Manufacturing of Polymeric Composite Resins - Introduction to Polymeric Composites with Rice Hulls - page 112

mix and the whole mass compounded well. This mass then goes through an extruder, in
which further mixing takes place, and is extruded through a multi-cavity die placed in
the extruder head. A set of fine mesh placed just before the die prevents foreign matter
going through the die. This is important for the uniformity and purity of the extruded
strands. The multiple strands emerging from the die go through a cooling process and then
through a pelletising machine and are cut into predetermined sized pellets. The resulting
pellets are opaque to slightly yellowish in colour or can be fully coloured.
9.3 Raw Materials
As mentioned above, the basic raw materials for manufacturing PCRH are a polymer
matrix, rice hull powder, additives and colourant (optional). Each formula will differ
in that it will be designed to meet a particular end application. Surface additive agents
can produce matt or high-gloss finishes as desired. In general, polymeric composites
with reinforcements are on the hard and heavy side and, if lighter products are
desired, a blowing agent can be incorporated. For example, hard and heavy products
will be ideal for flooring and building construction industries, whereas lighter and
aesthetically pleasing products will be ideal for automotive and indoor applications.
9.3.1 Polymers
In general, thermoplastic polymers such as PE; PP; PVC; acrylonitrile-butadiene-
styrene; and polystyrene are the easiest to work with. They could be in pellet or powder
form, free of atmospheric moisture. Some formulations may use virgin material or
recycled material. In the latter, if multiple coloured materials are recycled together,
achieving a final single colouring is affected and the formulation has to be adjusted
accordingly. If mixtures of polymers are used, it is important to use compatible
polymers starting with the melt-flow properties. Polymers are usually available in
bags of 25 kg or in bulk ('Gaylords') containing ≈400 kg. The proportions of the
polymer matrix to the reinforcing agents will vary with each formula, but a practical
base guide would be 40:60. Higher rice hull contents are possible.
9.3.2 Rice Hulls
Rice hulls are available in abundance in many countries. Until recently, the full
potential of these humble gifts of nature has not been exploited fully. The possibility
of PCRH as suitable substitutes for natural wood is exciting and challenging.
Rice hulls are different to wood fibres in that the fibre length of rice hulls is only
≈0.3 mm as compared with 6-7 mm for wood ibres. Moreover, a substantial part