Rice Hulls for Composites - Introduction to Polymeric Composites with Rice Hulls - page 57

5 Rice Hulls for Composites
5.1 Introduction
Rice is the staple diet of many people all over the world. More than 100 million metric
tonnes of rice are produced annually, especially in Asia and the Far East. Rice hulls
are the protective coatings of the rice seeds or grains. These hulls are formed by a
hard material and are unique products of nature. They contain ≈20% opaline silica
in combination with a large amount of structural polymer called lignin . These hard
coatings protect the rice seeds during the growing season until harvest.
Previously, separation of rice grains from hard coverings was a laborious process.
However, modern processing employs many advanced machines that make it much
easier, faster and more efficient. During milling, hulls are removed from raw grains
to reveal whole brown rice grains, which may then be milled further to remove the
brown (bran) layers, resulting in white rice. This abundant natural agricultural
waste has posed problems with regard to disposal. The few traditional uses for
these leftover hulls have not fully eliminated this problem. However, once the true
nature and properties of these hulls (especially the high content of silica) had been
discovered, more sustained efforts were made through research and development to
find end uses for these versatile hulls. Two of the lucrative uses of these rice hulls
are the production of polymeric composite resins with rice hulls for the processing
of plastic products (cheaper resins) and the production of excellent lumber as an
ideal substitute for natural timber. One may even say that these products have better
properties than the naturally occurring ones. Also, they can be considered to be
'green' products with tremendous potential to ease global environmental concerns
by reducing the felling of trees.
5.2 Properties of Rice Hulls
Rice hulls are unique within nature. This abundant agricultural waste has all the
properties one could expect of some of the best insulating materials and, due to diligent
research, its true potential is emerging. Tests carried out have shown that rice hulls:
do not flame or smoulder very easily; are highly resistant to moisture penetration and
fungal decomposition; do not transfer heat very well; are not odorous; do not emit
gases; are not corrosive with respect to metals such as aluminium, copper or steel.