One-Bath Dyeing of Polyester/Cotton Blends with Reactive Disperse Dyes in Supercritical Carbon Dioxide
One-bath dyeing of polyester/cotton blends with reactive disperse dyes is investigated using supercritical carbon dioxide (SC-CO2) as a solvent in the range of 353 to 393 K and 10 to 20 MPa. One-bath dyeing of polyester/cotton blends is successful. The dyeing behavior is compared with the thermosol dyeing method using the same dye. Samples are subjected to a color fastness test and colorimetric measurements. Good color intensity and wash fastness are obtained by the SC-CO2 dyeing method at 393 K and 20 MPa. The color fastness properties of fabrics dyed in SC-CO2 are superior to those of fabrics dyed by the thermosol dyeing method.
Recently, supercritical fluids, especially supercritical carbon dioxide (SC-CO2), as dyeing media have been investigated extensively because of environmental problems and increasing costs of water in the normal dyeing processes where a large amount of waste water needs to be treated [1-5, 8, 11, 31, 33-36, 44, 45]. CO2 is expected to be useful on an industrial scale due to its characteristics: it is inexpensive, essentially nontoxic, nonflammable, and easily accessible in critical conditions, i.e., T^sub c^ = 31°C and Pc = 7.37 MPa. It has also been used as an environmentally benign solvent substitute for hydrocarbons, chlorofluorocarbons, and other organics in other fields [6, 9, 10, 12, 13, 22-29, 46].
The hydrophobicity of CO2 is useful for dyeing polyester fibers and fabrics with disperse dyes because such hydrophobic dyes can dissolve in SC-CO2 and penetrate the hydrophobic polyester fibers [14-16, 31, 33-35, 43]. Similarly, Bach et al. extended SC-CO2 dyeing to polyolefin [1, 2]. Guzel et al. dyed wool fibers with mordant dyes dissolved in SC-CO2 . Accordingly, the solubility data of dyes in SC-CO2 has been researched considerably as basic important information [3, 7, 10, 17, 19, 32, 40, 42].
The influence of treatment temperature and media on fibers and fabrics has also been studied experimentally for SC-CO2 dyeing [37, 41]. Some research groups have constructed pilot plant systems for dyeing polyester using SC-CO2 [21, 29], and the technical and economic feasibility of SC-CO2 dyeing has been discussed.
Some interesting topics still remain concerning the SC-CO2 dyeing process. For example, in dyeing more hydrophilic fibers and fabrics such as nylon 66 and cotton, hydrophobic disperse dyes are not suitable for SC-CO2 dyeing. Protein fabrics like silk and wool have been satisfactorily dyed even in deep shades with conventional acid dyes with a nonionic surfactant reverse micellar system in SC-CO2 . SC-CO2 dyeing of cotton modified with the fiber reactive group 2,4,6-trichloro-l,3,5-triazine has been examined . Reactive disperse dyes have been used in the SC-CO2 dyeing process [18, 38]. These papers reported that natural fibers can be dyed without pre-treatment, but a high temperature (160°C) is needed to obtain the highest color yield. At that high temperature, fiber degradation occurs. In our previous paper, we reported that cotton fibers can be dyed with fluoro-triazinyl reactive disperse dyes in SCCO2 at 120°C , which were developed for dyeing polyester/cotton blends in a pad-thermosol process by DyStar Japan Ltd. . They reported that reactive disperse dyes could not achieve exhaustion when dyeing polyester/cotton blends, but cotton fabrics could be dyed in SC-CO2, which is considered to be an exhaustion dyeing method.
The textile industry produces a huge number of polyester/cotton blends. To dye these fabrics, two process are generally needed. However, in order to reduce the amount of waste water from dyeing, it is desirable to develop a one-bath dyeing method such as SC-CO2 dyeing for blended fabrics.
In this paper, we attempt to dye polyester/cotton blends using fluoro-triazinyl reactive disperse dyes as dyestuffs in SC-CO2 as an extension of our previous work.
The dye used in the experiments, was Reactive Disperse Blue, shown in Figure 1, supplied as test samples from DyStar Japan, Ltd. [3OJ. This dye has a triazine group for reaction with a hydroxyl group of the cotton fibers. The body structure of Reactive Disperse Blue is similar to that of a disperse dye, and this portion can fix to uncrystal line region of polyester. N-methyl-2-pyrtOlidinone (NMP) used as the solvent for pretreating polyester/cotton blends was purchased from Kanto Chemical Co., Inc. Its purity is believed to be more than 98%. Carbon dioxide (CO2) was purchased from Higashi Tyugoku Air Water Co., Ltd., and its purity was greater than 99.9%. The polyester/cotton blend (65/35 blends) fabrics used in these experiments were purchased from Nakao Filter Kogyo Co., Ltd.
Details of the apparatus in this study were shown in our previous paper [2O]. The fabric to be dyed was wrapped around a dyeing beam, and the dyes were placed in a vessel preheated to the desired dyeing temperature. After loading in a certain amount of fabric and dyes, the vessel was then sealed. A known amount of CO2 was charged into the dyeing vessel with a CO2 pump, and the vessel was pressurized to the desired pressure. Stirring in the dyeing vessel then began. Ethanol (special grade of Nakalai Tesque Co., Ltd.) was added to the dyeing vessel as the cosolvent at the same time by a cosolvent pump. After 1 hour, a stop valve was slowly opened to release the CO2 until the pressure of the dyeing vessel reached atmospheric pressure. After the dyeing, the fabric was removed, washed with 2 g/L anionic soaping agents (CM-2, Nissin Kagaku Co., Ltd.) at 353 K for 5 minutes, then rinsed in acetone at ambient temperature to confirm dye fixing.
PRETREATMENT OF POLYESTER/COTTON BEEND FABRICS
In order to achieve efficient dyeing in SC-CO2, polyester/cotton blend fabrics should be immersed in the aqueous solution including 10% NMP as a pretreatment. It is necessary to swell the cotton fibers for dyeing in SC-CO2, and this solvent is well known to be good for swelling cotton. Alkaline conditions were required to fix the reactive disperse dyes on the cotton fibers. Therefore, sodium carbonate (special grade of Kanto Chemical Co., Inc.) was employed.
First, the polyester/cotton blends were treated in an aqueous alkaline solution containing 1% sodium carbonate and 10% NMP for 1 hour· at room temperature for the swelling. The fabrics were then squeezed and dried at 373 K.
When dyeing with Reactive Disperse Blue, hydrogen fluoridc may be produced from the dyeing reaction, but the amount is very little. Sodium carbonate also exists in a dye bath, and is expected to prevent hydrogen fluoride corrosion. After the expansion of the SC-CO2 solution, hydrogen fluoride produced from the dyeing reaction passed through an aqueous solution containing calcium hydroxide and was recovered as calcium fluoride, which exists in nature as fluorite and is stable and harmless. MEASURING DYE UPTAKE, COEOR, AND COEOR FASTNESS
The Kubelka-Munk values (K/S), the color strength of the dyed samples, and the colorimetric data were measured with a spectrophotometer (Kurabou Co. Ltd., Color 7). The color yields of the washed fabrics were evaluated by the total K/S, which is the sum of K/S through 380 to 720 nm in 10 nm steps.
A washing fastness test was performed according to the JIS L 0844 standard method. Color change was evaluated according to the grey scale regulated in JIS L 0804. A light fastness test used the JIS L 0842 standard method.
Results and Discussion
Figure 2 shows the influence of temperature and pressure on the Reactive Disperse Blue dyeing behavior of the polyester/cotton blend fabrics pretreated with 10% NMP. The blends were dyed at 353, 373, and 393 K over a pressure range from 10 to 20 MPa. The amount of dye applied was 5% owf, and 2.5 ml ethanol was added as the cosolvent. The volume of the dyeing vessel was 50 ml. The ratio of ethanol to CO2 was estimated to be 0.08 to 0.1 at the mole fraction base. The error bar shows the standard deviation of the total K/S of the dyed fabric. The standard deviation of the total K/S indicates the degree of dyeing speck. The total K/S drastically increases with increased pressure at 393 K. The values of total K/S at 353 and 373 K gradually increase, and the maximum values of K/S are smaller than at 393 K. To evaluate the dyeing behavior of the polyester side of the blends, polyester fabrics dyed with Reactive Disperse Blue using the same dyeing conditions are shown in Figure 3. The total K/S values increase abruptly at 393 K. Adding ethanol as a cosolvent increases the total K/S because the dye solubility in SC-CO2 becomes larger. This tendency is consistent with the dyeing behavior of polyester with disperse dyes in SC-CO2 [16, 31, 33-35]. The reactive disperse dye behaves as a disperse dye for dyeing the polyester side of the polyester/cotton blends. The dyeing behavior of the polyester in SC-CO2 with or without NMP is similar. The dyeing behavior of polyester is not affected by NMP (pretreatment solvent) in our experimental conditions. The dyeing behaviors of cotton and polyester fabrics in our experimental conditions are shown in Figure 4. The total K/S values of cotton fabrics increase drastically at 393 K compared with polyester fabrics. We believe that the reaction of cotton with reactive disperse dye is accelerated at 393 K. Therefore, the dyeing properties of the cotton side in polyester/cotton blends are dominant for SC-CO2. The error bars at 393 K decrease with increasing pressure. This means that the degree of dye speck decreases at the higher pressure. Both polyester and cotton fabrics could not dye sufficiently below 373 K and 15 MPa [20, 31, 33-35]. This is the reason for low dye-uptake at 373 and 353 K, and the large error bar below 15 MPa at 393 K. In our experiment, the optimum conditions are 393 K and 20 MPa.
Fastness properties of dyed polyester/cotton blends under several dyeing conditions are listed in Table I. For comparison, the polyester/cotton blends dyed with Reactive Disperse Blue by the thermosol dyeing method are shown in Table I. In thermosol dyeing, the recommended conditions are used for dyeing polyester/cotton blends with reactive disperse dyes . Light and washing fastness are almost satisfactory for normal usage except at the temperature of 353 K. The color fastness of the blends dyed below 373 K is weaker than that of blends dyed at 393 K. We believe that fixation of the reactive disperse dye is not enough for polyester and cotton fibers. In addition, the color yield on cotton fibers of the blends is also low at 353 K.
The light fastness of polyester/cotton blends dyed by the thermosol method is weaker than that by the SC-CO2 method. In thermosol dyeing, the dye is sublimed or dissolved by heating and penetrating into the fibers. The surface of the fibers is selectively dyed. In supercritical fluid dyeing, however, the dye is dissolved in SC-CO2 and transported into the swollen fibers. Homogeneous dyeing is achieved in SC-CO2 compared with the thermosol dyeing method. These insufficient fixation values affect the colorimetric data listed in Table II. The dyed fabrics have almost the same color yield. The chroma (C*) value decrease along with a decrease in the dyeing temperature, and the C* value of polyester/cotton fabrics dyed by the thermosol method is lower than that with SC-CO2 dyeing. A decrease in C* is explained by aggregation of unfixed dyes in the fibers and their influence on C*.
One-bath dyeing of polyester/cotton blend fabrics with reactive disperse dyes is successful with SC-CO2. The optimum dyeing temperature and pressure are about 393 K and 20 MPa, respectively. The dyeing behavior of polyester/cotton blends is strongly affected by the dyeing characteristics of the cotton side. The color fastness of dyed fabrics is almost satisfactory, but color fastness becomes weak with a decrease in the dyeing temperature. In addition, the color fastness of fabrics dyed in SC-CO2 is better than that with the thermosol dyeing method.
We are grateful to Nishinokinryou Co. Ltd. for supplying the polyester/cotton blend fabrics dyed by the thermosol method.
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Manuscript received September 22, 2003: accepted December 19, 2003.
SHINGO MAEDA1 AND KATSUSHI KUNITOU
Industrial Technology Center of Okayama Prefecture, Okayama 701-1296, Japan
DyStar Japan Ltd. Technical Center, Chuo-ku, Osaka, 541-0052, Japan
Department of Chemical Engineering, Faculty of Engineering, Fukuoka University, Jonan-ku, Fukuoka 814-0180, Japan
1Corresponding author: phone +81-(0)86-286-9612, fax +81(0)86-286-9632, email firstname.lastname@example.org
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