I
J. REID SHELTON and WILLIAM T. WICKHAM, Jr.' Department of Chemistry and Chemical Engineering, Case Institute of Technology, Cleveland 6, Ohio
Carbon Black in the Oxidation of ButadieneStyrene Vulca nizates Here is a study indicating that carbon black added to GR-S vulcanizates may function in many ways to alter the rate of oxidation of carbon black on the oxidation of rubber vulcanizates, as reported by this laboratory (22), clearly showed a n increase in oxidizability, as measured by the volumetric oxygenabsorption technique (4, 73, 74))with an increase in carbon black content and in surface area of the black. I n the range of carbon content from 25 to 75 parts of black per 100 parts of rubber for a given type of black (either channel or furnace), the dependence of the observed rate (during the constant-rate period) on surface area of the black was linear. Hence, the effect of carbon black on oxidation was termed to be a catalytic one. Much information on the effect of carbon black upon the oxidation of rubber has been reported by other laboratories. Sweitzer and coworkers have observed an inhibitory effect (6, 77, i s ) . Kuz'minski: and his group found an acceleration, but explained it on a different basis (70). Van Amerongen has shown the relationship of solubility of oxygen in rubber to the presence of carbon black ( 7). Because of the many conflicting statements concerning carbon black and oxidizability of rubber, this laboratory re-examined the problem in an attempt to answer some of the questions raised by the more recent studies. Present address, Owens Illinois Glass Go., Toledo, Ohio
T H E EFFECTS
Experimentat Procedures
The oxygen-absorption technique (73, 74) was used to measure the susceptibility of the vulcanizates to oxidation. Oxidations were always conducted in duplicate or triplicate, and in pure oxygen. The temperatures of the oxidations were selected to give optimum resolution of the data within a reasonable period of time. Uninhibited vulcanizates were oxidized a t 70' C. in most cases, and inhibited vulcanizates at 100' C. The data are reported on the basis of oxygen absorbed per gram of polymer (rubber) in the vulcanizate. For each comparison study, vulcanizates were prepared from one sample of polymer, cure curves were prepared,
Table II.
Typical Recipe Parts by Weight Polymer (as indicated) 100 Zinc oxide 3 Stearic acid 2 N-Cyclohexyl-2-benzothiazole sulfenamide 1 Sulfur 1 Carbon black (as indicated) Variable Cures at 307' F.
Accelerating Effects
I n some instances carbon black appeared to act as a n antioxidant for rubber oxidation (6, 77, 19) and not as a promotor of oxidation as reported (22). This investigation involved new but similar oxidation studies. I n this case, furnace blacks weion of oxygen absorption of GR-S 1000 vulcanizate with Statex B at 100' C., 760 mm.
diene-styrene, GR-S 1000, polymer to form tread-type vulcanizates. Carbon blacks used in this study were Furnex, Statex M, and Statex B, which have three different surface areas. Surface areas, recipes, and oxidation data of
1278
stocks containing 10, 20, 40, and GO parts of E'urnex and Statex M, and 5, 10, 30, and 60 parts of Statex B per 100 parts of rubber are presented (along with data for the gum stock) in Tables I, 11. and 111, respectively.
INDUSTRIAL AND ENGINEERING CHEMISTRY
llilliliters of oxygen per gram of polymer (25' C.)
Hours a t 1000 c. 3.0 12.0 19.75 24.0 36.25 48.0 60.25 72.0 Cure at 307' B.3 Min.
Statex B Concu., PHR 0
0.3 1.4
2.3 2.7 3.7 4.9 6.6
9.4
50
10 0.4 1.8
60
2.9 3.3
3.6 4.2 6.6
0.3 2.2
5.0 7.5 12.2 19.2
13.5 18.9
45
35
9.5
OR-S V U L C A N I Z A T E S will increase in effectiveness with increase in surface area. A test might be made of this idea to compare the oxidatiqn curves for varying black loadings with curves of vulcanizates containing varying amounts of antioxidant (8). Superficially the curves show small differences in character which may indicate a difference in mechanism; but a worth-while study from this direction should include physical testing to evaluate effectiveness of the antioxidant included (75). A simpler and more direct analysis was attempted-examination of the effect of varying carbon black in rubber vulcanizates containing no antioxidant. This was attempted in two fashions, the first of which is described here: Commercial GR-S 1000 (butadienestyrene) was extracted in a Soxhlet extractor with acetone until no amine could be detected with the highly sensitive test of Burchfield ( 5 ) . T h e butadiene-styrene polymer was broken into small particles for more efficient extraction by freezing in liquid nitrogen and breaking with a hammer. T h e Burchfield test, a spot test, was performed on fresh portions of the extract which were concentrated to a few drops. T h e test was so sensitive that at least 2 weeks
Table VI. Effect of Carbon Black Content on Oxidation of Uninhibited GR-S 1000 Vulcanizate (Latex 1) Milliliters of oxygen per gram of polymer (25' C.) Statex B: Concn., PHR Hours at 70' C. 9.0 26.0 65.5 115.25 139.25 168.0 189.25 235.0
0 0.4 0.9 1.8 3.0 4.0 5.6 7.1 10.8
10
Cure at 307' F., Min. 30
0.6 1.2 2.5 5.4 7.1 9.2 11.2 15.7
30 0.6 1.3 2.8 5.2 6.6 8.4 9.7 13.1
60 0.8 1.7 4.0 6.8 8.2 10.0 11.3 14.1
25
22
20
Table VII. Effect of Carbon Black Content of Oxidation of Uninhibited Peroxamine GR-S 1500 Vulcanizate (Latex 2) Milliliters of oxygen per gram of polymer (25' C.)
H~~~~at 70" C. 11.0
22.0 48.0 92.5 117.5 142.0 170.5 220.75
3.2
4.2 5.8 10.5
Cure at 307' F., Min. 35
5
10
0.4 0.6 1.1 1.8 2.2 2.8 3.3 4.4 34
+
of continuous extraction were necessary to remove all traces of amines. T h e rubber so prepared was stored in the dark under acetone until used, then dried, compounded, cured, and tested immediately. The data for this study are recorded in Table IV. It may be argued that a vulcanizate prepared in such a manner will have residual amounts of antioxidant, regardless of the extent of extraction and testing. Nevertheless, the antioxidant concentration is greatly reduced, and any carbon black antioxidant interaction would be negligible. Then, if this were the only factor involved in the acceleration, this material should give a rate of oxidation independent of black loading. This is not the case (Table IV). The considerable difference in rate between the blackloaded and black-free stocks (Figure 4) indicates that a n accelerating effect of carbon black must be involved and cannot be explained as merely due to re25
I
I
moval of antioxidant by adsorption on the black. The technique of acetone extraction was laborious and the data obtained with the extracted rubber failed to give reproducible oxidation rates. Consequently, a check was made on this study from a different route. Three samples of uninhibited latex were obtained for this purpose (Table V). Latex 1 is a hot rubber polymer, latex 2 is a cold rubber "peroxamine" polymer (prepared without added iron), and latex 3 is a cold rubber polymer made in a high iron recipe, but with a sequestrant added. I n all cases no antioxidant was added, and either sodium or potassium dimethyldithiocarbamate was used as a shortstop, in order that all ingredients would have a minimum of antioxidant character. These latices were purged with and stored under lamp-grade nitrogen in the dark, and removed for use as needed. Data are presented in Table VI for I
20 -
0 6 0 phr STATEX B N,
a W
5150
f W
Statex B Concn., PHR 0 0.5 0.8 1.5 2.6
Table V. Polymerization Recipes of Special Latices Latex number 1 2 3 Total solids, 70 25 24.9 11 71/29 71/29 BD f S charge ratio 71/29 KFA soap Dresinate 214a Emulsifier NaFA soap p-menthane hydroCatalyst Potassium persulfate p-menthane hydroperoxide peroxide Activator Tetraethylpentamine FeS04.7HzO K1P207 Modi5er Tert-dodecyl mercaptan Shortstop Sodium dimethylSodium dimethylPotassium dimethyldithiocarbamate dithiocarbamate dithiocarbamate Antioxidant None None None Polymerization temp., O F . 122 41 41 Sequestrant None None Versene Fe-3 Stabilizer Tamol Nb Electrolyte K O H and KCI Iron in dry polymer, % 0.006 0.004 a A Potassium disproportionated rosin soap. A complex product of formaldehyde and naphthalenesulfonic acid.
0.5 0.8 1.3 2.1 2.8 3.2 4.2 6.7
30 0.7 1.0 1.8 3.3 4.1 5.7 7.5 11.6
60 0.6 1.0 1.5 3.1 3.9 4.9i 6.2 8.9
33
30
27
w n ON
5
80 I 40 HOURS 6o Figure 4. Effect of Statex B on oxidation of acetone-extracted GR-S 1000 vulcanizate at 100' C., 760 mm.
VOL. 49, NO. 8
AUGUST 1957
1279
Effect of iron
Figure 5. Effect of carbon black on oxidation of uninhibited GR-S 1000 vulcanizate a t 70' C., 760 mm.
oxidation of vulcanizates made from latex 1. the hot rubber, at 70' C. and are plotted in Figure 5. This study shows a clear dependence of rate of oxygen absorption on carbon black content. Again. the adsorption of antioxidant by the black is not the determining factor in the acceleration of oxidation by carbon black, but pla!s only a minor role in this case.
Inhibiting Effect Table VI1 and Figure 6 record a similar study performed on vulcanizates made from latex 2, the cold peroxamine rubber, at 70' C. In this material, a dependence of rate of oxidation on carbon content was also found. but in this case small amounts of carbon retarded the rate of oxidation, as reported with unvulcanized cold rubber (6). Such an effect was not observed in the hot rubber. The third latex used in this study was
an iron-recipe cold rubber. Results of oxidation studies of vulcanizates of this polymer, reported in Table VIII, show a sharp reversal of form, as the unloaded control stock oxidized very rapidly and addition of 5 parts of black per 100 parts of rubber sharply curtailed the rate. With 10 parts of black per 100 parts of rubber the rate decreased even more, but addition of further black (30 and 60 parts) did not lower the rate beyond that of 10 parts of black per 100 parts of rubber. In no case was there an increase in oxidizability of this vulcanizate with increase in carbon content. This material showed some similarity to that from latex 2, in that small amounts of black decreased oxidizability, and a similarity might be expected, since the two have somewhat similar molecular structures. However, the fact that the vulcanizates from latex 3 did not show a n increase in oxidizability with carbon content requires further explanation.
I t was postulated that there might be involved in the action of carbon black, as described above, some interrelationship with the iron present in the original polymer. Hence, a series of vulcanizates prepared from latex 1,with the addition of 0.2 parts of F e S 0 4 . 7 H z 0 per 100 parts of rubber, in the compounding, was studied for oxidizability (Table IX and Figure 7 ) . It can be seen by comparison with Table VI and Figure 5: the introduction of ferrous sulfate markedly diminished the increase in oxidizability normally produced by adding small amounts of carbon black, in spite of the fact that iron is a well-known oxidation accelerator. This indicates a carbon black-iron interaction. Such an interaction has been Observed in another laboratory (7) The combined effect of carbon black and iron upon the oxidation reaction was studied further by preparing another group of vulcanizates from latex 2 (the uninhibited cold rubber prepared without iron in the recipe) with the addition of ferrous sulfate and carbon black. alone and in combination. Iron alone in-
Table VIII. Effect of Carbon Black Content on Oxidation of Uninhibited Iron Recipe GR-S 1500 Vulcanizate (Latex 3) Milliliters of oxygen per gram of polymer (25'
H~~~~ at 7O'F. 4.5 15.5 32.75 51.25 74.5
Cure 1Il)'70
1-.
0
0.6 2.4 4.3 5.9 8.4 10.8 16.2
90.0 113.5
C.)
Statex B Concn., PHI< 5 10 30 60 0.3 1.6 3.0 4.1
5.5 6.5 8.6
0.1 0.6 1.8 2.6 3.6 4.1 5.0
0.1 0.9 1.9 2.6 3.6 4.1 5.0
0.05
1.0 1.9 2.6 3.5 4.0 4.8
at
B -.,
Min.
45
50
45
40
35
Table IX. Effect of Carbon Black Content on Oxidation of Uninhibited GR-S 1000 Vulcanizate with 0.2 phr FeS04.7H20 Added (Latex 1 )
Rillhhters of oxygen per gram of polymei (25' C.) Hours a t 70' C 9.5
23.25 48.0 78.0 118.75 142.0 191 .O 217.75 HOURS
Figure 6.
Effect of carbon black on oxidation of uninhibited peroxamine
GR-S 1500 vulcanizate a t 70' C., 7 6 0 mm. 1280
INDUSTRIAL AND ENGINEERING CHEMISTRY
Statex R Concn., PHR0 5 10 30 60 0.5 0.9 1.5 2.2 3.4 4.8 9.1 12.6
Cure at 307' F., Min. 27
0.6 1.0
0.5 0.9 1.5 2.4 4.2 5.6 9.2 11.6
2.6 4.5 5.9 9.5 11.9
25
25
1.6
0.6 0.8 1.1 1.4 2.0 2.5 2.9 3.9 4.3 6.1 5.4 7.5 8.1 10.5 9.9 12.4
24
23
OR-S V U L C A N I Z A T E S Table X. Effect of Carbon Black and of Iron on Oxidation of Uninhibited Peroxamine GR-S 1500 Vulcanizate (Latex 2) Milliliters of oxygen per gram of polymer (25' C.)
Hours at 70' C. 10.75 48.5 3
71.5 95.75 119.25 165.75 216.0
Cure 307'
Min. a
1-1'
I-2*
1-3"
1-4d
0.4 1.5 2.1 2.7 3.3 4.9 8.8
0.4 1.7 2.3 3.0 3.8
1.2 3.8 6.6 13.2 25.8
0.6 1.7 2.2 2.7 3.2 4.3 5.8
7.0 12.6
at
F., 35
28
35
28
Control (no black; no iron). 25 phr Statex B, no iron. No black,; 0 . 2 phr FeSOc7H20. 25 phr Statex B ; 0.2 phr FeS04.7H20.
creased the rate of oxidation considerably (Table X and Figure 8). Carbon black alone increased the rate only a little above the control, confirming the behavior previously observed (Figure 6) for a comparable concentration of black with this particular latex. When both carbon black and iron were added, the rate was initially about the same as for the control sample and in the later stages the control oxidized faster. Interaction between carbon black and iron with a mutual deactivation of the usual catalytic effect of each is clearly shown by the two sets of data. Figures 5 and 7 clearly show the reduced effectiveness of the carbon black in the presence of iron, while Figure 8 demonstrates the reduced effectiveness of iron in the presence of carbon black. Discussion
*
Throughout the study carbon black increased the rate of oxidation of most vulcanizates. Suitable loadings produced this effect in every oxidation but one (Table VIII). The general increase in oxidizability with carbon black may be due to more than one factor. Adsorption of antioxidant, which leaves the rubber more subject to oxidative attack according to Kuz'minskif (70), may be involved to some extent; but the results of this study indicate that this explanation is inadequate for the activity of black, since the catalytic effect of the carbon surface is observed even when no antioxidant is present. The effect of carbon black on solubility, and diffusibility of oxygen in rubber has been thoroughly discussed by van Amerongen ( 7 , Z ) . He suggests that the increased oxidation rate of rubber with increased carbon surface is related to the increased solubility of oxygen when
I
25
0
50
I
75
100
HOURS
125
150
I75
ZOO
J
225
Figure 7. Effect of carbon black on oxidation of uninhibited GR-S 1000 vulcanizate containing 0.2 phr FeS04.7HzO at 70' C., 760 rnm.
carbon black is present in the rubber vulcanizate. The oxygen is probably adsorbed on the available free surface not occupied by the rubber. T h e increased rate of oxidation could then be attributed to the higher concentration of available oxygen for reaction with the rubber. This interpretation is supported by the data and is probably an important factor in the accelerating effect of carbon black, particularly a t room temperatures. This effect of carbon black on solubility of oxygen in the rubber decreases a t higher temperatures, however, and the results reported by the authors for oxidation a t 100' C. would apparently require additional explanation. A pos-
2c
I
I
f
3 O
16
Lo
(u
L (L
Y>-
/
II
sible alternative to the concentration effect of higher solubility of oxygen would be a surface catalysis of the oxidation reaction which by adsorption of oxygen and/or rubber on the carbon would provide more reactive forms. Carbon black also promotes decomposition of peroxides, and since hydroperoxides play a major role in the oxidation mechanism, a material which increases the rate of peroxide decomposition to form chaininitiating free radicals would catalyze the over-all reaction. Some deactivation of rubber toward oxidation probably occurs in all cases, along with the activating effects, but in low loadings in cold rubber the deactivating effect predominates and I
0 CONTROL,
I
I
NO BLACK, NO IRON
CONTROL 25 PHR BLACK,NO I A (
h O . n O PHR F e 8 ~ . 7 H 2 O 1 NO BLACK
A
0 . e O PHR FeSO,,. 7Hp0 2 5 PHR BLACK
J
x i
a a W a cu
€
O
L
i
. z1
C 120 160 200 240 HOURS Figure 8. Effect of carbon black and iron on oxidation of uninhibited peroxamine GR-S 1500 vulcanizate at 70' C., 760 mm.
0
40
80
VOL. 49. NO. 8
AUGUST 1957
1281
the rate is reduced. The similarity in effect of two entirely different cold rubber recipes contrasted with the behavior in hot rubber indicates that the difference in the effect of the carbon black in the two types of butadiene-styrene may be due to differences in the molecular structure of the polymers. Carbon-black particles can function as free-radical acceptors. Watson (27) in an extensive study on the nature of the carbon black-polymer interaction explains the chemical combination of rubber and carbon black on the basis of carbon blacks ability to function as a free-radical acceptor. Garten ( 8 ) also shows that carbon blacks can combine with organic free radicals. His results indicate that reinforcing carbon blacks can act as single electron donors in the presence of free radicals or radical acceptors. As a result of this mechanism, not all of the new bond formation will be polymer-polymer cross linking, but the black will be similarly involved ( 3 ) . I n the presence of antioxidants carbon black has another inhibitory effect, as indicated in an earlier kinetic study (74) in which it was suggested that carbon black enhances the ability of amine antioxidants to destroy peroxides by promoting decomposition to stable products. Thus, carbon black can promote peroxide decomposition in two ways: only one of which produces chain-initiating free radicals. In the interaction of iron and carbon black more than one mechanism may be involved. The rate of oxygen absorption is lower for the vulcanizate with ferrous sulfate heptahydrate and Statex B than for the vulcanizate with carbon or iron sulfate alone. Thus, a mutual deactivation is indicated. The mechanism by which iron accelerates oxidation probably involves its known ability to induce decomposition of hydroperoxides. Several studies of the surface of carbon blacks (76. 77, 20, 27) indicate that functional groups are present, such as hydroxyl, hydroperoxide, carboxyl, and carbonyl, with a large content of 1,4-quinoid oxy-functional groups (78). These functional groups probably play a vital role in the mechanism of carbon black activity. For example, Sweitzer (79) has shown that the inhibitory power of carbon black is a function of the oxy-functional group content of the black, the decomposition of peroxides by black being reduced or halted by deoxidizing the black. Interaction of iron and black result3 in a lessening of effectiveness of the activity of each ingredient by the other. One possible explanation is that the iron is removed by the black, either as a surface absorption effect, or as a result of some sequestrant activity by chelation with the oxy-functional groups on the black. This would explain the loss of activity of the black, since ferrous or
1282
ferric ions attached by chelation would remove the groups to which they are attached from availability. The effect of varying the concentration of the iron ions may be important. Nicoll (72) in studying a very different system found that although ferric salts catalyze the, decomposition of hydrogen peroxide, very small concentrations ( 1 0 - 5 ~ 4 ) actually stabilize the peroxide. Nicoll suggests, as an explanation, the formation of stable peroxide ions with ferric salts. It is possible that such a concentration dependence is in effect here and adsorption or chelation affects the concentration sufficiently to alter the action of the iron. The actual nature of the interaction of iron and carbon black which results in mutual deactivation is not clear. As both ferric and ferrous ions normally decompose hydroperoxides to give free radicals ( 9 ) . the iron must be removed from availability, or the carbon black may act in some way in the ferric-ferrous oxidation-reduction reaction to alter the usual effect of iron upon peroxide decomposition ( 7 ) . Summary
The role of carbon black in the oxidation of butadiene-styrene vulcanizates is many sided. These phenomena have been observed in certain cases: An increase or decrease in rate of oxidation when carbon black is added: and a decrease in the accelerating effect of carbon black on the rate of oxidation when iron is present and vice versa. The net effect of the presence of carbon black upon the oxidation reaction depends upon the relative magnitude of the many ways in which carbon black may function in the oxidation reaction. Variations have been observed with different loadings of carbon black, a change in the polymer from hot to cold butadiene-styrene, and the addition of iron to the polymer. The observed behavior in this study, together with other evidences from the literature, suggests that carbon black can function to alter the rate of oxidation by :
1. Acceleration of oxidation due to surface catalysis ; catalysis of peroxide decomposition to free radicals; adsorption of antioxidant; and increased solubility of oxygen-and 2. Inhibition of oxidation due to activity as a free-radical acceptor; induced decomposition of peroxides to stable products; and deactivation of metals which catalyze oxidation. Combinations of these possible ways, in which the carbon black can function, may explain conflicting evidences in the many studies reporting on the effect of carbon black upon the oxidation reaction.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Acknowledgment
The authors are indebted to Case Institute of Technology for the original financial support of this work, and to Timothy McDonel for the data of Table X and Figure 8. All carbon blacks used in this study and the data in Table I were supplied by the Columbian Carbon Co. The special uninhibited latices (Table V) Ivere prepared at the Government Laboratories of the University of Akron. Portions of the study were sponsored by the Office of Ordnance Research, U . S. Army. Literature Cited
Amerongen G. J. van, I N D . ENG. CHEM.45, 377 (1953). Amerongen, G. J. van, Kautschuk u. Gummi 7, 132 (1954); Rubber Chem. and Technol. 28, 821 (1955). Barton, B. C., Smallwood, H. M., Ganzhorn, G. H., Ibcd., 28, 202 (1955). Blum, G. W., Shelton, J. R.. LVinn. H., IND.ENG.CHEM. 43,464 (1951) Burchfield. H. P.. Judy. J. Tu’., IND. ENG. CHEM.,.ANAL. ED. 19, 786 (1947). Burgess, K. A., Sweitzer, C . W., IND.ENG.CHEM.47, 1820 (1755). Cole, J. O., Goodyear Tire and Rubber Co., private communication. Garten, V. A , , Nature 173, 997 (1948); Rubber Chem. and Technol. 28, 596 (1755). Kharasch, M. S., Pauson, P., Nudenberg, W., J . Org. Chem. 18, 322 (1953). Kuz’minskil, A. S., Lyubshanskyaya, L. I., Khitrava. N. G., Bass, S.I., Rubber Chem. and Technol. 26, 858 (1753); trans. from Doklady Acad. A’auk S. S. S. R. 8 5 , 131 119521 Lyon, F., Burgess, I
