Rubber Latex Recent Scientific and Technical Developments V. N. MORRISAND H. W. GREENUP Firestone Tire & Rubber Co., Akron, Ohio
The more or less scientijic work which has been carried out on the rubber plantations, and the problems relating to the production of articles from latex in the factories of America and Europe are discussed. A review is given of the actual and proposed uses for latex and products made from latex.
T
EX years ago, latex, the white liquid obtained from the rubber tree, was as much a stranger t o manufacturers in Europe and America as it is to the public today. At the Fifth International Rubber Exhibition in London, in 1921, latex is said to have been gazed upon as a curiosity even by scientists connected with rubber-manufacturing concerns. In the past dccade, however, the direct utilization of this liquid in rubber and other industries has had such a rapid advance that 9,190,362 pounds of rubber, in the form of latex, were imported into the United States in the first ten months of 1931. It is significant t o note that the importation of latex was 19.3 per cent greater in 1930 than in 1929, despite the fact that the consumption of crude rubber decreased 20 per cent (16).
The opportunity for revolutionizing a major industry does not often occur. Given the proper economic conditions, it is not outside the realm of possibility that the direct use of latex could accomplish such a revolution in the rubber industry. The earliest manufacturers of rubber articles (the natives of the Amazon district) used latex as their raw material. The difficulty formerly encouritered in the transportation of latex without coagulation and the necessity for transporting so much water, combined to retard interest in its direct application in the more industrialized countries. The rubber in a modern manufactured article has usually, therefore, been collected a t the tree in the form of latex, coagulated with formic or acetic acid, washed, sheeted, dried and smoked, packed, shipped to this country, unpacked, plasticized by mechanical working in heavy machinery, compounded by the mechanical introduction of the desired ingredients, and vulcanized in a mold of suitable shape. The history of the rubber in an article manufactured from latex, on the other hand, is that of collection of the latex; shipment to this country in the presence of a preservative; compounding by stirring the ingredients into the liquid latex in the presence of suitable protective agents; forming by dipping, deposition, or other methods; and finally drying and vulcanizing. The simplicity of the latter procedure as compared with the former is obvious. Simplicity in manufacture is by no means the only advantage claimed for products made from latex. Investigators in the field are generally agreed that latex products, if properly made, are superior in physical properties, since no mechanical deterioration of the rubber occurs as a consequence of milling. Further advantages claimed for latex products include great saving in power consumption in the mixing procedure, greater uniformity in product, better dispersion of compounding ingredients. better aging, lower accelerator costs, and elimination of fire and health hazards in processes formerly requiring rubber cements (made with toxic and inflammable solvents).
I t is true that all of the advantages claimed have not been accepted without dispute. The writers have found, for instance, that, despite the supposed presence of naturally occurring antioxidants, products made from preserved latex do not always withstand aging well (particularly if the latex remains in the unvulcanized state). Furthermore, in order to obtain the high resistance-to-tear desired of certain latex products it usually seems necessary to maintain a somewhat “undercured” state, which results in high permeability and high permanent set. The present extraordinarily low price of crude rubber also places latex (with its high cost of transportation) at a disadvantage. Great as has been the commercial adoption of latex in recent years, the increase in the literature pertaining to latex seems to have been even greater. The direct utilization of this material so appeals to the imagination that it has stimulated both inventive and literary effort. The literature has been increasing a t such a rate, in fact, that even in 1924 van Rossem (186) considered it inadvisable to attempt to review it all in one paper. Early in 1927 Hauser published his book on the subject of Latex ( 7 1 ) . I n view of the adequate manner in which this book covers developments prior to that time, the present article has been confined very largely t o the scientific and technical developments of the past five years. Since the writers have listed hundreds of references to patents and journal articles pertaining to latex which have appeared in this period (about two hundred in 1930 alone), no attempt has been made to cover every reference in this review. Only those which seem to be of most interest have been discussed. Although some investigations of a strictly scientific nature have been reviewed, emphasis has been placed on the developments having possible practical application. SCIESTIFIC INFLUENCE O F
IKiVESTIGATIOKS O F
-4NATOMY
OF
LATEX SYSTEMS
TREESON
YIELD O F
L.4TEX.
h means of predicting the future yield of young Hevea trees has been the object of many researches. Such a method would avoid the planting of poor yielding trees, or would enable them to be detected and replaced before the tapping age was reached. For this reason the method suggested by Ashplant (17) has created widespread interest. It depends upon the determination of the bore of the latex tubes and the number of latex rings. Trees with latex tube bores below a certain average are said to be poor yielders, and good yielding trees are said to possess latex tubes above average bore. Whether or not a tree with large tube bore is a good yielder depends on the number of latex rings. Some workers have failed to confirm Ashplant’s results. With respect to the data of Frey-Wyssling (6S), Ashplant has claimed that they support (as far as they go) his results, but that too few trees and
I l a o w , has been sttttoil by XtcKtiy (102) to be uiirefiable in iwtain rangcs. Tlia diifiision of indicator outward from the gelatin foil is ariotlrer olijectioostde feature. The use of the glass clcctride in latex has been described b,, Itulilb(J~d(i10) wlio found it to be sufiicient y accurate over l,lte rang(, inrcstigated (pII 8 to 111% 12). Van Harpen ( M ) Itas iised the quinlydrone method in a tlioroiigh study of the hydrogen-ion mmnt,ration of frmh Intw and the adsorption scr
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