February 1953
377
INDUSTRIAL A N D ENGINEERING CHEMISTRY
the same period. The patches lacked some of the adhesive qualities of the original lining. Nearly all the patched areas, however, were located on the tank floors where adhesion was of less importance than was the case for roof and wall linings. It is not known whether this reduction in adhesiveness was a property of the particular batch of latex which was prepared for the lining repair program or was due to the addition of the pentachlorophenol. Direct microscopic examination of two patches which had been in place for 4 years revealed no evidence of mold growth or of bacterial attack. Swab cultures of the patches on nutrient agar plates having pH of 7.0 and 4.5 gave no mold growth. Minor and inconsistent growth of nonspore-forming bacteria was observed. It is suspected that these particular microorganisms were air-borne contaminants since these latter samples were not obtained or shipped under aseptic conditions. Spore-forming bacteria were isolated in only one of twelve cultures of the lining patches which had been mascerated with sand. These inspection and experimental results on samples of typical Thiokol M X latex and fabric tank lining patches after 4 years of service indicate that the fungus attack can successfully be controlled by the addition of pentachlorophenol to the Thiokol MX latex dispersion. These latter findings and the observations throughout this study suggest that the fungi are probably more deleterious than the bacteria in the polysulfide polymer latex-lined tanks, although both types of microorganisms were always found together in the areas of the linings which suffered from the observed biodeterioration. CONCLUSIONS
The polysulfide polymer linings in underground concrete storage tanks which are used for the storage of liquid fuels are subject to attack by molds and bacteria under conditions of
tropical temperature and-humidity. This attack can be inhibited by treating the polysulfide linings with an appropriate fungicide, notably pentachlorophenol. Severe biodeterioration was also encountered in the polysulfide polymer linings of tanks which were operated by water displacement systems. Bacteria of the genera, Bacillus, Flavobacterium, Achromobacter, and Sporovibrio, and molds of the genera, Acremonium, Cladosporium, Alternaria, Fusarium, Trichoderma, and Penicillium, were isolated from samples of the deteriorated linings. ACKNOWLEDGMENT
The writers wish to acknowledge the help and interest of the late Morris Spamer of the Navy Bureau of Yards and Docks for his continued help and interest in this project. F. X. Kerr and R. J. Hardy, Jr., assisted wholeheartedly in the phases of the work which were conducted in the Canal Zone. Further thanks are due to D. D. Ritchie, USNR, of this laboratory, to the Naval Medical Research Institute, Bethesda, Md., to the Miraflores Testing Laboratories of the Panama Canal Administration, and to the Research Institute of the University of Houston for laboratory facilities and assistance in isolating and identifying the microorganisms encountered in this study. W. D. Dunn, USNR, and Harold Drooz, USNR, of this laboratory, assisted with much of the laboratory work. LITERATURE CITED
(1) Fettes, E. M., and Jorczak, J. S., IND.ENG. CHEM.,42, 2217 (1944). ( 2 ) Spamer, M. A., J . Am. Cmc-reteInst., 40, 417-28 (1944). (3) Zobell, C. A., and Beckwith, J. D., J. Am. Water W o r k s Assoc., 36, 439-52 (1944). RECEIVED for review December 31, 1981.
A C C E P T E D September
11, 1952.
Influence of Carbon Black on the Oxidation of Natural Rubber G. J. VAN AMERONGEN Rubber-Stichting, Deut, Holland
I
N THIS investigation an attempt was made to determine to
what extent thesolubility of oxygen in natural rubber and the aging resistance are influenced by the incorporation of carbon blacks of various particle sizes in the rubber. The influence of the particle size of carbon black on the oxidizability of GR-S loaded with carbon black had already been proved by Winn, Shelton, and Turnbull (IO). In their explanation of this effect, carbon black was considered to be a catalyst for the oxidation reaction of rubber, although nothing was known about the nature of this catalytic action. Moreover, measurements with natural rubber were lacking. METHODS
The following methods were used in this investigation : MEASUREMENT OF THE RATEOF OXIDATION.Usually the aging properties of rubber are determined by the Geer Evans oven test or the Bierer Davis bomb test. These tests, however, give no quantitative data as to the amount of oxygen absorbed during aging. An apparatus was therefore used with which it was possible to measure the rate of oxidation of 12 small samples of rubber simultaneously. This apparatus, which has been described in detail elsewhere (W),works on the principle of measuring volumetri-
cally the amount of oxygen absorbedduring oxidation in an atmosphere of oxygen. During the test the rubber is thermostated at 80" or 110" C. and the oxygen pressure remains virtually constant a t 1 atmosphere. DIRECT MEASUREMENT OF THE SOLUBILITY OF OXYGEN. The amount of oxygen which is reversibly absorbed by an amount of finely cut rubber was measured in a specially designed apparatus ( 1 ) for measuring the solubility of gases in rubber and consisting of a container connected with a gas volume meter. This physical absorption of oxygen contrasts with the irreversible chemical absorption (oxidation) measured under the first procedure. T o avoid chemical absorption as far as possible, these measurements are carried out only at the relatively low temperatures of 25" and 50' C. INDIRECT MEASUREMENT OF THE SOLUBILITY OF OXYGEN. The solubility was calculated (1) from measurements of the permeability and diffusivity of rubber to gases, using the equation Permeability = diffusivity X solubility MEASUREMENT OF THE ADSORPTIVE POWER OF CARBON BLACK. Iodine adsorption was used to measure the adsorptive power of carbon blacks. One gram of carbon black was shaken for 1 hour with 100 ml. of a 570 potassium iodide solution in which various
INDUSTRIAL AND ENGINEERING CHEMISTRY
378
TABLE I. Surf. Area, Carbon Black i n Compound Thermax (MT) P 33 (FT) Sterling SO (FEF) Statex K (VFF) Vulcan 3 (HAF) Spheron 9 ( E P C ) Spheron 6 ( M P C ) Spheron 4 (HPC) Voltex (CC)
%?yG. 19 21 41 75 86 106 120 142 200
OXIDATION PROPERTIES OF NATURAL RUBBER-CARBON BLACK COMPOUNDS
Ir Adsd., G./G. a t 0.2 G. rZ/ioo All. S o h .
IZAdsd., G./G. at 2 G. 1~/10o 341. S o h .
0.015 0.033 0.046 0.08 O.OS3 0.06
0.07 0.10 0.13 0.12 0.17
0.085
0 .le 0.20 0.36
0.00 0.20
0.15
02 Solub., cO./cc. in VU'C. Compounds 25' C. 50' C. 0.088 0.098 0.14 0.19 0.35 0.61 0.78 0.99 0.62
-
090
080 -
-
0 7 0
0.60 -
0
050
-
X
040
-
0 3 0
-
0 2 0
-
0 IO;
0
I
cc
I
j MY
Furnace
40
Vol. 45, No. 2
120
0.41
0.47 0.58 0.47
0,027
0,025 0.032 0.044 0.045 0 . 065 0.075 0.076 0.079
0,021 0,022 0.026 0.033 0.037 0.047 0.053 0 0.51 0,062
160
200
Figure 1. Oxygen Solubility i n Natural RubbeiCarbon Black Vulcanizates as a Function of Carbon Black Surface Area
0.32 0.28 0.39 0.47 0 . eo 0.i2 0.66
0.27
0.23 0.28 0.35 0.45 0 . .io 0.4ti 0.54
0.08
0.011 0,015 0,014
0.09 0.12 0.11 0.14 0.19 0.18 0.18 0.24
0.019 0,023 0,038 0.031 0.034 0.031
0.75 0.81
In X = -H/R!l'
!
C a r b o n s u r f a c e area, m ? / g r a m
0.28
cont.aiiiing lionreinforcing blacks- The rerliarkabla point here is that the normal power of carbon black of adsorbing oxygen (or gases in general) is maintained even when mixed with rubber. Apparently carbon black in rubber has still a great amount, of free surface, probably in narrow pores, into which the rubber molecules cannot enter. It is interesting to note that carbon black with a suriace area of 100 square meters per gram will adsorb 22.8 cc. of nitrogen per gram of carbon black at - 195" C. in a monolayer, or 6.0 cc. calculated on the carbon black per ml. of rubber compound ( 3 ) . The highest value obtained a t 25' C. of nitrogen solubility in rubber with Spheron 4 loading was 0.61 cc. per ml. of rubber compound, but. this will increase considerably on reduction of temperature. It can be assumed that the dependence of gas solubility, S, on temperature T , follows the equation'
I
Channel
80
0.090 0.092 0.12 0.16 0.25
0 2 Absd., Cc./G./Hr. in Vulc. 0 2 Absd., Cc./G./FIr i n Compounds Crepe-Black Compounds 8OoC., llO°C., llO°C., 80' C., 110" C , 16-40 hr. 0-4 hr. 4-7 hr. 16-41 hr. 5-21 hr.
80" C., 0-16 hr.
+C
where H is the heat of solution and C and R are constants. From this equation it follows that by plotting t'he oxygen solubility against 1 / T , this solubility can be found a t any temperature, e.g., a t 70",SO", or 110" C. (see Figure 2 ) . Oxygen solubilities a t these high temperatures cannot be measured directly because of the chemical react.ion of rubber :znd oxygen.
amounts of iodine had been dissolved. From the adsorption graphs thus obtained, the amount of iodine adsorbed in equilibrium with a 0.2 and 2% iodine solution was determined. RESULTS
Table I gives a comprehensive survey of results. Measurements were taken with unvulcanized compounds of 100 parts of crepe and 50 parts of carbon black and v-ith vulcanized compounds of 100 parts of crepe, 50 parts of carbon black, 2 parts of sulfur, 5 parts of zinc oxide, 1part of Santocure, 1 part of stearic acid, and 1 part of phenyl-p-naphthylamine, vulcanized 15 t o 20 minutes, depending on the black used, a t 142" C. Column 2 in Table I gives the surface area of the carbon black. These data, which are determined from nitrogen adsorption measurements, are borrowed from the literature ( 6 - 7 ) . Columns 3 and 4 give data on iodine adsorption of carbon black in equilibrium with the two concentrations mentioned. Generally this adsorption increases with increasing surface area. Columns 5 and 6 give results of direct solubility nieasurements at 25" and 50" C. of oxygen in the vulcanized carbon black compounds. The data are expressed in cc. of oxygen a t normal temperature and pressure, soluble in 1 ml. of rubber compound a t 1 atmosphere oxygen pressure. Calculation of the oxygen solubility from permeability and diffusivity data gave fundamentally the same values. For comparison it should be realized that the solubility of oxygen in a pure gum vulcanizate a t 25" and 50" C. is about 0.10 CC. per ml. of rubber or in a compound containing 33% of an inert, nonadsorbing filler, 0.067 cc. per ml. of rubber compound (1). Figure 1, derived from the data of Table I, shows that the solubility increases very strongly n-ith increasing surface area of the carbon black used. The concentration of oxygen in a compound containing a very fine black can be 10 times that in a compound
I
I
33 6 L
1
,
*=
I
I
310 I
I
28 4
1/7XlO3,I
, ,
7080 tnoc. .
1
1
261 I
I
IOOIIO
1 0 T F r n F ~-
Figure 2. Oxygen Solubility i n Natural Rubbercarbon Black Compounds Depending o n Temperature
Figure 2 also shows that the difference in oxygen solubility in various carbon black compounds a t higher temperatures is considerably less than a t lower temperatures. In rubber loaded with 50 parts of Spheron 4,for example, 10 times as much oxygen dissolves a t 25' c. as in the rubber loaded withThermax. A4t110" c.
IN D U S T R I A L A N D E N G I N E E R IN G C H E M I-ST R Y
February 1953
I
I
aooc. -.
/
.
c
m
0,
/
u
. -
I,"
0.06
=- 0.05
.
0
m 0.04
2 0.03
-
0.020 Thermal furnace
01
Combustion furnace Channel
0
c
0.02
a 0.01
m
l x
0.07
?
0.060
0
0.08
00
60.070
-
u
0
0
a
319
c
0
40
80
120
160
200
C a r b o n s u r f a c e area, m?/gram Figure 3. Rate of Oxidation of Natural RubberCarbon Black Vulcanizates as a Function of Carbon Surface iirea i t is only twofold because of the strong decrease in oxygen solubility, caused by temperature increase. Strongly decreased solubility such as this indicates great heat of solution. Great heat of solution can be interpreted in this case as heat adsorption. The heat of solution of oxygen in the rubber-Spheron 4 and 6 compounds amounts to -3600 calories per mole, in the Spheron 9 compound to -3000 calories per mole, in the Vulcan 3 compound -2600 calories per mole, in the Statex K compound -1300 calories per mole, and in the Thermax compound +150 calories per mole. Columns 7 to 10 in Table I give the chemical oxygen absorption of vulcanized carbon black compounds a t 80' and a t 110' C., while columns 11 and 12 do the same for the uncured stocks. These data are expressed in cc. of oxygen at normal temperature and pressure absorbed by 1 gram of rubber compound per hour a t 1 atmosphere of oxygen pressure. The readings are taken and averaged for a time of oxidation as indicated. Generally the rate of oxidation is slightly dependent on time, being higher a t the beginning of the oxidation period and after a long oxidation period. Figure 3, derived from Table I, shows that the rate of oxidation of natural rubber-carbon black vulcanizates increases considerably with increasing surface area of the carbon black used. It is a striking fact that this increase is slower a t 110' C. The obvious explanation of this dependence of rate of oxidation on carbon surface area and temperature is that the rate of oxidation is directly related to the oxygen concentration, i.e., oxygen solubility in the rubber, as demonstrated by Figures 1 and 2. Thesg views are corroborated by Figure 4,which shows that the rate of oxidation depends upon oxygen solubility. Such an increase should be compared with the well-known increase in rate of oxidation on increase of oxygen pressure ( 2 , 4). DISCUSSION
It is not unlikely that the abrasion properties of rubber are influenced by oxidation processes. It might be suggested that a considerable temperature rise-e.g., of up to 200' to 300" C.-will occur locally or on a molecular scale at critical hot spots of a motor-car tire in actual service-e.g., during slippage. In that case breakdown of the rubber under the influence of oxygen and pyrolysis might be important factors in explaining abrasive behavior of motor-car tires. From this point of view abrasion might have some factors in common with ordinary mastication of rubber on the mill. It is well known that rubber molecules are broken down on the mill under the influence of mechanical forces (friction) and oxygen a t apparently low temperatures; probably because of the local development of hot spots in the rubber.
m
OT
Channel
Furnace I
,
,
,
!
,
,
0 20 40 60 0, Solubility, cc./cc., at 5OoC. Figure 4. Rate of Oxidation of Natural R u b b e d a r b o n Black Vulcanizatee at 80" C. as a Function of Oxygen Solubility a t 50" C.
Even if i t is doubted that momentary breakdown is likely to take place during slippage, it is reasonable to assume that evolution of heat a t the tire's surface will weaken the rubber in the long run on account of oxidation. Recognition of the effect of oxidation on abrasion would readily explain the better abrasion properties (9) of furnace blacks in rubber compared with channel blacks (compare Figure 4). I n this connection an explanation of another fact might be suggested. It has been found (9) that tire treads of cold rubberfurnace black compounds sometimes show better resistance to abrasion than natural rubber-furnace black treads. The special circumstances under which this occurs seem to be high temperature and dry weather. Now it might be suggested that one of the reasons for this phenomenon is the greater resistance of cold rubber t o high temperature oxidation. The same argument would of course apply to standard GR-S, but this rubber is obviously inferior on other*grounds (8). Another interesting point is to be learned from Figure 2, It can be imagined that during sudden slippage the temperature of the tire increases locally from 25" to, say, 110' C. in a very short time. The oxygen solubility in a rubber with channel black loading should, according to Figure 2, in that case decrease to one fifth of its previous value. If the rate of heating is higher than the rate of diffusion, this results temporarily in the presence of gas under a pressure five times the normal pressure. Undoubtedly this will have some bearing on oxidation in actual progress ( 4 ) . LITERATURE CITED
(1) Amerongen, G.J. van, J . Applied Phys., 17,972 (1946);Rubber Chem. & Technol., 20, 494 (1947)). (2) Amerongen, G. J., van, Rev. gbn caoutchouc, 20, 136 (1943); Rubber Chem. & Technol., 19, 170 (1946). (3) Beebe, R. A.,Biscoe, J., Smith, W. R., and Wendell, C. B., J. Am. Chem. Soc., 69,95 (1947). (4) Carpenter, A. S.,IND.ENG.CHEX,39, 187 (1947). ( 5 ) Dannenberg, E. M., and Collyer, H. J., Ibid., 41,1607 (1949). (6) Drogin, I., "Developments and Status of Carbon Black," Charleston, W. Va., United Carbon Co., Inc., 1945. (7) Drogin, I., and Bishop, H. R., "Today's Furnace Blacks," Charleston, W.Va., United Carbon Co., Inc., 1947. (8) Fielding, J. H., IND.ENQ.CHEM.,41, 1560 (1949). (9) Mandel, J., Steel, M. N., and Stiehler, R. D., Ibid., 43, 2901 (1951). (10) Winn, H.,Shelton, J. R., and Turnbull, D., Ibid., 38, 1052 (1 946). RECEIVED for review M a r c h 26, 1952. ACCEPTED September 2 4 , 1952. Communication 166 of the Rubber-Stichting, Delft, Netherlands.
