Agriculture Reference
In-Depth Information
of the silica content is concentrated on the outer surfaces of the hull. If rice hulls
are used for the manufacture of composite resins, their special characteristics must
be taken into account. Research has found that the surface of rice hulls contain
materials that are only loosely bound to the fibre structure underneath. For the sake
of easy processability and strength of the final product, it is necessary to have these
loosely bound material removed before use. Such 'cleaning' may be accomplished by
physical means such as tumbling. Mixtures of raw rice hulls and a fine hard material
can be fluidised in a fluid stream such as air to achieve this end. In other words, it
is acceptable to subject rice hulls to the abrasive action of a hard material that is
sufficiently fine to reach all surfaces of the hulls. The exact method employed for this
'cleaning' is not important and it is generally accepted that this can also be achieved
by a grinding action, probably when the particle size is reduced.
To maximise the strength of the product, tests have shown that it is not enough to
just 'clean' the hulls but it is also necessary to remove the dust or any other foreign
material from the cleaned hulls. This can be achieved by any of the commonly used
methods of size separation, such as sieving, air cyclone or air floatation, As they
emerge from the rice mill, rice hulls may also comprise pods containing the two husk
leaves joined together. Tests have shown that grinding the rice hulls in a hammer mill
followed by sieving serves the dual purpose of 'cleaning' and separating the pods into
individual leaves with the hulls that can be broken into pieces so as to shorten their
length and width. The ideal situation would be to have the rice hulls in a fine powder
form for easy blending and mixing with the polymer matrix.
Rice hulls have an inherent moisture-content problem. They are an agricultural waste
and the volumes are large, so rice mills after processing may store part of rice hulls to
protect them from the elements. However, due to the sheer volume of a rice-producing
and rice-milling operation, it may not be possible to protect them from the weather.
Hence, supplies from a rice mill will contain a high content of water unless a buyer has a
special arrangement with the rice mill. Producers may opt for purchasing their rice hulls
from a professional source, where the rice hulls can be supplied in neat bags or in bulk
processed according to a producer's specifications. However, this will be at a nominal
additional cost. Depending on the end products targeted, a producer of composite resins
with rice hulls can specify a particular sieve size and moisture content. To facilitate a good
homogenous mixture as well as a smooth and easy-to-process composite compound,
the ideal moisture content should be ≤1% and certainly ≤2%. Higher moisture contents
will pose processing problems and also swelling of the extrudate at the die.
9.3.3 Additives
Additives are an essential part of making polymeric composites, some to enhance
properties, some to achieve aesthetically pleasing finishes, and some are needed
 
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