Page 272 - Handbook of Properties of Textile and Technical Fibres
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246 Handbook of Properties of Textile and Technical Fibres
vegetable fibers. In contrast, polyester fiber is a nonhygroscopic fiber. The two key
parameters characterizing the moisture level in a fiber are the moisture regain and
the moisture content (Booth, 1968; Hamby, 1949; Morton and Hearle, 1975; Hearle
and Peters, 1960). Moisture regain is defined as the weight of water in a material
(W) expressed as a percentage of the so-called “oven dry” weight (D), which is defined
as the constant weight obtained by drying at a temperature of 105e110 C
(220e230 F):
Moisture Regain ð%Þ¼ 100 W=D
Moisture content is the weight of water in a material expressed as a percentage of
the total weight (i.e., oven dry weight plus water weight):
Moisture Content ð%Þ¼ 100 W=ðD þ WÞ
The main factor influencing the regain or moisture content of cotton is the relative
amount of water in the surrounding air, known as the relative humidity. Relative
humidity is defined as the ratio of the absolute humidity of the air to that of air
saturated with water vapor at the same temperature and pressure.
Moisture is a close partner to cotton fiber from the field to the end product. Indeed, it
starts in the field when cotton is only a living plant cell as the growing cotton boll,
which is typically filled with water, the main reason for growth. After the boll opens,
seed cotton fibers will react immediately to the ambient humidity; it will initially lose
some water and then reach an equilibrium stage depending on the relative humidity of
the surrounding environment. Not only the fiber is hygroscopic, but the cotton seed is
also hygroscopic, but they have different moisture capacities. For instance, if the seed
cotton is subjected to a relative humidity of 50% and temperature of about 70 F
(21 C), the fiber moisture content will drop to about 6%, whereas the seed moisture
content will drop to about 9%. Accommodation of this difference in moisture capacity
represents one of the secret recipes for better ginning of cotton fibers. Earlier in this
chapter, the cellulosic structure of cotton fiber was described (over 95% cellulose).
It was also mentioned that cotton fiber contains about 60%e80% crystalline regions,
that are typically held together by the hydroxyl groups. Therefore, the water has to
penetrate the cotton fiber structure through the spacing between the crystal lattice
planes, or the amorphous regions. When cotton is initially dry, water molecules will
form hydrogen bonds with hydroxyl groups that are not already linked within crystal-
line regions. As humidity is increased, water will be attracted to the accessible hydrox-
yl groups, which they will normally be located on the surface of crystalline fibrils.
The standard relationships of moisture absorption are shown in Fig. 7.8. Fig. 7.8(a)
illustrates the moisture regainerelative humidity relationship. As shown in this figure,
the initially wet preconditioned cotton sample always exhibit higher moisture regain
than the initially dry preconditioned sample at all levels of relative humidity.
Fig. 7.8(b) shows the standard regainetime relationship for two cotton samples, one
is initially dry and the other is initially wet. The former absorbs water (its regain
increases) and the latter loses water (its regain decreases). Eventually, each sample
approaches equilibrium. Normally, the initially wet sample will exhibit higher regain