Synthesis, Characterization of sym -2,4,6-trisubstituted- s -Triazine Derivatives and Their E ﬀ ects on Flame Retardancy of Polypropylene Composites

: Polypropylene (PP) is ﬂammable material, which brings latent danger to the environment and human society. Therefore, developing new environmentally friendly and e ﬀ ective ﬂame-retardant is one of the most important ways to improve the ﬂame retardancy of PP and improve safety during its lifetime. Herein, we describe the synthesis of ﬁve sym -2,4,6-trisubstituted- s -triazine derivatives, namely, N 2 ,N 4 ,N 6 -triphenyl-1,3,5-triazine-2,4,6-triamine (TAT), N 2 ,N 4 ,N 6 -tris(4-bromophenyl)-1,3,5-triazine-2,4,6-triamine (TBAT), N 2 ,N 4 ,N 6 -tris(4-chlorophenyl)-1,3,5-triazine-2,4,6-triamine (TCAT), 4,4 (cid:48) ,4”-((1,3,5-triazine-2,4,6-triyl) tris(azanediyl)) triphenol (THAT), and N 2 ,N 4 ,N 6 -tris(4-methoxyphenyl)-1,3,5-triazine-2,4,6-triamine (TMAT), from the reaction of cyanuric chloride and p -substituted aniline employing conventional heating or microwave irradiation. The prepared compounds characterized by di ﬀ erent techniques, such as Fourier-transform infrared (FTIR), Ultra-Violet and Visible (UV-Vis), Nuclear Magnetic Resonance spectroscopy ( 1 H-NMR and 13 C-NMR), Thermogravimetric Analysis (TGA), and di ﬀ erential scanning calorimetry (DSC). The e ﬀ ect of substituent on the aniline moiety has great impact on its thermal stability, as observed from the TGA and DSC data. Based on the TGA and DSC results, three triazine derivatives TAT, TBAT, and TMAT were used as charring agents in the presence of di ﬀ erent proportions of ammonium polyphosphate (APP) to form an intumescent ﬂame-retardant (IFR) system, to improve the ﬂame retardancy of PP. The ﬂammability property of PP was investigated by a vertical burning test (UL94). The results of UL94 revealed that the TXAT / APP (IFR) system inﬂuence the PP and could improve the ﬂame retardancy of PP. Best results were obtained with the mass ratio of APP and TXAT 2:1. When the IFR loading was 25 wt%, it displayed great inﬂuence and passed V-0 with TMAT, and V-1 with both TAT and TBAT in the UL94 test.


Introduction
As one of the most widely used polymers, polypropylene (PP) plays a significant role in our daily lives, and is extensively used in numerous applications, such as in the electrical and insulation field, building materials, transportation aspects, etc. [1][2][3][4][5]. However, PP is also considered a flammable material, releasing smoke and toxic or corrosive gases, which brings latent danger to the environment and human society, and strongly limits its uses. Therefore, developing a new environmentally friendly and effective flame-retardant (FR) agent is one of the most important methods to improving the flame

General Method for the Synthesis of sym-2,4,6-trisubstituted-s-Triazine
For the synthesis of the target compounds, two methods were used, as follows: Method A: conventional heating Cyanuric chloride 1 (1.84 g, 10 mmol) in 100 mL acetonitrile was added dropwise to a mixture of aniline derivatives 2a-e (30 mmol), K 2 CO 3 (100 mmol) in acetonitrile (100 mL) at 0 • C. After complete addition, the reaction mixture stirred at 0 • C for 1 h, then at room temperature for another 1 h, and finally refluxed for 16-18 h. The solvent was removed under vacuum and then excess of water was added. The final products were isolated by filtration, washed with ethanol-water, and then dried at room temperature to afford the pure products.
Method B: microwave irradiation Cyanuric chloride 1 (10 mmol) in 25 mL dioxane was slowly added to a mixture of aniline derivatives 2a-e (30 mmol) and K 2 CO 3 (30 mmol) in 20 dioxane at 0 • C. After complete addition, the reaction mixture was stirred at room temperature for 20 min and then irradiated in a microwave oven at 600 W for 20-25 min using a Galanz microwave oven, connected with a refluxing condenser. The solvent was removed under reduced pressure and the solid obtained was washed with water, dried, and then recrystallized to afford the pure product.

Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC)
The TGA performed on a TA Q500 thermal analyzer (Waters LLC, New Castle, DE, USA) with flow rate of 60 mL/min, starting from room temperature to 800 • C. Around 6 mg of sample was placed in an alumina crucible and heated from 25 • C to 800 • C with the heating rate of 10 • C/min under N 2 atmosphere; data were collected in Table 1.
Differential scanning calorimetry (DSC) was conducted on TA Instruments DSC Q1000 (Waters LLC, New Castle, DE, USA) in the range between 80 • C and 400 • C under nitrogen atmosphere. The 5-8 mg samples were placed in the aluminum pans and measured under N 2 at the heating rate of 10 • C/min. Each sample was analyzed three times and readings were averaged.

Flammability Test UL94
The vertical burning test was investigated according to the UL94 test standard with the sample dimension of 130 × 13 × 3.2 mm. Before mixing PP, all additives were dried in vacuum oven at 70 • C for 24 h. All samples were prepared by mixing PP with different proportions of APP and TXAT using a two-roll mill mixing (Rheomixer XSS-300, Shanghai Ke Chuang, Shanghai, China) at a temperature range of 170 • C-180 • C for 10 min, and the roll speed was maintained 100 rpm. The formulations of prepared composites in the shape of vertical bars are listed in Table 2.

Synthesis of sym-2,4,6-trisubstituted-s-triazine Derivatives
The preparation of the target compounds were performed in one step reaction using conventional heating or microwave irradiation, as shown in Scheme 1. First, the reaction was carried out at 0 • C, due to the high reactivity of the first substitution, and then heated gradually to 80-100 • C, or irradiated by microwave at 600 W. Microwave irradiation always afforded the products in shorter reaction times (Section 3.1) with high yields and purities, as observed form their spectral data (Supplementary Materials).

Synthesis of sym-2,4,6-trisubstituted-s-triazine Derivatives
The preparation of the target compounds were performed in one step reaction using conventional heating or microwave irradiation, as shown in Scheme 1. First, the reaction was carried out at 0 °C, due to the high reactivity of the first substitution, and then heated gradually to 80-100 °C, or irradiated by microwave at 600 W. Microwave irradiation always afforded the products in shorter reaction times (3.1. Experimental Section) with high yields and purities, as observed form their spectral data (Supplementary Materials). Scheme 1. Synthesis of sym-2,4,6-trisubstituted-s-triazine derivatives.

FTIR Spectroscopy
The FTIR spectrum for compounds 3a-e illustrated in Table 1 and Supplementary Materials showed absorption peaks in the range 3370-3400 cm −1 , and were attributed to N-H bond and characteristic peaks in the range 2850-2930 cm −1 related to C-H sp 3 and sp 2 . The absorption peaks at 1569 and 1550 cm −1 were attributed to C=N bond of the s-triazine ring. The absorption peaks at 1642, 1495, and 1430 cm −1 were attributed to C=C of the phenyl ring, while the absorption peaks in the range 1240-1260 cm −1 attributed to C-N bond (Table 1).

UV-Vis Spectra
The UV-Vis spectroscopies of the prepared compounds 3a-e were measured in methanol ( Figure  1). The results showed that the prepared compounds have λmax in range at 275-283 nm, depending on the type of substituent attached to the aniline moiety, as shown in Table 1 ( Figure 1). The substituent effects had a clear impact on the λmax as shown in Table 1, where the electron-withdrawing substituents, such as chloro (Cl) and bromo (Br), shifts λmax to longer wavelength (Bathochromic shift) than electron-donating substituents, such as hydroxyl (OH) and methoxy (OMe) group. This behavior can be explained by considering that, the p-position favors the extension of p-conjugation and the formation of highly delocalized in the excited state [45].

FTIR Spectroscopy
The FTIR spectrum for compounds 3a-e illustrated in Table 1 and Supplementary Materials showed absorption peaks in the range 3370-3400 cm −1 , and were attributed to N-H bond and characteristic peaks in the range 2850-2930 cm −1 related to C-H sp 3 and sp 2 . The absorption peaks at 1569 and 1550 cm −1 were attributed to C=N bond of the s-triazine ring. The absorption peaks at 1642, 1495, and 1430 cm −1 were attributed to C=C of the phenyl ring, while the absorption peaks in the range 1240-1260 cm −1 attributed to C-N bond (Table 1).

UV-Vis Spectra
The UV-Vis spectroscopies of the prepared compounds 3a-e were measured in methanol ( Figure 1). The results showed that the prepared compounds have λ max in range at 275-283 nm, depending on the type of substituent attached to the aniline moiety, as shown in Table 1 (Figure 1). The substituent effects had a clear impact on the λ max as shown in Table 1, where the electron-withdrawing substituents, such as chloro (Cl) and bromo (Br), shifts λ max to longer wavelength (Bathochromic shift) than electron-donating substituents, such as hydroxyl (OH) and methoxy (OMe) group. This behavior can be explained by considering that, the p-position favors the extension of p-conjugation and the formation of highly delocalized in the excited state [45].

Thermogravimetric Analysis (TGA)
The thermal parameters of the prepared compounds 3a-e (TAT, TBAT, TCAT, THAT, and TMAT, respectively) were evaluated using the thermogravimetric technique (TGA). Degradation curves of 3a-e are shown in Figure 2 and summarized in Table 2. The results from thermal degradation data of 3a-e showed that all compounds have good thermal stability and started to degrade in range 240-350 • C, which is expected to have good char residue [27].

Thermogravimetric Analysis (TGA)
The thermal parameters of the prepared compounds 3a-e (TAT, TBAT, TCAT, THAT, and TMAT, respectively) were evaluated using the thermogravimetric technique (TGA). Degradation curves of 3a-e are shown in Figure 2 and summarized in Table 2. The results from thermal degradation data of 3a-e showed that all compounds have good thermal stability and started to degrade in range 240-350 °C, which is expected to have good char residue [27]. As shown in Table 2 the compounds (TAT, TBAT

Thermogravimetric Analysis (TGA)
The thermal parameters of the prepared compounds 3a-e (TAT, TBAT, TCAT, THAT, and TMAT, respectively) were evaluated using the thermogravimetric technique (TGA). Degradation curves of 3a-e are shown in Figure 2 and summarized in Table 2. The results from thermal degradation data of 3a-e showed that all compounds have good thermal stability and started to degrade in range 240-350 °C , which is expected to have good char residue [27]. As shown in Table 2 the compounds (TAT, TBAT  As shown in Table 2 the compounds (TAT, TBAT The results in Table 2 indicated that TMAT and THAT are thermally more stable and have higher char residue (CR) than TAT, TBAT, and TCAT. This attributed to the type of substituent at the aniline ring attached to the s-triazine moiety, where compounds with an electron-donating group, such as methoxy group in TMAT and hydroxy group in THAT showed higher CR at 600 • C and 800 • C (45 and 35, respectively). On the other hand, compounds with a weak electron-withdrawing group, such as bromine in TBAT and chlorine in TCAT showed lower CR, as shown in Table 2. The bromo derivative TBAT showed higher CR at 600 • C and 800 • C (35 and 22, respectively); compared to its analogs of the chloro derivative TCAT at the same temperatures (15 and 6, respectively); this might be due to the difference in the electronegativity property. While, the unsubstituted derivative TAT was in the middle, as shown in Table 2.
The CR could be utilized for the limiting oxygen index (LOI) evaluation. To find out the correlation between the CR and LOI, the van Krevelen equation indicated below was used [46,47].
LOI, defined as the minimum concentration of O 2 , supports the combustion to sustain in a mixture of O 2 and N 2 [43,44]. Materials displaying LOI values greater than 26 are classified as self-extinguishing material, while compounds with lower LOI than 26 are considered flammable material [47]. The LOI for TAT, TBAT, TCAT, THAT, and TMAT were calculated at 800 • C ( Table 2). The results showed that the type of substituent has a clear effect on LOI values, where the chloro derivative TCAT showed the least LOI at 800 • C (19.9) compared to its analogs of the bromo derivative TBAT (26.3). On the other hand, the methoxy TMAT and the hydroxy derivatives showed the highest LOI values, while the unsubstituted was in the middle, as shown in Table 2.

Differential Scanning Calorimetry (DSC)
The thermal transitions of 3a-e was studied using differential scanning calorimetry technique under nitrogen atmosphere. The DSC data were collected from the second run, as all samples were heated first to 100 • C to confirm the removal of any trace of solvent, then cooled down with the same rate to 30 • C. Then the second run was heated from 30 • C to 300 • C at a scan rate of 10 • C/min to determine the glass transition temperatures (Tg).
The results summarized in Table 2 and shown in Figure 3, reveals that the Tg of the tested compounds 3a-e were in the range of 140-145 • C and are slightly similar to each other.

Flammability of PP and Its Composites UL-94
Based on the CR and LOI data, three derivatives TAT 3a, TBAT 3b, and TMAT 3e were selected and mixed with APP in different weight ratios to afford several IFRs. These mixtures were used in PP to yield PP/IFRs composites. The UL94 test was used to evaluate the flame retardant performance of neat PP, PP/3a, PP/3b, or PP/3e and PP/IFRs to provide a qualitative classification of the samples used in this work (Table 3). Pure PP is highly combustible, and cannot pass the UL94 test. When the weight ratio was 25 wt% of the TAT 3a, TBAT 3b or TMAT 3e and 75 wt% PP, the composite (PPx, PPy, and PPz, respectively, Table 3) did not pass the UL94 rating. However, there was a remarkable enhancement of UL-4 rating of IFR-PP composite when APP was added to the TXAT/PP composite. When the weight ratio of APP to TAT was 1:1 and the IFR loading was 25 wt % (PPa), the UL94 rating was V-2. The same rating was noticed with the same ratio of TBAT/APP (PPb) and TMAT/APP (PPc), as shown in Table 3. For further increase of the weight ratio of TXAT: APP (1:2) and using loading 25 wt% with 75 wt% PP, the UL94 rating remarkably improved and the char layer formed during combing could be observed obviously, which is unlike from that in neat PP and other PP composites. When the weight ratio of APP to TAT was 2:1 and when the IFR loading was 25 wt% (PP1), the UL94 rating improved and reached the V-1 rating of the UL94 tests, which is better than pure TXAT/PP with the same ratio (PP, PPx and PPa, Table 3). As for APP with TBAT (PP2) with the same weight ratio of 2:1, the UL94 rating also improved and reached the V-1 rating of the UL94 test, which is better

Flammability of PP and Its Composites UL-94
Based on the CR and LOI data, three derivatives TAT 3a, TBAT 3b, and TMAT 3e were selected and mixed with APP in different weight ratios to afford several IFRs. These mixtures were used in PP to yield PP/IFRs composites. The UL94 test was used to evaluate the flame retardant performance of neat PP, PP/3a, PP/3b, or PP/3e and PP/IFRs to provide a qualitative classification of the samples used in this work (Table 3). Pure PP is highly combustible, and cannot pass the UL94 test. When the weight ratio was 25 wt% of the TAT 3a, TBAT 3b or TMAT 3e and 75 wt% PP, the composite (PPx, PPy, and PPz, respectively, Table 3) did not pass the UL94 rating. However, there was a remarkable enhancement of UL-4 rating of IFR-PP composite when APP was added to the TXAT/PP composite. When the weight ratio of APP to TAT was 1:1 and the IFR loading was 25 wt % (PPa), the UL94 rating was V-2. The same rating was noticed with the same ratio of TBAT/APP (PPb) and TMAT/APP (PPc), as shown in Table 3. For further increase of the weight ratio of TXAT: APP (1:2) and using loading 25 wt% with 75 wt% PP, the UL94 rating remarkably improved and the char layer formed during combing could be observed obviously, which is unlike from that in neat PP and other PP composites. When the weight ratio of APP to TAT was 2:1 and when the IFR loading was 25 wt% (PP1), the UL94 rating improved and reached the V-1 rating of the UL94 tests, which is better than pure TXAT/PP with the same ratio (PP, PPx and PPa, Table 3). As for APP with TBAT (PP2) with the same weight ratio of 2:1, the UL94 rating also improved and reached the V-1 rating of the UL94 test, which is better than pure PP, TBAT/PP (PPy) and TBAT/APP (1:1)/PP (PPb). As for APP with TMAT (PP3) with the same weight ratio of 2:1, the UL94 rating also improved and reached the V-0 rating of the UL94 test, which is better than pure PP, TMAT/PP (PPz) and TMAP/APP (1:1)/PP (PPc). Moreover, PP3 sample even reaches UL94 V-0 rating without dripping, while other flame retardant samples reach UL94 V-1 rating. This showed that PPc composite is highly efficient in improving the flame retardancy of PP among all PP composites.
These results demonstrated that the combination of TXAT and APP would cause huge enhancement of flame retardant performance of PP, especially with TMAT derivative. These results agreed with the results obtained in Table 2.
Photographs of PP1, PP2, and PP3 specimens after UL94 tests are shown in Figure 4, it shows that neat PP is in a melted form without obvious char residues; however, char residues were observed for PP1, PP2, and PP3 specimens.  Moreover, PP3 sample even reaches UL94 V-0 rating without dripping, while other flame retardant samples reach UL94 V-1 rating. This showed that PPc composite is highly efficient in improving the flame retardancy of PP among all PP composites.
These results demonstrated that the combination of TXAT and APP would cause huge enhancement of flame retardant performance of PP, especially with TMAT derivative. These results agreed with the results obtained in Table 2.
Photographs of PP1, PP2, and PP3 specimens after UL94 tests are shown in Figure 4, it shows that neat PP is in a melted form without obvious char residues; however, char residues were observed for PP1, PP2, and PP3 specimens.

Conclusions
The sym-2,4,6-trisubstituted triazine derivatives 3a-e were prepared, characterized, and used as charring agents along with ammonium polyphosphate (as acid source) to construct intumescent flame-retardant systems, which could enhance the flame retardant performance of polypropylene.

Conclusions
The sym-2,4,6-trisubstituted triazine derivatives 3a-e were prepared, characterized, and used as charring agents along with ammonium polyphosphate (as acid source) to construct intumescent flame-retardant systems, which could enhance the flame retardant performance of polypropylene.
The char residue (CR) and limiting oxygen index (LOI) values were calculated from the thermogravimetric analysis (TGA) data. The results showed that TMAT (3e) and THAT (3d) have CR 31.5% and 31.9%, respectively, at 800 • C with LOI (35 and 36, respectively); while the unsubstituted aniline TAT and the halogenated derivatives TBAT, and TCAT showed CR (20%, 22%, 6%, respectively) with LOI (25.5%, 26.3%, 19.9%, respectively). Accordingly, the three derivatives with highest LOI values (TAT, TBAT, and TMAT) were selected and mixed with ammonium polyphosphate (APP) in different weight ratio to construct the IFR system. Combustion behavior showed that PP/IFR blends could acquire significant LOI values, and pass the UL94 V-0 rating. The PP/IFR blends achieve the UL94 V-0, indicating the flame-retardant efficiency is improved. When the components mass ratio of APP: TXAT in the IFR system was (2:1, respectively), the IFR offered the most effective flame retardancy in PP. When the mass ratio of APP and TXAT is 1: 1, the PP/IFR has a UL94 V-2 rating with high dripping. This has satisfactorily proved that IFR is very effective in PP.
Finally, the electron-donating group on the aniline residue attached to the s-triazine ring has a great effect on the thermal stability and intumescent flame retardant behaviors, as observed from TGA data and UL94 V-0.
Efforts on the synthesis and characterization of different generation of sym-trisubstituted s-triazine derivatives are in progress in our lab, which might have special interest as flame retardant agents in the industrial field.