Fabrication of Polypyrrole-Decorated Tungsten Tailing Particles for Reinforcing Flame Retardancy and Ageing Resistance of Intumescent Fire-Resistant Coatings

Polypyrrole-decorated tungsten tailing particles (PPY-TTF) were prepared via the in situ polymerization of pyrrole in the presence of tungsten tailing particles (TTF), and then carefully characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and thermogravimetric analysis (TG) analyses. The effect of PPY-TTF on the flame retardancy, smoke suppression property and ageing resistance of intumescent fire-resistant coatings was investigated by a fire protection test, smoke density test and cone calorimeter test. The results show that PPY-TTF exerts excellent cooperative effect on enhancing the flame retardancy and smoke suppression properties of the intumescent fire-retardant coatings, which is ascribed to the formation of more cross-linking structures in the condense phase that enhance the compactness and thermal stability of intumescent char. The cooperative effect of PPY-TTF in the coatings depends on its content, and the coating containing 3 wt% PPY-TTF exhibits the best cooperative effect among the samples, showing a 10.7% reduction in mass loss and 35.4% reduction in flame-spread rating compared to that with 3% TTF. The accelerated ageing test shows that the presence of PPY-TTF greatly slows down the blistering and powdering phenomenon of the coatings, thus endowing the coating with the super durability of fire resistance and smoke suppression property. This work provides a new strategy for the resource utilization of tungsten tailing in the field of flame-retardant materials.


Introduction
Intumescent fire-retardant coatings are widely used in high-rise buildings, steel structures, ancient buildings, railway stations, airport terminals and other buildings for the advantages of good finish, small coating thickness, being lightweight, and excellent fire protection performance of the components [1][2][3][4], which are considered to be one of the most effective and economical materials to reduce fire hazards on buildings [5,6]. Intumescent fire-retardant coatings generally compose of an intumescent fire-retardant system, binders, synergists, adjuvants and additives, which interact to forge an expanding char layer during combustion that effectively delays the spread of flame and preventing the formation of fire, thus guaranteeing the structural integrity of buildings and minimizing the fire risk [7,8]. Although a large number of studies have concentrated on the enhancement of fire resistance, smoke suppression and reduction in the thickness of intumescent fire-retardant coatings [4,[9][10][11], which neglected the ageing degradation of intumescent fire-retardant coatings under the impacts of environmental factors such as certain humidity, UV radiation, temperature and oxygen.
The use of multifunctional additive is an important strategy to enhance the flameretardant and smoke suppression efficiencies of intumescent fire-retardant materials. Numerous efforts have investigated the effects of additive on fire resistance, smoke suppression  98 The intumescent flame retardant (IFR) was obtained by mixing APP, PER and MEL 99 with a mass ratio of 3:1.5:1, which was combined with PPY-TTF and deionized water ac-100 cording to the formula in Table 1 and then stirred at 1000 r/min for 20 min. Then, water-101 borne epoxy resin, defoamer and dispersant were put into the slurry stirring at 500 r/min 102 for 20 min. Finally, the waterborne epoxy hardener was added and stirred at 500 r/min for 103 20 min to obtain the intumescent fire-retardant coatings. The intumescent fire-retardant 104 coatings with different amounts of PPY-TTF are named IFRC0-IFRC5, and the specific test 105 formulations are shown in Table 1 Fourier transform infrared spectroscopy (FTIR) spectra were characterized by an 110 iCAN9 FTIR spectrometer using KBr pellets (Tianjin Energy Spectrum Technology Co., 111 Ltd., Tianjing, China).

Preparation of the Intumescent Fire-Retardant Coatings
The intumescent flame retardant (IFR) was obtained by mixing APP, PER and MEL with a mass ratio of 3:1.5:1, which was combined with PPY-TTF and deionized water according to the formula in Table 1 and then stirred at 1000 r/min for 20 min. Then, waterborne epoxy resin, defoamer and dispersant were put into the slurry stirring at 500 r/min for 20 min. Finally, the waterborne epoxy hardener was added and stirred at 500 r/min for 20 min to obtain the intumescent fire-retardant coatings. The intumescent fire-retardant coatings with different amounts of PPY-TTF are named IFRC 0 -IFRC 5 , and the specific test formulations are shown in Table 1. Fourier transform infrared spectroscopy (FTIR) spectra were characterized by an iCAN9 FTIR spectrometer using KBr pellets (Tianjin Energy Spectrum Technology Co., Ltd., Tianjing, China).

Scanning Electron Microscopy
The MIRA 3 LMU scanning electron microscopy (Tescan, Brno, Czech Republic) was used to characterize the micromorphology of the samples at a voltage of 20.0 kV.

X-ray Diffraction
X-ray diffraction (XRD) pattern was characterized by an Advance D8 XRD diffractometer at a range of 3 to 70 • , a Cu-Ka radiation and a rate of 5 • /min (Bruker, Fällanden, Switzerland).

X-ray Fluorescence Spectrometer
X-ray fluorescence spectrometer (XRF) analysis was performed with a ZSX Primus II X-ray Fluorescence Spectrometer (Rigaku, Japan). Fire protection properties of the samples were assessed by the big panel method, cabinet method, tunnel method tests according to GB12441-2018 standard procedure. The big panel method test was conducted on a MT-X multiplex temperature recorder (Shenzhen Shenhwa Technology Co., Ltd., Shenzhen, China) to obtain the backside-temperature curves of coating samples.
Cabinet method test was carried out on an XSF-1 apparatus, the weight loss (the difference in the sample mass before and after the test), char index and intumescent factor (the ratio of the thickness of the coatings after and before test) of the samples were determined. The char index was calculated by Formula (1): where a i is defined as the char length (cm), b i is defined as the char width (cm), h i is defined as the char depth (cm) and n is the number of samples. Tunnel method test was conducted on a SDF-2-type 2-foot flame tunnel instrument (Jiangning Analysis Instrument Company, Nanjing, China). The flame spread over the coating surface of the samples was evaluated when ignited under controlled conditions in a small tunnel, and the flame-spread rating of the samples was calculated by Formula (2): where L s is the mean of five flame advance readings of samples (mm), L a is the mean of five flame advance readings of asbestos board (mm) and L r is the mean of five flame advance readings of oak board (mm).

Adhesion Classification Test
The adhesion classification test was carried out on a QFH-HD600 adhesion tester to test the coating adhesion (Changzhou Edex Instruments Co., Ltd., Changzhou, China) on the basis of ASTM D 3359-09.

Pencil Hardness Test
The pencil hardness test was performed on a QHQ-A portable pencil scratch tester in accordance with ISO 15184-2012. The lead of a pencil was vertically erected and polished on sandpaper for a flat and round cross-sectioned end. The pencil hardness of the coating was tested progressively by placing a polished pencil on the coating at an angle of 45 • under a load of 750 g.

Cone Calorimeter Test
The cone calorimeter test was used to characterize the heat release rate (HRR) and total heat release (THR) curves of coatings with an external heat flux of 50 kW/m 2 .

Thermogravimetric Analysis
TG analysis was carried out on a TGA/SOTA 851 thermogravimetric instrument (Mettretoli Instruments Co., Ltd., Zurich, Switzerland) from 30 • C to 800 • C at a heating where W i (t) is the amount of residual char for i at t • C; χ i is the percentage of i, %.

Accelerated Ageing Test
The accelerated ageing test was performed on an UV-accelerated ageing tester (Shi Haoran Machinery Equipment Factory, Dongguan, China) according to ASTM G154-2006. One ageing cycle included 12 h, during which, the condensation time was 4 h at 50 ± 3 • C, and the UV exposure was 8 h at 60 ± 3 • C with an irradiation of 0.76 W/(m 2 ·nm). The samples were exposed to accelerated ageing tests for 2, 6 and 11 cycles, accompanying with the change of sample position every 12 h.

Morphology and Composition of TTF and PPY-TTF
The XRD patterns of TTF are presented in Figure 2. From Figure 2, the XRD patterns of TTF samples are generally consistent with the standard patterns of quartz (SiO 2 ) (PDF85-0794 The accelerated ageing test was performed on an UV-accelerated ageing tester (Shi 167 Haoran Machinery Equipment Factory, Dongguan, China) according to ASTM G154-2006. 168 One ageing cycle included 12 h, during which, the condensation time was 4 h at 50 ± 3 °C, 169 and the UV exposure was 8 h at 60 ± 3 °C with an irradiation of 0.76 W/(m 2 ·nm). The 170 samples were exposed to accelerated ageing tests for 2, 6 and 11 cycles, accompanying 171 with the change of sample position every 12 h.      Figure 3 exhibits the SEM images and EDS maps of TTF and PPY-TTF. From Figure 3a, it can be seen that the TTF sample shows irregular blocky structures with a size range of 10 and 50 µm. Compared to TTF, the size of PPY-TTF is obviously reduced, showing a smaller agglomeration phenomenon. From EDS maps, C and N elements from polypyr-   Figure 4 shows the FTIR spectra of TTF and PPY-TTF. In the spectrum of TTF, the characteristic peaks at 1005, 777 196 and 459 cm −1 are attributed to the symmetric and antisymmetric stretching vibration peaks of Si-O-Si [22,23]. As for 197 PPY-TTF, the new peaks of the C-H stretching vibration (2928, 2859 cm −1 ), C=C stretching vibration (1631 cm −1 ) and C-198 N and N-H stretching vibration (1551 cm −1 ) are observed [23][24][25], further confirming polypyrrole was successfully dec-199 orated on the surface of TTF particles.  Figure 4 shows the FTIR spectra of TTF and PPY-TTF. In the spectrum of TTF, the characteristic peaks at 1005, 777 and 459 cm −1 are attributed to the symmetric and antisymmetric stretching vibration peaks of Si-O-Si [22,23]. As for PPY-TTF, the new peaks of the C-H stretching vibration (2928, 2859 cm −1 ), C=C stretching vibration (1631 cm −1 ) and C-N and N-H stretching vibration (1551 cm −1 ) are observed [23][24][25], further confirming polypyrrole was successfully decorated on the surface of TTF particles.  Figure 5 shows the TG and differential thermo-gravimetric (DTG) curves of TTF and 203 PPY-TTF. From the TG and DTG curves of TTF, it can be seen that the pyrolysis process 204 of TTF is mainly from 420 °C to 800 °C concomitant with a strong DTG peak and a residual 205 weight of 96.0% at 800 °C. Meanwhile, the pyrolysis process of PPY-TTF is mainly divided 206 into four stages. The first stage corresponds to the temperature interval of 26-200 °C, 207 which is due to physics-absorbing water molecules, oligomer molecules and volatile im-208  process of TTF is mainly from 420 • C to 800 • C concomitant with a strong DTG peak and a residual weight of 96.0% at 800 • C. Meanwhile, the pyrolysis process of PPY-TTF is mainly divided into four stages. The first stage corresponds to the temperature interval of 26-200 • C, which is due to physics-absorbing water molecules, oligomer molecules and volatile impurities. The second stage at 200-430 • C is attributed to the degradation of low molecular-weight polymer chains. The third stage at 430-560 • C is the decomposition stage of polypyrrole. The fourth stage at 560-800 • C is the decomposition stage of TTF and polypyrrole, remaining an 81.8% residue at 800 • C.  Figure 5 shows the TG and differential thermo-gravimetric (DTG) curves of TTF and 203 PPY-TTF. From the TG and DTG curves of TTF, it can be seen that the pyrolysis process 204 of TTF is mainly from 420 °C to 800 °C concomitant with a strong DTG peak and a residual 205 weight of 96.0% at 800 °C. Meanwhile, the pyrolysis process of PPY-TTF is mainly divided 206 into four stages. The first stage corresponds to the temperature interval of 26-200 °C, 207 which is due to physics-absorbing water molecules, oligomer molecules and volatile im-208 purities. The second stage at 200 °C-430 °C is attributed to the degradation of low molec-209 ular-weight polymer chains. The third stage at 430-560 °C is the decomposition stage of 210 polypyrrole. The fourth stage at 560-800 °C is the decomposition stage of TTF and 211 polypyrrole, remaining an 81.8% residue at 800 °C.

215
The results of the tunnel method and cabinet method tests are given in Table 3. The 216 mass loss, charring index, flame-spread rating and intumescent factor of IFRC0 samples 217 are 3.7 g, 34.9 cm 3 , 20.5 and 25.0, respectively, while the addition of PPY-TTF can signifi-218 cantly enhance the fire resistance of the intumescent fire-retardant coatings. In particular, 219 IFRC3 presents the best heat-insulation performance, with a 32.9% reduction in weight 220 loss, 46.7% reduction in charring index, 74.3% reduction in flame-spread rating and 80.0% 221 increase in intumescent factor compared with IFRC0. The results indicate that the appro-222 priate amount of PPY-TTF obtained via the in situ polymerization of pyrrole can effec-223 tively strengthens the fire protection of coatings. In addition, an excessive amount of PPY-224 TTF may suppress the expansion and carbonization of the coatings, thus diminishing its 225 cooperative fire resistance in intumescent fire-retardant coatings.

Fire Protection Tests
The results of the tunnel method and cabinet method tests are given in Table 3. The mass loss, charring index, flame-spread rating and intumescent factor of IFRC 0 samples are 3.7 g, 34.9 cm 3 , 20.5 and 25.0, respectively, while the addition of PPY-TTF can significantly enhance the fire resistance of the intumescent fire-retardant coatings. In particular, IFRC 3 presents the best heat-insulation performance, with a 32.9% reduction in weight loss, 46.7% reduction in charring index, 74.3% reduction in flame-spread rating and 80.0% increase in intumescent factor compared with IFRC 0 . The results indicate that the appropriate amount of PPY-TTF obtained via the in situ polymerization of pyrrole can effectively strengthens the fire protection of coatings. In addition, an excessive amount of PPY-TTF may suppress the expansion and carbonization of the coatings, thus diminishing its cooperative fire resistance in intumescent fire-retardant coatings.  Figure 6 shows the backside temperature curves of the substrates coated with IFRC 0 -IFRC 4 coatings. As demonstrated in Figure 6, the backside temperature of the IFRC 0 sample without PPY-TTF rises rapidly, with a fire resistance time of 800 s, corresponding to a poor heat-insulation performance. The backside temperature of IFRC 1 -IFRC 4 samples containing PPY-TTF has a slower rise and becomes stable at about 500 s. The equilibrium backside temperatures of the samples at 900 s are 180.0, 167.7, 142.3 and 156.2 • C, respectively, indicating that the presence of PPY-TTF enhances the fire protection of the coatings. Among them, the IFRC 3 sample presents the best fire-retardant effect, which is consistent with the results in Table 3.
ple without PPY-TTF rises rapidly, with a fire resistance time of 800 s, corresponding to a 230 poor heat-insulation performance. The backside temperature of IFRC1-IFRC4 samples con-231 taining PPY-TTF has a slower rise and becomes stable at about 500 s. The equilibrium 232 backside temperatures of the samples at 900 s are 180.0, 167.7, 142.3 and 156.2 °C, respec-233 tively, indicating that the presence of PPY-TTF enhances the fire protection of the coatings. 234 Among them, the IFRC3 sample presents the best fire-retardant effect, which is consistent 235 with the results in Table 3. where the IFRC0 char presents the smallest expansion height and the poorest surface struc-240 ture, thus showing the worst fire-retardant properties. After the addition of PPY-TTF, the 241 char layer heights of IFRC1-IFRC4 are 8.5 mm, 11.5 mm, 14.5 mm and 12.5 mm, respec-242 tively, revealing that the presence of PPY-TTF enhances the carbonization and expansion 243 process of the coatings. In particular, the IFRC3 sample exhibits the highest intumescent 244 factor and the densest char. From the SEM images of Figure 8, it can be found that the 245 introduction of PPY-TTF facilitates the formation of a denser and more continuous char 246 layer structure without obvious holes or other defects that effectively blocks the transfer 247 of combustible materials and heat. From the EDS maps of Figure 8, the higher content of 248 P and Si elements in IFRC3 are observed, which indicate that the presence of PPY-TTF is 249 favorable to the formation of more phosphorus-rich and silicon-rich cross-linking struc-250 tures that enhance the heat-insulation performance of the char layer. Moreover, the char 251 layer of IFRC3 has a higher C/O mass ratio compared to that of IFRC0, resulting in a good 252 oxidation resistance and fire resistance [7].  After the addition of PPY-TTF, the char layer heights of IFRC 1 -IFRC 4 are 8.5 mm, 11.5 mm, 14.5 mm and 12.5 mm, respectively, revealing that the presence of PPY-TTF enhances the carbonization and expansion process of the coatings. In particular, the IFRC 3 sample exhibits the highest intumescent factor and the densest char. From the SEM images of Figure 8, it can be found that the introduction of PPY-TTF facilitates the formation of a denser and more continuous char layer structure without obvious holes or other defects that effectively blocks the transfer of combustible materials and heat. From the EDS maps of Figure 8, the higher content of P and Si elements in IFRC 3 are observed, which indicate that the presence of PPY-TTF is favorable to the formation of more phosphorus-rich and silicon-rich cross-linking structures that enhance the heat-insulation performance of the char layer. Moreover, the char layer of IFRC 3 has a higher C/O mass ratio compared to that of IFRC 0 , resulting in a good oxidation resistance and fire resistance [7].

258
The THR and HRR curves of the IFRC0-IFRC4 samples are presented in Figure 9 and 259 Figure 10, respectively. As depicted in the figures, the samples illustrate two peaks due to 260 the decomposition of the char layer and the plywood. The first peak heat release rate 261 (PHRR1) of IFRC0 is 108.5 kW/m 2 appeared at 21 s. With the addition of PPY-TTF, both 262 THR and PHRR values of IFRC1-IFRC4 coatings are significantly reduced. Compared with 263 IFRC0, the THR and PHRR1 values are reduced by 5.9% and 12.1% for IFRC1, 13.5% and 264 18.5% for IFRC2, 21.6% and 31.2% for IFRC3 and 10.9% and 27.2% for IFRC4, suggesting 265 that the presence of PPY-TTF can effectively improve flame retardancy of the coating. 266 Moreover, the coatings containing PPY-TTF show lower second peak heat release rate 267 (PHRR2) compared to the coating without PPY-TTF, indicating better flame inhibition ef-268 fect on wood substrates. In conclusion, the coatings containing PPY-TTF can significantly 269 reduce the THR and HRR of coating samples, among which, the IFRC3 has the best per-270 formance. This phenomenon is mainly attributed to the fact that PPY-TTF can effectively 271 promote the char formation of the coating and strengthen the structure of the char layer, 272

Cone Calorimeter Test
The THR and HRR curves of the IFRC 0 -IFRC 4 samples are presented in Figures 9 and 10, respectively. As depicted in the figures, the samples illustrate two peaks due to the decomposition of the char layer and the plywood. The first peak heat release rate (PHRR1) of IFRC 0 is 108.5 kW/m 2 appeared at 21 s. With the addition of PPY-TTF, both THR and PHRR values of IFRC 1 -IFRC 4 coatings are significantly reduced. Compared with IFRC 0 , the THR and PHRR1 values are reduced by 5.9% and 12.1% for IFRC 1 , 13.5% and 18.5% for IFRC 2 , 21.6% and 31.2% for IFRC 3 and 10.9% and 27.2% for IFRC 4 , suggesting that the presence of PPY-TTF can effectively improve flame retardancy of the coating. Moreover, the coatings containing PPY-TTF show lower second peak heat release rate (PHRR2) compared to the coating without PPY-TTF, indicating better flame inhibition effect on wood substrates. In conclusion, the coatings containing PPY-TTF can significantly reduce the THR and HRR of coating samples, among which, the IFRC 3 has the best performance. This phenomenon is mainly attributed to the fact that PPY-TTF can effectively promote the char formation of the coating and strengthen the structure of the char layer, thus effectively delaying the transfer of heat and mass between the flame and the substrates. However, the cooperative effect of PPY-TTF in the coatings depends on the amount of PPY-TTF. When the amount of PPY-TTF exceeds 3 wt%, the cooperative fire-retardant effect will be weakened, which is ascribed to an excessive content of PPY-TTF may solidify the molten char layer that inhibits the char formation process [20].

293
The TG and DTG curves of coatings under a nitrogen atmosphere are depicted in 294 Figure 12. As seen in Figure 12, the decomposition process of the coating is mainly divided 295 into four stages in the temperature range of 100-300 °C, 300-430 °C, 430-580 °C and 580-296 800 °C, respectively. The first stage is mainly the stage of dehydration, volatilization for 297 small molecules and low temperature decomposition of IFR with lower mass loss. As the 298 temperature continues to rise, the second stage appears a strong DTG peak accompanied 299 with a higher mass loss, which is mainly caused by the decomposition of the cured epoxy 300 resin and APP, PER and MEL. In this stage, APP decomposes to generate polyphosphoric 301 acid and phosphoric acid derivatives that promote the esterification of PER into char, 302 while polyphosphoric acid dehydrates to form a cross-linking structure. The NH3 and H2O 303 released from MEL and APP in this process can promote the expansion of the char layer. 304 In the third stage, the formed cross-linking structures and polypyrrole are degraded at a 305 large scale. The fourth stage is ascribed to the decomposition of unstable char layer at a 306 high temperature, accompanying with a mass loss of about 1.5%. 307 Figure 11. Light absorption curves and SDR values of IFRC 0 -IFRC 4 samples.

Thermal Stability Analysis
The TG and DTG curves of coatings under a nitrogen atmosphere are depicted in Figure 12. As seen in Figure 12, the decomposition process of the coating is mainly divided into four stages in the temperature range of 100-300 • C, 300-430 • C, 430-580 • C and 580-800 • C, respectively. The first stage is mainly the stage of dehydration, volatilization for small molecules and low temperature decomposition of IFR with lower mass loss. As the temperature continues to rise, the second stage appears a strong DTG peak accompanied with a higher mass loss, which is mainly caused by the decomposition of the cured epoxy resin and APP, PER and MEL. In this stage, APP decomposes to generate polyphosphoric acid and phosphoric acid derivatives that promote the esterification of PER into char, while

Thermal Stability Analysis
The TG and DTG curves of coatings under a nitrogen atmosphere are depicted in Figure 12. As seen in Figure 12, the decomposition process of the coating is mainly divided into four stages in the temperature range of 100-300 • C, 300-430 • C, 430-580 • C and 580-800 • C, respectively. The first stage is mainly the stage of dehydration, volatilization for small molecules and low temperature decomposition of IFR with lower mass loss. As the temperature continues to rise, the second stage appears a strong DTG peak accompanied with a higher mass loss, which is mainly caused by the decomposition of the cured epoxy resin and APP, PER and MEL. In this stage, APP decomposes to generate polyphosphoric acid and phosphoric acid derivatives that promote the esterification of PER into char, while polyphosphoric acid dehydrates to form a cross-linking structure. The NH 3 and H 2 O released from MEL and APP in this process can promote the expansion of the char layer. In the third stage, the formed cross-linking structures and polypyrrole are degraded at a large scale. The fourth stage is ascribed to the decomposition of unstable char layer at a high temperature, accompanying with a mass loss of about 1.5%. into four stages in the temperature range of 100-300 °C, 300-430 °C, 430-580 °C and 580-296 800 °C, respectively. The first stage is mainly the stage of dehydration, volatilization for 297 small molecules and low temperature decomposition of IFR with lower mass loss. As the 298 temperature continues to rise, the second stage appears a strong DTG peak accompanied 299 with a higher mass loss, which is mainly caused by the decomposition of the cured epoxy 300 resin and APP, PER and MEL. In this stage, APP decomposes to generate polyphosphoric 301 acid and phosphoric acid derivatives that promote the esterification of PER into char, 302 while polyphosphoric acid dehydrates to form a cross-linking structure. The NH3 and H2O 303 released from MEL and APP in this process can promote the expansion of the char layer. 304 In the third stage, the formed cross-linking structures and polypyrrole are degraded at a 305 large scale. The fourth stage is ascribed to the decomposition of unstable char layer at a 306 high temperature, accompanying with a mass loss of about 1.5%.  Table 4 illustrates the relevant thermal decomposition parameters.As seen in Table 310 4, T0 and Tm values of IFRC0 sample occur at 223.1 °C and 362.9 °C, respectively, and the 311 residual weight at 800 °C is 26.7%. With the addition of PPY-TTF, the coatings show lower 312 T0, Tm and mass loss, suggesting a reduction in the release of pyrolysis products and an 313 increase in the amount of residual char. Generally, the higher the ΔW value of the sample, 314 the stronger the interaction between the components. After adding PPY-TTF, the Wexp of 315  Table 4 illustrates the relevant thermal decomposition parameters.As seen in Table 4, T 0 and T m values of IFRC 0 sample occur at 223.1 • C and 362.9 • C, respectively, and the residual weight at 800 • C is 26.7%. With the addition of PPY-TTF, the coatings show lower T 0 , T m and mass loss, suggesting a reduction in the release of pyrolysis products and an increase in the amount of residual char. Generally, the higher the ∆W value of the sample, the stronger the interaction between the components. After adding PPY-TTF, the W exp of the specimens is significantly higher than their W theo . Among them, IFRC 3 shows the maximum ∆W value of 15.9% corresponding to the best char-forming efficiency. The results indicate that PPY-TTF can effectively improve the structure of the char layer and encourage the cross-linking reaction of degradation products to strengthen the thermal stability and char-forming ability of coatings, thus exhibiting a super cooperative effect.  Figure 13 presents the morphologies of the IFRC 0 and IFRC 3 samples before and after the ageing treatment. With the increased ageing time, the obvious blistering, powdering and yellowing phenomena appear on the surface of the IFRC 0 coating. The powdering phenomenon is caused by the precipitation and decomposition of IFR under the influence of UV irradiation and hydrothermal conditions, whereas the blistering phenomenon mainly results from the reduction of the adhesion of the coatings. Figure 14 exhibits the adhesion classification and pencil hardness of IFRC 0 and IFRC 3 coatings after the accelerated ageing treatment. As shown in Figure 14, the adhesion classification and pencil hardness of the coatings gradually weaken with the increase of ageing cycles. However, the IFRC 3 coating has a better performance than that of IFRC 0 under the same ageing treatment. This may be explained by the reason that the introduction of PPY-TTF can effectively strengthen the hardness, adhesion and shielding effect of the coating and weaken the ageing degradation of the coatings, thus exhibiting less bubbles and the precipitation of flame retardants, as seen in Figure 13. Therefore, the addition of PPY-TTF can effectively enhance the durability of the coating and slow down the blistering, powdering and yellowing of the coatings. The smoke emission characteristics of IFRC0 and IFRC3 coatings before and after age-347 ing treatment are presented in Figure 15. As depicted in Figure 15, the maximum light 348 absorption rating and SDR values of IFRC0 and IFRC3 samples gradually increase with the 349 growth of the ageing cycle, reflecting the increase of smoke production. Compared with 350 IFRC0, the IFRC3 sample still expresses a better cooperative smoke-suppression effect after 351 the same accelerated ageing treatment, which is ascribed to the enhanced integrity of the 352 coatings after introduction of PPY-TTF under ageing treatment. It can be concluded that 353 the presence of PPY-TTF can strengthen the durability of smoke-suppression effect of in-354 tumescent fire-retardant coatings. The smoke emission characteristics of IFRC0 and IFRC3 coatings before and after age-347 ing treatment are presented in Figure 15. As depicted in Figure 15, the maximum light 348 absorption rating and SDR values of IFRC0 and IFRC3 samples gradually increase with the 349 growth of the ageing cycle, reflecting the increase of smoke production. Compared with 350 IFRC0, the IFRC3 sample still expresses a better cooperative smoke-suppression effect after 351 the same accelerated ageing treatment, which is ascribed to the enhanced integrity of the 352 coatings after introduction of PPY-TTF under ageing treatment. It can be concluded that 353 the presence of PPY-TTF can strengthen the durability of smoke-suppression effect of in-354 tumescent fire-retardant coatings. 355 Figure 14. Adhesion classification (a) and pencil hardness (b) of IFRC 0 and IFRC 3 after different ageing cycles.

Accelerated Ageing Test
The smoke emission characteristics of IFRC 0 and IFRC 3 coatings before and after ageing treatment are presented in Figure 15. As depicted in Figure 15, the maximum light absorption rating and SDR values of IFRC 0 and IFRC 3 samples gradually increase with the growth of the ageing cycle, reflecting the increase of smoke production. Compared with IFRC 0 , the IFRC 3 sample still expresses a better cooperative smoke-suppression effect after the same accelerated ageing treatment, which is ascribed to the enhanced integrity of the coatings after introduction of PPY-TTF under ageing treatment. It can be concluded that the presence of PPY-TTF can strengthen the durability of smoke-suppression effect of intumescent fire-retardant coatings. The fire resistance of IFRC0 and IFRC3 samples after the ageing treatment are given 360 in Figure 16. As shown in Figure 16, IFRC0 and IFRC3 have a reduction in fire-retardant 361 time with the increase of ageing time, indicating that the degradation of fire protection 362 performance. Compared with IFRC0, the ageing process has a less negative effect on the 363 fire resistance of IFRC3 coating. The reason is that the addition of PPY-TTF is beneficial to 364 weaken the migration of flame retardants and morphology deterioration of the coatings, 365 thus imparting the coatings with a long-term durability of fire resistance. The fire resistance of IFRC 0 and IFRC 3 samples after the ageing treatment are given in Figure 16. As shown in Figure 16, IFRC 0 and IFRC 3 have a reduction in fire-retardant time with the increase of ageing time, indicating that the degradation of fire protection performance. Compared with IFRC 0 , the ageing process has a less negative effect on the fire resistance of IFRC 3 coating. The reason is that the addition of PPY-TTF is beneficial to weaken the migration of flame retardants and morphology deterioration of the coatings, thus imparting the coatings with a long-term durability of fire resistance.
Intumescent fire-retardant coatings are susceptible to photo-oxidation and thermal oxidation in daily use, and the evolution processes of coatings under ageing conditions are monitored by FTIR analysis. Figure 17 presents the FTIR spectra of IFRC 0 and IFRC 3 coatings, and Table 5 gives the functional groups of IFRC 0 and IFRC 3 samples after different ageing cycles. As observed in Figure 17 and Table 5, the absorption peaks of -NH 2 (3470, 3419, 1439 cm −1 ), C=N (1654 cm −1 ), N-H (1552, 3134 cm −1 ), P=O (1249 cm −1 ) and C-O (1016 cm −1 ) groups are obviously strengthened with the increase in the ageing cycle [24,[26][27][28][29][30][31][32], indicating that flame retardant migrates to the surface of the coating under the influence of ageing factors such as irradiation, oxygen, humidity and temperature. However, after eleven ageing cycles, the intensity of the absorption peaks for -NH 2 , C=N, N-H, P=O, and C-O functional groups decrease remarkably and the -NH 2 and N-H groups disappear, showing that the migration, hydrolization and oxidation of APP, PER and MEL under ageing conditions. Compared with IFRC 0 , the IFRC 3 coating has stronger absorption vibration peaks for the main functional groups under the same ageing treatment, corresponding to a weaker ageing degradation. This result shows that PPY-TTF can strengthen the structural stability of the coatings and reduce the migration, hydrolization and oxidation of the coatings during ageing conditions, thus endowing the coatings with a better ageing resistance. The fire resistance of IFRC0 and IFRC3 samples after the ageing treatment are given 360 in Figure 16. As shown in Figure 16, IFRC0 and IFRC3 have a reduction in fire-retardant 361 time with the increase of ageing time, indicating that the degradation of fire protection 362 performance. Compared with IFRC0, the ageing process has a less negative effect on the 363 fire resistance of IFRC3 coating. The reason is that the addition of PPY-TTF is beneficial to 364 weaken the migration of flame retardants and morphology deterioration of the coatings, 365 thus imparting the coatings with a long-term durability of fire resistance. Intumescent fire-retardant coatings are susceptible to photo-oxidation and thermal 370 oxidation in daily use, and the evolution processes of coatings under ageing conditions 371 are monitored by FTIR analysis. Figure 17 presents the FTIR spectra of IFRC0 and IFRC3 372 coatings, and Table 5 gives the functional groups of IFRC0 and IFRC3 samples after differ-373 ent ageing cycles. As observed in Figure 17 and Table 5  To further study the effect of PPY-TTF on the flame retardancy and smoke suppres-392 sion properties of intumescent fire-retardant coatings, the combustion processes of the 393 cured epoxy resin, IFRC0 and IFRC3 samples are analyzed using a cone calorimeter, and 394 the results are presented in Figure 18. As shown in Figure 18, a molten layer is formed at 395 about 60 s on the surface of the coating at a radiation flux of 50 kW/m 2 . As the temperature 396 increases, the components of the coatings interact to form an intumescent char layer, 397 which suppresses the further decomposition of the coatings. With the increase in burning 398 time, the formed char layers start to decompose and weaken the barrier effect on the inner 399 coating. The cured epoxy resin starts to burn at 125 s, while the addition of IFR delays the 400 Figure 17. FTIR spectra of IFRC 0 (a) and IFRC 3 (b) after different ageing cycles.

Flame Retardant and Smoke-Suppression Mechanisms
To further study the effect of PPY-TTF on the flame retardancy and smoke suppression properties of intumescent fire-retardant coatings, the combustion processes of the cured epoxy resin, IFRC 0 and IFRC 3 samples are analyzed using a cone calorimeter, and the results are presented in Figure 18. As shown in Figure 18, a molten layer is formed at about Polymers 2022, 14, 1540 15 of 18 60 s on the surface of the coating at a radiation flux of 50 kW/m 2 . As the temperature increases, the components of the coatings interact to form an intumescent char layer, which suppresses the further decomposition of the coatings. With the increase in burning time, the formed char layers start to decompose and weaken the barrier effect on the inner coating. The cured epoxy resin starts to burn at 125 s, while the addition of IFR delays the ignition time of the coating to 372 s. More importantly, the IFRC 3 char maintains its structural integrity at 900 s, which means that the incorporation of PPY-TTF can effectively strengthen the fire resistance of the coatings and provide better protection for the substrate. The FTIR spectra and digital photos of IFRC0 and IFRC3 after different treating tem-407 peratures are depicted in Figure 19 and Table 6. When the temperature reaches 300 °C, the 408 -NH2 (3469, 3419 cm −1 ), N-H (1552 cm −1 ), PO3 2-(1129 cm −1 ), C-O (1015 cm −1 ) and P-O-P 409 (669, 873 cm −1 ) groups disappear and the P-O-C group appears at 1043 cm −1 [23,[26][27][28][29][30][31][32][33][34], 410 indicating the decomposition of APP, PER, MEL and epoxy resin at a low temperature. 411 Besides, the decomposition temperature of the N-H group in the IFRC3 coating is 100 °C 412 lower than that of the IFRC0 sample due to the earlier char formation of the coating. As 413 the temperature continues to increase, the pictures reveal that the components of the in-414 tumescent fire-retardant coatings interact to form an intumescent char layer, and the main 415 functional group peaks of the coatings in the corresponding FTIR spectra basically disap-416 pear. When the temperature reaches 800 °C, the char residue of the IFRC3 sample appears 417 stronger stretching vibration peaks of P=O (1324 cm −1 ), C-O-C (1139 cm −1 ), P-O-C (921 418 cm −1 ) and aromatic C-H (739 cm −1 ) groups than those of IFRC0, indicating that the presence 419 of PPY-TTF causes the char residue to form more phosphorus-rich cross-linking structures 420 and aromatic structures, thus improving the heat insulation and thermal stability of the 421 char layer [35][36][37][38]. This result is consistent with the higher residual weight of IFRC3 sam-422 ple in TG analysis. The FTIR spectra and digital photos of IFRC 0 and IFRC 3 after different treating temperatures are depicted in Figure 19 and Table 6. When the temperature reaches 300 • C, the -NH 2 (3469, 3419 cm −1 ), N-H (1552 cm −1 ), PO 3 2− (1129 cm −1 ), C-O (1015 cm −1 ) and P-O-P (669, 873 cm −1 ) groups disappear and the P-O-C group appears at 1043 cm −1 [23,[26][27][28][29][30][31][32][33][34], indicating the decomposition of APP, PER, MEL and epoxy resin at a low temperature. Besides, the decomposition temperature of the N-H group in the IFRC 3 coating is 100 • C lower than that of the IFRC 0 sample due to the earlier char formation of the coating. As the temperature continues to increase, the pictures reveal that the components of the intumescent fire-retardant coatings interact to form an intumescent char layer, and the main functional group peaks of the coatings in the corresponding FTIR spectra basically disappear. When the temperature reaches 800 • C, the char residue of the IFRC 3 sample appears stronger stretching vibration peaks of P=O (1324 cm −1 ), C-O-C (1139 cm −1 ), P-O-C (921 cm −1 ) and aromatic C-H (739 cm −1 ) groups than those of IFRC 0 , indicating that the presence of PPY-TTF causes the char residue to form more phosphorus-rich cross-linking structures and aromatic structures, thus improving the heat insulation and thermal stability of the char layer [35][36][37][38]. This result is consistent with the higher residual weight of IFRC 3 sample in TG analysis.  During the combustion process, APP is heated and decomposed to release metaphos-429 phate, phosphate, inorganic acid and non-combustible gas, among which inorganic acid 430 can encourage the esterification of PER into char to form a molten layer. At this stage, the 431 introduction of PPY-TTF promotes the production of more cross-linking structures in the 432 condensed phase. At the same time, MEL will decompose and cyclize into triazine com-433 pounds, while a large amount of non-combustible gas is released to induce the expansion 434 of the char layer, diluting the fuel gas and weakening the burning intensity. The PPY-TTF 435 will promote the formation of a smoother and denser protective char layer that delays the 436 transfer of heat and mass between the fire and char layer, thus effectively suppressing the 437 further decomposition of the coating. With the rise in temperature, the PPY-TTF contrib-438 utes to the cross-linking reaction of degradation products that generate more cross-linking 439 and aromatic structures in the condensed phase, which enhance the thermal stability and 440 char-forming properties of coatings. However, the positive effect of PPY-TTF in the coat-441 ings is depended on its content. An excessive content of PPY-TTF may inhibit the char 442 formation and decrease the expansion rating of the char layer, thus causing a reduction in 443 the cooperative effect on the fire resistance and smoke suppression performance of intu-444 mescent fire-retardant coatings.

446
In this work, a preparation method for PPY-TTF was proposed via the in situ 447 polymerization of pyrrole on the surface of tungsten tailing fillers, and the structures and 448 properties of synthesized particles were characterized in detail by combining SEM-EDS, 449 Figure 19. FTIR spectrum and digital pictures of IFRC 0 (a) and IFRC 3 (b) after different treating temperatures. During the combustion process, APP is heated and decomposed to release metaphosphate, phosphate, inorganic acid and non-combustible gas, among which inorganic acid can encourage the esterification of PER into char to form a molten layer. At this stage, the introduction of PPY-TTF promotes the production of more cross-linking structures in the condensed phase. At the same time, MEL will decompose and cyclize into triazine compounds, while a large amount of non-combustible gas is released to induce the expansion of the char layer, diluting the fuel gas and weakening the burning intensity. The PPY-TTF will promote the formation of a smoother and denser protective char layer that delays the transfer of heat and mass between the fire and char layer, thus effectively suppressing the further decomposition of the coating. With the rise in temperature, the PPY-TTF contributes to the cross-linking reaction of degradation products that generate more cross-linking and aromatic structures in the condensed phase, which enhance the thermal stability and char-forming properties of coatings. However, the positive effect of PPY-TTF in the coatings is depended on its content. An excessive content of PPY-TTF may inhibit the char formation and decrease the expansion rating of the char layer, thus causing a reduction in the cooperative effect on the fire resistance and smoke suppression performance of intumescent fire-retardant coatings.

Conclusions
In this work, a preparation method for PPY-TTF was proposed via the in situ polymerization of pyrrole on the surface of tungsten tailing fillers, and the structures and properties of synthesized particles were characterized in detail by combining SEM-EDS, XRD, FTIR, XRF and TG analyses. Then, the PPY-TTF was applied to intumescent fire-retardant coatings as an adjuvant, and the effect of PPY-TTF on the fire resistance and anti-ageing properties of intumescent fire-retardant coatings was investigated by different analytical methods. The results reveal that the presence of PPY-TTF enhances the fire resistance, thermal stability, char formation and smoke suppression properties of intumescent fire-retardant coatings, exhibiting super cooperative flame-retardant and smoke suppression effects. The cooperative effect of PPY-TTF in intumescent coatings is ascribed to the formation of more cross-linking and aromatic structures in the condensed phase that enhance the barrier effect of char, as supported by digital photos and SEM images. However, an excessive content of PPY-TTF will weaken the char-forming ability of the coatings, thus diminishing the excellent cooperative efficiency. In particular, the IFRC 3 sample containing 3 wt% PPY-TTF presents the best fire resistance among all samples, and has a 74.3% reduction in flame-spread rating, 30.7% reduction in total heat release, 32.9% reduction in mass loss and 32.4% reduction in smoke density rating value compared with IFRC 0 . The TG analysis suggests that PPY-TTF can strengthen the char-forming ability of intumescent fire-retardant coatings, and the residual weights of IFRC 0 , IFRC 1 , IFRC 3 , IFRC 4 and IFRC 5 at 800 • C are 26.7%, 31.6%, 34.9%, 33.5% and 30.6%, respectively. The accelerated ageing test demonstrates that an appropriate amount of PPY-TTF can improve the shielding effect and structural stability of the coatings that effectively slow down the blistering and powdering phenomenon of the coatings, thus achieving a long-term durability of the fire resistance and smoke suppression properties of the coatings. In summary, PPY-TTF provides a new strategy to utilize tungsten tailing in the fields of flame-retardant materials.