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Synthesis of a novel triazine-based polymeric flame retardant and its application in polypropylene
Polymer Degradation and Stability 134 (2016) 202e210
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Synthesis of a novel triazine-based polymeric flame retardant and its application in polypropylene Panyue Wen a, b, Xiaming Feng a, b, Yongchun Kan a, **, Yuan Hu a, b, *, Richard K.K. Yuen b, c a
State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei 230026, PR China Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute of University of Science and Technology of China, 166 Ren'ai Road, Jiangsu 215123, PR China c Department of Architecture and Civil Engineering, City University of Hong Kong, Tat Chee Avenue, Hong Kong b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 June 2016 Received in revised form 28 September 2016 Accepted 9 October 2016 Available online 15 October 2016
In this research, a novel triazine-based polymeric flame retardant (TBMC) was synthesized. The intumescent flame retardant (IFR) system was formed by combining. TBMC with ammonium polyphosphate (APP), and is applied to retard the combustion of polypropylene (PP). PP/IFR samples (APP/TBMC ¼ 3/1 to 1/1) achieved increased limited oxygen index (LOI) values and passed the vertical burning (UL-94) V-0 rating. Herein, cone calorimeter tests show that the heat release rate (HRR) and fire growth index (FGI) of PP/APP/TBMC blends were obviously decreased compared to those of pure PP and PP with 25 wt% APP. The thermogravimetric analytical (TGA) results showed that the TBMC/APP system could improve the thermal and thermal-oxidative stability of the char residues. The char residues were further investigated by visual observation and scanning electron microscopy (SEM). The compact char residues formed hindered the transfer of gas and heat during combustion, ultimately endowed PP/IFR systems improved flame retardancy. © 2016 Published by Elsevier Ltd.
Keywords: Triazine-based polymeric flame retardant Flame retardancy Thermal stability
1. Introductions As one of the significant general commodity of polymers and thermoplastic material, the PP could be applied to building materials, automobiles, electronics, and electric materials, because of its admirable mechanical properties, low density, good performanceto-cost ratio, easy processability, and good chemical resistance [1,2]. However, the applications in many aspects wherever the suitable flame retardant performance was required are strongly restricted owing to their poor flame resistance with low limiting oxygen index (LOI) [3]. Recently, the researchers had not spared more efforts to improve the flame retardancy of PP in the literature [4e12]. Even though, during the research on environmental friendly flame retardants of PP, intumescent flame retardants (IFR) was promising halogen free flame retardants and had drawn considerable attention due to their advantages, such as low smoke and less toxic
* Corresponding author. State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei 230026, PR China. ** Corresponding author. E-mail addresses: [email protected] (Y. Kan), [email protected] (Y. Hu). http://dx.doi.org/10.1016/j.polymdegradstab.2016.10.003 0141-3910/© 2016 Published by Elsevier Ltd.
gasses released during the burning [13e19]. But in general, the traditional IFRs concluded the acid catalysts, charring agents, and blowing agents. In these, the conventional acid catalysts are mainly phosphate and a phosphate ester, such as ammonium polyphosphate (APP). The charring agents which currently used are mainly small molecular compounds, like, pentaerythritol (PER). However, some of the drawbacks still exited with these traditional IFR, such as lower flame retardant efficiency, poor thermal stability, unsatisfactory migration of resistance and water resistance compared with bromine-containing flame retardants. To solve all these problems it required more efforts to design and prepare the new IFRs. On the other hand in literature, cyanuric chloride was as such a raw material for the design and synthesizes different flame retardants with its high selectivity [20e22]. Accordingly, a number of triazine derivatives were used as efficient charring agents in IFR have been investigated to reduce or cancel the above hazards caused by the presence of tertiary nitrogen in the triazine rings and favorable charring ability had been found [20e35]. Traditionally, hyperbranched polymers were prepared mainly by polycondensation of an ABx type monomer that has one “A” functional group and x “B” functional groups [25,26]. But the
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Scheme 1. The route for the synthesis of TBMC via polycondensation of A2B3.
Table 1 LOI and UL-94 results of pure PP and PP/IFR systems. Sample
PP (wt%)
APP (wt%)
TBMC (wt%)
LOI(%) ±0.5
UL-94 rating (3.2 mm)
PP0 PP1 PP2 PP3 PP4 PP5 PP6
100 75 75 75 75 75 75
0 25 18.75 16.67 12.5 8.33 0
0 0 6.25 8.33 12.5 16.67 25
17.0 21 30.5 29.5 27.5 24.5 23.0
NRa NR V-0 V-0 V-0 V-1 V-1
a
NR, No rating.
Table 2 Elementary analysis data of TBMC. Sample
Elemental value
C (%)
H (%)
N (%)
TBMC
Calculated Found
53.63 52.32
7.26 6.98
39.11 37.24
Fig. 1. FTIR spectra of cyanuric chloride (a) and TBMC (b).
researchers inclined to focus on employment of polycondensation of A2 and B3 type monomers because of their commercial availability. Furthermore, those polymers could be prepared in a single
Fig. 2. Solid-state
13
C NMR spectrum of TBMC.
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Fig. 3. TG curves of TBMC under nitrogen and air atmosphere.
step process only and thus obtained in large scale at a reasonable cost, which made them more interesting for commercial applications [31]. In this connection, in our previous work, we reported a novel hyperbranched charring foaming agent (HCFA) via a condensation reaction of cyanuric chloride and piperazine [31]. The results showed that the addition of HCFA can improve the flame retardancy and water resistance of PP blends at the loading of 30 wt % IFR (APP/HCFA). Obviously, the loading of IFR is not too low to reach the UL-94 V-0 rating and high LOI value, so it was necessary to develop some more efficient intumescent flame retardants for PP. Therefore, by considering all these aspects at present work, we prepared a novel kind of intumescent flame retardant with carbon source and blowing agent by the condensation of cyanuric chloride and N-amino ethylpiperazine. Then the TBMC was used to flameretard PP together with APP at the loading of 25 wt% and find outstanding flame retardancy even at lower addition compared to our previous report [31]. 2. Experimental 2.1. Materials PP resin (F401) was provided from Yangzi Petrochemical Co. (China). The commercial products APP (phase II, the degree of polymerization >1000) was purchased by Shandong Shi'an chemical Engineering Corp (China). Cyanuric chloride and N-Aminoethylpiperazine were obtained from Aladdin Chemical Reagent Corp (China). Other chemicals were gained from China National Pharmaceutical Group (Shanghai, China). 2.2. Measurements FTIR spectra were measured using a Nicolet MAGNA-IR 750 spectrophotometer. The elemental analysis was conducted with the Vario EL III elemental analyzer. Thermogravimetric analysis (TGA) was measured by using a Q5000IR thermo-analyzer instrument (TA Co., USA) (3e10 mg sample, 20 C/min heating rate). 13 C solid-state NMR spectra were recorded on a Bruker AVANCE III 400 WB. Real-time Fourier transform infrared (RT-FTIR) spectra were measured at heating rate of 20 C/min by using the Nicolet 6700 FT-
Fig. 4. TGA (a) and DTG (b) curves of PP and its samples under a nitrogen atmosphere.
IR spectrophotometer instrument. The Limiting oxygen index (LOI) was evaluated by using an HC-2 oxygen index meter (Jiangning Analysis Instrument Co., China) based on the standard of ASTMD 2863. The vertical burning test was carried out by a CFZ-2 type instrument (Jiangning Analysis Instrument Co., China) according to the UL 94 test standard. Samples of 100 6.5 3.2 mm3 dimensions for LOI test and 130 13 3.2 mm3 dimensions for the vertical burning test were used respectively. The combustion tests were carried out on the cone calorimeter (FTT, UK) by the procedure of ISO 5660 standard. Horizontal heat flux of 35 kW/m2 was used to specimens of 100 100 3.2 mm3 dimensions. The microstructure of PP samples was studied by scanning electron microscopy (JSM6700F, 10.0 kV). The tensile strength and elongation at break were carried out according to GB13022-91 with a WD-20D electronic universal testing instrument (Changchun Intelligent Instrument Co. Ltd., China). Five parallel runs were performed for each sample. 2.3. Synthesis of TBMC N-Amino ethylpiperazine (0.32 mol) was added into a 1000 mL three-neck flask equipped. Cyanuric chloride (0.20 mol) dissolved in 1, 4-dioxane and was appended dropwise into the flask. The system was kept at 0 C for 3 h.
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Table 3 TGA data of PP and its samples at nitrogen and air atmosphere. Sample
PP0 PP1 PP2 PP3 PP4 PP5 PP6
Nitrogen
Air
Td ( C)
Tmax ( C)
Char residue at 800 C (%)
Td ( C)
Tmax1 ( C)
Tmax2 ( C)
Char residue at 800 C (%)
385 401 420 424 429 429 443
459 476 484 485 483 483 483
2.2 16.6 10.9 10.7 5.11 8.55 4.27
286 281 282 285 266 285 277
359 375 394 387 390 399 367
e 554 604 605 588 698 615
0.65 6.33 5.33 2.89 4.38 2.30 0.02
Fig. 5. TGA (a) and DTG (b) curves of PP and its samples under air atmosphere.
Soon afterwards, a solution of NaOH (0.61 mol) was added to the above mixture and the PH value of the system was maintained at 8e9 through and the reaction was heated at 50 C for 6 h more. Later, the reaction was heated up to 100 C and maintained for 12 h. Then the reaction system was cooled to room temperature, filtered and washed with distilled water twice. The obtained product was dried at room temperature in a vacuum oven for overnight and finally got the TBMC became white solid powder at high product yield (93.6%) (Scheme 1).
2.4. The preparation of samples Before mixing, PP and all additives were dried in vacuum oven at
Fig. 6. RTFTIR spectra of the condensed products of PP/APP (a) and PP/IFR (APP: TBMC ¼ 3:1) (b) at different temperatures.
80 C for overnight. All the samples were obtained using a two-roll mixing mill (Rheomixer XSS-300, Shanghai Ke Chuang China) at 180 C, and the roll speed was maintained 100 rpm for approximately 15 min respectively. The prescriptions of the PP samples were showed in Table 1.
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3. Results and discussion 3.1. Characterization of TBMC The FTIR spectra of TBMC and cyanuric chloride were presented in Fig. 1. A broad absorption peak at 3416 cm1 was attributed to the secondary amine NeH vibration. And two peaks at 2935 and 2816 cm1 were attributed to the CeH absorption peaks. A broad intensive absorption peak at 1540 cm1 was the characteristic absorption of triazine ring. The absorption bands at 1159 cm1 corresponded to CeN. No characteristic absorption sign for CeCl was observed at 630 cm1, which meant that the Cl atom attached on the triazine ring had been reacted with N-Aminoethylpiperazine. These characterization data indicated that the TBMC had been successfully synthesized. To further clarify the composition of TBMC, the elemental analysis was conducted and results were listed in Table 2. Excitingly, almost identical contents of C, H and N obtained from theoretical calculation and measurements (calcd. for TBMC: 53.63%, 7.26% and 39.11%; found: 54.32%, 6.98% and 37.24% respectively) indirectly confirmed the consumption of chlorine atoms. These combined characterizations verified the successful synthesis of TBMC by this simple one-pot method. To further confirm the structure of TBMC, 13C solid-state NMR spectrum of TBMC was provided. As shown in Fig. 2, the signals at 165.17 ppm, 54.75 ppm, 43.34 ppm were own to the carbon atoms in triazine ring, piperazine ring and ethyl, respectively, which was in accordance with the results of elemental analysis. Thermogravimetic analysis, occupied to analyze the thermal degradation behaviors of TBMC, was investigated by TGA under nitrogen and air atmospheres. Fig. 3 showed the TGA curves of TBMC under nitrogen and the whole degradation course of TBMC experienced at one stage. The decomposition occured from 304 C to 550 C and the main peak located nearby at 427 C, which was attributed to the decomposition of the macromolecular backbone and crosslinking. Finally, the remaining weight of TBMC was 26.5% at 800 C. However, the thermal degradation process of TBMC under air was divided into two steps, the first step took place from 275 to 567 C (attributed to the decomposition of the macromolecular backbone and crosslinking). The second step occurred from 567 C to 800 C, which was assigned to the degradation of final char. When the thermal degradation temperature reached 800 C, the degradation was still going on. The char residue of TBMC under air was 21.0% at 800 C. All of the above results showed TBMC exhibited poorer char formation ability compared with our previous work [31]. The reason for the phenomenon was that the thermal stability of piperazine was better than the ethyl due to its similar structure of benzene ring, while the ethyl in the N-Amino ethylpiperazine replaced a part of piperazine ring under the same dosage ratio [26,28].
Fig. 7. HRR (a) and THR (b) curves of PP and its samples.
385 C and by further increase of temperature, rapid elevation of weight loss occured and negligible char residue (2.2%) left above 500 C, due to the degradation of polypropylene backbone. The Td of PP1 with 30 wt% APP additions was higher than pure PP, and 16.6% char residue remains at 800 C. Compared to pure PP, PP/APP and PP/TBMC systems had better thermal stability with higher, T5%, T10% and T75% and the main thermal decomposition peaks obviously moved to a higher temperature. With the incorporation of APP and TBMC into PP, the PP/IFR systems had higher thermal stability between 400 C and 500 C, which could be ascribed to the outstanding char-forming ability of APP and TBMC.
3.2. Thermal degradation of PP and its samples in nitrogen atmosphere
3.3. Thermal oxidative degradation of PP and its samples in air atmosphere
Next, we utilized thermal gravimetric analysis (TGA) for preliminary assessment of the thermal stability of PP and its samples. For elucidating the retardant effect of TBMC on IFR-PP, the thermal degradation behavior and the amount of residual char under nitrogen condition were systematically studied. The results were shown in Fig. 4 and Table 3. As TGA curves in Fig. 4, the temperature at which the weight loss exhibited 5 wt% was defined as the initial decomposition temperature, which was denoted as Td; the temperature at which the degradation rate reached a maximum is defined as Tmax. For pure PP, one step decomposition from 350 C to 480 C was captured. The Td for the pure PP was approximately
We also utilized thermal gravimetric analysis (TGA) to assess the thermal oxidative degradation behavior of PP (Fig. 5). The summary of the results was presented in Table 3. In sharp contrast with the thermal stability under nitrogen condition (Fig. 4), it was clear to see a double effect on the degradation of PP blends, that was, the second step weight loss was retarded obviously whereas the first step degradation process are shift to low temperatures. Moreover, the second degradation stage in the high temperature was observed, ascribed to the oxidative decomposition of the char residues in the hot air. For instance, the Td, Tmax1, Tmax2 of PP2 which was containing 25 wt% APP/TBMC (3/1) were all shifted to higher
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Table 4 The cone data of PP and its samples. Sample
TTI(s)
Time to PHRR (s)
PHRR (kW/m2)
THR (MJ/m2)
FPIa (m2s/kW)
FGIb (kW/m2s)
Char residue (%)
PP0 PP1 PP2 PP3 PP4 PP5 PP6
48 43 36 38 44 44 38
152 206 427 94 113 56 66
988 652 113 117 123 241 504
88.3 80.0 73.9 23.2 73.3 77.2 86.6
0.05 0.07 0.32 0.32 0.36 0.18 0.08
6.5 3.1 0.2 1.2 1.1 4.3 7.6
1.6 22.9 25.1 65.2 19.4 11.7 8.7
a b
FPI ¼ TTI/PHRR. FGI ¼ PHRR/tPHRR.
Fig. 8. Mass loss curves of PP samples from cone calorimeter test.
temperatures relative to PP1. Besides, the increases in a number of remaining residues at high temperature was attributed to the interaction of the pyrolysis products during the heating process, which were effectively decreased the release of volatile products and enhanced the thermal stability and residues at higher temperatures. It should be noted that higher amount of char residues were obtained in the PP/IFR samples (APP/TBMC ¼ 3:1 and 2:1) before 600 C, while the char residues of PP/IFR systems further oxidative decomposed between 600 C and 800 C in the hot air, leading to lower char residue than that of PP/APP [27,29]. Fig. 5 showed the second decomposition peak of PP/IFR systems appeared at 630 C. In general, TG, a micro-scale test method, was controlled to slowly rise the temperature, leading to the gradually degradation of the matrix and the slow rate of char forming. Thus, it could not form the intumescent char layer, resulting in the further degradation of the char residue [23,27,29]. 3.4. Real time FTIR study To further understand the flame-retardant mechanism of PP/IFR systems in condense phase, the thermal oxidative degradation residues at different temperature were investigated by FTIR. The changes in the FTIR spectra of PP/APP and PP/APP/TBMC (APP:TBMC ¼ 3:1) were presented in Fig. 6. For PP/APP system (Fig. 6a), the characteristic absorption peak of PP could be observed at 2952, 2921, 2871, 2834, 1454, 1374 and 884 cm1 under the room temperature [36]. With increasing the pyrolysis temperature, the intensity of PP absorption peaks gradually decreased and disappeared on further heating to 350 C, indicating the complete decomposition of PP. The band at 1254 was budged to 1286 cm1,
suggesting the presence of dissociative PO 4 . Furthermore, the peaks at 3165 cm1 attributed to the vibration of NeH gradually disappeared with increasing the temperature to 300 C, implying the release of NH3 [37]. With the increase of pyrolysis temperature, two new peaks at 1013 and 870 cm1 appearing at 350 C were assigned to the absorptions of PeOeP groups. The FTIR spectra for PP/IFR(APP:TBMC ¼ 3:1) at different degradation temperatures were given in Fig. 6b. The peaks at 1563 and 1097 cm1 were attributed to the skeleton of the triazine ring and the CeN bonds in TBMC [31]. As the degradation temperature ascended, The peak intensity was gradually decreased, indicating the decomposition and reconstruction of the triazine ring. The main difference between PP/APP and PP/IFR (APP:TBMC ¼ 3:1) during the pyrolysis process is obviously observed. During the pyrolysis process, the peaks at 1097 and 997 cm1 still existed at 550 C, which indicated that the PeOeP and PeOeC structure were formed in the residue of PP/IFR(APP:TBMC ¼ 3:1) at this high temperature, while nothing was observed for PP/APP system in this case. Moreover, the characteristic absorption peak of PP was not observed until the temperature increased to 450 C, while the absorption disappeared at 350 C for PP/APP system. As these above mentioned, it could be explained that the PeOeC and PeOeP structure could improve the char formation of APP/TBMC, which could protect the matrix from decomposition [15,38,39].
3.5. LOI and UL-94 analysis of flame retarded PP samples The LOI and UL-94 results for PP samples were presented in Table 1. The LOI value of PP/APP with 25 wt% loadings was 21%, and could not reach to the UL-94 V-0 rating. But, there was a remarkable increment in LOI values of IFR-PP systems when TBMC was added to the PP/APP system. Herein, the experimental consequences of vertical burning tests showed that all the PP/TBMC/APP systems were obtained UL-94 V-0 rating with a weight ratio of APP to TBMC being 3/1 to 1/1. When the weight ratio of TBMC to APP reached 3/1, the IFR system showed the most effective flame retardancy with a maximum LOI value of 30.5%. However, for further increase of the weight ratio of TBMC/APP led to the decreased LOI value. These results demonstrated that the combination of TBMC would cause huge improvement of the flame retardant performance of APP in PP. As a matter of fact, TBMC was a typical IFR integrating acid, char and gas sources, which was expect to achieve outstanding fire retardant effect, thus, LOI values of PP/TBMC with 25 wt% loadings was 23%, and achieved UL-94 V-1 rating. Heat release rate (HRR) and total heat release (THR) curves of PP and its samples were presented in Fig. 7 and the corresponding data were given in Table 4. When the pure PP was being ignited, it could burn so fast and it quickly reached the PHRR (1387 kW/m2). But, herein when a 30 wt% APP was added, then the PHRR of PP1 decreased by 61.1% and a slight broad HRR curve came into being. The combination of APP with TBMC could significantly improve the
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Fig. 9. Front and side views of the residues of PP1, PP2, PP3, PP4, PP5, and PP6 after cone calorimeter test after cone calorimeter test.
flame retardancy of PP. Therefore, the PHRR of PP blends was reduced significantly. For example, PHRR of PP2 was 113 kW/m2, decreasing by 88.6% compared with that of pure PP and the time to PHRR was 427 s, a lag of 275 s relative to that of pure PP. Besides, the value of its THR was reduced compared to pure PP and PP/APP system. These all were indicated that a much better flame retardancy was contributed by APP/TBMC system. Moreover, cone calorimeter analysis, a bench-scale fire test to exhibit retardant capacity in a real fire disaster, directly exposing the material to the high temperature, thus leading to the fast pyrolysis of the matrix and rapidly formation of intumescent char layer. The intumescent char layer could protect the underlying materials from further burning. Based on the discussion of the TG results, that was why PP/IFR owned lower char residues at 800 C, while better flame retardancy (LOI, UL94 and cone calorimeter results). To judge the fire hazards of polymer materials more clearly, herein, we selected fire performance index (FPI) and fire growth index (FGI) [31]. The comparison between the values of FPI and FGI of PP and its samples were shown in Table 4. Apparently, when the weight ratio of APP to TBMC was 3/1, the fire risks of the corresponding PP blends were much smaller than others. The mass loss results after the cone test has been provided in Fig. 8 and the corresponding data were presented in Table 4. It appeared clearly that there were 1.6%, 22.9%, 25.1%, 65.2% and 8.7% of residual mass left at the end of burning, respectively, for pure PP,
PP1, PP2, PP3 and PP6. The PP2 and PP3 samples presented significantly higher residual mass compared to that of other systems, indicating that TBMC and APP facilitated the carbonization of PP samples [28]. The intumescent char layer prevented pyrolysis products and heat during the combustion process, which was in accordance with the PHRR behavior during cone calorimetry and the TGA results before 630 C under air atmosphere. 3.6. The morphology of char residues of PP samples It was universally acknowledged that the intumescent flame retardant system usually undergoes an intense expansion process and the formation of the protective char layer. Thus, the investigation of char residues could be benefit to understand the differences in the flame retardancy. Fig. 9 presented the digital photos of the residues of PP and its samples collected after cone calorimeter measurements. Herein, nothing was left for pure PP. Only one kind of flame retardant be used, could the residues of PP1 with 25 wt% APP and PP6 with 25 wt % APA/TBMC demonstrate a tiny residue without intumescence. In contrast to this, PP2, PP3, PP4 and PP5 with weight ratio of APP to TBMC being 3/1 to 1/2 engendered the highly intumescent char residues, respectively. These results indicated that PP samples containing 25 wt% APP/TBMC with weight ratio from 1:3 to 1:2 had impactful intumescent char layers as effective physical barriers against the transfer of the heat and
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Fig. 10. SEM images of the char residues of PP1 (a), PP2 (b), and PP6(c).
THR and higher char residue. The char residues left after the cone calorimeter test were analyzed by SEM, and the corresponding images were shown in Fig. 10. For PP2 with a compact and continuous intumescent char could effectively prevent the heat, combustible gases and mass from transferring, which indicated that the triazine-ring were probably crosslinked with PeOeP, and formed the thermostable char [21]. In contrast, the char layers of PP6 were broken up and porous, which probably was the reason why these char layers were low efficient and poor thermal stability. 3.7. Mechanical behaviors
Fig. 11. Stressestrain behaviors of PP and its samples.
Table 5 Mechanical properties of PP and its samples. Sample
Tensile strength (Mpa)
PP0 PP1 PP2 PP3 PP4 PP5 PP6
32.1 21.2 30.6 28.1 24.5 26.1 21.5
± ± ± ± ± ± ±
0.9 0.3 0.4 0.8 0.6 0.6 0.8
Elongation at break (%) 107.3 ± 8.9 33.1 ± 3.8 5.3 ± 0.5 4.5 ± 0.6 4.3 ± 0.5 4.4 ± 0.6 3.5 ± 0.3
combustible gasses, thus resulting in the much lower PHRR and
The mechanical behaviors of PP and its samples with various loadings of TBMC and APP were investigated by tensile tests. Fig. 11 showed the representative stressestrain curves of PP and its samples. The detailed data, including tensile strength and elongation at break were listed in Table 5. As can be seen, the addition of APP or TBMC resulted in a decrease in tensile strength relative to pure PP, which was mainly due to the poor dispersion in PP. while the tensile strength of APP/TBMC systems increased, might be ascribed to the complexation between APP and TBMC. It should be noted that the reinforcing effect of APP/TBMC systems on the mechanical strength of PP was not very marked. It was observed that pure PP displayed a ductile fracture with an elongation at break of approximately 107%, After the addition of APP, the elongation at break of PP1 was 33.1%, which was much lower than that of pure PP, and the same trend to that of PP/APP/TBMC systems. It indicated that the addition of TBMC and APP has a significant influence on the elongation at break of PP systems. 4. Conclusion In this paper, a novel intumescent flame retardant, TBMC, was prepared. In combination with APP, it was utilized to flame retard
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combustible polypropylene. Relieved thermal degradation and high temperature oxidation resistance of the char residues in air atmosphere before 600 C were obtained. Investigation of combustion behavior reveals that PP/IFR blends (APP/TBMC ¼ 3/1 to 1/1) could acquire appreciable LOI values, and pass the UL-94 V-0 rating. In addition, PP/APP/TBMC blends exhibited dramatically decreased HRR and FGI compared with pure PP and PP with 25 wt% APP. Besides, The PP2 and PP3 samples presented significantly higher residual mass compared to that of other systems from the cone calorimeter results. SEM results provided further evidence of the compact char residues, which reduced the transfer of gas and heat during combustion, and ultimately induced improved flame retardancy of PP/IFR composites. Acknowledgements The authors acknowledge the research grants from the National Key Research and Development Program of China (2016YFC0800605), National Natural Science Foundation of China (51323010, 21411140231) and Research Grants Council of the Hong Kong Special Administrative Region (contract grant number CityU 11301015). References [1] S. Bourbigot, M. Le Bras, S. Duquesne, M. Rochery, Recent advances for intumescent polymers, Macromol. Mater. Eng. 289 (2004) 499. [2] F. Zhang, Y. Wang, Y. Jin, J. Zhang, Simulated effects of physical parameters on heat transfer of intumescent fire-retardant polypropylene during burning, J. Macromol. Sci. Phys. B 50 (2011) 1185. [3] X.P. Hu, W.Y. Li, Y.Z. Wang, Synthesis and Characterization of A Novel nitrogen containing flame retardant, J. Appl. Polym. Sci. 94 (2004) 1556e1561. [4] Y. Liu, J. Zhao, C.L. Deng, L. Chen, D.Y. Wang, Y.Z. Wang, Flame-retardant effect of sepiolite on an intumescent flame-retardant polypropylene system, Ind. Eng. Chem. Res. 50 (2011) 2047e2054. [5] J. Zhan, L. Song, S.B. Nie, Y. Hu, Combustion properties and thermal degradation behavior of polylactide with an effective intumescent flame retardant, Polym. Degrad. Stab. 94 (2009) 291e296. [6] S. Bourbigot, M. Le Bras, R. Delobel, P. Breant, J.M. Tremillon, 4A zeolite synergistic agent in new flame retardant intumescent formulations of polyethylenic polymerseStudy of the effect of the constituent monomers, Polym. Degrad. Stab. 54 (1996) 275. [7] Z.B. Shao, C. Deng, Y. Tan, M.J. Chen, L. Chen, Y.Z. Wang, Flame retardation of polypropylene via a novel intumescent flame retardant: ethylenediaminemodified ammonium polyphosphate, Polym. Degrad. Stab. 106 (2014) 88e96. [8] A.R. Horrocks, Developments inflame retardants for heat and fire resistant textiles-the role of char formation and intumescence, Polym. Degrad. Stab. 54 (1996) 143e154. [9] G. Camino, L. Costa, L. Trossarelli, F. Costanzi, A. Pagliari, Study of the mechanism of intumescence in fire retardant polymers: Part VI-Mechanism of ester formation in ammonium polyphosphate pentaerythritol mixtures, Polym. Degrad. Stab. 12 (1985) 213e228. [10] D.Y. Wang, X.X. Cai, M.H. Qu, Y. Liu, J.S. Wang, Y.Z. Wang, Preparation and flammability of a novel intumescent flame-retardant poly(ethylene-co-vinyl acetate) system, Polym. Degrad. Stab. 93 (2008) 2186e2192. [11] Q. Li, H.F. Zhong, P. Wei, P.K. Jiang, Thermal degradation behaviors of polypropylene with novel silicon-containing intumescent flame retardant, J. Appl. Polym. Sci. 98 (2005) 2487e2492. [12] S.L. Gao, B. Li, P. Bai, S.Q. Zhang, Effect of polysiloxane and silane-modified SiO2 on a novel intumescent flame retardant polypropylene system, Polym. Adv. Technol. 22 (2011) 2609e2616. [13] S. Bourbigot, M.L. Bras, F. Dabrowski, J.W. Gilman, T. Kashiwagi, PA-6 clay nanocomposite hybrid as char forming agent in intumescent formulations, Fire Mater. 24 (2000) 201e208. [14] S.H. Chiu, W.K. Wang, Dynamic Flame Retardancy of Polypropylene Filled with Ammonium Polyphosphate, Pentaerythritol and Melamine Additives, Polymer 39 (1998) 1951e1955. [15] Y.W. Yan, L. Chen, R.K. Jian, S. Kong, Y.Z. Wang, Intumescence: An Effect Way to Flame Retardance and Smoke Suppression for Polystryene, Polym. Degrad. Stab. 97 (2012) 1423e1431. [16] B. Lecouvet, M. Sclavons, C. Bailly, S. Bourbigot, A comprehensive study of the synergistic flame retardant mechanisms of halloysite in intumescent
polypropylene, Polym. Degrad. Stab. 98 (2013) 2268e2281. [17] J. Li, C.H. Ke, L. Xu, Y.Z. Wang, Synergistic effect between a hyperbranched charring agent and ammonium polyphosphate on the intumescent flame retardance of acrylonitrile-butadiene-styrene polymer, Polym. Degrad. Stab. 97 (2012) 1107e1113. [18] X.Q. Su, Y.W. Yi, J. Tao, H.Q. Qi, D.Y. Li, Synergistic effect between a novel triazine charring agent and ammonium polyphosphate on flame retardancyand thermal behavior of polypropylene, Polym. Degrad. Stab. 105 (2014) 12e20. [19] K. Yang, M.J. Yang, B. Li, Synthesis of N-Ethyl Triazine-Piperazine Copolymer and Flame Retardancy and Water Resistance of Intumescent Flame Retardant Polypropylene, Polym. Degrad. Stab. 98 (2013) 1397e1406. [20] P.A. Song, Z.P. Fang, L.F. Tong, Z.B. Xu, Synthesis of a novel oligomeric intumescent flame retardant and its application in polypropylene, Polym. Eng. Sci. 49 (2009) 1326e1331. [21] S.S. Mahapatra, N. Karak, s-Triazine containing flame retardanthyperbranched polyamines: synthesis, characterization and propertiesevaluation, Polym. Degrad. Stab. 92 (2007) 947e955. [22] B. Li, M.J. Li, Effect of A Novel Charring-Foaming Agent on Flame Retardancy and Thermal Degradation of Intumescent Flame Retardant Polypropylene, Polym. Degrad. Stab. 91 (2006) 1380e1386. [23] C.M. Feng, Y. Zhang, S.W. Liu, Z.G. Chi, J.R. Xu, Flame retardancy and thermaldegradation behaviors of polypropylene composites with novel intumescentflame retardant and manganese dioxide, J. Anal. Appl. Pyrolysis 104 (2013) 59e67. [24] C.H. Ke, J. Li, K.Y. Fang, Q.L. Zhu, J. Zhu, Q. Yan, Y.Z. Wang, Synergistic effect between a novel hyperbranched charring agent and ammonium polyphosphate on the flame retardant and anti-dripping properties of polylactide, Polym. Degrad. Stab. 95 (2010) 763e770. [25] P.Y. Wen, X.F. Wang, W.Y. Xing, X.M. Feng, B. Yu, Y.Q. Shi, Synthesis of a novel triazine-based hyperbranched char foaming agent and the study of its enhancement on flame retardancy and thermal stability of polypropylene, Ind. Eng. Chem. Res. 52 (2013) 17015e17022. [26] C. Gao, D. Yan, Hyperbranched polymers: from synthesis to applications, Prog. Polym. Sci. 29 (2004) 183e275. [27] C.M. Feng, Y. Zhang, D. Liang, S.W. Liu, Z.G. Chi, J.R. Xu, Influence of zinc borate on the flame retardancy and thermal stability of intumescent flame retardant polypropylene composites, J. Anal. Appl. Pyrolysis 115 (2015) 224e232. [28] D. Enescu, A. Frache, M. Lavaselli, O. Monticelli, F. Marino, Novel phosphorous nitrogen intumescent flame retardant system. Its effects on flame retardancy and thermal properties of polypropylene, Polym. Degrad. Stab. 98 (2013) 297e305. [29] C.M. Feng, Y. Feng, S.W. Liu, Z.G. Chi, J.R. Xu, Flame retardancy and thermal degradation behaviors of polypropylene composites with novel intumescent flame retardant and manganese dioxide, J. Anal. Appl. Pyrolysis 104 (2013) 59e67. [30] G. Camino, L. Costa, L. Trossarelli, Study of the mechanism of intumescence in fire retardant polymers: part IIemechanism of action in polypropyleneammonium polyphosphate-pentaerythritol mixtures, Polym. Degrad. Stab. 7 (1984) 25e31. [31] P.Y. Wen, X.F. Wang, B.B. Wang, B.H. Yuan, K.Q. Zhou, L. Song, Y. Hu, R.K.K. Yuen, One-pot synthesis of a novel s-triazine-based hyperbranched charring foaming agent and its enhancement on flame retardancy and water resistance of polypropylene, Polym. Degrad. Stab. 110 (2014) 165e174. [32] X.Q. Su, Y.W. Yi, J. Tao, H.Q. Qi, D.Y. Li, Synergistic effect between a novel triazine charring agent and ammonium polyphosphate on flame retardancy and thermal behavior of polypropylene, Polym. Degrad. Stab. 105 (2014) 12e20. [33] S. Zhou, L. Song, Z.Z. Wang, Y. Hu, W.Y. Xing, Flame retardation and char formation mechanism of intumescent flame retarded polypropylene composites containing melamine phosphate and pentaerythritol phosphate, Polym. Degrad. Stab. 93 (2008) 1799e1806. [34] C.M. Feng, Z.W. Li, M.Y. Liang, J.G. Huang, H.B. Liu, Preparation and characterization of a novel oligomeric charring agent and its application inhalogenfree flame retardant polypropylene, J. Anal. Appl. Pyrolysis 111 (2015) 238e246. [35] Z.F. An, R.F. Chen, J. Yin, G.H. Xie, H.F. Xie, T. Tsuboi, Conjugated asymmetric donor-substituted 1,3,5-triazines: new host materials for blue phosphorescent organic light-emitting diodes, Chem. Eur. J. 17 (2011) 10871e10878. [36] F. Zhang, J. Zhang, D.X. Sun, Study on Thermal decomposition of intumescent fire-retardant polypropylene by TG/Fourier transform infrared, J. Thermoplast. Compos. 22 (2009) 681e701. [37] F. Laoutid, L. Bonnaud, M. Alexandre, J.M. Lopez-Cuesta, P. Dubois, New prospects in flame retardant polymer materials: From fundamentals to nanocomposites, Mater. Sci. Eng. R. 63 (2009) 100e125. [38] B.O. Mysen, F.J. Ryerson, D. Virgo, The structural role of phosphorus in silicate melts, Am. Mineral. 66 (1981) 106e117. [39] C. Nguyen, J. Kim, Thermal stabilities and flame retardancies of nitrogene phosphorus flame retardants based on bisphosphoramidates, Polym. Degrad. Stab. 93 (2008) 1037e1043.
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Synthesis of a novel triazine-based polymeric flame retardant and its application in polypropylene Panyue Wen a, b, Xiaming Feng a, b, Yongchun Kan a, **, Yuan Hu a, b, *, Richard K.K. Yuen b, c a
State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei 230026, PR China Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute of University of Science and Technology of China, 166 Ren'ai Road, Jiangsu 215123, PR China c Department of Architecture and Civil Engineering, City University of Hong Kong, Tat Chee Avenue, Hong Kong b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 June 2016 Received in revised form 28 September 2016 Accepted 9 October 2016 Available online 15 October 2016
In this research, a novel triazine-based polymeric flame retardant (TBMC) was synthesized. The intumescent flame retardant (IFR) system was formed by combining. TBMC with ammonium polyphosphate (APP), and is applied to retard the combustion of polypropylene (PP). PP/IFR samples (APP/TBMC ¼ 3/1 to 1/1) achieved increased limited oxygen index (LOI) values and passed the vertical burning (UL-94) V-0 rating. Herein, cone calorimeter tests show that the heat release rate (HRR) and fire growth index (FGI) of PP/APP/TBMC blends were obviously decreased compared to those of pure PP and PP with 25 wt% APP. The thermogravimetric analytical (TGA) results showed that the TBMC/APP system could improve the thermal and thermal-oxidative stability of the char residues. The char residues were further investigated by visual observation and scanning electron microscopy (SEM). The compact char residues formed hindered the transfer of gas and heat during combustion, ultimately endowed PP/IFR systems improved flame retardancy. © 2016 Published by Elsevier Ltd.
Keywords: Triazine-based polymeric flame retardant Flame retardancy Thermal stability
1. Introductions As one of the significant general commodity of polymers and thermoplastic material, the PP could be applied to building materials, automobiles, electronics, and electric materials, because of its admirable mechanical properties, low density, good performanceto-cost ratio, easy processability, and good chemical resistance [1,2]. However, the applications in many aspects wherever the suitable flame retardant performance was required are strongly restricted owing to their poor flame resistance with low limiting oxygen index (LOI) [3]. Recently, the researchers had not spared more efforts to improve the flame retardancy of PP in the literature [4e12]. Even though, during the research on environmental friendly flame retardants of PP, intumescent flame retardants (IFR) was promising halogen free flame retardants and had drawn considerable attention due to their advantages, such as low smoke and less toxic
* Corresponding author. State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei 230026, PR China. ** Corresponding author. E-mail addresses: [email protected] (Y. Kan), [email protected] (Y. Hu). http://dx.doi.org/10.1016/j.polymdegradstab.2016.10.003 0141-3910/© 2016 Published by Elsevier Ltd.
gasses released during the burning [13e19]. But in general, the traditional IFRs concluded the acid catalysts, charring agents, and blowing agents. In these, the conventional acid catalysts are mainly phosphate and a phosphate ester, such as ammonium polyphosphate (APP). The charring agents which currently used are mainly small molecular compounds, like, pentaerythritol (PER). However, some of the drawbacks still exited with these traditional IFR, such as lower flame retardant efficiency, poor thermal stability, unsatisfactory migration of resistance and water resistance compared with bromine-containing flame retardants. To solve all these problems it required more efforts to design and prepare the new IFRs. On the other hand in literature, cyanuric chloride was as such a raw material for the design and synthesizes different flame retardants with its high selectivity [20e22]. Accordingly, a number of triazine derivatives were used as efficient charring agents in IFR have been investigated to reduce or cancel the above hazards caused by the presence of tertiary nitrogen in the triazine rings and favorable charring ability had been found [20e35]. Traditionally, hyperbranched polymers were prepared mainly by polycondensation of an ABx type monomer that has one “A” functional group and x “B” functional groups [25,26]. But the
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Scheme 1. The route for the synthesis of TBMC via polycondensation of A2B3.
Table 1 LOI and UL-94 results of pure PP and PP/IFR systems. Sample
PP (wt%)
APP (wt%)
TBMC (wt%)
LOI(%) ±0.5
UL-94 rating (3.2 mm)
PP0 PP1 PP2 PP3 PP4 PP5 PP6
100 75 75 75 75 75 75
0 25 18.75 16.67 12.5 8.33 0
0 0 6.25 8.33 12.5 16.67 25
17.0 21 30.5 29.5 27.5 24.5 23.0
NRa NR V-0 V-0 V-0 V-1 V-1
a
NR, No rating.
Table 2 Elementary analysis data of TBMC. Sample
Elemental value
C (%)
H (%)
N (%)
TBMC
Calculated Found
53.63 52.32
7.26 6.98
39.11 37.24
Fig. 1. FTIR spectra of cyanuric chloride (a) and TBMC (b).
researchers inclined to focus on employment of polycondensation of A2 and B3 type monomers because of their commercial availability. Furthermore, those polymers could be prepared in a single
Fig. 2. Solid-state
13
C NMR spectrum of TBMC.
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Fig. 3. TG curves of TBMC under nitrogen and air atmosphere.
step process only and thus obtained in large scale at a reasonable cost, which made them more interesting for commercial applications [31]. In this connection, in our previous work, we reported a novel hyperbranched charring foaming agent (HCFA) via a condensation reaction of cyanuric chloride and piperazine [31]. The results showed that the addition of HCFA can improve the flame retardancy and water resistance of PP blends at the loading of 30 wt % IFR (APP/HCFA). Obviously, the loading of IFR is not too low to reach the UL-94 V-0 rating and high LOI value, so it was necessary to develop some more efficient intumescent flame retardants for PP. Therefore, by considering all these aspects at present work, we prepared a novel kind of intumescent flame retardant with carbon source and blowing agent by the condensation of cyanuric chloride and N-amino ethylpiperazine. Then the TBMC was used to flameretard PP together with APP at the loading of 25 wt% and find outstanding flame retardancy even at lower addition compared to our previous report [31]. 2. Experimental 2.1. Materials PP resin (F401) was provided from Yangzi Petrochemical Co. (China). The commercial products APP (phase II, the degree of polymerization >1000) was purchased by Shandong Shi'an chemical Engineering Corp (China). Cyanuric chloride and N-Aminoethylpiperazine were obtained from Aladdin Chemical Reagent Corp (China). Other chemicals were gained from China National Pharmaceutical Group (Shanghai, China). 2.2. Measurements FTIR spectra were measured using a Nicolet MAGNA-IR 750 spectrophotometer. The elemental analysis was conducted with the Vario EL III elemental analyzer. Thermogravimetric analysis (TGA) was measured by using a Q5000IR thermo-analyzer instrument (TA Co., USA) (3e10 mg sample, 20 C/min heating rate). 13 C solid-state NMR spectra were recorded on a Bruker AVANCE III 400 WB. Real-time Fourier transform infrared (RT-FTIR) spectra were measured at heating rate of 20 C/min by using the Nicolet 6700 FT-
Fig. 4. TGA (a) and DTG (b) curves of PP and its samples under a nitrogen atmosphere.
IR spectrophotometer instrument. The Limiting oxygen index (LOI) was evaluated by using an HC-2 oxygen index meter (Jiangning Analysis Instrument Co., China) based on the standard of ASTMD 2863. The vertical burning test was carried out by a CFZ-2 type instrument (Jiangning Analysis Instrument Co., China) according to the UL 94 test standard. Samples of 100 6.5 3.2 mm3 dimensions for LOI test and 130 13 3.2 mm3 dimensions for the vertical burning test were used respectively. The combustion tests were carried out on the cone calorimeter (FTT, UK) by the procedure of ISO 5660 standard. Horizontal heat flux of 35 kW/m2 was used to specimens of 100 100 3.2 mm3 dimensions. The microstructure of PP samples was studied by scanning electron microscopy (JSM6700F, 10.0 kV). The tensile strength and elongation at break were carried out according to GB13022-91 with a WD-20D electronic universal testing instrument (Changchun Intelligent Instrument Co. Ltd., China). Five parallel runs were performed for each sample. 2.3. Synthesis of TBMC N-Amino ethylpiperazine (0.32 mol) was added into a 1000 mL three-neck flask equipped. Cyanuric chloride (0.20 mol) dissolved in 1, 4-dioxane and was appended dropwise into the flask. The system was kept at 0 C for 3 h.
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Table 3 TGA data of PP and its samples at nitrogen and air atmosphere. Sample
PP0 PP1 PP2 PP3 PP4 PP5 PP6
Nitrogen
Air
Td ( C)
Tmax ( C)
Char residue at 800 C (%)
Td ( C)
Tmax1 ( C)
Tmax2 ( C)
Char residue at 800 C (%)
385 401 420 424 429 429 443
459 476 484 485 483 483 483
2.2 16.6 10.9 10.7 5.11 8.55 4.27
286 281 282 285 266 285 277
359 375 394 387 390 399 367
e 554 604 605 588 698 615
0.65 6.33 5.33 2.89 4.38 2.30 0.02
Fig. 5. TGA (a) and DTG (b) curves of PP and its samples under air atmosphere.
Soon afterwards, a solution of NaOH (0.61 mol) was added to the above mixture and the PH value of the system was maintained at 8e9 through and the reaction was heated at 50 C for 6 h more. Later, the reaction was heated up to 100 C and maintained for 12 h. Then the reaction system was cooled to room temperature, filtered and washed with distilled water twice. The obtained product was dried at room temperature in a vacuum oven for overnight and finally got the TBMC became white solid powder at high product yield (93.6%) (Scheme 1).
2.4. The preparation of samples Before mixing, PP and all additives were dried in vacuum oven at
Fig. 6. RTFTIR spectra of the condensed products of PP/APP (a) and PP/IFR (APP: TBMC ¼ 3:1) (b) at different temperatures.
80 C for overnight. All the samples were obtained using a two-roll mixing mill (Rheomixer XSS-300, Shanghai Ke Chuang China) at 180 C, and the roll speed was maintained 100 rpm for approximately 15 min respectively. The prescriptions of the PP samples were showed in Table 1.
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3. Results and discussion 3.1. Characterization of TBMC The FTIR spectra of TBMC and cyanuric chloride were presented in Fig. 1. A broad absorption peak at 3416 cm1 was attributed to the secondary amine NeH vibration. And two peaks at 2935 and 2816 cm1 were attributed to the CeH absorption peaks. A broad intensive absorption peak at 1540 cm1 was the characteristic absorption of triazine ring. The absorption bands at 1159 cm1 corresponded to CeN. No characteristic absorption sign for CeCl was observed at 630 cm1, which meant that the Cl atom attached on the triazine ring had been reacted with N-Aminoethylpiperazine. These characterization data indicated that the TBMC had been successfully synthesized. To further clarify the composition of TBMC, the elemental analysis was conducted and results were listed in Table 2. Excitingly, almost identical contents of C, H and N obtained from theoretical calculation and measurements (calcd. for TBMC: 53.63%, 7.26% and 39.11%; found: 54.32%, 6.98% and 37.24% respectively) indirectly confirmed the consumption of chlorine atoms. These combined characterizations verified the successful synthesis of TBMC by this simple one-pot method. To further confirm the structure of TBMC, 13C solid-state NMR spectrum of TBMC was provided. As shown in Fig. 2, the signals at 165.17 ppm, 54.75 ppm, 43.34 ppm were own to the carbon atoms in triazine ring, piperazine ring and ethyl, respectively, which was in accordance with the results of elemental analysis. Thermogravimetic analysis, occupied to analyze the thermal degradation behaviors of TBMC, was investigated by TGA under nitrogen and air atmospheres. Fig. 3 showed the TGA curves of TBMC under nitrogen and the whole degradation course of TBMC experienced at one stage. The decomposition occured from 304 C to 550 C and the main peak located nearby at 427 C, which was attributed to the decomposition of the macromolecular backbone and crosslinking. Finally, the remaining weight of TBMC was 26.5% at 800 C. However, the thermal degradation process of TBMC under air was divided into two steps, the first step took place from 275 to 567 C (attributed to the decomposition of the macromolecular backbone and crosslinking). The second step occurred from 567 C to 800 C, which was assigned to the degradation of final char. When the thermal degradation temperature reached 800 C, the degradation was still going on. The char residue of TBMC under air was 21.0% at 800 C. All of the above results showed TBMC exhibited poorer char formation ability compared with our previous work [31]. The reason for the phenomenon was that the thermal stability of piperazine was better than the ethyl due to its similar structure of benzene ring, while the ethyl in the N-Amino ethylpiperazine replaced a part of piperazine ring under the same dosage ratio [26,28].
Fig. 7. HRR (a) and THR (b) curves of PP and its samples.
385 C and by further increase of temperature, rapid elevation of weight loss occured and negligible char residue (2.2%) left above 500 C, due to the degradation of polypropylene backbone. The Td of PP1 with 30 wt% APP additions was higher than pure PP, and 16.6% char residue remains at 800 C. Compared to pure PP, PP/APP and PP/TBMC systems had better thermal stability with higher, T5%, T10% and T75% and the main thermal decomposition peaks obviously moved to a higher temperature. With the incorporation of APP and TBMC into PP, the PP/IFR systems had higher thermal stability between 400 C and 500 C, which could be ascribed to the outstanding char-forming ability of APP and TBMC.
3.2. Thermal degradation of PP and its samples in nitrogen atmosphere
3.3. Thermal oxidative degradation of PP and its samples in air atmosphere
Next, we utilized thermal gravimetric analysis (TGA) for preliminary assessment of the thermal stability of PP and its samples. For elucidating the retardant effect of TBMC on IFR-PP, the thermal degradation behavior and the amount of residual char under nitrogen condition were systematically studied. The results were shown in Fig. 4 and Table 3. As TGA curves in Fig. 4, the temperature at which the weight loss exhibited 5 wt% was defined as the initial decomposition temperature, which was denoted as Td; the temperature at which the degradation rate reached a maximum is defined as Tmax. For pure PP, one step decomposition from 350 C to 480 C was captured. The Td for the pure PP was approximately
We also utilized thermal gravimetric analysis (TGA) to assess the thermal oxidative degradation behavior of PP (Fig. 5). The summary of the results was presented in Table 3. In sharp contrast with the thermal stability under nitrogen condition (Fig. 4), it was clear to see a double effect on the degradation of PP blends, that was, the second step weight loss was retarded obviously whereas the first step degradation process are shift to low temperatures. Moreover, the second degradation stage in the high temperature was observed, ascribed to the oxidative decomposition of the char residues in the hot air. For instance, the Td, Tmax1, Tmax2 of PP2 which was containing 25 wt% APP/TBMC (3/1) were all shifted to higher
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Table 4 The cone data of PP and its samples. Sample
TTI(s)
Time to PHRR (s)
PHRR (kW/m2)
THR (MJ/m2)
FPIa (m2s/kW)
FGIb (kW/m2s)
Char residue (%)
PP0 PP1 PP2 PP3 PP4 PP5 PP6
48 43 36 38 44 44 38
152 206 427 94 113 56 66
988 652 113 117 123 241 504
88.3 80.0 73.9 23.2 73.3 77.2 86.6
0.05 0.07 0.32 0.32 0.36 0.18 0.08
6.5 3.1 0.2 1.2 1.1 4.3 7.6
1.6 22.9 25.1 65.2 19.4 11.7 8.7
a b
FPI ¼ TTI/PHRR. FGI ¼ PHRR/tPHRR.
Fig. 8. Mass loss curves of PP samples from cone calorimeter test.
temperatures relative to PP1. Besides, the increases in a number of remaining residues at high temperature was attributed to the interaction of the pyrolysis products during the heating process, which were effectively decreased the release of volatile products and enhanced the thermal stability and residues at higher temperatures. It should be noted that higher amount of char residues were obtained in the PP/IFR samples (APP/TBMC ¼ 3:1 and 2:1) before 600 C, while the char residues of PP/IFR systems further oxidative decomposed between 600 C and 800 C in the hot air, leading to lower char residue than that of PP/APP [27,29]. Fig. 5 showed the second decomposition peak of PP/IFR systems appeared at 630 C. In general, TG, a micro-scale test method, was controlled to slowly rise the temperature, leading to the gradually degradation of the matrix and the slow rate of char forming. Thus, it could not form the intumescent char layer, resulting in the further degradation of the char residue [23,27,29]. 3.4. Real time FTIR study To further understand the flame-retardant mechanism of PP/IFR systems in condense phase, the thermal oxidative degradation residues at different temperature were investigated by FTIR. The changes in the FTIR spectra of PP/APP and PP/APP/TBMC (APP:TBMC ¼ 3:1) were presented in Fig. 6. For PP/APP system (Fig. 6a), the characteristic absorption peak of PP could be observed at 2952, 2921, 2871, 2834, 1454, 1374 and 884 cm1 under the room temperature [36]. With increasing the pyrolysis temperature, the intensity of PP absorption peaks gradually decreased and disappeared on further heating to 350 C, indicating the complete decomposition of PP. The band at 1254 was budged to 1286 cm1,
suggesting the presence of dissociative PO 4 . Furthermore, the peaks at 3165 cm1 attributed to the vibration of NeH gradually disappeared with increasing the temperature to 300 C, implying the release of NH3 [37]. With the increase of pyrolysis temperature, two new peaks at 1013 and 870 cm1 appearing at 350 C were assigned to the absorptions of PeOeP groups. The FTIR spectra for PP/IFR(APP:TBMC ¼ 3:1) at different degradation temperatures were given in Fig. 6b. The peaks at 1563 and 1097 cm1 were attributed to the skeleton of the triazine ring and the CeN bonds in TBMC [31]. As the degradation temperature ascended, The peak intensity was gradually decreased, indicating the decomposition and reconstruction of the triazine ring. The main difference between PP/APP and PP/IFR (APP:TBMC ¼ 3:1) during the pyrolysis process is obviously observed. During the pyrolysis process, the peaks at 1097 and 997 cm1 still existed at 550 C, which indicated that the PeOeP and PeOeC structure were formed in the residue of PP/IFR(APP:TBMC ¼ 3:1) at this high temperature, while nothing was observed for PP/APP system in this case. Moreover, the characteristic absorption peak of PP was not observed until the temperature increased to 450 C, while the absorption disappeared at 350 C for PP/APP system. As these above mentioned, it could be explained that the PeOeC and PeOeP structure could improve the char formation of APP/TBMC, which could protect the matrix from decomposition [15,38,39].
3.5. LOI and UL-94 analysis of flame retarded PP samples The LOI and UL-94 results for PP samples were presented in Table 1. The LOI value of PP/APP with 25 wt% loadings was 21%, and could not reach to the UL-94 V-0 rating. But, there was a remarkable increment in LOI values of IFR-PP systems when TBMC was added to the PP/APP system. Herein, the experimental consequences of vertical burning tests showed that all the PP/TBMC/APP systems were obtained UL-94 V-0 rating with a weight ratio of APP to TBMC being 3/1 to 1/1. When the weight ratio of TBMC to APP reached 3/1, the IFR system showed the most effective flame retardancy with a maximum LOI value of 30.5%. However, for further increase of the weight ratio of TBMC/APP led to the decreased LOI value. These results demonstrated that the combination of TBMC would cause huge improvement of the flame retardant performance of APP in PP. As a matter of fact, TBMC was a typical IFR integrating acid, char and gas sources, which was expect to achieve outstanding fire retardant effect, thus, LOI values of PP/TBMC with 25 wt% loadings was 23%, and achieved UL-94 V-1 rating. Heat release rate (HRR) and total heat release (THR) curves of PP and its samples were presented in Fig. 7 and the corresponding data were given in Table 4. When the pure PP was being ignited, it could burn so fast and it quickly reached the PHRR (1387 kW/m2). But, herein when a 30 wt% APP was added, then the PHRR of PP1 decreased by 61.1% and a slight broad HRR curve came into being. The combination of APP with TBMC could significantly improve the
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Fig. 9. Front and side views of the residues of PP1, PP2, PP3, PP4, PP5, and PP6 after cone calorimeter test after cone calorimeter test.
flame retardancy of PP. Therefore, the PHRR of PP blends was reduced significantly. For example, PHRR of PP2 was 113 kW/m2, decreasing by 88.6% compared with that of pure PP and the time to PHRR was 427 s, a lag of 275 s relative to that of pure PP. Besides, the value of its THR was reduced compared to pure PP and PP/APP system. These all were indicated that a much better flame retardancy was contributed by APP/TBMC system. Moreover, cone calorimeter analysis, a bench-scale fire test to exhibit retardant capacity in a real fire disaster, directly exposing the material to the high temperature, thus leading to the fast pyrolysis of the matrix and rapidly formation of intumescent char layer. The intumescent char layer could protect the underlying materials from further burning. Based on the discussion of the TG results, that was why PP/IFR owned lower char residues at 800 C, while better flame retardancy (LOI, UL94 and cone calorimeter results). To judge the fire hazards of polymer materials more clearly, herein, we selected fire performance index (FPI) and fire growth index (FGI) [31]. The comparison between the values of FPI and FGI of PP and its samples were shown in Table 4. Apparently, when the weight ratio of APP to TBMC was 3/1, the fire risks of the corresponding PP blends were much smaller than others. The mass loss results after the cone test has been provided in Fig. 8 and the corresponding data were presented in Table 4. It appeared clearly that there were 1.6%, 22.9%, 25.1%, 65.2% and 8.7% of residual mass left at the end of burning, respectively, for pure PP,
PP1, PP2, PP3 and PP6. The PP2 and PP3 samples presented significantly higher residual mass compared to that of other systems, indicating that TBMC and APP facilitated the carbonization of PP samples [28]. The intumescent char layer prevented pyrolysis products and heat during the combustion process, which was in accordance with the PHRR behavior during cone calorimetry and the TGA results before 630 C under air atmosphere. 3.6. The morphology of char residues of PP samples It was universally acknowledged that the intumescent flame retardant system usually undergoes an intense expansion process and the formation of the protective char layer. Thus, the investigation of char residues could be benefit to understand the differences in the flame retardancy. Fig. 9 presented the digital photos of the residues of PP and its samples collected after cone calorimeter measurements. Herein, nothing was left for pure PP. Only one kind of flame retardant be used, could the residues of PP1 with 25 wt% APP and PP6 with 25 wt % APA/TBMC demonstrate a tiny residue without intumescence. In contrast to this, PP2, PP3, PP4 and PP5 with weight ratio of APP to TBMC being 3/1 to 1/2 engendered the highly intumescent char residues, respectively. These results indicated that PP samples containing 25 wt% APP/TBMC with weight ratio from 1:3 to 1:2 had impactful intumescent char layers as effective physical barriers against the transfer of the heat and
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Fig. 10. SEM images of the char residues of PP1 (a), PP2 (b), and PP6(c).
THR and higher char residue. The char residues left after the cone calorimeter test were analyzed by SEM, and the corresponding images were shown in Fig. 10. For PP2 with a compact and continuous intumescent char could effectively prevent the heat, combustible gases and mass from transferring, which indicated that the triazine-ring were probably crosslinked with PeOeP, and formed the thermostable char [21]. In contrast, the char layers of PP6 were broken up and porous, which probably was the reason why these char layers were low efficient and poor thermal stability. 3.7. Mechanical behaviors
Fig. 11. Stressestrain behaviors of PP and its samples.
Table 5 Mechanical properties of PP and its samples. Sample
Tensile strength (Mpa)
PP0 PP1 PP2 PP3 PP4 PP5 PP6
32.1 21.2 30.6 28.1 24.5 26.1 21.5
± ± ± ± ± ± ±
0.9 0.3 0.4 0.8 0.6 0.6 0.8
Elongation at break (%) 107.3 ± 8.9 33.1 ± 3.8 5.3 ± 0.5 4.5 ± 0.6 4.3 ± 0.5 4.4 ± 0.6 3.5 ± 0.3
combustible gasses, thus resulting in the much lower PHRR and
The mechanical behaviors of PP and its samples with various loadings of TBMC and APP were investigated by tensile tests. Fig. 11 showed the representative stressestrain curves of PP and its samples. The detailed data, including tensile strength and elongation at break were listed in Table 5. As can be seen, the addition of APP or TBMC resulted in a decrease in tensile strength relative to pure PP, which was mainly due to the poor dispersion in PP. while the tensile strength of APP/TBMC systems increased, might be ascribed to the complexation between APP and TBMC. It should be noted that the reinforcing effect of APP/TBMC systems on the mechanical strength of PP was not very marked. It was observed that pure PP displayed a ductile fracture with an elongation at break of approximately 107%, After the addition of APP, the elongation at break of PP1 was 33.1%, which was much lower than that of pure PP, and the same trend to that of PP/APP/TBMC systems. It indicated that the addition of TBMC and APP has a significant influence on the elongation at break of PP systems. 4. Conclusion In this paper, a novel intumescent flame retardant, TBMC, was prepared. In combination with APP, it was utilized to flame retard
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combustible polypropylene. Relieved thermal degradation and high temperature oxidation resistance of the char residues in air atmosphere before 600 C were obtained. Investigation of combustion behavior reveals that PP/IFR blends (APP/TBMC ¼ 3/1 to 1/1) could acquire appreciable LOI values, and pass the UL-94 V-0 rating. In addition, PP/APP/TBMC blends exhibited dramatically decreased HRR and FGI compared with pure PP and PP with 25 wt% APP. Besides, The PP2 and PP3 samples presented significantly higher residual mass compared to that of other systems from the cone calorimeter results. SEM results provided further evidence of the compact char residues, which reduced the transfer of gas and heat during combustion, and ultimately induced improved flame retardancy of PP/IFR composites. Acknowledgements The authors acknowledge the research grants from the National Key Research and Development Program of China (2016YFC0800605), National Natural Science Foundation of China (51323010, 21411140231) and Research Grants Council of the Hong Kong Special Administrative Region (contract grant number CityU 11301015). References [1] S. Bourbigot, M. Le Bras, S. Duquesne, M. Rochery, Recent advances for intumescent polymers, Macromol. Mater. Eng. 289 (2004) 499. [2] F. Zhang, Y. Wang, Y. Jin, J. Zhang, Simulated effects of physical parameters on heat transfer of intumescent fire-retardant polypropylene during burning, J. Macromol. Sci. Phys. B 50 (2011) 1185. [3] X.P. Hu, W.Y. Li, Y.Z. Wang, Synthesis and Characterization of A Novel nitrogen containing flame retardant, J. Appl. Polym. Sci. 94 (2004) 1556e1561. [4] Y. Liu, J. Zhao, C.L. Deng, L. Chen, D.Y. Wang, Y.Z. Wang, Flame-retardant effect of sepiolite on an intumescent flame-retardant polypropylene system, Ind. Eng. Chem. Res. 50 (2011) 2047e2054. [5] J. Zhan, L. Song, S.B. Nie, Y. Hu, Combustion properties and thermal degradation behavior of polylactide with an effective intumescent flame retardant, Polym. Degrad. Stab. 94 (2009) 291e296. [6] S. Bourbigot, M. Le Bras, R. Delobel, P. Breant, J.M. Tremillon, 4A zeolite synergistic agent in new flame retardant intumescent formulations of polyethylenic polymerseStudy of the effect of the constituent monomers, Polym. Degrad. Stab. 54 (1996) 275. [7] Z.B. Shao, C. Deng, Y. Tan, M.J. Chen, L. Chen, Y.Z. Wang, Flame retardation of polypropylene via a novel intumescent flame retardant: ethylenediaminemodified ammonium polyphosphate, Polym. Degrad. Stab. 106 (2014) 88e96. [8] A.R. Horrocks, Developments inflame retardants for heat and fire resistant textiles-the role of char formation and intumescence, Polym. Degrad. Stab. 54 (1996) 143e154. [9] G. Camino, L. Costa, L. Trossarelli, F. Costanzi, A. Pagliari, Study of the mechanism of intumescence in fire retardant polymers: Part VI-Mechanism of ester formation in ammonium polyphosphate pentaerythritol mixtures, Polym. Degrad. Stab. 12 (1985) 213e228. [10] D.Y. Wang, X.X. Cai, M.H. Qu, Y. Liu, J.S. Wang, Y.Z. Wang, Preparation and flammability of a novel intumescent flame-retardant poly(ethylene-co-vinyl acetate) system, Polym. Degrad. Stab. 93 (2008) 2186e2192. [11] Q. Li, H.F. Zhong, P. Wei, P.K. Jiang, Thermal degradation behaviors of polypropylene with novel silicon-containing intumescent flame retardant, J. Appl. Polym. Sci. 98 (2005) 2487e2492. [12] S.L. Gao, B. Li, P. Bai, S.Q. Zhang, Effect of polysiloxane and silane-modified SiO2 on a novel intumescent flame retardant polypropylene system, Polym. Adv. Technol. 22 (2011) 2609e2616. [13] S. Bourbigot, M.L. Bras, F. Dabrowski, J.W. Gilman, T. Kashiwagi, PA-6 clay nanocomposite hybrid as char forming agent in intumescent formulations, Fire Mater. 24 (2000) 201e208. [14] S.H. Chiu, W.K. Wang, Dynamic Flame Retardancy of Polypropylene Filled with Ammonium Polyphosphate, Pentaerythritol and Melamine Additives, Polymer 39 (1998) 1951e1955. [15] Y.W. Yan, L. Chen, R.K. Jian, S. Kong, Y.Z. Wang, Intumescence: An Effect Way to Flame Retardance and Smoke Suppression for Polystryene, Polym. Degrad. Stab. 97 (2012) 1423e1431. [16] B. Lecouvet, M. Sclavons, C. Bailly, S. Bourbigot, A comprehensive study of the synergistic flame retardant mechanisms of halloysite in intumescent
polypropylene, Polym. Degrad. Stab. 98 (2013) 2268e2281. [17] J. Li, C.H. Ke, L. Xu, Y.Z. Wang, Synergistic effect between a hyperbranched charring agent and ammonium polyphosphate on the intumescent flame retardance of acrylonitrile-butadiene-styrene polymer, Polym. Degrad. Stab. 97 (2012) 1107e1113. [18] X.Q. Su, Y.W. Yi, J. Tao, H.Q. Qi, D.Y. Li, Synergistic effect between a novel triazine charring agent and ammonium polyphosphate on flame retardancyand thermal behavior of polypropylene, Polym. Degrad. Stab. 105 (2014) 12e20. [19] K. Yang, M.J. Yang, B. Li, Synthesis of N-Ethyl Triazine-Piperazine Copolymer and Flame Retardancy and Water Resistance of Intumescent Flame Retardant Polypropylene, Polym. Degrad. Stab. 98 (2013) 1397e1406. [20] P.A. Song, Z.P. Fang, L.F. Tong, Z.B. Xu, Synthesis of a novel oligomeric intumescent flame retardant and its application in polypropylene, Polym. Eng. Sci. 49 (2009) 1326e1331. [21] S.S. Mahapatra, N. Karak, s-Triazine containing flame retardanthyperbranched polyamines: synthesis, characterization and propertiesevaluation, Polym. Degrad. Stab. 92 (2007) 947e955. [22] B. Li, M.J. Li, Effect of A Novel Charring-Foaming Agent on Flame Retardancy and Thermal Degradation of Intumescent Flame Retardant Polypropylene, Polym. Degrad. Stab. 91 (2006) 1380e1386. [23] C.M. Feng, Y. Zhang, S.W. Liu, Z.G. Chi, J.R. Xu, Flame retardancy and thermaldegradation behaviors of polypropylene composites with novel intumescentflame retardant and manganese dioxide, J. Anal. Appl. Pyrolysis 104 (2013) 59e67. [24] C.H. Ke, J. Li, K.Y. Fang, Q.L. Zhu, J. Zhu, Q. Yan, Y.Z. Wang, Synergistic effect between a novel hyperbranched charring agent and ammonium polyphosphate on the flame retardant and anti-dripping properties of polylactide, Polym. Degrad. Stab. 95 (2010) 763e770. [25] P.Y. Wen, X.F. Wang, W.Y. Xing, X.M. Feng, B. Yu, Y.Q. Shi, Synthesis of a novel triazine-based hyperbranched char foaming agent and the study of its enhancement on flame retardancy and thermal stability of polypropylene, Ind. Eng. Chem. Res. 52 (2013) 17015e17022. [26] C. Gao, D. Yan, Hyperbranched polymers: from synthesis to applications, Prog. Polym. Sci. 29 (2004) 183e275. [27] C.M. Feng, Y. Zhang, D. Liang, S.W. Liu, Z.G. Chi, J.R. Xu, Influence of zinc borate on the flame retardancy and thermal stability of intumescent flame retardant polypropylene composites, J. Anal. Appl. Pyrolysis 115 (2015) 224e232. [28] D. Enescu, A. Frache, M. Lavaselli, O. Monticelli, F. Marino, Novel phosphorous nitrogen intumescent flame retardant system. Its effects on flame retardancy and thermal properties of polypropylene, Polym. Degrad. Stab. 98 (2013) 297e305. [29] C.M. Feng, Y. Feng, S.W. Liu, Z.G. Chi, J.R. Xu, Flame retardancy and thermal degradation behaviors of polypropylene composites with novel intumescent flame retardant and manganese dioxide, J. Anal. Appl. Pyrolysis 104 (2013) 59e67. [30] G. Camino, L. Costa, L. Trossarelli, Study of the mechanism of intumescence in fire retardant polymers: part IIemechanism of action in polypropyleneammonium polyphosphate-pentaerythritol mixtures, Polym. Degrad. Stab. 7 (1984) 25e31. [31] P.Y. Wen, X.F. Wang, B.B. Wang, B.H. Yuan, K.Q. Zhou, L. Song, Y. Hu, R.K.K. Yuen, One-pot synthesis of a novel s-triazine-based hyperbranched charring foaming agent and its enhancement on flame retardancy and water resistance of polypropylene, Polym. Degrad. Stab. 110 (2014) 165e174. [32] X.Q. Su, Y.W. Yi, J. Tao, H.Q. Qi, D.Y. Li, Synergistic effect between a novel triazine charring agent and ammonium polyphosphate on flame retardancy and thermal behavior of polypropylene, Polym. Degrad. Stab. 105 (2014) 12e20. [33] S. Zhou, L. Song, Z.Z. Wang, Y. Hu, W.Y. Xing, Flame retardation and char formation mechanism of intumescent flame retarded polypropylene composites containing melamine phosphate and pentaerythritol phosphate, Polym. Degrad. Stab. 93 (2008) 1799e1806. [34] C.M. Feng, Z.W. Li, M.Y. Liang, J.G. Huang, H.B. Liu, Preparation and characterization of a novel oligomeric charring agent and its application inhalogenfree flame retardant polypropylene, J. Anal. Appl. Pyrolysis 111 (2015) 238e246. [35] Z.F. An, R.F. Chen, J. Yin, G.H. Xie, H.F. Xie, T. Tsuboi, Conjugated asymmetric donor-substituted 1,3,5-triazines: new host materials for blue phosphorescent organic light-emitting diodes, Chem. Eur. J. 17 (2011) 10871e10878. [36] F. Zhang, J. Zhang, D.X. Sun, Study on Thermal decomposition of intumescent fire-retardant polypropylene by TG/Fourier transform infrared, J. Thermoplast. Compos. 22 (2009) 681e701. [37] F. Laoutid, L. Bonnaud, M. Alexandre, J.M. Lopez-Cuesta, P. Dubois, New prospects in flame retardant polymer materials: From fundamentals to nanocomposites, Mater. Sci. Eng. R. 63 (2009) 100e125. [38] B.O. Mysen, F.J. Ryerson, D. Virgo, The structural role of phosphorus in silicate melts, Am. Mineral. 66 (1981) 106e117. [39] C. Nguyen, J. Kim, Thermal stabilities and flame retardancies of nitrogene phosphorus flame retardants based on bisphosphoramidates, Polym. Degrad. Stab. 93 (2008) 1037e1043.