646
I N D U S T R I A L A N D ENGINBERING CHEMISTRY
tember 25 and October 21 shoxed alumina lower than the silica requiring an increase of the aluminate dosage with a resultant increase in soluble alumina. A rather rapid increase in soluble alumina during January was due to the river water becoming much softer. A comparison of the curve on treated water with that for the boiler water shows the increase in soluble alumina and decrease in soluble silica due to the increase in aluminate treatment. The two peaks in the curve represent soft-water seasons. The gradual increase in silica content of the boiler water during the last few months was attributed directly to the decrease in hardness of treated water. Consequently there was less calcium and magnesium in the treated water to react with the silica and alumina, which therefore built up as soluble alumina and silica in boiler water. The relation of silica-scale formation to the silica content of the boiler was noticeable. To determine the amount of scale the boiler was turbined and the removed scaIe caught in the lower mud drum. This scale was measured and an aliquot dried to constant weight. The total amount of scale and the pounds of scale per million pounds of treated water
Vol. 23, No. 6
evaporated were then calculated. The initial point on the scale curve showed an average of scale formation for several months before the use of sodium aluminate. With the increase of soluble silica in the boiler water there was a corresponding increase in scde formation. After adjusting the silica-alumina ratios there was a noticeable decrease in silica and subsequent scale formation until a point was reached where it was possible to put a boiler back on the line without turbining, the first instance of such an occurrence within recent years. It was also noted that the suspended solids in the boiler water changed from a finely divided, milky precipitate to a loose, flocculent sludge when the soluble alumina content of the boiler water was increased. The influence of silica on scale formation cannot under present-day operating conditions be overemphasized. However, its harmful effect may be definitely prevented through proper treatment. Literature Cited (1) Thayer, IKD. ENG CHBM, Anal. E d ,
a, 276 (1930)
Some Properties of Carbon Black I-Adsorption1 W. B. Wiegand and J. W. Snyder BINNEY& SMITHCo., 41 EAST4
2
SI., ~ NBW ~ YORK, N. Y
The diphenylguanidine (D. P. G.) adsorption test cor- compounds containing a n excess of added fatty acid, howrelates well (0.8) with rubber-curing behavior in organi- ever, both heat-deactivated and high-adsorption blacks cally accelerated mixings, in respect t o the broad classes improve the cure, doubtless through removal of excess of carbon black, but only moderately (0.4) in respect to acid. In general, for carbon black-litharge compounds a standard rubber carbon black. The D. P. G. or similar low-adsorption carbon black is preferable because it peradsorption test, although correlating better than the mits a minimum dosage of fatty acid. volatile-matter test, is therefore not capable of predicting { For entirely unaccelerated rubber mixings heat-deactiwith precision the rubber-compounding behavior of stand- vated (low-adsorption) carbon black exhibits striking improvement in physical properties. ard (rubber) carbon black. ’ The adsorptive properties of carbon blacks have been Carbon-black adsorption is much more marked in the turned t o advantage in the fields of rubber insulation alkaline than in the acid range. While heat-activated carbon blacks show marked in- and insulating oils. High-adsorption, heat-activated carcrease in D. P. G. adsorption, heat-deactivated carbon bon blacks are shown t o advance the cure of rubber blacks show greatly reduced alkaline and slightly increased mixings vulcanized with rn-dinitrobenzene. Specially acid adsorption,a which probably explains the anomalous activated carbon blacks have been developed which show iodine adsorption of heated blacks noted in the literature. a higher (alkaline) adsorptive effect than the active I n litharge compounds without added fatty acid, high- chars now on the market. It has been found possible t o adsorption blacks spoil the cure by removing the fatty produce new grades of carbon black in which both the Heat-deactivated carbon magnitude and direction of adsorptive activity are under acid present in the rubber. black leaves t h e cure relatively unaffected. In litharge control.
.
.....
I
N 1925 Le Blanc, Kroger, and Kloz (8) reported that
they could find no relation between the properties in rubber and the adsorptive properties of lampblack. The latter, when suspended in an organic medium, showed no adsorptive capacity for sulfur, iodine, or mercaptans. Later in the same year Thies (1.2) showed that with clays there was a relation between the adsorption of basic dyes (malachite green) and rate of cure. The lower the adsorption of dye the faster was the rate of cure. On the other hand, it was shown that with some acid dyes the faster 1 Received April 6, 1931. Presented before the Division of Rubber Chemistry at the 81st Meeting of the American Chemical Society, Indianapolis, Ind.,March 30 t o April 3, 1931. True for aqueous and alcoholic hy?rochloric a n d benzoic acids.
curing clays had a greater dye adsorption. This difference in the rate of cure of the clays, however, was apparent only in organically accelerated rubber compounds. I n 1926 Twiss and Murphy (IS) came to the conclusion that the retardation in cure of an unaccelerated mixing, by carbon black, was attributable -to the adsorption of the natural accelerator in the rubber. Spear and Moore (10) found no relation between the stiffening power of carbon blacks in rubber and the adsorption of malachite green from aqueous solutions, Victoria blue from benzene, and hexamethylenetetramine from benzene. However, they found some evidence that differences in adsorption of hexa ran parallel with differences in time of cure.
1NDUSTRIAL AND ENGINEERIiVG CHEMISTRY
June, 1931
In 1928 Goodwin and Park (3, 6) showed clearly that high-color paint blacks, rubber blacks, and certain thermaldecomposition blacks gave different adsorption isotherms towards iodine and methylene blue solutions in water and carbon tetrachloride. They concluded that adsorption differences exert a profound influence upon the properties in rubber. Beaver and Keller (1) concluded that the rate of cure was not related to the adsorptive behavior. In 1929 Carson and Sebrell(2) showed in a very stimulating paper that, except for blacks that had been heat-treated, the adsorption behavior toward benzoic acid, iodine, mercapto, and diphenylguanidine was a measure of the rate of cure.
1
, I O
30
40 BOP0 A D S O R B E D
20
50
60
Figure 1-Diphenylguanidine Adsorption YS. Tensiles of Rubber, P a i n t , a n d Ink Carbon Blacks in Accelerated Rubber C o m p o u n d
Wiegand (14) employed the adsorption of diphenylguanidine in alcohol and of potassium hydroxide in water to illustrate the reduced activity and enhanced curing properties of some new carbon blacks. Johnson (7) reported no correlation between iodine adsorption and rate of cure, except in samples coming from the same factory, He found positive correlation between volatile content and the adsorption of two organic accelerators, Captax and diphenylguanidine. He also confirmed the “exception” already noted by Carson and Sebrellviz., that heated blacks showed decreased volatile together with increased iodine adsorption. Fromandi (6) determined the adsorptive isotherms of various blacks for acetic acid, but has thus far offered no correlation with rubber properties. Ditmar and Preusse (4) used methylene blue in aqueous solution. They state that the retardation of cure by carbon black is due solely to removal of sulfur. A glance over the published work on adsorption as a clue to the vulcanizing behavior of carbon blacks a t once invites the conclusion that there is some intimate relationship between these two properties. The adsorption of Captax, diphenylguanidine (D. P. G.), and potassium hydroxide has, for example, been stated t o run parallel t o volatile or to rubbercuring behavior. On the other hand, important exceptions have been noted; the iodine adsorption, especially for heated blacks, seems to have given anomalous results. I n this paper an attempt has been made to assess the validity of the adsorption test and to resolve some of its apparent anomalies. Adsorption as a Quality Test
THE TEST-In order to fix ideas, the test as worked out in these laboratories is given. Two grams of c. P. diphenylguanidine are dissolved in 1 liter of ethyl alcohol (U. S. denatured formula No. 1 has been successfully used), and 50 cc. of this solution are shaken with 1 gram of the
647
sample for 2 hours. After filtering, 25 cc. of the filtrate are titrated with 0.01 N hydrochloric acid using a mixture of bromophenol blue and methyl red. The result is expressed as the percentage of D. P. G. removed. ACCURACY-Areview of many duplicate tests shows a mean deviation of well under 5 per cent. This indicates a probable error distinctly lower than the variability due to differences in quality. In other words, the test is satisfactory as regards chemical accuracy. CORRELATION WITH RUBBERPROPERTIES-Figure 1 shows the relation between the D. P. G. test and the tensile strength of rubber stocks developed after 30 minutes’ cure a t 40 pounds (2.7 atm.) steam pressure in a laboratory press. The test formula is shown on the graph and the blacks tested included “Super” quality rubber black (de-oxygenated), standard rubber black, Class 11 rubber black, paint black, and ink black. The curve represents thirty-five pairs of determinations and is broadly hyperbolic. As such it is not suitable for statistical treatment by Pearson’s methods, for the coefficient of correlation requires a linear disposition. However, by omitting the two very high values for adsorption-which, in fact, belong to paint and ink blacks which are never used in rubber-the remaining data approximate a straight-line relationship and so are amenable to the calculation of Pearson’s coefficient. This value is, in fact, -0.8 * 0.04. Since a coefficient of 0.5 is commonly held t o constitute definite correlation and since the probable error (0.04) is only one-twentieth of the coefficient, it is manifest that, speaking generally, there is a very direct connection between the behavior of the black in a rubber compound accelerated with D. P. G. and the capacity of the black t o remove D. P.G. from its alcoholic solution. 0
3
.
0
I I , 3
1
G R A M S DPG per 50cc 01 SOLUTION
Figure 2-Adsorption I s o t h e r m s . Rubber Carbon Blacks in Alcoholic S o l u t i o n s of D i p h e n y l g u a n i d i n e
The total breakdown of this correlation when the rubber compound is inorganically accelerated will later be illustrated in detail. TESTAPPLIEDTO STANDARD (RUBBER)CARBON BLACKFor this series a set of one hundred control tests was taken a t random from the laboratory records. The samples were taken from various channel plants of similar construction and producing the same grade of carbon black. The black was of standard rubber quality (Micronex). In the interest of increased accuracy the tensile a t break was replaced by a weighted modulus as follows: In the standard test formula (vs) the stress a t 300 and a t 500 per cent was averaged for both a 30-minute and a 40-minute cure. Thus in effect the rate-of-cure index was the average of four modulus values (approximately equal to modulus at 400 per cent for 35minute cure). The D. P. G. adsorption test was carried out as described above. The results have been thrown into a frequency table (Table I),
INDUSTRIAL A N D ENGINEERING CHEMISTRY
648
T a b l e I-D. P. G. Adsorption US. M e a n M o d u l u s in Accelerated Rubber Mixing, U s i n g Standard (Rubber) Carbon Black MEANMODULUS I N RUBBER D.P.G. < l 6 9 9 1700- 1750- 1800- 1850- 1900- 1950- 2000TEST 1749 1799 1849 1899 1949 1999 2049
5% ,120
ii.5f0fi;i 11.0t011.4 10.5t010.9 10.0t010.4 9.5to 9.9 9.0to94 85tO89 8 Oto 8 4 7 5tO 7 9
2
1 2
2 1
-
1
i 1
-
1
1 -
1 1
-
5 2 2
2 1 2 4 3 4 1 -
-
5 5 6
7 1 1
-
-
1 1 2 4 3 4 1
-
2
1
6
3 -
1 -
of A c t i v a t i o n on D. P. G. Adsorption of Carbon Blacks Refore After CARBON BLACK activating activating
T a b l e 11-Effect
3 -
-
Casual inspection of Table I will show the presence of correlation, but a t the same time a marked degree of dispersion, for perfect correlation would require all the data to be concentrated along one of the diagonals of the table. The correlation coefficient for the above set of one hundred determinations was -0.46 * 0.05. This value may be regarded as indicating definite correlation, the degree of which, however, is much inferior, for example, to the value (-0.8) obtained over the wider spread of carbon-black qualities. These correlation values for the D. P. G. adsorption test as compared with the rubber-compounding and -curing properties led to the conclusion that this test may with advantage be included in any examination of a new sample of carbon black in order to obtain a preliminary idea of its curing characteristics in organically accelerated tread-type rubber compounds; that the test is quick, inexpensive, and of sufficient manipulative accuracy; but that, in its present form, it is not capable of predicting with precision the rubber-compounding behavior of successive samples of standard carbon black. Comparison with Volatile Test
The volatile test has also been proposed as a quality test for rubber carbon black, and it is hoped to discuss this test in detail in another place. For the present it will suffice to state that, for the same group of test data as is shown in Table I, the coefficient is -0.24 * 0.06, a value well below the lower limit for correlation.
Tn
% ,”
11.4 40.0 58.0
30.5 93.0 64.0
I ”
Standard rubber Standard paint Standard ink
-
-2
Vol. 23, S o . 6
Blacks heated in the absence of air have been found to show
at the same time a decreased volatile content and an increased adsorption of iodine. This peculiarity was noted by Sebrell and Carson and later by Johnson. This type of black will be hereafter designated as “deactivated.” Figure 4 shows the change in selective adsorption of carbon black deactivated by heat treatment in the absence of air. The alkaline adsorption is seen to have been cut in two, but the acid adsorption has been increased. I n other words, the effect of the heat treatment has been to alter the character of the surface in such a way as to correct the asymmetry of the adsorption as regards basic and acidic substances. Herein, the writers think, lies the explanation of the anomalous iodine adsorption noted above. The circumstance that the proper vulcanization of rubber takes place only in an alkaline medium offered a unique opportunity to capitalize this altered adsorption balance. The newer carbon blacks may therefore be described as having a corrected adsorptive tendency, the excessive distortion towards alkaline adsorption having been eliminated. Adsorption in Litharge Mixings
The totally different picture presented when high- and low-adsorptive blacks are compounded in litharge mixings is shown in Table 111. T a b l e 111-Adsorption Properties in a Litharge C o m p o u n d w i t h a n d w i t h o u t Added F a t t y Acid (Rubber, 100; black, 45; litharge, 13; and sulfur, 3 parts) 6 % STEARIC D. P. G, CUREAT WITHOUT ADDED ADSORPTION40 LBS. -FATTY ACID-ACID ADDEDL-300 Tensile Elong. L-300 Tensile Elong. Lbs./sq. in. Min. Lbs./sq. i n . % % % STANDARD CARBOX BLACK
11.4
Adsorption Isotherm for Alcoholic D. P. G.
10 20 30 40 60
1150 1450 1700 1800 1950
4100 4200 4100 3800 3600
600 550 520
470 440
625 925 1350 1550 1850
3900 4400 4500 4600 4300
710 680 610 560 520
675 1100 1400 1550 1900
3900 4600 4500 4500 4300
705
460 925 1350 1450 1500
3900 4700 4800 4600 4300
750 680 610 570 550
4200 4300 4200 3900 3400
550 460 425 425 380
HEAT-DEACTIVATED CARBOX BIzACK
Figure 2 shows two isotherms, that of a standard (rubber) carbon black and that of a slower curing grade. The proportions of black and D. P. G. adopted for the standard procedure correspond t o approximately 0.1 on the abscissa. Higher concentrations would involve lower percentages of D. P. G. removal; lower concentrations would lead to titration difficulties.
8.0
10 20 30 40 60
1100 1450 1700 1750 1900
4000 4300 4300 4000 3100
10 20 30 40 60
1230 1400 1500 1500 1550
3500 3400 3100 2800 2600
10 20 30 40 60
1800 1900 2000 2000 2000
610 570 550 505 400
660
580 580 520
P A I N T BLACK
40.0
550 510 460 430 415
HEAT-ACTIVATED P A I N T BLACK
Selective Character of Carbon-Black Adsorption
Carbon black usually assumes a negative charge when suspended in liquids. It is therefore peptized by negative ions and so would be expected t o exhibit selective adsorptive effects toward such negative ions. That such is in fact the case is illustrated in Figure 3, in which are shown the molar adsorptions for standard (rubber) carbon black on both sides of the neutral point. The marked asymmetry of the curves for acid and alkaline adsorption is an important characteristic, a clear conception of which was an essential step in the development of the newer grades of carbon black (14). Adsorption Characteristics of Heated Blacks
Blacks heated in the presence of air or steam become more active (9, 11). Blacks so treated will be designated as “activated.” The extent of such activation is shown in Table 11.
75.0
3000 2600 2700 2400 2200
410 370 365 330 320
1550 2450 2400 2300 2250
In those cases where the only available fatty acid was that present in the rubber, the high-adsorption blacks spoiled the cure just as they do in organically accelerated mixings except that the mechanism is, of course, removal of natural fatty acid. The heat-deactivated black did not remove enough acid to disturb the cure. I n the experiments with the mixing to which 6 per cent stearic acid has been added, the cure has been definitely improved both by the heatdeactivated (low-adsorption) ’and the high-adsorption color blacks. I n fact, the best cure of all was with the heatactivated paint black having a D. P. G. adsorption value of 7 5 per cent. The explanation seems to be that the blacks removed, in varying degrees, the excess of fatty acid present. A similar series with 10 per cent stearic acid showed the same general phenomenon.
INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1931
The conclusion seems warranted that in litharge mixings there is only one content of fatty acid which will give the best curing results. This value depends, of course, on the percentage of litharge present.
I
01 c NORMALITY
005 ADUfOUS
I
01
005 HCL
NORHALITY
Figure 3-Standard
ALCOHOL1 C DPG-
(rubber) carbon black
649
Low- and High-Adsorptive Blacks i n Nitrobenzene Mixings
The tests reported in Table V were made in the belief that if, as some think, the nitrobenzene cure is an “oxygen” cure, the paint black, which has an oxygen content of about 7.0 per cent, might behave differently. These data, which are fully confirmed by hand tear tests, indicate that the highly adsorptive black, which in an organically accelerated sulfur mixing would have been difficult, if not impossible, to cure no matter how long prolonged the heating, was i n j a c t badly overcured in 30 minutes. Table V-Curing Data on Blacks in Nitrobenzene Mixing8 (Pale crepe, 100.0; carbon black, 30.0; litharge, 9.5; and m-dinitrobenzene, 3.0 parts) D.P.G. CURE BLACK ADSORPTION A T 4oLBS. L-300 TENSILE ELONGATION T. P % Min. Lbs./sq. in. % Standard (rubber) 1 1 . 4 30 625 3600 690 248 carbon 40 675 4000 680 272 Heat-deactivated rubber Paint
8.0
30 40
700 825
3600 3900
680 650
245 253
75.0
30 40
1250 1450
3150 3300
500 480
155 158
The interpretation, practical as well as theoretical, of this result we leave to those who are proficient in this esoteric phase of nibber technology! Comparison with Active Chars, etc.
1
0 os
01 -NORMALITY
I -
I
005
AQUEOUS H C L
NORMALITY
01
A L C O H O I I C DPG-
Figure 4-Special heat-treated carbon black Selective Adsorption Isotherms. Acid (HCI) YS. Alkali (D. P. G.)
The adsorptive activity of carbon black having a t last yielded, in considerable measure, to artificial control, it may be of interest to compare its activity with that of the older forms of adsorptive or active carbon. Table VI-Comparison of Adsorption Activity of Carbon Black with T h a t of Other F o r m s of Active Carbon
BLACK HC1 REXOVED D. P. G. REMOVED The addition of carbon black will in general disturb this 95, LT, ._ ._ fatty acid equilibrium, so that the effect upon the cure of the 7 Heat-deactivated carbon 8 addition of, say, 25 volumes of black will depend upon both 3 12 Standard rubber carbon 4 0 12 Paint the dosage of fatty acid and the adsorption activity of the 0 I n k hhck 57 black. Thus, there is only one proper fatty acid dosage for 57 32 Li irco each grade of carbon black, and conversely, if the acid dosage 70 Suchar 71 is fixed, a black of suitable adsorption must be chosen in 11 80 Activated paint 54 Merck blood charcoal 95 order to get the best cures. Super-activated carbon 96 5 Since excessive additions of fatty acid are not practicable for present-day litharge compounds (footwear, etc.), it would In general, it will be noted that the (alkaline) adsorptive seem from the above data that a carbon black of low ad- activity of carbon black may be controlled between the sorptive activity should prove most satisfactory. In fact, .widest limits, surpassing in this respect even the most active the newer deactivated, low-adsorptive carbon blacks have chars. I n acid adsorption, however, the carbcln blacks already found application in this field of compounding. show a range of from 0 to only 12 per cent. It is suggested that these newly developed adsorptive Low-Adsorptive Blacks i n Unaccelerated Mixings properties may find application in the arts of decolorizing, The striking improvement in curing behavior shown in deodorizing, etc. The specificity of such “removal” effects Table IV may be ascribed to the non-adsorptive blacks’ not is noteworthy and seems to be unapproached by other forms having removed the natural accelerators present in the crude of carbon. rubber. Literature Cited
Table IV-Curing
D a t a on Low-Adsorptive Blacks in Cnaccelerated
Mixings
(Rubber, 9 3 , carbon black, 35; zinc oxide, 3; and sulfur, 5. Cure, 120 minutes at 40 pounds) D. P. G. CARBON BLACK TENSILE ELONGATION T P. ADSORPTION Lbs./sq. in. % % Standard (rubber) 2200 580 128 11.4 3600 Heat-deactivated 590 212 5.1
Carbon-Black Adsorption and Electrical Insulation
It has recently been shown that the “removal” activity of carbon black may be turned to advantage in rubber insulation for wires, etc. (15),and also in the improvement of insulating oils (16). The very property which is so damaging in modern organic rubber compounds is capable of increasing by one-half the electrical insulating properties of these substances.
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16)
Beaver and Keller, IND. ENG.CHEM.,20, 817 (19281. Carson and Sebrell, Ibid., 21,911 (1929). Cox and Park, I b i d . , 20, 1088 (1928). Ditmar and Preusse, Gummi-Ztg., 45, 243 (1930); I n d i a Rubber J . , 81, 24 (1931). Fromandi, Kautschuk, 6, 27-30 (1930); Rubber Chem. Tech., 3, 220 (1930). Goodwin and Park, IND.ENG.CHEX.,20, 621, 706 (1928). Johnson, Ibid., 21, 1288 (1929). Le Blanc, Kroger, and Kloz, Kolloidchem. Beiheffe, 20, 365-411 (1925). Rose, U. S. Patent 1,433,099 (Oct. 24, 1922). Spear and Moore, IND.ENG.CHEM.,18, 418-20 (1926). Spear and Moore, Rubber A g a (London), 0, 123-5 (1928). Thies, IND.ENG.CHEM.,17,1165-9 (1925). Twiss and Murphy, J . Soc. Chem. Ind., 46, 121T (1926). Wiegand, Can. Chem. M e t . , 12 (1929). Wiegand and Boggs, IND.ENG.CHEM.,22, 822 (1930). Wiegand, Boggs, and Kitchin, Ibid., 23, 273 (1931).
