TXDUSTRIAL AA'D ENGINEERISG CHEMISTRY
1288
Vol. 21. No. 12
Carbon Black 11-The Role of Oxygen' C. R. Johnson GODPREYL. CABOTI N C . , BOSTON, MASS.
The results reported in this paper confirm the indication of previous work t h a t a portion of the oxygen in carbon black exists in rather definite chemical combinations on the surface of a hydrocarbon with a high carbonhydrogen ratio. It is not clear yet whether all of the oxygen in carbon black exists in this state, as previous work has shown t h a t after heating for extended periods a t 955" C. there still remains a small amount of oxygen in the black. I t has been shown t h a t the oxygen attached as here described does not exist in the state of gaseous adsorption and cannot be removed by heat alone as oxygen, but can be removed by reaction with zinc dust a t temperatures below t h a t at which gaseous evolution usually takes place. The oxygen goes over to zinc oxide and the volatile matter of the residue is considerably reduced. No correlation between volatile matter (oxygen content) and iodine adsorption could be found, but a comparison
of blacks from any one factory with respect to iodine adsorption shows t h a t high iodine values follow high volatile matter content. Using two accelerators, it was found t h a t the so-called high volatile blacks, which retard cure, adsorb more organic accelerators than a normal black. Those blacks which have been heated after manufacture a t temperatures between 500' and 900' C. invariably show high iodine adsorption and lower accelerator adsorption, which accounts for their much more rapid vulcanization rate. J u s t why the iodine adsorption values are inconsistent with other phenomena is not clear a t present. An explanation of poorer physical tests in rubber with high volatile blacks is given. The development of a new carbon black with low accelerator adsorption and improved physical properties when used with rubber is included.
...... HE phenomenon of adsorption and the status of
T
oxygen in carbon black are very closely related. Any addition to our knowledge of this relationship will help to explain the properties of carbon black in its use with rubber, as well as in inks and varnishes. At several different periods the adsorptive properties of carbon black have been studied. Spear and Moore ( 7 ) showed that the adsorption of different carbon blacks for organic dyes was variable and found some correspondence between the oil adsorption properties of the several blacks and their behavior in rubber. Beaver and Keller (8) studied the iodine adsorption of carbon blacks in connection with their work on the influence of oxygen on the properties of carbon black. Coodwin and Park (S), in their complete work on the physical and chemical properties of carbon blacks and their relationship to the properties when used in rubber, emphasized the importance of adsorption. Sebrell and Carson (6) showed that the rate of cure of different blacks in rubber was roughly inversely as the iodine adsorption of the several blacks which were studied. They showed also that the black had the capacity to adsorb organic accelerators. In one instance, however, he reported an inconsistency which was difficult to explain. When a carbon black was heated to about 800" C. for a short period the adsorptive capacity was materially increased and a t the same time the rate of cure in rubber was notably increased. Sebrell and Carson's work is responsible in part for the work reported in this paper, as a number of points in their report were stimulative of further study. It has been shown (4) that oxygen is important in its bearing on the behavior of carbon black in rubber. While oxygen was revealed in an ultimate organic combustion of carbon black, whenever carbon black was heated to temperatures above 400' C. gas evolution took place, the gases evolved consisting principally of hydrogen, carbon monoxide, carbon dioxide, and a very small percentage of oxygen. This result is in conformity with Langmuir's findings ( 5 ) that whenever oxygen is adsorbed on the surface of a carbon lamp 1 Presented before t h e Rubber Division of t h e American Chemical Society, a t Atlantic City, N. J., September 26 t o 28. 1929.
filament the oxygen cannot be driven off as such by heating, but takes carbon atoms with it in the form of carbon monoxide and carbon dioxide. It is Langmuir's concept that the oxygen in this case is combined on the surface of the carbon in a little more definite chemical combination than is usual in adsorption, and that the surface is covered with a monomolecular layer of oxygen with primary valences attached to the carbon. It was shown by several experiments that the same rule probably held with respect to carbon black. One of these was the determination of the volatile matter of carbon black a t 2 mm. pressure in comparison with the same determination made a t atmospheric pressure. I n all cases the volatile matter a t low pressure was less than a t higher pressure, and the explanation which was suggested was that the oxygen was removed a t low pressure without combining with the carbon, thus making a lower volatile loss. This was supported by the fact that an analysis of the gases evolved when that black was heated in v?ccuo showed more hydrogen, more oxygen, and less carbon monoxide and carbon dioxide. It was the purpose of the investigation discussed in this paper to more definitely fix the status of oxygen and to find what influence, if any, it has on the properties of black when used in rubber. Effect of Oxygen on Carbon Black According to Freundlich ( I ) , in the case of true gaseous adsorption there exists an exact equilibrium between the amount of gas adsorbed on a surface and the concentration of the same gas in contact with that surface. Such being the case, if oxygen be present as a truly adsorbed gas on carbon black, then the concentration of adsorbed oxygen should be diminished if black be placed in an oxygen-free atmosphere. A sample of black was placed in an atmosphere of nitrogen, and after each 24 hours a portion of the sample was removed for volatile matter determination. At the same time the nitrogen atmosphere was renewed. It had already been determined that for all of the blacks used in rubber there was a definite ratio between the oxygen content and the volatile matter content, which averages 0.863. Table I
INDUSTRIAL A N D ENGILVEERINGCHEMISTRY
December. 1929
shows that there is no diminution of oxygen as a result of exposure to pure nitrogen. 1- The objection might be raised that the data in Table I are not conclusive because after exposure to nitrogen the black, for a short period, is exposed to the atmosphere and possibly could take on oxygen again before the volatile matter determination was made. As a check on this point another test was run in which the black was soaked in nitrogen gas in a quartz tube for 2 days. The volatile matter determination was made directly in the quartz tube without exposing it to the atmosphere. The same sample of black was used as in making the determinations reported in Table I. The volatile matter decreased from 5.80 per cent a t the start to 5.72;per cent after 2 days in nitrogen. Frorn this it can reasonably be assumed that the oxygen content of carbon black is in a more definite chemical relationship to the carbon than strictly gaseous adsorption. Table I
VOLATILE MATTER Per cent 5.75 5.63 5.84
PERIOD I N NITROGEN
Davs Start 1 2 4 7 9
5 , DO
5.7II 5.7i 5.76
13
Another experiment was made to determine whether or not the carbon black could be made to take on oxygen in a strong oxygen concentration. A sample was exposed to a sample of liquid air containing 50 per cent oxygen for 15 minutes and for 6 hours, with the results shown in Table 11. Table I1 VOLATILE MATTER AFTER EXPOSURE FOR: T R E A T J I E N T O F ~ A M P L E 15 minutes 6 hours Per cent Per cent Before exposure 3.99 3.99 4.20 4.31 After exposure After exposure and 20 hours in air 4.07 4.26 After exposure and 68 hours in air 4.18
..
It would appear from the results in Table I1 that carbon black is not completely saturated with respect to oxygen, and that if oxygen is added it is- in fairly stable condition, as the volatile matter loss is only slightly roduced after long exposure t,o air after the black has been in liquid oxygen for 6 hours. Ai Another study was made to throw more light on the possibility of the addition of oxygen. It had been observed that when carbon black was heated in the presence of air sometimes oxygen was added and sometimes it was removed from the carbon black. As a general rule, oxygen was removed a t temperatures above 500" C. and added at temperatures below 450" C. Two runs were made as follows:
TEST1-Heated for 5 minutes a t 90-150" C., passing air over crucible placed in electric muffle furnace. Removed to desiccator and weighed. Volatile matter determined. TEST2-Heated for 5 minutes a t 2 7 5 3 2 5 " C. Other conditions the same as in Test 1. The results of these two tests are given in Table 111. --BEFORE
Loss on Vola-
TEMPERAheat ing
TURE
"
C
.
Table IIIQ HEATING-- ---AFTER
HEATING--
tile loss
h-et Net vola- VolavolaMois- tile tile Mois- tile ture loss loss ture loss
%
%
%
%
%
%
%
%
Gain
Carin bon volaloss tile
in the presence of air. According to an equation derived by Duerr, the gain in volatile matter should be about 2 to 3 times the loss in carbon, thus: W'V' where W W' V V'
The results of these two tests indicate that whenever equilibrium conditions are favorable for the addition of oxygen it takes place, and that the loss in carbon calculated from volatile matter residues is about equal to the gain in volatile matter after heating to temperatures of 90-325' C.
-
(W
WV
- W' -
W'V')
-
IVY = 7 to 11 x
= = = =
dry weight of original sample dry weight after increasing oxygen content volatile matter before heating volatile matter after heating 7 to 11
3
The factor - takes into account the fact that the oxygen comes off as a mixture of CO and COz. The ratio of the gain in volatile matter to the loss in carbon is less than 1 and does not conform to the expectations of the equation, but the difference can be explained by the fact that there is a greater loss in weight during heating than can be accounted for by the difference in the moisture before and after heating. Apparently some volatile matter loss takes place even a t the low temperature of heating, and while the black was being oxidized and increasing in its volatile matter capacity there was a definite carbon loss. Having shown that it is possible to add oxygen to carbon black, it is now advisable to consider the possibility of removing it. It has already been shown (4) that it is possible to remove it in part by heating in the absence of air at temperatures above 500" C. The experiment was tried by treating the carbon black with a powerful reducing agent, zinc dust being chosen as the reagent. An intimate mixture of 20 per cent zinc dust and 80 per cent Cabot's black from the Bowers plant was treated as follows: (1) heated 1 hour under vacuum a t 370" to 400" C.; (2) heated 2 hours under vacuum a t 370" to 400" C.; (3) mixture (enough for rubber test) heated ll/z hours under vacuum at 370" to 400" C. The gain in weight after heating was 0.158 per cent in Test 1 and 0.135 per cent in Test 2. The volatile matter run under vacuum a t 800" C. for 7 minutes was, in Test 1: carbon black (without zinc dust), 3.23 per cent; unheated mixture, 0.85 per cent; heated mixture, 0.92 per cent. I n Test 2 it was: heated mixture, 0.54 per cent. I n Test 3 it was: unheated mixture, 1.04 per cent; heated mixture, 0.89 per cent. The results of the rubber test' on the original black and on the heated mixture are given in Table IV. Table IV PERIOD OF
HARDNESS MODULUS SHORE
ELONGATIONAT 400% DUROMETENSILE AT BREAK AT BRAKE ELONGATION TER
CURE Minutes Lbs. per sq. i n . Kg.per sp. cm. Per cent ORIGINAL BLACK
40 60 80
4100
4250
28.t. 2 288.7 299.3
40 60
3830 3900 4025
269.7 274.6 283.4
4036
640 623 603
1710 2060 2265
62 65 68
1750 2180 2130
62 67 68
HIEATED MIXTURE
SO
617 567 580
A mixture of 20 per cent zinc dust and 80 per cent Cabot's black from the Bowers plant was heated for 4'/2 hours a t 400" C. After ll/z hours the apparatus was disconnected, a small sample was removed, and the mixture was stirred, after which heating was resumed under vacuum. Table V
%
9 0 t o 1 5 0 8 . 5 2 6 . 4 5 1 . 1 6 5.29 5.99 0 . 4 7 5.52 0.37 0.18 2 7 5 t o 3 2 5 7.96 6 . 4 5 1 . 1 6 5 . 2 9 6.09 0 . 4 5 5 . 6 4 0 . 3 7 0.30 a I t was assumed t h a t t h e net change in carbon could be represented by the weight of t h e residue alter each volatile matter determination. This is substantially correct, except for a small amount of hydrogen in t h e residue.
1289
Carbon black (without zinc dust) Zinc dust Zinc oxide Unheated mixture Heated mixture (11/z hours) Heated mixture (4'1% hours) Heated mixture (41/2 hours) exposed t o air for 3 days 1
VOLATILE MATTER Per cent 3.15
..
IODINBPER GRAMBLACX Grams 0 130
2 ' 26
3 627 ?;one 0 715 0 419 0.376
2.06
...
1:34
In testing t h e heated mixture, it was assumed t h a t t h e zinc dust h a d
changed t o zinc oxide and correction was made in t h e test formula.
INDUSTRIAL A N D ENGINEERIA’G CHEMISTRY
1290
The volatile matter in the mixture run under vacuum a t 800” C. for 7 minutes and the adsorption of 0.1 N iodine by the mixture are shown in Table V. The combustion analyses are shown in Table VI. Table VI HEATEDMIXTURE Per cent 77.28 1.35 1 77 19.60 0 67
Carbon Hydrogen Oxygen Ash Moisture
ORIGINAL BLACK Per cent 93.18 1.06 5.76 None 1.20
From the data in Tables V and VI it is evident that the volatile matter is considerably reduced even before the mixture is heated, which shows that the oxygen has been removed from combination with the carbon and has reacted with the zinc dust to form zinc oxide in the process of volatile matter determination. I n the case of the iodine adsorption, the amount found on the unheated mixture is almost exactly 20 per cent of the amount for pure zinc dust, so that practically no iodine goes to the black in that instance. With the heated mixture, the iodine adsorption falls off, owing to the fact that some of the zinc dust has gone over to the oxide, which has no adsorptive capacity for iodine.
adsorption and the amount of volatile matter in black, or in the rate of cure of the black when used with rubber. When samples of black from the same factory are considered, however, it is usual to find that the iodine adsorption varies directly with changes in volatile content. Studies of adsorption of two accelerators, Captax and DPG, by a number of blacks indicate that accelerator adsorption varies directly with volatile matter (oxygen content). This applies not only to samples from a given factory but throughout a number of sources. The exception which Sebrell and Carson noted with heated blacks, namely, that iodine adsorption was increased and volatile matter decreased, accompanied by more rapid curing rate, is confirmed by the data in Table VII. Taking the ideas developed in this and other papers, it begins to be clear now why carbon black with high volatile matter (or oxygen content) gives poor physical properties when vulcanized with rubber. Such black clearly has the property of adsorbing more organic accelerator than a normal black, with the result that there are established innumerable foci of incompletely vulcanized points throughout the rubber mixture caused by the conjuncture of a black particle, rubber nucleus, sulfur particle, and reduced accelerator concentration. This leads to a reduced average strength throughout
Tablo VI1
SAMPLS Schafer 1651 Schafer 1691 T Elf 199 T Elf 16 Schafer 1078 Schafer 1078 Schafer 1691
QUALITY Good Poor Good Poor
Schafer 1078 Schafer 1654 Schafer 1078 Schafer 1692 Kinasmill 1823 Bowers 1821 T Elf 199
Good Good Good Good Good Good
TRELTMBNT
Original Heated 2 hours at 650’ C. “As is” Heated 5 minutes at 650’ C. Heated 20 minutes at 650’ C. “As is” Heated 120 minutes at 650” C .
-----IODINE VOLATILE 0.05 N I Gram Per cent 0.136 5.48 0.158 7.35 ... 5.13 ... 6.67 ... 5.02 2.80 o:ik 7.35 0.161 6.82 0.202 5.09 5.02 0.126 2.80 0.183 ... 5.48 5.02 ... ... 5 36 ... 3 99 ... 5.67 ... 5 13
It might be argued that the zinc oxide is formed by reaction of the zinc with the carbon dioxide, rather than directly with the oxygen. From a consideration of heats of formation, the facts that ordinarily gases are not evolved from carbon black a t temperatures as low as that used, that there was no appreciable loss in weight in the heated mixture, and that there was no appreciable gas evolution during heating indicate that this reaction involves the taking of oxygen from the carbon by the zinc, without the intermediate formation of carbon dioxide. The next step appears to be to try the reducing effect of hydrogen a t various temperatures and pressures. If hydrogen is effective, it might have a practical as well as a theoretical interest, from the possibility of changing the properties of carbon black in its various uses. At any rate, it can now be asserted that the oxygen of carbon black is not so firmly attached to the carbon that it cannot be removed without appearing as carbon monoxide or dioxide. Adsorption Phenomena
Sebrell and Carson’s work on iodine adsorption was concerned with blacks produced by different processes, and there is some question as to whether or not it is safe to generalize from data under such conditions. It was deemed advisable to study the adsorption of iodine and some organic accelerators by rubber blacks which were known to vary widely in the volatile matter content, and consequently in the rate of vulcanization of rubber. The data in Table VI1 show, in the case of unheated blacks, that there is no correlation between iodine
Vol. 21, No. 12
ABSOB BLACK (DRY)F R O M : ED PER GRAM 0.1 N I 0.2N I CAPTAX Gram Gram Gram 0.149 0.166 0.0317 0.171 0.211 0.0400 0.104 0,0284 ... 0.138 0,0377 ... ... 0.0266 0.0231
o:iii
o:iii
0.178 0.223 0.146 0,195 0.149 0.146 0.138 0.138 0.143 0.104
0.202 0.230 0.160 0.204
... ...
... ...
....
--. DPG Gram 0,0237 0.0524
...
....
0.0183 0,0150
.... .... .... ....
.... .... .... ....
....
.... .... .... .... ....
....
the mass, and consequently lower tensile and lower modulus. The lower rate of cure is clearly explained by the reduction in accelerator concentration. For over two years the writer has been studying the possibilities of overcoming the phenomenon of accelerator adsorption. The investigations here reported have been favorable, and concurrent examination of the new product by several rubber manufacturing companies has demonstrated that carbon black in which accelerator adsorption is largely inhibited can be made. Evtensive tests over a period of 8 months in a large manufacturing laboratory have indicated that this new black has physical properties 20 per cent better than normal channel black. Test tires are now on the road to confirm the preliminary abrasion results. Acknowledgment
Acknowledgment is due W. A. Duerr for the scheme followed in studying the addition and removal of oxygen a t various temperatures. Acknowledgment is also due H. P. Gurney, R. K. Estelow, and Jacob Gabry for assistance in the preparation of this paper. 1,iterature Cited (1) Alexander, “Colloid Chemistry,” Freundlich, p. 575. (2) Beaver and Keller, I N D . ENG.CHEM.,20, 817 (1928). (3) Goodwin and Park, Ibid., 20, 706 (1928). (4) Johnson, I b i d . , 20, 904 (1928). (5) Langmuir, J . Am. Chem. SOC.,37, 1154 (1915). (6) Sebrell and Carson, IND.END.CHEX, 21, 911 (1929). (7) Spear and Moore, Ibid., 18, 418 (1926).
