CHEMICAL RELATION OF TEXTILE FIBRES

The chemical relations of textile fibres are based on their structure of molecular chains. All the textile fibres are sensitive to oxidizing agent which must be used with care; this is important in connection with the use of washing powders in laundering. The native cellulose fibres such as cotton and linen are resistant to alkali, even hot alkali, but are sensitive to hot dilute acid. Regenerated cellulose fibres are more reactive, thus more susceptible to chemical attack. Wool and silk are susceptible to alkali, but are inert to dilute acid.

All the textile fibres are attacked by concentrated mineral acids.

Alkalis: Alkaline compounds are used as cleansing agents from time immemorial probably on account of their ability to split up fatty and greasy impurities; it is to consider the behavior of textile fibres to alkaline liquors from standpoint of purification and laundering.

Cotton, flax, rayon are resistant to alkali but acetate rayon is sensitive. Animal fibres e.g. wool are sensitive to alkali. Initial attack on the wool substance takes place at the disulfide bond and them in the main polypeptide chain. Silk is also sensitive of alkali, but less than wool. Alkaline attack on silk takes place in the main polypeptide chains. Cotton is mercerized by the action of alkali, thus diminishing the length and increasing the width of fibres.

Acids: Cotton is very sensitive to acids. The attack is acid hydrolyses of the molecular chain with reduction in chain length until glucose is formed. Rayon is more sensitive to acid attack than cotton. Dilute acid bring about some swelling of wool, but high concentration decompose it. Concentrated solutions of mineral acids attack wool which suffers a diminution in strength, it is probable that the attack takes place both at the polypeptide chain and the salt linkages.

Silk is attacked by mineral acids with loss of strength, but organic acids are less destructive. Nylon is sensitive to mineral acids, whereas vinyl fibres are resistant.

Oxidizing Agents: All textile fibres are sensitive to oxidizing agents, so for bleaching and cleansing care should be taken. Calcium hypochlorite may be used at room temperature with cotton, linen, regenerated cellulose rayons. Hypochlorites are not used for animal fibres as they are not bleached with these reagents; both wool and silk absorb very considerable amount of chlorine and are damaged thereby H₂O₂ is universal bleaching agent; silk and wool may be suitable but nylon is sensitive. With acidic oxidation aldehyde groups are produced by the opening of ring structure but without diminution in chain length.

Reducing Agents: Reducing agents have no effect on cellulosic fibres. Concentrated solutions of strong reducing agents e.g. sodium bisulphite at the boiling point attack wool at the disulphide bond and cause considerable weakening. Synthetic fibres are unaffected by reducing agents.

Inorganic Salts: Inorganic salts have their particular effects on textile fibre differently, especially in the process of dyeing. The salts of Pb, Zn, Cu, Al, Sn, Fe and Cr may be used as mordents in dyeing because their basic oxides are removed from solution by the cellulose fibres cotton, jute etc. Silk and wool possess considerable affinity for many metallic salts. Silk may be dissolved by warm concentrated solutions of salt e.g. ZnCl₂, NaI etc. Wool with its cross-linkages is more resistant to these salt solutions as it is first necessary to break the bridging linkage. Again rayon is more susceptible than native cellulose to inorganic salts.

Organic Solvents: The native fibres and also the regenerated fibres are unaffected by the common organic solvents; hence they may be ‘dry-cleaned’.

Cellulose acetate rayon dissolves in glacial acetic acid and also acetone, insoluble in pure chloroform. It does not dissolve in petrol, ether, spirit, benzene, MEK (methyl ethyl ketone), pyridine, formic acid etc. The following organic chemicals will dissolve cellulose acetate on warming: aniline, lactic acid, phenol etc. Nylon dissolves in hot glacial acetic acid 90% formic acid, phenol, cresol and similar compounds. Vinyl fibres except orlon swell or dissolve in certain specific solvents, particularly ketones.

Both formaldehyde and quinine may react with the amino groups in the side chains of wool to give extra cross-linkages. Phosgene will also react with the basic side chains of wool to give side chains containing the N:CO grouping or isocyanate grouping.