Relation of Paint Properties to Surface Areas of Carbon Black OWEN J. BROWN, JR., AND W. R. SMITH Godfrey L. Cabot, Inc., Boston, Mass.
monly used for determination of adsorption isotherms; it consists of an adsorption cell of about 5-cc. capacity connected to a measuring buret and manometer by means of which the volume of nitrogen adsorbed can be readily determined as a function of the pressure. The dead space in the apparatus is determined before each adsorption isotherm by means of helium. The adsorption is carried out a t -195.8" C. At this temperature adsorption is purely physical (2). Experiments have been performed to indicate that the adsorption at this temperature is entirely reversible ( I S ) , and that the nitrogen so adsorbed can be readily pumped off at this same temperature. The adsorption isotherms for carbon blacks have been found to be S-shaped. It has been established (4) that the first break in the adsorption isotherm corresponds t o the point a t which a sufficient number of nitrogen molecules have been adsorbed to form a complete unimolecular coating over the entire carbon surface. Assuming liquid packing, the absolute surface area of the carbon is readily calculated by multiplying the number of nitrogen molecules in the unimolecular film by the average area covered by a single molecule. VOLATILEMATTER. This may be determined by noting the loss in weight when the sample of black is heated for 7 minutes a t 1760" F. (955" C.) ( S , 1 4 ) ; it consists essentially of carbon monoxide and dioxide, and indicates the amount of chemisorbed oxygen on the surface of the black (10). Small amounts of hydrogen are also present and represent residual hydrogen chemicallycombined on the carbon as a consequence of the pyrolytic reactions occurring in the flame when the carbon black is formed.
EVERAL types of channel carbon blacks are available to the industry, each with its specific properties and the ability to fill different needs in coatings. Every paint company carries a stock of many different types of carbon black in order to be able to formulate various black coatings. While in the past the paint manufacturer has been able through practical experience to evaluate these pigments in an empirical fashion, in recent years techniques have become available to permit us to describe some of the properties of these pigments more exactly. Notable advances have been made in the past few years, particularly in techniques for evaluating particle size, size distribution, and surface areas. Foremost among these recent techniques are centrifuge methods for particle size distribution (7, 8, lb), the electron microscope for particle diameter measurements (Q), and lowtemperature adsorption isotherms for total surface area (4). Since the extreme fineness of carbon blacks has long been recognized, it has often been suggested that the values of carbon black as a pigment can best be judged by its surface area. The surface areas of pigments in the paint field are thought t o be of extreme importance since so many of their properties apparently depend upon this specific characteristic. Covering power, color, and other of the useful properties are definitely connected with the amount of surface of the pigment. In the present paper the surface areas and other properties of a number of commercial carbon blacks have been determined, and we have attempted t o translate these properties into a few terms of practical paint and lacquer performance.
S
The total surface areas of a number of carbon blacks of commercial importance to the coating industry have been determined from low-temperature nitrogen adsorption isotherms. In most cases the commercially significant properties appear to be direct functions of this surface. These properties include color, time of drying, and paste consistency. In those blacks where the correlation between surface area and properties is not as evident, it is proposed that the measured surface is not completely exposed to the vehicle.
BLACKNESS. The Cabot nigrometer was used (Figure 1) as one means of measuring blackness (6, 11). It determines jetness by noting how much light is absorbed by a given carbon black when dispersed in oil. That fraction of light which is not absorbed is reflected, and its intensity is measured empirically by the nigrometer. Consequently, lower nigrometer values indicate increasing blackness. To simplify the study, one general type of vehicle was used. Most of this work was done with Beckosol 1313, a pure alkyd with 44 to 46 per cent solids. The same general results were found with Rezyl solution 330-5 and Glyptal solution 2452,
Experimental Procedure
SURFACE AREA. The surface areas reported in Table I were determined by the low-temperature nitrogen adsorption isotherm technique as developed by Emmett and his coworkers ( 2 , 4). The method is reliable, and results can be reproduced to about 7 per cent. It has been applied successfully to a large number of finely divided materials (4, I S ) . The results represent the total surface area of the materials and are independent of the activity or nature of the surface involved (2). The apparatus employed ( I S ) is of the type com352
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353
TABLEI. SURFACE AREAS OB COMMERCIAL CARBON BLACKS
Sample Carbolac A Black Pearls 2 Black Pearls 3 Black Pearls 71 Super Carbovar P Black Pearls 74 Black Pearls 78 Elf 25
Surface, (Ma/ Qram)
Nigrometer Value
Volatile Matter,
%
PH
832 733 542 376 334 294 220 96
62.8
16.9 13.2 11.1 4.3 4.5 4.4 4.0 5.3
2.4 2.6 2.9 5.6 5.0 4.9 4.3 4.2
66.0
71.0 72.7 71.8 75.2 78.4 83.1
as well as typical automotive lacquer formulations. The same order of results would undoubtedly be obtained if the blacks were incorporated with any of the common vehicles used by the industry. All grindings were carried out in Trojan steel ball mills for approximately 50 hours. The black concentration was 5 per cent on the weight of the solid resin. The grinds were thinned with a solvent consisting of 3 parts coal tar naphtha to 1 part petroleum naphtha, and sprayed on steel panels at 55 pounds per square inch (3.9 kg. per sq. cm.) pressure. The panels were compared visually for color.
FOR MEASURING BLACKNESS FIGURE 1. NIGRONETER
RELATIVEPASTECONSISTENCY. Pastes were prepared by incorporating 25 parts of carbon black with 75 parts of raw linseed oil. The mixture was then given three passes through a three-roll laboratory mill. To facilitate grinding on the roll, the blacks were used in their original unpelletised state. No essential differences in the area of the blacks in these two physical conditions could be noted. The pastes were then put into a hollow cylindrical cup with an inside diameter of 3.8 cm. and a depth of 4.7 cm. A rotating solid disk with a diameter of 3 cm. and a depth of 4 cm. was placed inside the cup, and the relative consistency was measured by noting how much weight was required to rotate this solid cylinder through the paste at the rate of 100 revolutions per minute. The paste was kept a t 25” C. in all measurements. DRYING. To study the effect of various blacks on rate of drying, the blacks were first ground in the Trojan steel ball mills as described under “Blackness”. Immediately after grinding, 0.10 per cent cobalt and 0.15 per cent zinc, based on the solid resin, were added in the form of Nuodex driers. The enamels were poured on steel panels to a uniform thickness,
Blackness of panel 1 (blaokeat) 2 3 4 4‘/1
6
7 8 (grayest)
Relative Paste Consistency, (Grams) 675 627 586 450 450 387. 250 158
Drying, Hours
After grind- After 2 Ing weeks 9.5 8.5 8.0 7.5 7.0 6.5 6.0 6.0
16.0 14.5 13.5 12.5 12.0 10.5
9.0
8.0
and the time for drying at room temperature was noted. The drying times of these black enamels were determined immediately after grinding and after the enamels had been stored at room temperature for 2 weeks.
Results BLACKNESS. The jetness or blackness that carbon blacks impart to a coating is one of the most important properties of the material. As demonstrated in Figure 2 (left), a definite correlation exists between total surface area of the pigment and its color value as judged by the nigrometer. The blackest carbon blacks are those with the largest surface areas. When panels were sprayed and baked, the same correlation was found. Examination of the panels in transmitted light showed that in every instance the enamels prepared with carbon blacks of large surface area gave the blacker coating. RXLATIVEPASTECONSISTENCY.As Figure 2 (center) shows, the blacks with the largest surface areas gave stiffer, more viscous pastes. This was to be expected since, for equal loadings, the black with the largest surface area had the greatest adsorptive capacity for the vehicle. DRYING. The slowest drying black was found to have the largest surface area. Loss of drying power appears to be proportional t o extent of surface. As Figure 2 (right) shows, this relation persists even after the enamels are stored for 2 weeks. TIMEOF WETTINO. In addition, attempts were made to correlate surface area and the speed with which the carbon black was wet by the vehicle. This was done by noting the time required for 3 grams of black to disappear completely from the surface of the vehicle. No definite relation between wetting time and surface area was noted. With unpelletized blacks of comparable apparent density, those of high volatile content appeared to wet somewhat more readily. That this phenomenon is essentially a function of the amount of air occluded by the black (apparent density) is evidenced by the fact that the same blacks in the pelletized form wet almost instantaneously.
Discussion If the carbon black particles are assumed to be spherica? and uniform in size, then it is possible to calculate the particle diameter from the surface area value. For a number of blacks excellent agreement was obtained between diameters calculated in this manner and those obtained by electron microscope observation (4, 9, l a ) . The close agreement is somewhat surpri6ng in view of the fact that the electron microscope value is a measured mean diameter, while the diameter obtained from surface measurements is that of the average particle. This may be taken to indicate that black particles are, in fact, nearly spherical and very uniform in size, a concept supported by electron photomicrographs (9). Although all the blacks presented in Table I have not yet been observed with the electron micrascope, increasing surface of these blacks does appear to correspond to a definite decrease in particle size. Under these conditions the correlation
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FIQURE
2. RELATIOR. O F PAIXT
PERFORJIAXCE CHAR.4CTERISTICS T O
between surface and paint properties as discussed are valid. However, certain carbon blacks have a much greater surface than can be accounted for by their relative particle sizes. For example, a typical long-ink black used in the manufacture of lithographic ink has a surface area of 377 M 2 per gram, but was found to have a dark-field count diameter identical with that of another black whose surface was only 100 M 2 per gram. The properties of this long-ink black when compared with a fine particle black of comparable surface area, such as Black Pearls 71, are strikingly different: Sample Black Pearls 71 Long-ink black
Surface Area Mz/ Gr'arn 376 377
Nigrometer Value 71.2 81.0
P a s t e Consistenoy, Grams 450 93
The large surface of the long-ink black may be due to a n internal porosity of large particles or to some type of agglomeration of smaller discreet particles. Whichever may be the case, normal grinding and compounding procedures do not make this large internal surface available to the vehicle, and accordingly the usual relation between total measured surface and paint properties is not observed. However, where such blacks are subjected to more vigorous grinding in suitable vehicles, they can be made to develop further color ( I S ) . Further investigation of blacks of this type are planned with the electron microscope. It is hoped that this method will shed further light on the structure of those blacks whose surface area and apparent diameter are not in agreement. Where surface area and particle size of the black coincide, increasing surface areas give paste consistencies, viscosities, and oil absorptions much as would be expected. With the blacks of larger surface area more oil-pigment interface is created which no doubt results in the pigment taking up more of the vehicle than is the case with blacks of smaller surface area. The loss of drying power seems to be a direct fwction of the surface area of the black used. This relation is readily understood if it is assumed that the retarding effect is due to a specific adsorption of the drier molecules a t the carbon black surface, the amount adsorbed becoming larger with increasing area. The loss in ability of the drier molecules to function when adsorbed by the carbon black may be due to an oriented adsorption at the surface. The functional group of the drier molecules, being immobilized at the carbon surface, is no longer able to operate in the same manner as a functional
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TOTALSURFACE
group on an entirely free unadsorbed molecule. Examples of this phenomenon are the orientation of stearic acid on carbon black particles in rubber compounds (6), and the adsorption of organic accelerators in rubber compounds (1). The capacity of carbon black to alter the characteristics of adsorbed substances is further strikingly exemplified in the following series of experiments. It is well known that certain substances, such as oleic acid, reduce viscosity and increase the blackness of carbon black in many paint vehicles when the oleic acid is added to the mix as a separate entity. However, when the same amount of oleic acid or similar material is first incorporated with the dry black and the black containing the adsorbed material is then added to the vehicle, the effects mentioned previously are entirely absent. The curves for paint performance characteristics of the blacks as plotted against total surface in Figure 2 show a sharp change in slope above 400 .?Iz per gram. Table I indicates that the three blacks with surface in excess of 400 per gram have a markedly higher percentage of chemisorbed oxygen (volatile matter), TViegand made similar observations on carbon blacks of high volatile content with regard to their pH and adsorptive properties (15). Such blacks are often considered separately, and effects such as those described here are attributed to their high volatile content. From the present surface area values, however, it appears that on a unit area basis these blacks may be regarded as lowvolatile blacks. Assuming a liquid packing of oxygen atoms on the black surface, it was previously pointed out ( I S , 15) that a 5 per cent volatile matter content corresponds to sufficient oxygen to cover almost completely 100 square meters of surface with a unimolecular layer. On this basis only about 40 per cent of the surface of the higher volatile blacks is covered. Accordingly, it is difficult to describe the unique properties of these pigments in terms of an enhanced volatile content. The change in slope of the curves pointed out above is probably due to the fact that the total surface covered by the nitrogen molecules in the present measurements is not readily exposed to the vehicle by present grinding techniques. If they do possess a porous or agglomerated structure as suggested earlier, then by improved grinding techniques it may be possible greatly to enhance the valuable properties of these pigments. Electron microscope measurements, together with the present surface area values, should be of extreme value in determining the relative importance of extent of surface and nature
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of the surface in defining the paint performance characteristics of carbon black.
Acknowledgment We wish to acknowledge the valuable aid given by G. J. Duffy and F. S. Thornhill in collecting this data, and also the helpful assistance of F. H. Amon for his constructive suggestions.
Literature Cited (1) Amon, F. H., and Estelow, R. K., IND.ENQ.CHEM.,24, 579 (1932). (2) Brunauer, S., and Emmett, P. H., J. Am. Chem. SOC.,59, 2682 (1937). (3) Cabot, G. L., Inc., "Rubber Black Testing Procedures", 1940. (4) Emmett, P. H., and De Witt, Thomas, IND. ENQ.CHEM.,ANAL. ED., 13, 28 (1941).
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Gehman, S. D., and Field, J. E., IND.ENG.CHEM.,32, 1401 (1940).
Hardy, A. C. (to Godfrey L. Cabot, Inc.), U. 8. Patent 1,780,231 (April 11, 1930). Hauser, E. A., and Lynn, J. E., IND. ENG.CHEM.,32, 659 (1940). Hauser, E. A., and Reed, C. E., J . Phys. Chem., 40, 1169 (1936). Herring, H., Gizycki, I. v., and Kirseok, A., Kautschuk, 17, 55 (1941); Columbian Carbon Co. Research Lab., Rubber Chem. Tech., 14, 52 (1941) Johnson, C. R., IND. ENQ.CHEM.,20, 904 (1928). Johnson, C.R., India Rubber World, 77, No. 5 , 65 (1927). Parkinson, D., Trans. I m t . Rubber Ind., 16, 87 (1940). Smith, W. R., Thornhill, F. S., and Bray, R. I., IND.ENG. CHEM.,33, 1303 (1941). Stanton, F. M., and Fieldner, A. C., U. 9. Bur. Mines, Tech. Paper 8 (1912). Wiegand, W. B., IND. ENG.CHEM.,29, 953 (1937). PRESENTED before the Division of Paint, Varnish, and Plastios Chemistry at the 102nd Meeting of the AMERICAN CHEMICAL SOCIETY, Atlantic City, N. J .
Thermal Reaction of Ethylene with Acetylene J
J
ERNEST A. NARAGON', ROBERT E. BURK, HERMAN P. LANKELMA
AND
Following the indications of earlier work of Burlr, Baldwin, and Whitacre, acetylene has been shown to react more readily with ethylene than the latter reacts with itself. In this cross reaction butadiene is the preponderant gaseous product although the liquid product exceeds the gaseous. While conditions were chosen to favor the cross reaction, no attempt was made to ascertain the optimum yield of butadiene by this procedure; our aim was to establish with certainty the existence of the cross reaction. No butadiene is formed when acetylene reacts with itself under conditions used in these experiments. Diolefins and aromatics were prominent in the liquid products. UTADIENE has been obtained as a thermal reaction product of ethylene (4,26) and also of mixtures of ethylene and acetylene (1,4,6,8,11). Burk, Baldwin, and Whitacre (4) summarized the various mechanisms proposed for the pyrolysis of ethylene and pointed out that the experimental observations upon which they are based do not always agree. They studied the pyrolysis of deoxygenated ethylene a t 625' C. and from their results, together with previous work, proposed a dual mechanism:
B
,
1
Present address, The Texas Company, Beacon. N. Y.
Western Reserve University, Cleveland, Ohio
The reaction between ethylene and acetylene to form butadiene, first proposed by Berthelot (I), was supported by the fact that the addition of small amounts of acetylene (1.85-3.3 per cent) to the ethylene increased the amount of butadiene formed, affected only slightly the general distribution of products, and was consistent with the order of the reaction under conditions where butadiene is observed. A full investigation of the reaction was not made, however. The present work is designed to study the reaction between ethylene and acetylene as it pertains to the formation of butadiene and also to liquid hydrocarbons which might be formed from butadiene as an intermediate.
Experimental Procedure A mixture of deoxygenated ethylene-acetylene, containing a trace of carbon monoxide added for the purpose and diluted with deoxygenated steam, was heated to 610' C. at atmospheric pressure for reaction times varying between 16.5 and 24.47 seconds. From 12 to 85 liters of ethylene-acetylene were used in each experiment, and the extent of reaction varied between 4.13 and 12.2 per cent for the mixture. Anesthetic grade ethylene was employed. Analysis showed it to be 99.7 per cent pure. The acetylene was generated from calcium carbide and purified by passage through a series of absorption pipets containing reagents for the removal of the various impurities. The ethylene-acetylene mixture was deoxygenated by the method of Burk, Baldwin, and Whitacre (4). The carbon monoxide for the purpose was prepared by the method suggested by Thompson (16). The water for the steam diluent was deoxygenated by the method of Kobe and Gooding (9). Deoxygenated nitrogen was used to displace the air from the reaction system. The flow system consisted of a feed system, a reaction vessel, and traps for collecting the products. The feed system was divided into two parts, one for feeding the gaseous mixture and the other for simultaneously feeding the steam.
