CHAPTER III.
PROPERTIES OF UNVULCANIZED AND VULCANIZED INDIA RUBBER.
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).
| Caoutchouc | 30.70 |
| Albuminous, extractive, and saline matter, etc. | 12.93 |
| Water | 56.37 |
| 100.00 |