Colour scouring

Fastness against scouring. The sample is rubbed ten times up and down energetically with a piece of white cotton materal stretched between the fingers to ascertain if the white cotton becomes coloured and to what extent. No standards are fixed for this test. 

The type of alkali used for scouring of cotton depends on the quality of goods. For example, if coloured yams present in the fabric, sodium carbonate is ideally suited because of its low pH. Cotton yams to be dyed in dark shade should be scoured with 1-2% sodium carbonate solution for 30 min in presence of wetting agent. 

Hard water is also objectionable because the insoluble soaps associated with its use tend to be deposited on the goods during scouring, when they give rise to faults. The soaps are somewhat sticky and adhere quite tenaciously to the materials, picking up adventitious dirt or impurities, thereby causing discoloured patches. They also form spots impervious to dye liquors, with the result that the final colour is not uniform. Some dyestuffs form calcium compounds which are insoluble, and hard water can, therefore, cause serious interference with the application of such dyes. 

M ht scouring operations in which these surface-active compounds are commonly used varv according to the nature of the fibre and the amount and composition of the impurities to be removed. In the case of cotton the cellulose of w hich it is composed is stable to dilute solutions of alkali at the boil. I he imphrities which must be removed are natural oils and waxes, proteins, pectic substances, natural colouring matter, and adventitious dirt. The basic principle of cotton scouring is to boil the goods for several hours at atmospheric, or under elevated, pressure with a 2 per cent solution of sodium h) droxide. 

The cotton or cellulosic goods, after thorough scouring or keir boiling, are entered into the dye bath at 40° to 50°C (100° to 122°F) and the liquid is raised to the boil over a period of 30 to 40 minutes, after which dyeing is continued for f to 1 hour. At the end of this time the distribution of the colour should be uniform, but if it is not any defects can be cured by further boiling. 

The dyed cotton requires the equivalent of 0-75 to 1 per cent of the weight of the goods of diazotized paranitroaniline. Coupling will be completed after running in a cold liquor for J hour. When the process is finished any uncoupled base should be removed by thorough rinsing, and finally a scour in soap solution is necessary to remove any loose colour. 

After the goods have been entered the dye liquor is raised to the boil and dyeing continued at this temperature for 60 to 90 minutes. When exhaustion is complete the load is rinsed and scoured at 50°C (122 F) to remove any surface-deposited colour. The dispersed dyes are classified according to affinity as follows by Mellor and Olpin J.S.D.C., 1955, 71, 819) ... 

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