Mineral and Clay Based textile bleaching

 

Now a days many commercial products based on innovative clay or mineral chemistry  are available for discontinuous bleaching of cotton and its blends with hydrogen peroxide. The clay or mineral technology have several advantages over conventional peroxide bleaching process.

What are these clay or mineral based products

(http://dyeingworld1.blogspot.com)

The clay or mineral based products are a composition of naturally occurring mineral or clays such as  montmorillonite and/or bentonite in combination with suitable wetting , sequestering agents  and viscosity control agents . The clay is finely dispersed in the form of a non settling, non separating emulsion which can be diluted with water and can be used easily. The clay structure is based on sheets. By separating these sheets e.g. by stirring, a very big surface is created This surface (till 800 M 2 per gram) makes it possible to absorb all kind of components like dirt, oil, greases etc. This gives an unique detergency effect .Two properties of montmorillonite crystals enable its use as a highly effective bleaching agent:

•On the one hand, impurities are adsorbed from the fabric and the liquor, due to the high specific surface area.

•On the other hand, montmorillonite has the property to replace disturbing heavy metal ions in the bleaching bath by Na-ions

The result is a dual effect as bleaching stabiliser and extractant.

How these products acts
 (http://dyeingworld1.blogspot.com)

In water the clay particles are anionic charged and show a large adsorption effect for:

• Hydrophobic waxes and oils

• Metal ions (cation exchange capacity)

• Hydrophilic materials and polymers

It is believed that the clay component provides both a scouring and softening effect, in essence by adsorbing organic contaminants and acting as an ion exchanger, in effect having a contaminant-suspending capacity. The powder or agglomerate particles of the clay divide into primary crystal plates caused by the swelling of the clay. The average size of the primary crystal plates is approximately 1.mu.. After being dispersed in the treatment liquor, the contaminants detached from the treated textile material are adsorbed by the clay particles, resulting in a greatly reduced redeposition of the contaminants. Textile incrustation is reduced due to the hardness elements released during washing having either become attached to the dispersed clay crystals or having been deactivated by the sequestrant. Clay, a natural product, washes off the treated textile material and forms an effluent having low BOD and COD values. The clay does not contribute to the BOD or COD values at all.(Ref: Clay-containing textile material treating composition and method by Brown; Harold M. (Lyman, SC))

The higher whiteness index of the treated textile materials is attributed to the fact that in this bleaching system the hydrogen peroxide is consumed to the maximum extent ,resulting into very low or zero residual peroxide.

Advantages of these single-component multifunctional auxiliaries are

(http://dyeingworld1.blogspot.com)

1.No need of Organic peroxide stabilizers,

2.No need of Dye bath Lubricants,

3.No need of Wetting and scouring agents

4.No need of Emulsifiers in most applications,

5.No need of Antifoaming agent.

6.No need of Machine cleaning aids.

7.Very low or no need of peroxide killer treatment.

8.Shortened bleaching cycle , less water and steam consumption.

9. Low loss of degree of polymerisation, due to controlled peroxide degradation

10.Better tensile strength compared with conventionally bleached fabric

11.Lower weight loss.

12, Better removal of oil or silicone

And since these products are derived from clay or natural resources these are eco friendly in nature , having low COD and BOD values

A typical bleaching recipe for cellulosic material is

(http://dyeingworld1.blogspot.com)

Best suitable for package dyeing machines , jets and winches .

1-2 ml/l Clay product

2-3 g/l Caustic flakes

2-5 ml/l Hydrogen peroxide 35%

At 95 -98 oC for 45-60 min

Test the peroxide content at equal intervals and ensure maximum utilization of peroxide , to optimize the process timing.

Followed by hot and cold wash, do peroxide killing if necessary.

How to Test pH of Textile material

Testing of pH by water extraction method

Purpose of the test

The pH of the processed textiles must be neutral and within the prescribed limits for a particular end use. The textiles with higher pH value may exhibit yellowing tendencies ,create change of shade upon storage and during shipment, result into poor dye pick up , patchy dyeing and poor colorfastness. very high and very low pH results into degradation of textiles during storage and use , may result into poor softness and harsh feel. pH of apparel fabrics which are in direct contact with the skin or used for baby wear, must be controlled not to make the skin itchy. 

The pH control is very important for in process quality as well as quality of the finished material.

Test method

Boil 250 ml distilled water or deionized water for 10 minutes and put 10+/- 0.1 gm of test specimen in it boil for another 10 minutes. Allow it cool down to room temperature , remove the specimen and squeeze back the excess water in the beaker. 

Test the pH by using a calibrated pH meter within an accuracy of +/-0.1 on pH scale.

The pH of water extract must be tested after prior to dyeing if you are getting uneven dyeing and to be tested after completion of dyeing and soaping to avoid any problems in finishing such as softening and dye fixing etc.  

This information is generalized for day to day in house process control, for detailed information please follow AATCC 81-2006

and ISO-3071. 
Search here for more information on the subject

Custom Search

Scouring

SCOURING:

Natural fibers contain oils, fats, waxes, minerals, leafy matter and motes as impurities that interfere with dyeing and finishing. Synthetic fibers contain producer spin finishes, coning oils and/or knitting oils, Mill grease used to lubricate processing equipment, mill dirt, temporary fabric markings and the like may contaminate fabrics as they are being produced. The process of removing these impurities is called Scouring. Even though these impurities are not soluble in water, they can be removed by Extraction, dissolving the impurities in organic solvents, Emulsification, forming stable suspensions of the impurities in water and Saponification, Converting the contaminates into water soluble components.

Scouring is usually carried out by means of continuous or discontinuous systems, with the same machines used for downstream treatments; temperature, processing time, pH, concentration of reagents, depend on the fibre,whiteness required and on the machine used.Incomplete scouring processes usually originate dyeing and printing defects due to different degrees of wettability and to inconsistent affinity for dyes of the material.

Types of scouring treatments
1.Scouring By Chemicals Means
2.Bio scouring or scouring with Enzymes
Adertisement

Your Ad Here

Scouring of Cotton

In the scouring process the cotton cellulose material is treated with a solution containing alkali ( soda ash and caustic soda), an anionic and/or nonionic detergent, a wetting agent, a complexing and sequestering for the removal of metal ions and polyacrylates or polyphosphonates as special surfactant free dispersing agents, at high temperature.

The scouring operation was conventionally done in kier machines and hence the process was called kier boiling , but now a days the scouring is done mostly in the dyeing machines itself as a part of combined or single operation such as scouring and bleaching.

I. Saponification  of fats into water soluble soap and water miscible glycerin  under alkaline conditions.

II. Hydrolysis of proteins into water soluble degradation products.

III. Dissolution of amino compounds.

IV. Solubilising of pectose and pectins  by converting into soluble salts .

V. dissolution and extraction of mineral matter.

VI. Emulsification and solubilisation of natural oils and waxes.

VII. removal and dispersion of dirt particles and kitty by the action of alkali and detergent.

The material after scouring is more absorbent , free from natural impurities and coloring matter.This treatment can be carried out on filaments, yarns and fabrics.

Scouring of Silk

The scouring of pure silk is a degumming process used to remove sericin (silk gum) from
fibroin floss. Sericin is the gummy element which keeps together the fibroin floss and gives the
silk a hard hand and dull appearance. It is carried out on yarn, on dyed yarn, piece-dyed fabric or on products ready for printing. The treatment, which causes a loss of weight ranging between 24 and 28%, gives the degummed silk a lustrous appearance and a soft hand; the treatment is
carried out with soapy solutions or with buffer dissolving agents. It is also possible to use enzymes (protease), which hydrolyses sericin.

Scouring of Wool

On wool, the scouring process removes oils and contaminants accumulated during upstream
processing steps and can be carried out on slivers, yarns and fabrics with solutions containing
sodium carbonate with soap or ammonia, or anionic and non-ionic surfactants, which carry out a
softer washing to avoid any damage to the fibres.

Scouring of Synthetics

The scouring process applied to synthetic fibres removes oils, lubricants and anti-static substances, dust, contaminants and can be carried out on yarns and fabrics (when warp yarns
have been bonded, the treatment is called debonding). It is carried out by means of surfactants,
detergents and emulsifying agents.

Scouring and Bleaching of Cotton knits

Reductive Bleaching

Sodium Hydrosulphite

Formula: Na2S2O4

Molecular weight:174.10
sodium dithionite is also used as a bleaching agent in reductive bleaching processes.
Examples are bleaching of mechanical paper pulp and the bleaching of cotton and wool. It especially reduces carbonyl and alcohol functional groups, which are responsible for the colour of the textile fabric. Unfortunately these processes are not 100 % effective. Therefore a reductive bleaching process is often preceded by an oxidative one (with hydrogen peroxide, sodium hypochlorite or sodium chlorite).
Suphur Dioxide

Dry sulphur dioxide does not have any bleaching action. However, if the gas is moist, or if the object to be bleached is moist, then bleaching takes place. Usually only delicate articles such as coloured flowers, silk, wool etc., are bleached by sulphur dioxide.

The bleaching action takes place in two steps.

First step

1.Sulphur dioxide reacts with water to liberate nascent hydrogen.

2.Second step

The nascent hydrogen so formed, then adds to the coloured matter. As a result the colour is removed and bleaching takes place.

Coloured matter + Nascent hydrogen -------------------> Colourless matter

The bleaching by sulphur dioxide is not permanent. The reaction is a unstable and reversible one; when exposed to the atmosphere, the bleached matter gets oxidized, and regains its original color.

Bleached matter + Oxygen -------------> Original colour

Dipping the bleached article in dilute sulphuric acid can also restore the original colors.

Bleaching With Hypochlorites

Hypochlorites

Hypochlorites are salts, chiefly important as powerful oxidizing and bleaching agents; not, however, when pure, but when containing chlorides. The chief hypochlorites, or at least the complex substances which contain hypochlorites, are bleaching-powder, and the bleach liquors made with potash and caustic soda/soda.

Bleaching Powder

Calcium hypochlorite is a chemical compound with formula Ca(ClO)_2,Molecular weight: 143 .Calcium hypochlorite is usually a white or grayish-white powder free from lumps , contains about 35-37% active chlorine, but it is also available as tablets. It is a strong oxidizer and is considered more stable than sodium hypochlorite. It is also believed to provide more chlorine.

Bleaching powder is partially soluble in water, the hypochlorite dissolving and being responsible for oxidizing and bleaching powers.Available chlorine is a measure of the oxidizing power of the hypochlorite ion.

Chemical properties of bleaching powder

Reaction with Water

Ca(OC)Cl + H₂O = Ca(OH)₂ + Cl₂
With HCl:
Ca(OCl)Cl + 2HCl = CaCl₂ + Cl₂ + H₂O
With CO₂:
2Ca(OCl)Cl + CO₂ + H₂O = CaCO₃ + CaCl₂ +2HOCl

Sodium Hypochlorites

Sodium Hypochlorite is a greenish-yellow liquid commonly referred to as "Bleach." The chemical compound formula for Sodium Hypochlorite is NaOCl.Sodium Hypochlorite is prepared by reacting dilute caustic soda solution with liquid or gaseous chlorine, accompanied by cooling. It is used extensively as a bleaching agent in the textile, detergents, and paper and pulp industries.

The active ingredients in hypochlorite bleaches vary with pH. At pH <2,cl>2 is the main component in solution; at pH 4 to 6, HOCl is the dominant species; at pH > 9, OCl ⁻ is the only component present. It is the hypochlorite ion in basic solution that is the active ingredient in household bleach, which is typically about 5 to 6 percent NaOCl. The OCl ⁻ ion oxidizes chromophores in colored materials, and is itself reduced to chloride and hydroxide ions.

Hypochlorite Bleaching process

The conditions of the bleaching agent varies according to the result required, the concentration depends upon the fabric quality, degree of whiteness required, types of machine and next operation.

Quantity Required:-

Normally 2.5-3.0 gpl of available chlorine is sufficient for good bleaching , but it is necessary to optimize it on a possible lower  value for safe bleaching process.

pH :-General PH range is 10-11 or 10.5-11.5 during bleaching if PH reaches 9 then it is a danger level and at 7 PH the bleaching is worse and causes extreme damage to the cloth.The PH value is maintained by adding sod ash or by buffering agent.

Temperature:-The suggested temperature for hypochlorite bleaching is 37-40oC. Generally reaction is accelerated with  increases of temperature.

Water Quality:-water for bleaching should be soft and even hard water can be used but should be free from Cu++ and Fe++.

Effect of metals

The bleaching equipment should be made of stainless steel , to avoid the catalytic degradation of the cellulose in the presence of copper and iron.

Substrate preparation

The substrate must be pre scoured in the presence of chelating agents , it should be free from rust spots and traces of metallic impurities when bleaching with hypochlorites.

Bleaching Time :-The time factor depends upon the following consideration.
(a) Concentration.

(b) PH value.

(c) Degree of Whiteness.

(d) The type of machine used in bleaching.

Roughly for normal machine the time is 2-3 hrs is required for completion of bleaching process.
antichlor and proper neutralization treatments should be followed by proper wash for removal of reagents after hypochlorite bleaching process.

Testing of Available Chlorine in bleaching powder

Two methods are in common use for the determination of the available chlorine. In the first, the hypochlorite solution or suspension is treated with an excess of a solution of potassium iodide, and strongly acidified with acetic acid:

ClO^- + 2I^- + 2H⁺ = Cl^- + I₂ + H₂O

The liberated iodine is titrated with standard sodium thiosulphate solution. The solution should not be strongly acidified with hydrochloric acid, for the little calcium chlorate which is usually present, by virtue of the decomposition of the hypochlorite, will react slowly with the potassium iodide and liberate iodine:

ClO₃^- +6I^- + 6H⁺ = Cl^- + 3I₂ + 3H₂O

In the second method, the hypochlorite solution or suspension is titrated against standard sodium arsenite solution; this is best done by adding an excess of the arsenite solution and then back-titrating with standard iodine solution.

Procedure

1.Available chlorine in bleach liquor (iodometric method)

Reagents required
1.70 gpl solution of sodium thiosulphate
2.10 % potassium iodide solution
3.400 gpl acetic acid solution
4.Starch indicator solution
Procedure
Take 10 ml of bleach liquor solution in a conical flask , add 20 ml KI solution and 20 ml acetic acid solution , the color will turn to amber brown , shake it well and then titrate it with 70 gpl sodium thiosulphate solution to a light brown color , now add few drops of starch indicator and titrate it to a colorless end point.
Result:- gpl of available chlorine is equal to  ml of thiosulphate consumed.
2.Sodium Hydroxide and Sodium Carbonate Determination
Take 10 ml of sample and add equal(of av. chlorine gpl) ml of 70 gpl sod.thiosulphate to neutralise the chlorine , shake well and titrate the content with N/2 sulphuric acid using phenolphthalien indicator.
End point:-Pink to color less.
caustic Gpl will be equal to ml of sulphuric acid consumed.
3.Available chlorine in Bleaching Powder
About 2 gm of sample , accurately weighed , is pasted up with a little water and made up 500 ml .Take 50 ml of this , add 20 ml of 10% KI , and solution is acidified with dilute acetic acid , titrate it with 0.1 N Sodium thiosulphate using starch indicator , to a colorless end point.
1 ml of 0.1 Sod. thiosulphate = 0.003546 g chlorine.
other tests of bleaching powder are alkalinity and chlorates.
for detailed analytical procedure click here 

Textile Pretreatment

Pretreatment

Means any treatment, which is done before actual (dyeing and printing) process.

Natural fibers and synthetic fibers contain primary impurities that are contained naturally, and secondary impurities that are added during spinning , knitting and weaving processes.Textile pretreatment is the series of cleaning operations.All impurities which causes adverse effect during dyeing and printing is removed in pretreatment process.
Pretreatment processes include desizing, scouring, and bleaching which make  subsequent dyeing and softening processes easy. Uneven desizing, scouring, and bleaching in the pretreatment processes might cause drastic deterioration in the qualities of processsed products, such as uneven dyeing and decrease in fastness.

Objective of Pretreatment

*  To Convert  fabric from hydrophobic to hydrophilic state.
*  To remove dust, dirt etc from the fabric.
*  To achieve the degree of desire whiteness.

Steps in Pretreatment Process of cotton and natural fibers

Major steps involved in textile pretreatment are,
1.Singeing
2 Desizing,
3. Scouring,
4. Mercerization
5. Bleaching.
6.Singeing

Introduction to Textile preparation : Slide Show

Steps in pretreatments for  wool are:

1.Raw wool scouring; aqueous and/ or solvent washing 

2.Carbonizing

3.Scouring (desizing)

4.Fulling /crabbing/thermo fixing

5.Easy-care treatments

6.Anti-felting anti-shrinking treatments

7.Wool Bleaching

Steps in pretreatment of Silk

To prepare a silk yarn for dyeing and silk fabrics for dyeing and printing, it is necessary to partially or completely remove sericin, as well as natural oils and organic impurities. Depending on the percentage of sericin removed during scouring (sericin is present in raw silk in a ratio between 20 % to 25 %), the end-product is defined as unscoured (used only for shirts and suits), `souple' or degummed.

1.Degumming

2.Scouring

3.Bleaching

Pretreatment Of Synthetic Textile Materials

Although mots of the synthetics do not need to be given a very strong pretreatment however the possible steps in pretreatment of synthetics are 

1.Desizing

2.Heat setting

3.Washing

4.Bleaching if necessary.

Read articles
Better ways to pretreat fabrics
New trends in continuous pretreatment
Enzymatic pretreatment of cotton wovens
Economic impact analysis of pretreatment standards for the textile industry
Pretreatment of textile wastes: Seminar for Upgrading Textile Operations to Reduce Pollution, Atlanta, Georgia, September 25 and 26, 1973
Economic impact analysis of proposed effluent limitations guidelines, new source performance standards and pretreatment standards for the Textile Mills Point Source Category (EPA440/279020)
Economic impact analysis of proposed effluent limitations guidelines, new source performance standards and pretreatment standards for the Textile Mills Point Source Category (EPA440/279020)

Textile Bleaching

Bleaching of Textiles.

Bleaching is chemical treatment employed for the removal of natural coloring matter from the substrate. The source of natural color is  organic compounds  with conjugated double bonds , by doing chemical bleaching  the discoloration takes place by the breaking the chromophore , most likely destroying the one or more double bonds with in this conjugated system. The material appears whiter after the bleaching.

Bleaching Agent.

A bleaching agent is a substance that can whiten or decolorize other substances.Bleaching agents essentially destroy chromophores (thereby removing the color), via the oxidation or reduction of these absorbing groups. Thus, bleaches can be classified as either oxidizing agents or reducing agents .


Type of Bleaching agents

a.Oxidative Bleaching Agents

b.Reductive Bleaching Agents

c.Enzymatic Bleaching Agents

Bleaching with hypochlorites.

Bleaching Powder

Calcium hypochlorite is usually a white or grayish-white powder, but it is also available as tablets. It is a strong oxidizer and is considered more stable than sodium hypochlorite. It is also believed to provide more chlorine.

Sodium Hypochlorite

Sodium Hypochlorite is a greenish-yellow liquid commonly referred to as "Bleach." The chemical compound formula for Sodium Hypochlorite is NaOCl.Sodium Hypochlorite is prepared by reacting dilute caustic soda solution with liquid or gaseous chlorine, accompanied by cooling. It is used extensively as a bleaching agent in the textile, detergents, and paper and pulp industries.

The active ingredients in hypochlorite bleaches vary with pH. At pH <>2 is the main component in solution; at pH 4 to 6, HOCl is the dominant species; at pH > 9, OCl ⁻ is the only component present. It is the hypochlorite ion in basic solution that is the active ingredient in household bleach, which is typically about 5 to 6 percent NaOCl. The OCl ⁻ ion oxidizes chromophores in colored materials, and is itself reduced to chloride and hydroxide ions.

 
Hypochlorite Bleaching process

The conditions of the bleaching agent varies according to the result required, the concentration depends upon the fabric quality, degree of whiteness required, types of machine and next operation.

Quantity Required:-

Normally 2.5-3.0 gpl of available chlorine is sufficient for good bleaching , but it is necessary to optimize it on a possible lower  value for safe bleaching process.

How to Test available chlorine content in a bleaching bath

ANALYSIS METHOD
Determination of available Chlorine.
Method	:Iodometric(Standard)
Apparatus	:Erlemeyer flask(100 ml)  -I  Burette stand  -I  Pipette(50 ml)  -I  Spatula  -I  Measuring Cylinder  -I  Micro burette(10ml.)  -I  Measuring Cylinder.  -I
Reagent	Sodium Thio Sulphate Solution(.1N) Glacial acetic acid Pottassium Iodide(KI crystal) Starch indicator solution.

Procedure :- 1) Collect sample solution ( Sodium hypo chlorite ) in an air-tight bottle. NOTES / CAUTION a) Keep sample solution away from excessive light and do not agitate the sample solution. b) Start chlorine determination IMMEDIATELY after sampling. ( i.e. do not store sample to be analysed for chlorine ). 2) Introduce 10ml of glacial acetic acid and about 0.5 g of Pottassium Iodide (Spatula full ) crystal into a 100ml erlemeyer flask. 3) Withdraw y ml  of sample solution with bulb pipette and introduce into above Solution. 4) Swirl the flask to well mix the solution. 5) Titrate with 0.1N Sodium Thio Sulphate Solution from upto a light Yellow Colour. 6) Add 1 ml of starch indicator solution and continue to titrate until the last drop of titrant causes the solution to turn colourless. % active chlorine = 3.545 x N x V/y   (i;e 3.545 x N x V divided by y) Where : V = Volume of (N ) Na2S2O3 N = Normality of Na2S2O3 y = Sample amount

How to test caustic gpl in bleaching solution

pH :-General PH range is 10-11 or 10.5-11.5 during bleaching if PH reaches 9 then it is a danger level and at 7 PH the bleaching is worse and causes extreme damage to the cloth.The PH value is maintained by adding sod ash or by buffering agent.

Temperature:-The suggested temperature for hypochlorite bleaching is 37-40oC. Generally reaction is accelerated with  increases of temperature.

Water Quality:-water for bleaching should be soft and even hard water can be used but should be free from Cu++ and Fe++.

Effect of metals

The bleaching equipment should be made of stainless steel , to avoid the catalytic degradation of the cellulose in the presence of copper and iron.

Substrate preparation

The substrate must be pre scoured in the presence of chelating agents , it should be free from rust spots and traces of metallic impurities when bleaching with hypochlorites.

Bleaching Time :-The time factor depends upon the following consideration.
(a) Concentration.

(b) PH value.

(c) Degree of Whiteness.

(d) The type of machine used in bleaching.

Roughly for normal machine the time is 2-3 hrs is required for completion of bleaching process.
antichlor and proper neutralization treatments should be followed by proper wash for removal of reagents after hypochlorite bleaching process.

More About Sodium Hypochlorite

Sodium hypochlorite FAQ

 
Bleaching with peroxides.

The bleaching bath is composed of hydrogen peroxide (35% or 50% by wt.) as the bleaching agent, an activator (usually alkali) and stabilizers.

Bleaching Process with Hydrogen Peroxide

Hydrogen peroxide bleaching can be done by

1.Batch wise, 

2.Continuous 

3.Semi continuous method.

a.Quantity of peroxide required in Bleaching

b.Temperature

Cotton and Bast fibers are bleached at 80 - 95°C in bath processes, while blends of cotton and regenerated cellulose fibers are bleached at 75 - 80°C. The bleaching time is generally between 2 and 5 hours. In a pressurized high temperature (HT) apparatus cotton can also be bleached at temperatures of 110 - 130°C in only 1 to 2 hours.

c.Time

During the impregnation processes the temperature and as well the retention time varies widely. During a cold bleach process a dwell time of 18 to 24 hours is necessary. In the pad steam process under atmospheric pressure the bleaching time is generally between 1 to 3 hours. The above mentioned processes describe batch processes. Today a lot of continuously, intelligent finishing equipment exists in which the bleaching step is only one of some other treatments and the reaction time of the impregnated material in such steamer is only between 7 to 20 minutes. In general these bleaching process correspond to a preliminary bleach.

d.pH:-The pH value depend on the fibres to be bleached and pre-treatment.

NaOH is used in case of H2O2 bleaching. This is used to bring the PH upto 9-10 because H2O2 become active at this PH or oxidation is start at this PH.

For the bast fibres, such as linen, weaker alkaline or soda alkaline baths are used in order to avoid a cottonizing. Regenerated cellulose fibres are more sensitive. Therefore, they are only bleached in weak alkaline baths.
Alkali sensitive animal fibers must be bleached in very weak alkaline solutions. Phosphates and ammonia are most widely used as alkalization source. With tetrasodium pyrophosphate simultaneously a stabilization of the bleaching liquor can be attained.

e.Water Quality

Soft Water free of iron and copper impurities is recommended for peroxide bleach treatment.

f.Peroxide Stabilizers

High pH and temperature lead to the faster decomposition of peroxide bleaching liquor and degradation of cellulose.The role of the stabilizer is simply to control or regulate these effects the
act as buffers, sequestrates and in special cases, enhancing performance of the surfactant used in the bleach bath.

For caustic alkaline bleach sodium silicate, organic stabilizers or the combination of both are suitable. In weak alkaline baths the addition of tetrasodium pyrophosphates can be used alone or together with an organic stabiliser.

Advantages of Peroxide Bleaching:
1.Among the oxidizing bleaching agents, only hydrogen peroxide provides a high bleaching effect     at reasonable costs, especially if modern short-term bleaching processes are used with only a        few minutes bleaching time.
2.Peroxide bleaching keeps the fibre quality intact.
3.Cotton can be bleached with peroxide in a single stage. Other processes require two or three
   bleaching stages,(desize with scour, scour with bleach and desize with scour and bleach).
4.No separate pre treatment is necessary because hot, alkaline bleaching has not only a bleaching but also a cleaning effect, it therefore combines  the advantages of an alkaline extraction with the bleaching treatment.
5.Animal fibres can only be bleached with peroxide to a high and stable degree of whiteness.
- Corrosion of stainless steel equipment does not occur during peroxide bleaching.
6.The spent peroxide baths still contain residuals of hydrogen peroxide which fever the                  degradation of the organic impurities in the effluent, and this helps to decrease the chemical        oxygen demand (COD).

 Bleaching of wool with hydrogen peroxide.

After scouring, wool may be bleached by immersion or pad and dry techniques, using alkaline or acid solutions. 

Bleaching of silk with hydrogen peroxide.

Prior to bleaching, silk is usually degummed. Hydrogen Peroxide addition assists this process and it is universally used as the bleaching agent for natural silk, usually in an alkaline solution.

Bleaching of synthetic fibres with peroxide

When used alone, synthetic fibers do not normally require bleaching. However, blends of synthetic fibers with natural or regenerated fibers, e.g. cotton-polyester are frequently bleached. The most popular bleaching agent is Hydrogen Peroxide and it is used in both batch and continuous processes.
 

Advantages and disadvantages of peroxide over hypochlorite bleaching.

Bleaching with sodium perborate

Sodium perborate (PBS, NaBO₃.nH₂O where n=1 or 4) can readily be incorporated. It has been described as a stable, solid form of hydrogen peroxide allowing its introduction into the wash at the same time as the detergent. Sodium perborate is a gentler bleach than sodium hypochlorite, causing less damage to fabrics and dyes, but by itself is only effective at high (>60ºC) temperatures. Although solid chlorine bleaches exist, they are rarely used in laundry detergents.
Click Here for detailed chemistry stabilization pH solubilization

Bleaching with sodium chlorite.(NaClO₂ )

The sodium chlorite is available as a powder , and it is applied under strongly acidic conditions to textiles. Its application produce a toxic and corrosive gas.

a. bleaching mechanism

b. effect of pH.

c. effect of temperature.

d. effect of metals

Application process of chlorite bleach on cellulosics and synthetics

Persulphates

Ammonium ,potassium and sodium  Persulphates are oxidizing agent that find use in textile bleaching are

a.       for bleaching of protein fibers in combination with hydrogen peroxide.

b.      Activated cold bleaching of cellulosics.

c.       To oxidize vats and sulphur dyes

d.      To bleach fur and skins in combination with hydrogen peroxide.

Bleaching with peracetic acid.

Peracetic acid is produced by the chemical reaction of acetic acid and hydrogen peroxide. It works in a very narrow pH range of 7 to 8. Below pH 7.0 the bleaching is not proper and above pH 9.0 fiber degradation takes place. Peracetic acid is used a bleaching agent for nylon and acetate where hydrogen peroxide can not be used.

Reductive bleaching systems

Reductive bleaches work by reducing colored impurities into colorless forms.

1.sodium hydrosulphite.

Sodium hydrosulphite is available as free flowing powder and a strong reducing agent. This is  explosive in nature  when come into contact with water. It is available in different purity ranges.

2.sodium sulphide.

Sodium sulphide is also a strong reducing agent

3.sulphur dioxide

Sulphur dioxide was used as a bleaching agent in early 20^th century for bleaching of wool.

Reductive bleaching of wool.

Bleaching of silk with reducing agents.

Commonly Sodium hydrosulphite, Sulfurdioxide and sodium sulphoxylates are the reductive bleaching agents which are used for silk.

Reductive bleaching of nylon

Oxidative bleaching isn’t suitable for polyamides as H2O2 attacks the polymer, instead
reductive dyeing using sodium hydrosulphite is used.

Peroxygen bleaching compounds.

A number of solid peroxygen compounds that release hydrogen peroxide when dissolved in water exist. These include sodium perborate (NaBO 3 z 4H 2 O or NaBO 2 z H 2 O 2 z 3H 2 O) and sodium carbonate peroxyhydrate (2Na 2 CO 3 z 3H 2 O 2 ). The structure of sodium perborate contains the peroxoanion B 2 (O 2 ) 2 (OH) 4 2− , which contains two O–O linkages that join two tetrahedral BO 2 (OH) 2− groups. These peroxygen compounds are used in detergents, denture cleaners, and tooth powders.

Bleaching of jute.

Bleaching of linen.

Bleaching of blended fabrics.

Bleaching of cotton weft knitted fabrics.  

 

Enzymatic Scouring and bleaching

Aside from cellulose, raw cotton contains impurities such as complex organic compounds and complex substances. The purpose of cotton preparation is to remove the non-cellulosic impurities from the cellulosics fibers and increase the wettability and whiteness of the fabric. This improves dyeing performance and subsequent processing steps such as printing and finishing. Conventional time consuming methods used in scouring and bleaching of cellulose-based textile material use large amounts of chemicals, energy, and water. To avoid environmental damages, the effluents require neutralization or dilution prior to discharge. The greatest problem occurring during bleaching with peroxide is in the radical reactions of the bleaching compounds with the fiber. This can  lead to a decline of polymerization and eventually to a decrease in tensile strength, especially in the presence of metal ions that act as activators for hydrogen peroxide. As a remedy, the use of enzymes for bioscouring has been suggested.

The use of enzyme technology is attractive because enzymes are highly specific and efficient, and work under mild conditions. Furthermore, the use of enzymes results in reduced process times, energy and water savings; improved product quality; and potential process integration.

As the purpose of scouring is to remove natural impurities — such as polymeric substances like pectins,waxes and xylomannans, among others —from cotton or other natural fibers, there are plenty of enzymes that can act on such impurities.Alkaline pectinase, which loosens fiber structure by removing pectins between cellulose fibrils and eases the wash-off of waxy impurities, is the key enzyme for a bioscouring process. Compared to the conventional alkaline boil-off, an efficient bioscouring process provides many advantages, such as reduced water and wastewater costs, reduced treatment time and lower energy con-sumption because of lower treatment temperature. Moreover, the weight loss in fabric is reduced, and fabric quality is improved with a superior hand and reduced strength loss.

However, the current enzymatic scouring processes are not effective enough to prepare the fabric for dyeing in batch and especially in continuous processes as the enzyme needs reasonably long reaction times, and controlling its performance in a continuous process is difficult.

As the enzyme does not break down waxes, a high-temperature treatment with surfactants is needed for a sufficient degree of hydrophilicity to guarantee even dyeing or printing.

The whiteness of the bioscoured material has remained lower than the whiteness obtained by using the traditional processes including alkaline wash.

Pectinase has  inability to remove motes — the remainders of cottonseed fragments. Thus, a separate bleaching step would be needed after the bioscouring process. On the other hand, the alkaline boil-off can be combined with simultaneous peroxide bleaching to efficiently remove the motes. As motes are not acceptable on fabrics other than those that will be dyed to dark shades, bioscouring will have limited usage unless a simultaneous mote-removal process is developed.

An efficient bio preparation process should be based on a combination, preferably simultaneously, of enzymes for desizing, scouring and bleaching in one bath. Success in developing such a process would result in a simple process, including savings in water, time and energy consumption. The environmental impact would be minimized by low to no use of harsh chemicals and lowered waste burdens, rendering a fabric of higher quality.

Accordingly, the attempts to combine the bioscouring and bleaching steps are not fully satisfactory, because they still involve alkaline conditions, thus causing strength losses and environmental damage

Full White Process

Bleaching Process for Full Whites and OBA application 
Introduction.

The coloring matter ,whether it is natural or present as a contaminant in the fiber is generally decolorized by different bleaching methods. However the appearance of the textile substrate is somewhat creamish after the bleaching, therefore chemical treatments are become necessary to neutralize the yellow tint of the textile fibers.

There are two methods ,which are generally used for this purpose,

1. By using a blue tinting agent , which absorbs the yellow part of the light and  reflected light appears to be of bluish tint. The total light reflected by this mean is less than the total incident light.
2. By using fluorescent optical brightening agents :- The OBA s (optical brightening agents ) are most widely used in textiles , paper, detergents  and plastics . The optical brightening effect is obtained by the addition of light , which means that the amount of light reflected by the substrate is more than the incident light , due to which the object appears brighter.

 
Desired properties of Good OBA

Before selecting an optical brightener for textile application we must look for following properties,

1. it should have good solubility , should not have its own color and good substantivity for the textile substrate under OBA application.
2. OBA’s should have good light as well as wet fastness properties.
3. Its rate of strike on the substarte.
4. build up and exhaustion properties.
5. requirement of electrolytes and its sensitivity towards different exhausting agents.
6. Effect of temperature on the exhaustion and build up properties.
7. Application pH range and sensitivity towards change in pH.
8. Effect of water hardness.
9. It should have good leveling and penetrating properties.
10. Should not decompose to colored products on exposure to atmospheric conditions as well as storage , and it should not absorb light in the visible region.
11. it should be compatible and stable  with finishing chemicals, auxiliary and process such as heat and temperature.
12. It should be stable and fast to the common oxidative and reductive bleaching chemicals and bleaching systems.

Chemical constitution of optical brighteners.

Optical brighteners are usually derivatives of  

Triazine-stilbenes (di-, tetra- or hexa-sulfonated)

Coumarins

Imidazolines

Diazoles

Triazoles

Benzoxazolines

Biphenyl-stilbenes

Brighteners can be "boosted" by the addition of certain polyols like high molecular weight polyethylene glycol or polyvinyl alcohol. These additives increase the visible blue light emissions significantly. Brighteners can also be "quenched". Too much use of brightener will often cause a greening effect as emissions start to show above the blue region in the visible spectrum. Besides the formation of cis isomer in stilbene-containing brighteners (only the trans isomer is optically active), continued exposure to UV-containing light will actually cleave the molecule and start the process of degradation.

Mechanism of fluorescent whitening.

Fluorescent Whitening Agents (also called optical brightener) absorb high energy radiation in the ultraviolet to violet region (330nm-380nm) on the part of characteristic molecules and emit lower energy radiation in blue region in visible spectrum (400nm-450nm), which yields the counteracting the yellowing appearance. FWA should be transparent on the substrate and should not absorb the visible region of the spectrum. The OBAs are effective only when the incident light has a significance proportion (such as daylight) of UV rays. When material treated with OBAs are exposed to UV black light source , it glows in the dark.Anionic OBA’s exhaust on cotton, wool and silk., cationic OBA’s  exhaust on acrylic and certain polyesters and nonionic OBA’s are exhaust on all synthetics.

Application of textile optical brighteners.  

Textile Bleaching Agents

Bleaching Agent.

A bleaching agent is a substance that can whiten or decolorise other substances.The purpose of bleaching agents is to bleach the natural substances in cotton fibres and cellulose fibres that make the fabric yellowish. 

Bleaching agents essentially destroy chromophores (thereby removing the color), via the oxidation or reduction of these absorbing groups.Bleaching agents also bleach textile colors and hence should be avoided for colored textiles.

Thus, bleaches can be classified as either oxidizing agents or reducing agents .

Type of Bleaching agents

a.Oxidative bleaching agents

Generally oxidative bleachings are carried out using hypochlorites,sodium chlorite or peroxy compounds. Natural fibres like cotton,ramie , jute, wool, bamboo are all generally bleached with oxidative methods.
The double bond breaks in the oxidative bleaching, which makes the substrate colorless,

b.Reductive bleaching agents
Reductive method of bleaching is done with sodium hydrosulphite,sodium sulpfide and sulfur dioxide are powerful reducing agents. Fibres like polyamide, Polyacrylics and Polyacetates can be bleached using reductive bleaching technology.
Double bond is converted into single bond in case of reductive bleaching ,

 c.Enzyme aided Bleaching 

The use of enzyme technology is attractive because enzymes are highly specific and efficient, and work under mild conditions. Furthermore, the use of enzymes results in reduced process times, energy and water savings; improved product quality; and potential process integration. The process of bioscouring(which remove all non-cellulosic components from native cotton  completely or partially.) when combined with bleaching process reduces the process time , cost as well as improves the quality.

for information about enzymes in pretreatment click here

For More information on Hypochlorites Click Here 

Chemistry of bleaches
Bleach action