THE INDIA RUBBER WORLD 



determining tlie change of spccilic gravity. This method, which 

 avoided the construction of any special apparatus, and was very 

 accurate, was the suggestion of Mr. W. B. VViegand, whose 

 inspiring cooperation in this and other rubber researches the 

 writer takes pleasure in acknowledging. 



( 1 ) Barvtes : The test results are shown by that part of 

 each curve in full line. It was found impossible to obtain higher 

 elongations than 200 per cent for the large rings, owing to their 

 failure by slow tearing when stretched above this value. The 

 curves approximate closely to straight lines, showing that the 

 volume increase varies almost directly with the elongation. Also 

 for a constant elongation, the volume increases progresses with 

 the percentage of barytes in a roughly proportionate manner. 

 There appears to be no adhesion whatever (or very little) be- 

 tween the rubber and the particles of barytes, because there is a 

 volume increase in the compound contain- 

 ing only live volumes of barytes. The 

 particles are certainly not crowded in a 

 five per cent mi.x, and their surfaces do 

 not necessarily transmit the whole local 

 stress, but on account of this early separa- 

 tion of the pigment particles from the 

 rubber, the particles take no important 

 part in the stress-strain curve for tlie 

 body, since the stress is transmitted 

 through the rubber only. 



It was thought tliat possibly the large vol 

 be due to the pigment entering the rubber in agglomerated masses, 

 and that most of the increase in volume might be due to the 

 ready separation from one another of particles which were in 

 dry contact. Accordingly, two tests were made to determine this, 

 one test of a sample of compound milled for 60 minutes instead 

 of 21 minutes for the normal mix, and another test upon a sample 

 which was softened to cement consistency in gasoline, thoroughly 

 mixed, dried, and press-cured as usual. The excessively milled 

 sample showed a very slight increase of percentage volume in- 

 crease, but the decrease of this property in the cement sample 

 was more considerable. The entry of the gasoline into the com- 

 pound presumably softened the rubber and allowed it to flow 

 around each particle, wetting the whole mass of pigment thor- 

 oughly; so that, although part of the volume increase under 

 stretch is due to the separation of pigment surfaces in dry con- 

 tact, the larger part of the increase is due to rubber separation 

 from the surface of the particles. 



The curves were extended as shown by the dotted lines to the 

 point corresponding to the ultimate percentage elongation as 



obtained from 

 standard break- 

 ing tests on a 

 tensile testing 



mcreases 



ight 



higher elongations could be obtained on the tensile testing 

 machine. 



(2) Whiting. The curves for this series of corapo'inds show 

 that up to 40 volumes of whiting, the percentage increase is com- 

 paratively small, then there is a sudden jump in volume increase 

 with further addition of pigments, until at 150 volumes the whit- 

 ing gives results almost equal to those with barytes. This prob- 

 ably is due to agglomeration of the pigment particles when pres- 

 ent in large amounts. It should be noted that these curves are 

 plotted on a more open scale of per cent volume increase than 

 that of the barytes. 



(3) Zinc Oxide.— The trend of the zinc oxide curves up to 30 

 volumes is identical with that of the basic mix, although the 

 ultimate elongation is reduced as much as 250 per cent. This 

 shows that there is a strong adhesion of the rubber to the parti- 

 cles of zinc oxide, which imparts addi- 

 tional strength to the basic mix, but 

 which reduces the ultimate elongation on 

 account of the dilution of the rubber. 

 This phenomenon classifies zinc oxide 

 phj'sically with the finer pigm.ents. lamp- 

 black and carbon black. The upward shift 

 of the curves for volumes above 30 in- 

 dicate a rapid growth of agglomerated 



"'°"°" masses of particles which greatly reduce 



the tensile strength of these compounds. 

 The shape of the almost complete 125-volume curve indicates 

 that the large extrapolations of the other curves are approxi- 

 mately correct. 



(4) China Clay. — The range of volumes experimented upon 

 is not so large, but there is indicated an increase in volume for 

 the low percentages of pigment. The open scale of "volume in- 

 crease" to which these curves are plotted should be noted. The 

 trend of these low volume curves away from the basic mix curves 

 shows that the addition of china clay to rubber lowers the tensile 

 effect upon the compound. 



(5) Red Oxide.— The red oxide, being a f^.ner pigment than 

 china clay, shows less volume increase under strain. Also, de- 

 parture from the curve of the basic mix shows a weakening 

 effect upon the compound. 



(6) Lampbl.\ck. — The volume increase of the lampblack com- 

 pounds is graduated proportionally to the content of lampblack 

 up to SO volumes. The 75-volume compound shows a consider- 

 able rise of volume increase, which indicates a rapidly increasing 

 agglomeration of the particles. This means that the limit of abil- 

 ity of the rubber to wet each particle of lampblack has 

 been reached at 50 volumes, and above this volume the rub- 

 lier surrounds groups of particles instead of embedding 

 each particle in- 

 dividually. 



Fig. 5. i-'f 



machine. The full-line curve joining these points is of special in- 

 terest. The discontinuity above 50 volumes is due to the for- 

 mation of local contractions in the lest ring, so that the whole 

 body of rubber does not receive the same proportional elonga- 

 tion. The neck is similar to that formed on metal specimens. 

 By mechanically working the rubber rings until the stififness is 

 removed, greater volume increases are obtained, i. c, we obtain 

 points on the true extension of these curves. In the same way 



6, Fig. 7. 



(7) Carbon Black.— A 30-volume content of carbon black low- 

 ers the ultimate elongation of the rubber to 530 per cent, while 

 the same volume of lampblack lowers it to 420 per cent. This 

 indicates a greater weakening effect of the lampblack on the rub- 

 ber, although the voluiue increase under equal strain is less. 



Curves for SO Per Cent and 100 Per Cent Strength.— Figs. 

 8 and 9 show the property of volume increase in a different light. 

 Here the variation of percentage volume increases is shown on 



