ANALYTICAL CHEMISTRY
1646 result in more reliable tread wear evaluations, more economical utilization of the tire test fleet, and a more rapid turnover of ideas. ACKNOWLEDGMENT
The authors wish to thank the Gates Rubber CO.for the
LITERATURE CITED
(1) Am. SOC. Testmg Materials, “A.S.T.;LI. Manual Control of AMaterials,”Part 3, 1951.
011
Quahty
(2) Buist, J. *M., India Rubber
J.,121,180 (Aug. 4,1951). (3) Buist, J. M., et al., T r a m . I n s f . Rubber I d . , 26,288 (1950). (4) Freeman, H. A., “Industrial Statistics,” New York, John I T dey
Migration of Materials During Accelerated Aging by the Oxygen Pressure Method RlARION B. FACKLER
AND
JOHN S. RUGG, The Gates Rubber Co.. Denver, Colo.
This work was undertaken in an attempt to find the causes of unexpected variations in data on oxygen aging of vulcanized rubber and to show quantitatively the effects of a phenomenon which had been recognized previously but had received little study. It was shown that materials such as sulfur, accelerators, antioxidants, and metallic salts migrate from stock to stock during accelerated aging by the oxygen-pressure method, the effect is large enough to
T
HE oxygen-preesure method of accelerated aging is well established among the test techniques used by the rubber industry. A great deal of work has been done in evaluating the effect of various factors which influence the test data (4-,Q) and standard test procedures have been established ( 2 ) . I n spite of this, variations which are beyond normal experimental error still occur, especially n-hen unlike stocks are aged together in a bomb. AE a result of this type of teqt discrepancy this invefitigation was undertaken.
A I
I
B I
I/
I I
!
I
I
1860 23dOi 2600 lOd0 2400 TENSILE (psi) Figure 1. Comparison of Group and Individual Aging Characteristics of Typical Natural Rubber Tread Formulation Three diffsxent antioxidants
Several workers (8, 7, 9) have recognized the fact that when a heterogeneous group of stecks is aged in a single bomb, some materials migrate from one stock to another and act to change the apparent age resistance of all the stocks in the group. This action may serve either to increase or decrease t,he apparent resist-
be significant, and the results are unpredictable. The oxygen-pressure method of accelerated aging is well established in the rubber industry. A great deal of work has been done, which now must be. w n sidered of doubtful value because more than one stock was aged in a bomb at a time. Many specifications permit “community aging.” I t is rewmmended that specifications and practice be ohanged to specify individual bombs for each unlike stock. ance to oxidation, depeiiding upon the nature of the migratory material and the stocks involved. .-i search of the 1iterat)ureon the subject failed to disclose specific materials which shon- this effect or the ext,ent to which the migration affected physical properties. This work shows that migration is not limited to any one material and that t,he effect,s on the stock:, are large enough to cause significant errors in test data. Because a primary purpose of an accelerated aging t,est is to rate the relative reeistancc to oxidation Jvhich is afforded to a stock hy an antioxidant. the majority of the data in this report are concerned with antioxidant. migration. However, the data show that materials such as sulfur, accelerators, anti copper salt8 also migrate and change appawnt age resistancrl. It is probable that the migratory tendencies are not limited to the relatively few materials mentioned in this report, but these are representative and probably cause the niofit drastic changes in physical properties. A very extensive investigat,ion would be necessary to classify d l conipounding materials into migrat,ory or nonmigratory p u p s . TEST METHODS
All aging wm carried out in Bierer-Davis oxygen bombs (2). -411 agings were made st 48 hours a t 80’ C. under 300 pounds per square inch ox gen ressure. Six samples of each stock were aged under e a c i con&tion, unless otherwise noted. All 8am Ies were from the same mix and the same cure. The bomb chamgers were thoroughly cleaned to remove any residual material from previous tests. Each stock was aged alone in the bomb and also In combination with other stocks. Tensile strength, per cent elongation, and Shore A durometer valuea were obtained. ANTIOXIDANT MIGRATION
Figure 1 shows the age resistance of a group of natural rubber tread-type stocks, identical except for antioxidant. To aeaure that the differences in test data were not due to accidental teat variations, repeated tests were made from a aingle mix and cure
V O L U M E 23, N O . 11, N O V E M B E R 1 9 5 1 'Fable I.
(;roup aged .\sed alonr
B 100.00 4.0 2.4 2.9 0.45 3.0 . . I .
0.25
....
,...
42.3
42.5
C 100,00 4.0 2.4
2.9 0.45 3.0
.... ....
0.25 42.5
Tensile t o Break, L h . per Square Inch 2600 2400 2600 1800
2300 1000
Elongation to Break, % 450 150 450 420
450 350
Gro!I~, um:d .\ged
Group and Individual Aging .1 100.00 4.0 2.4 2.9 0.45 3.0 1.0 ....
Rubber ZnO Pine t a r Stearic acid Santocure Sulfur Antioxidant la Antioxidant 2b Antioxidant 3c I,:PC black
1647
alone
as a consequence of the practice of placing unlike stocks together in a bomb. Stocks K and 11 show no diffeience in their apparent age resistance when aged together. When aged alone, stock K melted and stock 11 retained about 60% of its original properties. Apparently the antioxidants from one or more of stocks L, ?and *I, N migrated to the stock not protected by antioxidant (K) when all four veie exposed together in the bomb. I n Figure 2, stoch 11 shows less apparent aging resistance when aged nith other stocks than when aged alone. Possibly this antiovidant was nearly consumed by the parasitic action of stock I
