colour change is not noticeable. Polypropylene,
generally down to the
climate. polyethylene and ABS are the plastics most severely affected. PVC, however, is
unaffected by oxidative degradation.

UV degradation
Most plastics are vulnerable to degradation by the effects of direct exposure to
the UV part of the daylight spectrum. UV solar radiation is present even on cloudy
days. When UV attack occurs, the colour of the plastic may change and its surface
will become brittle and chalky. This can happen over a very short time period and
will lead to cracking and eventual failure.
Polypropylene waste pipes and MDPE water pipes are adversely affected by UV
degradation, with ABS pipework and fittings being severely compromised by
prolonged exposure to the UV daylight spectrum.

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