Ind. Eng. Chem. Res. 2001, 40, 1879-1883
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Properties and Structure of Soy Protein Isolate-Ethylene Glycol Sheets Obtained by Compression Molding Qiangxian Wu and Lina Zhang* Department of Chemistry, Wuhan University, Wuhan 430072, China
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 (b) 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 b ) 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. Introduction The market of petroleum-based plastic will be limited in applications because of its inevitable increase in price and pollution caused by nondegradability in the future.1,2 Hence, the research and development of biodegradable plastics from renewable resources including cellulose,3-5 starch,6,7 and protein8 have attracted much attention. Soy protein isolate (SPI) is abundant and relatively low cost and has been studied as a polymer from a materials science perspective.9-11 Thermoplastic SPI was paid attention to because thermomechanical processing techniques are a simple and effective way to make full biodegradable materials. The processing conditions of thermoplastic SPI including temperature, pressure, and heating time,11,12 plasticizers such as water11,13 and glycerin,14-16 and blends with starch17 have been widely studied. The processing techniques mainly include pressure molding11 and extrusion.18,19 Usually, using compression molding in a laboratory is convenient to obtain sheets for study. A rather weak water resistance is one of important reasons why wide applications of cellulose- and starchbased plastics were limited.20 To improve the water resistance of thermoplastic protein, blending SPI with polyphosphate fillers21 and improving its process conditions22 have been reported. The thermoplastic SPI is nontoxic and safe; hence, it provides a potential application as package materials in the food field. However, the water resistance for a thermoplastic sheet or film from SPI has been scarcely published. An understanding of the effect of a plasticizer such as ethylene glycol (EG) on the properties, especially its water resistance, of thermoplastic SPI is essential for the development of materials. In this work, a series of thermoplastic sheets from SPI with various EG contents were compression-molded from commercial SPI (CS). The effects of the content of EG plasticizer and the * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +86-27-87882661.
thermoplastic process on structure, morphology, mechanical properties, and water resistance were investigated and discussed. Experiment Materials. SPI with a protein of 91-95% (to dry basis) was purchased from Hubei Yunmeng Protein Technologies Co. in China, and vacuum-dried at 60 °C for 4 h to contain 2% water content. EG (supplied by Shanghai Chemical Co.) was analytical grade and was used without further treatment. Compression Molding. A heat press device was made by our laboratory, and the sketch map is shown in Figure 1. The press device was equipped with a mold, a pair of steel blocks, a jack with a pressure meter, and a thermoregulator. SPI was mixed with EG, and every premix was pestled in a mortar for 30 min, then sealed, and stored in a refrigerator at 5 °C for 2 days. Each sheet was molded according to the following procedure: 3 g of premix was placed into the mold and covered with a polished stainless steel plate at both sides. The temperature of the mold was controlled at 150 °C, and then the pressure was quickly increased from 0.5 to 15 MPa for 1 min. After compression molding, the mold was cooled below to 50 °C with a fan at a rate of 10 °C/min. A slight yellow and transparent sheet was released from the mold and stored in a desiccator. With change of the EG content such as 20, 30, 40, 50, 60, and 70 g/100 g of dry SPI, a series of thermoplastic sheets SPI (ES series) were prepared and coded as ES20, ES30, ES40, ES50, ES60, and ES70, respectively. The premix of ES50 coded as PES50 was used to study the effect of compression molding on its structure of thermoplastic SPI. Characterization. Infrared (IR) spectra of the samples were recorded with a FTIR spectrometer (Spectrum one, Perkin-Elmer Co.) using an ATR cell at a resolution of 4 cm-1. The X-ray diffraction (XRD) was measured with an X-ray diffractometer (D/MAX-1200, Rigaku Denki, Japan). The X-ray diffraction patterns
10.1021/ie000694k CCC: $20.00 © 2001 American Chemical Society Published on Web 03/17/2001
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Figure 2. Effect of the EG content on the tensile strength (σb) of ES sheets.
Figure 1. Sketch map of a heat press (a) and its mold (b).
with Cu KR radiation (λ ) 1.5405 × 10-10 m) at 40 kV and 30 mA were recorded in the range of 2θ ) 3-50°. The degree of crystallinity (χc) was calculated according to the usual method.23 The sections of sheets were observed at room temperature by a scanning electron microscope (SEM S-570, Hitachi). The sheets were frozen under liquid nitrogen to fracture and coated with gold under 0.1 τ vacuum conditions before the SEM experiment. The differential scanning calorimetry (DSC) analysis for the sheets was performed with a thermal analyzer (DSC-204, Netzsch Co.) under a nitrogen atmosphere. The glass transition temperatures (Tg) were determined by heating samples (10 mg) from 20 to 150 °C at a rate of 20 °C/min, followed by cooling to -150 °C at a rate of 50 °C/min, and rescanning at a rate of 20 °C/min to 100 °C. Measurement of Mechanical Properties. The mechanical properties of the samples were measured on a universal testing machine (CMT6503, Shenzhen SANS Test Machine Co. Ltd.) according to ISO62391986 (E) with a tensile speed of 5 mm/min to obtain the tensile strength (σb) and breaking elongation (b). The dimension of the sheets was 100 × 10 × 0.08 mm3. The rigid sheet ES20 was heated to 60-80 °C on a heated board to obtain a flexible one, and then the required dimension was prepared by cutting, to avoid fragmentation of the sheet. Every sample above was measured at least three times to obtain an average value. To study their water resistance, the sheets were immersed in water at 5 °C for 24 h.3 The water resistance (Rσ) of the sheets was evaluated from a σb(dry) value in the dry state and a σb(wet) value in the wet state by the following ratio:
Rσ ) σb(wet)/σb(dry)
(1)
In addition, the sheets were also immersed in boiling water (100 °C) for 10 min to further determine their
Figure 3. Effect of EG on the breaking elongation (b) and Young’s modules (E) of ES sheets.
water resistance. Corresponding values of Rσ were calculated as in eq 1. Results and Discussion Effect of the EG Content on Mechanical Properties. The EG content dependences of the tensile strength (σb) and of Young’s modulus (E) and breaking elongation (b) of the thermoplastic SPI are shown in Figures 2 and 3, respectively. With an increase of the EG content, the tensile strength first showed a maximum of 18.5 MPa for the sheet containing 20% EG, then decreased dramatically to 3.8 MPa for an EG content of 40%, and finally almost remained steady. This also was expressed in Young’s modulus. However, the breaking elongation increased sharply from 20% to 166% in the range from 20% to 40% of the EG content and then showed high values in the range from 175% to 230% with a further increase of EG. These results indicated that thermoplastic SPI was flexible when the EG content was up to 40%. This is due to a number of groups, which could form interchain hydrogen binding in protein chains, was broken by EG molecules,21 and the mobility of the SPI chain was improved with an increase of the EG content. To understand the properties of the ES sheets after the diffusion and evaporation of EG molecules, the sheets were vacuum-dried at 2 mmHg and 25 °C for 12 h. The effect of the EG content on the value of σb, b, and E of vacuum-dried sheets is displayed in Figure 4. For all vacuum-dried specimens, values of σb and E were much higher than those of the originals, but the value of b was lower than that of the originals, indicating that thermoplastic SPI became stiff, owing to the decrease
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Figure 4. Effect of the EG content on the tensile strength (σb), breaking elongation (b), and Young’s modules (E) of vacuum-dried ES sheets.
Figure 6. Effect of the EG content on the water resistance of ES sheets at 5 °C for 24 h (2) and in boiling water for 5 min (9).
Figure 5. Residual EG content in the sheets ES20-70 after vacuum-drying at 25 °C and 2 mmHg for 12 h.
of the plasticizer in SPI. Interestingly, the sheet ES50 had a higher elongation than the others. It can be explained that a maximum interaction force between EG molecules and chains of SPI occurred, and EG molecules in this case were difficult to extract by vacuum-drying. The residual EG in the sheets ES2070 after vacuum-drying is shown in Figure 5. Compared with the original plasticizer, about 50-70% EG in original EG remained in the SPI sheets after vacuumdrying, and the residual EG remained steady to about 40% in dry SPI for ES50-70, indicating that a strong intermolecular force exists between the protein and EG. Effect of the EG Content on Water Resistance. The effect of the EG content on the water resistance (Rσ) of the ES sheets immersed in 5 and 100 °C water is shown in Figure 6. The average value of Rσ of the samples immersed in 5 °C for 24 h was 0.21 ( 0.07, and the maximal Rσ even reached 0.54 for the sheet containing 70% EG. As was reported, the Rσ value for thermoplastic starch immersed for only 1 h was 0.01,24 and it was 0.05 for cellophane (a regenerated cellulose film) immersed for 24 h, which were plasticized with water, and all were much lower than that of thermoplastic SPI. In addition, normal starch films speedily dissolved in water. Furthermore, the sheets immersed in boiling water for 5 min have yet to have a value Rσ of 0.12. Therefore, thermoplastic SPI exhibited relatively high water resistance among the main renewable resources, and EG plays a key role in the enhancement of water resistance and mechanical properties. This can be explained by the fact that first the high temperature
Figure 7. IR spectra of the sheets ES50 and PES50.
and pressure of the heat-compression-molding process provided to the protein molecules a much higher mobility, which helped to strengthen the interchain hydrogen bonding, and then the EG molecules with their two hydroxyl groups would have interacted (in competition against the -NH groups) with the carbonyl groups of two neighboring protein chains and created a type of “physical cross-linking” between protein chains. This kind of apparent cross-linking was difficult to destroy even by vacuum-drying a as was observed, and it helps to resist the water adsorption for the SPI sheets. Structure Analysis. The IR spectrum of sheet ES50 and its premix PES50 are shown in Figure 7. The peaks at 1084 and 1040 cm-1 assigned to the absorption of EG molecules and the absorptions at 1623 and 1535 cm-1 attributed to carbonyl and amide groups of SPI, respectively. Obviously, after compression molding, the absorption intensity of EG molecules in ES50 became weaker and that of carbonyl became stronger than those of the original PES50. This indicated that strong intermolecular hydrogen bonding occurred between carbonyl groups and N-H groups of SPI and hydroxyl groups in the sheet SPI, when the PES50 powder was heat-compressed into a ES50 sheet. The XRD patterns of sheets ES30, ES50, and ES70 and premix PES50 are shown in Figure 8. The crystal-
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Figure 10. DSC scanning curves for ES70, ES50, and ES20.
Figure 8. X-ray diffractograms of sheets PES50, ES30, ES50, and ES70.
Figure 9. Effect of the EG content on the degree of crystallinity of ES sheets: (a) 2θ ) 21.5-23.3°, (b) 2θ ) 19.6°, (c) 2θ ) 7.210.7°.
linity degree (χc) of ES50 was 0.53, much more than that of PES50 (0.32), suggesting that molecular chains of a thermoplastic sheet arranged more ordered than that of the SPI powder. The result implied that heatcompression molding promoted the crystallinity degree (χc) of sheet SPI. A new diffraction peak of the sheet containing EG appeared at 19.6°, whose χc value was 0.164, indicating that a kind of new crystal formed. This novel crystal, called the “A” type, has not been reported. This can be explained by th fact that a part of EG used, under the effects of heat-compressing, would have aligned by themselves to create a new type A crystal, which may have contributed to the yield of the relatively high breaking elongation of the ES sheets but not their tensile strength and Young’s modulus because of the remaining amorphous phase of EG. It is necessary to make a further study on the new crystal. From the X-ray diffractograms of sheets ES20-70, the crystallinity degree (χc) was obtained. The effect of the EG content on crystal type and χc of thermoplastic SPI is presented in Figure 9. All specimens show the same total crystallinity degree from 0.53 to 0.55, indicating that heatcompression had a consistent effect on the crystallinity. Generally, the proportion of amorphous block in the polymer would increase, after EG was mixed as a
plasticizer, resulting from the fact that the order of the macromolecules would be destroyed by plasticizers.25 However, in this case, the total crystallinity of thermoplastic SPI did not decrease with an increase of the EG content, implying that EG was not destructive to the arrangement of macromolecules. χc values for the sheets at 7-10° slightly decreased with an increase of the EG content (Figure 9c), owing to the destruction of the original crystal of the SPI. The crystal at 21.5-23.3° exhibited an increasing trend with an increase of EG (Figure 9a), and the sheet ES50 showed a maximum at 21.5-23.3°, indicating a dense architecture between molecules, which resulted in enhancement of the mechanical strength and water resistance, as shown in Figures 2 and 6. The values of χc for the novel crystal at 19.6° increased with an increase in the EG content, contributing to interchain hydrogen bonds formed from EG molecules with protein. The DSC thermograms of sheets ES70, ES50, and ES20 are presented in Figure 10. Because all samples were quenched by liquid nitrogen into amorphous materials, there were not any melting-endotherm peak in the curves. The sheets ES70 and ES50 showed a slight thermal transition in the range from -90 °C to -70 °C, corresponding to the Tg temperature of thermoplastic SPI, which has been proved with dynamic mechanical thermal analysis and DSC by Zhang et al.16 The results implied that the mobility of molecular chains increased with an increase of the EG content, which resulted in increasing the breaking elongation of the sheet ES as shown in Figure 3. The light transmittance of sheet ES50 with a thickness of 0.08 mm in the visible light range was measured to be 82%, implying an absence of phase separation. Figure 11 shows SEM of the cross sections of sheets ES50 and ES80, respectively. As is known, SPI is a kind of globular or granular protein.26 In this case, they both exhibited much smooth sections, indicating that the granular structure of SPI was broken up and formed a dense and homogeneous structure in the heat-compressing process. Conclusion Thermoplastic sheets SPI with EG as the plasticizer were successfully compression molded under a pressure of 15 MPa at 150 °C for 1 min. The transparent sheets have good tensile strength, breaking elongation, water resistance, and thermostability. Therefore, the SPI as
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Figure 11. SEM of the sections of sheets ES50 (A) and ES20 (B).
film or sheet materials can be used in food package and medical fields. When the EG content reached 50% (g/ 100 g of SPI), the sheet (ES50) exhibited relatively high tensile strength (4.23 MPa) and breaking elongation (220%), and its water resistance was higher than that of thermoplastic starch or cellulose film. Interestingly, the water resistance increased with an increase of the EG content, owing to the fact that interaction of EG with protein resulted in hydrogen bonding and novel crystal. SEM and X-ray analysis indicated that the heatcompressed sheets from SPI have a dense homogeneous structure, resulting in the transparency. Acknowledgment This work was supported by the National Natural Science Foundation of China (59773026 and 59933070) and the Laboratory of Cellulose and Lignocellulosics Chemistry, Guangzhou Institute of Chemistry, Chinese Academy Sciences. Literature Cited (1) Mo, X.; Sun, X. S.; Wang, Y. Effects of molding temperature and pressure on properties of soy protein polymers. J. Appl. Polym. Sci. 1999, 73 (13), 2595. (2) Huang, S. J. Polymer waste management-biodegradation, incineration, and recycling. J. Macromol. Sci., Pure Appl. Chem. 1995, A32 (4), 593. (3) Zhang, L.; Zhou, Q. Effect of molecular weight of nitrocellulose on structure and properties of polyurethane/nitrocellulose IPNs. J. Polym. Sci., Part B: Polym. Phys. 1999, 37, 1623. (4) Zhang, L.; Zhou, J. Solubility of cellulose in NaOH/Urea aqueous solution. Polym. J. 2000, 32, 866. (5) Hosokawa, J.; Nishiyarna, M.; Yoshihara, K.; Terabe, A. Reaction between chitosan and cellulose on biodegradable composite film formation. Ind. Eng. Chem. Res. 1991, 30, 788. (6) VanSoest, J. J. G.; Kortleve, P. M. The influence of maltodextrins on the structure and properties of compression-molded starch plastic sheets. J. Appl. Polym. Sci. 1999, 74 (9), 22. (7) VanSoest, J. J. G.; Benes, K.; De Wit, D. The influence of starch molecular mass on the properties of extruded thermoplastic starch. Polymer 1996, 37 (16), 3543. (8) Sun, X.; Kim, H.-R.; Mo, X. Plastic performance of soybean protein components. J. Am. Oil Chem. Soc. 1999, 76 (1), 117.
(9) Lin, Y.; Hsieh, F.; Huff, H. E. Physical, mechanical, and thermal properties of water-blown rigid polyurethane foam cotaining soy protein isolate. Cereal Chem. 1996, 73 (2), 189. (10) Lin, Y.; Hsieh, F.; Huff, H. E. Water-blown flexible polyurethane foam extended with biomass materials. J. Appl. Polym. Sci. 1997, 65, 695. (11) Paetau, I.; Chen, C. Z.; Jane, J.-L. Biodegradable plastic made from soybean products. 1. Effect of preparation and processing on mechanical properties and water absorption. Ind. Eng. Chem. Res. 1994, 33 (7), 1821. (12) Liang, F.; Wang, Y.; Sun, X. S. Curing process and mechanical properties of protein-based polymers. J. Polym. Eng. 1999, 19 (6), 383. (13) Cuq, B.; Gontard, N.; Guilbert, S. Thermoplastic properties of fish myofibrillar proteins: Application to biopackaging fabrication. Polymer 1997, 38 (16), 4071. (14) Wang, S.; Sue, H. J.; Jane, J. Effects of polyhydric alcohols on the mechanical properties of soy protein plastics. J. Macromol. Sci., Pure Appl. Chem. 1996, A33 (5), 557 (15) Wang, S.; Zhang, S.; Jane, J.; Sue, H.-J. Effects of polyols on mechanical properties of soy-protein plastics. Polym. Mater. Sci. Eng. 1995, 72 (2-7), 88. (16) Zhang, J.; Mungara, P.; Jane, J. Effects of plasticization and cross-linking on properties of soy protein-based plastics. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1998, 39 (2), 162. (17) Otaigbe, J. U.; Goel, H.; Babcock, T.; Jane, J. Processability and properties of biodegradable plastics made from agricultural biopolymers. J. Elast. Plast. 1999, 31 (1), 56. (18) Whitaker, J. R.; Tannenbarm, S. R. Food proteins; Avi Publishing Company, Inc.: Westport, CT, 1977; p 498. (19) Huang, H. C.; Hammond, E. G.; Reitmeier, C. A.; Myers, D. J. Properties of fibers produced from soy protein isolate by extrusion and wet-spinning. J. Am. Oil Chem. Soc. 1995, 72 (12), 1453. (20) Vikman, M.; Hulleman, S. H. D.; van der Zee, M.; Myllarinen, P.; Feil, H. Morphology and enzymatic degradation of thermoplastic starch-polycaprolactone blends. J. Appl. Polym. Sci. 1999, 74, 2594. (21) Otaigbe, J. U.; Adams, D. O. Bioabsorbable soy protein plastic composites: effect of polyphosphate fillers on water absorption and mechanical properties. J. Environ. Polym. Degrad. 1997, 5, 199. (22) Paetau, I.; Chen, C.-Z.; Jane, J.-L. Biodegradable plastic made from soybean products. 1. effect of preparation and processing on mechanical properties and water absorption. Ind. Eng. Chem. Res. 1994, 33 (7), 1821. (23) Rabek, J. F. Experimental Methods in Polymer Chemistry: Applications of wide-angle X-ray diffraction (WAXS) to the study of the structure of polymers; Wiley-Interscience: Chichester, U.K., 1980; p 505. (24) Wu, Q.; Zhang, L. Preparation and Characterization of thermoplastic Starch Mixed with Waterborne Polyurethane. Ind. Eng. Chem. Res. 2001, 40 (2), 558. (25) Larena, A.; Pena, M. C.; Pinto, G. Effect of glycerine and water on the crystallinity of regenerated cellulose obtained by the viscose method. J. Mater. Sci. Lett. 1994, 13 (17), 1235. (26) Bresnahan, D. P.; Wolf, J. C.; Thompson, D. R. Potential for utilizing 11S soy globular protein to study texture formation. J. Food Proc. Eng. 1982, 5 (2), 113.
Received for review July 31, 2000 Revised manuscript received November 3, 2000 Accepted January 30, 2001 IE000694K
