Cu ternary nanohybrids for fire safety application

Journal of Colloid and Interface Science 539 (2019) 1–10

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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis

Regular Article

Sodium alginate-templated synthesis of g-C3N4/carbon spheres/Cu ternary nanohybrids for fire safety application Yongqian Shi a,⇑, Liancong Wang b, Libu Fu c, Chuan Liu a, Bin Yu d,⇑, Fuqiang Yang a, Yuan Hu e,⇑ a

College of Environment and Resources, Fuzhou University, 2 Xueyuan Road, Fuzhou 350116, PR China State Key Laboratory of Coal Mine Safety Technology, CCTEG Shenyang Research Institute, Fushun, Liaoning 113122, PR China c College of Civil Engineering, Fuzhou University, 2 Xueyuan Road, Fuzhou 350116, PR China d Department of Architecture and Civil Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China e State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei 230026, PR China b

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 22 September 2018 Revised 12 December 2018 Accepted 13 December 2018 Available online 14 December 2018 Keywords: Graphitic carbon nitride Ternary nanohybrid Flame retardancy Pyrolysis gaseous products suppression Interfacial adhesion

a b s t r a c t Constructing novel graphic carbon nitride (g-C3N4)-based nanohybrids via a facile method for highperformance polymeric materials which exhibit enhanced fire safety properties are highly desirable. Here, the g-C3N4/carbon sphere/Cu (denoted as CSACS-C) nanohybrid was prepared by metal ionsinduced gel reaction using sodium alginate as a green template, and thereafter introduced into thermoplastic polyurethane (TPU) matrix to prepare nanocomposites. Microstructure analyses indicated the successful synthesis of the ternary nanohybrid, where both the g-C3N4 nanosheets and Cu nanoparticles were well-dispersed in the carbon material. Moreover, the ternary nanohybrid showed good distribution in TPU matrices, and exhibited strong interfacial adhesion with the polymeric host. It is worth noting that addition of these nanohybrids led to significantly improved fire safety. Particularly, remarkably reduced pyrolysis gaseous products generation and peak of heat release rate were achieved for TPU/CSACS-C system. This enhanced fire safety was due to the interpretations that the SACS and g-C3N4 nanosheets retarded the heat and mass propagation, while the incorporation of Cu nanoparticles induced earlier thermal decomposition into more char residues and less pyrolysis gaseous products. The work provides a new, simple strategy to prepare nanohybrids for polymeric materials with enhanced fire safety. Ó 2018 Elsevier Inc. All rights reserved.

1. Introduction ⇑ Corresponding authors. E-mail addresses: [email protected] (Y. Shi), [email protected] (B. Yu), [email protected] (Y. Hu). https://doi.org/10.1016/j.jcis.2018.12.051 0021-9797/Ó 2018 Elsevier Inc. All rights reserved.

Carbon micro- and nano-spheres have received increasing interest, because of their tunable intrinsic properties by controlling bulk structure, size and chemical composition. Pristine carbon

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spheres have been utilized as high-performance cathode material and safe adjuvants, inducing excellent lithium-sulfur batteries and Th2-biased immune response, respectively [1,2]. In recent years, investigating into carbon sphere based hybrids with a variety of nanostructures has been under the spotlight. Doping strategy endows carbon sphere with multifunctional properties [3,4]. Additionally, decoration of carbon sphere by metal or nonmetal nanoparticles not only broadens its application the fields of catalysis, adsorption, electronic engineering, and optical detection, etc., but also enhances the dispersibility of such nanoparticles [5–10]. Graphitic carbon nitride (g-C3N4) as one of artificial polymers was firstly reported in 1834 [11]. Due to abundant carbon and nitrogen elements, and versatile surface feature, polymeric gC3N4 has received great attention from researchers worldwide. The surface of g-C3N4 contains residual ANH2 or ANH groups which can act as anchoring sites for other functional groups [12]. This is an advantage in formation of g-C3N4 based hybrids, leading to efficient charge separation in the heterojunction structure [13,14]. It is generally accepted that the catalytic property of g-C3N4 has been significantly improved by loading of metal nanoparticles, carbonaceous nanomaterials, semiconductors and so on [15–18]. Recently, increasing researchers are focused on exploration of g-C3N4 based hybrids/polymer composites. Shi et al. reported the alternate deposition of g-C3N4 nanosheets and multi-wall carbon nanotubes on polystyrene (PS) spheres by using a layer-by-layer self assembly technique [19]. Peak of heat release rate (PHRR) and total heat release (THR) were decreased by ca. 45% and 47%, respectively, for the ternary assembled systems. For g-C3N4/polymer systems, the direct distribution of hydrophobic g-C3N4 without dispersants remains a great challenge. It is reported that organic modified montmorillonite/g-C3N4 nanohybrids were successfully prepared and then added into PS matrix to achieve nanocomposites [20]. These nanohybrids were not only evenly distributed in PS, but also exhibited obviously enhanced fire safety. The phosphorus-containing compounds acting as highly effective flame retardants, were used as modifiers to impart the g-C3N4 nanosheets with excellent flame retardancy. Combining g-C3N4 with aluminium hypophosphite, aluminum phosphinates or ammonium polyphosphate contributed to striking reduction in PHRR, THR and smoke production rate of polymers [21–23]. Owing to outstanding wear-, oil- resistance, high chemical stability and excellent mechanical properties, thermoplastic polyurethanes (TPUs) are widely employed in the fields of national defense, health care and food, etc [24]. However, TPUs are highly flammable and release large quantities of toxic gas when a fire occurs [25,26]. Therefore, performing fire resistant and toxic gas suppressed treatment to TPUs becomes a necessity. It is well known that nano-additives could simultaneously reduce fire hazards and improve other performances [27–30]. In recently years, TPU nanocomposites containing nano-additives, such as nanoCuO, carbon nanotubes, multifunctional boron nitride (fBN), graphene and its nanohybrids, modified molybdenum disulfide, organic frameworks nanosheets, polyphosphazene nanotubewrapped mesoporous silica@bimetallic phosphide nanostructures, have been extensively studied for improved fire resistant, mechanical, electromagnetic shielding properties, due to physical barrier effect and catalytic effect of nano-additives, and trapping effect of evolved pyrolysis products [31–40]. Incorporating 2.0 wt% cerium dioxide decorated reduced graphite oxide (CeO2/rGO) into TPU matrix could suppress heat release and smoke release [41]. In addition, CuCo2O4/g-C3N4 (C-CuCo2O4) nanohybrids were successfully synthesized to significant reduce the CO generation, HRR and THR values of TPU [42]. Despite some advances in the flame retardancy of the polymer, there is still lack of a simple method to prepare highly effective nanohybrids for TPU or other

polymer materials with simultaneous decline in HRR and pyrolysis gaseous products evolution. In the present work, we synthesized the g-C3N4/carbon sphere/ Cu (named CSACS-C) ternary nanohybrid, and then added it into TPU host. The microstructure of as-prepared nanohybrids were dissected. In addition, the combustion properties of as-prepared TPU systems were evaluated. The mechanisms to illustrate reduced fire hazards of the TPU nanocomposites were also proposed. 2. Experimental 2.1. Materials Thermoplastic polyurethane (TPU, 85E85) was purchased from Baoding Bangtai Chemical Industry Co., Ltd. (Baoding, China). All of N,N-dimethyl formamide (DMF), isopropanol (IPA), anhydrous ethanol, urea and sodium alginate (SA) were afforded by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Cupric acetate monohydrate (Cu(CH3COO)2
H2O) was provided by Red Rock Reagent Factory (Tianjin, China). 2.2. Synthesis of ternary nanohybrids Bulk g-C3N4 was obtained according to our previous work [43]. Then, ultrathin g-C3N4 nanosheets were obtained by adding as-prepared bulk g-C3N4 into IPA via ultrasonic-assisted exfoliation (Fig. 1a). 80 mg of the resulting g-C3N4 nanosheets was casted into 80 mL of 10 mg mL 1 SA solution under ultrasound and agitation for 4 h. Subsequently, the g-C3N4/SA dispersion was added dropwise into 80 mL of 50 M Cu2+ solution to form gel spheres. The obtained gel spheres re-dispersed in deionized water were thrown to a 100 mL of Teflon-lined autoclave, followed by heating at 180 °C for 12 h. The sample was collected by filtration and washed with deionized water and anhydrous ethanol, and finally dried at 60 °C. The as-synthesized nanohybrid was labelled as CSACS-C. The schematic illustration for the solvothermal synthesis of the nanohybrids was described in Fig. 1b. For comparison, the same strategy was adopted to prepare SACS-C, meaning the presence of SA and Cu2+ solution without g-C3N4. In addition, the SACS was prepared using the same approach in the presence of SA alone. The compositions of as-prepared nano-additives are listed in Table 1 according to the TGA results obtained from Fig. S1. 2.3. Fabrication of TPU nanocomposites The TPU solution was obtained by dissolving the polymer in DMF at 80 °C. The certain content of CSACS-C dispersion was added to the TPU solution above. Ultrasonically-assisted stirring was adopted to further treat the mixture for 1 h. The hot mixture was poured into deionized water to precipitate TPU nanocomposite containing 2 wt% CSACS-C nanohybrid. Finally, this sample was separated by filtration, and dried at 80 °C. The same approach was used to manufacture TPU/SACS, TPU/SACS-C and TPU/g-C3N4 nanocomposites containing 2 wt% SACS, 2 wt% SACSC and 2 wt% g-C3N4, respectively for comparison. The TPU nanocomposites were hot-pressed for characterization. Accordingly, fabrication process of TPU nanocomposites was illustrated in Fig. 1c. 2.4. Instruments and measurements Fourier transform infrared (FTIR) spectrums were achieved by a Nicolet 6700 FTIR (Nicolet Instrument Company, USA) in the range of 500–4000 cm 1. Raman spectroscopy was performed with using

Y. Shi et al. / Journal of Colloid and Interface Science 539 (2019) 1–10

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a) Liquid exfoliation

g-C3N4 Nanosheets

Bulk g-C3N4

b) Cu2+ solution

180 oC, 12 h

Gel reaction

g-C3N4/SA suspension

Carbonized gel spheres g-C3N4 nanosheet

Gel sphere

c) 1) Precipitation Deionized water

2) Moulding

TPU/Nanohybrid suspension

TPU nanocomposites

Fig. 1. Schematic diagrams for (a) liquid exfoliation of bulk g-C3N4, (b) preparation of nanohybrids and (c) fabrication of TPU nanocomposites.

Table 1 The compositions of the as-synthesized nano-additives. Sample No.

SACS (wt.%)

Cu (wt.%)

g-C3N4 (wt.%)

SACS SACS-C CSACS-C g-C3N4

100.0 19.7 12.4 —

— 80.3 50.6 —

— 37.0 100.0

an Invia Reflex laser Raman spectrometer (Renishaw Co., UK.) at 532 nm. X-ray diffraction (XRD) profiles were obtained by a Japan Rigaku Dmax X-ray diffractometer equipped with graphite monochromatized high-intensity Cu Ka radiation (k = 1.54178 Å). Transmission electron microscope (TEM) was afforded by a JEOL 2010 instrument with an accelerating voltage of 200 kV. The dispersions of SACS, SACS-C and CSACS-C in anhydrous ethanol were achieved by ultrasound, and then distributed onto molybdenum grids for observation. The dispersion of nanohybrids in TPU matrix was available by using an Ultratome (Model MT-6000, Du Pont Company, USA). Thermogravimetric analysis (TGA) of the nano-additives and nanocomposites was conducted using a Q5000 thermal analyzer (TA Co., USA) ranging from 30 to 800 °C at 20 °C min 1 under N2 or air condition. The combustion properties of TPU nanocomposites were investigated via a cone calorimeter (FTT, UK) according to ISO 5660/ASTM E1354 [44,45]. The specimens with size of 100  100  3 mm3 were radiated by heat flux of 35 kW m 2. The specimen was placed in an aluminum foil. All of these processes were performed three times for each specimen. Thermogravimetric analysis/infrared spectrometry (TG-IR) of TPU and its nanocomposites was conducted by a TL-9000 device. Thermal analyzer was performed in the range from room temper-

ature to 800 °C with a rate of 20 °C min 1 in helium atmosphere. The morphologies of TPU nanocomposites were studied by scanning electron microscopy (SEM) with an AMRAY1000B type (Beijing R&D Center of the Chinese Academy of Sciences, China). These samples were fractured and sputter coated with a gold layer before observation. 3. Results and discussion 3.1. Microstructure characterization of nanohybrids Fig. 2 shows the FTIR spectra of pure SACS, g-C3N4, SACS/Cu (SACS-C) binary nanohybrid and g-C3N4/SACS/Cu (CSACS-C) ternary nanohybrid. The absorption bands at 1640, 1320 and 1235 cm 1 are attributed to the stretching vibration of linkage units (CAN(AC)AC or CANHAC), and the absorption band at ca. 810 cm 1 is assigned to the vibration of the tri-azine ring, revealing the existence of g-C3N4 [46]. For SACS, the two peaks at 2924 and 2852 cm 1 indicate that CAH groups are abundant [47]. In addition, the signals of 1701 cm 1, 1591 cm 1 and 1000–1300 cm 1 occur, demonstrating presence of the oxygen-rich groups [48]. After hybridization, the band assigned to CAOH groups moves gently towards 1616 cm 1, indicating that the SACS is subjected to weak coordination with Cu species during the synthesis process. In the case of CSACS-C, these absorption bands are assigned to both SACS and g-C3N4. Besides, Raman spectra shows that the band located at 1594 cm 1 corresponding to G band, appear, revealing the presence of SACS in SACS and its hybrids (see Fig. S2). XRD technique was employed to further analyze the structural phase of pure carbon sphere, g-C3N4 and their nanohybrids, as plotted in Fig. 3. For SACS, the broad peak centered at 21.4° is asso-

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3.2. Morphology dissection of nanohybrids

Fig. 2. FTIR spectra of SACS, SACS-C, CSACS-C and g-C3N4.

ciated with the amorphous carbon [49]. As can be clearly observed from Fig. 3b, a strong diffraction peak at 2h = 27.7° corresponds to the stacking structure of the conjugated aromatic group, and a peak of 13.0° is attributed to in-planar repeating unit, suggesting the presence of g-C3N4 [43]. Fig. 3c depicts two characteristic peaks at 43.4° and 50.5°, assigned to the obvious indices of (1 1 1) and (2 0 0), respectively. These typical peaks demonstrate the generation of a face-centered cubic copper phase [50]. It is noted that hybridizing SACS with Cu and g-C3N4 species can be successfully achieved at 27.7°, 43.4° and 50.5° (see Fig. 3d).

The morphology of as-prepared samples was observed by TEM. It is clearly observed from Fig. 4a that sand-like particles are large carbon spherical clusters. This may be a consequence of long period of solvothermal treatment during continuous carbon supply. After the hybridizing reaction, homogeneous spherical nanoparticles are obtained (see Fig. 4b). However, the carbon sphere/Cu binary nanohybrid shows smaller size, rivalled only by SACS, due to the shrinkage of alginate polymer during the carbonization progress. In the case of the ternary nanohybrid, these nanosheets are plicated and lamellar structure with size of approximately one micrometer (see Fig. 4c). In addition, there exist a lot of nanoparticles decorated on the nanosheets. The corresponding EDS spectrum was performed, as shown in Fig. 4d. The occurrence of C, O, N and Cu elements further demonstrates the successful preparation of SACS, g-C3N4 nanosheets and Cu nanoparticles. In order to investigate the distribution of Cu nanoparticles in g-C3N4 nanosheets, the TEM mapping of CSACS-C is considered (see Figs. S3c–e). It is clearly found that C, N, and Cu elements are uniformly dispersed in the specific portion selected for the mapping studies. 3.3. Thermal property assessment of TPU nanocomposites TG and derivative thermogravimetry (DTG) profiles of TPU and its nanocomposites versus temperature under air condition are given in Fig. 5, and the related thermal data are listed in Table 2. The Tonset is defined as the temperature corresponding to 10% weight loss, and Tmax means the temperature at maximum weight loss rate. As shown in Fig. 5a, thermal degradation process of these

Fig. 3. XRD patterns of (a) SACS, (b) g-C3N4, (c) SACS-C and (d) CSACS-C.

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Fig. 4. TEM images of (a) SACS, (b) SACS-C and (c) CSACS-C. (d) and (e) show corresponding EDS spectrograms of colored areas of (b) and (c), respectively.

Fig. 5. (a) TGA and (b) DTG curves of TPU and its nanocomposites in air.

Table 2 Related TGA data of TPU and its nanocomposites under air condition. Sample No.

Tonset (°C)

Tmax1 (°C)

Tmax2 (°C)

Tmax3 (°C)

Char yield (wt.%)

TPU TPU/SACS TPU/SACS-C TPU/CSACS-C TPU/g-C3N4

330.8 328.1 325.5 321.4 329.6

344.7 335.1 335.1 332.4 340.0

411.7 405.2 407.2 405.7 410.4

576.0 577.5 541.9 516.1 583.6

0.3 0.8 2.4 2.0 0.6

nanocomposites is analogous to that of pure TPU. Fig. 5b presents that all of samples show three-stage degradation process, different from the previous report, implying the existence of oxidation effect during thermal degradation [42]. For nanocomposites, both Tonset and Tmax slightly shift to lower values, as compared to those of native TPU (see Table 1). The addition of SACS results in increased

Tmax3 and char yield. These improvements are attributed to high thermal resistance (see Fig. S1) [51]. Hybridizing of SACS with Cu and/or g-C3N4 can further improve the thermal stability of SACS, leading to increased char residues of TPU nanocomposites. It is noted that the ternary nanohybrid shows the lowest Tonset and Tmax, as well as increased char residues, indicating catalytic effect

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Fig. 6. (a) HRR and (b) THR curves of TPU and its nanocomposites at 35 kW m

2

.

of Cu nanoparticles [52,53]. In comparison with the residual content of TPU nanocomposites obtained from Fig. 5, their content of char residues under N2 (see Fig. S4) are obviously increased. Moreover, the hybrids containing Cu nanoparticles show higher char yield, further suggesting the catalytic effect of Cu component. 3.4. Combustion behavior evaluation of TPU nanocomposites The combustion properties of polymeric materials were often investigated by the cone calorimeter. The curves of HRR as a function of time of TPU and its nanocomposites are shown in Fig. 6a, and their corresponding cone data are given in Table 3. Pristine TPU burns rapidly upon ignition, showing a sharply ascending profile with PHRR value of 1031 kW
m 2. It is evident that SACS or SACS-C nanohybrid renders TPU slightly reduced PHRR. A decrease of 23.2% in the PHRR is available for TPU/g-C3N4 nanocomposite, compared with virgin TPU. It is worth noting that significantly decreased PHRR is available once the TPU matrix is decorated by CSACS-C nanohybrid, i.e. decrease of 34.6% for TPU/CSACS-C (see Table 3). The THR is also a key parameter for estimating flame retardance of materials. As can be observed from Fig. 6b and Table 3, the THR value of TPU is 77.6 MJ m 2. Incorporating g-C3N4 and nanohybrids into TPU matrix leads to reduced THR, i.e. 72.0, 66.4 and 66.4 MJ m 2 for TPU/g-C3N4, TPU/SACS-C and TPU/CSACS-C, respectively. However, TPU nanocomposite shows a regime change at 2 wt% loading of SACS, where a high THR value (80.1 MJ m 2) is

Table 3 Related cone calorimeter data for TPU and its nanocomposites at 35 kW m Sample No.

PHRR (kW m

TPU TPU/SACS TPU/SACS-C TPU/CSACS-C TPU/g-C3N4

1051 ± 23 986 ± 30 830 ± 19 674 ± 21 792 ± 33

2

)

2

.

THR (MJ m

2

)

77.6 ± 1.4 80.1 ± 1.2 66.4 ± 1.3 66.4 ± 1.2 72.0 ± 0.4

Fig. 7. FTIR spectra of pyrolysis gaseous products emitted from (a) TPU and (b) TPU/ CSACS-C at the maximum decomposition rate.

Table 4 Comparisons in different work on flame-retardant TPU nanocomposites. Sample No.

Additives Content

PHRR Reduction

THR Reduction

Ref.

TPU/Co3O4-rGO TPU/PZS@M-SiO2@CoCuP TPU/fBN TPU/Covalent organic frameworks TPU/GNS@PDA@HPTCP TPU/CeO2/rGO TPU/CTAN-BN TPU/CSACS-C

2 wt% Co3O4-rGO 3.0 wt% PZS@M-SiO2@CoCuP 3.0 wt% fBN 1.6 wt% Covalent organic frameworks 2.0 wt% GNS@PDA@HPTCP 2.0 wt% CeO2/rGO 4.0 wt% CTAN-BN 2.0 wt% CSACS-C

16.4% 58.2% 35.6% 14.3% 59.4% 41.1% 57.5% 35.9%

— 19.4% 19.8% 2.3% 27.1% — 17.7% 14.4%

[33] [36] [37] [39] [57] [58] [59] This work

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achieved. It is interesting that SACS-C and CSACS-C show the same THR. In comparison with the previous work, the flame-retardant system in this work imparts excellent flame-retardant and smoke suppressed properties to TPU at relative low loadings of additives (see Table 4). It can be inferred that g-C3N4 nanosheets and SACS exhibit physical barrier effect, leading to obvious reduction of PHRR values. Nevertheless, g-C3N4 nanosheets completely decompose into inert gases, which has no inhibiting effect on THR curves, while instable carbon spheres degrade into combustible gas in high temperature, resulting in increased THR value [54]. Fortunately, combination among g-C3N4, SACS and Cu nanoparticles contributes to the most reduced heat effect, where Cu component catalyzes TPU matrices and flammable gas into stable carbonaceous layer. 3.5. Pyrolysis gas products detection of TPU nanocomposites TG-FTIR measurement was usually employed to detect release of the gaseous products during the thermal degradation. Fig. 7 presents FTIR spectra collected at the maximum evolution rate during the thermal decomposition of TPU and TPU/CSACS-C system. Some gaseous decomposition products at 353 and 447 °C for the pure TPU are identified by characteristic strong absorption signals, such as Ph-NH2 (3573 cm 1), CH2-groups (2963 and 2881 cm 1), CO2 (2360, 2303 and 666 cm 1), NCO-groups (2278 cm 1), carbonylcontaining species (1757 cm 1) and CAOAC bond (1147 cm 1) [55,56]. The first step is due to thermal decomposition of the hard

segment into isocyanate, alcohol, olefin and CO2, while the second step is attributed to the thermal degradation of the soft segment [56]. TPU nanocomposite containing CSACS-C nanohybrid exhibits similar thermal decomposition process in the range of 348–424 °C, whereas its value of ID(2360 cm 1)/ID(2303 cm 1) is obviously increased at the second maximum decomposition stage, indicating that the CSACS-C nanohybrid catalyzes earlier thermal decomposition of TPU nanocomposite into more CO2 [23,42]. Combined with the cone results, it is deduced that CSACS-C nanohybrid exhibits catalytic effect that leads to increased char residues and decreased pyrolysis gaseous products. In order to demonstrate the inference, the evolution of total pyrolysis gas of TPU and its nanocomposites is portrayed in Fig. 8a. It is obvious that the incorporation of nano-additives, especially the CSACS-C reduces the formation of total evolved gas. The total organic volatiles are primarily originated from CO2, alkyl- and carbonyl-compounds, appearing at 2360, 2977 and 1765 cm 1, as shown in Fig. 8b–d. The combination of g-C3N4 nanosheets, Cu nanoparticles and SACS significantly reduces the formation of CO2, alkyl- and carbonyl-compounds, rivalled only by the monobasic components. 3.6. Interfacial adhesion behavior of TPU nanocomposites The interfacial adhesion between the nano-additives and the TPU matrix is an important factor, and was investigated by SEM (see Fig. 9). As can be seen from Fig. 9a and b, SACSs alone are

Fig. 8. (a) Gram-Schmidt curves, and intensities of characteristic peaks for pyrolysis gaseous products of TPU and its nanocomposites: (b) 2977 cm 1765 cm 1.

1

, (c) 2360 cm

1

and (d)

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Fig. 9. SEM images of fracture surface of (a) (b) TPU/SACS, (c) (d) TPU/SACS-C and (e) (f) TPU/CSACS-C.

coated by the polymer matrix. In contrast, the TPU/SACS system, the TPU nanocomposite shows a fracture morphology between mere carbon sphere/Cu binary nanohybrid and the polymer matrix (see Fig. 9c and d). In addition, severe aggregation of SACS-C occurs in the nanocomposite. It is found that fracture surface of TPU/ CSACS-C nanocomposite shows an extraordinary difference from TPU/SACS and TPU/SACS-C systems (see Fig. 9e and f). The protruding CSACS-C nanohybrid is thickly wrapped by the polymer, suggesting strong adhesion at TPU/CSACS-C interface.

3.7. Dispersion of ternary nanohybrid The distribution of the g-C3N4 nanosheets/SACS/Cu nanoparticles ternary nanohybrid in the polymer host was further dissected by TEM, as depicted in Fig. 10. It is clear that combination of g-C3N4 and SACS-C enables good distribution of the nanohybrid in the polymer matrix (see Fig. 10a). Most of the Cu nanoparticles and g-C3N4 nanosheets are attached to the surface of SACS, due to strong interaction among the three components. However, part

Fig. 10. Ultrathin TEM images of TPU/CSACS-C: (a) Low magnification; (b) High magnification.

Y. Shi et al. / Journal of Colloid and Interface Science 539 (2019) 1–10

of the g-C3N4 nanosheets fall from SACS for TPU/CSACS-C, because of ultrasound, stirring and shearing (see Fig. 10b).

4. Conclusions In this work, the g-C3N4/SACS/Cu ternary nanohybrid was successfully prepared as flame retardant and decomposed gaseous products suppressant for TPU. Morphological analysis revealed the coexistence of g-C3N4 nanosheets, SACSs and Cu nanoparticles in the nanohybrid, where both nanoparticles and the nanosheets were dispersed in the carbon host. Moreover, the ternary nanohybrid was evenly dispersed in TPU matrices, and displayed strong interfacial adhesion with the polymer. Cone results indicated that addition of these nanohybrids resulted in significantly reduced fire hazards. Extraordinarily, remarkable decrease in pyrolysis gaseous products release and PHRR (34.6% reduction) was obtained for TPU/CSACS-C. It was proposed that on one hand, the g-C3N4 nanosheets and carbon spheres retarded the heat and mass permeation at low temperature; on the other hand, the incorporation of Cu nanoparticles catalyzed the thermal decomposition of the matrix in advance into more char residues and less evolved gases. The work opens a route for reducing fire hazards of other polymers. Acknowledgements This work was supported by the Natural Science Foundation of Fujian Province, China (Grant No. 2018 J05078), the Opening Project of State Key Laboratory of Fire Science of University of Science and Technology of China (Grant No. HZ2017-KF02), the Opening Research Fund of State Key Laboratory of Coal Mine Safety Technology (Grant No. SKLCMST101), and the Natural Science Foundation of China (Grant No. 51803031, 71804026 and 51741402). The authors thank Mr Xinqi Zhang for assisting with TEM measurement.

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