Influence of Valence and Structure of Phosphorus-Containing

Article pubs.acs.org/IECR

Influence of Valence and Structure of Phosphorus-Containing Melamine Salts on the Decomposition and Fire Behaviors of Flexible Polyurethane Foams Ming-Jun Chen, Ying-Jun Xu, Wen-Hui Rao, 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, Sichuan, China S Supporting Information *

ABSTRACT: In this work, 2-carboxyethyl(phenyl)phosphinic acid melamine salt (CMA, phosphorus valence +1), melamine hypophosphite (MHPA, phosphorus valence +1), melamine phosphite (MPOA, phosphorus valence +3), and melamine pyrophosphate (MPyP, phosphorus valence +5) were respectively used as flame retardants for flexible polyurethane foam (FPUF), and fire performance as well as pyrolysis behaviors of FPUF were investigated systematically. CMA and MHPA were found to have better flame retardance than MPOA and MPyP for FPUF, which mainly played a role in the gas phase due to the release of many phosphorus-containing volatiles and melamine derivatives. The types of phosphorus-containing gaseous pyrolysis products of CMA were much more than that of MHPA due to the different chemical moieties surrounding the phosphorus atom, which made it have the highest flame-retardant efficiency for FPUF. Besides this, the possible thermal degradation mechanisms of CMA and MHPA were proposed. resulted in the superior fire performance.13 However, Mariappan et al. found that phosphite additive was more suitable for epoxy resin than phosphine oxide and phosphate additives, because the formation of nonconventional intumescent char caused by the transesterification between the phosphite ester and the hydroxyl group present in the epoxy resin.14 Mariappan et al. also investigated the effect of different oxidation states of phosphorus additives (phosphine oxide, phosphite, and phosphate) on the thermal and flame-retardant properties of polyurea, and their results showed triphenylphosphate mainly played in the condensed phase was more effective for polyurea.14 Lorenzetti and her co-workers15 selected two liquid flame retardants (dimethylpropanphosphonate DMPP and triethylphosphate TEP) and two solid flame retardants (aluminum phosphinate IPA and polyphosphate APP) to study the influence of phosphorus valency on the thermal behavior of flame retarded rigid polyurethane foams. They found that the main mechanism of flame retardant action occurred in the vapor phase regardless of whether the liquid additive was DMPP or TEP. However, a combined gas and solid phase action took place in an IPA filled foam, while only solid phase action was observed in an APP filled system.15 It is a pity that they did not compare the fire performance of these flame retarded foams. From the published literature, it can be concluded that the predominance of the different mechanisms actually operating in

1. INTRODUCTION Nitrogen and phosphorus compounds have many advantages as flame retardants for polymers.1−6 Nitrogen compounds, especially melamine, mainly play a role in the gas phase due to the release of NH 3 which can dilute the oxygen concentration surrounding the matrix.7 It also makes a small effect on the condensed phase since the formation of more thermally stable polymeric products, such as melam (∼350 °C), melem (∼450 °C), and melon (∼600 °C).8 Phosphorus compounds also work on both the condensed phase and the gas phase. In the condensed phase, the generated polyphosphoric acid catalyzes the formation of char which protects the underlying material from heat and acts as a barrier to the fuel gases release from the surface.9−11 In the gas phase, the released phosphorus-containing gases may produce phosphorus-containing free radicals, which can scavenge H• and OH• in the flame, cutting off the decomposition radical reactions to decrease the supply of combustible volatiles, and consequently exhibit flame inhibition in the gaseous phase.12 The influence of phosphorus valence and structure of phosphorus compounds on the thermal and flame-retardant behaviors of epoxy resin, polystyrene, polyurea, and rigid polyurethane foam have already been reported. Braun and her co-workers compared the fire behavior of epoxy resins containing phosphine oxide-based, phosphinate-based, phosphonate-based, and phosphate-based hardeners, respectively, and found that with increasing the oxidation state of phosphorus the thermally stable residue increased, whereas the release of phosphorus-containing volatiles decreased. Their results indicated that the composite containing phosphine oxide-based hardener was mainly active in the gas phase, which © 2014 American Chemical Society

Received: Revised: Accepted: Published: 8773

February 18, 2014 April 23, 2014 May 9, 2014 May 9, 2014 dx.doi.org/10.1021/ie500691p | Ind. Eng. Chem. Res. 2014, 53, 8773−8783

Industrial & Engineering Chemistry Research

Article

Scheme 1. Chemical Structures of CMA, MHPA, MPOA, and MPyP

Table 1. Formulations of Neat FPUF and FPUF with the Same Loadings of CMA, MHPA, MPOA, and MPyPa

a

sample

TMN3050 (php)

FR (php)

H2O (php)

DABCO (php)

DBTDL (php)

SZ580 (php)

TDI80/20 (php)

neat FPUF FPUF/CMA20 FPUF/MHPA20 FPUF/MPOA20 FPUF/MPyP20

100 100 100 100 100

0 20 20 20 20

3.0 3.0 3.0 3.0 3.0

0.10 0.24 0.80 0.60 0.80

0.18 0.48 0.38 0.50 0.42

1.0 2.0 2.0 2.0 2.0

41 41 41 41 41

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

Chemical Regent Factory, China. Surfactant (SZ 580, technical pure grade) was supplied by Beijing Wanbo Huijia Technology and Trade Co., Ltd., China. Distilled water was used as a chemical blowing agent. The flame retardants were 2carboxyethyl(phenyl)phosphinic acid melamine salt (CMA, phosphorus valence +1), melamine hypophosphite (MHPA, phosphorus valence +1), melamine phosphite (MPOA, phosphorus valence +3), and melamine pyrophosphate (MPyP, phosphorus valence +5). CMA, MHPA, and MPOA were synthesized in our laboratory, and their synthetic methods were very similar.18 MPyP was analytical reagent grade and supplied by Shanghai Titanchem Co., Ltd. The chemical structures of CMA, MHPA, MPOA, and MPyP are shown in Scheme 1. 2.2. Preparation of Flame-Retardant FPUF. The flameretardant FPUF samples were prepared by a one-pot and freerise method. A typical preparation process for FPUF/CMA20 is shown as follows: polyether polyols (TMN 3050, 200 g), blowing agent (distilled water, 6.0 g), catalysts (DABCO, 0.48 g and DBTDL, 0.96 g), surfactant (SZ 580, 4.0 g), and flame retardant (CMA, 40 g) were well mixed in a 1 L plastic beaker. Then, toluene diisocyanate (TDI 80/20, 82 g) 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 (FPUF/CMA20). The foam was cured for 24 h under ambient conditions. The molar ratio of NCO (from TDI 80/20) to OH (from TMN 3050 and water) was 1.05. The formulations of all the FPUFs are shown in Table 1. 2.3. Measurements. Limiting oxygen index (LOI) tests were performed at room temperature according to ISO 45891:1996 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) is a suitable standard for evaluating the flame

a particular system depends on not only the phosphorus valence but also the nature of chemical moieties surrounding the phosphorus atom.6 Besides, the chemical structure of polymer and the physical state of phosphorus compound are also significant factors in terms of the level of flame retardance achieved.16,17 As far as we know, the influence of phosphorus valence and structure on the decomposition and fire behavior of flexible polyurethane foam (FPUF) has not been reported. In our previous work, FPUF containing only 12 wt % 2-carboxyethyl(phenyl)phosphinic acid melamine salt (CMA) can pass the requirements of California Technical Bulletin 117 Section A Part I (Cal T.B. 117A-Part I), which is more suitable for evaluating the flame retardance of resilient cellular materials.18 However, when melamine pyrophosphate was used as flame retardants for flexible FPUF, the flame retardance was not satisfactory. Hence, in this paper we aim to compare the flameretardant efficiency of CMA with other phosphorus-containing melamine salts (melamine hypophosphite, melamine phosphite, and melamine pyrophosphate) with different valence and structure and make a valuable contribution toward a better understanding of the influence of phosphorus valence and structure on the decomposition and fire behavior of flameretardant FPUF as well as provide a theoretical foundation for the design of more efficient new flame retardants for FPUF.

2. EXPERIMENTAL SECTION 2.1. Material. Polyether polyols (TMN 3050, numberaverage molecular weight 3000, average functionality of 3.0, OH content 56 mg KOH/g, technical pure grade) was obtained from the Third Oil Refinery of Tianjin Petrochemical Company, China. Toluene diisocyanate (TDI 80/20, technical pure grade) was obtained from Chongqing Weiteng Polyurethane Products Factory, China. Triethylenediamine hexahydrate (DABCO) and dibutyltin dilaurate (DBTDL) were analytical reagent grade and supplied by Chengdu Kelong 8774

dx.doi.org/10.1021/ie500691p | Ind. Eng. Chem. Res. 2014, 53, 8773−8783


Article

Table 2. LOI Values and Flammability Behaviors of Neat FPUF, FPUF/CMA20, FPUF/MHPA20, FPUF/MPOA20, and FPUF/ MPyP20a afterflame time (s)

a

afterglow time (s)

char length (inch)

sample

av

max

av

av

max

test results (pass or fail)

neat FPUF FPUF/CMA20 FPUF/MHPA20 FPUF/MPOA20 FPUF/MPyP20

36.7 1.2 51.5 95.3 86.1

56.2 2.5 97.6 129.6 116.1

0.0 0.0 0.0 0.0 0.0

no 5.6 7.4 0.0 0.0

no 6.1 10.2 0.0 0.0

fail pass fail fail fail

LOI (%) 17.0 23.5 22.5 21.5 21.5

± ± ± ± ±

0.5 0.5 0.5 0.5 0.5

According to Cal T.B. 117A-Part I standard. av represents average; max represents maximum.

resistance, glow propagation, and tendency to char of flexible polyurethane foam.19 During the test, a vertical specimen (12 × 3 × 1/2 in.) was suspended vertically in the cabinet, and the lower end of the specimen was 0.75 in. above the top of the burner. The burner flame was applied vertically at the middle of the lower edge of the specimens for 12 s. If the average afterflame time, afterglow time, and char length as well as the maximum afterflame time and char length of all the specimens did not exceed 5 s, 15 s, 6 in., 10 s. and 8 in., respectively, the material could pass the criteria. 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 specimen was 100 × 100 × 25 mm3 (length × width × thickness). FTIR spectra of samples were recorded on a Nicolet FTIR 170SX spectrometer over the wave numbers range from 500 to 4000 cm−1 using KBr pellets. A NETZSCH 209F1 TA Instruments 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 gases evolved during TGA tests were analyzed by coupling FTIR. Residues obtained in TGA by heating the samples to a desired temperature (maximum weight loss temperatures) and 600 °C were subjected to the FTIR analysis. The actual phosphorus contents of FPUFs containing phosphorus-containing melamine salts and their residual chars were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES; IRIS Advantage, TJA solution). About 9 mg foam or 3 mg char was completely combusted into gases under sufficient oxygen atmosphere; the gases were absorbed by 25 mL of 0.001 mol/L KMnO4/KOH solution and then diluted to 100 mL using deionized water. The phosphorus content in the char residue after cone calorimeter test was detected by X-ray photoelectron spectroscopy (XPS). XPS spectra were recorded by a XSAM80 (Kratos Co, UK), using Al Kα excitation radiation (hv-1486.6 eV). Py-GC/MS tests were performed in a Pyroprobe (CDS5000). The pyrolysis chamber was full of He, the relevant samples (300 μg) were heated from ambient to 400 °C at a rate of 1000 °C/min and kept for 20 s. The pyrolyzer was coupled with DANI MASTER GC-TOF-MS Systems, and the carrier gas was He. For the operation, the temperature program of the capillary column (DN-1701 FAST 10 m 0.10 mm 0.10 mm) of GC was as follows: 2 min at 45 °C, temperature increased to 280 °C at a rate of 10 °C/min, and then kept at 280 °C for 5 min. The injector temperature was 300 °C. MS indicator was operated in the electron impact mode at electron energy of 70 eV, and the ion source temperature was kept at 180 °C. The detection of mass spectra was carried out using a NIST library.

3. RESULTS AND DISCUSSION 3.1. Fire Performance. In our previous work, CMA exhibited good flame retardance for FPUF, and the best addition content was 20 php.18 To compare the effect of different phosphorus-containing melamine salts on the flame retardance of FPUF, in this work, these salts (CMA, MHPA, MPOA, and MPyP) with the same contents (20 php) were added, and the fire behavior of the foams was tested under the same conditions. LOI, flame propagation, and cone calorimeter measurements were used to investigate the flame retardance of FPUF containing CMA, MHPA, MPOA, and MPyP, respectively. The LOI values and flame propagation test results are displayed in Table 2 and Figure 1. It can be seen from Table 2 that FPUF/CMA20 had the highest LOI value, and only it passed the flame propagation test according to the standard of Cal T.B. 117A-Part I. However, the other foams failed in the flame propagation test because of the rapid fire growth, and they were burned out except FPUF/MHPA20. One reason for this difference was that more CMA and MHPA particles dispersed in the struts and strut junctions of the foams than MPOA and MPyP (Supplementary Figure S1 a-2−d-2), which may be caused by their small particle size and better particle distribution (Figure S2). The more flame retardant particles dispersed in the foam skeleton bring the better flame retardancy to the foam, which has been demonstrated by M. Kageoka et al.20 They found if there is not enough melamine particles in the foam strut, FPUF cannot inhibit the burning. The other reason was that the air permeability of MPOA and MPyP filled foams were higher than that of CMA and MHPA filled foams (Supplementary Figure S1 a-1−d-1), and it has been demonstrated that ignition is more likely to occur with a high value of air permeability.21 Figure 1 clearly shows that only FPUF/CMA20 and FPUF/MHPA20 did not burn out after continuous ignition for 12 s. This indicated that CMA and MHPA had better flame-retardant efficiency on FPUF than MPOA and MPyP, in which CMA was the best one. This difference might be related to their different phosphorus valences and chemical structures. At present, cone calorimeter measurement is one of the most effective methods for assessing the fire behavior of materials. Cone calorimeter brings quantitative analysis to the flammability of materials by investigating some parameters, such as heat release rate (HRR), the peak of HRR (PHRR), time to PHRR (tP), total heat release (THR), total smoke production (TSP), the maximum average rate of heat emission (MARHE), and the average effective heat of combustion of volatiles (AvEHC). These results of cone calorimeter investigations are illustrated in Figure 2 and Table 3. Figure 2 shows that the addition of phosphorus-containing melamine salts brought the decrease of HRR and THR. Table 3 presents that the loading of 8775



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the other flame-retardant foams. THR, MARHE, and FIGRA data was introduced to assess the hazard of developing fires, and the lower these values the better.22 These data shown in Table 2 indicated that the foam containing 20 php CMA had better flame retardancy than the foam containing 20 php MHPA or MPOA or MPyP. The significant increase in TSP of FPUF/CMA20 signified that CMA probably mainly played a role in the gas phase. The above results means different phosphorus valence and chemical structure of compounds show different flame-retardant mechanisms on materials,13,15 which will be discussed in the later part. 3.2. Thermal Degradation Analysis. To better illustrate the decomposition of foams loaded with different phosphoruscontaining melamine salts, nitrogen atmosphere was chosen because it was very similar to the condition behind the flame in cone calorimeter test.23 The mass loss curves during the decomposition of CMA, MHPA, MPOA, and MPyP and their filled foams are presented in Figure 3, and the main results are summarized in Table 4. Figure 3 (a, b) and Table 4 show that the decomposition of CMA, MHPA, and MPOA was complex and presented more stages (four or five decomposition regions) than MPyP (two decomposition regions). In addition, the 5% weight loss temperature (T5%) of CMA (225 °C), MHPA (241 °C), and MPOA (226 °C) was lower than that of MPyP (327 °C). This illustrated that different valence and chemical structures of phosphorus-containing compounds caused different thermal behaviors.15 Our previous study indicated that the first maximum weight loss temperature (Tmax1, 121 °C) of CMA was due to the elimination of adsorbed water, the second maximum weight loss temperature (Tmax2, 229 °C) was caused by the breakage of the P−C bond in CMA, the third maximum weight loss temperature (Tmax3, 313 °C) belonged to the breaking of the ionic bond between −COO− and RNH3+ as well as the further formation of cross-linked 2-carboxyethyl(phenyl)phosphinic acid and melamine derivatives, and the fourth maximum weight loss temperature (Tmax4, 440 °C) was attributed to the formation of polyphosphoric acid.18 Similarly, the Tmax1 of MHPA and MPOA was also due to the elimination of adsorbed water, their Tmax2 was caused by the breakage of the ionic bond between P−O− and RNH3+, their Tmax3 was ascribed to the further decomposition of hypophosphorous acid, phosphorous acid, and melamine, and the Tmax4 and Tmax5 were also attributed to the formation of polyphosphoric acid and the evolution of NH3. MPyP had the highest T5%, Tmax, and

Figure 1. Digital photos of neat FPUF (a-1, a-2, a-3), FPUF/CMA20 (b-1, b-2, b-3), FPUF/MHPA20 (c-1, c-2, c-3), FPUF/MPOA20 (d-1, d-2, d-3), and FPUF/MPyP20 (e-1, e-2, e-3) burned at different times (1 s, 10 s, 25 s).

CMA, MHPA, MPOA, and MPyP caused not only the reduction of PHRR, THR, MARHE, Av-EHC, and FIGRA but also the enhancement of TSP and char yield. Especially, FPUF/CMA20 had the lowest PHRR and FIGRA values as well as the highest tP, TSP, and Av-EHC values compared to

Figure 2. HRR (a) and THR (b) curves of neat FPUF, FPUF/CMA20, FPUF/MHPA20, FPUF/MPOA20, and FPUF/MPyP20 under 25 kW/m2 external heat flux. 8776



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Table 3. Effect of CMA, MHPA, MPOA, and MPyP on Fire Performance of FPUF sample neat FPUF FPUF/CMA20 FPUF/MHPA20 FPUF/MPOA20 FPUF/MPyP20

tP (s)a 75 100 75 70 80

± ± ± ± ±

10 15 10 15 5

PHRR (kW/m2) 424 359 393 388 385

± ± ± ± ±

20 17 20 9 24

THR (MJ/m2) 34.2 26.6 28.6 26.6 24.3

± ± ± ± ±

MARHEb (kW/m2)

0.6 1.0 6.7 4.6 0.3

263 222 220 223 235

± ± ± ± ±

7 8 4 8 7

TSP (m2) 1.49 5.46 3.30 3.34 3.04

± ± ± ± ±

0.33 0.22 0.07 0.16 0.22

Av-EHCc (MJ/kg) 28 23 18 23 22

± ± ± ± ±

1 1 6 1 1

FIGRAd 5.65 3.60 5.24 5.55 4.81

± ± ± ± ±

0.88 0.17 1.15 1.67 0.56

char yield (%) 0 8 7 7 6

± ± ± ± ±

0 1 1 1 1

a

tP denotes the time to peak of heat release rate (PHRR). bMARHE denotes the maximum average rate of heat emission. cAv-EHC means the average effective heat of combustion of volatiles. dFIGRA is calculated by dividing the PHRR by the time to PHRR (tP).

Figure 3. TG and DTG curves of phosphorus-containing melamine salts (a, b) and flame-retardant FPUF (c, d) at the heating rate of 10 °C/min in N2.

polyphosphate (MPP).25 The increase of Tmax4 probably attributed to the dilution effect of NH3 decomposed from melamine and the barrier effect of char promoted by polyphosphoric acid. However, the incorporation of MHPA, MPOA, and MPyP had no obvious effect on the shifting of Tmax2 and Tmax4. This may be due to that there was not much interaction between the pyrolysis products of these salts and FPUF. Besides, from Figure 3 (a, c), we also can see that CMA had the highest weight loss rate (Figure 3a) than the other phosphorus-containing melamine salts (MHPA, MPOA, and MPyP); however, CMA filled foam had the lowest weight loss rate compared to the other foams (Figure 3c). This indicated that there may be some interactions taken place between the pyrolysis products of CMA and neat FPUF, or the pyrolysis products of CMA can prevent the further decomposition of FPUF.18 This obvious interaction was one of the reasons for why FPUF/CMA20 had better flame retardance than FPUF/ MHPA20, FPUF/MPOA20, and FPUF/MPyP20. In addition,

residual mass (wtR600), which was related to its highest oxidation state of phosphorus. These results will be demonstrated by the following analysis of the residual chars and the evolved gases. Figure 3 (c, d) and Table 4 display that the decomposition of neat FPUF and FPUF respectively containing CMA, MHPA, MOPA, and MPyP mainly took place in two stages (Tmax2 and Tmax4). The second maximum mass loss process of the foams began with the degradation of hard segments, resulting in the formation of isocyanate, alcohol, primary or secondary amine, and olefin as well as carbon dioxide.24 The fourth maximum degradation step corresponded to the thermal decomposition of the soft segments.4 From Table 4, it can be seen that the addition of CMA caused the decrease of Tmax2 and the increase of Tmax4. The decrease of Tmax2 may be due to the formation of phosphinic acid derivatives, as a strong Lewis acid catalyst accelerated the decomposition of FPUF. This phenomenon was also found in the rigid polyurethane foam containing melamine 8777



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X-ray photoelectron spectroscopy (XPS) was utilized to investigate the atomic concentration of carbon (C), nitrogen (N), oxygen (O), and phosphorus (P) for the solid products of FPUF after the cone calorimeter test. In all the foam chars, C, N, O, and P with different intensity can be found (Figure 4 a). For FPUF/CMA20, its P intensity in the char residue is the weakest, which can be seen clearly in Figure 4 b. Figure 4 c displays a decrease in the content of C but an increased tendency in P and O with increasing the phosphorus valence. This means a more and more steady phosphocarbonaceous structure containing P−O−P, P−O−C, and aromatic groups was formed as the increase of phosphorus valence.11 The intensity and content of phosphorus in the char of FPUF/ CMA20 was not only the weakest but also the lowest compared with those of FPUF/MHPA20, FPUF/MPOA20, and FPUF/ MPyP20. This indicated that CMA mainly played a role in the gas phase. The phosphorus contents of CMA, MHPA, MPOA, and MPyP filled foams and their foam chars after the cone calorimeter test were examined by ICP-AES, respectively. Table 5 displays the detailed data of the ICP-AES test. It can be seen that the weight percentage of phosphorus in the char (Pmc) has the same tendency with that of the XPS test. In addition, the percentage of phosphorus retained in the char compared to that in the foam (P%) was 11%, 28%, 51%, and 65% for FPUF/ CMA20, FPUF/MHPA20, FPUF/MPOA20, and FPUF/ MPyP20, respectively. On the contrary, the percentage of phosphorus released into the gaseous phase compared to that in the foam was 89%, 72%, 49%, and 35%, respectively. This indicated that with increasing the valence of phosphorus the charring effect increased, whereas the release of phosphoruscontaining volatiles decreased. Considering the flame-retardant efficiency of these salts, it can be confirmed that the flame retardant mainly plays a role in the gas phase that is more favorable for FPUF. This means that melamine salts with low phosphorus valence (+1) are more favorable for enhancing the flame retardance of a flexible PU foam than that with high phosphorus valence (+5). However, another phosphoruscontaining melamine salt (melamine polyphosphate, MPP)

Table 4. TGA Data of CMA, MHPA, MPOA, MPyP, FPUF, and Flame Retardant FPUF in N2a sample CMA MHPA MPOA MPyP neat FPUF FPUF/ CMA20 FPUF/ MHPA20 FPUF/ MPOA20 FPUF/ MPyP20

T5% (°C)

Tmax1 (°C)

Tmax2 (°C)

Tmax3 (°C)

Tmax4 (°C)

Tmax5 (°C)

wtR600 (%)

225 241 226 327 253 243

121 89 136 394 / 213

229 265 201 533 287 280

313 308 308 / / 300

440 541 335 / 369 381

/ / 551 /

37 37 33 43 0.1 4.3

236

247

288

/

363

3.3

252

/

290

/

363

3.0

261

/

289

/

369

5.8

a T5% denotes the 5% weight loss temperature; Tmax denotes the maximum weight loss temperature; wtR600 denotes residual mass at 600 °C.

the Tmax1 of FPUF/CMA20 and FPUF/MHPA20 was caused by the elimination of adsorbed water in CMA and MHPA. Tmax3 of FPUF/CMA20 was ascribed to the formation of crosslinked 2-carboxyethyl(phenyl)phosphinic acid and melamine derivatives. The loadings of CMA, MHPA, MPOA, and MPyP increased the residual mass of the foams. Because of the interactions occurring between the decomposition products of CMA and neat FPUF, FPUF/CMA20 had higher wtR600 (4.3%) than FPUF/MHAP20 (3.3%) and FPUF/MPOA20 (3.0%). However, it was lower than that of FPUF/MPyP20 (5.6%), which was ascribed to the charring effect increase with the increasing oxidation state of phosphorus.26 3.3. Analysis of Residual Chars. To further investigate the flame-retardant effect of CMA, MHPA, MPOA, and MPyP mainly on the gas phase or the condensed phase of FPUF, the composition of foam chars after the cone calorimeter test were examined by XPS and ICP-AES. The solid pyrolysis products of CMA and MHPA were monitored by means of real-time FTIR.

Figure 4. Survey scan XPS spectra (a), binding energies of P2p (b), and atomic concentrations (c) for solid products of flame-retardant FPUF after cone calorimeter test. 8778



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Table 5. ICP-AES Data of Flame Retardant Foams and Their Charsa sample FPUF/CMA20 FPUF/MHPA20 FPUF/MPOA20 FPUF/MPyP20

mf1 (g) 11.03 8.82 10.62 9.59

mf2 (mg)

Cpf (ppm)

mc1 (g)

mc2 (mg)

Cpc (ppm)

Pmf (wt %)

Pmc (wt %)

P% (%)

9.4 5.1 9.0 8.0

0.96 0.79 0.83 0.76

0.76 0.62 0.73 0.57

3.0 3.3 2.6 4.3

0.47 2.02 1.79 4.43

1.0 1.6 0.9 1.0

1.6 6.1 6.9 10.3

11 28 51 65

a

Quantities measured: mf1, mass of foam for ICP-AES 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%.

Figure 5. FTIR spectra of CMA (a) and MHPA (b) residues obtained in TG by heating the samples to a desired temperature (Tmax and 600 °C).

Figure 6. FTIR spectra of pyrolysis products of CMA and MHPA at the maximum weight loss temperature.

Real-time FTIR was utilized to analyze the solid pyrolysis products of CMA and MHPA. Residues obtained in TG by heating the samples to a desired temperature (Tmax and 600 °C) were subjected to FTIR analysis. The FTIR spectra of the initial CMA, MHPA, and their solid pyrolysis products are shown in Figure 5. The peak of RNH3+ (3145 and 1508 cm−1), COO− (1657 cm−1), and PO (1270 cm−1) disappeared after 229 °C in Figure 5 (a), which indicated that the ionic bond (COO −NH3 +) was broken, and the pyrolysis product (phosphinic acid) was further decomposed above this temper-

with the highest phosphorus valence (+5) was proved to exhibit good flame retardance in a rigid PU foam.25 It indicates that phosphorus-containing melamine salts with the same phosphorus valence will exhibit different flame-retardant efficiency on flexible and rigid polyurethane foams. The above results also show that CMA can endow FPUF with better flame retardance than MHPA even if both of them have the same phosphorus valence. Therefore, it is necessary to investigate the thermal degradation mechanism of CMA and MHPA to find the difference. 8779



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Figure 7. Ion chromatogram of gaseous products from CMA (a) and the mass spectra of peak 1 (b) and peak 19 (c) in (a) pyrolyzed at 400 °C.

Figure 8. Ion chromatogram of gaseous products from MHPA (a) and the mass spectra of peak 5 (b) and peak 10 (c) in (a) pyrolyzed at 400 °C.

Similarly, adsorbed water (3400 cm−1), NH3 (964 cm−1, 929 cm−1), and some gases containing the group of N−H (3154− 3514 cm−1), CN and NN (2284 cm−1, 2250 cm−1), CN (1600−1660 cm−1), C−N (1439 cm−1), PO (1237 cm−1), P−O (1117 cm−1), etc. were decomposed from MHPA. The main differences between the pyrolysis products of CMA and MHPA were the absorption peaks of phenyl (3057 cm−1, 1481 cm−1) and CO2 (2356 cm−1, 2312 cm−1) in CMA as well as the absorption peak of P−H (2400 and 2381 cm−1) in MHPA. This was due to the difference of the chemical structure between CMA and MHPA. The release of nonflammable gases (NH3, CO2) made a dilution effect on the concentration of oxygen and flammable gases surrounding materials. To further illustrate the component of pyrolysis products, CMA and MHPA were pyrolyzed at 400 °C by Py-GC/MS. Figure 7 (a) shows that many gaseous products were decomposed from CMA, and the analysis results of the marked peaks were listed in Supplementary Table S1. These products were classified as 2-carboxyethyl(phenyl)phosphinic acid derivatives and melamine derivatives. These derivatives probably worked in two ways. On the one hand, some pyrolysis products containing the active groups of −OH and −NH2 possibly reacted with isocyanates decomposed from neat FPUF to reduce the contents of flammable gases. A similar result was reported by Price using melamine to improve the flame retardance of FPUF.7 On the other hand, these phosphorus-containing substances may produce phosphoruscontaining free radicals, which could scavenge H• and OH• in

ature. The generated polyphosphoric acid catalyzed the formation of the phosphocarbonaceous structure (CC, 1631 cm−1; P−O−C, 1123 cm−1).9 However, a minority of phosphorus (10.54%) was decomposed into polyphosphoric acid, because the PO peak around 1270 cm−1 could not be seen and the P−O−C peak at 1123 cm−1 became weaker. It can be seen from Figure 5 (b) that the P−H peak (2400 cm−1) disappeared and the PO peak shifted from 1220 to 1250 cm−1 after 265 °C. The peak of RNH3+ (3145 and 1507 cm−1) as well as P−O− (2673 cm−1) disappeared above 308 °C. This indicated that the P−H bond in MHPA was broken before the breakage of the ionic bond (between P−O− and RNH3+). At high temperature (541 °C, 600 °C), the phosphocarbonaceous structure (PO, 1250 cm−1; CN or CC, 1631 cm−1) was also formed. The absorption peak at 2170 cm−1 may be due to the formation of graphitic carbon nitride.8 3.4. Analysis of Evolved Gases. TG-FTIR and Py-GC/ MS were used to analyze the gaseous products of CMA and MHPA. FTIR spectra of pyrolysis products evolved from CMA and MHPA at the maximum weight loss temperature (Tmax) are shown in Figure 6. It can be seen from Figure 6 (a) that the adsorbed and combined water (3407 cm−1) was eliminated from CMA at lower temperature (121 °C, 229 °C). With continued heating, CO2 (2356 cm−1, 2312 cm−1), NH3 (964 cm−1, 929 cm−1), and some products containing the group of −NH2 (3404 cm−1, 3327 cm−1), phenyl (3057 cm−1, 1481 cm−1), CN (2284 cm−1, 2250 cm−1), CN (1600−1658 cm−1), PO (1200 cm−1), etc. were released from CMA. 8780



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Figure 9. Schematic diagram of the proposed thermal degradation mechanism model of CMA.

the flame, cutting off the decomposition radical reactions to decrease the supply of combustible volatiles and consequently exhibiting flame inhibition in the gaseous phase. It was in accordance with the effect of aluminum hypophosphite (AP) and aluminum isobutylphosphinate (APBu) on polyamide 6.12 From Table S1, it can be concluded that these fragments were not broken directly from CMA except benzene (Figure 7b) and melamine (Figure 7c), which indicated that possible chemical reactions occurred during pyrolysis or secondary pyrolysis at 400 °C. However, low molecular weight gases such as H2O and NH3 could not be detected because of a lower detection limit of 30. The pyrolysis products of MHPA presented in Figure 8a were less than CMA, and more melamine and its derivatives, but fewer kinds of hypophosphorous acid derivatives, were decomposed from MHPA (Supplementary Table S2). This illustrated simple chemical moieties surrounding phosphorus

atom led to the decomposition of little kinds of phosphoruscontaining fragments, which made the flame-retardant efficiency of MHPA lower than CMA. Interestingly, white phosphorus (P4) was detected from the pyrolysis products of MHPA (shown in Figure 8b), which was probably attributed to the hypophosphorous acid structure in MHPA.12 3.5. Decomposition Models. According to the above analysis, the proposed thermal degradation mechanism models for CMA and MHPA were illustrated in Figure 9 and Figure 10, respectively. The ionic bond (COO− NH3+) of CMA was broken up into its precursors (2-carboxyethyl(phenyl)phosphinic acid and melamine) above 229 °C. With continued heating, 2-carboxyethyl(phenyl)phosphinic acid was decomposed into many phosphorus-containing gaseous products, benzene and carbon dioxide (CO2), but a small amount of polyphosphoric acid was retained in the condensed phase. 8781



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Figure 10. Schematic diagram of the proposed thermal degradation mechanism model of MHPA.

these phosphorus-containing substances may produce phosphorus-containing free radicals, which could scavenge hydrogen and hydroxyl radicals (H• and OH•) in the flame, cutting off the decomposition radical reactions to decrease the supply of combustible volatiles.

Similarly, melamine was also pyrolyzed into many nitrogencontaining gaseous products (NH3, melamine, etc.) and few melamine condensation products (such as melem, melon).4 However, the types of phosphorus-containing gaseous products of MHPA were less than that of CMA due to the different chemical moieties surrounding the phosphorus atom. The P−H bond of MHPA was first broken up into phosphoric acid melamine salts at the temperature range of 89−265 °C, with concomitant evolution of phosphine (PH3) and white phosphorus (P4). At high temperature (above 308 °C), the ionic bond (PO− NH3+) of MHPA was broken up into phosphoric acid and melamine. Phosphoric acid was further decomposed into polyphosphoric acid and retained in the condensed phase. Melamine was broken up into many different kinds of nitrogen-containing gases (NH3, melamine, etc.) and a few melamine condensation products (such as melem, melon, graphitic carbon nitride). These gaseous products decomposed from CMA and MHPA probably work in two ways. On the one hand, these pyrolysis products containing the active group of −OH and −NH2 possibly reacted with isocyanates decomposed from neat FPUF to reduce the contents of flammable gases. On the other hand,

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CONCLUSIONS In this work, four different valence and structure of phosphorus-containing melamine salts (CMA, MHPA, MPOA, and MPyP) with the same content of 20 php (12 wt %) were respectively used to enhance the fire performance of FPUF. CMA and MHPA showed better flame retardance than MPOA and MPyP for FPUF, in which CMA was the most effective one. It was found that with increasing the oxidation state of phosphorus the thermally stable residues increased, whereas the release of phosphorus-containing volatiles decreased. As far as their flame-retardant mechanism is concerned, these phosphorus-containing melamine salts played a different role. CMA and MHPA with monovalent phosphorus valence (+1) mainly played a role in the gas phase of FPUF, MPOA with intermediate phosphorus valence (+3) played an equal role in the gas phase and condensed phase of FPUF, and 8782



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(8) Lotsch, B. V.; Schnick, W. New Light on an Old Story: Formation of Melam during Thermal Condensation of Melamine. Chem.Eur. J. 2007, 13, 4956. (9) Duquesne, S.; Le Bras, M.; Bourbigot, S.; Delobel, R.; Camino, G.; Eling, B.; Lindsay, C.; Roels, T.; Vezin, H. Mechanism of Fire Retardancy of Polyurethanes using Ammonium Polyphosphate. J. Appl. Polym. Sci. 2001, 82, 3262. (10) Schartel, B. Phosphorus-based Flame Retardancy MechanismsOld Hat or a Starting Point for Future Development? Materials 2010, 3, 4710. (11) Yan, Y. W.; Chen, L.; Jian, R. K.; Kong, S.; Wang, Y. Z. Intumescence: An Effect Way to Flame Retardance and Smoke Suppression for Polystryene. Polym. Degrad. Stab. 2012, 97, 1423. (12) Zhao, B.; Chen, L.; Long, J. W.; Chen, H. B.; Wang, Y. Z. Aluminum Hypophosphite versus Alkyl-Substituted Phosphinate in Polyamide 6: Flame Retardance, Thermal Degradation, and Pyrolysis Behavior. Ind. Eng. Chem. Res. 2013, 52, 2875. (13) Braun, U.; Balabanovich, A. I.; Schartel, B.; Knoll, U.; Artner, J.; Ciesielski, M.; Döring, M.; Perez, R.; Sandler, J. K. W.; Altstädt, V.; Hoffmann, T.; Pospiech, D. Influence of the Oxidation State of Phosphorus on the Decomposition and Fire Behaviour of Flameretarded Epoxy Resin Composites. Polymer 2006, 47, 8495. (14) Mariappan, T.; Zhou, Y.; Hao, J. W.; Wilkie, C. A. Influence of Oxidation State of Phosphorus on the Thermal and Flammability of Polyurea and Epoxy Resin. Eur. Polym. J. 2013, 49, 3171. (15) Lorenzetti, A.; Modesti, M.; Besco, S.; Hrelja, D.; Donadi, S. Influence of Phosphorus Vvalency on Thermal Behaviour of Flame Retarded Polyurethane Foams. Polym. Degrad. Stab. 2011, 96, 1455. (16) Price, D.; Bullett, K. J.; Cunliffe, L. K.; Hull, T. R.; Milnes, G. J.; Ebdon, J. R.; Hunt, B. J.; Joseph, P. Cone Calorimetry Studies of Polymer Systems Flame Retarded by Chemically Bonded Phosphorus. Polym. Degrad. Stab. 2005, 88, 74. (17) Price, D.; Cunliffe, L. K.; Bullett, K. J.; Hull, T. R.; Milnes, G. J.; Ebdon, J. R.; Hunt, B. J.; Joseph, P. Thermal Behaviour of Covalently Bonded Phosphate and Phosphonate Flame Retardant Polystyrene Systems. Polym. Degrad. Stab. 2007, 92, 1101. (18) 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. (19) Cal T. B. California Technical Bulletin 117: Requirements, Test Procedure and Apparatus for Testing the Flame Retardance of Resilient Filling Materials Used in Upholstered Furniture; 117-2000; 2000. (20) Kageoka, M.; Tairaka, Y.; Kodama, K. Effects of Melamine Particle Size on Flexible Polyurethane Foam Properties. J. Cell. Plast. 1997, 33, 219. (21) Zammarano, M.; Krämer, R. H.; Matko, S.; Smith, M.; Mehta, S.; Gilman, J. W.; Davis, R. D. Factors Influencing the Smoldering Performance of Polyurethane Foam; NISTTN: MD, 2012. (22) Schartel, B.; Hull, T. R. Development of Fire-Retarded MaterialsInterpretation of Cone Calorimeter Data. Fire Mater. 2007, 31, 327. (23) Lyon, R. E. Plastics and Rubber. In Handbook of Building Materials for Fire Protection; Harper, C. A., Ed.; McGraw-Hill Handbooks: New York, 2004; Chapter 3, pp 3.1−3.51. (24) Levchik, S. V.; Weil, E. D. Thermal Decomposition, Combustion and Fire-Retardancy of PolyurethanesA Review of the Recent Literature. Polym. Int. 2004, 53, 1585. (25) 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. (26) Hergenrother, P. M.; Thompson, C. M.; Smith, J. G.; Connell, J. W.; Hinkley, J. A.; Lyon, R. E.; Moulton, R. Flame Retardant Aircraft Epoxy Resins Containing Phosphorus. Polymer 2005, 46, 5012.

MPyP with the highest phosphorus valence (+5) mainly made an effect in the condensed phase of FPUF. After examining the gaseous products of CMA and MHPA by TG-FTIR and PyGC/MS, the kind and amount of phosphorus-containing gaseous products of CMA were proved to be more than those of MHPA due to the different chemical moieties surrounding the phosphorus atom, and this made CMA have the highest flame retarded efficiency. Based on our results, we can draw a conclusion that melamine salts with low phosphorus valence (+1) is more favorable for enhancing the flame retardance of flexible PU foam than that with high phosphorus valence (+5). In addition, the chemical moieties surrounding phosphorus and ionic bond type will also play an important role on their flame retardance when both of them had same monovalent phosphorus.

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ASSOCIATED CONTENT

S Supporting Information *

SEM microphotographs of FPUF/CMA20, FPUF/MHPA20, FPUF/MPOA20, and FPUF/MPyP20 are displayed in Supplementary Figure S1. The particle size distribution of CMA, MHPA, MPOA, and MPyP are shown in Supplementary Figure S2. The retention time, m/z, and assigned structure of pyrolysis products of CMA and MHPA are listed in Supplementary Tables S1 and S2, respectively. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Phone/Fax: +86-28-85410755. E-mail: [email protected] (X.-L. Wang). *E-mail: [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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Influence of Valence and Structure of Phosphorus-Containing

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