I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
2462
effect on the rate of change of the calculated viscosity with temperature. In the study of greases sufficient data have been supplied to show that both the soap concentration and the type of siloxane fluid determine the flow characteristics of these materials. The type of siloxane fluid in the grease is the determining factor in the effect produced b y temperature changes on apparent viscosities a t higher shear rates. NOMENCLATURE
9
qa
M P R
L
u/t
R, Rb
= =
= = = = = = =
v
=
la = 8 = r.p.m. T =
Vol. 42, No. 12
Radius of rotating part of viscometer, cm. Height of bob, em. (l/R,”- l/R:)/4rh = Revo]utions of bob per minute Torque, dyne-em. x 10-6 LITERATURE CITED
(1) Arveson, M. H., IND.ENG.C H E M .24,71 , (1932). (2)Zbid., 26,638 (1934). (3) Barry, A.J.,J. Applied Phys., 17,1020 (1942). (4) Buckingham, E., Am. SOC. Testing Materials, Proc., 21, 1164
(1921). (6) O’Connor, B. E., Machine Design, 19,155 (1949). (6) Pigott, R. J. S., Inst. Spokesman, 11, 4 (December 1947). (7) Roehner, T.G., and Robinson, R. C., Zbid., 10,4(March 1947). (8) Smith, J. W., and Applegate, P. D., Paper Trade J., 126, 60 (1948). (9) Weltman, R. N.,IND.ENG.CHEM.,40,272(1948). (IO) Weltman, R. N., IND.ENG.CFKEM., ANAL.ED.,15,424 (1943).
Newtonian or absolute viscosity at 25’ C., cp. Ap arent viscositp, poises Morecular weight Pressure, dynes/square cm. Radius of capillary, em. Length of capillary, em. Flow rate, ml./sec. Radius of cu , cm. Radius of bog, cm.
RECEIVED April 10, 1950.
THERMAL AND OXIDATION STABILITY
OF POLYMETHYLPHENYLSILOXANES C. M. M U R P H Y , C. E. SAUNDERS’, AND D. C. S M I T H Naval Research Laboratory, Washington PO, D. C. Thermal oxidation studies reveal that polymethylphenylsiloxanes are more stable than polymethylsiloxanes. No appreciable oxidative changes in the methylphenyl silicones were observed at 225’ C. A t 250’ C. viscosity increases and the evolution of volatile oxidation products were significant. Increasing the temperature accelerated these changes. Of the metals investigated (antimony, copper, duralumin, lead, nickel, selenium, silver, steel, tellurium, tin, and zinc) only lead, selenium, and tellurium affected the rate of oxidation, accelerating the reaction. The increase in viscosity is attributed to the condensation of two or more siloxane radicals from which methyl groups have been ruptured. Infrared studies reveal a progressive decrease in methyl group concentration with oxidation and no detectable change in the phenyl group concentration.
HE polymeric siloxanes or silicones, though a rather recent development, have an extensive patent and journal literature which has been summarized in three ( 6 , 25, 28) reviews. Polymethylsiloxanes have been studied more extensively to date, and their limitations and advantages for lubrication and hydraulic applications are known ( 2 , 4,6, Y, 10, 19-21, 27, 31, 33, 34). Investigations of thermal and oxidation stabilities (3) reveal that polymethylsiloxanes are unusually stable, showing only minor changes attributable to oxidation or pyrolysis a t 175’ C. A t 200’ C. atmospheric oxidative changes take place, and at 225’ C. polymerization to a gel occurs in 24 hours. Polymethylphenylsiloxanes are generally prepared by the hydrolysis and condensation of diorganosilanes of the general formuln
T
R-S-Xz
I
R‘ where R and R’ are methyl and phenyl radicals and X is usually an alkoxy or halogen radical (8, 13-18). Triorgano-substituted silanes are generally used to “end stopper” the chains. It is also possible to copolymerize the methylphenylsilanes with other 1 Present address, Naval Ordnance Laboratory, White Oaks, Silver Spring, Md.
organo-substituted silanes; thus the aromaticity of the resulting polymer may be varied considerably.. The properties of poIymethylphenylsiloxanes vary with the ratio of the phenyl to methyl substituents; increasing phenylation causes an increase in surface tension, refractive index, freezing point, density, viscosity, and temperature coefficient of viscosity (IO,11, 22, 29, 32). However, the variations in these properties cannot be used as a convenient analytical method for determining the precise structural changes occurring during hydrolysis or oxidation of such polymers. Recent spectroscopic studies of the silicones (26, 30, S6, 3 7 ) reveal that the infrared absorption spectra give more positive information on composition and structure than do the changes in the properties mentioned above. Characteristic bands for the methyl, phenyl, and other organic qubstituents were found to I)e remarkably constant for all compounds studied, thus affordiiig B method for their determination. .Infrared absorption spectra were used in this study both ab a means of characterizing the structure of the siloxanes and of determining the changes taking place on oxidation. Though the methylphenyl silicones have much larger temperature coefficients of viscosity and higher freezing points than the methyl silicones, they are reputed to be more resistant to heat and oxidation.
December 1950
INDUSTRIAL A N D ENGINEERING CHEMISTRY
I.
2463
tion of formaldehyde and formic acid, and the low weight losses that no significant changes attributable to oxidation ocL curred a t 200" C. Evaporation lossea DiMe at thie temperature are responsible for 40.5 22.02 100.0 the slight viscosity increases observed. 128 0 149 144 0.695 0.828 The presence of metals immersed in 0.233 polymethylphenylsiloxane H, e x c e p t -65 - 100.493 0.966 1. O m possibly lead, selenium, and tellurium, 1.403 1,524 0.0 0.67 had no effect on the rate or extent of oxidation a t 200" C. as judged by these criteria. For a contrast of the relative stabilities of polymethylphenylsiloxaoes and polymethylsiloxanes the data obtained on polymethylsiloxane C-4 from the previous investigation (9)under the same conditions are included in Table 11. This silicone had a viscosity increase of 48% as compared to the negligible increase for silicone H. There was an appreciable evolution of formaldehyde and formic acid from C-4 which is ipdicative of oxidative effects, and no significant evolution of these products occurred from H. At 225' C. the viscosity increases observed with the control runs are primarily due to evaporation, not oxidation, as the small amount of formaldehyde and formic acid evolved (3 X 10-6 * 1 moles) is close to the experimental uncertainty. At this X temperature duralumin, cold rolled steel, copper, silver, tin, itntimony, and zinc were also found to be inactive.. Viscosity changes attributable to the presence of lead, selenium, and tellurium aTe more apparent a t 225" C.; however, there was no increase in the evolution of formaldehyde and formic acid, Weight losses in the presence of tellurium and lead, especially the former, were much higher than in the control runs. T h e high viscosity increases observed in the presence of lead, Belenium, and tellurium are believed to be due to the catalytic action of these metals or their oxides in breaking the siloxane chain with the subsequent recombining of the heavier fragments (3) as well as t o their pro-oxidant effect. Evaporation of the low molecular weight fragments from the ruptured siloxane chain is believed responsible for the considerable weight losses observed with lead and tellurium. Polymethylsiloxane C-4 evolved 36 X 10-5 moles of formaldehyde plus formic acid per gram of fluid and gelled within 24 hours of exposure to these conditions (3). Hence, polymethylsiloxane C-4 is much less stable to oxidation a t 225" C. than is the methylphenyl silicone E. Results of the dynamic oxidation studies on fluid H at 250" C. are given in Table 111. As in the earlier work ( 3 ) air, oxygen, and helium gases were bubbled through the silicone. Differences
Properties of PolymethylphenylriloxanerCompared with Typical Polymethylsiloxanes Identification F G H K
Table
Siloxane type Vmosity, os. at 210° F. Viscosity. cs. at looo F. Kinematic viscosity index (1) Viscosity temp. coeff. (96) A.S.T.M. slope Pour polnt F. Density d m l . at 77O F. Refractive Index, ItD 77O F. Phenyl to methyl ratioC
MePha 6.84 34.2 152 0,800 0.628 - 30 1.081
1.517 0.62
DiMe 13.9 34.0 177 0.591 I
73
49 41 95 90 44 56 368 62 59 124 109 117 45 178 184
41
200 46 13
--
77 I i>
229 16,300 153 187
..
2 17 25 41 18 23
io
13 68 .
I
1.6 14 19 31 13 17
128.0 22.02
14 15 14 14 14
..
5 13 5 8 32 8 12 11 14 12
15 18 19 17 19
ii
4 4
3 3 2.5 3.6 3.1 4.1
.. 4
3
...
11
2 3 3
..
2.1 1.9 11.0 12.1
50 17 19 16 17 14
3 3
4.8
20 10
1.6 1.6 16.9 3.6 3.5
3 2 3 4
4.7 3.8 3.9 4.1 4.8 3.7 6.6 5.3 20.6 20.3 6.7 5.8 6.2 25.2 3.8 5.2
0.8 0.8
..
0.8 0.7
...
1.6 0.6 8.6 7.2 0.4 1.0 0.8 4.0 0.8 qo.9
... ... .
.
I
77 L r* 2,>
...
112
, . .
97
95.4, 20 68
23 17 104
C. for 168 Hours
L
Polyrnethylphenylsiloxane Ff 113 34 25 96 18 13 77 .. 95 id 19 122 18 14 84 98 119 199 82 1300 111 159 4400 109 130
N ~ 12 .
...
steel.
in the results obtained on check runs itre attributed to variations oxidation products evolved were the same as obtained in the absence of metals. There was no appreciable change in the in temperature control and gas flow-. The reproducibility conipares favorably with that ot)tained with similar oxidation procolor of the silicone after the oxidation tests with any of the cedures on petroleum oils. Pule ouygen caused more rapid oxidametals except selenium. I n the presence o f selenium the silition than air: the viscosity increase measured a t 100' F. was cone turned a pale straw yellow. N o chemical evidence of nearly 30y0 greater, volatile o\itlation products increased from t,he evolution of benzene was obtained on any of these runs, but 14 X 10-6 t o 17 X 10-6 niolrr per gram of charge; and the unpublished investigations of this laboratory proved that benzene, average weight losseq fiom 3.1 to 4.1%. When the ineit gas biphenyl, and other aromatic hydrocarbons are evolved a t temhelium was used, the viscosity increases were much smaller, and peratures much above 275' C. onl?. a dight amount of formuldehydc and formic acid was proOxidation experiments on silicone L, which has a higher aroduced, the greater proportion being the latter. matic content than Ht were also made, and the results are given The effect of metals on oxidation at 250' C. is also shown in in Table 111. Much smaller quantities of formaldehyde and Table 111. Viscosity increases after oxidation, the evolution of formic acid were evolved from fluid L than H. As L has a much volatile products, and the weight losses are so nearly equal to higher aromatic content (a phenyl to methyl infrared absorption the iesults obtained in the absence of metals that the possibility ratio of 0.67 as compared to 0.42 for H ) i t is probable that the of any catalytic activity In- duralumin, copper, cold rolled steel, methyl groups on the chain units of silicone L are better shielded silver, tin, antimony, or zinc was excluded. Duplicate Oxidation than those on H, and the terminal methyl groups account for the runs in the presence of lead gave rather variable results as regards viscosity changes. Similar difficulties were experiTable IV. Dynamic Oxidation Experiments with Polymethylphenylsiloxanes H and L at enced with lead in the earlier work ( 3 ) 975' and 300' C. for 168 Hours on the methyl silicones. The amounts of (Initial viscosity, cs. a t looo F.: H , 95.4; L, 128.0) formaldehyde plus formic acid evolved Moles X 10s of were considerably lower than obtained in HCOOH Plus Gelation l-ime visi. ~ ~ l HCHO/Gram i ~ ~ Sample ~ , Weight Loss, %_ the absence of metals bvhich may be due Metals Present of Air, Hours" a t 100" F., CY. Airb Helium Air* Helium to their oxidation t o carbon dioxide. Polymethylphenylsiloxane H a t 275' C. Weight losses were more than quadrupled 48 6 0.3 1.5 None (control) 80-96 130.6 74-90 36 ... 10.1 ... in the presence of lead. Even when Average (control) 77-93 13016 42 6 9.7 1.5 helium was used there vias :tn abnormal 125.6 41 14.2 1.3 Copper 100-116 129.8 37 10.3 1 .ti Duralriminc 80-96 increase in viscosity and weight loss. 6.4 1.5 125.0 47 4 Steelb 80-96 Using air with selenium an abnormal Polymethylphenylsiloxane L a t 279' C. Pvolution of volatile Oxidation productNonr (control) 152-168 267.2 42 2 8.1 1.6 152-168 48 ... 8.0 was observed though the viscwsity .4verage (cuntrol) 1R2- 168 267 :2 45 2 8.3 l:i 24 5 11.4 1.2 >168d 247.4 Copper changes were normal. With oxygeii there Durafiimin( 96-112 216.6 55 4 8.7 1.4 Steeld 96-112 262.9 46 4 10.2 I .R was a tenfold greater viscosity increase Polyn~ethylphenylsiloxane FI a t 300' C. and evolution o f volatile o\-idation prodNone (control) 8-24 320.6 18 6 2.4 6.7 ucts was several times more rapid. With Polymethylr~tienylsiloxancL at 300' C. helium the viscosity changes w e i e much None (control) 8-24 739.0 16 2.1 .i. 2 smaller than nTith air or o\ygen. Tellua Viscosity a t 100" F.--18.420 rs. Values given were obtained at tiiiie of gelation. rium caused much greater Viscosity Duralumin 24ST. changes than did any of the other d S A E 1020 cold-rolled steel. metals, but here the amounts of volatile
:
December 1950
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
greater proportion of these volatile produrts. .4\so it is possible that a grcitter proportion of the forni:tldehyde and formic acid was oxidized to carbon dioxide as there would be more oxygen available to oxidize the small quantities ot these products evolved. The viscosity increases were greater for L than H, and this appears contradictory to the evidence that smaller quantities of volatile oxidation products were evolved. However, the thickening effect caused by the formation of higher polymers in a mixture of low polymers is fiomplicated as it is a function of the molecular weights, molecular weight distribution, configuration, and solubility of the polymer formed. The temperature at which the viscosity is determined is another variable. Although no general relationship is known, i t has been observed experimentally that the thickening for the same amount of condensation or polymerization is always greater in the more viscous fluids. Tho Sam(' metals, lead, selenium, and tellurium, found to he active on the polymethylsiloxanes and polyincthylplit~n~lsiloxane H cbaused abnormal changes with L. An attempt was made to study fluid F under the same dyrianiir oxidation conditions a t 250" C., but the evaporation of this silicone and the subsequent condensation in the potaseium hydroxide trap was so great t h a t the teste were discontinued. As the evaporation losses a t 225" C . in the static tests, to be described, were quite high and only small viscosity changes were observed, it was doubtful that any information could he obtained from dynamic tests a t 225" C., and they were not attempted. I n order to obtain information on the maximum temperature to which the silicones H and L could be exposed without rapid decomposition or gelation, these fluids were studied at 275" anti 300" C . Only the metals which were inactive at 250" C. were studied a t these temperatures. The results are given in Table IV. Silicone H oxidized to a gel within approximately 96 hours with air a t 275" C. Since these metals tiid not influence the time of gelation, they are inactive a t this temperature also. The weight losses and the evolution of volatile oxidation products obtained agree within the limits of reproducibility. When helium was bubbled through the sample for 168 hours a t 275" C. the viscosity increases at 100" F. were approximately 40%. Some acidic products were evolved, presumably formic acid, but no aldehydes could be detected. There was sufficient oxygen (approximately 0.1%) in the helium used to aocwunt for the acidity calculated as formic noid. The weight loiis (1.570) obtained when using helium can only ht. attributed to the evaporrttion of the more volatile components. As the weight loss attributable to the formation of formaldehyde, formic acid, and carbon dioxide from the run with air is small, much less than 0.1% the difference in weight losses between the runs with wir and helium is accounted for by the cracking of the silosane chain to form volatile siloxane compounds. Further confirmation of this mechanism was the collection of a considerable amount of a water-insoluble m:tterial in the potassium hydroside trap during the run with air. Approximately the same quantities of volatile oxidation products were evolved by both the H and L silicones, hut a longer exposure was required for the gelation of 1,. Copper apparently extended the time required for gelation, and duralumin and cold rolled steel reduced the time interval. These results are not conclusive evidence of the activity of these metals as the experimental uncertainty is 1 2 4 hours. The runs with helium conform to the comparable experiments on H. Slightly less acidic products were formed with the silicone I,. The viscosity increases were greater with L for comparable weight losscs, hut this again is a manifestation of the greater thickening action observed with more viscous fluids. At 300' C. both H and L gelled between 8 and 24 hours. Volatile oxidation products and weight losses at gelation were much smaller than those observed a t gelation in the 275" C. studies. The weight losses and viscosity changes with helium a t 300" C. are much greater than at 275" C., as would he expected.
2465
INFRARED ABSORPTION STUDIES O F OXIDATION CHANGES
Changes in the infrared ;tlisorption spectra should indic:itr changes in molecular s t r u r t i w a.5 tho rwult of oxidatioil. In order to observe the greatest structural c-hanges, samples ot silicone I T from the tellurium rntalywd runb a t 250" C. with :iii :tnd oxygen were examined sinw thcw had experienced tlir grcwtost viscosity changes after oxid:ttioii. The intensity o f tht. :hsorption band having its maxiniuni at 7.!)5p, which is due n i h l y to the dimethyl chain unit structure, was measured quaiititstively to determine the decrease in niethyl units on oxidation. The intensity o f adsorption c~:tlculatecl011 a relative basis is t:ik)ul:i tcd below: Sample Original silicone fluid l i After oxidation with air h f t e r oxidation with oxygen
Relative 1ntenxit.y
of 7 . 9 5 ~ Absorption 100.0
90.5
80.2
The intensity derrease, whicsh ib coniideratAy outside the expvt Imental uncertainty, 1I%, must he causcd hy cleavage of methyl groups from the silicone chain. T h r shapc of the absorption band having its maximum a t 7 . 9 5 ~:~lsochsnged in such a mannw that it revealed ( a )the trimethyl wid group roncentration h d decareased to a large exttwt if not cntirdy and ( b ) the formation ot branch chains was prohablr~but not ccvtain. The abwsorption in the 8.4~region due to vibrations of ('-4% honds was also measured quaiititti tivvly. The concentration of :tromatie C-H was constant M ithin cxpcrimental error whet ('as that of the aliphatic C--H (nirthyl grouph:) decreased as follow: Sample Original silicone fluid I i After oxidst!on with air After oxidation with oxygen
Relative Aliphatic C-FI Absorption a t 3 . 4 ~ 100.0 80.0 68 6
The measurement of the decrc:tsc in diphatic absorption u t 3 . 4 ~ is greater than at 7.95c(, the difference being due to the trimethyl end groups which are measured a t the former b u t not at the 1:ttter wave lengths. The ratio of the intensities of the aromatic : i t i d diphatic C-H bands w:m mt~tsurcti:md thr results follow-: #ample Original silicone fluid I1 After oxidation with air After oxidation with oyygen
Ratio of Aromatic to 4lipliatic C-H Absorption 0 43 0 51 0 59
From the spectroscopic, rheniical, and physical studies 01 the Oxidation of fluid H it is apptrent that the oxidation takes place primarily through the methyl groups which are ruptured from the silosttne chain and oxidized to formaldehyde, formic acid, :inti carbon dioxide. Viscometric inrreaws due to oxidation indicbate that two or more of the fragments from which methyl groups have bees ruptured comhine to form more viscous and higher molecular weight polymers. I t is probable, from the spectrowopic evidence, that of the nicthyl groups in the molecule, those in the triniethylsiloxy end groups are the more readily oxidized and ruptured. The shielding artion of the phenyl groups may cwntribute to the stability difference obwrved between the methyl end groups and those on the disubstituted d o x y groups. No chemical evidence of the rupture of phenyl groups a t 250" C. was obtained, but the methods used for the chemical identifirittion of benzene were not suffickntly scwsitive to be conclusive. The constancy of the phenyl concentration as determined by infrared analysis of the original and oxidized silicone is fui ther confirmation of the greater stability of the phenyl g r o u p to oxidation. Static Oxidation Experiments. 14xperiments were rut1 nt 225" C. 011 polymethylphenylsiloxitne F using the static-type procedure previously described ( 3 ) . Studies of the effect of metals were restricted to duralumin 218T and copper t o typify the innctive metals and lead and tellurium, the active metals. The visvosity increase6 and w i g h t loiws w e plotted in Figuirs 1.2 and
2466
INDUSTRIAL AND ENGINEERING CHEMISTRY
1B. After 168 hours of exposure the viscosity increase was only 25y0 in the absence of metals and in the presence of copper and duralumin, yet the evaporation loss was 16.57,, which is evidence that fluid H does not contain polymers of radically different molecular weights. As in the dynamic test, lead and tellurium caused much greater viscosity changes and accelcrated the weight losses. Compared to the polymethylsiioxane C-4 of the previous in5007
25
(A)
.*
20
&-400-
-
Vol. 42, No. 12
oxane chain to form products of lower molecular weights. In order to obtain some indication of the thermal stabilities of the polymethylphenylsiloxanes, samples were sealed in glass in the absence of and in the presence of metals under an atmosphere of helium gas as described previously (3). These samples were subjected t o temperatures of 250", 275', and 300" C. for 168 hours. The results of these experiments are given in Table V. Silicone H showed only negligible viscosity changes after tests a t 250' C. as compared to a 7 7 , drop in the viscosity of polymethylsiloxane C-4 ( 3 ) . After the 275" C. tests there was a 2 to 3% drop in the viscosity of H and a negligible increase in the viscosity of L. S o additional viscosity decrease occurred with fluid
0
0 1000 Y
'"t
CONTROL,Cu
(AI
re
I
AND DURAL
0
Figure 1.
48
96 144 TIME -HOURS
TIME - H O U R S
Influence of Metals on Static Oxidation Stability of Polymethylphenylsiloxane F at 225" C. Control = no metala present Cu = copper present Dural = durslumin P4ST present Pb = lead present To = tellurium present
vestigation (3)polymethylphenylsiloxane F is much more stable, as the former gelled between 120 and 168 hours. The higher weight losses of F are the result of its lower boilingpoint -430" C. a t 760 mm. as compared to above 250' C. a t 0.5 mm. for C-4 (9). Static oxidation runs were made on fluid F a t 250' C., but the evaporation and the accompanying foaming were so great that these experiments were discontinued. The results of the static runs on polymethylphenylsiloxane H a t 250' C. are plotted in Figures 2A and 2B. The weight losses and viscosity increases after 168 hours in the absence of metals and in the presence of the inactive metals are approximately the same as those observed in the dynamic-type experiments. The viscosity increaEta after 168 hours in the presence of lead, selenium, and tellurium are not greatly different from the results obtained by the dynamic method, but the weight losses are much greater by the static method. This is probably due to the partial condensation of the volatile siloxanes in the condenser used in the dynamic experiments. The visc.osity of the lead- and tellurium-catalyzed samples incrcased rapidly after 168 hours of exposure, the former causing the silicone to gel after approximately 288 hours. Copper and selenium also accelerated the rate of viscosity increases and weight losses after long exposure. No accelerative action was observed with copper in the dynamic test, and selenium was relatively inactive when air was used. However, both these metals appreciably accelerated the weight losses and viscosity increases after 168 hours of exposure in the static tests. Static test results a t 250' C. with fluid L (Figures 3A and 3 B ) are in substantial agreement with those obtained with H. Slightly higher percentage viscosity increases were observed for the uncatalyzed fluid, but the catalytic action of lead, selenium, and tellurium was not YO great as with H. The weight loss against time curves in the experiments with L are similar to those with HI and as would be expected from the viscosity graphs, the weight losses with lead, selenium, and tellurium were lower than in the comparable runs with H. Thermal Stability Experiments. It is known that the methylsilicones undergo thermal rearrangements a t temperatures of 250' C. and above ( 9 , 12, 24) because of the rupture of the sil-
6
Figure 2. I n h e n c e of Metals on Static Oxidation Stability of Polyrnethylphenylsi,loxane H at
450" C.
Control = no metals present Cu = copper present Pb = lead present So = selenium present Te = tellurium present Inactive melsis present: antimony, duralumin P4ST, nickel, silver, S A E I020 cold-rolled steel, tin, zinc
INDUSTRIAL A N D ENG INEERING CHEMISTRY
December 1950
H, but a viscosity increase of 4 to 5% was observed with L after being subjected to 300' C. The metals investigated had no apparent effect on the thermal stability of the polymethylphenylsiloxanes at 250' and 275" C. as judged by viscosity changes. Silicone H in the presence of selenium turned a straw-yellow color, but the other metals did nDt cause color changes, Bubbles of gas were observed to form in the fluids on cooling after an exposure a t 275" C. of 168 hours. It is believed that these gas bubbles were due to the decrease in the solubility of helium with decreasing pressure as there was little change in the viscosity of the fluids after these experiments. The surface of the lead specimen had darkened considerably whereas the steel specimens were unchanged. Neither fluid exhibited any change in coolor after the test.
2467
Table V. Thermal Stability Experiments with Polymethylphenylsiloxanes H and L (Sealed in glass under an atmssphere of helium) Visc. Change a t 100° F., % 188 Hours at Metal Present 250° C 2750 c. 300' C .
None (control) Copper Duralumina Lead Selenium Steel b Tellurium
I'olymethylphenylsiloxane H -1 -3 -1
-1 -1
Nil -1
. . ..
-2
-2
Brkken
-2
-2
..
...
..
Polyrnethylphenyldoxane L
None (control)
+1
Lead Steel b
+I
f 2
+.1
Broken +5
Polyiriethylsiloxrrne C-4
None (control)
-7
Duralumin 24ST. 8.4E 1020 cold-rolled.ateel.
Figure 3. Influence of Metals on Static O x i dation Stability of Polymethylphenylriloxane L at
--
250" C.
Control = no metals present Cu cop er present Dural = &alumin 24 ST present Pb = lead wesent Se selenium present Te tellurlum present
At 300" C. cold rolled steel had no effect on the thermal stabilities of H and L. Both vials containing the lead-catalyzed runs broke in the oven a t this temperature, and breakage also occurred in duplicate runs. It is presumed that low molecular weight silicones formed by "cracking" in the presence of lead raised the vapor pressure to the breaking point of the glass vials. CONCLUSIONS
The polymethylphenylsiloxanes are more stable to oxidation b u t more volatile than the completely methyl-substituted analogs of the same viscosity. No changes attributable to oxidation were observed a t 200' C. and only minor changes a t 225' C. A t 250" C. significant viscosity increases with the evolution of formaldehyde and formic acid occurred in the dynamic oxidation tests. No definite evidence of the rupture of phenyl groups from the siloxane chain were obtained. The viscosity increases, as in the polymethylsiloxanes, are attributed to the condensation of two or more siloxane fragments from which methyl groups
have been ruptured. As would be expected the rate of Oxidation of the silicones increased with the oxygen content of the atmosphere used. At 275" C. in an atmosphere of dry ail, gelation of the silicones occurred after approximately 90 to 150 hours of exposure, the more aromatic silicoues having the longer gelation time. At 300" C. with dry air, gelation of the silicones occurred between 8 and 24 hours. Using the inert gas, helium, at this temperature, no gelation occurred and the viscosity increases 01,served are attributed to the evaporation of the more volatile components of the fluid. Infrared spectroscopic studies confirmed the chemical evidence that the oxidation of the polymethylphenylsiloxanes proceeds primarily by the rupture and oxidation of methyl substitumk with the subsequent condensation of the resulting silowne fritgments or radicals, A comparison of the absorption intensity of the bands having maximum absorption a t 3.4 and 7 . 9 5 ~revealed a progressive diminution in the methyl content with increawig viscosity or extent of oxidation. The shape of the 7 . 9 5 ~ absorption band changed in a manner that indicated that the trimethyl end group concentration had decreased to a large extent if riot entirely. The concentration of the aromatic constituenk appeared to remain constant a t the temperatures used. The same metals found to accelerate the oxidation atid t l e composition of the polymethylsiloxanes in an oxidizing atmosphere (lead, selenium, and tellurium) were also active with the polymethylphenylsiloxanes. Duralumin, ?old rolled steel, c o p per, antimony, silver, tin, and zinc had a negligible effect. The static-type oxidation experiments were in s u b s t a n t d agreement 'with the results of the dynamic-type. Viscometric measurements revealed only minor evidence of the thermal instability of the polymethylphenylsiloxanes at temperatures up to 300" C. I n contrast, the polymethylsiloxanes gave viscvimetric evidence of "cracking" a t 250' C. LITERATURE CITED
(1) Am. SOC. Testing Materials, Method D 567-41. ( 2 ) Atkins, D. C., Baker, H. R., Fobes, H. F., Murphy, C . > I . , and Zisman, W. A,, Naval Research Lab., Was$ingtoii. D. C., Rept. P-2227 (January 1944). (3) Atkins, D. C., Murphy, C. M., and Saunders, C. E., IND. EN(;.
CHEM.,39, 1395 (1947). (4)Brophy, J. E., Larson, J., and Militz, R. O., Trans. A m . Soc. Mech. Engrs., 70, 929 (1948). (6) Brophy, J. E., Militz, R. O., and Zisman, W. A., Zbid., 68. 355 (1946). (6) Burkhard, C. E., Rochow, E. G., Booth, H. S., and Hartt. J , Chem. Revs., 41, 97 (1947).
INDUSTRIAL AND ENGINEERING CHEMISTRY
2468
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(11) Fox,
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(13) (14) (15) (16) Ibid., 2,441,320 (May 11, 1 9 4 8 ) . (17) Ibid., 2,457,677 (Dec. 28, 1948). (18) Hyde, J. F., and De Long, R. C.,J . .4m. Chem. So( 63, 1194 (1941). ( 1 9 ) Kauppi, T. A . , and Pedciwii, W . W.,Lubricatioii Eng., 3 , 17 (1947). ( 2 0 ) Kauppi, T. A . , and I’edrrsen, U’. IV., h’trtl. Pcfroleum ’ V e u Y , Tech. Sect., 37, 944 ( 1 9 4 5 ) . (21) Kauppi, T. A . , and Pedersen, LY. IV.,S.A.E. J o w n u l , 54, 121 (1946).
(23) Xeilsoii, J. Rudd, Crawford, 1.’. JV., arid Srnith, D. C.. J , Optical Soc. A m . , 37, 296 (1947). ( 2 4 ) Patnode, IT.,and n’ilcock. 1). I.‘., J . d r n . Chem. Sor.. 68, 358 (1946). (”5) Post, Howard \V., “Silicones and Other Organic Silicon C‘ompounds,” Sew York. Keinhold Publishing Corp., 1949. Richards, R. E., and Thompson, H . W . , J . Chem. Soc., 1949, p. 124. Itochow, E. G . , Clteni. Eng. S e w s , 23, 612 ( 1 9 4 5 ) . Rochow, E. G . , “Introduction to Chemistry of Silicones.” New York, John Wiley & Sons, 1946. Sauer, R. O., J . Am. Chem. Soc.. 6 8 , 954 (1946).
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