Article pubs.acs.org/IECR
Preparation, Polymerization, and Performance Evaluation of Halogen-Free Radiation Curable Flame Retardant Monomers for Cotton Substrates Brian Edwards, Stacy Rudolf, Peter Hauser, and Ahmed El-Shafei* Fiber and Polymer Science Program, Department of Textile Engineering, Chemistry and Science, North Carolina State University, 1000 Main Campus Drive, Raleigh, North Carolina 27695, United States ABSTRACT: Three halogen-free, phosphorus-based flame retardant monomers were synthesized with good yields and characterized using FT-IR, 1H NMR, and 31P NMR. Two of the monomers were novel phosphoramides and the third was derived from cyclotriphosphazene. Each monomer was coated onto cotton substrates with the aid of a UV flood curing system. The impacts of monomer concentration, photoinitiator concentration, UV exposure time and proximity of the specimen to the UV lamp on coating yield were evaluated by experiments designed with JMP Pro 10. Of the three monomers, the cyclotriphosphazene derivative was polymerized into a coating that was durable to Soxhlet extraction with acetone. Vertical burn testing showed that all three monomers are valuable flame retardants. These results agreed with thermogravimetric analysis findings that demonstrated quantitatively the effectiveness of each monomer at promoting char formation. For the cyclotriphosphazene derivative, the coating was easily visualized covering significant portions of the fabric using scanning electron microscopy.
■
INTRODUCTION Cotton textiles are preferred for a variety of domestic applications because they are soft and comfortable, highly absorbent, colorable, and easy to launder. However, because of their high flammability, cotton textiles are not an appropriate choice in situations where extreme thermal conditions are possible. Since cotton is arguably the most important textile fiber in the world, it is of critical importance under certain circumstances to seek ways to render it less flammable. One common approach to achieve this goal is to treat the cotton substrate with a flame retardant (FR) chemical. Halogenated FR finishes and additives have very successfully inhibited the combustion of polymeric materials and, undoubtedly, saved many lives. However, concerns about their bioaccumulation and persistence in the environment, as well as their release of toxic and corrosive gases during combustion have caused the industry to seek safer alternatives.1−5 Halogen-free flame retardants, such as those based on phosphorus, have received a considerable amount of attention and research. These FR finishes function via a condensed phase mechanism by altering the pyrolysis process of the cellulose chains to encourage the formation of noncombustible pyrolysis products, such as char, water, and carbon dioxide.6 In the case of cotton, the phosphoric acid that is released during the thermal decomposition of the FR chemical serves as a phosphorylating agent at the primary hydroxyl (C-6) groups to halt the formation of levoglucosan.7,8 Levoglucosan is the major pyrolysis product of unmodified cotton and is an important precursor in the generation of flammable volatiles. To synergize the effectiveness of the phosphorus-based FR agent, nitrogen has been incorporated into FR formulations. It is thought that the presence of nitrogen, which can be protonated, may aid in keeping the pH in the optimum range to © 2014 American Chemical Society
promote phosphorylation and cross-linking reactions and maximize char yield.9 There are many studies in the literature that discuss the superior performance of phosphorus−nitrogen flame retardants on cotton substrates.10−14 Durable flame retardant cotton can be prepared by the application of a chemical finish using traditional wet processing techniques. However, this approach is known to consume large quantities of water and energy. An attractive alternative would be to develop compounds that contain polymerizable groups in addition to their flame retardant functionality. Compounds of this type could then be applied to the substrate and cured using a low-energy radiation process. Recent examples of different types of radiation that have been used to fix phosphorus-based flame retardants to cellulosic materials include thermal,15 plasma,16−18 and ultraviolet (UV).19−21 Of these technologies, we are particularly interested in UV curing for several reasons. First, the necessary equipment has open perimeters and is conducive to continuous processing. Second, UV lamps impart a great deal of energy to the substrate and the curing process occurs very rapidly on commercial equipment. When these two factors are considered, it is easy to imagine that this technology has enormous potential for commercial feasibility because fabrics can be rendered flame retardant, while maintaining the high throughput that is preferred by the textile industry. With these thoughts in mind, we report here the synthesis, characterization, UV polymerization properties, and flame retardant performance of three phosphorus−nitrogen containing monomers. Received: Revised: Accepted: Published: 577
July 23, 2014 December 12, 2014 December 16, 2014 December 16, 2014 DOI: 10.1021/ie502915t Ind. Eng. Chem. Res. 2015, 54, 577−584
Article
Industrial & Engineering Chemistry Research Table 1. Processing Conditions and Coating Yield for Each Piece of Treated Fabric sample ID
[monomer] (% owb)
[photoinitiator] (% owb)
UV exp. time (s/side)
stage height (cm)
coating yield DAEP (%)
coating yield DADMPA (%)
coating yield HACTP (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
20 20 20 20 20 20 20 20 20 30 30 30 30 30 30 30 30 30 40 40 40 40 40 40 40 40 40
30 30 30 30 20 10 10 10 10 30 20 20 20 20 20 20 20 10 30 30 30 30 20 10 10 10 10
10 10 50 50 30 10 10 50 50 30 10 30 30 30 30 30 50 30 10 10 50 50 30 10 10 50 50
6 12 6 12 9 6 12 6 12 9 9 6 9 9 9 12 9 9 6 12 6 12 9 6 12 6 12
1.37 0.99 1.68 5.80 2.03 1.89 1.69 1.84 2.03 1.15 0.77 1.25 1.37 1.41 1.54 1.81 1.50 1.21 1.75 1.18 2.11 6.19 0.80 0.41 0.80 0.90 1.99
1.53 0.62 1.77 5.56 2.54 1.42 1.39 1.65 3.45 1.39 1.36 1.21 1.16 1.20 1.22 1.91 2.09 1.23 1.30 1.21 1.27 4.73 1.19 0.91 1.22 0.82 1.65
1.59 1.20 13.29 17.61 2.54 1.55 1.65 4.24 5.01 7.41 0.63 2.19 5.69 5.23 7.27 9.98 7.51 0.94 2.00 1.13 13.28 25.82 8.56 1.57 1.08 1.28 3.87
■
were added to a three neck round-bottom flask and stirred vigorously under an atmosphere of argon. The solution was chilled to 0 °C in an ice bath. To this, the phosphoruscontaining species (dissolved in 50 mL of THF) was added dropwise (for DAEP, 13.73 g (0.084 mol) of ethyl dichlorophosphate; for DADMPA, 13.70 g (0.085 mol) of N,N-dimethylphosphoramic dichloride; for HACTP, 10.00 g (0.029 mol) of hexachlorocyclotriphosphazene22−24). An exothermic reaction took place and a white salt began to form immediately. After 3 h at 0 °C, the reaction was allowed to attain room temperature and stir under argon for an additional 21 h. The solution was filtered to remove the precipitated allylamine hydrochloride salt, and the THF and unreacted allylamine were removed by rotary evaporation. The residue was then dissolved in dichloromethane and washed several times with deionized water to remove any remaining traces of allylamine hydrochloride. After the organic layer was collected and dried over anhydrous calcium sulfate, the dichloromethane was removed by rotary evaporation to give a colorless oily liquid for DAEP (15.2 g; 88.3%) and DADMPA (13.6 g; 79.2%) and a white waxy solid for HACTP (12.5 g; 92.1%). Application of Monomers. A central composite experiment was designed using SAS JMP Pro 10 to examine the effects of monomer concentration, photoinitiator concentration, UV exposure time, and proximity of the sample to the UV lamp on coating yield. The experiment had three center points and the order of the runs was randomized to improve the quality of the statistical model. Treatment baths were prepared using a known concentration of the monomer (% owb) and photoinitiator (% owb) with acetone serving as the solvent. A small swatch of fabric was weighed and then immersed in each
MATERIALS AND METHODS Materials. Tetrahydrofuran (Alfa Aesar) was dried over 4A molecular sieves (Fisher Scientific) prior to use. Hexachlorocyclotriphosphazene, ethyl dichlorophosphate, and N,Ndimethylphosphoramic dichloride were purchased from Sigma-Aldrich and were used as received. Allylamine and dichloromethane were obtained from Acros Organics and were used as received. Anhydrous calcium sulfate (W. A. Hammond) was used as supplied. The photoinitiator, Irgacure 819 [(2,4,6trimethylbenzoyl)phenylphosphine oxide], was provided by BASF. Acetone was obtained from BDH. An argon gas cylinder was purchased from Machine & Welding Supply Company (Raleigh, NC). Deuterated chloroform was obtained from Cambridge Isotope Laboratories. The cotton fabric utilized in this study was constructed with a 3 × 1 twill weave and had a density of approximately 266 g/m2. Characterization. Products were characterized by FT-IR spectroscopy and 1H and 31P NMR spectroscopy. FT-IR spectra were collected using a Nicolet Nexus 470 spectrophotometer equipped with an Avatar OMNI-Sampler using the attenuated total reflectance (ATR) sampling technique. A drop of each neat compound was placed directly onto the germanium crystal. 1H and 31P NMR spectra were collected using a Bruker 500 MHz spectrometer. CDCl3 was used as the solvent, and the solvent peak served as a reference for the proton spectra. 85% H3PO4 in a sealed capillary was placed in the NMR tube for use as a reference in phosphorus spectra. Preparation of Di(Allylamino)ethyl Phosphate (DAEP), Di(Allylamino)dimethyl Phosphoramide (DADMPA) and Hexa(Allylamino)cyclotriphosphazene (HACTP). One hundred milliliters of THF and 24.0 g (0.42 mol) of allylamine 578
DOI: 10.1021/ie502915t Ind. Eng. Chem. Res. 2015, 54, 577−584
Article
Industrial & Engineering Chemistry Research
Figure 1. Synthesis of DAEP, DADMPA, and HACTP.
with a 600 W lamp and was operated at 100% intensity. Treated fabrics were always cured in the same spot beneath the lamp in the region of highest UV intensity as determined by the manufacturer. Performance Evaluation of Treated Fabrics. An aluminum frame was constructed that allowed for burn testing in the vertical configuration. A specimen was placed in the holder and the flame from a butane torch was brought underneath the bottom center edge of the fabric and held there for a total of 12 seconds. The flame was then removed and a stopwatch was used to measure how long the fabric burned (if at all). Thermogravimetric analysis (TGA) was performed to examine the thermal decomposition of the treated fabrics. A PerkinElmer Pyris 1 TGA was utilized with a heating rate of 20 °C/min from 25 to 600 °C. Air was used as the purge gas (20 mL/min). Finally, the surface morphologies of the untreated and treated fabrics were explored using an FEI Phenom desktop scanning electron microscope (SEM).
bath with agitation for approximately 2 min. The fabric was then removed from the bath (wet pick-up was observed to be approximately 125% in all cases) and set aside for a few minutes to allow the acetone to evaporate. The saturated fabrics were then placed in the UV chamber and a lab jack was used to raise or lower them to the specified height. After the treated fabrics were cured for the required amount of time on each side, they were removed from the UV chamber and subjected to Soxhlet extraction for 4 h in acetone to remove any unreacted monomer and low molecular weight polymer. Finally, the samples were dried for 24 h on air and reweighed. Table 1 shows the processing conditions and coating yields for each piece of fabric that was treated. Coating yield was calculated by the equation coating yield (%) = ((Wf − Wi )/Wi ) × 100
where Wf is the final weight of the fabric after treatment and extraction and Wi is the initial weight of the untreated fabric. These calculated values were then input into JMP Pro 10 and the software was used to build a statistical model and optimize the processing conditions to achieve the maximum coating yield. To confirm the statistical model, three pieces of fabric were treated using the optimum conditions as determined by the software and their coating yields were compared to the theoretical ones predicted by the software. If all three coating yields fell within the predicted range, the statistical model was confirmed. Radiation Curing in UV Chamber. Radiation curing was carried out in a Uvitron IntelliRay 600 shuttered UV flood curing system. Energy from the UV lamp was used, in collaboration with the photoinitiator, to drive the free radical polymerization of the monomers. The system was equipped
■
RESULTS AND DISCUSSION Synthesis of DAEP. The target novel monomer, DAEP, was successfully prepared in one step by the dropwise addition of ethyl dichlorophosphate to an excess of allylamine in THF as depicted in Figure 1. The reaction involved consecutive nucleophilic attacks of two amines on the partially positive phosphorus reaction center to produce the desired bifunctional monomer and hydrochloric acid as a byproduct. The IR spectrum of DAEP confirmed the presence of all of the necessary functional groups for the target bifunctional monomer. A single peak at 3223 cm−1 indicated the presence of a secondary amine. Peaks at 1644, 1443, 998, and 917 cm−1 were indicative of a vinyl group. The peak for the PO 579
DOI: 10.1021/ie502915t Ind. Eng. Chem. Res. 2015, 54, 577−584
Article
Industrial & Engineering Chemistry Research
Figure 2. 31P NMR data for DAEP (top), DADMAP (middle), and HACTP (bottom).
stretching frequency for this monomer was 1202 cm−1. A C−N stretch was observed at 1098 cm−1 and a peak that was assigned to the P−OEt group is seen at 1041 cm−1. 1 H NMR analysis gave signals at 5.2 and 5.0 ppm that were assigned to the two H2CCH protons. The H2CCHCH2 proton was found at 6.0 ppm. A signal at 3.4 ppm was assigned to the CH−CH2−NH protons. The O−CH2−CH3 and O− CH2−CH3 protons of the ethoxy group were observed at 4.1 and 1.2 ppm, respectively. Finally, a 31P NMR singlet at 15.8 ppm was attributed to the appropriately substituted phosphorus (Figure 2). The 31P chemical shift for the unreacted ethyl dichlorophosphate was 7.1 ppm and this signal was completely absent in the spectrum of the target monomer. This, coupled with the integrations of the 1H NMR spectrum, shows that the reaction was conducted to completion and that both chlorines of the starting material were successfully substituted with allylamine. Synthesis of DADMPA. DADMPA, which is also novel, was easily prepared in one step by the dropwise addition of N,N-dimethylphosphoramic dichloride to a chilled solution of allylamine in THF as shown in Figure 1. All of the required absorbances for DADMPA were observed in the IR spectrum of the target monomer. The characteristic peak for a secondary amine was seen at 3248 cm−1. Peaks that suggested the presence of a vinyl group were found at 1644, 1458, and 915 cm−1. The PO stretching frequency for this compound was 1184 cm−1. The PO stretch of DADMPA occurs at a lower frequency than DAEP because the −N(Me)2 group of DADMPA is a stronger electron donor than the −OEt group of DAEP. This means that the PO bond of DADMPA is more weakened by resonance effects than the PO bond in DADMPA and less energy is needed for the bond to begin vibrating. A C−N stretch was present at 1090 cm−1 and an
absorbance at 989 cm−1 was attributed to the vibration of the P−N(Me)2 group. 1 H NMR signals at 5.2 and 5.1 ppm were indicative of the two terminal protons on the vinyl group. The CH2CH CH2 proton was observed at 6.0 ppm. The two CH−CH2−NH protons were seen at 3.4 ppm. A signal at 2.8 ppm was characteristic of the six protons of the −N(CH3)2 group. The 31 P NMR spectrum consisted of a singlet at 19.3 ppm (Figure 2). A 31P NMR spectrum of unreacted N,N-dimethylphosphoramic dichloride showed a singlet at 19.7 ppm. The signal associated with the unreacted starting material was absent in the spectrum of DADMPA. Its absence, combined with the integrations of the 1H NMR spectrum, is proof that the reaction was carried to completion and that both chlorine atoms were replaced by allylamine. The location of the 31P singlet of DAEP is slightly upfield of the singlet associated with DADMPA. With all other parts of the molecule being equal, it can be concluded that −OEt is a more shielding ligand than −N(Me)2. This is rationalized by the stronger electronegative nature of the oxygen directly attached to the phosphorus. As oxygen pulls electron density away from the phosphorus, the lone-pair electrons of the nitrogen atoms in the two allylamine groups are delocalized in the direction of the phosphorus. Therefore, the phosphorus experiences a net increase in shielding and an upfield chemical shift is induced.20 Synthesis of HACTP. The reaction of hexachlorocyclotriphosphazene with an excess of allylamine in THF went as shown in Figure 1. This reaction has been reported previously in the literature.22−24 All six chlorines on the ring were replaced. IR analysis of HACTP confirmed all of the required functional groups. A secondary amine was observed at 3215 cm−1. Peaks that were assigned to the vinyl group were present 580
DOI: 10.1021/ie502915t Ind. Eng. Chem. Res. 2015, 54, 577−584
Article
Industrial & Engineering Chemistry Research
Figure 3. Process optimization for each monomer.
at 1643, 1419, 997, and 914 cm−1. The C−N stretch was seen at 1087 cm−1. Most interestingly, the presence of six allylamine groups on the phosphazene ring induced a shift of the PN vibration to lower frequency (1176 cm−1) when compared to hexachlorocyclotriphosphazene (1186 cm−1). This is characteristic of fully substituted aminocyclotriphosphazenes25 and is perhaps explained by the amine’s stronger electron donating character. By donating electrons toward phosphorus, the πelectrons of the phosphazene ring drift in the direction of the skeletal nitrogen atoms. The result is a slight lengthening and weakening of the bonds within the ring and a need for less energy to get the bonds to vibrate. 1 H NMR gave signals at 5.2 and 5.0 ppm that are assigned to the two terminal vinyl protons. The CH2CHCH2 proton was found at 5.9 ppm and the two CH−CH2−NH protons were observed at 3.5 ppm. In the 31P NMR spectrum, a singlet was present at 17.1 ppm that was assigned to the three identical allylamine-substituted phosphori (Figure 2). In unreacted hexachlorocyclotriphosphazene, the chemical shift of this singlet is 20.1 ppm. The signal associated with the unreacted starting material is absent in the spectrum of the target monomer. This information, combined with the fact that the 31 P-chemical shift of HACTP is consistent with other fully substituted aminocyclotriphosphazenes,26 is taken as proof that the target monomer was formed. Application of Monomers and Process Optimization. Each monomer was applied to the cellulosic substrate according to the previously described procedure. The coating yields that are provided in Table 1 were input into the JMP Pro 10 software and a statistical model was generated to predict the optimum conditions for maximizing the coating yield of each monomer. The effect of each variable on coating yield is shown in Figure 3. For each monomer, it can be concluded that an increase in photoinitiator concentration, UV exposure time or proximity of the sample to the UV lamp induces an increase in coating yield. The ideal levels for these three parameters were 30% photoinitiator concentration, 50 s of UV exposure per side and a stage height of 12 cm (sample is 6 cm from UV lamp). For DAEP and HACTP, it is also true that an increase in
monomer concentration causes an increase in coating yield. These two monomers achieved their maximum coating yields at 40% monomer concentration. On the other hand, the coating yield on substrates treated with DADMPA seems to decrease with increasing monomer concentration and maximum coating yield was achieved at 20% monomer concentration. The statistical models were verified by treating three pieces of fabric with each monomer under the optimum conditions predicted by the software. For DAEP, the coating yields of these three fabrics were 5.12%, 6.44%, and 6.07%. For DADMPA, the coating yields were 5.73%, 4.95%, and 4.51%. For HACTP, coating yields were found to be 22.22%, 22.98%, and 23.59%. All three coating yields for each monomer fell within the range that was predicted by the software and the statistical models were confirmed. For DAEP (R2 = 0.86) and DADMPA (R2 = 0.89), the correlation between the statistical model and the actual data points is less strong than that of HACTP (R2 = 0.93). The maximum coating yield for DAEP and DADMPA under the optimum conditions is very low. This suggests that these two monomers do not polymerize efficiently under these conditions. When substrates that were treated with these two monomers were placed under the UV lamp, a vapor began to emanate from them. We suspect that the boiling point of these monomers is sufficiently low to allow them to rapidly evaporate from the substrate once they are placed under the hot UV lamp. Contrastingly, HACTP is persistent and the coating yield with this monomer under optimum conditions is much more desirable. Vertical Burn Testing. For DAEP and DADMPA, the coating yields when treated under optimum conditions were 5.63 ± 1.27% and 5.10 ± 0.95%, respectively. DAEP is 15.17% phosphorus by weight and DADMPA is 15.24% phosphorus by weight. Therefore, at the optimum coating yields that were achieved with these two monomers, we can expect phosphorus loadings of approximately 0.85% for DAEP and 0.78% for DADMPA. This is not enough phosphorus to inhibit combustion and the fabrics burned readily. To demonstrate the ability of DAEP and DADMPA to prevent the fabrics from igniting and burning, we needed a much higher loading of 581
DOI: 10.1021/ie502915t Ind. Eng. Chem. Res. 2015, 54, 577−584
Article
Industrial & Engineering Chemistry Research
DADMPA were padded to 30% add-on (without UV curing), while the fabric treated with HACTP was treated under the optimum conditions as described earlier (with UV curing). All four fabrics showed an initial weight loss that is attributed to the loss of water. Untreated cotton began to thermally decompose at around 257 °C and only 6.1% of the original fabric sample remained at 600 °C. The fabric treated with HACTP began its thermal decomposition at a similar temperature (255 °C). However, about 27.1% of the original fabric remained at 600 °C. This is taken as further evidence of the strong ability to HACTP to promote char formation and increase the fire resistance of cellulosic materials through a condensed phase mode of action. The TGA analysis of DAEP and DADMPA treated fabrics showed that both monomers were also very effective char promoting flame retardants. At 600 °C, 28.5% of the DAEP treated substrate and 29.3% of the DADMPA treated substrate remained. The slightly elevated performance of DADMPA may be evidence of the synergistic effect of replacing the oxygen atom of DEAP with an extra nitrogen atom. Both DAEP and DADMPA showed a weight loss beginning around 150 °C that was ascribed to the evaporation of the monomers, which were not polymerized, from the substrate. At approximately 230 °C, this weight loss leveled off and the monomers began to decompose and release their flame retarding functionality more quickly than they were evaporated. The thermograms, and rates of decomposition, for DAEP and DADMPA were nearly identical. For HACTP, which has a different and unique chemistry, the thermal decomposition process is clearly different than the phosphoramides. The char yields at 600 °C for all three treated samples are nearly the same. For DAEP, the loading of FR compound was 30% and the monomer is 15.17% phosphorus by weight. This means that the loading of phosphorus on the fabric is about 4.55%. For DADMPA, the loading of FR compounds was 30% and the monomer is 15.24% phosphorus by weight. Therefore, the loading of phosphorus on the fabric is approximately 4.57%. Finally, the loading of HACTP was about 23% and this monomer is 19.71% phosphorus by weight. Therefore, the loading of phosphorus on the fabric is approximately 4.53%. It turns out that all three treated samples have phosphorus loadings of about 4.53−4.57%. This explains the similar char yields. Scanning Electron Microscopy. Scanning electron microscopy (SEM) was utilized to investigate the morphology of the untreated and treated substrates. Because of their very low coating yields, fabrics treated using the optimum processing conditions with DAEP and DADMPA were not viewed. SEM images are provided as Figure 6 to give a comparison of untreated cotton fabric and the same cotton fabric treated using the optimum conditions with the HACTP monomer. From these images, it is clear that treatment with HACTP generates a polymeric layer that covers large portions of the fibers and yarns. At both 500× and 1000× magnification, the HACTP coated layer on the fabric has obvious cracks. This suggests that the coating is brittle, which is consistent with what one might expect from a highly branched and cross-linked cyclomatrix network.
phosphorus than we could achieve with UV curing. Therefore, we padded these monomers onto the fabrics to 30% add-on. This gave a sufficient loading of phosphorus to prevent ignition. A 100 mm × 70 mm piece of cotton fabric was padded to 30% add-on of the appropriate monomer (without UV curing). These treated fabrics were then attached to the specimen holder, suspended vertically in the burn test apparatus and subjected to the flame from a butane torch for 12 s. The burn test was conducted in normal ambient atmosphere and our aim was to replicate real world fired conditions that the fabrics may actually encounter. The results are shown in Figure 4. At 30%
Figure 4. Vertical burn test results of fabric treated with 30% add-on of DAEP (top left), DADMPA (top right), and HACTP (bottom left). Fabric treated with HACTP under optimum processing conditions is also shown (bottom right).
add-on, all three monomers were very effective flame retardants and each prevented the fabric from igniting. The formation of char in the region of the fabric that was heated by the flame indicates that the monomers are active in the condensed phase, which is consistent with the mode of action of phosphorusbased flame retardants. Similar results were achieved with fabric treated under the optimum conditions with HACTP (with UV curing). This fabric had a coating yield of 22.98% and did not ignite. Instead, a significant char structure formed in the area that was exposed to the flame. For all of the treated fabrics, there was no after glow, total time of burning or burn rate to report since the fabrics did not ignite or combust. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) was used to explore the behavior of the treated fabrics as they were exposed to harsh thermal conditions (Figure 5). An atmosphere of air was chosen for these analyses to simulate real world fire conditions. Fabrics treated with DAEP and
■
CONCLUSIONS Three halogen-free, phosphorus-based flame retardant monomers were prepared with acceptable yields using simple one-pot processes. The monomers were easily isolated and charac582
DOI: 10.1021/ie502915t Ind. Eng. Chem. Res. 2015, 54, 577−584
Article
Industrial & Engineering Chemistry Research
Figure 5. Thermograms of untreated cotton, DAEP, DADMPA, and HACTP.
nearly five times that of untreated cotton. Furthermore, fabric treated with HACTP under optimum processing conditions gave a char yield greater than four times that of untreated cotton. Finally, SEM demonstrated that treatment with HACTP under optimum conditions generated a visible coating on large portions of the substrate. From these observations, it can be concluded that DAEP, DADMPA, and HACTP are all effective flame retardants for cellulosic materials. A different polymerization technique is required for DAEP and DADMPA to create durable coatings. The polymeric layers of HACTP that are produced under these conditions are durable to Soxhlet extraction in acetone. For future work, we recommend exploring other polymerization strategies to increase the durability of DAEP and DADMPA.
■
Figure 6. SEM images of untreated cotton and cotton treated using optimum processing conditions with HACTP. 1
AUTHOR INFORMATION
Corresponding Author
31
terized using FT-IR, H NMR, and P NMR. After they were applied to cotton substrates with a photoinitiator, a UV curing system was used to drive their polymerization to form polymeric coatings. An experiment that was designed using JMP Pro 10 was conducted to determine the optimum processing conditions for each monomer to maximize coating yield. It was found that DAEP and DADMPA were not polymerized to the necessary degree to generate flame retardant coatings using this strategy. On the other hand, HACTP was polymerized to an adequate degree to generate the necessary coating for flame suppression. A coating yield of about 23% was possible with HACTP even after exposing the treated substrate to a lengthy period of Soxhlet extraction to remove unreacted monomer. Burn testing in the vertical orientation showed that DAEP, DADMPA and HACTP were all effective char promoting flame retardant additives at 30% add-on and were capable of preventing the fabrics from igniting. In addition, fabric treated with HACTP under the optimum processing conditions failed to ignite. The burn testing results were supported by the TGA findings, which proved quantitatively that all three monomers were strong condensed phase flame retardants. Fabrics that were padded to 30% addon with DAEP and DADMPA gave char yields at 600 °C of
*Telephone: (919) 515-6548. Fax: (919) 515-6532. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors wish to express their gratitude to Dr. Jennifer Sun of the NCSU Department of Chemistry NMR Facility for her assistance with NMR studies. This research was funded by the National Textile Center through project number C10-NS03.
■
REFERENCES
(1) Xiong, J. F.; Luo, S. H.; Wang, Q. F.; Wang, Z. Y.; Qi, J. Synthesis and Characterization of a Novel Flame Retardant, Poly(Lactic Acid-co3,3′-diaminobenzene). Des. Monomers Polym. 2013, 16, 389. (2) Liao, F.; Zhou, L.; Ju, Y.; Yang, Y.; Wang, X. Synthesis of A Novel PhosphorusNitrogen−Silicon Polymeric Flame Retardant and Its Application in Poly(Lactic Acid). Ind. Eng. Chem. Res. 2014, 53, 10015. (3) Chen, M. J.; Shao, Z. B.; Wang, X. L.; Chen, L.; Wang, Y. Z. Halogen-Free Flame-Retardant Flexible Polyurethand Foam with a Novel NitrogenPhosphorus Flame Retardant. Ind. Eng. Chem. Res. 2012, 51, 9769.
583
DOI: 10.1021/ie502915t Ind. Eng. Chem. Res. 2015, 54, 577−584
Article
Industrial & Engineering Chemistry Research (4) Xu, M.; Zhao, W.; Li, B. Synthesis of a Novel Curing Agent Containing Organophosphorus and Its Application in Flame-Retarded Epoxy Resins. J. Appl. Polym. Sci. 2014, 131, 41159. (5) Xiong, J. F.; Wang, Q. F.; Peng, P.; Shi, J.; Wang, Z. Y.; Yang, C. L. Design, Synthesis, and Characterization of a Potential Flame Retardant Poly(Lactic Acid-co-pyrimidine-2,4,5,6-tetramine) via Direct Melt Polycondensation. J. Appl. Polym. Sci. 2014, 131, 40275. (6) Lewin, M. Handbook of Fiber Science and Technology Vol. II: Chemical Processing of Fibers and Fabrics: Functional Finishes, Part B; Marcel Dekker: New York, 1984. (7) Kandola, B. K.; Horrocks, A. R.; Price, D.; Coleman, G. V. FlameRetardant Treaments of Cellulose and Their Influence on the Mechanism of Cellulose Pyrolysis. J. Macromol. Sci., Polym. Rev. 1996, 36, 721. (8) Franklin, W. E.; Rowland, S. P. Effects of Phosphorus-Containing Flame Retardants on Pyrolysis of Cotton Cellulose. J. Appl. Polym. Sci. 1979, 24, 1281. (9) Schindler, W. D.; Hauser, P. J. Chemical Finishing of Textiles; Woodhead Publishing: Cambridge, U.K., 2004. (10) Gaan, S.; Sun, G.; Hutches, K.; Engelhard, M. H. Effect of Nitrogen Additives on Flame Retardant Action of Tributyl Phosphate: Phosphorus−Nitrogen Synergism. Polym. Degrad. Stab. 2008, 93, 99. (11) Gaan, S.; Sun, G. Effect of Phosphorus and Nitrogen on Flame Retardant Cellulose: A Study of Phosphorus Compounds. J. Anal. Appl. Pyrol. 2007, 78, 371. (12) Wu, W.; Yang, C. Q. Correlation between Limiting Oxygen Index and Phosphorus/Nitrogen Content of Cotton Fabrics Treated with a Hydroxy-Functional Organophosphorus Flame-Retarding Agent and Dimethyloldihydroxyethyleneurea. J. Appl. Polym. Sci. 2003, 90, 1885. (13) Nguyen, T. M.; Chang, S.; Condon, B.; Slopek, R.; Graves, E.; Yoshioka-Tarver, M. Structural Effect of Phosphoramidate Derivatives on the Thermal and Flame Retardant Behaviors of Treated Cotton Cellulose. Ind. Eng. Chem. Res. 2013, 52, 4715. (14) Nguyen, T. M.; Chang, S.; Condon, B.; Uchimiya, M.; Graves, E.; Smith, J.; Easson, M.; Wakelyn, P. Synthesis and Characterization of a Novel Phosphorus−Nitrogen-Containing Flame Retardant and Its Application for Textile. Polym. Adv. Technol. 2012, 23, 1036. (15) Cheema, H. A.; El-Shafei, A.; Hauser, P. J. Conferring Flame Retardancy on Cotton Using Novel Halogen-Free Flame Retardant Bifunctional Monomers: Synthesis, Characterizations and Applications. Carbohyd. Polym. 2013, 92, 885. (16) Edwards, B.; El-Shafei, A.; Hauser, P.; Malshe, P. Towards Flame Retardant Cotton Fabrics by Atmospheric Pressure PlasmaInduced Graft Polymerization: Synthesis and Application of Novel Phosphoramidate Monomers. Surf. Coat. Technol. 2012, 209, 73. (17) Tsafack, M. J.; Levalois-Grutzmacher, J. Flame Retardancy of Cotton Textiles by Plasma-Induced Graft-Polymerization (PIGP). Surf. Coat. Technol. 2006, 201, 2599. (18) Tsafack, M. J.; Levalois-Grutzmacher, J. Towards Multifunctional Surfaces Using the Plasma-Induced Graft-Polymerization (PIGP) Process: Flame and Waterproof Cotton Textiles. Surf. Coat. Technol. 2007, 201, 5789. (19) Xing, W.; Jie, G.; Song, L.; Hu, S.; Lv, X.; Wang, X.; Hu, Y. Flame Retardancy and Thermal Degradation of Cotton Textiles Based on UV-Curable Flame Retardant Coatings. Thermochim. Acta 2011, 513, 75. (20) Yuan, H.; Xing, W.; Zhang, P.; Song, L.; Hu, Y. Functionalization of Cotton with UV-Cured Flame Retardant Coatings. Ind. Eng. Chem. Res. 2012, 51, 5394. (21) Chen-Yang, Y. W.; Chuang, J. R.; Yang, Y. C.; Li, C. Y.; Chiu, Y. S. New UV-Curable Cyclotriphosphazenes As Fire-Retardant Coating Materials for Wood. J. Appl. Polym. Sci. 1998, 69, 115. (22) Christova, D.; Ivanova, S. D.; Velichkova, R. S.; Tzvetkova, P.; Mihailova, P.; Uzunov, I.; Lahov, L.; Peshev, O. Organic−Inorganic Composites Based on Cyclotriphosphazene-Cross-Linked PHEMA Networks. Des. Monomers Polym. 2003, 6, 11.
(23) Allcock, H. R.; Forgione, P. S.; Valan, K. J. Phosphonitrilic Compounds. IV. Preparation and Polymerization of Allylaminophosphonitrile Compounds. J. Org. Chem. 1965, 30, 947. (24) Ahn, K. D.; Kim, U. Y.; Kim, C. H. Photocurable System of Polythiol and Polyphosphazene-Containing Allyl Groups. J. Macromol. Sci. A 1986, 23, 169. (25) Allcock, H. R. Phosphorus−Nitrogen Compounds: Cyclic, Linear, and High Polymeric Systems; Academic Press: New York, 1972. (26) Wu, H. S.; Meng, S. S. 31P NMR Analysis of Cyclotriphosphazenes. Ind. Eng. Chem. Res. 1998, 37, 675.
584
DOI: 10.1021/ie502915t Ind. Eng. Chem. Res. 2015, 54, 577−584
