March I, 1921 



THE INDIA RUBBER WORLD 



429 



What the Rubber Chemists Are Doing 



Caoutchouc Considered as a Colloid 

 By D. F. Twiss 



WHEN Graham established the ditTerciice between crystalloids 

 and colloids he realized that they are separated by no real 

 boundary, but joined by certain connecting links. Caout- 

 chouc is a typical colloid, although its colloidal nature is not easily 

 explained, as our knowledge of the colloids is still principally based 

 upon materials that have been produced by artificial means. 



Colloidal materials generally arc divided into suspcnsoids and 

 emulsoids. The suspensoid contains suspended in the body of 

 the material particles which can be made visible only by aid of 

 the microscope. Emulsoids contain particles only in fluid form, 

 which arc mostly dissolved in the body of the material. Natural 

 caoutchouc may be designated as a double colloid. The latex 

 forms a milky fluid which contains particles of colloidal char- 

 acter. Hinrichsen and Kindscher identify these particles as 

 caoutchouc suspended in a watery fluid, the serum of the latex. 

 The activity of these caoutchouc particles varies greatly accord- 

 ing to the origin of the latex. 



CAOUTCHOUC LATEX 



If latex is exposed to electrical influence the caoutchouc parti- 

 cles move to the anode, leaving the fluid clear in the neighbor- 

 hood of the cathode, proving that these particles are negative 

 and that the latex is a negative suspensoid. The separation of 

 the caoutchouc from the latex is very siinilar to that of clay in 

 aqueous colloidal suspension. The separation of suspensoids of 

 this sort is considerably facilitated by the assistance of acids, 

 hence, latex is coagulated usually by addition of small amounts 

 of acetic acid. -Alkalies, on the other hand, strengthen negative 

 suspensoids. The action of acids and alkalies establishes the con- 

 clusion that the presence of ketones regulates the speed of the 

 separation of caoutchouc from latex. 



By dialysis soluble salts may be extracted from the latex of 

 the Hcvea. Latex so treated cannot be coagulated by mono- 

 valent metallic salt solutions. Alkaline salts coagulate latex if 

 the concentration is not above normal, while heavy inetallic salts 

 effect coagulation at a concentration of one-twentieth normal. 

 In the case of caoutchouc latex containing mineral salts, con- 

 centration may be reduced proportionally. 



.According to the strength of the coagulating medium, raw 

 caoutchouc separates as a spongy mass or a firm elastic body. 

 The first form of separation may be regarded as the luicom- 

 pleted form of the second. The quality of the caoutchouc de- 

 pends largely on the form of coagulation. Selection of the 

 coagulating medium and the system of coagulation is a matter 

 of practical importance and has led to the attempt to introduce 

 recognized standard methods in plantation practice. 



It is possible to increase the stability of a suspensoid by the 

 presence of a second colloid or cmulsoid. This characteristic 

 is made use of in the manufacture of photographic plates. Latex 

 contains, besides water, the caoutchouc proteins, the presence of 

 which strengthens the suspensoid and acts as a protector. Dilu- 

 tion reduces this action, which is why the addition of water to 

 the latex aids the formation of the "cream" or spontaneous sepa- 

 ration of caoutchouc particles. .Addition of an agent neutralizing 

 the cfifcct of the protein also increases the separation of the sus- 

 pensoid. The protective agent is not always an albuminoid. 

 The lalex of Puntiimin chislicn contains a peptonoid which exerts 

 a similar effect. The neutralizer in this instance is formaldehyde, 

 which in the case of Hevea adds to the stability of the latex. 



If fresh latex has been sterilized by subjecting it to heat it 

 becomes immune against the action of acids, and the addition of 



>Le Cioutcliouc et la Gutta-Pcrcha, March \S, 1920, pages 10240-43. 



a small (|uantity of fresh latex becomes necessary to reestablish 

 the property of coagulation. This occurrence has been attributed 

 by Eaton to the presence of microbes, but the existence of mi- 

 crobic action is held to be not absolutely necessary to produce 

 this effect, since the presence of enzymes appears to be a suf- 

 ficient explanation. 



RAW CAOUTCHOUC 



When raw caoutchouc is sulijccted to a solvent the latter slowly 

 enters it, causing it to expand greatly and forming, with the 

 assistance of shaking, a viscous colloid solution. Carbon bi- 

 sulphide, chloroform and benzene yield ccmiparativcly clear 

 solutions of caoutchouc, while naphtha, gasoline or ordinary ether 

 produce milky solutions. The difference in the appearance of the 

 solution is due not only to variations in index of refraction of 

 the solvent and suspended particles, but is caused also by the 

 presence of materials of varying solubility. Caoutchouc which 

 dissolves practically entirely in the first named of these solvents 

 dissolves only partially in the others. The undissolved portion 

 is held colloidally suspended. Even in apparently clear colloidal 

 solutions, as of gelatine or agar-agar, one may discover sub- 

 microscopic particles by means of the ultra-microscope. Turbid 

 caoutchouc solutions compared to clear ones must be regarded 

 as colloidal solutions of a lower order. Solutions of technical 

 caoutchouc mi.xtures, which contain sulphur and other filling 

 materials, arc inferior as regards the size of the particles in 

 suspension. 



CAOUTCHOUC AND EMULSOID 



While latex may be classified as a simple suspensoid, caoutchouc 

 is an emulsoid. The viscosity of emulsoids is greater than that of 

 suspensoids. Such colloids as gelatin probably retain their col- 

 loid character permanently. They consist of a colloid substance 

 finely dispersed in a colloid medium, which causes it to be spongy 

 or cellular and unchanged by ordinary means. Raw caoutchouc, 

 however, is a substance in which the caoutchouc hydrocarbon 

 is very finely distributed in a medium formed probably by the 

 protein of the latex. Since deproteinated caoutchouc retains its 

 typical character, it is practically certain that its colloid nature 

 is caused by two forms of caoutchouc substance of different 

 molecular weight and molecular condition. The influence of 

 heat, light, or acids brings about remarkable changes in the 

 viscosity of caoutchouc solutions caused probably by changes in 

 either colloidal or molecular conditions or by both combined. 

 The lessened viscosity of gutta percha and balata solutions and 

 the ease with which they separate from solution permits the con- 

 clusion that the hydrocarbon of gutta percha and balata is less 

 complex in nature than that of caoutchouc. 



ABSORBENT EFFECT OF CAOUTCHOUC 



.A characteristic of all emulsoids is the possibility of condensing 

 upon their surface materials of a ditTerent kind, as shown in the 

 case of coloring material. This explains w'hy during coagulation 

 of latex part of the coagulation material is retained which can 

 be removed only with difiiculty. Coagulation materials, there- 

 fore, should consist only of volatile substances and the quantity 

 that is added should be small. If coagulated caoutchouc is al- 

 lowed to stand several days the protein undergoes partial de- 

 composition with formation of basic substances and acid amines 

 which cannot be removed completely from the caoutchouc by 

 wasliing and drying. Their presence exerts a marked catalytic 

 influence upon vulcanization. The difference in quality between 

 wild and plantation caoutchouc without doubt is caused by the 

 varying quantities of catalysers which the two varieties contain. 



