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Level dyeing problems can be divided into two broad categories [336]:

(1) Gross unlevelness throughout the material: this type of unlevelness is primarily related to the dyeing equipment or process; the substrate is often uniform in properties, both chemically and physically

(2) Localised unlevelness: this is primarily related to physical and/or chemical non-uniformity of the substrate; typical examples are barriness in nylon or polyester dyeing and skitteriness in wool dyeing.

There are also two fundamental mechanisms that can contribute to a level dyeing:

(1) Control of rate of exhaustion of dye so that it is taken up evenly (2) Migration of dye after initially unlevel sorption on the fibre.

Either or both of these mechanisms may operate to a greater or lesser extent in a given dye–

fibre system, although a general trend towards better fastness properties has dictated the use of dyes that show low, if any, propensity to migration, thus placing the emphasis for level dyeing on the control of exhaustion rate. Physical factors such as temperature and frequency of liquor/substrate contact (governed by rate of liquor circulation in a jet, beam or package machine) can be used to exert some degree of control over these mechanisms. Slower rates of heating usually favour more even uptake of dye and higher temperatures tend to increase migration or diffusion. In some cases level dyeing can be influenced by dyebath pH and/or the presence of electrolytes. This section, however, is more concerned with the control of levelness by means of chemical auxiliaries, generally known as levelling or retarding agents.

Since levelling agents are invariably surfactants, they may be anionic, cationic, nonionic or amphoteric in nature. Sometimes combinations of these are used. The chemical structure of commercial products is seldom revealed, however; hence only general principles can be covered here. The main mechanisms by which levelling agents operate [337–341] are as follows:

(a) nonionic agents usually form water-soluble complexes with the dye, some degree of solubilisation being involved

(b) ionic agents are primarily dye- or fibre-substantive; in the former case they tend to form complexes with the dye and there is competition between the levelling agent and the fibre for the dye, while in the latter case the competition is between levelling agent and the dye for the fibre.

In complex formation the principle, as far as levelling action is concerned, is usually the same irrespective of whether nonionic or ionic agents are used, although the mode of complexing is different. The attractive forces between agent and dye create a counterbalancing mechanism against dye–fibre attractive forces, restraining the uptake of dye by the fibre. As the temperature of the dyebath increases the complex gradually breaks down, progressively releasing the dye for more gradual sorption by the fibre. Clearly, for an effective levelling agent that functions by this mechanism the stability of the agent–dye complex, governed by forces of attraction between agent and dye, is crucial. If these forces are so weak that a relatively unstable complex is formed, restraining or levelling action may be inadequate. On the other hand, strong forces of attraction may result in a complex that is too stable to break down as the temperature rises, so that the dye is effectively entrapped by the agent in the solution phase and is not available for sorption by the fibre. The objective therefore is to formulate the levelling agent so that it forms a dye complex of optimum, rather than maximum, stability relative to the conditions of application. This is done by adjusting the hydrophilic–lipophilic balance of the surfactant. The problem lies in the fact that the dye–agent interaction is so specific that different members of a range of dyes may each require a different balance. Hence commercial levelling agents may contain more than one surfactant.

A difficulty that arises with ionic levelling agents is that they may form an insoluble precipitate with ionic dyes of opposite charge; this can be obviated in various ways. In the first instance attention should be paid to the concentration of the surfactant; where initial addition of surfactant to the dyebath causes precipitation of the agent–dye complex, further additions of surfactant often lead to its solubilisation. Alternatively, a further surfactant may be added to solubilise the complex; a nonionic agent will not itself react with either the dye or the original ionic surfactant to form a further insoluble complex, but its addition may further complicate the relationship between the hydrophobic–hydrophilic balance of the ionic agent and the dyes to be complexed. Due regard also needs to be paid to the cloud point of the nonionic agent under the conditions of use. This does not preclude the use of a relatively hydrophobic nonionic agent, since its cloud point may be effectively raised in the presence of the ionic agent (subject to possible interference from any electrolytes or solvents present in the dyeing system). Similarly, if there is any danger from the cloud point of a nonionic surfactant used as the primary levelling agent (as with disperse dyes, for example), a suitable anionic surfactant may be added to effectively raise the cloud point, again paying due attention to any effect the anionic agent may have on the complexing–liberating performance of the nonionic agent.

The third method of obviating precipitation of an ionic agent–ionic dye complex is to choose what effectively amounts to a ‘modified’ ionic agent. Ethoxylated anionic and ethoxylated cationic agents are particularly useful in this respect. The ethoxylation tends to

reduce the ionic character of the agent, thus giving rise to weaker but more controllable forces of attraction for dye ions, and the oxyethylene chain can further function as a dispersing–solubilising moiety for the agent–dye complex. In a sense this is basically similar to using a mixture of ionic and nonionic agents as described above except that a single agent is used, thus facilitating the aim of obtaining the optimum complexing–liberating balance.

Dye–agent complexes of lower net charge are formed when the ionic agent is added to the ionic dye solution. As the concentration of agent is increased a point is reached at which all the dye is complexed and its ionic charge has been neutralised. Beyond this point, as more agent is added, the agent–dye complex takes on the charge of the complexing agent (i.e. the opposite to that of the dye itself). This brings about a change in the partition coefficient of the complex between water and organic solvents [336], modifying the electrical and solution properties of the dye and so altering its affinity for the fibre.

Fibre-substantive levelling agents are usually of the same ionic type as the dye, that is anionic agents are used with anionic dyes and cationic agents with cationic dyes, the aim being to create a system in which levelling agent and dye both compete for the sorption sites in the fibre. Just as the complexing type of levelling agent has to be carefully chosen so as to obtain the optimum complexing–liberating properties, so must the competing type of levelling agent be chosen such that its ionic power gives the optimum level of competition relative to the dye–

fibre system concerned. If the ionic power is too weak, it will not function as an effective levelling agent; if it is too strong, it may exert blocking effects, preventing sorption of the dye.

Ideally the balance should be such that the smaller surfactant ions are adsorbed by the fibre more quickly than are the larger dye ions, but the agent–fibre interaction needs to be weak enough to permit subsequent displacement of the surfactant ions by the dye ions.

As the forces of dye–fibre interaction vary from one dye to another, the ionic power of the levelling agent must be suitably adjusted through its hydrophilic–hydrophobic balance to give the optimum properties. This can be done either by careful choice of a single surfactant or by the use of mixtures, which has gained prominence in recent times. For example, the strongly anionic character of a long-chain alkyl sulphate or sulphonate can be modified (toned down) by mixing it with a more weakly anionic poly(oxyethylene) sulphate or with a nonionic agent.

Some levelling agents operate both by complexing and by competition. For example, in the application of acid dyes a weakly cationic agent may be used to complex with the dye and an anionic agent may also be used as a competing agent. This combination is more versatile because unlevelness may arise from different mechanisms. Unlevelness arising from process or equipment variables can often be controlled by dye–agent competition, whereas localised dye uptake variations generally respond better to dye–agent complex formation.

Evidently, in this combined system the balance of properties is highly critical. In particular the oppositely charged surfactants must not mutually precipitate; hence the more weakly ionic ethoxylates are of particular interest, since the oxyethylene assists solubilisation of any complex so formed. A purely nonionic agent may also be used to prevent coprecipitation of the ionic types. Amphoteric agents, in a sense, fall within this combined system.

Theoretical considerations are clearly useful in formulating suitable levelling agents.

Nevertheless, a good deal of empiricism is always involved in formulating well-balanced agents for specific dye–fibre systems. Table 10.36 shows the general types of levelling agents now being offered and their uses; more detail is given in Chapter 12 relative to each class of dye.

Many, but not all, levelling agents promote migration of dye in addition to retarding dyeing, such agents will obviously be a further aid to level dyeing. In some cases, however,

higher concentrations of levelling agent are needed to obtain significant migration and this may interfere unduly with dye sorption. Levelling agents are also widely used as stripping agents, either alone for non-destructive desorption or together with reducing agents such as sodium dithionite for destructive stripping. When used for this purpose, their hydrophilic–

hydrophobic balance is not as critical as when they are used simply as levelling agents. Thus higher concentrations are often used in order to maximise rather than optimise desorption of the dye.

It should not be overlooked that electrolytes can play an important part in levelling and retardation. In recent times the use of bolaform electrolytes (section 10.1), cyclodextrins (section 10.3.1) and liposomes (section 10.3.4) as complexing agents has been proposed.

Table 10.36 Levelling agent types and their uses

Recommended for use with

Type of levelling agent Substrate Dye classes

Nonionic Cotton Direct, vat, azoic

Wool, nylon Milling acid, metal-complex Polyester Disperse

Nonionic/anionic Polyester Disperse

Wool, nylon Milling acid, metal-complex

Nonionic/cationic Wool Acid, metal-complex, reactive, chrome

Anionic Wool, nylon Acid

Cotton Direct

Polyester Disperse

Weakly anionic Polyester Disperse

Anionic/cationic Wool, nylon Acid, metal-complex

Cationic Acrylic Basic

Wool, nylon Acid, metal-complex, reactive Weakly cationic Wool, nylon Acid, metal-complex, chrome Cationic/polymeric Cotton Vat, sulphur

Amphoteric Wool Acid, metal-complex, reactive, chrome