Desizing

What is desizing in textile?

Desizing is the wet preparation step that removes the protective size (starch, PVA or CMC) applied to warp yarns before weaving, so dye, bleach and finishing chemistry can penetrate the cloth evenly. Without it, size blocks absorbency and causes uneven shade. The routes are enzymatic, oxidative, or alkali — and 100% polyester greige carries little size, so it often skips the step entirely.

What is desizing in textile? Desizing removes the warp size—starch, PVA, acrylate, or blends, applied on the sizing machine or the slasher before weaving—from woven cloth so water, dye, and chemistry reach the fiber evenly. Weaving needs the size; dyeing cannot have it. Desizing is the step that hands cloth from the loom to the dyehouse in a state the dyehouse can actually work with.

Why the warp is sized at all

The warp is the yarn that takes the beating. It is pulled forward and back through the reed thousands of times, rubbed by the heddle eyes and the reed, and it carries the whole weight of the shed. Bare filament yarn under that load raises fuzz, loses strength, and breaks at the loom—and a broken warp end is a broken pick for the rest of the piece.

Size solves that by forming a film around the yarn:

• it holds the fibers together and reduces hairiness and end drop
• it protects the yarn from abrasion at the reed and the drop
• it raises the yarn's strength—a sized yarn carries more than the same yarn bare, and the sizing house's own figures put the gain in the region of 15–50%
• it holds the ends in place, so the weave does not slip and the pattern does not distort

Size is applied in the sizing box, and the pick-up is typically 5–15% of the yarn's weight. More is not better: heavy size on the warp is a promise the dyehouse will have to keep.

What the size is made of, and how it comes off

Two words that sound alike and are not the same thing: a size that is removed (desizing) and a size that is fixed (heat-setting). Only the first is this page.

How to tell what is on the cloth

Four checks, in the order a buyer would run them:

1. Water-drop — a drop of water on the face. Sized cloth beads; desized cloth wicks in within a second or two. This is the absorbency test, and it is the one the dyehouse runs.
2. Burn test — on a thread end, not on the cloth. Starch chars and blackens; PVA melts into a bead; acrylate burns with a flame and leaves a hard bead.
3. Iodine — a starch size turns blue-black. A negative iodine test means there is no starch left; it says nothing about PVA.
4. Size spot test — the laboratory version: a quantitative residual-size reading on the actual cloth, in percent. This is the number that belongs on a PO.

None of the first three quantifies what is left on a non-starch size. The spot test is the one that does.

Where desizing sits in preparation

1. Singeing — burn off the protruding surface fibers for a clean face
2. Desizing — remove the warp size
3. Scouring — remove the oils, waxes, and residues that block wetting
4. Bleaching where specified — even the base color so the shade is not shifted

Skipping it does not cost you a bath. It costs you the dye: patchy or unlevel dyeing, print edges that bleed into a halo, and a hand that stays stiff because the size is still there. More dye will not cover a wetting failure.

The four routes, and what each one costs you

Name the size chemistry first. Starch responds to enzyme, PVA wants oxidant or hot water, and a heat-set size does not come off at all.

The polyester exception

This is the part that matters if you buy polyester rather than cotton.

On a 100% polyester programme the warps are sized with PVA or acrylate so the filament survives the loom, and that size is either washed out in the dyehouse pretreatment or fixed by the heat the cloth receives later. A dedicated desize bath buys a 100% polyester nothing. It is a cotton-and-blend step, and on a polyester route it is a bath you pay for in water, heat, time, and effluent.

So the honest position, from a polyester mill: desizing is not a service we quote. The question a polyester programme actually asks is not "will you desize it" but "how absorbent does this cloth have to be before dye or print" — because that number decides whether the next step works at all.

What skipping it costs, and what over-doing it costs

Over-desizing is the quieter failure. It does not show on the first look; it shows on the tensile report and on the customer's panel.

The checks that decide whether it worked

• Absorbency — water-drop seconds, measured on the actual cloth. The dyehouse's number, not a description.
• Residual size — the spot-test percentage, against a limit the buyer sets.
• Weight and GSM — wet preparation moves dimensions; compare against the greige weight and state the tolerance.
• Tensile and tear retention — the over-desize detector.
• Whiteness — CIE, where the route includes bleaching.
• pH — near neutral, the same 6.5–7.5 window the PFD route asks for, so the dyehouse does not start by correcting the bath.

These are discussion points, not universal guarantees: the numbers belong on your own dyehouse specification.

The bath is a cost, not a formality

Starch and PVA in the effluent are what make a desize bath one of the dirtiest in the route. The organic load shows up as BOD and COD, and it is discharged whether or not the bath was needed. A bath you do not need is a bath you do not pay for, and on a 100% polyester route it is usually the one you can leave out.

What to put on the PO

• The size chemistry if it is known (starch, PVA, acrylate), or a size spot-test requirement if it is not
• The absorbency target after preparation, with the test method and the number (water-drop seconds)
• A residual-size limit, if you have one
• The downstream route — scour, bleach, dye, print, finish — and which of those the next plant is expected to run
• Weight, width, and GSM after preparation, with a tolerance, because the wet process moves them
• A lab check on the actual construction before bulk

On Weaverine polyester programs, desizing is a preparation note on the greige-to-finish route—write the size type and absorbency target, not the word desizing alone. A PO that says only "desized" has not specified anything that can be tested.

What we quote is the base cloth and the final finish: greige polyester in programs of 55–120 GSM at 160–330 cm, bleached white with CIE whiteness 120+ from 2,000 m, or solid dyed from 3,000 m per colour. Desizing itself is not on that list.