Carbon Black Differentiation by Electrical Resistance of Vulcanizates JOHN E. McKINNEY AND FRANK L. ROTH National Bureau of Standards, Washington 25, D . C.
A
COMMON method of testing for quality control or specifications of any ingredient used in preparing a rubber vulcanizate is t o compare stress-strain properties of a vulcanizate containing all standard ingredients with those of one which contains the ingredient under test in place of the corresponding standard. For carbon blacke, stress-strain measurements of vulcanizates often fail to distinguish between blacks that are different in many other properties. Thus, there is a need for a supplementary test which will be sensitive t o small variations in the properties of the black and which is sufficiently simple in procedure t o make it suitable for specification purposes or quality control of production. Many tests dealing with hysteresis loss, heat build-up, resilience, and abrasion resistance have been used in research studies, but most of these tests are cumbersome to conduct or are not highly sensitive t o differences between various blacks. Previous investigations (8, 4 ) have shown that the electrical resistivity of a carbon black vulcanizate of natural rubber or GR-Ssynthetic iubber is highly sensitive t o variations in the grade or type of black. It has also been shown (2,3) that the resistivity of vulcanizates containing blacks of the same type is a function of the size of the carbon black particles.
tive rubbers, i t was necessary to modify it to measure higher resistances. The modification consisted chiefly of replacing the milliammeter with a set of standard resistors and using a much more sensitive galvanometer in the null balance system. By measuring the potential drops across the standard resistor and across the specimen the resistance of the specimen was calculated in terms of the standard. By replacing the null system of potential measurement with an electrostatic voltmeter the apparatus became sufficiently simple to make it appear suitable for control measurements without loss in sensitivity or accuracy. D.C Source ol Potential Reference Realetors
Figure 2. Electrical Circuit Used for Resistivity Measurements
Figure 1. Electrode Assembly A. B. C. D., E.
Specimen Potential eleatbdee Current electrodes IO-pound weight 2-pound weight F. Insulating material G. Guide H. Insulation
The purpose of the present investigation is t o determine whether electrical resistivity tests of GR-Svulcanizates containing channel black or high abrasion furnace black (HAF) can be used t o supplement the stress-strain tests in quality control and specifications of these types of blacks. Since the resistivity tests are t o be used in conjunction with stress-strain tests, it is desirable to make all the tests on the standard tensile test sheet. The tentative method ( I ) given by the American Society for Testing Materials under Designation D 991-48T employs standard m i l e test sheets for specimens. Furthermore, the Celectrode specimen assembly offers the advantage that the contact resistance between the electrodes and the specimen are not measured as 8 part of the volume resistance of the spepimen. As the electrical circuit given in the ASTM method was designed for use with the relatively low resistances encountered in electrically conduc-
The results of the present investigation indicate that when careful control is exercised in preparing the specimens from certain G R S vulcanizates, the reproducibility of resistivity determinations combined with the large differences in resistivity with variations in the black make the resistivity test quite suitable for specification testing of channel and high abrasion furnace blacks APPARATUS
The electrode assembly used in this investigation resembles that described in ASTM Designation D 991-48T (I). Figure 1 shows a sectional sketch of the assembly. The electrical circuit found t o be most convenient for use with this electrode assembly is shown in Figure 2. In this circuit a reference resistor and the current electrodes of the specimen assembly are connected in series t o a source of direct current potential. By means of a double-pole, double-throw switch an electrostatic voltmeter can be connected across either the reference resistor or the portion of the specimen between the potential electrodes. A shielded electrostatic voltmeter with a range of 0 t o 150 volts and the source of direct current voltage shown in Figure 3 have been found t o operate satisfactorily in this circuit. The determination of the resistance of the portion of the specimen betvieen the potential electrodes is based on the principle that the resistance of any part of g circuit is proportional t o the potential drop 159
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INDUSTRIAL AND ENGINEERING CHEMISTRY
across it. Therefore, the resistance, R,, of the portion of the specimen between the potential electrodes is given by the equstion
Vol. 44, No. 1
this investigation but an electrostatic voltmeter has been found t o be much more convenient t o use. SPECIMENS
where V Iand V oare the potential drops across the specimen and the reference resistor, respectively, and Ro is the resistance value of the reference resistor. This equation is true only with the following provisions: that the current through the voltmeter is very small compared t o that in the specimen-resistor circuit; and that any potential drop between the specimen and the potential electrodes is small compared to the voltmeter readings.
All specimens were standard 6-inch square tensile test sheets. They were prepared by the compounding and curing procedures given in sections D-5-a and D-7-a, respectively, of "Specifications for Government Synthetic Rubbers" ( 6 )with the following exceptions: the blocks were added within an interval of 12 minutes instead of 10; and the recipes in Table I were used.
TABLE I. COMPOUNDINQ RECIPES Ingredient
Parts by Weight
X-539 GR-S Channel black Furnace black Sulfur Zinc oxide Beneothiaeyl disulfide Tetramethylthiuram disulfide
I
100
100
2
50 2 6
50 5
1.75
0.125
In most cases the specimens were cured for 50 minutes at 292* F. In order to revent flexing of the specimens after curing and before testing, t f e y were placed on aluminum plates immediately upon removal from the press. They were stored on the aluminum plates 16 to 24 hours a t 30% relative humidity and 77" F. before testing. INFLUENCE OF FLEXING OR STRETCHING
Figure 3. Wiring Diagram for Rectifier and Voltage Regulator Uaed as Supply Voltage Because the present method applies t o specimens with resistances of less than 100 ohms and an electrostatic voltmeter has a direct current resistance in the order of 10% ohms, i t is evident that the steady-state current through the voltmeter is never appreciably greater than a millionth of that in the resistorspecimen circuit. Thus, the first provision is easily satisfied. With respect to the second provision, measurements of the resistance between the specimens used in this investigation and the potential electrodes have never yielded values greater than 109 ohms. Thus, when the voltmeter pointer has reached ita equilibrium position the potential drop at the contact is less than one millionth of the voltmeter reading. If the resistance between the potential electrodes and the specimen becomes large enough to affect the measurement of V,, it will be manifested by sluggish response of the voltmeter when it is connected t o the potential electrodes. I n this circuit, potential drops between the specimen and the current electrodes are not included in the measurements made with the voltmeter and, therefore, these resistances are not included in the calculated resistance of the specimen. Since the lowest readable voltage on the scale of usual electrostatic voltmeters is about one fourth of the full scale voltage, the ratio of V. t o Voor of V oto V ecannot be greater than 4. When the reference resistors are selected as in Figure 2 so that each is ten times as great as the next smaller one, the electromotive force should be adjusted so that either V , or V8,whichever is greater, will yield nearly full scale deflection. A potentiometric method similar t o that used in ASTM designation D 991481' for measuring the potential drop across the specimen may be used for measuring V oand V,. However, when the contact resistances between the specimen and the potential electrodes approach lo9 ohms the current sensitivity of the potentiometer must be in the order of 10-10 amperes. Also. the minimum current required t o balance the system must be less than 1% of the current in the reference resistor-specimen circuit. This type of method was used for earlier measurements made in
The factor having the greatest effect on the precision of resistivity determinations is the flexing of the specimens. The resistivities of specimens containing, respectively, an easy processing channel (EPC) black and three HAF blacks from different sources were found t o be increased about twofold when the specimens were bent around a mandrel 1.5 inches in diameter. If the specimen is flexed by rolling it between the hands, the resistivity may increase as much as 20-fold. Almost complete recovery is obtained, however, upon heating of the specimens in an oven for 30 minutes a t 100' C.
I /
1-1
20 E 15
0
Figure 4.
0
IO
20 30 40 Time minutes
-
50
60
0
10
Effect of Flexing and Annealing in the Resistivity of a Specimen
This effect of flexing and annealing a specimen containing
NBS black is shown in Figure 4. The resislivity of the specimen was first measured in the usual manner obtaining the value of 1 megohm cm. shown in the left of the graph. It was then removed from the assembly, rolled between the hands, and replaced. Resistance measurements during the 60 minutes immediately following replacement of the specimen in the assembly, yielded resistivity values which decreased monotonically with time from 17.5 to 13.6 megohm om. These values are plotted in the center of Figure 4. At the end of this 60-minute period the specimen was removed, heated for 30 minutes a t 100" C., cooled, and replaced in the assembly. The values of about 1.7 megohm cm., shown at the right, were obtained indicating approximately 95yQ recovery.
INDUSTRIAL A I D ENGINEERING CHEMISTRY
January 1952
161
lime- minuteo
Figure 5. Resistivity as a Function of Time of Cure
Figure 6. Strain at 400 Pounds per Square Inch as a Function of Time of Cure
The solid line in the center portion of Figure 4 is derived from a calculation of the linear regression of the resistivities with the log of time. The relation is of the form p
3
A - Blogt
INFLUENCE OF WLCANlZATION
Another factor having considerable influence on the resistivity of the specimens is the state of cure. In Figure 5 the resistivity of a vulcanieate containing NBS black is plotted against the time of cure. The solid line ib of the form
- P O ) (t -
to>
=1
(3)
where t is the time of cure; p, the resistivity for cure time, 1; O , the final resistivity (limiting value as t approaches infinity); to,the time of incipient cure; and C, the reaction rate conetant. This curve is similar t o the one in Figure 6, where the strain (elongation at a fixed load) is plotted against the time of cure. The solid line is again an equilateral hyperbola of the form
K(E
- E,)
(t
-
0
0 0.0 Sulfur
0
(2)
where p is the resistivity in megohm cm.; t, the time in minutes after the specimen was placed in the electrodes; and A and B, constants having values of approximately 17.6 and 2.27, respectively, for the data shown. According to the above relation, about 40 years would be required for the specimen t o recover to its original value of resistivity. From this curve it is evident that flexing of the specimens which occurred immediately before testing would be revealed by a rapid decrease in resistance during the measurements, but flexing which occurred several hours before might escape detection. In order t o prevent large discrepancies because of inadvertent flexing, the specimens may be annealed before testing or precautions may be taken t o avoid flexing after curing. In the present investigation the latter procedure was followed. The Specimens were placed on flat aluminum plates immediately after removal from the press and were not touched until te&s were to be made. Care was taken t o minimize flexing or stretching of the specimen in transferring it from the aluminum plate t o the electrode assembly. When the specimens were properly handled, the resistance did not change with time even when the electrical potential was applied for more than an hour.
C(P
0
to)
1
(4)
where E is the strain for cure time, t; E,, the final elongation; and K,the reaction rate constant. If tois interpreted as the time of incipient vulcanization, it should be independent of the property measured. The value of to calculated from resistivity data was 4.2 minutes. Strain data obtained from the same
9
I
B
I
I
IO
20
I x)
,
40
Time
t
I
60 - 50 minutes
,
70
#
ao
,
90
,
100
Figure 7. Resistivity as a Function of Time of Cure for Compounds Containing Various Amounts of Sulfur
vulcanizates yielded a value of 3.3 minutes which is considered t o be in good agreement. The above relations do not apply t o specimens in which the time of cure was less than or nearly equal to to. In resistivity measurements taken for tStoa series of values were obtained in the order of 20 megohm cm. Over the same cure time interval for strain data, the specimens broke or stretched indefinitely. Changes in resistivity of carbon black vulcaniaates with time of cure have been reported by other investigators (&6,7). One of these investigators ( 8 ) stated that a change in conductivity was not caused by the vulcaniegtion process. Another (7) found that for one type of black the relation between resistivity and time of cure was unchanged when the sulfur was omitted from the compound, whereas another type of black yielded lower resistivities for the compound containing no sulfur. A third investigator (6)reported no significant change in resistivity with t h e of cure for specimens cured for 30 minutes or more, but the values of resistivity reported for specimens cured 15 minutes were much greater than those for the longer cure times. I n order to determine whether the decrease in resistivity with increasing time of cure shown in Figure 5 is an effect of vulcanieation or an effectof prolonged heating in the press, compounds of X-639 GR-S and NBS black containing varying amounts of sulfur were prepared. Figure 7 gives the observed resistivity values plotted against the time of cure for compounds containing 0.0,0.5, 1.0, and 2.0 parts of sulfur. For those containing sulfur there is a definite decrease in resistivity with time of cure which
INDUSTRIAL AND ENGINEERING CHEMISTRY
162
TABLE 11. RESISTIVITIES OF IDENTlCAL VULCANIZATES PREPARED ON VmIous DAYS USINGNBS BLACK Day 1 2 3
Resistivity, Megohm Cm. 1.20 0.91 1.10 1.16
A
1.09 1.09 1.27 1.08 1.09 1.02 1.11 0.11
b
9 10
Mean Std. deviation
e
TABLE Iv. RESISTIVITY AND STRAIN DATAFOR VULCANIZATES CONTAININQ HAF AND R F BLACKSFROM VARIOUSSOURCES Source E F
Type RF RF
G
HAF
C
HAF
E
HAF
D
HAF
F
HAF
8
8 8
a
e 0
8
IO4
e
t
H
HAF
C
RF
8
e
& k B C A Type MPC
C B A D
EPC
F C RF
Day 7 2 4 5 Mean 1
3 5 Mean 1 4 6 Mean 1
3 5 Mean 1 4 6 Mean 2 3 6
88
G C E D F H HAF
Figure 8. Resistivity Values from Tables 11, 111, a n d IV Plotted on a Logarithmic Scale
approximates an equilateral hyperbola. No trend is observed however, for the compound containing no sulfur. The undercured compounds, including all of those containing no sulfur, warped considerably after removal from the press. This may account for the increased scattering of resistivity values for these compounds.
Vol. 44, No. 1
Mean 2 3 6 Mean 2 4 5 Mean
Resistivit Strain at 400 Lbs. per 6q.Inch Megohm , % Cured50min. 25 min. 50 min. 100 min. Greater than 106 220 144 128 1.58 212 167 157 1.78 210 166 155 1.42 207 162 151 1 59 210 165 154 0.445 211 166 150 1.12 215 168 155 0.455 205 164 149 0.673 210 166 151 0,223 218 162 144 0.233 218 163 146 0.207 230 169 150 0.221 221 168 147 0.110 261 189 165 0.071 254 188 166 0.079 258 190 168 0.0867 258 189 166 0.0554 248 187 165 0.0790 258 188 170 0.0704 250 182 166 0.0683 252 186 167 0.0216 232 176 159 0.0324 241 179 161 0.0333 238 176 155 0.0291 237 177 158 0.00883 248 190 175 0,00757 242 187 175 0.00892 245 187 179 0.00844 245 188 176 0.00244 258 206 187 0.00221 264 207 191 0.00241 260 202 186 0,00235 26 1 205 188
Zm.
black from the same can. The mean of the values in this table is 1.11 ,and the atandard deviation is 0.11, yielding a coefficient of variation of about 10%. However, black from different cans of NBS standard channel black yielded resistivities ranging from 1 to nearly 2 megohm cm. CHANNEL BLACKS
Table 111shows the resistivity and strain data for vulcanizates containing NBS channel black, EPC and medium processing In order to determine the reproducibility of the test procedure, channel (MPC) blacks produced by sources A, B, C, and D. vulcanizates of NBS standard channel black in X-539 GR-S were The resistivities of the vulcanizates containing EPC black were prepared on different days. These vulcanizates also served as a '30 to 250 times as great as those containing MPC black from the control for resistivity studies of other blacks. Table I1 shows the same sources. Although the various MPC blacks differ b y a resistivity values obtained for all of these vulcanizates containing factor of 4, the EPC blacks differ by a factor of a t least 25. Also, the resistivity of the NBS standard channel black lies closer to that of the MPC black of source B than t o the other EPC blacks. AND STRAIN DATAFOR VULCANIZATES TABLE 111. RESISTIVITY The strain data failed t o show any marked differentiation CONTAINING EPC A N D MPC BLACKSFROM VARIOUSSOURCES between the blacks except in the case of EPC from source C Strain a t 400 Lbs. per Sq. Inch, Resistivit and that difference is small in comparison to the corresponding Megohm Z m. . % 25 min. 50 min. 100 min. Source Type Day Cured 50 Min. resistivity data. .. PRECISION OF TESTS
A
EPC
MPC B
EPC
MPC
C D
EPC MPC EPC (NBS)
1 1 3 Mean 2 3 Mean 1
3 4 Mean 1
2 4 Mean 5 5 1
2 3 4 4 5 Mean
5.19 4.04 4.25 4.49 0.104 0.112 0.108 12.1 13.9 14.7 13.6 0.441 0.381 0.455 0.426 28.6 0.121 1.20 0.91 1.19 1.16 1.16 1.09
1.12
187 185 193 188 190 193 192 170 173 178 174 172 174 181 176 238 192 178 176 178 180 180 173 178
132 130 133 132 136 136 136 132 134 132 133 131 131 131 131 154 133 128 130 129 129 130 116 127
100 102 104 102 110 109 109 103 107 107 106 104 108 108 106 115 104 104 102 104 102 102 99 102
FURNACE BLACKS
Table IV gives the resistivity and strain data for vulcanizates containing high abrasion furnace (HAF) and reinforcing furnace (RF) blacks from different sources arranged in order of decreasing resistivities. The mean resistivites ranged from some value greater than 10'2 to 2350 ohm cm. The limiting resistivity of 1012 ohm cm. for the R F black from source E was obtained by placing the specimen, the source of electromotive force, and the galvanometer in series, and noting that the galvanometer showed no deflection. From the value of the electromotive force and the galvanometer constants, the minimum resistance in the circuit was calculated. Specimens prepared a t various times consistently yielded this high resistivity. R F blacks, sources E and F, were made from oil, while RF black, source C, was made from natural gas. The resistivity values of all of the H A F blacks varied over a range of 75-fold and fell between%those of the RF blacks (E and F) made from oil and that of the RF black (C)
January 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
made from natural gas. There is considerable differentiation indicated between all of the blacks by the resistivity data, but the strain data again failed to indicate any marked differentiation. CONCLUSIONS
Data in Figures 4 and 5 show that the resistivity of carbon black vulcanizates is markedly affected by flexing and state of cure. In spite of the sensitivity of the measurements to these good factors, reproducibility-coefficient of variation of lO%-was obtained for vulcanizates of the same rubber and black compounded on different days. Figure 8 shows a raphic arrangement of the resistivities from Tables 11, 111, and f V plotted on a logarithmic scale. The data are grouped according to the type of black, and the code letters refer to the source. It is evident from this figure that the resistivity test reveals differences between blacks of the same type. It appears, therefore, that resistivity is a simple and sensitive test for characterizing blacks and should be useful for quality control of production and for specification purposes.
163
LITERATURE CITED
(1) “ASTM Standards,” Part 6, ASTM Deaienation D 99148T, Philadelphia, American Society for Testing Materials, 1949. (2) Bulgin, D., Trans. Znat;RubberInd., 21, 188 (1945); reprinted in Rubber Chem. and TechnoZ., 19,667 (1946). (3) Cohan, L.H., and Steinberg, M., IND.ENQ.CEEM.,36,7 (1944).
“Electrical Conductivity Study Cabot Carbon Blacks,” Vol. 2. No. 5, Boston 10, Mass., Godfrey L. Cabot, Inc., 1949. (5) Lane, K.A., and Gardner, E. R., Trans. Inat. Rubber I d . , 24, 70 (1948); reprinted in Rubber Chem. and Technol., 22, 536 (4)
(1949).
“Specifications for Government Synthetic Rubbers,” revised ed., Washington, D. C., Reconstruction Finance Corp., Office of Rubber Reserve, 1951. (7) Waring, J. R. S., Trans. Znst. Rubber I d . , 16, 23 (1940); reprinted in Rubber Chent. and Technol., 14,449 (1941).
(6)
RECEIVED May 11, 1961. Presented before the Division of Rubber Chemiatry of the A U ~ R I C ACHEMICAL N SOCIETY, Washington, D. C.. 1951.
Lignin-Reinforced Nitrile, Neoprene, and Natural Rubbers J. J. KEILEN, W. K. DOUGHERTY, AND W. R. COOK West Virginia Pulp &Paper Co.,Development Department, Charleston, S . C .
T
HE use of coprecipitated lignin as a reinforcing agent for general purpose synthetic rubber, GR-S, has been reported previously. Pine wood lignin from the waste liquor resulting from pulping wood by the sulfate process was shown to give tensile strengths close t o those obtainable with easy processing channel black in GR-S (3). Later it was shown that not all lignins isolated directly from the sulfate waste liquor are equivalent-in some cases oxidation is required to reach the values first reported (6). Other articles have been written reviewing the above (1, 6 ) but no other new work on reinforcing has been presented. One article gives data showing that unoxidized lignin,is an efiective stabilizer for GRS ( 4 ) . There has been no information published on the reinforcement of other synthetic rubbers or of natural rubber with lignin, and i t is the purpose of this report to provide such information. Before going into detailed results it would be well first to review briefly the different main types of lignin available as well as the process which renders lignin a suitable reinforcing agent. TYPES OF LIGNIN
Sulfate lignin is used in both of the studies resulting j n reinforcement of GR-S. Sulfate lignin is a type of alkali lignini.e., a lignin prepared by acidification of waste liquors from either the kraft or soda processes. All of these lignins regardless of wood used or variations in the process, are similar, being soluble in alkali and insoluble in acid. Although all of them do not give equal reinforcement when coprecipitated with rubber, all do give some appreciable degree of increased strength. The materials broadly designated as sulfite lignin are recoverable from the waste liquors resulting from pulping wood by the acid sulfite process. The lignin has reacted with the cooking liquor and is present as lignosulfonates rather than as lignin itself. While this type of material has been mentioned in the literature as a processing aid ( d ) , it hae not been described as having any particular value as a reinforcing agent. In general, the lignosulfonates cannot be coprecipitated with rubber because they are soluble in acid media.
A third material which has received considerable attention is the insoluble ligneous residue resulting from the hydrolysis of wood for the manufacture of ethyl alcohol.. Although potentially this material is very cheap, its production is dependent upon production of alcohol from wood, which still seems t o be an emergency procedure only. Before being suitable as a reinforcing agent for rubber, the ligneous hydrolysis residue must be purified by solution in #odium hydroxide, filtration to remove insoluble substances Ruth as cellulose, and reprecipitation to recover the purified lignin. LIGNIN COPRECIPITATED WITH RUBBER
The process by which lignin is coprecipitated with rubber to achieve reinforcing is similar to that used for masterbatching carbon black or other pigments in GR-S. The main differences are that the lignin is mixed as a solution, rather than as a suapension, with the latex and that the order of mixing materials for coagulation is reversed. Following is a stepwise outline of n procedure for preparing a coprecipitate containing 50 pounds of lignin per 100 pounds of rubber. LIGNINSOLUTION. Suspend 50 pounds of lignin in 140 pounds of water. Add 10 pounds of 50% caustic soda solution with stirring. Solution of the lignin will result almost immediately. LATEX. Use 400 pounds GR-S latex, Type 1, 25% rubber solids; or 263 pounds GR-S latex, T y e 3, 38% rubber solids; or 263 pounds normal natural rubber Etex, 38% rubber solids; or 210 pounds butadiene-acrylonitrile latex, 26% acrylonitrile 47.6% rubber solids; or 200 pounds neoprene latex 842, 50% rubber solids. Acm SOLUTION. Add 23 pounds of 60” BE. sulfuric acid to, 330 gallons of water. Heat to 150” F. COPRECIPITATION. Add the lignin solution to the latex with stirring. Add the lignin-latex mixture to the acid solution with stirring; filter; and wmh the coprecipitate with water until the p H of the wash water is at least 5. Dry the coprecipitate in air a t 160’ to 170’ F. The quantities of sodium hydroxide and sulfuric acid must be varied if coprecipitates containing other proportions of lignin
