Mechanical and Thermal Properties of Sisal Fiber-Reinforced Green Composites with Soy Protein/Gelatin Resins

Natural sisal fiber-reinforced fully biodegradable, green composites were fabricated with soy protein resin modified with gelatin. Sisal fiber, modified soy protein resins, and composites were characterized for their mechanical and thermal properties. Fracture stress, fracture strain, and Young's modulus of the resin increased from 19.4 MPa, 2.4% and 1166 MPa for SG-0 (pure SPC) resin to 45.6 MPa, 8.9%, and 1370 MPa for SG-50 resin (50% gelatin), respectively. The effect of plasticizers (glycerol and sorbitol) on the tensile properties of the resins was also investigated. Composite I and composite II were fabricated with SG-0 resin and SG-20 (20% gelatin) resin, respectively. Tensile fracture stress and Young's modulus of composite II were 15.1% and 18.1% higher, respectively, compared to those of composite I. The improvement in flexural stress and flexural strain was even higher. Flexural stress and flexural modulus of composite II were higher by 31.6% and 61.9%, respectively, compared to those of composite I. These improvements in mechanical properties are due to the significantly higher resin properties as well as the improved fiber/resin bonding resulting from blending soy protein with gelatin. Higher fiber/resin interaction was confirmed by SEM photomicrographs of the composite fracture surfaces, which showed shorter fibers protruding for composite II and resin still adhering to the fractured fibers, while in the case of composite I longer fibers were observed at the fracture surfaces. Thermal degradation profiles of sisal fiber, SG-0 resin, SG-20 resin, and the composites were also investigated using a thermogravimetric analysis (TGA).