INDUSTRIAL A N D ENGINEERING CHEMISTRY
860
Vol. 17, No. 8
SYMPOSIUM ON THE OXYGEN AGING TEST These papers represent the major portion of a symposium under the leadership of J. M. Bierer before the Division of Rubber Chemistry a t the 69th Meeting of the American Chemical Society, Baltimore, Md., April 6 t o 10, 1928
Further Developments and Applications of the Bierer-Davis Oxygen Aging Test By J. hl. Bierer and C. C. Davis BOSTON WOVRN HOSE8r RUBBERCO., BOSTOX,h l A S S .
An additional year's study of a wide variety of rubber compounds from different manufacturers indicates more than ever that oxidation is the predominant factor involved in the deterioration of vulcanized rubber in the dark under natural conditions, and that several years of natural aging can be duplicated in a few hours merely by increasing the concentration of oxygen. The oxygen bomb test developed for this purpose has therefore become of great value in determining the influence on the aging of various ingredients and conditions of treatment and cure. Practical difficulties limit the conditions, particularly the pressure and the temperature, under which the test can be carried out, but it has been found that treatment with oxygen under a pressure of 300 pounds per square inch a t 60" C. is a reliable index
of the natural aging of a wide variety of rubber compounds. The data from which this conclusion is drawn are also used to develop a general method which will show graphically whether an aging test is a duplication of the natural aging of the same substance. That complications such as the influence of light must be avoided is shown by the fact that exposure to sunlight, even with the intervention of considerable glass, causes very rapid deterioration in rubber compounds which are otherwise very resistant. This in conjunction with the fact that temperature fluctuations cause discrepancies in the results emphasizes the necessity of standardizing the procedure. Only by such standardization can the oxygen aging test be placed on a quantitative basis comprehensible to all.
FTER the lapse of about a year since the original description of an aging test involving the use of oxygen under pressure,l indications point more than ever to the fact that oxidation is the predominant factor involved in the deterioration of vulcanized rubber during natural conditions of storage. To increase the rate of oxidation all that is necessary is to increase the concentration of oxygen. The attempt was made a year ago to show that the easiest means of increasing this concentration, and therefore of obtaining an increased rate of oxidation, was to increase the pressure of the oxygen in the atmosphere surrounding the rubber. Experiments described a t that time showed that vulcanized rubber compounds on exposure to oxygen a t a pressure of several hundred pounds per square inch a t slightly elevated temperatures deteriorate to practically the same extent and in the same manner that they do after several years of natural aging. A simple technic was also described by which such high-pressure aging tests may be carried out with maximum convenience and minimum danger. Owing to the facility and the precision with which natural aging may be duplicated, the oxygen aging test has grown from a small beginning to a point where about twenty-five companies have installed equipment for carrying on the test. Because of this general interest the present paper is limited to a general survey of results which can be obtained with the test, with the inclusion of only enough representative data to show the quantitative manner in which natural aging may be duplicated. I n conjunction with the symposium for which it serves as an introduction, the chief aim is therefore to de-. termine the present status of the oxygen aging test and to discuss any complications which may have developed during the year since its inception. A great variety of rubber compounds have been investigated, including various types of compounds manufactured
by a large number of prominent rubber goods manufacturers. The latter include companies that have installed the equipment for carrying on oxygen bomb tests as well as some that have as yet not done so. Systematic investigations have been carried out which deal with the influence on aging of various accelerators, numerous antioxidants, different types of rubbers, reclaimed rubber, softeners such as oils and asphalts, and, in general, ingredients, beneficial and harmful, which are in use at present or which may have potential value. Since the aim of the present symposium is to survey the progress attained in adapting the test to practical problems, no attempt is made to show these detailed results. Duplication of several years of natural aging in a few hours renders the determination of the aging properties of vulcanized rubber compounds very easy. Formulas and ingredients about which doubt has arisen concerning their influence on the aging of rubber can be investigated in short order. For this reason a few representative rubber compounds from different manufacturers are included merely to emphasize the value of the test in foreseeing the natural aging of various types of stocks.
.... .. . .. . .. ..
A
1
THISJOURNAL, 16, 711 (1924).
Aging of Stocks Reported Last Year Among the stocks for which a large amount of data under various conditions of cure are available, a stock vulcanized with diphenylguanidine and one vulcanized with litharge are given to show the relation between natural aging and oxidation in the bomb. These two stocks were reported in detail in the paper of last year and their accelerated oxidation can now be compared with their subsequent aging under natural conditions in the dark. These two stocks were: Smoked sheets Diphenylguanidine inc oxide 'recipitated whiting Cure 30 minutes a t 142" C.
100 4 0.5 4 50
Smoked sheets Sulfur Litha we Precipitate :d whiting Cure 25 minutes a t 142' C.
100 6 10 50
I-VDUSTRIA L AND EXGINEERING CHEMISTRY
August, 1925
COMPARISON OF NATURAL AGING AND IN THE
BOMB
O F T W O COMPOUNDS
I n Figure 2 the bomb tests on this new mixing are compared with the natural aging in the dark over a period of 5 years. The original tests and those a t the end of 2 years have already been published in a paper on the natural aging of various rubber compounds containing u 1tra-a c celera t ors.2 From Figure 2 it is evident that the bomb tests and natural aging in the dark closely parallel one another and that here again a period of time in the neighborhood of 9 hours under 300 pounds per square inch pressure of oxygen at 60' C . is e q u i v a l e n t t o o n e year of natural aging in lthe dark.
DETERIORATION
DESCRIBED PREVIOUSLY
800
4000
z
&
861
3000
30
600
5
2000
20
400
g
1000
IO
In
e -I
I
-
0
4
I-
8
12
16
20
24
0 a c
E 9
v
G
HOURS IN BOMB
W
6 200
2
t; a
0
4
G
8
\2
2 O W
16
HOURS IN BOMB
L b
2zsoo
High-Grade Carbon Black Stock
0-
5
IO
"
6
20 MONTHSOF NATURAL AGING
"
5
Choosing a highly accelerated carbon black stock, :the formula of which:
IO 15 20 OF NATURAL AGiNG
MONTHS
Figure 1
Choosing a single cure and a single set of conditions of oxidation to render the graphical comparison simpler, the results are shown in Figure 1 before and after oxidation under 300 pounds per square inch pressure of oxygen a t 60" C. The socalled diphenylguanidine stock and the litharge stock were widely different in their resistance to oxidation. I n fact, the litharge stock had deteriorated badly before the diphenylguanidine stock had changed to any great extent. If these results are compared with natural aging in the dark for the past 14 months, their similarity is a t once evident. The same relatively great deterioration of the litharge stock has already begun to manifest itself in tensile strength and in elongation. The graphs in Figure 1 are so located that the same degrees of deterioration in the bomb or under natural conditions lie vertically above and below one another. This renders a comparison of natural aging and oxidation in the bomb very simple. Such a comparison indicates that about 10 hours under 300 pounds per square inch pressure of oxygen a t 60" C. is equivalent to one year of natural aging in the dark.
Smoked sheets Sulfur Diphenylguanidine Ethvlidene-aniline Pine t a r Zinc oxide Carbon black 100 2
Rubbei Age, 13, 227 (1923); lndia Rubber J., 66, 303 (1923).
E
YEARS OF
@=
ul
NATURAL AGING IN THE DARK.
3 4000
b)
-;3000 -L
t; 2000 z W L
tVI
1000
W
2
Ultra-Accelerator Stock
YEARS OF NATURAL AGING
iN THE DARK.
As a representative compound of a different character, a stock vulcanized with an ultra-accelerator is shown in Figure 2, the formula of the compound being: Smoked sheets Sulfur Tetrarnethylthiurarndisulfide Zinc oxide Press cure 10 minutes at 134' C.
100 1
1 100
The stock was first mixed in January, 1920, before any results had been published about tetramethylthiuramdisulfide as an accelerator and some time before it had appeared in a commercial form and could be obtained on the market. The stock has been tested at intervals since, and when the oxygen aging test was developed the stock was remixed under conditions approaching as nearly as practicable to the original.
=)
s
'
IO
NATURAL AGING
20
30
40
50
HOURSIN B O M B AND OXYGEN BOMB TESTS OF A
COMPOUND CURED WITH AN ULTRA-ACCELERATOR. Figure 2
IiQDUSTRIAL A N D ENGINEERING CHEMISTRY
862
closely resembles tire tread stocks and similar stocks compounded for high abrasive resistance, Figure 3 will show its natural aging in the dark compared with oxygen bomb tests and with the ordinary 70" C. air oven test. The bomb and oven tests were made over a long range of cures and show the deterioration as a function of the cure. Natural aging tests N A T U R A L AGING, OXYGEN BOMB TESTS AND OVEN TESTS
OF A HIGH GRADE CARBON BLACK COMPOUND.
5
$ so00 K
W
e 4000
6
43
-1
-
z 3OOC z
5z
ZOOC
E
1000
w
0
$ v,
.z
I. I
F
0
20
30
40
50
MINUTESOF CURE
60
square inch pressure of oxygen at 60" C. One of the stocks was known beforehand to be an excellent aging stock and to be in good condition when 8 years old. The other stock was known to be inferior in its aging properties, though a stock which aged in a very satisfactory manner. Figure 5 shows the comparative aging properties of the two compounds when oxidized under pressure. The results are seen to corroborate the information already known about the two inner tubes. Examples of oxidation tests on many other types of stocks could be cited to show the duplication of natural aging which is possible by a standardized method of procedure. Thus a red sheet packing made by a local rubber manufacturer became brittle and completely lost its resiliency after only 16 hours of treatment in the bomb a t 60" C. under 300 pounds per square inch pressure of oxygen. This result indicated a very short life for the rubber packing, which was only a few days old when received, a prediction which was veriiied by the fact that after the sample had been stored for only four months it lost its resiliency and became almost as dry and brittle as it had after oxidation. These changes were duplicated by the changes in the acetone extract after artificial oxidation and after natural aging in the dark. Stocks Cured with Sulfur Chloride
70
AT 142'C
Figure 3
are available over a much smaller range of cures, but those for which definite and reliable data are available indicate how closely the oxygen bomb test will duplicate natural aging in the case of such a high-grade carbon black stock. Incidentally, the relatively great deterioration in the oven after 96 hours a t 70" C. should be noted. Similar results were obtained for the changes in ultimate elongation. Here, too, interpolation would point to a period in the region of 9 hours at 300 pounds per square inch pressure of oxygen a t 60" C. as the equivalent of one year of natural aging in the dark.
Vol. 17, No. 8
Likewise six different bathing-cap stocks from still another company, when submitted to 300 pounds of oxygen a t 60" C. for 24 hours, could be divided into four good and two bad stocks. After reporting that the bomb test had indicated that two of the six stocks were very poor in their aging, information was received that natural aging gave the same results, that the two stocks which had deteriorated so badly in the bomb were the two which did not age well under normal conditions. The stocks have been preserved and these two stocks have deteriorated much faster than the four which were more resistant in the bomb.
Fire Hose Tube
Among various commercial rubber compounds of different manufacturers which have been tested, Figure 4 will show a typical fire hose compound of the older type cured with litharge and having the following formula: Smoked sheets 100 Sulfur 4'/2 Litharge 20 Zinc oxide 70 Whiting 20 Cure in steam 10 minutes a t 142' C. -k 15 minutes a t 148' C .
The stock was tested when first vulcanized both before and after oxidation a t 300 pounds per square inch pressure of oxygen at 60" C. for increasing periods of time. By choosing the time of oxidation which corresponds to the deterioration after two years of natural aging in the dark and comparing all three stress-strain curves, the changes in the stock may be judged. The quality of the stock after oxidation for 18 hours under 300 pounds per square inch pressure of oxygen a t 60" C. is practically the same as that after 2 years of natural aging, whereas the 70" C. oven test for 96 hours shown for comparison gave a relatively great deterioration. Many more results of a similar character for this type of stock might be recorded. Red Inner Tubes Two red inner tube compounds of differing composition manufactured by a prominent rubber company were submitted to the oxygen bomb test for increasing periods of time, the maximum treatment being 8 days under 300 pounds per
0
200 400 600 % ULTIMATE ELONGATION
800
Figure 4-Relation b e t w e e n t h e Stress-Strain Curves w h e n New a n d a f t e r Natural Aging, Aging i n Oxygen, a n d Aging i n t h e 70° C. Oven
Such tests are of particular significance because the stocks had been cured with sulfur chloride and ordinary tests had failed, as with nearly all cold-cured compounds, to differentiate between good and bad natural aging. I n the brief space allotted to these typical examples of the aging of widely different types of rubber compounds, stressstrain relations have, for the sake of simplicity, been dispensed with except in one case. Furthermore, tensile products have not been utilized, because the very fact that they represent a combination of two variables means that changes
INDUSTRIAL A N D ENGISEERISG CHEMISTRY
August, 1925
in neither variable can be followed separately. Thus, a compound might decrease greatly in elongation without any great decrease in tensile strength, and thereby suffer a loss in tensile product of the same magnitude as a compound of
r
9 2 4000 .fi
4
5 3000
E
g
2000
LA
+ az ln
w
1000
='
ln
F
O
1
2
3
4
5
6
7
8
DAYS IN BOMB
20 15
L E
4
x
W
10 w
z
?
5 w
2 o s Figure &Aging
DAYS I N BOMB of T w o Inner T u b e s of Differing Composition
863
Increased Rate of Deterioration
I n describing in the previous paper the effect of increasing the temperature, the inference was made that there was a temperature range, differing for each stock, above which stocks become disproportionately sensitive to oxidation, though it was stated a t the time that this is probably not chiefly a heat effect. It is of significance that in the original experiments this break in the aging curve also occurred after a definite period of time a t a given temperature and concentration of oxygen. The phenomenon has since been found with many other stocks and is probably a general characteristic of vulcanized stocks. I n other words, whether or not the action is autocatalytic, each rubber compound reaches a point in its deterioration where the physical and chemical changes become more rapid. If the change in any property is plotted as a function of time, temperature, or concentration of oxygen, a break appears in the curve, as is evident in Figure 6. These graphs represent the deterioration in terms of the tensile strength of four different types of rubber compounds of differing degrees of resistance to oxidation. The break in the curve comes after different periods of treatment because the four compounds have differing lengths of life. Without digression to a discussion of the formulas and the reasons for their differing aging properties, the graphs merely indicate the general character of the compounds. The extremely resistant compound does not contain an antioxidant, but is simply a very carefully balanced formula which has been cured to just the right degree for maximum aging. The curves show clearly that after a definite time the rate of deterioration increases. The same effect could just as well have been shown for the ultimate elongation, acetone extract, or other physical or chemical property which is a function of the deterioration. Furthermore, this break in the curve might have been shown as a function of temperature or of pressure of oxygen. I n short, after a certain degree of deterioration is reached, the rate increases regardless of the agent, whether increasing time, temperature, or pressure. This serves to emphasize another point. If a rubber compound remains practically unchanged for some time and then deteriorates badly in a relatively short time, an artificial aging
the same original tensile product which decreased less in elongation but lost proportionately more in tensile strength. In other words, changes in tensile product do not reveal proportionate changes in elongation and strength and therefore do not show states of cure i o clearly as do the independent changes in elongation and in strength. OXYGEN AGING TEST5 O N FOUR TYPICAL RUBBER C O M P O U N D S Discrepancies which are found a t times in the stress-strain curves may be accounted for by the TO 5HOW INCREASED R A T E OF DETE'RIORATION AFTER A T I M E . fact that as deterioration advances the outer strata of the rubber become of poorer quality than the interior. In this way the stress-strain 5 3000 curve measured by the outside dimensions gives ei 3000 a false estimate of the true quality of the sample, v, 2000 2000 for the simple reason that the stress is not distributed over a cross section of uniform strength but 1000 1000 that the stronger interior is carrying a disproportionate share of the load. The surface layer, on the other hand, adds to the apparent cross 0 2 4 6 8 Z-4 6 8 section without contributing to the strength, with DAYS I N BOMB DAYS I N BOMB the result that the stress-strain curve after aging is in a different position relative to its original position than it would be if the deteriorated rubber were homogeneous throughout. This is particularly true if test pieces are oxidized in the bomb, whereas natural aging is carried out with large uncut sections. In this case oxidation in the bomb involves a more advanced surface deterioration on all four surfaces of the cross section, whereas only two surfaces out of four have been exposed in the case of test pieces which are cut out after the rubber has aged naturally. This 2 4 6 6 difference increases with decrease in width and DAYSI N B O M B DAYS IN BOMB thickness of the test pieces. Figure 6
ISDCSTRIAL A N D ENGINEERIXG CHEMISTRY
864
test may be stopped before this rapid deterioration occurs. I n this way the two stocks in the lower part of Figure 6 might be given an artificial aging test corresponding to 3 hours and the two compounds be pronounced of similar aging properties. But in one case the compound was on the verge of a deterioration so rapid that in another year or so it would be in bad shape, whereas the other compound would not reach this rapid decline until the ninth hour.
RELATION BETWEEN NATURAL AGING AND AGING I N OXYGEN CRITERION OF THE
"
2
4
VALUE OF AN AGING TEST
8
6
MONTHS OF
IO
12
14
NATURALAGING
16
I n order to obtain a fairly complete indication of the aging of a rubber compound it is therefore necessary to carry the oxygen aging test to the point where the rate of deterioration increases decidedly. This point can, of course, be measured by the changes in any of several physical or chemical properties, but only by this method can a true criterion of the aging be obtained. Relation between Bomb and Natural Aging
It has already been shown that about 10 hours' treatment in the bomb under 300 pounds per square inch pressure of oxygen at 60" C. causes deterioration to the same degree and of the same character as one year of natural aging. This same relation between the time in the bomb and of natural aging holds true for any length of time, a fact which can be derived most readily with the aid of Figure 1. AGING IN THE
FORCED AIR OVEN A T 70'C.
-5:=
0-
2
4
6
8
IO
MONTHSO F NATURAL
12
I4
Plotting the hours in the bomb against the months of natural aging corresponding to the same deterioration, a curve is obtained which shows the relation between natural aging and the bomb test. A similar curve can be obtained for the elongation, the acetone extract, or any other property which changes on aging. Figure 7 shows tensile strength and elongation curves, the principle and derivation of which are described above. Both curves, which turn out to be straight lines, show that about 9.5 hours in the bomb is equivalent to one year of natural aging in the dark. The point a t the origin represents the quality when new and as the lines are followed away from the origin deterioration becomes more advanced. For the oxygen bomb test to duplicate natural aging, both the tensile strength and the elongation should show the same relative changes as time goes on; in other words, according to the graph the two lines should be coincident. Within the errors inherent in mixing, curing, and testing rubber compounds this relation seems to be true. I n contrast to this, Figure 8 shows the method applied to the aging of the same litharge compound in the 70" C. oven test. I n this case the curves for the tensile strength and the elongation are neither straight lines nor coincident, indicating that this test does not duplicate natural aging, which has already been shown to be true. Criterion for Any Aging Test
'
Figure 7
RELATION BETWEEN N A T U R A L AGING AND
Vol. 17, No. 8'
T6
AGING
Figure 8
With the litharge compound, for instance, the tensile strength has dropped from 3050 to about 2800 pounds per square inch after 4 hours' treatment in the bomb or after about 5 months of natural aging in the dark. Likewise it has dropped to about 2400 pounds after 8 hours in the bomb or over 10 months of natural aging, and to about 2100 pounds after 10 hours in the bomb or 14 months of natural aging.
The value of any aging test may thus be expressed by the curvature and approach to coincidence of curves representing various properties of the stock. It may include changes in physical properties, such as tensile strength, ultimate elongation, or resilient energy, or changes in chemical properties as shown by the acetone extract and the formation of oxidation products. I n all cases true duplication of natural aging means straight lines which coincide throughout. On this basis it is evident that the bomb test is almost an exact duplication of natural aging. Necessity of Standardization
At present about twenty-five rubber companies are using the new test either in an experimental way or for factory control. If no agreement is reached among the various laboratories, a great many modifications of the test are likely to arise and there will be no understanding among the individuals of the significance of the particular combination of temperature and concentration of oxygen. The use of different tests might be defended on the ground that some rubber goods undergo considerable heating with relatively little exposure to air, while others are exposed a t all times to fresh air but are never warm, and that for this reason alone a different test is necessary in each case. If rubber boots and shoes are exposed to light they deteriorate much faster and crack much sooner than when stored in the dark. Likewise, automobile topping in ordinary use has a much shorter life than it would if it were stored indoors in darkness. To duplicate these conditions exactly requires a test which combines the action of light, heat, and oxygen, the relative intensity of which would have to be chosen to agree with the particular conditions encountered in natural aging. Aging in Light and in Darkness That these conditions would have to be carefully controlled owing to the powerful effect of sunlight may be seen from Figure 9, which shows the effect of aging stocks in the dark and exposed to sunlight. The diphenylguanidine and litharge stocks already mentioned were stored for 14 months both in darkness and in a
INUUSTRISL d N I 1 ENGINEI$RING CHEMISTIZY
August, 1925
NATURAL AGING
z
u
86.5
O F TWO RUBBER COMPOUNDS STORED FOR FOURTEEN MONTHS I N
COnPLETf DARKNESS AND FOR THE SAME PERIOD EXPOSED TO SUNLIGHT 5 E H l N D GLASS.
u?
MONTHS OF NATURAL AGING
MONTHS OF NATUCAL AGCNG
800 600 400
200
G
O
2
4
6
8
10
I?.
14
0
MONTHSOF NATURAL AGING
2
4
6
8
10
12
14
I6
MONTH5 OF NATURAL AGING Fisure 9
very light room lint behind glass. Tlie curves sliow that tbe deterioration is far rnorc rapid in the light. I n fact the U.P.G. stock had become worse than the litharge stock and showed cracks on the surface when stretched, though in darkness it changed hardly at all. This may he due to the fact that the U.P.G. stock was white and the litharge stock black, or it niay he diie to some deeper reason. Tbe fact that the stocks in the light were behind two layers of glass indicates that there must liave been a relatively small proportioir of ultra-violet light to attack the stocks. Similar results have been obtained with other rubber compounds, the experiments indicating that i t is of fundamental importance whether aging takes place in the light or in darkness. To try to duplicate tlie different combinations of light, heat, and air would lead to a different test for every type of rubber goods manufactured. Obviously, this is most undesirable. All that can be hoped for, at least for the present, is to standardize the test so that it represents the equivalent of ordinary storage or shelf aging conditions. In this way a langnage common to all can develop, and to assert that a stock shows little change after 48 hours "in the bomb" would instantly convey the perfectly definite information that it will remain in good shape for 5 years or so under natural conditions in t.he dark.
coinpounded stocks. Whetlier this ratio holds good for certain pure guin stocks there are fewer data by which to judge. Pure gnm stocks transinit inore light than coinpounded stocks, which may he tlre reason why stocks containing little or no pigments or tillers have tlre reputation of deteriorating faster than compounded stocks. The effect of light has just been indicated and it is probably still more intense a factor in tlre case of pure gum stocks. Control of Temperature Whatever procedure is adopt&, great attention musthe paid to the accurate control of temperature. Experiments by several manufacturers have shown that a fluctuation of two or
Standard Test The writers have become convinced, after accumulating considerable evidence and finding no discrepancies between the bomb t.est and natural aging, that a combination of 300 pounds per square inch pressure of oxygen at 60" C . is very effeetive in obtaining a deterioration in a reasonable number of hours in the bomb similar to sevcral years of natural aging. This combination has the advantage of convenience and safety as well as having proved itself a reliable index of the natural aging of a wide variety of rubber compounds. As has already been indicated, between 9 and 10 hours i s the equivalent of one year of natural aging, at least for
Figure 10
tliree degrees above or below that prescribed for the test results in considerable differences in the degree of deterioration after bomb tests corresponding to a few years of natural aging. The bomb tests involve chemical reactions which are accelerated or retarded by small changes in t.lie temperature,
Ih’D USTRIAL -4ND E1VGILI’EERING CHEMISTRY
866
and if the temperature varies more than an amount corresponding to a 5 per cent change in the rate of the reaction, the results of the test will be in error by some similar amount. Since it is known in an empirical way that an increase of 10” C. will a t least double the rate of a reaction, deterioration should be a t least twice as rapid a t 70” C. as a t 60” C. Therefore, to maintain an accuracy of 5 per cent, the temperature must vary not over one-half degree above or below 60’ C. Some installations are now equipped with constant-temperature ovens for maintaining the prescribed temperature in the medium surrounding the bomb, others are equipped with a temperature-controlled water bath. Whatever the means employed, a nearly constant temperature must be maintained throughout the test to avoid large errors and unexplained discrepancies in the results. B o m b of Larger Capacity
A sufficiently accurate temperature control and bombs of larger capacity are two of the difficulties which may confront some who are now using the test. As a departure from the present type of bomb, one of larger capacity has been designed. It is shown in Figure 10 in comparison with the present type. Its internal capacity is approximately three and one-half times that of the smaller bomb.
Vol. 17, No. 8.
DISCUSSION Application to Factory Control of Stocks By John D. Morron and Harold E. Webster UNITEDSTATESRUBBERCo., CLEVELAND, OHIO
ESTS of a large number of factory stocks show that the Bierer-Davis oxygen aging test checks actual life aging and is very valuable as a means of factory control of stocks because of the small amount of time needed to obtain results. Proper Compounding Materials The writers’ first experience with oxygen aging was with two stocks t h a t had been compounded for the same purpose, the actual aging of which was known. Both stocks were “nonblooming” and both gave about the same results in the heat aging test:
Reiore heat aging Aged 1 week at 150’ F. Aged’2 weeks a t 150’ F. Before heat aging Aged 1 week 150’ F. Aged 2 weeks 150” F.
Stretch Per cent Sock 1 280
Permanent elongation Tensile Per cent Lbs./sq. in.
40
4 2 1
428 318
Stock 2 320 160 100
12 3 1
749 446
60
321
385
No great difference would be expected in the natural aging of these two stocks, but actually Stock 1 after less than 2 years was either in a soft, gummy condition or else was so stiff and brittle that it cracked on bending. Stock 2, although actual experience had not run over such a long time, showed perfect aging qualities as far as known. When submitted t o the oxygen aging test these two stocks acted in exact accordance with expectations. Stock 1 came out of the bomb exactly in the condition of the stock which had been aged naturally for about 2 years, being so destroyed t h a t it was impossible to make any tensile tests. Stock 2 showed the following tests:
Before oxygen aging Oxygen aging 16 hours at 60’ C.
Figure 11
Typical Installation for Several Bombs
Figure 11, in turn, represents an installation now used with success with four bombs in operation simultaneously. All four bombs lead to a common header, which serves to charge each from a single tank of oxygen. The header is, of course. the same whatever the number of bombs. Each bomb is equipped with individual piping and a gage so that it can he completely disconnected from the remainder of the system after charging. Likewise, the oxygen tank can be disconnected from the remainder of the system after charging. I n this way one tank of oxygen serves to supply all bombs and when the bombs are under pressure there is no connection between any two. Acknowledgment
The time seems opportune for acknowledging the aid of A. M. Varney, who since the inception of the bomb test has assumed a major part of the burden of the experimental work.
Stretch Per cent 330
300
Permanent elongation Tensile Per cent Lbs./sq. in. 9 792 12
765
This of course indicates an extremely good aging stock. Stock 1 was cured with lime as an accelerator, Stock 2 with an. organic accelerator. Since that time the writers’ experience has led them to believe that it is impossible to make “nonblooming” goods with lime which will age satisfactorily. On the contrary, the presence of even comparatively large amounts of lime in stocks containing large amounts of sulfur does not necessarily cause poor aging, if there is no attempt to make the stock “nonblooming.” For instance, one stock, containing about 25 per cent of lime and 15 per cent sulfur based on the rubber, tested as. follows :
Before oxvgen aging Oxygen aging 16 hours at 60’ C.
Permanent Tensile Stretch elongation Lbs./sq. in. Per cent Per cent 340 29 875 280
19
892
This shows a natural, but not excessive, hardening. numerable instances confirm the results given for these ticular stocks. Another stock, with about 5 per cent of and 20 per cent of sulfur in an equal mixture of shoddy and ber, showed very good aging:
Inparlime. rub-
