Ind. Eng. Chem. Res. 2006, 45, 7491-7496
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Influence of Plasticizers on Thermal and Mechanical Properties and Morphology of Soy-Based Bioplastics Praveen Tummala,† Wanjun Liu,† Lawrence T. Drzal,† Amar K. Mohanty,‡ and Manjusri Misra*,† Composite Materials & Structures Center and School of Packaging, Michigan State UniVersity, East Lansing, Michigan 48824
Bioplastics from soy protein and biodegradable polyester amide with plasticizers including glycerol, D-sorbitol, and blends of these two have been made using extrusion and injection molding. The thermal properties, mechanical properties, and morphologies of the soy-based bioplastics were evaluated with a dynamic mechanical analyzer (DMA), a united testing system (UTS), and an environmental scanning electron microscope (ESEM). The influence of plasticizers on the physical properties of the soy-based bioplastics was determined. It was found that sorbitol-plasticized soy-based bioplastic (SSBP) had a higher tensile modulus and tensile strength than glycerol-plasticized soy-based bioplastic (GSBP). GSBP had the highest impact strength, whereas SSBP had the highest thermal stability. The mixed plasticized soy-based bioplastic (MSBP) that was obtained using a mixture of glycerol and D-sorbitol showed an intermediate range of tensile modulus and tensile strength values in comparison to those of SSBP and GSBP. The ESEM results suggested that SSBP had brittle fracture features, whereas GSBP had local ductile fracture features, which is consistent with the results obtained for the mechanical properties. The glass transition temperatures from DMA measurements of the soy protein and polyester amide in the soy-based bioplastics differed from those of the neat components. Using Fox equation calculations, this was interpreted as compatibility between the soy protein and polyester amide, which causes thermal and mechanical changes in different soy-based bioplastics. Introduction Soy proteins are generally regarded as one of the most important groups of natural biopolymers used to produce biodegradable materials, and their utilization will reduce the dependence and consumption of nonrenewable resources. Soy proteins are complex macromolecules containing 20 amino acids1 that supply available sites to interact with a plasticizer. Soy proteins can be converted to soy protein plastics through extrusion with a plasticizer or cross-linking agent.2-4 Common plasticizers used in the manufacture of soy protein plastics include glycerol, ethylene glycerol, propylene glycerol, 1,2butanediol, 1,3-butanediol, poly(ethylene glycol), sorghum wax, and sorbitol.3-6 Recently, Wang and Zhang7 used anionic waterborne polyurethane as a new plasticizer to prepare soy protein plastic that exhibited good mechanical strength, water resistance, and thermal stability. In addition, Zhong and Sun8,9 found that sodium dodecyl sulfate (SDS) and guanidine hydrochloride (GuHCL) acted as plasticizers for the soy protein 11S, hence leading to an improvement of its tensile strength, elongation, and water resistance. Although the mechanical properties of soy protein plastic can be controlled and optimized by adjusting processing parameters such as the molding temperature and pressure and the initial moisture content,10-12 the application of soy protein plastic is limited because of its low strength. Therefore, blending soy protein with biodegradable polyester is a way to form more effective soy-based bioplastics. Currently, the biodegradable polyesters being used in blends with soy plastics include polyester amide, polycaprolactone, and poly(tetramethylene * To whom correspondence should be addressed. Address: Composite Material & Structure Center, 2100 Engineering Building, Michigan State University, East Lansing, MI 48824. E-mail: misraman@ egr.msu.edu. Tel.: +1-517-353-5466. Fax: +1-517-432-1634. † Composite Materials & Structures Center. ‡ School of Packaging.
adipate-co-terephthalate),13-15 whose processing windows match that of soy protein plastic. Because soy protein and biodegradable polyester have different polarities, a compatibilizer is needed to reduce the domain size of the soy protein and increase the interfacial interactions between the soy protein and the biodegradable polyester. Zhong and Sun16 used methylene diphenyl diisocyanate (MDI) as a compatibilizer to produce blends of soy protein with poly(caprolactone) (PCL) because it could react with both soy protein and PCL. As a result, the mechanical properties and the water resistance of soy protein/PCL blends were enhanced. Mungara et al.17 used a poly(vinyllactam) (PVL) as a compatibilizer to blend soy protein and polyesters and observed that soy-proteinbased blends showed high tensile strength and modulus as well as low water absorption. John and Bhattacharya18 used maleic anhydride grafted polyesters as compatibilizers to enhance the compatibility of soy protein and biodegradable polyesters. As a result, the blended products showed enhanced mechanical properties as well as water and oil resistance. Natural polymer such as soy protein cannot form a plastic material without a plasticizer, but any plasticizer is expected to reduce the efficiency of the compatibilizer in the natural polymer and biodegradable polymer blends.19 Therefore, only plasticizers and no compatibilizers were used to prepare soy-based bioplastics in the present article. Soy protein is available in three different forms: soy flour (52% protein), soy concentrate (65% protein), and soy isolate (90% protein). The lower the protein content, the lower the cost. In the present article, soy flour was selected for study because it is very cost-effective and has properties comparable to those of the other forms of soy protein. The commercial biodegradable polyester used to blend with the soy flour was a polyester amide, a random copolymer of aliphatic polyester and Nylon-6 as shown in Chart 1, because of the potential compatibility between its amide groups and the soy protein plastic. To investigate the influence of different plasticizers on the thermal and mechanical
10.1021/ie060439l CCC: $33.50 © 2006 American Chemical Society Published on Web 10/04/2006
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Chart 1. Schematics of Glycerol, Sorbitol, Soy Protein, and Polyester Amide
Figure 1. Comparison of the tensile properties of (A) GSBP, (B) SSBP, and (C) MSBP. Table 1. Compositions and Heat Deflection Temperatures (HDTs) of Three Different Soy-Based Bioplastics
properties and morphology of the soy-based bioplastics, glycerol, sorbitol and a mixture of the two were used as plasticizers (as shown Chart 1). Experimental Section Materials. Soy flour (defatted soy flour no. 063-130) with 52% protein was obtained from Archer Daniels Midland Company (Decatur, IL). The other components of the flour included 30% carbohydrate (18% dietary fiber and 12% soluble carbohydrate), 9% moisture, 3% fat, and 6% ash. Glycerol and D-sorbitol were obtained from J.T. Baker (Phillipsburg, NJ) and Sigma-Aldrich (St. Louis, MO). The polyester amide (BAK 1095) was obtained from Bayer Corp. (Pittsburgh, PA). Extrusion of Soy-Based Bioplastic. All experimental materials except the plasticizer were predried in a vacuum oven at about 80 °C for at least 3 h to remove moisture. Soy flour was premixed with the plasticizer with a kitchen mixer in a 70:30 ratio by weight percentage. This mixture was fed to a ZSK 30 Werner & Pfleiderer co-rotating twin-screw extruder. The extruder was divided into six temperature zones, and the zone temperatures were maintained at 95, 105, 115, 125, 130, and 130 °C, respectively. The screw rotation speed on the extruder was maintained at 100 rpm, and the torque values ranged from 50% to 60% of full scale. The die used at the extruder was a two-strand die, and the diameter of the strands coming from the die was 2 mm. The strands of the extrudate were collected and chopped to form pellets using a pelletizer. This extruded mixture was blended in the extruder with the polyester amide in a weight ratio of 2:1. All zone temperatures were maintained at 130 °C during this blending because both soy protein and polyester amide are thermally stable at this temperature. Injection Molding of Soy-Based Bioplastics. The soy-flourbased biodegradable plastics (soy-based bioplastics) were injection molded into tensile coupons using an 85-ton CincinnatiMillacron injection molder with four temperature zones, the last one being the nozzle. The temperatures on all zones were maintained at 130 °C, and the screw rotation speed was constant
sample
soy flour (wt %)
glycerol (wt %)
sorbitol (wt %)
polyester amide (wt %)
HDT (°C)
SSBP GSBP MSBP
46.67 46.67 46.67
0 20 10
20 0 10
33.33 33.33 33.33
45 35 39
at 50 rpm. The mold temperature was maintained at around 15 °C. The materials were injection molded into standard tensile coupons and subsequently used for the evaluation of mechanical and thermal properties. Mechanical Property Measurements. The tensile properties and thermal properties of injection-molded soy-based bioplastics were measured with a United Testing System SFM-20 instrument according to standard method ASTM D638 and ASTM D790, respectively. System control and data analysis were performed using Datum software. The notched impact properties were measured with a Testing Machines Inc. 43-02-01 monitor/ impact machine according to standard method ASTM D256. Dynamic Mechanical Properties. A dynamic mechanical analyzer (2980 DMA, TA Instruments, New Castle, DE) was used to measure dynamic mechanical properties of the soy-based bioplastics. Tests were performed from -120 to 120 °C at a heating rate of 4 °C/min and a frequency of 1 Hz. Heat Deflection Temperature (HDT). The heat deflection temperatures (HDTs) of the soy-based bioplastics were measured with a dynamic mechanical analyzer according to standard method ASTM D648 with a load of 66 psi under a DMAcontrolled force and three-point bending modes. The heating rate was 2 °C/min. Thermogravimetric Analysis (TGA). A thermogravimetric analyzer (2950 TGA, TA Instruments, New Castle, DE) was used to measure the decomposition behavior of the soy-based bioplastics under nitrogen gas atmosphere. The weight of samples was 20 mg, and the tests were performed at a heating rate of 10 °C/min. The gas flow rates were 60 and 40 mL/min for sample purge and balance purge, respectively. Environmental Scanning Electron Microscopy (ESEM). The tensile and impact fracture surfaces of the soy-based bioplastics were observed by environmental scanning electron microscopy (ESEM) with a Phillips Electroscan 2020 instrument at an accelerating voltage of 20 kV. Results and Discussions Mechanical Properties. Figure 1 shows a comparison of the tensile properties of the three different soy-based bioplastics examined in this work. The compositions of the different soybased plastics are reported in Table 1. It was observed that sorbitol-plasticized soy-based bioplastic (SSBP) showed a higher
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Figure 2. Comparison of the stress-strain curves of (A) GSBP, (B) SSBP, and (C) MSBP. Figure 5. Dynamic mechanical properties of the soy protein and polyester amide.
Figure 3. Comparison of the flexural properties of (A) GSBP, (B) SSBP, and (C) MSBP.
Figure 6. Dynamic mechanical properties of (A) GSBP, (B) SSBP, and (C) MSBP.
Figure 4. Comparison of the impact strengths of (A) GSBP, (B) SSBP, and (C) MSBP.
tensile modulus and tensile strength than glycerol-plasticized soy-based bioplastic (GSBP), whereas MSBP exhibited a range of tensile modulus and tensile strength values intermediate to those of SSBP and GSBP. The tensile strengths of MSBP and SSBP were about 45% and 50%, respectively, higher than that of GSBP, and the tensile moduli of MSBP and SSBP increased by about 135% and 255%, respectively, compared to that of GSBP. Figure 2 shows the stress-strain plots of the soy-based bioplastics. GSBP exhibited higher elongation and lower stress, but SSBP showed higher stress and lower elongation. MSBP showed mild elongation and a tensile stress similar to that of SSBP. The flexural properties as shown in Figure 3 also follow the same trend as the tensile properties. The flexural strengths of MSBP and SSBP are about 70% and 160%, respectively, higher than that of GSBP, and the flexural moduli of MSBP and SSBP increased by about 100% and 235%, respectively, compared to that of GSBP. GSBP had the highest value of impact strength, with SSBP and MSBP exhibiting lower values (Figure 4). In summary, GSBP had higher elongation, higher impact strength, lower modulus, and lower strength, but SSBP
had lower elongation, lower impact strength, higher modulus, and higher strength. MSBP had values intermediate between these two. The differences among these soy-based bioplastics might be caused by the interactions between the plasticized soy protein and the polyester amide, as discussed in the next section. Thermal Properties. Heat deflection temperature refers to the maximum temperature at which a polymer can be used as a rigid material. Specifically, according to standard method ASTM D648, HDT is defined as the temperature at which the deflection of the sample reaches 250 µm under an applied load of 66 psi. The HDT values of the soy-based bioplastics examined in this work are listed in Table 1. It was found that SSBP plastic has the highest value of HDT (45 °C) among the three plastics. The HDT of a plastic reflects the transitions and relaxations of molecular segments of the plastic. As discussed below, the polyester amide phase in SSBP had a higher glass transition temperature because the polyester amide and soy protein penetrated each other, which led to the highest HDT. The storage modulus and loss factor of soy protein and polyester amide are shown as functions of temperature in Figure 5. It was found that soy protein had two main relaxation peaks in the loss factor plot. The peak at around 69 °C was the glass transition relaxation of the soy protein, and the other peak at around -50 °C was the β relaxation of the soy protein.20 For the polyester amide, there was sharp decrease in modulus in
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Table 2. Glass Transition Temperatures (Tg), Degrees of Inward Shifting of Tg, and Storage Moduli at 25 °C of Three Different Soy-Based Bioplastics
sample
Tg (°C) polyester soy amide protein
SSBP GSBP MSBP
29.0 0 10.0
degree of inward shifting of Tg (°C)
modulus at 25°C (GPa)
47.1 10.0 21.6
1.3 0.3 0.8
61.6 69.7 68.1
Table 3. Interactions between Soy Protein (SP) and Polyester Amid (PEA) in the Phase Structure of Soy-Based Bioplastics from the Fox Equation weight fraction in the PEArich phase (%)
weight fraction in the SP-rich phase (%)
weight fraction (%)
sample
PEA
SP
PEA
SP
PEA-rich phase
SP-rich phase
SSBP GSBP MSBP
44 84 69
56 16 31
7 1
93 100 99
70 40 47
30 60 53
the main transition region at around -10 °C that was attributed to the glass transition of the amorphous phase of the polyester amide. The results for the temperature dependence of the loss factor of the soy-based bioplastics are presented in Figure 6. A summary of the experimental investigations on the main relaxations is included in Table 2. It was clearly found that two relaxation peaks appeared for all of the soy-based bioplastics. The lower-temperature peak was attributed to the glass transition (Tg) of the polyester amide. The higher-temperature peak corresponded to the glass transition (Tg) of the soy protein. However, compared to Tg of the neat polyester amide, it was found that Tg of the polyester amide in the soy-based bioplastics shifted to higher temperature. This indicates that the soy protein and polyester amide are partially compatible, which might be due to the intramolecular interactions of the amino and carboxyl groups in the soy protein and the amide, carboxyl, and hydroxyl groups in the polyester amide. These interactions include hydrogen bonding and polar interactions. For GSBP, Tg of the polyester amide shifted to higher temperature, but the main transition of the soy protein did not change. This means that only the molecular segments of the soy protein can migrate into the domain of the polyester amide, but the molecular segments of the polyester amide cannot migrate into the domain of the soy protein. Therefore, the soy protein and polyester amide in GSBP had limited improvement in compatibility. However, for SSBP, Tg of the polyester amide shifted to higher temperature, and that of the soy protein shifted to lower temperature. This result reveals that the molecular segments of soy protein and polyester amide can migrate toward each other, indicating that the compatibility between soy protein and polyester amide in SSBP was further improved. The changes in Tg of MSBP followed the same trend as that of GSBP, but the degree of change in Tg of the polyester amide was greater for the former. The compatibility of the blends can be predicted from the glass transition temperature (Tg) of the blends, and the phase composition can be calculated according to the Fox equation21
1 Tg
)
W 1 W2 + Tg1 Tg2
(1)
where 1 and 2 represent the components and W is the weight fraction of the corresponding component. The calculation results of phase information related to the interactions between the soy protein and polyester amide are
Figure 7. Thermogravimetric analyses of (A) GSBP, (B) SSBP, and (C) MSBP.
reported in Table 3. It was found that there was about 56% soy protein in the polyester-amide-rich domain and about 7% polyester amide in the soy-protein-rich domain in SSBP. This result confirms the above statement about the compatibility and migration between soy protein and polyester amide. Also, this further points out the stronger interaction between the soy and polyester amide in SSBP. The enhancement in compatibility can be simply measured by the degree of inward shifting of the glass transition temperatures of the components. The improvement in compatibility in the soy-based bioplastics is reported in Table 2. It can be easily seen that the compatibility between the soy protein and polyester amide in SSBP was significantly improved and showed a broadened glass transition region, but that in GSBP was marginally improved, and that in MSBP was in the middle. This is consistent with the results in Table 3 from calculations using the Fox equation. The most likely reason for the significant improvement in compatibility is that sorbitol has more hydroxyl groups in its molecular structure. This results in a greater contribution to the interactions between sorbitol and soy protein and, hence, stronger hydrogen bonding between the sorbitolplasticized soy protein and the polyester amide. The compatibility between the soy protein and polyester amide determines the mechanical properties of the soy-based bioplastics. It is not surprising that SSBP had the best tensile and flexural properties among all of the bioplastics examined in this work. The loss factor provides damping and molecular mobility information, which is related to the impact strength of the plastic. Because of the stronger interaction between the soy protein and polyester amide in SSBP and the higher Tg, the molecular motion in this bioplastic is restricted. Therefore, this plastic cannot absorb more energy under an impact load, resulting in a lower impact strength. In contrast, GSBP exhibits higher energy absorption under an impact load because of the weak interactions between the soy protein and polyester amide and the higher molecular mobility. Figure 6 also shows the temperature dependence of the storage modulus of the soy-based bioplastics. SSBP had a higher storage modulus than GSBP and MSBP. The storage modulus at 25 °C (Table 1) indicated that SSBP had the highest modulus and GSBP had a lower modulus. This is consistent with the results for the tensile and flexural moduli. TGA measures the weight loss of a substance as a function of temperature and provides information on the decomposition behavior of a substance. The TGA results in this study (Figure 7) showed that SSBP had a higher initial decomposition temperature than the other two plastics, indicative of the highest thermal stability of the three plastics. This also shows that there
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Figure 8. Tensile fracture surfaces of (A) GSBP, (B) SSBP, and (C) MSBP.
is a stronger interaction between the sorbitol-plasticized soy protein and the polyester amide, which is also consistent with the DMA results. Morphology. The tensile fracture surfaces as observed by ESEM are shown in Figure 8. It was found that GSBP had ductile fracture features with a coarse surface. Some of the deformed materials on the fracture surface showed a plastic deformation band that did not recover after deformation. This indicates that this sample had higher elongation during stretching and absorbed more energy, which resulted in a higher impact strength. This is consistent with the results from mechanical testing. Also, some fiber was found on the fracture surface because soy protein contains 1-3% crude fiber. However, SSBP showed brittle fracture features with relatively smooth surfaces. This suggests that SSBP had higher strength, higher stiffness, and lower elongation and absorbed less energy during stretching; hence, it had a lower impact strength. MSBP showed a local ductile fracture feature. Therefore, MSBP had moderate tensile strength and stiffness, as well as elongation and impact strength. The morphology of the tensile fracture surfaces confirms the mechanical natures of the different plasticized soy-based bioplastics. These results demonstrate that sorbitol and glycerol are good plasticizers for soy protein. Through blending with plasticizers,
one can adjust the balance between the strength, modulus, and toughness of soy-based bioplastics. Conclusion A series of bioplastics from soy protein and polyester amide with plasticizers including glycerol, D-sorbitol, and glycerol/Dsorbitol blends have been prepared by extrusion and injection molding. It was found that SSBP was more rigid, with a higher tensile modulus and tensile strength than GSBP. MSBP resulted in an intermediate range of tensile modulus and strength values. GSBP had the highest impact strength, whereas SSBP had the highest thermal stability. These thermal and mechanical properties are dependent on the compatibility between the soy protein and the polyester amide. Sorbitol-plasticized soy protein exhibited a significant improvement in compatibility with polyester amide because of the possible hydrogen bonding between them. ESEM morphology observations showed that SSBP had brittle fracture features, whereas GSBP had local ductile fracture features. These results demonstrate that use of a plasticizer allows one to control the balance between the strength, modulus, and toughness of soy-protein-based bioplastics. These soy-based bioplastics can be used as packaging materials to substitute petroleum-based plastic materials.
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Acknowledgment Financial support for this research from USDA-NRI (Grant No. 2001-35504-10734) and GREEEN (Generating Research and Extension to Meet Economic and Environmental Needs; No. GR02-066) is gratefully acknowledged. The authors also thank ADM (Decatur, IL) and Bayer Corp. (Pittsburgh, PA) for their supplying the soy flour and polyester amide samples, respectively. Literature Cited (1) Catsimpoolas, N.; Kenney, J. A.; Meyer, E. W.; Szuhaj, B. F. Molecular weight and amino acid composition of glycinin subunits. J. Sci. Food Agric. 1971, 22, 448. (2) Paetau, I.; Chen, C. Z.; Jane, J. Biodegradable plastic made from soy products. II. Effects of cross-linking and cellulose incorporation on mechanical properties and water absorption. J. EnViron. Polym. Degrad. 1994, 2, 211. (3) Mo, X.; Sun, X. Plasticization of soy protein polymer by polyolbased plasticizers. J. Am. Oil Chem. Soc. 2002, 79, 197-202. (4) 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, 557-569. (5) Kim, K. M.; Marx, D. B.; Weller, C. L.; Hanna, M. A. Influence of sorghum wax, glycerin, and sorbitol on physical properties of soy protein isolate films. J. Am. Oil Chem. Soc. 2003, 80 (1), 71-76. (6) Wu, Q.; Zhang, L. Properties and Structure of Soy Protein IsolateEthylene Glycol Sheets Obtained by Compression Molding, Ind. Eng. Chem. Res. 2001, 40, 1879. (7) Wang, Niangui; Zhang, Lina. Preparation and characterization of soy protein plastics plasticized with waterborne polyurethane. Polym. Int. 2005, 54 (1), 233-239. (8) Zhong, Z. K.; Sun, X. S. Thermal and mechanical properties and water absorption of guanidine hydrochloride-modified soy protein (11S). J. Appl. Polym. Sci. 2000, 78, 1063. (9) Zhong, Z. K.; Sun, X. S. Thermal and mechanical properties and water absorption of sodium dodecyl sulfate-modified soy protein (11S). J. Appl. Polym. Sci. 2001, 81, 166.
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ReceiVed for reView April 7, 2006 ReVised manuscript receiVed August 9, 2006 Accepted August 29, 2006 IE060439L
