Composite Technology - Introduction to Polymeric Composites with Rice Hulls - page 38

propylene-ethylene as the matrix will give higher impact strength than those made
with PP-PE blends. If used as matrices, elastomers are liable to have poor tear strength
and are thus prone to ripping. The tear strength of these elastomers can be improved
by the addition of certain types of particle fillers, for example, well-dispersed carbon-
black filler in the matrix. Larger particles will act as accumulators of microscopic
stress and will lower the strength of the polymer composite.
3.3.4 Permeability and Sorption
Polymers, in general, due to their low density, are prone to permeability by gases
and liquids. In practice, polymeric composites are liable to come into contact
with various materials, with some of it diffusing through surfaces. Absorption of
these diffusing materials at the interface of the polymeric surfaces is also known
as adsorption . A good example of adsorption is in outdoor applications such as
decking, in which environmental materials are deposited on the surface of composite
lumber. Concentration of the permeating substances inside the materials results in
the transport of that substance, which is called 'molecular diffusion'. The cause of
molecular diffusion is the thermal motion of molecules that permit foreign molecules
to move along the concentration gradient using intermolecular and intramolecular
spaces. Diffusion is only one part of permeation. First, the permeating substance
must penetrate the surface of the polymeric membrane. Similarly, the permeating
substance must be desorbed on the opposite side of the membrane. The quantity
and surface area of diffusion is dependent upon the quality and uniformity of the
composite material. In practice, this can also be minimised by application of a suitable
preventive coating on the surface.
3.3.5 Weathering
If exposed directly to the elements, polymeric composites tend to show cracks due to
stress at levels significantly less than those not exposed to the environment. The main
causes for these cracks are ultraviolet (UV) rays, moisture and heat. Such exposure
can result in early decay of the composite material. It is difficult to assess which of
these weathering elements (or which combinations) affect the decay of composites
the most. The location and climate of a region can play a significant part on the
weathering of polymeric composites. Losses of strength, loss of aesthetic values, and
surface discolouration can be attributed to the action of UV rays, whereas moisture
is responsible for decay and heat for warping. Composite designers can make use of
standard weathering tests and make suitable adjustments to achieve maximum results
by the selection and incorporation of suitable additives and fillers.