A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction. The reaction between the dyes and fibres must take into account the various types of forces exerted by one molecule upon another. All dyeing mechanisms can be divided into following three groups:
- A specific bond between the dye and fibre owing to the covalent bond, hydrogen bonds or other directed bonds.
- Non- specific attraction between dye and fibre owing to ion-exchange or Van der Waals' forces.
- In absence of any interaction, dyes are only mechanically retained. This may be due to insolubilisation of the dye inside the fibre or may be due to self -association into the possibly quite large molecular aggregates following their entry into fibre. An aggregation is promoted by a high ratio of molecular weight to ionic group as well as by increase in length of the aromatic structure of the molecule, also by rise in concentration and the presence of inorganic salts, e.g. common salt.
Vander Waal's Forces:
These forces are so named because they were first recognized by Van der Waals in 1873. Very weak forces of attraction are always present between the electrons of one atom and the nucleus of another in close enough proximity. Individually these atoms are very weak forces, but collectively they are considered to be of sufficient strength to be the most important attractive forces between dye and fibre. These forces of attraction are known as Van der Waals' forces. Disperse dyes are held in a polyester fibre by means of Van der Waals' forces.
These forces of attraction are weak forces set up between certain atoms in the dyestuff molecule when they are close enough to other atoms in the fibre. One of these atoms is the hydrogen atom, hence the term "hydrogen bond". Some of the direct and vat dyes are "hydrogen bonded" in the cellulose fibres.