Article pubs.acs.org/JAFC
Development and Characterization of Thermoplastic Films from Sorghum Distillers Dried Grains Grafted with Various Methacrylates Narendra Reddy,† Zhen Shi,†,‡ Lisa Temme,† Helan Xu,† Lan Xu,§ Xiuliang Hou,‡ and Yiqi Yang*,†,‡,∥,⊥ †
Department of Textiles, Merchandising & Fashion Design, 234 HECO Building, East Campus, University of NebraskaLincoln, Lincoln, Nebraska 68583-0803, United States ‡ Key Laboratory of Eco-textiles, Ministry of Education, College of Textiles and Garments, Jiangnan University, Lihu Road #1800, Wuxi 214122, People’s Republic of China § Department of Agronomy and Horticulture, University of Nebraska-Lincoln, 279 Plant Science Hall, Lincoln, Nebraska 68583-0915, United States ∥ Department of Biological Systems Engineering and ⊥Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, 234 HECO Building, East Campus, Lincoln, Nebraska 68583-0803, United States ABSTRACT: Distillers Dried Grains (DDG) obtained during production of ethanol from grain sorghum were grafted with methacrylates and compression molded into films with good dry and wet tensile properties. Since sorghum DDG contains up to 45% proteins that are indigestible by animals, it is necessary to find alternative applications to make sorghum ethanol economically competitive. In this research, sorghum DDG was grafted with methyl, ethyl, and butyl methacrylates, the grafted DDG was compression molded into films, and the properties of the grafted DDG and films were studied. At a grafting ratio of 40%, butyl methacrylate (BMA) grafted films had a strength of 4.8 MPa and elongation of 1.8% when dry and 3.1 MPa and 8.1% when wet, indicating that the films had good strength and wet stability. Films developed from grafted DDG show the potential to overcome the brittleness and poor water stability of biopolymer-based films and be useful for various applications. KEYWORDS: sorghum, distillers grains, thermoplastics
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INTRODUCTION Cereal grains, such as corn, are being extensively used to produce ethanol. Sorghum, a crop that is less energy intensive and requires considerably lower amounts of water to grow is gaining attention as an alternative to corn for biofuel production.1,2 Grains from regular sorghum and the stems from sweet sorghum are both being used to produce ethanol.3 Although the share of grain sorghum in ethanol production is low, commercial operations are increasingly using grain sorghum as an alternative feed stock for ethanol production. Production of ethanol from grains results in coproducts such as distillers dried grain (DDG) consisting of proteins, carbohydrates and oil. A gallon of ethanol produced from cereal grains generates about 6 lbs of DDG as coproduct. Therefore, it is necessary to find value added applications for the coproduct to reduce cost and make grain ethanol competitive to other fuels. Unlike corn DDG, the proteins in sorghum DDG are difficult to be digested and sorghum DDG is therefore, not preferred for animal feed.4 Hence, it becomes imperative to find alternative applications for sorghum DDG. Currently, there are limited applications of DDG which is primarily used as animal feed, a limited market and low value application. Efforts have been made to utilize DDG and its components for high value applications. It has been shown that DDG can be directly used for composites or the components in DDG can be extracted and used for multiple applications.5−7 For instance, oil extracted from corn DDG was similar to that of commercial corn oil and zein extracted from DDG was better than that of commercially available zein extracted during wet © 2014 American Chemical Society
milling of corn. However, DDG are a mixture of carbohydrates, proteins, and oil and therefore difficult to be modified and used for industrial applications. In our previous research, we have demonstrated that corn DDG can be acetylated under acidic and alkaline conditions and made into thermoplastic films.8−10 Acetylation under acidic conditions was found to provide a higher degree of substitution and weight gain compared to acetylation under alkaline conditions. Corn DDG was also etherified using acrylonitrile and made into transparent thermoplastic films.8 Although acetylation and etherification provided thermoplasticity, the films developed were either brittle or had poor stability in aqueous environments. Grafting of synthetic monomers has been used to make biopolymers thermoplastic and provide films with good properties. Common biopolymers such as starch and cellulose and proteins, such as soyproteins have been grafted to develop thermoplastic products. Coproducts from biofuel production such as camelina meal were grafted with various acrylates, and the grafted products were compression molded to form thermoplastics.11 Since DDGs are a mixture of proteins and carbohydrates, grafting, which is a milder process compared to acetylation, helps to preserve the properties of the proteins and Received: Revised: Accepted: Published: 2406
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carbohydrates, and the thermoplastics could therefore have better properties. In this research, we have grafted sorghum DDG with three different types of methacrylates (methyl, ethyl, and butyl methacrylates) and the grafting conditions have been optimized. The influence of grafting conditions on the grafting parameters, properties of the grafted DDG and the thermoplastics developed from the grafted DDG have been studied.
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% grafting efficiency W − W2 − W3 = 1 × 100 W1 − W2 % grafting W = × 0 × 100 % monomer conversion W1
where Wb and Wa were the weight of the products before and after the extraction, respectively; W3 and W0 were the weight of the homopolymer and feathers, respectively, and W3 = Wb−Wa. Extracting Homopolymers. Since homopolymers affect the thermoplasticity and the properties of the products developed, the grafted samples used for compression molding were extracted with acetone to remove the homopolymers. Grafted samples were Soxhlet extracted in acetone for 24 h until all the homopolymers were removed.12 Samples were later dried and conditioned before using for compression molding. Characterizing Grafted DDG. Thermal Analysis. Grafted DDG was examined using a differential scanning calorimeter (DSC) (Mettler Toledo D822e, Columbus, OH) to observe the melting behavior. DSC studies were done by placing the samples in sealed aluminum pans and heating them up to 240 °C at a rate of 20 °C/min. 13 C NMR. Solid state 13C NMR studies were done on a Bruker Avance 400 MHz spectrometer (Bruker Daltonics Ins, Fremont, CA) consisting of a 1H/13C/15N triple-resonance, 3.2 mm magic angle spinning probe. Spectra were collected at room temperature using a standard cross-polarization experiment with high-power 1H-decoupling, and spinning the samples at 20 kHz. Developing Thermoplastic Films. Grafted DDG with and without any homopolymers was compression molded to form films. About 10 g of the grafted samples were placed between aluminum foils and compression molded in a Carver Press (Carver Inc., Wabash, IN) at 360 °F for 5 min at 40 000 PSI. After compression molding, cold water was run-in to cool the press and the samples were removed. Films obtained were conditioned under standard atmospheric conditions of 21 °C and 65% relative humidity for at least 24 h before being tested. At least 8 films were compression molded and samples were cut from the films and randomly selected for testing. Thickness of the films varied from 0.3 to 0.7 mm depending on the extent of grafting and type of monomer used. Wet stability of the films was tested by immersing the films in 21 °C water for 30 min. After 30 min in water, the samples were immediately tested for their tensile properties according to ASTM standard D 882. Film strips measuring 15 ×1.5 cm2 were tested on an MTS tensile tester (Model Q test 10, Eden Prairie, MN) using a gauge length of 5 cm and crosshead speed of 50 mm/min. At least 25 samples were tested for each condition and the average and ± one standard deviation are reported.
MATERIALS AND METHODS
Materials. Sorghum DDG was obtained from an ethanol producer in Kansas, U.S.A. The three methacrylates, methyl methacrylate (99.8%, TCI America, Portland, OR), ethyl methacrylate (98%, Alfa Aesar, Ward Hill, MA), and butyl methacrylate (99%, Acros Organics, New Jersey) and the initiator (Potassium persulfate, TCI America, Portland, OR), reducing agent (sodium bisulfite, Sigma Aldrich, St. Louis, MO), and other chemicals required for the study were reagent grade and used as-received. Grafting. Grafting was done in a four-necked flask under nitrogen atmosphere. A schematic of the grafting process is shown in Figure 1.
Figure 1. Schematic of the grafting process.
Sorghum DDG was dispersed in water and the monomer concentrations ranging from 20 to 180% on weight of the DDG were added into the flask, along with the required amount of initiators and reducing agents. Grafting was done by heating the flask to 70 °C for 2 h. Paradioxybenzene (2%) was added into the flask to terminate the grafting reaction and the grafted DDG was collected. Any unreacted monomers were removed by thoroughly washing the grafted DDG with water. The amount of DDG obtained after grafting was weighted and used to calculate the % grafting and other grafting parameters. The grafting parameters that were determined include % monomer conversion, molar grafting ratio, % grafting, and % homopolymers using the equations given below.12 In this report, % grafting describes the % by weight of the monomers grafted on to the meal, grafting efficiency (%) indicated the weight % of monomers grafted to the initial weight of the monomers used and includes both the grafted polymers and the homopolymers.
% monomer conversion =
% grafting =
W1 − W2 × 100 W1
W1 − W2 − W3 × 100 W0
(3)
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RESULTS AND DISCUSSION Influence of Monomer Concentration on % Monomer Conversion. Figure 2 reveals that the % of monomers that were converted into polymers increased with increasing concentration of the monomers used during the reaction. Monomer conversion was considerably lower at monomer concentration of 20% but substantially increased to about 95% when the monomer concentration was increased to 60% or higher. Further increase in monomer concentration did not show any significant increase in the monomer conversion, and the monomer conversion was very similar for all three monomers studied. At low monomer concentration (20%), the molecules have a lower chance of reacting with each other and with the DDG and form polymers, and therefore had a low % conversion. Increasing monomer concentrations above 60% would result in excessive monomers that react with each other and form homopolymers. Since excessive concentration of monomers would add to the cost and does not substantially
(1)
(2) 2407
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Figure 4. Increase in % grafting with increase in monomer concentration when grafted with the three monomers at 70 °C for 120 min.
Figure 2. Influence of monomer concentration on the % monomer conversion. Grafting reaction was done at 70 °C for 120 min.
increase monomer conversion, a monomer concentration of 60% would be sufficient to get good % grafting and lower % homopolymers. Influence of Monomer Concentration on % Grafting Efficiency. Grafting efficiency was substantially lower at high monomer concentrations as seen from Figure 3. At monomer
monomer concentration was increased, the probability of the monomers reacting with themselves and with the DDG increased. This led to an increase in the amount of grafted chains on DDG and therefore the % grafting also increased. As seen with % grafting efficiency, MMA showed a higher % grafting than EMA and BMA because MMA are shorter polymers and readily reacted with the DDG and themselves. Longer polymer chains formed by EMA and BMA cannot penetrate easily into DDG and attach themselves to the grafting sites. Therefore, BMA had the lowest % grafting compared to the other two monomers. Molar grafting ratio data also showed that MMA had higher amounts of grafted monomers than EMA or BMA due mainly to the shorter chain length and lower stearic hindrance of MMA. Influence of Monomer Concentration on % Homopolymers. Amount of homopolymers formed increased linearly with increasing monomer concentration as seen from Figure 5. At any particular monomer concentration, MMA had the lowest % homopolymers compared to EMA or BMA. Since MMA could react more easily with DDG, it had a lower chance to react with itself and form homopolymers. Increasing the
Figure 3. Influence of monomer concentration on the % grafting efficiency. Grafting reaction was done at 70 °C for 120 min.
concentrations of 20%, the grafting efficiency ranged from 66 to 91%, with the % grafting efficiency being highest for MMA, followed by EMA and BMA. Monomers containing longer side chains are more difficult to penetrate into DDG and attach onto the grafting sites. In addition, longer alkyl chains offer stearic hindrance and are more difficult to react with the DDG compared to a shorter chain monomers such as MMA. Therefore, BMA had a lower % grafting than MMA or EMA as seen from Figure 2. Changes in % grafting efficiency with increasing monomer concentration are consistent with observations made for the same monomers when grafted onto starch, soyprotein and corn DDG. Influence of Monomer Concentration on % Grafting. Monomers (%) grafted onto DDG showed an opposite trend to the % grafting efficiency, as seen from Figure 4. Increasing monomer concentration steadily increased the % of polymers grafted onto DDG for all the monomers studied. When
Figure 5. Changes in the amount of homopolymers formed with increasing ratio of monomers when the grafting was done at 70 °C for 120 min. 2408
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concentration of monomers increased the reaction between the monomers and led to the formation of higher amounts of homopolymers. At monomer concentrations above 50%, the monomers predominantly form homopolymers than graft onto the DDG. Saturation of grafting sites in DDG that are accessible to the monomers decreased and that would also make the monomers to form homopolymers. Nuclear Magnetic Resonance. NMR spectra of ungrafted and DDG grafted with EMA show distinct differences as seen from Figure 6. Since sorghum DDG contains about 25−45%
Figure 7. Differential scanning calorimetry curves for the ungrafted and DDG grafted with the three monomers shows that EMA and BMA provided better thermoplasticity compared to MMA.
in Tables 1−3 reveal some interesting features. For all three monomers, increasing the grafting ratio increased strength, except for DDG grafted with 53% BMA, where the strength decreased compared to the films obtained at 40% BMA grafting. At a grafting ratio of about 23%, films made from all three monomers had strengths between 2 and 2.3 MPa but films grafted with longer lengths of carbon chains had higher elongation. For instance, MMA-grafted films had an elongation of 0.5% compared to 1.1% for BMA grafted films. Increasing the grafting ratio from 23 to 30% more than doubled the strength and elongation of the MMA-grafted films, whereas the EMA and BMA grafted films showed a marginal increase in strength. Further increase in the grafting ratio to 40% and above led to increased strength of the films. MMA-grafted films had the highest strength but low elongation compared to EMA and BMA films, whereas BMA grafted films had higher elongation than the MMA and EMA films. The modulus of the films was highest for MMA-grafted DDG, followed by EMA- and BMA-grafted DDG. Tensile properties of the films mainly depend on the thermoplasticity of the grafted DDG. At low grafting ratios, the DDG partially melts, and the films contain considerable amounts of proteins and carbohydrates that do not melt leading to poor tensile properties. Increasing grafting ratio improves the thermoplasticity, and therefore the films have higher strength and elongation. Monomers with longer chains offer higher extensibility and better thermoplasticity. Therefore, the EMA- and BMA-grafted films had higher elongation than the MMA grafted films. Since the EMA- and BMA-grafted films are more flexible, the modulus is lower than that of the MMA films. Wet tensile properties of the films showed a different trend than the dry properties of the films grafted with the three monomers. Films grafted with MMA showed strength retention ranging from 43 to 59% compared to 33−69% for EMA and 15−61% for BMA with increasing grafting ratio resulting in higher strength retention. Although there was no particular trend in the wet strength retention with the increasing chain length of the monomers, elongation of the films was considerably higher for BMA followed by EMA and MMA. MMA- and EMA-grafted films did not show significant change in the wet elongation but the BMA-grafted films had at least 100% increase in elongation when wet. Increasing the grafting ratio adds higher amounts of the hydrophobic polymer onto
Figure 6. NMR spectrum depicting the differences between the ungrafted DDG and DDG grafted with EMA.
protein, 20% hemicellulose and 9−16% cellulose, the NMR spectra of sorghum DDG displayed major peaks belonging to the proteins, hemicellulose and cellulose. In sorghum DDG, the peak at about 175 ppm reflected amino group in protein,13 the peak at about 75 ppm and 105 ppm represented C4 and C1 in cellulose and hemicellulose, respectively.14,15 After EMA grafting, peaks at 22 ppm reflecting methyl groups (−CH3), 45 ppm representing tertiary carbon and 55 ppm representing methoxy group (−OCH3) in methacrylate are seen. The difference between the two spectra clearly indicated that EMA has been successfully grafted onto sorghum DDG. Thermal Behavior of Grafted DDG. Figure 7 shows that the ungrafted and DDG grafted with the three monomers had distinct melting peaks. The peak at 130 °C seen in the ungrafted DDG should be due to the melting of the proteins. MMA grafted DDG had a broad melting peak suggesting poor thermoplasticity. Compared to MMA, the EMA- and BMAgrafted DDG showed considerably sharp melting peaks at 205 °C. BMA grafted DDG had an additional melting peak at about 180 °C, most likely due to the different melting behaviors of the proteins and carbohydrates in the DDG. As can be inferred from Figure 7, BMA grafted DDG had the highest melting enthalpy of 108 J/g, compared to 80 and 55 J/g for EMA and MMA, respectively. DSC curves clearly suggested that BMA imparted better thermoplasticity to DDG than MMA and EMA. Although the melting peaks for EMA and BMA were observed at 205 °C, the grafted DDG could be compression molded into films at 190 °C under pressure. Tensile Properties of Grafted DDG films Without Homopolymers. Tensile properties of the films made from the grafted DDG after extraction of the homopolymers shown 2409
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Table 1. Dry and Wet Tensile Properties of Thermoplastic Films Made from Sorghum DDG Grafted with Various Levels of MMA after Extraction of the Homopolymers tensile strength, MPa grafting ratio, % (mmol/g) 23 30 40 53
dry
(2.01) (2.63) (3.50) (4.64)
2.3 4.1 5.2 5.9
± ± ± ±
breaking elongation, %
wet 1.2 1.1 1.2 1.2
1.0 2.0 2.6 3.5
± ± ± ±
dry 0.4 0.6 0.6 0.7
0.5 1.1 1.1 1.2
± ± ± ±
Young’s Modulus, MPa
wet 0.2 0.4 0.4 0.3
0.8 0.9 1.2 1.2
± ± ± ±
dry
0.2 0.2 0.3 0.3
895 984 1227 1287
± ± ± ±
wet 278 103 175 129
330 440 491 672
± ± ± ±
96 121 106 124
Table 2. Dry and Wet Tensile Properties of Thermoplastic Films Made from Sorghum DDG Grafted with Various Levels of EMA after Extraction of the Homopolymers tensile strength, MPa grafting ratio, % (mmol/g) 23 30 40 53
dry
(2.01) (2.63) (3.50) (4.64)
2.4 2.9 4.4 4.6
± ± ± ±
breaking elongation, %
wet 0.7 0.8 0.8 0.9
0.8 0.84 2.3 3.2
± ± ± ±
dry 0.3 0.2 0.4 0.6
1.0 0.9 1.5 1.6
± ± ± ±
Young’s Modulus, MPa
wet 0.4 0.3 0.2 0.1
1.3 1.6 1.7 2.0
± ± ± ±
dry 0.4 0.3 0.3 0.4
656 764 848 842
± ± ± ±
wet 94 151 86 70
164 158 304 388
± ± ± ±
45 29 54 66
Table 3. Dry and Wet Tensile Properties of Thermoplastic Films Made from Sorghum DDG Grafted with Various Levels of BMA after Extraction of the Homopolymers tensile strength, MPa grafting ratio, % (mmol/g) 23 30 40 53
dry
(2.01) (2.63) (3.50) (4.64)
2.0 2.8 4.3 3.1
± ± ± ±
wet 0.4 0.9 0.8 0.9
0.3 0.84 1.84 1.9
± ± ± ±
0.09 0.2 0.4 0.8
1.1 1.6 2.3 2.3
DDG, and therefore the wet stability increased with increasing grafting ratio. Water acts as a plasticizer and allows the molecules to move more freely therby increasing the elongation. Although increasing chain length should increase hydrophobicity and improve wet tensile properties, BMAgrafted films had similar or lower strength retention than the MMA or EMA films. This should be due to the lower amounts (molar weight) of BMA on the films compared to MMA and EMA, although the % grafting ratios were similar. Tensile Properties of Grafted DDG Films with Homopolymers. Since homopolymers are formed during the grafting reaction and the homopolymers are part of the grafted DDG, we investigated the properties of DDG films containing homopolymers. As seen from Table 4, the dry tensile properties of the films containing homopolymers were slightly different but there were considerable variations in the wet strength of the films compared to those developed from the same monomers but without the homopolymers. For instance, at 40% grafting, EMA grafted DDG without homopolymers had
MMA EMA BMA
breaking elongation, %
dry
wet
dry
wet
4.5 ± 1.2 6.3 ± 1.1 4.8 ± 1.0
3.2 ± 1.0 4.7 ± 1.1 1.8 ± 0.4
1.4 ± 0.1 1.8 ± 0.1 3.1 ± 0.6
1.3 ± 0.4 2.3 ± 0.2 8.1 ± 1.2
Young’s Modulus, MPa dry
± ± ± ±
wet 0.3 0.4 0.5 0.6
2.5 4.4 4.0 4.3
± ± ± ±
0.5 0.9 0.9 1.3
700 427 550 580
± ± ± ±
wet 80 79 50 10
46 60 155 170
± ± ± ±
15 20 57 91
dry strength of 4.4 MPa compared to 6.3 MPa with homopolymers and the % elongation was 1.5% compared to 1.8%. Addition of homopolymers will increase the thermoplasticity and the hydrophobicity of the grafted DDG leading to increased strength and elongation. Also, the properties of the homopolymers will influence the properties of the DDG films. Our previous research has shown that films made from homopolymers with longer carbon chains have lower strength but higher elongations. BMA films with a dry strength of 4.8 MPa and elongation of 3.1% show excellent wet strength and elongation retention and could be useful for various applications. Sorghum DDG grafted with methacrylates could be used to develop films with good dry and wet tensile properties. Changing the concentration of monomers resulted in higher grafting ratios and a higher percentage of homopolymers. Also, increasing grafting ratio generally increased the strength and elongation of the films. Similarly, the wet strength of the films was also higher at higher grafting ratios. Films containing homopolymers exhibited better tensile properties compared to films made from the same monomers but without the homopolymers. It was also observed that increasing the length of the alkyl chain resulted in films with lower strength but higher elongation since the monomers with longer chains are more flexible. Among the three monomers studied, BMAgrafted DDG had better thermoplasticity and also formed films with good dry and wet tensile properties. Grafting appears to be a more viable approach to develop thermoplastics from DDG compared to other common types of chemical modifications.
Table 4. Tensile Properties of Sorghum DDG Films Grafted with MMA, EMA, and BMA under the Optimum Grafting Conditions and Including the Homopolymersa breaking strength, MPa
breaking elongation, % dry
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a
Grafting % for the three monomers was about 40% and homopolymers (%) were 54, 58 and 28% for MMA, EMA, and BMA, respectively.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 2410
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors wish to acknowledge the financial support from Agricultural Research Division at the University Nebraska Lincoln and Multistate project S1054 (NEB37-037). Financial support for Zhen Shi from the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1135) at Jiangnan University, Scientific Support Program of Jiangsu Province (BE2011404), the Graduate student innovation plan of Jiangsu Province (CX10B_222Z), the Doctor Candidate Foundation of Jiangnan University (JUDCF10004) and the China Scholarship Council is also thankfully acknowledged.
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