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Modifications of physical and mechanical characteristics induced by heat treatment: case study on Ayous wood (Triplochiton scleroxylon K. Schum)

Wood is a material of biological origin of fundamental importance for artisan and industrial uses. Its physical and mechanical properties make it suitable for the most diverse uses. In outdoor environments it is very attractive. However, being a material of biological origin, it is subject to degradation if exposed to the main degradation agents such as water, sunlight, temperature variations, biological attacks. To improve the durability of wood materials, preservatives are generally used, which can limit the effects caused by wood degradation agents. These treatments with toxic compounds to enhance natural durability have a high impact in terms of environmental pollution and human health, as well as significant costs. A valid alternative to chemical preservatives, which is increasingly established and widespread, is the thermal modification of the wood material. It consists in treating the wood at high temperature for exposure time depending on the desired objectives but limited to a few hours. The effects consist in the modification of the chemical structure of the wood, due to the degradation of hemicelluloses and cellulose, which results in a variation of the physical-chemical characteristics that make it more resistant to degradation.

The aim of this study is to evaluate the Ayous wood, (Triplochiton scleroxylon K. Schum), which was industrially subjected to thermal modification, in order to emphasize the influence of heat treatment on selected physical and mechanical characteristics. The thermally modification was conducted on planks of Ayous in an industrial system that used a slight initial vacuum in an autoclave (Maspell WDE Model TVS 6000) and a treatment temperature of 215 °C for three hours.

As a result of the heat treatment, the physical characteristics are generally reduced: the density in natural wood (TQ) was 379 kg/m3, in heat treated wood (TT) 319 kg/m3; the basal density in TQ was 327 kg/m3 in TT 299 kg/m3; shrinkage was significantly reduced in heat treated wood. Similarly, there was a general reduction in the mechanical properties of heat-treated wood compared to natural wood: the axial compression strength of heat-treated wood was reduced by 19%; the static bending strength of the heat-treated product was reduced by 41%. In addition, the samples, under the same environmental conditions in the laboratory, reached the equilibrium moisture content of 10% on TQ and 4% on TT. The Brinell hardness was reduced in TT. On the basis of the results of tests conducted on industrially heat treated wood, a decrease in the values of the selected physical and mechanical parameters was found, mainly due to deterioration of the structure and degradation of the cell wall compounds, evidence also confirmed by the equilibrium moisture content under laboratory conditions which was much lower than that of natural wood.

Keywords: heat treatment; physical characteristics; mechanical properties; Ayous wood; thermal modification
Comments on this paper
Ivan Moses Okuni
Application of heat treated Wood
Dear Authors, I wanted to find out the feasibility of using heat treated wood in manufacturing cross laminated timber for structural purposes. Can't heat treatment affect the structural applications of wood in wooden building construction?
Emiliano Gennari
Thank you for your question. I think any assortment is suitable when it complies with parameters for structural purposes; but generally the thermal modification induce the reduction of the main mechanical properties and some physical properties of the wood. This must be considered.