Pad-steam process

Cottoa—acryHc fiber bleads are also used for high quaHty upholstery pile fabrics. Besides the oae-bath exhaust dyeiag procedure involving a very high ratio of Hquor to fabric, a coatiauous pad-steam process is used to dye these fabrics. After padding, the goods are steamed for 7—15 min at 98—100°C. The material thea must be riased warm and cold before dryiag. 

Figure 12.20 Typical dye build-up curves for pad-steam process variants A and B on mercerised and unmercerised cotton [158]...

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). 

Continuous Processes. Vat dyes are especially suitable for continuous application. Dyeing is performed according to the pad steam process or the wet-steam process. The continuous processes are used almost exclusively for dyeing woven fabrics and only to a small extent for knitwear. 

The wet-steam process is used mainly for voluminous open fabrics with a high liquor-carrying capacity (e.g., toweling, cord, velvet). Unlike the pad steam process, this process does not require drying after dye application in the padder. 

Absorption of Dyes. In the exhaustion process, the dye is generally absorbed at 60-110°C in 30-60 min, and in the pad steam process at ca. 102-105 °C in 30-60 s. Dyeing with reducing agents is always performed by the exhaustion process. The fixing yield depends greatly on the liquor ratio. Optimal fixation and dye penetration are achieved with the pad roll process, in which the dye is applied from a concentrated liquor. 

Belts for technical purposes (safety belts) can be dyed most efficiently by the pad steam process in continuous systems. 

The most important continuous dyeing methods are the thermosol pad steam process and the thermo sol pad batch process. In all continuous processes, the sub-stantivity of the reactive dyes should be taken into account, i.e., the reactive dye can exhaust from the pad liquor or accumulate to a greater or lesser extent even at room temperature depending on the pH value and salt content. 

The thermosol pad steam process is especially important. Disperse and reactive dyes are padded together, and alkali and salt are padded after the thermosol passage. If a steamer is not available, the reactive dye can be fixed continuously by means of alkali shock in a roller vat with a relatively high alkali concentration. 

Disperseand Sulfur Dyes (see also Section 4.7.3). The thermosol pad steam process is the primary method for application of disperse and sulfur dyes. The sulfur dyes that are used preferentially in the ready-for-dyeing soluble form, and are especially inexpensive for muted shades, must be applied after the thermosol step because of their incompatibility with disperse dyes. 

Dyeing is performed mainly by exhaustion processes (batchwise). Another dyeing method with increased importance is the gel dyeing process. Dyeing in the gel state takes place during production of the fiber, i.e., after the fiber has been extruded and the solvent has been washed out, but before the fiber is stretched and dried (see Section 4.14.6). Continuous dyeing of stock, cable, and tops is also possible according to the pad steam process. 

Stock, cable, and top can be dyed on special machines according to the pad steam process [172], preferably with pressurized steam at temperatures above 100°C to obtain short fixing times. To avoid drying and condensed water spots, the steam should be neither superheated nor wet. A typical pad liquor contains acetic acid (pH 4.5) and dye solvent in addition to steam-resistant, readily water soluble cationic dyes (usually liquid brands) [173],... 

Continuous dyeing of PAC-cotton plush with cationic and direct dyes by the pad steam process plays an important role. The choice of dyes must take into account liquor stability, reservation of PAC or CEL fiber, and solubility. Precipitation of cationic and anionic dyes present in the pad liquor at relatively high concentrations cannot be avoided solely by dye selection. Suitable auxiliary systems have been developed. Differently charged dyes are kept in solution separated from each other in two phases by the combination of anionic and nonionogenic surfactants. With the help of fixing accelerators, good penetration of PAC fibers can be achieved in 10-15 min with saturated steam at 98-100°C. 

The most effective method of bleaching polyester/cotton is sodium chlorite, which may be followed by peroxide bleaching. Chlorite bleaches the husk, but does not destroy them completely. Polyester/cotton blends may be bleached with sodium chlorite in long liquors and also by pad-steam process [Table 6.10]. The... 

In the pad-steam process the goods after padding with a liquor pick-up of about 80% are steamed at 100-101 C for 2-3 h or under pressure (1-1.5 kg/cm-) for 25-30 min. Alternatively, the goods may be thermofixed by superheated steam at 150-170°C for 3-5 min. 

In the pad-steam process, the fabric is padded with a solution containing 70 g/1 caustic soda, 3 g/1 sodium persulphate and 10 g/1 non-ionic detergent at 40°C with a wet pick-up of about 80% and then the fabric is steamed for 12 min at 102 C. The fabric is then washed thoroughly in an open soaper with a temperature of about 98 C in the first and second, 80°C in the third and 60 and 50 C in the fourth and fifth compartments respectively. 

A greater measure of continuity is achieved in the Pad-steam process in which the cloth is first pigment-padded, dried, padded with sodium hydroxide and hydrosulphite, passed through a steam chamber to reduce and fix the vat dye, and finally finished off in a soaping and rinsing range. 

Urea is used in the pad-dry-bake dyeing process to improve the extent of the dye-fiber reaction. Inorganic salt is used in pad-steam dyeing to improve dye levelness into the fiber. Salt is also used in the pad-dry-chemical pad-steam process to minimize dye bleeding during chemical padding. 

The first volume-yardage continuous process was the continuous pad-steam process for vat dyes on cotton. The vat dye dispersion was padded onto the cloth and dried this was followed by passage through a reducing bath, steaming for 30 seconds, an oxidizing bath... 

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