S-Containing Flame Retardant and Its

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Synthesis of a novel P/N/S-containing flame retardant and its application in epoxy resin: thermal property, flame retardance and pyrolysis behavior Rong-Kun Jian, Pan Wang, Weisen Duan, JunSheng Wang, Xuelin Zheng, and Jiabao Weng Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03416 • Publication Date (Web): 21 Oct 2016 Downloaded from http://pubs.acs.org on October 24, 2016

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Industrial & Engineering Chemistry Research

Synthesis of a novel P/N/S-containing flame retardant and its application in epoxy resin: thermal property, flame retardance and pyrolysis behavior Rongkun Jian,* † Pan Wang,† Weisen Duan,† Junsheng Wang,* ‡ Xuelin Zheng,† Jiabao Weng† †

Fujian Key Laboratory of Polymer Materials, College of Materials Science and Engineering,

Fujian Normal University, Fuzhou 350007, China. ‡

Tianjin Fire Research Institute of the Ministry of Public Security, Tianjin 300381, China

ABSTRACT: The combination of DOPO and 2-aminobenzothiazole (ABZ) was designed to develop P/N/S-containing flame retardant DOPO-ABZ, and its chemical structure was confirmed by HRMS, FTIR,

1

H and

31

P NMR. The reduced thermal-stability of EP/DOPO-ABZ

formulations were found through DSC and TGA, as compared to that of EP. Fire properties were evaluated by LOI, UL-94, and cone calorimeter tests, respectively. The results indicated that DOPO-ABZ imparted flame retardance to EP, that EP/7.5wt%DOPO-ABZ passed V-0 rating, and acquired a LOI value of 33.5%; moreover, when the loading of DOPO-ABZ increasing to 10 wt%, it could further suppress the heat release and smoke release of the curved epoxy resin. Finally, the flame-retardant mechanism was studied by TG-FTIR and py-GC/MS, disclosing that DOPO-ABZ exerted predominant gaseous phase activity of fire inhibition via generating phosphorus-containing free radicals and nitrogen/sulfur-containing volatiles.

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1. INTRODUCTION Epoxy resin as one member of thermosetting resins, due to its outstanding properties, such as good chemical resistance, satisfactory electrical insulation, excellent mechanical and adhesive property, is generally utilized as coatings, adhesives and composites

1-4

. Nevertheless, epoxy

resin is easily combustible, which is one of the main drawbacks to its applications in transportation, building construction, electric and electronic industries, and so on. Hence, it is necessary to incorporate flame retardants into epoxy resins. In recent years, a great amount of investigation on flame-retarding epoxy resins has been reported. Among them, halogen-containing compounds have been broadly used thanks to their effective gas-phase quenching activity. However, up to now, some halogenated compounds are restrained for their proven or suspected adverse effect on the environment and ecological health 5, 6

. Therefore, ever-increasing requirement for developing environment-friendly alternatives to

the halogenated compounds has been put forward. So far, halogen-free flame retardants containing phosphorus 7-10, silicon 11-13, nitrogen 14-17 and sulfur

18, 19

, and functionalized nanomaterials including layered double hydroxide and double-

walled Al-P-Si nanorods

20-25

have been widely developed and applied to flame retard epoxy

resins. Among these, phosphorus based systems, especially for DOPO and DOPO based derivatives are found to play a crucial role in high flame retardant efficiency and low toxicity of the evolved gases during combustion. Since then, DOPO and its derivatives are continuously explored for the applicability in the epoxy resins 26-30. As expected, inspired results are obtained, when adding DOPO or its derivatives to the epoxy resins. Moreover, synergistic effect is discovered when integrating DOPO with other elements, including nitrogen

31-34

, silicon

35-37

,

and sulfur 18, 19, and with the help of synergistic effect it can further reduce the loading of flame


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retardant additives and keep the high flame-retardant efficiency simultaneously. Thus, we design a P/N/S-containing flame retardant synthesized by combining 2-aminobenzothiazole with DOPO through Atherton-Todd reaction. In this paper, we have successfully synthesized a novel flame retardant named as DOPO-ABZ with DOPO and ABZ via Atherton-Todd reaction. The chemical structure and thermal stability of DOPO-ABZ were also characterized. Then EP/DOPO-ABZ samples were prepared through thermal treatment, and EP as well as EP/DOPO was also prepared as the control sample. Besides, the performance and flame-retardant mechanism of the prepared epoxy thermosets were investigated and disclosed. 2. EXPERIMENTAL 2.1. Materials The epoxy resin, diglycidyl ether of bisphenol A (DGEBA, E-44) was sourced from Nantong Xingchen Synthetic Material Co., Ltd. (Nantong, China). 4, 4′-Diamino diphenyl methane (DDM) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). 9, 10Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) was supplied by Huizhou Sunstar Technology Co., Ltd. (Huizhou, China). 2-aminobenzothiazole (ABZ) was provided by J&K Chemical Ltd. (Shanghai, China). Carbon tetrachloride, triethylamine and dichloromethane were obtained from Tianjin Zhiyuan Chemical Reagent co., Ltd. (Tianjin, China), and were used without further purification. 2.2. Synthesis of DOPO-ABZ DOPO (47.6 g, 0.22 mol), 2-aminobenzothiazole (33.0 g, 0.22 mol) and triethylamine (33.6 ml, 0.24 mol) dissolved in 200 ml of dichloromethane were fed into a 500 ml, four-neck and round-bottom glass flask equipped with a mechanical stirrer, reflux condenser, thermometer, and


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additional funnel. Thereafter, the mixture was stirred and cooled to 0 °C. Subsequently, carbon tetrachloride (23.2 ml, 0.24 mol) was added dropwise to the mixture, and the reaction temperature should not exceed 15 °C. As the addition was completed, the solution was allowed to warm up to room temperature and continued stirring overnight. Finally the solution was filtered and then the filtrate was evaporated under reduced pressure. The obtained viscous compound was washed successively with saturated NaHCO3 solution, water, acetone and ethanol to remove the impurities, and dried under vacuum at 80 °C for 12 h to give a white solid product. The reaction formula is shown in Scheme 1. FTIR (KBr, cm-1), 3439.8 (N-H), 3064.6 (Ar-H), 1468.3 (P-Ph), 1198.0 (P=O), 1146.1 (P-O-Ar), 972.0 (P-N); 1H NMR (400 MHz，DMSO-d6, ppm): 12.73 (s, N-H), 8.15-8.19 (m, 2H), 7.84 (dd, J=14 Hz, 7.2 Hz, 1H), 7.71-7.77 (m, 2H), 7.56 (td, J=7.6 Hz, 3.2 Hz, 1H), 7.44 (t, J=7.6 Hz, 1H), 7.26-7.33 (m, 3H), 7.14-7.21 (m, 2H); 31P NMR (400 MHz, DMSO-d6, ppm): 12.00. HRMS (EI+): cal for C19H13N2O2PS [M+H] +365.0514, found 365.0519.

Scheme 1. Synthetic route of DOPO-ABZ. 2.3. Preparation of EP/DOPO-ABZ samples EP and EP containing different proportions of DOPO-ABZ, as listed in Table 1, were prepared as follows. DGEBA and DOPO-ABZ were first mixed together and stirred at 140 °C until DOPO-ABZ was completely dissolved in DGEBA. Afterwards, the temperature was cooled down to 90 °C, and stoichiometric DDM was fed into the mixture, and then the mixture


4

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continued stirring till a homogeneous solution was obtained. Finally, the solution was poured rapidly into a preheated mold and thermally cured in an oven at 100 °C for 2 h and post-cured at 150 °C for 3 h. The as-prepared EP/DOPO-ABZ samples for testing were labeled as EP/5.0% DOPO-ABZ, EP/7.5% DOPO-ABZ, EP/10.0% DOPO-ABZ, and EP/12.5% DOPO-ABZ, respectively. Besides, the neat EP and EP with DOPO was also prepared by the same procedure, except heat-treated at 140 °C. Table 1. Formulations of epoxy thermosets and the results of UL-94 and LOI tests. DGEBA DDM DOPO-ABZ DOPO P-content

UL-94

LOI

(3.2 mm)

(%)

Samples (g)

(g)

(g)

(g)

(wt %)

EP

100

25

0

-

0

N.R.a

26.0

EP/4.5% DOPO

100

25

-

5.9

0.64

V-1

31.5

EP/5.0% DOPO-ABZ

100

25

6.6

-

0.43

V-1

31.5

EP/7.5% DOPO-ABZ

100

25

10.1

-

0.64

V-0

33.5

EP/10.0% DOPO-ABZ

100

25

13.9

-

0.85

V-0

33.5

EP/12.5% DOPO-ABZ

100

25

17.9

-

1.06

V-0

32.0

a

: no rating.

2.4. Characterization Fourier transform infrared spectra (FTIR) were recorded in the range of 400-4000 cm-1 (KBr pellets) on a Thermo Nicolet 5700 FT-IR instruments. 1

H and 13P NMR spectra were obtained with a Bruker AVANCE AV II-400 NMR instrument,

and DMSO-d6 was used as the solvent. The glass transition temperature (Tg) was determined under nitrogen atmosphere by using a Mettler-Toledo DSC822e apparatus. The thermal history of samples were eliminated by heating to 180 °C and maintained for 3 min and then cooling down to 30 °C at a cooling rate of 10


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°C/min. The Tg of samples were then determined by heating to 200 °C at a heating rate of 10 °C/min. Thermogravimetric analyses were carried out on a METTLER TGA/SDTA 851 thermal analyzer from 30 to 800 °C at a heating rate of 10 °C/min under nitrogen atmosphere. Flame retardance of the materials were measured by LOI and UL-94 vertical burning tests. LOI values were evaluated according to ASTM D2863-97 with the three-dimensional size of 130.0×6.5×3.2 mm3; UL-94 vertical burning ratings were assessed according to ASTM D3801 with the three-dimensional size of 130.0×13.0×3.2 mm3. Combustion behaviors of the samples with three-dimensional size of 100.0×100.0×3.0 mm3 were measured through a FTT cone calorimeter device exposing to radiant cone at a heat flux of 35 kW/m2 according to ISO 5660-1. The tests for each sample were duplicated up to three times, depending on whether the deviation between the results was

S-Containing Flame Retardant and Its

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