By: Pavan S. Chinta and Sapna Dhar
The art of processing of textiles is not new aspect. As the time passes new processes, methods and machines have been in use to improve the productivity and quality of the processed fabric. In the era of energy and water saving and eco-friendly processing, to maintain the quality and cost has become tough task. It is always found that 70% problems of dyeing are attributed to poor treatment of the fabric. Hence it becomes necessary to give emphasis right from pretreatment to ultimate dyeing steps to achieve best quality dyed goods. "RIGHT FIRST TIME AND ALL THE TIME RIGHT" dyeing quality can be easily achieved by strictly adhering to process control parameters of each and every step of fabric processing.
In this paper an attempt has been made to highlight the various problems encountered due to men, machine and other factors of processing for damage about the possible causes and remedies of these dyeing problems helps the processes as a ready recockener for avoiding these problems.
The art of processing of textiles today is well known to all processors. In the past the organized sector dominated the textile scene; whereas today the decentralized process houses are predominant. There is a shift in emphasis from the requirements related to finished products to environment related requirements. Requirements related to the finished product fastness & other properties of dyestuffs & chemicals, processing etc. shift of emphasis environment related requirements, cost reduction, security, energy & water saving, toxicology, hygiene legal regulations, and competitiveness.
Reduction in costs and utilities are achieved by the trend to telescope processes. In order to be competitive processors would have to give equal if not better quality then the existing material at the same price. Considering the energy and labour costs, the only means of maintaining a competitive edge is to lay emphasis on 'Quality'. Good quality does not necessarily mean increased prices, but in fact would mean "Doing it right the first time" thereby reduce the costs.
Thus with this theme in mind of "Doing it right the first time" we would have to first understand the problems in the process of dyeing. So, this paper mainly discusses "Problems in Dyeing & its remedies".
The ultimate law of accuracy states, "when working towards a solution of a problem, it always helps if you know the answer provided, of course you know there is problem".
Every manufacturing unit has a number of problems, however problem solving cannot be considered to be astrology or palmistry, but it needs the fullest knowledge of basic facts.
The stages in the processing of textiles are:
The faults of textile processing are divided as follows:
- Pretreatment faults :21%
- Dyeing & Printing faults :23%
- Finishing faults :11%
- Biological & machine faults :24%
- Diverse faults :22%
70% of the faults can be attributed to the preparatory pretreatment, because it is the first stage in processing and all other processes depends on its effectiveness and efficiency. Some of faults due to pretreatment can be listed as under:
- Poor absorbency
- Catalytic damage/poor fluidity
- Moire effect
- Shade change from selvedge to selvedge
- Creasing/chafe marks
- Inferior brightness/luster
- Cloudy dyeing
- Pale areas
Most of the above faults could be easily corrected with the following precautions:
- Use of speciality chemicals
- Select suitable heat setting temperatures
- Thorough relaxation of the material
- Controlled tension and uniformity of batching during pretreatment
- Checking for rough patches in the machine
The dyeing process which follows pretreatment depends on a number of parameters like:
- Conc. of chemicals and auxiliaries selection
- pH of the process etc.
Remedies to above mentioned dyeing problems
- Select dyes with better dispersion stability, use efficient dispersants.
- Optimizing a dyeing process means adjusting the process parameters in such a way that the entire operation takes place in the shortest possible time without exceeding the tolerance limits with regard to reproducibility and levelness. This means rapid heating in the temperature ranges in which the dye exhausts slowly or is already exhausted; controlled heating when the dye exhibits a particularly high rate of exhaustion. The individual steps for an optimum dyeing process are:
- Heat at the maximum rate to the threshold of the critical temperature rangeg t1(phasea)
- Control the rate of heating in the critical temperature range t1-t2(phase b)
- On reaching t2(upper limit of the critical temperature range) heat again at the maximum rate to the fixation temperature (phase c)
- Select optimum conditions and chemicals
- Adjust the machine more carefully
- Use antifoamers, e.g. Antimussol brands
- Use special chemicals, e.g. Welting agent/defoamer combinations, or greater pressure
- Adapt dye selection
- select higher dyeing temperature
- select suitable carriers and chemical
- possibly set fibres at a higher temperature (caution!)
- Pay attention to perfect pretreatment
- Pay particular attention to the appropriate dyeing process and programme
- Select special dyes (smaIl-molecule)
- Pay attention to stability of dye to electrolytes
- Pay attention to consistent dyeing conditions
- Carry out lab tests
- Carry out preliminary tests with material
- Check and adjust dyeing conditions
- Pay special attention to dispersing instructions
- Pay attention to perfect liquor circulation
- Pay attention to cleanness in the machine I
- Select suitable carriers, use overhead heating in the machine
- Use silicone-free defoamers
- Drop dyebath hot, shorter dyeing time, use special chemicals
- Complete removal of solvents
- Do not store material near dyes;use low dusting dyes
- Select more suitable dyeing m/c
- Straight seams along weft 'and stitches
- Avoid prolonged contact with the hot machine
- Select suitable setting temperature
- Check injection pipe or mount a distribution plate
- Check rollers (pickup) regularly
- Check temperature and air current over the entire width in the drier
- Avoid migration by using lowest possible pickup and a suitable migration inhibitor.
- Check fixation units regularly with thermo paper or temperature sensors.
- Set roller in motion
- Check shore hardness, if necessary re-cover rollers
- Check temperature and air current in drying machine
- Check the air jets
Trouble shooting during yarn dyeing:
- Strength loss
- Harsh handle
- Channel building
- Unlevel dyeing
- Poor batch to batch reproducibility
- Package deformation & yarn abrasion
- Shade variation in Inner, Middle &.Outer layer
Remedies for problems in yarn dyeing:
It is said that "well wound is half dyed". A
uniform package density has to be ensured both within the package & among
the package to avoid channeling desirable density is approximately 300 g/1.
Angle of winding:
The standard of winding affects the quality of dyeing. Good
winding produces a package of even densities with good running off properties & minimum of end breakage. Package with cross winding are preferable.
Uniformity of package size should be ensured before
Perforation of spindle:
Choking of perforation on spindle hinders smooth &
uniform flow of liquor leading to uneven results.
It is defined as the ratio of dry material to liquor volume
employed. When calculating liquor ratio, one should not forget to include any
addition made during dyeing process which may increase the volume. A proper MLR has to be maintained during dyeing for consistency of batches.
The change in pressure after reversal of cycle should not be significantly high. Necessary action has to be taken for
smooth reversal & prevention of choking of pumps.
Complete air removal has to be ensured prior to pressure build up for uniform results.
pH, Temp. & Time:
Optimum values of those parameters have to be maintained during every stage of processing for best results.
From the foregoing, it can be said that process control is one of the most effective managerial tool. The extent of reprocessing on account of vigilance of process control in maintaining the processing parameters at desired level can be reduced thereby maintaining quality & increasing the margin between production cost and sales price.
- Process control in yarn dyeing - R. Shrivastava
- Towards zero defects - Amubhai Patel
- Major fabric defects during chemical processing
- Quality and Process control S.M. Doshi & HA Shah
- Chemical technology of fibrous materials - F. Sadov
- Technology of Dyeing V. A. Shenai
About the Authors:
Pavan S. Chinta and Sapna Dhar are associated with D.K.T.E.'s Textile and Engineering Institute, Ichalkaranji.
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