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%

Desizing using Low-Temperature Plasma treatment: Recent development in plasma treatment of textile material has revealed that it has enormous application as an alternative to textile processing in terms if saving in energy, water etc.

(Figure source from Indian Journal of Fibre and Textile Research, Vol 36, March 2011, p42) 

 Plasma is considered as fourth state  of matter (Three states :Solid, liquid and gas). Plasma treatment is a ecological viable desizing process. In this treatment special type of equipment is used to generate Plasma. The textile material is exposed to the Plasma in an evacuated chamber. The removal of size depends upon radio frequency power, dwell time and plasma gas composition.

In this process, starch sized cotton textiles are subjected to 100-200w oxygen plasma and exposed for 8-10 min. This process convert 65% of all organic sizing materials to gaseous compounds which are evacuated. Subsequent rinsing in water at room temperature removes 30-35% of left out size. There is one draw back associated with this process. Like solvent treatment size recovery and recycling are not possible in this treatment.  

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

Preparation of textiles for wet processing i.e Bleaching or Dyeing or Printing:

The preparation of textiles for bleaching/dyeing/printing is just like same as we do preparation for examination. Why we do preparation for examination? The answer may be  to pass the exam. Therefore when we have certain goal in mind, we do prepare our self to achieve the same. In similar way, in the textile, our goal may be to get bleached or dyed or printed fabric. To achive this goal we ahve to prepare textile material to make it ready to bleach/dye/print.

Following are the processes for preparation of textile material for bleaching/dyeing/printing:

Desizing: Removal of sizing chemicals such as starch/poly vinyl alcohol (PVA) etc from the fabric . To understand desizing we must know sizing process.

           Sizing: Sizing is a process in which we apply starch or PVA or other ingredient on the warp                 yarn (Just like a coating) to improve warp yarn strength without affecting its elongation                       properties much so it can stand stress and strain during weaving process. Please note that                  sizing is carried out only in weaving not in Knitting. 

Desizing process: There are four methods for desizing. These are:

1. Rot steeping
2. Acid steeping
3. Oxidative desizing
4. Enzymatic desizing