Objective  To address the problem of dimensional instability of wood due to dry shrinkage or wet rise when the ambient temperature and humidity change, we proposed to modify wood by vacuum pressure impregnation and high temperature curing using citric acid-sorbitol mixture solution as modifier. The effect of in-situ polymerization esterification of citric acid and sorbitol inside the wood on the dimensional stability of the wood was investigated to optimize the modification process.

  Method  Using Scots pine as raw material, the solution mass fraction, curing temperature and curing time as the factors, water absorption anti-swelling efficiency of the modified wood and the leachability as the response values, the Box-Behnken response surface design method in Design-Expert software was adopted. The mathematical model between the process and the properties of modified wood was established. The optimum process level of citric acid-sorbitol in-situ polymerization esterification of Scots pine was obtained by means of variance analysis of the significance and interaction of each factor, and the fitting equation was solved. On this basis, the changes of dimensional stability, micro morphology and chemical composition of unmodified and modified materials were compared and analyzed.

  Result  Curing temperature had an extremely significant effect on water absorption anti-swelling efficiency of the samples (P < 0.01), and the interaction between solution mass fraction and curing temperature had a significant effect on water absorption anti-swelling efficiency of the samples (P < 0.05). The effect of curing temperature on leachability was extremely significant (P < 0.01). According to the obtained response value, the secondary model of influencing factors and the actual situation, the process conditions were predicted and analyzed. The optimum process conditions were as follows: solution mass fraction was 30%, curing time was 16 h, curing temperature was 160 ℃, the water absorption anti-swelling efficiency value of the modified wood obtained was 58.47%. Scanning electron microscopy results showed that the polyester had swollen the cell wall and filled partof the cell lumen. Fourier transformation infrared spectroscopy showed the formation of ester bonds.

  Conclusion  The modification of wood by in-situ polymerization esterification using citric acid-sorbitol can significantly improve the water repellency and dimensional stability of wood. This modification method is a guide to improve the natural defects of wood and achieve efficient use of wood.