How to Wash Viscose and Rayon

The Chemistry

Viscose (also called rayon in North America) is a regenerated cellulose fibre — wood pulp cellulose dissolved and re-spun into fibre form. The production process is the xanthate process: cellulose is steeped in sodium hydroxide (NaOH, caustic soda), which converts it to alkali cellulose. This is then reacted with carbon disulphide (CS₂) to form cellulose xanthate, which is dissolved in dilute NaOH to produce a thick, viscous liquid — "viscose." This solution is pushed through fine holes in a spinneret and extruded into a sulphuric acid bath, which reconverts the xanthate back to cellulose, precipitating the fibre. The resulting viscose fibre is chemically cellulose, like cotton and linen. However, the structure is fundamentally different. Native cotton cellulose (cellulose I) has a high degree of crystallinity — the polymer chains are well-ordered in crystalline regions with limited spaces for water absorption. Viscose cellulose has a lower crystallinity — the dissolution and re-precipitation process creates a more amorphous, disordered polymer structure with many more sites available for water absorption. It is this lower crystallinity that explains viscose's distinctive and problematic wet behaviour. When viscose absorbs water, the water molecules penetrate the disordered, amorphous regions of the cellulose and intercalate between the polymer chains, disrupting the hydrogen bonds that hold the structure together. The viscose fibre swells substantially — more than cotton, more than linen, and more than any of the synthetic fibres. This swelling causes the fibre to elongate and to lose its structural rigidity. The technical measurement of this effect is wet tensile strength: viscose loses approximately 40–50% of its dry tensile strength when fully wet. Cotton, by comparison, loses about 20%. Linen actually gains strength when wet. Polyester and nylon retain essentially full strength when wet. Viscose is uniquely, dramatically weak when wet. The practical consequence of this wet weakness is that any mechanical stress applied to a wet viscose garment — rubbing, wringing, twisting, machine drum rotation — is working on a fibre that is half as strong as usual. The forces that would be harmless on dry viscose or on wet cotton are sufficient to distort the weave geometry, stretch individual threads, and break fibre bundles. Machine washing viscose, even on a gentle cycle, subjects the wet, weakened fabric to mechanical forces that permanently distort its structure. This manifests as shrinkage, loss of the original drape, puckering, and sometimes actual tearing at seams. Viscose also shrinks in water even without any mechanical stress — pure immersion causes shrinkage. The mechanism is fibre relaxation: the viscose spinning process leaves the fibres under tension (elongated beyond their equilibrium length). When the fibres absorb water and swell, they also relax laterally and contract longitudinally toward equilibrium, reducing the length of each fibre. In a fabric, this manifests as overall garment shrinkage — typically 3–8% in both length and width on first immersion. Subsequent washes cause less shrinkage, but the first cold soak is the most dramatic. Viscose should not be confused with Modal and Lyocell (Tencel), though all three are regenerated celluloses from wood pulp. Modal is produced via a modified viscose process that produces a fibre with better wet strength than standard viscose. Lyocell is produced via the NMMO solvent process and has the highest wet strength of the three, though it has the fibrillation sensitivity described in the Tencel guide. Standard viscose (rayon) has the worst wet strength of the three and requires the most careful handling. Many "bamboo fabric" garments are actually viscose made from bamboo pulp — not a bast fibre extracted mechanically from bamboo stalks (true bamboo fibre exists but is not commercially common). Bamboo viscose has the same structural properties and care requirements as standard wood-pulp viscose, despite different marketing language about "natural bamboo fabric." Acetate is a related but distinct fibre: cellulose acetate, produced by acetylating cellulose with acetic anhydride. Acetate is not a true regenerated cellulose — the chemical structure has been modified. Acetate has different chemistry from viscose: it is highly sensitive to acetone (found in nail polish remover), which dissolves it. Acetate also requires specific care and should not be confused with viscose.