Part Three: How to Reduce Variables in Fiber Reactive Dyeing
We’ve covered quite a bit in this series on reducing variables in fiber reactive dyeing. We looked at monitoring liquor usage, maintaining absorbency, and consistent color properties. In Part Two, we explored preparation of the dye bath, maintaining a good liquor ratio, and monitoring salt concentration levels.
Now we’re going to look at finishing the process with rinsing, soaping, and drying. In these steps, we rinse the electrolyte, or salt, and unfixed dye from the fabric. We also neutralize the alkaline and after-soaping of the fabric. Our end goal is for the fabric to match fastness requirements, be prepped for any further finishing processes, and to cut the water and energy consumption.
To wrap up this process, we’ll need to pull salt from the fabric, neutralize it to a stable pH, and give it a thorough after-soaping. This conventional method has many areas of opportunity to increase efficiencies, but these are the basics to understand the process.
Removing Electrolytes (Salt Removal)
Why is it important to remove salt from the fabric? The unfixed or hydrolyzed dye will act like a direct dye in the presence of electrolytes. This is due to the action of Van der Waals forces, or the repelling of the dye molecules from the fabric resulting in low washfastness. Thus, we’ll need to lower the salt level before after-soaping.
Cotton fabric will retain 200 – 300% wet pick-up after dropping the dyebath. In new, low-liquor machinery (LR: 1-6/1-8), 30 – 40% of the dyebath remains after the first drop. If there were 80 g/l salt in the original dye bath, 25 – 35 g/l will remain in the first rinse. You’ll need to do 2 – 3 rinses to reduce the salt volume. These rinses are typically for 8 – 12 minutes at 60° C (140° F). This will depend on weight and length of the fabric. The goal is to reduce the electrolyte content of the bath. With this, we can see the largest benefits from after-soaping.
Neutralizing the Fabric
It’s essential to neutralize the fabric to not impact the shade. For most reactive dye chemistries, the ideal pH for after-soaping is 6.5 – 7.5.
Acetic Acid is the most common product for this process. To use, add the acetic acid to the last rinse before soaping. The desired pH range will depend on the dyestuff, so check with a technician before carrying it out. You can also find pH recommendations in supplier’s catalogs.
Some fiber reactive dyes will change color properties if the pH is not in the recommended range. If the pH of the fabric in the soaping bath is too high, the fixed dye strip from the fabric. This will result in a weak final shade as compared to the standard. It can also alter the shade to the green or red side of the spectrum. Certain dyes are more sensitive than others.
A pH level that’s too low can also cause similar results. For example, Reactive Blue 21 and Reactive Blue 38 shades will be greener and duller. This can also affect the soaping efficiency due to pH issues.
After-Soaping the Fabric
The previous steps impact the success of the soaping steps. Only after proper preparation for after-soaping can we see good results with this process. We will use a low-foaming surfactant with or without a soaping agent. Depending on the water hardness and dyes you plan to use, you may also need a chelating agent. The soaping step can range in temperature from 60 – 90°C (140 – 195°F), for 9 – 15 minutes. In deep shades, especially blacks and reds, you may need another soap-off.
After cooling and dropping the bath, rinses will follow. One or two rinses in warm water between 50 – 60°C (120 – 140°F). The number of rinses will depend on the color of the rinse bath, equipment, and liquor ratio – as they all play a key role in the outcome.
Follow with rinse at 40°C with acetic acid to neutralize fabric. In the final rinse, you can add softeners.
This is a conventional procedure that serves as a base to start developing or reevaluating your processes. New chemistries exist which can reduce time, water, and energy consumption. Find areas of bottleneck or poor performance to begin investigating alternative improvements with newer processes. We can help you find more information on additional formulations, chemistries, and innovations to improve the preparation of your fiber materials. Regardless, in the end, having a set procedure for preparation will set you up for success down the line in creating exquisite fabrics.