A. S. CARPENTER1
D unlop R ubber C om pany, L td., Erdington, B irm ingham , England
T he rate o f the ru bber-oxygcn reaction bas been m eas
ured at con sta n t pressure. In th e absence o f antioxidant the reaction is autocataly.tic and in accord an ce w ith a free radical ch ain m ech an ism . In the presence o f antioxidant,
proximate relation is deduced betw een oxidizability, specimen size, and u n iform ity o f oxidation for rubbers containing antioxidant.
S
EVERAL workers have investigated the reaction between rubber and oxygen, in the important initial stages over which the rubber retains its rubbery qualities, by following the rate of disappearance of gaseous oxygen in a closed system. Particular attention has been directed toward establishing the effect of oxygen pressure upon the rate of the reaction. This is fundamentally important. Also, in the surface of the rubber the oxygen concen
tration consistent with the pressure of oxygen in the gas phase is maintained by dissolution, whereas in the interior, oxygen can be supplied only by the diffusion of already-dissolved oxygen. At any point in the interior the concentration which can be built up is determined by the rate at which oxygen diffuses in from higher concentration regions and out to lower concentration regions and by the rate at which it is used up by combination. In the in
terior, therefore, the oxygen concentration, and hence the reaction rate, corresponds to a gas pressure different from that of the surrounding free gaseous oxygen.
One method which was used, notably by Williams and Neal (14) and Morgan and Naunton (IS), for the fundamental investi
gation of the rubber-oxygen reaction has the advantages of simplicity and ease of operation. It consists in confining the rubber specimen in an oxygen-filled vessel connected to a vertical tube dipping into mercury. Oxygen pressure falls spontaneously as oxygen combines, and mercury ascends the vertical tube. At any particular stage in the experiment the position of the mer
cury meniscus in the tube gives the oxygen pressure, and its rate of movement gives the rate of combination of oxygen with the rubber. Williams and Neal, using finely subdivided acetone- extracted rubber specimens, conclude from their experiments that rate of oxygen combination is independent of oxygen pressure over most of the absorption. Morgan and Naunton, using similar specimens and continuing the work of Williams and Neal, chiefly with regard to the effect of temperature, put forward a chain re
action theory for the mechanism of the reaction, on the basis of the same conclusion.
The work of the investigators mentioned, however, is at variance with that of others using different experimental tech
niques ( /, 8,11,12) and,furthermore, their experimental method is not free from criticism. For example, Kohman (11), working at constant oxygen pressure over the whole range of oxidation up to résinification, showed that the reaction is autocatalytic;
conse-1 Present address, C ourtaulds, L td., Foleshill R oad, C ov en try , E ngland.
quently time effects other than those arising from the diffusion process may be operative and may vitiate the results of mano
metric experiments. Also, investigators in this field are generally agreed that the higher oxygen pressures favor the more rapid oxidation (usually judged by the decay of tensile properties), as shown by the common practice of assessing the oxidation re
sistance of technical products by accelerated aging in the Bierer- Davis pressure bomb. Furthermore, although a quantity of oxygen sufficient to cause a considerable modification of the physical properties of the rubber combines during an experiment (10,11) direct comparisons are made between the initial and final stages without evidence that the changed degree of oxidation has no effect upon oxidation rate.
The investigation outlined in the present account was com
menced with the following aims in view: (a) to try out a modi
fied manometric apparatus which was believed would have ad
vantages over the simple Williams and Neal type; (b) using this apparatus to investigate the value of the manometric method as a tool for a fundamental investigation (?) and as a means of de
termining and comparing the resistance to oxidation of specimens of technical rubbers; and (c) to investigate the physical chemistry of the reaction between rubber and oxygen. A preliminary note on some of the results of this investigation has been published previously (4).
M O D IF IE D M A N O M E T R IC APPARATUS
The apparatus differed considerably in detail from the simple Williams and Neal apparatus but was identical in principle. It was so designed that absorptions could be followed over pressure changes of about 15 cm. of mercury, starting from any desired initial pressure less than about 3 atmospheres. It was independ
ent of the pressure of the atmosphere and its fluctuations. The apparatus w'as described previously (S) in connection with ex
periments to determine the solubilities and diffusion coefficients of gases in rubbers. By the use of this apparatus the rate of oxygen absorption at any pressure within the chosen range may be cal
culated from readily ascertainable data.
E X P E R IM E N T S W IT H M A N O M E T R IC APPARATUS
The state of subdivision necessary for substantially uniform oxygen concentration and, hence, oxidation throughout a rubber specimen is dependent upon the susceptibility of the latter to oxidation, the more readily oxidizable rubber requiring the finer subdivision. It was considered possible that, with quite large specimens of more slowly oxidizing rubbers, oxidation might be substantially uniform and absorption rates little affected by dif
fusion.- An experiment was carried out with this possibility in mind. The specimens were made from the following mixture, in parts by weight:
The mixture was vulcanized by heating for 30 minutes at 148 ° C.
in closed molds. The specimens were in the form of cylindrical rods, about 5-mm. in diameter and 15 cm. long (specimens A and B), square sectioned rods about 1 X 1 mm. in cross section and not less than 5 cm. long (specimen C), and cubes of about
1.5-188 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 39, No. 2
nun. side (specimen D). The surface areas of the specimens were, approximately, 25 cm.2 (A and B), 75 cm.2 (C), and 120 cm.2 (D).
The experiment was carried out in darkness at 45° C., starting at a pressure of about 80 cm. of mercury. The period allowed for solution equilibrium was 24 hours. (A calculation based on the known solubility and diffusion coefficient of oxygon in the rubber showed that, if no chemical combination had occurred between the oxygen and the rubber, the rate of solution in the cylindrical specimens, at a pressure of 1 atmosphere, would have been about 4.2 X 10“ 8 gram oxygen per gram rubber per day after 12 hours, and about 0.9 X 10“ 12 gram oxygen per gram rubber per day after 24 hours.) In each case the mercury ascended the capillary at a constant rate over the 15-em. range. The absorption rates, calculated for the conditions when the pressure of oxygen in con
tact with the specimen was 1 atmosphere, were as follows:
Specim en A B C D
A bsorption rate (g. O i/g . ru b b e r/d a y ) X 1 0 -» 4 .9 2 5 .3 7 5 .0 7 4 .5 1
The experiment was repeated using fresh specimens, one of the cylindrical specimens being given an 18-hour aging pretreatment in the Geer oven. Again in all cases the mercury ascended the capillaries at a constant rate. The absorption rates, calculated as before for an oxygen pressure of 1 atmosphere, were as follows:
S pecim en A B C D
A bsorption rate (g. 02/ g . ru b b e r/d a y ) X 1 0 -» 5 .4 0 5 .2 7 9 .5 4 6 .0 6
The specimen subjected to Geer oven aging was B.
Although the agreement between the results of these experi
ments is poor, the results show that there was no consistent effect of subdivision. Thus with this slowly oxidizing rubber it appears that oxidation under these conditions is substantially uniform throughout all of the specimens. Several other investigators {10, 11, IS) showed that, with slowly oxidizing rubbers, it is not necessary to go to an extreme state of subdivision— for example, crumb— in order to eliminate diffusion as a rate-determining process. Calculations confirm that diffusion effects had negli
gible influence on the results.
In the apparatus of Williams and Neal the change in internal free volume of the specimen tube during the spontaneous pressure change was negligible. Their conclusion that the rate of oxygen combination is independent of pressure depended upon this ex
perimental arrangement. The design of the present modified ap
paratus was such that the internal free volume change was an ap
preciable fraction of the total. In these experiments, therefore, the rate of oxygen combination was lower, the lower the oxygen pressure.
An experiment was carried out to determine the effect of oxy
gen pressure upon absorption rate over a wide pressure range, us
ing five identical cylindrical specimens of 5-mm. diameter. The specimens were made from the mixture described and were kept for 3 days under vacuum in darkness before test. In this experi
ment the pressure of the oxygen in contact with the specimens was reduced alternately in one of two ways: (a) It was allowed to fall spontaneously over a small pressure range as absorption proceeded or (5) it was artificially reduced at intervals by with
drawing oxygen from the apparatus. In this way, starting from a pressure of about 180 cm. of mercury, absorption rates were followed over spontaneous pressure decrease steps of about 10
cm. down to about 12 cm. of mercury. When the pressure was changed by withdrawing oxygen, 24 hours were allowed for solu
tion equilibrium. The experiment was carried out in darkness at 45° C. The results are given in Table I. The specimens were kept for 3 days under vacuum in darkness, and a repeat experi
ment was carried out on them. The results were in general agree
ment with those of the first experiment. of the spontaneous pressure changes.
It is seen that absorption rate is markedly dependent upon oxygen pressure. This is in agreement with the findings of Ing- manson and Kemp (5); by following decay of physical proper
ties, they showed that, for oxygen pressures less than about 4 atmospheres, absorption rate increases with oxygen pressure.
The results, however, are unsatisfactory, because a graph shows that, in any one case, the short curves corresponding with the spontaneous pressure decreases do not lie on one continuous curve. Very roughly, absorption rate is proportional to the square root of the oxygen pressure, a result in agreement with the findings of van Amerongen (1) and of Milligan and Shaw { I S ) . These ex
periments show that results with the manometric apparatus are not independent of the arbitrary conditions of the experiment.
The progressive change in the condition of the rubber at each step, due to oxidation during the preceding steps, does not af
fect the absorption rate at that particular step, because the repeat experiment on the same specimens gave similar results. The re
producibility of results with the same specimen, however, does not exclude the possibility that the initial stages of oxidation in a particular step might affect the later stages of that step.
In all the experiments with this modified manometric appara
tus the agreement was poor among results of experiments differ
ing only in insignificant detail. This was considered surprising in view of the known reliability of the apparatus used in a slightly different connection (S), and in view of the fact that the result of each individual determination could be quoted with considerable accuracy— that is, having regard only to the accuracy with which the relevant experimental data could be ascertained.
From the nature of the results of these experiments it appears that they are affected by an unappreciated factor inherent in the method. Moreover, it is fundamentally unsound to allow both of the interdependent variables, oxygen pressure and oxygen com
bination rate, to change without control. All further work was carried out at constant oxygen pressure.
These criticisms of the manometric method do not necessarily apply to the work of Dufraisse (6), who used it under carefully standardized conditions as a routine test for the estimation of the oxidizability of various rubbers.
C O N S T A N T P R E S S U R E A P P A R A T U S
Milligan and Shaw {IS ) and van Amerongen {1) described ap
paratus for measuring the oxygen absorption of rubbers at more or less constant total gas pressure by methods involving the man
ual adjustment of the pressure from time to time. These methods were unsatisfactory for the present purpose. Kohman {11) de
scribed apparatus in which the pressure was automatically' kept constant. It was, however, designed to measure the total absorp
tion of oxygen over the complete range up to resinification, and was not considered capable of sufficient refinement to allow accu
rate measurement of very small oxygen absorption rates.
Figure 1 shows the apparatus designed and used for measuring oxygen absorption rates at constant pressure. It consisted of bulb B, of about 700-cc. capacity, with an extension, D, of 2-cm.
February 1947 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 189 diameter and 6-cm. length connected to a horizontal capillary,
C, 14 cm. long and uniform in bore (0.0104 cc. per cm.), marked off in centimeters by etched lines. The end of the capillary far
thest from bulb extension D was connected directly to specimen bulb A (which was sealed on afresh for each new specimen) and continued through tap T2 to the top of bulb B. The system con
tained a side tube with a tap, Ti. Bulb extension D served as a reservoir for a light paraffin oil freed from readily volatile constit
uents. The quantity of oil used was such that, when the capil
lary was horizontal, it rose into the capillary and the equilibrium position of its meniscus was at the end near the reservoir. Be
cause of the great cross-sectional area of the reservoir compared with that of the capillary, movement of the oil meniscus along the capillary caused only a very small change in oil level in the reser
voir. The oil meniscus in the capillary showed little tendency to move toward the reservoir when it was placed at the end of the capillary farthest from the reservoir and was free to move. Ad
justment of the oil meniscus to any desired position in the capil
lary could be accomplished by tilting the apparatus with tap T2 open.
For normal use the apparatus was connected by means of the side tube and tap T\ to a mercury manometer, a vacuum pump, and an oxygen cylinder. During an experiment the whole of the apparatus shown in the diagram was contained in a thermostat of temperature constant to within 0.02° C. The oxygen absorption rate of a rubber specimen was deduced from the movement of the oil meniscus in the capillary when, with the apparatus filled with oxygen at the desired pressure, gaseous connection between the specimen bulb and the large bulb was severed by turning off tap T2. The disappearance of oxygen from the gaseous phase in the specimen bulb caused the oxygen in the large bulb to force the oil along the capillary. The volume swept out by the oil men
iscus was a measure of the volume of oxygen absorbed.
Calculation shows that using the following equation as an ap
proximation :
where Vi = internal free volume on large bulb side of apparatus from oil surface in reservoir to tap T2
internal free volume of specimen bulb and connec
tions from oil meniscus in capillary to tap T2 cross-sectional area of capillary
volume of oxygen (measured at pressure of experi
ment) absorbed by specimen
movement of oil meniscus along capillary resulting from absorption surrounding the specimen, as the oil meniscus moves 10 cm. along the capillary, is about 0.02% of the original pressure. This was considered to be negligible.
The use of oil instead of mercury as the indicating liquid in the apparatus needs justification. Its main advantages and the reasons for which it was chosen are twofold: (a) The oil shows no tendency to stick in capillaries; (6) gas pressure errors arising from incorrect leveling of the capillary and from the change in liquid level in the reservoir due to movement of the meniscus in the capillary are minimized because of the low specific gravity of the oil. The disadvantages are its volatility at low pressures and its solvent power for gases. The former was overcome by never using pressures lower than 0.2 cm. of mercury. When the complete removal of oxygen from the apparatus was necessary, it was displaced by nitrogen.
The apparatus was tested in several ways to ensure that the rate of movement of the oil meniscus in the capillary was an ac
curate measure of the rate of disappearance of gas through the specimen tube side arm. All of the tests were satisfactory and showed that the apparatus was capable of giving trustworthy re
sults. The only correction necessary was that arising from the dis
solution of oxygen in the oil of the reservoir; the correction was
Oxygen Pressure
necessary only in determinations with rubbers giving very slow meniscus movement rates (of the order of 10 cm. per day) during the first few hours after the pressure change. In the calculation of the results the appropriate correction, determined by experiment, was applied where necessary. The method of using the apparatus for the determination of oxygen absorption rate of rubber was as follows:
The rubber specimen was sealed into a glass bulb with as little free space left as was conveniently possible. This specimen bulb was then sealed onto the apparatus, and, when required, an opaque covering was placed around it. The completed apparatus with tap Ti open was placed in the thermostat and, together with all connecting tubes, alternately evacuated and filled with oxygen until all gases other than oxygen were removed. The apparatus was then filled with oxygen at the desired pressure. Oxygen be
gan to dissolve in the rubber and was used up by oxidation. After a time interval determined by experiment (Table II), absorption was entirely due to chemical combination of oxygen. Tap Tt was turned off, and the rate of disappearance of oxygen from the gas phase in the specimen bulb was measured by the rate at which the oil moved along the horizontal capillary. When the oil reached the specimen tube end of the capillary, tap T2 was opened and the meniscus returned to the reservoir end ready for a subsequent determination. Similar determinations at other pres
sures could be made either by the evacuation procedure or by simply introducing or withdrawing oxygen.
The work carried out with the constant pressure apparatus and recorded here is in the nature of a preliminary investigation searching out the field for future, more rigid, treatment. Never
theless, a broad interpretation confirms the essential invalidity of deductions based on work carried out with the manometric type of apparatus, indicates the reasons, gives the required background for a general investigation of the oxygen-rubber reaction, and gives a new basis for the theoretical treatment of the problem.
R A P ID L Y O X ID IZ IN G V U L C A N IZE D RUBBER
In order to check the reproducibility of results on a rubber speci
men, an experiment was carried out with a rapidly oxidizing rubber prepared by heating the following mixture for 40 minutes at 145° C. in a closed mold (in parts by weight):
The 8-mm.-thick slabs of the vulcanized material were kept at laboratory temperature in the dark and, when required for testing were reduced to a crumb of particle diameter about 0.3 mm. by pas
sage through a cold friction mill at a tight nip. The temperature