Chemical Processing I: Impurities, Preparatory Processes, Mercerisation, Wool and Silk, Man-made Fibres and Optical Brightening

This chapter opens the Engineering Sciences (XE) paper's treatment of Section XE9.5 of XE9 Textile Engineering and Fibre Science, one of the nine optional sections, any two of which a candidate chooses in the exam. It answers the first paragraph of the heading “Chemical Processing”: “Impurities in natural fibre; Singeing; Chemistry and practice of preparatory processes for cotton; Preparatory processing of wool and silk; Mercerization of cotton; Preparatory processes for manmade fibres and their blends; Optical brightening agent.” It teaches what the impurities are, why each preparatory step is needed and the chemistry of desizing, scouring and bleaching, mercerisation, wool and silk preparation, preparation of man-made fibres and blends, and optical brightening. The numericals are liquor-ratio and recipe calculations (per cent on weight of fabric, g/L), size add-on, chemical dosing from stock strength, dilution of caustic soda and degumming loss.

1. Impurities in natural fibres and singeing

Preparation (“pre-treatment”) makes fabric absorbent, white or evenly pale, clean and dimensionally ready, so that dyes, print pastes and finishes act uniformly. Raw cotton is roughly 90% cellulose; the rest is non-cellulosic: wax and fatty matter on the cuticle, pectic substances and proteins in the primary wall, mineral ash, natural colouring matter (a creamy-yellow flavone-type pigment) and seed-coat fragments (motes). Grey woven cloth adds size (starch, PVA, CMC), spinning and weaving oils and dirt picked up in handling.

Cotton impurities and how each is removed
ImpurityEffect if leftRemoved mainly by
Wax, fatsWater-repellent; patchy dyeingHot NaOH with detergent (saponify, emulsify)
Pectins, proteinsHold other impurities, slow wettingAlkali (soluble pectate; hydrolysis)
SizeBlocks penetration, uneven shadeDesizing (enzyme, oxidant or hot water)
Natural colour, motesDull cream cast; specksBleaching (oxidative destruction)

Singeing burns off the protruding fibre ends (fuzz) by passing the dry grey fabric quickly over gas flames, heated copper plates or rollers, and then quenching it at once, usually in the desizing bath, so no sparks remain. It gives a clean, smooth face, sharper print outlines, less pilling and less lint in later steps, and it is done before desizing and wet processing because dry fibre ends burn off cleanly. Too slow a speed scorches the cloth; uneven singeing leaves streaks that show later as dye-depth differences. Blends containing thermoplastic fibres melt rather than burn, leaving hard beads, so the flame settings are tuned for them.

2. Cotton preparation: desizing, scouring and bleaching

Desizing removes warp size. Starch is hydrolysed to soluble sugars by amylase enzymes (mild, specific, no fibre damage; bacterial amylases tolerate higher temperatures than malt amylase) or degraded by oxidants (persulphate, peroxide) or dilute acid (risk of tendering). Completion is checked with the iodine test: residual starch turns blue-violet. PVA and CMC are not attacked by amylase; they are washed out in hot water. Scouring boils the desized cloth in sodium hydroxide (a few per cent on weight of fabric, abbreviated owf) with a detergent, a sequestrant and a reducing agent. Alkali saponifies and emulsifies wax, converts pectin to soluble sodium pectate, hydrolyses protein and dissolves minerals; the reducing agent prevents air oxidation of cellulose (oxycellulose) at the boil. Machines are the kier (batch, pressure), the J-box and pad-steam ranges (continuous). Bio-scouring uses pectinase, a milder, lower-effluent route. Success is judged by absorbency (a water drop must be taken up quickly) and weight loss.

Bleaching destroys the natural colour by oxidation. Hydrogen peroxide dominates: in alkali, H₂O₂ + OH⁻ ⇌ HOO⁻ + H₂O, and the perhydroxyl anion HOO⁻ is the bleaching species, best at pH 10 to 11. Transition-metal ions (Fe, Cu) catalyse wild decomposition to radicals that tender the cellulose locally, so a stabiliser (sodium silicate or an organic one) and a sequestrant are used. Sodium hypochlorite works cold in slightly alkaline bath (about pH 9 to 10); near neutral pH hypochlorous acid is most active and attacks cellulose, so that region is avoided. Sodium chlorite acts in acidic bath (pH about 3.5 to 4), releasing chlorine dioxide: a high white with little damage, but toxic fumes and corrosion mean titanium or stainless equipment. Damage is monitored by cuprammonium fluidity or degree of polymerisation, which fall as chains break.

Cotton bleaching agents
AgentBathMain risk
H₂O₂ (HOO⁻)pH 10–11, hot or pad-steamCatalytic damage by metal ions
NaOClpH about 9–10, coldOxycellulose near neutral pH; yellowing of some fibres
NaClO₂pH about 3.5–4, hotClO₂ fumes; corrosion

Recipe arithmetic. “% owf” means kg of chemical (as 100%) per 100 kg of dry fabric, so the quantity is fixed by the fabric mass. The liquor ratio (L:R, litres of bath per kg of fabric) then sets the bath volume and the concentration in g/L: 50 kg of fabric at 1:8 needs 400 L, and 3% owf of a chemical is 1.5 kg, that is 3.75 g/L. A commercial product is converted to a volume with its stated strength: volume = mass of 100% chemical ÷ strength in g/L. Always check whether a quantity is quoted owf, in g/L or on a product basis.

⚠️ Order and pH matter
Sequence for woven cotton is singe, desize, scour, bleach, mercerise (or bleach after mercerising). Do not call H₂O₂ an acid-bath bleach, and do not run hypochlorite bleaching near pH 5 to 7 to “speed it up”: that is the range of fastest cellulose attack. Whatever the agent, an antichlor/peroxide-kill step and thorough washing follow.

3. Mercerisation of cotton

Cotton treated with concentrated caustic soda (about 20 to 25% w/w, roughly 270 to 300 g/L, in the cold, for under a minute) swells: the flat, twisted, bean-shaped fibre becomes round, the lumen shrinks, and the crystal lattice changes from cellulose I to cellulose II after washing out the alkali. John Mercer observed the swelling in 1844; holding the material under tension is what gives lustre, and that is the modern process. The sodium hydroxide is washed out while the cloth is still held taut to a low level, then the fabric is neutralised with dilute acid and washed.

Effects of mercerisation under tension
PropertyChangeReason
LustreIncreasesRound, smooth cross-section reflects light regularly
StrengthIncreasesBetter chain orientation, even stress sharing
Dye uptake, reactivityIncreases (deeper shade, less dye needed)Lower crystallinity, more accessible OH groups
DimensionsShrinkage if not heldSwelling pulls fibres into a relaxed state

Degree of mercerisation is tested by the barium activity number (BAN): the ratio of baryta (barium hydroxide) taken up by the sample to that of untreated cotton, times 100. Untreated cotton is 100 and well-mercerised cotton is distinctly higher. Liquid ammonia treatment (at about −33 °C) gives similar swelling with a softer, more resilient handle. The caustic is recovered by evaporation from the wash liquor.

⚠️ Tension is the whole point
Strong caustic without tension only shrinks the cloth (slack mercerisation gives stretch and a harsh hand, not lustre). The higher dye uptake is also real for yarn mercerised slack, because it comes from crystallinity loss; the lustre needs the tension. Do not confuse mercerisation (concentrated, cold, tension) with scouring (dilute, boiling, no need of tension).

4. Preparation of wool and silk

Wool (keratin protein) carries wool grease (wax), suint (dried sweat salts), dirt and vegetable matter (burrs, seeds). Scouring uses a non-ionic detergent with a mild alkali such as sodium carbonate, at a moderate temperature (about 50 °C), pH about 9 to 10: hot caustic soda would hydrolyse and dissolve keratin, so strong alkali is never used. Carbonising removes cellulosic burrs: the wool is soaked in dilute sulphuric acid and baked, when the acid dehydrates cellulose to brittle hydrocellulose while keratin survives; the char is crushed out and the wool neutralised with soda. Bleaching is by alkaline-mild or acid peroxide, or by a reducing agent (sodium dithionite); a reductive white is not permanent, as air re-oxidises it and the yellow creeps back. Chlorine is never used on wool because it chlorinates the protein, yellows and damages it.

Raw silk is fibroin coated with sericin (silk gum), roughly 20 to 30% of the raw weight. Degumming (boil-off) removes it in a hot (about 90 to 95 °C) soap and soda ash bath, or with a protease enzyme, or with mild acid; the fibre becomes soft, lustrous and absorbent. Weight loss on degumming is therefore the sericin removed, about a quarter of the raw weight. Silk is bleached with H₂O₂ (not chlorine). Historically the loss was made good by weighting with metal salts.

5. Man-made fibres, blends and optical brightening

Man-made fibres are free of natural wax, pectin and ash, so preparation is milder: remove spin finish (lubricant and antistatic oil), knitting oils, size and dirt. Viscose needs only a mild scour (it is weak when wet, so tension is kept low). Cellulose acetate is scoured gently without strong alkali, because alkali saponifies the surface back towards cellulose. Polyester is scoured with detergent and a little soda or caustic; oligomers (mainly the cyclic trimer) migrate out in hot dyeing and are a source of deposits and specks; heat setting before dyeing evens out earlier thermal history. Nylon is scoured with soap or non-ionic detergent; hypochlorite is avoided, as the amide N–H is chlorinated and the fibre yellows, while chlorite or peroxide may be used. Acrylic takes mild scouring and, if needed, acid chlorite bleaching.

Blends need a compromise. Polyester/cotton is singed, desized, scoured and peroxide-bleached for the cotton part (alkali the polyester tolerates at the boil), and heat-set; the polyester is often brightened separately. Polyester/wool is treated mildly, as wool dislikes alkali and heat. Optical brightening agents (OBA) are colourless fluorescent compounds that absorb ultraviolet (roughly 340 to 380 nm) and re-emit blue-violet light (roughly 420 to 470 nm), which cancels yellow and makes the fabric look whiter than a pure reflector could. Stilbene derivatives (diaminostilbene disulphonic acid triazines) suit cellulose, and disperse-type brighteners suit polyester. They are applied after bleaching, a saturation level exists, and too much causes concentration quenching and a greenish dull cast.

⚠️ A brightener is not a bleach
OBA add fluorescent blue light and do not destroy colour, so they cannot replace bleaching on cotton with brown motes. A fabric under a light source with no ultraviolet will show no brightening effect. Always bleach first, then brighten.

Key takeaways

  • Raw cotton is about 90% cellulose; wax, pectin, protein, ash, pigment and motes plus size and oils are removed in sequence: singe, desize, scour, bleach. Singeing burns fuzz off dry grey cloth first.
  • Amylase removes starch only; PVA and CMC are washed out in hot water. Hot NaOH scouring saponifies wax and makes pectin soluble, with a reducing agent against oxycellulose.
  • Peroxide bleaches through HOO⁻ at pH 10 to 11 with a stabiliser against metal-catalysed damage; hypochlorite is run alkaline, not near neutral pH; chlorite works acidic and needs resistant plant. Fluidity monitors damage.
  • Mercerisation is concentrated cold NaOH (about 20 to 25%) on cotton held under tension: cellulose I becomes II, lustre, strength and dye uptake rise; BAN measures it.
  • Wool is scoured with mild alkali and carbonised with sulphuric acid, never treated with strong alkali or chlorine; silk loses about a quarter of its raw weight in degumming of sericin.
  • Man-made fibres need only spin finish and oil removal; avoid alkali on acetate and hypochlorite on nylon. OBA absorb UV and emit blue, are applied after bleaching, and over-dosing lowers whiteness.

Practice questions (17)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

  1. Grey cotton fabric that has been scoured only lightly sinks slowly in water, although cellulose itself is hydrophilic. Which non-cellulosic constituent is mainly responsible?

    1. The mineral ash left in the fibre, which forms an insoluble film on the surface
    2. The natural flavone pigment, which closes the pores between the cellulose chains and so stops water entering the fibre
    3. The wax and fatty matter on the cuticle, which repels water until it is saponified and emulsified
    4. The pectic substances, which are strongly hydrophobic until they have reacted with an acid in the bath
    Show answer

    Answer: C — The wax and fatty matter on the cuticle, which repels water until it is saponified and emulsified

    The cuticular wax and fats make raw cotton water-repellent; hot alkali with detergent saponifies and emulsifies them. Pectins are the tempting wrong choice, but they are water-swellable and are removed as soluble pectate; ash and pigment do not control wetting.

  2. A woven cotton fabric is sized with polyvinyl alcohol (PVA) only. Which desizing treatment is appropriate?

    1. Washing in hot water, since PVA is water-soluble and is not a substrate for amylase
    2. A bacterial amylase bath, since all film-forming sizes are polysaccharides
    3. Cold dilute sulphuric acid, since acid hydrolyses the vinyl alcohol chain to glucose
    4. A malt extract bath at 60 °C, which is selective for synthetic polymer sizes
    Show answer

    Answer: A — Washing in hot water, since PVA is water-soluble and is not a substrate for amylase

    PVA is a synthetic water-soluble polymer and amylase acts only on starch; so hot-water washing removes PVA (and CMC). An amylase bath is the tempting choice for starch sizes but leaves PVA behind. PVA is not a polysaccharide and gives no glucose on acid hydrolysis.

  3. A mill runs a woven cotton/polyester fabric through singeing. The singeing machine is set for an all-cotton fabric at the same speed and flame. Which result is most likely and why?

    1. The polyester fuzz is burnt cleanly, because thermoplastic fibres ignite at the same rate as cotton
    2. No singeing occurs, because polyester is flame-retardant and cannot lose its fuzz
    3. The cotton is damaged but the polyester is unaffected, because polyester reflects the flame heat
    4. The polyester fuzz melts into hard beads that stay on the surface and later show as specks and dye-uptake differences
    Show answer

    Answer: D — The polyester fuzz melts into hard beads that stay on the surface and later show as specks and dye-uptake differences

    Thermoplastic fibre ends melt and fuse into beads rather than burn off, so singeing settings for blends must be reduced or adjusted. The belief that blends burn like cotton is wrong; polyester does not reflect the flame and is not non-flammable in a way that prevents melting.

  4. Alkaline hydrogen peroxide bleaching of cotton works best at pH 10 to 11 because

    1. peroxide is itself a strong acid and the alkali only neutralises the acid by-products formed during the bleaching
    2. the alkali forms HOO⁻, the active bleaching species, while a stabiliser limits uncontrolled decomposition
    3. peroxide is only stable above pH 10, and below that pH it decomposes explosively in the bath
    4. the alkali oxidises the natural pigment directly while the peroxide merely acts as a solvent for the pigment
    Show answer

    Answer: B — the alkali forms HOO⁻, the active bleaching species, while a stabiliser limits uncontrolled decomposition

    In alkali H₂O₂ + OH⁻ ⇌ HOO⁻ + H₂O, and HOO⁻ is the bleaching species; silicate or organic stabilisers hold back the metal-catalysed breakdown that would tender the fibre. Peroxide is stable in mildly acidic solution, so the idea that it is stable only above pH 10 is wrong, and it is the peroxide anion, not the alkali, that oxidises the pigment.

  5. A mill wants to speed up hypochlorite bleaching of cotton and lowers the bleach bath to pH 6. What is the most likely consequence?

    1. A safer bath, because hypochlorous acid is inert towards cellulose and attacks only the natural pigment
    2. No change, because the bleaching rate of chlorine-based agents is independent of the pH of the bath
    3. Faster whitening but strong oxidative attack on cellulose, giving oxycellulose and loss of strength
    4. Conversion of the cellulose to cellulose II, increasing lustre
    Show answer

    Answer: C — Faster whitening but strong oxidative attack on cellulose, giving oxycellulose and loss of strength

    Near neutral pH the active hypochlorous acid is present in large amount and it attacks cellulose, which is why the bath is held alkaline. It is not inert and the rate does depend on pH. Cellulose II formation is a result of mercerisation, not of bleaching.

  6. Two identical cotton yarns are treated in 24% NaOH in the cold. Yarn P is held under tension while the caustic is washed out; yarn Q is allowed to shrink freely. Compared with yarn Q, yarn P has

    1. higher lustre and a rounder section; both yarns are less crystalline than untreated cotton
    2. lower lustre but a higher dye uptake than yarn Q, because tension closes the lumen completely
    3. the same lustre, because lustre depends only on the NaOH concentration and not on the tension
    4. lower strength, because tension breaks the cellulose chains in strong alkali
    Show answer

    Answer: A — higher lustre and a rounder section; both yarns are less crystalline than untreated cotton

    Swelling in strong alkali lowers crystallinity (cellulose I goes to II) in both yarns, which raises dye uptake, but only the yarn held taut keeps the round, smooth section that reflects light regularly, so lustre needs tension. The idea that lustre is independent of tension is the common trap; tension does not break chains, it improves orientation.

  7. For scouring raw wool a mill proposes boiling in 5% caustic soda to remove grease quickly. This is rejected because

    1. caustic soda cannot saponify wool grease at all, so the grease would stay on the fibre
    2. hot strong alkali hydrolyses and dissolves the fibre keratin, so mild alkali is used
    3. caustic soda converts the cellulosic burrs to hydrocellulose, which stains the wool
    4. wool is mercerised by caustic soda and becomes too lustrous and too weak to use
    Show answer

    Answer: B — hot strong alkali hydrolyses and dissolves the fibre keratin, so mild alkali is used

    Keratin is attacked by hot strong alkali, so wool is scoured with mild alkali (soda ash) and non-ionic detergent. Caustic soda can saponify grease, so the claim that caustic soda cannot saponify grease is false; hydrocellulose formation is the carbonising (acid) chemistry, and mercerisation is a cotton process.

  8. Why is sodium hypochlorite not used for bleaching silk and wool, and what is used for silk instead?

    1. It dissolves sericin but leaves the fibroin untouched; so silk is bleached with dilute sulphuric acid instead
    2. It is too weak an oxidant; so silk is bleached with a stronger hypochlorite solution
    3. It makes the fibre dissolve in water at once; so silk is bleached with sodium carbonate alone
    4. It chlorinates the protein, which yellows and weakens the fibre; so silk is bleached with hydrogen peroxide
    Show answer

    Answer: D — It chlorinates the protein, which yellows and weakens the fibre; so silk is bleached with hydrogen peroxide

    Chlorine agents attack the protein (chlorination of side chains and amide groups), giving yellowing and loss of strength; peroxide, under mild conditions, is the usual choice. Sulphuric acid and soda ash are not bleaches, and the suggestion of a stronger hypochlorite makes the damage worse.

  9. A white polyester/cotton fabric is brightened with a stilbene-type optical brightener at ten times the recommended dose. Compared with the recommended dose, the whiteness is most likely

    1. ten times higher, because the fluorescence emission is strictly proportional to the dose at all levels of dose
    2. unchanged, because the brightener only reflects visible light in the same way as a white pigment does
    3. lower and greener or duller, because beyond a saturation level concentration quenching and absorption of the emitted light set in
    4. permanent, because the excess brightener slowly converts to a bleaching agent on the fibre surface
    Show answer

    Answer: C — lower and greener or duller, because beyond a saturation level concentration quenching and absorption of the emitted light set in

    OBA show a saturation level; excess dye quenches its own fluorescence and absorbs some of the emitted blue light, so the effect falls and the shade turns greenish or dull. Linear gain with dose is the tempting but false view, and OBA never turn into a bleach.

  10. Which of the following statements about the scouring of cotton in sodium hydroxide are correct?

    1. Wax is saponified and emulsified, which restores the ability of the cloth to wet quickly
    2. Pectic substances are converted into soluble salts and washed out
    3. A reducing agent is added to prevent oxidation of cellulose by air at the boil
    4. Scouring is done before singeing so that the fuzz is wet when it is burnt
    Show answer

    Answer: A — Wax is saponified and emulsified, which restores the ability of the cloth to wet quickly; B — Pectic substances are converted into soluble salts and washed out; C — A reducing agent is added to prevent oxidation of cellulose by air at the boil

    Alkali with detergent removes wax and pectin, and a reducing agent protects cellulose from air oxidation. The sequence is singe first on dry grey cloth, since wet fuzz would not burn off cleanly, so scouring before singeing is wrong.

  11. Which of the following are correct for mercerised cotton (concentrated cold NaOH, held under tension) compared with untreated cotton?

    1. The barium activity number is higher
    2. The lattice changes from cellulose I towards cellulose II
    3. The fibre takes up less dye at the same dye concentration
    4. The crystallinity is higher because the chains pack more tightly
    Show answer

    Answer: A — The barium activity number is higher; B — The lattice changes from cellulose I towards cellulose II

    Mercerisation lowers crystallinity and opens the structure, so the BAN and the dye uptake rise and the lattice goes from cellulose I to II. The claims of lower dye uptake and higher crystallinity are the reverse of what happens.

  12. Which statements about preparing man-made fibres and blends are correct?

    1. Spin finish on polyester is removed by a detergent scour, as the synthetic fibre has no natural wax or pectin
    2. Cellulose acetate is scoured without strong alkali because alkali saponifies its surface
    3. Hypochlorite is the preferred bleach for nylon because it does not affect amide groups
    4. Acid sodium chlorite releases chlorine dioxide, so the plant must be corrosion resistant
    Show answer

    Answer: A — Spin finish on polyester is removed by a detergent scour, as the synthetic fibre has no natural wax or pectin; B — Cellulose acetate is scoured without strong alkali because alkali saponifies its surface; D — Acid sodium chlorite releases chlorine dioxide, so the plant must be corrosion resistant

    The statements on polyester spin finish, on acetate and alkali, and on chlorite and corrosion are correct. Hypochlorite chlorinates the amide N–H of nylon and yellows it, so it is avoided; it is chlorite or peroxide that is used.

  13. A grey woven cotton fabric specimen, dry, weighs 216 g. Its size add-on is 8%, calculated on the dry weight of the size-free (desized) fabric. The mass of size in the specimen, in grams, is ____.

    Numerical answer — type the value.

    Show answer

    Answer: 16

    Let W be the size-free dry weight. Then 216 = W + 0.08 W = 1.08 W, so W = 200 g and size = 216 − 200 = 16 g. Taking 8% of 216 g (17.28 g) is the trap, because the percentage is on the size-free weight, not on the grey weight.

  14. A bleach bath is made for 120 kg of cotton fabric. The recipe calls for 2% hydrogen peroxide on weight of fabric (owf), as 100% H₂O₂. The stock solution contains 350 g of H₂O₂ per litre. The volume of stock solution needed is ____ litres (answer to two decimal places).

    Numerical answer — type the value.

    Show answer

    Answer: 6.86

    H₂O₂ needed = 0.02 × 120 kg = 2.4 kg = 2400 g. Volume = 2400 g ÷ 350 g/L = 6.857 L, which is 6.86 L to two decimals. The liquor ratio does not matter for the amount of chemical when the recipe is given owf.

  15. A mercerising range needs 1000 litres of caustic soda liquor at 270 g/L NaOH. It is made by diluting a stock liquor containing 450 g/L NaOH with water; assume the volumes are additive. The volume of stock liquor needed is ____ litres.

    Numerical answer — type the value.

    Show answer

    Answer: 600

    NaOH needed = 270 g/L × 1000 L = 270,000 g. Stock volume = 270,000 ÷ 450 = 600 L, with 400 L of water added. The mass of solute is conserved on dilution: C₁V₁ = C₂V₂, so V₁ = 270 × 1000 / 450 = 600.

  16. A lot of 20 kg of raw silk contains 24% sericin by mass of raw silk. Degumming removes 95% of the sericin and none of the fibroin. The weight loss of the lot, as a percentage of the original raw weight, is ____ (answer to one decimal place).

    Numerical answer — type the value.

    Show answer

    Answer: 22.8

    Sericin = 0.24 × 20 = 4.80 kg; removed = 0.95 × 4.80 = 4.56 kg. Loss = 4.56 / 20 × 100 = 22.8%. The final weight is 15.44 kg. Quoting 24% would assume complete removal.

  17. Scouring is carried out on 50 kg of fabric at a liquor ratio of 1:8 (1 kg fabric to 8 litres liquor). The recipe uses 3% NaOH on weight of fabric, taking the NaOH as 100% pure. The concentration of NaOH in the bath is ____ g/L (two decimal places).

    Numerical answer — type the value.

    Show answer

    Answer: 3.75

    NaOH = 0.03 × 50 kg = 1.5 kg = 1500 g. Liquor volume = 50 × 8 = 400 L. Concentration = 1500/400 = 3.75 g/L. Equivalently, 30 g NaOH per kg of fabric divided by 8 L per kg gives 3.75 g/L.