Maya Trading's deep-dyeing auxiliary for polyester received many enthusiastic enquiries from local dyers when exhibited at China Interdye.

Tastes vary for black shade of chador

Even among women in the Islamic world who are accustomed to wearing black chadors to cover up their skin, nowadays, not just any old black will do. More and more women want a splendid shade of black.

For bringing out the depth and luster in black that can win the approval of Islamic women, neutral to acidic pH levels will be ideal in disperse dyeing of polyester. Rather than using alkaline conditions which require great care as seen in some dyeing plants, we recommend the use of neutral to acidic conditions together with the Builder RN deep-dyeing auxiliary.

Three kinds of deep-dyeing auxiliary are recommended for the production of an ideal deep black:

• Builder Deepart BL for an elegant bluish black
• Builder RN for dyes that produce a general deep color
• Builder RN-1 for stable, high-quality results with deep-color alkaline dyeing

Comparison of process outlines of neutral to acidic and alkaline dyeing

(1) Neutral to acidic dyeing method

Alkali reduction
Rinsing
Neutralization
Acidic dyeing
Rinsing
Reduction clearing
Rinsing
Neutralization
Drying
Deep dyeing – padding (with Deepart BL or Builder RN)
Drying
Finishing – padding (with softeners and anti-static agents)
Curing

(2) Alkaline dyeing method

Alkali reduction
Rinsing
Alkaline dyeing
Rinsing
Drying
Deep dyeing – padding (with Builder RN-1)
Drying
Finishing – padding (with softeners and anti-static agents)
Curing

Comparison of problems in dyeing methods 1 and 2

Let us compare processes 1 and 2. All treatments prior to the deep-dyeing stage are carried out under alkaline conditions in process 2 in order to simplify the process and reduce costs. However, there are several problems with this method that make it more likely that trouble will occur. Although the stock liquor is first adjusted to a PH level of 4.5 – 5.5 before being automatically added to the padding bath to match the acidic pH level of the deep-dyeing auxiliary, it is easily contaminated with unavoidable alkaline residues from the substrate dyed under alkaline conditions. What is more, problems are likely to arise in the final stage of dyeing due to changes in the pH level of the padding liquor, resulting in a reddish hue. These problems can be avoided through the use of neutral or acidic deep-dying techniques which have been proven safe.

Prevention of foam production in padding bath is the key to deep-dyeing

In order to produce fine results in polyester deep-dyeing, it is necessary to prevent foaming in the padding bath. Foaming easily occurs as fabric enters the padding bath at high speeds. To prevent this, adding a silicone-based de-foaming agent might be the first thing that comes to mind. However, this has a disadvantage: the agent, being an emulsifier, makes the padding liquor more prone to gumming up, and ultimately, this results in uneven dyeing. Alternatively, foaming can be safely prevented by making the treatment liquor acidic. Judging by this, neutral to acidic conditions are again preferred to alkaline conditions for deep-dyeing of polyester, with respect to avoiding risks.

Drying after padding and selection of softener also important

With fabrics treated with a deep-dyeing auxiliary under neutral to acidic conditions, drying with no tension applied and using a net dryer will bring out ideal results. Though the drying temperature depends on the texture and weight of the substrate as well as the pickup rate in the preceding stage, it will require temperatures above 140°C. For curing, a temperature above 160°C will prevent alterations of the substrate over time. 160-170°C is suitable for the final setting process. In neutral-to-acidic dyeing method 1, a softener and an anti-static finishing agent are used in the finishing (also known as “padding)” stage and in the subsequent drying stages that follow deep-dyeing treatment. While anti-static finishing agents do not affect the deep-dyed fabric, some softeners may have adverse effects on it. Preliminary assessment for the selection of an appropriate softener is necessary in order to maintain superior deep-dyeing results. Maya Trading Co. offers Siltop EXP-1000 and Beater SP-370 for improved compatibility between deep-dyeing auxiliaries and softeners. The following are seven points to bear in mind in order to ensure outstanding polyester deep-dyeing results.

1. Check the condition of polyester yarn.
2. Check the weave structure.
3. Check the quality of the disperse dyestuff used.
4. Check the quality of water before use in dyeing. Avoid the use of hard water.
5. Improve control over rinsing and reduction clearing that follow dyeing.
6. Manage an appropriate drying temperature after the padding (deep-dyeing) treatment.
7. Manage the curing temperature.

Ignoring the above points will result in slight shrinkage or alteration of shades in the substrate. There are cases in which the shade of the deep-dyed substrate turns from a bluish to yellowish black. The yarn structure and the curing temperature should be checked to prevent this from occurring.