Starch-based plastics use chemical reactions to chemically modify starch to reduce the hydroxyl group of starch and change its original structure, thereby changing the corresponding properties of starch and turning raw starch into thermoplastic starch.
Starch is a natural high-molecular polymer with a large number of hydroxyl groups in its molecule (molecular structure is Ⅱ and Ⅲ), so the strong interaction between starch macromolecules makes the original starch difficult to melt process, and it is coexisting with other polymers Compatibility with other polymers during blending is also poor. However, these hydroxyl groups can undergo chemical reactions such as esterification, etherification, grafting, and crosslinking. These chemical reactions are used to chemically modify the starch to reduce the hydroxyl group of the starch and change its original structure, thereby changing the corresponding properties of the starch and turning the original starch into a thermoplastic starch.
1. Esterification, transesterification or etherification reaction In order to change the hydrophilicity of the starch to hydrophobicity, the hydroxyl groups on the starch molecules and fatty acids, fatty acid esters, etc. undergo esterification and transesterification reactions to generate starch esters.
2. Cross-linking reaction
In order to improve the strength and heat resistance of the starch material, the starch is subjected to a crosslinking reaction with a compound having two or more functional groups (such as a polybasic acid, etc.) to make the starch moderately crosslinked.
3. Independent innovation of blending, modification and copolymerization Most starch plastics are prepared by blending starch with other high-molecular materials. The products obtained in this way have limited strength due to poor compatibility and limited use. Starch-based plastics also undergo starch copolymerization during starch modification to obtain graft copolymers of starch with other polymers, thereby improving the compatibility of starch with other polymer materials. Through chemical modification, three starch modifications were achieved: hydrophilicity was changed to hydrophobicity; heat sensitivity was changed to temperature resistance; hard and brittleness was changed to plasticity. This modification solves the worldwide problem of starch modification and lays a good foundation for subsequent processing.
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