Hot mercerisation

Classic mercerisation, developed by John Mercer in 1844, treats cotton with 20–30 % sodium hydroxide, causing the cellulose fibre to swell, lose its spiral structure and straighten into a rounder, smoother form. Horace Lowe recognised in 1889 that the famous silk-like lustre appears only when mechanical tension is applied at the same time. Hot mercerisation takes this further: the fabric is run through a hot caustic bath (typically 40–70 °C) under especially high warp and weft tension via a stenter chain, followed by cooling and progressive washing-out of the alkali.

The results: extremely high lustre, up to 25 % deeper colour, 15–20 % higher tensile strength, markedly reduced shrinkage and an improved hand. The process is more expensive and energy-intensive than cold mercerising but is used for premium cotton shirting (poplin, popeline, twill), cotton sewing threads, embroidery yarn and high-end cotton knitwear. For dyed goods, hot mercerisation means that a deeper, more brilliant shade is achieved with less dye — a double gain in economy and appearance.

• Process parameters: 20–30 % NaOH, 40–70 °C, high tension
• Effects: +25 % colour depth, +15–20 % tensile strength, high lustre
• Applications: premium shirting, sewing thread, high-end cotton knits
• Advantage vs. cold merc: more intense lustre, more uniform treatment