Enhancing the mechanical properties and hydrophobicity of heat-treated wood by migrating and relocating sulfonated lignin

Abstract 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 Na 2 SO 3 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.