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MoS2 hybrid sheets on reducing fire hazards of polyamide 6 composites
Journal of Hazardous Materials 320 (2016) 252–264
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Integrated effect of supramolecular self-assembled sandwich-like melamine cyanurate/MoS2 hybrid sheets on reducing fire hazards of polyamide 6 composites Xiaming Feng a,b , Xin Wang a,∗∗ , Wei Cai a , Ningning Hong a , Yuan Hu a,b,∗ , Kim Meow Liew b,c a
State Key Laboratory of Fire Science, University of Science and Technology of China, Anhui 230026, PR China Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute for Advanced Study, University of Science and Technology of China, Suzhou, Jiangsu 215123, PR China c Department of Architectural and Civil Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong b
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Novel
melamine cyanurate/MoS2 sheets were prepared by supramolecular self-assembly. • MoS2 sheets function as a template, synergist and smoke suppressor in MCA/MoS2 /PA6. • Incorporating MCA/MoS2 sheets obviously reduced the fire hazards of PA6 composites. • Integrated effect is crucial for the enhancements on fire safety of PA6 composites.
a r t i c l e
i n f o
Article history: Received 4 July 2016 Received in revised form 2 August 2016 Accepted 4 August 2016 Available online 5 August 2016 Keywords: Supramolecular self-assembly Molybdenum disulfide Sandwich-like hybrid sheets Fire hazards Catalytic activity
a b s t r a c t A novel strategy of using supramolecular self-assembly for preparing sandwich-like melamine cyanurate/MoS2 sheets as the hybrid flame retardants for polyamide 6 (PA6) is reported for the first time. The introduction of MoS2 sheets function not only as a template to induce the formation of two-dimensional melamine cyanurate capping layers but also as a synergist to generate integrated flame-retarding effect of hybrid sheets, as well as a high-performance smoke suppressor to reduce fire hazards of PA6 materials. Once incorporating this well-designed structures (4 wt%) into PA6 matrix, there resulted in a remarkable drop (40%) in the peak heat release rate and a 25% reduction in total heat release. Moreover, the smoke production and pyrolysis gaseous products were efficiently suppressed by the addition of sandwich-like hybrid sheets. The integrated
∗ Corresponding author at: State Key Laboratory of Fire Science, University of Science and Technology of China, Anhui 230026, PR China. ∗∗ Corresponding author. E-mail addresses: [email protected] (X. Wang), [email protected] (Y. Hu). http://dx.doi.org/10.1016/j.jhazmat.2016.08.012 0304-3894/© 2016 Elsevier B.V. All rights reserved.
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functions consisting of inherent flame retarding effect, physical barrier performance and catalytic activity are believed to the crucial guarantee for the reduced fire hazards of PA6 nanocomposites. Furthermore, this novel strategy with facile and scalable features may provide reference for developing various kinds of MoS2 based hybrid sheets for diverse applications. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Polyamide 6 as an important engineering plastic has been widely used because of its excellent performances, such as superior mechanical property, abrasion resistance, electrical characteristic and oil proof performance [1,2]. Therefore, it stimulated dramatical interest in reducing the high fire risk of PA6 products [3]. Moreover, large quantities of dense smoke and toxic gaseous products will generate during the combustion of PA6 materials, generally, inhalation of which is the main cause of death in fires [4,5]. Therefore, the high-efficiency but harmful halogen-containing flame retardants have been replaced with more environment-friendly flame retardants in many applications by the legal compulsion. Over the past decades, a massive efforts have been made to enhance the fire resistance of PA6 by incorporating halogen-free flame retardants [6–8]. As a kind of environmentally friendly, non-corrodible and high nitrogen-containing flame retardants, melamine cyanurate (MCA) has drawn more and more attention in the development of flame retarded PA6 composites [9–11]. In general, MCA are synthesized by a supramolecular selfassembly process in aqueous phase, which result in an extensive two-dimensional (2D) network of hydrogen bonds between melamine (MA) and cyanuric acid (CA) [12]. Limited by the oversimplified flame retardant mechanism, the effect of MCA on flame resistance of PA6 matrix is actually modest. In previous reports [10], about 15 wt% MCA are needed to achieve the flame retarded PA6 composite with UL94 V-0 rating. Moreover, the high addition level of MCA always leads to a damage on the physical properties of PA6. Hence, improving the flame retardant efficiency of MCA is quite necessary. MCA with different morphologies has been prepared to reinforce the flame retardancy of PA6 [13]. The authors indicate that nanoscale 2D MCA sheets exhibited a better performance in contrast to other morphologies due to the more hydrogen-bonding interactions between well dispersed MCA and PA6, which derived from the high specific surface area of 2D structure. However, it still cannot overcome the inherent shortcoming of MCA. As well known, integrating two or more constituents could always display a synergistic effect and result in an extraordinary improvement in properties of polymeric composites [14–16]. As a typical inorganic 2D flame retardant, montmorillonite (MMT) was formulated with MCA to fabricate the flame retarded of PA6, in which the synergistic effect between MCA and MMT were proposed [10]. Bulk MoS2 , a classic layered compound, has served as solid lubricants for decades [17–19]. In recent years, 2D MoS2 sheet has drawn much attention for the outstanding semiconductor performances and excellent catalytic properties [20,21]. As reported before [22–24], MoS2 sheets do not only possess the integrated flame-retarding effect but also exhibit a positive effect on the other properties of polymeric materials, which is regarded as a promising candidate to traditional inorganic layered flame retardants. In our previous reports [14,25,26], MoS2 sheets were performed to enhance the mechanical behaviors, thermal stabilities and crystallization performances, as well as the fire safety of polymer nanocomposites. It indicates that the dispersion, interfacial compatibility and inherent characters of MoS2 sheets play an important role to achieve the satisfactory improvements in properties of poly-
mer nanocomposites. More recently, Wu et al. demonstrated the synergistic flame-retarding effect between P-N flame retardants and commercial MoS2 within thermoplastic poly(ether ester) elastomers, in which the catalytic effect of Mo element through a cycloaddition interaction of gaseous decomposition products acts as a key factor in char forming process [27]. In previous report [28,29], the authors indicated that nano-MoS2 was an efficient filler to reinforce the wear performance and mechanical property of PA6 composites. To our knowledge, there is no report yet about the synergistic effect between MCA and 2D MoS2 sheets as well as the flame retardant properties enhancements of PA6 nanocomposites. In the present paper, a novel yet very effective strategy to achieve above goal through in situ fabrication of sandwich-like MCA/MoS2 hybrid sheets by a supramolecular self-assembly process has been demonstrated, as shown in Scheme 1. Benefiting from the integrated effect of sandwich-like structure, the hybrid sheets present an impressive performance in reducing fire hazards of PA6 materials. In addition, this simple approach is appropriate for mass production, implying the substantial promotion in developing MoS2 based flame retardants.
2. Experiment section 2.1. Materials Melamine, cyanuric acid, MoS2 , n-hexane and formic acid were supplied by Sinopharm Chemical Reagent Co., Ltd. (China). PA 6 (1003NW8) was supplied by Yubu Company (Japan). n-Butyl lithium (2.5 M in hexane) was provided by Aladdin Industrial Corporation (China).
2.2. Preparation of MA/MoS2 and MCA/MoS2 materials MA attached on MoS2 sheets (MA/MoS2 ) were fabricated by a simple solution mixing of MA and exfoliated MoS2 . In brief, 1 g MoS2 powder was reacted with n-butyl lithium in hexane (8 mL, 2.5 M) in a solvothermal synthesis reactor (100 mL) at 95 ◦ C for 5 h to prepare the lithium intercalated MoS2 (Lix MoS2 ). 0.5 g Lix MoS2 was hydrolyzed in 1000 mL deionized water by sonication to obtain MoS2 suspension [22]. Then, 1 g MA was dissolved in the MoS2 suspension under stirring at 80 ◦ C for 12 h. The products were collected through vacuum filtration and washed by hot water to remove the non-reacted reactants, further dried in an oven at 80 ◦ C. Sandwich-like MCA/MoS2 hybrid sheets (MCA/MoS2 ) were synthesized by supramolecular polymerization of MA and CA in the MoS2 suspension. In order to get the sandwich-like sheets rather than the other morphologies, we have tried different raw material ratios to define the best ration. In detail, 0.58 g melamine was added into the above MoS2 suspension with continuous stirring at 80 ◦ C. Then, 100 mL CA aqueous solution (6 mg mL−1 ) was poured in the above suspension and further stirred at 95 ◦ C for 10 h. At last, the products were filtered, washed and dried at 80 ◦ C overnight. In addition, MCA was synthesized by the same procedure without the incorporation of MoS2 sheets.
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Scheme 1. (a) Schematic for preparation of MCA by supramolecular polymerization of MA and CA. (b) Schematic illustration of the fabrication procedure of sandwich-like MCA/MoS2 hybrid sheets.
2.3. Preparation of MCA/MoS2 /PA6 nanocomposites MCA/MoS2 /PA6 nanocomposites were fabricated through a solution blending technique. For example, 0.8 g MCA/MoS2 was dispersed in formic acid assisting by sonication to obtain a uniform dispersion. Then, 19.2 g PA6 was added into the MCA/MoS2 dispersion with vigorous stirring for 60 min, and dried in a plate at 80 ◦ C for 12 h in the vacuum oven to completely remove solvents.
2.4. Characterization Fourier transform infrared (FT-IR) spectra were performed by a Nicolet 6700 spectrometer (Nicolet Instrument Corporation, U.S.). X-ray photoelectron spectroscopy (XPS) was conducted using a VGESCALB MK-II electron spectrometer (Al K␣ excitation source at 1486.6 eV). X-ray diffraction (XRD) was monitored using a Japan Rigaku D/Max-Ra rotating-anode X-ray diffractometer equipped with a Cu-K␣ tube and a Ni filter ( = 0.1542 nm). Transmission electron microscopy (TEM) (JEM-2100F, Japan Electron Optics Laboratory Co., Ltd.) was employed to study the micro-morphologies and the accelerating voltage was 200 kV. Morphologies of samples were observed by a PHILIPS XL30E scanning electron microscope (SEM). The specimens were cryogenically fractured in liquid nitrogen first and then sputter-coated with a conductive layer. Differential scanning calorimetry (DSC) (Q2000 TA Instruments Inc., USA) was conducted to characterize crystallization behaviors of samples. The PA6 composites were heated from 30 to 280 ◦ C (for 10 min), then cooled to 30 ◦ C at a rate of 10 ◦ C min−1 . The heatingcooling cycle was performed again.
Thermogravimetric analysis (TGA) was carried out using a Q5000 thermoanalyzer instrument (TA Instruments Inc., U.S.) under different flows of 25 mL min−1 and heated from 30 to 800 ◦ C at a heating rate of 20 ◦ C min−1 . Microscale combustion colorimeter (MCC) was performed to test the heat release of PA6 nanocomposites according to ASTM D7309-07. 5 mg PA6 composites were heated at a heating rate of 1 ◦ C s−1 from 30 to 650 ◦ C. Thermogravimetric analysis-infrared spectrometry (TG-IR) (TGA Q5000IR thermogravimetric analyzer linked to a Nicolet 6700 FTIR spectrophotometer) was performed from 30 to 700 ◦ C at 20 ◦ C min−1 (nitrogen atmosphere). The fire hazard was monitored by a steady state tube furnace (SSTF) (ISO TS 19700), as shown in Scheme 2, which is a small-scale fire apparatus designed to replicate the different stages of fire. Fuel and air are introduced into the furnace at a certain ratio, based on the knowledge of the material composition and its stoichiometric oxygen requirement [30,31]. About 15 g granular samples were put in the tube furnace at a rate of 3 cm min−1 . A stream of air was injected into the furnace over the specimens to support combustion. The exhaust gases were extruded from the tube furnace to the mixing chamber and diluted by a secondary air flow, where gaseous product concentration and smoke production would be monitored.
3. Results and discussion 3.1. Characterization of MCA/MoS2 materials Scheme 1 outlines the overall preparation procedures for sandwich-like MCA/MoS2 sheets by supramolecular self-assembly method. First, MoS2 was exfoliated by an intercalation-hydrolysis
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Scheme 2. The diagrammatic illustration of the steady state tube furnace.
Fig. 1. XRD patterns of MA, MoS2 , MA/MoS2 , MCA/MoS2 hybrid sheets and standard XRD data for MCA (JCPDS file no. 00-005-0127).
method to obtain exfoliated MoS2 suspension. Our previous papers indicated the easily re-stack of MoS2 sheets during filtration and drying [32]. Then, melamine as the surface modifier and precursor, was spontaneously absorbed on MoS2 sheets through intermolecular and hydrogen-bonding interactions, which can protect MoS2 sheets from re-stacking and induce the formation of MCA layer wrapping on MoS2 sheets. Finally, the hydrogen bonding between MA and CA induces the supramolecular self-assembly process, causing the formation of sandwich-like MCA/MoS2 hybrid sheets. In the previous works [10,13], both the raw flake-like MCA and simply mixing with MMT cannot achieve the integrated flame retardant mechanism. On clear contrast, our proposed process could combine the flame retardant elements (N, Mo and S) and the hierarchical structures (physical barrier effect of MoS2 sheets and more hydrogen-bonding interactions between MCA and PA6) to achieve excellent flame retardancy of polymer composites. Systematic characterizations were conducted to measure the proposed strategy. The product was first examined by powder XRD, as depicted in Fig. 1. In contrast, the MA/MoS2 displays a significantly decreased peak around 14.5◦ corresponding to (002) reflection of MoS2 , indicating that MoS2 is well exfoliated instead of re-stacked with the assistance of absorbed MA molecules. After in situ polymerized with CA, the major diffraction peaks of MCA/MoS2 exhibit a crystalline structure, which is consistent with the standard card of MCA (JCPDS file No. 00-005-0217). The peak intensity of (002) reflection of MoS2 in MCA/MoS2 materials is obviously decreased, demonstrating that face-to-face stacking of MoS2 flakes is obviously prevented by the wrapped MCA molecules.
The morphologies and microstructures of obtained materials were observed by TEM. The exfoliated MoS2 displays the flaky texture and wrinkled structure (Fig. 2a). As for the MA/MoS2 (Fig. 2b), the dense black dots decorated on the surface of MoS2 sheets confirmed the successful attachment of MA molecules. TEM and SEM image of MCA (Fig. 2c and e) shows the rod-like MCA supramolecular crystals with quite smooth surfaces, the sizes of which range from hundreds of nanometers to several micrometers. Notably, these rod-like supramolecular crystals disappear in the TEM and SEM images of MCA/MoS2 materials (Fig. 2d and f), instead, the MCA/MoS2 exhibits a lamellar structure similar with that of exfoliated MoS2 . Since the MA molecules were attached on the surfaces of MoS2 sheets, the MA/MoS2 sheets acted as a template to synthesis the sandwich-like MCA/MoS2 hybrid sheets during the supramolecular self-assembly. Moreover, the EDS spectrum confirmed the characteristic element composition of MCA/MoS2 sheets (Fig. 2g). Fig. 3 displays the FTIR spectra of these samples. As for raw MoS2 , there is only weak water bonding at 1635 cm−1 and wide stretching band of OH around 3440 cm−1 could be detected. After modified by MA, some new absorption peaks assigned to MA molecules can be observed, indicating the successful attachment of MA molecules on MoS2 sheets. After in situ polymerization, the MCA/MoS2 sheets exhibit the exactly similar spectrum to that of pure MCA. In detail, the strong band at 3388 cm−1 is ascribed to NH2 symmetric stretching of triazine groups; the peaks at 3222 and 3038 cm−1 are due to the formation of hydrogen-bond within NH2 /NH moieties. The peaks related to C N and C N groups can be recognized by the frequencies of 1446 cm−1 and 1533 cm−1 . The strong peaks at 1736 and 1658 cm−1 are corresponding to NH2 scissoring and NH2 bending vibration, respectively [9,33]. The FTIR results confirmed the successful preparation of MCA molecules by this method. XPS analysis was performed to examine the surface compositions and atomic-level interaction of MCA/MoS2 sheets. The XPS spectra in Fig. 4a demonstrates that the hybrid sheets mainly consist of C, O, N, S and Mo elements without impurity elements. By comparing the high-resolution N 1s XPS spectra (Fig. 4b), an obvious upshift for N 1s peak of MCA/MoS2 sheets can be observed. In general, the formation of hydrogen-bond or protonated nitrogen always results in an upshift of the binding energy of N 1s because of the reduction of electron density in nitrogen atoms [34]. In this case, it is unlikely to be ionic interaction because of the far less than 2.0 eV change for the ionic interaction in a protonated entity. Therefore, the peak shift of MCA/MoS2 hybrid sheets compared with MCA can be attributed to hydrogen bonding between MCA and MoS2 sheets. Thermal stability of MCA/MoS2 sheets and the content of MCA in the hybrid sheets were evaluated by TGA. In Fig. 5a, the pristine MoS2 is thermally stable under nitrogen atmosphere. The MCA
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Fig. 2. TEM images of (a) bare MoS2 sheets, (b) MA/MoS2 , (c) MCA and (d) MCA/MoS2 hybrid sheets. SEM images of (e) MCA, (f) MCA/MoS2 and (g) EDS result of MCA/MoS2 hybrid sheets.
exhibits an obvious weight loss from 300 to 400 ◦ C, caused by the elimination of NH3 . The TG curve of MCA/MoS2 exhibits a similar weight loss feature to that of MCA, due to the thermal degradation of MCA capsular layer. After calculation, the content of MCA on the surface of MoS2 sheets is about 70 wt%. The influence of MoS2 on the thermal oxidative stability of MCA was recorded in Fig. 5b. The weight loss of MoS2 at 650 ◦ C is well consistent with the transform of MoS2 to MoO3 and SO2 in air atmosphere. The MCA displays a main weight loss step corresponding to the release of CO2 , water and gas products arising from the decomposition of the MCA supermolecules. For the resulting MCA/MoS2 sheets, the mass loss before 385 ◦ C can be ascribed to the degradation of wrapped MCA. In addition, a slight delay of the maximum decomposition temperature from 367.2 to 368.6 ◦ C can be observed after incorporating MoS2 sheets, which is within the experimental error of TGA test. 3.2. Fractured surface characteristic of MCA/MoS2 /PA6 nanocomposites In general, interfacial compatibility of nanofiller within polymeric molecule displays a crucial role in enhancing the perfor-
Fig. 3. FTIR spectra of MA, MoS2 , MA/MoS2 , MCA and MCA/MoS2 hybrid sheets.
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Fig. 4. (a) XPS spectra of MCA, MoS2 and MCA/MoS2 hybrid sheets and the high-resolution spectra of N 1s regions of MCA/MoS2 hybrid sheets. Table 1 Crystallization characteristics of PA6 and its various nanocomposites. Samples
Tc (◦ C)
Tm (◦ C)
Hm (J/g)
Xc (%)
PA6 PA6-MCA/MS-1 PA6-MCA/MS-2 PA6-MCA/MS-4 PA6-MCA-4
186.0 193.3 192.3 191.6 189.2
221.0 221.1 223.1 225.9 221.3
71.6 89.3 98.1 90.6 78.5
37.7 47.5 52.7 49.7 43.0
mances of composites. The fractured surfaces of PA6 composites were monitored by SEM. There is no obvious difference between the surfaces of PA6 and MCA/PA6 composites in Fig. 6a1 and b1, both of them presented the rough and large-grained morphologies. It indicates that adding MCA has little effect on the fractured surface characteristic of PA composites. By contrast, the surface became relatively smooth with small irregularity after the addition of MCA/MoS2 sheets (Fig. 6c1). No aggregations and pull-out of MCA/MoS2 sheets were observed, demonstrating the well dispersed and tightly embedded sheets within PA6 matrix. As stated above, the hydrogen bonding between MCA and PA6 matrix is the primary cause for the strong interfacial adhesion, which always results in the improvement for properties of polymer composites. Moreover, at high resolution, some well-defined lamellae and looming spherulites structure of PA6 and MCA/PA6 composites can be clearly seen in Fig. 6a2 and b2 (yellow cycles). However, similar characteristic became difficult to identify with the incorporation of MCA/MoS2 sheets in Fig. 6c2. The spherulite was quite small and rare, revealing the destruction and imperfection of the spherulites structure caused by incorporating MCA/MoS2 sheets. 3.3. Properties of MCA/MoS2 /PA6 nanocomposites 3.3.1. Crystallization behavior In general, the crystallization behavior plays an important role on the physical properties of semi-crystalline polymers. Therefore, it is necessary to further determine the influence of MCA/MoS2 sheets on the crystallization properties of PA6 nanocomposites. The non-isothermal crystallization behavior of MCA/MoS2 /PA6 nanocomposites were evaluated by DSC. The corresponding curves during second loop of PA6 and its nanocomposites are plotted in Fig. 7. Obviously, the melting temperature (Tm ) of PA6 composites gradually increased as raising the contents of MCA/MoS2 sheets, the maximum increment is 4.9 ◦ C (Table 1). In contrast, the addition of MCA does not produce visible effects on the melting tempera-
Fig. 5. TGA curves of MCA, MoS2 and MCA/MoS2 hybrid sheets under (a) nitrogen atmosphere and (b) air atmosphere.
ture of PA6. The lamellar effect of MCA/MoS2 sheets together with the strong interfacial adhesion could effectively hinder the motion of polymer chain segments to retard the melt of PA6 matrix. The crystallization temperature (Tc ) obtained from the exothermic peak (Fig. 7b) increased from 186.0 ◦ C of pure PA6 to 193.3 ◦ C of 1 wt% MCA/MoS2 sheets (Table 1). This considerable increase in Tc can be attributed to the heterogeneous nucleation induced by MCA/MoS2
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Fig. 6. SEM images of the fractured surfaces of (a) neat PA6, (b) PA6-MCA-4 and (c) PA6-MCA/MS-4 at different magnification respectively (for interpretation of the references to colour in the text, the reader is referred to the web version of this article.).
Fig. 7. DSC (a) the second melting and (b) cooling curves of MCA/MoS2 /PA6 nanocomposites.
sheets. As the loadings of MCA/MoS2 sheets increasing, there is a slight reduction in the Tc values of PA6 composites, which is caused by the opposite hindering effect of MCA/MoS2 sheets, especially at high additive amount. Notably, all of these characteristic parameters of PA6-MCA/MS-4 sample are superior to those of PA6MCA-4 sample, indicating the high efficiency of MCA/MoS2 sheets as nanoreinforcers in PA6 materials.
3.3.2. Thermal stability Thermal stability of PA6 composites were evaluated with TGA, the corresponding curves are shown in Fig. 8. All samples follow a main decomposition process according to DTG profile, which correspond to fission occurring in the backbone of PA6 chains. The temperature related to 10 wt% weight loss (T−10% ) is often applied to study the early degradation of polymeric composites. The T-10% and the temperature of maximum weight loss (Tmax ) for bare PA6 were 343.2 and 430.1 ◦ C, respectively. When the MCA was incor-
Table 2 TGA data for PA6 and its various nanocomposites in air. Samples
T−10% (◦ C)
Tmax (◦ C)
Char residue at 600 ◦ C (wt%)
PA6 PA6-MCA/MS-1 PA6-MCA/MS-2 PA6-MCA/MS-4 PA6-MCA-4
343.2 369.8 371.6 347.9 322.8
430.1 430.2 430.9 435.3 425.8
0.5 0.8 0.9 1.1 1.3
porated, the T−10% and Tmax values of PA6 composites obviously decreased to 322.8 and 425.8 ◦ C (Table 2). This results is mainly attributed to the weak bond-breakage decomposition between amide groups of PA6 and melamine or cyanuric acid [11]. Inclusion of 2 wt% MCA/MoS2 sheets led to a 28.4 ◦ C increase in T−10% of PA6 nanocomposites, indicative of the physical barrier effect of MCA/MoS2 sheets during decomposition. Compared to the PA6MCA-4 sample, an obvious improvement on the Tmax of PA6-MCA-4
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Fig. 8. (a) TG and (b) DTG curves of MCA/MoS2 /PA6 nanocomposites under air atmosphere.
Fig. 9. (a) HRR and (b) THR curves of neat PA6 and its nanocomposites.
sample can be observed (from 425.8 to 435.3 ◦ C), which demonstrate the superiority of 2D sandwich-like MCA/MoS2 sheets in contrast to the traditional rod-like MCA flame retardant. 3.3.3. Flammability characteristic MCC was applied to directly measure the flammability of PA6 nanocomposites. The peak heat release rate (PHRR) is one of the most important index on evaluating the flame resistance, and a low value of PHRR is an indication of high flame retardancy [35,36]. In Fig. 9, the HRR curves of PA6 nanocomposites presented a single step, which is consistent with the degradation process of PA6 composites. In contrast, PA6 nanocomposites acquire reduction in flammability. Notably, the addition of 4 wt% MCA/MoS2 sheets results in the maximum reduction in PHRR (40%) and total heat release (THR) value (25%), respectively. In contrast, incorporating 4 wt% MCA within PA6 matrix reduces the flammability in limited, only 14% reduction in PHRR and 9% reduction in THR. This result reveals the high efficient of sandwich-like MCA/MoS2 sheets as flame retardants for PA6 materials. The specific gaseous products during thermal degradation process of PA6 nanocomposites was clarified by TG-IR technique. FT-IR spectra of PA6 composites at the maximum evolution rate were contrasted in Fig. 10. It is clearly shown that these three samples exhibit the similar absorption peaks, such as hydrocarbons,
CO2 and olefinic compounds [5]. It indicates that the addition of these two flame retardants has no effect on the thermal degradation behavior of PA6 material. However, after the incorporation of flame retardants, the intensities of major characteristic peaks for PA6 nanocomposite are decreased in contrast with that of neat PA6. Moreover, the absorption curves of characteristic gaseous products for PA6 and its two composites versus temperature are depicted in Fig. 11. Obviously, the absorbance intensities of flammable products for neat PA6 are higher than those of flame retarded PA6 composite, including hydrocarbons (Fig. 11a) and carbonyl compounds (Fig. 11c). The maximum reduction in the flammable volatiles was obtained by incorporating sandwich-like MCA/MoS2 sheets. Meanwhile, the release of CO2 was also suppressed by the addition of flame retardants (Fig. 11b), especially in MCA/MoS2 /PA6 nanocomposites. These results are ascribed to the synergistic fire retarding effect between the two components of MCA/MoS2 sheets. Moreover, absorbance intensity related to nonflammable NH3 at 964 cm−1 of PA6 composites obviously increased in contrasted to that of neat PA6 (Fig. 11d), demonstrating the flammable products might be diluted by NH3 in gas phase. 3.3.4. Fire hazards SSTF is a specifically developed technique to monitor the smoke density and toxic concentration in real-time. The significant advan-
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Fig. 10. FT-IR spectra of pyrolysis products for PA6, PA6-MCA-4 and PA6-MCA/MS-4 at the maximum decomposition rate.
tage of this apparatus over other techniques is its capability to replicate each stage of fire development [30]. The O2 concentration and the transmittance of smoke product versus time curves of PA6 nanocomposites are displayed in Fig. 12. As it is well known,
the O2 consumption is directly related to the heat release during the combustion process of polymeric materials [37,38]. The lower O2 consumption is an indicative of higher flame retardancy. Obviously, PA6-MCA/MS-4 sample exhibits the maximum reduction of O2 consumption and smoke density among these three samples. Compared to solely changing the thermal decomposition pathway of PA6 matrix caused by adding MCA, synergistic effect with the physical barrier effect of characteristic 2D structure is the primary reason for the enhancement on the flame retardancy of PA nanocomposites. To clarify the integrated flame retarding effect of sandwichlike MCA/MoS2 sheets on PA6 materials, the morphologies of char residues were studied by macro and micro analysis, and the corresponding digital photos and SEM images are shown in Fig. 13. The constant samples are placed in a muffle furnace in air at 600 ◦ C for 15 min. As presented in Fig. 13a, the neat PA6 material has been almost burned out and a small amount of fragile and flaky char residue can be observed by SEM. As for the MCA/PA6 composites, there are some char granules left in the crucible, but which is still loose and cracked (Fig. 13b) in high resolution. In the case of MCA/MoS2 /PA6 composites, the char residue obviously increased, which exhibits a whole layer without holes left on the surface in Fig. 13c. Moreover, a quite dense and smooth surface can be observed at high resolution, confirming the combined char-forming effect between MCA and MoS2 sheets.
Fig. 11. Comparison of the absorbance of pyrolysis products between neat PA6 and its nanocomposites.
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Fig. 12. The curves of (a) O2 concentration, (b) total O2 consumption, (c) transmittance of smoke and (d) total smoke production versus time of PA6 and its composites.
3.3.5. Properties reinforcing mechanism Based on the aforementioned analysis, a possible route of the reinforcing mechanism of sandwich-like MCA/MoS2 sheets within PA6 matrix is postulated in Fig. 14. Compared to the rod-like MCA, the sandwich-like MCA/MoS2 sheets not only possessed the intrinsic flame retardant effect of MCA, but also exhibited the integrated flame retardant mechanism in PA6. The high specific surface area of 2D MCA/MoS2 sheets brought more hydrogenbonding interactions between MCA and PA6, thus leaded to the improved fire retarding effect of MCA. Moreover, the well dispersed MCA/MoS2 sheets with strong interfacial adhesion to PA6 matrix dramatically hindered the thermal motion of PA6 molecular chains, which can effectively delay the thermal decomposition of PA6 composites, thus improving the fire safety of PA6 composites. In addition, during the predominant weak bond-breakage degradation of MCA/PA6 system, the remaining thermal stable MoS2 sheets acted as a physical barrier to restrict the evolution of flammable gases, and held the smoke particles together to form a compact and adiathermic char shield on the surface of inner materials, the suppression of which on heat and mass transfer can considerably reduce the flammability and smoke yield of PA6 materials. Furthermore, the catalytic performance of Mo element within MoS2 sheets also exhibited the positive effect on reducing the smoke production, demonstrating the indispensability in the integrated
flame retarding effect of MCA/MoS2 hybrid sheets on PA6 composites. 4. Conclusions In this work, sandwich-like MCA/MoS2 hybrid sheets were successfully synthesized by utilizing a supramolecular polymerization of MA and CA on 2D MoS2 sheet templates. With this design, MCA/MoS2 hybrids do not only possess the high specific surface area of 2D structure but also exhibit the integrated flame-retarding effect in PA6 materials. Compared to bare rod-like MCA, more hydrogen-bonding interactions between well dispersed MCA/MoS2 hybrid sheets and PA6 matrix leaded to the improved thermal oxidative stability of PA6 nanocomposites. In addition, MCA/MoS2 hybrid sheets exhibited considerable fire hazards suppression performance in terms of reduced PHRR value (decreased by 40%) and low smoke yield (decreased by 24%), as well as formation of protective char layer. These excellent enhancements on fire safety of PA6 can be attributed to the integration of inherent flame retarding effect, physical barrier performance and catalytic activity of sandwich-like MCA/MoS2 hybrid sheets. Moreover, the crystallization behaviors of PA6 nanocomposites also were positively influenced by adding sandwich-like hybrid sheets, highlighting their promising potential in actual industrial demand. What’s more,
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Fig. 13. Digital photos and SEM images of the char residues of (a) neat PA6, (b) PA6-MCA-4 and (c) PA6-MCA/MS-4 at different magnification respectively.
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Fig. 14. Illustration for the integrated flame-retardant mechanism for the sandwich-like MCA/MoS2 hybrid sheets in PA6.
this facile approach may be favorable to purposeful design of functionalized MoS2 -based hybrids for achieving their whole potential in polymer nanocomposites.
Acknowledgements The work was financially supported by the National Basic Research Program of China (973 Program) (2012CB719701), the National Natural Science Foundation of China (No. 21374111), the Fundamental Research Funds for the Central Universities (WK2320000029) and the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. 9042047, CityU 11208914).
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Integrated effect of supramolecular self-assembled sandwich-like melamine cyanurate/MoS2 hybrid sheets on reducing fire hazards of polyamide 6 composites Xiaming Feng a,b , Xin Wang a,∗∗ , Wei Cai a , Ningning Hong a , Yuan Hu a,b,∗ , Kim Meow Liew b,c a
State Key Laboratory of Fire Science, University of Science and Technology of China, Anhui 230026, PR China Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute for Advanced Study, University of Science and Technology of China, Suzhou, Jiangsu 215123, PR China c Department of Architectural and Civil Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong b
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Novel
melamine cyanurate/MoS2 sheets were prepared by supramolecular self-assembly. • MoS2 sheets function as a template, synergist and smoke suppressor in MCA/MoS2 /PA6. • Incorporating MCA/MoS2 sheets obviously reduced the fire hazards of PA6 composites. • Integrated effect is crucial for the enhancements on fire safety of PA6 composites.
a r t i c l e
i n f o
Article history: Received 4 July 2016 Received in revised form 2 August 2016 Accepted 4 August 2016 Available online 5 August 2016 Keywords: Supramolecular self-assembly Molybdenum disulfide Sandwich-like hybrid sheets Fire hazards Catalytic activity
a b s t r a c t A novel strategy of using supramolecular self-assembly for preparing sandwich-like melamine cyanurate/MoS2 sheets as the hybrid flame retardants for polyamide 6 (PA6) is reported for the first time. The introduction of MoS2 sheets function not only as a template to induce the formation of two-dimensional melamine cyanurate capping layers but also as a synergist to generate integrated flame-retarding effect of hybrid sheets, as well as a high-performance smoke suppressor to reduce fire hazards of PA6 materials. Once incorporating this well-designed structures (4 wt%) into PA6 matrix, there resulted in a remarkable drop (40%) in the peak heat release rate and a 25% reduction in total heat release. Moreover, the smoke production and pyrolysis gaseous products were efficiently suppressed by the addition of sandwich-like hybrid sheets. The integrated
∗ Corresponding author at: State Key Laboratory of Fire Science, University of Science and Technology of China, Anhui 230026, PR China. ∗∗ Corresponding author. E-mail addresses: [email protected] (X. Wang), [email protected] (Y. Hu). http://dx.doi.org/10.1016/j.jhazmat.2016.08.012 0304-3894/© 2016 Elsevier B.V. All rights reserved.
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functions consisting of inherent flame retarding effect, physical barrier performance and catalytic activity are believed to the crucial guarantee for the reduced fire hazards of PA6 nanocomposites. Furthermore, this novel strategy with facile and scalable features may provide reference for developing various kinds of MoS2 based hybrid sheets for diverse applications. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Polyamide 6 as an important engineering plastic has been widely used because of its excellent performances, such as superior mechanical property, abrasion resistance, electrical characteristic and oil proof performance [1,2]. Therefore, it stimulated dramatical interest in reducing the high fire risk of PA6 products [3]. Moreover, large quantities of dense smoke and toxic gaseous products will generate during the combustion of PA6 materials, generally, inhalation of which is the main cause of death in fires [4,5]. Therefore, the high-efficiency but harmful halogen-containing flame retardants have been replaced with more environment-friendly flame retardants in many applications by the legal compulsion. Over the past decades, a massive efforts have been made to enhance the fire resistance of PA6 by incorporating halogen-free flame retardants [6–8]. As a kind of environmentally friendly, non-corrodible and high nitrogen-containing flame retardants, melamine cyanurate (MCA) has drawn more and more attention in the development of flame retarded PA6 composites [9–11]. In general, MCA are synthesized by a supramolecular selfassembly process in aqueous phase, which result in an extensive two-dimensional (2D) network of hydrogen bonds between melamine (MA) and cyanuric acid (CA) [12]. Limited by the oversimplified flame retardant mechanism, the effect of MCA on flame resistance of PA6 matrix is actually modest. In previous reports [10], about 15 wt% MCA are needed to achieve the flame retarded PA6 composite with UL94 V-0 rating. Moreover, the high addition level of MCA always leads to a damage on the physical properties of PA6. Hence, improving the flame retardant efficiency of MCA is quite necessary. MCA with different morphologies has been prepared to reinforce the flame retardancy of PA6 [13]. The authors indicate that nanoscale 2D MCA sheets exhibited a better performance in contrast to other morphologies due to the more hydrogen-bonding interactions between well dispersed MCA and PA6, which derived from the high specific surface area of 2D structure. However, it still cannot overcome the inherent shortcoming of MCA. As well known, integrating two or more constituents could always display a synergistic effect and result in an extraordinary improvement in properties of polymeric composites [14–16]. As a typical inorganic 2D flame retardant, montmorillonite (MMT) was formulated with MCA to fabricate the flame retarded of PA6, in which the synergistic effect between MCA and MMT were proposed [10]. Bulk MoS2 , a classic layered compound, has served as solid lubricants for decades [17–19]. In recent years, 2D MoS2 sheet has drawn much attention for the outstanding semiconductor performances and excellent catalytic properties [20,21]. As reported before [22–24], MoS2 sheets do not only possess the integrated flame-retarding effect but also exhibit a positive effect on the other properties of polymeric materials, which is regarded as a promising candidate to traditional inorganic layered flame retardants. In our previous reports [14,25,26], MoS2 sheets were performed to enhance the mechanical behaviors, thermal stabilities and crystallization performances, as well as the fire safety of polymer nanocomposites. It indicates that the dispersion, interfacial compatibility and inherent characters of MoS2 sheets play an important role to achieve the satisfactory improvements in properties of poly-
mer nanocomposites. More recently, Wu et al. demonstrated the synergistic flame-retarding effect between P-N flame retardants and commercial MoS2 within thermoplastic poly(ether ester) elastomers, in which the catalytic effect of Mo element through a cycloaddition interaction of gaseous decomposition products acts as a key factor in char forming process [27]. In previous report [28,29], the authors indicated that nano-MoS2 was an efficient filler to reinforce the wear performance and mechanical property of PA6 composites. To our knowledge, there is no report yet about the synergistic effect between MCA and 2D MoS2 sheets as well as the flame retardant properties enhancements of PA6 nanocomposites. In the present paper, a novel yet very effective strategy to achieve above goal through in situ fabrication of sandwich-like MCA/MoS2 hybrid sheets by a supramolecular self-assembly process has been demonstrated, as shown in Scheme 1. Benefiting from the integrated effect of sandwich-like structure, the hybrid sheets present an impressive performance in reducing fire hazards of PA6 materials. In addition, this simple approach is appropriate for mass production, implying the substantial promotion in developing MoS2 based flame retardants.
2. Experiment section 2.1. Materials Melamine, cyanuric acid, MoS2 , n-hexane and formic acid were supplied by Sinopharm Chemical Reagent Co., Ltd. (China). PA 6 (1003NW8) was supplied by Yubu Company (Japan). n-Butyl lithium (2.5 M in hexane) was provided by Aladdin Industrial Corporation (China).
2.2. Preparation of MA/MoS2 and MCA/MoS2 materials MA attached on MoS2 sheets (MA/MoS2 ) were fabricated by a simple solution mixing of MA and exfoliated MoS2 . In brief, 1 g MoS2 powder was reacted with n-butyl lithium in hexane (8 mL, 2.5 M) in a solvothermal synthesis reactor (100 mL) at 95 ◦ C for 5 h to prepare the lithium intercalated MoS2 (Lix MoS2 ). 0.5 g Lix MoS2 was hydrolyzed in 1000 mL deionized water by sonication to obtain MoS2 suspension [22]. Then, 1 g MA was dissolved in the MoS2 suspension under stirring at 80 ◦ C for 12 h. The products were collected through vacuum filtration and washed by hot water to remove the non-reacted reactants, further dried in an oven at 80 ◦ C. Sandwich-like MCA/MoS2 hybrid sheets (MCA/MoS2 ) were synthesized by supramolecular polymerization of MA and CA in the MoS2 suspension. In order to get the sandwich-like sheets rather than the other morphologies, we have tried different raw material ratios to define the best ration. In detail, 0.58 g melamine was added into the above MoS2 suspension with continuous stirring at 80 ◦ C. Then, 100 mL CA aqueous solution (6 mg mL−1 ) was poured in the above suspension and further stirred at 95 ◦ C for 10 h. At last, the products were filtered, washed and dried at 80 ◦ C overnight. In addition, MCA was synthesized by the same procedure without the incorporation of MoS2 sheets.
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Scheme 1. (a) Schematic for preparation of MCA by supramolecular polymerization of MA and CA. (b) Schematic illustration of the fabrication procedure of sandwich-like MCA/MoS2 hybrid sheets.
2.3. Preparation of MCA/MoS2 /PA6 nanocomposites MCA/MoS2 /PA6 nanocomposites were fabricated through a solution blending technique. For example, 0.8 g MCA/MoS2 was dispersed in formic acid assisting by sonication to obtain a uniform dispersion. Then, 19.2 g PA6 was added into the MCA/MoS2 dispersion with vigorous stirring for 60 min, and dried in a plate at 80 ◦ C for 12 h in the vacuum oven to completely remove solvents.
2.4. Characterization Fourier transform infrared (FT-IR) spectra were performed by a Nicolet 6700 spectrometer (Nicolet Instrument Corporation, U.S.). X-ray photoelectron spectroscopy (XPS) was conducted using a VGESCALB MK-II electron spectrometer (Al K␣ excitation source at 1486.6 eV). X-ray diffraction (XRD) was monitored using a Japan Rigaku D/Max-Ra rotating-anode X-ray diffractometer equipped with a Cu-K␣ tube and a Ni filter ( = 0.1542 nm). Transmission electron microscopy (TEM) (JEM-2100F, Japan Electron Optics Laboratory Co., Ltd.) was employed to study the micro-morphologies and the accelerating voltage was 200 kV. Morphologies of samples were observed by a PHILIPS XL30E scanning electron microscope (SEM). The specimens were cryogenically fractured in liquid nitrogen first and then sputter-coated with a conductive layer. Differential scanning calorimetry (DSC) (Q2000 TA Instruments Inc., USA) was conducted to characterize crystallization behaviors of samples. The PA6 composites were heated from 30 to 280 ◦ C (for 10 min), then cooled to 30 ◦ C at a rate of 10 ◦ C min−1 . The heatingcooling cycle was performed again.
Thermogravimetric analysis (TGA) was carried out using a Q5000 thermoanalyzer instrument (TA Instruments Inc., U.S.) under different flows of 25 mL min−1 and heated from 30 to 800 ◦ C at a heating rate of 20 ◦ C min−1 . Microscale combustion colorimeter (MCC) was performed to test the heat release of PA6 nanocomposites according to ASTM D7309-07. 5 mg PA6 composites were heated at a heating rate of 1 ◦ C s−1 from 30 to 650 ◦ C. Thermogravimetric analysis-infrared spectrometry (TG-IR) (TGA Q5000IR thermogravimetric analyzer linked to a Nicolet 6700 FTIR spectrophotometer) was performed from 30 to 700 ◦ C at 20 ◦ C min−1 (nitrogen atmosphere). The fire hazard was monitored by a steady state tube furnace (SSTF) (ISO TS 19700), as shown in Scheme 2, which is a small-scale fire apparatus designed to replicate the different stages of fire. Fuel and air are introduced into the furnace at a certain ratio, based on the knowledge of the material composition and its stoichiometric oxygen requirement [30,31]. About 15 g granular samples were put in the tube furnace at a rate of 3 cm min−1 . A stream of air was injected into the furnace over the specimens to support combustion. The exhaust gases were extruded from the tube furnace to the mixing chamber and diluted by a secondary air flow, where gaseous product concentration and smoke production would be monitored.
3. Results and discussion 3.1. Characterization of MCA/MoS2 materials Scheme 1 outlines the overall preparation procedures for sandwich-like MCA/MoS2 sheets by supramolecular self-assembly method. First, MoS2 was exfoliated by an intercalation-hydrolysis
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Scheme 2. The diagrammatic illustration of the steady state tube furnace.
Fig. 1. XRD patterns of MA, MoS2 , MA/MoS2 , MCA/MoS2 hybrid sheets and standard XRD data for MCA (JCPDS file no. 00-005-0127).
method to obtain exfoliated MoS2 suspension. Our previous papers indicated the easily re-stack of MoS2 sheets during filtration and drying [32]. Then, melamine as the surface modifier and precursor, was spontaneously absorbed on MoS2 sheets through intermolecular and hydrogen-bonding interactions, which can protect MoS2 sheets from re-stacking and induce the formation of MCA layer wrapping on MoS2 sheets. Finally, the hydrogen bonding between MA and CA induces the supramolecular self-assembly process, causing the formation of sandwich-like MCA/MoS2 hybrid sheets. In the previous works [10,13], both the raw flake-like MCA and simply mixing with MMT cannot achieve the integrated flame retardant mechanism. On clear contrast, our proposed process could combine the flame retardant elements (N, Mo and S) and the hierarchical structures (physical barrier effect of MoS2 sheets and more hydrogen-bonding interactions between MCA and PA6) to achieve excellent flame retardancy of polymer composites. Systematic characterizations were conducted to measure the proposed strategy. The product was first examined by powder XRD, as depicted in Fig. 1. In contrast, the MA/MoS2 displays a significantly decreased peak around 14.5◦ corresponding to (002) reflection of MoS2 , indicating that MoS2 is well exfoliated instead of re-stacked with the assistance of absorbed MA molecules. After in situ polymerized with CA, the major diffraction peaks of MCA/MoS2 exhibit a crystalline structure, which is consistent with the standard card of MCA (JCPDS file No. 00-005-0217). The peak intensity of (002) reflection of MoS2 in MCA/MoS2 materials is obviously decreased, demonstrating that face-to-face stacking of MoS2 flakes is obviously prevented by the wrapped MCA molecules.
The morphologies and microstructures of obtained materials were observed by TEM. The exfoliated MoS2 displays the flaky texture and wrinkled structure (Fig. 2a). As for the MA/MoS2 (Fig. 2b), the dense black dots decorated on the surface of MoS2 sheets confirmed the successful attachment of MA molecules. TEM and SEM image of MCA (Fig. 2c and e) shows the rod-like MCA supramolecular crystals with quite smooth surfaces, the sizes of which range from hundreds of nanometers to several micrometers. Notably, these rod-like supramolecular crystals disappear in the TEM and SEM images of MCA/MoS2 materials (Fig. 2d and f), instead, the MCA/MoS2 exhibits a lamellar structure similar with that of exfoliated MoS2 . Since the MA molecules were attached on the surfaces of MoS2 sheets, the MA/MoS2 sheets acted as a template to synthesis the sandwich-like MCA/MoS2 hybrid sheets during the supramolecular self-assembly. Moreover, the EDS spectrum confirmed the characteristic element composition of MCA/MoS2 sheets (Fig. 2g). Fig. 3 displays the FTIR spectra of these samples. As for raw MoS2 , there is only weak water bonding at 1635 cm−1 and wide stretching band of OH around 3440 cm−1 could be detected. After modified by MA, some new absorption peaks assigned to MA molecules can be observed, indicating the successful attachment of MA molecules on MoS2 sheets. After in situ polymerization, the MCA/MoS2 sheets exhibit the exactly similar spectrum to that of pure MCA. In detail, the strong band at 3388 cm−1 is ascribed to NH2 symmetric stretching of triazine groups; the peaks at 3222 and 3038 cm−1 are due to the formation of hydrogen-bond within NH2 /NH moieties. The peaks related to C N and C N groups can be recognized by the frequencies of 1446 cm−1 and 1533 cm−1 . The strong peaks at 1736 and 1658 cm−1 are corresponding to NH2 scissoring and NH2 bending vibration, respectively [9,33]. The FTIR results confirmed the successful preparation of MCA molecules by this method. XPS analysis was performed to examine the surface compositions and atomic-level interaction of MCA/MoS2 sheets. The XPS spectra in Fig. 4a demonstrates that the hybrid sheets mainly consist of C, O, N, S and Mo elements without impurity elements. By comparing the high-resolution N 1s XPS spectra (Fig. 4b), an obvious upshift for N 1s peak of MCA/MoS2 sheets can be observed. In general, the formation of hydrogen-bond or protonated nitrogen always results in an upshift of the binding energy of N 1s because of the reduction of electron density in nitrogen atoms [34]. In this case, it is unlikely to be ionic interaction because of the far less than 2.0 eV change for the ionic interaction in a protonated entity. Therefore, the peak shift of MCA/MoS2 hybrid sheets compared with MCA can be attributed to hydrogen bonding between MCA and MoS2 sheets. Thermal stability of MCA/MoS2 sheets and the content of MCA in the hybrid sheets were evaluated by TGA. In Fig. 5a, the pristine MoS2 is thermally stable under nitrogen atmosphere. The MCA
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Fig. 2. TEM images of (a) bare MoS2 sheets, (b) MA/MoS2 , (c) MCA and (d) MCA/MoS2 hybrid sheets. SEM images of (e) MCA, (f) MCA/MoS2 and (g) EDS result of MCA/MoS2 hybrid sheets.
exhibits an obvious weight loss from 300 to 400 ◦ C, caused by the elimination of NH3 . The TG curve of MCA/MoS2 exhibits a similar weight loss feature to that of MCA, due to the thermal degradation of MCA capsular layer. After calculation, the content of MCA on the surface of MoS2 sheets is about 70 wt%. The influence of MoS2 on the thermal oxidative stability of MCA was recorded in Fig. 5b. The weight loss of MoS2 at 650 ◦ C is well consistent with the transform of MoS2 to MoO3 and SO2 in air atmosphere. The MCA displays a main weight loss step corresponding to the release of CO2 , water and gas products arising from the decomposition of the MCA supermolecules. For the resulting MCA/MoS2 sheets, the mass loss before 385 ◦ C can be ascribed to the degradation of wrapped MCA. In addition, a slight delay of the maximum decomposition temperature from 367.2 to 368.6 ◦ C can be observed after incorporating MoS2 sheets, which is within the experimental error of TGA test. 3.2. Fractured surface characteristic of MCA/MoS2 /PA6 nanocomposites In general, interfacial compatibility of nanofiller within polymeric molecule displays a crucial role in enhancing the perfor-
Fig. 3. FTIR spectra of MA, MoS2 , MA/MoS2 , MCA and MCA/MoS2 hybrid sheets.
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Fig. 4. (a) XPS spectra of MCA, MoS2 and MCA/MoS2 hybrid sheets and the high-resolution spectra of N 1s regions of MCA/MoS2 hybrid sheets. Table 1 Crystallization characteristics of PA6 and its various nanocomposites. Samples
Tc (◦ C)
Tm (◦ C)
Hm (J/g)
Xc (%)
PA6 PA6-MCA/MS-1 PA6-MCA/MS-2 PA6-MCA/MS-4 PA6-MCA-4
186.0 193.3 192.3 191.6 189.2
221.0 221.1 223.1 225.9 221.3
71.6 89.3 98.1 90.6 78.5
37.7 47.5 52.7 49.7 43.0
mances of composites. The fractured surfaces of PA6 composites were monitored by SEM. There is no obvious difference between the surfaces of PA6 and MCA/PA6 composites in Fig. 6a1 and b1, both of them presented the rough and large-grained morphologies. It indicates that adding MCA has little effect on the fractured surface characteristic of PA composites. By contrast, the surface became relatively smooth with small irregularity after the addition of MCA/MoS2 sheets (Fig. 6c1). No aggregations and pull-out of MCA/MoS2 sheets were observed, demonstrating the well dispersed and tightly embedded sheets within PA6 matrix. As stated above, the hydrogen bonding between MCA and PA6 matrix is the primary cause for the strong interfacial adhesion, which always results in the improvement for properties of polymer composites. Moreover, at high resolution, some well-defined lamellae and looming spherulites structure of PA6 and MCA/PA6 composites can be clearly seen in Fig. 6a2 and b2 (yellow cycles). However, similar characteristic became difficult to identify with the incorporation of MCA/MoS2 sheets in Fig. 6c2. The spherulite was quite small and rare, revealing the destruction and imperfection of the spherulites structure caused by incorporating MCA/MoS2 sheets. 3.3. Properties of MCA/MoS2 /PA6 nanocomposites 3.3.1. Crystallization behavior In general, the crystallization behavior plays an important role on the physical properties of semi-crystalline polymers. Therefore, it is necessary to further determine the influence of MCA/MoS2 sheets on the crystallization properties of PA6 nanocomposites. The non-isothermal crystallization behavior of MCA/MoS2 /PA6 nanocomposites were evaluated by DSC. The corresponding curves during second loop of PA6 and its nanocomposites are plotted in Fig. 7. Obviously, the melting temperature (Tm ) of PA6 composites gradually increased as raising the contents of MCA/MoS2 sheets, the maximum increment is 4.9 ◦ C (Table 1). In contrast, the addition of MCA does not produce visible effects on the melting tempera-
Fig. 5. TGA curves of MCA, MoS2 and MCA/MoS2 hybrid sheets under (a) nitrogen atmosphere and (b) air atmosphere.
ture of PA6. The lamellar effect of MCA/MoS2 sheets together with the strong interfacial adhesion could effectively hinder the motion of polymer chain segments to retard the melt of PA6 matrix. The crystallization temperature (Tc ) obtained from the exothermic peak (Fig. 7b) increased from 186.0 ◦ C of pure PA6 to 193.3 ◦ C of 1 wt% MCA/MoS2 sheets (Table 1). This considerable increase in Tc can be attributed to the heterogeneous nucleation induced by MCA/MoS2
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Fig. 6. SEM images of the fractured surfaces of (a) neat PA6, (b) PA6-MCA-4 and (c) PA6-MCA/MS-4 at different magnification respectively (for interpretation of the references to colour in the text, the reader is referred to the web version of this article.).
Fig. 7. DSC (a) the second melting and (b) cooling curves of MCA/MoS2 /PA6 nanocomposites.
sheets. As the loadings of MCA/MoS2 sheets increasing, there is a slight reduction in the Tc values of PA6 composites, which is caused by the opposite hindering effect of MCA/MoS2 sheets, especially at high additive amount. Notably, all of these characteristic parameters of PA6-MCA/MS-4 sample are superior to those of PA6MCA-4 sample, indicating the high efficiency of MCA/MoS2 sheets as nanoreinforcers in PA6 materials.
3.3.2. Thermal stability Thermal stability of PA6 composites were evaluated with TGA, the corresponding curves are shown in Fig. 8. All samples follow a main decomposition process according to DTG profile, which correspond to fission occurring in the backbone of PA6 chains. The temperature related to 10 wt% weight loss (T−10% ) is often applied to study the early degradation of polymeric composites. The T-10% and the temperature of maximum weight loss (Tmax ) for bare PA6 were 343.2 and 430.1 ◦ C, respectively. When the MCA was incor-
Table 2 TGA data for PA6 and its various nanocomposites in air. Samples
T−10% (◦ C)
Tmax (◦ C)
Char residue at 600 ◦ C (wt%)
PA6 PA6-MCA/MS-1 PA6-MCA/MS-2 PA6-MCA/MS-4 PA6-MCA-4
343.2 369.8 371.6 347.9 322.8
430.1 430.2 430.9 435.3 425.8
0.5 0.8 0.9 1.1 1.3
porated, the T−10% and Tmax values of PA6 composites obviously decreased to 322.8 and 425.8 ◦ C (Table 2). This results is mainly attributed to the weak bond-breakage decomposition between amide groups of PA6 and melamine or cyanuric acid [11]. Inclusion of 2 wt% MCA/MoS2 sheets led to a 28.4 ◦ C increase in T−10% of PA6 nanocomposites, indicative of the physical barrier effect of MCA/MoS2 sheets during decomposition. Compared to the PA6MCA-4 sample, an obvious improvement on the Tmax of PA6-MCA-4
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Fig. 8. (a) TG and (b) DTG curves of MCA/MoS2 /PA6 nanocomposites under air atmosphere.
Fig. 9. (a) HRR and (b) THR curves of neat PA6 and its nanocomposites.
sample can be observed (from 425.8 to 435.3 ◦ C), which demonstrate the superiority of 2D sandwich-like MCA/MoS2 sheets in contrast to the traditional rod-like MCA flame retardant. 3.3.3. Flammability characteristic MCC was applied to directly measure the flammability of PA6 nanocomposites. The peak heat release rate (PHRR) is one of the most important index on evaluating the flame resistance, and a low value of PHRR is an indication of high flame retardancy [35,36]. In Fig. 9, the HRR curves of PA6 nanocomposites presented a single step, which is consistent with the degradation process of PA6 composites. In contrast, PA6 nanocomposites acquire reduction in flammability. Notably, the addition of 4 wt% MCA/MoS2 sheets results in the maximum reduction in PHRR (40%) and total heat release (THR) value (25%), respectively. In contrast, incorporating 4 wt% MCA within PA6 matrix reduces the flammability in limited, only 14% reduction in PHRR and 9% reduction in THR. This result reveals the high efficient of sandwich-like MCA/MoS2 sheets as flame retardants for PA6 materials. The specific gaseous products during thermal degradation process of PA6 nanocomposites was clarified by TG-IR technique. FT-IR spectra of PA6 composites at the maximum evolution rate were contrasted in Fig. 10. It is clearly shown that these three samples exhibit the similar absorption peaks, such as hydrocarbons,
CO2 and olefinic compounds [5]. It indicates that the addition of these two flame retardants has no effect on the thermal degradation behavior of PA6 material. However, after the incorporation of flame retardants, the intensities of major characteristic peaks for PA6 nanocomposite are decreased in contrast with that of neat PA6. Moreover, the absorption curves of characteristic gaseous products for PA6 and its two composites versus temperature are depicted in Fig. 11. Obviously, the absorbance intensities of flammable products for neat PA6 are higher than those of flame retarded PA6 composite, including hydrocarbons (Fig. 11a) and carbonyl compounds (Fig. 11c). The maximum reduction in the flammable volatiles was obtained by incorporating sandwich-like MCA/MoS2 sheets. Meanwhile, the release of CO2 was also suppressed by the addition of flame retardants (Fig. 11b), especially in MCA/MoS2 /PA6 nanocomposites. These results are ascribed to the synergistic fire retarding effect between the two components of MCA/MoS2 sheets. Moreover, absorbance intensity related to nonflammable NH3 at 964 cm−1 of PA6 composites obviously increased in contrasted to that of neat PA6 (Fig. 11d), demonstrating the flammable products might be diluted by NH3 in gas phase. 3.3.4. Fire hazards SSTF is a specifically developed technique to monitor the smoke density and toxic concentration in real-time. The significant advan-
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Fig. 10. FT-IR spectra of pyrolysis products for PA6, PA6-MCA-4 and PA6-MCA/MS-4 at the maximum decomposition rate.
tage of this apparatus over other techniques is its capability to replicate each stage of fire development [30]. The O2 concentration and the transmittance of smoke product versus time curves of PA6 nanocomposites are displayed in Fig. 12. As it is well known,
the O2 consumption is directly related to the heat release during the combustion process of polymeric materials [37,38]. The lower O2 consumption is an indicative of higher flame retardancy. Obviously, PA6-MCA/MS-4 sample exhibits the maximum reduction of O2 consumption and smoke density among these three samples. Compared to solely changing the thermal decomposition pathway of PA6 matrix caused by adding MCA, synergistic effect with the physical barrier effect of characteristic 2D structure is the primary reason for the enhancement on the flame retardancy of PA nanocomposites. To clarify the integrated flame retarding effect of sandwichlike MCA/MoS2 sheets on PA6 materials, the morphologies of char residues were studied by macro and micro analysis, and the corresponding digital photos and SEM images are shown in Fig. 13. The constant samples are placed in a muffle furnace in air at 600 ◦ C for 15 min. As presented in Fig. 13a, the neat PA6 material has been almost burned out and a small amount of fragile and flaky char residue can be observed by SEM. As for the MCA/PA6 composites, there are some char granules left in the crucible, but which is still loose and cracked (Fig. 13b) in high resolution. In the case of MCA/MoS2 /PA6 composites, the char residue obviously increased, which exhibits a whole layer without holes left on the surface in Fig. 13c. Moreover, a quite dense and smooth surface can be observed at high resolution, confirming the combined char-forming effect between MCA and MoS2 sheets.
Fig. 11. Comparison of the absorbance of pyrolysis products between neat PA6 and its nanocomposites.
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Fig. 12. The curves of (a) O2 concentration, (b) total O2 consumption, (c) transmittance of smoke and (d) total smoke production versus time of PA6 and its composites.
3.3.5. Properties reinforcing mechanism Based on the aforementioned analysis, a possible route of the reinforcing mechanism of sandwich-like MCA/MoS2 sheets within PA6 matrix is postulated in Fig. 14. Compared to the rod-like MCA, the sandwich-like MCA/MoS2 sheets not only possessed the intrinsic flame retardant effect of MCA, but also exhibited the integrated flame retardant mechanism in PA6. The high specific surface area of 2D MCA/MoS2 sheets brought more hydrogenbonding interactions between MCA and PA6, thus leaded to the improved fire retarding effect of MCA. Moreover, the well dispersed MCA/MoS2 sheets with strong interfacial adhesion to PA6 matrix dramatically hindered the thermal motion of PA6 molecular chains, which can effectively delay the thermal decomposition of PA6 composites, thus improving the fire safety of PA6 composites. In addition, during the predominant weak bond-breakage degradation of MCA/PA6 system, the remaining thermal stable MoS2 sheets acted as a physical barrier to restrict the evolution of flammable gases, and held the smoke particles together to form a compact and adiathermic char shield on the surface of inner materials, the suppression of which on heat and mass transfer can considerably reduce the flammability and smoke yield of PA6 materials. Furthermore, the catalytic performance of Mo element within MoS2 sheets also exhibited the positive effect on reducing the smoke production, demonstrating the indispensability in the integrated
flame retarding effect of MCA/MoS2 hybrid sheets on PA6 composites. 4. Conclusions In this work, sandwich-like MCA/MoS2 hybrid sheets were successfully synthesized by utilizing a supramolecular polymerization of MA and CA on 2D MoS2 sheet templates. With this design, MCA/MoS2 hybrids do not only possess the high specific surface area of 2D structure but also exhibit the integrated flame-retarding effect in PA6 materials. Compared to bare rod-like MCA, more hydrogen-bonding interactions between well dispersed MCA/MoS2 hybrid sheets and PA6 matrix leaded to the improved thermal oxidative stability of PA6 nanocomposites. In addition, MCA/MoS2 hybrid sheets exhibited considerable fire hazards suppression performance in terms of reduced PHRR value (decreased by 40%) and low smoke yield (decreased by 24%), as well as formation of protective char layer. These excellent enhancements on fire safety of PA6 can be attributed to the integration of inherent flame retarding effect, physical barrier performance and catalytic activity of sandwich-like MCA/MoS2 hybrid sheets. Moreover, the crystallization behaviors of PA6 nanocomposites also were positively influenced by adding sandwich-like hybrid sheets, highlighting their promising potential in actual industrial demand. What’s more,
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Fig. 13. Digital photos and SEM images of the char residues of (a) neat PA6, (b) PA6-MCA-4 and (c) PA6-MCA/MS-4 at different magnification respectively.
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Fig. 14. Illustration for the integrated flame-retardant mechanism for the sandwich-like MCA/MoS2 hybrid sheets in PA6.
this facile approach may be favorable to purposeful design of functionalized MoS2 -based hybrids for achieving their whole potential in polymer nanocomposites.
Acknowledgements The work was financially supported by the National Basic Research Program of China (973 Program) (2012CB719701), the National Natural Science Foundation of China (No. 21374111), the Fundamental Research Funds for the Central Universities (WK2320000029) and the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. 9042047, CityU 11208914).
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