There are two broad divisions to which all varieties of india rubber can be assigned—unvulcanized and vulcanized rubber. Speaking with a certain amount of license it may be said that more properties characterize the former than the latter. The vulcanized article is very slightly affected by ordinary changes of temperature, cannot to any considerable extent be changed by heat short of absolute destruction or decomposition, cannot be united or moulded except in simple forms, is highly elastic, and is insoluble in almost every solvent for ordinary caoutchouc.

Unvulcanized caoutchouc possesses very interesting and peculiar properties. The first part of the present chapter is devoted to this substance. Those who have never seen the crude gum as imported are familiar with the article almost pure in the form of sheet rubber and black rubber articles generally. These are of nearly pure caoutchouc, though recently the tendency is to vulcanize them to a considerable degree.

A piece of pure gum containing no combined sulphur, iodine, or other vulcanizing constituent will be found to exhibit a very striking peculiarity. Two freshly cut surfaces when placed in contact will adhere. This is not in consequence of any viscous or sticky coating. When india rubber is cut the surface is perfectly dry and non-adherent except to itself.

The writer once had this property of adhesion brought strongly to his attention. In some analytical investigations of coal gas he had proposed to use finely divided india rubber as an absorbent of sulphur. This constituent it absorbs from gas, and it seemed that a basis for a quantitative determination of sulphur might be found in such property. Accordingly some raw india rubber was procured and with some trouble was cut up into little pieces which were put into a bottle. A day or two afterwards the pieces united wherever they were in contact, and an irregular cavernous lump was the result. This involved no melting or softening or change of shape. Each little piece was there intact and distinct but firmly attached to its neighbors.

The analogy of this action is seen in lead. Two fresh surfaces brought together, preferably with a twisting or wrenching pressure, adhere quite firmly. The adherence of india rubber and of lead each to itself is often exhibited by physical lecturers as an illustration of cohesion. The cohesion of india rubber is however far more perfect than that of lead, probably because of its comparatively great resistance to oxidation, and because, owing to its elasticity larger areas can be brought in contact. Comparatively great though this resistance to oxidation is, oxygen, especially in the allotropic modification known as ozone, may act quite powerfully on the gum. Sunlight also can affect it injuriously.

A more familiar illustration of the uniting of two pieces of the same material is seen in the welding of iron. The blacksmith heats two pieces of iron until they are nearly white hot and are pasty in consistency. On placing them in contact and hammering to force them together they unite so firmly as to be practically one. It is necessary that the surfaces of clean metal should be brought together. If the pressure induced by the hammering is insufficient to bring this about, a flux is added which dissolves the oxide and causes the metal to come in contact with metal and to weld. The analogy with india rubber in its cohesive action is evident. Surfaces long exposed or which are dusty do not cohere. The relegation of ice is similar in effect.

The cohesion of india rubber is important and should be thoroughly appreciated. It is not saying too much to assert that the entire treatment of the raw gum depends upon this interesting property. The great lumps of gum are torn to pieces and washed free from gravel and dirt without going to powder, because owing to their elasticity they yield and as fast as torn apart the pieces tend to reunite. Again india rubber is mixed with pigments and vulcanizing reagents by a method practically one of grinding or masticating, but the material while it changes its shape, and by the admixture of the various ingredients becomes less strong or easier torn, still remains intact, as it welds together or coheres as fast as disintegrated.

As regards its chemical constitution the sap of a Para rubber tree has been analyzed with the following general results: (Faraday).

Its specific gravity is 1.012.

Caoutchouc itself or raw india rubber is a mixture of several hydrocarbons of the following composition in general:

Its specific gravity is from .912 to .942.

The hydrocarbons composing it are isomeric or polymeric with turpentine. This fact brings it well within the range of familiar vegetable products. As will be seen the products of its distillation fall among the same polymers and isomers.

When pure it is nearly colorless, the dark color being due to impurities. In thin sheets it is almost or quite transparent. It burns readily, and with a very luminous, smoky flame, as might have been anticipated from its composition. The action of heat and cold on it is dependent on the degree of the temperature. At ordinary temperature it is elastic and firm. It can be stretched and will return almost to its original size when released from tension. Yet the return to its shape is so liable to be incomplete, especially after long sustained stretching, that pure unvulcanized india rubber is considered imperfectly elastic.

Any elasticity it possesses is principally elasticity of shape as distinguished from elasticity of volume. In other words when pressed or stretched it may change shape to a great extent but hardly change its volume at all. A cube of 2½ inches under a weight of 200 tons lost 1-10 of its volume only. This is largely due to the fact that it represents an approximately solid body, or one destitute of considerable physical pores. Solids and liquids are very slightly compressible. Whatever degree of compressibility caoutchouc possesses is due principally to its minute pores.

If the temperature is reduced to the freezing point of water a piece of raw india rubber becomes rigid and stiff. On application of heat it returns to its former pliable condition. The same return to flexibility may be brought about by stretching it mechanically. This may be rather a fallacy. Stretching india rubber warms it, so that in this mechanically imparted rise of temperature we may find at least a probable cause of the softening.

If the temperature is raised several effects are produced, according to circumstances. A piece which has been stretched and held stretched, has its tension increased by a degree of heat considerably less than that of boiling water. Some offer the theory that it contains air enclosed in its pores which, expanding, produces this effect. As the boiling point is reached the material softens and becomes somewhat plastic, so that it can be moulded into shape to a considerable extent and stretched to threads of great fineness. Its elasticity also disappears as the heat is maintained. These effects increase in extent up to a heat of 248° F. (120° C.). The return to its original state is not immediate however. Some time is required before the reduction of temperature will have full effect.

If now a still higher degree of heat is applied, 392° F. (200° C.) the india rubber softens to a viscous body, or melts. From this state it cannot be restored. It remains permanently “burned” or melted whatever is done to it. Some attempt at hardening may be made by the use of vulcanizing chemicals, but the result will be very imperfect.

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.