Dyeing Methods

Dyeing Methods

The  art  of  dyeing  dates  back  to  pre-historic times. It is practised from ancient time  and  it  is  as  old as  human  civilization.

Dyeing is the process of imparting colour to the fibre/yarn or fabric by the application of dyes or pigments. Colour is produced between the dye molecule and the fibre or fabric. The bond between dye molecule and fibre may be strong or weak depending on the dye used. Dyeing enhances the aesthetic look of the fabric. It is one of the surface ornamentation methods.

1. Stages of Dyeing

Dyeing can be done at any stage of textile production process such as at fibre stage, yarn stage, fabric or finished product.

1. Dyeing at Fibre Stage

The process of dyeing at fibre stage is called as fibre dyeing. It includes stock dyeing, top dyeing and dope dyeing.

Stock Dyeing

Fibres are dyed by the process of stock dyeing. In this method, dyeing is carried out in a large enclosed vessel called kiers. Fibres are placed loosely in perforated containers and kept inside the kiers. The dye liquor is circulated through the fibres at high temperature until the desired colour is obtained. Excess dye solution is removed after dyeing process. The dyed fibres are washed and dried.

Advantages

·               Produces varied colour effects by blending different coloured fibres

·               Large quantities of fibres can be dyed at one time

·               Produces uniform colour

·               Colour fastness ranges from good to excellent

·               Dye easily penetrates the fibres and hence crocking is prevented.

Disadvantages

·               Expensive and time consuming

·               Fibre wastage if not consumed immediately

·               Dyed fibres loses its flexibility and hence not readily spin as undyed fibre

·               Cannot withstand rapid changes in fashion

Top Dyeing

Wool that has been combed to remove short fibres is called as top. Top is wound on perforated spools and dye solution is passed through it. Top dyeing method results in even dyeing.

Advantages

·               Uniform dye uptake

·               Dye penetrates into the fibre therefore good colour fastness

Disadvantages

·               Flexibility is reduced

·               Time consuming

Dope dyeing

Manmade fibres such as polyester and polypropylene are dyed by this method. In this method, dye is mixed with the spinning solution before the filament is extruded through the spinneret.

Advantages

·               Dyeing is uniform

·               Dyed textiles have good to excellent colour fastness to washing and light

Disadvantages

·               The strength of solution dyed filaments are slightly lesser

·               Expensive method

2.  Dyeing at Yarn Stage

The process of dyeing at yarn stage is called as yarn dyeing. Yarn dyeing is preferred to create interesting checks, stripes and plaids in the fabrics. Yarn dyed fabrics are deeper and richer in colour. Fabric with dyed warp and undyed weft are example for chambrays. This includes techniques like skein dyeing, package dyeing, warp-beam dyeing and space dyeing.

Skein Dyeing

In this method, the yarns are loosely wound in hanks or skein form. The hanks or skeins are hung on perforated rods and immersed in a dye bath. The dye solution is circulated in and out of the yarn to achieve an even shade. This method is commonly used for dyeing acrylic and wool yarns.

Advantages

·               Loose arrangement of yarn permits excellent dye penetration

·               Yarns retain softer feel

Disadvantage

·               Most expensive yarn dyeing method

Package Dyeing

Yarns are wound on cones, spools or similar units. These yarn packages are arranged on perforated rods in a rack and immersed in a tank. The dye solution is forced outside from the rods and forced back to the centre under pressure through the packages so that, the dye penetrates the entire yarn.

Advantages

·               Dyeing capacity is higher (550 kg) when compared to skein dyeing (200 kg)

·               Material to liquor ratio is less

·               Uniform dyeing

Disadvantages

·               Package dyed yarns do not retain softness and loftiness feel

·               Not suitable for high twist yarns which will not allow dye to penetrate

Warp-Beam Dyeing

This method is similar to package dyeing. In warp-beam dyeing, the yarn is wound onto a perforated warp beam and placed in a tank containing dye solution. Dyeing is carried out under pressure for deeper and uniform penetration of dye. This method is used for the manufacturing of denims, in which warp yarns are dyed with indigo and weft yarns are left undyed.

Advantage

Economical than other yarn dyeing methods

Disadvantage

May not produce high fashion fabric since warp yarn will be dyed in specific colour

Space Dyeing

In this method yarn is not completely dyed, but it is dyed at intervals. Space dyeing is done by two methods namely, Knit-de-knit method and OPI space -dye applicator method. In knit-de-knit method, the knitted fabric is dyed and de-knitted, which results in appearance of alternating dyed and un-dyed spaces. In OPI space-dye applicator method, the yarns are allowed to pass through space dye baths at very high speed which results in intermittent dyeing.

Advantages

·               Produces brighter and deeper shades.

·               Create interesting checks, stripes and plaids.

Disadvantages

·               Costly and time consuming

·               May not create designs to match current fashion

3.  Dyeing at Fabric Stage

The process of dyeing woven or knitted fabric is called as piece dyeing. Piece dyeing methods include beam dyeing, beck or winch dyeing, jig dyeing, jet dyeing and pad dyeing.

Advantages

·               Uniform dyeing

·               Inexpensive than fibre or yarn dyeing

·               Versatile and flexible in accommodating changing fashion trends.

·               Faster and easier

Disadvantage

·               The ends of the fabric does not have some colour as the remaining fabric

Beam Dyeing

This process is similar to warp -beam dyeing of yarns. In this method, the woven or knitted fabrics are wound on perforated beams and immersed in dye solution. Dye is circulated through the fabric

Beck / Winch Dyeing

This method is done to dye the fabrics of longer length. The fabric is circulated through the dye bath in tensionless rope form. The ends of the fabric are tied together to form a loop. The fabric moves over a rail onto a winch reel which immerses it into the dye bath and draws the fabric up. The process is repeated till the fabric is dyed uniformly and to the desired colour shade. About 1000 m or 900 kgs of fabric can be dyed at a time. The dyed fabrics retain its original softness feel. This method is widely used for woolen woven and knitted fabrics.

Jig Dyeing

In this method, the fabric is held on rollers at full width and passed through the dye bath. The fabric is let off from one roller into the dye bath and wound on to the other roller. After one cycle, the process is reversed and the fabric is let off from the second roller to the first. The process is repeated till the desired colour strength is achieved. About 250 kg of fabric can be dyed at a time. This method is preferred for heat-sensitive thermoplastic fabrics that cannot be dyed in rope form as they will form permanent crease or colour streaks in the fabric. This method is not suitable for knitted or stretch fabrics which should not be subjected to tension and also to fabrics where a soft feel is required.

Jet Dyeing

In this method the fabric is dyed in a rope form. It is placed in a heated tube or column where jets of dye solution are thrown on the fabric at high pressure. The fabric also moves along the tube. The dye solution is circulated faster than the fabric while colouring it thoroughly. About 500 kg of fabrics can be jet-dyed at a time. This method of dyeing is suitable for both woven and knitted fabrics made up of wool, cotton, viscose rayon, polyester and its blends.

Pad Dyeing

In pad dyeing, the fabric at full width is passed through a trough containing dye bath. The fabric is then passed between two heavy rollers which force the dye to penetrate in to the fabric and squeeze out the excess dye solution. For the dye to set, the fabric is passed through a heat chamber. Then the dyed fabric is rinsed and dried.

4. Dyeing at Product Stage

This is the last stage of dyeing. When the finished product such as garment is dyed, it is called as product dyeing or garment dyeing. The fabric undergoes pre-treatments such as desizing, bleaching and mercerization before dyeing. The most widely used method for garment dyeing is paddle dyeing. In this method, garments are packed loosely in net bags. Ten to fifty of such bags are kept in large tubs containing dye solution. The dye is agitated by a motor driven paddle which increases the penetration of dye. Garment dyeing is suitable for products such as sweaters, T-shirts, casual clothing and hosiery.

Advantages

·               Economical

·               No shade variations

Disadvantages

·               Colour bleeding and fading.

·               Not suitable for products with zippers, sewing, threads, lining etc.

2. Dyeing of Blended Fabrics

When a fabric is made of one type of fibre/ yarn, dyeing process is simple but when it is composed of more than one kind of yarn as in a blend, dyeing process is complex. Special procedures are carried out where different dyes that are suitable for each fibre need to be selected.

1.  Union Dyeing

In this method, blended fabrics are dyed to the same shade to get a solid-coloured fabric. Fabrics can be dyed by one or two bath process. In one bath process, dyes appropriate for each fibre is added in same dye bath. In two bath process, dyes are added separately to each fibre subsequent to one another.

2. Cross Dyeing

In this method, fabrics are dyed in such a way that different fibres produce different shades with the same colour. Sometimes, one of the fibre components is left undyed. Cross dyeing results in checks, plaids, striped and multi-coloured fabrics. Cross dyed fabrics are similar to fibre or yarn dyed textile materials. If one of the yarns of the fabric is vegetable fibre and the other is from animal fibre, then the fabric is dyed in two separate baths, suitable for each fibre. This produces colourful effects.