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THE INDIA RUBBER WORLD 



[June 1, 1912. 



The Significance of Gravity in Rubber Manufacture. 



By Lothar E. Weber, Ph.D. 



IT is in large measure due to the ease of its execution that the 

 determination of the specific gravity test is so largely favored 

 by rubber manufacturers. Not only is great importance at- 

 tached to the gravity of a manufactured article, but in the case 

 of raw? materials — compounding ingredients, shoddies, substitutes, 

 etc. — it is the criterion, more so than any other whereby judg- 

 ment is passed on the goods, provided the samples in question 

 are not chemically analyzed. 



Unfortunately, the determination of the gravity as usually car- 

 ried out. suffers from a basic error, so that the values obtained, 

 if not altogether worthless, are apt to be very misleading. The 

 error referred to is incurred owing to the porosity of the sample. 

 It is obvious that an article which is porous weighs less per 

 unit volume, than if it were entirely free from pores, and ac- 

 cordingly in the former case its apparent gravity is lower. The 

 actual gravity, however, is identical in both cases, although the 

 common method of determining gravity does not reveal it to be 

 so. 



From the standpoint of a cost department the actual gravity 

 of an article is of considerable importance. If the goods are 

 to be sold by bulk or linear measure, a minimum gravity is de- 

 sirable. If, on the other hand, the goods are disposed of by 

 weight, a maximum gravity is aimed at. I am not aware that 

 any manufacturer of compounding materials has succeeded in 

 raising the gravity, actual or apparent, above its normal value. 

 The task would indeed be a difficult one. But it is. on the other 

 hand, a very simple matter to lower the gravity of a sample of 

 whiting, zinc, etc. Needless to say, it is the apparent gravity 

 that is affected in this case, the porosity of the sample having 

 been increased. 



In actual practise, this lowering of the apparent gravity is 

 accomplished by bringing the article to a very fine state of sub- 

 division. The number of particles per unit volume is thereby 

 increased, and as the particles are always separated from each 

 other by an air space, the number of air spaces per unit volume 

 is likewise increased. In other words, the porosity is increased, 

 and the apparent gravity lowered. It is therefore to be expected, 

 since the question of porosity is usually disregarded, that a 

 manufacturer of. say, whiting lauds his product simply because 

 the latter is of a lower gravity than ordinary whiting. The sales- 

 man will put forth the very plausible argument that by using 

 this particular whiting, the gravity of the mixing will, as a re- 

 sult, be lower than if whiting of ordinary gravity were employed. 

 Whiting, however, being a chemical compound can possess only 

 one gravity, as a fundamental law of chemistry states that every 

 compound has a definite and constant gravity, and chemical laws 

 do hold good, even if manufacturers are somewhat tardy in ac- 

 cepting them. 



Since then, this particular whiting owes its lower gravity 

 merely to enclosed air, what will be the fate of the latter in act- 

 ual use? Under normal conditions, the enclosed air will all be 

 forced out on the mixing mill and the resulting compound will 

 have the identical gravity as if ordinary whiting had been used. 

 The sceptic can readily convince himself on this point by a very 

 simple experiment. Let him mix two compounds consisting in 

 each case of 50 per cent, rubber, 46 per cent, whiting and 4 per 

 cent, sulphur. In one case, let him use whiting of ordinary grav- 

 ity, in the other case, the whiting of the lower gravity. After 

 mixing and vulcanizing it will be found that both compounds 

 have the identical gravity, in spite of the fact that in one case 

 whiting of lower gravity was employed. 



It is fortunate, that under normal conditions, the enclosed air 



is all forced out. Otherwise, the manufactured article would 

 still retain the porosity due to the whiting, and in this condition 

 it is very prone to decay owing to oxidation and consequent 

 hardening. It is very common among the cheaper reclaimed 

 rubbers, and not uncommon among the more expensive ones, that 

 they exhibit a pronounced porosity. The minuteness of the pores 

 often prevents their perception by the unaided eye, but they are 

 readily discernible by means of a lens. It is to the interest of 

 the shoddy manufacturer to make this porosity as large as pos- 

 sible, as consequent low gravity results. The rubber manufac- 

 turer believes that a low gravity indicates a large percentage of 

 rubber, and a comparatively small amount of mineral matter 

 (loading). It will readily be seen in the case of a porous shoddy, 

 that this assumption is somewhat erroneous. A shoddy low in 

 mineral matter must necessarily have a low gravity, but the re- 

 verse is only true if the shoddy is non-porous. 



Apart from the question of porosity, it is quite impossible to 

 estimate the rubber content of a shoddy from the gravity, in spite 

 of the fact that many manufacturers, as already stated, use the 

 gravity as the only criterion whereby the composition of a shoddy 

 IS judged. Litharge having a gravity of over 9, whiting some- 

 what more than 2. and most of the substitutes being floating, it 

 will readily be seen that by employing these substances in vari- 

 ous proportions the gravity of the shoddy can vary between wide 

 limits, and the amount of rubber present remain absolutely con- 

 stant. If we add to this the error liable to occur, owing 

 to porosity, the doubtful value of the gravity test becomes ap- 

 parent. In the case of a shoddy, porosity is liable to be a more 

 serious drawback than in the case of mineral fillers, as regards 

 its influence on the resulting compound. It is much more diffi- 

 cult to break up the pores in a shoddy than in a mineral filler, 

 as the latter usually yields quite readily to the action of the 

 mixing mill. 



It is very desirable, therefore, to have a ready means of deter- 

 mining the porosity of a sample. From what has already been 

 said it will be perceived that the porosity is really the ratio be- 

 tween the actual and the apparent specific gravities. The ap- 

 parent specific gravity, always the lower of the two, is determined 

 in the usual way by weighing the sample in air and in water. 

 From these two weighings the volume is calculated, and sub- 

 sequently the apparent gravity. For the determination of the 

 actual gravity, the sample must be in as a fine a state of sub- 

 division as is possible. This can usually be accomplished bj' 

 grinding the sample on the mill. If the sample resists this treat- 

 ment, however, it must be ground by means of a grater or coarse 

 file. A weighed sample of the finely ground product is then 

 introduced into a specific gravity bottle and the latter almost 

 filled with distilled water. Suction is then applied by means of 

 an air-pump till all the enclosed air has been sucked out. The 

 sample is now absolutely non-porous. Subsequently the bottle 

 is completely filled with water and weighed. The porosity p is 

 then expressed by the following formula : 



factual specific gravity ] 

 : --' \ 

 apparent specific gravity J 



For an absolutely non-porous sample, this would give p = o.* 

 There is one inorganic filler, the gravity of which might, 

 at first sight, legitimately vary. The filler referred to is 

 lithopone. As is well known, lithopone is a mixture of barium 

 sulphate (barytes) and zinc sulphide. The gravity of barium 



*l-"or further details regarding this determination, see C. O. Weber. "The 

 Chemistry of India Rubber," p. 227. 



