Chemical Processing III: Printing, Finishing, and Effluent Pollution Control
2. Direct styles of printing
In a direct style the colour is printed on undyed (or pale) cloth and fixed by a treatment: drying, then steaming or heat, then washing off the thickener and unfixed dye. Cotton with reactive dyes: a paste of dye, sodium alginate, urea and bicarbonate is printed and dried; saturated steam at about 100 to 102 °C for several minutes fixes the dye as the bicarbonate turns into carbonate, giving the alkali. A hot wash-off is essential to remove hydrolysed dye, otherwise white grounds are stained and wet fastness is poor.
Wool, silk and nylon with acid or metal-complex dyes: the paste uses a guar or locust bean gum thickener, urea or thiodiglycol as solvent and an acid donor such as ammonium sulphate or tartrate, which releases acid in steaming so that the fibre protonates and takes up the anionic dye; wool needs longer steaming than silk or nylon. Polyester with disperse dyes: the paste uses locust bean gum or alginate, an acid donor and a dispersing agent; the dye is fixed by high-temperature steaming (about 175 °C, several minutes), thermofixation (dry heat of around 200 °C for about a minute) or pressure steaming, then reduction cleared. In each case the fixation step is what pushes the dye into the fibre, as in dyeing.
3. Resist, discharge, pigment, transfer and inkjet printing; faults
Resist printing: a resist is printed first (or the pattern is protected) and the whole cloth is then dyed or overprinted, so the dye does not fix where the resist is. Resists may be physical (wax, as in batik, or clay) or chemical (an acid to block an alkali-fixed reactive dye, or an oxidant against a vat dye). Discharge printing: the cloth is dyed with a ground colour and the printed paste destroys the colour in the pattern. The usual agent is sodium formaldehyde sulphoxylate, a reducing agent that cleaves azo groups to colourless amines, with alkali and steaming; adding a discharge-resistant dye gives a coloured discharge. It is used on cotton, silk and polyester grounds, with grounds chosen to be dischargeable.
- Pigment printing: pigments have no affinity for fibres, so a binder (a film-forming acrylic copolymer) glues them on, with a synthetic thickener and a crosslinker; drying then curing (about 140 to 150 °C, a few minutes) crosslinks the binder. No steaming and no wash-off, and it suits every fibre, but the hand is stiffer and rubbing fastness (especially wet) is the weak point.
- Transfer printing of polyester: disperse dyes that sublime are printed on paper; the paper is pressed against the fabric at about 200 °C for under a minute, the dye turns to vapour and diffuses into the polyester, and no washing is needed. It gives sharp, fine designs, but only sublimable disperse dyes on polyester (and similar synthetics) can be used and the paper is consumed.
- Inkjet printing fires tiny drops of low-viscosity ink from nozzles (drop-on-demand, piezo or thermal, or continuous) directly from a digital file: no screens, quick sampling, unlimited colours and short runs, with less water. Inks are reactive for cotton, acid for wool, silk and nylon, disperse for polyester, or pigment; the cloth is often pre-treated with thickener and chemicals so the drops do not spread. It is slower than rotary screens for long runs.
| Fault | Usual cause |
|---|---|
| Off-register, mis-fit | Poor screen or roller setting, fabric stretch or skew |
| Flushing, bleeding, halo | Paste too thin or too much paste, wet cloth |
| Blade streaks, sharp lines | Damaged doctor blade or squeegee, lumps in paste |
| Poor rubbing fastness (pigment) | Low binder, under-curing |
| White ground stained | Poor wash-off of hydrolysed reactive dye |
4. Finishing of cotton: mechanical and chemical
Mechanical finishing changes surface and dimensions by machine action: calendering (heated rollers press the cloth for smoothness or gloss; embossing and schreinering give patterns or a silk-like lustre), raising (wire clothing pulls fibre ends into a nap), stentering (the cloth is stretched to width and heat-dried), and compressive shrinkage (sanforising), which pushes the damp cloth along against a rubber belt to remove the residual shrinkage.
| Finish | Agent and mechanism | Price paid |
|---|---|---|
| Stiff | Starch, PVA or resin film on and between fibres | Washes out unless crosslinked |
| Soft | Cationic softener adsorbs on anionic cotton and lubricates; silicones give a durable slick hand | Can cut water absorbency and cause yellowing |
| Wrinkle resistant | N-methylol resin (such as DMDHEU) with an acid catalyst crosslinks cellulose OH groups when cured at about 150 °C | Lower tensile, tear and abrasion strength; formaldehyde release |
| Water repellent | Wax, silicone or fluorochemical lowers surface energy so that water beads up, while pores stay open | Breathable but not waterproof; coating would seal it |
| Flame retardant | Phosphorus-nitrogen finishes (such as a THPC-ammonia polymer) promote char and cut flammable volatiles | Strength loss; add-on needed for durable result |
| Enzyme (bio-polishing) | Acid cellulase hydrolyses protruding microfibrils, which mechanical action then removes | Weight and strength loss if over-treated |
Bio-polishing gives a smooth, clean surface and permanent protection against pilling, because the fuzz that forms pills is removed rather than glued. Add-on arithmetic for padding: add-on (% owf) = bath concentration (g/L) × wet pick-up fraction ÷ 10, where the pick-up is in kg of liquor per kg of dry fabric. A bath of 40 g/L at 70% pick-up gives 28 g/kg, that is 2.8% owf. A flame-retardant is judged by the limiting oxygen index (LOI), the minimum percentage of oxygen in an oxygen–nitrogen mixture that supports burning; ordinary cotton, below air’s 21%, burns freely.
5. Wool finishing, antistatic and soil release, heat setting, minimum application
Milling (fulling) deliberately felts wool: in warm, wet, soapy or acid conditions, with mechanical pressure, the scaled fibres creep in one direction (the directional frictional effect), so the cloth shrinks, thickens and becomes compact with a fuzzy face; it is stopped when the desired area shrinkage is reached. Decatizing sets the cloth with steam (or hot water) under tension on a perforated roller, giving lustre, a smooth hand and dimensional stability. Shrink-resist finishing stops felting by degrading the scales (oxidative chlorination, for example with chlorine or dichloroisocyanurate) or by adding a polymer that masks the scales and sticks fibres together; the two are often combined, as in chlorine–polymer treatment.
Antistatic finish: synthetics build up charge because they absorb little moisture; hydrophilic or ionic finishes (quaternary ammonium compounds, ethoxylates) raise surface conductivity. Soil-release finish: a hydrophilic polymer (a polyester–polyether type) makes the surface wettable, so oily soil is lifted by water in washing. Heat setting of synthetic fabric uses dry or steam heat under dimensional control at a temperature above the highest later process temperature (and well above the glass transition) but below the melting point, for a short time; it sets the shape against shrinkage and creasing, but it also changes dye uptake, so the same set temperature must be used throughout a lot.
Minimum application techniques apply the chemical in a small volume of liquor instead of padding at about 60 to 80% pick-up: foam finishing (the finish in a controlled foam), kiss-roll, spray and print-on applicators. Lower pick-up means less water to evaporate, less energy, less migration of the finish during drying (more even) and less effluent. The recipe is then stated as the add-on wanted, and concentrations are raised in step.
6. Pollution control and treatment of effluents
Textile wet processing uses much water and its effluent is a mixture: desizing gives a high organic load (size), scouring and mercerising give strong alkalinity and organics, bleaching gives oxidants and chlorinated compounds, dyeing gives colour, salt (a large amount in reactive dyeing, hard to remove) and metals from metal-complex and chrome dyes, and printing gives thickeners and solvents. Monitored parameters: pH, temperature, suspended and dissolved solids, colour, BOD, COD, heavy metals and oil and grease.
BOD₅ is the oxygen microbes consume in 5 days at 20 °C while oxidising the biodegradable organics. For a dilution test, BOD = ΔDO × (bottle volume ÷ sample volume). With first-order decay, BOD_t = L₀ (1 − e^(−kt)), where L₀ is the ultimate demand and k the rate constant. COD uses a strong chemical oxidant (dichromate) and counts almost all organic matter, so COD ≥ BOD; a low BOD/COD ratio shows the organic matter, such as many dyes and synthetic sizes, is not readily biodegradable, which calls for chemical or advanced treatment rather than plain biology.
| Stage | Operations | Removes |
|---|---|---|
| Preliminary and primary | Screening, equalisation, neutralisation, coagulation with alum, ferrous sulphate or lime, flocculation and settling | Solids, part of colour (disperse, vat, sulphur dyes), pH correction |
| Secondary (biological) | Activated sludge, aerated lagoon, trickling filter; anaerobic stage reduces azo dyes to amines, which an aerobic stage then degrades | BOD, biodegradable COD |
| Tertiary and advanced | Activated carbon, ozonation and other oxidation, membrane filtration and reverse osmosis | Residual colour, dissolved salts; water for reuse |
Key takeaways
- Roller printing uses engraved copper rollers with a doctor blade; screen printing forces paste through mesh with a squeegee, flat-bed or rotary. A paste is dye plus thickener, urea, alkali or acid donor, and auxiliaries; alginate is preferred for reactive dyes.
- Direct styles are print, dry, fix (steam or heat) and wash off: reactive on cotton with bicarbonate and steam; acid donor for wool, silk and nylon; high-temperature steaming or thermofixation for disperse on polyester.
- Resist blocks dye fixation; discharge destroys ground colour with a reducing agent; pigment printing uses binder and curing; transfer printing sublimes disperse dye from paper; inkjet is screenless and digital.
- Cotton finishes: crosslinking resins raise crease recovery but cut strength; cellulase bio-polishing removes fuzz permanently; add-on (% owf) = g/L × pick-up fraction ÷ 10; minimum application cuts the water to evaporate.
- Wool is milled to felt, decatized to set and made shrink-resistant by degrading scales or adding polymer; heat setting is above later process temperatures and below melting, and alters dye uptake.
- BOD = ΔDO × bottle ÷ sample volume and BOD_t = L₀(1 − e^(−kt)); COD ≥ BOD and a low BOD/COD ratio means poor biodegradability. Treat in stages: neutralise, coagulate, biological treatment, then advanced polishing.
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.
Sodium alginate is preferred over starch as the thickener for printing cotton with reactive dyes because
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Answer: C — it has few hydroxyl groups that react with the dye, so less dye is wasted and the print is brighter
Starch and cellulose-type thickeners offer many reactive hydroxyl groups that can bond with the reactive dye and waste it, which dulls the print; alginate has few, so more dye reaches the cotton. It is a natural polysaccharide, not a reducing agent, and the alkali is still supplied separately.In printing polyester with a disperse dye, the dried print is treated with high-temperature steam (about 175 °C) or dry heat of about 200 °C. The purpose is to
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Answer: C — make the dye diffuse out of the paste into the swollen, mobile polyester, where it dissolves
Fixation is diffusion of the dye into the fibre at a temperature well above the glass transition, as in dyeing; the thickener still has to be washed off. Disperse dyes do not become reactive or ionic, and polyester has no ionic sites.A white discharge on a cotton ground dyed with a suitable azo dye is obtained with sodium formaldehyde sulphoxylate in an alkaline paste and steaming. The chemical basis is that
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Answer: C — the reagent is a reducing agent that cleaves the azo link into colourless amines
A reductive discharge breaks the chromophoric –N=N– group to amines that have no visible colour. Oxidation of cotton would tender the fabric, and dissolving the dye or polymerising it is not the mechanism.A pigment print is dried and then cured at about 140 to 150 °C. What does curing achieve?
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Answer: D — It crosslinks the binder into a film that holds the pigment on the fabric surface
Pigments have no affinity for the fibre; the binder forms a crosslinked film on curing and holds them mechanically, with no steaming or wash-off. Sublimation is transfer printing, and leuco formation is the vat route.Transfer printing in which a paper carrying the design is pressed against the fabric with heat is suited mainly to
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Answer: A — polyester, using sublimable disperse dyes
Sublimable disperse dyes vaporise from the paper and dissolve in polyester; the other fibre–dye pairs need water-based fixation and do not transfer in the dry state.A cotton shirt fabric is finished with an N-methylol crosslinking resin and cured. Compared with the untreated fabric, which pair of changes is expected?
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Answer: D — Higher crease recovery and a lower tensile and abrasion strength
Crosslinks between cellulose chains lock the structure so that it springs back from creasing, but they also make it less able to share stress and more brittle, so tensile, tear and abrasion strength fall. Gain in both properties together is the tempting but wrong choice.A polyester fabric is heat set at 190 °C and then dyed at 130 °C. A second lot is heat set at 170 °C and dyed in the same bath. What is the most likely outcome?
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Answer: B — Different shades, because the set temperature changes the fibre structure and so changes dye uptake
The heat-set temperature changes crystallinity and free volume, so uptake of disperse dye differs between lots, which is why a lot is set at one uniform temperature. Both settings are above the 130 °C dyeing temperature, so neither lot is expected to shrink from the dyeing step, and heat-set polyester can be dyed.A dyehouse effluent has BOD₅ = 60 mg/L and COD = 900 mg/L. The best interpretation is that
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Answer: D — the organic matter is largely non-biodegradable, so advanced treatment is needed
COD counts almost all oxidisable organics while BOD counts only what microbes use in 5 days, so COD exceeds BOD; a ratio of about 0.07 means mostly recalcitrant matter such as dyes and synthetic auxiliaries. Concluding that a low BOD means a clean or easily biotreated effluent ignores the COD.Compared with padding a finish at 70% wet pick-up, foam finishing at a much lower wet pick-up of the same finish mainly gives
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Answer: A — less water to evaporate, lower energy use and less finish migration
The finish is applied in a small volume of liquor, so far less water has to be dried off, saving energy and giving a more even finish with less migration. The add-on is set by the recipe and is not higher by nature, and foam does not itself bond covalently.Which statements about printing styles and faults are correct?
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Answer: A — Rotary screen printing runs continuously with a squeegee placed inside a seamless perforated cylinder; B — In roller printing the doctor blade wipes excess paste from the engraved roller surface; C — A paste that is too thin causes flushing and a loss of sharp outlines
The statements on the rotary screen, the doctor blade and the over-thin paste describe the standard machine action and a standard fault. Poor wash-off leaves hydrolysed dye which stains the ground and makes it less white.Which statements about finishing are correct?
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Answer: A — Cellulase bio-polishing removes the protruding fuzz of cotton and so reduces pilling durably; B — Milling of wool depends on the directional friction of the scaled fibres under heat, moisture and pressure
Removing fuzz gives lasting pill resistance, and wool felting arises from the scales and friction. A repellent finish lowers surface energy but leaves pores open, so it is not waterproof, and decatizing is a steam setting step, not milling.Which statements about effluent from textile wet processing are correct?
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Answer: A — Desizing of starch-sized cloth contributes a high organic load that raises BOD and COD; B — An alkaline effluent is neutralised before biological treatment since microbes work best near neutral pH
Size from desizing is biodegradable organic matter, and neutral pH suits microbes. Hydrolysed reactive dyes are water-soluble and removed only partly by coagulation, and COD, which counts nearly all organics, is not lower than BOD.A reactive print paste of 200 kg is made from 6 kg of dye, 10 kg of urea, 2 kg of sodium bicarbonate and 100 kg of stock thickener, with water making up the rest of the 200 kg. The mass of water added is ____ kg.
Numerical answer — type the value.
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Answer: 82
Water = 200 − (6 + 10 + 2 + 100) = 200 − 118 = 82 kg. A paste recipe is a mass balance: the ingredients, including the thickener stock, must sum to the batch mass.A wrinkle-resistant resin product contains 50% active solids by mass. The target add-on is 6% solids on weight of dry fabric. The wet pick-up of the pad mangle is 75% (75 kg of liquor per 100 kg of dry fabric) and the liquor density is taken as 1.00 kg/L. The concentration of the resin product needed in the pad bath is ____ g/L.
Numerical answer — type the value.
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Answer: 160
Per kg of dry fabric: solids needed = 0.06 kg = 60 g. Liquor taken up = 0.75 kg = 0.75 L. Solids concentration = 60/0.75 = 80 g/L. The product is only 50% solids, so product = 80/0.50 = 160 g/L. Stopping at 80 g/L forgets the solids content of the product.In a BOD test, 6.0 mL of effluent is added to a 300 mL bottle and filled with dilution water that has negligible oxygen demand. The dissolved oxygen of the diluted sample is 8.2 mg/L at the start and 3.7 mg/L after 5 days at 20 °C. The BOD₅ of the effluent is ____ mg/L.
Numerical answer — type the value.
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Answer: 225
ΔDO = 8.2 − 3.7 = 4.5 mg/L. Dilution factor = 300/6.0 = 50. BOD₅ = 4.5 × 50 = 225 mg/L. Quoting 4.5 mg/L forgets to scale back to the undiluted effluent.An effluent has an ultimate BOD of L₀ = 400 mg/L and the BOD follows first-order kinetics, BOD_t = L₀ (1 − e^(−kt)), with k = 0.20 per day (natural-logarithm base) at the test temperature. The BOD₅, to the nearest whole number in mg/L, is ____.
Numerical answer — type the value.
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Answer: 253
kt = 0.20 × 5 = 1.0. BOD₅ = 400 (1 − e⁻¹) = 400 × 0.63212 = 252.85, which is 253 mg/L. Using 400 × e⁻¹ (147 mg/L) would give the demand remaining, not the demand exerted.1000 m³ of an alkaline effluent has pH 11. Treat it as a dilute solution of a strong base with [OH⁻] = 1.0 × 10⁻³ mol/L (Kw = 1.0 × 10⁻¹⁴) and ignore buffering. It is brought to pH 7 with 100% sulphuric acid (molar mass 98 g/mol, two H⁺ per molecule). The mass of acid required is ____ kg, to the nearest whole number.
Numerical answer — type the value.
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Answer: 49
Volume = 1000 m³ = 10⁶ L. OH⁻ = 10⁻³ mol/L × 10⁶ L = 1000 mol, which needs 1000 mol H⁺. H₂SO₄ = 1000/2 = 500 mol × 98 g/mol = 49,000 g = 49 kg. Taking one H⁺ per molecule would give 98 kg.