Properties of wool

Wulfhorst et al. (2006) stated that wool is one of the most commonly used fibres in clothing, and has been in existence since the second half of the second millennium B.C. Some of the major wool producing countries in the world include Australia, New Zealand, Russia, Argentina, South Africa, Uruguay, UK, USA, Turkey, India, Brazil and Bulgaria (Ghosh, 2004).

Figure 25 illustrates the structure of a wool fibre (Wulfhorst et al. 2006). Wool fibres generally have a natural crimp, which results in bulky fabrics for improved insulation (IWTO, n.d.). The diameter of wool fibres vary from about 16 microns in superfine Merino wool, to more than 40 microns in coarse hairy wool (IWTO, n.d.). The broad range in wool fibre diameter, fineness, or grade, makes the fibre suitable for a range of products, whether clothing, household or technical textiles (IWTO, n.d. and Kott, n.d.). There are different methods used across the world for grading wool, such as the American system, the Spinning Count system, or the Micron system;

however, all three systems relate to the fibre diameter, which largely determines the price and value of the wool (Kott, n.d.). In addition, the more uniform the fibres are within a particular quantity of wool, the higher the value (Kott, n.d.). The colour of the wool fibre also contributes to the value of the fibre. If off-colour fibres

69 are found in white wool fibres, the value of the wool reduces (Chappell, n.d.).

Foreign material, such as seeds, reduce the value of the fibre, and in some cases might even require the wool to be discarded (Chappell, n.d.). Other factors which have an impact on the price of wool, include length, strength and medullation.

Wool fibres are hygroscopic, which indicates that they absorb moisture in the form of vapour (Zhou et al., 2007). In contrast to this, they repel liquid moisture, due to the scales on the cuticle which are hydrophobic (Zhou et al., 2007). Kadolph et al.

(1993) add that, due to the hygroscopic nature of wool fibres, they can absorb a great deal of moisture without feeling wet. Wool has the ability to absorb up to 30% of its own weight in moisture without feeling wet (Zhou et al., 2007).

Therefore, wool has the highest moisture absorption rate of any natural fibre or general synthetic fibre (Ghosh, 2004 and Zhou et al., 2007). As a result, wool clothing will generally keep the wearer dry, as it will take a while for water or rain to penetrate the fabric (Zhou et al., 2007). According to Ghosh (2004), the epicuticle, which is the outer layer of the cuticle, is the only non-protein part of the fibre that protects the fibre by acting as a water-repellent membrane, see Figure 25. Water vapour, as opposed to liquid water, however, can transfer from the external environment into the fibre, and vice versa through microscopic pores located on the epicuticle (Ghosh, 2004 and Zhou et al., 2007). As a result, wool fabrics have the ability to absorb perspiration in the form of vapour, without feeling wet or having a wet appearance, and will release the absorbed water vapour into the air for the purpose of maintaining a balance in moisture (Ghosh, 2004).

70 Figure 25: The structure of a wool fibre

(Source: The Woolmark Company, 2012)

The crimp of wool fibres prevent them from being aligned too close to one another which results in the entrapment of air in the air pockets (Ghosh, 2004). The insulation capability of wool fabrics is essentially due to these air pockets, as opposed to the properties of the wool fibre per sé (Ghosh, 2004). Mishra (2002) mentions that the crimp and cross-sectional shape of wool fibres will differ according to the different wool breeds and types.

Zhou et al. (2007) commented that wool fabric is versatile, being worn in both cold and warm environments. Mehta and Harnett (1981) state that the thermal insulation of a fibre decreases when it takes up moisture, as is the case with wool fibres. Nevertheless, it is important to note that this will not necessarily have a major impact on the thermal insulation of a garment, since the thermal insulation of a fabric or garment depends on the air trapped in the fabric (Mehta and Harnett, 1981).

Garments containing wool act as a buffer between the wearer and the external environment, due to their high heat releasing and heat absorption properties, which

Cuticle Cell membrane complex

Macrofibril

Matrix Microfibril

Left-handed coiled coil rope

Right-handed -helix

Para cell and ortho cell cortex

71 protect the wearer from sudden changes in climatic conditions (Mehta and Harnett, 1981 and Zhou et al., 2007). According to Mehta and Harnett (1981), the evaporation of perspiration absorbed by wool fibres is relatively slow, due to the fact that wool does not wick water readily. Therefore, heat loss is spread out over an extended period, which allows enough time for the regulatory system of the human body to adapt (Mehta and Harnett, 1981).

Mehta and Harnett (1981) summarised the main reasons for using wool to ensure comfort in both summer and winter clothing, as follows. Firstly, wool prevents damp sensations of clothing during hot summer days, due to the moisture absorbency of the fibre. In addition, the fabric removes perspiration and ensures cooling of the body by its evaporation at the skin. During winter months, thick wool fabrics, with a tightly woven construction, provide not only insulation, but wind protection as well.

Furthermore, the hygroscopic nature of the wool fibre facilitates the escape of perspiration. One of the major advantages of wool garments is that it protects the wearer during sudden changes in climatic conditions.