Thermoplastic starch composites and blends

The central theme of this thesis was the development of thermoplastic starch materials and composites, their characterization and analysis of mechanical properties. X-ray diffraction methodology was developed for the quantitative interpretation of molecular structures within starch materials, and their effects on retrogradation properties. An iterative smoothing process was used to estimate and subtract the amorphous scattering contribution from X-ray spectra and thus estimate the crystallinity of both granular starches and thermoplastic starch films. The background subtraction technique was used to monitor the retrogradation of a freshly produced high amylose starch film over a five day period. The X-ray diffraction areas considered to represent both the amorphous phase and crystalline phases were separated and independently analyzed for changes over time. There appeared to be some short range ordering of the ‘amorphous’ components that enhanced scattering intensity and peak shape indicating that some dynamic molecular re-arrangements were taking place within the amylose fraction as water was lost. The mechanical properties of films made from starches of various botanical origins and one industrial chemically hydroxypropylated starch was examined. Stress-strain and dynamic mechanical analysis indicated that the hydroxypropylated and potato starch create films with the lowest modulus, highest elongation and best overall film characteristics. A high amylose starch was modified using commercially available reactive dyes to try to create a film with reduced retrogradation and similar properties to a hydroxypropylated starch film. Commercial reactive dyes reduced film retrogradation and crystallinity and improved elongation and lowered film modulus. The chemical phosphorylation of three starches, hydroxypropylated, high amylose and potato starch was conducted, and the modified starches made into thermoplastic films and mechanical properties examined. Phosphorylation efficiency was found to be dependent on starch type and phosphate concentration used. Film modulus and elongation was reduced with increasing phosphorous content. The effect of low concentrations of glycerol, borax and a combination of both glycerol and borax on hydroxypropylated starch-PVOH film mechanical properties was examined. A combination of borax and glycerol together provided thermoplastic starch polymers with increased elongation, decreased modulus and enhanced creep recovery and tensile strength with respect to a control. Due to the molecular orientation observed in extruded starch films, the addition of a plasticiser or cross-linker did not have a homogeneous effect on mechanical properties; rather it affected properties in the machine and transverse directions differently. The reactive extrusion of TEOS and thermoplastic starch to produce starch–silica composites was achieved. Reaction efficiency was determined by X-ray florescence measurement of film Si content, and efficiency was observed to improve with increasing TEOS concentration. The base-catalysed hydrolysis of TEOS in situ produced well dispersed silicon dioxide particles and agglomerates that directly affected film mechanical properties. The composites produced had enhanced tensile strength but lower elongation and visco-elasticity. A wide variety of native starches, chemical modified starches and additives were trialed to create thermoplastic starch films and composites that featured a diverse range of mechanical properties.