Page 22 - Handbook of Properties of Textile and Technical Fibres

Introduction to the science of fibers 3

for many applications because of their inherent properties, such as water uptake mak-
ing clothes more comfortable, but also because they are renewable and do not depend
on nonrenewable material sources, which is usually the case for synthetic fibers. This
latter, environmental friendly characteristic, is turning peoples’ attention to the use of
natural fibers to replace, in some cases, synthetic fibers as reinforcements for some fi-
ber composite structures such as the use of flax in body panels for some cars. These
changes are changing the fundamental nature of textile technology, which is once
again at the forefront of industrial development.

1.2 Units of measure for fibers and their structures

The small diameters of most fibers has presented particular challenges to the fiber in-
dustry that have led to ways of defining fibers and their properties that are different
from those used in traditional engineering materials. The concerns are the same.
How is it possible to normalize characteristics, such as strength and stiffness, so
that fibers can be compared? With most engineering materials it is the Hooke’s law
that shows the way of comparing the failure stresses and intrinsic stiffnesses given
by the Young’s moduli of materials. Specimens can be compared by normalizing
the applied force by dividing the cross-section of the specimen to obtain the stress
and relating it to the strain, induced by the stress, which is the increase in length
divided by its original length. This cannot easily be done with fibers as they are
very fine and often, particularly in the case of many natural fibers, of irregular
cross-section, so their cross-sections cannot be easily measured. Even the best optical
microscopes are of little help because their resolving powers are largely limited by the
wavelength of light, about half a micron. Advances in optical microscopy are offering
ways of overcoming these limitations as is described in Chapter 2. Nevertheless, it is
the scanning electron microscope that is most often used to observe the fibers in great
detail. This is possibly due to the very short wavelengths of electrons, compared to
those of visible light, when they act as waves. However, observation by scanning elec-
tron microscopy is not always possible and because the specimens have to be prepared
for observation, it is not a very rapid technique.
The textile community developed units that avoided measuring the cross-section of
the fiber. The traditional unit of definition for fibers has been the “denier,” which is the
weight of the fiber or fiber assembly as a function of length. One denier is 1 g/9 km.
The denier is still in wide use but has been replaced as an international unit by the
“tex,” which is 1 g/km. This means that the tex is a less fine unit than the earlier denier
and for this reason the unit that is often used is the decitex (dtx), 1 g/10 km, not so
different from the denier.
Strength is given as the force to produce failure (in grams) per textile unit (denier or
tex). This can be seen to be related to traditional engineering units of strength as it is
equal to the force multiplied by the length and divided by the weight:

Force length/weight ¼ Force length/(volume density)
¼ Force length/(length cross-section density)
¼ Force/(cross-section density).