Ind. Eng. Chem. Res. 2006, 45, 6477-6482
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MATERIALS AND INTERFACES Durable and Regenerable Biocidal Polymers: Acyclic N-Halamine Cotton Cellulose Song Liu and Gang Sun* DiVision of Textile and Clothing, UniVersity of California, DaVis, California 95616
As a continued study on halamine biocidal materials, acyclic halamine cotton cellulose was prepared by graft reaction of amide monomers onto cellulose and subsequent chlorination of the grafted materials. Two acyclic vinyl amide monomers, acrylamide (AM) and methacrylamide (MAA), were employed in the reaction successfully. The grafting reaction was implemented by a free radical polymerization process. Several radical initiators were able to abstract hydrogen atoms on the polymers and generate macromolecular radicals to react with the monomers. Similar to cyclic halamines, the resulted acyclic ones were able to provide durable and rechargeable biocidal functions. However, acyclic halamine structures are more vulnerable to hydrolysis compared to the cyclic halamines. Introduction
Experimental Section
The past 10 years have witnessed renewed interest in research on antimicrobial polymers and textiles due to increased concern for contact transmissions of infectious diseases and personal protection.1-5 As a result, durable and rechargeable biocidal polymers and textiles have been developed based on halamine chemistry, i.e., antibacterial properties of cyclic N-halamine (N-Cl) structures.6,7 These cyclic structures mostly contain imide, amide, and amine halamine bonds possessing certain structural features as discussed in the literature.6,8 One of the features is the absence of R-hydrogen next to the halamine bonds, which can prevent formation of hydrochloric acid (HCl) as a result of an elimination reaction of the R-hydrogen and chlorine on the N-halamine (N-Cl) bonds. The cyclic halamines have demonstrated many advantages as biocidal materials, such as great stability in a wide range of temperatures and pH values, rapid inactivation of a broad spectrum of microorganisms, and being rechargeable using chlorine bleach.9-11 According to the structural features, some acyclic imide, amide, and amine should also form stable acyclic N-halamines. If these acyclic Nhalamines can demonstrate the same potency against a broad spectrum of microorganisms, they could be another group of new biocides that can be employed on polymeric and textile materials. To prove the above hypothesis, several vinyl monomers containing amide bonds such as acrylamide and methacrylamide were employed in grafting modifications of cotton cellulose. The grafted products were chlorinated in diluted chlorine bleach, and acyclic halamine structures were produced. The acyclic halamines demonstrated durable and regenerable biocidal functions against Escherichia coli (E. coli), similar to the cyclic halamines. The formation and characterization of the acyclic halamines were studied and are presented in this paper. Potent antibacterial properties of these halamines are demonstrated as well.
Materials. Pure cotton print cloth no. 400 was purchased from TestFabrics Inc. (West Pittiston, PA). Potassium persulfate (PPS) was supplied by Acros, Pittsburgh, PA, and was recrystallized from distilled water. Acrylamide (AM), methacrylamide (MAM), and 2,2′-azobis-(2-methylpropionamideine) (AMPDH) were purchased from Aldrich (St. Louis, MO) and used as received. Structure Analysis. Fourier transform infrared (FTIR) spectra were taken on a Nicolet 6700 spectrometer (Thermo Electron Corporation) using KBr pellets. The samples were made thin enough to ensure that the Beer-Lambert law was fulfilled. Nitrogen contents on fabric samples were examined following a total Kjeldahl nitrogen analysis and were conducted by the DANR analytical laboratory at the University of California, Davis. Graft Polymerization. Chemical finishing baths were prepared by dissolving all chemicals (monomers and the initiator) in distilled water according to different formulations. Fabrics were dipped in the chemical baths and padded at a required expression. This “dip-pad” process was repeated twice. The padded fabrics were dried at 60 °C for 10 min, cured at an elevated temperature for a certain period of time, and then washed in a large amount of water. Afterward, the fabrics were tumble dried and stored in a conditioning room (21 °C, 65% relative humidity) for over 72 h to reach constant weights. Percentage graft was calculated from the following equation:
* To whom correspondence should be addressed. Tel.: (530) 7520840. Fax: (530) 752-7584. E-mail:
[email protected].
graft % ) (W2 - W1)/W1 × 100
(1)
where W1 and W2 are the weights of the original and grafted fabrics, respectively. Chlorination. The amide grafted samples were converted to halamine structures in a simple chlorination process: immersing the samples in a diluted chlorine bleach solution (300 ppm available chlorine) at room temperature for 30 min. The liquid to fabric (liquor) ratio was 50:1 (w/w). The fabrics were then rinsed in distilled water and air-dried.
10.1021/ie060253m CCC: $33.50 © 2006 American Chemical Society Published on Web 08/18/2006
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Figure 1. Subtracted FTIR spectra (A) between AM-g-cotton and untreated cotton cellulose, (B) between chlorinated AM-g-cotton and untreated cotton cellulose, and (C) between pH 4 acetic acid buffer solution soaked AM-g-cotton and untreated cotton cellulose.
Antibacterial Assessment. Antibacterial properties of the grafted samples were examined according to a modified American Association of Textile Chemist and Colorists (AATCC) test method 100 against a nonpathogenic Gram-negative bacterium Escherichia coli K-12 (E. coli, UC Davis Microbiology Laboratory). The fabrics were cut into four small pieces (ca. 4 cm2), and two pieces of the samples were put together in a sterilized container. A 1.0 mL volume of an aqueous suspension containing 105-106 colony forming units (CFU)/mL E. coli was placed onto the surfaces of the fabrics. After variable contact times, the inoculated samples were placed into 100 mL of 0.03% sodium thiosulfate aqueous solution to neutralize any active chlorine. The mixture was then vigorously shaken for 5 min. An aliquot of the solution was removed from the mixture and then serially diluted, and 100 µL of each dilution was placed onto a nutrient agar plate. The same procedure was also applied to the bleached ungrafted and grafted but unbleached cotton as controls. Viable bacterial colonies on the agar plates were counted after incubation at 37 °C for 24 h. Bacterial reduction is reported according to the equation
percentage reduction of bacteria (%) ) (A - B)/A × 100 (2) where A is the number of bacteria counted from untreated fabrics, and B is the number of bacteria counted from treated fabrics. Results and Discussion Two vinyl amide monomers (AM and MAM) were grafted onto cellulose by using a free radical polymerization process. Both initiator and monomers were padded onto the fabric in an aqueous solution; the samples were dried at 60 °C and then heated to an elevated temperature to initiate the radical grafting reaction. This process is very similar to a regular wet fabric finishing operation, and the removal of water before the polymerization prevents side reactions between initiators and water. Grafting acrylamide onto cotton cellulose has been conducted extensively by using solution polymerization processes for applications of soil releasing, dye uptake, and moisture regain.12-16 However, the solution graft polymerization could easily form long chain grafting products, which may significantly affect cotton properties. Conversion of the amide N-H into N-Cl was conducted by a chlorine bleaching process. The treated cotton fabrics were
Table 1. Chlorination AM and MAM Grafted Cotton Cellulose under Different pH Conditions N% in samples AM-g-cotton pH 8 bleached AM-g-cottona pH 11 bleached AM-g-cotton MAM-g-cotton pH 8 bleached MAM-g-cotton pH 11 bleached MAM-g-cotton a
0.445 0.392 0.275 0.354 0.354 0.272
% reduction of N
chlorine (ppm)
11.9 38.3
136 ( 6 444 ( 32
0 23.2
145 ( 2 521 ( 12
Available chlorine, 300 ppm, bleached for 30 min.
Scheme 1. Grafting AM and Chlorination of AM Grafted Polymers
immersed a diluted chlorine bleach solution for a duration of 30 min. The pH of chlorination solutions was adjusted to achieve high conversion yields (Table 1). Both AM and MAM showed higher active chlorine contents under alkaline conditions. However, the amounts of active chlorine were 14-18 times less than that of the available amide (NH2). Such a result is possibly due to imidization and oxidization reactions that occurred on amide structures during the radical grafting. A liquid chromatographic-mass spectrometric analysis of water extracted compounds from the grafted cotton has found products resulted from the imidization of the amide (N-H), which serves as evidence of the proposed side reactions. Detailed studies are continuing in this laboratory, and results will be reported soon. The expected grafting and chlorination reactions are shown in Scheme 1. FTIR Study. To confirm the expected reactions, vinyl amide grafted cellulose was characterized by FTIR spectroscopy. Figure 1 shows three FTIR spectra. Spectrum A is a result of subtracting the FTIR spectrum of untreated cotton from that of acrylamide grafted cotton cellulose (AM-g-cotton), curve B is the subtraction of untreated cotton cellulose from chlorinated (at pH 4) AM-g-cotton, and spectrum C is a subtracted result of pH 4 acetic acid soaked AM-g-cotton and untreated cotton cellulose. Spectrum A shows characteristic peaks of 1671 cm-1
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Figure 2. Influence of initiator on acrylamide grafting on cellulose. Acrylamide 2.486% (0.35 M) solution, dried at 60 °C for 20 min and cured at 105 °C for 30 min.
(amide band I due to CdO stretch) and 3200 cm-1, which is similar to the IR spectrum of polyacrylamide. After chlorine bleaching at pH 4 the band of 1670 cm-1 shifted to 1695 cm-1, a change similar to the formation of cyclic halamine structures.17 As the amide structure -C(O)-NH2 changed to -C(O)-NHCl, the decreased electron density on the nitrogen atom could reduce its contribution to the resonant structure -C(O-)dN+HCl, and consequently increase the CdO double bond compositionshifting FTIR band to higher wavenumbers. Besides chlorination, hydrolysis reaction could occur at the same time, and the hydrolysis is heavily affected by the pH condition. To investigate the hydrolysis, the AM-g-cotton was soaked in pH 4 acetic acid buffer solution for 90 min. The amide structure was partially hydrolyzed into carboxylic acid under that condition, as evidenced by a peak at 1710 cm-1 in the subtracted spectrum (C) in Figure 1. Alkaline hydrolysis showed similar hydrolysis results. These findings indicate that the acyclic amide structures are vulnerable to hydrolysis, which may affect the washing durability of acyclic halamines. The treated cotton samples were thoroughly washed to remove any potential homopolymers of acrylamide before the infrared analysis; any amide or carboxylic structures shown in the FTIR of treated cotton samples confirmed grafting reactions on cotton cellulose. The band (CdO) shifting after chlorination further proved the formation of acyclic halamines. Methacrylamide grafted cotton (MAM-g-cotton) and chlorinated MAM-g-cotton showed similar FTIR spectra. Influence of Initiators and Monomers. Selection of proper initiators is extremely important in radical graft polymerization since it determines the grafting efficiency and possibly grafting sites. When the temperature is raised to above the decomposition temperatures of radical initiators, initiator radicals will be produced. To maximize the grafting efficiency, the initiator
radicals are expected to abstract hydrogen atoms from cellulose rather than undergo radical addition to the monomers.18 In consideration of practical applications of the process, watersoluble and inexpensive radical initiators such as 2,2′-azobis(2-methylpropionamideine) and potassium persulfate were employed in the grafting modification of cotton cellulose. Both were not considered the best radical initiators for hydrogen abstraction.18 Figure 2 shows grafting yields of using AMPDH and PPS as initiators; the yields increased with increase of initiator concentration in both cases. AMPDH was more effective than PPS in initiating the graft modification on cellulose at high concentration. Both initiators could initiate two possible radical reactions (Scheme 2). One is a hydrogen abstraction from cellulose, possibly from the sulfate radical anion, resulting in the formation of tertiary carbon radicals on cellulose chains; the other is a radical addition to any vinyl monomer, producing homopolymers. The tertiary carbon radical on cellulose can proceed in a radical addition reaction with vinyl monomers, which leads to the grafting modification of cotton cellulose. The grafting efficiencies on the cellulose were quite high, indicating the predomination of hydrogen abstraction initiation. Any homopolymer formed could be removed in the extensive washing process since polyacrylamide and polymethacrylamide are soluble in water. When the initiator concentration was above 0.1 M in the solution, the grafting efficiencies of the monomers were in a range of 52-81%, much higher than those obtained in solution graft polymerization (below 50% generally, less than 20% for the grafting yield under 10%12,13). Due to the high grafting efficiencies, the amount of homopolymers formed as byproducts was quite low. Monomer concentration in the formulation affects grafting yields as well. Figure 3 indicates grafting yields of acrylamide in varied concentrations of 0.05-0.65 M. As the monomer concentration increases, grafting yields show a steady increase, which denotes a growing grafting chain size due to the higher monomer/initiator ratio. Nitrogen analysis on the treated cotton fabrics showed a good agreement between the measurements of weight increase and nitrogen contents of the fabrics. Influence of Curing Temperature. Curing temperature and time may affect graft polymerization of monomers on the cellulose. The graft polymerization was conducted over a temperature range of 80-135 °C, which was chosen according to decomposition temperatures and rates of the initiators. Based on the activation energy and coefficient factor of the initiator PPS, the half-life time t1/2 was calculated at different temperatures. The reaction durations at different temperatures were set to be at least 5 times t1/2 to ensure 97% decomposition of the initiator. Thus, under a higher temperature the curing time
Scheme 2. Potential Grafting and Homopolymerization Reactions of AM
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Figure 3. Grafting yield under different concentrations of acrylamide. PPS 4.73% (0.175 M) solution, dried at 60 °C for 20 min and cured at 105 °C for 30 min.
Figure 4. Effect of temperature and time on grafting reaction.
was shorter and vice versa for lower temperature. The results are shown in Figure 4. Considering the melt point of AM at 84.5 °C, the reaction was initially set at temperatures much higher than the melting point. However, the results revealed that there was no significant difference in AM grafting yields in the temperature range of 80-135 °C. Based on consideration of reaction time and energy consumption, the curing temperature was set at 120 °C and the curing time was about 5-10 min. Durability of Acyclic Halamine. After formation of acyclic halamines, the durability of the amide and amide halamine structures to washing and hydrolysis was explored. Two different possible hydrolyses might happen to the acyclic halamines: one is the hydrolysis of halamine to precursor amide and the other
is hydrolysis to carboxylic acid. The hydrolysis of halamine/ amide to carboxylic acid could be monitored by nitrogen content changes of the cotton cellulose, which are shown in Table 1. AM and MAM grafted cotton samples were employed in different chlorination conditions. The original nitrogen contents of AM-g-cotton and MAM-g-cotton samples were 0.445% and 0.354%, respectively. After chlorination at pH 8, the nitrogen losses for these two samples were 11.91% and 0%, while at pH 11 the losses were 38.3% and 23.2%, respectively, much higher than those at pH 8. The AM-g-cotton sample always demonstrated a higher loss of nitrogen than MAM-g-cotton, an indication of easier hydrolysis of AM-g-cotton. The methyl group next to carbonyl may create some steric hindrance to the hydrolysis of amide.
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Figure 5. Storage stability of chlorinated AM-g-cotton. Table 2. Antibacterial Results of Treated Cotton Samplesa reduction of E. coli at contact times (%)
grafted cotton
chlorine (ppm)
5 min
15 min
30 min
60 min
AM-g-cotton MAM-g-cotton MAM-g-cotton
478 ( 36 541 ( 20 296 ( 23
99.2 99.999 99.999
99.999 99.999 99.999
99.999 99.999 99.999
99.999 99.999 99.999
a AATCC test method 100-1999, E. coli concentration 105-106 colony forming units (CFU)/mL.
One interesting point is that although AM-g-cotton contains more nitrogen, or more amide, the amounts of active chlorine resulted were always lower than that of MAM-g-cotton. Such a result might be caused by an imidization reaction occurring at amide sites. The imidization reaction could be caused by further hydrogen abstraction on the grafted amide hydrogen (NH), and the resulted nitrogen radical will have an addition reaction with monomers. The consumption of N-H bonds on the grafted amide leads to fewer sites for conversion to halamines. Detailed reactions will be addressed in a following study. Alkaline conditions make formation of acyclic halamine more efficient, but also less durable since the hydrolysis reaction. Thus, to increase regenerability of acyclic halamine, a less alkaline bleaching condition is favored. To further prove the washing durability and regenerability of acyclic halamines, AM-grafted cotton was continuously laundered according to an accelerated method, AATCC test method 61-2003, followed by bleaching with diluted sodium hypochlorite. The active chlorine contents on samples did not decrease after two or three accelerated washes as compared with one without accelerated washing. One accelerated washing is considered to be equivalent to five machine washings. The storage stability of the acyclic halamine was evaluated by measuring the active chlorine contents of the bleached AMg-cotton and MAM-g-cotton over a 50 day period of storage in a conditioning room (21 °C, 65% relative humidity), and the results are shown in Figure 5. The active chlorine on the halamine structures is quite stable, and the storage stability is comparable with cyclic halamine structures.19 The reduction rate of the active chlorine on both Am-g-cotton and MAM-g-cotton was in a range of 2-3 ppm/day. Antibacterial Assessment. The chlorinated AM-g-cotton and MAM-g-cotton samples were tested against E. coli following
the AATCC test method 100. Table 2 shows the antibacterial efficacy of two samples with different active chlorine contents. In all cases, when active chlorine contents were in a range of 296-541 ppm, the samples demonstrated rapid antibacterial properties. A 5-logarithmic reduction of E. coli in a contact time of 5-15 min makes the acycilic halamine cotton as powerful as cyclic halamine cotton.17 The results are consistent with the active chlorine contents on the fabrics and similar to the power of cyclic amide halamines.20 Such products can be used in biological protective clothing and medical use textiles. Conclusions Acrylamide and methacrylamide were grafted onto cotton cellulose in high yields using a radical graft polymerization process. The grafted products could be easily converted to acyclic halamines in a diluted chlorine bleach solution. The chlorinated AM-g-cotton and MAM-g-cotton provided rapid inactivation against E. coli, similar to cyclic halamines. However, the acyclic halamine structures are relatively vulnerable to alkaline conditions, and the storage stability was comparable to that of cyclic halamine. Acknowledgment The authors are grateful for the financial support from the National Science Foundation (DMI 0223987) and National Textile Center (C02-CD06). Literature Cited (1) Emerson, D. W.; Grigorian, C.; Hess, J. W.; Zhang, Y. Probing the Structures of Redox Polymers. React. Funct. Polym. 1997, 33, 91-101. (2) Tweden, K. S.; Cameron, J. D.; Razzouk, A. J.; Bianco, R. W.; Holmberg, W. R.; Bricault, R. J.; Barry, J. E.; Tobin, E. Silver Modification of Polyethylene Terephthalate Textiles for Antimicrobial Protection. ASAIO J. 1997 43, 475-481. (3) Sun, G.; Xu, X.; Bickett, J. R.; Williams, J. F Durable and Regenerable Antimicrobial Finishing of Fabrics with a New Hydantoin Derivative. Ind. Eng. Chem. Res. 2001, 41, 1016-1021. (4) Sun, Y.; Chen, Z.; Braun, M. Antimicrobial Polymers Containing Melamine Derivatives. I. Preparation and Characterization of Chloromelamine-based Cellulose. Ind. Eng. Chem. Res. 2005, 44, 7916-7920. (5) Chen, Z.; Sun, Y. Antimicrobial Polymers Containing Melamine Derivatives. II. Biocidal Polymers Derived from 2-Vinyl-4,6-Diamino-1,3,5Triazine. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 4089-4098.
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(6) Worley, S. D.; Williams, D. E. Halamine Water Disinfectants. Crit. ReV. EnViron. Control 1988, 18 (2), 133-175. (7) Sun, G.; Worley, S. D. Chemistry of Durable and Regenerable Biocidal Textiles. J. Chem. Educ. 2005, 82, 60. (8) Qian, L.; Sun, G.. Durable and Regenerable Antimicrobial Textiles: Chlorine Transfer among Halamine Structures. Ind. Eng. Chem. Res. 2005, 44 (4), 852-856. (9) Worley, S. D.; Wheatley, W. B.; Kohl, H. H.; Burkett, H. D.; Van Hoose, J. A.; Bodar, N. A New Water Disinfectant; A Comparative Study. Ind. Eng. Chem. Prod. Res. DeV. 1983, 22 (4), 716-718. (10) Sun, G.; Xu, X. Durable and Rregenerable Antibacterial Finishing of Fabrics: Biocidal Properties. Text. Chem. Color. 1998, 6, 26. (11) Sun, G.; Xu, X. Durable and Regenerable Antibacterial Finishing of Fabrics: Fabric Properties. Text. Chem. Color. 1999, 31, 21. (12) Ghosh, P.; Dev, D.; Samanta, A. K. Graft Copolymerization of Acrylamide on Cotton Cellulose in A Limited Aqueous System Following Pretreatment Technique. J. Appl. Polym. Sci. 1995, 58, 1727-1734. (13) Sahoo, P. K.; Samantaray, H. S.; Samal, R. K. Graft Copolymerization with New Class of Acidic Peroxo Salts as Initiators. I. Grafting of Acrylamide onto Cotton-Cellulose Using Potassium Monopersulfate, Catalyzed by Co(II). J. Appl. Polym. Sci. 1986, 32, 5693-5703. (14) Elkharadly, H.; Fattah, S. H. A.; Nasr, H. Chemical Factors Affecting Soiling and Soil Release from Cotton DP Fabric: Part XIV. Grafting with Polyacrylamide. Am. Dyest. Rep. 1983, 72 (9), 48-55.
(15) Zohdy, M. H.; Sahar, S. M.; Hassan, M. S.; Khalil, E. M.; ElHossamy, M.; El-Naggar, A. M. Selective Properties of Polyester and Cotton/Polyester Fabrics Gamma Radiation Grafted with Different Binary Mixtures of Vinyl Monomers. Polym. Int. 1999, 48 (6), 515-525. (16) Abdel-Hafiz, S. A.; El-Sisi, F. F.; Helmy, M.; Hebeish, A. Concurrent Grafting and Dyeing of Cotton with An Acrylamide/Potassium Permanganate/Citric Acid System. J. Soc. Dyers Colour. 1996, 112 (5/6), 162-166. (17) Sun, Y.; Sun, G. Novel Regenerable N-halamine Polymeric Biocides. II. Grafting Hydantoin-Containing Monomers onto Cotton Cellulose. J. Appl. Polym. Sci. 2001, 81, 617-624. (18) Moad, G.; Solomon, D. H. The Chemistry of Free Radical Polymerization; Elsevier Science: Oxford, U.K., 1995. (19) Qian, L.; Sun, G. Durable and Regenerable Antimicrobial Textiles: Improving Efficacy and Durability of Biocidal Functions. J. Appl. Polym. Sci. 2004, 91, 2588-2593. (20) Sun, Y.; Sun, G. Durable and Regenerable Antimicrobial Textile Materials Prepared by a Continuous Grafting Process. J. Appl. Polym. Sci. 2002, 84, 1592-1599.
ReceiVed for reView March 2, 2006 ReVised manuscript receiVed June 18, 2006 Accepted July 20, 2006 IE060253M
