Dye-Fibre Interaction
Q.What is Dye-Fibre interaction?
In dyeing, dye is attached with fibre by some sort of forces, may be physical or chemical in nature.
◆This is essential for desired fastness of dyeings during domestic use. The forces may be:- H-bond, van der Waals force, ionic bond, covalent bond, binder.
◆Dye should migrate and diffuse throughout freely; this depends on size and shape of fibre pores which are to give adequate passage to adsorbed dye molecules.
◆Few basic concepts to explain dye-fibre interaction, called theories, those predict this attachment are divided into six categories:-
1. Physical theory
2.Chemical theory
3.Physico-chemical theory 4.Fibre-complex theory
5.SolÃd solution theory and
6.Mechanical or pigment theory
1. Physical theory
2.Chemical theory
3.Physico-chemical theory 4.Fibre-complex theory
5.SolÃd solution theory and
6.Mechanical or pigment theory
1.Physical theory
◆Dyes after application are physically retained by fibre through hydrogen bond and van der Waals forces.
◆Fastness of dyeings depend on size of dye molecule and its solubility in water. Examples are dyeing of cellulosics with direct, vat, sulphur and solubilised vat dyes; dyeing of man-mades with disperse dyes.
◆Direct dyes are water-soluble causing poor wash fastness, whereas sulphur and vat dyes are water insoluble and larger in size showing good to excellent wash fastness.
2. Chemical theory
◆Dye and fibre, both possess required reactive groups to develop dye-fibre attachment through chemical bonding.
◆Nature of bond is mostly ionic (electrovalent), though in some cases, covalent bonds are also formed.
◆Fastness of dyeings depends upon number of reactive sites attached to the fibre -the greater the number of sites attached, the better the wash fastness.
◆After half-time dyeing, electrolyte is added for further exhaustion of bath, e.g., dyeing of cotton with reactive dyes.
◆ Examples of ionic attachment are dyeing of silk with acid or basic dye, acrylic with basic dye, nylon with acid dye, cationic dyeable polyester with basic dye and anionic dyeable polyester with direct, vat, sulphur and reactive dyes, while dyeing of cotton with reactive dyes is a case of covalent bond formation between dye and fibre.
3. Physico-chemical theory
◆Dyes are applied to form physical bonds with fibre. To improve fastness or dyed textile, size of dye molecule is increased by reacting the dye (while on extile) chemically with some other external chemical or dye known as after-treatment.
◆One component must be a dye and the second component is either a dye or a chemical, e.g, after-treatment of direct dyeings with either of metal salt, formaldehyde, cationic dye fixing agent, basic dye, resins or diazotized base.
◆Other examples are back tanning of dyed pretein fibres, mordanting of cotton for dyeing with basic dye and mordant dyeing.
4. Fibre complex theory
◆Dye is incapable of entering into fibre matrix by itself due to larger structure and lack of affinity and is produced in situ by means of reacting two different chemicals under favourable conditions.
◆The first component is applied to the textile by padding or exhaust method, followed by reacting
previously treated textile with second component.
previously treated textile with second component.
◆In some cases, the two components are mixed and simultaneously applied- reaction takes place when the treated fabric is after-treated under favourable conditions.
◆The desired colour is developed only after coupling the coloured complex (lake or ingrain dye) is formed in situ and can not come out of fibre phase, e.g., dyeing of cotton with insoluble azoic, mineral, oxidation and phthalocyanine colours.
5. Solid solution theory
◆Both the phases are solid -dyestuff and fibre, but when the dye is applied under suitable condition, it gets passage inside fibre resulting a solid- solid solution mechanism.
◆Dyestuffs are retained within the fibre with physical forces. Fastnesses of dyeings are good in some cases (polyester) and not good in other cases (light fastness of disperse dyed nylon).
◆ Dyeing is done at higher temperature, when the fibre structure is opened up paving adequate passage to the dye.
◆The dye gets mechanically deposited on fibre surface, diffuses inside and is trapped due to collapse in fibre structure during cooling, e.g, dyeing of man-mades with disperse dye.
6. Pigment or mechanical theory
◆The colouring material possesses no reactive site, has no affinity for fibre and is insoluble in water and other solvents.
◆Dyeing is generally carried out in aqueous emulsion along with a binder.
◆ The latter on curing establish cross-link with textile and trap pigments on fibre surface.
◆Though fastness properties are good, but it depends strictly on the longevity of the film.
◆Fabrics become stiffer, e.g. dyeing of any fibre with pigment colours. The colour is retained by the fibre mechanically and no pigment-fibre attachment exists.
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