Some of these solutions are better suited than others for the deposition of thin layers by evaporation. Turpentine gives a very sticky and unmanageable solution, which dries very slowly. Payen’s solution and the chloroform and the benzole solutions may be cited as especially adapted for this purpose. Careful vulcanization by the cold curing method can be applied to articles made by such deposition from evaporation.
In the case of all of them some form of mastication for the india rubber is needed. The simple mortar grinding of the gum swelled by the solvent is the only practical treatment without special apparatus.
When it is remembered that fixed oils are destroyers of vulcanized or unvulcanized india rubber it will be obvious how important it is to use pure solvents. Too great care cannot be taken to preserve the liquids pure and free from such matter.
A solid hydrocarbon may be used. Thus paraffin wax, such as candles are made of, when melted acts as a solvent. The resulting liquid solidifies when it cools, retaining an almost greasy feel.
Boiling oil of turpentine is recommended by some for the solution of vulcanized india rubber. Phenyle sulphide, it is stated, will soften it so as to render it workable. The latter discovery is credited to Dr. Stenhouse.
It is stated that a solution or pasty mixture of one part of caoutchouc in eleven parts of turpentine with one half part of a hot concentrated solution of sulphur (potassium sulphide) gives on evaporation a film neither tacky nor soft, a species of vulcanization taking place.
It is of much interest to note that an aqueous solution of india rubber has been proposed in which the vehicle is a solution of borax in water. This is well known to be a solvent for shellac and other resins. It has been recommended often as a vehicle for rubbing up india ink. The ink made by mixing lampblack with the shellac solution is nearly waterproof. A shellac varnish is given by the plain solution.
The experiments upon india rubber were published in a recent trade paper. One method of making the solution is as follows.
A solution of borax two fifths saturated is made by adding to two volumes of saturated solution three volumes of water. To this is added a solution of india rubber in benzole or other hydrocarbon of such strength and in such quantity as to contain from three and one-half to four and one-half per cent. of india rubber referred to the borax solution. It is now vigorously shaken and heated to 120°-140° F. (49°-60° C.) and the agitation, not too violent, is continued until it cools. Ceara or Madagascar rubber answers best; Para is not so good for this formula. This may be termed the indirect or emulsion method.
For direct solution from two to three volumes of water may be added to three volumes of saturated borax solution. The india rubber is added in extremely thin shavings and the solution is heated. For weak solutions the boiling point need not be reached. For strong solutions the heating should be done under pressure so as to bring up the pressure to one to three atmospheres.