Ind. Eng. Chem. Res. 2003, 42, 5417-5422
5417
APPLIED CHEMISTRY Durable and Oxygen Bleach Rechargeable Antimicrobial Cellulose: Sodium Perborate as an Activating and Recharging Agent Louise K. Huang and Gang Sun* Division of Textiles and Clothing, University of California, Davis, California 95616
Durable and rechargeable antimicrobial cotton was prepared from polycarboxylic acid (PCA)finished cellulose. Sodium perborate solution was employed as an activating and recharging agent to generate peroxyacid structures from cross-linked PCA moieties existing on the PCAtreated fabrics. The peroxyacid structures on the fabrics, characterized by using FTIR and active oxygen analysis, could demonstrate desired antimicrobial functions against Escherichia coli. The antimicrobial functions on the cellulose can be refreshed by using similar sodium perborate solutions in subsequent procedures. But the refreshable functions on the cotton seem to be limited by the peroxyacid formation process. The mechanisms and conditions of the formation of peroxyacid structures on PCA-cross-linked cellulose are discussed in this paper. Introduction
Experimental Section
Durable and rechargeable antimicrobial functions on textile materials have been developed by incorporating halamine structures and using chlorine bleach as an activating and recharging agent.1-4 More recently, carboxylic moieties on cellulose also demonstrated rechargeable antimicrobial functions with a commercial active oxygen bleach as a recharging agent.5 The carboxylic acid groups can be converted to biocidal peroxyacid structures upon reaction with the oxygen bleach. In this paper we will present another type of oxygen bleaches that can produce the biocidal peroxyacid structures from cross-linked polycarboxylic acids (PCAs). Sodium perborate is a nontoxic and stable oxidizing agent, used extensively as a major component in bleaching agents, detergents, and disinfecting agents.6-7 It contains a peroxo salt with the anionic formula [B2(O2)2(OH4)]2-. Sodium perborate rapidly yields hydrogen peroxide in aqueous solutions. Sodium perborate contains two peroxidic bonds (O-O) which are considered active or capable of oxidizing iodide ion to free iodine under acidic conditions.8 Perborate is an effective oxidizing agent in organic synthesis; it has been reported to be capable of oxidizing sulfides to sulfoxides, oxidatively rearranging imines to foramides, and decarbonylating β-arylpyruvic acids.9-11 Sodium perborate is also considered as an oxygen bleaching agent which can convert carboxylic groups to peroxyacid moieties under basic aqueous solutions. This paper will discuss the formation and characterization of peroxyacid structures on 1,2,3,4-tetracarboxylic acid-cross-linked cotton fabrics, and their rechargeable antimicrobial properties. The mechanism of the formation of peroxyacid moieties by using sodium perborate under basic conditions is explored as well.
Materials. Cotton fabrics were bleached, and desized cotton print cloth (no. 400) purchased from Testfabrics, Inc. (West Pittston, PA). 1,2,3,4-Butanetetracarboxylic acid (BTCA), sodium perborate tetrahydrate (NaBO3‚ 4H2O), sodium hypophosphite, and sodium silicate (SS, 27%) were purchased from Aldrich Chemical Co. (Milwaukee, WI). All other reagents were purchased from Fisher Scientific (Pittsburgh, PA) or VWR Scientific (South Plainfield, NJ). Methods. The cotton fabrics, cut in sizes of around 30 cm × 90 cm, were immersed in a finishing solution containing 6% BTCA, 3% sodium hypophosphite, and 0.2% Triton X-100 solution and then padded through a laboratory padder to control the fabrics to have a wet pick-up rate around 125%. Pick-up rate is defined as the percentage of weight increase per dry weight of the fabric. The treated fabrics were dried at 80 °C for 5-10 min and cured at 175 °C for 1.5 min, after which the fabrics were washed in a home washer (Maytag) at 50 °C for 30 min. The fabrics were then air-dried in a conditioning room (21 °C, 65% relative humidity) for at least 24 h. These finished fabrics were then treated with a sodium perborate solution at different concentrations, temperatures, and treatment duration using a finishing bath: fabric ratio of 50:1 (liquor ratio) at pH 10-11. A common oxygen bleach stabilizer, sodium silicate (5%), was added into the sodium perborate solution. These treated fabrics were rinsed in distilled water several times and then air-dried and stored at standard conditions prior to further testing, analysis, and characterization. Structures of the modified fabrics were analyzed by using a FTIR spectrophotometer (Nicolet Magna IR560). All of the spectra were generated by an absorbance mode from sample pellets containing 0.0030 g of ground fabric powder and 0.200 g of KBr. Each spectrum was based on 120 scans with a resolution of 4.0 cm-1.
* To whom correspondence should be addressed. Tel.: (530) 752-0840. Fax: (530) 752-7584. E-mail:
[email protected].
10.1021/ie030323e CCC: $25.00 © 2003 American Chemical Society Published on Web 09/25/2003
5418 Ind. Eng. Chem. Res., Vol. 42, No. 22, 2003
Quantitative analysis of the percentage active oxygen on the modified cellulose was carried out by using a modified titration method according to the American Association of Textile Chemists and Colorists (AATCC) standard test method 172, section 12.8 (AATCC 1997). Three-gram ((0.001 g) portions of treated and untreated samples were used in the titrations. Prior to titration, all samples were ground into fine powder form in a small laboratory mill (Arthur Thomas Scientific, Philadelphia, PA). Each finely ground sample was acidified by using a mixture containing 100 mL of 1.5 N sulfuric acid, 15 mL of 15.0% potassium iodide solution, and 10 mL of dichloromethane in the dark for 15 min. This mixture was then titrated in the dark, using a 0.005 N sodium thiosulfate solution with 5% starch solution as an indicator. The percent active oxygen of each sample was calculated using eq 1. Each fabric sample was titrated at least three times.
% available oxygen ) mL Na2S2O3 × normality of Na2S2O3 × 0.8 weight of sample (g)
(1)
The biocidal properties of the modified fabrics were quantitatively evaluated against the bacterium Escherichia coli (American Type Cell Culture no. 15597) according to AATCC standard test method 100. Four pieces of circular swatches in a diameter of 2.54 cm were challenged by 1.0 ( 0.1 mL of a bacterial inoculum containing 105-106 colony forming units (CFU) of E. coli in a sterilized Petri dish, and then were transferred into a sterilized 250-mL container. The fabric swatches were maintained in the container for a duration of 90 min before they were quenched by 100 mL of 0.03% sodium thiosulfate. The duration of 90 min is called the contact time for the fabric samples with the bacterial suspension. Each test was repeated at least three times. Repeated laundering tests were also carried out in a Launder-Ometer (Atlas Electric Devices Co., Chicago, IL) at room temperature according to AATCC standard test method 124. Results and Discussion Formation of Peroxyacid Structures. Sodium perborate is a solid oxygen bleach that can produce hydrogen peroxide when dissolved in water. Under basic conditions, sodium perborate will produce perhydroxide ions that can hydrolyze derivatives of carboxylic acids to form active peroxylate moieties. This reaction occurs at a temperature relatively lower than that of the formation of hydrogen peroxide; thus, some esters and amide compounds are often employed as accelerators for sodium perborate bleach.12 Polycarboxylic acids such as BTCA react with cellulose to form ester bonds, and multiple ester bonds that form on the PCA lead to crosslinking the cellulose. Dissolution of sodium perborate in water under basic conditions will produce nucleophilic perhydroxyl anions, which then attack the ester bonds in the PCA-cross-linked cellulose to form percarboxylates (Scheme 1). This serves as the major chemical reaction during bleaching. This reaction was confirmed from FTIR analysis of the resulted cotton fabrics (Figure 1). BTCA-cross-linked cotton exhibits strong ester bonds (1720 cm-1), and the repeated oxygen bleaching significantly reduced the intensity of the ester band, indicating a reduction of ester cross-linking. Although FTIR cannot reveal per-
Scheme 1
carboxylate structures on the bleached fabrics, the analysis of active oxygen on the fabrics demonstrated the amount of the peroxyacid moieties. Since the reaction involves the hydrolysis of ester bonds, it has a major drawback; i.e., the peroxyacid moieties formed on the treated cotton are a result of the loss of cross-linking ester bonds. This obviously will cause some reduction of wrinkle-free properties of the cotton fabrics, and the rechargeable antimicrobial properties cannot be recharged without limit. Because PCAs contain more than one carboxyl group, they have a tendency to form multiple cross-linkages, often resulting in a network of cross-linking. In the case of BTCA, which contains four carboxylic groups and affords efficient triplicate crosslinking of cellulose, the formation of three ester linkages can be attained.13 The possibility of BTCA multiple cross-linking to two and three esters on cellulose is 83% and 17%, respectively.14 Consequently, after the BTCAtreated cotton was modified with sodium perborate, the subsequent ester hydrolysis cleft one cross-linkage, leaving at least one or two other cross-links with cellulose. However, these remaining cross-links are subjected to similar cleavage upon continuing perborate treatments. Hence, repeated treatments could lead to irrecoverable loss to the modified chemical structure. Concentration of Sodium Perborate. According to Scheme 1, the concentration of sodium perborate could affect the hydrolysis of the ester bonds and, thus, the formation of percarboxylate structures. The resulting percarboxylate moieties can be analyzed by measuring active oxygen on the modified fabrics. Shown in Figure 2 are the amounts of active oxygen on the fabrics after they were bleached with different concentrations of sodium perborate at different temperatures. As the bleach concentration was increased, the amount of active oxygen on the fabrics also increased. A high concentration of sodium perborate in the treatment solution increased the amount of perhydroxyl anions formed, which would then increase the formation of peroxyacid moieties and the oxidative potential on the fabrics.7 As can be seen from the FTIR (Figure 1), ester bond intensity decreased as the concentration of sodium perborate increased, indicating a loss of cross-linking. This mechanism is in agreement with the perhydrolysis reaction of bleach activators such as tetraacetylethylenediamine (TAED) and sodium nonanoyloxybenzene sulfonate (NOBS) in the generation of peroxyacid during bleaching. The rate of such perhydrolysis was found to be directly related to the leaving group. More facile leaving groups have been shown to increase the rate of perhydrolysis.15 In this case, since cellulose is the only leaving group, there is no option to create a more facile leaving group. Therefore, the rate of perhydrolysis would depend solely on the amount of perhydroxyl anions that are available for the reaction. Bleaching Temperature and Time. Since the formation of perhydroxyl anions is dependent on tem-
Ind. Eng. Chem. Res., Vol. 42, No. 22, 2003 5419
Figure 1. FTIR spectra of cotton fabrics treated with 6% BTCA at 60 °C, 10 min: (a) no bleach, (b) 3% SPB, (c) 5% SPB, and (d) 10% SPB. Table 1. Effect of Treatment Time and Temperature on the Percentage of Active Oxygen of 6% BTCA Cotton Modified with SPB active oxygen (%), ×103 bleaching time (min) 5 10 30
Figure 2. Effect of treatment conditions on the percentage of active oxygen of cotton treated with 6% BTCA and bleached with SPB (10 min).
perature, the increase in treatment temperature of sodium perborate (SPB) on BTCA-treated cellulose was expected to contribute to the increase in peroxyacid formation, and thus lead to a higher percentage of active oxygen on the modified fabrics. Three treatment temperatures were used to mimic the three temperature selections in domestic washing machines: cold wash (30 °C), warm wash (45 °C), and hot wash (60 °C). At all three SPB concentrations, the percentage of active oxygen on the treated fabrics increased from about 0.002% to above 0.006% as the treatment temperature was raised from 30 to 60 °C (Figure 2). This increase is particularly substantial at the 10% SPB level. At all three concentrations, the increase in percentage of oxygen was most pronounced at temperatures above 30 °C. At the treatment temperature of 60 °C, the percentage of active oxygen has been shown to be superior to that in the other samples treated at lower temperatures. In practice, textile bleaching using sodium perborate is performed at high temperatures under alkaline conditions, based on the rapid perhydroxyl anion formation for effective bleaching under such conditions.16 Sodium perborate produces hydrogen peroxide in aqueous solution. At high temperatures, the decomposition
3% SPB 45 °C 60 °C
5% SPB 45 °C 60 °C
10% SPB 45 °C 60 °C
4.47 5.30 8.93
4.34 6.05 10.5
5.97 8.88 11.5
5.34 6.02 7.56
6.77 7.50 8.27
9.44 14.0 11.0
of hydrogen peroxide has been reported to accelerate. However, the use of sodium silicate stabilizes the hydrogen peroxide through the formation of peroxysilicates, thus reducing the decomposition of hydrogen peroxide.7 The effect of sodium perborate treatment time on BTCA-treated cellulose was also examined. Since peroxyacid moieties are formed via the perhydroxyl anions, the duration in which the species is allowed to react could affect the formation of peroxyacids. At 45 °C, samples that were treated at prolonged durations of 10 and 30 min showed a higher percentage of active oxygen on the fabrics at all three concentration levels (Table 1). Furthermore, a similar trend is observed at 60 °C (Table 1). Extended treatment time improved the percentage of active oxygen at all concentrations. However, at higher temperatures, extended bleaching time was not necessary and it could lead to a low amount of active oxygen at high sodium perborate concentration. This might be caused by the decomposition of peroxyacid structures, since the rate constant for peroxide decomposition was found to have units of the reciprocal of the reaction time, temperature, and concentration.7 Correlation between the Percentage of Active Oxygen and Biocidal Properties. Since the biocidal properties of the modified cellulose are closely related to the amount of active oxygen on the fabrics, attempts were made to determine the correlation between the two. As shown in Figure 3, at the lower treatment temperature of 45 °C, it was found that in order to attain a high percentage (over 90%) reduction of E. coli,
5420 Ind. Eng. Chem. Res., Vol. 42, No. 22, 2003
Figure 3. Percentage of active oxygen and relative biocidal efficacy of cotton fabrics treated with 6% BTCA and bleached with SPB at 45 °C, 10 min.
Figure 4. Percentage of active oxygen and relative biocidal efficacy of cotton fabrics treated with 6% BTCA and bleached with SPB at 60 °C, 10 min.
the percentage of active oxygen on the modified fabrics had to be over 0.009% (10% SPB). To obtain over 70% reduction of E. coli, the percentage of active oxygen had to be at the 0.006% level (5% SPB). A level below 0.006% active oxygen would only reduce less than 40% of E. coli (3% SPB). The correlation seems to be slightly different at the higher treatment temperature of 60 °C (Figure 4). Over 80% reduction of E. coli was attained at 0.008% active oxygen level (3% SPB). To attain over 90% reduction of E. coli, the percentage active oxygen was required to be over 0.01% (5% SPB); to attain over 99% reduction of E. coli, the relative percentage active oxygen was required to be over 0.014% (10% SPB). Although the correlations at the two different temperatures are slightly different, it is evident that in order to attain over 95% reduction of E. coli, the corresponding active oxygen on the modified samples has to be over 0.01%. This would be an indicative active oxygen level for other similar modifications in attaining effective biocidal properties. Biocidal Properties of Treated Cotton. On the basis of the analysis of the active oxygen, the BTCAtreated cellulose modified with sodium perborate exhibited promising oxidative and biocidal properties. The biocidal properties of these fabrics prepared under different treatment conditions were therefore examined against E. coli, a representative bacterium. Similar to the observation made from the effect of increased sodium perborate concentration, the percentage reduction of E. coli increased as the treatment concentration was increased (Figure 5). Consequently, concentration
Figure 5. Bleaching conditions on biocidal properties.
Figure 6. Effect of laundering and rebleaching on the biocidal efficacy of cotton fabric treated with 6% BTCA and bleached with SPB at 45 °C, 30 min.
of sodium perborate in bleaching solutions, higher treatment temperatures, and longer times contributed to higher amounts of active oxygen on the fabrics, which then led to better antimicrobial functions. At the highest concentration of 10%, samples that were treated at 45 and 60 °C exhibited the highest percentage reduction of E. coli. This is consistent with the findings from the active oxygen analysis. An increased amount of sodium perborate present in the treatment solution generated an increase in perhydroxyl anions in the treatment solution, which then led to an increase in peroxyacid formation, thus contributing to the increase in oxidative activity, and subsequently resulting in the enhanced biocidal properties of the modified cellulose. The same is true for the treatment duration, as seen in the active oxygen analysis. Prolonged treatment time resulted in improved biocidal properties of the modified cellulose. At 60 °C, as the treatment time was extended from 5 to 10 min at all three concentrations, the percentage reduction of E. coli also increased (Figure 5). This, however, is least pronounced at the 10% SPB level. A similar trend is observed at 45 °C. As the treatment time was extended from 10 to 30 min, the percentage reduction of E. coli was increased at all three concentration levels (Figure 5). This effect is most pronounced at the 5% SPB level. At the extended treatment time of 30 min, the samples modified with 5% SPB exhibited biocidal properties equivalent to those of the samples modified with 10% SPB. Durability of Antimicrobial Functions. To investigate the durability of the function of this modified cellulose, a series of laundering tests were carried out. Cotton samples that were treated with 6% BTCA and further modified with 10% sodium perborate at 45 °C
Ind. Eng. Chem. Res., Vol. 42, No. 22, 2003 5421
Figure 7. FTIR spectra of cotton fabrics treated with (a) 6% BTCA only and with (b) 6% BTCA and bleached with 10% SPB, no washing, (c) one wash, (d) two washes, (e) three washes, and (f) four washes at 45 °C, 30 min, and sterilized.
for 30 min were subjected to repeated laundering tests in a Launder-Ometer at room temperature. Each Launder-Ometer wash is generally considered equivalent to five domestic machine washes. After each wash, the samples were subjected to E. coli for biocidal activities, repeated up to four cycles (Figure 6). Apparently, repeated laundering reduces the biocidal properties of the modified cellulose. Samples that were not laundered at the beginning demonstrated over 99% reduction of E. coli. This level of percentage reduction gradually decreased as the samples were subjected to additional washing, finally dropping to 85% after four washes. This trend was also observed previously in the use of commercial oxygen bleach.5 For most chemically finished textiles, it is almost inevitable to undergo some physical damage during the treatment and even washing.8 However, the nonreplenishable reduction of antimicrobial functions is due to a loss of cross-linking ester bonds after repeated perhydrolysis (eq 1). The reduction of cross-linking ester bonds can be confirmed by a measurement of the ester band intensity on the fabrics. Figure 7 shows that the ester band intensities gradually decrease after each laundering with sterilization. This is in agreement with earlier findings that laundering induces a physical impact in the loss of BTCA as well as the peroxyacid moieties on cellulose, consequently reducing the oxidative potential and biocidal effectiveness on the treated fabrics. This result also indicates that this recharging mechanism can be employed only for limited usages on textile materials. Conclusions This study has demonstrated that the cross-linked ester bonds formed between butanetetracarboxylic acid and cellulose can be converted to peroxyacids moieties using sodium perborate. The reaction involved the hydrolysis of the cross-linking ester bonds with perhydroxyl ions, as confirmed by FTIR spectroscopy, and the relative oxidative activity of the treated fabrics has been
examined quantitatively using active oxygen analyses. The concentration of sodium perborate (SPB) in the solution greatly affects the formation of peroxyacid structures on the modified cellulose, with the highest concentration of 10% SPB-treated samples yielding the highest percentage of active oxygen at various treatment temperature. Bleaching temperature and duration also play important roles in the amount of active oxygen on the fabrics. The biocidal properties of the peroxyacid moieties on cellulose were consistent with the content of active oxygen, and the production of peroxyacid moieties on BTCA-treated cellulose could be repeated at least three times. Acknowledgment This research was supported by a CAREER award from the National Science Foundation (DMI 9733981) and was financially co-sponsored by Vanson-HaloSource Inc., Redmond, WA. Literature Cited (1) Sun, G.; Xu, X. Durable and regenerable antibacterial finishing of fabrics. Biocidal properties. Text. Chem. Color. 1998, 30, 26-30. (2) 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. (3) Sun, Y.; Sun, G. Durable and Regenerable Antimicrobial Textile Materials Prepared by A Continuous Grafting Process. J. Appl. Polym. Sci. 2002, 84, 1592-1599. (4) Lin J.; Winkelman C.; Worley, S. D.; Broughton, R. M.; Williams, J. F. Antimicrobial treatment of nylon. J. Appl. Polym. Sci. 2001, 81, 943-947. (5) Huang, L. K.; Sun, G. Durable and Regenerable Antimicrobial Cellulose with Oxygen Bleach: Concept Proofing. AATCC Rev., in press. (6) Karunakaran, C.; Anandhy, K.; Ramachandran V. Formation of peracetic acid upon aging of perborate in acetic acid. Kinetics of the oxidation of S-phenylmercaptoacetic acids. Monatsh. Chem. 2000, 131, 1025-1029.
5422 Ind. Eng. Chem. Res., Vol. 42, No. 22, 2003 (7) Varennes, S.; Daneault, C.; Parenteau, M. Bleaching of Thermomechanical Pulp with Sodium Perborate. Tappi J. 1996, 79, 245-250. (8) Vigo, T. Textile Processing and Properties; Elsevier Science: Amsterdam, 1994. (9) Morrow, N.; Ramsden, C. A.; Sargent, B. J.; Wallett, C. D. Oxidative Decarbonylation of BETA-Arylpyruvic Acids Using Sodium Perborate. Tetrahedron 1998, 54, 9603-9612. (10) Muzart, J. Sodium Perborate and Sodium Percarbonate in Organic Synthesis. SynthesissStuttgart 1995, 11, 1325-1347. (11) Nongkunsarn, P.; Ramsden, C. A. Oxidative Rearrangement of Imines to Formamides Using Sodium perborate. Tetrahedron 1997, 53, 3805-3830. (12) Wang, J. P.; Washington, N. M. Hydrophobic Bleach Systems and Textile Preparation: A discontinuity in Fabric Care. AATCC Rev. 2002, 2, 21-24.
(13) Schramm, C.; Rinderer, B. Hypophosphites as catalysts in durable press finishing with polycarboxylic acids. Text. Chem. Color. Am. Dyest. Rep. 2000, 32, 50-54. (14) Shank, D. Non-formaldehyde wrinkle-free finishing: A commercial update. AATCC Rev. 2002, 2, 29-32. (15) Grime, K.; Clauss, A. Chem., Ind. 1990, 647-651. (16) Cai, J. Y.; Evans, D. J.; Smith, S. M. Bleaching of Natural Fibers with TAED and NOBS Activated Peroxide Systems. AATCC Rev. 2001, 1, 31-34.
Received for review April 16, 2003 Revised manuscript received July 30, 2003 Accepted August 2, 2003 IE030323E
