Structure and properties of composites compression-molded from polyurethane prepolymer and various soy products

A series of protein composites was successfully prepared from 30 to 50 wt % polyurethane prepolymer (PUP) with soy dreg (SD), soy whole flour (SWF), and soy protein isolate (SPI), by a compression-molding process at 120 degreesC without addition of any plasticizer. The structure and properties of the sheets were characterized by Fourier transform infrared spectroscopy, wide-angle X-ray diffraction, scanning electron microscopy, differential scanning calorimetry, thermogravimetry analysis, dynamic mechanical analysis, tensile test, and biodegradability test. The results indicated that the -NCO groups in PUP reacted with -NH2, -NH-, and -OH groups in soy products to form a certain degree of grafting and cross-linking, showing a new glass transition temperature (T-g) at -32 to -25 degreesC, compared with raw materials. Moreover, the toughness, thermal stability, and water resistivity of the composite sheets significantly increased. By increasing PUP content, the elastomer materials blended PUP and soy protein could be obtained. The protein component in the soy products plays a role in enhancement of the adhesivity, processability, and biodegradability. In addition, with an increase of cellulose content in the system, the tensile strength and water resistivity of the composite sheets increased. The tensile strength, elongation at break, and water resistivity were 6.9 MPa, 100%, and 0.55 for SD-U50 sheet from SD with 50 wt % PUP, and 4.8 MPa, 140%, and 0.50 for SPI-U50 sheet from SPI with 50 wt % PUP, respectively. Therefore, composite materials could be prepared by controlling the content of PUP and changing the types of soy products to obtain desired properties.