Sizing and mercerizing
Sizing adds starch or gums for body and stiffness; it is temporary. Mercerizing treats cotton with strong caustic soda under tension, giving permanent lustre, strength and better dyeing.
Sizing as a finish
Sizing is the application of starch (rice, maize, tapioca, wheat), gums, gelatine, synthetic binders (such as PVA or CMC), fillers like china clay, softeners and a little preservative, to give cloth body, stiffness, weight, smoothness and a crisp hand. The cloth runs through a trough of size and is squeezed between rollers (padding), then dried on a tenter. Sizing is temporary: it washes out. Light sizing improves look and handling; heavy sizing (back-filling) with clay can make a thin, loosely woven cloth look heavier. To test, rub the cloth between the hands or hold it to the light: powder falls out and gaps show. Separately, warp sizing before weaving protects warp yarns from rubbing on the loom and is removed later by desizing.
Mercerizing
John Mercer found the effect of caustic soda on cotton in 1844; Horace Lowe later (about 1890) showed that holding the cotton under tension gives a silky lustre.
- 1Scoured cotton yarn or fabric is held under tension on a frame or chain.
- 2★It is treated for a short time with cold, concentrated sodium hydroxide (caustic soda, about 18-25%).
- 3★The fibres swell: the flat, twisted ribbon becomes a smooth, round rod and the lumen nearly closes.
- 4Still under tension, the caustic soda is washed out with water.
- 5The cloth is neutralised with dilute acid, rinsed and dried.
★ one-way, controls the pace
Effects of mercerizing on cotton
| Property | Change | Why |
|---|---|---|
| Lustre | Silky, lasting shine | The round, smooth fibre reflects light evenly |
| Strength | Increases (by about a fifth) | Molecules are better aligned and the fibre more even |
| Absorbency and dyeing | Higher; brighter colours with less dye | The swollen fibre opens up more space for water and dye |
| Smoothness | Smoother, softer hand | Convolutions are removed |
| Shrinkage | Shrinks if treated without tension | Swelling shortens fibres; tension prevents it |
| Permanence | Permanent | The fibre's structure itself is changed |
Sizing and mercerizing
Starch is both a nutrition and a textile topic: rice kanji (rice water) or a maida paste is the home size for cotton saris and uniforms. Mercerized cotton thread and tablecloths stay bright through many hot washes, which suits hotels, hospitals and canteens.
2 marks: “What is back-filling?” Heavy sizing with fillers such as china clay that makes a thin, loosely woven cloth look heavier and firmer; it washes out.
5 marks: “Explain sizing as a finish.” Definition; materials (starch, gums, fillers, softeners); padding and drying; temporary; light vs heavy sizing; test for back-filling; warp sizing vs finishing sizing.
10 marks: “What is mercerization? Explain the process and its effects on cotton.” Mercer 1844 and Lowe's tension; process steps; fibre change (round, no twists, lumen closes); effects on lustre, strength, absorbency, dyeing; uses.
Mercerizing = cold caustic + tension, and cotton gains L-S-A-D: Lustre, Strength, Absorbency, Dye uptake.