W. Africa and the Sudan, Landolphia Heudelotii of W. Africa, and Landolphia Kirkii and L. Dawei, which are found in the forests of E. Africa. Other species of Landolphia, including Landolphia florida, abundant in both E. and W. Africa, furnish rubber of inferior quality.
Fig. 7.—Landolphia owariensis. 1, twig with flowers; 2, fruit.
Among other shrubs and vines which yield rubber of fair quality may be mentioned Willughbeia edulis and Urceola elastica and Parameria glandulifera, which occur in Burma and Malaya.
The Sapiums of Colombia and Guiana are large trees resembling Hevea, and certain species furnish good rubber, especially the Sapium Jenmani of Guiana. Most of the native Sapiums have been destroyed by reckless tapping, and the merits of this genus have been somewhat overlooked and deserve reinvestigation. The same applies to certain species of Hevea, other than H. brasiliensis, which are known to produce good rubber in tropical America.
Pernambuco or Mangabeira rubber is obtained from Hancornia speciosa, Gom., an apocynaceous tree common on the S. American plateau in Brazil from Pernambuco to Rio de Janeiro, at a hei ht of 3000 to 5000 ft. above the sea. It is about the size of an ordinary apple tree, with small leaves like the willow, and a drooping habit like a weeping birch, and has an edible fruit like a yellow plum called “mangaba,” for which, rather than for the rubber, the tree is cultivated in some districts. Only a small quantity of this rubber comes to England, and it is not much valued, being a “wet” rubber. It is produced in “biscuits” or “sheets.” The caoutchouc is collected in the following manner: about eight oblique cuts are made all round the trunk, but only through the bark, and a tin cup is fastened at the bottom of each incision by means of a piece of soft clay. The cups when full are poured into a larger vessel, and solution of alum is added to coagulate the latex. In two or three minutes coagulation takes place, and the rubber is then exposed to the air on sticks, and allowed to drain for eight days. About thirty days afterwards it is sent to market. Pernambuco rubber, as is the case with most rubbers coagulated by saline solutions, contains a large quantity of water. The tree has been planted in other countries, but has so far not received much attention. It will grow on a dry sandy soil, dislikes much moisture, and needs no shade.
Forsteronia gracilis of Guiana is a climbing plant which also belongs to the Apocynaceae. Like the Forsteronia floribunda of Jamaica it yields rubber of good quality. Ficus Vogelii of W. Africa yields rubber of variable quality. The production of rubber by this tree merits further investigation, as it grows readily in nearly every district of W. Africa and the Sudan.
Specimens of the best known and of many of the lesser known rubbers are included in the Colonial and Indian Collections and Sample Rooms of the Imperial Institute, and many of the authentic specimens have been chemically and technically examined in the Scientific and Technical Department of the Institute and commercially valued. Reports on many of the lesser known rubbers have been published in the Bulletin of the Imperial Institute.
Chemistry of Rubber.
Rubber is chiefly composed of the soft, solid, elastic substance known as caoutchouc. It is usually assumed that this substance is present as such in the latex. The globules in the' latex, however, consist more probably of a distinct liquid substance which readily changes into the solid caoutchouc. The coagulation of the latex often originates with the “curding” of the proteids present, and this alteration in the proteid leads to the solidification of the globules into caoutchouc. The latter, however, is probably a distinct effect. Under certain conditions, as when latex is allowed to stand or is centrifugalized, a cream is obtained consisting of the liquid globules, which may be washed free from proteid without change, but, either by mechanical agitation or by the addition of acid or other chemical agent, the liquid gradually solidifies to a mass of solid caoutchouc. The phenomenon therefore resembles the change known to the chemist as polymerization, by which through molecular aggregation a liquid may pass into a solid without change in its empirical composition. The effect may, however, also be due to chemical change known as condensation, and be accompanied by the elimination of the elements of water. So far the chemical nature of the liquid globules of the latex is unknown, and the exact character of the change into solid caoutchouc remains to be determined. The watery liquid known as rubber milk or latex is an emulsion consisting chiefly of a weak watery solution of proteids, carbohydrates and salts holding the liquid globules in suspension. In connexion with the production of rubber the most important factor is the proportion of caoutchouc it contains. In a good rubber this ranges from 70–90% and over. The proportion and nature of the proteids or albuminous materials varies considerably in different latices. The proteids should be as far as possible removed during the preparation of the rubber, as these substances are chiefly responsible or the objectionable smell and colour of “native” rubbers, and their presence leads to subsequent change in the commercial material. All crude rubber contains more or less proteid, and in the opinion of some technical experts its presence even affords strength to the material, but this cannot be accepted as proved. The dissolved salts (potassium, sodium, ammonium, calcium, magnesium, &c.) of the latex are generally nearly entirely absent from the well prepared rubber. Of considerable importance to the value of the rubber is the absence of the resinous constituents which are present in greater or smaller proportion in all latices. The presence of more than a small percentage of resin in the latex leads to the production of rubber containing much resin, which seriously depreciates its commercial value for most purposes. The percentage of resin in a good rubber should be as small as possible, and should in any case be less than 10%. There is no feasible method at present known of preventing the inclusion of the resin of the latex with the rubber during coagulation, and although the separation of the resin from the solid caoutchouc by means of solvents is possible, it is not practicable or profitable commercially. A complete examination of a series of different latices has shown that, in many cases, e.g. Hevea and Castilloa, the resin is present in large proportion in the latex derived from young trees, and diminishes in amount as the tree ages. This is one reason why young trees should not be tapped. The composition of latex and of typical rubbers is given below:-
| Para Latex (Ceylon). |
Para Rubber (Ceylon). |
Ceara Rubber (Ceylon). |
Castilloa Rubber (Ceylon). |
Ficus Elastica (Bengal). |
Landolphia Kirkii (E. Africa). | ||
| % | % | % | % | % | % | ||
| Water | 55.15 | Caoutchouc | 94.6 | 76.25 | 86.19 | 84.3 | 80.1 |
| Caoutchouc | 41.29 | Resin | 2.66 | 10.04 | 12.42 | 11.8 | 6.9 |
| Proteids | 2.18 | Proteids | 1.75 | 8.05 | 0.87 | . . | 0.3 |
| Sugar, etc. | 0.36 | Ash | 0.14 | 2.46 | 0.20 | 0.8 | 7.7 |
| Ash (salts) | 0.41 | Moisture | 0.85 | 3.20 | 0.32 | 0.8 | . . |
The chemical analysis of crude rubber is an important guide to its value. At present, however, the methods of analysis usually employed are not sufficiently delicate to afford all the necessary information as to the intrinsic value of the higher grades of rubber, and do not go much beyond the exclusion of inferior rubber. The tests of the physical properties of crude rubber usually applied to determine its value in the market are also very rough and cannot be relied upon. The development of the rubber industry has now reached a stage at which more exact methods of determining the chemical composition and physical properties (strength and elasticity) of rubber are required. At present the caoutchouc present in crude rubber is usually estimated indirectly, and it is possible that what generally passes as caoutchouc may be in some instances a mixture of similar chemical substances, which if separated would be found to differ in those physical properties on which the technical value of rubber depends.
It is already certain that some commercial rubbers contain a variable proportion of a substance of the nature of caoutchouc, but having different properties.
True caoutchouc, the principal constituent of all rubbers, is probably essentially one and the same substance, from whatever botanical source it may have been derived. This is an elastic solid, almost transparent in thin sheets, composed entirely of carbon and hydrogen, the empirical composition of which is represented by
