Preparatory Processing and Dyeing

Acid dyeing of nylon

Acid dyes are water-soluble anionic dyes, usually sodium salts of sulfonic acids, and they are applied to nylon (polyamide) and wool from an acidic dyebath. Nylon has terminal amino groups, and in an acid bath these groups are protonated to -NH_3^+ sites, which attract the anionic dye by electrostatic (ionic) bonding. Acidity therefore controls both the amount of dye taken up and the rate of exhaustion. Example: nylon dyed at pH 4 takes up much more anionic acid dye than nylon dyed at pH 8, where most amino groups are not protonated.

Sodium chloride or sodium sulphate is used as a levelling (retarding) agent in the acid dyeing of polyamide. Its sodium ions compete with the anionic dye for the same protonated amino sites, which slows uptake and gives even exhaustion across the fabric. The same salt used with direct dyes on cotton or with reactive dyes on cotton acts as an exhaustion electrolyte, not as a levelling agent, so the fibre-dye pair must be matched correctly.

Dye-fibre pairs are also matched to pH. Basic dyes on acrylic, acid dyes on wool and disperse dyes on polyester are all applied from acidic or weakly acidic baths, whereas vinylsulphone reactive dyes on cotton need alkaline conditions, because cellulose must be ionised to the cellulosate anion before it reacts covalently with the dye.

Dye partition coefficient (isotherms)

At equilibrium a dye distributes between the fibre and the dyebath. The partition coefficient is defined as K = C_f / C_s, where C_f is the dye concentration in the fibre (for example g/kg of fibre) and C_s is the dye concentration in the solution (g/L) at the same equilibrium. A plot of C_f against C_s is the dyeing isotherm, and in the linear (Nernst) range its slope is the partition coefficient, so a steeper isotherm means higher affinity of the dye for the fibre. Example: if the fibre holds 10 g/kg of dye when the solution holds 0.1 g/L at equilibrium, then K = 100 L/kg.

An isotherm describes only the equilibrium state. It gives no information on the rate of dyeing, which depends on kinetic factors such as diffusion of the dye into the fibre.

Temperature changes the equilibrium position. Adsorption of vat dyes onto cotton is exothermic, so the equilibrium affinity and the exhaustion fall as the temperature rises. A higher temperature increases dye diffusion and migration within the fibre, which allows the dye to redistribute and improves levelness even though exhaustion is lower.

For disperse dyes on polyester, a higher dyeing temperature raises both the solubility of the dye in the bath and the saturation uptake, which appears as a higher plateau of the isotherm on the fibre axis.

Scouring and mercerisation of cotton

Scouring removes natural impurities such as waxes, pectins, fats and seed coat fragments from raw cotton. It uses dilute sodium hydroxide at high temperature, near boiling, for a comparatively long treatment, and it makes the fabric hydrophilic and absorbent. Cold mercerisation uses concentrated sodium hydroxide, about 25 to 30 per cent, at low temperature. Scouring and cold mercerisation therefore differ in alkali concentration, temperature and treatment time, so all three parameters change between the two processes.

Mercerisation with concentrated caustic causes the cotton fibre to swell and converts cellulose I into cellulose II. This improves lustre, dye affinity and dimensional stability.

A fabric that is desized and bleached but not scoured still contains residual natural impurities. Seed coat fragments remain on the fabric, and the residual waxes and pectins make it hydrophobic, so its water absorbency is poor. Bleaching removes the colouring matter and gives a high whiteness index. Cotton has low crease recovery, so high crease recovery is not a property of the fabric in this state.

In continuous scouring, the fabric passes through a saturator (alkali bath) before the J-box. Only the bath liquor picked up contributes alkali, so the concentration on the exiting fabric is C_{exit} = C_{bath} \times (WE_f - WE_i)/WE_f, where WE is the wet expression as a fraction of fabric weight. Example: if the wet expression rises from 50 % to 100 % and the fabric must leave with 4 % alkali liquor, the bath must contain 4/0.5 = 8 %.

Reactive dyeing of cotton

Reactive dyes form covalent bonds with the hydroxyl groups of cellulose, usually under alkaline conditions that ionise cellulose to the cellulosate anion. Each anhydroglucose unit of cellulose carries three hydroxyl groups (at C2, C3 and C6), so a cellobiose repeat unit carries six. A monofunctional reactive dye has one reactive group and so reacts with one hydroxyl group per dye molecule. The unreacted hydroxyl fraction is calculated from moles of dye reacted and moles of accessible OH groups, using moles = mass / molar mass.

A dichlorotriazine (DCT) group carries two chlorine atoms on one triazine ring, but the dye is not regarded as homo-bifunctional in this classification, because at room temperature only one chlorine is displaced by cellulose. The second chlorine reacts only at higher temperature, after the ring has been deactivated, so the two chlorines do not react together at room temperature.

In continuous padding with 100 % wet expression, the fabric retains solution equal to its own weight. For a solution of specific gravity 1, 1 L weighs 1 kg. Example: a padding liquor of 15 g/L at 80 % wet expression carries 15 \times 0.8 = 12 g of dye per kg of fabric, which is 1.2 % owf. The percentage of dye fixed is the fixed dye (after removal of unreacted and hydrolysed dye) divided by the applied dye, times 100.

Cotton bleaching agents

Sodium chlorite (\mathrm{NaClO_2}) bleaches cotton in an acidic bath, usually buffered with acetic acid at about pH 3 to 4, where it releases chlorine dioxide as the active bleaching species. Sodium hypochlorite is an alkaline bleach and is not the agent used for acidic bleaching. Magnesium chloride and sodium chlorate have no role as cotton bleaching agents in this sense.

Hydrogen peroxide bleaches in alkaline conditions, and alkaline peroxide bleaching also scours the fabric by removing waxes, fats and pectins. A partly scoured cotton fabric bleached with peroxide therefore absorbs water better than one bleached with acidic chlorite, because chlorite does not remove these hydrophobic impurities. The scouring action of peroxide is the reason for the difference in absorbency.