September 1, 1920.] 



THE INDIA RUBBER WORLD 



809 



What the Rubber Chemists Are Doing. 



THE COLLOIDAL VIEWPOINT OF RUBBER CHEMISTRY.' 



Till; lAitK is divided into two main topics. The lirst is an 

 analysis of the mechanism by which the particles in- 

 crease or decrease in size in rubber and other colloidal 

 systems. Under this head crystallization, condensation, 

 polymerization and coagulation will be discussed. The second 

 topic is concerned with some applications of selective ad- 

 sorption to rubber. 



CaTSTAIilZATlON. 



For the present purpose little need be said of the process 

 known as crystallization except with regard to a single phase 

 of this phenomenon. For instance, when very dilute solu- 

 tions of gold chloride are treated with weak reducing agents, 

 microscopic nuclei are formed from which colloidal crystals 

 of gold may be made to grow, if the reducing action is kept 

 suflSciently low, so that the molecules of gold have time to 

 become orientated on the crystal faces. Here the growth of 

 the particles occurs by a purely crystallization process, and 

 will cease as soon as the supply of molecular gold or reducing 

 agent is exhausted. 



CONDENSATION. 



The term condensation is often employed in a somewhat 

 loose sense. Strictly speaking it refers to an increase in the 

 size of the particles through the agency of a chemical change, 

 whereby two or more molecules unite by the splitting of? of 

 water or other substance. A classical example is the forma- 

 tion of starch from sugar according to the equation 

 n CJ-I^ 0.—> {CJi», 0,). + nH,0 



POLYMERIZATION. 



Polymerization is another well-known method involving the 

 growth of particles. This phenomenon differs from con- 

 densation only by the fact that nothing is split off when two 

 or more molecules unite to form a larger complex. The 

 molecular weight of the complex is, therefore, « times that 

 of the original substance if n represents the number of 

 molecules that have united to form the new and larger unit. 

 Organic chemistry is replete with examples of this kind of 

 chemical action. For instance the change of the aldehydes 

 into the para or meta modifications belongs in this category. 

 From the evidence at hand it seems very probable that in 

 polymerization the redistribution of major valencies is in- 

 volved, because those substances which exhibit this property 

 to a pronounced degree are possessed of double bonds. 



It should be emphasized in connection with a discussion of 

 condensation and polymerization that these two types of 

 changes are essentially chemical in the strictest sense of the 

 term. The products of these two reactions have properties 

 very different from the original, and the alterations of prop- 

 erties are sudden, not gradual. 



COAGtTLATION. 



Coagulation, in contradistinction to condensation or poly- 

 merization, involves a gradual change of properties and there 

 is no sharp line of demarcation between the original and the 

 final substance. Unfortunately, the term is employed to 

 designate four distinctly different physical processes. 



The first of these is well illustrated by the union of the 

 colloidal gold crystals referred to in a previous paragraph. 

 If during the growth of these tiny crystals the process is 

 hurried, or if an electrolyte is added to the colloidal solution, 

 several of the tiny crystals unite by surface contact, a loose 

 mass is formed and eventually a powder will be precipitated. 



'Published by courtesy of the American Chemical Society. Paper by 

 Ellwood B. Spear, chemical department, Massachusetts Institute of Tech- 

 nology, read at the St. I-ouis meeting, April. 192U. 



This is, of course, an irreversible process and is a typical 

 example of coagulation. It should be noted, however, that 

 the particles touch the surface of one another and do not 

 flow together. 



A second form of coagulation is manifested when, for 

 example, the particles of an oil-in-water systeni gather to- 

 gether. Here the tiny droplets touch one another, flow into 

 a larger drop which finally rises to the top or sinks to the 

 bottom, according to the differences in the specific gravity 

 of the two phases. This kind of coagulation is distinguished 

 from all others in that the resulting products are two dis- 

 tinct, molecularly dispersed and therefore homogeneous 

 phases. There is no colloidal oil dispersed in the water, nor 

 yet is there any colloidal water in the oil. ■ 



A third type is exemplified by the coagulation of gelatin 

 in a hydrosol by an excess of an electrolyte, whereby a 

 flocculent and perhaps a somewhat stringy mass is obtained. 

 It is conceded by most writers that the electrolyte to a large 

 extent desolvates the gelatin, that the particles are at first 

 actually reduced in size because of the loss of solvent, but 

 that tlie desolvated particles unite to form larger clumps 

 which appear as flocks. 



There is also another possibility that should not be over- 

 looked. Not only may the particles unite and flow together, 

 but the molecules of which these particles are composed 

 may unite chemically to some extent and form a polymerized 

 substance. In such a case we should have both a chemical 

 and physical reaction going on simultaneously or subse- 

 quently to each other. It is altogether probable that in the 

 coagulation of rubber latex where an elastic mass is formed, 

 there occurs both a physical union between the particles and 

 polymerization between the molecules in these particles. It 

 is quite conceivable that these two reactions might proceed 

 to a different degree in the same system under slightly dif- 

 ferent conditions. If this were true we should not expect 

 the two products to have identical properties. This may 

 explain why two samples of raw rubber from the same latex 

 may differ materially from each other if the conditions dur- 

 ing coagulation are varied in the two cases. 

 GELATION. 



A fourth physical change, that of gelation, is usually re- 

 ferred to as coagulation. When a sufficiently concentrated 

 solution of gelatin in water is allowed to cool the entire mass 

 sets as a gel. The process in this case is the reverse of 

 number three previously described. As the cooling proceeds 

 the particles become more and more solvated, the size of 

 the particles increases until they touch one another and the 

 gelatin eventually forms the continuous phase, while the 

 water is now dispersed in the gelatin. This phenomenon is 

 known as a reversal of phase. Gels may be formed by evap- 

 orating off the solvent, and here the same reversal of phase 

 may occur. It is also not impossible that polymerization 

 may take place during the formation of a gel by either of 

 these processes. 



CHAKACTEMSTICS OF GELATION. 



Gelation differs from the three previously mentioned forms 

 of coagulation because in a gel both the disperse phase and 

 the disperse medium are coextensive throughout the entire 

 mass. This expression is employed in preference to the term 

 homogeneous because the latter has a well-defined thermo- 

 dynamic meaning. Homogeneous solutions contain a solute 

 in a molecular state of subdivision and exhibit a lowering of 

 the freezing point or raising of the boiling point from which 

 molecular weights may be calculated. Gels are coextensive 



