Introduction
In textile dyeing, synthetic fibres need the right dye class to achieve the shade, levelness and fastness expected in production. The choice is guided by the physical and chemical nature of the fibre, so the same dye cannot be used equally well on every material. Disperse dyes, basic dyes and acid dyes each have their own place in synthetic fibres dyeing. The process conditions also matter, especially temperature, pH, carriers and after-treatment.
Disperse Dyes for Synthetic Fibres
What Disperse Dyes Are
Early disperse dyes were developed for dyeing cellulose diacetate. The dyes had a small molecular size, and although the fibre could be dyed quite easily, fastness properties were limited. With the introduction of polyester fibre, similar dyes gave much better wet fastness. Disperse dyes are only slightly water soluble, and they form a dispersion in water. This limited water solubility is essential for their application by exhaustion methods. The rate of dyeing is determined by the speed at which the dye is transferred from the aqueous dispersion to the solution and then into the fibre.
Disperse dyes are normally classified according to their degree of sublimation. Class A dyes are of small molecular size and readily sublime, and therefore show low heat fastness, whereas Class B, C and D dyes have increasingly better sublimation properties. In the case of dyeing with disperse dye, a reduction clearing process should be carried out to remove any dyestuff which remains unfixed and any dyeing aids, including carrier residues, remaining on the surface of the fibre.
Disperse Dyeing of Cellulose Acetate and Triacetate
This fabric has traditionally been dyed using batchwise methods, often jig dyed. In the case of jig dyeing, the fabric is passed end to end through a trough containing the dye. The temperature for the fabric on the rollers should be around 80°C, and dyeing is continued for up to 1 h. The conditions for dyeing cellulose triacetate are very similar, but a higher temperature of around 120°C is needed. If this is not possible, then dyeing should be at the boil with the aid of a carrier.
A number of carriers may be used, including diethyl phthalate and butyl benzoate. Normally, carriers are effective in low concentrations, but in the case of jig dyeing at low liquor ratios, higher levels can be effective at up to 20 g/L. Additional auxiliaries used in dyeing of disperse dyes on triacetate include buffers that are required to control pH and sequestering agents to reduce any problems associated with water hardness. In the case of certain blue and violet shades, it is sometimes necessary to use gas-fume fading inhibitors.
Disperse Dyeing of Polyamide
Disperse dyes can be used for dyeing polyamide if levelness of shade is more important than fastness. Compared to acid dyes on polyamide, the hues produced are often duller. Application conditions are similar to those used on triacetate. Temperatures of 95°C to 100°C or up to 120°C are employed for times of up to 45 min. The pH of the dye bath is buffered to 5.5 to 6.5.
Disperse Dyeing of Polyester
For polyester, the less-volatile dyes are the most suitable, particularly for dyeing fabrics which have to withstand high-temperature processing. There are two conventional batchwise methods for dyeing polyester with disperse dyes: at the boil with the aid of a carrier and at 130°C under pressure.
The dye bath must be buffered to maintain a slightly acid pH. The dye bath is set with water, disperse dye, dispersing agent and buffers. If a carrier is to be used, this is added as required. The temperature is raised to the required level, and dyeing is continued for 30 to 60 min, after which the bath is cooled and drained. The material is then rinsed, soaped, reduction cleared, rinsed again and then dried. Cooling the bath is essential before discharging when temperatures above 100°C are used.
Basic Dyes for Synthetic Fibres
Basic dyes are among the oldest known dyes, and some are still known by their original names such as methyl violet, malachite green and rhodamine magenta. Dyes of this type dissolve in water to give positively charged coloured ions which can dye any fibre that contains a negatively charged group. Thus in addition to being used on natural fibres such as cotton, wool and silk, they may also be used for dyeing polyamide, although this is not a popular commercial choice.
Their main application for synthetic fibres comes in dyeing modified acrylic fibres, which give better fastness than other dye classes. Basic dyes are adsorbed rapidly; thus it is necessary to control carefully their dyeing rate with the aid of retarding agents, which can be negatively or positively charged. It is their respective charge which determines their mode of action. A positively charged retarding agent will complex with the fibre, slowing down the dye uptake, whereas a negatively charged product reacts with the dye to control dye uptake. During heating in the dye bath, this association breaks down to allow dyeing of the fibre to take place.
Acid Dyes for Synthetic Fibres
Acid dyes can be used for dyeing natural fibres such as wool and silk. They are also used for dyeing polyamide. Acid dyes are anionic, and in solution they will ionise into sodium ions and large negatively charged coloured ions. For this reason, they will dye any fibres which have positively charged sites. In comparison to wool, polyamide has fewer sites available for dyeing, and thus the shades produced on polyamide are generally weaker than on wool but of higher fastness.
Conclusion
Dyeing of synthetic fibres depends on matching the dye class with the fibre type and the process conditions. Disperse dyes are widely used for cellulose acetate, triacetate and polyester, while basic dyes are mainly used for modified acrylic fibres. Acid dyes remain important for polyamide, where they produce shades with good fastness. Temperature, pH, carriers and after-treatment all influence the final result. With careful selection and control, synthetic fibre dyeing can continue to deliver reliable and consistent results.
