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.
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.
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 re placed by alcohol, and the chloride of lime by chloride of zinc, or manganese fused or dry. For economy's sake, a small quantity of light spirits from coal may be mixed with the solvent, but it is not desirable. 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.
A practical difficulty attending the use of the above process is that the solvents employed are so volatile. Large masses of celluloid may be pre pared better, quicker, and with less consumption of solvent by adopting nitrobenzol, aniline, or glacial acetic acid, and the celluloid may then be worked in the open air. The ordinary volatile solvents are improved by the addition of camphor.
When using nitrobenzol, the commercial article should be distilled with hydrochloric acid or chloride of lime, say 6 lb. of either to 1 gal. of nitrobenzol, which is thus rendered purer and sweeter. One hundred parts of pyroxyline are then moistened with ordinary solvent - preferably naphtha distilled chloride of lime - and the excess of solvent is removed by hydraulic pressure. The other solvent is then added in the proportion of 10 to 50 parts of prepared nitrobenzol or aniline, together with 10 to 50 parts of camphor, and 150 to 200 parts of oil, preferably cotton-seed or castor. This mixture is forced between rolls, heated by steam being admitted into them, till the whole forms a well-combined dough or paste, which will be more or less stiff according to the quantity of solvent used. For a hard compound, the oil should be less than the pyroxyline; for a soft one, it should exceed the latter - say, 150 to 200 oil to 100 pyroxyline. In making celluloid with glacial acetic acid, 100 parts of pyroxyline are dissolved in 50 parts of the acid, for a stiff paste; or 100 to 300 or more parts, for a semi-fluid consistency.
Usually the pyroxyline requires to be dried before dissolving it. The conduct of this operation on large quantities requires much care and time and a very large space of drying room, so that great advantages, on the score of cost, ease, and safety, are to be derived from dissolving it in a moist state. For this purpose, the pyroxyline is prepared in the usual way, and when rendered soluble by the addition of hydrocarbon solvents, it is taken out of the acids and placed in a hydraulic machine, by which as much as possible of the acid is expressed. The cake of pyroxyline is then taken out of the press, opened out, put into a centrifugal washing machine, and washed with water until clean; then the rotation of the machine is contined, to throw out the surplus water. Or the pyroxyline, after conversion, may be placed in the centrifugal machine, and there deprived of the acids, and, without removal, be thoroughly washed by admitting a copious supply of water, the operation occupying from a few minutes to an hour. When the pyroxyline does not contain more than 5 to 10 per cent, of water, it is dry enough for solution in naphtha, etc.
Instead of evaporating the sol vent used in making the celluloid, it may be removed by precipitating the pyroxyline by means of water, mineral naphtha, etc. There is thus obtained a semi-solid mass, containing a small quantity of the solvent, which is passed through grinding rolls or other disintegrating machinery, and then worked up as usual. The celluloid is placed in a vessel containing a revolving agitator or beater, together with water or mineral naphtha in the proportion of 1 lb. of celluloid to 1 qt. of liquid, and the agitator is set in motion. After a short time, the celluloid is let out in a curd-like form, and submitted to pressure (not exces sive), to separate the liquid. It is convenient to place it in a vessel of cylindrical form, and about 12 in. in diameter, provided with a movable and perforated bottom, covered with several layers of wire gauze. This is filled with the curd-like celluloid, upon which a plunger is forced down, and a cheese-like block is produced. This is rolled down between rollers heated by steam, and any pigment, etc., is worked in by them at the same time, the mixture being passed through and through till perfected. The solvent used is preferably mineral naphtha, as free from smell as possible. The solvent taken up by the liquid is recovered by distillation, if water has been used; but in the case of naphtha, the greater part will separate on standing, and may then be decanted off.
Camphor is now mostly used as the solvent of the pyroxyline. The latter is first reduced to a fine pulp by grinding it in water in a machine such as is used for grinding paper pulp, and to the pulp thus prepared pulverised camphor is added in the proportion of 1 part by weight of camphor to 2 parts pyroxyline when dry. At the same time is added any desired material for colouring the celluloid or modifying its specific gravity. The camphor is comminuted by grinding in water, trituration, or solution and precipitation. The camphorated mass is placed in a mould, and heated to a sufficient temperature to liquefy or vaporise the solvent, and is then subjected to heavy pressure. The temperature should never exceed 300° F. (149° C.), or the pulp in contact with the mould will become charred; sometimes 150° F. (66° C.) suffices. The mixture should remain in the mould under heat and pressure till the conversion of the pyroxyline is completed; it is then left to cool under pressure in the mould. When first taken out, it has the consistency of sole leather; but is easily softened by heat till the camphor has evaporated, when it grows as hard as horn.
The following process is adopted in practice to dissolve the pyroxyline in camphor, eliminate the solvent, and form a solid mass of celluloid at one operation. The prepared mixture of soluble pyroxyline and camphor is first dried, by compressing the moist pulpy compound into convenient sized cakes about ¼ in. to ½ in. thick, and arranging them in a pile with intermediate layers of paper, or other absorbent material, and subjecting the pile to pressure in a hydraulic press. By this means, the material is uniformly and sufficiently deprived of its moisture, while the compression of the material and exclusion of the air prevent all danger of ignition when exposed to the sun or the heated air of a drying-room. The mixture of pyroxyline and camphor is subjected to pressure by means of a plunger in a heated cylinder provided with a discharge nozzle or pipe, the cylinder being of sufficient length to cause the conversion of the pyroxyline to take place while the material is being gradually forced through it, so that by replenishing it as it becomes partially empty, a gradual discharge of the celluloid is effected, in the form of a continuous bar or sheet. The cylinder is unequally heated, in such a manner that the mixed material will first be compacted in the colder portion, before the solvent is melted and the process of transformation commences. The air is thus allowed to escape more freely, and is more completely expelled, while the conversion of the pyroxyline is effected in another and hotter portion of the cylinder, as the mass is forced through it. The upper or receiving end of the cylinder is cooled by being surrounded by a cold-water jacket; and the lower or discharging end is heated by a steam or hot-water jacket. The former is supplied by the escape pipe of the hydraulic press. In the discharge end of the converting cylinder is a central heating and distributing case, constructed with radial pins or projections, by which the material, before it escapes from the cylinder, is caused to pass through the annular space around the central core, and in contact with the heated surface of the cylinder, while the spurs or pins divide and mix the material, and at the same time serve to conduct the heat from the cylinder to the central core. The discharge pipe is passed through an equalising warm-water vessel, which keeps it sufficiently warm to prevent the material in contact with the inner surface cooling faster than the central portion, as the unequal cooling, and consequent unequal consistency, of the different portions of the material would cause the central and softer portion to move faster than the outer and harder portion, thus destroying the homogeneity of the mass, and rendering the surface rough and broken. The soluble pyroxyline is first comminuted in a wet condition, and the excess of water is pressed out. The camphor and colours, as required, are then thoroughly incorporated with it by the mixing rollers. The compound, thus prepared, is formed into cakes by means of a mould and follower, the bottom of the mould being made separate, and serving to transform the formed cake to the pile. These cakes are preferably made about 12 in. square and ¼ to ½ in. thick; it would be difficult to properly absorb the moisture from thicker cakes. These are laid up in a pile with layers of blotting-paper between them, and are then placed in a hydraulic press to remove the water as far as necessary. During this process, the compound is protected from the air, to prevent evaporation of the camphor and to avoid the chance of ignition. The rapidity with which this drying is effected ensures great saving of time and space. The dried material is ready for conversion into celluloid, for which purpose it is transferred, with the solvent, to the converting cylinder. The heat from the steam-jacket surrounding the lower portion of this cylinder brings about the conversion of the pyroxyline to a homogeneous mass of celluloid, which is then forced through a discharge nozzle, constructed according to the desired form of the product, e.g. in bars or sheets, or directly into a mould of the article to be manufactured.
The use of various solvents and combinations of solvent materials has been attempted or proposed; e.g. a mixture of camphor and oils in about the following proportions, viz.:
Camphor, camphor oil, or liquid camphor
20 pts. by wt.
Oil, such as castor or linseed, before or after boiling.
40 pts. by wt.
40 pts. by wt.
These will give a consistency suitable for covering telegraph wires, or for moulding or spreading. For material with greater or less flexibility, or greater or less fluidity, the proportion or character of the oil must be changed. In producing very hard or rigid material, it is preferable to use oils which will themselves harden by exposure to air, as those which have been boiled. Camphor may also be used in about equal proportions with hydrocarbons having a boiling point at 220° to 400° F. (104°-204° C.), or with alcohol or spirits of wine; or hydrocarbons in equal proportions with alcohol; or castor-oil in equal proportions with alcohol; or a distillate of a mixture of camphor oil and hydrocarbons, or of camphor and bisulphide of carbon in conjunction with alcohol; or aldehyde, either alone or with alcohol. Either of these solvents may be employed with the other ingredients in about the following proportions to produce a semi-fluid celluloid:
27 pts. by wt.
27 pts. by wt.
6 pts. by wt.
Either of the foregoing solvents
6 pts. by wt.
The consistency will depend chiefly on the proportions of the oil, as before.
Parkes suggests the use of a solution of carbon tetrachloride and camphor, either alone or with gums, resins, oils, dyestuffs, etc. He also proposes to use carbon bichloride and camphor, when the solution takes place under the aid of heat and pressure. Camphor, too, is a good solvent when heated to its melting point, at this temperature and under pressure, it dissolves the nitro-cellulose as fast as it can be mixed with the melted camphor, until it forms a stiff mass. This mass, to which other substances may be added, can be rolled and pressed into moulds. To lower the melting-point, he adds oil, paraffin, turpentine, alcohol, benzol, ether, etc., whereby thinner solutions are obtained. Another powerful solvent for nitro-cellulose can be made by conducting sulphurous acid gas through granulated camphor, or by dissolving camphor in sulphurous acid. A solution of camphor in benzol, of such quality that no unpleasant odour is left when the compound is done, works very rapidly with the aid of heat and pressure. Oils, gums, resins, and dyes can be added according to requirements. Turpentineand camphor also dissolve it with heat and pressure very quickly. Nitro-cellulose softens rapidly if sprinkled with alcohol, ether, or other solvents of gun cotton, and then pressed into hot moulds.