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 Vat Yellow-molecular structure. Application of Vat Dyes
Pre-pigmentation: Because the chemicals necessary for for Vatting make the leuco-vat anions in solution highly substantative, there is always the possibility of unevenness, if circulation of the dye liquor is inadequate. But what if dispersions of vat pigments, with no substantivity, were t be circulated and uniformly distributed at the fiber surfaces before alkali and reducing agents were added? Once the pigments were deposited, Vatting will result in anion formation just where it is needed, at the fiber surfaces. it would certainly require very fine dispersions, with low particle size distributions, but this problem had already been overcome, and vat pigments are indeed available in such finely subdivided forms. Pre-pigmentation (before reduction) is a particularly useful alternative for light and medium shades where levelness might be a problem. Circulation of a pigment can begin at any time and at any temperature between ambient plant temperature and approximately 80C after about 20 minutes the chemicals are added to reduce the dye. Dyeing can then be completed with whatever temperature, time and salt conditions to achieve the best results. Semi-pigmentation: This is the name give to a process which is a hybrid of the leuco-vat and pre-pigmentation process. All the chemicals and dyes required for leuco-vat process are added and circulated through the good s at plant temperature for 20 minutes before raising the temperature to 60 - 80C for 30 to 40 minutes. The pigmentary dispersion is low to reduce at plant temperatures and is simultaneously but gradually deposited while it is reduced as the temperature is raised. Variations on this theme include the programmed metering of the Vatting chemicals in to the dye bath, to have total control of the rate of formation of the leuco-vat anion and consequently of leveling. Batch After Treatments: Regardless of the vat dyeing process used, once the dye bath has been dropped, the after treatments include the following steps: rinsing, oxidation, soaping and neutralization. Softening may be included is not fundamental.
Continuous dyeing process: The main economic advantage of continuous dyeing are to be derived from the rapid production of goods with very consistent shade. Speeds of 160 meters per minute are possible. But, the potential benefits have been increasingly curtailed by the shrinking size of individual dye lots, despite successful efforts by machinery manufacturers to make changing colors as painless as possible. When the threaded up length of a normal continuous dye range may be 400 meters, or even more, and the length of a particular dye lot may be only 1800 meters, this spells potential shade consistency problems for the continuous dyer. A conventional, fully continuous dye range suitable for dyeing polyester/ cotton , cotton and other blended fabrics in open width consists of 11 separate pieces of dyeing equipment strung together in line. Ideally, goods can be taken up between these pieces, and any piece may be bypassed. The pieces, in order, are: padder, pre-dryer, dryer, thermosol oven, padder, steamer, washer units and dryer. Only polyester/Cotton blends require thermosol oven treatment in which disperse dyes are fixed on to polyester fibers at 200-215C. The thermosol process will be discussed later on. The principle dye range configurations used for cellulosic fiber dyeing are called pad-steam or pad-dry-steam and wet-on-wet or pad-pad-steam process. Pad-Steam Process: The Pad-Steam process for continuously dyeing vat dyes is suitable for most flat cellulosic fabrics. Here a vat pigment dispersion is padded on to the goods and dried. A wet pick up of 60 to 70% might be suitable for many cotton and polyester/cotton flat goods. This process would be unsatisfactory for corduroy and terry cloth because the pigment will migrate to the fabric surfaces during drying. In addition to the issue of migration is the adverse surface effect ( flattening, distortion), if goods are not run wet-on-wet, avoiding pre-drying and drying cans. The goods are then padded with alkali and reducing agent, enter the steamer, where Vatting and leuco-vat anion diffusion take place, and are then successfully rinsed, oxidized, soaped and rinsed through the four sections of washing units, followed by drying.
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