Basic Technology of Plastics - Introduction to Polymeric Composites with Rice Hulls - page 22

surfaces. If dealing with polymers, the process of two surfaces sliding past each other
is complicated by the enormous of heat that can be generated and stored near the
surfaces due to the low thermal conductivity of the materials. The analysis of friction
between polymer surfaces is complicated further by environmental effects such as
relative humidity and the likelihood of a polymer surface to deform if stressed.
Temperature influences the coefficient of friction significantly. For example, in the
case of PE, the friction first decreases with temperature. At 100 °C, the friction
increases because the polymer surface becomes tacky. The friction coefficient starts
to drop as the melt temperature is approached. Another example of a similar pattern
would be seen in PA. Wear is also affected by temperature conditions. Wear rates
will increase rapidly as the surface temperature of the polymer increases, causing it
to become softer and tacky.
2.3.4 Environmental Effects
The environment in contact with a loaded or unloaded component of a composite plays
a significant part in its properties, durability and mode of failure. The environment
can be a natural one such as rain, hail, ultraviolet (UV) radiation, detergents, and
high-temperature environments. Any damage in a polymeric composite due to the
influence of the natural environmental is usually referred to as weathering . These
conditions can be simulated in laboratory tests, so some designers of composites carry
out these tests in advance and make the necessary adjustments to ensure a better end
product, depending on the importance of its end applications.
2.3.5 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 will cause a polymer to swell, serving as a plasticiser, consequently lowering its
performance in terms of electrical and mechanical behaviours. Increases in temperature
will also result in an increase of free volume between the molecules, allowing the
polymer to absorb more water.
2.3.6 Weathering Effects
Weathering effects are of special interest to this topic because a major product of PCRH
comprises the manufacture of extruded board for outdoor applications. Common
problems encountered with polymeric composites if exposed to the elements are