Saturday, June 25, 2011

DIRECT COTTON DYE STUFFS

          These are dye stuffs which dye cotton from a neutral o rsilghtly alkaline bath. The syeing is an adsorption process and requires a certain segree of aggregation of the dye stuff in the dye bath and a certain special shape of the molecule .It is usual to increase the natural state of aggregation by the addition of salt or Sauber's salt. many direct cotton aye stuffs aye wool and silk at the boil as strongly as or more strongly than they sye cotton . A few leave wool undated especially in alkaline baths a low temperatures or after treatment of the wool with formal dehydrate or tannin . Many combine with formaldehyde copper or chromium salts to give dyeings of greatly improved fastness to either light or washing or both.


SULPHUR DYE STUFFS :
                                            This type of dye stuff is insoluble in water but is reduced by mild reducing agents such as sodium sulphide to a water soluble eleucocompound which is absorbed by cotton subsequent oxidation con averts the LegCo compound into the original insoluble form and thus "dyes " the fiber. The dyeing are usually very fast to wet processing .
Several sulphide dye stuffs are improved in fastness by after treat meant with chromium or copper salts and are brightened in shade by oxidation with hydrogen peroxide or other oxidizing agent . They always contain proportions of sulphur in a form easily oxidized on storage into sulfuric acid and consequently heavy shades such as black may produce sufficient acid over a period of time to cause tendering of vegetable fibers on which they are dyed . They can be brightened in shade by after treatment with basic colors for which they act as a mordant.

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