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Applying reactive dyes to cellulose is a 3 stage process.
1. Exhaustion (It involves the exhaustion of the dye onto the
fiber in the presence of electrolyte).
2. Fixation (Addition of alkali to provide the chemical reaction between dye and cellulose).
3. Washing (Last but not the least, removal of unfixed &
hydrolyzed dye from the material in order to achieve the desired results).
When reproducibility is the aim of any dyeing process, the following
factors should be taken in to account.
- Quality of water supply
- Preparation of substrate
- Dye ability of substrate
- Prior to start dyeing - uniformity in absorbency, whiteness and pH of the fabric have to be tested and uniform conditions have to be maiatained for all batches/processes.
- Weight of substrate
- Everywhere it is generally taken for granted and the weight is calculated according to the meterage or number of cheeses or number of bundles instead of actually weighing the materials ready for processing. In lab, we use RFD material for dyeing, where as in batch dyeing machines, we use only greige fabric that would be affected by up to 10 to 15% weight loss after scouring and bleaching. The correct method is to find out the actual average weight loss after scouring and bleaching and use this figure as a correction factor for predicting the recipe for dyes and chemicals.
- Weighing of dyes and chemicals
- A important phase of operation - first the recipe calculation part - recheck once or twice; second is the weighing balance caliberation and its worthiness in weighing small quantities. Third phase is actutal weighing. A proper supervision is a must in this phase.
- Selection of dyes (strength / hue)
- Moisture content of substrate
- In padding operations, the moisture content, temperature of fabric should be maintained uniformly as a constant. You should find out the mositure content of fabric before starting the operation (padding).
- Dye bath additives
- Check the compatibility of dyebath additives in a blank bath, with dyes and after raising the temperature to dyeing temperaure. There should not be any turbidity or precipitation.
- Liquor ratio
- Liquor ratio, though not affecting the shades, maintaining a constant figure would yield reproduciable results.
- pH of the dye bath
- Machine flow and sequence
- The circulation speed, jet flow, timing cycles (in-out/out-in or forward/backward rotation of fabric or flow of liquid) should be uniform.
- Time and temperature profile
- Time: Allowing the correct timings for each process results in reproducialble results.
- Temparature: Check frequently the calibration of the thermomter. Maintaining the correct temperature for every process is very important. Dyes, enzymes and some inorganic chemical treatment too require specific temperature. Maintaining the correct temperature gives better repetitive results.
The above mentioned parameters are considered very important for Right
First Time dyeing concept.
Usually, at first, test dyeing for color matching is conducted in a laboratory, and the data thus gained is important for reproducibility in practical dyeing because it is used to decide the practical dyeing conditions. It is difficult to perfectly reproduce the
dyeing conditions of the laboratory in practice, but it is important to try to get them as close as possible. Lab
to bulk reproducibility constraints - points to be considered:
- Due to mechanical restrictions, matching the liquor ratios and the
agitation speeds of the dye baths can be difficult, but the liquor
ratios should be made as close as possible.
- The material to be dyed should be the same as that dyed in the
laboratory.
- It is advisable that the test in the laboratory should be conducted
with same lot of dyes as what is being used in production,
particularly for the light shades.
- If manual pipetting is used for measuring the dyestuff solution:
- Use calibrated pipettes.
- Use Smaller volume pipettes, wherever necessary; eg. if you have to pipette 9 ml of a solution, use a 5 cc pipette for 5 plus 4 ml. For 11 ml use 2 measures of 5 cc and 1 measure of 1 cc pipette.
- Do not use pipettes, if the tips are broken.
- Good housekeeping in laboratory for dyes storage in order to prevent
them from extreme conditions of heat and moisture is also crucial.
- The dyeing auxiliaries that are used should be consistent and same
as what being used in bulk.
- The quality of water should be
same for both parameters.
- The temperature control of the dyeing machines should be accurate in
laboratory
Color matching equipments make real ease towards the above concept. The
results of the laboratory dyeing and the practical dyeing can be measured
and a coefficient for the degree of difference each time the dye is used
can be calculated. If this coefficient is not constant, the
reproducibility of results just within the laboratory or in the practical
environment should be verified before the reproducibility of results
between the laboratory and the factory is considered. If this coefficient
shows a constant trend for all of the dyestuff, it can be used as a
correction coefficient and crossed with the laboratory results in order to
decide the practical dyeing conditions.
In the case of certain dyes that show large deviation, a dye may have
bad compatibility, and a change of dye class should be considered.
Most of the exhaust dyers prefer the bifunctional and vinyl
Sulphone type of dyes. However, HE (High Exhaust) dyes have recently made their
entrance due to various advantages over the conventional dyeing system.
Nevertheless, hetro-bifunctional and vinyl Sulphone still dominates the
cellulosic dyeing in exhaust system. |
