The action of alkalies on cellulose or cotton is one of great importance in view of the universal use of alkaline liquors made from soda or caustic soda in the scouring, bleaching and dyeing of cotton, while great interest attaches to the use of caustic soda in the "mercerising" of cotton.
Dilute solutions of the caustic alkalies, caustic soda or caustic potash, of from 2 to 7 per cent. strength, have no
action on cellulose or cotton, in the cold, even when a prolonged digestion of the fibre with the alkaline solution takes place. Caustic alkali solutions of from 1 to 2 per cent. strength have little or no action even when used at high temperatures and under considerable pressure—a fact of very great importance from a bleacher's point of view, as it enables him to subject cotton to a boil in kiers, with such alkaline solutions at high pressures, for the purpose of scouring the cotton, without damaging the fibre itself.
Mercerised Cotton Fibre.
Solutions of caustic soda of greater strength than 3 per cent. tend, when boiled under pressure, to convert the cellulose into soluble bodies, and as much as 20 per cent. of the fibre may become dissolved under such treatment. The action of strong solutions of caustic soda or caustic potash upon cellulose or cotton is somewhat different. Mercer found that solutions containing 10 per cent. of alkali had a very considerable effect upon the fibre, causing it to swell up and become gelatinous and transparent in its structure, each individual cotton fibre losing its ribbon-like appearance, and assuming a rod-like form, the central canal being more or less obliterated. This is shown in Fig. 2 and 2A, where the fibre is shown as a rod and the cross section in Fig.
2A has no central canal. The action which takes place is as follows: The cellulose enters into a combination with the alkali and there is formed a sodium cellulose, which has the formula C6H10O52NaOH. This alkali cellulose, however, is not a stable body; by washing with water the alkali is removed, and hydrated cellulose is obtained, which has the formula C6H10O5H2O. Water removes the whole of the alkali, but alcohol only removes one half. It has been observed that during the process of washing with water the fibre shrinks very much. This shrinkage is more particularly to be observed in the case of cotton. As John Mercer was the first to point out the action of the alkaline solutions on cotton, the process has become known as "mercerisation".
Solutions of caustic soda of 1.000 or 20° Tw. in strength have very little mercerising action, and it is only by prolonged treatment that mercerisation can be effected. It is interesting to observe that the addition of zinc oxide to the caustic solution increases its mercerising powers. Solutions of 1.225 to 1.275 (that is from 45° to 55° Tw. in strength) effect the mercerisation almost immediately in the cold, and this is the best strength at which to use caustic soda solutions for this purpose. In addition to the change brought about by the shrinking and thickening of the material, the mercerised fibres are stronger than the untreated fibres, and at the same time they have a stronger affinity for dyes, a piece of cloth mercerised taking up three times as much colouring matter as a piece of unmercerised cloth from the same dye-bath.
The shrinkage of the cotton, which takes place during the operation of washing with water, was for a long time a bar to any practical application of the "mercerising" process, but some years ago Lowe ascertained that by conducting the operation while the cotton was stretched or in a state of tension this shrinkage did not take place; further, Thomas and Prevost found that the cotton so treated gained a silky
lustre, and it has since been ascertained that this lustre is most highly developed with the long-stapled Egyptian and Sea Island cottons. This mercerising under tension is now applied on a large scale to produce silkified cotton. When viewed under the microscope, the silkified cotton fibres have the appearance shown in Fig. 3, long rod-like fibres nearly if not quite cylindrical; the cross section of those fibres has the appearance shown in Fig. 3A. This structure fully accounts for the silky lustre possessed by the mercerised fibres. Silky mercerised cotton has very considerable affinity for dye-stuffs, taking them up much more readily from dye-baths, and it is dyed in very brilliant shades.