Optimization of oil palm biomass superheated steam treatment for improvement of fiber characteristics and polypropylene biocomposite performance

Superheated steam (SHS) treatment is an effective method for hemicellulose removal from oil palm biomass (OPB) fiber, which leads to the surface modification of the fiber and hence improved the compatibility of the fiber with polymer matrix in biocomposite production. Current SHS treatment is conducted at temperature around 210 - 230ºC for 60 - 180 min. The long treatment time caused defect to the fiber, in which some portion of cellulose will be removed during SHS treatment, resulting in fiber with reduced crystallinity; and this condition would affect biocomposite mechanical properties. This study was hence conducted with the aim to optimize the SHS treatment temperature and retention time so that only hemicellulose will be removed and not cellulose, during the treatment. Three types of OPB fibers were used: oil palm mesocarp fiber (OPMF), oil palm empty fruit bunch (OPEFB), and oil palm frond (OPF). Optimization was done by conducting statistical analysis using a type of response surface methodology (RSM), i.e. central composite design (CCD). Hemicellulose content and cellulose content were used as the responses. Results showed that the optimal SHS treatment temperature and retention time to remove hemicellulose and retain cellulose in the fiber were 265°C/5 min, 280°C/5 min, and 300°C/9 min for OPMF, OPEFB, and OPF, respectively. The removal of hemicellulose at these temperatures was in the range of 60% to 70%, while the cellulose degradation was maintained below 5%. The results showed that SHS treatment time can be shortened to only 5 min to 9 min, which is 18 to 20 times shorter than previously reported. PP/OPB biocomposite was then produced by melt-blending using optimized SHS-treated OPB fibers at PP:OPB weight ratios of 30:70 and 50:50. PP/SHS-OPMF30 in this study had tensile strength and flexural modulus of 26 MPa and 1739 MPa respectively, which is 21 - 23% higher than those reported in the literature. Results obtained in this research showed that optimizing the SHS treatment conditions will not only making the treatment method favorable for large scale application due to reduced treatment time, but also improved the fiber and biocomposite properties. Of all the OPB tested, it is interesting to note that OPF biocomposites had the best mechanical properties, which is believed to be contributed by its high cellulosic content which in turn contributed to its higher crystallinity compared to the other two OPB. This result suggests the potential use of OPF as filler in biocomposite.

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