Usage of direct dyes

Typical Application Method of direct dyes (Taking Cotton Fabrics as an Example)

1. Process Flow:

Pretreatment (scouring, bleaching) → Dyeing → Washing → (Fixed Color Post-treatment) → Dehydration → Drying

2. Example Dyeing Recipe:

· Direct Dye: X% (relative to fabric weight, i.e., o.w.f.)

· Anhydrous Sodium Sulfate (Gallium Sulfate) or Salt: 10-30% (o.w.f.) (Acts to promote dyeing)

· Soda Ash: 0-2% (o.w.f.) (Adjusts pH to slightly alkaline, approximately 8-9, aids in leveling and dye dissolution)

· Liquor Ratio: 1:20 ~ 1:50 (Ratio of dye liquor volume to fabric weight)

3. Operating Steps:

* Chemical Preparation: Dissolve the dye thoroughly in warm water (≤80℃). It can be prepared in advance with a small amount of cold water to form a paste, then diluted with hot water.

* Dyeing: Immerse the cleaned fabric in a dye bath at 40-50℃ for 10 minutes to ensure even wetting.

* Heating: Slowly increase the temperature to 95-100℃ at a rate of 1-2℃/minute.

* Dyeing at Boiling Point or 95-98℃: Dye at this temperature for 45-60 minutes. Add dissolved sodium sulfate in stages (e.g., after 15 and 30 minutes).

* Washing: After dyeing, drain the dye bath. Wash first with hot water, then with cold water to remove any excess dye.

* Fixing Treatment (Key Step): To improve wet fastness, a color-fixing treatment is usually required.

* Fixing Agent Y or M: Treat at 50-60℃ and pH 5-6 for 20-30 minutes. This is the most traditional and commonly used method.

* Catonic Fixing Agent: Can bind to the anionic dye, blocking water-soluble groups.

* Crosslinking Fixing Agent: Forms crosslinks between the dye and fiber, resulting in better color retention.

* Dehydration and Drying: Used for subsequent finishing processes.

Precautions and Key Technical Points in the dyeing process by using direct dyes

1. Dyeing Promotion Effect of Salt: Electrolytes (sodium sulfate/salt) reduce the negative charge on the fiber surface, helping negatively charged dye anions approach the fiber. They must be added in stages to prevent uneven dyeing (color spots).

2. Temperature Control: Direct dye uptake is temperature-sensitive. Slow heating is key to ensuring even dyeing. Boiling dyeing can improve uptake and penetration.

3. Water Hardness: Calcium and magnesium ions in hard water combine with dye to form precipitates, causing color spots and light color. Soft water should be used or a water softener (such as soda ash or sodium hexametaphosphate) should be added.

4. Fixing is Essential: For textiles requiring washing, color fixing is an essential step, significantly improving wash and rubbing fastness by 1-2 grades. 5. Post-dyeing washing: Thorough washing to remove excess dye is fundamental to achieving good fastness.

6. Dye selection: Select appropriate dyes based on the fastness requirements of the final product. Currently, there are also some direct lightfast dyes (with good light fastness) and direct copper salt dyes (treated with copper salts after dyeing to improve light and wet fastness).