Low Temperature Behavior of Silicone and Organic -5-
IC. E. POLRIANTEER, P. C. SERVAIS, i i K D ' G . 31. I(ION1CEE Dou; Corning Corp., Midland, Mich.
T
H E low temperature behavior of elastomers is complex and results from a composit,e of superimposed phenomena. These have been studied by many authors working principally with organic elastomers, both natural and synthetic. Boyer and Spencer ( 4 ) treat second-order transition effects in rubber and include a large bibliography covering the subject. Kood (9) thoroughly covers the subject of crystallization phenomena in rubbers and also includes an excellent bibliography. BeattJ- and Davies ( 2 )discuss time and stress effects in the behavior of rubber a t low temperature. They include a brief review of the general principles involved. Because of the inherent tendency of organic elastomers to solidify and become brit,tle a t low temperatures, invest,igations below dry-ice temperatures have not been necessary. Hon-ever, the development of silicone rubbers or polysiloxane elastomers necessitat,ed ext.reme low temperature investigations to gain knowledge of the character of this new elastomer. IVeir, Leser, and Wood ( 8 ) investigated crystallization and second-order transitions in silicone rubbers by interferometric methods. Kood et al. found a second-order transition temperature of about -189' F. for all types of silicone rubber. Silicone rubber is based on an inorganic skeleton of alternate silicon and oxygen atoms with heat stable organic groups bonded to each of the unoccupied valences of the silicon atoms yielding a sat,uratedpolymer. This paper reports on t x o types of silicone rubber stocksi.e., Type I that crystallizes a t about -76" F. and Type I1 t,hat crystallizes a t about -112' F. Type I is exemplified by Silast'ic 160 and Type I1 by Silastic 250 and 6-160. The purpose of this paper is t o describe briefly the special apparatus used in obtaining stress-strain curves a t temperatures down t o -130" F. or belon.. The modulus data so obtained give a more complete picture of the behavior of silicone rubber a t low temperatures. These data are compared with those for natural rubber and GR-S determined on the same apparatus. The difference in low temperature behavior between silicone rubber and organic stocks leads to a suggested short-term aging history technique for silicone rubber to replace long-term aging.
foam with aluminum foil between the polyst,yrene foam and the plywood exterior. A door with a triple-pane window ( l Z 3 / , X 13/a inches) allowed observation of the specimens during the test. Vaporized liquid nitrogen was used as a coolant for the test chamber. Circulat'ion was maintained bj- a fan placed in a partition which ext,ended almost the full length of the chamber, about 1 inch from the back wall. The partition diyided the test chamber in two long cells, one wide cell and one narrow cell. The fan IWP positioned in a round opening a t one end of the part,ition and a rectangular opening (the height of the chamber) was in the opposite end. The fan circulated the air and coolant' down the chamber around the partition and then through the fan opening for rccirculation. This gave uniform circulation and maintained a uniform temperature t,hroughout the chamber. A pressure IYas created in t,he liquid nitrogen flask wit,h the aid of a dry nitrogen cylinder, thus forcing the liquid nit'rogen up a tube leading t,o the cold chamber. The liquid nitrogen vaporized while en route from the liquid nitrogen flask to the cold chamber and therefore entered the chamber as a gas and not as a liquid. This had a definite advantage since it allowed the cold nitrogen to be circulated readily and also sxept, moisture out of the chamber so that t,he equipment, particularly the window, did not frost over on the inside of the chamber. The temperahre in the cold chamber was controlled by the pressure over t,he liquid nitrogen, which determined the amount of vaporized nitrogen entering the chamber.
APPARATUS AND EXPERI.MENT4L PROCEDURE
il special cold chamber was designed for a Scott tester (Model IP-4 Tensilgraph) which records the load in pounds and actual stretch on a 101/2 X 12 inch tensilgram (Figures 1 and 2 ) . The cold chamber was 22 inches long, 7 inches high, and 9 inches wide. The insulation consisted of 1 i n c h of p o l y s t y r e n e
Figure 1. Apparatus for Measuring Stress-Strain Properties a t Low Temperatures
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INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1952
TABLE I.
FORhlULAS FOR
Natural Rubber (R-860) No. 1 smoked sheet 100.00 Zinc oxide 20.00 Stearic acid 3.00 Sulfur 3.00 Santocure 1.25 EPC black 45.00 Indonex 6 381/2 2.00 Cumar M H 2 1 / 1 5.00 Heliozone 1.50 Agerite resin D Total 182.75 Cure 12 min./307O F.
CROSS SECTION OF PLAN VIEW A
2.00
I
ELEVATION
I
TESTRUBBERS
GR-S (S-9602) GR-S-50 Zinc oxide Stearic acid Sulfur Santocure E P C black DBP Heliozone Argerite resin D Total Cure 20 min./307'
100.00 4.80 2.00 2.00 1.50 55.00 10.00 3.00 1.50 179,OO
-
F.
perature for 15 t o 45 minutes t o stiffen the samples. The test temperature was then held for a t least one-half hour more before testing. The organic samples were not given a special history but instead were held a t test temperature until temperature equilibrium was attained. This was from a minimum of 15 minutes to a maximum of 1 hour before testing. Following this short-term history for Silastic the actual tensile test was performed according to the prescribed method for the Scott IP-4 tester. The load in pounds was read directly from the tensilgram; the elongation was recorded as actual stretch, which included some elongation of the shoulder as well as t h e neck of
OF COOLING SYSTEM 1.
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Cooling System
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The temperature was measured with a copper-constantan thermocouple in conjuction with a direct. temperature reading potentiometer. The thermocouple was positioned very near the test sample. A low temperature thermometer was used as a constant check on the thermocouple. The test specimens were tensile bars cut according t o ASTM D 412-491'. The bars were mounted in the jaws with the aid of long-nosed tongs and a screw driver. Approximately 30 seconds were required for the entire process. Speed in changing samples was essential t o minimize gain of temperature in the chamber. The silicone rubber stocks tested were Silastic 250, cured 24 hours at 480" F., Silastic 160 and 6-160, both cured 4 hours a t 480" F. The other stocks tested were natural rubber (R-860) and GR-S (S-9602), obtained from the Yale Rubber Mfg. Co., and a stock (R-10567) formulated by the Connecticut Hard Rubber Co. t o meet Aeronautical Materials Specification 3204B. The formulations for the natural rubber and GR-S stocks are given in Table I. A special short-term history, which imparts to the sample the near equivalent t o long-term history at a particular temperature, was given t o each Silastic sample before testing at a designated temperature. Also, this special history gave added assurance that the samples had reached temperature equilibrium. Justification of this short-term history will be considered later. The short-term history consisted of holding the samples at a conditioning temperature of whatever was necessary in the range of - 130" to - 166' F. until they were stiff before raising them t o the test temperature. It was necessary to maintain the conditioning tem-
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Figure 3. Stress-Strain Curves a t Various Temperatures for Silastic 160
the tensile bar. This was corrected for by a conversion factor, which was determined for each type of material by observing and measuring the per cent elongation of the neck of the tensile bar corresponding t o varying degrees of actual stretch on a representative number of samples. EXPERIMENTAL DATA
Stress-Strain Data. Figures 3 t o 8 give the stress in pounds per square inch versus strain in per cent elongation at different temperatures for Silastic 160, 6-160, 250, natural rubber (R-860), GR-S (S-9602), and R-10567, respectively. These curves give a quick survey of the effect of low temperatures on the modulus. I n all the Silastics the tensile strength increases without sacrificing elongation at low temperatures that are above the extreme stiffening temperature of the stock. For the organic stocks the
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 44, No. 7
TABLE 11. MODULL-s RATIOS ~ ~ 2 OF 5 % Alaxrmnr ELONGATION AT 4-75' F. Temp.,
'
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+75
-22 - 40 -58 - 67 - 76 - 85
Natural Rubber 1.00 1.83 2.30 3.03 4.58 Stiff
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Figure 4.
Stress-Strain Curves a t Various Temperatures for Silastic 6-160
limit of the Scott IP-4 was often reached before the samples reached their ultimate tensile elongation properties. The Scott L-6 was used when needed to obtain ultimate room temperature properties. X o extrapolations of these curves were attempted. Because of the difference in tensile elongation properties between the silicone elastomers and organic elastomers, i t was decided to choose an approximately equivalent elongation value a t which the relative stiffening of these stocks might be compared. One quarter of the ultimate elongation a t +75" F. for each stock was chosen as a basis of comparison. These fractional elongation values then served as the elongations to determine the modulus and then in turn the modulus ratios. The value of the stress in pounds per square inch of original cross-sectional area required to give an intermediate elongation is defined as the modulus a t that elongation. hlodulus ratio may then be defined as the ratio of the modulus a t test temperature t o the modulus a t +75" F. The niodulus ratio serves as a measure of the relative stiffening and is sometimes referred to as relative modulus rather than modulus ratio. The modulus ratios determined in this manner are given in Table 11. A comparison of the relative stiffening ning be made by comparing the modulus ratios of the different stocks. For example, a t -67' F. the modulus of the natural rubber has increased 4.58 times the modulus a t +75' F. The GR-S has increased 7.23 times; Silastic 160, 2.47 timrs; Silastic 250, somewhat less than 1.50 times; and Silastic 6-160, 1.72 times their respective modulus values a t 175'' F. This demonstrates the fact that the Silastic samples do not stiffen to the extent that natural rubber and GR-S do even a t the temperature of the above example. V h e n the data in Table I1 are plotted (Figure 9), the temperature range a t which the respective stocks stiffen to an appreciable extent may be observed. This extreme stiffening region for Silastic 250 appears to be between - 112' and - 130" F., for Silastic 160 between -60' and -SOo F.; for natural rubber between -55" and -70' F.; and for GR-S between -25' and -70' F. In obtaining these results the Silastic samples were given the special short-term history previously mentioned. It is believed
Silastic 160
Silastic 250
Silastic
1.00
1.00
1 .00
. .. ...
..
..
i:i7 5:60 7.23 Stiff
2.47 4.36
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.. ..
.. .. *.
-112 130
0 '
GR-8 1.00 2.57
1.95
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1:s9
2.06 5.86
2.32 7.61
that these data for Silastic are indicative of results that would be obtained on long-time aging at these temperatures. On the other hand the natural rubber and GR-S samples were short-term aged a t test temperature only and do not demonstrate time effects on either crystallization or plasticizer t o an appreciable extent. Hence, the Silastic data are indicative of long-term aging while the data on the organic stocks are only indicative of short-term aging. Justification of the use of the short term history technique is based on comparisons of extended aging data a t some of these temperatures. Silastic samples were aged 36 days a t -110" F. without increasing the modulus over values obtained from special short-term history. Similar results were obtained when samples were aged 4 days a t -85' F., 1 day a t -67' F., and 1 day at -22" F. Weir, Leser, and Wood ( 8 ) , using an interferometric method, reported that Silastic does not exhibit crystallization or melting when alternating cooled and heated cyclically a t temperatures slightly above optimum crystallization temperature. This is in contrmt to the behavior of natural rubber under similar circumstances (9, I O ) . The data presented by Weir, Leser, and Wood further indicate that crystallization of Silastic is a rapid process in contrast to the crystallization of natural rubber studied by Bekkedahl and Wood ( 3 , I O ) . These facts help explain why the special short-term aging technique for Silastic appears to be an
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Figure 5.
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Stress-Strain Curves at Various Temperatures for Silastic 250
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Figure 6. Stress-Strain Curves at Various Temperatures for Natural Rubber R-860
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Figure 7 . Stress-Strain Curves a t Various Temperatures for GR-S S-9602
adequate substitute for long-term aging. First, it points out that crystallization in Silastic is a very rapid process compared to that in natural rubber. Secondly, it points out that the range of crystallization is more restricted than that of natural rubber. It would probably be adequate then to hold the Silastic samples a t test temperature from 30 to GO minutes, omitting the lower temperature preliminary step. The reason for adding the preliminary low temperature stiffening step was to ensure that crystallization effects would be as complete as possible. Torsional Stiffness Data. The stress-strain data do not show the speed with which silicone rubber crystallizes. A torsional test may be used to illustrate this trait. A Gehman cold flex test as described in (6) was used to obtain low temperature data on Silastic 250. The Gehman apparatus when used on short-term tests (5-minute induction times) indicated the vicinity of the second-order transition temperature and also gave an indication of the speed of crystallization in silicone elastomers. The results substantiate the discussion in the foregoing paragraph. Only one specimen was run a t a time so that the 5-minute induction period a t each temperature could be held accurately and the tests kept on a short-term basis. Similar runs were made on natural rubber (R-860) and GR-S (S-9602). These results are shown in Figure
10. The Silastic 250 sample was cooled rapidly to a temperature of -202' F., held for 5 minutes, and the torsional stiffness measured. The temperature was then raised to -166' F., held for 5 minutes, and the stiffness measured. Both temperatures yielded a degree twist of one indicating that the sample was extremely stiff. This is what would be expected since these temperatures are close to the second-order transition temperature, Tm,found by Weir, Leser, and Wood (8) to be around -189.4' F. This is the temperature region in which rotation of polymer segments is a t a minimum. As the temperature was then raised quickly again t o -148' F. and held for 5 minutes, the sample had relaxed a great deal. The temperature had been increased so that the polymer segments had more freedom t o rotate, and the Silastic had then taken on rubberlike characteristics. The temperature was next raised to -130" F., and the sample, when tested as be-
0: NO BREAK 0
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Figure 8. Stress-Strain Curves at Various Temperatures for R-10567 (AMS 3204B)
fore, had relaxed more. As the temperature was again raised, this time to -112' F., the Silastic had stiffened considerably, even in the short 5-minute induction period. Evidently, this temperature was close to the optimum crystallization temperature for Silastic 250 as demonstrated by the speed of crystallization. The next temperature demonstrated that a little more crystallization and probably some melting of crystals had taken place. As the temperature was raised to -76' F., melting
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Figure 9. JIoclulus Ratio
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of the crystallites approached completion since a t -67" F. t,he Silast,ic had relaxed considerably. I t is important to realize that if 30-minute induction periods had been used instead of the &minute periods, the described curve would not be obtained. Instead of a n effective supercooling with little crystallization, allowing a relaxat,ion before the optimum crystallization t,emperature was reached, the Silast,ic 250 sample gave continuously low twist values a t the extreme low temperatures and relaxed only after the melting of the cryst:ils, as shown by the curve following the dotted line in Figure 10. This is true because the sample had enough time during t,he 30-minute induction periods for enough cr@als t o be formed t o render the sample stiff even below the optimum temperature of crystallization. Hence, time of induction is important, in Silastic to observe the phenomenon. These torsional test results compare fairly well with the modulus data from tensile elongation curves. For example, t,he torsion curve indicates optimum crystallization temperature to be around -112' F. Figure 9 indicates that somewhere between - 112' and -130" F. Silastic 250 has a rapid increase of modulus. The two sets of data also agree fairly vie11 for natural rubber and
GR-S. Weir, Leser, and Wood ( 8 ) found a n opt,iinum temperature of
TEMPERATURE IN 'f.
Figure 10. Twist
Z.S.
Temperature
Vol. 44, No. 7
crystallization of around -103" F. (-75" C.) for Silastic 250as compared to - 112' F. (-80' C.) found by the authors of this paper. Since K e i r et al. completed their 1%ork an improvement in the low temperature properties of Silastic 260 has been made, and this accounts for the lower optimum crystallization temperature reported here for Silastic 250. Recovery Data. It would be helpful to design engineers to know something about the resilience of Silastic at low temperatures compared t o natural rubber and GR-S.One method of obtaining resilience evaluation Tvould be to compress the samplrs. subject them to low temperatures for 24 hours, and measure the recovery after release a t test temperature. Juve and Marsh ( 7 ) reported results of this nature for pol>butadiene, and butadiene-styrene giving per cent compression after recover) periodb of 2, 24, and 168 hours. Hot?ever, per cent recovery after verT short release periods-i.e., 10 seconds-and also a 30-minute value as a measure of the slow recovery was accomplished by the follom ing technique: Vise grips \\ere modified to give two parallel plates with such separation that the samples were given 2547, compression. The vise grips gave the instantaneous release that was desired. The results for the 10-second release and 30-minute release are shown in Figures 11 and 12, respectively. The curves showing the per cent recovery 10 seconds after I elease give an excellmt indication of the retained resilience of the stocks a t low temperatures. Silastic 250 has SSYc recovery a t -62" F. as compared to 947, at 75' F. Hence, it has lost verj little of its room temperature resilience. Natural rubber and GR-S, on the other hand, both show a decrease in per cent recovery of from 85 to 90 percentage points. n'one of the curves of Figures 11 and 12 has been corrected for thermal effects on the vise grips and samples. The short time release values may be compared with the usual 30-minute release period in Figure 12. After 30 minutes release natural rubber and GR-S have recovered somea hat but still do not compare v d h the Silastic 250. THEORETICAL DISCUS S I 0 li
The foregoing data indicate that silicone rubber, particularly the special low temperature Type I1 exemplified by Silastic 250 and the Silastic 6000 series, is not affected over the low temperature region nearly so much as natural rubber or GR-S. This is undoubtedly due t o the difference in the molecular structure of silicone and organic polymers. The silicon t o oxygen t o silicon chain structure leads t o a polymer that is not affected by temperature as much as corresponding organic polymers. For most of the silicone rubbers and specifically most of the Silastic compounds, methyl groups constitute the heat stable organic groups attached to the silicon atoms. This gives a uniform, well-ordered, saturated polymer. However, when resilient behavior a t extreme low temperature is desired some of these methyl groups must be replaced by more bulky groups such as ethyl, phenyl, or chlorophenyl. ThePe bulky groups break up the regularity of the polydimethylsiloxane chain and lower the crystallization temperature. Certainly the ease of rotation of organic groups attached to the silicon atom play an integral part in low temperature characteristics. The energy barrier hindering internal rotation of methyl groups attached to the silicon atom in tetramethyl silane was shown by Aston ( 1 ) to be 1300 calories as compared to 4540 calories for analogous rotation in neopentane. The rapid crystallization in silicone rubber suggests that the well-ordered silicone polymer may consist of many like kinetic units, most of which require about the same free energy change associated with the formation of nuclei. This effectively limits any extensive nucleation to a relatively
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1952 100,
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Figure 11. Per Cent Recovery vs. Temperature Determined 10 Seconds after Release
Figure 12. Per Cent Recovery us. Temperature Determined 30 Minutes after Release
narrow temperature range. When the proper conditions are reached, nucleation takes place on a relatively large scale. From there the growth of the crystallites progresses very rapidly but is soon stopped by interference of neighboring crystallites on chain segments trying to align themselves into a crystal structure. A very important feature of silicone elastomers that allows such rapid formation of nuclei and subsequent crystal growth is the inherent low temperature coefficient of viscosity. By this is meant simply that the viscosity of a silicone polymer is not only low compared to other elastomeric polymers of the same molecular weight (6) but does not vary markedly over a wide temperature range, even down t o very low temperatures. This low temperature coefficient of viscosity allows the chain segments t o rotate easily, facilitating alignment into a crystal structure; this accounts for the rapid crystal growth.
down the formation of nuclei and subsequent crystal growth. Above T,, the rate of melting is too high to favor the formation of nuclei. SUMMARY
1. Modulus, torsional stiffness, and per cent recovery data all demonstrated t h e su erior low temperature characteristics of two types of silicone rufber as compared with natural rubber and GR-S. Type 11, extreme low temperature silicone rubber, as represented by Silastic 250 and the Silastic 6000 series stocks, did not show appreciable stiffening until some temperature between -112’ and -130’ F. was reached. Both the natural rubber and GR-S were stiff at -76” F. as would be expected from brittle oint and second-order transition measurements ( 4 ) . 2. was shown by carefully controlling the induction period in conjunction with the Gehman cold flex apparatus, t h a t silicone rubber Type I1 demonstrates both a supercooling effect and very rapid crystallization. 3. Reasons for the good low temperature characteristics of silicone rubber were given in a theoretical discussion. 4. A special short-term aging technique for silicone rubber was suggested as a substitute for long-term aging. 5. A low temperature modulus apparatus waR described.
6
LITERATURE CITED
(1) Aston, J. G., IND. ENG.CHEM.,34,514 (1942).
(2) Beatty, J. R.,and Davies, J. M., Rubber Chem. and Technol., 23, NO.1,54-66(1950). (3) Bekkedahl, N., J . Research Nall. B u r . Standards, 13,411 (1934). (4)Boyer, R.F.,and Spencer, R. S., “Advances in Colloid Science,” I\
I
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TEMPERATURE
Figure 13. Viscosity and Rate of Melting us. Temperature
If viscosity and rate of melting of crystals are plotted as a function of temperature, curves similar to those of Figure 13 should be obtained. There should be some optimum temperature, T,, where the rate of melting is low and viscosity is also low which allows ample freedom oi rotation and movement of chain segments in an energy region ideal for the formation of nuclei. At this temperature, T,, nuclei are formed and proceed to grow rapidly. At some temperature below T, the rate of melting is lower, but this is overshadowed by a n increased viscosity that limits rotation and movement of chain segments, thus slowing
Vol. 11, Chap. I, Mark, I. H., and Whitby, G. S., New York, Interscience Publishers, Inc., 1946. (5) Gehman, 9. D., and Woodford, D. E., Wilkinson, C. S., Jr., IND. ENG.CHEM..39. 1108 (1947). (6) Hunter, M. J.,’Wa;riok, E. L., Hyde, J. F., and Currie, C. C., J . Am. Chem. SOC.,68,2284 (1946). (7) Juve, R. D., and Marsh, J. W., Rubber Chem. and l’echnol., 23, 760 (1950). (8) Weir, C. E., Leser, W. H., and Wood, L. A., J . Research Natl. Bur. Standards, 44, 367-72 (1950). (9) Wood, L. A., “Advances in Colloid Science,” Vol. 11, Chap. 11, Mark, I. €I.,and Whitby, G. S., New York, Interscience Publishers, Inc., 1946. (10)Wood, L.A,, and Bekkedahl, N., J . Research Natl. Bur. Standards, 36, 489 (1946). RECEIVED for review October 31, 1951. ACCEPTED March 20, 1952. Presented at the Gordon Research Conference on Elastomers, J u l y 5, 1951.
