The influence of thermal modification and surface charring to different levels on wettability of Norway spruce wood
J Žigon, C Gerardin, M E P Wålinder, P Gerardin
The wood can alternatively be protected from water absorption by thermal modification and charring. Both techniques cause certain chemical and physical changes in the wood, the extent of which depends on the level of temperature applied. The objective of this study was to determine how the combination of charring to different levels and thermal modification affects the chemical, morphological, and wettability properties of the Norway spruce wood. By charring the wood to four different levels, the carbonization began in the earlywood, continued in the latewood, and finally led to the formation of the cracked, in-depth surface char layer. The study of chemical properties showed that both thermal modification and charring of the wood resulted in dehydration of cellulose and hemicelluloses and promoted the presence of chemical units typical of lignin. The roughness of the natural and thermally modified wood surface increased with the increased level of charring. The surface wettability study showed that a higher level of charring gradually decreased the polar part of the surface free energy for both types of wood. Consequently, the contact angle formed between the wood surface and water increased from 70° to 87° for natural spruce wood and from 75° to 101° for thermally modified spruce wood when the degree of charring was increased. These results provide a good basis for further studies on the influence of the level of charring on the properties of wood surfaces, as well as for future research of this technique for protecting wood.
Keywords: charring, contact angle, roughness, spruce wood, surface, water