Author’s Accepted Manuscript A Facile Way to Prepare Phosphorus-NitrogenFunctionalized Graphene Oxide for Enhancing the Flame Retardancy of Epoxy Resin Fang Fang, Pingan Song, Shiya Ran, Zhenghong Guo, Hao Wang, Zhengping Fang www.elsevier.com
PII: DOI: Reference:
S2452-2139(18)30077-9 https://doi.org/10.1016/j.coco.2018.08.001 COCO127
To appear in: Composites Communications Received date: 5 July 2018 Revised date: 15 August 2018 Accepted date: 15 August 2018 Cite this article as: Fang Fang, Pingan Song, Shiya Ran, Zhenghong Guo, Hao Wang and Zhengping Fang, A Facile Way to Prepare Phosphorus-NitrogenFunctionalized Graphene Oxide for Enhancing the Flame Retardancy of Epoxy Resin, Composites Communications, https://doi.org/10.1016/j.coco.2018.08.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A Facile Way to Prepare Phosphorus-Nitrogen-Functionalized Graphene Oxide for Enhancing the Flame Retardancy of Epoxy Resin Fang Fang a,b, Pingan Song c, Shiya Ran b, Zhenghong Guo b, Hao Wang c, Zhengping Fangb* 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, QLD4350, Australia
* Corresponding author. E-mail: [email protected]
Abstract In this paper, we have reported a facile way to functionalize graphene oxide (GO) via assembling a supermolecular aggregate of piperazine (PiP) and phytic acid (PA) onto the GO surface (PPGO) without using any organic solvent. The functionalization of GO is confirmed by the X-ray photoelectron spectrum (XPS), transmission electron micrographs (TEM) and Raman spectrum. The introduction of 3 wt% PPGO into epoxy resin (EP/PPGO3) results in notable suppression on the fire risk of epoxy resin. In addition, cone calorimeter tests showed that the peak heat release rate (pHRR) was decreased from 727.4 kW/m2 to 367.5 kW/m2 (49%), and the peak smoke production rate (pSPR) was decreased from 0.2316 m2/s to 0.1379 g/s (40%). The improved flame-retardant performance of EP nanocomposites is most likely due to a tripartite cooperative effect from the key components (piperizine, phytic acid, and GO). This strategy demonstrates a facile and efficient approach for fabricating highly effective graphene-based flame retardants for polymers.
Keywords: Epoxy resin; Self-assembled supermolecular aggregate; P, N-containing flame retardants; Graphene; Flame retardancy
1. Introduction Over the past two decades, many efforts have been done for the design of polymer/nanocomposites and develop flame-retardants, with the goal of enhancing thermo-stability and flame resistance of polymers. Nanofillers such as graphene, fullerene (C60), carbon nanotubes (CNTs) have attracted a lot of attention due to their remarkable improvements on flammability, thermal, mechanical and electric properties of the polymer matrix with a very low loading (generally less than 5 wt%) [1,2]. Among these additives, graphene with the unique structure of sp2-bonded one atom-thick two-dimensional carbon layer has shown outstanding physical properties such as super-large specific surface area, excellent thermal conductivity and mechanical properties [3,4]. To meet the common fire-retardant requirements, Layered lamellae structure of graphene can act as barriers to reduce oxygen access, delay the heat transfer between interfaces and resist the escape of pyrolysis products and prevent the spread of combustion gases in the material [5,6]. Although graphene has many advantages, its strong tendency to form irreversible aggregation and
significantly hinders the homogeneous dispersion within the polymer matrix [7,8].
In this work, we have developed a facile way to fabricate phosphorus-nitrogen-functionalized graphene oxide (PPGO) by creating a supermolecular aggregate of piperazine, phytic acid and graphene oxide via self-assembly. Phytic acid is a biocompatible, and nontoxic compound with six phosphate groups, which can be promising as an effective flame retardant, while piperazine and its derivatives is widely used as flame retardants [9,10]. However, there is a lack of understanding the flame retardancy effectiveness by combining phosphate, piperazine and graphene. It is expected that they are capable of playing different roles in improving flame retardancy of epoxy resin.
2. Experimental 2.1. Materials Phytic acid (70% in H2O) was provided by Aladdin Chemistry Co., Ltd. (Shanghai, China). Piperazine was purchased from Energy Chemical Co., Ltd. (Shanghai, China). Graphene oxide (10 mg/mL in H2O) was obtained from Hengqiu Graphene Company (Suzhou, China), with single layer
thickness of 0.7-1.2 nm and lateral dimension of 0.5-5 μm. Diglycidyl ether of bisphenol-A epoxy (EP, E51) with an epoxide equivalent weight of 185-200 was provided by Wuhuigang Adhesive Co., Ltd (Hangzhou, China). Hexahydro-4-methylphthalic anhydride (MHHPA), provided by Energy Chemical Co., Ltd. (Shanghai, China), was selected as a curing agent. 2.2. Fabrication of PPGO Piperazine (5.00 g) was dissolved in deionized water (250 mL) in a 500 mL beaker equipped with a mechanical stirrer. GO (100 mL, 10.0 mg/mL) was slowly added into piperazine solution and stirred overnight. The precipitate [email protected]
(PGO) was collected by centrifugation, washed with deionized water to neutral. Then 5.0 g phytic acid was dissolved into deionized water (300 mL) in a 500 mL beaker equipped with a mechanical stirrer. PGO was gradually added into the phytic acid solution and stirred overnight. Subsequently, dark brown precipitate phytic [email protected]
@GO (PPGO) was
collected by centrifugation, washed with deionized water to neutral. Figure 1 illustrates the synthesis of PPGO. 2.3. Preparation of EP Nanocomposites Briefly, EP nanocomposites were prepared as follows: PPGO with different proportions (1, 2 and 3 wt% relative to composites) were firstly dispersed in 30 mL acetone by ultrasonication for 30 mins, then E51 was added into PPGO suspension with mechanical stirring at room temperature for 1 day, the mixture was then dried at 60 oC under vacuum to remove acetone thoroughly. Next, curing agent (MHHPA) was added into the mixture with mechanical string at room temperature for 6 hours to homogeneous. The mass ratio of E51 and MHHPA is 1:0.85. 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 programmed-temperature of 80 oC for 2 h, 150 oC for 3 h. EP composites with 1, 2 and 3 wt% PPGO are called as EP/PPGO1, EP/PPGO2, EP/PPGO3, respectively. For comparison, EP/GO3 composite was prepared using the same process. 2.4. Characterization X-ray photoelectron spectroscopy was performed on an ESCALAB 250 spectrometer (XPS, ThermoVG Scientific, U.K.). Infrared spectroscopy was recorded on a Vector-22 FT-IR spectrometer using KBr
pellets (IR, Bruker, Germany). Raman spectra were collected on a Jobin-Yvon LabRam HRUV Raman spectroscope equipped with a 514.5 nm laser source. X-ray diffraction (XRD) tests were conducted using a Rigaku X-ray generator (Cu Kα radiation) in the reflection mode at room temperature. Transmission electron micrographs were collected using a JEM-2010 operated at an accelerating voltage of 200 kV (TEM, JEOL, Japan). Combustion behavior was performed in a cone calorimeter (Cone, Fire Testing 3
Technology, U.K.) with the dimension of the sample sheets of 100×100×3 mm at a heat flux of 35
kW m-2 according to ISO 5660 standard. 3. Results and discussion 3.1. Characterization of PPGO XPS was measured to evaluate diversified information about elemental compositions of GO and PPGO. As depicted in Figure 2a, the XPS spectrum of pure GO only shows the presence of carbon and oxygen, while the XPS survey of PPGO clearly exhibits the existence of phosphorus and nitrogen apart from carbon and oxygen. The corresponding C 1s spectra of GO and PPGO, obtained from XPS
testing are shown in Figure 2b and c. The high resolution C1s band of GO is deconvoluted into four peaks corresponding to C-C (284.6 eV), C-O (286.6 eV), C=O (287.8 eV), C(O)OH (288.7 eV) . However, after aggregating with piperazine and phytic acid, the peak of C(O)OH (288.7) disappeared, whereas an additional peak located at 286.0 eV, assigned to C-N  which is belonged to piperazine , indicating that most piperazine has aggregated with GO and phytic acid. IR analysis reveals the essential structural information of GO, PGO and PPGO. As shown in Figure 3, the typical stretching vibrations of GO appear, such as the stretching vibration of ubiquitous O-H (ca. 3400 cm-1), C=O carbonyl originated from COOH (ca. 1725 cm-1), adsorbed
water molecules (ca. 1620 cm-1), and carbonyl (or carboxyl) C-O-C (ca. 1055 cm-1) . After assembling with piperazine, the spectrum of PGO shows an additional peak located at 1554 cm-1 is found assigned to bending vibration of -NH- (amide II) , which is belonged to piperazine. Moreover, the peak of C=O carbonyl becomes smaller and almost disappears, revealing that most piperazine has aggregated with GO. In comparison with the PGO, the peak of -NH- in the spectrum of
PPGO becomes nearly neglectable, and instead an additional peak located at 1066 cm-1 appears, which is assigned to PO42-  originated from phytic acid. This indicates the successful self-assembly between the imide groups of piperazine and the PO42- groups in phytic acid. Raman spectroscopy is performed to characterize the graphitization degree of carbon materials. Figure 4 displays the Raman spectra of GO, PGO and PPGO. The Raman spectra of GO exhibits two broad peaks at 1355 cm-1 and 1593 cm-1 corresponding to the D and G bands, which is consistent with the previous report . The D peak is connected with disordered structure of graphite while the G peak is connected with the first order E2G mode . ID/IG is the relevant intensity ratio of D and G bands, which is regarded as an efficient measure to evaluate the disorder degree of carbon materials . By comparing the ID/IG values of GO (1.36), PGO (1.22), PPGO (1.26), the slight decreased I D/IG of PPGO suggests that the self-assembly process with piperazine and phytic acid enhance the graphitization degree of GO. Moreover, the G and D peaks of PPGO show obvious shift compared with GO. There are two reasons for raman shift in GO, one is the carrier density modulation induced by charge transfer during the self-assembly process and the other is the introduction of the mechanical strain by topmost piperazine and phytic acid layers. Figure 5 shows the XRD patterns of GO, PGO and PPGO, the first basal reflection (002) peak of GO appears at 2θ=10.14° , corresponding to an interlayer distance of 0.872 nm. The weak peak at a 2θ of ~ 42º is ascribed to the (100) reflection of GO . For PGO, the location of the (002) reflection peak is the same as GO, indicating that after aggregating with piperazine, there is no changes in interlayer distance. For PPGO, the (002) reflection peak shifts to 2θ=8.96°, revealing an increase of the interlayer distance to 0.986 nm. The increase of the interlayer distance is caused by the relatively lager molecular size of phytic acid than piperazine. Therefore, XRD results agree well with IR and Raman results. The morphologies of GO and PPGO were examined by TEM. Figure 6a shows a layered structure of individual nanosheet with wrinkled shape of GO nano-platelet  which is several hundreds of nanometers large. Figure 6b displays that the edges of PPGO sheets are much fuzzier and the sheets show more wrinkles as compared with the pure GO. This can also provide a further evidence for the assembly of piperazine and phytic acid on the GO sheet surface. 3.2. Flammability of EP Nanocomposites Cone calorimeter test is a standard bench-scale technique to examine the burning behaviors of EP/PPGO nanocomposites. Peak heat release rate (pHRR) and smoke production rate (SPR) vs time curves of EP composites are depicted in Figure 7a and b. Neat EP shows a high pHRR (727 kW/m2) and pSPR (0.2316 m2/s), presenting high risks of both heat and fire hazards. With the addition of 3 wt%
PPGO, EP/PPGO3 generates a 49 % maximum decrease in pHRR (368 kW/m2, Figure 5a), a 40% maximum decrease in pSPR (0.1379 m2/s, Figure 5b) relative to the EP. While the pHRR (634 kW/m2) and pSPR(0.1786 m2/s) of EP/GO3 are respectively reduced by 13% and 23% than those of neat EP, indicating that with the same loading of PPGO or GO, PPGO shows better flame-retardant effectiveness.
Figure 8a-c presents the digital photos of digital photographs of the residual chars of EP, EP/GO3 and EP/PPGO3 after cone calorimetry tests. For neat EP, the char residue is very thin, whereas EP/GO3 leaves a little more residue after the test. By contrast, 3 wt% of PPGO leads to a more compact and continuous char surface and the amount of char residue are remarkably increased. It seems that such thick and compact char residue is primarily responsible for the improved flame retardancy. Phytic acid can promote the char-forming ability in the condensed phase, acting as a carbonizing agent while piperazine shows excellent performance as a foaming agent, the nonflammable gases (water vapor and NH3) could play a very important role in promoting the formation of intumescent porous char structure. They work together to form an effective intumescent flame-retarding system . Meanwhile, GO can perform as physical barrier and provide a so-called "tortuous path" effect to further decrease the heat release and to prevent the heat and mass transfer between the combustion zone and the underlying polymer bulk . Moreover, during the degradation process and the platelet morphology of PPGO can also provide the potential application as an alternative to nano-clay that could improve the barrier performance of EP .
4. Conclusions To conclude, we have demonstrated a facile way to functionalize graphene oxide (GO) via self-assembled supermolecular aggregate of piperazine (PiP) and phytic acid (PA) onto GO denoted as PPGO. Then PPGO was incorporated into epoxy to fabricate flame retardant composites. EP/PPGO3 composite exhibited a significantly reduced pHRR (49%) and pSPR (40%) compared to that of the neat EP. The improved flame-retardancy of EP nanocomposites may be attributed to a combined effect from the key components (P, N flame retardant elements and GO).
Acknowledgments This work was supported by the National Natural Science Foundation of China (grant numbers 51628302, 51703197, 51673173) and Natural Science Foundation of Zhejiang Province, China (grant number Q15C160013).
Author Contributions Fang Fang and Pingan Song contribute equally to this work as the co-first author.
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Captions of Figures Figure 1. Illustrations of synthesis routes of PPGO. Figure 2. (a) XPS spectra of GO and PPGO; High resolution C 1s XPS spectra of (b) GO, (c) PPGO. Figure 3. FTIR spectra of GO, PGO and PPGO. Figure 4. Raman spectra of GO, PGO and PPGO. Figure 5. XRD spectra of GO, PGO and PPGO. Figure 6. TEM images of (a) GO and (b) PPGO. Figure 7. (a) HRR and (b) SPR vs time curves of EP and its nanocomposites from a cone calorimeter. Figure 8. Digital photos of the char residues of (a) EP, (b) EP/GO3 and (c) EP/PPGO3
Figure 1. Illustrations of synthesis routes of PPGO.
Figure 2. (a) XPS spectra of GO and PPGO; High resolution C 1s XPS spectra of (b) GO, (c) PPGO;
Figure 3. FTIR spectra of GO, PGO and PPGO
Figure 4. Raman spectra of GO, PGO and PPGO.
Figure 5. XRD spectra of GO, PGO and PPGO
Figure 6. TEM images of (a) GO and (b) PPGO. 10
Figure 7. (a) HRR and (b) SPR vs time curves of EP and its nanocomposites from a cone calorimeter.
Figure 8. Digital photos of the char residues of (a) EP, (b) EP/GO3 and (c) EP/PPGO3
Highlights: 1. A facile way to functionalize graphene oxide (PPGO) via self-assembling supermolecular aggregate of piperazine and phytic acid onto GO was developed. 2. Introduction of 3 wt% PPGO into epoxy resin resulted in a 49% reduction of the peak heat release rate and a 40% reduction of the peak smoke production rate from cone calorimetric analysis. 3. The improved flame-retardant performance of EP nanocomposites may be assigned to a tripartite cooperative effect from the key components.