Effect of heat treatment on the weathering and hardness properties of some wood species

Abstract

Thermal modification is known as a potential method to improve dimensional stabilization of wood and enhance its biological resistance. Scots pine (Pinus sylvestris), spruce (Picea orientalis), ash (Fraxinus spp.), tali (Erythrophleum ivorense), and iroko (Chlorophora excelsa) timbers were exposed to heat treatment at at a temperature of 180 °C for 1.5 hr and at 210 °C for 2 hr based on Thermowood process in the presence of steam. The treated timbers were cut in to small pieces for color measurement as well as brinell hardness and accelerated weathering performance. The total color change was found similar in Scots pine, spruce and ash when the samples heat treated at 180 °C, whereas it was found highest I tali and lowest in iroko. The color change re increased by the increasing temperature up to 210 °C. The color change was affected by accelerated weathering depends on the wood species, heating temperature and duration of weathering. The increasing in temperature led to higher color change especially for heat treated Scots pine and spruce after weathering. Heat treated ash samples (180 °C and 210 °C) were more affected by weathering than other woods with regard to color change. There was no significant difference between Scots pine (1,72 N/mm2) and spruce (1,78 N/mm2) in relation to the brinell hardness of the untreated samples. However, the reduction in hardness was found higher in spruce than Scots pine during the heating process at 180 °C and 210 °C respectively. The hardness of iroko and tali was increased by thermal treatment about two times at 210 °C in comparison to their controls.