CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201300749
Synthesis and Properties of a Bio-Based Epoxy Resin with High Epoxy Value and Low Viscosity Songqi Ma, Xiaoqing Liu,* Libo Fan, Yanhua Jiang, Lijun Cao, Zhaobin Tang, and Jin Zhu*[a] A bio-based epoxy resin (denoted TEIA) with high epoxy value (1.16) and low viscosity (0.92 Pa s, 25 8C) was synthesized from itaconic acid and its chemical structure was confirmed by 1 H NMR and 13C NMR spectroscopy. Its curing reaction with poly(propylene glycol) bis(2-aminopropyl ether) (D230) and methyl hexahydrophthalic anhydride (MHHPA) was investigated. For comparison, the commonly used diglycidyl ether of bisphenol A (DGEBA) was also cured with the same curing agents. The results demonstrated that TEIA showed higher
curing reactivity towards D230/MHHPA and lower viscosity compared with DGEBA, resulting in the better processability. Owing to its high epoxy value and unique structure, comparable or better glass transition temperature as well as mechanical properties could be obtained for the TEIA-based network relative to the DGEBA-based network. The results indicated that itaconic acid is a promising renewable feedstock for the synthesis of bio-based epoxy resin with high performance.
Introduction Owing to their superior mechanical properties, excellent thermal resistance and good processability, epoxy resins are extensively used as coatings, adhesives, and structural composites as well as electronic materials.[1–3] Diglycidyl ether of bisphenol A (DGEBA) is by far the most widely used monomer in the formulation of epoxy networks.[4] Bisphenol A, accounting for more than 67 % of the molar mass of DGEBA, is fully dependent on fossil resources. In addition, bisphenol A appears to be both an estrogen receptor agonist and an androgen receptor antagonist.[5] Resins containing bisphenol A have been deemed to be toxic to living organisms[6] and their application has been strictly limited in many countries. Under the concept of green chemistry[7, 8] and as a result of the increasing concern over the depletion of fossil reserves and growing greenhouse gas emissions, there is an increasing interest in finding a biobased feedstock to synthesize epoxy resins without bisphenol A. Until now, a number of bio-based epoxy resins derived from plant oils,[9–13] lignin,[14–18] and rosin[19–21] have been reported. But, as a result of their low epoxy value, low curing reactivity, as well as unsuitable chemical structures, the cured resins rarely exhibit satisfactory properties. The epoxy value is one of the determining factors in regulating the properties of epoxy resins. A higher epoxy value usually leads to better performance. Recently, several epoxies with high epoxy value were
[a] Dr. S. Ma, Prof. Dr. X. Liu, L. Fan, Y. Jiang, L. Cao, Dr. Z. Tang, Prof. Dr. J. Zhu Ningbo Key Laboratory of Polymer Materials Ningbo Institute of Material Technology and Engineering Chinese Academy of Sciences Ningbo 315201 (PR China) Fax: (+ 86) 574-86685925 E-mail: [email protected] [email protected]
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synthesized from bio-based feedstock. For example, Pan et al.[22–24] prepared a series of highly functional epoxy compounds from sucrose and fatty acids. The property investigation demonstrated that the higher epoxy value could lead to a higher crosslink density, which resulted in a higher glass transition temperature (Tg), higher tensile strength, and modulus relative to the commercial epoxidized soybean oil. However, their overall properties were still not comparable with those of the conventional DGEBA cured with the same curing agent because of the flexible aliphatic chain of fatty acids.[25] Shibata et al.[26, 27] investigated the properties of bio-based glycerol polyglycidyl ether, polyglycerol polyglycidyl ether, and sorbitol polyglycidyl ether using e-poly(lysine) and tannic acid as the curing agents. Chrysanthos et al.[6] studied isosorbide-based epoxy resins. Although these epoxy compounds commonly possess high epoxy value, especially glycerol triglycidyl ether with the epoxy value of 1.15, the cured networks still exhibited low Tg values as a result of the flexible ether bonds. In order to overcome the disadvantage of the long flexible aliphatic chain, Aouf et al.[28, 29] synthesized a multifunctional bio-based epoxy resin from gallic acid that showed a much higher value of Tg (233 8C) than DGEBA (160 8C) using isophorone diamine as the curing agent. But the gallic-acid-based epoxy network was brittle as a result of the rigid benzene ring and exorbitantly high crosslink density. Clearly, selecting the bio-based platform chemicals and designing their structures are the keys to synthesizing bio-based epoxy resins with satisfactory properties. Itaconic acid (IA), one of the top 12 potential bio-based platform chemicals selected by the U.S. Department of Energy,[30] possesses two carboxyl groups and one carbon–carbon double bond, plus a short molecular chain with no flexible bond or rigid ring. It has been proved to be suitable for the synthesis of high performance bio-based epoxy resin.[25] In our previous work, itaconic-acid-based epoxy resin with double bonds (EIA) ChemSusChem 2014, 7, 555 – 562
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CHEMSUSCHEM FULL PAPERS was synthesized. Compared with DGEBA, EIA exhibited comparable or better properties after curing with methyl hexahydrophthalic anhydride. The carbon–carbon double bonds could be utilized for copolymerization with divinyl benzene and acrylated epoxidized soybean oil so as to manipulate its properties further. However, EIA was a mixture of different oligomers and the remaining carbon–carbon double bonds had the tendency to self-polymerize unless polymerization inhibitor was added, which increased the viscosity of EIA and affected its properties. In this paper, we exploited the unique chemical structure of itaconic acid to synthesize a trifunctional epoxy monomer (TEIA, as shown in Figure 1). Based on its structure, we expect
www.chemsuschem.org acid (AIA) was obtained from dehydrobromination between IA and allyl bromide, and (ii) epoxidation of AIA. The chemical structures of AIA and TEIA were characterized by means of 1H NMR and 13C NMR spectroscopy. Figure 2 shows the 1H NMR spectra of AIA and TEIA. The spectrum of
Figure 2. 1H NMR spectra of a) AIA and b) trifunctional epoxy resin from itaconic acid (TEIA). Figure 1. Chemical structures of TEIA, DGEBA, D230, and MHHPA.
it to possess a high epoxy value, low viscosity, and satisfactory properties after curing. For easy evaluation, DGEBA was also employed here as the reference and cured with the same curing agents D230 and MHHPA (Figure 1).
Results and Discussion Characterization of allylated itaconic acid (AIA) and TEIA As shown in Scheme 1, a trifunctional epoxy resin from itaconic acid (TEIA) was synthesized in two steps: (i) allylated itaconic
AIA shows the characteristic peaks of protons H1, H2, H3, H12, H13, and H14 on the double bonds CH2 = CHCH2 at 5.21– 5.34 and 5.84–5.96 ppm, the peaks of protons H8 and H9 on the double bond CH2 = C at 5.73 and 6.37 ppm, the peaks of protons H4, H5, H10, and H11 on the carbon atoms next to the double bonds CH2 = CHCH2 at 4.58–4.67 ppm, and the peaks of protons H6 and H7 on the carbon atom between the double bond and ester bond CH2=CCH2COO at 3.38 ppm. In the spectrum of TEIA, these proton peaks all disappeared but the characteristic peaks of protons H1, H2, H3, H8, H9, H12, H13, and H14 on the glycidyl epoxides CH2(O)CH and pendant epoxide CH2(O)C, protons H6 and H7 on the carbon between the pendant epoxide and ester bond CH2(O) CCH2COO at 2.63–3.24 ppm, the peaks of protons H4, H5, H10 and H11 on the carbon atoms next to the glycidyl epoxides CH2(O)CHCH2 at 3.94–4.05 and 4.45–4.54 ppm were observed. In addition, the integrated area ratios of the characteristic signals for protons were all in good agreement with the theoretical values. In order to further identify their structures, the 13C NMR spectra of AIA and TEIA were also displayed in Figure 3. The spectrum of AIA presents the characteristic peaks of carbon atoms C1, C2, C6, C7, C10, and C11 on the the double bonds CH2 = CHCH2 and CH2 = CCOO at 118– 134 ppm. The spectrum of TEIA shows the disappearance of double bonds and the appearance of carbon atoms C1, C2, C6, C7, C10, and C11 on the glycidyl epoxides CH2(O)CH and pendant epoxide CH2(O)C at 45–78 ppm. These results demonstrated that the target compounds were synthesized successfully.
Scheme 1. Synthesis of trifunctional epoxy resin from itaconic acid (TEIA).
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Figure 3. 13C NMR spectra of a) AIA and b) trifunctional epoxy resin from itaconic acid (TEIA).
Viscosity and processability The viscosity of epoxy resin is one of the most important factors in determining its end use because it is closely related to its processability. During the process of practical application, there are two methods to reduce the viscosity of epoxy resins: (i) synthesis of epoxy resins with low viscosity, and (ii) mixing with reactive diluents.[31] The addition of reactive diluents usually decreases the crosslink density and affects the thermal and mechanical properties of the cured resins. Thus, it is preferable to synthesize an epoxy resin with low molecular weight and ultralow viscosity. The viscosity of epoxy monomers (DGEBA and TEIA) as a function of time were examined by the isothermal rheological analyzer at 25 8C and the data are shown in Figure 4. As can be seen, the viscosity of TEIA (about 0.92 Pa s) was much lower than that of DGEBA (about 10.2 Pa s). Rheokinetics is a good way to study the processability of epoxy resins. Figure 5 presents the variation of the viscoelastic properties during the curing reaction. The dynamic viscosity (h), storage modulus (G’) and loss modulus (G’’) as a function of temperature were recorded for analysis. As shown in Figure 5 a, the h of the TEIA/D230 and TEIA/MHHPA systems at
Figure 5. a) Dynamic viscosity (h) and b) storage modulus (G’), loss modulus (G’’) as a function of temperature.
25 8C (0.016 and 0.164 Pa s, respectively) were lower than those of the DGEBA/D230 and DGEBA/MHHPA systems (0.53 and 1.95 Pa s, respectively), which was in accordance with the lower h of TEIA. As shown in Figure 5 b, the gelation occurred at lower temperatures for the TEIA/D230 and TEIA/MHHPA systems compared with the DGEBA/D230 and DGEBA/MHHPA systems, indicated by the intersection of G’ and G’’ curves;[32] consequently, the whole curing process for the TEIA system could be completed at a lower process temperature or in a shorter time relative to the DGEBA systems. Based on the above results, it can be concluded that TEIA with low viscosity has better processability and is convenient for the end use. Curing behavior and curing activity
Figure 4. Viscosity of DGEBA and TEIA as a function of time.
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Non-isothermal (dynamic) curing behaviors of the DGEBA/ D230, TEIA/D230, DGEBA/MHHPA, and TEIA/MHHPA systems were studied by differential scanning calorimetry (DSC). Figure 6 shows the DSC curing curves for these systems. The DGEBA/D230 and DGEBA/MHHPA systems both displayed a single exothermic peak which corresponded to the ringopening reaction of epoxide groups of DGEBA with amino groups of D230 and anhydride group of MHHPA, whereas, for the TEIA/D230 and TEIA/MHHPA systems, double exothermic ChemSusChem 2014, 7, 555 – 562
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www.chemsuschem.org ln q ¼ 1:052 E a =RT p þ ln ðAE a =RÞln FðxÞ5:331
ð2Þ
where F(x) is a conversion dependent term. Therefore, Ea could be calculated from the slope of a linear fitting plot of ln q versus 1/Tp. Figure 7 shows the plots of ln (q/Tp2) versus 1/Tp based on Kissinger’s equation (a) as well as ln q versus 1/Tp based on Ozawa’s theory (b) for the DGEBA/D230, TEIA/D230, DGEBA/ MHHPA, and TEIA/MHHPA systems. TEIA/D230(L) and TEIA/
Figure 6. DSC thermograms of non-isothermal curing behaviors of the DGEBA/D230, TEIA/D230, DGEBA/MHHPA, and TEIA/MHHPA systems at a heating rate of 2.5 8C min1.
peaks appeared. The peaks at lower temperature corresponded to the ring-opening reaction of glycidyl epoxides (CH2(O) CH, C1, C2, C10, and C11 in Figure 3) with D230 and MHHPA, whereas the peaks at higher temperature were attributed to the curing reaction of pendant epoxide (CH2(O)C, C6 and C7 in Figure 3) with D230 and MHHPA. Under the same curing condition, the peak temperature of the DSC exothermic curve is often taken as an indicator to evaluate the reactivity of the compounds in curing reactions. The lower the temperature of the peak, the higher the reactivity.[33, 34] Clearly, during the curing reaction with D230 and MHHPA, the glycidyl epoxides (the temperatures of the exothermic peaks are about 58.1 and 146.0 8C for D230 and MHHPA, respectively) were more active than that of DGEBA (the temperature of the exothermic peaks are about 98.9 and 168.5 8C for D230 and MHHPA, respectively). Although the pendant epoxide of TEIA (the temperature of the exothermic peak is about 96.0 8C) had a curing reactivity with D230 similar to that of DGEBA, the pendant epoxide of TEIA (the temperature of the exothermic peak is about 176.6 8C) had a curing reactivity with MHHPA lower than that of DGEBA. This might be because the steric hindrance of the pendant epoxide CH2(O)C in TEIA had a determining effect on its curing reactivity toward different curing agents. The activation energy of the curing reaction was determined by both Kissinger’s[35] and Ozawa’s[36] methods for accuracy of the results. Based on Kissinger’s theory, the activation energy could be obtained from the peak temperatures at different heating rates using equation (1): ln ðq=T p 2 Þ ¼ E a =RT p ln ðAR=E a Þ
ð1Þ
where q is the heating rate, Tp is the exothermic peak temperature, Ea is the activation energy, R is the gas constant, and A is the pre-exponential factor. A plot of ln(q/Tp2) versus Ea/RTp is obtained and the apparent activation energy can be calculated from the slope of linear fitting plot. Ozawa’s method can be expressed by equation (2): 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 7. Linear plot of a) ln (q/Tp2) versus 1/Tp based on Kissinger’s equation and b) ln q versus 1/Tp based on Ozawa’s theory.
MHHPA(L) denote the exothermic peaks that appeared at lower temperatures, and the exothermic peaks that appeared at higher temperatures are represented by TEIA/D230(H) and TEIA/MHHPA(H). According to the above analysis, the Ea values of the curing reaction of DGEBA with D230 and MHHPA were 65.2/68.1 and 81.8/79.9 kJ mol1 as shown in Table 1. The Ea values of the curing reaction of the glycidyl epoxides of TEIA with amino groups in the TEIA/D230 system and anhydride group in the TEIA/MHHPA system were calculated to be about 61.4/63.8 and 61.1/65.0 kJ mol1, respectively; these values were lower than those of the DGEBA systems. Consequently, the glycidyl epoxides of TEIA possessed higher reactivity than those of DGEBA when cured with D230 and MHHPA. However, the Ea of the curing reaction between the pendant epoxide of ChemSusChem 2014, 7, 555 – 562
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Table 1. Composition and activation energy (Ea) for the different systems. Samples
DGEBA/TEIA/D230/MHHPA weight ratio
Ea[a] [kJ mol1] Kissinger Ozawa
DGEBA/D230 TEIA/D230
100:0:30.8:0 0:100:66.8:0
DGEBA/MHHPA 100:0:0:67.6 TEIA/MHHPA 0:100:0:146.6
65.2 61.4(L) 64.8(H) 81.8 61.1(L) 97.4(H)
68.1 63.8(L) 67.6(H) 79.9 65.0(L) 99.8(H)
[a] L and H indicate the activation energy of the reactions occurring at the lower and higher temperatures, respectively, during DSC scanning.
TEIA and the amino group of D230 in the TEIA/D230 system (64.8/67.6 kJ mol1, Table 1) was similar to that of the DGEBA/ D230 system, and the Ea of the curing reaction between the pendant epoxide of TEIA and anhydride group of MHHPA in TEIA/MHHPA system (97.4/99.8 kJ mol1, Table 1) was higher than that of DGEBA/MHHPA system. This demonstrated that the pendant epoxide of TEIA exhibited a curing reactivity toward D230 similar to that of the epoxide groups of DGEBA and a curing reactivity toward MHHPA lower than that of the epoxide groups of DGEBA. These results were in good agreement with the results obtained from non-isothermal curing curves shown in Figure 6.
Table 2. Thermal properties of the cured epoxy resins. Sample DGEBA/D230 TEIA/D230 DGEBA/MHHPA TEIA/MHHPA
Tg [8C] 97.9 61.6 109.5 135.2
Td5% [8C] In N2 In air
R800 [%] In N2 In air
In N2
In air
366.0 268.6 368.3 329.1
4.4 6.7 4.5 3.4
185.0 153.4 194.9 179.2
174.8 156.6 187.6 175.9
322.3 266.3 340.1 319.7
0 0 0.2 0
Ts
ments, the chain segments of the TEIA/D230 could move more easily, corresponding to a lower Tg than for DGEBA/D230, whereas the rigid segment of MHHPA hindered the movement of the chain segments of epoxy network, resulting in a higher Tg of TEIA/MHHPA than for DGEBA/MHHPA. According to the above analysis, it could be concluded that the Tg of the TEIA systems could be regulated easily by selecting different curing agents. The thermal stability and degradation behaviors of the cured epoxy resins were evaluated by thermogravimetric analysis (TGA). The TGA curves under nitrogen and air are shown in Figure 9. The values of the initial degradation temperature for 5 % weight loss (Td5%) and the residual weight percent at 800 8C (R800) are presented in Table 2. Based on Figure 9 and Table 2, the Td5% values of the TEIA/D230 and TEIA/MHHPA systems were lower than those of the DGEBA/D230 and DGEBA/
Thermal properties DSC was used to determine the Tg of the cured resins. The DSC heating curves and data are shown in Figure 8 and Table 2. Clearly, the Tg of TEIA/D230 (61.6 8C) was lower than that of DGEBA/D230 (97.9 8C), whereas the Tg of TEIA/MHHPA (135.2 8C) was higher than that of DGEBA/MHHPA (109.5 8C). This might be because TEIA has a higher epoxy value (1.16) than DGEBA (0.536). As shown in Table 1, to achieve the desired stoichiometric ratio between epoxy resin and curing agents, the TEIA/D230 and TEIA/MHHPA systems require more D230 and MHHPA than the DGEBA/D230 and DGEBA/MHHPA systems, respectively. Therefore, with more flexible D230 seg-
Figure 8. DSC curves of the cured epoxy resins.
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Figure 9. TGA curves of the cured epoxy resins under a) nitrogen and b) air.
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MHHPA systems. This could be because TEIA/D230 and TEIA/ MHHPA contained more easily thermally degradable polyether segments and cleavable ester bonds[37] than DGEBA/D230 and DGEBA/MHHPA. To determine the thermal stability of the cured resins, the statistic heat-resistant index (Ts) was used. It is determined from the temperature of 5 % weight loss (Td5%) and of 30 % weight loss (Td30 %) of the sample, as measured by means of TGA. The statistic heat-resistant temperature (Ts) is calculated by equation (3):[28, 38] T s ¼ 0:49½T d5% þ 0:6ðTd30 % T d5% Þ
ð3Þ
From Table 2, either in N2 or air, the Ts values of the TEIA systems were lower than those of the DGEBA systems, and the difference in Ts values could be reduced when the relatively stable curing agent MHHPA was used. These results demonstrated that the TEIA systems with D230 and MHHPA as the curing agents possessed a lower thermal stability than the DGEBA systems, and that the thermal stability of the TEIA systems could be regulated by different curing agents. Mechanical properties The flexural stress–strain curves for the cured epoxy resins are shown in Figure 10, and the data of flexural strength, flexural modulus, as well as the strain at break are listed in Table 3. As
Table 3. Flexural properties of the cured epoxy resins. Samples
Flexural strength [MPa]
Flexural modulus [MPa]
Strain at break [%]
DGEBA/D230 TEIA/D230 DGEBA/ MHHPA TEIA/MHHPA
121.0 0.7 117.8 2.0 131.6 23.2
2950 18 3600 77 3390 34
12.7 1.7 20.8 0.9 4.4 1.3
157.2 17.6
3640 32
5.0 0.9
lar. For the TEIA/MHHPA system, the flexural strength, flexural modulus and strain at break (157.2 MPa, 3640 MPa, and 5.0 %) were all higher than those of DGEBA/MHHPA system (131.6 MPa, 3390 MPa, and 4.5 %). These results indicated that high performance materials could be obtained by selecting different curing agents for TEIA systems.
Conclusions Itaconic acid was an excellent alternative feedstock to prepare a bio-based epoxy resin with outstanding properties. Compared with the diglycidyl ether of bisphenol A, TEIA had a higher epoxy value (1.16), a lower viscosity (0.92 Pa s, 25 8C), and higher curing reactivity toward D230 and MHHPA. As a result of the high epoxy value, its properties could be manipulated and high performance resins could be obtained by selecting different curing agents. As a result of the low viscosity, the TEIA systems had better processability than the DGEBA systems. Itaconic acid has already shown great potential to act as a promising renewable feedstock to synthesize high performance epoxy resin without bisphenol A.
Experimental Section Raw materials
Figure 10. Flexural stress–strain curves of the cured epoxy resins.
shown in Figure 10, the stress of DGEBA/D230 and TEIA/D230 increased linearly at the initial stage before yielding occurred and a maximum value was reached. After that, the stress began to decrease with the strain until final rupture, exhibiting the typical feature of ductile fracture.[39] In contrast, the stress of DGEBA/MHHPA and TEIA/MHHPA increased almost linearly with strain to the maximum value without any yielding, implying the nature of brittle fracture.[39] The flexural modulus and strain at break of TEIA/D230 (3600 MPa and 20.9 %) were much higher than those of DGEBA/D230 (2950 MPa and 12.8 %), and the flexural strength of TEIA/D230 and DGEBA/D230 was simi 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Itaconic acid (IA) was purchased from Zhejiang Guoguang Biochemistry Co., Ltd, China. Allyl bromide, poly(propylene glycol) bis(2-aminopropyl ether) (D230), and hexadecyl trimethyl ammonium bromide (HTAB) were obtained from Aladdin Reagent, China. M-chloroperoxybenzoic acid (m-CPBA) (85 %) and methyl hexahydrophthalic anhydride (MHHPA) were obtained from Sinopharm Chemical Reagent Co., Ltd, China. Diglycidyl ether of bisphenol A (DGEBA) with epoxide equivalent weight of 182–192 g eq1 was purchased from DOW Chemical Company. All the chemicals were used as received.
Preparation of AIA IA (50 g), allyl bromide (187 g), acetone (500 g), N,N-dimethylformamide (100 g), and potassium carbonate (117 g) were placed in a three-necked round-bottomed flask with a magnetic stirrer, a thermometer, and a reflux condenser. After the reactants were mixed vigorously at room temperature for 10 min, the mixture was heated and refluxed for 12 h. After removing the acetone and unreacted ally bromide, the raw product was diluted with dichloromethane and washed by deionized water. AIA was obtained with a yield of 97 % after removing the water and dichloromethane on ChemSusChem 2014, 7, 555 – 562
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CHEMSUSCHEM FULL PAPERS a rotary evaporator. The synthetic scheme is shown in Scheme 1. 1 H NMR (400 MHz, CDCl3): d = 6.37 (s, H), 5.91 (m, 2 H), 5.73 (s, H), 5.28 (m, 4 H), 4.64 (m, 4 H), 3.38 ppm (s, 2 H). 13C NMR (400 MHz, CDCl3): d = 170.3 (1C), 166.7 (1C), 133.7 (1C), 131.9 (2C), 128.7 (1C), 118.3 (2C), 65.6 (2C), 37.7 ppm (1C).
Preparation of TEIA AIA (71 g), dichloromethane (311.7 g), and m-chloroperoxybenzoic acid (m-CPBA, 312 g) was placed in a three-necked round-bottomed flask with a mechanical stirrer, a thermometer, and a reflux condenser. The reactants were mixed and kept at 40 8C for 7 d. The solution was filtered after cooling and washed with a solution of 10 % sodium sulfite followed by 10 % sodium carbonate and distilled water. The organic layer was then dried over anhydrous sodium sulfate. The trifunctional epoxy resin from itaconic acid (TEIA) was obtained with a yield of 60 % after removing the dichloromethane by means of a rotary evaporator. The epoxy value of TEIA determined by the hydrochloric acid–acetone method was about 1.16. The synthetic Scheme is shown in Scheme 1. 1H NMR (400 MHz, CDCl3): d = 4.50 (m, 2 H), 4.00 (m, 2 H), 3.22 (m, 3 H), 3.10 (m, 1 H), 2.93 (d, J = 5.7 Hz, 1 H), 2.82 (m, 3 H), 2.65 ppm (m, 2 H); 13 C NMR (400 MHz, CDCl3): d = 169.1 (2C), 66.0 (3C), 53.4 (1C), 51.8(1C), 48.9 (1C), 44.5 (2C), 37.3 ppm (1C).
Preparation of epoxy networks D230 and MHHPA were used as the curing agents in this study. For the systems using D230 as the curing agent, the predetermined DGEBA or TEIA and a stoichiometric amount of D230 (the molar ratio of N–H to epoxy group was 1:1) were stirred together until a homogeneous mixture was obtained. After that, it was degassed in a vacuum oven at 30 8C for at least 10 min. Then the gas free mixture was poured into a preheated stainless steel mold and cured at 80 8C for 2 h, 150 8C for 2 h, and 180 8C for 2 h to obtain a completely cured resin. For the systems using MHHPA as the curing agent, the necessary amount of DGEBA or TEIA, a stoichiometric amount of MHHPA (the molar ratio of MHHPA to epoxy group was 0.75:1) and 0.1 wt % HTAB (on the basis of the total weight of curing agent and epoxy resin) were stirred together until a homogeneous mixture was obtained. After that, it was degassed in a vacuum oven at 80 8C for at least 10 min. Then the gas free mixture was poured into a preheated stainless steel mould and cured at 150 8C for 2 h, and 190 8C for 2 h to obtain a completely cured resin. Their compositions and codes are listed in Table 1. All the samples were cured under the same conditions.
www.chemsuschem.org TOLEDO, Switzerland) under a nitrogen atmosphere. A heat scan ranging from 25 to 225 8C was performed at heating rates of 2.5, 5, 8, and 10 8C min1. The curves of heat flow as a function of time were recorded for activation energy analysis. The cured epoxy resins were cut into small pieces and about 8 mg of sample was used for the Tg measurement. It was heated to 200 8C and held there for 3 min to eliminate any thermal history. Then, it was cooled to 25 8C at a cooling rate of 50 8C min1, followed by heating again to 200 8C at a rate of 20 8C min1. The Tg was obtained from the peak temperature of the differential curve of the second heating curve of the cured epoxy resins. TGA was performed on a Mettler-Toledo TGA/DSC1 thermogravimetric analyzer (METTLER TOLEDO, Switzerland) with high purity nitrogen or air as purge gas at a scanning rate of 20 8C min1 from 50 to 800 8C. Flexural properties were evaluated by means of a three-point bending test conducted on an Instron 5567 Electric Universal Testing Machine (Instron, America) with a span length of 48 mm at a cross-head speed of 2 mm min1. Specimens of 60 10 3.5 mm3 were used for this evaluation. The data was obtained from the average of at least five specimens for accuracy.
Acknowledgements The authors acknowledge financial support from Project 51203176 supported by the National Natural Science Foundation of China, Project 51003116 supported by the NSFC, the National Basic Research Program of China (973 Program, 2010CB631100), the China Postdoctoral Science Foundation funded project (2013M540504), the Postdoctoral Science Foundation of Zhejiang province (Bsh1201011) and the Director Funds of Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences (Y20224QF06). Keywords: biomass · epoxidation · itaconic acid · renewable resources · sustainable chemistry [1] [2] [3] [4] [5] [6] [7] [8]
Characterization 1
H NMR and 13C NMR spectroscopy was performed on a 400 MHz AVANCE III Bruker NMR spectrometer (Bruker, Switzerland) with CDCl3 as a solvent. The rheological behavior of the epoxy resin was studied by means of dynamic oscillation using a dynamic analyzer Physica MCR-301 of Rheometric (Anton Paar, Austria) with parallel plate tools. The plate diameter and its gap were 25.0 and 0.30 mm, respectively. The test was performed with a heating rate of 3 8C min1, an angular frequency of 1 Hz, and an initial ramp strain of between 10 % and 0.5 %. The curing temperature ranged from 25 to 200 8C and G’, G’’, and h were measured as a function of temperature. The viscosity of TEIA and DGEBA was measured at 25 8C for 2 min with a shear rate of 5 s1. The non-isothermal curing reaction was monitored on a Mettler–Toledo MET DSC (METTLER 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
C. B. Bucknall, A. H. Gilbert, Polymer 1989, 30, 213 – 217. M. C. Lee, T. H. Ho, C. S. Wang, J. Appl. Polym. Sci. 1996, 62, 217 – 225. S. T. Lin, S. K. Huang, Eur. Polym. J. 1997, 33, 365 – 373. J. M. Raquez, M. Delglise, M. F. Lacrampe, P. Krawczak, Prog. Polym. Sci. 2010, 35, 487 – 509. S. Flint, T. Markle, S. Thompson, E. Wallace, J. Environ. Manage. 2012, 104, 19 – 34. M. Chrysanthos, J. Galy, J.-P. Pascault, Polymer 2011, 52, 3611 – 3620. R. Hçfer, M. Selig in Polymer Science: A Comprehensive Reference (Eds.: M. Krzysztof, M. Martin), Elsevier, Amsterdam, 2012, pp. 5 – 14. R. Hçfer, Sustainable Solutions for Modern Economies, RSC Publ., Cambridge, 2009, pp. 1 – 11. H. Miyagawa, A. K. Mohanty, M. Misra, L. T. Drzal, Macromol. Mater. Eng. 2004, 289, 629 – 635. H. Miyagawa, M. Misra, L. T. Drzal, A. K. Mohanty, Polym. Eng. Sci. 2005, 45, 487 – 495. A. P. Gupta, S. Ahmad, A. Dev, Polym. Eng. Sci. 2011, 51, 1087 – 1091. V. Fombuena, L. Sanchez-Nacher, M. D. Samper, D. Juarez, R. Balart, J. Am. Oil Chem. Soc. 2013, 90, 449 – 457. M. Stemmelen, F. Pessel, V. Lapinte, S. Caillol, J. P. Habas, J. J. Robin, J. Polym. Sci. Part A J. Polym. Sci. Pol. Chem. 2011, 49, 2434 – 2444. K. Hofmann, W. Glasser, Macromol. Chem. Phys. 1994, 195, 65 – 80. K. Hofmann, W. G. Glasser, J. Adhes. 1993, 40, 229 – 241. N. E. El Mansouri, Q. Yuan, F. Huang, BioRes. 2011, 6, 2492 – 2503. N. E. El Mansouri, Q. Yuan, F. Huang, BioRes. 2011, 6, 2647 – 2662. T. Koike, Polym. Eng. Sci. 2012, 52, 701 – 717. X. Q. Liu, J. W. Zhang, Polym. Int. 2010, 59, 607 – 609.
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CHEMSUSCHEM FULL PAPERS [20] X. Q. Liu, W. Huang, Y. H. Jiang, J. Zhu, C. Z. Zhang, Express Polym. Lett. 2012, 6, 293 – 298. [21] L. L. Deng, M. M. Shen, J. Yu, K. Wu, C. Y. Ha, Ind. Eng. Chem. Res. 2012, 51, 8178 – 8184. [22] X. Pan, D. C. Webster, Macromol. Rapid Commun. 2011, 32, 1324 – 1330. [23] X. Pan, P. Sengupta, D. C. Webster, Green Chem. 2011, 13, 965 – 975. [24] X. Pan, P. Sengupta, D. C. Webster, Biomacromolecules 2011, 12, 2416 – 2428. [25] S. Q. Ma, X. Q. Liu, Y. H. Jiang, Z. B. Tang, C. Z. Zhang, J. Zhu, Green Chem. 2013, 15, 245 – 254. [26] M. Shibata, K. Nakai, J. Polym. Sci. Part B 2010, 48, 425 – 433. [27] Y. Takada, K. Shinbo, Y. Someya, M. Shibata, J. Appl. Polym. Sci. 2009, 113, 479 – 484. [28] C. Aouf, H. Nouailhas, M. Fache, S. Caillol, B. Boutevin, H. Fulcrand, Eur. Polym. J. 2013, DOI:10.1016/j.eurpolymj.2012.1011.1025. [29] C. Aouf, J. Lecomte, P. Villeneuve, E. Dubreucq, H. Fulcrand, Green Chem. 2012, 14, 2328 – 2336. [30] T. Werpy, G. Petersen, Top Value Added Chemicals from Biomass. Volume I—Results of Screening for Potential Candidates from Sugars and Synthesis Gas, N. R. E. Laboratory Report DOE/GO-102004-1992, U.S. Department of Energy, 2004.
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www.chemsuschem.org [31] Q. Tian, Y. C. Yuan, M. Z. Rong, M. Q. Zhang in Multi-Functional Materials and Structures, Pts 1 and 2, Vol. 47 – 50 (Eds.: A. K. T. Lau, J. Lu, V. K. Varadan, F. K. Chang, J. P. Tu, P. M. Lam), 2008, pp. 290 – 293. [32] P. CaÇamero-Martnez, M. Fernndez-Garca, J. L. de La Fuente, React. Funct. Polym. 2010, 70, 761 – 766. [33] X. Q. Liu, W. B. Xin, J. W. Zhang, Green Chem. 2009, 11, 1018 – 1025. [34] Z. Q. Tao, S. Y. Yang, J. S. Chen, L. Fan, Eur. Polym. J. 2007, 43, 1470 – 1479. [35] H. E. Kissinger, J. Ind. Natl. Bur. Stand. 1956, 57, 217 – 221. [36] T. Ozawa, J. Therm. Anal. 1976, 9, 369 – 373. [37] E. Khosravi, O. M. Musa, Eur. Polym. J. 2011, 47, 465 – 473. [38] Y.-C. Chiu, I. C. Chou, W.-C. Tseng, C.-C. M. Ma, Polym. Degrad. Stab. 2008, 93, 668 – 676. [39] F. H. Liu, Z. G. Wang, Y. L. Wang, B. Zhang, J. Polym. Sci. Part B 2010, 48, 2424 – 2431.
Received: July 24, 2013 Published online on October 17, 2013
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Synthesis and Properties of a Bio-Based Epoxy Resin with High Epoxy Value and Low Viscosity Songqi Ma, Xiaoqing Liu,* Libo Fan, Yanhua Jiang, Lijun Cao, Zhaobin Tang, and Jin Zhu*[a] A bio-based epoxy resin (denoted TEIA) with high epoxy value (1.16) and low viscosity (0.92 Pa s, 25 8C) was synthesized from itaconic acid and its chemical structure was confirmed by 1 H NMR and 13C NMR spectroscopy. Its curing reaction with poly(propylene glycol) bis(2-aminopropyl ether) (D230) and methyl hexahydrophthalic anhydride (MHHPA) was investigated. For comparison, the commonly used diglycidyl ether of bisphenol A (DGEBA) was also cured with the same curing agents. The results demonstrated that TEIA showed higher
curing reactivity towards D230/MHHPA and lower viscosity compared with DGEBA, resulting in the better processability. Owing to its high epoxy value and unique structure, comparable or better glass transition temperature as well as mechanical properties could be obtained for the TEIA-based network relative to the DGEBA-based network. The results indicated that itaconic acid is a promising renewable feedstock for the synthesis of bio-based epoxy resin with high performance.
Introduction Owing to their superior mechanical properties, excellent thermal resistance and good processability, epoxy resins are extensively used as coatings, adhesives, and structural composites as well as electronic materials.[1–3] Diglycidyl ether of bisphenol A (DGEBA) is by far the most widely used monomer in the formulation of epoxy networks.[4] Bisphenol A, accounting for more than 67 % of the molar mass of DGEBA, is fully dependent on fossil resources. In addition, bisphenol A appears to be both an estrogen receptor agonist and an androgen receptor antagonist.[5] Resins containing bisphenol A have been deemed to be toxic to living organisms[6] and their application has been strictly limited in many countries. Under the concept of green chemistry[7, 8] and as a result of the increasing concern over the depletion of fossil reserves and growing greenhouse gas emissions, there is an increasing interest in finding a biobased feedstock to synthesize epoxy resins without bisphenol A. Until now, a number of bio-based epoxy resins derived from plant oils,[9–13] lignin,[14–18] and rosin[19–21] have been reported. But, as a result of their low epoxy value, low curing reactivity, as well as unsuitable chemical structures, the cured resins rarely exhibit satisfactory properties. The epoxy value is one of the determining factors in regulating the properties of epoxy resins. A higher epoxy value usually leads to better performance. Recently, several epoxies with high epoxy value were
[a] Dr. S. Ma, Prof. Dr. X. Liu, L. Fan, Y. Jiang, L. Cao, Dr. Z. Tang, Prof. Dr. J. Zhu Ningbo Key Laboratory of Polymer Materials Ningbo Institute of Material Technology and Engineering Chinese Academy of Sciences Ningbo 315201 (PR China) Fax: (+ 86) 574-86685925 E-mail: [email protected] [email protected]
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synthesized from bio-based feedstock. For example, Pan et al.[22–24] prepared a series of highly functional epoxy compounds from sucrose and fatty acids. The property investigation demonstrated that the higher epoxy value could lead to a higher crosslink density, which resulted in a higher glass transition temperature (Tg), higher tensile strength, and modulus relative to the commercial epoxidized soybean oil. However, their overall properties were still not comparable with those of the conventional DGEBA cured with the same curing agent because of the flexible aliphatic chain of fatty acids.[25] Shibata et al.[26, 27] investigated the properties of bio-based glycerol polyglycidyl ether, polyglycerol polyglycidyl ether, and sorbitol polyglycidyl ether using e-poly(lysine) and tannic acid as the curing agents. Chrysanthos et al.[6] studied isosorbide-based epoxy resins. Although these epoxy compounds commonly possess high epoxy value, especially glycerol triglycidyl ether with the epoxy value of 1.15, the cured networks still exhibited low Tg values as a result of the flexible ether bonds. In order to overcome the disadvantage of the long flexible aliphatic chain, Aouf et al.[28, 29] synthesized a multifunctional bio-based epoxy resin from gallic acid that showed a much higher value of Tg (233 8C) than DGEBA (160 8C) using isophorone diamine as the curing agent. But the gallic-acid-based epoxy network was brittle as a result of the rigid benzene ring and exorbitantly high crosslink density. Clearly, selecting the bio-based platform chemicals and designing their structures are the keys to synthesizing bio-based epoxy resins with satisfactory properties. Itaconic acid (IA), one of the top 12 potential bio-based platform chemicals selected by the U.S. Department of Energy,[30] possesses two carboxyl groups and one carbon–carbon double bond, plus a short molecular chain with no flexible bond or rigid ring. It has been proved to be suitable for the synthesis of high performance bio-based epoxy resin.[25] In our previous work, itaconic-acid-based epoxy resin with double bonds (EIA) ChemSusChem 2014, 7, 555 – 562
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CHEMSUSCHEM FULL PAPERS was synthesized. Compared with DGEBA, EIA exhibited comparable or better properties after curing with methyl hexahydrophthalic anhydride. The carbon–carbon double bonds could be utilized for copolymerization with divinyl benzene and acrylated epoxidized soybean oil so as to manipulate its properties further. However, EIA was a mixture of different oligomers and the remaining carbon–carbon double bonds had the tendency to self-polymerize unless polymerization inhibitor was added, which increased the viscosity of EIA and affected its properties. In this paper, we exploited the unique chemical structure of itaconic acid to synthesize a trifunctional epoxy monomer (TEIA, as shown in Figure 1). Based on its structure, we expect
www.chemsuschem.org acid (AIA) was obtained from dehydrobromination between IA and allyl bromide, and (ii) epoxidation of AIA. The chemical structures of AIA and TEIA were characterized by means of 1H NMR and 13C NMR spectroscopy. Figure 2 shows the 1H NMR spectra of AIA and TEIA. The spectrum of
Figure 2. 1H NMR spectra of a) AIA and b) trifunctional epoxy resin from itaconic acid (TEIA). Figure 1. Chemical structures of TEIA, DGEBA, D230, and MHHPA.
it to possess a high epoxy value, low viscosity, and satisfactory properties after curing. For easy evaluation, DGEBA was also employed here as the reference and cured with the same curing agents D230 and MHHPA (Figure 1).
Results and Discussion Characterization of allylated itaconic acid (AIA) and TEIA As shown in Scheme 1, a trifunctional epoxy resin from itaconic acid (TEIA) was synthesized in two steps: (i) allylated itaconic
AIA shows the characteristic peaks of protons H1, H2, H3, H12, H13, and H14 on the double bonds CH2 = CHCH2 at 5.21– 5.34 and 5.84–5.96 ppm, the peaks of protons H8 and H9 on the double bond CH2 = C at 5.73 and 6.37 ppm, the peaks of protons H4, H5, H10, and H11 on the carbon atoms next to the double bonds CH2 = CHCH2 at 4.58–4.67 ppm, and the peaks of protons H6 and H7 on the carbon atom between the double bond and ester bond CH2=CCH2COO at 3.38 ppm. In the spectrum of TEIA, these proton peaks all disappeared but the characteristic peaks of protons H1, H2, H3, H8, H9, H12, H13, and H14 on the glycidyl epoxides CH2(O)CH and pendant epoxide CH2(O)C, protons H6 and H7 on the carbon between the pendant epoxide and ester bond CH2(O) CCH2COO at 2.63–3.24 ppm, the peaks of protons H4, H5, H10 and H11 on the carbon atoms next to the glycidyl epoxides CH2(O)CHCH2 at 3.94–4.05 and 4.45–4.54 ppm were observed. In addition, the integrated area ratios of the characteristic signals for protons were all in good agreement with the theoretical values. In order to further identify their structures, the 13C NMR spectra of AIA and TEIA were also displayed in Figure 3. The spectrum of AIA presents the characteristic peaks of carbon atoms C1, C2, C6, C7, C10, and C11 on the the double bonds CH2 = CHCH2 and CH2 = CCOO at 118– 134 ppm. The spectrum of TEIA shows the disappearance of double bonds and the appearance of carbon atoms C1, C2, C6, C7, C10, and C11 on the glycidyl epoxides CH2(O)CH and pendant epoxide CH2(O)C at 45–78 ppm. These results demonstrated that the target compounds were synthesized successfully.
Scheme 1. Synthesis of trifunctional epoxy resin from itaconic acid (TEIA).
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Figure 3. 13C NMR spectra of a) AIA and b) trifunctional epoxy resin from itaconic acid (TEIA).
Viscosity and processability The viscosity of epoxy resin is one of the most important factors in determining its end use because it is closely related to its processability. During the process of practical application, there are two methods to reduce the viscosity of epoxy resins: (i) synthesis of epoxy resins with low viscosity, and (ii) mixing with reactive diluents.[31] The addition of reactive diluents usually decreases the crosslink density and affects the thermal and mechanical properties of the cured resins. Thus, it is preferable to synthesize an epoxy resin with low molecular weight and ultralow viscosity. The viscosity of epoxy monomers (DGEBA and TEIA) as a function of time were examined by the isothermal rheological analyzer at 25 8C and the data are shown in Figure 4. As can be seen, the viscosity of TEIA (about 0.92 Pa s) was much lower than that of DGEBA (about 10.2 Pa s). Rheokinetics is a good way to study the processability of epoxy resins. Figure 5 presents the variation of the viscoelastic properties during the curing reaction. The dynamic viscosity (h), storage modulus (G’) and loss modulus (G’’) as a function of temperature were recorded for analysis. As shown in Figure 5 a, the h of the TEIA/D230 and TEIA/MHHPA systems at
Figure 5. a) Dynamic viscosity (h) and b) storage modulus (G’), loss modulus (G’’) as a function of temperature.
25 8C (0.016 and 0.164 Pa s, respectively) were lower than those of the DGEBA/D230 and DGEBA/MHHPA systems (0.53 and 1.95 Pa s, respectively), which was in accordance with the lower h of TEIA. As shown in Figure 5 b, the gelation occurred at lower temperatures for the TEIA/D230 and TEIA/MHHPA systems compared with the DGEBA/D230 and DGEBA/MHHPA systems, indicated by the intersection of G’ and G’’ curves;[32] consequently, the whole curing process for the TEIA system could be completed at a lower process temperature or in a shorter time relative to the DGEBA systems. Based on the above results, it can be concluded that TEIA with low viscosity has better processability and is convenient for the end use. Curing behavior and curing activity
Figure 4. Viscosity of DGEBA and TEIA as a function of time.
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Non-isothermal (dynamic) curing behaviors of the DGEBA/ D230, TEIA/D230, DGEBA/MHHPA, and TEIA/MHHPA systems were studied by differential scanning calorimetry (DSC). Figure 6 shows the DSC curing curves for these systems. The DGEBA/D230 and DGEBA/MHHPA systems both displayed a single exothermic peak which corresponded to the ringopening reaction of epoxide groups of DGEBA with amino groups of D230 and anhydride group of MHHPA, whereas, for the TEIA/D230 and TEIA/MHHPA systems, double exothermic ChemSusChem 2014, 7, 555 – 562
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www.chemsuschem.org ln q ¼ 1:052 E a =RT p þ ln ðAE a =RÞln FðxÞ5:331
ð2Þ
where F(x) is a conversion dependent term. Therefore, Ea could be calculated from the slope of a linear fitting plot of ln q versus 1/Tp. Figure 7 shows the plots of ln (q/Tp2) versus 1/Tp based on Kissinger’s equation (a) as well as ln q versus 1/Tp based on Ozawa’s theory (b) for the DGEBA/D230, TEIA/D230, DGEBA/ MHHPA, and TEIA/MHHPA systems. TEIA/D230(L) and TEIA/
Figure 6. DSC thermograms of non-isothermal curing behaviors of the DGEBA/D230, TEIA/D230, DGEBA/MHHPA, and TEIA/MHHPA systems at a heating rate of 2.5 8C min1.
peaks appeared. The peaks at lower temperature corresponded to the ring-opening reaction of glycidyl epoxides (CH2(O) CH, C1, C2, C10, and C11 in Figure 3) with D230 and MHHPA, whereas the peaks at higher temperature were attributed to the curing reaction of pendant epoxide (CH2(O)C, C6 and C7 in Figure 3) with D230 and MHHPA. Under the same curing condition, the peak temperature of the DSC exothermic curve is often taken as an indicator to evaluate the reactivity of the compounds in curing reactions. The lower the temperature of the peak, the higher the reactivity.[33, 34] Clearly, during the curing reaction with D230 and MHHPA, the glycidyl epoxides (the temperatures of the exothermic peaks are about 58.1 and 146.0 8C for D230 and MHHPA, respectively) were more active than that of DGEBA (the temperature of the exothermic peaks are about 98.9 and 168.5 8C for D230 and MHHPA, respectively). Although the pendant epoxide of TEIA (the temperature of the exothermic peak is about 96.0 8C) had a curing reactivity with D230 similar to that of DGEBA, the pendant epoxide of TEIA (the temperature of the exothermic peak is about 176.6 8C) had a curing reactivity with MHHPA lower than that of DGEBA. This might be because the steric hindrance of the pendant epoxide CH2(O)C in TEIA had a determining effect on its curing reactivity toward different curing agents. The activation energy of the curing reaction was determined by both Kissinger’s[35] and Ozawa’s[36] methods for accuracy of the results. Based on Kissinger’s theory, the activation energy could be obtained from the peak temperatures at different heating rates using equation (1): ln ðq=T p 2 Þ ¼ E a =RT p ln ðAR=E a Þ
ð1Þ
where q is the heating rate, Tp is the exothermic peak temperature, Ea is the activation energy, R is the gas constant, and A is the pre-exponential factor. A plot of ln(q/Tp2) versus Ea/RTp is obtained and the apparent activation energy can be calculated from the slope of linear fitting plot. Ozawa’s method can be expressed by equation (2): 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 7. Linear plot of a) ln (q/Tp2) versus 1/Tp based on Kissinger’s equation and b) ln q versus 1/Tp based on Ozawa’s theory.
MHHPA(L) denote the exothermic peaks that appeared at lower temperatures, and the exothermic peaks that appeared at higher temperatures are represented by TEIA/D230(H) and TEIA/MHHPA(H). According to the above analysis, the Ea values of the curing reaction of DGEBA with D230 and MHHPA were 65.2/68.1 and 81.8/79.9 kJ mol1 as shown in Table 1. The Ea values of the curing reaction of the glycidyl epoxides of TEIA with amino groups in the TEIA/D230 system and anhydride group in the TEIA/MHHPA system were calculated to be about 61.4/63.8 and 61.1/65.0 kJ mol1, respectively; these values were lower than those of the DGEBA systems. Consequently, the glycidyl epoxides of TEIA possessed higher reactivity than those of DGEBA when cured with D230 and MHHPA. However, the Ea of the curing reaction between the pendant epoxide of ChemSusChem 2014, 7, 555 – 562
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Table 1. Composition and activation energy (Ea) for the different systems. Samples
DGEBA/TEIA/D230/MHHPA weight ratio
Ea[a] [kJ mol1] Kissinger Ozawa
DGEBA/D230 TEIA/D230
100:0:30.8:0 0:100:66.8:0
DGEBA/MHHPA 100:0:0:67.6 TEIA/MHHPA 0:100:0:146.6
65.2 61.4(L) 64.8(H) 81.8 61.1(L) 97.4(H)
68.1 63.8(L) 67.6(H) 79.9 65.0(L) 99.8(H)
[a] L and H indicate the activation energy of the reactions occurring at the lower and higher temperatures, respectively, during DSC scanning.
TEIA and the amino group of D230 in the TEIA/D230 system (64.8/67.6 kJ mol1, Table 1) was similar to that of the DGEBA/ D230 system, and the Ea of the curing reaction between the pendant epoxide of TEIA and anhydride group of MHHPA in TEIA/MHHPA system (97.4/99.8 kJ mol1, Table 1) was higher than that of DGEBA/MHHPA system. This demonstrated that the pendant epoxide of TEIA exhibited a curing reactivity toward D230 similar to that of the epoxide groups of DGEBA and a curing reactivity toward MHHPA lower than that of the epoxide groups of DGEBA. These results were in good agreement with the results obtained from non-isothermal curing curves shown in Figure 6.
Table 2. Thermal properties of the cured epoxy resins. Sample DGEBA/D230 TEIA/D230 DGEBA/MHHPA TEIA/MHHPA
Tg [8C] 97.9 61.6 109.5 135.2
Td5% [8C] In N2 In air
R800 [%] In N2 In air
In N2
In air
366.0 268.6 368.3 329.1
4.4 6.7 4.5 3.4
185.0 153.4 194.9 179.2
174.8 156.6 187.6 175.9
322.3 266.3 340.1 319.7
0 0 0.2 0
Ts
ments, the chain segments of the TEIA/D230 could move more easily, corresponding to a lower Tg than for DGEBA/D230, whereas the rigid segment of MHHPA hindered the movement of the chain segments of epoxy network, resulting in a higher Tg of TEIA/MHHPA than for DGEBA/MHHPA. According to the above analysis, it could be concluded that the Tg of the TEIA systems could be regulated easily by selecting different curing agents. The thermal stability and degradation behaviors of the cured epoxy resins were evaluated by thermogravimetric analysis (TGA). The TGA curves under nitrogen and air are shown in Figure 9. The values of the initial degradation temperature for 5 % weight loss (Td5%) and the residual weight percent at 800 8C (R800) are presented in Table 2. Based on Figure 9 and Table 2, the Td5% values of the TEIA/D230 and TEIA/MHHPA systems were lower than those of the DGEBA/D230 and DGEBA/
Thermal properties DSC was used to determine the Tg of the cured resins. The DSC heating curves and data are shown in Figure 8 and Table 2. Clearly, the Tg of TEIA/D230 (61.6 8C) was lower than that of DGEBA/D230 (97.9 8C), whereas the Tg of TEIA/MHHPA (135.2 8C) was higher than that of DGEBA/MHHPA (109.5 8C). This might be because TEIA has a higher epoxy value (1.16) than DGEBA (0.536). As shown in Table 1, to achieve the desired stoichiometric ratio between epoxy resin and curing agents, the TEIA/D230 and TEIA/MHHPA systems require more D230 and MHHPA than the DGEBA/D230 and DGEBA/MHHPA systems, respectively. Therefore, with more flexible D230 seg-
Figure 8. DSC curves of the cured epoxy resins.
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Figure 9. TGA curves of the cured epoxy resins under a) nitrogen and b) air.
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MHHPA systems. This could be because TEIA/D230 and TEIA/ MHHPA contained more easily thermally degradable polyether segments and cleavable ester bonds[37] than DGEBA/D230 and DGEBA/MHHPA. To determine the thermal stability of the cured resins, the statistic heat-resistant index (Ts) was used. It is determined from the temperature of 5 % weight loss (Td5%) and of 30 % weight loss (Td30 %) of the sample, as measured by means of TGA. The statistic heat-resistant temperature (Ts) is calculated by equation (3):[28, 38] T s ¼ 0:49½T d5% þ 0:6ðTd30 % T d5% Þ
ð3Þ
From Table 2, either in N2 or air, the Ts values of the TEIA systems were lower than those of the DGEBA systems, and the difference in Ts values could be reduced when the relatively stable curing agent MHHPA was used. These results demonstrated that the TEIA systems with D230 and MHHPA as the curing agents possessed a lower thermal stability than the DGEBA systems, and that the thermal stability of the TEIA systems could be regulated by different curing agents. Mechanical properties The flexural stress–strain curves for the cured epoxy resins are shown in Figure 10, and the data of flexural strength, flexural modulus, as well as the strain at break are listed in Table 3. As
Table 3. Flexural properties of the cured epoxy resins. Samples
Flexural strength [MPa]
Flexural modulus [MPa]
Strain at break [%]
DGEBA/D230 TEIA/D230 DGEBA/ MHHPA TEIA/MHHPA
121.0 0.7 117.8 2.0 131.6 23.2
2950 18 3600 77 3390 34
12.7 1.7 20.8 0.9 4.4 1.3
157.2 17.6
3640 32
5.0 0.9
lar. For the TEIA/MHHPA system, the flexural strength, flexural modulus and strain at break (157.2 MPa, 3640 MPa, and 5.0 %) were all higher than those of DGEBA/MHHPA system (131.6 MPa, 3390 MPa, and 4.5 %). These results indicated that high performance materials could be obtained by selecting different curing agents for TEIA systems.
Conclusions Itaconic acid was an excellent alternative feedstock to prepare a bio-based epoxy resin with outstanding properties. Compared with the diglycidyl ether of bisphenol A, TEIA had a higher epoxy value (1.16), a lower viscosity (0.92 Pa s, 25 8C), and higher curing reactivity toward D230 and MHHPA. As a result of the high epoxy value, its properties could be manipulated and high performance resins could be obtained by selecting different curing agents. As a result of the low viscosity, the TEIA systems had better processability than the DGEBA systems. Itaconic acid has already shown great potential to act as a promising renewable feedstock to synthesize high performance epoxy resin without bisphenol A.
Experimental Section Raw materials
Figure 10. Flexural stress–strain curves of the cured epoxy resins.
shown in Figure 10, the stress of DGEBA/D230 and TEIA/D230 increased linearly at the initial stage before yielding occurred and a maximum value was reached. After that, the stress began to decrease with the strain until final rupture, exhibiting the typical feature of ductile fracture.[39] In contrast, the stress of DGEBA/MHHPA and TEIA/MHHPA increased almost linearly with strain to the maximum value without any yielding, implying the nature of brittle fracture.[39] The flexural modulus and strain at break of TEIA/D230 (3600 MPa and 20.9 %) were much higher than those of DGEBA/D230 (2950 MPa and 12.8 %), and the flexural strength of TEIA/D230 and DGEBA/D230 was simi 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Itaconic acid (IA) was purchased from Zhejiang Guoguang Biochemistry Co., Ltd, China. Allyl bromide, poly(propylene glycol) bis(2-aminopropyl ether) (D230), and hexadecyl trimethyl ammonium bromide (HTAB) were obtained from Aladdin Reagent, China. M-chloroperoxybenzoic acid (m-CPBA) (85 %) and methyl hexahydrophthalic anhydride (MHHPA) were obtained from Sinopharm Chemical Reagent Co., Ltd, China. Diglycidyl ether of bisphenol A (DGEBA) with epoxide equivalent weight of 182–192 g eq1 was purchased from DOW Chemical Company. All the chemicals were used as received.
Preparation of AIA IA (50 g), allyl bromide (187 g), acetone (500 g), N,N-dimethylformamide (100 g), and potassium carbonate (117 g) were placed in a three-necked round-bottomed flask with a magnetic stirrer, a thermometer, and a reflux condenser. After the reactants were mixed vigorously at room temperature for 10 min, the mixture was heated and refluxed for 12 h. After removing the acetone and unreacted ally bromide, the raw product was diluted with dichloromethane and washed by deionized water. AIA was obtained with a yield of 97 % after removing the water and dichloromethane on ChemSusChem 2014, 7, 555 – 562
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CHEMSUSCHEM FULL PAPERS a rotary evaporator. The synthetic scheme is shown in Scheme 1. 1 H NMR (400 MHz, CDCl3): d = 6.37 (s, H), 5.91 (m, 2 H), 5.73 (s, H), 5.28 (m, 4 H), 4.64 (m, 4 H), 3.38 ppm (s, 2 H). 13C NMR (400 MHz, CDCl3): d = 170.3 (1C), 166.7 (1C), 133.7 (1C), 131.9 (2C), 128.7 (1C), 118.3 (2C), 65.6 (2C), 37.7 ppm (1C).
Preparation of TEIA AIA (71 g), dichloromethane (311.7 g), and m-chloroperoxybenzoic acid (m-CPBA, 312 g) was placed in a three-necked round-bottomed flask with a mechanical stirrer, a thermometer, and a reflux condenser. The reactants were mixed and kept at 40 8C for 7 d. The solution was filtered after cooling and washed with a solution of 10 % sodium sulfite followed by 10 % sodium carbonate and distilled water. The organic layer was then dried over anhydrous sodium sulfate. The trifunctional epoxy resin from itaconic acid (TEIA) was obtained with a yield of 60 % after removing the dichloromethane by means of a rotary evaporator. The epoxy value of TEIA determined by the hydrochloric acid–acetone method was about 1.16. The synthetic Scheme is shown in Scheme 1. 1H NMR (400 MHz, CDCl3): d = 4.50 (m, 2 H), 4.00 (m, 2 H), 3.22 (m, 3 H), 3.10 (m, 1 H), 2.93 (d, J = 5.7 Hz, 1 H), 2.82 (m, 3 H), 2.65 ppm (m, 2 H); 13 C NMR (400 MHz, CDCl3): d = 169.1 (2C), 66.0 (3C), 53.4 (1C), 51.8(1C), 48.9 (1C), 44.5 (2C), 37.3 ppm (1C).
Preparation of epoxy networks D230 and MHHPA were used as the curing agents in this study. For the systems using D230 as the curing agent, the predetermined DGEBA or TEIA and a stoichiometric amount of D230 (the molar ratio of N–H to epoxy group was 1:1) were stirred together until a homogeneous mixture was obtained. After that, it was degassed in a vacuum oven at 30 8C for at least 10 min. Then the gas free mixture was poured into a preheated stainless steel mold and cured at 80 8C for 2 h, 150 8C for 2 h, and 180 8C for 2 h to obtain a completely cured resin. For the systems using MHHPA as the curing agent, the necessary amount of DGEBA or TEIA, a stoichiometric amount of MHHPA (the molar ratio of MHHPA to epoxy group was 0.75:1) and 0.1 wt % HTAB (on the basis of the total weight of curing agent and epoxy resin) were stirred together until a homogeneous mixture was obtained. After that, it was degassed in a vacuum oven at 80 8C for at least 10 min. Then the gas free mixture was poured into a preheated stainless steel mould and cured at 150 8C for 2 h, and 190 8C for 2 h to obtain a completely cured resin. Their compositions and codes are listed in Table 1. All the samples were cured under the same conditions.
www.chemsuschem.org TOLEDO, Switzerland) under a nitrogen atmosphere. A heat scan ranging from 25 to 225 8C was performed at heating rates of 2.5, 5, 8, and 10 8C min1. The curves of heat flow as a function of time were recorded for activation energy analysis. The cured epoxy resins were cut into small pieces and about 8 mg of sample was used for the Tg measurement. It was heated to 200 8C and held there for 3 min to eliminate any thermal history. Then, it was cooled to 25 8C at a cooling rate of 50 8C min1, followed by heating again to 200 8C at a rate of 20 8C min1. The Tg was obtained from the peak temperature of the differential curve of the second heating curve of the cured epoxy resins. TGA was performed on a Mettler-Toledo TGA/DSC1 thermogravimetric analyzer (METTLER TOLEDO, Switzerland) with high purity nitrogen or air as purge gas at a scanning rate of 20 8C min1 from 50 to 800 8C. Flexural properties were evaluated by means of a three-point bending test conducted on an Instron 5567 Electric Universal Testing Machine (Instron, America) with a span length of 48 mm at a cross-head speed of 2 mm min1. Specimens of 60 10 3.5 mm3 were used for this evaluation. The data was obtained from the average of at least five specimens for accuracy.
Acknowledgements The authors acknowledge financial support from Project 51203176 supported by the National Natural Science Foundation of China, Project 51003116 supported by the NSFC, the National Basic Research Program of China (973 Program, 2010CB631100), the China Postdoctoral Science Foundation funded project (2013M540504), the Postdoctoral Science Foundation of Zhejiang province (Bsh1201011) and the Director Funds of Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences (Y20224QF06). Keywords: biomass · epoxidation · itaconic acid · renewable resources · sustainable chemistry [1] [2] [3] [4] [5] [6] [7] [8]
Characterization 1
H NMR and 13C NMR spectroscopy was performed on a 400 MHz AVANCE III Bruker NMR spectrometer (Bruker, Switzerland) with CDCl3 as a solvent. The rheological behavior of the epoxy resin was studied by means of dynamic oscillation using a dynamic analyzer Physica MCR-301 of Rheometric (Anton Paar, Austria) with parallel plate tools. The plate diameter and its gap were 25.0 and 0.30 mm, respectively. The test was performed with a heating rate of 3 8C min1, an angular frequency of 1 Hz, and an initial ramp strain of between 10 % and 0.5 %. The curing temperature ranged from 25 to 200 8C and G’, G’’, and h were measured as a function of temperature. The viscosity of TEIA and DGEBA was measured at 25 8C for 2 min with a shear rate of 5 s1. The non-isothermal curing reaction was monitored on a Mettler–Toledo MET DSC (METTLER 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
C. B. Bucknall, A. H. Gilbert, Polymer 1989, 30, 213 – 217. M. C. Lee, T. H. Ho, C. S. Wang, J. Appl. Polym. Sci. 1996, 62, 217 – 225. S. T. Lin, S. K. Huang, Eur. Polym. J. 1997, 33, 365 – 373. J. M. Raquez, M. Delglise, M. F. Lacrampe, P. Krawczak, Prog. Polym. Sci. 2010, 35, 487 – 509. S. Flint, T. Markle, S. Thompson, E. Wallace, J. Environ. Manage. 2012, 104, 19 – 34. M. Chrysanthos, J. Galy, J.-P. Pascault, Polymer 2011, 52, 3611 – 3620. R. Hçfer, M. Selig in Polymer Science: A Comprehensive Reference (Eds.: M. Krzysztof, M. Martin), Elsevier, Amsterdam, 2012, pp. 5 – 14. R. Hçfer, Sustainable Solutions for Modern Economies, RSC Publ., Cambridge, 2009, pp. 1 – 11. H. Miyagawa, A. K. Mohanty, M. Misra, L. T. Drzal, Macromol. Mater. Eng. 2004, 289, 629 – 635. H. Miyagawa, M. Misra, L. T. Drzal, A. K. Mohanty, Polym. Eng. Sci. 2005, 45, 487 – 495. A. P. Gupta, S. Ahmad, A. Dev, Polym. Eng. Sci. 2011, 51, 1087 – 1091. V. Fombuena, L. Sanchez-Nacher, M. D. Samper, D. Juarez, R. Balart, J. Am. Oil Chem. Soc. 2013, 90, 449 – 457. M. Stemmelen, F. Pessel, V. Lapinte, S. Caillol, J. P. Habas, J. J. Robin, J. Polym. Sci. Part A J. Polym. Sci. Pol. Chem. 2011, 49, 2434 – 2444. K. Hofmann, W. Glasser, Macromol. Chem. Phys. 1994, 195, 65 – 80. K. Hofmann, W. G. Glasser, J. Adhes. 1993, 40, 229 – 241. N. E. El Mansouri, Q. Yuan, F. Huang, BioRes. 2011, 6, 2492 – 2503. N. E. El Mansouri, Q. Yuan, F. Huang, BioRes. 2011, 6, 2647 – 2662. T. Koike, Polym. Eng. Sci. 2012, 52, 701 – 717. X. Q. Liu, J. W. Zhang, Polym. Int. 2010, 59, 607 – 609.
ChemSusChem 2014, 7, 555 – 562
561
CHEMSUSCHEM FULL PAPERS [20] X. Q. Liu, W. Huang, Y. H. Jiang, J. Zhu, C. Z. Zhang, Express Polym. Lett. 2012, 6, 293 – 298. [21] L. L. Deng, M. M. Shen, J. Yu, K. Wu, C. Y. Ha, Ind. Eng. Chem. Res. 2012, 51, 8178 – 8184. [22] X. Pan, D. C. Webster, Macromol. Rapid Commun. 2011, 32, 1324 – 1330. [23] X. Pan, P. Sengupta, D. C. Webster, Green Chem. 2011, 13, 965 – 975. [24] X. Pan, P. Sengupta, D. C. Webster, Biomacromolecules 2011, 12, 2416 – 2428. [25] S. Q. Ma, X. Q. Liu, Y. H. Jiang, Z. B. Tang, C. Z. Zhang, J. Zhu, Green Chem. 2013, 15, 245 – 254. [26] M. Shibata, K. Nakai, J. Polym. Sci. Part B 2010, 48, 425 – 433. [27] Y. Takada, K. Shinbo, Y. Someya, M. Shibata, J. Appl. Polym. Sci. 2009, 113, 479 – 484. [28] C. Aouf, H. Nouailhas, M. Fache, S. Caillol, B. Boutevin, H. Fulcrand, Eur. Polym. J. 2013, DOI:10.1016/j.eurpolymj.2012.1011.1025. [29] C. Aouf, J. Lecomte, P. Villeneuve, E. Dubreucq, H. Fulcrand, Green Chem. 2012, 14, 2328 – 2336. [30] T. Werpy, G. Petersen, Top Value Added Chemicals from Biomass. Volume I—Results of Screening for Potential Candidates from Sugars and Synthesis Gas, N. R. E. Laboratory Report DOE/GO-102004-1992, U.S. Department of Energy, 2004.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org [31] Q. Tian, Y. C. Yuan, M. Z. Rong, M. Q. Zhang in Multi-Functional Materials and Structures, Pts 1 and 2, Vol. 47 – 50 (Eds.: A. K. T. Lau, J. Lu, V. K. Varadan, F. K. Chang, J. P. Tu, P. M. Lam), 2008, pp. 290 – 293. [32] P. CaÇamero-Martnez, M. Fernndez-Garca, J. L. de La Fuente, React. Funct. Polym. 2010, 70, 761 – 766. [33] X. Q. Liu, W. B. Xin, J. W. Zhang, Green Chem. 2009, 11, 1018 – 1025. [34] Z. Q. Tao, S. Y. Yang, J. S. Chen, L. Fan, Eur. Polym. J. 2007, 43, 1470 – 1479. [35] H. E. Kissinger, J. Ind. Natl. Bur. Stand. 1956, 57, 217 – 221. [36] T. Ozawa, J. Therm. Anal. 1976, 9, 369 – 373. [37] E. Khosravi, O. M. Musa, Eur. Polym. J. 2011, 47, 465 – 473. [38] Y.-C. Chiu, I. C. Chou, W.-C. Tseng, C.-C. M. Ma, Polym. Degrad. Stab. 2008, 93, 668 – 676. [39] F. H. Liu, Z. G. Wang, Y. L. Wang, B. Zhang, J. Polym. Sci. Part B 2010, 48, 2424 – 2431.
Received: July 24, 2013 Published online on October 17, 2013
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