Characterization of nanocomposites based on plasticized starch and kenaf bast nanofibers

Plants cell walls are biological nanocomposites which exhibit high mechanical properties at a light weight. The secret of this rigidity and strength lies in its main structural component; cellulose. Native cellulose exists as highly-ordered microfibrils, which are just a few nanometers wide and have been found to be stiffer than many synthetic fibers. Many critical issues associated with global warming, environmental pollution and the amount of plastic wastes, synergistic with the scarcity and non-renewability of petroleum and petroleum-based products are driving material and composite scientists to focus on development of renewable and sustainable biomaterials. In this quest, using cellulose microfibrils from plant materials as renewable alternatives to conventional reinforcement materials such as glass fibers and carbon fibers is generating particular interest. Cellulosic nanofibers have the potential to reinforce and enhance properties of polymeric matrices, significantly, even at very low volume fractions. According to objectives of this research, by applying a chemi-mechanical approach both unbleached and bleached cellulose nanofibers were extracted from the cell wall of kenaf bast fibers and characterized. Then, the reinforcement performances of these two nanofillers were studied and compared using the starch as a matrix material, to identify the necessity of bleaching procedures in nanofiber isolation process. Subsequently, according to result, nanocomposite with different fiber loadings were prepared with unbleached nanofibers. Structural and mechanical performances of these materials were studied and compared with the composite prepared with micro scale cellulosic fibers from kenaf bast, unders the same conditions, to fully understand the effect of fibers loading and dimension on properties of fabricated materials. In order to undertake the research objectives, cellulosic fiber hierarchy from kenaf bast were isolated in three stages. Initially raw kenaf bast fibers were subjected to soda-AQ pulping process to eliminate lignin and hemicellulose. Unbleached fibers then undergone a bleaching process and finally both pulped and bleached fibers were mechanically separated into their constituent nanoscale cellulosic fibers. The influence of each treatment on the chemical composition, morphology, functional groups, crystallinity, and thermal stability of fibers was invest igated. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR/FTIR) showed that lignin and hemicellulose were almost entirely removed during the alkali and bleaching treatments. Morphological studies of fiber hierarchy were done by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). X-Ray diffraction (XRD) analysis revealed that the crystallinity of fibers increased from 62.9 to 81.5% as subsequent treatments were applied. Comparison with other works demonstrated the effectiveness and superior yield of 62.5% for unbleached nanofibers and 58% for bleached nanofibers, in the applied approach for producing nanocellulose from kenaf bast fibers. The isolated nanofibers displayed about 10% enhancement in thermal stability compare to that of raw kenaf bast fiber. The results of study on the properties of isolated bleached and unbleached nanofibers indicated the probable redundancy of the bleaching process as no significant difference was observed in their characteristics. The surface chemistry of the unbleached and bleached nanofibers is different and it has led to different behavior when these nanofibers are incorporated into polymeric matrices. In order to evaluate this issue both types of nanofibers were used to reinforce thermoplastic starch matrix and properties of obtained nanocomposite films were characterized using SEM, ATR/FTIR, XRD, thermogravimetric analysis (TGA), tensile and water uptake experiments. The results showed superior overall performance of nanocomposite films made from unbleached nanofibers. Thus the redundancy of bleaching procedures in nanofiber isolation process was cleared. In the subsequent work, unbleached nanofibers with different loading (0-10 wt%) were incorporated into thermoplastic starch matrix. Thin bionanocomposite films were made by casting and evaporating the mixture of aqueous suspension of nanofibers, starch and glycerol which underwent gelatinization process at the same time. In order to elucidate the importance and the effect of nanoscale cellulosic fibers on the performance of fabricated materials, starch based composite films with the exact same conditions were made by using micro sized pulverized kenaf bast fibers. The resulting composite and nanocomposite films were characterized by SEM, TEM, ATR/FTIR, XRD, TGA, scanning differential calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), tensile and water uptake experiments. Among tested materials, the nanoreinforced composites had significantly greater performance, particularly; increased mechanical strength and reduced water sensitivity. Tensile strength and Young‘s modulus increased by 313% and 343% and water uptake reduced by 21% with addition of 10% nanofibers. Overalls results demonstrated that applied nanofibers strongly interacted by hydrogen bonding with the starch matrix and showed a favourable compatibility which can be attributed to their chemical similarities.

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