超声水/NaOH协同DMDHEU处理对臭椿材吸湿尺寸稳定性的影响机制研究

[Objective]The objective of this study was to investigate the impact and contribution mechanism of ultrasonic water/NaOH synergistic DMDHEU treatment on the hygroscopic dimensional stability of Ailanthus altissima wood.[Method]Ailanthus altissima wood,approximately 20 years old,was selected and made into 2 cm×2 cm×2 cm(tangential×radial×longitudinal)blocks.The samples were pretreated with NaOH(1 wt%,4 wt%)and ultrasonication(320 W,25 kHz,2 h)and then impregnated with 1,3-dimethylol-4,5-dihydroxy ethylene urea(DMDHEU).The absolute dry density,the weight percent gain,and the hygroscopic dimensional stability(11.3%RH,25 °C)were measured before and after DMDHEU impregnation.Scanning electron microscopy,attenuated total reflection Fourier transform infrared spectroscopy,and X-ray photoelectron spectroscopy were applied to investigate the microstructure,resin distribution,and chemical group changes of the samples.[Result]The weight percent gain and the absolute dry density of DMDHEU-impregnated samples were higher than those of DMDHEU-impregnated-untreated samples.The peak intensity of the free hydroxyl group of DMDHEU-impregnated samples decreased significantly,while the intensity of the peaks representing C=O and C-N at 1710 cm-1 and 1230 cm-1 increased.The valence state of C in DMDHEU-impregnated samples changed when resin entered into the wood.The DMDHEU resin was chemically bonded to the cell wall,converting the hydrophilic free hydroxyl groups into hydrophobic ester and ether bonds.After curing of the resin,uniform film distribution was observed on the inner walls of DMDHEU-ultrasonic-water-treated samples.Resin deposits were found in the pores of the ultrasonic-1%NaOH-treated samples,while film-like resin deposits were observed on the inner wall of the vessel in the ultrasonic-4%NaOH-treated samples.The tangential and radial hygroscopic dimensional stability of DMDHEU-impregnated samples improved to varying degrees compared with the untreated samples.The tangential and radial swelling rate of the untreated samples was 2.01%and 1.22%,respectively,while the tangential and radial swelling rate of the DMDHEU-ultrasonic-4%NaOH-treated samples was reduced to 0.55%and 0.38%,respectively.[Conclusion]The impregnation effect of DMDHEU resin was improved by both ultrasonic-water pretreatment and ultrasonic-alkali pretreatment.Ultrasonic pretreatment facilitated the binding of DMDHEU resin with the cell wall,and reduced the content of free hydroxyl groups.Spot-like resin distribution with a densely structured resin film formed inside the vessel of DMDHEU-ultrasonic-NaOH-treated samples,which could partially block the moisture absorption channels of the wood.In this study,the hygroscopic dimensional stability of DMDHEU impregnation with ultrasonic-4%NaOH-treated Ailanthus altissima wood was the best.