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High Performing and Fire Resistant Biobased Epoxy Resin from Renewable Sources Jinyue Dai, Yunyan Peng, Na Teng, Yuan Liu, Chuanchuan Liu, Xiaobin Shen, Sakil Mahmud, Jin Zhu, and Xiaoqing Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00439 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018
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ACS Sustainable Chemistry & Engineering
High Performing and Fire Resistant Bio-based
1
Epoxy Resin from Renewable Sources
2 3 4
Jinyue Dai 1,2, Yunyan Peng 1,2, Na Teng 1,3, Yuan Liu 1,2, Chuanchuan Liu1,2,
5
Xiaobin Shen 1, 2, Sakil Mahmud 1,2, Jin Zhu 1,3, Xiaoqing Liu 1,3*
6 7 1
8
Ningbo Institute of Materials Technology and Engineering, Chinese Academy of
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Sciences, 1219 Zhongguan West Road, Zhenhai District, Ningbo, Zhejiang Province
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315201, People’s Republic of China
11
2
University of Chinese Academy of Sciences, 19 Yuquan Road, Shijingshan District, Beijing, People’s Republic of China
12 13
3
Key Laboratory of Bio-based Polymeric Materials of Zhejiang Province, 1219
14
Zhongguan West Road, Zhenhai District, Ningbo, Zhejiang Province 315201,
15
People’s Republic of China
16
Corresponding Author: Dr. Xiaoqing Liu, E-mail: liuxq@nimte.ac.cn;
17
北京市石景山区玉泉路 19 号
18 19 20 21 22
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ABSTRACT
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Epoxy resins with high thermal and mechanical performance as well as good
25
resistance to fire are difficult to synthesize. In this work, a high performance
26
intrinsically flame retardant epoxy resin (diglycidyl ether of daidzein: DGED) was
27
synthesized from renewable daidzein using an efficient one-step process, without the
28
addition of additional flame retardants. The structures of DGED were confirmed by
29
FTIR, 1H-NMR and 13C-NMR before it was cured with 4, 4'-diaminodiphenylmethane
30
(DDM). A commercial diglycidyl ether of bisphenol A (DGEBA) was cured with the
31
same curing agent. Results indicated that the cured DGED/DDM system possessed
32
glass transition temperature (Tg) of up to 205 °C (172 °C for DGEBA/DDM), the
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tensile strength, tensile modulus, flexural strength and flexural modulus of 83, 2972,
34
131 and 2980 MPa respectively, all much higher than those of cured DGEBA/DDM.
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The cured DGED/DDM system demonstrated excellent flame-retardant properties,
36
showing a residual char of 42.9% at 800 °C, the limiting oxygen index (LOI) of 31.6 %
37
and flammability rating of V-0 in UL94 test. This work provides us an efficient
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method to prepare high-performance epoxy resin from renewable resource.
39 40
KEYWORDS: Bio-based, Epoxy resins, High performance, Flame retardant
41 42 43 44
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INTRODUCTION
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Epoxy is one of the most versatile thermosetting resins, which plays a significant role
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in the fields of electronics, 1 aerospace, 2 adhesive 3 and coatings
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statistics, the global annual production of epoxy has reached 3 million tons by 2017 6
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and it will keep rapid growth in the future. Nowadays, most of the commercially
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available epoxy resins are derived from petroleum resources and among them,
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DGEBA is the predominant one (more than 90%).7 The main raw material, bisphenol
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A (BPA), accounting for more than 67% of the molar mass of DGEBA, is suspected to
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be an endocrine disrupting and nephrotoxic compound
54
products have already been banned in the food packaging industry in some European
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Union countries, 10 considering the risk of free BPA releasing. Therefore, there is an
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urgent demand to explore new compounds to replace BPA for the synthesis of epoxy.
8, 9
4,5
. According to the
and the BPA-based
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In recent years, with the expanding shortage of petroleum resources and
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exacerbating environmental influence, much more attention has been paid on the
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epoxy resins derived from renewable resources. 11-13 Up to now, numerous bio-based
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compounds, including diphenolic acid, 14 plant oils, 15 cardanol, 16-18 lignin, 19, 20 rosin
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acid,
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5-furandicarboxylic acid (FDCA), 32, 33 2, 5-furandimethanol
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36-38
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compared with the commercial epoxy resin DGEBA, the comprehensive
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performances of many bio-based epoxy resins still need further improvement; great
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progress on the synthesis of epoxy from bio-based feedstock has been made. For
21,
22
isosorbide,
23,
24
eugenol,
25-30
4-hydroxybenzoic 34, 35
acid,
31
2,
and itaconic acid
have been employed as the starting materials for epoxy preparation. Although,
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example, the epoxy resins derived from a diphenolic acid, 14 rosin acid 21 and FDCA 32
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could demonstrate better mechanical properties when compared with DGEBA. Using the
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multifunctional groups of eugenol 25-30 and itaconic acid, 36-38 the bio-based epoxy resins
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with unique properties have been reported. And in addition, the epoxy derived from plant
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oil 15 and cardanol
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though the platform building block selection and structure design, the synthesis of
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bio-based epoxy with satisfied thermal or mechanical properties could be realized.
16-18
were usually used as bio-based toughening agents. In one word,
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However, like the traditional petroleum-based epoxy, the epoxy resins derived from
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biomass are also highly flammable and a large number of toxic smoke will release out
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during the combustion. How to improve the flame retardant property of bio-based
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epoxy is also a significant subject, while the research in this field is far from enough.
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As we know, the most effective way to improve the flame retardancy of epoxy resin is
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to embed intrinsic flame retardant elements, such as the halogen atoms, silicon unit,
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and phosphorus-containing groups into their chemical architectures or blending with
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fire retardants. 39-45 After some of the halogenated flame retardants were banned by
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European Union because of the corrosive and toxic gases during combustion,
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silicon and phosphorus-based flame retardants have attracted more and more attention
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due to their low toxicity and high flame-retardant efficiency. 40, 41 In Karak’ work, 42 a
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bio-based flame retardant epoxy was prepared via blending vegetable oil-based epoxy
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with inorganic clay, and the cured resin showed self-extinguishing property. However,
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bad compatibility and poor processability were noticed when the content of clay was
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increased to meet the flame-retardant requirement. Deng et al
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39
synthesized a series
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of rosin-based siloxane epoxy resins; good flame retardancy was achieved by
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sacrificing their thermal properties. For the introduction of the phosphorus-containing
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group, the glass transition temperature (Tg) of cured resin is usually decreased due to
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the lower cross-link density. 44, 45 For example, David and his coworker 44 reported a
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phosphorus-containing bio-based epoxy monomer (P2EP1P) from phloroglucinol and
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triethyl phosphite. 44 In Gao’s group, a diphenolic acid-based epoxy resin containing
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the phosphorous group (PCDGEDP) was prepared and the cured PCDGEDP
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demonstrated LOI of 29.6 % and the flammability rating of V-0 in UL94 test.
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However, the Tg of this DDM cured system was only 127 °C, relatively lower than the
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phosphorous-free counterparts. Based on these literature researches, it was easy to
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find that the epoxy resins combining high thermal and mechanical performance as
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well as excellent flame resistance were difficult to be achieved via a simple and
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efficient synthesis strategy, neither from the petroleum resource nor form the
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bio-based feedstock, especially for the pure hydrocarbon systems. Usually, the
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introduction of anti-flammable additives was inevitable, which would negatively
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affect the processability, thermal or mechanical properties of resulted resins. In
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addition, the synthetic route was sometimes complicated and the consequent
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environmental impact could not be ignored. 44, 45
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Numerous facts have proved that utilizing the structural diversity of renewable
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feedstock to prepare the bio-based materials with surprising properties is worthy of
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expectation. Daidzein (7-hydroxy-3-(4-hydroxyphenyl)-4H-chromen-4-one) is a
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natural polyphenol monomer, which can be obtained from soybeans. 46 It has been
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regarded as a healthy product and widely used in the food and pharmacy industry
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because of its various biological effects including lowering cholesterol, preventing
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cardiovascular diseases and reducing the risk of cancers. 47 In scheme 1, the chemical
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structure of daidzein is illustrated, which has a certain similarity to those of 4,
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4’-bishydroxydeoxybenzoin (BHDB) and BPA. In previous literature,
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resistances of BHDB-containing polymeric materials have been reported, and their
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high char formation and low heat release rates are attributed to the presence of the
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deoxybenzoin structure. However, BHDB is usually prepared by the demethylation of
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desoxyanisoin in the harsh process condition. It is noted that the deoxybenzoin
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structures also existed in daidzein. Inspired by this fact, the DGED was synthesized
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from daidzein and epichlorohydrin in this work. For comparison, the commercial
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DGEBA was taken as a control and cured with the same curing agent, DDM. The
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thermal and mechanical properties, especially the flame retardant properties of cured
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resins were studied thoroughly. The first and foremost aim of this research is to ensure
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us a new strategy for the preparation of high-performance epoxy resin combining
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outstanding flame retardancy and excellent thermo-mechanical properties from a
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renewable resource using an efficient one-step process, without the addition of any
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additional flame-resistant elements.
48-53
129 130
Scheme 1 The chemical structure of daidzein, BHDB, and BPA
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EXPERIMENTAL SECTION
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Materials. Daidzein, 4, 4-diaminodiphenylmethane (DDM), tetrabutylammonium
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bromide (TBAB) and epichlorohydrin (ECH) were purchased from Aladdin-reagent
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Co., China. Dichloromethane, magnesium sulfate and sodium hydroxide were
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obtained from Sinopharm Chemical Reagent Co., Ltd., China. Epoxy resin (DGEBA,
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trade name DER331, epoxy value about 0.53) was supplied by DOW Chemical
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Company. The above chemicals were all used as received from commercial suppliers.
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Synthesis of diglycidyl ether of daidzein (DGED). Daidzein (20 g, 0.079 mol)
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was dissolved in epichlorohydrin (73 g, 0.79 mol) with TBAB (0.93 g, 1 wt % of the
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mixture) in a 500ml three-necked flask equipped with a magnetic stirrer, a reflux
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condenser and constant pressure dropping funnel. After the mixture was agitated at
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100 °C for 2 h, therefor it was passed through a cooling operation to room
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temperature and 16 g of 50 wt% aqueous sodium hydroxide solution (0.40 mol NaOH)
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was added drop wise into the flask and further reacted for another 5 h. The double
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extraction of the mixture was done with distilled water after diluting with
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dichloromethane and dried over by magnesium sulfate. The organic layer was
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concentrated in vacuum to remove the dichloromethane and residual epichlorohydrin.
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The obtained light yellow solid product DGED was dried at 60 °C for 12 h in a
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vacuum oven and its total yield was calculated to be 88.5%. The synthetic route as
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illustrated in Figure S1.
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FT-IR (KBr, cm−1): 1626 (C-C); 1598 (C=O); 1410, 1510 (benzene ring); 1030, 912
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(oxirane ring).
154
1
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(d, H), 6.97, 6.95 (d, 2H), 6.86, 6.85 (d, H), 4.97-3.93 (m, 4H), 3,38-3.35 (m, 4H),
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2.94-2.88 (m, 2H), 2.79-2.75 (m, 2H).
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13
158
152.2 (s, 1C), 130.1 (s, 1C), 127.8 (s, 1C), 124.7, 124.6 (d, 2C), 118.6 (s, 1C), 114.7
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(s, 1C), 114.6 (s, 1C), 101.1 (s, 1C), 69.2 (s, 1C), 68.8 (s, 1C), 50.1 (s, 1C), 49.7 (s,
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1C), 44.6 (s, 1C), 44.5 (s, 1C).
H-NMR (CDCl3, δ, ppm): 8.19, 8.16 (d, H), 7.89 (s, H), 7.48, 7.46 (d, 2H), 7.01, 6.99
C-NMR (CDCl3, δ, ppm): 175.7 (s, 1C), 162.7 (s, 1C), 158.4 (s, 1C), 157.7 (s, 1C),
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Formation of the epoxy networks. Epoxy resins (DGEBA or DGED) and curing
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agent (DDM) in the stoichiometric ratio (epoxy group: N-H= 1:1) without solvent was
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well mixed in a beaker at 100°C in an oil bath for about 10 minutes. Then the
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mixtures were quickly transferred to a preheated stainless steel mold at 140°C. After
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that, the thermal curing reaction was performed in an air-circulating oven at the
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temperatures of 140°C for 2h, 160°C for 2h and 180°C for 2h. Finally, the mold was
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slowly cooled down to room temperature and the cured resins were removed carefully
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from the mold and left still at room temperature for 24 h before mechanical and DMA
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testing.
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Measurements. The infrared spectrum (FT-IR) analysis of synthesized epoxy resin
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was performed on a NICOLET 6700 FT-IR using the KBr pellet method with
172
transmittance mode. The spectra were recorded ranging from 400 to 4000 cm-1 with a
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resolution of 4 cm-1 at room temperature. For experiment accuracy, each sample was
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scanned for 32 times. 1H-NMR and
13
C-NMR spectra were recorded with a Bruker
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AVANCE III 400MHz NMR spectrometer using TMS (Tetramethyl silane) as the
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internal standard. The measurement was performed at room temperature and CDCl3
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was used as a solvent. The curing behavior was investigated using a Mettler-Toledo
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MET DSC under a high purity nitrogen atmosphere with a flowing rate of 60 mL/min.
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Each sample weighing approximately 10 mg was sealed in an aluminum crucible and
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then heated from 25 to 250 °C with multiple heating rates of 5, 10, 15 and 20 °C/min.
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All the heating curves were recorded for curing reaction analysis. Dynamic
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Mechanical Analysis (DMA) tests were carried on a TA Instrument (TA Q800) in a
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tension fixture mode. All the samples with the dimension of 20 mm × 5 mm × 0.5 mm
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were tested from 0 to 250 °C at a heating rate of 3 °C/min and a frequency of 1 Hz.
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For accuracy, each sample was tested for five times. Thermogravimetric analysis
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(TGA) w carried out on a Mettler-Toledo TGA. All the samples were heated from 50
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to 800 °C with a heating rate of 20 °C/min under nitrogen and air atmospheres,
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respectively. The mechanical properties of cured resins were evaluated using a
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Universal Mechanical Testing Machine (Instron 5569A). The crosshead speed is 5
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mm/min for tensile properties and 2 mm/min for flexural properties. All the samples
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with the dimension of 60 mm × 10 mm × 3.5 mm were conditioned at room
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temperature for 24 h with the relative humidity of 50% before testing. The average of
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at least five measurements of each sample was taken to report the tensile or flexural
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properties. Limiting oxygen index (LOI) was determined by an HC-2 Oxygen Index
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Instrument (Jiangning Analytical Instrument Co. Ltd. China) according to ASTM
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D2863-2008. The torch burning test was similar to the UL-94 procedure. The Bunsen
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burner flame height was set to be about 30 mm, and the size of the sample was 30 mm
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× 10 mm × 1 mm. Vertical burning tests were performed on a CZF-3 Vertical Burning
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Tester (Jiangning Analytical Instrument Co. Ltd. China) following the procedure
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described in ASTM D3801 and the sample size was according to the standard.
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TGA-IR spectra were recorded on a Mettler-Toledo TGA connected with a NICOLET
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6700 FT-IR. The temperature of transferring line between TGA and FT-IR was
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determined to be 200 °C. About 10 mg samples were heated from 50 to 800 °C under
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a nitrogen atmosphere (50 mL/min) at the heating rate of 20 °C/min. FT-IR spectra
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were collected every 40 s during the whole testing process. The isothermal curing
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behavior of epoxy resin was characterized by a dynamic rheological analyzer (Anton
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Paar Physica MCR-301). The diameters of upper and lower plates were 25 mm and 50
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mm, respectively. The measurement was performed at 100 °C/min in a steady shear
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mode, with the frequency of 1 Hz and strain of 0.1%. The morphology of the residual
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char after UL-94 burning was recorded using a scanning electron microscope. All the
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samples for SEM were sputtered with a thin layer of gold before testing. X-ray
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photoelectron spectroscopy (XPS) spectra of cured resins and the char residue were
213
recorded with an AXIS ULTRA apparatus (Kratos, England).
214 215
RESULTS AND DISCUSSION
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Synthesis and Structural characterization of DGED. Figure S1 illustrated the
217
synthesis of DGED. The routine glycidylation reaction between daidzein and excess
218
epichlorohydrin was conducted in the presence of sodium hydroxide. And a small
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quantity of TBAB was employed as the phase transfer catalyst. In addition, the total
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yield was determined to be high up to 88.5%. Those all indicated the simplicity and
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efficiency of this reaction. Before curing reaction, the chemical structure of DGED
222
was confirmed by FT-IR, 1H NMR, and 13C NMR. In the FT-IR spectrum (Figure S2),
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the sharp peaks showing at 1626 cm-1 and 1598 cm-1 corresponded to the vibration of
224
C=C–C=O group 54 and benzene ring, respectively. The absorption band at 1197 cm-1
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indicated the presence of C-O-C in pyrone ring. Specifically, the peaks appeared at
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829, 912, 1027 and 1246 cm-1 evidenced the successful formation of oxirane ring 37.
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As seen from the 1H NMR and 13C NMR spectra of DGED in Figure S3, the chemical
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shift and integral area of all the peaks matched well with the protons and carbons of
229
the predicted chemical structure for DGED. Combining the results from FT-IR, 1H
230
NMR, and
231
good agreement with the predicted one.
13
C NMR, the chemical structure of DGED was confirmed and it was in
232
Curing behaviors investigation. The non- and isothermal curing behaviors of
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DGED/DDM and DGEBA/DDM were investigated by DSC and rheological
234
measurements. As shown in Figure 1 (a) and (b), both the DSC heating curves for
235
DGED/DDM and DGEBA/DDM system displayed a single exothermic peak, which
236
corresponded to the ring-opening reaction between epoxy and amine groups. For the
237
endothermic peaks in the range of 70-80 oC in Figure 1 (a), they were associated with
238
the melting of DGED. It is well known that under the same conditions, the peak
239
temperature in DSC exothermic curve can be taken as an indicator to evaluate the
240
curing reactivity of epoxy resins. The lower the peak temperature is, the higher the
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reactivity is. 36 In Figure 1 (a), the DGED/DDM system respectively showed the peak
242
temperatures of 122.2, 139.2, 147.2 and 156.0 °C, when the heating rate was
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increased from 5 to 20 K/min. While in Figure 1 (b), the peak temperatures under the
244
same heating rates for DGEBA/DDM system were obviously higher than those for
245
DGED/DDM, which indicated the relatively higher curing reactivity of DGED when
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compared with DGEBA, using the same curing agent DDM.
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In order to further evaluate the curing reactivity of DGED/DDM and
248
DGEBA/DDM systems, their viscosities as a function of curing time at 100 °C was
249
investigated by a dynamic rheological analyzer. In Figure 1 (c), the viscosity of
250
DGED/DDM was increased much faster than that of DGEBA/DDM. Usually, the
251
gelation point, at which the steady shear viscosity reaches 103 Pa·s and the resin
252
ceases to flow,
253
Figure1 (c), the gelation points of DGED/DDM and DGEBA/DDM at 100 °C were
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determined to be 12.9 and 28.8 min, respectively. Obviously, the curing reaction of
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DGED was faster than that of DGEBA when DDM was used as curing agent,
256
consistent with the results from non-isothermal DSC analysis.
55
could be taken as an indicator for curing rates comparison. From
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Figure 1. DSC curves for DGED/DDM (a) and DGEBA/DDM (b) at different heating
259
rates; (c) Viscosity as a function of curing time for DGED/DDM and DGEBA/DDM
260
isothermally cured at 100 °C; (d) FT-IR spectrum of DGED/DDM system before and
261
after curing reaction
262 263
According to DSC results, the curing reaction of DGED/DDM and DGEBA/DDM
264
was performed at 140 °C for 2 h, 160 °C for 2 h and 180 °C for 2 h, which was a
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normal curing procedure for epoxy resins. Figure 1 (d) represents the FT-IR spectra of
266
DGED/DDM before and after curing reaction. For abbreviation, only the absorption
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bonds between 500 and 1800 cm−1 were shown. Before curing reaction, the
268
characteristic absorption bonds for oxirane ring showing at 912, 1027 and 1256 cm−1
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were noticed. And the peaks appeared at 862 and 1631 cm−1 were assigned to the C=C
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in benzopyrone cycle. After curing reaction, they were all disappeared, which
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indicated the consumption of epoxy groups and carbon-carbon double bonds. As
272
reported in previous literature, 56 the coumarin ring, similar to that of benzopyrone
273
ring could take the cycloaddition reaction and the dimerization was confirmed by
274
FT-IR and NMR. Therefore, it was easy to figure out that, besides the ring opening
275
reaction of oxirane ring, the C=C in benzopyrone cycle could also participate in the
276
crosslinking reaction in the DGED/DDM system and the proposed structures were
277
illustrated in Scheme 2, which would increase its crosslink density dramatically.
278
279 280
Scheme 2. Proposed crosslinked structure of DGED/DDM after curing reaction
281 282
Dynamic
mechanical
properties.
The
viscoelastic
properties of
cured
283
DGED/DDM and DGEBA/DDM were studied by DMA, respectively. Figure 2
284
presents their storage modulus and tan δ as a function of temperature and the detailed
285
values for storage modulus and glass transition temperature (Tg) were summarized in
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Table 1. As seen in Figure 2, the cured DGED/DDM exhibited higher storage
287
modulus (E′) than that of DGEBA/DDM system over the entire experimental
288
temperature range. At 25 °C, DGED/DDM showed the E′ of 2778 MPa and it was
289
2398 MPa for DGEBA/DDM. When the temperature was increased up to 100 °C, the
290
E′ of DGEBA/DDM was decreased to 1844 MPa, while the DGED/DDM
291
demonstrated the higher E′ of 2169 MPa. That indicated the higher stiffness of
292
DGED/DDM compared with DGEBA/DDM.
293
The glass transition temperature (Tg) is a very important parameter that determines
294
the application filed for thermosetting resins. In Figure 2, the cured DGED/DDM and
295
DGEBA/DDM respectively showed Tg of 205 and 172 °C, which was determined by
296
the peak temperature of α-transition observed in the tan δ vs temperature curve. In
297
addition, the peak width at half-height of tan δ for DGED/DDM was broader than that
298
of DGEBA/DDM, indicating the lower segmental mobility in DGED/DDM. That was
299
consistent with its relatively higher Tg. Based on the literature research, 14, 18, 21, 23, 27, 32,
300
33, 37
301
Wang's work, 27 a eugenol-based epoxy monomer with three functional groups was
302
synthesized and cured with 3,3’-diaminodiphenyl sulfone (DDS). The resultant resin
303
showed the Tg of 207 °C and the authors believed that it was the highest one among
304
the already reported epoxy resins derived from a renewable resource, in which the
305
bio-based content was higher than 60 wt%. It is true because the more bio-based
306
component usually leads to the decreased thermomechanical properties and the
307
bio-based epoxy with Tg higher than 200 °C was seldom reported. In this work, the
205 °C is claimed to be an extremely high Tg for the bio-based epoxy resins. In
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308
two functional DGED was cured by DDM (much softer than DDS) and the bio-based
309
content of the cured product was calculated to be high up to 69 wt%. This result
310
strongly supported the outstanding thermal properties of DGED.
311
Generally speaking, Tg corresponds to the segmental mobility of polymer networks
312
and it is mainly affected by the crosslink density (νe) of cured resins and monomer
313
architectures.
314
higher Tg. Based on the rubber elasticity theory; the νe could be obtained from the
315
plateau of the elastic modulus in the rubbery state based on the equation (1): 37-38
316
35
The higher crosslink density and the more rigid segment will lead to
= ⁄3
(1)
317
Where E′ is the storage modulus of the cured resins in the rubbery plateau region, T
318
is the absolute temperature, as well as R, is the gas constant. The νe of cured
319
DGED/DDM was calculated to be 6.4 mol m−3, much higher than that of
320
DGEBA/DDM (3.5 mol m−3). This result supported the higher storage modulus and
321
Tg of DGED/DDM. In Scheme 2, the construction of a crosslinked structure in cured
322
DGED/DDM was illustrated. And it was the unsaturated double bond in DGED that
323
provided additional crosslinking points and then led to the increased νe in the cured
324
resin. In addition, compared with DGEBA, the molecular rigidity of DGED was
325
absolutely higher, which also enhanced the Tg.
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326
Figure 2. DMA curves for cured DGED/DDM and DGEBA/DDM
327 328 329
Table 1. Storage modulus at different temperatures, glass transition temperature and
330
crosslink density of cured DGED/DDM and DGEBA/DDM system E’a (MPa)
E’b (MPa)
Tg
Width of tan δ peaks
νe/103
@ 25 °C
@ 100 °C
(oC)
(half height) (oC)
(mol m−3)
DGED/DDM
2778
2169
205
27
6.4
DGEBA/DDM
2398
1844
172
13
3.5
Samples
331 332
Mechanical properties. Figure 3 (a) shows the tensile and flexural properties
333
comparison between the cured DGED/DDM and DGEBA/DDM. It was observed that
334
the DGED/DDM system showed average tensile strength and modulus of 83.0MPa
335
and 2972.6 MPa, respectively. While the DGEBA/DDM system possessed the
336
relatively lower tensile strength of 73.4 MPa and modulus of 2374.8 MPa were found.
337
For the flexural properties, the flexural strength and modulus of DGED/DDM were 31%
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338
and 27% higher than those of DGEBA/DDM, 131.8 MPa vs 100.6 MPa for strength
339
and 2980.1 MPa vs 2340.4 MPa for modulus. The thermal and mechanical properties
340
of thermosetting resins have a close tie with their crosslink density and molecular
341
rigidity. As discussed above, the crosslink density and molecular segmental rigidity of
342
DGED were much higher than that of DGEBA/DDM, which was responsible for their
343
higher mechanical properties.
344
Excellent mechanical properties combining with high Tg might be the common goal
345
for bio-based epoxy design and synthesis, especially in the high-value application
346
filed. In Figure 3 (b), the literature reported bio-based epoxy resins possessing
347
relatively high Tg and mechanical properties were collected. It was noted that starting
348
from the varied renewable resource, the bio-based epoxy systems demonstrating Tg in
349
the range of 50 to 190 oC and flexural/tensile strength ranged from 20 to 140 MPa
350
were extensively synthesized. As we know, besides the chemical structures of epoxy
351
resin, the curing agent also plays a significant role in determining the properties of
352
cured resins. Although the curing agents in these systems were different from each
353
other, varied from soft D230 to rigid IPDA or MHHPA, the cured resins
354
simultaneously demonstrating Tg higher than 200 oC and strength higher than 120
355
MPa was hardly reported. In this work, the DDM cured DGED system showed the Tg
356
of 205 oC. At the same time, its tensile and flexural strength was high up to 83.0 MPa
357
and 131.8 MPa. In Figure 3 (b), DGED/DDM was located at the upper right corner,
358
which obviously illustrated its better performance.
359
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361 362
Figure 3. Mechanical properties comparison between the cured DGED/DDM and
363
DGEBA/DDM (a) and mechanical properties as well as Tg of DGED/DDM compared
364
with literature’ results (without *: flexural strength, with*: tensile strength).
365 366
Thermal stability of the cured resins. The TGA and DTG curves of cured
367
DGED/DDM and DGEBA/DDM under N2 were shown in Figure 4(a, b) and the
368
related data, including Td5% (temperature at which the 5 % degradation occurred), Td30%
369
(temperature at which the 30 % degradation occurred), Tmax (temperature at which the
370
maximum degradation rate was observed) and R800 (residual char at 800 °C), was
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371
marked. As shown in Figure 4 (a), before 300 °C, no apparent thermal degradation
372
was observed for DGED/DDM and it exhibited the Td5% of 335 °C and Td30% of
373
393 °C, respectively. They were both lower than those of DGEBA/DDM, 384 °C for
374
Td5% and 403 °C for Td30%. In addition, as shown in Figure 4 (b), in the range of
375
350-500 °C, both the cured resins decomposed rapidly and the maximum degradation
376
rate was observed at 383 °C for DGED/DDM and 402 °C for DGEBA/DDM. The
377
lower Td5% and Td30% of DGED/DDM might be attributed to the thermal degradation
378
of pyrone ring in DGED. In Ishida’s work, 57 the thermal degradation occurred below
379
300 °C for coumarin-containing polybenzoxazine was attributed to the labile covalent
380
bond between the aromatic ring and the carbon from the C=C double bond. Although
381
no further details about the thermal degradation mechanism of benzopyrone ring have
382
been reported, the relatively lower thermal degradation temperature for resins
383
containing similar units was often noticed. 58, 59 In Figure 4 (a), DGED/DDM showed
384
the residual char of high up to 42.9 % at 800 °C, three times higher than that of
385
DGEBA/DDM (15.5 %). In fact, such a high char yield for the epoxy systems without
386
any extra flame retardant fillers or elements was hardly reported. Even for the epoxy
387
resins containing phosphorous or silicone moieties, R800 of 42.9 % was also high
388
enough. For instance, Sun and his co-worker
389
phosphazene-based epoxy resin and its residual char at 750 °C was found to be 31 %
390
after curing with DDM. Wang et al
391
vanillin, diamine and diethyl phosphite. And the cured resins showed the char yield of
392
44.7% at 700 °C. As we know, the char yield in nitrogen is related to the flame
61
60
synthesized a novel spirocyclic
synthesized a flame retardant epoxy from
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393
retardancy of thermosets. 62 Usually, the char formation will insulate the polymer-air
394
interface, reduce the heat conduction and starve the combustion process of
395
decomposition products. 53 In previous literature, 48-52 the deoxybenzoin-containing
396
polymers have demonstrated good flame retardant properties due to their high yield of
397
carbonization and low heat release rates. The reason was ascribed to the dehydration
398
of deoxybenzoin moieties at high temperature to form diphenylacetylene, and then
399
carbonization via cyclization and aromatization.
400
The thermal degradation behaviors of cured DGED/DDM and DGEBA/DDM
401
under air were also studied and the related TGA/DTG curves were shown in Figure
402
4(c, d)). Different from the results under nitrogen, both the cured DGED/DDM and
403
DGEBA/DDM showed two thermal degradation stages. In Figure 4(c), the relatively
404
higher carbonization capacity of DGED/DDM was certified by char yield of 64.5% at
405
500 °C. In Figure 4(d), the DGED/DDM system demonstrated the maximum
406
degradation rate temperatures at 332 and 564 °C for the separated two degradation
407
stages, respectively. As for DGEBA/DDM, the maximum degradation rates were
408
observed at 398 and 583 °C, a little higher than those of DGED/DDM. This result
409
indicated the relatively poor thermal stability of DGED/DDM and it might be ignited
410
earlier during the flammability. However, the carbonization capacity of DGED/DDM
411
was higher than that of DGEBA/DDM, which was indicted by its higher char yield of
412
64.5% at 500 °C, while it was only 36.1% for DGEBA/DDM (Figure 4(c)). Based on
413
these results, DGED/DDM might demonstrate good fire resistance, which would be
414
investigated in the following section.
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415
416 417
Figure 4. TGA (a) and DTG (b) curves of the cured DGED/DDM and DGEBA/DDM
418
resins under a nitrogen atmosphere; TGA (c) and DTG (d) curves of the cured
419
DGED/DDM and DGEBA/DDM resins under an air atmosphere
420 421
Flammability of cured resins. In order to visually recognize the burning
422
characteristics of DGED/DDM and DGEBA/DDM, they were applied to the torch
423
burning test at first and Figure 5 demonstrated the typical burning procedures. It was
424
noted that DGEBA/DDM exhibited a high flammability and it was burned out
425
completely in 60 seconds. However, DGED/DDM did not show an aggressive
426
combustion and the flame was much smaller. The surprising characteristic was that
427
the burning DGED/DDM was self-extinguished within 3 seconds after ignition and
428
the surface of residual resin was covered with expanded char layer.
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429 430
Figure 5. Digit photographs of the torch burning test: (a) for DGED/DDM and (b) for
431
DGEBA/DDM
432
For the more detailed investigation, the flame-retardant performance of cured resins
433
was further studied in terms of limit oxygen index (LOI) and UL-94 vertical burning
434
tests. The vertical burning classifications (UL-94) and LOI results were collected in
435
Table 2. Obviously, DGEBA/DDM showed no combustion grade (UL-94 NR) and the
436
LOI
437
flame-retardant grade of UL-94 V-0 rating by the vertical burning test (t1 + t2 = 2.9 s,
438
no dripping rating) and its LOI was as high as 31.4%. This result was similar to the
439
epoxy resins containing a high content of phosphorus or other flame retardant
440
elements.
441
spiro-cyclotriphosphazene and the cured resin was rated as UL-94 V-0, showing the
442
LOI of 32%. Wang
443
high up to 7% and finally obtained the UL-94 V-0 thermosets with the LOI of 32%.
444
As for the cured DGED/DDM in this work, the UL-94 V-0 grade and high LOI were
was
approximate
In
Sun’s
61
24.5%.
work,
60
While
they
DGED/DDM
reported
an
achieved
epoxy
the
derived
highest
from
developed an epoxy monomer with the content of phosphorus
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445
Page 24 of 38
achieved without the addition of any extra flame retardant elements.
446 447
Table 2. The UL-94 rating and LOI of the cured epoxy resins. Samples
LOI (%)
t1 + t2 (s)
UL-94
Flaming drips
DGED/DDM
31.6
2.9
V-0
None
DGEBA/DDM
24.5
---
NR
Yes
448 449
Figure 6 shows the digital photographs of cured DGED/DDM and DGEBA/DDM
450
before and after the UL-94 test. The length of DGEBA/DDM specimen became much
451
shorter after vertical burning because of the severe melt-dripping during combustion.
452
Usually, not only the yield but also the morphology of the residual chars formed
453
during combustion can reflect the flammability characteristics of thermosetting resins
454
to some extent
455
residues after UL-94 test for DGED/DDM and DGEBA/DDM were shown. The
456
difference between Figure 6 (a) and (b) was in evidence. As seen in Figure 6 (a), the
457
integrated and dense char layers were formed on the surface of the residues for
458
DGED/DDM, which would protect the matrix inside and prevent the heat
459
transmission during the combustion process. However, as seen in Figure 6 (b), the
460
loose porous char layer was observed for DGEBA/DDM and this porous structure was
461
difficult to inhibit the heat transfer and flame spread after ignition. This morphology
462
observation was in line with their flammability.
63
. In Figure 6, the SEM images of the outside aspects of the char
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464 465 466
Figure 6.
Digital photographs and SEM images for (a) DGED/DDM and (b) DGEBA/DDM before and after UL-94 test
467 468
XPS technique was applied to determine the chemical components of DGED/DDM
469
and its residual char after the vertical burning test. The XPS spectra were illustrated in
470
Figure S4 and related element contents were collected in Table S1. Obviously, the
471
surface oxygen content (O) was decreased from 25.23 wt% for DGED/DDM to 16.90
472
wt% for the residual char, while the surface nitrogen content (N) was increased
473
slightly from 2.09 wt% to 2.51 wt%. This result implied that a large number of
474
oxygen (O) was consumed and nitrogen (N) was transferred to the surface of the char
475
residues during the combustion. Remarkably, the surface content carbon (C) was
476
increased from 72.68 wt% to 80.59 wt % and the surface C=C and C-C content of
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477
char residue where both higher than those of DGED/DDM resin before combustion.
478
This result supported the excellent char forming ability of DGED/DDM again.
479
In order to get more insight into the thermal degradation of cured resins,
480
TGA-FTIR technology was employed to monitor the gaseous products during the
481
thermal degradation process. Figure 7 displays the 3D image of TGA-FTIR results (a,
482
b) and the total absorptions of the gaseous products (c) for DGED/DDM and
483
DGEBA/DDM resins throughout the whole thermal degradation process, respectively.
484
In comparison, the total absorptions spectra method was more intuitive than the 3D
485
infrared spectra method. Due to the consistency of test specimens, the intensity of the
486
absorption peak can intuitively reflect the quantity of degradation products. 63 Based
487
on Figure 7 (c), it was quite obvious that DGED/DDM released much fewer gaseous
488
products than DGEBA/DDM during the whole thermal degradation process. Figure 7
489
(d) presents the FT-IR spectra of the pyrolysis products of DGED/DDM and
490
DGEBA/DDM at the maximum decomposition rate. As shown in Figure 7 (d), similar
491
gaseous were produced from the cured resins during thermal degradation, indicated by
492
the aromatic alcohols showing characteristic absorption at 3651 cm−1, methane at
493
3015 cm−1, CO2 at 2358 and 2308 cm−1, CO at 2181 cm−1 as well as aromatic
494
compounds at 1604 cm−1. 33 Compared with DGEBA/DDM, the pyrolysis products of
495
DGED/DDM contained fewer organic flammable gases, but more non-flammable gas
496
CO2, which was indicated by the varied intensity of characteristic absorption. It was
497
easy to understand that more incombustible gas would make a contribution to prevent
498
further combustion, and thus led to a better flame retardancy of DGED/DDM.
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499 500
Figure 7. 3D image of TGA-FTIR result for cured DGED/DDM (a); cured
501
DGEBA/DDM (b); total absorptions spectra of the gaseous products for cured
502
DGED/DDM and DGEBA/DDM (c); and the FT-IR spectra of the pyrolysis products
503
of the cured resins at the maximum decomposition rate (d).
504 505
CONCLUSIONS
506
An intrinsically flame retardant bio-based epoxy DGED was successfully synthesized
507
from renewable daidzein via a one-step reaction. After curing reaction, DGED/DDM
508
demonstrated the Tg of up to 205 °C, and the tensile strength, tensile modulus, flexural
509
strength and flexural modulus of 83, 2972, 131 and 2980 MPa respectively, all much
510
higher than those of petroleum-based counterpart DGEBA/DDM. The bio-based
511
epoxy resin possessing such excellent thermal and mechanical properties was seldom
512
reported elsewhere and the reason was ascribed to the high crosslink density of cured
513
DGED caused by the dimerization of benzopyrone ring. More impressively, the cured
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DGED/DDM without the addition of any extra flame retardant elements showed
515
excellent flame-retardant properties, indicated by the limiting oxygen index (LOI) of
516
31.6 % and the flammability rating of V-0 in UL94 test. The excellent char forming
517
ability of DGED/DDM, supported by the char yield of 42.9 % at 800 °C and
518
formation of integrated and dense char layers after combustion, as well as the less
519
organic flammable gases produced during thermal degradation was responsible for its
520
outstanding fire resistance. Daidzein was proved to be an ideal bio-based compound
521
for synthesis of high-performance epoxy. And especially, the excellent flame retardant
522
properties could be achieved without the addition of any flame retardant elements.
523 524
ASSOCIATED CONTENT
525
Supporting Information
526
Synthesis of diglycidyl ether of daidzein (Figure S1); FT-IR and NMR spectra of
527
DGED (Figure S2, S3); XPS scan for the cured DGED/DDM and its char residue
528
(Figure S4); Element content of cured DGED/DDM and its char residue after vertical
529
burning determined by XPS (Table S1).
530 531
NOTES
532
The authors declare no competing financial interest.
533
*Corresponding authors: Xiaoqing Liu
534
*E-mail address: liuxq@nimte.ac.cn
535
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536
ACKNOWLEDGMENTS
537
The authors are grateful for the financial support from National Natural Science
538
Foundation of China (Grant No.51373194), National Key Technology Support
539
Program (2015BAD15B08) and the project co-funded by Chinese MIIT Special
540
Research Plan on Civil Aircraft with the Grant No. MJ-2015-H-G-103.
541 542
REFERENCES
543
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energy storage density of polymer composites. Energy Environ. Sci. 2017, 10 (1),
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nanotubes/polysulfone interleaves. Compos. Sci. Technol. 2017, 140, 8-15.
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(3) Yaphary, Y.; Yu, Z.; Lam, R.; Hui, D.; Lau, D., Molecular dynamics simulations
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on adhesion of epoxy-silica interface in salt environment. Composites, Part B 2017,
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wave-transparent, light-weight composites combined with superior dielectric
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on migration of bisphenol A (BPA) and bisphenol A-diglycidyl ether (BADGE) to
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elastomers derived from bio-based phenolic acids with varying functionality.
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For Table of Contents Use Only
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A bio-based epoxy with high performance and excellent fire resistance without extra
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