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Halogen-Free Flame-Retardant Flexible Polyurethane Foam with a Novel Nitrogen−Phosphorus Flame Retardant Ming-Jun Chen, Zhu-Bao Shao, Xiu-Li Wang,* Li Chen, 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), Sichuan University, Chengdu 610064, China S Supporting Information *

ABSTRACT: A novel nitrogen−phosphorus flame retardant, 2-carboxyethyl(phenyl)phosphinic acid melamine salt (CMA), was synthesized by the reaction of 2-carboxyethyl(phenyl)phosphinic acid with melamine in aqueous solution, and it was characterized by Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), inductively coupled plasma-atomic emission spectrometry (ICP-AES), elemental analysis, and thermogravimetric analysis (TGA). Halogen-free flame-retardant flexible polyurethane foams (FPUF) were prepared successfully by using CMA as a flame retardant. The effects of CMA content on the mechanical, thermal, and flame-retardant properties of FPUF were investigated by tensile test, TGA, limiting oxygen index (LOI), flame propagation test (Cal T.B. 117A-Part I standard), and cone calorimeter. The foam morphology of FPUF was studied via scanning electron microscopy (SEM). The results showed that CMA endowed FPUF with good flame-retardant properties. FPUF containing only 12 wt % CMA can pass Cal T.B. 117A-Part I standard, and its LOI value was increased from 18.2 (for FPUF without CMA) to 24.1. Particularly, the addition of CMA almost did not decrease the mechanical properties of the resulting flame-retardant FPUFs. diluent in the flame.13,14 Andersson et al. compared the flame retardance of FPUF containing the intumescent additives (aluminum hypophosphite and melamine) with the one containing the commercial halogenated flame retardant (TCPP), and found that the flame-retardant properties of FPUF were also improved without using halogenated species.15 Melamine salts are excellent intumescent flame retardants, such as melamine phosphate, melamine polyphosphate, dicyclic phosphorus-melamine compound, melamine salt of pentaerypolyol phosphoric acid, and melamine salt of bis-(pentaerythritol phosphate)phosphoric acid. It can act as acid source, carbonizing and foaming agent simultaneously.16−20 However, there are few reports on the application of melamine salts in FPUF. Thirumal et al.21 prepared flame-retardant rigid polyurethane foam with melamine polyphosphate (MPP), and reported that the MPP-filled polyurethane foam had good flame retardance due to the formation of intumescent char layer on the surface and the synergistic effect of nitrogen and phosphorus in MPP.21 However, incorporation of only 5 php (about 2 wt %) MPP into the foam would cause the density and the compressive strength at 10% strain to decrease by 28% and 46%, respectively. To overcome the deterioration of the physical and mechanical properties of the foams caused by the incorporation of flame-retardant additives, a suitable flame retardant for FPUF should be used, and it will not affect the foaming processing and the mechanical properties of flame-retarded FPUF foams.

1. INTRODUCTION Flexible polyurethane foam (FPUF) is one of the most important thermoset polymers, which has a wide range of applications in mattresses, furniture cushioning, and automotive seating.1,2 However, FPUF is highly flammable due to its porous, open-cell structure and low density, which limits its application in many fields.3−5 Therefore, it is essential to endow FPUF with good flame retardancy. Incorporation of flame-retardant additives into foams by simply mechanical mixing at the compounding stage is the most popular approach to improve the flame retardancy of FPUF.6 Generally, flame-retardant additives are mostly based on halogen, phosphorus, and nitrogen. Halogen-based additive is a kind of very effective flame retardant for FPUF. However, the European Community proposed to restrict the usage of brominated diphenyl oxide flame retardants due to the highly toxic and potentially carcinogenic brominated furans and dioxins formed during combustion.7 Besides this, one of the commonly used chlorinated flame retardants for FPUF, tris(2chloroisopropyl) phosphate (TCPP), is being reviewed by the European Union Risk Assessment Body.8 More attention has been paid recently to developing halogen-free flame retardants to replace the halogenated ones. Especially, intumescent systems containing phosphorus and nitrogen are welcomed. Formation of an intumescent system usually consisted of an acid source, a carbonizing agent, and a foaming agent. Phosphorus in intumescent species can promote the formation of an expanded carbonized layer on the surface of polymer during the thermal degradation or combustion, which can reduce the heat transfer between the heat source and the polymer surface, and limit the diffusion of oxygen into the polymeric material.9−12 Normally, nitrogen can provide an inert © 2012 American Chemical Society

Received: Revised: Accepted: Published: 9769

April 17, 2012 June 28, 2012 July 6, 2012 July 6, 2012 dx.doi.org/10.1021/ie301004d | Ind. Eng. Chem. Res. 2012, 51, 9769−9776

Industrial & Engineering Chemistry Research

Article

2.3. Preparation of FPUF. The flame-retardant FPUF samples were prepared by the one-pot and free-rise method. Polyols (TMN 3050), distilled water, catalysts (DABCO and DBTDL), surfactant (SZ 580), and flame retardant (CMA) were well mixed in a 1 L plastic beaker. Next, TDI 80/20 was added into the beaker with vigorous stirring for 5 s. The mixture was immediately poured into an open plastic mold (30 × 20 × 15 cm3) to produce free-rise foam. The foam was cured for 24 h under ambient conditions. The formulations of FPUF were shown in Table 1. The NCO/OH ratio was 1.05. 2.4. Measurements. Average particle size of CMA was determined by Mastersizer 2000, using absolute ethyl alcohol as a dispersing agent. FT-IR spectra of the samples were recorded on a Nicolet FT-IR 170SX spectrometer over the wavenumber range from 500 to 4000 cm−1 using KBr pellets. 31P NMR spectra were performed on a Bruker AV II-400 MHz spectrometer, using tetramethylsilane (TMS) as a reference and DMSO-d6 as a solvent. The actual phosphorus content of CMA was determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES; IRIS Advantage, TJA solution). The actual nitrogen, carbon, and hydrogen contents in CMA were measured by elemental analysis (EA) on a CARLO ERBA1106 instrument. A NETZSCH 209F1 TA Instruments was employed for the thermogravimetric analysis (TGA), and the samples were heated to 600 °C at a heating rate of 10 °C/min under a dynamic nitrogen flow of 60 mL/ min. The density of FPUF samples was measured according to ISO 845: 2006. The size of the specimen was no less than 100 cm3, and the average values of five samples were recorded. The morphology of various FPUF samples was carried out with an INSPECT F scanning electron microscopy (SEM) at the accelerating voltage of 5 kV. The foam samples were fractured after immersion in liquid nitrogen, and the surfaces were coated with a thin gold layer before observation. Tensile measurements were performed with a ZBC1400-2 testing machine according to ISO 1798: 2008 at a crosshead speed at 500 mm/ min. Sample sheets were cut to a dumbbell shape with the size of 40 mm × 10 mm × 10 mm (length × width × thickness). The fire test was performed using a cone calorimeter (FTT, UK) instrument according to ISO 5660-1 under a heat flux of 25 kW/m2. The size of the specimen was 100 × 100 × 25 mm3 (length × width × thickness). Limiting oxygen index (LOI) tests were performed at room temperature according to ISO 4589-1984 using a HC-2C oxygen index instrument, and the size of the specimen was 150 × 10 × 10 mm3 (length × width × thickness). California technical bulletin 117 Section A-Part I (Cal T.B. 117A-Part I), which is suitable for evaluating the flame resistance, glow propagation, and tendency to char of the

In this study, a novel intumescent flame-retardant additive (CMA) based on melamine was synthesized and used as a flame-retardant additive for FPUF. The effects of CMA on the morphological, mechanical, thermal, and flame-retardant properties of FPUF were well investigated. All of the results showed that CMA had little negative influence on the mechanical properties of flame-retarded FPUF especially for the FPUF foams with low CMA loadings.

2. EXPERIMENTAL SECTION 2.1. Materials. Polyether polyols (TMN 3050, average functionality 3.0, OH content 56.1 mg KOH/g) were obtained from Third Oil Refinery of Tianjin Petrochemical Co., China. Toluene diisocyanate (TDI 80/20) was obtained from Chongqing Weiteng Polyurethane Products Factory, China. 2-Carboxyethyl(phenyl)phosphinic acid (CEPP) was supplied by Chengdu Weili Flame Retardant Chemical Industrial Co., Ltd., China. Melamine (MA) and catalysts (triethylenediamine hexahydrate, DABCO; dibutyltion dilaurate, DBTDL) were supplied by Chengdu Kelong Chemical Regent Factory, China. Surfactant (SZ 580) was supplied by Beijing Wanbo Huijia Technology and Trade Co., Ltd., China. 2.2. Synthesis of CMA. CEPP (21.4 g) was heated and dissolved in 300 mL of distilled water. MA (12.6 g) then was added into the solution and reacted for several hours. Finally, the solution was immediately filtrated, and the white product was obtained. The product, named CMA, was dried under a vacuum until constant weight was reached. Yield, 95%; mp, 241−243 °C; average particle size, 9 μm. The particle size distribution of CMA was shown in Figure 1.

Figure 1. The particle size distribution of CMA.

Table 1. Formulations of FPUF without CMA and Flame-Retardant FPUF Containing Different CMA Contentsa

a

sample

TMN 3050 (php)

DABCO (php)

DBTDL (php)

SZ 580 (php)

distilled water (php)

CMA (php)

TDI 80/20 (php)

FPUF-0 FPUF-5 FPUF-10 FPUF-15 FPUF-20 FPUF-25 FPUF-30

100 100 100 100 100 100 100

0.09 0.12 0.15 0.18 0.21 0.24 0.27

0.18 0.24 0.30 0.36 0.42 0.48 0.54

1.0 1.2 1.4 1.6 1.8 2.0 2.2

3 3 3 3 3 3 3

0 5 10 15 20 25 30

41 41 41 41 41 41 41

php: parts per hundred of polyol by weight. 9770

dx.doi.org/10.1021/ie301004d | Ind. Eng. Chem. Res. 2012, 51, 9769−9776

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resilient cellular materials, was used for flame-retardant FPUF.22

3. RESULTS AND DISCUSSION 3.1. Characterization of CMA. 3.1.1. Chemical Structure of CMA. The FT-IR spectra of MA, CEPP, and CMA are shown in Figure 2. In the range of 3000−3500 cm−1, MA had

Figure 3. 31P NMR spectra of CEPP and CMA.

Table 2. Actual and Theoretical Contents of Elements in CMA sample theoretical content found content

P (%)

N (%)

C (%)

H (%)

9.12 9.18

24.71 25.36

42.35 42.50

5.00 5.80

Figure 2. FT-IR spectra of MA, CEPP, and CMA.

four vibration absorption peaks of −NH2 at 3468, 3418, 3332, and 3130 cm−1, respectively. CEPP had one peak at 3038 cm−1 (the vibration absorption of (OC)−OH). CMA had two absorption peaks at 3364 and 3149 cm−1, which were belonged to the vibration absorption of −NH2 and −NH3+, respectively.19,20 Among these, the characteristic peak at 3149 cm−1 was wide and strong. In addition, another absorption peak for −NH3+ was found at 1505 cm−1. The stretching vibration absorption of OC(−OH) in CEPP appeared at 1733 cm−1 has shifted to 1665 cm−1 due to the formation of −COO− in CMA. The appearance of −NH3+ and −COO− in CMA proved the formation of the melamine salt between −NH2 and −COOH. Besides this, P(O)−OH of CEPP had a weak and wide absorption at 2565 cm−1; however, it shifted to 2507 cm−1 in CMA,23,24 which could be ascribed to the inductive and steric effect caused by the melamine salt formation between −NH2 of melamine and −COOH of CEPP. 31 P NMR spectroscopy provided more evidence for the determination of the structure of CMA. The 31P NMR spectra of CEPP and CMA are shown in Figure 3. Only a single peak at 35.45 and 30.12 ppm was found for CEPP and CMA, respectively, suggesting that CEPP has reacted with MA to produce CMA. To further confirm the structure of the product, ICP-AES was used to measure the actual phosphorus content, and the contents of nitrogen, carbon, and hydrogen were measured by EA. The results are shown in Table 2. The actual contents of phosphorus, nitrogen, carbon, and hydrogen were almost coincident with their theoretical contents. On the basis of the above results, it can be concluded that CMA has been synthesized successfully. 3.1.2. Thermal Degradation Behavior of CMA. The TG/ DTG thermogram of CMA under nitrogen atmosphere is shown in Figure 4. It can be seen that the decomposition of CMA owned four stages, in which the temperatures of maximum weight loss were found at 121, 229, 313, and 440 °C, respectively. The 5% weight loss temperature (T5%) of

Figure 4. TG/DTG thermogram of CMA under a nitrogen atmosphere.

CMA was 225 °C, and the amount of residue (char yield) at 600 °C was 37.4%. To investigate the thermal decomposition process of CMA, CMA decomposition products obtained in muffle furnace at the four maximum weight loss temperatures and 600 °C, respectively, were investigated by FT-IR spectrometer (Figure 5). From their FT-IR spectra, it is clearly seen that the decomposition products at 121 °C were just the same as that of CMA, indicating that only adsorbed water in CMA has been eliminated at this temperature.25 In addition, the absorption peak of −NH3+ (3149 cm−1, 1505 cm−1) and −COO− (1665 cm−1) became gradually weaker from 121 to 313 °C, which can be ascribed to the fact that CMA was gradually degraded into cross-linked 2-carboxyethyl(phenyl)phosphinic acid derivatives and melamine. Similar results on the decomposition of melamine polyphosphate were also reported by Song.26 When the temperature was over 440 °C, the absorption peak of −NH3+ completely disappeared, which indicated that CMA was no longer decomposed over 440 °C; 9771

dx.doi.org/10.1021/ie301004d | Ind. Eng. Chem. Res. 2012, 51, 9769−9776

Industrial & Engineering Chemistry Research

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smooth; however, there were some CMA particles embedded in the struts and strut joints of flame-retardant FPUF (Figure 6b′,c′,b″,c″). In addition, from Figure 6b′,b″, it is clear that CMA particles were well dispersed in the struts and strut junctions because of its smaller size (9 μm) and homogeneous distribution (Figure 1). However, incorporation of a higher amount of CMA (30 php) in FPUF caused the conglomeration of CMA particles (Figure 6c″), and this was one of the reasons for the rupture of struts and the collapse of cellular structure. 3.2.2. Mechanical Properties of FPUF. The densities of FPUF with and without CMA are shown in Table 3. Foam Table 3. Mechanical Properties and LOI Values of FPUF with and without CMA

Figure 5. FT-IR spectra of CMA decomposition products at different temperatures.

however, melamine decomposed from CMA could be further linked to each other with concomitant evolution of ammonia.27 3.2. Characterization of FPUF. 3.2.1. Cell Morphology. It is known that the cell structure of foam can greatly affect the mechanical properties of FPUF. The SEM photographs of the surface of FPUF with and without CMA are shown in Figure 6. The major areas in the foam structure are cell window, strut, and strut joint. From Figure 6a−c, it is observed that the structure of the foam containing 20 php (about 12 wt %) CMA was almost the same as that of the foam without CMA, but incorporation of a higher amount of CMA (30 php) caused the rupture of struts, which would induce the collapse of cellular structure and the increase of cell windows. It was well understood that CMA existed as hard particles in the foam and higher amounts of CMA particles would lead to the foam shrinkage or even to crack.28 Figure 6a′−c′ and a″−c″ shows the corresponding microphotographs of a−c under 800 and 2000 magnification, respectively. It can be seen in Figure 6a′,a″ that the struts and strut joints of FPUF without CMA were

sample

CMA content (php)

density (kg/m3)

1 2 3 4 5 6 7

0 5 10 15 20 25 30

32 35 36 38 38 39 40

tensile strength (MPa) 0.10 0.11 0.11 0.11 0.11 0.09 0.10

± ± ± ± ± ± ±

0.02 0.03 0.04 0.02 0.01 0.03 0.02

elongation at break (%)

LOI (%)

± ± ± ± ± ± ±

18.2 21.2 22.6 23.6 24.1 25.0 25.6

168 165 166 164 163 143 145

10 13 7 6 9 8 6

density depends on the amount of blowing agent. The higher is the amount of blowing agent, the lower the foam density becomes. In this work, distilled water was used as blowing agent, and its concentration was kept constant. From Table 3, we can see that there was a slight increase in foam density with the increase of CMA contents, which was probably due to the increase of CMA particles embedded in the cell wall (Figure 6b′,c′).29 FPUF products, such as mattresses, furniture cushioning, and automotive seating, require sufficient strength in their application. However, incorporation of fillers into polyurethane foam usually caused inferior physical and mechanical properties.6 Table 3 presents the mechanical properties of the

Figure 6. Microphotographs of FPUF with different CMA loadings under 100 magnification (a−c), 800 magnification (a′−c′), and 2000 magnification (a″−c″): (a,a′,a″) without CMA, (b,b′,b″) containing 20 php CMA, and (c,c′,c″) containing 30 php CMA. 9772

dx.doi.org/10.1021/ie301004d | Ind. Eng. Chem. Res. 2012, 51, 9769−9776

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without CMA. The degradation of flame-retardant FPUF at 180−220 °C was attributed to the initial weight loss of CMA (T5% of CMA was 225 °C). With the increase of CMA contents in FPUF, the initial degradation peak of FPUF became wider and wider (Figure 8). The second degradation step was mainly due to the depolymerization of polyurethane to form isocyanate, polyol, primary or secondary amine, olefin, and carbon dioxide.5 The third degradation step at 290−320 °C was caused by the decomposition of the FPUF foam without CMA14 and CMA, in which this peak became wider with the increase of CMA loadings in FPUF. The last step of degradation was due to the dimerization and trimerization reaction of isocyanates.33 Table 4 shows the detailed thermal decomposition temperatures of samples determined from Figures 7 and 8. We can see

obtained FPUF. When CMA did not exceed 20 php in the formulations, the elongation at break was decreased slightly as compared to that of FPUF without CMA. This was because the cell structure of the foam containing less than 20 php CMA was almost the same as that of the foam without CMA. However, the elongation at break of the foam containing more than 20 php CMA decreased obviously, which was attributed to the fact that the amount and size of voids in FPUF matrix increased sharply when the content of CMA particles surpassed 20 php (Figure 6c). In addition, the tensile strength initially increased and then decreased when the loadings of CMA exceeded 20 php (shown in Table 3). This phenomenon was ascribed to the fact that the incorporation of a small amount of hard particles (CMA) increased the stiffness of FPUF, and higher CMA (more than 20 php) contents increased the frailability of FPUF, which would result in the crack formation. 3.2.3. Thermal Stability. It is reported that the thermal degradation of polyurethanes occurs in a two- to three-step process.30−32 Figures 7 and 8 show that the FPUF without

Table 4. TGA Data of FPUF without CMA and FlameRetardant FPUFa sample

CMA content (php)

T5% (°C)

Tmax1 (°C)

Tmax2 (°C)

Tmax3 (°C)

Tmax4 (°C)

char residue at 600 °C (%)

1 2 3 4

0 10 20 30

259 248 238 237

215 209 211

289 282 273 255

304 299 289 284

375 379 379 383

1.2 1.6 4.2 6.6

a

T5%: 5% weight loss temperature. Tmax: Maximum weight loss temperature.

clearly that with the increase of CMA loadings, the first maximum weight loss temperature (Tmax1) was almost kept in the range of 210−215 °C, but there was a slight decrease of the onset decomposition temperature (T5%) and the second and third maximum weight loss temperature (Tmax2 and Tmax3). The slight variation of Tmax1 is related to the thermal stability of CMA in this temperature range. The decrease of T5%, Tmax2, and Tmax3 was attributed to the breakage of the weak P−C bond of CMA and formation of polyphosphoric acid, which will accelerate the decomposition of flame-retardant FPUF as a strong Lewis acid catalyst. This phenomenon was also found in the rigid polyurethane foam containing MPP.21 On the contrary, there was an increase in the fourth maximum weight loss temperature (Tmax4), which was due to promoting the char formation by CMA at this temperature. As far as the residue was concerned, it was found that the char residue at 600 °C was increased with the increase of CMA contents (Table 4). We calculated the theoretical TG curves of FPUF-20 and compared it to actual TG curves (Supporting Information) and found that the actual residue (4.2%) of FPUF-20 at 600 °C was higher than the calculated residue (1.6%). This proved that the phosphorus and nitrogen in CMA may have a synergistic effect on char formation, which can help to improve the flame retardancy of FPUF. Besides this, CMA can form phosphoric anhydrides or the related acids at higher decomposition temperature, and these resultants also promote the char formation. This char with phospho-carbonaceous structure was more stable than that with carbonaceous structure obtained from FPUF without CMA.11 3.2.4. Flame-Retardant Behavior. The flame-retardant properties of FPUF filled with CMA were characterized by cone calorimeter measurement, LOI, and flame propagation test. At present, the cone calorimeter measurement is a widely used method for assessing the fire behavior of materials. The

Figure 7. TG curves of FPUF-0, FPUF-10, FPUF-20, and FPUF-30.

Figure 8. DTG curves of FPUF-0, FPUF-10, FPUF-20, and FPUF-30.

CMA had a three-step degradation process at 220−300 , 300− 320, and 320−420 °C, respectively. However, the decomposition of flame-retardant FPUF took place at four stages, 180−220, 220−290, 290−320, and 320−420 °C, respectively. The last three degradation processes of flame-retardant FPUF were in accordance with the degradation processes of the FPUF 9773

dx.doi.org/10.1021/ie301004d | Ind. Eng. Chem. Res. 2012, 51, 9769−9776

Industrial & Engineering Chemistry Research

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cone calorimeter brings quantitative analysis to the flammability of materials by investigating parameters such as heat release rate (HRR), time to ignition (TTI), and total heat release (THR). The HRR curves of FPUF containing different CMA loadings are presented in Figure 9. The HRR of flame-retardant

Table 5. Effect of CMA on the Cone Calorimeter Data of FPUF sample

CMA content (php)

peak HRR (kW/m2)

THR (MJ/m2)

char yield (%)

TTI (s)

0 1 2

0 10 20

240 237 214

17 16 14

4.3 11.4 17.2

0 1 5

The major task for a flame retardant is preventing or delaying flashover from the surface of combustibles; therefore, a flame retardant is not designed to prevent the polymer from ignition but to minimize the flame spread rate and prevent sustained burning.38 The resistance of FPUF to flame and glow propagation and the tendency to char formation were determined using Cal T.B. 117A-Part I standard and are shown in Table 6. It was shown that the foam without CMA Table 6. Flame Retardancy of FPUF with Different CMA Loadingsa afterflame time (s)

afterglow time (s)

char length (inch)

average

maximum

average

average

maximum

test results (pass or fail)

43.1 1.0

59.6 2.0

0.0 0.0

no 5.2

no 6.3

fail pass

0.0

0.0

0.0

4.1

4.9

pass

Figure 9. Heat release rates of (a) FPUF-0, (b) FPUF-10, and (c) FPUF-20 at 25 kW/m2. sample

FPUF was lower than that of the FPUF without CMA, which was due to the dilution effect of ammonia and other noncombustible gases decomposed from CMA on the oxygen concentration surrounding FPUF. For this reason, the time to peak HRR was delayed for 10−20 s and the peak HRR was decreased by 25 kW/m2 as compared to FPUF without CMA. Moreover, CMA-filled FPUF had a lower HRR than that of FPUF without CMA after 110 s, which was ascribed to the char formation. The more the char formed, the better was the flame retardancy. It is clearly seen from Figure 10 that the

pure FPUF10 FPUF20 a

According to Cal T.B. 117A-Part I standard.

had a long afterflame time, and none char layer was formed on the surface until it was burned out. However, FPUF with CMA had a significant improvement in afterflame time and char length, proving that the propagation of flame was hindered efficiently. Especially, FPUF added with 20 php CMA would self-extinguish after moving away the flame and form a char layer, not exceeding 6 inches. Besides this, Table 6 shows that there is no afterglow time for all of the samples, which was ascribed to the fact that the liquid molten drops were not able to emit sparks. The higher LOI value represents the better flame retardancy.34 The effect of CMA on the LOI values of FPUF is shown in Table 3. The results show that the LOI values of FPUF increased with the increase of CMA contents. When the loadings of CMA exceeded 10 php, the resulting flameretardant FPUF would achieve self-extinguishing level. The LOI results had good correlation with the results obtained from the cone calorimeter test and flame propagation test.

Figure 10. The charred residue pictures of (a) FPUF-0, (b) FPUF-10, and (c) FPUF-20.

intumescent char yields increased with the increase of CMA contents in FPUF. This is strong evidence for the better flame retardance of FPUF containing CMA, in which the intumescent char can act as a barrier to heat, air/O2, and pyrolysis products.34,35 TTI is another important flame-retardant parameter for materials. If a material can withstand an ignition for a long time, this will indicate that it has better flame retardance. Modesti et al.36 reported that the time to ignition of foam was always very low because of the cellular structure of the foam.36 From Table 5, it can be seen clearly that there was an increase in TTI with the increase of CMA content. Although the increase was not remarkable, the flame-retardant effect of CMA was proven. THR decreased with the increase of CMA contents. There existed a synergistic effect between nitrogen and phosphorus in CMA, which played the role both in the gas phase and in the condensed phase.37

4. CONCLUSIONS FT-IR, 31P NMR ICP-AES, EA, and TGA results indicate that CMA has been synthesized successfully via the reaction of CEPP with MA. CMA exhibited negligible negative influence on the mechanical properties of the flame-retardant FPUF particularly when the content of CMA did not exceed 20 wt %. Higher CMA loadings in FPUF would lead to the collapse of cell structure and the rupture of cell walls. TGA results showed that the onset decomposing temperature of the CMA-added foams decreased but the char yields increased. CMA endowed FPUF with good flame retardancy, which was mainly due to the synergistic effect of nitrogen and phosphorus in CMA. 9774

dx.doi.org/10.1021/ie301004d | Ind. Eng. Chem. Res. 2012, 51, 9769−9776

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Therefore, CMA was believed to act as a heat sink to lower the surface temperature, an inert diluent to prevent the ingress of air, and a dehydrating agent to promote the char formation.

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ASSOCIATED CONTENT

S Supporting Information *

The calculated TG curve and the actual TG curve of FPUF-20. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel./fax: +86-28-85410259. E-mail: [email protected] (X.L. Wang); [email protected] (Y.-Z. Wang). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (50933005, 51121001), the Excellent Youth Foundation of Sichuan (2011JQ0007), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT1026).

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REFERENCES

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