Microwave assisted moulding of starch-based foams

Abstract

This thesis reports a fundamental study on microwave assisted moulding (MAM), a novel technology where expandable starch based pellets made from extrusion are expanded within a mould cavity into blocks using microwave heating. Foamability or degree of expansion of starch-based pellets during microwave heating was studied comprehensively in terms of: the variation of raw natural materials, the extrusion conditions, the additives and the preconditioning of pellets before foaming. The expansion behaviour, foamed cell structures and mechanical properties of expanded pellets were characterized together with the characterization of microstructure of the extrudate materials and physical properties. Characteristics in microwave assisted moulding of the expandable pellets were then studied in terms of: the interfacial bonding and fusion between foamed pellets, the achievement of uniform heating through a moulded block, the loading of pellets and mould filling during foaming. It has been found that the degree of free expansion during microwave heating of the starch-based pellets is dependent on the degree of cook of starch during extrusion, the better the distructurization of the native starch granules, the higher the foamability in microwave heating. The maximum expansion ratio achieved in this work is 14, corresponding a foam porosity of 93%. Hydrophilic additives such as PYA and glycerol have adverse effect on the foamability due to combination effects of the melting point of the materials, degree of cook of starch in the pellets and water molecular mobility during foaming. Nucleation agents such as talc powder can refine cell structure of the foams and enhance elastic modulus, strength and energy absorption during compression. Addition of salts enhances microwave heating rate, expansion ratio and plasticization of the foam. Foamed blocks can be made using the MAM technology by adequate mould material, pre-treatment of the pellets, and control of initial loading in the mould cavity. This study paved the way for further development of the MAM technology for moulding of starch-based foams.