Functions of Dyeing Auxiliaries
There are different dyeing auxiliaries. They will be classified according to their functions and various uses. Dyeing auxiliaries provide high dyeing quality, process safety and increased reproducibility on all kinds of substrates and machines, in a variety of processes and with all classes of dyestuffs.
Function of different dyeing auxiliaries are given below:
The most undesirable impurities in Fibre, Common salt, Glauber salt, Caustic Soda and Soda ash are the di- and tri-valent cations, e.g., Ca++, Mg ++ Cu ++, Fe+++ etc. These ions increase hardness of the process bath and generate iron oxides in the bath. Calcium and Magnesium reacts with alkali and precipitates as a sticky substance on the textile material, which creates patchy dyeing and discoloration of the fibre. The ferric oxide with cellulose and creates small pinhole on the fibres also damages the machinery by scale formation in the nozzles and base.
To overcome these deleterious effects in the scouring and bleaching bath adequate amount of sequestrant must be used. Sequestrants prevent di-and tri-valent metal ions, e.g., Cu++, Fe +++ , Mn ++, Ca++, Mg++ etc from interfering with the chemical processing of the textile material. It prevents catalytic damage of cellulosic fibres in bleaching hath during hydrogen peroxide bleaching.
In Dye-bath Ca++, Mg ++, Fe +++ attack the dye molecules and forms aggregates of molecules which deposits on the fabric as dye spots and also they prevent the reaction process. Dye bath sequestrant should be of different strength than that of the scouring and bleaching baths. Because some dyes have metal compounds and if powerful chelating agents are used than it will attack the metal compound of dye.
Lubricating / Anticreasant.
Wet processing and dye-bath lubricants are used in any operation in which rope marks, creases, crows feet buffing, abrasion can occur on textiles. In low liquor ratios, full-loaded winches and jets when heavy materials re processed, fiber lubricant is essential. The basic requirement is that is it should from a thin uniform protective coating around the fiber to lower the surface friction and flexural rigidity, thus minimizing the formation of durable creases during high temperature processing. The most suitable lubricant should have the properties so that it helps to emulsify, it does not undergo phase separation with extreme changes in pH and temperature, e.g. it is stable in high temperature and over a wide range of pH; and it should have excellent compatibility with all the chemicals in treatment bath, Suitable products are relatively hydrophobic surfactants, many of which also contain a proportion of solubilized or emulsified oil or wax.
Levelling and Dispersing Agent:
Unleveled dyeing problems can be of two categories: Gross unlevelness hroughout the material or localized unlevelness e..g. barriness, skitteriness. There are two fundamental mechanisms that can contribute to a dyeing.
- Control of the exhaustion dye so that it is taken up evenly.
- Migration of dye after initially unleveled absorption on the fibre.
- Non-ionic agent usually from water soluble complexes with the dye, some degree of solubilization being involved.
- Ionic agent are primarily dye-of fibre-substantive ; in the former case they tend to form complexes with the dye and there is competition between the leveling agent and the fibre for the dye, while in the latter case the competition is between the leveling agent and the dye for the fibre.
Sequestering, Dispersing and Levelling Agent for Reactive Dyeing :
It increases the solubility of dyes and controls the exhaustion of dyes so that these are taken up evenly on the substrate. It effectively chelates Ca, Mg ions which otherwise create while spots on the substrate. It also sequesters other di-and tri-valent metal ions without impairing the metal complex dyes, and thus yiels uniformly dyed fabric.
Where the present and persistence of foam is a nuisance, defoaming agents or antifoams are used. For maximum efficiency deform should be added to baths when necessary. There two groups of defoamers one that is water-soluble surfactants and other is water insoluble emulsions of silicones of organic-based the most effective defoamers are where the water-soluble compounds are used as vehicles for silicones of organic emulsions.
pH Control and buffer system :
Many disperse dyes required controlled 4.5-5 and practically all disperse dyes give good results in this pH range and are much more sensitive to minor changes in pH. Thus there is greater awareness of the actors that not only determine pH but also helps to stabilize it against interferences, in particular, from any acids or alkalies carries over from previous process. A buffered system is necessary for precise control of pH. By the use of that tends to resist changes arising from contaminations entering y way of the substrate of the water supply.
Desizing is essentially a part of the scouring process, and rapid removal of size is very important in the present trend towards continuous preparation processes. Starch-based products and especially solubilised starches are still the sizes, most commonly used on cellulosic goods. The are most frequently removed y enzymes treatment, which since effect it affects only the starch product ensure that the cellulosic fibre is undamaged; baterial amkulase id increasingly replacing malt and pancreatic enzymes. The efficiency of desizing is often taken for granted, whereas incomplete of uneven size removal is just likely to be the cause of uneven dyeing or printing as inadequate scouring.
In the textile industry, a smooth production process is essential for perfect results and smooth production needs yarn and sewing thread lubricants. Yarn lubricant are essential for dyed yarn used in modern knitting machine. They significantly improve the slip properties of yarn and sewing thread, increased heat resistance, better sewabilitym, reduced thread breakage and increase the lustre.
The objective of mercerizing is to oil the cotton fibre, increase its lustre strength and dyeability. Traditionally a cold solution of 25-26% by mass of sodium hydroxide is used, although better penetration and more even treatment is obtained with the more recent hot mercerizing technique. The addition of a wetting agent to the mercerizing liquor give better penetration and more even treatment, the main equirements being for a combination of stability and powerful wetting action.
Dye Fixing Agent:
Properties of a good fixing agent includes good capability with cross linking agents without promoting yellowing effect; good leveling and migration properties, does not affect the shade, has good affinity for the fiber, stable to steaming and dry heat, and improved all round fastness properties of the fabric or yarn. Fixative is generally used after completing the dyeing or in printing paste.
An Optical Brightening Agent (OBA), also called Fluorescent Brightening Agent (FBA) or Fluorescent whiteners (FW), are strongly fluorescent substance that emits light in the blue-violet region of the visible spectrum. In the Color Index, OBA or FBA’s are classified and are given C.I. number, i.e. they are treated as dyes. When present on a substrate, OBA increase the apparent reflectance of the substrate and makes it appear “whiter than white”. To evaluate an OBA it is necessary to apply the product and measure the whiteness, preferably with a spectrophotometer.
Soaping Agent / Washing off Agent:
The important of proper washing after dyeing is painfully known to all dyers, particularly the difficulty in washing off the unfixed dye in order to meet the required fastness standard. The main functions of soaping agent are to:
- Disperse the unfixed substanted hydrolyzed dyes, and solubilise and emulsify these into the aqueous medium and keep these substances in suspension.
- Prevent redeposition of hydrolyzed dyes and other particles of colloidal substance into the textile material.
- Removes the alkaline earth metal ions which have already deposited on the dyed material.
- Prevent the precipitation of insoluble calcium or magnesium salt.
The Quality of softness is very difficult to define, although it is often confused with only lubricity. Softness is more than lubricity, but these other attributes are difficult to define scientifically. The various products used soften are divided into silicone, anionic, non-ionic, amphoteric and reactive types, all of which falls under two basic chemical classes:
- Fatty acid derivatives and
- Silicon derivatives, by far the most important softness today are the silicon derivatives and fatty acid cationic types; however non-ionic softeners is used for textile materials because of its non-yellow effect and compatibility with OBA. Relevant properties of softeners include aqueous solubility, exhaustion, durability to washing, and compatibility with other additives to the textile materials.