The effect of steam pre-conditioning treatment on the dimensional stability, moisture response behaviour, and durability against white-rot fungus P. chrysosporium of acetylated Hevea brasiliensis and Mitragyna ciliata wood species were assessed. Defect-free specimens of both species from the top, middle, and base positions were selected, prepared according to ASTM D143-09 & ASTM D-2017 standards and acetylated following a catalyst-free method. The primary source of variation in this study is the steam-conditioning treatment of specimens at holding periods of 30 minutes (H1), 60 minutes (H2), and 90 minutes (H3) before acetylation modification. Acetylated unsteamed specimens were used as treatment controls (T-CONTROL), while neither steam-conditioned nor acetylated (N-CONTROL) specimens were maintained for comparison. The experiment was a 2×3×5 factorial experiment in Randomised Complete Block Design (RCBD). Results revealed that steam pre-conditioning treatment resulted in a significant weight loss for the H. brasiliensis and M. ciliata, respectively. Effect of steam pre-conditioning treatment measured on the anti-swelling efficiency, volumetric swelling and reduced anti-shrinkage efficiency (ASE) values after repeated cycles of water-soaking revealed significant (P < 0.05) improvements on the dimensional stability and moisture response behaviour of both species, respectively as a function of their weight percent gain (WPG). The decay resistance of the acetylated (steamed and unsteamed) H. brasiliensis and M. ciliata wood was significantly improved and are classified as “highly resistant” to decay caused by P. chrysosporium white-rot fungus with weight loss values >10 %. The results prove that steam pre-conditioning treatment improved reagent accessibility and the acetyl weight gain of acetylated H. brasiliensis and M. ciliata wood. Consequently, their durability, dimensional stability, and moisture response behaviour also improve.

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