High Performing and Fire Resistant Bio-based Epoxy Resin from

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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

9

Sciences, 1219 Zhongguan West Road, Zhenhai District, Ningbo, Zhejiang Province

10

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: [email protected];

17

北京市石景山区玉泉路 19 号

18 19 20 21 22

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

24

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

33

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.

35

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

38

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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45

INTRODUCTION

46

Epoxy is one of the most versatile thermosetting resins, which plays a significant role

47

in the fields of electronics, 1 aerospace, 2 adhesive 3 and coatings

48

statistics, the global annual production of epoxy has reached 3 million tons by 2017 6

49

and it will keep rapid growth in the future. Nowadays, most of the commercially

50

available epoxy resins are derived from petroleum resources and among them,

51

DGEBA is the predominant one (more than 90%).7 The main raw material, bisphenol

52

A (BPA), accounting for more than 67% of the molar mass of DGEBA, is suspected to

53

be an endocrine disrupting and nephrotoxic compound

54

products have already been banned in the food packaging industry in some European

55

Union countries, 10 considering the risk of free BPA releasing. Therefore, there is an

56

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

57

In recent years, with the expanding shortage of petroleum resources and

58

exacerbating environmental influence, much more attention has been paid on the

59

epoxy resins derived from renewable resources. 11-13 Up to now, numerous bio-based

60

compounds, including diphenolic acid, 14 plant oils, 15 cardanol, 16-18 lignin, 19, 20 rosin

61

acid,

62

5-furandicarboxylic acid (FDCA), 32, 33 2, 5-furandimethanol

63

36-38

64

compared with the commercial epoxy resin DGEBA, the comprehensive

65

performances of many bio-based epoxy resins still need further improvement; great

66

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

68

could demonstrate better mechanical properties when compared with DGEBA. Using the

69

multifunctional groups of eugenol 25-30 and itaconic acid, 36-38 the bio-based epoxy resins

70

with unique properties have been reported. And in addition, the epoxy derived from plant

71

oil 15 and cardanol

72

though the platform building block selection and structure design, the synthesis of

73

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,

74

However, like the traditional petroleum-based epoxy, the epoxy resins derived from

75

biomass are also highly flammable and a large number of toxic smoke will release out

76

during the combustion. How to improve the flame retardant property of bio-based

77

epoxy is also a significant subject, while the research in this field is far from enough.

78

As we know, the most effective way to improve the flame retardancy of epoxy resin is

79

to embed intrinsic flame retardant elements, such as the halogen atoms, silicon unit,

80

and phosphorus-containing groups into their chemical architectures or blending with

81

fire retardants. 39-45 After some of the halogenated flame retardants were banned by

82

European Union because of the corrosive and toxic gases during combustion,

83

silicon and phosphorus-based flame retardants have attracted more and more attention

84

due to their low toxicity and high flame-retardant efficiency. 40, 41 In Karak’ work, 42 a

85

bio-based flame retardant epoxy was prepared via blending vegetable oil-based epoxy

86

with inorganic clay, and the cured resin showed self-extinguishing property. However,

87

bad compatibility and poor processability were noticed when the content of clay was

88

increased to meet the flame-retardant requirement. Deng et al


43

39

synthesized a series

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89

of rosin-based siloxane epoxy resins; good flame retardancy was achieved by

90

sacrificing their thermal properties. For the introduction of the phosphorus-containing

91

group, the glass transition temperature (Tg) of cured resin is usually decreased due to

92

the lower cross-link density. 44, 45 For example, David and his coworker 44 reported a

93

phosphorus-containing bio-based epoxy monomer (P2EP1P) from phloroglucinol and

94

triethyl phosphite. 44 In Gao’s group, a diphenolic acid-based epoxy resin containing

95

the phosphorous group (PCDGEDP) was prepared and the cured PCDGEDP

96

demonstrated LOI of 29.6 % and the flammability rating of V-0 in UL94 test.

97

However, the Tg of this DDM cured system was only 127 °C, relatively lower than the

98

phosphorous-free counterparts. Based on these literature researches, it was easy to

99

find that the epoxy resins combining high thermal and mechanical performance as

100

well as excellent flame resistance were difficult to be achieved via a simple and

101

efficient synthesis strategy, neither from the petroleum resource nor form the

102

bio-based feedstock, especially for the pure hydrocarbon systems. Usually, the

103

introduction of anti-flammable additives was inevitable, which would negatively

104

affect the processability, thermal or mechanical properties of resulted resins. In

105

addition, the synthetic route was sometimes complicated and the consequent

106

environmental impact could not be ignored. 44, 45

107

Numerous facts have proved that utilizing the structural diversity of renewable

108

feedstock to prepare the bio-based materials with surprising properties is worthy of

109

expectation. Daidzein (7-hydroxy-3-(4-hydroxyphenyl)-4H-chromen-4-one) is a

110

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

113

cardiovascular diseases and reducing the risk of cancers. 47 In scheme 1, the chemical

114

structure of daidzein is illustrated, which has a certain similarity to those of 4,

115

4’-bishydroxydeoxybenzoin (BHDB) and BPA. In previous literature,

116

resistances of BHDB-containing polymeric materials have been reported, and their

117

high char formation and low heat release rates are attributed to the presence of the

118

deoxybenzoin structure. However, BHDB is usually prepared by the demethylation of

119

desoxyanisoin in the harsh process condition. It is noted that the deoxybenzoin

120

structures also existed in daidzein. Inspired by this fact, the DGED was synthesized

121

from daidzein and epichlorohydrin in this work. For comparison, the commercial

122

DGEBA was taken as a control and cured with the same curing agent, DDM. The

123

thermal and mechanical properties, especially the flame retardant properties of cured

124

resins were studied thoroughly. The first and foremost aim of this research is to ensure

125

us a new strategy for the preparation of high-performance epoxy resin combining

126

outstanding flame retardancy and excellent thermo-mechanical properties from a

127

renewable resource using an efficient one-step process, without the addition of any

128

additional flame-resistant elements.

48-53

129 130

Scheme 1 The chemical structure of daidzein, BHDB, and BPA


the fire

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131

EXPERIMENTAL SECTION

132

Materials. Daidzein, 4, 4-diaminodiphenylmethane (DDM), tetrabutylammonium

133

bromide (TBAB) and epichlorohydrin (ECH) were purchased from Aladdin-reagent

134

Co., China. Dichloromethane, magnesium sulfate and sodium hydroxide were

135

obtained from Sinopharm Chemical Reagent Co., Ltd., China. Epoxy resin (DGEBA,

136

trade name DER331, epoxy value about 0.53) was supplied by DOW Chemical

137

Company. The above chemicals were all used as received from commercial suppliers.

138 139

Synthesis of diglycidyl ether of daidzein (DGED). Daidzein (20 g, 0.079 mol)

140

was dissolved in epichlorohydrin (73 g, 0.79 mol) with TBAB (0.93 g, 1 wt % of the

141

mixture) in a 500ml three-necked flask equipped with a magnetic stirrer, a reflux

142

condenser and constant pressure dropping funnel. After the mixture was agitated at

143

100 °C for 2 h, therefor it was passed through a cooling operation to room

144

temperature and 16 g of 50 wt% aqueous sodium hydroxide solution (0.40 mol NaOH)

145

was added drop wise into the flask and further reacted for another 5 h. The double

146

extraction of the mixture was done with distilled water after diluting with

147

dichloromethane and dried over by magnesium sulfate. The organic layer was

148

concentrated in vacuum to remove the dichloromethane and residual epichlorohydrin.

149

The obtained light yellow solid product DGED was dried at 60 °C for 12 h in a

150

vacuum oven and its total yield was calculated to be 88.5%. The synthetic route as

151

illustrated in Figure S1.

152

FT-IR (KBr, cm−1): 1626 (C-C); 1598 (C=O); 1410, 1510 (benzene ring); 1030, 912



153

(oxirane ring).

154

1

155

(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),

156

2.94-2.88 (m, 2H), 2.79-2.75 (m, 2H).

157

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

159

(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,

160

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),

161

Formation of the epoxy networks. Epoxy resins (DGEBA or DGED) and curing

162

agent (DDM) in the stoichiometric ratio (epoxy group: N-H= 1:1) without solvent was

163

well mixed in a beaker at 100°C in an oil bath for about 10 minutes. Then the

164

mixtures were quickly transferred to a preheated stainless steel mold at 140°C. After

165

that, the thermal curing reaction was performed in an air-circulating oven at the

166

temperatures of 140°C for 2h, 160°C for 2h and 180°C for 2h. Finally, the mold was

167

slowly cooled down to room temperature and the cured resins were removed carefully

168

from the mold and left still at room temperature for 24 h before mechanical and DMA

169

testing.

170

Measurements. The infrared spectrum (FT-IR) analysis of synthesized epoxy resin

171

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

173

resolution of 4 cm-1 at room temperature. For experiment accuracy, each sample was

174

scanned for 32 times. 1H-NMR and

13

C-NMR spectra were recorded with a Bruker


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175

AVANCE III 400MHz NMR spectrometer using TMS (Tetramethyl silane) as the

176

internal standard. The measurement was performed at room temperature and CDCl3

177

was used as a solvent. The curing behavior was investigated using a Mettler-Toledo

178

MET DSC under a high purity nitrogen atmosphere with a flowing rate of 60 mL/min.

179

Each sample weighing approximately 10 mg was sealed in an aluminum crucible and

180

then heated from 25 to 250 °C with multiple heating rates of 5, 10, 15 and 20 °C/min.

181

All the heating curves were recorded for curing reaction analysis. Dynamic

182

Mechanical Analysis (DMA) tests were carried on a TA Instrument (TA Q800) in a

183

tension fixture mode. All the samples with the dimension of 20 mm × 5 mm × 0.5 mm

184

were tested from 0 to 250 °C at a heating rate of 3 °C/min and a frequency of 1 Hz.

185

For accuracy, each sample was tested for five times. Thermogravimetric analysis

186

(TGA) w carried out on a Mettler-Toledo TGA. All the samples were heated from 50

187

to 800 °C with a heating rate of 20 °C/min under nitrogen and air atmospheres,

188

respectively. The mechanical properties of cured resins were evaluated using a

189

Universal Mechanical Testing Machine (Instron 5569A). The crosshead speed is 5

190

mm/min for tensile properties and 2 mm/min for flexural properties. All the samples

191

with the dimension of 60 mm × 10 mm × 3.5 mm were conditioned at room

192

temperature for 24 h with the relative humidity of 50% before testing. The average of

193

at least five measurements of each sample was taken to report the tensile or flexural

194

properties. Limiting oxygen index (LOI) was determined by an HC-2 Oxygen Index

195

Instrument (Jiangning Analytical Instrument Co. Ltd. China) according to ASTM

196

D2863-2008. The torch burning test was similar to the UL-94 procedure. The Bunsen



197

burner flame height was set to be about 30 mm, and the size of the sample was 30 mm

198

× 10 mm × 1 mm. Vertical burning tests were performed on a CZF-3 Vertical Burning

199

Tester (Jiangning Analytical Instrument Co. Ltd. China) following the procedure

200

described in ASTM D3801 and the sample size was according to the standard.

201

TGA-IR spectra were recorded on a Mettler-Toledo TGA connected with a NICOLET

202

6700 FT-IR. The temperature of transferring line between TGA and FT-IR was

203

determined to be 200 °C. About 10 mg samples were heated from 50 to 800 °C under

204

a nitrogen atmosphere (50 mL/min) at the heating rate of 20 °C/min. FT-IR spectra

205

were collected every 40 s during the whole testing process. The isothermal curing

206

behavior of epoxy resin was characterized by a dynamic rheological analyzer (Anton

207

Paar Physica MCR-301). The diameters of upper and lower plates were 25 mm and 50

208

mm, respectively. The measurement was performed at 100 °C/min in a steady shear

209

mode, with the frequency of 1 Hz and strain of 0.1%. The morphology of the residual

210

char after UL-94 burning was recorded using a scanning electron microscope. All the

211

samples for SEM were sputtered with a thin layer of gold before testing. X-ray

212

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

216

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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219

quantity of TBAB was employed as the phase transfer catalyst. In addition, the total

220

yield was determined to be high up to 88.5%. Those all indicated the simplicity and

221

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),

223

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

225

indicated the presence of C-O-C in pyrone ring. Specifically, the peaks appeared at

226

829, 912, 1027 and 1246 cm-1 evidenced the successful formation of oxirane ring 37.

227

As seen from the 1H NMR and 13C NMR spectra of DGED in Figure S3, the chemical

228

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

233

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



241

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

243

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

246

compared with DGEBA, using the same curing agent DDM.

247

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

254

determined to be 12.9 and 28.8 min, respectively. Obviously, the curing reaction of

255

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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257 258

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

265

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

267

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



Page 14 of 38

269

were noticed. And the peaks appeared at 862 and 1631 cm−1 were assigned to the C=C

270

in benzopyrone cycle. After curing reaction, they were all disappeared, which

271

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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286

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



Page 16 of 38

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.


Page 17 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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%



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


Page 18 of 38

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360

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



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


Page 20 of 38

Page 21 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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.



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.


Page 22 of 38

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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



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


Page 25 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


463

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



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.


Page 26 of 38

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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



514

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: [email protected]

535


Page 28 of 38

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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

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730 731 732 733



734

For Table of Contents Use Only

735 736

A bio-based epoxy with high performance and excellent fire resistance without extra

737

flame retardant elements.


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