Composite Technology - Introduction to Polymeric Composites with Rice Hulls - page 39

3.3.6 Water Absorption
All polymers absorb water to some degree, but some are sufficiently hydrophilic
that they absorb large quantities of water to significantly affect their performance.
Water causes polymers to swell, lowering their performance, particularly mechanical
properties. Increases in temperature results in the increase of free volume between the
molecules, making the polymers absorb more water. Absorption of water also causes
decay and warping when subject to drying. The environment or the media in contact
with the loaded or unloaded composites plays a significant part on the properties of
a composite, lifespan and mode of failure. The most common environmental issue is
rain, hail, sleet or other types of spills and are most significant in outdoor applications
(though indoor applications also sometimes experience this hazard).
3.3.7 Friction and Wear
Friction can be defined as the resistance that two surfaces experience as they slide or
try to slide past each other. Friction can take place in a dry state (direct to direction
surface interaction) or in a 'wet' state, in which the surfaces are separated by a thin
film of lubricating fluid. Several tables are available that show the coefficient of
friction of different polymers that composite designers can use. However, if dealing
with polymers, two surfaces sliding past each other is complicated by the fact that
an enormous amount of frictional heat can be generated and stored near the surfaces
due to the low thermal conductivity of polymers. The friction between polymers
can be complicated further by environmental effects such as relative humidity and
the possibility of a polymer surface deforming under stress. In polymeric composite
lumber in outdoor applications, for example, surface deterioration take places due
to constant human activity requiring periodic maintenance. In PCRH, hardly any
maintenance is required, as discussed in later chapters.
3.3.8 Fatigue
Loading of any composite material that finally leads to failure after a certain number
of cycles is called fatigue. 'Dynamic fatigue' is of extreme importance because a cycle
or fluctuating load will cause a composite product to fail at much lower stresses than
under monotonic (continuous) loads. Fatigue in polymeric composites is dependent
largely on the environment, temperature, frequency of loading a surface, and stress
factors. For example, due to surface irregularities, scratches and cracks on the surface,
failure due to fatigue is more likely in extruded composite products than in injection-
moulded or compression-moulded products.