Woven fabrics may be rendered waterproof in a variety of ways, one of the commonest methods being to apply a coating of rubber solution and then to vulcanise the film of rubber remaining after the evaporation of the solvent, by the waterproofing method of Hime & Node, zinc is added to a solution of cellulose in an ammoniacal copper solution; copper is precipitated, and the fabric to be proofed is immersed in the remaining colourless viscid solution of ammonium, zincate, and cellulose. The impregnated fabric is pressed, dried, and wet-calendered, that is, passed between rollers. By another method, a fabric having a close texture is treated with sulphuric acid (115 Tw.), the fibres being partly parehmentised thereby, and the interstices closed without the texture of the cloth being in any way injured. The excess of acid is washed out, with or without previous treatment with alkali, and the fabric is passed between calendering rolls, which complete the closing of the interstices. Holfert's process is to pass the fabric through a bath of gelatine and then expose it to the action of gaseous formaldehyde, the gelatine becoming insoluble.
Another method of treatment is to apply to the fabrics boiled linseed oil, paints, varnishes, asphaltum, etc., as in the production of oilskin, tarpaulin, etc. (see p. (69). But one of the best of the waterproofing processes is explained below, in which the fabric is treated with an alumina soap. The word " soap " refers generally to a material used in removing dirt, and this it does by attacking grease and by removing the harshness or "hardness" of the water in use. But there are soaps which are insoluble in or quite incompatible with water, and these have their use in rendering fabrics waterproof. The ordinary soap of commerce is in one of two classes - " hard " or "soft" - and is formed by boiling fats with alkalis. With soda as the alkali a hard soap results, with potash a soft soap, these products being the alkaline salts of certain fatty acids - oleic, palmitic, stearic, etc. - derived from the fats used. When a solution of the salt of any other metal is added to a solution of either of the above soaps, a precipitate of an insoluble soap of that metal is formed, because all but the alkaline soaps are insoluble in water. In this manner it is possible to produce soaps of lead, copper, iron, aluminium, etc.
Alumina soap, so largely used in waterproofing, is formed from alum and soap in the manner above descrihed. In waterproofing fabrics with an alumina soap, one of two different methods may be employed. For the first method two solutions are required. (1) llb. of alum in 1 gal. of boiling water; (2) l1b. of ordinary soap in lgal. of boiling water. Keep these solutions in separate tubs or troughs. The best soaps to use are palm-oil or white-curd soap, but common yellow soap answers very well. The soap must be dissolved entirely or the coating will be patchy. When the solutions have cooled slightly, but while they are still warm, the cloth to be waterproofed should be immersed in the soap bath for about fifteen minutes, so that the soap sinks into the fibre. The cloth previously should have been soaked in water and wrung out. After wringing out the excess of soap solution, immediately plunge the cloth into the alum bath, in which it may remain for an equal period, and, being removed, excess of alum solution may be wrung out also. If a thick coating of the alumina soap is required, the cloth may be put through this treatment two or three times, and, after steeping in clean water, it may be hung out to dry.
The cloth on drying will be rather stiff and white, and somewhat rough, but will be quite waterproof; if the roughness is objected to, pass over the surface a hot iron, or calender the cloth between rollers. Any kind of cloth may be treated by this method, but the most suitable kinds are those that are closely woven, no matter how coarse the fibre is. Fabrics waterproofed in this way are but little altered; their feel is, -however, somewhat harsh, and water poured over them will run off without wetting any part, the alumina soap having filled up all the interstices, and formed over the fibres a protective coat, which prevents the water touching the cloth. The second method of applying the alumina soap is in the form of a solution in petroleum ether. The alumina soap is formed by mixing together the boiling alum and soap solutions as previously prepared; for complete precipitation 2i lb. of soap will be required to every 1 lb. of alum. The alumina soap separates out as a large cake, which should be collected on a piece of cloth, and the water squeezed out.
The cake may be broken up into small pieces, thoroughly dried at a low temperature, put into a dry, wide-mouthed bottle, and covered with petroleum spirit (benzoline); paraffin oil is unsuitable, because it fonns an unmanageable stringy mass. As the soap absorbs the benzoline it swells and should be stirred from time to time so that it is mixed thoroughly. The paste thus formed may be diluted as required with benzoline, but care should be taken not to add too much of it at any one time, because on standing the mass becomes unaccountably fluid, and possibly too thin; if this should occur, a little of the alumina soap is added. The waterproofing solution made in this manner may be laid on the cloth with a brush or, better, by passing the material through rollers fed with the solution. After treatment, the cloth should be hung out for a short time in the open air to allow the benzoline to evaporate. If a thicker dressing is required, the cloth may be coated two or three times; for ordinary purposes, however, once is quite enough.
The alumina soap may be coloured reddish-brown by the addition of a little perchloride of iron in place of some of the alum, and green by the addition of sulphate of copper (blue vitriol). It is also possible to obtain other colours by employing solutions of other metals, but these are more or less expensive. The common colours, yellow and black, may be imparted by stirring in yellow ochre or lampblack with the soap solution in the first method, or by kneading it with the alumina soap in the second.