Cyanuric Chloride Derivatives for Cotton Textile ... - naldc - USDA

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Cyanuric Chloride Derivatives for Cotton Textile Treatment—Synthesis, Analysis, and Flammability Testing By Michael Easson, Brian Condon, Megumi Yoshioka-Tarver, Stephanie Childress, Ryan Slopek, John Bland, Thach-Mien Nguyen, SeChin Chang, and Elena Graves, USDA, SRRC Abstract Two cyanuric chloride derivatives were synthesized in good yields and analyzed by proton, carbon, and phosphorus nuclear magnetic resonance spectroscopy as well as high performance liquid and gas chromatography mass spectrometry. Treated cotton fabric was tested for flame retardant properties using standardized thermogravimetric, limiting oxygen index, and vertical flame methods. Scanning electron microscopy images revealed the level of flame retardant protection. The results are reported herein and indicate cyanuric chloride derivatives have a potential use in flame retardant applications to cotton textiles. Key Terms Cotton, Cyanuric Chloride, Flame Retardant, LOI, SEM, Thermogram, Vertical Flame

Introduction

Industrial, governmental, and academic research groups have long sought to synthesize flame retardant compounds for application to textiles. There are many reviews on this subject.1,2 Recently, legislative action in the European community has restricted the use of halogen-based flame retardants, causing the focus of research to shift to more environmentally-friendly nitrogen and phosphorus-based compounds. Our current research effort is focused in the area of organophosphorus flame retardants due to their non-brominated character, ease of synthesis, and performance in standardized flammability testing. Unlike bromine-based compounds, which retard flame in the gaseous phase by a free-radical scavenging mechanism, phosphorus-nitrogen based compounds retard in the condensed phase by synergistically promoting the phosphorylation of cotton fabric and the formation of protective char.3-5 In this laboratory, mono- and di-substituted diethyl-2-aminoethylphosphonate derivatives of cyanuric chloride were chosen as a covalent linker with bleached and mercerized cotton. The choice of cyanuric chloride stems from its affordability and long time use in the cotton industry as a reactive dye linker.6 Using existing linker chemistry between cyanuric chloride and cotton,

cyanuric chloride was derivatized with flame retardant properties in a similar manner as dyes. Two derivatives (Compounds 4 and 5) presented in Fig. 1 represent the latest efforts to provide effective flame retardant treatments to cotton textiles.

Fig. 1. Synthesis of Compounds 4 and 5.

The mention of different products and/or their manufacturer is solely for the purpose of research. The mention does not imply any endorsement by USDA. 60 | AATCC Review November/December 2011 www.aatcc.org



Experimental

Materials and Methods

Cyanuric chloride (Compound 3, Aldrich), diethyl cyanomethylphosphonate (Compound 1, Alfa Aesar), Adam’s catalyst (Strem), NaOH (J. T. Baker), NaHCO3 (J. T. Baker), dichloromethane (DCM), and anhydrous ethanol (Fisher) were used as received. All reactions were conducted under ultra-high purity (UHP) argon and monitored using silica gel IB2-F thin layer chromatography (J. T. Baker) viewed under short wave ultraviolet light (254 nm, Spectroline) and in a glass chamber containing iodine (Aldrich). NMR spectra were recorded on a Bruker 400 MHz instrument using CDCl3 as solvent. 1H- and 13 C-NMR data were reported as chemical shifts (δ) relative to tetramethylsilane (TMS). 31P-NMR spectra were reported as chemical shifts (δ) relative to external 85% aqueous H3PO4. Compound 2 was analyzed by gas chromatography/ mass spectrometry (GC/MS) using a Leco GC/timeof-flight (TOF) mass analyzer. The sample (1 µL) was injected at 270°C in splitless mode. The GC oven was held at 60°C for 1 min and then increased to 300°C at a rate of 10°C/min. Helium was used as the carrier gas with a 30 m × 0.25 mm, 0.25-µm particle size, DB-5 capillary column (J & W Scientific). Compounds were ionized by electron ionization (EI) at 70 eV. Individual spectra were collected and summed to give an effective scan rate of 10 scans/s. Data were collected and stored from m/z 40 to 400 Daltons. High performance liquid chromatography-mass spectrometry (HPLC-MS) analyses were carried out with an Alliance 2695 HPLC, with column heater, 996 Photodiode Array Detector, and LCT Premier XE time-of-flight mass spectrometer (TOFMS, Waters). A Luna C18, 3-µm particle size, 2.0 × 50 mm column (Phenomenex) was used at ambient temperature. The TOFMS was equipped with an electrospray ionization (ESI) source and parameters set as: capillary voltage, 3,000V; cone voltage, 30V; desolvation heater, 300°C; source temp, 100°C; desolvation gas, 500 L/h; cone gas flow, 50 L/h;, V mode (resolution = 6,500); positive ionization; and scan range m/z 150-1,000. For Compound 4, a 5-μL sample (10 μg/mL in acetonitrile (ACN)) was injected onto the column with a gradient of ACN in water from 20% to 50% in 19 min, and 50% to 100% in

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1 min at a flow rate of 0.3 mL/min. For Compound 5, a 20-μL sample (1 μg/mL in ACN) was injected onto the column with a gradient of ACN in water from 10% to 30% in 10 min and 30% to 100% in 10 min at a flow rate of 0.3 mL/min. Crude Compounds 4 and 5 were purified using column chromatography (60-200 mesh silica gel, 50 × 300 mm column). Twill fabric, 258 g/m2 (Style 423, Testfabrics Inc.) was bleached and mercerized. Three concentrations of Compounds 4 and 5 (5%, 10%, and 20% w/w in acetone) were prepared. Three fabric samples were immersed in each solution for one hour and cured in a vacuum oven at 160°C for 5 min prior to thermogravimetric analyses (TGA). The use of acetone may be a concern in a full-scale production environment. Additionally, some companies do not want chlorine in any form in the production process. Efforts to address these concerns will continue as research progresses. TGA was performed using a TA Instruments Q500 under a nitrogen atmosphere. Analyses were monitored between 25°C and 600°C at a rate of 10°C/min. Limiting oxygen index (LOI) tests were conducted on treated fabrics according to ASTM D2863-00 protocol. Vertical flame testing was performed on treated fabrics according to ASTM D6413-99. Surface morphology photographs of burned and unburned cotton samples were taken on a Phillips XL30 environmental scanning electron microscope (SEM) operating at 12 kV. To improve the image quality and reduce sample charging, cotton samples were coated with a gold-palladium alloy via vacuum sputtering.

Cyanuric Chloride Derivative Syntheses

Compound 2, diethyl 2-aminoethylphosphonate, was synthesized using a modified procedure of Jakeman, et al.7 A 500-mL reaction vessel was charged with Adam’s catalyst (0.75 g, 2.2 mmol) and covered with 20 mL of absolute ethanol. Diethyl cyanomethylphosphonate (Compound 1, 7.50 g, 42.3 mmol), 280 mL of absolute ethanol, and 7.5 mL of concentrated HCl were added to the reaction vessel, which was placed on a Parr Shaker and purged with hydrogen gas three times at 40 psi. The reaction vessel was refilled with hydrogen gas to 40 psi and then the Parr Shaker was switched on and allowed to run overnight. Once thin-layer chromatography (TLC, AATCC Review November/December 2011 www.aatcc.org

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1:9 MeOH/DCM) confirmed complete product formation, the reaction was gravity filtered and the filter paper rinsed with absolute ethanol. Filtered catalyst could be collected and reused, though activity was diminished. The filtrate was removed under reduced pressure to yield a light yellow semisolid that solidified when placed under high vacuum. The yield was quantitative. 1H-NMR (CDCl3) δ (ppm): 4.14-4.09 (m, 4H), 2.98-3.06 (m, 2H), 1.90-1.98 (m, 2H), 1.56 (bs, 2H), 1.33 (t, J = 6.5 Hz, 6H). 13C-NMR (CDCl3) δ (ppm): 61.58 (d), 36.40 (d), 30.76 (d), 16.51 (d). 31P-NMR (CDCl3) δ (ppm): 30.95-31.32 (m). MS using EI detection gave a peak at m/z 181 (M+ ion). The light yellow hydrochloride salt form of the compound was stable for several weeks when placed under high vacuum and shielded from light. However, the neutralized form of the light yellow primary amine partially degraded within 72 h to form orange-colored impurities. Additionally, once the HCl salt or the neutralized primary amine forms of Compound 2 were dissolved in an aqueous medium, extraction with organic solvents was extremely difficult. This is contrary to the procedure as reported by Jakeman, et al. Compound 4, diethyl 2-(4,6-dichloro-1,3,5-triazin2-ylamino)ethylphosphonate, was synthesized using a modified procedure of Yang, et al.8 Compound 2 (9.37 g, 43.1 mmol) was dissolved in 250 mL of deionized water and cooled to 0°C. Cyanuric chloride (7.94 g, 43.1 mmol) was dissolved in 250 mL of acetone in a 1-L three-necked round bottom flask and cooled to 0°C. The solution containing Compound 2 was slowly added by addition funnel to the three-necked reaction vessel at 0°C while rapidly stirring under argon. The pH was carefully monitored and adjusted to 6.0 with solid NaHCO3 as needed throughout the experiment. When the addition was complete, the ice bath was removed and the reaction was allowed to warm to room temperature (RT). After 1 h, TLC (1:9 MeOH/DCM) confirmed formation of the desired product. Acetone was removed under reduced pressure to yield an immiscible yellow oil in aqueous solution. Saturated brine (250 mL) was added to the crude mixture which was extracted with 250 mL DCM three times. The organic phase was dried over Na2SO4, filtered, and removed under reduced pressure to form a semi-solid which solidified overnight under high vacuum to produce a light yellow solid. The crude was dissolved in DCM and chromatographed using a solvent system consisting of DCM, followed by 2% MeOH/

DCM solution. The desired compound eluted as a clear liquid that was removed under reduced pressure to obtain a white solid in 76% yield. Compound 4 is stable to nucleophilic attack provided it is maintained as a solid, or not dissolved in a solution with a pH > 6. 1H-NMR (CDCl3) δ (ppm): 8.02 (s, 1H), 4.19-4.12 (m, 4H), 3.76 (dd, J = 14.5, 6.9 Hz, 2H), 2.23-2.14 (m, 2H), 1.24 (t, J = 6.5 Hz, 6H). 13C-NMR (CDCl3) δ (ppm): 170.91 (d), 165.79 (s), 62.31 (d), 35.85 (d), 25.93 (d), 16.56 (d). 31P-NMR (CDCl3) δ (ppm): 28.93-28.77 (m). MS using ESI detection gave a peak at m/z 328.8 (M+ ion). Compound 5, tetraethyl 2,2’-(6-chloro-1,3,5triazine-2,4-diyl)bis(azanediyl)bis(ethane-2,1-diyl) diphosphonate, was synthesized using a modified procedure of Koopman, et al.9 Compound 2 (9.49 g, 43.6 mmol) was dissolved in 250 mL of deionized water and cooled to 0°C. Cyanuric chloride (Compound 3, 4.02 g, 21.8 mmol) was dissolved in 250 mL of acetone in a 1-L three-necked, round bottom flask and cooled to 0°C. The solution containing Compound 2 was added by addition funnel to the three-necked reaction vessel at 0°C while rapidly stirring under argon. The pH was carefully monitored and adjusted to 9.0-10.0 with 1 M NaOH as needed throughout the experiment. After the addition was complete, the ice bath was removed and the reaction was allowed to warm to RT. After 5 h, TLC (1:9 MeOH/DCM) confirmed the formation of the desired product. Acetone was removed under reduced pressure to yield an immiscible yellow oil in aqueous solution. Brine (250 mL) was added to the mixture and the crude product was extracted with 250 mL of DCM three times. The organic phase was dried over Na2SO4, filtered, and solvent removed under reduced pressure to form a light yellow semi-solid. The crude was dissolved in DCM and chromatographed using a DCM solvent system, followed by a 2% MeOH in DCM. The desired compound eluted as a clear liquid that was removed under reduced pressure to obtain a white solid in 81% yield. Compound 5 is stable to nucleophilic attack provided it is maintained as a solid or not dissolved in a solution with a pH > 9. 1H- NMR (CDCl3) δ (ppm): 7.20 (s, 0.2H), 6.79 (t, J = 5.9 Hz, 1H), 6.66 (s, 0.4H), 6.04 (t, J = 5.6 Hz, 0.4H), 4.164.08 (m, 8H), 3.79-3.59 (m, 4H), 2.10 (dq, J = 12.7, 6.9 Hz, 4H), 1.38-1.30 (m, 12H).13C-NMR (CDCl3) δ (ppm): 167.83 (s), 165.46 (s), 61.20 (d), 34.62 (d), 26.51 (d), 16.50 (d). 31P-NMR (CDCl3) δ (ppm): 28.85-29.16 (m). MS using ESI detection gave a peak at m/z 474.14 (M+1 ion).

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Results and Discussion Synthesis

Fig. 2. 1H-NMR spectrum of Compound 4.

Compounds 4 and 5 were synthesized in good yields of 76% and 81%, respectively. Reaction temperature, time, and pH were important factors in the successful synthesis of the desired products. For the mono-substituted product, it was important to maintain 0°C and pH 6 during the amine addition. Once the addition was complete, careful monitoring of the reaction pH was necessary throughout the reaction. If the pH dropped below 6, the reaction did not proceed, even when the temperature was raised to RT. For Compound 5 synthesis, it was important to maintain 0°C and pH 9-10 during the amine addition. A pH < 9 resulted in the formation of the mono-substituted product.

Spectroscopic Analyses

Fig. 3. 1H-NMR spectrum of Compound 5.

All reaction products were analyzed using 1H-, 13C-, and 31P-NMR spectroscopy, and HPLC-MS. Of particular interest is a comparison of the monoand di-substituted cyanuric chloride derivatives (Compounds 4 and 5, respectively) 1H-NMR spectra (Figs. 2 and 3, respectively). The spectrum of Compound 4 had a sharp singlet at δ 8.02 (s, 1H) which was assigned to the secondary amine on the phosphonate ester chain. This same amine in Compound 5 underwent tautomerization, resulting in the formation of an imine and protonation of the aromatic triazine ring (Fig. 4). Spectral assignments for the two protons of the

Fig. 4. Two prototropic tautomeric structures of Compound 5.

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Table I. LOI Results for Mono- and Di-substituted Cyanuric Chloride Derivatives Name

Sample

LOI (%)

Time to Burn to 5 cm (s)

Compound 4 (Mono-substituted)

1

2

did not burn

2

2

did not burn

3

26

did not burn

4

27

did not burn

5

28

did not burn

6

29

62

7

29

58

8

29

73

9

30

55

1

25

did not burn

2

26

did not burn

3

26

58

4

26

72

5

26

63

6

27

50

7

27

48

8

27

61

9

27

67

Compound 5 (Di-substituted)

TGA Analysis

Table II. Vertical Flame Test Results Using ASTM D6413-99 Name

Sample

Afterflame (s)

Afterglow (s)

Char Length (cm)

Char Width (cm)

Compound 4

1

5.1

5.2

17

3.5

2

7.0

7.1

21

4

3

4.9

none

17.5

3.5

4

1.2

none

11.9

2.5

5

0.6

none

10.8

2.2

Average

3.8



15.6

3.1

1

4.2

none

14.9

3

2

2.9

none

16.8

2.7

3

1.9

none

10.6

2.4

4

7.8

none

21

3.9

5

0.9

none

9.1

2.5

Average

3.5



14.5

2.9

Compound 5

secondary amines on the phosphonate ester chain of Compound 5 were δ 7.20 (s, 0.2H), 6.79 (t, J = 5.9 Hz, 1H), 6.66 (s, 0.4H), 6.04 (t, J = 5.6 Hz, 0.4H) ppm and reflect prototropic tautomerization. A slight upfield shift was observed in the methylene

linker (δ 3.79-3.59 (m, 4H)) adjacent to the tautomeric secondary amine in the disubstituted Compound 5. A comparative thermogram of test fabrics after treatment with 5%, 10%, and 20% solutions of Compound 4 is shown in Fig. 5. In all three concentrations, less than 5% weight loss due to water occurred prior to reaching 200°C. Between 200°C and 225°C, a weight loss of 5% to 15% occurred, which was inversely related to increasing concentration. Two intersecting tangents from the slopes of the thermograms represented the first onset temperature of degradation for Compound 4 at 202.54°C (5%), 204.93°C (10%), and 208.29°C (20%). A second onset temperature occurred at 296.97°C (5%), 299.22°C (10%), and 310.68°C (20%) and was accompanied by a weight loss of 50%, 35%, and 38%, respectively. Upon reaching 600°C, fabrics treated with Compound 4 formed char and had residual weights of 25% (5% solution) and 31% (10%, 20% solutions). Fabrics treated with 5% to 20% solutions of Compound 5 did not result in any significant differences in their thermograms. Compound 5 exhibited a single onset temperature at 325°C and had a residual char of 36% at 600°C. A comparative thermogram of 20% solutions of Compounds 4 and 5 is given in Fig. 6. Of note was the lack of a second onset temperature in Compound 5, and the higher percentage of char formation at 600°C in fabric treated with a 20% solution of Compound 5 (36%) versus Compound 4 (31%).

Limiting Oxygen Index (LOI) Analysis

Twill fabric samples treated with 20% solutions of Compounds 4 and 5 as previously described had add-ons of 18.2% to 19.7% (Compound 4) and 15.25% to 16.33% (Compound 5). Both compounds’ flame retardant properties in LOI testing were superior to those of untreated twill fabric samples. Com-

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Vertical Flame Testing

Fig. 5. TGA comparison of 5%, 10%, and 20% w/w fabrics treated with Compound 4.

Vertical flame testing was performed according to ASTM D6413-99 on five fabric samples measuring 3 × 12 in. that were treated with 20% solutions of Compounds 4 and 5. Results are listed in Table II. For Compound 4, the monosubstituted cyanuric chloride derivative, the afterflame ranged from 0.6 s to 7.0 s and averaged 3.8 s. The afterglow ranged from 5.2 s to 7.1 s in two samples and was recorded as “none” in three other samples. The char length range was 10.8-21 cm and averaged 15.5 cm. The char width range was 2.2-4.0 cm and averaged 3.1 cm. For Compound 5, the di-substituted cyanuric chloride derivative, the afterflame range was 0.9-7.8 s and averaged 3.5 s. The afterglow was recorded as “none” in all samples. The char length ranged from 9.1 cm to 21.0 cm and averaged 14.5 cm. The char width ranged from 2.4 cm to 3.9 cm and averaged 2.9 cm.

SEM Analysis

Fabric samples treated with 20% solutions of Compounds 4 and 5 were subjected to a vertical flame as described above. SEM images were taken of the treated burned fabric, an untreated/burned fabric sample, and an untreated/ unburned fabric sample. All SEM images had a magnification of 1500× as shown in Fig. 7.

Fig. 6. TGA comparison of 20% fabrics treated with Compounds 4 (dashed) and 5 (solid).

pound 4 treated samples gave an LOI value of 29% (compared to 18% for untreated twill samples) in three of the nine samples tested and was classified as a self-extinguishing burning material (Table I).10-12 The time required to burn to the 5-cm line ranged from 58 s to 73 s when a 29% LOI test was performed. When Compound 4 treated samples were subjected to 24% to 28% oxygen, the treated twill fabric did not burn to the 5-cm line. Compound 5 treated samples had an LOI value of 26% to 27% and were classified as slow burning (Table I). The time required to burn to the 5-cm line under these conditions ranged from 50 s to 72 s. Two Compound 5 treated samples treated did not burn to the 5-cm line when 25% and 26% oxygen was applied.

From these images, superior performance of the treated/burned samples (Figs. 7A and B) was observed when compared to the untreated/ burned sample (Fig. 7D). Both samples treated with either Compound 4 or 5 retained their fiber appearance while the untreated/burned fabric sample looked entirely withered. Fibers from the treated/burned samples (Figs. 7A and B) looked similar in size and shape to the untreated/unburned material in (Fig. 7C). Furthermore, the evidence of blistering (Fig. 7A) and of globular formation (Fig. 7B) indicates that flame retardant properties were imparted to the cotton textile by the applied compounds.

Conclusion

Compounds 4 and 5 were synthesized in 76% and 81% yields, respectively, and exhibited flame retardant properties in standardized thermogravimetric, LOI, and vertical flame testing. SEM images of the treated/burned textiles indicated that they had improved flame retardant AATCC Review November/December 2011 www.aatcc.org

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Fig. 7. SEM images (1500×) of treated/ burned fabric samples treated with Compound 4 (A) and 5 (B), untreated/ unburned (C), and untreated/burned (D) fabric samples.

properties when compared to untreated/burned fabric. Given these results, cyanuric chloride derivatives are promising flame retardant compounds in applications on cotton textiles. Further durability testing is presently underway, and the results will be reported at a later date.

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

Weil, Edward and Sergei Levchik, Journal of Fire Sciences, Vol. 26, No. 3, May 2008, pp243-281. Bajaj, P., Handbook of Technical Textiles, edited by A. Horrocks and S. Anand, CRC Press, New York, N.Y., USA, 2000, pp223-263. Hendrix, James, George Drake, and Robert Barker, Journal of Applied Polymer Science, Vol. 16, No. 2, February 1972, pp257-274. Pandya, H. B. and M. M. Bhagwat, Textile Research Journal, Vol. 51, No. 1, January 1981, pp5-8. Kubota, Shizuo, Polymeric Materials Encyclopedia, Vol. 4, edited by Joseph Salamone, CRC Press, New York, N.Y., USA, 1996, pp2389-2396. Lewis, David, AATCC Olney Award Lecture, 2008, www.aatcc.org/awards/Olney2008.pdf, accessed August 2011. Jakeman, David, et al., Journal of Medicinal Chemistry, Vol. 41, No. 23, October 1998, pp4439-4452. Yang, Xiaoping and Christopher Lowe, Tetrahedron Letters, Vol. 44, June 2003, pp1359-1362. Koopman, H, Recueil, Vol. 77, No. 3, March 1958, pp235-240.

10. Fenimore, C. P., Flame-retardant Polymeric Materials, Vol. 1, edited by M. Lewin, S. Atlas, and E. Pearce, Plenum Press, New York, N.Y., USA, pp371-397. 11. Horrocks, A., M. Tunc, and D. Price, Textile Progress, Vol. 18, Nos. 1-3, January 1986, pp1-205. 12. Nelson, M., Combustion Theory and Modelling, Vol. 5, No. 1, January 2001, pp59-83.

Author

Michael Easson is a research chemist with the US Department of Agriculture in New Orleans, La., USA. He graduated from Louisiana State University with a PhD in organic chemistry and previously worked as a research scientist at Albany Molecular Research Inc. (AMRI) and as a process chemist at Albemarle Corp. His research interests include organophosphorus flame retardant applications to cotton textiles and ultrasound-assisted enzymatic conversions of biomass to biofuels. Michael Easson, USDA, Southern Regional Research Center, 1100 Robert E. Lee Blvd., New Orleans, LA 70124, USA; phone +1 504 286 4493; [email protected]

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