Effects of different pretreatment methods on the pore structure of wood cell wall

Objective This paper explores the changes in pore structure of wood cell walls induced by water and compares this method with the pretreatment methods such as microwave treatment and delignification, in order to provide a scientific basis for wood modification.

Method Poplar (Populus euramericana) and Chinese fir (Cunninghamia lanceolata) specimens of 20 mm (radial) × 20 mm (tangential) × 5 mm (longitudinal) were treated with water soaking for 1 h and 1 month, respectively, microwave treatment at 500 W for 18 min, as well as sodium chlorite under acidic conditions. The microscopic morphology and pore structure were characterized by scanning electron microscopy and nitrogen sorption test, and the effects of different treatment methods on cell wall thickness, specific surface area, pore size distribution and pore volume were compared.

Result The cell walls of poplar and fir thickened after water treatment, and the specific surface area of the water-treated wood for 1 h increased from 1.535 and 1.154 m²/g to 2.488 and 2.336 m²/g, and that of water-treated wood for 1 month further increased to 2.822 and 2.940 m²/g, respectively. For water-treated poplar, new pores were formed in the micropore and mesopore range (pore formation), and there was an increase in pore size (pore enlargement). For water-treated Chinese fir, the change in pore volume in the micropore range mainly reflected a pore enlargement effect, and there were both pore formation and enlargement phenomena with the increase of treating time. Larger pores were generated in the mesopore range due to the removal of water-soluble extractives, which led to a redistribution of pore size. The changes in cell wall thickness and specific surface area of microwave-treated poplar and Chinese fir were similar with those of water treatment, and some micropores were produced in poplar after microwave treatment, and the pore size distribution of mesopores in poplar and both pores in Chinese fir showed enlargement. The cell wall thickness of poplar and fir treated with delignification decreased by 1.79% and 0.53%, respectively, while the specific surface area increased, the micropores in Chinese fir rose, and the other changes of pore size distribution were due to the enlargement of pore size.

Conclusion The effect of water treatment on poplar and Chinese fir for 1 h and 1 month was comparable to that of microwave treatment with mass loss percentage of 1.29% and 3.33%, and delignification treatment at removal rate of 10.94% and 8.06%, respectively, presenting a unique pore size distribution change pattern. The pore volume of three treatments varies in different pore size ranges for different tree species due to different treating mechanisms. This study provides a reference for selecting a scientific and efficient wood pretreatment method.