Common Polymers and Additives used for Processing - Introduction to Polymeric Composites with Rice Hulls - page 55

• Migration : Most pigments or dyes can migrate to the surface. Also called blooming
or bleeding .
• Abrasion : Many inorganic pigments are very abrasive, thereby causing long-term
damage to processing equipment.
• Chalking : If too much pigment is used, the surface of a part degrades during
weathering.
4.7.12 Fillers
Fillers play an important part in the processing of plastics. They can be classified
as those that: reinforce a polymer matrix and improve its mechanical performance;
take up space and thus reduce the amount of polymer to make a product (sometimes
called an 'extender'); are used to lower costs as well as to increase the density of the
product. Polymers that contain fillers that improve their mechanical properties are
referred to as 'reinforced plastics' or 'composites'. Composites can be dived broadly
into composites with high-performance reinforcements and composites with low-
performance reinforcements. High-performance composites are those in which the
reinforcement is placed inside the polymer so that optimal mechanical behaviour is
achieved, such as in the case of glass fibres in an epoxy resin (fibreglass products).
Low-performance composites are those in which the reinforcement is sufficiently
small that it can be dispersed well into the matrix. These materials can be processed
in the same way as their unreinforced counterparts.
Probably the most common filler used to reinforce polymeric materials is glass fibre.
However, modern technology is using wood fibre and other biomass fibres with exciting
prospects for rice hulls, which are now being used to produce polymeric composite
resins and extruded profiles with improved properties. These biomasses, which are
being used as reinforcements and extenders, also increase the stiffness and mechanical
properties of some thermoplastics. To improve the bonding between the polymer
matrix and reinforcement, coupling agents such as silanes and titanates can be added.
Extenders used to reduce the cost of composite products are most often particulate
fillers. The most common are calcium carbonate, silica flour, clay, and wood flour.
The additives discussed so far are the general ones required for converting raw resins
into polymers for processing. For PCRH, as with wood flour polymer composites,
additional special additives must be incorporated into the polymer matrix and the rice
hulls mix to get whichever finishes are desired for the end application (e.g., outdoor,
indoor, furniture applications). These additives will be discussed in later chapters
with regard to the processing of PCRH.