CARBON BLACKS Comparison of a Fully Reinforcing Furnace Black and Easy Processing Channel Black C. A. STOKES AND E. M. DANNENBERG Godfrey L. Cabot, Inc., Boston, Mass. Furnace blacks which are as effective in their reinforcing ability in rubber as the widely used easy processing type of channel black now are available commercially. These fully reinforcing furnace blaclrs eventually may replace the channel blacks which are made a t lower yield by a relatively inflexible process that causes a smoke nuisance and a waste of natural gas. The literature contains no detailed or critical comparison of a selected fully reinforcing furnace black with a standard grade of channel black. In order to make this information available to prospective iisers of the new furnace blacks, this paper presents a detailed and critical comparison of Sterling 105, a fully reinforcing furnace black, with Spheron 9, a n easy processing channel black. The data show remarkable similarities in
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1 IPORTAST current technological problem facing both the rubber industry and the carbon black nianufacturer is the determination of the fundamental physical and chemical constitution and the rubber compounding properties of the newer reinforcing furnace blacks. These low particle size furnace blacks have been developed to provide replacements for channel blacks because it seems evident that rising natural gas prices will increase gradually the cost of the low yield channel blacks and force them off the market for all but specialty uses. Besides these economic factors, the furnace process can produce also various grades of rubber blacks possessing a combination of processing and rubber properties unattainable with the channel process. The fact that the furnace process has greater flexibility than the channel process and can be operated with no smoke nuisance points inescapably to the eventual replacement of the channel process by the more technologically advanced furnace process. The development of furnace blacks has occurred in the order of increasing power of reinforcement. The first commerc a1 varieties of furnace blacks introduced to the rubber trade were called semireinforcing and were designated as SRF. This relatively low reinforcing type was followed by high modulus (FILL?) and fine furnace (FF) blacks. Each of these blacks was characterized by smaller particle she and greater reinforcing power than its predecessor. The type of black that is here termed fully reinforciiig is the most recent development in the furnace black field and represents a definite step up the scale of reinforcement to the easy processing channel black level. These newer furnace blacks have been called fully reinforcing furnace blacks in this discussion to distinguish them from furnace blacks hithert,o known as fine furnace blacks. Fine furnace blacks give 80 to 90% of easy processing (EPC) channel black road wear whereas the fully reinforcing furnace blacks give road wear approximately equal to, and in some cases greater than, easy processing blacks (16). It would be somewhat misleading to call these furnace blacks channel type furnace blacks because of the different surface characteristics and some significant rubber property differences, compared to channel blacks, as will be shown by the data presented here.
the rubber properties of tho furnace black and the channel black. This is to be expected in view of their closely similar particle size and particle size distribution. Rubber tire treads containing these blacks gave nearly identical road wear performance. The distinct difference in the surface characteristics of these blacks is shown by a 100,000-fold greater electrical conductivity of the furnace black rubber stoclrs. This differcncc in surface constitution is a major contributing factor in the greater tendency of the furnace black stocks to scorch. The tendency of the furnace black to promote scorch is the only outstanding difference when it is used as a channel black replacement. Further research aimed a t overcoming this defect is underway.
A number of comprehensive evaluations of the rubber properties of various carbon black types in natural rubber (1, 11), G12-S (lb), Butyl (18). and neoprene ( 1 ) have been published. Drogin and Bishop (6) recently have made a complete summary of the rubber properties of the various new commercial fully reinforcing furnace blacks. Cnrr and Wiegand (4)have published a summary of the properties of a fully reinforcing furnace black compared with an easy processing channel black. The literature does not contain any comprehensive comparison between the fundamental physical and chemical properties and rubber properties of a fully reinforcing furnace black and a definite grade of channel black, probably because fully reinforcing furnace blacks have only recently come on the market in carload quantities. I t 15 the purpose of this paper to present such a critical comparison between a selected fully reinforcing furnace black and an easy processing channel black and to relate their fundamental chemical and physical differences to the practical properties of rubber-black compounds. This paper can establish only a sound basis of comparison and present certain fairly comprehensive supporting data; further significant conclusions will he reported when research now under way in these laboratories has hcen completed. The blacks selected for study were Sterling 105 (fully reinforcing furnace) and Spheron 9 (easy processing channel black). In the presentation following, these two blacks will be referred to simply as furnace or channel black since only two blacks are under consideration. The different characteristics of furnace and channel blacks are the result of different combustion conditions inherent in the two processes of manufacturc. Therefore, it is pertinent to consider briefly some of thcse conditions. A. The furnace process employs only about 50% of the air theoretically required to burn all of the natural gas raw material to carbon dioxide and water. All of this air is consumed rapidly by combustion to carbon mono- and dioxides and water in thc first 10 to 20% of the furnace length. The channel process employs an unlimited supply of air, although obviously combustion conditions are so regulated that the fuel (natural gas) is not consumed entirely.
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t"
Furnace
(Sterling 105)
Channel (Spheron 9)
Figure 1. Electron Micrographs of Furnace and Channel Blacks
13. The black made in the furnace process is exposed on the average to temperatures a t the 2000" to 2500" F. level for a t least one order of magnitude longei than channel black. At this temperature level rapid after-treatment of furnace black by reaction with carbon dioxide and water can occur, but little combined oxygen can be added to the black since degassing is rapid and practically complete in this temperature range. C. After 1 to 5 seconds a t the 2000" to 2500' E". level, furnace black is quenched in a fraction of a second to temperatures a t which further reaction with carbon dioxide and water (no residual oxygen is present) is negligibly dow. On the other hand, channel black after deposition on the channels is exposed to air for many seconds a t the 900" F. level. At 900" F. aftertreatment by reaction with oxygen in the air is iapid and takes place with addition of combined oxygen to the surface of the black. L). The flames employed in the furnace process are longer and thicker than those used in the channel process and may be described a5 nonpremixed flames burning in a mildly turbulent insulated enclosure. Channel black flames are relatively thin true diffusion flames burning in a practically still and relatively uninsulated enclosure. E. Furnace blacks in practically all cases are contaminated not only by traces of iron rust and scale but also by the dissolved solids deposited by total evaporation of the water used for quenching the carbon-laden flue gases. Channel blacks are contaminated only by deposition of wind-carried sand and scale and rust from the steel channels and conyeyer tubes. Each o f these differences between the channel and the furnace black processes can be related to properties of the respective blacks. It is conceded generally that particle size and particle size distribution of carbon black are important variables; these determine their performance in such applications as rubber, ink, paint, and the like. As a first step in this investigation, electron micrographs were obtained. Duplicate dispersions were prepared for each black. The channel black mounts were prepared by sparking the dry black on a precast film of collodion. The furnace black mounts were prepared as follows: 490 grams of nitrocellulose and 3809 grams of butyl acetate were mixed together and allowed to stand 36 hours to dissolve all of the nitrocellulose. To 650 grams of this solution 14.9 grams of black were added and ground on the ball mill for 24 hours. An additional 100 grams of butyl acetate r e r e added to thin the dispersion. Three different fields were photographed in each mount. I n obtaining particle size counts two or more independent counts mere made on each black by different observers who took different mounts for counting. Figure 1shows representative fields for the channel and furnace blacks a t the same magnification. While other evidence cited indicates that the furnace black possesses more of the property
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called structure than the channel black, it is difficult t o reach this conclusion by inspection of the micrographs, particularly when the possible effect of the dispersion techniques on structure is considered. Figure 2 shows the two particle size distribution ourves. The nioan (number average) diameter for the furnace black is an average of the mean diameters from four counts of 500 part,iclos each, and the mean diameter of the channel black is an average of two counts of 500 particles each. I t will be seen from the curves that the channel black has the narrower particle size distribution and the lower mean diameter. The broader particle size distribution for furnace blacks can be interpreted readily in light of the differences in flames described under D above. I n the channel flame little time is available for particle growth after nucleuses formation has occurred. The entire process is over in less than 0.1 second. In the flames of the furnace process, on the other hand, nucleuses can continue t o form all during the period in which particles are growing in size. This period, in turn, is something like ten t o fifty t.imes as great as the corresponding period in the channel process.
PARTICLE SIZE DISTRIBUTION ELECTRON MICROSCOPE MEASVREMENT
40
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30
LL
0 PARTICLE
DIAMETER
,
A'
Figure 2
Parkinson ( 1 1 ) has estimated from a correlat,ion of rubber rebound values v j t h particle size that the mean particle size ,of Spheron 9 is 300 A. This compares well with the value of 330 A. obtained in this work. Particle size measurements by a centrifuge method (10) have indicated that the rubber grade channel blacks (HPC,oMPC, EPC) have mean diameters in the range o l 250 to 350 A. Continental AA, an easyoprocessing typc, is reported t o have a mean particle size of 320 A. (28). In connection with the electron microscope studies reported here, fine furn5ce blacks have been found to have mean diameters of 420 to 450 A . These values are an average of the mean diamcters obtained in four independent counts on each of two different commercial fine furnace blacks, studied along with the easy processing and the fully reinforcing furnace black. Table I gives the specific surface areas of the two blacks calculated from the electron microscope particle size distribution data. For comparison the areas by the low temperature nitrogen adsorption method are shown in the same table. The large discrepancy between the specific surface areas oi the furnace black calculated from particle size distribution data and the specific surface measured by low temperature nitrogen adsorption may be explained by the differences in after-treating conditions as described under B and C above.
That the calculated surface area for channel black does not check any better mrith the low temperature nitrogen adsorption surface area measurement is not surprising in view of the limits
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derstanding of the nature of furnace blacks, particularly those of high specific surface area. (Electron microscope and nitrogen adsorption methods) Data obtained in this laboratory have shown repeatedly that Furnace Channel i t is possible to produce carbon blacks, either directly in a furnace Sterling 105 Spheron 9 procesd or indirectly by after-treating existing blacks having Surface area calcd. from particle size dis66 84 practically identical mean particle sizes, as measured by electron tributiona, gram microscope counts or nigrometer index (3) but having varying surSurface area by low temp. nitrogen adsorptionb, 99 99 face areas, as measured by low temperature nitrogen adsorption. gram These discrepancies might be explained by assuming differences 60,000 Efd2 a Surface area, ' E = __ in particle size distribution, but available evidence in the form gram p Zfdfda of particle distribution counts from electron microscope measuref = frequency of particle of dia,b ments does not substantiate this assumption. For example, d = diam. of particle, A. the data in Table I1 (15) show that the surface of a lampblack p = density, grams per cc. = 1.86 can be extended almost tenfold with less than 10% decrease in b Brunauer, Emmett, Teller method. mean diameter. Medium processing black treated similarly showed negligible change in mean diameter when its surface was extended threefold. TABLE 11. EFFECTOF AFTER-TREATMENT ox MEAN PARTICLE It is concluded that the furnace black is mildly after81ZB .4ND SURFACE AREA O F BLOCKS treated-that is, its surface has been extended with no apSurface Area, Sq. M./G. preciable change in particle size distribution by subjection of Calcd. Mean from Calod. the black to the conditions described under B in the discussion Diam. Particle from of after-treatment. This degree of after-treatment is probably from EM" sire Nz Blaok Count, A. Distribution Adsorption not sufficient to have any measurable effects on the rubber properLampblack T (av. of 4 determinations) 1030 26 24 ties of the black exoept those properties associated with structure. Same lampblack,, air after-treated 910 28 205 It is speculated that prolonged exposure to high temperatures (av. of 3 determinations) M P C (Spheron 6) (av. of 3 deter(2500" F. level) rapidly destroys that property of a black deminations) 270 108 MPC (Spheron 6) air after-treated 260 log 114 336 scribed as structure. Thus a furnace black showing evidence of after-treatment would show less structure as evidenced by Eleotron microscope. oil absorption and modulus in rubber than a non-after-treated black, provided the blacks are made from the same raw material and have approximately the same particle size distributions. in precision of the two methods. The calculation of surface A number of other characterizing measurements that are comarea from particle size distribution gives the poorer precision. monly made on blacks are shown in Table 111. The nitrogen surface area value is also subject t o a minus 20% Oil absorption increases with degree of structure and with uncertainty because of the assum tion that adsorbed nitrogen decrease in particle size (16). As the data show, the oil ahsorpoccupies an area corresponding to t i e liquid state rather than the solid state. tions of the furnace and channel black are the same. The nigrometer index, which compares the relative reflectance Bfter-treatment is defined here as the reaction of carbon black of light from a dispersion of carbon black in a vegetable or mineral with carbon dioxide, water, or oxygen with resulting increase oil vehicle, is a good measure of particle size when camin specific surface, presumably due to etching of the carbon surface paring blacks of a similar nature. Unfortunately, the method to produce surface irregularities. After-treatment in the presfor obtaining particle size comparisons between two distinctly ence of oxidizing gases like oxygen, water, and carbon dioxide different types of blacks, such as a furnace and a channel black, can take place in two ways: is not known with any degree of certainty. Good evidence At lower temperatures, addition of oxygen complexes preindicates that a furance black with the same mean diameter as a dominates over loss of carbon as monc- and dioxide. Apparently channel black may show a nigrometer index two to three points oxygen is added until the surface is almost saturated with oxygenhigher than the channel black when tested under identical condicarbon complexes. Until this point is reached, relatively little tions. The effect of the vehicle used in preparing the black disextension of surface takes place; above the saturation value persion for the nigrometer measurement is shown by the comextension of surface is marked and the oxygen content of the parison between the heavy mineral oil and the linseed oil vehicle. black continues to increase. At higher temperatures reaction of the surface with TABLE 111. ANALYTICAL DATA ON BLACKS oxidizing gases is rapid but hleasurement Method Sterling 105 Spheron 9 oxygen complexes tend to be Oil absorption, lb./100 lb. black Modified Gardner-Coleman driven off almost as fast as 82 82 Nigrometer index Boiled linseed 3 cc. 90 85.5 formed carrying carbon away Heavy minerd oil (Nujol), 3 00, 84.5 80.5 as mono- and dioxides. ExTinting strength Equal Extractable matter Too low to measure tension of surface is rapid in Iodine adsorption, mg. 1% per this case as is the loss of cargram 142 177 Diphenylguanidine adsorption, % bon. 12 31 9.1 3.8 These two general types of %?h, % by weight I p i i i h n t o const~iiiw t . 0.42 0.03 after-treatment are extremes. on wt of black Spectroscopic analy3is %Calcium 0.1-0.001
