Review of Laboratory and Outdoor Exposure Efficacy Results of Organic Biocide: Antioxidant Combinations, An Initial Economic Analysis and Discussion of a Proposed Mechanism
Abstract
As previously reported, the combination of various organic biocides with relatively high levels of commercial antioxidants always increased the biocides' efficacies against wood-destroying fungi in short-term laboratory decay tests. The two principal antioxidants examined, propyl gallate (PG) and butylated hydroxytoluene (BHT), are low cost and benign. In reviewing ground-contact outdoor exposure results, samples treated with the biocide chlorothalonil and the antioxidant BHT had 2-3-fold enhanced efficacy after four years of exposure against decay and termite degradation compared to stakes treated with only the biocide. In above-ground outdoor exposure tests after three years of exposure, lap-joint samples treated with only BHT at the relatively low retention of 0.94 kgm-3 had significantly less fungal decay than untreated samples. In addition, mini lap-joint samples treated with a quaternary formulation and 2.56 kgm-3 BHT and exposed in Hilo, HI, for two years, had more than 3-fold greater decay resistance than samples without BHT. However, relatively poor results were observed for both ground-contact and above-ground samples treated with various biocides and the antioxidant PG. Less BHT may be required to protect wood outdoors than the initial laboratory decay tests indicated. The antioxidant concept appears to be an economical option for totally organic systems. A mechanism by which BHT protects wood against fungal degradation is proposed.References
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