Moisture Absorption, Desorption and Swelling

Moisture Absorption, Desorption and Swelling

Moisture sorption is a physicochemical phenomenon. The hydroxyl groups of the carbohydrate fraction attract water molecules and attachment occurs due to hydrogen bonding. In a bone-dry fiber, water molecules can diffuse in to form such bonds ini-tially. Late-arriving molecules can attach themselves either to existing water molecules (indirect attachment) or to the other hydroxyl groups (direct attachment). Diffusion of water molecules does not take place in the crystalline regions of native cellulose in the fiber because of the relatively tight packing of the molecules therein. Thus, absorp¬tion of water is proportional to the extent of the fiber’s noncrystalline, less-oriented regions. Since hemicelluloses and polyuronides are not crystalline, their presence in the fiber will increase its regain. On the other hand, the presence of lignin decreases moisture absorption, since lignin is hydrophobic. In addition, layers of lignin in the inner middle lamella and close to the fiber surface will hinder penetration of moisture into the cellulosic cell wall.

According to Lewin, the small affinity of the lignin for moisture was, according to these investigators in accordance with the small changes in swelling behavior of the jute fiber after a considerable part of the lignin was removed. This is also the obvious explanation of the high moisture absorption in flax in spite of its well known high degree of crystallinity. If it is assumed that the regain of nonlignin middle lamella components in jute and flax is similar, it follows that, in the light of the much lower lignin content of flax, a higher regain should be expected for flax than jute. The lower crystallinity of a-cellulose in jute explains this discrepancy. It should be pointed out that the information on the contribution of the middle lamella and its constituents toward moisture absorption in bast and leaf fibers, other than flax and jute, is very scarce and limited.
Conventionally, the moisture sorption characteristics of fibers are depicted by ab-sorption/desorption isotherms. In practice, it is difficult to compare isotherms of dif-ferent fibers over the whole range of rh (relative humidity) and temperature values. It is often sufficient to compare their regains at the standard conditions of 65% rh and 21 °C (70°F). List gives absorption regains for a few fibers, together with the differences between absorption and desorption regains.


Fiber Absorption regain (%)
At 65% rh 700 F Difference between
desorption and absorption regains 65% rh 700 F
Abaca 9.5 --
Sisal 11.0 --
Cotton 7-8 0.9
Hemp 8 --
Jute 12 1.5
Kapok 10 --
Ramie (bleached) 6 --
Wool (scoured) 14 20
Coir 10 --
Banana 15.2 --
Okra Bast Fiber 9 1.2