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MATERIALS AND INTERFACES Continuous Dyeing of Cotton with Reactive Dyes Using Infrared Heat Arthur D. Broadbent,* Julienne Bissou-Billong, Miriem Lhachimi, Youssef Mir, and Serge Capistran De´ partement de ge´ nie chimique, Faculte´ de ge´ nie, Universite´ de Sherbrooke, Sherbrooke, Que´ bec, Canada J1K 2R1
Fabrics containing cotton fibers were dyed in a continuous process by impregnating the fabric with an alkaline solution of reactive dyes and then drying and heating it using electrically generated infrared radiation followed by hot air. Optimal fixation of the dyes required a strongly alkaline dye solution and heating the fabric to as high a temperature as possible consistent with avoiding thermal damage to the fibers. Color consistency and quality were well-controlled along the fabric length during both pilot- and industrial-scale continuous dyeing and the color differences, relative to commercial products obtained by batch dyeing procedures with the same recipes, were small. The infrared process offers reduced pollution loads in the washing liquors from reactive dyeing because fixation yields were greater than those for the cold pad-batch dyeing procedure, and no electrolytes or urea were needed in the initial dye solutions. Introduction In previous papers,1,2 we had shown that cotton fibers can be effectively dyed with reactive dyes using infrared heat to promote fixation, i.e., the reaction of the dye with the cotton cellulose. The process involved impregnating a cotton fabric with an alkaline solution of a reactive dye and then passing it through an infrared oven where drying and heating promoted fixation of the dye. The dye fixation yield is the ratio of the amount of the dye that has reacted with the cotton cellulose to the amount initially present in the impregnated fabric. The best fixation yields were obtained when the dye solution was strongly alkaline (5.0 g/L NaOH) and the dried fabric was heated to the highest permissible oven exit temperature consistent with avoiding any thermal damage to the fibers. It was established that infrared heating gave higher fixation yields than those obtained under similar conditions but using hot air as the drying and heating medium. We think that the better penetration of the radiant energy into the wet fabric, and the more uniform internal heating that this provides, is responsible for the higher fixation yields. During the first phase of drying with hot air there is considerable migration of the dye solution to the yarn surfaces where evaporation is occurring. The use of infrared heat suppresses such migration, and the ring-dyeing of the yarns that it causes, particularly if short-wave infrared sources are used with maximum emission at around 1 µm.2 The purpose of this paper is to describe further work on this process related to its industrial application. The * To whom correspondence should be addressed. Tel.: (819) 821-8000 Ext. 2172. Fax: (819) 821-7955. E-mail:
[email protected].
following points were considered in relation to the previous studies: 1. In this study, we used industrial dyeing recipes typically containing three reactive dyes. In the previous work only solutions of single dyes were used. The dyeing of deep shades that are difficult to produce with high efficiency in an industrial operation was given priority. 2. Because of the significance of cotton/polyester fiber blends in the marketplace, the process was studied using both cotton and cotton/polyester fabrics. 3. Our industrial partner’s equipment consisted of an electric infrared predryer situated in front of a conventional gas-heated hot air drying frame. We therefore studied a dyeing process in which the fabric was initially impregnated with alkaline reactive dye solution and then dried and preheated in an infrared oven. It then passed immediately into a hot air oven to complete dye fixation at a safe but constant temperature. 4. In this study, particular emphasis was placed on dyeing fabric continuously, on both a pilot and industrial scale, over an extended period to show that color consistency and quality could be achieved. The dyeings obtained were compared with those from actual commercial production using the same dye recipes but different dyeing procedures. 5. Alkaline reactive dye solutions for pad-batch dyeing invariably have considerable amounts of added salt (2050 g/L) and, in thermal fixation processes, of urea (100 g/L) to aid fixation.3 In this study, these chemicals were not used, taking advantage of the higher fixation obtainable when using infrared heat, and thus allowing a process with a greatly reduced pollution load in the post-dyeing washing liquor.
10.1021/ie040288r CCC: $30.25 © 2005 American Chemical Society Published on Web 04/21/2005
Ind. Eng. Chem. Res., Vol. 44, No. 11, 2005 3955 Table 1. Dyeing Recipes burgundy on cotton/polyester
forest green on cotton/polyester
navy blue on 100% cotton
Levafix Yellow EG 12.0 g/L (Dyestar) Levafix Brilliant Red ERN 23.3 g/L (Dyestar) Drimarene Blue K-2RL 4.70 g/L (Sandoz)
Remazol Yellow RNL 4.47 g/L (Dyestar) Remazol Red RB 0.935 g/L (Dyestar) Remazol Navy Blue GG 6.16 g/L (Dyestar)
Triactive Gold Yellow DF-RL 3.83 g/L (Tritex) Triactive Red DF-4BL 6.15 g/L (Tritex) Cibacron Dark Blue WR 20.4 g/L (Ciba)
Experimental Procedures
Figure 1. Schematic of the pilot-scale equipment used for continuous dyeing.
Materials. Our industrial partner provided all dyes and chemicals, and the dye recipes and fabrics. The white bleached 100% cotton fabric used for the navy blue dyeing was a simple woven fabric with a superficial weight of 144 g/m2. The cotton/polyester fabrics used for the burgundy and forest green shades were both woven with a 100% polyester multifilament warp and a 50/50 blended cotton/polyester filling. The superficial weights were 185 g/m2 (burgundy) and 183 g/m2 (forest green). The polyester fibers in both fabrics were predyed with a mixture of disperse dyes at 130 °C under pressure in a jet dyeing machine at the finishing plant. Three recipes for deep shades, each containing three reactive dyes to dye the cotton, were used in this study (Table 1). The amounts of the dyes used are in terms of the bath concentrations. These recipes and dyes were provided by our industrial partner and the dye supplier’s names are given in parentheses. Equipment. The pilot-scale continuous dyeing equipment (Figure 1) consisted of a set of drive rollers, a small impregnation unit (pad) with a 1.0 L dye bath, an electric infrared oven that has been described previously,1,2 and an electrically heated (12 kW) hot air oven (1.3 m3) with forced air circulation (19 m3/min). The infrared oven was fitted with tubular electric sources placed 5 cm above and below the fabric that passed straight through the heating zone. Each tube consisted of an iron/aluminum filament surrounded by a quartz envelope and gave maximum emission at around 3 µm at the maximum operating voltage of 450 V. Since the fabric entering the hot air oven was dry, most of the hot air was recycled. This unit was capable of heating a dry fabric to a surface temperature of over 200 °C in less than 30 s. The fabric speed (2.3 m/min) determined the residence time in the infrared oven (35 s) and the length of fabric passing over the adjustable rollers in the air oven regulated the time in the hot air (60 s). Dyeing Operations. The dyeing process consisted of three stages: (1) impregnating the fabric with the alkaline solution of the reactive dyes in the pad bath and then squeezing out the excess solution to give a water retention of 72-75% (on a dry basis, measured by weighing and drying), (2) running the fabric at a given speed through the infrared oven with each of the four modules of emitters operating at the same voltage (around 400 V, total power 13.1 kW), and finally (3) passing the fabric through the air oven typically running with the air at 150 °C. Preliminary trials to establish the optimum operating conditions for the best fixation were conducted using fabric samples (200 × 25 cm) sewn into a leader cloth (in the fabric accumulator) that then passed through the infrared oven and the hot air unit. Once optimum operating conditions had been established, continuous
dyeing trials were carried out under those conditions. This involved dyeing from 80 to 120 m of fabric at a speed of 2.3 m/min. The bath volume was maintained constant by addition of freshly prepared dye-alkali solution (5.0 g/L NaOH). The bath had a volume of 1.0 L and therefore the volume of solution in it would be totally replaced in about 13-14 min. Dye reactive group hydrolysis was shown not to be problematic over this time period. Fabric samples for analysis were cut before, during, or after the dyeing operation. Those taken after the completion of dyeing were washed four times to remove any unfixed dyes using the following protocol: 60 s at 20 °C, 30 s at 60 °C, 90 s at 90 °C, and 60 s at 40 °C using stirred water at a 100:1 liquor-to-fabric ratio. These conditions provided a reasonable simulation of those encountered in the finishing mill where a continuous washing unit was used. After the final washing all dyed samples were air-dried. For comparison purposes, the cotton was also dyed using a semi-continuous dyeing operation (cold padbatch) either in the laboratory or in the finishing plant. This involved impregnating the fabric with the reactive dye solution, containing 20 g/L Na2CO3 and 20 g/L NaCl, and storing the fabric, wrapped in plastic film to avoid evaporation, for 24 h at room temperature and finally washing and drying. Success in continuous dyeing on the pilot scale led to full-scale continuous dyeing trials in the finishing mill. The latter were only carried out for the burgundy and forest green shades. This involved dyeing of 400 m rolls of the same cotton/polyester fabrics, both of which gave a dye solution pickup of 58%. The impregnated fabric passed through the infrared oven and then through a conventional gas-fired hot air drying frame at about 1920 m/min. The infrared unit consisted of 6 banks each with 8 tubular emitters (7 kW each). The fabric temperature at the exit to the infrared oven, registered with a manual pyrometer, was 120-140 °C, with most variations across the fabric width. The motion of the fabric made precise temperature measurements difficult. The dyed fabric was washed at the mill in an eight-box continuous washing unit, numerous samples being cut along the 400 m length before and after washing. All of the dyes used in the project have been used in cold pad-batch dyeing and have the low to moderate substantivity required by this process. The pad bath solution was always at ambient temperature but, unlike that used in cold pad-batch dyeing, contained no added salt. Bath solution replacement times were about 14 and 16 min for pilot- and industrial-scale continuous dyeing trials, respectively, so the processes never attained a
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true equilibrium. Thus, neither type of trial would reach a true equilibrium in the pad bath but any tailing caused by dye substantivity would have been visible over the 2-3 bath replacements involved in the overall trials. Dye Fixation Yields. Measurements of the reflectance spectra of the dyeings (380-700 nm with a 10 nm wavelength interval) were performed using a Diano Match Scan II double-beam recording spectrophotometer. This allowed calculation of the absorption spectra of the dyeings (Kubelka-Munk K/S values as a function of wavelength) and the CIELAB color coordinates for CIE D65 Illuminant and the CIE 1964 supplementary standard observer.4 In almost all cases, measurement of the CIELAB color difference was with respect to the appropriate commercial product obtained by jet dyeing of the polyester and subsequent cold pad-batch dyeing of the cotton, using identical recipes to those given in this paper. Because a mixture of reactive dyes was used, and because the polyester component was already colored in two of the cases, the following equation was used to evaluate the degree of reactive dye fixation, 700
K/S(λ))W - (∑K/S(λ))P ∑ 380 380 700
(
dye fixation yields % type and concentration of alkali temp at the IR oven exit, °C
20 g/L Na2CO3
1.0 g/L NaOH
20 g/L Na2CO3 2.5 g/L NaOH
5.0 g/L NaOH
90 105 120
66.7 76.8 81.7
77.2 81.7 83.8
80.1 87.9 93.6
88.6 92.2 96.5
unexposed half of the sample. Despite the obvious variations in daylight intensity, this comparison of identically exposed samples provided information on any effect of the thermal dyeing process on the light fastness of the dyeings. Finally, because a high-energy infrared heating process involves possible thermal damage to both the cotton and polyester fibers, the handle of the fabric was evaluated manually by comparing it with the handle of standard fabrics dyed by the cold pad-batch procedure. Results
700
( % fixation )
Table 2. Dye Fixation Yields for the Navy Blue Dyeing of 100% Cotton with Solutions Containing Various Alkalis at Different Concentrations
700
× 100
K/S(λ))U - (∑K/S(λ))P ∑ 380 380
where the subscripts W, U, and P refer to dry samples of the washed dyeing, the unwashed dyeing, and the dyed polyester of the initial fabric, respectively. For determining the dye fixation on fabrics of 100% cotton, the summations for the polyester component were ignored. This is only an approximate measure and its validity is dependent upon the absence of ring-dyed yarns that would produce a stronger absorption of light by the fabric.5,6 However, fixation yields for the reactive dyes were also measured by extracting the unfixed dyes from the fabric before and after heating and determination of the concentrations of the solutions and the amounts of dye was done by spectrophotometry.1,2 The fixation yields determined by extraction were within 2% of those obtained using the above equation. Dyeing Quality Evaluation. The degree of dye bleeding during subsequent washing of the final dyed samples from the infrared process was compared to that from samples dyed by the cold pad-batch procedure in the laboratory. Samples of the final dyeings (5.00 g) were washed at 50 °C for 30 min in a dilute aqueous detergent solution (50 mL of 0.5 mL/L of Sandoclean PCL (Sandoz)) and the absorption spectrum of the washing solution determined (380-700 nm with a 10 nm wavelength interval). The degree of unfixed dye desorption was evaluated by summation of the absorbance values for the 33 wavelengths. In addition, the CIELAB color differences4 between the initial and tested samples were measured. Samples dyed using the thermal process and appropriate comparison standards, dyed by a cold padbatch procedure, were inclined at 45° to the horizontal and exposed to daylight under glass with half of each sample covered to prevent fading. The sum of the K/S values at 33 wavelengths was recorded each week and the color loss of the dyed fabric compared to that of the
Preliminary Trials. The initial work involved optimizing both the physical and chemical variables used in the thermal dyeing operation so as to maximize the reactive dye fixation (maximum K/S values). The key physical operational variables were the fabric speed (2.3 m/min) that determined the residence time in the infrared oven (36 s), the voltage supplied to the electrically heated tubular infrared sources (around 400 V), and the length of fabric in the hot air oven operating at 150 °C (2.3 m). The only important chemical variable was the strength and concentration of the alkali added to the reactive dye solution just prior to impregnation. This phase of the work, for each of the recipes, quickly confirmed what had been established previously.1,2 Optimum fixation required a relatively high concentration of a strong alkali (5 g/L NaOH) in the dye bath. This concentration is a compromise between maximizing activation of the cotton toward reaction with the dyes and minimizing hydrolysis of the reactive groups of the dyes during their residence in the dye bath. In addition, high levels of dye fixation required heating the fabric in the infrared oven so that drying was complete and the fabric temperature increased to 110 °C for the cotton/polyester and cotton fabrics, respectively. Heating to higher exit temperatures than these caused increased fabric stiffness and a deterioration of the fabric handle. Table 2 shows data for the navy blue dyeing of the 100% cotton fabric illustrating the effects of alkali type and concentration, and of heating to higher temperatures, on the dye fixation yields. Similarly, for the burgundy dyeing on cotton/polyester using dye baths containing 20 g/L NaHCO3, 20 g/L Na2CO3, 1.0 g/L NaOH, or 5.0 g/L NaOH, and with a fabric temperature at the infrared oven exit of 110 °C, the dye fixation yields were 10, 65, 62, and 72%, respectively. For the burgundy dyeing using 5.0 g/L NaOH in the dye bath, the CIELAB color difference was 1.85 (slightly yellower) with respect to the production standard for this shade obtained from the mill. This was a most satisfactory result. This dyeing was barely influenced by the presence of salt in the dye bath, the dye fixation yield only increasing from 72 to 75% with an addition of 50.0 g/L of anhydrous Na2SO4. The
Ind. Eng. Chem. Res., Vol. 44, No. 11, 2005 3957 Table 3. Continuous Dyeing Trials on the Pilot and Industrial Scale burgundy on cotton/ polyester
navy blue on 100% cotton
forest green on cotton/ polyester
Pilot-Scale Dyeings dye fixation yield % 94.1 ( 0.8 72.0 ( 3.5 74.0 ( 5.0 integrated K/S values 302.1 ( 1.6 295.8 ( 5.4 293.4 ( 6.0 a CIELAB color 6.05 ( 0.11 2.87 ( 0.38 2.84 ( 0.49 difference 1.60 ( 0.32a Mill Dyeings dye fixation yield % integrated K/S values CIELAB color difference
72.0 ( 1.4 65.7 ( 3.5 293.7 ( 4.06 305.3 ( 5.95 2.00 ( 0.05a 2.19 ( 0.20
a Measured with respect to a sample dyed in the laboratory by the cold pad-batch process.
reactive dyes in the burgundy recipe are less reactive than those in the navy blue recipe. The high concentration of NaOH required to obtain maximum dye fixation was of concern because the pH value of the bath was high enough to possibly cause rapid hydrolysis of the reactive groups of the dyes. Trials carried out over a period of time with the same alkaline dye solution (5.0 g/L NaOH) showed that there was no significant loss of color strength of any of the dyeings provided that the solution was not used for a period exceeding 20 min. One striking result of the optimization study was that the hot air oven played almost no role in dye fixation. We had initially assumed that 40-45 s of infrared heating would not be sufficient to drive the fixation to its optimum value and therefore chose to follow the infrared heating with hot air at a safe, constant high temperature. The optimum dye fixation depended only on how high a temperature the cotton fabric reached in the infrared oven. Even for dyeings where the fixation yield at the infrared oven exit was lower, as when using 20 g/L Na2CO3 in the dye bath, further heating for 1 min at 150 °C in the hot air oven did not increase the yield by more than about 2%. Continuous Dyeing Trials on a Pilot and Industrial Scale. For the pilot-scale and industrial-scale continuous dyeing trials, we measured the dye fixation yield, the integrated K/S values determined from the reflection spectrum, and the CIELAB color difference with respect to a cold pad-batch dyeing from the finishing mill, unless otherwise indicated. Measurements were taken at frequent intervals along the length of dyed fabric, and also across the fabric width for dyeings conducted at the mill. The results in Table 3 give the average values and the standard deviations of each measured quantity over all the samples of a particular dyeing. They show that the process was wellcontrolled and that, in all cases, any variations of the color difference along the fabric were negligible, a conclusion confirmed by detailed visual inspection by our industrial partner. Note that the standard deviations of the CIELAB color differences are all below 0.5. Despite some temperature variations across the fabric width at the exit of the infrared dryer at the mill, there was no measurable or visible color variations across the fabric width. The CIELAB color differences were determined with respect to the appropriate industrial dyeings provided by our partner that were prepared by jet pressure dyeing of the polyester with disperse dyes followed by cold pad-batch dyeing of the cotton with
reactive dyes, using exactly the same recipes as given in this paper. For the navy blue dyeing, the average CIELAB color difference with respect to the laboratory cold pad-batch dyeing was quite high because of the much lower dye fixation in the latter case (68%). The dyeings on the pilot scale in the laboratory and those at the mill were carried out under different conditions. The solution pickup at the mill (58%) was lower than that in the laboratory (72%), but despite this, the dyed fabrics produced at the mill had colors only slightly different than those of the standard shades. Washing and Light Fading Tests. The bleeding of residual unfixed dyes during subsequent washing and the light stability of the dyed samples were examined and compared to those of identically treated samples dyed by the cold pad-batch procedure. For the burgundy production standard from the mill, a similar laboratoryscale cold pad-batch dyeing, and a pilot-scale burgundy dyeing obtained by the infrared process, the values of the integrated absorbance values for the washing solutions were 7.30 ( 0.97, 2.61 ( 0.06, and 2.74 ( 0.27, respectively. Both the sample dyed using infrared radiation and the laboratory-scale cold-pad batch dyeing contained slightly more than a third of the amount of unfixed dyes as the sample produced by our industrial partner. They also had much smaller variations in the amounts of residual unfixed dyes among the samples. For the navy blue shade on the 100% cotton fabric dyed using the infrared procedure or by a cold pad-batch method, the values of the integrated absorbance values were 7.2 and 28.9 (single test of each sample). The very high value for the cold pad-batch sample is a consequence of the much lower fixation yield for this dyeing (62%) and the inadequacy of the post-dyeing washing procedure in removing the large amounts of residual unfixed dyes in this case. Color measurements and visual inspections on the washed and unwashed samples showed that the washing test did not result in any color difference. Exposure to daylight over periods of 3-6 weeks showed no decrease in the integrated K/S values for either infrared or cold pad-batch dyeings. The thermal process thus had no influence on the color stability of the dyed fabrics to light exposure. At each stage in the project, fabric samples were examined to evaluate their handle. Provided that the maximum temperatures at the infrared oven exit were not exceeded, the dyeings produced by the thermal process were of identical handle to those produced by the cold pad-batch method. Conclusion The use of infrared radiation for drying and heating fabrics containing cotton and impregnated with a strongly alkaline solution of reactive dyes gave effective fixation and less residual unfixed dye in the final fabric. The thermal continuous dyeing process was well-controlled, did not result in end-to-end or side-to-side color variations, and had no influence on color fading in daylight. The absence of electrolyte and urea from the reactive dye solution gave washing liquors with a much reduced pollution load. Acknowledgment This project was funded by a Co-operative R&D grant from the University-Industry program of the Natural Sciences and Engineering Research Council of Canada
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and by Hydro-Que´bec and Consoltex Inc. The authors also gratefully acknowledge the collaboration of the Bureau de Liaison Entreprise-Universite´ de Sherbrooke. Literature Cited (1) Broadbent, A. D.; The´rien, N.; Zhao, Y. Effects of Process Variables on the Fixation of Reactive Dyes to Cotton Using Infrared Radiation. Ind. Eng. Chem. Res. 1995, 34, 943. (2) Broadbent, A. D.; The´rien, N.; Zhao, Y. Comparison of Thermal Fixation of Reactive Dyes on Cotton Using Infrared Radiation or Hot Air. Ind. Eng. Chem. Res. 1998, 37, 1781. (3) Broadbent, A. D. Basic Principles of Textile Coloration; Society of Dyers and Colourists: Bradford, UK, 2001.
(4) McDonald, R. Colour Physics for Industry, 2nd ed.; Society of Dyers and Colourists: Bradford, UK, 1997. (5) Motamedian, F.; Broadbent, A. D. The Effects of Dye Distribution in Nylon Filaments on the Dyeing Color Yield and Fastness Properties. Ind. Eng. Chem. Res. 1999, 38, 4656. (6) Motamedian, F.; Broadbent, A. D. Modeling the Influence of Dye Distribution on the Perceived Color Depth of a Filament Array. Textile Res. J. 2003, 73, 124.
Received for review December 6, 2004 Revised manuscript received March 30, 2005 Accepted March 30, 2005 IE040288R
