Bleaching of yarn using cabinet and cheese/package processing machine:

Bleaching of cotton yarn can be carried out using cabinet or cheese/package processing machine. In the cheese/package processing machine, the yarn are wound onto perforated stainless-steel cone or cheese. Now-a-days spiral spring type of cheese as shown in the figure, is mainly used. Before winding yarn on it, it is covered with a sheet of porous paper to make manupulation during winding and the removl of any residue of unwound yarn, easier. The soft winding of yarn on cheese is preferred. The cheese or cone are loaded onto the perforated vertical tubes (Spindles) of the carrier and this carrier is drop in the dye vessel. In the dye vessel, when the processing is started, the liquor move either from inside to out side or out side to inside. The    direction of movement of liquor is reversed periodically for uniform treatment. 

                                                           Yarn wound on Spring

                                                                              Spindle

The selection of bleaching receipe depends upon the requirement of whiteness. Following chemicals may be used:

1. Hydrogen peroxide
2. Sodium hydroxide
3.tablizer (Sodium silicate)
4. Wetting agent

Cuprammonium fluidity test:

This is a senstive index for the determination of chemical degradation of cotton cellulose during scouring, bleaching, souring, etc. In this test, the condtional cotton sample required to make 0.5% solution is exactely weighed and dissolve in cuprammonium hydroxide solution. The flow time of this solution between two fixed marks on a calibrated viscometer (fluidity tube) is measured at specific temperature. The fluidity value "F" is then calculated from the flow time "t" and the viscometer constant "C" by using the fomula F=C/t. For flow time of 200 seconds or less, a kinetic energy correction k has to be applied which is given in the calibrated certificate. The results are expressed as rhes (poise^-1) , which is the reciprocal of the unit of viscisity. Fluidity values between 5-8 are considered to be satisfactory for normal bleached fabric. Determination of cuprammonium fluidity (which in turn indicates the degree of polymerisation, DP) is a senstive test for fabrics of 100% cotton.

In the case of 100% viscose fabric, a 2% solution in cuprammonium hydroxide (instead of 0.5% for cotton) may be used which is likely to show a fluidity value of about 8.  

Preparation of Cuprammonium Solution: A solution containing 240±5 g of ammonia (NH3) and 15±0.1 g of copper per 1,000 ml is satisfactory for the dispersion of cellulose of different forms, including cotton fibers which have not been subjected to any drastic chemical processing.

Bleaching in Jet and Soft flow dyeing machine: In a jet dyeing machine, a very efficient contact between the liquor and the fabric is established since both are in constant movement. This improved effective treatment and also reduce the treatment time. Jet dyeing machine is similar to winch in that the fabric is circulated through the liquor/dye bath in the rope form. However, in a jet, the transportation of the fabric occurs by circulating the liquor through a venturi jet, instead of the mechanical pull of the reel in a winch. (The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe.) The fabric is pulled out of the main dyeing chamber by means of a high speed flow of dye-liqour that passes through the venture opening. The venture tube has a narrowed passage through which the pumped dye-liquor is forced to move much faster. The jet flow of the dye-liquor formed causes a suction that forces the fabric to pass through the jet tube that lead back into main chamber. Although jet dyeing machine is mainly used for dyeing of Polyester fabric at higher temperature (around 130^oC) with material to liquor ratio ( MLR) 1:5 to 1:10. Because it has high speed in which the fabric moves, creases changes their positions rapidly and there is not enough time to set them in one location. Therefore, creasing is minimal.

Advantages of Jet Dyeing Machine:

The jet dyeing machine is very useful for dyeing polyester fabric. However it can be used for bleaching of cotton fabric also. Following are the main advantages of jet dyeing machine: 

1. Low consumption of water
2. Short dyeing time
3. Can be easily operated at high temperatures and pressure
4. Comparatively low material to liquor ratios, typically ranges between 1:4 and 1:20
5. Fabrics are handled carefully and gently

The next development in the jet dyeing machine is a soft flow dyeing machine.

Soft Flow Dyeing Machine:

Soft flow dyeing machine is very useful for dyeing delicate fabrics like knits. In the soft flow dyeing machine water is used for keeping the fabric in circulation. The conception difference of this equipment from a conventional jets that operates with a hydraulic system is that the fabric rope is kept circulating during the whole processing cycle (right from loading to unloading). There is no stopping of liquor or fabric circulation for usual drain and fill steps. The principle working behind the technique is very unique. There is a system for fresh water to enter the vessel via a heat exchanger to a special interchange zone. At the same time the contaminated liquor is allowed channel out through a drain without any sort of contact with the fabric or for that matter the new bath in the machine.

Key Features of Soft flow Dyeing Machine:

       1.   Significant savings in processing time

       2.   Savings in water

       3.   Excellent separation of different streams results in optimum heat recovery              and a distinct possibility of further use or a dedicated treatment

Following recipe can be taken for bleaching cotton fabric in soft flow dyeing machine:

Hydrogen peroxide (50% concentration):  2-4%

Sodium carbonate: 0.5-1.5%

Caustic flakes: 0.5-1.0%

Sodium silicate: 1.5-4.0%

Wetting agent: 0.2-0.5%

Sequestering agent: 0.02-0.05%

Detergent: 0.5-1.0%

The process parameters are given below:

MLR: 1:4 to 1:5

Temperature: 90-95^oC

Time: 2-4 hours

pH value: 10.3-10.8

3. Bleaching in winch and softflow:

The Winch:The winch is one of the oldest dyeing machines knowns. It consists of a tub containing the liquor, and an elliptical winch or reel which is located horizontally above the liquor bath. Numbers of fabric pieces can be treated in this machine simultaneously. Each piece is drawn over the winch, and itstow ends sre sewn together to form an endless rope. The ropes ar kept in the winch machine side by side, separated from each other by rods to prevent them from tangling. During processing, the reel rotates, pulling the ropes out of liquor bath and drpping them back into the bath at opposite side. In this way almost all the fabric is kept inside the liquor bath. A vertical perforated partition is located a few inches from the front of the machine, enclosing the steam pipes, so as to prevent them coming in direct contact with the fabric. The partition also provides a space for adding chemicals in their concentrated solutions form in such a way that they donot come directly in contact of fabric. 

Winch machine is very useful for treatment of light weight fabric and knits as these fabric can easily folded into a rope form without causing damage.  If the fabric is very light weight, it should not be treated in winch as these fabric may float on the liquor bath and tangle.

In winch machine treatment is carried out at higher liquor ratio (MLR::1:20 to 1:50) snf since the tension applied by the pool of the winch is low, the treated fabrics acquire a softer hand. For bleaching of cotton following recipe may be taken:

1. MLR: 1:40

2. Hydrogen peroxide: 3 g/l

3. Sodium silicate: 4g/l

4. Sodium hydroxide: 1 g/l

5. Wetting agent : 0.2 g/l

The above chemicals are added into liquor bath of the winch at around 40 degree C and then temperature is raised to 90-95 degree C over around 45 minutes and maintained for 60 to 120 minutes as per the requirement of whiteness.

2. Bleaching in Jigger:

Jigger machine can be used for desizing, scouring, bleaching, dyeing and finishing purposes. In this machine fabric is treated in open width. The jigger machine consists of a relatively small tub and drawing rollers located above the dye/treatment bath. The fabric is wound around one of the rollers. During treatment of fabric in this machine, fabric is passed through the liquor bath and rewind onto the second roller. When all the fabric is passed through the treatment/dye-bath the direction of movement is reversed, and in this way the fabric is passed back and forth throughout the treatment/dye-bath until the treatment is completed. Following receipe may be taken for bleching of cotton fabric with hydrogen peroxide:

1. Hydrogen peroxide (35% concentration) : 2-4%

2. Stablizer: 0.5-1.5%

3. Sodium hydroxide: 0.5-1%

4. Wetting Agent: 0.1%

The above chemicals are added in the jigger bath and temperartue is raised slowly to 85-95oC. The bleaching is continue 1 to 3 hours depending upon the type of fabric.Then fabric is hot wash followed by several cold washes. Finally if needed peroxide killer may be used to remove residual peroxide.

In the jigger bleaching, stabilser can be removed easily as the fabric is treated in open bath. The materila to liquor ratio is also low in jigger processing, therefore saving of water is also expected.

Technology of bleching:

Bleaching cab be carried out using following three ways:
i) Batch process
ii) Semi-continuous process
iii) Continuous process

Let us discuss these processes one by one:

i) Batch process: 
We can devide batch process in two categories:
a) Fabric bleaching: Kier bleaching, Bleaching in Jigger, Bleaching in winch & Softflow
b) Bleaching of yarn using cabinet and cheese machine

a) Fabric bleaching:

1. Kier Bleaching: Most common process in batch wise process is kier bleaching.  In such process fabric is first desized and then scoured and bleached (Combined process) in kier. Soem time desized and scoured fabric is also taken to kier for bleaching. Hydrogen peroxide is used as bleaching agent. Folling steps may be taken in kier bleaching:

- Loading fabric in kier
- Raisong temperature of bleach liquor slowly to 90-95 degree C with an intermediate holding of 10 minutes at 70-75 degree C to allow any trapped air escape
- Bleaching for about 3 hours  at 90-95 degree C
- Wahing with hot water on rope washing machine

The application of pressure kier can reduce time 1-2 hours as the process temperature is 120 degree C.

After the peroxide bleaching the fabric is thoroghly washed and treated with peroxide killer to remove residual peroxide.

Following recipe may be used for kier bleaching:

Fabric which is desized only:
Hydrogen peroxide (35% Conc.) for desized only: 3-0-5.0 %
Sodium Silicate for desized only: 2.0-3.0 %
Sodium Hydroxide for desized only: 0.6-1.4%
Wetting agent for desized only 0.1-0.2%

Fabric which is desized & Scoured:
Hydrogen peroxide (35% Conc.) for desized & scoured : 1.0-2.0 %
Sodium Silicate for desized & scoured : Not required
Sodium Hydroxide for desized & Scoured: 0.4-0.8%
Wetting agent for desized and scoured: 0.1-0.2%

Mercerization using Ammonia:

In this process cotton is treated with ammonia (NH3) to get similar effect as obtain using higher concentration of sodium hydroxide. The use of ammonia in mercerization has been known since the 1930’s and was developed commercially for fabric processing around 1970 in what was originally known as the Tedesco process, which was subsequently acquired by the Sanforized company and made well-known as the Sanfor-Set®process. Similar to mercerization process (using caustic soda), it also act as a swelling agent for  cotton and also bring changes in crystalline structure of cotton. Liquid ammonia penetrate cotton (Cellulose) very rapidly and form a complex with the hydroxyl group after breaking the hydrogen bonds in the crystalline and semi crystalline region. As ammonia molecule is very small it allows it to penetrate completely into every fiber of the fabric/yarn, unlike caustic which causes such rapid fiber swelling that the caustic is blocked at the surface of yarns, preventing further penetration. The surface tension of ammonia used in the mercerizing process is much lower than caustic soda cocentration (using mercerization). The lower surface tension overcomes the enormous resistance of air trapped in the fabric which is a characteristic of heavy cotton fabrics. Additionally, at normal mercerization concentrations of caustic (23%), sodium hydroxide forms a hepta-hydrate with water (NaOH·7H₂O), which results in a slow-moving, bulky group.

In the process of ammonia mercerization, the Cellulose I (native cellulose) is converted into ammonia cellulose complex, then to Cellulose III on heating and removal of ammonia. The main difference between Cellulose II (Obtain in caustic mercerization) and Cellulose III is that the cellulose III is recovered into Cellulose I, while Cellulose II could not be reconverted besides minor changes in the spacing and angle between crystalline lattices. The transition from Cellulose II to Cellulose I is possible through the application of water and heat, steam or by prolonged soaking in water at ambient conditions. 

Ammonia mercerization can be carried out on yarn as well as on fabric, which are discuss below:

Treatment of cotton yarn with ammonia:

Under this cotton yarns are first moist (30% moisture regain) and continuously treated in a liquid ammonia bath at -35oC (approximately 0.7 second contact time), then enter into hot water bath (95 degree C, 0.1 sec contact),  where it is stretched 10-30%. Subsequently, the yarn is would on a spool and dried continuously with hot water.

Treatment of cotton fabric with ammonia:

 Ammonia treatment is carried out in three steps as given below:

i) Padding fabric, ii) Tension control and iii) Removal of ammonia

In this dry fabric is impregnating for around 10 second in liquid ammonia bath to 100% wet pick up using padding mangle. After exposer, fabric is immediately dried using indirect steam under tension. As the removal of ammonia from the fabric is extremely sensitive to the applied tension, it is necessary to run over blanket. The small amount (5-10%) residual ammonia present in the fabric after drying is removed by steaming. The ammonia vapours are recovered, liquified and reuse.