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Carbonization mechanism of polypropylene catalyzed by Co compounds combined with phosphorus-doped graphene to improve its fire safety performance
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Carbonization mechanism of polypropylene catalyzed by Co compounds combined with phosphorus-doped graphene to improve its fire safety performance Yuanyuan Zhan 1, Sheng Shang 1, Bihe Yuan *, Shasha Wang *, Xianfeng Chen , Gongqing Chen School of Safety Science and Emergency Management, Wuhan University of Technology, Wuhan, 430070, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Thermal stability Residual char Catalytic carbonization Flame retardant
Polypropylene (PP), as a non-charring polymer, is highly flammable without obvious char residue after com bustion. Catalyzing carbonization of the polymer during combustion has become a valid method for improving its flame retardancy. In this work, two novel combined catalysts were designed and their effects on the thermal stability and flame retardancy of PP were researched. Thermogravimetric analysis results indicated that the combination of Co compounds and P atom-doped reduced graphene oxide (PRGO) markedly improved the thermal stability of PP. Besides, the char residue of PP was increased with the addition of combined catalysts. The char yield was enhanced from 1.09 wt % for pure PP to 7.10 wt % for PP/Co-MOF/PRGO. Combustion behaviors of PP and its composites were investigated by the cone calorimeter test. The incorporation of combined catalyst resulted in the reductions of peak heat release rate, total heat release, CO and CO2 releases of PP during com bustion. The presence of carbon spheres in the residual char has been confirmed by X-ray diffraction and scanning electron microscope. The flame-retardant mechanism was summarized based on the results of gas chromatography-mass spectrometry and char analysis. This research extends the method of catalyzing the for mation of high-quality carbonaceous protection layers to improve the flame-retardant performance of polymers.
1. Introduction Polypropylene (PP) has been one of the most widely used thermo plastic resins due to its low cost, easy processing and outstanding me chanical properties [1]. However, inflammability and serious melt dripping restrict its application in some fields for safety consideration. Therefore, it is extremely necessary to improve the flame retardancy of PP. Exploring halogen-free and highly effective methods for enhancing flame retardancy of polymer receives widespread attention due to growing safety and environmental awareness [2–5]. PP is a non-charring polymer, and its degradation products are volatile fragments and there is little char residue left after combustion [6]. It has been proved that the formation of carbonaceous protection layers during combustion is an effective method for enhancing flame retardancy of the polymer [6,7]. Char formation of polymer itself during combustion is a key factor for improving flame-retardant performance [8]. The formation of carbonaceous protection layers by catalyzing carbonization of polymer itself during combustion has become more popular than the addition of
an external carbon source. It shows more advantages, such as high flame-retardant efficiency, long-term stability and little impact on me chanical properties of the polymer matrix [6]. Transition metal (such as Ni and Co) compounds are widely used in catalysis and they have been proved to catalyze carbonization of degradation products of polyolefin [9,10]. Wen et al. found that when 5 wt % Ni2O3 was introduced, carbon residue of a poly(L-lactide)/Ni2O3 composite reached 2.4 wt % [6]. Besides, a combined catalyst (degradation catalyst and carbonization catalyst) has been investigated more extensively than a single carbon ization catalyst, and the composite system can further improve the carbonization conversion of degradation products [11–13]. Song et al. reported that the combined system of Ni2O3 and solid acid can further enhance the carbon deposition of degradation products and flame retardancy of polyolefin. The solid acid can catalyze the degradation of polyolefin at high temperatures to generate small molecule and aromatic compounds, which are beneficial to the formation of carbon material. Degradation products are catalyzed to produce carbon material by Ni2O3 via in-situ catalysis, which leads to the formation of a carbon protective
* Corresponding authors. E-mail addresses: [email protected] (B. Yuan), [email protected] (S. Wang). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.mtcomm.2020.101792 Received 21 September 2020; Received in revised form 20 October 2020; Accepted 20 October 2020 Available online 4 November 2020 2352-4928/© 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: Yuanyuan Zhan, Materials Today Communications, https://doi.org/10.1016/j.mtcomm.2020.101792
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layer [13]. Moreover, the structure of the catalyst also affects the degradation reaction of polymer and carbonization reaction of degra dation products [11,14]. Jiang et al. reported that the proton acidic sites from the thermal degradation of organically-modified montmorillonite (OMMT) layers can catalyze the degradation of PP. Meanwhile, the lamellar structure of OMMT can delay the diffusion of PP degradation products, prolong the carbonization reaction time, and then it will improve the yield of multi-walled carbon nanotubes [11]. Graphene, a two-dimensional carbon nanomaterial, has been widely used in various fields, such as supercapacitors, batteries, sensors and polymer composites [15–22]. Numerous studies have confirmed that graphene can improve the thermal stability and flame resistance of polyolefin due to its unique structure [23–25]. In our previous study, P atom-doped reduced graphene oxide (PRGO) was synthesized by high-temperature annealing [26]. The doping of P causes the presence of “acidic sites” on the surface of reduced graphene oxide (RGO), which may promote the thermal degradation of PP [27]. Considering the catalytic performance of Co-based compounds and the acidic site characteristic of PRGO, the combined catalysts (Co-based compounds and PRGO) may have excellent performance in improving the char yield and flame retardancy of polymer. The present paper aims to explore a novel and efficient combined catalyst system to enhance the thermal stability and flame retardancy of PP, and to clarify the carbonization mechanism. In this work, Co-based compounds including its phenylphosphonate (Co-PPOA) and metal-organic framework hybrid (Co-MOF) were pre pared. The effects of two novel combined catalysts (Co-PPOA and PRGO; Co-MOF and PRGO) on the thermal stability and flame retardancy of PP were investigated. Through the analyses on thermal stability, combus tion behavior and char residue of pure PP and its composites, the mechanism of improving the flame-retardant performance of PP by the combined catalysts was put forward.
Table 1 Constituents of PP composites. Sample PP PP/Co-PPOA PP/Co-PPOA/RGO PP/Co-PPOA/PRGO PP/Co-MOF PP/Co-MOF/RGO PP/Co-MOF/PRGO
Composition (wt%) PP
Co-PPOA
Co-MOF
RGO
PRGO
100 95.0 95.0 95.0 95.0 95.0 95.0
– 5.0 4.0 4.0 – – –
– – – – 5.0 4.0 4.0
– – 1.0 – – 1.0 –
– – – 1.0 – – 1.0
temperature, the final product was filtered and washed with ethanol several times. The product was dried at 80 ◦ C for 24 h to obtain Co-MOF. 2.4. Preparation of PP composites PP composites filled with different ratios of Co-PPOA (Co-MOF) and RGO (PRGO) were melt-mixed for 10 min in a QE-70A internal mixer (Wuhan Qien Technology Development Co., Ltd.) at 180 ◦ C and 60 rpm. After mixing, the composites were hot-pressed at 185 ◦ C and 10 MPa for 6 min to mold into the sheets by a YF-8017 Plate Vulcanizer (Yangzhou Yuanfeng Experimental Machinery Factory). The detailed formulations of PP composites are shown in Table 1. 2.5. Characterization Fourier transform infrared (FTIR) spectrum between 4000 and 400 cm− 1 was measured by using a Nicolet 6700 spectrophotometer (Thermo Nicolet Co.). X-ray diffraction (XRD) pattern was performed on a D8 Advance Xray diffractometer (Bruker Co.) using Cu Kα radiation. Scanning electron microscope (SEM) images of microstructures were observed by using a JSM-IT300 scanning electron microscope (JEOL Ltd.). Thermogravimetric analysis (TGA) was carried out using a STA6000 simultaneous thermal analyzer (PerkinElmer Inc.). Approximately 12 mg sample was weighed and heated from 35 to 700 ◦ C at a rate of 20 ◦ C/min under N2 atmosphere. The burning behavior of samples was tested on a cone calorimeter (Motis Fire Technology Co., Ltd.) according to ISO 5660-1 standard. The sample with a dimension of 100 × 100 × 3 mm3 was ignited and com busted under an irradiation flux of 35 kW/m2. The char residues after combustion were collected for further Raman and SEM analysis. Raman spectra of carbon materials were conducted on an InVia Raman microscope spectrometer (RENISHAW) by using a 514.5 nm laser line. Pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) was carried out by using an ENTECH 7100-CDS 5150-Agilent 6890 N/5975 (ENTECH company-CDS company-Agilent Technologies Co., Ltd.). Approximately 50 μg sample was weighed, placed in the pyrolysis tube and heated in an atmosphere of high-purity helium. The temperature rose rapidly to 700 ◦ C at a rate of 10 ◦ C/ms and kept for 15 s. The chromatographic column Agilent 222-5532LTM DB-5 ms (G390063004) (30 m × 0.25 mm × 0.25 μm) was used for GC separation. The inlet temperature was 250 ◦ C, and the split ratio was 6.45:1. The oven temperature was programmed as follows: initially held at 50 ◦ C for 1 min, a residence time of 0.5 min, and finally increased at a rate of 10 ◦ C/min to 280 ◦ C and held for 10 min. Helium was used as the carrier gas, and the flow rate was set to 1.0 mL/min. MS conditions were as follows: electron impact mode at 70 eV, ion source temperature of 230 ◦ C, quadrupole temperature of 150 ◦ C and a mass scan range of 40− 1000 m/z.
2. Experimental section 2.1. Materials PP (F401) was provided by Sinopec Yangzi Petrochemical Co., Ltd. RGO was synthesized according to our previous work [26]. H3PO4 (85 %, analytical reagent (AR)), HCl (37 %, AR), CoCl2⋅6H2O (AR), phenyl phosphonic acid (PPOA, AR), NaOH (AR), terephthalic acid (AR), acetic acid (AR), N, N-dimethylformamide (DMF, AR) and ethanol (AR) were supplied by Sinopharm Chemical Reagent Co., Ltd. The water used was deionized water produced in the laboratory. 2.2. Preparation of PRGO PRGO was prepared according to the method described in our pre vious study [28]. 2.3. Preparation of Co-PPOA and Co-MOF Co-PPOA was prepared based on the literature [29]. 2.38 g CoCl2⋅6H2O, 0.8 g NaOH and 100 mL deionized water were added into a 500 mL three-neck flask, and electromagnetic stirring was performed for 30 min to obtain a uniform solution. Then the three-neck flask was transferred to an oil bath and heated to 70 ◦ C. 0.4 mol PPOA was dis solved in 50 mL deionized water. The solution was added to the three-neck flask within 30 min, and then it was treated by a reflux condensation reaction for 3 h. The product was filtered, washed with water and ethanol several times, and the filter cake was dried in an oven at 80 ◦ C for 24 h to obtain Co-PPOA. 2.30 g terephthalic acid, 3.29 g CoCl2⋅6H2O, 4.15 g acetic acid and 60 mL DMF were mixed in a 100 mL beaker and dispersed for 30 min. The mixture was transferred into a 500 mL Teflon lined stainless steel autoclave and reacted at 150 ◦ C for 2 h. After cooling to room 2
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Fig. 1. (a) TGA and (b) DTG curves of PP and its composites containing Co-PPOA and RGO (/PRGO).
Fig. S1b, typical absorption peaks are detected at 1578 cm− 1 and 1356 cm− 1, which are assigned to the –COO− asymmetric and sym metric stretching vibration, respectively [35]. The peak at 571 cm− 1 is attributed to Co-O stretching vibration [29]. Morphologies of Co-PPOA and Co-MOF are investigated by SEM and the corresponding images are revealed in Fig. S2 (see the supplementary material). Fig. S2a presents SEM images of Co-PPOA, which displays a lamellar structure. Moreover, the SEM image of Co-MOF shows irregular morphology with different crystalline sizes. XRD is a powerful tool to study the crystal structure of materials [36]. As shown in Fig. S3a (see the supplementary material), the XRD pattern of Co-PPOA exhibits four significant diffraction peaks ascribing to (010), (020), (110) and (030) planes of Co-PPOA [30]. The charac teristic diffraction peaks of Co-MOF are almost consistent with its XRD pattern reported in the literature, indicating that Co-MOF is successfully synthesized [37].
Table 2 TGA data of PP and its composites. Sample
Tinitial (◦ C)
Tmax (◦ C)
Char residue (700 ◦ C, wt%)
PP PP/Co-PPOA PP/Co-PPOA/RGO PP/Co-PPOA/PRGO PP/Co-MOF PP/Co-MOF/RGO PP/Co-MOF/PRGO
408 487 493 498 434 496 494
459 530 533 538 514 535 533
1.09 3.96 3.93 4.62 3.38 3.51 7.10
3. Results and discussion 3.1. Characterization of Co-PPOA and Co-MOF FTIR spectra of Co-PPOA and Co-MOF are shown in Fig. S1 (see the supplementary material). As observed in the FTIR spectrum of Co-PPOA, an absorbing signal at 3060 cm− 1 corresponds to C–H in benzene rings –C [30,31]. The band at 1425 cm− 1 is ascribed to the aromatic C– stretching vibrations [32,33]. Besides, the peaks at 1285 cm− 1 – O), 1113 cm− 1 (P–O symmetric stretching (stretching vibration of P– vibration), 1019 cm− 1 (symmetric vibration of PO2 and PO3), and 881 cm− 1 (P–O asymmetric stretching vibration) are observed [2,31]. Peaks at 731 cm− 1 and 566 cm− 1 are found, which are ascribed to the adsorption of P–C and Co-O bonds, respectively [29,34]. As shown in
3.2. Thermal stability analysis TGA and differential thermogravimetric (DTG) are widely used to analyze the thermal stability of materials. The related curves of PP and PP/Co-PPOA composites are shown in Fig. 1 and the detailed data are summarized in Table 2. The temperature at 5 % weight loss is defined as initial decomposition temperature (Tinitial), and the temperature at maximum weight loss rate is named as Tmax. As presented in Fig. 1a, PP and its composites exhibit one-step decomposition. From TGA curves, it
Fig. 2. (a) TGA and (b) DTG curves of PP and its composites containing Co-MOF and RGO (/PRGO). 3
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Fig. 3. (a) HRR, (b) THR, (c) COPR, (d) CO2PR curves of PP and its composites containing Co-PPOA and PRGO.
can be observed that pure PP is decomposed by a rapid step between 408 ◦ C and 459 ◦ C, and char residue is only 1.09 wt % at 700 ◦ C. The presence of 5 wt % Co-PPOA in PP shifts the Tinitial and Tmax to higher temperatures, which are 79 ◦ C and 71 ◦ C higher than those of pure PP. This may be attributed to the lamellar structure of Co-PPOA, which can obstruct the exchange of heat and delay the pyrolysis of PP. The Tinitial and Tmax of PP/Co-PPOA/RGO are approximately 493 ◦ C and 533 ◦ C, which are higher than those of PP (the increments are 85 ◦ C and 74 ◦ C, respectively). The char residue is enhanced from 1.09 wt % for PP to 3.93 wt % for PP/Co-PPOA/RGO. When RGO is substituted by PRGO, PP/Co-PPOA/PRGO has excellent thermal stability, and its Tinitial and Tmax are increased by 90 ◦ C and 79 ◦ C compared with pure PP, respec tively. Except that, it is observed that the char residue of PP/Co-PPOA/ PRGO is much higher than those of PP and other PP/Co-PPOA com posites. Based on these facts, it can deduce that PRGO and Co-PPOA have a combined effect on improving thermal stability and char res idue of PP. Furthermore, PRGO performs better than RGO on the enhancement in char yield. From TGA (Fig. 2a) curves and Table 2, it can be found that the addition of Co-MOF leads to the enhanced thermal stability of its cor responding composite. The Tinitial and Tmax of PP/Co-MOF are enhanced by 26 ◦ C and 55 ◦ C, respectively. Char residue of PP/Co-MOF is 3.38 wt %, which is 2.29 wt % higher than that of PP. As presented in Table 2, it can be found that the Tinitial of PP/Co-PPOA is higher than that of PP/CoMOF, which may be due to the stronger lamellar barrier effect of the former one. Tinitial and Tmax values of PP/Co-MOF/RGO are 62 ◦ C and 21 ◦ C higher than those of PP/Co-MOF. Besides, char residue of PP/CoMOF/RGO is also increased compared with PP/Co-MOF. These im provements may be attributed to the barrier effect and peroxy radicals absorbing capability of RGO [38]. The char yield of PP/Co-MOF/PRGO
is 3.59 wt % higher than that of PP/Co-MOF/RGO, indicating that PRGO possesses higher catalytic performance. The main reason is probably ascribed to the fact that “acidic sites” on the surface of PRGO can pro mote the degradation of PP to generate small molecules and aromatic compounds that are beneficial to the char formation, thus increasing the char formed by the catalysis of Co-MOF. 3.3. Combustion behavior analysis Cone calorimetry test is regarded as one of the most effective tool to evaluate the combustion behavior of flame-retarding polymeric mate rials [39–45]. Heat release rate (HRR), total heat release (THR), CO and CO2 production rate (COPR and CO2PR) curves of PP and its composites are presented in Figs. 3 and 4, and the relevant parameters are sum marized in Table 3. Fig. 5 presents the line charts of peak heat release rate (PHRR) and THR values of PP and its composites. As shown in Fig. 3 and Table 3, pure PP burns promptly after being ignited, and the HRR curve reaches its peak value of 832 kW/m2. When 5 wt % Co-PPOA is incorporated, the PHRR value of its PP composite is 681 kW/m2, which is significantly lower than that of PP. The THR value of PP/Co-PPOA is 6.2 MJ/m2 lower than that of neat PP. It can be observed from Table 3 that PP/Co-PPOA/PRGO has a lower time to ignition (TTI) than that of PP/Co-PPOA, which may be due to the high thermal absorption of PRGO and catalytic decomposition effect of its surface“acidic sites”. When PRGO is introduced, further decreases in PHRR and THR are achieved. The average effective heat of combustion (av-EHC) of PP/Co-PPOA/PRGO is 33.88 MJ/kg, which is also lower than of PP and PP/Co-PPOA. This is mainly attributed to the fact that PRGO and Co-PPOA have a better combined effect on catalyzing the decomposition products of PP to form carbonaceous protection layers, preventing 4
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Fig. 4. (a) HRR, (b) THR, (c) COPR, (d) CO2PR curves of PP and its composites containing Co-MOF and PRGO.
MOF. A similar trend can also be observed in the THR curves (Fig. 4b). Compared with neat PP, the THR values of PP/Co-MOF and PP/CoMOF/PRGO are decreased by 4.9 MJ/m2 and 8.3 MJ/m2, respectively. Besides, the av-EHC values of PP/Co-MOF and PP/Co-MOF/PRGO are 2.29 MJ/kg and 2.83 MJ/kg lower than PP, respectively. These results confirm that Co-MOF and PRGO present an excellent combined effect on reducing the combustion extent of volatiles in gas phase and heat release of PP. It can be seen that the composite system of Co-MOF and PRGO also performs remarkable suppression on the releases of CO and CO2 during combustion.
Table 3 Cone calorimetry combustion data of PP and its composites. Sample
TTI (s)
PHRR (kW/ m2)
THR (MJ/ m2)
PCOPR (10− 3g/s)
PCO2PR (10− 1g/s)
av-EHC (MJ/ kg)
PP PP/CoPPOA PP/CoPPOA/ PRGO PP/CoMOF PP/CoMOF/ PRGO
37 31
832 681
85.2 79.0
4.87 3.61
3.88 3.07
35.02 34.35
21
611
74.8
3.92
2.71
33.88
37
791
80.3
4.47
3.61
32.73
30
619
76.9
3.49
2.80
32.19
3.4. Analysis of residual char Analysis of residual char is used to further study the combined effect of PRGO and Co-PPOA (Co-MOF) on improving the flame-retardant performance of PP. The residual char was collected from the cone calorimetry test of composite samples. Digital photographs of residual char of PP composites are shown in Fig. 6. From the comparison of Fig. 6, it can be observed that the addition of PRGO increases the char residue of PP composites. Fig. 6b and d present that the aluminum foil is covered by dense black char, implying that the combined effect between Co-PPOA (Co-MOF) and PRGO results in the formation of higher quantity and quality char. SEM images of char layers of PP composites before and after pickling are shown in Figs. 7 and 8. It can be seen from Fig. 7a that the residual char of PP/Co-PPOA consists of plentiful amorphous aggregates. When 1 wt % of Co-PPOA is replaced by PRGO, wide ranges of diameter spheres on the surface of PRGO can be seen, which may be attributed to the fact that the lamellar structure of PRGO can serve as a template for carbon growth. Besides, the SEM image of char layers of PP/Co-PPOA/
further combustion of PP. It is important to evaluate the toxic gases produced during the combustion of polymer, as these toxic fumes often cause asphyxiation [46,47]. The peak COPR and CO2PR (PCOPR and PCO2PR) of PP/Co-PPOA/PRGO are also lower than those of PP, which may be because that the barrier effect of char residue inhibits the re leases of CO and CO2 during the combustion process of PP. As revealed in Fig. 4a, it can be seen that a decrease in PHRR is obtained in PP/Co-MOF and PP/Co-MOF/PRGO. Compared to neat PP, the PHRR value of PP/Co-MOF decreases from 832 kW/m2 to 791 kW/ m2, revealing that Co-MOF is effective in inhibiting the heat release of PP during combustion. As shown in Fig. 5 and Table 3, when 1 wt % PRGO is introduced into PP/Co-MOF composite, the PHRR value of PP/ Co-MOF/PRGO presents a 21.7 % reduction compared to that of PP/Co5
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Fig. 5. Line charts of (a) PHRR and (b) THR values of PP and its composites.
Fig. 6. Digital photographs of residual char: (a) PP/Co-PPOA, (b) PP/Co-PPOA/PRGO, (c) PP/Co-MOF and (d) PP/Co-MOF/PRGO.
PRGO composite indicates that its char residue is more continuous and flat. Co compounds are reduced to metallic cobalt catalyst, which is an essential process for the char formation [48]. To further observe the formation of carbon spheres, char residue is pickled to remove Co-based materials. As shown in Fig. 7c, the residue of PP/Co-PPOA after pickling is composed of small spherical particles. Fig. 7d displays that many small
size carbon spheres are clustered on the surface of PRGO. Similar results can be observed in Fig. 8. As shown in Fig. 8a, the spheres with uniform size aggregate together to form an irregular network structure. Spheres can be also observed in the residual char of PP/Co-MOF/PRGO (Fig. 8b). The aggregated spheres are scattered on a flat structural surface. SEM image of char residue of PP/Co-MOF after 6
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Fig. 7. SEM images of char layers of PP/Co-PPOA and PP/Co-PPOA/PRGO composites: (a, b) before and (c, d) after pickling.
Fig. 8. SEM images of char layers of PP/Co-MOF and PP/Co-MOF/PRGO composites: (a, b) before and (c, d) after pickling.
pickling presented that the char is relatively dense and composed of vast small spheres. SEM image of char layers of PP/Co-MOF/PRGO after pickling is similar to PP/Co-PPOA/PRGO. The surface of the char layer becomes more wrinkled. After the removal of Co catalysts, the fold structure of PRGO is observed. Meanwhile, there are still gathered spheres on the surface of pleated char, which may be because that PRGO can be used as a template to promote the growth of carbon spheres.
XRD patterns of the char layer are exhibited in Fig. 9. In the XRD pattern of residual char of PP/Co-PPOA, the diffraction peaks are attributed to metallic cobalt (2θ = 44.5◦ , 51.8◦ and 76.2◦ ), indicating that Co-PPOA catalyst was reduced into metallic cobalt by the degra dation products of PP during the combustion [49]. The metallic cobalt is considered to be a real active site for the formation of carbon spheres [48]. Besides, the peak attributing to the graphite (002) crystal plane 7
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Fig. 9. XRD patterns of char layers: (a) PP/Co-PPOA(/PRGO) and (b) PP/Co-MOF(/PRGO) composites.
Fig. 10. Raman spectra of residual char of PP/Co-PPOA(/PRGO) composites: (a, b) before and (c, d) after pickling.
can also be observed at approximately 2θ = 25.9◦ [50]. When PRGO is introduced, the characteristic peaks of graphene (002) crystal plane and metallic cobalt can also be found in the XRD pattern of char. Similar results can be obtained in the XRD patterns of the composite containing Co-MOF and PRGO. Raman spectroscopy is a useful tool to analyze crystal size in the microstructure of carbonaceous materials [51]. Raman spectra of
residual char of PP composites before and after pickling are presented in Figs. 10 and 11, respectively. The spectra for all testing samples show two characteristic peaks at around 1350 cm− 1 (D band) and 1590 cm− 1 (G band), which are related to typical graphitic structures [52–54]. The integrated intensity ratio of D to G bands (ID/IG) is used to measure the microcrystalline size of carbon-based materials [55]. A higher ratio of ID/IG means a smaller microcrystalline size of carbon material [56,57]. 8
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Fig. 11. Raman spectra of residual char of PP/Co-MOF(/PRGO) composites: (a, b) before and (c, d) after pickling.
Fig. 12. GC–MS profiles of degradation products from PP and PP/PRGO.
Raman spectra of char residues of PP/Co-PPOA and PP/Co-PPOA/PRGO before pickling are presented in Fig. 10a and b, respectively. The ID/IG value of PP/Co-PPOA/PRGO is 1.22, which is higher than that of PP/Co-PPOA (1.09). After pickling, the ID/IG value of PP/Co-PPOA/PRGO is also higher than that of PP/Co-PPOA, indicating that a smaller microcrystalline size of char is formed by the addition of Co-PPOA and PRGO. Similar results are achieved in Fig. 11 for Co-MOF
and PRGO. Due to the promoted degradation effect from “acidic sites” and template effect, PRGO provides plenty of nucleating sites for char ring reactions, thus smaller microcrystalline size of char is achieved. 3.5. Degradation product analysis Fig. 12 displays GC–MS profiles of degradation products from PP and 9
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Fig. 13. Catalytic carbonization and flame-retardant mechanisms of Co compounds and PRGO.
thermal stability and flame retardancy of PP. The TGA results showed that the combination of Co-PPOA (Co-MOF) and PRGO can significantly improve the thermal stability of PP. Compared with pure PP, the Tinitial values of PP/Co-PPOA/PRGO and PP/Co-MOF/PRGO were increased by 90 ◦ C and 86 ◦ C, respectively. Moreover, the char residue of PP was increased with the addition of a combined catalyst. Char residue values of PP/Co-PPOA/PRGO and PP/Co-MOF/PRGO were 3.53 wt % and 6.01 wt % higher than that of PP, respectively. From the cone combus tion experiment, compared with PP, PHRR values of PP/Co-PPOA/ PRGO and PP/Co-MOF/PRGO were decreased by 26.6 % and 25.6 %, respectively. THR values of PP/Co-PPOA/PRGO (74.8 MJ/m2) and PP/ Co-MOF/PRGO (76.9 MJ/m2) were lower than that of PP (85.2 MJ/ m2). These results showed that the addition of a combined catalyst can effectively inhibit the production and release of heat and toxic gases. The possible flame-retardant mechanism of Co-PPOA (Co-MOF) and PRGO was proposed through the analysis of char residue and degrada tion products. This research extends the application of combined cata lytic carbonization in the field of flame-retardant polymer.
PP/PRGO at 700 ◦ C. Table S1 (see the supplementary material) shows the main components of pyrolysis products. The main degradation products from PP are olefins, and its content of aromatic is scarce, which is only 0.28 area %. When PRGO is added, it can be observed that the content of aromatics is increased to 15.37 area %. Previous studies have confirmed that aromatic products are conducive to be catalyzed to the formation of high-quality char [58,59]. Thus, it is suggested that PRGO can promote the aromatization of degradation products, which are conducive to the char formation. 3.6. Catalytic carbonization and flame-retardant mechanism Based on the above results, the mechanism of the combined catalyst on improving the flame-retardant property of PP is proposed and shown in Fig. 13. Co-PPOA (Co-MOF) and PRGO have remarkable combined effect in the catalytic carbonization of PP. During combustion, the PP matrix is promoted degradation by “acidic sites” on the surface of PRGO to decompose to generate small molecule aromatic compounds that are conducive to the growth of carbon materials. Co-PPOA (Co-MOF) is in situ reduced to metallic cobalt by PP degradation products. Metallic cobalt is the real active site for catalytic carbonation, which catalyzes those degradation products of PP to form carbon materials. The lamellar structure of PRGO can slow down the diffusion of PP degradation products and prolong the time to participate in the carbonization reac tion. It is equivalent to a micro-reactor role in the matrix, thus increasing the yield of carbon spheres. Besides, PRGO also serves as a template for the growth of carbon spheres on its surfaces, which can be confirmed from SEM images of char layers. Combined catalytic carbonation of CoPPOA (Co-MOF) and PRGO promotes the formation of a smaller microcrystalline size of char. Therefore, the flame retardancy of PP is enhanced by the combined catalyst of Co-PPOA (Co-MOF) and PRGO.
CRediT authorship contribution statement Yuanyuan Zhan: Data curation, Methodology, Writing - original draft. Sheng Shang: Software, Formal analysis, Data curation. Bihe Yuan: Supervision, Writing - review & editing. Shasha Wang: Conceptualization, Methodology, Supervision. Xianfeng Chen: Inves tigation, Visualization. Gongqing Chen: Formal analysis, Methodology. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
4. Conclusions
Acknowledgments
In this study, two novel combined catalysts of Co-PPOA (Co-MOF) and PRGO were prepared successfully and conducted to enhance the
This research was funded by the National Natural Science 10
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Foundation of China (51703175). The authors are grateful for the lan guage revision provided by Mr. Huidong Zhao and Mr. Quan Fang.
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Hu, Solid acid-reduced graphene oxide nanohybrid for enhancing thermal stability, mechanical property and flame retardancy of polypropylene, RSC Adv. 5 (51) (2015) 41307–41316. [17] X.M. Feng, X. Wang, W.Y. Xing, B. Yu, L. Song, Y. Hu, Simultaneous reduction and surface functionalization of graphene oxide by chitosan and their synergistic reinforcing effects in PVA films, Ind. Eng. Chem. Res. 52 (36) (2013) 12906–12914. [18] B. Zirnstein, W. Tabaka, D. Frasca, D. Schulze, B. Schartel, Graphene / hydrogenated acrylonitrile-butadiene rubber nanocomposites: dispersion, curing, mechanical reinforcement, multifunctional filler, Polym. Test. 66 (2018) 268–279. [19] Y.Q. Shi, C. Liu, L.B. Fu, F.Q. Yang, Y.C. Lv, B. Yu, Hierarchical assembly of polystyrene/graphitic carbon nitride/reduced graphene oxide nanocomposites toward high fire safety, Compos. Part B-Eng. 179 (2019), 107541. [20] B. Dittrich, K.A. Wartig, D. Hofmann, R. Mulhaupt, B. 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Carbonization mechanism of polypropylene catalyzed by Co compounds combined with phosphorus-doped graphene to improve its fire safety performance Yuanyuan Zhan 1, Sheng Shang 1, Bihe Yuan *, Shasha Wang *, Xianfeng Chen , Gongqing Chen School of Safety Science and Emergency Management, Wuhan University of Technology, Wuhan, 430070, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Thermal stability Residual char Catalytic carbonization Flame retardant
Polypropylene (PP), as a non-charring polymer, is highly flammable without obvious char residue after com bustion. Catalyzing carbonization of the polymer during combustion has become a valid method for improving its flame retardancy. In this work, two novel combined catalysts were designed and their effects on the thermal stability and flame retardancy of PP were researched. Thermogravimetric analysis results indicated that the combination of Co compounds and P atom-doped reduced graphene oxide (PRGO) markedly improved the thermal stability of PP. Besides, the char residue of PP was increased with the addition of combined catalysts. The char yield was enhanced from 1.09 wt % for pure PP to 7.10 wt % for PP/Co-MOF/PRGO. Combustion behaviors of PP and its composites were investigated by the cone calorimeter test. The incorporation of combined catalyst resulted in the reductions of peak heat release rate, total heat release, CO and CO2 releases of PP during com bustion. The presence of carbon spheres in the residual char has been confirmed by X-ray diffraction and scanning electron microscope. The flame-retardant mechanism was summarized based on the results of gas chromatography-mass spectrometry and char analysis. This research extends the method of catalyzing the for mation of high-quality carbonaceous protection layers to improve the flame-retardant performance of polymers.
1. Introduction Polypropylene (PP) has been one of the most widely used thermo plastic resins due to its low cost, easy processing and outstanding me chanical properties [1]. However, inflammability and serious melt dripping restrict its application in some fields for safety consideration. Therefore, it is extremely necessary to improve the flame retardancy of PP. Exploring halogen-free and highly effective methods for enhancing flame retardancy of polymer receives widespread attention due to growing safety and environmental awareness [2–5]. PP is a non-charring polymer, and its degradation products are volatile fragments and there is little char residue left after combustion [6]. It has been proved that the formation of carbonaceous protection layers during combustion is an effective method for enhancing flame retardancy of the polymer [6,7]. Char formation of polymer itself during combustion is a key factor for improving flame-retardant performance [8]. The formation of carbonaceous protection layers by catalyzing carbonization of polymer itself during combustion has become more popular than the addition of
an external carbon source. It shows more advantages, such as high flame-retardant efficiency, long-term stability and little impact on me chanical properties of the polymer matrix [6]. Transition metal (such as Ni and Co) compounds are widely used in catalysis and they have been proved to catalyze carbonization of degradation products of polyolefin [9,10]. Wen et al. found that when 5 wt % Ni2O3 was introduced, carbon residue of a poly(L-lactide)/Ni2O3 composite reached 2.4 wt % [6]. Besides, a combined catalyst (degradation catalyst and carbonization catalyst) has been investigated more extensively than a single carbon ization catalyst, and the composite system can further improve the carbonization conversion of degradation products [11–13]. Song et al. reported that the combined system of Ni2O3 and solid acid can further enhance the carbon deposition of degradation products and flame retardancy of polyolefin. The solid acid can catalyze the degradation of polyolefin at high temperatures to generate small molecule and aromatic compounds, which are beneficial to the formation of carbon material. Degradation products are catalyzed to produce carbon material by Ni2O3 via in-situ catalysis, which leads to the formation of a carbon protective
* Corresponding authors. E-mail addresses: [email protected] (B. Yuan), [email protected] (S. Wang). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.mtcomm.2020.101792 Received 21 September 2020; Received in revised form 20 October 2020; Accepted 20 October 2020 Available online 4 November 2020 2352-4928/© 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: Yuanyuan Zhan, Materials Today Communications, https://doi.org/10.1016/j.mtcomm.2020.101792
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layer [13]. Moreover, the structure of the catalyst also affects the degradation reaction of polymer and carbonization reaction of degra dation products [11,14]. Jiang et al. reported that the proton acidic sites from the thermal degradation of organically-modified montmorillonite (OMMT) layers can catalyze the degradation of PP. Meanwhile, the lamellar structure of OMMT can delay the diffusion of PP degradation products, prolong the carbonization reaction time, and then it will improve the yield of multi-walled carbon nanotubes [11]. Graphene, a two-dimensional carbon nanomaterial, has been widely used in various fields, such as supercapacitors, batteries, sensors and polymer composites [15–22]. Numerous studies have confirmed that graphene can improve the thermal stability and flame resistance of polyolefin due to its unique structure [23–25]. In our previous study, P atom-doped reduced graphene oxide (PRGO) was synthesized by high-temperature annealing [26]. The doping of P causes the presence of “acidic sites” on the surface of reduced graphene oxide (RGO), which may promote the thermal degradation of PP [27]. Considering the catalytic performance of Co-based compounds and the acidic site characteristic of PRGO, the combined catalysts (Co-based compounds and PRGO) may have excellent performance in improving the char yield and flame retardancy of polymer. The present paper aims to explore a novel and efficient combined catalyst system to enhance the thermal stability and flame retardancy of PP, and to clarify the carbonization mechanism. In this work, Co-based compounds including its phenylphosphonate (Co-PPOA) and metal-organic framework hybrid (Co-MOF) were pre pared. The effects of two novel combined catalysts (Co-PPOA and PRGO; Co-MOF and PRGO) on the thermal stability and flame retardancy of PP were investigated. Through the analyses on thermal stability, combus tion behavior and char residue of pure PP and its composites, the mechanism of improving the flame-retardant performance of PP by the combined catalysts was put forward.
Table 1 Constituents of PP composites. Sample PP PP/Co-PPOA PP/Co-PPOA/RGO PP/Co-PPOA/PRGO PP/Co-MOF PP/Co-MOF/RGO PP/Co-MOF/PRGO
Composition (wt%) PP
Co-PPOA
Co-MOF
RGO
PRGO
100 95.0 95.0 95.0 95.0 95.0 95.0
– 5.0 4.0 4.0 – – –
– – – – 5.0 4.0 4.0
– – 1.0 – – 1.0 –
– – – 1.0 – – 1.0
temperature, the final product was filtered and washed with ethanol several times. The product was dried at 80 ◦ C for 24 h to obtain Co-MOF. 2.4. Preparation of PP composites PP composites filled with different ratios of Co-PPOA (Co-MOF) and RGO (PRGO) were melt-mixed for 10 min in a QE-70A internal mixer (Wuhan Qien Technology Development Co., Ltd.) at 180 ◦ C and 60 rpm. After mixing, the composites were hot-pressed at 185 ◦ C and 10 MPa for 6 min to mold into the sheets by a YF-8017 Plate Vulcanizer (Yangzhou Yuanfeng Experimental Machinery Factory). The detailed formulations of PP composites are shown in Table 1. 2.5. Characterization Fourier transform infrared (FTIR) spectrum between 4000 and 400 cm− 1 was measured by using a Nicolet 6700 spectrophotometer (Thermo Nicolet Co.). X-ray diffraction (XRD) pattern was performed on a D8 Advance Xray diffractometer (Bruker Co.) using Cu Kα radiation. Scanning electron microscope (SEM) images of microstructures were observed by using a JSM-IT300 scanning electron microscope (JEOL Ltd.). Thermogravimetric analysis (TGA) was carried out using a STA6000 simultaneous thermal analyzer (PerkinElmer Inc.). Approximately 12 mg sample was weighed and heated from 35 to 700 ◦ C at a rate of 20 ◦ C/min under N2 atmosphere. The burning behavior of samples was tested on a cone calorimeter (Motis Fire Technology Co., Ltd.) according to ISO 5660-1 standard. The sample with a dimension of 100 × 100 × 3 mm3 was ignited and com busted under an irradiation flux of 35 kW/m2. The char residues after combustion were collected for further Raman and SEM analysis. Raman spectra of carbon materials were conducted on an InVia Raman microscope spectrometer (RENISHAW) by using a 514.5 nm laser line. Pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) was carried out by using an ENTECH 7100-CDS 5150-Agilent 6890 N/5975 (ENTECH company-CDS company-Agilent Technologies Co., Ltd.). Approximately 50 μg sample was weighed, placed in the pyrolysis tube and heated in an atmosphere of high-purity helium. The temperature rose rapidly to 700 ◦ C at a rate of 10 ◦ C/ms and kept for 15 s. The chromatographic column Agilent 222-5532LTM DB-5 ms (G390063004) (30 m × 0.25 mm × 0.25 μm) was used for GC separation. The inlet temperature was 250 ◦ C, and the split ratio was 6.45:1. The oven temperature was programmed as follows: initially held at 50 ◦ C for 1 min, a residence time of 0.5 min, and finally increased at a rate of 10 ◦ C/min to 280 ◦ C and held for 10 min. Helium was used as the carrier gas, and the flow rate was set to 1.0 mL/min. MS conditions were as follows: electron impact mode at 70 eV, ion source temperature of 230 ◦ C, quadrupole temperature of 150 ◦ C and a mass scan range of 40− 1000 m/z.
2. Experimental section 2.1. Materials PP (F401) was provided by Sinopec Yangzi Petrochemical Co., Ltd. RGO was synthesized according to our previous work [26]. H3PO4 (85 %, analytical reagent (AR)), HCl (37 %, AR), CoCl2⋅6H2O (AR), phenyl phosphonic acid (PPOA, AR), NaOH (AR), terephthalic acid (AR), acetic acid (AR), N, N-dimethylformamide (DMF, AR) and ethanol (AR) were supplied by Sinopharm Chemical Reagent Co., Ltd. The water used was deionized water produced in the laboratory. 2.2. Preparation of PRGO PRGO was prepared according to the method described in our pre vious study [28]. 2.3. Preparation of Co-PPOA and Co-MOF Co-PPOA was prepared based on the literature [29]. 2.38 g CoCl2⋅6H2O, 0.8 g NaOH and 100 mL deionized water were added into a 500 mL three-neck flask, and electromagnetic stirring was performed for 30 min to obtain a uniform solution. Then the three-neck flask was transferred to an oil bath and heated to 70 ◦ C. 0.4 mol PPOA was dis solved in 50 mL deionized water. The solution was added to the three-neck flask within 30 min, and then it was treated by a reflux condensation reaction for 3 h. The product was filtered, washed with water and ethanol several times, and the filter cake was dried in an oven at 80 ◦ C for 24 h to obtain Co-PPOA. 2.30 g terephthalic acid, 3.29 g CoCl2⋅6H2O, 4.15 g acetic acid and 60 mL DMF were mixed in a 100 mL beaker and dispersed for 30 min. The mixture was transferred into a 500 mL Teflon lined stainless steel autoclave and reacted at 150 ◦ C for 2 h. After cooling to room 2
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Fig. 1. (a) TGA and (b) DTG curves of PP and its composites containing Co-PPOA and RGO (/PRGO).
Fig. S1b, typical absorption peaks are detected at 1578 cm− 1 and 1356 cm− 1, which are assigned to the –COO− asymmetric and sym metric stretching vibration, respectively [35]. The peak at 571 cm− 1 is attributed to Co-O stretching vibration [29]. Morphologies of Co-PPOA and Co-MOF are investigated by SEM and the corresponding images are revealed in Fig. S2 (see the supplementary material). Fig. S2a presents SEM images of Co-PPOA, which displays a lamellar structure. Moreover, the SEM image of Co-MOF shows irregular morphology with different crystalline sizes. XRD is a powerful tool to study the crystal structure of materials [36]. As shown in Fig. S3a (see the supplementary material), the XRD pattern of Co-PPOA exhibits four significant diffraction peaks ascribing to (010), (020), (110) and (030) planes of Co-PPOA [30]. The charac teristic diffraction peaks of Co-MOF are almost consistent with its XRD pattern reported in the literature, indicating that Co-MOF is successfully synthesized [37].
Table 2 TGA data of PP and its composites. Sample
Tinitial (◦ C)
Tmax (◦ C)
Char residue (700 ◦ C, wt%)
PP PP/Co-PPOA PP/Co-PPOA/RGO PP/Co-PPOA/PRGO PP/Co-MOF PP/Co-MOF/RGO PP/Co-MOF/PRGO
408 487 493 498 434 496 494
459 530 533 538 514 535 533
1.09 3.96 3.93 4.62 3.38 3.51 7.10
3. Results and discussion 3.1. Characterization of Co-PPOA and Co-MOF FTIR spectra of Co-PPOA and Co-MOF are shown in Fig. S1 (see the supplementary material). As observed in the FTIR spectrum of Co-PPOA, an absorbing signal at 3060 cm− 1 corresponds to C–H in benzene rings –C [30,31]. The band at 1425 cm− 1 is ascribed to the aromatic C– stretching vibrations [32,33]. Besides, the peaks at 1285 cm− 1 – O), 1113 cm− 1 (P–O symmetric stretching (stretching vibration of P– vibration), 1019 cm− 1 (symmetric vibration of PO2 and PO3), and 881 cm− 1 (P–O asymmetric stretching vibration) are observed [2,31]. Peaks at 731 cm− 1 and 566 cm− 1 are found, which are ascribed to the adsorption of P–C and Co-O bonds, respectively [29,34]. As shown in
3.2. Thermal stability analysis TGA and differential thermogravimetric (DTG) are widely used to analyze the thermal stability of materials. The related curves of PP and PP/Co-PPOA composites are shown in Fig. 1 and the detailed data are summarized in Table 2. The temperature at 5 % weight loss is defined as initial decomposition temperature (Tinitial), and the temperature at maximum weight loss rate is named as Tmax. As presented in Fig. 1a, PP and its composites exhibit one-step decomposition. From TGA curves, it
Fig. 2. (a) TGA and (b) DTG curves of PP and its composites containing Co-MOF and RGO (/PRGO). 3
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Fig. 3. (a) HRR, (b) THR, (c) COPR, (d) CO2PR curves of PP and its composites containing Co-PPOA and PRGO.
can be observed that pure PP is decomposed by a rapid step between 408 ◦ C and 459 ◦ C, and char residue is only 1.09 wt % at 700 ◦ C. The presence of 5 wt % Co-PPOA in PP shifts the Tinitial and Tmax to higher temperatures, which are 79 ◦ C and 71 ◦ C higher than those of pure PP. This may be attributed to the lamellar structure of Co-PPOA, which can obstruct the exchange of heat and delay the pyrolysis of PP. The Tinitial and Tmax of PP/Co-PPOA/RGO are approximately 493 ◦ C and 533 ◦ C, which are higher than those of PP (the increments are 85 ◦ C and 74 ◦ C, respectively). The char residue is enhanced from 1.09 wt % for PP to 3.93 wt % for PP/Co-PPOA/RGO. When RGO is substituted by PRGO, PP/Co-PPOA/PRGO has excellent thermal stability, and its Tinitial and Tmax are increased by 90 ◦ C and 79 ◦ C compared with pure PP, respec tively. Except that, it is observed that the char residue of PP/Co-PPOA/ PRGO is much higher than those of PP and other PP/Co-PPOA com posites. Based on these facts, it can deduce that PRGO and Co-PPOA have a combined effect on improving thermal stability and char res idue of PP. Furthermore, PRGO performs better than RGO on the enhancement in char yield. From TGA (Fig. 2a) curves and Table 2, it can be found that the addition of Co-MOF leads to the enhanced thermal stability of its cor responding composite. The Tinitial and Tmax of PP/Co-MOF are enhanced by 26 ◦ C and 55 ◦ C, respectively. Char residue of PP/Co-MOF is 3.38 wt %, which is 2.29 wt % higher than that of PP. As presented in Table 2, it can be found that the Tinitial of PP/Co-PPOA is higher than that of PP/CoMOF, which may be due to the stronger lamellar barrier effect of the former one. Tinitial and Tmax values of PP/Co-MOF/RGO are 62 ◦ C and 21 ◦ C higher than those of PP/Co-MOF. Besides, char residue of PP/CoMOF/RGO is also increased compared with PP/Co-MOF. These im provements may be attributed to the barrier effect and peroxy radicals absorbing capability of RGO [38]. The char yield of PP/Co-MOF/PRGO
is 3.59 wt % higher than that of PP/Co-MOF/RGO, indicating that PRGO possesses higher catalytic performance. The main reason is probably ascribed to the fact that “acidic sites” on the surface of PRGO can pro mote the degradation of PP to generate small molecules and aromatic compounds that are beneficial to the char formation, thus increasing the char formed by the catalysis of Co-MOF. 3.3. Combustion behavior analysis Cone calorimetry test is regarded as one of the most effective tool to evaluate the combustion behavior of flame-retarding polymeric mate rials [39–45]. Heat release rate (HRR), total heat release (THR), CO and CO2 production rate (COPR and CO2PR) curves of PP and its composites are presented in Figs. 3 and 4, and the relevant parameters are sum marized in Table 3. Fig. 5 presents the line charts of peak heat release rate (PHRR) and THR values of PP and its composites. As shown in Fig. 3 and Table 3, pure PP burns promptly after being ignited, and the HRR curve reaches its peak value of 832 kW/m2. When 5 wt % Co-PPOA is incorporated, the PHRR value of its PP composite is 681 kW/m2, which is significantly lower than that of PP. The THR value of PP/Co-PPOA is 6.2 MJ/m2 lower than that of neat PP. It can be observed from Table 3 that PP/Co-PPOA/PRGO has a lower time to ignition (TTI) than that of PP/Co-PPOA, which may be due to the high thermal absorption of PRGO and catalytic decomposition effect of its surface“acidic sites”. When PRGO is introduced, further decreases in PHRR and THR are achieved. The average effective heat of combustion (av-EHC) of PP/Co-PPOA/PRGO is 33.88 MJ/kg, which is also lower than of PP and PP/Co-PPOA. This is mainly attributed to the fact that PRGO and Co-PPOA have a better combined effect on catalyzing the decomposition products of PP to form carbonaceous protection layers, preventing 4
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Fig. 4. (a) HRR, (b) THR, (c) COPR, (d) CO2PR curves of PP and its composites containing Co-MOF and PRGO.
MOF. A similar trend can also be observed in the THR curves (Fig. 4b). Compared with neat PP, the THR values of PP/Co-MOF and PP/CoMOF/PRGO are decreased by 4.9 MJ/m2 and 8.3 MJ/m2, respectively. Besides, the av-EHC values of PP/Co-MOF and PP/Co-MOF/PRGO are 2.29 MJ/kg and 2.83 MJ/kg lower than PP, respectively. These results confirm that Co-MOF and PRGO present an excellent combined effect on reducing the combustion extent of volatiles in gas phase and heat release of PP. It can be seen that the composite system of Co-MOF and PRGO also performs remarkable suppression on the releases of CO and CO2 during combustion.
Table 3 Cone calorimetry combustion data of PP and its composites. Sample
TTI (s)
PHRR (kW/ m2)
THR (MJ/ m2)
PCOPR (10− 3g/s)
PCO2PR (10− 1g/s)
av-EHC (MJ/ kg)
PP PP/CoPPOA PP/CoPPOA/ PRGO PP/CoMOF PP/CoMOF/ PRGO
37 31
832 681
85.2 79.0
4.87 3.61
3.88 3.07
35.02 34.35
21
611
74.8
3.92
2.71
33.88
37
791
80.3
4.47
3.61
32.73
30
619
76.9
3.49
2.80
32.19
3.4. Analysis of residual char Analysis of residual char is used to further study the combined effect of PRGO and Co-PPOA (Co-MOF) on improving the flame-retardant performance of PP. The residual char was collected from the cone calorimetry test of composite samples. Digital photographs of residual char of PP composites are shown in Fig. 6. From the comparison of Fig. 6, it can be observed that the addition of PRGO increases the char residue of PP composites. Fig. 6b and d present that the aluminum foil is covered by dense black char, implying that the combined effect between Co-PPOA (Co-MOF) and PRGO results in the formation of higher quantity and quality char. SEM images of char layers of PP composites before and after pickling are shown in Figs. 7 and 8. It can be seen from Fig. 7a that the residual char of PP/Co-PPOA consists of plentiful amorphous aggregates. When 1 wt % of Co-PPOA is replaced by PRGO, wide ranges of diameter spheres on the surface of PRGO can be seen, which may be attributed to the fact that the lamellar structure of PRGO can serve as a template for carbon growth. Besides, the SEM image of char layers of PP/Co-PPOA/
further combustion of PP. It is important to evaluate the toxic gases produced during the combustion of polymer, as these toxic fumes often cause asphyxiation [46,47]. The peak COPR and CO2PR (PCOPR and PCO2PR) of PP/Co-PPOA/PRGO are also lower than those of PP, which may be because that the barrier effect of char residue inhibits the re leases of CO and CO2 during the combustion process of PP. As revealed in Fig. 4a, it can be seen that a decrease in PHRR is obtained in PP/Co-MOF and PP/Co-MOF/PRGO. Compared to neat PP, the PHRR value of PP/Co-MOF decreases from 832 kW/m2 to 791 kW/ m2, revealing that Co-MOF is effective in inhibiting the heat release of PP during combustion. As shown in Fig. 5 and Table 3, when 1 wt % PRGO is introduced into PP/Co-MOF composite, the PHRR value of PP/ Co-MOF/PRGO presents a 21.7 % reduction compared to that of PP/Co5
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Fig. 5. Line charts of (a) PHRR and (b) THR values of PP and its composites.
Fig. 6. Digital photographs of residual char: (a) PP/Co-PPOA, (b) PP/Co-PPOA/PRGO, (c) PP/Co-MOF and (d) PP/Co-MOF/PRGO.
PRGO composite indicates that its char residue is more continuous and flat. Co compounds are reduced to metallic cobalt catalyst, which is an essential process for the char formation [48]. To further observe the formation of carbon spheres, char residue is pickled to remove Co-based materials. As shown in Fig. 7c, the residue of PP/Co-PPOA after pickling is composed of small spherical particles. Fig. 7d displays that many small
size carbon spheres are clustered on the surface of PRGO. Similar results can be observed in Fig. 8. As shown in Fig. 8a, the spheres with uniform size aggregate together to form an irregular network structure. Spheres can be also observed in the residual char of PP/Co-MOF/PRGO (Fig. 8b). The aggregated spheres are scattered on a flat structural surface. SEM image of char residue of PP/Co-MOF after 6
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Fig. 7. SEM images of char layers of PP/Co-PPOA and PP/Co-PPOA/PRGO composites: (a, b) before and (c, d) after pickling.
Fig. 8. SEM images of char layers of PP/Co-MOF and PP/Co-MOF/PRGO composites: (a, b) before and (c, d) after pickling.
pickling presented that the char is relatively dense and composed of vast small spheres. SEM image of char layers of PP/Co-MOF/PRGO after pickling is similar to PP/Co-PPOA/PRGO. The surface of the char layer becomes more wrinkled. After the removal of Co catalysts, the fold structure of PRGO is observed. Meanwhile, there are still gathered spheres on the surface of pleated char, which may be because that PRGO can be used as a template to promote the growth of carbon spheres.
XRD patterns of the char layer are exhibited in Fig. 9. In the XRD pattern of residual char of PP/Co-PPOA, the diffraction peaks are attributed to metallic cobalt (2θ = 44.5◦ , 51.8◦ and 76.2◦ ), indicating that Co-PPOA catalyst was reduced into metallic cobalt by the degra dation products of PP during the combustion [49]. The metallic cobalt is considered to be a real active site for the formation of carbon spheres [48]. Besides, the peak attributing to the graphite (002) crystal plane 7
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Fig. 9. XRD patterns of char layers: (a) PP/Co-PPOA(/PRGO) and (b) PP/Co-MOF(/PRGO) composites.
Fig. 10. Raman spectra of residual char of PP/Co-PPOA(/PRGO) composites: (a, b) before and (c, d) after pickling.
can also be observed at approximately 2θ = 25.9◦ [50]. When PRGO is introduced, the characteristic peaks of graphene (002) crystal plane and metallic cobalt can also be found in the XRD pattern of char. Similar results can be obtained in the XRD patterns of the composite containing Co-MOF and PRGO. Raman spectroscopy is a useful tool to analyze crystal size in the microstructure of carbonaceous materials [51]. Raman spectra of
residual char of PP composites before and after pickling are presented in Figs. 10 and 11, respectively. The spectra for all testing samples show two characteristic peaks at around 1350 cm− 1 (D band) and 1590 cm− 1 (G band), which are related to typical graphitic structures [52–54]. The integrated intensity ratio of D to G bands (ID/IG) is used to measure the microcrystalline size of carbon-based materials [55]. A higher ratio of ID/IG means a smaller microcrystalline size of carbon material [56,57]. 8
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Fig. 11. Raman spectra of residual char of PP/Co-MOF(/PRGO) composites: (a, b) before and (c, d) after pickling.
Fig. 12. GC–MS profiles of degradation products from PP and PP/PRGO.
Raman spectra of char residues of PP/Co-PPOA and PP/Co-PPOA/PRGO before pickling are presented in Fig. 10a and b, respectively. The ID/IG value of PP/Co-PPOA/PRGO is 1.22, which is higher than that of PP/Co-PPOA (1.09). After pickling, the ID/IG value of PP/Co-PPOA/PRGO is also higher than that of PP/Co-PPOA, indicating that a smaller microcrystalline size of char is formed by the addition of Co-PPOA and PRGO. Similar results are achieved in Fig. 11 for Co-MOF
and PRGO. Due to the promoted degradation effect from “acidic sites” and template effect, PRGO provides plenty of nucleating sites for char ring reactions, thus smaller microcrystalline size of char is achieved. 3.5. Degradation product analysis Fig. 12 displays GC–MS profiles of degradation products from PP and 9
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Fig. 13. Catalytic carbonization and flame-retardant mechanisms of Co compounds and PRGO.
thermal stability and flame retardancy of PP. The TGA results showed that the combination of Co-PPOA (Co-MOF) and PRGO can significantly improve the thermal stability of PP. Compared with pure PP, the Tinitial values of PP/Co-PPOA/PRGO and PP/Co-MOF/PRGO were increased by 90 ◦ C and 86 ◦ C, respectively. Moreover, the char residue of PP was increased with the addition of a combined catalyst. Char residue values of PP/Co-PPOA/PRGO and PP/Co-MOF/PRGO were 3.53 wt % and 6.01 wt % higher than that of PP, respectively. From the cone combus tion experiment, compared with PP, PHRR values of PP/Co-PPOA/ PRGO and PP/Co-MOF/PRGO were decreased by 26.6 % and 25.6 %, respectively. THR values of PP/Co-PPOA/PRGO (74.8 MJ/m2) and PP/ Co-MOF/PRGO (76.9 MJ/m2) were lower than that of PP (85.2 MJ/ m2). These results showed that the addition of a combined catalyst can effectively inhibit the production and release of heat and toxic gases. The possible flame-retardant mechanism of Co-PPOA (Co-MOF) and PRGO was proposed through the analysis of char residue and degrada tion products. This research extends the application of combined cata lytic carbonization in the field of flame-retardant polymer.
PP/PRGO at 700 ◦ C. Table S1 (see the supplementary material) shows the main components of pyrolysis products. The main degradation products from PP are olefins, and its content of aromatic is scarce, which is only 0.28 area %. When PRGO is added, it can be observed that the content of aromatics is increased to 15.37 area %. Previous studies have confirmed that aromatic products are conducive to be catalyzed to the formation of high-quality char [58,59]. Thus, it is suggested that PRGO can promote the aromatization of degradation products, which are conducive to the char formation. 3.6. Catalytic carbonization and flame-retardant mechanism Based on the above results, the mechanism of the combined catalyst on improving the flame-retardant property of PP is proposed and shown in Fig. 13. Co-PPOA (Co-MOF) and PRGO have remarkable combined effect in the catalytic carbonization of PP. During combustion, the PP matrix is promoted degradation by “acidic sites” on the surface of PRGO to decompose to generate small molecule aromatic compounds that are conducive to the growth of carbon materials. Co-PPOA (Co-MOF) is in situ reduced to metallic cobalt by PP degradation products. Metallic cobalt is the real active site for catalytic carbonation, which catalyzes those degradation products of PP to form carbon materials. The lamellar structure of PRGO can slow down the diffusion of PP degradation products and prolong the time to participate in the carbonization reac tion. It is equivalent to a micro-reactor role in the matrix, thus increasing the yield of carbon spheres. Besides, PRGO also serves as a template for the growth of carbon spheres on its surfaces, which can be confirmed from SEM images of char layers. Combined catalytic carbonation of CoPPOA (Co-MOF) and PRGO promotes the formation of a smaller microcrystalline size of char. Therefore, the flame retardancy of PP is enhanced by the combined catalyst of Co-PPOA (Co-MOF) and PRGO.
CRediT authorship contribution statement Yuanyuan Zhan: Data curation, Methodology, Writing - original draft. Sheng Shang: Software, Formal analysis, Data curation. Bihe Yuan: Supervision, Writing - review & editing. Shasha Wang: Conceptualization, Methodology, Supervision. Xianfeng Chen: Inves tigation, Visualization. Gongqing Chen: Formal analysis, Methodology. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
4. Conclusions
Acknowledgments
In this study, two novel combined catalysts of Co-PPOA (Co-MOF) and PRGO were prepared successfully and conducted to enhance the
This research was funded by the National Natural Science 10
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Foundation of China (51703175). The authors are grateful for the lan guage revision provided by Mr. Huidong Zhao and Mr. Quan Fang.
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