The vast majority of polymers are substantially weaker
than mild steel. Tensile strengths up to 20 - 50 MPa are common in
some commodity plastics, up to around 50 -
80 MPa for engineering plastics and in some
high performance plastics up to 150+ MPa.
Due to effects such as stress relaxation, creep and
stress induced crazing and cracking B&B recommend that structural
plastic parts have working Factors of Safety on tensile strengths of
between 6:1 and 10:1 (ratio of actual working stress to material tensile
Fibre and particulate fillers can affect tensile
strengths positively so check details for particular grades offered by
In the references -
B&B, table 6-4 provide approximate tensile strengths for
Strong, table 7.1 shows strength data for some
Engineering & High
CC&F, table 6.5 shows short term tensile strength for
To show some effects of fibre reinforcement CC&F, table
6.7 shows tensile strengths for some reinforced
Nylons, Polycarbonates and
Polyesters. Note also the effect on
elongation (ductility), whereas strength generally improves with
reinforcement ductility reduces.
CC&F, table 3.4 shows some properties of a number of
polymers and their blends.
The ability of the polymer to maintain its properties
after exposure to the environment may be significant. Particular aspects
of this that are of interest are -
- Behaviour on exposure to UV, for exterior use.
- Moisture absorption, effect on properties and
- Chemical exposure, special resistance needs.
Exposure to UV Light
Prolonged exposure to UV light can cause deterioration of
properties including discolouration and embtrittlement. Polymers
reported to be susceptible are -
PE Polyethylene - degrading
PP Polypropylene - ditto
PVC Polyvinylchloride -
however UV resistant grades are available
PS Polystyrene - yellows
PI Polyimides - stability
PPS Polyphenylene Sulphide
It is reported that most polymers do not deal well with
prolonged UV exposure. There are some that do better -
Strong, table 7.1 indicates UV resistance od some
Water absorption can be marked in some polymers. It can
result in dimensional changes and loss of mechanical properties. The
dimensional instability can be detrimental to mechanism components such
as gears, bearings etc.
Polymers particularly susceptible are -
B&B, table 6-6 gives moisture absorption data
B&B, table 6-12 shows more data plus general
Plastic exposed to chemicals, for example in fuel tanks,
medical equipment, processing equipment etc will be additionally
B&B, table 6-12 indicates the effects of chemicals.
Some polymers are resistant to many chemicals, they are -
Vinyl Polymers eg CPVC
+ some other high performance polymers