Cotton - Scouring and Bleaching - Pretreatment

Pretreatment - A simple way

The current problem that mostly worries the modern dyers could be short-listed as

1. Shade-matching in buyer defined illuminants.

2. Shade tolerance limits for shade passing.

3. Strict limits of various Color-fastness properties.

4. Lowest Processing cost.

5. Lowest load in effluent water

6. Meeting the various buyer-defined pre-requisites like Oko-Tex standards

The survival of the modern dyer would therefore lie in the ability to

1. Prepare Right Fabric for dyeing

2. Select Right Dyes and Chemicals for various fastness requirements.

3. Maximize the Right First Time (RFT) dyeing both in lab and bulk dyeing.

The “Garbage in, Garbage out”, “what we sow that we reap” – all these sayings would be proved true in the case of quality pretreatment. It has been told that dyeing is the mirror image of preparatory treatment. Unless the preparatory fabric is excellent, one cannot achieve proper dyeing and finishing.

Bleaching Technique:

 The main aim of preparatory process is,

1. To remove natural pectin, wax, protein, coloring matter and unwanted minerals from the fiber,

2. To make the substrate uniformly absorbent throughout its length and breadth,

3. To make the substrate uniformly white for pure white finishes with optical brighteners or for subsequent dyeing, mercerizing etc.

The ideal process of combined Scouring and Bleaching for cotton knits and yarn should have the following contents.

S.No	Material Details	Recipe
1	Non-Ionic detergent with high cloud point, low foaming and good rewetting property or a nofoaming Anionic wetting agent	0.5 to 0.75% (o.w.f.)
2	Anti-crease lubricating-agent	0.3 to 0.50% (o.w.f.)
3	Soda Ash	0.5 to 1.0% (o.w.f.)
4	Hydrogen Peroxide (50%)	2.0 to 4.0gms/liter
5	Peroxide Stabilizer	0.5 to 1.0 gm/liter
6	Caustic Soda flakes	0.7 to 1.0% (o.w.f.)

If the water hardness is above 100ppm, then a suitable sequestering agent should also be added to the above recipe (0.5 to 1.0g/l).

Check the pH of the bleach bath. It should be 10.5~11.0

1. Hot wash at 80°C for 15 minutes

2. Treatment with 0.5% peroxide-killer at the pre-defined temperature

3. Hot wash at 80°C for 15 minutes

4. Neutralize with 2 gms/liter Acetic Acid.

So, this is the optimum process requirement for good quality fabric/yarn.

Bleaching Machines:

Jigger, winch, soft-flow, cabinet and continuous bleaching ranges such as Beninger are some of the routine machines used for textile bleaching.

Some useful hints in Hydrogen Peroxide bleaching:

The maximum bleaching activity from hydrogen peroxide is obtained generally at about pH 11.5. In practice this alkalinity level is altered to suit fiber sensitivity and bleaching process requirements; e.g. wool, cotton, amount of seed etc. Selection of the alkali to be used in peroxide bleaching is dependent therefore on the fibers or the blend being bleached. Sodium Hydroxide and Sodium carbonate will be used generally on cellulose fibers, whilst ammonia and tetra sodium pyrophosphate are used when bleaching protein fibers. 

        Basic Requirements for Hydrogen Peroxide Bleaching

        Water Hardness - How does it affects Textile Processing?

        Determination of Water Hardness - How to Test details?

        Industrial Water Softening Treatment - A detailed Ion Exchange Method

        A simple pretreatment of cotton - Useful tips and recipes of Peroxide Bleaching

        Bio-scouring, technique, tips and benefits

        Assessment of Bleached Goods

        Useful Links on Water