A LABORATORY LATEX'
BY HSI-CHING CHEN
The raw material which forms the basis of the rubber industry is a gum imported from the tropical regions of Asia, Africa, and America. It is obtained in the form of an aqueous emulsion or latex secreted in the cambium of certain trees and creepers. From this latex the rubber is obtained as a tenacious, springy, self-adhesive mass by methods of coagulation which vary somewhat widely with botanical origin, local custom, and other factors. An average analysis of rubber latex shows it to contain the rubber hydrocarbons as it's chief constituents to the extent of 25-4076 rubber with varying amounts of resins, sugars, proteins, and mineral salts, the balance being water. In plantation rubber from Ceylon the rubber content is usually put a t about 40% and the water content at about 55%. The globules in the emulsion are said to be about 3p in diameter for Ficus elastica, 2p for Hevea brasiliensis and less than I,U for F u n t u m i a elastica. While there is no reason to suppose that the rubber latex differs in principle from other emulsions, there is a wide-spread belief that the rubber latex is in a class by itself because it is apparently the only emulsion with which rubber chemists have ever worked. It seemed desirable therefore tosee whether one could make a substitute latex which could be studied easily in the laboratory and which would duplicate many or perhaps most of the reactions of the real latex. Evidently the first requisite for a laboratory latex is that the emulsified material should be liquid but should solidify when the emulsion cracks. This was easy to do, though the method is not that adopted by nature in the rubber latex. Cazeneuve2 has shown that acetaldehyde liquefies camphor and that the resulting liquid floats on water. By emulsifying the mixture of camphor and acetaldehyde, we can get an emulsion. When the emulsion cracks, the acetaldehyde will evaporate leaving solid camphor behind, The acetaldehyde used in these experiments was made from paraldehyde by distillation from concentrated sulphuric acid, Only freshly prepared samples were used. Since the camphor could not be ground, one gram of it was cut into small pieces and placed in a test-tube into which was run 4cc H2O and 2cc aldehyde. Two liquid layers are formed with the camphor solution floating on top. About 0 . 0 2 grams of sodium oleate were added and the whole shaken intermittently3 until an emulsion was formed. Based on a thesis for the M.Sc. degree in Chemistry presented to the Faculty of the Graduate School of Cornel1 University in May, 1922. Bull. ( 2 ) 36, 650 (1881). Briggs: J. Phys. Chem. 24, IZO (1920).
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HSI-CHING CHEN
The methods of coagulation of the rubber may be classified’ as follows:I. Smoking: by injecting steam and acid smoke. Smoking with Urucury nuts in the lower Amazon regions and with hard woods above Manaos. The smoke is said to contain acetone, acetic acid, creosote, and tarry matters. 2. Evaporation. The latex dries to ‘scrap’ if allowed to run down the bark. It may be poured into saucer-shaped stumps in which case the wood soaks up the water or the water evaporates into the air. 3. Boiling. This method of coagulation can be used for Castilloa, Ficus, and Funtumia latices but does not work satisfactorily with Hevea. 4. Dilution. This causes coagulation with Castilloa, Landolphia, and Funtumia but not with Hevea. j . Addition of acids, salts, alcohol, acetone, etc. Over goy0 of the plantation rubber latex is coagulated with acetic acid in some form. Alcohol and acetone give an admirable coagulation but are too expensive. Hubener points out that alcohol works only in presence of salts which is exactly what one would expect from what one knows about negatively charged sols2. Whitby3 states that dialyzed latex is not precipitated by methyl, ethyl, or amyl alcohol but is by acetone. t 6. Beating, churning, and centrifuging. Beating worked with Manihot rubber. Beadle and Stevens4 state that churning or shaking has no effect on normal Hevea latex. “If, however, sufficient of a coagulant be added to produce creaming and flocculation on standing, the effect is hastened and the flocks are more dense if the liquid be shaken vigorously. It is natural to suppose that with shaking or churning me have merely the mechanical effect of bringing the aggregates into frequent contact with each other, causing them to adhere and join up, forming larger and more compact aggregates.” Centrifuging5 works well with Castilloa (3p), not very well with Hevea ( 2 p ) , and not at all with Funtumia ( { ~ p which ), is exactly what one would expect so far as creaming is concerned. I n order to duplicate to some extent the South American method of smoking, acetic acid and steam were distilled into the emulsion. Cracking and solidification occurred. Of course this is really coagulating with hot acetic acid; but people have always made the distinction, If the laboratory latex is allowed to evaporate, the emulsion cracks and solid, white camphor is obtained as the acetaldehyde passes off. Slight warming speeds up the cracking considerably. When the laboratory latex is boiled, the emulsion breaks down. The differences observed with different natural latives are undoubtedly due to variations in the amount and nature of the Clouth: “Rubber, Gutta Percha and Balata,” 66 (1903); Spence: “Lectures on India Rubber,” 25 (1909); Schidrowitz: “Rubber,” 127 (1911):Hubener: Kolloid-Z. 16, 5 (19151. 2Bancroft: Rec. Trav. chim. (4) 3, 733 (1923); Bancroft and Gurchot: J. Phys. Chem. 28, 1279 (1924); Gurchot: 3 0 , 83 (1926). 3 “Plantation Rubber and the Testing of Rubber,” 34 (1920). 4 Eighth Congress Applied Chemistry, 9, 37 (1912). Beadle and Stevens: Xolloid-Z. 13, 210 (1914).
A LABORATORY LATEX
7=5
protecting colloid. When the laboratory latex is diluted with about I O cc of water, the emulsion is destroyed. Freezing1 also gives camphor. With more emulsifying agent the emulsion would not break down on dilution or on freezing and would then behave like the Hevea latex. It seemed unnecessary to make cataphoresis experiments because the liquid drops would certainly go to the anode in a soap solution. Centrifuging seemed also to be a mere waste of time. It is evident that the laboratory latex duplicates the real one in many respects. One could give additional stability against heating and evaporation by using a less volatile liquid than acetaldehyde; but it would take correspondingly longer before the camphor solidified, Some people may be horrified by the use of soap; but it is perfectly possible to substitute any other protecting colloid so long as it does not reverse the emulsification. It would also be an easy matter if one wished to duplicate or simulate the bacterial action which occurs in the natural latex.
Summary Practically all the reactions of a rubber latex may be duplicated with an emulsion made up of one part camphor, four parts water, 0 . 0 2 parts sodium oleate and 1.5 parts acetaldehyde. On coagulation the camphor is obtained as a white solid. The following methods of coagulating rubber have been duplicated with the laboratory latex : smoking, evaporation, boiling, dilution, and addition of acetic acid, oxalic acid, tannic acid, tartaric acid, formic acid, citric acid, hydrochloric acid, sulphuric acid, hydrofluoric acid, alum, and alcohol. No coagulation was obtained with acetone or phenol; but this could perhaps have been secured if another emulsifying agent had been used. I wish to extend my thanks to the various members of the staff of the Department of Chemistry for help which I have received and I wish particularly to thank Professor W. D. Bancroft, who suggested the investigation and whose kindly interest has assisted me over many difficulties. Cornell University. Zsigmondy-Spear: “The Chemistry of Colloids,” 8 (1917).
