Article pubs.acs.org/IECR
Development and Characterization of Thermoplastics from Corn Distillers Grains Grafted with Various Methacrylates Zhen Shi,†,‡ Narendra Reddy,‡ Xiuliang Hou,† and Yiqi Yang*,†,‡,§,∥ †
Key Laboratory of Eco-textiles, Ministry of Education, College of Textiles and Garments, Jiangnan University, Lihu Road #1800, Wuxi 214122, China ‡ Department of Textiles, Merchandising & Fashion Design, §Department of Biological Systems Engineering, and ∥Nebraska Center for Materials and Nanoscience, University of NebraskaLincoln, Lincoln, Nebraska 68583-0802, United States ABSTRACT: A new approach has been developed to add high value to corn Distillers Dried Grains with Solubles (DDGS) by developing thermoplastics with controllable dry and wet tensile properties. Although attempts have been made to develop thermoplastics from DDGS, the presence of carbohydrates, proteins, and oil makes it difficult to use or modify DDGS for various applications and obtain useful thermoplastics. In this research, DDGS was grafted with individual and a combination of four vinyl monomers (methyl, ethyl, butyl, and hexyl methacrylate; MMA, EMA, BMA and HMA, respectively) and the grafted DDGS was compression molded into films. The type of monomer and amount of homopolymers in the films considerably influenced tensile strength and water stability. At similar grafting ratios, HMA provided higher thermoplasticity, but films with BMA homopolymers had higher elongations (up to 19%). The addition of homopolymers was necessary to obtain films with good strength, elongation, and water stability.
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INTRODUCTION Biothermoplastics that can replace the traditional plastics derived from synthetic polymers such as polypropylene, polyethylene, and polystyrene have gained considerable attention in the past decade mainly due to the nonbiodegradability of the synthetic polymer based materials and related environmental concerns.1 Starch and cellulose are the two most common biopolymers that are widely available and used but are nonthermoplastic. Various approaches including chemical modifications such as acetylation, etherification, and grafting have been used to convert natural biopolymers into thermoplastics. Cellulose and starch acetates are commercially available in various forms for thermoplastic applications. Although a polyester, poly(lactic acid) derived from corn starch is another common biothermoplastic that has been used to make fibers, films, extrudates, and other products. Another approach to develop biothermoplastics or bioplastics is to use microorganisms. Synthetic bioplastics commonly called poly(alkaonates) are currently being used in commodity products.2 Although considerable progress has been made to develop renewable and sustainable sources to replace traditional plastics, high cost and relatively inferior performance properties have limited the use of biothermoplastics for commodity applications. Cereal grains such as corn, soybeans, and camelina have been commercially used as sources for biofuels. However, the production of biofuels from cereal crops generates up to 50% of the cereal as coproducts. Value addition to these coproducts is a key to have economically competitive and sustainable biofuels compared to fossil fuels.3 Currently, corn ethanol is the predominant biofuel in the United States. Approximately 37.3 billion L of ethanol and consequently about 33.4 × 109 kg of distillers dried grains with solubles (DDGS) were produced as coproducts in the United States in 2009−2010. DDGS, the © XXXX American Chemical Society
coproduct of corn ethanol production, contains up to 10% oil, 25% proteins, and 45−50% carbohydrates, which are valuable raw materials.4,5 To date, the major use of coproducts of biofuel production, including DDG, is animal feed, which is a low value and limited market application. Substantial amounts of coproducts are exported due to oversupply in the domestic market. Currently, DDGS sells at about $125 per 103 kg, nearly 10 times lower than the price of synthetic polymers such as polyethylene, polyester, and polystyrene. In addition to the low cost and large availability, DDGS is derived from corn, a renewable and sustainable source, and is biodegradable. Therefore, attempts have been made to utilize DDGS as a source for various products.6 However, DDGS is nonthermoplastic which limits its industrial applications. In addition, DDGS is a mixture of proteins, carbohydrates, and oil, which makes it difficult to be chemically modified. Many studies have been done to utilize DDGS with or without chemical modifications. For instance, DDGS without any modifications was mixed with phenolic resin and extruded as composites.7 It has been shown that acetylation and etherification can be used to make DDGS thermoplastic. Etherified DDGS was reported to have good tensile strength and elongation even without the use of any plasticizers, but the thermoplastic films developed had poor water stability.8 DDGS acetylated under alkaline and acidic conditions was also compression molded into thermoplastic films but required the use of plasticizers.9,10 Instead of using the complete DDG, the oil in DDGS was extracted, esterified, and converted into biodiesel that could meet U.S. and European specifications after Received: May 17, 2014 Revised: August 13, 2014 Accepted: August 18, 2014
A
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Scheme 1. Graft Polymerization of DDGS with Four Vinyl Monomers (Methyl, Ethyl, Butyl, and Hexyl Methacrylate) through NaHSO3/K2S2O8 Redox System
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adding some additives.11 Similar to DDGS, other coproducts such as wheat gluten, soy proteins, camelina meal, soymeal, and poultry feathers have also been studied for potential thermoplastic applications.1,12,13 Although considerable efforts have been made to develop biothermoplastics, low elongation, poor water stability, and the need for expensive chemical modifications are some of the major deficiencies of thermoplastics developed from natural biopolymers.14,15 Considerably high amounts of plasticizers are necessary to improve the elongation and make biothermoplastics useful.14 Unfortunately, plasticizers decrease the tensile strength and increase the susceptibility of the products to water. Among the various types of chemical modifications, grafting is more appropriate to develop thermoplastics form biopolymers. Grafting avoids damage to the main chain of the biopolymers, and therefore the products developed can have better tensile properties and also retain biodegradability. Grafting is also performed under mild conditions which avoid the degradation of the proteins in DDGS that could help to obtain products with good tensile properties. Acrylic monomers such as methyl, ethyl, butyl acrylates, and methacrylates are widely used for grafting because they are comparatively inexpensive and provide good thermoplasticity, tensile properties, and water stability to the grafted products.12,13 Recently, we have shown that camelina meal grafted with methyl methacrylate, ethyl methacrylate, and butyl methacrylate had good tensile strength and elongation even under wet conditions.16 In the previous study, it was found that the presence of homopolymers affected the tensile properties and water stability of the films to a large extent and that an optimum level of homopolymers was necessary to obtain good tensile properties.16 In this research, DDGS was grafted with four types of methyacrylates. The grafted DDGS was compression molded into films, and the effect of grafting conditions on the dry and wet tensile properties of thermoplastics was investigated.
MATERIALS AND METHODS Materials. Distillers Dried Grains with Solubles (DDGS) was supplied by Abengoa Bioenergy Corporation, York, Nebraska. DDGS was obtained during the fermentation of the corn into ethanol and was used as received. The DDG used in this research had about 40% proteins, 30−35% carbohydrates, and 10% moisture. Raw DDG did not melt even with the addition of glycerol but decomposed above 230 °C when compression molded. Monomers used in this study were methyl methacrylate (99.8% MMA, TCI America), ethyl methacrylate (98% EMA, Alfa Aesar), butyl methacrylate (99% BMA, Acros Organics), and hexyl methacrylate (98% HMA, TCI America). The oxidant used was potassium persulfate, and the reducing agent was sodium bisulfite. DDGS was Soxhlet extracted for 12 h using acetone to remove oil and other substances. The extraction was done to ensure that the acetone extractables did not interfere with the calculation of grafting parameters such as grafting efficiency and % grafting ratio. The extracted DDGS was dried and later used for grafting. Grafting DDGS with Methyl Acrylates. The acetone extracted DDGS was grafted with MMA, EMA, BMA, and HMA based on our initial studies, which showed that methyl acrylates were found to provide films with better tensile properties than those grafted with acrylates.12,16 Grafting was done in closed four-necked glass bottles after deoxygenating the flask by passing nitrogen gas. Potassium persulfate was used as the initiator, and sodium bisulfite was used as the reducing agent. Grafting temperature and time were chosen based on our previous studies on grafting camelina meal and chicken feathers.12,16 In this research, the grafting temperature was 70 °C, and grafting time was 60 min. The ratio of monomer to DDGS was varied from 0.4:1 to 1:1, and the weight of the initiator and reductant based on weight of DDGS used was 5% and 1.9%, respectively. Monomers were added slowly into DDGS, and the addition was completed in 30 min. The grafting reaction was continued for 1 h under constant stirring using a mechanical stirrer (Talboys Engineering Corporation, Model T B
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180 °C) for a predetermined time at a pressure of 270 MPa. Different temperatures were used for compression molding because the samples grafted with different monomers had different thermoplasticity. Thermoplastics with Homopolymers. To understand the effect of homopolymers on tensile properties and water stability, a known amount (25, 50, or 75%) of homopolymer from the corresponding monomer was mixed with the grafted samples. Homopolymers were first dissolved in acetone and then mixed with the grafted DDGS in various ratios. Acetone was evaporated at a low temperature (50 °C) in an oven, and the samples were conditioned until a constant weight was obtained. Samples containing the homopolymers were compression molded as described above. The pure (100%) homopolymers (except HMA which was too soft) were also compression molded into films and tested for their tensile properties. Dry and Wet Tensile Properties of Thermoplastics. Films obtained after compression molding were conditioned at 21 °C and 65% relative humidity before testing. Samples were tested according to ASTM standard D882 using a crosshead speed of 10 mm min−1 and gauge length of 50 mm. To study the wet stability, samples were immersed in 20 °C water for 30 min and then tested immediately. About 20 samples from three different films were tested for each condition, and the average and standard deviations are reported.
line 134-1). After the desired reaction time, the graft polymerization was terminated by adding 8 mL of 2% paradioxybenzene. Grafted DDGS was filtered, neutralized by washing in water, and later dried in an oven at 105 °C. Scheme 1 depicts the grafting process and the possible reaction mechanism. DDGS was also grafted with two monomers MMA, EMA, or BMA with HMA. For grafting with the mixed monomers, the two monomers were added together and the grafting reaction was completed as described above. After grafting, the grafted products were extracted with acetone to remove the homopolymers, and the samples were then used to develop films. Homopolymer Synthesis. The amount of homopolymers affects the thermoplasticity of the grafted samples and tensile properties of the products developed. To understand the effect of homopolymer content on tensile properties and water stability, homopolymers were separately synthesized from each of the monomers studied. Homopolymers were synthesized under the same conditions used to graft the monomers except that there was no DDGS in the reaction. Synthesized homopolymers were dried and collected. Determining Grafting Efficiency. Grafting parameters such as % grafting, molar grafting ratio, and % grafting efficiency were determined using equations described in our previous research.16 The molar grafting ratio and % grafting describe the amount of substance and the weight of the monomers grafted onto DDGS. The grafting efficiency describes the weight of monomers grafted onto DDGS compared to the initial weight of the monomer used for the reaction. The % monomer conversion was determined by titrating the double bonds in the final product (grafted DDG) obtained.16 Grafting (%) was calculated using the equation below. %grafting =
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RESULTS AND DISCUSSION Effect of Monomer Concentration on % Grafting. The amount of monomer used during grafting was varied to obtain different levels of grafting. As seen from Figure 1, an optimum
W1 − W2 − (Wb − Wa) × 100 W0
W0 was the weights of raw DDGs. W1 was the weights of total monomer added. W2 was the residual monomer. Wb was the weights of the products before extraction. Wa was the weights of the products after extraction. Fourier Transform Infrared (FTIR). Ungrafted and grafted DDGS was characterized in an attenuated total reflectance FTIR (Thermo Electron, Model iS10) with a diamond cell. Samples compressed into pellets were scanned 32 times at a resolution of 8 cm−1. Thermal Analysis. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to study the thermal behavior of the ungrafted and grafted samples. For TGA, about 10 mg of samples were heated from 30 to 600 °C under a nitrogen atmosphere in a Sigma T300 analyzer at a heating rate of 20 °C min−1. DSC studies were conducted in sealed aluminum pans on a Mettler Toledo (Model D822e) differential scanning calorimeter by heating about 7−10 mg of samples from ambient temperature to 250 °C at a heating rate of 20 °C min−1 under a nitrogen atmosphere. Developing Thermoplastics. Thermoplastics without Homopolymers. Ungrafted and samples grafted with similar grafting ratios but without any homopolymers were compression molded into films on a Carver press. Glycerol (10% w/w) was added to improve thermoplasticity and reduce brittleness. About 10 g of the samples were placed between two aluminum foils and compression molded at different temperatures (150−
Figure 1. Influence of % initial monomer concentration on % grafting. Grafting was done at 70 °C for 5 min with potassium persulfate as an initiator and sodium bisulfite as the reductant.
level of monomers was necessary to obtain high % grafting. MMA grafted DDGS had the highest grafting ratio at a lower concentration of monomer (40%), but 60% monomer was required for EMA and BMA to obtain a similar grafting ratio as that of MMA. Increasing the concentration of MMA did not show any appreciable increase in % grafting, and in fact, the % grafting was low at a monomer concentration of 80 and 100%, especially for MMA. Increasing the concentration of monomers influences the amount of homopolymers and the % monomer conversion, both of which affected % grafting. As seen from Figure 2, increasing the % monomers increased the % homopolymers continually. Monomer conversion initially C
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may have modified the properties of the DDGS components leading to faster degradation. The overall weight loss was 75% for ungrafted DDGS and for HMA grafted DDGS and 81% for the DDGS grafted with MMA, EMA, and BMA. Higher weight loss of the grafted samples should be due to the relatively unstable grafted polymers compared to the carbohydrates and proteins in unmodified DDGS. DSC curves in Figure 5 show that HMA had a considerably lower melting peak at 164 °C with a melting enthalpy of 40 J g−1. MMA, EMA, and BMA have progressively higher melting peaks at 177, 192, and 202 °C. The corresponding melting enthalpies were 29.5, 19.2, and 14.7 J g−1, respectively. Higher melting temperatures and lower melting enthalpies for MMA, EMA, and BMA compared to HMA suggest that HMA is more thermoplastic than DDGS grafted with the other monomers. The ability of the HMA grafted DDGS to melt and form films at considerably lower temperature (150 °C) compared to MMA, EMA, or BMA during compression molding corroborates the DSC results. Figure 6 showed that DDGS grafted with mixed monomers had similar melting behavior when grafted with MMA-HMA and EMA-HMA and showed a small melting peak at about 160 °C and another sharp and larger peak at about 190 °C. The two distinct peaks in the samples containing the mixed polymers should be due to the differential melting of HMA and the other polymer. The initial melting at 160 °C should be due to the partial melting of HMA polymers, and the other larger peak at 190 °C should be due to the combined melting of HMA and MMA/EMA. In Figure 6, MMA showed a broad melting peak between 170 and 200 °C. When combined with HMA, the grafted DDGS sample had a relatively sharper melting peak at 190 °C. Samples grafted with BMA−HMA showed a broad melting peak between 170 and 220 °C compared to the melting peak seen at 200 °C for samples grafted only with BMA. It appears that grafting of HMA and BMA resulted in some of the DDGS that did not melt when grafted with BMA alone being able to melt and produce the broad melting peak. The DSC results of the samples grafted with the two monomers suggest that the monomers are partially compatible and that cografting could help to decrease the melting temperature and produce better thermoplastics. Comparison of Tensile Properties. Tables 1 and 2 provide the tensile properties of the films made from DDGS grafted with various monomers and at different ratios of homopolymers that were added into the grafted DDGS. The addition of homopolymers considerably changed the strength and elongation of the films. Without homopolymers, films made from the DDGS grafted with different monomers had similar strength. However, the elongation of the films increased with increasing length of the monomer side groups. MMA had only 50% of the elongation compared to films grafted with HMA. Increasing the amount of homopolymers had varying effects on the tensile properties for each of the monomers studied. For MMA grafted DDGS, adding 25 and 50% homopolymers decreased the strength, but the DDGS films containing 75% homopolymers had considerably higher strength, similar to the films made from 100% MMA homopolymers. On the contrary, the strength of the EMA and BMA grafted films with 25% homopolymers was higher than that of the films containing 50 and 75% homopolymers. Even the 100% EMA and BMA homopolymer films had considerably lower strength than the films containing 25% homopolymers. HMA homopolymers were very soft and sticky,
Figure 2. Influence of % initial monomer concentration on the homopolymer and monomer conversion.
increased with increasing monomer concentration but then decreased except for HMA where increasing monomer concentration increased the % monomer conversion. However, high % monomer conversion does not necessarily mean good grafting because monomer conversion includes the polymers grafted onto the sample and also homopolymers formed during the reaction. High homopolymers would mean lower grafting onto the sample and therefore decrease in grafting efficiency. In addition to higher monomer conversion, it would be desirable to achieve a higher grafting ratio and lower homopolymers. Higher % grafting for HMA compared to the other monomers is due to the longer side chains in HMA that provide a better opportunity for the HMA monomers to be grafted onto DDGS. Due to the longer side chains, a higher number of reaction sites will be available for grafting, and hence, higher % grafting can be achieved.16 FTIR. The peak at about 1700 cm−1 in DDGS grafted with more monomers as seen in Figure 3a is due to the vibrations from the CO stretch of the ester group.17,18 The absence of intense peaks at 3040−3010 cm−1 and 1690−1635 cm−1 due to the stress of the (C−H) group and CC flexion of the unsaturated carbonyl compounds indicated the absence of any unreacted monomers.19 Another intense peak is observed for the grafted samples at about 1400 cm−1 but is not present in the ungrafted sample. This peak is due to the C−O stretching of the ester group of the polyacrylates.20 In the fingerprint region (1500−500 cm−1) seen in Figure 3b, peaks belonging to C−H stretching of the alkyl group are seen at about 1380 cm−1 in MMA, EMA, and BMA but absent in the HMA grafted DDGS. Also, the small peaks seen in all four spectra at 720 cm−1 confirm the presence of CH2 groups. The additional peaks present in the grafted samples confirm grafting of the acrylates onto DDGS. Subtle differences in the fingerprint regions suggests that the grafted samples have slightly different arrangement of polymers on the DDGS. Thermal Analysis. Grafted DDGS showed better thermal stability than ungrafted DDGS up to a temperature of about 390 °C as seen from the thermal degradation curves in Figure 4. MMA and EMA grafted DDGS have very similar degradation behavior and show better stability than the HMA and ungrafted DDGS. Above a temperature of 400 °C, the grafted samples showed faster degradation than the ungrafted DDGS due to the instability of the ester group. In addition, the grafting reaction D
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Figure 3. Comparison of the FTIR spectrum of ungrafted and DDG grafted with MMA, EMA, BMA, and HMA. (b) FTIR peaks in the fingerprint region show subtle differences in configuration of the four polymers on DDG.
and it was not possible to prepare films containing HMA homopolymers higher than 25%. Without homopolymers, the HMA grafted films had similar strength but much higher elongation compared to the MMA, EMA, and BMA grafted films. Adding the homopolymers increased the water resistance of the films as seen from Table 2. Without the homopolymers, films made from all four monomers had considerably low wet strength. MMA and EMA grafted films had nearly twice higher strength than BMA and HMA whereas BMA and HMA grafted films had 2−5 times higher elongation than MMA and EMA
grafted films. At 25% homopolymers, the wet strength of the films increased 3−4 times compared to the strength of the films without homopolymers. A further increase in homopolymers did not show a major increase in strength for EMA and BMA, but the MMA grafted films with 75% homopolymers had a strength of 4.5 MPa, more than twice the strength at lower % homopolymers. Tensile properties and water stability of the grafted films are mainly dependent on the thermoplasticity and the amount and properties of homopolymers in the films. Without any or at 25% homopolymers, DDGS does not melt completely, and the E
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homopolymers makes the DDGS to melt better and form uniform films. It should be noted that the highest strength for the DDGS films grafted with MMA was obtained at 75% homopolymers, whereas similar strength was obtained for EMA and BMA grafted DDGS at 25% homopolymers. This indicated that a certain level of thermoplasticity was necessary for the DDGS to provide high strength to the films. Elongation of the films was related to the elongation of the homopolymers. EMA homopolymer was considerably brittle, and therefore films containing EMA had low elongation. Contrarily, BMA had very high extension, and BMA grafted DDGS films even without adding homopolymers had higher elongation than MMA and EMA grafted films. Although the addition of 25 and 50% homopolymers did not increase the elongation of the BMA grafted films compared to films without homopolymers, films containing 75% BMA homopolymers had substantially high elongation of 19%. Inclusion of homopolymers generally improved the wet tensile properties, particularly strength as seen from Table 2. All the homopolymers had lower wet strength, especially EMA, but similar elongations compared to their respective dry strength and elongation. HMA grafted DDGS films had low wet strength but more than twice the elongation of the dry films. Wet tensile strength of the films increased several fold even with 25% homopolymers because of the hydrophobicity of the homopolymers that prevented the DDGS from being swollen and losing strength. Tensile properties of the films in Tables 1 and 2 show that a certain level of homopolymers is necessary to form films with adequate strength (wet) and elongations, which was also observed when methacrylates were grafted onto camelina meal.16 Although increasing the length of the side chains of the monomers should provide higher hydrophobicity, HMA had lower wet strength than the MMA or EMA grafted DDGS. This is because of the differences in the amount of grafted polymers on the DDGS. Samples in Table 2 had similar % grafting, but due to the differences in molecular weights between the monomers, the amount of grafted polymers will be lower for HMA compared to EMA or BMA. The actual amounts of grafted polymers were 2.0 mmol g−1, 1.4 mmol g−1, 1.8 mmol g−1, and 1.2 mmol g−1 for MMA, EMA, BMA, and HMA, respectively. In addition to the higher amount of polymers for MMA and BMA, the smaller the size of the monomers, the higher will be the number of monomers that are available to react and block the hydrophilic groups in DDGS. Therefore, HMA will have a lower possibility to block the hydrophilic groups, and the DDGS grafted with HMA will have higher moisture sorption than MMA or EMA. Overall, HMA grafted DDGS had high thermoplasticity, but the homopolymers were too sticky to use and BMA homopolymers provided better elongation to the films. Properties of Films with Mixed Monomers. Tables 1 and 2 showed that EMA was considerably brittle but provided good strength, whereas HMA grafted DDGS was flexible but had lower tensile strength. We therefore attempted to combine the advantages of the individual monomers and grafted DDGS with a combination of two monomers as seen in Table 3. Except for MMA blended with HMA, the addition of HMA improved the tensile strength compared to the DDGS grafted with the individual monomers. Elongation of the DDGS films was higher for MMA/HMA, similar for EMA/HMA, but lower for BMA/HMA compared to the DDGS films grafted with the individual monomers without any homopolymers. Modulus of the films was considerably lower for MMA/HMA compared to
Figure 4. Comparison of the TGA curves of ungrafted and DDG grafted with MMA, EMA, BMA, and HMA.
Figure 5. Comparison of the DSC curves of DDG grafted with MMA, EMA, BMA, and HMA containing 10% glycerol and with a grafting ratio of about 28%.
Figure 6. DSC thermogram of DDG grafted with random copolymers formed by the polymerization of the two monomers with a combined grafting ratio of about 32%.
films formed are like composites. These films will have high strength but low elongation as seen for the EMA and BMA films with 25% homopolymers. Increasing the percentage of F
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Table 1. Dry Tensile Properties of Films Made from DDG Grafted (20%) with Various Monomers and at Different Levels of Homopolymersa breaking stress, MPa % homo polymers 0 25 50 75 100
MMA 3.9 3.1 3.6 6.0 12.8
± ± ± ± ±
EMA
0.8 0.4 0.5 1.3 1.0
3.9 6.3 2.9 3.4 8.5
± ± ± ± ±
0.5 1.0 0.5 0.9 0.9
breaking elongation, % b
BMA
HMA
± ± ± ± ±
2.9 ± 0.8 1.3 ± 0.2
2.8 6.6 2.4 2.1 5.8
0.4 0.9 0.6 0.3 0.6
MMA 2.0 2.7 2.1 2.0 2.3
± ± ± ± ±
0.1 0.6 0.5 0.4 0.3
EMA ± ± ± ± ±
1.5 1.9 1.6 1.5 3.4
0.2 0.2 0.6 0.4 0.6
BMA
HMA
± ± ± ± ±
3.4 ± 0.9 11.2 ± 4.2
3.3 3.0 3.1 19 70.3
0.9 0.8 1.0 2.9 4.2
MMA, EMA, and BMA films were compression molded at 180 °C for 5 min, and HMA films were compression molded at 150 °C for 5 min at a pressure of 270 MPa. bHMA with 50 and 75% homopolymers were too soft and could not be compression molded. a
Table 2. Wet Tensile Properties of Films Made from DDG Grafted (20%) with Various Monomers and at Different Levels of Homopolymersa breaking stress, MPa % homo polymers 0 25 50 75 100
MMA 1.1 2.9 2.1 4.5 4.7
± ± ± ± ±
0.1 0.6 0.3 0.8 0.3
EMA 0.9 2.6 2.5 3.3 2.0
± ± ± ± ±
0.2 0.4 0.5 0.8 0.7
breaking elongation, %
BMA
HMA
± ± ± ± ±
0.5 ± 0.05
0.5 2.3 2.0 2.5 2.8
0.1 0.3 0.2 0.4 0.4
MMA 1.9 2.7 2.1 2.4 2.0
± ± ± ± ±
0.2 0.6 0.5 0.4 0.4
EMA 1.5 1.9 1.6 1.5 1.3
± ± ± ± ±
0.2 0.2 0.6 0.4 0.2
BMA
HMA
± ± ± ± ±
9.5 ± 1.3
4.7 5.7 5.7 11.2 129
1.5 1.4 1.4 3.1 34
MMA, EMA, and BMA films were compression molded at 180 °C for 5 min, and HMA films were compression molded at 150 °C for 5 min at a pressure of 270 MPa. a
Table 3. Comparison of the Dry and Wet Tensile Properties of DDG Grafted with Two Monomers (Combined Grafting Ratio of about 32%)a dry tensile properties
a
wet tensile properties
type of monomers
breaking stress, MPa
breaking elongation, %
modulus, MPa
breaking stress, MPa
breaking elongation, %
modulus, MPa
MMA/HMA EMA/HMA BMA/HMA
2.8 ± 0.4 5.2 ± 1.1 4.8 ± 0.9
3.4 ± 0.5 1.9 ± 0.2 1.9 ± 0.1
285 ± 32 800 ± 69 752 ± 102
0.8 ± 0.2 1.1 ± 0.2 1.1 ± 0.4
7.8 ± 1.2 4.0 ± 0.8 2.4 ± 0.4
39 ± 14 86 ± 8.0 138 ± 59
Homopolymers were extracted from the samples that were compression molded at 160 °C for 5 minutes.
polyethylene, polypropylene, and polystyrene, which have current selling prices ranging from $1.50 to $2.50 kg−1. Biothermoplastics such as starch acetate, cellulose acetate, and poly(lactic acid) have selling prices higher than $4.40 kg−1. We therefore anticipate that grafted DDGS would be competitive to the synthetic and biopolymers currently used for thermoplastic applications.
EMA/HMA or BMA/HMA. Lower strength and modulus but higher elongation for the MMA/HMA films was due to the better melting of this blend during compression molding. MMA/HMA films retained about 28% strength when wet compared to 20% for EMA/HMA and BMA/HMA films. Elongation of all the films increased when wet but the increase was relatively low for the BMA/HMA because BMA and HMA had considerably lower moisture sorption than the other two monomers. Grafting DDGS with a blend of two monomers would be useful to improve the strength and/or water stability of the grafted samples. Economics of Developing DDGS Thermoplastics and Potential Value addition. DDGS currently sells at about $125 per 103 kg, which is equivalent to about $0.12 kg−1. Monomers such as methyl methacrylate used in this study have a current selling price between $2.1 and 2.4 kg−1 (http://www. icis.com/chemicals/methyl-methacrylate/#?tab=tbc-tab2&_ suid=1359469164536009902992989747022). Based on the ratio of monomers to DDGS used, monomer conversion and grafting efficiency, and cost of other chemicals used for grafting, we estimate that it would cost about $1.35−$1.60 to produce a kilogram of grafted DDGS on a laboratory scale assuming that other overhead and production costs are negligible. The cost of producing DDGS could be considerably cheaper for bulk production. Even at $1.35−$1.60 kg−1, thermoplastic DDGS would be competitive to common synthetic polymers such as
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CONCLUSIONS
The type of monomer and the amount of homopolymers were found to control the tensile properties, especially elongation and wet strength of DDGS films grafted with various acrylic monomers. Among the four monomers studied, HMA provided the highest thermoplasticity, and films containing BMA homopolymer had better elongation, whereas EMA homopolymers and films from EMA grafted DDGS were considerably brittle. HMA homopolymers were too soft, and samples containing higher than 25% HMA homopolymers had poor dry and wet tensile properties. Grafting DDGS with two monomers showed that the two monomers were partially compatible and generally increased the thermoplasticity and would be helpful to develop thermoplastics with higher strength and elongation. At about $0.12 kg−1 and the ability to become thermoplastic at low grafting ratios, thermoplastics from grafted DDGS will be inexpensive and show potential to replace the traditional G
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Industrial & Engineering Chemistry Research
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synthetic polymers or expensive biopolymers for thermoplastic applications.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 402-472-5197. Fax: 402-472-0640. E-mail: yyang2@ unl.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank 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), agricultural research division, USDA-HATCH act, and Multistate project S1054 (NEB 37-037) at the University of NebraskaLincoln for their financial support; the China scholarship council is also acknowledged for providing the living expenses for the visiting scholar (Z.S.).
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REFERENCES
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dx.doi.org/10.1021/ie501987n | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
