How Garments are Dyed Commercially

The garments can be dyed by using pigment dyes. Previously it was condsidered that pigment dyes were non chemically reactive to any fiber. Hence padding or printing with a binder was used. However, now a cationic binder is exhausted onto a garment. This creates an affinity for the garment by the pigment. Then pigment dyestuff is added.

 

Once the pigment dyeing is completed, the garment is rinsed. Then a low temperature or air curable binder is applied to the garments to improve the colorfastness to rubbing.

 

The fastness to rubbing for these colors is satisfactory. However, the colors will washdown during the life of the garments. The higher the concentration of the color, the poor is the colorfastness to washing. However, they have excellent colorfastness to light.

 

Please see the complete process here.

 

Why some prints smell of Kerosene

This smell is normally observed in pigment printing. 

In pigment printing, thickener is used. Two types of thickeners are common. One is emulsion thickener, which has zero solid content in it. This is obtained by emulsification of two immiscible liquids with the help of the emulsifier. Generally Oil-in-water emulsions are used.

A typical recipe of pigment contains 100 parts of binder, 100-150 parts of water, 20-parts emulsifier,  Kerosene or Mineral Turpentine Oil ( MTO) is used which is 750-800 parts. Apart from this 20 parts Urea is added  as hygroscopic agent and 4-6% paste of CMC (10 parts ) is added which acts as a protective colloid. The recipe is ideal for pigment printing. However it suffers from demerits such as inflammable fire hazards, air pollution, high costs and most important of all is the smell of the fabric which is due to residual Kerosene Oil or MTO. To counter these synthetic thickeners are used. These are high weight copolymers of acrylic or methacrylic acid. They come in acid or neutralized form. They do not suffer from drawbacks of the emulsion thickeners, however, they suffer from dull prints and harsh fabric feel.  Also the drying time is longer.  

There are some printers who feel that with synthetic thickeners, there is always a concern of colors spilling over when working with very fine intricate designs and they prefer to use MTO or kerosene. 

How curing is important in pigment printing

A pigment has no affinity to fiber. It is insoluble in water. It needs binder for fixation onto fiber.  A binder is a prepolymer available in the form of aqueous emulsion. Chemically it is copolymer of UTYLACRYLATE-N-METHYLOL ACRYL AMIDE.  Mechanism of binding involves the following sequence: PRINT > DRY >CURE. During curing, the binder polymerises and forms a strong film.  The film embeds pigment color and also strongly adheres to the fiber. Curing is done at 150 degrees for 4-5 minutes. When curing is not proper the poor wash fastness and poor colorfastness will result. Assuming sufficient binder was added to the color paste, these problems are usually resolved by repeating the heat exposure ( Re curing)

Some Notes about Bleaching

Effect of Metals on Hypochlorite Bleaching

Copper and Iron catalyze the oxidaton of cellulose by Sodium hypochlorite degrading the fiber. Fabric must be free from rust spots and traces of metals otherwise bleaching will damage the fabric. 

Stainless stell equipments should be used and care must be taken that the water supply is free from metal and rust from pipes. Prescouring from chelating agents become an important step when bleaching with sodium hypochlorite. 

Weight Loss of Fabric in Bleaching

After bleaching operation the weight loss in the material takes place and it depends on different types of bleaching agents that are used. Due to the removal of coloring matters and fiber damage in the bleaching, textile material may lose considerable weight. In a study it was found that in plain weave fabrics, the weight loss was upto 11% for Sodium hypochlorite bleach and upto 8% in Hydrogen Peroxide bleach.

(Source:http://dspace.daffodilvarsity.edu.bd:8080/xmlui/bitstream/handle/123456789/519/Determination%20of%20weight.pdf?sequence=1)