Influence of plastificer selection on starch-based polymer properties

Abstract

Synthetic plastics have become widely used in everyday life and industry but, thus, have become one of the biggest factors in environmental pollution. One of the ways to reduce pollution is to replace synthetic plastics with their natural counterparts. The most common raw materials for the production of natural plastics are starch and cellulose. Starch bioplastics are interesting because they are easily modified and can be processed employing techniques and the same equipment that is used for synthetic plastics.
 The peculiarity of obtaining bioplastics from starch is that the original starch is a dry granular material that cannot be molded in its original form. Starch can be molded at elevated temperatures using plasticizers. When heated together with the plasticizer, starch loses its crystallinity. This happens in two stages. The first stage is characterized by swelling and the second involves gelatinization and destruction of the starch granules.
 The properties of starch biopolymers directly depend on the starch structure, production parameters and composition of the final additives used. Starches of different botanical origins have different "amylose-amylopectin" ratio, which influences the final characteristics of the product. Good film-forming ability is inherent in starch from ahipa, cassava and corn. Corn starch films contain more amylose and, therefore, have greater moisture resistance but lower modulus of elasticity. Starch with a higher amylose content has higher values of elongation and tensile strength but lower modulus of elasticity. This is due to different sensitivities of amylose and amylopectin to plasticizers. Amylose is less susceptible to the plasticizing effect than amylopectin, so plasticizer molecules interact more efficiently with starch containing more amylopectin. Therefore, a starch film with a higher amylopentin content has better flexibility and extensibility. Amylose influences the gas-protective properties of films. We compared the values of vapor permeability of films depending on different botanical origins and concluded that vapor permeability was higher for films with higher amylopectin content.
 The type and amount of plasticizer are important in the production of thermoplastic starch (TPS). The introduction of a plasticizer breaks down hydrogen bonds and reduces the glass transition temperature of starch. At elevated temperatures and under shear forces, the starch in the presence of plasticizers turns into a liquid fluid mass that can be fed to the processes of extrusion, injection molding or blowing. TPS prepared only in an aqueous medium has low mechanical properties. Ethylene glycol, sorbitol, sucrose, fructose, glucose, urea, amides, amino acids and others are often used as plasticizers. The addition of glycerin increases the plasticity of TPS. Fillers plasticized with glycerin absorb much more moisture from the air than films plasticized with sorbitol. This is because sorbitol has the same hydrophilicity and hydroscopicity as pure starch. TPSs containing higher molecular weight plasticizers are stronger and have a higher glass transition temperature but are more brittle.
 The mechanical properties of TPS are equally dependent on the botanical origin of the starch and plasticizer used. We determined that these values can differ up to 88 times for tensile strength, up to 25 times for tensile strain and up to 83.2 for the modulus of elasticity. The highest tensile strength and modulus of elasticity among the considered materials are achieved in rice starch, plasticized with sorbitol. Corn starch in the composition with glycerin and stearic acid has the lowest values. Starch films plasticized with sorbitol have higher tightness than films plasticized with glycerin. Urea, formamide and ethanolamine work better as plasticizers than glycerin, promoting stronger and more homogeneous films. The strength of hydrogen bonds is correlated in the following order: urea> formamide> acetamide> polyols.
 We believe that one of the most important issues in the technology of producing thermoplastic material based on starch is the correct selection of plasticizers, their concentrations, and mode parameters, which are necessary for the material to acquire the specified physical and mechanical characteristics.