Effect of thermal treatment with water, H2SO4 and NaOH aqueous solution on color, cell wall and chemical structure of poplar wood

As an efficient method for wood modification, hydrothermal technology is employed to deposit various metal oxides on wood surface, aiming at fabricating multi-functional wood, such as dimensional stability, anti-UV, hydrophobic, decomposition of formaldehyde, etc1,2,3,4,5. Be different than traditional wood hydrothermal methods, recently technique is sealing wood with water or other chemical solvents into a Teflon-lined autoclave and heating at relatively low temperature from 80 °C to 120 °C1,2. One of the important reasons for selecting of this temperature is wood tissue decomposed drastically when over 160 °C hydrothermal treatment6. Although the relatively low temperature range was selected during the process of thermal treatment, the autogenous pressure in a small closed chamber make the liquid into the wood cell gaps or through the grooves, which may cause more cracks in the intercellular layer and the pits7,8. Combination of moisture with exposure to heat will be impact on the degradation of all components of wood9,10.To our knowledge, more attention focus on the synthesis of various ingredients on wood and its function while limited to know the effect of this treatment condition on wood itself.

On the other hand, the wood cell wall structure and wood color will be changed when facing various modification conditions, such as the heat treatment, at the acid or alkali liquid environment, and light or CO2 laser irradiation. Wood color is one of the important properties related to the wood utilization and modification. Several studies11,12,13,14,15,16 reported that the cleavage of chemical bonds in polymers causes the changes in chemical structures of lignin and hemicellulose, and the formation of new chromophore substances can be responsible for color changes of wood. In addition, the changes of wood color in hydrothermal at low temperature may be caused by moving out of extractives in wood. Ji et al.17 investigated dilute acid and dilute alkali pretreatment of Miscanthus × giganteus resulting in partial degradation of the low degree substitution xylose in the cell wall. The connection bond between lignin and hemicellulose was cleaved. As a result, the cell wall porosity and the permeability increased. Saito et al.7 suggested that the ultrastructure of the inner surface of the fiber cell walls was changed after the dilute acid, dilute alklic or hydrothermal pretreatment. Ma18 found that the fiber cell wall of Triploid Populus tomentosa was destroyed after the NaOH thermal treatment, but no effect on cell corner middle lamella (CCML). The chemical pretreatment combined with disc refiner mechanical method was also employed for reducing energy and increasing enzymatic digestibility of wood19,20. Although these studies focused on the changes of wood color, chemical constituents and conversion efficiency of woody tissue, the cell wall structure variations in wood block during the thermal with different liquid treatment is still unclear.

Based on the previous reports, it can be assumed that there may be structure damage and composition changes on cell walls during the reaction process of wood thermochemical modification, which probably are closely related with the treatment temperature and solutions. Furthermore, wood color will be changed after facing different thermochemical modifications. As an explorative experiment, we selected the concentration and temperature of acid and alkali treatment refer to some wood degradation publishes. Herein, the objectives of this study were: 1) to observe and quantify the changes of color on wood surface and microstructure of cell walls after the thermal treatment with water, dilute acid and alkali; and 2) to describe the changes of chemical structure on wood surface after those pretreatments. The results of this work not only can provide theoretical foundation in wood thermochemical modification but also can be used as a method for the wood surface coloring.