Degradation of the mechanical properties of heat-treated wood is a significant problem that needs to be addressed. This study aimed to stabilize the mechanical strength of heat-treated spruce wood by adding gaseous ammonia during the heat treatment. Gaseous ammonia penetrates rapidly into wood and is expected to form ammonium hydroxide when combined with water in the wood. This modification strategy neutralizes the acids produced by the degradation of hemicelluloses and reduces the degradation of the wood polymer composition and cell-wall structure. The preservation of wood polymer composition and cell-wall structure increases the indentation modulus of the wood cell walls. This increases the strength of the wood cell walls, resulting in an improvement in the mechanical properties of the heat-treated wood. The heat-treated wood’s dimensional stability and equilibrium moisture content are only slightly affected by the weak alkalinity modification.
Surface fractal dimension evaluates the internal surface complexity of pores in a wide range of materials. Unfortunately, the scale-dependent property of surface fractal in the pore structure of natural and heat-treated wood remains unclear. In this study, derived from the Frenkel-Halsey-Hill (FHH) fractal model and the Neimark fractal model, a comprehensive surface fractal analysis of the pore structure of natural and heat-treated wood was carried out based on nitrogen adsorption/desorption data. The results showed that two regions were identified as surface fractal, i.e., the pores with diameters less than 10 nm (Region 1) and the pores with diameters larger than 10 nm (Region 2). The scale-dependent property of two fractal regions was not affected by the different heat treatment atmospheres. The FHH and Neimark surface fractal dimensions of the pores in Region 1 were 2.079–2.155 and 2.780–2.940, respectively, and showed an obvious difference. The FHH and Neimark surface fractal dimensions of the pores in Region 2 were 2.481–2.536 and 2.413–2.551, respectively, and showed a slight difference. In addition, the FHH surface fractal dimensions of the pores in Region 2 had a positive relationship with the rate of early-stage moisture absorption. These findings are expected to evaluate the relationship between the transport properties and the pore structure in wood cell walls through the surface fractal dimension.
Fractal geometry describes the complex pore structure in natural and heat-treated wood and the relationship between pore structure and wood properties, such as strength, heat conductivity, and transport properties. However, the fractal types and the scale-dependent properties of natural and heat-treated wood remain unclear. In this study, comprehensive fractal analysis of the pore structure of natural and heat-treated spruce wood was carried out based on mercury intrusion porosimetry data. Both the volume fractal and surface fractal of natural and heat-treated wood were determined. The results showed that the two fractal types had different scale-dependent fractal properties. Four regions were identified in the pore structures. A volume fractal region was identified for pores in the region of 2–90 μm, while a surface fractal region was identified for pores in the region of 90 nm–7 μm. The pore structure in the region of 2–90 μm that corresponded to the large pore (the lumina in the cell) range showed strong volume fractal properties, and the fractal dimensions were 2.645–2.884. The pore structure in the region of 90 nm–7 μm that corresponded to the small pore (voids on or in cell walls) range showed strong surface fractal properties, and the fractal dimensions were 2.323–2.999. The range of fractal regions was hardly affected by the heat treatment atmospheres. These results showed that fractal geometry can be used to characterize the pore structures of natural and heat-treated wood. These findings are expected to explain the differences in properties between natural and heat-treated wood in the future.
Heat-treated wood (HTW) has better dimensional stability but worse mechanical strength than untreated wood. This study aimed to overcome this shortcoming by sulfonating lignin in Balfour spruce (Picea likiangensis var. balfouriana) wood with sulfurous acid and Na2SO3 followed by heat treatment. The mass loss of as-prepared HTW decreased while the crystallinity index increased slightly compared with those of HTW without sulfonation pretreatment. The cellulose structure of the as-prepared HTW was not damaged by the sulfonation pretreatment. The as-prepared HTW showed a higher MOE, MOR, and compressive strength (CS) of 34, 32, and 22%, respectively, compared with the HTW without sulfonation treatment. The improved mechanical properties were attributed to the increase of the relative mass fraction of lignin in the secondary walls of wood, as sulfonated lignin could migrate with water from the compound middle lamellae into the secondary wall under the combined driving forces of a concentration difference and steam pressure. These findings provide a way to enhance the mechanical properties of HTW while gaining better hydrophobicity.
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