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Ind. Eng. Chem. Res. 2010, 49, 4720–4725
Improving the Resistance of Sulfur Dyes to Oxidation Wenlong Zhou† and Yiqi Yang*,‡ Key Laboratory of AdVanced Textile Materials and Manufacturing Technology (Ministry of Education), Zhejiang Sci-Tech UniVersity, Hangzhou, Zhejiang 310012, China, and Department of Textiles, Clothing & Design, Department of Biological Systems Engineering, UniVersity of NebraskasLincoln 234 HECO Bldg, Lincoln, Nebraska 68583-0802
A method that substantially improves the oxidative bleaching resistance of sulfur dyed textiles is reported. Sulfur dyes are known for their poor bleaching resistance due to the sensitivity of the sulfur linkages to oxidation. This disadvantage severely prohibits the broad application of sulfur dyes because of the existence of the activated oxidative bleaching agents in many commercial detergents. In this study, 1% (w/w) of lanthanum triacetate was applied to sulfur dyed cotton fabrics via a simple padding procedure to chemically stabilize the sulfur linkages. The treatment could render sulfur dyed cotton fabrics a reduction in color changes of about 50% after laundering with detergent containing a bleaching agent, an improvement in strength retention of almost 100% after aging and an improvement in staining resistance of at least 0.5-class for about 1/3 of the sulfur dyes examined. These improvements were proved durable to repeated launderings. Introduction Over 100 years have passed since the first sulfur dye, C. I. Leuco Sulfur Black 1, was commercialized.1-5 Sulfur dyes have a broad range of colors,4 and at one time were the most consumed dyes for cotton and cotton blends due to their low cost, good wet fastness, satisfactory light fastness and beautiful shades, especially dark shades like black, navy and brown.1-5 In the last two decades, sulfur dyes have been substituted gradually by reactive dyes mainly because of their sensitivity to oxidation and aging and the substantial price decrease of reactive dyes. Nevertheless sulfur dyes are still a major class of dyes for cellulosics.4 Sulfur blacks are by far the most consumed black dyes around the world.1,4 No other blacks can match the sulfur blacks in their color hues and chromas, wet fastnesses, and the cost of dyeing. The exact structures of sulfur dyes are unknown.1-11 During dye manufacturing, sulfur was introduced into dye structures to form heterocyclic rings and sulfur linkages between chromophores.1,5,11 The stability of the sulfur linkages influences the performance properties of sulfur dyes. These linkages can be cleaved by reduction to form leuco compounds in dyeing, in order to dissolve the dyes and provide the dyes substantivity to cellulose. Sulfur linkages in sulfur dyes can also be easily oxidized and then cleaved by hydrolysis, thus causing poor resistance of the dyed textiles to bleaching and aging. Aging occurs in fabrics dyed with sulfur dyes and stored under conditions of high temperature and humidity.1-12 Aging is the degradation of cellulose catalyzed by sulfur containing acids generated from the hydrolysis of oxidized sulfur linkages.2-4,9 Poor bleaching fastness and aging are still the two major problems of sulfur dyed materials, although they have been somewhat relieved by improving the methods of dyestuff manufacturing.2 Resistance of sulfur dyed fabric to oxidation and aging is also reportedly improved via finishing.7,9,12,13 Durable press (DP) finishing, for instance, has proven to be capable of inhibiting aging and improving fastness to oxidation of sulfur * To whom correspondence should be addressed. E-mail: yyang2@ unl.edu. Tel.: 402 472 5197. Fax: 402 472 0640. † Zhejiang Sci-Tech University. ‡ University of NebraskasLincoln.
dyed fabrics. However, DP finishing imparts a harsh hand to fabrics, dulls the shade in some cases, and severely damages the mechanical properties of the finished fabrics.14 Furthermore, this approach is limited to the products where DP finishing is applicable. The commercial solution to aging is the aftertreatment with an alkaline finish which can neutralize the released acid during textile storage. Sodium carbonate and sometimes sodium hydroxide are the common alkalis used.6-9,12 However, this approach is not durable to laundering. Poor colorfastness to oxidation of sulfur dyes becomes a serious problem in recent years, because detergents with activated oxidative bleaching agents are broadly used in home laundering. Such bleachers in the detergents are peroxide-based oxidizers with activators, making the bleachers safer than chlorine bleachers to many dye classes. Bleaching activators are necessary for the peroxide-based bleachers to function at a laundering temperature less than 60 °C.15 Bleaching activators are mainly acylated chemicals, which react with peroxide bleachers to form organic peroxy-acids under laundering pH between 9 and 12. As a result of their higher oxidation potentials relative to common peroxide, these organic peroxy-acids have effective low-temperature bleaching properties.15 Due to their excellent performance properties under low laundering temperatures and better color protection than chlorine bleaches, the peroxide bleaching agents with activators are common additives in today’s detergents. However, peroxide-based oxidizers still change the colors of sulfur dyed fabrics substantially.18 Efforts have been made to improve the resistance of sulfur dyed fabrics to color change in home laundering with detergents containing activated peroxides.13,16-19 Examples are the use of cationic agents to treat fabrics both before and after dyeing,13,16-19 treatment of sulfur dyed fabrics with copper sulfate together with potassium or sodium dichromate and acetic acid,3 and alkylation of the reduced dye anion, Dye-S-, with a large alkyl group.5,12 However, no simple treatment is available with less impact on other properties of the fabrics, and acceptable costs. The poor fastness to oxidation and aging of sulfur dyed fabrics is mainly related to the chemical properties of the sulfur linkages within the dyes. In this paper, the use of lanthanum triacetate was examined for the protection of sulfur dyes from being
10.1021/ie901901r 2010 American Chemical Society Published on Web 03/30/2010
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Table 1. Sulfur Dyes and Their Abbreviations Used trade name Sodyesul Navy GFCF Liquid Sodyesul Green NYCF Liquid Sodyesul Red 2B Liquid Sodyesul Black 4GCF Liquid Diresul Black RDT-4D Liquid Diresul Navy RDT-GF1 Liquid Diresul Yellow E Liquid
C.I. name Leuco Leuco Leuco Leuco Leuco Leuco Leuco
Sulfur Sulfur Sulfur Sulfur Sulfur Sulfur Sulfur
Blue 20 Green 2 Red 14 Black 1 Black 1 Blue 20 Yellow 22
abbreviation SN SG SR SB DB DN DY
attacked by oxidants, i.e. the improvement of antioxidation and antiaging ability of sulfur dyed fabrics. Experimental Section Materials. The fabric used in this study was a desized and bleached plain weave print cotton cloth with a weight of 102 g/m2 (Testfabrics Style 400, supplied by Testfabrics, Inc. West Pittston, PA). The sulfur dyes and their corresponding additives were commercial products generously supplied by Clariant. The trade and C. I. (Color Index) names and abbreviations of the dyestuffs used in this paper are summarized in Table 1. Lanthanum triacetate (La(Ac)3), sodium sulfate, and sodium carbonate were supplied by Aldrich. The detergent without bleaching agents (Standard detergent) was the AATCC (American Association of Textile Chemists and Colorists) Standard Reference Detergent (1993) without optical brighteners. The detergent with an activated oxidative bleach (NOBS detergent) was Tide With Bleach, a common powdered product without fluorescent brightening agent, containing built-in peroxide bleach with NOBS (nonanoyloxy-benzenesulphonate) purchased from
a War-Mart supermarket, NE. NOBS is an activator for peroxide bleaches to provide effective whitening and stain removal functions to the detergent at room temperature. Dyeing of Cotton Fabrics. Dyeing was carried out in a FRITZ dyeing machine of Equipment Service, Inc., USA. For the Sodyesul dyes, the dyeing method used is shown in Figure 1.Dyebath was heated from room temperature to 66 and 88 °C for nonblack and black dyes, respectively. Fabric-to-liquor ratio was 1:10. At the end of dyeing, the dyed fabrics were rinsed well with cold and warm tap water, and then oxidized at 65 °C for 10 min with a solution containing 2.00% owf (based on the weight of fabric) Clariant Oxidizer B Liquid and 1.12% owf acetic acid at a fabric-to-liquor ratio of 1:100. For the Diresul dyes, the dyeing method used is shown in Figure 2. Dyebath was heated from room temperature to 93 °C. Fabric-to-liquor ratio was 1:10. At the end of dyeing, the dyed fabrics were rinsed well with cold and warm tap water, and then oxidized at 65 °C for 10 min with a solution containing 1.00% owf Clariant Oxidizer A Powder and 1.00 g/L sodium carbonate at a fabric-
Figure 1. Dyeing method for Sodyesul dyes. A: Fabric (20 g); Dye (10, 15 or 20% owf); 0.50% owf Sodyeco Penetrant EH Mod Liquid; 8.00% owf Sodyefide B liquid and 20.00 g/L sodium sulfate.
Figure 2. Dyeing method for Diresul dyes. A: Fabric (20 g); Dye (15 or 20% owf); 0.50% owf Sodyeco Penetrant EH Mod Liquid, 8.00 g/L sodium carbonate, 2.00 g/L caustic, 12.00 g/L Sandozol Reducer RDT liquid and 20.00 g/L sodium sulfate.
to-liquor ratio of 1:100. All dyed fabrics after oxidation were washed-off at 82 °C for 10 min with 1.0% owf Sandopure RSK Liquid at a fabric-to-liquor ratio of 1:100, and rinsed thoroughly and dried. Sodyeco Penetrant EH Mod Liquid (Figure 1-2), Sodyefide B liquid (Figure 1), Sandozol Reducer RDT liquid (Figure 2), Clariant Oxidizer B Liquid, Clariant Oxidizer A Powder, and Sandopure RSK Liquid are the commercial additives of Clarient. The dyeing, oxidation, and washed-off conditions were recommended by Clarient. Lanthanum Triacetate Treatment. Lanthanum triacetate was used for this research, although other lanthanoid salts have the similar effects. The lanthanum triacetate treatment conditions used thereafter were the optimized conditions. The conditions were decided based on the balance of the properties of resistance of sulfur dyed fabrics to oxidation and aging, and the amount of lanthanum triacetate required to obtain such properties. An aqueous solution of 1.0% (w/w) of lanthanum triacetate was padded onto the dyed fabrics through two dips and two nips to reach a wet pickup of 100 ( 5% using an EVAC padding machine (L. & W. Machine Works). The padded fabrics were then dried at 90 °C for 3 min by a Flash unit air heater (Lawson Screen Products, Inc.). Sodium Carbonate Treatment. An aqueous solution of 1.0% (w/w) sodium carbonate was padded onto the dyed fabrics through two dips and two nips to reach a wet pickup of 100 ( 5% using an EVAC padding machine (L. & W. Machine Works). The padded fabrics were then dried at 90 °C for 3 min by a Flash unit air heater (Lawson Screen Products, Inc.). Property Evaluation. Aging of the dyed fabrics was evaluated according to Alternate Oven Test of AATCC Test Method 26-1994 (Aging of sulfur-dyed textiles: Accelerated). Two cycles of aging were performed in this study in order to better evaluate the property improvements. The warp yarn strengths were used to evaluate the strength changes of the dyed fabrics before and after aging. The tensile tests were carried out on the Instron 4444 (Instron single column materials testing system) with a gage length of 50 mm at a rate of 60 mm/min. The strength reported in the paper was an average of 30 measurements. The CIE 1976 L*a*b*(CIELab) formula was used to determine the total color difference, ∆E(Lab)* (∆E ) ((∆L*)2 + (∆a*)2 + (∆b*)2)1/2), using an UltraScan XE spectrocolorimeter of Hunter Associates Lab, Inc., using a D65 illuminant, 1 in. viewing area, and 10° observer. The accelerated LaunderOmeter washing method, described as LOW, was operated according to AATCC Test Method 190-2002 (Colorfastness to home and commercial laundering: Accelerated), which is equivalent to at least 10 home laundering cycles, for testing colorfastness to home laundering with detergent containing activated oxidative bleach. Both Standard detergent and NOBS detergent were used, in order to compare the effects of the oxidative bleacher in the detergent on color changes after laundering. The AATCC Evaluation Procedure 1, a standard
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Figure 3. Color changes of cotton fabrics after one LOW cycle using different detergents. The fabrics were dyed with 10% (owf) SR, 10% (owf) SN, or 15% (owf) DB.
assessment method described in 2002 AATCC Technical Manual, USA, was used to evaluate the gray scale rating for color changes. The color staining during laundering was examined according to AATCC Test Method 61-1996 (Test conditions 2A) using both the AATCC Standard detergent and NOBS detergents. The staining was rated according to AATCC Evaluation Procedure 2, also a standard assessment method described in 2002 AATCC Technical Manual, USA. The standard AATCC test method (61-2A) was used to evaluate whether our treatment affects the colorfastness and staining of the dyed fabrics under standard conditions.
Figure 4. Effect of lanthanum triacetate (1%, w/w) on resistance of sulfur dyed cotton fabrics to oxidation after one LOW cycle using NOBS detergent. The fabrics were dyed with 15% (owf) SR, SN, SG, DN, or DY or 20% (owf) DB or SB. Table 2. Comparison of K/S Values of Sulfur Dyed Fabrics (15% owf: SR, SN, SG, DN, and DY; 20% owf: DB and SB) with and without Lanthanum Triacetate Treatment (1.0%), before and after One LOW Cycle with NOBS Detergenta before laundering after laundering
dyes
detergent
treatment
K/S
K/S retention, K/S %
SG
standard detergent
control La(Ac)3 control La(Ac)3 control La(Ac)3 control La(Ac)3 control La(Ac)3 control La(Ac)3 control La(Ac)3 control La(Ac)3 control La(Ac)3 control La(Ac)3 control La(Ac)3 control La(Ac)3 control La(Ac)3 control La(Ac)3
6.6 6.4 6.3 5.9 21.6 20.9 21.3 21.6 10.5 9.9 8.5 8.3 11.7 11.2 11.2 11.6 9.5 10.1 9.6 9.2 25.4 15.8 15.5 15.6 23.3 23.9 23.1 23.2
6.0 6.4 2.3 4.8 20.9 20.7 20.2 21.5 9.7 9.8 6.8 7.7 11.1 11.0 7.8 10.8 9.0 10.0 6.9 9.0 24.7 15.7 14.6 15.3 22.8 23.6 22.1 22.6
NOBS detergent
Results and Discussion Effect on Antioxidation. Color changes of sulfur dyed fabrics after one LOW cycle laundering using detergents with and without activated bleacher are illustrated in Figure 3. As expected, the detergent with a bleacher (NOBS detergent) caused substantially more color changes (higher ∆E value) than that without a bleacher (Standard detergent) or without detergent, i.e., washed in water. Color changes (∆E value) due to the activated oxidative bleacher in the detergent were substantial for both SR and SN, indicating extremely poor resistance of sulfur dyed fabrics to oxidation. Although the ∆E values of the fabric dyed with DB and washed with NOBS detergent was only 0.9, it still had an increase of 50% and 80% compared to the launderings with water and with Standard detergent, respectively. Usually, when ∆E(Lab)* is larger than 1.0, a visual difference is easily observed. Some researchers consider a ∆E > 0.5 to be noticeable. For all three sulfur dyes presented, color changes after washing with water and Standard detergent was less than or equal to 1.0, indicating good washfastness of sulfur dyes if bleaching agents were not presented. The effect of lanthanum triacetate on resistance of sulfur dyed fabrics to laundering using detergent with an activated bleacher is depicted in Figure 4, in which the “control” means all but lanthanum triacetate. The comparison of ∆E values between fabrics with and without lanthanum triacetate treatment for all seven dyes demonstrated that the treatment with lanthanum triacetate did provide notable protections to sulfur dyed fabrics against activated oxidative bleaching. Without the treatment of lanthanum triacetate, most of the dyes investigated lost color enormously after just one LOW cycle of laundering with NOBS detergent except the blacks. The relatively less color loss of the blacks was probably because of the much higher shade depth of the blacks (Table 2) than the other colors. However, the percent protection against color change of the blacks with lanthanum triacetate treatment was still notable and comparable to the other dyes as depicted in Figure 5. Although the absolute
SB
standard detergent NOBS detergent
SN
standard detergent NOBS detergent
SR
standard detergent NOBS detergent
DN
standard detergent NOBS detergent
DY
standard detergent NOBS detergent
DB
standard detergent NOBS detergent
90.9 100 36.7 81.8 96.8 99.0 94.5 99.3 92.3 99.0 80.5 91.7 94.9 98.2 69.5 92.7 94.7 99.0 71.7 97.3 97.2 99.4 94.7 98.3 97.9 98.7 95.9 97.3
a *K/S retention ) (K/S value after laundering)/(K/S value before laundering) × 100%.
reductions in color change differed considerably from dye to dye (Figure 4), the percent reductions in color change for most of the dyes were at about 50% (Figure 5), indicating that lanthanum ion was similarly effective to all the dyes examined. The reduction in color changes of the fabrics treated with lanthanum triacetate could also be depicted by K/S values of various treatments as summarized in Table 2. Of the 7 dyes examined, SG showed the greatest K/S reduction after washing with NOBS detergent, and the improvement in K/S retention of SG after lanthanum triacetate treatment was also the best. Since K/S values are more sensitive to color changes in fabrics with low shade depths rather than with high shade depths, % K/S retention shown in Table 2 were not expected to be linear
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Table 4. Effect of Lanthanum Triacetate on Staining of Sulfur Dyes in Laundering (15% owf: SR, SN, SG, DN and DY; 20% owf: DB and SB) standard detergent dyes SG
SB
Figure 5. Effect of lanthanum triacetate (1%, w/w) on reduction in color changes of sulfur dyed cotton fabrics after one LOW cycle using NOBS detergent. The fabrics were dyed with 15% (owf) SR, SN, SG, DN, or DY or 20% (owf) DB or SB. *% reduction in color change ) [∆E(control fabric) - ∆E(LaAc3 treated fabric)/∆E(control fabric)] × 100. Table 3. Effect of Lanthanum Triacetate (1%, w/w) on Color Changes of Sulfur Dyed Fabrics (15% owf: SR, SN, SG, DN and DY; 20% owf: DB and SB) dyes
SG
SB
SN
SR
DY
DN
DB
*K/S value before treatment *K/S values after treatment ∆E gray scale rating
6.4
21.8
8.6
11.6
15.5
9.5
23.3
6.3
21.5
8.3
11.4
15.6
9.3
23.2
0.6 4-5
0.4 4-5
0.8 4-5
0.6 4-5
0.6 4-5
0.5 4-5
0.4 4-5
SN
SR
DY
* The K/S values were tested using the one fabric that was tested the K/S value before and after La(Ac)3 treatment, respectively.
to % reduction in color changes illustrated in Figure 5. However, data in Table 2 indicate that lanthanum triacetate treatment could substantially increase the K/S retention of the fabric after laundering with detergents containing oxidative bleachers. One concern for applying chemicals onto a fabric after dyeing is their impacts on color changes of the fabric. Color changes due to the use of lanthanum triacetate are presented in Table 3. Both the gray scale ratings and ∆E values indicated that only slight color changes were observed due to the application of lanthanum triacetate. These color changes should be easily manageable in the dyehouse during shade matching. The effect of lanthanum triacetate on staining of sulfur dyes onto other textiles during home laundering was investigated, and the results are summarized in Table 4. As expected, most of the staining was on cotton and nylon, followed by wool, acrylic, polyester and acetate. Comparing the stains from the fabrics with and without lanthanum triacetate treatment, 14 out of 42 cases after launderings with Standard detergent and 15 out of 42 cases after launderings with NOBS detergent showed at least 0.5 class improvement in staining resistance when lanthanum triacetate was used. Only 2 out of the 84 cases showed 0.5 class increase in staining. These results suggested that lanthanum triacetate could also decrease the staining of sulfur dyes onto other fabrics during laundering. Comparing the differences in staining of sulfur dyes to other textiles during launderings with Standard detergent and NOBS detergent, it could be found that NOBS detergent caused more staining than Standard detergent. There were 19 out of the 84 cases showed at least a 0.5-class increase in staining using NOBS detergent compared to the staining using Standard detergent, whereas only 5 cases showed the opposite. These results could be explained by the oxidation of sulfur dyes in laundering with the existence of bleachers. Staining is
DN
DB
staining fabrics worsted wool polyacrylic polyester polyamide bleached cotton diacetate worsted wool polyacrylic polyester polyamide bleached cotton diacetate worsted wool polyacrylic polyester polyamide bleached cotton diacetate worsted wool polyacrylic polyester polyamide bleached cotton diacetate worsted wool polyacrylic polyester polyamide bleached cotton diacetate worsted wool polyacrylic polyester polyamide bleached cotton diacetate worsted wool polyacrylic polyester polyamide bleached cotton diacetate
NOBS detergent
control
1.0% lanthanum triacetate
control
1.0% lanthanum triacetate
4-5 4-5 4-5 4-5 4-5 5 4-5 5 5 5 4-5 5 4-5 4-5 5 3 3-4 4 4-5 4 4 4 2-3 4-5 4-5 4-5 5 4 4 4-5 4-5 4-5 4-5 3-4 4 4-5 4-5 4-5 5 4-5 4-5 5
4-5 4-5 4-5 4-5 5 5 4-5 5 5 4-5 4-5 5 4-5 4-5 5 3-4 4 4 4-5 5 5 4 3-4 5 4-5 5 5 4 4-5 5 4-5 4-5 5 3-4 4-5 4-5 4-5 5 5 4-5 4-5 5
4-5 4-5 4-5 4 4 5 4-5 4-5 5 4-5 4 4-5 4-5 4-5 5 3 3 4 4-5 4-5 4 3-4 2-3 4-5 4-5 5 5 4 4 5 4-5 4-5 4-5 3 3-4 4 4-5 4-5 5 4-5 4 5
4-5 4-5 4-5 4 4 5 4-5 5 5 4-5 4 5 4-5 4-5 5 3-4 3-4 4 4-5 4-5 5 4 3-4 4-5 5 5 4-5 4 4-5 5 4-5 4-5 5 3-4 3-4 4-5 4-5 5 5 5 4 5
an indication of color removal from the original fabric and transfer to other textiles during laundering. As will be discussed later, lanthanum ion forms chemical linkages, probably chelating bonds, with sulfur dyes. The chemical linkages between lanthanum ions and sulfur dyes improve the substantivity of the dyes on the fabric, therefore, decrease the desorption of the dyes into the laundering bath and thus decrease the staining of the dyes onto other fabrics. Bleachers in the detergent cause oxidation of the sulfur linkages of the dyes and result in the cleavages of dye molecules. Such cleavages increase the solubility of sulfur dyes and result in more colorants in the bath and more staining onto the multifiber strips. Lanthanum ions might be capable of partial transfer of electrons away from sulfur linkages and, therefore, decreasing the oxidation and cleavage of the sulfur linkages in sulfur dyes. This effect of lanthanum ions result in less dissolution of the dye in the bath and thus less staining of the dyes onto the multifiber strips. The possible oxidation mechanism of sulfur dyes and the prevention of the oxidation by lanthanum ion will be discussed further in a later section. The durability of the treatment of lanthanum triacetate was also evaluated using the accelerated LOW method, in which one cycle was equivalent to at least 10 home launderings according to AATCC Test Method 190-2002. Fabrics dyed with SN and DB and treated with or without 1.0% lanthanum triacetate have been subjected to 1 to 3 LOW cycles of
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Figure 6. Durability of bleaching resistance of lanthanum triacetate treatment (1%, w/w) to repeated LOW launderings using NOBS detergent. The fabrics were dyed with 15% (owf) SN or 20% (owf) DB.
Figure 7. Effect of lanthanum triacetate (1%, w/w) on strength retention of sulfur dyed cotton fabrics (15% owf SN or 20% owf DB) after aging of the fabrics with and without one LOW cycle with NOBS detergent.
launderings, and their color changes are depicted as a function of washing cycles in Figure 6. As expected, color changes of the fabrics increased with increasing the numbers of LOW cycles. However, even after 3 LOW cycles of laundering, fabrics treated with lanthanum triacetate still showed substantially less color changes than the controls for both dyes. A 54% reduction in color change was still observed after 3 cycles of laundering, equivalent to 30 or more home launderings, for the fabric dyed with DB and treated with lanthanum triacetate. This reduction was very similar to the 47% reduction of color changes of the fabrics treated with lanthanum triacetate compared to the fabric without lanthanum triacetate after 1 LOW laundering. The effect of lanthanum triacetate on color retention of fabrics dyed with SN was weakened to a certain extent after three cycles of launderings, from 50% to 32% in comparison to the fabrics without lanthanum triacetate treatment. The comparison of color changes between fabrics with and without the treatment of lanthanum triacetate after repeated LOW cycles in Figure 6 demonstrated that the lanthanum triacetate treatment was durable to laundering. Aging Resistance. The aging resistance of sulfur dyed fabrics treated with and without La(Ac)3 were investigated. The aging performances were different from dye to dye. Sulfur blacks had the most substantial aging followed by SN. Presented in Figure 7 are the strength retentions of fabrics dyed with DB and SN and treated with 1.0% lanthanum triacetate after aging. The effect of laundering with the NOBS detergent on strength retention after aging is also presented. As shown, fabrics dyed with the sulfur black (DB) had more strength loss than that with
the sulfur blue (SN). However, fabrics dyed with both dyes and treated with lanthanum triacetate all showed an improvement in strength retention after aging. The sulfur black (DB), which had much more strength loss after aging than the blue (SN), had almost 100% improvement in strength retention after being treated with lanthanum triacetate. The fabrics dyed with SN and treated with lanthanum triacetate had close to 100% retention of the strength after aging. Even after the fabric was laundered (1 LOW cycle), the effect of lanthanum triacetate against aging was still substantial, indicating excellent laundering durability of lanthanum triacetate. Comparing strength loss of fabrics due to aging before and after laundering, it was obvious that laundering decreased sulfur aging as shown in Figure 7. This was probably due to the fact that laundering removed surface dyes, which were believed to contribute to aging more than the dyes having good interactions with the fibers.3 Laundering in the alkaline conditions also decreased the total dye concentration on fabrics and neutralized the strong acid generated from the sulfur dyes, and therefore, decreased aging caused by sulfur dyes. As shown in Figure 7, a treatment with1.0% sodium carbonate, the most common treatment to reduce aging in manufacturing, did give fabric significant aging resistance. However, after the fabric was subjected to 1 LOW laundering, tensile strength retention of sodium carbonate treated fabric was decreased from 79.7% to 55.7% after aging treatment of the DB dyed fabric. When the same fabric was treated with 1.0% lanthanum triacetate, its tensile strength retention has a slight increase from 74.3% to 76.1% after the fabric was subjected to 1 LOW laundering. These results further indicate that lanthanum triacetate treatment is durable, while sodium carbonate treatment is not. Possible Functions of Lanthanum Triacetate. It is broadly accepted that sulfur linkages (-Sn-) exist in sulfur dyes with the numbers of sulfur atoms between 2 and 7, depending on the manufacturing conditions.1,2,20,21 Therefore, the general structure of sulfur dyes can be expressed as D - Sn - D
(Scheme 1)
where 2 e n e 7 and D represents a chromophore. The divalent sulfur could be easily oxidized.11,22 The oxidized sulfur linkages are complicated because each of the sulfur atoms in the sulfur linkages is possible to be oxidized and the structures of oxidized sulfur linkages vary. The most common examples are those oxidized from a sulfur atom which is linked to the chromophore directly, such as formulas (a) and (b) in Scheme 2.11 The oxidized sulfur can be further hydrolyzed to form sulfur containing acids as shown in Scheme 3.11 These acids catalyze the hydrolysis of cellulose molecules, reduce the strength of Scheme 2
Scheme 3
Ind. Eng. Chem. Res., Vol. 49, No. 10, 2010 Scheme 4
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Division, funds through the Hatch Act and USDA Multi-State Research Project 1026, and funds from the Key Laboratory of Advanced Textile Materials and Manufacturing Technology (Zhejiang Sci-Tech University), Ministry of education, China (2009001) are gratefully acknowledged. The financial sponsors do not endorse the views expressed in this article. Literature Cited
the fabrics during storage especially in high temperature and humidity, and result in aging of fabrics. When an oxidative bleacher is present in the detergent, oxidation and hydrolysis occur during laundering, but the released hydrogen ions are diluted and neutralized in the alkaline laundering conditions, therefore, aging will not occur during the laundering. However, oxidation and hydrolysis of sulfur linkages result in disconnection of chromophores of the sulfur dye structure. The disconnected chromophores will be washed off causing color fading of the fabrics. Therefore, oxidation is the crucial step for both color fading and aging of fabrics dyed with sulfur dyes. Lanthanum triacetate treatment provides sulfur dyed cotton fabrics both the oxidation and aging resistances via its protection of the sulfur linkages from being oxidized. This protection of the sulfur linkages is probable achieved from the formation of stable chemical linkages between lanthanum triacetate and sulfur. As reported, sulfur atoms could form various complexes with lanthanoid metals such as lanthanum.23,24 In such complexes, sulfur atoms serve as the coordination to the metal.23 When lanthanum triacetate was applied to the sulfur dyed cotton fabrics, the lanthanum ion probably forms the chemical complex with sulfur atoms in the dyes. A lanthanum ion could form coordinate bonds with sulfur atoms from multiple dyes or a single dye as depicted in Scheme 4a,b. The coordinate bonds thus formed are capable of decreasing the electron density of the sulfur linkages in the dye, thus decreasing the oxidized capability of the sulfur dyes. The chemical structure of lanthanum triacetate also may provide steric hindrance which prevents the close contact between sulfur linkages and oxidizing agents, and decreases the oxidation. Conclusions Lanthanum triacetate applied to sulfur dyed fabrics through a common pad-dry process improved the resistance of sulfur dyed fabrics to oxidation and aging. Fabrics treated with lanthanum triacetate had up to 70% reduction in color changes after being washed with a detergent containing activated oxidant. This treatment could also improve strength retentions up to 100% after aging. The treatment had good durability to repeated launderings. The color changes due to the addition of lanthanum triacetate were minimal. The lanthanum triacetate treatment could also reduce the staining of sulfur dyes onto other fabrics during laundering. Lanthanum triacetate is probably chemically linked onto sulfur dyes to attract the electrons away from sulfur atoms and to provide steric hindrance to sulfur linkages, and thus protected sulfur linkages from being attacked by oxidants, consequently improved the resistance of sulfur dyed fabrics to oxidation and aging. Acknowledgment Financial support from The Procter & Gamble Company and from the University of NebraskasLincoln Agricultural Research
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ReceiVed for reView December 2, 2009 ReVised manuscript receiVed February 21, 2010 Accepted March 9, 2010 IE901901R
