Why do some fabrics wrinkle and others don't? Close examination of this question reveals a relationship between moisture absorption and wrinkling. Wool and cotton fabrics wrinkle, both fibers absorb water. Polyester and nylon fabrics are more restive to wrinkling; they absorb much less than the other two.
To
understand wrinkling of cellulose fibers, consider the stress and strain forces
within a smooth rod as it bent - see figure 43. The stretching stresses at the outer
reaches of the bend provide lateral forces to adjacent polymer chains which can
cause them to move. The inner polymer chains are under compression stresses –
also
providing lateral forces. A large diameter rod, because the radius of
curvature is bigger, will exhibit greater strains than a thinner rod.
Under
stress, the hydrogen bonds between adjacent cellulose chains can break allowing
the chains to slip past each other. New H-bonds can form as the hydroxide groups
re-associate with different partners. Having done so, there are no forces to pull
the neighboring chains back to their former position. The stressed shape of the
fiber is just as stable now as was the original shape.
Bending Forces in a Thin Rod
Bending forces in a Thick Rod
Water will facilitate the disruption of these
bonds by forming H-bonds between the water molecule and the cellulose hydroxides. Now the hydrated polymer chains can slide past each other more
easily so the strain level to deform the fiber is even lower than before. Water
acts as a lubricant allowing the chains to slide past one another easily.
Illustrative of this point is ironing wrinkles out of a cotton garment. It is
impossible to iron out wrinkles from a dry garment. However, when the garment is
dampened first or if a steam-iron is used, the wrinkles will iron out and the
fabric becomes flat and smooth.
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