UV-Curable Cellulose Nanofiber-Reinforced Soy Protein Resins for 3D Printing and Conventional Molding

Plastic wastes pose a serious threat to the environment and there has been strong interest in developing soy protein isolate (SPI)-based plastics to reduce the use of traditional petroleum-based ones. However, the use of SPI plastics has been hindered due to their inferior mechanical properties and poor water resistance compared with traditional plastics. In this work, SPI plastics with significantly improved mechanical properties and water resistance were developed using UV-curable SPI resin and UV-curable cellulose nanofibers. Three UV-curable materials, i.e., methacrylated SPI (MSPI), methacrylated cellulose nanofibrils (MCNF), and methacrylated cellulose nanocrystals (MCNC), were produced through methacrylation of SPI, CNF, and CNC, respectively. Methacrylation of the materials and UV curing of the resins were confirmed using Fourier-transform infrared spectroscopy. Uniform dispersion of CNC, CNF, MCNC, or MCNF in the resin was confirmed by scanning electron microscopy and X-ray diffraction. The tensile properties and water resistance of the resins were found to improve with increasing curing time and degree of methacrylation. The incorporation of CNC, CNF, MCNC, and MCNF further improved the mechanical and water resistance performance of the resins, with the MCNF leading to the largest improvement. UV curing and the cellulose nanofibers evidently worked synergistically to improve the properties of the resins. This study demonstrated a method and formulations to produce the SPI plastics with exceptional properties. Furthermore, the potential of this resin in 3D printing and conventional plastic molding was demonstrated.