Question

Explain De-sizing process.

Hint:

Size is a coating on warp yarns, and de-sizing removes it before dyeing and bleaching.

Answer 1

Step 1: What is sizing and why de-size:
Before weaving, warp yarns are coated with size such as starch, PVA or CMC.
Size makes the yarn smooth and strong, so it does not break on the loom.
After weaving, the size has no use. It blocks water, dye and chemicals from entering the fibres.
De-sizing is the process of removing this size from the grey (loom-state) fabric.

Step 2: Order of the process:
De-sizing is the first wet process. It comes before scouring, bleaching, dyeing and printing.
If size stays on the cloth, bleaching and dyeing become patchy and uneven.

Step 3: Main steps of de-sizing:
The fabric is first wetted with warm water so that the size swells and softens.
A de-sizing agent is then applied by padding or soaking. The cloth is batched (kept rolled and wet) for some hours so that the agent breaks the size.
Finally the cloth is washed well with hot water to wash away the broken-down size.

Step 4: Common methods:
Enzymatic de-sizing: amylase enzyme breaks starch into soluble sugars. It works at about 50-70 degrees C and is safe for the fibre.
Acid de-sizing: dilute sulphuric or hydrochloric acid is used on cotton to loosen starch.
Oxidative de-sizing: hydrogen peroxide or persulphate breaks the size quickly.
Hot water or rot steeping: water-soluble sizes like PVA and CMC wash out in hot water; in rot steeping, the cloth is kept damp so that bacteria break the starch.

Step 5: Checking the result:
A drop of iodine solution on starch-sized cloth turns it blue-black. If no blue colour appears, the size has been removed.

Final Answer:
De-sizing removes the starch or other size from woven grey fabric, using enzymes, acids, oxidising agents or hot water, so that the cloth can take scouring, bleaching, dyeing and printing evenly.