Fabric Shade Variation:
Shade variation or shading is one of the more obvious visual defects that can be found in fabric. Shade variation in fabric is the variation of shade (depth of color or hue) from roll to roll or piece to piece that were intended to match. Shading is a common and big problem for the dyeing industries. Shade variation may occur selvage to selvage (side to side) or from one end of the fabric roll to another end (or anywhere in between). Shade variation can take place during printing or dyeing processes. It is the most common when a new batch of dyes are mixed when attempting to create or re-create a desired color.
Fig: Fabric shade variation
There are various reasons for fabric shade variation in textile dyeing industries. Some prime causes are-
Variation in dyeing recipes between batches
Mixing of fabrics used in production
Poor lab-to-bulk correlation
Variations in the production process with regard to time and speed
Due to lacking of responsibility taken by concerned management in the dyeing floor along with some technical issues.
Preventive Measurement of Shade Variation in Fabric Maintained in Wet Processing
In case of batch to batch shade variation the following preventive measurement is maintained in wet processing-
Step-1: Preparatory Stage
Check the batch weight exactly for all batches. If all batch weight is not equal then should taken proper weight.
After Scouring, Bleaching and neutralization, before dyeing, take a sample, dry it, condition it and check the GSM.
Quality of water would have changed. Check for hardness, TDS and pH of water used in each batch.
Check the whiteness index of the RFD fabric and note on the Job card.
Check the absorbency of RFD fabric and note on the job card. Let the test results be noted in terms of seconds.
Check the remaining Core alkali in the fabric and the fabric pH.
Check for residual Peroxide on the fabric before dyeing.
Note down the difference in percentage as scouring and bleaching loss on the job card. compare this loss% between each batch. It should not deviate more than 2 to 3%.
If deviation is there then the following may be the reasons:
Material to Liquor Ratio difference during scouring/bleaching.
Process timing difference
Temperature may be kept high or low.
Low or better quality of grey material, ie. Higher staple length good quality fiber loses less weight rather than the poor quality short staple length fiber.
Alkali and Scouring auxiliaries’ concentration or quality may have changed.
Check the quality of water – pH, hardness and turbidity.
Check personally while dissolving the dyestuff, filtering the dyestuff solution and adding into the dye bath of the machine.
Check the quality of Salt.
Counter the hardness due to salt with proper dosage of a good quality-sequestering agent.
Check the quality of Soda Ash. Check for purity percentage.
Allow the same number of minutes or hours to run dyeing with salt alone – e.g. if the first batch – salt addition is done for exactly 30 minutes and after complete addition of salt, if the machine was allowed to run for 30 minutes, before dossing the soda solution, please follow the same procedure. It has its own effect in exhaustion percentage.
Similarly dossing the soda into the machine for the same length of time the first batch was done. If dossing time for the first batch was 30 minutes, do it for all subsequent batches for 30 minutes.
Allow running the batch for the same length of time in soda before raising temperature. If it is 20 minutes. Follow the same procedure for all batches.
Steam Raising time – if 60°C is reached in 20 minutes for the first batch – do it for 20 minutes for all batches.
Run at Specified temperature for the same timings for all batches.
Check the pH of the dye bath every 30 minutes from the starting of dyeing and note it the job card. After the final addition of Soda ash the pH should be 11 to 11.2. A well-calibrated good quality pH meter will aid you in this job.
Record the final pH of dye bath after every 30 minutes. Maintain the water level same throughout the dyeing operation and make sure that your are monitoring the level batch to batch.
Not only the preparation and dyeing operations are important, final finishing operation also is very important to avoid batch-to-batch variation.
After draining the dye bath, needed to cold wash and hot wash. Cold wash timings and hot wash timings and temperature are also important. Note this in the job card. Do not take this finishing part as unimportant. In fact here also there is every possibility to get shade variation. If temperature and timing of each batch are not same the washing off will also not be the same. Please maintain it.
Neutralize with Acetic acid. The quality of Acetic acid should be checked every time you get fresh lots.
The quality softener should be check.
Neutralize with the same quantity of accurately weighed Acetic acid volume. The final pH always determines the tone of the shade. So quality of Acetic acid, quantity of acetic acid and the treatment time are all-important and they should be identical between batches.
Step-4: Soaping
Use good Quality soaping agent.
Before adding the soap in to the bath, fill the machine with adequate volume of water, run blank for 5 minutes, take out some water and check for TDS. Let the TDS be not more than 100ppm of original water i.e before entering in to the machine.
If higher TDS observed is say 1000 ppm higher than original water, then it is understood that there is still the high residual salt present in the fabric.
The higher residual salt and soap at higher temperature of say 80° to 90°C would strip off the reactive dyestuffs from the fabric, instead of removing the unfixed dyestuff from the surface.
More over the properly not removed salt would result in a harsh feel of the finished fabric.
Higher dosages may sometimes cause white patches.
Maintain the timing, temperature; water level and dosage of soap, same for all soaping processes.
Check the Final pH of the fabric; let it be slightly acidic.
Finally the drying temperature. Same temperature and duration of exposure of the fabric is a must. Over dried fabric and under dried fabric will show a lot of shade variation.
Step-5: Machinery Problems for Shade Variation
Rope length should be maintained almost the same in all the loops.
Ensure that in each batch the fabric rope passes through the nozzle of the machine the same number of times during the actual dyeing process.
A change in rope speed – depending on the batch size may be necessary to do this.
Use the same standard program procedures for each batch.
Clean the filter for each batch.
If nozzle diameter can be adjusted in your machine, keep the same diameter for getting the reproducibility.
Pump pressure should be same batch to batch’
Conclusion: Shade variation or shading is considered to be a critical problem in fabric. Most quality assurance managers would have the topic of checking fabric color variances as an important step in their inspection process. Using good technology, high quality inks and dyes and appropriate fiber content and fabric quality is beneficial but does not guarantee 100% success. The above process should be check properly. Higher the precautions lower the change.
Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.
