Rice Hulls for Composites - Introduction to Polymeric Composites with Rice Hulls - page 59

Rice hulls (also called 'rice husks') are flame-retarding and self-extinguishing. A lighted
match thrown onto a pile of rice hulls will, in general, cause it to burn and smoulder
without producing a high flame. Production of a flame will require a flammable
starter to ignite the pile. Depending on the length of burning, the resulting ash will be
grey or black; both have many useful applications. Conventional cellulose insulation
requires the addition of large quantities of flame- and smoulder-retardants. The
concentration of flame- or smoulder-retardant chemicals such as boric acid, sodium
borate ammonium sulfate, and aluminium sulfate in conventional cellulose may be
≤40% by weight. These chemicals are expensive and incorporating them into the
cellulose fibre is not easy. Importantly, rice hulls do not require flame- or smoulder-
retardants and, if used in insulation applications, additional chemicals are not required.
In general, all organic materials absorb or release moisture until they come into
equilibrium with the relative humidity of the surrounding air. The high presence of
silica on the outer surface of rice hulls impedes the atmospheric transfer of moisture
into the hull. Also the presence of a small percentage of a bio-polyester in the rice hull
in combination with a wax produced by the rice plant forms a highly impermeable
barrier to moisture and heat. The high concentration of opaline silica on the outer
surface of the rice hull also strengthens and makes the outer shell hard. However, due
to lignin within the rice hull, this hardness is tempered with flexibility and elasticity.
The rice hull is hard and yet elastic, so it resists settling and compression far better
than some materials.
Rice hulls are available at minimal cost and in some cases may be even available
free. In applications where they are used as loose filling in their natural form for
insulation, costs would be very much less as compared with other commercially
available insulating materials, especially because rice hulls do not require shredding,
hammer-milling, fluffing, fiberising or binding for this application. Perhaps the most
significant cost associated with the utilisation of rice hulls is their transport. Having a
bulk density of ≈5.6 kg/m 3 may cost extra unless they are compressed and packed in
such a way as to reduce transport costs. In general, it is accepted that loose rice hulls
can be compressed to a bulk density of ≈16 kg/m 3 without destroying their properties.
Depending on the type of transport and costs, it may not be necessary to reach this
maximum compression, and a lesser compression may be acceptable. Although rice
hulls have many versatile and practical applications, this chapter details the use of
rice hulls as a reinforcing component in polymeric composites.
5.3 Chemistry of Rice Hulls
Rice hulls are a unique gift of nature and their full potential is being exploited only
now. Rice hulls do not need to be mined or manufactured in processes that generate