Synthesis of a Triazine-Based Hyper Branched Macromolecule Charring Agent and its Effect on Flame-Retardant Thermoplastic Polyester Elastomer to Improve Anti-Dripping


 A triazine-based charring agent (CDS) was synthesized and combined with diethyl aluminum hypophosphite (AlPi) to develop an intumescent flame retardant (IFR) system to improve thermoplastic polyester elastomer (TPEE) anti-dripping. The results showed that the limiting oxygen index (LOI) of the TPEE/15AlPi/5CDS composite reached 30.2%, and it passed the V-0 test in vertical combustion (UL-94). The results of thermogravimetric analysis (TGA) showed that CDS had good thermal stability and high char residue (50.8wt%) at 700°C. And when combined with AlPi added to TPEE, which can improve its char residue ranges from 1.68wt% to 23.52wt%. The structure evolution during the heating process was studied by heating infrared spectroscopy (IR), and the morphology and chemical structure of char residues were studied by scanning electron microscope (SEM) and laser Raman spectroscopy (LRS). The high-efficiency flame-retardant TPEE composite formed a continuous, dense and porous char layer containing triazine ring and aromatic ring structure after combustion.


Introduction
Thermoplastic polyester elastomer (TPEE) is a polymer that have the high elasticity of traditional vulcanized rubber at room temperature and can be plasticized and molded at high temperatures [1] .
Because of its many advantages in use and processing performance, TPEE have been widely used in the electrical industry. However, limited by the molecular structure, TPEE are ammable, and are accompanied by melting and dripping during the combustion process, and cannot self-extinguish from the re [2,3] . In recent years, with the continuous improvement of the requirements for material re resistance and environmental protection performance, the research of ame retardant TPEE has become a hot spot [4][5][6] .
Intumescent ame retardant (IFR) is a rapidly developing halogen-free ame retardant, usually composed of three components: acid source, gas source and carbon source and the ame-retardant mechanism is that the acid source catalyzes the dehydration of the carbon source into char [7] . The incombustible gas produced by the gas source expands the char layer and dilutes the concentration of combustibles in the combustion environment. Metal hypophosphite has been considered a friendly ame retardant and aluminum hypophosphite(AlPi) has proved to be effective in the re resistant of polyester in recent year [8,9] . In our previous research in order to solve the problem of melt dripping in the combustion process of thermoplastic polyester elastomer, it was found that the intumescent ame retardant composed of aluminum hypophosphite and melamine phosphate can be used to improve the combustion performance of thermoplastic polyester elastomer [10][11][12][13][14] . However, the system still needs to add some synergists, such as novolac [10] montmorillonite [11] graphite [12] , CNTs and MoS 2 [13] to improve the charring performance of TPEE and achieve the purpose of preventing combustion. However, excessive addition of inorganic components will undoubtedly worsen the mechanical properties of TPEE. Therefore, it's of great signi cance for developing a new type of charring agent with high e ciency and stability.
Triazine ame retardants are triazine derivatives in molecular structure, and nitrogen ame retardants in ame retardant elements, which have the advantages of low toxicity, low smoke, recyclability, and environmental protection [15] . The ame retardant mechanism of triazine ame retardants is to release non-combustible gases such as N 2 and NH 3 during the ame retardant process to achieve gas phase ame retardancy. At the same time, the tertiary carbon structure of the triazine ring is aromatic, thermally decomposed to form a stable graphitized char layer, which realizes the condensed phase ame retardant during the ame retardant process. Junfeng Zhou [16] , Caimin Feng [17,18] and others used cyanuric chloride and different organic diamines to copolymerize to form macromolecules with different triazines, which are compounded with acid sources to show excellent ame retardant properties in polymer materials, especially the ability to form char. And in our previous research, we found that the linear triazine charring agent can play a synergistic effect with diethyl aluminum hypophosphite, and can inhibit the droplet in the TPEE matrix to achieve the ame retardant effect [14] .
In our follow-up research, we strive to synthesize a new type of hyper branched macromolecular charring agent in order to achieve a better cross-linking into charcoal effect to suppress the droplet of TPEE in the combustion process. In this work, a triazine-based bulk macromolecular charring agent (referred to as CDS in this article) was successfully synthesized. Flame-retardant TPEE composites were prepared using CDS and aluminum hypophosphinate as phosphorus ame retardants. Compared with the analogous triazine charring agents reported by other researchers, the incorporation of diamine as a chain extender makes the relative content of polar end-capping reagent in CFA higher, which can effectively moderate the compatibility between the charring agent and the hard molecular chains in TPEE, and improve the carbonization e ciency of CDS in the intumescent ame retardant TPEE systems. The further research was conducted on the mechanism of char forming process and the synergistic effect of CDS with AlPi on ame retardant process. The re behavior, the thermal decomposition and the char formation mechanism of the ame retardant composites were studied by LOI test, UL-94 test, TGA, SEM and LRS.

Synthesis of CDS
As showed in Fig.1, the speci c preparation route of CDS are as follows. 4,4-Diaminodiphenyl sulfone (0.09mol) and 400ml 1, 4-dioxane were added to a 1000ml four-necked ask with a thermometer and mechanical stirring. And then the solution of cyanuric chloride (0.06mol) dissolved in 200 ml 1, 4-dioxane was drop-wise added into the ask in 2 h at 25℃. After all the ingredients were added in the reaction system, the reacting mixture was stirred at 95℃for 10 h. The by-products Cl generated during the reaction are neutralized by the reaction with Na 2 CO 3 . Finally, the reaction mixture was ltered and washed several times to remove the solvent, NaCl, NaHCO 3 and the excessive Na 2 CO 3 .After drying in a vacuum oven at 120℃ for 4 h, light yellow product was obtained and the yield is 92.2%.

Preparation of ame retardant TPEE samples
TPEE, AlPi, CDS were dried in a 90℃ constant temperature drying oven (DHG-9055A, Shanghai Hecheng Instrument Manufacturing Co., Ltd.) for 8 hours, and weighed according to the research formula. All the components were melt-mixed in a torque rheometer (160 Nm/Polab QC, Germany Haake Company) at 205℃and 60 r/min rotor speed. Speci cally, the premix of TPEE and CDS was rstly added into torque rheometer and mixed for 3.5 min, and then AlPi were added and melt-mixed for another 2.5 min. After that, all the components were press molding on a at vulcanizer (QLB-50, Shanghai Rubber Machinery First Factory) at 205℃. After hot-pressed for 5 min and holding pressure for 4 min, all samples were coldpressed to room temperature to obtain suitable test specimens.

Characterization
The Fourier transform infrared was obtained with a Nicolet FTIR 6700 infrared spectrophotometer where the samples were prepared with KBr pellets.
The 13 C solid-state NMR spectra were measured on a Thermo Varian INOVA500NB spectrometer at 500 MHz.
The Limiting oxygen index (LOI) was performed by an oxygen index instrument (JF-3, Jiangning Analysis Instrument Factory, China) with specimen size of 130.0 mm× 6.0 mm × 3.0 mm according to ASTM D2863-17.
The thermogravimetric analysis (TGA) was performed using a STA 409 PC/PG thermogravimetric analyzer (Netzsch, Germany) at a heating rate of 10 ℃/min, heating from room temperature to 700 ℃ under air atmosphere (40 mL/min).
The microstructure of char residues was examined with eld emission scanning electron microcopy (SEM S-3400, Japan) with accelerating voltage was 15.0 KV. The sample were sputter-coated with a conductive layer of gold before analysis.
The graphitic degree of char residues was characterized by Laser Raman spectroscopy (SPEX, USA) with a 532 nm argon laser line at room temperature, the scanning range was 400 ~ 2000 cm -1 region.

Characterization of CDS
The structural characterization of CDS was performed by FTIR and 13 C-NMR. The FTIR spectra of DDS, CDS and CNC were presented in Fig.2. Clearly, several functional groups from DDS and CNC, such as N-H(3320-3530cm -1 ), C=C (1620-1450cm -1 ), C=N(1530cm -1 ), can be observed in the FTIR spectra of CDS.
The absorption peak of C-N(1265cm -1 ) appeared, and the absorption peak of C-Cl(850cm -1 ) disappeared, which indicated the completion of polymerization. The structures of CDS was further con rmed by 13 C solid-state NMR in Fig.3. The signals at 165.75 ppm, 142.49 ppm, 136.08ppm, 128.85 ppm, 121.30 ppm were attributed to C (a, δ tr-C ), C (b, δ-C=C -in para-point of benzene ring), C (e, δ-C=C -in para-point of benzene ring), C (d, δ-C=C -in meta-point of benzene ring), C (c, δ-C=C -in ortho-point of benzene ring), respectively.

Flame retardancy of TPEE composites
The formula and the results of LOI values and UL-94 ratings of neat TPEE and TPEE composites are shown in Table1 and Table 2. Neat TPEE was very ammable and burned to the clamp after the rst ignition, and was accompanied by severe droplets with the LOI value of 17.9. CDS was acted as both charring and blowing agents in the intumescent ame retardant system. There was hardly any improvement for the LOI values and UL-94 ratings by the addition of 20% CDS separately. It revealed that the lack of acid source for the intumescent ame retardant system is not e cient enough. When 15wt% or 20wt% of AlPi was added, the TPEE sample were extinguished in about 5 seconds after the Bunsen burner is ignited for 10 seconds, and then during the secondary combustion, droplets dripped into the re Bunsen burner constantly. Therefore, it reaches the Ul-94 V-2 grade. In addition, compared with neat TPEE, the LOI values of TPEE/15AlPi and TPEE/20AlPi increased to 26.8 and 24.5, which showed that aluminum hypophosphite was able to play a ame retardant effect in both condensed phase and gas phase. In the gas phase, free radical capture leaded to ame suppression, and in the condensed phase, it initiated the formation of char or inorganic residues [19] .
The formulation of TPEE composites and results of LOI and UL94 test of all the investigated samples were shown in Table 1 and Table 2 respectively. As the mass ratio of the acid source and the char source increased, it can be seen that when 2 wt% CDS was added into the intumescent ame retardant system, the LOI of the TPEE/18AlPi/2CDS was increased to 28.2. However, droplets were still produced during the secondary combustion and V-2 rating was reached, which meant less char agent was unable to form a thick and dense char layer to prevent melt dripping. With the increase in the proportion of CDS, TPEE/15AlPi/5CDS can stop burning during the rst combustion in 2.2 seconds and the UL-94 rating was enhanced to V-0 from V-2 with inhibited dripping during the secondary combustion. However, with the further increase of the charring agent, it can be found that the LOI value of TPEE/10AlPi/10CDS was decreased to 26.2 and dripped again within 10s in the ignition process of secondary ame. The reason was that the char layer formed after expansion and combustion was too much to cover the surrounding of the sample, making the heating area larger. It can be concluded that an appropriate ratio of AlPi and CDS in the intumescent ame retardant system was particularly important for the ame retardant performance of TPEE composites. LOI: limiting oxygen index; t 1 : average combustion time after the rst application of ame; t 2 , average combustion time after the second application of ame; BC: burns to clamp; NR: no rating.
b No/yes corresponds to the rst/second ame application. (corresponding to the chemical reaction of dehydration and release of SO 2 ) [20] , and the second step occurred at 400 to 550°C, which may be allocated to the decomposition of macromolecular framework.

Charring behavior and thermal degradation of CFA and AlPi
And nally, CDS degraded into expanded char during the pyrolysis process with the increase of temperature, and the char residue is about 50.8% at 700°C. These results showed that CDS had good thermal degradability, which can be attributed to the presence of triazine and benzene ring.
The initial decomposition temperature (T 5% ) of IFR was 394.3℃ and the char residue was 32.24% at 700℃, which proved that IFR had excellent thermal stability and charring performance. Compared with Calculation IFR, the calculated value of initial decomposition temperature was 409.2 ℃, which was higher than the actual measured temperature value. This result revealed that with the incorporation of CFA into AlPi, the overall thermal degradation process changed. Meanwhile, the char residue (28.44%) of calculation IFR was lower than the IFR, which indicated that AlPi can improved the charring performance of CDS and accelerated the formation of the char layer. Figure 6 and Figure 7 showed the TGA and DTG curves of TPEE and ame-retardant TPEE composites under N 2 atmosphere. Table 4 summarized detailed data such as 5% mass loss, 10% mass loss, maximum mass loss rate, and the experimental and calculation value of char residue at 700°C. Through these data, we tried to explore the mechanism of the condensed phase and clarify the thermal stability of different formulations. The decomposition of neat TPEE started at 370.2℃, and the highest loss rate at 403.2℃ was -2.53 wt%/min -1 . After the TGA test, TPEE was completely degraded into gaseous molecules, so it can be decomposed in one step with almost no residue. After AlPi was added to the TPEE matrix, the initial thermal decomposition temperature (T 5% ) and the maximum thermal decomposition rate temperature(T max ) of the TPEE composite did not change much, which indicated that AlPi can cooperate with the TPEE matrix well, and the experimental value of char residue of the composite was much higher than the calculated value, which indicated that the phosphorus in AlPi can catalyze TPEE to form a char layer during the combustion process. When the intumescent ame retardant composed of CDS and AlPi was added to TPEE, the initial thermal decomposition temperature of the composite material is slightly reduced, which was due to the lower initial thermal decomposition temperature of CDS. The proportion increased gradually, and it was found that when the addition amount of the charring agent CDS was 5%, the synergistic e ciency of AlPi and CDS was the highest at this time, and the char residue of the TPEE composite material was increased to 23%, which meant the surface can form a thick and dense char layer to prevent the TPEE composite from dripping so that the ame retardancy would be improved, which was consistent with the UL-94 measurement result. Comparing all formulas from TPEE/18AlPi/2CDS to TPEE/10AlPi/10CDS, the experimental values of the char residue rate were always much higher than the calculated values, which showed that the role of the intumescent ame retardant was not a simple linear addition, but was caused by the good synergy between the acid source, the charring agent and the matrix.

Dynamic FTIR of TPEE and TPEE composites
Through the characterization of the infrared spectrum of the TPEE composites during the heating process, the synergistic effect between ame retardants and the mechanism and form of action in the matrix can be studied. Figure.8-10 showed the infrared spectra of Neat TPEE, TPEE/AlPi and TPEE/AlPi/CDS at a heating rate of 10°C/min -1 . 2962cm -1 and 2878cm -1 (-CH 2 , stretching vibration), 1712cm -1 (-C=O, stretching vibration), 1458cm -1 (-CH 2 , fracture vibration), 1410cm -1 (aromatic) ring), 1274cm -1 (-CO-O-ester), 1106cm -1 (-CH 2 -O-CH 2 -ether) and 727cm-1 (-CH, bending vibration of aromatic ring) were the characteristic peaks of TPEE. With the increase of heating temperature, the band intensity at 2956 and 2861 cm -1 (-CH 2 ) dropped rapidly, which was caused by the scission of the α-methylene group in the soft segment, which was consistent with our previous research results [21] . Same as Neat TPEE, TPEE/15AlPi in Fig.9 was also decomposed on the soft segment due to the active amethylene group of the ether bond. In the hard segment (C=O), due to the presence of carboxyl groups, there was a shoulder gap at 1680cm -1 . As the temperature rose, due to the strong interaction between Al 3+ and carboxyl groups, electron migration occurred, which also improved the thermal stability of the composite. There was no P-O-H unit (3400-3800cm -1 ) from the curve at 450℃, which indicated that the diethyl aluminum hypophosphite may played a ame retardant effect in the gas phase after being released. The solid phase spectrum of TPEE/AlPi showed that the interaction between TPEE and AlPi improved thermal stability and increased the residue rate, and may formed intermediate products in the gas phase to suppress ames.
The infrared spectrum of TPEE/15AlPi/5CDS during the heating process was shown in Figure.10. It can be seen that the N-H peak at 3400cm -1 gradually weakened with the increase of temperature, which indicated that the triazine charring agent CDS decomposed non-combustible gases such as ammonia and nitrogen, thereby exerting a ame retardant effect in the gas phase. In addition, C-N (1265cm -1 ), C=N (1500cm -1 ) and characteristic bands of aromatic structure 737cm -1 and 1411cm -1 appeared in the residue at 450℃, which indicated that the addition of triazine charring agent CDS can make the TPEE composites form the char layer spliced by benzene ring and triazine ring after combustion, thereby increasing the strength of char.
3.6. Morphology and chemical structure of char residues Raman spectroscopy was widely used to study the crystal structure and molecular structure of carbon materials. In addition to analyzing the morphology of char residue at high temperature, the study of the structure of char residue was of great signi cance to the research of the ame retardant mechanism. . Generally speaking, the lower the I D /I G , the more graphitic carbon in the char layer, and the higher the quality of the char layer. The higher the degree of graphitization, the denser and stable the structure of the carbon layer. It can be seen from Figure 11 that the I D /I G ratio of TPEE/20AlPi (2.645) was higher than that of TPEE/18AlPi/2CDS (2.226), which indicated a decrease in the degree of graphitization in the residual carbon. However, compared to TPEE/18AlPi/2CDS, the I D /I G ratio of TPEE/15AlPi/5CDS (1.835) reduced, which suggested that the mass percentage of graphitized carbon in carbon was increased. The graphitized carbon formed in the combustion process was of great signi cance for controlling the release of heat and volatiles from the stable carbon structure at high temperatures. The I D /I G ratio followed the order of TPEE/20ALPi <TPEE/18AlPi/2CDS<TPEE/15AlPi/5CDS, which showed that TPEE/15AlPi/5CDS had the highest degree of graphitization and the best thermal stability. And this was also in line with the improvement in ame retardancy mentioned above. Figure.12 showed the SEM image of the residual carbon after calcination in a mu e furnace at 700°C for 15 minutes. It can be seen from the gure that the char residue of the TPEE/15AlPi had many obvious defects, and the surface was irregular. Such a structure cannot effectively prevent the heat transfer from being transferred to the substrate, nor can it prevent the molten combustion material from dripping. As for Figure.12(b), due to the addition of a small amount of the charring agent CDS, the carbonization of CDS accelerated the carbonization speed of the composites, thereby forming a thick carbon layer, which can slow down the heat transfer. However, due to the lack of carbon source, it was not enough to form a dense enough char layer to prevent dripping. Further adding CDS, it can be seen from Figure.12(c) that the residue of the TPEE/15AlPi composite was dense, porous, and folded structure char layers, which can deposit char residues on the surface while blocking heat insulation. In addition, CDS can be used as a gas source after combustion, and non-combustible gases such as N 2 , NH 2 and CO 2 were generated, which played the role of in ation, thereby diluting the concentration of oxygen on the surface of the composites and reducing the back diffusion of oxygen. These reasons had also prompted the composites to pass the V-0 UL-94 test. The above results showed that the intumescent ame retardant 15CDS/5AlPi had a good ame retardant effect and can play a role in the condensed phase and the gas phase.

Proposed ame-retardant mechanism
According to the described structure characterization and char formation behavior analysis, the possible ame retardant mechanism of TPEE / 15AlPi / 5CDS composite was shown in Fig.13 and Fig.14. Firstly, the charring agent CDS decomposed at about 336°C to release incombustible gas, and then cracks to generate triazine ring structures and aromatic structures, which were easy to assemble and splice into a char layer. Subsequently, the thermal oxidative degradation of the TPEE matrix occurred selectively in the soft segment of the polyether, and at the same time, the ester bond was formed. When the temperature continued to be oxidized, the polyester segment will also be broken. The chain scission reaction passes through a six-membered ring intermediate product, -COO abstracted the H atom from the methylene group at the β position to form an oligomer with a carboxyl group and an unsaturated double bond at the end, which continued to oxidize to produce tetrahydrofuran, butadiene, benzoic acid, water, CO2, etc. [22]. Subsequently, the Al 3+ provided by AlPi after the temperature raised and the carboxyl-containing oligomers produced by the degradation of TPEE formed a compound, and nally a variety of inorganic aluminum phosphate salts such as pyrophosphate were formed. This phosphate had a certain strength and can help to isolate hot oxygen exchange. In addition, the phosphate closely covered the surface of the substrate to further catalyze the carbonization of the TPEE substrate and the triazine charring agent CDS. Finally, the decomposition of CDS produced an aromatic structure that was assembled with the pyrolysis product of TPEE, and the graphitized char layer was formed on the surface of the composites to isolate heat and play a ame retardant effect.

Conclusions
In this paper, a new type of charring agent CDS was synthesized by CNC, DDS through condensation reaction, which can form an IFR system together with AlPi, and the IFR signi cantly improved the ame retardancy of TPEE. When only 20% IFR was loaded, the TPEE composite can obtain a high LOI value of 30.2% and passed UL-94 V-0 rating. Through the analysis of TGA, it can be seen that CDS has an extremely high char residue rate and had a certain synergistic ame retardant effect with AlPi. Combining the structure analysis of heating infrared and the analysis of SEM and LRS charring behavior, it can be