812
A N A L Y T I C A L CHEMISTRY
experimental work, Fred Bruton for the fabrication of the calculator, and Howard A. Bewick for helpful criticism in the preparation of the manuscript.
(11) Henderson-Hamilton, J. C., and Laurie, A . J., J . Sac. Chem. Ind. (London), 64, 309-12 (1945). (12) Hughes, H. K., and hfurphy, R. R., J . Optical SOC.Am., 39, 501 (1949). (13) Kaiser, H., Spectrochirn. Acta, 2, 1-17 (1941). (14) King, Carl, J . Optical Soc. Am., 32, 112 (1942). (15) Levy, Saul, Ibid., 34,447 (1944). (16) Muller, R. H., IND.ENG.CHEM.,ANAL.ED.,13, GG7 (1941). (17) op1inger,~ . , I b i d . ,19,444 (1947). Ibid., 10,664 (1938). (18) Owens, J. S., (19) Owens, J. S., “Proceedings of Fifth Summer Conference on Spectroscopy and Its Applications,” h’ew Tork, p. 17, John Wiley & Sons, 1938. (20) Sampson, -4. M., Ibid , p. 8. (21) Schmidt, R.7 Ret. truu. ch@tn.,679 737 (1948). (22) Silberstein, Ludwik, J . Optical SOC.Am., 32, 474 (1942). . ~ 34, 689 (1944). (23) Sinclair, D. ~ 4Ibid., (24) Vanselow, A. P., and Liebig, G . F., Jr., Ibid., 34, 219 (1944).
LITERATURE CITED
Baker, E. A., Proc. Rw.SOC.Edinburgh, 45, 166 (1925). (2) Breckpot, R., Spectrochim. Acta, 1 , 137 (1939). (3) Brommelle, N. s., and Clayton, H. R., J . Soc. Chem. I d . (1)
(London),63,83-9 (1944).
Carlason, C. G., Jernkontorets Ann., 132, 467-84 (1948). (5) Chong, E. Y., Metallw’rtschaft, 22, 562 (Dec. 20, 1944). (6) Churchill, J. R., IND. EXG.CHEM.,ANAL. ED., 16, 662-70 (4)
(1944).
(7) Crosswhite,H. H., Spectrochim. Acta, 4, 122 (1950). (8) Dieke, G. H., War Production Board, Office of Research and Development, Rept. E-54 (Sept. 24, 1943). (9) Dieke, G. H., and Crosswhite, H. H., J . Optical SOC.Am., 33, 425 (1943). (10) Hale, C., Eighth Pittsburgh Conference on Applied Spectroscopy, 1947.
RECEIVED for review August 20, 1931.
Accepted February 6, 1962.
Physical Properties of Natural and Synthetic Rubber Materials at low Temperatures J . Z. LICHTMAN
AND
C. K. CIIATTEN
Material Laboratory, New Y o r k Nazal Shipyard, Brooklyn 1 ,
Although a ,number of elastomers resistant to low temperatures have recently been developed, relatively little work has been done in standardizing the instruments and procedures used in evaluating their low temperature properties. Accordingly, a survey of all such equipment and methods was undertaken in order to standardize appropriate ones and to determine the degree of correlation in data obtained in the tests. A comparison of data derived in evaluating typical stocks exposed at low temperatures and tests using a torsion wire ap-
I
N T H E past few years both government and industrial rubber laboratories have been actively engaged in developing elastomer materials that will function properly under low temperature service conditions. Extensive research programs carried out by these laboratories have resulted in the formulation of a considerable number of low temperature-resistant stocks such as those used in hose, rubberized fabrics, and gaskets. At the same time many different types of apparatus and procedures have been devised for evaluating the low temperature properties of the stocks. Each of these teste was, naturally, favored by the organization responsible for its development. This condition resulted in the adoption of a large variety of testing instruments, many of which were designed for use in evaluating the same basic properties, although by slightly different means. I n view of the importance of standardization of evaluation procedures to be incorporated in procurement specifications, the Bureau of Ships, Department of the Navy, authorized the Material Laboratory, New York Naval Shipyard, to investigate all known devices and procedures used in evaluating the low temperature properties of rubber materials and, if considered desirable, to modify or revise appropriate ones The over-all objective of the program was to select and standardize equipment and procedures considered to be the most suitable for incorporation in military procurement specifications. An analysis of the manner of employing the major number of rubber items in shipboard low
N. Y
paratus and a hardness indentation device confirms the existence of a mathematical relationship between flexural modulus and hardness indentation. The iovestigations indicate the feasibility of using either instrument to evaluate the stiffness or the hardness properties of elastomers over a range of exposure conditions. The small T-50 type specimens used with the torsion wire apparatus are well adapted for use in evaluating changes in stiffness of elastomers due to exposing the materials to solvents.
temperature service 4iows the following physical properties to be the most significant: (1) flexibility, or the magnitude of stress required to produce an observed degree of deformation; (2) compression set, including the rate and amount of dimensional recovery of a material after being held under constant deformation; and (3) brittleness or structural failure of a material under rapid deformation. The significance of these properties may vary from one application to another, each specification thus requiring the evaluation of the property or properties that are pertinent to the service performance of the material. REVIEW OF TEST PROCEDURES
A review was made of the methods used in evaluating the three classes of properties. The second property, compression set, is usually evaluated after the specimens have been aged a t elevated temperatures under constant deflection (8). A basically similar method has been adopted and standardized by the Navy for carrying out low temperature evaluations, a modification being made in that the dimensional recovery of the specimen is evaluated a t the test temperature and a t two time intervals-Le., 10 seconds and 30 minutes after the specimen is released from the clamping plates, In this manner both the rate and amount of recovery of the specimen are evaluated and the data so obtained can be used in differentiating between first- and second-order transition effects. The evaluation of the third property, brittleness, has like-
813
V O L U M E 24, N O . 5, M A Y 1 9 5 2 wise been more or less stctndardieed, although there are a number of different instruments designed to conform to the procedural requirements (6). Despite the differences between the several brittleness testers (6, 8, 14, 15, $0, S l ) , such as the msnner of operating the impact component and the number of specimens evaluated a t one time, the basic features of these instruments are standard. These standard features are the velocity and dimensions of the impacting component and the amount and manner of deflection of the specimen.
described (1.9) and were recently adopted as a tentative ASTM method (7). Although the Gehman tester showed many advantages over other types of flexibility devices, i t was not considered entirely satisfactory. The instrument was therefore modified in order to eliminate some of the elaborate accessory equipment, t o simplify the test procedure, and to improve the accuracy of the test. The modified apparatus is shown in Figure 1. The specimen rack was replaced by a permanently fmed lower clamp assembly, the upper clamp or grip being suspended from the torque wire as in the original instrument.' By this arrangement the ~ e r opointer setting and specimen span length can be easily set and will remain properly adjusted during the course of a test, A check on the zero setting of the instrument is made by mounting a metal bar of rectangular cross section in the specimen grips, making mre that the rotating head and specimen angle indicator me set t o zero, and noting the indicator reading after removing the metal bar from the grips. If the angle indicator remains a t zero under these conditions, no twist wm present in the wire and the instrument is properly adjusted in this respect. The distance between the specimen grips, which determines the span length, can be accurately adjusted by means of gage blocks.
Figure 1. G e h m a n Torsional Test Apparatus Material Laboratory modification
A very considerable number of flexibility tests for evaluating the first property havk been developed and reported. These tests are based upon various methods far producing deformation. Some require deformation of the specimen by bending, either as a cantilever beam (6,10, $S) 01as a centrally loaded, end-supported beam (11, 17). Other tests require that the specimens be subjected to tensile elongation (16), compression-deflection ($), indentation under variable or constant loads (1, 4, 9, 1$), or torsional deformation ( 1 0 , ' l S ) . Some of the apparatus and procedures represent considerable improvement over earlier devices, while others are in use in individual laboratories on the hasis of precedence or by the grace of origin only. For these reasons, it was necessary to conduct individual investigations on flexibility, indentation, and similar specimen deformationd procedures. After this work was completed, most of the testing devices were eliminated from further consideration on the hasis of inaccuracy, complexity of operation, or excessive size or oost ofthe equipment. Of the remaining devices, two were selected for further study.
Figure 2.
Modified G e h m a n Torsional Test A p p a r a t u s
Aa used in immersion tests
TORSIONAL APPARATUS
A Dewar flask containing a mixture of methanol and dry ice chips waa used in cooling specimens exposed t o short-time conditioning tests, as shown in Figure 2. An accurately calibrated Weston thermometer was used in lieu of the original temperatwemeasuring equipment and a stop watch was substituted for the electrical timer-light signal. A spirit level was attached t o the base of the instrument to permit vertical alignment of the specimen grips and 8. mirror was mounted over the torsion head aasembly to facilitate reading the deflection soale when the tests are conducted in a thermostatically controlled low temperature conditioning cabinet. This cabinet, which is shown in Figure 3, is
The first of these instruments was the torsional apparLratus de. veloped by S. D. Gehman of the Goodyear Research Laboratory. The instrument and procedure &B originally designed have been
Ls shown in Figureh, the specimen-conditioning chamber is in the upper half of the cabinet, while the lower part contains wire-
814
A N A L Y T I C A L CHEMISTRY
mesh Screen trays for holding chunks of dry ice. The blower shown in the righehand part of the dry ice section is thermstatically controlled t o circulate air from the conditioning chamber down through the stack of trays on the left-hand side and up through the trays on the right. The fan looated toward the top of the specimen-conditioning chamber is operated continuously during a test in order to mmre attainment of thermal equilibrium conditions therein. The Gehman instrument was operated in the working chamber of the cabinet without the Dewar flask in making long-time specimen conditioning tests.
Equation 1 (23)expresses the basic relationship between the torque, T,the cross-seotional dimensions of the specimen. a and b,
t o be conitant and havine
a v d u e of 0.5 for elastomers. then'the
EXPERIMENTAL PROCEDURES AND CALCULATIONS
In order t o evaluate the effects of short-time law temperature exposure on different elastomers, a specimen of rectangular CIOSB section is mounted in the jaws of the instrument and the entire ~
~~~~~
~~~ ~~~~~
I
torsion head of the tester is rotated and the reaulting deformation of the specimen is observed. When the same sDecirncn is used
Figure 4,
Interior View of Low Temperature Test Chamber
Table I. Formulations of Mare Island Rubber Laboratory
Figure 3.
Low Temperature Test Chamber Material Isboretow deaipn
Comoounds GR-SStock
Hevsa Stoak Smokad sheets Zino oxide Stearic acid Cottonseed oil Helioaone Age Rite resin D Capta. Aitax Tu&& Sulfur P-33 (FT)
E-13-92 100.0 5.0 1.0
2.5 3.0 1.0
0.5 0.5 0.5 0.75 1.0
Total Perbunsn Stock
115.75 E-194488
E-162489 GR-S 100.0 Zinc oxide 5.0 Phiiblsck A (HMF) 6o.a Heliovone 3.0 Tributoayethyl phosphate 5.0 Plnaticirer SC 5.0 Diootyl phthalate 5.0 Diisobutyl adipste 5.0 Thionex 2.0 Diphenylguanidine 0.4 SulIur
Total
GR-M Stock
I
6 . 0
181.0
E-156-315
in a series of tests, the relative changes in stiffnessof the material at increasingly lower temperatures may he determined without reference t o the specimen dimensions. In order, however, t o determine numerical or relative values of moduli of different materials under the same conditions, the specimen dimensions must be measured and the moduli calculated BS shown below. Total
GRI Stock
E-34-104
155
0
Thiokol FA Stock E-53-17 Thiokol FA 100.0. Zino oxide 10.0 Pelletex (SRF) 65.0, Stcario acid 0.5 Plhstioirer SC 5.0 Tributaxyethyl phosphate 5.0 Altsx 0.3 DiDheylguanidine 0.1 Total
-
185.8
V O L U M E 2 4 , N O . 5, M A Y 1 9 5 2
815
A series d stocks including neoprene, Buna S, Perbunan-26, Hevea, Butyl, Thiokol-FA, and silicone-type polymers having formulations indicated in Table I was eva.lua.tted using the modified torsional tester. Short-time exposure tests were made in which specimcnr were conditioned in cold methanol a t various low temperatures down to -100" F. In the case of the longtime exposure evaluations, the specimens were conditioned a t -20 "F. in the dry ice cabinet, the tests heine: made after specimen exposure periods of from 1 t o 94 hours.
10.
TORSIONAL TEST RESULTS
The modulus proportionality factors for the aeversl materials were calculated from the data obtained and are shown in graph form in Figure 5. The factors platted here were determined in the shoretime exposure tests. The Perhunan%fi stock exhibits
(n a
u
i
2
1.0.
>
t
zF
x
(r
a 0.1 VI
2
0
z
-do .do
01
20
.A
b
-20 20 TEMPEHATURE,DEG. F
a, A
eA
Figure 5. Torsional Modulus Properties of Elastomers at Low Temperatures Conditioning time 3 to 5 minutes
in lieu of radians, E, Equation 4 is obtained. This equation w&p used in ealmlating the modulus of elasticity of stocks tested In the mesent inve8tieation. As DI'eViOUSh indicated, it may not he ..- ~~~~~.~~~~~~ ~. posure. Using the same or ide5ical ~spekmensin the various tests. Eauation 4 may he simplified as indicated in Equation 5, K ~
~~
~
~~~
by a simple cdcdatibn after n&ing the specimen twist,#. The change in modulus proportionality factors determined under different conditions using the 8ame or equivalent specimens would then indicate the change in stiffnessof the materials.
at
i , ,
SZ I'
BUNA-S
01
0
I 10
20
l
~
30 40 5 0 60 70 CDNMTIONING TIME, HOURS
Figure 6.
80 90
(
100
~
Torsional Modulus Properties of Elastomers at Low Temperatures Gnditiodng fsmpsr.fura -20'
F.
Figure 7.
Admiralty Rubber Meter
Specimen eubjected to major load indentstion
pronounced stiffening at a. temperature of ahout -20'F., while the neoprene and Thiokol-FA materials show a large increase in stiffness upon reaching a temperature of ahout -40'F. The Bun& and Butyl compounds are roughly midway in the scale of shoretime exposure serviceability, while the Hevea and silicone materials show very little increase in modulus until reaching exposure temperatures of shout -50" and -80" F., respectively. The results of the shoretime conditioning tests, however, do not serve as a satisfactory index of the suitability of an elastomer for continued u8e a t low temperatures. As shown in Figure 6, the Thiokol-FA stack exposed for 94 hours a t -20'F. exhibits a rapid increase in the modulus proportionality f a d o r within the firat 20 hours of exposure, the value being almost constant after 1 ~ ~ ~ ~ this time. The Hevea compound shows a very pronounced and continued increase in modulus during the entire exposure period, no doubt due t o the occurrence of crystallization of the material. Also, the Perhunan-26 stock exhibits a high hut relatively constant value for modulus proportionality factor, while the neo-
816
ANALYTICAL CHEMISTRY
,prene elastomer shows a fairly low but graduitlly increasing .modulus value during the entire exposure period. The remaining stocks-ie., silicone, Bun* S,and Butyl-do not show any kignifioant change in modulus under the stated conditions of test. The ratio of the modulus factor of a specimen determined a t a particular base temperature t o that determined after the speci-
men has been exposed a t Some low service temperature may be used as an index of the relative suitability of the material under low temperature servioe oonditions. The selection of the temperature and period of exposure will depend largely on the expected service requirements for the material. HARDNESS INDENTATION APPARATUS
The second and last instrument to be disoussed is the Admiralty rubber meter shown in Figure 7. This constant load indentation instrument was constructed in the Material Labor& tory and is in compliance with the British Standards method ( 9 ) . EXPERIMENTAL PROCEDURES AND CALCULATIONS
The relationship between oonstant load hardness indentation and the flex modulus of rubber materials has been investigated by Scott (18, 19) among others. Scott found that this relationship may be expressed by Equation 6.
In Equation 6, A is a constant determined by the physical characteristics of the instrumentnamely, the indentor and the major load. It follows from Equation 6 that, for n particular instrument using the same indentor and major load in all tests, n modulus proportionality factor equal to E / A may be expressed by Equation 7.
UOWLUS PROPORTIONALITY FACTOR.xIO? ADMRALTY TESTER
Figure 8. Relationship between Flex Modulus and I n d e n t a t i o n Modulus at 75' F.
'.\ \ -v-
D
Figure 10. C h a n g e o f FlexModulus with Conditioning T e m p e r a t u r e of Various Elastomers
Figure 9. Admiralty I n d e n t o m e t e r
Hardness indentation tests were made a t 75 'F.usingepecimens of the compounds previously discussed. The modulus proportionality factors obtained by use of the term (180+)/$ in the oane of the GehmtLn appmatus and the term l/H'.' in the case of the hardness tester ,were then calculated and plotted as shown in Figure 8. In view of the linearity of the graph, which c o n h a the
817
V O L U M E 2 4 , NO. 5, M A Y 1 9 5 2
sodium-catalyzed 75/25 butadiene-styrene material, an unplasticized Perbunan-26 compound (To. 394-S-7) and the 14°F. polybutadiene compound all showed considerably larger increases in modulus a t progressively lower temperatures than the other materials, which included a plasticized Perbunan-26 stock ( S o . 394-S-12), a 122 F. polybutadiene stock, and plasticized and unplasticized GR-S materials. The plasticized GR-S and the plasticized 122 F. 85/15 butadiene-styrene compounds showed the lowest increases in modulus.
106 I
-
CONOITIONING
T E M P E R A T U R E RANGE 75OTO 50?, I5 SEC INDENTATION VS IO SEC. F L E X MoouLus 6 0 S E C . INDENTATION VS. 60 SEC. FLEX MODULUS F L E X APPARATUS- GEHMAN TORSION TESTER I H D E N T O M E T E R MAT.. L A B . A D M I R A L T Y
d
O
O
v)
HARDNESS TEST RESULTS
102 0
0.4 0.6 0.8 1.0 1.2 1.4 HARDNESS INDENTATION, MM.
0.2
1.6
1.6
Figure 11. Relationship between Constant Load Hardness Indentation and Flexural Moduli of I-arious Elastomers
previously expressed relationship, further tests were conducted to investigate the existence of this relationship a t low temperatures. The Admiralty instrument was modified as shown in Figure 9. The major load was changed t o 1000 grams and the indentor to 0.125-inch diameter, the instrument thus being in conformity with ASTM standard method ( 2 ) and federal specification ( 1 2 ) . A cylindrically bored weight was placed on the specimen concentric with the indentor to ensure contact between the specimen and the support plate. .4 series of 12 stocks prepared by the Office of Rubber Reserve and having formulations indicated in Table I1 was used in the tests in which both Gehman torsional data and hardness indentation data were obtained using the modified Gehman and Admiralty instruments. Specimens were conditioned for 94 hours a t temperatures ranging from 75' down to -50' F. The values for flex moduli of the various stocks were calculated and are presented graphically in Figure 10. The
The flex modulus and hardness indentation data of the Office of Rubber Reserve compounds determined a t comparable intervals after deformation were plotted as shown in Figure 11. The semilog plot shows the relationship between flex modulus and hardness indentation to be represented by a hyperbolic function of the form y = czn. A log-log plot of the data over the range of indentations from 1.7 to 0.1 mm. and of flex modulus from 380 to 20,000 pounds per square inch is shown in Figure 12. Evaluation of the equation on the basis of the plot shows the relationship to be expressed by the equation E = 820/H1,36. This equation is similar in form to that determined by Scott, the exponent being very close, and it shows the basic equivalence of flex stiffness and resistance t o indentation. The relationship appears to be valid for various types of compounds and over a relatively wide range of specimen exposure conditions. CONCLUSION
A torsional apparatus and a hardness indentation tester have been found to be essentially equivalent for use in evaluating the stiffness characteristics of elastomers over a range of low teinperatures. The torsion apparatus, requiring the use of relatively 106
1
CONDITIONING
TEMPERATURE RANGE 75' T O -50-F, I5 SEC. INDENTATION VS. IO SEC. FLEX MODULUS 6 0 S E C . INDENTATION V s 6 0 S E C . F L E X MODULUS F L E X APPARATUS - GEHMAN TORSION TESTER INDENTOMERS- MAT. L A B . - ADMIRALTY
v)
Table 11.
Polymer Statex B Zinc oxide Sulfur Altax Stearic acid PBNA Final weight
Formulations of Office of Rubber Reserve Compounds 1
2
Xatural Rubber
GR-S X-539
100
100
40 5 3 0.75
Methyl Tuads Sulfur Final weight
XP-148
100 40 5
3
2
- 1-. 5 - _ _ _ 153.25
Polymer Ststex B Zinc oxide Sulfur .41t8X Stearic acid Final weight 10
3 1 2 2 O F. Polybd
40 5 2 1.75
148.75
6
GR-S Polymer Philblack A Zinc oxide Heliozone Flexol TOF
1
Na 75/25 Bd/S 100 40
5 2 1.25 3 151.25
7 Perbunan 26 100 40 5 1.25 1.75 148.00
167.8
Pol bd
SlP?4-6 100 40 5 2
2 3
3 1.5
1.25
1.5
151.5
3
151.5
8 85/15 Bd/S XP-138 122'F. 100 40 5
2 3 1.5 151.5
151.25 9
80/8/12 Bd/I/S 41'F. 100 40 5 2 3 1.5 151.5
- -
11 85/15 Bd/S (XP-138) 122' F. 100 Polymer 100 40 StatexB 75 5 Zincoxide 5 1 Plastolein 905C 10 20 Dicapryl sebacate 10 0 . 8 Stearic acid 1 1 Methyl Tuads 1 Sulfur 1
-
5 Xa
4
14O F. Polybd XP-169 100 40 5
12
Perbunan 26 Polymer 100 StatexB 75 Zincoxide 5 Adipol BCA 10 Dibutyl sebicate 10 Plasticizer 3425 10 Stearic acid 1 Thionex 0.3 Sulfur 1.5 203,O 212.8
-
I .l
.I3
I
l
l
I
I
I l l l l
.23 .3 .4 .5 .6 .7 -8.91.0 HARDNESS INDENTATION, MM. .¶
I I 1-3 2.0
Figure 12. Relationship between Constant Load Hardness Indentation and Flexural Moduli of Various Elastomers
small specimens, will facilitate carrying out various conditioning cycles on a material such as liquid immersion or atmospheric aging. The indentometer, on the other hand, permits the employment of larger size specimens or even samples such as relatively thick gasket stocks. There are, then, individual advantages in each apparatus which would determine the choice to be made in selecting a test method for a particular specification. ACKNOWLEDGiMENT
The authors wish to thank T . A. Werkenthin of the Bureau of Ships, Navy Department, Washington 25, D. C., for his continued interest and support in this and other programs of low temperature work on elastomers conducted by the Material Laboratory.
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., I n d . Eng. Chem., 16,
(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) (15) (16)
Graves, F. L., India Rubber World, 113, No. 4, 521 (1946) Graves, F. L., and Davis, A. R., Ibid., 109, No. 1, 41 (1943). Greene, H. L., and Loughborough, D. L., J . Spplied P h y s . . 16,
(17) (18) (19) (20)
Liska, J: Ind. Eng. Chem., 36, 40 (1944). Scott, J. R., J. Rubber Research, 17, No. 1, 1 (1948). Scott, J. R., Trans. Inst. Rubber Ind., 11, 224 (1935). Selker, M. L., Winspear, G. G., and Kemp, A. R., I n d . Eng.
No. 1. 3 11945).
\e.,
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).
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).
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 to 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 groups indicative of the results they yield-judgment, indirect 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 to 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
