Textile Fabrics and Their Preparation for Dyeing

TEXTILE FABRICS AND THEIR PREPARATION FOR DYEING.

CHAPTER I.

COTTON.

The Cotton Plant.—Cotton is the white, downy, fibrous substance which envelopes the seeds of various species of the cotton-plant, Gossypium, belonging to the natural

Fig. 1.—Cotton Plant.

order Malvaceœ. The seeds, to which the cotton fibres are attached, are enclosed in a 3- to 5-valved capsule, which bursts when ripe; the cotton is then collected and spread out to dry. The seeds are afterwards separated by the mechanical operation termed ginning, and the raw cotton thus obtained is sent to the spinner. The cotton-plant (Fig. 1) is cultivated with success only in warm climates. There are numerous varieties, of which the following are the principal:—

(1) Gossypium barbadense.—An herbaceous plant, bearing a yellow flower, and attaining a height of 4-5 m. (13-16 ft.). A variety of this species yields the Sea Island cotton, much prized on account of the great strength, length, and lustre of its fibres. It is grown in the North American States of South Carolina, Georgia, and Florida, and on the neighbouring islands of the West Indies.

(2) Gossypium hirsutum.—A hairy, herbaceous plant, about 2 m. high, with pale yellow or almost white flowers. It is grown in the States of Alabama, Louisiana, Texas, and Mississippi.

Fig. 2.—Appearance of Cotton under the Microscope.

(3) Gossypium herbaceum.—A small herbaceous plant, 1 m. high, and bearing yellow flowers. Varieties of this species are grown in India, China, Egypt, and America. The Madras, Surat, and short-stapled Egyptian cotton, also some American cottons, are obtained from this species.

(4) Gossypium peruvianum.—This species, a native of South America, grows to a height of 3-5 m. (10-16 ft.), and bears a yellow flower. It yields the long-stapled and much-esteemed Peruvian and Brazilian cottons.

(5) Gossypium religiosum.—This is a low annual shrub, about 1 m. high, and bearing a yellow flower. It is grown in China and India, and yields the so-called Nankin cotton, remarkable for its tawny colour.

(6) Gossypium arboreum.—This is a perennial tree, growing to a height of 6-7 m. (20-23 ft.), and bearing reddish-purple flowers. It is a native of India, and produces a good quality of cotton.

Physical Structure of Cotton.—If raw cotton is examined under the microscope, it is seen to consist of minute fibres. Their general appearance is that of spirally-twisted bands, having thickened borders and irregular markings on the surface (Fig. 2). In the better qualities of cotton, such as Sea Island, the spiral character is less prominent. Transverse sections of the fibres show them to be flattened tubes, having comparatively thick walls and a small central opening (Fig. 3).

A single cotton fibre is an elongated, tapering, and

Fig. 3.—Transverse Sections of Cotton Fibre.

collapsed plant cell, the thin end of which is closed, and the other (by which it was attached to the seed) irregularly torn. Sometimes broad ribbon-like fibres may be noticed, which are remarkably transparent, and possess irregular folds. Their transverse section exhibits no central opening (Fig. 4). They are unripe fibres, in which no separation of the thin cell walls has yet taken place. They refuse to be dyed like ordinary ripe fibres, and appear occasionally as white specks in indigo- and madder-dyed calicoes; hence the name of dead cotton has been given

Fig. 4.—Transverse Sections of Unripe Cotton Fibre.

to them. In half-ripe cotton fibres the cell walls are still so closely pressed together that the ultimate central canal is indicated in a transverse section only by a fine line. When steeped in water, however, such fibres gradually swell up and form hollow tubes. Cotton fibres vary in length 2.5-6 cm. (1-2 1/2 in.), and in breadth 0.017-0.05 mm. (.0007-.002 in.).

The spiral character of the fibre makes it possible to spin exceeding fine yarn, and also accounts for the elastic character of calico as compared with linen, the fibres of which are stiff and straight. The microscopic appearance of cotton serves to distinguish it from other vegetable and animal fibres.

Chemical Composition of Cotton.—The substance of the cotton fibre is called Cellulose. This is almost universal in vegetable cells, forming the so-called ligneous matter or woody fibre of plants, but whereas in woody fibre the cellulose is encrusted with a large proportion of foreign matter—such as dried-up sap, resin, etc.—in the cotton fibre it is in a tolerably pure condition. The impurities present amount to about 5 %, this being the loss sustained by raw cotton when submitted to the process of bleaching, the main object, indeed, of which is the total removal of these impurities. The principal bleaching operation consists in boiling the cotton with a solution of sodium carbonate or hydrate. From the dark brown solution thus obtained, acids throw down a voluminous light brown precipitate, which, when washed and dried, amounts only to about 0.5 % of the weight of cotton employed. This precipitate is found to consist of the following organic substances: Pectic acid, brown colouring matter, cotton wax, fatty acids (margaric acid), and albuminous matter. Pectic acid exists in the largest proportion, and it is not improbable that the 4.5 % loss by bleaching still unaccounted for, represents certain pectic matters, modified and rendered soluble by the action of alkalis, but not precipitated by acids.

In addition to the above-mentioned impurities of the cell wall, the raw cotton fibre seems to be covered with an exceedingly delicate membrane, or cuticle, which is not cellulose. If cotton, when under microscopical observation, be moistened with an ammoniacal solution of cupric hydrate, the fibre swells up under its influence, whereas the cuticle is unaffected and shows itself as band-like strictures or rings of various breadths. If a drop of sulphuric acid be then added, the cellulose separates out as a gelatinous mass, which, on adding a drop of iodine solution, becomes coloured blue, whereas the cuticle is coloured yellow. By moving the cover-glass aside a little, the cuticle rings are seen to be in the form of tubes, possessing apparently a spiral structure. Some observers state that during the bleaching process this cuticle is removed, while others say this is not the case. The standard of moisture in raw cotton is 8.5 %, so that, reckoning the 5 % impurities already alluded to, one may consider that raw cotton contains 86.5 % of pure dry cellulose.

When submitted to chemical analysis, cellulose is found to be composed of carbon, hydrogen, and oxygen, the formula assigned to it being C6H10O5. It is closely allied in composition to starch, dextrin, and glucose, and is classed along with them as a carbo-hydrate. It is colourless, possesses neither taste nor smell, and has a density of about 1.5. If heated above 130° C. it becomes brown, and begins to decompose. In contact with air it burns without emitting any very strong odour, a fact which may sometimes serve to distinguish it from wool and silk. It is quite insoluble in the ordinary solvents, water, alcohol, ether, etc., but, as already indicated, it dissolves in an ammoniacal solution of cupric hydrate; from this it is precipitated by acids as a gelatinous mass, which, when washed with alcohol, forms an amorphous white powder.

Action of Mildew on Cotton.—Owing to its comparative freedom from impurity, cotton may be stored for a long period without undergoing any change, more especially if it is bleached and kept dry. When, however, it is contaminated with added foreign organic matter, such as starch, gum, etc. (in finished calicoes), and then exposed to a moist, warm atmosphere, it is very liable gradually to become tender or rotten. This is owing to the growth of vegetable organisms of a very low order, generally called mildew. These fungi feed upon the starchy matters present, inducing their decomposition, and after some time the cotton fibres themselves are attacked. The simultaneous production of crenic, humic, ulmic, and other organic acids may possibly assist somewhat in the tendering process.

Action of Frost on Cotton.—It has been supposed by some that wet calico is tendered when it is frozen. Although the evidence on this point is conflicting, it is quite conceivable that the crystallisation might act injuriously in a mechanical way, and that the atmospheric ozone might also exercise some slight destructive influence. The popular notion probably arises from the fact that in their rigid state the cotton fibres are readily broken. A similar friable condition is obtained by excessive stiffening with starch or gum.

Action of Acids on Cotton.—Cold dilute mineral acids have little or no action, but if allowed to dry upon the cotton they gradually become sufficiently concentrated to corrode and tender the fibre. The physical structure of the fibre is not affected, but the chemical composition of the disintegrated fibre seems to be somewhat altered: it contains more oxygen and hydrogen. The same corrosive action soon takes place if cotton impregnated with such acids is heated. The process of extracting or carbonising woollen rags containing cotton (destroying and removing the cotton), by means of sulphuric or hydrochloric acid, is founded on this fact.

The action of strong acids varies considerably according to the nature, concentration, and temperature of the acid, as well as the duration of its contact with the fibre.

Very concentrated sulphuric acid causes cotton to swell up, and form a gelatinous mass, from which, on the addition of water, a starch-like substance termed Amyloid may be precipitated. A solution of iodine colours this amyloid blue. Vegetable parchment is paper (cellulose) superficially changed into amyloid by a short steeping in strong sulphuric acid 140° Tw. (Sp. Gr. 1.7), then washing and drying. An increased affinity for basic coal-tar colouring matters is said to be imparted to cotton by this treatment, even when the acid is diluted to 84° Tw. (Sp. Gr. 1.42), although its physical aspect then remains unchanged. Cotton completely disorganised by acid, and obtained as a fine powder, seems to contain one molecule of water more than ordinary cellulose, and the substance thus produced has been termed Hydro-cellulose.

If the concentrated sulphuric acid is allowed to act for a longer time, the cotton dissolves with the formation of a different substance of a gummy nature, called Dextrin (C6H10O5); when the solution is diluted with water and boiled for some time, this dextrin is further changed into Glucose (C6H12O6).

If cotton be heated with strong nitric acid it is entirely decomposed, producing oxalic acid and an oxidised cellulose soluble in alkalis. By the action of cold concentrated nitric acid, or, better still, a mixture of strong nitric and sulphuric acids, cellulose is changed into so-called Nitrocellulose. The physical structure of the cotton remains the same, although increased in weight by more than 5 %, but its chemical composition and properties are very much altered, certain elements of the nitric acid having replaced a greater or less proportion of the hydrogen of the cellulose. The most highly nitrated compound is Pyroxylin, or Gun-cotton (C12H14(NO2)6O10), produced by the short action of a very concentrated mixture of acids; it is very explosive, and insoluble in alcohol and ether. The less nitrated product, obtained by the longer action of more dilute acids, forms the so-called Soluble Pyroxylin. Its solution in a mixture of ether and alcohol constitutes Collodion, which on evaporation leaves the pyroxylin as a thin, transparent, horny film, insoluble in water. It was long ago noticed by Kuhlmann that gun-cotton had an increased affinity for colouring matters, but no practical use has been made of the fact.

Strong hydrochloric and phosphoric acids behave towards cotton like sulphuric acid, but their action is less energetic. The ultimate product of the action of hydrochloric acid on cotton is the same as that given by sulphuric acid, but there is no intermediate formation of amyloid.

Solutions of tartaric, citric, and oxalic acids have no destructive action on cotton if it is simply steeped in the liquid; but if cotton saturated with a solution containing 2 % of any of the above acids is dried and heated for an hour to 100° C., it becomes slightly tendered. With 4 % solutions the destructive action is very decided at 100° C., and perceptible even at 80° C. Oxalic acid has the most injurious effect in this respect. If the acid solutions are thickened with gum or starch, and if steaming be substituted for a dry heat, the corrosive action in each case is less marked, so that in the ordinary practice of the calico-printer, who frequently uses steam-colours containing 4 % or more of the above acids, there is little to fear. Still, it is well to bear in mind that even organic acids cannot under all circumstances be applied to cotton with impunity.

Acetic acid may be considered as having no perceptible action on cotton.

Action of Alkalis on Cotton.—Weak solutions of caustic potash or soda, when used cold, have, under ordinary circumstances, no action on cotton, although long-continued and intermittent steeping and exposure to air tender the fibre. Cotton may even be boiled for several hours with weak caustic alkalis, if care be taken that it remains steeped below the surface of the solution during the whole operation, but otherwise it is very liable to become rotten, especially if the exposed portions are at the same time under the influence of steam. Such exposure is to be guarded against during certain of the operations in bleaching cotton fabrics. The tendering action is probably due to oxidation. It is worthy of note that the disorganised fibre (oxy-cellulose) possesses an increased attraction for basic coal-tar colouring matters.

In the case of raw cotton, the action of boiling with weak caustic alkalis is simply to remove those natural impurities already referred to, which cause it to be water-repellent, and therefore difficult to wet by mere steeping in cold water.

The action of strong solutions of caustic potash or soda is very remarkable. If a piece of calico is steeped for a few minutes in a solution of caustic soda, marking about 50° Tw. (Sp. Gr. 1.25), it assumes quite a gelatinous and translucent appearance; when taken out and washed free from alkali, it is found to have shrunk considerably, and become much closer in texture. If a single fibre of the calico thus treated be examined under