Treatment of timber with permethrin in supercritical carbon dioxide to control termites

The solubility of permethrin in supercritical carbon dioxide (scCO2) was determined, with optimal solubility for the timber impregnation trial and the specific operating conditions examined occurring at 50 °C. Methanol as co-solvent improved solubility, but was not needed for timber impregnation. Eucalyptus obliqua heartwood (a refractory timber), Pinus radiata sapwood and P. radiata LVL were impregnated in a treatment cylinder, and their resistance to attack by termites was compared to that of material protected with standard permethrin treatments using light organic solvent preservative (LOSP). Termite field tests were conducted against Coptotermes acinaciformis and Mastotermes darwiniensis. The choice of solvent system did not affect permethrin efficacy in P. radiata sapwood against termites. P. radiata LVL specimens treated using scCO2 had slightly lower mean mass losses than those impregnated with LOSP, reflecting slightly higher permethrin uptakes in the former. E. obliqua heartwood was completely destroyed by M. darwiniensis when impregnated with LOSP, but was mostly sound when scCO2 was used. These results show that scCO2 did not alter the effectiveness of the insecticide, while giving the added benefit of allowing the impregnation of timbers previously considered refractory to treatment.