What is Direct Dye?
Direct dyes are water-soluble dyes that can color cellulose fibers (like cotton, rayon) directly without using a mordant. They are easy, cheap, and versatile for cellulosic fibers specially for cotton, but they fade and wash out more easily than reactive dyes. This class includes the first synthetic dyes to be manufactured which had affinity for cellulose. They are mainly high-molecular-weight azo compounds containing sulphonic acid groups which make the dye water soluble. A wide range of colors is available varying in cost and fastness properties.
The fastness properties of direct dyes on cellulosic fibers can vary: their light fastness ranges from poor to moderate, though some copper complex types show much better resistance to light. Generally, the deeper the color of the dyeing, the lower the fastness to wet treatments, and the higher the fastness to light. Various aftertreatment process of the dyeing improves the fastness to washing. In some cases, however, such aftertreatments decrease the light fastness.
Direct dyes will also dye wool, silk and polyamide, but as higher fastness properties can be achieved on such fabrics by using other dyes, the use of direct dyes is largely confined to cellulosic fibres.
Classification of Direct Dyes:
Generally speaking, direct dyes are classified into three categories:
- Class A: Self-levelling dyes have good migrating power and thus very good levelling properties.
- Class B: Salt-controllable dyes generally migrate less well. Their uptake can be controlled with gradual additions of salt (electrolyte).
- Class C: Temperature controllable dyes have poor migrating properties, and once they are taken up unevenly are difficult to level. To achieve level dyeing, it is necessary to use a gradual increase in temperature to help control dye uptake.
Objects:
- Cost saving.
- Comparative low time required to produce supply garments.
- No possibility of shade variation.
- Easy process.
Recipe:
- Shade ……………………5 %
- Wetting agent …………..1 g/l
- Sequestering agent ……..1 g/l
- Levelling agent …………0.5 g/l
- Soda ash ……………….3 g/l
- Gluber salt……………. 4 g/l
- pH……………………. 10-11
- Temperature ………….70
- Time ………………….30
- M:L ………………….1:30
- Sample Weight ………5 gm
Calculation:
………..10% x 5
Dye = ———————
…………..2%
= 25 cc
……………………….1 x 150
Wetting agent = —————-
………………………..1000
= 0.15 cc
………………………………1 x 150
Sequestering agent = ———————
……………………………..10000
= 0.15 cc
………………………..0.5 x 150
Leveling agent = ——————–
………………………….1000
= 0.07 cc
……………….3 x 150
Soda ash = —————–
………………..1000
= 0.45 cc
…………………….4 x 150
Glauber salt = —————–
…………………….1000
= 0.6 cc
Dyeing Procedure of Direct Dyes with Cotton Fabric:
Generally, dyeing of cotton fabric with direct dyes involves 2 processes: Preparation of dye bath and dyeing.
The dye bath is prepared by dissolving the required amount of direct dye in water, often by making a paste mixed with soda ash (sodium carbonate) to maintain an alkaline pH. Then, salt is added to control fiber charge and optimize dye fixation. Besides, NaCl or Na₂SO₄ is also increase dye absorption. Sometimes, small amounts of wetting agents or leveling agents are also added.
Dyeing process is point out below:
- Set the dye bath with substrate at room temperature
- Add dye solution with other auxilaries ad raise the temperature at 900c
- Run the bath for 15-20 min and add salt step according to dye bath concentration
- This is better to add salt after reaching the temperature to build point since at this period the maximum penetration is achieved all over the substrate.
- Run the dye bath for 30 to 5 minutes at 90-95°C for complete the dyeing cycle
- Cold down the bath temperature to 60-70°C
- Drop the bath and rise
- Carry on the aftertreatment process to improve wet fastness
Dyeing Curve:
After Treatment of Direct Dyes Goods:
Direct dyes typically have poor drying and washing fastness. Therefore, to improve the fastness properties after treatment based on principle like increasing molecular weight of dye and thus decreasing their solubility after dyeing or treatment with metallic salts may be performed. These treatments help lock the dye onto the fiber and reduce dye bleeding or fading during washing.
Precaution:
- All chemicals should be measured very carefully.
- Temp should be increased very carefully.
- Water should be added very carefully.
- Time & Temp should be maintained very sincerely.
Advantages of Direct Dyes for Cotton:
- Simple and economical dyeing process.
- Wide range of shades available, including bright colors.
- No need for mordants or complex chemical fixatives.
- Suitable for dyeing large fabrics and bulk production.
Limitations of Direct Dyes:
- Poor wash fastness compared to reactive dyes.
- Shades may bleed during washing.
- Aftertreatments may change the shade and reduce light fastness.
- Not very suitable for dyeing synthetic fibers or protein fibres without modifications.
- Less preferred for high-quality apparel and household textiles.
Conclusion:
Direct dyeing is the most popular dyeing process in Bangladesh. Direct dyes provide an easy and cost-effective means of dyeing cotton fabric. Today, reactive dyes are more popular for cotton, but direct dyes are still widely used for items where high fastness is not the primary requirement, such as inexpensive clothing, furnishings, and decorative fabrics.
Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. Mr. Kiron is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.
