Synthesis of starch-based bioplastics reinforced with lignocellulosic fibers / Yang Jianlei

Yang , Jianlei (2021) Synthesis of starch-based bioplastics reinforced with lignocellulosic fibers / Yang Jianlei. PhD thesis, Universiti Malaya.

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Abstract

Biodegradable starch-based bioplastics are environmentally friendly and have attracted considerable interest to replace the conventional petroleum-based plastics. As compared to synthetic plastics, starch-based bioplastics show low mechanical and water resistance properties that hamper their applications. Therefore, the current research work focused on synthesizing starch-based bioplastics with enhanced properties by incorporating lignocellulosic fiber reinforcements with crosslinking agents. In Malaysia, the abundant natural resources such as cassava starch, oil palm empty fruit bunch (EFB) fibers, and epoxidized palm oil (EPO) derived from palm oil enable us to synthesize these bioplastics. Lignin-containing lignocellulosic fibers were obtained by thermal and alkali treatments in order to improve their adhesion in the composites. The treatments removed spherical particles, fatty acid, and part of lignin or hemicellulose on the fiber surface. Thermal stability of fibers decreased after the treatment. The EFB fibers treated with 10 wt% NaOH (aq) at 180 °C demonstrated the best reinforcing effect on the physical properties of bioplastics. This was confirmed by more compact surface and higher thermal stability, higher tensile strength and increased water resistance as compared to the other bioplastics. These optimally treated fibers (TEFB) were applied to strengthen the bioplastics at varying loading levels. The contents of fibers had a positive effect on the tensile strength and water resistance of the bioplastics but reduced the melting behavior, thermal stability, and plasticization of the composites. Moreover, TEFB fibers above 10 wt% contents formed the aggregates in the bioplastics. The optimum addition level of fibers was 5 wt%. Due to the inherent hydrophilicity and possible incompatibility of starch and fibers, starch/TEFB-based bioplastics were further crosslinked by citric acid (CA), EPO or epoxidized soybean oil (ESO). Low contents of the crosslinkers promoted the compatibility between fibers and starch effectively and led to a significant enhancement of tensile strength, while higher contents of crosslinkers showed a negative effect. The tensile strength increased from 0.83 MPa of the control sample to 1.62, 3.92 and 5.42 MPa for the bioplastics with the incorporation of CA, EPO and ESO, respectively. EPO was selected as the effective modifier of the bioplastics because EPO was the most abundant commodity oil in Malaysia and the bioplastics with ESO were very brittle. However, the compatibility and reactivity between EPO and starch were low. Further, starch/TEFB-based bioplastics were modified with CA-EPO prepolymer (CEPO) using melt blending to improve the compatibility. As evidenced by the Fourier transform infrared spectroscopy, CEPO generated strong interactions with starch/fibers through CA-inspired esterification reaction. The compatibility between starch and CEPO was obviously improved. As a result, the blending of CEPO in the composites has contributed to a higher evolution of the tensile strength than EPO. The tensile strength of the control sample increased from 3.67 to 6.90 MPa after the addition of 0.75 wt% CEPO. The water sensitivity of the composites was also moderately reduced upon the addition of both oils. This study indicated that the starch-based bioplastics with superior tensile strength and biodegradability can be used to replace part of commercial low-density polyethylene for packaging application.

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