Eleostearic Acid

coatings

Article Bio-Based Composites with Enhanced Matrix-Reinforcement Interactions from the Polymerization of α-Eleostearic Acid

Amanda Murawski and Rafael L. Quirino *

Chemistry Department, Georgia Southern University, Statesboro, GA 30458, USA * Correspondence: [email protected]

Received: 25 June 2019; Accepted: 9 July 2019; Published: 17 July 2019

Abstract: Vegetable oil-based composites have been proposed as interesting bio-based materials in the recent past. The carbon–carbon double bonds in unsaturated vegetable oils are ideal reactive sites for free radical polymerization. Without the presence of a reinforcement, typical vegetable oil-based polymers cannot achieve competitive thermo-mechanical properties. Compatibilizers have been utilized to enhance the adhesion between resin and reinforcement. This work discusses the antagonist implications of polarity and crosslink density of an unprecedented polar α-eleostearic acid-based resin reinforced with α-cellulose, eliminating the need of a compatibilizer. It is shown that the polar regions of α-eleostearic acid can interact directly with the polar reinforcement. The successful isolation of α-eleostearic acid from tung oil was veriﬁed via GC-MS, 1H NMR, Raman, and FT-IR spectroscopies. The optimal cure schedule for the resin was determined by DSC and DEA. The composites’ thermo-mechanical properties were assessed by TGA, DSC, and DMA.

Keywords: bio-based materials; fatty acids; thermosets; tung oil; cellulose composites

1. Introduction In the past decade, vegetable oils have received a lot of attention as an attractive bio-renewable starting material for the preparation of bio-based polymers. Polymers prepared from vegetable oils have the advantage of being versatile due to their easily tunable properties, which can be simply adjusted with changes in the resin composition. Applications in biomedicine, construction, furniture, adhesives, and coatings can benefit from the development of vegetable oil-based polymers and composites. It is widely accepted that fossil fuels are limited and non-renewable in practical terms [1]. It is, therefore, imperative to continue research for more sustainable alternatives in order to alleviate economic stress and petroleum depletion impacts in the polymer and composites industry. Furthermore, the development of bio-renewable alternatives offers materials with the potential of exhibiting a positive environmental impact, especially with respect to current waste disposal concerns. Polyurethanes [2,3], polyester amides [4], polyolefins [5], and alkyd resins [6] are just some of the current resins on the market that can be prepared from vegetable oils. In order to improve mechanical properties, these resins have been reinforced with inorganic fillers, such as clays [7]. The difficulty with binding between vegetable oil-based polymers and reinforcement lies within typical incompatibility of common hydrophobic resins and hydrophilic fillers. In vegetable oil-based thermosets reinforced with rice hulls [8], and wood flour [9], mechanical property improvements have been made by enhancing the reinforcement-resin interactions with the addition of maleic anhydride as a co-monomer. In another study, asolectin, a phospholipid mixture isolated from soybeans previously used as a synthetic protein transport vessel [10,11], was employed as a bio-based compatibilizer between cellulose and a tung oil-based matrix [12]. The amphiphilic nature of asolectin, with long unsaturated carbon chains

Coatings 2019, 9, 447; doi:10.3390/coatings9070447 www.mdpi.com/journal/coatings Coatings 2019, 9, 447 2 of 14 and a highly polar phosphate group, ensures its role as an ideal bio-based compatibilizer. Indeed, an enhancement of the thermo-mechanical properties was observed with the addition of asolectin to tung oil/cellulose composites [12]. Industrially, vegetable oils find use in biofuels, drying agents, food, cosmetics, soap and detergents, and pharmaceuticals [13]. The chemical structure of vegetable oils consists of a triglyceride, which corresponds to glycerol esters connected to fatty acids. The carbon–carbon double bonds in unsaturated vegetable oils are ideal reactive sites for the chemical industry, offering a chemical handle for modifications leading to a variety of bio-based precursors [14]. Most unsaturated oils must be modified before free radical polymerization due to the low reactivity of non-conjugated carbon–carbon double bonds towards free radicals. In tung oil, approximately 83% of the fatty acid chains are α-eleostearic acid, a naturally triply-conjugated fatty acid. The naturally conjugated carbon–carbon double bonds in tung oil (at carbons 9-cis, 11-trans, and 13-trans)[15] can readily react with free radicals and polymerize in the presence of oxygen [14]. The co-polymerization of tung oil with vinyl co-monomers via cationic, thermal, or free radical polymerizations results in rigid, crosslinked polymers [16]. Other studies have further shown that a biopolymer can successfully be prepared from tung oil by cationic polymerization for self-healing applications in the curing of micro-cracks [17]. Due to the reactivity of tung oil and the promising thermo-mechanical properties obtained from tung oil-based/cellulose composites [12], it was hypothesized that an improvement in the compatibility of the matrix and the reinforcement can be obtained, without compromising the bio-based nature of the system, by using tung oil fatty acids as the major resin component. Cellulose is a hydrophilic polysaccharide composed of repeating D-glucose units linked through β-(1–4)-glycosidic bonds [18]. Due to celluloses’ biodegradability, accessibility, and mechanical properties, it has become increasingly popular for the production of low-cost materials. The work described in this manuscript proposes a bio-based composite system in which the need of a compatibilizer between the hydrophilic cellulose and a tung oil-based matrix is eliminated. By isolating the fatty acid, α-eleostearic acid, from tung oil, and subsequently polymerizing it, a polymeric network containing carboxyl groups with potential to hydrogen bond with polar reinforcements, such as cellulose, is obtained. It is suggested that intermolecular hydrogen bonding between the tung oil fatty acid and cellulose creates an interlocking polymer network, therefore enhancing the thermo-mechanical properties. The successful isolation of fatty acids in this work was verified via GC-MS and 1H NMR spectroscopy. Additionally, all materials were analyzed by Raman, and FT-IR spectroscopies. The polymerization of the resins was monitored by dielectric analysis (DEA) to obtain an optimal cure schedule. The morphology of the composites prepared was verified by scanning electron microscopy (SEM), and their hydrophilicity was assessed by means of contact angle and water absorption experiments. The final materials’ thermo-mechanical properties were assessed by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). When comparing cellulose composites prepared from tung oil fatty acids and tung oil methyl esters, it was found that the former exhibit higher binding affinity between matrix and reinforcement, which is reflected in their thermo-mechanical properties. The results presented here confirm the initial hypothesis that a better fiber-matrix interaction may be obtained with the use of fatty acids as co-monomers for the fabrication of biocomposites at the expense of a lower crosslink density when compared to tung oil triglycerides.

2. Experimental

2.1. Materials n-butyl methacrylate (BMA), tung oil (TO), α-cellulose, and di-tert-butyl peroxide (DTBP) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Methanol and acetone were procured from BDH Analytical Chemicals (Radnor, PA, USA). 50% w/w sodium hydroxide aqueous solution (NaOH), 38.0% w/w hydrochloric acid aqueous solution (HCl), and chloroform were obtained from Fisher Chemical Coatings 2019, 9, 447 3 of 14

(Fair Lawn, NJ, USA). Deuterated chloroform (chloroform-D) was purchased from Cambridge Isotope Laboratories, Inc. (Andover, MA, USA). Divinylbenzene (DVB) (composed of meta, para and ortho isomers) was procured from TCI America (Portland, OR, USA).

2.2. Methods

2.2.1. Fatty Acid Isolation from Tung Oil The saponification reaction used in this work was adapted from the literature as follows [17]. A 50% w/w sodium hydroxide aqueous solution was diluted to 20% w/w with deionized water. Two-hundred milliliters of the 20% sodium hydroxide solution was added to 100.0 g of tung oil, and the mixture was kept in a water bath under agitation with a magnetic stir bar at 70 ◦C for 1 h. Three-hundred milliliters of water at 70 ◦C were then added and the mixture was left undisturbed for 30 min before an additional 300.0 mL of water at 70 ◦C was added. After a final addition of 95.0 mL of the HCl solution (38%), the mixture was kept under agitation at 70 ◦C for 1 h. The precipitates formed were filtered and washed with 200 mL of deionized water, followed by 300 mL of methanol. The filtered material was then dried overnight at 70 ◦C under vacuum.

2.2.2. Transesterification of Tung Oil The procedure for the transesterification of tung oil with methanol was adapted from the literature and is described herein as follows [19]. Twenty grams of tung oil was refluxed with 30.0 mL of methanol in the presence of 0.5 g of 50% sodium hydroxide aqueous solution for 2 h. The reaction mixture was then decanted in a separatory funnel overnight. After phase separation, the bottom layer, containing glycerol and other aqueous by-products, was discarded. The upper layer, containing the methyl esters, was analyzed by proton nuclear magnetic resonance spectroscopy (1H NMR) and gas chromatography (GC) for purity and verification of successful isolation.

2.2.3. Preparation of Thermosetting Resins Fatty acid resins were prepared by heating 5.0 g of the fatty acids obtained from tung oil, 3.0 g of BMA, and 2.0 g of DVB in a capped 20 mL scintillation vial at 50 ◦C until the fatty acid was completely molten. The mixture was then vortexed for 30 s before 0.5 g of DTBP was introduced. The mixture was vortexed again for an additional 30 s. The scintillation vial was then placed in a convection oven for the designated temperatures and times, as discussed in the text. Thermosetting resins from tung oil and from tung oil methyl esters were prepared following an identical procedure, with the exception of the initial heating at 50 ◦C. Figure S1 shows a simpliﬁed free radical polymerization reaction mechanism for α-eleostearic acid. BMA and DVB were not included in the reaction mechanism presented in Figure S1 for clarity reasons.

2.2.4. Preparation of Cellulose-Reinforced Composites The aforementioned thermosetting resins were reinforced with α-cellulose by saturating the monomer mixture with 3.0 g (23 wt %) of the reinforcement prior to the addition of 0.5 g of DTBP. The solution was vortexed prior to cure in a convection oven (time and temperature cure schedules were evaluated and conﬁrmed by DEA, DSC, and Raman spectroscopy, as discussed in Section 3.2). Figure S2 displays a general schematic for the preparation of fatty acid-based composites.

2.3. Characterization Fourier-transformed infrared (FTIR) spectra were collected using a Thermo Nicolet Avatar 370-FTIR spectrometer (Thermo Scientific, Waltham, MA, USA) with an attenuated total reflectance 1 (ATR) accessory. The spectra were obtained in the 400–4000 cm− spectral region, with 64 scans, 1 1 and 4 cm− resolution. Proton nuclear magnetic resonance ( H NMR) spectra were obtained on an Agilent MR400DD2 spectrometer (Santa Clara, CA, USA) operating at 400 MHz. Raman spectra were Coatings 2019, 9, 447 4 of 14 collected on a DXR Raman Microscope (Thermo Scientific, Waltham, MA, USA) at a laser power of 1 mW. Gas-chromatography/mass spectrometry (GC-MS) analyses were completed on a Shimadzu GC–17A gas chromatograph equipped with a mass spectrometer detector Shimadzu GCMS-QP5050A (Kyoto, Japan) using a 30 m Rxi-5ms column (Restek, Bellefonte, PA, USA) with an internal diameter of 0.25 mm and a film thickness of 0.25 µm. Samples were dissolved in acetone, leading to an approximate concentration of 5% (v/v). Helium was used as the carrier gas at a total (column + split) flow rate of 10 mL/min. The temperature was initially held at 70 ◦C for 2 min, then increased to 300 ◦C at a heating rate of 15 ◦C/min, and finally held at 300 ◦C for 13 min. Dielectric analysis (DEA) measurements were completed on an Epsilon DEA 230/1 cure monitor (Netzsch Instruments North America LLC, Burlington, MA, USA). Differential scanning calorimetry (DSC) studies were conducted on a Q250 DSC instrument from TA Instruments (New Castle, DE, USA). 5–10 mg of sample were heated from 30 to 200 C at a rate of 10 C/min under N atmosphere in − ◦ ◦ 2 aluminum hermetic pans. Static and dynamic contact angles were measured using a goniometer (Ramé-hart, Succasunna, NJ, USA) equipped with an automatic drop dispenser. For the static contact angle measurements, 2.0 µL of water were dispensed onto a flat surface of the sample and the contact angle was measured every 30 s for a total of 10 measurements, and repeated for a total of three trials. The average angle from the three trials are reported. For the dynamic contact angle, measurements were generated by slowly enlarging the droplet size (advancing contact angle, θa) and then retracting it (receding contact angle, θr). For each expanding or retraction step, 0.25 µL of water was used. Water absorption experiments were conducted by weighing fully cured samples at their dry state (Wd), and then submerging them in water for 24 h. After 24 h, the samples were removed from water and their wet weight (Ww) was recorded after gently drying the excess water with a kimwipe. The morphology analyses were completed on a JSM-760F field emission scanning electron microscope (SEM) (JEOL, Peabody, MA, USA), using an accelerating voltage of 1.0 kV, secondary electron imaging (SEI) and lower detector scanning (LEI) modes, and an emission current of approximately 81 mA. All samples were sputter-coated with gold prior to imaging. Thermal degradation profiles were obtained on a Q50 thermal gravimetric analyzer (TA Instruments) using samples of approximately 10 mg. The experiments were run under air, from 30 to 500 ◦C, with a heating rate of 10 ◦C/min. Thermo-mechanical properties were assessed on a Q800 DMA (TA Instruments) using a three-point bending fixture with 15 mm gap. Rectangular samples (1.5 mm 10.0 mm 17.0 mm—thickness width length) were cut and subjected to an iso-strain × × × × experiment with an amplitude of 14 µm. The temperature was increased from 60 to 150 C at a − ◦ heating rate of 3 ◦C/min.

3. Results and Discussion

3.1. Starting Materials Tung oil and its corresponding fatty acids and methyl esters were analyzed by FTIR, 1H NMR, and Raman spectroscopies, and by GC-MS in order to check for purity and to confirm the structure of the bio-based starting materials used for the preparation of polymers and composites in this work. When comparing the FTIR spectra of tung oil, isolated α-eleostearic acid, and tung oil methyl esters (Figure1), it can be observed that the main structural features of the fatty acid chain in tung oil are unaltered after saponification and transesterification. Signals, such as the strong peak at approximately 1 1 1750 cm− , representing the carbonyl C=O stretch, and the strong peak at approximately 1000 cm− , which corresponds to the C–O stretch in the ester (tung oil and methyl esters) or carboxyl (α-eleostearic 1 acid) functional groups, can be seen in all three spectra (Figure1). Likewise, the peak at 990–980 cm − , representing the C–H wagging vibrations of conjugated carbon–carbon double bonds are also observed in all three compounds (Figure1)[ 20]. It’s interesting to note the absence of the typical signal for the Coatings 2019, 9, 447 5 of 14

O–H stretch in the spectrum of isolated α-eleostearic acid (Figure1). It is believed that, due to the basic reaction conditions employed during saponification, the final fatty acid is obtained in its salt form as a sodium carboxylate,Coatings 2019, 8, whichx FOR PEER does REVIEW not affect its subsequent polymerization and/or future5 of 15 interactions with the reinforcement.

Coatings 2019, 8, x FOR PEER REVIEW 5 of 15

Figure 1. FTIRFigure spectra 1. FTIR spectra of (a )of tung (a) tung oil oil methyl methyl esters, esters, (b) (isolatedb) isolated α-eleostearicα-eleostearic acid, and (c) acid, tung oil. and (c) tung oil.

1 The 1H NMRThe H spectra NMR spectra of the of the bio-based bio-based starting materials materials (Figure (Figure 2) and the2) peak and integration the peak of integration of their corresponding key signals provide further insight into reaction completion and product purity. Figure 1. FTIR spectra of (a) tung oil methyl esters, (b) isolated α-eleostearic acid, and (c) tung oil. their correspondingThe characteristic key signalssignals for provide the methylene further (4.1–4.3 insight ppm, intosignal reaction6) and the completionmethyne (~5.25 and ppm, product purity. 1 The characteristicsignal 7)The hydrogens signals1H NMR ofspectra for the the glycerol of methylenethe bio-basedbackbone canstarting (4.1–4.3 be clearly materials ppm, seen (Figure in signal the H2) NMRand 6) andthe spectrum peak the integration of methyne tung oil of (~5.25 ppm, (Figure 2a). Their absence from the spectra of tung oil methyl esters and isolated α-eleostearic acid signal 7) hydrogenstheir corresponding of the glycerol key signals backbone provide further can insight be clearly into reaction seen completion in the 1H and NMR product spectrum purity. of tung oil (FiguresThe characteristic 2b,c, respectively) signals confirmsfor the methylene successful (4.1–4.3separation ppm, of anysignal residual 6) and glycerol the methyne from the (~5.25 desired ppm, Commented [M6]: Please check if the citation format is (Figure2a).products. Their absence The intense from singlet the at spectra3.5 ppm in of the tung spectrum oil methyl of the tung esters oil methyl and isolatedester (Figureα 2b)-eleostearic acid signal 7) hydrogens of the glycerol backbone can be clearly seen in the 1H NMR spectrum of tung oil correct. (Figure2b,c,corresponds(Figure respectively) 2a). to Their its newlyabsence conﬁrms formed from successful themethyl spectra group. ofseparation tung The oil absence methylof of esters anysuch and residualpeak isolated from the glycerolα-eleostearic spectrum from acidof the desired isolated α-eleostearic acid (Figure 2c) suggests successful synthesis, purification, and drying of the products. The(Figures intense 2b,c, respectively) singlet at confirms 3.5 ppm successful in the sepa spectrumration of any of residual the tung glycerol oil from methyl the desired ester (FigureCommented2b) [M6]: Please check if the citation format is desiredproducts. fatty The acid. intense As explained singlet atpreviously, 3.5 ppm inthe the fatty spectrum acid is obtainedof the tung in itsoil saltmethyl form, ester therefore, (Figure the 2b) correct. correspondsprotoncorresponds to its of newly the carboxyl to formedits newly group methylformed isn’t detected methyl group. for group. the The fatty The absence acid. absence of of such such peak from from the the spectrum spectrum of of isolated α-eleostearicisolated acid (Figureα-eleostearic2c) acid suggests (Figure 2c) successful suggests successful synthesis, synthesis, puriﬁcation, purification, andand drying drying of the of the desired fatty acid. Asdesired explained fatty acid. previously, As explained the previously, fatty acid the fatty is obtained acid is obtained in its in saltits salt form, form, therefore, the the proton of proton of the carboxyl group isn’t detected for the fatty acid. the carboxyl group isn’t detected for the fatty acid.

Figure 2. 1H NMR spectra of (a) tung oil, (b) tung oil methyl esters, and (c) α-eleostearic acid.

Figure 2. 1HFigure NMR 2. 1 spectraH NMR spectra of (a )of tung(a) tung oil, oil, ( b(b)) tung oil oil methyl methyl esters, esters, and (c) α and-eleostearic (c) α-eleostearic acid. acid.

It is important to note that all peaks related to the fatty acid carbon backbone of tung oil (signals 1–5 and 8, Figure2a) are virtually unchanged after either transesterification (Figure2b), or saponification (Figure2c), indicating that the carbon chain is intact, including the set of three naturally conjugated carbon–carbon double bonds. Indeed, the integration of vinylic hydrogens (5.3–6.4 ppm, signal 8) with respect to the terminal methyl group (~0.8 ppm, signal 1) in each spectrum confirms that the Coatings 2019, 8, x FOR PEER REVIEW 6 of 15

It is important to note that all peaks related to the fatty acid carbon backbone of tung oil (signals Coatings 2019, 91–5, 447 and 8, Figure 2a) are virtually unchanged after either transesterification (Figure 2b), or 6 of 14 saponification (Figure 2c), indicating that the carbon chain is intact, including the set of three naturally conjugated carbon–carbon double bonds. Indeed, the integration of vinylic hydrogens (5.3–6.4 ppm, unsaturationssignal were 8) with not respect affected to the by terminal the reactions methyl grou performed.p (~0.8 ppm, signal In each 1) in each case, spectrum the integration confirms indicates a that the unsaturations were not affected by the reactions performed. In each case, the integration total of 6 vinylicindicates hydrogens a total of 6 vinylic per hydrogens fatty acid per chain. fatty acid Another chain. Another indication indication thatthat the the unsaturations unsaturations are not compromisedare not by compromised either transesterification by either transesterification or saponification or saponification of of tung tung oil oil can can be obtained be obtained by a by a quick analysis of thequick Raman analysis spectra of the Raman of the spectra three samplesof the three (Figure samples3 (Figure). Indeed, 3). Indeed, the intense the intense peak peak at at approximately approximately 1630 cm−1, associated to C=C vibrations, is clearly observed in all three spectra (Figure 3). 1 1630 cm− , associated to C=C vibrations, is clearly observed in all three spectra (Figure3).

Figure 3. RamanFigure 3. spectra Raman spectra of (a) of tung (a) tung oil, oil, (b (b)) isolatedisolated α-eleostearicα-eleostearic acid, and acid, (c) tung and oil ( cmethyl) tung ester. oil methyl ester.

The purity of the bio-based starting materials was verified by GC-MS. The chromatograms obtained The purityare presented of the in bio-basedFigure 4. In the starting chromatograms materials of tung oil was and verifiedisolated α-eleostearic by GC-MS. acid (Figure The 4a,b, chromatograms obtained arerespectively), presented two in signals Figure are 4observed. In the at chromatograms15–17 min. The existence of of tung these oilsignals and in the isolated chromatogramα-eleostearic acid (Figure4a,b,of respectively), tung oil (Figure 4a) two is believed signals to arerelate observed to the various at possible 15–17 triglyceride min. The structures existence of the of oil these based signals in the on its fatty acid composition (~83% of α-eleostearic acid, 9% of linoleic acid, 4% of oleic acid, and 4%–6% chromatogramof palmitic of tung acid) oil [21], (Figure with a predominance4a) is believed of triglycerides to relate bearing to the α-eleostearic various possibleacid accounting triglyceride for the structures of the oil basedmajor on signal. its fattyA similar acid scenario composition is reflected in (~83% the chromatogram of α-eleostearic of isolated α acid,-eleostearic 9% of acid linoleic (Figure 4b). acid, 4% of oleic acid, and 4%–6%The absence of palmiticof other relevant acid) signals [21 suggests], with that, a predominancebesides the mixture of of fatty triglycerides acids, no other by-products bearing α-eleostearic are detected. For the methyl esters (Figure 4c), a better separation of the different methyl ester chains is acid accountingachieved, for most the likely major due signal.to their independent A similar structure scenario when iscompared reflected to triglycerides, in the chromatogram and their lower of isolated α-eleostearicpolarity acid (Figurein comparison4b). to The free fatty absence acids. Three of other signals relevant are indeed signals obtained in suggests the 15–17 min. that, range besides with the mixture of fatty acids,one no major other signal by-products attributed to theare methyl detected. ester of α-eleostearic For the methylacid (Figure esters 4c). (Figure4c), a better separation When comparing the retention times of the major signal in each one of the chromatograms (Figure of the different4a–c), methyl a clear and ester logical chains trend in is retention achieved, times can most be observed. likely due On one to han theird, while independent tung oil’s major structure when compared tosignal triglycerides, displays a retention and time their of 16 lower min and polarity 45 s (Figure in 4a), comparison the major signal to for free the fatty fatty acid acids. sample Three signals exhibits a slightly longer retention time (16 min and 54 s, Figure 4b) that can be attributed to its higher are indeed obtainedCoatings 2019, 8 in, x FOR the PEER 15–17 REVIEW min. range with one major signal attributed to the7 of methyl 15 ester of polarity and, therefore, better affinity for the GC column. On the other hand, the methyl ester’s major α-eleostearicsignal acid exhibits (Figure a shorter4c). retention time (16 min and 16 s, Figure 4c) in comparison to that of tung oil’s major signal. Despite their similar polarities, independent methyl ester chains move more freely through the column than triglycerides, resulting in the observed shorter retention time. Based on the FT-IR, 1H NMR, Raman, and GC-MS results, it can be concluded that the preparation of methyl esters and the isolation of fatty acids from tung oil were successful.

Figure 4. Chromatograms of (a) tung oil, (b) isolated α-eleostearic acid, and (c) tung oil methyl esters. Figure 4. Chromatograms of (a) tung oil, (b) isolated α-eleostearic acid, and (c) tung oil methyl esters.

When comparing3.2. Cure Evaluation the retention times of the major signal in each one of the chromatograms (Figure4a–c), a clear and logical trend in retention times can be observed. On one hand, while tung In order to establish an optimum cure schedule for the fatty acid resins prepared, the cure oil’s major signalevolution displays of a crude a resin retention containing time 50 wt of % 16 of min tung andoil fatty 45 acids, s (Figure 30 wt %4a), of BMA, the majorand 20 wt signal % of for the fatty acid sampleDVB exhibits was monitored a slightly through longer dielectric retention analysis time(DEA) (16at different min and temperatures 54 s, Figure as shown4b) in that Figure can 5A, be attributed where the log of ion viscosity (an artificial property derived from the permittivity of the resin) is plotted against time at each chosen temperature. This resin composition was selected based on previous studies with similar bio-based systems [12]. The initial decrease in ion viscosity observed in the first 50 min of each run (Figure 5A) corresponds to the initial increase in temperature before polymerization starts. Indeed, the increase in temperature prior to extensive polymerization leads to greater molecular mobility, which translates into a decrease in ion viscosity (Figure 5A). At approximately 50 min, the polymerization process reaches a significant rate, resulting in an increase in ion viscosity due to a decrease in molecular mobility, as the crosslinked polymer network grows. Once the ion viscosity plateaus, it is accepted that no further changes occur in the material’s electrical and physical properties, indicating cure completion. When the fatty acid resin is cured at 140 °C, a significant amount of shrink cracks can be easily noticed in the sample, indicating an inappropriate cure temperature. Additionally, the DEA curve obtained at 140 °C (Figure 5A) displayed a secondary decrease in ion viscosity at approximately 12 h, which could be attributed to exposure of the DEA sensor to air after formation of shrink cracks in the polymer, affecting the permittivity measured.

Coatings 2019, 9, 447 7 of 14 to its higher polarity and, therefore, better aﬃnity for the GC column. On the other hand, the methyl ester’s major signal exhibits a shorter retention time (16 min and 16 s, Figure4c) in comparison to that of tung oil’s major signal. Despite their similar polarities, independent methyl ester chains move more freely through the column than triglycerides, resulting in the observed shorter retention time. Based on the FT-IR, 1H NMR, Raman, and GC-MS results, it can be concluded that the preparation of methyl esters and the isolation of fatty acids from tung oil were successful.

3.2. Cure Evaluation In order to establish an optimum cure schedule for the fatty acid resins prepared, the cure evolution of a crude resin containing 50 wt % of tung oil fatty acids, 30 wt % of BMA, and 20 wt % of DVB was monitored through dielectric analysis (DEA) at different temperatures as shown in Figure5A, where the log of ion viscosity (an artificial property derived from the permittivity of the resin) is plotted against time at each chosen temperature. This resin composition was selected based on previous studies with similar bio-based systems [12]. The initial decrease in ion viscosity observed in the first 50 min of each run (Figure5A) corresponds to the initial increase in temperature before polymerization starts. Indeed, the increase in temperature prior to extensive polymerization leads to greater molecular mobility, which translates into a decrease in ion viscosity (Figure5A). At approximately 50 min, the polymerization process reaches a significant rate, resulting in an increase in ion viscosity due to a decrease in molecular mobility, as the crosslinked polymer network grows. Once the ion viscosity plateaus, it is accepted that no further changes occur in the material’s electrical and physical properties, indicating cure completion. When the fatty acid resin is cured at 140 ◦C, a significant amount of shrink cracks can be easily noticed in the sample, indicating an inappropriate cure temperature. Additionally, the DEA curve obtained at 140 ◦C (Figure5A) displayed a secondary decrease in ion viscosity at approximately 12 h, which could be attributed to exposure of the DEA sensor to air after formation of shrink cracks in the polymer, affecting the permittivity measured. Coatings 2019, 8, x FOR PEER REVIEW 8 of 15

Figure 5. (A) DEA curves of fatty acid resin cured at 120, 130, and 140 °C for 20 h; (B) Derivative of Figure 5. (A) DEA curves of fatty acid resin cured at 120, 130, and 140 ◦C for 20 h; (B) Derivative of the the DEA curves of the fatty acid resin cured at 130 and 120 °C with respect to time. DEA curves of the fatty acid resin cured at 130 and 120 ◦C with respect to time. For the DEA curve of the fatty acid resin cured at 120°C (Figure 5A), the ion viscosity For the DEAcontinuously curve increases of the fatty throughout acid resinthe duration cured of at the 120 experiment◦C (Figure (20 h).5A), In order the ionto obtain viscosity a clearer continuously increases throughoutcomparison thebetween duration the cure ofat 120 the and experiment 130 °C, the derivative (20 h). of In ion order viscosity to obtainwith respect a clearer to time comparison was plotted against cure time in Figure 5B. Close inspection of Figure 5B reveals that the derivative between theapproaches cure at 120 zero and (indicating 130 ◦ C,the thechange derivative in electrical of and ion physical viscosity properties with is zero respect and the to cure time is was plotted against cure timecompleted) in Figure at approximately5B. Close 18 h inspection (dashed line added of Figure as a guide5B revealsto the eye,that Figure the 5B) derivativewhen the resin approaches zero (indicatingis heated the at change 130 °C. When in electrical the resin is and heated physical at 120 °C, propertiesthe derivative isof the zero DEA and curve the never cure reaches is completed) at zero for the duration of the experiment, indicating an incomplete cure after 20 h (Figure 5B). approximately 18A comparison h (dashed of linethe Raman added spectra as a of guide a crude to fatty the acid eye, resin Figure and a cured5B)when sample the(Figure resin 6A) is heated at provides further evidence that cure completion was achieved for the fatty acid resin after heating at 130 °C for 18 h. Indeed, the presence of a signal at approximately 1630 cm−1 in the spectrum of the crude resin (Figure 6A) is indicative of unreacted carbon–carbon double bonds from the various resin components. That signal is absent in the spectrum of the cured resin (Figure 6A), indicating that the polymer was fully cured as previously supported by the DEA results. In order to validate the DEA and Raman results and to confirm cure completion, a tung oil fatty acid resin cured at 130 °C for 18 h was subjected to a DSC experiment (Figure 6B). The same cure schedule was applied to similar resins prepared from tung oil and from tung oil methyl esters. The DSC curves of all three samples are presented in Figure 6B(a–c). The DSC results indicate that the cure schedule used is adequate for tung oil and fatty acid resins since no exothermic peaks are observed for these two samples (Figure 6B). However, the methyl ester resin is not fully cured when subjected to that same cure schedule, as indicated by a large exothermic peak centered at approximately 140 °C (Figure 6B). In order to ensure cure completion, the tung oil methyl ester resin was subjected to a longer (24 h) cure schedule at 130 °C. The increase in cure time by an additional 6 h resulted in a fully cured sample as evidenced by the absence of any exothermic peaks in the corresponding DSC curve (Figure 6B). In view of these results, the cure schedule at 130 °C for 18 h is considered to be optimal for the fatty acid and tung oil resins, whereas the tung oil methyl ester resin requires heating at 130 °C for 24 h. These cure schedules will, therefore, be employed with their corresponding resins in the remainder of this work.

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130 ◦C. When the resin is heated at 120 ◦C, the derivative of the DEA curve never reaches zero for the duration of the experiment, indicating an incomplete cure after 20 h (Figure5B). A comparison of the Raman spectra of a crude fatty acid resin and a cured sample (Figure6A) provides further evidence that cure completion was achieved for the fatty acid resin after heating at 1 130 ◦C for 18 h. Indeed, the presence of a signal at approximately 1630 cm− in the spectrum of the crude resin (Figure6A) is indicative of unreacted carbon–carbon double bonds from the various resin components. That signal is absent in the spectrum of the cured resin (Figure6A), indicating that the polymer was fully cured as previously supported by the DEA results. Coatings 2019, 8, x FOR PEER REVIEW 9 of 15

Figure 6. (A) Raman spectra of (a) tung oil fatty acid crude resin, and (b) tung oil fatty acid resin cured Figure 6. (A) Raman spectra of (a) tung oil fatty acid crude resin, and (b) tung oil fatty acid resin cured at 130 ◦C forat 18 130 h. °C( forB) 18 DSC h. (B curves) DSC curves of (a) of (a) tung tung oiloil resin resin cured cured at 130 at °C 130 for ◦18C h, for (b) tung 18 h, oil (b) fatty tung acid oil fatty acid resin cured at 130 °C for 18 h, (c) tung oil methyl ester resin cured at 130°C for 18 h, and (d) tung oil resin cured at 130 ◦C for 18 h, (c) tung oil methyl ester resin cured at 130◦C for 18 h, and (d) tung oil methyl ester resin cured at 130 °C for 24 h. methyl ester resin cured at 130 ◦C for 24 h. 3.3. Resin Hydrophilicity In order to validate the DEA and Raman results and to conﬁrm cure completion, a tung oil fatty Contact angle measurements are arguably the most widely used surface analysis technique acid resin cureddirectly at related 130 ◦toC polymer for 18 surface h was hydrophilicity, subjected co toating a DSC abilities, experiment and is strongly (Figure representative6B). The of same cure schedule wassurface applied adhesion to similar properties resins [22,23]. prepared Static contact from angle tung measurements oil and from along tung with oil their methyl standard esters. The DSC curves of alldeviations three samples for resins prepared are presented with tung inoil, Figuretung oil 6fattyB(a–c). acids, and The tung DSC oil methyl results esters indicate are listed that the cure in Table 1. As anticipated, the tung oil fatty acid resin exhibited the most favorable interactions with schedule usedwater, is adequate reflected in forthe tunglowest oil contact and angles fatty acidmeasured. resins These since results no indicate exothermic an increase peaks in arethe observed for these two sampleshydrophilicity (Figure of the6B). polymer However, when tung the oil methyl is replaced ester with resin a more is polar not fullymolecule, cured α-eleostearic when subjected to acid. Not surprisingly, resins prepared from tung oil and tung oil methyl esters exhibited comparable that same cure schedule, as indicated by a large exothermic peak centered at approximately 140 C results, associated with the similar chemical structure and polarity of their main bio-based ◦ (Figure6B). Incomponents. order to ensure cure completion, the tung oil methyl ester resin was subjected to a In order to limit the interference of factors, such as surface roughness and sample heterogeneity, longer (24 h) cure schedule at 130 ◦C. The increase in cure time by an additional 6 h resulted in a fully a dynamic analysis was also conducted (Table 1), revealing a similar trend to that of the static contact cured sampleangles. as evidenced The overall higher by the dynamic absence contact of angles any exothermic in comparison peaksto static inangles the measured corresponding is most DSC curve (Figure6B). Inlikely view the ofresult these of water results, absorption the cure during schedule static measurements. at 130 ◦C Water for 18 absorption h is considered is also believed to be optimal for to be the reason for the high hysteresis value observed for the fatty acid resin in comparison to the the fatty acid and tung oil resins, whereas the tung oil methyl ester resin requires heating at 130 ◦C other resins evaluated (Table 1). This is further supported by water uptake/absorption studies (Table 1). for 24 h. These cure schedules will, therefore, be employed with their corresponding resins in the remainder of thisTable work. 1. Static and dynamic contact angles, and water absorption measurements of fully-cured resins.

Resin Bio-Based Static Receding Advancing Hysteresis Ww b Wd c Ratio d 3.3. Resin HydrophilicityComponent Angle (°) Angle (°) Angle (°) a (g) (g) (%) α-eleostearic acid 36.70 41.1 80.8 39.7 0.0448 0.0236 90 ContactTung angle oil methyl measurements ester 100.77 are arguably 106.6 the 121.4 most widely 14.8 used 0.0395 surface 0.0276 analysis 43 technique Tung oil 97.93 117.5 130.6 13.1 0.0412 0.0384 7 directly related to polymer surface hydrophilicity, coating abilities, and is strongly representative of a Hysterisis = advancing angle − receding angle; b Ww = wet weight; c Wd = dry weight; d Ratio = ×100. surface adhesion properties [22,23]. Static contact angle measurements along with their standard deviations for resinsWater uptake prepared experiments with tungwere used oil, to tung determine oil fatty the polymer’s acids, andability tung to absorb oil water, methyl which, esters are listed consequently, can be used to make inferences about its ability to biodegrade. The ratio reflects the in Table1. Aspercentage anticipated, of water theabsorbed/uptake. tung oil fatty The sample acid prepared resin exhibited from fatty acids the displayed most favorable 90% water interactions with water, reﬂecteduptake (Table in 1), the indicating lowest favorable contact interaction angles with measured. water and a These high potential results for indicatebiodegradability. an increase in the The sample prepared from methyl esters also exhibited a significant amount of water uptake (43%)α hydrophilicitydespite of the its polymerhydrophobic when nature, tung displaying oil is good replaced potential with for abiodegradability. more polar molecule, The considerably-eleostearic acid. Not surprisingly,smaller resins water uptake prepared observed from for tungthe sample oil and prepared tung from oil tung methyl oil (7%) esters can be exhibited related to its comparable more results, associated with the similar chemical structure and polarity of their main bio-based components.

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Table 1. Static and dynamic contact angles, and water absorption measurements of fully-cured resins.

Resin Bio-Based Static Angle ( ) Receding Angle ( ) Advancing Angle ( ) Hysteresis a W b (g) W c (g) Ratio d (%) Component ◦ ◦ ◦ w d α-eleostearic acid 36.70 41.1 80.8 39.7 0.0448 0.0236 90 Tung oil methyl ester 100.77 106.6 121.4 14.8 0.0395 0.0276 43 Tung oil 97.93 117.5 130.6 13.1 0.0412 0.0384 7

a b c d Ww Wd Hysterisis = advancing angle receding angle; Ww = wet weight; Wd = dry weight; Ratio = − 100. − Wd ×

In order to limit the interference of factors, such as surface roughness and sample heterogeneity, a dynamic analysis was also conducted (Table1), revealing a similar trend to that of the static contact angles. The overall higher dynamic contact angles in comparison to static angles measured is most likely the result of water absorption during static measurements. Water absorption is also believed to be the reason for the high hysteresis value observed for the fatty acid resin in comparison to the other resins evaluated (Table1). This is further supported by water uptake /absorption studies (Table1). Water uptake experiments were used to determine the polymer’s ability to absorb water, which, consequently, can be used to make inferences about its ability to biodegrade. The ratio reflects the percentage of water absorbed/uptake. The sample prepared from fatty acids displayed 90% water uptake (Table1), indicating favorable interaction with water and a high potential for biodegradability. The sample prepared from methyl esters also exhibited a significant amount of water uptake (43%) despite its hydrophobic nature, displaying good potential for biodegradability. The considerably smaller water uptake observed for the sample prepared from tung oil (7%) can be related to its more cross-linked structure, preventing swelling and water absorption, which can significantly impact its biodegradability and recycling potential.

3.4. Fiber-ResinCoatings Interaction 2019, 8, x FOR PEER REVIEW 10 of 15 cross-linked structure, preventing swelling and water absorption, which can significantly impact its Fatty acid,biodegradability tung oil, andand recycling methyl potential. ester resins were reinforced with α-cellulose and the resulting composites were imaged by SEM at 2000 and 2500 magnifications (Figure7) in order to explore × × fiber-matrix interactions.3.4. Fiber-Resin Interaction After inspection of Figure7A, it can be concluded that the resin prepared with tung oil fattyFatty acids acid, adherestung oil, and well methyl to theester surfaceresins were of reinforced the α-cellulose with α-cellulose reinforcement, and the resulting as indicated by composites were imaged by SEM at 2000× and 2500× magnifications (Figure 7) in order to explore the significantfiber-matrix amount interactions. of resin residue After inspection covering of Figure the fiber.7A, it can For be aconcluded composite that the prepared resin prepared with the tung oil methyl ester resinwith tung (Figure oil fatty7B), acids the adheres lack ofwell adhesion to the surface of of the the resin α-cellulose to the reinforcement, reinforcement as indicated is evidenced by by the clean exposedthe cellulose significant amount fiber. of These resin residue results covering align the well fiber. with For a thecomposite contact prepared angle with and the tung water oil uptake data methyl ester resin (Figure 7B), the lack of adhesion of the resin to the reinforcement is evidenced by obtained, andthe can clean be exposed considered celluloseindicative fiber. These results of the align polarity well with of the the contact resins angle prepared. and water Foruptake the composite prepared withdata tung obtained, oil triglycerides and can be considered (Figure indicative7C), some of the resin polarity residue of the canresins be prepared. noticed For on the the surface of the cellulose fiber,composite despite prepared the with predominant tung oil triglycerides hydrophobic (Figure 7C), nature some resin of the residue resin can and be noticed the hydrophilic on the nature surface of the cellulose fiber, despite the predominant hydrophobic nature of the resin and the of cellulose. Thishydrophilic result nature shows of cellulose. a good This correlation result shows witha good the correlation static with contact the static angle contact data angle obtained data (Table1), but at this stage,obtained the (Table fundamental 1), but at this reason stage, the for fundamental why the tungreason oilfor why resin the exhibits tung oil resin a better exhibits adhesion a to the fiber’s surfacebetter than adhesion the methyl to the fiber’s ester surface resin than is still the methyl unclear. esterHowever, resin is still unclea it is worthr. However, noting it is worth that the intent of noting that the intent of this work is to maximize fiber-resin interactions by preparing a novel polar this work is totung maximize oil fatty acid fiber-resin resin. interactions by preparing a novel polar tung oil fatty acid resin.

Figure 7. SEMFigure images 7. SEM of images (A) compositeof (A) composite prepared prepared withwith tung tung oil fatty oil fatty acid resin, acid (B resin,) composite (B) prepared composite prepared with tung oilwith methyl tung oil ester methyl resin, ester andresin, (andC) ( compositeC) composite prepared with with a tung a oil tung resin. oil resin. 3.5. Thermo-Mechanical Properties The thermogravimetric analysis (TGA) of tung oil, tung oil fatty acid, and tung oil methyl ester resins (Figure 8a–c) shows a clear trend for their thermal stability. It is indeed obvious that the onset degradation temperature (Tonset) for the tung oil methyl ester resin (~100 °C) is much lower than that of samples prepared with tung oil fatty acids (~180 °C) or tung oil triglycerides (~300 °C). This trend can be explained as a consequence of the chemical structure of the major resin components. For example, the markedly higher Tonset observed for the tung oil resin is the result of a highly crosslinked structure, due to the fact that the tung oil triglyceride monomer consists of three fatty acid esters connected to a glycerol backbone. In both fatty acids and methyl esters, the polymerizable fatty chains are independent from each other, resulting in a much less crosslinked polymer network. The difference observed in the Tonset of fatty acid and methyl ester polymers can be associated to their polarity. The polar fatty acid resin exhibits stronger hydrogen bonding interactions between polymer chains, resulting in a more compact arrangement of the overall polymer structure and, therefore, constituting a more thermally stable material in comparison to its methyl ester analogue.

Coatings 2019, 9, 447 10 of 14

3.5. Thermo-Mechanical Properties The thermogravimetric analysis (TGA) of tung oil, tung oil fatty acid, and tung oil methyl ester resins (Figure8a–c) shows a clear trend for their thermal stability. It is indeed obvious that the onset degradation temperature (Tonset) for the tung oil methyl ester resin (~100 ◦C) is much lower than that of samples prepared with tung oil fatty acids (~180 ◦C) or tung oil triglycerides (~300 ◦C). This trend can be explained as a consequence of the chemical structure of the major resin components. For example, the markedly higher Tonset observed for the tung oil resin is the result of a highly crosslinked structure, due to the fact that the tung oil triglyceride monomer consists of three fatty acid esters connected to a glycerol backbone. In both fatty acids and methyl esters, the polymerizable fatty chains are independent from each other, resulting in a much less crosslinked polymer network. The diﬀerence observed in the Tonset of fatty acid and methyl ester polymers can be associated to their polarity. The polar fatty acid resin exhibits stronger hydrogen bonding interactions between polymer chains, resulting in a more compact arrangement of the overall polymer structure and, therefore, constituting a more thermally stable material in comparison to its methyl ester analogue. Coatings 2019, 8, x FOR PEER REVIEW 11 of 15

Figure 8. TGAFigure curves 8. TGA of curves (a) tung of (a) oiltung resin, oil resin, (b ()b) tung tung oil oil fatty fatty acid acid resin, resin, (c) tung ( coil) tungmethyl oil ester methyl resin, ester resin, (d) composite prepared with tung oil resin, (e) composite prepared with tung oil fatty acid resin, and (d) composite(f prepared) composite prepared with tung with oil tung resin, oil methyl (e) composite ester resin. prepared with tung oil fatty acid resin, and (f) composite prepared with tung oil methyl ester resin. It is also apparent, from the TGA results (Figure 8a–c), that the bulk of the thermal degradation It is alsoprocess apparent, occurs fromfrom T theonset to TGA approximately results 480 (Figure °C for8 thea–c), resins that prepared the bulk with tung of the oil thermaland methyldegradation esters (Figure 8a,c), while the resin prepared with fatty acids clearly exhibits an additional secondary process occursstep from startingTonset at approximatelyto approximately 450 °C (Figure 480 ◦8b).C forThe themajor resins degradation prepared process with is believed tung oilto and methyl esters (Figurecorrespond8a,c), while to the the breakdown resin prepared of the cross-link with network fatty of acids the thermoset. clearly The exhibits additional an degradation additional secondary step startingstep at approximatelyobserved for the resin 450 preparedC (Figure with fatty8b). acids The could major be associated degradation to the final process oxidation isof believed to residues and char formation, but◦ its definitive origin can’t be reliably inferred from the TGA results correspond toalone, the lying breakdown outside the of scope the of cross-link the work described network in this of manuscript. the thermoset. The additional degradation step observed forA very the similar resin preparedtrend in thermal with stability fatty can acids be observed could befor associatedthe composites to (Figure the final 8d–f) oxidation of residues andcompared char formation, to their corresponding but its definitive resins (Figure origin 8a–c). can’t All composites be reliably exhibit inferred a very clear from thermal the TGA results degradation profile, with two distinct stages that can be associated with the degradation of the resin alone, lying outsideand the degradation the scope of ofthe the reinforcement, work described although it in isthis unavoidable manuscript. that the degradation of these A verytwo similar components trend (resin in thermaland reinforcement) stability occur can with be some observed overlap. When for thecorrelating composites the results (Figure 8d–f) compared toobserved their corresponding with fiber-matrix interactions, resins (Figure one would8a–c). expect All that compositesa stronger binding exhibit between a resin very and clear thermal reinforcement would result in a better fiber protection, and ultimately in a delayed degradation, degradationespecially profile, withtowards two the distinct end of the stages thermal that degr canadation be associated process. Indeed, with when the comparing degradation the of the resin and the degradationtemperature of at the 80 wt reinforcement, % degradation (T80 although) for resins and it is their unavoidable corresponding that composites, the degradation respectively, of these two components (resinan increase and from reinforcement) ~430 to ~450 °C occur can be with seen somefor samples overlap. prepared When with correlating fatty acids, whereas the results a observed decrease from ~445 to ~425 °C is observed for samples prepared with methyl esters. This behavior is with fiber-matrixreflective interactions, of the better one fiber-matrix would interactions expect that obtained a stronger with fatty binding acids betweenin comparison resin to methyl and reinforcement would resultesters. in a betterIn the case fiber of protection,tung oil triglyceride, and ultimately the increase in T a80 delayedfrom ~455 degradation,to ~475 °C, for resin especially and towards composite, respectively, can be associated to the highly crosslinked structure of the oil offering the end of thethermal thermal protection degradation to the fiber despite process. the expected Indeed, poor when fiber-matrix comparing interaction. the temperature at 80 wt % degradation (T80DMA) for was resins used and to assess their thermo-mechanical corresponding properties composites, of the respectively,resins and composites an increase studied. from ~430 to These results are summarized in Figure 9 and Table 2. At temperatures below 20 °C, the resin ~450 ◦C can be seen for samples prepared with fatty acids, whereas a decrease from ~445 to ~425 ◦C is prepared with fatty acids displays a higher storage modulus than the resin prepared with tung oil observed for samples prepared with methyl esters. This behavior is reflective of the better fiber-matrix triglycerides (Figures 9a,b), exhibiting a behavior that is reflective of the expected, more prevalent Commented [M7]: Please check if the citation format is interactions obtainedhydrogen bonding. with fatty Above acids 20 °C, init is comparisonbelieved that chain to mobility methyl overcomes esters. hydrogen In the case bonding, of tung and that oil triglyceride, correct. the more crosslinked structure of the resin prepared with tung oil accounts for the higher storage modulus the increase in T80 from ~455 to ~475 ◦C, for resin and composite, respectively, can be associated to the observed in comparison to the resin prepared with fatty acids. At the rubbery plateau (Tg + 50 °C), above all phase transitions of the materials, the storage modulus for the resin prepared with tung oil is approximately 4.7 times higher than that of the resin prepared with fatty acids (68.9 MPa vs. 14.6 MPa, respectively, Table 2). Indeed, that same trend is also clear in the calculated crosslink density, with 16.2 × 10−4 mol/cm3 for the fatty acid resin and 77.7 × 10−4 mol/cm3 for the tung oil resin (Table 2). Cross- link densities were calculated based on the rubbery elasticity theory, according to Equation (1) [24,25]:

Coatings 2019, 9, 447 11 of 14 highly crosslinked structure of the oil offering thermal protection to the fiber despite the expected poor fiber-matrix interaction. DMA was used to assess thermo-mechanical properties of the resins and composites studied. These results are summarized in Figure9 and Table2. At temperatures below 20 ◦C, the resin prepared with fatty acids displays a higher storage modulus than the resin prepared with tung oil triglycerides (Figure9a,b), exhibiting a behavior that is reflective of the expected, more prevalent hydrogen bonding. Above 20 ◦C, it is believed that chain mobility overcomes hydrogen bonding, and that the more crosslinked structure of the resin prepared with tung oil accounts for the higher storage modulus observed in comparison to the resin prepared with fatty acids. At the rubbery plateau (Tg + 50 ◦C), above all phase transitions of the materials, the storage modulus for the resin prepared with tung oil is approximately 4.7 times higher than that of the resin prepared with fatty acids (68.9 MPa vs. 14.6 MPa, respectively, Table2). Indeed, that same trend is also clear in the calculated crosslink density, with 16.2 10 4 mol/cm3 for the fatty acid resin and 77.7 10 4 mol/cm3 for the tung oil resin × − × − (Table2). Cross-link densities were calculated based on the rubbery elasticity theory, according to Equation (1) [24,25]:

Coatings 2019, 8, x FOR PEER REVIEW Crosslink density = E’/RT 12 of 15 (1) where E’ is the storage modulus at Tg + 50 C, R is the gas constant, and T is the absolute temperature Crosslink◦ density = E’/RT (1) (K). Crosslink density was not calculated for composites (Table2) because the numbers would be where E’ is the storage modulus at Tg + 50 °C, R is the gas constant, and T is the absolute temperature artiﬁcially inﬂated(K). Crosslink by the density natural was increase not calculated in E’ forresulting composites from (Table the 2) because addition the ofnumbers a reinforcement, would be therefore masking theartificially true crosslink inflated densityby the natural value. increase It is in also E’ resulting worth from noting the thataddition DMA of a specimensreinforcement, could not be produced fromtherefore theresin masking prepared the true crosslink with methyldensity value. esters It is duealso worth to its noting exceeding that DMA fragility. specimens could not be produced from the resin prepared with methyl esters due to its exceeding fragility.

(A) (B)

Figure 9. (A)Figure Storage 9. (A) modulus Storage modulus (E’) vs.(E’) temperaturevs. temperature and and (B) ( Btan) tanδ curvesδ curves for (a) a forresin (a) prepared a resin with prepared with tung oil, (b) a resin prepared with fatty acids, (c) a composite prepared with tung oil, (d) a composite tung oil, (b) aprepared resin preparedwith fatty acids, with and fatty (e) a co acids,mposite (c) prepared a composite with methyl prepared esters. with tung oil, (d) a composite prepared with fatty acids, and (e) a composite prepared with methyl esters. Table 2. DMA results for samples prepared with tung oil, tung oil fatty acids, and tung oil methyl esters. Table 2. DMA results for samples prepared with tung oil, tung oil fatty acids, and tung oil methyl esters. Cross Link Major Resin Tg a ( E’ at 25 °C E’ at Tg + 50 °C Reinforcement Density Major Resin Component a ℃) E’ at(MPa) 25 ◦ C E’(MPa)at Tg + 50 𝐦𝐨𝐥Cross Link Density Reinforcement Tg (°C) ×𝟏𝟎mol𝟒 4 Component (MPa) C (MPa) 𝟑 ( 10 ) ◦ 𝐜𝐦 cm3 × − Tung oil – 30 338.4 68.9 77.7 Tung oil – 30 338.4 68.9 77.7 Fatty acid – 39 284.8 14.6 16.2 Fatty acid – 39 284.8 14.6 16.2 Tung oil α-cellulose 38 524.0 131.2 – Tung oil α-cellulose 38 524.0 131.2 – Fatty acid α-cellulose 56 459.5 31.7 – α Fatty acid Methyl ester-cellulose α-cellulose 56 −6 459.520.2 11.8 31.7 – – Methyl ester α-cellulose 6 20.2 11.8 – a Tg was −obtained from the peak of the tanδ curve. a Tg was obtained from the peak of the tanδ curve. In the presence of the cellulose reinforcement, the composite prepared with methyl esters exhibits significant lower storage modulus than the composite made from fatty acids (Figure 9d,e), Commented [M8]: Please check if there is an explanation of In the presenceindicating ofthat the a stronger cellulose reinforcing reinforcement, effect is obta theined compositefor the more polar prepared resin, as with expected methyl due to esters exhibits Figure 9d in Figure 9 caption significant lowerits increased storage affinity modulus with the than reinforcement. the composite For the composite made from prepared fatty with acids tung oil (Figure triglyceride9d,e), indicating that a stronger(Figure reinforcing 9c), the storage effect modulus is obtained is consistently for the hi moregher than polar all other resin, composites, as expected revealing due that, to its increasedCommented [M9]: Please check if there is an explanation of contrary to expected, the crosslink density of the resin is a greater contributor towards final Figure 9c in Figure 9 caption affinity withmechanical the reinforcement. properties than For fiber-matrix the composite interactions. prepared It is interesting with tung to note oil triglyceridethat at lower (Figure9c), the storage modulustemperatures, is the consistently fatty acid resin (Figure higher 9b) than exhibits all higher other storag composites,e modulus than revealing the other systems, that, contrary to surpassing the tung oil resin (Figure 9a) below 24 °C as discussed previously, the fatty acid composite (Figure 9d) below 23 °C, and the tung oil composite (Figure 9c) below 3 °C. This trend indicates that Commented [M10]: Please check if there is an explanation below the Tg of the polar resin, hydrogen bonding between the resin and reinforcement is limited, of Figure 9d in Figure 9 caption therefore, the ligno-cellulosic filler particles represent weak points within the sample, contributing to the lower mechanical performance observed at lower temperatures in comparison to the pure resin. Commented [M11]: Please check if there is an explanation For all samples investigated, the tanδ curves (Figure 9) exhibit a single major peak, representing of Figure 9c in Figure 9 caption the glass transition of the system evaluated. The presence of a single transition is indicative of a

Coatings 2019, 9, 447 12 of 14 expected, the crosslink density of the resin is a greater contributor towards final mechanical properties than fiber-matrix interactions. It is interesting to note that at lower temperatures, the fatty acid resin (Figure9b) exhibits higher storage modulus than the other systems, surpassing the tung oil resin (Figure9a) below 24 ◦C as discussed previously, the fatty acid composite (Figure9d) below 23 ◦C, and the tung oil composite (Figure9c) below 3 ◦C. This trend indicates that below the Tg of the polar resin, hydrogen bonding between the resin and reinforcement is limited, therefore, the ligno-cellulosic filler particles represent weak points within the sample, contributing to the lower mechanical performance observed at lower temperatures in comparison to the pure resin. For all samples investigated, the tanδ curves (Figure9) exhibit a single major peak, representing the glass transition of the system evaluated. The presence of a single transition is indicative of a homogeneous polymer with no evident phase separation. It is worth noting that the use of vegetable oils with less carbon–carbon double bonds typically results in polymers with more than one Tg [8,9], and this behavior has been attributed to a micro-phase separation within the polymer network due to a significant difference in reactivity of the co-monomers used. The presence of a single tanδ peak for the composite samples (Figure9c–e) also suggests that cellulose fibers have been evenly distributed throughout the polymer, providing universal thermo-mechanical properties for the entire sample. Another indicator of resin heterogeneity is the breadth of the tanδ peak. Indeed, the narrower the peak, the more homogeneous the system. It is interesting to notice how the addition of reinforcement changes the breadth of the tanδ peak for the samples prepared with fatty acids (Figure9b,d). The same effect is not observed for the other resins, therefore revealing greater interaction between resin and reinforcement on the composite prepared with fatty acids. Indeed, as observed previously in similar systems, a greater interaction of one of the resin components with the reinforcement contributes to composite heterogeneity at the molecular level [8]. The greater interaction between fatty acids and cellulose is also reflected in the Tg values of the samples (Table2). Indeed, there is only a small change in the Tg of samples prepared with tung oil upon addition of cellulose, revealing that the presence of cellulose has little effect on the polymerization of the resin and its intrinsic properties. For the fatty acid resin, a significant increase of the Tg from 39 to 56 ◦C (Table2) is observed upon the addition of cellulose. This increase in Tg suggests a stronger affinity between resin and reinforcement, resulting in more restricted chain mobility. Among all samples, the composite prepared with methyl esters exhibits the lowest Tg ( 6 C, Table2), indicating − ◦ the combination of a weak resin with a poor adhesion between the resin and reinforcement. Finally, the maximum value in the tanδ curve (tanδmax) can be associated to the damping properties of the material. It is noteworthy that all composites exhibit tanδmax values lower than their corresponding unreinforced resins (Figure9), with a more pronounced decrease (from approximately 0.44 to approximately 0.23, Figure9b,d) for the fatty acid system. This greater decrease is most likely the result of limited ability of the composite in dispersing shock/sound waves into molecular/chain movements, due to the greater interaction between resin and reinforcement, effectively interlocking the polymer chains through hydrogen bonding. When comparing the different composites, it is clear that the sample prepared with methyl esters exhibits the highest tanδmax, which can be associated to a better ability to dampen vibration through dissipation of the energy into polymer chain movement.

4. Conclusions In conclusion, this manuscript discusses the successful isolation and purification of α-eleostearic acid from tung oil via saponification, and its subsequent co-polymerization with DVB and BMA, with the purpose of preparing bio-based composites with enhanced resin-reinforcement interactions. After optimization of the novel fatty acid-based system and establishment of an appropriate cure schedule, a comparison of the morphology, hydrophilicity, and thermo-mechanical properties of cellulose-reinforced composites prepared with α-eleostearic acid, tung oil triglycerides, and tung oil methyl esters revealed trends that can be associated to resin-reinforcement interactions and the resin’s crosslink density. SEM imaging indicated stronger resin-reinforcement interactions for composites Coatings 2019, 9, 447 13 of 14 prepared with α-eleostearic acid, confirming the trends obtained with the contact angle experiments, which suggested that the fatty acid resin exhibits the most favorable interactions with water due to its higher polarity in comparison to tung oil and tung oil methyl esters. From the thermo-mechanical analyses performed (TGA and DMA), it was observed that the tung oil resin had superior properties due to its higher crosslink density, whereas samples prepared with tung oil methyl esters displayed the worst thermo-mechanical properties, as expected based on its lower crosslink density, lower polarity and, therefore, poor compatibility with the reinforcement. The results presented herein indicate the viability of enhancing resin-reinforcement interactions at the cost of crosslink density. Such an approach can find use in applications that require high water absorption, swelling, and degradability, such as naturally decomposable flower pots, for example.

Supplementary Materials: The following are available online at http://www.mdpi.com/2079-6412/9/7/447/s1, Figure S1: Reaction mechanism for the free radical polymerization of α-eleostearic acid with di-tert butyl peroxide (DTBP) as the free radical initiator, Figure S2: General schematic for the preparation of thermosetting composites. Author Contributions: Conceptualization, Writing—Review and Editing, Supervision, and Funding Acquisition, R.L.Q.; Methodology and Writing—Original Draft Preparation, A.M. Funding: This research was funded by Herty Advanced Materials Development Center through the 2016 Herty Fellowship. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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