Oxidation of GR-S Vulcanizates J. REID SHELTON AND HUGH WINN Case School of Applied Science, Cleveland, Ohio
The effect of surface area, cure, and temperature on the rate of oxygen absorption by a tread type GR-S vulcanizate has been measured by a volumetric method at substantially constant pressure, and changes in physical properties have been correlated with oxygen absorption. Under the conditions employed, 'chemical reaction and not diffusion is the rate controlling factor. Three stages are involved in the oxidation: an initial rapid reaction of apparent first order, involving a limited number of ccactive" centers, which is completed within a few hours at
100" C.; a slower constant-rate reactibn of apparent zero order, which accounts for most of the degradation of properties normally encountered in service; and an autocatalytic reaction starting after the absorption of approximately 5% of oxygen based on GR-S in the vulcanisate, which rapidly converts the sample into a hard brittle condition. The use of oxygen absorption methods offers certain advantages over the usual physical property methods for evaluating the effect of compounding ingredients upon aging resistance.
G
sults are more readily interpreted than with the manometria method which involves a simultaneous change in volume and pressure. Dufraisse (1) preferred the manornetnc method for routine comparisons of oxidizability in a practical study of rubber; however, he also pointed out t h a t the volumetric method is more convenient for studying the kinetics of the oxidation reaction, and t h a t the data are more reliable. The manometric method does offer certain advantages of simplicity and ease of operation, but the volumetric method is best suited for studies of the type reported in this paper.
R-S VULCANIZATES are, in general, quite resistant t o aging at ordinary temperatures, but a t elevated tempera-
tures they undergo rapid changes resulting in increased modulus, decreased elongation, and poor flex-cracking resistance (8, 6-9). This general hardening a t temperatures comparable to those encountered in service seriously impairs the usefulness of GR-S for certain purposes-for example, in heavy duty tires. It is now generally accepted t h a t the changes which occur in natural rubber during aging are primarily the result of oxygen attack, although the question was debated for many years and was the subject of numerous papers, which have been reviewed elsewhere (1). It OXYGEN ABSORPTION MEASUREMENTS was t o be expected, therefore, t h a t the aging of GR-S likewise The equipment is similar t o that used by Kohman (4) in would involve reaction with oxygen. experiments with rubber. The assembled apparatus (Figure 1) The authors (8) previously demonstrated t h a t more than half consists of eight independent absorption units, each with its of the hardening of GR-S vulcanizates (as measured by the inown sample tube, gas buret, and leveling bulb. Figure 2 is a Dicture of the s a m d e tube. crease in modulus obtained showing the m e r c u 6 seal foi after 5 days in air at 100' C.) the large ground-glass joint is caused by oxygen, since a and the hooks for suspending much smaller increase was the samples a t a point well below the level of the liquid in observed i n a nitrogen atthe constant-t e m p e r a t u r e mosphere. Oxygen was also bath. Capillary tubing conshown t o be one of the factors nects the sample tube t o its involved in the flex cracking w a t e r - jac k e t e d gas buret through a three-way stopcock of GR-S (9). The present inwhich also joins it to a manivestigation was undertaken fold for evacuation and int o study the mechanism of troduction of oxygen. the oxidation of GR-S and the For the study of GR-S vulcanizates as reported here, factors which influence the the following procedure was reaction. It was felt that the employed: The assembled abbest approach t o the probsorption bulbs (containing the lem walj to determine quantiappropriate weight of sample suspended from the hooks and tatively the rate at which a small lump of calcium oxide oxygen combined with the t o absorb water vapor and vulcanizate. carbon dioxide) are placed in The methods employed in the constant- t e m p e r a t u r e bath and connected to the gas analogous studies with burets. All tubes are then natural rubber fall into three evacuated si m u l t a n e o u s l y general types-gravimetric, through the manifold to remanometric, and volumetric. move the air from the system and as much as possible of the The latter method, which dissolved and adsorbed gases follows oxygen absorption by from the samples. Oxygen is volume change at constant then admitted, and the altertemperature and pressure, nate evacuation and admiswas selected for this study besion of oxygen repeated three times. The three-way stopcause i t is apparently more cocks at the top of each buret sensitive t o small amounts of are then turned so as to disabsorption than the graviconnect the manifold while almetric method, and the relowing the absorption tube Figure 1. Absorption Apparatus 11
~
72
I N D U S T R I A LA N D E N G I N E E R I N G C H E M I S T R Y
and gas buret of each independent unit to remain connected. The leveling bulbs are set to give the proper differential of mercury so t h i t the total pressure in each tube is 760mm. The initial volume is read and recorded, t'ogether with t,he barometric pressure and jacket temperatures. Periodically the leveling bulbs are adjust'ed t o maintain the desired pressure. This need not be exact, for experience has shown that small variations of pressure have only a negligible effect upon the rate of absorption. After appropriate time intervals, the pressure is adjusted to 760 mm. by setting the leveling bulb a t t'he proper differential before the next set of readings is taken. -4llvolume changes are calculated t o a reference temperature of 25' C. and are reported as milliliters of oxygen absorbed per gram of G1E-S in the sample. For this purpose the observed volume of oxygen in each buret as measured a t water jacket temperature is corrected t o 25" C. The rest of the system remains a t constant volume and temperature except, for the upper portion of the absorption tubes and the capillary connecting tubes which are exposed to the temperature of the room. Small variations due to such factors are adjusted by the use of control tubcs. Any change in t,he corrected volumes of the control tubes may then be applied directly to the corrected volumes of each sample tube, and the difference between the resulting volume for two successive readings represents the volume absorbed. All samples are run in duplicate t,ubes and the results averaged. With the apparatus described, three different samples can be studied simultaneously with the remaining two units reserved for controls. 'ITlit,hthis procedure, satisfactory reproducibility is obtained (Figure 3), These data represent two completely separate runs, not only with respect to t,he oxygen absorption measurements but also separate compoundFigure 2. Absorption inn and vulcanization. Tube The tread t y p e stock used had the following compositioll (in 25.0 parts by weight) :
Vol. 38, No. 1
strength, stress a t 20070 and 3007, elongat'ion, and ultimate elongation as a function of the amount of oxygen absorbed. The general shape of the curves is similar to that, usually obtained by plotting changes in physical properties against time of aging. After the initial rapid change, the properties vary proportionally with the amount of oxygen absorbed. The plot ends a t a value corresponding to an absorption of approximately 3.7 yooxygen by weight based on the weight of GR-S in the sample. The init,iai rapid change in both the stress a t a given elongation and ultimate elongation would seem to indicate eit'lier that a small amount of oxygen in the early stages results in structural changes which greatly affect the properties or that factors other than oxygen are also involved. The authors showed previously J6)that approximately 45y0 of the increase in stress a t 2007, elongation obtained by aging 2 days in a n air oven a t 100" C . can definitely be attributed to the action of oxygen, since only 55y0 of the observed change in air took place in a nitrogen at'mosphere. If we assume that the modulus increases regularly from t'ne beginning with the amount of oxygen absorbed, then t h a t portion of the observed increase which is due to other factors is represented by the intercept of the extrapolated linear portion of the curve in Figure 4. On this basis, roughly 60% of the observed increase in the 2007, modulus after 2 days may be regarded as due to oxygen. Considering the change from air to oxygen and the experimental variation in tensile, stress, and elongation measurements, this value is not inconsistent with the previously published work. It seems probable, therefore, that the degradation of physical properties actually rcsulting from oxidation is a t least roughly proportional to the amount of oxygen absorbed by the vulcanizate. During the first few days of aging, however, other changes (such as aftervulcanization) also occur and, thus produce a greater total change in both modulus and ultimate elongation, and a smaller change in tensile strength than would otherwise be obtained. The close correlation between oxygcn absorption data and aging data based on changes in physical properties m a l m possible the use of oxygen absorption as a measure of the relative aging propert,ics of various stocks.
-
GR-S
Bardol F a t acid Channel black
100.0 5.0 1.5 50.0
Zincoxide Santocure Sulfur
5.0 1.2 2.0
d 20.0
0%
3000
. I
All samples were vulcanized 50 minutes a t 298" F. (50 pounds steam) unless otherwise designated. EFFECT ON PHYSICAL PROPERTIES
If oxygen absorption data are to be interpreted in terms of aging resistance, i t is first necessary to correlate changes in physical properties with oxygen absorption. For this purpose rectangular specimens, 1 X 4 X (approximately) 0.026 inch were aged i n the absorption apparatus a t 100" C. in oxygen a t 760 mm. pressure. Samples (in triplicate) were removed after 1, 2, 4, and 8 days, and tensile strips were cut from each for testing. Figure 4 is a plot of tensile
29
z
15.0
2400
5..
m
B a
N
O
i 2
10.0
m
1800
_1
m 5.0
g m +
1200 D A Y S AGED
0 0
50
100
150
HOURS
Figure 3. Reproducibility of Measurements (100' C., 760 Rlm., Cured 50 Rlinutes a t 298" F.)
600 0
8
16
24
32
ML. 0 2 / GM. G R - S , 2 5 O C .
Figure 4. Effect of Oxygen Absorption on Physical Properties (100" C., 760 Mm.)
January, 1946
INDUSTRIAL A N D ENGINEERING CHEMISTRY
13
EFFECT OF SURFACE AREA
The rate of oxygen absorption a t constant temperature and pressure could be limited by either of two factors: the rate of reaction of oxygen with the vulcanizate or the rate of diffusion of oxygen into the sample. If diffusion is a factoy, the rate of oxygen absorption should increase, as the area-toweight ratio of the sample is increased, until a point is reached where the diffusion of oxygen into the sample is rapid enough t o maintain the chemical reaction a t its maximum rate. Since each of,these factors would be influenced differently by temperature, the effect of varying surface area for a given sample weight was investigated at 80", go", loo', llOo, and 120' C. Figure 6. Effect of Surface Area on Oxygen Figure 5. Effect of Surface Area o n Oxygen I n each instance samples Absorption at 110' and 120' C. (760 Mm., Absorption at 80°, 90°, and 100' C. (760 Mm., were cut from vulcanized, Cured 50 Minutes at 298' F.) Cured 50 Minutes at 298' F.) sheets. of three different t h i c k n e s s e s averaging 0.078, 0.039, and 0.026 inch, respectively. The range of surface limitation of oxygen absorption by rate of diffusion, i t is therefore areas thus obtained was approximately 9, 18, and 27 sq. cm. per necessary t o employ thin samples (less than 0.040 inch) for gram, respectively, so that the thin samples had three times as studies at temperatures above 100' C., but at or below this temmuch surface for a given sample weight as those cut from ordinary perature samples cut from tensile sheets of standard thickness tensile sheets. may be used as a matter of convenience. The absorption data are plotted in Figure 5 for studies at 80°, EFFECT OF CURE go", and 100' C. All the' curves show the same general type of absorption-initial rapid rise followed by a slower linear rate. It is well known that the initial state of cure has a pronounced At 80' the data for all three sample thicknesses follow the same effect upon aging as measured by changes in physical properties. curve. At 90" and 100" C. the samples with lower ratio of surSince it has also been shown (6) that oxygen plays a major part in face area t o weight absorbed more oxygen during the initial stage aging, it seemed desirable to determine the effect of cure upon the than either of those with greater surface areas. After the first rate of oxygen absorption. few hours, however, all samples absorbed oxygen a t the same The samples for this study were cut from tensile sheets of standlinear rate at a given temperature. Subsequent work has shown ard thickness, vulcanized for 20, 40, and 80 minutes a t 298" F. that diffusion does limit the rate of absorption during the initial The physical properties of these vulcanizates showed t h a t 20 minrapid rise a t both 90" and 100' and, conseqfiently, the samples of utes gave a n undercure while 80 minutes produced a n overcure. greater surface area had absorbed more oxygen before the initial Figure 7 gives the oxygen absorption data obtained at 100' C. reading was taken. I n these early studies the system was alThe effect of cure was unexpectedly small as compared to the lowed to stand for about a n hour to attain temperature equilibmarked effect observed by Kohman (4) with natural rubber, i n rium before the first reading was taken. I n subsequent work the which case the rate of oxygen absorption increased with time of initial reading has been recorded immediately, and any error due cure. With GR-S, on the other hand, the effect is not only small, t o lack of temperature equilibrium is corrected by the correspondbut the overcure absorbed oxygen at a slightly lower rate than the 40-minute cure. This is in line with the greater resistance of ingly small changes observed with the control tubes. For example, the data for the 100' C. curve in Figure 8 was obtained GR-S overcures t o oven aging as observed by Harrison and Cole in this manner using thin samples, and the observed absorption (9). It is of interest t o note (as further evidence t h a t the unusual corresponded t o the curve for the thick samples in Figure 5 . It order shown i n Figure 7 is nevertheless real) t h a t the curve obis clear that, over the range of sample thicknesses studied, diffutained for a 50-minute cure of comparable surface area (Figure 5 ) sion is not a n important factor a t or below 100' C. will lie between the curves for the 40- and 80-minute cures, if The absorption curves obtained a t 110' and 120' C. (Figure 6) plotted on the same graph. appear at first glance to be somewhat different in type from those Since time of cure produces so little effect upon the rate of oxydescribed above. These differences are discussed later under the gen absorption, this apparatus and technique may well prove t o effect of temperature. be superior t o the more conventional methods for evaluating antiDiffusion appears t o limit .the absorption rate of the thicker oxidants and the effect of various other compounding ingredients samples during the later stage of oxidation. There is little difupon aging. Such studies usually involve the measurement of ference, however, between the medium and the thin samples u p changes in physical properties, and the effect of variations in comt o a n absorption of about 100 ml. per gram of GR-8. T o avoid pounding upon cure frequently obscures the effect upon aging
INDUSTRIAL AND ENGINEERING CHEMISTRY
14 4
0
1
~
-
0 20
MINUTES
7
Vol. 38, No. 3.
inates in natural rubber, while the latter predominates i n GR-S, and the over-all rate of oxidation of natural rubber is much the faster. THEORETICAL CONSIDERATIONS
I
1
50
IO0
150
200
250
HOURS
Figure 7. EiTect of Cure on Absorption a t 100' C. and 760 Mm. (Platen Temperature, 298' F.)
resistance. If oxygen absorption methods were used, however, such difficulties would be avoided since cure has so little effect upon rate of oxygen absorption by GR-S. EFFECT O F TEMPERATURE
The effect of temperature upon the rate of oxygen absorption by GR-S is demonstrated in Figure 8. The curves were plotted from the data obtained with the thin samples so that diffusion is not a factor in this case. The curve for 100' C. is typical for a vulcanized GR-S tread stock since it includes all the special features of the curves obtained a t both lower and higher temperatures. This curve indicates that three distinct stages are involved: an initial rapid rise, a slower constant rate period, and a final upswing which approaches a linear rate of greater magnitude. The initial rapid rise observed a t 80', go", and 100" C. is not detectable a t the two higher temperatures, but apart from this initial rise all the curves show a linear absorption rate up to the point a t which the upswing occurs. This upswing in the curves for looo, 110', and 120' C. occurred after each had absorbed approximately 40 ml. of oxygen per gram, or roughly 5% by weight, based on the weight of polymer in the compounded stock. The absorption a t 80" and 90" C. would, in all probability, show a similar upswing if the measurements were continued for a sufficient time. The fact that the break occurs after a constant amount of oxygen has been absorbed is consistent with either the building up of a sufficient concentration of peroxides t o start an autocatalytic reaction or with the depletion of the antioxidant a t that point. The general form of these curves is similar to that observed by Kohman (4) with natural rubber except that his curves do not show a n initial rapid rise, and the linear rate is much more rapid than with GR-S. Kohman's experiments were carried to a more advanced stage of oxidation in which the rate decreased as the oxidizable rubber concentration diminished. A similar behavior could probably be demonstrated with GR-S by continuing the measurements for a longer period. A suggestion of such a decrease in rate was, in fact, indicated by the last point (not shown) i n the case of the 120" C. curve of Figure 8. However, the material was so hard and brittle as to be completely worthless a t the termination of the runs reported here, and it was therefore decided t o confine the present study to the earlier stages of oxidation. The difference between the oxidative behavior of GR-S and natural rubber is marked; however, the authors have come to the conclusion t h a t the difference is primarily a matter of degr'ee and relative rates rather than a fundamental difference in the nature of the reactions involved. Reactions resulting in chain scission (as shown by tensile breakdown) and in hardening (as shown by increased modulus) occur in both cases; but the former predom-
The preceding section showed that there are three distinct stages in the oxygen absorption curve for a typical GR-S tread stock. The physical properties of the stock are degraded, as oxidation proceeds, t o such an extent as to be of little practical value even before the final upswing is reached. For this reason the following discussion is limited t o the first two stages. It should be pointed out, however, t h a t the third stage is apparently an autocatalytic reaction similar to t h a t observed with natural rubber. The rapid absorption observed in the initial stage could be caused by either chemical reaction or physical absorption resulting from increased solubility of oxygen In GR-S when the sample is transferred from air to an oxygen atmosphere. T o test the latter theory, samples were placed in oxygen under a gage pressure of 300 pounds per square inch and left overnight a t room temperature. The samples were thus supersaturated with oxygen when removed from the bomb and placed in the absorber a t 100" C. If physical absorption were the true explanation, no initial rapid rise would be expected under these conditions, but rather a lower initial rate followed by the constant rate characteristic of the second phase of the reaction. Figure 9 shows the actual behavior observed for these samples as compared with control samples handled in the usual fashion. After the first hour, during which the excess oxygen escaped, the curves are almost identical in form. The initial rapid absorption of oxygen must, therefore, represent a chemical reaction. Since this initial rate soon levels off as i t approaches the slower constant-rate reaction, i t would appear that the particular reaction responsible for the initial rapid absorption is completed within the first f e a hours at 100' OF days a t 80" C. For the purpose of developing a theoretical equation for the absorption of oxygen by GR-S, we shall assume the existence of
HOURS
Figure 8. Oxygen Absorption at Various Temperatures (760 Mm., Cured SO Minutes at 298' F.)
&
15
INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1946
a limited number of particularly reactive centers which are attacked rapidly by oxygen and thus are responsible for the initial rapid absorption. Such active centers might result from certain epecial structural relations between double bonds and phenyl groups due to: (a) the uneven distribution of styrene units, (b) the polymerization of butadiene in the 1,2 as well as the 1,4position, and (c) the probable existence of both cis and trans isomers (3). Whatever their nature, however, once these especially reactive centers have been oxidized, the absorption rate becomes linear as oxidation about the normally situated double bonds proceeds at a steady rate. The above theory is only one possible explanation for the presence of "active" EM well as normally reactive centers in the vulcanizate. Other plausible mechanisms include the activation of certain preferentially absorbed groupings on the surface of the carbon black, the presence of sulfur in certain readily oxidizable forms, and the combination of oxygen with the antioxidant. The following theoretical analysis is based only upon the existence of both particularly active and normally reactive centers and is valid regardless of the reason for the greater than normal reactivity. If the normal constant-rate reaction proceeds from the start, then the total volume of oxygen absorbed a t any timg, 1, will be the sum of that required for reaction with the normal and the active structures. I n Figure 10 the extrapolation of the straightline portion of the curve to zero time, therefore, gives the volume of oxygen required to react completely with all active centers. If we assume :
V = volume of 0 2 (25"C. and 760 mm.) absorbed a t time, t VI = volume of 02 absorbed by reaction with active centers vn = volume of O2absorbed by reaction with normal structures V , = total volume of Oz required to react completely with all
HOURS
Figure 9. Effect of Prior Saturation on Oxygen Absorption (100' C., 760 Mm., Cured 50 Minutes at 298' F.)
vation was based and establishes the apparent order of the reactions involved in the first two stages. Under the conditions of the test (constant oxygen pressure) the initial reaction with the active centers behaves as a first-order reaction. The observed linear rate for the second stage justifies the assumption that the change in concentration of the remaining oxidizable groups is too small to affect the rate over the range involved. Thus, a t constant oxygen concentration i t takes the form of a zero-order reaction. The following tabulation summarizes the constants evaluated from the data obtained a t the five temperatures:
active centers
+
then V = VI Vz V , VI = effective concentration of active groups remaining a t any time, t d V i / d t = Ki(Vo Vi) (1) dVz/dt = Kz (2)
-
-
where K 1and K z are the rate constants for the active and normal reactions, respectively. By integration we obtain:
V1 = V , (1 - e-Kit) Va = Kat V = V , (1 - e--K11) Kzt
+
(3) (4) (5)
Temp., ' C.
80
eo
100
110 128
Yo a. 18 3.72 a. 20
.. ..
K1
@.a48 O.lb7 0.618
... ...
Rz 0.018 0.066 0.144
0.382 8.817
Theoretically V., is a constant for a given stock; however, the experimental value may vary somewhat, depending on the amount of oxidation which the active centers have undergone at the time of the initial reading. It should be independent of temperature except as the increased rate of reaction a t the higher temperatures makes i t increasingly difficult t o obtain an initial reading a t the true zero of time, and the increased rate of the second stage (Figure 8) causes i t to merge with the first. At
To test the validity of Equation 5, the constants were evaluated from the experimental data and the calculated ,absorption was compared with that observed a t 80" C. in Figure 10. On this plot the actual experimental 10.0 absorption curve is drawn as a solid line, the experimental points are circled, and the extrapolated portion of the straight line (the slope of 6.0 6 which is IC2) is shown as a dashed line whose 10 intercept gives the value of V,. The curve for N VI was determined graphically by subtracting Vz from V (where V , IS the difference between the extrapolated portion of V and V,,). This curve W approaches V. as a limit. At various values of i (3 , 4.0 t, corresponding values of V , Vl, and V 2 were determined from the curve and K 1was calculated according to Equation 3. The average of these values was employed t o calculate the points 2.0 plotted as crosses. Similar comparisons of observed and calculated oxygen absorption are shown in Figure 11 for absorption a t 90° and 100" c. 0 40 80 I20 160 200 240 280 320 The close agreement between the calculated HOURS values and the experimental data clearly justifies Figure 10. Analysis of Oxygen Absorption Curve (80' C., 760 Mm., the theoretical assumptions on which the deriCured 50 Minutes a t 298" F.)
1
INDUSTRIAL AND ENGINEERING CHEMISTRY
76
temperatures above 100' C., Vz becomes so much greater than 'VI t h a t the first term of Equation 5 may be neglected. The plot of log K against the reciprocal of absolute temperature in Figure 12 is a straight line for both K 1 and K z ,indicating that single reactions are rate controlling in both cases. The temperature coefficients are 3.9 and 2.6, respectively, for a 10" C. change in temperature over the range 80' to 100' for K1,and SO' to 120' for Kz. The figure 2.6 is in good agreement with values previously reported for the deterioration of physical properties for both natural and synthetic rubber during aging, which range from approximately 2.0 to 2.6 (8). The much higher value of the temperature coefficient for K1indicates a higher energy of activation for the initial reaction. The energy of. activation, E, may be calculated by the integrated form of the Arrhenius equation:
K'
l n z
=
E
E
Vol. 38, No. 1
2 . The rate of oxygen absorption increases with temperature. The temperature coefficient for the constant rate reaction is 2.6 for a 10" C. change in temperature over the range 80" to 120 C. 3. Diffusion is not rate controlling, provided the specimen thickness does not exceed 0.080 inch at 100" C. or 0.040 inch a t 120" C. 4. The effect of cure upon the rate of oxygen absorption by GR-S vulcanizates is slight, and consequently, the technique is particularly useful for studying the effect of changes in compounding upon aging resistance. 5. Oxygen absorption data can k w batisfactorily correlated mith changes in physical properties during aging.
10.0
9.6
Y
9.2
3
+
0 8.8
8.4
I
I
I
1 1200
1100
I/TO
1000
K.
x
900
8OOC.
\\'bile the fundamental nature of the mechanism of the oxidation of GR-S is
has been obtained with respect to some of the factors involved, and the gcvwral Figure 12. Loglo K PloLted against has been estab; ~ ~ ~ of Absolute ~ Temperature i ~ nature ~of the reaction ~ ~ lished. This information., together with -_ the work st,ill in progress, should be of value in pointing the way toward possible methods of improving TI- T the resistance of GR-S to aging. 103
(r)
For the first reaction-that is, the initial rapid rise-the erieigy of activation is 36,000 calories; for the second or linear reaction, i t is 26,000 calories. These values are too large for a phSsica1 process and clearly represent a chemical reaction of oxygen with the GR-S vulcanizate. It should be recognized that the absorption rates reported in this paper apply only to the particular stock employed. HOWever, the effect of the variables studied should be similar for any GR-S vulcanizate.
ACKNOWLEDGMENT
The authors desire t o express their t,hariks to The Firestone Tire & Rubber Company for sponsoring t'his work and for pcrmission to publish this portion of the data; Hugh Winn is Firestone Fellow a t Case School of Applied Science. The authors are particularly gra.t,eful t o 0. D. Cole of Firestone and to C. F. Prutton and David Turnbull of Case School of Applied Science for helpful suggestions and cooperation. LITERATURE CITED
SURIMARY AhD CONCLUSIONS
Oxygen absorption studies on a tread type GIG3 vulcanizate show-:
1. There are three stages involved in the oxidation: ( a ) a n initial rapid absorption, of apparent first order at conatant pressure, which is of short duration and appears t o involve a limited number of especially reactive centers in the vulcanizate; ( b ) a constant-rate reaction, of apparent zero order a t constant oxygen pressure, which would extend over most of the useful life of the vulcanizate in service; (c) an autocatalytic reaction, beginning after a given stock has absorbed a definite amount of oxygen t h a t is independent of the temperature a t which the absorption takes place.
Dufraisse, in Davis and Blake's "Chemistry and Technology of Rubber", New York, Reinhold Pub. Corp., 1937. (2) Harrison and Cole, IND.ENG.CHEM., 36, 702 (1944). (3) Kemp and Straitiff, Ibid., 36, 707 (1944); Rubber Chem. Tech., 18, (1)
41 (1945). (4) Kohman, J . Phps. Chem., 33, 226 (1929); Rubber Chem. Tech., 2, 390 (1929). (5) Neal and Ottenhoff, IND.ENG.CHEM.,36, 352 (1944). (6) Shelton and Winn, Ibid., 36, 728 (1944); Rubber Chem. Tech., 18, 116 (1945). (7) Sturgiu, Baum, and Vincent, IND.EKG.CHEM., 36, 348 (1944). (8) Vila, Ibid., 34, 1269 (1942); Rubber Chem. Tech., 16, 184 (1943). (9) Winn and Shelton, IND.EBG. CHEM.,37, 67 (1945); Rubber Chem. Tech., 18,407 (1945).
