Inherently Flame-Retardant Flexible Polyurethane Foam with Low

Article pubs.acs.org/IECR

Inherently Flame-Retardant Flexible Polyurethane Foam with Low Content of Phosphorus-Containing Cross-Linking Agent Ming-Jun Chen, Chun-Rong Chen, Yi Tan, Jian-Qian Huang, 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 ABSTRACT: A halogen-free phosphorus-containing triol named phosphoryltrimethanol (PTMA) was synthesized and used as a cross-linking agent and a reactive-type flame retardant to prepare inherently flame-retardant flexible polyurethane foam (FPUF). Incorporation of a low content (7.8 wt %) of PTMA into the polyurethane chains can increase the flame retardance of FPUF because of good char formation. The residual chars and evolved gases of PTMA-cross-linked FPUFs were analyzed by SEM, FTIR spectroscopy, inductively coupled plasma-atomic emission spectrometry (ICP-AES), energy-dispersive X-ray (EDX) spectroscopy, and thermogravimetric analysis (TGA) coupled with FTIR spectroscopy. The results indicated that more than 60% of the phosphorus in PTMA-cross-linked FPUF was decomposed into polyphosphoric acid or its derivatives and retained in the char residue. This shows that PTMA mainly played a role in the condensed phase of flame-retardant FPUF. Based on the results, a possible thermal degradation mechanism of PTMA-cross-linked FPUF is proposed.

1. INTRODUCTION Flexible polyurethane foam (FPUF) is a type of polymeric material consisting of a chain with repeating units containing the characteristic urethane group and is normally manufactured from isocyanates and polyols.1 It has been widely used in mattresses and automotive and furniture cushions, where fire safety is compulsory.2−4 In terms of fire safety, it is essential to endow FPUF with good flame retardancy, which can be realized through the introduction of additive- or reactive-type flame retardants into FPUF.5,6 During the past decade, various flame-retardant additives, such as melamine and its derivates,7−9 organophosphorus compounds,10−13 and intumescent flame retardants,14−16 have been used to improve the fire retardance of polyurethane foams. However, the common defects of introducing additivetype flame retardants into polyurethanes cannot be ignored, such as poor compatibility, easy leaching, and reduced mechanical properties. Compared with additive-type flame retardants, the use of reactive-type flame retardants has many advantages including increasing the compatibility between the flame retardants and the polymers, providing permanent flame retardance, and furnishing high flame-retardant efficiency.17−19 In general, reactive-type flame retardants are mostly based on halogens and phosphorus.20−22 Recently, selected halogencontaining flame retardants have been banned because they have poor environmental profiles, along with some human health and toxicity effects. For this reason, there is a strong desire to develop effective nonhalogenated flame retardants with superior environmental profiles and the ability to react with foams and not leach into the environment.23,24 It has been widely demonstrated that the most suitable reactive flame retardants for rigid polyurethane foams are mainly phosphoruscontaining polyols, such as 2,2′-((bis(hydroxymethyl) phosphoryl)methylazanediyl)diethanol, tris(dipropylene © 2013 American Chemical Society

glycol)phosphonate, diethyl-N,N′-bis(2-hydroxyethyl)aminomethylphosphonate, diethyl-N,N′-diethanolaminomethylphosphate, and morpholinophosphoryldimethanol.20,25,26 Several new phosphorus-containing polyols, based on tetrakis(hydroxymethyl)phosphonium chloride (THPC), have been used for rigid polyurethane foams, being introduced into the polyurethane main chain in the chain-extension step.25,27−29 Moreover, the modified rigid polyurethane foams showed good flame resistance. Usually, these phosphorus-containing polyols are used as chain extenders or reactive monomers for rigid polyurethane foams; however, they are rarely used for FPUF. In this work, a halogen-free phosphorus-containing polyols, phosphoryltrimethanol (PTMA), which is based on tetrakis(hydroxymethyl)phosphonium sulfate (THPS), was synthesized, and it was used as a cross-linking agent and reactive-type flame retardant for FPUF. A low-functionality nitrogencontaining polyether polyol (CPOP-3628H) was also used to help PTMA play the role of cross-linker for FPUF. In addition, because CPOP-3628H can release gas (NH 3 ) during combustion, it was expected that there might be some synergistic effect with PTMA. The effects of PTMA on the morphology and mechanical, thermal, and flame-retardant properties of FPUF were well investigated. In addition, a possible flame-retardant mechanism for FPUF modified with PTMA is also proposed.

2. EXPERIMENTAL SECTION 2.1. Material. Polyether polyols (TMN 3050, numberaverage molecular weight of 3000, average functionality of 3.0, Received: Revised: Accepted: Published: 1160

October 31, 2013 December 15, 2013 December 27, 2013 December 27, 2013 dx.doi.org/10.1021/ie4036753 | Ind. Eng. Chem. Res. 2014, 53, 1160−1171

Industrial & Engineering Chemistry Research

Article

phosphorus-containing cross-linking agent (10 g of PTMA) were well mixed in a 1 L plastic beaker. Then, 96 g of TDI 80/ 20 was added to 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 (FPUF-PTMA5). The foam was cured for 24 h under ambient conditions. The molar ratio of NCO (from TDI 80/20) to OH (from TMN 3050, CPOP-3628H, PTMA, and water) was 1.05. The formulations of all of the FPUFs are listed in Table 1. The densities of all samples were controlled at 33 ± 1 kg/m3. A possible FPUF chain structure formed between TDI 80/20 and TMN3050, CPOP-3628H, and PTMA is shown in Figure 1. The actual phosphorus contents in the FPUF-PTMAs were determined by ICP-AES as follows: FPUF-PTMA5, 0.75 wt % (calcd 0.71 wt %); FPUF-PTMA10, 1.22 wt % (calcd 1.27 wt %); FPUF-PTMA15, 1.55 wt % (calcd 1.72 wt %). 2.4. Measurements. FTIR spectra of samples were recorded on a Nicolet FTIR 170SX spectrometer over the wavenumber range from 500 to 4000 cm−1 using KBr pellets. 1 H NMR and 31P NMR spectra were obtained on a Bruker AV II-400 MHz spectrometer, using tetramethylsilane (TMS) as the reference and DMSO-d6 as the solvent. The density of FPUF was measured according to standard method ISO 845:2006, in which the size of each specimen was no less than 100 cm3 and the average values of five samples were recorded. Tensile measurements were performed with a ZBC1400-2 testing machine according to standard method ISO 1798:2008 at a crosshead speed at 500 mm/min. Sample sheets were cut to a dumbbell shape with dimensions of 40 mm × 10 mm × 10 mm (length × width × thickness). Compression set (CS) was examined according to standard method ISO 1856:2000, in which the size of each specimen was 50 × 50 × 25 mm3 (length × width × thickness) and the average values of five samples were recorded. The sample was compressed to its half-thickness and fixed, and then it was placed into an oven with continuous heating at 70 °C for 22 h. After thermal aging, the sample was allowed to recover for 0.5 h at ambient temperature. The compression set at 50% was calculated by the equation

OH content of 56 mg of KOH/g, technical pure grade) was obtained from Third Oil Refinery of Tianjin Petrochemical Company, Tianjin, China. Nitrogen-containing polyether polyols (CPOP-3628H, modified with dicyandiamide and melamine, number-average molecular weight of 5000, average functionality of 2.5, OH content of 28 mg of KOH/g, nitrogen content of 14.24%, technical pure grade) was supplied by Huzhou Innovative Polyurethane Technology Corporation, China. Toluene diisocyanate (TDI 80/20, technical pure grade) was obtained from Chongqing Weiteng Polyurethane Products Factory, Chongqing, China. Catalysts (triethylamine, TEA; dibutyltin dilaurate, DBTDL) were analytical reagent grade and were supplied by Chengdu Kelong Chemical Reagent Factory, Chengdu, China. Surfactant (SZ 580, technical pure grade) was supplied by Beijing Wanbo Huijia Technology and Trade Co., Ltd., Beijing, China. Distilled water was used as a chemical blowing agent. Tetrakis(hydroxymethyl)phosphonium sulfate (THPS, 75% solution, technical pure grade) was supplied by Hubei Xingfa Chemicals Group Co., Ltd., Hubei Province, China. Barium hydroxide (chemical pure grade), hydrogen peroxide (30% solution, analytical reagent grade), and chloroform (analytical reagent grade) were supplied by Chengdu Kelong Chemical Regent Factory, Chengdu, China. 2.2. Synthesis of PTMA. Ba(OH)2 (0.3 mol) was dissolved in 600 mL of distilled water at 60 °C. Then, THPS (0.3 mol) was added dropwise to the solution and reacted at 60 °C for 4 h. After BaSO4 had been separated by centrifugation, hydrogen peroxide (30% solution, 0.6 mol) was slowly added to the solution and reacted at room temperature for 5 h. After that, the impurities were extracted by chloroform, and then the product (PTMA) was obtained by removing water under reduced pressure. The detailed synthetic route of PTMA is shown in Scheme 1. Yield: 70%. 1H NMR (DMSO-d6, 400 Scheme 1. Synthetic Route of PTMA

CS =

MHz): 3.81 ppm (s, 6H, PCH2O), 5.29 ppm (s, 3H, C OH). 31P NMR (DMSO-d6, 400 MHz): 44.04 ppm (s, 1P, O PC). FTIR (KBr): 3390 (OH), 2898 (CH2), 1267 (PO), 1112 (CO), 882 cm−1 (PC). 2.3. Preparation of Flame-Retardant FPUF. The flameretardant FPUF samples were prepared by a one-pot and freerise method. A typical preparation process for FPUF-PTMA was as follows: Polyether polyols (140 g of TMN 3050, 60 g of CPOP-3628H), 6 g of distilled water, catalysts (0.5 g of TEA and 0.3 g of DBTDL), surfactant (SZ 580, 2 g), and

d 0 − dr × 100% d0

(1)

where d0 denotes the initial thickness before aging and dr denotes the final thickness after aging. The volume percentage of open cells was examined according to Chinese standard GB/ T 10799-2008 and performed on a 3H-2000 PS1 automatic analyzer (Beishide Instrument Technology Co., Ltd., Beijing, China), in which the size of each specimen was 25 × 25 × 50 mm3 (length × width × thickness) and the average values of three samples were recorded. The morphologies of the tensile cross sections were observed by scanning electron microscopy (SEM) on an an INSPECT F spectrometer at an accelerating

Table 1. Formulations and Densities of FPUF-CPOP and FPUF-PTMAsa sample

TMN 3050 (php)

CPOP-3628H (php)

PTMA (php)

H2O (php)

TEA (php)

DBTDL (php)

SZ 580 (php)

TDI 80/20 (php)

density (kg/m3)

neat FPUF FPUF-CPOP FPUF-PTMA5 FPUF-PTMA10 FPUF-PTMA15

100 70 70 70 70

− 30 30 30 30

− − 5 10 15

2.5 3.0 3.0 3.1 3.3

0.10 0.15 0.25 0.45 0.65

0.14 0.12 0.15 0.25 0.40

1.0 1.0 1.0 1.5 2.0

35 39 48 59 71

33 32 33 34 33

a

Units of php represent parts per hundred of polyol by weight. 1161

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Figure 1. Possible FPUF chain structure formed between TDI 80/20, TMN3050, CPOP-3628H, and PTMA.

structure. Interestingly, when the magnification was ×2000 (Figure 2a-2−e-2), it was found that the cross sections of the strut joints in neat FPUF, FPUF-CPOP, and FPUF-PTMAs were very different from each other, and this difference was further demonstrated by the images with a magnification of ×10000 (Figure 2a-3−e-3). It is clear from Figure 2a-3−e-3 that the tensile cross-sectional surfaces of FPUF-CPOP and FPUF-PTMAs were rougher than that of neat FPUF. This is due to the effects of CPOP-3628H and PTMA on the urea segments and the soft- and hard-segment blocks, which altered the foam morphology of the solid state. When diethanolamine was used as the chain extender, the interconnectivity of the hard domain was reduced, which also changed the morphology of the foam.32 In addition, some spherical objects were found in the FPUF-PTMAs (Figure 2c-3−e-3), whose amounts increased with increasing PTMA content. This is because the addition of PTMA increased the contents of hard segments in FPUF, which altered the anisotropic morphology of the hard domain.33 3.2. Mechanical Properties. It has been reported that the density, microstructure, chemical structures of the chain extender and cross-linking agent, and type of polymeric polyol have influences on the mechanical properties of polyurethane.34−41 To eliminate the effects of other factors, FPUF samples with the same density (33 ± 1 kg/m3) prepared with the same polyether polyols (TMN 3050 and CPOP-3628H) but with different PTMA contents were investigated. Table 2 presents the test data of tensile strength, elongation at break, and compression set at 50% (50% CS) for neat FPUF, FPUFCPOP, and FPUF-PTMAs. As can be seen, FPUF-CPOP had the best mechanical properties because of the increase in tensile strength and elongation at break despite the slight deterioration of 50% CS. The improvement in elongation at break can be ascribed to the high molecular weight of CPOP-3628H. It has been demonstrated that a high molecular weight of the softsegment domain increases the flexibility of foams.37 Because CPOP-3628H contains a triazine ring, it increased the stiffness of the polyurethane, which caused the tensile strength to be enhanced and the 50% CS to deteriorate. When 5 php PTMA (3.2 wt %) was added, the tensile strength and the elongation at break of FPUF increased compared with those of the neat sample, although both were lower than the corresponding values for FPUF-CPOP. With a further increase in the PTMA content, both the tensile strength and elongation at break of the foam decreased. Shutov found that the cell structure of a foam can greatly affect its mechanical properties and that the tensile strength and elongation at break of FPUF decreased as the cell diameters increased.42 The SEM

voltage of 5 kV, and the surfaces were coated with a thin gold layer before observation. A NETZSCH 209F1 instrument was employed for thermogravimetric analysis (TGA), and the FPUF samples were heated to 600 °C at a heating rate of 10 °C/min under a dynamic nitrogen flow of 50 mL/min. The actual phosphorus contents of FPUF containing PTMA and its char were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES; IRIS Advantage, TJA Solutions Corporation, Franklin, MA). About 9 mg of foam or 5 mg of char was completely combusted into gases under sufficient oxygen atmosphere, and then the gases were absorbed in 25 mL of 0.001 mol/L KMnO4/KOH solution and diluted to 100 mL using deionized water. The gases evolved during TGA tests were analyzed by coupling TGA with FTIR spectroscopy. Limiting oxygen index (LOI) tests were performed at room temperature according to standard method ISO 4589-1:1996 using an HC-2C oxygen index instrument, and the size of each specimen was 150 × 10 × 10 mm3 (length × width × thickness). The fire test was performed using a cone calorimeter [Fire Testing Technology (FTT), East Grinstead, West Sussex, U.K.] instrument according to standard method ISO 5660-1 under a heat flux of 25 kW/m2. The size of each specimen was 100 × 100 × 25 mm3 (length × width × thickness). The phosphorus content in the char residue after the cone test was detected by energy-dispersive X-ray (EDX) spectroscopy (INCA, Oxford Instruments, Abingdon, Oxfordshire, U.K.).

3. RESULTS AND DISCUSSION 3.1. Cell Morphology. The microstructure of the foams was mainly its cellular structure.30 Hence, the cellular structures of cross sections of the foams were evaluated by SEM. The SEM micrographs of neat FPUF, flame-retardant FPUF with only CPOP-3628H (FPUF-CPOP), and FPUF cross-linked with different PTMA contents (FPUF-PTMAs) are shown in Figure 2. The major areas in foam structures are cell windows, struts, and strut joints. From Figure 2, it can be observed that the number of cell windows and the cell size of FPUF-CPOP were consistent with those of neat FPUF. However, for FPUFPTMAs, the number of cell windows decreased sharply, and the cell size obviously increased with increasing PTMA content. This was due to the increase in the number of closed foam cells caused by the cross-linking action of PTMA.31 These results can be illustrated by the volume percentage of open cells in Table 2, which shows that the number of open cells initially decreased because of the cross-linking action of PTMA but the amount of open-cell foam subsequently increased with increasing PTMA content because of the collapse of the foam 1162



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Figure 2. Tensile cross-sectional images of (a-1−a-3) neat FPUF, (b-1−b-3) FPUF-CPOP, (c-1−c-3) FPUF-PTMA5, (d-1−d-3) FPUF-PTMA10, and (e-1−e-3) FPUF-PTMA15 with different magnifications [(1) ×50, (2) ×2000, (3) ×10000].

to hard segments in FPUF decreased, causing the foam to have a low elongation at break and a high compression set at 50%. 3.3. Thermal Degradation Analysis. Many studies in the literature have shown that polyurethanes have a two- or threestep thermal degradation process and that the decomposition product is usually their precursor, such as isocyanate, amine, and hydroxyl compounds.43 Polyurethanes with different backbone structures have different thermal stabilities and char yields.44 It was found that the first decomposition process of polyurethanes begins with the degradation of hard segments, resulting in the formation of isocyanate, alcohol, primary or

images of the FPUF-PTMA samples (Figure 2c-1−e-1) show that the number of open foam cells and the strut joints decreased and the diameters of the cells increased with increasing PTMA content. All of these changes in cell structure made the mechanical properties of the FPUF-PTMAs deteriorate. Moreover, PTMA had a high hydroxyl equivalent of 46.7 and consumed more TDI 80/20 than TMN 3050 or CPOP-3628H. This means that more hard segments were formed by the reaction between PTMA and TDI 80/20, especially at high PTMA contents. As a result, the ratio of soft 1163



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Table 3. TGA Dataa for Neat FPUF, FPUF-CPOP, and FPUF-PTMAs in N2

Table 2. Mechanical Properties and Volume Percentages of Open Cells of Neat FPUF, FPUF-CPOP, and FPUF-PTMAs sample neat FPUF FPUFCPOP FPUFPTMA5 FPUFPTMA10 FPUFPTMA15 a

tensile strength (kPa)

elongation at break (%)

50% CSa (%)

volume percentage of open cells (%)

79 ± 5

165 ± 10

1.0 ± 0.1

91.95

128 ± 6

210 ± 20

4.0 ± 0.2

90.18

98 ± 9

101 ± 6

6.6 ± 0.2

89.76

91 ± 8

91 ± 5

10.4 ± 0.3

90.41

76 ± 7

74 ± 5

14.7 ± 0.3

93.97

sample neat FPUF FPUFCPOP FPUFPTMA5 FPUFPTMA10 FPUFPTMA15

T5% (°C)

Tmax1 (°C)

RTmax1 (%/min)

Tmax2 (°C)

RTmax2 (%/min)

C600 °C (%)

255 252

293 286

6.5 6.6

368 375

15.2 17.7

0.3 2.3

246

282

4.0

384

14.1

10.2

236

282

4.1

383

12.1

11.4

235

282

4.1

383

11.4

12.1

a T5% denotes the 5% weight loss temperature. Tmaxdenotes the maximum weight loss temperature. RTmax is the weight loss rate at Tmax. C600 °C represents the char residue at 600 °C.

50% CS represents compression set at 50%.

residues were obtained, which shielded the polymeric substrate from the flame.47 When only CPOP-3628H was added, the T5% and Tmax1 values of FPUF-CPOP were slightly lower than those of neat FPUF, which was ascribed to the decomposition of CPOP3628H. Compared with that of neat FPUF, the second maximum weight loss temperature (Tmax2) for FPUF-CPOP was increased, which also can be ascribed to the enhancement of char residue caused by the nitrogen-containing segments. 3.4. Fire Performance. At present, cone calorimeter measurements provide one of the most effective methods for assessing the fire behavior of materials. The cone calorimeter brings quantitative analysis to the flammability of materials by investigating parameters such as the heat release rate (HRR), especially the peak HRR (PHRR); the time to ignition (TTI); the total heat release (THR); the maximum average rate of heat emission (MARHE); and the average effective heat of combustion of volatiles (Av-EHC). A heat flux of 25 kW/m2 is ordinarily chosen for flexible polyurethane foam, as the foam cannot withstand high heat flux because of its open-cell structure.2,3,8,11,12 Moreover, to make our results comparable with those of others, in this investigation, the same heat flux was also applied. Figure 4 shows the HRR and THR curves of neat FPUF, FPUF-CPOP, and FPUF-PTMAs. The corresponding cone calorimetry data and LOI values are collected in Table 4. It has

secondary amine, and olefin, as well as carbon dioxide. The second and third degradation steps correspond to the thermal decomposition of the soft segments.6 The thermogravimetry/ differential thermogravimetry (TG/DTG) curves of neat FPUF, FPUF-CPOP, and FPUF-PTMAs are shown in Figure 3, and the detailed thermal decomposition data are listed in Table 3. To better illustrate the decomposition of foams linked with PTMA, a nitrogen atmosphere was chosen as the pyrolyzing environment because it is very similar to the conditions behind the flame in the cone calorimeter test.45 It was observed that the thermal degradation of each of the FPUFs occurred in two stages. When PTMA was introduced into FPUF, the onset degradation temperature (T5%) and the first maximum weight loss temperature (Tmax1) were shifted to lower temperatures, and the weight loss rate of Tmax1 (RTmax1) also decreased. This occurred because the urethane group formed between PTMA and TDI 80/20 is less stable than the corresponding groups formed from TMN 3050, CPOP-3628H, and TDI 80/20. Other researchers have also found that phosphorus-containing polyurethanes are thermally less stable than neat polyurethane.46 Regarding the second decomposition step, the second maximum weight loss temperature (Tmax2) of FPUF-PTMA was shifted to higher temperature, and the corresponding weight loss rate was decreased. In addition, as the PTMA loading was increased, increasing amounts of char

Figure 3. TG/DTG curves of neat FPUF, FPUF-CPOP, and FPUF-PTMAs in N2. 1164



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Figure 4. HRR and THR curves of neat FPUF, FPUF-CPOP, and FPUF-PTMAs.

Table 4. Cone Calorimeter Testing Data for Neat FPUF, FPUF-CPOP, and FPUF-PTMAs

a

sample

TTI (s)

PHRR (kW/m2)

time to PHRR (s)

THR (MJ/m2)

MARHEa (kW/m2)

Av-EHCb (MJ/kg)

char yield (%)

LOI (%)

neat FPUF FPUF-CPOP FPUF-PTMA5 FPUF-PTMA10 FPUF-PTMA15

0 1 2 5 2

381 373 345 307 301

45 65 60 60 35

27 22 19 15 12

267 243 233 219 204

26.4 28.3 23.9 23.4 22.9

0 6 18 22 19

17.0 19.0 20.5 22.5 23.0

MARHE denotes the maximum average rate of heat emission. bAv-EHC represents the average effective heat of combustion of volatiles.

been reported that fire properties are improved with increasing foam density.48,49 Therefore, to eliminate the effects of density on flame retardancy, in this work, FPUFs with the same density were prepared and investigated. From Figure 4 and Table 4, one can clearly see that FPUF is a flammable polymeric material, as indicated by the high PHRR value (381 kW/m2) that was measured. The addition of CPOP-3628H and PTMA led to a decrease in the PHRR, as well as an increase in the time to PHRR, especially when the PTMA content reached 10 php (5.8 wt %), for which the PHRR was decreased to 307 kW/m2 and the time to PHRR was prolonged by 15 s. The reduction of the PHRR can probably be attributed to the char formation promoted by PTMA and the inert gases produced from CPOP3628H. It can be seen from Figure 5a-1−c-1 that the introduction of PTMA into the polyurethane chain obviously increased the char residue, but almost no char was generated for neat FPUF. When the PTMA content was increased further to 15 php (7.8 wt %), the PHRR and THR were reduced by 27% and 56%, respectively. These reductions were higher than our previously reported values of 11% and 18%, respectively, for FPUF containing 20 php (12 wt %) additive flame retardants.8 However, the reduced level of the PHRR and the time to PHRR for FPUF-PTMA15 were lessened. When the content of PTMA exceeded 10 php, the foam structure was seriously ruptured (Figure 2e-1), and the volume percentage of open cells was obviously increased (Table 2), which increased the inner surface area and air permeability. It has been demonstrated that a high air permeability and high inner surface area of the foam structure cause flexible polyurethane foam to be highly flammable.50 This might be one of the reasons for FPUF-PTMA15 to lack any flammability reduction. Moreover, the reaction activity of PTMA with TDI 80/20 was higher than that of the polyether polyols (TMN 3050 and

CPOP-3628H), which gave the PU chain uneven phosphoruscontaining segments. This caused the uneven interaction early in the fire: It still had an effect on the THR, but not as much on the PHRR, time to PHRR, and LOI. In addition, with increasing PTMA content, the THR and MARHE were gradually reduced, and the char yield of FPUF was increased, which indicated that the polyurethane chain formed between PTMA and TDI 80/20 participated in the carbonization process. The higher Av-EHC value represents the greater amount of gases decomposed from the material. The FPUFCPOP foam had a higher value of Av-EHC than neat FPUF, whereas the FPUF-PTMA foams had lower values than the neat sample. This is more evidence that PTMA mainly played a role in the condensed phase. In addition, it also indicates that CPOP-3628H mainly worked in the vapor phase and that it could provide a gas source for the formation of intumescent char. Because of the low weight percentage of PTMA and the open-cell structure of the foam, the improvement of TTI was not significant, but at least the flame-retardant effect of PTMA was confirmed. Moreover, with increasing PTMA content, the LOI value was accordingly enhanced (Table 4). When the loading of PTMA surpassed 10 php, the LOI value of flame-retardant FPUF increased slowly, which correlates well with the results of the cone calorimeter tests. It also can be seen from the photograph (Figure 5c-2) after the LOI test that the introduction of PTMA into the PU chain obviously increased the char residue. 3.5. Analysis of the Residual Chars and Evolved Gases. To further illustrate the effects of PTMA on the char formation of flame-retardant FPUF, the microstructures and composition of the char after combustion were examined by SEM, FTIR spectroscopy, ICP-AES, and EDX spectroscopy. Figure 6 shows the char morphologies of FPUF-PTMA15 1165



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Figure 5. Char residue photographs of (a-1,a-2) neat FPUF, (b-1,b-2) FPUF-CPOP, and (c-1,c-2) FPUF-PTMA15 after cone and LOI tests.

PTMA, illustrating that PTMA mainly played a role in the condensed phase of flame-retardant FPUF. EDX spectroscopy can provide information about the elemental composition and content of the char layer, and the detailed results for FPUF-PTMA15 char are shown in Figure 8. It was found that the weight percentages of phosphorus in the char residue of FPUF-PTMA15 reached 5.94 and 7.19 wt % in its external and internal char layer surfaces, respectively. The average phosphorus content in the char of FPUF-PTMA15 (6.57 wt %) was much higher than the content added in the FPUF-PTMA15 foam (1.55 wt %), which also indicates that the phosphorus derived from PTMA mainly stayed in the char residue. In addition, the ratio of phosphorus to oxygen can be calculated as

obtained after cone calorimeter testing. A compact and dense char layer was observed on the external surface of FPUFPTMA15 (Figure 6a-1,a-2), which acted as a barrier to inhibit the transmission of heat and oxygen during combustion. In addition, it also can be seen from Figure 6b-1,b-2 that the internal surface of char had a swollen charred layer, which effectively prevented the inside pyrolysis products from transmitting into the flame zone. These images indicate that PTMA had the effect of promoting the char formation in the condensed phase. An FTIR spectrum of FPUF-PTMA15 char is shown in Figure 7. It can be observed that CH2 or CH3 groups (2919, 2850 cm−1), CC groups (1621 cm−1), PO groups (1228 cm−1), and POC groups (1082 cm−1) existed in the char. The presence of PO and POC groups indicates that the PTMA segments in the FPUF probably decomposed into cross-linked phosphoric acid derivatives, which promoted char formation and retention. To further illustrate the decomposition of PTMA mainly releasing gas or being retained in the condensed phase, PTMAcross-linked foams and their chars after cone calorimeter testing were examined by ICP-AES. Table 5 displays the detailed data from the ICP-AES tests. It can be seen that the percentage of phosphorus retained in the char compared to that retained in the foam (P%) was 60.6%, 85.9%, and 59.8% for FPUF-PTMA5, FPUF-PTMA10, and FPUF-PTMA15, respectively. This means that most of the phosphorus was retained in the char for

P/O =

Pwt % /31 Owt % /16

(2)

where Pwt % is the average weight percentage of phosphorus (6.57 wt %) in the char residue, Owt % is the average weight percentage of oxygen (14.02 wt %) in the char residue, 31 is the relative atomic weight of phosphorus, and 16 is the relative atomic weight of oxygen. According to eq 2, the ratio of phosphorus to oxygen was about 1:4. This means that polyphosphoric acid or its derivative (i.e., the decomposition product of PTMA) existed in the char residue. TG-FTIR analysis was used to analyze the gas products during the thermal degradation process. FTIR spectra of the 1166



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Figure 6. SEM microphotographs of FPUF-PTMA15 char residue (a-1,a-2) external surface and (b-1,b-2) internal surface at different magnifications [(1) ×500, (2) ×1000].

pyrolysis products evolved from neat FPUF, FPUF-CPOP, and FPUF-PTMA15 at T5%, Tmax1, and Tmax2 are shown in Figure 9. It can be seen that all three of the foams evolved HCN or gases containing NCO groups (2274 or 2309 cm−1) at T5% and Tmax1. This was due to the decomposition of urethane groups (NHCOO) in the polyurethane chains at these temperatures.5 Furthermore, PTMA-cross-linked FPUF (FPUFPTMA15) released CO2 (2349 cm−1) and some compounds containing PH (2377 cm−1) before the first maximum weight loss temperature (282 °C). This was probably due to the cleavage of PC bonds and urethane groups formed between PTMA and TDI 80/20. At Tmax2, all foams released gases containing CH2 or CH3 groups (2978, 2938, and 2891 cm−1), CO groups (1743 cm−1), and CO groups (1104 cm−1). It has been reported that the first thermal degradation step of polyurethanes is due to the degradation of the hard segments whereas the second step corresponds to the thermal

Figure 7. FTIR spectrum of FPUF-PTMA15 char residue.

Table 5. ICP-AES Dataa for FPUF-PTMAs and Their Chars after the Cone Calorimeter Test sample

mf1 (g)

mf2 (mg)

Cpf (ppm)

mc1 (g)

mc2 (mg)

Cpc (ppm)

Pmf (wt %)

Pmc (wt %)

P% (%)

FPUF-PTMA5 FPUF-PTMA10 FPUF-PTMA15

7.72 6.33 5.41

8.9 8.8 9.1

0.6697 1.075 1.408

1.37 1.42 1.02

4.5 5.3 5.7

1.152 2.477 2.802

0.75 1.22 1.55

2.56 4.67 4.92

60.6 85.9 59.8

a Quantities measured: mf1, mass of foam plate for cone calorimeter test; mf2, mass of foam for ICP-AES test; mc1, mass of char after cone calorimeter test; mc2, mass of char for ICP-AES test; Cpf, mass concentration of phosphorus in the foam′s ICP-AES testing solution; Cpc, mass concentration of phosphorus in the char′s ICP-AES testing solution; Pmf, weight percentage of phosphorus in the foam, which was calculated as Pmf = 0.1(Cpf/mf2) × 100%; Pmc, weight percentage of phosphorus in the char, which was calculated as Pmc = 0.1(Cpc/mc2) × 100%; and P%, percentage of phosphorus retained in the char compared to that in the foam, which was calculated as P% = (Pmcmc1)/(Pmfmf1) × 100%.

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precursors, such as TDI, TMN 3050, and CPOP-3628H. These isocyanates and polyether polyols can further form carbodiimides and dienes with the evolution of CO2, NH3, H2O, and THF. Meanwhile, in this temperature range, the decomposition product of PTMA was dehydrated to form polyphosphoric acid, which is a strong Lewis acid and can catalyze the cross-linking reaction of dienes to form stable char containing polyaromatic structures at higher temperatures (383−600 °C).

4. CONCLUSIONS Phosphorus-containing triol (PTMA) was synthesized and used as a cross-linking agent and a reactive-type flame retardant to successfully prepare inherently flame-retardant FPUF-PTMAs. FPUF with the same density can be obtained in the presence of polyether polyols (TMN 3050 and CPOP-3628H), TDI 80/20, and PTMA, but the foam morphology, such as the number of cell windows and cell size, was altered, especially for that with high PTMA content. Compared with neat polyurethane foam, the addition of PTMA enhanced the tensile strength except for that of FPUF-PTMA15. This illustrates that, although the ratio of hard to soft segments increased as the PTMA content was increased, which improved the tensile strength, the deterioration of the cell microstructure of the foams made the PTMAs lose this positive action. Even when the weight ratio of PTMA used in FPUF was low, the resulting FPUF-PTMAs exhibited good char formation. The PHRR, THR, and MARHE values of FPUF-PTMAs decreased with increasing PTMA content. Moreover, the char residues of the FPUF-PTMAs obviously increased, which acted as a barrier to inhibit the transmission of heat, oxygen, and pyrolysis products during combustion. It can be deduced that the decomposition product of PTMA, polyphosphoric acid or its derivatives, catalyzed the crosslinking reaction of dienes to form a stable char at high temperature. This illustrates that PTMA mainly plays a role in the condensed phase of flame-retardant FPUF.

Figure 8. EDX spectra of FPUF-PTMA15 char residue after the cone test.

decomposition of the soft segments.51 Therefore, the gases released at the second step can be attributed to the scission of the polyether segments of TMN 3050 and CPOP-3628H. In addition, the addition of nitrogen-containing polyether polyols (CPOP-3628H) helped to evolve a small amount of NH3 (931, 913 cm−1), which acted as an inert diluent in the flame. According to the above results combined with the reports in the literature,6,43,52−54 a possible thermal degradation mechanism of PTMA-cross-linked FPUF is proposed (Figure 10). First, at 40−282 °C, the urethane groups formed between PTMA and TDI 80/20 are broken and form some isocyanate and amine groups and PTMA released gases such as CO2 and HCN simultaneously. Second, at 282−383 °C, the urethane groups formed between polyether polyols (TMN 3050 and CPOP-3628H) and TDI 80/20 were decomposed into their

Figure 9. FTIR spectra of FPUF, FPUF-CPOP, and FPUF-PTMA15 pyrolysis products at T5%, Tmax1, and Tmax2 during TGA in nitrogen atmosphere. 1168



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Figure 10. Possible thermal degradation mechanism of PTMA-cross-linked FPUF.

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(3) Lefebvre, J.; Bastin, B.; Bras, M. L.; Duquesne, S.; Paleja, R.; Delobel, R. Thermal stability and fire properties of conventional flexible polyurethane foam formulations. Polym. Degrad. Stab. 2005, 88, 28. (4) Price, D.; Liu, Y.; Hull, T. R.; Milnes, G. J.; Kandola, B. K.; Horrocks, A. R. Burning behaviour of foam/cotton fabric combinations in the cone calorimeter. Polym. Degrad. Stab. 2002, 77, 213. (5) Singh, H.; Jain, A. K. Ignition, combustion, toxicity, and fire retardancy of polyurethane foams: A comprehensive review. J. Appl. Polym. Sci. 2009, 111, 1115. (6) Chattopadhyay, D. K.; Webster, D. C. Thermal stability and flame retardancy of polyurethanes. Prog. Polym. Sci. 2009, 34, 1068. (7) Czerwonka, D. G. Polyurethane foams from melamine solutions in reactive solvents based on ethyl methyl ketone. J. Appl. Polym. Sci. 2013, 128, 3465. (8) Chen, M. J.; Shao, Z. B.; Wang, X. L.; Chen, L.; Wang, Y. Z. Halogen-free flame-retardant flexible polyurethane foam with a novel nitrogen-phosphorus flame retardant. Ind. Eng. Chem. Res. 2012, 51, 9769. (9) Thirumal, M.; Khastgir, D.; Nando, G. B.; Naik, Y. P.; Singha, N. K. Halogen-free flame retardant PUF: Effect of melamine compounds on mechanical, thermal and flame retardant properties. Polym. Degrad. Stab. 2010, 95, 1138.

AUTHOR INFORMATION

Corresponding Authors

*Tel. and Fax: +86-28-85410755. E-mail: xiuliwang1@163. com. *Tel. and Fax: +86-28-85410755. [email protected]. 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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