Ind. Eng. Chem. Res. 1998, 37, 1781-1785
1781
Comparison of the Thermal Fixation of Reactive Dyes on Cotton Using Infrared Radiation or Hot Air Arthur D. Broadbent,* Normand The´ rien, and Yifang Zhao De´ partement de ge´ nie chimique, Faculte´ des sciences applique´ es, Universite´ de Sherbrooke, Sherbrooke, Que´ bec, Canada J1K 2R1
A 100% cotton fabric was impregnated with an alkaline solution of a reactive dye and reaction with the cellulose achieved by heating using electric infrared emitters or hot air. The conditions gave temporal variations of fabric humidity and temperature that were similar for each heating method. Fixation yields and color strengths for the dyed fabrics were measured as a function of heating time. Infrared dye fixation yields were higher, and obtained in much shorter times, than those for dyeings produced by heating in air, particularly for dyes of lower reactivity. When using hot air for fixation, evaporation of water at the yarn surfaces during the initial phase of drying causes dye solution migration to the surface and an increased color yield of the final dyeing. The lower color yields of dyeings produced by infrared fixation were interpreted in terms of the suppression of this type of migration. Reaction of dye with the cotton at room temperature, under conditions preventing any drying, was assumed to occur without any migration of the initially unfixed dye. Fully continuous dyeing trials showed that the infrared fixation process gave high fixation yields with no visible color variation. Infrared fixation of reactive dyes on cotton could be valuable for reducing the environmental impact of unfixed dyes and dyeing assistants in the dyehouse effluent. Introduction In the continuous dyeing of cotton fabrics with reactive dyes, the cloth is impregnated with a solution of the reactive dye, salt, and an alkali. The formation of the covalent bond to the fiber by reaction of a chloride leaving group of the dye (Dye-Cl) and the cellulosate ion (Cell-O-) is then promoted by heat, using either hot air or steam (Shore, 1995). In other cases, the reactive group is a vinylsulfone that gives similar reactions by nucleophilic addition of cellulosate or hydroxide ions (Zhao and Broadbent, 1993).
Cell-OH + HO- ) Cell-O- + H2O cellulose alkali cellulosate -
Dye-Cl + Cell-O- ) Dye-O-Cell + Cl fixed dye reactive dye cellulosate + Cl Dye-HO Dye-Cl + HO- ) reactive dye alkali hydrolyzed dye
-
Hydrolysis of the reactive group always competes with the fixation reaction. The hydrolyzed dye is unable to react with the cellulose, and its formation decreases the amount of dye bonded to the cellulose. In current industrial practice, the efficiency of the fixation reaction in hot air or steam is often less than 75%, particularly for some of the older types of reactive dyes, even under optimum conditions. After dyeing, any unreacted or hydrolyzed dye must be washed from the material to avoid desorption of the color in wet treatments during use. Large amounts of salt and excessive color in the dyehouse effluent are environmentally undesirable, and * Author to whom correspondence is addressed. Telephone: (819) 821-8000 ext. 2172. Fax: (819) 821-7955. E-mail:
[email protected].
much effort is being devoted to developing high fixation/ low salt dyeing procedures for this type of dye. In addition, for dry heat fixation methods, large quantities of urea (100 g/L) in the dye solution are often recommended and must also be removed from the fabric in the final washing, contributing significantly to pollution of the effluent. In a previous paper, we described the dyeing of cotton by impregnation of the fabric with an alkaline solution of a reactive dye followed by infrared heating. We established the dependence of the dye fixation yield on the controllable process variables by means of factorial plans of experiments (Broadbent et al., 1995). For the low reactivity dye studied, a surprisingly high fixation yield of 92% was found under optimum conditions. This paper describes the fixation of various reactive dyes on cotton by using either infrared radiation or hot air to promote reaction of the dye with the cellulose. The objective was to compare the fixation yields and color strengths of the dyeings from the two types of processes and to examine the reasons for the high fixation yields obtained using infrared heating. Experimental Section Materials. The plain weave cotton fabric was scoured and bleached and had a superficial density of 164 g/m2. The four reactive dyes studied, of widely differing reactivity toward cellulose, are listed in Table 1. All other chemicals were reagent grade. Equipment. The padding and infrared drying equipment used in this study and the general dyeing procedure have already been described in detail (Broadbent et al., 1995; Zhao and Broadbent, 1993). The dryer for the hot air fixation process was a 9 kW Benz (Zurich) WMLTF 97085 textile laboratory dryer. The sample of cotton fabric, impregnated with an alkaline dye solution, was mounted on the dryer pin
S0888-5885(97)00570-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/20/1998
1782 Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 Table 1. Reactive Dyes Used in the Experimental Work
a
dye
The Colour Indexa name
supplier
reactive group
reactivity
Remazol Black B Remazol Blue R Drimarene Red X-6BN Procion Blue HE-RD
Reactive Black 5 Reactive Blue 19
Dystar Dystar Clariant BASF
bis(vinylsulfone) vinylsulfone monochlorotriazine bis(monochlorotriazine)
high medium low low
Reactive Blue 160
The Colour Index, 1964-1988.
frame. This automatically slid into the dryer and out again after a preset time. The air jet plates were situated at a distance of 5 cm from the fabric and directed preheated air at a speed of about 2.0 m/s at both faces of the material. A rapid response pyrometer (Raytak Inc.) recorded the maximum fabric surface temperature about 5 s after leaving the heating zone. The differences between the highest pyrometer readings and the air temperatures in the dryer (Table 3) indicated that the maximum temperature of the fabric was 15-16 °C higher than the pyrometer reading. This difference would be much less for fabric temperatures below 100 °C. Dyeing and Fixation Procedures. Solutions for dyeing were freshly prepared by mixing an aqueous dye solution containing NaCl with the NaOH solution. They were used immediately to minimize premature hydrolysis of the reactive dye. The solutions always contained 20.0 g/L of reactive dye, 25.0 g/L of NaCl, and 5.00 g/L of NaOH. For impregnation, a piece of cotton was passed through the pad bath containing the dyeing solution and squeezed by the pad rollers to remove the excess liquid. Samples were then cut immediately, one for determination of the dye content and the other for mounting on the pin frame ready for fixation. For infrared fixation, the pin frame holding the wet fabric was placed in the hole on the mesh conveyer and passed through the infrared oven at speeds giving residence times in the range from 4 to 27 s. Two types of electric tubular infrared sources were used in the experimental work. The solid quartz T-3 tube sources, operating at a total power of 18 kW and a temperature of around 2200 °C, give a short-wave infrared emission with a maximum output at about 1 µm. The quartz tube sources (iron/aluminum filament inside a transparent quartz envelope), operating at a total power of 15 kW and a temperature of about 760 °C, give a medium-wave infrared emission with a maximum output at around 3 µm. On leaving the heating zone, the maximum fabric surface temperature reading was recorded using the pyrometer, as before. For hot air fixation of the dyes, a sample of impregnated fabric was mounted on the pin frame and heated in the Benz dryer for a predetermined time from 5 to 60 s using air at 172 or 214 °C. Impregnated fabric samples were also predried from 68% to about 30% water content by heating in the dryer at 75 °C for 30 s and then at 172 °C to promote fixation. The predried sample was stored in a plastic bag with minimum air space while the dryer was heated from 75 to 172 °C (10 min). The objective of milder predrying was to decrease the extent of migration of the dye solution to the yarn surfaces during the initial stages of drying. For cold batch fixation, impregnated cotton samples were sealed in plastic bags, excluding air, and stored for 24 h at room temperature. Analytical Procedures. The amount of dye reacted with the cotton and the fixation yields was determined by the extraction and spectrophotometric procedures
outlined in the previous papers (Broadbent et al., 1995; Zhao and Broadbent, 1993). The fixation yield of a reactive dye F is given by
F)
Di - Df Di
where Di and Df are the amounts of unfixed reactive dye extracted from the cotton (g of dye/g of cotton) before and after fixation. The yields were corrected slightly for the initial rapid reaction of the dye with the cotton as described previously (Broadbent et al., 1995). The color yield of a dyeing is the depth of color obtained for a given amount of dye. This was evaluated in terms of the slope of the graph of the Kubelka-Munk K/S value at the wavelength of minimum reflectance of the dyeing versus the amount of fixed dye in the cotton
K/S(λ) )
[1 - R∞(λ)]2 2R∞(λ)
where R∞ is the reflectance at a given wavelength (λ) of an infinite thickness of the dyed material, measured using a Diano Match Scan II double-beam reflectance spectrophotometer fitted with an integrating sphere. The amount of dye that had reacted with the cotton (Wf) was calculated as FDi. The same spectrophotometer was also used for determining the reflectance spectra of specimens cut from cotton dyed continuously by passage through the heating zone of the infrared oven. These reflection spectra were used to calculate the CIE L*, a*, and b* color coordinates. Comparison of the color coordinates of each sample with the average values for all the samples allowed calculation of the variability of the color along the length of the dyed fabric in the form of the CIELAB color difference (∆E) (ASTM, 1996b). All measurements were repeated, and the fixation yields presented are the average of two measurements (reproducibility (3%). Water contents of wet fabrics were determined by a standard method (ASTM, 1996a). Results and Discussion The influence of the drying and heating methods on the degrees of dye fixation of four different reactive dyes on cotton was examined. Dyeings were produced by six different fixation procedures: (1) infrared heating using medium-wave quartz tube sources, (2) infrared heating using short-wave T-3 tube sources, (3) hot air fixation at 172 °C, (4) hot air fixation at 214 °C, (5) hot air fixation at 172 °C after predrying in warm air at 75 °C for 30 s, and (6) fixation by storing the fabric in a sealed bag for 24 h at room temperature. Fabric Surface Temperature and Humidity Profiles. Tables 2 and 3 show the typical fabric water contents (dry basis) and final temperatures for the various heating methods. The fabric temperatures during the constant rate drying phase were all about
Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1783 Table 2. Cotton Water Contents and Surface Temperatures after Infrared Heating quartz tube
T-3 tube
time (s)
% water
temp (°C)
% water
temp (°C)
0 4 9 18 22 27
59.9 55.9 44.9 17.7 6.7 2.3
22 43 51 51 62 142
58.8 56.4 44.1 10.5 4.2 2.4
22 41 51 52 68 109
Table 3. Cotton Water Contents and Surface Temperatures after Hot Air Heating air temperature 172 °C
214 °C
time (s)
% water
temp (°C)
% water
temp (°C)
0 5 10 15 20 25 30 35 40 45 50 55 60
68.1 56.9 47.1 30.8 18.2 11.5 3.2 1.4 0.7 0.3 0 0 0
22 43 46 50 51 57 68 120 150 152 155 157 157
68.1 52.8 34.7 18.1 6.5 0 0 0 0 0 0 0 0
22 43 48 55 69 131 170 191 195 197 198 198 198
50-55 °C. For both types of infrared source and for drying in hot air at 214 °C, the drying rates (humidity/ time profile slope) and the duration of the constant rate drying phase (about 20 s) were similar. For drying in hot air at 172 °C, drying was somewhat slower, with the end of the constant rate drying period occurring after about 25 s. Influence of the Fixation Method on the Dye Fixation Yield. For dyeings with Black B, the most reactive of the dyes, obtained with either type of infrared source, dye fixation yields of 95% were obtained in periods as short as 10 s, even before the impregnated cotton was completely dried. Hot air fixation at either 172 or 214 °C was much slower, taking as long as 40 s to reach a yield of 90%. The reexamination of the fixation of the dye Black B confirmed the remarkable results on the fixation of this dye obtained in our preliminary study (Zhao and Broadbent, 1993). Figure 1 illustrates the data for another dye (Blue R) of medium reactivity toward cotton. The rates and extents of infrared fixation were markedly higher than those obtained when using the hot air dryer. For the two dyes of low reactivity (Red X-6BN and Blue HE-RD) the fixation time profiles were similar (Figure 2 for Red X-6BN). The dye Blue HE-RD reacted more slowly and only gave fixation yields of 70 and 55% using infrared or hot air heating, respectively. For both of these low reactivity dyes, significant fixation (>20%) only occurred after the drying phase when the fabric temperature began to increase. Again, hot air fixation was slower and gave lower final fixation yields. None of the reported infrared fixation yields necessarily represent the maximum attainable values. In particular, no attempt was made to optimize either the alkali concentration in the dyebath or the heating conditions. Infrared fixation by the short-wave T-3 tubes tended to be faster during the initial stages of drying, possibly because this type of radiation penetrates more effectively into the wet web. For infrared heating, the
Figure 1. Fixation yield as a function of heating time for dyeings with Blue R. See Figure 4 for legend.
Figure 2. Fixation yield as a function of heating time for dyeings with Red X-6BN. See Figure 4 for legend.
short-wave T-3 tubes gave dyeings with color strengths very close to those of the cold batch dyeings. This implied effective dye migration suppression during infrared drying. The influence of the predrying method for the impregnated fabrics before subsequent heating in hot air was not significant. This may be a consequence of the time required to increase the dryer temperature from 75 to 172 °C (10 min) and the unavoidable cooling of the sample during the pause. Unfortunately, the physical separation of the infrared and hot air ovens prevented the possibility of infrared predrying, followed immediately by hot air curing. Influence of the Fixation Method on the Color Yield of the Dyeings. Infrared drying of wet textiles is known to suppress the migration of water to the yarn surfaces along with unfixed dyes and chemicals. The migration of soluble dyes during the initial stages of hot air drying results in much higher dye concentrations in fibers near the yarn surfaces, leading to an increased color yield, the color intensity per unit amount of dye in the material. For comparison purposes, it was assumed that the cold batch dyeings, in which fixation occurred without any drying, would have zero migration and thus the lowest color yields. This was, in fact, the case (Figures 3 and 4). In all cases, hot air drying gave
1784 Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 Table 4. Average Values of Fabric Temperature, K/S, Fixation Yield, and Color Difference over 1 h of Continuous Dyeinga trial
additives
temperature °C
K/S
% fixation
∆E
1 2
salt, no urea urea, no salt
130 (3) 163 (3)
14.1 (0.3) 15.2 (0.2)
78 (2) 89 (1)
0.42 (0.2) 0.44 (0.2)
a Values in parentheses are standard deviations for the ten samples examined over 100 m of dyed fabric.
Figure 3. Color strength (K/S) as a function of the amount of dye fixed in the cotton for dyeings with Blue HE-RD. See Figure 4 for legend.
Continuous Dyeing Trials. To demonstrate that the process of infrared fixation is capable of producing high fixation yields with good color fidelity, continuous dyeing trials were conducted using the pilot-scale infrared oven fitted with quartz tube sources. The fabric was continuously impregnated to 80% solution pickup and passed through the preheated oven at a speed ensuring that the final temperature was sufficiently high to give adequate fixation. The dye solution (20 g/L of Red X-6BN, 5.0 g/L of NaOH, and 25 g/L of NaCl or 100 g/L of urea) was freshly prepared and continuously fed into the padding trough, using a metering pump, at a rate that ensured a constant liquid level in the bath. The only other requirement was a compensator roller to compensate for the slightly different speeds of fabric during impregnation and during winding by the takeup roller at the oven exit. Dyeing at a fabric speed of around 2.0 m/min was continued for over an hour, with fabric samples being cut every 10 m. The results are shown in Table 4. The fixation yields were not fully optimized, but they and the measured K/S values were remarkably constant. The CIELAB color differences, calculated for each sample relative to the average values of the color coordinates for all samples (ASTM, 1996b), had values ∆E < 0.5 that agreed with the visual conclusion that over 100 m of fabric was dyed without any visible color variations. Discussion and Conclusion
Figure 4. Color strength (K/S) as a function of the amount of dye fixed in the cotton for dyeings with Blue R: 9, infrared heat with T-3 tubes: 2, infrared heat with quartz tubes; f, heating with air at 172 °C; [, heating with air at 212 °C; 0, cold batch dyeings.
higher color yields, with these being somewhat less for drying at 214 °C and also for the dyeing that had been predried at 75 °C before heating at 172 °C. For infrared fixation, the short-wave T-3 tubes gave dyeings with color strengths very close to those of the cold batch dyeings. We conclude that this demonstrates the effectiveness of this type of emitter in migration suppression despite the lack of a direct experimental confirmation of this. The medium-wave quartz tube sources, for which the radiation is more strongly absorbed at the wet fabric surface, had higher color strengths but lower than those obtained for dyeings in hot air. The color strength data represent the K/S values for dyeings with the same initial amount of dye but obtained for different heating periods. The assessment of color strength of a different group of dyes was also carried out on dyeings with different amounts of dye but was fixed by heating for the same time under optimum conditions. Again, the color yields of dyeing obtained by heating with shortwave infrared radiation (T-3 tubes) were indistinguishable from those of dyeings produced by cold fixation under conditions preventing any drying and migration.
The reaction of reactive dyes with cotton by means of infrared heating produces dyeings of high dye fixation yield often exceeding 85% and in some cases over 90%. Fixation yields exceeding 80% are even possible without additions of salt and urea to the dyebath. For the dye Drimarene Red X-6BN, reducing the concentration of salt in the dyebath from 40 to 10 g/L has a negligible effect on the optimum 92% fixation yield, obtained when urea was present, and only decreased the fixation from 86 to 81% when urea was absent (Broadbent et al., 1995). The use of electric infrared sources in this type of fixation process would allow a compact, efficient continuous dyeing operation, with rapid color changes for small lots of material. Such a high-fixation continuousdyeing method would have beneficial environmental consequences because of the reduced amounts of unfixed dyes in the dyehouse effluent. The benefits would be even greater if the marginal effect of reduced salt on the fixation yields proves to be general for a majority of dyes and also if the use of urea can be avoided. Infrared radiation, particularly of shorter wavelengths, is able to penetrate into wet cotton fibers. This effect is at least partly responsible for the suppression of dye solution migration to the yarn surfaces during the initial stages of drying. Infrared fixation produced dyeings with higher fixation yields than when using hot air alone but having color yields close to those obtained
Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1785
by cold fixation without any drying, for which migration is presumably absent. It is therefore tempting to relate the higher fixation yields obtained using infrared radiation to the suppression of the dye migration indicated by the lower color yields. It seems obvious that migration of the initial dye solution out of the cotton fiber pores toward the yarn surfaces, where most of the water evaporation is occurring during the constant rate drying period, would result in less dye reacting with the cotton and a higher proportion of the fixed dye at the yarn surface. In a preliminary dyeing trial using Drimarene Red X-6BN, using infrared predrying followed by hot air fixation, the fixation yield was 76%, intermediate between those for fixation exclusively by infrared (82%) or hot air (72%). This result suggests that migration suppression is, in fact, at least partly responsible for the higher fixation yields obtained by infrared heating. Further studies on migration suppression, and on fullscale commercial use of this process, are in progress. Acknowledgment The authors thank Hydro-Que´bec (Laboratoire des technologies e´lectrochimiques et des e´lectrotechnologies) and the Natural Sciences and Engineering Research Council of Canada for financial support and Dystar
Canada Inc., Clariant Canada Inc., BASF Canada Inc., Vivatex Inc., and Monterey Textile Inc. for materials. Literature Cited ASTM. Standard Test Method for Quantitative Analysis of Textiles. D 629-88, Section 8. Annual Book of ASTM Standards; ASTM: Conshohocken, PA, 1996a; Vol. 07.01. ASTM. Computing the Colors of Objects by Using the CIE System. E 308, Calculation of Color Differences From Instrumentally Measured Color Coordinates. D 2244. Annual Book of ASTM Standards; ASTM: Conshohocken, PA, 1996b; Vol. 6.01. 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, 943-947. Shore, J. Dyeing With Reactive Dyes. In Cellulosics Dyeing; Shore, J., Ed.; Society of Dyers and Colourists: Bradford, U.K., 1995. The Colour Index, 3rd ed. (and supplements); Society of Dyers and Colourists and American Association of Textile Chemists and Colorists: Bradford, U.K., and Research Triangle Park, NC, 1964-1988. Zhao, Y.; Broadbent, A. D. Fixation of Reactive Dyes on Cotton Using Infrared Radiation. Can. Textile J. 1993, 110 (3), 3238.
Received for review August 18, 1997 Revised manuscript received January 23, 1998 Accepted February 2, 1998 IE970570G
