Textile Soaps.
In the woollen, cloth, and silk textile industries, the use of soap for detergent and emulsifying purposes is necessary in several of the processes, and the following is a brief description of the kinds of soap successfully employed in the various stages.
1. Woollen Industry.—The scouring of wool is the most important operation—it is the first treatment raw wool is subjected to, and if it is not performed in an efficient manner, gives rise to serious subsequent troubles to manufacturer, dyer, and finisher.
The object of scouring wool is to remove the wool-fat and wool perspiration (exuded from the skin of sheep), consisting of cholesterol and isocholesterol, and potassium salts of fatty acids, together with other salts, such as sulphates, chlorides, and phosphates. This is effected by washing in a warm dilute soap solution, containing in the case of low quality wool, a little carbonate of soda; the fatty matter is thereby emulsified and easily removed.
Soap, to be suitable for the purpose, must be free from uncombined caustic alkali, unsaponified fat, silicates, and rosin.
Wool can be dissolved in a moderately dilute solution of caustic soda, and the presence of this latter in soap, even in small quantities, is therefore liable to injure the fibres and make the resultant fabric possess a harsh "feel," and be devoid of lustre.
Unsaponified fat denotes badly made soap—besides reducing the emulsifying power of the liberated alkali, this fat may be absorbed by the fibres and not only induce rancidity but also cause trouble in dyeing.
Soaps containing silicates may have a deleterious action upon the fibres, causing them to become damaged and broken.
By general consent soaps containing rosin are unsuitable for use by woollen manufacturers, as they produce sticky insoluble lime and magnesia compounds which are deposited upon the fibres, and give rise to unevenness in the dyeing.
A neutral olive-oil soft soap is undoubtedly the best for the purpose of wool scouring, as, owing to its ready solubility in water, it quickly penetrates the fibres, is easily washed out, and produces a good "feel" so essential in the best goods, and tends to preserve the lustre and pliability of the fibre.
The high price of olive-oil soap, however, renders its use prohibitive for lower class goods, and in such cases no better soap can be suggested than the old-fashioned curd mottled or curd soaps (boiled very dry), as free as possible from uncombined caustic alkali. The raw wool, after this cleansing operation, is oiled with olive oil or oleine, prior to spinning; after spinning and weaving, the fabric, in the form of yarn or cloth, has to be scoured to free it from oil. The soap in most general use for scouring woollen fabrics is neutral oleine-soda soap. Some manufacturers prefer a cheap curd soap, such as is generally termed "second curd," and in cases where lower grades of wools are handled, the user is often willing to have soap containing rosin (owing to its cheapness) and considers a little alkalinity desirable to assist in removing the oil.
Another operation in which soap is used, is that of milling or fulling, whereby the fabric is made to shrink and thus becomes more compact and closer in texture. The fabric is thoroughly cleansed, for which purpose the soap should be neutral and free from rosin and silicates, otherwise a harsh feeling or stickiness will be produced. Curd soaps or finely-fitted soaps made from tallow or bleached palm oil, with or without the addition of cocoa-nut oil, give the best results. All traces of soap must be carefully removed if the fabric is to be dyed.
The woollen dyer uses soap on the dyed pieces to assist the milling, and finds that a good soap, made from either olive oil, bleached palm oil, or tallow, is preferable, and, although it is generally specified to be free from alkali, a little alkalinity is not of consequence, for the woollen goods are, as a rule, acid after dyeing, and this alkalinity would be instantly neutralised.
2. Cotton Industry.—Cotton fibres are unacted upon by caustic alkali, so that the soap used in cleaning and preparing cotton goods for dyeing need not be neutral, in fact alkalinity is a distinct advantage in order to assist the cleansing.
Any curd soap made from tallow, with or without the addition of a small quantity of cocoa-nut oil, may be advantageously used for removing the natural oil.
In cotton dyeing, additions of soap are often made to the bath, and in such cases the soap must be of good odour and neutral, lest the colours should be acted upon and tints altered. Soaps made from olive oil and palm oil are recommended. The same kind of soap is sometimes used for soaping the dyed cotton goods.
The calico-printer uses considerable quantities of soap for cleansing the printed-cloths. The soap not only cleanses by helping to remove the gummy and starchy constituents of the adhering printing paste, but also plays an important part in fixing and brightening the colours. Soaps intended for this class of work must be quite neutral (to obviate any possible alteration in colour by the action of free alkali), free from objectionable odour and rosin, and readily soluble in water. These qualities are possessed by olive-oil soaps, either soft or hard. A neutral olive-oil soft soap, owing to its solubility in cold water, may be used for fibres coloured with most delicate dyes, which would be fugitive in hot soap solutions, and this soap is employed for the most expensive work.
Olive-oil curd (soda) soaps are in general use; those made from palm oil are also recommended, although they are not so soluble as the olive-oil soaps. Tallow curd soaps are sometimes used, but the difficulty with which they dissolve is a drawback, and renders them somewhat unsuitable.
3. Silk Industry.—Silk is secured to remove the sericin or silk-glue and adhering matter from the raw silk, producing thereby lustre on the softened fibre and thus preparing it for the dyer.
The very best soap for the purpose is an olive-oil soft soap; olive-oil and oleine hard soaps may also be used. The soap is often used in conjunction with carbonate of soda to assist the removal of the sericin, but, whilst carbonates are permissible, it is necessary to avoid an excess of caustic soda.
Tallow soaps are so slowly soluble that they are not applicable to the scouring of silk.
The dyer of silk requires soap, which is neutral and of a pleasant odour. The preference is given to neutral olive-oil soft soap, but hard soaps (made from olive oil, oleine, or palm oil) are used chiefly on account of cheapness. It is essential, however, that the soap should be free from rosin on account of its frequent use and consequent decomposition in the acid dye bath, when any liberated rosin acids would cling to the silk fibres and produce disagreeable results.
Patent Textile Soaps.—Stockhausen (Eng. Pat. 24,868, 1897) makes special claim for a soap, termed Monopole Soap, to be used in place of Turkey-red oils in the dyeing and printing of cotton goods and finishing of textile fabrics. The soap is prepared by heating the sulphonated oil (obtained on treatment of castor oil with sulphuric acid) with alkali, and it is stated that the product is not precipitated when used in the dye-bath as is ordinary soap, nor is it deposited upon the fibres.
Another patent (Eng. Pat. 16,382, 1897), has for its object the obviating of the injurious effects upon wool, of alkali liberated from a solution of soap. It is proposed to accomplish this by sulphonating part of the fat used in making the soap.
Miscellaneous Soaps.—Under this heading may be classed soaps intended for special purposes and consisting essentially of ordinary boiled soap to which additions of various substances have been made.
With additions of naphtha, fractions of petroleum, and turpentine, the detergent power of the soap is increased by the action of these substances in removing grease.
Amongst the many other additions may be mentioned: ox-gall or derivatives therefrom (for carpet-cleaning soap), alkali sulphides (for use of lead-workers), aniline colours (for home-dyeing soaps), pumice and tripoli (motorists' soaps), pine-needle oil, in some instances together with lanoline (for massage soaps), pearl-ash (for soap intended to remove oil and tar stains), magnesia, rouge, ammonium carbonate, chalk (silversmiths' soap), powdered orris, precipitated chalk, magnesium carbonate (tooth soaps).
Soap powders or dry soaps are powdered mixtures of soap, soda ash, or soda crystals, and other chemicals, whilst polishing soaps often contain from 85 to 90 per cent. siliceous matter, and can scarcely be termed soap.