Study On The Effect Mechanism Of Ultrasonic/NaOH Synergistic DMDHEU Modification On The Dimensional Stability Of Ailanthus Altissima

Improving dimensional stability of fast-growing wood is one of the important scientific problems in wood processing industry.In order to improve the dimensional stability of fast-growing wood,such as Ailanthus altissima,ultrasound and sodium hydroxide（NaOH）were used to prepare pretreated Ailanthus altissima substrate,and then DMDHEU was used to impregnate the pretreated substrate.The main research results could be summarized as follows:（1）The larger the integral area of ultrasound intensity（UIA）,the lower the relative content of hemicellulose,the higher the crystallinity,the greater the degree of detachment of the pit film from the pit,and the lower the swelling rate.The ultrasonic power and frequency corresponding to the optimal UIA value of the ultrasonic cleaning machine used in this study were 320 W and 25 k Hz.（2）Ultrasonic-water treatment had the most excellent effect on the hemicellulose in cell wall.The longer the sonication time,the lower the FTIR bands intensity representing hemicellulose and lignin,and the lower the lignin and carbohydrate intensities observed by confocal Raman spectroscopy.After 5 h ultrasonic treatment,the lignin distribution of cell wall showed speckle,while the distribution of carbohydrate showed speckle deletion.For the ultrasonic-treated samples,the oven-dried density,thermal stability,porosity,and macropore ratio increased,and the swelling rate was reduced.（3）The extractives could be observed both in the early and latewood conduits of Ailanthus altissima,and it migrated during the ultrasonic process.The content of the benzene alcohol extractives and hot water extractives decreased after ultrasonic treatment.The relationship between the extractives concentration of Ailanthus altissima and the absorbance was A=0.79829C.It was found that the migration process of extractives was mainly a dynamic compounding process of increasing"extractable area extractives",extractives moving inside the wood,and extractives moving out of wood during ultrasonic treatment by using the gravimetric method and absorbance method.（4）The relative content,structure,and distribution of chemical components in cell wall were affected by ultrasonic-NaOH treatment.Compared with the alkali-treated samples,the extractives content,the relative hemicellulose content,and the carbohydrate and lignin strength of the cell wall of the ultrasonic alkali-treated samples decreased significantly,while the crystallinity increased.The uniform shrinkage Ailanthus altissima substrate with high oven-dried density,small porosity,relatively large pore size,was good for impregnation after ultrasonic-NaOH treatment.（5）DMDHEU impregnation could reduce the swelling rate of Ailanthus altissima.The DMDHEU impregnated ultrasonic-assisted substrate had a better impregnation effect than that of untreated substrate,and its inner conduit wall had a film-like resin distribution.The results of ATR-FTIR and XPS showed that the DMDHEU resin was chemically bonded to the Ailanthus altissima substrate,the free hydroxyl groups of the impregnating samples decreased,and the relative proportion of C₂ and C₃ increased.Compared with untreated samples,the swelling rate of DMDHEU impregnated samples decreased to different degrees.In addition,DMDHEU impregnated ultrasonic-4%NaOH sample showed the best dimensional stability,and the tangential and radial swelling rates decreased to 0.55%and 0.38%,respectively.