Page 251 - Handbook of Properties of Textile and Technical Fibres

Tensile properties of cotton fibers: importance, research, and limitations 225

In a recent book authored by Dr. Elmogahzy (2009a), the concept of durability in
both traditional and industrial textiles was discussed from an engineering design view-
point, and the trade-off between comfort and strength for some natural fibers including
cotton fibers was addressed. The most significant finding of this book was that natural
fibers exhibit numerous inherent properties that are yet to be utilized, and the use
of chemical additives to finish products made from these fibers should be based on
avoiding any form of obscuring these inherent properties. Historically, humans were
attracted to natural fibers primarily because of their inherent nature, which could not
be entirely described using traditional testing methods. In addition, cotton textiles
have traditionally been designed and produced using craft approaches that are based
on the know-how and long experience followed by many trials and errors and exten-
sive revisions. In today’s market, more engineering approaches should be taken to
make products from 100% cotton or cotton/synthetic blends of optimum functional
characteristics.

7.2 The structural integrity of cotton fiber

Any study of the tensile behavior of cotton fibers should be based on understanding the
internal fiber structure of cotton. In general, cotton fiber is composed mostly of long-
chain carbohydrate molecule cellulose; about 95% cellulose (Duckett, 1975; El Gaiar
and Cusick, 1976; Cotton Fibers, Handbook of Fiber Chemistry, 1998). The internal
structure of the cotton fiber consists of cell walls in which crystalline microfibrils
are arranged in concentric multilayers of closely packed cellulosic substance (Duckett,
1975). Typically, the crystallinity of cotton fiber may range from 60% to 80% (by
X-ray diffraction analysis). This means that cotton is not entirely crystalline as a
certain fraction of cellulose molecules do not necessarily associate into crystallites
but rather into disordered or amorphous regions. A combination of crystalline and
amorphous regions typically provides good durability imposed by the ordered crystal-
line regions, and good flexibility and absorption characteristics imposed by the
presence of the amorphous regions.
Most studies divided cotton fiber structure into six main parts as shown in Fig. 7.1.
The first is the cuticle, or the “skin” of the fiber. This waxy and smooth layer contains
pectin and proteinaceous materials. The presence of this layer has a significant impact
on the smoothness and the handling of the cotton fiber during processing. It also
influences the interfiber friction in a bundle strength test and in the yarn. However,
it is a very thin layer, only a few molecules thick, which makes it vulnerable to
environmental effects, such as heavy rain and high temperature. On scouring, this layer
is removed, which explains the increase in fiberefiber friction and the increase in
the bundle strength of a scoured fiber bundle (Cotton Fibers, Handbook of Fiber
Chemistry, 1998). The second part of the structure is the primary wall, which is mainly
a cellulosic substance made up of a network of fine fibrils. The primary wall may be
visualized as a sheath of spiraling fibrils where each layer spirals about 20e30 degrees
around the fiber axis. The thickness of this wall correlates with the extent of maturity of
cotton fiber; the thicker the wall, the higher the maturity. In general, highly mature