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Dyeing Polyester Fabrics with Indigo KATSUSHI KUNTTOU,1 SETSUAKI HONGYO,

AND

SHINGO MAEDA,

Industrial Technology Center of Okayama Prefecture, Okayama 701-1296, Japan

KENJI MISHIMA Department of Chemical Engineering, Faculty of Engineering, Fukuoka University, Jonan-ku, Fukuoka 814-0180, Japan ABSTRACT Polyester fabrics are successfully dyed with indigo dye by controlling the ratio of sodium hydrosulfite and sodium hydroxide concentration in the dyebath solution, because non-ionic reduced indigo has a higher affinity for polyester fabrics than ionic reduced indigo. The value of total K/S increases with increased dyeing temperature. The color difference of indigo dye in the polyester and cotton fabrics is discussed interms of a reflection spectroscopic analysis. The difference in the maximum wavelength of indigo color for polyester and cotton fabrics is about 55 nm. Rub fastness on polyester dyed with indigo is superior to that on cotton.

Classified as a historical dye, indigo has retained its importance up to the present day. From practical indigodyed clothes such as aizome to modern fashionable jeans, indigo has always had a wide field of application [2, 12, 14]. Moreover, the chronic and acute toxicity of indigo is very low [5]. Indigo, which is a vat dye, has an affinity for cotton, wool, and silk fabrics in its leuco form [10], but it has a low affinity for synthetic fabrics such as polyester. Therefore, few dyeing methods of indigo on polyester fabrics have been reported. Hongyo et al. reported dyeing several cotton garments with indigo dyes and described the difficulty of dyeing synthetic fabrics such as polyester with the cotton dyeing method [6 –9]. Moreover, even at high temperatures (up to 160°C) polyester did not dye well with the cotton dyeing method. Several investigators attempted to dye polyester fabrics with indigo or several vat dyes to seek a part of the large market share [3, 6 –9, 15]. Sugawara examined vapor phase dyeing on polyester fabrics [15], which requires very high temperatures— up to 220 –250 °C—almost the limit of polyester’s melting point. Chevli and Lewis proposed a leuco vat acid method, in which organic acids such as citric acid were added to bring the pH below 7 [3]. However, this method required additions of organic acids, which increased the cost of treating a huge amount of wastewater.

1 Corresponding author: phone ⫹81-(0)86-286-9600, fax ⫹81(0)86-286-9632, email: [email protected]

Textile Res. J. 75(2), 149 –153 (2005)

In this paper, we demonstrate the possibility of dyeing polyester fabrics with indigo by controlling the ratio of sodium hydrosulfite and sodium hydroxide concentration in the dyebath solution at a mild temperature. We also compare dyeing performance for color for the polyester and cotton fabrics using a reflection spectroscopic analysis.

Experimental Polyester (70 g/m2; number of threads in the warp 210/5cm and in the weft 191/5 cm, warp 8.3 tex, weft 8.3 tex) and cotton (100 g/m2, number of threads in warp, 141/5 cm and in the weft 135/5 cm, warp 20 tex, weft 16 tex) described in Japanese Industrial Standards (JIS) L 0803 were used as the samples. The mousseline knit polyester (Tetron mousseline, warp 20 S, weft 20 S) purchased from Nakao Filter Co. Ltd. was also used to check the effect of dyeing factors, such as the temperature and concentration of the dyebath solution. Indigo, shown in Figure 1, was provided as a test sample from Mitsui BASF Co. Ltd., and was used without further purification. DYEING PROCEDURES

FOR

POLYESTER

AND

COTTON

Indigo dye was reduced in aqueous solutions of 3–10 g/L sodium hydrosulfite (Na2S2O4) and 0.5–2.5 g/L sodium hydroxide (NaOH) at 80, 100, and 120°C for 30 minutes. The bath ratio was 40:1 and the dye concentration was 0.05–3 % owf. The typical dyeing procedure for 0040-5175/$15.00

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TEXTILE RESEARCH JOURNAL against the appropriate gray scale. The data for cotton fabrics were cited in our previous paper [9].

Results and Discussion

FIGURE 1. Structure of indigo (oxidized form).

polyester was 8g/L sodium hydrosulfite, 1g/L sodium hydroxide, and 1 % owf indigo at 120°C for 30 minutes. The dyeing machine was a Mini-colour, manufactured by Texam Giken Co. Ltd. After dyeing, the reduced indigo in the polyester fabrics was air-oxidized at 100°C for over 10 minutes. Indigo dye was reduced in aqueous solutions of 4 g/L sodium hydrosulfite (Na2S2O4), 2.88 g/L sodium hydroxide (NaOH), and 30 g/L sodium sulfate (Na2SO4) at 50°C. Cotton fabrics were put into the reduced indigo solution where the bath ratio was 50:1 and the dye concentration was 0.05– 0.2 % owf. The cotton fabrics were dyed at 50°C for 30 minutes in the Mini-colour. After dyeing, the reduced indigo in the cotton was airoxidized at room temperature for over 10 minutes, then washed with an aqueous solution of 5 g/L anionic soaping agent (Marcel soap, Kao Co. Ltd.) at 80°C for 10 minutes.

The effect of sodium hydrosulfite concentration on polyester fabrics dyed with indigo in a dyebath solution including sodium hydrosulfite and sodium hydroxide at 120°C for 30 minutes is shown in Figure 2. At a constant concentration of 1g/L sodium hydroxide, the total K/S value increased with an increasing concentration of sodium hydrosulfite. This increase almost saturated over the concentration of 8 g/L sodium hydrosulfite. The effect of the sodium hydroxide concentration on polyester fabrics dyed with indigo is shown in Figure 3. The total K/S value decreased with increasing concentration of sodium hydroxide over the range of 0.5 g/L. The effect of the sodium hydroxide concentration was not enough at 0.2 g/L sodium hydroxide. Concentrations of sodium hydrosulfite and sodium hydroxide were important for indigo dyeing of polyester fabrics. These phenomena are explained by the molecular structure and reduced form of indigo dye, described below.

COLOR MEASUREMENTS Kubelka-Munk values (K/S) and the color strength of the dyed samples were spectrophotometrically measured. The color yields of each sample were determined by measuring the total K/S (␭ ⫽ 380 to 720 nm in 10 nm steps) by a spectrophotometer (Color 7, Kurabo Co. Ltd.). The reflection spectra (␭ ⫽ 500 to 800 nm) of the indigo-dyed polyester and cotton fabrics were measured by a reflection spectrophotometer (UV3100PC, Shimadzu Co. Ltd.). COLOR FASTNESS TESTS Fastness was tested on the polyester fabrics for rubbing, washing, and light. Test samples for polyester fabrics were dyed in 3% owf. Dry and wet rub fastness data were obtained with JIS L 0849, which is based on ISO 105-X12:1993; wash fastness data were obtained with JIS L 0844, which based on ISO 105-C02:1989; and light fastness data were obtained with JIS L 0842:1996. Test samples for light fastness were exposed to a carbon arc light at 63°C for 20 hours, using a Suga UV fade meter. Color changes in the samples were assessed

FIGURE 2. Effect of sodium hydrosulfite (Na2S2O4) concentration for dyeing polyester fabrics at 120°C (1g/L sodium hydroxide (NaOH) concentration 1%owf, indigo, 40:1 bath ratio, 30 minutes dyeing time).

The typical forms of reduced indigo dye are illustrated in Figure 4. Indigo (Figure 1, oxidized form) is normally insoluble in water, alkaline, or acid solutions and the oxidized form has no affinity for the cotton fabrics. Sodium hydrosulfite can reduce indigo and produce the three different forms of reduced indigo by carefully controlling the dyebath pH [4]. When sodium hydroxide exists, the di-sodium arylenolate form (Figure 4a, di-

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FIGURE 3. Effect of sodium hydroxide (NaOH) concentration for dyeing polyester fabrics at 120°C. (8g/L sodium hydrosulfite (Na2S2O4), 1%owf indigo, 40:1 bath ratio, 30 minutes dyeing time).

ionic form) and the mono-sodium arylenolate form (Figure 4b, mono-ionic form), which are water-soluble, are present in the dyebath. The nonsodium arylenolate form (Figure 4c, non-ionic form) is present in the low pH dyebath. The first and second ionization steps of the reduced form of indigo have already been measured using spectrophotonic technology [1]. The ionic forms, which are more hydrophilic than the non-ionic form, have a high affinity for cellulose fabrics such as cotton [4]. However, the ionic forms have a low affinity for polyester fabrics, which are more hydrophobic than cellulose fabrics [11]. The hydrophilic cotton fabrics could be dyed with indigo dissolved in an aqueous solution including a small amount of sodium hydrosulfite and sodium hydroxide. The polyester fabrics could not be dyed well with the ionic forms under the same conditions as shown in Table I. When the smaller amount of sodium hydroxide was added, the pH value, which was measured after dyeing, inside the range neutral to acid. The excess amount of sodium hydrosulfite (for example 8 g/L) and a smaller amount of sodium hydroxide (for example 0.5 g/L) decreased the amount of ionic forms and increased the amount of the non-ionic form. The non-ionic form should have a higher affinity for polyester fabrics than the ionic forms [3], so the polyester fabrics were dyed with indigo using the aqueous solution including the excess amount of sodium hydrosulfite and the smaller amount of sodium hydroxide. The existence of the indigo dye in polyester fabrics was confirmed by sectional observation of the polyester fiber with a microscope. In normal cotton dyeing, the fabric is often ring-dyed with indigo. With polyester, however, the fabric is dyed into the center of the fiber structure. This polyester dyeing

FIGURE 4. Structures of reduced indigo: (a) di-ionic form, (b) mono-ionic form, (c) non-ionic form. TABLE I. Influence of sodium hydroxide (NaOH) concentration on pH value for dyeing polyester fabrics at 120°C (Indigo, 1%owf, bath ratio, 40:1, dyeing time, 30 minutes). Na2S2O4 , g/L

NaOH, g/L

pHa

Total K/S

Form of reduced indigob

8 8 8

0.2 1 2.5

5.45 7.01 11.70

56.4 83.8 4.9

non-ionic non-ionic ionic

a Measured after dyeing process. value from Etters’ paper [4].

b

Assumed structure by pH

method is also expected to be useful for dyeing polyester/cotton blend fabrics in one batch, because cotton fabrics are dyed in the aqueous solution with the sodium hydrosulfite and sodium hydroxide. The effect of the dyebath temperature on indigo dyeing of polyester fabrics is shown in Figure 5. The total K/S value increased with increased dyeing temperature. We considered that at a higher temperature, the solubility of the indigo dye in the aqueous solution increased, and the amorphous regions of the polyester fabrics expanded, so the polyesters were dyed with indigo. Dye molecules cannot penetrate the highly ordered crystalline regions of polyester; their adsorption can only take place in the amorphous regions, which are not highly ordered [13]. Because of the extension in the amorphous region of polyester, a high temperature over 100°C should be necessary, even for disperse dyes. This temperature effect is consistent with the result of dyeing polyester with disperse dyes.

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FIGURE 5. Effect of dyeing temperature for polyester fabrics (8g/L sodium hydrosulfite (Na2S2O4), 1g/L sodium hydroxide (NaOH) 2%owf indigo 40:1 bath ratio, 30 minutes dyeing time).

The polyester fabrics had richer blue color than the cotton fabrics when they were dyed with the indigo. We investigated the color difference between cotton and polyester using reflection spectroscopic analysis with the visible spectra (500 – 800 nm), as shown in Figure 6. The maximum wavelength of the indigo blue color in the cotton fabrics was about 660 nm and that in polyester was about 605 nm. The reflection spectra of the indigo dye in the polyester fabrics apparently have characteristics different from those in the cottons. Within a concentration ranging between 0.05 and 0.2% owf, dye concentration has no effect on the maximum wavelength of both fabrics. It was very difficult to get the maximum wavelength at high owf, such as 1% owf, because of the too-high signal. The difference of the maximum wavelength of the indigo color for polyester and cotton fabrics was about 55 nm. This small difference in the spectra was not caused by the molecular structure, such as the reduced and oxidized forms. The association of the indigo molecules appears to cause this difference in the spectra. Weinstein and Wyman reported that the maximum wavelength of indigo dye varied with the association of indigo molecules, which are held together by intermolecular hydrogen bonds [16]. For the oxidized form of indigo, the associated molecules have a bathochromic shift of reflection spectra from the spectra of isolated indigo molecules. Association of indigo molecules requires space in the fabrics. Cotton fabrics have larger amorphous regions than polyester [11]. The reflection spectra of polyester dyed with indigo were similar to the spectra of indigo dyes dissolved in dimethylforamide. These spectra indicate the isolation of each indigo dye

TEXTILE RESEARCH JOURNAL

FIGURE 6. Visible spectra of indigo in cotton and polyester fabrics.

molecule, and there was no association of indigo molecules on polyester fabrics. The reflection spectra of the cotton fabrics dyed with the indigo were almost the same as the spectra of the solid state of the indigo dyes. The dyeing fastness properties for polyester and cotton fabrics dyed in 3% owf with indigo are shown in Table II. Dry and wet rub fastness for polyester was superior to that for cotton. On the other hand, light fastness for polyester was inferior to that for cotton. Wash fastness was the same for both fabrics. TABLE II. Fastness properties of indigo-dyed polyester fabrics. Washb

Ruba

Polyester Cottond a

Dry Wet

Staining of adjacent white

3–4 3

4 (cotton) 4 (cotton)

JIS L0849.

3–4 1–2 b

JIS L0844.

Shade change Lightc

4–5 (polyester) 4 (wool) c

JIS L0842.

5 4–5 d

3 ⬎4

Reference 9.

Conclusions Dyeing polyester fabrics with indigo dyes is successful under certain conditions, which coordinate the rate of sodium hydrosulfite and sodium hydroxide. When the sodium hydrosulfite concentration is 8 g/L, the optimum dyeing concentration of sodium hydroxide is 0.5–1.0 g/L in the dyebath solution at 120°C. It seems that the role of non-ionic reduced indigo formation is important in this dyeing method. The dyeing temperature is effective, and well-dyed polyester fabrics are obtained at 120°C. The maximum wavelength of polyester fabrics dyed with indigo is clearly different from that of cotton fabrics.

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Polyester has a richer blue color than cotton when they are both dyed with the indigo. The rub fastness of polyester dyed with indigo is superior to that of cotton.

9.

Literature Cited

10.

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thetic Fibers with Vat Dyes, Senshoku Kogyo 48, 3– 8 (2000). Hongyo, S., and Moriwaki, H., Dyeing of Cotton Garments and Synthetic Fibers with Indigo Dyes, Senshoku Kogyo 47, 55– 62 (1999). Koshida, H., and Nakayama, T., New Application Technologies of Vat Dye (5), Senshoku Kogyo 35, 199 –225 (1987). Kuroki, N., “Senshokurironkagaku,” 1st ed., Makisyoten, Japan, 1966, pp. 41– 42. Parmar, M. S., Satsangi, S. S., and Prakash, J., Indigo Dyed Warp Yarn—An Essential Requirement for Manufacturing Denim, Colourage 46, 45–50 (1996). Remington, W. R., Principal of Dyeing Dacron Polyester Fiber, Am. Dyest. Rep. 41, 859 – 861 (1952). Sakagawa, T., Koshida, H., and Nakayama, T., Proposal for New Application Methods of Indigo to Dye Fashionable and Sensuous Beauty (1), Senshoku Kogyo 39, 210 – 220 (1991). Sugawara, H., Dyeing Polyester Fiber Products with Indigo Dyes with High Color Depth and Brightness, Japanese patent 09132879, 1997. Weinstein, J., and Wyman, G. M., Spectroscopic Studies on Dyes, I: The Association of Indigo Dyes in the Solid Phase, J. Am. Chem. Soc. 78, 2387–2390 (1956). Manuscript received September 23, 2003; accepted January 5, 2004.