Weathering of Wood Modified with the N-Methylol Compound 1,3‑dimethylol-4,5-dihydroxyethyleneurea (DMDHEU)
C Mai, Yanjun Xie, A Krause, K Urban, P D Evans, K Richter, H Militz
Scots pine (Pinus sylvestris L.) sapwood boards were treated with a methylated 1,3‑dimethylol-4,5-dihydroxyethyleneurea (mDMDHEU) causing weight gains of approx. 25% and exposed to weathering for 18 months. Compared to untreated boards, treatment with mDMDHEU reduced surface discoloration mainly caused by staining fungi. Boards treated with mDMDHEU exhibited clearly lower moisture content throughout the exposure time and lower water uptake in periodical submersion tests. The treatment also reduced deformation (cupping) and crack formation of the boards due to weathering (assessed as waviness and surface roughness). When coated boards were compared, prior treatment with mDMDHEU resulted in lower water uptake in periodical submersion tests, less discoloration, minor deformation (cupping) and less crack formation (assessed as waviness and surface roughness). Oil-based coatings did not peel off the mDMDHEU treated board surfaces as observed for the untreated board surfaces.
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. The weight losses of all treated veneers during 144 h of weathering, however, were similar to those of the unmodified controls. 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.