Recycling rice hull ash: A filler material for polymeric composites?

Born more out of necessity, the "awareness-attitude" of our society has caused the channeling of technological developments toward immediate economic and ecological benefits. The polymer industry has a newfound interest in fillers from industrial by-products and other waste materials having potential "recyclability" This new class of fillers includes fillers from natural sources (e.g., natural fibers), industrial by-products (e.g., saw dust, rice husks), and a recent entry in the form of rice husk ash-an industrial waste material-obtained by burning rice husks. Their recyclability and utilization has become a major driving factor in their acceptance and employability, as well as low cost and abundant availability. However, their performance in composites and processing requirements has hindered their applicability within the current economic framework. The present article reviews the performance of rice husk ash, or silica ash, in polymeric composites. Silica ash, composed mainly of silica, is obtained after burning rice husks and is a major industrial waste material in rice growing countries. Optimism surrounds the application of silica ash as a potential filler in a variety of polymeric composites, however, its performance has been limited by its inherent characteristics. This paper emphasizes the need for better characterization of silica ash to obtain an in-depth understanding of its behaviour with the view to identifying suitable modifications to improve its performance as a filler. It is emphasized that poor understanding of silica ash as a filler is linked to the lack of surface characterization, since its behaviour is significantly linked with its surface properties. Based on this analysis, a new approach to silica ash modification is proposed.