The influence of starch molecular mass on the properties of extruded thermoplastic starch

Abstract

The mechanical properties of a low and a high molecular mass thermoplastic starch (TPS) were monitored at water contents in the range of 5–30% (w/w). The granular starches were plasticized by extrusion processing with glycerol and water. The low molecular mass starch was prepared by partial acid hydrolysis of potato starch. The extruded TPS materials were stored at 60% relative humidity for 12 months to level out differences in starch structure due to retrogradation. The water content was then varied by an additional storage period at various humidities. The average molecular masses of the TPS materials, composed of native starch or of hydrolysed starch, were 37 000 and 1900 kg mol−1, respectively. The apparent amylose contents of the high and low molecular mass materials were 25% and 11%, respectively. Differences were observed in thermal properties and crystallinity between the two types of materials, as a function of water content but not as a function of molecular mass. The stress-strain properties of the materials were dependent on the water content. The materials showed a viscoelastic behaviour characteristic of a semicrystalline polymer. Materials containing less than 9% water were glassy with an elastic modulus between 400 and 1000 MPa. For the materials a transition from brittle to ductile behaviour occurred at a water content in the range of 9–10%, which is in accordance with the observed glass transition temperature at this water content. The rubbery materials, with a water content of 9–15%, were tough and an optimum in ultimate elongation was observed. Above a water content of 15% the materials became weak and soft and the strain at break decreased. No significant differences in brittle-to-ductile transition as a function of water content were observed between the low and high molecular mass TPS materials. In the rubbery state with 14% water, the elongations at break of the high and low molecular mass materials were 100–125% and 30–50%, respectively. The tearing energy of the materials showed a maximum at a water content of 9–10%. The energies at this maximum of the high and low molecular mass materials were 0.15 and 0.1 J mm−2, respectively. The lower strain and tearing energy of the low molecular mass materials in the rubbery state were attributed to the reduced amylose chain length as well as the molecular mass and the degree of branching of the amylopectin molecules. This resulted in a material with a less effective entangled starch matrix. The entanglements were described as a complex network of the linear amylose chains and the outer chains of the amylopectin molecules in which hydrogen bonding plays an important role.