Cellular structure and mechanical properties of starch-based foamed blocks reinforced with natural fibers and produced by microwave heating

• Starch foamed blocks were produced by microwave heating of thermoformed sheets.
• The foams stiffness and strength are higher than in foams produced from pellets.
• The natural fillers reinforcement reduced the cell size and increase the strength.
• The toughness of these foams was increased by using barley straw fibers.
• The stiffness of the foams is higher than that of the solid precursors.

The cellular structure and mechanical properties in compression of starch-based foams filled with natural reinforcements, such as grape wastes, cardoon wastes and barley straw fibers, have been studied in this work. The foams were produced by a microwave foaming process in which water is the plasticizer and at the same time the blowing agent. The use of thermoformed sheets as solid precursors for foaming allowed the production of foamed blocks with cells elongated in the expansion direction and with better properties in terms of rigidity and strength than foams produced in previous works by microwave heating of pellets. Moreover, the natural reinforcements increased not only the rigidity and strength, but also the toughness of these foams. Finally, the modeling of the compressive modulus using scaling laws shows how the stabilization of the cellular structure by the drying of the polymer matrix increases the rigidity of the solid cell walls. The flexible solid thermoplastic starch based precursor turns while foaming into a rigid starch-based foam, which could be suitable either for structural applications, due to its high stiffness and strength, or for packaging due to its complete biodegradability under controlled conditions.

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