Biobased Heat Resistant Epoxy Resin with Extremely High Biomass

Research Article pubs.acs.org/journal/ascecg

Biobased Heat Resistant Epoxy Resin with Extremely High Biomass Content from 2,5-Furandicarboxylic Acid and Eugenol Jia-Tao Miao, Li Yuan, Qingbao Guan, Guozheng Liang,* and Aijuan Gu* State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Materials Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 199 Ren’Ai Road, Suzhou 215123, China S Supporting Information *

ABSTRACT: Preparing a biobased (biomass-based) high performance epoxy resin with extremely large biomass content is of great importance for sustainable development. Herein, a new epoxy resin with a precise structure, bis(2-methoxy-4-(oxiran-2ylmethyl)phenyl)furan-2,5-dicarboxylate (EUFU-EP), was synthesized from two biobased green and low toxic compounds (2,5furandicarboxylic acid and eugenol) and the biomass content of EUFU-EP is as large as 93.3%. In addition, a new biobased epoxy resin, EUFU-EP/MHHPA, was prepared by using methyl hexahydrophthalic anhydride (MHHPA) as the curing agent and 2ethyl-4-methylimidazole as the curing accelerator. The curing reactivity and integrated performances including thermal and mechanical properties as well as flame retardancy of the cured resin were systematically researched and compared with those of petrochemical resource-based epoxy resin (DGEBA/MHHPA) consisting of commercial diglycidyl ether of bisphenol A (DGEBA), MHHPA and 2-ethyl-4-methylimidazole. Results show that EUFU-EP/MHHPA and DGEBA/MHHPA have similar curing reactivity, but cured EUFU-EP/MHHPA resin shows better thermal properties, rigidity, and flame retardancy than cured DGEBA/MHHPA resin. Specifically, the glass transition temperature (Tg) of EUFU-EP/MHHPA resin is as high as 153.4 °C, the storage modulus at 50 °C increases by 19.8%; meanwhile, both peak heat release rate and total heat release reduce by 19.0%. The nature behind these outstanding integrated performances is attributed to the unique structure of EUFU-EP, which is not only rich in aromatic structure but also has a furan ring. The especially large biomass content and outstanding thermal, mechanical, and flame retarding performances clearly show that EUFU-EP resin has a great potential in actual applications. KEYWORDS: Biomass, Epoxy resin, Thermal property, Flame retardancy, Structure

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INTRODUCTION It is arguable that the petrochemical resources are indispensable; however, energy depletion is one of the greatest challenges of the world today.1,2 Unlike petrochemical resources, biomass that can translate into petrochemical resources through hundreds of millions of years is characterized to be abundant, renewable, and high annual output.3−7 To meet the challenge of energy crisis and the requirement of sustainable development, it is urgent to pay increasing attention to biomass resources.8−11 Thermosetting resins, strongly dependent on petroleum resources, have been key and basic materials in industry since the birth of the first synthetic resin in 1872.12 They play an indispensable role from basic (chemical industry,13 coating14) to strategic (aerospace,15 new energy,16 information17) fields. © 2017 American Chemical Society

Among them, epoxy resin occupies about 70% of the whole thermosetting resin market due to its outstanding integrated performances.18 Note that more than 90% of epoxy resin is bisphenol A epoxy resin (DGEBA),18,19 which is synthesized from bisphenol A and epichlorohydrin in the presence of sodium hydroxide.17 Unfortunately, bisphenol A is strongly dependent on petrochemical resources. In recent years, attempts have been made to introduce some biomass-based (biobased) units into epoxy resins instead of petroleum-based units. Various biomasses have been reported (such as vegetable oil,20 itaconic acid,21,22 cardanol,1 rosin,23 Received: April 19, 2017 Revised: June 8, 2017 Published: June 22, 2017 7003

DOI: 10.1021/acssuschemeng.7b01222 ACS Sustainable Chem. Eng. 2017, 5, 7003−7011

Research Article

ACS Sustainable Chemistry & Engineering eugenol,6,24,25 2,5-furandicarboxylic acid (FDCA),26 lignin,7,27 etc.) to act as raw materials in synthesizing biobased epoxy resins, of which the biomass content of epoxy ranges from 40.1% to 84% as summarized in Table S1 in Supporting Information. FDCA and eugenol, both of which are common renewable resources in nature, have aroused worldwide concern in recent years due to their rigid aromatic structure. FDCA is classified as a top ten green chemical by U.S. Department of Energy.28 Eugenol, occupying about 80% of clove oil, is a renewable biomass material with low toxicity and relatively low cost.29 FDCA and eugenol have been used for synthesizing biobased epoxy resins, respectively. Zhang et al. reported a eugenol-based epoxy resin with a biomass content of 62.7%, and the glass transition temperature (Tg) of epoxy resin cured with hexahydrophthalic anhydride was only 114 °C.25 Wang et al. reported two kinds of eugenol-based epoxy resins that link eugenol molecules together with biomass-free α,α-dichloro-pxylene24 and cyanuric chloride,6 respectively. The former (melting point, 124 °C) cured with 4,4′-diaminodiphenylmethane has a biomass content of 70.2% and a Tg value of 114.4 °C, whereas the latter cured with 3,3′-diaminodiphenylsulfone has a higher biomass content of 80% and higher Tg (207 °C). Liu’s group prepared an epoxy resin based on FDCA, the Tg of the epoxy resin cured with methyl hexahydrophthalic anhydride (MHHPA) was 152 °C, the flexural strength was 96 MPa and the biomass content was 65.2%.26 These works have significantly promoted the use of eugenol and FDCA in the field of biobased epoxy resins. However, all synthesis routes of them use dichloromethane as solvent; moreover, the biomass contents and integrated performances of the above biobased epoxy monomers still need to increase. Herein, a unique biobased epoxy resin monomer, bis(2methoxy-4-(oxiran-2-ylmethyl)phenyl)furan-2,5-dicarboxylate (EUFU-EP), with extremely large biomass content (93.3%), high thermal and mechanical properties as well as good flame retardancy, was designed and synthesized through building a precise structure from FDCA and eugenol. The curing behavior and relationships between structure and properties of EUFUEP/MHHPA resins were intensively studied and compared with its petrochemical resource-based DGEBA counterpart.

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stirring to get a solution B, into which a solution of FDCDCl (50 mmol, 9.65 g) in ethyl acetate (100 mL) was added dropwise at 0 °C within 20 min, followed by maintaining at 25 °C for 30 min. Then, the reaction solution was filtered. After that, solvent was removed through rotary evaporation, followed by washing with water and filtering to get a white solid (20.5 g, yield: 91.4%), which was EUFU. 1H NMR (400 MHz, CDCl3): δ 7.45 (s, 2H), 7.06 (d, J = 8.0 Hz, 2 H), 6.85−6.77 (m, 4H), 5.98 (m, 2H), 5.16−5.08 (m, 4H), 3.82 (s, 6H), 3.40 (d, J = 6.6 Hz, 4H). 13C NMR (151 MHz, CDCl3): δ 156.07, 151.04, 146.82, 139.79, 137.37, 137.13, 122.59, 120.93, 119.98, 116.46, 113.06, 56.05, 40.30. HRMS (ESI+) m/z calcd for [C26H24O7Na]+: 471.1414. Found: 471.1402. Anal. Calcd for C26H24O7: C 69.63, H 5.39. Found: C 69.67, H 5.607. Synthesis of EUFU-EP. mCPBA (12.2 g, 60 mmol) was slowly added into a solution consisting of EUFU (8.97 g, 20 mmol) and ethyl acetate (100 mL) at 0 °C; the reaction mixture was slowly heated and reacted at 40 °C for 48 h. After that, the resultant solution was filtered to get the filtrate, which was then washed with 10% Na2SO3 solution, 5% NaHCO3 solution, and deionized water successively. The organic layer was dried with anhydrous sodium sulfate, and ethyl acetate was rotary evaporated to give a yellow solid. The solid was then washed with ethanol to give a white solid, coded as EUFU-EP (6.10 g, yield: 63.5%). Melting point: 99.7 °C (differential scanning calorimetry, DSC). 1H NMR (400 MHz, CDCl3): δ 7.47 (s, 2H), 7.09 (d, J = 8.0 Hz, 2H), 6.91 (s, 2H), 6.87 (d, J = 8.0 Hz, 2H), 3.84 (s, 6H), 3.20− 3.14 (m, 2H), 2.88 (d, J = 5.3 Hz, 4H), 2.83 (t, J = 4.2 Hz, 2H), 2.60− 2.54 (m, 2H). 13C NMR (151 MHz, CDCl3): δ 155.97, 151.12, 146.76, 137.84, 137.05, 122.74, 121.33, 120.04, 113.51, 56.10, 52.46, 47.00, 38.86. HRMS (ESI+) m/z calcd for [C26H24O9Na]+: 503.1313. Found: 503.1306. Anal. Calcd for C26H24O9: C 65.00, H 5.04. Found: C 64.71, H 5.176. Preparation of Cured Resins. EUFU-EP and MHHPA with an equivalent molar ratio as well as 2-ethyl-4-methylimidazole (0.5 wt % of the total weight of EUFU-EP and MHHPA) were mixed thoroughly at 80 °C for 30 min to get a clear liquid (prepolymer). The prepolymer was then transferred to a preheated mold and degassed under vacuum at 80 °C for 30 min, followed by putting into an oven for curing using the procedure of 130 °C/2 h + 150 °C/2 h + 170 °C/ 2 h. After that, the mold was naturally cooled to room temperature to obtain cured resin, coded as EUFU-EP/MHHPA. Similarly, EUFU-EP was replaced by DGEBA to prepare cured DGEBA/MHHPA resin. Characterizations. 1H NMR spectra of FDCDCl, EUFU, and EUFU-EP, 2-D HHCOSY spectra of EUFU and EUFU-EP were recorded with a Bruker AVANCE III 400 MHz superconducting magnetic resonance spectrometer (USA). 13C NMR spectra of EUFU and EUFU-EP were recorded with an Agilent DD2-600 MHz spectrometer (USA). CDCl3 and tetramethylsilane were applied as the solvent and internal standard, respectively. Fourier transform infrared (FT-IR) spectra were obtained using a Bruker a Vertex 70 spectrometer (USA) over the wavenumber ranging from 600 to 4000 cm−1. High-resolution mass spectra (HRMS) were tested on a Bruker UltiMate 3000 high performance liquid chromatography (HPLC) system (Germany). The elemental analysis (C and H) was performed on a Vario EL III elemental analyzer (Germany). DSC curves were recorded on a TA Instruments Q200 (USA) under a nitrogen atmosphere with a flowing rate of 50 mL min−1. Each sample was weighed and sealed in an aluminum crucible, and heated from room temperature to 250 °C at a heating rate of 5, 10, 15, or 20 °C min−1. Density was measured on a Shanghai Sunny Hengping FA1104J electronic balance with density device (China). Dynamic mechanical analyses (DMA) were performed using TA DMA Q800 apparatus (USA) by multifrequency-strain model in a single cantilever clamp at a heating rate of 3 °C min−1. The tests were performed at a frequency of 1.0 Hz, a deflection amplitude of oscillation of 20 μm, and a poisson’s ratio of 0.44. The dimensions of specimen were (35 ± 0.02) mm × (13 ± 0.02) mm × (3 ± 0.02) mm.

EXPERIMENTAL SECTION

Raw Materials. Eugenol, 3-chloroperoxybenzoic acid (mCPBA, 85%), and MHHPA were purchased from Energy Chemical, China. FDCA was bought from Sichuan Dagaote Technology Co., Ltd., China. Oxalyl chloride, 2-ethyl-4-methylimidazolet, triethylamine, sodium sulfite, sodium carbonate, tetrahydrofuran (THF), ethyl acetate, N,N-dimethylformamide (DMF), and other reagents were obtained from Sinopharm Chemical Reagent Co., Ltd., China. Diglycidyl ether of bisphenol A (DGEBA) used herein has an epoxy value of 0.44 mol/100 g, which was obtained from Nantong Xingchen Synthetic Material Co., Ltd., China. Synthesis of Furan-2,5-dicarbonyl dichloride (FDCDCl). To a 100 mL round-bottomed flask was added a mixture of FDCA (15.6 g, 100 mmol), THF (100 mL), and DMF (catalyst, 0.05 mL); to above suspension was added dropwise oxalyl chloride (19.04 g, 150 mmol) at 0 °C. The reaction mixture was slowly heated and reacted at 25 °C for 3 h until the solid was completely dissolved. Then, the resultant solution was rotary evaporated to obtain a white solid (19.14 g, yield: 99.2%), which is FDCDCl. 1H NMR (400 MHz, DMSO-D6): δ 7.31 (s, 2H) (Figure S1 in Supporting Information). Synthesis of Bis(4-allyl-2-methoxyphenyl)furan-2,5-dicarboxylate (EUFU). Eugenol (16.4 g, 100 mmol) and triethylamine (16.9 mL, 120 mmol) were dissolved in ethyl acetate (200 mL) with 7004


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Figure 1. Synthesis of EUFU-EP.

Figure 2. 1H NMR spectra of EUFU (top) and EUFU-EP (bottom). Tg is regarded as the peak temperature of the tan δ-temperature curve.30 Flexural strengths were measured according to Chinese standard GB2570-95 using an electronic universal testing machine (SUST, Zhuhai, China) at a speed of 2 mm min−1. Thermogravimetric (TG) analyses were performed on a TA Discovery TGA Instrument (USA) under a nitrogen atmosphere (10 mL min−1) with a heating rate of 10 °C min−1. Microcombustion calorimetery (MCC) was performed on a Govmark MCC-2 microscale combustibility calorimeter (USA). A 5 mg sample was heated to 700 °C at a heating rate of 60 °C min−1 in a mixed stream of oxygen and nitrogen flowing at 21 and 79 cm3 min−1, respectively. Thermogravimetric analysis infrared (TG-IR) spectra were recorded using a Netzsch TGA F1 thermogravimetric analyzer (Germany) that was interfaced to a Bruker TENSOR 27 Fourier transform infrared spectroscopy (FT-IR) spectrophotometer (Germany). Ten milligrams of a sample was put in an alumina crucible and heated from 30 to 800 °C with a heating rate of 10 °C min−1 under a nitrogen atmosphere.

method (Method I) is derived from bisphenol A and epichlorohydrin under sodium hydroxide (Figure S2a in Supporting Information).33 Note that this method produces complex oligomers but not epoxy resin monomer with precise structure. Typically, DGEBA monomers are marketed with average repeat unit (n) in the range of 0.03−10.18 In addition, the obtained oligomers are hardly purified and separated. Obviously, this is not conducive to control structure and performances of epoxy resin. The second method (Method II) for synthesizing DGEBA contains two steps. An intermediate (allyl ether compound) from the reaction between bisphenol A and 3-bromopropene with the aid of sodium hydroxide was synthesized, and then the double bond of allyl group is oxidized by an oxidizing agent to obtain an epoxy resin monomer (Figure S2b in Supporting Information).34 Different from Method I, Method II can synthesize an epoxy monomer with precise structure. However, when Method II is used to synthesize biobased epoxy monomer, the reactant (3bromopropene) obtained from petrochemical resources will reduce the biomass content of epoxy monomer to some extent. On the other hand, there are two major problems of bisphenol A: besides its dependence on the petrochemical

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RESULTS AND DISCUSSION Design, Synthesis and Characterization of EUFU-EP. It is well-known that there are two methods to synthesize epoxy resin.22,31,32 Taking DGEBA as the example, the first synthesis 7005


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Figure 3. 13C NMR spectra of EUFU (top) and EUFU-EP (bottom).

double bond of allyl in EUFU, the characteristic peaks at 3.40 ppm (Hf), 3.82 ppm (He), and 7.45 ppm (Ha) represent protons on methylene group of allyl adjacent to the double bond, methoxy group and furan ring, respectively. In the spectrum of EUPU-EP, the characteristic peaks of proton on epoxy group are observed, such as 2.54−2.60 ppm (Hi′), 2.83 ppm (Hh′) and 3.14−3.20 ppm (Hh′), whereas all peaks of protons (Hg, Hh, and Hi) on double bond are not found. Other peaks in the spectrum of EUFU-EP are similar to those of protons of EUFU, indicating that double bonds of allyl groups in EUFU have been successfully oxidized to epoxy groups. These assignments of protons have been also confirmed by the 2-D HHCOSY NMR spectra (Figure S3 in Supporting Information). Above statement is further confirmed by 13C NMR spectra of EUFU and EUFU-EP shown in Figure 3. The characteristic peaks at 139.79 and 116.46 ppm in the spectrum of EUFU indicate C12 and C13 on allyl groups, respectively. These peaks do not appear in the spectrum of EUFU-EP, instead, characteristic peaks representing carbon atoms on epoxy groups appear at 52.46 ppm (C12′) and 47.00 ppm (C13′), also proving that double bonds of allyl groups in EUFU have been successfully oxidized. FT-IR spectra of EUFU and EUFU-EP are shown in Figure S4 (in Supporting Information). The characteristic peak standing for CC bonds of EUFU at 1639 cm−1 disappears. Instead, the characteristic peak standing for the oxirane rings of EUFU-EP at 930 cm−1 appears, further proving that double bonds of allyl groups in EUFU have been successfully oxidized. In addition, the HRMS spectrum of EUFU-EP (Figure S5 in Supporting Information) shows that the experimental [M + Na+] is 503.1306, which is consistent with the theoretical value

resources, bisphenol A has a similar structure with estrogen, which will reduce fertility and increase the risk of developing cancers and other diseases.35,36 The U.S. Federal Drug Administration has banned the use of bisphenol A-based materials for packing infant formula,37 and the demand for bisphenol A-free products is booming. Therefore, derived from less toxic raw materials, a new biobased epoxy monomer with precise structure and extremely large biomass content should be designed and synthesized. Figure 1 illustrates the route to synthesize such a biobased epoxy monomer, EUFU-EP, with precise structure and extremely large biomass content. Specifically, eugenol with allyl group itself is taken as the reactant, so it is not necessary to use bromopropene anymore, endowing EUFU-EP with very large biomass content (93.3%). The specific route is composed of three steps. First, FDCDCl was obtained from acylation of FDCA with oxalyl chloride in the presence of catalytic DMF; FDCA was fully converted to FDCDCl and directly used for the next step without purification. Second, triethylamine was used as base, FDCDCl and eugenol were esterified to produce intermediate (EUFU) with a high yield (91.4%). Third, the target product EUFU-EP was obtained by oxidizing allyl group in EUFU by mCPBA; the yield of this step is 63.5%, and it was not necessary to purify EUFU-EP by column chromatography, showing great potential for large-scale production. No bisphenol structure existed in the target product (EUFU-EP); moreover, no bisphenol structure was used and formed during the whole synthesis process, so EUFU-EP is a healthier substitute compared with DGEBA. Figure 2 shows 1H NMR spectra of EUFU and EUFU-EP. In the spectrum of EUFU, the characteristic peaks at 5.98 ppm (Hg) and 5.08−5.16 ppm (Hh, Hi) correspond to protons on 7006


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ACS Sustainable Chemistry & Engineering ⎛ β ⎞ ⎛ AR ⎞ E ⎟ − ln⎜⎜ 2 ⎟⎟ = ln⎜ ⎝ ⎠ E RT T p ⎝ p ⎠

(503.1313). The DSC curve of EUFU-EP (Figure S6 in Supporting Information) shows a clear endothermic melting peak at 99.7 °C. Above characterizations fully prove that a unique biobased epoxy resin with precise structure has been facilely synthesized, its theoretical molecular weight is 480.47, and the epoxy value is 0.416 mol/100 g. Curing Behavior and Aggregation Structure of EUFUEP/MHHPA. Figure 4a shows DSC curves of EUFU-EP/

(1)

where Tp is the peak temperature, β is the heating rate, E is the activation energy of the curing reaction, A is the preexponential factor, and R is the universal constant (8.314 J mol−1 K−1). EUFU-EP was then cured by MHHPA using the curing procedure of 130 °C/2 h + 150 °C/2 h + 170 °C/2 h. To make sure that the samples were fully cured under this condition, DSC and FT-IR techniques were used for cured EUFU-EP/ MHHPA resin. As shown in Figures S7 and S8 in Supporting Information, there is no exothermic peak in DSC curve, and no characteristic peak of epoxy groups in FT-IR spectra, verifying that EUFU-EP/MHHPA resin has been completely crosslinked. Cross-linking density is an important index for characterizing the aggregation structure of thermosetting resins,38 which is often calculated according to the classical rubbery elasticity theory as shown in eq 2.26 E′ = 3υeRT

(2)

where E′ is the storage modulus of the thermosetting resin in the rubbery plateau region, R is the universal constant, and T is the temperature at which the storage modulus is minimum.30 Cross-linking densities of EUFU-EP/MHHPA and DGEBA/ MHHPA resins are found to be 1373 and 2081 mol m−3, respectively. These results are attributed to the fact that EUFUEP has higher molecular weight, smaller epoxy value, and lower reactivity than DGEBA. Thermomechanical Properties. DMA thermograms of cured EUFU-EP/MHHPA and DGEBA/MHHPA resins are shown in Figure 5. It can be seen that EUFU-EP/MHHPA has

Figure 4. DSC thermograms (a) and fitting curves of exothermic peak temperature vs heating rate (b) of EUFU-EP/MHHPA and DGEBA/ MHHPA prepolymers.

MHHPA and DGEBA/MHHPA prepolymers at different heating rates. Each curve displays a single exothermic peak corresponding to the ring-opening reaction between epoxy and acid anhydride. From fitting curves of heating rate vs exothermic peak temperature of prepolymers (Figure 4b), the actual peak temperature (142.1 °C) of EUFU-EP/MHHPA is found to be slightly higher than that of DGEBA/MHHPA (139.6 °C). At the same time, according to the Kissinger equation (eq 1), dynamic curing parameters of prepolymers were calculated. It is found that EUFU-EP/MHHPA has a higher activation energy (71.15 kJ mol−1) than DGEBA/ MHHPA (61.21 kJ mol−1), so the former has lower reaction activity than the latter. This is because epoxy groups of DGEBA connect with oxygen atoms, increasing the polarization degree of epoxy groups due to the inductive effect, and thus is more conducive to the ring opening reaction of epoxy groups.24

Figure 5. DMA thermograms for storage modulus and tan δ against temperature of EUFU-EP/MHHPA and DGEBA/MHHPA resins.

higher storage modulus (E′) at 50 °C (2229 MPa) than DGEBA/MHHPA, which is expected because E′ in the glassy state is affected synthetically by chemical structure and chain packing. It is well-known that the use of furan building block usually increases the stiffness of the resins,39,40 whereas higher density27 and hydrogen bonding27 result in a greater chain packing, both of which are good for increasing E′. These two factors exist in EUFU-EP/MHHPA resin. Specifically, on one hand, EUFU-EP/MHHPA resin has rigid rod-like aromatic ester and furan structures. On the other hand, EUFU-EP/ 7007


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ACS Sustainable Chemistry & Engineering MHHPA has higher density (1.25 g/cm3) than DGEBA/ MHHPA resin (1.18 g/cm3) reflecting a greater chain packing; moreover, the hydrogen and oxygen atoms of methoxy substituents on the aromatic rings have the ability to form hydrogen bond with carbonyl of anhydride and methylene resulting from the epoxy-anhydride reaction, respectively.41 Generally, Tg is regarded as the peak temperature of the tan δ-temperature curve.30,42 EUFU-EP/MHHPA has higher Tg (153.4 °C) than DGEBA/MHHPA (144.1 °C). Tg depends on combined effects of chemical structure and aggregation structure. Specifically, an increased cross-linking density and a structure with chemical bonds that are hard to rotate can lead to a higher Tg. Compared with DGEBA/MHHPA, EUFU-EP/ MHHPA has lower cross-linking density, tending to get reduced Tg; whereas EUFU-EP/MHHPA has rigid rod-like aromatic ester and furan structures that are very difficult to rotate. The combined effect of above opposite and positive factors determines the final Tg. Figure 5 suggests that the positive factor plays the domain influence. Note that for previously reported biobased epoxy resins cured with MHHPA, their Tg values are within 101.9−152 °C (Table S1 in Supporting Information), meaning that EUFUEP/MHHPA reported herein shows high Tg that falls in the high end range of resins reported. This is very attractive because T g is the upper temperature of service for thermosetting resins;43 that is, EUFU-EP has the biggest ability of applying in strict environment requiring high temperature among all biobased epoxy resins so far. Mechanical Performance. Figure 6 gives flexural properties of EUFU-EP/MHHPA and DGEBA/MHHPA resins.

Flammability and Mechanism of Cured Resins. Figure 7 shows curves of heat release rate (HRR) versus temperature;

Figure 7. Heat release rate-time curves from MCC tests for cured EUFU-EP/MHHPA and DGEBA/MHHPA resins.

the characteristic parameters including longer time-to-ignition (TTI), peak heat release rate (PHRR), and total heat release (THR) temperature are listed in this figure. Different from about 70 s shortened TTI in eugenol-based epoxy resins,6,24 EUFU-EP/MHHPA resin has a similar TTI as that of DGEBA/ MHHPA; in addition, EUFU-EP/MHHPA has significantly reduced PHRR and THR compared with DGEBA/MHHPA, and the reduction is about 19.0%, indicating that EUFU-EP/ MHHPA has better flame retardancy than DGEBA/MHHPA. This attractive result is attributed to the fact that the char yield (Yc) at 800 °C of EUFU-EP/MHHPA resin is 10.9 wt %, about 1.7 times of that of DGEBA/MHHPA (Figure 8). This is

Figure 6. Flexural moduli and strengths of EUFU-EP/MHHPA and DGEBA/MHHPA resins.

Analogical with storage modulus in the glassy state, flexural modulus reflects the rigidity of resins. The flexural modulus of EUFU-EP/MHHPA is 3.33 GPa, higher than that of DGEBA/ MHHPA (3.05 GPa). This is consistent with the storage moduli in the glassy state. The flexural strength is usually used to characterize overall mechanical properties of a material because the flexural loading contains multiple types of loadings such as bending, stretching, compression, etc.44 Although EUFU-EP/MHHPA has slightly lower flexural strength than DGEBA/MHHPA, the flexural strength of EUFU-EP/MHHPA is as high as 129.2 MPa, comparable with that of reported epoxy resin with high mechanical properties,45 and is at a high level in biobased epoxy resins reported (Table S1 in Supporting Information), so EUFU-EP/MHHPA still has excellent mechanical properties.

Figure 8. TGA curves of cured EUFU-EP/MHHPA and DGEBA/ MHHPA resins.

benefited from the highly compact aromatic rings of EUFU-EP, which can promote carbonation,6 resulting in more residual carbon, and thus displaying condensed phase mechanism of flame retardation. To investigate further the flame retarding mechanism, TG-IR technology was used to detect components directly in the gas phase during the degradation process. Figure 9 shows threedimensional IR spectra of pyrolysis products for EUFU-EP/ MHHPA and DGEBA/MHHPA resins throughout the whole thermal degradation process. Because the same size, morphology, and weight were taken in tests, the intensity of the 7008


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Figure 9. Three-dimensional FT-IR spectra of pyrolysis products of cured EUFU-EP/MHHPA and DGEBA/MHHPA resins.

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CONCLUSION Starting from green biobased FDCA and eugenol, a new epoxy resin (EUFU-EP), of which the biomass content is as high as 93.3%, was successively synthesized using an eco-friendly route. Compared with DGEBA/MHHPA, EUFU-EP/MHHPA expresses 9 °C and 19.9% enhancement in Tg and storage modulus at 50 °C, respectively, as well as considerably high mechanical properties and better flame retardancy. Abundant amounts of aromatic and furan structures in the main chain endow cured EUFU-EP resin with much better integrated performances than petrochemical resource-based DGEBA resin. Because of these attractive properties as well as the advantages of nontoxic and renewably source, EUFU-EP holds a great potential as a sustainable alternative for DGEBA.

absorption peak in the three-dimensional spectra can reflect the amounts of degradation products. Figure 9 shows that the absorption peaks of EUFU-EP/MHHPA resin are much lower than those of DGEBA/MHHPA resin, proving that EUFU-EP/ MHHPA resin releases much less pyrolysis products than DGEBA/MHHPA resin. Figure 10 shows FT-IR spectra of pyrolysis products of EUFU-EP/MHHPA and DGEBA/MHHPA at the maximum

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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/acssuschemeng.7b01222. Figure S1 gives 1H NMR spectra of FDCA and FDCDCl, Figure S2 shows synthesis routes of commercial DGEBA, Figure S3 gives 2-D HHCOSY NMR spectra of EUFU and EUFU-EP, Figure S4 gives FT-IR spectra of EUFU and EUFU-EP, Figure S5 gives the high-resolution mass spectrum (HRMS) of EUFU-EP, Figure S6 gives DSC curve of EUFU-EP, Figure S7 gives DSC curve of cured EUFU-EP/MHHPA resin, Figure S8 gives FT-IR spectra of EUFU and EUFU-EP/MHHPA resin, Table S1 summarizes biomass contents and integrated performances of biobased epoxy resins in literature (PDF)

Figure 10. FT-IR spectra of pyrolysis products of cured EUFU-EP/ MHHPA and DGEBA/MHHPA resins at their maximum decomposition rates.

decomposition rates. It can be seen that most pyrolysis products of DGEBA/MHHPA are organic combustible gases, including various hydrocarbons (2968 cm−1), carbonyl compounds (1807 cm−1), aromatic compounds (1604 and 1510 cm−1), and C−O−C (1221 and 1176 cm−1);46,47 whereas a small amount of water (3656 cm−1), CO2 (2351, 2308, and 669 cm−1) and CO (2180 cm−1) were also found in pyrolysis products. Differently, pyrolysis products of EUFU-EP/ MHHPA show fewer organic combustible gases (various hydrocarbons, carbonyl compounds, and aromatic compounds) and more none-flammable CO2; this is beneficial to retard further combustion, and thus EUFU-EP/MHHPA resin has a better flame retardancy.

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +86 512 65880967. Fax: +86 512 65880089. E-mail address: [email protected] (Guozheng Liang). *E-mail address: [email protected] (Aijuan Gu). ORCID

Jia-Tao Miao: 0000-0001-6454-067X Li Yuan: 0000-0003-2059-5235 Qingbao Guan: 0000-0002-3384-3229 Guozheng Liang: 0000-0001-9690-7931 Aijuan Gu: 0000-0002-2235-1018 7009


Research Article

ACS Sustainable Chemistry & Engineering Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank National Natural Science Foundation of China (21274104), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), China, and Scientific Innovation Research of College Graduate in Jiangsu Province of China (KYLX16_0120), for financially supporting this project.

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Biobased Heat Resistant Epoxy Resin with Extremely High Biomass

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