GATE Textile Engineering Chemical Processing Syllabus

What are vat dyes, and what is the essential feature of their application to cotton?

Vat dyes are water-insoluble dyes (e.g. indigo, anthraquinone types) that must be reduced (vatted) with sodium dithionite in alkali to a soluble leuco form, absorbed by the fibre, then re-oxidized to the insoluble pigment inside the fibre, giving excellent fastness.

Write the reduction (vatting) reaction central to vat dyeing.

The insoluble keto dye is reduced by sodium dithionite in caustic soda to the soluble leuco (enolate) form: $$\ce{Dye(C=O) + Na2S2O4 + NaOH -> Dye(C-ONa)\;(leuco) }$$ The leuco form is substantive to cotton; subsequent oxidation regenerates the insoluble dye.

What is the role of sodium dithionite (sodium hydrosulphite) in vat dyeing?

Sodium dithionite ($\ce{Na2S2O4}$) is the reducing agent that converts the insoluble vat pigment into its soluble leuco form, which has affinity for cellulose, enabling the dye to penetrate the fibre.

What class of dyes is used to dye polyester, and why are ordinary water-soluble dyes unsuitable for it?

Disperse dyes are used for polyester. Ordinary water-soluble (ionic) dyes are unsuitable because polyester is hydrophobic, highly crystalline, and lacks reactive/ionic sites; only small, non-ionic, sparingly soluble disperse dyes can diffuse into it.

By what mechanism do disperse dyes dye polyester?

Disperse dyes dye polyester by a solid-solution (dissolution) mechanism: the slightly water-soluble dye monomer is absorbed onto the fibre surface and then diffuses into the polymer matrix where it dissolves, held by Van der Waals forces and hydrogen bonding.

What is the 'glass transition temperature' significance in disperse dyeing of polyester?

Polyester must be heated above its glass transition temperature ($T_g \approx 80\,^{\circ}\text{C}$) so that the amorphous regions become mobile and segmental chain motion opens free volume, allowing the disperse dye to diffuse into the fibre. Below $T_g$ diffusion is negligible.

Describe the three main methods of applying disperse dyes to polyester.

(1) Carrier (atmospheric) dyeing at ~100 °C using carriers to swell the fibre; (2) High-temperature high-pressure (HTHP) dyeing at ~130 °C under pressure; (3) Thermosol (continuous) process: pad–dry–bake at ~190–220 °C for dry heat fixation.

What is the function of a carrier in disperse dyeing of polyester, and name an example?

A carrier swells the compact polyester structure and lowers its effective $T_g$, accelerating dye diffusion at atmospheric boil (~100 °C). Examples include o-phenylphenol, methyl naphthalene, and chlorinated benzenes (e.g. trichlorobenzene).

What is the thermosol process and its typical temperature?

The thermosol process is a continuous disperse-dyeing method for polyester: the fabric is padded with disperse dye, dried, then baked (thermofixed) with dry heat at about $190\text{–}220\,^{\circ}\text{C}$ for 30–60 s, during which the dye sublimes/diffuses into the fibre.

What class of dyes is used to dye wool, silk and nylon, exploiting their amino groups?

Acid dyes are used to dye wool, silk and nylon; these protein/polyamide fibres have basic amino ($\ce{-NH2}$) end groups that protonate in acid and bind the anionic acid dye.

What type of bond chiefly holds an acid dye to wool or nylon?

Primarily an ionic (electrostatic/salt) bond forms between the anionic sulphonate group of the dye ($\ce{-SO3^-}$) and the protonated amino group ($\ce{-NH3^+}$) of the fibre, supplemented by Van der Waals and hydrogen bonding.

Write the simplified ionic binding reaction of an acid dye with wool in an acidic dyebath.

In acid, the fibre amino group is protonated and then forms a salt link with the dye anion: $$\ce{Wool-NH2 + H^+ -> Wool-NH3^+}$$ $$\ce{Wool-NH3^+ + ^-O3S-Dye -> Wool-NH3^+ \, ^-O3S-Dye}$$

Why is acid (low pH) required when dyeing wool with acid dyes?

Low pH protonates the amino end-groups of wool to form cationic $\ce{-NH3^+}$ sites that attract the anionic dye, and it controls dissociation so dye-fibre ionic bonds form. The strength of acid needed depends on the dye class (strongly, weakly, or milling acid dyes).

Classify acid dyes by their levelling behaviour and pH of application.

Acid dyes are classed as: (1) strongly acidic / levelling (equalising) dyes — applied at low pH (sulphuric acid), good levelling, poor wet fastness; (2) weakly acidic / half-milling dyes — applied at moderate pH; (3) milling (neutral-dyeing) dyes — applied near neutral, good wet fastness, poor levelling.

What are metal-complex (premetallized) dyes, and how do they relate to acid dyes?

Metal-complex dyes are acid dyes in which a metal atom (usually chromium or cobalt) is coordinated with the dye molecule(s). They are applied like acid dyes to wool/silk/nylon and give superior light and wet fastness due to the stable metal chelate.

Distinguish 1:1 and 1:2 metal-complex dyes.

In 1:1 metal-complex dyes one metal atom is bound to one dye molecule; they are applied at strongly acidic pH (~1.8–2.0, sulphuric acid). In 1:2 metal-complex dyes one metal atom coordinates two dye molecules; they are applied at weakly acidic to neutral pH and give better fastness with gentler conditions.

What metal is most commonly used in metal-complex dyes for wool, and what fastness benefit does it give?

Chromium (Cr) is the most common metal; chromium complexation greatly improves the light fastness and wet (washing) fastness of the dyeing by forming a stable, large, insoluble chelate within the fibre.

What are mordant (chrome) dyes for wool, and how is chrome applied in the afterchrome method?

Mordant dyes require a metal mordant (usually chromium) to develop fast shades on wool. In the afterchrome method the wool is first dyed with the mordant dye, then treated with sodium or potassium dichromate ($\ce{Na2Cr2O7}$/$\ce{K2Cr2O7}$), which forms an insoluble dye-chromium complex inside the fibre for excellent fastness.

Why must wool be dyed under milder conditions than cotton (regarding temperature/pH/time)?

Wool is a sensitive protein fibre; prolonged boiling, strong alkali, or harsh oxidants degrade the keratin (yellowing, strength loss, felting). Hence wool is dyed under acidic to neutral pH, controlled temperature, and limited boiling time to protect fibre integrity.

Why is alkali avoided when wet-processing wool?

Keratin in wool is readily attacked by alkali, which hydrolyses peptide and cystine disulphide bonds, causing yellowing, loss of strength, and dissolution. Therefore wool processing uses acidic or neutral conditions, never strong alkali.

Compare the dyeing of cotton, silk, and polyester in terms of preferred dye class.

Cotton (cellulose) is best dyed with reactive, direct, or vat dyes; silk (protein) with acid, metal-complex, or reactive dyes; polyester (hydrophobic) only with disperse dyes. The choice follows fibre chemistry: anionic sites for silk, hydroxyls for cotton, hydrophobic matrix for polyester.

What is roller (machine) printing, and what carries the design?

Roller printing is a continuous machine-printing method in which the design is engraved (etched) into the surface of copper rollers; each engraved roller, supplied with one colour of print paste, transfers its pattern to the fabric pressed against it. One roller is used per colour.

In roller printing, how is the print paste held on the engraved roller and excess removed before printing?

The engraved (intaglio) cells of the copper roller pick up print paste from a furnisher/colour box; a sharp steel doctor blade scrapes excess paste off the smooth (non-engraved) surface, leaving paste only in the engraved design cells, which then transfer to the fabric.

What is the function of the 'lint doctor' (cleaning blade) in roller printing?

The lint doctor is a second blade positioned after the printing point on the roller; it removes lint, loose fibres, and any paste picked up from the fabric, keeping the roller surface clean to prevent smudging/streaks on subsequent repeats.

State a key advantage and a key limitation of roller printing.

Advantage: very high production speed for long runs of continuous repeating designs with fine detail. Limitation: high engraving cost and setup time make it uneconomical for short runs; the repeat size is limited by the roller circumference, and the number of colours is limited by the number of rollers.

What is a print paste, and what are its essential components?

A print paste is the viscous medium that carries colourant onto fabric in printing. Essential components are the dye/pigment (colourant), a thickener (to control viscosity and prevent spreading), water/solvent, and auxiliaries such as binders, fixing agents, hygroscopic agents, and pH controllers.

What role does the thickener play in a print paste?

The thickener gives the paste the right viscosity so it stays sharply within the design boundaries without bleeding, holds the dye in intimate contact with the fibre during drying/steaming, and acts as a carrier; examples include sodium alginate (for reactive prints), starch, and gums.

Why is sodium alginate the preferred thickener for reactive-dye printing on cotton?

Sodium alginate is anionic; its carboxylate groups are repelled by (and do not react with) the anionic reactive dye and cellulose, so it does not bind the reactive dye. This prevents loss of colour value and allows easy wash-off, giving bright, well-fixed prints.

In dye printing, what post-printing step fixes the dye, and what does it provide?

Steaming (ageing) is the post-printing fixation step: it provides moisture and heat so the dye dissolves, the fibre swells, and the dye diffuses and bonds (e.g. reactive dye reacts with cellulose, disperse dye diffuses into polyester). It is followed by washing-off to remove thickener and unfixed dye.

Compare exhaust (batch) dyeing with continuous (pad) dyeing in terms of principle.

In exhaust dyeing the fabric is immersed in a finite dyebath and dye gradually transfers (exhausts) onto the fibre over time at a fixed liquor ratio (batch process). In continuous dyeing the fabric is padded through a concentrated liquor, squeezed to a fixed pick-up, then dried and fixed continuously — economical for long, level runs.

Define wet pick-up (expression) in pad dyeing and give its formula.

Wet pick-up is the amount of liquor retained by the fabric after padding/squeezing, expressed as a percentage of dry fabric mass: $$\text{Pick-up}\% = \frac{W_{wet} - W_{dry}}{W_{dry}} \times 100$$ It is controlled by the mangle (padder) nip pressure and determines the depth of shade in continuous dyeing.