Working of  bleaching agents: Let us discuss working of following bleaching agents:


 i) Sodium hypochlorite (NaOCl)

ii) Hydrogen peroxide (H2O2)

iii) Sodium Chlorite (NaClO2)

      Before bleaching, cotton fabric should be prepared well i.e it should be desized (if woven and having sizing chemicals) and scoured. If  material is not prepared well then there are chances of uneven bleaching effect.

i) Sodium Hypochlorite (Manufacture, Reaction and Mechanism): It is a stronger oxidative bleaching chemicals used in textile industry. This manufacture by bubbling chlorine into a solution of sodium hydroxide.

2NaOH + Cl2 --------------   NaOCl  + NaCl

when acid is added to a hypochlorite   solution,   chlorine gas is liberated

NaOCl + HCl---------------- NaCl  +  Cl2    +  H2O

        Commercial NaOCl mostly contains 12-15% active chlorine while household bleach contain around 5% active chlorine. NaOCl is strong alkaline having pH around 12 and free caustic present in the solution act as a stablizer. To make it stable, it should be kept in dark room. 

NaOCl  +  H2O----------------- Na + +  OCl- (Hypochlorous ion)

OCl- + H2O-------------------- HOCl  +  OH-

Hypochlorous acid (HOCl) is a active bleaching agent.

Bleaching: This is a process of whitening-fibers, yarns, fabrics or garments having natural colour. Bleaching can be carried out by oxidative bleaching and reductive bleaching agents.

1. Oxidative bleaching agents: These can be divided in to two system- peroxide system & chlorine system. 

Peroxide system: Hydrogen peroxide, sodium peroxide, sodium perborate, potassium permangnate, peracetic acid, ozone 

Chlorine system: Bleaching powder, sodium hypochlorite, lithium hypochlorite, sodium chlorite, chloramine, isocynual trichloride

2. Reductive bleaching agents: Sulhur dioxide, sodium hyposulphite, sulphoxylates, acidic sodium sulphite, sodium bisulpphites


The most popular bleaching agents used in the textile industry are:

•Sodium hypochlorite (NaOCl)

•Hydrogen peroxide (H2O2)

•Sodium Chlorite (NaClO2)

It is important to optimize the whitening of fibre using these oxidative bleaching agents as they may also degrade cellulose.

Action of Oxidative and Reductive bleaching agents:

Oxidative bleaching agent is a chemical reagent which decomposes in alkali solution and produce active oxygen. The active oxygen in fact the intrinsic bleaching agent as it will further destroy partly or completely the coloring matter present in the textile materials. 

In the case of reductive bleaching agent destroy the colouring matter by reductive reaction of SO2. Whenever bleaching is carried out following auxiliaries are used:

Stablizers, activators, wetting agents, sequestering agent etc.

Colour producing agents:

The colour producing agents in natural fibers are often organic compounds containing  conjugated double bonds. (Conjugated double bonds in a molecule, mean that the single and double bonds alternate.) 

It is known in dye chemistry that conjugation is necessary for an organic molecule to perform as a dyestuff.

Discolouration can occur by breaking up the  chromophore, most likely destroying one or more of the double bonds within the conjugated system.
(A chromophore is the part of a molecule responsible for its colour. The colour arises when a molecule absorbs certain wavelengths of visible light and transmits or reflects others. It is a Greek word: Chroma= "colour" and Phoros =Bearer)  

How does bleaching agents works? Oxidative bleaches oxidize color bodies into colourless compounds. For example, double bonds are known to be oxidize into epoxides which easily hydrolyse into diols. 

Enzymatic reaction mechanism:

The speed of the chemical reactions depends on the energy barrier between the substrate and the product. This barrier is known as the ‘activation energy’ and for molecules to react, they must possess the energy to overcome the barrier. 

However, enzymes do not increase the energy levels of substrate molecules, but provide an alternative low-energy route for the reaction to proceed. They achieve this by forming an intermediate enzyme-substrate complex, which alters the energy of the substrate such that it can be quite readily converted to the  product. The enzyme itself is released unaltered at the end of the reaction, thus acting as a catalyst. 

It can be schematically represented by the following equation: 

E (Enzyme) + S (Substrate) ------→    ES -------→            E + P (Product)

 (Initial state)                             (Intermediate state)         (Final state)

Enzymes being highly specific catalysts, the substrate must fit precisely into the active site of the enzyme just like a key fitting into lock. 

The formation of enzyme substrate complex also requires very little energy. Consequently, enzymes are very effective catalysts, enhancing reactions up to several thousand-fold more than the most effective chemical catalysts. 

The substrate & reaction specificity is determined by the structure of enzyme. The primary structure is determined by the amino acid sequence, the secondary structure by the specific conformation of protein chain and the tertiary structure by the arrangement of chain segments.

Most enzymes have a maximum activity at an optimum temperature and optimum pH. The reaction rate increases with increasing temperature until the optimum temperature is reached and activity decreases sharply on both sides of the optimum pH range. 

Enzymatic Desizing:

Desizing process involves impregnation of fabric with the desizing agents allowing it to degrade or solublise the size and finally to wash off the degraded products. Desizing process involves impregnation of fabric with the desizing agents allowing it to degrade or solublise the size and finally to wash off the degraded products. The most important enzyme, which is used to hydrolyse starch is alpha-amylase. Alpha-amylases are found in microorganisms, plants and animals. Alpha-amylases are relatively small proteins, of molecular weight between 50 - 60 k daltons. How does alpha-amylase wotk on starch are given below:

• The enzymes alpha-amylases hydrolyse starch in an 'endo' fashion, cleaving at random internal alpha-1,4 glycosidic bonds in both amylose and amylopectin, showing little or no activity on the alpha-1,6 bonds. 
• These enzymes degrade starch to shorter polymeric fragments, known as dextrins and maltose, a disaccharide, which contain two glucose residues. 

Enzymatic desizing consists of three main steps: 

• application of the enzyme, 
• digestion of the starch and 
• removal of the digestion products. 

To facilitate this, the components of enzymatic desizing bath are as follows:

• Amylase enzyme.
• pH stabiliser.
• Chelating agent.
• Salt.
• Surfactant.

The general recipe of desizing process using enzyme is given below:

1. Take 0.5% to 2.0% desizing enzyme (on weight of fabric) in water

2. Add require quantity of Common salt 

3. Maintain pH around neutral

Desizing in a jigger is a simple method where the fabric from one roll is processed in a bath and rewound on another roll. First, the sized fabric is washed in hot water (80 - 95°C) to gelatinize the starch. The desizing liquor is then adjusted to pH 5.5 - 7.5 and a temperature of 60 - 80°C depending on the enzyme. The amylase is added in an impregnation stage and degraded starch in the form of dextrins is then removed by washing at 90 - 95°C for two minutes. The jigger process is a batch process. In contrast, in modern continuous high-speed processes, the reaction time for the enzyme may be as little as 15 seconds.

Desizing on pad rolls is continuous in terms of the passage of the fabric. However, a holding time of 2 - 16 hours at 20 - 60°C is required using low-temperature alpha-amylases before the size is removed in washing chambers. With high-temperature amylases, desizing reactions can be performed in steam chambers at 95 - 100°C or even higher temperatures to allow a fully continuous process.

One of the important property of enzyme is that it  targets only on the starch, it does not negatively affect on the cellulose as in the case with acidic or oxidative methods of desizing (in which degardation of cellulose is expected) . Thus, the specific advantages are:

• Effective solubilisation and removal of starches.
• Excellent biodegradability.
• No aggressive chemicals needed, thus maintaining the tensile strength of the substrate.
• Safe handling and operation.
• Improved wettability.
• Improved fabric quality.
• Reproducible performance and ease of use.

Factors affecting enzyme activity:

1. Substrate concentration - the rate of enzyme activity increases with substrate concentration at lower level up to a certain point and then slows down.
2. pH value - the amino acids and other ionisable groups in enzyme may get ionised at lower or higher pH effecting its activity.
3. Temperature - with increase in temperature, the reaction rate increases due to "thermal energy", but with further increase in temperature, the rate decreases due to thermal denaturation.
4. Activators - presence of specific bivalent metal cation can activate enzyme reaction. Such metal ions stabilise the structure of enzyme-substrate complex or sensitize the substrate to the attack of enzyme or take part in the ion exchange process.
5. Inhibitors - certain alkalis, acids and antiseptic tend to inhibit enzyme activity.

Effect of inadequate desizing:

Following problem may arise, if there is a inadequate:

• Reduced absorbency
• Inadequate whiteness
• Paler dyeing
• Spotty dyeing
• Reserved marks in dyeing and printings
• Increased tendency to creasing due to hard handle
• Reduced tear strength

Checking effectiveness of desizing:

The effectiveness of desizing can be checked using TEGEWA solution. This solution is prepare using 10 g potassium iodide (KI) and 0.635 g iodine in 1000 ml distilled water.

A drop of TEGEWA solution is placed on the fabric and the color change observed and assessed by TEGEWA scale. The fabric is not spotted randomly but from side-centre-side at different points along the length of the fabric. A dark bluish violet color indicates excessive starch and if so, the fabric or garments must be reprocessed. 

Oxidative steeping:Starch and other added impurities are hydrolyzed through oxidation process followed by washing process. Following oxidising agents may be used:

Sodium Hypochlorite: 2.0-5.0 gm/lit. at room temperature at pH 7.0

Hydrogen peroxide:3.0-6.0 gm/lit of H2O2 and 7.0-15.0 gm/lit Sodium Hydroxide, 40 degree C for 12-16 hours

Drawback: It is a time consuming process and some time bleaching of fabric occured which other wise not required. As in the process, oxidising agents are used , there are chance of fibre damage.

Enzymatic desizing: The enzymatic desizing is the most widely practiced method of desizing starch based sizing material. Enzymes are high molecular weight protein biocatalyst that are very specific in their action. This means that a given enzyme only will work on a certain substrate and only do a very particular job. Secondly it should be remembered that, although they take part in a chemical (enzymatic) reaction, they do not change during that reaction. They are what we call biological catalysts that accelerate or facilitate chemical reactions.Enzymes are named after the compound they break down. Some of the examples are given below:

• Amylase breaks down amylose and amylopectin,
• Maltase breaks down maltose and
• Cellulase breaks down cellulose.

The starch based desizing is carried out using amylase and maltase enzymes.

Rot Steeping: This is an oldest method of desizing. In this method no chemical is used. Under this, the fabric which need to be desized, is stored in water at 40-60 degree C for overnight. Due to reaction with water, starch based sizing material get dissolved. 

                                   Padding            Storing                Washing

In this process fabric is first padded with warm water for 100% pickup and then kept for 12-24 hours. During that process microorganism present in water get multiplied and generate starch hydrolyzing enzymes. These enzymes break down starch and dissolve it in to the water. Finally cloth is washed. This process has following two main draw backs. 

1. Process is time consuming and 
2 One can not be sure that in the overnight dipping in water would remove sizing material completely.
3. Requirement of big space 
4. Some time mildew generated during processing attack the cloth and leaves stains

Acid Steeping: In this process acid is used to hydrolyze starch in to lower molecular weight glucose. The most common acids used in this process are Sulphuric and hydrocholoric acids. Fabric is treated 0.25 to 0.50% acid at room temperature around 30 to 40 degree C. To perform this process fabric is padded in acid and then stored for around 12 hours after that fabric is washed.

This process is also economical. There is no need for adjusting pH. The sizing material is removed completely. It take less time than Rot steeping.

The main disadvantage  associated with this process:

• If the material dried during storage the concentration of acid concentration increases which results tendering of cellulose material.

Desizing:Desizing is the process of removing the size material from the warp yarns in woven fabrics. Before understanding desizing process, one has to understand sizing process.

Sizing/Slashing: Slashing is the process where Size chemicals are applied to the warp yarns, to make the yarns weavable. The purpose of size  is to protect the yarn from the abrasive action of the loom parts. The process is carried out on a Slasher machine and the application procedure is called Sizing or Slashing.

Sizing agents: 

1. Natural sizing agents: Starch and starch derivatives; native starch, degradation starch and chemical modified starch products such as Cellulose derivatives, carboxymethylcellulose (CMC), methyl cellulose and oxyethylcellulose etc.

2. Synthetic sizing agent: Polyacrylates,  Modified polyesters, Polyvinyl alcohols(PVA) etc.

Desizing Starch: Once a starch solution dries, the resulting film will not readily re-dissolve in water; therefore, to completely remove starch from a fabric, the polymer must be chemically degraded to make it water  soluble. Following four chemical methods can be used to convert/degrade starch into water soluble compounds.  These are:

•Rot steeping

•Acid steeping

•Oxidative desizing

•Enzymatic desizing

Essential conditions for good GAS singeing:

There are three essential conditions for good singeing. These are:

• A flame with high mechanical & thermal energy quickly burn thermoplastic protruding fibres (e.g. polyester) without any molten beads formation  
• A homogeneous flame with uniform mechanical & thermal energy results in uniform singeing
• An optimal flame - fabric contact time to neither result in incomplete not over-singeing

Testing  effectiveness of singeing:

The effectiveness of singeing process can be checked by one or more of the following methods:

• By looking at the singed fabric with magnifying glass and comparing its hairiness with that of the un-singed fabric. A well-singed fabric shows less hairiness.
• By testing the singed fabric for pilling performance and comparing it with that of the un-singed fabric. A well-singed fabric gives less pilling.
• By sticking and removing a sticking tape on the singed fabric and observing the number of fibres attached to the sticking side of the tape. A well-singed fabric results in less number of fibres sticking on the tape.
• Noticing the feel or handle of the singed fabric. An over-singed fabric may give a harsher feeling.

Singeing Positions:

There are three singeing positions. These are:

1. Singeing on to free guide fabric:

This is the most intensive singeing position with highest efficiency. In this position, the flame bounces onto the free-guided fabric at right angles. This position is usually recommended for singeing of fabrics with all natural fibres (e.g. cotton), regenerated fibres and blended fabrics, which have been tightly woven and have weights over 125 g/m2

2. Singeing onto water-cooled roller:

In this position, the flame bounces at right angles onto the fabric while the fabric passes onto water-cooled guide roller. This position avoids the penetration of the flame into the fabric. The flame does not pass through the fabric, and because of the fabric passing onto water-cooled roller, any thermal damage of temperature-sensitive synthetic fabrics is avoided.  This position is usually recommended for all blended and synthetic fabrics as well as for fabrics having weights less than 125 g/m2 and fabrics with open structure.

3. Tangential Singeing :

In this position, the singeing flame falls on the fabric tangentially. The flame touches only the protruding fibres without having any significant contact with the main fabric body. This position is usually recommended for very light weight and sensitive fabrics as well as fabrics with broken filaments.

Distance between Flame Burner and Fabric:

As the energy content of the flame is lower the farther it is from the burner, the singeing efficiency is consequently decreased by increasing the burner-fabric distance. The distance between the burner and the fabric is usually in the range of 6-8mm but it can be adjusted in a range from 6-20mm.

Flame Width:

All good singeing machines come with a provision of flame width adjustment according to the width of the fabric. This is essential to optimise the gas economy.

Important Gas singeing parameters :

Following are the important gas singeing parameters:

• Flame intensity
• Fabric speed
• Singeing position
• Distance between flame burner and fabric
• Flame width

Flame intensity: The flame intensity of the singeing burners is based on the amount and the outlet speed of the gas-air mixture leaving the burner slots. Besides having high thermal energy, flame also has considerable mechanical energy. All the thermal and mechanical energy of the flame is directed onto the fabric during singeing. The temperature of the flame at the mouth of the burner is in the range of 1250 to 1300oC. The speed of the flame at the burner outlet may be between 15 and 35 meter per second. The flame intensity usually lies between 5 and 20 mbars.

    

      Fabric speed:The fabric speed in the singeing machine is usually in the range of 50-160 m/min depending on fabric weight (gram per square meter)  and fibre blend. For heavier fabrics, the speed is kept slower as compared to lighter weight fabrics.

     

       

F

Plate singeing: 

In this type of singeing machine, the cloth passes over and in contact with one or two heated curved copper plates. The thickness of the plates ranges from 1 to 2 inches. The heating of the plates is done by a suitable burning arrangement of gas mixed with air. The plates are heated to bright red and the cloth passes over and in contact with these plates at a speed ranging from 150 to 250 yards per minute, the protruding fibres are burnt during the passage. Due to contact between the cloth and a hard and hot surface of metal a certain amount of luster in the cloth is produced.

This types of singeing has some drawbacks as given below:

• The fibres end present in the interstices of warp and weft are not singed, since they are not made accessible. 
• As the maintenance of uniform temperature is difficult in the plates, uneven singeing may result.

This singeing process can be explained with the following figure:

Gas singeing machine: 
In this type of singeing machine, the fabric passes over a burning gas flame at such a speed that only the protruding fibres burn and the main body of the fabric is not damaged by the flame. This is the most common type of machine used for singeing fabrics as well yarns.

The line diagram is given below of gas singeing process:

Singeing: 

Singeing is a process of burning protruding fibres from a textile structure. Following figure shows the unsinged and singed yarn. In the first yarn, it is clear that there are protruding fibres on the surface of yarn and in the second yarn, the protruding fibres are removed by applying singeing process.

Similarly following two figures indicate unsinged and singed fabric.

Unsinged fabric                                                                  Singed fabric 

Singeing process brings in fabric smooth surface, less soiling and reduce pilling problems.

Singeing can be carried out using any of the following techniques:

Rotary (Roller)-cylinder singeing m/c:

In this type of singeing machine, the cloth passes over and in contact with a heated rotary cylinder made of copper or cast iron.The rotary cylinder has internal firing and revolves slowly so that constantly a fresh surface of the roller comes in contact with the cloth. The direction of rotation of the cylinder is opposite to the direction of the fabric so that the protruding fibers or nap of the fabric is raised.This type of machine is particularly suitable for the singeing of velvets and other pile fabrics. If the singeing of both sides of the fabric is required, then two cylinder are employed, one for each side of the fabric. Following line diagram shows the principle of rotary singeing:  

Scouring of Wool:
Wool fibre contains 20-50% impurities like dirt, dust, vegetable matter, wool grade, dried sweat etc.

Wool is a protein fibre and sensitive to base. It can be scoured by using detergent at pH up to 7  at 60oC.  The detachment of  oil from wool by the action of detergent are shown below in the figure:    

     Carbonization: The vegetable impurities from the wool is removed using carbonizing process.In this process wool is treated with sulfuric acid. Wool is resistant to acid and vegetable matter dissolves in the sulfuric acid.

Detachment of Oil by the action of detergent

Scouring of Silk:

Silk is a protein fibre and consists of solidified fluid excreted from the glands of silk worm. When the silk worm is matured it consists large number of these glands. The liquid comes out from each glands in the form of fine stream and as soon it comes in contact with air it is solidified in the form of fine filament. The filament are wrapped in the oval shape due to spinning of the silk worm during the excretion of the fluid. This is known as silk cocoon in which silk worm (as shown in the adjacdent figure) eventually get enclosed.Silk cocoon may contain 300 meters of silk multi -filaments. 

For getting around 100g of silk, 1000 cocoons are required. Spinning of cocoon takes 304 days. When this process is completed the silk worm get converted into pupa (as shown in the adjacdent figure) which remains inside the cocoon. The silk filament is surrounded by natural gum known as silk gum. Both silk and gum are protein. Seracin is silk gum protein and fibron is silk fibre protein. Seracin is more amorphous and easily attacked by chemicals.   

There are four varieties of silk. These are Mulberry, Tussar, Eri and Muga. Mulberry is a cultivated silk while others are wild silk.

Following is the composition of Mulberry silk:

i) Fibroin: 70-75%

ii) Sericine: 25-30%

iii) Waxy substances: 2-3%

iv) Naturak colours: 1-1.5%

v) Mineral matter: 0.5-1%

The gum content in various varieties are given below:

• Mulberry silk: 25-30%
• Tussar: 5-15%
• Muga:7-10%
• Eri: 3-5%

Degumming of silk:

Just boiling with water in the presence of detergent, removes silk gum (Sericine).  The presence of  sericene acts as protective coating during weaving, thus improve weaving performance. Therefore it is removed after weaving to make it dyeable. If the gum is not removed following problems are observed:

-No lustre

-Brown colour

-Harsh feel

After removal of gum, silk become very shiny and soft to feel. 

For degumming of silk, silk is treated with olive oil soap 25% on weight of material at 90-95 degree Celsius for 1-2 hours followed by washing with hot and cold water to remove hydrolysed gum. After degumming  silk lost around 25-30% weight. 

The degumming of silk can also be carried out using Protease enzyme. It breaks  peptide/amide linkage of gum and converted into simple amino acids.