Wood products with ground and direct water contact, but without sufficient biological durability, have to be protected against biological degradation by fungi and other microorganisms. Due to European legislation (Biocidal Products Regulation 528/2012), the selection of possible chemical agents, which are still allowed to be used, is diminishing rapidly. The use of previously widely applied, very effective substances, as creosote, organic chlorine or hexavalent chromium fixed water soluble cuprous salts e.g., is no longer allowed or strongly limited. Hence, it makes sense to develop new approaches for hydrophobizing wood, which solve the problem of agents leaching in the meantime. The paper presents a method to improve the natural durability of beech wood by means of a vacuum/pressure procedure using on a modified vegetable oil. The oil can be hardened directly in the inner wood zone by means of microwaves and oxygen treatment. This shall prevent the commonly occurring disadvantages of oil treatments which frequently cause bleeding by time, affection with microorganisms as bacteria and algae or staining and simultaneously reduce the leaching of cuprous salts. Firstly, as biocide component an aqueous cuprous salt (Cu acetate) solution was embedded before the oil treatment. Secondly, an organic oil soluble copper soap (Cu octanoate) was incorporated directly into the oil component. It could be verified, that the oil can be hardened completely in 1cm depth of beech wood specimens after a 24-hours lasting treatment in an laboratory autoclave with oxygen at 70°C. Thus, the biological durability of beech wood could be enhanced from durability class 5 to class 1-3 (depending on the treatment and tested microorganisms according to CEN/TS 15083-1 and 2). The oil impregnation causes a significant decrease of copper leachability and enhancement of the biological resistance against the basidiomycetes Coniophora puteana and Trametes versicolor after accelerated ageing according to EN 113. Maximal mass loss varies from 1 % to 10 % and allows the utilisation of the treated wood up to use classes 2 and 3 according to EN 335. Tests with soft rot fungi show that the mass loss, which normally varies from 21 % to 36°% for natural beech, could be decreased down to 6 % for treated beech. The resulting x-values (acc.to CEN/TS 15083-2) allow a categorisation into durability class 2 for copper containing variants and 3 for pure oil treatments. The addition of siccatives like cobaltous or manganous salts causes an enforced polymerizing process with higher resistance against water and biological affection. However, in some combinations the copper compounds cause liquefaction of the oils, as there act also as siccatives. In order to find highly efficient receipts, detailed formulation work is necessary according to the choice of the oil, the siccatives and their combinations as well as the hardening parameters.

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