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N-containing oligomer on flame retardancy and thermal degradation of intumescent flame-retardant epoxy resin
Polymer Degradation and Stability 162 (2019) 129e137
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Influence of a novel P/N-containing oligomer on flame retardancy and thermal degradation of intumescent flame-retardant epoxy resin Zong-Min Zhu a, *, Luo-Xin Wang a, Liang-Ping Dong b, ** a
State Key Laboratory of New Textile Materials and Advanced Processing Technology, College of Materials Science and Engineering, Wuhan Textile University, Wuhan, 430200, Hubei, PR China b Institute of Electronic Engineering, China Academy of Engineering Physics, Mianyang, 621000, Sichuan, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 November 2018 Received in revised form 13 February 2019 Accepted 17 February 2019 Available online 20 February 2019
In this work, a novel P/N-containing oligomer poly(piperazine phenylphosphamide) (BPOPA) was synthesized via the polymerization reaction of phenylphosphonic dichloride with piperazine. Its structure was characterized by Fourier transformation infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), elemental analysis (EA) and gel permeation chromatography (GPC). It was used to enhance the flame retardancy of epoxy resin/ammonium polyphosphate (APP). As expected, improvement in flame retardancy of EP/APP was observed. In detail, EP containing 10 wt% APP alone possessed a LOI value of 30.2% but UL-94 no rating. When both 7.5 wt% APP and 2.5 wt% BPOPA were added into EP, EP/7.5APP/ 2.5BPOPA passed a UL-94 V-0 rating with a LOI value of 33.1%. Moreover, EP/7.5APP/2.5BPOPA presented much lower values of the peak of heat release rate (PHRR) and total smoke production (TSP) compared with EP/10APP. The flame-retardant mechanism was analyzed by scanning electron microscopy (SEM), Xray photoelectron spectroscopy (XPS) and thermogravimetric analysis/infrared spectrometry (TG-IR). Results illustrated that EP/7.5APP/2.5BPOPA mainly functioned in condensed phase which was associated with the formation of more compact char layer. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Epoxy resin Flame retardancy P/N-containing oligomer Flame-retardant mechanism
1. Introduction Epoxy resin (EP) is one of the most extensively used thermosetting resins for various industrial fields such as paints and electronic appliances, because of its good chemical resistance and mechanical properties, etc [1e4]. However, EP material is very flammable, which restricts its application in many fields to some extent. Therefore, it is critical to improve flame retardancy of EP. Owing to the function of flame suppression in gaseous phase, halogen-containing compounds are effective and economic flame retardants for EP. Nevertheless, some of them have been banned due to the harms to environment and ecological development [5,6]. Accordingly, many halogen-free flame retardants (HFFR) are widely used to replace halogenated one for improving the flame retardancy of EP. Among various HFFR, intumescent flame retardant (IFR) has
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L.-P. Dong).
(Z.-M.
https://doi.org/10.1016/j.polymdegradstab.2019.02.021 0141-3910/© 2019 Elsevier Ltd. All rights reserved.
Zhu),
[email protected]
been considered to be promising for improving the flame retardancy of polymeric materials. Generally, IFR consists of an acid source, a charring source and a gas source. When polymer composites with IFR are ignited, intumescent char layer will form by synergistic interaction among the three components, which isolates the transfer of oxygen and heat and interrupts combustion reaction [7e11]. APP is a common flame-retardant additive that provides both acid and gas source for IFR [12e14]. Due to the good charring capacity of EP, IFR-EP system can be constructed directly with the addition of APP. The flame-retardant mechanism of APPbased EP system (EP/APP) is explained as follows: Firstly APP decomposes to generate polyphosphoric acid and NH3 under heating, which then is involved in the dehydration reaction to form carbonaceous layer. Secondly, the char layer is blown by NH3 to intumescent structure [15,16]. However, the high loading content of APP is the major problem for IFR-EP system [17,18]. To improve flame-retardant efficiency of IFR-EP system, it is preferable to add synergistic agents such as charring agent or other additives. Zhao et al. synthesized a new charring agent poly(4,40 -diamino diphenyl sulfone 2,6,7-trioxa-1- phosphabicyclo[2.2.2]octane-4-methanolsubstituted phosphoramide (PSA)), and revealed that the
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simultaneous addition of APP and PSA significantly improved flame retardancy of EP composites [19]. Flame-retardant mechanism analysis illuminated APP/PSA functioned in the condensed phase, accompanied by the formation of intumescent chars. Wang et al. prepared intumescent flame-retardant EP containing APP and a new charring agent 1,3,5-triazine-2,4,6-tri(2,6,7-trioxa-l-phosphabicyclo-[2.2.2]octane-4)methyl (PEPATA). EP composite containing 5 wt% APP and 5 wt% PEPATA possessed LOI value of 34% and UL-94 V-0 rating with a condensed-phase flame-retardant mechanism [20]. However, some of these charring agents were synthesized in multi-steps, which resulted in the preparation complexity [21,22]. Thus, we designed a P/N-containing oligomer (charring agent) by reaction between phenylphosphonic dichloride with piperazine for improving the flame retardancy of EP/APP. In this manuscript, we successfully synthesized a novel P/Ncontaining oligomer (BPOPA). Besides, the fire behavior and flame-retardant mechanism of EP composites were investigated and disclosed. 2. Experimental 2.1. Materials Diglycidyl ether of bisphenol A (DGEBA, E-44, with an epoxy value of 0.44 mol/100 g) was provided by Nantong Xingchen Synthetic Material Co., Ltd. (Nantong, China.). 4, 40 - Diamino diphenylmethane (DDM), acetonitrile (99%) and triethylamine (99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Phenylphosphonic dichloride (99%) was supplied by Wuhan ge'ao Chemical Technology Co., Ltd. (Wuhan, China). Anhydrous piperazine (99%) was provided by Energy Chemical (Shanghai, China). Ammonium polyphosphate (APP, form II, with degree of polymerization of 1500e2000) was supplied by Taifeng Fire Retardants Co., Ltd. (Sichuan, China). Distilled water was provided by our Lab. 2.2. Synthesis of poly(piperazine phenylphosphamide) (BPOPA) The synthetic route of BPOPA was shown in Scheme 1. In N2 atmosphere, anhydrous piperazine (0.1 mol, 8.6 g), triethylamine (0.2 mol, 20.2 g) and 250 mL of acetonitrile were poured into a 500 mL four-neck flask equipped with a mechanical stirrer and reflux condenser. Then the mixture was stirred until a transparent solution was obtained. Afterwards, the temperature was heated to 95 C, and phenylphosphonic dichloride (0.1 mol, 19.5 g) in 15 mL of acetonitrile was dripped into the mixture over 1.5 h. After finishing, the mixture was still stirred for 12 h. The crude product was filtered and washed by distilled water several times to remove triethylamine hydrochloride and other impurity substance. Finally, the brown product was obtained and dried under vacuum at 80 C for 12 h (yield: 70%) 2.3. Preparation of the cured epoxy resins The preparation of EP and flame retardant EP samples were as
follows: DGEBA and APP or BPOPA were first mixed together and stirred at 95 C. Then DDM was added to the mixture, once mixture became a homogeneous solution, it was rapidly poured into PTFE mold. After cooling down to room temperature, then EP mixture was firstly cured at 100 C for 2 h and heated to 150 C for 3 h. In addition, the neat EP was also prepared by similar procedure, but without APP or BPOPA. Formulas of epoxy samples were shown in Table 1. 2.4. Measurements Fourier transform infrared spectra (FTIR) of BPOPA was recorded between 400 and 4000 cm1 (KBr pellets) at room temperature from a Nicolet 6700 FTIR instrument. 1 H NMR and 31P NMR spectra were collected by a Bruker Ascend 400 spectrometer using DMSO‑d6 as the solvent. The contents of elements (C/H/N) were measured by a Vario MACRO cube elemental analysis instrument (EA). Gel permeation chromatography (GPC) was performed on a HLC-8320 system with a 2414 refractive index (RI) detector and a 1515 Waters HPLC pump. Dimethyl sulfoxide was used as the eluent at a flowing rate of 1.0 mL/min, and sample concentration was 2.5 mg/mL. Thermal behavior of BPOPA and EP samples were investigated by thermogravimetric analyzer (METTLER, Switzerland). About 10 mg samples were heated from 40 to 800 C with a heating rate of 10 C/min under nitrogen flow of 50 mL/min. The volatile pyrolysis products decomposed from TGA were connected by coupling FTIR. Glass transition temperatures (Tg) of EP composite were investigated by a TA Q200 differential scanning calorimeter (DSC) (TA, USA), operating at a heating rate of 10 C/min from 40 to 250 C under N2 atmosphere. Limiting oxygen index (LOI) and UL-94 vertical burning rating tests were performed to investigate flame retardancy of samples. The LOI value and UL-94 rating were obtained by a HC-2C oxygen index meter (Jiangning, China) according to ASTM D2863-97 and a CZF-4 instrument (Jiangning, China) according to ASTM D3801, respectively. The size of samples in LOI test were 130 mm 6.5 mm 3.2 mm and the sizes of samples in UL-94 vertical test were 130 mm 13 mm 3.2 mm. Cone calorimeter test was carried out to investigate fire behavior of samples using a cone calorimeter apparatus (Fire Testing Technology, UK) according to ISO5660-1. Each sample with a size of 100 mm 100 mm 3.2 mm was mounted on aluminum foil, and only the upper face of samples was exposed to a radiant cone at a heat flux of 35 kW/m2. The distance between the bottom surface of the cone heater and the top of the specimen was adjusted to be about 25 mm. Scanning electron microscopy (SEM) was obtained to observe the char morphologies using a JEOL JSM-5900LV instrument. The char layers needed to coat with gold before testing and then were tested under a high vacuum at a voltage of 20 kV. Raman spectroscopy was recorded to analyzed carbon structure using a LabRAM HR800 laser Raman spectrometer (SPEX Co.) with a 532 nm helium-neon laser line under room temperature condition. X-ray photoelectron spectroscopy (XPS) was recorded to investigate the component of chars using a K-Alphaþ (Thermo fisher Scientific Co., USA) with Al Ka excitation radiation (hv ¼ 1486.6 eV) under ultrahigh vacuum conditions. 3. Results and discussion 3.1. Characterization of BPOPA
Scheme 1. Route for synthesis of BPOPA.
Firstly, the chemical structure of piperzine and BPOPA was characterized by FTIR spectra. In Fig. 1, both the piperzine and
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Table 1 LOI and UL-94 results of the neat EP and its composites. Sample
EP (%)
DDM (%)
APP (%)
BPOPA (%)
APP: BPOPA
UL-94
LOI (%)
EP EP/10APP EP/20APP EP/20BPOPA EP/7.5APP/7.5BPOPA EP/10APP/5BPOPA EP/11.25APP/3.75BPOPA EP/12APP/3BPOPA EP/7.5APP/2.5BPOPA EP/6APP/2BPOPA
80 72 64 64 68 68 68 68 72 73.6
20 18 16 16 17 17 17 17 18 18.4
0 10 20 0 7.5 10 11.25 12 7.5 6
0 0 0 20 7.5 5 3.75 3 2.5 2
e e e e 1:1 2:1 3:1 4:1 3:1 3:1
NRa NR NR NR V-0 V-0 V-0 V-0 V-0 NR
26.2 ± 0.2 30.2 ± 0.3 37.5 ± 0.3 28 ± 0.3 36.3 ± 0.2 35 ± 0.3 37 ± 0.2 36 ± 0.2 33.1 ± 0.3 31.2 ± 0.3
NRa: No rating.
Fig. 2. 1H NMR and 31P NMR spectra of BPOPA.
Fig. 1. FTIR spectra of piperazine and BPOPA.
BPOPA had the characteristic absorption peaks at 2800-3000 cm1, which belonged to the stretching vibration of -CH2- bonds, suggesting that piperazine took part in the reaction between phenylphosphonic dichloride and piperazine. Meanwhile, for BPOPA, both the disappearance of -N-H- bond (3206 cm1) and the appearance of P-N- bond (720 cm1) meant that BPOPA was successfully prepared. Secondly, 1H NMR and 31P NMR tests were also carried out. In Fig. 2a, the peak at 2.8 ppm was attributed to -CH2- groups of piperazine ring. The signal peaks at 7.5 and 7.6 ppm were ascribed to H atoms of the benzene ring. The 31P NMR spectrum of BPOPA was shown in Fig. 2 b, it displayed a single peak at 23.9 ppm, corresponding to one kind of P atom in BPOPA. All above results of FTIR and NMR spectra indicated that BPOPA had been synthesized successfully. Moreover, the molecular weight of BPOPA was measured by GPC test. The number-average molecular weight (Mn) and weight-average molecular weight (Mw) of the oligomer were 2011 and 3871 g/mol, respectively. Therefore, on the basis of chemical structure, the degree of polymerization of BPOPA was deduced ca. 10. Table 2 presented the elemental contents of BPOPA. The experimental contents of C, H and N were approximately equal to calculated value, which further proved that successful preparation of the target product. The thermal degradation behavior of BPOPA was also investigated by Thermogravimetric analysis (TGA) under N2 atmosphere. Fig. 3 showed thermogravimetric analysis (TGA) curve and differential thermogravimetry (DTG) curves of BPOPA. As seen in Fig. 3,
Table 2 Elemental contents of BPOPA. Sample
Cal. (wt%)
Exp. (wt%)
C
H
N
C
H
N
BPOPA
57.7
6.3
13.5
56.5
6.4
13.8
Fig. 3. TG and DTG curves of BPOPA under N2 atmosphere.
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below 100 C, a small mass loss appeared, corresponding to the vaporization of adsorbed water. The onset thermal degradation temperature, defined as the temperature at which 5 wt% mass loss, of BPOPA reached to 286 C, and the residue mass of BPOPA at 800 C was 19 wt%, which indicated that BPOPA possessed high thermal stability and good charring ability at high temperature. In DTG curve, the degradation process of BPOPA was divided into three-steps. The first weight loss at 220e320 C was attributed to the breaking of a few C-N and C-C bonds in piperazine ring, the second weight loss at 320e420 C involved mainly the dehydrogenation of piperazine ring, the breaking of P-C bond and a part of C-C and C-N bonds in piperazine ring, and the third weight loss at 450e700 C was attributed to the breaking of the unsaturated bonds [23,24]. 3.2. Thermal analysis The glass transition temperatures (Tg) of EP and flame-retardant EP were investigated by DSC. As shown in Fig. 4, the all samples exhibited a single Tg. Addition of APP slightly decreased Tg compared with that of neat EP, which was ascribed to the decrease of cross-linking density of epoxy thermosets caused by the relatively large free volume and stereo-hindrance of APP [25]. For EP/ 7.5APP/2.5BPOPA, the addition of BPOPA further decreased Tg value compared to EP/10APP, this was because -N-H- groups in BPOPA reacted with epoxy groups of EP, thus further reducing crosslinking degree of EP [26,27]. Fig. 5 showed TG and DTG curves of samples, and the relevant test data were presented in Table 3. EP started to decompose at 355.8 C, and exhibited one-step degradation stage with a single maximum weight loss at 380.5 C (Tmax). The thermal decomposition of EP was mainly at about 300e500 C which involved dehydration of the epoxy thermosets and the formation of residue char containing polyaromatic structures [28]. By comparing with EP, EP/ 10APP and EP/7.5APP/2.5BPOPA samples presented one-step thermal decomposition process as well. Unlike EP, T5% and Tmax of EP/ 10APP were shifted to lower temperature, suggesting that the incorporation of APP accelerated the decomposition of EP composites in initial stage (~420 C) attributed to the earlier decomposition of APP. With the increase of temperature (420 Ce700 C), the residual mass of EP/10APP were much higher than that of neat EP. The enhanced char yield was due to the charring reaction between the degradation products of APP and EP at lower
Fig. 4. DSC curves of EP and flame-retardant EP.
temperature [17]. For EP/7.5APP/2.5BPOPA, its T5% and Tmax was further decreased with the addition of BPOPA, due to the lower thermal stability of BPOPA compared with that of APP. 3.3. Flammability 3.3.1. LOI and UL-94 tests To investigate the influence of BPOPA on flame retardancy of EP/ APP, LOI and UL-94 tests were carried out. The LOI and UL-94 tests results of EP and flame-retardant EP were shown in Table 1, and the relevant video snapshots of EP and flame-retardant EP during UL94 test were presented in Fig. 6. EP had a LOI value of 26.2% and no rating in UL-94 test. Neat EP did not extinguish spontaneously within 60 s after first ignition. By incorporation of 10 wt% APP alone resulted in the increase of LOI value of EP/10APP but no enhancement on UL-94 rating. The addition of 20 wt% APP (EP/20APP) showed no UL-94 rating. In comparison, it was notable that adding BPOPA significantly improved the flame retardancy of EP/APP. All EP composites with 15 wt% of both APP and BPOPA (APP: BPOPA ¼ 1:1, 2:1, 3:1 and 4:1) passed V-0 rating. Therein, the LOI value of EP/APP/BPOPA at 3:1 was highest and reached to 37%, which indicated that 3:1 was the best mass ratio for improving the flame retardancy of EP. With the mass ratio of APP to BPOPA at 3:1, the EP composite with 7.5 wt% APP and 2.5 wt% BPOPA still passed V-0 rating and reached a LOI value of 33.1%. In addition, EP/APP/ BPOPA composites were more likely to form a foamed char layer during LOI and UL-94 test. These results were attributed to that the addition of BPOPA promoted earlier decomposition of EP/APP and formed intumescent compact char layer. According to the above analysis, it was concluded that adding BPOPA greatly increased the flame retardancy of EP/APP composites. Evidently, there was a synergistic effect between APP and BPOPA on improving the flame retardancy of EP. 3.3.2. Cone calorimeter analysis Cone calorimeter (CC) is one of the most effective apparatus to quantitatively evaluate the fire behavior of materials. The test results provide some critical parameters, including the time to ignition (TTI), peak of heat release rate (PHRR), total smoke product (TSP), total heat release (THR), total smoke production (TSP), peak of smoke produce rate (PSPR), time to peak of heat release (tp) and the average effective heat of combustion of volatiles (av-EHC), etc [26]. As shown in Table 4, compared to TTI of neat EP, TTI of flameretardant EP samples increased slightly. According to the result of TGA, the early decomposition of APP or BPOPA generated a stable char layer, which restrained the release of flammable gases derived from decomposition of EP. In Fig. 7, EP had a sharp PHRR value of 1063 kW/m2, addition of 10 wt% APP alone decreased the PHRR value of EP to 754 kW/m2. After combining both APP and BPOPA, the PHRR value of EP/7.5APP/2.5BPOPA was decreased to 576 kW/ m2. As seen in Fig. 8, the EP sample was nearly burnt out, and a small amount of char residues with a fractured structure. Unlike EP, its flame-retardant composites had extremely expanded char residues with high char yields, which confirmed the existence of condensed phase flame-retardant mechanism. Relative to EP/ 10APP, the lower PHRR value of EP/7.5APP/2.5BPOPA was ascribed to the formation of more compact char layer. In parallel, FIGRA was lower for EP/7.5APP/2.5BPOPA. The lowering of FIGRA was related to a reduction in flame spread, which increases escape time in reallife fire scenarios [29e31]. Meanwhile, the lower PSPR and TSP values of EP/7.5APP/2.5BPOPA were due to the formation of more stable and compact char layer. In addition, the av-EHC of flameretardant composites (EP/10APP and EP/7.5APP/2.5BPOPA) was decreased compared with EP, which was mainly attributed to the fuel-dilution by non-flammable gas, e.g., NH3, CO2, H2O, N2 [32].
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Fig. 5. TG (a) and DTG (b) curves of cured EP, APP and flame-retardant EP under N2 atmosphere.
Table 3 The results of TG and DTG of EP, APP and flame-retardant EP. Sample
T5% ( C)
Tmax ( C)
Rate at Tmax (%/ C)
Residue at 700 C (wt%)
EP APP EP/10APP EP/7.5APP/2.5BPOPA
355.8 327.2 326.0 314.9
380.5 325.4/626 348.5 344.9
1.4 0.3/-0.63 1.7 1.4
20.2 30.1 28.8 26.2
Fig. 6. Video screenshots of EP (a), EP/10APP (b) and EP/7.5APP/2.5BPOPA (c) during UL-94 test and photos of samples after LOI test (d, left: EP/10APP; right: EP/7.5APP/2.5BPOPA).
3.4. Flame-retardant mechanism
Table 4 The relevant results of neat EP and its composites after cone calorimeter test. Sample
EP
EP/10APP
EP/7.5APP/2.5BPOPA
TTI (s) PHRR (MJ/m2) tp (s) FIGRA (kW/m2$s) THR (MJ/m2) PSPR (m2/s) TSP (m2) av-EHC (MJ/kg) Residue (wt %)
59 ± 1 1063 ± 35 130 ± 4 8.2 76.1 ± 2 0.55 ± 0.02 71.4 ± 2.5 23.5 ± 1 11.9 ± 0.5
63 ± 2 754 ± 27 105 ± 2 7.18 42.8 ± 1.5 0.43 ± 0.01 30.6 ± 1.6 21.1 ± 0.8 45.7 ± 1.5
61 ± 1 576 ± 30 100 ± 2 5.76 42.6 ± 2 0.28 ± 0.01 25.9 ± 1 21.3 ± 1 47.2 ± 2
3.4.1. Gaseous products analysis TG-FTIR technique was used to analyze gaseous products of EP and flame-retardant EP during thermal decomposition process [33,34]. Fig. 9 showed absorbance curves and the IR spectra of gaseous products from the decomposition of EP, EP/10APP and EP/ 7.5APP/2.5BPOPA obtained at T5%, Tmax and 600 C, respectively. As seen in Fig. 9 (a), the absorbance intensity of EP was stronger than EP/10APP and EP/7.5APP/2.5BPOPA, implying that EP released much more gaseous products than the others. After addition of APP or APP/BPOPA in EP, the production of gaseous products was depressed evidently. As seen in Fig. 9 (b), the main pyrolysis products of EP included hydrocarbons (2974 cm1), CO2 (2360 cm1), aromatic compounds (1613, 1511 cm1) and ether
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Fig. 7. The HRR (a), THR (b), SPR (c) and TSP (d) curves of EP, EP/10APP and EP/7.5APP/2.5BPOPA after cone calorimeter test.
Fig. 8. Digital photos of EP (a), EP/10APP (b, d) and EP/7.5APP/2.5BPOPA (c, e) after cone calorimeter test.
compounds (1256, 1174 and 828 cm1) [35]. For EP/10APP and EP/ 7.5APP/2.5BPOPA, the gaseous products were similar to EP. However, a new signal peak appeared in Fig. 9 c and d, belonged to NH3 (930, 964 cm1), which was attributed to the decomposition of APP [36]. Moreover, a certain amount of NH3 was produced from the onset temperature (326 and 315 C) of EP/10APP and EP/7.5APP/ 2.5BPOPA, which diluted the flammable gases during burning and slowed down combustion.
3.4.2. Char analysis To better understand the flame-retardant mechanism, the chars
of materials after cone calorimeter test were further investigated by SEM, Raman and XPS. Fig. 10 showed SEM images of char residues for EP and flame-retardant EP composites after cone calorimeter test. As shown in Fig. 10, the surface of char layer of EP had many apparent crevices while the char layer of EP/10APP was relatively compact. Unlike EP and EP/APP, the char layer of EP/7.5APP/ 2.5BPOPA was smoother and compacter. It was speculated that some kind of chemical interaction between APP and BPOPA occurred, which improved the quality of char residues. According to the literature [15,37], APP catalyzed the decomposition of BPOPA to form abundant structures of C-N-P, C¼C and C¼N, etc. These
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135
Fig. 9. Absorbance curves (a) and FTIR spectra of volatile pyrolysis products for EP (b), EP/10APP(c) and EP/7.5APP/2.5BPOPA (d) during TGA in N2 atmosphere.
Fig. 10. SEM images of external char residues of EP (a-1, a-2), EP/10APP (b-1, b-2) and EP/7.5APP/2.5BPOPA (c-1, c-2) after cone calorimeter test.
structures were conducive to the formation of a stable and compact char layer. Thus, the char layer of EP/7.5APP/2.5BPOPA provided an effective barrier for the underlying material during combustion. Raman spectroscopy is an effective method to characterize the different carbonaceous types after combustion. As shown in Fig. 11, there were two peaks at 1360 and 1580 cm1 in all spectra, which belonged to D and G bonds respectively. The D bond corresponded to disordered graphite or glassy carbon, whereas the G bond
represented to vibration mode with E2g symmetry in the aromatic layers of the graphite crystalline. Thus, the ratio of the integrated intensities of D to G bond (ID/IG) was used to measure the degree of graphitization. The lower ratio of ID/IG meant the higher graphitization degree of char, which was related to the better flame retardancy [38]. As seen in Fig. 11, EP/7.5APP/2.5BPOPA had a lowest ID/IG value compared to EP and EP/APP, indicating the char layer of EP/7.5APP/2.5BPOPA was more compact than those of EP and EP/ 10APP. This was in agreement with the discussion of SEM. The concentration and status of elements of the chars were further studied by XPS. Fig. 12 showed C1s, O1s, N1s and P2p XPS spectra. The relevant element concentration of the chars was summarized in Table 5. As shown in Fig. 12, EP/10APP and EP/ 7.5APP/2.5BPOPA exhibited same distinct peaks in C1s, O1s, N1s and P2p XPS spectra. For C1s XPS spectra, three peaks appeared in spectra for EP/APP and EP/APP/BPOPA. The peak at 284.7 eV was assigned to the C-C and C-H bonds in aliphatic and aromatic groups. The peak at 285.9 eV was attributed to the C-N, C-O-C and C-OH groups. The peak at 288.9 eV was corresponded to C¼O and C¼C groups [17]. However, the peak intensities at 285.9 eV for EP/7.5 APP/2.5BPOPA evidently increased compared to EP/10APP, indicating that chars with cross-linking structure rich in C-O or C-N were formed. For O1s XPS spectra, three peaks appeared in both samples. The first one (531.3 eV) was attributed to C¼O or P¼O groups; the second one (532.1 ev) was ascribed to P-O or C-O groups while the last one (533.2 eV) was determined as C-OH, C-OO and COOR groups [39]. As shown in N1s XPS spectra, The peak at 398.6 eV was attributed to C-N or P-N groups, and the peaks at 400.2 and 401.4 eV were assigned to N¼C and N-H bonds, respectively. It was noted that the peak intensity at 398.6 eV increased to a certain extent for EP/7.5APP/2.5BPOPA, which was attributed to the addition of BPOPA [40]. For P2p XPS spectra, two characteristic peaks appeared in EP/APP or EP/APP/BPOPA. The peaks at 133.8 eV and 134.7 eV were assigned to P-O and P¼O groups in pyrophosphate or polyphosphate compounds.
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Fig. 11. Raman spectra of char residues for EP (a), EP/10APP (b) and EP/7.5APP/2.5BPOPA (c) after cone calorimeter test.
Fig. 12. C1s (a), O1s (b), N1s (c), and P2p (d) spectra of external char residues for EP/10APP and EP/7.5APP/2.5BPOPA.
Table 5 Element composition of the external char residues for EP/10APP and EP/7.5APP/ 2.5BPOPAP. Sample
C (wt%)
O (wt%)
N (wt%)
P (wt%)
EP/10APP EP/7.5APP/2.5BPOPA
70.02 79.02
20.02 12.89
4.87 5.40
5.09 2.69
Table 5 showed element composition of the external char residues for EP/10APP and EP/7.5APP/2.5BPOPA. For EP/APP composite, APP dehydrated with EP to form chars during combustion, which enriched P-O- and P¼O bonds on the surface of char residues. Therefore, the contents of phosphorus and oxygen were higher than that of EP/7.5APP/2.5BPOPAP. For EP/7.5APP/2.5BPOPA, the contents of carbon and nitrogen was higher than that of EP/10APP,
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suggesting abundant C-N and C-N-P bonds resulted from the degradation of BPOPA existed on the surface of char residues [15,40]. 4. Conclusions A novel P/N-containing oligomer (BPOPA) was successfully synthesized and characterized. It was used to enhance the flame retardancy of epoxy resin/ammonium polyphosphate (APP). With adding 10 wt% APP alone, the EP/10APP composite failed to pass vertical burning (UL-94) test. In contrast, with adding both 7.5 wt% APP and 2.5 wt% BPOPA, the EP/7.5APP/2.5BPOPA composite passed UL-94 V-0 rating with high LOI value of 33.1%. Cone calorimeter test results showed the PHRR, SPR and TSP values of EP/7.5APP/ 2.5BPOPA were evidently decreased compared with those of EP/ 10APP. The analysis of flame-retardant mechanism that addition of BPOPA resulted in the formation of more compact char layer in combustion process, which was responsible for the better flame retardancy of EP/7.5APP/2.5BPOPA.
[17]
[18]
[19]
[20]
[21]
[22]
[23]
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant 51803195).
[24]
[25]
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Influence of a novel P/N-containing oligomer on flame retardancy and thermal degradation of intumescent flame-retardant epoxy resin Zong-Min Zhu a, *, Luo-Xin Wang a, Liang-Ping Dong b, ** a
State Key Laboratory of New Textile Materials and Advanced Processing Technology, College of Materials Science and Engineering, Wuhan Textile University, Wuhan, 430200, Hubei, PR China b Institute of Electronic Engineering, China Academy of Engineering Physics, Mianyang, 621000, Sichuan, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 November 2018 Received in revised form 13 February 2019 Accepted 17 February 2019 Available online 20 February 2019
In this work, a novel P/N-containing oligomer poly(piperazine phenylphosphamide) (BPOPA) was synthesized via the polymerization reaction of phenylphosphonic dichloride with piperazine. Its structure was characterized by Fourier transformation infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), elemental analysis (EA) and gel permeation chromatography (GPC). It was used to enhance the flame retardancy of epoxy resin/ammonium polyphosphate (APP). As expected, improvement in flame retardancy of EP/APP was observed. In detail, EP containing 10 wt% APP alone possessed a LOI value of 30.2% but UL-94 no rating. When both 7.5 wt% APP and 2.5 wt% BPOPA were added into EP, EP/7.5APP/ 2.5BPOPA passed a UL-94 V-0 rating with a LOI value of 33.1%. Moreover, EP/7.5APP/2.5BPOPA presented much lower values of the peak of heat release rate (PHRR) and total smoke production (TSP) compared with EP/10APP. The flame-retardant mechanism was analyzed by scanning electron microscopy (SEM), Xray photoelectron spectroscopy (XPS) and thermogravimetric analysis/infrared spectrometry (TG-IR). Results illustrated that EP/7.5APP/2.5BPOPA mainly functioned in condensed phase which was associated with the formation of more compact char layer. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Epoxy resin Flame retardancy P/N-containing oligomer Flame-retardant mechanism
1. Introduction Epoxy resin (EP) is one of the most extensively used thermosetting resins for various industrial fields such as paints and electronic appliances, because of its good chemical resistance and mechanical properties, etc [1e4]. However, EP material is very flammable, which restricts its application in many fields to some extent. Therefore, it is critical to improve flame retardancy of EP. Owing to the function of flame suppression in gaseous phase, halogen-containing compounds are effective and economic flame retardants for EP. Nevertheless, some of them have been banned due to the harms to environment and ecological development [5,6]. Accordingly, many halogen-free flame retardants (HFFR) are widely used to replace halogenated one for improving the flame retardancy of EP. Among various HFFR, intumescent flame retardant (IFR) has
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L.-P. Dong).
(Z.-M.
https://doi.org/10.1016/j.polymdegradstab.2019.02.021 0141-3910/© 2019 Elsevier Ltd. All rights reserved.
Zhu),
[email protected]
been considered to be promising for improving the flame retardancy of polymeric materials. Generally, IFR consists of an acid source, a charring source and a gas source. When polymer composites with IFR are ignited, intumescent char layer will form by synergistic interaction among the three components, which isolates the transfer of oxygen and heat and interrupts combustion reaction [7e11]. APP is a common flame-retardant additive that provides both acid and gas source for IFR [12e14]. Due to the good charring capacity of EP, IFR-EP system can be constructed directly with the addition of APP. The flame-retardant mechanism of APPbased EP system (EP/APP) is explained as follows: Firstly APP decomposes to generate polyphosphoric acid and NH3 under heating, which then is involved in the dehydration reaction to form carbonaceous layer. Secondly, the char layer is blown by NH3 to intumescent structure [15,16]. However, the high loading content of APP is the major problem for IFR-EP system [17,18]. To improve flame-retardant efficiency of IFR-EP system, it is preferable to add synergistic agents such as charring agent or other additives. Zhao et al. synthesized a new charring agent poly(4,40 -diamino diphenyl sulfone 2,6,7-trioxa-1- phosphabicyclo[2.2.2]octane-4-methanolsubstituted phosphoramide (PSA)), and revealed that the
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simultaneous addition of APP and PSA significantly improved flame retardancy of EP composites [19]. Flame-retardant mechanism analysis illuminated APP/PSA functioned in the condensed phase, accompanied by the formation of intumescent chars. Wang et al. prepared intumescent flame-retardant EP containing APP and a new charring agent 1,3,5-triazine-2,4,6-tri(2,6,7-trioxa-l-phosphabicyclo-[2.2.2]octane-4)methyl (PEPATA). EP composite containing 5 wt% APP and 5 wt% PEPATA possessed LOI value of 34% and UL-94 V-0 rating with a condensed-phase flame-retardant mechanism [20]. However, some of these charring agents were synthesized in multi-steps, which resulted in the preparation complexity [21,22]. Thus, we designed a P/N-containing oligomer (charring agent) by reaction between phenylphosphonic dichloride with piperazine for improving the flame retardancy of EP/APP. In this manuscript, we successfully synthesized a novel P/Ncontaining oligomer (BPOPA). Besides, the fire behavior and flame-retardant mechanism of EP composites were investigated and disclosed. 2. Experimental 2.1. Materials Diglycidyl ether of bisphenol A (DGEBA, E-44, with an epoxy value of 0.44 mol/100 g) was provided by Nantong Xingchen Synthetic Material Co., Ltd. (Nantong, China.). 4, 40 - Diamino diphenylmethane (DDM), acetonitrile (99%) and triethylamine (99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Phenylphosphonic dichloride (99%) was supplied by Wuhan ge'ao Chemical Technology Co., Ltd. (Wuhan, China). Anhydrous piperazine (99%) was provided by Energy Chemical (Shanghai, China). Ammonium polyphosphate (APP, form II, with degree of polymerization of 1500e2000) was supplied by Taifeng Fire Retardants Co., Ltd. (Sichuan, China). Distilled water was provided by our Lab. 2.2. Synthesis of poly(piperazine phenylphosphamide) (BPOPA) The synthetic route of BPOPA was shown in Scheme 1. In N2 atmosphere, anhydrous piperazine (0.1 mol, 8.6 g), triethylamine (0.2 mol, 20.2 g) and 250 mL of acetonitrile were poured into a 500 mL four-neck flask equipped with a mechanical stirrer and reflux condenser. Then the mixture was stirred until a transparent solution was obtained. Afterwards, the temperature was heated to 95 C, and phenylphosphonic dichloride (0.1 mol, 19.5 g) in 15 mL of acetonitrile was dripped into the mixture over 1.5 h. After finishing, the mixture was still stirred for 12 h. The crude product was filtered and washed by distilled water several times to remove triethylamine hydrochloride and other impurity substance. Finally, the brown product was obtained and dried under vacuum at 80 C for 12 h (yield: 70%) 2.3. Preparation of the cured epoxy resins The preparation of EP and flame retardant EP samples were as
follows: DGEBA and APP or BPOPA were first mixed together and stirred at 95 C. Then DDM was added to the mixture, once mixture became a homogeneous solution, it was rapidly poured into PTFE mold. After cooling down to room temperature, then EP mixture was firstly cured at 100 C for 2 h and heated to 150 C for 3 h. In addition, the neat EP was also prepared by similar procedure, but without APP or BPOPA. Formulas of epoxy samples were shown in Table 1. 2.4. Measurements Fourier transform infrared spectra (FTIR) of BPOPA was recorded between 400 and 4000 cm1 (KBr pellets) at room temperature from a Nicolet 6700 FTIR instrument. 1 H NMR and 31P NMR spectra were collected by a Bruker Ascend 400 spectrometer using DMSO‑d6 as the solvent. The contents of elements (C/H/N) were measured by a Vario MACRO cube elemental analysis instrument (EA). Gel permeation chromatography (GPC) was performed on a HLC-8320 system with a 2414 refractive index (RI) detector and a 1515 Waters HPLC pump. Dimethyl sulfoxide was used as the eluent at a flowing rate of 1.0 mL/min, and sample concentration was 2.5 mg/mL. Thermal behavior of BPOPA and EP samples were investigated by thermogravimetric analyzer (METTLER, Switzerland). About 10 mg samples were heated from 40 to 800 C with a heating rate of 10 C/min under nitrogen flow of 50 mL/min. The volatile pyrolysis products decomposed from TGA were connected by coupling FTIR. Glass transition temperatures (Tg) of EP composite were investigated by a TA Q200 differential scanning calorimeter (DSC) (TA, USA), operating at a heating rate of 10 C/min from 40 to 250 C under N2 atmosphere. Limiting oxygen index (LOI) and UL-94 vertical burning rating tests were performed to investigate flame retardancy of samples. The LOI value and UL-94 rating were obtained by a HC-2C oxygen index meter (Jiangning, China) according to ASTM D2863-97 and a CZF-4 instrument (Jiangning, China) according to ASTM D3801, respectively. The size of samples in LOI test were 130 mm 6.5 mm 3.2 mm and the sizes of samples in UL-94 vertical test were 130 mm 13 mm 3.2 mm. Cone calorimeter test was carried out to investigate fire behavior of samples using a cone calorimeter apparatus (Fire Testing Technology, UK) according to ISO5660-1. Each sample with a size of 100 mm 100 mm 3.2 mm was mounted on aluminum foil, and only the upper face of samples was exposed to a radiant cone at a heat flux of 35 kW/m2. The distance between the bottom surface of the cone heater and the top of the specimen was adjusted to be about 25 mm. Scanning electron microscopy (SEM) was obtained to observe the char morphologies using a JEOL JSM-5900LV instrument. The char layers needed to coat with gold before testing and then were tested under a high vacuum at a voltage of 20 kV. Raman spectroscopy was recorded to analyzed carbon structure using a LabRAM HR800 laser Raman spectrometer (SPEX Co.) with a 532 nm helium-neon laser line under room temperature condition. X-ray photoelectron spectroscopy (XPS) was recorded to investigate the component of chars using a K-Alphaþ (Thermo fisher Scientific Co., USA) with Al Ka excitation radiation (hv ¼ 1486.6 eV) under ultrahigh vacuum conditions. 3. Results and discussion 3.1. Characterization of BPOPA
Scheme 1. Route for synthesis of BPOPA.
Firstly, the chemical structure of piperzine and BPOPA was characterized by FTIR spectra. In Fig. 1, both the piperzine and
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Table 1 LOI and UL-94 results of the neat EP and its composites. Sample
EP (%)
DDM (%)
APP (%)
BPOPA (%)
APP: BPOPA
UL-94
LOI (%)
EP EP/10APP EP/20APP EP/20BPOPA EP/7.5APP/7.5BPOPA EP/10APP/5BPOPA EP/11.25APP/3.75BPOPA EP/12APP/3BPOPA EP/7.5APP/2.5BPOPA EP/6APP/2BPOPA
80 72 64 64 68 68 68 68 72 73.6
20 18 16 16 17 17 17 17 18 18.4
0 10 20 0 7.5 10 11.25 12 7.5 6
0 0 0 20 7.5 5 3.75 3 2.5 2
e e e e 1:1 2:1 3:1 4:1 3:1 3:1
NRa NR NR NR V-0 V-0 V-0 V-0 V-0 NR
26.2 ± 0.2 30.2 ± 0.3 37.5 ± 0.3 28 ± 0.3 36.3 ± 0.2 35 ± 0.3 37 ± 0.2 36 ± 0.2 33.1 ± 0.3 31.2 ± 0.3
NRa: No rating.
Fig. 2. 1H NMR and 31P NMR spectra of BPOPA.
Fig. 1. FTIR spectra of piperazine and BPOPA.
BPOPA had the characteristic absorption peaks at 2800-3000 cm1, which belonged to the stretching vibration of -CH2- bonds, suggesting that piperazine took part in the reaction between phenylphosphonic dichloride and piperazine. Meanwhile, for BPOPA, both the disappearance of -N-H- bond (3206 cm1) and the appearance of P-N- bond (720 cm1) meant that BPOPA was successfully prepared. Secondly, 1H NMR and 31P NMR tests were also carried out. In Fig. 2a, the peak at 2.8 ppm was attributed to -CH2- groups of piperazine ring. The signal peaks at 7.5 and 7.6 ppm were ascribed to H atoms of the benzene ring. The 31P NMR spectrum of BPOPA was shown in Fig. 2 b, it displayed a single peak at 23.9 ppm, corresponding to one kind of P atom in BPOPA. All above results of FTIR and NMR spectra indicated that BPOPA had been synthesized successfully. Moreover, the molecular weight of BPOPA was measured by GPC test. The number-average molecular weight (Mn) and weight-average molecular weight (Mw) of the oligomer were 2011 and 3871 g/mol, respectively. Therefore, on the basis of chemical structure, the degree of polymerization of BPOPA was deduced ca. 10. Table 2 presented the elemental contents of BPOPA. The experimental contents of C, H and N were approximately equal to calculated value, which further proved that successful preparation of the target product. The thermal degradation behavior of BPOPA was also investigated by Thermogravimetric analysis (TGA) under N2 atmosphere. Fig. 3 showed thermogravimetric analysis (TGA) curve and differential thermogravimetry (DTG) curves of BPOPA. As seen in Fig. 3,
Table 2 Elemental contents of BPOPA. Sample
Cal. (wt%)
Exp. (wt%)
C
H
N
C
H
N
BPOPA
57.7
6.3
13.5
56.5
6.4
13.8
Fig. 3. TG and DTG curves of BPOPA under N2 atmosphere.
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below 100 C, a small mass loss appeared, corresponding to the vaporization of adsorbed water. The onset thermal degradation temperature, defined as the temperature at which 5 wt% mass loss, of BPOPA reached to 286 C, and the residue mass of BPOPA at 800 C was 19 wt%, which indicated that BPOPA possessed high thermal stability and good charring ability at high temperature. In DTG curve, the degradation process of BPOPA was divided into three-steps. The first weight loss at 220e320 C was attributed to the breaking of a few C-N and C-C bonds in piperazine ring, the second weight loss at 320e420 C involved mainly the dehydrogenation of piperazine ring, the breaking of P-C bond and a part of C-C and C-N bonds in piperazine ring, and the third weight loss at 450e700 C was attributed to the breaking of the unsaturated bonds [23,24]. 3.2. Thermal analysis The glass transition temperatures (Tg) of EP and flame-retardant EP were investigated by DSC. As shown in Fig. 4, the all samples exhibited a single Tg. Addition of APP slightly decreased Tg compared with that of neat EP, which was ascribed to the decrease of cross-linking density of epoxy thermosets caused by the relatively large free volume and stereo-hindrance of APP [25]. For EP/ 7.5APP/2.5BPOPA, the addition of BPOPA further decreased Tg value compared to EP/10APP, this was because -N-H- groups in BPOPA reacted with epoxy groups of EP, thus further reducing crosslinking degree of EP [26,27]. Fig. 5 showed TG and DTG curves of samples, and the relevant test data were presented in Table 3. EP started to decompose at 355.8 C, and exhibited one-step degradation stage with a single maximum weight loss at 380.5 C (Tmax). The thermal decomposition of EP was mainly at about 300e500 C which involved dehydration of the epoxy thermosets and the formation of residue char containing polyaromatic structures [28]. By comparing with EP, EP/ 10APP and EP/7.5APP/2.5BPOPA samples presented one-step thermal decomposition process as well. Unlike EP, T5% and Tmax of EP/ 10APP were shifted to lower temperature, suggesting that the incorporation of APP accelerated the decomposition of EP composites in initial stage (~420 C) attributed to the earlier decomposition of APP. With the increase of temperature (420 Ce700 C), the residual mass of EP/10APP were much higher than that of neat EP. The enhanced char yield was due to the charring reaction between the degradation products of APP and EP at lower
Fig. 4. DSC curves of EP and flame-retardant EP.
temperature [17]. For EP/7.5APP/2.5BPOPA, its T5% and Tmax was further decreased with the addition of BPOPA, due to the lower thermal stability of BPOPA compared with that of APP. 3.3. Flammability 3.3.1. LOI and UL-94 tests To investigate the influence of BPOPA on flame retardancy of EP/ APP, LOI and UL-94 tests were carried out. The LOI and UL-94 tests results of EP and flame-retardant EP were shown in Table 1, and the relevant video snapshots of EP and flame-retardant EP during UL94 test were presented in Fig. 6. EP had a LOI value of 26.2% and no rating in UL-94 test. Neat EP did not extinguish spontaneously within 60 s after first ignition. By incorporation of 10 wt% APP alone resulted in the increase of LOI value of EP/10APP but no enhancement on UL-94 rating. The addition of 20 wt% APP (EP/20APP) showed no UL-94 rating. In comparison, it was notable that adding BPOPA significantly improved the flame retardancy of EP/APP. All EP composites with 15 wt% of both APP and BPOPA (APP: BPOPA ¼ 1:1, 2:1, 3:1 and 4:1) passed V-0 rating. Therein, the LOI value of EP/APP/BPOPA at 3:1 was highest and reached to 37%, which indicated that 3:1 was the best mass ratio for improving the flame retardancy of EP. With the mass ratio of APP to BPOPA at 3:1, the EP composite with 7.5 wt% APP and 2.5 wt% BPOPA still passed V-0 rating and reached a LOI value of 33.1%. In addition, EP/APP/ BPOPA composites were more likely to form a foamed char layer during LOI and UL-94 test. These results were attributed to that the addition of BPOPA promoted earlier decomposition of EP/APP and formed intumescent compact char layer. According to the above analysis, it was concluded that adding BPOPA greatly increased the flame retardancy of EP/APP composites. Evidently, there was a synergistic effect between APP and BPOPA on improving the flame retardancy of EP. 3.3.2. Cone calorimeter analysis Cone calorimeter (CC) is one of the most effective apparatus to quantitatively evaluate the fire behavior of materials. The test results provide some critical parameters, including the time to ignition (TTI), peak of heat release rate (PHRR), total smoke product (TSP), total heat release (THR), total smoke production (TSP), peak of smoke produce rate (PSPR), time to peak of heat release (tp) and the average effective heat of combustion of volatiles (av-EHC), etc [26]. As shown in Table 4, compared to TTI of neat EP, TTI of flameretardant EP samples increased slightly. According to the result of TGA, the early decomposition of APP or BPOPA generated a stable char layer, which restrained the release of flammable gases derived from decomposition of EP. In Fig. 7, EP had a sharp PHRR value of 1063 kW/m2, addition of 10 wt% APP alone decreased the PHRR value of EP to 754 kW/m2. After combining both APP and BPOPA, the PHRR value of EP/7.5APP/2.5BPOPA was decreased to 576 kW/ m2. As seen in Fig. 8, the EP sample was nearly burnt out, and a small amount of char residues with a fractured structure. Unlike EP, its flame-retardant composites had extremely expanded char residues with high char yields, which confirmed the existence of condensed phase flame-retardant mechanism. Relative to EP/ 10APP, the lower PHRR value of EP/7.5APP/2.5BPOPA was ascribed to the formation of more compact char layer. In parallel, FIGRA was lower for EP/7.5APP/2.5BPOPA. The lowering of FIGRA was related to a reduction in flame spread, which increases escape time in reallife fire scenarios [29e31]. Meanwhile, the lower PSPR and TSP values of EP/7.5APP/2.5BPOPA were due to the formation of more stable and compact char layer. In addition, the av-EHC of flameretardant composites (EP/10APP and EP/7.5APP/2.5BPOPA) was decreased compared with EP, which was mainly attributed to the fuel-dilution by non-flammable gas, e.g., NH3, CO2, H2O, N2 [32].
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Fig. 5. TG (a) and DTG (b) curves of cured EP, APP and flame-retardant EP under N2 atmosphere.
Table 3 The results of TG and DTG of EP, APP and flame-retardant EP. Sample
T5% ( C)
Tmax ( C)
Rate at Tmax (%/ C)
Residue at 700 C (wt%)
EP APP EP/10APP EP/7.5APP/2.5BPOPA
355.8 327.2 326.0 314.9
380.5 325.4/626 348.5 344.9
1.4 0.3/-0.63 1.7 1.4
20.2 30.1 28.8 26.2
Fig. 6. Video screenshots of EP (a), EP/10APP (b) and EP/7.5APP/2.5BPOPA (c) during UL-94 test and photos of samples after LOI test (d, left: EP/10APP; right: EP/7.5APP/2.5BPOPA).
3.4. Flame-retardant mechanism
Table 4 The relevant results of neat EP and its composites after cone calorimeter test. Sample
EP
EP/10APP
EP/7.5APP/2.5BPOPA
TTI (s) PHRR (MJ/m2) tp (s) FIGRA (kW/m2$s) THR (MJ/m2) PSPR (m2/s) TSP (m2) av-EHC (MJ/kg) Residue (wt %)
59 ± 1 1063 ± 35 130 ± 4 8.2 76.1 ± 2 0.55 ± 0.02 71.4 ± 2.5 23.5 ± 1 11.9 ± 0.5
63 ± 2 754 ± 27 105 ± 2 7.18 42.8 ± 1.5 0.43 ± 0.01 30.6 ± 1.6 21.1 ± 0.8 45.7 ± 1.5
61 ± 1 576 ± 30 100 ± 2 5.76 42.6 ± 2 0.28 ± 0.01 25.9 ± 1 21.3 ± 1 47.2 ± 2
3.4.1. Gaseous products analysis TG-FTIR technique was used to analyze gaseous products of EP and flame-retardant EP during thermal decomposition process [33,34]. Fig. 9 showed absorbance curves and the IR spectra of gaseous products from the decomposition of EP, EP/10APP and EP/ 7.5APP/2.5BPOPA obtained at T5%, Tmax and 600 C, respectively. As seen in Fig. 9 (a), the absorbance intensity of EP was stronger than EP/10APP and EP/7.5APP/2.5BPOPA, implying that EP released much more gaseous products than the others. After addition of APP or APP/BPOPA in EP, the production of gaseous products was depressed evidently. As seen in Fig. 9 (b), the main pyrolysis products of EP included hydrocarbons (2974 cm1), CO2 (2360 cm1), aromatic compounds (1613, 1511 cm1) and ether
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Fig. 7. The HRR (a), THR (b), SPR (c) and TSP (d) curves of EP, EP/10APP and EP/7.5APP/2.5BPOPA after cone calorimeter test.
Fig. 8. Digital photos of EP (a), EP/10APP (b, d) and EP/7.5APP/2.5BPOPA (c, e) after cone calorimeter test.
compounds (1256, 1174 and 828 cm1) [35]. For EP/10APP and EP/ 7.5APP/2.5BPOPA, the gaseous products were similar to EP. However, a new signal peak appeared in Fig. 9 c and d, belonged to NH3 (930, 964 cm1), which was attributed to the decomposition of APP [36]. Moreover, a certain amount of NH3 was produced from the onset temperature (326 and 315 C) of EP/10APP and EP/7.5APP/ 2.5BPOPA, which diluted the flammable gases during burning and slowed down combustion.
3.4.2. Char analysis To better understand the flame-retardant mechanism, the chars
of materials after cone calorimeter test were further investigated by SEM, Raman and XPS. Fig. 10 showed SEM images of char residues for EP and flame-retardant EP composites after cone calorimeter test. As shown in Fig. 10, the surface of char layer of EP had many apparent crevices while the char layer of EP/10APP was relatively compact. Unlike EP and EP/APP, the char layer of EP/7.5APP/ 2.5BPOPA was smoother and compacter. It was speculated that some kind of chemical interaction between APP and BPOPA occurred, which improved the quality of char residues. According to the literature [15,37], APP catalyzed the decomposition of BPOPA to form abundant structures of C-N-P, C¼C and C¼N, etc. These
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135
Fig. 9. Absorbance curves (a) and FTIR spectra of volatile pyrolysis products for EP (b), EP/10APP(c) and EP/7.5APP/2.5BPOPA (d) during TGA in N2 atmosphere.
Fig. 10. SEM images of external char residues of EP (a-1, a-2), EP/10APP (b-1, b-2) and EP/7.5APP/2.5BPOPA (c-1, c-2) after cone calorimeter test.
structures were conducive to the formation of a stable and compact char layer. Thus, the char layer of EP/7.5APP/2.5BPOPA provided an effective barrier for the underlying material during combustion. Raman spectroscopy is an effective method to characterize the different carbonaceous types after combustion. As shown in Fig. 11, there were two peaks at 1360 and 1580 cm1 in all spectra, which belonged to D and G bonds respectively. The D bond corresponded to disordered graphite or glassy carbon, whereas the G bond
represented to vibration mode with E2g symmetry in the aromatic layers of the graphite crystalline. Thus, the ratio of the integrated intensities of D to G bond (ID/IG) was used to measure the degree of graphitization. The lower ratio of ID/IG meant the higher graphitization degree of char, which was related to the better flame retardancy [38]. As seen in Fig. 11, EP/7.5APP/2.5BPOPA had a lowest ID/IG value compared to EP and EP/APP, indicating the char layer of EP/7.5APP/2.5BPOPA was more compact than those of EP and EP/ 10APP. This was in agreement with the discussion of SEM. The concentration and status of elements of the chars were further studied by XPS. Fig. 12 showed C1s, O1s, N1s and P2p XPS spectra. The relevant element concentration of the chars was summarized in Table 5. As shown in Fig. 12, EP/10APP and EP/ 7.5APP/2.5BPOPA exhibited same distinct peaks in C1s, O1s, N1s and P2p XPS spectra. For C1s XPS spectra, three peaks appeared in spectra for EP/APP and EP/APP/BPOPA. The peak at 284.7 eV was assigned to the C-C and C-H bonds in aliphatic and aromatic groups. The peak at 285.9 eV was attributed to the C-N, C-O-C and C-OH groups. The peak at 288.9 eV was corresponded to C¼O and C¼C groups [17]. However, the peak intensities at 285.9 eV for EP/7.5 APP/2.5BPOPA evidently increased compared to EP/10APP, indicating that chars with cross-linking structure rich in C-O or C-N were formed. For O1s XPS spectra, three peaks appeared in both samples. The first one (531.3 eV) was attributed to C¼O or P¼O groups; the second one (532.1 ev) was ascribed to P-O or C-O groups while the last one (533.2 eV) was determined as C-OH, C-OO and COOR groups [39]. As shown in N1s XPS spectra, The peak at 398.6 eV was attributed to C-N or P-N groups, and the peaks at 400.2 and 401.4 eV were assigned to N¼C and N-H bonds, respectively. It was noted that the peak intensity at 398.6 eV increased to a certain extent for EP/7.5APP/2.5BPOPA, which was attributed to the addition of BPOPA [40]. For P2p XPS spectra, two characteristic peaks appeared in EP/APP or EP/APP/BPOPA. The peaks at 133.8 eV and 134.7 eV were assigned to P-O and P¼O groups in pyrophosphate or polyphosphate compounds.
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Fig. 11. Raman spectra of char residues for EP (a), EP/10APP (b) and EP/7.5APP/2.5BPOPA (c) after cone calorimeter test.
Fig. 12. C1s (a), O1s (b), N1s (c), and P2p (d) spectra of external char residues for EP/10APP and EP/7.5APP/2.5BPOPA.
Table 5 Element composition of the external char residues for EP/10APP and EP/7.5APP/ 2.5BPOPAP. Sample
C (wt%)
O (wt%)
N (wt%)
P (wt%)
EP/10APP EP/7.5APP/2.5BPOPA
70.02 79.02
20.02 12.89
4.87 5.40
5.09 2.69
Table 5 showed element composition of the external char residues for EP/10APP and EP/7.5APP/2.5BPOPA. For EP/APP composite, APP dehydrated with EP to form chars during combustion, which enriched P-O- and P¼O bonds on the surface of char residues. Therefore, the contents of phosphorus and oxygen were higher than that of EP/7.5APP/2.5BPOPAP. For EP/7.5APP/2.5BPOPA, the contents of carbon and nitrogen was higher than that of EP/10APP,
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suggesting abundant C-N and C-N-P bonds resulted from the degradation of BPOPA existed on the surface of char residues [15,40]. 4. Conclusions A novel P/N-containing oligomer (BPOPA) was successfully synthesized and characterized. It was used to enhance the flame retardancy of epoxy resin/ammonium polyphosphate (APP). With adding 10 wt% APP alone, the EP/10APP composite failed to pass vertical burning (UL-94) test. In contrast, with adding both 7.5 wt% APP and 2.5 wt% BPOPA, the EP/7.5APP/2.5BPOPA composite passed UL-94 V-0 rating with high LOI value of 33.1%. Cone calorimeter test results showed the PHRR, SPR and TSP values of EP/7.5APP/ 2.5BPOPA were evidently decreased compared with those of EP/ 10APP. The analysis of flame-retardant mechanism that addition of BPOPA resulted in the formation of more compact char layer in combustion process, which was responsible for the better flame retardancy of EP/7.5APP/2.5BPOPA.
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Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant 51803195).
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