Thermal and mechanical properties of environment-friendly 'green' plastics from stearic acid modified-soy protein isolate

Most plastics, at present, are petroleum-based and do not degrade over many decades under normal environmental conditions. As a result, efforts towards developing environment-friendly and biodegradable 'green' plastics for various commercial applications have gained significant momentum in recent years. Soy protein isolate (SPI)-based 'green' plastics have been shown to suffer from high moisture sensitivity and low strength. These properties have limited their use in most commercial applications. They are also difficult to process into sheets without any plasticizer. The commonly used plasticizer, glycerol, tends to leach out over time producing time-dependent properties, which is highly undesirable for commercial applications. The objectives of the current research are to reduce the moisture sensitivity and simultaneously improve the tensile properties of SPI by incorporation of stearic acid without affecting its biodegradability. The effect of stearic acid and glycerol on the tensile and thermal properties of SPI has been characterized using various techniques to determine the interaction mechanisms between stearic acid and soy protein. Mechanical properties were characterized using Instron tensile tester. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, differential scanning calorimetry (DSC), thermo-gravimetric analysis (TGA) and X-ray diffraction (XRD) techniques have been used to determine the effects of stearic acid and glycerol on the surface chemistry, thermal transitions and thermal degradation of the stearic acid modified SPI plastic (resin). The tensile test results show that Young's modulus increased on increasing the stearic acid content, reaching the maximum value at about 25% (by weight of SPI powder) stearic acid. Further increase in stearic acid content from 25 to 30% led to a reduction in Young's modulus. The moisture content, fracture stress, strain, and energy at break decreased steadily on increasing the stearic acid from 0 to 30% for SPI containing 30% glycerol. At 25% stearic acid content, the modulus and the fracture stress increased significantly, whereas the fracture strain, energy at break and the moisture content decreased on reducing glycerol content. Scanning electron microscopy photomicrographs of fractured surfaces showed a layered structure for stearic acid modified-SPI resin. TGA measurements showed that the thermal degradation of stearic acid modified-SPI resin initiated at higher temperature than the SPI resin. DSC scans indicated that stearic acid modified-SPI resin had a small degree of crystallinity, which was confirmed by X-ray diffraction patterns. Modifying SPI resin with stearic acid has been successful in obtaining better tensile and thermal properties as well as reduced moisture sensitivity without any processing problems.