Tall oil is a by-product in pulping of resinous wood by the sulphate process. Tall oil contains a complex mixture of wood extractives. Some of these extractives act as natural protection against wood decaying fungi while other serve as nutrition for the fungi. This report describes a screening of the efficacy of four refined tall oils with different chemical composition on wood decaying fungi. Testing was performed as filter paper assay and mini-block assay. In the filter paper assay growth rates of the white rot fungus Coriolus versicolor and the brown rot fungus Poria placenta were inhibited by the tall oils. None of the oils caused total inhibition of the fungi but there was a clear pattern towards increased efficacy with increased portion of resin acids in the oils. Impregnated mini-blocks with approximately 200 kg/m3 retention of tall oil after leaching showed an evident effect against Coniophora puteana and Poria placenta compared to untreated control samples. However, using the criteria from EN 113 requiring less than 3% mass loss, tall oil failed. The results indicate that decay resistance of tall oil impregnated Scots pine sapwood to the retention level used in this study is comparable with the decay resistance of Scots pine heartwood. It was expected that the efficacy of the tall oils was related to chemical composition of the oils. This was confirmed for the filter paper assay where the efficacy increased with increasing amount of resin acids. However, this pattern was not found for the mini-block assay. The protective effect of the tall oils in wood seems therefore to be more related to their hydrophobic properties than to their fungicidal properties.

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