November 1, 1920 



THE INDIA RUBBER WORLD 



101 



In order to ascertain the effect of the different fillers on the 

 rubber, the device of figuring tensile strength, ultimate elonga- 

 tion and tensile product back to the actual volume of rubber 

 present was tried out and found to be of value. 



The present state of compounding demands a simple pro- 

 cedure whereby fillers can be compared with regard to their 

 effect on rubber regardless as to how that effect is produced. 



Assuming that the effective area is that obtained by sub- 

 tracting the area occupied by the particles of filler from the 

 total area of the test piece one can refer the test back to the 

 proportional quantity of rubber present by dividing the difference 

 by the percentage by volume of rubber and multiplying by 

 100. For example, referring to Table IV we find that the stock 

 containing 25 vohmies of magnesium carbonate to 100 volumes 

 of rubber (80 per cent rubber and 20 per cent MgCOj by volume) 

 has a tensile strength of 2,670 pounds per square inch. The 

 ultimate elongation is 630 per cent and the tensile product 168. 

 Correcting these values to the relative quantity of rubber present 

 by multiplying them by 100 -t- 80 gives 3,340 pounds per square 

 inch as the corrected tensile, 788 per cent corrected elongation, 

 and 210 as the corrected tensile product. The term "corrected" 

 which is used throughout the paper always indicates that the 

 value has been corrected back to a basis of 100 volumes of rubber. 



Complete data of the various experiments are recorded in tables 

 I to VI, inclusive. This information has been reduced to curves. 

 Figs. 1 to 7 inclusive. 



Barvtes — You will note that this filler causes a continuous 

 decline in tensile until 35 volumes to 100 volumes of rubber is 

 reached. From this point the curve comes to a "flat." 



LiTHOPONE — This filler also causes a decided falling off in 

 tensile. 



Tripoli — Infusorial earth, etc., shows a maximum above 

 3,000 pounds per square inch at 3 volumes to 100 volumes of 

 rubber after which there is a decided falling off, due perhaps to 

 cutting action of the sharp particles. 



Zinc Oxipe — The variety used here was New Jersey Red XX. 

 You will note that its curve comes up to a flat at 5 volumes to 100 

 volumes of rubber, remains constant until IS volumes and then 

 falls gradually until 35 volumes is reached. Beyond this point 

 the fall is rapid. 



Magnesium Carbonate — This filler comes up to a maximum at 

 6 volumes to 100 volumes of rubber and then falls off gradually. 



Gas Black — The curve rises gradually to 20 volumes, remains 

 constant until 30 volumes and falls off slowly. 



ULTIMATE ELONGATION 



Barvtes — This filler stands out over all the others as having 

 least effect on the elongation. 



LiTHOPONE — Is next to barytes. 



Magnesium Carbonate, Gas Black and Tripoli — These all 

 produce about the same falling off. 



Zinc Oxide — This produces a somewhat greater drop in elonga- 

 tion, at the beginning of the curve. 



TENSILE PRODUCT 



Magnesium Carbonate — Shows the highest values but rapidly 

 falls off when over 15 volumes are employed. 



Gas Black — Holds practically constant until 20 volumes and 

 then falls away. 



LiTHOPONE AND Barytes — Both are quite low. 



Tripoli — Comes up and then rapidly goes down, indicative that 

 both tensile and elongation are markedly affected by increase in 

 filler. 



Zinc Oxide — Shows a rather steady falling off. 



The above curves are satisfactory only as far as they go. The 

 methods employed do not permit one to analyze the effects of 

 the filler and to differentiate between simple decrease in tensile 

 with decrease in rubber and increase in tensile due to some 

 peculiar property or action of the filler. By using the corrected 



values we eliminate the effect of decreasing the actual rubber 

 content of the stock. 



TENSILE STRENGTH, CORRECTED VALUES 



Barvtes — It is remarkable how well the evidence supports the 

 view that this filler has no effect on the stock. The straight line 

 curve is not absolutely accurate considering the data but it is be- 

 lieved to be very close to the truth. 



LiTHOPONE— Falls off to about 2,400 pounds per square inch 

 and then very gradually declines. 



Tripoli — Shows the same behavior as in other curves which 

 would indicate some peculiar behavior of the filler, probably 

 a cutting action by the siliceous skeletons of the diatoms. 



Zinc Oxide — Comes up to a maximum at 15 volumes and re- 

 mains constant until 35 volumes is reached. At this point we 

 liave a decided falling off in corrected tensile. This value can 

 be taken as the maximum quantity which may be added without 

 overloading. 



Magnesium Carbonate — Shows a maximum value from 6 to 

 15 volumes beyond which it shows a marked decline. 



Gas Black — Shows a continued increase until 30 volumes is 

 reached. Beyond this point the curve remains constant. Ap- 

 parently black has a stiffening or toughening action on rubber. 

 ULTIMATE ELONGATION 

 Barvtes and Lithopone— Have very little effect on the elonga- 

 tion. The values obtained are not far from those secured with 

 pure gum. 



Tripoli, Gas Black and Magnesium Carbonate— Show about 

 the same effect, namely, a gradual decrease with increased filler. 

 Zinc Oxide— Gives a more or less flat curve which does not 

 show such a marked decline as the others. 



TENSILE PRODUCT (CORRECTED) 

 These curves require considerable study. The previous con- 

 clusions are substantiated. 



Magnesium Carbonate — Is shown to give excellent results up 

 to 15 volumes. 



Gas Black — Increases the corrected tensile product up to 

 20 volumes after which the curve declines, thus indicating that 

 the increase in tensile, as ordinarily figured, is more or less at the 

 expense of elongation. 



Zinc Oxide — Comes to a maximum at 11 volumes to 100 vol- 

 umes of rubber. 

 LiTHOPONE, Barvtes and Tripoli — Function as before. 



PERMANENT SET 

 On Fig. 7 data as to the relative permanent sets of the re- 

 spective stocks are plotted. The method of obtaining permanent 

 set was worked out by E. L. Davies and the writer and was de- 

 scribed by my colleague in a letter accompanying the methods 

 proposed by the Rubber Testing Committee. 



The curves all show a decided increase in set, namely, plasticity, 

 with increased filler. The very high set obtained with 20 vol- 

 umes of magnesium carbonate explains why American compound- 

 ers have not used this filler to any marked extent. 

 NETWORK STRUCTURE OF RUBBER 

 It is desirable for the better understanding of compounding 

 phenomena to form a mental picture of the probable internal 

 structure of rubber. The conception presented below is given 

 solely as a vehicle fur thought. 



As a working hypothesis let us assume that vulcanized rubber 

 consists of plastic material and elastic fibers. There is evidence 

 that some such condition actually exists. For instance, elastic 

 fibers are indicated by the following: 



(1) It is well known that high-grade stocks have a noticeable 

 grain when calendered, that is, they tear in the direction the 

 stock has been run. This is as true for pure gum as for com- 

 pounded stocks. Grain in rubber is somewhat analogous to 

 grain in wood. See Table VII. 



