Improving gas permeability and characterizing the multi-scale pore size distribution of radiata pine (Pinus radiata D. Don) treated via high-intensity microwave

The effects of different microwave energy densities (0, 60, 80, and 100 kWh/m³) on the gas permeability of sapwood and heartwood of radiata pine in the longitudinal, radial, and tangential directions were investigated. The multi-scale pore size distribution was systematically analyzed with various analytical methods. High intensity microwave (HIMW) treatment significantly improved the gas permeability of the wood in three directions, and the gas permeability was positively correlated with the microwave energy density. The gas permeability of the treated sapwood was 1.6–2.5 times higher, and that of the treated heartwood was more than 6 times higher than the untreated wood at the microwave energy density of 100 kWh/m³. The HIMW treatment significantly affected the pore size distribution. In the micropore–mesopore (< 50 nm) range, the surface area and average pore diameter of the treated sapwood samples were 10.68% and 17.12% higher, respectively, and the total pore volume was 31.50% lower than that of the untreated sample. However, the specific surface area and total pore volume of the heartwood samples were 14.92% and 26.06% lower, respectively, and the average pore diameter was 35.55% higher at the microwave energy density of 100 kWh/m³. Treated wood mainly had slit-shaped mesopores (< 10 nm). For macropores (> 50 nm), treated sapwood displayed 20.57% total pore volume increase and 11.66% porosity rise; treated heartwood showed 31.24% total pore volume rise and 20.78% porosity increment. Furthermore, SEM results showed HIMW treatment damaged weak structures like the compound middle lamella and ray parenchyma cells.