Balancing of machines for complete process house :Part-3 Scouring and Bleaching

For this purpose one Pre-treatment Range (PTR) is required.

Machine working speed: 60 m/min, M/c utilization= 80%

Total production per shift (8 hours or 480 min)= (speed x utilization x 480)/100

= (60 x 80 x 480)/100 = 23,040 meter

Total fabric to be pretreated= 50,000/-

23,040 meter fabric treated in 8 hours

50,000 meter…………= (8 x 50,000)/23040 = 17.36 hrs

Balancing of machines for complete process house :Part-1 (Introduction)

•      Machine balancing is a most important part of any process house to run production smoothly.

•      Let us take example of dyeing of 50,000 meter 100% cotton medium weigh quality (150 gsm) fabric to understand this

Process flow:

To start it, let us have following process flow:

Grey cotton fabric------Singeing & Desizing------Scouring------Bleaching-------mercerizing-----dyeing-----finishing----finish folding

Let us have following type of machines to carry out processing of 100% cotton fabric:

•      Singeing cum Desizing

•      Scouring & Bleaching (Pretreatment renage-PTR)

•      Mercerizing

•      Continuous dyeing range (CDR)

•      Softening & Drying on stenter

•      Sanforzing (Zero-zero)

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

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.

3. Bleaching with Sodium Chlorite (NaClO₂):

Bleaching process with Sodium Chlorite is carried out in acidic conditions. During the process yellow-brownish ClO₂ gas is released which is toxic in nature. The bleaching action is supposed to be carried out by this gas. When a solution of NaClO₂ is acidified, ClO₂, hypochlrous acid (HClO₂), sodium chlorate (NaClO₃) and NaCl are formed. ClO₂ and HClO₂ are used as bleaching species. 

The optimum pH for bleaching with sodium chlorite is 3 to 5. Which can be achieved using formic, acetic or phosphoric acid.  Below pH 3, there may be severe damage to the cotton fibre. Bleaching rate increases if the temperature is around 90^oC. Below 50 ^oC, there is a negligible bleaching effect.

This bleaching agent can be used for acrylic, polyester and rayon. It removes all kitties, wax etc from cotton and also the feel of the fabric is very soft. It gives better whiteness than hydrogen peroxide. 

There are some disadvantages also associated with this bleaching agent. It creates pollution as ClO₂ liberate during process. It also corrodes equipment. This bleaching agent cannot be used for continuous process as H₂O₂. 

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.