Objective  In order to improve the use value of fast-growing wood Chinese fir, this study investigated the effects of citric acid pretreatment and thermal compression treatment on the compression set of Chinese fir, so as to improve its shortcomings such as low density, poor dimensional stability and poor mechanical properties.

  Method  In this study, Chinese fir pretreated with an aqueous solution of citric acid with varying mass fractions was evaluated at different hot-pressing temperature, then the set-recovery for water absorption and the set-recovery for moisture absorption of Chinese fir compressed wood were measured. Stress relaxation and Fourier infrared spectroscopy were used to investigate the deformation and fixation mechanism of Chinese fir compressed wood pretreated with citric acid, and physical and mechanical properties of Chinese fir compressed wood were characterized.

  Result  The set-recovery for water absorption of Chinese fir compressed wood decreased with the citric acid mass fractions and then increased slightly, while decreased with the hot-pressing temperature. The lowest set-recovery for water absorption and moisture absorption was 10.78% and 1.38%, respectively. This could be attributed to the fact that some components of wood were degraded by citric acid, inducing partial cleavage of connections among Chinese fir components, therefore the internal stress of Chinese fir compressed wood was released. In addition, citric acid may react with a small amount of hydroxyl of Chinese fir to form a cross-linked network. The polymer formed by the esterification of citric acid filled in the intercellular space of cell wall during the process of compression. At higher temperature, the polymer was soft and flexible, which promoted the release of the internal stress and the fixation of Chinese fir compressed wood. The density of Chinese fir compressed wood increased with the compression ratios, and reached a maximum value of 0.717 g/cm³ when the compression ratio was 50%. Compared with the control, its density, bending strength, flexural elastic modulus and the hardness of compressed wood increased by 94.8%, 70.6%, 278.2% and 52.5%, respectively.

  Conclusion  The deformation of Chinese fir compressed wood is fixed effectively by citric acid pretreatment, and the physical and mechanical properties of Chinese fir compressed wood are improved significantly. The results of this study provide theoretical support and technical reference for adding high-value to application of fast-growing wood.