Effects of Dimethyl Methylphosphonate, Aluminum Hydroxide

Publication Date (Web): May 15, 2015. Copyright © 2015 ... In the presence of ATH and APP, dense spherical substances were produced. When EG was adde...
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Effects of Dimethyl Methylphosphonate, Aluminum Hydroxide, Ammonium Polyphosphate, and Expandable Graphite on the Flame Retardancy and Thermal Properties of Polyisocyanurate− Polyurethane Foams Yanlin Liu, Jiyu He,* and Rongjie Yang National Laboratory of Flame Retardant Materials, School of Materials, Beijing Institute of Technology, 5 South Zhongguancun Street, Haidian District, Beijing 100081, P. R. China S Supporting Information *

ABSTRACT: For the first time, a new flame-retardant formula based on dimethyl methylphosphonate (DMMP), aluminum hydroxide (ATH), ammonium polyphosphate (APP), and expandable graphite (EG) was applied to polyisocyanurate− polyurethane foams and found to exhibit a high flame-retardant efficiency and low cost, to be environmentally friendly, and to allow for the reduction of the amount of solid flame retardants added. The multiple effects were evaluated based on thermal conductivity tests, compressive strength tests, limiting oxygen index (LOI) measurements, cone calorimetry tests, thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The results showed that ATH can form “villi”like substances during the combustion process. In the presence of ATH and APP, dense spherical substances were produced. When EG was added to the system, a wormlike carbon layer that adsorbed phosphate-containing acid resulting from APP decomposition was formed during the decomposition process, so that the carbon layer was denser. The combined effects of the villi-like and spherical substances as well as the wormlike carbon layer can block heat and flame propagation from being transferred to the unburned foam.

1. INTRODUCTION Rigid polyurethane foams (RPUFs) are important synthetic materials with relatively low densities because of their high porosities. Flexible polyurethane foams, high-resilience polyurethane foams, and rigid polyurethane foams have been produced using different raw materials or formulations. However, their applications are limited because of their poor flame retardancy. In recent years, more and more research has been focused on a modified polyurethane material, namely, polyisocyanurate−polyurethane (PIR−PUR) foams. When compared with conventional polyurethane foams, PIR−PUR foams have distinct advantages. First, PIR−PUR foams have lower thermal conductivities, typically between 0.019 and 0.021 W/m·K. That is, PIR−PUR foams exhibit superior thermal insulation properties. Second, PIR foams have a better flame retardancy than RPUFs. In general, it is difficult for the flame resistance of RPUFs to achieve level B2 (GB 8624-2012), whereas that of PIR−PUR foams can reach level B2 (GB 86242012) and even level B1 (GB 8624-2012). According to Chinese Standard GB 8624-2012, level B2 means that building materials and products are combustible, whereas level B1 means that they are flame-retardant. In addition, because of the higher closed-cell rate, PIR foams have less water absorption. The main reactions taking place during the preparation are as follows:1 (1) Reaction between hydroxyl and isocyanate groups to give PUR linkages © 2015 American Chemical Society

(2) Reaction between isocyanate and water to give urea linkages

(3) Cyclotrimerization of the isocyanates to give PIR rings

where

PIR−PUR foams have been widely used in wall thermal insulation boards, cold insulation materials, electrical engineering materials, and so on because of their excellent sealing performance and thermal insulation properties. However, the Received: Revised: Accepted: Published: 5876

March 19, 2015 May 13, 2015 May 15, 2015 May 15, 2015 DOI: 10.1021/acs.iecr.5b01019 Ind. Eng. Chem. Res. 2015, 54, 5876−5884

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Industrial & Engineering Chemistry Research flame retardancy of PIR−PUR foams should be improved because of the use of flammable blowing agents, due to the banning of chlorofluorocarbons and hydrochlorofluorocarbons, and because of the greater attention paid to the fire safety of materials. Improvements in flame retardancy are commonly achieved through the addition of flame retardants. The common flame retardants used in PIR−PUR foams to date are phospho-halogenated compounds. However, these retardants generally cause very dense and toxic smoke during burning.2,3 To decrease the density and toxicity of the smoke, there is an urgent demand to develop effective halogen-free flame retardants for PIR−PUR foams.1,4,5 Dimethyl methylphosphonate (DMMP) is a highly efficient halogen-free flame retardant that can significantly improve the limiting oxygen index (LOI). It is unreasonable to increase the amount of DMMP because it is a liquid flame retardant, which, when added in large amounts, can result in a migration to the surface of the PIR−PUR foams and cause the loss of the flame retardant and a reduction of the flame retardancy of the foams. In addition, DMMP is very expensive, so it is uneconomical to increase the amount of DMMP added to obtain satisfactory flame retardancy. Hydroxide flame retardants,6−8 such as aluminum hydroxide (ATH), magnesium hydroxide (MH), and layered double hydroxides (LDHs), are inexpensive, nontoxic, and smoke-suppressing halogen-free flame-retardant additives. To some extent, ATH can inhibit the generation of smoke but does not cause a significant change in the LOI value.9 Ammonium polyphosphate (APP) is an effective intumescent fire retardant for several kinds of polymer-based materials10,11 and, in particular, for PU. It is a high-molecularweight chain phosphate. Its efficiency is generally attributed to an increase in char formation through a condensed-phase reaction.12−14 Expandable graphite (EG) has been widely used in RPUFs in recent years. It is obvious for EG to enhance the LOI value, but it has little effect on the heat release.14 In addition, a large amount of EG is incorporated into RPUFs to obtain better flame retardancy of the matrix, which not only makes the foaming process harder but also sacrifices the mechanical properties and thermal conductivity of the materials. Therefore, in this work, foams containing 10 phr of DMMP (where phr is parts per 100 g of polyol) and different amounts of ATH were prepared to find the optimal ratio of DMMP and ATH. In addition, we also introduced the solid phosphoruscontaining flame-retardant ammonium polyphosphate (APP) and the intumescent flame-retardant EG into the system. The purpose of this study was to maximize the flame retardancy of the PIR−PUR foams at the lowest cost, while maintaining their physical and mechanical properties, as well as the thermal conductivity. An additional aim was to improve the durability of flame-retardant foams.

(3) 2,4,6-Tri(dimethylaminomethyl)phenol (DMP-30) was used as the trimerization catalyst for isocyanurate rings. Triethanolamine was used as a cross-linking agent for polyurethane. (4) 1,1-Dichloro-1-fluoroethane (141b), supplied by Hangzhou Fushite Chemical Industry Co. (Zhejiang, China), was used as the blowing agent. (5) The flame retardants used were dimethyl methylphosphonate (DMMP), aluminum hydroxide (ATH), and expandable graphite (EG). Dimethyl methylphosphonate (DMMP) with a purity of 99% was supplied by Beijing Donghua Rio Tinto Technology Development Co. (Beijing, China). Aluminum hydroxide (ATH) particles, with a particle size of 1−20 μm, were purchased from Beijing Chemical Factory and used without further purification. Expandable graphite (EG), ADT 350, was produced by Shijiazhuang ADT Carbonic Material Factory (Hebei, China). Its main properties were as follows: moisture, 0.56%; pH, 7.0; expansion rate, 350 mL g−1; volatile content, 17.1%; ash content, 4.8%; particle size (not less than 300 μm), 83%; purity, not less than 95%. Ammonium polyphosphate (APP) was purchased from Yantai Wanhua Polyurethanes (Yantai, China). (6) The polymethylene polyphenylene isocyanate (PM-200) was produced by Yantai Wanhua Polyurethanes. It had an NCO content of 31.3 wt % and a viscosity of 197 mPa·s−1 (25 °C). 2.2. Foam Preparation. PUR−PIR foams were prepared using a one-pot and free-rise method. First, components 1−5 were mixed with a high-speed stirrer for 1 min at room temperature until a uniform mixture was achieved. Then, component 6 was added to the reaction mixture, which was stirred for an additional 10 s at an elevated stirring speed, after which the mixture was quickly poured into an open paper mold (250 × 250 × 60 mm3) to obtain a free-rise foam. Finally, the PUR−PIR foams were kept in an incubator at 60 °C for 20 min to accelerate the curing process. After preparation, the samples were cut into the desired shapes and sizes according to the corresponding standards for the evaluation of different properties. Flame retardants were added at different concentrations, reported in units of parts per 100 g of polyol (phr). The reference sample, containing no flame retardants is denoted as Ref. First, 10 phr of DMMP and various amounts of ATH (i.e., 0, 5, 10, 15, and 20 phr) were used to prepare foams; these samples are denoted as ATH-0, ATH-5, ATH-10, ATH-15, and ATH-20, respectively. The second set of foams contained 10 phr of DMMP, 5 phr of ATH, and various amounts of APP (10, 15, 20, and 25 phr) and are denoted as APP-10, APP-15, APP20, and APP-25, respectively. The third set contained 10 phr of DMMP, 5 phr of ATH, 15 phr of APP, and various amounts of EG (10, 20, 25, and 40 phr) and are denoted as EG-10, EG-20, EG-25, and EG-40, respectively (Supporting Information, Table S1). 2.3. Measurements and Characterization. 2.3.1. Physical−Mechanical Characterization. The compressive strength was tested according to Chinese Standard GB/T 8813-2008 using samples with sheet dimensions of 100 × 100 × 100 mm3. The apparent density was measured according to standard method ISO 845. The thermal conductivity of samples was determined according to standard method ASTM C518-04 using specimens with dimensions of was 300 × 300 × 40 mm3.

2. EXPERIMENTAL SECTION 2.1. Raw Materials. The following components were used for the preparation of the PUR−PIR foams: (1) Polyol 380A, made from poly(propylene oxide) and a sucrose/glycerin base, was purchased from QingdaoLianmei Chemical Co. (Shandong, China). The main properties are as follows: density, 1.15 g cm−3; typical hydroxyl number, 380 mg of KOH/g; functionality, 5.8. (2) Silicone glycol copolymer 8811, used as a surfactant, was purchased from Shanghai Chemical Reagent Co. (Shanghai, China). 5877

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Industrial & Engineering Chemistry Research 2.3.2. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) was performed using a Netzsch 209 F1 thermal analyzer at a heating rate of 10 °C/min under a nitrogen atmosphere at temperatures ranging from room temperature to 800 °C. 2.3.3. Fire Behavior. The limiting oxygen index (LOI) was obtained according to Chinese Standard GB/T 2406.2-2009. An oxygen index instrument (Rheometric Scientific Ltd.) was used on barrels with dimensions of 10 × 10 × 80 mm. Combustion experiments were performed using a cone calorimeter device. Samples with dimensions of 100 × 100 × 30 mm were exposed to a radiant cone (45 kW/m2). The heat release rate (HRR), total heat release (THR), total smoke release (TSR), and other parameters were recorded simultaneously. 2.3.4. Morphological Characterization. Scanning electron microscopy (SEM) images were obtained using a Hitachi S4800 scanning electron microscope. The micromorphology of the residual char after combustion with a conductive gold layer was observed by low-temperature fracturing under a high vacuum at a voltage of 15 kV.

3. RESULTS AND DISCUSSION 3.1. Effects of ATH on the Properties of PIR−PUR Foams. 3.1.1. Physical−Mechanical Characterization. Figure

Figure 2. TG and DTG curves for the PIR−PUR/DMMP/ATH composites.

Figure 1. Thermal conductivity and compressive strength curves for the PIR−PUR/DMMP/ATH composites.

1 shows the thermal conductivity and compressive strength curves for foams with no flame retardants (Ref), with only 10 phr of DMMP (ATH-0), and with 10 phr of DMMP and different proportions of ATH. It can be seen that the thermal conductivity and compressive strength are negatively correlated in terms of the effect of ATH. Upon addition of 10 phr of DMMP to PIR−PUR foam, the thermal conductivity changed from 0.0266 to 0.0282 W/m·K, and the compressive strength decreased from 0.052 to 0.01 MPa. This indicates that DMMP can reduce the compressive strength and increase the thermal conductivity of PIR−PUR foams to some degree. Therefore, on this basis, a series of ATH proportions were added to foams containing 10 phr of DMMP. As shown in Figure 1, after addition of ATH, the compressive strength of the foams increased, and the thermal conductivity decreased. 3.1.2. Thermal Stability. The TG and DTG curves of foams with no flame retardants (Ref), with only 10 phr of DMMP (ATH-0), and with 10 phr of DMMP and different proportions

Figure 3. Limiting oxygen index curve for the PIR−PUR/DMMP/ ATH composites.

of ATH under a nitrogen atmosphere are shown in Figure 2. The peak weight losses appear at 330−360 and 450−510 °C and correspond to the decomposition of the urethane bonds and the thermal depolymerization reaction of polyisocyanates and polyols, releasing some gaseous products, respectively.15−23 The initial decomposition temperatures of the foams decreased after addition of DMMP and ATH, because the boiling temperature of DMMP is just 181 °C, so that it volatilizes easily, and the crystal water is removed when ATH is heated. 5878

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Figure 4. HRR curves for the PIR−PUR/DMMP/ATH composites.

Figure 6. Thermal conductivity and compressive strength curves for the PIR−PUR/DMMP/ATH/APP composites.

When compared with the reference sample, in the first stage, the samples containing flame retardants exhibited a significantly lower rate of weight loss, which can be seen from the DTG curves. This is due to the decomposition of DMMP. PO free radicals are released under a nonoxidative atmosphere during the DMMP degradation process.24 The PO free radicals produced by DMMP can quench the flammable active species away from the matrix and inhibit the decomposition intensity of the matrix. Therefore, DMMP plays an important role in the gas phase during decomposition. 3.1.3. Fire Behavior. Figure 3 displays the limiting oxygen index (LOI) curve for foams with no flame retardants (Ref), with only 10 phr of DMMP (ATH-0), and with 10 phr of

DMMP and different proportions of ATH, indicating that the limiting oxygen index increased from 22.4% to 25.2% upon the addition of 10 phr of DMMP. However, it is obvious that the addition of ATH did not have a significant effect on the limiting oxygen index, which is consistent with the literature.25 Figure 4 shows the heat release rate curves for foams with no flame retardants (Ref), with only 10 phr of DMMP (ATH-0), and with 10 phr of DMMP and different proportions of ATH. In addition, the data for the cone calorimeter tests are listed in Table S2 of the Supporting Information. The results show that the peak heat release rate was reduced by 20−30 kW/m2 upon addition of 5−25 phr of ATH. The total heat release decreased most significantly after addition of 5 phr of ATH, when

Figure 5. SEM images of the PIR−PUR/DMMP/ATH composites. 5879

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Figure 9. HRR curves for the PIR−PUR/DMMP/ATH/APP composites.

DMMP (ATH-0), which further confirms that DMMP plays a role in the gas phase and does not significantly contribute to the char. In addition, these results are consistent with those of TGA. Furthermore, ATH effectively produced “villi”-like particles, which are useful for the formation of dense char and can prevent heat and flame from spreading to unburned polymer. 3.2. Effects of APP on the Properties of PIR−PUR Foam. 3.2.1. Physical−Mechanical Characterization. The thermal conductivity and compressive strength curves of PIR− PUR foams containing 10 phr of DMMP, 5 phr of ATH, and different proportions of APP are shown in Figure 6. The data show that the thermal conductivity and compressive strength are negatively correlated in terms of the effect of APP. In short, the thermal conductivity first decreased and then increased, with the latter occurring at a minimum value of 15 phr of APP. In contrast, the compressive strength initially increased upon addition of APP and then decreased, reaching a maximum value at 15 phr of APP. This might be due to the relatively small size of APP powder. When the amount of APP added was less than 10 phr, APP performed as a nucleating agent; it was not sufficient to destroy the cell structure of the foam. Therefore, APP can play a supporting role in the system. When the amount of APP added was excessive, APP destroyed the cell structure, so that the compressive strength decreased and the thermal conductivity increased. This view can be confirmed from the SEM image shown below (Figure 10). 3.2.2. Thermal Stability. Figure 7 shows the TG and DTG curves of the samples containing 10 phr of DMMP, 5 phr of ATH, and different proportions of APP investigated under a nitrogen atmosphere. The sample without APP exhibited two visible degradation stages. Similarly, the samples with APP also exhibited two steps of decomposition, as reported previously.26,27 The first step, occurring at approximately 300 °C, consists essentially of elimination of NH3 and H2O, resulting in the formation of highly cross-linked polyphosphoric acid. The second step, at temperatures higher than 550 °C, corresponds to polyphosphoric acid evaporation and/or dehydration to P4O10, which sublimes.12 It can be seen from the DTG curves that, upon addition of APP, the decomposition of the first stage appeared in advance and the peak weight loss rate increased, which is due to the decomposition of APP and DMMP. The dense polyphosphoric acid formed from the decomposition of APP covered the

Figure 7. TG and DTG curves for the PIR−PUR/DMMP/ATH/APP composites.

Figure 8. LOI values for the PIR−PUR/DMMP/ATH/APP composites.

compared with the reference sample, specifically falling from 33.42 to 8.47 MJ/m2. At the same time, the total smoke release, CO emissions, and CO2 emissions changed from 738.47 to 248.91 m2/m2, from 0.206 to 0.008 kg/kg, and from 3.218 to 0.121 kg/kg, respectively. Therefore, ATH can significantly reduce smoke emissions and CO/CO2 release from PIR−PUR foams. 3.1.4. SEM Analysis of Chars. SEM images of the char of PIR−PUR/DMMP/ATH foams after cone calorimetry tests are presented in Figure 5. It can been seen that the chars were still relatively loose even after addition of only 10 phr of 5880

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Figure 10. SEM images of APP-15 (10 phr of DMMP, 5 phr of ATH, 15 phr of APP) after cone calorimeter tests.

Table 1. Contents (at. %) of Different Elements in Regions A and B element

region A

region B

C N O Al Si P

56.70 15.68 23.75 0.17 0.18 3.52

53.43 17.29 26.48 0.11 0.37 2.33

Figure 11. Thermal conductivity and compressive strength curves for the PIR−PUR/DMMP/ATH/APP/EG composites.

surface of the foam. Therefore, in the second step, the decomposition of the foam was significantly delayed, and the peak value was significantly reduced. 3.2.3. Fire Behavior. Figure 8 shows the changes in the LOI value of the foams with 10 phr of DMMP, 5 phr of ATH, and different amounts of APP. It can been seen that the addition of APP significantly improved the LOI value of the foams when

Figure 12. TG and DTG curves for the PIR−PUR/DMMP/ATH/ APP/EG composites.

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shown in Figure 4. Conversely, the peak heat release rate of the foams can be dramatically reduced by the addition of APP; in particular, with an APP amount of 15 phr, the value can be reduced from 179.58 to 76.83 kW/m2. Meanwhile, the total smoke release was also greatly reduced, reaching its lowest value at a dosage of 40 phr from 738.47 to 300.25 m2/m2. In addition, the total heat release and CO and CO2 emissions during combustion also decreased. Consequently, the addition of both ATH and APP can significantly improve the flame retardancy of PIR−PUR foams. 3.2.4. SEM Analysis of Chars. Figure 10 displays SEM images of the char from sample APP-15 after cone calorimetry tests. It is known that the morphology of char influences the flame retardancy of materials during burning. Figure 10 shows that the chars become much denser upon the addition of APP. Each piece of char was covered by a layer of villi-like substances, produced by ATH, that are useful in forming denser char. In addition, a layer of spherical particles was formed on the surface of the chars, which has never been reported in the literature. Furthermore, with increasing amount of APP added, the spherical particles became denser. Energy-dispersive X-ray spectroscopy (EDS) was used to analyze the composition of the spherical particles (Table 1). The EDS results indicate that the spherical particles contained the elements C, N, O, and P, which are consistent with the nonspherical particle area. The spherical particles might be the decomposition product of APP in synergy with ATH. 3.3. Effects of EG on the Properties of PIR−PUR Foam. 3.3.1. Physical−Mechanical Characterization. The thermal conductivities and compressive strengths of foams with 10 phr of DMMP, 5 phr of ATH, 15 phr of APP, and different amounts of EG are reported in Figure 11. The EG size (150 μm) was larger than the wall thickness of the foams. EG may penetrate the walls of the cells, which would result in a nonuniform porous structure. Therefore, the thermal conductivity gradually increased, and the compressive strength decreased. 3.3.2. Thermal Stability. Figure 12 shows the TG and DTG curves for the PIR−PUR/DMMP/ATH/APP/EG composites. The initial decomposition temperature of the foams rose after the addition of EG, because the decomposition of EG occurs at a higher temperature than those of the foams and the other flame retardants added. In addition, the residues at 800 °C did not change significantly (except for that with 10 phr of EG added). The DTG curves show that the peak weight loss rate decreased from 4.2% to 2.7% per minute and the residue at 800 °C increased from 36% to 59% when 10 phr of EG was added. Therefore, these results further support the existence of a synergistic effect when the flame retardants are 10 phr of DMMP, 5 phr of ATH, 15 phr of APP, and 10 phr of EG. 3.3.3. Fire Behavior. The LOI curve for foams with 10 phr of DMMP, 5 phr of ATH, 15 phr of APP, and different amounts of EG is shown in Figure 13. When 10 phr of EG was added, the LOI value increased from 28% to 30.2%. EG can promote the formation of larger-volume char of foams and shows a condensed-phase action. The heat release rate curves for foams with 10 phr of DMMP, 5 phr of ATH, 15 phr of APP, and different proportions of EG are shown in Figure 14, with the cone calorimetry test data listed in Table S4 of the Supporting Information. The results show that the addition of EG can reduce the peak heat release rate, total heat release, smoke emissions, and CO and CO2 emissions when compared with

Figure 13. LOI curve for the PIR−PUR/DMMP/ATH/APP/EG composites.

Figure 14. HRR curves for the PIR−PUR/DMMP/ATH/APP/EG composites.

Figure 15. SEM image of EG-40 (10 phr of DMMP, 5 phr of ATH, 15 phr of APP, 40 phr of EG) after the cone calorimeter test.

the amount added was less than 15 phr. However, the LOI value was almost unchanged when the amount added was greater than 15 phr. Figure 9 shows the heat release rate curves of foams containing 10 phr of DMMP and 5 phr of ATH after the addition of different proportions of APP. In addition, the cone calorimetry test data are listed in Table S3 of the Supporting Information. Previously, it was shown that the addition of ATH can slightly decrease the peak heat release rate (20 kW/m2), as 5882

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the reference sample. However, these values are not significantly reduced when compared with those of the sample containing 10 phr of DMMP, 5 phr of ATH, and 15 phr of APP with no EG. Therefore, we can speculate that the role of EG in foams is mainly to increase the oxygen index. EG contributes little to the heat release. 3.3.4. SEM Analysis of Chars. It can be seen from the SEM image of the char combusted after the cone calorimetry test on sample EG-40 that EG expanded to form tight, wormlike segments (Figure 15). This behavior is due to the expansion of EG, which has been reported in the literature. The expansion of EG is related to the redox process between sulfuric acid and EG that occurs between the graphite layers, according to the equation28

The wormlike segments both lie on the surface of the foams and are interspersed in the foams. Thus, it is possible to stop heat and combustible gases from passing into the combustion zone of the flame. Therefore, EG shows a flame-retardant effect. When compared with the sample without EG, the addition of EG significantly improved the oxygen index, which can be attributed to a synergistic effect among ATH, APP, and EG. EG and ATH can adsorb oxygen acids of phosphorus by the decomposition of APP in the combustion process, which leads to the formation of a denser char.

4. CONCLUSIONS As flame retardants, dimethyl methylphosphonate, aluminum hydroxide, ammonium polyphosphate, and expanded graphite were simultaneously added to PIR−PUR foams for the first time. Ultimately, the simultaneous addition of 10 phr of DMMP, 5 phr of ATH, 15 phr of APP, and 10 or 20 phr of EG to the foams results in the best foam because the mechanical properties and thermal insulation performance did not decrease and the flame-retardant properties significantly improved. ATH forms a villi-like substance covering the surface of the foams during the combustion process. Meanwhile, under the synergistic effect between ATH and APP, a dense spherical substance is produced on the surface of the char. In addition, after EG is added to the system, a wormlike carbon layer is formed, which can adsorb the phosphate-containing acid arising from APP decomposition, so that the carbon layer becomes denser. Under the combined effects of the villi-like substance, spherical substance, and wormlike carbon layer, heat and flame propagation are obstructed from being transferred to the unburned regions of the foam. ASSOCIATED CONTENT

S Supporting Information *

Table of the formulations of the flame retardants (per hundred grams of polyols). Parameters for each PIR-PUR foam investigated by cone calorimeter test. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b01019.

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C + 2H 2SO4 → CO2 + 2H 2O + 2SO2

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 5883

DOI: 10.1021/acs.iecr.5b01019 Ind. Eng. Chem. Res. 2015, 54, 5876−5884

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DOI: 10.1021/acs.iecr.5b01019 Ind. Eng. Chem. Res. 2015, 54, 5876−5884