The art of rendering fabrics impervious to moisture has attained considerable importance, especially in the case of clothing materials. A few simple processes are briefly described in the First Series of 'Workshop Receipts,' but many others are now in vogue, some conducted on an industrial scale.
Since supplies of indiarubber have been regular and abundant, that useful vegetable secretion has been very largely employed as a waterproofing agent for wearing apparel. The crude rubber, which is received at rubber works in the condition in which it is imported, varies greatly in appearance and quality, as well as in the amount of impurities it contains, and which must be removed before it can be manufactured.
The preliminary processes, therefore, to which the crude material is subjected, have for their object this cleansing of the rubber from impurities, and at the same time the softening of it, and its reduction to a form in which it is best fitted for subsequent operations. (1) The crude material is introduced into a vat or tank with water, and boiled by throwing in free steam. This operation, which has the effect of softening the crude masses, is sometimes carried on in the open air, but in other works within some part of the building. Some makers find it sufficient to soak the crude rubber in water at a temperature not higher than 120° F. (49° C). (2) When thus softened, the masses of rubber are passed between powerful rollers, the surfaces of which are indented with flat, square indentations, while a stream or jets of water flow upon the rubber from a perforated pipe above. By this operation the rubber is flattened out into a thin sheet, and more or less torn and disintegrated, while the water works out the foreign substances, dirt and impurities, which the boiling has failed to remove. This process is repeated until the mass is thoroughly cleansed.
The result is the production of a thin crumpled sheet, full of holes, which is then hung up in a room warmed by hot air to dry. (3) The rubber thus washed and cleansed is introduced into the "masticator," which consists of a strong cylindiical box, containing a stout deeply-fluted iron drum, which revolves within the box; steam is introduced into the interior of the drum, and a current of water is kept running between the drum and the cylinder. It is then introduced dry into another similar "masticator," the flutings of the drum of which are made sharp and chisel-shaped. The rubber is here torn to pieces, rendered homogeneous, and the last traces of air or water are expelled. At some works this process of "mastication" is omitted. (4) If it be desired to cut up the rubber into sheets, etc, the rubber thus prepared is made into blocks of the requisite size by compression in a strong screw-press iron box, the inside of which is smeared with French chalk to prevent sticking, and the sides of which arc hollow and filled with steam.
A uniform block is thus obtained, which when cold or frozen can be cut into thin sheets or smaller blocks as may be desired. (5) When it is intended to prepare the rubber for the process of "vulcanisation," the vulcanising material is either kneaded with the rubber in a masticator, or by means of mixing rollers heated by steam and kept for the purpose. In either instance it is in the form of the rough sheets above described that the rubber is introduced, and there is a shallow tray beneath to catch such of the material as falls through. The vulcanising material consists generally of finely-sifted flowers of sulphur, together with colouring powders such as lampblack, zinc oxide, or antimony sulphide (orange). These are thoroughly incorporated with the rubber in the apparatus used. (6) After this stage of preparation, the Goodyear or American process, and the Hancock or English process diverge. (a) By the Goodyear process, the rubber thus prepared is rolled out into sheets by causing it to pass between strong iron rollers heated by steam, in what is known as the "calendering" machine, and these sheets may be subsequently manipulated for the construction from them of various kinds of articles.
As the sheet passes from between the rollers it is received upon linen, and the linen and indiarubber sheets are rolled up together upon a roller. (6) In the Hancock process some solvent is used to soften the rubber and convert it into a thick paste. The solvent now universally used is that product of the fractional distillation of light tar oil known in the trade as "solvent naphtha." The solution is sometimes effected in a close cylinder, where the rubber and naphtha are, by an arrangement provided within the cylinder, wrought up together, and from which the pasty product is drawn off by means of a tap below. The paste is received into iron pots provided with covers, in which pots it can be reduced, by hand mixing with more naphtha, to any condition of liquidity that may be- desired. In other works the solution is effected in another way. The rubber, calendered out into a thin sheet, is passed between a pair of hot rollers into an open vessel containing naphtha, by the side of which vessel a workman sits and presses down the sheet, softened by the heat, into the naphtha. When rubber enough has thus been put in, the vessel is covered up and set aside for the completion of the solvent action. In other works the rubber is simply mixed with the naphtha with a spade.
In order to, make the thick paste into a sheet, what is termed a " spreading machine " is used. This consists of a table formed of a hollow steam-chest, along, but not touching, which a sheet of linen previously sized is rolled off from a roller at the spreading end to a roller at the farther end, and from this to other rollers underneath. By an arrangement devised for the purpose, the softened rubber or thick paste is spread in a thin layer on this linen as it passes to the top of the steam-chest; and the rubber thus laid on is carried upon the linen as if it had been painted on it. As the sheet passes slowly along the top of the steam-chest, the heat causes the solvent to volatilise. This operation is repeated until the required thickness of rubber is obtained. It is conducted in a room where many similar machines are in use at the same time. The rubber is then stripped from the linen by rolling each off in different directions upon different rollers, with the aid of water. As the rubber is rolled off, it is evenly coated with a wash of French chalk, to prevent sticking, and the roll being bound round with a wet cloth bandage, is ready for the "vulcaniser." In making rubber tubing, the solution of rubber above mentioned, is used to smear the edges of the strips of rubber which are to be made to cohere; the solution is also used in the manufacture of mixed linen and rubber tubing, and also in the joining of surfaces of material in the manufacture of articles of dress, etc.
Reference may now be made in greater detail to the treatment of fabrics which are to be "proofed" by spreading. This consists in passing them through a pair of calenders, with the object of pressing down knots, and giving a smooth and even surface; after this, they are passed over a steam-chest, to expel moisture, when they are ready to receive the first coat. This is usually a different mixture from the bulk of the proofing, and is called a "sticking-coat," its object being to secure adhesion between the fabric and rubber; it is generally incorporated with colouring pigments, white or black, so as not to allow the general mixture to show through the cloth, or alter its appearance. A little zinc oxide, or whiting, is used for white or light-coloured goods; Frankfort and other blacks are used for dark goods. The coats, as applied, are dried by passing over a steam-chest, when the fabric is again brought to the front of the machine for another coat, and so on. Some descriptions of goods have a finishing coat of better quality or mixture, in some cases containing no sulphur, nor any pigment whatever.
The number of coats varies from 3 to 7, according to the class of goods and the weight of material which is to be put on.
Machines are now employed which work on the continuous principle; but as they require more space, so as to allow each coat to dry in time to receive another, it is not certain that there is much gain in using them.