Semicontinuous Dyeing Processes

Thermosol Process. The most important continuous dyeing process for PES fibers is the thermo sol process. It is applied primarily to PES-CEL blends (see Section 4.12.4). The thermosol process consists of four individual steps (1) Padding of the dye liquor on the fabric, (2) Drying, (3) Fixing of the dyes in the fiber, (4) Aftertreatment. The four individual steps usually follow one another in one pass. Systems also exist that include the subsequent overdyeing of the cellulose component in blended fabrics [e.g., by using the pad steam technique (see Section 4.1.1)]. 

The quality of thermo sol dyeing is assessed according to its levelness over large yardages. For good quality, uniform pretreatment of the material and a very uniformly operating thermosol system are required. 

Pretreatment ensures, above all, that the material is free of burls and wrinkles, contains no dirt or preparation stains, and is uniformly wettable. It includes careful singeing, washing, and bleaching. 

Drying is carried out contact free up to a residual moisture of 20-30% and then on any equipment available. 

The fixing time and temperature depend on the aggregate, the dyes used, and the depth of shade (see Table 4.5 for guide values). 

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Continuous or semicontinuous dyeing processes are uncommon because dry heat treatment (thermosol) hardens wool. Besides, application of the wool dye requires dyeing in aqueous liquor or dye fixation by steaming. 

Manu B, Chauhari S (2003) Decolorization of indigo and azo dyes in semicontinuous reactors with long hydraulic retention time. Process Biochem 38 1213-1221... 

Maas R, Chaudhari S (2005) Adsorption and biological decolourization of azo dye Reactive Red 2 in semicontinuous anaerobic reactors. Process Biochem 40 699-705... 

Pad Processes. In a continuous or semicontinuous processes, the fabric is first padded with the dye solution. The substantive adsorption of dyes during padding is avoided by employing minimum amounts of pad liquor at ambient temperatures. For even penetration of the dye, the material is then steamed without intermediate drying for 1-3 min in saturated steam (pad steam process ). Alternatively, it is heated to 80-85 °C and rolled up in a closed chamber where it is slowly rotated for 2-8 h (pad roll process). 

In the semicontinuous pad roll process, after impregnation with the pad liquor, the material is batched up and stored in an air-free steam chamber (dry and wet temperature ca. 98°C) where it is rotated slowly for about 1 h. By this procedure, the dye is almost completely fixed. Excellent penetration is obtained. 

In the thermosol pad batch process, a semicontinuous process, alkali-sensitive disperse dyes can be used if the alkali is applied after the thermosol process. Conversely, if the thermosol process is carried out after the pad batch step, the pH must be lowered before the thermosol process so that the disperse dye is not destroyed. 

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