Effect of Recycling on Thermomechanical Properties of Zein and Soy Protein Isolate Bioplastics

Bioplastics are an alternative to reduce the environmental damage caused by petroleum-based plastics. However, the effect of primary recycling (reprocessing) of bioplastics from biomass resources has not yet been well studied. If successful, this would boost the landing of recyclable and biodegradable bio-based materials to the market. In order to meet the challenge of recycling bioplastics, it is necessary to study the reprocessing of bio-based materials that potentially behave as thermoplastics. This study investigated the primary recyclability of Zein- and soy protein isolate (SPI)-based bioplastics by reprocessing. Protein powders were initially mixed with glycerol (Gly), which acts as a plasticizer, and the blends were subjected to injection moulding. Initial specimens were reprocessed by injection moulding up to five times. The effect of reprocessing was evaluated by dynamic mechanical analysis (DMA), tensile test, and water uptake capacity (WUC). Finally, the property-structure relationship was assessed by scanning electron microscopy (SEM). The results showed that the recycled SPI-based bioplastics reduced elongation at break (i.e., eMax decreased from 0.8 to 0.3 mm/mm), whereas the parameters from tensile tests did not decrease upon recycling for Zein-based bioplastics (p < 0.05). The results obtained confirm that it is possible to reprocess protein-based bioplastics from two different renewable sources while maintaining the mechanical properties, although the loss of Gly was reflected in tensile tests and WUC. These results highlight the possibility of replacing petroleum-based plastics with bio-based materials that can be recycled, which reduces dependence on natural biopolymers and contributes to sustainable development.