Introduction

With the rapid social and economic development, functional materials have become increasingly important. Particularly, flame-retarding polymers have attracted more and more interest, due to their wide application (such as in aerospace engineering, information technology, construction and textile) [1,2,3]. Numerous studies have shown that the flame retardancy of polymers can be improved by adding nano-inorganic flame retardants [4,5,6]. Therefore, the development of nano-inorganic flame retardants is important for flame-retarding polymers.

Hexagonal boron nitride (h-BN) has a layered structure similar to graphite and is called “white graphite.” Each of its layers is composed of B atoms and N atoms. The B atoms and N atoms are covalently bonded by sp2 hybrid to produce unlimited extension of hexagonal ring networks, and interlayers combined by the van der Waals forces. In addition, h-BN is widely used in machinery, metallurgy, electronics, aerospace and other high-tech fields, due to its excellent physical and chemical properties, such as high temperature resistance, oxidation resistance, chemical stability, low coefficient of friction and good processability [7,8,9]. Compared with carbon nanomaterials, boron nitride nanomaterials have better thermal stability, chemical stability and oxidation resistance. Therefore, h-BN has remarkable advantages in improving the mechanical properties, thermal stability and oxidation resistance of polymers. Yu et al. obtained BNO by calcination of h-BN and then added BNO to modified EP (MEP) to study the effect of flame retardancy on the BNO/MEP composite. They found that BNO exhibited good flame retardancy and that the incorporation of 1 wt% BNO reduced the PHRR by 47.2% [10]. Gu et al. chemically modified h-BN with hexachlorocyclotriphosphazene and p-phenylenediamine and added it to bismaleimide resin to study the thermal and flame retardant properties of the composites. Their results showed that modified h-BN could effectively improve the thermal stability and flame retardancy of resin; the PHRR was reduced from 383 to 207 kW m−2, down by 45.9% [11]. Qu et al. [12] found that PCB-BN synthesized by hexachlorocyclotriphosphazene, bisphenol-A and BN could effectively improve the flame retardancy of EP. Hexagonal boron nitride could improve the flame retardancy of the polymer due mainly to its good thermal stability and physical barrier effect.

As is known, polymers can produce a lot of toxic gas and smoke during the combustion process. In fact, these harmful volatile substances and smoke are the greatest threats to the safety of human life in a fire. Therefore, it is a major challenge to effectively reduce the generation of smoke for flame-retarding polymers. Numerous studies have shown that molybdenum-containing compounds and copper-containing compounds can well inhibit the generation of smoke during the combustion of polymers [13, 14]. Zhou et al. [15] found that putting layered double hydroxides (LDHs) on the surface of molybdenum disulfide (MoS2) could not only effectively reduce the PHRR of EP, but also reduce the production of hydrocarbons, CO and CO2, and effectively reduce the TSP during the combustion of EP. Xu et al. studied the application of MoO3 and graphene in polyurethane elastomer. They found that MoO3 and graphene hybrids exhibited good smoke suppression and that the incorporation of 2 wt% hybrids reduced the smoke density by 29.5%. That was because MoO3 promoted the formation of a dense char layer during the combustion of the composites, hindering the diffusion of oxygen and transfer of heat [16]. In addition, Chen et al. studied the effect of Cu2O on the smoke suppression performance of EP composites containing ammonium polyphosphate (MAPP). Their results showed that Cu2O could effectively reduce the production of smoke, which was due mainly to the fact that Cu2O could promote the formation of a compact char layer during the combustion process. The compact char layer had the effect of impeding the exchange of heat and isolating oxygen [17]. In view of the above-mentioned findings, copper molybdate (CuMoO4) was selected in the present work to modify h-BN because it contained two elements of molybdenum and copper. It was expected that loading CuMoO4 on the surface of h-BN could not only improve the flame retardancy of the polymer more effectively, but also significantly improve its smoke suppression performance.

Polyurethane elastomer (PUE) is widely used for various products, such as sporting goods, toys and decorative materials, because of its good abrasion resistance, oil resistance and elasticity [18,19,20]. Unfortunately, PUE can rapidly burn and generate large amounts of harmful volatile substances and smoke while it is continuously heated [21, 22]. As a result, it is very important to effectively improve the flame retardancy and smoke suppression performance of PUE to expand the range of its application.

In this work, a green flame retardant (CuMoO4@h-BN hybrid) was prepared by the co-precipitation method, and the application of CuMoO4@h-BN in PUE was studied. The effects of CuMoO4@h-BN on the flame retardancy and smoke suppression performance of PUE were investigated by using TGA, cone calorimeter and smoke density. Moreover, the flame retardant and smoke suppression mechanism were studied by the analysis of char residue after the cone calorimeter test.

Experimental

Materials

CuCl2·2H2O, Na2MoO4·2H2O, H2SO4 (98%) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. HNO3 was purchased from Shanghai Yihai Chemical Reagent Co., Ltd., China. h-BN was purchased from Weifang Ruida Ceramic Materials Co., Ltd., China. Polyester alcohol (Mn = 1975) was purchased from Shandong Xin Dezhou Huarun Polyurethane Industry Co., Ltd., China. 3,3′-Dichloro-4,4′-diaminodiphenylmethane (MOCA) was purchased from Jinan Haiwu Chemical Co., Ltd., China; Toluene diisocyanate (TDI) was purchased from Mitsui Chemical Co., Ltd., Japan.

Preparation of CuMoO4@h-BN

First, in order to improve the surface inertia and poor wettability, h-BN was treated with H2SO4 and HNO3 [12]. h-BN was added into the mixed solution of H2SO4 and HNO3 and then sonicated 5 h to generate a sticky and pale yellow suspended solution. After that, it was transferred to a four-necked flask, and the oil bath was refluxed at 80 °C for 72 h. Finally, the supernatant was neutralized by washing with deionized water and then freeze-dried to obtain surface-functionalized h-BN, referred to as a-BN.

CuMoO4@h-BN was prepared as follows: First, 0.25 g a-BN was sonicated in 50 ml of deionized water for 30 min. Then, 0.171 g CuCl2·2H2O was dissolved in deionized water and added dropwise to the above liquid of a-BN with continued ultrasonic agitation for 1 h. The mixture was transferred to a four-necked flask, with oil bath at 60 °C for 12 h. Then, 0.242 g Na2MoO4·2H2O aqueous solution was added dropwise to the above mixture with continued stirring for 2 h. After standing for a few hours, the supernatant was removed and then washed with deionized water. Finally, the above was freeze-dried to obtain a green CuMoO4@h-BN (0.48 g).

Preparation of PUE composites

PUE composites were prepared by simple blending. Typically, the PUE composite containing 2 wt% CuMoO4@h-BN was prepared as follows: Polyester polyol (44.4 g) was heated to 110 °C, stirred, and vacuumed for 2 h to remove trace water. Then, polyester polyol was cooled to 70 °C. TDI was added and stirred at 75 °C for 2 h and then degassed under vacuum at 80 °C for 30 min to obtain NCO group-terminated prepolymer. CuMoO4@h-BN (1 g) was dispersed into acetone by ultrasonication, poured into the prepolymer and stirred under ultrasound at 75 °C to form a homogeneous mixture. Then, molten MOCA (4.6 g) was added into the above blend and stirred fully. Thereafter, the mixture was casted on a Teflon mold at 80 °C for 8 h and placed in an oven at 120 °C for 2 h. Finally, the PUE/CuMoO4@h-BN composite was prepared. Pure PUE, PUE/h-BN and PUE/h-BN/CuMoO4 composites were prepared via a similar procedure. The filler loading of PUE/h-BN was kept at 2 wt%. The fillers of PUE/h-BN/CuMoO4 were 1 wt% h-BN and 1 wt% CuMoO4 (Scheme 1).

Scheme 1
scheme 1

Illustration for the modification of h-BN and preparation of PUE composites

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Characterization

X-ray diffraction (XRD) patterns were performed with a Bruker D8 X-ray diffractometer (Germany). Fourier transform infrared (FTIR) spectra were carried out with a Nicolet 6700 FTIR spectrophotometer (Thermo Fisher Scientific, USA). Laser Raman spectroscopy (LRS) measurements were taken with a SPEX-1403 laser Raman spectrometer (SPEX Co., USA). X-ray photoelectron spectroscopy (XPS) was performed with an ESCALAB 250 spectrometer (Thermo Scientific Ltd., USA). Transmission electron microscope–energy-dispersive X-ray spectroscopy (TEM-EDS) images were obtained using a JEM-2100 instrument (JEOL Co., Japan). Thermogravimetric analysis (TGA) was carried out on a Q50 thermo-analyzer instrument (TA Instruments, USA) under an air flow of 20 °C min−1. Differential scanning calorimetry (DSC) was performed on a Q20 (TA Instruments, USA) from 20 to 150 °C at a linear heating rate of 20 °C min−1. The cone calorimeter combustion test was performed on a JCZ-2 cone calorimeter (Jiangning Analytical Instrument Company, China) following the procedures given in the ISO5660 standard with heat radiation of 50 kW m−2, and the size of each specimen was 100 × 100 × 4 mm3. The smoke density test was carried out in a JSC-2 smoke density test chamber (Jiangning Analytical Instrument Company, China) according to ISO 5659-2 with heat radiation of 50 kW m−2, and the size of each specimen was 75 × 75 × 3 mm3.

Results and discussion

Characterization of CuMoO4@h-BN

XRD is known to be a useful technique to examine the structure of inorganic materials. Figure 1 shows the XRD patterns of h-BN, CuMoO4 and CuMoO4@h-BN. As shown in the figure, the characteristic absorption peak of h-BN appears at 26.6°, 41.4°, 43.5°, 49.9°, 54.9° and 75.8° corresponding to (002), (100), (101), (102), (004) and (110) plane diffraction peaks, respectively [23]. The diffraction peaks of CuMoO4 in the XRD pattern conform to the standard CuMoO4 diffraction peaks [24]. Compared with CuMoO4 and h-BN, the diffraction peaks of CuMoO4 and h-BN appear in the XRD spectrum of CuMoO4@h-BN, indicating that CuMoO4 is successfully supported on the surface of h-BN.

Figure 1
figure 1

XRD patterns of as-prepared samples

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Figure 2 shows the FTIR spectra of h-BN, CuMoO4 and CuMoO4@h-BN. From the spectrum of h-BN, two obvious characteristic peaks are observed: the stretching vibration peak of B–N (1372 cm−1) and the deformation vibration peak of B–N (805 cm−1), which are typical of h-BN absorption peaks [25]. From the spectrum of CuMoO4, two obvious absorption peaks can be observed between 3300 and 3500, which are the stretching vibration peak of O–H in adsorbed water and the stretching vibration peak of Mo = O at 972 cm−1, the stretching vibration peak of Mo·O·Mo at 815 cm−1 and the absorption peak of Cu–O bond at 454 cm−1 [26]. Compared with h-BN and CuMoO4, almost all absorption peaks of h-BN and CuMoO4 are observable in the CuMoO4@h-BN spectra, indicating that the hybrid of CuMoO4@h-BN has been successfully prepared.

Figure 2
figure 2

FTIR spectra of as-prepared samples

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TEM is used to further study the morphology and composition of CuMoO4@h-BN. Figure 3a shows that h-BN nanosheets are round or oval, stacked on top of each other, and their size is relatively uniform, about 100 nm. In Fig. 3b, we can see that a lot of CuMoO4 sheets appear on the surface of h-BN, indicating that CuMoO4 is loaded on the sheets of h-BN. The elemental composition of CuMoO4@h-BN is obtained from Fig. 3c. It is clear that the B and N elements belong to the h-BN nanosheet. Cu, Mo and O all belong to CuMoO4, while C belongs to the carbon film on the stage. EDS results further confirm that CuMoO4@h-BN has been prepared.

Figure 3
figure 3

TEM images of a h-BN; b CuMoO4@h-BN; c EDS analysis of CuMoO4@h-BN

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Figure 4 presents the TGA curves of h-BN, CuMoO4 and CuMoO4@h-BN treated for 2 h in a vacuum oven at 60 °C. It can be seen from the figure that there is almost no mass change in h-BN, and the residual is as high as 99.8% at 700 °C, indicating the excellent thermal stability of h-BN. For CuMoO4, there is almost no mass loss before 150 °C. After 150 °C, the weight loss is due to the decomposition of CuMoO4 into CuO and MoO3, and the residual is 95.7% at 700 °C. The weight loss trend of CuMoO4@h-BN is similar to that of CuMoO4, and the residual at 700 °C is obviously higher than that of CuMoO4, reaching 97.1%.

Figure 4
figure 4

TGA curves of h-BN, CuMoO4 and CuMoO4@h-BN

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The thermal properties of pure PUE and its composites

Thermogravimetric analysis is used to investigate the thermal stability of pure PUE and its composites. Figure 5 shows the TGA and DTG curves for pure PUE, PUE/h-BN, PUE/h-BN/CuMoO4 and PUE/CuMoO4@h-BN composites under air atmosphere. As can be seen from the figure, all the samples show a similar tendency of thermal decomposition under air conditions. First of all, the first decomposition occurs below 350 °C, which is mainly the process of hard segment decomposition to produce isocyanates, polyols and volatile materials. Then, a second decomposition takes place between 350 and 470 °C, which is attributed mainly to the decomposition of soft segments. Finally, there is a third decomposition after 470 °C, which is due to the degradation of metastable char.

Figure 5
figure 5

TGA (a) and DTG (b) curves of pure PUE and PUE composites

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As shown in Fig. 5, T5% of pure PUE (corresponding to a sample with a mass loss of 5 wt%) is 275.4 °C and Tmax (corresponding to a maximum thermal decomposition rate) is 384.2 °C. Compared with pure PUE, there is a slight decrease in the T5% of PUE/h-BN/CuMoO4 and PUE/CuMoO4@h-BN composites. It is probably because that the metal oxides generated during the decomposition of CuMoO4 catalyze the degradation of PUE [27]. The slight increase in the Tmax of PUE/h-BN/CuMoO4 and PUE/CuMoO4@h-BN composites is attributed to the physical barrier effect of h-BN and char residue. In addition, the residual char yield of pure PUE is less than 0.1% at 700 °C, while those of PUE/h-BN, PUE/h-BN/CuMoO4 and PUE/CuMoO4@h-BN are 2.1, 6.9 and 9.8%, respectively. It is noteworthy that the PUE/CuMoO4@h-BN composite exhibits the most significant increase in its yield of char residue, which may be attributed to a better barrier effect with better dispersibility of h-BN and a catalytic carbonization effect from the metal oxides produced by the decomposition of CuMoO4@h-BN (Table 1).

Table 1 Data of TGA and DTG of pure PUE and PUE composites
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In general, the glass transition temperature (Tg) is an important parameter to characterize thermal properties of polymers and is frequently used to represent the segmental mobility of macromolecular chains. The Tg of PUE and its composites are characterized by DSC, and the results are shown in Fig. 6. Compared with pure PUE, the Tg of the PUE composites are all increased in some degrees. It may be because the physical barrier effect of h-BN and CuMoO4 helps hinder the movement of molecular chains to a certain extent [28].

Figure 6
figure 6

DSC thermograms of pure PUE and PUE composites

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The flame retardancy of pure PUE and its composites

Figure 7 shows the heat release rate (HRR) and total heat release (THR) curves of pure PUE and its composites. The related data are shown in Table 2. As can be seen from the figure, pure PUE can ignite rapidly in combustion, with a peak heat release rate (PHRR) of 1106 kW m−2 and total heat release (THR) of 61.7 MJ m−2. Compared with pure PUE, the PHRR and THR of PUE/h-BN, PUE/h-BN/CuMoO4 and PUE/CuMoO4@h-BN are all decreased to some extent. Among them, the PHRR and THR of PUE/CuMoO4@h-BN are decreased most obviously, by 73.6 and 52.4%, respectively. This is due mainly to the physical barrier effect between the h-BN nanosheets and the char layer produced during the combustion process, suppressing the volatilization of the flammable gas generated during the decomposition of the polymer, isolating oxygen and reducing heat radiation. Meanwhile, CuMoO4 can decompose and generate Cu2O and MoO3 during the heating process. Cu2O and MoO3 have the function of catalyzing the formation of a compact char layer [16, 29]. More compact char layers can produce a better physical barrier effect, thereby enhancing the flame retardancy of the composite.

Figure 7
figure 7

HRR (a) and THR (b) curves of pure PUE and PUE composites

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Table 2 PHRR, THR, SPR and TSP data of pure PUE and PUE composites
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Smoke suppression performance of pure PUE and PUE composites

The smoke generation rate (SPR) and the total smoke generation (TSP) are two important parameters in the cone calorimetry test to assess the smoking behavior of polymers during combustion. Figure 8 shows the SPR and TSP curves for the cone calorimetric test of PUE and its composites. The SPR and TSP of pure PUE reach 0.14 m2 s−1 and 8.9 m2, respectively. When different flame retardants are added, the SPR and TSP of the composites all decrease to some degrees. Among them, the SPR and TSP of PUE/CuMoO4@h-BN are decreased most, indicating that CuMoO4@h-BN has a better smoke suppression effect. This is because that, on the one hand, the physical barrier effect of h-BN is in play; on the other hand, Cu2O and MoO3 generated during the decomposition of CuMoO4 have a catalytic carbonization effect, enhancing the densification of the residual char and suppressing the combustion of the material, thereby reducing the release of smoke.

Figure 8
figure 8

SPR (a) and TSP (b) curves of pure PUE and PUE composites

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The smoke density test under flameless combustion conditions is used to further investigate the influence of CuMoO4@h-BN on the smoke suppression performance of PUE. The results are shown in Fig. 9 and Table 3. Pure PUE has the largest smoke density value at 600 s compared with the PUE composites. When 2 wt% h-BN is added, the smoke density of PUE/h-BN is 349, only 7.9% lower than that of pure PUE. Such a slight reduction in smoke density may be attributed to the barrier effect of h-BN. Meanwhile, it shows that the barrier effect is not strong in smoke suppression. The smoke density of PUE/h-BN/CuMoO4 is decreased to 303 at 600 s and that of PUE/CuMoO4@h-BN is decreased to 272 at 600 s. It indicates that CuMoO4 loaded on the surface of h-BN has a more active smoke suppression effect, which is consistent with the cone calorimetric test results.

Figure 9
figure 9

Smoke density curves of pure PUE and PUE composites

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Table 3 Smoke density data of pure PUE and PUE composites
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Char residue analysis of PUE and its composites

Figure 10 presents the char residue mass curves of PUE and its composites after the cone calorimeter test. As can be seen from the figure, the char yield of pure PUE is 11.4%. The char yield of the PUE composites is all increased after the addition of different flame retardants. Notably, the char yield of PUE/CuMoO4@h-BN is increased most obviously, indicating that CuMoO4@h-BN has a better effect of catalytic carbonization. This result is consistent with the TGA test results.

Figure 10
figure 10

Char residue mass curves of PUE and its composites after cone calorimeter test

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Figure 11 displays the digital photographs of PUE and its composites after the cone calorimeter test. They intuitively show that pure PUE has a little residual char after combustion. It is worth noting that the residual char of PUE/CuMoO4@h-BN is thicker. It is attributed mainly to the physical barrier effect of CuMoO4@h-BN and the catalytic carbonization effect of its decomposed metal oxides. It is consistent with the results of the TGA test.

Figure 11
figure 11

Digital photographs of the samples after cone calorimeter test: pure PUE (a, e), PUE/h-BN (b, f), PUE/h-BN/CuMoO4 (c, g), PUE/CuMoO4@h-BN (d, h)

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The Raman spectra of the char residue of PUE and its composites after the cone calorimeter test are shown in Fig. 12. It can be seen from the figure that the Raman spectra of the four samples have a similar shape, with two peaks at 1360 and 1600 cm−1, which correspond to D and G band (D band represents the symmetrical carbon atoms of amorphous carbon vibration, and G band represents two-dimensional sp2 hybrid graphite carbon atoms in the symmetrical stretching vibration), respectively. The graphitization degree of char can be estimated by the relative intensity ratio of the D and G bands (ID/IG). The lower the ratio of ID/IG, the higher the graphitization degree of the char [30, 31]. The ID/IG value of pure PUE is 4.35. In comparison, the ID/IG values of PUE/h-BN, PUE/h-BN/CuMoO4 and PUE/CuMoO4@h-BN all decrease to some extent. Among them, PUE/CuMoO4@h-BN has the smallest ID/IG value, which indicates that CuMoO4@h-BN can effectively promote the formation of graphitized carbon, improving the graphitization degree of residual char, thereby enhancing the flame retardancy and smoke suppression performance of PUE.

Figure 12
figure 12

Raman spectra of char residue of pure PUE and PUE composites

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Figure 13 shows the C 1 s spectra of PUE and its composites after the cone calorimeter test, with the corresponding data listed in Table 4. The thermal oxidation resistance of polymer is investigated by calculating the Cox/Ca value (Cox: oxidized carbons and Ca: aliphatic and aromatic carbons). The smaller the value of Cox/Ca, the higher the thermal oxidation resistance of the composite formed during the combustion process [32]. The char layer with high thermal oxidative resistance could effectively inhibit the heat and mass transfer between the flame and matrix, delay the degradation of inner polymer and thus retard the combustion. As listed in Table 4, the Cox/Ca value of pure PUE is 0.86. Those of PUE/h-BN and PUE/h-BN/CuMoO4 are reduced to 0.75 and 0.66, respectively. However, the Cox/Ca value of PUE/CuMoO4@h-BN is further decreased to 0.56, indicating that CuMoO4@h-BN could effectively improve the thermal oxidation resistance of the char residue, and that PUE/CuMoO4@h-BN has higher flame retardancy and smoke suppression properties.

Figure 13
figure 13

C 1s spectra of char residue of pure PUE and PUE composites

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Table 4 Results of C 1s XPS of char residue of pure PUE and PUE composites
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Figure 14 shows the Mo 3d, Cu 2p3/2 and Cu LMM spectra of PUE/CuMoO4@h-BN residual char after the cone calorimeter test. As shown in Fig. 14a, the Mo 3d peak can be fitted into three peaks. Two peaks at 229.45 and 232.7 eV are attributed to the binding energies of Mo4+ 3d5/2, Mo4+ 3d3/2 and Mo6+ 3d3/2, respectively, indicating that +4 and +6 valences of molybdenum compounds are present in the char residue of PUE/CuMoO4@h-BN [33]. The Cu 2p3/2 peak is fitted to four peaks, and the two peaks at 934.5 and 932.8 eV belong to the binding energy of Cu2+, Cu+ or Cu0. In addition, there are two satellites of Cu2+ between the 938 and 945 eV peak (S1 and S2). Since the binding energies of Cu+ and Cu0 are extremely small in the Cu 2p3/2 spectrum, it is difficult to distinguish the existence of Cu+ and Cu0. Therefore, the Auger (LMM) spectrum of Cu is used to distinguish the existence of Cu+ and Cu0 [34]. It can be seen that the Cu LMM peak can be fitted into three peaks belonging to Cu0, Cu+ and Cu2+, respectively, indicating that copper, + 1 and + 2 valence copper-containing compounds coexist in the residual char of the PUE/CuMoO4@h-BN composite.

Figure 14
figure 14

Mo 3d (a), Cu 2p3/2 (b) and Cu LMM (c) spectra of char residue of PUE/CuMoO4@h-BN

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XRD is used to further analyze the residual char after the cone calorimeter test of PUE/CuMoO4@h-BN to explore the types of Mo and Cu compounds. The results are shown in Fig. 15. The diffraction peak of h-BN can be seen in the figure, indicating that the residual char still contains h-BN. The h-BN sheet has good thermal stability and can play a very good role with a barrier effect in the polymer’s combustion process. In addition, the diffraction peaks of MoO3 and MoO2 can also be found in the spectrum, as the result of thermal decomposition of CuMoO4. Among them, MoO3 as a Lewis acid in the decomposition of the matrix through the promotion of Friedel–Crafts alkylation reaction to form more residual char inhibits further combustion of the matrix [35]. In addition, Cu element in the residual char in the form of single element Cu, CuO and Cu2O exists. Cu2O can catalyze the coupling reaction between polymer molecular chains to form a continuous dense char layer. At the same time, Cu2O can be reduced to elemental Cu by reductive gases such as CO and H2, and elemental Cu can be oxidized by oxygen to Cu2O, thereby continuing to exert its catalytic carbonization effect, promoting the formation of more char residue and reducing the smoke emission of the polymer [36].

Figure 15
figure 15

XRD pattern of the char residue after the cone calorimeter test of PUE/CuMoO4@h-BN

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Based on the above, the mechanism of flame retardancy and smoke suppression properties of PUE/CuMoO4@h-BN is illustrated in Scheme 2. The reason for the improved flame retardancy and smoke suppression of the PUE composite, on the one hand, is ascribed to the physical barrier effect of h-BN, and on the other hand, is that CuMoO4 generates MoO3 and Cu2O during the combustion process. MoO3 and Cu2O have the effect of promoting the formation of a compact char layer. The compact char layer can effectively inhibit heat transfer in the system during the combustion of the polymer, slow down the volatilization and diffusion of flammable gas and retard the further degradation of the composites.

Scheme 2
scheme 2

Schematic illustration of the flaming PUE/CuMoO4@h-BN

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Conclusions

In this work, CuMoO4 was loaded on the surface of h-BN to synthesize a novel flame retardant (CuMoO4@h-BN). The structure of CuMoO4@h-BN was systematically characterized. Subsequently, CuMoO4@h-BN was added to PUE to study the influence of CuMoO4@h-BN on its thermal, flame retardancy and smoke suppression properties. The results of TGA showed that the char yield of PUE/CuMoO4@h-BN reached 9.8% at 700 °C, indicating that CuMoO4@h-BN had a good effect of catalytic carbonization. In addition, the PHRR, THR and Ds,max of CuMoO4@h-BN/PUE decreased by 73.6, 52.4 and 28.2%, respectively. CuMoO4@h-BN exhibited an excellent flame retardancy and smoke suppression performance. CuMoO4@h-BN could improve the flame retardancy and smoke suppression properties of PUE due to the physical barrier effect of h-BN and the catalytic charring effect of CuMoO4. The char layer acted as a physical barrier, inhibiting the exchange of heat between the fire and the matrix, isolating O2, and thereby retarding the further degradation of the polymer. Hopefully, the approach demonstrated herein will be a promising pathway for improving the flame retardancy and smoke suppression properties of polymer composites.