While supercritical fluid impregnation offers tremendous potential for impregnating wood species that resist conventional liquid preservative treatments, the resulting treatments are often not uniform among samples in a charge or between charges. One factor that influences treatment is the dynamic change in pressure that occurs during introduction of the biocide laden supercritical fluid into the treatment vessel. Subcritical conditions during these time periods sharply reduce biocide solubility, setting the stage for more variable treatment results. One approach to limiting these variations would be to deliver a large percentage of the biocide into the wood prior to SCF treatment. This would place the biocide closer to their intended locations with in the wood as it was solubilized. The potential for using a dip treatment to deliver biocide into the wood surface followed by SCF impregnation was assessed using Douglas-fir heartwood blocks and cyproconazole. Dipping in biocide tended to produce slightly more uniform internal retentions, however, inclusion of otherwise untreated samples in the vessels indicated that biocide also diffused from the blocks into the SC-CO2 where it was available for uptake into other wood. The results suggest that surface loading of wood prior to SCF treatment produced only marginal gains in uptake and is probably not practical.

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