This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
Caoutchouc, India rubber, is the produce of several trees of tropical countries which yield a milky juice, hardening by exposure to the air. In a pure state it is nearly white, the dark color of commercial caoutchouc being due to the effects of smoke and other impurities. Its physical characters are well known; in the mutual state it is softened, but not dissolved, by boiling water, hardening again to extreme rigidity at low temperature; it is also insoluble in alcohol. In pure ether, rectified native naphtha, and coal tar oil, it dissolves, and is left unchanged on the evaporation of the solvent. Few chemical agents have an effect upon it; hence its great practical use in the mineral-water manufactory.
Caoutchouc combines with variable proportions of sulphur; the mixture thus obtained after subjection to vulcanization forms what is called vulcanized India rubber, ebonite, vulcanite, etc.
India rubber is imported from various localities, that from South America, known as "para," being of the finest quality; none other should be employed in the preparation of India rubber of a soft elastic nature, which is to be used directly in contact with carbonated or other fluids, where delicacy of flavor and purity is of importance. The inferior rubbers possess an objectionable odor and flavor, highly intensified by the process of vulcanization, which is readily imparted to saline solutions.
India rubber in the process of manufacture, contrary to the general impression, is not in all cases dissolved, but is kneaded or masticated between massive iron rollers in the dry state, the various pigments, sulphur, etc., being incorporated at this stage of the manipulation. This grinding is continued till the India rubber assumes a putty-like consistency, it is then removed to still larger rollers, known as calenders, which are heated by steam, where it is rolled out into sheets of the desired thickness.
So far the physical properties of the India rubber have not been materially changed; it is still affected by variations of temperature, and must undergo the process of vulcanization to render it equally unchangeable at temperatures ranging from zero to 300° F.; this process consists of subjecting the compounds of India rubber and sulphur for several hours in closed steam chambers to a pressure of 40 lbs. to 50 lbs. per square inch, equal to a temperature of 280° to 300° F., or more, as may be required, either to produce the soft elastic India rubber, or that of a harder nature known as ebonite or vulcanite.
Rubber is introduced to take the place of the cork, and many of the patent stoppers consist of it. It is used either as a permanent cork by attaching it with some device to the neck of the bottle, or, when cheap enough, is thrown away like corks after using. Rubber stoppers, either external or internal, if unprotected from contact with the contents of the bottle, will contaminate the beverage, whether carbonated or fermented; and careful analysis has demonstrated the presence, in the beverage, of deleterious substances absorbed from the rubber. Carbonated or fermented beverages will retain their purity of taste and quality but for a short time in contact with unprotected rubber stoppers of the usual forms.
The vulcanizing process of the rubber is not yet thoroughly understood, but, according to the knowledge that we now possess, it cannot be considered a chemical process, in its restricted sense, in which an atomic combination in definite proportions takes place. This is shown, among others, by the fact that not sulphur alone, but other compounds containing sulphur, metallic sulphides, e.g., mercury sulphide, lead sulphide, etc., will perfectly vulcanize rubber, in which case it is not to be assumed that the sulphur is completely or even partly removed from the sulphur compounds.
According to all appearances, vulcanized rubber is a molecular compound of indefinite proportion, similar to the alloys.
The quantity of sulphur present in vulcanized rubber varies from 10 to 24 per cent., while but 6 to ? per cent., after other statements bu 1 to 2 per cent., are sufficient to effect a perfect vulcanization. The sulphur used in excess of this quantity simply serves as a mechanical admixture. According to the investigations made by scientists, it is certain that in practice the addition of sulphur is always more or less in excess of the actual requirements, and numerous experiments have shown that the excess of sulphur, which is only present as a mechanical mixture, is the cause of many disagreeable properties and disadvantageous changes of the rubber articles. Other mechanical admixtures of vulcanized rubber which chemical analysis has revealed, are chalk, zinc oxide, calcium sulphate, barium sulphate and magnesium silicate; for colorings vermilion, ferric oxide, also sulphurate of antimony by change (for red), and zinc oxide (for white). Organic admixture consists of cork meal, leather scrap, paper pulp, etc. The addition of some mineral matters, aside from its use for coloring, is made for the purpose of giving greater solidity and hardness. From statements made regarding these additions, and from, the results of investigations, it is certain that the admissible limit in this direction is far exceeded in most manufactures.
Rubber corks, made out of such mixed material, are naturally unfit for bottles containing carbonated or any other beverages, and unless most carefully prepared rubber is used and coated with a non-corrosive and protecting substance, they should be entirely discarded from the sealing of any bottle containing a beverage.