One-Step Toughening of Soy Protein Based Green Resin Using

Apr 22, 2017 - Cross-link density was calculated using eq 1(43) (1)where E′ represents the elastic modulus above Tg, ρ the density of network or cr...
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Research Article pubs.acs.org/journal/ascecg

One-Step Toughening of Soy Protein Based Green Resin Using Electrospun Epoxidized Natural Rubber Fibers Joo Ran Kim and Anil N. Netravali* Fiber Science & Apparel Design, Cornell University, Ithaca, New York 14853, United States ABSTRACT: Epoxidized natural rubber (ENR) fibers (ENRFs) were electrospun to obtain diameters ranging from a few hundred nanometers to a few micrometers by changing the ENR solution concentration. ENRFs were used to toughen soy protein isolate (SPI) based resin. A facile 1-step process was developed for wet electrospinning and blending the ENRFs directly into SPI resin. As the ENR concentration increased from 0.1% to 5%, the surface topography of ENRFs changed from irregular to somewhat bumpy. The average diameter of ENRFs also increased from 250 nm to 17 μm. Increased ENRF (electrospun from 3% concentration) loading from 0 to 20% in SPI resin increased the fracture strain significantly from 1.7 to 18.8% and increased the toughness by a factor of 10. Interestingly, tensile strength and Young’s modulus decreased only slightly compared to the increase in the toughness. Crosslinking between the epoxy groups in ENR and amine and/or carboxylic groups in SPI as well as the high aspect ratio of the ENRFs contributed to increased toughness of SPI resin while retaining its tensile strength and Young’s modulus. This research opens up possibilities of using fully green and toughened SPI resins in many applications including green composites. KEYWORDS: Green resin, Toughening, Epoxidized natural rubber fiber, Electrospinning, Soy protein isolate

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INTRODUCTION The diameters of polymeric fibers generally range from a few nanometers to several micrometers.1 Fibers of smaller diameters have many advantages such as very high surface area to volume ratio (this ratio for nanofibers can be over 100 times that of microfibers), flexibility in obtaining surface functionalities combined with greater mechanical performance compared with any other known forms of material.1,2 On the basis of the applications, many methods have been used to produce nanofibers including drawing, self-assembly, gelspinning, and electrospinning.3 Among them, electrospinning seems to be an economical and useful way to spin nanofibers because the process parameters can be easily altered to obtain nanofibers with different diameters or properties.4 Electrospinning consists of three basic components: a high voltage supply, a pipet or a small diameter needle, and a metal collector.5 One of the electrodes from the high voltage supply is connected to the polymer solution being metered through the needle and the other electrode is connected to the collector. As electrostatic repulsion overcomes the surface tension of the polymer solution jet coming out of the needle tip, it is pushed toward the collector and stretched to finer diameters. In addition, the vigorous motion of the polymer solution jet also splits it into finer jets. This splitting and stretching of the polymer solution allows fast evaporation of the solvent to form solid fibers as they get deposited onto the collector.6,7 Electrospun fibers can be drawn further into ultrafine fibers with diameters ranging from below 3 nm to about 1 μm.3 © 2017 American Chemical Society

Many process variables can influence the surface topography and diameter (size) of the fibers during electrospinning. These variables include (i) polymer concentration and solvent type and solution viscosity, elasticity, surface tension, and conductivity; (ii) applied voltage, needle diameter size, and the distance between the needle tip and the collector; and (iii) surrounding variables such as temperature, humidity, and air velocity in the electrospinning chamber.3 For example, increase in the electrical field results in higher elongational forces imposed on the polymer solution which ultimately results in smaller fiber diameters and/or irregular surface.8 In terms of the morphology of electrospun fibers, Deitzel et al. reported that the increase in the electrical potential results in rougher poly(ethylene oxide) (PEO) nanofibers.9 If the distance between the needle tip and the collector is short, the electrospun fibers tend to stick to the collector as well as to each other because of incomplete solvent evaporation.10 Biodegradable resins such as PEO,9 poly(D,L-lactic acid) (PLA),11 poly(lactic-co-glycolic acid) (PLGA),12 poly(vinyl alcohol) (PVA),13 gelatin,14 dextran,15 and many other plant derived resins such as proteins and starches16,17 show high moisture sensitivity, brittleness, and/or poor processability. In order to overcome their brittleness and improve the toughness, addition of ductile or rubber-modified materials, hybrid fibers, Received: February 4, 2017 Revised: April 11, 2017 Published: April 22, 2017 4957

DOI: 10.1021/acssuschemeng.7b00347 ACS Sustainable Chem. Eng. 2017, 5, 4957−4968

Research Article

ACS Sustainable Chemistry & Engineering

an (MTHF) as the solvent. MTHF is a green solvent produced by the acid-catalyzed digestion of pentosan sugars and polysaccharides obtained from renewable biomass rich in cellulose, hemicellulose, lignin, and other agricultural wastes.37 Further, the biodegradable, toughened SPI resin containing ENRFs was prepared in a facile 1-step wet electrospinningblending process. The new 1-step process is convenient to process water-based biodegradable resins without separately collecting and dispersing fibers into resins. ENRF loading was varied from 0 to 20% (by wt), and its effect on SPI toughness and other mechanical properties was characterized.

as well as various chemical modifications have been carried out.18,19 Biobased silicone phenolic composites reinforced with sisal fibers (SPF) have demonstrated improved toughness with impact strength of 4.2 kJ m−2, over 50% higher than that of the neat composites.20 Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) composites with 20% (by wt) miscanthus were toughened by adding 35% poly(butylene adipate-co-terephthalate) and 15% epoxidized natural rubber (ENR).21 These modified composites achieved impact strengths of up to 240.5 J m−1 compared to neat PHBV composites which had impact strengths of only 29.2 J m−1.21 To improve the toughness of the brittle resins without significantly losing their tensile properties can be difficult. However, by obtaining good interfacial bonding via chemical reactions between the resin and the toughening agent maintaining tensile properties can be possible. As a result, this has been an area of active research.22−28 For example, epoxy resin, containing just 0.5% dodecane functionalized tannic acid as the toughening agent, increased the impact strength by 196% compared to that of neat epoxy.22 This positive change was a result of good interfacial bonding, as the terminal hydroxyl groups of dodecane functionalized tannic acid reacted with the epoxy resin.22 Glycerol (10%), as plasticizer, has also been used to improve the toughness of PLA resin, resulting in a fracture strain of up to 93% and fracture stress of 48 MPa, compared to that of neat PLA resin with a fracture strain of 2.3% and a fracture stress of 64 MPa. In this case, the hydroxyl groups of glycerol were able to react with the carboxylic groups in PLA forming covalent bonds.23 The immiscible poly(L-lactic acid)/acrylonitrilebutadiene-styrene system (PLA/ABS) at a 50:50 ratio with a 3% reactive comb polymer, which consisted of a random terpolymer of methyl methacrylate (MMA) and glycidyl methyl acrylate (GMA), was studied by Dong et al.24 The blending of terpolymer increased the chain mobility of both PLA and ABS phases and resulted in 7 times higher fracture strain than that of neat ABS resin.24 The presence of epoxy groups in epoxidized natural rubber (ENR) has been found to be effective in causing desired reactions with amine groups present in some resins.25 One explanation provided by Nghia et al. suggests that the epoxy groups present in ENR can react with functional chemical groups such as amino, glycidyl, hydroxyl, and carboxyl groups of resins offering good ENR-resin interfacial bonding in the resin systems.26 In another example, ENR was blended with biodegradable PLA resin where chemical reactions between epoxy groups and carboxylic groups resulted in improved toughness and tensile properties.27 Raw ENR used with brittle soy protein isolate (SPI) resin also showed improved fracture toughness from 0.13 to 1.02 MPa, while maintaining the tensile properties of the resin.28 Synthetic and nonbiodegradable electrospun nanofibers such as polyurethane,29 styrene−butadiene,30 2-hydroxyethyl methacrylate,31 chloroprene rubber,32 or polysulfone33 have been developed for toughening purposes. Partially green copolymers such as ethylene-vinyl acetate,34 polylactide/natural rubber,35 or ENR/polycaprolactone36 have also been applied as toughening agents in textiles, composites, sensors, biomaterials, and membranes. No study, however, has been published in open literature on using electrospun elastic fibers of ENR as toughening agents for any resins. In this article, epoxidized natural rubber fibers (ENRFs) were electrospun to obtain diameters ranging from a few hundred nanometers to a few micrometers using 2-methyltetrahydrofur-

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EXPERIMENTAL SECTION

Materials. SPI, PRO FAM 974, was provided by Archer Daniels Midland Co., Decatur, IL. Epoxidized natural rubber (ENR, 50 mol % conversion) was provided by Malaysian Rubber Board as a research sample. Ignite brand fluorescent dye was purchased from Smooth-on Inc. to stain ENR. (Macungie, PA, USA). Sodium hydroxide pellets (≥97.0%), MTHF, and glutaraldehyde (GA, 25% solution in water) were purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA). All chemicals were used as received. Wet Electrospinning. ENR was dissolved in MTHF at different concentrations of 0.1, 1, 2, 3, 4, 5, and 10% (w/v) and stirred for 3 h at room temperature (RT). The solution was put into a 5 mL syringe and electrospun into fibers (ENRFs) using Gamma power supply with a high voltage of 13 kV. The syringe feeding rate used was 50 μL min−1, and the distance between the syringe needle tip and the collector was 10 cm. SPI resin was prepared and poured into a Teflon mold (127 × 127 mm) above the metal collector. A piece of cardboard was placed between the metal collector and the hot plate to separate them and avoid any electrical contact. ENRFs were directly deposited into the SPI resin. When the ENRFs were electrospun into SPI solution without stirring, significant aggregation of ENRFs was observed. In order to prevent the aggregation of ENRFs, the water-based SPI resin was continuously stirred at 50 rpm while depositing electrospun ENRFs. Figure 1 shows the one-step wet electrospinning-blending

Figure 1. One-step wet electrospinning-blending system to prepare the water-based SPI resin. Electrospun ENRFs are directly deposited into the SPI slurry (resin) during the electrospinning process. system developed in this study for ENRF fiber production and simultaneously blending them with the SPI resin. Compared to the basic electrospinning system, the wet electrospinning system is based on a usage of a liquid reservoir collector instead of solid flat metallic collector.38,39 Preparation of ENRF Toughened SPI Resin. SPI was mixed with deionized (DI) water at a 1:20 ratio, and the mixture was stirred for 1 h at 400 rpm at RT to form a slurry. Since SPI consists of large molecules (about 250 kDa), a high amount of water is necessary to prepare a workable solution. The excess water can be easily evaporated prior to the hot pressing of a resin sheet. The pH of the slurry was adjusted to 11 ± 0.01 using 1 M NaOH solution to unfold (denature) the polypeptide chains. SPI has an isoelectric point at about 4.5 pH. 4958

DOI: 10.1021/acssuschemeng.7b00347 ACS Sustainable Chem. Eng. 2017, 5, 4957−4968

Research Article

ACS Sustainable Chemistry & Engineering

Figure 2. SEM images of electrospun ENRFs at different ENR concentrations (a) 0.1%, (b) 1%, (c) 2%, (d) 3%, (e) 4%, (f) 5%, and (g) 10% in MTHF (w/v). Raising its pH to 11 using NaOH opens up the molecules making it easier for the reactive groups to react with the cross-linker, GA in this case. The cross-linking reaction between amine groups in soy protein and aldehyde groups of GA have been reported in many studies.40,41 All percentages of ENR, GA, and ENRFs in this study are on a w/w basis. GA (10% of SPI), the cross-linking agent, was added to the SPI slurry, and the composition was stirred at 400 rpm for 1 h at 50 °C. Electrospun ENRFs using the 3% ENR polymer concentration were directly deposited into the SPI slurry (resin) to obtain 5, 10, 15, or 20% loading. Additionally, ENRFs were electrospun using 0.1, 0.5, 1, 2, 3, 4, 5, and 10% ENR concentrations and were directly deposited into the SPI slurry (10% ENRF loading) to investigate how ENRF surface topography and diameters influence the tensile properties of the resin. SPI resins containing different loadings of ENRFs (prepared at 3% concentration) or different sizes (prepared at different ENR concentrations) with 10% loading were cast, in the form of sheets, separately, in a Teflon mold (127 × 127 mm) in an air circulating drying oven at 35 °C for 12 h (precured). SPI resin sheets were then cured by hot pressing at 90 °C under a pressure of 0.43 MPa for 15 min and then conditioned at 21 °C and 65% RH for 72 h prior to their characterization. Tensile Properties of SPI/ENRF Resins. Conditioned resin sheets were cut using a laser cutter to obtain the final tensile specimen dimensions of 50 × 10 × 1 mm. Neat SPI resin, SPI/ENRF resins (95/5, 90/10, 85/15, and 80/20) and SPI/ENRF 90/10 resins

containing ENRFs electrospun using 0.1, 0.5, 1, 2, 3, 4, 5, and 10% ENR concentrations were characterized for their tensile properties according to the ASTM D882-02 standard at a strain rate of 0.6 min−1 and gauge length of 30 mm. At least 10 specimens were tested from different resins of each type prepared at three different times. All tests were performed on an Instron universal tensile tester (Instron, Model 5566, Instron Co., Canton).42 Dynamic Mechanical Analysis of SPI/ENRF Resins. Dynamic mechanical properties of neat SPI resin, SPI/ENRF resins, and neat ENR resin were analyzed using DMA (DMA Q800, TA Instruments, New Castle, DE) in 3-point bending mode. Resin sheets were cut into the final specimen dimensions of 30 × 10 × 2 mm and scanned from −40 to 60 °C at a ramp rate of 10 °C min−1. All tests were performed at 5 μm amplitude and sinusoidal tensile stress (frequency, 1 Hz), and conducted on specimens prepared at three different times. Cross-link density was calculated using eq 143

ρ=

E′ ΦRT

(1)

where E′ represents the elastic modulus above Tg, ρ the density of network or cross-link density (mol cm−3), Φ the front factor (equal to three), R the gas constant (8.314 J mol−1·K−1), and T the absolute temperature in the rubbery region. Moisture Content (%) of Resins. Moisture content (%) of neat SPI resin, all SPI/ENRF resins, and neat ENR was measured using a 4959

DOI: 10.1021/acssuschemeng.7b00347 ACS Sustainable Chem. Eng. 2017, 5, 4957−4968

Research Article

ACS Sustainable Chemistry & Engineering

provide desirable mechanical “lock and key” type bonding when mixed with any resin. However, too rough a surface may not be ideal as it can reduce the strength of the fibers because of the stress concentrations. Viscosity of the ENR Solution. The ENR solutions with concentrations varying from 0.1 to 10% in MTHF were measured for their viscosities at 21 °C. Polymer viscosity, a function of the polymer concentration, has been found to be one of the biggest factors determining the fiber diameter and morphology during electrospinning.29 Increasing the solution viscosity, whether by a higher molecular weight polymer or by higher concentration, will increase the resulting fiber diameter. The fiber surface morphology has also been shown to be related to elongational viscosity which, in fact, is related to the shear viscosity by the Trouton ratio, which is the ratio of the elongation to the shear viscosity.48,49 Figure 3 depicts the effect

gravimetric method and using eq 2 where W1 is the weight of the conditioned resin specimen, and W2 is the weight of the resin specimen dried in an air circulating oven at 105 °C for 24 h. Three specimens were tested to confirm the reproducibility and obtain average values.

⎛ W⎞ Moisture content (%) = ⎜1 − 2 ⎟ × 100 W1 ⎠ ⎝

(2)

Morphological and Chemical Analyses. Differential scanning calorimeter (TA-DSC 2000, TA Instruments Inc., New Castle, DE) was used for evaluating the thermal properties of SPI/ENRF resins. Resin specimens were scanned from −60 to 300 °C at a heating rate of 10 °C min−1 while maintaining a nitrogen flow rate of 100 mL min−1. Microstructures of the tensile fracture surfaces of nest SPI resin and all SPI/ENRF resins (95/5, 90/10, 85/15 and 80/20) were analyzed using Tescan Mira3 FESEM (Kohoutovice, Czech Republic). ATRFTIR (Magna 560, Nicolet Instrument Technologies, Fitchburg, WI) was used to confirm cross-linking reactions of neat SPI resin, SPI/ ENRF resins (95/5, 90/10, 85/15 and 80/20), and neat ENR resin. Resin specimens were scanned from 4000 to 800 cm−1 wavenumbers with a total of 300 scans and a resolution of 2 cm−1. ENR solution in MTHF was stained with 0.5% Ignite brand fluorescent dye of ENR and then electrospun directly into SPI resin. The resin specimens were analyzed using a confocal laser scanning microscope (CLSM; Zeiss LSM710, Thornwood, NY) and 554 nm excitation filter with a 10× objective lens. In order to measure the intrinsic viscosity of ENR polymer solutions, an advanced rheometer (AR2000, TA Instruments, New Castle, DE) was equipped with a 60 mm diameter steel parallel plate. The gap between the plates was 1000 μm, and the shear rate was 20 s−1. The intrinsic viscosities of ENR solutions prepared in 0.1 to 10% MTHF were measured at 21 °C. Three replicate tests were conducted to ensure reproducibility.

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RESULTS AND DISCUSSION ENRF Diameters and Surface Topography. Figure 2 shows typical SEM images of ENRFs obtained at different ENR solution concentrations. The average diameter of ENRFs varied between 0.25 to 250 μm depending on the ENR concentration. At 0.1% ENR concentration, the irregular surface of fibers were observed as can be seen in Figure 2a with diameters between 250 to 500 nm. As the concentration increased from 0.1% to 1%, the shape changed to somewhat elongated and spindle-like, and the diameter of the fibers increased to about 3.5 μm, as shown in Figure 2b. At ENR concentrations of 2 and 3%, ENRFs showed a smoother surface with average diameters of 3.8 and 5.9 μm, respectively, as shown in Figure 2c and d. When the polymer concentration increased to 4 and 5%, ENRFs showed both bumpy and irregular surfaces with larger diameters of 13 and 17 μm, respectively, as shown in Figure 2e and f. At an ENR concentration of 10%, the diameter of ENRFs increased up to about 250 μm as can be seen in Figure 2g. The shape of ENRFs produced using 10% ENR concentration seems like a bundle of smaller fibers that are attached to one another. The morphology of electrospun fibers is closely related to how rapid the solvent evaporation occurs. Also, higher viscosity of the solution does not allow the fiber to split effectively during electrospinning, resulting in higher diameter.44 The lower fiber surface makes it harder to evaporate the solvent from ENRFs within a very short time of electospinning. However, partial surface evaporation of the solvent creates a rougher surface which gets elongated as the fiber travels to the collector plate. Identical results have been reported by other researchers.44 These results clearly indicate that the increase in ENR concentration from 0.1 to 10% increases the diameter of ENRFs as well as the surface topography. A rougher surface can

Figure 3. Viscosity−polymer concentration plot showing their exponential relationship. SEM images of electrospun ENRFs are presented for all concentrations.

of viscosity and ENR concentration on the fiber surface characteristics and diameter. When ENR was dissolved in MTHF, the solution viscosity and the polymer concentration showed an exponential relationship as demonstrated in the plot in Figure 3. Deitzel et al. indicated a power law relationship between polymer concentration and fiber diameter.9 Table 1 summarizes the average diameter size of ENRFs and viscosities used for wet electrospinning along with average fiber diameters. These results show a very similar trend as observed by Dietzel et al.9 At the lowest ENR concentration of 0.1%, the solution Table 1. ENR Solution Concentration, Viscosity, and Average Diameters and Ranges Obtained for ENRFs polymer concentration (%)

viscosity (cP)

fiber diameter (μm)

diameter range (μm)

0.1 1 2 3 4 5 10

1.4 (0.3)a 18 (1.2) 31 (3.9) 153 (13) 287 (24) 498 (88) 1591 (96)

0.25 3.5 3.8 6.1 13 16 250

0.11−0.43 1.1−5.3 1.6−6.7 2.2−8.2 9.8−15 12−21 180−330

a

4960

Values in parentheses represent standard deviation. DOI: 10.1021/acssuschemeng.7b00347 ACS Sustainable Chem. Eng. 2017, 5, 4957−4968

Research Article

ACS Sustainable Chemistry & Engineering

Figure 4. ATR-FTIR spectra of SPI/ENRF resins of all compositions (100/0, 95/5, 90/10, 85/15, 80/20, and 0/100).

of the solution. Meanwhile, droplet-like structures formed when the viscosity was too low (