The manufacture of coated tissues is effected in several ways. The following is a typical process. A mixture of one part washed and sheeted india rubber with one part zinc white, one fourth part sulphur, and about one third part naptha is mixed into a dough-like mass and is spread upon the cloth by machinery. The latter is simple. It consists of a bare board arranged to move under a scraping bar. The cloth is placed on the board and carried under the bar. The coating mixture is fed on one side of the bar upon the surface of the cloth. As it passes under, a regulated amount, according to the set of the bar, adheres. It is then dried by steam heat and recoated, until ordinarily six coats, each about one one-hundredth of an inch in thickness, have been given. Three coats are given in each direction with intermediate drying. The fabric is then cured by heat in vulcanizers.
Sometimes the sulphur is omitted from the mixture and cold curing, as described later, is adopted. When the goods are made up the seams are secured with rubber cement, a thick solution of masticated gum. Such seams have to be vulcanized.
Sometimes two such fabrics before curing or vulcanization, are placed face to face and allowed to adhere and are then cured or vulcanized.
Enough has been said in this outline of the manufacturer’s treatment of india rubber to show that the first treatment requires machinery. Very little can be done with mortar and pestle, although in making up solution these simple instrumentalities are available. As a starting point for making small articles masticated sheet rubber and mixed sheet rubber are the staple materials. The preceding steps are best accomplished in the factory.
CHAPTER V.
INDIA RUBBER STAMP MAKING.
We have seen that india rubber cannot be cast in moulds. Except in special cases deposition from solution is not available. It has to be shaped by a combination of heat and pressure. When gently heated it softens and can be pressed in a mould. As it cools it retains the shape thus given and is moulded. This applies to all unvulcanized india rubber. If mixed rubber is moulded and heated to a higher temperature without removal from the mould the curing process is brought about and the rubber may be not only moulded but cured and the product is moulded vulcanized india rubber. The mixed sheet whose manufacture is described in chapter IV. ([page 42]) is the starting point in rubber stamp making. It is made for this purpose by the manufacturers.
When the material is examined it looks like ordinary white india rubber, being firm in texture and quite strong. On heating to 280° F. to 290° F. (137° C. to 143° C.) it begins to become “cured,” and if in a thin sheet one to ten minutes are sufficient for the process. As the heat is applied the india rubber first softens and becomes much like putty. It can now be pressed through the smallest orifice and will fill up the finest details of anything it is pressed against. It is at this point that pressure must be applied to drive it into the interstices of the mould.
As the heat continues it begins to lose its doughy or putty-like consistency. This marks the reaction of the vulcanizing materials. They gradually combine with and change the nature of the caoutchouc. The rubber while still quite soft is elastic. If pressed by the point of a knife it yields, but springs back to its shape when released from pressure. The india rubber is vulcanized.
On removal from the mould it will be found to reproduce its smallest detail. The color and appearance have not changed much, but its nature and properties are now those of vulcanized rubber. It is unaffected by heat or cold within ordinary ranges of temperature, and if the india rubber is of good quality and made by a proper formula it will last for years.