29/06/2025
Why Wool Swells and Shrinks When Washed and Not in Dry Cleaning
Wool is made of protein, primarily keratin, which is a long-chain molecule composed of repeating amino acids. These amino acids have polar side groups, such as:
–COOH (carboxyl group)
–NH₂ (amino group)
–OH (hydroxyl group)
These polar groups can form hydrogen bonds with water molecules.
Water (H₂O) is a polar solvent, meaning it has a slightly negative oxygen atom and slightly positive hydrogen atoms. These charges allow water molecules to form hydrogen bonds with the polar groups in wool. This interaction allows water to pe*****te the wool fiber, disrupting internal bonds and causing the wool to swell. Upon drying or agitation, the fibers can shrink and felt due to fiber movement and re-bonding.
On the other hand, the dry cleaning solvent perchloroethylene (Perc, C₂Cl₄) is a non-polar solvent, meaning its electrons are evenly distributed with no charge separation. As a result, it cannot form hydrogen bonds with wool and does not pe*****te the fiber. Perc remains mostly on the surface, allowing it to clean without causing swelling or shrinkage.
