October, 1929
I N D U S T R I A L .AND ENGINEERING CHEMISTRY
911
Some Observations on Carbon Black’ C. M. Carson and L. B. Sebrell GOODYEAR Y I R 6
& RUBBER COMPANY, -4KRON. OHIO
H E peculiarities of this interesting, unpleasant, but wholly necessary material have been thoroughly studied by its producers and consumers. Its related substances, charcoal and lampblack, have been the subject of theoretical research for many years. I n the course of some work recently carried out in the writers’ laboratories some interesting observations have been made. These have not been studied exhaustively, nor can definite conclusions applicable to all types and classes of blacks be drawn, but the results seemed to be of sufficient importance to record in this paper.
T
indicative of differences in physical properties of the cured stock. Experimental Procedure
I n the study of adsorption by carbon blacks the method of Bartell and Miller (1) using benzoic acid was first followed; later the use of iodine solutions, as suggested by Davis (S), was adopted. I n the course of the work a number of variables were found which might affect the results. The effects of time, temperature, concentration of solutions,. weight - of sample, presence of inorganic salts, and pH of water Historical used in making up standard Different carbon blacks have certain definite effects solutions have all been taken It has been noticed for upon the vulcanized rubber stocks with which they are into account. H o w e v e r , several years. in compoundmixed. The experiments described herein may be for purposes of comparing ing carbon blacks, that difgrouped under four main heads: (1) adsorption, (2) commercial blacks it was ferent rates of cure of the effect of heat, (311 reaction with sulfur and zinc oxide, found possible s i m p l y t o rubber stock are obtained and (4) dispersion or rate of settling. establish a standard procewith different blacks. The The adsorptive capacity of the black is a measure of dure and follow it in all general opinion is that these the rate of cure of the rubber mix, low-adsorptive cases. differences are caused by adblacks giving a faster curing stock than high-adsorpBENZOIC ACIDMETHODsorption of sulfur and active blacks. A n indication of the stiffening action of a 0.25 gram of the black was celerator by the blacks, and carbon black may be secured by measuring the amount weighed into a bottle, 50 cc. that a determination of the of carbon dispersed in a thin rubber cement which of standard benzoic a c i d a d s o r p t i o n v a l u e would cannot be centrifuged out in a given time. The effect solution added, and adsorpshow the approximate action of temperatures from 500” to 1200” C. on carbon blacks tion allowed to take place of a black on the cure of a is to render them highly adsorptive and also to give a with shaking from 10 to 30 rubber mix. rubber mix which cures faster and has a higher moduminutes. The solution was Leblanc, K r o e g e r , a n d lus. The reaction of carbon blacks with sulfur and filtered quickly into a buret, Kloz (6) concluded that it zinc oxide in boiling xylene liberates a substance having 25 cc. run out into a beaker was not possible to predict accelerating properties in a pure gum mix. and titrated with sodium hythe properties of a rubberdroxide solution of approxiblack mix from a study of mately the same normality, the crude black. b e a r and Moore (?) decided that adsorptive capacity and other tests on using phenolphthalein as an indicator. To correct for the black were not sure indications of the quality of a stock. benzoic acid adsorbed by the filter paper, the blank was also Recently Goodwin and Park ( 4 ) have stated that “adsorption filtered, and the difference between the titration figure of is the one property of carbon blacks that exerts the profound- blank and sample was a measure of the acid adsorbed by the est influence upon the character of a rubber mix,” while Beaver black. The result was expressed in grams of benzoic acid per and Keller ( 2 ) state that ‘‘no correlation could be found be- gram of black. The titrations were made a t three or more tween iodine adsorption and the effect of these blacks on the different concentrations and the adsorption curves of four rate of cure.” The results of the present work are in agree- different blacks are plotted in Figure 1. ment with the opinion of Goodwin and Park. IODINE METHOD-Iodine was found to be somewhat more I n regard to the effect of heat on blacks, Johnson (6) has easily adsorbed than benzoic acid, giving a larger scale to found that blacks having a low percentage of volatile material work over. Hence the rest of the experiments were made a t 900-1000° C. are of higher quality in a rubber mix than with iodine. Iodine solutions were made up in several conthose having high volatile matter. Likewise, Beaver and centrations using a little potassium iodide to aid solution, Keller have found that blacks of low oxygen content give a 0.25 gram of black was weighed into an oil sample bottle, 50 fast curing stock. cc. of standard iodine solution added, and the bottle shaken The writers are not aware of any published work backed by for 10 to 20 minutes. It was then centrifuged for 5 minutes experimental data on the relation of dispersion of carbon to throw all the black to the bottom. 25 cc. were then pipetr blacks to any properties of the rubber with which it is mixed. ted out and titrated with sodium thiosulfate solution of about However, an article of Stamberger (8) on the rate of settling the same normality as the iodine. The initial concentration and dispersion of various pigments (including carbon black) of the iodine was determined by a blank, the difference being in dilute rubber cements led to an investigation along similar the iodine adsorbed by the black. The result was calculated lines. It is the purpose of this paper to show, among other to grams of iodine per gram of black. The adsorption of things, that there is a remarkable relationship between degree iodine by several blacks plotted against end concentration is of dispersion and modulus, but that rate of settling is seldom shown in Figure 2 . The logarithmic diagram of a true adsorption curve is a straight line, according to Freundlich. Presented before t h e Division of Rubber Chemistry a t the 77th Figure 3 shows the logarithmic curves of the blacks plotted Meeting of the American Chemical Society, Columbus, Ohio, April 29 t o M a y 3, 1929. in Figure 2.
INDUSTRIAL AND ENGINEERING CHEMISTRY
912
BLACKS STUDIED-A variety of blacks was secured-two channel blacks, two grades of Thermatomic, one varnish black, and three high-temperature blacks. These blacks were compounded in the following standard test formula: rubber 100; black 36; zinc oxide 5; stearic acid 4; sulfur 3.5; Captax 1.2. The standard black was taken as Micronex, which gave a best cure in 60 minutes a t 125' C. in the above stock. In Table I an average iodine adsorption figure is taken for each black and compared with the_time of best cure of each stock. The figures for modulus and tensile are also added. Table I-Properties of Blacks Studied IODINE BEST ADSORBED TECHNICAL PER GRAM CUREAT ~~ODL-LUS BLACK BLACK 12.5' C. AT 5 O O C b TENSILE Gram Minules Kg./sq. cm. Kg./sq. cm. Thermatomic regular 0.05 30 110 275 0.06 30 110 255 Thermatomic special 0.07 30 Goodwin 207 290 0.09 40 235 Acetylene 200 0.09 60 Micronex 194 305 0.10 50 195 Cabot 320 0.20 IO,? 165 295 Super-Spectra 0.13 Experimental black 40 186 310
Figure 1
Adsorption
The relationship between adsorption and, rate of cure is. obvious, while the lack of regularity between either modulus or tensile and adsorption needs no further discussion. The experimental black failed to line up with the others, possibly because it was simply a laboratory sample and not a perfected factory product. Blacks from the same source showed no difference in rate of cure so that no figures comparing adsorption and cure are presented. The varying adsorption of accelerator by different blacks is shown by the following experiment: A solution of mercaptobenzothiazole in I part of alcohol and 2 parts of water of about 0.01 normality was prepared. The amount of accelerator was determined by titration with 0.01 N sodium hydroxide using phenolphthalein indicator. A similar solution of diphenylguanidine was made up and its concentration determined by titration with alcoholic hydrochloric acid using bromophenol blue indicator. The adsorption of four blacks was then determined, with the results shown in Table 11. Table 11-Adsorption BLACK S uper-Spectra Micronex Goodwin Thermatomic
of Accelerator by Different Blacks ADSORPTION PER GRAMOF BLACK Mercapto D P G. Gram Gram 0 131 0 101 0 030 0 039 0 009 0 008 0 004 0 008
These figures indicate that differences in rate of cure are a t least partly due to adsorption of accelerator.
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Effect of Heat
It has been stated previously that the work of Johnson in differentiating between high- and low-quality blacks by their percentage of volatile combustible matter led to the opinion that heat treatment might give better quality. ACTIVATION BY S T E A M - T ~ ~ original idea had been to activate black by superheated steam, thus removing air and adsorbed hydrocarbons. An apparatus was set up consisting of an iron pipe heated by an electric combustion furnace set yertically. Steam was superheated in a copper coil and passed in at the bottom of the pipe. A thermocouple was inserted in the upper half of the black and temperatures of 550-750' C. mere obtained. Micronex, when so treated for 3 hours, was found t o have twice the adsorptive capacity for iodine that unactivated Micronex had. The rate of cure was somewhat faster and the modulus slightly higher than the control. Thermatomic blacks could not be activated at these temperatures. As about ten runs of the small activating unit were required to supply enough black for compounding purposes, this experiment was abandoned for one in which a large sample could be prepared in one run. HEATIXG UKDER REDUCED PRESSURE-The object of this experiment was t o remove adsorbed gases or moisture and to protect the black from re-adsorption of other substances during subsequent milling and curing operations. Two hundred and fifty grams of Micronex were evacuated a t 10 mm. for 3 hours at 200-220' C. After cooling, and without releasing the vacuum, a solution of 7 5 grams of mineral rubber in xylene was added. After thoroughly mixing, the xylene was distilled off under reduced pressure. When compounded in a rubber stock, this black was not different from the untreated control, containing mineral rubber mixed on the mill. EFFECT O F HIGH TEIIPERATURE-T~~ amount Of volatile combustible matter in different blacks varies according to the temperature a t which the black was made, as would be expected. Table I11 shows results on several blacks, the determination being made by the standard A. s. T. hl. method for coal. Table 111-Effect
of High Temperature on Volatile Matter
BLACK Acetylene Thermatomic regular Thermatomic special Goodwin Experimental black Super Spectra Cabot, high-quality Cabot, low-quality Micronex, high-quality Micronex, low-quality
VOLATILEMATTER P e r cent 0 40 0 75 1 30 1 82 2 16 5 17 4 33 7 08 4 40 6 82
E = S P E C I R LB L R C H
Figure 2
JVithout considering any other causes of variation, it seems that blacks made a t higher temperatures (1000-1200" C.) are ordinarily better stiffening agents than channel blacks. A comparison of the complete stress-strain curve shows this
. INDUSTRIAL .LVD EiVGIiVEERISG CHEMISTRY
October, 1929
distinctly. That the high-temperature Thermatoinic blacks do not give increased stiffening to a rubber mix must be due to increased particle size. The possibility and means of improving low-quality carbon blacks by heat treatment should be considered. For channel blacks, the miters agree with Johnson that the test is a measure of the quality of blacks for use in rubber. But, for other types of black the difference would probably be too small to serve as an indication of quality. Micronex was heated in large covered alundum crucibles in an electric furnace a t 900-1000" C. for various periods from 10 to 60 minutes. Heating mas found to increase the iodine adsorption in all cases, evidently by the removal of adsorbed material. However, instead of giving a slon-er curing stock, due to adsorption of sulfur and accelerator, it was found that the rate of cure was markedly increased and in some cases a slightly stiffer stock was obtained. The black itself, after heating, had a dry, cindery appearance and feel in contrast to the velvety feel of ordinary black. It settled very rapidly in organic liquids and seemed to be badly agglomerated. When milled into rubber it v-as very difficult t o disperse. Table IV presents t h r data in regard to this experiment. The stock is the standard test stock, with Captax acveleration. Table 11'-Effect
TECHNICAL CL-RB .\T I\.fODULVS ELOSG4125' C A T .500'; TEXSILE TIOX M i n u t e s Kg./sg. cm. Kg./sq. cm. Per cenl i5 185 324 690
Control Heated 60 minutes a t 950' C . Heated 10 minutes a t 950' C. Check on above one week later
Table V-Reaction Products a s Shown b y Extract BLACK TIMEOF HEATIXG COLOROF EXTRACT H0117S 4 Micronex Dark reddish brown Cabot 8 Reddish brown Goodwin Pale yellow 4 Activated Micronex 4 Very dark red-brown 4 Red-brown Acetylene
From the fact that only a pale yellow extract mas obtained when sulfur, zinc oxide, or black was omitted or when the black was extracted alone, it seems that a reaction of the three took place, liberating an absorbed hydrocarbon which probably reacted with part of the sulfur. On concentrating the extract most of the sulfur crystallized out in needles and was removed. Removal of all the solvent left a gummy resin which still contained 77 per cent total sulfur. This resin, when compounded in an unaccelerated stock (allowance made for sulfur in the resin), shortened the time of cure from 2 hours t o ll/zhours a t 141" C. and gave a somewhat stiffer stock. The experiment was repeated nith acetylene black and a similar result wah noted.
of H e a t T r e a t m e n t on L o w - Q u a l i t y Blacks REST
STOCK
IODIXE ADSORPTION
Gram 0.09
60
I95
312
665
0.13
45
197
3"i
660
0.16
45
191
316
675
0.15
It will be seen that there is a decided increase in rate of cure and a corresponding increase of adsorptive capacity of the black with some difference in modulus or tensile of the stock as shown in Figure 4. This is, of course, a glaring exception to the previous statement, that high adsorption means slow cure. The writers have no explanation for this behavior. Reactions with Sulfur and Zinc Oxide
The reaction products of accelerators with sulfur and zinc oxide have often been isolated by boiling in xylene a t 138" C., which is about the temperature of the 40-pound (2 1 kg. per sq. cm.) cure. Twenty-five grams of black, 10 grams zinc oxide, and 5 grams sulfur were boiled in 300 cc. of xylene for
I JFILN 1 IT I I IF 1 I I NCR AT O N
Figure 3
4 hours.
913
The xylene was then decanted, fresh solvent added, and the process repeated several times. Some of the blacks gave very highly colored extracts, others showed only the color of sulfur in xylene. The results obtained from several blacks follow in Table V.
60
30
90
120
Figure 4
This experiment is proof of a peculiarity in rate of cure noted in the writers' laboratories and best expressed in tabular form (Table VI). Table VI-Effect RUBBER Per cenl
of Zinc Oxide a n d of Black on R a t e of Cure
SULFUR Per cenl
ZnO Per c r i i l
BLACK Per c e n l
TIMEOF BEST CrRE A T 141' C . Minute&
The addition of black to a rubber-sulfur mix slows down the time of cure, but the addition of black to a rubber-sulfur-zinc oxide mix accelerates, evidently owing to the liberation of an accelerator. No change in iodine adsorption or in rate of cure in a Captax stock was found in the black from which the material n a s extracted. I t s amount was very small, pmhaps 0.25 per cent of the black. Black which had been boiled with sulfur and zinc oxide in xylene for several days retained about 1.0 per cent sulfur after repeated extractions with benzene, carbon disulfide, and finally 16 hours with acetone. While boiling in xylene, large quantities of hydrogen sulfide were evolved, probably due to the Same reaction by which hydrogen sulfide is formed when paraffin and sulfur are heated together. Another experiment was made which shows the strong adsorptive power of black for sulfur. A channel black and sulfur were heated in the activation apparatus for 2 hours at 540-590" C. A large amount of sulfur dioxide was evolved but after repeated and continuous evtraction with carbon
INDUSTRIAL A N D ENGINEERING CHEMISTRY
914
disulfide and finally with acetone, the black still contained
7 per cent adsorbed sulfur. Rate of Settling
A series of experiments on rate of settling was undertaken in an attempt to show some relation between this property and the action of the blacks in a rubber mix. One gram of black was ground in 2.5 grams of linseed oil on a glass plate. This paste was then diluted with benzene and, after shaking out all lumps, was allowed to settle in a stoppered test tube. The data showed a large difference between blacks but was not related to any physical quality of the black-rubber mix. Stamberger investigated the action of various materials in rubber cements, especially their rate of settling. If we suppose that in a mixture of black and rubber in benzene, certain blacks are able to bind themselves more firmly to rubber than others, owing to fineness of subdivision, deflocculation, or degree of dispersion, then some quantitative measurement should give us an indication of the quality of the black-rubber mix. Such a measurement is found in the actual determination of carbon in suspension in a 0.5 per cent rubber cement after centrifuging. One gram of black was ground into a paste with toluene or xylene on a glass plate. A few cubic centimeters of 4 per cent rubber cement were added and the grinding continued until the black and rubber were thoroughly mixed. The paste was then transferred to a bottle containing 50 cc. of 4 per cent rubber cement and all lumps were broken up. The cement was then gradually diluted to 400 cc. with benzene, with constant stirring, and was then uniform and free from lumps. The bottle was placed in a shaking machine for an hour or shaken by hand occasionally over an 8-hour period. The solution was then allowed to settle overnight or for 24 hours or a portion of it was centrifuged for 15 to 30 minutes. The supernatant liquid was analyzed for carbon by evaporating off the solvent on the steam bath, dissolving the rubber in cold concentrated nitric acid (preferably overnight), then heating to boiling, and filtering hot through a Gooch crucible packed with acid-washed asbestos. By this means it was found possible, not only to differentiate between blacks made by different processes, but also to classify blacks from the same source as to their value in a rubber mix. I n practically all cases a high-modulus black showed a high carbon-in-suspension figure. The test applied only to stiffness as determined by modulus in the standard test formula and was not a measure of hardness or plasticity of the uncured stock. Table VI1 is a comparison of modulus with amount of carbon held in suspension after 30 minutes’ centrifuging. Table VII-Comparison BLACK Thermatomic regular Thermatomic special Super-spectra Micronex, low-quality Micronex, medium-quali Micronex, high-quality Experimental black Goodwin Acetylene
of Modulus with R a t e of Settling CARBON I N hf0DULC.S AT SUSPEXSION 500% Gram1100 cc. Rg./sq. cm. 110 0 110 0 0.011 160 0,0096 125 0.026 160 0.035 194 186 0.062 207 0,055 210 0.067
There is overlapping of figures for different groups, but in general Thermatomic carbons have no affinity for rubber, zero carbon being held in suspension; paint or varnish blacks will be less than 0.020 gram; channel blacks run betveen 0.02 and 0.04 gram, except the poor ones which will run lower; thermal decomposition blacks will run over 0.050 gram per 100 cc. cement. Rate of settling and degree of dispersion after centrifuging are different phenomena, as there was no relation between settling experiments and dispersion results. Regular Thermatomic and Super-spectra are highly dispersed
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in linseed oil-benzene but are not retained in suspension by a rubber cement. It is possible to distinguish between high- and low-quality blacks from the same shipment by this method, as a difference of 10 kg. in modulus will show up in amount of carbon held in suspension. The modulus a t 500 per cent of several different samples of Rlicronex was determined in the standard test formula and compared with the amount of carbon held in suspension. It mill be seen that the amount of carbon decreases with a decrease in modulus. Table VIII-Comparison of Blacks from Same Shipment CARBON IN SUSPENSION MODULUS A T 5007, Gram/100 cc. Kg./sq. cm. 54 0,0195 200 59 0.016 193 56 0.014 190 55 0.013 183 53 0,012 178
LOT
The figures in Table VIII are not isolated instances, but have been noted in several series. It is of interest that the first four blacks listed would be classified as high-quality by Johnson’s volatile matter test while the fifth would be lowquality. Note-The figures for grams of carbon in suspension in Table VI11 are somewhat lower than figures obtained for the same kind and quality of black and shown in Table V I I , since a somewhat thinner rubber cement was used in the second series. T o be comparable, all tests must be run on a cement made from one batch of rubber and over a period of 3 or 4 days, since a rubber cement in benzene does not maintain the same viscosity or rubber content indefinitely.
The union of black and rubber is a powerful force as is shown by the fact that high-quality blacks cannot be totally centrifuged from a rubber cement even by further dilution. This is true of no other pigment. When a black is milled into rubber, the union is still more difficult to break by centrifuging. The difference between blacks is still observed, but the scale is narrower. Other pigments when milled into rubber can be centrifuged out. The accuracy of the test is remarkable when we consider that the quality of the black may be determined by 1 t o 5 per cent of the weight of sample taken, and yet we have seldom found a black that could not be classified as to modulus in the cured stock by this method. Acknowledgment
Thanks are due t o R. P. Dinsmore for permission to publish this paper, to C. R. Park and C. R. Johnson for helpful criticism, and to the Godfrey L. Cabot Company and to Binney & Smith for samples of carbon black. Literature Cited (1) Bartell and Niller, J . A m . Chem. Soc., 46, 1106 (1923). (2) Beaver and Keller, IND.ENG.CHEI., 20, 817 (1928). (3) Davis, J . Chem. SOC.,91, 11, 1666 (1907). . , 706 (1928). (4) Goodwin and Park, IND.EXG.C H E M 20, (5) Johnson, Ibid., 20, 904 (1928). (6) Leblanc, Kroeger, and Kloz, Kolloidchem. Beihefte, 20, 356 (1925); C. A . , 19, 3398 (1925). (7) Spear and Moore, IND.ENG CHEX.,18, 418 (1926). (8) Stamberger, Kolloid-Z., 42, 295 (1927).
Vitrified Brick for Pavements to Be Discussed The twenty-fourth annual meeting of the Xational Paving Brick Manufacturers Association will be held a t the Palmer House, Chicago, Ill., December 4 to 6, 1929. The program for the last two days will include papers and discussions of the latest practices in the design and construction of streets and highways, particularly the vitrified brick type. These sessions will be open to the general public and any one interested in street and highway development is invited to attend.
