The Thermosol process of polyester fyeing with disperse dyes

The Thermosol process with disperse dye is a continuous dyeing process introduced by Du Pont in 1949. A dispersion of the disperse dyes is padded onto the polyester fabric. The material is then dried using a hot flue air dryer or by infrared radiation, the latter usually giving much less migration of the dye. The use of a migration inhibitor in the pad bath is usually recommended. Even then, dye migration during drying of materials of 100% polyester is difficult to eliminate and such materials tend to dye more deeply on the yarn surface. Final drying of the padded material takes place using heated cylinders. Section 10.5 discusses padding and migration in continuous dyeing.

The dry fabric is then heated in air, or by contact with a hot metal surface, to a temperature in the range of 190–220 °C for 1–2 min. In hot air, at least 50% of the time is for heating the polyester to the maximum temperature. The specific conditions depend on the type of equipment, the dyes and the fabric. As the fabric approaches the maximum temperature, the disperse dyes begin to sublime and the polyester fibres absorb their vapours. (Sublimation is the transformation of a solid directly into a gas without forming the liquid phase. A common example is the evaporation of ice on a cold day.

At about 200 °C, sublimation of the solid dye, transfer of its vapour into the fibres, and penetration into the fibres by diffusion are all quite rapid. Commercial disperse dyes for the Thermosol process are usually classified according to their ease of transfer by sublimation. This is related to the their fastness to heat in hot pressing and pleating. It is imperative that as much of the vaporised dye as possible be absorbed by the polyester fibres. If the rate of sublimation is too low, dye particles will remain in the fibre matrix and the colour yield will be low. When the rate of sublimation is too high, the dye vapour builds up faster than it can be absorbed by the polyester and escapes from the proximity of the fibres, usually depositing on the machine walls. The temperature and time of heating must therefore be carefully controlled to provide the appropriate rate of sublimation and the optimum colour yield.

Despite the simple dyeing mechanism, there are a number of technical problems that can result in inferior dyeings. The fabric must initially contain a uniform distribution of dye particles if the final dyeing is to be level. Therefore, uniform dispersion and padding are crucial. Migration must be minimised, particularly if it leads to more dye on one face of the fabric than the other. During the sublimation stage, it is essential to provide conditions that allow a balance between the rate of dye vaporisation and absorption of the vapour by the fibres. The Thermosol process is widely used for narrow fabrics of 100% PET such as ribbons and belts. The vapour dyeing technique also applies to transfer printing.

The Thermosol method is popular for dyeing the polyester component in cotton/polyester fabrics. In this case, the absorbent cotton fibres in the fabric hold almost all the initial dye dispersion padded onto the material. This helps to reduce dye migration during drying. During subsequent heating, the dye vaporizes and transfers from the cotton into the polyester fibres. Since two types of fibres are being continuously dyed, each with a separate fixation step, the dyeing ranges for cotton/polyester materials tend to be very complex. Both the dyes for the polyester and for the cotton are initially padded onto the fabric. The polyester is dyed in the Thermosol unit. After additional padding of the cooled fabric with a solution of the other required chemicals for dyeing the cotton, it passes through a steamer. This promotes diffusion and fixation of dyes on the cotton. The second ­pad contains NaOH and salt solution for dyeing with reactive dyes NaOH and Na2S2O4 for vat dyes, NaS or NaSH for sulphur dyes and simply salt solution for direct dyes. A thorough washing of the dyed fabric completes the process. This includes rinsing, an oxidation step in the case of vat and sulphur dyes, soaping to remove surface colour and final rinsing.