Understanding Wool Fiber,Structure and it's Composition

Wool Fiber/ Protein Fiber

Animal hair consists of complex proteins. The hair of sheep and goats is particularly important for textile. Wool is the animal fiber with outstanding importance. It comes from the fibroins, covering of the sheep, goat, camel etc.

For thousands of years, wool has been used for clothing and other textiles by different tribes and nations around the world. Till today wool fiber has great importance because of its unique natural properties that are present in other natural and synthetic fibers.

The entire range of protein fibers accounts for only 6% of world fiber consumption, where wool fiber contributes 90% of total protein fiber consumption.

Structure of Wool

The morphological structure of wool fiber is extremely complex. The wool fiber exhibits wide variations in physical characteristics such as diameter and length. Its diameter can vary from 18-40 μm along its length, primarily due to nutrition. The staple length of fibers removed from sheep varies to a great extent depending on a large number of factors such as breed of sheep and the position of the fibers on the skins.

From the morphological standpoint, each individual wool fiber is divided into three sections:
• The root: It is the living part situated beneath the surface of the skin.
• The shaft/stem: It is cylindrical and tapers to a point at its free end.
• The tip: The free end of the fiber and it is naturally pointed. While the tip from the
mature fleece is flat indicating that it has been previously shorn.

The fine homogeneous wool fiber consists of two layers:
       a. The outermost scaly layer- cuticle
       b. The inner layer- cortex
While the coarser heterogeneous wool fiber has third core layer-medulla.
Cuticle:
The outermost surface of the individual hairs, generally known as scales, varies in relative size and arrangement from one type of hair to the next. The individual scales appear to be connected to one another resulting from intercellular cement or the interlocking fibrils which protrude from individual cells.

The scaly layer is an external sheath protecting the fibrous layer within it from the action of
exterior factors. The friction they generate hinders the movement of wool fiber in the direction of the tip. This is called 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 epicuticles. It is a part of cell membrane complex and covers all the fiber, except the tip where it has been worn away by weathering. The epicuticle is, however permitted by many microscopic pores through which water vapor may penetrate into the interior of the fiber. Wool fabric will thus absorb water vapor from the body without feeling damp and will release it again slowly into the air.

Cortex:
Cortex is the main part of the wool fiber underlying directly the scaly layer. It constitutes the principle body and determines many mechanical properties of the fiber. Cortical cells are long, slightly flattered, more or less twisted and spindle shaped and consist of intermediate filaments (micro fibrils) of keratin, in a matrix of high Sulphur proteins and surrounded by the cell membrane complex. The average cells vary from 100-200 μm in length, 2-5 μm in width and 1-3 μm in thickness. These cells are found in lesser quantity in fine wool fibers (about 20 cells) than the coarser wools (about 50 cells) across the diameter of their cross-section.
The cortex of wool fiber is composed of two distinct sections, namely Orth cortex and
paracortex, which differ from each other slightly in their physical and chemical characteristics:

Difference on dye-uptake. Ortho is more intensively dyed.
Ortho is less resistant to the action of chemical agents and enzymes than paracortex.

In fine wool, both of this cortex appear as like two half cylinders stuck together along their
axes. They spiral around each other along the fiber, following the crimp. The ortho cortex
always being oriented towards the outside of the crimp wave.The characteristics and the art of wool fiber crimping are determined alone by the natural quantitative proportion of ortho and paracortex. With same portion of the para and ortho-cortex develops intensive crimp. During the growth of wool fiber hardening (keratinization) of
paracortex begins earlier than the hardening of ortho-cortex, thus results in a crimp. Besides, the composition of ortho-cortex is more elastic and flexible than that of the paracortex, which leads to absorb higher amount of water by the ortho-cortex components. Therefore, as the wool gets wet, the crimp increases.

Medulla:
A central stream of cells along the length of the wool fiber is termed the medulla. It does not run the entire length of the fiber and there may be hollow spaces. Although not all wool fibers have medulla some of them show the presence of medulla as a third layer within the cortical layer. It is found mostly in coarse wool fiber.
The presence of medulla contributes to thermal insulation and also increases the light scattering properties of fibers. However, it also causes some deterioration of fiber quality.
These are-

medullated fiber suffers from lower bundle strength
they cause lower spinning properties
they produce lighter shade than the true wool fibers. This may be due to the presence of    medulla causing lower thickness of cortex to take up the dye.

Chemical composition of Wool

Raw wool contains a variety of different chemicals and dirt that must be removed. These
impurities may comprise as much as 60% of raw wool mass. This include wool wax, suint and a range of vegetable and mineral debris. Wool wax is complex mixture of hydrophobic fatty alcohols and fatty acids, whereas suint consists of water-soluble carboxylic acids and their salts from dried animal sweat. To eliminate these impurities, wool is subjected to special treatment e.g. scouring, carbonizing etc.

Approximate composition of raw wool

Composition

Percentage (%)

Wool Keratin

45

Wool wax

18

Suint

10

Water

12

Debris

15

 

The wool fiber is chiefly composed of protein called keratin. The basic composition of wool
keratin is more or less same in different types of wool, but they differ widely in respect of sulfur content. Sulfur is distinctly characteristic of wool and all hair fibers. Wool keratin contains the following elements:

Elements

Percentage (%)

Carbon

50

Oxygen

22-25

Nitrogen

16-17

Hydrogen

7

Sulfur

3-4