Page 133 - Basic Principles of Textile Coloration

122 PROTEIN FIBRES

material. During washing, water absorption softens and lubricates the fibres so that
they can return to their original unstrained condition. This effect is called
relaxation shrinkage. It is reversible and can be partly rectified by stretching and
drying, although shrinkage will occur again on wetting. For some knitted materials,
extension rather than shrinkage may occur on washing as the water releases the
strain in the yarn loops and they deform more easily.

Washing of woollen articles causes irreversible shrinkage and felting.
Mechanical compression and relaxation of the fibres in a woollen fabric during
washing cause fibre displacement. This is promoted by the wool fibre’s elasticity
and the lubricating action of the detergent. The scales on the surface of the wool
fibres allow them to move only in the direction of the root, and their friction
prevents their return to the original positions. This irreversible process is called
felting. It closes up the fabric structure, making it much more compact and of
increased rigidity. The separate yarns are frequently much less easily distinguished.
Although the mechanism of unidirectional fibre movement is probably an over-
simplification, it provides a partial explanation of this important effect. Shrinkage
and felting are obviously undesirable in a finished article that is going to be
repeatedly washed.

On the other hand, felting is often deliberately carried out during the finishing
of many woollen fabrics, either before or after dyeing. The process is called fulling
or milling. In a fulling mill, the wet, woollen fabric is pounded and turned around
by reciprocating hammers. Rotary milling machines are frequently used. These
hold a continuous rope of fabric that passes round and round, driven by a pair of
rollers that force the fabric through a compression funnel. The compressed fabric
relaxes as it exits the funnel and as it passes round the machine for another cycle.
Felting is achieved by repeated compression and relaxation. Rotary milling
machines allow combined milling, scouring and desizing before dyeing. Dirty liquor
initially squeezed from the material passes into the trough below the rollers and
out to the drain. Later, once the oils have been removed, this liquor is retained in
the machine.

7.4 WOOL PROCESSING [4]

7.4.1 Scouring wool fabric

Woven and knitted wool materials and yarns must be scoured before dyeing to
remove lubricating oils that were added to aid processing. To minimise felting,