Fast-growing Chinese fir wood has shortfalls such as loose structure and low strength because it grows faster than natural trees. Resin impregnation is a great way to increase the strength of fast-growing fir. However, the resin used for impregnation is a kind of urea-formaldehyde resin, phenolic formaldehyde resin, melamine formaldehyde resin, and the like, which introduce harmful substances such as formaldehyde or phenolic into the wood. In this paper, Chinese fir wood was impregnated with natural shellac polymer, and the effects of impregnation variables on the mechanical properties of the wood were examined. The increase in strength in compression perpendicular to grain (SCPG) of wood samples impregnated with 15% shellac solution achieved a maximum value of 39.01%, but the modulus of rupture (MOR) was slightly reduced. The effects of the impregnation pressure, time, and their interaction were investigated by the response surface method (RSM). ANOVA analysis revealed that the impregnation pressure and time and the interaction between the two seemed to have a significant effect on ∆SCPG. Based on the response face model, the corresponding optimal parameters obtained are 1.0 MPa and 16.0 min for impregnation pressure and time, respectively. By impregnating fir wood with the above optimal conditions, the SCPG increased by 85.78%, whereas the MOR decreased by the least amount.

To improve the mechanical properties of fast-growing Chinese fir (Cunnighamia lanceolate), expand its range of application, increase its value, and avoid the environmental pollution caused by impregnation with synthetic resin, Chinese fir was impregnated with a shellac solution. Since the shellac solution was difficult to penetrate into fast-growing Chinese fir, so microwave pretreatment was used to irradiate the wood to improve the permeability. This study investigated the effects of four factors, including the content of moisture in the wood before it was microwaved, the chamber pressure of microwave, the time of microwaving and the vacuum impregnation on the mechanical properties of Chinese fir wood. When the moisture content of wood before microwave was approximately 50%-60%, after microwaving and impregnation, the ultimate strength in static bending (modulus of rupture [MOR]) and strength in compression perpendicular to the grain (SCPG) of the wood increased significantly. A microwave time of 100 seconds was more effective at improving the MOR and SCPG of the wood. If the wood was microwaved for too short or long period of time, the microwave pretreatment was not effective. When the samples were immersed in shellac for a longer period, the MOR and SCPG of Chinese fir gradually increased, but when the wood was impregnated for more than 12 hours, the increases were not significant. After the shellac penetrated the Chinese fir wood, it spread on the inner wall surface of tracheid to form a shellac film and easily formed plug-like deposits in microcapillaries. The use of Fourier-transform infrared spectroscopy, scanning electron microscopy and scanning electron microscopy-energy dispersion x-ray indicated that the microwave pretreatment can destroy the pit membranes on tracheids and facilitate the ability of shellac to penetrate the channel.