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UNIVERSITY OF KARCHI CHEMICAL ENGINEERING BLEACHING Bleaching Definition !! " Mechanism of Bleaching !# $% & %!'( )** Bleach goods: +$ Objectives of Bleaching • , • , • -$ • !. • /. • 0 Bleaching Agent Oxidative bleaching !# ,! ! ' Reductive bleaching -' 1 Auxiliaries used for bleaching: * 234removes ion from a solution system by forming a ring which does not have the chemical reactions of the ion which is removed. It can be a complexing or a chelating agent.) +56 2 Bleaching with Hypochlorites 74#8*9' :;<=>'?#8(;>@' 0 !* Sodium hypochlorite (NaOCl) Bleaching: 2* ?#870A ) ! B ) 8?#8;C;D@7D@ Bleaching Mechanism: ?#84#8*947#89 ?#847E;;DD9 2F> G8 ?#8H7C#I? H H#8 * 49 #8 * H7C#I7#8H#7 J [...]... require bleaching, but when cotton is present bleaching is usually necessary •Bleaching treatments of such blends are normally required to remove the natural colours of cotton, sighting colours and if the polyester portion is turned yellow at the time of heat-setting operation •Chlorine bleaching, peroxide bleaching and chlorite bleaching are employed widely •If the polyester portion requires bleaching,. .. does not interfere • Aqueous solution is extremely stable • Chlorite bleaching results in a degree of whiteness which cannot be achieved with hydrogen peroxide in a single stage process Disadvantages: • Treatment under acidic conditions so all alkali must be removed before hand • ClO2 liberation causes pollution and corrodes equipment • Continuous methods cannot be used • More expensive than H2O2 • Bleaching... Filament viscose rayon may not require bleaching since this is normally carried out during manufacture However, viscose in staple form requires bleaching as it may not necessarily include a bleaching treatment during its manufacture •The same reagents as those used for bleaching linen and cotton fabrics are useful for these fibers For very good whiteness, rayon may be bleached on a jigger with alkaline... is required and care must be taken that the water supply be free of metal ions and rust from pipes Antichlor • Fabrics bleached with hypochlorite will develop a distinctive chlorine odor • An antichlor treatment with sodium bisulfite and acetic acid removes any residual chlorine from the cloth Uses: Hypochlorite is used mainly to bleach cellulosic fabric • It cannot be used on wool, polyamides (nylon),... are employed widely •If the polyester portion requires bleaching, then chlorite bleaching is used, as this bleaching agent bleaches both polyester and cellulose •If the polyester portion does not need bleaching, then peroxide bleaching is more convenient •Alkaline hydrogen peroxide bleaching is the most preferred system for polyester/cotton blends Polyester/Wool Blends: • In general, blends containing