Studies On Structures And Properties Of Soy Protein Plastics

The increasing price of petroleum and the ever-increasing pollution from non-degraded plastic waste have directly threatened human being's survival, health and development. Therefore, the natural renewable plant resources have the potential to be used as an alternative to petroleum and become one of the primary chemical materials in the 21st century, which has been one of the fronts of polymer science and material science. Soy protein, a by-product of the production of soy bean oil is a kind of widespread and abundant natural polymers. In the latest decade, developing biodegradable soy protein plastics has been a new topic in material science. However, the condensed structure for soy protein plastics lacks systemic understanding. Meanwhile, the water resistance of soy protein plastics is relative low, which limited their research, development and application. The main objectives of this thesis are to study the glass transition behaviour, microstructure and interaction of glycerol plasticized soy protein isolate (SPI) plastics, and to prepare SPI/montmorillonite (MMT), SPI/alumina hydrate (AlOH) and SPI/ hydroxypropyl alkaline lignin (HPL) nanocomposites via solution intercalation, in situ synthesis and solution blending methods, respectively. The interaction mechanism and the structure-properties correlation for these novel materials are clarified.The innovative achievements of this thesis are as follows. (1) For the first time, we suggested that there were two glass transition temperatures assigned to glycerol-rich and protein-rich domains in glycerol-plasticized SPI plastics. (2) It was clarified that the moisture obviously plasticized the glycerol-rich domains, leading to the occurrence of protein-water domains and the obvious changes of the material properties. (3) The protein-rich and glycerol-rich domains were of compact disk-like structure and loose chain-like structure, respectively. (4) We revealed that there were strong hydrogen bonding and electrostatic interaction between SPI and montmorillonite. These two interactions resulted in the high exfoliation of MMT layers and the significant improvements of mechanical properties and thermal stability. (5) A novel way to prepare the SPI/AlOH nanocomposites in aqueous media via in situ synthesis has been brought out, and it was revealed that the dispersion status of AlOH nanoparticles obviously influenced the material properties. (6) Theε-caprolactone (CL)/glycerol plasticized SPI plastics were successfully prepared through an extruding and compression molding process. It has been shown that grafting and crosslinking reactions occur among CL, glycerol and SPI.The main research contents and conclusions are divided into the following parts. Firstly, two glass transition temperatures (Tg1 and Tg2) of SPI plasticized with glycerol were clearly observed by differential scanning calorimetry (DSC). The results revealed that when glycerol content was in the range from 25 to 50 wt.-%, the obvious microphase separation occurred in the SPI/glycerol system. In the meanwhile, Tg1 at -28.5 ~ -65.2 oC and Tg2 at about 44 oC coexisted, assigned to glycerol-rich and protein-rich domains in the SPI sheets, respectively. The results from DSC with O-ring-sealed capsule and thermogravimetric analysis-Fourier transform infrared spectroscopy (TGA-FTIR) evidenced that the endothermal peaks at 100 ~ 120 oC in the DSC curves were assigned to the evaporation of the residual moisture, rather than denature of proteins. The values of gyration radii (Rg) of protein-rich domains estimated by small angle X-ray scattering (SAXS) were around 60 nm when the glycerol contents were higher than 25 wt%, indicating the amorphous protein-rich domains were composed of the compact protein chains. All of the results suggested that there were two kinds of protein chains with relatively high or low compatibility to glycerol in the SPI, leading to two glass transitions as a result of the existence of glycerol-rich and protein-rich domains.On the basis of the results from DSC with the aluminum pan and O-ring-sealed stainless steel capsule, there were three glass transition temperatures (Tg1, Tg2 and Tg3) at -12.7 ~ -83.8 oC, 65.8 ~ 53.1 oC and 3.7 ~ 1.5 oC in glycerol plasticized SPI plastic sheets at high relative humidity (RH). They were assigned to the glycerol-rich, protein-rich and protein-water domains, respectively. Tg3 of the protein-water domain occurred when the RH value was higher than 35%. Moreover, the absorbed water mainly dispersed in the glycerol-rich domain composed of loose protein chains with high glycerol content. With an increase of RH, the slow decrease of the Tg2 and Tg3 values as well as the relative stability of the Rg values indicated that the protein-rich and protein-water domains were relatively stable protein aggregates. The results from DMTA, TGA-FTIR and tensile testing further confirmed that the glycerol-plasticized soy protein sheets easily absorbed the moisture in atmosphere into the glycerol-rich and protein-water domains, leading to the obvious decrease of Tg1, the occurrence of Tg3 and the evident change of mechanical and thermal properties.The domain structure and interaction in the glycerol plasticized SPI plastics were carefully investigated using DSC, atomic force microscropy (AFM) and solid-state 13C nuclear magnetic resonance (13C NMR). The results from DSC indicated that there were exactly two glass transitions in SPI/glycerol systems at 1.5 ~ -47.6 oC and around 70 oC, assigned to glycerol-rich and protein-rich domains, respectively. The protein-rich domains were compact disk-like protein aggregates, and the glycerol-rich domains were loose chain-like structures. With an increase of the glycerol content in the protein plastics, the protein molecules were rapidly diluted, and the free spaces around the molecules are obviously enlarged. In the meanwhile, the protein-rich domains maintained their compact structures. The results from 13C NMR revealed that there was strong hydrogen bonding between glycerol and protein. Such bonding was significantly influenced by the amino acid residues. The protein molecular segments with non-polar residues had a low compatibility with glycerol, which formed protein-rich domains. Namely, the protein-rich domains were composed of the amino acids bearing long alkyl or aromatic groups, and the glycerol-rich consisted of those having polar or short non-polar groups.The biodegradable SPI/MMT plastics with highly exfoliated or intercalated structures were successfully prepared via a solution intercalation process in neutral aqueous medium. The structures of both the SPI/MMT nanocomposites powders and the plastics were strongly depended on the MMT content. When the MMT content was lower than 12 wt%, the MMT were highly exfoliated into single layers with a thickness of approximately 1~2 nm, whereas the intercalation structure predominated in the SPI/MMT nanocomposites when the MMT content was higher than 12 wt.%. The electrostatic surface potential calculation revealed that the heterogeneous distribution of the surface positive charges provided the possibility for negatively charged soy protein to intercalate and exfoliate MMT. In view of the results from theζ-potential measurement and Fourier transform infrared spectroscopy (FTIR), two kinds of interactions existed in this protein/MMT system, that is, the surface electrostatic interaction between the positive-charge-rich domains of soy protein and the negatively charged MMT layers as well as the hydrogen bonding between the–NH and Si–O groups. Such two interactions resulted in the intercalation and delamination of the MMT layers in the soy protein matrices. The mechanical strength and thermo-stability of the SPI/MMT plastics were significantly improved as a result of the fine dispersion of the MMT layers and the strong restriction effects on the interfaces.The SPI / AlOH nanocomposites were successfully prepared via an in situ reaction between AlCl3 and NH3?H2O in aqueous media. The hydrogen bonding interaction between the peptide bond and alumina hydrate played a key role in the high affinity between AlOH and SPI matrices as well as the homogeneous dispersion of nanoparticles. The structures and properties of the SPI/AlOH nanocomposites were strongly depended on the amount of AlCl3 addition. The results from TEM and tensile testing suggested that the local network-like dispersion of AlOH nanoparticles resulted in high transparence, good mechanical performance and elevated water resistance for the SA-8 sheets with the AlCl3 addition of 8 wt%. When the AlCl3 content was lower than 8 wt%, the alumina hydrate were homogenously dispersed in soy protein matrices with a dimension of about 10~50 nm, whereas the phase separation occurred in the nanocomposites when the AlCl3 addition was more than 8 wt%. Additionally, the increase of the glass transition andα-relaxation temperature evidenced the effective confinement of the protein molecules by the strong interfacial adhesion. Meanwhile, this kind of confinement significantly lowered the water uptake, and enhanced the tensile strength and modulus of the composite plastics even at high RH environments. Especially, the composite plastics still maintain relatively good biodegradability.The hydroxypropyl alkaline lignin (HPL) has been homogeneously dispersed in soy protein isolate (SPI) as nano-particle with glutaraldehyde (GA) as compatibilizer to improve the mechanical properties. The H-6 plastic sheets with 3.3 wt% GA and 6 wt% HPL exhibited the best mechanical properties in this case. The results from FTIR and X-ray diffraction (XRD) indicated that SPI/HPL composites were of amorphous network structure resulted from the physical crosslinking between HPL and SPI as well as the chemical crosslinking caused by GA. TEM micrographs demonstrated that the compaibilization of GA led a small dimension of the dispersed HPL particles to be about 50 nm in diameter. The cross section structures of the plastic sheets confirmed that there was a good interfacial adhesive in the SPI/HPL composites. The restricting effects of nano-scale HPL particles on the network structures in the SPI/HPL sheets are responsible for the increase of Tg from 62.5 to 70.4 oC when HPL content increase from 0 to 6 wt%. On the whole, the coexistence of the physical and chemical crosslinking networks significantly improved the mechanical and thermal properties of the SPI plastics.The SPI plastics with CL and glycerol as plasticizers were successfully prepared through reactive extruding and compression molding. The results of FTIR and SEM revealed that the high temperature and high shearing process could cause the occurrence of the grafting and crosslinking reactions among CL, glycerol and SPI, which was responsible for the good compatibility among each component. When the CL content was relatively low (< 25 wt%), the CL was mainly dispersed in the glycerol-rich domains and formed crosslinking with glycerol. When the CL content was relatively high (> 25 wt%), the dispersion of CL was mainly in protein-rich domains and grafted onto the protein chains. The crosslinking and grafting polymerization reactions built the network structures in soy protein matrices, leading to the increase of the glass transition andα-relaxation temperatures. Such chemical reactions enhanced the tensile strength, Young's modulus and water resistance. Furthermore, the thermal stability was elevated, and the evaporation of glycerol and the emission of NH3 and CO2 during heating were retarded.Biodegradable plasticses (GSD-3) were prepared from soy dreg (SD) with 25 wt% glycerol as the plasticizer and 6.8 wt% GA as the cross-linker under a pressure of 20 MPa at 120 oC. Compared with the sheets based on SPI, the GSD-3 sheets exhibited good tensile strength, breaking elongation and thermostability because of the strong interaction between cellulose, polysaccharide, and protein in the SD. Moreover, the GSD-3 sheets were fully biodegraded by the strains of Fusarium moniliforme, Chaetomium divaceum and Trichoderma viride. The water resistant SD plastics were prepared by coating the castor-oil-based polyurethane/nitrochitosan interpenetrating polymer networks (IPNs) on GSD-3 plastic sheets. The strong interfacial bonding between the GSD-3 sheet and the coating layer elevated the tensile strength and water resistance of the GSD-3 sheets.The achievements mentioned above not only brought out new viewpoints for the microstructure and interaction in soy protein plastics, but also provided convenient ways to prepare soy protein nanocomposites. And the correlation of structure and properties for the novel soy protein nanocomposite plastics were established. Additionally, a novel renewable recourse, soy dreg has been applied to prepare eco-friendly materials via green process. Therefore, this thesis possesses academic value and application perspective, and well accords with the strategy of sustainable development.