August 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
with such a material to counteract capillarity, which might conceivably cause a film of electrolyte between the gasket and the cap to reach the outer edge of the cap, a gasket that will not itself conduct an ionic current. Probably one of the polyethylenes or a film of paraffin would be most effective. Merely treating the surfaces with one of the many detergents may be effective, by virtue of its effect on surface tension. 3. Avoid entrapment of a film of food in the seam. ACKNOWLEDGMENT
Acknowledgment is made to A. D. Bowers, chief chemist, control laboratory, Campbell Soup Company, for the microdeter-
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mination of chloride and t o Jackson B. Hester, soil technologist, Campbell soup Company, for the determination of sodium by the flame photometer method* LITERATURE CITED
(1) Kohman, E. F., and Sanborn, N. H., IND. ENG.CHEIM., 20, 76
(1928).
(2) Stevenson, A. E., and Flugge, S. L., U. S. Patent 2,168,107 (Aug. 1, 1939). IND* ENG* CHEM** 754 (1909)* Ws (3) ' 1
R~~~~~~~jUly 19, 1949.
Cold Compression Set of ~
Elastomer Vulcanizates ROSS E. MORRIS, JOSEPH W. HOLLISTER, AND ARTHUR E. BARRETT Rubber Laboratory, Mare Island Naval Shipyard, Vallejo, Calif.
T h e significance of the cold compression set of elastomer vulcanizates as regards internal viscosity, second order transition, and tendency to crystallize is discussed. It is shown experimentally that the ability of a gasket to maintain a seal when compressed for extended periods at low temperatures can be foretold from compression set tests.
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'RAL years ago the Mare Island Rubber Laboratory proposed a cold compression set test for elastomer vulcanizates ( 1 2 ) in the course of work on t,he low temperature properties of rubbers for the Bureau of Ships. Subsequently, other governmental and industrial laboratories have adopted this test for the evaluation of rubbers a t low temperatures (8), and the test has appeared in a military specification for gaskets (14). The purpose of the present paper is to review the significance of c o l i compression set and to demonstrate the relation between cold set and the sealing ability of gaskets a t low temperatures. PROCEDURE FOR COLD SET TEST
The cold compression set test follows the procedure of the A.S.T.M. hot compression set test, method B ( I ) , except for time intervals, temperatures of conditioning and recovery, and load on the presser foot of the dial micrometer used for the thickness measurements. In the cold compression set test, the specimen is compressed at room temperature between chromium-finished plates and held a t constant deflection by means of spacer bars. The clamped specimen is immersed in a cold methanol bath or placed in a cold chamber within 5 minutes after compression. The specimen is released from the clamps a t the end of the conditioning period without removal from the cold conditioning medium. After allowing the specimen to recover in the cold medium for a definite period, its thickness is rapidly measured with a dial micrometer gage. The time intervals and temperatures of conditioning and recovery have not been definitely established for the cold compression set test, except that the temperature of recovery is always the same as the temperature of conditioning, and both are generally below 40' F. A 30-minute recovery period was used for the work reported here. It was found advisable to debrease the load on the hemispherical presser foot of the dial micrometer used for the thickness measurements from 3 ounces to l/z ounce in order to lessen the indentation of the rubber specimens by the foot. For example, a l/n-inch thick specimen of 35 Shore A hardness was indented 0.010 inch more when the 3-ounce load was used than when the '/$-ounce load was used. This greater indentation, of course, reduced the accuracy of the thickness measurement by a corresponding amount. It follows that the present A.S.T.M. hot compression set test would be more accurate if the thickness measurements were made with less weight on the presser foot.
THEORY
Cold compression set is not caused by the same phenomena in the rubber as hot compression set. Hot compression set is caused by plastic flow and sometimes by further vulcanization of the rubber while compressed ( 2 ) . Cold compression set is caused by slow rate of recovery due to high internal viscosity, and also may be caused by crystallization or by second order transition if conditions are favorable for either of these phenomena. In the case of rubbers which do not crystallize, cold compression set is due only to slow rate of recovery, which will be referred t o as the viscosity effect, or to second order transition. A test specimen of such a rubber, rpleased after compression, strives to recover its original unstressed shape during the recovery period at, low temperature. The effort put forth increases with the temperature of the rubber. I n other words, the recovery stress is a manifestation of the entropy of the rubber. The recovery of the specimen is retarded by its own high viscosity, but eventually the specimen recovers its original shape unless it is below i t s srcond order transition point. An understanding of this behavior can be gained by referring to Figure l , which shows a highly simplified mechanical model of rubber known as the Maxwell unit. The viscosity of the fluid in the dashpot of the Maxwell unit retards the recovery of the compressed spring, and the viscosity of this fluid, of course, rises as the temperature falls. When the fluid in the dashpot is frozen, the compressed spring cannot recover and a condition similar to rubber below its second order transition temperature results. In the case of rubbers which crystallize, the combined effects of high viscosity and crystallization are responsible for cold c o m p r e s s i o n set. Crystallization blocks full recovery of the deformed elastic network a s long - as the rubber is kept at a sufficiently low temFigure 1. Maxwell Unit perature. The ex-
t
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Figure 2.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Navy-Type Scuttle, Welded t o Tank, in Refrigerated Chamber
tent of recovery up t o the poiiit of blocl\ing depends upon the e\tent of crystallization; thri e is no recoveiy when crystallization has progressed sufficiently Of course, when the crystallized rubber is warmed t o a high enough temperature t o melt the crysI d s , the compressed specimen recovers its original shape, Crystallization can be visualized as the nelding together of some 01 all of the coils of the compressed spring in the h l a x ~ e l lunit. Crystallization is always faster when the rubber specimen is compressed or otherwise distorted (10, I d ) . In order to understand the relation between cold comparison set and sealing ability of gaskets a t low temperatures, one must first analyze the mechanism of gasket action. This mechanism is not coinplicated; it is axiomatic that the recovery stresi: everted by a compressed gasket against the bearing surfaces is iesponsible for its ability to seal fluid pressure. When this iecovery stress relaxes, sealing ability is lost to the same extent. If cold compression set can be correlated with stress relaxation, it follows that the cold compression set test can be used t o evaluate gaskets for low temperature service. I t should be emphasized that the stress relaxation referred to here is the gradual relavation which occurs after practical attainment of stress equilibrium in a cornpressed gasket. Theie is a temporary and comparatively rapid stress relavation whic,h begins immediately after a gasket is compressed, and which is not considered here. This latter phenomenon is particularly evident when the gasket is compressed while cold, and is related diiectl\to the viscosity of the rubber ( 3 ) . The recovery stress of a compressed rubber gasket is enliielv an entropy effect, as was pointed out previously. Any change 111 the compressed rubber which reduces its entropy, reduces thc sealing ability of the gasket. A gasket already compressed a t room temperature suffers stress relaxation while being cooled to L: low temperature because of decreased entropy. If the compressed rubber crystallizes, a major decrease in entropy occurs, arid therefore, a serious reduction in sealing ability takes place. The second order transition does not produce the same abrupt entropy decrease as does crystallization; but, nevertheless, the kiscosity of the compressed rubber rises to such a high level when its temperature falls below the second order transition point, that the effective recovery stress of the rubber becomes negligible. The viscosity of compressed rubber above its second order transition point does not interfere with its recovery stress, although of course it does interfere with its recovery rate. Thus, compressed gaskets, similar in all respects except for viscosity,
Vol. 42, No. 8
seal equally well unless, as is frequently the case, the bearing surfaces of the fluid container are not rigidly fixed and separate too much when the fluid pressure inc,reases. l n that event, a compressed gasket having high viscosity might not be able l o recover fast enough to maintain recovery stress against the bearing surfaces. The foregoing considerations lead to the conclusion that a rubber having high cold compression set, n.1iether causeil 1)y viscosity, second order transition, or ( stallization, is unsuitable for gasket service a t low temperatures. In this connection, it, is important to realize that the cold compression set value depends upon the length of time that the specimen is allowed to recover beforc measurement. If the thickness measurement n-erc rna.d instantaneously after release of the specimen from the clamping plates, all rubbers n-ould exhibit lOO'j', cold coinpression sct because all rubbers require time to start recovery. The authors have used a 30-minute recovery period in this work in order to permit differences in the recovery tendencies of the various rubhers t,o be clearly defined. There are other phenomena which become evident whcn compressed rubber has lost all tendency to recover because of second order transition or crystallization; these phenomena are a decrease in volume by cooling and a decrease in volume bv crj-stallizntiori ( 6 ) . They cause the cold compression set to rise above 100% and cause 3 coinpressed gasket t o leak a t practically atmospheric pressure. TESTS USIYG SCUTTLE
GR-S GASKETS. The inference that high viscosity, as measured by the cold compression set test, does not directly produce stress relaxation was confirmed by the results of sealing tests performed on two GR-S gaskets a t -20" and a t -35' F. One of these gaskets was manufactured commercially and had a Shore hardness of 46; the other gasket was made in the authors' laboratory and had a hardness of 47. (All of the hardness measurements made in this investigation employed the Shore A Durometer and m r e instantaneous readings.) The latter gasket had the following composition and cui
