Dyebath water

Perkins, W.S. Walsh, W.K. Reed, I.E. Namboodri, C.G. A demonstration of reuse of spent dyebath water following color removal with ozone. Text. Chem Color. 1995, 28, 31-37. 

It has also been reported that spent dyebath water can be re-used by decolouri-sation with ozone. Such decolourised liquors could be successfully used in the dyeing of polyester. The dyeing of cotton fabric with reactive dyes can be carried out using a modified dyeing method. Ozonated dyebaths can be re-used for the bleaching of cotton fabric and even for fabric whitening and rinsing, provided the effluent is extensively decolourised. 

Uses Encrustation aid, sequestrant, process aid for detergents (powd. laundry, liq. laundry, hard surf., auto dish liq., auto dish powd., auto dish rinse), textiles (scours, rinse aids, dyebaths, water softeners), pigments (mining, pigment slurries)... 

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. 

Phenylphenol was one of the earliest carrier-active compounds used industrially. Originally it was used as its water-soluble sodium salt (4). By lowering the pH of the dyebath, the free phenol was precipitated in fine form and made available to the fiber. However, proprietary Hquid preparations containing the free phenol are available that afford a greater ease of handling. 

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).

Many proprietary carries are available as soHds (flakes or pellets) or in preemulsified form. These present some difficulties in the dyehouse. The former require dispersion in water through steam injection and addition to a preheated dyebath. The latter suffer from short storage life owing to separation of the emultion. Currently the industry prefers clear products easily emulsified by premixing with water at the time of use. 

Emulsion stabiUty refers to the stabiUty of the emulsion in water. It must withstand various dyebath conditions. 

The Dilution Factor of the Dyebath. The amount of water in a dyebath may vary from 5—25 times the weight of fiber to be dyed, according to the capacity and type of equipment employed. The larger water-to-fiber ratio dilutes the emulsifier, thus reducing its effect. 

Elevated Temperatures of the Dyebath. Emulsions that are stable in cold or warm water lose their stabihty at higher temperatures. The carrier-emulsifier equiUbrium undergoes stress, particularly when the time at high temperature is prolonged. 

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. 

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. 

Influence of the Fiber. In order for a dye to move from the aqueous dyebath to the fiber phase the combination of dye and fiber must be at a lower energy level than dye and water. This in turn implies that there is a more efficient, lower energy sharing of electrons or intramolecular energy forces, and there are a number of mechanisms that allow this to happen. 

Hydrophobic fibers are difficult to dye with ionic (hydrophilic) dyes. The dyes prefer to remain in the dyebath where they have a lower chemical potential. Therefore nonionic, hydrophobic dyes are used for these fibers. The exceptions to the rule are polyamide and modified polyacrylonitriles and modified polyester where the presence of a limited number of ionic groups in the polymer, or at the end of polymer chains, makes these fibers capable of being dyed by water-soluble dyes. 

Zeta Potential. When a textile is immersed in water a negative charge is developed on its surface. This is caked the 2eta potential. This happens even with ionic fibers in neutral dyebaths. Negatively charged dyes therefore are coulombicaUy repeUed. 

Internal and External Phases. When dyeing hydrated fibers, for example, hydrophUic fibers in aqueous dyebaths, two distinct solvent phases exist, the external and the internal. The external solvent phase consists of the mobile molecules that are in the external dyebath so far away from the fiber that they are not influenced by it. The internal phase comprises the water that is within the fiber infrastmcture in a bound or static state and is an integral part of the internal stmcture in terms of defining the physical chemistry and thermodynamics of the system. Thus dye molecules have different chemical potentials when in the internal solvent phase than when in the external phase. Further, the effects of hydrogen ions (H" ) or hydroxyl ions (OH ) have a different impact. In the external phase acids or bases are completely dissociated and give an external or dyebath pH. In the internal phase these ions can interact with the fiber polymer chain and cause ionization of functional groups. This results in the pH of the internal phase being different from the external phase and the theoretical concept of internal pH (6). 

Cold Exhaust Dyeings Fiber-Reactive Dyes. Start at 25—30°C optionally with a sequestrant and maintain. The dye is added over 5 min, then there is portionwise addition of salt every 10—15 min, increasing the size of the addition each time over 1 h. The amount of salt used (10—100 g/L) depends on the depth of shade. After the final addition of salt, wait 15 min, portionwise add soda ash (10—20 g/L) over 15 min, and continue dyeing for 30—45 min. Drop dyebath, cold water rinse, and use a sequence of hot washes to remove all loose "unfixed" dye. 

Alkoxylated polysiloxanes are a relatively new class of dyebath lubricants. They have practically no substantivity for the substrate, yet combine adequate lubrication with water solubility and easy rinsability. If the silicones contain primary hydroxy groups, these can be modified by esterification, phosphation, phosphonation, sulphation, sulphonation or carboxylation. These anionic substituents confer substantivity for various substrates without losing rinsability. Anionic organic sulphates and sulphonates probably offer the best overall properties for dyebath lubricants, whilst other types can be more suitable for selected applications [464]. 

Currently the most suitable system, that will generate potentials up to -1050 mV, is the iron-triethanolamine complex prepared from either iron(II) or iron(III) salts. Using iron(III) sulphate penta- or hexahydrate, for example, dyebaths are prepared by first dissolving sodium hydroxide in a small amount of water, to which is added the triethanolamine. Hydrated iron (III) sulphate is separately dissolved in a small amount of water and then added to the alkaline triethanolamine solution until the initially precipitated iron(III) hydroxide redissolves, after which the solution is diluted to full volume to give ... 

Measurements of the surface tension of aqueous solutions of various sulphonated and unsulphonated phenylazonaphthol dyes showed that the degree of surface activity (that is, the lowering of surface tension) tended to increase progressively with the degree of alkyl substitution in the series of dyes [7]. The surface-active behaviour of such alkylated dye ions ensures that they become more concentrated at the interface between the dyebath and the fibre surface, just as they do at the air-water interface of the dye solution. Foaming of dyebaths can be a serious practical problem with relatively hydrophobic dye structures solubilised by means of a single ionised group. 

Complete miscibility with water and other liquid products used in dyebath or print-paste formulations... 

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