Intumescent Phosphorus and Triazole-Based Flame-Retardant

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Article Cite This: ACS Omega 2019, 4, 1086−1094

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Intumescent Phosphorus and Triazole-Based Flame-Retardant Polyurethane Foams from Castor Oil Kesavarao Sykam,†,‡ Kiran Kumar Reddy Meka,† and Shailaja Donempudi*,†,‡ †

Polymers & Functional Materials Division, Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad 500007, Telangana, India ‡ Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, Uttar Pradesh 201002, India

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S Supporting Information *

ABSTRACT: Synthesis of a novel phosphorus and triazolefunctionalized flame-retardant (FR) monomer (PTFM) using azide−alkyne “click” reaction between triprop-2-ynyl phosphate and 2-azidoethanol that can impart intumescent FR property to polyurethane foams (PUFs) has been reported. Polyurethane triazole foams (PUTFs) were prepared using the as-synthesized PTFM and a hydroxylated castor polyol with a hydroxyl value of ∼310 mg KOH/g for application as reactive FR rigid foams. PTFM and the castor polyol were characterized for structural elucidation using Fourier transform infrared and 1H, 13C, and 31P NMR. PUTFs with a varying loading content of PTFM were subjected to the labscale flame test, cone calorimetry test, Underwriters Laboratory 94 Vertical burning test (UL 94V), and limiting oxygen index (LOI) test. A significant increase in the char yields, reduction in heat release rates, V-1 rating, and 27% of LOI were observed for PUTFs compared to PUFs and proportional to the percentage loading of PTFM. The cumulative effect of nitrogen and phosphorus in PUTFs on their intumescent behavior was evident from the thermogravimetric analysis and scanning electron microscopy micrographs, which were further supplemented by X-ray photoelectron spectroscopy studies, indicating expulsion of N2 and overall improvement in compression strength as well. Such environment-friendly reactive FRs can be good replacements to the halogenated ones.

1. INTRODUCTION

epoxy nanocomposites and silicon-based polyborazine with phenol-formaldehyde resin in FRs was also reported.9,10 Phosphorus-based reactive FRs with nitrogen are reported to offer synergistic effects and enhance flame retardancy,11,12 such as 2,2-diethyl-1,3-propanediol phosphoryl melamine, azideterminated phosphonate, alkyl/aryl phosphonates, and alkyneterminated polyols in rigid PU foams, 13,14 hexa-[4(glycidyloxycarbonyl)phenoxy]cyclotriphosphazene in polyamide 6 polymers,15 amino-functionalized, aromatic-substituted tricyclophosphazenes in poly(butylene terephthalate),16 and phosphorus, nitrogen, and silica FR polypropylene systems.17 Further advantages of these compounds emanating less smoke, vapors, and carbon monoxide gas under combustion were highlighted.18,19 In order to achieve this synergistic impact of flame retardancy, one can design a new class of monomers with both phosphorus and nitrogen, which can be incorporated in the backbone of the polymer by adapting suitable synthetic organic chemistries. Among them, azide−alkyne “click” chemistry is well recommended for its simple reaction

In recent years, development of nonhalogenated flame retardants (FRs) has been an emerging area of research because of the blanket ban on halogenated FRs in view of the environmental and health concerns.1,2 Organophosphorusbased FRs find scope as one of the alternatives to halogenated ones.3 FRs can be either added to the polymer as an additive or inserted into its backbone during the course of polymerization popularly known as reactive FRs, which offer the advantage of nonleaching. Typical reactive organophosphorus-based FRs that have been reported are 2-carboxyethyl(methyl)phosphonic acid and 2-carboxyethyl(phenyl)phosphonic acid in polyesters,4 bis(4-carboxyphenyl)phenyl phosphine oxide in polyamides,5 and diethyl(methacryloyloxy) methyl phosphonate and diethyl p-vinylbenzene phosphonate in polyacrylates and vinylic group of polymers.6 Similarly, oligo(1,3-phenylenemethyl phosphonate), bis(aminophenyl)alkyl phosphine oxide, and hyperbranched poly(aminomethyl phosphine oxideamine) were reported for use in epoxy resins,7,8 whereas in polyurethanes, bis(2-hydroxyethyl)methyl phosphonate and 3,3′-(butylphosphoryl)dipropan-1-ol have been used.3 Further, use of polypyrrole-functionalized nanomagnetite for making © 2019 American Chemical Society

Received: October 27, 2018 Accepted: December 31, 2018 Published: January 14, 2019 1086

DOI: 10.1021/acsomega.8b02968 ACS Omega 2019, 4, 1086−1094

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The TON values of PUTFs were found to be low compared to the TON of PUF. This is due to the fact that phosphoruscontaining polyurethanes are thermally less stable than pure polyurethane as the phosphorus segment present in PUTFs is easily decomposable. In the next step, a thermally stable polyphosphate layer is formed as a molten layer that reduces the rate of decomposition of the second step corresponding to urethane segment decomposition.26 The phosphate layer formed acts as a physical obstruction that avoids heat and mass transfer between the condensed and gas phase.13 Even though the difference in TON between PUTF 50 and PUTF 100 is not significant, the increase in char yields and the decrease in total weight loss were found to be proportional to the percentage of PTFM. The decomposition curve between 300 and 350 °C is observed in all the foam specimens and corresponds to depolymerization of polyurethane, which involves cleavage of urethane linkages wherein the production of moderately less volatile polyol constituents and production of nitrogen-rich volatiles take place.27 The decomposition between 300 and 400 °C is attributed to the decomposition of polyol.18,28 The rate of decomposition pattern for the PUTFs was noticed to be lower than for PUF, which may be attributed to the formation of phosphoric acid derivatives from PTFM and their interaction with polyurethane to form stable intermediates. The char yields are found at 3.68, 20.84, and 30.82 wt % corresponding to PUF, PUTF-50, and PUTF-100, respectively. The findings clearly indicate a significant rise in the char residue for PUTFs compared to PUF and support their FR performance with respect to PTFM loading. 2.2. Lab-Scale Flame Test. The lab-scale flame test was performed on foam specimens prior to ASTM standards of flammability tests such as UL 94V, LOI, and cone calorimetry. Foam specimens with dimensions 40 × 40 × 40 mm3 (length × width × thickness) were taken, and a butane flame was applied continuously over all the foam specimens for two times, viz., first ignition (t1, 15 s) and second ignition (t2, 15 s). In the case of control foam, there was no second ignition (t2) as it had shown melt dripping after the first ignition, whereas PUTF-50 and PUTF-100 were ignited two times (t1, t2) and it was observed that after t1, both the PUTFs self-extinguished immediately. After t2, PUTFs started to burn and there was no dripping observed for both PUTF-50 and PUTF-100. It was observed that the burning took place on the periphery of the foams but not in the internal parts. Moreover, it was observed that the burned thickness of the foam was lowered for PUTF100 than for PUTF-50, which could be due to the higher weight percentage of PTFM in PUTF-100. The dripping pattern of PUF and peripheral burning of PUTF-50, as well as PUTF-100, can be clearly seen from Figure 3. 2.3. UL 94 Vertical Burning Test and LOI Test. The flame-retarding behavior of PUF and PUTFs was studied by UL 94V analysis. According to UL 94V, materials are classified into three major types such as V-0, V-1, and V-2.29 In the present study, all the PUTFs are classified as V-1, whereas no rating is given to PUF as it shows complete melt dripping over combustion. Thus, the control PUF does not exhibit FR property. The total time taken for flaming combustion was recorded as 46 s for PUF with single ignition (t1), whereas PUTFs were subjected to two ignitions t1 and t2 and the average values of flaming combustion times for five samples of PUTF-50 and PUTF-100 are 44 and 38 s, respectively. This pattern of combustion behavior with less than 30 s for PUTFs per ignition complies with the requirement for V-1 rating and

conditions, ease of solvent selection, higher yields, nil or fewer byproducts, and so on.20,21 The 1,2,3-triazole moiety helps in exclusion of nitrogena noncombustible gas upon combustion that is useful in flame retardancy.22 Hence, the design of a monomer with P and N that can be inserted into the polymer chain will be beneficial in contributing to the FR properties of polymer foams.23,24 In the present work, we aimed to prepare a novel monomer [phosphorus- and triazole-functionalized FR monomer (PTFM)] with P- and N-containing moieties for imparting FR properties to polyurethane triazole foams (PUTFs) for application as FR rigid foams. PTFM was prepared via azide− alkyne click reaction under thermal conditions in the absence of a catalyst yielding 1,4 and 1,5 isomers of triazole. The objective behind the design of PTFM (Figure 1) has been (a)

Figure 1. Structure of PTFM.

phosphate ester group for promoting char, (b) nitrogen-rich 1,2,3-triazole moiety for the swelling of char because of the release of N2 gas and (c) hydroxyl groups for polymerization. PTFM with varying contents was reacted with hydroxylated castor oil and polymeric methylene diphenyl diisocyanate (MDI) to prepare PUTFs. Polyurethane foam (PUF) with nil PTFM was also prepared for comparative studies. All the foams were evaluated for FR performance by subjecting to FR testing by thermogravimetric analysis (TGA), UL 94V, limiting oxygen index (LOI) tests, cone calorimetric tests, and lab-scale flame tests.

2. RESULTS AND DISCUSSION 2.1. Thermal Analysis. The thermal stability of the PUTFs and control foam (PUF) was evaluated by using TGA. The onset decomposition temperature (TON), 25 and 50% weight loss temperatures, weight loss at 500 °C, and percentage of weight residue at 700 °C of all the samples are tabulated in Table 1, and the decomposition pattern can be seen in Figure 2. The small decomposition with negligible weight loss is observed at 50−100 °C, which corresponds to the removal of adsorbed moisture and traces of other solvents.25 The TON of PUF is noticed at 277 °C, whereas the TON values of PUTF-50 and PUTF-100 are noticed at 237 and 240 °C, respectively. Table 1. TGA of Foam Specimens

sample code

TON (°C)

Td 25% (°C)

Td 50% (°C)

char yield wt % at 700 °C

weight loss % at 500 °C

PUF PUTF-50 PUTF-100

266.90 237.26 240.02

320.12 301.57 293.71

421.82 401.69 404.52

3.68 20.84 30.82

89.2 76.5 64.74 1087


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Figure 2. TGA and derivative thermogravimetry curves of foam specimens.

whereas it is found that the control PUF has the LOI value of 19% and is flammable. The increase in LOI values for PUTFs reflects enhanced FR property with respect to the higher quantity of PTFM in the foam. 2.4. Flammability and Cone Calorimeter Studies. The flammability of PUF and PUTFs was studied with the help of cone calorimeter experiments. The data of the peak heat release rate (PHRR), total heat release (THR), average heat release rate (AHRR), time to ignition (TTI), carbon monoxide (CO) yield, carbon dioxide (CO2) yield, total smoke release (TSR), and total smoke production (TSP) were obtained from cone calorimeter studies and are tabulated in Table 3. The time taken for igniting the sample is one of the parameters to evaluate the flame retardancy of a material.31 Materials with higher TTI values have good FR property. The TTI of the control PUF is 3 s, whereas the TTI values for PUTF-50 and PUTF-100 are 8 and 12 s, respectively. The delay in TTI was solely credited to the loading of PTFM into PUTFs and was directly related to the loading percentage of PTFM. Further, the decrease in PHRR values from PUF to PUTFs also indicates the contribution of PTFM to FR property. The reduction in PHRR of PUTF-50 and PUTF-100 is about 60 and 61%, respectively, compared to that of the control PUF. From the HRR profile of Figure 4, it can be observed that PUTF-50 releases heat up to 200 s, whereas PUTF-100 releases heat up to 100 s only. This indicates that PUTF-100 extinguishes the flame in less time compared to PUTF-50 and is attributed to the high loading of PTFM into PUTF-100. In addition, the AHRR values of PUTFs are found to be low compared to that of PUF and are directly proportional to the weight percentage of PTFM loading. The overall reduction in AHRR of PUTF-50 and PUTF-100 compared to that of the control PUF is about 70 and 86%, respectively. The THR of PUTF-100 is found at 13.0 MJ/m2, whereas PUTF-50 and PUF are found at 28.3 and 59.5 MJ/m2, respectively. From the values, it is found that the reduction of THR is 77% for PUTF100 and 53% for PUTF-50 with reference to PUF. The THR patterns of the foam specimens can be seen from Figure 4. The reduction in PHRR and THR values of PUTF-100 is due to the higher loading of PTFM, in which the phosphate group, as well as triazole, has shown an intumescent and cumulative effect in firefighting mechanism (Figure 5). The yields of combustible products, viz., CO and CO2, are found to be directly proportional to HRR. From Table 3, it is evident that the yields of CO and CO2 were decreased with an increase in the loading of PTFM from control to PUTF-100. The reduction in TSR and TSP of PUTFs over control PUF is attributed and is directly proportional to the percentage

Figure 3. Lab-scale flame test patterns of (a) PUF, (b) PUTF-50, and (c) PUTF-100.

hence implies their superior FR performance compared to PUF. The observations and ratings of UL 94V are tabulated in Table 2. According to LOI standards, the materials with less than or equal to the oxygen content value 21% are flammable.30 The measured LOI values of PUF and PUTFs are listed in Table 2, and it is found that the PUTFs have LOI values of 23 and 27% corresponding to PUTF-50 and PUTF-100, respectively, Table 2. UL 94V Analyses of Foam Specimens average burning time (s) (t1 + t2)

dripping

PUF

46a

yes

PUTF-50

44

no

PUTF-100

38

no

sample code

flame type yellow and sooty yellow and sooty yellow and sooty

rating

LOI

no rating

19

test

V-1

23

pass

V-1

27

pass

a

Flaming combustion time after t1. 1088


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Table 3. Cone Calorimetry Data of the Foams sample code

density (kg/m3)

PHRR (kW/m2)

AHRR (kW/m2)

CO yield (kg/kg)

CO2 yield (kg/kg)

TSR (m2 /m2)

TSP (m2)

THR (MJ/m2)

TTI (s)

PUF PUTF-50 PUTF-100

30.29 50.65 82.65

611.15 238.09 235.80

201.43 58.91 25.18

0.0549 0.0146 0.0091

1.01 0.19 0.18

1497.6 1258.0 761.7

13.2 11.1 6.7

59.5 28.3 13.0

3 8 12

Figure 4. PHRR and THR profiles of foam specimens.

Figure 5. Photographs of char residues of foams after the cone calorimeter test of (a) PUF, (b) PUTF-50, and (c) PUTF-100.

forms a foaming char at the cell walls of the foam upon burning. This could reduce the pore size of foam and retards the permeability of combustible gases as well as the product gases (CO, CO2). This mechanism is supplemented by scanning electron microscopy (SEM) micrographs and X-ray photoelectron spectroscopy (XPS) analysis of foam specimens before and after the cone calorimeter test (Figures 7 and 8). Photographs of char residues of foams after the cone calorimeter test can be seen from Figure 5. 2.5. XPS Analysis. The N (1s) and P (2p) signals were recorded by XPS for the foam specimens [before (A) and after burning (B)] with their corresponding binding energies and intensities shown in Figure 7, and the values of N (1s) are tabulated in Table 4. The spectrum for all A foams exhibits the peaks at the binding energy ∼398 eV corresponding to N (1s) of urethane (−NH−) and triazole at (N−).32,33 The exclusion of nitrogen gas from PUTF upon combustion was confirmed by the significant decrease in the intensity of N (1s) peak in PUTF’s B samples (PUTF-50 = 21 824.13 cps and PUTF-100 = 17 577.16 cps) proportional to the content of PTFM in them, while negligible change is seen for the B sample of PUF. Thus, the XPS findings clearly correlate the advantage of the triazole ring in the PUTFs in support of the cumulative contribution from both N and P on the intumescent behavior and FR property. 2.6. Morphology and Compression Strengths. From the SEM micrographs, it is observed that the control PUF exhibits an open cell structure, whereas PUTFs exhibited no specific cell structure having both open cell and closed cells. This could be due to the incorporation of PTFM into PUTFs.

loading of PTFM. The TSR profile of foam specimens is depicted in Figure 6, and the values are found to be 1497.6,

Figure 6. TSR profiles of foam specimens.

1258.0, and 761.7 m2/m2 corresponding to PUF, PUTF-50, and PUTF-100, respectively. Moreover, a similar trend has been observed in TSP values wherein 13.2 m2 is attributed to control PUF, whereas 11.1 and 6.7 m2 are accredited to PUTF50 and PUTF-100, respectively. It is noted that the percentage of reduction in TSR compared to that in control PUF of PUTF-100 is 49.14% and of PUTF-50 is 15.99%. The percentage of reduction in TSP of PUTF-100 and PUTF-50 compared to that of PUF is 49.24 and 15.90%, respectively. This could be due to the intumescent effect of PTFM that involves phosphate char formation as well as exclusion of nitrogen gas from the triazole moiety,22 which in sequence 1089


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Figure 7. XPS spectra of foam specimens (A = before burning, B = after burning).

PUTFs was clearly observed by SEM analysis and can be seen in Figure 8. The thickness of strut of PUTFs was increased upon burning, and the thickness is highest for PUTF-100 over PUTF-50. This observation reiterates the concept of the cumulative effect of nitrogen and phosphorus as explained earlier. The control foam exhibited no intumescent behavior, which is due to the melt dripping upon exposure to flame in which shrinkage of cells was observed. SEM micrographs of all foam specimens are depicted in Figure 8. The densities of the foams were measured at foam specimens with 40 × 40 × 40 mm3 (length × width × thickness) dimensions by weight to volume ratio. The densities of the foams are 30.29, 50.65, and 82.65 kg/m3 corresponding to PUF, PUTF-50, and PUTF-100, respectively, and are directly proportional to the loading of PTFM into PUTFs. This is due to the dual role of PTFM that acts not only as the FR monomer but also acts as the cross-linker in the foam formation. Thus, the increasing density and flame retardancy are solely credited to PTFM only. From Figure 9, it can be seen that the compression strengths of the foam specimens increased and are directly proportional to the loading of PTFM. The stress values experienced at 10% deformation of the foam samples are 0.076, 0.134, and 0.144 MPa corresponding to PUF, PUTF-50, and PUTF-100, respectively. The density effect on stress has been reflected as 0.175 and 0.138 MPa for PUTF-100 and PUTF-50, respectively, at 25% deformation while insignificant at 10%. The typical stress (σ) and strain (ε %) curves of foam specimens are shown in Figure 9.

Figure 8. SEM micrographs of foam specimens (a) before exposure to flame and (b) after exposure to flame.

The higher loading of PTFM into the polyurethane network led to the rupture of the cell wall, which results in an increase in a cell window. It was also observed that the size of the cell window of PUTF-100 is greater than that of PUTF-50 and PUF. The cell window of PUF was noticed at ∼150 μm, whereas PUTF-50 and PUTF-100 have the cell window at ∼300 and ∼450 μm, respectively. The intumescent behavior of

3. CONCLUSIONS A novel phosphorus- and triazole-containing FR reactive monomer (PTFM) was prepared via azide−alkyne “click” chemistry, and its flame retardancy was studied with respect to its loading into PU foams. Increase in char yields for PUTFs 1090


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Table 4. XPS Analysis of the Foam Specimensa N (1s) PUF

PUTF-50

PUTF-100

sample code

A

B

A

B

A

B

binding energy (eV) intensity (cps)

398.04 41 552.85

398.04 41 656.65

398.00 43 402.51

397.46 21 578.37

399.00 53 011.47

398.00 35 434.30

a

Sample code: A = before burning, B = after burning.

Scheme 2. Schematic Representation of the Preparation of Castor Polyol

Figure 9. Stress−strain curves of foam specimens.

Table 5. Recipe for the Preparation of PUF and PUTFs

above 700 °C was inferred from the characteristic char formation by the phosphate group upon combustion. The intumescent behavior due to the cumulative impact of N in the 1,2,3-triazole moiety and phosphorus in PTFM was clearly observed in terms of reduction in THR, PHRR, and HRR. The intumescent behavior of PUTFs was further supplemented by the foaming char formation and was also observed in SEM micrographs and XPS analysis. The lab-scale flammability experiments also substantiated the results obtained from both SEM and cone calorimetry. The flame retardancy of PUTFs was supported by the V-1 rating of UL 94V and 27% of LOI. Hence, the synthesized novel reactive FR monomer (PTFM) opens a wide scope of its application as an eco-friendly alternative to FR additives and halogenated FRs in rigid PU foams.

ingredients castor polyol PTFM polymeric MDI (1.2 equiv) TEGOSTAB DBTL water n-pentane

PUF 10 g 8.8 g 0.18 g 0.28 g 0.18 g 2 mL

PUTF-50

PUTF-100

10 g 5g 13.8 g 0.28 g 0.43 g 0.28 g 3 mL

10 g 10 g 18.8 g 0.38 g 0.58 g 0.38 g 4 mL

purchased from AVRA Chemicals. Castor oil and cetyltrimethylammonium bromide were purchased from SD Fine Chemicals. Sodium sulfate, n-pentane, toluene (sulfur-free), ethyl acetate, chloroform, and hydrogen peroxide solution 30% w/v were purchased from Finar. Triethylamine, formic acid, and sulfuric acid were purchased from RANKEM. All the reagents and chemicals were used without further purification. 4.2. Measurements. Fourier transform infrared (FTIR) spectra of PTFM and castor polyol were recorded on a Thermo Nicolet Nexus 670 spectrometer. 1H NMR, 13C

4. EXPERIMENTAL SECTION 4.1. Materials. Propargyl alcohols and dibutyltin dilaurate, 95%, were purchased from Aldrich Chemicals. Phosphorus(V) oxide chloride, 2-chloroethanol, and sodium azide were Scheme 1. Schematic Representation of the Synthesis of PTFM

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Scheme 3. Schematic Representation of Foam Preparation of (a) PUF, (b) PUTF-50, and (c) PUTF-100

NMR, and 31P NMR of the compounds were recorded on a Bruker-300 MHz spectrometer using CDCl3 and dimethyl sulfoxide as solvents and tetramethylsilane as a reference. Phosphoric acid was chosen as an external reference for 31P NMR. Electrospray ionization mass spectrometry (ESI-MS) analysis was performed on a Waters Micromass Quattro Micro, API Instrument. TGA of PUF and PUTFs was performed on TGA Q500 Universal TA Instruments (UK). All the foams were characterized with a ramp at 5 °C per minute under a continuous nitrogen atmosphere. The UL 94V test of all the foams was carried out on HVUL 2 (Atlas) according to the ASTM D 3801 standard. Five specimens of each sample with dimensions 150 × 50 × 10 mm3 (length × width × thickness) were taken for the analysis. The specimen was clamped vertically, and the distance between the lower end of the specimen and cotton was ca. 300 mm. The distance between the Bunsen burner and specimen lower end was ca. 10 mm in which ca. 20 mm length of the butane flame was applied. Every specimen was exposed to butane flame for two times, and each time ignition was carried out for 10 s and the time of burning with flaming combustion after first and second (t1 and t2) ignition was noted. The cone calorimetric test was performed on the foams on Fire Testing Technology Ltd., UK, with specimen dimensions 100 × 100 × 10 mm3 (length × width × thickness). All the specimens were covered with an aluminum foil at bottom and sides. The distance between the cone heater and the specimen surface is ca. 20 mm, and the radiant heat flux of 50 kW/m2 was applied to all the foams. The LOI test was executed on Atlas Electrical Devices Company, USA instrument according to the ASTM-D 2863 standard. An average of five samples with specimen dimensions 150 × 10 × 40 mm3 (length × width × thickness) was tested. XPS analysis of foam specimens for elemental analysis was performed on a Kratos AXIS 165 X-ray photoelectron spectrometer (UK) at room temperature. The X-ray gun was operated at 15 kV voltages and 20 mA current. SEM experiments were performed on foam specimens before and after the cone calorimeter test by using JEOL JSM6550F with a magnification of ×50. Whereas the compression strength of foam specimens with dimensions 40 × 40 × 40 mm3 (length × width × thickness) was measured by a universal testing machine (Dac System Inc series 7200) with a cross-head speed of 1 mm/min at room temperature. The test was performed in opposite to foam rising direction. An average of three specimens for each sample was tested, and the specimens were compressed up to 25% and the compression strength of foams was studied at 10% of strain. 4.3. Synthesis of Triprop-2-ynyl Phosphate. In a 500 mL round-bottom flask, propargyl alcohol (10.9 g, 0.195 mol) and triethyl amine (19.7 g, 0.195 mol) were taken along with

100 mL of toluene. Then, the reaction mixture was heated at 50 °C for 30 min with continuous stirring to get a clear solution. Later, the reaction mixture was cooled to 0 °C, and phosphoryl chloride (10 g, 0.065 mol) was added dropwise along with 20 mL of toluene. Then the reaction mixture was left overnight with stirring at room temperature. The resultant mixture was filtered, and toluene was removed from the filtrate under reduced pressure. The orange colored liquid product with 65% yield was collected (Scheme 1). 4.4. Synthesis of 2-Azidoethanol. In our previous publication,34 we reported the synthesis of 2-azidoethanol. In brief, in a round-bottom flask, 2-chloro ethanol and sodium azide were dissolved in water. Sodium azide must be added in small portions while stirring to avoid accidents as it is an exothermic reaction. The reaction mixture was refluxed for 24 h with continuous stirring. Later, the product was extracted with dichloromethane, and the solvent was removed under reduced pressure. Once the compound was isolated, it was immediately stored in dark and below room temperature to avoid accidents as the product has high N/C ratio (3:2). 4.5. Synthesis of PTFM. In a 500 mL round-bottom flask, triprop-2-ynyl phosphate (10 g, 0.0471 mol) and 2azidoethanol (12.31 g, 0.1415 mol) were dissolved in toluene. The reaction mixture was heated at 85 °C for 12 h. The product settled at the bottom of the flask was separated by decanting the solvent, and the viscous product was washed with ethyl acetate, followed by chloroform. The trace amounts of solvents were removed by high vacuum, and the product was collected with ∼98% yield. The reaction was monitored by FTIR as the peak at 2100 cm−1 (azide stretching frequency) disappeared (Scheme 1). 4.6. Hydroxylation of Castor Oil. In a three-neck roundbottom flask equipped with a mechanical stirrer, dry castor oil (10 g, 0.033 mol) was taken. Then, a mixture of formic acid (0.759 g, 0.0165 mol) with sulfuric acid (0.042 g, 2 wt % of HCOOH + H2O2) was added at 0 °C. After 10 min, the required amount of hydrogen peroxide (1.3503 g, 0.0397 mol) was added dropwise. After addition, the temperature increased to 55 °C, and the stirring was continued for 5 h. The reaction temperature was then raised to 90 °C and maintained for 5 h. The hydroxylated castor oil was extracted with ethyl acetate and washed with brine solution. Then the combined organic layer was dried over sodium sulfate. Finally, ethyl acetate was removed from hydroxylated castor oil under reduced pressure (Scheme 2). The hydroxyl value (OHV) of castor polyol was calculated by ASTM standard D 4274-94, and the OHV of the castor polyol was 310 mg KOH/g. It is known from the literature that castor oil is a triacylglycerol comprising 90% of ricinoleic acid and 10% of a variety of fatty acids and glycerol.35,36 The ester of ricinoleic acid and glycerol has been reported as the castor oil structure in Scheme 2 and Figures S7 and S8. 1092


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4.7. Preparation of PUF and PUTFs. In a beaker, castor polyol was taken along with PTFM (0, 50, and 100 wt % with respect to castor polyol, and resulted foams were named as PUF, PUTF-50, and PUTF-100, respectively). The above mixture was heated at 50 °C for 10 min to get a uniform mixture. The mixture was cooled to room temperature, and a required amount of dibutyltin dilaurate (DBTL) (catalyst), TEGOSTAB (surfactant), and water was added. To the above mixture, the calculated amount of polymeric MDI (corresponding to NCO/OH 1.2:1 ratio) followed by n-pentane was added and mixed thoroughly with a mechanical stirrer at 2000 rpm. Then the mixture was poured into molds to get free-rise foams, and the resulted foams were cured at ambient temperature for 24 h. Recipe for the preparation of PUF and PUTFs is tabulated in Table 5, and free-rise foams in paper cups are shown in Scheme 3.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02968. FTIR spectra of 2-azidoethanol, triprop-2-ynyl phosphate, PTFM, castor oil, and castor polyol; 1H, 13C, and 31 P NMR spectra of triprop-2-ynyl phosphate; ESI-MS of triprop-2-ynyl phosphate; 1H, 13C, and 31P NMR spectra of PTFM; ESI-MS of PTFM; 1H and 13C NMR of castor oil and castor polyol; and solubility of PTFM (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 040-27193992 (S.D.). ORCID

Kesavarao Sykam: 0000-0003-2362-5751 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.S. would like to express his gratitude to University Grants Commission (UGC) for the fellowship and Academy of Scientific and Innovative Research (AcSIR), and the authors thank CSIR-XII Five year plan programme INTELCOATCSC0114 for financial support.

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REFERENCES

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Intumescent Phosphorus and Triazole-Based Flame-Retardant

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