Sheet rubber is made from the blocks of masticated gum by slicing. A machine is used for the purpose which carries a knife which works back and forth in the direction of its length at high speed, making two thousand cuts a minute. The knife is kept wet by a stream of water, and about sixty cuts are made per inch. In many articles made from this sheet the marks of the cuts can be seen as a fine ribbing. The appearance is familiar to many readers.
The sheet is often cut from rectangular blocks, but cylindrical blocks are also used. The latter are rotated in front of the knife edge and a long, continuous sheet can thus be obtained.
The sheet rubber can be cut into threads on webbing and braid. Everyone has noticed that these threads are usually square. The method of preparation accounts for it. Vulcanized sheet is now almost universally used for threads.
Round threads however can be made by forcing softened or partly dissolved gum through a die.
It is from unvulcanized masticated sheet that toy balloons, tobacco pouches, etc., are made. It is the starting point for india rubber bands. For the usual form of the latter article the sheet is cemented into a long tube which is afterwards cut transversely, giving bands of any desired width. To make any of these articles satisfactory vulcanization is imperative. Unvulcanized rubber for many years was used, but it is now completely displaced by the vulcanized product. Sheet rubber is made as above; is vulcanized by some of the absorption processes described in the chapter on vulcanization.
We now come to the second product: regularly mixed and cured rubber. Its starting point is the washed india rubber from the washer and sheeter.
We have seen that the pure gum or caoutchouc is very sensitive to changes of temperature. At the freezing point of water it is hard and rigid, and at the boiling point is like putty in consistency. There are several substances which can be made to combine with the gum and which remove from it this susceptibility to change of temperature. The process of effecting this combination is called vulcanization, and the product is called vulcanized india rubber. Sulphur is the agent most generally employed.
Making Mixed Rubber.
In the factory the normal vulcanization is carried out in two steps, mixing and curing. The washed sheet india rubber which has not been masticated and which must be perfectly dry is the starting point, and the mixing rolls shown in the cuts are the mechanism for carrying out the first step. These are a pair of powerful rollers which are geared so as to work like ordinary rolls, except that one revolves about three times as fast as the other. They are heated by steam, which is introduced inside of them. The sheet is first passed through them a few times to secure its softness, and then the operative begins to sprinkle sulphur upon it as it enters the rolls. This is continued, the rubber passing and repassing until perfect incorporation is secured. About ten per cent. of sulphur is added, and a workman can take care of thirty pounds at a time.