Q1: Process of cotton fibres

Question 1

Cotton fabric preparation

Answer 1

Singeing
Desizing
Scouring
Bleaching
Mercerising

Question 2

Singeing

Answer 2

The processing of cotton fabrics often starts with brushing and singeing. The surface of the fabric is brushed to raise and remove lint, loose yarns and surface dirt.

The protruding fibre ends are then burned off by passing the fabric over a small gas flame. This is known as singeing.

Singeing provides a smooth surface for printing, improves wettability, prevents a frosty appearance after dyeing and also reduces the formation of pills on the fabric.

Cotton woven fabrics would normally be singed but knitted fabrics would not.

Question 3

Desizing

Answer 3

Sizing agents are applied to the warp yarns of woven fabrics to assist in the weaving process (sizing agents are not found on knitted fabrics).

The sizing agents must be removed prior to dyeing and printing in a desizing process.

After singeing, the fabric is saturated with a desizing agent. This can be an oxidising agent such as hydrogen peroxide or enzymes.

The fabric is allowed to steep in the desizing agent for anything from a few minutes to overnight.

Afterwards the fabric is thoroughly washed to remove all sizing.

Question 4

Scouring

Answer 4

Hot alkaline solutions are used to remove all desizing products, pectin, oils and wax from the fabric.

Knitted fabrics are typically scoured under milder conditions than those used for woven fabrics.

Question 5

Bleaching

Answer 5

The scouring process removes the wax and the majority of impurities from the fabric.

The bleaching process completes the purification by removing seed and husk remnants and colouring matter.

Both chlorine and peroxide bleaches are used to obtain a uniform white surface on the fabric.

Care must be taken not to weaken the fabric during bleach- ing.

Most woven cotton fabrics are bleached before dyeing and finishing.

Knitted cotton goods are usually not bleached before dyeing or printing, but when they are, bleaching is often combined with the scouring process.

Question 6

Mercerising

Answer 6

The cotton fabric is saturated with a 16 to 24% caustic (sodium hydroxide) solution while being held under tension.

The cotton fibre swells and untwists, the lumen becomes smaller, and because the fibre is now rounder, it gains lustre.

Mercerised fabrics are stronger and smoother than unmercerised fabrics, and also accept dyes more easily.

Cotton and cotton blends (excluding blends with fibres that are susceptible to alkali, say, wool) can be mercerised in both yarn and fabric forms.

Knitted cotton fabrics are generally not mercerised because of difficulties in applying tension to the knit without distorting the fabric structure.