Starch is the most abundant sustainable biodegradable material that can be obtained from renewable resources using biotechnological procedures. Its applications are diverse ranging from pharmaceutics, textiles to adhesives, and interestingly, as a plastic: either as a basic material or additive for natural or synthetic biodegradable plastics. For starch to be used as a plastic, it must be process-able using standard plastic technologies. Presently, many studies investigating thermoplastic modification of starch by mixing natural starch with plasticizers under elevated temperatures and certain shear stress are underway. Unfortunately, such treatment results in the loss of crystallinity and destruction of intra- and intermolecular hydrogen bonds thereby keeping the overall solid structure of native starch together. Preparing thermoplastic starch involves an inevitable step of plasticizer addition. Water is the most common plasticizer but the resultant thermoplastic starch exhibits poor mechanicals strength.
To this regard, source of starch and selection of plasticizer are crucial when considering the physical properties of thermoplastic starch yielded which are critical for all aspects of application. Therefore, there is need to find a suitable plasticizer or design an optimal plasticizer system to ensure that the resultant thermoplastic starch is of desirable tensile strength, preferred Young’s modulus of elasticity and of acceptable elongation at break.
Researchers led by Professor Ivan Chodák from the Polymer Institute of the Slovak Academy of Sciences in Slovakia investigated the effect of a mixture of two plasticizers, namely glycerol as commonly used additive for thermoplastic starch and urea, which apparently forms rather strong hydrogen bonds with starch on the ultimate properties of resulting thermoplastic starch. They hoped that their plasticizer mixture would help develop a thermoplastic of superior mechanical properties and high resistance towards moisture uptake. Their research work is now published in European Polymer Journal.
Briefly, the researchers obtained native corn starch Meritena 100 for use as the basic polymer and determined its water content. The team applied commercial plastizicers glycerol and urea. They then went ahead and prepared the plasticized starch where distilled water, plasticizers and dried starch were added in predetermined ratios. The prepared thermoplastics were subjected to mechanical properties tests where the tensile strength was determined. Eventually, rheological measurements and dynamic mechanical analysis were undertaken.
The authors noticed that the rheological measurements were critical and provided useful information on the exact temperatures when the starch mixture with plasticizer changed from dispersion to gel. They observed that the tensile strength was much higher for material plasticized with urea compared to starch with glycerol, urea-glycerol mixture resulted in modest increase but even at the ratio 2:1 the increase in tensile strength was modest and much lower in comparison with the sample plasticized using urea only. More so, the glass transition temperature of the starch plasticized with urea determined by dynamic mechanical analysis was seen to be much higher when compared to the material plasticized by glycerol.
Herein, an exemplary study on the in depth analysis of the potential of applying a mixture of two plasticizers: glycerol and urea, in the preparation of thermoplastic starch, as proposed by Ivan Chodák research team has been presented. Crucial observations have shown that this technique yields thermoplastic starch of superior and desirable qualities. Therefore, the urea-glycerol plasticizer mixture technique is viable and should be adopted for thermoplastic starch fabrication.
The dependence of storage modulus G’ on temperature for starch with plasticizers mixture given as % of glycerol in the mixture with urea at constant starch/plasticizers ratio being 100/60.
František Ivanič, Daniela Jochec-Mošková, Ivica Janigová, Ivan Chodák. Physical properties of starch plasticized by a mixture of plasticizers. European Polymer Journal 93 (2017) 843–849.
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