Effect of different polyols as plasticizers in soy based bioplastics

A suitable selection of the plasticizer in the formulation of plastic materials is a fundamental step to achieve the convenient physical and thermomechanical properties for each final application. Polyols have been extensively used as plasticizers in the production of many bioplastics, mostly glycerol. The present work assesses the influence of the presence of four polyols (glycerol (GLY), ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG)) on the properties of bioplastics based on a soy protein isolate (SPI), which is obtained as by-product of the soy oil industry. All bioplastics studied contained the same mass fraction (45 wt.%) of plasticizer, being injection moulded at the same processing conditions (120 degrees C, 600-700 bar). Glass transition temperature (T-g) was estimated for both the blends prior injection and the resulting bioplastics, observing generally a slight decrease in its value after the injection moulding. T-g values estimated from the peak in the loss tangent, tan delta, for bioplastics showed a clear dependence on the molecular weight of the plasticizer, being around -5, 20, 60 and 80 degrees C for the materials containing EG, DEG, GLY and TEG, respectively. As a consequence, different physical and mechanical properties were found at room temperature. In this sense, TEG-bioplastics were opaque, brittle, and also with a higher water uptake capacity, while EG-bioplastics were more ductile and translucent, absorbing much less water when immersed. Only GLY and TEG remained in the bioplastic after 9 days of storage at 50 degrees C, pointing out the volatility of EG and DEG causing a major ageing effect.