810 



THE INDIA RUBBER WORLD 



[September 



1920. 



in the sense that every submicroscopic portion contains both 

 the solute and the solvent. 



Caoutchouc dissolved in benzol may be separated from 

 the solvent either by the addition of acetone or by the evap- 

 oration of the benzol. In the first case a lumpy, stringy 

 mass is obtained, doubtless because the acetone dcsolvates 

 the raw rubber. In other vi-ords, the acetone robs the 

 caoutchouc of its benzol, the particles are attracted to one 

 another and therefore coagulate in masses or flocks. When, 

 however, the benzol is removed slowly by evaporation the 

 particles are forced closer and closer together as desolvation 

 proceeds. Finally they touch, flow into one another and a 

 single coextensive mass is formed. Just as in the case of 

 gelatin, it is quite possible that polymerization occurs during 

 the coagulation. 



In accordance with this point of view an elastic rubber 

 mass is a coagulum permeated with capillaries, the walls of 

 which are made Up of more or less polymerized particles of 

 different sizes stuck together at several portions of their 

 surfaces. If a solvent is introduced the particles become 

 solvated and separate into submicrons that differ greatly in 

 size. Moreover, if polymerization occurs during coagulation 

 it naturally follows that depolymerization must take place 

 while dissolution is in progress. 



This same increase in dispersion and possibly depolymeriza- 

 tion may be brought about by other agencies than solvents. 

 Mechanical work and rise of temperature have this effect to 

 a marked degree. Doubtless the function of "milling" is 

 precisely of this nature, but whether the paramount change 

 is chemical depolymerization or merely a physical reduction 

 in the size of the particles cannot be decided at present. It 

 seems very probable that both occur at the same time. It 

 is of interest to note in this connection that a tire on the road 

 is being constantly subjected to a milling treatment which 

 should tend to depolymerize and decrease the size of the 

 particles. This effect should be offset wholly or in part by a 

 slow vulcanization in case free sulphur is present, because 

 the temperature rise in a tire under load is often very con- 

 siderable. 



The subject of coagulation and the antithesis, dispersion. 

 should not be dismissed without reference to the important 

 part these reactions play in the preparation of such com- 

 pounding materials as zinc oxide, gas black and sublimed 

 lead. A recent article on carbon black by Perrott and Thies- 

 sen,° makes it quite clear that not only the size of the particles 

 but also the structure is vital. Both of these are without 

 doubt influenced by the method of preparation. 

 SELECTIVE AUSORPTION. 



Selective adsorption, the second topic of this paper, is a 

 colloidal conception that promises to become more and more 

 useful as it is better understood. By the term is meant that 

 certain substances mutually attract each other with very 

 considerable force, although no chemical action according to 

 the law of definite proportions may result. When the sub- 

 stances are microscopic in size this attraction is called ad- 

 hesion, but if one or both are of colloidal dimensions the 

 term adsorption is appHed. May it not be that this phenom- 

 enon is the vital factor in the compounding of rubber? Cer- 

 tainly on purely chemical grounds we cannot account for 

 the very great change of properties occasioned by the mixing 

 of gas black with raw rubber, nor yet for the fact that oil 

 black is not so efiicacious as gas black. Selective adsorption, 

 on the other hand, predicts very pronounced differences of 

 behavior between substances having the identical chemical 

 composition but a different physical structure. In order that 

 carbon black may be adsorbed by raw rubber to the greatest 



degree and with the strongest bonds, the former must be in 

 a very fine state of division, and doubtless the particles must 

 have a particular structure. These conditions are not met so 

 well by the oil black as they are by the gas black. 



This theory also predicts other important facts familiar to 

 the rubber chemist. For example, it ought to be possible to 

 substitute for carbon other substances which are in a suitable 

 physical state. The determining factor is whether or not the 

 compounding material and rubber mutually adsorb each 

 other to a high degree. This is doubtless the reason that 

 zinc oxide may be employed instead of gas black. 



Many other systems are known where mutual adsorption 

 changes the properties fundamentally. As an instance may 

 be cited a mixture of sand and mud, or clay. Roads made of 

 either of these materials alone are nearly impassable, but if 

 the two are mixed in the rifjht proportions very fair surfaces 

 result. 



CONCLUSION. 



From tlie colloidal point of view the particles of the com- 

 pounding material of a compounded vulcanized rubber mass 

 are surrounded by a thin film of rubber. The latter is there- 

 fore the continuous phase and is held securely to the surface 

 of the compounding material by the forces of adsorption. 

 Not only will the rubber films offer resistance to efforts 

 tending to deform the mass, but the particles of the com- 

 pounding material hold the films of the rubber so firmly that 

 the resistance is greatly increased. A good example of this 

 last action is shown by the surfaces of two glass plates which 

 have been wet and brought together. As long as the plates 

 are not very close they can be moved across each other with 

 ease. When, however, the surfaces are pressed together so 

 that the water forms a thin film between them, the adsorp- 

 tion forces become so strong that the plates may be moved 

 in different directions only with great difficulty. 



CARBON BLACK. 



The following interesting facts are taken from an address de- 

 livered before the National Association of Printing Ink Manu- 

 facturers liy Godfrey L. Cabot, a well-known manufacturer of 

 carbon black. 



PRODUCTION. 



The makers of carbon black are straining every nerve and 

 have been making the utmost effort for several years to increase 

 their output, and it has greatly increased. In 1915 The B. F. 

 Goodrich Co. made the remarkable discovery that carbon black 

 incorporated in rubber increased its tensile strength fifteenfold, 

 giving it greater tractile effect on a smooth and slippery pave- 

 ment, less wear and afforded greater resistence to the oxidizing 

 effect of the atmosphere than any other material yet tried for 

 these purposes. The result was that the demand for carbon 

 black increased more than twofold, and more than half the carbon 

 black now made is used in the rubber trade, and the amount 

 available is not enough to satisfy it. 



Many different causes have increased the difficuhies of sup- 

 plying the demand for carbon black. During the war pressure 

 was brought to bear by the Fuel Administration to divert gas 

 from its use in the manufacture of carbon black to use for fuel, 

 and two factories have been shut down in part by this pressure. 



The great scarcity of steel, its high price and the difiiculties 

 of transportation have greatly enhanced the expense and difficulty 

 of building black factories, and it is difficult, expensive and 

 tedious to get such material to-day. 



Many of you have probably noticed old signs on 

 country roads that had been painted in black on a background 

 of white lead, and this in turn on a wooden board— the white 

 lead almost gone, the wood beneath it rotted away to a very 

 appreciable extent, and the black lettering of the original sign 

 standing out in bas relief by reason of its having protected the 



