AbstractThe effect of time-dependent moisture absorption on the surface roughness in natural fiber/polypropylene composites after a hot-press molding was studied. The results showed that the moisture absorption in both bagasse and oil palm natural fiber composites correlated closely with time-dependent surface roughness in the composites. The surface roughness in all natural fiber composites increased with an increase of moisture absorption up to 50 d. The fibers absorbed moisture and swelled due to hydroxyl groups of celluloses in the natural fibers, which caused an increase in surface roughness. Time-dependent tests found that the surface roughness in long fiber composites was larger than that in short fiber composites due to inhomogeneous dispersion of long fibers in the vicinity of the surface of composites. The increase in surface roughness of oil palm composites was 55% that of the bagasse composites due to the lower temperature of thermal decomposition in oil palm fibers than in bagasse fibers. Thermal decomposition decreased the number of hydroxyl groups in fibers during heating and resulted in a decrease in moisture absorption in the palm fibers. Furthermore, the effect of the carbodiimide treatment on bagasse fibers was confirmed to reduce moisture absorption for both the fibers and the composites.