Apkil 1, 1921 



THE INDIA RUBBER WORLD 



491 



Vulcanized Rubber Energy — IF 



By William B. Wiegand- 



VOLS. OF ACTIVE PIGMENT MIXE 

 100 VOLS OF RUBBER 



Fig. 4 



THE EFFECT OF COMPOUNDING INGREDIENTS 



THIS PRESENTS 311 eiiormous held of research, and reference 

 will be confined to a brief outline of the basic facts. 



Fig. 4 shows hysteresis plotted against the volume per- 

 centage of active pigment associated with 100 parts of rubber. 



The first point on the 



curve shows a pure gum 



compound, the second, a 



lightly loaded breaker 



compound containing about 



4.5 parts by volume of 



active pigment. The third •j) 



point represents a very - 



high-grade tread com- £■*"•> 



pound containing about 15 aaoo- 



volumes of active pigment: .^Jjoc 



the last, another tread = 



stock containing nearly 24 



volumes. By active pig- 

 ment is meant a pigment 



which definitely increases 



the energy storage capa- 

 city of the compound and includes pigments such as carbon 

 black, lampblack, zinc oxide, the finer clays, etc. It will be noted 

 that for the particular stocks used there is a linear relationship 

 between the amount of hysteresis and the amount of such pig- 

 ment present. It is also important to note that the effect of 

 the addition of a highly dispersed phase upon hysteresis is much 

 greater than moderate changes in the state of cure of a com- 

 pound. It is unnecessary to emphasize the importance of this 

 result from the standpoint of practical compounding. 



Here again, however, one must use caution not to overlook 

 the importance of heat conductivity, and it is entirely within 

 the realm of possibility that a pigment, although markedly in- 

 creasing the hysteresis and so also the frictional heat, may at 

 the same time compensate for this by a greatly enhanced heat 

 conductivity. Thus, for example, carbon black not only causes 

 high frictional heats, but is also a bad conductor, whereas zinc 

 oxide, although producing similarly high hysteresis values, has 

 a very much better heat conductance. 



It may be of some interest to indicate roughly the actual 

 percentages of energy which are degraded into heat in these 

 various types of rubber compounds. A pure gum friction or skim 

 coat stock when led through a hysteresis loop to an elongation 

 of 200 per cent degrades about four per cent of the total energy 

 into heat. A stock containing about five volumes of zinc oxide 

 degrades about eight per cent, w-hereas a tread stock containing 

 20 volumes of zinc o.xide degrades in the neighborhood of 14 

 per cent of the total energy input in each cycle. 



FABRIC ENERGY LOSSES 



We have dealt thus far with the degradation of energy into 

 frictional losses in and by the rubber substance itself. These 

 are of paramount importance in the case of solid tires, for ex- 

 ample. However, in the case of pneumatic tires, which consist 

 primarily of layers of fabric held together and waterproofed by 

 rubber, we have to consider the extent to which frictional heat 

 is developed by the carcass fabric itself. It is true that the 

 hysteresis loss of an inflated casing taken as a whole can be 

 accurately determined by the electric dynamometer. This, how- 

 ever, is an expensive machine, and has the further disadvantage 



'Continued from The India Rubber World. March 1. 1921, pages 425-427. 

 Presented before the Rubber Division at the meetine of the American 

 Chemical Society, ChicaRO, Illinois. September 6-10, 1920. 



*Ames Holden McCready, Limited, Montreal, Canada, 



of not being able to determine in what proportion the various 

 constituent parts of the casing contribute to the integral result. 

 The writer has therefore applied the principle of the damped 

 pendulum to the study of casing energy losses. Briefly, the 

 method consists in inserting a one-inch carcass section in the 

 arm of a pendulum which is allowed to swing from a fixed posi- 

 tion until it comes to rest. The more perfectly resilient the 

 carcass wall, the longer will such a pendulum swing. In order 

 to analyze the elastic properties of the various structural com- 

 ponents of the carcass, it is necessary merely to strip off the 

 tread and breaker and repeat the series of vibrations with the 

 carcass alone. In order to ascertain the effect of the number 

 of plies of fabric the carcass is stripped down ply by ply and 

 the total period of the pendulum redetermined in each case. 



Fig. S shows the simplicity of the set-up. The inch section 

 is gripped by two clamps, the upper one rigidly fastened to the 

 wall, the lower attached to the pendulum arm, consisting of 

 thick piano wire about 2 feet long, weighted down by a 

 cylindrical bob of convenient mass, say O.S-pound. Space will 

 not permit description of the minute 

 experimental details, some of which 

 are of considerable importance to 

 the accuracy of the results obtained, 

 but, briefly, the practice was to 

 start the pendulum from a position 

 60 degrees from the vertical, and 

 take shadow readings on an arc 

 background by means of a fine 

 needle axially inserted in the bob. 

 The "total period" of the pendulum 

 is the number of seconds required 

 for the amplitude to fall from the 

 fixed arbitrary value, viz., when the 

 shadow of the needle reaches the 

 point C until the shadow reaches 

 the point D, which is preferably a small distance removed from 

 the position of rest. The length of the carcass strip between the 

 clamps may be varied at will, but is preferably about two inches. 



Significance of Total Period. The total period, viz., the 

 time required for the pendulum to damp down from the position 

 C to the position D is clearly a measure of the time required 

 for the potential energy of the pendulum system to fall from 

 that corresponding to the height of its center of gravity when 

 the pointer is at C to that corresponding to D. It is therefore 

 inversely proportional to the rate of generation of frictional heat 

 through the various internal energy losses in the casing section. 

 If the tire were of theoretically perfect resilience the pendulum 

 would keep on swinging forever, except, of course, for external 

 losses due to air resistance, etc. 



A typical series of determinations will serve to fix our ideas. 

 A 3.S-inch plain casing gave a total period of six minutes 42 

 seconds. After removing the band ply of the carcass, the period 

 increased to seven minutes 37 seconds ; after removing the 

 second ply, to eight minutes ; after removing the third ply, to 

 10 minutes 55 seconds. When all the carcass plies had been 

 removed and the tread and breaker inserted, the pendulum swung 

 for 21 minutes four seconds. As a matter of fact, it was found 

 in many hundreds of tests that the total period of the pendulum 

 when plotted against the number of plies of fabric in the carcass 

 lay on a smooth curve, shown in Fig. 6. 



This curve is of the exponential type, the equation of which is 

 TP = K, X K^N, 

 where TP is the total period, Ki and K2 are empirical constants, 



Fig. 5 



