Cooperative Effect of ZIF-67-Derived Hollow NiCo-LDH and MoS2 on Enhancing the Flame Retardancy of Thermoplastic Polyurethane

In this work, a novel three-dimensional (3D) hollow nickel-cobalt layered double hydroxide (NiCo-LDH) was synthesized using zeolitic imidazole framework-67 (ZIF-67) as a template, and then utilized to functionalize molybdenum disulfide (NiCo-LDH/MoS2) via electrostatic force. Flame retardant thermoplastic polyurethane (TPU) composites were prepared by the melt blending method. Compared to pure TPU, NiCo-LDH/MoS2 filled TPU composite was endowed with a decrease of 30.9% and 55.7% of the peak heat release rate (PHRR) and the peak smoke production rate (PSPR), respectively. Furthermore, the addition of NiCo-LDH/MoS2 can significantly improve the thermal stability and char yield of the TPU composite. The catalytic carbonization effect and dilution effect of NiCo-LDH, and the barrier effect of MoS2 nanosheets enable TPU composites with excellent flame retardancy and toxic gas suppression ability.


Introduction
As an important engineering thermoplastic, thermoplastic polyurethane (TPU) has been widely used in the fields of cable, automotive, building and medical devices because of its good tensile strength, superior mechanical properties, excellent chemical stability and adjustable flexibility [1][2][3][4]. Nevertheless, TPU itself is flammable and releases a large amount of black smoke and toxic gases during the combustion process, posing a great threat to human life and property safety [5]. Therefore, it is highly desirable to find efficient and halogen-free flame retardants to improve the flame retardant and smoke suppression properties of TPU. In recent years, nanoscale fillers such as molybdenum disulfide (MoS 2 ) [6], layered double hydroxide (LDH) [7] and graphene (GO) [8] have been used as flame retardant additives for TPU and other polymers.
Layered double hydroxide (LDH) is a layered material similar to brucite, also known as hydrotalcite or anionic clay, mainly composed of a positively charged layer and interlayer anion [9]. The two-dimensional (2D) layered structure of LDH can not only hinder the heat and mass transfer between the combustion zone and the polymer matrix, but also dilute the flammable gases by releasing water vapor and carbon dioxide during the polymer decomposition process [10]. Nevertheless, LDH with higher surface energy tends to aggregate together, which is not conducive to improving the flame retardancy of the ultrasonically dispersed in 30 mL of anhydrous ethanol, which was recorded as liquid B. Finally, mix liquid A and liquid B uniformly and then transfer them into a 100 mL Teflon-lined autoclave, and reacted at 90 • C for 2 h. After cooling to room temperature, the obtained product was washed three times with ethanol in a centrifuge, and dried in an oven at 60 • C for 10 h to obtain hollow NiCo-LDH.

Synthesis of 3D Hollow NiCo-LDH/MoS 2 hybrid material
Preparation of exfoliated MoS 2 nanosheets by Li-ion intercalation [23]. The 3D hollow NiCo-LDH/MoS 2 hybrid material was also synthesized by the hydrothermal method. Figure 1 is the preparation process diagram of the 3D hollow NiCo-LDH/MoS 2 hybrid material.
ZIF-67 was synthesized according to previous reports in the literature [21,22]. 3D hollow NiCo-LDH was fabricated using the hydrothermal method. Typically, 200 mg of ZIF-67 was dissolved in 50 mL of anhydrous ethanol and ultrasonically dispersed for 30 min, which was recorded as liquid A. Then 600 mg of nickel nitrate hexahydrate was ultrasonically dispersed in 30 mL of anhydrous ethanol, which was recorded as liquid B. Finally, mix liquid A and liquid B uniformly and then transfer them into a 100 mL Teflon-lined autoclave, and reacted at 90 °C for 2 h. After cooling to room temperature, the obtained product was washed three times with ethanol in a centrifuge, and dried in an oven at 60 °C for 10 h to obtain hollow NiCo-LDH.

Synthesis of 3D Hollow NiCo-LDH/MoS2 hybrid material
Preparation of exfoliated MoS2 nanosheets by Li-ion intercalation [23]. The 3D hollow NiCo-LDH/MoS2 hybrid material was also synthesized by the hydrothermal method. Figure 1 is the preparation process diagram of the 3D hollow NiCo-LDH/MoS2 hybrid material.

Synthesis of TPU Composites
Under the mixing conditions of 180 °C and 30 rpm, 58 g of TPU was added to the mixer, and then the flame retardants with a mass fraction of 2 wt% were blended into the TPU matrix and stirred for 10 min. The specific formulations of TPU composites are shown in Table 1. Afterwards, the prepared TPU composites were put into a tablet press (180 °C, 10 MPa), and the TPU composites were hot-pressed for 10 min and cold-pressed for 3 min to obtain a size of 100 mm × 100 mm × 3 mm of TPU composites.  TPU0  100  0  0  0  TPU1  98  2  0  0  TPU2  98  0  2  0  TPU3 98 0 0 2

Synthesis of TPU Composites
Under the mixing conditions of 180 • C and 30 rpm, 58 g of TPU was added to the mixer, and then the flame retardants with a mass fraction of 2 wt% were blended into the TPU matrix and stirred for 10 min. The specific formulations of TPU composites are shown in Table 1. Afterwards, the prepared TPU composites were put into a tablet press (180 • C, 10 MPa), and the TPU composites were hot-pressed for 10 min and cold-pressed for 3 min to obtain a size of 100 mm × 100 mm × 3 mm of TPU composites.

Characterization
X-ray diffraction (XRD) was recorded on an X-ray diffractometer equipped with Cu-Kα tube and Ni filter (λ = 0.1542 nm), and the diffraction angle (2θ) ranged from 5 • to 80 • . Fourier transform infrared (FTIR) spectra of the prepared samples were performed by a Nicolet 6700 FTIR spectrophotometer in the range of 4000 cm −1 to 400 cm −1 . The morphology of the prepared samples was observed using a scanning electron microscope (SEM), accompanied by an accelerating voltage of 9 kV. Transmission electron microscopy-energy dispersive spectrometer (TEM-EDS) images were performed with a JEM-2100Plus instrument at 200 KV. X-ray photoelectron spectroscopy (XPS) characterizes the elemental chemical states of the prepared samples. The thermal stability of the samples was tested using a thermogravimetric analyzer (TGA). About 10.0 mg of the sample was placed in an alumina crucible and heated from 40 • C to 800 • C at a linear heating rate of 20 • C/min, setting the N 2 flow rate to 20 mL/min. The combustion tests were carried out on a cone calorimeter according to the ISO 5660 test standard. All TPU composites (100 mm × 100 mm × 3 mm) were wrapped with aluminum foil and measured under an external heat flux of 50 kW/m 2 . Analysis of pyrolysis products by Thermogravimetric-Fourier transform infrared spectrometer (TG-FTIR).

Characterization of Hollow NiCo-LDH and Its hybrid material
The structural information of the prepared samples was analyzed by XRD, as shown in Figure 2. As demonstrated in Figure 2a, the XRD pattern of MoS 2 exhibits an obvious peak at 2θ = 14.2 • , corresponding to the (002) plane. In addition, the characteristic peaks of ZIF-67 are in agreement with the previously reported work [21]. For NiCo-LDH, reflections at 2θ = 11.3 • , 22.9 • , 34.1 • and 60.7 • can be indexed as (003), (006), (009), and (110) diffraction peaks of NiCo-LDH, respectively, while the diffraction peaks are sharper, indicating that NiCo-LDH has a good crystalline form [24]. It can be seen from the figure that the NiCo-LDH/MoS 2 hybrid material has similar characteristic peaks to NiCo-LDH, and there is no diffraction peak corresponding to MoS 2 , indicating a high dispersion state of MoS 2 in the hybrid material. This is mainly attributed to the loss of face-to-face stacking structure of MoS 2 nanosheets for the growth of NiCo-LDH on MoS 2 surfaces [25]. Compared with NiCo-LDH, the diffraction peaks of NiCo-LDH/MoS 2 are blunt, which is caused by the disorder of the stacked structure between NiCo-LDH and MoS 2 nanosheets.
Kα tube and Ni filter (λ = 0.1542 nm), and the diffraction angle (2θ) ranged from 5° to 80°. Fourier transform infrared (FTIR) spectra of the prepared samples were performed by a Nicolet 6700 FTIR spectrophotometer in the range of 4000 cm −1 to 400 cm −1 . The morphology of the prepared samples was observed using a scanning electron microscope (SEM), accompanied by an accelerating voltage of 9 kV. Transmission electron microscopy-energy dispersive spectrometer (TEM-EDS) images were performed with a JEM-2100Plus instrument at 200 KV. X-ray photoelectron spectroscopy (XPS) characterizes the elemental chemical states of the prepared samples. The thermal stability of the samples was tested using a thermogravimetric analyzer (TGA). About 10.0 mg of the sample was placed in an alumina crucible and heated from 40 °C to 800 °C at a linear heating rate of 20 °C/min, setting the N2 flow rate to 20 mL/min. The combustion tests were carried out on a cone calorimeter according to the ISO 5660 test standard. All TPU composites (100 mm × 100 mm × 3 mm) were wrapped with aluminum foil and measured under an external heat flux of 50 kW/m 2 . Analysis of pyrolysis products by Thermogravimetric-Fourier transform infrared spectrometer (TG-FTIR).

Characterization of Hollow NiCo-LDH and Its hybrid material
The structural information of the prepared samples was analyzed by XRD, as shown in Figure 2. As demonstrated in Figure 2a, the XRD pattern of MoS2 exhibits an obvious peak at 2θ = 14.2°, corresponding to the (002) plane. In addition, the characteristic peaks of ZIF-67 are in agreement with the previously reported work [21]. For NiCo-LDH, reflections at 2θ = 11.3°, 22.9°, 34.1° and 60.7° can be indexed as (003), (006), (009), and (110) diffraction peaks of NiCo-LDH, respectively, while the diffraction peaks are sharper, indicating that NiCo-LDH has a good crystalline form [24]. It can be seen from the figure that the NiCo-LDH/MoS2 hybrid material has similar characteristic peaks to NiCo-LDH, and there is no diffraction peak corresponding to MoS2, indicating a high dispersion state of MoS2 in the hybrid material. This is mainly attributed to the loss of face-to-face stacking structure of MoS2 nanosheets for the growth of NiCo-LDH on MoS2 surfaces [25]. Compared with NiCo-LDH, the diffraction peaks of NiCo-LDH/MoS2 are blunt, which is caused by the disorder of the stacked structure between NiCo-LDH and MoS2 nanosheets.    Figure 3a shows the FTIR spectra of MoS 2 , NiCo-LDH and NiCo-LDH/MoS 2 . It can be seen from Figure 3a that the broad absorption peak at 3443 cm −1 of the three materials originates from the stretching vibration of the -OH group. The characteristic peak at 1627 cm −1 is ascribed to the bending vibration of water molecules. For NiCo-LDH, the stretching vibration of NO 3 − is also detected at around 1380 cm −1 . Due to the lattice vibration of metal-O, NiCo-LDH exhibits characteristic absorption peaks at 500-800 cm −1 [26]. The BET surface area of hollow NiCo-LDH was detected by N 2 adsorption/desorption isotherm, as shown in Figure 3b. Qin et al. reported that the BET surface area of solid NiCo-LDH is about 34 m 2 /g [27]. Interconnected NiCo-LDH nanosheets are loosely stacked on the precursor surface to form a highly porous structure, therefore NiCo-LDH has a high BET surface area of 80.3768 m 2 /g, which helps the NiCo-LDH to form more interfaces with the polymer matrix, thus improving the interaction between the two. Figure 3c shows the TG curves of MoS 2 , NiCo-LDH and NiCo-LDH/MoS 2 . It is noted that MoS 2 nanosheets have high thermal stability, and the char yield of MoS 2 is 90.29%. NiCo-LDH undergoes three thermal degradation processes by which the loss of interlayer H 2 O, decomposition of metal hydroxide, and collapse of metal organic framework. NiCo-LDH/MoS 2 and NiCo-LDH have similar thermal decomposition trends, which might be attributed to the lower content of MoS 2 in the hybrid material. In addition, the char yield of NiCo-LDH/MoS 2 (58.28%) is higher than that of NiCo-LDH (57.64%), indicating that NiCo-LDH/MoS 2 has better high temperature thermal stability. In Figure 3d, MoS 2 nanosheets exhibit a sheet-like structure. The dark parts in the sample can be attributed to the partial aggregation of MoS 2 nanosheets.
surface area of hollow NiCo-LDH was detected by N2 adsorption/desorption isotherm, as shown in Figure 3b. Qin et al. reported that the BET surface area of solid NiCo-LDH is about 34 m 2 /g [27]. Interconnected NiCo-LDH nanosheets are loosely stacked on the precursor surface to form a highly porous structure, therefore NiCo-LDH has a high BET surface area of 80.3768 m 2 /g, which helps the NiCo-LDH to form more interfaces with the polymer matrix, thus improving the interaction between the two. Figure 3c shows the TG curves of MoS2, NiCo-LDH and NiCo-LDH/MoS2. It is noted that MoS2 nanosheets have high thermal stability, and the char yield of MoS2 is 90.29%. NiCo-LDH undergoes three thermal degradation processes by which the loss of interlayer H2O, decomposition of metal hydroxide, and collapse of metal organic framework. NiCo-LDH/MoS2 and NiCo-LDH have similar thermal decomposition trends, which might be attributed to the lower content of MoS2 in the hybrid material. In addition, the char yield of NiCo-LDH/MoS2 (58.28%) is higher than that of NiCo-LDH (57.64%), indicating that NiCo-LDH/MoS2 has better high temperature thermal stability. In Figure 3d, MoS2 nanosheets exhibit a sheetlike structure. The dark parts in the sample can be attributed to the partial aggregation of MoS2 nanosheets. Morphologies of ZIF-67, NiCo-LDH and NiCo-LDH/MoS2 were observed by TEM and SEM, as shown in Figure 4. It can be seen from Figure 4a,d that the precursor ZIF-67 exhibits a solid regular dodecahedron morphology with uniform size (about 100 nm), and its surface is considerably smooth. As shown in Figure 4b,e, NiCo-LDH still maintains the morphology of the precursor and has a hollow interior, and NiCo-LDH nanosheets grown on the surface of the ZIF-67 precursor. From Figure 4c, it can be seen that the NiCo- Morphologies of ZIF-67, NiCo-LDH and NiCo-LDH/MoS 2 were observed by TEM and SEM, as shown in Figure 4. It can be seen from Figure 4a,d that the precursor ZIF-67 exhibits a solid regular dodecahedron morphology with uniform size (about 100 nm), and its surface is considerably smooth. As shown in Figure 4b,e, NiCo-LDH still maintains the morphology of the precursor and has a hollow interior, and NiCo-LDH nanosheets grown on the surface of the ZIF-67 precursor. From Figure 4c, it can be seen that the NiCo-LDH/MoS 2 hybrid material fails to detect MoS 2 , which may be caused by the complete coverage of MoS 2 nanosheets by NiCo-LDH. Figure S1 gives the EDS spectrum and plan scan image of NiCo-LDH/MoS 2 . It can be seen from Figure S1 that Co, Ni and O elements are uniformly distributed on the ZIF-67derived hollow dodecahedral framework. At the same time, the detected Mo element also confirms the existence of MoS 2 , indicating the successful preparation of NiCo-LDH/MoS 2 hybrid material.   Figure S1 gives the EDS spectrum and plan scan image of NiCo-LDH/MoS2. It can be seen from Figure S1 that Co, Ni and O elements are uniformly distributed on the ZIF-67derived hollow dodecahedral framework. At the same time, the detected Mo element also confirms the existence of MoS2, indicating the successful preparation of NiCo-LDH/MoS2 hybrid material.
XPS was used to test the chemical composition of NiCo-LDH/MoS2 and the valence states of the elements, and the results are presented in Figure 5. Figure 5a is the XPS survey spectrum of NiCo-LDH/MoS2, indicating the presence of C, O, Ni, Co and Mo elements of hybrid material. This further proves the successful hybridization of NiCo-LDH and MoS2. In the high-resolution Ni 2p spectrum, the peaks at 853.2 ev and 870.8 ev can be attributed to Ni 2p3/2 and Ni 2p1/2. Another two peaks of 858.8 ev and 877.2 ev correspond to the satellite shake-up peaks of Ni 2p3/2 and Ni 2p1/2, respectively [27]. These peaks prove that the valence state of the Ni element is divalent. As shown in Figure 4c, the binding energies of Co 2+ 2p3/2 and Co 2+ 2p1/2 at fitting peaks 779.3 ev and 795.2 ev, while the binding energy peaks at 778.2 ev and 793.7 ev refer to Co 3+ 2p3/2 and Co 3+ 2p1/2 [28]. XPS was used to test the chemical composition of NiCo-LDH/MoS 2 and the valence states of the elements, and the results are presented in Figure 5. Figure 5a is the XPS survey spectrum of NiCo-LDH/MoS 2 , indicating the presence of C, O, Ni, Co and Mo elements of hybrid material. This further proves the successful hybridization of NiCo-LDH and MoS 2 . In the high-resolution Ni 2p spectrum, the peaks at 853.2 ev and 870.8 ev can be attributed to Ni 2p 3/2 and Ni 2p 1/2 . Another two peaks of 858.8 ev and 877.2 ev correspond to the satellite shake-up peaks of Ni 2p 3/2 and Ni 2p 1/2 , respectively [27]. These peaks prove that the valence state of the Ni element is divalent. As shown in Figure 4c, the binding energies of Co 2+ 2p 3/2 and Co 2+ 2p 1/2 at fitting peaks 779.3 ev and 795.2 ev, while the binding energy peaks at 778.2 ev and 793.7 ev refer to Co 3+ 2p 3/2 and Co 3+ 2p 1/2 [28].

CCT Analysis
It has been demonstrated that the results of the CCT correlate well with the results obtained in the large-scale fire tests and can be used to predict the burning behavior of polymers in real fires [29]. Therefore, the effects of MoS2, NiCo-LDH and NiCo-LDH/MoS2 on the flame retardant and smoke suppression properties of TPU composites were further

CCT Analysis
It has been demonstrated that the results of the CCT correlate well with the results obtained in the large-scale fire tests and can be used to predict the burning behavior of polymers in real fires [29]. Therefore, the effects of MoS 2 , NiCo-LDH and NiCo-LDH/MoS 2 on the flame retardant and smoke suppression properties of TPU composites were further investigated by the cone calorimeter.
Heat release rate (HRR) is an important indicator to describe the fire hazard of polymers and can predict the behavior of polymers under real combustion conditions [30]. The HRR results for pure TPU and TPU composites are given in Figure 6 and Table 2. In Figure 6, pure TPU burns fiercely after being ignited and has the highest peak heat release rate (PHRR) of 1135 kW/m 2 . The addition of 2 wt% NiCo-LDH and MoS 2 nanosheets slightly decreases the PHRR of TPU composites to 804 kW/m 2 and 734 kW/m 2 , respectively, indicating that MoS 2 and NiCo-LDH nanosheets can inhibit the heat release of TPU composites and improve the flame retardancy of TPU composites to a certain extent. It is worth noting that the PHRR value of NiCo-LDH/MoS 2 filled TPU composite is lower than that of single MoS 2 or NiCo-LDH filled TPU composites, indicating that NiCo-LDH and MoS 2 have synergistic flame retardant effect. On the one hand, the transition metals nickel and cobalt in NiCo-LDH have catalytic carbonization effects. Coke can form a barrier effect on the polymer surface, slow down heat and mass transfer between the gas phase and the condensed phase, and protect the underlying material from further combustion [31]. On the other hand, two-dimensional MoS 2 nanosheets have nano-barrier effect, which can hinder the release of volatile products including hydrocarbons, so that less volatile products form fuel into the combustion zone, thereby reducing the heat release rate [32]. It is not difficult to see from Figure 6 that the ignition time of TPU composites is longer than that of pure TPU, which is related to the decomposition of MoS 2 and NiCo-LDH at low temperatures. Of note, The addition of MoS 2 nanosheets or NiCo-LDH/MoS 2 shortens the ignition time of TPU composites, which is attributed to the early decomposition of MoS 2 nanosheets or NiCo-LDH/MoS 2 .    Figure 7 exhibits the total heat release (THR) curves of pure TPU and TPU composites. Pure TPU has the highest THR value of 118.8 MJ/m 2 . After the incorporation of 2 wt% NiCo-LDH and MoS2 separately, the THR values of TPU composites are decreased to 104.6 MJ/m 2 and 100.7 MJ/m 2 , respectively, this is mainly because the uniformly dispersed NiCo-LDH and MoS2 nanosheets in the TPU matrix can inhibit the release of combustible gases during the combustion process, thereby promoting carbonization. Meanwhile, NiCo-LDH will release water vapor and reduce the surface temperature of TPU substrate during the combustion process, so as to achieve better flame retardant effect [33]. However, when incorporating 2 wt% NiCo-LDH/MoS2 hybrid material into TPU, the THR value of TPU3 increased to 106.1 MJ/m 2 , which may be due to the combination of MoS2 and NiCo-LDH further reducing the exfiltration rate of combustible gas, leading to more complete oxidative combustion of combustible volatiles such as hydrocarbons and thus   Figure 7 exhibits the total heat release (THR) curves of pure TPU and TPU composites. Pure TPU has the highest THR value of 118.8 MJ/m 2 . After the incorporation of 2 wt% NiCo-LDH and MoS 2 separately, the THR values of TPU composites are decreased to 104.6 MJ/m 2 and 100.7 MJ/m 2 , respectively, this is mainly because the uniformly dispersed NiCo-LDH and MoS 2 nanosheets in the TPU matrix can inhibit the release of combustible gases during the combustion process, thereby promoting carbonization. Meanwhile, NiCo-LDH will release water vapor and reduce the surface temperature of TPU substrate during the combustion process, so as to achieve better flame retardant effect [33]. However, when incorporating 2 wt% NiCo-LDH/MoS 2 hybrid material into TPU, the THR value of TPU3 increased to 106.1 MJ/m 2 , which may be due to the combination of MoS 2 and NiCo-LDH further reducing the exfiltration rate of combustible gas, leading to more complete oxidative combustion of combustible volatiles such as hydrocarbons and thus generating more heat. The amount of heavy smoke released during the combustion process is an im parameter to evaluate the fire hazard of TPU. The SPR (smoke production rate) cu pure TPU and its composites are shown in Figure 8. The PSPR (peak smoke prod rate) value of pure TPU reaches 0.113 m 2 /s, indicating the highest smoke emissio ertheless, the addition of MoS2 has little effect on the PSPR value of the TPU com indicating that MoS2 alone could not achieve satisfactory smoke suppression ef contrast, the PSPR value of NiCo-LDH filled TPU2 is further reduced to 0.056 m 2 /s is reduced by 50.4% as compared to that of pure TPU. It is ascribed that the tra metals Ni and Co have the effect of catalytic carbonization, and the formed carbo can reduce the combustible gas and smoke-forming materials in the gas phase. I tion, the porous structure of 3D hollow NiCo-LDH can absorb organic volatiles ge by the thermal decomposition of TPU, which are the main source of smoke partic When TPU composite is reinforced with NiCo-LDH/MoS2 hybrid material, its PSP is further reduced to 0.05 m 2 /s, clearly revealing the significant enhancement of suppression performance of TPU composites. This is mainly attributed to NiCo-LD MoS2 decomposed nickel, cobalt and molybdenum compounds can catalyze the for of carbon, which can suppress the smoke production rate of TPU composites [32]. The amount of heavy smoke released during the combustion process is an important parameter to evaluate the fire hazard of TPU. The SPR (smoke production rate) curves of pure TPU and its composites are shown in Figure 8. The PSPR (peak smoke production rate) value of pure TPU reaches 0.113 m 2 /s, indicating the highest smoke emission. Nevertheless, the addition of MoS 2 has little effect on the PSPR value of the TPU composite, indicating that MoS 2 alone could not achieve satisfactory smoke suppression effect. In contrast, the PSPR value of NiCo-LDH filled TPU2 is further reduced to 0.056 m 2 /s, which is reduced by 50.4% as compared to that of pure TPU. It is ascribed that the transition metals Ni and Co have the effect of catalytic carbonization, and the formed carbon layer can reduce the combustible gas and smoke-forming materials in the gas phase. In addition, the porous structure of 3D hollow NiCo-LDH can absorb organic volatiles generated by the thermal decomposition of TPU, which are the main source of smoke particles [34]. When TPU composite is reinforced with NiCo-LDH/MoS 2 hybrid material, its PSPR value is further reduced to 0.05 m 2 /s, clearly revealing the significant enhancement of smoke suppression performance of TPU composites. This is mainly attributed to NiCo-LDH and MoS 2 decomposed nickel, cobalt and molybdenum compounds can catalyze the formation of carbon, which can suppress the smoke production rate of TPU composites [32]. The TSP (total smoke production) curves for pure TPU as well as TPU composites are given in Figure 9. Obviously, pure TPU releases the most smoke during combustion, with a TSP value as high as 12.3 m 2 . It is worth noting that the TSP value of TPU1 is 11.9 m 2 , which is basically the same as that of pure TPU. Compared with pure TPU, the TSP value of TPU2 is 8.8 m 2 , corresponding to a decrease of 28.5%. This can be explained that organic volatile is the main source of smoke particles, and the presence of NiCo-LDH makes TPU molecules more retained in the condensed phase without being converted into an organic volatile [35]. The TSP value of the TPU composite with NiCo-LDH/MoS 2 hybrid material is further reduced to 8.2 m 2 , corresponding to a 33.3% reduction compared to pure TPU. The above results indicate that the combination of two additives imparts better smoke suppression to TPU.
The amount of heavy smoke released during the combustion process is an importan parameter to evaluate the fire hazard of TPU. The SPR (smoke production rate) curves o pure TPU and its composites are shown in Figure 8. The PSPR (peak smoke production rate) value of pure TPU reaches 0.113 m 2 /s, indicating the highest smoke emission. Nev ertheless, the addition of MoS2 has little effect on the PSPR value of the TPU composite indicating that MoS2 alone could not achieve satisfactory smoke suppression effect. In contrast, the PSPR value of NiCo-LDH filled TPU2 is further reduced to 0.056 m 2 /s, which is reduced by 50.4% as compared to that of pure TPU. It is ascribed that the transition metals Ni and Co have the effect of catalytic carbonization, and the formed carbon layer can reduce the combustible gas and smoke-forming materials in the gas phase. In addi tion, the porous structure of 3D hollow NiCo-LDH can absorb organic volatiles generated by the thermal decomposition of TPU, which are the main source of smoke particles [34] When TPU composite is reinforced with NiCo-LDH/MoS2 hybrid material, its PSPR value is further reduced to 0.05 m 2 /s, clearly revealing the significant enhancement of smoke suppression performance of TPU composites. This is mainly attributed to NiCo-LDH and MoS2 decomposed nickel, cobalt and molybdenum compounds can catalyze the formation of carbon, which can suppress the smoke production rate of TPU composites [32].  The TSP (total smoke production) curves for pure TPU as well as TPU composites are given in Figure 9. Obviously, pure TPU releases the most smoke during combustion with a TSP value as high as 12.3 m 2 . It is worth noting that the TSP value of TPU1 is 11.9 m 2 , which is basically the same as that of pure TPU. Compared with pure TPU, the TSP value of TPU2 is 8.8 m 2 , corresponding to a decrease of 28.5%. This can be explained that organic volatile is the main source of smoke particles, and the presence of NiCo-LDH makes TPU molecules more retained in the condensed phase without being converted into an organic volatile [35]. The TSP value of the TPU composite with NiCo-LDH/MoS2 hybrid material is further reduced to 8.2 m 2 , corresponding to a 33.3% reduction compared to pure TPU. The above results indicate that the combination of two additives imparts better smoke suppression to TPU.

Thermal Stability Analysis
TGA is a widely used technique to rapidly evaluate the thermal stability of materials and can also reveal the thermal degradation behavior of polymers at different temperatures [36]. To profoundly understand the influence of MoS2, NiCo-LDH and NiCo-LDH/MoS2 hybrid material on the thermal stability of TPU composites, the thermal oxi- Figure 9. Total smoke production curves of TPU composites.

Thermal Stability Analysis
TGA is a widely used technique to rapidly evaluate the thermal stability of materials, and can also reveal the thermal degradation behavior of polymers at different temperatures [36]. To profoundly understand the influence of MoS 2 , NiCo-LDH and NiCo-LDH/MoS 2 hybrid material on the thermal stability of TPU composites, the thermal oxidative degradation behavior and carbon residues of different TPU composites were compared using TGA.
The TG and DTG profiles for pure TPU and TPU composites under N 2 atmosphere are displayed in Figure 10, and the related data are summarized in Table 3. From Figure 10b, it can be seen that pure TPU mainly presents two decomposition stages. More precisely, the first decomposition stage corresponds to the removal of CO 2 , and the second decomposition stage is mainly attributed to the dehydration carbonization reaction [37]. In contrast, TPU composites exhibit only one thermal decomposition stage, the rapid decomposition stage of TPU composites occurs between 250 • C and 450 • C. From Figure 10a, it can be easily observed that the initial decomposition temperature (T −5% , temperatures at 5% weight loss) of the TPU composites is lower than that of pure TPU, which is mainly attributed to the early degradation of MoS 2 and NiCo-LDH. In addition, the T −5% of TPU3 is 6 • C higher than that of TPU2, demonstrating that the NiCo-LDH/MoS 2 hybrid material improved the thermal stability of TPU composites. As can be seen, the char yields of TPU1 and TPU2 at 800 • C are 7.93% and 8.02%, which are 1.36 and 1.37 times than that of pure TPU, respectively. The enhanced char yields may be due to the addition of MoS 2 or NiCo-LDH, implying the formation of effective barrier layers in the TPU composites. Furthermore, the char yield of TPU3 reaches 11.87% at 800 • C. demonstrating that NiCo-LDH and MoS 2 nanosheets jointly promote the improvement of char yield of TPU composite from the above analysis, the catalytic carbonization effect of NiCo-LDH/MoS 2 hybrid material can enhance the thermal stability of TPU composites, which is beneficial to improve the fire safety of TPU composites [38]. LDH, implying the formation of effective barrier layers in the TPU composites. Furthermore, the char yield of TPU3 reaches 11.87% at 800 °C. demonstrating that NiCo-LDH and MoS2 nanosheets jointly promote the improvement of char yield of TPU composite from the above analysis, the catalytic carbonization effect of NiCo-LDH/MoS2 hybrid material can enhance the thermal stability of TPU composites, which is beneficial to improve the fire safety of TPU composites [38].

Char Residue Analysis
In order to study the flame retardant mechanism of NiCo-LDH/MoS2 in the condensed phase, the digital photos of carbon residues of TPU composites were firstly investigated. Figure S2 gives digital photos of carbon residues of pure TPU of TPU composites after the cone calorimeter test. It can be clearly seen from Figure S2 that the pure TPU burns very completely, and there is no remaining carbon residue. For TPU1 added with MoS2, the carbon layer is not complete, which also leads to the inability of TPU1 to effectively suppress mass and heat transfer during the combustion process. Although the carbon residue of TPU2 covers the entire aluminum foil, the carbon residue is loose and fragile. On the contrary, by incorporating NiCo-LDH/MoS2 hybrid material into TPU, the

Char Residue Analysis
In order to study the flame retardant mechanism of NiCo-LDH/MoS 2 in the condensed phase, the digital photos of carbon residues of TPU composites were firstly investigated. Figure S2 gives digital photos of carbon residues of pure TPU of TPU composites after the cone calorimeter test. It can be clearly seen from Figure S2 that the pure TPU burns very completely, and there is no remaining carbon residue. For TPU1 added with MoS 2 , the carbon layer is not complete, which also leads to the inability of TPU1 to effectively suppress mass and heat transfer during the combustion process. Although the carbon residue of TPU2 covers the entire aluminum foil, the carbon residue is loose and fragile. On the contrary, by incorporating NiCo-LDH/MoS 2 hybrid material into TPU, the amount of carbon residual increases significantly, and the carbon layer is thicker and harder, indicating that NiCo-LDH/MoS 2 hybrid material has excellent catalytic carbonization effect.
The SEM images of the carbon residues of the TPU composite are shown in Figure 11. It is clear that there are obvious cracks and a large number of holes in the carbon residual of TPU1. When NiCo-LDH is added to TPU, the carbon residue of TPU2 has a more continuous structure, but there are still some cracks and holes. In comparison, the application of NiCo-LDH/MoS 2 hybrid material in TPU gives a denser and less porous carbon residue. The dense carbon layer structure can not only effectively inhibit the transfer of heat and volatiles, but also protect the underlying polymer, thereby significantly improving the thermal stability and flame retardant properties of TPU composites [39]. The carbon residues of TPU1 and TPU3 were analyzed by XRD and Raman tests, and the results are shown in Figure 12. It can be found in Figure 12a that diffraction peaks of MoO3 and MoS2 appeared in the XRD pattern of the carbon residue of TPU1. As shown in Figure 12b, the XRD pattern of the carbon residue of TPU3 not only has characteristic peaks of metal oxides such as MoO3, NiO, Ni2O3 and Co2O3, but also has a diffraction peak of graphite crystallite around 2θ = 25°. The degree of graphitization of carbon residues was determined by Raman spectra, as shown in Figure 12c,d. It is clear from both Raman spectra that obvious peaks are observed at 1360 cm −1 and 1598 cm −1 , representing the D and G bands, respectively. The area ratio of the D band and G band (ID/IG) is widely used to determine the degree of graphitization of carbon residues. The lower the value of ID/IG, the higher the degree of graphitization of the carbon residues [40]. The ID/IG value of TPU3 is 2.71, lower than that of TPU1 (3.19), which indicates that the addition of NiCo-LDH/MoS2 is beneficial to improving the degree of graphitization of carbon residues. The carbon residues of TPU1 and TPU3 were analyzed by XRD and Raman tests, and the results are shown in Figure 12. It can be found in Figure 12a that diffraction peaks of MoO 3 and MoS 2 appeared in the XRD pattern of the carbon residue of TPU1. As shown in Figure 12b, the XRD pattern of the carbon residue of TPU3 not only has characteristic peaks of metal oxides such as MoO 3 , NiO, Ni 2 O 3 and Co 2 O 3 , but also has a diffraction peak of graphite crystallite around 2θ = 25 • . The degree of graphitization of carbon residues was determined by Raman spectra, as shown in Figure 12c,d. It is clear from both Raman spectra that obvious peaks are observed at 1360 cm −1 and 1598 cm −1 , representing the D and G bands, respectively. The area ratio of the D band and G band (I D /I G ) is widely used to determine the degree of graphitization of carbon residues. The lower the value of I D /I G , the higher the degree of graphitization of the carbon residues [40]. The I D /I G value of TPU3 is 2.71, lower than that of TPU1 (3.19), which indicates that the addition of NiCo-LDH/MoS 2 is beneficial to improving the degree of graphitization of carbon residues. was determined by Raman spectra, as shown in Figure 12c,d. It is clear from both Raman spectra that obvious peaks are observed at 1360 cm −1 and 1598 cm −1 , representing the D and G bands, respectively. The area ratio of the D band and G band (ID/IG) is widely used to determine the degree of graphitization of carbon residues. The lower the value of ID/IG, the higher the degree of graphitization of the carbon residues [40]. The ID/IG value of TPU3 is 2.71, lower than that of TPU1 (3.19), which indicates that the addition of NiCo-LDH/MoS2 is beneficial to improving the degree of graphitization of carbon residues. Figure 12. XRD patterns of carbon residues of TPU1 (a) and TPU3 (b); Raman spectra of TPU1 (c) and TPU3 (d).

Thermal Decomposition Products Analysis
In order to obtain information about the variation of gaseous products with temperature during the thermal decomposition of TPU composites, 3D TG-FTIR spectra of TPU0 and TPU3 are given in Figure S3. As shown in Figure S3, the thermal decomposition process of TPU0 and TPU3 is similar, which indicates that the addition of NiCo-LDH/MoS 2 hybrid material has little effect on the inherent properties of TPU. In addition, it can be obviously seen that the temperature at which TPU3 releases thermal decomposition products is lower than that of TPU0, which is primarily because the addition of NiCo-LDH/MoS 2 hybrid material makes the initial decomposition temperature of TPU composite earlier. Figure 13a presents the FTIR spectrum of the pyrolysis products of TPU0 and TPU3 at the maximum decomposition rate. The characteristic peak at 3548 cm −1 is ascribed to the vibration of the O-H bond in H 2 O. The absorption peak appearing at 2980 cm −1 is assigned to the symmetrical stretching vibration of the C-H bond in hydrocarbons. The peaks at 2360 cm −1 and 1766 cm −1 are typical absorption peaks of CO 2 and carbonyl compounds, respectively. The absorption peaks of aromatic hydrocarbons and HCN are located at 1510 cm −1 and 678 cm −1 , respectively. As shown in Figure 13b,c, the presence of NiCo-LDH/MoS 2 hinders the release of HCN and CO 2 , which is mainly attributed to the formation of high-quality carbon residues and the barrier effect of MoS 2 nanosheets [41]. From Figure 13d, it is observed that the amount of H 2 O released in the pyrolysis product of TPU3 is significantly higher than that of pure TPU, which is beneficial to diluting the combustible gas.
pounds, respectively. The absorption peaks of aromatic hydrocarbons and HCN are located at 1510 cm −1 and 678 cm −1 , respectively. As shown in Figure 13b,c, the presence of NiCo-LDH/MoS2 hinders the release of HCN and CO2, which is mainly attributed to the formation of high-quality carbon residues and the barrier effect of MoS2 nanosheets [41]. From Figure 13d, it is observed that the amount of H2O released in the pyrolysis product of TPU3 is significantly higher than that of pure TPU, which is beneficial to diluting the combustible gas. Figure 13. FTIR spectra of TPU0 and TPU3 pyrolysis products at maximum decomposition rate (a), the absorbance intensities of HCN (b), CO2 (c) and H2O (d) of TPU0 and TPU3. Figure 13. FTIR spectra of TPU0 and TPU3 pyrolysis products at maximum decomposition rate (a), the absorbance intensities of HCN (b), CO 2 (c) and H 2 O (d) of TPU0 and TPU3.

Flame Retardant Mechanism
Based on the flame retardant properties and condensed phase-gas phase analysis of TPU/NiCo-LDH/MoS 2 composite above, a possible flame retardant mechanism was proposed. In the condensed phase: (1) The transition metals Ni, Co and Mo have the effect of catalytic carbonization. The formed carbon layer can block the transfer of heat and combustible gas between the combustion zone and the TPU matrix, and protect the unburned TPU matrix. (2) During combustion, MoS 2 and NiCo-LDH act as physical barriers, slowing down the escape of combustible, hindering the permeation of oxygen and inhibiting the exudation of toxic substances. (3) The metal oxides generated by the decomposition of NiCo-LDH/MoS 2 not only improve the degree of graphitization of the carbon layer, but also can absorb the flue gas generated during the combustion process [34]. In the gas phase, NiCo-LDH releases non-combustible gases (H 2 O, CO 2 ) that can dilute the concentration of combustible gases to some extent.

Conclusions
In conclusion, 3D hollow NiCo-LDH was assembled on MoS 2 nanosheets by the principle of electrostatic self-assembly to form 3D hollow NiCo-LDH/MoS 2 hybrid material. Characterization of the structure and morphology of the NiCo-LDH/MoS 2 by XRD, FTIR, SEM, TEM, BET and XPS. Then the NiCo-LDH/MoS 2 was mixed with TPU by melt blending. With the addition of NiCo-LDH/MoS 2 , PHRR, PSPR and TSP values of the obtained TPU composite were remarkably decreased by 42.9%, 55.7% and 33.3%. Meanwhile, the TPU composite filled with NiCo-LDH/MoS 2 hybrid material had higher char yield and thermal stability. In addition, SEM, XRD and Raman spectroscopy revealed that the NiCo-LDH/MoS 2 filled TPU composite has dense carbon residue with enhanced graphitization degree, which is able to protect the underlying TPU matrix. TG-FTIR results showed NiCo-LDH/MoS 2 hybrid material also exhibits excellent toxic gas (HCN) suppression performance. In summary, the catalytic carbonization effect and dilution effect of NiCo-LDH, and the barrier effect of MoS 2 nanosheets enable TPU composites with excellent flame retardancy, thermal stability and toxic gas suppression ability.