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The functional group effect on char formation, flame retardancy and mechanical properties of phosphonate and triazine based compound as flame retardant in epoxy resin Geyun You, Zhiquan Cheng, Yuying Tang, and Hongwu He Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b00315 • Publication Date (Web): 07 Jul 2015 Downloaded from http://pubs.acs.org on July 10, 2015

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Industrial & Engineering Chemistry Research

1

The functional group effect on char formation, flame

2

retardancy and mechanical properties of phosphonate and

3

triazine based compound as flame retardant in epoxy resin

4

Geyun You, Zhiquan Cheng, Yuying Tang, Hongwu He,

5

Ministry of Education, College of Chemistry, Central China Normal University, Wuhan

6

430079, China

7

ABSTRACT:

8

A series of novel flame-retardant thermosets were prepared by melt blending of

9

phosphonate-triazine based compound TNTP, triazine-based compound TN, and

10

phosphonate-based compound TP respectively. The curing systems were consisted of

11

diglycidyl ether of bisphenol-A (DGEBA) and 4,4’-diamino-diphenyl sulfone (DDS). The

12

thermal behaviors and flame retardancy of these flame-retardant thermosets were

13

investigated in terms of thermogravimetric analysis (TGA), limiting oxygen index (LOI),

14

vertical burning test (UL-94) and cone calorimeter tests. TGA results showed that the char

15

formation of flame-retardant thermosets could be significantly improved due to the

16

presence of phosphonate moiety rather than triazine unit. It was found that the excellent

17

flame retardant effect of TNTP was not contributed by either single group of phosphonate

18

or triazine. An obvious synergic-effect on flame retardant produced by a combination of

19

phosphonate and triazine moiety. The LOI value of TNTP-3/DGEBA/DDS could achieve

20

32.4% and reach UL 94 V-0 rating, while that of TN-3/DGEBA/DDS was 29.0% and

21

failed in UL94 test, and TP-3/DGEBA/DDS with a LOI value of 31.8% just reach UL 94

22

V-1 rating. Moreover, cone calorimeter test revealed that the incorporation of TNTP to

1

ACS Paragon Plus Environment


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23

epoxy thermoset with 1.5 wt % phosphorus content could result in a decrease of peak heat

24

release rate (PHRR), total heat release (THR), average mass loss rate (AMLR), total smoke

25

release (TSR), average CO yield (ACOY), and average CO2 yield (ACO2Y) compared with

26

DGEBA/DDS control. The results from TGA data, scanning electronic microscopy (SEM),

27

fourier transform infrared spectroscopy (FT-IR) indicated TNTP modified thermosets had a

28

comprehensive flame retardant mechanism, including the gas phase, condensed phase and

29

phosphorus-nitrogen synergism mechanism. Furthermore, the mechanical properties of all

30

the thermosets were also investigated by Izod impact strength and flexural property tests.

31

Key words: phosphonate-triazine based; flame retardancy; synergic effect; flame

32

retardant mechanism; mechanical property.

33

1. INTRODUCTION

34

Epoxy has a wide range of industrial applications in metal coatings, electronic and

35

electrical components, high tension electrical insulators, fibre-reinforced plastic materials,

36

and structural adhesives.1-3 However, fire hazards associated with the use of flammable

37

epoxy resins are particular concern to consumers and manufactures, especially in electric

38

and electronic products. Simply, flame-retardant epoxy resins are urgently needed.

39

A widely known approach to achieve nonflammability for epoxy resins is addition of

40

flame retardants to the base resin. However, the use of halogen-containing flame retardants

41

is

42

Phosphorus-containing flame retardants have become one of the most promising candidates

43

to replace the halogen-containing flame retardants, because of their advantages such as

44

high efficiency, less smoke, and low toxicity.7-9 Phosphonates are one of most important

45

phosphorus-containing flame retardants, now used in several modifications of polymers as

restricted

due

to

perceived

environmental

and

2


toxicological

issues.4-6

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46

a reactive or additive one.10-14 Effective flame retardancy can be achieved with the addition

47

of phosphonates into epoxy resins, because phosphonates can degrade at a relatively low

48

temperature and form a protective layer of char to prevent the combustion of underlying

49

resins.15 Thus a condensed phase action can be considered as a primary flame-retarded

50

mechanism for the phosphonate flame-retarded epoxy resins.

51

Nowadays, triazine based polymers or macromolecules have received much attention in

52

the field of flame retardant.16-19 It is reported that triazines and their derivatives have

53

excellent charring effect because they possess abundant nitrogen content.20, 21 Meanwhile,

54

most reported work demonstrated that flame retardants containing phosphorus-nitrogen

55

could exhibit excellent flame retardant efficiency due to their nitrogen-phosphorus

56

synergistic effect.22

57

These fractions containing nitrogen could produce a number of non-flammable gases

58

without toxic smoke or fog under degradation at high temperature. Not only the heat

59

produced during combustion could be taken away from the surface of materials but also

60

the phosphorus-containing char residues could be foamed due to the release of gas,

61

producing an intumescent char layer.23 This intumescent char layer can act as an effective

62

protective shield against heat and combustible gases during combustion which can

63

enhance the flame retardant property. Both of the gas phase action by nitrogen and the

64

phosphorus-nitrogen synergistic effect are exhibited in this process of inflaming retarding.

65

In our previous work, a phosphonate-triazine based compound TNTP was synthesized

66

and applied in DGEBA epoxy resin.24 TNTP could produce a desirable char layer and

67

showed excellent flame retardancy. As mentioned above, both of phosphonates and 3



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triazines could promote polymers to obtain good char yield during combustion. In order to

69

better understand which is the major factor for charring effect and what is the dominant

70

action of triazine in compound TNTP, further study on TNTP is still needed.

71

Phosphonate-triazine based compound TNTP was synthesized based on our reported

72

method24. Meanwhile, corresponding phosphonate-based compound TP and triazine-based

73

compound TN as controls were prepared. We hope to have a better understanding about the

74

role of chemical structure for improving the efficiency of flame retardants by investigating

75

the thermal performance and flame retardancy of these three compounds modified

76

thermosets. The flame retardant mechanism and the mechanical properties of these

77

flame-retardant thermosets were also studied.

78

2. EXPERIMENTAL SECTION

79

2.1. Materials.

80

Cyanuric chloride, phenol, 4-hydroxy benzaldehyde, triethylamine, benzaldehyde and

81

4,4'-diaminodiphenylsulfone (DDS) of reagent grade were purchased from Sinopharm

82

Chemical Reagent Co., Ltd. Acetone, dichloromethane, and toluene were used as received. ?

83

Sodium hydroxide was purchased from Tianjin Fuchen chemical reagents factory in China.

84

DGEBA epoxy resin (commercial name: YD-128) with an epoxy equivalent weight (EEW)

85

of 188 g/eq showed a viscosity of 11500-13500 mPas in room temperature, which was

86

obtained from Yuehua organic chemical plant of Yueyang General Petroleum Refining and

87

Petrochemical Works.

88

Intermediate

5,5-dimethyl-1,3,2-dioxaphosphinane-2-oxide

(M-1)

and

2,4,6-tris

89

(4-formylphenoxy)-1,3,5-triazine (M-2) were synthesized according to the existing

90

methods.24 Analytical thin-layer chromatography (TLC) was conducted on silica gel GF254

91

(400 mesh). 4


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92

2.2. Synthesis of 2-(hydroxy(phenyl)methyl)-5,5-dimethyl-1,3,2-dioxaphosphinane

93

2-oxide (TP)

94

A solution of benzaldehyde 1.06 g (10.00 mmol) in 15 mL dichloromethane was added

95

dropwise to a stirred mixture of M-1 1.58 g (10.50 mmol) and triethylamine 0.10 g (1.00

96

mmol) in 20 mL dichloromethane at 0 oC. Then the mixture was stirred at room

97

temperature for 10 h. The solution was evaporated at reduced pressure to obtain crude

98

product, which was recrystallized from ethyl acetate to obtain pure white solid TP. 79%

99

yield, m.p.: 144-146 oC. 1H NMR [400 MHz, DMSO-d6, ppm]: δ 0.84 (s, 3H, CH3), 1.15 (s,

100

3H, CH3), 3.95 (d, J = 11.8 Hz, 2H, CH2O), 4.52-4.34 (m, 2H, CH2O), 5.24 (d, JP-H = 12.5

101

Hz, 1H, PCH), 6.41 (s, 1H, OH), 7.19-7.52 (m, 5H, Ar-H). 13C NMR [101 MHz, DMSO-d6,

102

ppm]: δ 19.9, 21.4, 31.9, 32.0, 69.2, 70.8, 77.2, 77.7, 127.1, 127.2, 127.5, 127.9. 138.4;

103

31

104

found: 256.22.

105

2.3 Synthesis of 2,4,6-triphenoxy-1,3,5-triazine (TN)

P-NMR [162 MHz, DMSO-d6, ppm]: δ 12.78. GC-MS: calcd. for C12H17O4P: 256.09,

106

Phenol 2.82 g (30.00 mmol) and sodium hydroxide 1.2 g (30.00 mmol) dissolved in 30

107

mL water, which was added dropwise to a stirred mixture of cyanuric chloride 1.84 g

108

(10.00 mmol) in actone (60 mL) at 0 oC. The resultant mixture was stirred for 2 h at room

109

temperature and then heated under reflux until the reaction was complete based on TLC

110

monitoring. Then the white precipitate was collected by filtration, washed several times

111

with water and acetone, and dried at 60 °C for 2 h. Desired white solid compound TN

112

could be obtained as high as 91% yield. m.p.: 237-239 oC; 1H NMR [400 MHz, DMSO-d6,

113

ppm]:δ 7.41 (s, 6H, Ar-H), 7.24 (s, 9H, Ar-H); GC-MS: calcd. for C21H15N3O3: 357.11,

114

found: 357.27.

115

2.3.

Preparation

of

[4-(2,4,6-Tris{4-[(5,5-dimethyl-2-oxo-

5


-[1,3,2]


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116

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dioxaphosphinan-2-yl) hydroxymety]phenoxy}-(1,3,5)-triazine (TNTP)

117

A solution of M-1 4.50 g (30 mmol), corresponding M-2 4.41 g (10.00 mmol) and

118

triethylamine 0.30 g (3.00 mmol) in toluene (40 mL) was heated under reflux. Analytical

119

thin-layer chromatography (TLC) was used to monitor the progress of the reaction by using

120

developing solvent of ethyl acetate and petroleum ether (2:1), and the target compound had

121

a Rf value of 0.52. After the reaction was completed (as indicated by TLC), the resultant

122

light yellow solid product was filtered and dried at atmospheric pressure. Then the crude

123

products were purified by crystallization with DMF/DMSO (4:1, v/v) to give the desired

124

white solid compound TNTP. Yield: 94%,

125

DMSO-d6, ppm]: δ 0.86 (s, 9H, CH3), 1.16 (s, 9H, CH3), 3.98 (d, J = 8.8 Hz, 6H, CH2O),

126

4.61-4.37 (m, 6H,CH2O), 5.28 (d, J = 8.9 Hz, 3H, PCH), 6.46 (d, J = 18.3 Hz, 3H, OH),

127

7.23 (d, J = 8.2 Hz, 6H, Ar-H), 7.49 (d, J = 8.1 Hz, 6H, Ar-H);

128

DMSO-d6, ppm] δ 19.8, 21.4, 32.0, 68.7, 70.2, 77.3, 77.8, 120.9, 128.3, 136.2, 150.6, 173.1;

129

31

130

found: 891.58.

131

2.4. Curing Procedure.

m.p.: 225-228 oC; 1H NMR [400 MHz,

13

C NMR [101 MHz,

P-NMR [162 MHz, DMSO-d6, ppm]: δ 12.80. GC-MS: calcd. for C39H48N3O15P3: 891.23,

132

The phosphonate-triazine based compound TNTP and phosphonate-based compound TP,

133

as organophosphorous flame retardants were used in DGEBA epoxy resins, respectively.

134

Different amount of TNTP (or TP) was applied in 70.0 g DGEBA into a 250 mL flask,

135

depending on the control of phosphorus content in epoxy thermosets (Table 1). The mixture

136

was heated at 160 oC with frequent agitation to completely dissolve, and then 23.0 g DDS

137

severed as curing agent was added to the mixture, stirred at 160 oC until DDS dissolved

138

completely. Thereafter, the resin-hardener mixture was kept in a vacuum oven again to

139

eliminate air bubbles, and then rapidly poured into LOI mold, UL 94 mold, cone

140

calorimeter test mold, and mechanical property test mold with preheating at 120 oC, 6


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141

respectively (Figure 1). The resin-hardener mixtures were cured at 120 oC for 2 h, at 150

142

o

143

were allowed to cool slowly to room temperature to prevent cracking. The epoxy

144

thermosets TNTP/DGEBA/DDS and TP/DGEBA/DDS with phosphorus content 0.5 wt%,

145

1.0 wt% and 1.5 wt% were obtained, respectively.

C for 2 h, at 180 oC for 2 h to make epoxy thermosets, respectively. The epoxy thermosets

The triazine-based compound TN was applied in DGEBA epoxy resin with the same

146 147

nitrogen

148

TN/DGEBA/DDS with nitrogen content 2.9 wt%, 3.0 wt% and 3.1 wt% were also prepared

149

respecitively in the same curing process.

150

2.5. Measurements.

151 152 153 154 155 156 157 158 159 160 161 162

content

according

to

TNTP/DGEBA/DDS.

The

epoxy

thermosets

Melting point (m.p.) was measured on an electrothermal melting-point apparatus and uncorrected. 1

H, 13C and 31P NMR spectra were recorded in DMSO-d6 on a Varian Mercury Plus-400

NMR spectrometer (Varian, USA). Fourier transform infrared (FT-IR) spectra were recorded in potassium bromide pellets with a Nicolet Avatar 360 Fourier transform infrared spectrophotometer. Gas chromatography-mass spectromete (GC-MS) was obtained on a Finnigan Trace MS 2000 spectrometry. Thermogravimetric analysis (TGA) was performed on a STA 409 PC instrument from room temperature to 700 oC at a heating rate of 10 oC /min under N2 atmosphere. The limiting oxygen index (LOI) was measured according to ASTMD 2863-97 standard with test sample bars 120.0 mm × 6.5 mm × 3.0 mm on a JF-3 oxygen index meter.

163

Vertical Burning Test (UL-94) was carried out according to the ASTM D 3801 standard

164

with test sample bars 125.0 mm × 12.7 mm × 3.2 mm on a CZF-3 type level vertical flame

165

detector. 7



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166

The cone calorimeter tests with 100.0 mm × 100.0 mm × 4.0 mm size sample was

167

investigated using a Fire Testing Technology cone calorimeter in conformance with ISO

168

5600-1:2002 at heat flux of 50 kW/m2.

169 170

Scanning electron microscopy (SEM) was performed on a JSM-6700F SEM at an accelerating voltage of 10 kV. All samples were coated with a conductive gold layer.

171

Flexural strength was measured using a CMT6503 tensile tester with sheet dimensions

172

of 80.0 mm×10.0 mm×4.0 mm and completed in accordance with the procedures in GB/T

173

9314-2000 at a crosshead speed of 1.7 mm/min.

174

Izod impact strength was measured by a ZBC1400-1 impact tester with sheet dimensions

175

of 80.0 mm×10.0 mm×4.0 mm according to GB/T 1043-1993. All specimens were held in

176

a vertical cantilever beam and broken by a pendulum (4 J).

177

3. RESULTS AND DISCUSSION

178

3.1. Synthesis and Characterization.

179

The synthetic routes of TP, TN, and TP

could

be

TNTP are shown in

successfully

180

Phosphonate-containing

181

nucleophilic addition reaction of M-1 and benzaldehyde in dichloromethane using

182

triethylamine as catalysts at room temperature. Triazine-containing TN was synthesized by

183

the nucleophilic substitution reaction of phenol and cyanuric chloride under alkaline

184

conditions. The solution of sodium phenol was dropwise added into reaction system, and

185

three active chloride atoms in cyanuric chloride molecule were stepwise replaced at 0 oC,

186

room temperature, and reflux condition, respectively. Then, tri-substituted 1,3,5-triazine

187

was obtained in 91 % yield. Phosphonate-triazine based compound TNTP was synthesized

188

by the nucleophilic addition reaction of M-1 and M-2. Compared with reported method,24

189

the present method improved the yield more than 8%, and shorten the reaction time nearly

190

3h under the catalysis of triethylamine.

8


prepared

Scheme 1. through

the

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191

All of the three compounds were fully confirmed, and the corresponding data were

192

presented in Experimental Section. The GC-MS spectra of TN was presented in Figure S1

193

in supporting information. TN with M/Z 357.11 was found in GC-MS to confirm a

194

completely substitution of chloride atoms in cyanuric chloride. TP or TNTP was

195

synthesized by the nucleophilic addition of phosphinane M-1 and benzaldehyde or M-2.

196

The signal of methylidyne of TP or TNTP in 1H NMR spectrum was split to two peaks due

197

to a 2JP-H coupling with the corresponding coupling constant of 12.5 and 8.9 Hz,

198

respectively (Figure S2 and S3 in supporting information). In the

199

signal of methylene carbon of TP was split into two peaks at 69.2 and 70.8 ppm with a

200

coupling constant of 156.7 Hz due to the 1J

201

TNTP was split into two peaks at 68.7 and 70.2 ppm with a coupling constant of 157.6 Hz

202

(Figure S4 and S5 in supporting information). These characterized results were considered

203

to be an evidence that phosphinane M-1 reacted successfully with benzaldehyde or M-2,

204

respectively. That was also in agreement with the 31P NMR spectra of TP and TNTP, where

205

only sharp singlet was showed at 12.78 or 12.80 ppm, which were consistent with the

206

characterization of phosphonate.25

P-C

13

C NMR spectrum, the

coupling, and the corresponding signal in

207

Compounds TN, TP, and TNTP were incorporated to DGEBA/DDS curing system with

208

different phosphorus or nitrogen content, respectively. Three flame-retardant thermosets of

209

TN-3/DGEBA/DDS, TP-3/DGEBA/DDS, TNTP-3/DGEBA/DDS, and neat DGEBA/DDS

210

thermoset were analyzed by FT-IR spectroscopy, and the spectra are presented in Figure S6

211

in supporting information. Compared with neat DGEBA/DDS, TN-3/DGEBA/DDS and

212

TNTP-3/DGEBA/DDS showed the intensive absorption bands at 1571-1579 cm-1 and 1380

213

cm-1 which were attributed to the characteristic absorption of triazine ring.20 The peaks at

214

1065 cm-1 (TP-3/DGEBA/DDS) and 1068 cm-1 (TNTP-3/DGEBA/DDS) could be assigned

215

to the stretch vibrations of P-O-C in phosphonate moiety. These results indicated that TN, 9



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Page 10 of 40

216

TP, and TNTP were incorporated to DGEBA/DDS curing systems successfully. The

217

dispersion of compound TN, TP and TNTP was further analyzed by SEM. The

218

cross-sectional

219

TNTP-3/DGEBA/DDS and neat DGEBA/DDS are shown in Figure S7 in supporting

220

information. As shown in Figure S7, TN, TP and TNTP disappeared in the thermoset

221

matrix, all the three flame-retardant thermosets as well as neat DGEBA/DDS show an

222

evenly continuous phase system. It indicated that TN, TP, and TNTP had good

223

compatibility with DGEBA/DDS curing system.

224

3.2. Thermal Properties.

morphologies

of

TN-3/DGEBA/DDS,

TP-3/DGEBA/DDS,

225

The TGA and DTG curves of TP, TN, TNTP modified epoxy thermosets with the control

226

of phosphorus or nitrogen content under nitrogen atmosphere are shown in Figure 2. The

227

onset degradation temperature (Tonset) of these flame-retardant thermosets which was tested

228

when 5 wt% weight was lost, and the char yield (Rc) at 700 oC were obtained from the

229

TGA curve. The temperature of the maximum weight loss rate (Tmax) of these

230

flame-retardant thermosets was obtained from the DTG curve. These data are list in Table

231

2.

232

As shown in Figure 2(1A, 2A, 3A), all the thermosets showed only one step of

233

decomposition. TN modified thermosets showed a similar thermal degradation behavior

234

with neat DGEBA/DDS. However, TNTP and TP modified thermosets were decomposed at

235

a lower temperature than that of DGEBA/DDS. It indicated that the incorporation of TNTP

236

or TP could lower the thermostability of the thermosets. Meanwhile, as seen from the DTG

237

curves in Figure 2(1B, 2B, 3B), the temperature of maximum weight loss rate (Tmax) of

238

TNTP or TP modified thermosets was obviously lower than that of neat DGEBA/DDS.

239

However, all the flame-retardant thermosets showed a decreased maximum weight loss rate,

240

especially TNTP and TP. It suggested that the incorporation of TNTP or TP could 10


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241

accelerate the initial decomposition of thermosets, and decreased the maximum weight loss

242

rate obviously under high temperature, which could promote the formation of protective

243

char layer during combustion. In Table 2, the Tonset and Tmax for neat DGEBA/DDS thermoset were 374.5 oC and 408.0

244 245

o

C, respectively, and the char yield (Rc) was 16.4 wt% at 700 oC. When DGEBA/DDS was

246

modified by triazine-based compound TN with nitrogen content about 2.9, 3.0, and 3.1

247

wt% respectively, no significant effect on Tonset and Tmax was observed. However, the Rc

248

values of TN/DGEBA/DDS thermosets at 700 oC were in the range of 13.1 - 14.9 wt%,

249

which was lower than that of neat DGEBA/DDS. The char yield of TN/DGEBA/DDS

250

thermosets might even decrease with the increasing amount of TN addition due to a large

251

number of nitrogen-containing gas released during the thermal degradation process, which

252

could take mass away from the material matrix.23 It revealed that TN could not enhance the

253

char yield of DGEBA/DDS thermoset matrix either.

254

The incorporation of TNTP or TP into DGEBA decreased Tonset of thermosets, which

255

could be attributed to the fact that phosphonate-containing TNTP and TP were less stability

256

than that of DGEBA/DDS thermosets.26,27 Thus, with the increasing of phosphorus content,

257

TNTP/DGEBA/DDS and TP/DGEBA/DDS thermosets also experienced a drop in the Tmax

258

values. However, TNTP/DGEBA/DDS and TP/DGEBA/DDS thermosets had a significant

259

increase in char yield compared with DGEBA/DDS and TN/DGEBA/DDS thermosets. The

260

char yield of TNTP/DGEBA/DDS thermosets at 700 oC was in the range of 25.1 – 29.2

261

wt%, and that of TP/DGEBA/DDS thermosets in the range of 24.4 – 28.0 wt%. These

262

results showed that the phosphonate unit in the structure of TNTP played an important role

263

in charring effect during the degradation of resin matrix. With the same phosphorus content,

264

the char yield of TNTP/DGEBA/DDS thermoset was higher than that of TP/DGEBA/DDS,

265

which could be attributed to different nitrogen content in the thermosets. It indicated that 11



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Page 12 of 40

266

there was a synergistic effect between phosphorus and nitrogen in TNTP/DGEBA/DDS

267

thermosets. As a result, TNTP could improve the residual-carbon of thermosets at high

268

temperature due to its phosphonate moiety, and obtain a strengthened effect on charring by

269

the phosphorus-nitrogen synergistic effect due to the combination of phosphonate and

270

triazine units in its structure.

271

3.3. Flame-retardant performance

272

3.3.1. LOI and UL-94 Tests.

273

To investigate the flame retardancy of flame-retardant thermosets and neat

274

DGEBA/DDS, the LOI tests and the vertical burning tests (UL-94) were carried out at

275

room temperature. The LOI values, the burning time after each ignition (t1, t2), and UL-94

276

ratings are listed in Table 3. The combustion phenomena of TNTP-3/DGEBA/DDS,

277

TN-3/DGEBA/DDS, and TP-3/DGEBA/DDS were photographed and shown in Figure 3.

278

Table 3 showed that the neat DGEBA/DDS thermoset was a highly flammable material

279

with a low LOI value of 22.5%. As seen from Figure 3, DGEBA/DDS could not extinguish

280

spontaneously at time more than 50 s after the first 10 s-ignition and accompanied with

281

drippings during combustion. Thus, DGEBA/DDS thermoset failed in UL-94 test and

282

obtained no rating. Even DGEBA/DDS was extinguished by artificial factor, little chars

283

were resided on the surface of the tested specimen (Figure 3: DGEBA/DDS, Test Over).

284

When triazine-based compound TN was added into DGEBA/DDS curing system, the LOI

285

values of TN/DGEBA/DDS thermosets with 2.9 wt%, 3.0 wt%, 3.1 wt% nitrogen content

286

could be increased to 25.4%, 27.3%, and 29.0% respectively. However, TN/DGEBA/DDS

287

thermosets was no rating in UL-94 test, because of these thermosets could not

288

self-extinguish after first 10 s ignition, and even left several droppings (Figure 3:

289

TN-3/DGEBA/DDS, tb = 50 s). These results showed that TN containing triazine moiety

290

could not improve the flame retardancy of DGEBA epoxy thermosets significantly. 12


Page 13 of 40

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291

When phosphonate-based compound TP was incorporated into DGEBA/DDS curing

292

system with different phosphorus content, clear inflaming retarding effect could be

293

observed by increasing LOI values, eliminating melt dripping, and decreasing burning time.

294

As shown in Table 3, the LOI values of TP/DGEBA/DDS increased with increasing of

295

phosphorus content, and reached 30.6 - 31.8% when the phosphorus content was in range

296

of 0.5 - 1.5 wt%, showing an obvious positive effect on LOI values. However,

297

TP-3/DGEBA/DDS with 1.5 wt% phosphorus content could not extinguish in 10 s after the

298

first ignition (Figure 3: TP-3/DGEBA/DDS, tb = 15 s), and only reached UL-94 V-1 grade,

299

though its LOI value was as high as 31.8%. The results indicated that phosphonate-based

300

compound TP could not impart excellent flame retardancy to thermosets.

301

The flame retardancy of DGEBA/DDS curing system could be obviously improved by

302

introducing phosphonate-triazine based compound TNTP. When TNTP was incorporated

303

into DGEBA/DDS curing system with 0.5 wt%, 1.0 wt%, and 1.5 wt% phosphorus content,

304

the LOI value of corresponding thermosets could be increased to 30.3%, 31.5%, and 32.4%,

305

respectively. TNTP-2/DGEBA/DDS could pass the UL-94 V-0 rating with only 1.0 wt %

306

phosphorus content, while TP-2/DGEBA/DDS just reached UL 94 V-1 rating. As shown in

307

Figure 3 (TNTP-3/DGEBA/DDS, tb = 10 s), TNTP-3/DGEBA/DDS with 1.5 wt%

308

phosphorus content could extinguish in 10 s after fire source was taken away, and

309

accompanied with less smoke and no dripping, which could also reach UL 94 V-0 rating.

310

As shown in Table 3, TNTP/DGEBA/DDS showed more excellent flame retardancy than

311

TP/DGEBA/DDS with same phosphorus content or TN/DGEBA/DDS with same nitrogen

312

content. That is to say, the excellent flame retardant effect of TNTP was not contributed by

313

either single group of phosphonate or triazine. These results indicated that an obvious

314

synergic-effect on flame retardant produced by a combination of phosphonate and triazine

315

moity. 13



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Page 14 of 40

316

3.3.2 Cone calorimetric analyses

317

The cone calorimeter test as one kind of fire testing technology is widely used to

318

evaluate the fire hazards for a polymeric material. In order to investigate the effects of

319

TNTP, TN and TP on the combustion behaviors, the neat DGEBA/DDS and three

320

representative

321

TNTP-3/DGEBA/DDS) were tested by cone calorimeter at a heat flux of 50 kW/m2. A

322

series of parameters was obtained including time to ignition (TTI), heat release rate (HRR),

323

total heat release (THR), mass loss rate (MLR), total smoke release (TSR), the analysis of

324

CO and CO2. The corresponding test data are listed in Table 4, and the HRR, TSR, MLR,

325

and TSR curves as functions of combustion time of thermosets are shown in Figure 4.

326

Digital photographs of the char residues obtained at the end of cone calorimeter tests are

327

also shown in Figure 5.

thermosets

(TN-3/DGEBA/DDS,

TP-3/DGEBA/DDS,

and

328

TTI is the time required for entire surface of a sample to burn with a sustained flame

329

under a constant heat flux, which can be measured from the onset of a HRR curve. As

330

shown in Table 4, the TTIs of the modified thermosets were much shorter than that of neat

331

DGEBA/DDS,

332

additives not only decomposed ahead of the time but also promoted the degradation of

333

resin matrix at lower temperature.

which

was

mainly

due

to

these

334

TTI of TNTP-3/DGEBA/DDS was 9 s and 11 s longer than that of TN-3/DGEBA/DDS

335

and TP-3/DGEBA/DDS, respectively. It showed that the combination of triazine and

336

phosphonate could obviously improve the flame retardancy of epoxy thermosets.

337

Heat release rate (HRR) is believed to have the greatest influence on the fire hazard. The

338

primary parameter responsible for peak heat release rate (PHRR) is the mass loss rate

339

(MLR), which in turn can affect the char formation. As shown in Figure 4 and Table 4, the

340

neat DGEBA/DDS showed a rapid combustion giving only one PHRR of 872.8 kW/m2, 14


Page 15 of 40

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341

total heat release (THR) of 88.5 MJ/m2, and average mass loss rate (AMLR) of 0.057 g/s,

342

which remained only a little char residues after test [Figure 4(1A)]. Compared with neat

343

DGEBA/DDS, the introduction of TN with 3.1 wt % nitrogen content resulted in an earlier

344

and higher PHRR value of 943.3 kW/m2, but a decreased THR value of 78.4 MJ/m2, and an

345

increased AMLR value of 0.078 m2/m2. As seen in Figure 4(2A), much less char residues

346

were remained by TN-3/DGEBA/DDS, which was consistent with the results of TGA tests.

347

It indicated that triazine based compound TN could not effectively promote thermoset to

348

construct a protective char layer during combustion.

349

When TP and TNTP were incorporated to DGEBA/DDS resin system with 1.5 wt%

350

phosphorus content respectively, two decreased peak heat release rate (PHRR1 and PHRR2)

351

were observed in HRR curves in Figure 4(A). The PHRR1 appeared at 50 s, which was due

352

to the thermal degradation of phosphonate moieties and generated initial carbon residue.

353

The PHRR2 was mainly attributed to the destruction of carbon residue due to a long-time

354

exposure to high temperature and yield a protective char yield, which was a little higher

355

than PHRR1. Although the PHRR2 value of TNTP-3/DGEBA/DDS (253.0 kW/m2) was

356

7.7 % less than that of TP-3/DGEBA/DDS (312.6 kW/m2) as shown in Figure 4(B), the

357

THR value of TNTP-3/DGEBA/DDS was higher than that of TP-3/DGEBA/DDS at the

358

same burning time. After combustion, TP-3/DGEBA/DDS and TNTP-3/DGEBA/DDS

359

showed the THR value of 59.0 MJ/m2 and 65.8 MJ/m2 respectively, which were lower than

360

that of DGEBA/DDS. Meanwhile, as shown in Figure 4(C), mass loss rate (MLR) of

361

thermosets was obviously decreased by the incorporation of TP and TNTP. The AMLR

362

data is related to the char yield, and carbonaceous residue that remains at the end of the test

363

after flame extinguish. As shown in Table 4, compared with DGEBA/DDS, the AMLR

364

value of TNTP-3/DGEBA/DDS and TP-3/DGEBA/DDS was reduced by 24.6 % and

365

35.1 %, respectively. Thereafter, a significant increase of char residues for both 15



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Page 16 of 40

366

TP-3/DGEBA/DDS and TNTP-3/DGEBA/DDS could be observed after test [Figure 5(3B),

367

Figure 5(4B)]. However, the char layer of TNTP-3/DGEBA/DDS was thicker and more

368

intumescent obviously than that of TP-3/DGEBA/DDS, which was consistent with char

369

yield in TGA tests. It further confirmed the existing of synergistic effect between

370

phosphonate and triazine on enhancement of char formation and flame retardancy for

371

epoxy thermosets.

372

It have been found that organophosphorus flame retardants generate less smoke and

373

toxic gas than halogen-containing flame retardants. As expected, the incorporation of

374

phosphonate-containing TNTP and TP into DGEBA epoxy resin with 1.5 wt % phosphorus

375

content could decrease the total smoke release (TSR) at least 37.6 % and 48.0 % relative to

376

neat DGEBA/DDS thermoset (3508.1 m2/m2), respectively (Table 4). However, compared

377

with the neat DGEBA/DDS, the TSR value of TN modified thermoset with 3.1 wt%

378

nitrogen only reduced by 3.9%. The TSR value of TNTP-3/DGEBA/DDS (2187.8 m2/m2)

379

was a little higher than that of TP-3/DGEBA/DDS (1823.3 m2/m2), which might be caused

380

by the release of non-combustible gases produced by the thermal degradation of triazine

381

moieties. Although, a certain number of non-combustible gases from triazine moieties

382

could benefit the construct an intumescent char layer, volatile fragments could be brought

383

away from the material surface by the released gases. It might be a reason for higher

384

AMLR value of TNTP-3/DGEBA/DDS than that of TP-3/DGEBA/DDS. Meanwhile, a gas

385

phase mechanism was proposed to explain the flame retardancy which produced by the

386

action of triazine moieties.

387

GC-MS spectra of neat DGEBA/DDS thermoset and three flame retardant thermoset of

388

TN-3/DGEBA/DDS, TP-3/DGEBA/DDS, and TNTP-3/DGEBA/DDS are supplied in

389

Figure S8 in supporting information. Through the analysis of these four thermosets, it was

390

found that all these four thermosets could release NH3, H2O, CO, N2, and CO2 during 16


Page 17 of 40

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391

degradation. Among these released gases, CO is the main toxic gas. As shown in Table 4,

392

TNTP-3/DGEBA/DDS and TN-3/DGEBA/DDS presented an average carbon monoxide

393

yield (ACOY) of 0.0596 kg/kg and 0.0700 kg/kg respectively, which were lower than that

394

of TP-3/DGEBA/DDS (0.1206 kg/kg) and neat DGEBA/DDS (0.0726 kg/kg). Moreover,

395

TNTP/DGEBA/DDS showed the lowest average carbon dioxide yield (ACO2Y) among

396

these four thermosets, which reduced by 32.9% relative to the neat DGEBA/DDS.

397

Therefore TNTP would be a potential environmental friendly flame retardant for epoxy

398

resins, due to its advantage of less smoke and low toxic.

399

3.4. Flame Retardant Mechanism

400

In order to further reveal the flame retardant mechanism of TNTP, the TGA and DTG

401

curves of triazine-based TN, phosphonate-based TP, phosphonate-triazine based TNTP and

402

neat DGEBA/DDS under nitrogen atmosphere are recorded and shown in Figure 6. The

403

char

404

TNTP-3/DGEBA/DDS after UL-94 tests were analyzed by SEM and shown in Figure 7.

405

The chars obtained from the corresponding thermosets were also analyzed by FT-IR

406

spectroscopy, and their spectra are presented in Figure 8.

morphologies

of

TN-3/DGEBA/DDS,

TP-3/DGEBA/DDS,

and

407

From Figure 6(A), the initial decomposition temperatures of compound TN, TP, and

408

TNTP, were lower than that of neat DGEBA/DDS thermoset, which was favor to promote

409

the formation of protective carbon residue before the thermal degradation of thermoset

410

matrix. Meanwhile, the initial decomposition temperatures (Tonset) of these three

411

compounds were in order of Tonset (TP) < Tonset (TNTP) < Tonset (TN), which indicated that

412

the introduction of triazine moiety could improve the thermal stability of TNTP. At 700 oC,

413

TNTP expressed much more char residues than that of TN and TP, which might be

414

attributed to the combination of phosphonate unit with high aromatic content. As shown in

415

Figure 6(A), a two-step thermal degradation was observed in TGA curve of TNTP. The 17



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Page 18 of 40

416

first-step degradation was in the range of 180 – 350 oC, which may attribute to the cleavage

417

of the P-O-C bonds that could be further confirmed by the only one-step degradation of TP

418

with the maximum decomposition at 230 oC. The phosphorus-containing char residues

419

were produced during the first stage. The second-step degradation may attribute to the

420

degradation of the main chain of triazine moiety, and residual chars formed at the first

421

stage. The second maximum decomposition temperature of TNTP was at 495 oC, which

422

was higher than the maximum decomposition temperature of TN and DGEBA/DDS at 368

423

o

424

TNTP could release nonflammable gases during combustion to dilute the hot atmosphere,

425

cool the pyrolysis zone at the combustion surface, and cut off the supply of oxygen. Thus,

426

the mechanism of flame retardancy might belong to a gas phase mechanism by importation

427

of triazine moieties. Meanwhile, phosphorus-containing char residues could be foamed

428

during the release of nonflammable gases and reinforced the condensed phase flame

429

retardation. Above observation showed a phosphorus-nitrogen synergistic effect.

C and 408 oC, respectively [Figure 6(B)]. The delayed decomposition of triazine moiety in

430

It is well known that the charring structure is one of the most important factors

431

determining flame retardancy. Figure 7 illustrates the SEM images of outer surface and

432

inner surface of the residual chars obtained after the UL-94 tests. Although a smooth outer

433

surface and compact inner surface can be seen from Figure 7(1A, 1B), the char layer was

434

so thin that was not sufficient to prevent mass loss and heat transfer [Figure 5(1A)]. When

435

TN was added to thermosets, the inside char of TN-3/DGEBA/DDS showed a high

436

expanding and lacunaris structure [Figure 7(2B)]. However, its char skeleton was weak,

437

along with a very gassy outer surface with several pores [Figure 7(2A)], which was caused

438

by the release of nonflammable gases. Thus, based on above observation, char residues

439

could not be sufficiently formed on the surface of TN-3/DGEBA/DDS thermoset during

440

combustion [Figure 5(2A)], so resulting in poor flame retardancy. On the contrary, 18


Page 19 of 40

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441

phosphonate-based TP modified thermoset was slightly expanding, producing char residues

442

with rigid-skeletal-structure and sealing surface [Figure 7(3A, 3B)]. This carbon layer

443

could prevent molten drops and result in the extinguishment of fire during combustion.

444

Although the nonflammability of TP/DGEBA/DDS was increased by increasing

445

phosphorus content it still did not reach the V-0 rating (Table 3). A remarkable flame

446

retardancy could be achieved when phosphonate-triazine based TNTP was incorporated

447

into the thermosets.

448

In this case, a cohesive and intumescent char layer with sealing surface could be formed

449

on the surface of modified thermosets during combustion [Figure 7(4A, 4B)]. This

450

well-defined intumescent char layer was thicker than that of TP modified thermoset [Figure

451

5(3B, 4B)], which could effectively inhibit the transmission of heat and oxygen during

452

combustion. As a result, an intumescent mechanism by the incorporation of triazine moiety

453

was exhibited in the flame-retardant mechanism of TNTP.

454

FI-IR spectroscopy was used to characterize the functional groups of the char residues.

455

From Figure 8(A, B), it can be observed that TN-3/DGEBA/DDS shows similar FT-IR

456

spectra of char residues to DGEBA/DDS. The peaks at 1593, 1510, and 1460 cm -1 are

457

accounted for stretching vibration of aromatic ring. However, significant difference in

458

FT-IR

459

phosphonate-containing thermosets (TP-3/DGEBA/DDS and TNTP-3/DGEBA/DDS) can

460

be observed in Figure 8(B, C, D). In Figure 8(C) (TP-3/DGEBA/DDS) and Figure 8(D)

461

(TNTP-3/DGEBA/DDS), the new peaks appear at 1220, 1229 cm-1, at 1086, 1093 cm-1,

462

and at 896, 901 cm-1 are, respectively, ascribed to stretch vibrations of P=O, P-O-C and

463

P-O-P.9,

464

phosphonate

465

thermosets to produce char residues containing rich in phosphorus.

spectrogram

15

of

residual

chars

between

TN-3/DGEBA/DDS

and

It indicated that a condensed phase flame-retardant mechanism caused by units,

which

made

TP-3/DGEBA/DDS

and

TNTP-3/DGEBA/DDS

466

These results together with the char yields characterized by TGA test confirmed that the

467

phosphonate units in TNTP also play a major role in the formation of char during thermal 19



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Page 20 of 40

468

degradation. These results indicated that the flame retardancy of TNTP modified DGEBA

469

thermosets was not due to any single mechanism but rather a complex combined

470

mechanisms, including the gas phase mechanism caused by triazine moieties, the

471

condensed phase mechanism caused by phosphonate units, and the phosphorus-nitrogen

472

synergism mechanism

473

3.5. Mechanical properties

474

The mechanical properties of the modified thermosets are also very important for the

475

application of flame retardant. In our work, the mechanical properties of TN/DGEBA/DDS,

476

TP/DGEBA/DDS, TNTP/DGEBA/DDS, and neat DGEBA/DDS thermoset were further

477

investigated. Their flexural strength and Izod impact strength were examined and the

478

results are presented in Figure 9. The addition of TNTP, TN, or TP to DGEBA epoxy

479

decreased the flexural strength. With the increase of additives content in modified

480

thermosets, the flexural strength of modified thermosets gradually decreased. When TNTP

481

was imported into DGEBA epoxy resins, it showed less decline in flexural strength relative

482

to TP with the same phosphorus content. It was noteworthy that TN/DGEBA/DDS showed

483

Izod impact strength in the range of 7.98 – 9.20 kJ/m2, which was higher than that of neat

484

DGEBA/DDS thermosets (7.92 kJ/m2). It might be attributed to the incorporation of rigid

485

aromatic structures to thermoset matrix composites. The Izod impact strengths of

486

TNTP/DGEBA/DDS was also higher than that of TP/DGEBA/DDS with the same

487

phosphorus content. The results suggested that the thermosets modified by TNTP with the

488

combination of phosphonate moiety and triazine unit showed better mechanical strengths

489

than that of the thermosets modified by phosphonate-based TP.

490

4. CONCLUSIONS

491

An efficient and environment-friendly flame retardant TNTP containing phosphonate

492

and triazine units were synthesized and characterized. A triazine-based compound TN, and

20


Page 21 of 40

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493

a phosphonate-based compound TP were also synthesized as controls. In order to study the

494

charring effect and flame retardancy of TNTP on epoxy thermosets, TNTP, TN, and TP

495

were applied to DGEBA with controlling phosphorus or nitrogen content. The char yield of

496

TN/DGEBA/DDS was in the range of 13.1 - 14.9 wt % at 700 oC, which was lower than

497

that of neat DGEBA/DDS thermoset (16.4 wt %). Whereas, TP/DGEBA/DDS showed a

498

char yield in the range of 24.4 - 28.0 wt %, which was similar to that of

499

TNTP/DGEBA/DDS (25.1 - 29.2 wt %). It was found that the charring effect of TNTP on

500

DGEBA epoxy resin might be caused by phosphonate units rather than triazine moiety, and

501

there was a synergistic effect on charring between phosphonate and triazine groups. TNTP

502

modified thermosets exhibited not only the enhancement effect on char formation but also

503

excellent flame retardancy compared with its TN and TP counterparts. The LOI value of

504

TNTP-3/DGEBA/DDS was 32.4%, while that of TN-3/DGEBA/DDS was 29.0%, and

505

TP-3/DGEBA/DDS was 31.8%. The flammability rating of TNTP/DGEBA/DDS could

506

reach UL 94 V-0 rating with only 1.0 wt% phosphorus content, while that of

507

TP/DGEBA/DDS could just reach UL 94 V-1 rating with 1.5 wt% phosphorus content, and

508

TN/DGEBA/DDS was no rating. Moreover, the incorporation of phosphonate-containing

509

TP and TNTP could decrease the peak heat release rate (PHRR), total heat release (THR),

510

and average mass loss rate (AMLR). TNTP-3/DGEBA/DDS produced a thicker and

511

tougher char layer than that of TP-3/DGEBA/DDS after cone calorimeter test. The

512

phenomenon of intumescent and phosphorus-containing char layer showed there was a

513

comprehensive flame retardant mechanism, including the gas phase mechanism, the

514

condensed phase mechanism, and the phosphorus-nitrogen synergistic mechanism during

515

the combustion of TNTP/DGEBA/DDS. Especially TNTP could result in a decrease of the

516

total smoke release (TSR), average CO yield (ACOY), and average CO2 yield (ACO2Y) of

517

modified thermosets. These indicated that TNTP as a potential environmental friendly 21



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Page 22 of 40

518

flame retardant could be applied in epoxy resins.

519

■ ASSOCIATED CONTENT

520 521

Supporting Information GC-MS spectrum of compound TN (Figure 1S); 1H NMR spectrum of compound TP

522

(Figure S2); 1H NMR spectrum of compound TNTP (Figure S3);

523

compound TP (Figure S4);

524

spectra of neat DGEBA/DDS and flame-retardant thermosets (Figure S6); SEM images for

525

cross section of DGEBA/DDS and flame-retardant thermosets (Figure S7); GC-MS spectra

526

of DGEBA/DDS and flame-retardant thermosets (Figure S8). This information is available

527

free of charge via the Internet at http: //pubs.acs.org.

528

■ AUTHOR INFORMATION

529

Corresponding Author

530

 E-mail: [email protected]. Tel.: +86-027-67867960.

531

Notes

532 533 534

13

13

C NMR spectrum of

C NMR spectrum of compound TNTP (Figure S5); FT-IR

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS Financial support from the National Engineering and Technology Center for the

535

Development &Utilization of Phosphor Resources (No. 2012k011) is acknowledged.

536

■ REFERENCES

537 538 539 540 541 542 543 544 545 546 547

(1) Sun, D. C.; Yao, Y. W. Synthesis of three novel phosphorus-containing flame retardants and their application in epoxy resins. Polym. Degrad. Stab. 2011, 96, 1720. (2) Liang, S. Y.; Neisius, N. M.; Gaan, S. Recent developments in flame retardant polymeric coatings. Prog. Org. Coat. 2013, 76, 1642. (3) Wang, X. D.; Zhang, Q. Synthesis, characterization, and cure properties of phosphorus-containing epoxy resins for flame retardance. Eur. Polym. J. 2004, 40, 385. (4) Lin, C. H.; Chang, S. L.; Wei, T. P.; Ding, S. H.; Su, W. C. Facile, one-pot synthesis of phosphinate-substituted bisphenol A and its alkaline-stable diglycidyl ether derivative. Polym. Degrad. Stab. 2010, 95, 1167. (5) Ryan, J. J.; Rawn, D. F. K. The brominated flame retardants, PBDEs and HBCD, in Canadian human milk samples collected from 1992 to 2005; concentrations and trends. Environ. Intern. 2014, 70, 1. 22


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548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591


(6) Qi, H.; Li, W. L.; Liu, L. Y.; Song, W. W.; Ma, W. L.; Li, Y. F. Brominated flame retardants in the urban atmosphere of Northeast China: Concentrations, temperature dependence and gas–particle partitioning. Sci. Total. Environ. 2014, 491, 60. (7) Ye, J. H.; Liang, G. Z.; Gu, A. J.; Zhang, Z. Y.; Han, J. P.; Yuan, L. Novel phosphorus-containing hyperbranched polysiloxane and its high performance flame retardant cyanate ester resins. Polym. Degrad. Stab. 2013, 98, 597. (8) Zhang, W. C.; He, X. D..; Song, T. L.; Jiao, Q. J.; Yang, R. J. The influence of the phosphorus-based flame retardant on the flame retardancy of the epoxy resins. Polym. Degrad. Stab. 2014, 109, 209. (9) Zhu, S. W.; Shi, W. F. Thermal degradation of a new flame retardant phosphate methacrylate polymer. Polym. Degrad. Stab. 2003, 80, 217. (10) Chang, S. S.; Condon, B.; Graves, E.; Uchimiya, M.; Fortier, C.; Easson, M.; Wakelyn, P. Flame retardant properties of triazine phosphonates derivative with cotton fabric. Fib. Polym. 2011, 12, 334. (11) Wo, S.; Shamblee, D. Flame retardant phosphonate additives for thermoplastics. EP1651737B1, 2014. (12) Failla, S.; Consiglio, G.; Finocchiaro, P. New Diamine Phosphonate Monomers as Flame-Retardant Additives for Polymers. Phosphorus, Sulfur Silicon Relat. Elem. 2011, 186, 983. (13) Shu, W. J.; Perng, L. H.; Chin, W. K. Synthesis and Characteristics of Phosphonate-Containing Maleimide Polymers. Polym. J. 2001, 33, 676. (14) Zhao, X. Synthesis and application of a durable phosphorus/silicon flame-retardant for cotton. J. Text. Inst. 2010, 101, 538. (15) Hoang, D. Q.; Kim, J.; Jang, B. N. Synthesis and performance of cyclic phosphorus-containing flame retardants. Polym. Degrad. Stab. 2008, 93, 2042. (16) Liu, Y.; Wang, Q. Synthesis of in situ encapsulated intumescent flame retardant and the flame retardancy in polypropylene. Polym. Compos. 2007, 28, 163. (17) Li, B.; Xu, M. J. Effect of a novel charring–foaming agent on flame retardancy and thermal degradation of intumescent flame retardant polypropylene. Polym. Degrad. Stab. 2006, 91, 1380. (18) Nie, S. B.; Hu, Y.; Song, L.; He, Q. L.; Yang, D. D.; Chen, H. Synergistic effect between a char forming agent (CFA) and microencapsulated ammonium polyphosphate on the thermal and flame retardant properties of polypropylene. Polym. Adv. Technol. 2008, 19, 1077. (19) Kaya, İ.; Yıldırım, M.; Kamacı, M.; Avcı, A. New Poly(azomethine-urethane)s including melamine derivatives in the main chain: Synthesis and thermal characterization. J. Appl. Polym. Sci. 2011, 120, 3027. (20) Dai, J. F.; Li, B. Synthesis, thermal degradation, and flame retardance of novel triazine ring-containing macromolecules for intumescent flame retardant polypropylene. J. Appl. Polym. Sci. 2010, 116, 2157. (21) Ke, C. H.; Li, J.; Fang, K. Y.; Zhu, Q. L.; Zhu, J.; Yan, Q. W.; Wang, Y. Z. Synergistic effect between a novel hyperbranched charring agent and ammonium polyphosphate on the flame retardant and anti-dripping properties of polylactide. Polym. Degrad. Stab. 2010, 95, 763. (22) Gu, L. Q.; Chen, G. A.; Yao, Y. W. Two novel phosphorus–nitrogen-containing halogen-free flame retardants of high performance for epoxy resin . Polym. Degrad. Stab. 2014, 108, 68. (23) Chen, H.; Zhang, K.; Xu, J., Synthesis and characterizations of novel phosphorous–nitrogen containing poly(ether sulfone)s. Polym. Degrad. Stab. 2011, 96, 197. (24) You, G. Y.; Cheng, Z. Q.; Peng, H.; He, H.W. The synthesis and characterization of a novel phosphorus–nitrogen containing flame retardant and its application in epoxy resins. J. Appl. Polym. Sci. 2014, 131, 1. (25) Jin, C. F.; He, H. W., Synthesis and Herbicidal Activity of Novel Dialkoxyphosphoryl Aryl Methyl 2-(4,6-Dimethoxypyrimidin-2-yloxy) Benzoate Derivatives. Phosphorus, Sulfur Silicon Relat. Elem 2011, 23



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598

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Page 25 of 40

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


599

Caption of Tables and Figures

600

Table 1. The formula of flame-retardant thermosets and neat DGEBA/DDS.

601

Table 2. Thermal properties of flame-retardant thermosets and neat DGEBA/DDS under

602

N2 atmosphere.

603

Table 3. Flame-retardant properties of flame-retardant thermosets and neat DGEBA/DDS.

604

Table 4. Cone calorimetry data for flame-retardant thermosets and neat DGEBA/DDS.

605

Scheme 1. The synthesis of TP, TN and TNTP.

606

Figure 1. The images of different molds with resin-hardener mixtures for thermosets

607

preparation: A, LOI mold; B, UL 94 mold; C, cone calorimeter test mold; D, mechanical

608

property test mold.

609

Figure 2. TGA (A) and DTG (B) curves of flame-retardant thermosets and neat

610

DGEBA/DDS.

611

Figure 3. Photographs of the specimens during UL-94 burning tests of DGEBA/DDS,

612

TN-3/DGEBA/DDS, TP-3/DGEBA/DDS, and TNTP-3/DGEBA/DDS: ta, the first 10s

613

ignition; tb, the time after first ignition.

614

Figure 4. Cone calorimetry tests of the thermosets: A (Heat Release Rate, HRR), B (Total

615

Heat Release, THR), C (Mass Loss Rate, MLR), D (Total Smoke Release, TSR).

616

Figure 5. Digital photographs of residues after cone calorimeter tests: A, front view; B,

617

side view.

618

Figure 6．TGA (A) and DTG (B) curves of compound TP, TN, TNTP, and DGEBA/DDS

619

thermoset.

620

Figure 7. SEM images of outer surface (A) and inner surface (B) of the residual chars 25



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

621

obtained from the UL-94 tests for flame-retardant thermosets and neat DGEBA/DDS.

622

Figure 8. FT-IR spectra of char residues after UL-94 tests.

623

Figure 9. The mechanical properties of flame-retardant thermosets and neat DGEBA/DDS.

624

26


Page 26 of 40

Page 27 of 40

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

625


Table 1. The formula of flame-retardant thermosets and neat DGEBA/DDS. Thermoset DGEBA/DDS TN-1/DGEBA/DDS TN-2/DGEBA/DDS TN-3/DGEBA/DDS TP-1/DGEBA/DDS TP-2/DGEBA/DDS TP-3/DGEBA/DDS TNTP-1/DGEBA/DDS TNTP-2/DGEBA/DDS TNTP-3/DGEBA/DDS

626 627

P (wt%) 0 0 0 0 0.5 1.0 1.5 0.5 1.0 1.5

N (wt%) 2.8 2.9 3.0 3.1 2.7 2.6 2.5 2.9 3.0 3.1

TNTP (g) 0 0 0 0 0 0 0 4.7 9.4 15.6

TN (g) 0 1.1 2.2 3.3 0 0 0 0 0 0

TP (g) 0 0 0 0 4.0 8.4 13.2 0 0 0

P: the phosphorus content in thermoset system; N: the nitrogen content in thermoset system;

628

27


DGEBA (g) 70.0 70.0 70.0 70.0 70.0 70.0 70.0 70.0 70.0 70.0

DDS (g) 23.0 23.0 23.0 23.0 23.0 23.0 23.0 23.0 23.0 23.0


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

Page 28 of 40

629 630

Table 2. Thermal properties of flame-retardant thermosets and neat DGEBA/DDS under N2 atmosphere. Thermoset P (wt %) N (wt %) Tonset (oC) Tmax (oC) Rc (wt%) DGEBA/DDS 0 2.8 374.5 408.0 16.4 TN-1/DGEBA/DDS 0 2.9 374.9 408.6 14.9 TN-2/DGEBA/DDS 0 3.0 372.0 411.8 13.5 TN-3/DGEBA/DDS 0 3.1 366.2 411.0 13.1 TP-1/DGEBA/DDS 0.5 2.7 334.5 375.2 24.4 TP-2/DGEBA/DDS 1.0 2.6 332.7 373.7 25.8 TP-3/DGEBA/DDS 1.5 2.5 306.3 352.6 28.0 TNTP-1/DGEBA/DDS 0.5 2.9 340.0 379.2 25.1 TNTP-2/DGEBA/DDS 1.0 3.0 331.0 371.7 26.3 TNTP-3/DGEBA/DDS 1.5 3.1 312.8 359.4 29.2

631 632 633

Tonset: the onset degradation temperature; Tmax: the maximum decomposition temperature; Rc: char residue at 700 oC.

634

28


Page 29 of 40

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

635

636 637 638 639


Table 3. Flame-retardant properties of flame-retardant thermosets and neat DGEBA/DDS. Thermoset

P (wt%)

N (wt%)

LOI (%)

DGEBA/DDS TN-1/DGEBA/DDS TN-2/DGEBA/DDS TN-3/DGEBA/DDS TP-1/DGEBA/DDS TP-2/DGEBA/DDS TP-3/DGEBA/DDS TNTP-1/DGEBA/DDS TNTP-2/DGEBA/DDS TNTP-3/DGEBA/DDS

0 0 0 0 0.5 1.0 1.5 0.5 1.0 1.5

2.8 2.9 3.0 3.1 2.7 2.6 2.5 2.9 3.0 3.1

22.5 25.4 27.3 29.0 30.6 31.0 31.8 30.3 31.5 32.4

Flammability test of UL-94 t1 (s) >50 >50 >50 >50 30.75 15.69 13.72 9.43 9.07 7.38

NR: no rating in UL-94 test t1: the burning time after the first ignition; t2: the burning time after the second ignition.

29


t2 (s) / / / / 0 0 0 1.3 0 0

Rating NR NR NR NR V-2 V-1 V-1 V-1 V-0 V-0

Dripping Yes No Yes Yes No No No No No No


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

Page 30 of 40

640 641

642 643

Table 4. Cone calorimetry data for flame-retardant thermosets and neat DGEBA/DDS. Thermoset

TTI (s)

PHRR (kW/m2)

THR (MJ/m2)

AMLR (g/s)

TSR (m2/m2)

ACOY (kg/kg)

ACO2Y (kg/kg)

DGEBA/DDS TN-3/DGEBA/DDS TP-3/DGEBA/DDS TNTP-3/DGEBA/DDS

60 25 23 34

872.8 943.3 312.6 253.0

88.5 78.4 59.0 65.8

0.057 0.078 0.037 0.043

3508.1 3369.7 1823.3 2187.8

0.0726 0.0596 0.1206 0.0700

1.49 1.34 1.19 1.00

TTI: time to ignition; PHRR: peak heat release rate; THR: total heat release; AMLR: average mass loss rate; TSR: total smoke release; ACOY: average carbon monoxide yield; ACO2Y: carbon dioxide yield.

30


Page 31 of 40

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


644

645 646

Scheme 1. The synthesis of TP, TN and TNTP.

647

31



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

Page 32 of 40

648

A

C

B

D

= 15 s 649

A

650

Figure 1. The images of different molds with resin-hardener mixtures for thermosets

651

preparation: A, LOI mold; B, UL 94 mold; C, cone calorimeter test mold; D, mechanical

652

property test mold.

32


Page 33 of 40

0

100

1A

-2

Deriv. Weight (% / min)

90

Weight (%)

80 70 60 50 TN-1/DGEBA/DDS TP-1/DGEBA/DDS TNTP/DGEBA/DDS DGEBA/DDS

20 100

200

300

400

500

Temperature (oC)

600

700

100


Weight (%)

60 50

-2

300 400 o 500 Temperature ( C)

600

700

500

600

700

500

600

700

2B

-4 -6 -8

20 10 100

200

300

400

TN-2/DGEBA/DDS TP-2/DGEBA/DDS TNTP-2/DGEBA/DDS DGEBA/DDS

-12


30

-14 -16

o

500

Temperature ( C)

600

100

700

200

300

400

Temperature (oC)

0

100


3A

80 70 60 50

-2

3B

-4 -6 -8

-10


-12

100

200

300

400


-14 -16

10

653

200

-10

40

20


0

2A

70

30

-8

-16

80

40

-6

-14

100

90

-4

-12

10

90

1B

-10

40 30

Weight (% )

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


500

Temperature (oC)

600

700

100

200

300

400

Temperature (oC)

654

Figure 2. TGA (A) and DTG (B) curves of flame-retardant thermosets and neat

655

DGEBA/DDS.

33



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

DGEBA/DDS ta = 10s

DGEBA/DDS tb= 50s

DGEBA/DDS tb = 15s

Page 34 of 40

DGEBA/DDS Test Over

TN-3/DGEBA/DDS ta = 10s

TN-3/DGEBA/DDS tb = 15s

TN-3/DGEBA/DDS tb = 50s

TN-3/DGEBA/DDS

TP-3/DGEBA/DDS ta = 10s

TP-3/DGEBA/DDS tb = 5s

TP-3/DGEBA/DDS

TP-3/DGEBA/DDS

Test Over

Test Over

TNTP-3/DGEBA/DDS TNTP-3/DGEBA/DDS TNTP-3/DGEBA/DDS TNTP-3/DGEBA/DDS Test Over tb = 10s tb = 5s ta = 10s

656 657

Figure 3. Photographs of the specimens during UL-94 burning tests of DGEBA/DDS,

658

TN-3/DGEBA/DDS, TP-3/DGEBA/DDS, and TNTP-3/DGEBA/DDS: ta, the first 10s

659

ignition; tb, the time after first ignition. 34


Page 35 of 40

800


600 400 200

Total Heat Release (MJ / m2)

100

A

B 80 60 40 TN-3/DGEBA/DDS TP-3/DGEBA/DDS TNTP-3/DGEBA/DDS DGEBA/DDS

20

0

0 0

100 200 300 400 500 600 700 800 900 1000

Time (s)

0.5

C 0.4 0.3


0.2 0.1 0.0

0

100 200 300 400 500 600 700 800 900 1000

Time ( s)

4000

Total Smoke Release (m2/m2)

2 Heat Release Rate (kW / m )

1000

Mass Loss Rate (g / s)

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


D

3500 3000 2500 2000 1500


1000 500 0

0

660

100 200 300 400 500 600 700 800 900 1000

0

100 200 300 400 500 600 700 800 900 1000

Time (s)

Time (s)

661

Figure 4. Cone calorimetry tests of the thermosets: A (Heat Release Rate, HRR), B (Total

662

Heat Release, THR), C (Mass Loss Rate, MLR), D (Total Smoke Release, TSR).

35



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

1A: DGEBA/DDS

2A: TN-3/DGEBA/DDS

3A: TP-3/DGEBA/DDS

4A: TNTP-3/DGEBA/DDS

3B: TP-3/DGEBA/DDS

4B: TNTP-3/DGEBA/DDS

663 664 665

Figure 5. Digital photographs of residues after cone calorimeter tests: A, front view; B, side view.

36


Page 36 of 40

Page 37 of 40

666 90

TN TP TNTP DGEBA

A

80 70 60 50

0 -2

Deriv. weight (%/min)

100

Weight (%)

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


-4 -6 -8

-10

40

-12

30

TN TP TNTP DGEBA/DDS

-14

20

-16

10

-18

0 100

667

B

200

300

400

500

Temperature (oC)

600

700

100

200

300 400 o 500 Temperature ( C)

600

700

668

Figure 6．TGA (A) and DTG (B) curves of compound TP, TN, TNTP, and DGEBA/DDS

669

thermoset.

37



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

1A: DGEBA/DDS

1B: DGEBA/DDS

2A: TN-3/DGEBA/DDS

2B: TN-3/DGEBA/DDS

3A: TP-3/DGEBA/DDS

3B: TP-3/DGEBA/DDS

4B: TNTP-3/DGEBA/DDS

4A: TNTP-3/DGEBA/DDS

670 671

Figure 7. SEM images of outer surface (A) and inner surface (B) of the residual chars

672

obtained from the UL-94 tests for flame-retardant thermosets and neat DGEBA/DDS.

38


Page 38 of 40

Page 39 of 40

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


D

C

TNTP-3/DGEBA/DDS

TP-3/DGEBA/DDS

B TN-3/DGEBA/DDS A DGEBA/DDS

4000 3500 3000 2500 2000 1500 1000

673 674

500

Wavenumbers (cm-1)

Figure 8. FT-IR spectra of char residues after UL-94 tests.

675

39



80 70 60 50 40 30 20

TP-1/DGEBA/DDS TP-2/DGEBA/DDS TP-3/DGEBA/DDS

TN-1/DGEBA/DDS TN-2/DGEBA/DDS TN-3/DGEBA/DDS

2

90

DGEBA/DDS TNTP-1/DGEBA/DDS TNTP-2/DGEBA/DDS TNTP-3/DGEBA/DDS

Izod impact strength (kJ/m

100

10

676 677

14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

)

110

Flexural strength (Mpa)

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

0

Page 40 of 40

DGEBA/DDS TNTP-1/DGEBA/DDS

TP-1/DGEBA/DDS

TN-1/DGEBA/DDS

TNTP-2/DGEBA/DDS

TP-2/DGEBA/DDS

TN-2/DGEBA/DDS

TNTP-3/DGEBA/DDS

TP-3/DGEBA/DDS

TN-3/DGEBA/DDS

Figure 9. The mechanical properties of flame-retardant thermosets and neat DGEBA/DDS.

40


Functional Group Effect on Char Formation, Flame ... - ACS Publications

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