Dynamic strength properties and structural integrity of wood modified with cyclic N-methylol and N-methyl compounds

Abstract

Abstract Cyclic N-methylol compounds have been used for cell wall impregnation modifications of wood. Besides an improved decay resistance and dimensional stability, the modifications resulted in a decrease of wood’s dynamic strength properties. However, the mechanisms behind a significant loss in dynamic strength are not fully understood yet. In this study, wood blocks were treated with the N-methylol compounds 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) and methylated DMDHEU (mDMDHEU) and the N-methyl compound 1,3-dimethyl-4,5-dihydroxy-ethyleneurea (DMeDHEU). In order to study the factors that control the changes of wood performance under dynamic loads, single (impact bending strength, IBS) and multiple dynamic impact (resistance to impact milling, RIM) tests were applied. It became evident, that reductions in IBS and RIM increased with increasing solid content, formaldehyde content and catalyst concentration of the impregnation solutions, but were not affected by a cold-water leaching. Differences in structural integrity of wood modified with N-methylol and N-methyl compounds were more pronounced than those of IBS. Therefore, RIM appeared more sensitive to changes on cellular level, as a higher degree of co-condensation of the N-methylol compounds with cell wall polymers was expected in comparison with the N-methyl compound.