Measuring dimensional stability of thermally modified ayous and iroko wood: comparison of the three test methods

The dimensional stability and moisture content of wood are important physical properties that influence the end-use of wood. These properties can be greatly improved by thermal modification of wood. In this study, three different test methods are studied on thermally modified iroko (Milicia excelsa) and ayous (Triplochiton scleroxylon) wood to compare the methods of measuring dimensional stability behavior. Test methods include (1) repeated water-soaking until saturation, (2) test cycles of water-soaking and drying, and (3) exposure to relative humidity cycles in the air. Test methods revealed enhanced dimensional stability in thermally modified wood samples relative to the unmodified samples. However, the relative dimensional stability behavior varied between the methods. Water-soaking until maximum swelling (Method-1) showed differences in the swelling rate for each wood material. Thermally modified ayous wood swelled more slowly (similar to 44%) than thermally modified iroko wood. Interestingly, subsequent test methods (Methods 2 and 3) showed decreasing dimensional stability especially for modified iroko wood as the wetting-drying cycles increased and iroko wood was kept in high relative humidity such as 85%. The cracks in the cross-section were more in modified iroko samples than in the controls, whilst modified ayous wood had no visible cracks in the cycles of wetting and drying in Method-2. The anti-swelling efficiency of thermally modified ayous wood was found as similar to 45%, 30%, and 40-60% in Method 1, 2 and 3, respectively. The results of the three different methods clearly showed that thermal modification ensured good dimensional stability for ayous wood, whereas in iroko wood insufficient dimensional stability was obtained, especially in Methods 2 and 3. Thermal modification may not be a good option to ensure dimensional stability for iroko wood. For a complete understanding of dimensional stability in wood modification studies, long-term wetting and drying cycles and/or air humidity cycles are recommended especially for extractive-rich wood species.