Effect of aluminum sulfate impregnation heat joint treatment on color changes of Chinese fir

Changes in the extract composition and lignin structure are the main factors that affect color changes in wood during heat treatment. GC–MS analysis revealed that changes in the aromatic and ketone containing compounds in the extractives followed the same trend as changes in the ΔE* of the wood. 2D-HSQC NMR analysis showed that the lignin structure of heat-treated Chinese fir pretreated with aluminum sulfate changed significantly, and β-O-4 bonds were broken in the side chains. Additionally, condensation reactions occurred on the benzene ring, indicating that sulfate ion obviously affected the heat treatment of wood. Further analysis of lignin with XPS, UV–Vis, and FTIR showed that pretreatment with 0.2 mol/L aluminum sulfate produced more –C=O bonds in heat-treated Chinese fir, more hydroxyl groups at a dosage of 0.4 mol/L, and more ether bonds when the dosage reached 0.6 mol/L. These structural changes are thought to be related to the H+ produced by the hydrolysis and absorption-bridge reactions of aluminum ions in wood during heat treatment. At the same impregnation dosage, as the heat treatment temperature increased, the ΔE* gradually increased.