Highly Effective Flame Retardancy of a Novel DPPA-Based Curing

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Highly effective flame retardancy of a novel DPPAbased curing agent for DGEBA epoxy resin Qinqin Luo, Yanchao Yuan, Chunlei Dong, Haohao Huang, Shumei Liu, and Jianqing Zhao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02083 • Publication Date (Web): 30 Sep 2016 Downloaded from http://pubs.acs.org on October 9, 2016

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Industrial & Engineering Chemistry Research

Highly effective flame retardancy of a novel DPPA-based curing agent for DGEBA epoxy resin Qinqin Luo1,2, Yanchao Yuan1, Chunlei Dong1, Haohao Huang1*, Shumei Liu1, Jianqing Zhao1* 1. School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China; 2. School of Chemistry and Chemical Engineering, Lingnan Normal University, Zhanjiang 524048, China.

Abstract The flame retardancy, thermal and mechanical properties of the cured epoxy resins are difficult to be simultaneously improved. In the present work, a novel DPPA-based curing agent, 10-[(4-hydroxyphenyl)(4-hydroxyphenylimino) methyl] - 5,10-dihydro - phenophosphazine -10 oxide

(H-DPPA), is successfully synthesized to serve as a co-curing agent of 4,

4'-diaminodiphenylmethane (DDM) for bisphenol A diglycidyl ether（DGEBA） epoxy resin. With the aid of 3.0 wt% of H-DPPA, the cured epoxy resin, in which the phosphorus content is as low as 0.22%, passes V-0 rating of UL-94 test with limiting oxygen index (LOI) of 31.8%. The high flame retardancy of epoxy resin modified by H-DPPA is originated mainly from the formation of intumescent char layer during combustion. This new type of flame retardant containing both phosphine oxide structure and nitrogen moieties provides epoxy resins with excellent integrated performances.

1

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1. Introduction Epoxy resin has been employed in many fields for their strong adhesion, excellent chemical resistance, high tensile strength, good dimensional stability, and insulation characteristics.

1,2

However, its flammability limits its application where high flame retardancy is required.3,4 Although halogenated compounds have been often employed to prepared flame retarded epoxy, some of them produce harmful substances during combustion, leading to environmental pollution and human health hazard.5,6 Therefore, halogen-free flame retardants for epoxy resins has been paid much attention in recent decades. Various organophosphorus flame retardants have attracted intensive interests due to their high reactivity and applicability in epoxy resins.7-10 However, incorporation of flame retardants containing P–O–C bond, such as phosphinates or phosphonates, into epoxy matrix usually seriously decreases the thermal stability and mechanical properties of epoxy resins.11-13 Braun et al

14

discussed the influence of oxidation state of phosphorus-containing aminic hardeners on the

decomposition and flame behavior of epoxy resins. The thermal stability and flame retardancy of phosphine oxides are higher than that of the compounds involving P-O-C bond. Moreover, phosphine oxides have little impact on the thermal stability of epoxy resins.14-18 During burning, phosphine oxides may release radical scavengers, such as PO·, which can react with H· or ·OH active radicals to extinguish the flame in the gas phase. Meanwhile, they may provide acid sources, which promote the dehydration of the matrix, leading to a char barrier in the condensed phase.19, 20 It was found that the flame retardancy could be significantly improved by a synergistic effect through the co-working of phosphorus and nitrogen groups in the flame retardants.21, 22 The incombustible gases released by nitrogen moieties can not only dilute the concentration of oxygen

2


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but also facilitate to the expansion of char layers. Therefore, it is desirable to exploit flame retardants containing both phosphine oxide structure and nitrogen moieties to obtain high-performance epoxy resins with satisfactory flame retardancy. 5,10-Dihydro-phenophosphazine-10-oxide (DPPA), containing both phosphine oxide structure and nitrogen moieties, and its derivative synthesized by the addition reaction of DPPA and Schiff base

have

been

found

to

endow

bisphenol

A

diglycidyl

ether

(DGEBA)/4,

4'-diaminodiphenylmethane (DDM) system with high flame-retardant efficiency.23, 24 For flame retardant DPPA, 0.36 wt% of phosphorus content was required make epoxy resin pass UL-94 V-0 rate with a LOI value of 33.6%. However, for its derivative, much lower phosphorus content at only 0.19 wt% could make epoxy resin pass UL-94 V-0 rate with a LOI value of 31.3%. In addition, the as-prepared epoxy resin with the derivative displayed improved mechanical properties. These results suggest that DPPA-based reactive monomers may exhibit different flame-retarding efficiency and other properties by adjusting functional groups and the active sites. In order to obtain better flame retardancy, thermal and mechanical performance of epoxy resin, the present paper prepares a more effective novel derivative, 10-[(4-hydroxyphenyl) (4-hydroxyphenylimino)

methyl]-5,10-dihydro-phenophosphazine-10-oxide

(H-DPPA),

by

introducing aromatic rings, two hydroxyl group, and one secondary amino sites via the high addition reactivity of P-H in DPPA molecule. As co-curing agent and flame retardant, H-DPPA is covalently introduced into DGEBA epoxy resin. With much less H-DPPA used than DPPA, the cured epoxy resin possesses superior flame retardancy as well as excellent integrated performances when the phosphorus content is only 0.22%. The flame-retardant and thermal properties are investigated, and flame-retardant mechanism of the epoxy resin is also proposed.

3



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2. Experimental Section

2.1 Materials 4-Hydroxybenzaldhehyde and 4,4'-diaminodiphenylmethane were provided by Aladdin Reagents

(Shanghai)

Co.,

Ltd.

5,10-Dihydro-phenophosphazine-10-oxide

(DPPA)

was

synthesized by us as described in our previous report.21 Epoxy resin (DGEBA, trade name: CYD128, epoxy value: 0.51) was purchased from Momentive Specialty Chemicals Inc. Absolute ethanol and dichloromethane were supplied by Guangzhou Chemical Reagent Factory.

2.2 Synthesis of 10-[(4-hydroxyphenyl) (4-hydroxyphenylimino) methyl]-5,10-dihydrophenophosphazine 10-oxide (H-DPPA) 4-Hydroxybenzaldhehyde (0.1mol), 4-aminophenol (0.1mol), and absolute ethanol (100 mL) were introduced into a 500 mL three-necked round-bottomed glass flask equipped with a condenser, a mechanical stirrer, and a nitrogen inlet. The mixture was refluxed at 80 °C for 7 h under N2 and then cooled to the room temperature. Subsequently, DPPA (0.1mol) and absolute ethanol (150 mL) were introduced into the mixture. The mixture was stirred at 50 °C for 12 h under N2 and then was cooled to the room temperature. The resulting precipitate was filtered and washed twice with dichloromethane, and then was dried at 65 °C in a vacuum oven for 12 h. 39.30 g creamy white powder (88% yield) was obtained. 1

H NMR (600 MHz, DMSO-d6) δ (ppm): 9.97 (s, 1H, NHb'), 9.20 (s, 1H, OHa'), 8.37 (s, 1H,

OHa), 7.76 (dd, 1H, H15, J1 = 7.8 Hz, J2 = 10.2 Hz), 7.49 (t, 1H, H24, J = 7.2 Hz), 7.42 (t, 1H, H17, J = 7.2 Hz), 7.31 (dd, 1H, H22, J1 = 8.4 Hz, J2 = 10.2 Hz), 7.09 (dd, 1H, H1, J1 = 6.0 Hz, J2 = 8.4 Hz), 7.01-7.05 (m, 2H, H5,16), 6.85 (t, 1H, H23, J = 7.8 Hz), 6.72 (d, 2H, H12,10, J = 7.8 Hz), 6.45 (d, 4


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2H, H13,19, J = 8.4 Hz), 6.35-6.40 (m, 4H, H18,21,2,4), 5.30 (t, 1H, NHb, J = 7.8 Hz), 4.46 (dd, 1H, H7, J1 = 7.8 Hz, J2 = 14.4 Hz). 13

C NMR (600 MHz, DMSO-d6) δ (ppm): 156.78 (C3), 149.47 (C11, 8), 143.06 (C19), 142.95

(C20), 140.63 (C15), 140.54 (C24), 133.15 (C17), 132.97 (C22), 132.04 (C1), 131.70 (C5), 129.61 (C6), 126.86 (C14), 120.11 (C25), 119.84 (C18), 116.12 (C21), 115.91 (C16), 115.67 (C23), 115.62 (C12), 114.84 (C10), 110.09 (C2), 109.64 (C4), 109.45 (C13), 109.00 (C9) , 61.14 (C7). 31

P NMR (600 MHz, DMSO-d6）δ (ppm): 14.19.

HRESI-MS：m/z = 429.13 (M + H)+, 451.12 (M + Na)+ , C25H21N2O3P (anal. calcd for C25H21N2O3P: 428.13). FT-IR (KBr, cm-1): 3571 and 3342 (-OH), 3219, 3190, 3161, 3138 and 3106 (N-H), 1612, 1580, 1515, 1466 and 359 (C=C), 1247(C-N), 1153(P=O).

2.3 Preparation of flame-retarded epoxy resins with H-DPPA and DGEBA H-DPPA and DGEBA were mixed under N2 at 175 °C for 2 h. DDM was added after the formed homogenous liquid was cooled to 80 °C. The mixture was stirred until DDM was completely dissolved. The sample was vacuum-degassed in a mold and then was cured sequentially at 80 °C for 2 h, 110 °C for 1 h, 150 °C for 2 h, 180 °C for 2 h, and 200 °C for 3 h.

2.4 Characterization and measurement 1

H,

13

C and

31

P NMR spectra were obtained with a Bruker DRX 600 spectrometer using

deuterated dimethyl sulfoxide solution as a solvent, tetramethylsilane as internal standard, and 85% H3PO4 as references. High resolution electrospray ionization mass spectrometry (HRESIMS) was conducted on a Bruker maXis Impact mass spectrometer. Scanning electron microscopy 5



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(SEM) was carried out on NOVA NANOSEM 430. Raman spectroscopy measurement was conducted with a Lab RAM Aramis (HORIBACJOBIN YVON Co., France) at room temperature. X-ray photoelectron spectroscopy (XPS) were collected by using a PHI X-tool system ( UIVAC-PHI, Inc. Janpan) with Al Kα excitation radiation at 51W, 15kV. Fourier transform infrared (FTIR) spectroscopy was recorded on a Netzsch 870 FT-IR spectrophotometer with KBr pellet. Thermogravimetric analysis (TGA) in nitrogen was carried out at a heating rate of 10 °C /min in the range of 25-700 °C on a Netzsch 2209F1 thermogravimetric analyzer. TGA-FTIR was performed on a Bruker STA449C/3MFC/G thermograimetry-Fourier transform infrared spectrometry. Differential scanning calorimeter (DSC) analysis was carried out on a Netzsch DSC 200F3A01 in nitrogen. Dynamic mechanical analysis (DMA) was performed on Netzsch DMA 242C with a sample size of 40×10×4 mm3. The storage modulus E' and tan δ could be determined when the temperature scan mode was employed at a heating rate of 3 °C/min and a frequency of 1 Hz in air atmosphere. In this test, the amplitude is 30 µm for bending mode. LOI test used the sample with the sheet dimensions of 150×6.5×3.2 mm3 following ASTM D2863-97. Vertical burning test was conducted on a UL 94 flammability meter (Fire Testing Technology Co. Ltd., UK) in the dimension of 130×13×3.2 mm3 according to ANSL UL 94-1985. Cone calorimeter measurement was performed on a FTT cone calorimeter and the sample size is 100×100×4mm3 with the incident radiant flux of 35 kW/m2 following ISO5660. For each specimen, the measurements were repeated three times, and the reported values were the average of three results. From the test, time to ignition (TTI), heat release rate (HRR), total heat release (THR), and char residues could be determined.

6


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The tensile property was tested by Instron-5967 according to ASTM D638-08 and the tensile strength was the average of six sheets within error range. The Charpy impact strength was obtained on a SUNS ZBC 7000 according to ISO 179-1982 and was the average of six sheets within error range.

3. Results and Discussion 3.1 Synthesis and characterization of H-DPPA H-DPPA was synthesized through the electrophilic addition reaction between P-H bond in DPPA and C=N bond in Schiff base obtained from the condensation reaction of 4-hydroxybenzaldhehyde with 4-aminophenol. The synthetic route of H-DPPA is illustrated in Scheme 1. 1H,

13

C and

31

P NMR, and FT-IR spectra (Figure 1) were employed to verify the

chemical structure of H-DPPA. The chemical shifts at 9.97 and 5.30 ppm are assigned to the protons of NHb' and NHb, respectively. The peaks for the protons of OHa' and OHa appear at 9.20 and 8.37 ppm, respectively. The chemical shift at 4.46 ppm is attributed to the proton of P-CH. The chemical shifts of sixteen aromatic protons are observed in the range from 6.35 to 7.76 ppm, whose assignment is explicated in the insert. The 13C NMR spectrum also confirms the structure of H-DPPA by the well-assigned locations of carbon atoms. The

31

P NMR spectrum of H-DPPA

exhibits an intensive singlet resonance signal at 14.19 ppm. The disappearance of P-H absorption at 2371 cm-1 in FT-IR spectrum of H-DPPA verifies the complete reaction of DPPA with Schiff base. The absorption peaks at 3571 and 3342 cm-1 (-OH); 3219, 3190, 3161, 3138 and 3106 cm-1 (N-H); 1612, 1580, 1515, 1466 and 1359 cm-1 (C=C); 1247 cm-1 (C-N); 1153 cm-1 (P=O) are consistent with the structure of H-DPPA. In addition, the mass spectrum shows M+ at m/z 429.13,

7



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which are consistent with the molecular weight of C25H21N2O3P formula. H-DPPA contains two phenolic hydroxyl groups and two secondary amino groups, which can cure epoxy resins.

Scheme 1 Synthetic route of H-DPPA. TGA-FTIR analysis of H-DPPA in air was investigated to understand its thermal decomposition process. 5% weight loss of H-DPPA occurs at 301.3 °C and a rapid weight loss follows, with a char yield of 38.2% at 700 °C (Figure 2). FTIR spectra of its evolved volatile components at 268 °C, 415 °C, and 600 °C are presented in Figure 2b. Except for phenol derivatives or water (3652 cm-1), aromatic rings (3055, 1595, and 1505 cm-1), and N-Ar (1303 cm-1), some phosphorus species evolved from the decomposition of H-DPPA. At 268 °C, the characteristic absorption peaks for PO· free radicals (1251 cm-1) and P-O-Ar (1174 cm-1) 25, 26 were detected. At 415 °C, the absorption peaks for PO· and P-O-Ar become stronger and the absorptions for P-O-C (1075 and 871 cm-1) also appear. Meanwhile, the absorptions for PO·, P-O-Ar, and P-O-C become weaker at 600 °C. It is well known that phosphorus-containing gases can inhibit the combustion in the gas phase via scavenging H· or OH· active radicals.21, 22

8


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Figure 1. 1H NMR(a), 13C NMR (b), 31P NMR (c), and FT-IR spectra (d) of H-DPPA.

Furthermore, the residue at 700 °C was characterized by FTIR analysis (Figure 2c). The appearance of OH (3685 cm-1), NH (3272, 3227, 3183, 3133 cm-1), C=O (1717 cm-1), aromatic rings (3086, 1442, 1372, 1331 cm-1), and P-O-Ar (1138 cm-1) groups suggests the presence of phosphorus- and nitrogenous-containing carbonized aromatic networks. The acid species could promote the dehydration and esterification of epoxy resin to generate a char barrier during combustion.19,

20

Therefore, H-DPPA can play a flame-retardant role both in the gas and

condensed phases when incorporated into epoxy resin matrix.

9



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Figure 2. TGA curve (a), FTIR spectra of evolved gases (b) and residue at 700 °C (c) of H-DPPA in air

3.2 Flame retardancy of cured epoxy resins H-DPPA containing four active protons serves as both the co-curing agent of DDM and the flame retardant for DGEBA epoxy resin. An epoxide/active proton equivalent ratio of 1:1 is maintained among DGEBA, H-DPPA, and DDM. A series of flame-retarded epoxy resins were prepared by varying the amount of HD-DPPA (1.0 wt%, 1.5 wt%, 2.0 wt%, 2.5 wt%, 3.0wt%, and 3.5wt%). Their formulas and flame-retardant properties are listed in Table 1. The pure epoxy resin (EP-H0) exhibits a LOI value around 24.1%. The LOI value of epoxy resins is dramatically increased with the amount of H-DPPA and it achieves 32.6% at 3.5 wt% of H-DPPA. The sample of EP-H0 sustains burning after the first ignition and flaming drips occur during UL-94 test. The addition of only 1.0 wt% H-DPPA obviously improves the

10


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flame retardancy of epoxy. After the first ignition, the airflow and char layer formed on the surface of the igniting end make the fire difficultly spread upward. The average burning

Table 1 Formulation and the results of LOI and UL-94 of the flame-retardant epoxy resins H-DPPA DGEBA DDM Samples

(wt%)

(wt%)

(wt%)

P

N

LOI

(wt%)

(wt%)

(vol %)

UL-94 Average burning

Drips Rating

time a (s) EP-H0

0

79.82

20.18

0

2.85

24.1

Lasting burning

Yes

NR

EP-H1.0

1.0

79.39

19.61

0.07

2.84

28.4

44.4±3.3

No

NR

EP-H1.5

1.5

79.18

19.32

0.11

2.83

28.9

26.0±1.9

No

V-1

EP-H2.0

2.0

78.97

19.03

0.15

2.82

29.8

17.8±1.2

No

V-1

EP-H2.5

2.5

78.75

18.75

0.18

2.81

30.8

11.1±1.5

No

V-1

EP-H3.0

3.0

78.54

18.46

0.22

2.81

31.8

2.8±1.0

No

V-0

EP-H3.5

3.5

78.32

18.18

0.25

2.80

32.6

3.3±1.7

No

V-0

a

The average value of the burning time after the first and second ignition for six specimens.

time is about 45s. Unexpectedly, a V-0 rating is reached when the amount of H-DPPA goes up to 3 wt%, in which the content of phosphorus is only 0.22%. The phosphorus content to reach UL-94 V-0 rating for H-DPPA decreases by 39% compared with that for DPPA23, indicating a higher flame-retarded efficiency for H-DPPA. The above results verify that H-DPPA provides excellent flame retardancy to DGEBA epoxy. It has been reported that a derivative from the reaction of DOPO with Schiff base only 11



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Page 12 of 29

helps the LOI value of epoxy resin reach 26% with a high phosphorus content of 3.6 wt%.25 Therefore, the derivative of DPPA endows epoxy resins with higher flame-retardant efficiency than the above derivative of DOPO. This conclusion is in agreement with the result obtained by Braun et al14 that phosphine oxide acted mainly as flame inhibitor and showed the higher LOI value and flame-retardant rating of UL-94 than phosphinate. The cone calorimeter was employed to examine the effect of H-DPPA on the fire behaviour of epoxy resin. The combustion parameters, including the time to ignition (TTI), peak of heat release rate (PHRR), total heat release (THR), and residual weights for EP-H0 and EP-H3.0, are compared in Table 2. The heat release rate (HRR) curves and total heat release THR curves of EP-H0 and EP-H3.0 are plotted in Figure 3. The TTI value of EP-H3.0 is higher than that of EP-H0, verifying that the incorporation of H-DPPA makes epoxy resin more difficult to be ignited. Compared with EP-H0, EP-H3.0 shows lower PHRR and THR values. Furthermore, the residual weight of EP-H3.0 dramatically increased from 12.1% for EP-H0 to 20.2%. It indicated that H-DPPA could effectively promote epoxy resins to produce a protective char layer during combustion. Table 2 The results of the cone calorimeter of EP-H0 and EP-H3.0 Samples TTI (s) PHRR (kW/m2) THR (MJ/m2) Yc (%) EP-H0

48

862

175

12.1

EP-H3.0

57

752

164

20.2

12


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Figure 3. HRR curves (a) and THR curves (b) of EP-H0 and EP-H3.0. The thermal degradation behaviors of EP-H0 and EP-H3.0 were investigated by TGA in nitrogen. Figure 4 shows the TGA and DTG curves in nitrogen. The thermal degradation properties, including 5% weight loss temperature (T5%), 10% weight loss temperature (T10%), char yield at 650 °C (Yc), and the first maximum weight loss rate (Rmax1) are given in Table 3. EP-H3.0 shows a one-stage thermal degradation process similar to EP-H0, but it starts to degrade at a slightly lower temperature and degrades more quickly than EP-H0. Its T5% and T10% are a little lower than those of EP-H0. When the temperature exceeds 381 °C, the degradation rate of EP-H3.0 becomes slower and the amount of its residue above 416 °C is larger than that of EP-H0. Rmax1 decreases significantly from 19.3 %/min for EP-H0 to 14.3 %/min for EP-H3.0, and the residue at 500 °C is 26.1% for EP-H3.0, higher than 20.5% for EP-H0. Yc of EP-H3.0 is higher than that of EP-H0. This occurs because that the decomposition product of H-DPPA promotes the degradation of epoxy resin matrix in the early stage and participates in charring, leading to enhancement of the thermal stability of residue above 416 °C.

13



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Figure 4. TGA curves (a) and DTG curves (b) of EP-H0 and EP-H3.0 in N2. Table 3 The thermal properties of EP-H0 and EP-H3.0 Tg a

Tg b

Crosslinking

(°C)

(°C)

density (103 mol/m3)

TGA, in N2 T5%

T10%

(°C)

(°C)

Rmax1 (%/min）

Yc (%)

EP-H0

164.4

162.4

5.37

367.4

375.9

19.0

15.8

EP-H3.0

160.5

156.9

5.30

360.3

369.0

14.3

22.3

a

Measured by DSC.

b

Measured by DMA.

3.3 Flame retardant mechanism

3.3.1 TGA-FTIR analysis TGA-FTIR analysis of EP-H0 and EP-H3.0 in air was conducted to investigate the function of H-DPPA for epoxy resin. The FTIR spectra of evolved volatile components at 378 °C (5.2% and 16.2% weight loss for EP-H0 and EP-H3.0, respectively) and 392 °C (23.3% weight loss for EP-H0 and 33.4% for EP-H3.0) are presented in Figure 5. The temperature of 378 °C is the 14


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strongest release temperature for EP-H3.0, and the spectrum at 392 °C is a characteristic spectrum in the 3D (temperature-time) surface graph for EP-H3.0. Main volatile gases detected for EP-H0 and EP-H3.0 are similar and contain phenol derivatives or water (3649 cm-1), aromatic compounds (3035, 1603, 1509, and 1336 cm-1), alkane components (2972, 2937 and 2884 cm-1), ester or ether components (1744, 1259, and 1176 cm-1), CO2 (2359 and 2311 cm-1), and CO (2176 and 2114 cm-1).29 As for EP-H3.0, the strong absorptions can be detected while absorption bands for EP-H0 are quite weak at 378 °C, indicating that the decomposition of EP-H3.0 is earlier than EP-H0. Furthermore, the distinct absorptions in the region 1094-967 cm-1 and absorption at 886 cm-1 attributed to P-O-C and P-O-P are detected in the volatile products from EP-H3.0 at 378 °C and 392°C. It is believed that the high flame retardancy of EP-H3.0 is related to these phosphorus-containing compounds in the gas-phase flame inhibition.26

Figure 5. FTIR spectra of evolved volatiles from EP-H0 and EP-H3.0 at 378 °C (a) and 392 °C (b). In order to clearly discern the change of evolved volatile components from EP-H0 and EP-H3.0, the relationships between intensities of characteristic peaks and temperatures for ether 15



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components (1176 cm-1), aromatic components (1603 cm-1), CO2 (2359 cm-1), hydrocarbons (2972 cm-1 ), and phenol derivatives or water (3649 cm-1), respectively, are illustrated in Figure 6. The initial gas evolution temperature of monitored characteristic peaks from EP-H3.0 is lower than that of EP-H0. The absorbance intensities of ether, aromatic components, alkane components and compounds containing OH from EP-H3.0 are much higher than those from EP-H0 in the early stage. Its maximum release rates of ether, aromatic components, hydrocarbons, and phenol derivatives/water in the absorbance intensity are 78%, 75%, 85% and 73%, respectively, larger than those of EP-H0. The results suggest that the thermal degradation of epoxy resin matrix is accelerated and evolved gas amount of epoxy resin matrix is increased by H-DPPA in the early stage. CO2 can be detected and the intensity of its absorbance peak is the strongest among all evolved gases during the whole thermal degradation process of EP-H3.0. The absorbance intensities of ether, aromatic components, hydrocarbons and compounds containing OH are smaller than those of EP-H0 in the later stage, indicating that the interaction between the derivatives from H-DPPA and epoxy resin matrix results in the reduction of volatile gases and enhancement of char residue in the later degradation.

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Figure 6. Variation of absorbance intensities of various components from EP-H0 and EP-H3.0 with temperature. (a) ether components (1176 cm-1); (b) aromatic components (1603 cm-1); (c) CO2 (2359 cm-1); (d) hydrocarbons (2972 cm-1); and (e) phenol derivatives or water (3649 cm-1).

Figure 7. Digital Photographs of residues after the cone calorimeter test. 3.3.2 Analysis of residues EP-H0 almost completely burned and its residue was incoherent and very fragile after the cone calorimeter test. However, EP-H3.0 exhibited a coherent intumescent rigid residue as shown in Figure 7. SEM image of the external residues of EP-H3.0 is presented in Figure 8. Clearly, the char layer of 17



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EP-H3.0 is continuous and compact without any broken bubbles. The visual observations suggest that only loading of 3 wt% H-DPPA can induce epoxy matrix to form the intumescent char layer, which is closely related to the excellent flame-retardant performance.

Figure 8. SEM micrograph of the external char residues of EP-H3.0 after the cone calorimeter test. In order to further investigate the role of H-DPPA in the flame retardancy of epoxy resin, the char structure of ignited ends of EP-H1.0 and EP-H3.0 samples after the UL-94 test was examined by SEM micrographs. The results are shown in Figure 9. A rigid surface char layer with some gas pores and bumps can be observed. Interestingly, a large cavity, honeycomb-like char layer, and thick char layer at break were observed after the surface char layer was partially peeled, as shown in Figure 10. This is due to the expansion of char layer and migration of degradation products to surface char layer during combustion. The unique char layer structure of ignited ends effectively retards the heat and mass transfer and protects the matrix inside. During the ignition of samples, the acid substances, which are derived from H-DPPA, can catalyze the degradation reaction and make epoxy resin quickly generate a char layer on the surface. The matrix under the char layer degrades to produce gaseous products, such as P-O-C, P-O-P, CO2, H2O, phenol derivatives, and aromatic components. As a result, gas pores and

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Figure 9. SEM micrographs of the surface char structure of ignited ends after the UL-94 test.

Figure 10. SEM micrographs of the char structure of ignited ends after their char layer was partially peeled.

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bumps on the surface char layer are left. Therefore, the formation of the intumescent char layer plays an important role in the excellent flame retardancy of epoxy resins modified by H-DPPA. Raman spectroscopy is a powerful tool to characterize chemical structure of carbonaceous materials. Raman spectra of the external char layer (for EP-H0 and EP-H3.0) from the cone calorimeter test are given in Figure 11. Both EP-H0 and EP-H3.0 possess two broad peaks with the maximum intensities at about 1585 and 1360 cm-1. The band at 1585 cm-1 (called G band) corresponds to the crystalline graphite, however the other one (called D band) is attributed to disordered graphite or glassy carbons. The graphitization degree of char is often characterized by the ratio (ID/IG) between the integrated intensities of the D and G bands. The lower the ratio of ID/IG, the more ordered structure of the char.30, 31 According to Figure 11, the ID/IG ratios follow the sequence of EP-H3.0 (1.93) < EP-H0 (2.53), indicating higher graphitization and more stable char structure of EP-H3.0.

Figure 11. Raman spectra of the external chars from EP-H0 (a) and EP-H3.0 (b) after the cone calorimeter test.

The chemical components of external char layer of EP-H3.0 were analysed by XPS (Figure 12). The char layer mainly contains carbon, oxygen, nitrogen, and phosphorus elements, and these elements account for 82.10 wt%, 11.16 wt%, 4.78 wt%, and 1.96 wt% respectively. The N/P ratio is 2.44, much

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lower than N/P ratio of 12.93 before the cone calorimeter test. It is because that a large number of nitrogen-containing compounds produced during the combustion are released into the gas phase while most phosphorous compounds retain in the condensed phase. To further study the state of various elements, the peaks corresponding to the elements are resolved using the peak analysis software XPS Peak4.1. The C1s peak can be split into three peaks. The peak at 284.6 eV is assigned to C–C in aliphatic and aromatic species, the peak at around 286.0 eV to C–O (ether and/or hydroxyl group), and the other peak at around 288.5 eV to carbonyl groups.32 Three peaks at 530.8, 532.2, and 533.5 eV are observed in O1s spectra. The peak at 530.8 eV is due to =O in phosphate or carbonyl groups, and the ones at around 532.2 and 533.5 eV are due to −O− in C−O−C, C−O−P, and C−OH groups. In N1s spectra, two peaks at around 398.5 and 400.5 eV are present, which may be attributed to the nitrogen functionality in pyridinic and pyrrolic groups. The single peak at 133.3 eV in P2p spectra can be assigned to pyrophosphate and/or polyphosphate.26,33 In a word, EP-H3.0 produces phosphorus and polyphosphorus acid during combustion, which promote epoxy matrix to form the carbonaceous protective layer by dehydration and esterification.

Figure 12. (a) C1s, (b) O1s, (c) N1s, and (d) P2p XPS spectra of the external char of EP-H3.0 after the cone calorimeter test.

3.4 Other properties of cured epoxy resins 3.4.1 Glass transition temperature

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The glass transition temperature (Tg) of EP-H0 and EP-H3.0 is measured by DSC and DMA, and the measurement results are presented in Table 3. DMA thermograms are shown in Figure 13. Tg of EP-H3.0 appears at 160.5 °C (DSC) and 156.9 °C (DMA), which is only 3.9 °C (DSC) and 5.5 °C (DMA) lower than that of EP-H0. Therefore, EP-H3.0 retains the excellent thermal resistance of pure epoxy resin. The crosslinking density is calculated as described in the reference 28 and the results are presented in Table 3. Compared with EP-H0, a slightly lower value of crosslinking density for EP-H3.0 is found, indicating that the introduction of H-DPPA slightly decreases the crosslinking density of epoxy resins. The lower crosslinking density of EP-H3.0 leads to its decrease in Tg.

Figure 13. DMA thermograms of EP-H0 and EP-H3.0. 3.4.2 Mechanical properties The effect on mechanical properties of epoxy resin is also important for the application of H-DPPA. EP-H3.0 shows the tensile strength of 85.9±1.2 MPa and impact strength of 36.6±3.1 kJ/m2. The tensile strength and impact strength of EP-H0 are 75.1±1.2 MPa, 36.7±1.5 kJ/m2, respectively. Compared with EP-H0, the tensile strength of EP-H3.0 is increased by 12.0%, and the impact strength remains almost unchanged. The improvement in tensile strength occurs because that large volume of rigid rings hinders segmental motion and the strongly polar C-N bond increases the interaction between segments.

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4. Conclusions The

novel

phenophosphazine-containing

(4-hydroxyphenylimino)

compound,

methyl]-5,10-dihydro-phenophosphazine-10-oxide

10-[(4-hydroxyphenyl) (H-DPPA),

was

successfully synthesized and characterized. The DPPA-based derivative provides epoxy resins good flame retardancy. With only 3.0 wt% H-DPPA, the epoxy resin passed V-0 rating of UL-94 test and achieved a high LOI value of 31.8%, where the phosphorus content is as low as 0.22%. Incorporation of H-DPPA reduced the value of peak heat release rate and total heat release and significantly increased char yield. The high flame retarancy of DGEBA epoxy resin modified by H-DPPA was mainly resulted from the formation of a unique intumescent char layer. Furthermore, incorporation of H-DPPA had a positive impact on the tensile strength of epoxy resin, and keeps its thermal properties. Therefore, derivatives of DPPA can be another important flame-retardant category similar to derivatives of DOPO. AUTHOR INFORMATION *Corresponding authors. E-mail address: [email protected] (H. H.), [email protected] (J. Z.); TEL.: +86 2022236818

ACKNOWLEDGEMENTS

We gratefully acknowledge support from Joint Fund of NSFC with Guangdong Provincial Government (U1201243), the National Natural Science Foundation of China (51173047, 21606121).

REFERENCES (1) You, G.; Cheng, Z.; Peng, H.; He, H. Synthesis and performance of a novel nitrogen-containing

23



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

cyclic phosphate for intumescent flame retardant and its application in epoxy resin. J. Appl. Polym. Sci. 2015, 132, 41859. (2) Xu, G.-R.; Xu, M.-J.; Li, B. Synthesis and characterization of a novel epoxy resin based on cyclotriphosphazene and its thermal degradation and flammability performance. Polym. Degrad. Stab. 2014, 109, 240. (3) Li, Y.-J.; Gu, X.-Y.; Zhao, J.; Jiang, P.; Sun, J.; Wang, T. Flame retardancy effects of phosphorus-containing compounds and cationic photoinitiators on photopolymerized cycloaliphatic epoxy resins. J. Appl. Polym. Sci. 2014, 131, 40011. (4) Tian, N.; Gong, J.; Wen, X.; Yao, K.; Tang, T. Synthesis and characterization of a novel organophosphorus oligomer and its application in improving flame retardancy of epoxy resin. Rsc. Adv. 2014, 4, 17607. (5) Lin, H. T.; Lin, C. H.; Hu, Y. M.; Su, W. C. An approach to develop high-Tg epoxy resins for halogen-free copper clad laminates. Polymer 2009, 50, 5685. (6) Klinkowski, C.; Wagner, S.; Ciesielski, M.; Döering, M. Bridged phosphorylated diamines: Synthesis, thermal stability and flame retarding properties in epoxy resins. Polym. Degrad. Stab. 2014, 109, 240. (7) Liu, W. C.; Varley, R. J.; Simon, G. P. A phosphorus-containing diamine for flame-retardant, high-functionality epoxy resins. I. Synthesis, reactivity, and thermal degradation properties. J. Appl. Polym. Sci. 2004, 92, 2093. (8) Sponton, M.; Mercado, L. A.; Ronda, J. C.; Galia, M.; Cadiz, V. Preparation, thermal properties and flame retardancy of phosphorus- and silicon-containing epoxy resins. Polym. Degrad. Stab. 2008, 93, 2025.

24


Page 24 of 29

Page 25 of 29

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


(9) Qian, X.; Song, L.; Yuan, B.; Yu, B.; Shi, Y.; Hu, Y.; Yuen, R. K. K. Organic/inorganic flame retardants containing phosphorus, nitrogen and silicon: Preparation and their performance on the flame retardancy of epoxy resins as a novel intumescent flame retardant system. Mater. Chem. Phys. 2014, 143 , 1243. (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) Xiao, L.; Sun, D.; Niu, T.; Yao, Y. Syntheses and characterization of two novel 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide-based flame retardants for epoxy resin. High. Perform. Polym. 2014, 26 , 52. (12) Huan, L.; Kai, X.; Hualun, C.; Jiangxun, S.; Xin, L.; Zien, F.; Mingcai, C. Thermal properties and flame retardancy of novel epoxy based on phosphorus-modified schiff-base. Polym. Advan. Technol. 2012, 23, 114. (13) Wang, X.; Hu, Y.; Song, L.; Xing, W.; Lu, H.; Lv, P.; Jie, G. Effect of a triazine ring-containing charring agent on fire retardancy and thermal degradation of intumescent flame retardant epoxy resins. Polym. Advan. Technol. 2011, 22, 2480. (14) Braun, U.; Balabanovich, A.I.; Schartel, B.; Knoll, U.; Artner, J.; Ciesielski, M.; Dӧering, M.; Perez, R.; Sandler, J.K.W.; Altstӓedt, V.; Hoffmann, T.; Pospiech, D.. Influence of the Oxidation State of Phosphorus on the Decomposition and Fire Behaviour of Flame-Retarded Epoxy Resin Composites. Polymer, 2006,47, 8495. (15) 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. Advan. Technol. 2009, 20 (9), 753.

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

(16) Toldy, A.; Szolnoki, B.; Csontos, I.; Marosi, G. Green synthesis and characterization of phosphorus flame retardant crosslinking agents for epoxy resins. J. Appl. Polym. Sci. 2014, 131, 40105. (17) Mariappan, T.; You, Z.; Hao, J.; Wilkie, C. A. Influence of oxidation state of phosphorus on the thermal and flammability of polyurea and epoxy resin. Eur. Polym. J. 2013, 49, 3171. (18) 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. (19) Lv, Q.; Huang, J.-Q.; Chen, M.-J.; Zhao, J.; Tan, Y.; Chen, L.; Wang, Y.-Z. An effective flame retardant and smoke suppression oligomer for epoxy resin. Ind. Eng. Chem. Res. 2013, 52, 9397. (20) 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. (21) Xiong, Y.; Jiang, Z.; Xie, Y.; Zhang, X.; Xu, W. Development of a DOPO-containing melamine epoxy hardeners and its thermal and flame-retardant properties of cured products. J. Appl. Polym. Sci. 2013, 127, 4352. (22) Jirasutsakul, I.; Paosawatyanyong, B.; Bhanthumnavin, W. Aromatic phosphorodiamidate curing agent for epoxy resin coating with flame-retarding properties. Prog. Org. Coat. 2013, 76, 1738. (23) Luo, Q.; Yuan, Y.; Dong, C.; Liu, S.; Zhao, J. Intumescent flame retardancy of DGEBA epoxy resin based on 5,10-dihydro-phenophosphazine-10-oxide. RSC. Adv. 2015, 5, 68476. (24) Luo, Q.; Yuan, Y.; Dong, C.; Liu, S.; Zhao, J. High performance fire-retarded epoxy imparted by a novel phenophosphazine-containing antiflameing compound at ultra-low loading. Mater. Lett. 2016, http://dx.doi.org/10.1016/j.matlet.2016.01.083. (25) Lijun, Q.; Longjian, Y.; Yong, Q.; Shuren, Q. Thermal degradation behavior of the compound

26


Page 26 of 29

Page 27 of 29

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


containing phosphaphenanthrene and phosphazene groups and its flame retardant mechanism on epoxy resin. Polymer 2011, 52, 5486. (26) Bai, Z.; Jiang, S.; Tang, G.; Hu, Y.; Song, L.; Yuen, R. K. K. Enhanced thermal properties and flame retardancy of unsaturated polyester-based hybrid materials containing phosphorus and silicon. Polym. Advan. Technol.2014, 25, 223. (27) Xiong, Y.-Q.; Zhang, X.-Y.; Liu, J.; Li, M.-M.; Guo, F.; Xia, X-N.; Xu, W.-J. Synthesis of novel phosphorus-containing epoxy hardeners and thermal stability and flame-retardant properties of cured products. J. Appl. Polym. Sci. 2012, 125, 1219. (28) Henna, P. H.; Larock, R. C. Rubbery thermosets by ring-opening metathesis polymerization of a functionalized castor oil and cycloodene. Macromol. Mater. Eng. 2007, 292, 1201. (29) Zhang, W.; Li, X.; Yang, R. Blowing-out effect and temperature profile in condensed phase in flame retarding epoxy resins by phosphorus-containing oligomeric silsesquioxane. Polym. Advan. Technol. 2013, 24, 951. (30) Tai, Q.; Hu, Y.; Yuen, R. K. K.; Song, L.; Lu, H. Synthesis, structure-property relationships of polyphosphoramides with high char residues. J. Mater. Chem. 2011, 21, 6621. (31) Qian, X.; Song, L.; Jiang, S.; Tang, G.; Xing, W.; Wang, B.; Hu, Y.; Yuen, R. K. K. Novel flame retardants containing 9,10-Dihydro-9-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. (32) 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. (33) Bai, Z.; Song, L.; Hu, Y.; Yuen, R. K. K. Preparation, flame retardancy, and thermal degradation of

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unsaturated polyester resin modified with a novel phosphorus containing acrylate. Ind. Eng. Chem. Res. 2013, 52, 12855.

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Highly Effective Flame Retardancy of a Novel DPPA-Based Curing

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