It has now been 200 years since the invention of chlorine in 1774 by Scheele, and ensuing studies on its bleach�ing action developed in the earlier methods for bleaching for linen and cotton. With an improvement in both, the mechanical handling of fabric and technical control, the basic methods for chlorine-based bleaches were developed during the 19th century. However, they are multistage processes with many drawbacks. These include low unit labor productivity, greater utility requirement of energy as well as water and lengthy processing times.

Bleaching of textiles is being done since a very long time. Even now, fabrics made from the natural cellulosic fabrics are bleached to better its whiteness. The only difference is that the bleaching methods have changed extremely with the progression of many kinds of bleaching agents. Despite these methods, whiteness would not have been possible without optical brighteners.

Though, the level of whiteness is not the only key criterion for appraising bleaching, but also absorbency. Besides, cellulosic fibres also consist of motes, naps, pectin, wax and fragments of seed coat.

The bleaching process targets several goals that need to be achieved:

.High and standardized absorptivity of fabric for water, dyestuff and finishing agent, achieved by eliminating hydrophobic impurities from the natural fibres.
.Adequately sharp and standardized level of whiteness, which is stable to storage.
.The fabric should not be spoiled with the level of polymerization remaining high.

Of the two processes oxidative bleaching and decreasing bleaching, the latter method is greatly used for the treatment of wool while the former is generally used for cotton.

Normally, the following agents are applied for bleaching:

.Sodium hypochlorite
.Sodium chlorite
.Hydrogen peroxide

Sodium hypochlorite
After World War-I, sodium hypochlorite was substituted by bleaching powder as a major bleaching agent. It does not give preci�pitate of calcium or confer a harsh handle on the fabrics as the bleaching powder does. It has self-decomposition as mentioned below:

2 NaOCI NaCI NaClO2

NaOCI NaCIO2 NaCI NaCIO3

3 NaOCI 2 NaCI NaCIO3

Another reaction, which is slow, is:

2 NaOCI 2 NaCI O2

The relative proportion of the above mentioned reaction depends on the pH. Maximum decomposition was seen at pH 7. A large part of hypochlorite decomposed results in chlorate formation. This chlorate formation reaction is highly seen in almost all the hypochlorite decomposition on the alkaline side, where the oxygen development is almost absent. While on the acidic side, oxygen development outweighs chlorate formation.The most excellent conditions for the storage of sodium hypochlorite are about 2N NaOH at 0.5�C in the absence of light. As pH of sodium hypochlorite drops from 12 to 4.6, all the sodium hypochlo�rite is transformed to hypochlorous acid, and while below a pH of 4.6 chlorine gas is released. Therefore bleaching in the acidic side is not done.

The following are the drawbacks of chlorine bleaching:
.Though this process is low-priced, but the obtained whiteness of the cloth is not satisfactory.
.Cellulosic fabrics may be damaged to a large extent as compared to hydrogen peroxide bleaching.
.Corrosion resistance equipment is required.
.There is always an unpleasant odor in the factory.

Bleaching carried out by Sodium Chlorite Sodium chlorite is an oxidising agent consisting chlorine as available chlorine and it bleaches cellulose on the acidic side at the boil without degrading it.

Sodium chlorite decomposes mostly into sodium chlorate than into gaseous oxygen.

3 NaClO2 2 NaCIO3 NaCI
NaClO2 NaCI O2

Over a pH range 1.6 - 5.95 at 40 - 80 �C, the decomposition products of sodium chlorite include sodium chloride, sodium chlorate, chlorine dioxide and oxygen. Formation of Chlorine dioxide is the key reaction and is maximum at pH 2.5 -3:
5 CIO2 2H 4ClO2 CI� - 2 OH��-

Oxygen evaluation is done maximum at pH 2.4:

ClO2- CI- O2

Chlorate formation commences below pH 4.8:
3 ClO2- 2 ClO3- CI�-

It is noted that the amount of sodium chlorite decomposed reduces with rising pH. At 100�C in fairly acidic solutions (at pH 4 most) about 95 per cent of the chlorite is decomposed within 2 hours of boiling. The best conditions for using acidified chlorite solution for bleaching should be those that support maximum formation of sodium chloride and minimum formation of chlorine dioxide and chlorate. At pH less than 4, chlorine dioxide evolution reaches its peak, which results to a loss of the oxidising power of the solutions. This loss can be prevented by managing pH in the range of 4-7.

The drawback of sodium chlorite is the evolution of chlorine dioxide, which is toxic in nature and possesses corrosive qualities. Organic acids like acetic acid or formic acid used for activating chlorite solutions lead to evolution of chlorine dioxide gas. A reduction of corrosion in bleaching unit is possible by the addition of sodium nitrate to bleach liquor. A variety of acid generating agents like organic ester, e.g., diacetin, ethyl-lactate or triethanol-amine-hydrochloride have been suggested. Mag�nesium chloride, zinc 'nitrate and aluminium- sulphate are found to be effective activators.

The benefits of sodium chlorite have earned a huge acceptance despite its high cost. They are:

.Sodium chlorite decomposition is not catalyzed by metal ions, nor is there any degradation of cellulose.
.Because of lesser weight, failure and partial removal of waxes, chlorite bleached fabric is softer against hypochlorite or peroxide bleached fabric.
.The bleached fabrics have low residual alkaline, which helps removing chemical residue.

The disadvantages of sodium chlorite are:

.This process is comparatively expensive.
.The level of whiteness of the cloth is good, but not long lasting. There is slight risk of damage for cellulosic fabrics, and negligible risk for synthetic fabrics.
.There is danger to the corrosion of equipment.
.Processing methods are limited to discontinuous and semi discontinuous procedures.

Cold pad chlorite methods depend on generation of chlorite dioxide in the roll and, in order to manage the irregularities in formation, it is usual to use a fan at or near floor level at the pad mangle and the still age. If fumes are created in the pad box, it should be vacated at once and washed out well before proceeding. Chlorite formaldehyde pad liquor possesses limited life, and an economical way of providing continuity is to use two-storage tanks alternately.

Another cheaper use of chlorite is possible, if it is applied in proportionate feed, in a wet-�on- wet pad box, to cloth that has been rinsed well: if the components are measured independently from two tanks, more concentrated mixtures can be used safely. It is necessary to arrange the pad liquor cautiously, by mixing each component roughly with plenty of water in a bucket and fripper them in order shown into the bulk of the water, with capable stirring. It is essential that the temperature be set below 30�C. Nearly all the brass or copper parts be permitted to come into contact with the permanganate - persulphate �chlorite solution, as they catalyze decomposition of persulphate, resulting in generation of gas. Stainless steel padding boxes are not decomposed by alkaline chlorite solutions.

After padding with chlorite, the batches should be enclosed with polythene, many layers of wet cloth and the outer layer being saturated with weak bisulphate. For attaining maximum whiteness, peroxide bleached batches need hot soda boil. Cloth bleached with chlorite formaldehyde may retain fumes and must be unrolled under the surface of bisul�phate solution. Cloth treated with permanga�nate persulphate chlorite requires to be cleaned with hot bisulphate, oxalic acid or neutral hydrosulphite; together with sequestering agent, this is mainly applicable where an alkali scald confer wettability follows or where a size is present, which coagulates in acid solutions.

Activation of NaCIO2 by formaldyde was done with alkaline medium (pH 10) to avoid the problems related to evolution of chlorite dioxide. Depending on the reheats obtained, combined desizing scouring and bleaching of loom state cotton fabric could be received by making the latter at 70�C for 60 minutes with aqueous solution including with 3 gpl NaCIO2, 0.5 gpl HCHO, 2 gpl wetting agent at pH 10 by using a material to liquor ratio 1 :20.

Loom state or starch sized cotton fabric was treated with sodium chlorite/potassium permanganate co-oxidant in various conditions. It was noted that the combined bleaching effect depends on the permanganate strengths, pH of the bleaching medium and duration of the bleaching treatment. Outcome of bleaching of chlorite is measured by the whiteness index. Loss in fabric weight wettability, copper number, carboxyl content and tensile strength of the cured fabric is preferred at acidic pH (4-6) by applying 0.01 per cent and at alkaline pH (8-10) by applying 0.04 per cent permanganate.

The increase in the duration of the treatment from 15 to 120 minutes is accompanied with an increase in the bleaching effect of both chlorite and chlorite perman�ganate co-oxidant, which in return increases the bleaching effect considerably. Based on the results, a new blea�ching formulation could be developed for combined desiz�ing, scouring and bleaching of loom state cotton fabric. The treatment process based on these formulations includes treatment of loom state cotton fabric with a solution including 0.3 per cent chlorite, 0.2 per cent wetting agent at pH 4-6 by applying 0.01 per cent permanganate for 60 minute or at pH 8-10 by applying 0.04 per cent permanganate.

Bleaching done through hydrogen peroxide
The bleaching of textile fabric with hydrogen peroxide is dependent on many aspects such as pH, temperature, time, stabilizer type and presence of metallic impurities. The evidence of peroxide is associated with the strength of alkali and stabilizer and the temperature of the reaction. For logical reasons, peroxide bleaching is done with higher temperature of 80-90�C for 2 to 4 hours.

Hydrogen Peroxide is beneficial in the following ways:

1) It produces a stable white color.
2) Its reaction products are non�toxic and innocuous, hence decreasing pollution problems.
3) Cellulose is not degraded by it under maximum conditions.
4) Greater absorbency of finished goods is gained.
5) Minimum tendency of after yellowing of goods bleached with H2O2.
6) It is applied for both open-width and rope for bleaching.
7) There is no risk of corrosion to the equipment as against the toe processes.
8) The most significant feature is that it has no air pollution problem.

The drawback is that the bleaching is slow unless high temperature is maintained. Therefore, the costs of energy are also high. The presence of iron, nickel, copper, cobalt and lead hydroxides in the bleach may lead to the catalytic decomposition of H2O2.
Hydrogen peroxide decomposition Peroxide solutions are unstable in even less bound alkaline solutions and the decomposition is catalyzed even by residue of particular metallic salts. Copper ions are further useful catalysts than iron and normally the catalytic process rises with increasing alkalinity. On the other hand, the stabilizing possessions are noticeable for phosphates, pyrophosphates, silicates, tartarates and borates. The catalytic effects of the metal ions are widespread to the fibres and this effect is increased if the catalytic impurity already exists in fibres.

To neutralize this catalytic process, a variety of stabilizers buffer the pH. It has been noted hat sodium silicates are the most excellent stabilizers for hydrogen peroxide. They are economical, useful, possess a detergent reaction and reduce corrosion of metal material. Further more, they offer a buffering action, which holdback the strength of free sodium hydroxide, further offering a decrease in fibre damage.

The main drawback of silicate stabilizer is their application to create hard scales on the processing equip�ment. These impede the free running of the cloth, though the equipment grazes the cloth and decreases the efficiency of heat exchangers. There is a tendency of deposition of silicate onto the fibre. This may result in the cloth having a poor handle problem with dyeing and printings have been accredited to silicate deposition.

A lot of sequestering agents have also been set up in stabilizer formulations for bleaching. Many amides can increase the speed of decomposition of hydrogen peroxide, when included with the bleaching bath, thereby decreasing the bleaching time. Urea, benzamide, formamide and N, N dimethyl formamide can be used. While amid them, benzamide provides the greatest whiteness.

Cold bleaching by applying hydrogen peroxide

There are two different cold bleaching methods by using hydrogen peroxide. In the first method, the scoured fabric is bleached with peroxide at room temperature. This method has been developed mainly for non-power sector, where workers stand inside the open tank and carry out all functions manually. Higher alkalinity of the bath is rejected without a doubt for expected problems of skin irritation. The bleaching formula has to be used in such a manner, so as to have no effect on the skin. On the other hand, greater alkalinity is needed for decomposition of peroxide at room temperature.

In the second method, scouring and bleaching works are done together at room temperature using hydrogen peroxide with desized and gray chemicked fabric. Higher alkalinity is used in this method for scouring action and this method can be adapted easily to kiers with liquor circulation.

Peroxide cold bleaching is an extremely mild, useful and energy saving bleaching method, which needs less machine expenses. In fact, it can be done in an open cement tank. Despite growth of several rapid and contin�uous bleaching methods, cold bleaching method is attaining much more acceptance in recent years due to energy crisis every�where. The formula for cold peroxide bleaching process is mentioned in Table 1.

In case of soda silicate combination (formula I), the primary rate of decomposition of peroxide is much quicker than the other formula (II). A greater control in the decomposition of peroxide is seen in the case of formula II. Though, residual peroxide (25 per cent) is more or less same in both cases. The residual peroxide to the extent of 20 per cent after bleaching depicts higher stabilization of the bath and this gives better whiteness.

Process done by solvent

A solvent-based combined method has been developed by ICI (UK) and is promoted under the trade name Markel process. It has been attained by utilizing a unique machine in which the fabric is padded through a solution including a chlorinated organic solvent, a surfactant and an enzyme suspended in the media. After the treatment, the used solvent can be regained and recycled. This method claims that 80 per cent of steam consumption can be saved.

In, another solvent base combined method, sodium hydroxide has been substituted by self emulsifier solvent com�position. This composi�tion is useful at a very large pH range of 4 to 12. The self emulsifiable solvent formulation includes pine oil as wetting agent, non-ionic emulsifier and perchloroethylene, combined in the ratio of 5:4:1. Since the solvent content is very low, regaining the utilized solvent is not necessary. This scouring agent (2-�3 per cent solution) is combined with hydrogen peroxide, silicate, sequestering agent and wetting agent. The fabric is padded in this formula and steamed for 1 hour at 95�C. The pH (close to to 10.5) of the system is used by appropriate buffering compounds. The method has been declared to be cheaper than the old process.

Based on a nonyl phenol �ethylene oxide condensate, the non-ionic emulsifying agent applied in these procedures, when one mole of nonyl phenol is clutched with 7-8 moles of ethylene oxide, the product shows the exclusive property of being soluble in both organic solvent and water. A 1:1 blend of nonyl phenol ethylene oxide condensate and a chlorinated solvent perchleroethylene or trichloroethylene provides extremely high-quality emulsifying properties. Contaminations like wax, protein, etc present in cotton can be emulsified easily with the assistance of this combined method even under cold situations.

Enzyme applied in combined process

Bleaching is one of the basic essential wet processes before dyeing textiles. At present, the more general industrial bleaching agent for cotton is hydrogen peroxide, which is used under boiling settings and makes an alkaline medium. Since the construction, composition and the binding position of the natural colorants in cotton have not yet been fully exposed, a non-specific bleaching agent such as hydrogen peroxide is quite useful. Though, large sum of rinse water is required because of the alkaline condition in this bleaching method. Furthermore, hydrogen peroxide can cause damage through radical reaction, particularly if metal ions exist.

Three major substitutions to bleaching based on non-toxic options to bleaching, environ�mentally benign enzyme have been explored in both the textile and pulp and paper industries -Peroxidases, laccase/mediator method and glucose oxidases. Normally, these three enzyme methods work on somewhat different bases. Amid other catalytic works, peroxidases can promote the reaction of oxidising agents such as hydrogen peroxide.

Laccases are a set of rather non-specific enzymes that are capable to catalyze the breakdown of the chromophore of colored com�ponents. Because of their un-specified mode of action, organic mediators direct the transfer of electrons between the active site of the enzyme and the substrate over moderately long distances. Laccases collectively with manganese peroxidases have been utilized for making lignin-containing fibres such as kenaf and flax.

Conclusion

Since a huge amount has been invested in research and development in these areas in the developed counties, it is expected that very soon all-season outfits will be mass-produced. For example, in Britain, scientists have designed an acrylic fibre by integrating microcapsules covering Phase Change Materials. These fibres have been used for producing lightweight all-season blankets.

Many garment making companies in USA are now producing many of such garments, like thermal underwear and socks for inner layer, knit shirt or coated fleece for insulating layer; and a jacket with PCM interlines for outer layer, beside helmets, other head gears and gloves. Such clothing can maintain warm and comfortable temperatures in the extreme of both weathers. There is no doubt that textile which integrate PCMs will find their way into several uses in the near future.

Bleaching done by peracetic acid

Quite a number of substances used in textile chemical pro�cessing are identified to pose a variety of environmental problems. Chlorite bleaching is suggested to be keeping away from the environmentalists as residual chlorite in wastewater and chloro derivatives of organic contamination in cellulosic natural fibres direct to the cause of absorbable organic halogen.

Peracetic acid has an extremely superior oxidation potential and thus is very reactive. Normally, all organic proxy acids possess an acid reaction and their stability reduces with rising PH and temperature. The resultant products of peracetic acid are hydrogen peroxide, oxygen, water and acetic acid. In the existence of heavy metal and ions such as copper, manganese or iron, this acid experiences catalytic decom�position.

Peracetic acid bleaching is an environmentally friendly method, a good substitute for old bleaching methods. Bleaching can be achieved in neutral or mildly alkaline settings with an acid (10 per cent) concentration of 30 gpl. For materials that experience frequent dyeing, the bleaching method can be carried out in nearly 30 min at 50�C, while for an OBA treatment; it continues for approximately an hour or so at 60 to 70�C. The quality of the bleached material is in comparison with that of H2O2 bleaching.

At tempera�ture as low as 30�C, Peracetic acid can be catalyzed to bleach cotton fibres by incorporating 2, 2 bipyridine and sodium lauryl sulphate in the bleach solution. The 2, 2, bipyridine is sorbed from solution and forms the tris 2, 2, bipyridine ferrous com�plex (trischelates) with ferrous ions in the fibres, and it is the trichelate linked with the fibres that catalyzes bleaching. Sodium lauryl sulphate decreases peracetic acid decomposition in the bath by ionic linkages with trichelates.

Bleaching agent - Potassium permanganate

Potassium permanganate is a strong oxidizing agent. Potassium permanganate is the potassium salt of permanganic acid. Permanganic acid is tremendously unbalanced, which decomposes into growing oxygen and manganese dioxide.

Potassium permanganate uses bleaching action in alkaline as well as acidic medium. In alkaline medium rate, the extent of discharge of oxygen is much lower and thus it has very limited application as an oxidizing agent under these circumstances.

Conclusion

Bleaching procedure is a vital pre-treatment and it should be done without any errors in order to gain the total absorbency and unique level of whiteness. The purpose of bleaching includes the elimination of a variety of natural, added or acquired contaminations from the grey cloth as efficiently as possible, with less or no damage to the fibre and leaving the fabrics in a perfectly white state. The harm to the cloth refers to the decrease in the tensile strength of the fabric caused by lessening the degree of polymerization of the cellulose chains because of molecular chain breakdown. Bleaching can also be specified as a procedure of producing the fabric whiter, regardless of whether the purification is done for making of dyeing, printing or in the procedure of goods to be finished as white.