In a reactive dye a chromophore contains a substituent that is activated and allowed to directly react to the surface of the substrate. Reactive dyes have good fastness properties owing to the bonding that occurs during dyeing. Reactive dyes are most commonly used in dyeing of cellulose like cotton or flax, but also wool is dyeable with reactive dyes.
Dyeing Process of Reactive Dye:
There are different types of dyeing process of reactive dye. In this article, three popular dyeing process of reactive dye are discussed below.
- Pad batch.
The following chart gives a comparison between batch dyeing and continuous dyeing:
Batch Dyeing Process is the most popular and common method used for dyeing of textile materials. Batch dyeing is also sometimes referred to as Exhaust dyeing.
Variable factors in reactive dyeing:
A higher temperature in dyeing with reactive dyes results in,Higher rate of dyeing,Lower colour yield. Better dye penetration, Rapid diffusion and Lower substantivity.
pH influences the primary site of reaction on the fabric,Best dyeing rate is achieved at pH of 11-12. Further increase in pH will reduce the reaction rate as well as the efficiency of fixation.
Electrolyte increases the rate & extent of exhaustion,Electrolyte increases the dye aggregation. Electrolyte decreases the diffusion and Mostly used KCl, Na2SO4, NaCl.
4. Liquor to material ratio:
Advantages of Lower liquor ratio,Higher colour strength and Higher exhaustion.
5. Surfactants and other chemicals being added:
Some anions may enhance the colour yield, Some non-anions decrease the exhaustion and Some non-anions may slow down the dye hydrolysis
Washing off after dyeing:
Effective washing, to remove un-reacted dye, If un-reacted dye not removed it gives false indication of low wash fastness, Rinse 2-3 times with fresh water at 50-600C, Wash with soap for 10-15 minutes using easily rinseable detergents.Rinse with cold water until rinsed water is colourless.
- Exhaustion from aqeous water bath containing electrolyte under neutral conditions.
- Addition of alkali to promote further uptake and dye fiber reaction at optimal pH and temperature.
- Rinsing and soaping at boil to remove electrolyte, alkali and un-reacted dye.
|Reactive groups||Dyeing temp.||Common salt g/L||Soda ash g/L|
There are three general types of batch dyeing machines. The first type is the one where there is circulation of fabric. Second type is the one where the dyebath gets circulated while the material that is being dyed remains stationary, and finally the third type where both the bath and material to be dyed gets circulated. Examples of dyeing machines that utilizes batch dyeing process are Beck, Jet, Jigs, Beam Package dyeing machines etc.
- In the dyeing of deep shades or inefficient washing equipment there may be incomplete removal of unfixed dye.
- After treatment with cationic dye fixing agent.
- Insolublises the unfixed dye.
- Improves wash fastness.
- Treated with 5-10 g/L cationic dye fixing agent at 50-60º C for 10-30 mints.
- Treatment with dye fixing agent is not substitute of wash off process.
Some important parameters to be considered in Exhaust dyeing:
- Liquor Ratio (textile that is dyed/water used in dyeing bath)
- Dyeing Method (temperature/time curves; Double bath or single bath process in case of fiber mixtures)
- Nature of Dyestuff, auxiliaries
- Degree of exhaustion degree of dyestuffs
- Number of rinsing bathes and nature of after treatments required
The working of a continuous dyeing process is described here.
- The textile substrates are feeded continuously into a dye range. The speeds can vary between 50 to 250 meters per minute. According to Industry estimates Continuous dyeing is a popular dyeing method and accounts for around 60% of total yardage of the products that are dyed.
- High productivity.
- Dye fixation achieved in few seconds to minutes
- High capital cost.
A Continuous dyeing process typically consists the following. Dye application, dye fixation with heat or chemicals and finally washing. Continuous dyeing has been found to be most suitable for woven fabrics. Mostly continuous dye ranges are designed for dyeing blends of polyester and cotton. The step of padding plays a key role in the operation of continuous dyeing. Sometimes Nylon carpets are also dyed in continuous processes, but the design ranges for them is unlike that for flat fabrics. Warps are also dyed in continuous process. Very good examples of such warp dyeing are long chain warp dyeing and slasher dyeing using indigo.
Continuous dye range has been found useful and economically sustainable for dyeing long runs of a given shade. One important factor that separates continuous dyeing from batch dyeing is the tolerance factor for color variation. That is more for continuous dyeing as compared to batch dyeing. This is so because of two reasons a) the speed of the process. b) presence of a large number of process variables which affects dye application. The process that is illustrated below is designed for dyeing of blended fabric of polyester and cotton.
- Known as the pad batch dyeing.
- Able to dye fabric length of 1000-2000m at economic cost.
In the process of semi-continuous dyeing that consists of pad-batch, pad-jig, pad-roll the fabric is first impregnated with the dye-liquor in, what is called a padding machine. Then it is subjected to batch wise treatment in a jigger. It could also be stored with a slow rotation for many hours. In the pad-batch this treatment is done at room temperature while in pad-roll it is done at increased temperature by employing a heating chamber. This helps in fixation of the dyes on to the fiber. After this fixation process, the material in full width is thoroughly cleansed and rinsed in continuous washing machines. There is only one point of difference between Continuous and semi-continuous dyeing process is that in semi-continuous dyeing, the dye is applied continuously by a padding. The fixation and washing remaining discontinuous. Liquor Ratio in semi-continuous dyeing is not of much importance and is not taken as a parameter. One of the widely used techniques for semi-continuous dyeing process is the Pad Batch Dyeing a schematic diagram is given here for the semi-continuous dyeing process.
The following table shows some of the important machineries for semi-continuous and continuous dyeing processes.
- A Review of Textile Dyeing Processes
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