BLEACHINGBLEACHING

Bleaching --- What and Bleaching --- What and When??When??

When fibres are to be dyed to very dark shades, where fibres brightness is not so important no bleaching is requiredno bleaching is required

For light and medium shades light and medium shades , fibre brightness is important bleaching is an essential operationessential operation before dyeing and

printing

In case of man-made fibres if the process sequence causes slight dis-colourationslight dis-colouration that sometimes cannot be accepted as full white, bleaching is requiredbleaching is required

A process to remove

• The natural colouring material

• Any other colouring material from natural fibres or man-made fibres

The natural colouring material can be destroyed by oxidationoxidation by reductionreduction

How ????How ????

• OxidationOxidation bleaching agents– Chlorine– Hydrogen Peroxide

• ReductionReduction bleaching agents– Sodium sulphite– Sodium hydrosulphite

Bleaching AgentsBleaching Agents

Oxidizing bleaching agentsOxidizing bleaching agents

Use oxygen directly or indirectly for the bleaching

Nascent oxygen Nascent oxygen decomposes colouring matter completely into simpler compounds

Simpler compounds are washed away washed away with water during washing

Do not give any trouble at the later stagetrouble at the later stage

• Reducing bleaching agentsReducing bleaching agents

– The nascent hydrogen combines with the colouring matter in the fibre to produce a colourless compound

– The compound however remains in the fabricremains in the fabric and gets oxidized back to the original colouring matter upon exposure to the air

• Hence the whiteness produced is not permanentnot permanent

Sodium Hypochlorite (NaOCl)Sodium Hypochlorite (NaOCl)

It is the oldest industrial method of bleaching cotton

Sodium hypochlorite is the strongest agentstrongest agent It is capable of giving oxygen at room temperature

It is also economical in use

Prior to bleaching with hypochlorite it is necessary to thoroughly scour fabrics to remove fats, waxes and pectin impurities This impurities will reduce the available hypochlorite,

reducing its effectiveness for whitening fabrics

MechanismMechanism

NaOCl + H2O NaOH + HOCl

HOCl HCl + [O]

NaOCl + HCl NaCl + HOCl

HOCl + HCl H2O + Cl2

Sodium hypochlorite dissolves in water to give the hypochlorous acid (HOCl)

HOCl is unstable and dissociates to yield active oxygen and

hydrochloric acid (HCl)

To avoid the fibre damage it is advisable to maintain a pH of 10.5 to 11 and for that Sodium carbonate is added Small amount of NaOH is also added as pH stabiliser

Cellulose textiles are to be bleached very carefully with hypochlorite

The four factors affecting the bleaching efficiencyfactors affecting the bleaching efficiency are: Concentration of the bleaching liquor Its temperature Its pH (pH between 10 – 11 is to be maintained; pH

below 9 will accelerate the degradation of cellulose) The time of bleaching

The greater the concentration of the bleach liquor, the better the bleaching result but the greater the risk of chemical damage to the cellulose

The time and temperature must be standardized to minimize the chemical damage

The pH should be controlled in between 10 and 11, a fall in pH to below 9 accelerating the degradation of the cellulose material through the formation of oxy-cellulose

Advantages Low chemical cost Low energy output Low cost of equipment

Disadvantages The formation of high level of AOX(Absorbable organic

halogens) results in environmental hazards Rapid bleaching process is not possible with hypochlorite Danger of tendering of the material if pH and

temperature is not controlled Cotton goods must be scoured before hypochlorite

bleaching Imparts harsh feel to the bleached fabric

SOURINGSOURING

It is an acid treatment generally given to hypochlorite bleached goods

Dilute hydrochloric acid at room temperature is generally used It is very difficult to remove traces of alkali even after

thorough washing. The alkali needs to be neutralized, otherwise it may get concentrated and result in the formation of oxy-cellulose during drying

In case of bleaching powder, the calcium carbonate formed during the bleaching is deposited on the fabric and cannot be removed by simple washing. It imparts a harsh feel to the cloth

Sulphuric acid may also be used for souring when bleaching is carried out with sodium hypochlorite

But it is not suitable when bleaching with calcium hypochlorite because of the formation of calcium sulphate, which will deposited on the fabric yielding a harsh feel The reaction with the HCl will give the soluble chloride

Complete removal of acid is required after souring before the cloth is dried

Any traces of acid will cause degradation of the cellulose material because of the increased concentration of the acid on drying

Also the goods should not contain any hypochlorite because over bleaching may take place during souring Presence of hypochlorite is souring will cause intensive action

of hypochlorite by decreasing the pH of the solution to the neutral level

To avoid this the goods may be given “antichlor” treatment with a solution of sodium bisulphite or sodium thiosulphite, which destroys the residual hypochlorite in the fabric

Precautions during souring Concentration of acid solution to be controlled Control of process temperature Intermediate local drying of the fabric should never be

allowed Optimum dwell period should be standardized

Antichlor TreatmentAntichlor Treatment

An antichlor treatment with reducing agent may be given after the hypochlorite treatment to ensure removal of any residual chlorine from the bleached fabric

Chloramines are formed which cause after-yellowing of the material

Reducing agents used Sodium bisulphite (NaHSO3)

Sodium thiosulphate (Na2S2O3)

Sodium hydrosulphite (Na2S2O4)

Peroxide Bleaching (HPeroxide Bleaching (H22OO22))

Universal bleaching agent Applicable for almost all textile material Including wool, silk and manmade fibres

Bleaching is carried out at raised temperature Hence can be utilised for simultaneous scouring and

bleaching

H2O2 is colourless liquid, soluble in water, highly stable in acidic condition but stability decreases as the alkalinity increases

Stabilizers H2O2 will decompose readily to form the bleach active

nascent oxygen

To ensure a fairly uniform availability of bleaching action, stabilizers are used

The commonly used stabilizer is sodium silicate (NaSiO3)

The use of slightly hard water is preferred for hydrogen peroxide bleaching as the presence of magnesium salts in water improves the effectiveness of sodium silicate stabilizer

Recipe for peroxide bleaching

Hydrogen peroxide = 2 – 8%

Sodium silicate = 3%

Soda ash = 0.8 to 6%

Sodium hydroxide = 0.4 – 0.6%

Initial pH – 10.3 – 12

Temperature – 90 degree celcius

Peroxide Killers After peroxide bleaching and rinsing, if traces of

peroxide are left on the fabric, it will affect the dye uptake and give rise to uneven dyeing

Hence peroxide killers are used to remove the residual peroxide from the fabric

The various methods for removal of peroxide are Vigorous rinsing Treatment with inorganic reducing agents Enzyme treatment

E.g – Finostab, Basopal etc.

Advantages

There is generally no need for thorough scouring before peroxide bleaching

It is environmentally acceptable as it has no absorbable halogen

Hydrogen peroxide is compatible with optical brightening agents

It involves low risk of chemical damage to cotton

Disadvantages

High cost

High risk of local damage to cotton and wastage of peroxide due to possible metallic contamination in water

Sodium Chlorite (NaClOSodium Chlorite (NaClO22))

It is versatile

Can be used for cotton as well as other fibres and mixtures with manmade fibres

Cannot be used for silk and woolCannot be used for silk and wool

More expensive than peroxide

Little tendering due to the presence of metallic ions

MechanismMechanism

Sodium chlorite is soluble in water to give a stable solution with a pH of about 10

An acidified solution of sodium chlorite contains chlorous (HClO2) acid, the amount of which depends upon the pH and the temperature of the bath

It is to be noted that acidified chlorite solutions are very reactive and cause corrosion to commonly used metallic vessels For this reason vessels lined with resistant materials such

as glass or ceramics are used Corrosive inhibitorsCorrosive inhibitors like sodium nitrate sodium nitrate can also be used

in the bleaching solution

Bleaching process The bleaching liquor may be made up with the sodium

chlorite ( 1 – 2 %) and a wetting agent and brought to a pH around 4.0 with addition of acetic or formic acid at 800C for 2-3 hours of bleaching action

5ClO2- + 2H+ 4ClO2 + Cl- + 2OH-

3ClO2- 2ClO3 + Cl-

ClO2- 2Cl + 2(O)-

AdvantagesAdvantages Pre-scouring is not required Effectively bleaches the lignin present in the woody

matter Soft fabric handle and good sewability due to non-

removal of fats Least risk of chemical damage to cotton in chlorite

bleaching Least sensitive to metallic contamination in the process

water

DisadvantagesDisadvantages Possibility of liberation of toxic chlorine dioxide gas Equipment is expensive because of the need of corrosion

resistant material Chlorite bleached fabric has poor absorbency and thus is

not suitable for further dyeing and printing Sodium chlorite is very expensive It is incompatible with optical whitening agents

OPTICAL WHITENING

• Certain organic compounds possess the property of fluorescence which means that they can absorb shorter wave length light and re-emit longer wave-length. A substance can absorb invisible ultraviolet rays and re-emit them within the visible spectrum. Therefore, a surface containing a fluorescent compound can emit more than the total amount of daylight that falls on it, giving an intensively brilliant white. Compounds that posses these properties are known as OBA.

• When OBA treated fabric are exposed to UV light bulbs, the fabric glow in the dark; thus conclusion can be drawn that the fabric is treated with OBA

• There are several classes of chemicals whose structure serve the purpose. These structures have dye like properties and in essence are colourless dyes. They can exhaust onto various fibres.

• Anionic OBA’s will exhaust onto cotton, wool and silk.

• Cationic ones exhaust onto acrylic and certain polyesters.

• Nonionic exhaust onto all synthetics

• OBA,s are not a substitute for bleaching. They are used to obtain brilliant market whites. These "white" whites can be obtained without over bleaching and damaging the fiber.

• On cellulose, they have poor wash fastness but most commercial laundry detergents contain OBA's so they are constantly replenished.

• Some OBAs have poor washfastness - some nonionics types have excellent light fastness.

• Fiber producers can include OBA,s in their spinning process. These can have excellent durability to both light and washing.

BIO-POLISHING

The purpose of bio-polishing or bio-finishing is to get a soft handle and attractive clean appearance of the fabric

Cotton or cotton blend fabric contains small cellulosic protruding fibres or microfibrils that make the surface of the fabric rough and fuzzy

The problems associated with fuzz can also be eliminated by Singeing Applying silicon softner to soften the surface of the

fabric By enzymatic bio-polishing

Drawback of singeing Chance of fire hazards

Drawbacks of silicon softners Reduces the water absorbency They are washed out and the fabric becomes rough again

Enzymatic removal of fuzz is absolutely safe, efficient and permanent as it is carried out in mild chemical and physical condition with accurate control

Cellulase enzymes are usually classified by the pH range in which they are most effective viz. acid stable, neutral stable and alkaline stable

The enzyme cellulase degrades the cellulosic fuzz on the surface thus increases the lustre and smoothness of the fabric

Certain properties are however adversed Weight loss of the fabric Loss in fabric strength

Recipe:Recipe:

Enzyme dosage = 1 – 5% on fabric weight (depending upon the activity of enzyme

Liquor ratio = 1: 5 to 1: 15

Time = 60 – 120 minutes (depending upon amount of hydrolysis required)

Temperature = 50 – 600C

pH = 4.5 – 5.5 (for acid stable cellulase)