Here I am going to share information about fabric dyeing, printing & finishing process
Dyeing, printing & finishing are critical processes in the manufacture of textiles because they impart colour, appearance, and handle to the final product. The processes depend on the equipment used, the constituent materials and the structure of the yarns and fabrics. Dyeing, printing & finishing may be carried out at various stages in textile production.
Natural fibres such as cotton or wool may be dyed before being spun into yarns and yarns produced in this way are called fibre-dyed yarns. Dyes could be added to the spinning solutions or even in the polymer chips when synthetic fibres are spun, and, in this way, solution-dyed yarns or spun-dyed yarns are made. For yarn-dyed fabrics, yarns need to be dyed before weaving or knitting takes place. Dyeing machines are designed for dyeing yarns in the form of either loosely wound hanks or wound into packages. Such machines are referred to as hank dyeing and package dyeing machines respectively.
Finishing processes my also be performed on the assembled garments. For example, denim clothing washed in many ways, such as stone washing or enzyme washing, is very popular these days. Garment dyeing might also be used for some types of knitwear to produce garments so as to avoid colour shading within them.
However, in most cases dyeing, printing & finishing are carried out on fabrics, whereby cloths are woven or knitted and then these grey or " greige" state fabrics, after preliminary treatments, are dyed, and/or printed, and chemically or mechanically finished.
In order to achieve " predictable and reproducible" resuluts in dyeing and finishing, some preliminary treatments are necessary. Depending on the process, fabrics may be treated as single pieces or batches, or sewn together using chain stitches, easily to be removed for post-processing, to create long lengths of different batches for continuous processing.
Singeing is the process to burn off fibres or nap on the fabric surface to avoid uneven dyeing or printing blotches. Generally speaking, woven cotton grey cloths need to be singed before other preliminary treatments are started. There are several types of singeing machines, such as the plate singer, to roller singer and the gas singer. The plate singeing machine is the simplest and oldest type. The cloth to be singed passes over one or two heated copper plates at high speed to remove the nap but without scorching the cloth. In the roller singeing machine, heated steel rollers are used instead of the copper plates to give better control of the heating. The gas singeing machine, in which the fabric passes over gas burners to singe the surface fibres, is the most commonly used type nowadays. The number and position of the burners and the length of the flames can be adjusted to achieve the best result.
For warp yarns, especially cotton, used in weaving, sizing, usually using starch, is generally necessary to reduce the yarn hairiness and strengthen the yarn so that it can withstand the weaving tensions. However the size left on the cloth may hinder the chemicals or dyes from contacting the fibres of the cloth. Consequently the size must be removed before scouring starts.
The process to remove the size from the cloth is called desizing or steeping. Enzyme desizing, alkali desizing or acid desizing may be used. In enzyme desizing, the cloths are padded with hot water to swell the starch, an then padded in enzyme liquor. After being stacked in piles for 2 to 4 hours, the cloths are wahsed in hot water. Enzyme desizing requires less time and causes less damage to the cloths, but if chemical size instead of wheat starch is used, enzymes may not remove the size. Then, the widely used method for desizing is alkali desizing. The fabrics are impregnated with a weak solution of caustic soda and piled into a steeping bin for 2 to 12 hours, and then washed. If after that, the cloths are treated with dilute sulphuric acid, better results can be achieved.
For knitted fabrics, desizing is not needed since yarns used in knitting are not sized.
For the grey goods made of natural fibres, impurities on the fibres are inevitable. Taking cotton as an example, there could be waxes, pectin products as well as vegetable an mineral substances in them. These impurities may give the raw fibres a yellowish colour and make them harsh to handle. The waxy impurities in the fibres and oil spots on fabrics are likely to affect the dyeing results.
Furthermore, waxing or oiling might be necessary to make the staple yarns soft and smooth with lower frictional coefficients for winding or knitting. For synthetic filaments, especially those to be used in warp knitting, surface active agents and static inhibitors, which are usually a specially formulated oil emulsion, should be used during warping, otherwise the filaments may carry electrostatic charges, which will severely disturb the knitting or weaving actions.
All impurities including oils and waxes must be removed before dyeing and finishing, and scouring can, to a great extent, serve the purpose. One of the most common methods of scouring for cotton grey cloth is kier clothing. The cotton cloth is packed evenly in a tightly sealed kier and boiling alkaline liquors are circulated in the kier under pressure. Another commonly used way in scouring is continuous steaming and the scouring is processed in serially arranged apparatus, which gengerally comprises a mangle, a J-box and a roller washing machine.
The alkaline liquor is applied onto the fabric through the mangle, and then, the fabric is fed into the J-box, in which saturated steam is injected through the steam heater, and afterwards, the fabric is piled uniformly. After one or more hours, the fabric is delivered to the roller washing machine.
Although most of the impurities in cotton or linen cloths can be removed after scouring, the natural colour still remains in the cloth. For such cloths to be dyed to a light colour or to be used as the ground cloths for prints, bleaching is necessary to remove the inherent colour.
The bleaching agent is actually an oxidizing agent. The following bleaching agents are commonly used.
Sodium hypochlorite ( calcium hypochlorite may also be used ) may be the commonly used bleaching agent. Bleaching with sodium hypochlorite is generally performed under alkaline conditions, because under neutral or acidic conditions the sodium hypochlorite will be severely decomposed and the oxidization of the cellulosic fibres will be intensified, which may make the cellulosic fibres become oxidized cellulose. Furthermore, metals such as iron, nickel and copper and their compounds are very good catalytic agents in the decompositon of sodium hypochlorite, therefore equipment made of such materials cannot be used in the process.
Hydrogen peroxide is an excellent bleaching agent. There are many advantages for bleaching with hydrogen peroxide. For example, the bleached fabric will have a good whiteness and a stable structure, and reduction in fabric strength is less than that when bleached with sodium hypochlorite. It is possible to combine the desizing, scouring and bleaching processess into one process. Bleaching with hydrogen peroxide is gengerally performed in a weak alkali solution, and stabilizers such as sodium silicate or tri-ethanolamine should be used to overcome the catalytic actions caused by the metals mentioned above and their compounds.
Sodium chlorite is another bleaching agent, which can impart a good whiteness into the fabric with less damage to the fibre and is also suitable for continuous processing. Bleaching with sodium chlorite has to be performed in acidic conditions. However as the sodium chlorite is decomposed, chlorine dioxide vapour will be release, and this is harmful to human health and is strongly corrosive to many metals, plastics and rubber. Therefore titanium metal is generally used to make the bleaching equipment, and necessary protection against the harmful vapours would have to be taken. All these make this method of bleaching more expensive.
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