Aged thermoplastic starch (TPS), plasticized with hydrophilic plasticizers such as glycerol, shows brittle property. It is critical to develop tough TPS without using hydrophilic plasticizers. In this work, polyurethane prepolymer (PUP) was synthesized and mixed reactively with TPS in an intensive mixer to prepare modified TPS. Structural and morphological analyses showed that polyurethane (PU) microparticles were formed in situ and dynamically cross-linked to the starch matrix through urethane linkages. The modified TPS without hydrophilic plasticizer become tough. The elastic polyol soft segments in PU played the role of impact modifier, improving the toughness of the modified TPS. Almost 100% of PU was cross-linked to starch, indicating high efficiency of the modification. Formation of multifunctional PU microparticles was essential to achieve the high reaction efficiency. The dynamically cross-linking modification is a novel, green, and efficient method for preparing tough TPS.
Corn gluten meal (CGM)/wood fiber composites, plasticized by glycerol, water and ethanol, were extruded into pellets, and then the pellets were compression-molded into sheets for evaluation of water resistance, thermal stability and morphology. Pellets were also injection-molded to prepare plant pots for developing low cost, biodegradable containers used in agriculture. Ethanol played an important role for preparing homogeneous pellets under a condition of low specific mechanical energy. Extrusion, compression and injection-molding showed a similar trend that melt viscosity increased with the increasing wood fiber content and with decreasing water content. The trend led to a decrease in melt fluidity. The acceptable injection molding temperature range is from 125 to 160 °C, depending on the water content and wood fiber content in the pellets. Flexural strength of molded sheets was improved by 10-30% wood fiber but reduced by 40-50% wood fiber, which was a result of a change in the breaking mechanism. Visual observations showed that fracture occurred in matrix for sheets with low fiber content but in the interface for high fiber content composite. The former breaking mechanism can improve composite strength but the latter cannot. Compression-molded sheets and injection-molded pots showed a medium water resistance, suggesting possible commercial potential.
Thermoplastic sheets prepared from soy protein isolate (SPI) with ethylene glycol (EG) as the
plasticizer were obtained by compression molding under a pressure of 15 MPa at 150 °C. The
effects of the glycol content on the structure and properties of thermoplastic SPI were investigated
by using infrared, X-ray diffraction, differential scanning calorimetry, scanning electron
microscopy, and tensile test. The results showed that, with increasing EG content, the tensile
strength (σb) and Young's modulus decreased and breaking elongation (εb) increased. The water
resistance of the thermoplastic sheet of SPI increased with an increase of the EG content and
was much higher than that of thermoplastic starch sheets or a cellulose film. Further
investigation were carried on the SPI sheet containing 50% EG, which displayed a maximum
water resistance in boiling water, good mechanical properties (σb = 4.23 MPa and εb = 220%)
caused by interaction of SPI with EG, and light transmittance of 82% at 800 nm owing to
interchain hydrogen bonds and novel crystal. Therefore, the thermoplastic materials from SPI
provided a potential application as film and materials in food package and medical fields.
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