492 



THE INDIA RUBBER WORLD 



April 1, 1921 



NO. OF PLIES 



Fic. 6 



and X is the number of plies of fabric. An interesting deduction 



from this curve is that the 

 frictional losses in a cas- 

 ing are not a linear func- 

 tion of the number of plies 

 of fabric. As a matter of 

 fact, the total period for a 

 5-ply carcass bears the 

 same ratio to that of a 

 4-ply carcass, as that of a 

 4-ply carcass bears to that 

 for a 3-ply carcass. In 

 other words, as the num- 

 ber of plies of fabric is 

 increased the frictional 

 lieat increases not in 

 arithmetic but in geometric 

 progression. This con- 

 stant ratio we have called the '"ply factor," and its value in a 

 typical square fabric casing lies very close to 0.7 for ranges of 

 from 2 to 7 plies. If the total period for a 6-ply section is 100 

 minutes, that tor a 7-pIy section will be 70 minutes. If there 

 were no fabric friction, this factor would of course become unity, 

 except for the small losses due to the skim coat between the plies. 

 Influexce of Gum Stocks on Casing Energy Losses. — It was 

 at first thought that the condition of the skim coat and friction 

 between the plies of fabric might profoundly influence the casing 

 energ>- losses, and a series of tire sections was therefore prepared 

 of various degrees of under and over-cure. To our great surprise 

 the effect of these exaggerated under and over-cures upon the 

 total period of swing was entirely negligible in every case. 



Effect of Tread and Breaker. — Our results, furthermore, 

 showed that, for example, in the case of a 3.S-inch 4-ply casing, 

 the total period of swing for, the complete section was almost 

 exactly the same as that for a 4-inch 5-pIy casing, stripped of its 

 tread and breaker. We thus see that the entire tread and breaker 

 of a casing contribute no more to the energy losses than does a 

 single ply of carcass fabric. 



Cord Construction.— These remarkable results made it at once 

 desirable to ascertain the effect of cord construction, the advan- 

 tages of which, from the standpoint of internal chafing, seemed 

 obvious. Our e.xperiments fully bore out this idea, and in fact 

 we found that a 5-inch cord carcass swings almost exactly three 

 times as long as a square fabric carcass of the same size. Cord 

 fabric is therefore three times as efficient as a transmitter of 

 energ)- as square fabric. Oui purpose in thus briefly describing 

 the pendulum method of investigation is not to expound the 

 behavior of the various structural elements of a casing, but rather 

 to illustrate the usefulness of a simple, convenient, cheap, and yet 

 accurate physical apparatus in helping to solve the pressing prob- 

 lems of our industry. 



EFFECT OF PIGMENTS ON ENERGY STORAGE CAPACITY 

 Of equal interest is the study of tlie total energy storage 

 capacity of vulcanized rubber and the profound changes in this 

 quantity which can be induced through the admixture of suitable 

 ingredients. The experimental details of this work have been 

 published elsewhere.' The fundamental facts are as follows: 



1 — A pure gum stock is totally unsuitable for some of the most 

 important technical applications of rubber by reason of its 

 inability to stand abrasive wear. 



2— The addition in suitable amounts of certain compounding 

 ingredients enormously improves the wear-resisting power of 

 rubber. Our investigation as to the reasons underlying these 

 facts naturallv began with a quantitative study of the effect of 

 the various compounding ingredients upon the mechanical prop- 

 erties of the stock. These properties are very largely expressed 

 by the stress-strain curve, and on selecting a suitable basic mix 

 and adding to it regularly spaced increments by volume of the 

 most important inorganic compounding ingredients, it was at 

 once discovered that profound changes in the character of the 



stress-strain curve were thereby induced. These changes may be 

 divided into two classes. 



One class comprises merely a foreshortening of the curve. 

 Thus, for example, the addition to the basic mixing of increasing 

 percentages by volume of barytes produces a stock which, when 

 gradually stressed to the failure point, preserves the same values 

 of elongation and load as in the case of the pure mixing. The 

 only ditTerence is that failure occurs earlier. In other words, 

 this pigment simply dilutes or attenuates the mechanical proper- 

 ties of the mixing. It plays a passive role. 



In the other class the stress-strain relationships are profoundly 

 altered. Thus, for example, if glue or zinc oxide or one of the 

 blacks be added to the basic mix in increasing amount, the 

 mechanical properties of the resultant vulcanisate show the fol- 

 lowing changes : 



First, the curvature of the stress-strain curve is diminished and 

 at suitable pigment concentrations actually disappears. That is 

 to say, rubber can be so compounded as to display the same kind 

 of stress-strain relationship as in the case of steel and the other 

 rigid structural materials i. e., Hooke's law obtains. Again, 

 certain of these same pigments, if not added in excessive 

 amounts, produce compounds, the tensile strength of which at 

 rupture remains undiminished or even increased over large com- 

 pounding ranges. In these cases the final elongation is, however, 

 markedly reduced. In the other cases, although linear stress- 

 strain relationships are induced, both tensile strength and elonga- 

 tion fall off more or less equally. 



It has been thought justifiable in view of these striking dif- 

 ferences in behavior to call pigments of the second class active 

 pigments and those of the former class inert pigments. 



Table II 



Displacement Total \'olume 



Apparent of S. S. Energy of Increase 



Pigment Surface Curve Resilience at 200% El. 



( arbon blade 1,905. OHO 42 640 1.46 



Lampblack 1,524,000 41 480 1.76 



rhina clay 304,800 38 405 



Red oxide 152,400 29 355 1.9 



Zinc oxide 152,400 25 530 0.8 



'.lue 152,400 23 344 



I ithopone 101,600 .. ... .... 



Whiting 60,390 17 410 4.6 



Ft&sil flour 50,800 14 365 3.5 



Barytes 30,480 8 360 13.3 



Base 

 450 



In Table II are brought together, along with the energy storage 

 capacities which are here designated, the total energy of resilience, 

 the dispersoid characteristics of the pigments in question, and 

 also the increase in total volume of the compounded rubber when 

 stressed to 200 per cent elongation. These volume increases, for 

 the details of which you are referred to a recent paper' by my 

 colleague, Mr. Schippel, prove beyond any doubt that particularly 

 in the case of the inert pigments the application of stress causes 

 a partial separation of the pigment from the rubber with resultant 

 development of vacua at the poles. In the active pigments, those 

 which show a positive eflfect upon the energy storage capacity, 

 this separation from the rubber matrix is very slight. Column 

 2, which gives the square inch of surface per cubic inch of pig- 

 ment, indicates that the extraordiuarj- differences in behavior 

 are without doubt attributable to differences in surface energy. 

 When a stock containing one of the active pigments is stressed 

 to rupture, the energy required to do so goes partly towards dis- 

 torting the rubber phase and partly towards tearing apart the rub- 

 ber from the pigment particle. 



.Again, the fact that in the case of the active pigments the 

 rubber remains more nearly adhesive to each particle means more 

 uniform stress on the rubber phase, and so enhanced tensile prop- 

 erties and energy capacity. 



Surface energy has, of course, two factors. The capacity 

 factor is represented by the specific surface, and it is the varia- 

 tions in this factor which appear to predominate in the behavior 

 of the various pigments. The other factor, the intensity factor, 

 which is represented by the interfacial surface tension, is also 

 doubtless of importance, as is shown by the fact that zinc oxide 



'Canac'ian Chemical Journal, 4 (1920), 160; see also abstract in Thi 

 India Rubber World, 63 (1920). 18. Both references give curves illu*. 

 trating the effect of various pigments on the energy storage capacity of the 

 rubber. 



•Canadian Chemical Journal, 12 (1920). 33. 



