A further increase of heat brings about a destructive distillation. India rubber treated in a retort to a heat exceeding 400° F. (204° C.) evolves volatile hydrocarbons of oily consistency, and it distills almost completely, a small residue of gummy matter or of coke if the final heat has been pushed far enough being left. The distillate is called caoutchoucin. According to Mr. Greville Williams it consists of two polymeric hydrocarbons: one, caoutchin C10H16, boiling point 340° F. (171° C.); the other, isoprene C5H8 (in formula equal to one-half of caoutchin), boiling point 99° F. (37° C.). The mixture has a strong naptha-like odor and has won considerable reputation as being the best solvent for india rubber. How far it deserves its reputation is a matter open to discussion.
The solution of india rubber like its fusion is a vexed point. There is little question that it can be dissolved by proper treatment. Usually naptha, carbon disulphide or benzole are used as solvents, the choice being guided by motives of cheapness and efficiency.
It is worthy of remark that the formula given for caoutchoucin is the same as that of the principal constituent of oil of turpentine, and that the latter is often recommended as a solvent. Turpentine is slightly more volatile than caoutchoucin, its boiling point being 322° F. (161° C.) Other hydrocarbons have been recognized in the distillate by Bouchardat, Himly and G. Williams, varying in boiling point from 32° F. (0° C.) to 599° F. (315° C.), and in specific gravity from 0.630 to 0.921.
Although it has been spoken of as approximately solid it does possess microscopic pores, to which its limited amount of elasticity of volume is mostly due. Thus it is found to absorb water, in which it is quite insoluble. As it does this it acts like a dry sponge and increases in volume a little, owing to dilation of these minute pores. The water absorbed may be as much as 18.7 to 26.4 per cent. with an increase of volume of the gum of 15/1000 to 16/1000. When it has once absorbed water it is very hard to get rid of it. Although the minute surface orifices communicate with the entire system of capillary vessels and pores, the surface pores on drying contract and seal up the absorbed water within the mass. This is a clew to the impracticability of the gatherer shipping dry rubber, and to the great difficulty the manufacturer experiences in drying his washed and sheeted stock before working it up by masticating or mixing and curing.
By proper manipulation caoutchouc may be made inelastic. This can be done by the freezing process or by keeping it stretched for two or three weeks. In this way threads can be made to extend and to remain extended to seven or eight times their original length. They can then be woven into a fabric. On gentle heating their original elasticity reappears and they contract. In this way fluted braids can be made which will have a high capacity for stretching.
The solution of caoutchouc is difficult often to bring about. We have seen that in water it swells a little without dissolving. In benzole it does the same, but swells to a greater extent, to 125 times its original volume or even more. Some authorities (Watts) go so far as to assert that no solvent completely dissolves it. Acting on it repeatedly with benzole or other solvent and taking care not to break up the swelled mass, from 49 to 60 per cent. of soluble matter can be extracted. On evaporation this is deposited as a ductile adherent film. The swelled up residue which remains undissolved is assumed to be the constituent giving strength and elasticity, and is only sparingly soluble. If the gum is masticated or kneaded at the temperature of boiling water a change occurs not well understood, by which its solubility is greatly increased. As solvents many liquids have been named. Oil of turpentine, caoutchoucin, coal-tar, naptha, benzole, petroleum-naptha, coal-tar-naptha, anhydrous ether, many essential oils, chloroform, bisulphide of carbon, pure, or mixed with seven or eight per cent. of alcohol, are among the solvents recommended. A mixture of fifty parts of benzole and seventy parts of rectified turpentine has been given as a solvent for twenty-six parts of the gum. Mastication before or after immersion in the solvent is to be advised. More will be said on this subject in a succeeding chapter.
Vulcanized india rubber is unaffected by changes of temperature within ordinary range. It softens a little on heating. Even hard vulcanite when heated can be bent and will retain the bend on cooling. It is exceedingly elastic with elasticity of shape but far less compressible as regards absolute change of volume than the raw gum. It melts at 392° F. (200° C.) It cannot be made to cohere, and no cement has yet been discovered that will satisfactorily unite two surfaces. It is unaffected by light, by ordinary acids and rubber solvents. In contact with the latter solvents it swells sometimes to nine times its original volume, but on heating returns to its original volume and shape. Of water it will absorb no more than four per cent. and often much less. If it is maintained at a high temperature 266° to 302° F. (130° to 150° C.) for a long time it gradually loses its flexibility, especially if in contact with metals. Often the escape of sulphuretted hydrogen may be observed under these conditions. A small admixture of coal tar operates to prevent this action.
Its composition and specific gravity vary widely as the most varied mixtures are added by the manufacturer. Its relation of carbon to hydrogen is unaffected by the mixtures added. While it may contain twenty per cent. or more of sulphur it is believed that but a very small quantity is combined with it, although the excess of sulphur or some equivalent, such as sulphide of antimony is essential to vulcanization. The combined sulphur is from one to two per cent. Some or all of the excess of sulphur is mechanically retained, and as the rubber in ordinary use is worked about, keeps escaping and forms a whitish dust upon the surface. By treatment with alkali some of the excess of sulphur can be removed when the rubber acquires the power of absorbing a little more water, up to six and four-tenths per cent.
Boiling oil of turpentine is given as its solvent.