Thermal Stability, Combustion Behavior, and Toxic Gases in Fire

Article pubs.acs.org/IECR

Thermal Stability, Combustion Behavior, and Toxic Gases in Fire Effluents of an Intumescent Flame-Retarded Polypropylene System Jun-Sheng Wang,†,* Guo-Hui Wang,† Yun Liu,‡,* Yun-Hong Jiao,§ and Dan Liu† †

Tianjin Fire Research Institute of the Ministry of Public Security, Tianjin 300381, China College of Chemistry and Chemical Engineering, Wuhan Textile University, Wuhan 430073, China § Flame retardant Laboratory, College of Chemistry and Environmental Science, Hebei University, Baoding 071002, China ‡

S Supporting Information *

ABSTRACT: Flame-retarded polypropylene (FRPP) was prepared with pentaerythritol and melamine phosphate as flame retardant, organic montmorillonite (OMMT) as synergistic agent, and PP by reactive extrusion. TGA testing indicated that the Tonset temperatures of FRPPs are lower than that of pure PP; however, the Tmax temperatures of FRPPs are higher than that of pure PP. The LOI and UL-94 tests indicate that the flame retardancy of FRPPs could be enhanced obviously with a low dosage of OMMT. The cone calorimeter coupled with online FTIR test shows that the loading of intumescent flame retardant (IFR) significantly reduces the heat release rate, smoke production rate, total heat release, total smoke production, mass loss rate, and toxic gases in fire effluents of the FRPPs, and the low dosage of OMMT in FRPPs further reduces the above corresponding values. Moreover, the addition of OMMT could decrease the production of HCN and NOx in fire effluents of FRPPs.

1. INTRODUCTION Polypropylene (PP) is widely utilized in the building, transport, electrical, and furniture industries.1 However, a drawback of PP is that it is inherently combustible; thus, many studies have focused on improving the flame retardancy of PP.2,3 As is wellknown, recently, intumescent flame retardant (IFR) has been usually utilized for polymer materials due to its advantages of low heat release, low smoke, and antidripping during burning.4−7 However, compared with halogen-containing flame retardants, it has some shortcomings such as low flameretardant efficiency, poor compatibility, and moisture sensitivity.8,9 In reactive extrusion, a novel IFR system is prepared with pentaerythritol (PER) and melamine phosphate (MP) as flame retardants and PP as the carrier resin during the process of extrusion in a twin-screw extruder, to improve the compatibility of IFR and the water resistance of PP/IFR.10 To improve the flame-retardant efficiency, many synergistic agents have been added to IFR systems, such as zeolites,9,11 metal compounds,12−19 organically modified montmorillonite (OMMT),20−22 and layered double hydroxide (LDH).23−25 A lot of papers have demonstrated that a low dosage of synergistic agents can effectively enhance the flame retardancy of flameretarded materials.26,27 The major cause of death or permanent injury in fires results from inhalation of toxic gases.28 Recently, it has been shown that online Fourier transform infrared spectroscopy (FTIR) is indeed a useful technique to analyze concentrations of the main toxic gases in fire effluents.29,30 A cone calorimeter coupled with online FTIR (CC−FTIR) is a useful technique to investigate burning behavior and toxic gases in fire effluents of materials, which could lead to the evaluation of the potential fire risk of materials including heat risk and fire effluent hazard.31−33 The studied IFR contains the elements of carbon, nitrogen, and phosphorus, which may be turned into toxic gases such as CO, CO2, HCN, and NOx or lead to toxic gas yields © 2014 American Chemical Society

during burning. However, there are few reports on the toxic gases in fire effluents of flame-retarded PP with IFR. In this paper, a flame-retarded master batch obtained by reactive extrusion with IFR as flame retardant and PP as a carrier is used to enhance the flame retardacny of PP along with OMMT. The thermal stability, flame retardancy, burning behavior, smoke production, and toxic gases in fire effluents of PP and flame-retarded PP were systematically investigated by thermogravimetric analysis (TGA), the limiting oxygen index (LOI), the vertical burning test (UL-94), and CC−FTIR.

2. EXPERIMENTAL SECTION 2.1. Materials. Industrial pentaerythritol (PER) was supplied by Bazhou Shengfang United Chemical Co., Ltd. (Bazhou, China). Industrial melamine phosphate (MP) was supplied by Shandong Shian Chemical Co., Ltd. (Dezhou, China). The OMMT, I.44P, was supplied by Nanocor (USA). Commercial polypropylene (PP, T30S) was provided by Petrochina Dagang Petrochemical Company (Tianjin, China). 2.2. Preparation of Flame-Retarded PP Samples. The flame-retarded (FR) master batch was prepared by reactive extrusion by mixing MP, PER (nMP:nPER = 1.6:1), OMMT, and PP (as a carrier) in a counterrotating twin-screw extruder (diameter 20.5 mm, length/diameter 44, Model CTE 20, Kebeilong Keya Nanjing Machinery Co., Ltd., Nanjing, China) at a temperature profile of 240, 250, 260, 260, and 250 °C and at 100 rpm. The extruded strands were cut into pellets. The obtained FR master batch was blended with PP in the desired proportions in a twin-screw extruder at a temperature profile of Received: Revised: Accepted: Published: 6978

January 19, 2014 April 1, 2014 April 2, 2014 April 2, 2014 dx.doi.org/10.1021/ie500262w | Ind. Eng. Chem. Res. 2014, 53, 6978−6984

Industrial & Engineering Chemistry Research

Article

170, 190, 195, 200, 195, and 185 °C. The extruded strands were cut into pellets. Then the composites were injected into testing bars according to the standards. The formulations of the FRPP matrix composites are summarized in Table 1. Table 1. Formulations of FRPPs sample

PP (%)

IFR (%)

OMMT (%)

auxiliaries (%)

PP FRPP1 FRPP2 FRPP3 FRPP4

100 79.0 74.0 79.0 74.0

0 20 25 19.0 24.0

0 0 0 1.0 1.0

0 1.0 1.0 1.0 1.0

2.3. Measurements. Thermal Decomposition Behaviors. Thermogravimetric analysis (TGA) was performed with a TG 209 F3 (NETZSCH, Germany) thermogravimetric analyzer at temperatures ranging from 40 to 800 °C with a constant heating rate of 10 °C/min, and under N2 atmosphere at a flowing rate of 50 mL/min. Flame Retardance and Burning Behaviors. The LOI tests were conducted on a JF-3 limiting oxygen index meter (Jiangning, China) according to ASTM D2863-97 at room temperature, and the dimensions of the samples were 130 × 6.5 × 3.2 mm3.Vertical burning tests (UL-94) were measured by using a horizontal and vertical burning test instrument (CZF-2) (Jiangning, China) with samples dimensions of 130 × 13 × 3.2 mm3 according to ASTM D3801.The combustion behaviors of PP and FRPPs were measured on a cone calorimeter (FTT, U.K.) under a heat flux of 50 kW/m2 with the sheet dimensions of 100 × 100 × 6 mm3 according to ISO 5660-1. The components and concentrations of toxic gases of fire effluents during fire testing were characterized according to ISO 19702 by Fourier transform infrared spectroscopy (FTIR) coupled with a cone calorimeter by a heat line; the flow rate of fire effluents was 4 L/min and the temperature of the sampling line and sample cell was 180 °C.

Figure 1. TGA curves of PP, FRPP1, FRPP2, FRPP3, and FRPP4 in N2.

Figure 2. DTG curves of PP, FRPP1, FRPP2, FRPP3, and FRPP4 in N2.

Table 2. TGA Data of PP, FRPP1, FRPP2, FRPP3, and FRPP4 in N2

3. RESULTS AND DISCUSSION 3.1. Thermal Degradation Behaviors. In order to understand the effect of OMMT on the flame retardancy of FRPP, the thermal degradation behaviors of PP and FRPPs were investigated by thermogravimetric analysis (TGA). TGA and derivative thermogravimetric (DTG) curves are shown in Figures 1 and 2, respectively, and the relevant data are summarized in Table 2. It can be seen that the onset degradation temperature (Tonset, defined as the temperature at which 5% mass loss occurs) of PP is 421 °C, which is much higher than those of the FRPP samples, 326, 353, 345, and 311 °C for FRPP1, FRPP2, FRPP3, and FRPP4, respectively. TGA behaviors of FRPPs are similar under the experimental condition. However, it can be seen that much more residues are obtained for the FRPPs than for pure PP above 434 °C. Figure 1, Figure 2, and Table 2 show that the loading of IFR decreases the Tonsets of the FRPP samples and the loading of 1 wt % OMMT further decreases the Tonsets of the FRPP samples, which suggests that the low loading of OMMT accelerates the thermal degradation of FRPPs. However, the maximaldegradation-rate temperatures, Tmaxs, of the FRPPs are higher than that of pure PP and Tmaxs of FRPP3 and FRPP4 are higher than those of FRPP1 and FRPP2, which could be primarily attributed to the formation of the intumescent char and the synergic effects of OMMT on charring.

residues (%) sample

Tonset (°C)

Tmax (°C)

rate of Tmax (%/min)


421 326 353 345 311

456 461 461 464 466

32.87 22.68 23.43 21.90 19.10

500 °C 600 °C 700 °C 0.51 10.77 10.98 12.81 16.67

0 10.04 10.29 11.46 14.48

0 10.04 10.29 11.46 14.42

Figure 1, Figure 2, and Table 2 show that the loading of IFR reduces the maximum degradation rate of the FRPP samples and the maximum degradation rates of FRPPs with 1.0 wt % OMMT samples are smaller than those of FRPPs with only IFR, which demonstrates that the residues formed under heat could protect the matrix effectively and the addition of OMMT could enhance the effect. Obviously, after 480 °C, the FRPP samples with 1 wt % OMMT are more stable than the FRPP samples without OMMT, with more residues remaining. At 700 °C, the amounts of the residues for pure PP, FRPP1, FRPP2, FRPP3, and FRPP4 are 0, 10.02, 10.29, 11.46, and 14.42%, respectively. This suggests that the addition of OMMT could promote FRPP to form char and enhance the stability of the 6979

dx.doi.org/10.1021/ie500262w | Ind. Eng. Chem. Res. 2014, 53, 6978−6984


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char, and there exists an obvious synergistic flame-retarded effect between OMMT and IFR. 3.2. Burning Behaviors. LOI and UL-94 Tests. The LOI values and UL-94 ratings of pure PP and the FRPPs are summarized in Table 3. The LOI value of FRPP1 with the Table 3. Flammability of PP and Flame-Retarded PP sample

PP (wt %)

IFR (wt %)

OMMT (wt %)

LOI (%)

UL-94


100 79 74 79 74

0 20 25 19 24

0 0 0 1 1

18.1 29.0 34.0 31.5 35.0

NR V-2 V-0 V-0 V-0

loading of 20 wt % IFR is 29.0; that is quite high for PP. The LOI value of FRPP2 is 34 with a UL-94 V-0 rating. From Table 3, it can be seen that the LOI values of samples with 1 wt % OMMT increase, from 29.0 to 31.5 for FRPP3 and from 34.0 to 35.0 for FRPP4, respectively. A UL-94 V-2 rating and melt drippings with fire are observed for FRPP1; however, the addition of 1 wt % OMMT could prevent melt dripping and the rating of UL-94 test is enhanced from V-2 to V-0. The low loading of only OMMT could not enhance the flame retardancy of polyolefin.34 Therefore, from the results of LOI and UL-94 tests, the conclusion that there exists a synergistic effect between OMMT and IFR could be drawn, which is in agreement with the TGA tests. Heat Release. The combustion behaviors of pure PP and FRPPs were investigated with a cone calorimeter (CC) to identify the effect of OMMT on the fire behavior of FRPPs, and values such as the time to ignition (TTI), peak heat release rate (PHRR), average heat release rate (Av-HRR), total heat release (THR), and peak values of mass loss rate (PMLR) are summarized in Table 4. The heat release rate (HRR) is a measurement of the heat release per unit surface area of a burning sample in the CC test, and is one of the most important parameters as a predictor of fire hazard. The measured HRR curves for pure PP and FRPPs are presented in Figure 3. It can be observed that pure PP burned fast after ignition, and the HRR curve achieves a sharp peak with a PHRR value of 1290 kW/m2. The PHRR values of FRPP1 and FRPP2 with IFR are 383 and 265 kW/m2, respectively, and the combustion time of these composites was prolonged in comparison with that of pure PP, which indicates that the addition of IFR could greatly reduce the heat release of FRPPs. Compared with FRPP1 and FRPP2, the PHRR values of FRPP3 and FRPP4 were 228 and 197 kW/m2, respectively, which were further reduced, and the combustion time was further prolonged with the addition of 1 wt % OMMT. The barrier effect of OMMT inhibits mass/heat transfer, which is the reason for the reduction in PHRR values of OMMT-based polymer composites, acting as a condensed-phase mechanism.35 Figure 3 and Table 4 show that the time to PHRR (TTPHRR)

Figure 3. Heat release rate curves of PP and FRPPs.

of the FRPP samples increased significantly with the addition of IFR. Furthermore, the above results show other two clear characteristics caused by the loading of OMMT and IFR. First, the TTI values of FRPPs decrease with the addition of IFR, which may be due to initial combustion of the IFR before they could play their role in the matrix,36 or it may be due to more severe thermal decomposition of FRPPs with increasing applied heat flux as the intumescent layer approaches the cone heater.16 Second, Av-HRR values of the FRPP samples decrease significantly with the addition of IFR, and the Av-HRR values of FRPP3 and FRPP4 further decrease with the addition of 1 wt % OMMT, which could be attributed to the effectively protective effect of the compact char formed during the burning. Moreover, PHRR and Av-HRR values of FRPP3 with 19 wt % IFR and 1 wt % OMMT are further less than those of FRPP1 with 25 wt % IFR. As shown in Figure 4, the chars of

Figure 4. Photographs of residue of samples after CC test.

FRPP3 and FRPP4 with 1.0 wt % OMMT are very compact and strong, while the chars of FRPP1 and FRPP2 are more fragile and cracked. Thus, the effectiveness of the chars of FRPP1 and FRPP2 in reducing mass/heat transfer is less than that of the chars of FRPP3 and FRPP4. The char strength may also play an important role in retaining flammable degradation products, which may be the reason for a prolonged burning time of the FRPP samples with 1 wt % OMMT. As shown in

Table 4. Combustion Parameters Obtained from Cone Calorimeter Test sample

TTI (s)

PHRR (kW/m2)

TTPHRR (s)

Av-HRR (kW/m2)

THR (MJ/m2)

PSPR (m2/s)

TSP (m2)

mass (%)

PMLR (g/s)


42 25 24 23 24

1290 380 265 228 197

255 455 560 455 495

386 212 182 130 122

228 212 206 207 205

0.870 0.174 0.118 0.065 0.051

187 96 87 57 43

0 4.55 8.61 10.23 11.68

0.369 0.104 0.086 0.076 0.071

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for FRPP1 and FRPP2. The peak values of mass loss rate (PMLR) are 0.369, 0.104, 0.086, 0.076, and 0.071 g/s, respectively, for pure PP, FRPP1, FRPP2, FRPP3, and FRPP4. The above results indicate that the char formed could prevent the PP matrix from thermal decomposition and blocks heat/mass transfer, and that the addition of 1 wt % OMMT could enhance the effect. Smoke Production and Toxic Gases. It is well-known that, in the case of fire, most deaths and fire injuries result from fire effluents. The smoke production and toxic gases in fire effluents of the samples were investigated with a cone calorimeter coupled with FTIR, which could be powerful for indicating the fire risk of fire effluents for flame-retarded materials under wellventilated conditions. The smoke production release (SPR) and total smoke production (TSP) curves of PP and FRPPs are shown in Figures 7 and 8, respectively. The SPRs of FRPP1 and FRPP2

Figure 4, there is no residue left for pure PP, but there are much more residues for the FRPP samples. Figure 5 presents the total heat release (THR) curves of PP and FRPPs. From Table 4, at the end of the CC test, pure PP

Figure 5. Total heat release curves of PP and FRPPs.

has released a total heat of 228 MJ/m2, while the FRPP samples have released 212, 206, 207, and 205 MJ/m2 for FRPP1, FRPP2, FRPP3, and FRPP4, respectively. This suggests that the effects of addition of IFR and OMMT on reducing THR values at the end of burning are very small. However, as can be observed in Figure 5, the loading of IFR slows the THR growth of FRPP1 and FRPP2, and the loading of 1 wt % OMMT further slows the THR growth of FRPP3 and FRPP4. This indicates that the char formed could prevent the heat/mass transfer and the addition of 1 wt % OMMT could enhance barrier effects of the char. Mass loss curves of the samples during the CC test are shown in Figure 6. There are 0, 4.55, 8.61, 10.23, and 11.68% of

Figure 7. Smoke production rate of PP and FRPPs.

Figure 8. Total smoke production of PP and FRPPs. Figure 6. Mass loss (ML) of PP and FRPPs.

with addition of IFR decreased significantly, and the SPRs of FRPP3 and FRPP4 further decrease with addition of 1 wt % OMMT. The peak SPR (PSPR) values for FRPP1, FRPP2, FRPP3, and FRPP4 are 0.174, 0.118, 0.065, and 0.051 m2/s, respectively, which are lower than that for pure PP, 0.870 m2/s. This is in agreement with other parameters obtained from cone calorimeter test. From Figure 8 and Table 4, it can be seen that the TSP values of PP and the FRPP samples are 187, 96, 87, 57,

residues left at the end of burning, respectively, for pure PP, FRPP1, FRPP2, FRPP3, and FRPP4. From Table 4 and Figure 6, it can be observed that the loading of IFR increases the amount of residues for FRPP1 and FRPP2, and the loading of 1 wt % OMMT further increases the amount of residues for FRPP3 and FRPP4. The rates of mass loss for FRPP1 and FRPP2 are lower than that for PP at the same burning time, and those for FRPP3 and FRPP4 are further lower than those 6981



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and 43 m2, respectively, for neat PP, FRPP1, FRPP2, FRPP3, and FRPP4. The above results demonstrate that the loading of IFR largely reduces the SPR and TSP values of the FRPP samples, and the loading of OMMT further reduces the values mentioned above of FRPP3 and FRPP4, which indicates that OMMT has a more obvious effect than IFR does on smoke suppression for the FRPP samples. The toxic gases in fire effluents of the samples were examined by online FTIR during cone calorimeter testing. The main toxic gases detected in fire effluents of PP and FRPP are CO2, CO, HCN, and NOx. The concentration curves are shown in Figures 9, 10, 11, and 12, respectively, and the maximum

Figure 11. Concentration of HCN in fire effluents of PP and FRPPs.

Figure 9. Concentration of CO2 in fire effluents of PP and FRPPs.

Figure 12. Concentration of NOx in fire effluents of PP and FRPPs.

Table 5. Maximum Concentration Values of Toxic Gases in Fire Effluents of PP and FRPPs during Fire Testing sample

CO2 (%)

CO (μL/L)

HCN (μL/L)

NOx (μL/L)


2.84 0.65 0.44 0.4 0.4

618 156 106a 107a 111a

5 12 10 6 8

11 41 29 18 19

a

The values are the maximum concentration values near the end of burning.

Figure 10. Concentration of CO in fire effluents of PP and FRPPs.

neat PP, which suggests that only a little of the nitrogen element in IFR transforms into HCN. From Figure 12, it is shown that the addition of IFR containing nitrogen badly increases the NOx in fire effluents of FRPP; however, the addition of OMMT could significantly decrease the NOx release during burning. Compared with Figures 10 and 12, the toxic gases containing nitrogen in fire effluents of FRPPs are mainly NOx, which is much less toxic than HCN. From the results of toxic gases, it is indicated that IFR could significantly decrease the concentrations of CO2 and CO and slightly increase the concentrations of HCN and NOx in fire effluents of FRPPs, and the addition of OMMT could significantly decrease the concentrations of CO2, CO, HCN, and NOx in fire effluents of FRPPs.

concentration values of the toxic gases are summarized in Table 5. As shown in Figure 9, the loading of IFR could decrease the CO2 release rate of FRPPs significantly, and the addition of OMMT further decreases the CO2 release rate of FRPPs. The curve of CO concentration in fire effluents of FRPPs in Figure 10 is very similar to that of CO2 in Figure 9, increasing near the end of burning, which is the result of decomposition of residues formed during burning. As is well-known, HCN and nitrogen oxides (NOx) are the important toxic gases in fire effluents. Thus, it is worth noting the potential fire hazard of fire effluents yielded from the nitrogen-containing compounds. As shown in Figure 11, the HCN maximum concentration values of FRPPs with only IFR are about 9−12 μL/L (ppm) and maximum concentration values of FRPPs with OMMT are close to that of 6982



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From the results of fire effluents, it is clear that FRPPs with only IFR have low potential fire risk, and that OMMT could decrease the potential fire risk of FRPPs significantly.

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4. CONCLUSIONS The combustion behaviors of PP are enhanced by introducing IFR and are further improved by the loading of 1 wt % OMMT. It is clear from the TGA test that the onset thermal degradation temperatures of the FRPP samples with IFR are lower than that of pure PP; however, the maximal-degradation-rate temperatures of the FRPP samples are higher than that of pure PP. The LOI values of FRPP3 and FRPP4 with 1.0 wt % OMMT are 31.5 and 35, respectively, which are higher than those of FRPP1 and FRPP2, whose LOI values are 29 and 34, respectively. Meanwhile, the UL-94 rating of FRPP3 with 1.0 wt % OMMT is improved from V-2 to V-0. The cone calorimeter test demonstrates that the loading of IFR significantly reduces the heat release rate, total heat release, and mass loss rate of the FRPP samples, and the loading of 1.0 wt % OMMT further reduces the above corresponding parameters of FRPP3 and FRPP4 with the same total loading of IFR. From the results of smoke production and toxic gases analysis, the addition of IFR could decrease the SPR, TSP, and CO2 and CO in fire effluents of FRPPs significantly; meanwhile, the addition of OMMT could further decrease the SPR, TSP, and toxic gases concentration in fire effluents of FRPPs. All results mentioned demonstrate that OMMT has a significantly synergistic effect on the flame retardancy of FRPP, and also has the effect of smoke suppression and toxic gases reduction in fire effluents for the FRPP samples.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

FTIR spectra of samples obtained by Gasmet FTIR during fire testing for 10 s, for 180 s, and at the end. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Authors

*Tel.: +86-22-23383501-2501. Fax: +86-22-23950119. E-mail: [email protected]. *Tel.: +86-27-59367434. Fax: +86-27-59367434. E-mail: yliu@ wtu.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to acknowledge the financial support of the China Postdoctoral Science Foundation funded project (2012M520565), National Basic Research Program of China (973 Program) (2012CB719701), and National Science Foundation of China (51203126).

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REFERENCES

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