Last updated on August 7th, 2023 at 01:05 am
Definition of Bleaching
A process of whitening fibres or fabrics or depriving a colored material. This is brought about by using various bleaching-agents. Generally chlorine is used for cotton and other vegetable fibres and peroxide and sulphur dioxide for animal fibres.
Bleaching of Cotton
Cotton is bleached in the raw state (as yarns) or in piece. The impurities present are cotton wax, fatty acids, peptic substances, coloring matters and albuminoids (another mineral matter). Although these are do not amount to more than 5% of the weight.
Bleaching of cotton is comparatively a simple process, the main operations being:
- Boiling with alkali.
- Bleaching the organic matters by some bleaching agents.
- Souring treatment with dilute H2SO4 or HCl.
In this process, the necessary operations are:
- Steeping in warm solution of soda for some hours.
- Bleaching powder treatment or sodium hypochlorite.
- Souring with dilute acids.
- Washing free of acids.
In principle the process employed for yarns is the same as for loose cotton, but this requires different machinery:
- Boiling in alkaline lye. This is carried out in kier-boil (6 to 8 hours) in alkaline lye (3% soda ash or 2% caustic soda on weight of cotton).
- Bleaching powder solution 2% Tw treatment contained in stone cistern which is provided with a false bottom.
- Souring with dilute acids HCl about 2% Tw.
- Washing to remove traces of acids.
- Optical bluing.
The bleaching of cotton yarns is a very straight-forward process and with reasonable care, is almost foolproof.
By far the largest bulk of cotton-goods are bleached in the piece as this happens to be the most convenient form, the principle is similar to yarn-blending but here some alter-actions and additional operations have to be performed. These are necessary to remove factors like sizing and other chemical substances which are generally added in sizing or weaving operations. The sizing materials are starches, farinas with other substances like tallow, ZnCl2, soaps, MgCl2, etc., and sometimes some mineral oils. All these materials have to be completely removed if perfect results are desired. So two steps viz. singeing, and desizing become necessary.
The surface of the fabric, coming from weaving dept has a downy appearance as it is covered with nap and loose fibres. A clean surface of the fabric is absolutely necessary for further operations. The napping must be removed
first, and this is achieved by singeing.
The cotton-pieces are run at full width through the singeing machines, over a non-luminous gas flame.
The operation simply consists of running pieces through ordinary washing machines.
In order to wet these out on leaving the machines, these are piled in heaps and left as such overnight when fermentation sets in which results in hydrolysis of starch and renders it soluble in water.
In this operation the pieces are first run through alkaline of strength of about 4% of the weight of charge. They are run into kier. After this the pieces are run through continuous washing machine with plentiful supply of water.
In this operation, the goods are run through a washing machine containing HCl of strength of 20 Tw. Object of this is to remove the lime completely. The goods are then well washed.
The pieces are then washed free of alkali and the bleaching proper or chemicking follows. This operation is effected in various ways. The most efficient or effective being to run the goods in a washing machine through bleaching powder solution (10 Tw), and allow them to lie loosely piled up overnight. The goods are then washed and run through very dilute solutions of acids (at 20 Tw), called white-sour and again washed thoroughly to free them completely from acids as otherwise this results in tendering of fibres.
The next operation of bleaching is scotching or opening the cloth out from rope form to its full width. After scotching the opened cloth is dried on drying machine.