For certain purposes, a bath containing a mixture of cuprammonium and the analogous zinc-ammonium hydroxide solutions may be used with advantage; the zinc compound does not of itself sufficiently pectise cellulose to give good results, but when used in conjunction with cuprammonium hydroxide, pectising is brought about by the copper solution, whilst certain advantages are gained by the simultaneous presence of zinco-celiulose and cupro-cellulose in the finished goods. The manufacture of cuprammonium and zinc-ammonium solutions is effected by the simultaneous action of air and ammonia water on metallic copper (or brass, if a mixture of cupro- and zinco-ammonium hydroxides is required), due attention being paid to the recovery of the large amount of ammonia necessarily carried away by the "spent" air during the operation. The manufacture of fabrics, notably paper and canvas, treated with cuprammonium solutions so as to waterproof them and render them rot-proof, and practically free from the attacks of insects and mould, has been recently commenced on the large scale, at Willesden, by the Patent Waterproof Paper and Canvas Co.
The process by which these fabrics are manufactured may be described as essentially consisting of the preparation of a concentrated solution of cuprammonium hydroxide, and the passing of the goods to be treated through a bath of this material at just such a rate as will permit of the pectising and gelatinising of the exterior of the fibres composing the paper or canvas, etc., without wholly disintegrating the mass; so that the material on emerging from the bath retains coherence sufficient to enable it to be passed over and under the usual drums, etc., of a paper mill, and so to be dried in the ordinary way. This drying converts the film of pectised cellulose coating each filament and fibre into an insoluble solid varnish which cements the whole together. In order to build up thick cards, 2 or more reels of paper are employed, passed simultaneously through the bath, and then pressed together and dried as a whole, 2 thicknesses thus treated forming 2-ply card. The best kinds of thick cards are made by passing 2 rolls of 2-ply simultaneously through the bath ft second time, and pressing them together and drying, thus giving rise to 4-ply card, the thickness mostly adopted for roofing.
By repeating this process with 2 batches of 4-ply, 8-ply is obtained; and similarly to any required degree of thickness necessary for special purposes.
A noteworthy point in connection with these processes is that, when certain precautions are taken, the copper present in the ammoniacal fluid imbibed by the material passed through the bath is wholly converted during drying into a compound with the pectised cellulose of an agreeable green tint, and is not deposited as black copper oxide, as it would be on evaporation of the solution without cellulose in a porcelain dish, etc. It is largely the presence of copper in this form that renders Willes-den fabrics so free from growths of mould, mildew, rot, and fungoid vegetation generally, and from the attacks of insects. The compound is so stable as to be wholly unaffected by water, when once dry. Of course, mineral acids dissolve out copper to some extent, but this is not the case with ordinary water, apparently not even with London rain.
Instead of cuprammonium hydroxide alone, in certain cases a mixture of cuprammonium and zincammonium hydroxides may be used. The pectised cellulose then contains both zinc and copper, indicating apparently that a zinc cellulose compound has also been formed. Zincammonium hydroxide alone, however, does not pectise paper sufficiently to give good results. In order to pectise paper, etc, thoroughly, when the materials are passed through the baths at a convenient manufacturing speed, it is essential to use a liquid containing 100-150 lb. of ammonia per 100 gal.* (about as much ammonia as is present in solution of ammonia, sp. gr. 940 to 960). Such a fluid, when nearly saturated with copper present as pure .cuprammonium hydroxide, will contain 20-25 lb. copper (reckoned as metal) per 100 gal. Considerably larger amounts of copper, * 1 lb. per 100 gal. = 1 grm. per litre. however, may be taken into solution in the form of cuprammonium salts, or when certain forms of organic matter are also present in the liquor. According to the textbooks, ammoniacal solutions of cuprammonium salts dissolve cellulose, but such fluids are found to be unsuitable for the manufacture of Willesden goods, for a variety of reasons.
In the first place, for a given quantity of copper in solution an ammoniacal solution of cuprammonium hydroxide appears to possess a considerably higher pectising power than a similar solution of a cuprammonium salt; next, when a cuprammonium salt, e. g. the sulphate, is used, there is not only a tendency to form a little copper sulphate, which can be washed out of the finished fabric by water (thus rendering the material unsuitable for many purposes, e. g. cattle drinking-troughs, portable sheep-pens, &c), but further, much ammonium sulphate is formed in the body of the fabric, thus giving rise to a double disadvantage : first, because the ammonia thus fixed is wholly lost, whereas the ammonia in cuprammonium hydroxide is wholly volatilised during the drying, and can be recovered and used over again; secondly, because when the fabric is wetted, the ammonium sulphate is washed out, thus partially opening the texture, and rendering the mass more porous and less impervious to moisture; beside which, the ammonium sulphate sometimes effloresces as an unsightly saline film.
For these and other reasons cuprammonium hydroxide, and not a cuprammonium salt, is employed in the manufacture of the so-called "Willesden" goods.
These goods are divisible into 2 classes, viz. (a) round or made-up, such as rope, cordage, netting, etc, and (6) rolled or flat.
Goods of the first class (a) are prepared by simply dipping the made-up materials to be treated into a bath of cuprammonium solution, using certain precautions as to the mode of immersion and its duration, and the strength of the solution. On subsequently drying the dipped fabrics, they are obtained coated and impregnated with cupro-cellulose, which thus not merely forms a kind of varnish-like surface dressing, but further adds strength to the fibres by more or less intimately cementing them together. The freedom from liability to mildew and rot of these products is remarkable, whilst they possess many advantages as compared with similar goods protected by tarring, or dipping in the bark vat, or treatment with other preservative compositions.