Nitrogen-Containing Additive with

Article pubs.acs.org/IECR

Synthesis of a Phosphorus/Nitrogen-Containing Additive with Multifunctional Groups and Its Flame-Retardant Effect in Epoxy Resin Shuang Yang,* Jun Wang, Siqi Huo, Mei Wang, and Liufeng Cheng School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, People’s Republic of China ABSTRACT: A novel additive, tri(phosphaphenanthrene-maleimide-phenoxyl)-triazine (DOPO-TMT), was successfully synthesized. The chemical structure was characterized by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance. DOPO-TMT was blended with epoxy resins to prepare flame-retardant thermosets. The flame-retardant properties were evaluated using limited oxygen index (LOI), vertical burning (UL94), and cone calorimeter tests. The results indicated that DOPO-TMT exhibited excellent flame-retardant effect. The flame-retardant mechanism was studied by thermogravimeric analysis (TGA), pyrolysis-gas chromatography/mass spectrometry, and thermogravimetric analysis/infrared spectrometry (TGAFTIR) coupled with the morphology and chemical analysis of the char residues. The results disclosed that DOPO-TMT exerted biphase flame-retardant effect. In gaseous-phase, DOPO-TMT released phosphorus- and nitrogen-containing free radicals with quenching effect under thermal decomposition. The morphologies of the char residues exhibited intumescent and honeycombed structure with a small number of holes on the surfaces. The honeycombed char structure served as an excellent protective layer. A few number of holes on the surface facilitated the concentrated release of free radicals to implement a strong quenching effect. The functional groups of DOPO-TMT synergistically interacted to endow epoxy resin with excellent flame retardancy. flame-retardant groups were integrated into one molecule.42 Hence, the addition of flame retardant with multiflameretardant functional groups is a promising way to develop flame-retardant epoxy resins. In this paper, maleimido-substituted aromatic s-triazine (TMT) was synthesized using a new method with high yield. Then a novel additive DOPO-TMT with multiflame-retardant groups in one molecule was successfully synthesized via the addition reaction between DOPO and TMT. The chemical structure of DOPO-TMT was characterized by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR). The LOI, vertical burning (UL94) and cone calorimeter tests were used to evaluate the flame-retardant properties of the EP thermosets. The flame-retardant mechanism was studied by a series of methods.

1. INTRODUCTION Epoxy resins are widely used as advanced matrix resin in the electronic and electrical industries due to their attractive characteristics of high tensile strength and modulus, high adhesion to substrates, good chemical and corrosion resistance, excellent dimensional stability, and superior electrical properties.1−5 However, conventional epoxy resins are combustible, which limits its application in high flame-resistance requirement of advanced materials. Up to now, research efforts on enhancing the flame retardancy of epoxy resins have attracted increasing attention. Traditionally, halogen-containing compounds have been widely used to improve the flame retardancy of epoxy resins. Currently, the applications of halogenated compounds are restrained for environmental reasons.6,7 Therefore, there is a trend to develop and apply halogen-free flame retardants. In recent years, nonhalogen flame retardants, such as phosphorus-,8−13 silicon-,14−18 and boron-based19−22 have been widely developed and applied to epoxy resins. Among them, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and its derivatives have received a great deal of attention due to their high reactivity, high thermal stability, and flame-retardant efficiency.23−28 However, conventional DOPObased epoxy resins show great decline in thermal properties; moreover, single flame retardant composition limits the further enhancing of flame retardancy of epoxy resins.29−31 Therefore, there is a trend to develop a compounded epoxy resin system with multiflame-retardant components. On the basis of some previous reports, multiple flame-retardant compositions containing phosphaphenanthrene and other functional groups with certain flame-retardant effects, such as maleimide,32,33 striazine,34,35 triazine-trione,36,37 phosphazene,38 and silsesquioxane,39−41 can endow epoxy resins with higher flame-retardant properties compared with those only containing DOPO. Furthermore, synergistic effect was observed when different © XXXX American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. Diglycidyl ether of bisphenol A (DGEBA) with an epoxide equivalent weight (EEW) of about 188 g/equiv was provided by Yueyang Baling Huaxing Petrochemical Co., Ltd. N-(4-hydroxyphenyl) maleimide (HPM) was obtained from Puyang Willing Chemicals Co., Ltd. 9,10-Dihydro-9-oxa10-phosphaphenanthrene-10-oxide (DOPO) was purchased from Huizhou Sunstar Technology Co., Ltd. Cyanuric chloride, diglyme, and triethylamine were obtained from Aladdin Reagents (Shanghai) Co., Ltd. 4,4′-Diamino-diphenyl sulfone (DDS) was purchased from Sinopharm Chemical Reagent Co., Ltd. Received: June 3, 2015 Revised: July 24, 2015 Accepted: August 3, 2015

A

DOI: 10.1021/acs.iecr.5b02026 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Scheme 1. Synthesis Route of TMT

Scheme 2. Synthesis Route of DOPO-TMT

2.2. Synthesis of TMT. HPM (31.2 g, 0.165 mol), triethylamine (16.7 g, 0.165 mol), and diglyme (200 mL) were introduced into a 500 mL, three-neck and round-bottom glass flask equipped with a mechanical stirrer, reflux condenser, thermometer, and dry nitrogen inlet. The mixture was heated to 85 °C under nitrogen atmosphere and stirred until HPM dissolved completely. Cyanuric chloride (9.225 g, 0.05 mol) dissolved in 80 mL of diglyme was added over 2 h. After that, the mixture was further heated to 110 °C and stirred for 6 h. Then the mixture was distilled to remove diglyme; the crude product was washed successively for 3 times with deionized water and ethyl acetate, and then vacuum-dried at 60 °C for 24 h. The reaction formula is shown in Scheme 1. Yield: 29.3 g (91.3%). Elemental analysis: C, 60.94 (cal 61.68), N, 12.87 (cal 13.08), H, 2.83 (cal 2.8). FTIR (KBr, cm−1): 3101 and 691 (CC), 1713 (CO), 1508 (triazine ring skeleton). 1H NMR (DMSO-d6, ppm): 7.2 (CHCH, 6H), 7.38 and 7.39 (Ar−H, 12H). 2.3. Synthesis of DOPO-TMT. TMT (25.7 g, 0.04 mol), DOPO (43.2 g, 0.2 mol), and diglyme (200 mL) were introduced into a 500 mL, three-neck and round-bottom glass flask equipped with a mechanical stirrer, reflux condenser, thermometer, and dry nitrogen inlet. The mixture was stirred at 130 °C for 8 h. Then the mixture was distilled to remove diglyme, and the crude product was washed with ethyl alcohol 3 times and then vacuum-dried at 90 °C for 6 h. The reaction

formula is shown in Scheme 2. Yield: 42 g (81.4%). Elemental analysis: C, 62.05 (cal 64.2), N, 6.64 (cal 6.5), H, 3.48 (cal 3.5). 2.4. Preparation of EP/DOPO-TMT Hybrids and the Control Samples. DGEBA and DOPO-TMT were dispersed in ethyl alcohol under ultrasonic dispersing. After that, the mixture was distilled to remove the solvent with continuous stirring. Then the mixture was heated to 135 °C and stoichiometric DDS was added. After a homogeneous solution was obtained, the mixture was degassed under vacuum and poured directly into a preheated mold and thermally cured in an air convection oven at 160 °C for 2 h and then at 180 °C for 5 h. The preparation process of control sample EP/TMT was similar to that of EP/DOPO-TMT samples. The contents of maleimide and triazine groups in the EP/TMT sample are the same as those in the EP/DOPO-TMT-1.0 sample. The control samples, neat EP and EP/DOPO, were prepared as follows. DDS with or without DOPO was mixed with DGEBA at 135 °C. After well mixed, the mixture was degassed and then poured directly into preheated mold and thermally cured in an air convection oven at 160 °C for 2 h and then at 180 °C for 5 h. The phosphorus content of the EP/DOPO sample is the same as that of the EP/DOPO-TMT-1.0 sample. All the compositions of composites are listed in Table 1. 2.5. Characterization. Fourier Transform Infrared (FTIR) spectra were obtained using a Nicolet 6700 infrared B


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Industrial & Engineering Chemistry Research Table 1. Formulas of the Cured Epoxy Resins sample code EP EP/DOPO EP/TMT EP/DOPOTMT-0.5 EP/DOPOTMT-0.75 EP/DOPOTMT-1.0 EP/DOPOTMT-1.25 EP/DOPOTMT-1.5

DGEBA (g)

DDS (g)

DOPO (g)

TMT (g)

DOPOTMT (g)

P content (wt %)

100 100 100 100

33 33 33 33

0 10 0 0

0 0 10 0

0 0 0 10

0 1.0 0 0.5

100

33

0

0

15.5

0.75

100

33

0

0

21.5

1.0

100

33

0

0

28

1.25

100

33

0

0

35

1.5

spectrometer. The powdered samples were thoroughly mixed with KBr and then pressed into pellets. 1 H and 31P NMR spectra were obtained on a Bruker AV400 NMR spectrometer using DMSO-d6 as the solvent. Elemental analysis (EA) was performed on a Vario EL cube Elemental Analyzer. The LOI values were measured at room temperature on a JF3 oxygen index meter (Jiangning Analysis Instrument Company, China) according to the ISO4589-1984 standard and dimensions of all samples were 130 × 6.5 × 3 mm3. Vertical burning (UL-94) tests were carried out on the NK8017A instrument (Nklsky Instrument Co., Ltd., China) with the dimension of 130 × 13 × 3 mm3 according to the UL94 test standard. Cone calorimeter measurements were performed on a FTT cone calorimeter according to the ISO 5660 standard under an external heat flux of 50 kW/m2. The dimension of samples was 100 × 100 × 3 mm3. The measurement for each specimen was repeated three times, and the error values of the typical cone calorimeter data were reproducible within ±5%. Thermogravimetric analysis (TGA) was performed using NETZSCH STA449F3 at a heating rate of 10 °C/min under nitrogen atmosphere from 40 to 800 °C. Py-GC/MS analysis was carried out with an Agilent 7890/ 5975 GC/MS. The injector temperature was 250 °C, 1 min at 50 °C then the temperature was increased to 280 °C at a rate of 8 °C/min. The temperature of the GC/MS interface was 280 °C, and the cracker temperature was 500 °C. Thermogravimetric analysis/infrared spectrometry (TGAFTIR) was performed on TGA 209F1 NETZSCH that was interfaced to the Nicolet 6700 FTIR spectrophotometer. About 6.0 mg sample was put in an alumina crucible and heated from 40 to 700 °C at a heating rate of 20 °C/min under N2 atmosphere. Morphological studies on the residual chars were conducted using a JSM-5610LV scanning electron microscope (SEM) at an acceleration voltage of 25 kV.

Figure 1. FTIR spectra of DOPO, TMT, and DOPO-TMT.

absorptions of CO and triazine ring skeleton, respectively. The absorption peak at 1370 cm−1 is attributed to the stretching vibration absorptions of C−N. The absorption peaks at 925 and 755 cm−1 are assigned to the stretching vibration of P−O−Ph. In addition, the absorption peaks of CC at 3110 and 694 cm−1 and P−H at 2434 cm−1 disappear. The above discussion indicates that DOPO has completely reacted with TMT. The 1H and 31P NMR spectra of DOPO-TMT are shown in Figures 2 and 3, respectively. As shown in Figure 2, DOPO-

Figure 2. 1H NMR spectrum of DOPO-TMT.

3. RESULTS AND DISCUSSION 3.1. Synthesis of DOPO-TMT. As an intermediate of DOPO-TMT, TMT was first prepared via nucleophilic substitution reaction between cyanuric chloride and HPM using a new method, and a higher yield of TMT was obtained compared with the previous report.35 Then DOPO-TMT was synthesized through the addition reaction of DOPO and TMT. As shown by FTIR spectra in Figure 1, the absorption peaks at 1705 and 1504 cm−1 are attributed to the stretching vibration

TMT shows the chemical shifts of aromatic hydrogen, including the benzene ring and phosphaphenanthrene group at 6.6−8.5 ppm, CH at 4.0−4.3 ppm, and CH2 at 3.2 ppm. As shown in Figure 3, the 31P NMR spectrum of DOPO-TMT shows a single peak at 29.5 ppm. These results further confirm that DOPO-TMT is successfully prepared. 3.2. Flame Retardancy. 3.2.1. LOI and UL94 Tests of the Cured Epoxy Resins. The flame-retardant properties of EPC


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Industrial & Engineering Chemistry Research

DOPO-TMT-1.0 are higher than those of EP/DOPO with the same phosphorus content and EP/TMT with the same contents of maleimide and triazine groups. It is evident that there is a synergistic effect between different functional groups of DOPO-TMT. 3.2.2. Cone Calorimeter Analysis of the Cured Epoxy Resins. The flame-retardant behavior of DOPO-TMT on epoxy resin was further investigated using the cone calorimetry test. The characteristic parameters, such as the average of heat release rate (av-HRR), peak of heat release rate (pk-HRR), average of effective heat of combustion (av-EHC), total heat release (THR), time of flameout (TOF), char yields at 400 s, average CO yield (av-COY), and average CO2 yield (av-CO2Y) are summarized in Table 3. The curves of heat release rate are shown in Figure 4. As illustrated in Figure 4, the neat EP burns rapidly after ignition

Figure 3. 31P NMR spectrum of DOPO-TMT.

based thermosets were initially investigated using limited oxygen index (LOI) and UL94 vertical burning tests. As shown in Table 2, the LOI value of neat EP is only 22.5%, Table 2. LOI and UL94 Test Results of the Cured Epoxy Resins sample code

P content (%)

LOI (%)

UL-94 (3 mm)

EP EP/DOPO EP/TMT EP/DOPO-TMT-0.5 EP/DOPO-TMT-0.75 EP/DOPO-TMT-1.0 EP/DOPO-TMT-1.25 EP/DOPO-TMT-1.5

0 1.0 0 0.5 0.75 1.0 1.25 1.5

22.5 34 25.5 29.5 33 36.2 37.5 38.4

NR V-1 NR NR V-1 V-0 V-0 V-0

Figure 4. HRR curves of the cured epoxy resins.

and the HRR reaches a sharp peak with a pk-HRR of 1208 kW/ m2. In addition, the neat EP has the highest av-HRR of 177 kW/m2. With the addition of DOPO, TMT, and DOPO-TMT, the av-HRR and pk-HRR of the modified EP decrease sharply. Notably, the av-HRR and THR of EP/DOPO-TMT-1.0 are lower than those of the control samples EP/DOPO and EP/ TMT, indicating the synergistic effect between different functional groups of DOPO-TMT on the decrease of heat release. Av-EHC, which is the ratio of average of heat release rate (av-HRR) to the average mass loss rate from the cone

whereas those of EP/TMT and EP/DOPO are increased to 25.5% and 34%, respectively. For the EP/DOPO-TMT, the LOI value of EP/DOPO-TMT-0.5 is 29.5% with the phosphorus content of only 0.5%. However, EP/DOPOTMT-0.5 fails to pass the UL94 test. When the phosphorus content reaches 1.0%, the LOI value of EP/DOPO-TMT-1.0 increases to 36.2%, and the sample achieves the UL94 rating of V-0. With the further increase of phosphorus content, the LOI performance is promoted without degrading the UL94 rating. It is worth noting that the LOI value and UL94 rating of EP/

Table 3. Combustion Parameters of the Cured Epoxy Resins Obtained from Cone Calorimeter Test sample code EP EP/DOPO EP/TMT EP/DOPO-TMT-0.5 EP/DOPOTMT-0.75 EP/DOPO-TMT-1.0 EP/DOPOTMT-1.25 EP/DOPO-TMT-1.5

TOF (s)

av-HRR (kW/m2)

pk-HRR (kW/m2)

av-EHC (MJ/kg)

THR (MJ/m2)

av-COY (kg/kg)

av-CO2Y (kg/kg)

char yields at 400 s (%)

430 360 425 311 315

177 155 163 157 141

1208 833 858 919 694

22.2 19.6 21.7 20.4 18.9

80.6 66.3 73.5 71.2 63.7

0.063 0.095 0.072 0.092 0.096

1.589 1.255 1.544 1.413 1.274

14.8 21.5 22.8 19.0 21.0

322 344

135 125

776 556

18.3 17.2

60.6 56.5

0.105 0.11

1.235 1.11

22.3 23.1

339

132

674

17.4

59.6

0.122

1.125

21.7

D


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Industrial & Engineering Chemistry Research calorimetry test, discloses the burning rate of volatile gases in gaseous-phase flame during combustion. As shown in Table 3, neat EP has the highest av-EHC of 22.2 MJ/kg. The EP/TMT sample shows only a slight decrease in av-EHC, suggesting that the flame-retardant effect of TMT mainly functions in the condensed phase. The av-EHC obviously decreases with the incorporation of DOPO and DOPO-TMT. Further, the avEHC of EP/DOPO-TMT-1.0 is lower than those of EP/ DOPO and EP/TMT, indicating that the different functional groups of DOPO-TMT synergistically function in the gaseous phase. The less combustion heat generated from the burning of volatile gases in the gaseous phase is ascribed to the quenching effect from the gaseous-phase pyrolysis products of DOPOTMT. This is further proved by TOF, av-COY, and av-CO2Y. As presented in Table 3, the neat EP has the highest TOF of 430 s. The TOFs are shortened to some extent with the incorporation of DOPO-TMT. In addition, the av-COY increases, whereas the av-CO2Y decreases with the addition of DOPO-TMT, indicating the flame-retardant quenching effect in the gaseous-phase, since more av-COY and less avCO2Y mean more incomplete combustion products (CO) and less complete combustion products (CO2).37 It is inferred that DOPO-TMT decomposes to release pyrolysis fragments with quenching effect in the gaseous-phase during combustion. The char yields at 400 s obtained from the cone calorimeter test are shown in Table 3. For the neat EP, the char yield at 400 s is just 14.8%. The char yields increase with the addition of DOPO, TMT, and DOPO-TMT, indicating that all the functional groups of DOPO-TMT can exert flame-retardant effect in the condensed phase. In accordance with the above discussion, it is deduced that the enhanced flame retardancy of EP/DOPO-TMT is ascribed to the synergistic interaction between different functional groups of DOPO-TMT, which simultaneously function in the gaseous and condensed phase. Further discussions on the action mechanism of DOPO-TMT are given in the subsequent sections. 3.3. Flame-Retardant Mechanism of DOPO-TMT on Epoxy Resin. 3.3.1. Thermal Analysis of the Cured Epoxy Resins. The weight loss behavior was detected by TGA for exploring the basic degradation information. The TG and DTG curves of DOPO-TMT, neat EP, EP/DOPO, EP/TMT, and EP/DOPO-TMT hybrids in the N2 atmosphere are shown in Figures 5 and 6, respectively. The characteristic thermal decomposition data, such as temperature at 5% weight loss (T5%), temperature at maximum weight loss rate (Tmax), and char yields at 800 °C are listed in Table 4. DOPO-TMT exhibits two weight loss stages, and accordingly, there are two differential thermogravimetric (DTG) peaks, as seen from Figure 5. The first stage is in the temperature range of 300−450 °C, corresponding to a strong DTG peak at 409 °C (Tmax1), and the weight loss is 41%. The first stage reflects the main decomposition process, and the main weight loss occurs during this stage. The weight loss results from the decomposition of DOPO, maleimide, and phenoxyl groups. The second stage is in the temperature range of 450−565 °C, corresponding to a small and blunt DTG peak at 495 °C (Tmax2) with a weight loss of 12%, due to the dissociation of triazine rings.43 The char yield of DOPO-TMT at 800 °C is 40%, exhibiting good charring performance. The decomposition route of DOPO-TMT will be further investigated in the subsequent discussion.

Figure 5. TG and DTG curves of DOPO-TMT under N2 atmosphere.

For the neat EP, EP/DOPO, EP/TMT, and EP/DOPOTMT hybrids, the TG and DTG curves show only one weight loss stage, which are different from that of DOPO-TMT. As can be calculated, the percentage content of triazine groups in EP/ TMT and EP/DOPO-TMT hybrids is less than 1.26%. So the weight loss caused by the dissociation of triazine groups is inconspicuous, resulting in only one weight loss stage. The T5% and Tmax of EP/DOPO and EP/DOPO-TMT hybrids decrease with the addition of DOPO and DOPO-TMT. The less stable DOPO group induces the decomposition of EP matrix in advance, which is responsible for the decreased T5% and Tmax. The char yields at 800 °C increase with the incorporation of DOPO, TMT, and DOPO-TMT, indicating that all the functional groups of DOPO-TMT can promote the char formation. The simple addition of char residues produced by the EP matrix and DOPO-TMT are lower than those obtained from the TGA tests. It is disclosed that there are chemical reactions between the EP matrix and the pyrolytic products of DOPO-TMT, which promotes the formation of char residues. It is worth noting that the decomposition rate of EP/DOPOTMT thermosets is between those of EP/DOPO and EP/TMT thermosets during the main decomposition stage (300−450 °C), indicating that the maleimide and triazine groups do the opposite compared with the phosphaphenanthrene group during the pyrolysis process of EP thermosets. The phosphaphenanthrene group promotes the decomposition of EP matrix in advance, whereas the thermal stable maleimide E


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Figure 7. FTIR spectrum of pyrolysis products for DOPO-TMT at 500 °C.

account. The main absorption peaks are observed at 3652 cm−1 (H2O), 3056, 1592, and 1511 cm−1 (aromatic rings), 1740 cm−1 (carboxylic acid RCOOH), 1376 cm−1 (C−N), 966 and 925 cm−1 (nitrogen-containing products and P−O−Ph), 1267 cm−1 (PO), 1174 cm−1 (C−O), 1118 cm−1 (−P−O−P−O), and 754 cm−1 (phenol). On the basis of the molecule structure of DOPO-TMT (Scheme 2), it is sure that the phosphoruscontaining products are derived from the degraded phosphaphenanthrene group and the nitrogen-containing products from the dissociation of maleimide and triazine groups. As a result, the fragments at gas phase are mainly composed of phosphorus- and nitrogen-containing products, which exert a quenching effect in the gaseous phase during the combustion of EP thermosets. 3.3.3. Py-GC/MS Analysis. The degradation process of DOPO-TMT was disclosed to explain why DOPO-TMT enhanced the flame retardancy of the epoxy resin. The pyrolysis temperature is chosen at 500 °C because it is close to the ignition temperature of the epoxy resin. The typical fragment flows with some characteristic ionic peaks selected, and the results are illustrated in Figure 8. The deduced pyrolysis route of DOPO-TMT is illustrated in Figure 9. As shown in Figures 8 and 9, the m/z of fragments at 78, 79, 80, and 81 are considered as the triazine free radicals. The m/z of fragments at 107, 108, 135, and 136 are identified as nitrogen-containing free radicals produced by the decomposition of maleimide and phenoxyl groups. The m/z of fragments at 168, 169, and 170 are determined as dibenzofuran, o-phenylphenoxyl free radical, and o-phenylphenol, respectively.42 The m/z of fragments at 63 and 64 are considered as PO2 and HPO2 free radicals produced by the PO free radicals from the broken phosphaphenanthrene group combining with OH, H, or O free radicals.42 The m/z of fragment at 139 and 141 are considered as the OP−O−Ph free radical. The m/z of fragment at 115 is considered as the carboxylic acid compound derived from the ring-opening reaction of the maleimide group. The Py-GC/MS results match well with those of TGA-FTIR. From the pyrolysis route of DOPO-TMT (Figure 9), it is obvious that the generated free radicals can capture and quench the H, O, and OH free radicals, and subsequently inhibit the free radical chain reaction of

Figure 6. TG and DTG curves of the cured epoxy resins under N2 atmosphere.

Table 4. Thermal Parameters of the Cured Epoxy Resins sample code DOPO-TMT EP EP/DOPO EP/TMT EP/DOPO-TMT0.5 EP/DOPO-TMT0.75 EP/DOPO-TMT1.0 EP/DOPO-TMT1.25 EP/DOPO-TMT1.5

T5% (°C)

Tmax1 (°C)

Tmax2 (°C)

char yields at 800 °C (%)

368 384 342 372 375

409 410 368 403 406

495 − − − −

40 17.6 26.5 26.3 31.8

357

393

−

32.5

348

388

−

32.0

342

382

−

30.3

347

384

−

32.9

and triazine groups retard the decomposition process. The thermal decomposition mode will significantly affect the flame retardancy of EP thermosets. 3.3.2. TGA-FTIR Analysis. The TGA-FTIR analysis was utilized to find out the main compositions of the gas products from the thermal degradation of DOPO-TMT. The FTIR spectrum of DOPO-TMT at 500 °C is shown in Figure 7. Some characteristic absorption peaks should be taken into F


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Figure 8. Typical Py-GC/MS spectra of DOPO-TMT at different retention time.

free radicals also function in the gaseous phase as EP/DOPOTMT-1.0 has lower av-EHC than that of EP/DOPO (Section 3.2.2). In addition, the incorporation of triazine and maleimide groups promotes the char yields of epoxy resins (Section 3.3.1). Hence, we think the inert free radicals produced by the fragmentation of triazine and maleimide groups also function in the condensed-phase through combining with the molecular chains of matrix containing active terminal free radical and then prevent the matrix from further degradation.38 Further evidence of the flame-retardant effect in the condensed phase will be provided in the following discussion. 3.3.4. FTIR Study of the Char Residues. The FTIR spectra of the char residues of the cured epoxy resins after UL94 test are shown in Figure 11. The absorbance peaks of neat EP at 1590 and 1510 cm−1 indicate the formation of polyaromatic carbons, which are observed in other spectra curves as well. For the EP/DOPO-TMT samples, except the peaks at 1590 and

combustion and reduce the combustion intensity in the gaseous phase. Figure 10 shows the Py-GC/MS spectrum of EP/DOPOTMT-1.0. Compared with the Py-GC/MS spectrum of DOPOTMT, the m/z of fragments at 63, 64, and 141 and 168−170 recur in that of EP/DOPO-TMT-1.0. It is confirmed that the free radicals with the quenching effect are still produced by DOPO-TMT during the combustion of EP/DOPO-TMT thermosets. On the basis of the above discussion, it is concluded that a series of free radicals with quenching effect are produced from the decomposition of DOPO-TMT. However, we think the quenching effect in the gaseous phase is mainly attributed to the phosphorus-containing free radicals, such as PO and PO2 free radicals, due to the fact that the DOPO-containing samples have much lower av-EHC values than those of neat EP and EP/ TMT samples as discussed in Section 3.2.2. Of course, other G


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Figure 9. Deduced pyrolysis route of DOPO-TMT.

Figure 10. Py-GC/MS spectrum of EP/DOPO-TMT-1.0. Figure 11. FTIR spectra of the char residues after UL94 test.

1510 cm−1, new absorbance peaks at 1764, 1706, 1382, 935, 756, and 717 cm−1 appear and become more and more obvious with the increasing content of DOPO-TMT. The absorbance peaks of CO at 1764, 1706, and 717 cm−1 and C−N at 1382 cm−1 are ascribed to the fragments resulting from the degradation of maleimide groups, further confirming the flame-retardant effect of inert free radicals in the condensedphase (Section 3.3.3). In addition, we think the triazine structure remains in the char residues, and its absorption peak at 1510 cm−1 is overlapped with that of polyaromatic carbons. The new absorbance peaks at 935 and 756 cm−1 indicate the existence of P−O−Ph in the char residue, proving the flameretardant effect of the DOPO groups in the condensed phase.

The DOPO group decomposes to form phosphate and polyphosphate, which promote the charring of EP matrix to form the phosphorus-rich viscous char layer. The triazine and maleimide groups decompose to produce inert free radicals which react with molecular chains of EP matrix containing active terminal free radical and then retard the matrix from further degradation. The different functional groups of DOPOTMT synergistically act in the condensed phase to form highly cross-linked and thermal stable char layers. 3.3.5. Morphological Study of the Char Residues. Morphological study of the char residues was conducted by H


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Figure 12. SEM and digital images of char residues after the cone calorimeter test.

4. CONCLUSIONS A novel flame-retardant additive, DOPO-TMT, with maleimide, phosphaphenanthrene, and triazine functional groups was successfully synthesized. DOPO-TMT was then blended with DGEBA to prepare flame-retardant epoxy resins. The results of combustion tests indicated that the flame retardancy of EP/ DOPO-TMT thermosets was remarkably promoted with the incorporation of DOPO-TMT. The research on the flameretardant mechanism of DOPO-TMT on epoxy resin disclosed that DOPO-TMT exerted biphase flame-retardant effect on the epoxy resin. The flame-retardant effect is achieved by means of carbonization, swelling, and high concentration of free radicals. The functional groups of DOPO-TMT synergistically worked to exert the barrier and quenching effects, thus endowing EP/ DOPO-TMT thermosets with excellent flame retardancy.

visual observation and SEM. The SEM and digital images of the char residues after the cone calorimeter test are shown in Figure 12. The char of neat EP shows a small amount of residue with a fragmentary structure which is unable to serve as a protective layer, whereas those of the EP/DOPO-TMT samples exhibit higher char yields with cross-linked and rigid char layers. From the top views of the char layers, it is evident that the EP/DOPO-TMT samples exhibit a more continuous and compact surface with some holes on the surfaces. From the side views of the char residues, the expansion ratios of the EP/ DOPO-TMT samples are markedly increased. The char residues were further investigated by SEM. The interior and exterior char layers of the EP/DOPO-TMT-1.0 and EP/DOPO-TMT-1.25 were studied by SEM as shown in Figure 12. The exterior char layers of EP/DOPO-TMT-1.0 and EP/DOPO-TMT-1.25 show continuous and compact structures with just several holes. The interior structures of EP/ DOPO-TMT-1.0 and EP/DOPO-TMT-1.25 chars present intumescent and honeycombed structures with numerous bubbles separated by very thin layers. The continuous and compact surface of the char residue prevent the release of free radicals which are finally concentratedly released to exert a strong quenching effect, thus resulting in some holes on the surfaces. The honeycombed structure of char layer reduces the efficiency of heat- and oxygen-exchange. The barrier effect of the char layer and strong quenching effect of high concentration of free radicals lead to the promoted flame retardant property. 3.3.6. Flame-Retardant Mechanism. From the above discussion, it is obvious that DOPO-TMT exerts biphase flame-retardant effect on the epoxy resin. During combustion, DOPO-TMT decomposes to release phosphorus- and nitrogen-containing free radicals, which are accommodated by the viscous char layer to form an intumescent and honeycombed structure. When the amount of pyrolytic products exceeds the holding capacity of the char layer, the free radicals are concentratedly released to exert a strong quenching effect. The barrier effect of the honeycombed char layer and quenching effect of high concentration of free radicals are responsible for the enhanced flame-retardant performance of the EP/DOPO-TMT thermosets.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 15827136966. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Wang, D.; Zhou, K.; Yang, W.; Xing, W.; Hu, Y.; Gong, X. Surface Modification of Graphene with Layered Molybdenum Disulfide and Their Synergistic Reinforcement on Reducing Fire Hazards of Epoxy Resins. Ind. Eng. Chem. Res. 2013, 52, 17882. (2) Perez, R. M.; Sandler, J. K. W.; Altstädt, V.; Hoffmann, T.; Pospiech, D.; Ciesielski, M.; Döring, M.; Braun, U.; Balabanovich, A. I.; Schartel, B. Novel Phosphorus-Modified Polysulfone as a Combined Flame Retardant and Toughness Modifier for Epoxy Resins. Polymer 2007, 48, 778. (3) Schartel, B.; Balabanovich, A. I.; Braun, U.; Knoll, U.; Artner, J.; Ciesielski, M.; Döring, M.; Perez, R.; Sandler, J. K. W.; Altstädt, V.; et al. Pyrolysis of Epoxy Resins and Fire Behavior of Epoxy Resin Composites Flame-Retarded with 9,10-Dihydro-9-Oxa-10-Phosphaphenanthrene-10-Oxide Additives. J. Appl. Polym. Sci. 2007, 104, 2260. (4) Kandola, B. K.; Biswas, B.; Price, D.; Horrocks, a. R. Studies on the Effect of Different Levels of Toughener and Flame Retardants on Thermal Stability of Epoxy Resin. Polym. Degrad. Stab. 2010, 95, 144. (5) Wang, Z.; Wei, P.; Qian, Y.; Liu, J. The Synthesis of a Novel Graphene-Based Inorganic−organic Hybrid Flame Retardant and Its Application in Epoxy Resin. Composites, Part B 2014, 60, 341. I


Article

Industrial & Engineering Chemistry Research (6) Zang, L.; Wagner, S.; Ciesielski, M.; Müller, P.; Döring, M. Novel Star-Shaped and Hyperbranched Phosphorus-Containing Flame Retardants in Epoxy Resins. Polym. Adv. Technol. 2011, 22, 1182. (7) Liang, B.; Cao, J.; Hong, X.; Wang, C. Synthesis and Properties of a Novel Phosphorous-Containing Flame-Retardant Hardener for Epoxy Resin. J. Appl. Polym. Sci. 2013, 128, 2759. (8) Deng, J.; Zhu, S.; Shi, W. Effect of Molecular Chain Structure of the Cured Epoxy Resin Containing Hyperbranched (3-Hydroxyphenyl) Phosphate on Expansion and Flame Retardance. J. Appl. Polym. Sci. 2004, 94, 2065. (9) Toldy, a.; Anna, P.; Csontos, I.; Szabó, A.; Marosi, G. Intrinsically Flame Retardant Epoxy Resin − Fire Performance and Background − Part I. Polym. Degrad. Stab. 2007, 92, 2223. (10) Hergenrother, P. M.; Thompson, C. M.; Smith, J. G.; Connell, J. W.; Hinkley, J. a.; Lyon, R. E.; Moulton, R. Flame Retardant Aircraft Epoxy Resins Containing Phosphorus. Polymer 2005, 46, 5012. (11) Liu, H.; Xu, K.; Ai, H.; Zhang, L.; Chen, M. Preparation and Characterization of Phosphorus-Containing Mannich-Type Bases as Curing Agents for Epoxy Resin. Polym. Adv. Technol. 2009, 20, 753. (12) Li, X.; Ou, Y.; Shi, Y. Combustion Behavior and Thermal Degradation Properties of Epoxy Resins with a Curing Agent Containing a Caged Bicyclic Phosphate. Polym. Degrad. Stab. 2002, 77, 383. (13) Xu, M.; Zhao, W.; Li, B. Synthesis of a Novel Curing Agent Containing Organophosphorus and Its Application in Flame-Retarded Epoxy Resins. J. Appl. Polym. Sci. 2014, 131, 41159. (14) Wu, C. S.; Liu, Y. L.; Chiu, Y. S. Epoxy Resins Possessing Flame Retardant Elements from Silicon Incorporated Epoxy Compounds Cured with Phosphorus or Nitrogen Containing Curing Agents. Polymer 2002, 43, 4277. (15) Liu, Y. L.; Chou, C. I. The Effect of Silicon Sources on the Mechanism of Phosphorus−silicon Synergism of Flame Retardation of Epoxy Resins. Polym. Degrad. Stab. 2005, 90, 515. (16) Liu, Y.; Wu, C.; Chiu, Y.; Ho, W. Preparation, Thermal Properties, and Flame Retardance of Epoxy-Silica Hybrid Resins. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 2354. (17) Kumar, S. A.; Denchev, Z. Development and Characterization of Phosphorus-Containing Siliconized Epoxy Resin Coatings. Prog. Org. Coat. 2009, 66, 1. (18) Cheng, X.; Shi, W. Synthesis and Thermal Properties of SiliconContaining Epoxy Resin Used for UV-Curable Flame-Retardant Coatings. J. Therm. Anal. Calorim. 2011, 103, 303. (19) Unlu, S. M.; Dogan, S. D.; Dogan, M. Comparative Study of Boron Compounds and Aluminum Trihydroxide as Flame Retardant Additives in Epoxy Resin. Polym. Adv. Technol. 2014, 25, 769. (20) Yang, H.; Wang, X.; Yu, B.; Song, L.; Hu, Y.; Yuen, R. K. K. Effect of Borates on Thermal Degradation and Flame Retardancy of Epoxy Resins Using Polyhedral Oligomeric Silsesquioxane as a Curing Agent. Thermochim. Acta 2012, 535, 71. (21) Martín, C.; Ronda, J. C.; Cádiz, V. Novel Flame-Retardant Thermosets: Diglycidyl Ether of Bisphenol A as a Curing Agent of Boron-Containing Phenolic Resins. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 1701. (22) Dogan, M.; Murat Unlu, S. Flame Retardant Effect of Boron Compounds on Red Phosphorus Containing Epoxy Resins. Polym. Degrad. Stab. 2014, 99, 12. (23) Wang, X.; Hu, Y.; Song, L.; Xing, W.; Lu, H.; Lv, P.; Jie, G. Flame Retardancy and Thermal Degradation Mechanism of Epoxy Resin Composites Based on a DOPO Substituted Organophosphorus Oligomer. Polymer 2010, 51, 2435. (24) Wang, X.; Hu, Y.; Song, L.; Yang, H.; Xing, W.; Lu, H. Synthesis and Characterization of a DOPO-Substitued Organophosphorus Oligomer and Its Application in Flame Retardant Epoxy Resins. Prog. Org. Coat. 2011, 71, 72. (25) Qian, X.; Song, L.; Jiang, S.; Tang, G.; Xing, W.; Wang, B.; Hu, Y.; Yuen, R. K. K. Novel Flame Retardants Containing 9,10-Dihydro9-Oxa-10-Phosphaphenanthrene-10-Oxide and Unsaturated Bonds: Synthesis, Characterization, and Application in the Flame Retardancy of Epoxy Acrylates. Ind. Eng. Chem. Res. 2013, 52, 7307.

(26) Liao, S.; Liu, P.; Hsiao, M.; Teng, C.; Wang, C.; Ger, M.; Chiang, C. One-Step Reduction and Functionalization of Graphene Oxide with Phosphorus-Based Compound to Produce FlameRetardant Epoxy Nanocomposite. Ind. Eng. Chem. Res. 2012, 51, 4573. (27) Lin, C. H.; Wu, C. Y.; Wang, C. S. Synthesis and Properties of Phosphorus-Containing Advanced Epoxy Resins. II. J. Appl. Polym. Sci. 2000, 78, 228. (28) Xiong, Y.; Zhang, X.; Liu, J.; Li, M.; Guo, F.; Xia, X.; Xu, W. Synthesis of Novel Phosphorus-Containing Epoxy Hardeners and Thermal Stability and Flame-Retardant Properties of Cured Products. J. Appl. Polym. Sci. 2012, 125, 1219. (29) Wang, C. S.; Lin, C. H. Synthesis and Properties of Phosphorus Containing Advanced Epoxy Resins. J. Appl. Polym. Sci. 2000, 75, 429. (30) Wang, X.; Zhang, Q. Synthesis, Characterization, and Cure Properties of Phosphorus-Containing Epoxy Resins for Flame Retardance. Eur. Polym. J. 2004, 40, 385. (31) Wang, C. S.; Lin, C. H. Synthesis and Properties of PhosphorusContaining Epoxy Resins by Novel Method. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 3903. (32) Chen, X.; Gu, A.; Liang, G.; Yuan, L.; Zhuo, D.; Hu, J. Novel Low Phosphorus-Content Bismaleimide Resin System with Outstanding Flame Retardancy and Low Dielectric Loss. Polym. Degrad. Stab. 2012, 97, 698. (33) Yang, S.; Wang, J.; Huo, S.; Cheng, L.; Wang, M. The Synergistic Effect of Maleimide and Phosphaphenanthrene Groups on a Reactive Flame-Retarded Epoxy Resin System. Polym. Degrad. Stab. 2015, 115, 63. (34) Qian, L.; Qiu, Y.; Liu, J.; Xin, F.; Chen, Y. The Flame Retardant Group-Synergistic-Effect of a Phosphaphenanthrene and Triazine Double-Group Compound in Epoxy Resin. J. Appl. Polym. Sci. 2014, 131, 39709. (35) Wang, Y.; Zhao, J.; Yuan, Y.; Liu, S.; Feng, Z.; Zhao, Y. Synthesis of Maleimido-Substituted Aromatic S-Triazine and Its Application in Flame-Retarded Epoxy Resins. Polym. Degrad. Stab. 2014, 99, 27. (36) Wang, X.; Hu, Y.; Song, L.; Xing, W.; Lu, H. Preparation, Flame Retardancy, and Thermal Degradation of Epoxy Thermosets Modified with Phosphorous/nitrogen-Containing Glycidyl Derivative. Polym. Adv. Technol. 2012, 23, 190. (37) Qian, L.; Qiu, Y.; Sun, N.; Xu, M.; Xu, G.; Xin, F.; Chen, Y. Pyrolysis Route of a Novel Flame Retardant Constructed by Phosphaphenanthrene and Triazine-Trione Groups and Its FlameRetardant Effect on Epoxy Resin. Polym. Degrad. Stab. 2014, 107, 98. (38) Qian, L.-J.; Ye, L.-J.; Xu, G.-Z.; Liu, J.; Guo, J.-Q. The NonHalogen Flame Retardant Epoxy Resin Based on a Novel Compound with Phosphaphenanthrene and Cyclotriphosphazene Double Functional Groups. Polym. Degrad. Stab. 2011, 96, 1118. (39) Zhang, W.; Li, X.; Yang, R. Novel Flame Retardancy Effects of DOPO-POSS on Epoxy Resins. Polym. Degrad. Stab. 2011, 96, 2167. (40) Zhang, W.; Li, X.; Yang, R. Study on Flame Retardancy of TGDDM Epoxy Resins Loaded with DOPO-POSS Compound and OPS/DOPO Mixture. Polym. Degrad. Stab. 2014, 99, 118. (41) Zhang, W.; Li, X.; Jiang, Y.; Yang, R. Investigations of Epoxy Resins Flame-Retarded by Phenyl Silsesquioxanes of Cage and Ladder Structures. Polym. Degrad. Stab. 2013, 98, 246. (42) Qian, L.; Ye, L.; Qiu, Y.; Qu, S. Thermal Degradation Behavior of the Compound Containing Phosphaphenanthrene and Phosphazene Groups and Its Flame Retardant Mechanism on Epoxy Resin. Polymer 2011, 52, 5486. (43) Lv, Q.; Huang, J.; Chen, M.-J.; Zhao, J.; Tan, Y.; Chen, L.; Wang, Y. An Effective Flame Retardant and Smoke Suppression Oligomer for Epoxy Resin. Ind. Eng. Chem. Res. 2013, 52, 9397.

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