Weathering of wood modified with the N-methylol compound 1,3-dimethylol-4,5-dihydroxyethyleneurea

Abstract

N-methylol compounds are used as a wrinkle-resistant finish in the textile industry. They are expected to enhance the resistance of wood to weathering because they can cross-link the cell wall and dimensionally stabilise wood. Scots pine veneers were modified with 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) to weight percent gains (WPG) of 10%, 27% or 48% and exposed to artificial weathering. Initially, weight losses of unmodified veneers were significantly greater than those of DMDHEU treated specimens even though DMDHEU was leached from wood at a higher rate than loss of wood substance. The weight losses of all treated veneers after 144 h of weathering, however, were similar to those of the unmodified controls. Therefore we conclude that in the short term DMDHEU treatment can restrict weight losses of wood during weathering, which occur due to degradation of lignin and hemicellulose and loss of degraded wood fragments from wood. Infrared spectroscopy suggested that treatment of wood veneers with DMDHEU to high WPG (48%) stabilised lignin to some extent. Tensile strength losses of DMDHEU treated veneers during weathering were lower than those of untreated veneers. DMDHEU treatment, however, had a deleterious effect on the tensile strength of the veneers, possibly associated with the presence of magnesium chloride catalyst in the treatment solution. Scanning electron microscopy revealed that DMDHEU treatment was highly effective at preventing the degradation of the wood cell wall during weathering. Tracheids in unmodified veneers became distorted within 48 h of weathering exposure, whereas cells in modified veneers, especially those reacted to higher weight percent gains, retained their shape even after 144 h weathering.