Rate of dyeing

If diffusion through the fiber is not carried out efficiendy then not only will the rate of dyeing be slow, with a chance that equihbrium between dye and fiber is not reached, but also the fibers will be dyed unevenly and possibly be ring dyed leading to poor fastness properties. Diffusion through the fiber is dependent on the actual dye and fiber chain molecular stmcture and configuration, and also, especially with hydrophobic fibers, the mobiUty of the chemical chain (7). 

Temperature. No dye transfer takes place below 40°C, above which, because the fiber swells, the rate of dyeing increases. 

The compatibihty value is mainly related to the affinity of the dye for the particular fiber because for basic dyes on modified acryhc fibers there is htde possibihty for migration and therefore this does not play a significant part in determining compatibihty. The rate of dyeing of a specific mixture of dyes of the same compatibihty value is not determined by the value itself. The adsorption of cationic dyes is induenced by the presence of others in the dyebath the presence of cationic retarding agents and electrolytes also induences the rate of exhaustion. It is therefore possible to have a combination of dyes with a compatibihty value 5 that under specific dyebath conditions exhausts more rapidly than a combination based on dyes of compatibihty value 3. 

Level Dyeing Techniques. It is exceptionally difficult to obtain level dyeings on acryhc, and temperature and pH control depend on fiber type and are not always adequate. Sodium sulfate in limited amounts can be used to some effect. The sulfate ions compete for the dye with the fiber SO3 sites and so retard the rate of dyeing by forming a dye complex with the ions. The effect of sodium sulfate is best with dyes having the lowest... 

The process of textile print coloration can be divided into three steps. First, the colorant is appHed as pigment dispersion, dye dispersion, or dye solution from a vehicle caUed print paste or printing ink, containing in addition to the colorant such solutions or dispersions of chemicals as may be required by the colorant or textile substrate to improve and assist in dye solubUity, dispersion stabUity, pH, lubricity, hygroscopicity, rate of dye fixation to the substrate, and colorant-fiber bonding. The required viscosity characteristics of a print paste are achieved by addition of natural or synthetic thickening agents or by use of emulsions. 

Some auxiliaries fulfil more than one of the above functions. For example, an auxiliary to improve dye solubility may also accelerate (or retard) a coloration process, or an emulsifying agent may also act as a thickening agent pH-control agents may both stabilise a system and also affect the rate of dye sorption. 

In addition to having an effect on the rate of dyeing, cationic retarders will assist migration to an extent that depends on the fibre and the substantivity of the dyes. Retarders tend to diffuse more quickly than dyes and to be absorbed at lower temperatures (typically 65-70 °C, compared with 80-85 °C), although the magnitude of these effects will depend on the structure and properties of both retarder and dye. In some cases, such as hank dyeing on machines with poor circulation or inadequate temperature control, it may be preferable to use a retarder that almost totally restrains the uptake of dye until the top temperature has been reached, after which dye sorption takes place gradually. 

Figure 12.4 Equilibrium absorption isotherms at 34 °C and rate of dyeing curves at 40 °C for Cl Direct Blue 1 on viscose in the presence of electrolytes singly and in binary mixtures [70]...

Figure 12.11 Rate of dyeing curves for Cl Disperse Blue 56 and Yellow 42 in admixture on polyester at various concentrations of a nonylphenol 20 EO levelling agent [107]...

The rate of dye decomposition should be negligible at exposure energies up to several thousand times the energy absorbed during the read process and increase rapidly thereafter. Dye compositions which exhibit this type of threshold behavior must have relatively high photochemical and thermal stability and decompose by a free radical process above the melting point of the substrate.199... 

Rate of dye degradation, pH, BOD, and enzymes involved, and survival of test organisms... 

The assessment of reaction kinetics by means of batch tests may be strongly affected by dye adsorption on the biophase and supports. The relevance of the adsorption phenomena of dyes on biophase has been addressed in studies regarding free cells [41], granular support biofilm [24], entrapped cells [11, 18], anaerobic sludge [10,24,31,34] and biological activated carbon (BAC) [42,45,47,48]. They have pointed out that the kinetics may be overestimated if the assessment of the adsorption contribution to the dye removal is not taken into account. Under batch conditions, the dye is fastly split between the liquid phase and the biophase, resulting in a sharp reduction of the dye concentration in the liquid phase until adsorption equilibrium is approached. The rate of dye adsorption must be estimated and ruled out in the kinetic assessment. 

As a general rule, the lower the dyebath pH the more rapid is the rate of initial adsorption by the amide fibre and the subsequent approach towards equilibrium exhaustion. Consequently, the attainment of a safely controllable rate of dyeing becomes easier the higher the pH at which dyeing begins. Eventually, however, a pH is reached at which the dyebath is no longer exhausted within a reasonable time. 

Dyeing conditions are usually controlled to give a moderate initial rate of dyeing for optimum levelling, followed by a gradual lowering of pH by acid release to achieve as much exhaustion as possible at the end of the process. Selection of the preferred initial pH and rate of acidification is determined by the affinity of the dyes at neutral pH values. Neutraldyeing affinity is dependent on the structural features and hydrophilic-lipophilic balance of the dye molecule. 

The rate of dyeing of acid dyes of relatively small molecular size decreases with increasing degree of sulphonation. Simple monoazo monosulphonate types are rapidly absorbed but readily desorbed again. Thus they migrate easily but show extremely poor fastness to wet treatments. There are two main types of levelling acid dye ... 

A much more decisive factor in determining the rates of dyeing of many direct dyes from aqueous salt solutions is their tendency to aggregate (section 3.1.2), since this greatly retards their diffusion into the water-swollen voids of the substrate. Ureido-linked ] acid residues... 

The kinetic behaviour of anionic dyes on amide fibres tends to be much more closely related to molecular size and hydrophilic-lipophilic balance. Thus within a related series of dye structures it is possible to discern more specific relationships. For example, in the series of p-substituted aniline—>R acid dyes (3.91 R = H, methyl, n-butyl or n-dodecyl) the logarithm of the rate of dyeing on wool is inversely proportional to the molecular volume [110]. 

The successful and possibly rapid degradation of toxic dye residues is of considerable importance form both a human and a veterinary toxicological point of view and for environmental protection. Because of the high amount of dyes loaded in the environment, much effort has been devoted to the development of physical, physicochemical and microbiological methods obtaining the degradation of dyes to non-toxic (even nutritive) derivatives. As has been previously mentioned, visible spectroscopic methods are widely applied for the measurement of the decomposition rate of dyes. However, when more than one dye molecule is simultaneously present or the primary decomposition products also absorb on... 

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