Popular Science Monthly/Volume 17/October 1880/The India-Rubber Industries
By THOMAS BOLAS, F. C. S.
INDIA-RUBBER, or caoutchouc, possesses properties so widely different from those of most other substances that it became an object of very great interest as soon as it made its appearance in the civilized world, and its industrial importance has rapidly increased as the knowledge of its remarkable characters and manifold applicability has become more extended. At the present time, caoutchouc holds such an important position with regard to the economy of modern arts and manufactures, that, were it suddenly to be withdrawn from circulation, many minor industries would in consequence cease to exist; while numerous large and important branches of handicraft would languish until arrangements could be made to adapt their operations to the altered circumstances.
It is, however, during the last forty years that India-rubber has enjoyed its greatest triumphs as an industrial agent—that is to say, since the art of vulcanization was discovered and perfected by the labors of Charles Goodyear, Thomas Hancock, and others.
The earliest rumor of the existence of caoutchouc reached Europe nearly five hundred years ago; the first visit of Columbus to Hayti having brought to light the fact that the natives of this island were in the habit of making playing-balls of an elastic gum. Nothing more appears to have been heard of India-rubber until Torquemada, rather over two hundred and fifty years ago, described the Mexican Indians as not only making playing-balls of India-rubber, but also as fabricating helmets, shoes, water-proof fabrics, and other articles of elastic gum. This writer gives some details as to the collection of the juice and the making of various articles from it, thus giving us the first view of the India-rubber manufacture as a branch of industry. We do not hear, Soon after La Condamine's communication to the Academy of Sciences, samples of India-rubber frequently reached Europe, and scientific men began to make investigations regarding this remarkable body. Between 1760 and 1770 we find Fresneau and Macquer studying the subject, and the last-named investigator made tubes and other articles of caoutchouc by dissolving it in ether and coating molds with the solution, so that a solid skin of caoutchouc should remain adherent to the mold on the evaporation of the solvent., of samples of India-rubber reaching Europe until long after this, and little more appears to have been learned regarding the substance until the celebrated French naturalist, La Condamine, made a communication to the Academy of Sciences at Paris concerning caoutchouc, he having had ample opportunities of studying the subject in Para. In the memoir in question, La Condamine gives very detailed particulars regarding the Para India-rubber tree, the collection and treatment of the juice, and the methods made use of by the natives for the production of various articles of caoutchouc. He tells us that the substance in question was used for making torches, these being only an inch and a half in diameter by two feet long, and yet burning for twelve hours. Again we hear of the use of India-rubber for the making of playing-balls, and it appears that the natives were in the habit of using enema or injection bottles made of caoutchouc.
From this time until the end of the eighteenth century, the India rubber industry may be considered to have been undergoing its period of gestation, and to have been born with the dawn of the present century. Among the first of the important patents regarding the utilization of caoutchouc is that granted in 1823 to Charles Macintosh, for dissolving the substance in coal-oil, or coal-naphtha, and the use of this solution as a water-proofing agent. I have here a specimen of such a solution, as now manufactured by Messrs. Charles Macintosh and Co., of Manchester, together with some examples illustrating its uses.
About the same time, elastic webbing was first made with threads cut from the raw rubber, and other minor applications of caoutchouc to the industrial arts were adopted from time to time, until the great discovery of vulcanization inaugurated a new epoch in this branch of industry, rendering it possible to so far alter caoutchouc as to make it capable of resisting, to a great extent, the action of heat on the one hand and cold on the other hand.
The milky sap of many plants contains caoutchouc, suspended in the form of minute transparent globules, these being frequently as small as 20000 to 50000 of an inch in diameter; but comparatively few plants contain sufficient caoutchouc to render them important sources of this body.
The trees which yield the largest supply of the best quality of caoutchouc consist of various species of hevea, which flourish in the northern districts of South America, especially in the province of Para, some portions of the valley of the Amazon being crowded to an extraordinary extent with heveas. The abundance of the India-rubber trees in Para may be judged of by the fact that this province alone exported 7,340 tons of caoutchouc in the year 1877, more than half of this being sent to Liverpool.
Among the heveas most productive of caoutchouc may be mentioned the Hevea Brasiliensis, which flourishes in Para, and yields some of the finest caoutchouc, and often attains a height of sixty to seventy feet, with a diameter of nearly three feet; the Hevea Guianensis, a similarly magnificent tree, likewise abundantly productive of caoutchouc; and the Hevea spruceana, a smaller tree, which grows almost exclusively in the province of Para. Fig. 1 represents the flowers and foliage of Hevea Guianensis.
In the operation of collecting the juice several cuts are made through the bark of the tree, and either shells or clay vessels are attached to receive the exuding milky sap. When sufficient of this has been collected, the operation of drying it is performed as follows: A kind of
Fig. 1.—Hevea Guianensis (Flower and Foliage).
wooden bat, thinly covered over with clay, is dipped into a pail filled with the juice, and the bat, thus coated, is held over a fire, fed with certain wild nuts, which, in burning, give off abundance of aromatic smoke. Fig. 2 represents this operation, and you will see that a kind of short chimney is fixed over the fire to lead the smoke compactly upward. As soon as the first layer of juice has become indurated, the bat is again dipped, and the drying operation is repeated, layer after layer being thus dried on the bat, until a thickness of nearly an inch is attained. A knife-cut is now made in the bottle or biscuit of caoutchouc thus obtained, so that it can be removed from the wooden bat, and exposed to the air to become still further indurated. Para caoutchouc, prepared in this manner, has a fragrant, aromatic odor, which you can study for yourselves in the samples now before you.
The residues of juice left in the various vessels employed, the scrapings of the incisions, together with other materials, which the ingenious native thinks he can shuffle off on the unsuspecting merchant as caoutchouc, are made into balls, and sold as "negro-head." The negro-head rubber is frequently made into crude representations of animals, and there are several such works of native art on the table—as, for example, this specimen, which will pass about equally well for a horse, a pig, or a crocodile.
Here is a piece of Para bottle-rubber, which has been boiled for some hours in water, and you see that it is now so far softened as to render it easy to pull asunder the several layers of which it is composed, its laminated structure being thus very well illustrated.
The milky juice of the Para rubber trees, of which you see a specimen before you, has approximately the following composition:
As a rubber-producing tree, the Ficus elastica stands next in importance of the heveas. The Ficus elastica grows abundantly in India and the East Indian Islands, one district in Assam, thirty miles long by eight miles wide, being said to contain 43,000 trees, many of them attaining a height of a hundred feet. This tree also grows freely in Madagascar, and it is well known to us as a greenhouse plant. Fig. 3 represents a Ficus elastica now growing out of doors in the Pare Monceau at Paris.
The juice of the Ficus elastica contains notably less caoutchouc than that of the American trees, the proportion very often falling as low as ten per cent, of the juice.
A vine-like plant, the Urceola elastica, which grows abundantly in Madagascar, Borneo, Singapore, Sumatra, Penang, and other places, yields a considerable amount of caoutchouc of very good quality. Africa yields a considerable quantity of caoutchouc, but generally soft and of inferior quality. It is believed to be yielded by various species of landolphia, ficus, and toxicophlea. Here are some specimens of African rubber—this specimen, representing the quality known as African ball, being tolerably firm in consistency, while the African flake, which you see here, and the African tongue, represent the lowest and most viscous qualities of commercial rubber.
The commercial value of the various qualities of rubber may be estimated, to a certain extent, by noting the loss which the samples undergo during the operation of washing, and also by noticing how far the various samples are softened by a long-continued gentle heat. Here are some samples which have been heated for some hours in this water-oven; you will notice that the African tongue has become almost as soft as treacle, while the Para rubber still retains its form and much of its consistency.
Caoutchouc is nearly colorless, and when in thin leaves tolerably transparent. It, like very many other substances, contains nothing but carbon and hydrogen, but its properties differ very widely from those of other hydrocarbons almost identical in composition. It has been found to contain, in one hundred parts, 12·5 of hydrogen and 87·5 of carbon. Caoutchouc, as might be supposed, burns very readily and leaves no residue; if I set fire to a few ounces, you see how it blazes up. It is soft, and very imperfectly elastic, in the true sense of the term—that is to say, it does not return to its old dimensions after having been considerably stretched. Here is a strip of pure (i. e., unvulcanized) caoutchouc a foot long; you see that I have stretched it to a length of three feet, and, after holding it stretched for a few seconds, I relax it. It now measures, as you see, several inches over the foot. The elasticity of caoutchouc may be enormously increased by vulcanization.
As regards the stretching of India-rubber, there is a point at which it requires a greatly increased force to stretch it, and at this point it seems to become fibrous in texture, as you may perceive by examining this extended sample by the aid of a magnifying-lens. India-rubber has valuable electrical properties, as you are no doubt aware, it being an admirable insulator, and having a great tendency to become electrical by friction.
Freshly cut surfaces of India-rubber cohere very strongly when brought into contact, and this is well illustrated by the old way of making a tube of unvulcanized caoutchouc. You see that I wrap a sheet of caoutchouc round a mandrel, so that the edges project parallel to each other. These parallel edges being cut off by means of scissors, the freshly cut edges adhere, and a perfect tube is the result. Toy balloons are made in a somewhat analogous manner, and are cold vulcanized afterward.
Either French chalk or soapy water is of constant use in the rubber factories, to prevent the adhesion of new surfaces of caoutchouc to each other, or to other substances.
Cold has a remarkable effect on caoutchouc, rendering it rigid and inelastic, and this circumstance considerably detracts from the value of unvulcanized India-rubber. Here is a strip of India-rubber; you see that it is quite soft and pliable. I will now expose it for a few minutes to a temperature of 0° Centigrade, or the freezing-point of
Fig. 2.—The Operation of drying the India-rubber Juice.
water. It becomes, as you see, rigid and stiff, but its original pliability may be restored, either by warming, or by applying sufficient tensile strain to it, to extend it to three or four times its length. One half of this strip I will warm in water, heated to 50° Cent., and the other I will stretch. In each case you see that the caoutchouc is restored to its original condition. In the case of the stretching it is very likely that the effect is due to the heat evolved during that operation. It is easy to illustrate the fact that heat is produced when India-rubber is subjected to tension. Here are some strips of India-rubber, arranged side by side on a board. I bring them in contact with the bulb of an air-thermometer, and you see that there is no indication of either heat or cold. The strips of India-rubber being now stretched to four or five times their previous length, the air-thermometer indicates a considerable rise of temperature. Here is a similar set of strips, which were stretched some hours ago, and which on trial by the air-thermometer we now find to have cooled down to the temperature of the surrounding objects. Note the effect of releasing the tension and allowing the rubber strips to contract. You see that they have become so cold as to influence the air-thermometer to a very considerable extent.
The effects of heat on India-rubber present many points of interest, and, in the first place, I wish to illustrate to you the effect of moderate heat on a stretched band of caoutchouc. Here is such a band, one end being attached to an index, pointing, at the present time, to the zero of this paper scale. Notice the consequence of applying a gentle heat to the caoutchouc band—it contracts as regards its length, but expands in a transverse direction, causing the index to move rapidly through a space of several degrees. This property, which stretched caoutchouc possesses, of contracting by heat, may be described by saying that, within certain limits, the tensile elasticity of caoutchouc is increased by an elevation of temperature. Caoutchouc, however, if heated to 100° Cent., softens considerably, and almost entirely loses its elasticity, as you will perceive by examining this sample, which has been heated for some hours; while a heat of 120° Cent, produces a most decided softening effect on caoutchouc of the best quality, but after exposure to this temperature, it recovers its pristine state by exposure to cold for a moderate period. If, however, the action of heat has been pushed still further, say to 200 Cent., the caoutchouc becomes converted into a permanently viscous body, which has little or no tendency to harden again. This viscous substance possesses the same composition as unaltered caoutchouc, and is of value as a medium for making air-tight joints, which can be easily undone. This glass jar has its top edge ground level, and, after applying a little of the heated caoutchouc to the ground edge, the jar may, as you see, be hermetically closed by a disk of plate-glass. A joint of this kind may be broken and remade with the utmost facility and rapidity.
When caoutchouc is subjected to a temperature somewhat above 200 Cent., it becomes converted into a variety of volatile hydrocarbons, which present many points of interest, and you will find a tolerably full account of them in the manuals of chemistry. In this retort, the dry distillation of caoutchouc is being carried on, and in time very nearly the whole of the India-rubber will be converted into the mixture of oily hydrocarbons, only an insignificant carbonaceous residue remaining in the retort. The mixture of volatile hydrocarbons, often referred to as caoutchoucine, forms a very good solvent for caoutchouc and certain resinous bodies.
India-rubber is subject to two kinds of deterioration and decay. In one instance it tends to become soft, and loses its elasticity, while in the other it becomes friable, yellowish, and resinous in its nature. Examples of each kind of deteriorated rubber are on the table, and you will notice that, in the case of this specimen, we have a well-marked instance of both kinds of deterioration going on side by side. The last-mentioned kind of deterioration has been clearly and indubitably traced to an oxidation of the caoutchouc. This oxidation is tolerably rapid when the caoutchouc exists in a finely divided state, and when it is exposed to damp at the same time; but the alternate damping and drying of the caoutchouc tends more toward its rapid oxidation than does a continual state of dampness. The resinous matter resulting from the oxidation of caoutchouc has been carefully studied by Spiller, who found that a sample of felt, originally composed of cotton fibers and India-rubber, had become so far changed during six years as to contain no trace of caoutchouc; but in its place he found a resinous substance resembling shellac. This resinous body, of which a sample is before you is easily soluble in alcohol, and also dissolves in benzole. Alkalies dissolve it readily, and acids precipitate it from the alkaline solution. It contains 27·3 per cent. of oxygen.
The conditions under which the softening of the India-rubber takes place are not so well understood, but there is some reason to believe that this is due to incipient oxidation.
Ozone oxidizes caoutchouc with extreme rapidity, as Warren pointed out in 1877, and I have arranged a simple experiment to illustrate this fact. Through the open end of this glass passes a slow stream of air which has been slightly ozonized; that is to say, a portion of its oxygen has been converted into ozone. When the stream of ozonized air is allowed to impinge on a surface of India-rubber, you see that the surface is instantly corroded and roughened. Again, note the effect of allowing: the ozonized air to act on the surface of a distended caoutchouc balloon—you see that it bursts immediately. I should mention, by the by, that in the case of these balloons the caoutchouc is slightly vulcanized, but the action of ozone on vulcanized India-rubber is similar to its action on the unvulcanized material.
It is extremely probable that the rapid deterioration of caoutchouc, which is known to take place under conditions which are not perfectly understood, is frequently due to the corrosive and oxidizing action of ozone.
Ozone, or some agent nearly resembling it, is often produced when oil of turpentine is exposed to the air, and this circumstance may perhaps
Fig. 3.—Ficus Elastica; a Specimen growing in the Open Air at Paris.
explain the destructive influence which oil of turpentine occasionally exercises on India-rubber.
Exposure to sunlight often causes the destruction of India-rubber, either converting it into a soft and sticky substance, or into a hard body, less soluble in benzole than unaltered caoutchouc; and, it is quite possible to obtain a photographic print by exposing a film of India-rubber under a negative, and then dissolving away, by means of benzole, those parts on which the light has not acted. Here is such a photograph made by Mr. Woodbury. I now project it on the screen, so that you may all see it. It is generally a discreet thing to keep India-rubber where it will not be exposed to the prolonged action of a powerful light, although there are cases in which exposure to light is a useful aid to the process of vulcanization. India-rubber is, to a certain extent, porous and cellular in its texture, as may be seen by a microscopical examination of a thin section. Again, if a thin leaf of caoutchouc is boiled for a long time in water, it absorbs a considerable proportion of this liquid. You see that this piece of caoutchouc has become quite milky and translucent from the absorption of water, and it probably holds, at the present time, as much as ten or fifteen per cent, of water. The amount absorbed may, in some cases, rise as high as twenty-five per cent. In a similar manner alcohol is absorbed by India-rubber, more readily than is the case with water.
Now, we pass on to a more important matter, namely, the action of such liquids as benzole or coal-naphtha on caoutchouc. Here are two cubes of Para rubber, each being three eighths of an inch across the face. One of these I will preserve as a pattern, and the other I will suspend in a bottle containing benzole. The cube suspended in the benzole will immediately begin to swell, and will continue to do so until it has attained a bulk about one hundred times as large as its original size. During the time that the cube is swelling in the benzole, a certain proportion of the caoutchouc will become dissolved out and incorporate itself with the bulk of the solvent. Now, as a matter of fact, every kind of natural India-rubber contains two distinct modifications of caoutchouc, one of which tends to swell up in such a liquid as benzole, while the other dissolves and forms a true solution. The first mentioned of these bodies may be referred to as the fibrous constituent of caoutchouc, while the second may be spoken of as the viscous constituent. The proportions in which these two bodies occur in raw rubber vary extremely, Para rubber, of good quality, containing only a small proportion of the viscous constituent, while African tongue, on the other hand, consists principally of the viscous modification of caoutchouc. The viscous constituent of caoutchouc is the agent principally concerned in the joining together of freshly cut edges of India-rubber; and, as we proceed with the study of caoutchouc, we shall see that, under certain conditions, the fibrous caoutchouc can be more or less perfectly changed into the viscous form. The treatment of the juice of the India-rubber trees is often of such a nature as to greatly deteriorate the caoutchouc obtained; a considerable proportion being thus changed from the fibrous to the viscous condition. This kind of injury to the caoutchouc can be obviated by coagulating the milky juice, and carefully drying the clot after it has been subjected to pressure. For experimental purposes, alcohol may be employed as a coagulating agent; while, on an industrial scale, alum has been tried with apparently an excellent result. The milk is strained to remove solid impurities, after which a small proportion of alum solution is added. The clot which separates is next drained or pressed, after which it is dried. Caoutchouc dissolves more or less perfectly, according to its condition in various liquids, among which may be mentioned
Fig. 4.—Cube of Para Caoutchouc unswelled.
Fig. 5.—Similar Cube swelled by the Prolonged Action or Benzole.
the various fixed and hydrocarbon oils, chloroform, ether, and carbon disulphide. Unless, however, the caoutchouc has been masticated or otherwise degenerated, it is doubtful whether a true solution is obtained. When a clear limpid solution is required, one of the best solvents is that proposed by Payen, namely, carbon disulphide, mixed with five per cent, of absolute alcohol. If one part of masticated caoutchouc is dissolved in thirty parts of the above solvent, a solution is obtained which can be filtered through paper, and may be employed in covering the most delicate molds with successive layers of caoutchouc.
Caoutchouc may be utterly ruined by the use of impure solvents, and those experimenting with India-rubber solutions should, in cases where it is desirable to regenerate the caoutchouc by allowing the solvent to evaporate, take the utmost care not to employ any solvents which contain fatty or greasy matter.
Weak or diluted acids have little or no action on caoutchouc in the majority of cases, but strong sulphuric acid slowly acts on it, the action becoming rapid if heat be applied. Strong nitric acid acts on it with some energy, causing its entire destruction, and in a similar manner it is destroyed by the prolonged action of chlorine, bromine, or iodine; although these reagents, when their action is kept under control, produce a vulcanizing or strengthening effect.—Abridged from Journal of the Society of Arts.