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 1. Moisture absorption behavior of Textile fibres  
1.1 Introduction  
Most fibers tend to absorb moisture (water vapor) when in contact with the 
atmosphere. The amount of water absorbed by the textile fiber will depend on the 
chemical and physical structure and properties of the fiber, as well as the 
temperature and humidity of the surroundings. The percentage absorption of water 
vapor by a fiber is often expressed as its moisture regain. The regain is determined by 
weighing a dry fiber, then placing it in a room  set to standard temperature and 
humidity (210 ± 10C and 65% relative humidity [RH] are commonly used). From these 
measurements, the percentage moisture regain of the fiber is determined:  
                  (                             )
                        Equation 1 
 
Percentage moisture content of a fiber is the percentage of the total weight of the 
fiber which is due to the moisture present, and is obtained from the follow ing 
formula:  
                                                           
                  x 100%   Equation 2 
The percentage moisture content will always be the smaller of the two values.  
Fibers vary greatly in their regain, with hydrophobic (water -repelling) fibers having 
regains near zero and hydrophilic (water -seeking) fibers like cotton, rayon, and wool 
having regains as high as 15% at 21°C and 65% RH.  
The moi sture condition of the material is one of the most important factors in 
determining its electrical properties; ‘static’ is much less likely to occur in damp 
conditions. The above examples show the technological importance of moisture 
absorption in fibres. There is also a direct commercial interest. In 100kg of raw 
cotton, for example, there may be up to 12kg of water. Since it is expensive to pay 
for this at the price of raw cotton, it must be allowed for in calculating the weight to 
be charged.  
The ability  of fibers to absorb high regains of water affects the basic properties of the 
fiber in end -use. Absorbent fibers are able to absorb large amounts of water before 
they feel wet, an important factor where absorption of perspiration is necessary. 
Fibers with  high regains will be easier to process, finish, and dye in aqueous solutions, 2 
 but will dry more slowly. The low regain found for many man -made fibers makes them 
quick drying, a distinct advantage in certain appl ications.  
Fibers with high regains are ofte n desirable because they provide a "breathable" fabric 
which can conduct moisture from the body to the outside atmosphere readily, due to 
their favorable moisture absorption -desorption properties. The tensile properties of 
fibers as well as their dimension al properties are known to be affected by moisture . 
The moisture absorbency and comfort of a fiber is related to its chemistry and 
morphology and to the way it absorbs, interacts with, and conducts moisture. In 
addition, comfort is related to the yarn and fabric structure into which the individual 
yarns have been made.  
1.2 Moisture regain of different types of fibres  
 
cotton  
The superior absorbency of cotton, coupled with its ability to desorb moisture, makes 
it a very comfortable fiber to wear. This absorbency  permits cotton to be used in 
applications where moisture absorption is important, such as in sheets and towels. 
Mildewing of cotton under hot moist conditions and its slowness of drying are 
undesirable properties associated with its high affinity for wate r. 
Jute 
Jute fibers are stiff, but it does have an unusually high moisture regain .  
Rayons  
Owing in part to the more extensive networks of amorphous regions  found in rayons, 
the moisture regain of rayon is significantly higher than  that of cotton. The regains of 
rayons under standard conditions range from  11% to 13%. The lower crystallinity and 
degree of polymerization of rayons  also affect the way water acts on the fibers. 
Rayons as a consequence are  swollen to varying degrees due to increased 
susceptibility  to water penetration. During wetting rayon may increase up to 5% in 
length and swell up  to double its volume. Rayon possesses excellent moisture 
absorbency characteristics, but the weaker rayons sw ell and deform somewhat in the 
presence of moisture.  
Acetate  
Acetate and triacetate have moisture regains of 6.0% and 4.5%, respectively. On 
heating, the moisture regain of triacetate drops further to 2.5%.  
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 Wool 
The moisture regain of wool is very high an d varies between 13% and 18% under  
standard conditions. At 100% RH, the regain approaches 40%. Wool fibers are highly 
absorbent and have excellent moisture transmission properties.  
Nylon  
Nylons are moderately hydrophil ic with a  moisture regain of 4% -5% und er standard 
conditions and a regain of 9% at  100% relative humidity.  
Aramid  
The aramids have moisture  regain comparable to the other polyamide fibers and are 
in the 3.5% -7% range. The fibers are swollen and dissolved in polar aprotic solvents or 
strong aci ds. The aramids have high heat and electrical resistivity , have excellent 
insulative capabilities, and are unaffected by heat up to 250°C.  
Polyester  
Polyester  fiber is quite hydrophobic, with a moisture regain of 0.1% -0.4% under 
standard conditions and 1.0 % at 21°C and 100% RH. It is swollen or dissolved by 
phenols, chloroacetic acid, or certain chlorinated hydrocarbons at elevated 
temperatures. The fiber exhibits moderate heat conduct ivity and has high resistivity, 
leading to extensive static charge buildu p. On heating, the fiber softens in the 210° -
250°C range with fiber shrinkage and melts at 250° -255°C.  
Acrylic  
Acrylic fibres have low moisture regains of 1.0% - 2.5% under standard temperature 
and humidity conditions. The fiber is soluble in pol ar aprotic solvents such as N -N-
dimethylformamide. The fiber exhibits good heat and electrical insulation properties. 
Acrylic fibers do build up moderate static charge and soften at 190° -250°C.  
Spandex  
Owing to the more hydrophilic nature of spandex, its mois ture regain varies between 
0.3% and 1.3% under standard conditions. Although  spandex is swollen slowly by 
aromatic solvents, it is generally unaffected  by other solvents. Spandex is a heat 
insulating material and shows poor  heat conductivity. It softens be tween 150°C and 
200°C and melts between  230°C and 290°C. It has moderate electrical resistivity and 
builds up some  static charge under dry conditions.  
 
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 Glass  
The surface of glass fibers can be wetted by water, but otherwise the fibers have 
essentially no af finity for water. As a result the moisture  regain of these fibers is 0.5% 
or less. Glass fibers are not soluble in common organic solvents but can be slowly 
dissolved by concentrated aqueous base and more rapidly by hydrofluoric acid. Glass 
fibers exhibit excellent heat and electrical insulation properties and are not affected 
by heat up to their melting point of 750°C or above.  
1.3 Swelling  
When fibres  absorb water, they change in dimensions, swelling transversely and 
axially. This has technical consequences in the dimensional stability of fabrics, the 
predominant transverse swelling usually resulting in shrinkage of twisted or interlaced 
structures. It  also means that the pores of closely woven fabrics will be completely 
blocked when the fibres are swollen, and they may then be impermeable to water. 
This principle is utilised in hosepipe materials and the Ventile fabrics, which were 
developed in the 194 0s for showerproof garments. Swelling is also an important factor 
in crêpeing, due to the increased twist angle in a swollen yarn, and in drying and 
dyeing. Swelling is akin to solution in that there is an interchange of position between 
fibre molecules an d water molecules, but in swelling this occurs only to a limited 
extent, whereas in solution it continues until there is a uniform mixture of the two 
substances.  
The swelling may be expressed in terms of the increase in diameter, area, length or 
volume as illustrated in Figure 1. This leads to the following quantities : 
 
 
Figure 1 Changes in fibre dimensions on swelling.  
Transverse diameter swelling = fractional increase in diameter  
 
                                 
    Equation 3         
5 
 Transverse area swelling = fractional increase in area of cross -section  
 
                                 
      Equation 4    
Axial swelling = fractional increase in length  
 
                                 
      Equation 5    
 
Volume swelling = fractional increase in volume  
 
                            =   
      Equation 6   
 
In practice, these quantities are often  expressed as percentages rather than fractio n. 
Relations between them reduce the number of independent parameters to two, 
though these relations may be affected by the fibre shape. For instance, in a fibre 
that is uniform along its length, we have:  
                                                                                                                                Equation 7 
     (    )(     )                                                                                    Equation 8 
   =   
  = [      
  ]    
    
      
                        Equation 9 
Where V = volume, A = area of cross -section and l = length.  It can be similarly shown 
that, for a fibre of circular cross -section : 
                                                                                                                         Equation 10 
 
1.4 Moisture  sorptio n 
Adsorption in a non -swelling medium, for example, the adsorption of gases on 
charcoal, is a comparatively simple process, and so is the solution of one substance in 
another, for example the solution of sugar in water, but the absorption of water by 
fibres is an example of a process that comes midway between these two and partakes 
of some features of each. It encompasses not only the relation between regain and 
humidity but also associated phenomena, such as hysteresis, heat effects, the 
variation of regai n with temperature, the influence of moisture on physical 
properties, and all the complicated factors arising from the interaction of moisture 
and mechanical effects owing to the limited swelling of fibres.  We shall  look at a 
broad description of the way in which water is absorbed.  6 
 1.4.1  The effect of hydrophilic groups  
In considering absorption, we must take account of the interaction between the water 
molecules and the molecules of the fibre substance. All the natural animal and 
vegetable fibres (and the fibres regenerated from natural materials) have groups in 
their molecules that attract water. For example, the cellulose molecule contains 
three hydroxyl groups for each glucose residue, and hydrogen bonds can be formed 
between water molecules and the hydr oxyl groups (see Fig. 2). 
 
Figure 2  Absorption of water by hydrogen bonding to hydroxyl groups in a cellulose molecule.  
The molecular weight of water is 18, and that of the glucose residue is 162, so that if 
one water molecule wer e attached to each hydroxyl group the re gain would be 33.1%. 
In fact,  not all the hydroxyl groups are involved, and there may be more than one 
water molecule per hydroxyl group.  
In cellulose acetate, all or most of the hydroxyl groups have been replaced b y the 
comparatively inert acetyl (CH3·COO —) groups. These groups do not attract water 
strongly, so the absorption of water by acetate is low. In particular, there is no rapid 
rise at low humidities owing to the initial absorption on strongly attractive gro ups. 
The protein fibres contain amide groups ( —NH—) in the main chain, to which water 
can be hydrogen bonded, and other water -attracting groups such as —OH, —NH , 3 + —
COO–, —CO·NH2, in the side chains. Wool contains many active groups in the side 
chains, but silk contains only a few . 
All the synthetic fibres so far produced contain few if any water -attractive groups, 
and this accounts for their low moisture absorption. The polyamide fibres, nylon 6.6 
and 6 and aramids, contain one amide ( —NH—) group for e very six carbon atoms in the 
chain, which would give a regain of 16% of each amide group held one water 
molecule.  
The polyester fibres , polyethylene terephthalate, are composed only of benzene 
rings, —CH2— groups, and —CO·O — groups, none of which attracts water strongly. 
Polyethylene is simply a -CH2-chain, polypropylene has additional –CH3 side groups, 
and the vinyl fibres are similar e xcept for the substitution of —Cl, —O·CO·CH3, or 
other comparatively inert groups for some of the hydrogen atoms, and consequently 
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 these fibres absorb little water. Acrylic fibres, containing —CN groups and other 
groups from the minor components, absorb sl ightly more than the other vinyl fibres, 
and polyvinyl alcohol, containing some —OH groups, absorbs still more.  
Inorganic fibres, including carbon, do not attract water absorption.  
1.4.2  Directly and indirectly attached water  
The first water molecules must be ab sorbed directly onto the hydrophilic groups, but, 
there is a choice for those absorbed after the first. They may be attracted to other 
hydrophilic groups, or they may form further layers on top of the water molecules 
already absorbed. The resulting effect is illustrated in Fig. 3.  
 
 
 
Figure 3 Direct and indirect absorption of water molecules by a polymer molecule.  
The directly attached water molecules will be firmly fixed, fitting closely to the 
structure of the molecules. They will be limited in their movement. The indirectly 
attached water molecules will be more loosely held. Their arrangement is uncertain, 
but the dielectric propert ies of fibres suggest that they are not as free as the 
molecules in liquid water and are probabl y restrained to about the same extent as the 
water molecules in ice . 
1.4.3  Absorption in crystalline and non -crystalline region  
In crystalline regions, the fibre molecules are closely packed together in a regular 
pattern. The active groups form crosslinks betwee n the molecules, for example by 
hydrogen bonding in cellulose and keratin. Thus it will not be easy for water 
molecules to penetrate into a crystalline region, and, for absorption to take place, 
the active groups would have to be freed by the breaking of c rosslinks. In native 
cellulose, with the crystalline arrangement known as cellulose I, the Xray diffraction 
pattern is unchanged during the absorption of water by the fibre, which indicates that 
no water is absorbed in the crystalline regions. In regenerat ed cellulose, with a 
slightly less compact crystal structure known as cellulose II, there is a change of 
crystal structure on absorption. This is due to the formation of a hydrate, which 
probably contains one water molecule to every three glucose residues.  This would 
correspond to a regain of about 3.7% in the crystalline region (about 1% regain in the 
whole fibre). When the regenerated cellulose is wet, there is a further modification 
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 of the crystal structure owing to the formation of a hydrate with about three water 
molecules to every two glucose residues. The material easily accessible to moisture 
will be either the non -crystalline regions  or the surfaces of crystalline regions . It 
would then be expected that the regain at any particular relative humidity  would be 
proportional to the amount of this effectively non -crystalline material. Consequently, 
the ratios of regain at the same relative humidity for any two cellulosic fibres should 
be independent of the relative humidity, that is, the curves of regain against relative 
humidity should be the same shape and differ only in scale . 
The moisture absorption thus offers one way of estimating the effective crystalline/ 
non-crystalline ratio in cellulosic fibres.