Fibre Behaviour
Moisture absorption causes natural and regenerated fibers to expand in volume when exposed to water. The transverse swelling represents the percentage increase in a fiber’s diameter or cross-sectional area as it absorbs water molecules into its amorphous regions. This expansion occurs primarily perpendicular to the fiber axis because the crystalline regions prevent longitudinal expansion.
It is a fundamental property of hydrophilic fibers like cotton, viscose, and wool. This expansion can cause the yarns to become thicker, which alters the air permeability and thickness of the entire fabric during wet conditions.
Moisture Absorption
Water molecules penetrate the intermolecular spaces of the fiber, pushing the polymer chains apart. This action increases the fiber’s thickness without significantly changing its length. Regenerated cellulose fibers such as viscose exhibit high swelling rates compared to synthetic fibers like polyester, which absorb almost no water.
This difference in behavior affects how different textiles react to washing.
Fabric Structure
Yarn diameter increases as the individual fibers swell, which reduces the space between yarns in a woven or knitted fabric. This phenomenon blocks the pores in the fabric, making it more windproof and water-resistant when wet. Activewear and outerwear designs sometimes utilize this behavior to create self-sealing fabrics that protect against rain.
The change in fabric density also affects the drying rate.
Performance Limit
Finishing mills must account for this behavior when applying chemical coatings or dyes. Excessive swelling during wet processing can cause the fabric to become stiff and distort its shape. Consequently, mills must control the tension on the fabric to prevent permanent distortion during drying.
