Wool.pdf

546 WOOD COMPOSITES

Vol. 12

WOOL

Raw Wool Speciﬁcation

Wool is the ﬁbrous covering from sheep (1) and is by far the most important

animal ﬁber used in textiles. It appears to have been the earliest ﬁber to be spun

and woven into cloth. In 2000–2001, world greasy wool production was 2.3

10 9

kg from 1

10 9

sheep, which is equivalent to 1.4

10 9

kg of clean wool (2)

10 9 kg in 1989–1990.

Wool belongs to a family of proteins, the keratins, that also includes hair

and other types of animal protective tissues such as horn, nails, feathers, beaks,

and outer skin layers. The relative importance of wool as a textile ﬁber has de-

clined over the past decades with the increasing use of synthetic ﬁbers for textile

products. Wool, however, is still an important ﬁber in the middle and upper price

ranges of the textile market. It is also an extremely important export commodity

for several nations, notably Australia, New Zealand, South Africa, and Argentina,

and commands a price premium over most other ﬁbers because of its outstanding

natural properties. These include soft handle (the feel of the fabric), water absorp-

tion (and hence comfort), and superior drape (the way the fabric hangs). Table 2

shows wool production and sheep numbers in the world’s principal wool-producing

countries.

The principal characteristics of clean wool types are average diameter, mea-

sured in micrometers (referred to as microns), and average length, measured in

millimeters. Essentially all ﬁne diameter wool is produced by merino sheep or

merino crossbreeds. Over 75% of the sheep in Australia (the world’s largest wool

producer) are merino sheep, which are also bred in large numbers in South Africa,

Argentina, and the former USSR. The softness, ﬁneness, and lightness of fabrics

is determined primarily by ﬁber diameter, and so the price is very sensitive to the

mean diameter (3) (Fig. 1).

(Tables 1 and 2). This is down from a peak of 2.0

Vol. 12

WOOL 547

Table 1. World Production (10 6 kg) of Wool (2000/2001) a

Source

Fiber diameter, µ m

Greasy

Clean

Merino

24.5

932

572

Crossbred

24.6–32.5

540

308

Other (carpet)

> 32.5

852

494

Total

2324

1374

a From Ref. 2.

Medium diameter wool includes sheep breeds of English origin, eg, south-

down, hampshire, dorset, and cheviot, as well as crossbreds, eg, columbia, targhee,

corriedale, and polwarth, from interbreeding with merinos. Coarse diameter wool

comes from sheep chieﬂy bred for meat, eg, lincoln, cotswold, and leicester.

Raw wool from sheep contains other constituents considered contaminants

by wool processors. These can vary in content according to breed, nutrition, en-

vironment, and position of the wool on the sheep. The main contaminants are a

solvent-soluble fraction (wool grease), protein material, a water-soluble fraction

(largely perspiration salts, collectively termed suint ), dirt, and vegetable matter

(VM) (eg, burrs and seeds from pastures).

In buying raw wool, wool processors are concerned about its quality, the

quantity of pure ﬁber present, and its freedom from contamination (4). For the

ﬁne and medium wools used for apparel, the major characteristics are average

ﬁber diameter, yield (ie, the percentage content of pure ﬁber), content and type of

VM, average ﬁber length, strength of ﬁber staples, and the position of any weak

spot along the ﬁbers. For very ﬁne wools, the frequency and clarity of waves (crimp)

in the staples has a signiﬁcant effect on price. The range of ﬁber diameters, color

of the clean wool, and the number (if any) of naturally colored ﬁbers present can

also be important. For carpet-type wools (long wools), the important properties

Table 2. Wool Production and Numbers of Sheep in Principal Wool-Producing

Countries (2000/2001) a

Wool production (greasy)

Country/region

10 6

No. of sheep, (10 6 )

Australia

652

28.1

113

China

291

12.5

135

New Zealand

258

11.1

45.3

South Africa

2.2

17.5

Argentina

2.7

13.4

Uruguay

2.5

13.0

United Kingdom

2.7

27.6

Turkey

3.0

30.2

Iran

3.2

53.9

Former USSR

128

5.5

49.2

Others

620

26.5

508

Total

2324

100

1006

a From Ref. 2.

548 WOOL

Vol. 12

Fig. 1. Dependence of Australian wool prices on ﬁber diameter (1999/2000) (3).

(5) are yield, ﬁber diameter, ﬁber length, color, bulk (the volume occupied by the

ﬁbers in a yarn), and VM content. Also important for coarse wools is the degree

of medullation. This is associated with cells containing air, arranged along the

ﬁber axis. The presence of medulla cells increase light scattering by the ﬁbers,

restricting the use of these wools for some purposes.

Until the early 1970s, the characteristics of different wools were largely eval-

uated visually by wool classers and valuers. With the development of sampling

techniques and equipment capable of rapid and economical measurement of yield,

diameter, and VM (6,7), objective measurement and the sale of wool by sample

became dominant in major wool-exporting countries. In sale by sample, cores are

drawn from each lot and tested for yield, diameter, and VM content in accordance

with international standards (8,9). Measurements of staple length, strength, and

position of weakness are also now in routine commercial use. In addition, a full-

length display sample, representative of each lot and obtained by standard proce-

dure (10), is available for buyers to appraise other characteristics. Sale by sample

decreased costs by reducing the handling of bulk wool in wool-brokers’ stores and

selling operations. It also enabled the processing performance of wool to be pre-

dicted in topmaking (11) and spinning (12).

Fiber Characteristics

New instrumentation for measuring ﬁber diameter (13,14) has meant that data

on the range of diameters present (CV D ) and ﬁber curvature (related to crimp

frequency) are now available. The impact of these is fairly well established (15).

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WOOL 549

These instruments have also been introduced on-farm so that the ﬂeece quality of

each animal can be assessed from a mid-side (16) or whole-ﬂeece (17) sample. In

some cases it is possible to gain increased returns from separating out the ﬁnest

ﬂeeces but bigger gains are possible from accelerating the rate of genetic progress.

A remaining objective of research is to facilitate the introduction of a system of

sale of raw wool by description in which a sale sample will not be required for

inspection.

Fiber Growth. Wool ﬁbers are produced from multicellular tube-like struc-

tures known as follicles. These follicles are located in the skin layers (dermis and

epidermis) of sheep, and two types of follicles, primary and secondary, are usu-

ally identiﬁed. Primary and secondary follicles are described from the order of

their initiation and development in foetal skin. The primary follicles develop ﬁrst,

in the unborn lamb. Secondary follicles develop later and in ﬁner wooled sheep

derived secondary follicles subsequently form by branching from the original sec-

ondaries, with which they share a common oriﬁce (18). Each primary follicle has

a sebaceous gland and a sweat gland together with an arrector muscle, whereas

secondary follicles usually have only an associated sebaceous gland (19).

Fiber Morphology. Wool ﬁbers consist of cells, where ﬂattened overlap-

ping cuticle cells form a protective sheath around cortical cells. A scanning elec-

tron micrograph of a clean merino wool ﬁber is shown in Figure 2. In some coarser

ﬁbers, a central vacuolated medullary cell type may be present.

Fig. 2. Scanning electron micrograph of merino ﬁber, showing overlapping cuticle cells.

550 WOOL

Vol. 12

Fig. 3. Schematic of the structure of a ﬁne merino wool ﬁber.

m) and usually constitutes

about 10% by weight of the total ﬁber. Sections of cuticle cells show an internal

series of laminations (Fig. 3), comprising outer sulfur-rich bands known as the

exocuticle and inner regions of lower sulfur content called the endocuticle (20).

On the exposed surface of cuticle cells, a membrane-like proteinaceous band (epi-

cuticle) and a unique lipid component form a hydrophobic-resistant barrier (21).

These lipid and protein components are the functional moieties of the ﬁber surface

and are important in ﬁber protection and textile processing (22).

The cortex comprises the main bulk and determines many mechanical prop-

erties of wool ﬁbers (Fig. 3). Cortical cells are polyhedral, spindle-shaped, and

approximately 100

0.5

m long. They consist of a class of biological ﬁlaments known

as intermediate ﬁlaments (23) embedded in a sulfur-rich matrix. The intermediate

ﬁlaments (originally termed microﬁbrils), together with the matrix, are organized

into large macroﬁbrillar units and these are often observed in sections of cortical

cells. In ﬁne merino wool, two main types of cortical cell, known as ortho- and

para-, are arranged bilaterally. Orthocortical cells show different intermediate

ﬁlament/matrix packing arrangements from those of paracortical cells (24). The

arrangement of ortho- and paracortical cells differs among wool types. For ex-

ample, in lincoln wool an annular (orthocortical core surrounded by paracortex)

cellular arrangement is present. Merino ﬁbers possess a characteristic crimp and

in these ﬁbers the orthocortex is located on the outer side of the crimp curvature.

A continuous intercellular material is present between cuticle and cortical

cells which, despite being a relatively minor fraction of the total ﬁber weight, is of

In ﬁne wool, such as that obtained from merino sheep, the cuticle is nor-

mally one cell thick (approximately 20

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