Phosphoramidate-Containing Flame-Retardant ... - ACS Publications

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Phosphoramidate-Containing Flame-Retardant Flexible Polyurethane Foams Matthias Neisius,† Shuyu Liang,† Henri Mispreuve,‡ and Sabyasachi Gaan*,† †

Additives and Chemistry Group, Advanced Fibers, Empa Swiss Federal Laboratories for Materials Science and Technology, Lerchenfeldstrasse 5, 9014 St. Gallen, Switzerland ‡ Foampartner, Fritz Nauer AG, Oberwolfhauserstrasse 9, 8633 Wolfhausen, Switzerland S Supporting Information *

ABSTRACT: In this study, we have investigated the structure−flame-retardant (FR) efficacy relationships of phosphoramidates (PRs) on flexible polyurethane foam (FPUF). FPUFs that contain model monosubstituted secondary PRs, monosubstituted tertiary PRs, and trisubstituted secondary PRs were prepared and evaluated for FR efficacy and thermal decomposition characteristics. The fire test results indicate that methyl ester PRs exhibit better FR behavior, compared to analogous phenyl ester derivatives at equal weight percentage in polyurethane (PU) foams. Within the same class of PRs, the monoallyl derivatives exhibited the highest level of flame retardancy. The multiallyl PR derivatives did not offer any advantage, in terms of improved flame retardancy. Evolved gas analysis from the thermal degradation of PU foams indicates that the PRs are volatilized in the first stage of thermal decomposition and are primarily active in the gas phase with the exception of the triallyl-PR derivative, which is primarily active in the condensed phase.

1. INTRODUCTION Over the past decade, there has been an increasing impetus for the development of novel flame retardants for application in polymeric systems. In recent years, the nitrogen−phosphorus (P−N)-based compounds such as phosphoramidates (PRs) have been attracting more and more attention in the scientific community. Compared to the halogenated and some phosphorus-based compounds, their main advantages are their relatively low volatility, ease of synthesis, and low evolution of toxic gases and smoke in the event of fire.1−4 The versatility of flame retardation of various PRs for application in different substrates (cotton,5 poly(butylene terephthalate),6 epoxy resins,7,8 cellulose,9 cellulose acetate,10 etc.) has been demonstrated by researchers. An efficient single additive-type PR that is suitable for a wide range of polymeric materials, from highly charrable (polycarbonate), moderately charrable PBT to two noncharrable matrices (EVA and ABS)11 has been developed. A novel PR intumescent FR system for noncharrable and highly flammable ABS materials has been reported.12 A new ultraviolet (UV)reactive PR monomer has been reported, and its effect on the flame-retardant (FR) properties of ethyl-type UV-cured polyurethane coatings has been studied.13 A series of organophosphorus flame retardants based on phosphate, phosphonate, and PR structure for the flame retardation of flexible polyurethane foams (FPUFs) has been described. It was found that some PR and phosphonate compounds exhibit significantly better flame retardancy than their phosphate analogues.4 A detailed systematic study of the thermal and FR behavior of the phosphoramidate compounds could provide guidance for the development of an efficient halogen-free FR system for FPUFs. Thus, the focus of this work was to evaluate, in detail, the effect of various factors, such as molecular weight and the © 2013 American Chemical Society

type of functional group attached to the P atom of a PR moiety, which might contribute to its flame retardance and thermal behavior. Structurally diverse PRs (i.e., monosubstituted secondary PRs, monosubstituted tertiary PRs, and trisubstituted secondary PRs) have been synthesized and evaluated. Most of the compounds were prepared using the well-known and established Atherton−Todd reaction (ATR), using CCl4 as a chlorinating agent and triethylamine as a base.14−20 These compounds were incorporated in the polymerization of TDIbased FPUFs and were subsequently investigated for their thermal stability and flame retardancy. Several testing including thermogravimetric analysis (TGA), pyrolysis combustion flow calorimetry (PCFC), limiting oxygen index (LOI), VKF-BKZ (vertical burn test), and UL94-HB were used. Evolved gas analysis from the decomposition of PR containing PU foams using direct insertion probe mass spectrometry (DIP-MS) has provided insight into possible modes of action of the model PR compounds.

2. EXPERIMENTAL SECTION 2.1. Materials. Dimethyl phosphite, diphenyl phosphite, phosphoryl chloride, triethylamine, methylene chloride, carbon tetrachloride, tetrahydrofuran (THF), n-propylamine, allylamine, benzylamine, diallylamine, and diethylamine were purchased from Sigma−Aldrich, Switzerland. All reagents and chemicals were used without further purification. Materials used in the synthesis of FPUFPO 56 (a propylene oxide polyol with a molecular weight of ∼3000 g/ mol and a hydroxyl value of 56 KOH/g), TDI 80 (a 80:20 Received: Revised: Accepted: Published: 9752

March 21, 2013 June 19, 2013 June 24, 2013 June 24, 2013 dx.doi.org/10.1021/ie400914u | Ind. Eng. Chem. Res. 2013, 52, 9752−9762

Industrial & Engineering Chemistry Research

Article

Table 1. List of Monosubstituted Secondary Dimethyl/Diphenyl Phosphoramidates (PRs)

Table 2. List of Dimethyl Monosubstituted Tertiary Phosphoramidates (PRs) and Trisubstituted Secondary PRs

gas. Aliquots (1 μL) from the crimp-sealed vials were injected by use of an automated sampler. The split/splitless injection inlet was maintained at 250 °C, and samples were separated on a 30-m capillary column coated with a 0.25 μm film of (5% phenyl)methylpolysiloxane. A 10:1 split ratio was used. The temperature program consisted of an isothermal separation at 280 °C for 30 min. The NMR analyses were performed on a Bruker machine. The 1H and 31P NMR analyses were measured with a frequency of 400 MHz, while the 13 C NMR measurements were run at a frequency of 100 MHz. 2.2.1. Synthesis of Monosubstituted Secondary Dimethyl Phosphoramidates. To a stirred solution of dimethyl phosphite (0.5 mol), carbon tetrachloride (0.55 mol) in anhydrous THF (350 mL) under nitrogen gas at 0−5 °C, was added dropwise a mixture of appropriate amine (0.5 mol) and triethylamine (0.55 mol) in anhydrous THF (150 mL). The resulting mixture was then allowed to warm to room temperature and left stirring for 6−12 h. The mixture was then filtered off and the filtrate concentrated at reduced pressure. The products (excluding DMBPR) were distilled under

mixture of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate), urea, demineralized water, tin(II) octanoate catalyst, A-1tertiary amine blowing catalyst, silicone surfactant, and sodium alkane sulfonatewere supplied by FoamPartner AG, Switzerland and were used without further purification. A standard commercially available flame retardant, tris-(1-chloro-2-propyl) phosphate (TCPP), was also supplied by FoamPartner AG. 2.2. Synthesis of Phosphoramidates. Four classes of compoundsi.e., monosubstituted secondary dimethyl PRs, monosubstituted secondary diphenyl PRs, monosubstituted tertiary dimethyl PRs, and trisubstituted secondary PRswere synthesized according to the general approaches outlined in Schemes S1 and S2 in the Supporting Information. The chemical structures, as well as their abbreviations used afterward, are presented in Tables 1 and 2. All compounds were analyzed by gas chromatography/mass spectroscopy (GC/MS) and 1H, 13C, and 31P NMR to ensure the purity and confirm the structure of such. For GC/MS analyses, a gas chromatograph coupled with mass spectrometer was used (Agilent GC6890/MS 5973). Helium was used as the carrier 9753

dx.doi.org/10.1021/ie400914u | Ind. Eng. Chem. Res. 2013, 52, 9752−9762


Article

purity of the final products were confirmed using GC/MS, as well as 1H, 13C, and 31P NMR analyses. DMDAPR.27 Yield: 86%, light yellow liquid. 1H NMR (400 MHz, CDCl3) δ (ppm): 5.62 (mc, 2H), 5.09−5.02 (m, 4H), 3.58 (d, J = 11.2 Hz, 6H), 3.48 (mc, 4H); 13C NMR (100 MHz, CDCl3) δ (ppm): 134.2, 117.8, 52.9, 47.5; 31P NMR (400 MHz, CDCl3) δ (ppm): 16.5. MS (EI 70 eV) m/z (%) = 206 (19), 190 (27), 178 (23), 164 (64), 138 (42), 109 (100), 96 (12), 95 (8), 79 (13), 56 (11), 47 (3), 41 (63). 2.2.4. Synthesis of Trisubstituted Secondary Phosphoramidate. To a stirred solution of phosphoryl chloride (0.25 mol) in anhydrous THF (400 mL) under N2 at 0−5 °C was added, dropwise, a solution of allylamine (0.75 mol) together with triethylamine (0.83 mol) in anhydrous THF (250 mL). The mixture was then allowed to warm to room temperature and was stirred for 10−12 h. The precipitate was removed by filtration, and the filtrate concentrated at reduced pressure to produce the required compound. The structure and purity of the final products were confirmed by GC/MS as well as 1H, 13 C, and 31P NMR analyses. TATPR.28 Yield: 87%, light yellow liquid. 1H NMR (400 MHz, CDCl3) δ (ppm): 5.76 (mc, 3H), 5.00 (ddq, J = 1.7, 17.2, 60.5 Hz, 6H), 3.49−3.30 (m, 6H), 3.44−3.37 (m, 6H), 2.82 (q, J = 7.3 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ (ppm): 137.0, 114.7, 43.4; 31P NMR (400 MHz, CDCl3) δ (ppm): 17.3. MS (EI 70 eV) m/z (%) = 215 (3), 186 (5), 159 (21), 145 (10), 131 (16), 119 (21), 104 (24), 76 (4), 64 (7), 56 (100), 47 (4), 41 (34). 2.3. Foam Preparation. FPUFs, modified with various PR compounds at different concentrations (0, 1, 2, 5, 10 wt %, based on the weight of applied PO 56), were prepared via the laboratory-scale mixing of FPUF components: 97 parts propylene oxide polyol (PO 56), a softening agent, and the appropriate flame retardant (PR/TCPP) were mixed by stirring at 1500 rpm (mechanical stirrer) for 1 min. Then, 0.97 parts water, 0.8 parts emulsifier, 0.5 parts silicone surfactant, 0.8 parts urea, 0.1 parts tertiary amine, and 0.25 parts tin(II) octanoate were added and the entire mixture was stirred for 10 s. A final addition of 22.3 parts TDI 80 followed. Stirring was continued for 15 s. The resultant mixture was immediately poured into a 250 mm × 100 mm × 80 mm container to produce free-rise foams. After preparation, the foams were cured in an oven at 80 °C for 1.5 h. After conditioning, several samples were cut for further characterizations. 2.4. Characterization. The apparent density of FPUFs was measured according to ISO 845 standard, using a specimen bar cut to the dimensions of 150 mm × 50 mm × 13 mm (length (L) × width (W) × thickness (T)). The thermal stability of FPUF samples was studied using a Netzsch TG 209 F1 instrument and a sample mass of 2−5 mg in a nitrogen environment at a heating rate of 10 °C/min from 50 °C to 800 °C. Three measurements were conducted for each sample system, to ensure the reproducibility of the results. The LOI values of the FPUF samples were measured according to the testing procedure described in Standard ASTM-2863-00, using a Fire Testing Technology (FTT) instrument (UK). The dimensions of the specimen were 150 mm × 50 mm × 13 mm (L × W × T). Pyrolysis combustion flow calorimetry (PCFC) was used to determine the heat-release rate and total heat of combustion for the samples. The actual principle for PCFC measurement has been described in detail.29 Each measurement was performed for a 4 ± 1 mg sample in the PCFC apparatus from FTT UK,

reduced pressure to provide the desired compounds. The structure and purity of the final products were confirmed using GC/MS, as well as 1H, 13C, and 31P NMR analyses. DMPPR.21 Boiling point (bp): 129 °C at 20 mbar. Yield: 91%, colorless liquid. 1H NMR (400 MHz, CDCl3) δ (ppm): 3.60 (d, J = 11.2 Hz, 6H), 3.20 (s, 1H), 2.73 (m, 2H), 1.40 (mc, 2H), 0.80 (t, J = 7.3 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ (ppm): 52.7, 42.9, 24.6, 10.9; 31P NMR (400 MHz, CDCl3) δ (ppm): 12.0. MS (EI, 70 eV) m/z (%) = 168 (7), 138 (81), 136 (2), 109 (100), 95 (6), 79 (10), 65 (8), 58 (6), 47 (5), 41 (11). DMAPR.22 Boiling point (bp): 126 °C, at 20 mbar. Yield: 93%, light yellow liquid. 1H NMR (400 MHz, CDCl3) δ (ppm): 5.75 (mc, 1H), 5.12 (dq, J = 1.8, 17.5 Hz, 1H), 4.98 (dq, J = 1.4, 10.8 Hz, 1H), 3.58 (d, J = 11.2 Hz, 6H), 3.44 (bs, 1H), 3.39 (mc, 2H); 13C NMR (100 MHz, CDCl3) δ (ppm): 135.7, 114.9, 52.7, 43.1; 31P NMR (400 MHz, CDCl3) δ (ppm): 12.5. MS (EI, 70 eV) m/z (%) = 165 (14), 138 (15), 134 (3), 109 (21), 95 (8), 79 (19), 56 (100), 47 (8). DMBPR.23 Yield: 90%, colorless liquid. 1H NMR (400 MHz, CDCl3) δ (ppm): 7.30−7.17 (m, 5H), 4.02 (mc, 2H), 3.67 (bs, 1H), 3.61 (d, J = 11.2 Hz, 6H); 13C NMR (100 MHz, CDCl3) δ (ppm): 139.5, 128.3, 127.1, 127.0, 52.8, 45.0; 31P NMR (400 MHz, CDCl3) δ (ppm): 11.9. MS (EI, 70 eV) m/z (%) = 215 (31), 109 (11), 106 (100), 95 (3), 91 (2), 79 (23), 65 (5), 47 (3). 2.2.2. Synthesis of Monosubstituted Secondary Diphenyl Phosphoramidates. All the diphenyl PRs were prepared using a procedure similar to that for DMBPR, and diphenyl phosphite and n-propyl- (allyl- or benzyl-) amine as starting materials. In this case, the reaction mixture was left for 6−8 h, instead of 6− 12 h, after the completion of the addition. The precipitate was removed by filtration and the filtrate washed with a saturated aqueous NaHCO3 solution (50 mL) and then distilled water (50 mL). The organic layer was dried over anhydrous Na2SO4 and concentrated at reduced pressure to afford the required compound. The structure and purity of the final products were confirmed by 1H, 13C, and 31P NMR analyses. DPPPR.24 Yield: 89%, white solid. 1H NMR (400 MHz, CDCl3) δ (ppm): 7.36−7.11 (m, 10H), 3.67 (mc, 1H), 3.01 (mc, 2H), 1.47 (mc, 2H), 0.85 (t, J = 7.4 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ (ppm): 151.0, 129.5, 124.7, 120.1, 43.4, 24.5, 11.0; 31P NMR (400 MHz, CDCl3) δ (ppm): 0.1. MS (EI, 70 eV) m/z (%) = 291 (40), 262 (100), 215 (5), 198 (1), 183 (14), 156 (3), 140 (1), 94 (13), 77 (36), 58 (10), 47 (3). DPAPR.25 Yield: 89%, white solid. 1H NMR (400 MHz, CDCl3) δ (ppm): 7.36−7.12 (m, 10H), 5.77 (mc, 1H), 5.16 (ddq, J = 1.7, 17.1, 49.7 Hz, 2H), 4.00 (mc, 1H), 3.67 (mc, 2H); 13C NMR (100 MHz, CDCl3) δ (ppm): 150.4, 153.0, 129.2, 124.6, 120.1, 115.7, 43.9; 31P NMR (400 MHz, CDCl3) δ (ppm): 0.2. MS (EI, 70 eV) m/z (%) = 289 (9), 245 (1), 213 (1), 196 (2), 183 (2), 156 (1), 140 (2), 94 (57), 77 (40), 56 (100), 47 (4). DPBPR.26 Yield: 89%, white solid. 1H NMR (400 MHz, CDCl3) δ (ppm): 7.36−7.12 (m, 15H), 4.29−4.23 (m, 2H), 4.10−4.02 (m, 1H); 13C NMR (100 MHz, CDCl3) δ (ppm): 150.7, 138.7, 129.2, 128.4, 127.3, 124.8, 120.2, 115.4, 45.7; 31P NMR (400 MHz, CDCl3) δ (ppm): 0.2. MS (EI, 70 eV) m/z (%) = 339 (22), 262 (4), 246 (4), 182 (43), 140 (2), 106 (100), 94 (27), 91 (32), 77 (22), 47 (3). 2.2.3. Synthesis of Monosubstituted Tertiary Dimethyl Phosphoramidate. DMDAPR was prepared using a procedure similar to that for DMPPR, using diallyl amine together with dimethyl phosphite as starting materials. The structure and 9754



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with the pyrolysis temperature of 75−800 °C at a heating rate of 1 °C/s and a combustion temperature at 900 °C. For each sample, three measurements were conducted. UL94-HB testing was carried out to provide another indication of the flame retardancy of FPUFs. The test specimen was oriented in a horizontal position. In the test, the flame is applied to the free end of the specimen for 60 s and then removed, while cotton was placed under the specimen. The after-flame time is noted as tA. The 150-mm-long test specimen is marked at the 25 mm, 60 mm, and 125 mm positions and the burning rate is measured over a span of 100 mm. The dimensions of specimen bar are 150 mm × 50 mm × 13 mm (L × W × T). The appropriate material classifications are provided in Table S1 in the Supporting Information. The vertical burn test, according to Swiss Standard VKFBKZ (which is a classification of flammable materials by the Swiss Association of Cantonal Fire Insurances), was used. The flammability of FPUF samples was also evaluated according to the Swiss flammability standard (VKF-BKZ), using a sample of specific dimensions (160 mm × 60 mm × 6 mm (L × W × T)). In this test, the air-dried specimen was placed in a vertical position and subjected to a standardized flame from the lower front edge. A flame height of 20 mm was maintained (burning with sharp outlines). The burner position was adjusted to 45° so that the flame hit the specimen vertically in the middle of the lower front edge. The flame was brought into contact with the foam for 15 s and was positioned such that the foam bottom was approximately 4 ± 1 mm inside the flame tip. The test was considered to be passed when two conditions are satisfied: burned length, LBV < 150 mm; burning duration, tBV < 20 s. Direct Insert Probe Mass Spectrometry (DIP-MS). DIP-MS was used to evaluate the degradation processes for the FPUF containing various PRS and further understand their FR action. The sample was introduced into a quartz microcup in contact with a heating probe, the tip of which was inserted into the ionization chamber maintained at a strongly reduced pressure.30 In this study, DIP-MS analyses were conducted for a 1−2 μg sample of FPUF, using a ThermoQuest FINNIGAN apparatus. The probe was heated from 30 °C to 480 °C at a rate of 50 K/ min. The pressure was 10−6 mbar.

production, bceause of their compatibility with the production process. Solid additives have been shown to influence the physical properties31 of the PU foam32 and further create complication in actual industrial production. In a normal industrial production of PU foams, the FR additives are mixed with the polyol component, which reacts with TDI to form the polymer. Solid additives may alter the viscosity and the flow behavior of the polyol; uneven dispersions of solid particles will create unreliable physical and fire properties. To obtain more insight into the effect of presence of PR on physical properties of the FPUF, the densities for all of the manufactured foams were calculated. It can be seen from the data presented in Table S2 in the Supporting Information that the addition of liquid PR compounds increases the density of PU foams by ∼1.3%, compared to that of the virgin foam. However, in the case of solid PR compounds, a significant increase in density was observed for higher PR loadings, up to 51.1 kg/m3 at 10 wt % DPPPR, 50.8 kg/m3 at 10 wt % DPAPR, and 51.8 kg/m3 at 10 wt % DPBPR. The foaming kinetics during the polymerization process for PU is believed to be affected by the presence of solid PRs, which could lead to viscosity variation in the initial stage of the foaming process and, subsequently, an increase in the density of the entire system.33 Unlike other liquid PRs, there was a slightly larger increase in the foam density for materials generated in the presence of TATPR. This implies that the incorporation of TATRP may interfere with the foaming process and finally change the cellular structure of the resulting matrix. Overall, a relatively good compatibility between the synthesized PR and the polymer matrix, in most of the cases, could be realized.34,35 3.2. Flammability Test. The FR efficacy of the synthesized PR model compounds was evaluated using LOI, VKF-BKZ, and UL94-HB tests for the combustion of foams containing these compounds. To ensure the repeatability of data, for each flameretarded foam system, five samples were tested. The results are collected in Tables 3 and 4. The allyl group has previously been shown to be important for achieving good flame resistance in PU foams. Furthermore, it has been reported that phosphoramidate compounds are active primarily in the gas phase.4 Thus, it is important to evaluate, in detail, the effect of various factors, such as molecular weight and type of functional group linked to the P atom of a PR moiety, which might contribute to flame retardancy and thermal behavior. The impact of different classes of phosphoramidates on the fire performance of PU foams has been evaluated and results are presented in Tables 3 and 4. The fire results of TCPP (commercial flame retardant) containing PU foams are also presented in Table 3 and 4. It can be seen from these tables that the fire results of PR-containing PU foams are similar or better than those of TCPP-containing PU foams. Class 1 and class 2 (Table 1) PRs differ in the ester group linked to the phosphorus. The methyl ester PRs have relatively higher phosphorus content than the phenyl ester derivatives. Because of a relatively lower molecular weight, it might be expected to be volatile and, consequently, to function primarily in the gas phase. Table 2 contains structures for the PRs with more than one allyl group (i.e., class III and class IV flame retardants). It can be obviously seen from this table that the increased concentration of FR in the PU foam increases the fire performance of the foams. The LOI values of the PRcontaining foams are somewhat better than those for the virgin foam, and the best results are obtained for DMAPR-containing foam. It is remarkable that the LOI value of the PU foam with

3. . RESULTS AND DISCUSSIONS 3.1. Choice of Phosphoramidates. Various PRs synthesized in this research are shown in Tables 1 and 2. The various methyl (class I) and phenyl ester PRs are presented in Table 1. The methyl ester derivatives, being smaller in structure, are expected to be more volatile and possibly active in the gas phase. The phenyl ester derivatives are expected to be less volatile and possibly display more condensed-phase action. It was previously reported that monoallyl secondary derivatives (phosphonate and PR) have superior FR behavior in PU foams.4 To further understand the effect of the allyl group monosubstituted diallyl tertiary PRs have been synthesized and evaluated (class III, Table 2). The triphosphoramidate (class IV, Table 2) was chosen to permit an evaluation of the influence of multiple P−N bonds on FR action. It is important to note that the methyl ester PRs synthesized in this work are liquid, whereas the phenyl ester PRs are solid. Liquid and solid FR additives may offer advantages or pose challenges in processing and subsequent use of the PU foams. 3.2. Density of Foams. The physical properties of flexible PU foams greatly depend on the type and level of additives. Miscible liquid additives are preferred for industrial PU foam 9755



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Table 3. Flammability of FPUFs Containing Various Concentrations of Methyl Ester PR Compounds

Table 4. Flammability of FPUFs Containing Various Phenyl Ester and Triphosphoramidate Compounds

VKF-BKZ type of flame retardant

flame retardant concentration

limiting oxygen index, LOI (O2 %)

none

virgin FPUF

TCPP TCPP TCPP TCPP

VKF-BKZ UL94HB

type of flame retardant

flame retardant concentration

limiting oxygen index, LOI (O2 %)

PASS/ NO

burning velocity (mm/s)

UL94HB

7.3

NO

none

virgin FPUF

20.5

NO

7.3

NO

PASS PASS PASS PASS

4.2 4.9 4.9 5.4

NO HBF HBF HF2

TCPP TCPP TCPP TCPP

1% 2% 5% 10%

21.7 21.8 22.5 23

PASS PASS PASS PASS

4.2 4.9 4.9 5.4

NO HBF HBF HF2

21.6 21.8 22.2 22.8

PASS PASS PASS PASS

5.2 4.9 4.3 3.9

HBF HBF HBF HF2

DPPPR DPPPR DPPPR DPPPR

1% 2% 5% 10%

20.9 21.2 21.9 22.3

PASS PASS PASS PASS

5.7 6.1 5.2 4.5

NO HBF HBF HBF

1% 2% 5% 10%

21.8 22.3 22.8 23.9

PASS PASS PASS PASS

4.8 4.2 3.8 3.1

HBF HBF HF2 HF1

DPAPR DPAPR DPAPR DPAPR

1% 2% 5% 10%

21.2 21.5 22.1 22.5

PASS PASS PASS PASS

6.1 5.4 4.5 4.2

HBF HBF HBF HF2

DMBPR DMBPR DMBPR DMBPR

1% 2% 5% 10%

21.2 21.6 21.9 22.5

PASS PASS PASS PASS

5.6 5.2 4.9 4.1

HBF HBF HBF HF2

DPBPR DPBPR DPBPR DPBPR

1% 2% 5% 10%

20.8 21.0 21.3 21.9

NO PASS PASS PASS

6.3 6.2 5.5 4.7

NO NO HBF HBF

DMDAPR DMDAPR DMDAPR DMDAPR

1% 2% 5% 10%

20.8 21.3 21.5 22.1

PASS PASS PASS PASS

6.3 5.8 5.4 4.5

NO HBF HBF HF2

TATPR TATPR TATPR TATPR

1% 2% 5% 10%

20.5 20.8 20.9 21.1

NO NO NO NO

7.5 7.7 7.3 7.6

NO NO NO HBF

PASS/ NO

burning velocity (mm/s)

20.5

NO

1% 2% 5% 10%

21.7 21.8 22.5 23

DMPPR DMPPR DMPPR DMPPR

1% 2% 5% 10%

DMAPR DMAPR DMAPR DMAPR

10% DMAPR is almost 1% higher than the one with 10% of TCPP. All methyl ester PR-containing foams pass the VKFBKZ test qualification, and a higher concentration of flame retardants decreases the burning velocity of the foams. DMAPR with a similar phosphorus content (Table 1) has a greater influence on the fire performance (better UL 94 HB rating) of the foam than DMPPR. It is furthermore much more efficient than TCPP. The enhanced FR performance of DMAPRcontaining foams may be attributed to the presence of the allyl group and efficient release of the PO radical.4 Increasing the molecular weight of the PR by incorporating a benzyl group (DMBPR) reduces the phosphorus content (Table 1) and also reduces the FR efficacy per weight of flame retardant in the foam. It is important to note that, among all the methyl ester PRs, DMAPR exhibits the highest UL 94-HB rating of HF1 at 10 wt % in PU foam. In contrast, phenyl ester derivatives (Table 4), because of their higher molecular weight (relatively lower phosphorus content), have a reduced FR effect at a similar weight percentage in foam, compared to methyl ester derivatives. The phenyl esters exhibit comparable or even less FR efficiency than TCPP. From the data in Tables 3 and 4, it can be estimated that the FR efficiency of phenyl ester derivatives might be quite similar to methyl ester derivatives if an equal weight percentage of phosphorus in PU foam is taken into consideration. For example, foam with 10% DPAPR is quite similar in FR behavior (UL94 HB rating) to foam with 5% DMAPR. DPAPR (10.7% P) has ∼43% lower P content than DMAPR. Comparison within the phenyl ester derivatives reveals that DPAPR is more effective as a flame retardant than

others, which may be due to the presence of the allyl group. It is also quite notable that foams containing DPBPR do not pass the VKF-BKZ rating at a loading of 1 wt %. From the data presented in Tables 3 and 4, it can be seen that the presence of more than one allyl group (DMDAPR, TATPR) in a PR structure does not markedly enhance flame retardancy over the monoallyl derivative (DMAPR). In fact, TATPR displays poor FR behavior, compared to that of the other PRs investigated. Even PU foams with 10 wt % TATPR do not pass the VKF-BKZ test. 3.3. Thermogravimetric Analysis. TGA studies were carried out to determine the possible effect of the addition of PR compounds on the thermal stability of the FPUF and to gain further insight into their mode of action in PU systems. Figure 1, as well as Figures S1, S2, and S3 in the Supporting Information show the TGA data for virgin foam and the same materials containing methyl and phenyl ester (5 wt %)-containing foams. It may be seen that the degradation of virgin foam takes place in two different stages, i.e., in the range of 200−300 °C and 300−400 °C. The data are similar to that previously reported.36,37 The first stage is characterized by the cleavage of urethane bonds by depolymerization or rearrangement reactions to form TDI, diaminotoluene, and polyols. The second stage is characterized by decomposition reactions mostly involving the polyether polyol.38 The addition of flame retardants to PU foams is expected to interfere with both stages of decomposition. 9756



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all flame-retardant-containing foams decompose completely with no residue left after ∼400 °C. This is due to limited condensed phase activity of the flame retardants, as well as a lack of free hydroxyl groups in PU chains, which could be dehydrated to form stable char. The thermal decomposition behavior of multiallyl PRcontaining foams is shown in Figure 1. DMAPR-containing foam is the least thermally stable foam, with thermal decomposition starting at 150 °C. The DMDAPR-containing foam is thermally quite similar in the 289−380 °C region, compared to the DMAPR. TATPR-containing foams show quite different thermal decomposition characteristics in the region of 300−380 °C. Their rate of decomposition is higher in the region 300−380 °C, unlike most other PR-containing foams. For diphenyl ester PR-containing foams (see Figures S1, S2, and S3 in the Supporting Information), two well-defined steps of decomposition (5 wt % DPPPR, DPAPR, or DPBPR) can also be observed, with the onset of thermal decomposition temperature ∼15−25 °C lower than that of virgin foam. Moreover, the weight loss rates in the first stage for phenyl ester PR containing FPUFs are relatively lower than those of their methyl ester analogues. This could be attributed to the low volatility of the phenyl ester PRs (solid compounds). Furthermore, a gradual slope (first stage) seen for the PRcontaining FPUF (compared to virgin foam) in the TGA curve (280−320 °C) also suggests the possible formation of a thermally stable intermediate. Unlike other FR foams, TATPR-containing foams (Figure 1) exhibit a higher rate of thermal decomposition in the temperature range of 270−320 °C. Subsequently, the second stage decomposition of the trisubstituted PR-containing FPUFs is further accelerated and occurs at lower temperature (∼320 °C), compared to that of virgin and other FR foams (∼350 °C). As a result, a different FR action for trisubstituted secondary PR compounds may be suggested. The accelerated decomposition of the second stage could be due to acidcatalyzed depolymerization of the polyether chains to form alcohols.43 Further decomposition studies using PCFC and direct insertion probe MS (DIP-MS) were carried out in an attempt to understand the burning behavior of the PRs.

Figure 1. Thermogravimetric analysis (TGA) curves of virgin FPUF and FPUF containing 5 wt % PRs.

As can be seen from the data presented in Figure 1, as well as Figures S1, S2, and S3 in the Supporting Information, that the addition of methyl and phenyl ester PRs reduces the onset temperature of the first stage of decomposition of FPUF by ∼15−40 °C. This could be due to the volatilization of PRs or decomposition of FR leading to the formation of volatile species and nonvolatile acidic species.39 Formation of acidic species from the FR may reduce the onset of decomposition temperature of FPUF.40 Meanwhile, it is also interesting to note that all these monosubstituted secondary PR containing foams display a lower weight loss rate than the virgin foam in the range of 270−320 °C. This may be attributed to possible interaction of phosphorus acid derivatives formed from the decomposition of PRs with PU to form thermally stable intermediates.40−42 However, because of the low density and poor char formation capabilities (low aromatic content and absence of free hydroxyl groups) of the PU foams, this thermally stable intermediate undergoes further thermal decomposition without being able to form highly heat-resistant carbonaceous residue that may act as an efficient thermal barrier. The foams containing methyl ester PRs show reduced decomposition temperatures, compared to foams that contain analogous phenyl ester derivatives. This is due to the lower boiling point of methyl ester PRs, compared to the phenyl ester derivatives. It can be seen from the TGA data (see Figure 1, and Figures S1, S2, and S3 in the Supporting Information) that

Figure 2. Heat-release rate (HRR) curves of FPUFs containing 5 wt % PRs. 9757



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Table 5. PCFC Data for Virgin FPUF and FPUF Containing 5 wt % of PRs Stage 1

Stage 2

Total Heat of Combustion (J/g)

sample

TM 1a (°C)

MHRR 1b (W/g)

TM 2c (°C)

MHRR 2d (W/g)

stage 1, THC 1

stage 2, THC 2

THC 1 + THC 2

chare (%)

virgin FPUF 5% DMPPR 5% DMAPR 5% DMBPR 5% DPPPR 5% DPAPR 5% DPBPR 5% DMDAPR 5% TATPR

305 286 289 287 282 283 288 287 281

84 74 74 72 77 78 84 75 77

406 404 405 405 407 406 406 403 396

546 599 593 608 595 584 614 615 488

4939 4877 4721 4917 4649 5060 5111 4440 5116

19250 18799 18085 18845 18198 17252 18332 19111 18839

24089 23676 23706 23762 22847 22312 23443 23551 23955

0 0 0 0 0 0 0 0 0

a

Temperature for maximum heat release rate (stage 1). bMaximum heat release rate (stage 1). cTemperature for maximum heat release rate (stage 2). dMaximum heat release rate (stage 2). eChar residue after the test (Tmaximum = 750 °C).

Figure 3. Extracted ion chromatograms of phosphoramidate (PR) foams: DMAPR foam (m/z 165.1), DPAPR foam (m/z 289.1), DMBPR foam (m/z 215.1), DPBPR foam (m/z 331.1), DMDAPR foam (m/z 205.1), and TATPR foam (m/z 57.1).

earlier. Stage 1 (200−350 °C) is a low HRR region, which may be due to the possible release of TDI and DAT formed from the decomposition of urethane bonds. Stage 2 (350−450 °C) reflects a higher HRR, which corresponds to the decomposition of polyether polyol to release of propene, formaldehyde, acetaldehyde, C3H6O isomers, and high-molarmass polyol chain fragments of various structures.38 Addition of class I and class II PRs interferes with the thermal decomposition of the PU foams. As seen from the data presented in Figure 2, Figures S4−S6 in the Supporting Information, and Table 5, the temperature of maximum heat release rate, stage 1 (TM 1) is lower for the class I and class II PR-containing foams. This may be due to the volatilization of flame retardants and possible acid-catalyzed decomposition of urethane bonds to release TDI and DAT. A closer look at the beginning (∼350 °C) of the second stage of decomposition (Figure 2, Figures S4−S6 in the Supporting Information) reveals that the PR FR-containing PU foams have reduced HRRs but subsequently exhibit higher peak HRR (at ∼405 °C) than the blank PU foams. Comparison of the total heats of

3.4. Pyrolysis Combustion Flow Calorimetry. PCFC is a common method for evaluating the combustibility of materials in milligram quantities by separately reproducing the solidphase and gas-phase processes of combustion in a nonflaming test. By measuring the heat of combustion of pyrolysis products evolved from the decomposition of material at constant heating rate, useful information about the mode of action of additives which proceeds in the condensed phase can be gained. PCFC measurements for 5 wt % PR-containing foams were conducted. In Figure 2, as well as Figures S4, S5, and S6 in the Supporting Information, the heat-release rates (HRR) for FPUFs are plotted against temperature and various thermal data obtained from such curves are summarized in Table 5. The HRR profiles for foams containing analogues of methyl and phenyl ester PRs (class I and class II), in comparison with that for virgin material, are shown in Figure 2 and Figures S4− S6 in the Supporting Information. It can be seen that the HRR profile for virgin PU foam can be characterized by two main stages and is quite similar to the TGA data (see Figure 1 and Figures S1−S3 in the Supporting Information) discussed 9758



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Scheme 1. Hydrolysis and Decomposition Reactions: (A) Hydrolysis of TATPR and (B) Acid-Catalyzed Decomposition of Urethane Bond To Release DAT

Figure 4. Extracted ion chromatograms of DAT (m/z 122) release from PU foams. (PA Foam: blank foam containing phosphoric acid.)

combustion stage 2 (THC 2) for blank foams and class I and class II PR-containing foams shows no significant difference. Furthermore, the lack of significant difference between the total heat of combustion of (THC 1 + THC 2) and the absence of any residue for PR-containing foams (class I and class II) clearly indicate their gas-phase action. The HRR profile of multiallyl PR derivative-containing foams (class III and class IV) is shown in Figure 2. The TATPRcontaining foam shows a clear shift in the second stage of thermal decomposition to lower temperature. The TM 2 and MHRR 2 for the TATPR foam is lower than that of blank foam

and other phosphoramidate-containing foams which could be due to condensed-phase action. 3.5. Direct Insertion Probe Mass Spectrometry Measurements. The TGA and PCFC analysis for PRcontaining foams clearly indicate their gas-phase action. To further analyze their FR behavior, direct insertion probe mass spectrometry (DIP-MS) measurements were performed on the foams to identify the volatile products being evolved during their thermal decomposition. As seen in previous section in the TGA and PCFC studies, the first stage of decomposition of PU foams is affected by the presence of PRs. Thus, evolved gas 9759



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4. CONCLUSION A series of model monosubstituted secondary phosphoramidates (PRs), monosubstituted tertiary PRs, and trisubstituted secondary PRs have been synthesized and incorporated into polyurethane (PU) foams. Overall, the PR compounds had good compatibility with the PU matrix, with only some increase in the density of foam in the case of solid PRs (phenyl derivatives) at high concentration (5% and 10%). Fire test results indicate that methyl ester PRs, because of their higher phosphorus content, exhibit better flame-retardant (FR) behavior, compared to analogous phenyl ester derivatives at equal weight percentage in PU foams. Within the same class of PRs, the monoallyl derivatives (i.e., DMAPR and DPAPR) exhibited the highest level of flame retardancy. The multiallyl PR derivatives (i.e., DMDAPR and TATPR) did not offer any advantage, in terms of improved flame retardancy, compared to monoallyl PR derivatives. TATPR foams could not pass the VKF-BKZ test and exhibited the worst FR behavior. The thermogravimetric analysis (TGA) and PCFC results indicated that the PRs are primarily active in the gas phase, with the exception of TATPR. Evolved gas analysis of PR-containing PU foams using DIP-MS indicated that the PRs are volatilized in the first stage of thermal decomposition and are primarily gasphase active, with the exception of TATPR, which may act primarily in the condensed phase. TATPR becomes hydrolyzed in PU foams and releases acidic species, which catalyze the decomposition of PU to release diaminotoluene at low temperature. The methyl ester PR derivatives are volatilized at lower temperatures, compared to phenyl ester PR derivatives. The application of phenyl ester PR derivatives may be more suitable where the fogging requirements are stricter.

analysis for the decomposition of PU foams over the temperature range of 30−300 °C was carried out using DIPMS. The total ion chromatogram for the decomposition of the foams revealed the release of major components such as TDI and DAT in the first stage. In case of FR foams, the further release of PR compounds was observed in most cases. The extracted ion chromatogram for respective PR compounds being evolved during the thermal decomposition of the PU foams is displayed in Figure 3. The volatilization of respective PR from the foam in the first stage of thermal decomposition could clearly be observed. Low-molecular-weight PRs such as DMAPR and DMDPR start to volatilize at a much lower temperature than do higher-molecular-weight FRs such as DPAPR and DPBPR. Low-pressure conditions prevailing in the DIP-MS reduces the onset temperature for the release of the PRs and other decomposing components from the PU foam. Nevertheless, it is important to note that, irrespective of molecular weight of the PRs, they are released into the gas phase in the first stage of thermal decomposition of PU foams (up to 350 °C). The PRs thus released could further decompose to form PO* radicals, which would prevent the oxidation of H* and OH* radicals.4 It is quite interesting that the release of TATPR into the gas phase is not detected. Only the release of allylamine could be observed (Figure 3), which indicates that TATPR is not released into the gas phase; instead, it decomposes thermally/hydrolytically to form allylamine and possibly nonvolatile acidic PRs. Hydrolysis of TATPR may happen during the foaming process, because of the presence of water and an acid catalyst, tin(II) octanoate (see Scheme 1A). The release of DAT from the foams was further investigated to explore the influence of phosphoramidates on the acid-assisted catalytic decomposition of urethane bonds of the PU. The extracted ion chromatogram of DAT (m/z 122) released during the thermal decomposition of foams in DIP-MS experiments in shown in Figure 4. The DAT release profile (onset and peak release temperature, ∼350 °C) for all PR foams (except TATPR) is similar to that for the blank foam. For TATPR foam, the peak release of DAT is reduced to a lower temperature (300 °C). This could be due to the acid-catalyzed decomposition of urethane bonds of PU to release DAT, as shown in Scheme 1B. To further verify the possible acid catalysis of urethane bonds, PU foams containing phosphoric acid were prepared. The DAT release profile of phosphoric-acid-containing PU foam (PA Foam) is also shown in Figure 4. The onset of DAT release and the peak release temperature is lower, compared to the same value for other foams. Acid in the PU foam could protonate the carbonyl oxygen. Subsequent rearrangement and bond cleavage would lead to formation of carbamic acid and an alkene. Since carbamic acid is unstable, it would further decompose to release carbon dioxide and DAT (Scheme 1B). The inferior FR behavior of TATPR could thus be attributed to the absence of TATPR in the gas phase during the combustion of foams. Other PRs investigated in this work are relatively more stable hydrolytically than TATPR, volatilize preferentially in the first stage of decomposition of PU, and release PO*, thus contributing to gas-phase action. It is important to point out that the condensed phase action of PR on PU is limited due to the unavailability of free hydroxyl groups, which can be dehydrated to form char. Thus, a flame retardant that is primarily active in the gas phase will perform better in fire tests (VKF-BKZ and UL94 HB) for PU foams.

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ASSOCIATED CONTENT

* Supporting Information S

Scheme S1, general synthetic approach of monophosphoramidates by ATR; Scheme S2, general synthetic approach of triphosphoramidates. Figure S1, TGA curves of virgin flexible polyurethane foam (FPUF) and FPUF containing 5 wt % PRs; Figure S2, TGA curves of virgin FPUF and FPUF containing 5 wt % PRs; Figure S3, TGA curves of virgin FPUF and FPUF containing 5 wt % PRs; Figure S4, heat-release rate (HRR) curves of FPUFs containing 5 wt % PRs; Figure S5, heat-release rate (HRR) curves of FPUFs containing 5 wt % PRs; Figure S6, heat-release rate (HRR) curves of FPUFs containing 5 wt % PRs. Table S1, UL94-HB flammability test specifications; Table S2, density of FPUFs containing different concentrations of PRs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +41 58 765 76 11. Fax: +41 58 765 78 62. E-mail: [email protected]. URI: www.empa.ch. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Foampartner, Switzerland, for helpful technical assistance and financial support. This research was also funded by Commission for Technology and Innovation (CTI), Switzerland. Finally, the authors are thankful to Mrs. 9760



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Elisabeth Michel for her help in series of analytical measurement and Dr. Daniel Rentsch for all NMR measurements.

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