Article pubs.acs.org/IECR
Reaction of Zein with Methylenediphenyl Diisocyanate in the Melt State: Thermal, Mechanical, and Physical Properties David J. Sessa, Gordon W. Selling,* and Atanu Biswas Plant Polymer Research, National Center for Agricultural Utilization Research, U.S. Department of Agriculture-Agricultural Research Service, Peoria, Illinois 61604, United States ABSTRACT: Corn protein (zein) was melt-processed with methylenediphenyl 4,4′-diisocyanate (MDI) using triethylamine (TEA) as a catalyst to facilitate the reaction of the isocyanate groups with the nucleophilic moieties present on zein. The product of the reaction was examined using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) to monitor changes in molecular weight. Techniques used to evaluate property changes after reaction included FTIR, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and evaluation of mechanical properties. Our findings demonstrated that zein reacts with MDI in the melt state generating higher molecular weight compounds that after compression molding have improved physical properties and solvent resistance.
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INTRODUCTION Zein, a prolamin, is the predominant protein in the coproducts corn gluten meal (wet-milling) and distillers dried grains (dry grind) from the corn-ethanol industry. Current production of ethanol is approximately 15 billion gallons/year and is expected to increase to 36 billion gallons by 2022. The coproducts of the corn-ethanol industry can be better utilized to enhance the economics of ethanol production. A recently published review article1 discusses zein plasticization modifications, cross-linking, and its applications. Based on zein’s plasticization capabilities as discussed in that review publication, zein can be used as a renewable resource to replace petroleum-based plastics. To meet those needs zein must be modified to deliver improved physical properties. Isocyanates have been studied to a lesser extent than other reagents and may provide advantages over other techniques in providing zein articles with improved properties. Isocyanates react with molecules containing an active hydrogen, such as alcohols, amines, and carboxylic acids.2−7 Reaction of the isocyanate group with these functionalities will give carbamates, ureas, and amides, respectively. These reactions can be catalyzed with tertiary amines (such as triethylamine, TEA) which may coordinate with either the isocyanate or the nucleophile to increase reactivity.2 In those investigations involving reactions of proteins with diisocyanates,8−11 the proteins were dried prior to reaction with the diisocyanate to reduce the amount of reaction of methylenediphenyl 4,4′-diisocyanate (MDI) with water present in the system. Melt extrusion, based on its cost efficiency, processing ease, and high throughput capabilities, is the method of choice to provide the lowest cost article. Soy protein isolate and polycaprolactone (PCL) were treated with MDI in the melt state to generate materials with enhanced elongation properties and water resistance; however, tensile strength decreased.8,9 Modification of PCL with hexamethylene diisocyanate generated a PCL diisocyanate prepolymer that was then allowed to react with zein in solution to provide a product with improved mechanical properties and water resistance.10,11 This article not subject to U.S. Copyright. Published 2012 by the American Chemical Society
Melt-processing of zein with PCL yielded an incompatible product with poor interfacial interaction between the polymers.12 Commercially produced MDI is readily available and very reactive. The use of melt processing to derivatize a material brings many economic advantages. There are no publications on melt-processing zein with MDI. The amino acids Glu, Gln, Tyr, His, Arg, Asp, Asn, Thr, Ser, and Cys are those amino acids in zein that can potentially react with MDI.13 Because water can react with MDI,13 the impact of moisture was also investigated using as-is and dried zein.
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MATERIALS AND METHODS Materials. Freeman zein lot F40009101C13 was purchased from Flo-Chemical Corp., Ashburnham, MA. The percent protein (dry basis) for that lot was 87.7% based on % Dumas N x 6.25 and residual lipid was 1.72%. Methylenediphenyl 4,4′diisocyanate (MDI) was obtained from TCI America (Portland, OR). All other chemicals used in this investigation were reagent grade and purchased from Sigma-Aldrich (St. Louis, MO) Torque Rheometry/Compression Molding. Mixtures of zein with various levels of MDI from 0 to 15% and TEA (0.5%) based on the dry weight of zein were crudely mixed. One series of experiments used as-is zein (4.3% moisture) and a second series consisted of zein with moisture of 1.1% (zein vacuumdried for 48 h at 60 °C). These series are labeled MPZ (4.3) and MPZ (1.1), respectively, throughout the text. Fifty-five grams of each formulation was used to fill the chamber of a Haake Fisons, Rheocord 90 using a 600 series mixing bowl and roller rotors (Thermo-Fisher, Newington, NH). The initial wall temperature was set to 110 °C and rotor speed was set at 50 rpm. After 1 min at 110 °C the wall temperature was increased to 140 °C. The sample temperature was measured, but not
Received: Revised: Accepted: Published: 9199
July 14, 2011 May 24, 2012 June 12, 2012 June 12, 2012 dx.doi.org/10.1021/ie201501s | Ind. Eng. Chem. Res. 2012, 51, 9199−9203
Industrial & Engineering Chemistry Research
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Article
RESULTS AND DISCUSSION Preparation and Structure of Zein Melt-Processed with MDI. The impact of moisture, MPZ (4.3) versus MPZ (1.1) was investigated at various levels of MDI. Melt-processing for each study involved reactions of zein with 2, 5, 7, 9, 12, and 15% MDI where each reaction was catalyzed with 0.5% TEA. In all cases the final temperature increased more than the wall temperature of the rheometer due to viscous heating. With increasing amounts of MDI for both the MPZ (1.1) and MPZ (4.3) series, the final temperature and torque (an indicator of viscosity) in general increased with the amount of MDI (Table 1). For MPZ (1.1) at the highest amount of MDI, the torque
controlled. The sample temperature increased beyond the wall temperature due to viscous heating. After an additional 4 min of mixing, the processing was terminated. Two replicates were made for each blend. The melt-processed blends of zein were removed from the rotors and cooled to room temperature. They were placed in open bags stored in a hood for 7 days to ensure complete reaction. Samples were ground in a Thomas-Wiley mill equipped with a 1-mm screen. The ground material was compression molded into D638 type V tensile bars using a Carver Press. Ground material was placed into a six-pocket mold and placed under a pressure of 0.015 MPa 5 min at 170 °C to equilibrate the mold and distribute the material. The mold was then placed under 40.2 MPa pressure at 170 °C for 15 min. The mold was cooled to 93 °C under pressure. After being stored for 2 days at 50% RH, the molded tensile bars (two replicates of six samples) had their physical properties (tensile strength (MPa), elongation %, and Young’s modulus (MPa)) assessed using an Instron Universal Testing Machine (model 4201, Canton, MA) with a crosshead speed of 10 mm/ min. Thermal Analyses. Differential scanning calorimetry (DSC) measurements were performed with ground samples, using hermetically sealed T Zero pans and a Q2000 with a RC500 cooling unit (TA Instruments, New Castle, DE). The instrument was calibrated against an indium standard. Samples were heated from −20 to 150 °C at a rate of 5 °C/min for one cycle followed by cooling to −20 °C. Then a second heating cycle from −20 to 210 °C at 5 °C/min was performed. The glass transition temperature (Tg) representing the midpoint between the onset temperature (Ti) and the final temperature (Tf) was measured on the second heating cycle. Thermal gravimetric analysis (TGA) was performed on a 2050 TGA (TA Instruments, New Castle, DE) by heating each sample at a heating rate of 10 °C/min from 25 to 800 °C under nitrogen (90 mL/min). Sodium Dodecyl Sulfate Polyacrylamide Gel Electrophoresis (SDS-PAGE). Gel electrophoresis was performed with NuPAGE 4−12% gradient bis-tris gels (Invitrogen, Carlsbad, CA). Samples electrophoresed consisted of a broad range molecular weight standards (Bio-Rad Laboratories, Hercules, CA) and 2.5 mg each of the desired sample dispersed in 250 μL sample buffer containing 0.055 M Tris, 2.0% SDS, 7.0% glycerol, 4.3% mercaptoethanol, and 5 M urea. These dispersed samples were boiled for 5 min, and then centrifuged at 10 000 rpm for 5 min. Supernatants were loaded into the wells of the gel. A Bio-Rad Mini-PROTEAN II electrophoresis cell was used with a running buffer of 0.025 M Tris, 0.2 M glycine, and 0.5% SDS. Electrophoresis was performed at room temperature with 200 V. The gel was stained with Coomassie Brilliant Blue R-250 and then destained with a mixture of 18% ethanol and 8% acetic acid. Fourier Transform Infrared (FTIR) Spectroscopy. Spectra (200 scans and 1.9 cm−1 resolution) were collected using an ATR Smart DuraSamp II R accessory in a Thermo Nicolet Avatar 370 spectrometer equipped with EX OMNIC software, CsI beam splitter, and DTGS-CsI detector. Ethanol Dissolution. Samples were added to 80% ethanol/ water to provide solutions containing 8% solids. After stirring 24 h at room temperature, the mixtures, run in triplicate, were centrifuged and the solid was collected. After drying at 100 °C for 24 h, the amount of insoluble material was determined and the relative insolubility was determined gravimetrically.
Table 1. Final Temperature and Torque for Reaction of MPZ (1.1) and MPZ (4.3) with Various Amounts of MDI MPZ (1.1)
MPZ (4.3)
% MDI
final temp (°C)
final torque (Nm)
final temp (°C)
final torque (Nm)
0 2 5 7 9 12 15
185 185 190 195 200 210 215
35 40 50 50 50 50 45
155 155 165 170 175 190 205
20 23 26 36 40 50 55
value decreased, however, the temperature increased. The increase in temperature will decrease the torque measure by the instrument, so that it is difficult to determine what the viscosity would be for this formulation at the same temperature. The viscosity increase was caused by increased molecular weight that resulted from the reaction of zein proteins with MDI. As the amount of MDI was increased, the number of zein proteins that are connected through these branch points increases, yielding high molecular weight (vide inf ra) and high viscosity. When the melt temperatures exceeded 200 °C, evidence of thermal degradation of the resulting products were observed. The material removed from the melt rheometer was a dark crumbly mass. In addition there was evidence of increased low molecular weight proteins present (vide inf ra).14 The temperature of the melt was always less for the samples made using MPZ (4.3). This could be caused by the water acting as plasticizer or the water may react with some of the MDI reducing the degree of branching that can take place. Results of analysis of the protein molecular weights using SDS-PAGE of MPZ (4.3) and MPZ (1.1) MDI reaction products are shown in Figure 1. Higher molecular weight material is located higher up on the gel. This analysis examines only that material which is soluble in the denaturing buffer. The amount of insoluble material appeared to be proportional to the relative amount of MDI reacted. With increased amounts of MDI there was an increase in the amount of material that had molecular weights too high to penetrate the gel. On comparing MPZ (1.1) with MPZ (4.3) after reaction with MDI, it can be seen that MPZ (1.1) has a broader molecular weight distribution, with more high and low molecular weight material. This demonstrates that water present in the zein reacts with some amount of the added MDI, reducing the concentration of MDI that can bond zein proteins. The higher molecular weight proteins present in the MPZ (1.1) samples will increase viscosity (torque) and temperature (due to viscous heating) as seen in Table 1. The increased temperature drove protein 9200
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Figure 1. SDS-PAGE patterns. Lane 1, MPZ (4.3) control; lane 2, MPZ (4.3) 7% MDI; lane 3, MPZ (1.1) 7% MDI; lane 4, MPZ (4.3) 9% MDI; lane 5, MPZ (1.1) 9% MDI; lane 6, MPZ (4.3) 12% MDI; lane 7, MPZ (1.1) 12% MDI; lane 8, MPZ (4.3) 15% MDI; lane 9, MPZ (1.1) 15% MDI.
degradation, leading to more low molecular weights protein being present.14 This demonstrates that the amount of water present in the zein is critical to selection of the amount of MDI and the reaction conditions. Given that both low and high molecular weight species are generated during the reaction, sample solubility was investigated using 80% ethanol/water as solvent. Improved solvent resistance may be important in certain applications. In this test unreacted zein and the lower molecular weight degraded protein will go into solution. Table 2 shows the solubility of the
Figure 2. IR spectral difference spectra of MPZ (4.3) processed with 12% MDI minus MPZ (4.3) control normalized at 1647 cm−1 between 2500 and 3500 cm−1 (A) and 800 and 1800 cm−1 (B).
with 12% MDI and MPZ (4.3) control. The FTIR spectrum was normalized with regard to the amide 1 band for zein at 1647 cm−1. The spectrum shows peaks indicative of reaction of MDI with zein. The absorption at 3305 cm−1 can be attributed to NH or NH2 stretching vibrations.15 Absorptions at wave numbers 2926 and 2866 cm−1 can be attributed to the aromatic C−H stretch.15 The peak at wavenumber 1660 cm−1 is due to CO stretching.15 The peaks at wavenumber 1595, 1540, and 1229 cm−1 result from aromatic C stretch, the carbamate C−N stretch, and the carbamate C−O stretch.15 The aromatic group from the MDI also gives rise to absorptions at 1510, 1409, 1304, 861, and 818 cm−1.15 Thermal Analyses of Zein Melt-Processed with MDI. Thermal events of select formulations using DSC techniques are detailed in Table 3 and shown in Figure 3. All other samples also showed a single Tg. One glass transition is one means of indicating the presence of a compatible blend.16,17 Reactions of zein with increased percent of MDI caused an increase in Tg. Increases in Tg are attributed to lower chain mobility which may be caused by branching or cross-linking.18,19 Thermogravimetric analysis (TGA) was used to determine the impact of derivitzation of zein with MDI on thermal stability. Samples of MPZ (1.1) and MPZ (4.3) zein using 12% MDI and control were tested (Figure 4). On examination of the first derivative of weight loss, all samples experienced the maximum rate of degradation at approximately the same temperature with the maximum rates for control, MPZ (4.3)/ 12% MDI, and MPZ (1.1)/12% MDI occurring at 318, 319,
Table 2. Relative Insolubility of MPZ (4.3) or MPZ (1.1) after Reaction with MDI % MDI
MPZ (4.3)
MPZ (1.1)
0 2 5 7 9 12 15
0.0 25.8 37.2 50.2 70.1 70.3 55.3
0.0 10.4 24.5 25.1 41.2 46.1 26.0
various MPZ (4.3) and MPZ (1.1) formulations. It was found that the relative insolubility was generally proportional to the amount of MDI used. The amount of insoluble material was always more when using the MPZ (4.3) zein. Formulations using MPZ (1.1) undergo more degradation during processing (Figure 2) providing larger amounts of low molecular weight soluble proteins. Using 15% MDI with either MPZ (1.1) or (4.3), the amount of insoluble material decreases. Given that these formulations were exposed to higher temperatures during processing, increased degradation would provide higher amounts of soluble material. The FTIR spectra in Figure 2A 2500−3500 cm−1 and 2B 800−1800 cm−1 (region between 1800 and 2600 cm−1 removed for clarity) is the difference spectrum of MPZ (4.3) 9201
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Table 3. DSC of Zein Melt-Processed with MDI sample zein (4.3) 5% MDI 9% MDI 12% MDI 15% MDI zein (1.10) 5% MDI 9% MDI 12% MDI 15% MDI
Ti 133.4 124.5 138.8 142.0 144.4 141.6 142.3 143.6 144.7 146.5
(2.7) (2.2) (2.6) (1.0) (0.7) (1.2) (2.2) (1.5) (1.1) (0.6)
Tg 137.9 138.0 141.5 147.3 149.7 146.6 147.5 149.4 149.9 153.5
ΔCp J/g °C
Tf
(1.2) (1.1) (1.6) (0.8) (0.9) (0.8) (1.7) (0.5) (1.8) (1.5)
141.5 141.8 143.8 151.0 154.2 151.9 153.0 153.4 155.4 158.7
(1.4) (1.2) (1.2) (0.4) (0.3) (0.8) (1.2) (1.3) (1.2) (0.7)
0.150 0.292 0.139 0.138 0.208 0.339 0.351 0.269 0.251 0.344
(0.035) (0.023) (0.041) (0.055) (0.023) (0.051) (0.026) (0.062) (0.042) (0.060)
Mechanical Properties of Zein Melt-Processed with MDI. As shown in Table 4, the tensile bars prepared from MPZ Table 4. Mechanical Properties of Zein Melt-Processed with MDIa % MDI
tensile stength (MPa)
MPZ (4.3) 0 2 5 7 9 12 15 MPZ (1.1) 0 2 5 7 9 12 15
Figure 3. DSC scans of MPZ (4.3) control and MPZ (4.3) or MPZ (1.1) processed with 12% MDI. MPZ (4.3) reduced by 0.2 W/g for clarity.
a
elongation %
Young’s modulus (MPa)
26.4 32.5 34.3 34.3 33.1 36.0 31.9
(0.1) (0.1) (2.9) (2.7) (3.2) (1.6) (0.5)
6.5 7.8 7.5 8.0 7.7 7.9 6.2
(0.5) (0.6) (0.8) (0.2) (1.1) (0.9) (0.3)
449.9 (31.8) 476.0 (0.7) 471.6 (11.0) 464.9 (4.4) 477.7 (22.7) 510.1 (32.0) 561.9 (19.1)
28.7 26.9 22.2 21.2 18.2 19.4 18.3
(2.3) (1.0) (1.2) (0.8) (1.9) (0.6) (1.7)
6.0 5.7 5.3 4.2 4.4 4.0 4.0
(0.2) (0.6) (0.4) (0.6) (0.5) (0.1) (0.2)
517.7 (4.3) 525.3 (62.9) 485.5 (28.4) 585.3 (35.6) 496.6 (20.4) 542.2 (1.1) 519.6 (2.5)
Standard deviations in parentheses, all samples contained 0.5% TEA.
(4.3) modified with 2−12% MDI all showed improvements in both tensile strength and elongation due to reaction of MDI with zein. The differences between samples produced using MDI between 2 and 12% are not statistically significant. As expected with the formation of branch points, the stiffness, Young’s modulus, is higher after incorporating the highest amounts (12 and 15%) of MDI. The physical properties of formulations using MPZ (1.1) were deficient relative to control. As detailed previously thermal degradation appeared to occur during processing of the MPZ (1.1) samples which could lead to poorer properties.14 As the amount of MDI was increased using MPZ (1.1), the temperature of the melt increased (Table 1) driving degradation (Figure 2) and giving poorer physical properties.
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Figure 4. TGA results of MPZ (4.3) control, MPZ (4.3)/12% MDI, and MPZ (1.1)/12% MDI. Mass loss is reported using solid line, first derivative is reported using dashed lines.
CONCLUSIONS
The data presented suggest that the mechanical properties and solvent resistance of melt processed zein can be enhanced by cross-linking zein with MDI. The presence of some moisture in zein proved beneficial for melt processing and gave articles with improved physical properties. It has been shown that batch melt processing of zein using cross-linking reagents can be used to predict results from zein extrusion processing.18,21,22 The mechanical properties obtained in this study were similar to
and 323 °C respectively. These values are similar to the literature value.20 The rate of degradation of the control is somewhat higher between the temperatures of 260 and 295 °C, suggesting that reaction with MDI reduces the rate of thermal degradation at lower temperatures. 9202
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those obtained from using glyoxal to modify zein21 but deficient relative to when glutaraldehyde was used.18 Additional studies on the reaction of zein with MDI, where the zein will have small amounts of water, will be studied.
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(16) Brostow, W.; Chiu, R.; Kalogeras, I. M.; Vassilikou-Dova, A. Prediction of glass transition temperatures: Binary blends and colpolymers. Mater. Lett. 2008, 62, 3152−3155. (17) Fried, J. R.; Karasz, F. E.; MacKnight, W. J. Compatibility of poly(2,6-dimethyl-1,4-phenylene oxide (PPO)/poly(styrene-co-4chlorostyrene) blends. I. Differential scanning calorimetry and density studies. Macromolecules 1978, 11, 150−158. (18) Sessa, D. J.; Mohamed, A.; Byars, J. A. Chemistry and physical properties of melt-processed and solution-cross-linked corn zein. J. Agric. Food Chem. 2008, 56, 7067−7075. (19) Sessa, D. J.; Mohamed, A.; Byars, J. A.; Hamaker, S. A. H.; Selling, G. W. Properties of films from corn zein reacted with glutaraldehyde. J. Appl. Polym. Sci. 2007, 105, 2877−2883. (20) Magoshi, J.; Nakamura, S.; Murakami, K.-I. Structure and physical properties of seed proteins. 1. Glass transition and crystallization of zein protein from corn. J. Appl. Polym. Sci. 1992, 45, 2043−2048. (21) Woods, K.; Selling, G. Melt reaction of zein with glyoxal to improve tensile strength and reduced solubility. J. Appl. Polym. Sci. 2008, 109, 2375−2383. (22) Selling, G.; Woods, K.; Biswas, A.; Willett, J. Reactive extrusion of zein with glyoxal. J. Appl. Sci. 2009, 113, 1828−1835.
AUTHOR INFORMATION
Corresponding Author
*Tel.: 3096816338. Fax: 3096816691. E-mail: gordon.selling@ ars.usda.gov. Notes
Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Gary Kuzniar, Mardell Schaer, and Kelly Utt for their technical assistance and Jason Adkins for performing TGA. REFERENCES
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