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Wood modification with N-methylol and N-methyl compounds: a case study on how non-fixated chemicals in modified wood may affect the classification of their durability

Abstract Chemical modification is increasing the durability of wood against biological deterioration. Usually, the effect of a new treatment on the durability of wood is screened in laboratory decay tests, where durability classes are assigned on the basis of the mass loss (ML) caused by degrading fungi. The aim of this study was to demonstrate how non-fixated chemicals in modified wood may affect fungal ML measurements and corresponding durability classification when wood samples are incubated under humid conditions for long periods. Wood blocks were treated with solutions of 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU), methylated DMDHEU (mDMDHEU) and 1,3-dimethyl-4,5-dihydroxyethyleneurea (DMeDHEU) and subjected to consecutive cold-water leaching cycles. Significant amounts of non-fixated chemicals were removed from the wood by three leaching cycles and might lead to ML mistaken as response of fungal decay. Consequently, the treated material was assigned erroneously by up to four durability classes (DC) worse than material which did not include leachable, non-fixated chemicals. Thus, for a reliable durability classification of chemically modified wood, prolonged leaching procedures are recommended to assure that the measured ML is entirely attributed to fungal decay.