ANALYTICAL CHEMISTRY
818 Thanks are also due Lt. Comdr. Robert S.Burpo, USNR, of the Material Laboratory staff for his helpful criticisms in reviewing this article. LITERATURE CITED
Am. Soc. Testing Materials, Standard Method D 531-49. Indentation of rubber by Pusey and Jones plastometer. Ibid., D 676-46. Compression-deflection characteristics of vul-
canized rubber. Am. Soc. Testing Materials, Tentative Method D 395-49T. Compression set of vulcanized rubber. Ibid.,D 67-649T. Indentation of rubber by durometer. Ibid., D 746-44T. Test for brittle temperature of plastics and elastomers. Ibid., D 747-481'. Stiffness in flexure of plastics. Ibid., D 1053-49T. Measuring low-temperature stiffening of rubber and rubberlike materials by Gehman torsional apparatus. Bimmerman, H. G., and Keen, W. N., Ind. Eng. Chem., 16, 558 (1944). British Standards Institution, hlethod 903-1940. Methods of testing vulcanized rubber. Clash, R. F., Jr., and Berg, R. M., Modern Plastics, 21, 119 (1944). Conant, F. S.,and Liska. J. W.,J . Applied Phys.. 15, 767 (1944).
(12) Federal Speclfication ZZ-R-6Ola
(June 25, 1940). Rubber goods, general specifications. (13) Gehman, S. D., Foodford, D. E., and Wilkinson, C. E., Jr.. Ind. Eng. Chem., 39, 1108 (1947). (14) Graves, F. L., India Rubber World, 113, No. 4, 521 (1946) (15) Graves, F. L., and Davis, A. R., Ibid., 109, No. 1, 41 (1943). (16) Greene, H. L., and Loughborough, D. L., J . Spplied P h y s . . 16, No. 1. 3 11945). (17) Liska, J: Ind. Eng. Chem., 36, 40 (1944). (18) Scott, J. R., J. Rubber Research, 17, No. 1, 1 (1948). (19) Scott, J. R., Trans. Inst. Rubber Ind., 11, 224 (1935). (20) Selker, M. L., Winspear, G. G., and Kemp, A. R., I n d . Eng. Chem., 34, 157 (1942). (21) Smith, E. E., and Dienes. G. J.. S S T M Bull. 46 (TP 204) (October 1948). (22) Stechert, D. G., Ibid., 157 (TP 539, 61 (March 1949). (23) Trayer, G. W.,and hlarch. H. W.,Natl. Advisory Pornm. Aeronaut., Repl. 334 (1929).
\e.,
RECEIVED for review March 2, 1951. Accepted February 15, 1952. Presented before the 58th Meeting of the Division of Rubber Chemistry, AMERICANCHEMICALSOCIETY, Washington, D. C., March 1951. The opinions or assertions contained herein are the private ones of the writers and are not to be construed as o5cial or representing the views of the Department of the Navy or the Military Establishment at large.
Ozone Crack Depth Analysis for Rubber JOHN S. RUGGl The Gates Rubber Co., Denver, Colo.
I
N T H E past few years, much attention has been given t o the
weathering and ozone resistance of natural and synthetic elastomers. This has been due, in large part, to the advent of chemical rubbers in commercial manufacturing and acceptance of ozone as the component of natural weatheraThich causes cracking of stretched rubber. The synthetic elastomers, with apparently poorer weathering resistance than natural rubber, have presented rubber manufacturers with a serious problem in weather cracking, which has wsumed greater proportions than with natural rubber. Confirmation of ozone as the agency that causes cracking of stretched mbber has provided the basis upon which accelerated tests for simulated weather cracking have been designed ( 1 ) . Being confronted with a serious problem and having a rapid, convenient method of testing, the rubber industry is searching for methods of controlling or preventing weather and ozone cracking. Much of the development work directed toward improving weather resistance uses ozone cracking as a guide. .4s a result, ozone testing and interpretation of results have become very important. .4 major deterrent to the wider use of ozone testing and of natural weathering data is the lack of a satisfactory quantitative method of analyzing the test results. Quantitative data have many recognized advantages and appear to be a necessary requirement for exploratory development work in this field. PRESENT METHODS
The present methods of data analysis can be divided into three indirect groups indicative of the results they yield-judgment, quantitative, and direct quantitative. Most of the methods in common use today are the judgment type, where the cracked sample is examined visually, and the rating is developed in the mind of the observer. In this group fall photographic ratings (6),ink impressions, and arbitrary visual ratings. The major dimdvantage of this type of analysis is that it depends upon the judgment and experience of the examiner for the values obtained; 1
Present address, Rubber Laboratory, E. I. du Pont de Nemours & Co..
Inc., Wilmington, Del.
the conclusions may or may not be sound and the ratings itirty or may not be reproducible. The indirect quantitative methods measure a known property of the exposed rubber and relate the change in that property to the extent of ozone cracking. An example of this type of test is measurement of modulus of the rubber after ozone exposure which relates to reduction of cross section due to ozone cracking and indirectly to the extent of cracking ( 7 ) . A similar test recently has been proposed using changes in electrical conductivity ae the criterion, The objection to these types of test is that other influences concurrent with weathering or ozone attack may also affect the properties being measured. A direct quantitative method measures directly the changes wrought by ozone exposure. There have been few such methods proposed; an example is the measurement of the volume of ozone cracks by filling them with a heavy material, measuring the change in apparent specific gravity, and calculating the volume of the cracks from the difference in the apparent specific gravity. The results of this method are reproducible with difficulty because of the delicate handling required for accuracy. Consequently, a search was made for a method that would yield direct quantitative results. The method should be easily performed, not require specialized equipment, be independent of human factors, and measure an important property in terms of ozone life. The development of the crack depth method of analysis is the result of this search. THE METHOD
Crack depth is used as a measure of ozone and weather cracking results. This method is based on the premise that the growth of ozone cracks is inseparably bound to an increase in crack depth. Pon-ell and Gough (6) have shoxn that crack length and depth progress together, so that either would be an acceptable criteria; however, depth is more easily measured. Crack length is d&cult t o measure because in advanced stages of cracking the cracks grow together and lose their identity and personal judgment is required to identify each crack length. Crack depth is easily
V O L U M E 24, NO. 5, M A Y 1 9 5 2
819
T h e faetors that contml weathering and ozone resistance are not fully understood. A major deterrent to a hetter understanding of these factors is the la& of a satisfactory quantitative method of analyzing;the results of owne and weathering tests. A method has heen developed, which follows the change in era& depth as the criterion of the extent of damage that has taken place. It has been shown that when crack gmwth takes place, the depth increase~proportionally. The depth of the crack is measured with the 2OX miemswpe in aomsssection made normal to the length of the craeks. The major cracks, which have the directing influence on the extent of cracking, are the deepest ones and are all
nearly the same length. This method has been used to analyze results of owne craoking in whioh wmmonly r-gnized variables were employed. The results wnfirrn some previously reported trends with quantitative data. This method can he used in measuring the effect of compounding variables, where results are desired in terms of a dimension of actual oEone eraeking. This method is performed with moderate ease, does not require specialized equipment, is independent of human judgment, and is directly related to the gmwth of owne eracks. It should contribute to a hetter understanding of the factors that control weathering and ozone resistance of natural r u b b e r and other elastomers.
measured in cross section, requiring no personal judgment in the determination of results.
of exposure and ozone concentration were usually varied, and
Crack depth is measured by using a 2OX microscope having a scale divided in millimeters, in the field, A relaxed longitudinal seqtianed sample containing cracks is scanned, and the deepest cracks are measured, because they are the active ones-the shallower cracks have ceased t o have a directing influence on oracking ( 8 ) . The deepest crack and those within 10% of the deepest are averaged to obtain the reported value. The data are reported a8 crack depth in millimeters. Reproducibility on replicate samples has been found to be =t0.05mm. with the equipment and test methods used.
Crack Depth us. Exposure Time. Crack depth increased directly, but not proportionally, to exposure time, confirming previous work (5). The curve rises rapidly during the fist few a relatively hours of exposure, after which it flattens and -me8 constant slope. The change in slope may be explained by the hypothesis that a t the beginning of exposure, the oame a t the rubber surface is being constantly changed and is in close contact with the rubber, while as the cracks become deeper, the ozone at thedepthof thecraokis changedlessoften and beoomesexhausted, so that cracking is decelerated. Also, as cracking progresses, local relaxation occurs around the cracks, which probably contributes to a slackened rate of cracking.
Figure 1 s h o w a sample as viewed through the microscope. In Figure 1 the majority of the visible cracks are the same depth, varying less than 0.10 nun. There is no doubt in the viever's mind which orsoks are the indicative ones. In order to demonstrate the usefulness of this crack depth method, it is used in the experiments that follow as a means of analyzing the results of ozone testing, in which 8ome of the variables commonly aasociated with ozone testing are employed. Many of these variables have been reported in the literature previously, but have not had the benefit of quantitative data.
they are indicated in each experiment below.
EXPERIMENTAL
Except as otherwise stated, the following procedure was used: Base Formulation
The formulation was presemred 20 minutes a t 280" F. in 4 X 6 X 0.10 inoh sheets. Specimens measuring 1 X 6 inches were cut from the plate, placed in tension clamps, and elongated to 10%. Two Specimenswere mounted in eaob clamp, one auper-
The mecimens were allowed to oonditidn under tension in an
~~~
~~
loed differences in ozone c&ce&rationwithkthe cabinet. The apparatus also contains a variable damper system far more precise control of oBone concentration. All tests were run st 70" to 80" F. A check ofozone concentration by the potassium iodide decompositionmethod(8)wasmadethreetimesduringeachmn. When concentration was changed, concentration checks were made five times to be certain of stabilized ozone concentration The time [&Is
Figure 1. Sample as Viewed for Analysis
Crack Depth us. Ozone Concentration and Exposure Time. Ozone concentration and exposure time are both controlling factors in crack depth; therefore, knowledge of the interaction between the two would be helpful in designing experimental test conditions. The effect of variahle exposure time and ozone concentration on crack depth is shown in a trifunetional graph in Figure 3.
ANALYTICAL CHEMISTRY
820 Figure 3 shows that as ozone concentration is increased, the time to a given crack depth is markedly reduced. If 0.4 mm. is considered a crack depth a t which significant cracking has occurred and the time vs. ozone concentration necessary to obtain that crack depth is plotted (Figure 8), it is evident that, in the range of 7 to 30 parts per hundred million ozone concentration, the time is reduced one half as the ozone concentration doubles.
CONDITIONS 10%ElongOIlOn 2% v h m Ozone
Y
0.0
Figure 3 also shows the crack depth to be expected under a wide variety of exposure conditions. This information is useful in determining ozone exposure conditions to approximate natural weather cracking when the depth of weather cracks is knoan under given conditions. The laboratory ozone conditions may be adjusted to a given crack depth in a variety of exposure periods, depending upon the ozone concentration used. Per Cent Elongation us. Crack Depth. In Figure 4 the importance of strain on the exposed samples can be seen. As the per cent elongation of the sample at the time of exposure increases, the crack depth decreases. This phenomenon has been explained by Powell and Gough ( 6 ) , who say in effect that the fewer cracks obtained a t smaller elongations grow deeper, because the stress is concentrated a t fewer points and less relaxation occurs from neighboring cracks.
pointed out that as cracks grow, local relaxation of strain occurs, so that some cracks grow more rapidly than others. This condition is shown in the scale plot of Figure 5, showing crack location os. crack depth. Figure 5 is a profile of cracks in a sample for a length of 7 mni. The length of each line in the figure represents the depth of an actual crack and its position on the horizontal coordinate indicates its position in the actual sample. I t is evident from an examination of Figure 5 that some of the cracks have grown much more rapidly than others. After 100 hours’ exposure, used in this test, there were 35 cracks visible, of which only 13, or 377& had grown to a depth of 0.4 to 0.5 mm. If the sample had been examined under higher magnification, it is possible that many small cracks mould have been observed and thus the percentage of cracks which had grown to appreciable crack depth would he further reduced. This finding, that only a small percmtage ot the cracks originated in a sample on exposure to ozone grow to any great depth, is in agreement with the earlier work of Powell and Gough (6).
9; ELONGATION
Figure 4.
Elongation
Depth of Crack
In the method proposed here, for evaluating the results of ozone exposure, only those cracks which have reached the greatest depth are being considered. In all the experiments reported here there Iyere always a number of cracks in each sample which were nearly equal in crack depth and considerably deeper than all the other cracks in the specimen. Constant Load us. Constant Deflection. Samples measuring 1 x 6 inches were cut from a single molded plate. One set of samples was placed in a fixture which clamped one end of the piece rigidly. The other end of the sample was free. A small metal container was attached to the free end of the sample and
I
__
-
SCALE-PLOT OF CRACK LOCATION v s CRACK DEPTH CONDITIONS
0
cs.
103 Hours I09 Elongation 2 5 b p h m OIoi
5 IO 15 20 OZONE CONCENTRATION [ p.p.h.m)
Figure 3.
Crack Depth us. Ozone Concentration
Parts par hundred million at various exposure times
This experiment was not designed to show a ‘(critical elongation” where cracking is more severe, but to show the general trend. Separate samples were exposed in increments of 5% from 5 to 50%. This curve cannot be extrapolated to zero because there is a peak in the curve, the exact location of which is not known. The region between 0 and 5 % elongation is an interesting one in which to do further work. Crack Distribution. I t is generally well known that strain is necessary for ozone and weather cracking. Newton (6)and others
0
1
Figure 5.
CRA(
i
.
4 5 LOCATION (m.nJ
7
Scale Plot of Crack Location us. Crack Depth
V O L U M E 24, NO. 5, M A Y 1 9 5 2
82 1
sufficient lead dust wm placed in the container to stretch the sample 10%. This method of preparation was compared with a method in which a sample was stretched 10% and clamped in that position.
C C o n s l a n t Load m C o n r t a n t Defl'n.
1 ~
Figure 7 shows that ozone cracking and actual weather exposure as measured by the crack depth method are related. The shapes of the curves a+re very similar, but the values of crack depth are different because of the differences in ozone concentration in contact with the samples. It was shown previously that in the areas of low ozone concentration, a small change in ozone concentration makes a large change in the crack depth. -4s a result, the difference between actual weathering and laboratory ozone exposure was marked. At the time of weather exposure, the average atmospheric ozone content was 3.5 parts per hundred million of air. This lower ozone concentration has resulted in reduced weather cracking compared to the results obtained in the ozone chamber, where the concentration was 5.03 parts per hundred million of air. In order to show a correlation, the samples exposed to sunlight must not be compared to ozone-exposed samples. Sunlight exposure exerts a protective influence on weather-exposed samples which is not present in laboratory ozone tests. Shaded weatherexposed samples show a correlation with accelerated ozone tests. Exposure temperature must be taken into account when attempting to correlate accelerated exposure with service exposure ( 3 ) .
HMF BLACK
Figure 6.
Constant Load Deflection
TS.
Constant
The sample under constant load exhibited creep and a t the end of the exposure time showed greater extension than the sample
held a t constant deflection. Figure 6 shows the crack depth obtained by using the two methods on stocks containing equal volume loadings of different carbon blacks. Constant load application during exposure is more severe than constant deflection, BS shown by deeper cracking. However, the same general trends apply, regardless of which method is used. As cracking progresses under constant load, the local stress around the cracks increases as a result of reduction in cross section. Using the constant load method, the effect of stress relaxation is avoided and cracking proceeds more rapidly to a more severe end point. The constant load method of sample exposure has been proposed as 3 more sensitive test method for ozone exposure ( 4 ) .
0
10
I
20
I5
25
OZONE ( p . p h m )
Figure 8. Time and Ozone Concentration Required to Produce Ozone Cracking Equivalent to Actual Weathering for 3 Weeks' Exposure
The correlation of the laboratory method with actual service conditions is a great asset in reducing experimental findings to practice. A correlation exists between ozone crack depth produced in the laboratory and weather crack depth produced under natural Tveather conditions. Figure 8 shows a constant crack depth curve for a range of time and ozone concentration. The crack depth curve is for 0.4 mm., which is the depth obtained on the same stock exposed to natural weather for 3 weeks. This curve has proved valuable in determining laboratory ozone exposure conditions necessary to duplicate weather exposure conditions. ACKNOWLEDGMENT
TIME, HOURS Figure 7. Correlation of Ozone and .ictual Weathering
The author wishes to thank The Gates Rubber Co. for permiesion to present this work, E. F. Haeger for help with the experimental work, and Margaret Seaman for preparation of the graphs. LITERATURE CITED
RIost commercial rubber products operate under constant deflection in the areas exposed to ozone or weather. Stress relaxation playa an important part in ozone resistance of most products, and the beneficial effects of this condition should not be eliminated by the test method used. CORRELATION OF WEATHER AND ACCELERATED OZONE TESTING
iin experiment was made to determine the correlation between exposure in an ozone chamber and actual weathering,
(1) Crabtree, J., and Kemp, A. R., Ind. Eng. Chem., 38, 278 (1946). ( 2 ) Crabtree, J., and Kemp, A. R., ISD. EXG.CHEM.,ANAL.ED.,18, 769 (1946).
(3) Cuthbertson, G. R., and Dunnom, D., Ind. Eng. Chem., 44, 534 (1952).
International Organization for Standardization (190)Draft Report, 3rd Meeting. Akron, Ohio, October 1950. (5) Kewton, R. G., Rubber Chem. and Technol., 18, 504 (1945). (6) Powell, E. F., and Cough, V. E., Ibid., 19, 406 (1946). (7) Shaw, R. F., and Adams, 9. R., ha^. CHEM.,23, 1649 (1951).
(4)
RECEIVED for review September
1 2 , 1951.
.4ccepted Maroh 15, 1952.
