Self-healing green composites based on soy protein and microfibrillated cellulose

Self-healing soy protein isolate-microfibrillated cellulose (MFC-SPI) composites that incorporated poly(d,l-lactide-co-glycolide) microcapsules containing SPI (SPI-PLGA-MCs) as the healing agent were developed. SPI-PLGA-MCs prepared using a green solvent, ethyl acetate, had a protein loading of over 50%. Self-healing MFC-SPI (10 wt%) composites containing SPI-PLGA-MCs (15 wt%) had Young's modulus of about 970 MPa and strength of over 15 MPa whereas neat SPI resin had Young's modulus of 326 MPa and strength of about 8 MPa. The significantly higher tensile properties of composites compared to neat SPI resin was due to the inherent high tensile properties of MFC and excellent hydrogen bonding with SPI resin. Self-healing mechanism, i.e., healing agent (SPI) bridging the fracture surfaces of the microcracks, was observed through SEM imaging. Composites with no SPI-PLGA-MCs showed no self-healing whereas self-healing SPI composites showed 27% healing efficiency after 24 h healing. The self-healing efficiency was noticeably lower than 48% obtained earlier for self-healing SPI resin using the same SPI-PLGA-MCs. This was because the self-healing mechanism occurs only for the SPI resin component and not for the MFC fibrils. Self-healing of green composites can extend their useful life and make it easier for them to replace conventional composites derived from petroleum.