Continuous dyeing of cellulosics and its blends

In older days cotton fabric (both woven and knitted) was dyed using jigger, winch, etc. then synthetics and cotton synthetics were dyed in jet dyeing machines, beam dyeing machines, etc. One limitation in these machines were small batch sizes.

Then batch to batch color difference was not accepted by the garment manufacturer. Today’s market driver’s focus on reduced lead times to support speed to market, higher productivity and elimination of non-conformance for cost-effectiveness.

To overcome this continuous dyeing ranges were started which includes enhanced productivity (as high processing speeds of 50 – 80 m/min are possible), better process control leads to better reproducibility, and production flexibility because the lot size could be from 200 m to 10,000 meters and more. Besides these followings are the advantages of continuous dyeing.

1. Cost-efficient process.
2. Uniformity of dyeing.
3. Enhanced fabric brilliance and luster.
4. Improved levelness and solidity of shade.
5. Short through time.

Continuous dyeing processes are suitable for lightweight to heavyweight fabrics. Semi batch for knitted fabric is pad batch whereas semi-batch for woven fabric is pad batch and pad jigger. Semi-batch and continuous dyeing processes are used to produce uniform shades with quality and economy for longer lengths of production. The cold pad batch (CPB) is the most effective method of dyeing both woven and knitted fabric.

The continuous dyeing is a two-stage operation which involves continuous preparation and continuous dye application and fixation which means instantaneous physical application of dye by padding. The dye application padding mangle is the most important component for shade uniformity without giving selvage to selvage or center to selvage shade variation.

In India, knit fabrics are commonly dyed in rope form on winches, jet or air/soft flow machines. This process while being relatively simple and scalable poses several disadvantages and problems which are familiar to any dyer experienced in his field. Those are e.g.

1. Formation of crease marks.
2. Uneven dyeing (especially on viscose).
3. Hairiness of surface ‘pilling’ from mechanical friction.
4. Tension in fabric causing high rest – shrinkage values.
5. High consumption of water/steam/chemicals/dyestuffs.
6. Usage of salt causing problems in effluent treatment.

An alternative to the dyeing in rope is the process of dyeing in open width, also called cold pad batch process (CPB). It has been successfully operated on knits in Europe for more than 20 years and in India for more than 10 years.

This method allows a low-cost and high-quality dyeing while avoiding some of the problems experienced in rope dyeing. The global market demands are supported by the use of CPB processing foe woven fabrics.

The key factors are continued pressure on cost, improved quality (both appearance and fastness) and reduced lead times in the global textile supply chain. These requirements combined with shorter production runs – due to reduced lead time or multi-phased seasons – means the dyehouse must be capable of the excellent lab to bulk transferability.

There are five main methods of piece dyeing

1. Beam dyeing
2. Beck dyeing
3. Jet dyeing
4. Jig dyeing
5. Pad dyeing

Pad dyeing is nowadays known as continuous dyeing. The fabric is passed through large pad rollers which squeeze the dye onto the fabric. The material passed through the liquor and squeeze rollers would be expected to absorb 50% to 100% of their weight in the dye liquor. The lower the percentage pick-up, the higher must be the concentration of dye liquor. Pad dyeing is usually done in a continuous dyeing range (CDR) and is useful for dyeing large yardage of fabric. The method is particularly suitable for the dyeing of 100% cotton and polyester /cotton blended fabrics.

Benninger, Kusters, Harish are the main manufacturers of CDR. Presently CDR working in the world are as follows.

1. Asia – 50%
2. Europe -20%
3. Africa – 10%
4. South America – 10%
5. North America – 20%

Dyes used for continuous dyeing of cellulosic are as follows

1. Reactive dyes – 45%
2. Vat dyes – 25 %
3. Sulfur dyes – 15%
4. Direct dyes – 10%
5. Naphthol dyes – 5%

A major contribution to the reactive dye application procedure has been the development of pad – batch method. Today the short – time pad – batch is one of the most important of all padding process. With the right choice of dyes this process becomes economically more significant since it saves energy, labor and time. Dyestuff with relatively lower affinity and high reactivity make them most suited for cold pad – batch technique. The reactive dyes suitable for cold pad – batch method is

1. Reactofix Supra Dyes (Vinyl Sulphone based)
2. Reactofix ME dyes (Bi-functional based)

Cold pad batch method 

This dyeing process has gained wide acceptance due to the following reasons:

1. Maximum color yield as compared to other methods
2. Less requirement of energy, since fixation is carried out at room temperature
3. Good levelness of dyeing
4. Longer yardages possible to dye

The flow chart is as below

Following are the steps in CPB.

1. Padding

The dye and alkali solutions are normally mixed in a ratio of 4:1 and added to the padding trough immediately before the start of padding. Dye solution and alkali solutions are prepared separately and mixed into the trough through metering and mixing device.

Thus, dyestuff and the required amount of urea are prepared in 80% and sodium silicate and caustic soda solution in 20% of total liquor. Only the required quantity of boiling water (approx. 2 to 4 times of total quantity of dyestuff) is used for dissolving dyestuff.

This concentrated solution is poured through a fine-mesh sieve into the required quantity of cold water. Addition of the required quantity of urea (50 to 200 gm./l) is done at this stage to improve the dye solubility. Urea should not be dissolved together with the dyes.

Urea is sprinkled in the solid form directly into the cooled padding liquor and dissolved with stirring. Special equipment has been devised for dye and alkali dissolution and mixing which greatly ensure the reproducibility of the dyeings. Benninger uses a self – optimizing proportional dosing system with a centrifugal pump to avoid dyeing errors.

The requirements for this function are a flow meter and a control valve for dye and chemical feeding. Modern pad – batch systems provide proportionating units that synchronize with the speed of the machine and the liquor off-take to continuously mix dye and alkali and feed the pad – box in a linear fashion.

A padder or padding mangle is required with the lowest possible trough volume and highest possible speeds of goods, especially dyeing lightweight fabrics. This ensures high liquor exchange. Pre – scoured, cooled goods are padded at room temperatures (25-degree c to 30 degrees c).

As it is desirable to replace the liquor at a faster rate. The liquor pick –up should be 60 – 70 % for cotton and 90 – 100% for viscose fabric. At higher padding temperatures the pad liquor stability declines, leading to a reduction in dye yield and a risk of tailing. Make sure the goods are cooled before dying. Impregnate with the pad liquor at a constant speed.

2. Fixation

After padding, the goods are batched, care should be taken that the selvedges are perfectly aligned on the roller. The distance between padding mangle and the batching roller should be as short as possible.

Large batches should be kept in rotating condition to avoid liquor seepage. The goods are batched up with the sides aligned after padding, covered with plastic film and tied to exclude air. This prevents partial neutralization of the fixation alkali with carbon dioxide from the air.

The selvedges should also be properly covered otherwise effect of alkali is weakened due to action of atmospheric carbon dioxide. Make sure the minimum fixation time for the recommended temperature is observed. The roll must be rotated during fixation.

The minimum fixation time recommended is for 6 hours per batch. Practically it varies from 16 to 20 hours. Longer fixation times of up to 36 hours do not normally have an adverse effect providing the maximum fixation temperature is not exceeded. If the fixation temperature is significantly lower than the recommended temperature, the fixation time must be increased.

3. WASHING OFF

Continuous washing – off is carried out on an open – width machine with 7 compartments. The normal process of neutralizing the reactive dyed goods with acetic acid prior to soaping should be strictly avoided for pad – batch application.

Continuous washing units based on counter – flow technology are particularly economical as they save water and energy.

Washing sequence on open width soaper is as follows.

Continuous dyeing ranges (CDR)

A great variety of fully continuous dyeing processes have been developed. Which is described as below.

1. Pad –dry – pad steam process

It is a double pad process. Classic procedure for dyeing of heavyweight woven fabrics. Primarily for cotton goods and long yardages. Main advantage of the pad – dry – pad – steam process are

1. High productivity
2. Good appearance
3. High color yield

Pad liquor temperature is 20 – 30 degrees c. Modified padding mangles are used in both cold brand and hot brand reactive dyes can be used in this process.

Liquor pick up 50 – 80%

Drying at 110 -130 degrees c. in hot flues

Chemical pad (salt, alkali) on a padder

Pad liquor temperature is 20 – 25 degrees c

Liquor pick up 70 – 80%

Steaming 45 – 90s in saturated steam. It is done in a dyehouse steamer. Dye fixation takes place during steaming. The goods are then washed off and scoured as they move down the range.

1. Pad – dry – steam process

This is a one-bath continuous process without any addition of salt. It is mainly used for cotton goods and long yardages. Following are the advantages of the pad – dry – steam process

1. High productivity
2. High color yield
3. Salt – free dyeing
4. Lower migration of dyes
5. Good washing off

Pad with dye and alkali àdrying à steaming à  wash –off.

In this process reactive dye solution containing urea (50 gpl), ludigol/resist salt (10 gpl) (to protect dye from reducing fumes) and padded at room temperature and the padded goods are then dried in a hot flue at 100 -120 degrees c., after which the goods are steamed for 6-8 min at 100 – 105 degrees c. The steamed goods are then washed – off.

Following are the precautions

1. Soda ash should not be added to the dye – liquor until immediately before dyeing starts
2. The dyes have less tendency to migrate because of intermediate drying operation
3. Steaming should be adequately carried out with saturated steam to ensure uniform moisture absorption by the dry goods
4. Pad – steam process

It is a one-bath continuous process without intermediate drying. It is particularly suitable for heavy woven fabrics for which intermediate drying would not be economical and there would be a risk of dyestuff migration. Following are the advantages of the pad – steam process.

1. high productivity.
2. suitable for mainly light to medium shades.
3. suitable for terry towels and cord.

Padding with alkaline dye –solution -àsteaming –à washing off

This is also called wet – steam process. , where the fabric is padded with dye solution and alkali, steamed without intermediate drying and then washed –off.

The dyes are completely fixed by steaming for a minute at 102 -105 deg c. to avoid dye hydrolysis the main requirements are cooled fabric, cold pad-liquor, small trough, rapid liquor turn over, etc. sulfur dyes can also be used in this process.

4. Pad – pad steam process

This process is only for specific heavy fabric qualities and for very absorbent fabric – like towels.  Good color yields are obtained even when dyeing dark shades. A special pad liquor applicator is required. This is a” wet – on wet “process without intermediate drying.

In this process, the cloth is first padded with dye solution and consequently padded with alkali (wet-on-wet) without intermediate drying. The alkali padded material is then steamed. Reactive dyes having high reactivity and substantivity are suitable for this process with limitation of dyeing pale shades only.

5. Pad –air /steam process

The pad – air /steam process is one – bath continuous dyeing method for dyeing light to dark shades. It is mainly used for cord, terry towels and regenerated cellulose.  The advantages of pad –air /steam process include

1. high color yields.
2. salt and urea not necessarily required

6. Pad – dry – thermofix process

This is a salt – free, continuous one – bath method that ensures satisfactory coverage of dead cotton.  Drawbacks include lower fastness properties, pollution of exhaust air and effluent and soiling of equipment by urea.

This process is usually recommended for dischargeable dyeing and shades of turquoise and brilliant green, where no other process is suitable.

In this process, reactive dye is padded on to cellulose in the presence of sodium bicarbonate and urea (150 – 200 gpl). After drying dye fixation is carried out in a curing chamber at 125 – 160 deg c  for 2-5 min depending on the depth of the shade. Then washing –off is carried out.

7. Pad thermosol pad steam process

It is the process for continuous dyeing of polyester-cotton blends. It may be either disperse reactive combination or disperse vat combination.

Following is the flow chart

Pad with disperse & reactive dyes – infra-red drying – thermosol -pad -steam -washing –off – drying.

The temperature in infra-red drying is 120 deg c, time 45-60 sec, residual moisture 30%

The temperature in thermosol is 210-220 deg c and time is 15-30 sec.

The second padding is done by using Glauber salt – 250 gpl, caustic soda – 15 gpl, soda ash – 20 gpl.

The temperature in the steaming is 102-104 deg c and time is 45-75 secs.