October 1, 1920.] 



THE INDIA RUBBER WORLD 



21 



the load axis, indicating greater and greater toughness. And 

 yet the breaking tensile holds up splendidly. A stock containing 

 20 volumes of lampblack possesses stress-strain properties re- 

 sembling in type those of steel and other rigid bodies; the 

 curve is practically linear, that is, Hooke's Law applies. There 

 is none of the usual flabbiness at low elongations. It is no 

 wonder such a stock wears better as a tire tread than one 

 made up even of zinc oxide or the finest grade of china clay. 



Beyond 20 volumes, however, aggregation again supervenes 

 and the pigment reverts to the barytes class. 



CARBON BLACK 



We come now to the king of pigments. The re-inforcing 

 qualities of lampblack are here displayed in superlative degree. 

 Instead of being diminished or at best maintained, the breaking 

 tensile is markedly improved. Linear (Hooke's Law) stress- 

 strain conditions begin early and continue unabated up to 40 

 volumes. 



Particle aggregation, with resultant collapse of the reinforcing 

 effect is postponed to 40 volumes, which is, of course, un- 

 approached by any other pigment, 



NUMEIRICAL MEASURE OF REINFORCING ACTION 



The question now arose as to a suitable quantitative means of 

 assessing the toughening effect of these various pigments on 

 compounds containing them in varying amounts. One method 

 consisted in measuring the rotation or displacement of the curve 

 toward the load axis by simply taking the height (or elongation) 

 at a definite load, say of 16,000 grams per square millimeter. 

 The trouble with this method was, of course, that it took no 

 account of the lowering of the tensile at rupture, which property 

 varies greatly with different pigments. 



The method finally chosen was developed by a consideration 

 of the conditions governing the phenomenon of abrasive wear. 

 Take for example an automobile casing tread. "Wear" here 

 consists in the gouging or tearing out of small masses of gum, 

 due to impact upon the road surface. Now a numerical meas- 

 ure of impact is the work done on each little mass of rubber. 

 If this work can be stored up without stressing the rubber sub- 

 stance past its rupture point the mass will stay in place. The 

 less energy it can so absorb, the easier it will be torn from its 

 moorings. 



Now the energy absorption is in each case represented by the 

 area contained between the .stress-strain curve and the elonga- 

 tion axis. This area was therefore measured by a planimeter 

 and the results calculated to foQt-pounds per cubic inch of 

 original stock. 



The curves in the graphs show the remarkable results obtained. 

 Foot-pounds per cubic inch are plotted against volume percen- 

 tages of pigment added to the base. 



The base mix stored up 450 foot-pounds. The addition of 

 barytes continuously diminished the energy content. Fossil flour, 

 glue, whiting and red oxide all behave in essentially the same 

 manner. China clay, however, is capable of slightly increasing 

 the energy content. Zinc oxide and lampblack run neck and 

 neck, showing marked increases. Carbon black is again the 

 winner, and if not added in excess of 25 volumes may increase 

 the energy content up to nearly 150 per cent of its original value. 



SPECIFIC SURFACE 



These facts point at once to the conclusion that the presence 

 within the rubber matrix of a disperse phase, such as carbon 

 black, which must be regarded as chemically inert, may never- 

 theless profoundly alter the characteristics of the system. The 

 subjoined table indicates almost beyond a doubt that these effects 

 run parallel with the specific surfaces developed by the various 

 pigment phases. 



The particle diameters here shown were determined micro- 

 scopically and are of course only approximate, particularly in the 

 case of the finer pigments. The surface developed per cubic 



inch of pigment was in each case calculated from the observed 

 average diameter of the particles. The values range from 30,000 

 (barytes) to 2,000.000 (carbon black), and if, for simplicity, 

 we assume that the adhesive force between the rubber sub- 

 stance and the pigment is the same in all classes, the enormous 

 differences in the area of contact are alone sufficient to account 

 for the striking differences in physical properties. 



As a matter of fact zinc oxide increases the energy absorption 

 of a compound to a greater degree than would be accounted 

 for by its specific surface, and it is safe to assume that in this 

 case there is also an exceptional surface tension behavior. 



WORK OF H. F. SCHIPPEL 



'llic fundamentally important work done by my colleague, 

 Mr. SchippeP, showed that, contrary to general assumptions, com- 

 pounded rubber under strain undergoes relatively large volume 

 increases which must be attributed to a separation of each pig- 

 ment particle from its rubber matrix, doubtless forming a 

 vacuum at each pole. He found increases, at, for example, 200 

 per cent elongation, ranging from 1.5 per cent for carbon black 

 to over 13 per cent for barytes. The volume increases ran 

 roughly parallel with the size of the pigment particles, zinc 

 oxide again occupying an anomalous position. 



Schippel's results throw a clear light on the mechanism of 

 the reinforcing action of the finer pigments. These resist the 

 increase of the free surface energy necessary to separate them 

 from their rubber matrix. When a carbon black stock is stressed 

 to rupture, the work done on the rubber phase must be increased 

 by an amount representing the increase in surface energy re- 

 quired to separate each particle of carbon from its surrounding 

 bed of rubber. In the case of a coarse pigment, such as barytes, 

 this increase in surface energy is negligible. 



The fact that with the finer pigments the rubber remains 

 nearly uniformly anchored, instead of pulling free along the 

 poles of each particle, must also result in a more uniform 

 stress on the pure rubber phase and so contribute materially 

 to the enhanced tensile properties and "energy capacity" of the 

 compound. 



Displace- Total Volume 



Apparent ment of Energy of Increase 



Surface Stress-strain Resilience & 200% EI. 

 Pigment. Sq.in per cu.in Curve Foot-pounds Percentage 



Carbon blacli 1.905.000 



Lamp black L524,000 



China clay 304,800 



Red oxide 152.400 



Zinc oxide 152.400 



Glue 152,400 



Lithopone 101,600 



Whiting 60,950 



Fossil flour 50,800 



Barytes 30.480 



In the above table are brought together for convenience the 

 various properties already referred to, for mixings containing 

 in each case 20 volumes of pigment. 



Taken in conjunction with the well-known wearing properties 

 of the various compounds, this table will bring out the funda- 

 mental casual connection between toughness or abrasive power, 

 capacity for storing work, and bond between particle and rubber 

 matrix ; all three of these being, in the main, functions of the 

 degree of dispersion of the rigid pigment phase. 



One interesting deduction from this work is that perhaps the 

 most direct and accurate way of determining the average fine- 

 ness of an unknown pigment is to take its stress-strain curve 

 in a standard mixing, measure its area with a planimeter, and 

 compare the energy content with that of a known pigment. 

 The application of this method to a glue compound gave u 

 particle diameter, which was later confirmed by direct micro- 

 scopic examination after staining. 



'The India Rubber World. January 1. 1920, 



