CHAPTER XII.
THE SOLUTION OF INDIA RUBBER.
India rubber presents some difficulties in its solution. If a piece of pure gum just as received by the factory is placed in hot water it will swell and whiten after a while, but will not dissolve. If a similar piece is placed in benzole a similar but greatly exaggerated action takes place. The piece if left to soak for a day or more swells enormously, but very little solution is effected.
The swollen india rubber can be removed from the benzole in a single piece. It will display all the layers and marks of the original piece which was perhaps of not one hundredth part of its volume. Some parts will be a perfect transparent jelly.
It has been found that masticated india rubber dissolves with comparatively little difficulty. If the experimenter will place in a porcelain mortar, the jelly-like mass obtained as above detailed, and will rub it up thoroughly, it will be effectually masticated. This requires a little patience, as the slippery material seems to elude the pestle. Yet eventually it will all be reduced to a perfectly homogeneous mass. Its action while being rubbed up is very peculiar. At first no progress seems to be made. After a little the lumps yield to the friction. The rubber then begins to attach itself to the pestle and mortar, and begins to be drawn out into ever changing webs and threads. As the operation approaches completion the material makes a snapping, crackling noise familiar to all rubber workers. When complete there will be no lump left, and the whole will be a uniform pulp.
If benzole or a volatile solvent has been used, the rubber will easily be removed from the mortar with a spatula or palette knife. If turpentine was the solvent it will be impossible to remove the last traces except after long standing or by solution.
If replaced in the original solvent it will now come into nearly or quite perfect solution. This is the best way of masticating on the small scale. It is almost impossible to masticate untreated gum in an ordinary mortar.
The dealers sell a special india rubber for the manufacture of cement and solutions. This is so treated by mastication that it dissolves with great readiness. It is also said that heating under pressure is used to dissolve it in some factories.
Many solvents have been used and none work without some difficulty. Benzole, coal tar naptha, petroleum naptha, carbon disulphide, ether and chloroform, oil of turpentine and caoutchoucin are the best known. The naptha best suited for its solution is termed solvent naptha. It has a specific gravity of .850 at 60° F. (15½° C.); it boils at from 240° F. (115½° C.) to 250° F. (121° C.) and on evaporation should leave no more than ten per cent. of residue at 320° F. (160° C.)
Payen recommends a mixture of 95 parts bisulphide of carbon with 5 parts of absolute alcohol.
Commercial chloroform is apt to be too impure to act as a good solvent. It is apt to contain alcohol mixed with it as a preservative, which impairs its effectiveness.