Superwash
Also searched as: superwash wool, shrink-resist finish, shrink-resistant wool, machine-washable wool, Chlorine-Hercosett, chlorine-resin treatment, anti-felting finish
A shrink-resist treatment that degrades or coats the cuticle scales of wool so that the fiber can be machine washed with little felting shrinkage.
In detail
Wool felts because its cuticle scales create a directional friction effect: fibers slide more easily one way than the other, so agitation in warm, wet conditions ratchets them into a denser mat. Superwash processes reduce that effect in two ways. An oxidative step, classically acid chlorination, degrades the outer cuticle surface and makes it more hydrophilic; a polymer step then deposits a thin resin film that masks the scale edges. The combined chlorine-resin route is the traditional industrial method and can be applied to tops, yarn or garments.
The science and numbers
In the chlorine-resin route, wool is chlorinated under acidic conditions, which oxidizes the disulfide-rich surface proteins and strips part of the fatty outer layer; a cationic polyamide-epichlorohydrin resin is then applied and cured as a film that masks scale edges and narrows the friction difference between sliding with and against the scales. Because of its halogenated byproducts, research targets alternatives. Proteases alone act poorly on the hydrophobic outermost cuticle and can attack the cell membrane complex inside the fiber, costing strength. Pairing them with disulfide-reducing agents such as sodium thioglycolate, sodium sulfite or TCEP has given laboratory felting shrinkage of about 4 to 6% with limited strength loss. Performance is judged by area shrinkage after a specified number of machine-wash cycles.
A practical example
A sock yarn is sold in two versions from the same Merino top, one untreated and one superwash. A student washes a swatch of each in a warm machine cycle and measures area shrinkage. The untreated swatch shrinks and firms visibly, while the treated one stays close to size. The exercise shows that the finish, not the fiber diameter, is doing the work.
What to distinguish
Superwash describes a finish, not a breed or grade, and it changes more than washability: treated wool usually feels smoother, dyes differently and cannot be deliberately felted or fulled. The chlorine route generates environmentally harmful organic halogen byproducts, and chlorine-free alternatives carry their own trade-offs. A machine-washable label depends on a tested care regime, not simply on the word superwash.
Origins and history
Shrink-resist research intensified as domestic washing machines spread in the mid-twentieth century. The chlorine-resin method called Chlorine-Hercosett, named for the Hercosett resin used in its polymer step, became the traditional industrial process. Superwash became the trade shorthand for wool treated to survive machine washing, and recent work on enzymes combined with reducing agents aims to replace the chlorine step without sacrificing fiber strength.
Related terms
Sources & further reading
- International Journal of Biological Macromolecules: Thioglycolic acid compounds and proteases for anti-felting finishing of wool (Li et al., 2025)
- International Journal of Biological Macromolecules: Shrink-resist finishing of wool with a disulfide reducing agent and papain (Li et al., 2024)
- Woolmark: Wool processing
Technical references reviewed 2026-09-22. Examples are illustrative. Industry organizations and manufacturers describe their own fields; their references are not independent product endorsements. Figures are approximate and depend on the stated test conditions.