Adding gaseous ammonia with heat treatment to improve the mechanical properties of spruce wood

Abstract 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.