Synergistic Flame Retardant Effect of Barium Phytate and Intumescent Flame Retardant for Epoxy Resin

Recently, widespread concern has been aroused on environmentally friendly materials. In this article, barium phytate (Pa-Ba) was prepared by the reaction of phytic acid with barium carbonate in deionized water, which was used to blend with intumescent flame retardant (IFR) as a flame retardant and was added to epoxy resin (EP). Afterward, the chemical structure and thermal stability of Pa-Ba were characterized by Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA), respectively. On this basis, the flammability and flame retardancy of EP composites were researched. It is shown that EP/14IFR/2Ba composite has the highest limiting oxygen index (LOI) value of 30.7%. Moreover, the peak heat release rate (PHRR) of EP/14IFR/2Ba decreases by 69.13% compared with pure EP. SEM and Raman spectra reveal the carbonization quality of EP/14IFR/2Ba is better than that of other composites. The results prove that Pa-Ba can cooperate with IFR to improve the flame retardancy of EP, reducing the addition amount of IFR in EP, thus expanding the application range of EP. In conclusion, adding Pa-Ba to IFR is a more environmentally friendly and efficient method compared with others.


Introduction
In recent years, safety and environment protection requirements have become higher and higher for many materials used in industry and daily life. Epoxy resin (EP) plays a significant role in mechanical properties, electrical insulation, heat resistance, corrosion resistance and so on, thus becoming one of the most indispensable resins. It is extensively used in coatings, electronic and electrical industries, handicrafts, and photoelectric industries. However, EP is composed of chains of hydrocarbon with high flammability, which will produce comparatively great toxicity in the process of combustion. Therefore, there is an urgent need to improve the flame retardancy of EP [1][2][3][4][5].
In order to improve the flame retardancy of EP [6][7][8][9], there are several commonly used methods, such as surface modification [10], superrefining [11][12][13], complex cooperation [14,15], and cross-linking. A lot of research works have been carried out, and the direct addition of flame retardants has the advantages of convenience and economy, thus becoming the most chosen approach. Halogen flame retardant has advantages of high flame retardant efficiency [16], low dosage and good compatibility with materials, however, a large amount of smoke and poisonous and corrosive gases, such as dioxins, will be produced during combustion, causing great harm to the environment. Moreover, metal hydroxide is also an available flame retardant synergist, which is non-toxic and has good stability, while plenty of additive amounts and poor flowability will reduce the mechanical properties of materials. Therefore, we consider IFR as an environmentally friendly flame retardant [17], which is halogen-free and has low smoke. The most familiar and commercial IFR system is ammonium polyphosphate (APP) [18] and pentaerythritol (PER) [19][20][21], but only improve the effect of a high content of flame retardant on EP matrix, but also become more economical because of the reduction in the amount of APP originating from nonrenewable resources and the increase in Pa-Ba and make EP widely used in various fields.

Preparation of Pa-Ba
The synthesis diagram of Pa-Ba prepared from Pa and barium carbonate is shown in Scheme 1. First, 11.96 g (0.06 mol) BaCO3 was suspended in 100 mL deionized water under 35 °C, and 9.43 g (0.01 mol) Pa (70% aqueous solution) was dissolved into 50 mL of deionized water. After they dissolved completely, the Pa solution was placed in a constant pressure droplet funnel, which was added to the BaCO3 suspension at the rate of 5 drops 10 s within 30 min, with mechanical stirring. The reaction was kept at a constant temperature for 3 h, until there was no precipitation. Afterwards, the white precipitation yielded was filtered and rinsed with deionized water no less than 5 times until PH was equal to 7. Finally, the product was dried at 75 °C for 10 h, and a white powder, namely Pa-Ba, was obtained. The yield of Pa-Ba is about 85.88%. Scheme 1. Synthesis route of Pa-Ba.

Preparation of EP Composites
The composition of EP is shown in Table 1. At the very beginning, EP systems were slowly stirred for 30 min under 75 °C after the participation of IFR and Pa-Ba, so that the flame retardant was uniformly dispersed in the epoxy resin. The curing agent PA651 (the mass ratio of EP to PA651 was 3:1) was added into EP composites, stirring until the mixture was uniform. Afterwards, the blends were dried in a vacuum oven at 100 °C for 3 h and injected slowly into the mold which was preheated in 10 min and cured in a constant temperature drying box by the curing system of 110 °C/3 h + 130 °C/3 h + 150 °C/2 h. The EP composites were obtained after natural cooling. Scheme 1. Synthesis route of Pa-Ba.

Preparation of EP Composites
The composition of EP is shown in Table 1. At the very beginning, EP systems were slowly stirred for 30 min under 75 • C after the participation of IFR and Pa-Ba, so that the flame retardant was uniformly dispersed in the epoxy resin. The curing agent PA651 (the mass ratio of EP to PA651 was 3:1) was added into EP composites, stirring until the mixture was uniform. Afterwards, the blends were dried in a vacuum oven at 100 • C for 3 h and injected slowly into the mold which was preheated in 10 min and cured in a constant temperature drying box by the curing system of 110 • C/3 h + 130 • C/3 h + 150 • C/2 h. The EP composites were obtained after natural cooling.

Measurements
The FTIR was performed with the Thermo Fisher Nicolet ls10 spectrometer (Beijing Ruili Analytical Instrument Co., Ltd., Beijing, China) by recording the frequency of 16 scans, and the region was 400-4000 cm −1 . The sample functional groups were tested by KBr pressing method.
Thermogravimetric analysis (TGA) (Netzsch, Germany) was responded under the heating rate of 20 • C/min from 40-800 • C in N 2 atmosphere.
The synthesized samples and the residual chars after burning were observed by ZEISS EV0 MA15 scanning electron microscope(Carl Zeiss, Germany).
With the laser wavelength 532 nm, the Raman spectra were recorded in the range 200-2000 cm −1 by using the Thermo Fisher Dxr2xi Confocal Raman spectrometer (REN-ISHAW plc, Wotton-under-Edge, UK).

Characterization of Pa-Ba
In accordance with Scheme 1, Pa-Ba was produced. Figure 1 indicates the FTIR spectra of Pa, BaCO 3 and Pa-Ba [43]. As shown in Pa, 3416.67 cm −1 is associated with the O-H absorption of H 2 O, the appearance of the O-P-O telescopic vibration is shown in 1639.30 cm −1 , and CH 2 meets the conditions of characteristic vibration peak at 2846-2942 cm −1 , the position of 2820.32 cm −1 is attributable to the vibration shrinkage range of P-OH. It is found that BaCO 3 shows the absorption peak of C=O at 1447.34 cm −1 , 692.28 cm −1 is the characteristic band of barium salt. As for Pa-Ba, some characteristic peaks are observed from barium salt and Pa, for example, the peaks at 692.28 cm −1 , just as the same with BaCO 3 and the absorption for (PO 3 ) 2− at 1007.18 cm −1 shifts to 1072.20 cm −1 , which means the interaction between ions is changed. Moreover, the vibration of P-OH at 2820.32 cm −1 and the absorption peak of C=O at 1447.34 cm −1 both disappeared. The results above suggested that Pa-Ba was synthesized.
Barium phytate was characterized by Scanning electron microscopy (SEM) and energydispersive spectroscopy (EDS) as shown in Figure 2 [44]. Figure 2a shows the SEM morphology of Pa-Ba which has an irregular granular shape. The EDS data of Pa-Ba are depicted in Figure 2b-e, which demonstrate the composition and distribution of main elements in Pa-Ba. It is evident that there are four main elements oxygen (O), phosphorus (P), carbon (C), and barium (Ba) in Pa-Ba. In addition, the four main elements are distributed homogeneously, which further confirms that Pa-Ba was synthesized. Barium phytate was characterized by Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) as shown in Figure 2 [44]. Figure 2a shows the SEM morphology of Pa-Ba which has an irregular granular shape. The EDS data of Pa-Ba are depicted in Figure 2b-e, which demonstrate the composition and distribution of main elements in Pa-Ba. It is evident that there are four main elements oxygen (O), phosphorus (P), carbon (C), and barium (Ba) in Pa-Ba. In addition, the four main elements are distributed homogeneously, which further confirms that Pa-Ba was synthesized.   Barium phytate was characterized by Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) as shown in Figure 2 [44]. Figure 2a shows the SEM morphology of Pa-Ba which has an irregular granular shape. The EDS data of Pa-Ba are depicted in Figure 2b-e, which demonstrate the composition and distribution of main elements in Pa-Ba. It is evident that there are four main elements oxygen (O), phosphorus (P), carbon (C), and barium (Ba) in Pa-Ba. In addition, the four main elements are distributed homogeneously, which further confirms that Pa-Ba was synthesized.

Thermal Stability
To investigate the reactions among Pa-Ba, EP and IFR, their thermal decomposition behaviors are researched using TGA in the N 2 atmosphere as displayed in Figure 3. The details are shown in Table 2.
composites both show downward trends. EP/IFR/2Ba composite has the lowest maximum thermal decomposition rate, corresponding to a temperature of 347 °C, which is 67 °C lower than that of pure EP. It is proved that IFR and Pa-Ba can reduce the Tmax of EP, generating more residues and forming more stable carbon layers.
Lower initial decomposition temperature, the reduction in the Tmax and the increase in the residues demonstrate that Pa-Ba can be used as a carbon source, which can protect the matrix degradation, having the effect of heat insulation and oxygen insulation, thus producing more stable carbon layers.  The degradation of pure EP can be divided into the following two stages. The first stage occurs from 350 to 500 • C, which is also the main thermal degradation stage of EP. The degradation of EP can release a large amount of heat and produce CO, CO 2 , CH 4 and other thermal degradation gases. In the second stage, after 500 • C, EP continuously degrades and carbonizes to form a carbon layer. With the addition of IFR and Pa-Ba, the carbon layer increases ulteriorly, which can better isolate the exchange of heat and gas and prevent combustion. Furthermore, there are some exothermic and endothermic events in TGA. The endothermic reaction occurs when APP is heated to 300-330 • C, and the side-chain structure decomposes and removes part of the amino groups to form hydroxyl groups. The exothermic reaction occurs after the thermal decomposition of APP, which reacts with PER to form a homogeneous carbon layer. The char formation of IFR is instrumental in restraining the combustion of EP composites, thus improving thermal stability. In addition, Pa-Ba reacts with IFR exothermically at 350-600 • C to form a more stable carbon layer.
As exhibited in Figure 3a and Table 2, the initial decomposition temperature (T 5 wt% ) of pure EP occurs at 372 • C, and there is obvious weightlessness at 300-500 • C. After adding Pa-Ba and IFR, the weightlessness stage of EP composites is identical to that of pure EP. However, the T 5 wt% of EP composites decreases in different degrees, for example, the T 5 wt% for EP/16IFR and EP/14IFR/2Ba decreases from 372 • C to 320 • C and 313 • C, respectively, which reveals that IFR and Pa-Ba advance the decomposition of EP [43]. The residue of EP/IFR with 23.27 wt% at 800 • C enhances remarkably in comparison with that of pure EP with 7.25 wt%, which means that a stable carbon layer was generated by the reaction of APP and PER. After adding Pa-Ba, the residue of EP composites increases further, especially, EP/14IFR/2Ba composite has the highest residue of 25.90 wt%.
From the DTG curves in Figure 3b, the maximum weight loss rate and the T max of EP composites both show downward trends. EP/IFR/2Ba composite has the lowest maximum thermal decomposition rate, corresponding to a temperature of 347 • C, which is 67 • C Polymers 2021, 13, 2900 7 of 14 lower than that of pure EP. It is proved that IFR and Pa-Ba can reduce the T max of EP, generating more residues and forming more stable carbon layers.
Lower initial decomposition temperature, the reduction in the T max and the increase in the residues demonstrate that Pa-Ba can be used as a carbon source, which can protect the matrix degradation, having the effect of heat insulation and oxygen insulation, thus producing more stable carbon layers.

Flame Retardancy
As demonstrated in Table 3, the pure EP shows high flammability when the LOI reaches 19.1%. Obviously, the adjunction of IFR increases the LOI of EP from 19.1% to 24.3%, which availably improves the flame retardancy of EP, revealing IFR can be considered as a productive flame retardant. With Pa-Ba added into EP composites, the LOI values exhibit a trend of increasing at first and then decreasing, especially, EP/14IFR/2Ba composite has the highest LOI reaching 30.7%. The results demonstrate that Pa-Ba can increase the flame retardant efficiency of IFR in EP, thus making EP widely used in various fields. This section may be divided into subheadings. It should provide a concise and precise description of the experimental results, their interpretation, as well as the experimental conclusions that can be drawn. Cone calorimeter test (CCT) is adopted to test the flammability of polymer materials, the combustion behavior in a fire can be assessed in accordance with the experimental data [41]. There are many combustion parameters of combustible materials obtained by CCT in a fire. The flame retardancy in an actual fire can be evaluated by heat release rate (HRR), peak heat release rate (PHRR), and total heat release (THR). Meanwhile, the smoke suppression properties can be estimated by smoke production rate (SPR), peak smoke production rate (PSPR), total smoke production (TSP).
Curves of HRR and THR are displayed in Figures 4 and 5, respectively. It can be recognized that pure EP has high flammability, showing the HRR is a single peak and changes rapidly over time, and the PHRR is as high as 794.09 kW/m 2 . The HRR curves of EP composites are analogous to those of pure EP. The addition of IFR slows down the heat release and decreases the PHRR value. After the addition of Pa-Ba, the value of PHRR decreases further, especially, the PHRR of EP/14IFR/2Ba reduces to the lowest level of 245.15 kW/m 2 , a decrease of 548.94 kW/m 2 , which is only 30.87% of that of pure EP. As displayed in Figure 5, pure EP has a THR of 95.45 MJ/m 2 . The THR of EP/IFR is remarkably lowered, which attains to 41.18 MJ/m 2 . After adding Pa-Ba, the value of THR decreases further, especially, the THR of EP/14IFR/2Ba drops to the lowest level of 27.16 MJ/m 2 , a decrease of 68.29 MJ/m 2 , which is only 28.45% of that of pure EP. From the HRR and THR curves, the addition of IFR and Pa-Ba can effectively reduce the PHRR and THR values, which attests that the flame retardancy of EP was improved [45].
6.48 m 2 /m 2 , a decrease of 22.14 m 2 /m 2 . From the SPR and TSP curves, the addition of IFR and Pa-Ba can availably decrease the SPR and TSP values and the total smoke volume of materials, which proves that EP composites have excellent smoke suppression performance. All these indicate that in the process of combustion, the existence of Pa-Ba can promote the cross-linking of IFR into carbon more efficiently, and the formed carbon layer can be used as an obstacle to block the transmission of heat and combustible gas, thus playing a better role of flame retardant.   6.48 m 2 /m 2 , a decrease of 22.14 m 2 /m 2 . From the SPR and TSP curves, the addition of IFR and Pa-Ba can availably decrease the SPR and TSP values and the total smoke volume of materials, which proves that EP composites have excellent smoke suppression performance. All these indicate that in the process of combustion, the existence of Pa-Ba can promote the cross-linking of IFR into carbon more efficiently, and the formed carbon layer can be used as an obstacle to block the transmission of heat and combustible gas, thus playing a better role of flame retardant.   Curves of SPR and TSP are exhibited in Figures 6 and 7, respectively. As shown in Figure 6, the PSPR value of pure EP is 0.255 m 2 /s, which is much higher than that of EP composites. The addition of IFR and Pa-Ba slows down the smoke production of EP and decreases the PSPR value, especially, the PSPR of EP/14IFR/2Ba reduces to the lowest level of 0.109 m 2 /s, a decrease of 0.146 m 2 /s. From Figure 7, the TSP of EP composites decreases remarkably. Pure EP can produce smoke constantly and promptly, and the TSP is as high as 28.62 m 2 /m 2 . After adding IFR and Pa-Ba, the TSP of EP composites decreases dramatically, especially EP/14IFR/2Ba, the TSP value cuts down to the lowest level, which is only 6.48 m 2 /m 2 , a decrease of 22.14 m 2 /m 2 . From the SPR and TSP curves, the addition of IFR and Pa-Ba can availably decrease the SPR and TSP values and the total smoke volume of materials, which proves that EP composites have excellent smoke suppression performance. All these indicate that in the process of combustion, the existence of Pa-Ba can promote the cross-linking of IFR into carbon more efficiently, and the formed carbon layer can be used as an obstacle to block the transmission of heat and combustible gas, thus playing a better role of flame retardant.

Residual Char
Consecutive and dense residual char can have the effect of heat insulation and oxygen insulation, thus forestalling secondary combustion of the EP matrix. Figure 8 displays the digital photos of EP, EP/16IFR and EP/14IFR/2Ba after CCT. It is shown that pure EP is nearly burnt out and rarely produces residual char. Moreover, EP/16IFR has generated an expanded carbon layer, which is not compact enough and has a relatively low expansion height of 3.8 cm. However, the carbon layer generated by EP/14IFR/2Ba with the expansion height of 5.0 cm is relatively dense and consecutive, which means that the carbonization quality of EP/14IFR/2Ba is better than that of EP/16IFR.

Residual Char
Consecutive and dense residual char can have the effect of heat insulation and oxy gen insulation, thus forestalling secondary combustion of the EP matrix. Figure 8 displays the digital photos of EP, EP/16IFR and EP/14IFR/2Ba after CCT. It is shown that pure EP is nearly burnt out and rarely produces residual char. Moreover, EP/16IFR has generated an expanded carbon layer, which is not compact enough and has a relatively low expan sion height of 3.8 cm. However, the carbon layer generated by EP/14IFR/2Ba with the ex pansion height of 5.0 cm is relatively dense and consecutive, which means that the car bonization quality of EP/14IFR/2Ba is better than that of EP/16IFR.

Residual Char
Consecutive and dense residual char can have the effect of heat insulation and oxygen insulation, thus forestalling secondary combustion of the EP matrix. Figure 8 displays the digital photos of EP, EP/16IFR and EP/14IFR/2Ba after CCT. It is shown that pure EP is nearly burnt out and rarely produces residual char. Moreover, EP/16IFR has generated an expanded carbon layer, which is not compact enough and has a relatively low expansion height of 3.8 cm. However, the carbon layer generated by EP/14IFR/2Ba with the expansion height of 5.0 cm is relatively dense and consecutive, which means that the carbonization quality of EP/14IFR/2Ba is better than that of EP/16IFR. Polymers 2021, 13, x FOR PEER REVIEW 10 of 14 Furthermore, the SEM of char residues is shown in Figure 9. For pure EP, although the surface is relatively smooth, numerous pores and cracks were found on the discontiguous residual char surface, making it impossible to delay the combustion of the underlying EP. In contrast, the continuance of the carbon layer of EP/16IFR is remarkably improved, with few pores produced and a compact and continuous carbon layer. Compared with EP/16IFR and pure EP, the addition of Pa-Ba forms a denser wrinkled carbon layer, which can be considered as a more effective protective barrier, not only to prevent molten droplets and the escape of combustible gas, but also have great effects on heat insulation and oxygen insulation. The residual chars after cone calorimeter testing are further researched as demonstrated in Figure 10. Based on the Raman spectra, we analyzed the characteristic peak of D-band and G-band in graphite, which appeared at 1348 and 1590 cm −1 in turn [46]. The D-band mainly corresponds to the defect of the graphitized layer, while the G-band is mainly equivalent to the ordered graphite layer. The ratio of R intensity of the D band to the G band (ID/IG) reflects the graphitization degree. Moreover, ID/IG is inversely proportional to graphitization degree. The R values of pure EP, EP/16IFR and EP/14IFR/2Ba are 3.93, 3.52 and 3.15, respectively, indicating that the R value of EP/16IFR is lower and the degree of graphitization is higher compared with pure EP. After adding Pa-Ba, the R value decreases further, which means that the graphitization degree of the carbon layer is higher than that of EP/16IFR. Therefore, the carbon layer formed by EP/14IFR/2Ba after combustion is more orderly and dense, and its quality is better than that of the other two materials, Furthermore, the SEM of char residues is shown in Figure 9. For pure EP, although the surface is relatively smooth, numerous pores and cracks were found on the discontiguous residual char surface, making it impossible to delay the combustion of the underlying EP. In contrast, the continuance of the carbon layer of EP/16IFR is remarkably improved, with few pores produced and a compact and continuous carbon layer. Compared with EP/16IFR and pure EP, the addition of Pa-Ba forms a denser wrinkled carbon layer, which can be considered as a more effective protective barrier, not only to prevent molten droplets and the escape of combustible gas, but also have great effects on heat insulation and oxygen insulation. Furthermore, the SEM of char residues is shown in Figure 9. For pure EP, although the surface is relatively smooth, numerous pores and cracks were found on the discontiguous residual char surface, making it impossible to delay the combustion of the underlying EP. In contrast, the continuance of the carbon layer of EP/16IFR is remarkably improved, with few pores produced and a compact and continuous carbon layer. Compared with EP/16IFR and pure EP, the addition of Pa-Ba forms a denser wrinkled carbon layer, which can be considered as a more effective protective barrier, not only to prevent molten droplets and the escape of combustible gas, but also have great effects on heat insulation and oxygen insulation. The residual chars after cone calorimeter testing are further researched as demonstrated in Figure 10. Based on the Raman spectra, we analyzed the characteristic peak of D-band and G-band in graphite, which appeared at 1348 and 1590 cm −1 in turn [46]. The D-band mainly corresponds to the defect of the graphitized layer, while the G-band is mainly equivalent to the ordered graphite layer. The ratio of R intensity of the D band to the G band (ID/IG) reflects the graphitization degree. Moreover, ID/IG is inversely proportional to graphitization degree. The R values of pure EP, EP/16IFR and EP/14IFR/2Ba are 3.93, 3.52 and 3.15, respectively, indicating that the R value of EP/16IFR is lower and the degree of graphitization is higher compared with pure EP. After adding Pa-Ba, the R value decreases further, which means that the graphitization degree of the carbon layer is higher than that of EP/16IFR. Therefore, the carbon layer formed by EP/14IFR/2Ba after combustion is more orderly and dense, and its quality is better than that of the other two materials, The residual chars after cone calorimeter testing are further researched as demonstrated in Figure 10. Based on the Raman spectra, we analyzed the characteristic peak of D-band and G-band in graphite, which appeared at 1348 and 1590 cm −1 in turn [46]. The D-band mainly corresponds to the defect of the graphitized layer, while the G-band is mainly equivalent to the ordered graphite layer. The ratio of R intensity of the D band to the G band (I D /I G ) reflects the graphitization degree. Moreover, I D /I G is inversely proportional to graphitization degree. The R values of pure EP, EP/16IFR and EP/14IFR/2Ba are 3.93, 3.52 and 3.15, respectively, indicating that the R value of EP/16IFR is lower and the degree of graphitization is higher compared with pure EP. After adding Pa-Ba, the R value decreases further, which means that the graphitization degree of the carbon layer is higher than that of EP/16IFR. Therefore, the carbon layer formed by EP/14IFR/2Ba after combustion is more orderly and dense, and its quality is better than that of the other two materials, which can be conducive to preventing the formation of cracks during and after combustion. which can be conducive to preventing the formation of cracks during and after combustion. Figure 10. Raman spectra of EP, EP/16IFR and EP/14IFR/2Ba after combustion.
Curves of the FTIR spectra of residual chars are exhibited in Figure 11. As shown in EP/16IFR, 3462.01 cm −1 is associated with the O-H absorption, the appearance of the C-H telescopic vibration is shown in 2933.42 and 2863.26 cm −1 , C=C meets the conditions of characteristic vibration peak at 1637.82 cm −1 , the position of 1401.27 cm −1 is attributable to C-N stretching and N-H bending vibration absorption peak, and 1085.91 cm −1 is the characteristic band of P=O. It can be recognized from EP/14IFR/2Ba that there is no obvious difference between the two FTIR curves, demonstrating that the addition of Pa-Ba does not change the degradation products, only accelerating or delaying the reactions. Figure 11. FTIR spectra of EP/16IFR and EP/14IFR/2Ba after combustion.

Conclusions
Barium phytate (Pa-Ba) was prepared from phytic acid and barium carbonate and characterized by FTIR and SEM. EP composites were produced by the addition of the synthesized Pa-Ba and IFR. The thermal stability was studied by TGA. EP/14IFR/2Ba has the Curves of the FTIR spectra of residual chars are exhibited in Figure 11. As shown in EP/16IFR, 3462.01 cm −1 is associated with the O-H absorption, the appearance of the C-H telescopic vibration is shown in 2933.42 and 2863.26 cm −1 , C=C meets the conditions of characteristic vibration peak at 1637.82 cm −1 , the position of 1401.27 cm −1 is attributable to C-N stretching and N-H bending vibration absorption peak, and 1085.91 cm −1 is the characteristic band of P=O. It can be recognized from EP/14IFR/2Ba that there is no obvious difference between the two FTIR curves, demonstrating that the addition of Pa-Ba does not change the degradation products, only accelerating or delaying the reactions. which can be conducive to preventing the formation of cracks during and after combustion. Figure 10. Raman spectra of EP, EP/16IFR and EP/14IFR/2Ba after combustion.
Curves of the FTIR spectra of residual chars are exhibited in Figure 11. As shown in EP/16IFR, 3462.01 cm −1 is associated with the O-H absorption, the appearance of the C-H telescopic vibration is shown in 2933.42 and 2863.26 cm −1 , C=C meets the conditions of characteristic vibration peak at 1637.82 cm −1 , the position of 1401.27 cm −1 is attributable to C-N stretching and N-H bending vibration absorption peak, and 1085.91 cm −1 is the characteristic band of P=O. It can be recognized from EP/14IFR/2Ba that there is no obvious difference between the two FTIR curves, demonstrating that the addition of Pa-Ba does not change the degradation products, only accelerating or delaying the reactions. Figure 11. FTIR spectra of EP/16IFR and EP/14IFR/2Ba after combustion.

Conclusions
Barium phytate (Pa-Ba) was prepared from phytic acid and barium carbonate and characterized by FTIR and SEM. EP composites were produced by the addition of the synthesized Pa-Ba and IFR. The thermal stability was studied by TGA. EP/14IFR/2Ba has the Figure 11. FTIR spectra of EP/16IFR and EP/14IFR/2Ba after combustion.

Conclusions
Barium phytate (Pa-Ba) was prepared from phytic acid and barium carbonate and characterized by FTIR and SEM. EP composites were produced by the addition of the synthesized Pa-Ba and IFR. The thermal stability was studied by TGA. EP/14IFR/2Ba has the highest residue of 25.90 wt%, which is much higher compared with pure EP (7.25 wt%).
Afterwards, the flame retardancy was analyzed by LOI and CCT. The results show that Pa-Ba can cooperate with IFR to flame retardant EP, and EP/14IFR/2Ba has the highest LOI value of 30.7%. The PHRR value of EP/14IFR/2Ba decreases dramatically, from 794.09 kW/m 2 to 245.15 kW/m 2 ; meanwhile, the PSPR value reduces from 0.255 m 2 /s to 0.109 m 2 /s. From the residue char of EP composites after combustion, the expanded carbon layer generated by EP/14IFR/2Ba is dense and continuous, with a height of 5.0 cm. SEM and Raman spectroscopy were adopted to investigate the residue char further. They reveal that the carbonization quality of EP/14IFR/2Ba is better than that of other composites, which is conducive to preventing the formation of cracks during and after combustion.
The results demonstrate that Pa-Ba can be used as a carbon source, which can protect matrix degradation, prevent the escape of combustible gas, and have significant effects on heat insulation and oxygen insulation, thus forming more stable carbon layers. Meanwhile, Pa-Ba can improve the flame retardant efficiency of IFR in EP and reduce the total smoke volume of materials, so as to cooperate with IFR to improve the thermal stability, flame retardancy and smoke suppression performance of EP, thus playing a better role in reducing the probability of fire as well as expanding the applicable scope of EP.

Data Availability Statement:
All the data will be available to the readers.