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Role of Leveling agents in Textile Wet-Processing

Level dyeing is achieved by i) controlled exhaustion of the dye so that it is taken up evenly and ii) migration of dye after initially unlevel sorption on the fiber. In some cases level dyeing can be influenced by dye-bath pH and/or presence of electrolytes. However, to produce better yields with correct depth, leveling agent is a must.

Mechanism of Leveling agents :

Since leveling agents are surfactants, they may be anionic, cationic, non-ionic or amphoteric in nature. The main mechanism of non-ionic agents is to form water soluble complexes with the dye. Two main groups of ionic agents can be identified. They are either dye-substantive (products which have an affinity for the dye) or fiber-substantive (products which have an affinity for the fiber). In case of dye-substantive leveling agent, there is competition between the leveling agent and the fiber for the dye. Products which have an affinity for the fibre compete with the dye for dye-sites on the fiber. The attractive forces between agent and dye, create a counter balancing mechanism against dye-fiber attractive forces, restraining the uptake of dye by the fibre. As the temperature increases, the complex gradually breaks down releasing the dye for uniform sorption by the fiber.

Truly speaking, for an effective leveling agent that functions by this mechanism, the stability of dye-agent complex is crucial. If these forces create relatively unstable complex, then leveling action may be ineffective. On the other hand if these forces create too stable complex, then it will not be broken down with the rise in temperature and will not release the dye for uniform sorption by the fiber. Thus the object is to make a leveling agent such that it forms an optimum complex, rather than maximum complex. This is done by adjusting HLB (Hydrophilic-Lipophilic Balance) of the surfactant.

The dye-agent complex varies with the nature of dyes, nature of the substrate and nature of the leveling agent. Hence there is a need of different leveling agents for different dyeing processes. Fibre-substantive leveling agents must be of the same ionic nature as that of the dye. i.e. anionic agents are used with anionic dyes and cationic agents are used with cationic dyes. While selection of appropriate leveling agent, care must be taken that the ionic charge is not too weak. Otherwise it will not function as an effective leveling agent. If the ionic charge is too strong then, it may exert blocking effects, preventing sorption of the dye. Thus an ideal leveling agent is the one  who is adsorbed by the fibre quickly than the dye, but the agent-fibre bond must be weak enough to allow displacement with dye ions.

Leveling for Cellulosic dyeing :

During exhaust dyeing of cellulosics, pad batch or continuous dyeing with direct, vat, sulphur, naphthol and reactive dyes as well as indigo types, it is necessary to prevent the disturbing effect of inactivated ions or any prematurely oxidized dye in order to prevent stains, markings and unlevelness.

Matlevel DLR liquid is designed to satisfy the needs which effectively combines dispersing and leveling properties to uniformly dye the fabric. It allows the dyer to simplify the process to shorten the dyeing time by
i) proper handling and introduction of the totality of the electrolyte previously to the dyes and
ii) linear addition of alkali, without a dispensing system at a selected temperature in max. 30 mins.

Leveling for Polyester dyeing :
Polyester dyeing at 130°C in beam and package dyeing machines has placed especially great demands on aspects of both, initial dispersion quality and subsequent leveling under adverse conditions. The crux of the problem lies in the inherent thermodynamic instability of all dye dispersions, there being an overall tendency of fine particles to undergo Ostwald ripening with consequent formation of large particles.

Matlevel DLP is such a dispersing and leveling agent, which disperses the dye molecules evenly and maintains an equilibrium state in water.

Leveling for Texturised Polyester dyeing :

In the dyeing of 100% texturised polyester material, barriness is observed due to dye-ability differences in the material. The difference arises due to variation of temperature and tension during texturisation. The temperature can be controlled to avoid variation, but it is difficult to control the variation in tension. This is revealed only after dyeing particularly when dyes of lower migration properties are used.

Matlevel DLTP is the most suitable dispersing & leveling agent which not only covers barriness effectively but also promotes the migration thereby achieving level dyeing.

Leveling for Polyamide dyeing :

Polyamides refer to various natural (polypeptides) and synthetic materials containing free amino groups. Examples of polyamides include nylons, wool, and silk. Nylon fiber is commonly dyed with acid dyes which are anionic in character, including premetallized acid dyes, in a batch process referred to as exhaust dyeing. For example, nylon fiber which has been made into fabric may be dyed in a jet-dyeing machine, whereby a continuous loop of the fabric is circulated throughout the dye bath by impinging the dye bath liquor against the fabric in a venturi nozzle. Care must be taken during the dyeing process to obtain a uniform distribution of dye on the fabric, referred to as leveling.

Since acid dyes are negatively charged, the dyes are attracted to positive dye sites appearing in the targeted substrate. With respect to nylon, positive dye sites can be created by exposing the free amino groups contained within the polymer matrix to an acid. In particular, when exposed to acidic conditions, the amino groups are activated by protonation and become positively charged and cationic. Once positively charged, the acid dyes are strongly attracted to the cationic sites.

In general, acid dyes have a high affinity for protonated polyamide materials, meaning that the dyes have a strong tendency to quickly bind to the polymer. Unfortunately, however, once in contact with the cationic polymer surface, acid dyes have a tendency to poorly diffuse into the polyamide. In other words, acid dyes exhibit such a high rate of strike that they do not diffuse evenly into polyamides. Thus, if the dye is absorbed by the polymer too quickly, the polyamide material can absorb the dye unevenly and not exhibit a constant shade or color. In such cases, anionic leveling agents act by competing for the dye sites and are mainly used to counter-act fiber-oriented unlevelness due to physical and chemical irregularities in the fibre.

Matlevel DLN liquid due to its substantivity for nylon fibre, enforces a slow and even rate of absorption which results in excellent penetration and level dyeing.

Anionic leveling agents act by competing for the dye sites and are mainly used to counter-act fibre-oriented unlevelness due to physical and chemical irregularities in the fibre. As acid dyes are negatively charged, cationic leveling agents form complexes with acid dyes and may precipitate when used alone. Hence in order to counteract both the types of unlevelness it is necessary to use amphoteric leveling agent.

Matlevel DLMC is an amphoteric leveling agent which, due to its substantivity for 1:2 metal complex dyes enforces a slow and even rate of absorption which results in excellent penetration and level dyeing.

Leveling for acrylic dyeing :

For acrylic dyeing, basic dyes, also called as cationic dyes are used by exhaustion techniques. Basic dyes being cationic in nature, it is advisable to use a leveling agent that will compete with it. i.e it must be a cationic retarder rather than a leveling agent. This is because, in addition to having an effect on the rate of dyeing, cationic retarders also assist migration to an extent that depends on the fiber and the substantivity of the dyes. They tend to have higher diffusion rates than dyes and to be adsorbed at lower temperatures (65-70°C)

Acrylic fibres vary significantly in the number of anionic sites available for sorption of cations, but level dyeing accrues when the number of cations in the system (retarder as well as dyes) is just enough to saturate the anionic sites in the fibre. This means more retarder will be needed for fibres of high saturation value and for paler shades.

Matlevel CAR liquid due to its substantivity for acrylic fibres is most suitable for dyeing of all types of acrylic fibres by any of the normal methods. While Matlevel CAR liquid retards the build up of cationic dyestuffs it does not block the fibre to any appreciable extent, so the bath is well exhausted during the boiling process.

Many, but not all leveling agents promote migration of dye in addition to retarding dyeing, and such agents will obviously be a further aid to level dyeing. In some cases, however, higher concentrations of leveling agents are needed to obtain significant migration. Leveling agents are also widely used as stripping agents, either alone or or other reducing agents for destructive stripping.

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