Optimization of Natural Rubber and PMMA Grafted Natural Rubber Contents in Soy Protein Isolate Film and Its Characterization

The growing demand for biodegradable alternatives to synthetic plastics has shifted attention to protein-based bioplastics. This study focused on developing and characterizing films made from soy protein isolate (SPI) blended with natural rubber (NR) and polymethyl methacrylate grafted natural rubber (PMMA-g-NR), with and without a curing agent (CA). The ideal curing time for NR was found to be 30 h at 60°C, resulting in films that are non-sticky and uniform. Visual assessments reveal that all films have semi-transparent, glossy surfaces, with minor color alterations from heat exposure. Fourier Transform Infrared (FTIR) analysis confirmed successful interactions between SPI and both additives, enhanced cross-linking, and network formation in the sample treated with the curing agent. UV-Visible spectroscopy demonstrated that PMMA-g-NR incorporated SPI films have better transparency than NR films, especially when cross-linked with a curing agent. NR-incorporated films clearly outperform PMMA-g-NR films in mechanical properties, highlighting NR's effectiveness in enhancing durability and performance. Water uptake studies revealed that NR increased the hydrophilicity of the films, whereas PMMA-g-NR with curing agent significantly reduced water absorption, i.e., PC25 showed the lowest water uptake (~70%). Additionally, Water Vapor Transmission Rate (WVTR) analysis indicated that the incorporation of additives (especially with curing agent) enhanced barrier properties, and NC25 and PC25 showed the lowest permeability. However, antibacterial assays showed no inhibitory effect against Escherechia coli and Listeria monocytogenes. In summary, the findings indicate that PMMA-g-NR, particularly with a curing agent, outperforms NR in improving the physical and barrier properties of SPI films. This makes them suitable as eco-friendly packaging options.