Attempts to turn rice straw, the inedible remains of the rice crop, into a wood substitute are beginning to shed light on a host of properties that might exceed even those of humble wood.

Attempts to turn rice straw, the inedible remains of the rice crop, into a wood substitute are beginning to shed light on a host of properties that might exceed even those of humble wood.

Rice straw can already be processed into medium density fibre board (MDF), but particle board presents more of a challenge. Finding out why - by investigating the chemical composition, morphology and surface properties of this agricultural by-product - is leading US researchers into unexpected territory.

Maria Inglesby of the Bioproduct Chemistry and Engineering unit at the Agricultural Research Service in California, US, is investigating whether the SiO2 content of the straw underlies the particle-board problem. Wheat straw, which can be processed in this way, has a much lower SiO2 content (4-5 per cent based on dry weight) compared with rice straw (11 per cent), she says.

So silica content might be part of the problem, but it could also represent a decided advantage for other uses.

’The building industry is interested in the SiO2 because there may be potential benefits regarding flame retardancy,’ Inglesby told Chemistry World. ’Our basic research may lead to some answers there.’

And there’s more. Homes are already being built out of complete rice-straw bales and appear to have improved moisture resistant properties compared with homes built out of wheat straw bales. Whether that is related to SiO2 content is unknown, but worth investigating. ’SiO2 is a solid, amorphous, hydrated material, I’d be so curious to know how that plays out in buildings,’ said Inglesby.

Her research, presented at the 2004 American Chemical Society meeting in Anaheim, California, is based on processed rice straw from local firm California Agriboard (CalAg). With a patented technique to produce rice straw-MDF, CalAg is poised to start mass production (265 000 m3 pa) at a ~$150 million (?84m) facility.

The firm is interested in everything that can be developed from rice straw, says CEO Jerry Uhland. ’Being an aquatic plant, the high quality wax make-up, and the use of the pMDI [polymeric methylene diphenyl diisocyanate] resin binder, gives our MDF product very high moisture resistance properties,’ he said, ’about 35 per cent more moisture resistant than wood-based MDF.’

CalAg is not researching the product’s natural fire-resistance potential, but it’s an area of ’extreme interest’, says Uhland. Currently, only wood-based products achieve flame-retardant certification. ’We believe that less additives will be required to achieve [certification] with our rice straw MDF,’ he predicts.

Bea Perks