Journal of Thermal Analysis and Calorimetry https://doi.org/10.1007/s10973-018-7325-5 (0123456789().,-volV)(0123456789().,-volV)
Improved thermal stability of phenolic resin by grapheneencapsulated nano-SiO2 hybrids Juan Chen1 • Wenbo Zhang1 • Jing Liu2 • Heyi Ge1
•
Moufeng Tian2 • Jianye Liu2 • Min Jing3
Received: 7 January 2018 / Accepted: 22 April 2018 Ó Akade´miai Kiado´, Budapest, Hungary 2018
Abstract Phenolic resin (PR) modified with hybrids of reduced graphene oxide (RGO)-encapsulated nano-SiO2 (SiO2–RGO) was prepared by a simple method. The synergistic effect of RGO and nano-SiO2 was achieved in the thermal decomposition of the modified PR. Thermal stability of SiO2–RGO and the modified PR was evaluated by thermogravimetric analysis (TG). With 1 mass% loading of SiO2–RGO, the maximum decomposition temperature (Tdmax) of the modified PR was increased by 32.50 °C and the residual mass at 800 °C was increased by 6.54%. The structure of the resin char was characterized to study the mechanism of ameliorative thermal stability. SiO2–RGO hybrids were conducive to induce the PR to form graphitized carbon in the pyrolysis process. Thus, SiO2–RGO can facilitate the application of PR in the fields of heat insulation and ablation resistance. Keywords Graphene Nanostructures Synergistic effect Thermal analysis
Introduction High thermal stability, flame retardancy, excellent insulation characteristic, and char retention capacity make phenolic resin (PR) attractive for ablative system applications [1–4]. PR has high decomposition temperature. There are low smoke and low toxicity in the decomposition process. Moreover, char is formed by carbonization, which is mechanically stable even at high temperature and prevents the spread of fire [5]. However, the antioxidation property of methylene and phenol groups is poor, which limits the application of PR [6]. In order to meet the requirements of the ablative materials, the modified PR with improved thermal resistance has been developed by incorporating inorganic particles or carbon materials [7–11]. & Heyi Ge
[email protected] 1
School of Materials Science and Engineering, University of Jinan, Jinan 250022, People’s Republic of China
2
Beijing Composite Materials Co., Ltd., Beijing 102101, People’s Republic of China
3
School of Materials Science and Engineering, Shandong Jianzhu University, Jinan 250000, People’s Republic of China
Nanosized inorganic particles, such as nano-SiO2, nanoTiO2, have oxidative stability, flame retardancy, and low volume shrinkage. They are often used for facilitating heat resistance, thermal stability, and dimensional stability of polymer matrixes [6, 12–14]. Srikanth et al. [13] prepared carbon fabric/PR with different amount of nano-SiO2. Both the ablative property and the performance of heat transfer coefficient were enhanced. Periadurai et al. [14] prepared modified PR with nano-SiO2 by in situ polymerization. The thermal decomposition and the flame-retardant property of nanocomposite were enhanced with 5 mass% nano-SiO2. Li et al. [6] prepared modified PR with silicon and boron. In air, the maximum decomposition rate (Dmax) decreased about 2% min-1 and the residual mass at 900 °C (R900) increased 16.7% compared with pure PR. However, the nanosized inorganic particles are difficult to disperse uniformly in polymer matrix. The expected improvements in processabilities and properties by incorporating nanosized particles are not fully realized. Therefore, it is necessary to find a way to improve the compatibility of nanoparticles with polymer matrix. Graphene and graphene oxide (GO) have inspired much interest owning to the unique structure and excellent performance [15, 16]. There are many reports about improving thermal stability of PR with GO [10, 17–20]. Si et al. [17]
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prepared GO/PR composites by incorporating GO into PR. The peak degradation temperature of the PR was increased by about 14 °C with heat-treated GO. The char yield of GO/PR composite at a GO mass fraction of 0.5% was about 11% greater than that of PR. Wang et al. [18] prepared GO/ PR composites by Steglich esterification process. The result revealed that the GO was absolutely exfoliated and covalent linked GO/PR composite was obtained. The thermal stability of PR was remarkably improved by modification with GO. Noparvar-Qarebagh et al. [19] prepared GO modified with furfuryl alcohol and (3aminopropyl) triethoxysilane, and then graphene-containing silica aerogel was used as an additive in novolac resin matrix. The char yield was 70.6% for the modified novolac resin at 700 °C. (For comparison, the char yield of GO/ novolac resin is 60.7%.) Liao et al. [20] prepared 9,10dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)-functionaized reduced GO (DOPO-RGO) by one-step method. DOPO-rGO/epoxy nanocomposites with phosphorus and graphene layer structures were found to contribute to excellent flame retardancy compared to that of neat epoxy. The synergestic effect of DOPO-rGO was quite useful. Zhang et al. [21] developed flame-retardant, tough, and heat-resistant thermosetting composites through building cross-linked network based on cyanate ester (CE) and unique hybridized graphene oxide (FGO) with phosphorus and silicone. With the same loading of fillers, FGO/ CE composite had much better integrated performances than GO/CE composite. It was found that the origins behind attractive properties could be attributed to unique cross-linked structure induced by the presence of FGO. The aforementioned studies involving modification of PR have focused solely on inorganic nanoparticles or GO (RGO). Few have investigated the combination of them to enhance thermal stability of PR. In this work, the synergestic benefit was obtained by coupling oxide nanoparticles with RGO as fillers. Meanwhile, we developed an environmentally friendly method to prepare SiO2–RGO/PR by incorporating SiO2–RGO into PR using ultrasonic dispersion and mechanical agitation. The effect of SiO2–RGO hybrids on thermal stability of PR was evaluated, and the origin of heat resistance was investigated as well. Additionally, the effects of SiO2, GO, SiO2–GO on PR were also investigated as contrasts.
solid content 57.57 mass%, 137.7 mPa.s) was supplied by Beijing Composite Materials Co., Ltd. 3-Triethoxysilylpropylamine (APTES) was kindly provided by Taishan Glass fiber INC. Sodium borohydride (NaBH4) was obtained from Shanghai Zhanyun chemical Co., Ltd.
Preparation of SiO2–RGO hybrids GO was synthesized by the modified Hummers’ method, which was described in detail in our previous research [22]. Nano-SiO2 was prepared by Sto¨ber process [23]. The preparation procedure of SiO2–RGO is shown in Fig. 1. In a typical process, APTES (0.25 g) was added into an ethanol–water solution with pH 5–6 dropwise. The solution was stirred for 1.5 h for the hydrolysis of APTES. Next, nano-SiO2 (5 g) was immersed into it. The grafting reaction of APTES on nano-SiO2 surface was realized by sonication for 50 min. Then, the APTES-modified nanoSiO2 (amino-SiO2) was taken out from the mixture, washed with ethanol and deionized water for several times, and dried under vacuum. Next, 5 mL GO dispersion (1 mg mL-1) was added into 5 mL amino-SiO2 nanoparticles (1 mg mL-1). After 30-min stirring, the hybrids of GO encapsulated nano-SiO2 through electrostatic adsorption were obtained. The chemical reduction of SiO2–GO was performed at 80 °C with NaBH4 solution (50 mmol mL-1, pH 12) for 3 h to obtain SiO2–RGO hybrids. Then the SiO2–RGO hybrids were washed by distilled water thrice to remove residual solution. Finally, the hybrids were desiccated in a vacuum oven at 50 °C for 6 h.
Preparation of the modified PR Hybrids ethanol suspension was added into the PR through ultrasonic vibration and mechanical agitation. The mass fraction of SiO2–GO or SiO2–RGO hybrids was 1.0%. The modified PR was coated to the surface of glass sheet forming a thin sheet which was cured in an oven under the temperature of 120 °C for 2 h, 150 °C for 2 h, and 170 °C for 1 h. Then SiO2–GO/PR or SiO2–RGO/PR was obtained, respectively. For comparison, the samples of pure PR, 1 mass% GO/PR, and 1 mass% SiO2/PR were prepared in the same process.
Characterizations
Experimental Materials Graphite powder (8000 mesh, 99.95%) and tetraethyl orthosilicate (TEOS) were purchased from Aladdin Industrial Co., Ltd. Phenolic resin (ammonia-catalyzed PR,
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The morphology of hybrids and the fracture surface of phenolic composites were characterized by scanning electron microscopy (SEM, HITACHIS-2500) and transmission electron microscopic (TEM, JEM-1200EX, JEOL). The distributions of carbon, oxygen, silicon elements were conducted on an energy-dispersive spectrometer (EDS).
Improved thermal stability of phenolic resin by graphene-encapsulated nano-SiO2 hybrids
NH2
NH2
OH
OH
Si
HO
Si
SiO2
Si
OH HO
Si
O
O
Si
Si
OH
SiO2
APTES
Amino–SiO2
Nano–SiO2
GO
Electrostatic self–assembly
Chemical reduction NaBH4 80 °C/3 h
SiO2–RGO SiO2–GO Fig. 1 Schematic of SiO2–RGO preparation process
Fourier transform infrared (FTIR) measurement was performed on a Nicolet 380 infrared spectrometer (Thermo electron corporation, USA). The specimens were prepared by potassium bromide pellet technique. Raman spectra of the samples were recorded from 500 to 3500 cm-1 on a LabRAM XploRA INV laser confocal Raman spectrometer (Jobin–Yvon Co., Ltd., France) using a 532-nm argon laser line. Thermogravimetric analysis (TG) was carried out at a TGA/DSC thermo-analyzer (Mettler-Toledo) with a heating rate of 10 °C min-1 in the range of 35–1000 °C under nitrogen atmosphere or air atmosphere. The powder X-ray diffraction (XRD) patterns were recorded on Bruker D8 ADVANCE X-ray diffractometer using Cu Ka radiation (k = 0.154178 nm) with 40 kV scanning voltage, 40 mA scanning current and scanning range from 10° to 60°.
Results and discussion Morphology of SiO2, SiO2–GO, and SiO2–RGO Figure 2a shows the zeta potentials of amino-SiO2 and GO in aqueous solution at different pH values. It can be found that the surface of GO is negatively charged in the pH range 2.0–12.0, and amino-SiO2 is positive in the pH range 2.0–7.0. When amino-SiO2 and GO are oppositely charged, the mutual assembly can be triggered. Thus, it needs to control the pH below 7.0 to drive the self-electrostatic assembling process. The morphologies and structures of nano-SiO2, SiO2– GO, and SiO2–RGO hybrids were characterized by SEM analysis, as shown in Fig. 2b–d. Based on the SEM images, the surface of nano-SiO2 is bright and clean. The diameter of the nano-SiO2 ranges from 100 nm to 200 nm. For SiO2–GO hybrids, the SiO2 particles are firmly encapsulated on the surface of GO nanosheets. The ultrasonic washing process did not separate SiO2 particles from GO nanosheets. After chemical reduction, SiO2–RGO hybrids
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(a)60 GO SiO2
Zeta potential/mv
40 20 0 –20 –40 2
4
6
8
10
12
pH
Fig. 2 a Zeta potentials of amino-SiO2 and GO; b, c, d SEM images of nano-SiO2, SiO2–GO and SiO2–RGO hybrids
demonstrate an obviously different morphology. The morphology of SiO2–RGO hybrids displays slight reaggregation phenomenon. They reunite into small groups about 1–2 lm. The TEM images of GO encapsulated nano-SiO2 are shown in Fig. 3. Spherical nano-SiO2 particles are generated on the smooth graphene sheet by electrostatic adsorption. The processing method effectively prevented nanoparticles from agglomeration and enabled them to have a good dispersion. Based on the above results and discussion, the hybrids have been successfully synthesized through electrostatic adsorption.
FTIR analysis of SiO2, amino-SiO2, GO, SiO2–GO, and SiO2–RGO Fig. 3 TEM image of SiO2–GO hybrid materials
FTIR spectra of SiO2, amino-SiO2, GO, SiO2–GO, and SiO2–RGO are shown in Fig. 4. For GO, the peaks at 3420, 1730, 1620, 1380, and 1073 cm-1 are corresponding to – OH, C=O, C=C and C–O stretching vibrations, respectively
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[24]. The FTIR spectrum of nano-SiO2 shows absorption band around 1100 cm-1, attributed to the asymmetric stretching of linear Si–O–Si. The absorption band located
Improved thermal stability of phenolic resin by graphene-encapsulated nano-SiO2 hybrids 1380 cm–1 1073 cm–1
GI /I (0.95) D G
D GO 1730 cm–1
1620 cm–1
Transmittance/a.u.
SiO2–RGO
Intensity/a.u.
3420 cm–1 SiO2–GO
1370 cm–1 Amino–SiO2
2936 cm–1
1562
ID/IG(1.24) SiO2– GO
1465 cm–1
cm–1
SiO2– RGO
SiO2
1100 cm–1 1640 cm–1
0
–1 945 cm–1 799 cm
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Wavenumber/cm–1 Fig. 4 FTIR spectra of SiO2, amino-SiO2, GO, SiO2–GO and SiO2– RGO
at 799 cm-1 is the characteristic band for network Si–O–Si [13]. The peak at 945 cm-1 is assigned as Si–OH stretching vibration. It can be found that the bands of amino-SiO2 have changed markedly compared with pure nano-SiO2. Three additional peaks appear in the wave number range from 1562 to 1370 cm-1 in the spectrum of amino-SiO2, corresponding to the N–H, C–H, and C–N stretching or bending vibrations. This indicates that APTES component has been successfully grafted onto the surface of nano-SiO2 [25]. The spectrum of SiO2–GO inherits characteristic bands of GO and SiO2, such as peaks at 3420, 1620, 1100, and 945 cm-1, indicating that SiO2 particles have been encapsulated with GO sheets successfully [26]. After the chemical reduction process, the broad peak at 3420 cm-1 becomes almost flat, and the intensity of bands corresponding to the oxygen functional groups of GO decreases dramatically in the spectrum of SiO2–RGO, indicating the removal of oxygen functional groups [24].
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shift/cm–1
Fig. 5 Raman spectra of SiO2–GO and SiO2–RGO
literatures [29, 30] and was discussed in our previous work [24].
Thermal property of SiO2–GO and SiO2–RGO For thermal stability test, thermogravimetric analysis (TG) is an indispensable representation. The thermal stability of SiO2–GO and SiO2–RGO was assessed by TG in the range of 35–1000 °C under argon atmosphere. Based on the TG curves (Fig. 6), SiO2–GO is thermally unstable and exhibits a severe mass loss from 35 to 300 °C compared with SiO2–RGO, due to the removal of adsorbed water by oxygen-containing functional groups on the GO surface and labile oxygen functional groups that lead to the release of CO, CO2 and steam [31]. Compared to SiO2–GO, SiO2– RGO demonstrates the improved thermal stability due to the chemical reduction of GO by NaBH4. The amount of oxygen functional groups on the RGO surface was decreased. Therefore, the mass loss of SiO2–RGO was 100
Raman analysis of SiO2–GO and SiO2–RGO Residual mass/%
The Raman spectra of SiO2–GO and SiO2–RGO are shown in Fig. 5. It can be seen that the characteristic D and G bands are centered at approximate 1350 and 1560 cm-1, respectively. The G band is a characteristic feature of all sp2 hybridized carbons [27]. Furthermore, the D band is attributed to an increase in the disorder. The intensity ratio of D/G (ID/IG) is used according to the Tuinstra–Koenig relation as a measure for the size of sp2-domains [28]. The ID/IG value increases obviously for SiO2–RGO (1.24) compared with that of SiO2–GO (0.95). The average size of the sp2-domains decreased after reduction process, which led to the increase in ID/IG. This result is in accord with the
95 90
SiO2–RGO
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SiO2–GO
70 65 200
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Temperature/°C Fig. 6 TG curves of SiO2–GO and SiO2–RGO
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(a)
(b)
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0.0000
5–SiO2–RGO/PR
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Derivative mass
Residual mass/%
4–SiO2–GO/PR
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5
–0.0005
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–0.0010 1–PR 2–GO/PR 3–SiO2/PR
–0.0015
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–0.0020
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Fig. 7 TG (a) and DTG (b) curves of pure PR, SiO2/PR, GO/PR, SiO2–GO/PR and SiO2–RGO/PR under nitrogen atmosphere Table 1 Summary of TG results under nitrogen atmosphere
Sample name
T5%/°C
T10%/°C
Tdmax/°C
Residual mass at 800 °C/%
PR
310.29
407.47
529.65
64.78
GO/PR
382.83
446.33
547.45
67.73
SiO2/PR
438.16
497.50
549.36
68.74
SiO2–GO/PR
447.17
494.12
558.47
69.24
SiO2–RGO/PR
425.33
484.37
562.15
71.32
1–PR
100
2–SiO2–GO/PR
Residual mass/%
3–SiO2–RGO/PR
80
Thermal property of the modified PRs
1 60 2
40
3
20 0 200
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600
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Temperature/°C Fig. 8 TG curves of pure PR, SiO2–GO/PR and SiO2–RGO/PR under air atmosphere Table 2 Summary of TG results under air atmosphere Sample name
T5%/°C
T10%/°C
Residual mass at 600 °C/%
PR
211.51
399.10
14.88
SiO2–GO/PR
306.56
426.01
43.43
SiO2–RGO/PR
299.91
426.17
51.34
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much lower and the mass percent residual after major degradation at 1000 °C was much higher than that of SiO2– GO.
TG analysis under nitrogen atmosphere was investigated to understand the thermal resistance of the cured PR, GO/PR, SiO2/PR, SiO2–GO/PR, and SiO2–RGO/PR (Fig. 7). The corresponding data are summarized in Table 1. During the thermal degradation of PR, the beginning stage is up to 200 °C and the following is 200–400 °C. The water in PR retained from the production process is released up in the beginning stage. From 200 to 400 °C, the postcuring and the oxidative degradation occur with the evolution of formaldehyde, water and carbon dioxide [8]. The mass loss in both stages is about 8%. The main degradation of the PR takes place with maximum reaction rate, starts from approximately 400 °C [32]. Therefore, the thermal degradation properties include 5% mass loss temperature (T5%), 10% mass loss temperature (T10%), the maximum mass loss temperature (Tdmax) and the residual mass rate at 800 °C. The T5% and T10% of all modified PRs are higher than those of pure PR. Although the T5% and T10% of SiO2–RGO/PR are not the highest among the modified PRs, the Tdmax of it reaches to the highest 562.15 °C. Moreover, the residual mass rates at 800 °C of the modified PRs are higher than
Improved thermal stability of phenolic resin by graphene-encapsulated nano-SiO2 hybrids
PR. Thus, the thermal resistance of PR can be improved by incorporating nanoparticles at a low loading. Among of these nanoparticles, the SiO2–RGO has the best effect.
Compared with PR, the Tdmax of SiO2–RGO/PR goes up by 32.50 °C, and the residual mass rate increases by 6.54%. As shown in Fig. 7b, the maximum mass loss temperature of the modified PR is shifted to a higher temperature by introducing the SiO2–RGO. Literature have indicated that the incorporation of GO or SiO2 can enhance the thermal resistance of PR [13, 14, 33]. Silica particles could enhance the char formation of the matrix material and form a protective surface barrier to prevent the immediate damage of substrate materials [14]. GO could facilitate the formation of char and promote graphitization of PR [33]. However, nanoparticles and GO sheets themselves were difficult to disperse well in the PR matrix. The processing method in this work effectively prevented nanoparticles from agglomeration and enabled them to have a good dispersion. The introduction of SiO2 nanoparticles on GO could alleviate the phenomena of crimp and stack for GO sheets in PR. On the other hand, the amount of oxygen-containing functional groups of GO were decreased after chemical reduction. The proper reduction of GO could further enhance the heat resistance [17, 20]. Therefore, the residual
Transmittance/a.u.
SiO2–RGO/PR
SiO2–GO/PR
GO/PR SiO2/PR PR
4000
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Wavenumber/cm–1 Fig. 9 FTIR spectra of pure PR, GO/PR, SiO2/PR, SiO2–GO/PR and SiO2–RGO/PR
(a)
(b) 800 °C
600 °C 400 °C PR
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Transmittion/%
Transmittion/%
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SiO2–RGO/PR
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(c)
Transmittion/%
400 °C, air atmosphere
PR
SiO2–RGO/PR
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Wavenumber/cm–1 Fig. 10 FTIR spectra for the samples that were heat-treated to a specified temperature under N2 atmosphere a pure PR and b SiO2–RGO/PR, c 400 °C under air atmosphere
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Fig. 11 SEM images of a pure PR; b SiO2–RGO/PR; c, d SiO2/PR
mass rate of SiO2–RGO/PR at 800 °C is higher than that of SiO2–GO/PR. The synergistic effect between RGO and nano-SiO2 has been achieved in the thermal decomposition process. Thermal properties of the cured PR, SiO2–GO/PR, and SiO2–RGO/PR in air atmosphere were also tested. The results presented in Fig. 8 and Table 2 reveal that in air atmosphere the decomposition rates of SiO2–GO/PR and SiO2–RGO/PR are slower than that of PR, especially the T5%, which are increased by 95.05 and 88.4 °C, respectively. The residual mass rates of the cured SiO2–GO/PR and SiO2–RGO/PR are 43.43 and 51.34% at 600 °C, respectively. But the residual mass rate of PR is only 14.88%. The introducing SiO2–RGO hybrids into PR effectively improves the thermal stability.
FTIR analysis of the modified PRs FTIR was used to characterize the chemical structure of the cured pure PR and the modified PRs. As shown in Fig. 9, all the samples present the typical spectra of cured PR. The absorption peaks of five samples have unconspicuous deviation, which implies that the nanoparticles might have no new chemical bond reaction with PR. The peak at 3440 cm-1 is due to the O–H stretching vibration [34, 35]. The peaks at 2923, 2856, 1473, and 1432 cm-1 are attributed to C–H stretching and bending vibrations [36]. The C=C stretching vibration of phenolic ring appears around 1610 cm-1. The C–O stretching vibration of phenyl-OH is at 1510 and 1205 cm-1 [34]. The signals at about 752–754 cm-1 are the absorption of ortho substituted phenol. The signals at about 818–820 cm-1 are the
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characteristics peaks of para-position substituted phenol [36]. Although it is difficult to find the evidence of covalent bonding between SiO2–RGO and PR from the infrared spectra, noncovalent functionalization of RGO can also be accomplished through p–p stacking. The noncovalent interaction can restrict the movement of partial PR molecule chains and, therefore, improve the thermal decomposition temperature [37]. To investigate the chemical structure changes in the thermal degradation process of PRs, the FTIR spectra were obtained for the samples which were heat-treated in a furnace to a specified temperature under nitrogen atmosphere and air atmosphere (see Fig. 10). As can be seen in Fig. 10, with the increase in the heat treatment temperature, the peak intensities of the characteristic peaks decrease together with peak shift and/or peak merge and even disappear. Under nitrogen atmosphere, when the heat temperature is up to 400 °C, it can be noticed from Fig. 10a and b that the peaks at 3340, 1610, and 1205 cm-1 decrease dramatically,while the peaks at 1510 cm-1, 1473 and 1432 cm-1 merge. It suggests that the postcuring occurred concurrently with some extent of oxidative degradation with the evolution of formaldehyde, water, and carbon dioxide [8, 32]. In 400–600 °C, the peaks of C–H bending vibrations, C–O stretching vibration of phenyl-OH are almost disappeared, which indicates that the main degradation of PR took place in this temperature range [32]. Comparing PR with SiO2–RGO/PR, the curves of them are similar at 400 °C. However, the relative intensity of peaks in SiO2–RGO/PR curve is rather lower than PR at 600 °C. This suggests that SiO2–RGO would be conducive to induce the PR to form graphitized carbon. Besides, when the temperature rose to 800 °C, there are still –OH and benzene ring peaks in the curve of PR, but there is almost no peak in SiO2–RGO/PR curve. On the other hand, it is shown in Fig. 10c that when the samples were heated to 400 °C in air, the relative intensity of peaks in SiO2–RGO/ PR curve is higher than PR, which suggests that in the initial decomposition stage, SiO2–RGO/PR had a higher decomposition temperature than PR. The above analysis shows that SiO2–RGO can delay the decomposition rate of PR, promote the graphitized process, and improve the heat resistance of PR.
Morphology of PR, SiO2/PR, SiO2–GO/PR, and SiO2–RGO/PR Figure 11 shows the SEM images of PRs. Nanofillers are normally easy to reaggregate. Their uneven dispersion in resin matrix will influence the performance of composites. Therefore, uniform dispersion of hybrids in PR matrix is critical. As shown in Fig. 11a, the PR shows a smooth and dendritic morphology, implying a typical brittle feature.
Improved thermal stability of phenolic resin by graphene-encapsulated nano-SiO2 hybrids
Fig. 12 EDS images of a 1 mass% SiO2–RGO/PR electronic image; b carbon element mapping; c oxygen element mapping and d silicon element mapping
600 °C, 2 h, N2 atmosphere
Transmittance/a.u.
PR
1588 cm–1
SiO2–RGO–PR
3451 cm–1
3500
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2000
1500
1000
Wavenumber/cm–1
Fig. 13 FTIR spectra of PR char and SiO2–RGO/PR char
However, SiO2–RGO/PR and SiO2/PR show rough fracture surfaces (Fig. 11b–d), reflecting typical tough feature. The agglomeration of nano-SiO2 makes it difficult to disperse when mixed into the PR. The microstructure of SiO2/PR (Fig. 11c, d) shows that nano-SiO2 particles are apparently
agglomerated in chunks, resulting in the uneven microstructure. Compared with SiO2/PR,the existence of RGO (Fig. 11b) is a good protection to prevent SiO2 from agglomeration, which makes the hybrids dispersed evenly in the PR matrix and makes full use of the heat resistance of the fillers. The SiO2 particles spread evenly through the layers of GO and the GO layers obstruct heat with the aid of SiO2. Thus, the synergistic effect of RGO and nano-SiO2 was achieved in the thermal decomposition process. There were many irregular granules with a diameter of about 1–2 lm that are uniformly distributed in the PR matrix. These granules are SiO2–RGO hybrids, whose aggregation size corresponds to Fig. 2d. To further illustrate that the hybrids are good for preventing aggregation of nanofillers, the EDS images of 1 mass% SiO2–RGO/PR are shown in Fig. 12. Three elements carbon, oxygen, and silicon distribute evenly in PR, which indicates that graphene promoted the dispersion of nano-SiO2, and the good distribution of 1 mass% SiO2– RGO would be conducive to the thermal stability of PR in the pyrolysis process.
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(a)
(b) SiO2–RGO/PR
PR
1000 °C, 2 h, N2 atmosphere
Intensity/a.u.
Intensity/a.u.
1000 °C, 2 h, N2 atmosphere
20
30
40
50
60
20
30
40
50
60
2θ /°
2θ /°
Fig. 14 XRD patterns of a PR char and b SiO2–RGO/PR char
Structure of PR char and SiO2–RGO/PR char The FTIR spectra of PR char and SiO2–RGO/PR char treated at 600 °C under N2 atmosphere for 2 h are shown in Fig. 13. For both of them, the C=C stretching vibration of phenolic ring appears around 1588 cm-1. However, the O– H stretching vibration band peak at 3451 cm-1 is disappeared at SiO2–RGO/PR char spectrum. The comparison of the characteristic IR features of PR char and SiO2–RGO/ PR char indicates the improved formation of char in the pyrolysis process. In order to further illustrate the improved graphitization of SiO2–RGO/PR, the XRD analysis was employed to examine the PR char and SiO2–RGO/PR char treated at 1000 °C under N2 atmosphere for 2 h. The XRD patterns are shown in Fig. 14, and the distance (d) between char layers was calculated according to the Bragg’s law. The distance between PR char layers is 0.3696 nm (Fig. 14a) which is larger than that of pristine graphite (d = 0.3354 nm), implying the amorphous carbon structure of PR char. It can be observed that there are two peaks at 23.30° (d = 0.3815 nm) and 26.01° (d = 0.3423 nm) as shown in Fig. 14b, which suggests that SiO2–RGO is conducive to induce PR to form graphitized carbon.
Conclusions In summary, SiO2–RGO hybrids were synthesized from the chemical reduction of electrostatic self-assembly SiO2–GO hybrids. SEM, TEM and EDS indicated that nano-SiO2 particles were dispersed on the graphene sheet uniformly, and 1 mass% SiO2–RGO hybrids were evenly dispersed in the PR. SiO2–RGO had better thermal stability than SiO2– GO due to the chemical reduction of GO by NaBH4. The synergistic effect of RGO and nano-SiO2 was achieved in
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the thermal decomposition process of PR. TG analysis showed that the Tdmax of 1 mass% SiO2–RGO/PR was increased to 562.15 °C and the residual mass rate at 800 °C was significantly increased to 71.32% compared with pure PR (Tdmax and the residual mass rate: 529.65 °C and 64.78%, respectively) due to the introduction of SiO2– RGO hybrids. FTIR and X-ray diffraction showed that SiO2–RGO were conducive to induce the PR to form graphitized carbon. Thus, SiO2–RGO hybrids are potentially used as modifier for refractory materials. Acknowledgements This work is supported by the Foundation for the Youth Science and Technology Innovation of Beijing Composite Materials Co., Ltd., and A Project of Shandong Province Higher Educational Science and Technology Program (Grant No. J14LA05).
References 1. Bafekrpour E, Simon GP, Yang C, Chipara M, Habsuda J, Fox B. A novel carbon nanofibre/phenolic nanocomposite coated polymer system for tailoring thermal behaviour. Compos Part A Appl Sci. 2013;46(1):80–8. 2. Kim M, Choe J, Dai GL. Development of the fire-retardant sandwich structure using an aramid/glass hybrid composite and a phenolic foam-filled honeycomb. Compos Struct. 2016;158:227–34. 3. Ebrahimi H, Roghani-Mamaqani H, Salami-Kalajahi M. Preparation of carbon nanotube-containing hybrid composites from epoxy, novolac, and epoxidized novolac resins using sol–gel method. J Therm Anal Calorim. 2018;132:513–24. 4. Song SA, Lee Y, Kim YS, Kim SS. Mechanical and thermal properties of carbon foam derived from phenolic foam reinforced with composite particles. Compos Struct. 2017;173:1–8. 5. Choe J, Kim M, Kim J, Dai GL. A microwave foaming method for fabricating glass fiber reinforced phenolic foam. Compos Struct. 2016;152:239–46. 6. Li S, Chen F, Zhang B, Luo Z, Li H, Zhao T. Structure and improved thermal stability of phenolic resin containing silicon and boron elements. Polym Degrad Stab. 2016;133:321–9. 7. Chen Y, Hong C, Chen P. The effects of zirconium diboride particles on the ablation performance of carbon–phenolic
Improved thermal stability of phenolic resin by graphene-encapsulated nano-SiO2 hybrids
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
composites under an oxyacetylene flame. RSC Adv. 2013;3(33):13734–9. Asaro L, D’Amico DA, Alvarez VA, Rodriguez ES, Manfredi LB. Impact of different nanoparticles on the thermal degradation kinetics of phenolic resin nanocomposites. J Therm Anal Calorim. 2017;128:1463–78. Li C, Qi S, Zhang D. Thermal degradation of environmentally friendly phenolic resin/Al2O3 hybrid composite. J Appl Polym Sci. 2010;115(6):3675–9. Amirsardari Z, Aghdam RM, Salavati-Niasari M, Shakhesi S. Enhanced thermal resistance of GO/C/phenolic nanocomposite by introducing ZrB2, nanoparticles. Compos Part B Eng. 2015;76(21):174–9. Sang HY, Kim SH, Lee WI, Kim H. Improvement of ablation resistance of phenolic composites reinforced with low concentrations of carbon nanotubes. Compos Sci Technol. 2015;121:16–24. Nair S, Aswathy UV, Mathew A, Raghavan R. Studies on the thermal properties of silicone polymer based thermal protection systems for space applications. J Therm Anal Calorim. 2016;128:1731–41. Srikanth I, Daniel A, Kumar S, Padmavathi N, Singh V, Ghosal P. Nano silica modified carbon–phenolic composites for enhanced ablation resistance. Mater Scr. 2010;63(2):200–3. Periadurai T, Vijayakumar CT, Balasubramanian M. Thermal decomposition and flame retardant behaviour of SiO2-phenolic nanocomposite. J Anal Appl Pyrolysis. 2010;89:244–9. Bao C, Guo Y, Yuan B, Hu Y, Song L. Functionalized graphene oxide for fire safety applications of polymers: a combination of condensed phase flame retardant strategies. J Mater Chem. 2012;22(43):23057–63. Wang H, Hao Q, Yang X, Lu L, Wang X. Effect of graphene oxide on the properties of its composite with polyaniline. ACS Appl Mater Interface. 2010;2(3):821–8. Si J, Li J, Wang S, Li Y, Jing X. Enhanced thermal resistance of phenolic resin composites at low loading of graphene oxide. Compos Part A Appl Sci. 2013;54(4):166–72. Wang D, Gao J, Xu WF, Bao F, Ma R, Yan CJ. The synthesis and characterization of the graphene oxide covalent modified phenolic resin nanocomposites. Adv Mater Res. 2011;327(327):115–9. Noparvar-Qarebagh A, Roghani-Mamaqani H, Salami-Kalajahi M. Novolac phenolic resin and graphene aerogel organic-inorganic nanohybrids: high carbon yields by resin modification and its incorporation into aerogel network. Polym Degrad Stab. 2015;124:1–14. Liao SH, Liu PL, Hsiao MC, Teng CC, Wang CA, Ger MD. Onestep reduction and functionalization of graphene oxide with phosphorus-based compound to produce flame-retardant epoxy nanocomposite. Eng Chem Res. 2012;51(12):4573–81. Zhang Z, Yuan L, Guan Q, Liang G, Gu A. Synergistically building flame retarding thermosetting composites with high toughness and thermal stability through unique phosphorus and silicone hybridized graphene oxide. Compos Part A Appl Sci. 2017;98:174–83.
22. Chen J, Zhao D, Jin X, Wang C, Wang D, Ge H. Modifying glass fibers with graphene oxide: towards high-performance polymer composites. Compos Sci Technol. 2014;97:41–5. 23. Sto¨ber W, Fink A, Bohn E. Controlled growth of monodisperse silica spheres in the micron size range. J Colloid Interface Sci. 1968;26(1):62–9. 24. Chen J, Wu J, Ge H, Zhao D, Liu C, Hong X. Reduced graphene oxide deposited carbon fiber reinforced polymer composites for electromagnetic interference shielding. Compos Part A Appl Sci. 2016;82:141–50. 25. Yang S, Feng X, Ivanovici S, Mu¨llen K. Fabrication of grapheneencapsulated oxide nanoparticles: towards high-performance anode materials for lithium storage. Angew Chem. 2010;49(45):8408. 26. Chen J, Zhao D, Ge H, Wang J. Graphene oxide-deposited carbon fiber/cement composites for electromagnetic interference shielding application. Constr Build Mater. 2015;84:66–72. 27. Reich S, Thomsen C. Raman spectroscopy of graphite. Philos Trans. 1842;2004(362):2271–88. 28. Zalan Z, Lazar L, Fulop F. Chemistry of hydrazinoalcohols and their heterocyclic derivatives. Part 1. Synthesis of hydrazinoalcohols. Curr Org Chem. 2005;36(31):357–76. 29. Ren PG, Yan DX, Ji X, Chen T, Li ZM. Temperature dependence of graphene oxide reduced by hydrazine hydrate. Nat Nanotechnol. 2011;22(5):055705. 30. Noparvar-Qarebagh A, Roghani-Mamaqani H, Salami-Kalajahi M, Kariminejad B. Nanohybrids of novolac phenolic resin and carbon nanotube-containing silica network. J Therm Anal Calorim. 2016;128(2):1027–37. 31. Abdolmaleki A, Mallakpour S, Karshenas A. Facile synthesis of glucose-functionalized reduced graphene oxide (GFRGO)/poly(vinyl alcohol) nanocomposites for improving thermal and mechanical properties. Mater Sci Eng B Sold. 2017;217:26–35. 32. Kim YJ, Kim MI, Yun CH, Chang JY, Park CR, Inagaki M. Comparative study of carbon dioxide and nitrogen atmospheric effects on the chemical structure changes during pyrolysis of phenol-formaldehyde spheres. J Colloid Interface Sci. 2004;274(2):555–62. 33. Park JM, Kwon DJ, Wang ZJ, Gu GY, Devries KL. Interfacial, fire retardancy, and thermal stability evaluation of graphite oxide (GO)-phenolic composites with different GO particle sizes. Compos Part B Eng. 2013;53(5):290–6. 34. Blanco M, Vazquez A, Gabilondo N. Curing characteristics of resol-layered silicate nanocomposites. Thermochim Acta. 2007;467(1):73–9. 35. Krˇ´ıStkova´ M, Filip P, Weiss Z, Peter R. Influence of metals on the phenol–formaldehyde resin degradation in friction composites. Polym Degrad Stab. 2004;84(1):49–60. 36. Lin CT, Lee HT, Chen JK. Preparation and properties of bisphenol-F based boron-phenolic resin/modified silicon nitride composites and their usage as binders for grinding wheels. Appl Surf Sci. 2015;330:1–9. 37. Huang L, Zhu P, Li G, Lu D, Sun R, Wong C. Core–shell SiO2@RGO hybrids for epoxy composites with low percolation threshold and enhanced thermo-mechanical properties. J Mater Chem A. 2014;2(43):18246–55.
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