INDUSTRIAL AND ENGINEERING CHEMISTRY
392
LlTERATURE CITED
Bateman, L., and Bolland, J. L., Brit. Rubber Producers’ Research Assoo., Pub. 126. (2) Boeseken, J., and Soesman, A. C., Rec. Irav. chim., 52, 874 (1)
(1933). (3)
Bolland, J. L., and Gee, G., T r a n s . Faraday Soc., 42, 236, 244
(4) (5)
Farmer, E. H., el al., J. Chem. Soe., 1942, 121, 139. George, P., and Walsh, A. D., T r a n s . FaTaday Soc., 42, 94
(1946).
(1946).
(6) Lawrence, J. W., and Shelton, J. R., IND. ENG.CHEM.,42, 136 (1950)* (7) Marvel, C. S., et al., J . A m . Chem. SOC.,72, 2642, 3887 (1950). (8) Shelton, J. R., and Winn, H., unpublished work.
Vol. 45, No. 2
(9) Shreve, 0. D., Heether, M. R., Knight, H. B., and Swern, D., A n a l . Chem., 23, 277, 282 (1951). (10) Siggia, S., “Quantitative Organic Analysis via Functional Groups,” New York, John Wiley & Sons, 1949. (11) Swern, D., Findley, T. W., Billen, G. N., and Scanlan, J. T., A n a l . Chem., 19, 414 (1947). (12) Tobolsky, -4.V., Metz, D. J., and Mesrobian, R. B., J . Am. Chem. SOC.,72, 1942 (1950). (13) Wagner, C. D., Smith, R. H., and Peters, E. D., A n a l . Chem., 19, 976 (1947). (14) Warner, W. C., and Shelbon, J. R., IND.ENG.CHZX.,43, 1160 (1951).
RECEIVED f o r review May 20, 1952.
I
A c c m r m September 20, 1952.
Presented before ehe Division of Rubber CHEMICAL SOCIETY, Cinoinnati, Ohio, 19.52.
Chemistry of
the
. ~ N R I C A X
Effect of on Agin EFFECT OF PARTIAL PRESSURE OF OXYGEN ON RATE OF OXYGEN ABSORPTION J. REID SHELTON
AND
WILLIAM L. COX
Case I n s t i t u t e of Technology, Cleveland 6 , Ohio
HE influence of oxygen pressure in the aging atmosphere upon the aging of rubber compounds has been reported from time to time. It has been demonstrated by numerous workers ( 7 , 11, 19, 14, 17, 18, 20) that a n increase in the oxygen pressure generally increases the rate of deterioration of most stocks. It has also been reported, however, that the rate of oxygen absorption is nearly independent of oxygen pressure. Williams and Neal (R5), for example, reported that the rate of oxidation of samples ground on a mill and extracted with acetone showed no dependence on oxygen pressure above a. certain minimum value. Kinetic studies by Bolland and Gee (ci), Tobolsky (25), and others have shown that the principal initiation reaction in the autocatalytic oxidation of olefins and of rubber involves the &cornposition of peroxides, and that the rate is thus independent of oxygen pressure except at very low oxygen concentrations and in the very early stages of oxidation. However, work reported b y Milligan and Shaw ( I O ) , Carpenter ( 6 ) , and van ilmerongen ( l ) , and in previous publicat,ions from this laboratory (16, 2 6 ) , has demonstrated a dependence of rate of oxidation on oxygen pressure, although the quantitative nat,ure of the dependence was not clearly defined. The present investigation was undertaken to determine the effect of varying the partial pressure of oxygen in the aging atmosphere upon the rate of oxygen absorption of certain natural and synthetic rubber stocks.
The study has been confined t,o the constant-rate stage which precedes the start of t,he autocatalytic reaction, as this is the stage of great,est practical significance. Both the rate of oxidation in this stage, and the duration, appear t o be controlled by the nntioxidant. Only amine-type antioxidants have been included in this study. The effect of different oxygen pressures upon changes in physical properties accompanying the oxygen absorption is discussed in the second part of this paper, PROCEDURE
The volumetric oxygen-absorption technique described i n previous publications from this laboratory (4, 19) was used. Three tread-t>Te stocks Were employed, including both and uninhibited Hevea black stocks and a GR-S black stock. The f?rmulat,ions and cures are given in Table 1. The “uninhibited” stock is actually weakly inhibited, owing to the presence of natural antioxidants in the original ruhber, and t o the use of Santooure as an accelerator. This accelerator contains a n amine, and has been shown to have some antioxidant character (17). The effect of varying the partial pressure of o in combirmtion with gen, was determined by measuring the oxygen absorbed and periodically adding oxygen t o maintain the desired average concentration in the aging atmosphere. The oxygen concentrations were approximately 10, 20, and loo%, 0 8 16 24 32 40 48 56 64 w-ith a maximum variation HOURS of 1 t o 27,. The total presFigure 1. Oxygen Absorption of Inhibited Hevea Black sure was 1 atmosphere (760 Stock mm. of mercury) in each case. A t looo C. and various partial pressures of oxygen Three sample thicknesses Total pressure 760 mm.
INDUSTRIAL AND E N G INEERING CHEMISTRY
February 1953
TABLE I. COMPOUNDING OF STOGKS Hevea Black Inhibited Uninhibited 100 100
Smoked sheet GR-S ACa ... Parafluxb 4 Stearic acid 3 BardolC Zinc oxide '3 Santocured 1 Sulfur 3 Santoflex Be 1.5 EPC black 50 Cure time a t 280' F., min. 40 Contains 1.25% phenyl-%naphthylamine. Q
__ ___ -
-
... 4 3
GR-S Black Inhibited
...
100
4 2.4
4 2.4 1.2 2
'3
1 3
...
,..
45 60
50 40
393
At the end of the constant-rate period, however, the rates increase and the curves diverge, indicating that the rate of the reaction in this autocatalytic stage is partially limited by the rate of diffusion of oxygen into the samples. The experimentally determined rates are compared in Table 11 with the values obtained from the equation, Kz = kPo.6. Comparable data obtained with the same stock a t 90" C. and with a GR-S black stock a t 100' C. are also included. (The oxygenabsorption curves for the GR-S stock are shown in Figure 2 . ) There is good agreement between experimental and calculated values of KZin all three cases.
.
b C. P . Hall Cn .
Barrett Division, Allied Chemical and D e Corp. Reaction product of cyclohexylamine andY mercaptobenzothiazole e Reaction product of acetone and p-aminobiphenyl, mainly 2,2,4-trimethyl-8-phenyl-1 ,2-dihydroquinoline. 0
d
were used (approximately 0.010, 0.020, and 0.040 inch) as a check on possible limitation of the rate of oxidation by the rate of diffusion of oxygen into the sample. Each stock was studied a t three different temperatures.
TABLE 11. RATESOF OXYGENABSORPTION IN CONSTANT-RATE STAGEAT VARIOUS PARTIAL PRESSURES OF OXYGEN Inhibited Hevea black stock a t 90' a n d 100' C . GR-S black stock at looo C. Obsvd. Evaluation Calcd. 0% T%mp Pressure, Rat,e, of Rate, Stock Atm. KZ k = K2/P0*6 Kz = kP0.5 Hevea black 90 1.00 f 0.02 0.131 0.131 0.132 inhibited 0 . 2 1 i 0.02 0 . 0 5 8 0.127 0.060 0.10 f O . O 1 0.044 0.139 0.042
c:'
__
EXPERIMENTAL RESULTS
The oxygen-absorption curves obtained with three thicknesses of the inhibited Hevea black stock a t 100' C. and three different partial pressures of oxygen (1.0 f 0.02,0.21 i0.02, and 0.056 f 0.01 atm.) are shown in Figure 1. An increase in the oxygen concentration in the aging atmosphere results in an increased rate of oxygen absorption-for example, the rate of oxygen absorption in the constant-rate stage, K z , in oxygen, is slightly more than twice that observed with the oxygen-nitrogen ratio corresponding to air. Approximately the same effect was observed in comparing air-oven aging with aging in the oxygen-absorption apparatus. The ratio of hours in air to hours in oxygen to give the same degradation of tensile strength was approximately 2.0 for a stock of the same compounding (16).
GR-Sblack
&"I
100
Av. 1 . 0 0 f O . 0 2 0.292 0.21 1 0 . 0 2 0.128 0.056 =I= 0.01 0.067
0.132 0.292 0.283 0.301
100
Av. 1.00 f 0 . 0 2 0 . 0 9 5 0 . 2 1 1 0 . 0 2 0.042 0.10 f 0.01 0,028
0.292 0,095 0,092 0.089
Av.
0.092
-
0,292 0,134 0.069 0.092 0.042
0,029
I
AVE. ; H I C K N E S S 0.013"
0.023" 0~039"
A
1,OO 1000 ATM.
- 20 0
n
16
IL. Y
5 & 0
5
12
awz
8
o v
0
0
2
1
80
320
240
160
f
400
I 480
HOURS
1
Figure 3.
4
Oxygen Absorption of Uninhibited Hevea Black Stock
At 70' C. and various partial pressures of oxygen T o t a l pressure 760 mm.
0 0
40
80
I20
160
200
240
280
HOURS
Figure 2.
Oxygen Absorption of GK-S Black Stock
At 100° C. and various partial pressures of oxygen Total pressure 760 mm.
A quantitative relationship between the observed rates in the constant-rate stage and the oxygen pressure is given by
Kz = kP0.6
It is evident t h a t this pressure dependence cannot be due to diffusion limitation, as, with one exception, samples of three different thicknesses absorbed oxygen a t the same rate in the constant-rate stage. I n the case of the lowest oxygen concentration, data for the thickest sample deviate somewhat, but the two thinnest samples exhibited the same rate of oxygen absorption.
The "uninhibited" Hevea black stock a t 70' and 80" C. also showed a marked dependence of rate of oxidation on oxygen pressure. (The oxygen-absorption curves obtained a t 70" C. are shown in Figure 3 . ) The simple equation, K Z = kPO.6, did not give good agreement with the observed behavior, but an equation of the form
Kz
=
k(P
+
.)0.6
did give a satisfactory correlation, as shown in Table 111. A similar relationship was observed a t 120"C. with the inhibited Hevea stock and a t 110' and 132' C. with the GR-S stock. Representative oxygen-absorption data are shown in Figure 4 for the Hevea stock at 120' C. and in Figure 5 for the GR-S stock a t 132" C. Only the thinner samples were employed a t these higher temperatures. The simple equation which gave
INDUSTRIAL AND ENGINEERING CHEMISTRY
394
satisfactory correlation of the data for these stocks a t temperatures of 100" C. and below was not satisfactory for these data, but the equation IC2 = k ( P a)OJ was required. The experimental and calculated ratcs are compared on this basis in Table 111.
R
+
RO,.
+
Vol. 45. No. 2 1%2
0 2
---+
RO,
+ RH ---+ROO13 + &(or k8
R.')
(R.'could also be formed by addition of R.t o the double bond with the formation of a new radical, but such a step would not T A B L E 111. RATES O F O X Y G E N i i B S O R P T I O S IN COSSTANT-RATE change the concentration of free radicals, and would be indisSTAGE AT TT-4RIOUS PARTIAL PRESSURES OF O X Y G E S tinguishable in the experimentally observed rate of oxidation.) The possible termination steps include combinations of all Tininhibited Hevea black stock at 70' and SOo C. Inhibited Hevea black stock a t 120° C. types of radicals present to form stable products, but a t apprcciGI
