Page 79 - Handbook of Properties of Textile and Technical Fibres

60 Handbook of Properties of Textile and Technical Fibres

(1969a,b). Less effort has gone into failure properties, although Reis (1992) has
reviewed the variations in the strength (breaking stress) of wool fibers.
The first part of this chapter elucidates the complex chemical and physical struc-
tures of wool that was the focus of much of this early work. The next section summa-
rizes the models and theories of the strength of wool, which were developed during the
1960s and are being refined still today. This is followed by a section on methods of
measurement, focusing particularly on the specific challenges that wool brings, viz.,
its nonuniformity and moisture dependence. The heart of the chapter deals with the ef-
fect of processing and environmental conditions on the tensile failure properties of
wool, in particular the effects of torsion, abrasion, moisture, temperature, rate of
testing, ultra violet (UV) light, dyeing, and setting. Finally, there are sections dealing
with applications, sources of further information and advice, and a comprehensive list
of references.

3.2 Structure of wool

3.2.1 Chemical

Wool is a semicrystalline, proteinaceous polymer, part of the family of proteins called
a-keratins, which also include materials such as hooves, horns, claws, and beaks
(Rippon, 1992). The basic building blocks of proteins are amino acids, and extensive
research has shown that wool is made up of 18 a-amino acids with typical percentages
as shown in Table 3.1. Amino acids have the general structure H 2 NeCH(R)eCOOH,
where R represents the side group of the amino acid. Multiple amino acids condense by
reaction of adjacent amino and carboxyl groups to form proteins or polypeptides with a
general structure e(NHCHRCO) n e. In wool, the 18 amino acids combine in many
different ways leading to about 170 different types of polypeptides varying in relative
molecular mass from less than 10,000 to greater than 50,000 Da (Zahn and Kusch,
1981; Gillespie, 1990).
The side groups of the amino acids vary markedly in size and chemical nature and
play an important role in the physical and chemical properties of the wool fiber. The
low-sulfur proteins, containing a high proportion of amino acids that contribute to
a-helix formation (glutamic acid, aspartic acid, leucine, lysine, arginine), assemble
into rod-like intermediate filaments (microfibrils). These crystalline microfibrils
make up approximately 25%e30% of the dry fiber (Feughelman, 1989) and play a
dominant role in the tensile properties of the fiber, particularly when wet. The micro-
fibrils are embedded in a matrix that consists largely of high-sulfur proteins, rich in
cysteine, proline, serine, and threonine, and high-glycine, high-tyrosine proteins,
which are also rich in serine.
The large number of polar groups present in wool means that it has a strong affinity
for water, taking up 34%e37% (Speakman and Cooper, 1936; Warburton, 1947; Watt
and D’Arcy, 1979) by mass of water on going from completely dry to wet (Fig. 3.2).
The water is believed to go mainly into the amorphous matrix, the tightly packed crys-
talline microfibrils preventing water from entering, although some X-ray evidence has