Green Synthesis of Soy Protein Nanocomposites: Effects of Cross-Linking and Clay Nanoparticles on the Mechanical...

A green synthesis scheme was adopted for preparation of soy-protein-based clay nanocomposites, in which soy protein isolates (SPIs) were utilized as the biodegradable resin and clay nanoparticles (CNPs) were used as the nanoreinforcing phase. Cross-linking of the SPIs was realized through an aqueous reaction scheme with oxidized sugars (e.g., glucose and sucrose as the typical constituents of soy flours) as the cross-linkers. Toughening effects of the cross-linkers, process parameters, and CNPs on the mechanical properties (e.g., tensile strength, stiffness, strain at break, and toughness) of the resulting SPI-based clay nanocomposites were examined by micromechanical tensile testing. The cross-linking and toughening mechanisms of the SPI-based nanocomposites were evaluated by Fourier transform infrared spectroscopy, sol-gel and color characterization, scanning differential calorimetry, and transmission electron microscopy. Thermal stability of the cross-linked SPIs was evaluated by thermogravimetric analysis. Experimental results show that cross-linking can noticeably improve both the tensile strength and tensile modulus of the resulting SPI films, and a small quantity of CNPs can obviously alter the mechanical properties of the resulting clay nanocomposite films. The present study indicates that defatted soy flours can be directly utilized for developing low-cost, SPI-based nanocomposites without the need for external plasticizers, and the entire synthesis is completely green without involvement of any petroleum-based organic solvents, polymers, and metallic catalysts. Such biodegradable SPI-based green nanocomposites have the potential to substitute fossil-based plastics and polymer composites for use in various industrial products and house utilities.