Objective To improve the dimensional stability by lignin on fast-growing wood, this work proposed a compound modification method by alkali lignin and hyperbranched polyacrylate emulsion (HBPA) for fast-growing poplar (Populus cathayana).
Method By introducing HBPA emulsion into alkali lignin, compound emulsions (group 4HL and 8HL) composed of 1.31% alkali lignin and 4.00% and 8.00% HBPA emulsion were prepared, and then the alkali lignin, HBPA emulsion and their compound emulsions were used separately to treat fast-growing poplar. Mass percentage gain, bulk capacity and scanning electron microscope of the modified wood were used to explore the impregnation effect; the Fourier transform attenuated total reflection infrared spectroscopy was applied to analyze the polymerization of the HBPA emulsion and the composition change of the modified wood; dimensional stability was evaluated through water absorption and moisture adsorption; and longitudinal compressive strength of the samples was tested.
Result Microscopically, as the concentration of the emulsion increased, more emulsion was observed to adhere to the cell cavity after solidification, and the amount of vessels and wood fibers fully filled with the modifiers increased as well. Macroscopically, with emulsion mass fraction increased, the mass percentage gain and bulk capacity of the modified wood also rose. Specifically, the mass percentage gain and bulk capacity of the 4HL group were 8.14% and 3.14%, and those for the 8HL group were 15.05% and 3.36%, respectively, and the mass percentage gain of the compound emulsion modified samples was greater than that of the sum for alkali lignin and HBPA emulsion individual treatment. Compared with the alkali lignin, unmodified wood, water absorption rate at 192 h for the 4HL and 8HL groups of the compound emulsion modified wood reduced by 29.4% and 35.3%, and the corresponding volume swelling rate decreased by 28.5% and 29.7%, respectively. After 21 d of moisture adsorption under 84% relative humidity, moisture content reduced by 6.9% and 11.5%, and the corresponding volume swelling rate decreased by 21.3% and 26.0%, respectively. HBPA emulsion and compound emulsion effectively enhanced the longitudinal compressive strength of treated wood, but this change trend was not related to the mass fraction of HBPA emulsion.
Conclusion Compared with alkali lignin and HBPA emulsion modified wood, the mass percentage gain and bulk capacity of the compound emulsion modified wood were higher, the water absorption and moisture adsorption decreased significantly, and the dimensional stability was promoted greatly, confirming the compound modification effect of lignin and HBPA emulsion on fast-growing poplar.