Starch Esters and their Properties for Use in Plastics

Starch-based plastics were first explored through chemical modification of starch. However, the resulting films were brittle at low humidity. To improve the properties, esters of starch or isolated amylose were prepared. Plasticized starch triacetate films had similar tensile properties to cellulose triacetate, but amylopectin esters or whole starch esters were weak and brittle. The high cost of production and lack of outstanding merits were reasons why starch esters didn’t become popular.

Amylose mixed esters were made with different radicals like formate, acetate, propionate, butyrate, or benzoate. Clear disks with a tensile strength of 7000 psi and elongations of 12% to 20% were produced. Formate esters were brittle and aged quickly, while longer substituent esters had better properties. Amylose triacetate fibers were similar to cellulose triacetate but with lower properties. The use of amylose acetates would depend on the cost-effectiveness of separating and processing amylose.

Sagar and Merrill prepared esters of high-amylose starch with butyrate, valerate, and hexanoate radicals. As the chain length increased, the melting points of neat and plasticized esters decreased, along with the glass transition temperature. During tensile testing, butyrate and hexanoate esters formed stable necking at low strain rates, while the valerate ester did not propagate. At higher strain rates, the butyrate ester displayed brittle behavior, and the hexanoate ester showed a stable neck during extension. They also claimed blends of these modified starches with unmodified starch.

Rivard et al. made starch esters with different levels of substitution and chain lengths. When the substitution level or chain length increased, the time needed for biodegradation decreased. When they plotted the results, they found that the point at which biodegradation decreased shifted to lower levels of substitution as chain length increased.

Researchers have developed blends of water-repellent starch esters and biodegradable polyesters. They used high-amylose starch with DS values of at least 1.5 to prepare the starch esters, and added plasticizers to make them thermoplastically processable. The resulting materials showed good stability at high humidity levels. Starch propionate (DS 2.4) was blended with polycaprolactone or poly(hydroxy butrate valerate) to create materials with properties similar to general purpose polystyrene. Similarly, starch propionate (DS 1.7) was blended with PHBV and extruded into flexible, translucent films.

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