Mengqi Li,Yajun Chen,Zimeng Kong,Zhe Sun,Lijun Qian
Impact of a Novel Phosphoramide Flame Retardant on the Fire Behavior and Transparency of Thermoplastic Polyurethane Elastomers
In many application fields of thermoplastic polyurethane (TPU), excellent flame retardancy and transparency are required. However, higher flame retardancy is often at the expense of transparency. It is difficult to achieve high flame retardancy while maintaining the transparency of TPU. In this work, a kind of TPU composite with good flame retardancy and light transmittance was obtained by adding a new synthetic flame retardant named DCPCD, which was synthesized by the reaction of diethylenetriamine and diphenyl phosphorochloridate. Experimental results showed that 6.0 wt % DCPCD endowed TPU with a limiting oxygen index value of 27.3%, passing the UL 94 V-0 rating in the vertical burning test. The cone calorimeter test results showed that the peak heat release rate (PHRR) of the TPU composite was dramatically reduced from 1292 kW/m2 (pure TPU) to 514 kW/m2 by adding only 1 wt % DCPCD. With the increase of DCPCD contents, the PHRR and total heat release gradually decreased, and the char residue gradually increased. More importantly, the addition of DCPCD has little effect on the transparency and haze of TPU composites. In addition, scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy were carried out to investigate the morphology and composition of the char residue for TPU/DCPCD composites and explore the flame retardant mechanism of DCPCD in TPU.
Impact of a Novel Phosphoramide Flame Retardant on the Fire Behavior and Transparency of Thermoplastic Polyurethane Elastomers

In many application fields of thermoplastic polyurethane (TPU), excellent flame retardancy and transparency are required. However, higher flame retardancy is often at the expense of transparency. It is difficult to achieve high flame retardancy while maintaining the transparency of TPU. In this work, a kind of TPU composite with good flame retardancy and light transmittance was obtained by adding a new synthetic flame retardant named DCPCD, which was synthesized by the reaction of diethylenetriamine and diphenyl phosphorochloridate. Experimental results showed that 6.0 wt % DCPCD endowed TPU with a limiting oxygen index value of 27.3%, passing the UL 94 V-0 rating in the vertical burning test. The cone calorimeter test results showed that the peak heat release rate (PHRR) of the TPU composite was dramatically reduced from 1292 kW/m2 (pure TPU) to 514 kW/m2 by adding only 1 wt % DCPCD. With the increase of DCPCD contents, the PHRR and total heat release gradually decreased, and the char residue gradually increased. More importantly, the addition of DCPCD has little effect on the transparency and haze of TPU composites. In addition, scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy were carried out to investigate the morphology and composition of the char residue for TPU/DCPCD composites and explore the flame retardant mechanism of DCPCD in TPU.

Thermoplastic polyurethane (TPU) elastomer is a block polymer composed of soft and hard segments with excellent strength, impact resistance, and abrasion resistance. The soft segment is mainly composed of polyether or polyester and other oligomeric diols, accounting for 50–80% of the mass fraction of polyurethane. The hard segment is mainly composed of diisocyanate and chain extender, which mainly affects the high-temperature performance of the material. The main application fields of TPU include pipes, synthetic leather, wires and cables, automobile fields, etc. However, TPU is a flammable material, and its limiting oxygen index (LOI) is only about 20%. In addition, TPU emits a large number of toxic gases such as hydrocyanic acid and carbon monoxide during combustion and has a severe molten dripping phenomenon, which may further ignite other materials and cause rapid flame spread. Therefore, it will seriously endanger the safety of people’s lives and property. It is necessary to improve the flame retardancy of TPU. There are two main types of flame retardant methods currently applied to TPU. The first is the preparation of inherently flame-retarded TPU, which is difficult to industrialize. The second is the additive type, which is mainly used in the research and application field of flame retardant TPU. There are three types of additive flame retardants commonly used in TPU. First, inorganic traditional added flame retardants, such as aluminum hypophosphite, ammonium polyphosphate (APP), aluminum diethylphosphinate, melamine cyanurate and its derivatives, melamine polyphosphate, metal hydroxide, and expandable graphite. However, these flame retardants generally have many problems, such as large addition amounts, low flame retardant efficiency, and influence on the mechanical properties of materials. Second, the new modified nano-additive flame retardants, such as MXene, Si3N4 nanosheets, COF, MOF, black phosphorus, graphene, boron nitride nanosheets, montmorillonite nanosheets, and carbon nanotubes. Compared with inorganic conventional additive flame retardants, modified nano-additive flame retardants with a small additional amount are found to have a lower peak heat release rate (PHRR) and better smoke suppression in cone calorimetric tests with low impact on the mechanical properties of TPU composites. However, it is difficult to obtain a high LOI value and high UL 94 level by adding a nano flame retardant in TPU, which cannot meet the demand for industrial products. In addition, the high cost and price also make it difficult to industrialize. In recent years, researchers have successively developed a variety of organic flame retardants and achieved good flame retardancy in TPU, most of which were prepared by organic synthesis methods based on phosphorus and nitrogen compounds. Chen et al. designed a novel piperazine-containing additive with phosphorus and alkynyl named poly(2-butyne-1,4-diol piperazine bisphosphonate) (PPBPP) to fabricate high-performance flame-retarding TPU. It was found that the introduction of piperazine groups into PPBPP achieved flame retardation and high performance of TPU simultaneously via induced crystallization. They also synthesized another flame retardant poly(2-butyne-1,4-diol phenylphosphonate) (PPBP) containing alkynyl groups to fabricate flame-retarded TPU. PPBP played a crucial role in enhancing the char-forming ability of TPU. Yang et al. synthesized a Schiff-base polyphosphate ester (SPE) flame retardant as a novel flame retardant for TPU. Zhang et al. prepared a novel flame retardant (ACS) by cross-linking chitosan with bis-(4-formylphenyl)-phenyl-phosphonate (ABPO). When 10 wt % APP/ACS (APP/ACS = 1:1) was incorporated into TPU, the LOI value of the TPU/APP/ACS sample reached 28.6% and passed the UL 94 V-2 rating. Liu et al. synthesized a monocomponent intumescent flame retardant dimelamine pyrophosphate (DMPY) and incorporated it into the TPU matrix. When 15 wt % DMPY was incorporated, the samples of TPU/DMPY fulfilled UL 94 V-0 grade during vertical burning tests with the LOI of 27.3%. Jiao et al. prepared a phenolic resin hollow microsphere (PHM). It was found that comparing TPU/4%PHM with pure TPU, the PHRR was reduced by 61.6%, the total smoke release (TSR) by 23.2%, and the CO production by 86.2%. The new organic synthetic flame retardant has the advantages of high flame retardancy efficiency, low addition amounts, and little impact on mechanical properties. Besides, it can also endow TPU with some other functionality. It is a kind of flame retardant additive with a wide research scope and has been widely concerned. In this work, a novel flame retardant additive containing phosphoramide bonds based on diethylenetriamine and diphenyl phosphorochloridate was successfully synthesized and named DCPCD. The structure of DCPCD is characterized by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), and X-ray photoelectron spectroscopy (XPS). Then, DCPCD was introduced into TPU with different ratios for preparing flame-retarded TPU composites. The effect of DCPCD on the flame retardancy of TPU composites was investigated. The decomposition path of DCPCD and its flame retardant mechanism in TPU composites were illustrated by analyzing the condensed phase products.

Diethylenetriamine (AR, 99%), diphenyl phosphorochloridate (97%), and triethylamine (AR, 99%) were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China) and used without further purification. Tetrahydrofuran (THF, 99.9%, SuperDry, stabilized with 250 ppm BHT, J&KSeal) was obtained from Beijing J&K Scientific Co., Ltd. (Beijing, China). Ethanol (AR 99.7%) was provided by Beijing InnoChem Science & Technology Co., Ltd. (Beijing, China). Distilled water is made in our laboratory. TPU (9380A, Bayer) was purchased from Beijing Jinlis Technology Co., Ltd. (Beijing, China).

First, diphenyl phosphorochloridate (72.54 g, 0.27 mol) and triethylamine (38 mL, 0.27 mol) were dissolved in 165 mL of THF and transferred to the flask under nitrogen. Then, 60 mL of THF solution containing diethylenetriamine (9.29 g, 0.09 mol) was added dropwise into the flask by a constant-pressure dropping funnel at 0 °C under nitrogen. Afterward, the reaction mixture was stirred at 0 and 60 °C for 4 and 10 h sequentially. Next, triethylamine hydrochloride (TEA·HCl) was removed by suction filtration, and THF in the yellowish liquid product was removed by vacuum distillation at 60 °C to obtain the yellowish solid powder crude product. This is followed by washing repeatedly three times in the mixture of water and ethanol (water/ethanol = 1:1) to produce the white powder. Afterward, the white solid powder was dried in a vacuum oven at 80 °C for 5 h. The yield of the white solid powder (DCPCD) obtained is 83%. The synthesis route of DCPCD is shown in Scheme 1. FTIR (KBr, cm–1): 3210 (−NH−), 2890 (−CH2−), 1590, and 1490 (Ar-), 1310 (P=O), 1290 (P-NR2), 1250 (P-NHR), 936 (P–O); 1H NMR (700 MHz, DMSO-d6, ppm): 7.38 (m, 12H), 7.29–6.91 (m, 18H), 5.93 (m, 2H), 3.11–2.98 (m, 4H), 2.89 (m, 4H).

Before processing, all ingredients were dried in a vacuum oven at 100 °C for 8 h. DCPCD and TPU with the formulation in Table 1 were first dry-mixed (shaken in a bag to combine), respectively, followed by melt-extruding with a twin-screw extruder (LHFS1-271822, Lab Tech Engineering Co., Ltd., Thailand). Ten heating zones were set at 150, 160, 170, 175, 185, 185, 185, 175, 170, and 165 °C, and the screw speed was set at 80 r/min. The specimens for different tests were molded at 180 °C by a vertical injection molding machine (TY-400, Hangzhou Dayu Machinery Co., Ltd., China) and a vacuum-forming molding machine (VC-150-T-FTMO-3RT, Jiaxin Electronic Equipment Technology Co., Ltd., China).

Structural characterization: the chemical structure of DCPCD was characterized and verified by FTIR and 1H NMR. FTIR spectra were scanned on a Nicolet iN10MX-type infrared spectrometer. 1H NMR spectra were detected by a Bruker Avance III HD 700 MHz NMR spectrometer, by using dimethyl sulfoxide-d6 (DMSO) as the deuterated solvent. The LOI values were measured via a 300800 LOI instrument (Concept Equipment Ltd., UK) according to ASTM D2863 with a sample dimension of 100.0 × 6.5 × 3.2 mm3. The vertical burning tests for the UL 94 combustion level were conducted on an FTT 0082 instrument (Fire Testing Technology, UK) with sample dimensions of 130 mm × 13 mm × 3.2 mm according to the ASTM D3801 testing procedure. Five specimens were tested for each sample. Fire behavior was characterized on an FTT cone calorimeter (Fire Testing Technology, London, UK) according to ISO5660 under a 50 kW/m2 external heat flux with a sample dimension of 100.0 × 100.0 × 3.2 mm3. Time to ignition (TTI), total heat release (THR), TSR, and other typical parameters were collected and recorded synchronously, and the reported data were the average from three measurements. The thermogravimetric (TG) analysis was performed using a Perkin-Elmer instrument STA8000 (Perkin Elmer, USA) thermogravimetric analyzer. About 8 mg of the sample was heated from 50 to 800 °C at the rate of 20 °C/min in a N2 or air atmosphere. All tests were repeated two times. Differential scanning calorimetry (DSC) test: The glass transition temperature (Tg) was detected under a nitrogen (N2) atmosphere by using the Perkin-Elmer differential scanning calorimeter DSC 6000 (Perkin Elmer, USA). About 6 mg of the sample was quickly heated up to 250 °C and kept for 5 min to eliminate heat history, then cooled down to 30 °C at the rate of 20 °C/min and held at 30 °C for 5 min, and finally heated up to 250 °C at the rate of 20 °C/min. The microscopic morphology of the residual char after the cone calorimeter test was observed on a Phenom ProX SEM (SEM, Phenom World, Holland) under vacuum conditions with a voltage of 10 kV. XPS spectra were obtained on EscaLab Xi+ equipment (Thermo Scientific, America). The vacuum degree of the analysis chamber was 8 × 10–10 Pa, and the working voltage was 14.4 kV. The Raman data were measured by a laser Raman spectrometer (Renishaw inVia plus, UK) at ambient temperature by a 532 nm laser. The total transmittance spectra were measured in the range of 500–1500 nm using a UV–vis spectrophotometer (UV-3600, Shimadzu, Japan) with a sample thickness of 50.0 × 50.0 × 0.2 mm at room temperature. The values of transmittance and haze were tested by a transmittance haze tester (WGT-S, Shanghai Precision Scientific Instrument Co., Ltd) according to the determination of the luminous transmittance and haze of transparent plastics standard (GB/T 2410-2008) with a sample thickness of 50.0 × 50.0 × 0.2 mm at room temperature; all tests were repeated five times.

DCPCD was obtained by the nucleophilic substitution reaction between the chlorine atom of diphenyl phosphorochloridate and the amino group of diethylenetriamine. The chemical structure and thermal behavior of DCPCD were identified by FTIR, 1H NMR, XPS, TG, and DSC, which are shown in Figure 1. The FTIR spectra of diethylenetriamine, diphenyl phosphorochloridate, and DCPCD are presented in Figure 1a. Comparing the FTIR spectra of DCPCD with diphenyl phosphorochloridate, it can be seen that the absorption peak of P–Cl (541 cm–1) completely disappears in the spectra of DCPCD. It indicates that diphenyl phosphorochloridate and diethylenetriamine react completely. In addition, two absorption peaks belonging to −NH2 (3360 and 3280 cm–1) in diethylenetriamine disappeared in the spectrum of DCPCD. At the same time, compared with diphenyl phosphorochloridate, several peaks appear in the spectrum of DCPCD. The peak at 3210 cm–1 can be ascribed to the stretching vibration of −NH–, and the peak at 2890 cm–1 can be attributed to the stretching vibration of −CH2–. In particular, the peaks at 1290 and 1250 cm–1 are designated to the stretching vibration of P-NR2 and P-NHR, respectively. As displayed in Figure 1b, the influence of the shielding effect of the hydrogen atom will reduce the chemical shift so that the peak at 2.89 ppm (marked by e) is assigned to the protons of −CH2– near the secondary amine, and the peak at 3.11–2.98 ppm (marked by d) corresponds to the protons of −CH2– near the tertiary amine. Moreover, the chemical shifts at 5.93 ppm (N–H protons, marked by c), 7.38 ppm, and 7.29–6.91 ppm (the protons on the monosubstituted benzene ring, marked by b and a) are also found in the 1H NMR spectrum of DCPCD. In addition, the experimental proportion of the integral area according to different peaks is b:a:c:d:e = 11:18:2:4:4, which is consistent with the theoretical number of protons in different chemical environments. All the above results confirmed the chemical structure of DCPCD. Figure 1c shows the TG and DTG (from 50 to 700 °C) curves of the flame retardant DCPCD under a nitrogen atmosphere. To gain more information, distinctive data such as the temperature of 5% weight loss (T5%), the temperature of the maximum degradation rate (Tmax), and the char residue at 700 °C are marked in Figure 1c. As seen, the thermal degradation range of DCPCD occurs between 300 and 465 °C, the T5% is 343 °C, and the Tmax is 393 °C. After heating, the char residue at 700 °C is as high as 25 wt %. Figure 1d shows the DSC curve of the flame retardant DCPCD under a nitrogen atmosphere. The figure shows that the melting temperature (Tm) of DCPCD is 120 °C. As depicted in Figure 2, DCPCD is investigated by XPS. It can be observed from Figure 2a that the DCPCD powder contains four kinds of elements, including C, O, N, and P. The experimental value of element contents listed in Figure 2a is consistent with the theoretical value calculated according to the structural formula of DCPCD. According to the XPS-peak differentiating analysis of C 1s (Figure 2b), the main peaks contain C–C or C–H (284.8 eV) and C–O or C–N (286.5 eV). For the O 1s spectrum (Figure 2c), the peaks at 531.4 and 533.6 eV belong to the phosphorus–oxygen double bond (P=O) and single bond (P–O–C). Hence, it can be proved that DCPCD is successfully synthesized.

To explore the effect of DCPCD on the thermal properties of TPU, the thermal stability and Tg of TPU/DCPCD composites were studied by TG and DSC. The TG results of TPU and TPU/DCPCD composites under nitrogen and air atmospheres are shown in Figure 3. The detailed TG and DTG data are listed in Tables 2 and 3. There are two TPU decomposition stages under a nitrogen atmosphere. The first one is attributed to the degradation of the urethane bond of the TPU hard segments, and the main products at this stage are isocyanates, alcohols, amines, and carbon dioxide. The second stage is due to the thermal decomposition of the polyether polyol of the TPU soft segment. The temperature at 5 wt % weight loss is regarded as the initial degradation temperature (T5%). It is referred from Figure 3a,b that pure TPU starts to degrade at 317 °C with Tmax1 at 339 °C and Tmax2 at 425 °C. T5% of TPU composites is higher than that of pure TPU and increases slightly with the increase of the DCPCD weight because T5% of DCPCD is higher than that of pure TPU. In addition, Tmax1 of TPU composites shows little change with the addition of DCPCD, and Tmax2 of TPU composites decreases significantly with the increase of the DCPCD weight, which may be caused by the decomposition of DCPCD at about 343 °C. Moreover, the char residues of TPU composites under nitrogen increase rapidly with the increased weight loading of DCPCD. No matter how much DCPCD is added, the experimental value of the char residues of TPU composites is much higher than the theoretical value, indicating that the addition of DCPCD promotes the formation of the char residues of TPU composites. It is shown in Figure 3c,d that there are three weight loss stages of TPU under air. The third decomposition stage of TPU composites corresponds to the further thermal oxidative decomposition of the carbon residue in the previous stage. It indicates that TPU/DCPCD composites form an organic carbon layer during the heating process and further decompose under high temperatures in the air. The char residues of TPU composites are still higher than that of pure TPU. The above test results show that the addition of DCPCD improves the thermal stability and charring ability of the TPU matrix. The glass transition temperature (Tg) of pure TPU and TPU/DCPCD composites is shown in Figure 4a and Table 4. The Tg of pure TPU is 93.87 °C, and the Tg of TPU/DCPCD composites gradually move to high temperatures because of the hydrogen bond between DCPCD and TPU. The movement of the TPU molecular chain is limited due to hydrogen bonding (Figure 4d), so the Tg of TPU/DCPCD composites increase with the increase of the DCPCD content. The improvement of Tg can bring better heat resistance to TPU composites. The hydrogen bond between DCPCD and TPU was characterized by FTIR. From Figure 4b, the characteristic C=O peak of TPU at about 1690–1740 cm–1 can be seen. A curve-fitting procedure based on the Gaussian distribution was used to study the change of the C=O groups with the addition of DCPCD, as shown in Figure 4c. The peak near 1700 cm–1 represents the hydrogen-bonded C=O groups, and the peak near 1732 cm–1 represents the free C=O groups. With the increase of DCPCD, the intensity of the peak for the free C=O groups decreases, whereas the intensity of the peak for the hydrogen-bonded C=O groups increases. The ratios of the peak area of hydrogen-bonded C=O bands to the peak area of the free C=O bands calculated by integral software gradually increase from 1.27:1 for pure TPU to 2.54:1 for TPU/12%DCPCD, demonstrating the formation of a large number of new hydrogen bonds between the free C=O groups of TPU and the N–H groups of DCPCD.

The flame retardancy of pure TPU and TPU/DCPCD composites was evaluated by the UL 94 vertical burning test and the LOI measurement, as displayed in Figure 5a. The digital photos of the moment when the ignition source is removed in the 10th s of the first ignition during the vertical burning test are illustrated in Figure 5b. Figure 5a shows that pure TPU is flammable, which shows the LOI value of 22.3% and V-2 rating in the UL 94 vertical burning test, accompanied by severe melt dripping. Additionally, absorbent cotton was quickly ignited by molten drops, posing a severe secondary threat in actual fires. The LOI values of TPU/DCPCD composites gradually increase as the contents of DCPCD increase. When the amount of DCPCD is increased to 6%, the LOI value of TPU/6%DCPCD could reach 27.3%, passing the UL 94 V-0 rating. Although there was dripping, the absorbent cotton was not ignited. The LOI value of TPU/12%DCPCD raised to 30.8%, which is the highest among all the test samples. The combustion of the samples during the UL 94 vertical burning test can be observed in Figure 5b. It can be seen that after the first ignition, all samples dropped at the moment when the ignition source left. When the content of DCPCD is 1 and 3%, the drops can ignite the absorbent cotton. When the DCPCD content is equal to or higher than 6%, the dripping of melt was reduced, and the absorbent cotton was not ignited. In addition, it is observed that the drops could remove flame and heat and inhibit combustion, making the samples self-extinguish. The effect of DCPCD on the flame retardancy of TPU composites was further explored by cone calorimeter tests, and the result is shown in Figure 6 and Table 5. As shown in Figure 6a,b, it is worth noting that the PHRR of TPU can be reduced from 1292 to 574 kW/m2 by adding only 1% DCPCD, and the THR decreases from 78 to 62 MJ/m2, which drop by 55.6 and 20.5% compared with the PHRR and THR of pure TPU. With the increase of DCPCD, both the PHRR and THR decrease steadily, and the corresponding values of TPU/12%DCPCD are reduced by 67.7 and 38.5%, indicating that DCPCD effectively reduces the heat release of TPU during burning and has a positive effect in improving the fire safety of TPU. As shown in Figure 6c, there is a little residual char left after the combustion of pure TPU. As the mass percentage of DCPCD is increased, there is a dramatic higher trend in the residue char and a sharp decreasing trend in the average mass loss rate (av-MLR), which demonstrates that DCPCD might give substrates improved charring capabilities. Compared to pure TPU, the carbon residue of TPU/12%DCPCD increased by more than three times, which is consistent with the TG test results. Smoke generated in the process of fire is also one of the main risk factors of fire. Figure 6d and Table 5 reveal the TSR curves and data of pure TPU and TPU composites. The TSR of pure TPU is 1795 m2/m2. With the addition of DCPCD, the TSR values increase first and then decrease, and the TSR value of the TPU/12%DCPCD composite declined to 1501 m2/m2, which is 16.4% lower than that of pure TPU. When the amount of DCPCD added is low, the protective effect of the carbon layer is weak, and some decomposition products enter the gas phase in the form of smoke, leading to the increase of the TSR. With a large additional amount of DCPCD, more high-quality carbon layers are formed, and more decomposition products remain in the condensed phase, resulting in the decline of the TSR. The average effective heat of combustion (av-EHC) value reveals the burning degree of gas volatiles during combustion. With the addition of DCPCD, the av-EHC of TPU composites gradually decreases. This result further explains that DCPCD can effectively delay the decomposition of the TPU matrix, and the inhibition will lead to incomplete combustion and reduce the heat release during combustion. With the increase of DCPCD addition, the average carbon monoxide yield (av-COY) gradually increases, and the av-CO2Y gradually decreases, caused by less complete oxidation of the material during combustion, which illustrates that the addition of DCPCD can inhibit the combustion of materials. Fire performance index (FPI) (s m2 kW–1) is the ratio of TTI and the PHRR. A lower FPI value means the time to catch flashover is shorter and the fire risk is higher. With the increase of DCPCD addition, the FPI values of TPU composites gradually increase, which indicates that TPU composites have a higher safety level than pure TPU.

DCPCD participated in the carbonization process of TPU, which might not only increase the quantity of the residual char but also improve the quality of the residual char. Digital photos and SEM micrographs of the residues obtained from the cone calorimetry test for TPU and TPU/DCPCD are shown in Figure 7. From the macro residue digital photos, it can be seen that compared with pure TPU, when the addition of DCPCD is greater than or equal to 6%, the expansion degree of the residue of TPU/DCPCD composites is significantly increased. The SEM micrographs show that the char residue of pure TPU is broken and full of cracks and holes. When 1 wt % DCPCD is added, the cracked holes of the char layer become smaller. When 6 wt % DCPCD is added, the char residue of the TPU/DCPCD composite is further reduced, and char foaming is formed. Among all the samples, the residue of TPU/12%DCPCD is the most complete and compact, and char foaming can be observed. The elemental compositions of the char residues for TPU and TPU/DCPCD composites were further analyzed by XPS tests, and the corresponding spectra are presented in Figure 8. As shown in Figure 8a and Table 6, the elemental compositions of the char residue for TPU/DCPCD composites were C, O, N, and P, and the contents of P increased with the addition of DCPCD. Figure 8b,c displays XPS-peak differentiating analysis of C 1s for TPU and TPU/6%DCPCD. The C 1s peaks of the char residue of TPU at 284.8, 285.3, 286.3, and 289.4 eV are ascribed to the C–C, C–N, C–O, and C=O. For TPU/6%DCPCD, the peak area at 285.3 eV corresponds to the C–N/C–P bond, which increased because the char residue of TPU/6%DCPCD contains C–P. In addition, TPU/6%DCPCD has a new peak at 285.0 eV that is ascribed to the C–O–P.Figure 8d–f shows the XPS-peak differentiating analysis of O 1s, N 1s, and P 2p of the char residue for TPU/6%DCPCD. The N 1s peaks of the char residue of TPU/6%DCPCD at 398.6 and 400.6 eV are ascribed to the N–C and N–P. For the O 1s spectrum, the peaks at 530.9 eV belong to the phosphorus–oxygen single bond or carbon–oxygen single bond from P–O–P, P–O–C, or C–O–C groups. The peaks at 532.4 and 533.7 eV belong to the phosphorus–oxygen double bond (P=O) and carbon–oxygen double bond (C=O). In the high-resolution P 2p XPS spectrum, the fitting analysis presents four deconvoluted peaks at 132.5, 133.3, 134.0, and 134.7 eV corresponding to P–C, P–O, P–N–C, and P=O bonds, respectively. Combined with the above analysis, the results indicate that DCPCD could promote the formation of a phosphorus-containing carbon layer with higher cross-linking and strength in the TPU matrix during burning (Figure 9). To further illuminate the flame retardant mechanism of TPU/DCPCD composites, the char residues of TPU and TPU/DCPCD composites were analyzed by Raman measurement, as exhibited in Figure 10. Two distinctive peaks, the D peak and the G peak, are visible at 1360 and 1590 cm–1, respectively. Typically, the G peak relates to the structured graphite structure, whereas the D peak stands for the disordered graphite structure; the degree of graphitization of the char residue is often represented by the area ratio of the D peak to the G peak (ID/IG). If the ID/IG value is low, it means that the char residue has a high degree of graphitization and is dense and solid. As displayed in Figure 10, the pure TPU shows a high ID/IG value (3.04). With the increase of DCPCD addition, the ID/IG ratios of TPU composites decrease gradually. When the additional amount of DCPCD is 12 wt %, the ID/IG value of TPU composites decreases to 2.84, which is the lowest. It indicates that the graphitization degree of char residues is improved after the addition of DCPCD.

Transparency is an inherent excellent property of TPU materials. In many application fields of TPU, transparency is highly required, such as electronic and electrical enclosures, highly transparent adhesives, and film materials. However, higher flame retardancy is often at the expense of transparency. It is difficult to achieve high flame retardancy while maintaining the transparency of TPU. During the experiment, it was observed that the addition of DCPCD had little effect on the transparency of TPU. It can be seen from Figure 11 that the plastic pellets of TPU and TPU/DCPCD composites obtained by twin-screw extrusion granulation are all transparent. Moreover, the pictures under the pure TPU and TPU composite films can be seen clearly, which indicates that DCPCD almost has no influence on transparency. In order to quantitatively characterize the influence of DCPCD addition on the transparency of TPU, UV–vis spectroscopy, transmittance, and haze tester are introduced. As is shown in Figure 12a, in the wavelength range of 500–1500 nm, the total transmittance of pure TPU is from 75 to 85%. When the additional amount of DCPCD is 1 and 3 wt %, there is no effect on the total transmittance of TPU composites. Even 1 wt % DCPCD can slightly improve the transparency of TPU. The total transmittance of the TPU/6%DCPCD composite is from 51 to 78%, which increased with the increase of wavelength. TPU with 9 wt % DCPCD shows similar transmittance as that of TPU/6%DCPCD in the high-wavelength range, which decreases slightly in the low-wavelength range. When the amount of DCPCD is increased to 12 wt %, the total transmittance is greatly affected. The transmittance and haze test results are shown in Figure 12b. With the increase of the amount of DCPCD added, the transmittance first increases and then decreases. When the additional amount of DCPCD is 1 and 3 wt %, compared with pure TPU, the haze decreases from 70 to 60%. When the addition of DCPCD is greater than 3 wt %, the haze of the TPU composite shows little change with the increase of the amount of DCPCD, which is consistent with pure TPU.

A kind of TPU composite simultaneously possessing excellent flame retardancy and transparency was obtained by adding a novel flame retardant additive (DCPCD) containing phosphoramide bonds, which was synthesized through diethylenetriamine and diphenyl phosphorochloridate. The chemical structure of DCPCD was successfully confirmed by FTIR, 1H NMR, and XPS. The addition of DCPCD enhanced the char yield at 700 °C. With the incorporation of 6 wt % DCPCD, the UL 94 V-0 rating and the LOI value as high as 27.3% were achieved. The cone calorimetry test showed that the values of PHRR and THR were reduced after the incorporation of DCPCD. Compared with pure TPU, the PHRR and THR of the TPU/1%DCPCD composite were effectively reduced by 55.6 and 20.5%, and those for the TPU/6%DCPCD composite were further dramatically decreased by 59.1 and 30.8%. Meanwhile, the char residues of TPU with 6.0 wt % DCPCD increased by 100% from 8.0 wt % (pure TPU) to 16.0 wt %. Besides, TPU/6%DCPCD showed compact and continuous char layers, which function as a barrier to retard the combustion process. Raman spectroscopy suggested that DCPCD contributed to the formation of more stable char layers. SEM results showed that the presence of DCPCD caused obvious intumescence, which verified the condense phase flame retardant mechanism. The char residue of TPU/DCPCD composites after cone calorimetry was detected by XPS technology. XPS exhibited that DCPCD promoted the formation of a phosphorus-containing char layer with high cross-linking and strength in the TPU matrix after burning. In addition, the transmittance of TPU/DCPCD composites remained at a high level when the additional amount of DCPCD is less than 6 wt %. The haze of TPU composites shows little change with the increase of the amount of DCPCD, which is consistent with pure TPU. To sum up, DCPCD endowed TPU with high fire safety, and also the transparency was largely retained.