Abstract: The wood fiber was activated by sodium hydroxide/urea mixed aqueous solution under freezing treatment. The freezing activation effect of wood fiber was characterized by measuring the mechanical properties of the self-bonding fiberboard hot-pressed with activated wood fiber and implementing the analysis of Fourier tansform infrared spectroscopy and X-ray diffraction. The experimental results showed that the sodium hydroxide/urea mixed aqueous solution had a certain activation effect on the wood fiber under freezing treatment. The optimized activated process was described as the mass ratio of total sodium hydroxide and urea (the mass ratio of sodium hydroxide to urea 7:12) to wood fiber 1:12, freezing temperature -15℃, freezing time 1.0 h, and wood fiber moisture content 20%. The static bending strength and internal bonding strength of fibreboard prepared with wood fiber freezing-activated by sodium hydroxide/urea mixed aqueous solution increased from 32.16 and 0.29 MPa to 45.53 and 1.21 MPa, and the water absorption rate and the thickness swelling rate decreased from 53.95% and 22.57% to 43.62% and 10.34% respectively compared to those of the unactivated wood fiber. Microscopic characterization by Fourier tansform infrared spectroscopy and X-ray diffraction showed that the hydrogen bonding structure of cellulose in the wood fiber was destroyed under the freezing activation treatment with sodium hydroxide/urea mixed solution, the expansion of cellulose I lattice was promoted and a new crystalline modification was produced, which reduced the degree of molecular association and consequently enhanced the hydroxyl activity of the wood fiber surface.

Key words: wood fiber, self-bonding, freezing activation, sodium hydroxide, urea