Effect of protein content in soy protein resins on their interfacial shear strength with ramie fibers

Interfacial shear strength (IFSS) between ramie fiber and soy protein-based resins was investigated using the microbond technique. Three commercially available soy products: soy flour (SF), soy protein concentrate (SPC) and soy protein isolate (SPI) were processed into resins and their IFSS with a single ramie fiber (20-40 μm diameter) was determined. The IFSS increased with the protein content in the resins. The IFSS with SPI which contains 90% protein showed the highest value of 25.7 MPa, while the IFSS with SF which contains 53% protein and 32% carbohydrate showed the lowest value of 9.5 MPa. Effect of sorbitol, used to plasticize the soy resins, on the IFSS was also determined. With increase in sorbitol content, the adhesion strength of ramie fiber to soy protein resins decreased. With 30% sorbitol (on soy protein wt basis), the IFSS with SPI resin decreased by 63% while the IFSS with SPC resin and with SF resin decreased by 56% and 48%, respectively. The results suggest that the low molecular weight carbohydrates present in SF and SPC resins also act as plasticizer. A scanning electron microscope (SEM) and an electron microprobe analyzer (EMPA) were used to characterize the debonded surfaces of the fibers and map the residual resins on the fiber surface after the microbond tests.