Pretreatment in Textile Processing

Pretreatment in textile processing is the foundational wet-process step that removes natural and added impurities from greige (unfinished) fabric before dyeing, printing, or finishing. The sequence typically runs singeing → desizing → scouring → bleaching → mercerizing, with each step targeting a specific class of impurity and conditioning the fabric for the next. Done well, pretreatment is what makes a cotton fabric absorbent, white, and lustrous enough to take a uniform color in the dye house; done poorly, it is the single largest source of downstream defects (unlevel dyeing, poor hand, low fastness, pinholes, shade variation).

This article follows the standard pretreatment sequence used in modern cotton mills, drawing on process methodology published by accredited third-party textile testing laboratories and on the relevant ISO standards for whiteness, absorbency, and mercerization. The focus is woven cotton, the most common substrate, with notes where knit, wool, silk, and synthetic substrates differ. Wherever a load-bearing claim is made, the source is cited inline; sources are listed at the end.

Why pretreatment is necessary

Greige cotton fabric straight from the loom contains four classes of impurities that interfere with wet processing:

• Natural fiber impurities – cotton waxes, pectins, proteins, mineral salts, and natural pigments in the cuticle and primary wall.
• Added sizing – warp yarns are coated with starch, PVA, or acrylic size before weaving to survive loom tension; this size film must be removed before the fabric can absorb water.
• Loom oil and machine contamination – mineral oils, grease, and metal debris picked up during weaving.
• Seed-coat fragments – broken cottonseed shells (“motes”) that survive ginning and present as dark specks unless chemically or mechanically removed.

Each subsequent wet process, scouring, bleaching, dyeing, printing, only works if the fabric is first wettable. A fabric that cannot wet out cannot absorb dye uniformly, and any residual size or wax will produce resist spots and shade variation. The pretreatment sequence is engineered so that the output of one step is the right input for the next.

The standard sequence for cotton woven fabric

1. Singeing

Cotton is a short-staple fiber, and during spinning and weaving loose fiber ends project from the yarn and fabric surface as a nap. This nap makes the fabric surface fuzzy, interferes with printing clarity, and is prone to pilling in the finished garment. Singeing burns the nap off by passing the fabric at high speed over an open gas flame or heated plate.

The most common machine is the gas singeing machine, in which the fabric passes across a row of burner nozzles and is immediately quenched in a water bath or on a cooled roller. The effect is rated visually on a 1–5 scale under good light: Grade 1 is unsinged greige, Grade 3 is “basically no long nap” (the minimum for general fabric), Grade 4 is “only short nap, and neater” (the target for high-quality fabric), and Grade 5 is fully clean. The cost of over-singeing is fiber damage and strength loss, so the process is a balance.

2. Desizing

Desizing removes the warp size so the cotton can wet. The chemistry depends on the size:

• Enzyme (amylase) desizing – the standard for starch sizes. The enzyme hydrolyzes starch into water-soluble sugars at 50–70 °C, typically with a 30–60 min dwell. Mild and fiber-safe.
• Alkali (caustic) desizing – common for blended sizes and as a pre-soak. Less specific than enzyme, but effective on PVA-blended sizes.
• Acid desizing – for sizes sensitive to oxidative breakdown. Rare on cotton today.
• Oxidative (persulfate / peroxide) desizing – for sizes that resist hydrolysis.
• Plasma desizing – newer, low-water option using atmospheric plasma to break down the size film.

The desizing rate is the quality metric. Production targets a desizing rate above 80%, or residual size below 1% of fabric weight. Any residual size carries through to scouring and bleaching, where it is harder to remove and shows up as dye resist in the dye house.

3. Scouring

Desizing removes most of the size and part of the natural impurities, but scouring is what makes cotton absorbent. The workhorse reagent is sodium hydroxide (caustic soda), typically 2–4% on weight of fabric, with:

• Wetting agent / surfactant – to break the fabric’s surface tension and let the alkali penetrate.
• Sodium silicate – to sequester iron and other metal ions that would otherwise deposit on the fabric as brown stains.
• Sodium bisulfite or other reducing agent – in some recipes, to protect the fiber from oxidative damage.

Caustic soda does the work: it hydrolyses pectins and proteins into water-soluble fragments, saponifies the fatty-acid components of cotton wax into soap, and dissolves the cottonseed-shell fragments. Hot dwell (95–100 °C) for 60–90 min in a pad-batch, J-box, or continuous scouring range is typical. The output is a fabric that wets out instantly in cold water.

Scouring quality is measured by wicking height: a vertical strip of fabric is dipped in water and the height water climbs in 30 minutes is measured. The general requirement is 8–10 cm in 30 min. Anything below that is under-scoured and will produce unlevel dye uptake.

4. Bleaching

Scouring removes most impurities but leaves the natural cotton pigment, which gives greige cotton a cream to tan color. Bleaching destroys this pigment to give the fabric the stable white base it needs for pastel shades or for a clean ground before printing. The standard cotton bleach is hydrogen peroxide (H₂O₂), applied at 2–4% on weight of fabric under alkaline conditions at 95–100 °C, with sodium silicate as a stabilizer. Sodium hypochlorite (NaOCl) and sodium chlorite (NaClO₂) are older alternatives, but peroxide is now dominant because the effluent is easier to treat and the fiber damage is lower.

The relevant standard for whiteness on cotton is ISO 2469 / ISO 2470 (reflectance / whiteness measurement), with target Berger whiteness values typically 78–82 for a peroxide-bleached cotton woven fabric. Under-bleached fabric has a yellow cast and prints dull; over-bleached fabric has lost tensile strength and may fail downstream processing.

5. Mercerizing

Mercerizing is the optional final step that transforms cotton’s appearance and reactivity. The fabric is treated with concentrated (typically 22–26% w/w) sodium hydroxide solution at ambient temperature, under controlled tension, then washed and neutralized. Three things happen:

• The cotton fiber swells irreversibly, the cell wall shifts from a kidney-bean cross-section to a rounder, smoother one, and the fiber’s surface gains a silky luster.
• The internal crystalline structure converts from cellulose I to cellulose II, which has higher dye affinity (deeper shades from the same dye load) and better moisture regain.
• The fabric gains roughly 10–25% in tensile strength.

Liquid ammonia is an alternative mercerizing medium (used for high-twist and compact fabrics where the caustic doesn’t penetrate evenly), but it requires -33 °C operating temperature and is less common. The standard for mercerization effectiveness on cotton is ASTM D1926 (mercerization indicator) and AATCC TM 89 (mercerization in cotton).

Substrate-specific notes

Cotton knits

Knitted cotton skips the singeing and desizing steps (knit yarns aren’t sized, and the looser loop structure is less prone to surface fuzz). The sequence is scouring → bleaching → alkali decrement. The alkali-decrement (or “deweighting”) step is a relaxed-state treatment with dilute caustic that causes the cotton to shrink freely and gain density, hand, and drape, useful for jersey and interlock.

Wool

Wool pretreatment is a different chemistry: wool washing removes wool grease (lanolin) and suint, carbonizing removes vegetable matter ( burrs, seeds) by acid treatment, and bleaching is usually reductive (sodium bisulfite) to avoid fiber damage. ISO 3074 covers wool-solvent-extractable matter; AATCC TM 14 covers wool carbonizing.

Silk

Silk pretreatment is degumming, boiling off the sericin (the natural gum coating the fibroin fiber) with soap or alkali to reveal the soft, lustrous silk underneath. ISO 137 covers silk degumming loss.

Synthetic fibers

Most synthetics (polyester, nylon, acrylic) have no natural impurities and need only heat setting (for dimensional stability) and a mild scour to remove spin finish. The pretreatment step that matters for synthetics is heat setting on a stenter frame at 180–200 °C, well before any dyeing.

Quality control checkpoints

A pretreatment line has three go/no-go gates:

• After desizing – iodine drop test for residual starch. Blue-black color means size is still present.
• After scouring – wicking height (target 8–10 cm / 30 min) and residual wax extractables (target < 0.5% by Soxhlet).
• After bleaching – Berger whiteness (target 78–82 for cotton), residual alkalinity (target pH 6.5–7.5 in the rinse water), and tensile-strength retention (target ≥ 90% of greige).

Any failure at one of these gates means the fabric does not pass downstream to the dye house. Pretreatment is the cheapest place in the mill to fix a problem; letting a defective fabric reach the dye house multiplies the cost by an order of magnitude.

Frequently Asked Questions

What is the difference between scouring and bleaching?

Scouring removes hydrophobic impurities (waxes, pectins, oils, size) so the fabric becomes absorbent. Bleaching removes the natural pigment so the fabric becomes white. A fabric can be scoured but not bleached (e.g., dark-colored knitwear that just needs to be absorbent), and a fabric can be bleached without being properly scoured (rare and usually a defect, the residual wax produces unlevel dyeing downstream).

Why is hydrogen peroxide the standard cotton bleach instead of chlorine?

Peroxide is oxidative but produces water and oxygen as byproducts, which are easy to treat in effluent. Chlorine bleaches (sodium hypochlorite, sodium chlorite) produce adsorbable organic halogens (AOX) in the wastewater, which are regulated in most jurisdictions. From a fiber-damage standpoint, peroxide is also more controllable: a well-stabilized peroxide bath has a clear end point (when the peroxide is consumed, bleaching stops), whereas chlorine can over-oxidize cellulose to oxycellulose and cause permanent strength loss.

Is mercerizing necessary?

No. Mercerizing is an optional step that improves luster, dye affinity, and strength. It is essential for high-end shirting, sewing thread, and fabrics that will be dyed to deep shades; it is often skipped for utility fabrics, low-end knitwear, and fabrics that will be printed to pastel shades where the luster is not a feature.

How much water does cotton pretreatment use?

A conventional cotton pretreatment line uses 60–100 L of water per kg of fabric. Modern closed-loop systems with counter-current washing and enzymatic stages can bring this down to 20–30 L/kg, but most mills still operate well above that. Water and energy (mostly for heating the scour and bleach baths) are the largest operating costs after labor in a pretreatment range.

What is the relationship between pretreatment whiteness and the final dye shade?

Whiteness of the bleached substrate sets the floor for the achievable shade range. A fabric at Berger 75 can be dyed to a clean medium shade but not a true pastel; a fabric at Berger 80 can be dyed to pastels. Going below Berger 70 starts to limit the dye house’s ability to match buyer standards, particularly for pale and pastel shades, and is the most common reason a mill rejects a pretreatment batch.

References

• Testex (2022). Textile Pretreatment Processes: Singeing, Desizing, Scouring, Bleaching, Mercerizing… https://www.testextextile.com/textile-pretreatment-processes-singeing-desizing-scouring-bleaching-mercerizing/ – process methodology, sequence, and quality metrics.
• ISO 2469:2014, Paper, board and pulps, Measurement of diffuse reflectance (whiteness measurement methodology, applicable to bleached textile substrates).
• ISO 2470-1:2016, Paper, board and pulps, Measurement of diffuse reflectance, Part 1: Indoor daylight conditions (D65 brightness) (related whiteness standard).
• ASTM D1926-11, Standard Test Method for Mercerization Indicator in Cotton.
• AATCC TM 89, Mercerization in Cotton.

This article is the working reference for the textile pretreatment process. Editorial by Mohammad Yousuf (Anik), TextileTuts. Reviewed against accredited test lab process methodology and current ISO/AATCC/ASTM standards as of 2026.