or Measuring Stress Relaxation &eGoodyear
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of low shape factor, recesses in platens for ends of samples, and cyclic preloading of samples before starting the test. Accuracy of the instrument was
THE
value of observations of stress decay in strained rubber a$ a means of studying molecular Row processes and the scission of moleoular bonds has been demonstrated by Tobolsky and co-
Tire and Rubber Company, Akron 16, Ohio
cheeked by wmparing results with those obtained using a resistance wire strain gage. Experiments upon GR-S and Hevea compounds showed that the rate of sties8 relaxation was not appreeiahly affected by the degree Over the rar greater rates extreme high peratures. 'I S t r e 8 8 was al compounds 01 several types 01 bn-3 ana HeYea to determine the effect of o u m upon the rate of stress relaxation. In all instances longer cures gave lowe= rates of relaxation. Examples ilhistrate the usefulness of the apparatus in wmparing particular gasket-type compounds. Extensive tests were made to study possible correlation between stress relaxation results and wmpression set results. For some w m pounds, such a correlation may exist.
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w o r ~ e r s [ i , a ,1~-1x],wnonavepuollsnedanumb~~.~ot interesting papers describing experimental methods developed for this purpose. Stress decay in rubber bands in tension was shown by them to behave in FL manner that could be reasonably well accounted for by theoretical considerations based on Eyring's concepts of reaction rate processes. Recently, attention has been directed toward the str tion of rubber under oompressian. Interest is primarily m n d s upon compounds to be used as material for g: mountings over a wide range of temperature. Several irmw UIIIWUIJ
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mow ana rietcner (ai uescrmeu a simple Deam-type apparatus in which a sliding weight was shifted to obtain a balance st the time of observation.. Between observations, a constant eompression oi the test piece was maint,ained by means of a stop a t the scales. Beatt,y and Juve (1) have recently published extensive work using th is type of apparatus. !-..",A ^ _ A T T ^ L ^ I _ ^ P ,o, A"-:-..-> -" "a..p..-p-a'~..n"u u o"w"..-pLL ~ .e ~ Maedunmu uu V S L ~ W I I IY, YSULGLI~U recording stress decay of samples in compression, tension, or shear. They pointed out, however, that results of the three were similar, and that compression was to be preferred for the sake of simpIicity. A high degree of accuracy and reproducibility was claimed. Their results were plotted as the ratio of stress to initial stress (0.01 hour) us. log time. Labbe and Pbillips (6)elaborated on the above apparatus, adapting it to operation a t very low t,emperatures and incorporating a system for changing the amount of compression while the test was in progress. By this means, they were able to get a, measure of R sample's recovery properties. Their method of plotting results was the same as'that of Macdonald and Ushakoff. Morris, James, and Seegman (9)used an instrument designed by Morris for an extensive study of the effect of various accelerators in nittile gasket compounds. Stress decay measurements were made a t 194" F. and 30% compression, extending over 14 days' time. Interesting Compa&9OnSwere made with results from other trpes of tests upon the same stocks. Their instrument was unique in that air pressure was used to apply the cornpressingload, and an air gage w&s used to indicate the force. Results were plotted both as stress us. time and as the per cent decrease from 1 hour stress os. time, d l scales being linear. YI
r
ANALYTICAL CHEMISTRY
1440 The apparatus herein described incorporates a novel means for the automatic adjustment of the compression load to correspond to the decay of stress in the rubber. Data here presented are intended primarily to illustrate possibilities of the instrument and the general stress relaxation characteristics of typical polymer compounds rather than the development of specifically useful types of gasket‘compounds, DESCRIPTION OF APPARATUS
The apparatus, shown in Figures 1 and 2, is comparatively simple to make, easy to operate, accurate, and self-contained. This last feature is especially valuable when it is desired to use it a t different temperatures, necessitating transfer of the instrument from oven to refrigerator, etc. Its relatively small size is also advantageous in surh situations.
-------SYLPHON BEUOWS\
1
I
++,SYLPHON
BALANCING VAL BAR HE1 JUSTING CROSS
--
FAUDl
Air pressure is used to furnish the compressive force, and an ordinary tire valve serves to adjust this pressure continuously and automatically to the exact amount necessary just to equal the decreasing stress within the sample. A Sylphon bellows takes the place of the more conventional cylinder and piston, thus allowing considerable simplification in design. An air gage connected t o the bellow indicates the pressure a t all times. Readings of this gage are taken a t intervals throughout the duration of the test. A universal-type micrometer dial.gage, shown in Figure 1 but not in Figure 2, gives the compression of the sample a t any time. A “hole attachment” is used to transmit motion of the bellows to the gage. The principle of operation may be explained by referring to Figure 2. Air is introduced through the inlet valve to expand the bellows downward abainst the test pieces. They are thus compressed until the balancing valve makes contact with the height adjustment cross bar. This contact opens the valve, releasing air and arresting any further expansion of the bellows. A ceramic porous plug smooths out the cycling operation by retarding the air flow through the valve. When a balance between stress in the qample and pressure in the bejlows has been attained, the valve closes. But the balance is upset as the stress decays within the sample; the bellows expands, openin the valve. Air pressure is rrleased until a new balance is reacted and the valve closes. This cycling continues throughout the test. Motion of the bellows in opening and closing the valve is so slight, less than 0.001 inch, that the sample compression may be iegarded as essentially constant. All joints in the apparatus are soldered. Leakage with good valves is so small as to be negligihle, less than 1 pound per square inch in 2 or 3 days. No correction for leakage is required as long as the stress relaxation rate exceeds the leakage. Test pieces are cylinders 0.5 inch in diameter by 0.5 inch high. Three, four, or six pieces may be arranged in shallow recesses in the platens, the number depending on the stiffness of the stock. By using several pieces, each having a small shape factor (d), end effects are minimized. Samples may be molded to size or &t from a vulcanized slab with a rotating cutter. COMPOUNDS USED
Formulas for compounds used in this work are given in Table I. EXPERIMENTAL PROCEDURE
Samples to be tested are pre ared by lightly buffing their ends with Crocus cloth. Their heigit is measured with a micrometer and the necessary deflection for the desired ercentage of compression is calculated from this dimension. TEey are then placed in the apparatus and the whole assembly is put into the oven or low temperature cabinet for 2 hours in an unstressed state. A t the end of this conditioning period air is pumped into the bellows, using an ordinary tire pump, until the micrometer dial gage indicates the correct deflection. The cross bar is then carefully adjusted to make contact with the automatic balancing valve.
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Figure 2.
1
SCHEMATIC DIAGRAM TO SHOW ACTION OF BALANCING VALVE
I
Diagram of Apparatus
ACTUAL STRESS
140
a
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GR-S redox RllX349-15. Cure 35/275 Compression 30%. Test temperature 158’ F.
Before starting the test the bellows is alternately exhausted and refilled three times a t 1-minute intervals. This type.of prestressing has been found advantageous in obtaining reproducible results in compression-type tests (4) and performs this same useful function for stress relaxation tests. However, it reduces markedly the relatively lar e transient relaxation .usually observed a t the start of a test wit% the initial loading. The first reading of the air gage is taken 6 minutes after the start of the test, and subsequent readings are taken a t appropriate intervals. At the termination of the test the samples are removed from the‘apparatus and allowed to recover for 0.5 hour. Their height is then measured again with the micrometer for calculation of set. Typical curves are shown in Figure 3. The actual stress curve is based upon calculations which take into account air pressure, cross-section area of the bellows, stiffness of the bellows, and crosssertion area of the samples. The equation used is: S =
aP - bx
+c
d2N
where
S = stress, pounds per square inch a = cross-section area of bellows, square inches P = gage pressure, pounds per square inch b = spring constant of bellows, pounds per inch x = amount of compression, inches c = dead weight of coupling system, pounds
1441
V O L U M E 22, NO. 11, NOVEMBER 1 9 5 0 = diameter of a test piece, inches .V = number of test pieces being used
d
Here a, b, and c are constants for any particular apparatus. For any group of experiments z, d, and N are usually constants. In this case the equation may be simplified to:
S = -aP K+ C
time by the stress at 0.1 hour. This arbitrary choice of initial stress time differs from that of both Ushakoff and Macdonald and Morris, James, and Seegman. A few later experiments at 212" F. indicated that the initial time of 1 hour used by Morris et al. may be the more useful.
where C and K are constants for the fixed conditions. For example, with the resent apparatus and three test ieces 0.5 inch in diameter, a n 8 0.5 inch hig:, compressed 3 0 4 ) the equation becomes:
S =
-
2.14P 10.66 0.59
.4s a numerical example: The 0.1-hour reading of the pressure gage was 56.7 pounds per square inch for the compound used in obtaining data for the curve shown in Figure 3. Substituting this value for P in the above equation gives:
S =
(2.14)(56.7)
v)
W
- 10.66
0.59
= 187.6
pounds per square inch DURATION- HRS.
Alternatively, for definite testing conditions, the gage dial may be graduated to read directly the stress in the test pieces. The stress ratio curve is obtained by dividing the stress at any
Figure 5.
Effect of Amount of Compression
GR-S tread RllX400-6 Test temperature 1 5 8 O F.
Cure 20/275
1.0
-
0.9
0
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0.8
a 0.7 0.61
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1.0
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DURATION
Figure 6. 100' 0.I
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Figure 4.
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Table I. \laterials GH-S GR-S redox GR-9-10 Smoked. sheet Sulfur Captaxe DOTGd Zinc oxide Stearic acid Softener Pine tar
Compound Formulas Used"
R607D489 1 100 100 .. .. .. ..
..
Cure 100/275
Compound R36X175 6
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2 1.5
100' 2 3 1.5 1
5 2 3
5. . 2 3
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3:75
3:75
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RllX34915 16
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100' .. 2:5 1.5 50 . 3 5 2 3
different C Mercaptobensothiacole. run. d Di-o-tolylguanidine. e Paraflux. / Phenyl-%naphthylamine.
50'
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501
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10.0
HRS.
Effect of Amount of Compression
GR-S tread RllX400-6 Test temperature 158' F.
Cure 20/275
100'
2:s
1.5 05 . 3 5 2 3 ..
COMPARISON WITH RESISTANCE WIRE STRAIN GAGE RllX400-fib 100
.. 2:5 1.5 05 . 3 5 2 3
..
1 HAF black .. .. .. .. 30 a BJ93 and BJQ4 were supplied without formulations by C. K. Chatten, chairman of A.S.T.M. Section on Relaxationof Rubber in Compression. b R l l X 3 7 4 - 2 (Figure 4) same as R607D489, except polymer was obtained from a Anaxl EPC black
111111'
For convenience semilogarithmic paper is generally used in plotting results. Short-term tests extending over a few hours or several days are best plotted in this way. When tests extend over several weeks, curves are more descriptive when exhibited on linear graph paper.
Comparison of Pneumatic Gage with Resistance Wire Strain Gage G R - S tread RllX374-2.
-
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Stress relaxation tests were made using a Baldwin Southwark resistance wire strain gage incorporated into the apparatus on a stiff spring as in the apparatus of Macdonald and Ushakoff ( 6 ) . This gage was one arm of a conventional Wheatstone bridge circuit. The initial load was applied by means of the bellows as before, but brass spacers were immediately inserted to maintain a constant deflection and all air was then released. A check upon the pneumatic system by a more familiar method Gas thus made possible. Comparisons were obtained for 3070 compressions a t 158" F. The stress relaxation curves for three tests using the pneumatic method and three tests using the strain gage method are shown in the lower part of Figure 4. Points from t.he v
ANALYTICAL CHEMISTRY
1442
three curves by each method were averaged to use in calculating the two ratio curves shown in the upper part of the figure. New samples were of necessity used for each test, which introduced some variation.
140
HEVEA GUM
'
120
OPERA'NONAL VARlABLES
Two variations in conditions of operation hqve been investigated: the effect of the amount of compression and the effect of the temperature of testing. Figures 5 and 6 show results obtained from samples cut from the same vulcanized slab and tested at three different compressions. Although the actual stress curves have different shapes, when the data are plotted as ratio curves, they are similar. These results indicate that the degree of compression over a considerable range has little influence upon the rate of stress decay.
-20
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20
40
Figure 9.
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100
I2Q
140
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60 TEMP.
-
80 *F
Effect of Temperature on Initial Stress C o m p r e d o n 30 %
cn
c 4 a 0.7
78
-
0.6 0.I I
OF.
0 OF.
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1 ' 1 1 1 ' 1.0 1
I
10.0 DURATION- HRS.
Figure 7.
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Effect of Test Temperature
1.0
-
Figure 10.
The effect of the temperature at which the samples are tested was studied using GR-S gum and treat compounds. Figure 7 shows results from the gum compound. The lowest temperature curve indicates high initial stress, which decays rapidly during the early part of the test. This behavior is in agreement with that found in low temperature torsion flex tests (S),where the greatest rate of creep is found at about this temperature. At the higher temperatures the decrease in stress during the early period of the test is much less severe. The ratio of stress decay curves shows that the rate of stress decay increases regularly as the temperature decreases.
100.0
10.0 DURATION HRS.
I
GR-S gum R36X17-1 Compression 30%. Cure 100/275
Effect of Cure
Hevea tread R36X17-6 Cornpression 30%. Tent temperature 158' F.
Table 11. Correlation with Type B Compression Set I BJ93
BJ94
Temp.,
Set,
Decay at 20 hours, %
Set,
% '
Decay at 20 hours, 3' 6
158 212
7 29
8 6 7
9 43 72
42
O F 86
% '
9
6
15
rise in temperature, as might be expected from the Joule effect. The GR-S acts oppositely, probably because of higher internal friction at loner temperatures. EFFECT O F CURE
a 0.7
Hevea and GR-S tread compounds were tested a t 158' F. to determine to what extent stress relaxation is affected by amount of cure. The duration of these runs was generally 22 hours, although in ssvpral instances longer times were used. Figures 10
t
0.6 0.t
I .o DURATION- HRS.
Figure 8.
10.0
Effect of Test Temperature
CR-9 hepd R607JX89 Compreuion 30%. Cure 100/275
Results for the tread compound, tested over a wider range of temperatures, are similar with one exception. As shown in Figure 8, the rate of stress decay goes through a minimum at 78' F., being greater at both lower and higher temperatures. Figure 9 shows an interesting outcome incidental to this study. Here the initial stresses for a series of t c s t s m Hevea and GR-S gum compounds are plotted against temperature. The Hevea compound shows a practically proportional increase in stress with
cn
P
0.8
-
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I 0.6'
0.I
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100.0
Figure 11. Effect of Cure GR-S trend R607D489.
Corn m i o n 30%. Teat temperature F.
lb'
1443
V O L U M E 22, N O . 1 1 , N O V E M B E R 1 9 5 0 I .o
-6 0.9 14WLtS W TO
RI 1x549-15 0.0
so \ 3 0
1.0 DURATION- H R S
Figure 12. G r n p m s i o n 30%.
10.0
Effect of Cure
Test temperature 158' F.
and 11 show results from these experiments. In all cases longer cures gave lower rates of relaxation. Figure 12 shows results of similar tests on GR-S-10, and Redox tread compounds. COMPARISON OF COMPOUNDS
Usefulness of the apparatus in comparing particular compounds may be illustrated by referring to Figures 13 and 14. The stress curves show that a t all three temperatures BJ94 has a much higher initial stress. Under constant compression of 401%, however, the samples a t higher temperatures soon begin to fail rapidly. BJ93, on the other hand, although having a lower
220
86T
-
2 00
EI 180 ,a
F.
initial stress, maintains* it well at ail temperatures. The ratio curves emphasize this fact. Only a t the lowest temperature does BJ94 compare favorably with BJ93. In connection with this experiment extensive measurenients of compression set were made, upon both test .pieces used in the stress relaxation apparatus and samples run concurrently between polished plates. An idea of the degree of correlation between compression set and stress relaxation may be obtained from Table 11, which was prepared from averaged results of seventeen tests. 4 considerable amount of correlation exists for BJ94 but none for BJ93. A possible explanation of this widely different behavior may be based upon knowledge of other types of experiments. I t is t'heorized that at elevated temperatures strained rubber is constantly undergoing scission and formation of bonds. For each bond broken there is a corresponding decrease in stress; but when a bond is formed, there is no change in stress because the new bond is formed in an unstrained state. I n the stress relaxation test, primarily, scission of bonds is being measured. But in the compression set test combined effects due to both scission and formation of bonds are being measured, and possibly effects due to delayed elasticity. Applying this to the results here shown it may be said that in the case of BJ93 the rate of scission is practically unaffected by these temperatures, but the formation of bonds is increased a t higher temperatures. In the case of BJ94 both bond scission and bond formation are accelerated by increasing the temperature. CONCLUSIONS
.The results of these experiments may he summarized in the four following conclusions: The rate of stress relaxation is independent of the amount of compression over a considerable range. I t is affected by the test temperature; therefore, the test temperature should approximate the temperature a t which the product is to be used. The rate of stress relaxation decreases with increase in cure. It cannot be predicted from results of compression set tests, although for some compounds some degree of correlation exists. ACKWOWLEDGMENT
I
\212oF.
The authors wish to express their thanks to the Goodyear Tire and Rubber Company and t o H. J. Osterhof for permission to publish this work. This investigation was carried out under the sponsorship of the Office of Rubber Reserve, Reconstruction .Finance Corporatipn, in connection with the government synthetic rubber program. LITERATURE CITED
DURATION- HRS.
Figure 13.
Comparison of Compounds Compresmioa 40%
Figure 14. Comparison of Compounds Compression 40%
(1) Beatty, J. R.,and Juve, A. E., India Rubber World, 121, 537 (1950). (2) Blow, C. M., and Fletcher, W. P., India Rubber J., 106, 403 (1944); Rubber Chem. and Technol.. 17, 1000 (1944). (3) Gehman, S. D., Woodford, D. E., and Wilkinson, C. S., Jr., Ind. Eng. Chem., 39,1108 (1947). (4) Kimmich, E.G.,Rubber Chem. and Technol., 14,407 (1941). (5) Labbe, B. G . , and Phillips, W. E.. India Rubber World, 119,224 (1948) (6) Macdonald, W. S., and Ushakoff, A., ANAL. CHEM.,20, 713 (1948); Rubber Chem. and Technol., 22,828 (1949). '7) Mesrobian, R. B.,and Tobolksy,-A. V., J. Polymer Sci., 2, 463 (1947);Rubber Chem. and Technol., 21,398 (1948). (8) Mochulsky, M.,and Tobolsky, A. V., Ind. Eng. Chem., 40,2155 (1948); Rubber Ch,em. and Technol., 22,712 (1949). (9) Morria, R. E.,James, R. R., and Seegman, I. P., India Rbbber World, 119,466 (1949). (10) Stern, M. D., and Tobolsky, A. V., J . Chem. Phys., 14, 93 (1946); Rubber Chem. and Technol., 19, 1178 (1946). (11) Tobolsky, A. V., and Andrews, R. D., J . Chem. Phys., 13, 3 (1945); Rubber Chem. and Technol. 18,731 (1945). (12) Tobolsky, A. V.,Prettyman, I. B., and Dillon, J. H., J . Applied Phys., 15, 380 (1944); Rubber. Chem. and Technol., 17, 551 (1944). RECEIVED May 3, 1950. Presented before the Division of Rubber Chemistry at the 117th Meeting of the AMERICAN CHEMICAL SOCIETY, Detroit, Mich. Contribution 178 from the Research Laboratory of the Goodyear Tire & Rubber Company.
