Improving dimensional stability of <i>Ailanthus altissima</i> wood by ultrasonic alkali-assisted DMDHEU treatment

Abstract The effectiveness of DMDHEU (1,3-dimethylol-4,5-dihydroxy ethylene urea) modification is influenced by factors such as pore size distribution. In this study, ultrasonic-assisted-alkali pretreatment combined with DMDHEU impregnation was used to enhance the hygroscopic dimensional stability of Ailanthus altissima wood. The weight percentage gain (WPG), absolute dry density, tangential swelling, resin distribution, and chemical functional groups were analyzed. The results showed a marked reduction in the tangential swelling of DMDHEU-ultrasonic-4 % NaOH-pretreated samples, decreasing from about 2.0 % and 4.9 % in untreated samples (without any pretreatment or DMDHEU impregnation) to 0.6 % and 3.2 % at approximately 11.3 % and 75.3 % RH, Respectively. Compared to untreated samples, DMDHEU-ultrasonic-pretreated samples exhibited increased WPG, Absolute dry density, and dimensional stability. Ultrasound-assisted pretreatment also influenced resin distribution within vessels, with raised-membranous resin formations observed in the DMDHEU-ultrasonic-4 % NaOH-pretreated samples, in contrast to the sporadic deposition in untreated samples. Furthermore, XPS and ATR-FTIR results indicated ultrasonic pretreatment facilitated deeper DMDHEU resin penetration into the wood cell wall, converting hydrophilic hydroxyl groups into hydrophobic ester and ether bonds. These findings underscore the significance of ultrasonic alkali pretreatment in enhancing the efficacy of DMDHEU modification for A. altissima , contributing to improved durability and expanding potential applications for this fast-growing timber species.