Abstract
Alkali treatment is used for melt extrusion film formation with corn starch, but optimal conditions for this procedure are still unknown. In this study, the changes in properties and structure of high amylose corn starch (70%) films with different concentrations of sodium hydroxide (NaOH), prepared by melting extrusion, were investigated. With increasing sodium hydroxide concentrations, the tensile strength of the high-amylose starch film decreased gradually, while the elongation at break increased. The tensile strength of the high amylose starch (HAS) film with 2% NaOH-treatment was 10.03 MPa and its elongation at break was 40%. A 2% NaOH-treatment promoted the orderly rearrangement of starch molecules and formed an Eh-type crystal structure, which enlarged the spacing of the single helix structure, increased the molecular mobility of the starch, and slowed down the process of recrystallization; a 10% NaOH-treatment oxidized the hydroxyl groups of the high amylose corn starch during extrusion, formed a poly-carbonyl structure, and initiated the degradation and cross-linking of starch molecule chains.
Highlights
Petroleum-based plastics have caused serious environmental pollution [1], and research into alternatives is needed
As the concentration of NaOH increased, the melting extrusion process caused the color of the thermoplastic high amylose starch (HAS) to gradually darken from a translucent yellow to a dark brown (Figure 1)
With 10% NaOH concentration was rough, and irregular aggregation of starches was observed. This treated with 10% NaOH concentration was rough, and irregular aggregation of starches was may be caused by the oxidation of starch molecules, molecular chain breaks, and random cross-linking observed
Summary
Petroleum-based plastics have caused serious environmental pollution [1], and research into alternatives is needed Starch, one such alternative, is renewable, biodegradable, non-toxic [2], and is suited for manufacturing thermoplastic film [3]. The dense molecular arrangement and linear molecular structure of HAS means it has higher thermal stability [5], higher gelatinization temperature, higher melting viscosity, in addition to a faster aging and recrystallization rate [6,7]. These properties make gelatinizing high amylose corn starch difficult, and limit its utility in the preparation of starch films.
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