943
Ind. Eng. Chem. Res. 1996,34,943-947
Effects of Process Variables on the Fixation of Reactive Dyes to Cotton Using Infkared Radiation Arthur D. Broadbent,* Normand ThBrien, and Yifang Zhao Dipartement de ginie chimique, Faculti des sciences appliquies, Universiti de Sherbrooke, Sherbrooke, Quibec, Canada J1K 2Rl
A series of experiments on the furation of a reactive dye to a cotton fabric in a pilot-scale electric infrared dryer was designed and analyzed using two factorial plans. Equations were derived describing the influence of the controllable process variables on the dye furation yield and also on the final fabric temperature and water content. Fabric speed (residence time) and the power supplied to the infrared emitters were the most important variables influencing the above responses. The dye fixation yield was also strongly dependent on the type of alkali and its concentration in the dye bath. The effects of increasing the NaCl and urea concentrations were minor. The use of vacuum slot extraction to remove superficial dye solution from the impregnated material prior to heating gave more efficient drying (higher fabric speeds and lower power consumption) and increased the dye furation yield.
Introduction Drying thin sheets of materials such as paper and textile fabrics using medium-wave infrared radiation (2-5pm) involves a relatively high power density ( > l o W/m2) and high energetic efficiency >70% (Broadbent et al., 1994). As the water content of the material approaches low values, the infrared drying process must be carefully controlled because of the rapid increase in sheet temperature and risk of overheating and scorching. Several recent papers dealt with modeling and simulation and multivariate control of this process (ThBrien et al., 1990, 1991; Forget et al., 1993; Dhib et al., 1994). Since some wavelengths penetrate into the sheet, the initial temperature profile in the sheet is less pronounced than in hot air or hot cylinder drying and evaporation occurs both a t and below the sheet surface. Infrared drying therefore suppresses the migration of chemicals to the sheet surface during the constant rate drying period (Singleton, 1980). In a recent publication (Zhao and Broadbent, 19931, we described some preliminary results on the application of various reactive dyes t o cotton fabric by means of impregnation with an alkaline dye solution (padding) followed by infrared drying and heating. Even though the process was not optimized, the dye fixation yields obtained were considerably higher than those found in conventional processes where the impregnated fabric is dried in a hot-air unit. In addition, we established that the best fixation yields required rather forceful conditions, i.e., high dye bath pH and drying and heating the fabric to temperatures around 140 "C, using longer residence times in the dryer and higher emitter power settings. Reactive dyes are applied to cotton using an alkaline solution. Hydroxide ions from the aqueous solution penetrate into the cellulose and cause dissociation of some hydroxyl groups. The cellulosate ions (CELL-0in Figure 1)thus formed give a nucleophilic substitution or addition reaction with the reactive group of the dye which becomes covalently bonded to the polymer (Figure 1). The final color thus has excellent fastness to washing. Unfortunately, the hydroxide ions necessary for activation of the cellulose also give a similar reaction producing the hydrolyzed form of the dye which is incapable of reacting with the cellulose. This must be washed out of the material after the dyeing process to
DyEN / H-& . CI
(OH)
I
CELLO/
0-CELL FIXED DYE
/
\ CI
HO\
DyE,NH&Cl
(OH)
OH HYDROLYZED DYE Figure 1. Fixation and hydrolysis of a trichloropyrimidine fiberreactive dye.
achieve the best fastness to washing in use. The fixation yield is simply the mass of dye which covalently bonds to the cellulose relative t o the total amount added t o the dye bath. This paper analyzes the dependence of the dye fixation yield and also of the final sheet temperature and water content, on the controllable process variables. This was achieved using two factorial plans of experiments and statistical analysis of the results. The dye selected for study was of low reactivity and would not usually be recommended for application in such a pad/ heat process (Fox and Sumner, 1986). As initially hoped, the low reactivity of this dye gave a large variation in the fixation yield for the selected changes in the controllable process variables. The objective was to determine the optimum conditions for application of this dye to cotton using infrared heating of the cotton fabric impregnated with the dye solution.
Experimental Section The equipment used for the padding and infrared fixation process is shown in Figure 2. The bleached cotton fabric (plain weave, superficial density = 164 g/m2) was first passed through the pad bath containing the reactive dye (Drimarene Red X-GBN, Sandoz Canada Inc.), NaC1, NaOH, and possibly urea. It was then squeezed between the driven pad rollers to aid penetra-
0888-5885/95/2634-0943$09.00/00 1995 American Chemical Society
944 Ind. Eng. Chem. Res., Vol. 34,No. 3, 1995 INFRARED DRYER CUT VACUUM SAMPII S EXTRACTOR !' F
FLOW OF AIR
-
7
Figure 2. Schematic of the infrared process equipment.
tion and express surplus solution. A second set of driven rollers held the fabric with the correct tension and alignment over the vacuum extraction slot (Evac Inc., Spartanburg, SC) formed from two parallel moulded polyethylene plates. The vacuum tube on which the slot was mounted was connected to a 7.5 kW vacuum pump. The system was capable of generating pressure differentials of between 10 and 72 kPa. A cyclone separator was used to separate the extracted liquid and lint from the air flow to protect the pump. Details of this equipment have been published elsewhere (Broadbent, 1990; Broadbent et al., 1991). The dye bath solution was freshly prepared by mixing a predetermined volume of dye solution, having known concentrations of the reactive dye, NaC1, and urea, with a solution of NaOH just before padding. This avoided premature hydrolysis of the reactive group of the dye. A sample of the wet fabric was cut after padding and mounted on a 24 x 38 cm pin frame. This was supported in an opening in the stainless steel mesh conveyer and passed through the infrared dryer (Radiant Heat Inc., Coventry, RI). The effective heating zone was 1.44 m long. The tubular infrared sources (iron/ aluminum filaments inside 67.5 cm quartz envelopes) were installed in four independently operated modules (5 kW each) transversely placed above and below the conveyor in a uniform manner. At the maximum operating voltage of 460 V, the quartz sources have an operating temperature of about 760 "C, with maximum emission around 3 pm. Polished aluminum reflectors behind the tubes redirected as much radiation as possible towards the textile. In all the tests, the four modules were situated a t a distance of 5.1 cm from the fabric and operated a t the same power level. An infrared pyrometer (Modline 4, Ircon Inc., Niles, IL) situated a t the exit of the infrared oven was used to measure the temperature of the fabric surface as it left the oven. The ventilation system allowed introduction and evacuation of air using transversely placed slots, the flow being directed along the upper and lower faces in the same direction as the moving fabric. The air introduced was at ambient temperature with a flow rate of 4 kg/min and was evacuated a t a slightly higher rate. This allowed effective removal of steam from the fabric surface. To measure the water content of the cotton fabric, samples were cut after padding and again after drying. They were immediately sealed in preweighed plastic bags and reweighed before drying a t 110 "C for 90 min, cooling in a desiccator, and reweighing (ASTM, 1991). Fabric samples were also cut after padding, or after fixation in the dryer, to determine the dye fixation yield. Any unfured dye was removed from these by two 5 min extractions with boiling water, the first containing a small amount of HC1 to just neutralize the alkali in the fabric. The furation yield of the reactive dye was
Table 1. Values of Controlled Variables for Infrared Dye Fixation with and without Prior Vacuum Extraction variable symbol units lower limit upper limit Without Vacuum fabric speed S dmin 3.10 4.00 emitter power P kW 10.03 13.06 10 40 NaCl conc E & 0.5 5.0 NaOH conc A gA urea conc u gA 0 100 With Vacuum 3.80 4.60 fabric speed S dmin 7.84 10.55 emitter power P kW 0 39 vacuum level V kPa
determined from the calculated quantities of unfixed dye per gm of dry fabric using spectrophotometric measurements on the appropriately diluted aqueous extracts from fabric samples cut immediately after padding and after drying Di - Df furation yield = -x 10 Di At its wavelength of maximum absorption (520 nm), the commercial dye followed the Beer-Lambert law giving absorbance at 520 nm = 19.84 x concentration ( g m (2)
Even though the initial sample was cut within 10 s after padding, and immediately neutralized and extracted, some dye had already reacted and could not be removed by boiling water, so the cotton was a very pale pink. Fixation yields were therefore corrected by a method based on measurement of the reflectance and calculation of the Kubelka-Munk WS values of the extracted and dried initial and final fabric samples (Zhao and Broadbent, 1993). The corrections always increased the fixation yield by less than 2%. Factorial Plans. For trials in which the padded fabric was not vacuum extracted prior to heating, the five independent controllable process variables whose effects were examined were emitter power (P), fabric speed (S),NaOH (A), urea (U),and NaCl ( E )concentration (Table 1). All the remaining operational parameters such as the dye solution concentration (20 g/L), the initial temperature and solution pickup of the padded fabric (65%),the initial temperature, humidity, and flow rate of the ventilating air, and the source/web distance (5.1 cm) were held or were assumed constant. For tests of the effects of vacuum extraction, only three controllable variables were studied, namely, emitter power (P), fabric speed (S),and vacuum level (v> (Table 1). Because there was a considerable reduction of the water content of the impregnated wet fabric after vacuum extraction (65-45% water), the key operational parameters for drying ( P and S)had to be significantly different when vacuum extraction was used. Its effect was therefore studied in a separate factorial plan. All chemical concentrations were held constant for this second series of trials with dye a t 30, NaCl a t 10, and NaOH a t 5 g/L. For each factorial plan, two levels of operation were selected for each controllable variable. Thus 32 (25) trials without prior vacuum extraction and 8 (23)with vacuum were carried out, each trial being duplicated (Montgomery, 1991). The measured process output responses were the fabric water content and tempera-
Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 945 Table 2. Two-Level Factorial Plan and Process Responses for Infrared Dye Fixation without Prior Vacuum Extraction
Table 4. Coefficients for Empirical Relationships between the Process Responses and Controlled Variables without Prior Vacuum Extraction
coded values of input variables
RF.% A U - 1 -1 3.2 4.0 -1 -1 2.0 2.5 -1 -1 62.1 62.3 -1 -1 6.2 6.1 4.4 4.6 -1 -1 -1 +1 2.6 2.2 +l -1 -1 63.3 63.9 +1 -1 -1 6.6 6.5 -1 +1 -1 18.0 18.5 -1 +1 -1 3.5 4.8 -1 +1 -1 81.3 81.5 -1 +1 -1 31.6 31.4 +1 +1 -1 24.8 27.1 tl +1 -1 11.1 11.2 +1 +1 -1 86.4 86.8 $1 +1 -1 40.4 40.3 -1 -1 +l 3.1 3.2 -1 -1 +1 2.3 2.5 -1 -1 +1 36.4 35.0 6.8 7.4 -1 -1 +l +1 -1 +l 5.6 6.7 4.0 3.7 +1 -1 +1 +1 -1 +1 39.1 37.5 +1 -1 +1 8.5 8.3 -1 +1 +1 22.0 20.6 -1 $1 +1 5.6 6.4 -1 +1 +1 90.9 90.8 -1 +1 +1 52.1 54.2 +l +l +1 30.3 29.3 +l +1 +1 13.8 11.8 +1 +1 +1 92.1 92.2 +1 +1 +l 54.8 54.4
S P E 1 -1 -1 +l -1 -1 -1 $1 -1 +l +1 -1 -1 -1 +1 +1 -1 +l
-1 +1
+l
-1 +l
-1 -1 $1 +1
+1
-1 +1 -1 +1 -1 +l -1
-1
-1 +1 +1 -1
-1
+1
-1
+1
+1 -1
+1 -1 -1 +1
+1
+1
-1 +l
-1 +l -1 -1 +1 +1 $1 -1 -1 $1 -1 -1 +1 +1 +l -1
Rw, %
3.8 16.6 0.9 2.7 6.9 19.5 2.9 6.1 4.4 17.1 0.6 2.7 7.2 18.2 1.4 4.8 9.2 18.1 3.1 8.2 11.3 19.8 4.4 10.6 9.2 18.4 3.1 8.6 11.2 20.4 4.4 10.6
3.8 15.6 03 2.5 8.0 18.8 2.5 6.1 4.4 15.3 0.7 2.6 6.0 19.3 0.9 4.6 8.3 17.3 2.4 7.6 10.9 19.2 4.4 10.6 9.4 17.9 2.4 8.5 11.6 19.9 4.0 10.4
RT. "C
60 43 144 72 57 44 155 72 55 44 142 77 55 44 148 74 72 45 153 107 77 47 159 110 78 45 157 108 81 46 158 107
59 43 148 76 54 43 151 69 52 43 145 73 58 44 151 78 72 46 155 103 77 47 157 105 76 45 158 108 78 47 158 110
Table 3. Two-Level Factorial Plan and Process Responses for Infrared Dye Fixation Preceded by Vacuum Extraction coded values of input variables RF,% Rw,% RT,"C s P V ~
1
-1
-1
+l
-1
-1
-1 -1 +l $1 +l +1
-1
+1
+1 -1
$1 -1
+1 -1
-1 $1
$1
+1
7.6 8.2 14.5 15.4 40 4.5 4.9 20.5 19.2 40 12.9 12.9 4.9 6.7 49 8.8 9.8 11.0 9.6 45 25.1 28.1 1.0 1.0 68 14.0 13.8 2.5 2.3 52 83.6 85.1 0.3 0.2 136 48.8 44.1 0.6 0.6 105
40 40 47 46 69 54 138 100
ture on leaving the dryer and the dye fixation yield. The reproducibility was good (Tables 2 and 3). Multiple linear regression was used to obtain empirical relationships between the output responses and the controlled operational variables. These were then used for numerical simulation of the process results and comparison with experimental data.
Results and Analysis The experimental results are given in Tables 2 and 3 and showed the anticipated wide range of dye fixation yields as well as the conditions for optimum furation ('90% yield). The model for a given response Ri was written in terms of coded values of the controlled variables and coefficients describing the magnitude of their individual effects and second and higher order
coefficient
std dev of estimate of response coefficients neglected
estimates of coefficients R F ( % ) Rw(%) RT("C) 28.6 8.7 88.5 -12.8 3.8 -20.9 18.8 -4.1 33.0 1.2 12.6 1.6 8.1 -8.6 -1.6 -10.1 2.3 6.3 2.5 3.4 2.5 5.3 2.9 0.983 0.983 0.997 4.5 1.2 2.5 x2.0 -=LO x2.0
interactions (Montgomery, 1991). These have the form
Ri = Po + j3lS + By? + ... + PsU + P1#P
+ Pi3SE +
... + Pl5SU + /3123SPE + other high order
interactions (3) where S, P, E, etc., represent the coded value of each particular variable with values ranging from +1(upper limit) to -1 (lower limit): coded value of var; nhln =
V - (hi + 10)/2
~*,
Multiple linear regression was used to calculate the values of the coefficients &,pi, pij, etc.) for the principal effects and second and higher order interactions (eq 3) for the three responses dye fixation yield (RF), and final fabric water content (Rw)and temperature (RT). Equations such as eq 3, but including many third and higher order interactions, obviously gave higher values for the correlation coefficients but become rather cumbersome. We therefore performed regressions using only significant coef'Eicients having values above a certain level (Table 4). This allowed equations with good predictive capability which were easy to manipulate. Table 4 shows that the most important variables influencing the dye fixation yield were fabric speed, emitter power and NaOH concentration. We had previously established that higher fixation yields were obtained using NaOH as the cellulose-activating alkali rather than a weaker one such as Na2C03 (Zhao and Broadbent, 1993). The effects of the urea and NaCl concentrations were much less significant. The fabric speed and emitter power were also the principal variables influencing the final fabric temperature and water content. These responses were barely affected by the NaOH concentration but were both somewhat greater in the presence of urea, which retards loss of water during drying. When the cotton fabric was padded with dye solution, and then vacuum extracted to remove the bulk of the interstitial solution, the falling rate drying period began almost as soon as the fabric entered the dryer and RT was higher. The reaction of the dye with cellulose under alkaline conditions occurred mainly at temperatures above 100 "C, which were easier to attain if the impregnated fabric had been vacuum extracted. In the simpler factorial plan for evaluation of the effect of prior
946 Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 Table 6. Coefficients for Empirical Relationships between the Process Responses and Controlled Variables with Vacuum Extraction before Fixation estimates of coefficients coefficient R F ( % ) Rw(%) RT('C) 25.7 6.9 66.8 80(intercept) -7.2 1.4 -6.6 P1 (SI 12.5 -2.6 16.4 Pz (PI 17.1 -5.8 2.4 8 3 (v) -5.5 -5.9 813 (su 10.1 2.0 13.1 8 2 3 (pv) correlation. R2 0.969 0.980 0.989 std dev of estimate of response 8.9 1.6 6.8 coefficients neglected
