Effect of starch structure on the processing, mechanical properties and biodegradability of thermoplastic starch films

Abstract

Thermoplastic starch (TPS) materials have great potential to replace some conventional synthetic plastics, and have the advantage of being economical, biodegradable, renewable, and can usually be processed using conventional plastic processing equipment. An important requirement is that TPS materials should have acceptable mechanical and biodegradability properties as a given functional material. The structure of TPS materials, at the molecular, crystalline and granular levels, may be altered during processing, which in turn affect their mechanical properties and biodegradability. This dissertation encompasses a detailed understanding of starch structural changes resulting from an archetypal processing procedure, and also examines its effects on mechanical properties and biodegradability. The effects of the thermal and mechanical energies of extrusion on the starch degradation at multiple structural levels were quantitatively investigated. Waxy (WMS), normal (NMS), and high-amylose maize starch (HAMS) with different amylose contents of 0, 34 and 63%, were extruded with varying temperatures, screw speeds, and plasticizer contents. The size distributions of individual branches did not show any significant change after extrusion. The whole amylopectin molecules were degraded into smaller sizes during extrusion while whole amylose molecules were not significantly affected. The crystalline and granular structures were disrupted during extrusion, without changing the crystalline polymorph displayed, suggesting that the crystalline structure remaining mainly originated from ungelatinized starch, which was confirmed by polarized light microscope images. Starch structural degradation was more severe at lower plasticizer content due to the greater amount of mechanical energy input at the same screw speed. Higher processing temperature (thermal energy) did not have any significant effect on the crystalline structure. The effects of mechanical and thermal energies on starch structural degradation were analyzed separately using Pearson correlation tests to compare the effect of the different parameters, showing that mechanical energy caused more significant degradation on starch structure than thermal energy. The starch extrudates obtained previously were compression-molded and the crystalline structure of NMS films was further altered using a hydrothermal treatment (HTT). The mechanical properties of starch films with various molecular and crystalline structures were investigated. For WMS, which contains only amylopectin, the degradation at the molecular level did not affect the mechanical properties significantly. HAMS films, with a higher amylose content and longer branches, showed higher elongation at break, and tensile strength than WMS and NMS films. The effects of amylose content on the mechanical properties were not significant when the plasticizer content was low, probably because the starch chains were restrained in a more rigid network. As distinct from previous studies reporting that an increase in crystallinity enhanced some mechanical properties, the present study found that the crystallinity of different films prior HTT was not significantly correlated with their mechanical properties, which might be due to these crystalline structure from the remaining ungelatinized starch granules unable to form a continuous network. On the other hand, the alteration of TPS crystalline structure by HTT increased the tensile strength and Young’s modulus, while decreased the elongation at break. The results indicate that the crystallinity from the remaining ungelatinized starch granules has less significant effects on the mechanical properties of TPS than the crystalline structure formed from starch retrogradation, probably due to the leached-out amylose forming a stronger network surrounding the remaining starch granules. The effects of starch structures on the biodegradability of TPS films were investigated by hydrolyzing starch films using fungal α-amylase. The substrates comprised varied starch structures obtained by different degrees of acid hydrolysis, different granular sizes using size fractionation, and different degrees of crystallinity by aging for different times (up to 14 days). Two stages are identified for unretrograded films by fitting degradation data using first-order kinetics. Starch films containing larger molecules were degraded faster, but the rate coefficient was independent of the granule size. Retrograded films were degraded much slower than unretrograded ones, with a similar rate coefficient to that in the second stage of unretrograded films. Although initially the smaller molecules or the easily accessible starch chains on the amorphous film surface were degraded faster, the more ordered structure (resistant starch) formed from retrogradation, either before or during enzymatic degradation, strongly inhibits film biodegradation. Starch structural changes induced by processing at different levels can be inter-related with one another; for example, amylopectin molecules present in the rigid semi-crystalline conformation in native starch granules undergo severe shear scission by mechanical energy during extrusion, decreasing the degree of crystallinity and destroying the granular structure. Crystalline structure from the continuous network in TPS materials is dominant in improving the mechanical properties and decreasing the degradation rate of TPS. Although the molecular size does not influence the mechanical properties, it has a great impact on the biodegradability of starch films.