Introduction

Research on flame retardant polymer is primarily focused on the fire scenario and the development of new kinds of flame retardant additives [1]. Additive flame retardants, as one of the most effective ways, has been widely employed to protect a polymer against fire [2]. Polymer flame retardants are divided into halogen-containing (bromine/chlorine compounds [3, 4] and halogenated phosphates [5], etc.), inorganic (metal hydroxides/metal oxides [6], silicon-containing additives [7] and ammonium polyphosphate [8], etc.), organic phosphorus/nitrogen (aliphatic phosphate [9], aromatic phosphonates [10] and nitrogen-containing flame retardants [11], etc.) and nanosized materials (zero/one/two-dimensional nanomaterials [12]) flame retardants categories. Among them, two-dimensional nanomaterials, nanoplatelets with layered structures, have been widely applied to induce fire resistance and smoke suppression of polymeric materials [13, 14]. This flame retardant effect is primarily attributed to the so-called “tortuous path” effect of nanosheets, which can act as a barrier to heat and mass transfer, and delay the release of pyrolysis gases of polymers during thermal degradation [15, 16]. However, it is believed that the challenge in applying two-dimensional nanomaterials as flame retardant additives is the high cost of peeling. Meanwhile, due to the improved polymer performance, it will be a long-term challenge to select suitable two-dimensional nano materials for specific polymers and prepare two-dimensional nano additives that are functionally controllable by simple and universal methods, so as to overcome the multi-functionality of polymers.

TPU is a widely used plastic [17, 18]. Designing flame retardant additives for TPU not only requires flame retardant and smoke suppression performance but also certain self-healing performance [19]. To solve this challenge, we have designed flame retardants via in-situ growth of polypyrrole on the surface of two-dimensional carbon nitride nanosheets (g-C3N4). Two-dimensional graphitic carbon nitride, polyheptazine, has been employed in the flame retardant field due to its condensed-phase flame retardancy mechanism that forms a physical barrier in the polymer matrix. Polypyrrole (PPy) is an important heterocyclic conjugated polymer material. Many studies have shown that the incorporation of PPy can improve the thermal stability and flame retardancy of polymers [20,21,22]. In the PPy molecule, the introduction of P through the surface functionalization of phytic acid has proven to be a feasible method to enhance the flame retardant effects [17, 23]. Meanwhile, PPy and g-C3N4 nanosheet can form stable organic-organic heterojunctions through π-π stacking interactions [24,25,26], so that the conductive polymer can grow on the surface of the g-C3N4 nanosheet. It has been reported that PPy can be employed in the field of biomedicine [27] and self-healing [28] due to its excellent photothermal effects. It is speculated that in-situ growth of PPy coatings on exfoliated g-C3N4 nanosheets can improve the dispersion and interfacial interaction of g-C3N4 in the TPU matrix, aiding its flame retardancy, rheological and self-healing properties.

To achieve a multifunctional additive for TPU with both improved rheological and self-healing ability, we designed a synergetically-enhanced, multifunctional flame retardant using exfoliated carbon nitride as the core and phytic acid-doped PPy as the shell. Then, a multifunctional additive was incorporated into TPU, and the fire hazards, rheological behavior, mechanical and self-healing performances were investigated.

Experimental

Materials

Urea (99%) was obtained from J&K Scientific Ltd. Phytic acid (PA) solution (70%), pyrrole solution, (NH4)2S2O8 (APS), methanol, ethanol, and N,N-dimethylformamide (DMF) were provided by Macklin Ltd (China). All chemicals were used as received. TPU (S80A, BASF SE) with a density of 1.21 g/cm3 was used as received.

Synthesis of few-layered carbon nitride (ECN)

In a typical synthesis, 20 g urea was calcined at 550 ℃ in air for 4 h (heating rates: 8.7 K/min) to synthesize bulky carbon nitride (CN). Then, 0.7 g bulky CN powder was heated to 500 °C (heating rates: 5 K/min) for 4 h in a tube furnace under a water atmosphere using an Ar-carrying gas to synthesize ECN nanosheets (yield: 55%).

Synthesis of CNPPy

0.6 g ECN powder was dispersed in 600 mL of deionized water for 0.5 h to form a uniform suspension. Then, 0.369 g of pyrrole solution and 15.0 g of PA acid were added into the suspension under ultrasonication for 15 min. Next, 0.72 g of APS was dissolved into 50 mL of deionized water and the formed solution was slowly added into the above suspension dropwise. Pyrrole polymerization proceeded for 4 h, and the black product was collected by filtration, and washed with deionized water, methanol, and ethanol three times. The product of CNPPy was subsequently dried overnight at 90 ℃. The product of PA-doped polypyrrole particles (PPy) was prepared in the absence of ECN. The preparation procedure of multifunctional CNPPy was illustrated in Scheme 1.

Scheme 1
scheme 1

Schematic illustration for the preparation of the CNPPy

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Fabrication of pure TPU and its nanocomposites

Pure TPU and its nanocomposites were fabricated by a solution blending method. Typically, 3.0 g of CNPPy was dispersed in 200 mL of DMF with ultrasonication and stirring for 0.5 h. Then, the dispersion was heated to 50 ℃, and 97.0 g of TPU pellets was dissolved into the dispersion via stirring for 3 h. Then, the mixture was slowly poured into low temperature deionized water to sufficiently remove the solvent, which was further dried at a 100 ℃ oven for 12 h to remove the remaining solvent. The sample containing 3.0 wt% CNPPy was hot-pressed at 185 ℃ to obtain sheets of suitable and named as CNPPy-TPU. Other samples with 3.0 wt% ECN and 3.0 wt% PPy were defined as ECN-TPU and PPy-TPU, respectively. The control sample of pure TPU was also prepared by the process above.

Characterization

X-ray diffraction (XRD) patterns were obtained using an X-ray diffractometer (Empyrean, Malvern Panalytical, Netherlands) equipped with CuKα radiation. Fourier transform infrared (FT-IR) spectra were obtained on a Fourier transform infrared spectrometer (EQUINOX 55, Bruker) and scanned from 4000 to 400 cm−1. Transmission electron microscopy (TEM) were performed using a FEI Talos 200S FEI microscope integrated with an energy dispersive X-ray spectrometer (EDS). Thermogravimetric analysis (TGA) was performed on a NETZSCH TG209F1 libra thermogravimetric analyzer. Thermal analysis was carried out from room temperature to 800 °C at a heating rate of 10° C min−1 in N2 or air atmosphere. Cone calorimetry tests (CCT) were carried out by a FTT0007 cone calorimeter under radiation heat flux of 35 kW/m3. The sample dimension was 100 × 100 × 1 mm3 or 100 × 100 × 3 mm3. Small-scale combustion (MCC) performance was investigated by a Govmark MCC-2 microcalorimeter (New York, NY, USA). Tensile testing of three groups of composites (dumbbell specimens) were examined by a tensile testing machine (CMT4304GD, Zhuhai SUST Electrical Equipment Co., Ltd, China) in accordance with the GB/T 1040.2–2006 standard at room temperature. The assay was performed under a linear deformation loading rate of 30 mm/min until mechanical failure occurred. A laser system (LASER DIODE DRIVER-808, DS3-11,312–705) was used to generate near-infrared light at 808 nm, which was used to measure the near-infrared light response. A spectrophotometer (PE Lambda950) was used to record the ultraviolet–visible-near-Infrared (UV–vis-NIR) absorption spectrum of all nanocomposites. The rheological behavior was measured on a rheometer (ARES, TA Co., New Castle, USA) with a disk-like sample size of about 25 mm in diameter. Frequency sweeping was selected from 0.1 to 1000 rad s−1 with a strain of 0.1%, and the testing was carried out at 195 °C under nitrogen.

Results and discussion

Fourier transform infrared spectroscopy (FT-IR) and X-Ray diffraction (XRD) were employed to confirm the structures of ECN, CNPPy, and PPy. For the typical FT-IR spectrum of exfoliated carbon nitride (Fig. 1a), the peaks at 1245, 1330, 1415, 1573, and 1639 cm−1 correspond to the typical stretching mode of CN heterocycles. The broad peak at 3000–3500 cm−1 corresponds to the stretching vibration of N–H and C-H [14]. The physically adsorbed H2O molecules are also attributed to this broad peak [24]. Furthermore, the characteristic breathing vibration of the triazine unit at 810 cm−1 was observed. It was noted that the peak at 1562 cm−1 shown by the blue curve is attributable to the C–C and C = C tensile vibrations of the pyrrole ring [17], confirming the successful synthesis of PPy. The peaks at 1105 and 931 cm−1 are attributed to the C-H and N–H vibrations in pyrrole [29, 30]. The N–H bond vibration of a typical pyrrole of 931 cm−1 was also observed in CNPPy, indicating the successful loading of polypyrrole on carbon nitride. In addition, the two peaks of PPy at 1205 and 1055 cm−1 are attributed to the P = O and P-O bonds [31, 32], which prove the successful doping of PA on polypyrrole. For the XRD spectrum of ECN (Fig. 1b), two peaks were identified. The peak at 12.90° corresponds to the in-plane structure filling motif of tri-s-triazine units (100), and the peak at 27.76° corresponds to the interlayer stacking of aromatic planes (002) [33]. Compared with ECN, the XRD peak position and morphology of CNPPy were unchanged, indicating that the in-situ polymerization of PPy had no effect on the lattice structure of ECN.

Fig. 1
figure 1

(a) FT-IR spectra and (b) XRD patterns of ECN, CNPPy and PPy

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The morphology and structure of ECN were investigated by TEM. ECN showed a smooth surface without other nanoparticles attached (Fig. 2a). ECN exhibited a thin nanosheet structure, which showed obvious differences when compared with unexfoliated g-C3N4 that had shown a stacked sheet structure in the literature [14]. ECN nanosheets tend to be fragmented with a smaller area than g-C3N4 because of thermal steam reforming [34]. PPy showed a dark round structure in the TEM (Fig. 2c). The morphology of CNPPy was studied as shown in Fig. 2b. The PPy nanoparticles accumulated in the darker area were observed on the ECN surface, indicating that the PPy was successfully loaded on the surface of the ECN nanosheets. In addition, the EDS spectra of CNPPy showed the presence of C, N, O, and P elements, indicating the successful doping of PA on PPy. The digital photos of ECN and CNPPy are shown in Fig. 2d. ECN is typically light yellow, while CNPPy was obviously darkened to almost black due to the in-situ polymerization of PPy on the ECN surface. The distribution of the CNPPy nanosheets in the polyurethane host was further dissected by TEM, as depicted in Fig. 2f, g and h. As shown in Fig. 2g, the CNPPy can be uniformly dispersed in the matrix, presenting a better dispersion as compared with ECN (see Fig. 2f). From Fig. 2h, some particles with about 200 nm were dispersed in the TPU. This demonstrated that the combination of g-C3N4 nanosheets and PPy can further exfoliate ECN.

Fig. 2
figure 2

TEM images of (a) ECN, (b) CNPPy and (c) PPy, scale bar: 100 nm; (d) Digital photos of ECN and CNPPy; EDS spectra of (e) CNPPy. TEM ultrathin section images of TPU nanocomposites: (f) ECN-TPU; (g) CNPPy-TPU; (h) PPy-TPU. Scale bar: 200 nm

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Thermal properties of ECN, CNPPy, and PPy were characterized by thermogravimetric analysis (TGA) under nitrogen and air, as presented in Fig. 3 and Table 1. The temperatures at 5% and 50% weight loss were defined as T-5 and T-50, respectively, and the residues at 750 °C were investigated in Table 1. The first mass loss of PPy below 100 °C was attributed to the trapped moisture (Fig. 3a). However, the second mass loss from 230 °C could be attributed to the loss of PA and PPy chain degradation, as the char residues at 750 °C reached 60.1% in nitrogen. ECN exhibited a similar one-step degradation process whether under nitrogen or air. The char residues of CNPPy were 21.1% under nitrogen, which due to the high thermal stability of ECN, protecting PPy to a certain extent. PPy was almost completely degraded, leaving only 1.9% in air. However, the residues of CNPPy in the air were 4.1% (Fig. 3b), indicating that PA-doped PPy exhibiting a small degree of crosslinking to form char with ECN during thermal degradation.

Fig. 3
figure 3

TG curves of as-prepared flame retardants (a) TPU and its nanocomposites (c) in nitrogen. TG curves of as-prepared flame retardants (b) and TPU and its nanocomposites (d) in air

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Table 1 The related data of three kinds of flame retardants in nitrogen and air atomosphere
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In addition, the thermal stabilities of pure TPU and its nanocomposites in nitrogen and air were investigated, as shown in Fig. 3c, d, and Table 2. The T-5 of CNPPy-TPU was 313 °C in nitrogen, which is higher than the other three samples, indicating that CNPPy had good thermal stability in the polymer matrix. The T-5 of PPy-TPU was only 305 °C, which was due to the relatively poor thermal stability of the PA-doped PPy. The char residues of CNPPy-TPU at 500 ℃ and 750 ℃ were 11.9% and 8.6%, respectively, which were higher than those of the other three samples, indicating that CNPPy improved the flame retardance of TPU. The higher char residues of CNPPy-TPU at 500 °C were attributed to the high thermal stability of ECN. The char residues of CNPPy-TPU at 750 °C were the highest and likely attributed to the synergistic char formation of ECN and PPy during degradation. It is notable that the char residues of pure TPU and ECN-TPU were about 2% lower than that of CNPPy-TPU and PPy-TPU at 500 °C in air, which may be due to the presence of PA-doped PPy changing the decomposition process of the soft segment of TPU to a certain extent, leading to the formation of a small quality of unstable char.

Table 2 The related data of TPU and its nanocomposites in nitrogen and air atmosphere
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The mechanism of thermal degradation can be further elucidated by investigating the rheological properties of polymers [35]. Figure 4 shows the dependence of storage modulus (G') and complex viscosity (η*) on the shear frequency (ω) of TPU and its nanocomposites at 195 ℃ and 0.1% strain, respectively. For the binary ECN-TPU and PPy-TPU, the storage modulus trend with ω is similar to that of pure TPU. However, for ternary CNPPy-TPU, a much higher modulus in the low frequency region was clearly observed, which may be due to the formation of a network structure. The dependence of G' on ω weakens with an obvious plateau at low frequencies. It demonstrated that a transition from liquid to solid was accompanied by the formation of a mechanically stable network structure [36]. For CNPPy-TPU and PPy-TPU (as shown in Fig. 5), there was no intersection point between the storage modulus and loss modulus curves. PPy-TPU melts in the whole shear frequency range as exhibited by G' < G'', while CNPPy-TPU melts in the shear frequency range as exhibited by G' > G'', showing a solid-like behavior [37, 38]. The complex viscosity results of TPU and its nanocomposites also exhibited a similar conclusion (Fig. 4b). Therefore, the combination of carbon nitride and polypyrrole can form a more stable network in the TPU matrix, which is a prerequisite to improve the flame retardancy (reduce the peak heat release rate) of the polymer.

Fig. 4
figure 4

Effects of flame retardants on (a) storage modulus (G') and (b) complex viscosity (η*) as a function of shear frequency (ω) in TPU nanocomposites (195 °C)

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Fig. 5
figure 5

Storage modulus and loss modulus versus shear frequency curves for TPU and its nanocomposites: (a) TPU, (b) ECN-TPU, (c) CNPPy-TPU and (d) PPy-TPU

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Fire behaviors of TPU and its nanocomposites were investigated by CCT and MCC, and the results were shown in Figs. 6, 7 and 8 and Tables 3 and 4. Pure TPU possessed a pHRR value of 666.1 kW/m2, 872.3 kW/m2 and 347.7 W/g in CCT (1 mm-thick), CCT (3 mm-thick) and MCC test, indicating its high inflammability in fire. After the incorporation of 3 wt% ECN and PPy separately, the pHRR values of TPU nanocomposites decreased to varying degrees. When TPU was blended with 3wt% CNPPy, the pHRR values of corresponding nanocomposites experienced a 29.9%, 43.4% and 26.6% reduction in CCT (1 mm-thick), CCT (3 mm-thick) and MCC test compared with pure TPU, respectively, revealing further reduced in the peak heat release rate by the combined effects of carbon nitride and polypyrrole. It was worth noting that the law of the total heat release curve under different thicknesses was not consistent. The total heat release of CNPPy-TPU in the 1 mm-thick samples was the lowest among comparable samples, but it was not consistent in the 3 mm-thick samples. This may be attributed to the different products of thermal oxidative degradation and thermal degradation of TPU nanocomposites. The peak smoke production rate (pSPR) values of pure TPU (1 mm-thick) and pure TPU (3 mm-thick) were 0.101 m2/s and 0.176 m2/s during burning. The introduction of 3 wt% ECN, CNPPy, and PPy decreased the pSPR of TPU nanocomposites (1 mm-thick) to 0.068, 0.072, and 0.071 m2/s, respectively. The introduction of 3 wt% ECN, CNPPy, and PPy decreased the pSPR of TPU nanocomposites (3 mm-thick) to 0.129, 0.082, and 0.129 m2/s, respectively. When compared to pure TPU, the TSR (total smoke release) values (1 mm-thick) of ECN-TPU, CNPPy-TPU and PPy-TPU were reduced by 17.0%, 24.2%, and 13.2%, respectively, as shown in Fig. 6d. However, the TSP in the 3 mm-thick CCT test of the nanocomposites did not decrease compared to the pure TPU. The above results demonstrated that CNPPy possessed better smoke suppression performance than others, which would reduce the fire risk from smoke to a certain extent.

Fig. 6
figure 6

(a) HRR, (b) THR, (c) SPR and (d) TSP curves of pure TPU and its nanocomposites (1 mm-thick) obtained from the cone calorimeter test

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Fig. 7
figure 7

(a) HRR, (b) THR, (c) SPR and (d) TSP curves of pure TPU and its nanocomposites (3 mm-thick) obtained from the cone calorimeter test

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Fig. 8
figure 8

(a) HRR and (b) THR curves of pure TPU and its nanocomposites obtained from the Microcalorimeter test

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Table 3 The related data of TPU and its nanocomposites (1 mm-thick) obtained from cone calorimeter test
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Table 4 The related data of TPU and its nanocomposites (3 mm-thick) obtained from cone calorimeter test
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Figure 9 shows the tensile strength and elongation at break of the TPU nanocomposites. For pure TPU, the elongation at break and tensile strength were 2861.4% and 40.5 MPa, respectively. The elongation at break of ECN-TPU was almost equal as that of pure TPU. Unfortunately, all TPU nanocomposites had a lower tensile strength and shorter elongation at break than pure TPU. The tensile strengths of ECN-TPU and PPy-TPU were 33.4 MPa and 25.2 MPa, respectively, and reduced by 17.7% and 37.8% as compared with that of pure TPU. With 3 wt% CNPPy, the tensile strength and elongation at break of the TPU nanocomposites were 37.1 MPa and 2672.1%, respectively. Obviously, the effect of polypyrrole on the mechanical properties of TPU was effectively improved after the incorporation of ECN, compared with the corresponding values of PPy-TPU and ECN-TPU (Fig. 9b). This demonstrated that the formation of a stable network (the combination of carbon nitride and polypyrrole) in the TPU matrix could improve the tensile strength.

Fig. 9
figure 9

Mechanical properties of Pure TPU, ECN-TPU, CNPPy-TPU and PPy-TPU nanocomposites: (a) stress–strain curves; (b) tensile strength and elongation

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The polymer nanocomposites possess excellent photothermal conversion performance, so that it can reach the appropriate temperature in a short time and realize self-healing [28]. The process and mechanism of near-infrared (NIR) self-healing are shown in Fig. 10a. First, the sample was cut into a dumbbell shape, and then the dumbbell shape was completely cut off. Next, two of the cut surfaces were put back together, placed under a near-infrared laser light, and then irradiated to self-healing. After irradiation, the treated sample was placed in air for 2 days. Finally, a tensile test was performed to evaluate the repair efficiency. The self-healing efficiency was defined as: \(healing\;efficiency\;\mathrm{ \%}=\frac{\sigma\;of\;healing\;sample}{\sigma\;of\;original\;sample}\times 100\mathrm{\%}\), where σ is the tensile strength of the samples.

Fig. 10
figure 10

(a) Schematic illustration of NIR light-induced self-healing. The (b) DSC and (c) UV–vis-NIR absorption spectra of TPU and its nanocomposites. (d) The temperature elevation versus illumination time (808 nm, 1 W, distance from the sample surface to laser: 2 cm) curves of TPU and its nanocomposites. The (e) stress–strain and (f) healing efficiency curves of TPU and its nanocomposites after exposure to NIR light

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The DSC curves of the TPU nanocomposites are shown in Fig. 10b. The first heating scan of pure TPU showed two melting temperature peaks (Tm), 154.2 ℃ and 182.2 ℃, respectively. The peaks of Tm at lower and higher temperatures can be attributed to the melting of less and better ordered structures [39, 40], respectively. With the PPy and ECN incorporation into pure TPU, the first and second melting temperatures almost disappear, respectively. The melting peak of CNPPy-TPU was closer to that of ECN-TPU, which indicates that CNPPy still keeps the nanocomposite in a state similar to that of ECN-TPU. The results of UV–vis-NIR absorption spectra are given in Fig. 10c. Compared with pure TPU and ECN-TPU, CNPPy-TPU and PPy-TPU have obvious absorption at about 808 nm in the NIR region. Therefore, 808 nm was used to investigate the photothermal effect, so that it has excellent rapid heating performance. As shown in Fig. 10d, we analyzed the temperature increase of TPU and its nanocomposites after NIR light irradiation (808 nm, 1 W). The temperatures of pure TPU, ECN-TPU, CNPPy-TPU, and PPy-TPU could be elevated by 68.9 ℃, 51.4 ℃, 174.6 ℃, and 181.0 ℃, respectively. This demonstrated that the CNPPy-TPU has excellent photothermal effects compared with pure TPU. To further study the healing properties under NIR light, CNPPy-TPU was selected as the research object. Figure 10e, f present the stress–strain curves and mechanical healing efficiencies of CNPPy-TPU with different irradiation times. With increasing irradiation time, the healing efficiency of CNPPy-TPU gradually increased. The healing efficiency of CNPPy-TPU can reach 31.3% after 45 s. Unfortunately, when the irradiation time was 90 s, the healing efficiency of CNPPy-TPU drops to 19.7%. This shows that although prepared TPU nanocomposites can quickly heat up through NIR radiation to achieve self-healing, the effect is still limited. This is because overexposure will generate excessive heat, which may cause deformation of the TPU nanocomposites.

Conclusion

In this work, we designed and synthesized a multifunctional additive using exfoliated carbon nitride as the core and phytic acid-doped PPy as the shell. The microstructure of the TPU nanocomposites was characterized, and the distribution of phytic acid-doped polypyrrole on carbon nitride was further illustrated by TEM and EDS. Thermal analysis shows that the incorporation of PPy improves the thermal degradation residues of the prepared nano additive, and there was a synergistic effect between carbon nitride and PPy in the polymer matrix, which promoted the increased amounts of residues in the condensed phase. CNPPy has obtained the best dispersibility in TPU and can form an effective network structure in the polymer, but no such phenomenon was found when carbon nitride or phytic acid-doped polypyrrole were used alone. The formation of the network structure further reduces the peak heat release rate in the condensed phase and enables the TPU nanocomposites to obtain better flame retardant performance, as shown by the CCT test. Especially after the repair experiment, the introduction of CNPPy can make the composite material obtain self-healing effects under near-infrared light. The strategy of constructing a two-dimensional nanocomposite additive in this paper provides a simple and universal method for the preparation of multifunctional polymers.