batch-to-batch shade variation

 

Question:
I am getting batch-to-batch shade variation (color variation) for the same reactive shade, in Soft Flow dyeing machine. I am using the same manufacturer’s dyes and chemicals and almost same quality of fabric. What are the reasons and how to avoid it?

Answer:
Batch to batch shade variation is a general phenomena noticed almost everywhere in dye houses. There are so many factors that affect a particular dyeing operation. Please make the following checks; I hope you can eliminate 90 to 100% of the above problem.

Phase-1: Preparatory Stage.
Check the batch weight exactly for all batches. Find the exact GSM of the greige fabric and note down on your job card.
After Scouring, Bleaching and neutralization, before dyeing, take a sample, dry it, condition it and check the GSM.

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 greige 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.
  • 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.
Phase-2: Dyeing Stage.
  • Check the quality of input materials.
  • Dyestuff concentration to be checked drum to drum.
  • Cheque the quality of sequestering agents.
  • Check the quality of water – pH, hardness and turbidity.
  • Check personally while weighing the dyes and chemicals.
  • Check personally while dissolving the dyestuff, filtering the dyestuff solution and adding in to the dye bath in to the machine.
  • Cheque the quality of Salt. Find out in advance how much hardness is increased in the dye bath due to salt addition.
  • 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 injecting the soda solution, please follow the same procedure. It has its own effect in exhaustion percentage.

Similarly inject the soda in to the machine for the same length of time the first batch was done. If injection 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.

Cheque 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.

Phase-3: Finishing:
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, do one 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.

a. 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.
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.

Soaping:
Use good Quality Anionic Soap such as Lissopol D of ICI.

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 content in the water would act as exhausting agent for the unfixed dyestuffs and would not allow to come out from fabric, during soaping and impair the quality of soaping and finally the washing fastness.

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.

But in acidic pH condition the fabric should not be allowed to kept wet for long durations of say one day or two days, because there is every possibility of formation of fungus over the processed material that not only changes tone but also impairs the strength of fibers.

Final cationic fixing operation: Due to the presence of free formaldehyde in different dosages between manufacturers, the final shade’s tone changes to bluer or yellowier side. Select the best product and adhere to it. Do not change this product frequently for small cost benefits.

Final Cationic Softener Treatment: During this phase also there is every possibility of change in tone, depth and light fastness of the final product. Purchase the best product and maintain the same for all your production processes.

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.

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.

Conclusion: ‘Change is one that always remains changeless’ is true in all walks of life. Higher the precautions lower the change.