The internal surface of fibers and its importance
The nature fibres, i.e. the cellulosic and protein fibres have exceedingly large internal surfaces, which are the walls of the channels between the bundles of long-chain molecules composing the fibres. The number of such channels is immense, of the order of ten million in the cross-section of, e.g. cotton or a wool fiber, and the total surface of their walls is of the order of 100 m2 or five acres per lb. This is about one thousand times as the outer surface of the fiber.
Whets the fiber is wetted, water rapidly penetrates and swells a large proportion of these channels, and Dyes in solution are then able to diffuse into the channels or pores. They can however enter only a relatively s proportion of the total internal space, because the remainder is in pores too small to admit a dye molecule. Many of the synthetic polymer fibres have much less internal surface than the natural fibres, but the dye used with such fibres are able to penetrate between the fiber molecules even though water cannot always do so.
Dyes are surface-active substances, that is, when dissolved in water their molecules tend to concentrate more closely together at a surface than in the body of the solution .the surface (or interface) can be that between the solution and either air or a fibre. The first action in any dyeing operation is the concentration of dye molecules at as much of the
internal surface of the fibre as they can reach. The concentration so produced is not usually sufficient to give a useful deep coloration to the fibre, and for such coloration other factors must be brought into play. These are the chemical forces, which can operate between a dye molecule, and fibre molecule, which are classified below, and also those between the dye molecules themselves. Which can cause their association into larger units?
The main physical and chemical effects between fibres and dyes
Broadly, four main chemical effects subsequently responsible for the substantively of the dye for the fibre are list below:
These seldom act in isolation; usually at least two operate in any dyeing process.
· The hydrogen bond
This is the ‘secondary valency’ by which a hydrogen atom in e.g. a hydrogen group can form a weak association with another atom. Most fibres and dyes contain groups that can take part in this form of combination. There is evidence for the importance of hydrogen bonds in dyeing some man-made fibres. e.g. cellulose acetates and possible cellulosic protein fibres.
· Non-polar forces
This is a manifestation of the universal tendency, of atoms and molecules to attract one another. They seem to be particularly effective in attracting a dye to a fibre when the two have certain special characteristics, e.g. either when they both have long and fairly flat molecules, as with cellulose and direct or vat dyes and also with cellulose acetate and disperse dyes, or when they both contain a considerable proportion of purely hydrocarbon groups (aliphatic or aromatic) as with some dyes applied to wool and most dyes applied to polyester. In the latter cases the presence of the water of the dye bath assists the dye-fibre attraction because hydrocarbon groups tend to escape from water and associate together. This effect is known as ‘hydrophobic bonding’.
· Ionic forces
The third form of attraction between dye and fibre is due to difference of electric charge between them. In water, fibres become negatively charged and. Since most water soluble
dyes are anionic, their coloured ion carrying a negative charged, adsorption does not occur readily. It is then necessary to reduce or even reverse the charge on the fibre before the dye ion can approach closely enough, for the non-polar forces to become effective. (This does not apply with the u of cationic, i.e. positively charged dyeing of acrylic fibres.)
Adding salt to the bath can have the required effect with cellulose fibre, a suitable with protein fibres and nylon. In the latter case, the reaction for wool dyeing in presence of acid can be illustrated by a series of simple chemical equations.
· Covalent bonds
Only reactive dyes are attached to the fibre by a covalent bond, which is much stronger than the previously mentioned forces and difficult to break down. Some degree of breakdown, shown by bleeding of the dye from the fibre, can with some types of reactive dye be produced to small extent by drastic treatment with acid or alkali, and almost completely by a treatment For 3 h in boiling 49% aqueous hydrazine solution.