Vol. 18, No. 4
INDUSTRIAL A N D ENGINEERING CHEMISTRY
418
tions, but the data of thermochemistry have a most important part to play in the perfection of a theory of solutions and its applications. Need of Government Research
The writer desires to take this opportunity to point out the need and appropriateness of systematic research in this field by the National Government. Indeed, a properly equipped laboratory for carrying out this more or less routine but all the more necessary systematic accumulation of accurate and consistent thermochemical data seems to be one of the most pressing needs both of pure and applied science. The development of new industrial processes may
well be left to the industries themselves, but governmental laboratories should have as one of their prime functions the accumulation of scientific data which are of fundamental importance to science and industry as a whole. The national physical laboratories of the world have demonstrated that cooperation in the establishmeht of fundamental values in such fields as thermometry and electrical units is entirely feasible. A similar program of cooperative research in the establishment of the fundamental data of thermochemistry should also be possible, and it is to be hoped that our own Bureau of Standards will take the lead in bringing about the necessary arrangements.
High and Low Stiffening Carbon Blacks’ By Ellwood B. Spear and Robert L. Moore DEVELOPMENT LABORATORIES, THERMATOMIC CARBON CO.. PITTSBURGH, PA.
NTIL recently it was generally believed that carbon blacks exhibit a pronounced stiffening effect when incorporated in a rubber mix. In this article it is shown that carbon blacks differ greatly in this respect and that they may be divided roughly into two main classes, one of which has a regnforcing effect much greater than, and the other equal to or even less than, an equal volume of ordinary zinc oxide such as is used in the rubber industry. This difference is so great a t times that, although two rubber stocks, each containing 14 volumes of a different black on 100 volumes of rubber, may have approximately the same ultimate tensile, the tensile value at 500 per cent elongation may be twice as great in the one case as in the other. It follows, of course, that the per cent stretch a t break must be much greater in the stock having the lower tensile at 500 per cent elongation. This remarkable difference in stiffening effect is exhibited to an even greater degree when the respective carbons are ground into a mineral oil vehicle, such as is suitable for making news ink. Here the rate of flow from a given orifice may be almost five times as fast in one case as in the other. The wetting equivalent of carbon black, an arbitrary term which is defined later-is a fairly reliable index of the stiffening power in a rubber mix. These relations are by no means exact, but qualitatively they are often useful. The wetting equivalent values demonstrate in general that carbons may be divided by this means into a high and low stiffening class. There is no necessary relation between the stiffening power of carbons in rubber or oils and the adsorption of (1) malachite green in aqueous solution, (2) Victoria blue in benzene, and (3) hexamethylenetetramine in benzene. Some carbons with high stiffening powers for rubber adsorb the smallest amounts of these substances named. Finally, the writers’ observations on very thin films2 of rubber stocks containing carbon black lead to the conclusion that high tensiles are always associated with the colloidal dispersion of a t least a part of the carbon black in the rubber matrix, where the particles of the carbon are too small to be resolved by the microscopic objective. There is some evidence to show, however, that such relations do not necessarily obtain in the case of stiffening effect. Stocks
U
1
Presented before the joint meeting of the Division of Rubber Chem-
istry and the Akron Section of the American Chemical Society, Akron, Ohio,
February 22 and 23,1926. I Tms JOURNAL, 17, 936 (1925).
containing the highest regnforcing carbon given in the table and others containing the lowest appear to be identical when viewed in the microscope. Myriads of tiny black particles or aggregates with intervening spaces having a brownish color are discernible in both cases. Experimental Data
The experimental data are shown in the accompanying table. The following formulas for compounding the rubber stocks were used: -FORMULA Pale crepe Carbon Zinc oxide Sulfur DPG
-1 73.5 19.06 4.33 1.84 1.27
100.00
-FORMULA Pale crepe Carbon Zinc oxide Sulfur Hexa
2-7 72.23 18.80 4.25 3.60 1.12
100.00
The different carbons used in this work are given in code form. Six of them are widely known in the rubber industry. Some of the remainder are used extensively in the production of printer’s ink and news ink. Many of them are experimental carbons that have never appeared on the market. The list includes carbons produced by widely different methods. Some are collected on a surface as in the channel or disk processes and some are made by condensation in the gaseous phase. In making the inks, 5 per cent by weight of the respective carbon black was ground for 16 hours in a ball mill into a 95 per cent (by weight) mineral oil vehicle suitable for making news inks. These figures are only relative. The figures for time necessary for a given volume of the inks to flow through a given orifice of arbitrary dimensions (Column 6) have not been reduced to such terms that they may be connected with the usual expressions for viscosity on plasticity. The wetting equivalent values (Column 7) represent the number of cubic centimeters of a neutral, pale yellow, raw linseed oil that must be added to 100 grams of a given carbon so that the mass may be pressed into a coherent ball similar to a stiff, dry putty. The oil is added from a buret in small portions a t first and finally drop by drop toward the end of the titration. The small balls of carbon and oil are squeezed out and the mass pressed as much as possible with a flexible paint knife. The end point is fairly sharp, being 2 to 3 drops for 5 grams of carbon. These values are not identical
April, 1926
419
INDUSTRIAL AND ENGINEERING CHEMISTRY Carbon Black Data
Carbon (1)
Optimum cure Min. at 40 Ibs. steam (2)
A B
c
D E- 1 E-2 F
G
H
I- 1 1-2 No. 62 P-3 P-6 P-7 P-8 P-9 P-164 A
c
E-2 I-2 P-3 P-6 P-9 ( a ) Too small t o be
-T&NSILE
600 per cent elon ation Lbs.jsq. in. (3)
-TA
Break Lbs. / s q . in. (4)
45 55 25 35 35 25 35 45 35 35 45 35 35 25 35 35 35 35 75 60 60 75 60 75 75 measured b y
2770 3955 3967 1477 3387 4380 4232 3363 1415 4300 4552 1752 4245 1204 method employed.
Time of Per cent flow elongation a t of ink break Seconds (5) (6) Formula I 706 70 722 72 688 40 622 45 604 64 1 190 638 98 716 56 712 52 702 65 666 60 711 732 790 750 752 737 75s Fovm:uIa 2 625 702 600 590 737 719 735
with the oil absorption number so often used for paint pigments. The oil absorption number is often the same as the wetting equivalent when the carbon is very compact and has a very low oil absorption. The oil absorption number is entirely misleading, however, when the carbon is very fluffy, because it is impossible to avoid adding too much oil before the end point is reached. The little balls of carbon will remain dry on the outside, but the interior will be soaked with oil where the pigment is very light and fluffy, sometimes making the oil absorption number twice as great as the weting equivalent. The wetting equivalent is much more nearly indicative of the stiffening power in rubber than is the oil absorption number. The wetting equivalent is not an infallible guide, however, for P-16-b, with a wetting equivalent value of 88, is not a stiffening carbon. The adsorption figures for malachite green are the number of grams adsorbed by 100 grams of the carbon from 10 liters of an aqueous solution containing 0.5 gram of malachite green per liter. Thus carbon A takes out practically all of the malachite green under these conditions. For Victoria blue are given the number of grams of the respective carbons necessary to adsorb the same amount of Victoria blue from a definite volume of a benzene solution having an initial concentration of 0.29 gram of Victoria blue per liter. The concentration of the dye in the solution was, therefore, the same in each experiment when the carbon in question and the solution had come to an equilibrium. This is very important; otherwise the adsorption power of the various carbons would have been measured under different states of equilibrium with the dye. Thus, 100 grams of carbon A and 150 grams of carbon G will adsorb exactly the same amount of Victoria blue under identical conditions. Carbon A , therefore, has the greater adsorption power. The figures for hexamethylenetetramine represent the weight in grams that is adsorbed by 100 grams of the given carbon from a benzene solution where the initial concentration is the same in all cases-namely, 0.2 gram of hexamethylenetetramine per liter. The method of obtaining these values is by no means faultless, but they are a t least relatively correct. Discussion of Results The stress-strain curves of zinc oxide and carbon C stocks are very similar. This has been attested by many experimenters. Column 3 shows that carbons C, P-3, P-6, P-7, P-8, and P-9 exert a stiffening power not greater than an equivalent volume of zinc oxide. Carbons A , B, I-1 and
.
Wetting equivalent (7)
ADSORPTION : F O Malachite green Victoria in blue in water benzene (8) (Q)
Hun in
benzene (10)
144 162 36 119
5.0 3.5 0.5
100
325 218 137 123 155 132 7 5 .._ 39 38 41 44
2.0
(4
0.073
4.0 4.1 4.1 4.2 0.5
150 160
0.25 0.35
140
0.32
0.58
(0)
88
No. 62 have a medium stiffening effect; whereas the others have a very great stiffening effect. The pronounced difference between high and low classes is brought out especially well in the high-sulfur stock (Formula 2). The wetting equivalent is usually high for carbons that stiffen rubber to a pronounced degree and very low for those that do not. P-16-b is an exception and no doubt others will be found later on wider experience with the application of this method. The wetting equivalent of zinc oxide such as generally used in the rubber industry is about 30. Carbons E-2 and F have abnormally high wetting equivalents. These carbons it will be noticed also make very stiff inks. Carbon F is a high-grade lampblack. Carbon A is of special interest because its stiffening effect in rubber is only moderate, whereas its absorption for oil and dyes is consistently high throughout the series. Carbon E-2 exhibits the phenomenon of selective absorption in a very striking manner; its stiffening power for rubber is abnormally high, but its absorption for Victoria blue and hexamethylenetetramine is unusually low. Carbon E-2 requires the minimum curing time of 25 minutes. There may be some relation between this and the low absorption of hexamethylenetetramine, especially as carbon A , having a high absorption power, retards the cure and therefore requires 45 minutes. Carbon 62 and those of the P series are black and fluffy. Some of them have a comparatively high tinctorial power and a blue undertone. The significance of these facts will be apparent to the manufacturer of gray and black paints. Observations with the Microscope Work with the microscope using very thin sections of carbon-black rubber stocks indicates a definite relation between the distribution of the carbon in the rubber matrix and the ultimate tensile, but it fails to show that the size of the particles or aggregates of carbon has any bearing on the stiffening effect. This statement is based on microscopic observations of diphenylguanidine stocks containing various carbons. For instance, stocks containing carbon C show myriads of tiny, dark particles or aggregates which are easily discernible a t 1500 diameters magnification. The spaces between these particles are quite colorless if the section is very thin. It does not follow, of course, that there are no colloidal carbon particles in these intervening spaces, because the undertone of carbon C is violet blue.*
-4X-b:
ILVDUSTRIALA N D ENGINEERING CHEMISTRY
The slides of stocks containing carbons D and F are net g c a b o n C; but carbons effect in rubber, whereas g class. All three, however, give stocks with a comparatively low ultimate tensile. The microscopic picture of stocks containing carbon P-8 differs from that of carbon C in several respects. Tiny particles or aggregates of carbon can be seen in both cases, but in P-8 stocks they are very much finer. The most striking difference, however, is that the intervening matrix between the particles is colored brown in P-8 stocks. Here it will be*noticed the Wsile is higd but the stiffening power of P-8 is unusually low. If the brown color is due to colloidally dispersed carbon in the rubber between the larger carbon particles or aggregates, it apparently has no pronounced effect on the stiffening power. Carbon P-8 has a blue undertone, but it is not so dominant as that of carbon C. If the distribution of the carbon in rubber were a cardinal factor in the regulation of the stiffening effect, then stocks of carbons E-2 and P-8 should give very different pictures under the microscope. In point of fact it is quite difficult to distinguish between them. The resolved particles or aggregates are to all appearances identical in the two cases. The only noticeable difference is in the depth of color in the matrix between the particles; in P-8 stocks it is somewhat greater than in those containing E-2.
Vol. 18, No. 4
Finally, carbon A stocks show scarcely any particles. The matrix tmnsmits yellow, orange, red, brown, and even darker colors, depending upon the thickness of the film on the slide. Here again the aolored matrix is concomitant with high tensile, but quite fortuitous to the stiffening power, which is only medium. Conclusion
From these and other observations which the writers hope to publish later, it is concluded that the whole story of the stiffening power of pigments has not yet been told. Particle size in the rubber mix is undoubtedly a factor, but this does not predominate to the extent that other conditions yet to be satisfactorily explained may be disregarded. Carbon black is a complex substance physically, containing particles and aggregates which differ greatly in size and probably also in shape. That the portion of the carbon which causes the high tensile in a rubber mix is probably not the portion that is responsible for the stiffening effect. It is also quite possible that a third portion takes no part in either the reenforcing or the tensile, but acts merely in the capacity of a diluent. Perhaps work on the fractional precipitation, flotation, or dialysis of carbon black may throw some light on this interesting and important phase of the subject.
rhe Addition of Light to Accelerated Aging' By Frederick P. Jecusco CHATHAM RUBBERTHREAD Co., MIDDLETOWN, CONN.
T
WO accelerated aging tests have come into use within the past few years-namely, the Geer oven2and the Davis-Bierer oxygen b0mb.j These methods are accomplished without light and this paper attempts to present some preliminary data as to what might be expected to be gained from the addition of this factor. Asano4 proposed using this combination of heat and light but gives no data, except on raw rubber without heat. Test Sample
A pure rubber-sulfur mix was used for three reasons: (1) it resisted aging by other methods; (2) it is universally admitted to be the most sensitive to light; and (3) rubber and sulfur are the foundations of all technical mixes.5 Therefore, a vulcanizate was used which contained 100 parts of para rubber and 10 parts of sulfur, the optimum cure being 120 minutes. Apparatus
The oven used is of a standard construction, 3 feet deep, 5 feet high, and 4 feet wide, with walls filled with 2-inch asbestos air insulation. It has a specially constructed wooden reel 24 inches in diameter and 28 inches long, having eight arms and 11wooden cross rods 1 inch in diameter, forming an open reel of the squirrel-cage type. This reel revolves on a shaft entering through each side of the oven a t a slow speed 1 Presented before the joint meeting of the Division of Rubber Chemistry and the Akron Section of the American Chemical Society, Akron, Ohio, February 22 and 23, 1926. 2 Indra Rubber World, 56, 127 (1916); 64,887 (1921). a THIS JOURNAL, 16, 711 (1924); 17, 860 (1925). 4 I n d i a Rubbm J . , 70, No. 8, 11; No. 10, 11 (1925). 6 Whitby, "Plantation Rubber and the Testing of Rubber," 1920, Chap. XVI, p. 386. Longmans, Green & Company.
(26 r. p, m.) by means of a small electric motor and driving gear placed on one side of the oven. Underneath a perforated floor plate are placed sufficient electric heating units to give a range of temperature from 50" to 100" C. under automatic temperature control, operated by a thermometer thermostat having a range from 50" to 100' C. Underneath the floor plates on either side of the oven are large adjustable air shutters and baffle plates to admit the circulation of air through the heaters. There is also a top vent with damper control. Into this oven, 7 cm. above the reel which holds the samples of rubber, was placed an Uviarc (mercury vapor) quartz lamp. The lamp was so situated that its rays fell a t right angles to the narrow part of the dumb-bell samples, held by clips, which in turn were fastened on two metallic hoops. Maximum thermometers are fastened at various points on the reel and checked within 1' C., after the oven has operated 2 hours. The oven temperature is maintained a t 160' F. (71' (3.). Comparative Experiments with and without Addition of Light
Preliminary figures of breaking stresses obtained from samples exposed to the light both edgewise and flatwise showed that there was very little difference. The samples therefore were exposed flat, as they could then be quickly inserted and taken out of the clips giving speed to the handling of dumb-bells, and thus preventing too great an exposure to the Uviarc light, which causes a skin burn and an irritation to the eyes. The figures plotted for the stresses are the averages of three breaks obtained on a Scott machine. These breaks were not made until 24 hours after the samples had been removed. Samples were removed a t intervals of 24 hours,
