Of late years, the scarcity and dearness of genuine ivory have driven inventors to manufacture artificial compounds capable of replacing it for many industrial and domestic purposes. These compounds, which may almost without exception be included under the name "celluloid " (called also parkesine, xylonite, pyroxy-line, artificial ivory), will here be described.
In general terms, celluloid is formed of divided cotton waste, or similar substance, dissolved in one or more of the following solvents: - Vegetable naphtha, nitro-benzol, camphor, alcohol, and glacial acetic acid. Sufficient of these solvents is used to make a soft, plastic mass, which is then or subsequently subjected to hydraulic pressure, and mixed with oils, gums, and colours. By this means, any degree of hardness or flexibility can be given to it, and it can be made white and transparent or of brilliant colour. It can be made as hard as ivory, or retained in so soft a condition as to be spread in layers over textile fabrics much in the same way that paint is laid on. The substance is water-proof, acid-proof and air-proof. It can be worked in a soluble, plastic, or solid state. It can be pressed and stamped, planed as wood, turned in a lathe, cut with a saw, carved, inlaid, woven into fabrics, and, as already has been stated, applied as a varnish. It can be made either transparent or opaque, and is capable of bearing a high polish.
When dyed, the dye runs through the whole substance, and cannot, consequently, be rubbed or washed off.
The manufacture may be divided into two distinct stages: - 1. The production of the so-called " pyroxyline; " 2. The treatment of this compound with solvents, in order to make it plastic, and give it other desired qualities. The first stage of the process suffers but little variation. A convenient quantity of cellulose or woody fibre, such as disintegrated cotton waste, paper, etc, is fed into an open vessel called a " converter," and treated with an acid mix-ture composed of 1 part of nitric acid, sp. gr. 1.420, and 4 to 5 parts of sulphuric acid, sp. gr. 1.845, mixed in a separate vessel, and kept as cool as possible. The acid mixture is pumped or forced up into the converter, while the fibrous substance, previously placed in a hopper over the converter, falls gradually into it by an opening in the top. The charging of the cotton into the converter occupies about 10 minutes, and at the end of 20 to 30 minutes at most, it is chemically converted into the so-called pyroxyline or nitrocellulose. This, together with the excess of acids adhering, is then allowed to fall through an opening in the bottom of the converter, and is caught in a large box provided with a false bottom of perforated iron or wire gauze, at about 6 in. above the real bottom.
On this the wet mass remains for an hour, to admit of the excess of acids draining away as far as possible; the still remaining impregnations of acid are then expressed by placing the pyroxyline in a cylinder with a perforated bottom, and subjecting it to hydraulic pressure. The result is a hard cylinder of pyroxyline, containing about 5 to 20 per cent, of the acid mixture, in which state it is stored for future use. When required, the cylinders of pyroxyline are torn into dust by special machinery, such as that employed for grinding paper pulp, and the disintegrated mass falls into a large tank, where it is well washed with water, to remove the last traces of acid. It is then again placed in the cylinders with perforated bottoms, and pressed to remove the water, leaving in only 5 to 20 per cent. The solid cylinders of soluble pyroxyline are again broken up in the disintegrating machine, preparatory for the treatment with solvents, which forms the second stage of the manufacture.
This is performed in a variety of ways, chiefly according to the ulterior applications for which the product is intended, and differing less in the apparatus employed than in the ingredients and proportions of the dissolving agents.
(1) One of the first solvents employed on a large scale was wood naphtha, distilled with chloride of lime, in the proportion of 1 gal. of the naphtha to 2 to 6 lb. of fused chloride; the more of the latter used within these limits, the stronger will the solvent be. The first 3 qt. of the distillate are collected for use; the remainder is caught in a separate vessel so long as any spirit comes over, and is distilled again at the next operation with more fresh materials. The deposit remaining behind in the still is chloride of lime, dissolved in water and contaminated with some tarry matter. It is run into an open iron vessel, heated by a fire beneath, to evaporate away the water and fuse the chloride of lime ready for re-use.
(2) The solvent thus prepared is applied to the pyroxyline in such proportions as to make a pasty mass; but if used alone, the resulting celluloid would soon become hard and brittle. To avoid this, a certain quantity of oil is added to the mass, and kneaded up with it in the mixing machine. The proportion of oil will vary with the desired degree of toughness. To produce a consistency suitable for coating telegraph wires, or for spreading on textile fabrics, the proportion of oil may equal half the weight of the pyroxyline. If the oil used be first treated with chloride of sulphur, the compound is much more elastic. It is thus treated by mixing with 2 to 10 per cent, of liquid chloride of sulphur, according to the degree of elasticity required; but the chloride of sulphur should first be diluted with an equal bulk or more of mineral naphtha, or bisulphide of carbon, to prevent too violent action. The prepared oil is compounded with the dissolved pyroxyline in various proportions, but seldom exceeds 20 per cent.
(3) To increase the hardness and modify the colour of the product, sometimes a small portion of gum or resin, such as shellac or copal, is added, but seldom more than 5 per cent. The wood naphtha may be replaced by alcohol, and the chloride of lime by chloride of zinc, or manganese fused or dry. For economy sake, a small quantity of light spirits from coal may be mixed with the solvent, but it is not preferable. For the oil may be substituted balata gum treated with chloride of sulphur - usually not more than 5 per cent, of the chloride. The combustibility of celluloid thus made may be corrected by the addition of chloride of zinc or tungstate of soda. Ten per cent, of either effectually prevents burning; but usually much less will do, especially when pigments are used. The same end is attained by employing iodide of cadmium, oxalate of zinc or manganese, or gelatine dissolved in glacial acetic acid.