A Novel Linear-Chain Polyamide Charring Agent for the Fire Safety of

Article pubs.acs.org/IECR

A Novel Linear-Chain Polyamide Charring Agent for the Fire Safety of Noncharring Polyolefin Liang-Ping Dong, Sheng-Chao Huang, Ying-Ming Li, Cong Deng,* and Yu-Zhong Wang* Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, State Key Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Analytical and Testing Center, Sichuan University, Chengdu 610064, China ABSTRACT: A novel linear-chain polyamide charring agent named poly(piperazinyl oxalamide) (PPOA) was prepared successfully. Thermogravimetric analysis (TGA) proved that PPOA had a charring capability that was better than those of traditional polyamide charring agents. After incorporation of PPOA into ethylene-vinyl acetate copolymer/ammonium polyphosphate (EVA/APP), the flame retardancy of EVA/ APP was dramatically improved. At 25.0 wt % APP/ PPOA(3:1), the OI value of the EVA/APP/PPOA system was 31.5%; its UL-94 rating was V-0. Both the peak of heat release rate (PHRR) and the peak of smoke production rate (peak SPR) for the EVA/APP/PPOA system decreased remarkably compared with that of EVA containing 25.0 wt % APP. The analysis of the charring mechanism of PPOA demonstrated that abundant heterocyclic aromatic structures were formed during the decomposition of PPOA, which endowed the PPOA with excellent charring capability and resulted in the low risk of combustion of the EVA/APP/PPOA system.

1. INTRODUCTION Polyolefin has a wide range of applications in the cable industry, construction, packing material, etc., because of its excellent physical and mechanical properties.1−4 However, it is a flammable material, accompanied by heavy melt dripping; moreover, no residue is left after burning for polyolefin, which limits its application in the areas that require flame retardancy. Therefore, it is very necessary to flame retard polyolefin. Generally, the incorporation of an efficient flame retardant is a feasible method for reducing the flammability of polyolefin. Thus far, intumescent flame retardant (IFR) has attracted a great deal of attention because of its excellent flame-retardant effect, low level of smoke, and low hypotoxicity during burning. For IFR, some chemical reactions that contribute to the formation of an intumescent char layer occur during a thermal decomposition process, such as esterification, dehydration, carbonization, and expansion. The formed char layer acts as a barrier between the burning zone and the material underneath, so the unburned material is protected in this case.5 Camino et al. invented the original IFR system and also investigated the IFR mechanism.6,7 An IFR commonly consists of an acid source, a charring agent, and a blowing agent, and the charring agent is very vital for fabricating an efficient IFR. Different kinds of charring agents were reported in the past, including pentaerythritol,8 polyamide-6,6,9−15 triazine derivatives,16−28 spiral pentaerythritol derivatives,21,27,29 etc. However, the minimal loading for IFRs containing these charring agents is at least 30 wt % to make EVA achieve excellent flame retardancy;3,13,30,31 thus, the charring efficiencies for these © 2016 American Chemical Society

charring agents are still unsatisfactory. For example, Hu and coworkers13 found that flame-retarded EVA reached an oxygen index of 31% and a UL-94 V-0 rating when the loading of IFR was 30 wt %. Generally, the flame-retardant efficiency of IFR is improved through two methods. The first is based on the acid source.32−34 Shao et al.33 prepared the chemically modified monocomponent flame retardant based on the APP and found that the piperazine ring structure could efficiently facilitate the carbonization of APP and then lead to its high flame-retardant efficiency. The second is one in which a highly efficient charring agent is incorporated into an IFR system in a flame-retarding polymer.23,25,29 For example, Yang et al.25 prepared a triazinecontaining macromolecular charring agent (ETPC) and found that the IFR that consists of APP and ETPC showed outstanding flame retardancy in a PP matrix. In our previous study,35 a novel hyperbranched phosphamide charring agent (PPPA) was synthesized successfully, which shows high efficiency in promoting the flame retardancy of EVA/APP. The initial decomposition temperature (T5%) of PPPA was ∼310.8 or 271.8 °C under a nitrogen or air atmosphere, respectively; each of these is quite low, and thus, the prepared EVA/APP/PPPA has an initial decomposing temperature lower than that of EVA/APP, which led to the Received: Revised: Accepted: Published: 7132

April 5, 2016 June 8, 2016 June 14, 2016 June 14, 2016 DOI: 10.1021/acs.iecr.6b01308 Ind. Eng. Chem. Res. 2016, 55, 7132−7141

Article

Industrial & Engineering Chemistry Research

mixer (SX300) at 150 °C and 100 rpm, and the mixing time was ∼10 min. Finally, these blends were hot-pressed into different dimensions for combustion and mechanical tests through a plate vulcanizer (Qingdao Yadong Rubber Viachinery Co. Ltd.) at 150 °C. 2.3.2. Residual Char Samples for the Fourier Transform Infrared Spectroscopy (FTIR) Test. The PPOA was heated to 400, 500, 600, and 700 °C at a heating rate of 10 °C/min in TG 209 F1 (NETZSCH) under a nitrogen atmosphere and held for 10 min at these temperatures, and then the obtained residues were used for the FTIR test. 2.4. Measurements. 1H nuclear magnetic resonance (NMR) spectra were recorded with a Bruker AV II-400 MHz spectrometer. The solvent was dimethyl sulfoxide. Tensile properties were measured with a universal testing machine (CMT6503, Shenzhen SANS Test Machine Co., Ltd., Shenzhen, China) according to GB/T 1040.1-2006 at a speed of 200 mm min−1. For the pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) test, the temperature program of the capillary column was as follows: 2 min at 45 °C and then the temperature increased to 280 °C at a heating rate of 15 °C/ min. Other parameters are the same as those in our previous study.35 Fourier transform infrared spectroscopy, elemental analysis (EA), thermogravimetric analysis (TGA), the oxygen index test (OI), the UL-94 vertical burning test, the cone calorimeter test (CC), and scanning electron microscopy (SEM) are identical with the corresponding methods described in our previous study.35

dramatic decrease in the time to ignition to 40 s for EVA/APP/ PPPA from 65 s for EVA/APP when it encountered fire. Obviously, the EVA/APP/PPPA system has flame retardancy that is better than that of EVA/APP, but the time to ignition of the former is shorter than that of the latter. In this work, our aim is to simultaneously maintain a long time to ignition while improving the flame retardancy of EVA/APP. To achieve this, we designed a new kind of linear-chain polyamide flame retardant PPOA without a traditional flame-retardant phosphorus element, which is quite different from the hyperbranched phosphamide flame retardant PPPA containing a phosphorus element in our previous work. The chemical structure of PPOA was investigated with the aid of various measurements. In flame-retarding EVA (a typical noncharring polyolefin) using APP/PPOA, the flame retardancy of EVA/ APP/PPOA was measured with different combustible tests. More importantly, the mechanism for the effect of charring agent PPOA on the flame retardancy of EVA/APP was studied in detail.

2. EXPERIMENTAL SECTION 2.1. Materials. Piperazine (analytical reagent) was obtained from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). Oxalyl chloride (analytical reagent), acetonitrile (analytical reagent), and chloroform (analytical reagent) were obtained from Chengdu Kelong Chemical Reagent Co., Ltd. (Sichuan, China). Triethylamine (analytical reagent) was from Fuyu Chemical Reagent Co., Ltd. (Tianjin, China). Commercial APP (form II) supplied by Taifeng New-Type Flame Retardants Co., Ltd. (Shifang, China), has a degree of polymerization of 900−1100. EVA (UE630, Taiwan Polymer Chemical Co., Ltd., Taiwan, China) contains 16 wt % vinyl acetate. PA6 (YH-800) was purchased from Hunan Yueyang Baling Petrochemical Co., Ltd. (Hunan, China). PA66 (101L) and PA6T/DT (Zytel HTN 501) were obtained from DuPont China Co. Ltd. PA6T/DT is a kind of semiaromatic polyamide, which consists of a 6,T and a D,T segment with a substituted methyl group at the aliphatic chain. 2.2. Synthesis of PPOA. The synthetic route of PPOA is shown in Scheme 1. First, both 0.1 mol of oxalyl chloride (12.7

3. RESULTS AND DISCUSSION 3.1. Characterization of PPOA. The chemical structure of PPOA was confirmed by 1H NMR spectra, and the result is shown in Figure 1. The peaks at 2.59 and 3.34 ppm correspond

Scheme 1. Route for the Synthesis of PPOA

g) and 100 mL of acetonitrile were added in a 500 mL fourneck flask under a nitrogen atmosphere. Afterward, the mixture of piperazine (0.1 mol, 8.6 g) and triethylamine (0.2 mol, 20.2 g) dissolved in 200 mL of dry acetonitrile was poured into the solution described above at 0 °C while it was being stirred. Then, the mixture was heated to 80 °C and refluxed for 10 h. After that, the temperature of the mixture was decreased to room temperature, and the solvent was removed by spinning evaporation. The obtained solid was washed with plenty of chloroform. Finally, the clean solid was dried to a constant weight at 100 °C in a vacuum oven. The final yield was 90%. 2.3. Sample Preparation. 2.3.1. Sample for Combustion and Mechanical Tests. The first step is drying of EVA, APP, and PPOA at 80 °C for 12 h in a vacuum oven. Then three kinds of materials were blended at different ratios via a Banbury

Figure 1. 1H NMR spectra of (a) piperazine and (b) PPOA.

to the proton of methylene in piperazine and PPOA, respectively. The fact that the chemical shift of PPOA is larger than that of piperazine indicates that the piperazine reacted with oxalyl chloride, and the PPOA might be prepared successfully. To further confirm the structure of PPOA, FTIR measurement was employed. Figure 2 shows that the peaks at 2849 and 2926 cm−1 correspond to the −CH2− asymmetry stretching vibration of piperazine, and the two peaks are also observed for PPOA, indicating that the piperazine reacted with oxalyl chloride. In addition, both the disappearance of the peak at 3206 cm−1 (νN−H) and the appearance of the peak at 1636 7133

DOI: 10.1021/acs.iecr.6b01308 Ind. Eng. Chem. Res. 2016, 55, 7132−7141

Article


Table 2. TGA Data of PPOA, PA6, PA66, and PA6T/DT

cm−1 (νCO) are observed for PPOA, illustrating the successful preparation of PPOA. Next, the EA test was also used to demonstrate that the PPOA was obtained successfully from another aspect. Table 1 Table 1. EA Results for PPOA calcd (wt %)

exptl (wt %)

C

H

N

C

H

N

PPOA

51.4

5.8

20.0

50.7

6.3

20.0

T5%

Tpeak (°C)

char residue at 700 °C (wt %)

398.4 316.5 395.1 409.6 423.6

469.0 388.0 456.9 458.2 477.9

11.6 1.5 0.5 0.8 3.0

and 3.0 wt % for traditional polyamide charring agents PA6, PA66, and PA6T/DT, respectively, indicating that PPOA has excellent charring capability at high temperatures. Under air, the residue of PPOA at 700 °C was ∼1.5 wt %. Here, the following fact must be noted. The combustion of polymer material needs oxygen, but the oxygen mainly exists in the region of outer flame during burning; therefore, it is close to the anaerobic environment rather than the aerobic environment at the interface between the flame and the unburned material during actual combustion. In this case, it is closer to the real combustion situation, so we can discuss the charring capability of PPOA under a nitrogen atmosphere rather than an air atmosphere. 3.3. Effect of PPOA on the Flame Retardancy of EVA/ APP. The PPOA was used to improve the flame retardancy of EVA/APP. The OI and UL-94 results are listed in Table 3. In

Figure 2. FTIR spectra of piperazine and PPOA.

sample

sample PPOA (N2) PPOA (air) PA6 (N2) PA66 (N2) PA6T/DT (N2)

Table 3. OI and UL-94 Results for EVA and Its FlameRetardant Systems

shows that the experimental contents of C, H, and N of PPOA are 50.7, 6.3, and 20.0 wt %, respectively. Obviously, these experimental contents are close to the corresponding theoretical values. Here, these theoretical values were obtained according to the ideal structure of the PPOA.25,26 This result further illustrates that the PPOA was prepared successfully. 3.2. Thermal Behavior and Charring Capability of PPOA. The thermal behavior and charring ability of PPOA were investigated by TGA, and the results are shown in Figure 3 and Table 2. The initial thermal decomposition temperatures (T5%) of PPOA under nitrogen and air atmospheres were as high as 398.4 and 316.5 °C, respectively, which are 87.6 and 44.7 °C higher, respectively, than the corresponding value of PPPA reported in our previous study.35 The excellent thermal stability and thermo-oxidative stability of PPOA endow it with promising prospects of application for the flame-retardant modification of polymers with high processing temperatures. The char residue of PPOA was 11.6 wt % at 700 °C under a nitrogen atmosphere, while the char residues were only 0.5, 0.8,

UL-94 sample EVA

EVA (wt %) 100

APP (wt %)

PPOA (wt %)

OI (%)

0

0

19.0

rating

dripping yes

EVA/APP30

70.0

30.0

0

23.5

EVA/PPOA30

70.0

0

30.0

22.0

EVA/APP20.0/ PPOA10.0 EVA/APP22.5/ PPOA7.5 EVA/APP24.0/ PPOA6.0 EVA/APP18.8/ PPOA6.2 EVA/APP15.0/ PPOA5.0

70.0

20.0

10.0

34.0

no rating no rating no rating V-0

70.0

22.5

7.5

35.0

V-0

no

70.0

24.0

6.0

34.5

V-0

no

75.0

18.8

6.2

31.5

V-0

no

78.0

15.0

5.0

28.5

V-2

yes

yes yes no

Figure 3. TGA curves of (a) PPOA and (b) PA6, PA66, and PA6T/DT. 7134


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APP22.5/PPOA7.5, and these results are shown in Figure 4 and Table 5. The CC result shows that neat EVA burned rapidly after being ignited. A very sharp peak of HRR (PHRR) for EVA appeared at ∼180 s, which was ∼608.3 kW/m2, and its THR was 103.6 MJ/m2. After incorporation of 30.0 wt % APP, the PHRR decreased to 295.3 kW/m2 and the THR decreased to 83.3 MJ/m2. For EVA/PPOA30, its PHRR and THR were 458.7 kW/m2 and 102.4 MJ/m2, respectively. For EVA/ APP22.5/PPOA7.5, both PHRR and THR values were further reduced to 185.2 kW/m2 and 82.8 MJ/m2 compared with those of EVA/APP30, respectively. Undoubtedly, APP/PPOA has an efficiency much higher than that of APP or PPOA alone in reducing the amount of heat released by EVA. The synergistic efficiency (SE) for the EVA/APP/PPOA system could be calculated according to eq 1 proposed by Lewin and Horrocks.37,38

the UL-94 test, neat EVA had no rating; in the OI test, its value was ∼19.0%. Apparently, EVA is an easily flammable polymeric material. After incorporation of 30.0 wt % APP, its OI value increased to 23.5%, demonstrating that APP had an effect on the flame retardancy of EVA. However, APP failed to allow EVA to achieve the V-0 rating when it was used alone, illustrating that it showed poor efficiency in flame-retarding EVA. Similarly, when PPOA was added in EVA alone, the OI of EVA/PPOA30 was 22.0%, and it had no rating in the UL-94 test. After incorporation of PPOA into EVA/APP, the flame retardancy of EVA/APP was greatly improved. The OI value of EVA/APP22.5/PPOA7.5 was 35.0%, and the UL-94 V-0 rating could also be achieved. The optimal weight ratio of APP/PPOA for fabricating the flame-retardantsystem is ∼3:1 in the investigated range. Here, it should be noted that the numbers shown in sample names represent their weight percents. When the content of APP/PPOA decreased to 25.0 wt %, the UL-94 V-0 rating was also achieved for the flame-retardant EVA containing PPOA, and the OI was kept at 31.5%. According to previous studies,3,13,30,31 the minimal loading of IFR is usually not less than 30 wt % to make EVA achieve the V-0 rating, so APP/PPOA shows high flame-retardant efficiency for EVA. 3.4. Mechanical Properties of Neat EVA and Its FlameRetardant Composites. The tensile strength and elongation at break for neat EVA, EVA/APP30, and EVA/APP22.5/ PPOA7.5 are listed in Table 4. The incorporation of APP

SE = (PHRR EVA − PHRR EVA /APP/PPOA) /[(PHRR EVA − PHRR EVA /APP) × 0.75 + (PHRR EVA − PHRR PPOA) × 0.25]

According to eq 1, the value of SE was 1.55, which was above 1, indicating that there was an obvious synergistic effect between APP and PPOA.37 Notably, the time to ignition (TTI) of EVA/APP22.5/PPOA7.5 was scarcely decreased compared with that of neat EVA, which should be ascribed to the fact that PPOA has excellent thermal stability, and the NH3 and H2O produced by APP could play a role in diluting combustible gas. Thus, when the loading of PPOA was only 7.5 wt %, it had an only slight effect on the TTI value. Similar to the HRR curves, those of EVA/APP22.5/PPOA7.5 distinctly presented a lower peak of SPR compared with those of neat EVA, EVA/APP30, and EVA/PPOA30, indicating that the novel IFR fabricated in this work is an effective smoke retarder. Table 5 shows the total smoke release (TSR), the mean specific extinction area (SEA), the ratio of carbon monoxide to carbon dioxide (CO/CO2), and the average mass loss rate (MLR) of neat EVA and flame-retardant EVA. The average MLR values of EVA, EVA/APP30, and EVA/PPOA30 were 0.059, 0.034, and 0.052 g/s, respectively, while the value was only 0.018 g/s for EVA/APP22.5/PPOA7.5. Evidently, the MLR of EVA/APP was greatly slowed after incorporation of PPOA. Additionally, because of the incorporation of PPOA, EVA/APP22.5/PPOA7.5 could generate a protective char layer (as can be seen in Figure 5) during combustion, which could prevent oxygen and heat transfer to the undegraded EVA, which resulted in incomplete combustion. Therefore, the TSR, mean SEA, and CO/CO2 of EVA/APP/PPOA were the highest among the values of the three flame-retardant samples.

Table 4. Tensile Properties of Neat EVA and Its FlameRetardant Composites sample EVA EVA/APP30 EVA/ APP22.5/ PPOA7.5

EVA (wt %)

APP (wt %)

PPOA (wt %)

tensile strength (MPa)

elongation at break (%)

100 70.0 70.0

0 30.0 22.5

0 0 7.5

21.1 ± 0.7 13.1 ± 0.4 10.2 ± 0.2

760 ± 25 581 ± 12 532 ± 25

(1)

remarkably weakened the tensile strength of EVA. Moreover, the tensile strength and elongation at break for EVA/APP22.5/ PPOA7.5 further slightly decreased compared with the corresponding value of EVA/APP30, which might be ascribed to the low density of PPOA. Generally, the large volume of filler easily leads to a stress concentration and results in the deterioration of mechanical properties of the polymer matrix. 3.5. Effect of PPOA on the Combustible Performance of EVA. The cone calorimeter (CC) is an effective method for predicting the combustion behavior of flame-retarded polymer materials in a real fire.29,36 Here, the heat release rate (HRR), total heat release (THR), and smoke production rate (SPR) were tested for EVA, EVA/APP30, EVA/PPOA30, and EVA/

Figure 4. CC result of EVA and its flame-retardant systems: (a) HRR, (b) THR, and (c) SPR. 7135


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Industrial & Engineering Chemistry Research Table 5. CC Results of EVA and Its Flame-Retardant Composites sample

TTI (s)

PHRR (kW/m2)

THR (MJ/m2)

peak SPR (m2/s)

TSR (m2/m2)

mean SEA (m2 kg)

CO/CO2

average MLR (g/s)

EVA EVA/APP30 EVA/PPOA30 EVA/APP22.5/PPOA7.5

66 65 39 63

608.3 295.3 458.7 185.2

103.6 83.3 102.4 82.8

0.073 0.046 0.048 0.024

1245.3 1064.0 980.0 1231.9

462.1 460.9 321.7 526.2

0.001 0.015 0.012 0.015

0.059 0.034 0.052 0.018

Figure 5. Digital photos of the residues obtained from the CC test: (a) EVA, (b) EVA/APP30, (c) EVA/PPOA30, and (d) EVA/APP22.5/ PPOA7.5.

Compared with our previous work,35 the application of PPOA not only improved the flame retardancy of EVA/APP but also dramatically prolonged the time to ignition to 63 s for EVA/APP/PPOA, which is much longer than 40 s for the EVA/APP/PPPA system reported before; therefore, obvious progress was made in this work compared with the previous result. To study the role of the residue formed during the thermal decomposition process, the morphology of the residues after the CC test for neat EVA, EVA/APP30, EVA/PPOA30, and EVA/APP22.5/PPOA7.5 was investigated through digital photos. Figure 5 shows that there was no char left for neat EVA after the CC test. A gossamer and loose char layer with obvious cracks was formed for EVA/APP30. The char layer with poor quality is inadequate for isolation of the transfer of heat and flammable gas; thus, it cannot remarkably improve the flame retardancy of EVA. However, a tough and compact char layer with sufficient strength was observed after the combustion process for EVA/APP22.5/PPOA7.5, which acted as an efficient insulating barrier and prevented the heat and mass transfer between the material underneath and the flame zone.34 The digital photos directly illustrate that an intumescent char layer with a good barrier effect was formed for the EVA/APP/ PPOA system during the combustion process. To further confirm the improvement in the quality of the residue for EVA/APP22.5/PPOA7.5 compared with that of EVA/APP30, SEM measurement was performed for these flame-retardant composites. Figure 6 shows that there are a large number of defects and cracks for the char layer of EVA/ APP30, which is discontinuous, while the microstructure of the residue for EVA/APP22.5/PPOA7.5 is quite compact and continuous. The char layer with compact and continuous microstructures could prevent the combustible gases contacting oxygen and slow the external heat transfer from the burning area to the unburned EVA.39,40 The distinct difference for the microstructures of char layers may lead to the remarkable change in the flame retardancy of these materials. Judging from OI, UL-94, and CC results, we conclude that the distinct difference for the structures of char layers must be an important reason for the remarkable change in flame retardancy of different EVA systems. The formation of a

Figure 6. SEM micrographs of the residues of EVA/APP30 (a, 1000×; a1, 5000×) and EVA/APP22.5/PPOA7.5 (b, 1000×; b1, 5000×).

compact, tough, and intumescent char layer is very conducive to achieving the excellent flame retardancy of the EVA/APP/ PPOA system and decreasing its peak of SPR. 3.6. Charring Mechanism of PPOA and Its Influence on the Flame Retardation of EVA/APP. To explain the charring process of PPOA and its contribution to the flame retardancy of the EVA composite, its condensed phases formed at 400, 500, 600, and 700 °C were investigated by FTIR, and these results are shown in Figure 7. The peaks at 2926 and 2849 cm−1 are assigned to the stretching vibration of −CH2−;41 the peak at 1004 cm−1 corresponds to the stretching vibration of C−N. The three peaks disappeared at 500 °C, which should be ascribed to the dehydrogenation of the piperazine ring and the generation of pyrazine derivatives; meanwhile, the absorbing peak at 2169 cm−1 appeared at this temperature, indicating that a few structures containing −N CO or −C−N bonds were formed. Moreover, the peak at 1636 cm−1 corresponding to the stretching vibration of CO 7136


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Table 6. TG Data of APP, PPOA, and APP/PPOA under N2 and Air Atmospheres char residue (wt %) T5% (°C)

sample APP (N2) PPOA (N2) APP/PPOA(3:1) (N2) APP/PPOA(3:1) (N2) APP (air) PPOA (air) APP/PPOA(3:1) (air) APP/PPOA(3:1) (air)

Figure 7. FTIR spectra of PPOA and its char residue formed at different temperatures.

became weaker when the temperature was increased from 25 to 700 °C, meaning that the majority of the carbonyl gradually decomposed during the decomposition process. Additionally, the TGA test of APP/PPOA was employed to investigate the effect of PPOA on the flame retardation of EVA/APP. The TG results for APP, PPOA, and APP/ PPOA(3:1) are shown in Figure 8 and Table 6. The decomposition of APP shows two stages. The first stage is assigned to the elimination of NH3 and H2O; the second stage is ascribed to the formation of a stable cross-linked polyphosphoric acid,42−44 which could effectively catalyze the dehydration, dehydrogenation, and cross-linking reactions in the IFR system.17 To confirm whether the interaction occurred between APP and PPOA, the theoretical TG result for APP/ PPOA(3:1) was calculated according to eq 2.

400 °C 500 °C 600 °C 700 °C

exptl

318.6 398.4 323.4

85.3 94.9 78.0

80.1 14.8 63.7

53.5 12.4 29.1

20.5 11.6 22.6

calcd

−

87.7

63.7

43.2

18.3

exptl

316.8 316.2 316.5

84.7 59.9 75.5

79.5 22.4 69.0

48.2 11.3 47.1

23.2 1.5 11.4

calcd

−

78.5

65.2

39.0

17.8

On the basis of the analysis presented above, we conclude that some chemical interactions must occur between PPOA and APP during decomposition. To further illuminate the charring mechanism of PPOA and the interaction between PPOA and APP, the Py-GC/MS test was performed for APP, PPOA, and APP/PPOA(3:1), and these results are shown in Figure 9 and Table 7. With the

M(cal)APP/PPOA(3:1) = [M(exp)APP × 75 + M(exp)PPOA × 25]/100 (2)

Under a nitrogen atmosphere, the char residues of APP/ PPOA(3:1) at 400 and 600 °C were 78.0 and 29.1 wt %, respectively, both of which are lower than the corresponding calculated value. However, the char residue of APP/PPOA(3:1) at 700 °C was 22.6 wt %, which is higher than the theoretical value, implying that the dehydrogenation and decomposition of PPOA at a low temperature were accelerated by APP, while its decomposition at a high temperature was inhibited under a nitrogen atmosphere. Similarly, the char residue of APP/ PPOA(3:1) at 400 °C was lower than the corresponding calculated value under an air atmosphere, while the char residue at 600 °C was higher than the corresponding theoretical value.

Figure 9. Pyrograms of APP, PPOA, and APP/PPOA(3:1).

temperature increased to 600 °C, APP released NH3 and H2O; meanwhile, some cross-linked structures were also produced, such as polyphosphoric acid.45 Because the detection limit of this instrument is 30 (m/z ≥30) and the cross-linked products

Figure 8. Experimental and theoretical TG curves under (a) N2 and (b) air atmospheres. 7137


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Industrial & Engineering Chemistry Research Table 7. Compounds Identified from the Pyrograms of APP, PPOA, and APP/PPOA(3:1)

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Industrial & Engineering Chemistry Research Scheme 2. Charring Mechanism of PPOA and Flame-Retardant Mechanism of EVA/APP/PPOA

which further demonstrates that the chemical interaction between PPOA and APP indeed occurred. On the basis of FTIR, TGA, and Py-GC/MS results, the charring mechanism of PPOA and the flame-retardant mechanism of EVA/APP/PPOA are described as follows (Scheme 2). When the temperature increased to 400 °C, the partial carbonyl, amido, and C−N bonds in PPOA began to fracture; thus, some volatile products, including carbon dioxide, isocyanatoethane, aldehyde, amide, and urea, were produced. Meanwhile, the dehydrogenation, rearrangement of the piperazine ring, and cyclizing of its decomposition segments occurred, which resulted in the generation of a large number of heterocyclic aromatic structures that endowed the PPOA with excellent charring capability. Moreover, APP showed a catalyzing effect on the carbonization of PPOA, and some new structures containing benzene derivatives, pyrazine derivatives, pyrimidine derivatives, etc., were generated, which could further improve the charring capability of PPOA at high temperatures. Finally, the flame retardancy of EVA/APP was greatly improved because of the formation of an efficient protective char layer in the presence of PPOA, and excellent flame retardancy of EVA/APP/PPOA was achieved.

did not enter the gas phase, the pyrolysis product of APP was not detected. When PPOA was heated to 600 °C, different kinds of pyrolysis products were detected, in which carbon dioxide, isocyanatoethane, pyrazine, 4-(2-oxoacetyl)piperazine1-carbaldehyde, and 1,1,3,3-tetraethylurea, corresponding to peaks 1−3, 10, and 12, respectively, are the major components. The main pyrolysis process for PPOA is summarized as the following three steps. The conjugated carbonyl might directly break and then result in the generation of a mass of carbon dioxide and isocyanatoethane corresponding to peaks 1 and 2, respectively; because of the fracture of the partial amido bond and the C−N bond in the piperazine ring, some imidazolidinone, aldehyde, amide, and urea were produced, corresponding to peaks 7 and 10−12, respectively. Most important is the fact that the dehydrogenation and rearrangement reactions of piperazine rings occurred, which are accompanied by the two processes described above, leading to the generation of numerous heterocyclic aromatic structures, such as pyrazine derivatives, isoquinoline, and 2,2′-bipyrazine (peaks 3−6 and 8). Generally, aromatic structures may make a great contribution to the formation of a stable char layer,46 so the aromatic structures formed during the decomposition of PPOA can endow it with excellent charring capability. Here, it should be noted that the gas components detected in the Py-GC/MS test were derived from the pyrolysis products of samples, and part of them must be involved in the charring process; therefore, the analysis of the structures of the gas components directly reflects the structures of the char layer.35 Obviously, the Py-GC/MS result is in accordance with the FTIR result. For APP/PPOA(3:1), benzene, pyrrole, 6-ethyl-1-methylindolizine, etc., corresponding to peaks 13−26 were detected in its pyrolysis products besides these structures produced during the pyrolysis of individual forms of APP and PPOA, which must be from the pyrolysis of PPOA, implying that the APP indeed catalyzed the carbonization of PPOA during the thermal decomposition process, which led to the residue of APP/ PPOA(3:1) being higher than the corresponding theoretical value at a high temperature in the TG test. These new aromatic structures that contain benzene derivatives, pyrazine derivatives, pyrimidine derivatives, etc., can greatly promote the formation of abundant char residue and, furthermore, lead to the formation of a stable char layer during the combustion process. The Py-GC/MS result is in accordance with the TGA result,

4. CONCLUSION A novel polyamide charring agent PPOA with high thermal stability and thermo-oxidative stability was prepared successfully. The excellent charring capability of PPOA dramatically improved the flame retardancy of EVA/APP. The OI value of EVA with 25 wt % APP/PPOA(3:1) reached 31.5%, and the V0 rating was achieved in this case; in addition, its time to ignition was 63 s, which was much longer than that of an efficient EVA/APP system containing charring agent PPPA. The study of the effect of PPOA on the flame retardancy of EVA/APP demonstrated that numerous heterocyclic aromatic structures were produced during the thermal decomposition of PPOA. Additionally, the catalytic action of APP on the charring of PPOA resulted in the generation of some new aromatic structures. On the basis of these physical and chemical actions mentioned above, highly efficient charring of PPOA was achieved, which finally resulted in the excellent flame retardancy of the EVA/APP/PPOA system. 7139


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AUTHOR INFORMATION

Corresponding Authors

*Telephone and fax: +86-28-85410259. E-mail: dengcong@scu. edu.cn. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the National Natural Science Foundation of China (Grant 51421061) and the Program for Changjiang Scholars and Innovative Research Team of the University of Ministry of Education of China (IRT1026) is sincerely acknowledged.

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A Novel Linear-Chain Polyamide Charring Agent for the Fire Safety of

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