1. There are six main theories that explain the interaction between dyes and fibers: physical, chemical, physico-chemical, fiber-complex, solid solution, and mechanical or pigment theories. 2. The physical theory involves dyes being physically retained on fibers through hydrogen bonding and van der Waals forces, while the chemical theory involves chemical bonding between reactive groups on the dye and fiber. 3. The physico-chemical theory involves dyes first being attached physically and then chemically treated to further improve fastness.

1. BY
B. Venkatesh
Asst. Professor,
Dept. of Textile Technology
VFSTR University
*

2. *
*In dyeing, dye is attached with fibre by some sort of forces, may be
physical or chemical in nature.
*This is essential keeping in view desired fastness of dyeings during
domestic use.
*Various dyeings show wash, light, sublimation, rubbing fastness at
varying extents based on dye–fibre system.

3. Few basic concepts
to explain dye–fibre
interaction, called
theories, are
divided into six
categories
1. physical
theory
2.chemical
theory
3. physico-
chemical theory,
4. fibre-complex
theory
5. solid solution
theory
6. mechanical or
pigment theory

4. *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 – the larger the size and lesser the solubility, the better the
fastness.
*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,
*Sulphur and vat dyes are water insoluble and larger in size showing
good to excellent wash fastness.

5. *
*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.

6. *Somewhere, rate of dyeing is so fast that retarding or levelling agents are applied
at the start of dyeing to ensure level dyeing through reduction in rate of dyeing,
* e.g., dyeing of wool with acid dyes.
*Examples of ionic attachment:
*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
*Dyeing of cotton with reactive dyes is a case of covalent bond formation
between dye and fibre.

7. *
*Dyes are applied to form physical bonds with fibre.
*To improve fastness of dyed textile, size of dye molecule is increased by
reacting the dye (while on textile) 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 protein fibres, mordanting of
cotton for dyeing with basic dye and mordant dyeing of wool.

8. *
*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
*e.g., dyeing of cotton with insoluble azoic, mineral, oxidation and
phthalocyanine colours.

9. *
*Both the phases are solid – dyestuff and fibre
*When the dye is applied under suitable condition, it gets passage
inside fibre resulting a solid–solid solution mechanism.
*The term solid solution has been referred due to the fact that
location of dye inside hydrophobic fibre.
*It can not be ascertained by electron microscopy or x-ray
diffraction due to solubilisation of it in fibre matrix, looks like a
solid vs solid solution

10. *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.

11. *
*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

12. *