- Email: [email protected]
Improved flame resistance and thermo-mechanical properties of epoxy resin nanocomposites from functionalized graphene oxide via self-assembly in water
Composites Part B 165 (2019) 406–416
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Improved flame resistance and thermo-mechanical properties of epoxy resin nanocomposites from functionalized graphene oxide via self-assembly in water
T
Fang Fanga,b, Shiya Ranb,∗, Zhengping Fanga,b, Pingan Songc,d,∗∗, Hao Wangc a
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, 310027, China b Laboratory of Polymer Materials and Engineering, Ningbo Institute of Technology, Zhejiang University, Ningbo, 315100, China c Centre for Future Materials, University of Southern Queensland, Toowoomba, QLD, 4350, Australia d School of Engineering, Zhejiang A&F University, Hangzhou, 311300, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Epoxy resin P, N-containing flame retardants Graphene oxide Flame retardancy
The development of a green and facile strategy for fabricating ecofriendly, highly effective flame retardants has remain a major challenge. Herein, supermolecular aggregates of piperazine (PiP) and phytic acid (PA) have been self-assembled onto the graphene oxide (GO) surface in water to fabricate functionalized GO (PPGO). The chemical structure and morphology of PPGO are determined by the X-ray photoelectron spectroscopy, transmission electron microscopy and scanning electron microscopy along with the energy dispersive spectroscopy. Due to the introduction of organic component onto the surface of graphene oxide, the adhesion between PPGO and the epoxy resin (EP) matrix is enhanced. As a result, the storage modulus (E′) of EP composites is increased in addition to a better dispersion of PPGO. Compared with the pure EP, the flame resistance of EP/PPGO is significantly improved, exhibiting a 42% decrease in peak heat release rate (pHRR), 22% reduction in total heat release (THR). The reduced flammability of EP is attributed to the synergistic effects afforded by the gas dilution effect of piperazine, char-forming promotion effect of phytic acid and the creation of "tortuous path" barrier effect of GO during burning. This work offers a green and facile approach for creating highly effective graphenebased flame retardants.
1. Introduction As one of the most important thermosetting polymers, epoxy resin (EP) is used extensively in vehicles, construction, electrical appliance and aircrafts fields owing to its excellent moisture, good heat and solvent resistance, low shrinkage on curing, remarkable adhesive strength, good mechanical and dielectric properties [1,2]. However, EP suffers intrinsic flammability, extremely limiting its practical applications [3,4]. (see Scheme 1) Last decades have witnessed the application potential of nanofillers in polymeric materials in terms of enhancing the mechanical and flame retardancy as well as other performances of the later. Until now, the addition of carbon materials, such as carbon nanotubes (CNTs), exfoliated graphite nanosheets (GNS), fullerene (C60) and graphene oxide (GO), have shown significant improvement in the thermal stability and flame resistance of polymer matrix at a very low loading level [5–7].
∗
Among these nanoscale materials, graphene and its derivatives have been regarded as ideal functional fillers for polymers due to its large surface area and high aspect ratio two-dimensional structures [8–10]. Indeed, graphene nanosheets with single-atom thickness have been demonstrated to contribute to forming a continuous compact barrier that can decrease the heat release rate and prevent the transfer of pyrolysis gases into the burning surface [11–13]. However, graphene itself only exhibits a limited flame retardancy effect on the polymer due to its thermal barrier mechanism. For this reason, it is necessary to introduce external flame-retardant elements onto the surface of graphene to further strengthen its flame retardancy effects [14,15]. Decorating graphene with phosphorus and nitrogen is a facile but effective approach for enhancing the flame-resistant efficiency of graphene because phosphorus- and nitrogen-containing compounds are regarded as the highly efficient halogen-free flame retardants (FRs). Yu et al. [16] prepared phosphorus and nitrogen wrapped graphene via a
Corresponding author. Corresponding author. Centre for Future Materials, University of Southern Queensland, Toowoomba, QLD, 4350, Australia. E-mail addresses: [email protected] (S. Ran), [email protected] (P. Song).
∗∗
https://doi.org/10.1016/j.compositesb.2019.01.086 Received 4 December 2018; Received in revised form 14 January 2019; Accepted 17 January 2019 Available online 25 January 2019 1359-8368/ © 2019 Published by Elsevier Ltd.
Composites Part B 165 (2019) 406–416
F. Fang et al.
Scheme 1. Illustrations of synthesis routes of PPGO.
as a curing agent.
simple one-pot method with POCl3 and 4,4′-diaminodiphenylmethane (DDM). The functionalized graphene significantly improved the flame retardancy of epoxy resin at a low loading. Wang et al. [17] prepared a novel flame retardant through grafting synthesized polyphosphamide (PPA) onto the surface of graphene nanosheets (GNSs), and PPA-g-GNS was incorporated into epoxy resins matrix to enhance the flame resistance. Qian et al. [18] prepared a phosphorus, nitrogen and silicon containing hybrids flame retardants containing exfoliated rGO (FRsrGO) via sol-gel process to increase the fire resistance of epoxy resin. The flame-retardant mechanism was integrated with condensed phase and gas phase flame retardant strategies. Despite these great advances, the development of highly effective, low cost flame-retardants remained desired and attractive. Previously we have reported a green and facile methodology to prepare phosphorus-nitrogen-decorated graphene via self-assembly of piperazine, phytic acid and GO without using any organic solvent, holding enormous potential in the flame-resistance field [19]. Therefore, this work aims to further confirm chemical structures of this selfassembling product, especially the interactions among three components in detail. Then, the effects of phosphorus-nitrogen-decorated graphene on the thermal stability and flame retardancy of EP were systematically investigated and the mechanism of ternary flame-retardant system acting on the EP was also discussed. Our results show that the addition of 3 wt% PPGO reduces the peak heat release rate of EP by 42% in addition to a 44% reduction in the peak CO production. Moreover, the thermal stability and glass transition temperature of EP are also enhanced to different extents. This work provides a green and facile approach for creating highly effective graphene-based flame retardants.
2.2. Preparation of PPGO and EP/PPGO nanocomposites Piperazine was dissolved in deionized water and then GO was slowly added into piperazine solution and stirred overnight. The precipitate Piperazine@GO (PGO) was collected by centrifugation, washed with deionized water to neutral. After that phytic acid was dissolved into deionized water and then PGO was gradually added into the phytic acid solution and stirred overnight. Subsequently, dark brown precipitate phytic acid@piperazine@GO (PPGO) was washed with deionized water and collected by centrifugation, Fig. 1 illustrates the synthesis of PPGO. Briefly, EP nanocomposites were prepared as our previous work: PPGO with different proportions (1, 2 and 3 wt% relative to composites) were firstly dispersed in acetone and then E51 was added into PPGO suspension with mechanical stirring to homogeneous, the mixture was then dried at 60 °C under vacuum to remove acetone thoroughly. Next, curing agent (MHHPA) was added into the mixture with mechanical string to homogeneous. Subsequently, the mixture was evacuated under vacuum until no bubbles emerged. Finally, the mixture was poured into a stainless-steel mold and cured at a programmedtemperature of 80 °C for 2 h, 150 °C for 3 h. EP composites with 1, 2 and 3 wt% PPGO are designated as EP/PPGO1, EP/PPGO2, EP/PPGO3, respectively. For comparison, EP/GO3 composite was prepared using the same process. 2.3. Characterization X-ray photoelectron spectroscopy was performed on an ESCALAB 250 spectrometer (XPS, ThermoVG Scientific, U.K.). Transmission electron micrographs were collected using a JEM-2010 operated at an accelerating voltage of 200 kV (TEM, JEOL, Japan). An S-4800 scanning electron microscope was used to observe the micro-morphology of the nanocomposites (SEM, Hitachi, Japan). Elemental composition and mapping were performed using an energy dispersive spectroscopy (EDS) analyzer with a 20 mm2 SDD detector (Oxford Instruments). Raman spectra were collected on a Jobin-Yvon LabRam HRUV Raman spectroscope equipped with a 514.5 nm laser source. Thermal gravimetric analysis as performed on TGA 209 F1 at a heating rate of 20 °C/ min from 25 °C to 800 °C (TGA, Netzsch, Germany). Combustion behavior was performed in a cone calorimeter (Cone, Fire Testing
2. Experimental 2.1. Materials Phytic acid (70% in H2O) was provided by Aladdin Chemistry Co., Ltd. (Shanghai, China). Piperazine was purchased by Energy Chemical Co., Ltd. (Shanghai, China). Graphene oxide (10 mg/mL in H2O) was obtained by Hengqiu Graphene Company (Suzhou China). Diglycidyl ether of bisphenol-A epoxy (EP, E51) with an epoxide equivalent weight of 185–200 g/eq was provided by Wuhuigang Adhesive Co., Ltd (Hangzhou, China). Hexahydro-4-methylphthalic anhydride (MHHPA), provided by Energy Chemical Co., Ltd. (Shanghai, China), was selected 407
Composites Part B 165 (2019) 406–416
F. Fang et al.
Fig. 1. (a) XPS spectra of GO and PPGO; High resolution C 1s XPS spectra of (b) GO, (c) PPGO; XPS high-resolution spectra of (d) N 1s and (e) P 2p for PPGO.
3. Results and discussion
Technology, U.K.) with the dimension of the sample sheets of 100 × 100 × 3 mm3 at a heat flux of 35 kW m−2 according to ISO 5660 standard. Thermogravimetric analysis coupled to Fourier transform infrared spectroscopy (TGA-FTIR) measurement was performed using a TGA 209 F1 instrument (Netzsch, Germany), coupled to a Thermo Nicolet IS10 FTIR spectroscopy (Thermo Scientific, Germany). The volatiles evolved from TGA can be transferred into the gas cell of FTIR through the transfer line by a suitable gas flow. Each specimen with approximate 7.45 mg was placed to a ceramic crucible and heated from 30 °C to 700 °C at a heating rate of 20 °C/min under nitrogen atmosphere. Dynamic mechanical analysis (DMA) for the epoxy nanocomposites was using DMA (Q800, TA Instruments, U.S.A.) performed in a three-point-bending mode, which was operated at 1 Hz at a constant heating rate of 3 °C/min from 30 °C to 180 °C.
3.1. Characterization of PPGO XPS was measured to evaluate diversified information about elemental compositions and chemical state of GO and PPGO. As can be seen in Fig. 1a, the XPS spectrum of pure GO only displays the existence of oxygen and carbon, while PPGO shows the presence of nitrogen and phosphorus, besides carbon and oxygen. The corresponding C 1s spectra of GO and PPGO are shown in Fig. 1b and c. The high resolution C1s band of GO is deconvoluted into four peaks corresponding to C(O) OH (288.7 eV), C]O (287.8 eV), CeO (286.6 eV), CeC (284.6 eV) [20]. However, the peak of C(O)OH (288.7) disappeared in Fig. 1c, an additional peak located 286.0 eV appeared, assigning to CeN [21], which is belonged to piperazine, indicating the aggregation of piperazine and 408
Composites Part B 165 (2019) 406–416
F. Fang et al.
Fig. 2. TEM images of (a) GO and (b) PPGO; SEM-EDS element distribution maps of (c) GO, (d) PPGO.
electron microscopy combined with energy dispersive spectroscopy (SEM-EDS). To further explore the transfer of piperazine and phytic acid over the surface of graphene oxide, elemental distribution mapping was also carried out as shown in Fig. 2c–d and Fig. S1. The distribution of elements carbon and oxygen are homogeneous in all samples, reflecting the oxidation degree of graphene oxide is homogeneous. Moreover, the uniform dispersion of elements nitrogen and phosphorus as shown in the elemental mapping image of PPGO indicating that piperazine and phytic acid not only occupied on the edge of graphene oxide sheets but also took place in the whole bulk of the graphene oxide [25]. TG/DTG profiles for GO, PGO and GO as a function of temperature at the heating rate of 20 °C/min under N2 are shown in Fig. 3a and b and the data are collected in Table 1. Detailed parameters including T5, Tm, RW and Rm are defined as the temperature at 5% weight loss occurs, the temperature of maximum weight loss rate, the residual weight at 800 °C, and the maximum weight loss rate respectively. As shown in Fig. 3a, GO is thermal unstable and starts losing weight below 100 °C due to evaporation of water molecules held in samples, PGO and PPGO
GO. Moreover, high resolution N 1s and P 2p test have been done to further explore the structure of PPGO. As can be observed from the N 1s (Fig. 1d) spectrum of PPGO, the binding energies at 401.6 eV and 400.1 eV are ascribed to eNH2+- and eNHe respectively [22]. The obviously larger eNH2+- area and smaller eNHe area indicates that most piperazine molecules aggregate with GO and phytic acid. As shown in Fig. 1e, P 2p survey of PPGO is deconvoluted into two peaks at 133.6 eV and 134.2 eV, which are corresponding to P]O and PeOeC or PO32− in P2O5 [23,24]. TEM images of GO and PPGO were shown in Fig. 2a–b. GO sheets show a layered structure of individual nanosheets, which are overlayered together layer by layer with a size around several hundred nanometers [10]. Compared to GO, PPGO sheets are much more fuzzy and flexible with more wrinkles. These changes are attributed to the fact that piperazine and phytic acid co-doped onto GO sheets could facilitate the charge transfer to enhance the mechanical strain of GO sheets [26]. FTIR, XRD, Raman tests have been done and analyzed by our previous work. Elemental composition of GO and PPGO, including carbon, oxygen, nitrogen, phosphorus was analyzed by scanning 409
Composites Part B 165 (2019) 406–416
F. Fang et al.
Fig. 3. TGA and DTG curves of GO, PGO and PPGO.
PPGO3 and EP/GO3. Accordingly, Table 2 summarizes the storage modulus at 30 °C (glassy state), Tg + 30 °C (rubbery state) and glass transition temperature (Tg) [28]. With the addition of PPGO, near room temperature (30 °C), EP/PPGO3 shows an E′ value of 1944 MPa, 20% higher than that of neat EP (1614 MPa) and 27% higher than that of EP/GO3 (1599 MPa). The enhanced E′ demonstrated that EP/PPGO3 is more rigid than neat EP and EP/GO3, indicating that PPGO exhibits a better reinforcing effect than GO in this system. This may be due to the stack of GO sheets, after self-assembling with peperazine and phytic acid, this nanoscale roughness likely results in an enhanced mechanical interlocking with the polymer chains [29]. Relatively, higher storage modulus at rubbery state (Er) of EP/PPGO3 and EP/GO3 compared with EP is observed, reflecting a stronger bonding and interaction at the interface between graphene and matrix, which might be the reason for the better compatibility. Meanwhile, the higher Er of EP/PPGO3 than EP/GO3 further indicates the better dispersion of PPGO in EP matrix. Furthermore, the van der Waals' force between PPGO nanosheets may decreases, so that the dispersion of PPGO in the EP matrix is improved and the cohesive energy density among the aromatic ring-rich epoxy networks [30]. The temperature at the maximum of tan δ is usually consider as the glass transition temperature (Tg), which also follows a similar change in trend to the storage modulus. The result of DMA corresponds with the morphology of EP nanocomposites.
Table 1 Thermogravimetric properties of GO, PGO, PPGO and EP composites. Sample
GO PGO PPGO EP EP/PPGO3 EP/GO3
In N2 T5 (°C)
Tm (°C)
RWa (wt%)
Rm (%/°C)
76 85 76 313 346 329
204 166 194 432 430 427
41.1 50.8 49.5 5.8 11.5 7.7
14.5 11.3 11.9 31.3 31.8 34.0
a RW of GO, PGO and PPGO is at 800 °C, RW of EP, EP/PPGO3 and EP/GO3 is obtained at 700 °C.
perform similar degradation process below 100 °C. Moreover, GO degrades quickly at 204 °C (Tm) due to the decomposition of the oxygencontaining functional groups at the edge of GO sheets [27]. And the RW of GO at 800 °C is 41.1 wt%. Comparing to GO, PGO degrades earlier due to the poor thermal stability of piperazine, and the Rm of PGO and PPGO are both lower than GO. However, the self-assembling of piperazine and phytic acid in the presence of GO decreases the disordered structure of GO and thus enhances the thermal stability of GO. Furthermore, the RW of PGO (50.8 wt%) and PPGO (49.5 wt%) at 800 °C are both much higher than GO, according to the ionic interactions among carboxyl group, amino group and phosphate group, which enhance the thermal stability and char formation of GO as well.
3.3. Thermal stability and flammability of EP nanocomposites TGA was utilized to assess the thermal stabilities of EP and its composites. For exploring the influence of GO and PPGO on the thermal properties of EP, EP nanocomposites were characterized with TGA under N2 atmosphere. Fig. 6a and b exhibit the corresponding TGA and DTG curves. All the EP nanocomposites follow a one-stage decomposition process, presenting the similar degradation behaviors to bare EP, the detailed data are summarized in Table 1. As can be observed, there is only one degradation step for neat EP in the temperature range of 300–400 °C, the T5, Tm and RW at 700 °C are 313 °C, 432 °C and 5.8 wt%, respectively. Other EP composites exhibit the similar decomposition behaviors to the neat EP. For comparison, the incorporation of PPGO and GO could decrease T5 and increase char residues. PPGO has better effect on thermal stability at the early stage and char layer formatting of EP than GO, which is attributed to the better dispersion of PPGO in the EP matrix, ‘‘tortuous path’’ effect of graphene [31,32] and char layer promotion effect of piperazine and phytic acid. TG-FTIR analysis was performed to further investigate the influence of PPGO on the toxic and smoke gases released from EP nanocomposites during the thermal degradation process. In order to highlight the change of various volatile products, the absorbance of decomposed gases vs. time for EP and EP/PPGO3 is presented in Fig. 7a–f. Total
3.2. Morphology and thermo-mechanical properties of EP nanocomposites The interaction between graphene-based nanofillers and the EP matrix is the key factor for the dispersion of PPGO [14]. The TEM images of EP/GO3 and EP/PPGO3 are exhibited in Fig. 4a and b. As shown in Fig. 4, GO nanosheets are dispersed in the EP matrix presenting layered morphology along with a degree of agglomeration, while PPGO in EP matrix displays a better dispersive state with the same magnification. Furthermore, the color of GO nanosheets is dark with wrinkle surface, which is due to the stacking and agglomeration of GO nanosheets. However, as can be observed from Fig. 4b, the size of PPGO nanosheets is much smaller than GO and the exfoliation structure of PPGO sheets is observed clearly. It is indicated that with the selfassembling with piperazine and phytic acid, the interfacial interaction of GO with EP matrix is enhanced. In summary, the dispersion of PPGO in the EP matrix is more favorable. DMA is very subtle to physical and chemical structure of polymeric materials, which is utilized to explore the dynamic mechanical properties of epoxy resin networks. Fig. 5 shows the influence of temperature on storage modulus (E′) values and loss factor (tan δ) of EP, EP/ 410
Composites Part B 165 (2019) 406–416
F. Fang et al.
Fig. 4. TEM images of the freeze-fractured surface of EP composites (a) EP/GO3, (b) EP/PPGO3.
volatile products of EP/PPGO3 (Fig. 7a) are lower than that of pure EP, indicating the lower smoke toxicity. The intensity of the evolved volatile components is up to the maximum at approximately 21 min (about 425 °C), in accordance with the peak in DTG curves. Comparing with pure EP, with the incorporation of 3 wt% PPGO, the maximum absorbance intensities of decomposed products shift to lower values, including CO2, CO, hydrocarbons, carbonyl and aromatic compounds (Fig. 7b–f). The barrier effect of PPGO nanosheets further contributes to the decrease in flammable organic volatiles, the suppression of toxic smoke and the reinforcement on fire safety. Therefore, the incorporation of PPGO promotes the formation of char residues and has the potential on reducing the fire hazards of EP nanocomposites. Cone calorimetry is a standard bench-scale technique to examine the burning behavior of EP/PPGO nanocomposites. Several significant parameters, such as the time to ignition (TTI), peak heat release rate (pHRR), total heat release (THR), peak carbon monoxide production (pCOP), peak carbon dioxide production (pCO2P), total smoke production (TSP) are listed in Table 3. Among these parameters, HRR, THR, COP and CO2P vs time curves are displayed in Fig. 8a–d. Neat EP shows relatively high PHRR (707 kW m−2) and THR (82.1 MJ/m2) value, presenting a substantial risk of thermal and fire hazards. With the addition of 3 wt% PPGO, EP/PPGO3 reveals a 42% maximum decrease in PHRR (Fig. 8a), a 22% maximum decrease in THR (Fig. 8b). While the PHRR and THR of EP/GO3 are reduced by 8% and 4% than those of neat EP, indicating that with the same loading of PPGO or GO, PPGO shows higher flame-retardant efficiency. In fact, in a real fire, more people are killed because of poisonous gas rather than burning. Carbon monoxide (CO), carbon dioxide (CO2) and other toxic gases are
Fig. 5. Storage modulus values (E′) and loss-tangent (tan δ) at 1 Hz of cured EP, EP/GO3 and EP/PPGO3. Table 2 Storage modulus at 30 °C, 30 °C above the glass transition temperature (Tg) and Tg of EP, EP/GO3 and EP/PPGO3.
EP EP/PPGO3 EP/GO3
E′ at 30 °C (MPa)
Er (E′ at Tg + 30) (MPa)
Tg (°C)
1614 1944 1599
14 51 44
134 142 141
Fig. 6. (a) TGA and (b) DTG curves of EP, EP/GO3 and EP/PPGO3 under nitrogen. 411
Composites Part B 165 (2019) 406–416
F. Fang et al.
Fig. 7. Absorbance of decomposed products for EP and its nanocomposites vs time: (a) total decomposed products; (b) CO; (c) CO2; (d) hydrocarbons; (e) aromatic and (f) carbonyl compounds.
3.4. Condensed phase flame-retardation mechanism
released in the fire scene which can quickly asphyxiate people to death. In general, when the amount of CO in the air reaches 1%, the people will lose consciousness after several counts of inhalation. After 1–2 min, the people may die of poisoning. CO2, although with slight toxicity, it will reduce the oxygen content of air in the fire, also giving people a threat to life. In a normal fire, when CO2, concentration increases to 5%, people will die of choking. CO is the considerable poisonous gas released from EP composites during burning process. Accordingly, pCOP is also a non-negligible factor on evaluating the fire safety of EP composites. Fig. 8c and d exhibit COP and CO2P curves of neat EP and EP nanocomposite. The peak of COP of EP is reduced from 0.0157 to 0.0130 g/s after incorporation of 3 wt% GO. Moreover, it brings a much more significant decrease of EP from 0.0157 to 0.0086 g/s by addition of 3 wt% PPGO, indicating that the aggregation of piperazine and phytic acid works on flame inhibition [33]. Besides, it may be attributed to the physical barrier effect of the PPGO nanosheets and the compact char layer generated from the char forming effect of piperazine and phytic acid. As shown in Table 3, the pCO2P and TSP values for all samples are decreased, displaying a similar trend to that of COP, while EP/PPGO3 exhibits the lowest pCO2P and TSP value among them. These flame-retardant reinforcements are assigned to the inadequate combustion of EP and the compacter char layer.
Fig. 9a–c presents the digital photos of the residual char of EP, EP/ GO3 and EP/PPGO3 after cone calorimetry. For neat EP, the char residues are very weak, while EP/GO3 leaves a little more char residues. Particularly, 3 wt% of PPGO makes a more compact and continuous char surface and the amount of char residues are remarkably increased. SEM images allow us to observe microstructures of external char residues on a microscopic scale, in order to further elucidate the effect of PPGO on the char forming capability of EP composites. As shown in Fig. 9d–e, the char residues of EP/GO3 display more compact surface with smaller micro porous compared to the neat EP. Unlike the broken char residues of neat EP and EP/GO3, the char residues of EP/PPGO3 exhibit a more cohesive and compressed layer on the surface without obvious holes and cracks, indicating relatively slow transfer of heat and combustible gases during thermal degradation and combustion by introducing 3 wt% PPGO. Fig. 10a–c presents the Raman spectra of the char residues for EP and its nanocomposites after cone calorimeter tests. All the spectra depict two broad peaks at 1355 cm−1 and 1593 cm−1, corresponding to the D and G bands, respectively. The ratio of the integrated intensities of D to G band (ID/IG) is used to assess the graphitization degree of the char residues [34]. Obviously, the ID/IG ratio significantly decreases from 2.20 (neat EP) to 2.00 (EP/PPGO3), indicating that the char residues of EP/PPGO3 have the higher graphitization degree than EP and
Table 3 Detailed cone calorimetry data for EP and EP composites.
EP EP/PPGO1 EP/PPGO2 EP/PPGO3 EP/GO3
TTI (s)
pHRR (kW/m2)
THR (MJ/m2)
pCO2P (g/s)
pCOP (g/s)
TSP (m2/kg)
55 60 56 53 50
707 696 494 412 650
82.1 81.2 72.0 63.8 79.0
0.482 0.467 0.325 0.272 0.442
0.0157 0.0143 0.0108 0.0086 0.0130
25.5 24.4 22.7 19.3 23.7
± ± ± ± ±
3 2 3 4 5
± ± ± ± ±
21 49 32 44 16
± ± ± ± ±
2.9 0.3 3.2 0.9 4.4
Note: The parameters are the average values of one formula with two samples. 412
± ± ± ± ±
0.019 0.036 0.016 0.026 0.007
± ± ± ± ±
0.0010 0.0016 0.0011 0.0018 0.0001
± ± ± ± ±
0.7 0.9 1.5 1.0 2.1
Composites Part B 165 (2019) 406–416
F. Fang et al.
Fig. 8. (a) HRR and (b) THR vs time curves; (c) COP and CO2P (d) vs time curves of EP and its nanocomposites obtained from a cone calorimeter.
Fig. 9. Digital photos of the char residues of (a) EP, (b) EP/GO3 and (c) EP/PPGO3; SEM images of the char residues from (d) EP (e) EP/GO3 and (f) EP/PPGO3 after cone calorimeter test.
char residues after cone calorimeter. As displayed in Fig. 11a and b, C 1s of EP and EP/PPGO-3 both have the peaks at 284.6 eV (CeC or CeH in the aromatic and aliphatic structures), 286.2 eV (CeO and CeOeP groups) and 288.1 eV (C]O group) [20]. What's more, stronger peaks of CeO and C]O are found in the sample of EP/PPGO3 compared with
EP/GO3. Furthermore, the higher graphitization can be attributed to synergistic effect between the catalytic action of piperazine and phytic acid and the physical effect of graphene sheets, towards forming more stable char structures. XPS provides detailed information about the chemical structure of 413
Composites Part B 165 (2019) 406–416
F. Fang et al.
Fig. 10. Raman spectra of the char residues of (a) EP, (b) EP/GO3 and (c) EP/PPGO3.
Fig. 11. High resolution C 1s XPS spectra of (a) EP, (b) EP/PPGO3; O 1s spectra of (c) EP, (d) EP/PPGO3; (e) N 1s and (f) P 2p spectra of EP/PPGO3.
414
Composites Part B 165 (2019) 406–416
F. Fang et al.
Fig. 12. Schematic illustration of flame retardant mechanism.
Acknowledgments
those of EP. For O 1s spectra (Fig. 11c and d), two peaks are observed, which can be attributed to = O in phosphate or carbonyl groups (531.3 eV) and eOe in CeOeC or/and CeOeP groups (533.1 eV) [37]. As summarized, CeO/CeC and CeOeC&CeOeP/C]O&P]O ratios of EP/PPGO-3 are much higher than that of EP, indicating that with the addition of PPGO in to EP matrix, more CeO structures were retained in the char layers than in char residues of pure EP. The N 1s main peak of the char residues of EP/PPGO-3 in Fig. 11e at 400 eV is ascribed to quaternary nitrogen and to some formation of oxidized nitrogen compounds [35]. The P2p peak of EP/PPGO-3 in Fig. 11f appears around 133.8 eV attributed to PeOeC structure. The results above show that there are more organic crosslink structures bridged by CeO and CeOeP bonds presented in the char layers of EP/PPGO3 [36,37]. On the basis of the above analysis, a mechanism of piperazine and phytic acid functionalized graphene in EP matrix is proposed in Fig. 12. During the primary stage of combustion process, the temperature increases rapidly, PPGO starts to decompose and release water vapor from graphene oxide and phytic acid, at the same time, NH3 gas is generated from the decomposition of piperazine on the chemical structure of PPGO. These nonflammable gases (water vapor and NH3) can play a very important role in promoting the formation of char layer [38]. After that, piperazine has almost decomposed due to its poor thermal stability, so it is reasonable to presume that PPGO may convert to polyphytate graphene due to the self-crosslinking at elevated temperatures and it can further produce a compact char layer [39]. Moreover, the incorporation of PPGO nanosheets in EP can provide a so-called “tortuous path” barrier effect which can slow down the release rate of inflammable gases such as hydrocarbons, aromatic compounds, CO during the degradation process and the platelet morphology of PPGO also provides the potential applications as an alternative to nanoclay that could improve the barrier performance of EP [8].
This work was financially supported by the National Natural Science Foundation of China (Grant No. 51628302, 51703197, 51873196); the Ningbo Natural Science Foundation (grant number 2018A610112); the Ningbo Science and Technology Innovation Team (Grant No. 2015B11005), and K.C.Wong Education Foundation. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.compositesb.2019.01.086. References [1] Liu T, Nie YX, Chen RS, Zhang LD, Meng Y, Li XY. Hyperbranched polyether as an all-purpose epoxy modifier: controlled synthesis and toughening mechanisms. J Mater Chem 2015;3:1188–98. [2] Tan Y, Shao ZB, Chen XF, Long JW, Chen L, Wang YZ. Novel multifunctional organic-inorganic hybrid curing agent with high flame-retardant efficiency for epoxy resin. ACS Appl Mater Interfaces 2015;7:17919–28. [3] Müller P, Bykov Y, Döring M. New star-shaped phosphorus-containing flame retardants based on acrylates for epoxy resins. Polym Adv Technol 2013;24:834–40. [4] Jian RK, Wang P, Duan WS, Wang JS, Zheng XL, Weng JB. Synthesis of a novel P/N/ S-containing flame retardant and its application in epoxy resin: thermal property, flame retardance, and pyrolysis behavior. Ind Eng Chem Res 2016;55:11520–7. [5] Feng YZ, Hu J, Xue Y, He CG, Zhou XP, Xie XL, et al. Simultaneous improvement in the flame resistance and thermal conductivity of epoxy/Al2O3 composites by incorporating polymeric flame retardant-functionalized graphene. J Mater Chem 2017;5:13544–56. [6] Song PA, Zhao LP, Cao ZH, Fang ZP. Polypropylene nanocomposites based on C60decorated carbon nanotubes: thermal properties, flammability, and mechanical properties. J Mater Chem 2011;21:7782–8. [7] Song PA, Xu ZG, Wu YP, Cheng QF, Guo QP, Wang H. Super-tough artificial nacre based on graphene oxide via synergistic interface interactions of π-π stacking and hydrogen bonding. Carbon 2017;111:807–12. [8] Wang X, Kalali EN, Wan JT, Wang DY. Carbon-family materials for flame retardant polymeric materials. Prog Ploym Sci 2017;49:21–46. [9] Nine MJ, Tran DNH, Tung TT, Kabiri S, Losic D. Graphene-borate as an efficient fire retardant for cellulosic materials with multiple and synergetic modes of action. ACS Appl Mater Interfaces 2017;9:10160–8. [10] Sang B, Li ZW, Li XH, Yu LG, Zhang ZJ. Graphene-based flame retardants: a review. J Mater Sci 2016;51:8271–95. [11] Huang GB, Song PA, Liu L, Han D, Ge CH, Li RR, et al. Fabrication of multifunctional graphene decorated with bromine and nano-Sb2O3 towards high-performance polymer nanocomposites. Carbon 2016;98:689–701. [12] Guan FL, Gui CX, Zhang HB, Jiang ZG, Jiang Y, Yu ZZ. Enhanced thermal conductivity and satisfactory flame retardancy of epoxy/alumina composites by combination with graphene nanoplatelets and magnesium hydroxide. Compos B Eng 2016;98:134–40. [13] Pan YT, Wang JT, Zhao XL, Li C, Wang DY. Interfacial growth of MOF-derived layered double hydroxide nanosheets on graphene slab towards fabrication of multifunctional epoxy nanocomposites. Chem Eng J 2017;330:1222–31. [14] Tao SH, Liu PL, Hsiao MC, Teng CC, Wang CA, Ger MD, et al. One-step reduction and functionalization of graphene oxide with phosphorus-based compound to produce flame-retardant epoxy nanocomposite. Ind Eng Chem Res
4. Conclusions This work has demonstrated a green and facile way to functionalize GO via self-assembled supermolecular aggregate of piperazine (PiP) and phytic acid (PA) onto GO. PPGO/EP composites exhibited a significantly reduced PHRR (42%), THR (22%) and pCOP (44%) compared to that of the neat EP. Apart from the excellent fire-retardancy performance, the loading of 3 wt% PPGO also leads to the increase in both storage modulus in rubbery state and glass transition temperatures. The improved flame-retardant performance of EP nanocomposites is assigned to a tripartite cooperative effect from the key components (piperizine, phytic acid, GO). This work offers a green and facile approach for creating graphene-based flame retardants for EP, which can contribute to expanding the practical applications of EP. 415
Composites Part B 165 (2019) 406–416
F. Fang et al.
2012;51:4573–81. [15] Hu CQ, Yu T, Li Y. Novel DOPO-modified graphene: synthesis and characterization. J Nanosci Nanotechnol 2017;17:4894–900. [16] Yu B, Shi YQ, Yuan BH, Qiu SL, Xing WY, Hu WZ, et al. Enhanced thermal and flame retardant properties of flame-retardant-wrapped graphene/epoxy resin nanocomposites. J Mater Chem 2015;3:8034–44. [17] Wang X, Xing WY, Feng XM, Yu B, Song L, Hu Y. Functionalization of graphene with grafted polyphosphamide for flame retardant epoxy composites: synthesis, flammability and mechanism. Polym Chem 2014;5:1145–54. [18] Qian XD, Song L, Yu B, Wang BB, Yuan BH, Shi YQ, et al. Novel organic-inorganic flame retardants containing exfoliated graphene: preparation and their performance on the flame retardancy of epoxy resins. J Mater Chem 2013;1:6822–30. [19] Fang F, Song PA, Ran SY, Guo ZH, Wang H, Fang ZP. A facile way to prepare phosphorus-nitrogen-functionalized graphene oxide for enhancing the flame retardancy of epoxy resin. Compos Comm 2018;10:97–102. [20] Cui XY, Yang SB, Yan XX, Leng JG, Shuang S, Ajayan PM, et al. Pyridinic-nitrogendominated graphene aerogels with Fe–N–C coordination for highly efficient oxygen reduction reaction. Adv Funct Mater 2016;26:5708–17. [21] Pethsangave DA, Khose RV, Wadekar PH, Some S. Deep eutectic solvent functionalized graphene composite as an extremely high potency flame retardant. ACS Appl Mater Interfaces 2017;9:35319–24. [22] Song PA, Liu L, Huang GB, Yu YM, Guo QP. Largely enhanced thermal and mechanical properties of polymer nanocomposites via incorporating C60@graphene nanocarbon hybrid. Nanotechnology 2013;24:118–21. [23] Ke CH, Li J, Fang KY, Zhu QL, Zhu J, Yan Q, et al. Synergistic effect between a novel hyperbranched charring agent and ammonium polyphosphate on the flame retardant and anti-dripping properties of polylactide. Polym Degrad Stabil 2010;95:763–70. [24] Qu BJ, Xie RC. Intumescent char structures and flame-retardant mechanism of expandable graphite-based halogen-free flame-retardant linear low density polyethylene blends. Polym Int 2003;52:1415–22. [25] Wang WX, Liang SH, Yu T, Li DH, Li YB, Han XF. The study of interaction between graphene and metals by Raman spectroscopy. J Appl Phys 2011;109. 571-R. [26] Šimek P, Klímová K, Sedmidubský D, Jankovský O, Pumera M, Sofer Z. Towards graphene iodide: iodination of graphite oxide. Nanoscale 2015;7:261–70. [27] Turgut H, Tian ZR, Yu FJ, Zhou WZ. Multivalent cation cross-linking suppresses highly energetic graphene oxide's flammability. J Phys Chem C 2017;121:5829–35. [28] Liu Y, Babu HV, Zhao JQ, Goni-Urtiaga A, Saiñz R, Ferritto R, et al. Effect of Cu-
[29]
[30]
[31]
[32]
[33]
[34] [35]
[36]
[37]
[38]
[39]
416
doped graphene on the flammability and thermal properties of epoxy composites. Compos B Eng 2016;89:108–16. Wan JT, Gan B, Li C, Aldareguia JM, Li Z, Wang X, et al. A novel biobased epoxy resin with high mechanical stiffness and low flammability: synthesis, characterization and properties. J Mater Chem 2015;3:21907–21. Ramanathan T, Abdala AA, Stankovich S, Dikin DA, Herrera-Alonso M, Piner RD, et al. Functionalized graphene sheets for polymer nanocomposites. Nat Nanotechnol 2008;3:327–31. Cao YW, Feng JC, Wu PY. Preparation of organically dispersible graphene nanosheet powders through a lyophilization method and their poly(lactic acid) composites. Carbon 2010;48:3834–9. McLauchlin AR, Thomas NL. Preparation and thermal characterisation of poly (lactic acid) nanocomposites prepared from organoclays based on an amphoteric surfactant. Polym Degrad Stabil 2009;94:868–72. Qiu SL, Xing WY, Mu XW, Feng XM, Ma C, Yuen RKK, et al. A 3D nanostructure based on transition-metal phosphide decorated heteroatom-doped mesoporous nanospheres interconnected with graphene: synthesis and applications. ACS Appl Mater Interfaces 2016;8:32528–40. Xu Z, Gao C. In situ polymerization approach to graphene-reinforced nylon-6 composites. Macromolecules 2011;43:6716–23. Xie RJ, Qu BJ. Expandable graphite systems for halogen-free flame retarding of polyolefins. II. structures of intumescent char and flame-retardant mechanism. J Appl Polym Sci 2001;80:1190–7. Zhao W, Liu JP, Peng H, Liao JY, Wang XJ. Synthesis of a novel PEPA-substituted polyphosphoramide with high char residues and its performance as an intumescent flame retardant for epoxy resins. Polym Degrad Stabil 2015;118:120–9. Li J, Li B, Zhang XC, Su RZ. The study of flame retardants on thermal degradation and charring process of manchurian ash lignin in the condensed phase. Polym Degrad Stabil 2001;72:493–8. Feng XM, Wang X, Cai W, Qiu SL, Hu Y, Liew KM. Studies on synthesis of electrochemically exfoliated functionalized graphene and polylactic acid/ferric phytate functionalized graphene nanocomposites as new fire hazard suppression materials. ACS Appl Mater Interfaces 2016;8:25552–62. Zheng ZH, Liu SF, Wang BN, Yang T, Cui XJ, Wang HY. Preparation of a novel phosphorus- and nitrogen-containing flame retardant and its synergistic effect in the intumescent flame-retarding polypropylene system. Polym Compos 2015;36:1606–9.
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Improved flame resistance and thermo-mechanical properties of epoxy resin nanocomposites from functionalized graphene oxide via self-assembly in water
T
Fang Fanga,b, Shiya Ranb,∗, Zhengping Fanga,b, Pingan Songc,d,∗∗, Hao Wangc a
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, 310027, China b Laboratory of Polymer Materials and Engineering, Ningbo Institute of Technology, Zhejiang University, Ningbo, 315100, China c Centre for Future Materials, University of Southern Queensland, Toowoomba, QLD, 4350, Australia d School of Engineering, Zhejiang A&F University, Hangzhou, 311300, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Epoxy resin P, N-containing flame retardants Graphene oxide Flame retardancy
The development of a green and facile strategy for fabricating ecofriendly, highly effective flame retardants has remain a major challenge. Herein, supermolecular aggregates of piperazine (PiP) and phytic acid (PA) have been self-assembled onto the graphene oxide (GO) surface in water to fabricate functionalized GO (PPGO). The chemical structure and morphology of PPGO are determined by the X-ray photoelectron spectroscopy, transmission electron microscopy and scanning electron microscopy along with the energy dispersive spectroscopy. Due to the introduction of organic component onto the surface of graphene oxide, the adhesion between PPGO and the epoxy resin (EP) matrix is enhanced. As a result, the storage modulus (E′) of EP composites is increased in addition to a better dispersion of PPGO. Compared with the pure EP, the flame resistance of EP/PPGO is significantly improved, exhibiting a 42% decrease in peak heat release rate (pHRR), 22% reduction in total heat release (THR). The reduced flammability of EP is attributed to the synergistic effects afforded by the gas dilution effect of piperazine, char-forming promotion effect of phytic acid and the creation of "tortuous path" barrier effect of GO during burning. This work offers a green and facile approach for creating highly effective graphenebased flame retardants.
1. Introduction As one of the most important thermosetting polymers, epoxy resin (EP) is used extensively in vehicles, construction, electrical appliance and aircrafts fields owing to its excellent moisture, good heat and solvent resistance, low shrinkage on curing, remarkable adhesive strength, good mechanical and dielectric properties [1,2]. However, EP suffers intrinsic flammability, extremely limiting its practical applications [3,4]. (see Scheme 1) Last decades have witnessed the application potential of nanofillers in polymeric materials in terms of enhancing the mechanical and flame retardancy as well as other performances of the later. Until now, the addition of carbon materials, such as carbon nanotubes (CNTs), exfoliated graphite nanosheets (GNS), fullerene (C60) and graphene oxide (GO), have shown significant improvement in the thermal stability and flame resistance of polymer matrix at a very low loading level [5–7].
∗
Among these nanoscale materials, graphene and its derivatives have been regarded as ideal functional fillers for polymers due to its large surface area and high aspect ratio two-dimensional structures [8–10]. Indeed, graphene nanosheets with single-atom thickness have been demonstrated to contribute to forming a continuous compact barrier that can decrease the heat release rate and prevent the transfer of pyrolysis gases into the burning surface [11–13]. However, graphene itself only exhibits a limited flame retardancy effect on the polymer due to its thermal barrier mechanism. For this reason, it is necessary to introduce external flame-retardant elements onto the surface of graphene to further strengthen its flame retardancy effects [14,15]. Decorating graphene with phosphorus and nitrogen is a facile but effective approach for enhancing the flame-resistant efficiency of graphene because phosphorus- and nitrogen-containing compounds are regarded as the highly efficient halogen-free flame retardants (FRs). Yu et al. [16] prepared phosphorus and nitrogen wrapped graphene via a
Corresponding author. Corresponding author. Centre for Future Materials, University of Southern Queensland, Toowoomba, QLD, 4350, Australia. E-mail addresses: [email protected] (S. Ran), [email protected] (P. Song).
∗∗
https://doi.org/10.1016/j.compositesb.2019.01.086 Received 4 December 2018; Received in revised form 14 January 2019; Accepted 17 January 2019 Available online 25 January 2019 1359-8368/ © 2019 Published by Elsevier Ltd.
Composites Part B 165 (2019) 406–416
F. Fang et al.
Scheme 1. Illustrations of synthesis routes of PPGO.
as a curing agent.
simple one-pot method with POCl3 and 4,4′-diaminodiphenylmethane (DDM). The functionalized graphene significantly improved the flame retardancy of epoxy resin at a low loading. Wang et al. [17] prepared a novel flame retardant through grafting synthesized polyphosphamide (PPA) onto the surface of graphene nanosheets (GNSs), and PPA-g-GNS was incorporated into epoxy resins matrix to enhance the flame resistance. Qian et al. [18] prepared a phosphorus, nitrogen and silicon containing hybrids flame retardants containing exfoliated rGO (FRsrGO) via sol-gel process to increase the fire resistance of epoxy resin. The flame-retardant mechanism was integrated with condensed phase and gas phase flame retardant strategies. Despite these great advances, the development of highly effective, low cost flame-retardants remained desired and attractive. Previously we have reported a green and facile methodology to prepare phosphorus-nitrogen-decorated graphene via self-assembly of piperazine, phytic acid and GO without using any organic solvent, holding enormous potential in the flame-resistance field [19]. Therefore, this work aims to further confirm chemical structures of this selfassembling product, especially the interactions among three components in detail. Then, the effects of phosphorus-nitrogen-decorated graphene on the thermal stability and flame retardancy of EP were systematically investigated and the mechanism of ternary flame-retardant system acting on the EP was also discussed. Our results show that the addition of 3 wt% PPGO reduces the peak heat release rate of EP by 42% in addition to a 44% reduction in the peak CO production. Moreover, the thermal stability and glass transition temperature of EP are also enhanced to different extents. This work provides a green and facile approach for creating highly effective graphene-based flame retardants.
2.2. Preparation of PPGO and EP/PPGO nanocomposites Piperazine was dissolved in deionized water and then GO was slowly added into piperazine solution and stirred overnight. The precipitate Piperazine@GO (PGO) was collected by centrifugation, washed with deionized water to neutral. After that phytic acid was dissolved into deionized water and then PGO was gradually added into the phytic acid solution and stirred overnight. Subsequently, dark brown precipitate phytic acid@piperazine@GO (PPGO) was washed with deionized water and collected by centrifugation, Fig. 1 illustrates the synthesis of PPGO. Briefly, EP nanocomposites were prepared as our previous work: PPGO with different proportions (1, 2 and 3 wt% relative to composites) were firstly dispersed in acetone and then E51 was added into PPGO suspension with mechanical stirring to homogeneous, the mixture was then dried at 60 °C under vacuum to remove acetone thoroughly. Next, curing agent (MHHPA) was added into the mixture with mechanical string to homogeneous. Subsequently, the mixture was evacuated under vacuum until no bubbles emerged. Finally, the mixture was poured into a stainless-steel mold and cured at a programmedtemperature of 80 °C for 2 h, 150 °C for 3 h. EP composites with 1, 2 and 3 wt% PPGO are designated as EP/PPGO1, EP/PPGO2, EP/PPGO3, respectively. For comparison, EP/GO3 composite was prepared using the same process. 2.3. Characterization X-ray photoelectron spectroscopy was performed on an ESCALAB 250 spectrometer (XPS, ThermoVG Scientific, U.K.). Transmission electron micrographs were collected using a JEM-2010 operated at an accelerating voltage of 200 kV (TEM, JEOL, Japan). An S-4800 scanning electron microscope was used to observe the micro-morphology of the nanocomposites (SEM, Hitachi, Japan). Elemental composition and mapping were performed using an energy dispersive spectroscopy (EDS) analyzer with a 20 mm2 SDD detector (Oxford Instruments). Raman spectra were collected on a Jobin-Yvon LabRam HRUV Raman spectroscope equipped with a 514.5 nm laser source. Thermal gravimetric analysis as performed on TGA 209 F1 at a heating rate of 20 °C/ min from 25 °C to 800 °C (TGA, Netzsch, Germany). Combustion behavior was performed in a cone calorimeter (Cone, Fire Testing
2. Experimental 2.1. Materials Phytic acid (70% in H2O) was provided by Aladdin Chemistry Co., Ltd. (Shanghai, China). Piperazine was purchased by Energy Chemical Co., Ltd. (Shanghai, China). Graphene oxide (10 mg/mL in H2O) was obtained by Hengqiu Graphene Company (Suzhou China). Diglycidyl ether of bisphenol-A epoxy (EP, E51) with an epoxide equivalent weight of 185–200 g/eq was provided by Wuhuigang Adhesive Co., Ltd (Hangzhou, China). Hexahydro-4-methylphthalic anhydride (MHHPA), provided by Energy Chemical Co., Ltd. (Shanghai, China), was selected 407
Composites Part B 165 (2019) 406–416
F. Fang et al.
Fig. 1. (a) XPS spectra of GO and PPGO; High resolution C 1s XPS spectra of (b) GO, (c) PPGO; XPS high-resolution spectra of (d) N 1s and (e) P 2p for PPGO.
3. Results and discussion
Technology, U.K.) with the dimension of the sample sheets of 100 × 100 × 3 mm3 at a heat flux of 35 kW m−2 according to ISO 5660 standard. Thermogravimetric analysis coupled to Fourier transform infrared spectroscopy (TGA-FTIR) measurement was performed using a TGA 209 F1 instrument (Netzsch, Germany), coupled to a Thermo Nicolet IS10 FTIR spectroscopy (Thermo Scientific, Germany). The volatiles evolved from TGA can be transferred into the gas cell of FTIR through the transfer line by a suitable gas flow. Each specimen with approximate 7.45 mg was placed to a ceramic crucible and heated from 30 °C to 700 °C at a heating rate of 20 °C/min under nitrogen atmosphere. Dynamic mechanical analysis (DMA) for the epoxy nanocomposites was using DMA (Q800, TA Instruments, U.S.A.) performed in a three-point-bending mode, which was operated at 1 Hz at a constant heating rate of 3 °C/min from 30 °C to 180 °C.
3.1. Characterization of PPGO XPS was measured to evaluate diversified information about elemental compositions and chemical state of GO and PPGO. As can be seen in Fig. 1a, the XPS spectrum of pure GO only displays the existence of oxygen and carbon, while PPGO shows the presence of nitrogen and phosphorus, besides carbon and oxygen. The corresponding C 1s spectra of GO and PPGO are shown in Fig. 1b and c. The high resolution C1s band of GO is deconvoluted into four peaks corresponding to C(O) OH (288.7 eV), C]O (287.8 eV), CeO (286.6 eV), CeC (284.6 eV) [20]. However, the peak of C(O)OH (288.7) disappeared in Fig. 1c, an additional peak located 286.0 eV appeared, assigning to CeN [21], which is belonged to piperazine, indicating the aggregation of piperazine and 408
Composites Part B 165 (2019) 406–416
F. Fang et al.
Fig. 2. TEM images of (a) GO and (b) PPGO; SEM-EDS element distribution maps of (c) GO, (d) PPGO.
electron microscopy combined with energy dispersive spectroscopy (SEM-EDS). To further explore the transfer of piperazine and phytic acid over the surface of graphene oxide, elemental distribution mapping was also carried out as shown in Fig. 2c–d and Fig. S1. The distribution of elements carbon and oxygen are homogeneous in all samples, reflecting the oxidation degree of graphene oxide is homogeneous. Moreover, the uniform dispersion of elements nitrogen and phosphorus as shown in the elemental mapping image of PPGO indicating that piperazine and phytic acid not only occupied on the edge of graphene oxide sheets but also took place in the whole bulk of the graphene oxide [25]. TG/DTG profiles for GO, PGO and GO as a function of temperature at the heating rate of 20 °C/min under N2 are shown in Fig. 3a and b and the data are collected in Table 1. Detailed parameters including T5, Tm, RW and Rm are defined as the temperature at 5% weight loss occurs, the temperature of maximum weight loss rate, the residual weight at 800 °C, and the maximum weight loss rate respectively. As shown in Fig. 3a, GO is thermal unstable and starts losing weight below 100 °C due to evaporation of water molecules held in samples, PGO and PPGO
GO. Moreover, high resolution N 1s and P 2p test have been done to further explore the structure of PPGO. As can be observed from the N 1s (Fig. 1d) spectrum of PPGO, the binding energies at 401.6 eV and 400.1 eV are ascribed to eNH2+- and eNHe respectively [22]. The obviously larger eNH2+- area and smaller eNHe area indicates that most piperazine molecules aggregate with GO and phytic acid. As shown in Fig. 1e, P 2p survey of PPGO is deconvoluted into two peaks at 133.6 eV and 134.2 eV, which are corresponding to P]O and PeOeC or PO32− in P2O5 [23,24]. TEM images of GO and PPGO were shown in Fig. 2a–b. GO sheets show a layered structure of individual nanosheets, which are overlayered together layer by layer with a size around several hundred nanometers [10]. Compared to GO, PPGO sheets are much more fuzzy and flexible with more wrinkles. These changes are attributed to the fact that piperazine and phytic acid co-doped onto GO sheets could facilitate the charge transfer to enhance the mechanical strain of GO sheets [26]. FTIR, XRD, Raman tests have been done and analyzed by our previous work. Elemental composition of GO and PPGO, including carbon, oxygen, nitrogen, phosphorus was analyzed by scanning 409
Composites Part B 165 (2019) 406–416
F. Fang et al.
Fig. 3. TGA and DTG curves of GO, PGO and PPGO.
PPGO3 and EP/GO3. Accordingly, Table 2 summarizes the storage modulus at 30 °C (glassy state), Tg + 30 °C (rubbery state) and glass transition temperature (Tg) [28]. With the addition of PPGO, near room temperature (30 °C), EP/PPGO3 shows an E′ value of 1944 MPa, 20% higher than that of neat EP (1614 MPa) and 27% higher than that of EP/GO3 (1599 MPa). The enhanced E′ demonstrated that EP/PPGO3 is more rigid than neat EP and EP/GO3, indicating that PPGO exhibits a better reinforcing effect than GO in this system. This may be due to the stack of GO sheets, after self-assembling with peperazine and phytic acid, this nanoscale roughness likely results in an enhanced mechanical interlocking with the polymer chains [29]. Relatively, higher storage modulus at rubbery state (Er) of EP/PPGO3 and EP/GO3 compared with EP is observed, reflecting a stronger bonding and interaction at the interface between graphene and matrix, which might be the reason for the better compatibility. Meanwhile, the higher Er of EP/PPGO3 than EP/GO3 further indicates the better dispersion of PPGO in EP matrix. Furthermore, the van der Waals' force between PPGO nanosheets may decreases, so that the dispersion of PPGO in the EP matrix is improved and the cohesive energy density among the aromatic ring-rich epoxy networks [30]. The temperature at the maximum of tan δ is usually consider as the glass transition temperature (Tg), which also follows a similar change in trend to the storage modulus. The result of DMA corresponds with the morphology of EP nanocomposites.
Table 1 Thermogravimetric properties of GO, PGO, PPGO and EP composites. Sample
GO PGO PPGO EP EP/PPGO3 EP/GO3
In N2 T5 (°C)
Tm (°C)
RWa (wt%)
Rm (%/°C)
76 85 76 313 346 329
204 166 194 432 430 427
41.1 50.8 49.5 5.8 11.5 7.7
14.5 11.3 11.9 31.3 31.8 34.0
a RW of GO, PGO and PPGO is at 800 °C, RW of EP, EP/PPGO3 and EP/GO3 is obtained at 700 °C.
perform similar degradation process below 100 °C. Moreover, GO degrades quickly at 204 °C (Tm) due to the decomposition of the oxygencontaining functional groups at the edge of GO sheets [27]. And the RW of GO at 800 °C is 41.1 wt%. Comparing to GO, PGO degrades earlier due to the poor thermal stability of piperazine, and the Rm of PGO and PPGO are both lower than GO. However, the self-assembling of piperazine and phytic acid in the presence of GO decreases the disordered structure of GO and thus enhances the thermal stability of GO. Furthermore, the RW of PGO (50.8 wt%) and PPGO (49.5 wt%) at 800 °C are both much higher than GO, according to the ionic interactions among carboxyl group, amino group and phosphate group, which enhance the thermal stability and char formation of GO as well.
3.3. Thermal stability and flammability of EP nanocomposites TGA was utilized to assess the thermal stabilities of EP and its composites. For exploring the influence of GO and PPGO on the thermal properties of EP, EP nanocomposites were characterized with TGA under N2 atmosphere. Fig. 6a and b exhibit the corresponding TGA and DTG curves. All the EP nanocomposites follow a one-stage decomposition process, presenting the similar degradation behaviors to bare EP, the detailed data are summarized in Table 1. As can be observed, there is only one degradation step for neat EP in the temperature range of 300–400 °C, the T5, Tm and RW at 700 °C are 313 °C, 432 °C and 5.8 wt%, respectively. Other EP composites exhibit the similar decomposition behaviors to the neat EP. For comparison, the incorporation of PPGO and GO could decrease T5 and increase char residues. PPGO has better effect on thermal stability at the early stage and char layer formatting of EP than GO, which is attributed to the better dispersion of PPGO in the EP matrix, ‘‘tortuous path’’ effect of graphene [31,32] and char layer promotion effect of piperazine and phytic acid. TG-FTIR analysis was performed to further investigate the influence of PPGO on the toxic and smoke gases released from EP nanocomposites during the thermal degradation process. In order to highlight the change of various volatile products, the absorbance of decomposed gases vs. time for EP and EP/PPGO3 is presented in Fig. 7a–f. Total
3.2. Morphology and thermo-mechanical properties of EP nanocomposites The interaction between graphene-based nanofillers and the EP matrix is the key factor for the dispersion of PPGO [14]. The TEM images of EP/GO3 and EP/PPGO3 are exhibited in Fig. 4a and b. As shown in Fig. 4, GO nanosheets are dispersed in the EP matrix presenting layered morphology along with a degree of agglomeration, while PPGO in EP matrix displays a better dispersive state with the same magnification. Furthermore, the color of GO nanosheets is dark with wrinkle surface, which is due to the stacking and agglomeration of GO nanosheets. However, as can be observed from Fig. 4b, the size of PPGO nanosheets is much smaller than GO and the exfoliation structure of PPGO sheets is observed clearly. It is indicated that with the selfassembling with piperazine and phytic acid, the interfacial interaction of GO with EP matrix is enhanced. In summary, the dispersion of PPGO in the EP matrix is more favorable. DMA is very subtle to physical and chemical structure of polymeric materials, which is utilized to explore the dynamic mechanical properties of epoxy resin networks. Fig. 5 shows the influence of temperature on storage modulus (E′) values and loss factor (tan δ) of EP, EP/ 410
Composites Part B 165 (2019) 406–416
F. Fang et al.
Fig. 4. TEM images of the freeze-fractured surface of EP composites (a) EP/GO3, (b) EP/PPGO3.
volatile products of EP/PPGO3 (Fig. 7a) are lower than that of pure EP, indicating the lower smoke toxicity. The intensity of the evolved volatile components is up to the maximum at approximately 21 min (about 425 °C), in accordance with the peak in DTG curves. Comparing with pure EP, with the incorporation of 3 wt% PPGO, the maximum absorbance intensities of decomposed products shift to lower values, including CO2, CO, hydrocarbons, carbonyl and aromatic compounds (Fig. 7b–f). The barrier effect of PPGO nanosheets further contributes to the decrease in flammable organic volatiles, the suppression of toxic smoke and the reinforcement on fire safety. Therefore, the incorporation of PPGO promotes the formation of char residues and has the potential on reducing the fire hazards of EP nanocomposites. Cone calorimetry is a standard bench-scale technique to examine the burning behavior of EP/PPGO nanocomposites. Several significant parameters, such as the time to ignition (TTI), peak heat release rate (pHRR), total heat release (THR), peak carbon monoxide production (pCOP), peak carbon dioxide production (pCO2P), total smoke production (TSP) are listed in Table 3. Among these parameters, HRR, THR, COP and CO2P vs time curves are displayed in Fig. 8a–d. Neat EP shows relatively high PHRR (707 kW m−2) and THR (82.1 MJ/m2) value, presenting a substantial risk of thermal and fire hazards. With the addition of 3 wt% PPGO, EP/PPGO3 reveals a 42% maximum decrease in PHRR (Fig. 8a), a 22% maximum decrease in THR (Fig. 8b). While the PHRR and THR of EP/GO3 are reduced by 8% and 4% than those of neat EP, indicating that with the same loading of PPGO or GO, PPGO shows higher flame-retardant efficiency. In fact, in a real fire, more people are killed because of poisonous gas rather than burning. Carbon monoxide (CO), carbon dioxide (CO2) and other toxic gases are
Fig. 5. Storage modulus values (E′) and loss-tangent (tan δ) at 1 Hz of cured EP, EP/GO3 and EP/PPGO3. Table 2 Storage modulus at 30 °C, 30 °C above the glass transition temperature (Tg) and Tg of EP, EP/GO3 and EP/PPGO3.
EP EP/PPGO3 EP/GO3
E′ at 30 °C (MPa)
Er (E′ at Tg + 30) (MPa)
Tg (°C)
1614 1944 1599
14 51 44
134 142 141
Fig. 6. (a) TGA and (b) DTG curves of EP, EP/GO3 and EP/PPGO3 under nitrogen. 411
Composites Part B 165 (2019) 406–416
F. Fang et al.
Fig. 7. Absorbance of decomposed products for EP and its nanocomposites vs time: (a) total decomposed products; (b) CO; (c) CO2; (d) hydrocarbons; (e) aromatic and (f) carbonyl compounds.
3.4. Condensed phase flame-retardation mechanism
released in the fire scene which can quickly asphyxiate people to death. In general, when the amount of CO in the air reaches 1%, the people will lose consciousness after several counts of inhalation. After 1–2 min, the people may die of poisoning. CO2, although with slight toxicity, it will reduce the oxygen content of air in the fire, also giving people a threat to life. In a normal fire, when CO2, concentration increases to 5%, people will die of choking. CO is the considerable poisonous gas released from EP composites during burning process. Accordingly, pCOP is also a non-negligible factor on evaluating the fire safety of EP composites. Fig. 8c and d exhibit COP and CO2P curves of neat EP and EP nanocomposite. The peak of COP of EP is reduced from 0.0157 to 0.0130 g/s after incorporation of 3 wt% GO. Moreover, it brings a much more significant decrease of EP from 0.0157 to 0.0086 g/s by addition of 3 wt% PPGO, indicating that the aggregation of piperazine and phytic acid works on flame inhibition [33]. Besides, it may be attributed to the physical barrier effect of the PPGO nanosheets and the compact char layer generated from the char forming effect of piperazine and phytic acid. As shown in Table 3, the pCO2P and TSP values for all samples are decreased, displaying a similar trend to that of COP, while EP/PPGO3 exhibits the lowest pCO2P and TSP value among them. These flame-retardant reinforcements are assigned to the inadequate combustion of EP and the compacter char layer.
Fig. 9a–c presents the digital photos of the residual char of EP, EP/ GO3 and EP/PPGO3 after cone calorimetry. For neat EP, the char residues are very weak, while EP/GO3 leaves a little more char residues. Particularly, 3 wt% of PPGO makes a more compact and continuous char surface and the amount of char residues are remarkably increased. SEM images allow us to observe microstructures of external char residues on a microscopic scale, in order to further elucidate the effect of PPGO on the char forming capability of EP composites. As shown in Fig. 9d–e, the char residues of EP/GO3 display more compact surface with smaller micro porous compared to the neat EP. Unlike the broken char residues of neat EP and EP/GO3, the char residues of EP/PPGO3 exhibit a more cohesive and compressed layer on the surface without obvious holes and cracks, indicating relatively slow transfer of heat and combustible gases during thermal degradation and combustion by introducing 3 wt% PPGO. Fig. 10a–c presents the Raman spectra of the char residues for EP and its nanocomposites after cone calorimeter tests. All the spectra depict two broad peaks at 1355 cm−1 and 1593 cm−1, corresponding to the D and G bands, respectively. The ratio of the integrated intensities of D to G band (ID/IG) is used to assess the graphitization degree of the char residues [34]. Obviously, the ID/IG ratio significantly decreases from 2.20 (neat EP) to 2.00 (EP/PPGO3), indicating that the char residues of EP/PPGO3 have the higher graphitization degree than EP and
Table 3 Detailed cone calorimetry data for EP and EP composites.
EP EP/PPGO1 EP/PPGO2 EP/PPGO3 EP/GO3
TTI (s)
pHRR (kW/m2)
THR (MJ/m2)
pCO2P (g/s)
pCOP (g/s)
TSP (m2/kg)
55 60 56 53 50
707 696 494 412 650
82.1 81.2 72.0 63.8 79.0
0.482 0.467 0.325 0.272 0.442
0.0157 0.0143 0.0108 0.0086 0.0130
25.5 24.4 22.7 19.3 23.7
± ± ± ± ±
3 2 3 4 5
± ± ± ± ±
21 49 32 44 16
± ± ± ± ±
2.9 0.3 3.2 0.9 4.4
Note: The parameters are the average values of one formula with two samples. 412
± ± ± ± ±
0.019 0.036 0.016 0.026 0.007
± ± ± ± ±
0.0010 0.0016 0.0011 0.0018 0.0001
± ± ± ± ±
0.7 0.9 1.5 1.0 2.1
Composites Part B 165 (2019) 406–416
F. Fang et al.
Fig. 8. (a) HRR and (b) THR vs time curves; (c) COP and CO2P (d) vs time curves of EP and its nanocomposites obtained from a cone calorimeter.
Fig. 9. Digital photos of the char residues of (a) EP, (b) EP/GO3 and (c) EP/PPGO3; SEM images of the char residues from (d) EP (e) EP/GO3 and (f) EP/PPGO3 after cone calorimeter test.
char residues after cone calorimeter. As displayed in Fig. 11a and b, C 1s of EP and EP/PPGO-3 both have the peaks at 284.6 eV (CeC or CeH in the aromatic and aliphatic structures), 286.2 eV (CeO and CeOeP groups) and 288.1 eV (C]O group) [20]. What's more, stronger peaks of CeO and C]O are found in the sample of EP/PPGO3 compared with
EP/GO3. Furthermore, the higher graphitization can be attributed to synergistic effect between the catalytic action of piperazine and phytic acid and the physical effect of graphene sheets, towards forming more stable char structures. XPS provides detailed information about the chemical structure of 413
Composites Part B 165 (2019) 406–416
F. Fang et al.
Fig. 10. Raman spectra of the char residues of (a) EP, (b) EP/GO3 and (c) EP/PPGO3.
Fig. 11. High resolution C 1s XPS spectra of (a) EP, (b) EP/PPGO3; O 1s spectra of (c) EP, (d) EP/PPGO3; (e) N 1s and (f) P 2p spectra of EP/PPGO3.
414
Composites Part B 165 (2019) 406–416
F. Fang et al.
Fig. 12. Schematic illustration of flame retardant mechanism.
Acknowledgments
those of EP. For O 1s spectra (Fig. 11c and d), two peaks are observed, which can be attributed to = O in phosphate or carbonyl groups (531.3 eV) and eOe in CeOeC or/and CeOeP groups (533.1 eV) [37]. As summarized, CeO/CeC and CeOeC&CeOeP/C]O&P]O ratios of EP/PPGO-3 are much higher than that of EP, indicating that with the addition of PPGO in to EP matrix, more CeO structures were retained in the char layers than in char residues of pure EP. The N 1s main peak of the char residues of EP/PPGO-3 in Fig. 11e at 400 eV is ascribed to quaternary nitrogen and to some formation of oxidized nitrogen compounds [35]. The P2p peak of EP/PPGO-3 in Fig. 11f appears around 133.8 eV attributed to PeOeC structure. The results above show that there are more organic crosslink structures bridged by CeO and CeOeP bonds presented in the char layers of EP/PPGO3 [36,37]. On the basis of the above analysis, a mechanism of piperazine and phytic acid functionalized graphene in EP matrix is proposed in Fig. 12. During the primary stage of combustion process, the temperature increases rapidly, PPGO starts to decompose and release water vapor from graphene oxide and phytic acid, at the same time, NH3 gas is generated from the decomposition of piperazine on the chemical structure of PPGO. These nonflammable gases (water vapor and NH3) can play a very important role in promoting the formation of char layer [38]. After that, piperazine has almost decomposed due to its poor thermal stability, so it is reasonable to presume that PPGO may convert to polyphytate graphene due to the self-crosslinking at elevated temperatures and it can further produce a compact char layer [39]. Moreover, the incorporation of PPGO nanosheets in EP can provide a so-called “tortuous path” barrier effect which can slow down the release rate of inflammable gases such as hydrocarbons, aromatic compounds, CO during the degradation process and the platelet morphology of PPGO also provides the potential applications as an alternative to nanoclay that could improve the barrier performance of EP [8].
This work was financially supported by the National Natural Science Foundation of China (Grant No. 51628302, 51703197, 51873196); the Ningbo Natural Science Foundation (grant number 2018A610112); the Ningbo Science and Technology Innovation Team (Grant No. 2015B11005), and K.C.Wong Education Foundation. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.compositesb.2019.01.086. References [1] Liu T, Nie YX, Chen RS, Zhang LD, Meng Y, Li XY. Hyperbranched polyether as an all-purpose epoxy modifier: controlled synthesis and toughening mechanisms. J Mater Chem 2015;3:1188–98. [2] Tan Y, Shao ZB, Chen XF, Long JW, Chen L, Wang YZ. Novel multifunctional organic-inorganic hybrid curing agent with high flame-retardant efficiency for epoxy resin. ACS Appl Mater Interfaces 2015;7:17919–28. [3] Müller P, Bykov Y, Döring M. New star-shaped phosphorus-containing flame retardants based on acrylates for epoxy resins. Polym Adv Technol 2013;24:834–40. [4] Jian RK, Wang P, Duan WS, Wang JS, Zheng XL, Weng JB. Synthesis of a novel P/N/ S-containing flame retardant and its application in epoxy resin: thermal property, flame retardance, and pyrolysis behavior. Ind Eng Chem Res 2016;55:11520–7. [5] Feng YZ, Hu J, Xue Y, He CG, Zhou XP, Xie XL, et al. Simultaneous improvement in the flame resistance and thermal conductivity of epoxy/Al2O3 composites by incorporating polymeric flame retardant-functionalized graphene. J Mater Chem 2017;5:13544–56. [6] Song PA, Zhao LP, Cao ZH, Fang ZP. Polypropylene nanocomposites based on C60decorated carbon nanotubes: thermal properties, flammability, and mechanical properties. J Mater Chem 2011;21:7782–8. [7] Song PA, Xu ZG, Wu YP, Cheng QF, Guo QP, Wang H. Super-tough artificial nacre based on graphene oxide via synergistic interface interactions of π-π stacking and hydrogen bonding. Carbon 2017;111:807–12. [8] Wang X, Kalali EN, Wan JT, Wang DY. Carbon-family materials for flame retardant polymeric materials. Prog Ploym Sci 2017;49:21–46. [9] Nine MJ, Tran DNH, Tung TT, Kabiri S, Losic D. Graphene-borate as an efficient fire retardant for cellulosic materials with multiple and synergetic modes of action. ACS Appl Mater Interfaces 2017;9:10160–8. [10] Sang B, Li ZW, Li XH, Yu LG, Zhang ZJ. Graphene-based flame retardants: a review. J Mater Sci 2016;51:8271–95. [11] Huang GB, Song PA, Liu L, Han D, Ge CH, Li RR, et al. Fabrication of multifunctional graphene decorated with bromine and nano-Sb2O3 towards high-performance polymer nanocomposites. Carbon 2016;98:689–701. [12] Guan FL, Gui CX, Zhang HB, Jiang ZG, Jiang Y, Yu ZZ. Enhanced thermal conductivity and satisfactory flame retardancy of epoxy/alumina composites by combination with graphene nanoplatelets and magnesium hydroxide. Compos B Eng 2016;98:134–40. [13] Pan YT, Wang JT, Zhao XL, Li C, Wang DY. Interfacial growth of MOF-derived layered double hydroxide nanosheets on graphene slab towards fabrication of multifunctional epoxy nanocomposites. Chem Eng J 2017;330:1222–31. [14] Tao SH, Liu PL, Hsiao MC, Teng CC, Wang CA, Ger MD, et al. One-step reduction and functionalization of graphene oxide with phosphorus-based compound to produce flame-retardant epoxy nanocomposite. Ind Eng Chem Res
4. Conclusions This work has demonstrated a green and facile way to functionalize GO via self-assembled supermolecular aggregate of piperazine (PiP) and phytic acid (PA) onto GO. PPGO/EP composites exhibited a significantly reduced PHRR (42%), THR (22%) and pCOP (44%) compared to that of the neat EP. Apart from the excellent fire-retardancy performance, the loading of 3 wt% PPGO also leads to the increase in both storage modulus in rubbery state and glass transition temperatures. The improved flame-retardant performance of EP nanocomposites is assigned to a tripartite cooperative effect from the key components (piperizine, phytic acid, GO). This work offers a green and facile approach for creating graphene-based flame retardants for EP, which can contribute to expanding the practical applications of EP. 415
Composites Part B 165 (2019) 406–416
F. Fang et al.
2012;51:4573–81. [15] Hu CQ, Yu T, Li Y. Novel DOPO-modified graphene: synthesis and characterization. J Nanosci Nanotechnol 2017;17:4894–900. [16] Yu B, Shi YQ, Yuan BH, Qiu SL, Xing WY, Hu WZ, et al. Enhanced thermal and flame retardant properties of flame-retardant-wrapped graphene/epoxy resin nanocomposites. J Mater Chem 2015;3:8034–44. [17] Wang X, Xing WY, Feng XM, Yu B, Song L, Hu Y. Functionalization of graphene with grafted polyphosphamide for flame retardant epoxy composites: synthesis, flammability and mechanism. Polym Chem 2014;5:1145–54. [18] Qian XD, Song L, Yu B, Wang BB, Yuan BH, Shi YQ, et al. Novel organic-inorganic flame retardants containing exfoliated graphene: preparation and their performance on the flame retardancy of epoxy resins. J Mater Chem 2013;1:6822–30. [19] Fang F, Song PA, Ran SY, Guo ZH, Wang H, Fang ZP. A facile way to prepare phosphorus-nitrogen-functionalized graphene oxide for enhancing the flame retardancy of epoxy resin. Compos Comm 2018;10:97–102. [20] Cui XY, Yang SB, Yan XX, Leng JG, Shuang S, Ajayan PM, et al. Pyridinic-nitrogendominated graphene aerogels with Fe–N–C coordination for highly efficient oxygen reduction reaction. Adv Funct Mater 2016;26:5708–17. [21] Pethsangave DA, Khose RV, Wadekar PH, Some S. Deep eutectic solvent functionalized graphene composite as an extremely high potency flame retardant. ACS Appl Mater Interfaces 2017;9:35319–24. [22] Song PA, Liu L, Huang GB, Yu YM, Guo QP. Largely enhanced thermal and mechanical properties of polymer nanocomposites via incorporating C60@graphene nanocarbon hybrid. Nanotechnology 2013;24:118–21. [23] Ke CH, Li J, Fang KY, Zhu QL, Zhu J, Yan Q, et al. Synergistic effect between a novel hyperbranched charring agent and ammonium polyphosphate on the flame retardant and anti-dripping properties of polylactide. Polym Degrad Stabil 2010;95:763–70. [24] Qu BJ, Xie RC. Intumescent char structures and flame-retardant mechanism of expandable graphite-based halogen-free flame-retardant linear low density polyethylene blends. Polym Int 2003;52:1415–22. [25] Wang WX, Liang SH, Yu T, Li DH, Li YB, Han XF. The study of interaction between graphene and metals by Raman spectroscopy. J Appl Phys 2011;109. 571-R. [26] Šimek P, Klímová K, Sedmidubský D, Jankovský O, Pumera M, Sofer Z. Towards graphene iodide: iodination of graphite oxide. Nanoscale 2015;7:261–70. [27] Turgut H, Tian ZR, Yu FJ, Zhou WZ. Multivalent cation cross-linking suppresses highly energetic graphene oxide's flammability. J Phys Chem C 2017;121:5829–35. [28] Liu Y, Babu HV, Zhao JQ, Goni-Urtiaga A, Saiñz R, Ferritto R, et al. Effect of Cu-
[29]
[30]
[31]
[32]
[33]
[34] [35]
[36]
[37]
[38]
[39]
416
doped graphene on the flammability and thermal properties of epoxy composites. Compos B Eng 2016;89:108–16. Wan JT, Gan B, Li C, Aldareguia JM, Li Z, Wang X, et al. A novel biobased epoxy resin with high mechanical stiffness and low flammability: synthesis, characterization and properties. J Mater Chem 2015;3:21907–21. Ramanathan T, Abdala AA, Stankovich S, Dikin DA, Herrera-Alonso M, Piner RD, et al. Functionalized graphene sheets for polymer nanocomposites. Nat Nanotechnol 2008;3:327–31. Cao YW, Feng JC, Wu PY. Preparation of organically dispersible graphene nanosheet powders through a lyophilization method and their poly(lactic acid) composites. Carbon 2010;48:3834–9. McLauchlin AR, Thomas NL. Preparation and thermal characterisation of poly (lactic acid) nanocomposites prepared from organoclays based on an amphoteric surfactant. Polym Degrad Stabil 2009;94:868–72. Qiu SL, Xing WY, Mu XW, Feng XM, Ma C, Yuen RKK, et al. A 3D nanostructure based on transition-metal phosphide decorated heteroatom-doped mesoporous nanospheres interconnected with graphene: synthesis and applications. ACS Appl Mater Interfaces 2016;8:32528–40. Xu Z, Gao C. In situ polymerization approach to graphene-reinforced nylon-6 composites. Macromolecules 2011;43:6716–23. Xie RJ, Qu BJ. Expandable graphite systems for halogen-free flame retarding of polyolefins. II. structures of intumescent char and flame-retardant mechanism. J Appl Polym Sci 2001;80:1190–7. Zhao W, Liu JP, Peng H, Liao JY, Wang XJ. Synthesis of a novel PEPA-substituted polyphosphoramide with high char residues and its performance as an intumescent flame retardant for epoxy resins. Polym Degrad Stabil 2015;118:120–9. Li J, Li B, Zhang XC, Su RZ. The study of flame retardants on thermal degradation and charring process of manchurian ash lignin in the condensed phase. Polym Degrad Stabil 2001;72:493–8. Feng XM, Wang X, Cai W, Qiu SL, Hu Y, Liew KM. Studies on synthesis of electrochemically exfoliated functionalized graphene and polylactic acid/ferric phytate functionalized graphene nanocomposites as new fire hazard suppression materials. ACS Appl Mater Interfaces 2016;8:25552–62. Zheng ZH, Liu SF, Wang BN, Yang T, Cui XJ, Wang HY. Preparation of a novel phosphorus- and nitrogen-containing flame retardant and its synergistic effect in the intumescent flame-retarding polypropylene system. Polym Compos 2015;36:1606–9.