Colouring of Polymers and Composites - Introduction to Polymeric Composites with Rice Hulls - page 105

the holding capacity of the polymer matrix for these particle, but the tensile strength
of the composite decreases.
Although wood has been the preferred cellulose material for composites, not only
softwoods like pinewood and cedar wood but also hardwoods such as oak and maple
can be used. Examples of agricultural materials that are similarly useful are: rice hulls
as well as fibres of sisal; flax; hemp; jute; cotton; bamboo; reed; coir; banana fibres;
and peanut shells. Preferred polymer matrices come from thermoplastics but other
polymer resins may be equally useful depending on their compatibility. In general,
additives are used to increase the compatibility between the cellulose component
and polymer component of a composite or the interphase adhesion; or to enhance
the processability of the composite mix during extrusion, injection moulding or
compression moulding.
It is also customary to employ additives to modify the technical properties of a
composite. Some common areas are tensile strength, flexibility, density, impact
strength, and thermal stability for their mechanical and chemical properties, and
to enhance their aesthetic values. Such agents may be foaming agents to expand
the polymer matrix and lighten the composite; flow additives; thermal stabilisers;
biocides; insecticides; antioxidants; UV absorbers; antistatic agents; flame retardants;
fillers and colourants.
Some of the various colouring methods for polymer composites are:
• Granular colour concentrates comprising = 20% by weight of a pigment or dye and
80% by weight of a PE-wax. Concentrates are mixed into a mixture of polymer
rice hulls and mixed thoroughly before extrusion.
• An impression of a wood-grain effect on the surface of an extruded proile can
be obtained by adding a combination of pigment-olefin masterbatch and a liquid
colourant at the time of extrusion.
• Polymer composites based on polyvinyl chloride (PVC) and rice hulls can be
coloured with an aqueous dispersion of a pigment. These can include metal oxide
pigments such as iron oxide, depending on the final colour desired.
• For packaging materials based on transparent composites with low concentrations
of wood flour or rice hull flour, the transparent polymer can be coloured with dye
solutions or the constituents can also be coated with coloured material.
The advantageous properties of specially made colourant preparations also come to
the fore in the colouration of cellulose particles in cellulose polymer composites. The
cellulose particles are readily through-coloured regardless of their size, including even