Mass loss evaluation of wood; are the results correct?
2008
Larnoey, E., Norwegian Forest and Landscape Inst., Aas (Norway). Section of Wood Technology
To evaluate the decay resistance of wood, treated or untreated, for hazard classes 3, 4 and 5, the mass loss due to fungal exposure needs to be calculated. The standards for calculating mass loss in wooden test samples (i.e. EN 113 and ENV 807) require that the samples used in the test are pre-dried to 0% moisture content, m0, for determination of the initial dry mass. The standards describe that the samples should be dried at 103+-2 deg C for 18 hours. When drying wood samples to 103 deg C the samples are altered. Literature describes that extractives are influenced at temperatures as low as 60 deg C, and redistribution and/or evaporation of these, will change the characteristics of the wood samples. By a redistribution of extractives, some penetration paths may be clogged, while others again may be opened. A surface layer of extractives may also occur. The evaporation of extractives may alter the natural durability of the wood samples, and could influence a chemical reaction between a preservative and wood extractives. As the impregnation industry dry their lumber at only ca. 60 deg C before impregnation, the current test does not represent the real life conditions. This paper describes which extractives evaporate from the wood samples by drying at 103+-2 deg C, and different alternative drying methods are tested. The samples were then exposed to different drying regimes: vacuum drying at 50, 70 and 90 deg C, as well as freeze drying. This provided new information concerning the influences of different drying regimes.
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