Synthesis of New S-Triazine Bishydrazino and Bishydrazido-Based Polymers and Their Application in Flame-Retardant Polypropylene Composites

In this study six new s-triazine bishydrazino and bishydrazido-based polymers were synthesized via condensation of bishydrazino s-triazine derivatives with terephthaldehyde or via nucleophilic substitution of dichloro-s-triazine derivatives with terephthalic acid hydrazide. The synthesized polymers were characterized by different techniques. The new polymers displayed good thermal behavior with great values in terms of limited oxygen indexed (LOI) 27.50%, 30.12% for polymers 5b,c (bishydrazino-s-triazine based polymers) and 27.23%, 29.86%, 30.85% for polymers 7a–c (bishydrazido-s-triazine based polymers) at 800 °C. Based on the LOI values, these polymers could be classified as flame retardant and self-extinguishing materials. The thermal results also revealed that the type of substituent groups on the triazine core has a considerable impact on their thermal behavior. Accordingly, the prepared polymers were mixed with ammonium polyphosphate (APP) in different proportions to form an intumescent flame-retardant (IFRs) system and were introduced into polypropylene (PP) to improve the flame-retardancy of the composites. The best results were obtained with a mass ratio of APP: 5a–c or 7a–c of 2:1, according to the vertical burning study (UL-94). In addition, the presence of 25% “weight ratio” of IFR in the composite showed great impact and passed UL-94 V-0 and V-1 tests.


Introduction
Polypropylene (PP) is considered as a vital commodity in the thermoplastic family, and has been extensively employed in numerous diverse usages like transportation, aviation, building and construction materials, the electrical and electronics field, and medical and consumer applications [1][2][3] as a consequence of its chemical and mechanical characteristics, its processability and machinability, cost price ratio, etc. [3]. Hence, raising the fire-retardant activities of plastics material is a critical task for many forms of usage [4,5]. For this reason, employing flame-retardant (FR) material is essential [6,7]. Because of the flammability problem of PP due to its low limiting oxygen index (LOI value is around 17.5%), many scientists have displayed an interest in improving the fire retardancy of polypropylene. They performed intensive work to achieve retardancy of PP. Several researchers recommended low weight material to increase the fire retardancy together with the desired mechanical, thermal and electrical properties. The most popular efficient flame retardant is the one having halogens such as bromine (Br 2 ) and antimony trioxide (Sb 2 O 3 ) synergistic systems. This type offered a good flame-retardancy system of PP [8]. Implementing this type of flame retardant is limited, as a consequence of releasing a large amount of toxic gases and smoke whenever halogenated flame-retardant burns. An alternative fire-retardant additive is a metal oxide, such as magnesium hydroxide and aluminum hydroxide. However, to acquire the similar grade of halogenated fire retardant, huge amount of metal oxide is needed to annihilate the mechanical properties of polypropylene [9][10][11].
Materials having a heterocyclic moiety such as a s-triazine ring in their backbone chain have acquired special attention as a result of the significant influence on the end-product properties [12][13][14]. The sophisticated structure of the s-triazine ring as well as the exceptional reactivity of the starting material cyanuric chloride (TCT) has been employed as a basic core for preparation of an enormous number of derivatives, as a result of its commercial accessibility and the three chlorine atoms can be exchanged by distinct nucleophiles with the control of temperature that make it valuable in material [15][16][17][18][19][20][21] and industrial applications [22][23][24][25]. Recently, many scientists reported that polymer-based s-triazine has been used in the design and advancement of fire-retardant derivatives [26][27][28], metal adsorption [29], carbon dioxide capture [30][31][32] and covalent organic frameworks [24,25,28]. Also, the design of the structure of s-triazine derivatives could be controlled at different temperatures [33]. These s-triazine derivatives achieved excellent synergistic intumescent flame retardant effect with ammonium polyphosphate (APP) [34][35][36].
In this regards, and in continuation of our previous work [44,45], we describe herein the synthesis and characterization of two groups of charring agent based on polymers containing s-triazine with hydrazine and hydrazido linkage in their backbone chain. The described derivatives have aniline, p-bromoaniline, p-methoxyaniline, linked to the striazine ring. The change of thermal behavior of the prepared compounds based on different substitutes will be studied. Correspondingly, the synthesized charring agent was applied to fire retard PP jointly with APP at different percentages in order to figure out the fire-retardancy characteristics of the described compounds based on the UL-94 V0 rating.

Materials and Methods
Polypropylene (PP) was obtained from SABIC company (PP 595A, Riyadh, Saudi Arabia). APP was purchased from Hubei Jusheng Technology Co. Ltd (Wuhan, China). All chemicals and solvents were purchased from their commercial sources. FT-IR (cm −1 ) was recorded on spectrophotometer (8201-PC, Shimadzu, Ltd., Kyoto, Japan). Scanning electron microscopy (SEM) was performed on a JEOL (SEM 6380 LA, Tokyo, Japan) to investigate the morphology of the prepared polymers. Differential scanning calorimetry (DSC) was conducted on a DSC, TA instrument Q1000 (New Castle, DE, USA) in the range between 30 • C and 300 • C under N 2 atmosphere to investigate glass transition (T g ) of polymers. Thermogravimetric analysis (TGA) data for the prepared polymers were taken by using TA Q500 (New Castle, DE, USA) with a flow rate of 60 mL/min (N 2 gas), starting from 30 to 800 • C to studied thermal behavior of prepared polymers. The reported analysis for each sample is the average of three reading.
2.2. Synthesis of Polymer-Based S-Triazine Bishydrazino Derivatives, 5a-c 2,4-Bishydrazino-6-substituted s-triazine derivatives 3a-c (20 mmol) was added portion wise to a solution of terephthaldehyde 4 (2.68 gm, 20 mmol) in 100 mL ethanol containing three drops of acetic acid. The reaction mixture was allowed to cool at room temperature after being refluxed for 24 h. The obtained yellow solid was filtered, washed with cold ethanol, and then dried for 8 h at 60 • C to give the desired polymers 5a-c in yield 80-86%.

Synthesis of Polymer-Based S-Triazine Bishydrazido Derivatives, 7a-c
First, terephthalic acid hydrazide 6 was prepared according to the reported method [45] and then used for preparation of the desired polymers. Terephthalic acid hydrazide 6 (20 mmol) in 100 mL acetonitrile were added slowly to a solution of 2,4-dichloro-6-substituted s-triazine derivatives 2a-c (20 mmol) in 100 mL acetonitrile in a period of 15 min followed by addition of K 2 CO 3 (45 mmol) at 25 • C. The reaction mixture was refluxed for 48 h, then solvent was removed under vacuum, and more water was added. The precipitated product was filtered, washed with water, ethanol, and then dried for 8 h at 60 • C to afford the desired polymers 7a-c in yield 80-85%.

UL-94 Test for Flame Retardant of PP Composites
The vertical burning test was examined rendering to the UL-94 test standard with the sample dimension of 130 mm × 13 mm × 3.2 mm. The samples were prepared before compounding. PP and all the additive materials were dried in vacuum oven for 24 h at 70 • C. All tested samples were prepared by mixing PP, APP and 5a-c or 7a-c using a two-roll mill mixing (Rheomixer-XSS-300, Chuang, China) at range of 170 • C-180 • C for 10 min with a roll speed of 100 rpm.

FT-IR Spectrum Analysis
The FT-IR spectrum ( Figure 1) for polymers 5a-c showed absorption peaks related to N-H bond in the range 3320 to 3350 cm −1 . The C-H; sp 3 and sp 2 hydrogen have a characteristic peak in the range 2840-2950 cm −1 . The peaks related to C=N bond of triazine ring observed at 1590 and 1550 cm −1 . The peaks C=C of phenyl ring were observed at 1600, and 1490 cm −1 and the peak at 1270 cm −1 was related to the C-N bond of the triazine ring. In addition, the peaks at 757 and 688 cm −1 were attributed to the monosubstituted phenyl ring (Table 1). While FT-IR spectra ( Figure 2) for polymers 7a-c showed, absorption peaks related to the N-H bond. C-H in the range 3320 to 3330 cm −1 ; sp 3 and sp 2 hydrogen has a characteristic peak in the range 2830-2950 cm −1 . The peaks at 1590 and 1550 cm −1 were ascribed to the C=N of the triazine ring. The both peaks at 1630-1620 and 1490-1470 cm −1 were ascribed to C=O of the hydrazide group and C=C of the phenyl ring, respectively. The peak at 1270 cm −1 was assigned to the C-N bond of the triazine ring ( Table 1). The shift in C=N and C-N values was due to the influence of the replacement groups on the aniline ring bonded to the triazine ring. In addition, the elemental analysis for the prepared 5a-c and 7a-c samples agreed with the calculated values as shown in Table 2.        Figure 3 displays the XRD pattern of the 5a-c polymers; as observed from Figure 3, the spectra of 5c polymer does not exhibit any sharply peak due to its amorphous nature, which further supports its low compactness. While the two polymers 5a and 5c showed a mostly amorphous phase with few crystalline materials. In addition, the 5b polymer have more than five strong fairly sharp diffraction peaks appeared at 2θ = 5-30 • , giving a pattern typical of crystalline materials. Figure 4 displays XRD pattern of the 7a-c polymers, as observed from Figure 4 the polymers showed sharp peaks in the intensity versus scattering angle (2θ) plot. More than six strong fairly sharp diffraction peaks appeared at 2θ = 5-40 • for 7c, 7b and 7a, giving a pattern typical of crystalline materials. Based on the XRD spectrum, it was found that the crystallization of 7a-c was higher than that of 5a-c. This might be attributed to the presence of oxygenated functional groups in 7a-c, which make the polymer chains more aligned by hydrogen bonding. Figure 3 displays the XRD pattern of the 5a-c polymers; as observed from Figure 3, the spectra of 5c polymer does not exhibit any sharply peak due to its amorphous nature, which further supports its low compactness. While the two polymers 5a and 5c showed a mostly amorphous phase with few crystalline materials. In addition, the 5b polymer have more than five strong fairly sharp diffraction peaks appeared at 2θ =5-30°, giving a pattern typical of crystalline materials.   Figure 4 the polymers showed sharp peaks in the intensity versus scattering angle (2θ) plot. More than six strong fairly sharp diffraction peaks appeared at 2θ =5-40° for 7c, 7b and 7a, giving a pattern typical of crystalline materials. Based on the XRD spectrum, it was found that the crystallization of 7a-c was higher than that of 5a-c. This might be attributed to the presence of oxygenated functional groups in 7a-c, which make the polymer chains more aligned by hydrogen bonding.

Scanning Electron Microscope (SEM)
The surface morphologies of the polymers 5a-c and 7a-c were examined by using a SEM. The polymer 5a ( Figure 5) showed a globular, multidroplet morphology, and nonuniform porosity distribution with roughness compared to 5b, while the 5b showed less porosity and a rougher surface while 5c ( Figure 5) was uniform, smooth with small pores and a rougher surface compared to its analogs 5a and 5b ( Figure 5). On other hand, the polymers 7a-c ( Figure 5) showed more roughness surface with nanosphere morphology. This might be attributed to the different group (CO-NH-NH) rather than the hydrazine (CH=N-NH-)-containing functional groups of the polymers, where in 5a-c there a direct conjugation between the benzene moiety and the functional groups (CH=N-N) which is not the case in 7a-c [46].

Scanning Electron Microscope (SEM)
The surface morphologies of the polymers 5a-c and 7a-c were examined by using a SEM. The polymer 5a ( Figure 5) showed a globular, multidroplet morphology, and nonuniform porosity distribution with roughness compared to 5b, while the 5b showed less porosity and a rougher surface while 5c ( Figure 5) was uniform, smooth with small pores and a rougher surface compared to its analogs 5a and 5b ( Figure 5). On other hand, the polymers 7a-c ( Figure 5) showed more roughness surface with nanosphere morphology. This might be attributed to the different group (CO-NH-NH) rather than the hydrazine (CH=N-NH-)-containing functional groups of the polymers, where in 5a-c there a direct conjugation between the benzene moiety and the functional groups (CH=N-N) which is not the case in 7a-c [46].

Thermogravimetric Analysis (TGA)
TGA was used to investigated the thermal degradation performances of the prepared polymers 5a-c. Degradation curves were obtained for the prepared compounds of the 5ac ( Figure 6 and summarized in Table 3). As observed from Figure 6 and Table 3, the first degradation peaks in the range 50-100 °C for compounds 5a-c, with mass loss about 2-3%, are related to the loss of the solvent content. Compounds 5a-c showed one main degradation peak in the range 320-460 °C, 340-450 °C and 350-450 °C with mass loss 57.88%, 50.50% and 43.70%, respectively. At 800 °C, the residual weight of 5a, 5b and 5c was 16.52, 25 and 31.55 %, respectively. This result indicates that the nature of the substituent group in the Para site plays an important role in the thermal behavior of the prepared polymer. Where the electron-releasing group, as OCH3 in 5c and weak electron-attracting group as Br in 5b, is relatively more thermally stable than the unsubstituted one 5a.

Thermogravimetric Analysis (TGA)
TGA was used to investigated the thermal degradation performances of the prepared polymers 5a-c. Degradation curves were obtained for the prepared compounds of the 5a-c ( Figure 6 and summarized in Table 3). As observed from Figure 6 and Table 3, the first degradation peaks in the range 50-100 • C for compounds 5a-c, with mass loss about 2-3%, are related to the loss of the solvent content. Compounds 5a-c showed one main degradation peak in the range 320-460 • C, 340-450 • C and 350-450 • C with mass loss 57.88%, 50.50% and 43.70%, respectively. At 800 • C, the residual weight of 5a, 5b and 5c was 16.52, 25 and 31.55%, respectively. This result indicates that the nature of the substituent group in the Para site plays an important role in the thermal behavior of the prepared polymer. Where the electron-releasing group, as OCH 3 in 5c and weak electron-attracting group as Br in 5b, is relatively more thermally stable than the unsubstituted one 5a.

Sample ID Main Degradation
Step (℃)   Table 4 summarizes the thermal properties of the other series (bishydrazido derivatives) 7a-c ( Figure 7). As shown in Figure 7 and Table 4 the thermal degradation process of polymers 7a and 7c were classified into main three steps; their initial decomposition was between 280-235 °C and 250-330 ℃, with mass loss of 10% and 6%, respectively. The second degradation step started in a range of 335°C to 440℃ and 330 to 430 ℃ with mass loss of 29% and 23%, for 7a and 7c, respectively. The main third degradation step started in the range of 470 °C to 630 ℃ and 440 to 650 ℃ with mass loss of 24% and 31%, for 7a and 7c, respectively. On the other hand, it was observed that the thermal behavior of the 7b sample differs from that of samples 7a and 7c as it occurred in two main steps. The main first and second degradation steps started in a range of 250 °C to 430 ℃ and 430 °C to 620 °C with mass loss of 38% and 34%, respectively. This result revealed that polymersbased s-triazine 7a and 7b have highly thermal stability compared to their analogs bihydrazino derivatives 5a-b, while 5c showed higher thermal stability compared to 7c. The substituent functional groups on the phenyl ring also affected the thermal stability of the prepared polymers. The electron-releasing groups, as OCH3 in 7c and weak electron-attracting group as Br in 7b, are relatively more thermally stable than the unsubstituted one 7a. These data agreed with our previously reported data [43].   Table 4 summarizes the thermal properties of the other series (bishydrazido derivatives) 7a-c ( Figure 7). As shown in Figure 7 and Table 4 the thermal degradation process of polymers 7a and 7c were classified into main three steps; their initial decomposition was between 280-235 • C and 250-330 • C, with mass loss of 10% and 6%, respectively. The second degradation step started in a range of 335 • C to 440 • C and 330 to 430 • C with mass loss of 29% and 23%, for 7a and 7c, respectively. The main third degradation step started in the range of 470 • C to 630 • C and 440 to 650 • C with mass loss of 24% and 31%, for 7a and 7c, respectively. On the other hand, it was observed that the thermal behavior of the 7b sample differs from that of samples 7a and 7c as it occurred in two main steps. The main first and second degradation steps started in a range of 250 • C to 430 • C and 430 • C to 620 • C with mass loss of 38% and 34%, respectively. This result revealed that polymers-based s-triazine 7a and 7b have highly thermal stability compared to their analogs bihydrazino derivatives 5a,b, while 5c showed higher thermal stability compared to 7c. The substituent functional groups on the phenyl ring also affected the thermal stability of the prepared polymers. The electron-releasing groups, as OCH 3 in 7c and weak electron-attracting group as Br in 7b, are relatively more thermally stable than the unsubstituted one 7a. These data agreed with our previously reported data [43].
The formed char could be utilized for the limiting oxygen index (LOI) calculation. To find out the relationship between the limit oxygen index and char residue CR of halogenfree polymer, van Krevelen equation used [47][48][49]. The flame retardancy characteristic of any polymer will be enhanced with higher char percentage [46,47]. Materials displaying lower than 26% LOI values are considered flammable, where values more than 26% are classified as self-extinguishing materials [47]. Based on the calculated LOI values as shown in Tables 3 and 4, the polymer 5a can be classified as flammable material due to its LOI value less than 26%. In contrast, the polymers 5b,c and 7a-c can be classified as self-extinguishing materials as they have a high LOI value (5b = 27.50%, 5c = 30.12%, 7a = 27.23%, 7b = 29.86% and 7c = 30.85%, respectively) at 800 • C. The results in Tables 3 and 4 showed that the two 5c and 7c have better thermal stability than 5a,b and 7a,b. This indicated that the type of substituent on the phenyl ring attached to the s-triazine core within the polymeric chain, where the methoxy group (electron-donating group) gave the highest char residue and the highest LOI value at 800 • C.  The formed char could be utilized for the limiting oxygen index (LOI) calculation. To find out the relationship between the limit oxygen index and char residue CR of halogenfree polymer, van Krevelen equation used [47][48][49].
The flame retardancy characteristic of any polymer will be enhanced with higher char percentage [46,47]. Materials displaying lower than 26% LOI values are considered flammable, where values more than 26% are classified as self-extinguishing materials [47]. Based on the calculated LOI values as shown in Tables 3 and 4, the polymer 5a can be classified as flammable material due to its LOI value less than 26%. In contrast, the polymers 5b-c and 7a-c can be classified as self-extinguishing materials as they have a high LOI value (5b = 27.50%, 5c = 30.12%, 7a = 27.23%, 7b = 29.86% and 7c = 30.85%, respectively) at 800 ℃. The results in Tables 3 and 4 showed that the two 5c and 7c have better thermal stability than 5a-b and 7a-b. This indicated that the type of substituent on the phenyl ring attached to the s-triazine core within the polymeric chain, where the methoxy group (electron-donating group) gave the highest char residue and the highest LOI value at 800 ℃.

Differential Scanning Calorimetry (DSC)
DSC experiments for the prepared polymers 5a-c with different substituents were evaluated and conducted on a DSC, TA instrument Q1000 thermal analyzer under nitrogen. Scanning from -20 to 300 • C was used to estimate glass transition temperatures (T g ). The T g value of 5a, 5b and 5c was 88.50, 91.60 and 84.80 • C, respectively (Table 3, Figure 8, and Figure S4).

Differential Scanning Calorimetry (DSC)
DSC experiments for the prepared polymers 5a-c with different substituents were evaluated and conducted on a DSC, TA instrument Q1000 thermal analyzer under nitrogen. Scanning from -20 to 300 °C was used to estimate glass transition temperatures (Tg). The Tg value of 5a, 5b and 5c was 88.50, 91.60 and 84.80 ℃, respectively (Table 3, Figure  8 Figure S4). The thermal properties of 7a-c polymers with different substituent were evaluated and summarized in Table 4, Figure 9 and Figure S5. The Tg of the polymers were tested by DSC. The Tg values of 7a-c polymers were in the range of 129-150 ℃, which were found that Tg values of the polymers are slightly similar to each. There are no other obvious exothermic or endothermic peaks in the DSC scans were observed for 5a-c and 7a-c polymers, indicating the high purity sideways with a good stability of polymers. The thermal properties of 7a-c polymers with different substituent were evaluated and summarized in Table 4, Figure 9 and Figure S5. The T g of the polymers were tested by DSC. The T g values of 7a-c polymers were in the range of 129-150 • C, which were found that T g values of the polymers are slightly similar to each. There are no other obvious exothermic or endothermic peaks in the DSC scans were observed for 5a-c and 7a-c polymers, indicating the high purity sideways with a good stability of polymers. 3.2.7. Flammability of neat PP and its composites UL-94 The achieved polymers 5a-c and 7a-c were mixed with APP to form several intumescent flame retardants (IFRs), which were blended in PP to form PP composites. The UL-94 test was employed to investigate the flame-retardant behavior of pure PP, PP/5a-c,

Flammability of Neat PP and Its Composites UL-94
The achieved polymers 5a-c and 7a-c were mixed with APP to form several intumescent flame retardants (IFRs), which were blended in PP to form PP composites. The UL-94 test was employed to investigate the flame-retardant behavior of pure PP, PP/5a-c, PP/7a-c and PP/IFRs composites. The results in Table 5 show that neat PP is highly flammable, and did not pass the UL-94 test. Adding 5c, 5a and 5b at 25% with only 75% PP the composite did not pass the UL-94 rating ( Table 5, F1-5c, F1-5a and F1-5b). However, addition of APP to the system made a remarkable improvement in the UL-94 rating of IFR-PP composite. The UL-94 rating was V-2 when the ratio of APP to 5a was 1:1. The same rating was noticed with 5b and 5c ( Table 5, F2-5a, F2-5b and F2-5c). In addition, increasing the ratio of APP to 5a (5a 1:2 APP) led to a remarkable improvement in the UL-94 rating and the formed char-layer during combusting, which is unlike what happened in neat PP and other PP composites. For F3-5a (Table 5), the UL-94 rating was better and reached the V-1 rating of the UL-94 tests, which is better than PP itself, F1-5a and F2-5a. As for F3-5b, the UL-94 rating also improved and reached the V-1 rating of the UL-94 tests, which is better than PP itself, F1-5b and F2-5b. As for F3-5c (Table 5), the UL-94 rating also improved and reached the V-0 rating of the UL-94 tests, which showed better results compared to pure PP, F1-5c and F2-5c.
Moreover, F3-5c sample reached the UL-94 V-0 rating without dripping, while the other samples showed UL94 V-1 rating. This indicated that F3-5c composite has high efficiency in enhancing the flame retardancy behavior of PP amongst all PP-composites. These results established that the mixing of 5a-c with APP in ratio (1:2) improve the flameretardant behavior of PP. Figure 10 indicated the photographs of F3-5a, F3-5b and F3-5c specimens after UL-94 tests. As indicated in Figure 10, F3-5a, F3-5b, and F3-5c specimens showed char residues, while neat PP showed a melted form without clear char residues.
The 7a-c polymers were mixed with same portion, when the addition of the 7a, 7b and 7c was 25% with only 75% PP, and the composite did not pass the UL-94 rating (F1-7a, F1-7b and F1-7c). However, after APP was added to the system, the IFR-PP composite's UL-94 rating considerably improved. When the weight ratio of APP to 7a was 1:1, the UL-94 rating was V-2. The same rating was recorded with 7b and 7c (F2-7a, F2-7b and F2-7c). However, increasing the weight ratio of 7a-c (7a-c 1:2 APP) led to improved UL-94 rating and the formed char layer during combusting was observed compared to the neat PP and other PP composites. For F3-7a (Table 5), the UL-94 rating was better than PP itself, F1-7a and F2-7a and reached the V-1 rating of the UL-94 tests. As for F3-7a, the UL-94 rating was better than pure PP, F1-7b and F2-7b and reached the V-1 rating of the UL-94 tests. As for F3-7c, the UL-94 rating also better than pure PP, F1-7c and F2-7c and reached the V-0 rating of the UL-94 tests. Moreover, the F3-7c sample reached the UL-94 V-0 rating without dripping, while the other samples reached the UL-94 V-1 rating. This indicated that F3-7c composite is of high efficiency in enhancing the flame retardancy of PP among all PP composites. These results indicated that the combination of 7a-c with APP in weight ratio (1:2) would cause enormous improvement of flame-retardant behavior of PP. Figure 10 indicates photographs of F3-7a, F3-7b and F3-7c specimens after UL-94 tests. It can be seen from Figure 10 that char residues were observed for F3-7a, F3-7b and F3-7c, specimens, while neat PP is in a melted form without any char residues. Also, in comparison with some flame retardants, it seems clear that the prepared polymers can act as a flame retardant in PP/IFR systems as shown in Table 6.

Conclusions
Two types of polymer-based s-triazine bishydrazino and bishydrazido derivatives (5a-c and 7a-c) were synthesized, characterized and applied as charring agents together with APP (as acid source) to build an IFR system which could develop the thermal degradation performance of PP. The TGA data revealed that the achieved compounds have shown a good thermal stability and ability of char formation itself. The char residue of 5c, 5b and 5a reached 31.55, 25, and 16.55 wt% at 800 • C with LOI values of 30.12%, 27.50% and 24.11%, respectively; while char residue of 7c, 7b and 7a reached 33.38, 30.90, and 24.32 wt% at 800 • C with LOI values of 30.85%, 29.86% and 27.23%, respectively. Accordingly, replacing of the hydrazine group (C=N-NH-) in the series 5a-c by a hydrazido group (CO-NH-NH) in the series 7a-c increased the thermal stability of the polymers, and in addition the presence of the methoxy in the phenyl moiety in both series exhibited higher thermal stability. Therefore, the prepared derivatives (5a-c and 7a-c) were blended with APP to make an IFR system. Combustion testing revealed that PP-IFR blends could gain noteworthy LOI values, and pass the UL-94 V-0 rating and the flame-retardant efficacy is developed. When the mass ratio of the components for IFR are 75% APP and 25% 5a-c or 7a-c, the IFR represented the most efficient flame retardancy in PP. The PP-IFR has a UL-94 V2 rating with excessive dripping when the mass ratio of APP and 5a-c or 7a-c is 1:1. This has clearly established that the intumescent flame retardant is very effective in PP. Finally, the substituent effect also affected the thermal stability of the prepared polymers and IFR behaviors as shown from TGA data and UL-94 V0 as well. The electron-releasing group, as methoxy in 5c and 7c and weak electron-attracting group as bromine in 5b and 7b, are relatively more thermally stable than the unsubstituted one as in 5a and 7a. The hydrazido and hyrdazino derivatives with different substituted phenyl ring showed the same influence in the thermal stability and fire retardancy when blended with PP. The mechanism of flame-retardant PP by prepared polymers is still unknown, so more research will be needed in the future.
Finally, the reported polymers herein could be of interest as flame retardant agents and possess special interest for industrial chemistry.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/polym14040784/s1, (method S1, and method S2 are the general method used for the synthesis of 2a-c and 3a-c following the reported method in literates References [44,55] are cited in the supplementary materials; Figure S1 is the 1 H and 13 C NMR for 2a; Figure S2 is the 1 H and 13 C NMR for 2a; Figure S3 is the 1 H and 13 C NMR for 2c, Figure S4 is DSC full cycle of 5a-c samples and Figure S5 is DSC full cycle of 7a-c samples.  Data Availability Statement: Data sharing not applicable. No new data were created or analyzed in this study. Data sharing is not applicable to this article.