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TABLE IV First Latex Pale Crepe.. . . . . . 9 2 . 5 parts Sulfur.. . . . . . . . . . . . . . . . . . . . 7 . 5 parts Accelerator.. . . . . . . . . . . . . . . . . . . X parts Vulcanized for 90 min. a t 148' C. X = Parts -Acetone ExtractAccelerator Physical Per cent Per cent Added t o Prop(Uncor(CorODOROF CUREDSLAB the Mixture erties rected) rected) Control. 0.00 1 Standard 8.630 2 894 Phenyl Mustard Oil.. 0.59 -1 Phenyl mustard oil 8.443 2.729 Aniline ...................... 0.41 4 Decomposition products of aniline 7.065 3.137 Phenyl Mustard Oil and Aniline CBHshTCS = 0 . 5 9 3 Phenyl mustard oil and decomposition prod- 7.966 3.595 CoHsNHt = 0 . 4 1 ucts of aniline, the latter predominating Diphenylthiourea . . . . . . . . . . . . . . 1 .o 2 Phenyl mustard oil and decomposition prod- 7.429 3.119 ucts of aniline, the latter predominating Diphenylthiourea and Aniline.. .. (C6Hah")nCS = 1 . 0 6 Decomposition products of aniline predom- 7.301 3.963 CsHsNH2 = 0.41 inating Triphenylguanidine . . . . . . . . . . . . . 1.25 5 Strong, but indistinguishable. Phenyl mu+ 7.523 4.063 tard oil could be detected
...................... ..........
the preceding equations. The results obtained for t h e acetone extracts of the different mixtures, together with all t h e sulfur estimations made, are also recorded in detail in Table IV. T o facilitate comparison, t h e combined sulfurs, expressed as sulfur coefficients, are tabulated in Table V in t h e order of their relative magnitude. TABLE V First Latex Pale Crbpe.. 92.5 parts S u l f u r , ....................... 7 . 5 parts Accelerator. . . . . . . . . . . . . . . . . . . X parts Vulcanized for 90 min. a t 148" C. Combined Sulfur Sulfur CoefPer cent ficient ACCELERATOR Phenyl Mustard Oil . . . 1.726 1.877 1.937 Control (No Acceler 1.792 Diphenylthiourea.. . . . . 3.078 3.361 Phenyl Mustard Oil . . . . . . . . . 3.100 3.385 Aniline, . . . . . . . . 3.625 3.935 3.872 4.240 Triphenylguanidine , Diphenylthiourea an 3.959 4.340
.......
...
............
...........
Excess (+) Sulfur Coefficient -0,060
:
1 424 1.448 1.998 2.303 2.403
From Table V i t is seen t h a t t h e sulfur coefficients obtained confirmed very closely t h e estimates made upon t h e physical properties. Further, i t is shown t h a t t h e activity of diphenylthiourea is almost exactly equal t o t h a t obtained with a n amount of aniline corresponding t o t h e quantity formed in its decomposition products as expressed in Equation I . Likewise, triphenylguanidine was found t o have t h e same activity as obtained with an equivalent amount of diphenylthiourea a n d aniline, as required b y Equation 2 , when t h e reaction proceeds toward t h e left. It is evident t h a t t h e action of both diphenylthiourea and, triphenylguanidine as accelerators is due t o their tendency t o decompose, under vulcanizing conditions a n d temperatures, with aniline as one of t h e most probable degradation products. Moreover, i t would appear t h a t t h e aniline so formed is responsible for t h e acceleration effected, a n d owes its activity t o t h e pres'ence of an active group containing nitrogen, which functions as a sulfur carrier. T h a t in no case were we able t o recover t h e original amount of accelerator in t h e acetone extract, is a n indication-and an indication only-that a portion of t h e active principle may alsb remain bound t o t h e rubber as well as the sulfur. .'Final emphasis is laid upon the fact t h a t all of our results have been obtained with a mixture composed of rubber, sulfur, and accelerator only. The presence in t h e mixture of 'inorganic oxides has .been found t o have a marked influence on both t h e chemical a n d physical results obtained after vulcanization.
CornFree bined Sulfur Sulfur Per cent Per cent 5.736 1.792 5.714 1.726 3.928 3.625 4.371 3.100
Total Sulfur Per cent (By Addition) 7.528 7.440 7.553 7.471
4.310
3.078
7.388
3.338
3.959
7.297
3.460
3.872
7.332
CONCLUSIONS
I n view of t h e preceding results, we have been led t o t h e following conclusions: I-Comparisons of organic substances as accelerators should be made with molecularly equivalent amounts of t h e substances in question, and should be based on t h e values obtained for their excess sulfur coefficients over t h a t of a control which contains no acceler a t or. 2-The action of certain substances, such as diphenylthiourea, is due t o their tendency t o decompose under vulcanizing conditions and temperatures into simpler substances which contain a n active nitrogen group which is responsible for t h e acceleration effected. 3-Molecularly equivalent quantities of substances which contain t h e same active nitrogen group in their primary nucleus effect t h e same accelerating activity. 4-The replacement of t h e hydrogen in t h e active nitrogen group by other a n d larger groups, or radicals, decreases t h e activity of t h e parent substance. 5-The activity of t h e nitrogen in certain groups is most readily interpreted as due t o a change in valency f r o m three t o five, with t h e temporary addition of sulf u r ; t h e active nitrogen group would t h u s function as a sulfur carrier. CARBON BLACK-ITS PROPERTIES AND USES182 By G. St. J. Perrott and Reinhardt Thiessen CHEMICAL RESEARCHLABORATORY, BUREAUOB MINES STATION, PITTSBURGH, PA.
EXPERIMENT
An investigation of t h e carbon black industry has been undertaken by the United States Bureau of Mines as a result of economic issues brought up during t h e war. I n t h e present process of manufacture carbon black is made by burning natural gas with a supply of air insufficient for complete combustion a n d collecting t h e liberated carbon on a metal surface by actual contact of t h e flame on t h e surface. This process produces from 0.j lb. t o 1.5 lbs. of carbon black from 1000 cu. ft. of gas or 1.5 per cent t o 3.5 per cent of the total carbon in t h e gas. The process a t first sight seems most wasteful but examination of t h e problem shows t h a t i t is a t any rate t h e only process in practical operation which produces carbon black Published by permission of the Director, U. S. Bureau of Mines. Presented a t the 58th Meeting of the American Chemical Society, Philadelphia, Pa., September 2 t o 6 , 1919. 1 2
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T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
suitable for t h e ink trade a n d rubber industry consumers whose combined use is over 30,000,000 lbs. annually. T h e whole situation appears t o necessitate thorough investigation before any opinion can be given in regard to t h e wastefulness of t h e present process. Accordingly t h e industry has been carefully studied as t o methods of manufacture, properties, and uses of the finished product. Plants in Louisiana, Oklahoma, and West Virginia have been studied b y Bureau engineers. Other processes for making carbon black have been investigated. The uses of carbon black have been studied with t h e idea of determining t h e properties of the product which users of carbon black demand and with a n a t t e m p t a t designating in which of these uses carbon black is essential and in which a substitute material might be employed. Microscopic investigation of a large number of blacks has been made a n d test methods studied with a view t o finding the reason for t h e very different behavior exhibited by different blacks. METHOD O F MANUFACTURE
Carbon black, as known t o t h e American trade, is the fluffy, velvety black pigment produced by burning natural gas with a smoky flame against a metal surface. It is entirely different in physical characteristics from lampblack, which is made by burning oil or other carbonaceous material with insufficient air for complete combustion a n d collecting t h e smoke in settling chambers. Lampblack is gray in contrast t o the deep black of carbon black, often contains considerable quantities of empyreumatic matter, and when used in printing ink gives a product with very different properties from a n ink of similar composition made from carbon black. The process of manufacture used to t h e greatest extent at t h e present time is t h e so-called channel system, in which t h e black is deposited on t h e smooth under-surface of steel channels by lava-tip burners set a t a distance of 3 or 4 in. below the channel. T h e channel irons are usually built up in tables of eight, sometimes I O O f t . long, a n d are given a slow reciprocating motion which scrapes t h e black deposited on them into hoppers from which i t is carried by screw conveyors t o the packing house, where it is bolted and sacked. The mechanism is enclosed in sheet-iron buildings in order t h a t t h e amount of air may be regulated. Varying the amount of air, speed of scraping, and pressure of t h e gas controls t h e quality of t h e product. T h e shape of the burner and distance from the collecting surface also affects t h e quality of t h e black, b u t these are constant for any one plant. Other similar processes differ only in t h e nature of the collecting surface and burners. Godfrey L. Cabot, a pioneer in t h e carbon black industry, has described in a very able manner t h e history of the development of t h e manufacture of carbon black.' Mr. R. 0. Neal, of the Bartlesville Station of t h e Bureau of Mines, is preparing a technical paper describing in detail t h e present method of manufacture and discussing t h e economic side of t h e industry. 1 8 t h Intern. Cong. A p p l i e d Chem., 12, 13.
325
THEORY O F FORMATION O F CARBON BLACK
When natural gas burns in an incomplete supply of air, t h e carbon is liberated, not as a result of preferential combustion of hydrogen, b u t as a direct decomposition of unburned gas in t h e heat of t h e flame. According t o Bone' and co-workers, combustion takes place in steps as a result of hydroxylation: A B oxidation via C oxidation Hz: C : (OH), CHd---+
Ha j C.OH-
Hz : C : 0
CO
+ HZO
+ Hz
C'
I t is evident t h a t the tendency is always t o run from A t o C. When t h e proportion of methane t o oxygen is CH4 : O 2 t h e reaction goes from A t o B t o C t o C'. If the ratio is 2 CHd : 0 2 or higher, only a part of t h e methane can be oxidized through t h e reaction A t o C a n d part is decomposed a t A by t h e heat evolved in t h e A t o C reaction. The lowest per cent of oxygen in which a methane flame will burn is 15.6 per cent. Carbon will be evolved only in t h e inner p a r t of t h e flame, where t h e oxygen supply is low b u t where * ' -9 is sufficient heat t o break up t h e methane, anc per cent of carbon t o be expected by t h e incorr combustion of methane is low. Gases rich in el a n d t h e higher homologs produce greater yields b process. The function of t h e cold surface is t o cool t h erated carbon in t h e flame sufficiently t o prevc , combustion. This permits t h e use of a s d i c i e n t l y hot flame t o give carbon uncontaminated with hydrocarbons or their partial oxidation products a n d produces a finely divided material which has been prevented from agglomerating by t h e sudden cooling, It is evident t h a t there must be a n optimum temperat u r e a n d a n optimum position for t h e surface in t h e flame. Too cold a surface may prevent t h e maximum separation of carbon; too hot a surface will allow too much carbon to be burned and may change t h e properties of the carbon which remains unburned. T h e temperature of t h e channels in t h e present processes is about 300' C. Numerous methods for producing a larger yield of carbon black from natural gas have been proposed and patented. I n most of these processes t h e gas is broken up into carbon and hydrogen by passing through a retort filled with refractory material a t a temperature of 1200' C. or over. A much higher yield of carbon, sometimes as high as 40 per cent of t h e theoretical, is produced in this way, b u t i t has not yet been made of a grade commercially salable. T h e carbon, while often finely divided, is usually gray in color and contains particles of grit which may be agglomerates of carbon or particles of t h e refractory material. It seems highly possible, however, t h a t with properly 1
Tyans. Roy. SOC.London, 216 (1915), 275.
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controlled conditions a carbon suitable for use in rubber might be made by this method. T h e hydrogen produced can be sold a t a nice profit a n d even if t h e hydrogen were not used, t h e black could be produced for less t h a n 5 cts. a lb. For use in ink, carbon black made by t h e closed retort method up t o t h e present time has been of no value, due t o inferior color, grittiness, and low oil absorption. The defect with methods t h u s far developed has been t h a t t h e carbon after being liberated stayed in t h e heated zone long enough t o become partially agglomerated and changed into t h e gray modification of amorphous carbon. Experimental work along this line should aim t o crack t h e carbon on a thin layer of catalyst a t as low a temperat u r e as possible a n d t o get i t away from t h e heated zone immediately after formation, or t o catch i t on a cold surface near t h e catalyst. Other methods have been patented, e . g., chlorination of natural gas t o hydrochloric acid and carbon black, explosion of acetylene, rupture of hydrocarbon vapors with electric spark, exploding hydrocarbon vapors with CO, COz and 02, b u t to t h e authors’ knowledge none of t h e m have ever produced any quant i t y of a salable product in this country. USES OF C A R B O N BLACK
I n order of importance the uses of carbon black are
as
&.a$: 1 the
res and other rubber goods. y paint and enamel.
‘plete ;bane
y this
s-such as phonograph records, carbon miter ribbons, black and gray paper, glazed e liblack leather, bookbinders’ board, marking *nt,.%i Pubber sheeting and clothing, hard rubber, artificial stone and black tile, Chinese and India inks, fireworks, insulating materials, crucible steel, case hardening. T h e amounts used b y these industries in 1918 were approximately as follows: dlrb
._
........................ ..................... .I ........................
Printers’ ink.. Rubber goods.. Stove polish.. All others..
..........................
POUNDS 10,000,000-12,000,000
20,000,000 4,000,000-5,000,000 1,000,000
Besides this, in normal times probably 15,000,000 lbs. are exported. PRINTERS’ INK-Lampblack has been used as a pigment for printers’ ink ever since t h e invention of t h e printing press, a n d was used almost exclusively u p t o 1864. For certain sorts of printing where an extremely fine-grained ink was required, great trouble was taken t o purify t h e black, b u t aiter t h e advent of carbon black in 1864,and its increased production a n d diminished cost after 1880, lampblack became less a n d less used in t h e ink trade, and a t t h e present time only a small amount is used, and then only t o impart certain qualities t o an ink already containing carbon black. Carbon black has certain properties which make i t peculiarly suitable t o the needs of t h e modern printing art. It is well adapted t o the modern fast running presses a n d t o t h e fine half-tone screen work which enters into most of our illustrations. Certain carbon blacks
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give what is known as a short ink, t h a t is, an ink of buttery consistency which does not flow rapidly. This is especially desirable in lithographic and offset work, and in slow speed presses and most half-tone work. Lampblack does not give t h e right consistency for such work and is too gray in color. For fast running presses, carbon black made b y other processes is also good in t h a t it makes a very fluid long ink which a t the same time has sufficient opacity t o give a black letter obscuring the paper well. I n k manufacturers a n d users believe t h a t carbon black is absolutely essential t o their business. R U B B E R TIRES-Prior t o 1914 little carbon black was used by the rubber industry and then only in small amounts as a coloring material. Little distinction was made between carbon black and lampblack, the two compounds being used indiscriminately. At this time, due partly t o t h e stimulus afforded b y the rising price of zinc oxide, i t was found t h a t carbon black could be used in very large amounts as a filler for rubber, with a correspondingly smaller amount of zinc oxide. Carbon black is used in rubber in quantities of 3 per cent t o 2 0 per cent by weight, Many manufacturers claim very unusual properties for rubber so compounded. It is said t o increase t h e tensile strength very greatly and t o give increased toughness and resistance t o abrasion. I t is believed by some authorities t h a t t h e life of t h e rubber is increased. Other rubber chemists are more conservative in their praises of carbon black a n d do not admit t h a t i t possesses any properties which make i t irreplaceable. From the point of view of cost, carbon black is much cheaper t h a n zinc oxide. Carbon black, absolute specific gravity 1.8,suitable for compounding in rubber, can be procured as low as I O cts. a lb. a t the present time. Zinc oxide costs a t least a s much a n d has a specific gravity of 5.8. Carbon black evidently costs one-third as much as zinc oxide on a volume basis, supposing t h a t equal volumes were compounded with t h e rubber, b u t in practice a greater volume of carbon black is used t h a n of zinc oxide so t h a t t h e resulting mix with carbon black contains less rubber per unit volume t h a n the corresponding zinc oxide mix. From a theoretical consideration, carbon black should be a n ideal filler for rubber on account of its extremely fine state of division a n d t h e correspondingly large surface energy developed when intimately mixed with the gum rubber. It also serves t o protect t h e rubber substance from the effects of light and may possibly retard oxidation. Whatever may be t h e t r u e state of affairs as t o the irreplaceability of carbon black in rubber, a n enormous amount is now consumed by t h e rubber companies, probably zo,ooo,ooo lbs. annually, and 10,000,ooo lbs. exported for foreign trade in normal times. PAINT-carbon black is coming into extensive use in t h e p a i n t trade. I t has a higher tinting strength t h a n any other black and a given weight will obscure a greater area of surface. Carbon black is acknowledged superior in varnishes and enamels a n d evidently has many followers in making black and gray paints for general purposes. T h e U. S. War Department re-
Apr.,
1920
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quires t h e use of carbon black in black enamels and various black and gray paints for general purposes. Some authorities consider lampblack superior t o carbon black and i t is probably true t h a t in certain gray tints lampblack is superior, owing t o its bluish gray tones. OTHER usEs-For other purposes carbon black is employed chiefly because of its cheapness and high coloring power, It produces a carbon paper of very smooth surface which will give a large number of copies before t h e letters lose their opacity. For phonograph records it gives a smooth surface for recording t h e vibrations, although lampblack is often used for this latter purpose. TESTING METHODS
T h e final test of the suitability of a black for a given purpose is the actual trying out of t h e working qualities. I n rubber making a sample mix is made and t h e finished piece tested for tensile strength, per cent elongation, toughness, and resistance t o abrasion. I n ink making a sample batch of ink is made u p and t h e suitability of t h e black determined b y a n actual run o n t h e press for which t h e ink is intended, in which t h e working qualities of t h e ink and t h e amount used for a given number of impressions are noted.’ There are, however, a number of laboratory tests which are of use in matching a standard sample. The tests most commonly employed are for tinting strength, color, a n d grit. It is also desirable t o determine moisture, ash, and acetone extract. TINTING STRENGTH-ACCOrding t o t h e American Society for Testing Materials, tinting strength is “the power of coloring a given quantity of paint or pigment selected as a medium or standard for estimating such power.” Tinting strength, then, as applied t o carbon blacks is t h e measure of t h e ability of t h e black t o impart a color t o a definite weight of standard white. I t depends on the size of t h e particles a n d t h e specific gravity of t h e black. I n making t h e test, t h e black is always compared with a standard black. Weigh out accurately on a sensitive balance, 0.100g. of the black to be tested and 10.0g. of a standard zinc white kept especially for the purpose. Transfer to a glass or marble slab and add from a burette exactly 3.5 cc. refined linseed oil. Mix with a palette knife and rub out thoroughly with the palette knife (or, better, a glass muller) until no streakiness or difference of color is observed, when successive small portions are spread out on a clean piece of window glass and viewed from the upper side. It is important that the rubbing out be thorough; I O min. are usually sufficient. Follow the same procedure with the standard black. Then spread a small amount of each mix side by side on a clean glass (a microscope object glass serves the purpose very nicely). Examination of the samples from the other side of the glass, particularly at the line where they overlap, will show the difference in tinting strength. To make a quantitative estimation of the tinting strength of the sample as compared t o the standard, more white is added to the stronger mix until the colors match. A new sample of the stronger black is then weighed out, using the calculated 1 For a detailed discussion of printing inks see Norman Underwood and John V. Sullivan, “The Chemistry and Technology of Printing Inks,” Van Nostrand Co., 1915; “The Composition, Properties, and Testing of Printing Inks,” Bureau of Standards, Circular 58, Government Printing Office, Washington, 1915.
327
amount of zinc white, and the process repeated until mixes of the same color are obtained. If, for example, it was necessary to mix 15 g. of zinc white with 0.1g. of the standard to match a mixture of I O g. zinc white and 0.1g. of the sample, the latter has two-thirds the strength of the standard. COLOR-BY this t e r m is meant t h e relative blackness of t h e material when mixed in oil. To 0.3 g. of each of the blacks t o be compared add 1.3 cc. of refined linseed oil from a burette. Mix thoroughly with the palette knife and spread side by side on a slip of glass and compare the relative color by viewing from the upper side of the glass. GRIT-Presence of gritty matter is determined b y rubbing a portion of t h e black under t h e finger or by placing a small amount on t h e tongue and rubbing between t h e tongue and palate. CHEMICAL TEsTs-It is occasionally desirable t o make a few quantitative chemical tests of carbon black. A black containing more t h a n 0.2 per cent ash is probably adulterated with mineral black or charcoal. An acetone extract over 0.1 per cent indicates adulteration with a poorly calcined lampblack. Too great a percentage of moisture is undesirable from t h e point of view of working qualities. Certain blacks will absorb as much as 1 5 per cent of their weight of moisture, making a total moisture content of 2 0 per cent or more. Most blacks for ink making contain from 2 t o 4 per cent of moisture, although certain blacks may contain as high as 7 per cent. Moisture’-A I-g. sample of the black is placed in a weighed porcelain crucible and heated for one hour a t 105 ’C. in a constant temperature oven in circulating dry air. The crucible is then removed from the oven, covered, and cooled in a desiccator over sulfuric acid. The loss in weight multiplied by IOO is recorded as the percentage of moisture. A &-The crucible containing the residue from the moisture determination is heated gradually with a Meker burner (or better, in a muffle furnace) to cherry-red (about 750’ C.). Ignition is continued until all the particles of carbon have disappeared. The crucible is then cooled in a desiccator and weighed, after which it is heated again for 15 min., cooled in a desiccator, and reweighed. If the change in weight is more than 0.0002 g., the process is repeated until successive weighings are constant to this figure. The weight of the crucible and ash minus the weight of the crucible is taken as the weight of the ash. Acetolzf Extract-A 2 - 8 . sample is weighed into an alundum or paper extraction thimble of 20 cc. capacity and the extraction carried out for one hour, using any standard apparatus of the Soxhlet type. The weight of the residue after evaporation of the acetone is taken as the acetone extract. The extract for a pure carbon black is usually zero. SPECIFICATIONS
T h e Bureau has received a great many inquiries in regard t o tests which a carbon black must meet t o be suitable for use in printing ink or rubber. T h e following specifications represent a n attempt t o gather together t h e requirements adopted by t h e trade. I t should be realized t h a t there are no hard a n d fast specifications for carbon black, and t h a t t h e test on which a black stands or falls is t h e practical test. 1 For details of method, see F. M. Stanton and A. C. Fieldner, “Methods for Analyzing Coal and Coke,” Bureau of Mines, TeGhnical Paper 8.
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I-PRINTING INK Chemical Tests Moisture-Less than 5.0 per cent Ash-Less than 0.1 per cent Acetone Extract-Less than 0.1 per cent Physical Tests Color-Must match standard Tinting Strength-Must equal standard Grit-None Practical Tests The black when made into ink must have satisfactory working qualities as determined by a n actual run on the press for which the ink is intended. The ink must have satisfactory transfer, tack, drying properties, color, and The oil must not must print a sufficient number of pages per pound separate from the pigment and there must be no offset or smutting. 11-RUBBER Chemical Tests , Moisture-Less than 4 per cent Acetone Extract-Less than 0.5 per cent Ash-Less than 0.25 per cent Physical Tests Grit-None Tinting Strength-Not less than 90 per cent of the strength of standard Practical Tests Rubber mixes are made up containing equal weight of the sample to be tested and of the standard. Mixes are cured under exactly the same conditions. The finished sheet is tested for tensile strength, per cent elongation, toughness, and resistance t o abrasion. 111-PAINT Chemical Tests Moisture-Less than 5 per cent Ash-Less than 1.25 per cent Physical Tests Grit-None Tinting Strength-Not less than 95 per cent of the strength of standard
PRELIMINARY
WORK
ON
OTHER
LABORATORY
TESTS
Some preliminary work has been done on t h e problem of devising tests which will predict in a quantitative way the performance of t h e black when made into ink. Tests which suggest themselves are measurements of viscosity, cohesion, and adhesion of mixtures of black and oil. Determination of these three properties should throw light on the probable performance of t h e black in use. vIscosITY-Attempts t o measure t h e viscosity of mixtures of black and linseed oil in a Saybolt viscosimeter were unsatisfactory because an extremely dilute mixture was necessary. Accordingly, it was decided t o t r y out t h e MacMichael torsiQn viscosimeter1 which is used in t h e Petroleum Laboratory a t t h e Pittsburgh Station for determining t h e viscosity of heavy oils. I n this instrument a brass disk connected t o a graduated torsion head is suspended from a piano wire in a rotating cup containing the liquid t o be tested. Deflection times t h e time of one revolution is proportional t o t h e viscosity. By use of suitable wire, liquids of very high viscosity can be tested. The apparatus is calibrated with a liquid of known viscosity. Mixtures of equal weights of various blacks with t h e same amount of raw linseed oil were made up and t h e viscosity determined by means of the MacMichael apparatus. It was found t h a t carbon blacks prized b y ink makers on account of their “length” gave a lower reading on t h e viscosimeter t h a n other blacks. 1 THISJOURNAL,
7 (1915). 961.
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12,
No. 4
At this time t h e author’s attention was drawn t o an illuminating paper by Bingham and Green’ on measurements of the mobility and “yield value” of paint. Bingham distinguishes between the viscosity of t r u e liquids and the r i g i d i t y of plastic solids. Measurements of t h e flow through a capillary when different pressures are applied t o t h e liquid show t h a t the curve of pressure against t h e volume flowing through in unit time is different for true liquids a n d for plastic solids. I n a true liquid t h e curve passes through t h e origin; for a plastic solid t h e curve cuts t h e pressure axis at some distance on one side of t h e origin. This distance Bingham calls t h e yield value or force which must be applied t o t h e plastic solid before any deformation takes place. It appeared possible t o obtain similar curves using t h e torsion viscosimeter by measuring t h e deflection a t different speeds of rotation, t h e speed of rotation corresponding t o t h e volume flowing through t h e capillary and t h e deflection corresponding t o t h e pressure. Mixes of several blacks were made up, using I O g. of black t o I O O cc. of raw linseed oil. They were run a t different speeds in t h e torsion viscosimeter a t a temperature of 80” F. Curves are shown in Fig. I . It will be noticed t h a t a t t h e higher speeds t h e points lie on a straight line and t h a t t h e extrapolation of this line back t o zero r. p. m. does not pass through zero deflection. These curves are similar t o those obtained b y Bingham with t h e capillary method. It is possible t o obtain a relative figure for t h e rigidity or mobility a n d t h e yield value from these d a t a from the formula Rrel
= I(dt - 4 , )
where t is t h e time of one revolution in seconds, dt the deflection, and dt, t h e deflection at zero r. p. m. D a t a and calculated relative rigidity and mobility are given in Table I. TABLE I-VISCOSITY
MIXTURESOF CARBONBLACKA N D LAMPBLACK WITH LINSEEDOIL Time of One Revolution Relative Relative Rev per Deflection Seconds Rigidity Mobility itlin SHORT CARBON BLACK 124 90 0 48 15 4 0.0650 80 79 0 75 15 7 0 0637 47 70 1.28 15 4 0 0650 21 59 2 86 . . 11 54 5.45 Yield Value = 5 8 LAMPBLACK 0.0486 0.48 20.6 124 78 0.0481 20.8 0.67 90 66 0.0478 20.9 0.95 63 57 0.0575 1.58 17.4 38 46 Yield Value = 35 LONGCARBONBLACK 0.0650 15.4 124 55 0.48 0.0654 15.3 102 49 0.59 0.0658 15.2 67 40 0.90 0.0685 14.6 33 31 1.82 .... .. 11 22 5.45 Yield Value = 23 R A W LINSEEDOIL 0.0194 3.4 124 7 0.48 0.0194 3.4 70 4 0.86 Yield Value = 0 SuM31A R Y Yield Value Relative Relative Deg. Deflection Mobility Rigidity 59 0.067 Short carbon black plus linseed ojl 15.0 23 0.065 Long carbon black plus linseed oil 15.3 33 0.046 Lampblack plus linseed oil. 22.0 0 0.019 . 3.4 Raw linseed oil OF
....
. . .. . . .
..... . .. . . .. ...
1 “Paint, a Plastic Material and Not a Viscous Liquid,’’ Reprint, American Society for Testing Materials, 22nd Annual Meeting, June 24-27, 1919.
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Apr., 1920
Revofufions per minu fe FIG.~-“VISCOSITY”
OF MIXTURESOF BLACKAND LINSEED OIL MACMICHAEL TORSION VISCOSIMETER
USING
DISCUSSION O F RESULTS-It is at once seen both from t h e table and t h e curves t h a t t h e difference between the long and short blacks tested is a difference principally in yield value, and t h a t t h e mobilities of the two mixtures are nearly t h e same. The lampblack, with a steeper slope, has a lower mobility and a yield value intermediate between t h e long and short blacks. The yield value is apparently t h e resistance, due probably t o attractive forces between the particles of black, which the mixture offers t o deformation when t h e load is applied above a certain rate. If t h e load is applied slowly, a different condition obtains, as instanced b y t h e falling off of t h e curve from a straight line at t h e lower speeds of revolution. If t h e rotation of t h e cup is stopped while a mixture is being tested, t h e deflection drops back quickly to a point near t h e yield value, and then very slowly decreases nearly t o zero over a period of several minutes. It is hoped t o continue work with t h e torsion viscosimeter and t o compare results as obtained with it t o t h e results obtained b y t h e capillary method. Results are, of course, empirical, b u t might nevertheless be of practical value. coHEsIow--Cohesion is defined as t h e attraction between t h e particles of t h e same substance. It is t h e resistance which a substance offers t o deformation. An ink with low cohesion is a long ink, i. e . , i t allows itself t o be changed in shape and easily drawn out into strings. One with higher cohesion offers greater resistance t o deformation a n d breaks off when a n attempt is made t o draw it out into a string with t h e palette knife. Cohesion may be measured b y determining t h e force necessary t o draw a flat, circular plate away from t h e surface of t h e liquid on which i t is resting. I n order t o get duplicable results with a mixture such as ink, t h e force must be applied quickly. If it is applied slowly, t h e same indefinite results are obtained as in t h e viscosity determinations a t t h e lower speeds, due probably t o slippage. Results of a few measurements of this character with long and short black mixtures show long blacks t o have lower cohesion. ADHESION-The molecular attraction between t h e surface of bodies in contact is called adhesion. The
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adhesion of a n ink for t h e t y p e a n d paper must influence its working qualities. This is what t h e printer calls “tack.” Adhesion of a liquid may be measured b y dipping a weighed metal plate i n t h e liquid, allowing i t t o drain at a constant temperature, and reweighing. For a plastic material, such as ink or paint, t h e value for t h e adhesion so obtained would probably be modified b y t h e yield value of t h e mixture. I n making any tests of mixtures of carbon or lampblack in oil, care must be taken t h a t t h e black is thoroughly incorporated with t h e oil if duplicable results are t o be obtained. The mixture should be thoroughly ground in a paint mill or three-roller ink mill. An inert mineral oil will probably be better t h a n linseed oil for testing purposes. C H E M I C A L ANALYSIS-carbon blacks consist of from 85 per cent t o 95 per cent amorphous carbon, I per cent t o 7 per cent water, 0.5 per cent t o 0.8 per cent hydrogen, and from 2 per cent t o 8 per cent oxygen, present partly as CO and COZ, and partly as fixed oxygen. Chemical analysis serves t o show in a general way whether a carbon black will give a long or short ink. A black giving a long ink is usually low in carbon, and high in volatile matter and oxygen, while shorter blacks are correspondingly lower in t h e latter and higher in carbon. Typical analyses of long and short blacks are given in Table 11. 11-ANALYSES O F C A R B O N BLACKS Long Long Long Short Short CHARACTER Black Black Black Black Black NUMBER OF S A M P L E 1 2 3 1 2 PROXIMATE ANALYSIS: Moisture 3.56 7.13 5.30 2.25 3.02 Vol. M a t t e r . , ........... 11.99 13.41 10.40 5.60 5.48 Fixed Carbon . . . . . . . . . . . . 84.40 79.44 84.16 92.13 91.47 Ash 0.05 0.02 0.14 0.02 0.03 ULTIMATEANALYSIS: Hydrogen.. ............. 1.19 1.32 1.11 0.74 0.88 Carbon 88.17 84.56 87.98 94.78 93.50 Nitrogen ................ 0.04 0.04 0.08 0.09 0.04 Oxygen . . . . . . . . . . . . . . . . . 10.54 14.00 10.68 4.37 5.25 Sulfur 0.01 0.06 0.01 0.00 0.30 Ash ..................... 0.05 0.02 0.14 0.02 0.03 ULTIMATEANALYSIS (Moisture-free) : Hydrogen ............... 0.82 0.57 0.55 0.50 0.52 Carbon. . . . . . . . . . . . . . . . . 91.42 91.05 92.91 96.96 96.41 Nitrogen . . . . . . . . . . . . . . . . 0.04 0.04 0.08 0.09 0.04 Oxygen . . . . . . . . . . . . . . . . . 7.66 8.26 6.30 2.43 2.69 Sulfur . . . . . . . . . . . . . . . . . . . 0.01 0.06 0.01 0.00 0.31 Ash. .................... 0.05 0.02 0.15 0.02 0.03 T4BLE
................
.....................
.................
...................
Short
Black 3
3.12 5.58 91.22 0.08 1.05 93.63 0.05 5.19 0.00 0.08 0.72 96.64 0.05 2.51 0.00 0.08
A D S O R B E D GASES-carbon black contains considerable quantities of carbon monoxide, carbon dioxide, and oxygen. The oxygen is probably present as “fixed oxygen,” t h a t is, in some kind of combination with t h e carbon. Through t h e cooperation of Dr. G. A . Hulett and H. E. Cude, of Princeton University, t h e nature and amount of t h e adsorbed gases were determined. The apparatus used was t h a t designed b y Dr. Hulett t o determine t h e nature of t h e adsorbed gases in gas-mask charcoal. The gases are pumped off at any desired temperature b y means of a Topler pump and analyzed, t h e water being caught on a bulb containing solid carbon dioxide. At room temperature t h e composition of t h e gas which can be pumped off is practically t h a t of air and t h e volume about t h a t of t h e voids and capillary spaces. At 445’ C. a larger volume usually comes off, consisting chiefly of COz, CO, and Nz. Carbon dioxide may be present in as large amount as one per cent of t h e
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