Jlarch, 1934
I N D U S T R I A L A N D E N G 1 N E E K I N C;
4. These variations in carbonizing properties were not characterized by corresponding variations in the volatile matter content of the samples, as has been the case with some coals previously examined; the yolatile matter content of the two types of coal and of the mixture is nearly the same. 5. The bright coal alone apparently is too fusible to yield coke of the best quality; the splint cod, ordinarily deficient in fusing properties, has in thir coal sufficient translucent attritus to impart the optimum fusing properties for the bed and hence it produces a better coke than the other two iamples.
ACKNOWLEDGJIEST ~h~ coalsamples were collected by c. H.F ~prepara-~ tion manager of the Consolidat'ion Coal Coml)any. The analyses of the coal samples shown in Table 1 n-ere made by
C H E 21 I S T 11 \i
303
H. hl.. Cooper of the Coal Analysis Section of this station. Analyses of the tar distillate and light oil given in Table IX were made by L. P. Rockenbacli of this station. LITERATURE CITED (1) Fieldner, A . C . , DaT-ia, J. D., and Reynolds, D. A . , IND.ENG. CHEII.,22, 1113-23 (1930). (21 Fieldner, 4.C., Davis, J. D., Thiessen, R . , Kester, E. B., Selvig, TI-. A . ,. Revnolds. . " ~ D. h..June. F. TT.. a n d Snrunk. G. c':, Bur. Mines, Tech'. Paper' 542 (1932). ( 3 ) Holrog-tl, R., a n d Kheeler, R . T., P U P / 9, , 104-13 (1930). 14) Thiessen, Keinhardt, Bprunk, G. C., and O'Donnell, H. ,J., 1 % ~ ~~
~
~
Mines, Ttcii. Paper 506 (1931)
RECEIVED August 26, 1933. Presented before the Division
of Gas and Fuel ~ a t the ~ 86th >leetine_ ~ of the ~ American ~ Chemical, Society, Chicago, Chemistry Ill., September 10 to 1.5, 1933. Published by permission of the Dirertor, C . 8. Bureau of >lines. (?Jot subject to copyright.)
Thermal Properties of Rubber Compounds I. Thermal Conductivity of Rubber and Rubber Compounding Materials C. E. BARKETT, 'The New Jersey Zinc Company, Palmerton, Pa. An apparatus is described f o r measuring the thermal conductioity of rubber and rubber compounding materials using dectric crrrrent as the source of heat. I n the majority of cases the thermal conductivities check those f o u n d by Williams, using steam as the source of heat. As reported by Williams, no difference is f o u n d between smoked sheet and pale crepe rubber, and the state of cure of a compound has no egect on its heat conductivity. Variations in particle size within the pigment range hate no bearing on the heat conductivity of zinc oxide, but extremely coarse oxides have higher conductivities. Chemical constituents, such as are HE purpose of this series of papers is to present and
T
analyze the experimental data on the thermal behavior of rubber compounds which have been obtained during the past few years in the writer's laboratory. Because of the coniplex nature of the problem, i t is impossible to say exactly what a given test measures, much less to correlate the data of the various experiments and to predict results in other tests aud in performance. I n general, the factors recognized in this laboratory as variables controlling the thermal behavior of a rubber compound are: (I) heat conductivity, (2) heat resistance, and (3) heat generation. Heat conductivity is recognized as of great importance in vulcanization, in determining the running temperature of rubber tires, and, in general, wherever fairly large masses of rubber are subjected to repeated distortion. With relatively small test pieces, this factor does not have as great a bearing on the results as it does with larger masses of rubber, an important consideration in interpreting laboratory results in terms of performance. For the purpose of this investigation, heat resistance is considered as a measure of the ability of a rubber compound to withstand high temperatures without undergoing reversion
present in slow curing zinc oxides, hare no measurable effect on thermal conductioity. Particle shape has considerable effect o n thermal conductivify. Acicular pigments show higher conductivities when the calender grain is parallel to the direction of heat $ow rather than across it. Fast curing carbon black has the same heat conductivity as the sloiv curing variety; lampblack and graphite show increased thermal conduct iv it ies. The results of this work check fhose of Williams who showed that thermal conductitqity is a straightline function of the volume per cent of pigment and of rubber. or serious deformation under load. Thermal resistivity is frequently used to mean the reciprocal of thermal conductivity or the resistance which a substance offers to the conduction of heat. The third factor, heat generation, is of importance in all kinds of tests involving thermal problems. This is probably the most difficult of the three factors to examine in a performance test because of the difficulty of eliminating such variables as thermal conductance and resistance. It is generally accepted that the area of the hysteresis loop formed when rubber is put through a cycle of extension and retraction gives an indication of heat generation. The work of Cotton (4) indicates that the area of this loop varies directly with the entire area under the extension curve, but this conclusion may apply only to one particular set of experimental conditions. From the above discussion it follows that, so far as heat effects are concerned, the most satisfactory rubber compound would be one of high thermal conductivity and resistance and with low heat generation. Future papers are planned to discuss heat resistance and generation. The purpose of the present paper is to consider the first factor studied-heat conductivity.
304
IXDUSTRIAL AND ENGINEERING CHEMISTRY
The first practical work on the subject of the heat conductivity of rubber was t h a t of Somerville (Y), and adequate data enabling the calculation of the thermal conductivity and diffusivity of compounds were determined by Williams (8). For the present investigation, however, more detailed information was required on the effect of the variations obtainable in zinc oxide, covering a range of particle size and chemical analysis. Williams determ i n e d heat conductivity by covering a closed cylindrical vessel with the material to be s t u d i e d a n d measuring t h e water which collected when s t e a m was p a s s e d tlirough the vessel, the o u t s i d e t e m p e r a t u r e being JL__--lA controlled by immers i o n i n a constantFIGURE1. HEATING CIRCUIT temperature bath. It was felt t h a t the substitution of electric current as a source of heat might result in a n apparatus which n-ould be more conveniently operated than one using steam and, a t the same time, would offer a check on published data by a n independent method. APPARATUS AND PROCEDURE Figures 1to 3 are wiring diagrams for thc apparatus. I n Figure 1 the heating circuit is shown. HI is a compensating heater which surrounds H2, the calorimeter heater, to prevent lateral flow of heat. RI and RPare variable resistances controlling the heaters. A is an ammeter for measuring the current in the compensating heater. B is a shunt, C a potential divider, 8 a double-throw switch, and D a potentiometer for measurement of electrical energy entering the calorimeter heaters. In the case of alternating current, B , C, and D may he replaced by an a. c. wattmeter, hut, sinre such an instrument of sufficient accuracy was not available, it was necessary to use direct current and the rather complicated auxiliary circuit described above. Figure 2 shows the calorimeter and the location of the thermocouples. Plates 2 and 3 are heated with two circular elements in each late, one inside the other. The outer element is used as a guar2ring permitting the adjustment of the temperature so that there is no lateral heat flow, and only the area covered by the inner heater is used in the calculation of thermal conductivity. There are thirteen thermocouples on each of these two plates, one in the center and four in each of three circles radiating outward. The outer thermocouples (2B and 3 B ) are not read in determining heat conductivity but are used to measure the temperature differential between the compensating and the calorimeter heaters. The remaining thermocouples are connected with the selective switch, SI, shown in Figure 3, which in turn is connected to a potentiometer and sensitive galvanometer provided for recording the average temperature of that portion of the 4 D E
by means of the double pole-double throw switch, Sa, so that an opposing electromotive force is set up between 2B and 2C. Then, if these temperatures are not equal, a deflection will he shown on the sensitive galvanometer, GP, when the switch is thrown connecting these couples. With this arrangement it is possible to control the temperature differential between the calorimeter and guard ring within 0.03' C. The elimination of lateral heat flow is t h e most important factor in determining the accuracy of the apparatus. It was found that a groove cut almost through the hot plates between the calorimeter and guard ring and packed withinsulating material was effective in decreasing this error. Even before this refinement was made, if t h e differential was as much as 0.3' C., t h e average error would still be within 5 per cent. Plates 1 and 4 are equipped with thermocouples in exactly the same manner as described for t h e hot plates 2 and 3. This permits the measurement of the temperature gradient through the rubber and from this the calculation of the conductivity constant. Several experiments were made t o determine possible sources of error and the extent of their elimination in the apparatus used. Previous workers ( I ) have commented on the effect of air films between t h e sample and metal plates on the conductivity constant. The error caused by such an air film n-ould decrease as the thickness of the test specimen was increased, but experiments illustrated in Figure 4 show that with t h e present apparatus t h e thickness of the test piece could be varied from 0.4 t o 2.0 cm. without affecting the conductivity constant. d L D C 3 E
D C 0 2 8
C D
L I C
D F
2-
plate covered by the inner heater. It has been mentioned that the outer thermocouples (2B and 3 B ) on the hot plates are used in determining the temperature differential between the two heating elements. This is done by connecting these couples with those from the outer edge of the calorimeter heaters (2C and 3C)
.I
WITH SELECTIVE SWITCH FIGURE3. COXSECTIONS
I n order t o check the apparatus in general, as well as the effect of air films, a n experiment was designed which would be entirely independent. A small spherical heater was cured into the center of a sphere of rubber 4 inches (10.2 em.) in diameter. The location of this heater and of the two series of thermocouples for measurement of the temperature gradient is shown in Figure 5 . The conductivity constant for a ten-volume zinc oxide compound as measured in this experiment checked results obtained with the flat plate tester almost exactly.
FORMULA The electric current has been used widely as a source of heat in determining t h e conductivity of metals and in a few cases for insulating materials (5,6). In the case of good conductors a long rod of the material can be wrapped in a n insulator and the factor of lateral flow of heat neglected. To apply this method to t h e present problem, the rod is compressed t o a flat disk and the following conditions apply: Let two sides of the rubber disk of thickness 1 be kept a t constant temperatures, T I and Tf, T Ibeing larger t h a n Tz. Heat flo~vsthrough the sample from the higher t o the lower temperature. The quantity of heat, H , passing through any area, A , is proportional t o this area, t o the temperature gradient, and to the time, t , during which the heat flows. Thus we obtain: C.kLCUL.4TION
FIGURE2. CALORIMETER AND LOCATION OF THERMOCOUPLES
Vol. 26, No. 3
OF WORKINQ
K = a proportionnlity fticbor depending upon plate iuaterial. K is numerically eqiial t,o tlie lieat t.ransfcrrd i n iiriil time tliroiigli unit area of a plate of unit tlrickncss, if iiriil difyerericc oi temperature is maintained lietwipen its two faces, I! is equal to the prodiict of the number of watt,s, & I , applied to the heating cleiiients and the nimrlxr of calories irliiclr art equiralent tu one watt second or 0.238!). 'The working foriniila thus Iicconies: wlicre
1tii;ULTS
\viliiams 113s siiua.11 that the condiiotirity of a rulhrr cotiipound is t,lie sum [if tlie conductivities of its ingredients, each multiplied ti?. its vrdutiie per cent in the compoond. This means that the curve of volume per cent ~iigmentplotted against lieat conductivity is a straight line between tlie conductivity of robber and the value for the pigment iaed. Figure 6 sliows these curves for soveral zinc oxides. In Tables I and 11 tlie values for the conductivity of the diforcnt pigments were obt,aincd bv extraidation of curvcs similar to those in Figure F. Zinc oxides A, B. and 1) in Table 1 are fast-ciirinp oxides nith particle sizes of 0.10, 0.40, and 0.20 niicruii, respcct,ivcly; I" is t,lie same t,ype of oxide but is above the pippient range in particle size. It is apparent that particle size, xit,iiin tlie pigncnt range, has 110 effect on thermal ixinductivit~yhit that very coarse oxides have high conduet,ivities. Zinc oxide E is similar t,o 1) but is surface-treated for improved mixing properties; this lias no effect on its tliermal conductivity. The remaining nxidrs in Table I are of the slow eiiring variety with i m t i c k sizes lietmcn 0.30 and 0.40 micron exceirt for the last one wliicli is c o m p o s e d almost e n t i r e l y of acicular particles. The rcsiilts show that the slow ciwing oxides haw considerably higher condoctivities than fast curingoxides of correspondiiig particle size. This Ieitvcs chcmical co1riiJosition and i,article strape ab the diliore m i s iii these two tviicsoi ziiic oxide which could account for
CdBrie xilo
dire
10 :10 4s
8.8 22.8 8n.6
611
37.0
70 10 90 4.5
au in Bile
10 8n 45
en
70 eke
"
30
40.3 8.8 za . 8
SO.6
37. u 40.3 R.8
?A. 8
dn.6
37.u 40.3 22.8
n.onn4:is
u.onu843 0.00oi2i
n.onn7xo
u.uons71 0. 0ci ?orit suiiu,. 0.9 Per miit ruiiar.
Table 11 slioivs the results ihtained fur nilher and several ~r,tiipi~iinding ingredicnt,s. The figiircs given are not equivasliow as lent t o the eondoctiuity which the pigments w?;l~uld iiowlers in air but arc'siiitahle hor use in tlie calculation of the tliermal conductivities of rubber compoiinrls. Brown and Fiirna (9) have measured the thermal condiietivity of ~ X J W -
306
INDUSTRIAL AND ENGINEERING
CHEMISTRY
Vol. 26, No. 3
cr2 T center = 6K
.ooic
where c = rate at which heat is generated r = radius of sphere K = thermal conductivity of compound Ts = outside temperature
.ooos OOOE
+ Ts
Compounds of t h e type of A and B below have been used for a number of thermal tests in the writer's laboratory:
k 1 I-
U ,0007 0
Rubber
U
Slow curing zinc oxide
.OOOf -I SIZE
,0005
0 A
Fast curing zinc oxide Sulfur Carbon black Diphenylguanidine
A 500 750
... 20 4 r,
10
B
c
500
...
500 50
20
45
20 263
10
10
750
...
FAST CURING COARSE SLOWCURING ACICULAR
ACROSS G R A I N
K for compound A is 0.00065, for B, 0.00058; if most of the zinc oxide is replaced by an equal volume of carbon black as in C, K becomes 0.000405. If t h e time is taken in minutes, ,0003 these values must be multiplied by 60. IO 20 30 40 50 If i t is assumed that heat is generated V O L U M E P E R C E N T OF P I G M E N T throughout t h e sphere at the rate of one caloric FIGURE 6. HEATCONDUCTIVITY VS. VOLUME P E R CENT PIGhIENT F O R SEVERALZINC OXIDES per minute, t h a t t h e radius of the sphere is 5 cm. and the outside temnerature 67" C.. t h e dered ferric oxide and report values varying from 0.00092 equilibriuni temperatures a t t h e centers of the above comcalorie at 125" C. t o 0.00133 at 700", while Bidwell (2) work- pounds are 175' C. for A, 187' for B , and 235' for C. The ing with a rod of fused iron oxide reported conductivities figure given above for the rate of heat generation was deranging from 0.00130 calorie a t 160" C. t o 0.00255 a t '700"; rived from the time-temperature curve in a test using the the value obtained i n t h e present investigation was 0.00132 high-zinc compound, A. If the early part of this curve is calorie. taken, the temperature difference between the inside and outAs stated by Williams, no difierence was found for smoked side of the rubber is small, and consequently the amount of sheet, pale crepe, cured or uncured rubber. The values for heat conducted away is also small. Knowing the specific zinc oxide, carbon black, iron oxide, lithopone, blanc fixe, gravity and specific heat of the compound, the rate of heat and whiting checked closely. The conductivities obtained generation necessary t o produce the rise in temperature obfor antimony sulfide, sulfur, and talc are somewhat higher tained may be readily calculated. The largest error in the than t h e recorded values, while t h e values for clay and mag- above calculation must be t h a t of assuming the same heat nesium carbonate are almost exactly reversed. In this in- generation for fast and slow curing zinc oxides and for carbon vestigation three clays of varying properties were tested, but black, but a t least the temperatures obtained are compatible all were within a few points of the value given above for Dixie with experiments involving these fsctors. clay. No difference was found in the conductivities of fast and slow curing carbon blacks, but lampblack had an in, LITER.4TURE CITED creased conductivity while graphite was still higher. Y
SLOWCURING
PARTICLE
SIZE 3 5 ~
,0004
IMPORTASCE OF THERMAL COSDUCTIT-ITY DATA I n order to discuss the significance of the data on thermal conductivity and its importance in t h e broader problem of the thermal properties of rubber, it is necessary t o make assumptions regarding t h e heat generation of a rubber compound under some particular set of conditions. For instance, the equilibrium temperature at the center of a sphere when heat is generated at a constant rate throughout the sphere may be expressed by the equation:
IntenEXPLOITATION OF ITALY'S OIL RESOURCES PLANNED.
sive exploitation of Italy's oil resources is planned for the next five years, according to a report from the American consulate,
Genoa. All activities in connection with petroleum roduction and distribution, the report shows, are carried on bye!lt Azienda Generale Italiana Petroli, 60 per cent of whose stock was owned originally by the Italian Government. During the three years ended June 1933, this corporation was granted an annual government subsidy of 6,000,000 lire to carry on its researches. In August the government authorized it to continue exploring and drilling activities for a period of five years and appropriated for this purpose a total of 90 million lire. According to a recent report of the A. G. I. P., development of Italy's petroleum resources has thus far been encouraging. The company has not only peifected and consolidated its technical
Anonymous, J . Inst. Elec. Engrs. (London), 68, 1313 (1930). Bidwell, Phus. Rev., 10, 756 (1917). Brown and Furnas, Trans. Am. Inst. Chem. Engrs., 18, 309 (1926). Cotton, Inst. Rubber Ind. Trans., 7, 209 (1931-32). Hartman, Westmont, and Wienland, Pioc.Am. Soc. Teding Malerials, 28,Pt. 2, 820 (1928). (6) Heilman, Trans. Am. Inst. C h m . Engrs., 18, 283 (1926). (7) Somerville, Rubber Age (N. Y . ) ,9, 131 (1921). (8) Williams, IND.ENO.CHEM.,15, 154 (1923). (1) (2) (3) (4) (5)
RECEIVEDSeptember 20, 1933. Presented before the Division of Rubber Chemistry a t the 86th Meeting of the American Chemical Society, Chicaao, I l l , September 10 to 15, 1933.
organization for executing the vast and difficult task, but it has made detailed studies in all sections of Italy where oil-bearing beds would be likely to exist. From the commencement of its explorations until June 30, 1933, the A. G. I. P. had perforated one hundred twelve wells, of which fifty-nine have been abandoned. Thirty-seven wells are being worked, and sixteen are either in process of being installed with equipment or are being sunk to further depths. During the fiscal year 1932-33 the output of petroleum amounted to 2722 metric tons, compared with 2310 tons in the preceding fiscal period. The A. G. I. P. plans t o extend-its fields of explorations, hitherto confined almost entirely to the foothills of Emilia, t o a good part of the Padana Valley, the A ennine-Adriatic Coast, and to vast sections of south Italy and iicily. It is calculated that a total of ninety borings will be made, some of very great depths.
