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
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