Evaluation of hardness and surface quality of different wood species as function of heat treatment

• Hardness of the samples and their surface quality are function of heat treatment.
• An important impact on hardness and roughness is induced by the wood species itself.
• The best results may be obtained under typical heat treatment for each wood species.
• It seems that heat treatment would have potential to improve some wood properties.

The objective of this study was to evaluate the effect of heat treatment on surface roughness and hardness of four wood species, namely black alder (Alnus glutinosa L.), red oak (Quercus falcata Michx.), Southern pine (Pinus taeda L.) and yellow poplar (Liriodendron tulipifera). Samples were exposed to heat treatment schedules having two temperature and exposure levels of 120 °C and 190 °C for 3 and 6 h, respectively. Average hardness value of red oak samples exposed to a temperature of 190 °C for 6 h was 41.7% lower than that recorded before the heat treatment. Temperature of 190 °C produced 7.9% lower hardness values for black alder with the increased exposure time from 3 h to 6 h. No significant differences were found between same type of Southern pine and yellow poplar specimens before and after the heat treatment in terms of their hardness values. Among the four species considered in this study red oak having the most porous anatomical structure showed the roughest surface. An improvement in surface quality (Ra) with 7.46% with extending exposure time from 3 h to 6 h at the temperature level of 190 °C was noticed. However all four types of wood species kept in the oven at 190° for 6 h presented smoother surface quality. It was found that increased temperature from 120 °C to 190 °C for both exposure times showed significant differences from the surface quality of nontreated samples at 95% confidence level. The anatomical structure of samples was also observed by scanning electron microscope (SEM) and some damage of the cell wall was determined due to heat treatment. The findings of this study demonstrated that heat treatment resulted in adverse effect on hardness characteristics of the samples. It appears that strength losses can be limited through alternative modified heat treatment techniques. On the other hand, surface quality of the samples from all species was enhanced as a result of heat treatment. Therefore such heat treatment would be considered to improve surface quality of the sample for furniture applications where smooth surfaces are ideal adding potential value on wood material to be used more effectively in furniture manufacturing.