Characterization of wood cell walls treated by high-intensity microwaves: Effects on physicochemical structures and micromechanical properties

High-intensity microwave treatment is an environmentally friendly and time-saving method with a great potential to generate a platform for wood product innovations by increasing the permeability and providing an excellent precondition for wood modification. This work aimed to evaluate the influence of high-intensity microwaves on the physicochemical structures and micromechanical properties of wood cell walls. At the same time, the different response behavior of earlywood and latewood cell walls to microwaves was determined separately. With the help of the optical microscopic, imaging FTIR microscopy, X-ray diffractometer and nanoindenter, the characteristic of the treated cell walls, such as morphology, chemical structures, crystalline structures, hardness and elastic modulus, was thoroughly investigated. The results showed that the physicochemical structures and properties of cell walls were significantly affected by the initial moisture content and microwave intensity with the earlywood being more sensitive to microwaves than latewood. The most damaged cell wall morphology occurred with the moisture content of 20% and microwave power density of 80 kWh/m3. Compared with cellulose and lignin, the hemicellulose showed much more considerable changes in the molecular structures. The crystallinity in microwave-treated samples fluctuated and increased by 15.5% under 60 kWh/m3 microwaves. The cell wall hardness and modulus could both be enhanced, the maximum increase for the former being 48.3% and 36.5% and for the latter being 50.5% and 33.3% in earlywood and latewood, respectively. The findings obtained from this study could help elucidate the mechanism and precisely manipulate the effect of high-intensity microwave treatment, and further extend the application of the wood microwave treatment.