As we have seen, the dyeing of polyester with disperse dyes at the boil is slow because of the low rate of diffusion of the dyes into the fibre. The activation energy for diffusion is quite high and raising the dyeing temperature from 100 to 130 °C considerably increases the rate of dye diffusion. Dyeing at this higher temperature under pressure, without a carrier, considerably increases the rate of dyeing and gives better coverage of filament irregularities because of the improved migration of the dyes. Dyeing is then also possible using higher molecular weight dyes, whose rates of diffusion at 100 °C are unacceptable. This permits production of dyeings with better fastness to light and to sublimation during permanent pleating. For those fabrics and yarns that lose bulk when dyed at 130 °C, dyeing at a lower temperature (110–120 °C) in the presence of some carrier is preferred.
The dyebath is usually set at pH 4.5–5.5 using either ammonium sulphate plus formic or acetic acid, or acetic acid alone. The weakly acidic dyebath ensures neutralisation of any residual alkali from scouring, which readily catalyses hydrolysis of the polyester, decreasing its strength. Reduction of some azo disperse dyes can occur during dyeing at high temperatures, while others undergo hydrolysis. These effects are minimal when dyeing in weakly acidic solution. The concentrated dye dispersion is added to the bath at 50–60 °C. The bath may already contain a small amount of dispersant (0.5 g l–1), if required.
Lubricants in the dyebath avoid possible crack and crease marks in dyeing fabric in jet machines. The temperature of the bath is then slowly raised to 130 °C. A typical heating rate is about at 1.5 °C min–1. Dyeing continues at the maximum temperature for about 60 min.
Each particular dyeing will have an optimum temperature/time profile, depending upon the type of goods, the machine being used and the dyes in the formula. A set of generalised dyeing conditions is used, however, provided that the dyebath exhaustion, the colour uniformity, and the shade reproducibility from batch to batch are acceptable. Dyeing times can be kept to a minimum by temperature control of the rate of exhaustion that gives uniform dye absorption. In this way, long leveling times at the maximum dyeing temperature are not needed. The dyeing time should be long enough for the dyes with the lowest dyeing rate to approach equilibrium.
Disperse dyes do not generally interfere with each other and prevent their mutual absorption but they do have different dyeing rates. The dyeing rate is always higher at low dye concentrations in the bath. Some disperse dyes are deliberate mixtures of dyes of the same or different hue and about the same dyeing rate. They give fairly rapid dyeing because each dye is only present at low concentration.
PET fibre contains 1–4% of oligomers, mainly a cyclic trimer of ethylene terephthalate. It has a high melting point and is soluble enough in hot water during pressure dyeing to be extracted from the fibre. The oligomers also migrate to the PET fibre surface during steam heat setting, and to a lesser extent on dry setting. The oligomer can often be seen as a white dusty powder on the surface of the goods, or on the dyeing machine walls. Hydrolysis of oligomer deposits on machine surfaces by heating with an alkaline solution under pressure provides effective cleaning. Precipitated oligomer can cause nucleation of disperse dye crystal formation leading to coloured specks on the goods. In addition, oligomer particles reduce the rate of liquor flow through yarn packages and cause filament friction in spinning. The oligomer is much less soluble at temperatures below the boil. To avoid its precipitation once dyeing is concluded, the dyebath is drained at as high a temperature as possible, even above 100 °C. This can lead to problems in dyeing woven goods in rope form in jet machines since creases and crack marks can form while the polymer is still somewhat plastic. In these cases, draining at a lower temperature is necessary and the dyer must depend to a greater extent upon the subsequent rinsing and reduction clearing process to remove oligomer residues.
During dyeing, particularly of deep shades, there will invariably be some dye particles that adhere to the fibre surfaces, or are retained by yarns without penetration into the fibre. These mechanically held particles result in decreased fastness to washing, rubbing, sublimation and dry cleaning. Their presence also tends to dull the shade. Superficial dye particles can be detected by rinsing a dyed sample with a little cold acetone. This will dissolve the surface particles and produces a coloured solution but it does not remove any dye from within the PET fibres. For pale shades, scouring removes deposits of surface dye. Deep dyeing with disperse dyes on PET fibres will invariably require treatment by reduction clearing to give satisfactory crocking fastness. This process involves treatment with alkaline hydros (2 g l–1 NaOH, 2 g l–1 Na2S2O4.2H2O) and a surfactant ( 1 g l–1) for 20 min at 70 °C. The reduction clearing temperature is well below the glass transition temperature of the polyester. The ionic compounds do not therefore penetrate into the fibres and only reduce the dye on the fibre surface. The reduction of azo disperse dyes is relatively easy but anthraquinone derivatives are more difficult to remove. The latter must be reduced and washed off the surface before re-oxidation occurs. The less soluble oxidised form is then held in suspension by the surfactant in the bath.
Some disperse dyes, originally from ICI (now available through DyStar), allow easy clearing of surface deposits. These are methyl esters of carboxylic acids that readily hydrolyse under alkaline conditions. The free carboxylic acids formed by hydrolysis are soluble in alkaline solution. This allows clearing without a reducing agent. Since the alkali does not penetrate into the PET fibre at the clearing temperature, the dye within the fibres is unaffected.
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