Behavior of Silastic on Aging G . R i . KOKKLE, R . R. SELFRIDGE, ASDP. C. SERVAIS Dow Corning Corporation, Midland, Mich.
T
Silicone rubbers possess combined resistance to a wide HE general properties ot exposed t o h e a t , c o l d , variety of conditions which normally cause deterioration various silicone rubbers R-eat,hering,and oils. in organic elastomers. Under some of these conditions, have been described in other Most of the silicone rubber such as exposure to heat, silicone rubber is seriiceable at publications ( 7 , 8, 9 ) . The formulations tested shon a temperatures considerably above that of any other natural most important of these, on remarkable degree of resistor synthetic rubber formulation. Moreover, these tests which most commercial uses ance t.o all of these condigive further support to the hypothesis that stability is intions. The Silastic stocks depend, are heat resistance, herent in the silicone polymer because resistance to one used in the stability teste loir temperature flexibility, set of conditions is not developed by special formulation weather resistance, and oil were molded in a press at a t the expense of resistance to other hinds of aging. Both 500 pounds per square inch resistance. D a t a are prehigh and low temperature stabilitj, together M ith M eather pressure and ?60° F. for 5 sented here t o evaluate the and oil resistance, are found to be characteristic of silicone minutes. The Silastic was stability of silicone rubbers elastomers. then cured for 4 hours at after exposure to heat, cold, 482" F. in a circulating air weather, and oil. Silicone rubbers, in common with other silicone compounds, oven. The test strips and dumbbells were cut from these cured are based on molecular skeletons which are chains of alternate sheets. The test strips were approximately 4 inches long and 0.125 inch thick. silicon and oxygen atoms. I n contrast, most other elastomers are composed of carbon-to-carbon linked molecular chains. On the RESISTANCE TO HEAT basis of their bond energies, the structure of these silicone elasThe most significant characteristic of a silicone rubber is its tomers could be expected t o produce compounds of exceptional retention of properties after exposure t o heat. Tests were run at stability. The bond energy between carbon atoms, for example, elevated temperatures t o det,ermine the maximum temperature is 50 kg.-cal. per mole. The bond energy between t8hesilicon and at, which Silastic could be considered to be stable for extended oxygen at'oms in a silicone rubber chain is 89.3 kg.-cal. per mole, pcriods. approximately 50% great'er (6). That silicone elastomers bear Test samples Jvere aged in circulating air ovens held at 302", out this predict'ion of greater stability is shown by their abilit,y t'o 392", or 482" F. -4t intervals the samples were removed and withst,and conditions which cause carbon-based elastomers to t.ested for weight loss, shrinkage, hardness (3),and flexibility. undergo temporary or permanent change. The tests were discontinued only when the samples could no The inherent stability of the silicone polymer is further emphalonger be flexed about 60' without breaking. sized by the fact t h a t special compounding is not necessary to After 50-day heating a t 302" F., all the Silastic samples were produce stocks resistant to many conditions. It is, on the constill flexible enough t o be bent 180" without, cracking or breaking. trary, common practice t o vary the formulation of rubber stocks The average weight 108s of three representative formulations was to meet varying specifications. Certain desired properties are 2.5%. The average shrinkage was 1.7%. The increase in harddeveloped and enhanced, usually a t the expense of others less imness, as measured r i t h the Shore A durometer, averaged 15 port,ant to the particular application requiremenk Thus, carpoints. Tests are being continued on t,hese samples. bon-based natural and synthetic rubbers compounded for heat .4 commercial GR-11 stock, specially compounded for heat and resistance could not be expected t o be especially effective a t low oil resistance, was used as a basis of comparison of the stability of temperatures. Conversely, most low temperature rubber formuSilastic with that of more conventional elastpniers. This same lations are of h t l e use a t elevated temperatures. Rubber compounds can be alt,ered by a n almost infinite number of techniques and additions, but no single formulation combines resistance to more than a few of the conditions which usually cause elastomers to deteriorate. The properties of silicone rubber can also be varied to a certain extent by compounding. Silicone rubber contains a dimethylsiloxane polymer and suitable fillers. A11 of the present Silastic stocks contain the same polymer. The individual properties of the various stocks are gained by varying the kind and amount of filler. The fillers used in compounding the Silastic stocks referred to in the subsequent data are listed in the folloxing table; the filler constitutes approximately 60% by weight of each of the compounded stocks: Silastic Stock KO.
Filler
160
167
180
181
Although the fillers affect some properties oJ silicone rubbw, others are characteristic of the polymer itself. These propert ies are common t o all the various stocks and require no special dcvelopment. The most important are resistance t o change when
Figure 1.
1410
Silastic ( l e f t ) after 90 Days and GR-\I (right) after 1 Day a t 300' F.
November 1947
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1411
1 of exposure tests. GR-11 was exposed along with t,he Silastic samples at 302" F. and tcst,ed in the same way. Figure 1 s h o w the sample of GR-11 being bent after I-day exposure. The x h i t e sample is Silastic 160 exposed t'o 302" F. for 90 days. The organic rubber has cracked badly, rvhile the silicone rubber is nithstanding a much more severe bend with no visible effect. Figure '2 shons graphically the weight loss of three Silastic stocks and the GR-11 stock.' The weight loss of the GR-11 in 1 day is tn-ice t,he maximum w i g h t loss of any of the Silastic samples aft,er 75 days. XI1 the Silastic stocks have approximately the same rveight loss in spite of the difference in fillers. This indicates that tveight loss is independent of the filler and is the result only of slight decomposition of the polymer. Figure 3 shons the change in Shore -4 hardness 10 20 10 40 eo 10 M on aging at 302" F. The silica- and t,itania-Uled TIME, I N DAYS dilast'ic 180 increases least in hardness, evidently Figure 2. Weight Loss of Silastic and GR-M after Oven Aging at 300" F. because its original hardness n-as greater and, therefore, nearer the top of t'he Shore scale. All st,ocksfollow the same general pattern of hardness increase. T h e in a prescribed manner; any fracture or cracking ws original durometer reading of Silastic 160 and GR-;\I were about noted. TKOsamples of each stock tested were used at each temequal. .At t'he indicated rate of hardness increase, hon-ever, alperature, and both samples had t o show no signs of failure t,o pass most 4 months would he needed for Silastic 160 t o reach the same the test. Brittle points of three commercial types a t intervals of durometer reading attained by GR-11 in 1 day. 10" F. follow: Silastic 167, -80" F.; Silastic 180, -90" F.; Silastic 181, -80' F. Evidently t h e kind of filler has little inAfter 50-day heating at 392" F., all of the Silastic samples Tere &ill flexible enough t o be bent 180" without cracking or breaking. fluence on the brittle points of Silastic stocks. The conclusion is that Ion. temperature flexibility, like t,he high temperature beThe average weight loss was 6.3% and the shrinkage was 4.0%. The increase in hardness averaged 23 points. havior, is an inherent, property of Silastic. At 482' F. the Silastic samples failed the flexing test at 50 days, Several samples exposed t o the lowest temperature a t which it wasrpossible t o flex them tvere tested for such properties as tensile the 42-day inspection showing no failure. The weight, loss !vas l l . O % , the shrinkage 7.1%, and the hardness increase 29 points. strength ( I ) , elongation, and hardness after return to room temperature. I n all cases no change in these properties could be deSince all present Silastic stocks contain the same polymer and since the changes in properties on aging at, elevated temperatures tected, an indication t h a t flexure a t low temperature did not harm are evidently independent of the type of filler used, the behavior the elastomer until the brit,tle point was reached. The hardening of Silastic a t low temperat,ures Tvas measured by can be said t o be characteristic of all present Silastic stocks. I n operating the Shore durometer inside t'he test chamber. Exgeneral, silicone rubbers can successfully nithstand considerably higher temperatures than other natural or synthetic heat,-rcsistant posure periods a t each t,emperature were varied t o ensure that rubber formulations. These tests indicate the usefulness of aaniples were at equilibriuni by testing a t each t'emperature until check readings of hardness were obtained. The time required for Bilastic for long periods a t t,emperatures above 302" F. (150" C.) which is well beyond the useful range of other heat-resistant rubreadings t o become constant a t temperatures don-n t o -67" F. ber formulations. At temperatures near 392" F. (200' C.j a 3-6 n-as 30 minutes. Hon-ever, hardness equilibrium a t loner temperatures m a not reached until after 3.5 hours of exposure. month life can be expected for a silicone rubber. -4 temperature of 482" F. (250' C.) m u l d cause failure in 1 t o 2 months. These predict,ions are based on data obtained from aging t'est samples in circulating air ovcns. Actual service life depends, t'o a great, extent, on the conditions of exposure and the physical requirements of the rubber. I n less extreme service conditions, where the rubber is protected, the life of t , h r silicone rubber would be considerably longer.
GR-XI.stock was used throughout the ent'ire series
LOW TEMPERATURE BEHAVIOR
The same Silastic formulations yere tested at t'emperatures from f 3 2 " t o -100" F. (0" t o -73' C.) for brittleness ( 5 ) and hardness change as measured on the Shore durometer. Temperatures were obtained in a machine which recirculated air through solid carbon dioxide. Specified teniperatures were maintained within *2' F. throughout the entire range. I n the brittleness t,est, standard tensile dumbbell specimens a e r e exposed t o low temperatures for 5 hours and n-ere then bent double
2
0
10
20
30
Figure 3.
50
40
TIME,IN
10
80
DAYS
Effect of Oven Aging at 300' F. on Hardness of Silastic.
and GR-RI
Vol. 39, No. 11
INDUSTRIAL AND ENGINEERING CHEMISTRY
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TABLE I. RESULTSO F -Tensile Original, I b . / s q . in.
Sample
TESTOILS
IhiMERSIOS IS
StrengthFinal, % relb./sq. in. tained
FOR
-ElongationOriginal,
Final,
70
70
% re. mned
178 165 67 53 130
154 132 134 123 34
B.S.T.XI. KO. 1 Oil (LOWSwell) Silastic 160 Silestic 167 Silastic 180 Silastic 181
534 480 764 639 1772
Silastic 160
534
349
65
115
187
163
Silastic 167 Silastic 180 Silastic 181
480 764 639 1772
347 488 378 479
72 64
125 50 43 383
245 90 68 175
196 180 158 46
GR-Xi
574 485 741 624 952
107 100 97
115 125 jo 43 383
98 54
.I.S.T.M. 30.3 Oil (High Swell)
GR->I
27 59
I
1
1
-M
o
~
-80
-so
-40
TEMPERATURE
eo
a
BO
60
IN O F
Figure 4. Effect of Low Temperature on Hardness of Silastic
,
After the 3.5-hour exposure at -76" F. the temperature w-as raised t o -67" F. and held there for 1.5 hours. All samples tested a t that time showed a return t o the durometer readings previously obtained a t this higher temperature. This indicates that equilibrium conditions had been reached during t h e test. Figure 4 shows the effect of low temperatures on the hardness of three Silastic stocks. The rise in hardness is very gradual as the temperature is lowered t o -67" F. Below this temperature the rise is abrupt, and a t -76" F. all of the formulations shoi$ durometer readings of 90 to 100 points. I t is important t o observe that these hardness-temperature curves have the same slope. The filler in Silastic 167 is titanium dioxide. I n Silastic 160 half the titanium dioxide is replaced by zinc oxide p i t h a consequent reduction in hardness of the stock. I n Silastic 180, on the other hand, half the titanium dioxide is replaced by silicon dioxide that causes an increase in the hardness of the stock. Severtheless, the hardness curves of all three stocks follom- the same pattern and make a n abrupt rise be1oLv the same temperature (-67" F.). This indicates that the filler determines the degree of hardness but not the change of hardness with lowering of temperature. Aaain. the low
tensile dumbbells in a test tube of oil, heating in a controlled oven for the prescribed
period of time, and then breaking the samples in a tensile tester immediately after removal and % cooling. The tests were run using A.S.T.Rf. KO. 1 Increase (a low me11 oil) and A.S.T.M. No. 3 (a high swell oil). The volume change of t,he stocks was deter6.1 mined by a water displacement method before and 6.0 7.2 after exposure t o the test oil. Table I summarizes 6.3 the data obtained. The conditions were 70-hour 2.6 immersion at 350" F., a higher temperature than is 33 ordinarily used ip testing rubber. 55 il.S.T.11. KO. 1 oil has little effect on the tensile 41 strength or the volume of any of these silicone ruh38 81 ber formulations. The maximum decrease in tensile is 1 5 7 , the maximum snell 7.2%. The elongat,ion is actually increased by the oil immersion. Apparently the oil acts as a plasticizer. Variations in the formulation do not have a great effect on the over-all oil resistance of Silastic. The volume increase of GR-11 exposed under identical conditions is lower than that of Silastic. HoIvever, the tensile strength and elongation are appreciably lowered. I n -4.S.T.AI.S o . 3 oil the effects are more pronounced on all of the stocks tested. I n general, silicone rubber formulations shon-ed a 30 t,o 40T0 decrease in tensile strength. There Tvas, hoIvever, 50 t o 100% increase in the elongation. On the basis of the efficiency of elastic materials (tensile strength x elongation) this Jyould more than compensate for the loss in t,ensile. The increase in volume, betneen 38 and 55%, vas more than can be t,olerated for some applications. The GR-M !vas deteriorated t o a great,er extent in S o . 3 oil than Silastic. Its retenticn of t,ensile strength \vas only 27% of the original value. Insread of increasing, elongation decreased to less than half that of the original. The volume increasr also !vas almost twice that, of Silastic. Table I shows there ie little variation in the capacity of the various Silastic stocks t o resist the effects of hot oil. The titaniafilled stocks retain their original properties slightly better than do t8heother stocks. I n general, i t can be said that a t elevated temperature (350" F.) silicone rubber sIvells in oil, but t o a lesser extent than do most organic rubbers. I t deteriorates only slightly and does not discolor the oil.
70 HOCRSAT 350" F.
Figure 5.
WEATHERING RESISTAIVCE
Ultraviolet light, moisture, ozone, and oxygen all take part in the attack on rubber Tyhen it is exposed t o weather. To determine hon- they affect Silastic, several samples werr exposed on a
TABLE 11. EFFECTO F EXPOSURE O N SIL.4STIC 12 Months Out of Doors
Silastic N o . Toof original elongation retained ?' & of original tensile strength retained Original Shore hardness Final Shore hardness
160 81 92
5;
oo
167 82 74 63
68
180 71 81 76 77
50 Days in WeatherOmeter 167 98 73 67 68
181 68 98 80 82
Silastic (left) and Synthetic Rubber ( r i g h t ) Samples after Outdoor Exposure for One Tear
November 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
1413
indication that the two types of exposure are not exactly equiralmt. I n none oi the samples were there any changes which could be detected visually. Figure 5 is a photograph of Silastic and sj-nthetic rubber samples which have been held looped for a year out of doors. S o n e of the Silastic stocks has an?- tendmcy to crack if held under tension. Figure 6 s h o w some strips t h a t r e r e stretched 20% out of doors for a year and likewise did not check or crack during this period. The n-eights of t h r samples did not vary more than O . l % , and t,heri: were no measurable changes in dimensions. Both the outdoor exposure and the 50-day Weather-Ometer tests indicate that titanium dioxide in t h e filler of Silastic stocks contributes t o the retention of elongation. On the other hand, silicon dioxide contributes t o the retention of tensile strengt,h. Although t,he evidence is not conclusive, it, seems that the tiFigure 6. Stretched Silastic Samples after Outdoor Exposure for tania-filled stocks have slightly better over-all One Year weather stability than the silica-filled stocks. These test,s are being continued at Midland 60” exposure rack facing south at Nidland, X c h . Some other and any iurthrr changes will he recorded everv 6 months. samples were placed in a n Atlas single-arc Weather-Ometer deLITERATURE CITED signed t o intensify the conditions of moisture and ultraviolet light (1) .Im. doc. foi Testing Materials, Standards on Rubber Products, encountered in outdoor weathering. A4.S.T.lI.procedure ( 2 ) Method D412-41, pp. 43-9 (1946). was followed in testing flexed pieces and stretched pieces of (2) Ibid.. Method D51844. D D . 66-9. (3) Ibid., Method D67644T; b ~112-14. . Silastic. Flat moldings-for use in tensile testing were exposed at (4) Ibid.. Method‘D47146T, pp. 146-54. the same time. (5) Ibid., Method D73643T. pp. 158-60. I1 the effects o f Outdoor exposure On (6) Hunter, M.J., Hyde, J . F., Yarrick, E;. L., and Fletcher, H. J., J . Am. Chem SOC..68. 667 (1946). Both tensile strength and elongation are somewhat reduced, (7) Irish. E. M.,and Stirrat; J. R:, Product Eng., 18, 146-50 (1947). while durometer values increase- only slightly. Table 11 also (8) Servais, p. c.,I n d i a Rubber World, 114, No. 5, 659-62 (1946). shon-s t h a t the effects of 5 0 - d ~\i-eather-Ometcr ~ aging on t,ensile (9) Servais, P. C., Ibubber Age (N. Y . ) , 58, No. 5 , 579-84 (1946). strength and elongation are of t,he same order of magnitude as RECEITED J u n e 10, 1947. Presented before the Spring Meeting of the Divithose Obtained in year Of outdoor esposure’ Ho”-everl the sion of Rubber Chemistry, AMERIcas CHEMIC.AL SOCIETY,Cleveland, Weather-Ometer aging had little effect. on durometer hardness, a n Ohio, 1947,
Industrial Applications of Diatomite Filters E. G. KO3IINEK I~iJilcoZnc., Chicago, I l l . Diatomite filters are finding wide industrial application because of the adtantages of saving in space and weight and more effectite filtration. Operating rates as high as 8 gallons per minute per square foot can be employed, and, as precoagulation is not required, sparkling-clear filtrate can be obtained in handling many aqueous or nonaqueous solutions. operating costs are higher than the cost of sand filtration, but there are nianj cases where over-all etaluation favors installation of diatomite filters when the installed cost and the space requirements are taken into consideration.
D
I A T O M I T E filters, like many other new or improved types of industrial equipment, n-ere given their first tests on the battle fronts of World K a r 11. It Tvas these tests t h a t brought out the many advantages of diatomite filters xvhich are as important in many respects t o industry as t o the armed forces.
Tht, advantages can ht, suniniarized a!: savings in rpace and w i g h t and more effective filtration. I n some instances, as cited in test wsults which are later discussed, the diatomite filters afforded a solution to filtration problems which could not he satisfactorily handlrd by conventional methods. In others the considerably lower installed cost of a diatomite filter has led to its selection itistrad of that of sand filters for thc filtration of preclarified \vat csr . The f o h n - i n g tabulation compares the United States ;irniy’s pack and mobile units with pressure sand filters, and illustrates the saving in spacc and wc4ght rvhich resulted from their use: Pack Unit Filter area, sq. f t . 3.6 ~ ~ ~ e g d h t c , a P b a c i t ~ , g , p . n l .l 5 30 Diameter, in. 8 Over-all height, in. 2% ~ l ~ ~ ~ 10 ~ X 10 ~
Pressure Mobile Sand FiIter Unit 7.1 10 15 50 2860 350 18 36 30 73 ~37 X ~49 ~ 21 , X 24i
Pressure Sand Filter 19.6 50 8050 60 82 ~8 1 X .78
