November, 1929
INDUSTRIAL! A N D ENGINEERISG C H E X I S T R Y
ating the rate of deterioration of rubber caused by oxidation by increasing the concentration and the temperature of oxygen in the surrounding medium. Judgment must be used in choosing the particular conditions of pressure and temperature for a particular type of rubber mixture, but whatever intensities are chosen, the test is still a n "oxygen bomb test" if the presrure of oxygen exceeds that in the atmosphere. The test might better be known as a compressed oxygen test, for the bomb is only an incidental feature for safety. The question coiitiiiues to be asked-how many hours in a n oxygen bomb under given conditions correspond to one year of natural aging? $ssuming that oyidation is the predominant influence causing natural deterioration in the rubber mixture in question, there should be a certain number of hours in oxygen a t a definite pressure and temperature wliicli correspond to one year of natural aging in dry air in darkness a t a definite and constant temperature. B u t the difficulty lies in the fact that in natural aging the conditions, particularly the temperature, are not constant. l'lost chemicsal reactions proceed a t least twice as fast TT hen the temperature is raised 15" F. (8.3" C.), and assuming thc oxidation of lubber to be a typical chemical reaction, it is obvious that, n i t h vulcanized rubber which deteriorates chieflj from oxidation, deterioration should be a t least tnice as fast a t 75" F. 123.9" C.) a< at GO" F. (15.6" C.). I n other words, a certain number of hours in an oxygen bomb might represent one y t w at 73" F., but two years a t 60" F. There i, so much variation from place to place, and betnei,'11 summer and n-inter in each place, that the average temperature of rubber goods during a year in one place may varv far more than 15" I?. (8.3" C.) from tlie average temperature of rubber goods elsewhere. Therefore, from a practical point of view it is impoisible to designate so many hours in an oxygen bomb as the equivalent of so many yearb of natural aging. Because of this impossibility i t is necessary, as 111 so many other laboratory tests, to run control tests. In other words, instead of assuming that an oxygen bomb test of a certain number of hours represents definitely a certain period of natural aging, it is better to determine the relative aging of the sample in question u i t h a vulcanizate of known aginq properties. For example, an unknown sample is aged for 200 hours in oxygen under 150 pounds (10.6 kg. per sq. cm.) pressure a t 50" C., and a t the same time a control, the aging of which is known to be satisfactory for the use concerned, is also aged. The comparative aging in oxygen will then give the needed information about the unknown sample If in the oxygen bomb the aging of unknown samples is compared with the aging of known samples, then all the practical information which is ordinarily desired I $ obtained without the use of a fallacious comparison of number of hours in a bomb with number of years of natural aging. I n this way the oxygen bomb test may be depended upon t o give reliable information about the aging properties of rubber products nhich deteriorate chiefly from oyidation. I t s great utility is well illustrated by the story of the company which found discordant results between the 70" C. oven test and the oxygen bornh test in developing 3 new inner tube. The oven test indicated an excellent stock, while the oxygen bomb test gave warning that the tubes would deteriorate too soon. Trusting the mole favorable oven tebt, tlie inner tubes were sold, and nithin one year tuhcs valued a t $150,000 were returned because of had aging Another concern, d i i c h supplieq inner tubes to certain buq :ompanies, received the complaint that its tubes became wit and tacky after 5000 miles. By the aid of the oyygen bomb test this company was able to develop inlier tubes nliich after 20,000 miles were still in p o d condition.
1009
I t is unnecessary to cite further examples to show the value of the oxygen bomb test in foreseeing the natural aging of rubber products which deteriorate chiefly from oxid a t ion. ' But this test is not sufficient, for the importance which sunlight, for instance, plays in the aging of some of the most important of products demands that an artificial test be developed for this kind of aging. Until tests other than those involving oxidation in darkness are developed, the rubber industry will remain without any means of foreseeing the natural aging of some of its most important products.
Natural vs. Artificial Aging Stanley Krall
A
COSSIDERABLE number of types of n.rtificial aging are being used t'oday in an endeavor to determine thc aging properties of rubber stocks in a short period of time without nrxitiiig for natural aging result,s. The dnta reported here \yere taken from some thnt are being obtniiied by Sub-Committee XY,Committee D-11 of the Ainwican Society for Testing 1Iaterinls. Experimental
Three types uf pneumatic tire stocks were aged by two natural and two artificial methods: (1) Slabs were hung separately in the dark. (2) Slabs were hung separately exposed t o the weather. (3) One-inch (2.5-cm.) wide strips were hung separately in the Geer ( 2 ) oven a t 158" F. (70" (2.). (4) One-in-h (2.5-cm.) wide strips were hung separately in the Bierer ( 1 ) oxygen bomb a t 1%" F. (70" C.) and 300 pounds (21 kg. per sq. cm.) oxygen pressure.
The st'ocks tested were a pure gum stock, a first grade, and a reclaim tread st'ock as follows: Smoked sheets Whole tire reclaim D. 0. T. G. Sulfur Zinc oxide Carbon black Mineral rubher Pine tar
STOCK1 100.00
STOCK2
STOCK3
100.00
0.75 3.00 5.00
1.25 3.50 5.00 40.00 5.00 2.00
60.0 66.0 1.0 3.5 5.0
....
....
,...
35.0 5.0 2.0
.... . .. . --_
-__
_-
108 73
156. 75
177.5
The stocks were cured 45 and 60 minutes a t 287' F. (141.7' C.). The stoclcs were tested after 6 and 12 months' dark aging; 3, 6, 9, and 12 months' weather exposure; 3 and 7 days in the oven; 12 and 24 hours in the bomb. Results
The results were plotted on a scale ol 3 clays in the oven and 12 hours in the bomb, equivalent to one year's natural aging. Charts 1 and 2 show the load a t break values; 3 and 4 the load a t 500 per cent for stock 1 and a t 300 per cent for stocks 2 and 3; 5 and 6 the percentage elongation :it ljreak; 7 and 8 the tensile product. Chart 9 shows the relation between the days in the oven and hours in the bomb versus months in the dark. These results IT-ere obtained from the load a t break curves on Charts 1 and 2. The results indicate that one year of dark aging is equivalent to approximat,ely: STOCK
1 >
3
~ S - A I I N U TCURE B Days in oven Hours in bomb 2 15 3 9 6: 10
BO-AZINUTI~ C~JRF: Days in oven Hours in bomb 3 16 3 X 5 11
INDUSTRIAL A N D ENGINEERIRG CHEMISTRY
1010 I
LOADAT BREAK
CtiART
*1
Vol. 21, No. 11
LOADAT BREAK
CHART
*&
!IO00
i
I
CHART '4
MoDuius 13000
CuaE -60 M I N GZ
287'F.
,200
,
.zow
Chart 10 shows the relation between the days in the oven and hours in the bomb versus months weather exposure. The results were obtained from the load a t break curves on Charts 1 and 2. These results indicate that one year of natural aging is equivalent to approximately: STOCK
1
2
3
$&MINUTE C U R E Days in oven Hours in bomb ? ? 2-3 6-8 3-5 6-8
60-MINUTE C U R E Days in oven Hours in bomb ? ? 2-3 6-8 3-4 8-10
It is difficult to draw a comparison for stock 1, probably owing to the checking (alligator-skin effect) which developed on it after a few months of weather exposure. The curves show clearly the more rapid aging in the hot bright sunlight of the summer months as against the slower aging for the colder months (aging began June 1). Chart 11 shows the relation between days in the oven versus hours in the bomb. The results were obtained from the load a t break curves on Charts 1 and 2, and indicateithat one day in the oven is equivalent to approximately:
Kovember, 1929
I i D UXTRIAL SA-D ENGINEERING CHEMIXTR I' FINAL ELONGATION
--
C r A R T -5
I
1
T E N ~ I L PRODUCT E
-7 -~ CHAKT
1 2 3
I ~ - M I B U TCURE E Hours in bomb ? 2'/1 l'/r
CHART.6
CURE-GO MIN@ 287'F
TENJILC PRODUCT
CHAET
e0
CURE-OOMIN 61 207'F
C V R C -45 MIN (m 287'F
STOCK
FINALELONGATION
1011
~ O - M I N U T ECIJRE
Hours i n bomb ?
a
2
It is difficult to draw a comparison for stock 1 owing to the change in rate of aging for this stock. Conclusions
The results show that, for all stocks, there is no general correlation between either method of artificial aging and either method of natural aging. Therefore a general state-
nient cannot be made in regard to a time in the bomb 01 oven which will duplicate a definite period of natural aging. However, the trend of the natural aging-physical property curves can be duplicated by either artificial method, but the rate of artificial aging by either method must be determined for each particular stock. There is such a wide variation ~. .in . . the type of natural aging to which rubber products are subjected, such as heat, oxidation, light, etc., as well as a combination of any of them, that it is very important that the method of artificial aging be one that will best duplicate the aging and service conditions to which any particular stock is subjected.
LNDUSTRIAL I S D EA-GISEERISG CHEMISTRY
1012
O ~ Y G E NBOMB+. G r r a O v ~ ~
v5
CHART^^,
Vol. 21,
O X Y G t N B O M B 4- G E E R O V E N
KO.
11
cH*rrTilo
VJ
NATURALAGING
DARK
-
Cunr 60 C ?A?' P.
Accelerated Aging vs. Shelf
Aging
CHAKT*\\
OXYGEN BOMB
Everett M . Follansbee
V~C~EEROVCN
C u e E - 4 5 L-287.F.
SIXPLEXM'IRE & CABLECo., BOSTON, MASS. 24
AMPLE8 of rubber-insulated wire were aged by exposing
S
to the diffused light in the physical testing laboratory. The thickness of wall of rubber surrounding the tinned copper wire showed a variation from 0.030 t o 0.065 inch (0.76 to 1.65 mm.). The samples were tested for tensile strength and elongation during the aging interval and the results werd then compared with accelerated life teats obtained on the same samples when new. Average results on 480 different compounds, ranging i n quality from a low-grade compound to a compound containing 60 per cent by weight of crude rubber, show the following relationship between shelf aging and accelerated aging.
a
2
E q u i v a l e n t Y e a r s of Shelf Aging
Tensile strength Elongation
D A V 5 IN O V C N
Acknowledgment
Acknowledgment is due B. N. Larsen for his valuable assistance in preparing this paper. Literature Cited (1) Bierer. IND. ENG. CHEM.,16, 711 (1924). ( 2 ) Geer, I n d i a Rubber W o r l d , 64, 887 (1921).
HEATED 96 HOURSAT 70" C. IX AIR Years 1.43 1.18
HEATED24 HOURSIN OXY-
GEN AT 60' C. A N D 300 L B S (21.1 KG.) PRESSURE
Years 1.37 1.00
Both of theqe accelerated aging tests give a longer shelfaging time equivalent for tensile strength than is given for elon gat ion. The Geer test (96 hours a t 70" C. in air) on the average represents the same shelf-aging time regardless of the type of compound. The oxygen test, however, in the case of elongation, represents a much shorter shelf-aging time for the lower grade compounds. Both of the accelerated aging tests gave results that varied considerably in equivalent time for shelf aging, and it should be emphasized that the above equivalent aging results are only awrage figures.
