Electrical Conductivitv of RubberCarbon Black Vulcanizates B. B. S. T. BOONSTRA AND ELI M. DWNEYBERG Research and Development Laboratories, Godfrey L . Cabot, Znc., Cambridge, Mass.
C
ONDUCTIVITY of rubber-carbon black compounds has been the object of intensive study in the last 10 years (1-10). This interest has originated partly from the importance of such practical applications as antistatic tires, belts, and electric cables, and also from the fundamental scientific interrelationships of conductivity with carbon black dispersion and reinforcement. Conductivity is usually expressed in terms of its reciprocal, resistivity, the units of which are ohm-cm. Many of the older measurements have been unreliable because of the difficulty in making complete contact between the rubber sample and the electrodes, a situation which gives rise to an additional contact resistance. Errors are also introduced because of the nonhomogeneous current distribution through the surface and volume of the sample. These factors have resulted in resistivity nieasurementa which have been contradictory and lacking in reproducibility, making theoretical explanations extremely difficult. It is now generally accepted that the electric current is carried by conducting paths formed by continuous chains of caybon particles which form a more or less persistent structure in the rubber. This structure is destroyed partly by deformations and, with time, the destroyed structure reforms partly, the rate of reformation increasing with temperature. The type of carbon black used and its degree of dispersion (3-9)appear to be of great influence on the electrical conductivity of the compound, Carbon blacks of small particle size or a high degree of permanent "structure" generally give the low resistivity values. Increased degree of dispersion over a certain minimum will yield higher values. There are, however, other factors which are of importance to the understanding of the phenomena involved in electric conductivity. It is the object of the following study to compare various types of carbon blacks as to the resistivities they impart to rubber stocks and the changes in these resistivities with various parameters. One of the properties which is most important for practical applications is the retention of the conductivity during and after deformation of the conductive rubber sample. It will be shown that in natural rubber this property is primarily a function of the type of carbon black used in compounding. I n Figure 1 an arbitrary classification for practical purposes is shown.
freshly sanded brass electrodes. After curing, a strong bond (circa 600 pounds per square inch) between brass and rubber was formed (for natural rubber and GR-S stocks). It is generally conceded that after curing no measurable contact resistance between the brass and the rubber compound exists. However, these experiments have indicated that resistivity is still dependent on the dimensions of the test specimen. No satisfactory explanation of this effect is apparent. To minimize this effect, the same sample size (1 X 1/4 X 3l/4 inches) was always used and the same procedure followed in molding. This procedure is especially appropriate for measuring the low resistivity values of socalled highly conductive rubbers. For the study of the effect of compression on retention of conductivity, cylindrical specimens were used with brass disks molded on the top and bottom faces. Measurements of current flow were made with a sensitive ammeter at 1.5 volts for most specimens, at 20 volts for the more highly resistive samplps, and at higher voltages when the resistivity values ran extremely high. For the highest resistivity samples, a Leeds and Northrup micromicroampere amplifier with a sensitivity of ampere was used. All measurements, unless othemise stated, were made at 75" F. and 50% relative humidity. The blacks studied included the usual blacks recommended for the compounding of electrically conductive stocks. In addition, two newly developed oil-furnace blacks (Vulcan C and Vulcan SC) were studied. -4 list of the blacks used and their analytical properties are given in Table 11. The recipes used are listed in Table I. The properties of compounds containing 50 parts of black per 100 parts rubber in natural rubber and in GR-S 1500 are given in
MEASUREMENT OF SPECIFIC RESISTIVITY
COLDGR-S (Cure 60 minutes at 292O F.) GR-S 1500 100 Zinc oxide 3 1.5 Stearic acid 5 Paraflux 3 Circosol 2 X H 1 BLE powder 1.25 Santocure 1.75 Sulfur
The various methods of measurement of specific resistivity have been given ample attention the authors mentioned in the Introduction ( 1 , 2, 4-8, IO). I n order to avoid errors arising from the contact resistance between the rubber and externally applied electrodes, it was decided to use brass electrodes molded to the ends of the test specimen strips during vulcarhation. This method gives brass-rubber bonds of considerable strength and negligible contact resistance and is considered simple and reliable. In some cases, tests were also made using the electrode arrangement suggested by McKinney and Roth of the National Bureau of Standards ( 5 ) . Sometimes considerable difference was found between results from the two methods. These are believed to be due to differences in grain and surface effects especially measurable at higher resistivity values. Rubber strips were cured with their ends in contact with clean,
TABLE
I. FORMULATIOKS
NATURAL RUBBER (Cure 60 minutes at 280' F.) Smoked sheets Stearic acid Pine tar 600 Zinc oxide Agerite Hipar Sulfur Santocure
NEOPRENE (Cure 15 minutes a t 307' F.) Neoprene GN-A Accelerator 552 Neozone D Stearic acid XLC magnesia Heleozone Ciroo LP oil Zinc oxide XX-78 Permalux
218
Parts 100
3 3 5 1 2.5 0.5 (1.0for .Spheron N )
100 0 26
16 0 7
4
3
12 5 0.5
219
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1954
TABLE 11.
PROPERTIES O F C A R B O S BLACKS AT 60-PART LOADINGS I1\' ru'ATUR.4L RUBBER AND GR-S
Av. Palticle Diam., mp
Spheron N Shawinigan Vulcan 3 Vulcan C Vulcan SC Treated' Spheron 4)
S heron N SEawinigan Vulcan 3 Vulcan C Vulcan SC Treated Spheron 4
}
Spheron N Shawinigan Vulcan 3 Vulcan C Vulcan SC Treated Spheron 41 a
Iodine Absorption Surface Area NigromSq. M./ eter Gram Scale pH
Classification ANALYTICAL PROPERTIES Conductive channel 16 350 (approx.) Acetylene 55 70 High abrasion furnace 28 65 Conductive furnace 26 130 Superconductive furnace 23 200 Treated 24 160
72 5
5 0
92 90 87
5-6 9 0 9 0 8 5 9 5
85
82
NATURAL RUBBERPROPERTIES (Cure 60 minutes a t 280° F.) Modulus 300%, Tensile, ElongaHardReLb./Sq. Lb./Sq. tion, ness bound, Inch Inch % (Shore .4) % 1430 5000 620 75 .. 1850 3500 510 68 74 2300 3900 470 71 70 2350 4100 510 72 64 2000 4200 520 71 63 1080 4150 600 69 61 COLDGR-S PROPER TIE^ (Cure 60 minutes a t 292' F.) 720 4150 680 1500 2700 610 3800 530 1800 1750 4000 560 1700 3950 520 960 3050 590
75 68 66 70 70 67
..
BO 58 53 51 50
Heated 2 hours a t 1000' C.
Table 11. All carbon blacks were used in pelletized form except Vulcan SC and acetylene black, which were fluffy. PROCESSING VARIABLES AND RESISTIVITY
The effect of processing variables on resistivity was studied using the Vulcan C and Vulcan SC mixes. There are three parameters that have to be investigated to ensure reproducible preparation of samples. Two of these variables influence the degree of dispersion of black in rubber. The third variable is the pressure under which the samples are molded. The effects studied included time of mixing of the carbon-rubber compound, viscosity of the raw rubber, molding pressure, and time of cure. MIXINGTIME. It is well known and substantially shown by experiments on high abrasion furnace blacks in regular GR-S (Dannenberg, 3) that the mixing time has a considerable influence on the resistivity values obtained for the compound. Better dispersion results in higher resistivity. Electrical resistivity seems to be much more sensitive to dispersion than other physical properties. The small changes in physical properties due to dispersion effects are accompanied by large changes in electrical resistivity. The effect on electrical properties of varying the laboratory Banbury mixing times from 35 seconds to 8 minutes is shown in Table 111. These experiments were performed with the more conductive furnace blacks in natural rubber and cold GR-S. All ingredients were mixed in first, and the black added last. With the 4- to &minute natural rubber mixes the sulfur and accelerator were added in the last minute of black mixing to prevent scorching. The results indicate that with Vulcan SC there is a three- to fivefold increase in resistivity from the shortest to the longest mixing time in either rubber. With Vulcan C in natural rubber about the same change occurred, but Vulcan C in cold rubber gave about a tenfold increase. This effect occurs over a sixteenfold
t
8-l
-2 -4]
METALS
-6 Figure 1. Classification of Rubber Compounds According to Resistivity
change in mixing times, such as would not easily occur in practice. A more reasonable range of practical mixing times might be 4 to 8 minutes, which would give about a twofold variation in resistivity values. In GR-S, Vulcan SC gave less variation in resistivity values compared with Vulcan C. Comparatively speaking, this difference in resistivity due to increased mixing time is very small. I t does not compare with the hugedifferences reported by Dannenberg for Vulcan 3 (high abrasion furnace) in regular GR-S (3), where changes in resistivity by a factor of 500 were observed. Therefore, there is little danger in the case of Vulcan C and SC that these effects would interfere with the reproducibility of the test. VISCOSITY OF RAWRUBBER. The dispersion of carbon black in rubber is caused by the shearing stresses in the rubber which, in turn, are brought about by the mechanical action of the rotors in Banbury or roll mill mixing. A stiff rubber will set up higher shearing stresses in its interior than a soft rubber, so the dispersing action is stronger in the stiff rubber and this might result in differences in resistivity in stocks which have otherwise undergone identical treatment. To study this effect, natural rubber was premasticated to three different Mooney viscosities, 93, 63, and 34. From each of these three different lots, two compounds were made containing 50 parts of Vulcan C and Vulcan SC, respectively, using the same Banbury mixing procedure. Molded-
TABLE 111. EFFECTOF BANBCRY MIXINGTIMEON RESISTIVITY Mixing Time 35 1.25 4 8 See. Mip. Min. hlin. -Resistivity, ohm-cm.Natural rubber (cure 60 min. a t 280' F.) Vulcan SC Method Aa Method B Vulcan C Method A a Method B Cold GR-S 1500 (cure 60 min. a t 292' F.) Vulcan SC Method Aa Method B Vulcan C Method Aa Method B a I,
20 13
22 15
38 23
10.3 41
36 31
36 22
47 24
53 19
20 21
28 30
50 49
78 59
60 58
85 79
260 264
550 380
Brass electrodes molded in rubber strip. National Bureau of Standards method (6)as control.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 46, No. 1
t IO'
IO'
i
U
3
io5
6
IO2
102
IO
IO
IO
20
40
60
I
80
PARTS BLACK/100 PARTS RUBBER
Figure 2. Effect of Loading on Resistivity of Natural Rubber Compounds
Figure 3.
I
I
60
80
Effect of Loading on Resistivity of Cold Rubber Compounds
effect on resistivity is expected from the normal viscosity variations encountered in standardized laboratory mixing procedures. MOLDING PRESSURE. The pressure used in molding rubbercarbon black compounds is known to affect electrical resistivity (2), Insufficient molding pressure results in poor filling out of the mold, whereas too high a pressure causes the sample to expand on release of pressure and this deformation causes a considerable rise in resistivity. At medium pressures there exists a rather flat minimum in the plot of resistivity versus molding pressure. The object of this study was to investigate the effect of a reasonable variation in molding pressure on resistivity values. Natural rubber compounds containing 50 parts of Vulcan SC and Vulcan C, respectively, were compounded using the usual procedure. Resistivity samples were prepared by molding to brass electrodes under four different pressures and curing for 60 minutes a t 280" F. The results obtained are shown in Table V. These results indicate that there is little effect of molding pressure on the Vulcan SC compounds. With Vulcan C there is a moderate increase in resistivity over the entire pressure range. Within the pressure range of 30 to 60 tons there is little danger of
t
IO'
IO'
i
il
I
IO 20 40 PARTS/100 PARTS RUBBER
IOS
102
IO IO 20 40 PARTS/lOO PARTS RUBBER
Figure 4.
60
00
Effect of Loading on Resistivity of Neoprene GN-A Compounds
in brass electrode samples were vulcanized for 30 and 60 minutes at 280" F. The results of resistivity determinations are shown in Table IV. The 60-minute cure gives the most reliable results and definitely shows a tendency towards lower resistivity values with lower Mooney viscosity. This is to be expected on the basis of lower degreeof dispersionobtainedin the lower viscosityrubber, resulting in a larger number of conducting carbon paths. However, the effect is not very striking consideringthat a threefold decrease in viscosity only causes a 35% decrease in resistivity. Since such large changes in viscosity are not common in a test laboratory, no I
TABLE IV. EFFECTOF POLYMER VISCOSITY ON RESISTIVITY
Vulcan C Cure 30 min. Cure 60 min. Vulcan SC Cure 30 min. Cure 60 min.
Mooney Viscosity 93 63 34 -Resistivity, ohm-cm.a t 280' F. a t 280' F.
155 85
70 70
90 55
at 280' F. at 280" F.
55 40
42 28
35 33
TABLEV. EFFECTOF MOLDING PRESSURE ON RESISTIVITY (15 X 15 Inch mold) Molding Pressure, Tons 30 60 100 Resistivity, ohm-om. Vulcan SC Sample a Sample b Vulcan C
34 42 100
34 62 123
33 57 164
180 34 58 185
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1954
22 1
TABLE VI. EFFECT OF LOADIKG ON RESISTIVITY Loading, Parts/100 Parts Rubber Metiod
A
Bb
x 10s ... ...
Spheron N Shawinigan Vulcan 3 Vulcan C Vulcan SC Heat-treated Spheron 4
4 . 3 x 10s 3 . 0 X 1010 2 x 10'0 10 x 10'0 1 . 6 X 10'2
,..
510 'X' 10'2
Spheron N Shawinigan Vuloan 3 Vuloan C Vulcan SC Heat-treated Spheron 4
...
88 73 99 111 98 119
...
... ... I . .
...
1"
I"
Method
4.4
... x
x x x
x x
Method
Aa
2600 7 x 10'0 24 X 10'0 3 . 4 x 106 0 . 5 x 108 0 . 8 x 10s
10'20
10'20 10'2C 10'2C
1012; 10'2
... ,.. I
.
.
...
fin
rn
on
in 1"
Resistivity, ohm-om. Metlyd Aletpd
A
B
Natural Rubber 161
...
28,000 4,600 15,000 GR-S 1500 40,000 67 X 10'2c 85 X 10'2O 34 X 10'2O 37 x 10'20
...
110 2120 7800 244 116 380
196 1 . 3 X 1od 40 X 10s 2490 480 7000
Seoprene GN-.I Spheron N 0 . 7 x 1010 1 . 0 x 109 6 X 10'0 1 . 5 X 108 790 0.34 X l O Q 6 x 1019 0 . 3 x 108 4400 Shawinigan 0 . 7 X 1010 0 . 7 x 109 4 X 10'0 0 . 9 X 108 Vuloan 3 o 7 x 1010 0 . 9 X 108 1 . 1 x 109 4 x 1010 2160 Vuloan C 0 . 7 x 1010 0.3 x 109 180 Vulcan SC 0 . 7 X 1010 0.8 X l o 9 0 . 9 X 10'0 0 . 3 X 109 Molded-in electrodes. National Bureau of Standards method (6). e Special set-up for high resistivity using Leeds & Northrup micromicroampere amplifier,
interfering with the reproducibility of the resistivity tests of Vulcan C and Vulcan SC compounds. I t seems that the lower the original conductivity value in these 50-part black loaded compounds, the less it is affected by outside variables. This will also be shown to hold for the resistivity changes accompanying deformation. The change in the mechanical properties of the rubber stocks as a function of milling time and Mooney viscosity is very small, compared with the changes in electrical resistivity. CURING TIME.The effect of curing time was investigated with compounds containing 50 parts by weight of Vulcan SC, Vulcan C, and Spheron N, and acetylene black in 100 parts of natural rubber, cold GR-S, and Neoprene GN-A. Five different cures were used, a definite undercure, an overcure, the optimum cure, and two cures over the optimum. The resistivity was measured by both molded-in electrode and the National Bureau of Standards methods to compensate for possible irregularities in one of the tests. The results show a general trend for the resistivity values of all samples to be high for the shorter curing times and to come down to more or less constant values when the conditions of optimum cure are approached. The over-all conclusion from these data is that significant differences in resistivity values do not occur with small variations in curing time in the region of optimum cure. Only in the case of the definitely undercured samples are the resistivity values considerably higher than those of the properly cured stocks. These observations are in agreement with the general relation between resistivity and time of cure found by McKinney and Roth ( 5 ) . LOADING
Loading studies using 10, 20, 40, 60, and 80 parts of black per 100 parts of rubber were made in natural rubber, cold rubber, and Keoprene type GN-A. In neoprene some difficulty was experienced with the curing of the compound to the brass electrodes. In cases where the adhesion was entirely insufficient, the end of the strip was covered with Aquadag and an electrode clip was placed over the aquadag. This procedure was necessary for only a few of the lower loadings in neoprene. The resistivities of the various loadings are plotted in semilogarithmic graphs, Figures 2, 3, and 4, except for values over 108. The complete data are given in Table VI. The reproducibility of the tests depends on the value of the resistivity. The lower values are more reproducible than the very high ones. Also,
Q?
V"
Metlpd
B
192 210 2150 93 56
81 204 0 . 1 x 106 2 . 3 X 108 1110 340 11,000
260 230 14,000 280
70
"I
Method Method
Aa
Bb
Method Method
Aa
Bb
36 118 403 35 21 40
22 35 132 31 11 23
22 25 335 29 14 13
18 9.1 100 37
56 3,400 10,700 183 51 218
62 116 4500 136 43 197
31 134 1000 76 22 38
28 165 870 100 27 31
56 43 730 138 16
45 54 205 73 25
38 24 450 21 10
46 14 170 22 19
5
14
-
agreement between values obtained by different methods agree better a t lower resistivity. The average of National Bureau of Standards and molded-in electrode results is plotted in Figures 2, 3, and 4. In general, the plots all have the same appearance. Even though the resistivity is plotted logarithmically, there is a tremendous rise when the loading is decreased, and it is in the intermediate loading range that the electrical properties of the various blacks show their most striking differences. NATURAL RUBBER. At a loading of 80 parts, the resistivities of acetylene black and Spheron S vulcanizates are practically the same at about 25 ohm-cm. A4ta 40-part loading, Spheron N is still low (100 ohm-cm.), whereas acetylene black has risen to 2000. At 20 parts the figures are 2600 and 7 X 1O1Oor a factor of 3 X l o 7 difference. Even at a 10-part loading, Spheron N has a reasonable resistivity-i.e., circa 4 X lo6 ohm-cm. Acetylene black a t this loading has about the same resistivity (of about lolo) as its 20-part compound. This phenomenon was observed in a number of instances and, in some cases, the resistivity of the 10part compound was found even lower than for the 20-part compound. The values for the 20-part compound are believed to be the more reliable since, with only 10 parts, dispersion may be of a lower order and resistivity, therefore, not reproducible. A few fortuitous conductive paths then will make a large difference in resistivity. The comparatively low resistivity of the IO-part Spheron S compound may be due to its extremely fine particle size. Small particle size means a large number of particles per gram so that, even a t a loading as low as 10 parts per 100 of rubber, there may be a sufficient number of particles to form some conductive paths. Similar observations were made by Wack, Anthony, and Guth (IO). All the other blacks show much higher resistivities a t the low loadings than Spheron N. Vulcan SC imparts lowest resistivity at loadings higher than 40 parts. Between 20 and 40 parts it is still lower than acetylene black but higher than Spheron N. All blacks except Spheron N at a loading lower than 20 parts show resistivity values not much below the value of the pure gum stock. At the high loading side of the curve, it appears that the decrease in resistivity is comparatively small when the loading is raised from 60 to 80 parts. Rubberlike properties change considerably in this range and 50-part loading is about the highest that combines high elasticity with high electrical conductance. GR-S. In cold rubber the same general picture shows up a8 in
222
INDUSTRIAL AND ENGINEERING CHEMISTRY
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I
I
I
I
Voi. 46, No. 1
COLD RUBBER STOCKS
400
4
300
:
0
i
c
E 2
200
Mm
100
L
50
0
VULCAN' S C J I I
20
40
60 80 TEMPERATURE, C.
I
I
100
120
20
40
60
80
TEMPERATURE,
100 O
120
C.
Figure 5. Effect of T e m p e r a t u r e on Resistivity of N a t u r a l Rubber Stocks
Figure 6 . Effect of T e m p e r a t u r e on Resistivity- of Cold Rubber Stocks
natural rubber, except for the fact that the resistivity level is higher over the whole range of loadings. As a result, even with Spheron N no stock can be made up at a 10-part loading in cold GR-S with a resistivity lower than 106 ohm-cm. At a loading of 20 parts, only Spheron N gives a stock well below this figure. At 40-part loading acetylene black and the high abrasion furnace blacks still have resistivities over 500,000 ohm-em. These blacks form superconductive compounds only a t high loadings of 80 parts. The cold GR-S stocks differ from the natural rubber stacks in that the curves do not level off so much a t loadings over 60 parts, as is the case with the natural rubber. NEOPRENE. In Keoprene GN-A, the resistivity level is much the same as in natural rubber, except for the fact that the Spheron N stock is no longer the most conductive at lower loadings as it was in natural rubber and in GR-S. In Neoprene, Vulcan SC shows the lowest resistivity over the whole range of 30- to SO-part loadings. All blacks have useful resistivities at 60-part loading and, even at 40 parts, all can still be considered conductive, though high abrasion furnace is very near the borderline of 1,OOO,OOOohm-cm. At 20 parts, none of the blacks yields stocks with a resistivity below that value.
In the present study, the resistivity of a number of compounds with 50 parts of black per 100 parts of rubber was measured at temperatures ranging from 20" to 120" C. The resistivity was first measured at the highest temperature. The sample was then cooled down to 20" C., the resistivity measured again, the sample heated to the next lower temperature and measurements recorded, the sample cooled t p room temperature again, etc. Thus, between every two readings at succeeding temperatures, a room temperature measurement was taken. The samples were kept for 40 minutes at the desired temperature before the resistivity measurement was made. The curves obtained are shown in Figures 5 and 6. If a stable equilibrium was reached after heating to 120" C., the 20" C. readings should remain constant throughout the test. In some cases, however, the room temperature control readings taken between those at elevated temperatures show a tendency to decrease when the elevated temperatures are as low as 40' C. This may indicate that no definite equilibrium was reached at the first heating and that the readings at these high temperatures are still somewhat too high. In the natural rubber compounds this was noted with acetylene black and to a lesser degree with Spheron C. In these cases the temperature coefficient is probably more negative than actually wag found. The most consistent results were obtained with Vulcan SC, Spheron N, and Vulcan C compounds. The first shows the lowest resistivity and practically no change in resistivity over the whole temperature range. Spheron N has a slightly negative temperature coefficient between 80" and 120" C. and the results show more spreading than with Vulcan SC. Vulcan C gives reproducible results and small change in resistivity with temperature except above 100" C. where the temperature coefficient becomes slightly negative. The temperature coefficients in cold GR-S of even the low resistivity stocks containing Vulcan SC and Spheron N show some change with temperature. With acetylene black the temperature coefficient in GR-S is less negative than in natural rubber. There is no satisfactory explanation for the behavior observed with the different compounds. The temperature coefficient of
TEMPERATURE
The influence of temperature has been investigated by others. Newton (6) obtained results showing a negative temperature coefficientof resistivity of up to 5%. Wack, Anthony, and Guth (IO)report a negative temperature coefficient for loadings of channel black in GR-S below about 30 volume % and a positive one for loadings of 40 volume %. In natural rubber they found the temperature coefficient to be negative for all channel black loadings they tested. In the case of channel black in both natural rubber and GR-S, they found that the lower the loading the more negative the temperature coefficient. Acetylene black had a positive temperature coefficient at all loadings in both natural rubber and GR-S. The temperature coefficient of the dry blacks under 2000 pounds per square inch pressure was found negative.
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1954
the dry compressed black is negative and, if this were the determining factor, all compounds should exhibit a decrease in resistivity with increasing temperature. Even in the dry black the resistance consists primarily of a contact resistance between conducting aggregates or units forming chains through which the current is flowing. In rubber, carbon black at moderate loadings differs in two respects from dry compressed black. The number of paths passing through 1 square cm. is much smaller than in dry black. Also, the particles are imbedded in a medium with a high resistivity and where there is the possibility of displacement of the particles because of the thermal movement of rubber chain segments. The decrease in resistivity for the acetylene-natural rubber compound averaged between 40' and 120" C. is 0.62 ohm-cm. per ' C., or 0.2% per C. For the dry acetylene black this decrease is only 0.06% per ' C. (IO), so the 50-part compound changes more in conductivity than the dry black. The decrease for the Spheron N-natural rubber compound appears to be 0.5% per 'C. and that for the dry bIackO.21% per O C. Though the coefficients for dry black and rubber compound are of the =me order of magnitude, the difference still indicates different mechanisms for the change in conductivity. In GR-S the temperature coefficients for acetylene and Spheron N compounds (60-part loading) were 0.84 and 0.5, respectively, and, in this case, there is even a greater difference from the dry blacks. The observations of Wack, Snthony, and Guth that temperature coefficients of stocks with low loadings of acetylene and conductive channel blacks have lower (in absolute sense) temperature coefficients than highly loaded stocks indicate that there are a t least two opposing factors involved. One of these may be the resistivity of the matrix and the resistivity of the dry black which will decrease at higher temperature, and, opposing this, is the constant breaking and repairing of the conductive structure which will increase resistivity a t increasing temperature. The conductivity of the dry blacks is very much dependent on the degree of compression or apparent density, so that it is not possible to correlate it directly to the resistivity of the rubber compound. Table VI1 shows data comparing the resistivity of compressed dry black with the values obtained in rubber for a group of blacks. Such parallel behavior of rubber and dry black resistivity does not hold for blacks with different particle sizes. This may be illustrated in Table VII.
223
100,000
5Y
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f
0
O
TABLE VII.
1
1 -;-
R ~ l i j Y A F T E j E( L O N T O N
1
10 50
IO0
I50
ELONGATION, %
Figure 'I. Resistivity as Function of Elongation for Natural Rubber Vulcanizates
rium proceeds more rapidly at higher temperature, and generally these phenomena fit in with the picture of structure breakage and reformation. 4 test specimen with molded-in electrodes is particularly suitable for deformation studies, since the resistance can be followed during the deformation process. All deformation tests were done with natural rubber compounds containing 50 parts of black. The following three types of deformation were studied:
1. Slow elongation (250% per minute) of strip samples in the Scott tensile tester. Determination of resistivity in elongated condition and after release. 2. Repeated rapid elongation to about 25% of 8-inch strips on the De Mattia tester (7500'% per minute). Measurement during deformation and 1 minute after application of a definite number of extensions. RESISTIVITIES OF DRYBLACK AND CORRESPONDIXG 3. Repeated compression in the Goodrich Flexometer to RUBBERCOMPOCNDS about 67% on Flexometer samples cured with brass bottom and top electrodes at the frequency of the Goodrich Flexometer (30 (Apparent density of blacks, 0.52 gram/cc.) cycles per second). This means an average rate of deformation Resistivity a t of 54000% per minute. Particle 50-Part Loading in Diam.,
Black
Sterling S Sterling V Sterling SO Vuloan 3 Vuloan C Spheron 4 Heat-treated Spheron4 Spheron N Spheron C Vulcan SC hetyleneApparent density 0 . 7 5 Apparent density 0 . 6 4
}
~
100
~~~
P'Y
ma
Resistivity
80 50
2.05 0.8* 0.8 0.95 0.54 9.1
40 28 26 24 24 16 20 23 55 grem/oc. gram/cc.
0.3
Natural Rubber, Ohm-Cm. 1000
x
106
100 x 108 0.5 X 108 2500 80 0.2 x 106 30
1.4
90
2.2 0.7 03
1000 30
80
~
DEFORMATION
Earlier work has repeatedly described the changes of conductivity of carbon black-rubber vulcanieates on flexing, bending, elongation, or compression ( I , 2, 4, 6 ) . With such deformations the resistivity increases to high values and decreases slowly as won as conditions are stationary again. The return to equilib-
ELONGATION IN SCOTTTENSILE TESTER.The technique followed was to elongate the strip in steps of 25%, retracting after each elongation. Measurements were taken 15 seconds after reaching the required elongation and 15 seconds after return to the unstretched state on retraction. In this way, two curves were obtained, one for the resistivity in the elongated state and one for resistivity after retraction, both as functions of elongation. The first curve in all cases shows a maximum for all compounds; however, the elongation a t which the maximum occurs is different for the various blacks. (Practically the same curve is found when the elongation is not carried out in steps followed by retractions but continued without interruption.) For a number of black stocks these curves are shown in Figures 7 and 8. It also appears from the curves of Figures 7 and 8 that the resistivity after retraction is in all cases going up with increasing elongation, which is obviously due to increasing breakdown of conductive structure by the increasing stresses the rubber undergoes. The most desirable properties for a conductive black compound are low resistivity value and small change of resistivity with
INDUSTRIAL AND ENGINEERING CHEMISTRY
224 100,000
small increase), but there is comparatively little change in resistance in cases where a pronounced maximum in the resistivity is observed. This would indicate that after the first breakdown of chains there remains a more permanent structure which maintains about the same resistance during further elongation. The specific resistance then only changes because of change of dimensions of the specimen, which is proportional to l/a2( a = extension ratio). It would certainly be interesting to follow the resistivity up t o the breaking point; the present technique, however, does not allow elongations over 200%. It would be expected that the decrease in resistivity will not persist to the breaking point but will reach a minimum and rise quickly with elongation until rupture occurs.
---
I I RESISTIVITY IN ELONGATED STATE RESISTIVITY AFTER ELONGATION
10,000
i Y
I
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Vol. 46, No. 1
1,000
f flL
TABLEVIII. RESISTAXCE AND RESISTIVITY OF VULCAN SCNATURALRUBBERCOMPOUNDS AS FUXCTIOKS O F ELONGATION
100
Elongation
IO
! 0
I
I
1
I
50 100 ELONGATION, %
I 1
I
Resistance, Ohms
n
570
6 25 50 75
2800 9500 12500 13300 12500 13000
100
150
125
Resistivity, Ohm-Cm. 60 230 550 500 400 280 230
Figure 8. Resistivity as Function of Elongation for Natural Rubber Vulcanizates
elongation. As shown in Figure 7, some of the black compounds do give a low initial resistivity but a t relatively small elongations the resistivity value ( R , value) increases rapidly. This is the case with the Spheron C vulcanizate. Starting at 350 ohm-em., it reaches a maximum of almost 108 at about 120% elongation. To a lesser degree this is also the case with Vulcan 3, still less with acetylene black, and least of all with Vulcan SC (Figure 8). In the same sequence, the maximum R, values decrease and the elongation at which the maxima occur decreases except for the heat-treated Spheron 4, which does not show a very pronounced maximum in the range of elongations tested. The stock containing Vulcan SC has a maximum resistivity of 470 ohm-em. a t an elongation of about 3575, so a t any elongation the R, value would be well under 1000 ohm-em. The resistivity measured after elongation passes slightly over 1000 ohm-em. at an elongation of about 110%. The Vulcan C compound, with initial resistivity of about 75 ohm-cm., reaches its maximum value of 1500 ohm-em. at about 40% elongation. In this respect it differs from acetylene black stocks which run up to about 3500 ohm-em. a t 60% elongation. The phenomenon of decrease of resistivity with increasing elongation after passing through a maximum has been observed for acetylene black by Wack, Anthony, and Guth. From the study presented in this paper it appears to be a general property of carbon black stocks a t the loading of 50 parts by weight. Wack, Anthony, and Guth (IO) explain the phenomenon by assuming breakdown of carbon chains a t the lower elongations and realignment a t the higher elongations, lowering resistivity in the direction of stress but increasing it in the direction perpendicular to the stress. It has to be considered that the dimensions of the sample change on elongation, so that when the same resistance was maintained between the electrodes during elongation the specific resistance, R,, would still decrease, since R, = R X areal length and AIL is steadily decreasing on extension inversely proportional to the square of the extension ratio. If the resistance, R, during the deformation for a Vulcan SC, for instance, is observed, i t appears that at elongations above 40% the resistance is practically constant, as shown in Table VIII. The values shown in Table VI11 were obtained on a particular sample, the curve in Figure 8 is made from an average of determinations for a number of different samples. For some of the blacks there seems to be a small decrease in resistance (for acetylene black a
1
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,500
0
E
Li3 P
. 100 50
4)
10
1000 PO00 4000
6000
8000
10,000
24
NUMBER OF CYCLES TIME AFTER TEST, HR.
Figure 9. Resistivity as Function of Number of Extensions to 25% Elongation
REPEATED RAPIDELONGATION ON DE MATTIATESTER.The repeated elongation study was carried out with all stocks except those containing Vulcan 3 and Spheron 4 (50-part loading in natural rubber). The resistivity strips were clamped in the DeMattia flexing machine in such a way that a maximum stretch of about 25% was obtained with a small degree of bending when the jaws of the machine were closest together a t every stroke. The number of strokes per minute was about 300 and the resistivity was tested during the process of repeated deformation and after 100, 200, 500, 1000, 2000, 5000, and 10,000 cycles. These measurements were taken about '/g to 1 minute after stopping deformation, but there were no signs of any major changes during that time. After the final deformation, readings were also taken after
January 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
1 hour and after 24 hours to follow the electrical conductance reACETYLENE BLACK covery. STOCK The results are represented in Figure 9, where resistivity (measured minute after stopping the machine) is plotted against the 0 total number of cycles of deformation. There was no pro-rl 100,000 nounced difference between the readings during deformation and - I those after the process. The reading during deformation cycles A ! could not be taken accurately, since the needle of the ammeter moves forward and backward following the extension and retraci tion of the specimen. Figure 9 shows that the Vulcan SC stock gave the best perform10,000 ance of the black stocks tested. Its resistivity does not rise f: very much over the original value after the first 100 deformation 5 cycles and, after about 1000 cycles, it has reached an equilibrium value which changes only slightly during continued operation. There is not much change in resistance with standing, and the 1,000 whole range of values under these conditions lies between 15 and 35 ohm-cm. REST REST ' Spheron C and K, the conducting channel blacks, are in a different class. The resistivity of their natural rubber compounds is originally reasonably low. However, a t the first few strokes, 100 this value jumps to up over 1000 ohm-cm. Between 2000 and 10,000 cycles the resistivity lingers around a constant value. After termination of the extensions and 24-hour recovery, the resisFigure 10. Resistivity as Function of Compression tivity value still has not dropped considerably. in Goodrich Flexometer The difference in behavior between the channel black compounds and those with furnace blacks is probably due to a different degree or type of interaction between rubber a nd black Samples of compounds containing 50 parts of Vulcan C and surface. This must influence the stability of the conductive Vulcan SC, respectively, were cured into strips with molded-in paths, making them more persistent in the case of the furnace electrodes and subjected to repeated flexing and elongation in the blacks. De Mattia flex cracking machine. The change in resistivity was Spheron N gives a large improvement over Spheron C, its level recorded, as previously described. The results of the resistivity being about halfway on the logarithmic scale between Spheron C tests can be recorded in graphs similar to Figure 9. These are and Vulcan C. summarized in Table IX. The constant resistivity level after 2000 flexes may be attribThe data for natural rubber reveal one remarkable feature. uted to a certain equilibrium which is set up within the test strip. Though there is a considerable rise in value after 2000 deformaIt seems that there is a more persistent structure which is untion cycles, the resistivity levels during deformation are more or broken while the temporary structure is being broken and repaired less in proportion to the original resistivities measured before deat the same rate. Even when the breaking down stops, the reformation. In other words, resistivity rises in all cases by about pairing goes on and resistivity values drop. the same factor. If the resistivity values in the original state and Since the deformation generates heat, the values after about after 2000 deformation cycles are compared, the factor, Fn = 2000 cycles are read a t a considerably higher temperature than Rzaoe/R,, ranges between 1.7 and 3.3 and does not show a signifithe original ones. The deviation from the room temperature cant change with time of milling. This is illustrated by Table value is considered to be too small to change the general picture. IX. DEGREEOF DISPERSION AND RETEXTION OF COXDUCTIVITY. In the GR-S compounds, however, the situation is different and This same type of deformation was also applied to a number of there is a constant rise in the deformation factor, F d with insamples representing Vulcan C and Vulcan SC in different decreased degree of dispersion. For Vulcan C, this effect is more grees of dispersion. pronounced than for Vulcan SC. With the Vulcan C compound, R2Mo/R,changes from 3 for the 0.5-minute mixed compound to 24 for the 8-minute mixed compound. With the Vulcan SC comTABLEIX. RETENTIOROF CONDUCTIVITY OF COKDUCTIVE pounds, the figures are 3.5 and 10. These data show, contrary t o BLACKCOMPOUXDS AS A FUNCTION OF BANBURY MIXINGTIME what might be expected, that in GR-S the stocks with the miniResistivity, Ohm-Cm. mum mixing show the best retention of conductivity and the Av. overmilled compounds have the poorest retention of conductivity. Mixing Original level Time in (before after Factor In natural rubber this effect is not found, which may be explained 2000 Fd = Banbury, deformaby the fact that in natural rubber longer milling causes more Min. tion) cycles Raow/Ro Cold rubber breakdown of the polymer and, therefore, less intensive dispersVulcan SC 0 5 20 75 3 7 ing action than in GR-S. 1.25 28 80 3 4 50 450 9 The samples used COMPRESSION IN GOODRICH FLEXOMETER. 8 80 800 10 in the compression test mere Goodrich flexometer test cylinders Vulcan C 0.5 60 180 3 1.25 85 750 9 (1 X 7/s inch) cured with a top and bottom electrode of brass. 4 260 4500 17 The resistivity measured on these cylinders before any deforma8 550 12000 24 Natural rubber tion started was always somewhat higher than found for strips of Vulcan SC 0.5 20 45 2.3 the same compound. 1.25 22 70 2.2 4 40 120 3 Compression was adjusted to 33%; the static load was just 8 105 210 2 enough to keep the sample from jumping. Leads were connected to top and bottom electrodes and the test was carried out in a way somewhat different from the previous tests. Resistance was measured under the following conditions:
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In original state Compressed 33% and the change with time followed for a t least l m i n u t e After release; change with time After 10 compression cycles and release; change with time During 1000 compression cycles After 1000 compression strokes, uncompressed; change with time for 4 minutes During 10,000 cycles After 10,000 cycles, uncompressed; change with time ( 4 t o 5 minutes) The course of the resistivity with time and with the compression cycles is represented for an acetylene black stock in Figure 10. Since it is not practical to plot the curves of all the various black stocks in one graph, comparison is made by picking out a few points on the curve of Figure 10. Point A . For the resistivity during deformation, the value in the third minute of the 10,000-cycleoperation. This is about the average of the resistivity values during this deformation except for the first peak. Point B. For the resistivity after a small number of deformations, the value at the third minute after the 1000-cycleperiod. Point C. For resistivity after a prolonged period of deformation, the value at the third minute after the 10,000-cycle period of deformation. It appears that the height to which the resistance rises a t the first compression is dependent on the speed a t which the sample is compressed. The quicker the deformation, the higher the peak, but also the quicker the decrease to lower values. This is illustrated in Table X by the values for Vulcan C. AS FCKCTION O F RATE O F COMPRESSION x. RESISTAKCE
TABLE
(Vulcan C) Original Compressed by hand in approx. ] / a see. After 1 min. in compressed state Released in see. After 1 min. in relaxed state Compressed in 1/10 see. After 1 min. in relaxed state Compressed in ‘/So sec. After 1000 compressions in ‘/z min. Sfter 1/2 min. of relaxation
Resistance, Ohms 240 7,000 3,400 20,000 3,500 20,000 2,000 35,000 8,000 1,500
Even when the sample is constantly being compressed and released in 1 / 3 0 of a second, as happens in the Goodrich flexometer, some sort of an equilibrium is reached after about 1000 to 2000 cycles where the resistivity has a well-defined value. A survey of the characteristic values is given in Table XI.
TABLEXI. CHARACTERISTIC RESISTIVITIESOF XATURAL RUBBERSTOCKSSUBJECTED TO DEFORMATION IN GOODRICH FLEXOMETER
Original 95 280 490 85 1600 280
Resistivity. Ohm-Cm. Point B Point C Point A (3 m n . (3 min. (during after 1000 after 10,000 opera: deforma- deformation) tions) tions) 175 125 860 1,850 1550 28,000 14,000 1,125 1400 180 85 860 10,800 3700 7,600 1,200 600 2,700
Apparently compression is a much more destructive type of deformation to the conductive structure than elongation. The recovery of conductivity after completion of the test is speeded up by the heat built up in the sample. The lowest resistivity values are exhibited by the stocks containing Vulcan SC and heattreated Spheron 4. Their resistivities do not exceed 1000 ohmmi. during deformation and the values taken 3 minutes after
Vol. 46, No. 1
deformation are in between 100 and 200 ohm-cm. With this rigorous type of deformation, there appears to be no consistent difference between channel and furnace black compounds. The conductive structure of acetylene black seems to be most sensitive to breakage but also reforms rather rapidly. Of the black compounds tested, those containing Vulcan SC showed a desirable combination of low resistivity and retention of resistivity under the various types of deformation. Actually, Vulcan SC had the lowest conductivity in the undeformed state of all the black compounds tested. In two of the three deformation tests it exhibited the best retention of conductivity of the series tested; in the third test it shared the first place with heat-treated Spheron 4. This means that Vulcan SC is the most appropriate black for applications of conductive rubber which must retain its conductivity under all types of deformation. The results in general tie in with the theory that conductivity is caused by conductive paths which comprise areas of high carbon black concentration. This structure is easily destroyed by deformation and rebuilt by aggregation of the particles. These conductive paths do not necessarily consist of pure carbon chains linked together by carbon-carbon contacts. One cc. of compound a t 50-part loading contains about 0.4 gram of black which may represent a surface of 400,000 square em. This enormous area, if thought of in the shape of plates of a capacitor, would have a resistance as low as 1000 ohms when the layer in between has the volume of about l cc. of rubber. It is possible that condenser-like units are linked in between carbon chains and the whole structure forms the conductive paths. From the elongation experiments, it would appear that there is a part of the structure which is not broken a t all. The compression experiments, however, indicate that the rise in resistivity at deformation and the drop in the value once a stationary condition is maintained are more pronounced the more rapidly the deformation takes place. This would mean that part of the structure, a t least, is very rapidly rebuilt and that there is, practically speaking, never a moment in which all the chains are broken a t the same time unless the compression is carried out with extremely high rapidity. .MECHANICAL PROPERTIES
A summary of the physical properties of the various black vulcanizates is given in Table XII.
TABLEMI. MECHANICAL PROPERTIESOF CARBONBLACKNATURAL RUBBERVULCANIZATES
Spheron N Spheron C Spheron 4 Acetylene Vulcan 3 Vulcan SC Heat-treated Spheron 4 Vulcan C
(50 parts per 100 parts rubber) Modulus Tensile Hard- Reais300%, Strength, Elonga- ness tivity, tion, (Shore OhmLb./Sq. Lb./Sq. Inch Inch % -4) Cm. 1430 5000 620 74 80-180 610 1200 4400 73 480 1220 610 4200 69 100,000 1850 510 3500 68 80-120 2300 2,000 470 71 3900 2000 4200 71 20-30 520 1080 25-40 4150 69 600
2350
4100
,510
72
60-100
Retention of Conductivity a t Deformation Fair Bad Bad Bad Bad Very good Fair
The figures of Table XI1 clearly indicate that no parallel can be drawn between resistivity retention and modulus. Vulcan SC, one of the lowest resistivity blacks, has compounds with one of the highest moduli. Even within the group of channel blacks no such relationship exists. Spheron N shows higher modulus compounds than Spheron C, but its conductivity and retention of conductivity are far better than those of Spheron C. So the authors cannot satisfactorily explain the different electrical behaviors of rubber stocks containing different types of black. There is only a slight relationship with dry black resistivity and no relationship with mechanical properties.
January 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY SUWWARY
Apart from the percentage of black in the compound, a number of factors were observed to have a decided influence on the resistivity of the compound. DISPERSION.Over a certain minimum degree of dispersion of the black in the rubber, resistivity increases with better dispersion (9). Softer polymers, which may give a lover degree of dispersion, make better conducting compounds which, however, may not retain much conductivity under deformation.
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retention of conductivity on deformation, and Spheron C, in turn, is better than Spheron 4 or 6 with a surface area of about 120 to 140 square meters per gram. This influence may be explained by the part the capacitor-type units play in the conductance of the current. For oil furnace blacks of about 25 millimicrons in diameter this effect is demonstrated in Figure 11. The surface area increase is due to internal surface porosity or surface roughness. STRCCTURE.The conductivity of acetylene black in rubber ia explained by the high degree of structure which it possesses in natural rubber, This structure, however, is easily destroyed, so that retention of conductivity is not very good. In tougher polymers, like cold GR-S, the structure is already partly destroyed during the regular mixing procedure, so that a much higher resistivity is found than in natural rubber. CONDUCTIVITY OF BLACK. Graphitization or heat treatment, which reduces the volatile matter in the case of channel black, decreases the resistivity of the dry black considerably but decrease8 the resistivity of the compound of black in rubber even more. This is shown by the heat treatment of Spheron 4,which results in a decrease in the resistivity of the natural rubber compound (50part loading) by a factor 3000, whereas the resistivity of the dry black goes down by a factor of 30. Usually two or more of these factors change at the same time, making it difficult to establish quantitative relationships. Considerably more study will be necessary to increase the knowledge of the mechanism of conductance in rubber compounds. ACKNOWLEJXM ENT
SURFACE ARFA SQ. MJGRAM’
The authors are indebted to W. D. Schaeffer and M. S. Polley for providing the data on dry black conductivity.
Figure 11. Resistivity as Function of Surface Area of Blacks
LITERATURE CITED
Natural rubber 50-pert loading
Blanchard, A. F., and Parkinson, D., Proc. 2nd Rubber Tecknol. Cony., 1948, 414.
Bulgin, D., Trans. Inst. Rubber Ind., 21, 188 (1945); Rubber Chem. and Technol., 19, 667 (1947).
STATE OF CURE. During vulcanization the resistivity shows a large decrease which is very steep in the early stages of the cure, leveling off after optimum cure has been obtained. The type of acceleration, amount of sulfur, and other ingredients used in the compound all influence the final resistivity of the vulcanizate. PARTICLE SIZE OF BLACK. Smaller particle size favors lower resistivity. The smaller particles arrange themselves more easily into chains than do the coarser types. SURFACE AREA. For the same type of black, channel black for instance, resistivity will decrease with increasing surface area. This factor, of course, is, in most cases, difficult to separate from particle size influence. Spheron S, with a surface area of 400 square meters per gram has a better conductivity than Spheron C, with an area of about 250 square meters per grams and also better
Dannenberg, E. M.,IXD.ENG.CHEM.,44, 813 (1952). Lane, K. A., and Gardner, E. R., Trans. Inst. Rubber Ind., 24, 70 (1948); Rubber Chem. and Technol., 22, 535 (1949).
LlcKinney, J. E., and Roth, F. L., IND. ENG.CHEY.,44, 159 (1952).
Newton, R. G., J . Rubber Research, 15, 35 (1946). Norman, R. H., J . Sci. Instr., 27, 200 (1950); Trans. Inst. Rubber Ind., 27, 276 (1951).
Sperberg, L. R., Popp, G. E., and Biard, C. C., Rubber Age, 67, 561 (1950).
Sperberg, L. R., Swetlik, J. ,.!I
and Bliss, L. A., IRD. ENG. CHEM.,41, 1641 (1949). Wack, P. E., Anthony, R . L., and Guth, E., J . A p p l . P h y s . , 18, 456 (1947). RECEIVED for review Kovernber 7, 1952. ACCEPTED October 10, 1B53. Presented a t the meeting of the Division of Rubber Chemistry of the AMERICAN CHEMICAL SOCIETY, Buffalo, N. Y., 1952.
