Page 100 - Basic Principles of Textile Coloration

COTTON PROCESSING 89

the cellulose, giving a more open accessible structure. Obviously, dissociation of
the hydroxyl groups gives sufficient swelling for water and hydroxide ions to
penetrate into the crystalline regions. The cellulose chains then rearrange into a
new, more stable pattern, which is preserved after washing out the alkali.

Identification of mercerised cotton is quite easy by microscopic observation of
its characteristic rounder fibres, without convolutions. Alternatively, a small
region of a fabric sample may be wetted with 20% NaOH solution for a few
minutes. After washing, the whole sample is dyed with a direct cotton dye such as
Benzopurpurine 4B (CI Direct Red 3). If the treated zone dyes deeper than the
remainder of the sample, the cotton was not mercerised. The greater accessibility
of mercerised cotton is also evaluated from the increased absorption of chemicals
such as iodine and barium hydroxide.

Large quantities of bleached cotton fabric, and to a lesser extent cotton/
polyester, are mercerised by a technique called chain mercerisation. The wet fabric
coming directly from bleaching is mangled, and possibly vacuum extracted. The
objective is to remove as much water as possible from the goods, but to ensure a
uniform distribution of the residual water. The material then passes into the first
bath of the mercerisation unit, containing 22% NaOH. The solution may contain
an alkali-stable wetting agent to assist penetration of the solution into the cotton
yarns. It is squeezed, to remove excess liquid, and then passes over a series cold
metal cylinders to allow time for the solution to absorb into the fibres. This
impregnation is repeated with a second bath and series of ‘timing cans’. Then,
clips grip the selvages of the fabric and pull it to the required width. The clips run
down each edge of the fabric in continuous chains. The tension in the fabric is
determined by the speed of the chains and their separation. Further along the
chain, the washing process begins, using dilute NaOH solution originating from
the countercurrent flow of the wash water that enters at the chain end. Suction
boxes below the fabric pull the washing solution through it and circulate the liquid
upstream. The NaOH in the cotton is diluted to a concentration below 5% before
the clips release the fabric at the end of the chain. Further washing in a wash box,
neutralisation with acetic acid solution and final rinsing and drying complete the
series of operations. The entire process must be conducted under conditions giving
uniform mercerisation from side to side and end to end of the fabric. The residual
alkalinity should be negligible. Table 5.2 lists some of the effects of this process.

Cotton yarn, in hanks held by a pair of rollers, can also be mercerised under
tension. This is usually a batch operation. In the Prograde process, cotton yarn is
treated under tension with liquid ammonia at its boiling point (–34 °C), washed