Question 1

Describe how you could obtain ready for dyeing fabric starting with grey material and mention all the functions of the chemicals used in the recipe.

To get ready for dyeing fabric we have to complete number of steps inorder to grey material and the steps are shown below.

Lot Formation Shearing and Cropping Singeing Desizing Scouring Bleaching Mercerizing

After completing these steps the fabric is ready for dyeing.

1.Lot Formation

Grey fabrics are sorted into lots and we should take the grey fabric we need with the same weight per unit length and same width, so that we can process the fabric altogether.

The separated fabric packages (rolls) will be stitched end to end to get a continuous fabric length.

Now the grey fabric will go through a special machine and there it will detect damages, weaving faults, stains, etc.

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2.Shearing and Cropping

After lot formation shearing and cropping is done because after stitching process there will be small projected yarn remaining on thesurface of the fabric.

This can cause defects in dyeing and printing. They may also entanglewith machine parts during these processors and cause massive damage.

To remove this projected yarn we use an extruder type blade to cut off them.

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Shearing mechanism

3.Singeing

Cotton fabric is made from staple yarns and therefore we can see protruded fibers on the fabric surface.

These protruded fibers reduce the smooth lustrous appearance of the fabric and it can cause problems in dyeing.

Singeing is done to increase the marketability of the fabric.

In this process both sides of the fabric will be exposed into a gas flame in a high speed so that the protruded fibers will burn and removed from the fabric surface.

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4.Desizing

Removal of starch from grey fabric is called Desizing and the most popular and eco-friendly method is using an ENZYME.

Natural starch that is made from potatoes, rice is commonly used to size the grey fabric. But there is chemically modified starch too.

Starch is used in the fabric to increase the strength of warp sheet and the strength will be increased by 10 - 12%.

Enzymes are protein based structures and it can degrade starch into water soluble compound.

So that we can wash off the water soluble compound and improve its hand feel and dye ability.

Popular enzyme is Amylase.

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5.Scouring

Scouring is removal of impurities from the gray fabric by boiling with an Alkali and a Detergent.

Impurities of raw cotton is as follows,

Water 6 – 8% Nitrogenous Matter 1 – 2.8% Mineral Matter 1 – 1.8% Waxes 0.5 – 1% Residue of pigment resin 3.4 – 6%

The main scouring agent is Caustic Soda (NaOH) and it will remove oils, waxes, and neutralize acidic materials.

The detergent emulsifies oils, fats, waxes and removes stains from the fabric.

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6.Bleaching

Bleaching is removal of natural pigment by treating with a strong oxidizing agent.

Normally cotton fabric is off-white and it will be whiter after bleaching.

The main oxidizing agent is Hydrogen Peroxide (H2O2) and best results will be in 11 – 12 pH range.

After bleaching the fabric should be free of Ca++, Mg++, Fe+++ irons to have a very good absorbency of dyes.

Natural pigments are organic compounds that have double bonds and in the bleaching process this conjugated double bonds will break and discoloration takes place.

7.Mercerizing

Mercerizing is done after bleaching and the bleached cotton will be treated with highly concentrated solution of Caustic Soda (NaOH) under tension.

After mercerizing following changes will be seen.

The fiber swells and the lusture will increase. Fiber strength increase. Cotton fiber will be more cylindrical because of the

disappearance of convolutions. Dye absorbency will increase. Hand feel will improve.

In this process cotton fiber cell wall will swell and this will increase the surface area of the fiber and so that the reflectance will be increased and will be more lustrous.

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Chemicals used in all these steps and the main object of each chemical are shown below.

Caustic soda (NaOH) : Main scouring agent Wetting agent : Reducing the tension

of water Anti-creasing agent : Removal of crease

marks Sequestering agent : Water softener Hydrogen Peroxide (H2O2) 50% : Main bleaching agent Stabilizer : Delay the reaction of

the peroxide to bleach even

By completing the above steps we can obtain ready for dyeing fabrics from grey material.

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Question 2

What are the two popular printing systems for cotton material? Indicate all the functions of chemicals in the binder system.

There are two popular printing techniques for cotton materials and they are,

1. Reactive Printing2. Pigment Printing

1.Reactive Printing

In reactive printing we make the printing paste and we print the necessary design on the fabric either using a block or screen printing method.

Fixation will take place in Alkali medium.

Steaming will be done at 103°C for 8 – 10 minutes to better fixation.

Washing off with hot and cold water will be done next and boil with soap will be done to remove unfixed dye and thickener.

Chemicals and objectives of each chemical used in the printing paste is as follows,

Sodium Alginate (Thickener) 40% Adjusting the viscosity ofprinting paste

Urea 15% Dye solvent and moisture carrier Sodium Bicarbonate 3% Alkali Sodium Carbonate 1% Alkali Ludigol 1% Water softener Sodium Hexametaphosphate 1% Water softener Water X%

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Advantages of reactive printing

The dyes react with cellulose and so that the fastness properties are good.

Printed area won’t be stiff after washing is done, so it’s more comfortable to the wearer.

Durable

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Printing paste|

Printing|

Fixation – steaming 103°C to 8 – 10 minutes|

Washing (Hot and Cold)|

Soaping (95°C)|

Drying|

Finishing

2.Pigment printing

In pigment printing we make the printing paste and we print the necessary design on the fabric.

Fixation will take place in 4 – 4.5 pH level.

Curing is done at 160°C for 5 minutes for better fixation.

Testing fastness properties and finishing will be done.

No washing or soaping is required in pigment printing.

Chemicals and objectives of each chemical used in the printing paste is as follows,

Binder 10 – 20 g kg-1 Chemical which helps to keep the pigments on the fabric surface

Synthetic thickener 12 - 18 g kg-1 Adjusting the viscosity of the printing paste

Fixing agent 10 – 20 g kg-1 To accelerate the fixing action14 | p a g e

Softener 10 – 20 g kg-1 Improve the hand feel Anti-foam 3 g kg-1 To prevent air bubbles

in the printing paste Urea 20 g kg-1 Moisture carrier Water X g kg-1

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Advantages of pigment printing

Simple handling Easy to be fixed No washing is necessary No back staining Low cost production

Disadvantages of reactive printing

Print is on the fabric surface No reaction with fibers Fastness properties are low Not durable

Printing paste|

Printing|

Fixation – (4 – 4.5 pH) |

Curing at 160°C for 5 minutes|

Testing fastness properties|

Finishing

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