Mechanical and interfacial properties of ramie fiber-reinforced green composites with soy protein resins

Ramie fiber-reinforced green composites were fabricated with soy flour (SF), modified soy flour (MSF), and soy protein concentrate (SPC) as resins. Defatted soy flour (SF) was modified by a lab scale filtration system to improve its mechanical and interfacial adhesion properties through increasing the protein content and removing soluble sugars. Protein content of SF was increased from 53.1% to 67.5% by filtration. Tensile properties of modified soy flour (MSF) resin were characterized and compared to those of SF and SPC resins. Fracture stress and Young's modulus of MSF resins were 35.48 MPa and 1411.7 MPa, respectively, which were significantly higher compared to those (12.70 MPa and 379.30 MPa) of unmodified SF resin and very close to those (37.50 MPa and 1412.6MPa) obtained for SPC resin. Interfacial shear strength (IFSS) of single ramie fibers with MSF resin was 8.76 MPa which was about 40% higher than that obtained for SF resin but still 25% lower than that with SPC resin. Fracture stress and Young's modulus of ramie fiber-reinforced composites were 88.0 MPa and 2.94 GPa for SF resin, 103.8 MPa and 3.51 GPa for MSF resin, and 113.8 MPa and 3.59 GPa for SPC resin while the fracture strains for all composites were around 4.25%.