3.2 Fabric preparation processes

Question 1

Bleaching:

Answer 1

A process used to remove natural and artificial impurities in fabrics to obtain clear whites for finished fabric or to prepare fabrics for colouration

Question 2

Calendering:

Answer 2

A mechanical finishing process that uses heated rollers and high pressure to produce special effects such as lustre, moiré and embossing

Question 3

Carbonising:

Answer 3

A chemical process used to remove cellulosic matter from animal fibres

Question 4

Crabbing:

Answer 4

A process used to set wool, that is, to provide dimensional stability before further wet processing – hot or boiling liquids, rollers and tension are used

Question 5

Decatising (decating):

Answer 5

A process in which fabric, wound tightly on a perforated roller, has hot water or steam forced through it in order to improve hand or remove wrinkles

Question 6

Desizing:

Answer 6

A process that removes the sizing materials applied to warp yarns prior to weaving

Question 7

Fulling:

Answer 7

A compressive mechanical process applied to wool and wool-blend fabrics to produce a compact, closely constructed fabric

Question 8

Mercerising:

Answer 8

The application of a caustic soda treatment to cotton yarns or fabric to increase lustre and affinity for dyes; the process causes a permanent swelling of the fibre

Question 9

Scouring:

Answer 9

A cleaning process used to remove sizes, oils, soils and other foreign matter from textiles prior to further colouration or finishing processes

Question 10

Singeing:

Answer 10

A continuous high-speed process in which fabric is passed over a gas flame to remove protruding fibres ends

Question 11

3.2.1 Cotton fabric preparation

Singeing

Answer 11

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 12

cotton Desizing

Answer 12

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 13

cotton Scouring

Answer 13

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 14

cotton Bleaching

Answer 14

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 15

cotton Mercerising

Answer 15

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 mer-
cerised 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.

Question 16

Wool Scouring

Answer 16

Wool is scoured in loose-stock (fibre), yarn and fabric forms. This is done to remove im- purities that could hinder the next manufacturing stage. Detergents and alkalis (sodium carbonate) are used to remove the impurities. Woven fabrics are generally scoured in rope form, in a dolly scouring machine.

Question 17

Wool Carbonising

Answer 17

When a sheep moves around in the veld, small pieces of bush or other vegetable matter become entangled in its wool. All of this is not removed by carding. The woollen fibres or fabric are treated with sulphuric or hydrochloric acids. This reduces any vegetable matter left in the wool to carbon which can then be shaken from the wool.

Question 18

Wool Bleaching

Answer 18

Wool is generally only bleached for a white end use or if the intended dyed shade is par- ticularly bright or pale. Wool can be bleached using reducing or oxidising agents. The most common process uses hydrogen peroxide.

Question 19

Wool Crabbing (setting)

Answer 19

The woollen fabric is immersed first in hot and then in cold water, before passing it between rollers. The function of crabbing is to permanently set the filling yarns at right angles to the warp yarns. The fabric will be off grain if the crabbing was not done properly.

Question 20

Wool Decating

Answer 20

Woollen fabrics are treated with water and steam to develop and set the lustre of the fabric.

Question 21

Wool Fulling (milling)

Answer 21

A length of woollen fabric is lightly felted by placing it in warm soapy water. This is a method of controlling shrinkage and produces a thicker, more compact fabric. Boiled wool is a knitted woollen fabric that has been heavily fulled to produce a dense, compact fabric. Boiled wool is popular in countries with freezing cold climates such as certain European countries, but is extremely expensive and rarely obtainable in South Africa.