Efficient Toughening of Epoxy–Anhydride Thermosets with a Biobased

Oct 24, 2016 - Matthew Korey , Gamini P. Mendis , Jeffrey P. Youngblood , John A. Howarter ... Journal of Materials Chemistry A 2017 5 (46), 24299-243...
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Research Article pubs.acs.org/journal/ascecg

Efficient Toughening of Epoxy−Anhydride Thermosets with a Biobased Tannic Acid Derivative Xiaoma Fei, Wei Wei, Fangqiao Zhao, Ye Zhu, Jing Luo, Mingqing Chen, and Xiaoya Liu* The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi, Jiangsu 214122, P. R. China

ACS Sustainable Chem. Eng. 2017.5:596-603. Downloaded from pubs.acs.org by UNIV OF BRITISH COLUMBIA on 07/06/18. For personal use only.

S Supporting Information *

ABSTRACT: Research into toughening an epoxy resin using biobased modifiers without trade-offs in its modulus, mechanical strength, and other properties still remains a challenge. In this article, an approach to toughen epoxy resin with tannic acid, a common polyphenolic compound extracted from plants and microorganisms, is presented. First, dodecane functionalized tannic acid (TA-DD) is prepared and subsequently incorporated into epoxy/anhydride curing system. Owing to the modification of long aliphatic chain, TA-DD can induce epoxy matrix yielding phase separation, forming microscaled separated phases. In the meantime, the terminal hydroxyl groups of TA-DD can participate in the curing process, which offers a good interfacial interaction between TA-DD and epoxy matrix. With such a mechanism, the results show that TA-DD can significantly toughen the epoxy resin without trade-offs in its strength, modulus, and Tg. The thermoset with only 0.5 wt % TA-DD reaches highest impact strength, which is 196% increase of that of neat epoxy. This article opens up the possibility of utilizing the renewable tannic acid as an effective modifier for epoxy resin with good mechanical and thermal properties. KEYWORDS: Tannic acid, Epoxy toughener, Biobased, Hyperbranched

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groups.4 Liu et al. reported a series of TA based UV-curable antibacterial resins. The results showed that the antimicrobial effects of TA greatly depended on the content of phenolic hydroxyl groups. The resin with the highest phenolic hydroxyl groups content exhibited the highest antibacterial activity with 5 log reduction.5 Epoxy resin is one of the most important industrial materials with a long history.1 Owing to its outstanding properties, good processability, and low cost, epoxy resin is widely used as coatings, adhesives, structural composites, and electronic materials. However, its high cross-linking density leads to a relatively low impact resistance, which limits its use in many applications.11−14 Up to now, various modifiers, such as rubber, block copolymer, clay, hyperbranched polymers (HBPs), carbon materials, and thermoplastics have been developed to improve the toughness of the epoxy resin.15−22 What should be pointed out is that the key issue for these approaches is to toughen an epoxy resin without trade-offs in its modulus, mechanical strength, and thermal properties. In addition, developing a highly efficient toughener especially under a low loading amount is still a big problem. Recently, it has been proven that the incorporation of hyperbranched aromatic polyester toughener into epoxy resin

INTRODUCTION In recent years, the fast depletion of petroleum reserves and increasing environmental problems, in addition to the global political toward the principles of sustainable development, have led to a growing interest in the use of biobased sustainable feedstock in the synthesis of biobased chemicals and products. In this context, preparing the partially or full biobased materials is nowadays a real challenge from both academic and industrial points of view.1−3 Tannic acids (TAs) are water-soluble, high-molecular-weight, polyphenolic compounds mostly extracted from plants and microorganisms. The chemical formula for commercial TA is often given as C76H52O46, which corresponds with decagalloyl glucose (Figure S1). As for the aspects of its chemical structure, its architecture is similar to a hyperbranched aromatic polyester core with abundant reactive terminal phenolic hydroxyl groups. Owing to such a specific structure, TA has been widely used in many applications, such as coatings, adsorption and antibacterial materials, and nanomaterials.4−10 Gogoi et al. reported a simple approach to the synthesis of a sustainable and biodegradable waterborne hyperbranched polyurethane (WHPU) using TA as a biobased component. The synthesized WHPU exhibited pronounced thermal and mechanical performance as well as a potent antioxidant activity and cytocompatibility with erythrocytes. Furthermore, WHPU could be degraded by Pseudomonas aeruginosa. As is said above, TA contains abundant terminal phenolic hydroxyl © 2016 American Chemical Society

Received: August 16, 2016 Revised: October 8, 2016 Published: October 24, 2016 596

DOI: 10.1021/acssuschemeng.6b01967 ACS Sustainable Chem. Eng. 2017, 5, 596−603

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Synthesis of TA-DD and TA-DD Modified Epoxy Thermoset

Methylhexahydrophthalic anhydride (MeHHPA) used as anhydride hardener was supplied by Dow Chemicals. All reagents were used without further purification. Synthesis of Dodecane Functionalized Tannic Acid (TA-DD). TA (17 g, 0.01 mol), TPP (0.865 g, 1.5 wt % of 1,2-epoxydodecane), 50 mL of butyl acetate were mixed in a 250 mL three-neck roundbottom flask which was equipped with a thermometer, a dropping funnel, and a cold-water condenser. The reaction system was moved into an oil bath and heated to 100 °C. Then, 22 mL (0.10 mol) of 1,2epoxydodecane was slowly added into the mixture using a dropping funnel under gentle magnetic stirring. The reaction was kept at 100 ◦C for 48 h under N2 atmosphere. Once the reaction was completed, the solution was washed thrice with petroleum ether to remove the unreacted epoxydodecane. The mixture was then precipitated into water twice. Finally, the product was dried in a vacuum oven at 60 °C for 24 h to give a brown waxy product. Preparation of the Curing Mixtures. A desired amount of TADD and 50 g of epoxy monomer were added in a one-necked 250 mL flask. The mixture was heated and kept at 90 °C for 12 h with vigorous stirring until the TA-DD was dissolved and the solution became clear. Then MeHHPA and catalyst (ethyl triphenyl phosphonium bromide, 1 wt % of the total weight) were added to the mixture, followed by the degas process. Finally, the mixture was poured into mold. The samples for thermal and mechanical characterization were cured using the following profile: 80 °C for 1.5 h, 100 °C for 1 h, 120 °C for 1 h, and 140 °C for 2 h. Neat epoxy which contains epoxy resin, MeHHPA and catalyst was prepared following the same procedure as mentioned above. Characterization. Fourier transform infrared (FTIR) spectra were obtained using a Thermo Fisher Scientific Nicolet iS50 spectrometer at room temperature in the wavenumber range of 600−4000 cm−1. 13C NMR was measured using a Bruker Avance 400 MHz spectrometer equipped with autotunable BBO probe. Dynamic rheological measurements were performed on a TA Instruments Discovery DHR-2 rheometer at room temperature. Calorimetric analyses were carried out on a NETZSCH 204 F1 thermal analyzer. The samples of ∼5 mg in weight were placed in aluminum pans under nitrogen atmosphere.

may improve the toughness of epoxy resin by a phase-separated or nonphase-separated mechanism, meanwhile minimizing the deterioration of the Tg and strength.23−26 In this context, TA seems to be an attractive candidate for effective epoxy toughener. We had tried to add TA into epoxy formula without any modification. However, TA is immiscible in epoxy resin, leading to precipitation during curing as a result of intermolecular hydrogen bonds, van der Waals interactions, and π−π stacking of aromatic groups, as seen in ESI (Figure S2). Accordingly, a certain extent of chemical modification of TA is necessary to weaken the intermolecular interaction and improve the miscibility of TA in epoxy matrix. In this work, TA-DD was synthesized by a triphenylphosphine-catalyzed ring-opening reaction between TA and 1,2epoxydodecane, and subsequently used as an toughener for epoxy/anhydride system. Owing to the chemical modification, TA-DD could easily disperse in epoxy resin and induced epoxy matrix yielding phase separation during the curing process, forming microscaled separated phases with good interfacial interaction. The introduction of TA-DD could significantly improve the toughness of cured thermosets with a great increase in impact strength under a very low loading amount and simultaneously improve the Tg and strength. In addition, other thermal properties and toughening mechanisms were also studied.

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EXPERIMENTAL SECTION

Materials. TA and 1,2-epoxydodecane were purchased from Aladdin (Shanghai, China). Triphenylphosphine (TPP) was supplied by Hexion Specialty Chemical Management Co., Ltd. (Shanghai, China). All the organic solvents were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The diglycidyl ether of bisphenol A (DGEBA) with an epoxy equivalent weight of 171−175 g per equiv was supplied by Kukdo Chemical (Kunshan, China). 597

DOI: 10.1021/acssuschemeng.6b01967 ACS Sustainable Chem. Eng. 2017, 5, 596−603

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Figure 1. FT-IR (a) and 13C NMR (b) of TA and TA-DD. Dynamic mechanical analysis (DMA) was conducted on a TA Instruments DMA Q800 at a heating rate of 3 °C min−1 and a frequency of 1 Hz under an air atmosphere. The tests were carried out using the double cantilever mode. Thermal stability was measured using a METTLER TOLEDO 1100SF thermo gravimetric analyzer (TGA) from 50 to 800 °C at a heating rate of 10 K/min under nitrogen. The tensile strengths of the cured hybrids were characterized by an Instron 1185 test machine according to ISO 527:1993. Unnotched impact strength tests were performed on a Ceast Resil impact tester according to ISO 179:1982. For each composition, at least 5 samples were measured. The fracture surfaces from fracture toughness tests were investigated by a HITACHI S-4800 field-emission scanning electron microscope (FESEM). The samples were coated with gold. Atomic force microscopy (AFM) was performed by a Bruker Multimode 8 (Bruker, Germany) using mapping mode. For sample preparation, the uncured resin was first mixed and casted on a clean mica. Then it was cured in oven as the same procedure as preparing thermoset.

confirmed by FTIR, and the results are given in Figure 1a. The band in the spectrum of TA at 1720 and 1200 cm−1 are ascribed to carbonyl (CO) and C−O−C groups, respectively. TA shows a broad hydroxyl band in the region of 2500− 3500 cm−1, which is attributed to the great hydrogen bonding interaction of phenol groups. After modified with epoxydodecane, this broad band decrease and a new OH peak occurred attributed to the hydroxyl groups generated by the by ringopening reaction of epoxy group. In the meantime, The peaks at 2960 and 2880 cm−1 are observed which are attributed to the stretching of CH3 and CH2 from dodecane, respectively. The structure of TA-DD was further confirmed by NMR spectroscopy. Figure 1b showed the 13C NMR spectrum of TADD with the corresponding assignments. The core signals can be appreciated as a complex multiplet in the aromatic zone, which is consistent with that of TA (shown in Figure S1). The methylene and methyl signals of the dodecane unit appear at 13.5, 22.4, and 31.8. These above results were indicative of the reaction between TA and epoxydodecane and successful incorporation of dodecane groups to TA. In addition, owing to the modification of dodecane, TA-DD exhibits improved organic compatibility, which can be dissolved in common organic solvents. Effects of TA-DD on Rheological Properties and Curing Behaviors. Viscosity is very important for the processability of an epoxy resin, so, the rheological behaviors of uncured neat epoxy resin and the epoxy precursors with different TA-DD loadings are shown in Table 1. Typically, when an epoxy resin is toughened by a toughener, its viscosity increases to a great extent. Such a phenomenon is more distinct in the rubber toughen epoxy system. In our case, when TA-DD was added to the formula, all the samples shows similar and relatively low viscosity, although, the viscosity (η) slightly

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RESULTS AND DISCUSSION Preparation and Characterization of TA-DD. TAs are water-soluble, high-molecular-weight, polyphenolic compounds, mostly extracted from plants and microorganisms. TA contains abundant phenolic hydroxyl groups, which could serve as reactive sites and be exploited for the functionalization of TA through various reactions.4−7,27 As we said above, TA is immiscible with epoxy resin and could not be added into epoxy formula without modification. In this work, TA was modified by a TPP-catalyzed ring-opening reaction with 1,2-epoxydodecane, generating dodecyl modified TA (TA-DD). The structure and notation of TA-DD are represented in Scheme 1. The aim of such a strategy is the following: first, introducing an aliphatic chain could weaken the intermolecular interaction with TA, enhancing its miscibility in epoxy resin. Second, long aliphatic chains can induce the phase separation of TA-DD from the epoxy matrix during curing. The separated phase with an adequate size could increase the toughness of the TA-DD modified epoxy thermoset, to a certain extent. Last, TA-DD contains a few unreacted phenolic hydroxyl groups and hydroxyl groups generated by ring-opening reaction of the epoxy group. These hydroxyl groups could take part in the curing process, thus increase the interface interaction between TA-DD and the epoxy matrix. The epoxide equivalent (EEW) of the reaction system first increases with the reaction time and then reaches a stable value of about 15 000 (Figure S3). So the reactions were carried out for 48 h at 100 °C. The chemical structure of TA-DD was first

Table 1. Viscosity and DSC Results of the Neat Epoxy and TA-DD Modified Epoxy Resins sample

DGEBA:TA-DD (weight ratio)

ηa (Pa·s)

Tp (°C)

ΔH (J/g)

NEAT TA-DD0.25 TA-DD0.5 TA-DD1.0 TA-DD2.0

1:0 1:0.05 1:0.1 1:0.2 1:0.4

0.69 0.63 0.68 0.72 0.79

148.3

315.1

147.2 146.3

314.3 303.5

a

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Shear rate = 1.0 s−1 at room temperature. DOI: 10.1021/acssuschemeng.6b01967 ACS Sustainable Chem. Eng. 2017, 5, 596−603

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Figure 2. (a) DSC thermograms of neat epoxy and the TA-DD modified epoxy resins and (b) degree of conversion against temperature of the curing of neat epoxy and the TA-DD modified epoxy resins.

Figure 3. Impact (a) and tensile (b) properties of the neat epoxy and TA-DD modified epoxy resins.

and hydroxyl groups by a complex reaction mechanism. The terminal hydroxyl groups on the TA-DD can react with anhydrides by an esterification, giving rise to ester units and carboxyl groups which can subsequently react with epoxide, forming new ester and hydroxyl. Similar phenomena were also described by other researchers.28,29 What is more, such a mechanism allows the covalent incorporation of the TA-DD through hydroxyl groups to the epoxy matrix and leads to a good interface interaction. Mechanical Properties. It has been reported that hyperbranched tougheners could effectively toughen epoxy resins with a phase separated or an in-suit toughening mechanism.21−24,28−31 The reinforcing and toughening effects of the incorporated TA-DD into system were investigated by impact and tensile test. The results are shown in Figure 3. It can be observed that the impact strength was distinctly improved with the incorporation of TA-DD. Thermoset with 0.5 wt % TA-DD loadings shows the best toughening effect and the impact strength of cured hybrid reaches to 38.8 kJ/m2, which is almost twice higher than that of neat epoxy (13.1 kJ/m2). Such a result shows that TA-DD could effectively toughen DGEBA/ MHHPA system. What should be pointed out here is that the loading amount is relatively low. For general hyperbranched polymer toughen epoxy system, like Bolton toughen epoxy system, the loading amount at maximum of impact strength is usually around 10 wt %.30,31 However, in our study, the impact strength is enhanced with a very low loading amount, as low as 0.5%. Such an efficient toughen effect could be attribute to the specific structure of TA-DD with a rigid aromatic core and flexible aliphatic arms, which induce a microscaled phase separation during curing, as will be discussed with the SEM results. In the meantime, the terminal reactive hydroxyl groups offer enough

increases with the increase of TA-DD loading. The viscosity at room temperature (shear rate = 1.0 s−1) of sample with 2.0% TA-DD loading is still only 790 mPa s. Which seems to be an excellent candidate for toughening epoxy resin without much affecting the processability of the formulation. Such a good mobility could be explained in the following way: (a) The loading amount of TA-DD is relatively low. (b) It has been reported that hyperbranched toughener usually has a low melt viscosity because of its globular structure, which is favor of maintaining low viscosity. (c) The long terminal aliphatic chain could play a role of an inner lubricant, like a wax, enhancing the mobility. The influence of introducing TA-DD on the curing behaviors of DGEBA/MeHHPA system was also studied. Typically, adding nonreactive toughener into epoxy system will decrease the curing activity because of the increased viscosity. As we said above, the viscosity did not increase a lot. Thus, we would see the curing process would not be much affected. Figure 2a shows the DSC exothermic curves of neat epoxy and epoxy resins with different TA-DD loadings. Data of peak temperature and total enthalpy of curing a listed in Table 1. It can be seen that all the samples showed similar curing behavior and curing enthalpy. However, there is a slight difference should be pointed out, that is the peak temperature (given in Table 1) shifts to a lower temperature after adding TA-DD. This means TA-DD has a positive effect on the curing process. A similar result could also be achieved by the conversion against temperature curve (Figure 2b) where the addition of the TADD leads to a slight increase in the conversion at a given temperature. These results reveal that TA-DD could slightly accelerative the curing process. Such an experimental behavior could be explained as following: Anhydrides can react with both epoxide 599

DOI: 10.1021/acssuschemeng.6b01967 ACS Sustainable Chem. Eng. 2017, 5, 596−603

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Table 2. Thermomechanical Properties and Brittleness of the Neat Epoxy and TA-DD Modified Epoxy Resins

a

sample

Tga (°C)

T5% (°C)

Erb (MPa)

ρ (10−3 mol/cm3)

B (% Pa/1010)

NEAT TA-DD0.25 TA-DD0.5 TA-DD1.0 TA-DD2.0

136.8 150.6 148.4 148.1 147.5

373 357 358 365 378

13.9 15.4 15.8 15.9 15.1

1.27 1.31 1.35 1.35 1.28

1.73 1.33 1.42 1..48 1.13

Tg determined using DMA. bStorage modulus at Tg + 30 °C.

in Table 2, all the TA-DD modified epoxy resin showed higher rubbery plateau modulus (Er), the Er was first increased with increasing TA-DD loadings and then decreased at 2.0% TA-DD loading. According to the theory of rubber elasticity, the Er is proportional to average cross-linking density (ρ) of a cured epoxy network. The cross-linking density could be calculated using the equation:

interfacial interaction. As is reported by other researches that compared with the macro-scaled phase separation, the microscaled separated phase is in favor of enhancing modulus and lowing the loading amount.32 In typical epoxy materials, improving toughness is usually accompanied by sacrificing the stiffness. However, tensile test results showed that the tensile strength and Young’s modulus also increased after adding TA-DD. The tensile strength and Young’s modulus of the cured sample with 2.0 wt % TA-DD loadings reachs 73.5 and 2092 MPa respectively, which were 56% increase in tensile strength and 2.5% increase in Young’s modulus with respect to those of neat epoxy (47.1 MPa in tensile strength and 2042 MPa in Young’s modulus). Such increases could be explained by the microphase separation and introducing of rigid aromatic structure. Hagg Lobland et al. recently reported a definition of brittleness (B) and connected B to many other properties such as sliding wear and impact strength.33,34 B can be calculated according to following equation: 1 B= εbE′ (1)

ρ=

Er 3RT

(2)

Where ρ represents the cross-linking density per unit volume (mol cm−3), Er is the rubbery modulus (MPa), R is the gas constant, and T is the absolute temperature at Tg + 30 °C.35 The calculated cross-linking density is also given in Table 2. Theoretically, adding TA-DD may reduce the cross-linking density because the plasticizing effect of the long aliphatic chains. However, samples with different TA-DD loadings show higher cross-linking density than neat epoxy. This is presumably due to the presence of internal branching points in TA-DD structure and also the −OH groups outside of TADD could take part in the curing process, which makes the TADD as a chemical cross-linking points. The tan δ plot against temperature for all the sample with different TA-DD loadings are represented in Figure 4. The peak of the tan δ versus temperature curve is considered as Tg. It can be seen, all the TA-DD modified epoxy thermosets showed higher Tg than that of neat epoxy thermoset. As the TA-DD loading increases, the Tg of TA-DD modified epoxy thermosets decrease slightly. Tg was also measured by DSC and the variation of Tg at different TA-DD loadings is similar to the results of DMA. This is an unexpected result, because adding of biobased toughener will generally decrease the Tg.36,37 Although the long aliphatic chains in TA-DD have a plasticizing effect, the enhanced Tg in this study may could be explained as following: (a) The rigid inner aromatic core as well as the ester group of TA-DD can limit the chain mobility. (b) As is said above, TA-DD contains some unreacted phenolic hydroxyl group and hydroxyl groups generated by the ring-opening reaction of epoxide group. These groups may take part in the curing process and increase the cross-linking density to some degree.38 (c) Owing to the ester and hydroxyl groups, some hydrogen bonds may be formed between TA-DD and other oxygen-containing functional groups from the epoxy matrix.39 As for increase the TA-DD loading, Tg slightly decrease. This may be due to that at low TA-DD loading, the size of TA-DD phase is small, which shows a higher internal cohesion between the epoxy matrix and TA-DD. But at high TA-DD loading, the size of TA-DD phase increase as will be discussed in SEM section. This may lead to a decrease in cross-linking density and enhance the plasticizing effect. Thus, the Tg decrease. Toughening Mechanism. The toughness behavior of neat and TA-DD modified DGEBA/MeHHPA thermosets can be explained in terms of morphology observed by SEM. Figure 5

Where εb is the tensile elongation at break and E′ is the storage modulus as determined by DMA. Here, B of neat epoxy and TA-DD modified epoxy resin were calculated and given in Table 2. It can be seen, TA-DD modified epoxy resin showed lower B than neat epoxy, which is consistent with the results of impact strength testing, meaning that TA-DD can efficiently toughen the epoxy resin. All the impact and tensile tests indicated that TA-DD could simultaneously improve toughness and stiffness and serve as an all-purpose toughener. Dynamic Mechanical Properties. The dynamic mechanical behaviors of neat and TA-DD modified epoxy were measured to further realize the toughening and reinforcing effects. The results are given in Figure 4 and Table 2. As shown

Figure 4. tan δ versus temperature for the neat epoxy and the TA-DD modified epoxy resins. 600

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Figure 7. TGA curves of the neat epoxy and TA-DD modified epoxy resins. Figure 5. SEM images of impact fracture surfaces: (a and b) neat epoxy; (c and d) TA-DD modified epoxy resins.

thermogravimetric for the thermosets with different TA-DD loadings and the data are collected in Table 2. As we can see, both neat epoxy and TA-DD modified epoxy show only one degradation stage and without any apparent differences in thermal stability, indicating that a homogeneous structure of the matrix is formed and the breakage of bonds in the network structure occurs simultaneously. The only differences in the thermal stability can be observed in the initial degradation temperature (T5%) which is given in Table 1. It can be seen, compared to that of neat epoxy (373 °C), sample with 0.25% TA-DD loading shows a lower onset degradation temperature (357 °C). This is due to the lower thermal stability of ester groups and aliphatic chain, which are more degradable. The T5% of TA-DD modified epoxy increases with further increasing the content of TA-DD. The sample with 2.0% TA-DD loading shows an initial decomposition temperature at (378 °C) which is a little higher than that of neat epoxy. This fact can be attributed to the high content of aromatic components in TADD and the hydroxyl groups of the TA-DD provide miscibility during cure and allow reaction with other network functional groups, which in turn produces greater adhesion to the matrix and may prevent the elimination of volatile fragments.

presents the SEM micrographs of impact fractured surfaces of the thermosets prepared. First of all, the fracture surface of the neat epoxy exhibits a characteristic epoxy fracture−surface morphology, with minimal deformation observed from the smooth surface pattern, which can account for the low impact strength. For the TA-DD modified epoxy thermosets, it can be seen that the spherical domains of the TA-DD were formed and dispersed within the continuous epoxy resin phase, which indicates that phase separation occurred during the curing process. The size of these separation phases is relatively uniform. Uniform distribution of the separation phase throughout the matrix is very important for toughening. It allows the yielding process to operate throughout the matrix.40 On the other hand, the TA-DD phase did not undergo cavitations, which suggested a good adhesion between TA-DD and the epoxy matrix. Such a good interface interaction could efficiently transfer the stress between the TA-DD and epoxy matrix, induce the plastic deformation and crack deflection, which is benefit for toughness.41,42 In addition, the AFM images of the neat epoxy and epoxy resin with 2.0 wt % TA-DD loadings are shown in Figure 6. In the same way as in the SEM images, a microphase separation of TA-DD in the epoxy matrix is observed. The size of the TA-DD domains is about 100−200 nm. Such a microphase separation is benefit to enhancing the toughness. Thermogravimetric Analysis. The thermal stability of the thermosets was studied by TGA. Figure 7 shows the

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CONCLUSION The present study reported an approach to synthesize an efficient epoxy toughener using TA as a biobased feedstock. TA-DD was synthesized by a triphenylphosphine-catalyzed ring-opening reaction between TA and 1,2-epoxydodecane. The obtained TA-DD could be easily dispersed in epoxy resin and then was used as a modifier for epoxy/anhydride system. Owing to the chemical modification, TA-DD could induce epoxy matrix yielding phase separation during the curing process, forming microscaled separated phases. In the meantime, the terminal hydroxyl groups of TA-DD react with epoxy matrix, which offers a good interfacial interaction. With such a mechanism, TA-DD could efficiently toughen the epoxy resin under a low loading amount. The results showed the impact strength of the thermoset with 0.5 wt % TA-DD reached 38.8 kJ/m2, which is almost two times higher than that of neat epoxy (13.1 kJ/m2). What is more, introducing TA-DD into the epoxy matrix could simultaneously enhance modulus, mechanical strength, and thermal properties. We believe that this effective toughener coming from low-cost, renewable tannic acid has considerable potential for epoxy thermosets with high performance.

Figure 6. AFM mages of (a) neat epoxy and (b) the TA-DD modified epoxy resins. 601

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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.6b01967. 13 C NMR spectrum of TA; picture of TA modified epoxy thermosets with different loadings and change in the EEW of the reaction mixture as a function of time for the synthesis of TA-DD (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the EnterpriseUniversity-Research Prospective Program, Jiangsu Province (BY2013015-08 and BY2015019-08) and MOE & SAFEA for the 111 Project (B13025).

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DOI: 10.1021/acssuschemeng.6b01967 ACS Sustainable Chem. Eng. 2017, 5, 596−603