Sustainable Animal Protein-Intermeshed Epoxy Hybrid Polymers

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Article Cite This: ACS Omega 2018, 3, 14361−14370

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Sustainable Animal Protein-Intermeshed Epoxy Hybrid Polymers: From Conquering Challenges to Engineering Properties Xiaoyan Yu,†,‡ Sreeprasad Sreenivasan,†,§,∥ Kevin Tian,†,⊥ Ting Zheng,†,‡ Joseph G. Lawrence,§ and Srikanth Pilla*,†,‡,#

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†

Department of Automotive Engineering and ‡Clemson Composites Center, Clemson University, 4 Research Dr, Greenville, South Carolina 29607, United States § Polymer Institute, The University of Toledo, 2801 W Bancroft Street, Toledo, Ohio 43606, United States ⊥ Southside High School, Greenville, SC 29605, United States # Department of Materials Science and Engineering, Clemson University, Sirrine Hall, Clemson, SC 29634, United States S Supporting Information *

ABSTRACT: The presence of highly modifiable chemical functional groups, abundance of functional groups, and their biological origin make proteins an important class of biomaterials from a fundamental science and applied engineering perspective. Hence, the utilization of proteins from the animal rendering industry (animal protein, AP) for high-value, nonfeed, and nonfertilizer applications is intensely pursued. Although this leads to the exploration of proteinderived plastics as a plausible alternative, the proposed methods are energy-intensive and not based on protein in its native form, which leads to high processing and production costs. Here, we propose, for the first time, novel pathways to develop engineered hybrid systems utilizing AP in its native form and epoxy resins with mechanical properties ranging from toughened thermosets to elastic epoxy-based systems. Furthermore, we demonstrate the capability to engineer the properties of epoxy−AP hybrids from high-strength hybrids to elastic films through controlling the interaction, hydrophilicity, as well as the extent of cross-linking and network density. Through the facile introduction of cochemicals, a sevenfold increase in the mechanical properties of the conventional epoxy−AP hybrid is achieved. Similarly, because of better compatibility afforded by the similar hydrophilicity, AP demonstrated higher cross-linking capability with a water-soluble epoxy (WEP) matrix, resulting in an elastic WEP−AP hybrid without any external aid.

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INTRODUCTION The accelerated depletion of nonrenewable petroleum reserves and a significant concern over environmental problems such as “white pollution” have driven a universal movement toward the concept of “sustainable development”.1 The push toward such a sustainable cyclical economy will involve the production of biobased polymers and composites that will substitute their petroleum-based counterparts.1−3 Hence, renewable resourced polymers and composites are projected to be the future of green economy owing to their renewability, low cost, and tantamount characteristics.4−11 In most cases, biobased materials incorporate industrial products, while avoiding the use of food, feed, or materials produced from renewable agricultural and forestry feedstock, such as wood, wood wastes, and residues, grasses, or crop wastes. Among various plausible underutilized renewable and natural materials, proteins demand greater attention. The intricate internal structure, abundance of functional groups, and surface chemistry of proteins make them ideal candidates for a multitude of applications.12−14 However, the use of such proteinaceous materials, particularly the ones obtained from the rendering © 2018 American Chemical Society

industry, is mostly confined to low volume and economic value applications. This led to extensive research in recent times toward finding suitable, alternate, nonfeed, and nonfertilizer applications for such proteinaceous materials. A potential surrogate use for proteins derived from the rendering industry (termed animal protein or AP hereafter) has emerged in the form of producing plastics and composites. For example, the complex inter- and intramolecular couplings present in solidified protein residues can be capitalized on to designing materials and plastics with enhanced stiffness. Similar to carbon black, proteins are known to aggregate and form reinforcing networks when integrated into elastomers and rubbers.8 The interconnected matrix formation capacity of proteins leads to their use in packaging,13−15 composites,16 regenerated fibers,17 and adhesives.18 However, a complex interdigitated network of proteins erected through hydrogen bonding, electrostatic and hydrophobic interactions, as well as Received: June 13, 2018 Accepted: October 10, 2018 Published: October 30, 2018 14361

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Figure 1. (a) Scheme depicting the three primary modes of reaction between the protein and the epoxy matrix. (b) Schematic of the protein-crosslinked epoxy matrix. (c) FTIR spectrum of the epoxy matrix before and after the cross-linking reaction. Arrows indicate the important spectral changes (AP weight is 30 wt %). (d) Plot showing the variation of the reaction heat (enthalpy) with the increasing protein weight ratio in the epoxy−AP hybrid. (e) Typical DSC curve for the epoxy−AP hybrid containing 30 wt % AP by weight.

disulfide bonding also results in stiff and brittle structures.19 Although chemical modification and plasticization allow the creation of more processable and useful materials,20 they also lead to inferior mechanical properties (e.g., simultaneous reduction in stiffness and strength and enhancement in extensibility). Moreover, the high moisture absorption (which is aggravated by the use of hydrophilic plasticizers) and the inherent order associated with certain proteinaceous materials in conjugation with ordinary mechanical properties limit the widespread use of proteins, especially in highperformance structural applications.21 Epoxy resins, among various possible candidates for highperformance structural composite applications, have emerged as one of the most important industrial engineered plastics.22 Because of their superior chemical, electrical, heat resistance, adhesion, and mechanical properties as well as low cost, epoxy resins are widely employed in composites, coatings, adhesives, and automotive components.23 For epoxy-based structural composite applications, the system generally contains an epoxy resin and an external cross-linking agent that helps to form the cross-linked epoxy matrix, which results in the enhanced mechanical properties of the composite structures. The most widely used epoxy prepolymer is diglycidyl ether of bisphenol A (DGEBA) derived from petroleum products, whereas the primary cross-linkers employed are amine-based synthetic chemicals.24,25 The constant increase of oil prices and the nonsustainable nature coupled with its intrinsic poor impact resistance (due to the highly cross-linked structure) as well as relatively poor resistance to crack initiation and growth are the biggest challenges of cross-linked epoxy systems when considered for engineering applications. Although significant efforts have been made to developing toughened epoxy composites with and without the use of external fillers as well as biobased epoxies, developing a product that is viable for industrial applications is still an engineering pursuit.9,25−34 In the direction of enhancing the sustainability, various studies looked into the opportunity to find a sustainable replacement for typical epoxy resin components,25 including the incorpo-

ration of vegetable oils10,35 and natural rubber. However, an introduction of a foreign body into the epoxy matrix typically resulted in overall adverse effects on the thermal and mechanical properties. In this direction, proteinaceous materials that can simultaneously work as fillers as well as cross-linking agents provide an alternate potentially beneficial pathway. The rich surface functionality of AP and the possibility to chemically engineer their interaction with an epoxy system make them a viable candidate for futuristic biobased structural composite applications. Consequently, efforts were undertaken to investigate the properties and behavior of plastics and composites derived from APs. However, efforts to create a protein-based thermoplastic film by unmodified36 or surfacemodified proteins37,38 as well as to develop feather meal protein-based biodegradable polymers39 resulted in inferior mechanical properties. Moreover, the hydrophilic nature of the resultant polymers proved detrimental for their processing by expediting their thermal degradation, leading to inferior thermal properties, making it a challenge to obtain highperformance materials. Hence, such materials are limited to low performance and perhaps low economic value applications. More recently, because of the exciting economic value, much interest is expressed toward the preparation of protein-based thermoset materials. Most of the studies targeted protein functional groups, especially amino acid residuesprimary amines, carboxyl, hydroxyl, and sulfhydryls and their reactivity toward a particular chemical. Among these studies, aldehydes (formaldehyde, glutaraldehyde, and glyoxal) are the most commonly used agents for the chemical cross-linking of protein monomers.40−42 However, the release of toxic monomers41 from such cross-linked polymeric materials limits the use of protein-based plastics. More recently, Mekonnen et al.43,44 employed peptide molecules from the hydrolyzed AP as cross-linkers to construct cross-linked DGEBA thermoset resins. In addition to being an energy-intensive and expensive process, the resultant thermosets demonstrated poor cure 14362

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Figure 2. (a) DSC curves (exo up) at different heating rates for the epoxy−AP sample (ideal configuration: epoxy 70% and AP 30%). (b) Constructed Kissinger plots for the epoxy−AP hybrid polymer with a 7:3 weight ratio between DEGBA and AP.

temperatures, the functional groups on the AP react with the epoxy groups in the resin to form a cross-linked network, resulting in the proposed protein-intermeshed thermoset material (epoxy−AP). The hybrid was investigated using different spectroscopic, microscopic, and mechanical tests to understand the fundamentals of the epoxy−AP interaction and the resultant properties. First, the cured sample was analyzed using infrared (IR) spectroscopy. The IR spectra (Figure 1c) were obtained for the system before and after the cross-linking reaction to verify the successful cross-linking between AP and DGEBA as well as to understand the chemical changes for the functional groups during the cross-linking reaction. The emergence of a new feature at around 1728 cm−1 (due to the formation of an ester group through the curing reaction between DGEBA and protein functional groups) and a concurrent decrease in intensity of the peak at 859 cm−1 (corresponding to the epoxy groups) indicate the curing reaction through the coupling of epoxy groups in the resin and functional groups on the protein. The coupling reaction results in the formation of more ester linkages in the cured system and a reduction of epoxy groups, which are utilized to form a crosslinked structure. After establishing the coupling reaction between the AP and the epoxy, the ratio between the AP and the epoxy to get the best curing and kinetics was investigated. Most applications which use epoxy require the epoxy to undergo curing. It is proven that variations in the ratio between the curing agent and the resin can lead to different mechanical properties with very low or high amount of the cross-linking agent, leading to dramatic deterioration in the mechanical properties.46 Hence, it is prudent to investigate the optimal ratio of the epoxy resin and the cross-linking agent (AP) used. Differential scanning calorimetry (DSC) is a widely accepted technique to monitor the heat change during curing of thermoplastics47,48 and investigate the kinetics of curing reactions for thermoset epoxy resins.49 We employed DSC to monitor the extent of crosslinking in the epoxy systems having different AP contents (ranging from 5 to 50 wt %). Curing being exothermic, the heat of reaction corresponding to curing should be positive,50,51 and hence, we calculated the total area under the exothermal peak to understand the extent of curing. Figure 1d shows the heat of reaction obtained from DSC for epoxy samples containing different weight ratios of AP. We can observe that the maximum heat of reaction is for the sample containing 30 wt % protein and 70 wt % epoxy, which was fixed as the optimal ratio between the epoxy and the AP. A typical DSC curve for the sample with optimal composition (epoxy 70% and AP 30%) is shown in Figure 1e. An

kinetics and significant reduction in strength and strain-atbreakparameters critical for structural applications. As illustrated above, a lack of in-depth understanding about the protein−matrix interactions and an absence of energyefficient chemical processing methodologies for APs inhibited the complete utilization of the AP’s potential in polymer composite applications. In this study, we analyze and understand various challenges regarding the implementation of APs in epoxy-based composite applications and devise diverse mechanisms to overcome the challenges and engineer their properties. First, we devise an efficient process to extract and purify rendered APs and use them individually (up to 30%) or in combination with other additives as biobased crosslinking agents for developing high-strength, toughened APintermeshed thermoset polymers. Understanding the polarity difference between the matrix and the protein and hence the low compatibility, we demonstrate a novel hybrid cross-linked film containing a water-soluble epoxy resin (WEP) and AP. Leveraging a suite of spectroscopic and microscopic techniques, the compatibility, phase separation, curing reaction kinetics, and thermal properties were studied for each of the engineered products. Reaction kinetics study was also performed to investigate the activation energy of the curing process. Thus, various parameters and pathways to optimize the mechanical properties of the AP-based epoxy cross-linked matrix are elucidated. The fundamental understanding and various pathways reported here to design diverse epoxy systems and to engineer their properties are expected to catalyze the AP-based composite research especially targeted toward high-performance applications, such as in the automotive industry.

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RESULTS AND DISCUSSION Investigation of Cross-Linking Reactions of AP and Epoxy Resin and Optimization. Although curing the epoxy system using hydrolyzed proteins showed moderate success,43,44 the use of a native protein as a natural cross-linking agent for epoxy is comparatively less explored. The presence of a multitude of reactive functional groups in purified protein, including amine (−NH2), carboxyl (−COOH), sulfhydryl (−SH), hydroxyl (−OH), and carbonyl (−CHO) groups, should ensure the reaction between AP and DGEBA. The plausible reactions between the functional groups and epoxy are available in the literature.45 Among the various possibilities, the three major mechanisms of the reaction between the AP functional groups and epoxy are depicted in the scheme in Figure 1a. The proposed reaction of the AP-aided cross-linking of DGEBA is illustrated in Figure 1b. Thus, at elevated 14363

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Table 1. Calculated Kissinger Parameters and Initial Degradation Temperatures (Ti), Char Yields, and Temperature at Which First and Second DTG Peaks Are Observed for the Epoxy−AP, Epoxy−AP−HBP, Epoxy−AP−BMI, and Epoxy−AP−BMI− HBP Samples1 sample configuration

β (°C/min)

Ti (K)

TP (K)

DEGBA/protein (30 wt %) Ea = 28 267 (J/mol) A = 589 (s−1)

2

460.48

480.63

5 10 15 20

468.30 475.86 482.30 489.11

484.23 504.79 509.89 518.21

Ti (°C)

char yield

T1max

epoxy−AP

308.5

19.5

416.0

epoxy−AP−HBP epoxy−AP−BMI epoxy−AP−BMI−HBP

299.4 310.2 293.6

18.8 11.7 18.2

420.5 408.9 419.6

sample name

T2max

483.7

a

In all the samples, the weight ratio between the epoxy resin and the AP is 7:3.

Figure 3. (a) Schematic illustration of the reaction between BMI, epoxy resin, and AP to result in increased cross-linking. (b) DSC-based optimization of the BMI content. (c) Optimization of the HBP in the epoxy−AP−BMI system through the measurement of mechanical performance.

exothermal peak centered at ∼200 °C points to the crosslinking reaction between the resin and the AP, again confirming the AP-induced curing of the epoxy samples at elevated temperatures.52 Dynamic Curing Studies and Curing Kinetics. To investigate the curing kinetics of the epoxy−AP system, we conducted DSC studies of the sample at different curing rates. Figure 2a depicts curing peaks at different heating rates (2, 5, 10, 15, and 20 °C/min) for the sample with the ideal composition (epoxy 70% and AP 30%). We can observe that the temperature at which the highest reaction occurs (maximum of the exothermal peak) is affected by the heating rate, which is in accordance with the literature.53,54 As discussed earlier, curing of epoxy systems by the hydrolyzed protein and its cure kinetics was investigated using nonisothermal DSC measurements.43 Similarly, assuming the reaction rate to be proportional to the recorded heat flow, the maximum temperature value of the exothermal peak was taken as an indicator for the highest reaction rate.55 Hence, we calculated the activation energy (Ea) of the curing reaction

using the Kissinger equation method. Here, the curing reaction is assumed to be first-order, dependent on temperature, and independent of conversion. Hence, the Ea and the preexponential factor (A) can be calculated using the maximum exothermic peak temperature (TP) and heating rate (β) (obtained from Figure 2a)56 using the following expression (eq 1) ij β yz ij AR yz E − lnjjj 2 zzz = a − lnjjj zzz jT z j Ea z RTP k { k P {

(1)

where R is the universal gas constant (8.314 J/mol). A plot between ln(β/T P 2 ) and 1/RT P for all heating rates demonstrates a linear regression (Figure 2b), where the slope of the line is Ea and the y-intercept of the line is ln(AR/ Ea). From the experimental values and line fitting, we found the activation energy Ea to be 28.27 kJ/mol, which is much smaller compared to the curing activation energy reported with a similar animal protein/epoxy ratio (∼70 kJ/mol).55 Table 1 lists the Kissinger parameter values from our calculations. 14364

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Figure 4. (a) UTS; (b) elongation-at-break; and (c) Young’s modulus for the epoxy−AP, epoxy−AP−BMI, epoxy−AP−HBP, and epoxy−AP− BMI−HBP hybrid systems. (d) TGA and (e) derivative TGA of the epoxy−AP hybrids with and without BMI and/or HBP.

optimizing the ideal BMI content in the composite, the optimization of HBP was conducted by comparing the ultimate tensile strength (UTS) (Figure 3c). Samples were prepared by the addition of 5, 10, and 15 wt % of HBP to the epoxy−AP mixture, and the tensile tests indicated that the optimal HBP is around 10% at which the UTS is maximum. Thus, 20% BMI and 10% HBP were chosen to enhance the properties of the epoxy−AP networks. The improvement of the mechanical properties obtained by the addition of BMI (20 wt %) and HBP (10 wt %), either individually or in combination, is summarized in Figure 4a−c. Whereas the individual addition of BMI and HBP demonstrated increase in the UTS of the epoxy−AP hybrid polymer, the introduction of both HBP and BMI led to its UTS value reaching 14 MPa, which is comparable to that of soybean protein-based plastics.60 The addition of HBP was seen to increase the elongation-at-break (or strain-at-break), whereas the addition of BMI reduced its elongation-at-break (Figure 4b). This is understandable because the addition of BMI promotes the formation of intermolecular connections, which increases the cross-linking density and causes a smaller strainat-break with a higher UTS and modulus.57,58 Furthermore, the modulus of the epoxy−AP−BMI combination was observed to be the highest among all combinations. Thus, the addition of HBP resulted in enhancement in modulus, strength, and strain-at-break, and the introduction of BMI led to an increase in modulus and strength along with a decline in the strain-at-break of the epoxy−protein thermoset matrix because of increase in the cross-linking density and reduction in the free volume in the final polymer network.61 However, simultaneous addition of BMI and HBP was observed to result in complementary properties, that is, superior strength and strain-at-break. When compared to the study using hydrolyzed polymers, the UTS obtained here is inferior,43 but we were able to achieve strength comparable to that of soybean protein-based plastics using native proteins. This is a radical improvement because such enhanced mechanics has never been achieved using native proteins

Generally, a lower activation energy indicates a faster reaction. Thus, we conclude that the cross-linking process with native proteins reported in this study is more feasible compared to that with hydrolyzed proteins reported earlier. Engineering of Mechanical Properties. After establishing the cross-linking of the epoxy matrix by the purified native protein, we evaluated the mechanical properties of the cured materials. The cured material, although showing superior performance compared to the pure epoxy matrix, largely demonstrated inferior properties when compared to the existing composites (ones cross-linked with hydrolyzed proteins).43 The samples demonstrated a brittle nature and were not suitable for structural composite applications (Figure 4c). We infer this inferior performance to be due to (a) a large complex molecular structure of the protein, which makes it difficult to leverage all functional groups in the proteins, (b) inherent brittle nature of the proteins, and (c) the difference in polarity between the protein and the matrix, which limits the possible interactions. Understanding the challenges allowed us to overcome them via intelligent chemical strategies. For solving the complex molecular structure of APs and their significantly reduced reaction potential, we introduced bismaleimide (BMI), an agent with high cross-linking ability, high thermal stability, high glass-transition temperature, superior specific strength modulus, and excellent fire resistance.57,58 Figure 3a shows the reaction of BMI with the protein-infiltrated epoxy matrix. To bypass the brittleness induced by the protein, we introduced hyperbranched polymers (HBPs), which are ultrabranched polydisperse molecules at the nanometer scale. Because of their nano size, HBPs help to constrain matrix deformation as they interact better with the polymer microstructure on reaching nearly molecular dimensions.59 First, we conducted studies to identify the optimal ratio of BMI and HBP using the reaction heat calculations and mechanical property measurements, respectively. By varying the BMI content in the epoxy matrix and calculating the reaction heat (Figure 3b), we identified 20% BMI as the ideal content (highest reaction heat). After 14365

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Figure 5. (a) Scheme depicting the cross-linking reaction between WEP and AP in the WEP−AP sample. (b) FTIR spectra of the AP, WEP, and WEP−AP (5 wt %) hybrid polymer. (c) TGA spectra of the WEP−AP (5 wt %) hybrid polymer and the parent WEP. The inset shows the corresponding derivative spectra.

with significantly lower accessible functional groups compared to hydrolyzed proteins. The functional group density increases effectively when the AP is hydrolyzed to small peptides, which increases the cross-linking density of the polymer network. However, such hydrolysis also comes at an expense as it is an energy-intensive process because of the employment of high pressure and high temperature. As such, although the hydrolyzed protein leads to enhanced strength, it also leads to an increase in the process cost, thereby leading to an increase in the overall product cost. On the other side, the mechanical properties show a compromise with the replacement of a traditional hardener with AP. The investigation of a traditional epoxy−hardener system shows the UTS, elongation-at-break, and modulus as 52 MPa, 7.9%, and 420 MPa, respectively. However, the enhanced mechanics obtained here using the native protein through cochemical addition is more convenient and readily adaptable. The failure to obtain superior mechanics even in the presence of a cochemical that will enhance the cross-linking density and toughness leads to the conclusion that the difference in polarity or affinity to water could be the most influencing factor that controls the mechanical properties of the epoxy−AP polymer systems. Epoxy matrices are hydrophobic and are insoluble in water, whereas APs, due to the presence of various polar functional groups, are highly hygroscopic. Although this difference in the inherent chemical nature might not have any influence in the case of small molecules (e.g., in the case of hydrolyzed polymers) and at molecular level reactions, when employing high-molecularweight molecules such as native AP, even minute differences in polarities could be the governing factor. Unfortunately, this aspect was not investigated and is typically ignored. Our preliminary investigation of the epoxy−AP system revealed clear phase separation supporting our hypothesis. The presence of phase separation in the hybrid could lead to the observed inferior mechanical properties. To overcome the chemical polarity differences, we tested AP as a cross-linker for the WEP resin systems. The expected chemical cross-linking reaction between WEP and AP is given in the scheme (Figure 5a). Although both the amine and carboxyl functional groups

are present in AP under the present reaction condition, the amine groups are more active than the carboxyl groups toward WEP.62 During cross-linking, the epoxy ring in WEP opens and connects with the amine groups, along with the simultaneous formation of new hydroxyl groups in the final product. After cross-linking, water-based acrylic epoxy and AP formed a new thermoset network plastic that will be referred to as “WEP−AP hybrid polymer”. The Fourier transform IR (FTIR) spectra of pure AP, WEP−AP hybrid polymer (5 wt % AP), and pure WEP are shown in Figure 5b. Compared to the individual component spectra, the WEP−AP hybrid polymer shows a new peak at ∼1500 cm−1, indicating the formation of new ester groups through a reaction between AP and WEP, proving the cross-linking reaction. For AP, the FTIR spectra showed characteristic peaks of amide I, which are centered at around 1634 cm−1. The addition of WEP was found to lead to shifting of this peak to the higher wavelength of ∼1720 cm−1, indicating strong intermolecular interactions between WEP and protein, as reported earlier.63 Investigation and Comparison of Thermal Stability. Thermogravimetric analysis (TGA) measurements over the temperature range of 30−600 °C were performed under a N2 atmosphere to evaluate the thermal stability of the samples. Initial degradation temperature (Ti) was cited as the characteristic temperature to assess thermal stability. Here, Ti is defined as the temperature at which 5% of the sample undergoes degradation (Table 1). First, the samples prepared using conventional epoxy were investigated. A comparison of the Ti values of the four combinations using conventional epoxy shows that addition of HBP led to reduction in the thermal stability, whereas the addition of BMI did not lead to any compromise on the thermal stability (Figure 4d). It is wellknown that a longer polymer chain possesses lower thermal stability than shorter polymeric chains.64 This explains the easier degradation of the epoxy−AP−BMI−HBP combination compared to the unmodified epoxy−AP combination. Figure 4e shows a typical derivative thermal degradation pattern of the different epoxy−AP hybrid polymers with and without BMI−HBP. All the maximum degradation temperatures were close to each other for all chosen combinations and ranged 14366

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Figure 6. SEM image of (a) parent WEP and AP; (b) WEP−AP (1.5% by weight AP); and (c) WEP−AP (7.5% by weight AP). Variation of (d) UTS; (e) elongation (strain-at-break); and (f) modulus WEP−AP with the increasing weight ratio of AP in the hybrid polymers.

from 408.9 to 420.5 °C. Two peaks (T1max and T2max) were observed at around 400 °C in the DTG curves for epoxy−AP− BMI and epoxy−AP−BMI−HBP, whereas only one degradation peak was observed in the epoxy−AP and epoxy−AP− HBP samples. This indicates that BMI started to degrade initially at ∼385 °C, whereas the epoxy started to degrade at ∼415 °C. However, in epoxy−AP−BMI−HBP, a third degradation peak was observed at 483.7 °C, which can be attributed to the presence of unreacted AP in the system. However, the broadness of the degradation peaks for all the combinations indicates improved cross-linking in the system. On the other hand, WEP and WEP−AP only demonstrated one degradation temperature, indicating more compatibility and cross-linking even in the absence of cocross-linking agents. Figure 5c shows the dependence of weight loss on temperature upon heating the WEP and WEP−AP hybrid films in a N2 atmosphere. The residual weight demonstrates a concurrent decrease with an increase in temperature for both the films. Here, we measured Ti as well as the temperature at which 20% weight loss occurs. Both the temperatures demonstrated reduction for the WEP−AP hybrid polymer, mainly because of evaporation of the moisture present in the AP as described earlier (inset of Figure 5c). Similar to the case of traditional epoxy, both the pure WEP and hybrid polymer showed a significant weight loss above 250 °C, along with a sharp decrease in weight at 350 °C, indicating the initiation of their decomposition. However, compared to the parent WEP, the WEP−AP hybrid polymer has a larger residual weight, indicating the enhanced cross-linking between the AP and the WEP system.

Figure 6a−c shows the scanning electron microscopy (SEM) images depicting the morphology of the fracture surface of pure WEP, WEP−AP wt (AP1.5 wt %), and WEP−AP wt (AP7.5 wt %). The SEM images of AP powder demonstrated an aggregated particle with dimensions below 200 μm, which can be attributed to the grinding process after extraction from poultry materials. The pure WEP film was prepared by drying a water-based acrylic epoxy water solution and showed a porous and uneven structure. This morphology was possibly caused by the aggregation of noncross-linked molecules. Addition of AP to the water-based acrylic epoxy solution, followed by its subsequent curing, led to the films showing a more even, smooth cross section. This can be explained by the cross-linking action of AP in the final product matrix. The AP-based cross-linking through strong covalent bonds between AP and WEP results in a high density of interconnected molecules in the highly cross-linked network. Thus, after curing, newly formed hybrid thermoset films show smoother fracture surfaces than pure water-based acrylic epoxy solution-dried films. A summary of the mechanical data obtained from the WEP−AP hybrid polymer films is given in Figure 6d−f. We can see a clear increase in UTS and modulus of WEP films until the AP content reaches 5 wt %, followed by subsequent decrease (upon increase in the AP content). This could plausibly be due to nonhomogeneous dispersion of the AP at higher concentrations because of agglomeration caused by increased interaction and proximity between APs. The increase in UTS and modulus can be explained by the crosslinking of AP and WEP. Increase in cross-linking density results in a decrease in elongation-at-break and an increase in 14367

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UTS and modulus. Pure WEP dried films showed the lowest UTS, elongation, and modulus, which are in agreement with the SEM images that show a highly porous structure for the WEP films, leading to poor mechanical properties. The effect of thermal instability of the epoxy systems is negligible and is expected to be compensated by the enhancement of the mechanical properties obtained by cross-linking. Moreover, through precise process engineering, this inadequacy is supposed to be overcome to result in a bioresourced epoxy system with excellent mechanical properties.

were purchased from Sigma-Aldrich, WI, USA. Hexane and BMI were purchased from VWR Chemicals, GA, USA. The filter paper used in our study was obtained from Whatman (Whatman #4, D = 11 cm, pore size 20−25 μm). The dialysis membrane (d = 3000) was purchased from Thermo Fisher Scientific, MA, USA. The HBP (Boltorn H30) was purchased from Perstorp Inc, Sweden. The water-based acrylic epoxy was purchased from Vital Coat (Tallahassee, FL, USA). All chemicals purchased were used as received without any further processing. Proteinaceous Material Extraction. Protein was extracted from the poultry meal and salt solution mixture by controlling its pH value. Every 100 g of the poultry meal was treated with 450 mL of salt solution containing 18 g NaCl (or MgCl2), 4.1 g KH2PO4, and 4.3 g Na2HPO4, according to the method used by Park et al.40 and purified through dialysis. The protein/salt solution was filled into the dialysis membrane bag and then placed in distilled water for 4 h, during which the water was replaced every hour. Free α-amino group contained in the extracted protein was determined to be 3.35 × 10−4 mol/g (see the Supporting Information). Thermoset Polymer Preparation. Protein samples from the previous step were used to cross-link the DGEBA epoxy resin. The epoxy resin was preheated to 55 °C in the oven for 30 min. The protein was dried in a vacuum oven for 2 h at 100 °C and then mixed with the epoxy resin by stirring at 60 °C for 2 h in varying AP−DGEBA ratios. The prepared mixture was then cast in a silicone mold (consistent with ASTM D-638 type V tensile geometries) and cured at 150 °C for 5 h. Preparation of WEP−AP Hybrid Films. AP (3 g) and glycerol (1 g) were dissolved in distilled water (50 mL) by constant stirring at 70 °C for 30 min. The pH of the solution was adjusted to 10 ± 0.1 using 1 M ammonia solution. The solution was then heated to 90 °C for 30 min to denature the AP. The resultant protein solution was then cooled to room temperature for further use. About 50 mL of water-based acrylic epoxy solution was heated at 60 °C until no change in weight was observed, indicating the complete removal of water. The final solid part was then weighed, and the density of the solution was calculated. Calculated amounts of water-based acrylic epoxy solution and AP solution were mixed in a glass beaker, varying the net epoxy−protein ratio as 100/0, 98.5/1.5, 95/5, and 92.5/7.5. The mixture was thoroughly mixed by stirring on a hot plate for 1 h at 60 °C. The mixture was then degassed in a vacuum chamber (until no bubbles were observed) and then poured into a flat silicone mold. The mixture was subsequently dried at room temperature for 48 h and then dried at 60 °C for 2 h. The final membrane was cut into 1 cm × 5 cm specimen and kept in vacuum for testing. Characterization. Attenuated Total Reflectance−FTIR Spectroscopy. Attenuated total reflectance (ATR)−FTIR spectroscopy was performed using a Thermo-Nicolet Magna 550 FTIR spectrometer in combination with a Thermo-Spectra Tech Foundation Series Diamond ATR accessory with the angle of incidence being 50°. Differential Scanning Calorimetry. DSC was conducted to investigate the curing behavior of the protein−epoxy mixture using a thermal analyzer (Q2000, TA Instruments, USA) under a N2 atmosphere. Experiments were performed with sample weights ranging from 6 to 10 mg. In dynamic experiments, the samples were heated from 25 to 275 °C at various heating rates (i.e., 2, 5, 10, 15, and 20 °C/min). The

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CONCLUSIONS In conclusion, we devised strategies to create animal proteinbased epoxy hybrid structures with good mechanical properties using native AP for the first time. Our study indicated that the mechanical performance of native protein−epoxy hybrid systems can be tuned from a high-strength structural composite to an elastic epoxy-based system via intelligent introduction of participating hybrid components. Because of the polarity difference between AP and epoxy, the conventional epoxy systems need cocross-linkers to extract the complete potential of the native AP. Although the epoxy−AP hybrids showed inferior mechanical performance in the absence of externally added cross-linking agents, the introduction of coagents, such as BMI and HBP, resulted in ∼700% improved ultimate strength. Following the cure kinetics using the reaction rate, the curing activation energy was found to be 28.267 kJ/mol, which is much smaller than that reported in a similar epoxy−AP system using the hydrolyzed protein. The smaller Ea value indicates that the curing reaction in the present case is more facile compared to the cross-linking using the hydrolyzed protein. We also identified the optimal amount of BMI and HBP required in the hybrid polymers through following the reaction heat and mechanical properties, respectively. While 20% of BMI helped in enhancing the cross-linking, introduction of 10% HBP resulted in overcoming the brittle nature of the introduced protein and resulted in a hybrid with enhanced mechanical properties. Thermal characterization indicated partial crosslinking in the absence of BMI and HBP and increased crosslinking indicated by a single thermal degradation peak. On the contrary, the WEP−AP hybrid, owing to the enhanced affinity of AP toward the hydrophilic polymer matrices, demonstrated high cross-linking and mechanical properties without the aid of cocross-linking agents. Microscopic investigations revealed that the introduction of AP to WEP results in the reduction in porosity of WEP as a direct result of the enhanced crosslinking via the amine group of AP and the epoxide group on the WEP. The new ester-bond formation in the WEP−AP hybrid was confirmed through spectroscopy. This work is the first step in the direction of leveraging underutilized protein from the rendering industry in its native form to create biobased hybrid polymeric systems. This is expected to attract widespread attention and to help in developing and promoting the wider application of animal byproducts, thereby decreasing animal waste generation as well as promoting sustainability.

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EXPERIMENTAL SECTION

Materials. The APused in this studywas a poultry byproduct meal (feed grade) provided by GRO-MOR Inc, Adams, MA. The total protein content of the sample was >56% by dry weight. DGEBA, NaCl, Na2HPO4, KH2PO4, and MgCl2 14368

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thermal, wetting and viscoelastic properties of poly(butylene succinate) composites. Cellulose 2017, 24, 4313−4323. (4) Zhang, K.; Misra, M.; Mohanty, A. K. Toughened Sustainable Green Composites from Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) Based Ternary Blends and Miscanthus Biofiber. ACS Sustainable Chem. Eng. 2014, 2, 2345−2354. (5) Mohanty, A. K.; Misra, M.; Hinrichsen, G. Biofibres, biodegradable polymers and biocomposites: An overview. Macromol. Mater. Eng. 2000, 276−277, 1−24. (6) Mohanty, A. K.; Misra, M.; Drzal, L. T. Sustainable BioComposites from Renewable Resources: Opportunities and Challenges in the Green Materials World. J. Polym. Environ. 2002, 10, 19− 26. (7) Bhardwaj, R.; Mohanty, A. K.; Drzal, L. T.; Pourboghrat, F.; Misra, M. Renewable Resource-Based Green Composites from Recycled Cellulose Fiber and Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) Bioplastic. Biomacromolecules 2006, 7, 2044−2051. (8) Jong, L. Characterization of soy protein/styrene−butadiene rubber composites. Composites, Part A 2005, 36, 675−682. (9) Gandini, A. Epoxy Polymers Based on Renewable Resources. Epoxy Polymers; Wiley-VCH Verlag GmbH & Co. KGaA, 2010; pp 55−78. (10) Raquez, J.-M.; Deléglise, M.; Lacrampe, M.-F.; Krawczak, P. Thermosetting (bio)materials derived from renewable resources: A critical review. Prog. Polym. Sci. 2010, 35, 487−509. (11) Jin, F.-L.; Park, S.-J. Impact-strength improvement of epoxy resins reinforced with a biodegradable polymer. Mater. Sci. Eng. A 2008, 478, 402−405. (12) Mekonnen, T.; Mussone, P.; Bressler, D. Valorization of rendering industry wastes and co-products for industrial chemicals, materials and energy: review. Crit. Rev. Biotechnol. 2016, 36, 120−131. (13) Cuq, B.; Gontard, N.; Guilbert, S. Proteins as Agricultural Polymers for Packaging Production. Cereal Chem. 1998, 75, 1−9. (14) Zheng, T.; Yu, X.; Pilla, S. Mechanical and moisture sensitivity of fully bio-based dialdehyde carboxymethyl cellulose cross-linked soy protein isolate films. Carbohydr. Polym. 2017, 157, 1333−1340. (15) Schmid, M.; Dallmann, K.; Bugnicourt, E.; Cordoni, D.; Wild, F.; Lazzeri, A.; Noller, K. Properties of Whey-Protein-Coated Films and Laminates as Novel Recyclable Food Packaging Materials with Excellent Barrier Properties. Int. J. Polym. Sci. 2012, 2012, 7. (16) Zhao, R.; Torley, P.; Halley, P. J. Emerging biodegradable materials: starch- and protein-based bio-nanocomposites. J. Mater. Sci. 2008, 43, 3058−3071. (17) Poole, A. J.; Church, J. S.; Huson, M. G. Environmentally Sustainable Fibers from Regenerated Protein. Biomacromolecules 2009, 10, 1−8. (18) Kumar, R.; Choudhary, V.; Mishra, S.; Varma, I. K.; Mattiason, B. Adhesives and plastics based on soy protein products. Ind. Crops Prod. 2002, 16, 155−172. (19) Sothornvit, R.; Olsen, C. W.; McHugh, T. H.; Krochta, J. M. Tensile properties of compression-molded whey protein sheets: Determination of molding condition and glycerol-content effects and comparison with solution-cast films. J. Food Eng. 2007, 78, 855−860. (20) Hernandez-Izquierdo, V. M.; Krochta, J. M. Thermoplastic Processing of Proteins for Film FormationA Review. J. Food Sci. 2008, 73, R30−R39. (21) Chan, W. Y.; Bochenski, T.; Schmidt, J. E.; Olsen, B. D. Peptide Domains as Reinforcement in Protein-Based Elastomers. ACS Sustainable Chem. Eng. 2017, 5, 8568−8578. (22) Auvergne, R.; Caillol, S.; David, G.; Boutevin, B.; Pascault, J.-P. Biobased Thermosetting Epoxy: Present and Future. Chem. Rev. 2014, 114, 1082−1115. (23) Mark, H. Encyclopedia of Polymer Science and Technology; Wiley, 2014. (24) Pascault, J. P.; Williams, R. J. Epoxy Polymers: New Materials and Innovations; John Wiley & Sons: Germany, 2009; pp 1−37. (25) Ma, S.; Li, T.; Liu, X.; Zhu, J. Research progress on bio-based thermosetting resins. Polym. Int. 2016, 65, 164−173.

reaction heat was calculated based on the total area under the exothermal peak of the DSC graph. Thermogravimetric Analysis. TGA measurement was performed using a TGA Q5000 instrument (TGA Q5000) under N2 flow at a heating rate of 10 °C/min to study the thermal degradation behavior of the protein-based plastics. About 8 mg of the samples was heated at 10 °C/min over the temperature range of 30−600 °C under a N2 atmosphere. Scanning Electron Microscopy. Thin films at varying weight ratios of WEP−AP were frozen in liquid nitrogen and broken quickly. The fractured surfaces of the films were then coated with 60% gold and 40% palladium for 60 s at 45 mA current in a sputter coater (Desk II, Denton Vacuum, Moorestown, NJ). Hitachi S4800 SEM, Japan, was used to understand the miscibility and compatibility of epoxy and AP. Mechanical Properties. The tensile strength and elongation-at-break were measured using an Instron Universal Testing Machine (model 1125), Grove City, PA, USA, at a cross-head speed of 50 mm/min, with 30 mm grip separation. Testing was done as per the relevant ASTM standards.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b01336. Method for the free amino group quantification and TGA characterization of the extracted protein (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.P.). ORCID

Ting Zheng: 0000-0002-4115-4626 Joseph G. Lawrence: 0000-0001-5659-116X Srikanth Pilla: 0000-0003-3728-6578 Present Address ∥

Department of Chemistry and Biochemistry, The University of Texas at El Paso, 500 W University, El Paso, Texas 79968, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support provided by the Animal Coproducts Research and Education Center consortium and the office of the Vice-President for Research at Clemson University, Robert Patrick Jenkins Professorship, and Dean’s Faculty Fellow Professorship. T.Z. would like to acknowledge the financial support provided by the Cooper-Standard Postdoctoral Fellowship.

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