When spread cotton goods have become tolerably firm, or quite dry, they are wound upon hollow sheet-iron cylinders, for curing in open steam, or in a steam-jacketed heater. As the condensed steam spoils these goods, they are carefully wrapped up as air- and water-tight as possible. Since wool and silk are destroyed by the heat necessary to cure rubber in this way, the cold process is the only eligible method of vulcanising. Very frequently, however, cotton goods are treated in the same manner.
In packing the goods for the steam-heater, care must be taken that the fabrics are wound without creases, and are not stretched, as the fibres of; the cloth, after curing, will retaiu their distorted appearance. Double textures are simply wound up; but "surface" goods are first carefully brushed over with very fine French chalk, no excess or loose chalk being allowed to remain. They are then wound up; but, as this necessitates the rubber surface coming into contact with the cotton surface, whereby it is liable to be marked, it is more usual to run 2 pieces together, with the rubber surfaces against each other. This not only prevents marking, but secures an even surface; blisters, from dampness in the cotton, are also prevented.
Double textures are obtained by passing the proofed fabrics through a pair of rollers (the doubling-machine), whilst the surfaces are still sticky or adhesive; these are vulcanised, if required, by means of sulphur incorporated with the compounds, and steam heat. The doubling-rollers are of solid cast iron, with turned surfaces, 6 ft. long. One is fixed, while the other can be moved by a lever, so as to admit the fabrics to be doubled. As they revolve in opposite directions, they draw the fabric through, and, when tightened up, press the 2 coated surfaces together.
Parkes' process of vulcanising with sulphur chloride is extensively used for surface curing, such as single textures for garments, and sundry small articles manufactured from masticated sheet rubber, as tobacco-pouches, tubing, rings, etc. The chloride is mixed with 30-40 times its bulk of carbon bisulphide for ordinary fabrics; but for solid rubber goods, much more dilute solutions must be used, and a longer immersion allowed, than with stronger solutions, since the surfaces would be overcured, and crack. Sulphur chloride in vapour is preferable in many cases to the mixture in carbon bisulphide. The articles are then suspended in a lead-lined chamber, well varnished with shellac, and heated by steam-pipes; the chloride is gently evaporated, either by placing it in an open dish on the steam-pipes, or by using a small retort, the end of the tubulure of which passes into the chamber. The chloride is evaporated by a small gas-burner. Chlorine, bromine, hypochlo-rous acid, and several other vapours, can be used in the same way.
Although Parkes uses these vapours with solvents of rubber, they act equally well, and in many cases more certainly, without them.
Several improvements for curing double textures have been recently introduced, the most important of which is the Silvertown process. This consists in passing the rubber surface of each piece to be united over a roller, revolving in a mixture of sulphur chloride and carbon bisulphide; the acid mixture does not come into contact with the fabrics, so that no injury can happen either to the colour or the fibres, and the most delicate tissues can be treated. Another process, by Anderson and Abbott, effects the curing by suspending the fabrics or completed garments in a chamber, which is afterwards charged with the vapours of sulphur chloride; it is questionable how far this method can be depended upon, without injury to the fabrics. If the colours are discharged by the sulphur chloride, they are brought back by placing a dish of liquid ammonia in the drying-room.
Single textures arc cured by passing the coated surface over a roller, revolving in the curing-mixture, as above. The fabrics are run on to a large drum, and the cured surface, which is still sticky, is kept from coming into contact with the cloth surface, by making the drum pick up a roller whenever its arms pass the frame which supports them, so that between each 2 layers of material there is a space of about 2 in.; as soon as the bisulphide has nearly all evaporated, the fabrics are run on to a roller for hanging up.
Single textures, when cured, are well wiped over, and varnished with shellac dissolved with liquid ammonia in water. Lampblack is added for black goods; bleached shellac or seedlac is best suited for white or light-coloured goods. The varnishing is performed by passing the fabrics over a roller running in a trough of varnish, or better still, by letting the varnish fall on the rubber surface. It spreads of itself, the excess being removed by passing under a close-fitting scraper or pad. It is dried by running over a large drum or cylinder, heated by steam. Small articles are varnished by a soft sponge.
Cured or uncured fabrics are joined for garment-making and other articles by cementing together with thin solution. Camphene was largely used a few years ago for softening the edges of rubber for uniting. It leaves the rubber more sticky than any other solvent does. Its present price precludes its use on a large scale. Several coatings are applied, each being allowed to get nearly dry before the next is rubbed on; the 2 adhesive surfaces are then well rolled down by manual labour, and the excess of cement which oozes out is rubbed off, when nearly dry, by a piece of masticated block rubber. Double textures are stripped, so as to cement the rubber surfaces, by applying first a little solvent, which renders the stripping-off easier. In spreading, it is necessary to coat one of the fabrics with less pressure, so as not to drive the rubber into the meshes of the cloth. Such coatings are specially designated "stripping-coats." Without such arrangement, double textures could not be made with watertight seams.