|dc.description.abstract||The environmental menace associated with the existing eco-unfriendly conventional plastics prompted the exploration of natural polymers such as starch for the development of biodegradable plastics. These efforts have seen starch used in various ways, one of which is in the processing of thermoplastic starch (TPS). Thermoplastic starch (also known as plasticized starch) is the product of the interaction between starch and a plasticizer in the presence of thermomechanical energy. While starch blends with conventional plastics only yield products that biofragment, thermoplastic starch (TPS) offers a completely biodegradable option. However, it is limited in application due to its weak mechanical strength and poor moisture resistance. To this end, the objective of this study was to determine the effects of incorporating polycaprolactone (PCL) and flax fiber into glycerol-plasticized pea starch. The effects of processing moisture content on the physical properties of glycerol-plasticized pea starch were also evaluated. The physical properties investigated included morphology, tensile properties, moisture absorption, and thermal properties.
Accordingly, two thermoplastic pea starch mixtures containing 9.3 and 20% processing moisture contents were prepared while maintaining starch (pea starch) and glycerol in ratio 7:3 by weight (dry basis). Polycaprolactone was then compounded at 0, 10, 20, 30, and 40% by weight in the solid phase with the TPS mixtures to determine the effects of processing moisture content and PCL incorporation on the physical properties of glycerol-plasticized pea starch. This experiment was structured as a 2 x 5 factorial completely randomized design at 5% level of significance. Subsequently, PCL and flax fiber were compounded with the TPS mixture containing 20% processing moisture to determine the effects of PCL (0, 20, and 40% wt) and flax fiber (0, 5, 10, and 15% wt) incorporation on the physical properties of glycerol-plasticized pea starch. This was structured as a 3 x 4 factorial completely randomized design at 5% level of significance. All the samples were compressed at 140°C for 45 min under 25000-kg load. The compression-molded samples were characterized using scanning electron microscopy (SEM), tensile test, moisture absorption test, and differential scanning calorimetry (DSC) techniques.
The tensile fracture surfaces showed a moisture-induced fundamental morphological difference between the two TPSs. The TPS prepared at 20% processing moisture content revealed complete starch gelatinization, thus, exhibiting a rather continuous phase whereas the TPS prepared at 9.3% processing moisture content revealed instances of ungelatinized and partly gelatinized pea starch granules. Consequently, the tensile strength, yield strength, Young’s modulus, and elongation at break increased by 208.6, 602.6, 208.5, and 292.0%, respectively at 20% processing moisture content. The incorporation of PCL reduced the degree of starch gelatinization by interfering with moisture migration during compression molding due to its (PCL) hydrophobicity. At both processing moisture levels of 9.3 and 20%, PCL incorporation had significant impacts on the tensile properties of the plasticized pea starch. Flax fiber incorporation also increased the tensile strength, yield strength, and Young’s modulus while concomitantly reducing the elongation at break of the plasticized pea starch. In the TPS/PCL/flax fiber ternary composites, both PCL and flax fiber improved the tensile strength by acting as independent reinforcing materials as no PCL-fiber interfacial bonding was observed. Maximum tensile strength of 11.55 MPa was reached at 10% flax fiber and 40% PCL reinforcement. While the PCL-TPS interfacial interaction was poor, some degree of TPS-flax fiber interfacial bonding was noticed due to their chemical similarity.
TPS prepared at 20% moisture showed more moisture affinity than that prepared at 9.3% moisture. The moisture absorption of TPS dropped progressively with the addition of hydrophobic PCL. Fiber incorporation also reduced moisture absorption by the plasticized pea starch. PCL-fiber incorporation also yielded improved moisture resistance vis-à-vis pure TPS. Finally, the TPS processed at 9.3% moisture exhibited higher thermal stability than that processed at 20%. Individual components of the composites retained their respective thermal properties, thus, implying thermodynamic immiscibility.||en_US