A new preservative treatment method that uses supercritical carbon dioxide (CO2) to carry preservatives into wood is being developed in Australia.
"The technique employs 'supercritical' carbon dioxide to spread preservative through the wood," says Dr Abdul Qader, who leads the research at Australia's public research organisation, CSIRO.
"Like other gases, carbon dioxide enters a supercritical phase when subjected to pressures and temperatures beyond a 'critical' point. A key change is that it acquires a liquid-like ability to dissolve compounds. At the same time it retains a gas's ability to penetrate fine structures such as the micropores of wood, unrestricted by the high surface tension associated with liquids", says Dr Qader.
"Unlike conventional dipping and pressurised treatment procedures that use preservative dissolved in water or oil, it should allow effective treatment of hardwoods such as messmate eucalypts and difficult softwoods such as cypress pine," he says.
Research on the possible use of supercritical fluid to spread termite- and fungus-resistant preservatives through the fine structure of wood is now under way in the US and Europe, as well as Australia.
In small-scale tests, Dr Qader has found supercritical CO2 treatment gives much better preservative penetration and retention than conventional techniques. Samples treated include messmate eucalypt timber and the reconstituted products laminated veneer lumber (LVL) and medium density fibreboard (MDF).
Tests on larger samples are planned for early this year following the installation of new equipment. CSIRO is seeking industry involvement in further development of the technique.
"Because of the high pressures required - CO2 becomes supercritical above 72.9 atmospheres pressure (about 1070 psi) and 31.1°C - the capital cost of treatment facilities will be considerably higher than for conventional processes. However, operating costs will be lower," says Dr Qader.
CSIRO has applied for patent protection as the technique appears to offer significant environmental and economic benefits.
Major benefits are greater preservative penetration, the ability to provide effective treatments for wood that is not amenable to conventional application methods, no requirements for drying and stock holding after the treatment. The process also eliminates the problems associated with residual solvent in wood treated using solvent-based techniques. In the new process, the CO2 reverts to its gaseous state and dissipates when the pressure falls after treatment, leaving the preservative behind in the wood.
Qader says products likely to be prime candidates for the new treatment include window and house frames, LVL, decking, hardwood flooring and particleboard.