Dyebath dyeing

Isotherms. When a fibei is immersed, in a dyebath, dye moves fiom the external phase into the fibei. Initially the late is quick but with time this slows and eventually an equiUbrium is reached between the concentration of dye in the fiber and the concentration of dye in the dyebath. For a given initial dyebath concentration of a dye under given dyebath conditions, eg, temperature, pH, and conductivity, there is an equiUbrium concentration of dye in fiber, Dj and dye in the dyebath external solution, D. Three models describe this relationship simple partition isotherm, Freundhch isotherm, and Langmuir isotherm. 

Articles made of pure PES fibers are dyed almost exclusively in exhaustion processes. Disperse dyes are most stable at a pH of 4-5 (acetic acid). A dispersing agent is also added to the dyebath. Dyes sensitive to heavy metals (e.g., anthra-quinoid red products) are dyed in the presence of a complexing agent (EDTA type). 

Fiber optics cables Fiber optic waveguides Fiber-reactive dyebath Fiber reactive dyes... 

Traditionally, these dyes are appHed from a dyebath containing sodium sulfide. However, development in dyeing techniques and manufacture has led to the use of sodium sulfhydrate, sodium polysulfide, sodium dithionite, thiourea dioxide, and glucose as reducing agents. In the reduced state, the dyes have affinity for cellulose (qv) and are subsequendy exhausted on the substrate with common salt or sodium sulfate and fixed by oxidation. 

The conventional sulfur dye powder is made into a paste with a small amount of soft water and an alkah-stable wetting agent. Boiling for a few minutes in a strong solution of sodium sulfide reduces the dye. The dissolved dye is diluted to the requited dyebath volume. When dyeing pale shades, the final bath should contain at least 5 g/L sodium sulfide (60%), inrespective of the amount used to dissolve the dye. 

Liquids. Some Hquid dyes are made directly from the thionation melt by additions of caustic soda and sodium hydrosulfide. Hydrotropic substances are sometimes added, either at the initial thionation stage or after the polysulfide melt is finished in order to keep the reduced dye in solution. Pardy reduced Hquids are also available. They are usually more concentrated than fully reduced Hquids, thus saving packaging and transportation costs. However, they require a further addition of reducing agent to the dyebath in order to obtain full color value. On the other hand, fully reduced Hquids are... 

Cotton yam is dyed in package machines and the dye exhausted by increasing the temperature and adding salt. The dye must be completely dissolved when preparing the dyebaths to avoid contamination with undissolved dye in the yam package. The increased avaUabUity of the prereduced Hquid dyes and the improved quaHty of sodium sulfide have reduced this problem. Incorrectly dissolved dye was previously the cause of most faulty dyeings. 

YeUowing of wool can occur during dyeing, depending on pH, temperature and time, and chlorinated wools ate especially sensitive. Bleaching agents that can be added to the dyebath have been developed based on sodium bisulfite and hydroxylamine sulfate (108). Addition of hydrogen peroxide to the dyebath after exhaustion can also be effective. 

There are three general classifications of acid dyes depending on their method of apphcation acid dyes that dye direcdy from the dyebath, mordant dyes that are capable of forming metallic lakes on the fiber when aftertreated with metallic salts, and premetallized dyes. 

Most mordant dyes are monoazo stmctures. The most important feature of this class of dyes is excellent fastness to light and washing. Mordant dyes are available ia aU shades of the spectmm with the exceptioa of bright violets, blues, and greens. To be useful, the metal complexes must be stable, ie, must not demetallize when subjected to dyebath conditions and aU aftertreatment processes, especially repeated washings. Chromium forms stable chelate rings with mordant dyes which are not affected by treatment with either weak acid or alkaU (see Coordination compounds). 

Premetallized Dyes. Although discovered in 1912, the 1 1 chromium complexes known as Palatine Fast (BASF) and Neolan (Ciba) dyes had httie practical use as wool dyes until 1920 when it was found that a strongly acidic dyebath (pH ca 2.0) (51) was requited to obtain satisfactory dyeing and acceptable fastness properties. Dyes of this type exemplified by Neolan Blue 2G [6370-12-3] (57) (Cl Acid Blue 158A Cl 15050) are stiU in use despite the damage to the wool caused by the strong acid in the dyebath. 

Under the low pH dyebath conditions, 2 1 complexes of the sulfo-containing types, disproportionate iato a 1 1 complex and metal-free dye. At higher pH, 2 1 complexes are stable, but they do not yieldlevel dyeiags of dye to their ionized substituents (SO )-... 

Another class of metal complex dyes is derived from the formazan stmcture. These dyes are appHed to wool and nylon from a neutral or weakly acidic dyebath analogous to the 2 1 premetallized OjO -dihydroxyazo complexes. The bluish-gray dye Cl Acid Black 180 [11103-91-6] (61) (Cl 13710) is a 2 1 cobalt complex of the formazan type. 

There are many chemicals, by lowering suitable as carriers. Their bp is one of the principal criteria in selection. If bp is too low, the compound will evaporate from the dyebath at dyeing temperatures, and will be lost before it is effective in its role as a carrier. It may also steam distill (condense on the cooler parts of the equipment) and cause drips that will spot the fabric. On the other hand, if the bp is too high, the compound cannot be removed from the fabric under normal plant drying conditions and will affect lightfastness of finished goods, leave residual odor, and possibly cause skin irritation to the wearer. 

Table 2 Hsts the four main groups of compounds most commonly used as dye carriers. In order for these compounds to act effectively as carriers, they must be homogeneously dispersed in the dyebath. Because the carrier-active compounds have Httie or no solubiUty in water, emulsifiers are needed to disperse these compounds in the dyebath (see Emulsions).

Polyester (Textured or Filament) Dyed Under Pressure. The dyebath (50°C) is set with water conditioning chemicals as required, acetic acid to ca 5 pH, properly prepared disperse dyes, and 1—3 g carrier/L. The bath is mn for 10 minutes, then the temperature is raised at 2°C/min to 88°C and the equipment is sealed. Temperature is raised at l°C/min to 130°C, and the maximum temperature held for 1/2—1 h according to the fabric and depth of shade required. Cooling to 82°C is done at 1—2°C/min, the machine is depressurized, and the color sampled. The shade is corrected if needed. Slow cooling avoids shocking and setting creases into the fabric. Afterscour is done as needed. 

Acid Dyes. These are anionic dyes, usually containing sulfonic acid groups, that are substantive to wool, other protein fibers, and polyamides when dyed from an acidic dyebath. The lower the pH the more rapid the dyeing, and exhaustion efficiency is enhanced by increased acidity. 

Direct Dyes. These are defined as anionic dyes, again containing sulfonic acid groups, with substantivity for ceUulosic fibers. They are usually a2o dyes (qv) and can be mono-, dis-, or polya2o, and are ia general planar stmctures. They are appHed to ceUulosic fibers from neutral dyebaths, ie, they have direct substantivity without the need of other agents. Salt is used to enhance dyebath exhaustion. Some direct dyes can be appHed to wool and polyamides under acidic conditions, but these are the exception. 

Disperse Dyes. These are substantially water-insoluble dyes appHed from aqueous dyebath in a finely dispersed form. They are the most important class of dye for dyeing hydrophobic synthetic fibers such as polyester and acetates. 

Thus the necessary chemical properties of dyes are a chemical stmcture that imparts colors and chemical characteristics that result in the molecule having a lower chemical potential in a fiber than in the dyebath. 

In the dyeing process absorption from the dyebath solution to the fiber eventually stops when an equiHbrium exists between the dye in the fiber phase and the dye in the solution phase. At this point by definition (no movement of dye molecules), therefore... 

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