Natural fiber and fabrics even after scouring stillcontain naturally occurring coloring matter.
Natural fibers are off-white in color due to colorbodies present in the fiber
This yellowish and brown coloration may berelated to of the cotton flower.
The climate, soil, drought and frost can alsocause various degrees of yellowness.
Tips of leaves or stalks coming in contact withthe moist ball after opening will cause darkspots and coloration.
Coloration may also come from dirt, dust,insects or from harvesting or processingequipment in the form of oils and greases.
The objective of bleaching is to produce whitefabrics by destroying the coloring matter withthe help of bleaching agents with minimumdegradation of the fiber.
The bleaching agents eitherthe coloring matter which is washed out andwhiteness thus obtained is of permanent nature.
Chemical bleaching of textile fibers is furtheraided by addition of optical brighteners.
Mechanism of bleaching is very complicated and notcompletely understood.
One opinion is that the color producing agents in naturalfibers are often organic compounds containing conjugateddouble bonds.
It is known in dye chemistry that conjugation is necessaryfor an organic molecule to perform as a dyestuff.
Decoloration can occur by breaking up the chromophore,most likely destroying one or more of the double bondswithin the conjugated system.
The major bleaching agents used in textile preparation aresodium hypochlorite, hydrogen peroxide and sodiumchlorite.
Other bleaching agents, of lesser importance to textilepreparation but important in consumer laundry products, areperborates, percarbonates and peracetic acid.
All of these are oxidative bleaches.
Known to degrade cellulose so the objective in bleaching isto optimize whitening and minimize fiber damage.
Most commonly & widely used bleaching agent.
90 to 95 % of all cotton and cotton/syntheticblends are bleached with hydrogen peroxide
discovered hydrogen peroxide in 1818and called it .
It is available commercially as 35, 50 and 70 %solutions.
It is a corrosive, oxidizing agent which may causecombustion when allowed to dry out on oxidizableorganic matter.
Decomposition is accelerated by metal contaminationand is accompanied by the liberation of heat andoxygen, which will support combustion and explosionsin confined spaces.
The material is an irritant to the skin and mucousmembranes and dangerous to the eyes.
Hydrogen peroxide is a weak acid and ionizes in waterto form a and a . The
is the active bleaching agent.
H2O2 + H2O H+ + HOO-
Hydrogen peroxide can also decompose. This reactionis catalyzed by metal ions e.g. Cu++, Fe+++. Thisreaction is not desired in bleaching because it is anineffective use of hydrogen peroxide and causes fiberdamage.
H2O2 + H2O H2O + O2
Though hydrogen peroxide is stable in acidic medium, but bleaching occurs by the addition of alkali or by increased temperature. Hydrogen peroxide liberates perhydroxyl ion (HO2-) in aqueous medium and chemically behaves like a weak dibasic acid.
The perhydroxyl is highly unstable and in the presence of oxidisable substance (coloured impurities in cotton), it is decomposed and thus bleaching action takes place.
Sodium hydroxide activates hydrogen peroxide because H+ ion is neutralized by alkali which is favorable for liberation of HO2-.
However, at higher pH (above 10.8) the liberation of HO 2- ion is so rapid that it becomes unstable with the formation of oxygen gas which has no bleaching property.
If the rate of decomposition is very high, the unutilised HO2- may damage the fiber. A safe and optimum pH for cotton bleaching lies between
where the rate of evolution of perhydroxyl ion is equal to the rate of consumption (for bleaching).
At higher pH, hydrogen peroxide is not stable and hence a stabilizer is frequently added in the bleaching bath.
The process of regulation or control ofperhydroxyl ion to prevent rapiddecomposition of bleach and to minimizefiber degradation is described as
They could include any of the followings:
e.g. caustic soda/carbonate/silicate
e.g. magnesium salts
e.g. acrylic polymers
To control the decomposition of hydrogen peroxide.
The process of regulation or control of perhydroxyl ion to prevent rapid decomposition of bleach and to minimize fiber degradation is described as stabilization
Stabilisers for peroxide normally work by controlling the formation of free radicals
By providing buffering action to control the pH at the optimum level
To complex with trace metals which catalyze the degradation of the fibers
Stabilizers include sodium silicate, inorganic and organic compounds and phosphates
Sodium silicate is the most conventional, easily available and widely used stabilizer
It is very difficult to specify strict guidelinesfor optimum bleaching conditions forhydrogen peroxide as the operation isnormally affected by the nature and qualityof the goods to be bleached, the amount ofbleaching required and on the equipmentavailable.
However, the following general variables are considered to be important:
The stability of hydrogen peroxide depends on pH. At pH 1 to 3 it is stable ; but at highly alkaline pH 11.5 to 13 it has least stability. The bleaching takes place around 10.5 due to accumulation of perhydroxyl ions in the bleaching bath.
At neutral or weak alkaline media, hydrogen peroxide does not produce any whitening effect and may cause degradation of cellulose.
In practice cotton bleaching with hydrogen peroxide is carried outat 90-100C but the temperature may be increased to 120C inthe case of pressurized equipment with a corresponding reductionin process time.
The rate of bleaching increases with the increase in temperature,but at the same time solution becomes unstable and degradationof cotton increases.
Better bleaching occurs at 95 to 100 C. This feature makes it ideal for continuous operations using insulated J-
boxes or open-width steamers.
Below 80C the evolution of perhydroxyl ion is very slow so alsothe rate of bleaching.
The optimum concentration of hydrogenperoxide depends on number of factorsnamely:
Class of fiber
The time required to bleach with hydrogen peroxidedepends on:
Temperature Class of fiber Equipment used for bleaching
In general, the time of bleaching is inversely proportional to the temperature of the bleaching bath.
Usually 18-24 hrs for cold pad batch 10-20 min for continuous steaming
Bleaching with sodium chlorite is carried out underacidic conditions which releases chlorine dioxide, atoxic and corrosive yellow-brown gas.
Sodium chlorite is sold as an 80% free flowingpowder.
Chlorine dioxide is thought to be the activebleaching specie.
It is not used much for bleaching but it issometimes used to strip dyed goods and is oftendescribed as the bleach of last resort.
One advantage of sodium chlorite bleaching is thatit leaves the fabrics with a soft hand.
Because of the gaseous nature, toxicity andcorrosiveness of chlorine dioxide, special attentionmust be paid to the equipment.
It must be designed so as to not allow the gas toescape into the work place. Emissions into theatmosphere are of concern too.
The gas corrodes even stainless steel so specialpassivating treatments must be carried out toprolong the life of the equipment.
The corrosive nature of this chemical demandsexotic constructional materials such as titanium.
When a solution of sodium chlorite isacidified, chlorine dioxide (ClO2),hypochlorous acid (HClO2), sodium chlorate(NaClO3) and sodium chloride (NaCl) areformed.
Chlorine dioxide and hypochlorous acid arebleaching species, sodium chlorate andsodium chloride are not.
The reactions may be written:
Chlorite breaks down according to thefollowing mechanism:
Reaction (I) is always the most important ofthe two; reaction (II) is promoted by the dropin pH.
Chlorine dioxide is favored at low pH i.e., 1 to 2.5.
It is a more active bleaching agent than hypochlorousacid which is favored at pH 4 to 5.
Relatively unaffected by iron or copper contamination.
However chlorine dioxide is a corrosive and toxic gas.
When generated too rapidly, it escapes from thebleaching bath into the atmosphere creating anexplosion and health hazard.
Once the chlorine dioxide is out of solution, itseffectiveness as a bleaching agent is lost.
Little or no bleaching takes place at temperaturesbelow 50 C, however the bleaching rate increasesconsiderably up to 90C.
Going to the boil is not recommended because itleads to excessive loss of chlorine dioxide with thesteam.
Oxidation action occurs due to chlorineperoxide which develops by acidificationprocess.
Concentration 0.3-0.5 g/L
pH is around 4
Temperature is 95C
Time give 45-60 min
Cant be used for wool and silk
Chlorine dioxide is corrosive
Acidic media damages the cotton
Whiteness Measurement:The CIE Whiteness Index value (CIE WI) is determined using AATCC Test meth