Fundamental Investigation of Ultrasonic Effects in Textile Wet Processing

Transcription

1 C95-G13 Fundamental Investigation of Ultrasonic Effects in Textile Wet Processing W. W. Carr, S. Michielsen, and H. W. Beckham Georgia Institute of Technology Gary Mock and Bob McCall North Carolina State University Project Goals: To identify the mechanisms responsible for the observed advantages gained by employing ultrasound in textile wet processes and to assess the relative importance of the effects of ultrasonic irradiation on basic transport phenomena occurring in the dye solution, in the polymer, and in the textile. Abstract The use of ultrasound in textile wet processing offers many potential advantages. Previous work has shown that the use of ultrasonics in dyeing provides the following benefits: (1) energy savings and reduced processing times, (2) environmental improvements, (3) process enhancement, and (4) lower overall processing costs. A fundamental understanding of the operative processes are needed so that meaningful application of ultrasonics in textile processing may be made. This project is directed at determining the phenomena responsible for the effects of ultrasound in textile wet processing and the relative importance of phenomena occurring in the bulk solution and in the fiber. Our studies show that ultrasound increases the swelling of both unmercerized and mercerized cotton. Although the beneficial effects of ultrasound have been reported for dyeing polyester, no effect of ultrasound on the glass transition temperature was observed. An experimental approach has been developed that will enable a direct determination of the effect of ultrasound on the diffusion coefficient of the dye within polyester. Research including modeling is being conducted to determine the effect of ultrasound on the partition coefficient and enhancement of dye transport to the fiber surface by reducing the boundary layer thickness. The effect of ultrasound on the particles size of insoluble dyes is being

2 G studied. Ultrasound reduces the average particle size of vat dyes in their insoluble form suspended in water and eliminates the larger particles. Introduction The use of ultrasound in textile wet processing offers many potential advantages. Previous work by NCSU researchers [1,2] and others [3,4] has shown that the use of ultrasonics in dyeing provides the following benefits: (1) energy savings by dyeing at lower temperatures and reduced processing times, (2) environmental improvements by reduced consumption of auxiliary chemicals, (3) process enhancement by allowing real-time control of color shade, and (4) lower overall processing costs, thereby increasing industry competitiveness. A fundamental understanding of the relative operative processes is needed so that meaningful application of ultrasonics in textile processing may be made. Although several fundamental studies [5-9] on the effects of ultrasound in various physicochemical processes have been conducted, the mechanisms responsible for the effects of ultrasound on textile processes have not been established. This project is directed at determining the phenomena responsible for the effects of ultrasound in textile wet processing and establishing the relative importance of phenomena occurring in the bulk solution and in the fiber. The mechanisms under investigation include: 1.) increasing fiber swelling in water, 2.) reducing glass transition temperature of the fiber (dilation of amorphous regions), 3.) increasing the diffusion coefficient of dye in the polymer, 4.) increasing the fiber/dye bath partition coefficient, 5.) enhancing transport of the dye to the fiber surface by reducing the boundary layer thickness, and 6.) breaking up of micelles and high molecular weight aggregates into uniform dispersions in the dye bath. Cotton Fiber Swelling (G. Mock, R. McCall, D. Klutz) The swelling of both mercerized and unmercerized cotton fibers with water alone and with ultrasound has been studied. Ultrasound causes significant fiber swelling compared to water alone. Since cotton fibers are very non-uniform and hard to measure, the tests were repeated numerous times, and the results averaged. Several methods of measurement were used to confirm the results.

3 G For unmercerized cotton, the swelling with water alone ranges from 10% to 20%, but with ultrasound the range is from 25% to almost 50%. For mercerized cotton, the swelling with water alone is only about 3% but with ultrasound is about 35%. Remember, the mercerization process causes permanent swelling of the cotton fiber. Of interest here is that ultrasound still causes additional swelling. The fiber swelling is observed for at least an hour after the removal of the ultrasound, as long as the fiber remains in water. When removed from water and allowed to dry, the fiber returned to its original diameter. Dilation of Polymeric Amorphous Regions (W. Beckham, J. Good, and J. Zhan) Synthetic fibers such as polyester must be heated above their effective glass transition temperatures (T g ) to allow dye penetration of amorphous regions. Plasticizers are often added to lower the glass transition and thereby facilitate dyeing. Since it is known that ultrasound enables dyeing at lower temperatures, ultrasonication may cause "mechanical plasticization" of the polymeric fibers. This effect, dilation of polymeric amorphous regions, was studied using dynamic mechanical analyses of polyester films in a water bath as a function of ultrasonication. If incident ultrasound causes dilation of polymeric amorphous regions, one postulated effect is a lowering of the glass transition temperature, the temperature which governs the minimum dyeing temperature. The results indicate that ultrasound is not affecting the glass transition temperature. Diffusion and Partition Coefficient Studies (S. Michielsen, W. Carr, P. Das, and E. McFarland) Previously, we reported the adoption of Disperse Blue 79 to directly measure the diffusion coefficient and partition coefficient during dyeing of PET films. A well established recipe [10] was taken from the literature for dyeing PET films. A 1 L aqueous dye bath containing 10 mg/l of C.I. disperse blue 79, 10 g/l of benzoic acid, and 2 g/l of Triton X- 100 was prepared and heated to 100 C. Seven rectangular pieces of PET films (1 inch X 0.5 inch) were hung in the dye bath using a stainless steel wire. Each piece of film was separated by stainless steel washers to allow full exposure of the film to the dye solution. Minimal stirring

4 G was provided by magnetic stir-bars inside the bath. A solid PET film and an aqueous sample were removed from the bath at 0, 1, 2, 4, 8, 16, and 24 hours. The aqueous samples were diluted, and the dye concentration measured by absorption using a Spectronic -20 and shown in Figure 1. The dye concentration falls by more than 75% during the 24 hours of dyeing. Comparing 1a and 1b it is evident that the dye is being degraded. A multi-stage batch dyeing procedure was developed in order to circumvent this dye degradation (hydrolyzing?) problem. In this process, the bath volume was reduced to 250 ml, and the film samples were placed in a fresh dye bath that has just reached the appropriate dyeing temperature. This bath was replaced by an identical bath every hour and aqueous samples were removed from the bath before and after each 1-hr dyeing stage. The ratio of final to initial concentration of dye in each bath is illustrated in Figure 2. The dye degradation in each dye bath is minimal in this modified dyeing procedure. Using this procedure, the film samples are exposed to a (nearly) constant dye concentration. C inst. / C initial (%) (C instantaneous /C initial ), film present (C instantaneous /C initial ), no film Dyeing Time (hr) Figure 1. Dye depletion (a) in presence of film samples and (b) with no film present.

5 G (C Final /C Initial ), % (C final /C initial ), % Dyeing Time (hr) Figure 2. Dye depletion for multi-step dye process with film samples present. Degradation of the dye within the film may still be a problem, but we believe that it is much less likely than degradation in the aqueous solution. Clearly, earlier experiments with this dye are suspect whenever the dye time exceeds two hours. We are now attempting to measure the dye concentration, C(t,x), as a function of time t and of position x within the film using a combination of Raman and fluorescence microscopy. Once the technique for measuring the concentration C(t, x) is established, the mass diffusion equation for one-dimensional diffusion through a stationary membrane will be used to estimate the diffusion coefficient D [11], during transient dyeing experiments on thin PET films. The assumption will be made that D is independent of concentration. To calculate diffusion coefficient, the following equation will be used: C D = t 2 C 2 x The derivatives will be evaluated by using the central difference approximation.

6 G An analysis has been made to estimate the errors in calculating D associated with the central difference approximation and the uncertainty in position x. Concentration C(t,x) was calculated by using the one-term approximation to the series solutions for transient one-dimensional diffusion through a plane wall. The one-term approximation is valid for Fourier numbers greater than 0.2 [11]. All Fourier numbers used in the calculations were greater than 0.2, and a diffusion coefficient of 1 x m 2 /s was used. Calculations were made to show the effects of Fourier number, Biot numbers, and distance moved between successive pixel measurements (jump size). The initial analysis assumed that measured concentrations were equal to the calculated concentration. The effect of error in measurement of concentration at a given location will be discussed later. Since it is not possible to exactly determine the edge of the sample, one source of error is uncertainty in position x. The effect of uncertainty in position is largest in estimating C/ t since concentration profiles measured at different times are used for corresponding locations in the film. The uncertainty in position was assumed to be one half the distance moved between successive pixel measurements. The effect of positional uncertainty on the calculated value of D was evaluated. Each concentration in the equation for D was calculated for minimum and maximum position uncertainty. Since concentrations at different positions and time are used in calculating D, various combinations had to be evaluated. The rate of change of concentration with time, C/ t, was evaluated using times corresponding to Fourier numbers of 0.2, 0.3, and 0.4. Several values of Biot numbers and jump sizes were used. The maximum calculated % error in diffusion coefficient due to uncertainty in position is given in Table 1. As jump size increased, an error in the position measurement caused a greater increase in error in diffusion coefficient. The error in diffusion coefficient also increased with Biot number. The analysis discussed above assumed that the measured concentration equaled the theoretical value at the specified position. Since the measured concentration will differ from the

7 G Table 1. Error analysis associated with uncertainty in positional measurements. a) Bi: Biot Number, b) Jump: Jump size (µm), c) % Error: Maximum Percent Error in D with Uncertainty in Positional Measurements Bi a Jump b % Error c actual concentration for several reasons, the analysis is being extended to include this type of error. Initial calculations were made using random concentration errors up to 10%. For Biot numbers of 100 and a jump size of 2 microns, errors in diffusion coefficients as high as -92% were obtained. When Biot numbers was reduced to 10, errors as high as -90% were obtained. Thus the initial results indicate that errors in concentration measurements can have a big effect on diffusion coefficient when measured in transient diffusion. Once the errors associated with the technique being used to measure concentration are established, the analysis will be used to determine if measuring D from transient diffusion measurements is viable. An apparatus for studying the steady state diffusion of dye through a film is being constructed. The apparatus consists of a diffusion cell that is a standard electrochemical H-cell and a two part glass apparatus separated by the film. One side (side 1) of the glass apparatus will be filled with the dye solution prepared in a buffer solution. The other side (side 2) of the glass apparatus will be filled with a buffer solution only. By circulating the dye solution on side 1 through an expansion reservoir of dye solution, the concentration of dye in solution will remain constant. Side 1 will be stirred with a mechanical stirrer. A microtip transducer will be

8 G used for isonated dyeings. The solution on Side 2 will be circulated through a spectrophotometer to measure absorbance as a function of time. The results will be used to plot dye diffused through the film versus time. When steady state is reached in the system, the rate of dye transport through the film can be measured. The method discussed above will be used to determine the concentration gradient in the film. This will allow calculation of the diffusion coefficient and the mass transfer coefficient. Thus the relative importance of the phenomena occurring in the bulk solution and in the fiber can be established. Dye Particle Size (G. Mock, R. McCall, D. Klutz) Vat dyes were examined in their insoluble unreduced form suspended in water to determine if ultrasound changed the dye particle size. Five different vat dyes of known structure were examined, most of which had previously been used in ultrasound dyeing trials. A 5 g/l solution of each dye was prepared, then divided in half to give two samples for each dye. One sample of each dye was treated with 20 khz ultrasound at 25 o C for 60 minutes; the other sample was used as a control untreated sample. The dye samples were measured with a Honeywell Microtrac Particle Size Analyzer. This instrument measures each sample twice and averages the results to produce an average particle size and a particle size distribution. The results of the particle size analysis are shown in Table 2. The particle size distributions of all the untreated and treated samples are summed together and displayed in Figure 3. Ultrasound reduced the average size of the dye particles for each of the dyes tested. The effect ranged from virtually nothing in the case of Vat Black 25 to greater than 1.75 microns in the case of Vat Violet 1. The effect of ultrasound on particle size is most evident when examining the before and after graphic distributions drawn by the analyzer. Ultrasound had the greatest effect on vat dyes with a bimodal particle distribution. After these dye samples were treated with ultrasound the large particles were completely eliminated. Without ultrasound, vat dyes may contain particles larger than 14 µm, but when vat dyes are treated with ultrasound the largest dye particles are smaller than 2 µm.

9 G Table 2. The Average Particle Size Of Vat Dyes Dye Manufacturer Without Ultrasound (microns) With Ultrasound (microns) Avg. Size Decrease (microns) Vat Violet 1 Catawba Charlab Vat Black 25 Catawba Charlab Vat Black 25 Sunbelt Vat Green 3 (Dyeing) Catawba Charlab Vat Green 3 (Printing) Catawba Charlab Vat Green 1 Catawba Charlab Vat Violet 13 Sunbelt Total % of Particles In Each Size W/O US With US Particle Diameter (microns) Figure 3. The Summed Particle Size Distributions of the Untreated and Treated Dye Samples

10 G Figure 3 shows a combined plot of all the samples treated. The solid bars are untreated dyes and the open bars are the ultrasound treated dyes. It can be seen that ultrasound completely eliminates the larger particles (> 2 µm). Treating vat dyes with ultrasound shifts the particle size distribution toward smaller particles. References 1. Klutz, D., Master's Thesis, North Carolina State University, Thakore, K., Smith C., and Clapp, T., American Dyestuff Reporter, 30, October, (1990). 3. Thakore, K., Indian Journal of Textile Research, 13, 133 and 208 (1988). 4. Shimizu, Y., Yamamoto, R., Shimizu, H., Textile Research Journal, 59, 684 (1989). 5. Suslick, K., Ed. Ultrasound, Its Chemical, Physical, and Biological Effects VCH: New York, (1988). 6. Liu, L-S., Kost, J., D'Emanuele, A, Langer, R, Macromolecules, 25, 123 (1992). 7. Henglein, A., Gutierrez, M., J. Phys. Chem., 94, 5169 (1990). 8. Cost, M., Mills, G., Glisson, P., and Lakin, J., Chemosphere, 27, 1737 (1993). 9. Riesz, P., Kondo, T., Free Radical Biology & Medicine, 13, 247 (1992). 10. Tincher, W. C., Preparation, Coloration, and Finishing Laboratory Maunal, Georgia Institute of Technology, Atlanta, Georgia (1994). 11. Incropera, F. P. and DeWitt, D. P., Fundamentals of Heat and Mass Transfer, 3rd ed., John Wiley & Sons, New York (1990). Presentations/Publications 1. J. Good, J. Zhan, D. Klutz, G. Mock, and H. W. Beckham, Fundamental Investigations of Ultrasound-Enhanced Dyeing, AATCC International Conference and Exhibition, Atlanta, GA; 8 11 October Klutz, D. S., Ultrasound Aided Exhaustion Dyeing, MS Thesis, NCSU, 1993.