Effects of Thermo-Mechanical Pretreatments on the Mechanical Properties of Kenaf Fiber Bundles and Kenaf Fiber-Reinforced Polypropylene

This study investigates the effects of thermal treatment, high-humidity exposure, and tensile loading during heating on the mechanical properties of kenaf fiber bundles and kenaf fiber-reinforced polypropylene (kenaf-PP) composites. Kenaf fiber bundles were thermally treated at 140, 160, 180, and 200 °C for one hour, and their tensile properties were evaluated before and after exposure to a high-humidity environment. The crystallinity index (CrI) was determined by X-ray diffraction using Segal’s empirical method to investigate the relationship between cellulose crystallinity and the mechanical performance of the fibers. Heating at 140 °C increased the average tensile strength by approximately 11%, which coincided with the highest crystallinity index measured by X-ray diffraction, whereas higher treatment temperatures resulted in reduced mechanical performance, consistent with the onset of thermal degradation. The application of tensile loading during thermal treatment further enhanced the tensile strength of the kenaf fiber bundles, with the greatest improvement observed for fibers treated at 140 °C under a 48 N load. Microwave-assisted treatment (600 W) under tensile loading provided comparable improvements in tensile properties while reducing the treatment time from one hour to 60 s. Kenaf-PP composite sheets reinforced with thermally treated fibers exhibited enhanced flexural performance, with fibers treated at 140 °C producing the best overall results, including increases of approximately 15% in specific flexural strength and 31% in specific flexural modulus. These findings demonstrate that moderate thermal treatment, particularly when combined with tensile loading, is an effective strategy for improving the mechanical performance of kenaf fibers and kenaf-PP composites. Furthermore, microwave-assisted heating represents a promising alternative to conventional thermal treatment by significantly reducing processing time while maintaining the mechanical benefits.