Dimensional Property
Ratio of size change compared to the original dry dimensions of a wool or hair-based fabric. In textiles, wet hygral expansion occurs when the internal structure of the fibre swells, forcing the yarn to bend and the weave to expand. This movement is a predictable physical reaction to moisture absorption.
Moisture Response
Humidity levels in the atmosphere or direct immersion in water trigger this physical change. When wet hygral expansion takes place, the fabric can grow by several percentage points in both directions. This movement is a result of the cortical cells in the wool fibre absorbing water and expanding in diameter.
Internal Pressure
Force within the cortex pushes against the outer cuticle during hydration. As wet hygral expansion continues, the increase in fibre width reduces the space available between yarns in the fabric structure. This lack of space forces the yarns into a more crimped path, which increases the external dimensions of the piece.
The effect is measurable in both knitted and woven wool products.
Drying Recovery
Removing the water usually returns the fabric to its original dimensions as the fibres contract. However, wet hygral expansion can cause problems during garment assembly if the fabric is not conditioned to the factory environment. If pieces are cut while the fabric is expanded, they will shrink and become too small once they reach the dry air of a retail store.
