Yang, Yiqi
ABSTRACT
One concern associated with wet-on-wet processing is the changing chemical concentration in the pad bath. After processing for several hundred yards or so, chemical add-on considerably decreases, resulting in poor performance properties of finished goods or decreased shade depth. To obtain a constant bath concentration and add-on, an appropriate initial concentration and a reconstitution stream with higher concentration than the initial concentration should be used. The validity of a mathematical model relating parameters in wet-on-wet padding is proven by experimental data from both laboratory and production scales. Using that model, changes in pad bath concentration can be quantitatively described. For given processing conditions and add-on requirements, an initial pad concentration and a different reconstitution concentration can be obtained from the model to assure a constant add-on from the beginning to the end of the wet-on-wet process.
A conventional continuous textile wet process is to apply chemicals to a dry fabric by means of padding after the dry fabric has been wetted in a chemical bath. Such a process is called "wet-on-dry" (WOD). Padding the dry fabric (WOD) allows quick penetration, high add-on, and easy concentration control in the padding bath. "Wet-on-- wet" (wow) is a term used to describe a padding process starting with a wet fabric. This process has been quickly accepted in many textile plants as a padding procedure to substitute for conventional WOD processes from slashing [5, 10] to dyeing [1-4, 6] and finishing [7-9]. Some of the advantages of WOW over WOD are the requirement of smaller amounts of chemicals to obtain the same results as the WOD process, energy savings by eliminating fabric drying between two wet treatments, and decreasing or eliminating wetting agents.
One problem associated with the wow process is varying chemical concentration in the pad bath and chemical add-on of the finished fabric. A common mistake in the WOW practice is to assume that the chemical add-on is based on the net increase of the wet pickup of the fabric after being padded through the chemical pad. For example, if the pickup of the water pad is 50% and the pickup of the chemical pad is 100%, the chemical concentration should be doubled to obtain the same add-on as WOD for a pickup of 100%. If such a calculation is used, the chemical add-on is much higher than the targeted one at the beginning of the process, but decreases gradually during processing, leading to inconsistent performance properties in the finished goods.
When the wet fabric enters the chemical pad and is being nipped, some of the water on the wet fabric goes into the chemical bath. On the one hand, this increases the net pickup from the chemical pad and, on the other hand, dilutes the chemical concentration in the bath over time. Such a water interchange causes a higher chemical add-on in the beginning and a continuous decrease in add-on with processing.
To avoid diluting the chemicals, the interchange of water from the wet fabric into the chemical pad must be considered. Our objective in this work is to determine the quantitative relationship between the change in chemical concentration in the bath and padding conditions, so that constant bath concentration and chemical add-on can be achieved with the wow process.
Conclusions
We have discussed a mathematical model for wet-on-- wet padding for both laboratory and production scales. Using that model, changes in pad bath concentration can be quantitatively described. For given processing conditions and add-on requirements, the model provides an initial pad concentration and a different reconstitution concentration to assure a constant add-on from the beginning to the end of the wet-on-wet process.
The quality of the dyed goods from the wow pad-batch dyeing is the same as fabrics dyed with the conventional WOD technology, including color yield, shade variation, and colorfastness to light, wash, and crock.
A well-controlled dyeing process with constant bath concentration, shade depth, and color quality indicates that the wow technology developed in this work is appropriate for application to all prewet padding processes, including-but not limited to--prewet sizing, wow dyeing, and wow finishing.
ACKNOWLEDGMENTS
We wish to express our appreciation to Neil Stewart and Gary Moore of the Institute of Textile Technology for their assistance throughout this research. We also thank Milliken & Company for providing the machinery, materials, and technical assistance necessary for the study.
Literature Cited
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Manuscript received October 13, 1999; accepted December 22, 2000.
YIQI YANG1 AND STEVEN HENSLEY
Institute of Textile Technology, Charlottesville, Virginia 22903, U.S.A.
1 Current address: Dept. of Textiles, Clothing & Design, University of Nebraska-Lincoln, Lincoln, NE 68583-0802.
