Page 125 - Basic Principles of Textile Coloration

114 PROTEIN FIBRES

7.2.2 Morphology of wool fibres

Wool is a staple fibre from the fleece of various breeds of sheep. It is a
multicellular, protein hair that tapers from the root to a point. Fibre lengths vary
from 4 to 40 cm and diameters from 17 to 40 mm (1 mm = 1 ´ 10–6 m). Wool
fibres have complicated structures. The main components are the scaly cuticle,
the body of the fibre or cortex, and the cell membrane complex. The latter
surrounds the cells of both the cuticle and cortex in a continuous phase. Coarse
fibres may also have a medulla or inner core.

Microscopic examination shows the scales on the fibre surface that are
characteristic of most animal fibres. The overlapping scales point towards the fibre
tip. In a fine wool, they constitute about 10% of the fibre. The friction they
generate hinders the movement of the wool fibre in the direction of the tip. This is
called the directional friction effect and is partly responsible for the felting and
shrinkage of wool articles on washing. The scales consist of several layers and are
covered with a hydrophobic outer layer called the epicuticle. It is part of the cell
membrane complex and covers all the fibre, except the tip where it has been worn
away by weathering.

The cortex is composed of spindle-shaped cells made up of fibrils of keratin, in
a matrix of high-sulphur proteins, and surrounded by the cell membrane complex.
It comprises about 90% of the wool fibre. In fine wools, the cortex appears as two
distinct regions rather like two half-cylinders stuck together along their axes.
These two regions are called the ortho- and paracortex. They spiral around each
other along the fibre, following the crimp, the orthocortex always being oriented
towards the outside of the crimp wave. This structure is similar to that in a
bicomponent synthetic fibre. The crimp arises from the unequal rates of hardening
of the keratin that begins in the hair follicle, but occurs at different levels on each
side of the wool hair. These two cortical regions differ in structure and reactivity.
Basic dyes stain the more accessible orthocortex cells but acid dyes show no
preference for either cortex.

The cell membrane complex consists mainly of protein and lipid materials and
surrounds all types of cells in a continuous network throughout the fibre. It forms
a region of poor mechanical strength compared to the rest of the fibre and
probably fractures first during fibre abrasion, leading to fibrillation – splitting into
the individual cortical cells or even into protein fibrils within the cells. The
membrane complex is weakened by extended dyeing at the boil in acidic solution.
The medulla, if present, is a central core of cells that may contain coloured