Aging of Rubber F. LYON, K. A. BURGESS, and C. W. SWEITZER Columbian Carbon Co., 380 Madison Ave., New York 17, N. Y.
Dual Inhibition-Acceleration Role of Carbon Black in Rubber Oxidation Carbon black can function as an inhibitor or as an accelerator of oxidation and cure of rubber as well as of other polymers. The inhibiting action of carbon black on the oxidation of unvulcanized cold rubber appears to be related to, if not the cause of, the increased reinforcement of the heat-treated stock. In general, the effect of carbon black on the oxidation of cold rubber and natural rubber i s the same. With a more complete understanding of the role of carbon black in polymer oxidation and cure reactions, it should be possible to control the behavior of carbon black so that advantage can be taken of its inhibiting and accelerating properties in prolonging the life and increasing the usefulness of rubber goods.
THE
importance of minimizing oxidation reactions in prolonging the useful life of rubber products is well known.
T h e role of oxidation in the complex problem of tire wear is less well understood, but a number of investigations (3,5. 8) have indicated that a connection seems to exist between wear and oxidation. T h e factors which govern the influence of carbon black on rubber oxidation reactions. inhibiting as well as accelerating, are examined here, and their significance is discussed.
Experimental Procedure
Oxygen Absorption. Oxygen absorption was measured volumetrically a t constant temperature and pressure. T h e equipment and procedure have been previously described in detail (7). Physical Properties of Unvulcanized GR-S Films. Pure gum and carbonrubber films, 0.0035 to 0.0055 inch thick, were prepared by evaporating rubberbenzene cements on mercury; the concentration of the rubber cement was 2 grams of rubber prr 100 ml. T h e carbon black had been previously colloid-
ally dispersed in the rubber cement by tumbling the mixture for 16 hours. The undispersed carbon settled out on standing and the upper portion of the sample, diluted with rubber solution so that the final carbon loading was 10 parts of carbon per 100 parts of rubber (phi), was used in these studies. T-50 samples were died from the cast films and elongation was recorded \vith time under a 2gram load. Benzoyl Peroxide Gelation. T h e modified dry adsorption method, previously described in detail (.9% 10). \vas employed. Benzoyl peroxide was added to the rubber-carbon cement before the solvent was evaporated. T h e dried films were heated under nitrogen at 130' C. for the required time intervals. Following the heating period, the flasks were cooled and 100 ml. of benzene was pipetted into each flask. T h e flasks were allowed to remain for 24 hours at room temperature, after ivhich the gel content was determined gravimetrically. Experimental
Recipes for Vulcanized Stocks Sulfur Cure Parts by weight
..__
GR-S 1500 Bardol
100.0 5.0
Fatty acid Zinc oxide Sulfur Santocure Carbon black Cures at 298 ' F.
1.5 5.0
2.0
Dicumjl Peroxide_Cur? _______ __~____ Parts by weight
GR-S 1500 Di-Cup. (405; dicumyl peroxide) Carbon black
100
3.1
Variable
Cures at 290' F.
1.2
Variable Trade name of Hercules Powder Co.
All Stocks Healed in Oaygen at 140°C.
'?
All Stocks Heated in Oxygen of 140.C.
been apparent that carbon black can act as a n inhibitor as well as a n accelerator of rubber oxidation. O n the one hand work in this laboratory sho\ved that carbon black is a strong inhibitor of unvulcanized cold rubber oxidation (7, 70). O n the other hand several investigators have demonstrated that carbon black accelerates the oxidation of rubber vulcanizates (2, 73). This re-
2 8 0 -f
W
a
/ P'%n .
Dual Oxidation Role of Carbon Black. For a number of years it has
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T I M E OF HEATING
- hours
Figure 1. Effect of carbon black on oxidation of vulcanized 41 ' F. GR-S
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TIME OF HEATING
- hours
Figure 2. Effect of carbon black on oxidation of dicumyl peroxide-GR-S vulcanizates
INDUSTRIAL AND ENGINEERING CHEMISTRY
0
All 8to& thatod
In w
e n
al 140'12.
Control
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All Films Heated I Hour at 130° C. in Nitrogen
n 70 -
w
Peroxide
1.3% 0.6 %
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IO
30
40 CARBON LOADING- phr
20
Figure 4. 41 " F. GR-S. peroxide gelation
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Effect of carbon on benzoyl
IO
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40 CARBON LOADING- p h r
20
30
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50
60
1
Figure 5 . Acetone-extracted 41 ' F. GR-S. Effect of carbon black on aging of unvulcanized 4 1 ' F. GR-S
dicumyl peroxide as the curative (7), this ported acceleration effect in sulfur was the first nonsulfur svstem studied. vulcanizates has also been observed in T h e effect of carbon black loading on the recent investigations in this laboratory. oxygen absorption of dicumyl peroxideTemperatures u p to 140" C. were exGR-S vulcanizates is presented in Figure plored and carbon black was found to 2. The strong inhibiting action of caraccelerate oxidation of sulfur vulcanbon black in dicumyl peroxide vulcanizates over the complete temperature range. The accelerating effect of 50 izates strongly resembles the unvulcanized cold rubber series previously disparts of EPC carbon black per 100 parts cussed, in which the stocks became more of rubber on the oxidation of sulfur resistant to oxidation with increasing vulcanizates a t 140' C. is shown in Figure 1 . carbon black loading. Increasing cure time, from the optimum of 40 minutes to This dual behavior of carbon black is 1 00-minute overcures, did not signifinot limited to rubber oxidative systems. cantly change the inhibiting characterRecently this laboratory presented data istics of carbon black in these stocks. which showed that carbon black can act Highly reinforcing ISAF carbon blacks as a strong inhibitor as well as a n ultraexhibited similar antioxidant properties accelerator for the cure of unsaturated in dicumyl peroxide vulcanizates. polyester resins ( 7 7 ) . I n the relatively T h e inhibiting effect of carbon black simple polyester system it was clear that on the oxidation of dicumyl peroxide the dual behavior of carbon black was vulcanizates suggests that the acceleratdue to compounding changes. T h e effect of recipe and compounding ing action of carbon black in sulfur vulcanizates is associated with the comchanges on the oxidation of rubberpounding recipe employed. I t should carbon black systems was, therefore, be emphasized. however. that the investigated next. Factors Involved in the Dual Oxidicumyl peroxide gum vulcanizate oxidative Behavior. DICUMYLPEROXIDE dized much more rapidly than the sulfur VULCANIZATES. Since excellent carbon gum vulcanizate. while the carbon black vulcanizates can be prepared with black-loaded dicumyl stock oxidized at
300
I
I
60
50
a slightly slower rate than the carbon black-sulfur vulcanizate. Differences in the rate of oxidation of the gum vulcanizates are, therefore, responsible for carbon black's being classified as an inhibitor or as a n accelerator. One reason for the excellent aging properties reported for dicumyl peroxide vulcanizates may be associated with the strong inhibiting action of carbon black. ACETONE-EXTRACTED SULFURVULCANIZATES.A similar reversal in the effect of carbon black on the oxidation of sulfur vulcanizates can be demonstrated by oxidizing acetone-extracted vulcanizates. Sulfur vulcanizates were extracted in acetone for 24 hours at 25' C.. followed by another 24-hour extraction with fresh acetone; these stocks were dried under vacuum for 3 days and stored under nitrogen until tested. T h e oxygen absorption data in Figure 3 show that carbon black inhibited the oxidation of acetone-extracted sulfur vulcanizates. T h e change in classification of carbon black from a n accelerator in the unextracted vulcanizate to an inhibitor in the acetone-extracted stocks is once again primarily due to the fact that the extraction accelerated the
t
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1
Figure 6.
2
3 4 8 TIME minuler
-
6
7
Films aged in air at 25" C.; 2-gram load
8
1
2
3 4 5 TIME- minute:
6
7
8
Figure 7. Films shelf-aged 100 hours prior to heating for 1 hour at 125" C.; 2-gram load VOL. 48, NO. 9
SEPTEMBER 1956
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subsequent oxidation of the g u m vulcanizate considerably more than the carbon black vulcanizate. BENZOYLPEROXIDEGELATION. A third interesting example of the dual behavior of carbon black is the gelation of cold rubber films with benzoyl peroxide. Carbon black normally acts to accelerate and promote gelation of cold rubber as shown in Figure 4. But when acetoneextracted rubber is employed, carbon black represses gelation as indicated in Figure 5.
Physical Properties of Aged Unvulcanized Cold Rubber Films. T h e inhibiting action of carbon black on the oxidation of unvulcanized cold rubber has been previously studied by means of oxygen absorption, gelation, and intrinsic viscosity measurements. Data in these studies indicated that oxidative scission predominated when cold rubbrr films 'ivere heated in air a t 130' C . ; but when carbon-rubber films were heated under identical conditions, the intrinsic viscosity of the soluble fraction remained unchanged and the amount of rubber insolubilized by the carbon increased. I t was suggested that the carbon acts as a n inhibitor of the oxidation reactions by reacting with the free radicals and intermediates through which the oxidation reactions proceed. Such interaction should result in a marked difference between the physical properties of gum and carbon-rubber films which have been aged or heat treatrd. I n this study the effect of carbon black on the strain-time relationship of aged and heat-treated cold rubber films is presented. Figure 6 sho\vs that air aging a t room temperature stiffens the gum film considerably. while in the carbon black film no significant change in the strain-time curve is observed. Carbon black can be considered to have inhibited the oxidative cross-linking reactions. T h e aged films were then heat treated a t 130' C. for 1 hour in a n air atmosphere and tested as berore. 11is evident in Figure 7 that the gum film softened, indicating a predominance of scission
A l l Film8 Heated In Oaygen at 140°C.
Control No Carbon
300
Figure 8. oxidation rubber
1 546
Effect of carbon black on of unvulcanized natural
reactions, while the carbon black film became stiffer, suggesting that not only were the scission reactions curtailed, but that carbon black-rubber bonding increased. Effect of Carbon Black on Natural Rubber Oxidation. Studies of van Amerongen have demonstrated that reinforcing carbon blacks accelerate the oxidation of vulcanized natural rubber, Lvhile other studies based on gel measurements have indicated that carbon black inhibits the oxidation of unvulcanized natural rubber ( 9 ) . A more complete investigation of the effect of carbon black on the oxidation of unvulcanized natural rubber films cast from solution was made, using oxygen adsorption measurements. I t is evident from the data i n Figure 8 that carbon black is a strong inhibitor of oxidation of such films; increasing the carbon black loading decreases the rate of oxidation. I n general, the pattern of carbon black activity in rubber oxidation is the same for both GR-S and natural rubber.
Discussion The data obtained ivith the three vulcanized systems described-dicumyl peroxide vulcanizates, acetone-extracted sulfur vulcanizates, and benzoyl peroxide-vulcanized cold rubber-can be interpreted with partial success, according to the theory of Kuz'minskii ( 6 ) . This theory states that carbon black accelerates oxidation of rubbers containing antioxidant because the carbon black sorbs and inactivates the antioxidant, which is a stronger inhibitor than carbon black itself. I n rubbers without antioxidant carbon black itself functions as a n antioxidant. I n this study, dicurnyl and benzoyl peroxide, as well as carbon black; can be considered as inactivators of antioxidant. I n such systems carbon black can function as a n inhibitor in the loaded stocks. while the gum stocks. no longer containing active antioxidant! oxidize a t a more rapid rate. Similar reasoning can be applied to the acetone-extracted sulfur vulcanizates. from Ivhich a significant portion of the antioxidant has been removed by the solvent, leaving the carbon black in the loaded stocks as the principal inhibitor of oxidation. Such reasoning, however, cannor explain the inhibiting action of carbon black in unvulcanized cold rubber Lvhich contains antioxidant. I n this system previously described in detail (7): increasing the carbon black loading decreases the rate of oxidation. Kuz'minskii's theory predicts that increasing the carbon black loading \vould incrrase the oxidation rate. The inhibiting action of carbon black on the oxidation of unvulcanized rubber has been rationalized in terms of the ability of carbon black to react Lvith and
INDUSTRIAL AND ENGINEERING CHEMISTRY
inactivate rhe free radicals and intermediate reaction products through which oxidation proceeds (70). More recently additional evidence of the interaction of carbon black \vith free radicals has been presented (J? 72). This reaction ma)not only be one of the mechanisms 01 oxidative inhibition, but also one of tht. mechanisms of carbon-grl formation responsible for reinforcement. Thv strain-time relationships obtained with unvulcanized cold rubber films tend to support this theory. T h e role of carbon black in accelerating the oxidation of sulfur-vulcanized stocks appears to be far more complex. I n addition to the antioxidant sorption theory of Kuz'minskii, J . R. Shelton and his associates a t Case Institute h a w postulated a catalyric role for carbon. in \vhich thr carbon promotes thr decomposition of peroxide to free radicals capable of initiating oxidation chains, lvhile van &\merongen suggested that the greater solubility of oxygen in carbon-loaded stocks may account for the increased rate of oxidation. There is not sufficient evidence at present to arrive a t a complete understanding of the role of carbon black in oxidation reactions. Howevrr, the evidence presented here suggests that carbon black may be interacting with one or more of the compounding ingredients or cvith the products of vulcanization in such a way as to accelerate the subsequent Oxidation reactions in sulfur vulcanizates. literature Cited (1 ) .hnberg. L. 0.: LVillis, LV. D., meeting of Division of Rubber Chemistry, ACS, Detroit, Mich., May
1955. ( 2 ) Amerongen, G. J. van, I N D E N G
CHEM.45, 477 (1953). 13) Amerongen, G. J. van: Trans. Inst. Rubber Ind. 31, T-70 (1955). 14) Garten. V. A., Sutherland, J. K., Proc. 3rd Rubber Technol. Con!., London
.Trine 1954. (5) Humphrevs. K. C H.. Trans. I I I S ~ Rubber Ind. 31, T-103 11955). ( 6 ) Kuz'minskii, A . S.,Lyubchanskaya, L. I., Khitrova, S . G.: Bass, S. I . ,
Doklady A k a d . .\'auk
S.S.S.R. 82,
131 (1952). ( 7 ) Lyon, F.: Burgess. K . .\.. Sweitzer. C. LV.. IND.ENG. CHEM.46, 596 11954) \ - ' '3'
(81 Powell, E. F.. Gough, S. W.. Rubbei Tl'orld 132, 201 (1955). (9) Sweitzer. C . I%'.? Rubber A p~.e (.V. Y . ) 72, 55'(1932).' ( I O ) Sweitzer. C. W., Lyon. F., IND.E s c . CHEM.44, 125 (1952). (11) Sweitzer, C. \V.> Lyon, F.. Grahowski. T. S . , Ibid.? 47, 2380 (19552.( 1 2 ) \$ratson. LV. F.. Zbid.. 47, 1281 (1932). (13) Winn, H.. Shelton. J. R . , Turnbull. D., Ibid., 38, 1052 (1946).
RECEIVED for review Xovember 4, 1955 ACCEPTED March 3, 1956 Presented at meeting of Division of Rubber Chemistry, ACS, Philadelphia. Pa., November 1955.
