Structural Effect of Phosphoramidate Derivatives on the Thermal and

Article pubs.acs.org/IECR

Structural Effect of Phosphoramidate Derivatives on the Thermal and Flame Retardant Behaviors of Treated Cotton Cellulose Thach-Mien Nguyen, SeChin Chang,* Brian Condon, Ryan Slopek, Elena Graves, and Megumi Yoshioka-Tarver United States Department of Agriculture, Agricultural Research Service, Southern Regional Research Center, New Orleans, Louisiana 70124, United States S Supporting Information *

ABSTRACT: The present research is aimed at studying the structural effect of two phosphoramidate derivatives diethyl 3hydroxypropylphosphoramidate EHP and dimethyl 3-hydroxypropylphosphoramidate MHP as flame retardants for cotton fabric. EHP and MHP were obtained in very high yield and purity by one step procedures. Cotton twill fabrics treated with the two compounds at different add-ons (5−20 wt %) were characterized. Vertical flammability, limiting oxygen index, thermogravimetric, and microscale combustion calorimeter analyses were performed, and all resulted in better flame retardancy and thermal behavior for MHP compared to EHP. A study of the functional groups which appeared on the treated fabrics by attenuated total reflection infrared spectroscopy revealed different binding mechanisms between each compound and cotton cellulose. Analysis of the released gas products by thermogravimetric analysis-Fourier transform infrared spectroscopy showed some distinctive details in the degradation of the treated fabrics during the burning process.

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INTRODUCTION Cotton fiber is one of the most commonly used textile fibers in the world due to the comfort and breathability of garments made from it. However, cotton is susceptible to thermal decomposition and combustion which restricts its applicability for uses sensitive to fire risk. Therefore, it is of primary concern for public safety to research ways to render this material less flammable in an economically and environmentally friendly manner. Halogen-free phosphorus-based compounds are the flame retardants (FRs) most frequently used on cotton textiles. Numerous studies have shown that the action of phosphorus is enhanced in the presence of nitrogen,1−5 the synergism phenomena. The nitrogen could come from an external (additive such as urea or guanidine derivatives5,6) or internal source (indirect or direct linkage of nitrogen to the phosphorus atom7−9). When the nitrogen directly connects to the phosphorus, the compound is classified as a phosphoramidate, a member of the organophosphorus family. This class of compounds has great potential in flame retardant application for various polymeric systems including cotton cellulose.9−12 Besides being less volatile, phosphoramidates offer good thermal stability and enhanced char formation during the burning process. In a previous study, phosphoramidates have been shown to be a better flame retardant on cellulose than phosphates.4 Studies on phosphoramidates also include the structural effect on flame retardant performance. It was shown that increasing the number of nitrogen atoms linked to the phosphoryl group helped improve the flame retardancy of the phosphoramidates except the hexamethyl phosphoric triamide.13 The presence of a reactive terminal hydroxyl group in the alkyl substituent on the nitrogen atom of the phosphoramidate could catalyze its decomposition to produce This article not subject to U.S. Copyright. Published 2013 by the American Chemical Society

an acidic intermediate that could react with cellulose to alter its thermal decomposition and enhance flame retardancy.14 In this research, we have studied how alkyl substituents on the oxygen atom of two phosphoramidates, diethyl 3hydroxypropylphosphoramidate EHP and dimethyl 3-hydroxypropylphosphoramidate MHP, affects the flame retardancy and thermal behavior of treated cotton cellulose. Both compounds have the same alkyl chain linked to the nitrogen but a slightly different alkyl chain linked to the oxygen. EHP and MHP were synthesized and characterized by 1H, 13C, and 31 P nuclear magnetic resonance (NMR) spectroscopy. The flammability of fabrics treated by these two compounds at different add-on levels were studied by different methods: vertical flammability (ASTM D-6413-11),15 LOI (ASTM D2863-09),16 TGA, and MCC. Scanning electron microscopy (SEM) was applied to examine the surface morphology of unburned and burned areas of the control and treated fabrics with the highest add-ons. Analyses of evolved gases during the burning process by TGA-FTIR and functional groups of chemicals present on the surface of treated fabrics before the burning process by ATR-IR provided further insights into the thermal decomposition mechanisms of the two compounds.

1. EXPERIMENTAL SECTION 1.1. Materials. Dimethyl phosphite, diethyl chlorophophate, 3-amino-1-propanol, and triethylamine were purchased from Aldrich. Carbon tetrachloride (CCl4) was obtained from Mallinckrodt. Tetrahydrofuran (THF) solvent was purchased from Aldrich and was dried using a Solvent Purification System Received: Revised: Accepted: Published: 4715

January 16, 2013 February 28, 2013 March 8, 2013 March 8, 2013 dx.doi.org/10.1021/ie400180f | Ind. Eng. Chem. Res. 2013, 52, 4715−4724

Industrial & Engineering Chemistry Research

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sample between 4000 and 750 cm−1, and the spectra were analyzed using the Opus software. 1.4.4. Microscale Combustion Calorimeter (MCC). MCC using a pyrolysis combustion flow calorimeter (PCFC) is considered the most significant bench scale instrument in fire testing and is conducted according to ASTM D7309. In this test, heat release as a function of temperature is the key measurement to assess the fire development of materials and products, and the data have been shown to correlate with flammability results (LOI). This technique enables many flammability parameters to be quickly determined from milligrams of test specimens. All of the details about the principle of measurement for the instrument are described in the literature.17 In this experiment, the samples were heated in an inert atmosphere of nitrogen from 85 to 700 °C at a heating rate of 1 °C/s. The specimen is first heated in a pyrolyser and the degradation products are swept from the pyrolyser by the gas stream of nitrogen and oxygen to a combustor at 900 °C, where the decomposition products are completely oxidized. Oxygen concentrations and flow rates of the combustion gases are used to determine the oxygen depletion involved in the combustion process, and the heat release rates are determined from these measurements. Three repetitive measurements were taken for each sample, and the average values are reported. 1.4.5. Scanning Electron Microscopy (SEM). The surface morphology of unburned and burned areas of control twill and the highest add-on of MHP and EHP treated fabrics (after flammability test) was examined using the Philips XL 30 ESEM with magnifications of 500× and 1500× and a setting of 12 kV. All samples were coated with gold for analysis purposes. 1.5. Fabric Treatment. The required quantity of MHP or EHP was dissolved in a minimum quantity of 1:4 acetone/ water and poured into a shallow container that contained the twill fabric sample laying flat. The fabric was then soaked in this solution for 1 h, padded at 10 psi, dried at 100 °C for 5 min, and cured in air at 170 °C for 5 min. After removal from the curing oven, the fabrics were allowed to cool to room temperature in a desiccator and their weights were immediately obtained.

from Innovative Technology. The reaction was conducted under nitrogen atmospheric conditions and monitored using silica gel 60 F254 thin layer chromatography purchased from EMD Chemicals Inc. Cotton cellulose from 100% cotton fiber was obtained as twill fabric with the weight of 258 g/m2 (Testfabrics, Inc., Style 423). This fabric was desized (starches removed), and bleached, and was free of all resins and finishes. 1.2. Synthesis of Diethyl 3-Hydroxypropylphosphoramidate (EHP). To a solution of 3-amino-1-propanol (2.18 g, 29 mmol) in dry THF (10 mL), triethylamine (4.04 mL, 29 mmol) in dry THF (10 mL) was added and the mixture was cooled to 0 °C. A solution of diethyl chlorophosphate (5.0 g, 29 mmol) in dry THF (10 mL) was slowly added by addition funnel to the above mixture while stirring under nitrogen. After the addition, the reaction was allowed to warm to room temperature and monitored by TLC using 10% CH2Cl2/ MeOH as an eluent and iodine as staining reagent. When the reaction was complete, a white solid was filtered. The removal of solvent gave a yellowish oil as product in 95% yield with no purification needed. 1.3. Synthesis of Dimethyl 3-Hydroxypropylphosphoramidate (MHP). The procedure of this synthesis was based on a procedure of synthesis of diethyl derivative by Gann et al.14 Clean MHP was a clear colorless oil with 95% yield. 1.4. Measurements. 1.4.1. NMR. Structural analysis of EHP and MHP was conducted on a Varian 400 MHz instrument using DMSO-d6 as solvent. 31P was given in δ relative to external 85% aqueous H3PO4. 1.4.2. Vertical Flammability and Limiting Oxygen Index (LOI). The samples of control twill and all treated fabrics were subjected to the vertical flammability and limiting oxygen index tests. Vertical flame tests were performed on strips of fabric (30 cm × 7.6 cm). The LOI tests were conducted on strips of fabric (13 cm × 6 cm), and the average values of 4−6 repetitive measurements are reported. 1.4.3. Thermogravimetric Analysis (TGA), Thermogravimetric Analysis−Fourier Transform Infrared (TGA−FTIR), and Attenuated Total Reflection Infrared (ATR-IR). TGA on the control twill, all treated fabrics, and two synthesized compounds were conducted on a TA Instruments Q500 under nitrogen. Analysis was monitored on 5−8 mg samples that were heated between 20 and 600 °C at a rate of 10 °C/min and a flow rate of 60 mL/min. Onset of degradation and char content at 600 °C were obtained from TGA thermograms. The TGA-FTIR experiment was conducted by the TGA between 100 and 500 °C and a Bruker Tensor-27 FTIR at 4 cm−1 resolutions in the 800−4000 cm−1 region. The volatile decomposition products released by a sample that is exposed to a controlled temperature program were analyzed and the signals were measured by a liquid-nitrogen cooled MCT detector with ZnSe window. Both lines transferring the evolved gases from the TGA to the FTIR and the IR cell were maintained at 200 °C. The FTIR data obtained at every 5 degree increment during the experiment were analyzed using Opus software, which measures the intensity of an absorption band (representing a functional group) as a function of temperature. These FTIR spectra were supported by an OriginLab 8.6 software to retain their original three dimensions for analysis purposes. Functional groups on the control twill and treated fabrics before thermal degradation were examined on a Bruker Platinum Alpha ATR-IR spectrometer, A220/D01. Thirtyfour scans at a resolution of 4 cm−1 were recorded for each

2. RESULTS AND DISCUSSION 2.1. Syntheses and Structural Characterization of EHP and MHP. EHP and MHP were synthesized in one step, resulting in high yields of high purity (Scheme 1). This allowed both compounds to be used immediately in flame retardant testing, without further purification operations. The phosphoramidates can be easily prepared by the Atherton−Todd reaction of phosphites or an elimination reaction of chloroScheme 1. Synthesis of Diethyl 3Hydroxypropylphosphoramidate (EHP) and Dimethyl 3Hydroxypropylphosphoramidate (MHP)

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phosphates.14,18,9 (see Supporting Information for detailed NMR data of both including 1H, 13C, and 31P). In both 1H NMR, the multiplets at 1.55, 2.80, and 3.42 ppm are attributed to the −CH2− groups and two multiplets at about 4.42 and 4.80 ppm correspond to OH and NH protons, respectively. The −CH3 protons of the MHP appear as a doublet at 3.53 ppm and the −CH2CH3 protons of the EMP resonate at 1.21 ppm as a triplet and 3.89 ppm as a quintet. The 13C of EMP has three doublets at 16.1(J = 28 Hz), 34.5 (J = 20 Hz), and 61.1 ppm (J = 20 Hz), and two singlets at 38.0 and 58.4 ppm while MHP carbons appear as doublets at 34.5 (J = 24 Hz) and 52.2 ppm (J = 24 Hz) and as singlets at 38.0 and 58.3 ppm. 31P NMR shows a multiplet for both compounds, which is at 11.2 ppm for EHP and at 13.4 ppm for MHP. The difference in chemical shift between the two compounds can be explained by substituent effects. Branching and substitution on chains attached to P in many functional groups have effects in 31P NMR and this effect passes through heteroatoms on P, which act as the alpha atom. For EHP and MHP, the alpha atom is O. Replacement of H on a CH group beta- to P by methyl creates a gamma substituent. Rotation of the gamma-related groups about the C−C bond to which they are attached causes upfield shifting due to the steric compression. This electronic repulsion from bringing groups close together will increase electron density on the P nucleus, causing the shielding.19 2.2. Fabric Treatment. The treatment does not impart stiffness or discoloration on the fabric samples. These samples were weighed before and after treatment and the values were fitted to the eq 1 to obtain add-on percents.

Figure 1. Vertical flammability test results of control and treated fabrics.

test and Table 2 summarizes the test results for all treated samples. Overall, the flammability test shows the effectiveness of EHP and MHP as FRs on twill fabric at add-on levels of 10 to 20 wt % (Figure 1). There was no occurrence of afterflame or afterglow burning upon the removal of the flame. Also no melting or dripping during the burning was observed for all add-on levels though the 5 wt % fabrics did not pass the test and displayed afterflame burning. From Table 2, it is clear that both types of treated fabrics have a char length of less or equal to 3.5 cm or approximately 1.4 in., which is much lower than the required maximum char length, 10.0−15.0 cm or 3.9−5.9 in., to pass a vertical flammability test.23 Above 5 wt %, the MHP performed better than the EHP since all of its char length values are lower than those of EHP. In addition to the char length, afterflame, and afterglow time, the speed with which the fire is moving away horizontally from the origin per unit of time (usually refers to the head fire segment of the fire perimeter) is measured. The rate of flame spread was calculated using formula 2

add‐on (%) = [(weight afterdrying − weight beforetreatment) /weight beforetreatment] × 100

(1)

All add-on values of treated fabrics are summarized in Table 1. The loading levels used here are similar to those used in the Table 1. Add-Ons (wt %) Obtained from Twill Fabrics Treated with EHP and MHP treated compound

add-ons (wt %)

EHP

5 11 15 20 5 10 14 19

MHP

rate of flame spread (mm/s)

studies of the effectiveness of organophosphorus FRs for cotton textiles.20,21 They are also recommended by the industry for the goods treated with similarly structured compound to pass flammability testing.22 Surprisingly, after curing, the add-on percents of the MHP treated fabrics were always higher than those of the EHP treated ones even though both treatment solutions were made at the same concentration; therefore, a larger amount of EHP was required to achieve the same addons as on MHP fabrics. 2.3. Flame Retardant Performance and Flammability Properties of Treated Twill Fabric. To evaluate the flame retardant properties of treated fabrics, two important parameters were investigated: the vertical flammability and LOI. Figure 1 comprises the images taken after the flammability

= char length (mm)/[x + after flame (s)]

(2)

where x is flame application duration (12 s). As seen in Table 2, fire spread is faster on EHP treated fabrics as compared to MHP ones at all add-on levels. The average LOI values and time to burn to 5 cm line during the LOI test for all samples are provided in Table 2. The flame retardant action of EHP is inferior to MHP for most add-on levels. MHP treated fabrics achieve higher LOI values than EHP treated ones though the burning time to the 5 cm line of both types are almost the same except the lowest add-on. From Table 2 it is also clear that for both treated fabric types, only the 5 wt % add-on is classified as slow burning, the rest can be considered as self-extinguishing.25 24

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Table 2. Vertical Flammability (ASTM D-6413-11)15 and LOI Test (ASTM D-2863-09)16 for Different Add-Ons (wt %) of Treated Twill Fabrics vertical flammability

LOI

fabric

afterflame time (sec)

afterglow time (sec)

char length (cm)

rate of flame spread (mm/s)

EHP-5 EHP-11 EHP-15 EHP-20 MHP-5 MHP-10 MHP-14 MHP-19

21 0 0 0 25 0 0 0

0 0 0 0 0 0 0 0

25 3.5 3.0 2.5 21 2.5 2.0 1.0

7.6 2.9 2.5 2.1 5.7 2.1 1.7 0.8

average of LOI (vol%) (no. of trials) 25.8 31.5 31.5 33.4 27.0 29.5 34.2 37.2

± ± ± ± ± ± ± ±

0.5 1.0 0.8 0.9 1.5 1.0 1.2 1.3

(4) (4) (4) (5) (4) (4) (6) (5)

time (sec) to burn to 5 cm line 52.0 43.8 35.8 37.4 42.3 42.0 38.0 35.8

± ± ± ± ± ± ± ±

2.4 8.4 6.8 7.3 7.5 15.1 4.4 7.1

2.4. Heat of Combustion. MCC has been utilized to evaluate the flammability of various systems of flame retardant formulations containing different hydrated mineral fillers, phosphorus compounds, and carbon nanotubes.26−29 In our experiment, the control twill and all treated fabrics were subjected to the FAA Micro Calorimeter (Fire Testing Technology Limited) and the flammability parameters such as the peak of heat release rate (pHRR), temperature of maximum HRR (Tmax), and total heat release (THR) were determined and reported in Table 3. Also, comparison of the Table 3. Pyrolysis Combustion Flow Calorimeter Data. (All Reported Values Are Data from Three Observations on the Same Fabric Samples)a fabric control twill EHP-5 EHP-11 EHP-15 EHP-20 MHP-5 MHP-10 MHP-14 MHP-19

THR (kJ/g) 8.5 4.6 5.3 6.0 6.3 3.9 3.7 3.8 3.8

± ± ± ± ± ± ± ± ±

0.2 0.2 0.2 0.2 0.2 0.5 0.3 0.3 0.2

pHRR (w/g) 213.0 169.0 164.0 142.7 129.0 141.0 108.3 105.0 128.3

± ± ± ± ± ± ± ± ±

13.3 5.6 1.7 3.1 1.7 14.7 10.4 5.0 3.2

Tmax (°C) 364.0 294.3 288.0 294.0 293.7 294.7 286.0 280.0 278.7

± ± ± ± ± ± ± ± ±

3.5 0.6 1.0 1.7 1.5 1.5 1.0 1.7 1.2

a

Abbreviations: THR, total heat release; pHRR, peak of heat release rate; Tmax, temperature of maximum heat release rate.

heat release rate of all samples and the control is depicted in Figure 2. From Table 3, it is noted that all treated fabrics have lower THR, pHRR, and Tmax values as compared to the control twill and most values of MHP samples are lower than those of EHP. The different values between the control twill and treated fabrics can be attributed to the action of phosphorus, which lowers the dehydration and decomposition temperatures of the cotton. While the THR of MHP remains the same for all addon levels, the THR of EHP samples increases with increasing add-on. As shown in Figure 2, the control twill combusts slowly by exhibiting a single blunt peak of HRR at 366 °C. With the incorporation of phosphorus additives (EMP and MHP), twopeak behavior and lowering of the temperature are observed for all add-on levels. The first peaks at around 210−220 °C may be due to flame spread over the sample surface and to the process of dehydration and decomposition of FRs to form protective barriers. The difference in the intensity of the peaks stems from the difference in the concentration of FRs. The protective layer not only prevents the underlying cotton from igniting, but also reduces the normal thermal degradation of cotton and

Figure 2. The heat release rate curves of control and treated fabrics: EHP (a) and MHP (b).

structural disintegration of the char to release volatiles and gases. The heat release in the thermal degradation and decomposition of residual char processes attributes to the rise of the second peaks at around 280−290 °C. Thus, the intensity of the second peak increases with a decrease in the thickness of the protective layer. 2.5. TGA. Thermal decomposition curves of both compounds, control, and treated twill fabrics are shown in Figure 3. Summary of the char content values (% at 600 °C) and the beginning of weight loss (onset temperatures) are listed in Supporting Information. The control twill, untreated cellulose, has three stages of weight loss due to thermal degradation: initial stage (slow weight loss) between 100 and 120 °C (dehydration), main stage (rapid weight loss) between 320 and 375 °C (thermal decomposition of the cellulose cleavage of the glycosidic bond and evaporation of the products), and decomposition of residual char formed in the 4718



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Figure 4. Char residue images of EHP and MHP at 600 °C in TGA pans.

2.6. FTIR Analysis of Gas Products. Further study to understand the action of both compounds on cotton twill was carried out on TGA-FTIR. The real-time analysis of the FTIR spectra taken every 30 s, or 5 °C, in each experiment are plotted one on top of the other with a total of 80 scans to form the 3D-dimensional spectra showing the evolution of gas products in the course of thermal decomposition of control and treated fabrics, as a function of both wavenumber and temperature. Typical spectra output from the TGA-FTIR experiment of highest add-ons MHP-19 and EHP-20 as well as cotton twill are shown in Figure 5 (see Supporting Information for detailed band assignment). When comparing evolution profiles for MHP-19 and EHP20, as shown in Figure 5a,b it is obvious that both profiles are very similar in most absorption peaks: at 100−500 °C, CO2 (2270−2400 cm−1); at 115−500 °C, hydrocarbon OH (∼3060−3500 cm−1); at 225−500 °C, CO (∼2110 and 2185 cm−1); at 250−300 °C, water vapor (1800−1500 and 4000− 3500 cm−1) and some hydrocarbon CO (1850−1600 cm−1), C−O−C, C−H, C−C, and CC (1500−900 cm−1); at 425− 500 °C, vibrational stretches of PO-Me, PO-Et (1182 cm−1), PO (1321 cm−1), and P−OC (1027 and 1035 cm−1 of MHP and EHP, respectively).30,33−35 This is expected because they differ from each other only by one CH2 group. However, they do show some differences in characteristic absorption peaks of MeOH (2965, 2856, 1058, 1043, and 1044 cm−1) in MHP-19 and EtOH (2987, 2886, 2940, 1392, 1274, 1050, and 951 cm−1) in EHP-20 at 150−225 °C.36−38 Though the vibrational stretches of CH2 and CH3 both asymmetric and symmetric in both spectra appear almost at the same regions 2988−2650 cm−1 for EHP-20 and 2964−2686 cm−1 for MHP-19 at 250− 300 °C, it is clear that EHP-20 has more processes running in parallel and consecutively during its thermal decomposition than MHP-19. With respect to cotton twill thermal decomposition (Figure 5c), besides the evolution of water vapor, hydrocarbon OH, CO2, and CO gases, which are the same as in treated fabrics, other gases are released at temperatures between 300 and 400 °C including hydrocarbon CH2, CH3 (3014−2600 cm−1), hydrocarbon CO (1850−1600 cm−1), C−O−C, C−H, C−C and CC (1500−900 cm−1).30,33 These are the results of reduction of molecular weight, the appearance of free radicals, oxidation, dehydration, decarboxylation, and decarbonylation at about 300 °C and the decomposition of cellulose to tarry pyrolyzate-containing levoglucosan, which vaporizes and then decomposes at above 300 °C. In comparison with treated fabrics, pyrolysis products of the control start to evolve at higher temperatures and come mainly from the degradation of cellulose. The findings from FTIR are consistent with the TGA results. A small mass loss was observed at around 158 and 168 °C and a major mass loss occurred at around 259 and 239 °C for EHP20 and MHP-19, respectively, while a major mass loss took place at 313−400 °C for the control.

Figure 3. Degradation thermograms in nitrogen of EHP and different add-ons (a) and MHP and different add-ons (b).

main stage (slow weight loss) after 400 °C.30,31 It is known that phosphorus additives lower the onset temperature (the beginning of weight loss) of the second stage of treated cellulose by 50−150 °C.32 For all treated fabrics except 5 wt %, a small weight loss of first stage (155−168 °C) occurs slowly, which could be due to the decomposition of flame retardants after dehydration. This stage is then followed by a second stage (240 to 258 °C) which happens faster and covers 55 to 58% of the weight loss; that can be attributed to the evolution of most volatiles resulting from the depolymerization of the cellulose molecule. As seen in Figure 3, the onset temperature of EHP fabrics is lower for the first stage and higher for the second stage as compared to MHP. The 5 wt % of compounds have almost the same onset temperature, which is close to the onset temperature of the second stage of EHP treated fabrics. Overall, both types of treated fabrics have a similar onset temperature range for either first or second stage of thermal degradation and their onset temperatures of the second stage are still lower than that of untreated cotton twill (327 °C). It is observed that the MHP has two different onsets of degradation (though it has three stages of decomposition) with the first one the same as the only onset of degradation of the EHP. The thermogram associated with EHP is sharper with less slope when compared to MHP. The final appearance of their decomposition at 600 °C (char residue in TGA pan) is presented with the control (clean pan) in Figure 4. It is obvious that the two FRs have different effects on the thermal degradation since the decomposition of MHP turns it to black char while the decomposition of the EHP leaves no char behind. This explains why EHP fabrics achieve lower char residue at 600 °C when compared to MHP at all add-on levels, and thus proves its inferior flame retardancy on cotton. 4719



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the treated/burned sample surface may supply evolved decomposition products to the flame, which in turn compromises the FR effectiveness. These types of protective coatings on burned areas were also observed in earlier studies where the burned surface morphology was linked to the flame retardant ability of the phosphoramidate and phosphorus− nitrogen containing FR substances.7−9 Figure 6 also reveals the difference in mechanism between the FRs. The unburned MHP-19 contains slightly less amount of chemical but the burned MHP-19 shows more bubbles and swollen areas when compared to the EHP-20 ones. 2.8. Functional Groups Present on the Surface of Treated Fabrics before Thermal Decomposition. To learn more about the differences in the flame retardant process of both compounds, ATR-IR was utilized to examine the chemicals present on the treated fabrics before thermal degradation. Figure 7 shows the spectra of treated cotton fabrics at highest add-ons, EHP-20 and MHP-19, and untreated cotton fabric for comparison (The observed vibrational frequencies of absorption peaks together with their assignments are listed in Supporting Information). Although upon first inspection of the three spectra EHP-20 and MHP-19 look similar and moreover impossibly complicated as compared to the control, they contain in fact quite useful information. The peak characteristic of hydrocarbon CH2 and CH3 both asymmetric and symmetric vibrations at 2985, 2934, and 2948 cm−1, of PO-C at 1026 and 1029 cm−1, and of PO at 1204 and 1230 cm−1 in both treated fabrics are found, confirming the presence of P−O−C linkage in the chemicals.39 In addition, the stretch of the O−P−O group at 798 and 833 cm−1 present in the chemical layer is detected.39 Besides these common features, there are differences in absorption peaks found in both spectra. Spectroscopic features of the NH group such as free vibration in P-NHR at 3448 cm−1 and NH deformation in secondary phosphoramidate at 1394 cm−1 are observed in EHP-20.39,40 The second absorption is very useful in identifying NH in PNHR because primary and tertiary phosphoramidates show no such absorption in this region.40 Also in EHP-20 spectrum, an absorption peak at 972 cm−1 is a distinctive characteristic of the P−OH of phosphoric acid. In addition, the phosphoryl (PO) absorption of EHP-20 (secondary amide) is broadened and shifted to the lower frequency (at 1190 cm−1) as compared to the phosphoryl absorption of MHP-19 (at 1230 cm−1). This is probably a contributory factor of hydrogen bonding with the amide hydrogen atom.41 Finally, MHP-19 spectrum shows a small absorption at around 982 cm−1, which is a characteristic of the OC-C vibration in a chain comprising more than two aliphatic carbons.39 2.9. Possible Mode of Action of EHP and MHP. Initial thermal decomposition of untreated cellulose leads to depolymerization of the cellulose polymer to form various anhydrosugar derivatives, among which levoglucosan, the fuel of combustion, is the most prevalent (Scheme 2, reaction A).30 In treated cellulose, this process can be altered by the presence of phosphorus compounds due to their catalytic action: phosphorylating the cellulose unit at the C6 position to prevent the formation of levoglucosan (Scheme 2, reaction B). With this binding, the phosphorus compounds could promote the formation of acid intermediates that could further catalyze the dehydration of the cellulose and other flammable products of the cellulose to carbonaceous char.42

Figure 5. Volatiles released out during pyrolysis of EHP-20 (a), MHP19 (b), and control twill (c).

2.7. Char Study. SEM analysis of the control twill and highest add-on treated fabrics was carried out to further examine the surface morphology (Figure 6). The micrographs of the unburned area obtained for the MHP-19 and EHP-20 show uneven surfaces compared to that of the control twill. The mechanism of both FRs helping the cellulose to form a protective layer can be observed in B2 and B3. The introduction of MHP and EHP to the fabrics increased the thermal stability of the fibers during the burning while the burned control (B1) was destroyed completely leaving a gray ash without residue. The occurrence of the blisters (nodules) and swollen fibers on 4720



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Figure 6. SEM micrographs of unburned (A, 1500×) and burned (B, 500×) areas of control twill (1), MHP-19 (2), and EHP-20 (3) fabrics.

Different functional groups on treated fabrics before thermal decomposition (by ATR-IR) show that reactions between phosphorus compounds and cellulose had occurred in different ways (Scheme 3) resulting in different binding modes. These might be the main reasons of the appearances of some different functional groups in the spectra of evolution profiles of gas products (by TGA-FTIR) of EHP-20 and MHP-19. It is possible that water could play a significant role in the thermal decomposition of EHP since the absorption peak of P−OH suggests that water (absorbed in cotton cellulose) could react with EHP at high temperature to form an acidic compound. Thermal decomposition of EHP could begin by an intramolecule nucleophilic attack on the phosphorus atom to form a six-membered ring intermediate and release ethanol (reaction

C). The formation of the ring could lead to hydrolysis to release ring strain.43 Big molecules like cellulose could not bind covalently with EHP due to the steric hindrance of OEt groups. On the other hand, the thermal decomposition of EHP could simply be a nucleophilic attack of water on the phosphorus atom resulting in the cleavage of an OEt group (reaction D). In either case, the acidic phosphate intermediate formed could further catalyze the decomposition of cellulose to form a char as presented in Scheme 2. MHP, which proved to be a better flame retardant than EHP, could thermally decompose by a different mechanism as shown in Scheme 3, reaction E. Since MHP has the same amide chain as EHP, it might also go through a cyclization to form sixmembered phosphoramidate and release MeOH under thermal 4721



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Figure 7. ATR-IR spectrum of control and treated fabrics, EHP-20 and MHP-19 before pyrolysis: full spectra (a) and expanded region (b).

Scheme 2. Thermal Decomposition of the Untreated Cellulose and Cellulose Treated with Phosphorous FR

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Scheme 3. Proposed mode of action of EHP (a) and MHP (b)

Further study on durable property using Standard Laboratory Practice for Home Laundering will be carried out and data will be presented in future publication.

decomposition. Cellulose could form the covalent bond with MHP as the OMe group is less hindrance and the nitrogen could be protonated to make the ring susceptible to opening. The resulting amine could further react with decomposing cellulose or get volatilized to prevent more burning. The size of the O-alkyl in MHP could explain its higher efficacy as compared to EHP.

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ASSOCIATED CONTENT

S Supporting Information *

Multiple NMR spectra, TGA, and analysis for FTIR and ATRIR. This material is available free of charge via the Internet at http://pubs.acs.org.

3. CONCLUSION We have reported our characterization of the cotton twill fabric treated with two new phosphoramidate derivatives: EHP and MHP. In 31P NMR, EHP phosphorus resonated at lower frequency as compared to MHP phosphorus due to a substituent effect of O-alkyl on phosphorus. The results of vertical flammability showed that the char length ranged from 1.0 to 21.0 cm for MHP samples and from 2.5 to 25.0 cm for EHP ones. In addition to having a slower rate of flame spread, almost all MHP samples had higher LOI values (27.0−37.2%) as compared to EHP ones (25.8−33.4%). In the MCC test, the smaller values obtained for THR and pHRR in all MHP samples indicated the better reduction in heat of combustion. Furthermore, while the THR of EHP samples increased with increasing add-on (4.6−6.3 kJ/g), the THR of MHP samples slightly decreased with increasing add-on (3.9−3.8 kJ/g). Tests performed on TGA showed that the ranges of char yield for MHP and EHP fabrics were 33−36% and 28−31%, respectively. Among all the tests performed, the SEM did not show the differences in microstructure of the surfaces of the unburned and burned areas of highest add-on EHP and MHP fabrics. ATR-IR data revealed the presence of the P−OH group of an acidic intermediate on the EHP-20 and the disappearance of the P−N bond and appearance of the OC-C bond on the MHP-19. In addition, TGA-FTIR data of the MHP-19 showed more processes that took place during the thermal decomposition when compared with EHP-20. MHP compound on fabric could thus decompose to form a terminal amine that could further react with decomposing cellulose or get volatilized to prevent more burning. The superior action of MHP could be attributed to the ability to form covalent bond with cellulose arising from the smaller size of O-alkyl group.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 1-504-286-4487. Fax: 1-504-286-4390. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the U.S. Department of Agriculture for financial support. The authors would like to thank Crista Madison and Jade Smith for their assistance in LOI experiments.

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REFERENCES

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Structural Effect of Phosphoramidate Derivatives on the Thermal and

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