Influence of Natural Durability, Laboratory Weathering, Resin Content, and Ammoniacal Copper Arsenate Treatment on the Decay Resistance of African Hardwood Particleboard

Particleboards manufactured from ring-flaked Ghanaian hardwood chip mixtures were evaluated for decay resistance as measured by weight and internal bond (IB) strength losses after the ASTM soil-block test. Variables studied were ammoniacal copper arsenate (ACA) levels of 0.0 to 0.6 pcf, natural durability of wood species, laboratory weathering tests of ASTM accelerated aging (AA) and vacuum-pressure-soak (VPS), phenol-formaldehyde (PF) resin levels of 5 and 8 percent, and decay fungi (C. versicolor and G. trabeum). Twenty-two species, representative of the ranges of wood characteristics encountered in West African hardwood forests, were utilized. The species ranged in specific gravity from 0.24 to 0.81 and heartwood decay resistance from highly resistant to nonresistant (ASTM D2017). Both sapwood and heartwood were present in the mixtures averaging 55 percent heartwood and 45 percent sapwood by volume. VPS and AA reduced initial IB of all panels and significantly increased IB losses by the soil-block test particularly of the untreated and 0.2 pcf ACA treated panels. VPS and AA also resulted in increased weight losses by the two fungi in those panels that were susceptible to decay. G. trabeum was more tolerant of ACA than C. versicolor; the former caused significant weight losses at 0.2 pcf ACA whereas the latter did not. Treatment with ACA decreased initial IB, but also reduced weight and IB losses of weathered particleboards caused by the decay fungi. The higher PF resin level resulted in higher initial IBs and higher IBs after weathering and the soil-block test. When weight loss was less than approximately 5 percent, there was no correlation between IB and weight loss, and IB was the best indicator of particleboard performance in a decay situation. In the weight loss range of 5 to 40 percent, IB was inversely linearly proportional to percent weight loss. The addition of decay-resistant species to a particleboard furnish of less resistant species (sapwood being present in both cases) did not reduce the percent IB loss of untreated panels. The most decay-resistant species had the highest average SGs, and panels containing those species had lower IBs both before and after the soil-block test because of low compaction ratios. Panels from high SG species also swelled more in the culture bottles which appeared to result in increased accessibility of wood substance to the decay fungi.

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