Aging of Elastomers COMPARISON OF CREEP WITH SOME CONVENTIONAL AGING METHODS RI. C. THRODAHL Monsanto Chemical Company, Nitro, W. V a ,
It appears t h a t the chemical reactions in elastomers a t elevated temperatures caused by oxidation, and w-hich result in cross linking and chain scission, are fundamentally exhibited by creep and stress relaxation. These interrelated functions have been used in this paper as a convenient means of studying the behavior of antioxidants and accelerators in Hevea and GR-S rubbers. Tests conducted on several representative Hevea rubber stocks containing different pigments show t h a t creep differentiates more clearly between antioxidants than do conventional aging tests. Creep measurements show t h a t the relationships between the effectiveness of various antioxidants are independent of both accelerator and state of cure. Creep tests are shown also w-hich differentiate GR-S tread stocks containing various antioxidants, although conventional aging tests indicate them to be alike. The relationship of continuous creep behavior with continuous and intermittent stress relaxation is shown in a typical Hevea vulcanizate containing combinations of three antioxidants with three types of accelerators. By either method rating of antioxidant is the same for all three accelerators.
S
UBJECTION of elastomers to mechanical stresses results in unusually complicated behavior. Recent theoretical researches (2-10) have shown that this behavior cannot be described satisfactorily by either of the classical theories of elasticity or viscosity. The general molecular theories which describe the behavior of elastomers (8, 9, 10) have experimental verification manifested by three regions of temperature-stress relationship: (a) a low temperature region in which stiffening is observed due
to the stability of secondary bonds between network chains, ( b ) an intermediate temperature region in which the secondary bonds are sufficiently unstable so t h a t complete relaxation has occurred before measurements can be obtained; the scission of primary valence bonds is occurring at such a slow rate that no measurable effects are obtained during the course of the usual laboratory experiment, and (c) a high temperature region in xyhich the relaxation of stress with time is associated with a chemical reaction which, through breaking of primary valrnce bonds in the network, severs the chains rapidly enough t o be measured during the course of usual laboratory experiments. The high temperature region is t h a t in which elastonicrs soften and/or harden and finally lose their rubbery characteristics. Oxygen has been shown to be necessary for the chain-scission reaction. Several papers have described this fundamental experimental technique on stress-relaxation ( I , 6, 8, 10) and creep ( I , 8) for diffcrcnt elastomers. Well-known laboratory methods of artificially aging elastomers in oxygen and air bombs, and in circulating air atmosphere have selected conditions of test somewhat arbitrarily. I n exploratory searches for promising compounds to be used as antioxidants in elastomers and in evaluation of well-known antioxidants often i t has been found t h a t the conventional methods of aging do not differentiate among several antioxidants. It is the purpose of this paper to describe a n application of the previously described creep tcchnique as a convenient and precise means of studying the relative performance of antioxidants and accclerators in Hevea and GR-S rubbers. EXPERIMENTAL
A creep apparatus similar to t h a t used by Andrews, ILlesrobian and Tobolsky ( 1 ) was designed and built for operation in a con-
TIME
I N HOURS - 121~~.
Figure 2. Continuous Creep Curves for Carcass Type Stock, Antioxidant 1 Accelerators: 0 Ureka C-Guantul, 0 Thiofide, C S antocure
60-minute cure.
Figure 1. Creep Apparatus
2180
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1948
2181
6o
100
80
60I
O +
00’
1 0
TIME
20
IN H O U R S -
39
121 C.
40
Figure 3. Continuous Creep Curves for Carcass Type Stock, Antioxidant 3
Figure 4. Continuous Creep Curves for Carcass Type Stock, Antioxidant 4
60-minute cure. Accelerators: 0 Ureka CGuantal, 0 Thiofide, 9 Santocure
60-minute cure. Accelerators: Ureka CGuantal, 0 Thiofide, 0 Santocure
0
-
HOURS 1210C. Figure 5. Continuous Creep Curves for Carcass Type Stock, Control (No Antioxidant) 60-minute cure. Acoeleratom: Ureka C-Glrantal, 0 Thiofide, 0 Santocure
stant temperature oven (Figure 1). Ring specimens (1.875 outside diameter x 1.60 inches inside diameter) were cut from vulcanized sheets of the elastomer 0.025 to 0.030inchin thickness. Each load was adjusted to give equal tensile stress on all stocks-usually 30 pounds per square inch, Creep readings were taken a t hourly increments until the test was stopped. Agings in the air bomb were carried out a t 121O C. at 80 pounds per square inch air pressure or at 135’ C. a t 60 pounds per square inch air pressure. Agings in the circuIating oven were carried out a t the temperatures designated. All stocks shown in Table I were mixed and vulcanized according to conventional laboratory practices.
TABLE I. BASEFORMULAS Composition Elastomer E P C black Zinc oxide Lithopone Softener Sulfur Stearic acid Paraffin Accelerator Antioxidant
Carcass 100.0
Hevea Stocks High pigment 100.0
30.0
60.0 20.0
...
....
...
....
3.0 2.5 1.5
Tread
100
50.0
5.0
...
2.0
3.0
2.0
,...
...
0.25 1.5
0.7 1.0
1.o
I O
3.0
A S 6i;bwn
1.0
GR-S Tread Stock 100.0 40.0 3.0
....
8.0 1.75
...* ....
60-minute cure.
1.2 1.0
TABLE 11. CONTINUOUS STRESSRELAXATION, ANTIOXIDANT-
+
Santocure accelerator
TABLE 111. INTERMITTENT STRESSRELAXATION, ANTIOXIDANT-ACCELERATOR SERIES
ACCELERATOR SERIES Stress (Lb./Sq. In.) - Per Cent of 0.005 Hr. Stress Antioxidant a t 0.005 Hr. At 0.1 hr. A t 1 hr. At 3 hr. 0 . 3 % Ureka C 0 . 4 % Guantal Acceleration 79.0 36.4 13.8 1 111.4 81.1 35.4 9.0 2 115 8 5.6 74.8 28.9 3 112.1 75.1 30.1 5.6 4 118.8 None 107.1 68.2 19.6 2.0 0 . 7 % Santocure Acceleration 27.6 83.9 45.8 1 90.5 27.2 87.1 50.3 2 102.2 15.0 78.0 34.4 3 92.5 28.7 10.0 4 87.2 76.2 None 81.4 74.2 24.6 7.5 0.7% Thiofide Acceleration 1 106 5 83.6 47.5 28.5 85.3 50.0 30.4 2 110.1 17.0 80.3 37.4 3 94.0 80.2 35.6 13.8 4 98.6 None 92.2 75.2 27.4 9.7
IN HOURS- 12fC.
TIME
Figure 6. Continuous Creep Curves for Carcass Type Stock, 0 Antioxidant No. 1, A No. 2, No. 3, 0 No. 4 , 0 None
Stress
Antioxidant
1 2 3 4 None
(Lb./Sq. In.) at 0.1 Hr. 0.3% Ureka C 114.8 120 I 2 112.6 122.0 102.1
Per Cent of 0.1 Hr. Stress A t 1 hr. A t 3 hr. A t 8 hr.
+ 0.4%
Guantal Acceleration 99.4 82.6 72.6 96.8 62.0 94.9 57.8 89.4 79.2 35.0
32.2 27.3 7.2 5.7
...
0.7% Santocure Acceleration
None
97.5 101.8 94.6 84.0 84.7
113.4 104.4 92.0 82.4 64.0
111.8 103.8 64.1 47.4 27.2
1 2 3 4 None
0.7% Thiofide Acceleration 111.4 108.9 104.0 109.0 114.4 103.2 91.0 102.2 100.2 84.4 97.9 101.4 87.2 91.2 73.6
107.1 107.4 59.8 53.2 42.9
1
2 3 4
109.2 101.5 103.8 100.0 88.2
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
2182
Figure 7.
Continuous Creep Curves for Hevea Tread Stock, Antioxidant 3
-
Santo60-minute cure. 0 Thiotax, A Thiofide-Guantal, El Sixty-A-10 cure, XA-32, Ureka C-Guantal,
Figure 8.
Continuous Creep Curves for Hevea Tread Stock, Antioxidant 3
90-minute cure. 0 Thiotax, A Thiofide-Guantal, 0 Santocure, XA-32, tireka C-Guantal, I El Sixty-A-10
-
K
-x
0
0'
40
-
Figure 9.
Continuous Creep Curves for Hevea Tread Stock, Antioxidant 3
Accelerators:
A-32,
100
.I$@
3 m
pt
e
e
-
~
20 __ TIME IN HOURS- lOO*C.
Vol. 40, No. 11
a
.
'
1
0 tireka C-Guantal
'i
I
10 20 30 40 TIME IN HOURS- 135 C. Figure 11. Continuous Creep Curves for GR-S Tread Stocks, Antioxidants 5-A to 5°F 60-minute cure.
Santoaure accelerators
Figure 12. Continuous Creep Curves for GR-S Tread Stocks, Antioxidants 1, 3, 4, S A , 6 , 7, 8, 9 90-minute cure
INDUSTRIAL A N D ENGINEERING CHEMISTRY
November 1948
Stress relaxation d a t a shown in Tables I1 and 111 and the continuous creep data of Figures 2 to 6 were obtained from vulcaniiwd specimens prepared from the same compounded mix. The author is indebted to 0. D. Cole of the Chemical and Physical Research Laboratories, Firestone Tire & Rubber Company, for these data. Method and apparatus used were those described by Tobolsky, Prettyman, and Dillon (IO). Tobolsky and Andrews (8) plotted the creep and stress relaxation from the following equation (which was derived from theoretical considerations of the statistical theory of elasticity of rubberlike materials) :
where 5 = fraction of original stress a t time t SO 1 = length a t time t I, = initial length 1, = unstretched length in order t h a t both might have the same significance when plotted on the same set of axes. s/so was plotted arithmetically against t logarithmically. As the data in this paper were comparative, the creep plots were made on an arithmetic scale with the per cent creep versus time. DISCUSSlON OF RESULTS
P’igures 2 to 6 show continuous creep curves obtained with specimens prepared from Hevea carcass stocks containing combinations of three types of antioxidants and three types of accelerators. Different creep rates were observed with each accelerator but the relationship between antioxidants was the same irrespective of the accelerator used. The Santocure (cyclohexyl-2-benzothiaaolesulfenamide)stocks had the slowest creep rate followed by Thiofide (2,2’-benaothiazyl disulfide) and Ureka C (benzoyl-2-benzothiazyl sulfide)-GuantaI (diphenylguanidine phthalate) stocks in that order. The creep plot of Figure 6 when compared with the data obtained from air pressure aging shown in Table IV indicates the former to be more precise and consistent. Greater significant differences were observed with the creep data. Few significant differences between antioxidants shown in Table IV were observed a t the 121 C. aging temperature in the
TABLE IV. PERCENTRETENTION OB TENSILE STRENGTH OF HEVEACARCASS STOCKSAFTER EXPOSURE TO AIR-BOMBAGING, ~O-MINUTE CURES Antioxidant
1 hr.
Aged at 135P C., 60 Lb./Sq. In. 2 hr. 3 hr.
0 . 3 % Ureka C
67
1
2 3 4 None 1
2 3 4 None
4 hr.
Aged at 121O C., 80 Lb./Sq. In. 6 hr. 9 hr. 12 hr.
+ 0 4 % Guantal Aceeleration 28 15 5
5 4 4
air bomb. However, a rough approximation existed between creep rate and the agings at 135 C. Creep data indicated that a definite difference between antioxidants was significantly measurable although the air-bomb aging results did not agree with I creep for antioxidant 2. A fundamental relationship between creep and stress relaxation which has been established (8) is shown by comparison of data from Tables I1 and I11 with data of Figures 2 to 6. By either method-creep or stress relaxation-the “rating” of the antioxidant is the same. The creep data of Hevea tread stocks of Figures 7 to 9 compare more closely with the conventional air qomb aging data of Table V than any of the preceding comparisons. These data show the pronounced influence of the type of accelerator on the creep rate. Comparison of the conventional air bomb and circulating air oven aging data of Tables VI and VI1 with the respective continuous creep curves of Figures 10 and 11 indicate the latter to be more sensitive in both Hevea high-pigmented and GR-S tread stocks. Antioxidant 5-A is separated more markedly from the remainder of the antioxidants than one would determine from the data of Tables V and VI. Another series of antioxidants in G R S is shown in Table VI11 and Figure 12. The percentage increase in modulus of stocks shown in Table VI11 would not enable prediction of differences in antioxidant effectiveness shown in Figure 12. These data are O
TABLE V. PER CENT RETENTIONOF TENSILE STRENGTHOF HEVEATREAD STOCKSAFTER EXPOSURE TO AIR-BOMBAGINQ, ANTIOXIDAXT3 Accelerator Thiotax (1 .O%)
Cure 60 I20
6 Hr. 86 80 72
9 Hr. 75 66 57
Thiofide (0.75%) Guantal (0.30%)
60 90 120
68 71 66
47 50 50
Santocure (0.63%)
60 90 120
51 45 53
40 26 40
A-32 (1 .25Yo)
BO 90 120
33 29 37
2 2 2
90 120
eo
57 62 64
38 39 37
60 90 120
61 86 67
36 58 42
90
C‘reka C (0.757,) Guantal (0.50%) El Sixty (0.60%) A-10 (0.10%)
TABLE VI. AIR-BOMBAGINGDATAOF HEVEAHIQH PIGMENT STOCKS Antioxidant 5-A 5-B 5-C 5-D 5-E 5-F
Per Cent Tensile Strength Retained on 60-Minute Cure after Aging at 121° C . at 80 Lb./Sq. In. 9 hr. 12 hr. 69 62 55 50 60 58 67 62 62 59 59 60
69 59 33
60
0 . 7 % Santocure Acceleration 89 81 30 72 64 57 16 71 55 38 27 70 8 53 46 40 51 33 4 38
67 63 64 49 36
59 47 49 23 11
93 112 98 104 92
0 . 7 % Thiofide Acceleration 88 85 59 72 105 76 45 68 94 56 24 77 83 42 12 63 16 4 4 42
66 65 65 60 24
41 57 66 44 10
109 95 92 90
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TABLEVII. CIRCUL.4TIXG AIR-OVEN AGING DATA O F GR-S TREAD STOCKS, 6O-&IIXUTE CURE
Antioxidant
5-A
5-B 5-C 5-D 5-E 5-F
Per Cent Change after Aging in Circulating Air 24 Hours at looo C, 300% modulus Ultimate tensile lb./sq. in. strength, Ib./sq. in. 60 +31 - 5 ’ 74 82 +23 66 4-24 55 +16 - 2 +68
+++ ++
INDUSTRIAL AND ENGINEERING CHEMISTRY
2184
1001
generally in accord with published attempts t o establish relationships between modulus change and antioxidant effectiveness. T h e continuous creep data, however, clearly show significant differences among the various antioxidants-particularly between the control stock and Nos. 5-A and 9. Table IX demonstrates the relative precision of eleven specimens taken from the same stock and subjected to the same conditions of stress and temperature. T h e values of the mean and the standard deviation for the creep at each period of time are shown. This is excellent by comparison Figure 13. Effect of with precision ordinarily obSpecimen Thickness on Creep Rate, Hevea Cartained with standard aging tests. cass Stock Effect of the thickness of the 6 0 - m i n u t e cure. No anticreep specimen is shown in Figure oxidant 13. Complete failure of the 0.009-inch specimens occurred in times too short for good comparisons. Furthermore, preparation of uniform specimens of that order of magnitude is a difficult task. All the data shown in this paper were taken on specimens approximately 0.030 inch in thickness. It was found that increased stress (tensile load) on the specimen and higher temperature caused faster creep rate. Figure 14 demonstrates the change in creep rate with increase in tensile load for a Hevea tread type stock at 121" C. A stress of 30 pounds per square inch was selected to give a creep rate sufficicnt to minimize error and allow the specimens to remain exposed long enough to show definite differences. TTith Hevea rubber,
TABLE
Vol. 40, No. 11
I
CIRCUL.4TING AIR-OVEK AGING D A T A GR-S TREAD S T O C K S , ~ O - ~ ~ I S U TCURE E
\'III.
OF
Per Cent Change after Aging in Circulating Air 24 Hours a t 100' C. 3 0 0 7 modulus, Ultimate lb!//sq. in. elongation, 7"
-
Antioxidant
1 3
-13
+ + +a
+51 49 f44 +45 42
4
5-A 6 7
8
-11 - 7
-11
- 14 - 17 - 17 - 10 - 16
f67 52 4-87
+
9 Blank
I
tl W
w
a
0
01 a=--
0
Figure 14.
'
I
I
I
I
1
IO 15 20 26 30 TIME IN HOURS- 121'6. Effect of Stress Variable on Hevea Tread Stock , 5
Continuous creep curves. n o antioxidant, @ 15, 0 30, 3 45, - 0 75 pounds per square inch
0- 60.
tcmpcratures higher than 121 O C. caused excessively fast creep rates. This resulted in complete failure of the specimen in less than 5 to 7 hours. Lower teniperatures merely reduced the creep rate but did not aid in differentiating the stocks. With GR-5 the time to coniplete failure was of the order of 100 hours at 121" C. but a t 135" C. the cleep rate was increased to yield most useful information in 40 to 60 hours. Figure 9 shows the effect of state of cure on creep rate of Hevea tread stocks. Other unpublished data with Hevea and GR-S have indicated that the creep rate is faster for shorter timcs of curc; but all curves are of similar phape. LITERATURE CITED
(1) dndrews, R. D., Mesrobian, R. B., and Tobolaky, A. V., I n d i a Rubber W o r l d , 112, 181-4 (1945). (2) Andrews, R. D., Tobolsky, A. V., and Hanson, E. E., J . A p plied Phys., 17, 352-61 (1946) ; Rubber C h e m . a n d Technol., 19, 1099-1112 (1946). (3) Blatz, P. J., and Tobolsky, A. V., J . C h e m . Phys., 14, 113-14 (1946). (4) Green, M. S., and Tobolsky, A. V.,Ibid., 14,80-92 (1946). (5) IIulburt, H. M., Harman, R. A, Tobolsky, A. V., and Eyring, Henry, Ann. N. Y . A c a d . Sei., 44, 371-418 (1943). (6) Mooney, Xi., Wolstenholme. W. E., and Villers, D. S . , J . A p plied Phys., 15, 324-37 (1944) ; Rubber C h e m . a n d Technol., 17, 576-96 (1944). (7) Stern, M . D., and Tobolsky, A. V., J . C h e m . Phys., 14, 93100 1194633; Rubber C h e m . a n d Technol., 19, 1178-92 (1946). (8) Tobolsky, A . i 7 . , and dndrews, R. D., J . Chem. Phys., 13, 3-27 (1945) ; Rubber C h e m . a n d Technol., 18, 731-82 (1945). (9) Tobolsky, A,, and Eyring, H., J . C h e m . Phys., 11, 125-34 (1943). (10) Tobolsky, 9. \7., Prettymnn, I. H . , and Dillon, J. F I . , J . A p p l i e d Phys., 15, 380-95 (1944) ; Rubber C h e m . a7zd Techrml., 17, 581-78 (1944). RECEIVED June 6, 1947. Presented before the Division of Rubber Chemistry of the h I E R I C h N CIIEYICALSOCIETY at Cloveland, Ohio, hIaS 1947.
OF CREEPME&SURCJICKTS TABLE IX. PRECISION Hours
of
Creep 1 2 4
5 6 7 8 9
__ 1 2 .. 6 5 2 13.2 17.1 26.3 36.8 59.2 107.3
2
3
2 5 .. 26 13.2 17.1 26.3 36.8 59.2 llt.0
2.6 5,2 10.5 15.8 25.0 35.5 56.6 11t.l
4 2.6 5.2 15.8 21.1 28 6 40 7 83.7 122.4
10
C o m p u t e d o n basis of 10, deleting specimen 8 b E = arithmetic mean. 6 c = standard deviation. d 1' = coefficient of variability. 0 Broken.
5
Creep in Per Cent of Hevea Carcass Specimens a t 1 2 l 0 C. 5 6 7 8 9 10 2.6 2.6 2.6 2.6 2.6 2.6 5.2 5.2 5.2 7.9 5.9 5.9 13 2 13.2 13.2 15.8 13.8 13.8 19.7 17.1 19.7 23.6 19.4 19.4 36.5 28.9 27.6 30.3 27.6 28.9 40.4 40.8 40.8 51.3 40.1 43.4 61.1 57.1 83.1 84;2 63.6 69.7 12Ie.1 114.5 12:.4 11!.1 105.3 e
11 2.6
5.2 13.2 19.7 27.6 39.4 60.5 107.4
-
A-2 6 2.6 0.3 13.3 113.0 27.7 R9.j 62.4 113.7
...
CsC
V d
0.0 0.67
0.0 12.6 9.6 9.1 5.3 6.1 5.6 2.6
1.28
1.69 1.46 2.40 3.52 6.4
...
..
