Objective  Impregnation modification can effectively improve the physical-mechanical properties of Chinese fir plantation wood. Herein, the sodium silicate (Na₂SiO₃) and magnesium sulfate (MgSO₄) was selected as impregnation modifier and fixing agent, separately. A silica-magnesium gel modified Chinese fir wood was manufactured using a vacuum-pressure cyclic impregnation method. In detail, the impregnation process conditions including MgSO₄ content were further explored, which provided a theoretical basis for large-scale production of the silica-magnesium gel modified Chinese fir wood.

  Method  Through the single factor experiment, the effects of four factors, the molar ratio of magnesium sulfate to sodium silicate (n(Mg)∶n(Si)), impregnation time, impregnation pressure and the ratio of positive and negative pressure time, on the modification effect of Chinese fir wood were investigated. The L₉(3⁴) orthogonal experiment was designed based on these single factor experiments to optimize the impregnation parameters. Properties such as mass increasing rate, compressive strength along grain, hardness, water absorption rate, leach resistance, thermal stability and micro-morphology characteristics of the material were investigated. Moreover, the silica-magnesium gel modified Chinese fir wood prepared by the optimal technical condition was compared with the sodium silicate modified Chinese fir wood and the untreated Chinese fir wood.

  Result  According to the orthogonal and single-factor tests, the optimal process parameters for the silica-magnesium gel modified Chinese fir wood were followings: the MgSO₄ solution and Na₂SiO₃ solution with n(Mg)∶n(Si) of 1∶2, impregnation time of 2 h, impregnation positive pressure of 0.3 MPa and negative/positive pressure time ratio of 2∶1. Compared with untreated Chinese fir wood, the compressive strength, cross section hardness, tangential section hardness and radial section hardness of the silica-magnesium gel modified Chinese fir wood increased by 81.1%, 73.1%, 52.6% and 37.2%, respectively, and water absorption rate decreased from 129.3% to 73.3%. The results of scanning electron microscopy showed that the MgSO₄ in the silica-magnesium gel modified Chinese fir wood was successfully immersed into the tracheid of Chinese fir wood and reacted with Na₂SiO₃, resulting in different morphology deposits, with a leach resistance increased by 22.1% compared with that of sodium silicate modified Chinese fir wood. A decrease in the mass loss rate of silica-magnesium gel modified Chinese fir wood was observed, and the residual mass was increased by 27.09% due to the impregnation of the inorganic components.

  Conclusion  The physical mechanical properties including density, mechanical strength, water resistance, hardness, leach resistance and thermal stability are significantly increased compared with regular fir wood. Compared with the sodium silicate modified Chinese fir wood, it has more performance and application advantages.