'November 1947
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
methyl compound was transferred from 0.01 A- hydrochloric acid, it eshibited much higher contact angles (Table I). Hydrolysis, catalyzed by t,he acid, probably generated a great number of hydrosyls which, by condensation with those in the glass surface, more firmly fised t.he film. Of all the compounds discussed, those of the type (R,SiOI, appear t,o be the least reactive hoivever applied t,o the glass surface (Figure 9'1. -4marked degree of orient,ation is not manifest initially as observed from contact angle measurements. Subsequent heating in the case of the dimethl-lsiloxane film, for example (Figure TI, so011results in a niasimum value characteristic of t h e compound Presumably, the heating has facilitated displacement of t h e adsorbed v a t e r molecules on the glass surface t o allon- highly intimate contact and probably chemical reaction n-ith tlic silosane in an oriented position. Films applied t o glass using eir1ii.r Iidiiles ur their hydroll-sis products, aft,er propcxr thc.rnid tre:itiiieiiT. are not removed by solvents which ordiiisriljdissolw such hydrolysis products. It is t,herefore not unreasonable,t o consider that chemical attachment, plays an important part in cit,her case. I n the case of the organosilicon halides, orient:ttion may he considered as prerequisite t o the occurrence of a reartitin. -1sa result, plates so treated immediately show the charactc,i,iitics r ~ foriented films. ELr:c.rRi(. ..iL PROPERTIES. The surface resistances of the treatetl niicroacope slides were dcterniined a t 50y0 relative humidity hy nie:lsurements betnceii 2-em. silver bands 1.5 cm. apart at n potential of 500 volts. .1 number of treat,ments including 3t SiCl,, StMeSiCl,, niethylhydrogenpol>-siloxane,dimethylsilosane, diethylsilosane, and nieth~lplienylsilosanepolymers $iio\vetl surface resistances in the order of 10'2 t o 10'3 ohms against n value of 108 t o 109 for tlic untreated glass slide. On testing the treated slides after 15 minutes of heating a t various tivities persisted up t o 375temperatures, the high surface r 400' C., hut the films burned off above this temperature range. Although stearic acid films shorved resistivities of the same order of magrlituric. the effect \vas lost sharply above 300" C.
1395
I n general the data show that, when the contact angle of a treated surface drops below 50-60", the electrical resistivity diops by a factor of the order of lo4. There does not seem t o be any direct correlation between resistivity and contact angle for angles above 60". The resistivlties are pretty much the same ithether the contact angle is 70" or 110'. From Livingston (9) a contact angle of 50-60" would correspond t o about 75q0 of a complete layer adsorbed. On this basis three fourths of a complete layer is sufficient coverage such t h a t the film conductivity becomes quite small. -1possible conclusion, then, is t h a t for contact angles greater than 60°, the conductivity is due chiefly to the subsurface moisture which passes from t h e atmosphere through the layer of organic molecules and into the surface la\-ers of t h r glass. Thus, Ivhile the contact angle, determined by the characteristics of the top of the organic layer, is high, the resistivity reaches a limiting value, since the conductivity is deti~rminedby the moieture content just beloiv the organic layer. LITERATURE CITED
K., "Physics and Chemistry of Surfaces," 3rd ed., p, 182, London, Oxford Univ. Press, 1911. Adam, N. K., and Jessop, G., Proc. R o y . SOC.,110, 423 (1926). Hardy, IT.,and Biicumshaw, I., Ihid., A108, 2 (1925). Hunter, 11.J., and Fletcher, H. J. (to The Dow Chemical Co.), G. S. Patent 2,415,389 (Feb. 4 , 1947). Hunter, hl. J., Hyde, J. F., Warrick, E. L., and Fletcher, €1. J., J . Am. Chem. Soc., 68, 667 (1946); Hunter, SI. J., Tarrick, E. L., Hyde, J. F., and Currie, C. C., Ibid., 68, 2284 (1946). Johansson, 0 . K., and Torok, J. J., I'roc. Inst. R a d i o Eng., 34, 296 (1946). Langmuir, I . , J . Am. Chem. SOC.,39, 1848 (1917). Langmuir, I . , Trans. Faradag Soc., 15, 62 (1920). Liringston, H. K . , J . Phgs. Chem., 48, 120 (1944). Korton, F.J., Gen. Elec. Rev., 1 (Aug. 1944). Yorton, F. J. (to General Electric Co.), U. S.Patent 2,386,259 (Oct. 6 , 1945). Sauer, R . O . , Scheiber, W. J., and Brewer, S. D., J . Am. Chem. Soc., 68, 962 (1946).
(1) Adam, N.
(2)
(3) (4) (5)
(6)
(7) (8) (9) (10) (11) (12)
RECEIVED M a y 28, 1947.
POLYMETHYLSILOXANES..
..
Thermal and Oxidation Stabilities D. C. ATKINSI, C. M.
M U R P H Y ,A N D C. E. SAUNDERS
NAVAL R E S E A R C H L A B O R A T O R Y . W A S H I N G T O N . D . C.
. i R L 1 in the war the polvorganosiloxanes (19), or silicone fluids, were brought to the attention of the S a v y although they Tv-ere still in the development stage. Because of their small rates of decrease of viscosity with temperature, their lo^ vapoi pressures and low freezing points, and the vide range of viscosity grades available, these fluids were investigated as possible lubricants and poTver transmission fluids for unusual applications. -4s the polymethylsiloxanes have very low temperature coefficients of viscosity and adequately low freezing points and vapor pressures, this type of silicone polymer has been the most carefullvstudied t o date. The synthesis and many of the physical and chemical properties of the methyl-substituted polyorganosiloxanes have already been described ( 5 , 4 , 6 , 7 ,11, 12, 17, 18, 19, 81, 2 3 ) . Silicone polvmers may be prepared having linear, cyclic, branched, and cross-linked structures ( 4 , 11, 17, 18)) depending .on the methylchlorosilanes hpdrolvzed. The commercial polymethylsiloxanes ( 4 , 17, 18, 19, 21) are mixtures of essentiallv 1
Present address, The Cniversity of California at Loa Angeles, Calif
linear homologs with a more or less wide range of molecular weights, depending on the viscosity. The polymethylsiloxanes may be used as lubricants under certain conditions ( 5 ) and as hydraulic fluids in systems employing gear and piston type pumps (9). Other applications to lubrication have been discussed (6, 14, 15, 18, 19, 21,bd). These fluids were also found t o be much less flammable than commercial lubricants and hydraulic fluids (20). The polymethylsiloxanes have been reported to be very resistant to heat and thermal oxidation, but no information is available concerning the safe temperature range of operation, the nature and objectionability of the decomposition products, and possible catalytic effects of metals. This article is a summary of our work on the oxidative and thermal breakdown of polymethylsiloxanes and related catalytic effects. Table I lists the polymethylsiloxanes discussed and some of their viscometric properties. The viscosity-temperature coefficient (24, $5) is defined by the relation,
1396
INDUSTRIAL AND ENGINEERING CHEMISTRY
THE polymethylsiloxanes (or silicone fluids) are unusually stable to oxidation. No significant changes attributable to oxidation have been observed at 175" C. A t 200" C. oxidation occurs as revealed by- viscosity changes and the evolution of formaldehyde and formic acid. The increase in viscosity of the fluids after oxidation is attributed to the condensation of two or more siloxane radicals from w-hich methyl groups have been ruptured. The oxidation stability of the fluids decreases rapidly above 200' C., the maximum temperature at which they may advantageously he used in an oxidizing atmosphere. Copper, lead, and selenium inhibit the oxidation of the silicone fluids at ZOOo C. as reflected by decreases in evolution of formaldehyde and formic acid. Copper and selenium also inhibit the viscosity changes, and tellurium accelerates oxidation
~ T =C 77100'
F.
-
77lOOO
There
7 =
l121Qo F.
F.
kinematic viscosity
The fluids were made available by the Corning Glass Company Fellovahip a t Mellon Institute, the DOITCorning Corporation, and the Research Laboratories of General Electric Company. were prepared a t Mellon Institute early in the Fluids A, B, and \IT w r and are not typical of present commercial production. These fluids are believed to contain some unreacted hydroxyl groups, and increased in viscosit,y with time when stored in the dark at room temperature. The other fluids showed insignificant viscosity changes after one year under these conditions. Fluid E-3 TYas specially prepared t o have low temperature characteristirs and and contains branched structures. The other silicones are essentially open-chain polymers of polymethylsiloxane with trimethyl terminal groups, and probably differ only in the methods of synthesis. The infrared adsorption spectra of many of the silicones xere examined, and no branched or cyclic structures could be detected. STABILITY TESTS
Several different procedures were used in the investigation of the thermal and oxidation stabilit,ies of t,he silicones. The dynamic type of aeration apparatus ( 2 ) was used extensively. * -1 25-gram sample was used n i t h a gas flow of 20 nil. per minute. Runs were made at 175", 200°, and 225" C. (*l.5'), and lasted 168 hours. The effluent gases from the oxidation cell were bubbled through 10 ml. of 0.1 S potassium hydroxide solution, Ivhich was back-titrated a t intervals with 0.1 S hydrochloric acid t o determine the amount of volatile acidic products (calculated as moles of formic acid per gram of sample). The neutral solutions from the determination of volatile acids were tested for aldehydes, by means of the hydroxylamine hydrochloride method (15') with bromophenol blue as indicator. It n'ap difficult t o obtain reproducible results with this method and, as formic acid also reacts with hydroxylamine hydrochloride, the bisulfite method (1) was adopted. The viscosities of t,he fluids before and after each test, and in some instances at intervals during the 168-hour run, were determined in Cannon-Fenske modified Ostwald viscoiiiet'ers according t o 4.S.T.M. ;\Iet,hod D445-42T. The effect, of met?ls on the thermal and oxidation stabilities of a typical commercial silicone (C-4) was studied b y aerating it a t the test temperatures in the presence of clean, polished metal strips having dimensions inch. of 11/,X l / , X The metals investigated include antimony, cadmiuni, copper, lead, nickel, platinum, selenium, silver, tellurium, tin, and zinc; each was a t least 99.8% pure. The alloys were duralumin 24 ST, SAE 1020 cold-rolled steel, and SAE 30915 stainlcss steel. Experiments were made with silicone C-4 in the absence and presence of metals using air, oxygen, nitrogen, and helium. The air \?as purified as described ( 2 ) . The oxygen and nitrogen had a minimum purity of 99.5%; nitrogen comprised the bulk of t,he impurities in the oxygen, and oxygen was the principal impurity in the nitrogen. The heliuni was N a r y balloon grade, 98.2% pure; nitrogen was t,he principal impurity, and the oxygen con-
Vol. 39, No. 11
at this temperature. hone of the other metals in\estigated significantly affects the stabilits of the fluids at 200' C. i t 225O C. tellurium inhibits in the dynamic method, as do copper, lead, and selenium; ebaporation losses with these metals are abnorniall? high. IIexameth?lcyclotrisilolane and octamethylcyclotetrasiloxane are identified among the products evolved. A reaction of lead oxide with the siloxane is postulated to explain the high e\aporatioii losses obserbed with lead. The results of the static tests, although less accelerated, are in substantial agreement with those by the djnamic method. \iscometric eTidence of the thermal instabilitj of the polymeth31siloxanes at 250' C. has been obtained. Thus, at this and higher temperatures in an oxidizing atmosphere hoth cracking and oxidation take place.
tent \vas less thnn O . l c i . Traces of the lower hydrocarbons'and carbon dioxide may also have been present. All gases'nere dried by passing through anhydrous calcium chloride. Static-type oxidation experiments were also made on t,he same silicone fluid a t 225" and 250" C. This consisted in heating 25 grams of fluid in a 100-ml. beaker in a forced-draft oven from t o 168 hours. The oven temperature was controlled to = 1.5" Xetal specinlens of the same size were used in this procedure i n the dynaniic method. The change in viscosity and the evaporation loss were found t o be valuable criteria of st,abilit>;. The stability of this silicone in a closed system was also invcstigated. 1 2 5 - g r a m sample in the absence and presence of various metals was sealed in a 50-ml. Pyrex vial under atmospheres of air, nitrogen, and helium gases. and niaint,ained a t 250" C. for 2 4 , 72-, and 168-hour intervals. The dissolved air was replaced by bubbling the desired gas through the sample for 30 minutes before the glass vial was sealed. The change in viscosity was used as a criterion of thermal stability. DYNAMIC OXIDATION
The silicones listed in Table I were oxidized with air a t 200" C. a n all-glass system, as just described. The viscosity of t h e oxidized sample was determined a t 24-hour intervals, as were t h e moles of formaldehyde and formic acid produced. The presence of formaldehyde in the volatile oxidation products \?as shown by the reaction with chyomotropic acid (1,8-dihydroxynaphthaIene3,6-disulfonic acid) (8). This \vas confirmed by the melting point of thc diniedone (5,5-dimethylcyclohexane-1,3-dione) drrivative (IO). I n some runs the odor of formaldehyde was readily detectable. A white dcposit !vas also observed after in the condrnsers almvr the oxidation cells. Upon iIi
~
~~
~
TABLE I. Source hIellon Inst.
Identification n--2 w-3 A-1 A-2
n-i DUW
Corning
General Electric
B-2 B-3 B-4 1 C-2 c-3 0-4 D-1 D-2 D-3 D-4 E-1 E-2 E-3
c-
E-4
Viscosity Kinematic ViscosityTemp. at loo0 F., A.S.T.M. Viscosity Coefficient Centistokes Slope Index 0.584 138 . 0 238 1.14 0.585 232 0 219 0.641 22 Y 0 410 191 0.649 65 6 0 323 161 210 0,249 139 0.651 0 642 2-17 0 240 0 641 2i6 0 242 0 653 359 0 216 0 581 1Oi 19 i 0 311 0 591 169 43 -1 0 288 0.591 169 0 288 i4 9 0.594 156 ,2 1 0 219 153 0.594 82 5 0 253 162 0 221 140 0.595 2i 1 0 203 .. 0,695 640 0 183 .. 0 601 0.58S 0.594 0,622 0.597
November 1942
*
'
1397
INDUSTRIAL AND ENGINEERING CHEMISTRY
heating this deposit, foi~maldrhytlewa identified as a deconiposition product. I t also reduced Fehl g solution; therefore, i t was concluded to be parafornialtleliyde, a polyosymethylene. Even !There no deposit of paraformaldehyde was observed, it is possible that polymers of lon-er molecular n-eight were formed. As fornialdc.hyde is readily oxidized, it is likely that formic acid was present. Its presence n-as confirmed by the increase in aldehyde content after rrductinn of t,he solution containing the volatile products. Silicones -\, B, and 11- i n c r e a d i n viseusit>-rapidly n-ith time. .ill of them gelled within 72 hours of osidation with the exception of h-1, ivhich required 125 hours for gelation. 8iliconr B-3 required only 25 hours for gelation. These silicones are believed to contain some unreacted hydroxyl groups, since they reacted n-ith lead oxide (PbO) at room temperature to form a s e m i d i d mass. Patnode and Schmidt ( 1 6 ) diRcussed this reaction and showed t h a t trimethylsilanol reacts Jvith lead oxide to form an insoluble product. No definite evidence of a reaction with lead oside was found x i t h any of the other silicones listed in Table I. The viscosity changes observed for these siliconca on oxidation are, iri part, due to the condensation of the silanolc to form larger molecules and thus increase the vixosity. Figure I d s h o w viscosity changes with time fur the conipletely methyl-substituted silosanes. S o n e of the C or E series silicones oxidized to a gel within the 168-hour period. Fluid E-3 showed the greatest viscosity incrrase for this group. Only the leaat viscous of the D series remainrd liquid during the oxidation t w t ; the gelation time of the others in this series varied inversrly with initial viscosity. The number of moles of formaldehyde and formic acid produced is plotted against time in Figure 1B. This quantity is only approximately equivalent t o the number of nioles of methyl groups cracked off or oxidized froni the siloxancx chain, sirice paraformaldehyde is kno\vn t o be formed in some runs; ir is also likely that some formaldehyde is osidized to carbon dioxide. Figure 1 indicates that the lower the viscosity of the silicone, the snialler the changes in viscosity with times of oxidation. Thv viscosity changes in the methyl-substituted siloxanes may be accounted for by the condensation of tn-o or more of the silosane residues from n-hich the methyl groups were ruptured. The apparent stability of the less viscous silicones as revealrd b>-viscosity changes is misleading, for as much formaldehyde p l u ~formic acid n-a.q evolved by them as b!- the more viscous silicones. S o correlation between viscosity increases and the amounts of methyl oxidation products formed by the various silicones was obt,ained, except the generalization that the viscosity ir?creaseP for of formaldehyde and formic acid were grratcr us .silicones. I t seems unlikely that any simple general relation can be found to hold for a mixture of linear polymers, as the viscosity increase caused by the addition of a polymer t o a solvent is a function of t h r molecular weight, niolwular w i g h t distribution, configuration, solubility of the polymer, arid tht, viscosity and nature of the solvent. S o Si-Si linkages could he detected from the i n f r a r d a h o r p tion spectra of thrse oxidized ,silicones, and it seems prohable that the condensation products are linked through oxygen. Seithc.1. was there any definite indication of branched or cyclic structures in their spectra. l s the sensitivity of this method t o ruch structure decreases with increasing molecular w i g h t s , their complet(, absence is riot r.rahlished. Hon-ever, there n-as no marked increase in their concentration. This suggests that thc, methyl groups ruptured or oxidized from the siloxanr chairi are priricipally from the terminal silicon atoms. The only silicone (E-3) known t o contain branched structures showed abnornial viscosity increases for the aniounts of formaldehyde and formic acid produced, as compared to other fluids of comparable initial viFcosity. This may be attributed t o the greater thickening action of the branched-chain condensation products of oxidation. The early gelation of some of the D series fluids would contradict the h - p o t h 'esis just suggestrd, since no branchtd structures w r c detcctc,d
1
1000 ' ' o o [ D I GEL
IN 72 HRS.
900
24
'
0
k
1
1
48
72
1
1
I
1
96
120
1
144
,
l
168
i
GEL IN 72 HRS.
K
t-c
I
/GEL
IN
/ ,
4
TIME, HOURS
Figure 1. Comparison of Dynamic Oxidation Stabilities of Silicones at 200" C.
in those t~saniincd. 1Ion-c it is poscible that the coriceritration of the hranchetl-chain siloxanes wa.s too small for detection by infrared spectroscopy. The oxidation with air of commercial silicone C-4 a t 175" C. for 168 hours revealrd that the only significant viscosity changes took place within the first 24 hours. This is attributed t o the evaporation of thc lon--molecular-n-eight polymers present in t h e original fluid. Thew way no appreciable evolution of acids or formaldehyde. Table I1 gives results of the dynamic tests a t 200" C. on silicone C-4 with air, oxygen, nitrogen, and helium gascs. The effect of temperature variations d u r to the control of the thermostat are reflected in the results shown, since the check runs were not made simultaneously. Repeat runs showed that the viscosities of the silicone fluid, aftcr the 168-hour test with a given gas, agree reasonably n-ell and compare farorably with the reproducibility I,
INDUSTRIAL AND ENGINEERING CHEMISTRY
1398
obtained by similar oxidation methods on hydrocarbons. As would be expected, oxygen accelerated the viscosity changes more than air; the increase amounted to approximately 50%. With nitrogen and helium gases the viscosity changes were, respectively, approximately one third and one tenth that obtained with air. A small amount of a white deposit was again observed in the condenser after several of the oxygen runs and was identified as paraformaldehyde. For the reasons previously discussed, the number of moles of formaldehyde and formic acid produced is only approximately equal to the number of moles of methyl groups cracked or oxidized off the siloxane chain. Small amounts of formaldehyde were produced when nitrogen and helium gases were used. The acidity observed was presumably due t o formic acid. With the gas flow used, a 0.1% oxygen concentration in the helium is more than sufficient t o account for the formaldehyde and formic acid found. The viscosity increases observed with the different gases are roughly proportional to the number of moles of these products formed. Table I1 also gives detailed results of experiments on the effect of metals on the thermal and oxidation stabilities of fluid C-4. Table I11 was prepared from Table I1 in order to compare the effects of the various metals as reflected by the difference in viscosity changes, volatile oxidation products evolved, and evaporation losses. Duralumin, cadmium, silver, cold-rolled steel, tin, and zinc had no appreciable action on the stability of the silicone. The presence of tellurium caused increases in viscosities a i t h corresponding increases in the evolution of volatile oxidation products when oxidized with air or oxygen. The runs with nitrogen and helium w b e normal as compared to the control. Copper and selenium acted as inhibitors, reducing the evolution of volatile products and viscositv changes t o those obtained in the control runs with helium. I n the runs with nitrogen the volatile oxidation products evolved were also low when copper and selenium were present. During the course of these runs some of the selenium was sublimed and deposited as amorphous red powder in the cool condenser tube. The fluid had a pink tinge, probably due to the fact that the selenium was colloidally dispersed. The color of the fluid \vas more pronounced when hot but this may be
Vol. 39, No. 11
partially explained by the settling of the larger particles after the bubbling had ceased. The presence of lead in the silicone fluid caused larger viscosity increases than in the control runs. T h e total amount of formaldehyde and formic acid was low when air, oxygen, and nitrogen were used, but the evaporation losses were high in all these cases. The silicone fluid was turbid after all the runs except where helium was used. The turbidity was observed to increase with time of oxidation. Dynamic type tests a t 223" C. were also made on the same silicone fluid. I n the absence of metal the fluid gelled within 24 hours when air or oxygen x a s used. The amount of formaldehyde plus formic acid formed in 24 hours was twice as great as the amount formed in 168 hours a t 200" C. I n all these runs there was a heavy deposit of paraformaldehyde in the condenser tube after both air and oxygen runs. The evaporation losses a t gelation were approximately 2%. In the 168-hour runs with nitrogen and helium gases the viscosities and the evaporation losses were approximately t.ivice as great as after the tests a t 200 C. Runs Lvith air in the presence of various niet,als were also made, and (as in the control) the silicone fluid turned to a gel n-ithin 24 hours except \Then copper, lead, selenium, or tellurium were present. The fluids gelled between 72 and 96 and beta-een 96 arid 120 hours, respectively, in the presence of copper and lead. T h e fluid remaining after 168 hours in runs when selenium and tellurium were present was less viscous than the original fluid, having a viscosit?- of 60 centistokes a t 100" F. The weight loss amounted to approximately 70%, and only small amounts of iornialdehyde and formic acid viere produced. A considerable volume of a volatile water-insoluble liquid was collected in the aqueous solution through which the effluent gases were bubbled. h crvstalline deposit also collected in the condenser tube after each run n i t h air in the presence of lead, selenium, and tellurium. T h e liquid \vas ident,ified spectroscopically as being predoniinantly octamethylcyclotetrasiloxane, and the crystalline deposit a s
hexamethylcyclotrisiloxane.
From t,hese experiments i t is apparent that the polymcthyisiloxane fluid is much less stable to oxidation a t 223" than a t 200"' C. No accelerative action of metals oil t h e viscosity changes could be detected a t 225" because of the rapid gelation encountercd i n t h e cont'rol runs. The presence of copper, lead, TABLE 11. RESULTS OF DYNAMIC OXIDATION TESTS AT 200" c. O S FLUID C-4" selenium, and tellurium a t 223' inhibited t h e rate of change of viscosity and the evolution of formaldehyde and formic acid. h t 200" C. lead did not materially affect the ehangcs i n None 100 108 8 1 . 4 7 8 . 6 15 25 , . 2 3 2 1 1 viscosity, although i t reduced the evolution of (control) 105 137 9 2 . 7 7 6 . 0 17 21 . 1 2 . 1 1 formaldehyde and formic acid. Telluiium' a t 102 116 8 6 . 2 7 4 . 1 18 26 9 Trace 2 2 2 1 122 119 81.2 20 26 8 .. 2 2 2 the lower temperature accelerated the oxitlatiOD 9 4 . 1 130 79.2 16 30 8 .. 1 3 2 118 112 80.6 28 12 .. 2 1 1 rate as reflected by these criteria. All of these 120 83.6 7 6 . 2 17 26 Average 107 9 1 2 2 1.5 1 metals accelerated the evaporation of the sili119 8 5 . 2 7 8 . 8 25 Duralumin 114 23 9 2 2 2 .. 1 cone, as compared t o the control runs witb 24 ST 140 helium. This suggests that they react in some Cadmium 101 113 8 0 . 6 7 8 . 9 16 22 7 3 2 2 ,. 1 manner to break the siloxane chain with t h e Copper 82 6 7 9 . 6 88.3 78 0 6 4 2 3 .. 1 2 1 formation of lower molecular iveight and inorr 79.6 81.1 2 3 1 3 volat,ile products. I n the runs with; lead the silicone became turbid within the first 24 hours of oxidation, Selenium 78 1 7 6 . 4 7 8 . 2 7 8 . 4 2 2 1 2 2 2 2 1 and the turbidity increased with time. Xt the 7 7 . 5 76.9 7 8 . 1 7 4 . 4 2 Trace 1 Trace 2 1 1 2 end of the run a xhite deposit had settled in Silver 114 125 7 6 . 1 7 9 . 2 21 21 10 .. 3 2 2 1 the bottom of the oxidation cell. Patnode a n d 86 128 16 28 .. 1 Schmidt (16) showed t,hat a reaction takes Steel, cold- 112 127 105 7 8 . 6 15 28 . . 1 3 2 2 2 rolled place between trimethylsilanol and lead oxide Tellurium 114 145 7 8 . 2 7 7 . 6 16 32 . . Trace 1 1 , . 1 which they consider t o be as follows: 133 146 7 9 . 0 34 26 1 2 1 1 Tin Zinc 5
112 107
127 80.4 7 6 . 8 14 116 7 7 . 5 7 6 . 8 19 148 Initial viscosity, 7 2 . 4 centistokes a t 100° F.
21 22 20
11 9
2 1
2 2
2 2 2
1
1
2
1
2(CH&SiOH
+ PbO +[(CH&SiOl2+ H 2 0
It is possible t h a t under the influence of heat, lead oxide reacts with a siloxane t o break t h e
.
.
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1947
TABLE 111. AN.4LYSISa O F AND
EFFECT O F AIETALS O N THERMAL AT 200" C. FOR 168
OXIDATION STABILITY OF FLVID C-4
1
IOoo 900
HOCRS
700 k 600 cn w Y
1
1399
800
0":
Duralumin X N K Ii N K K N N N N 24 S T Cadmium S N N N +? N N N N Ii N Copper N S N N N N N N Lead +7 N - . y $ $ G N Selenium - ,. x ,' " Silver N X K N N N X N N N N Steel. coldN N X N N N N S N S N rolied Telluriuni S N . S N N Y Tin K N S X N N I i h ' N X S Zinc N i Y N N N - s X h - . \ T x S increase as compared to a S = normal a s compared t o control, trol, - = decrease a8 compared t o control.
+
+ +
+ A
+ +
+-
N
N N
X
E 500
-
N
'"
-
N
E
400 -
N N con-
I-
-
0
300-
-
cn 0
2-
200-
-
>
chain and thus form a compound with lead similar t o the preceding. T h e reaction may be represented as follows:
CONTROL RUNS e INACTIVE METALS
100
(CH3),Si[0-8i(CH&].O-Si(CH3~,$- PbO +
(CI13)3S1[O-Si(CH3)2],0
(CH,)&3i[0-Si(CH3),],O
)Pb
Thc turbidity may be due to the presence of such a compound. If this product is thermally unstable, the loiver-molecular-n.eight silicones formed mould evaporate; this would account for the losses in weight observed. The increase in evaporation losses with increasing temperature could be accounted for by the greater speed of the reaction or the thermal instability of the intermediate. hltliough no insoluble products were observed with the other metals causing large evaporation rates, it is possible that they or their oxides may react as postulated. Dynamic-type oxidation experiments a t 230' C. were started, but no further work was done after several explosions Tvhich were attributed t,o the spontaneous ignition of some of the silicone deconiposition products. STATIC OXIDATION
Static rests FTere used at the higher temperatures since they are 'simpler and less accelerative than the dynamic tests. The same silicone fluid, C-4, was used. It was heated a t 225" C. in the absence and presence of antimony, copper, lead, nickel, platinum, selenium, silver, tellurium, tin, and zinc; each m-as a t least, 99.8% pure. Alloys used Fvere duralumin 24 ST,SAE 1020 cold-rolled steel, and ShE 30915 stainless steel. T h e viscosity a t 100" F. and the evaporation loss were determined after exposures of 24, 48, 72, 96, 120, 'and 168 hours. The fluid gelled in all cases between the 120-168 hour interval except in the runs made in the presence of antimony, lead, and selenium. Antimony caused the gelation of the fluid between the 96-120 hour interval; when lead or selenium was present, t,he fluid did not gel after 168 hours of exposure. Figure 2 1 shows the variations in viscosity of the fluid with ' tiniu. The presence of most of the metals caused no abnormal viscosity changes a s compared to the control runs, as indicated by the narrow cross-hatched band in which these curves all lie. The samples containing lead and particularly those containing selenium caused much smaller viscosity changes than those found in the control runs. Tellurium and antimony increased the viscosity somewhat more than did the control. The shape of the viscosity-time curve indicates t h a t the effect mas greatly accelerated somewhere between 72 and 96 hours, the curve rising more rapidly than an exponential. Figure 2B gives evaporation-time graphs. S o great variations in evaporation rates were observed between the control runs and those in the presence of metals, as shown by the narrow cross-
4 2
0
24
48
72 96 120 TIME, HOURS
144
168
Figure 2. Influence of JIetals on Static Oxidation Stability of Polpmethjlsiloxane C-* at 223" C. 1 = antimony; 2 = lead; 3 = selenium; 4 = tellurium. Inactive metals: copper, nickel, platinum, silver, zinc, duralumin 24 ST, cold-rolled steel, stainless steel
hatched band. Only lead greatly accelerated the evaporation rate; antimony and tellurium caused much smaller increases. A s Kith the viscosity changes, the evaporation rate increased rapidly between 72 and 96 hours. The gradual decrease in evaporation rate after 120 hours is attributed to gelation. Metal specimens of lead, zinc, and tin having ten times the surface area of the standard specimens and 20 grams of the granular metals having even larger surface areas were run in the silicone Increasing the surface area of tin and zinc had no appreciable effect on the vicosity changes, and the evaporation rates were increased only slightly. Increasing the surface area of the lepd specimen by a factor of 10 increased the evaporation rate by a factor of 2. The granular lead of much larger surface area caused
1400
TABLE Iv. RESULTS O F sT.ITIC OXIDATIOS TESTS hT 250" C. O S FLUID C-4 Metal
Vol. 39, No. 11
INDUSTRIAL AND ENGINEERING CHEMISTRY
Viscosity a t 100O F., Cs. 24 hr. 48 hr.
Bone 104 None 101 102 Average Duralumin 24 SI' 104 Antimony 103 Copper 107 Lead 83.8 Kickel 90.0 Platinum Selenium 1;; 8 Silver 101 Steel, cold-rolled 110 Steel, stainless 105 Tellurium 88.4 Tin 116 Zinc 91.0 a Gelled within 72 hours.
Gel Gel Gel Gel Gel 1226= Gel Gel Gel
Gel
Gel Gel Gel Gel Gel Gel
Weight L o s , 5 24 hr. 48 hr. 2.5 2.5 2.5 2.7 2.7 2.4 10.3 2.6 2.5 3.6 2.4 2.6 2.4 2.4 2.8 2.4
5.6 5.4 5.5 5.5 5.2 4.9
'6.9
a. I
5.5 6.3 5.4 5.3 5.1 4.7 5.3 5 0
only a slight, additional increase in evaporation rate. These results indicate that, the reaction is not surfaee catalytic. T h e reaction previously postulakd t,o explain the action of l ~ a do n the silicone necessitates its oxidation t o the oxide (PbO). -4s the only source of oxygen is t h a t in solution, the rate of the oxidation of lead 11-ould be expected t o be sloy, anti this would govcm the evaporation rate and not t h e area of the lead surface exposed. Varying the surface area of the lead sprcimens caused only iiegligible changes in viscosity. This !vas expected. since t l i v tlynamic esperiments showed t h a t the prrsence of lead niateri:dly inhibited oxidation, as indicated by the decrcxascs in the aniouiits of formaldehyde and formic acid evolved. Therefore the predominant reaction a t 225' C. in t h e static test is the cracking of t,he siloxane chain caused b- lead oxide. .Is a result of the rvaporation of the low viscosity s losanes produced, oril>-sinall changes in viscosit'y are t o be expeckd. T h e dynamic test is more accelerative than the static t w t , as there is a more int,imate contact between the fluid and air or oxygen, Whereas a n esposure of over 120 hours vias r r q u i r d t o gcl t h e silicone fluid at 225' C. by the static method, le.-.* than 24 hours [vas required in t h e dynamic test. In the d>-namicmethod the prrsence of copper, lead, and especially selenium and tellurium greatly accelerated the evaporation effect and rc'tardcd t h r viscosity rise. I n the static test only lead causcd abnormally high weight losses. Tellurium accelerated the viscosity incwases in t h e static test while it caused a decrease in viscosity \)>- t h r ti?namic method at 225' C. If the oxides of these metals arcelerates the rupture of t,he siloxane chain, it is apparent 1 hat 1he air in solution is the only source of oxygen for this reaction. In the dynaniic test fresh air continually siveeps o m r the metal surfacr, agitates the liquid, and hastens the evaporation of the volatile reaction products. As in the dynamic test, t'he fluid containing lead v a s cloudy even after 24 hours of exposure t o the static test, and t,heturbidity increased wit,htime. Similarly, selenium caused a slight pink color t o develop in the fluid IThich faded as the fluid cooled. Table IT gives the results of t h e static oxidat,ion of siliconr' C-4 a t 250' C. At this temperature the silicone gelled somen-hew between 24 and 48 hours of exposure. As the silicone fluid gelled so rapidly at this temperature, i t iyas difficult t o ascertain the effect of metals. Copper was the only metal that inhibited gelation. ..lft,er 24-hour exposure only selenium, Icad, tellurium. nickel, and zinc inhibited the viscosity increase, the effect being very marked for the first txvo. No such effect n-as observed with nickel and zinc at the l o m r temperatures. .Is the osidntioii stability of the silicone is very sensitive t o temperature in this range, it is possible that the apparent inhibitive action observed is due t o variations in temperature control. The evaporation rate in the presence of lead was four times as great as in the control after 24 hours and was three times as great aftvr 48 hours.
The decrease in t h e evaporation rate after 24 hours is probably due t o the gelation of the sample. T h e presence of selenium also caused a n increase in the evaporation rate, but it was only 50% greater than the control. After each of the runs lasting 48 hours, a small amount of white powder was observed on the walls of t h e beakers and on the top of the gel. This was believed tmobe silica or a highly cross-linked siloxane network. Alt,hough the static oxidation experiments 'Ivere less accelerated the'results are in substantial agreement x i t h those of the dynamic type. Both show that t,he oxidation stability of the polymethylsiloxane fluid dccrcascs rapidly as the temperature is raised above 200' c'. THERMAL STABILITY
It has been shown (11,1 7 ) t h a t the polymethysilosancs undergo t,hermal rearrangements at 350" t o 400" C. with the rupture of the silosane chain t o form products which are predoininatcly cyclic siloxanes of low molecular weight. T h e dynamic experinients a t 225" C'. revealed t h a t the presence of some metals, notably lead, selenium, and tellurium, in an oxidizing atmosphere greatly accelerated the rupture of the siloxane chain with t h e formation of cyclic products. As the cyclic products of low molc>cular ivcight are volatile at 200" C. and above, visbomrtric evidence of tlir thcrinal instability of the silicone would probably be masked hy t,lic>viscosity increases duc t o oxidation and the cvaporation of t h c b lo\~-iiiolecular-n-eigllt.siloxanes originally present in t,hr fluid. T o obtain some indication of t h e thermal stability of the ~)olyiiictli~lgilt,ssnes,silicone ('-4 ivas sealed in a I'yrcu vial with the various gases, as described under "Stability Tests," and hvatrd at 250' C. The vixosities of t h e fluids were determined after 24, 72, and 168 hours of exposure (Table V). These data reveal that cracking does occur and t h a t equilibrium has not been ieac.liid at 168 hours. The viscosity decreasrs w r e greater for t h c aaniples under an atmosphere of air. Atmospheres of heliuni and nit rogen resulted in smaller rates of viscosity change. Probably t h e control runs under the inert gases v.-ould approach the same limiting value a t equilibrium. Under an atmosphere of helium nonf' of th(z nictals affectrd the rate of the viscopity change .;ignificantly, as compared t o the control run. ('opper, nickel, and cold-rollcd steel n w e also inactive under atmospheres of air :md nitrogen. The metals (lead, selenium, and tellurium) which iiccelcrat,ed t,he evaporation of the silicones and inhibited the visrosit,y increases in the dynamic experiments were also found t o ' ciffect the thermal stability of the silicones as reflected b y the dec'reases in viscosity. Lead caused only a slight acceleration in the viscosity decrease as compared t o the control run under these :itmospheres. In the presence of selenium the viscosity of the
Atmosphere Air
IIeliurii
2
Netal None Copper Lead Sickel Selenium Steel= Telliirium Sone Copper Lead Sickel 5eleniurii Steela Teliuriuni
d.lE 1020 cold-rolled.
Viscosity a t 100' F , Centistokes 2 1 hr. 7 2 hr 168 hr. 62.4 64 8 67.3 63.8 6.1.8 67.4 59.5 64 2 66.5 ... 64.9 ... 57.9 57 3 ;is.9 62.4 64.3 6i.6 52.1 64.9 65.9 71.4 70.6 67.4 65.4 71.0 70.4 71.9 70.1 67.3 Z0.8 G8 2 ... ,1,7 69 9 69.7 z1.7 70 3 66.0 #1,9 G7.4 63.6
November 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
silicone decreased greatly duiing the first 24 hours of exposure, and thereafter there was little change. With tellurium the great viscosity change generally occurred between 72 and 168 hours of exposure. However, variable results were obtained with tellurium under a n atmosphere of nitrogen. I n some expcriments the equilibrium viscosity was obtained after 24 hours. ACKNOWLEDGRIEKT
The authors n-ould like to express their appreciation t o C. E. C o s for many of the viscometric determinations and to D. C. Smith for his cooperation in making the essential spectroscopic analyses discused here. LITERATURE CITED Assoc. of Official Agr. Chemijts, Official and Tentative Methods of Analysis, 5th ed., p. 171 (1940). Atkins. D. C., Baker, H. R., Murphy, C.AI., and Zit.nian. I\-. .1.,IND. EKG.CHEX, 39, 491 (1947). Barry, A. J.. J . A p p l i e d Phys., 17, 1020 (1946). Bass, S. L., Hyde, J. F., and llcGregor, R . R., J . B m . Ceram. SOC.,29, 66 (1946). Brophy, J. E., Militz, R . O., and Zisnian, W. d.,Trans. A m . SOC.Mech. Engrs.. 6 8 , 353 (1946). (6) Collings, IT. K., Cham. Eng. S e w s , 23, 1616 (1943).
(7) Don, Corning Corp., “Silicones-Sen- Engineering Materials” (pamphlets), 1945-47. ( 8 ) Feigl, F.,“llaiiual of Spot Trats,” p. 193. S e w York, .Icademic Press Inc., 1943.
1401
19) Fitzsinnnons, V. G., Pickett, D. L., Milits, R. O., and Zisman, W,A., Trans. Am. SOC.Mech. Engrs., 68, 361 (1946). (IO) Hopkins and Williams, Ltd., Research Lab., “Organic Reagents for Organic .Lnalysis,” p. 47, London, 1944. (11) Hunter, hl. J., Hyde, J. F., Karrick, E. I,., and Fletcher, H. J., J . -4m. Chem. SOC.,68, 667 (1946). (12) IIurd, C. B., Ibid., 68, 364 (1946). (13) Jacobs, >I. B., “.lnalytical Chernistr~. of Industrial Poisons, Hazards and Solvents.” D. 536, Ken- York. Interscience Publishers, h e . , 1941. (14) Kauppi, T. .I., and Pederaen, W.W.,Luhricalion E ~ Q .2,, 158 (1946). f l 3 ) Kauppi, T. .I,, and Pedersen, JV. K., 8 . d . E . Journal, 54, 121 (1946). (16) Patnode, K , , and .Schmidt, F. C . , ,J, A m . Chem. Soc., 67, 2722 (1945). ( l i ) Patnode, \V., aiid Kilcock, D. F.,Ibid., 68,358 (1946). (18) Rochow, E. G., Chem. Eng. S e i c a , 23,612 (1945). (19) Rochow, E. G., “Cheniistry of Silicones,” p. 64, Nen- York, John Wiley 6- Sons, I n c . , 1916. (90) Sullivan, M. V,, Kolfe, J. K., a i d Zisinan, IT. A , , IXD,Eso. CHmi., t o be published. (21) Wilcock, D. F., Gen. Elec. Rei).,49,No.11, 14 (1946). (22) Ibid., 49, S o . 12, 25 (1946). ( 2 3 ) Xlcock, D. F.,J . Am. Chem. Soc.. 68, 691 (1916). 124) FVilcock. D. F., Jleeh. Eng., 66, 739 (1944). ( 2 5 ) Ibid., 67, 202 (1945). RECEIVED April 9, 1947. T h e opinions o r assertions contained in this paper are the authors’ and are not t o be construed as official o r reflecting the riews of the X a r y Department.
POLYORGANOSILOXANES..
..
Surface Active Properties H . W. F O X , P A U L A W . T A Y L O R , A N D W. A. Z I S M A N NAVAL RESEARCH LABORATORY, WASH I N a T O N . D . C.
A STUDY has been made of the densities, the surface tensions and their temperature coefficients, the interfacial tensions against water, the spreading pressures, and the force-area and potential-area relations of monolay ers on water of Farious types of linear polyorganosiloxanes. The RIcLeod constants and parachors hale been calculated, and their application to the type analysis of the silicones is discussed. Relations have been found between the critical spreading pressure, the spreading coefficient, arid the viscositj. 4 study of the force-area curtes revealed that the pol, methylsiloxanes aiid the related polymers containing a small proportion of phenyl substituents are able to coil relersibly into helices made up of about six monomers per turn. Conclusions relative to the molecular structures in thin films hale been carried oler to the three-
A S I T of the physical and chemical properties of t,he polyorganosiloxanes (or silicones) have been described recently (1,2, 4, 9, 10, 11, 12, 17, 19,2 2 ) . Relatively few data have appeared concerning the surface active properties and their relation t o molecular structure. This discussion deals with some of the properties of polyorgano.;iloxane films when adsorbed a t the gas-nilicone and water-silicone interfaces. The linear polymethylsiloxanes from the dimer to the heptadecanier m r e carefully purified compounds having t,rimethylsiloxy end groups. 1 I a n y of the properties of the dimer through the octamer have been drscribed by Hunter et al. (10). T h e DC 500 series fluids arc commercial mixtures of homologous linear
dimensional liquid state. I t is shown that a qualitathe explanation can be giten of the variation with substituents in the viscosity indices of the different linear pol~organosiloxanes and of the unusually high values of the methjlsubstituted compounds. I t is concluded that the larger diameter of the silicon atom as compared with the carbon atom i i reaponsible for the greater ability of the polyniethylsilo~anes to coil, as compared with analogous linear polymers of the hjdrocarbon or ether types. At low film preqsures each helix uncoils, and the molecule adsorbs Mith the long axis in the water. The length of the helix iiicrea3es with the temperature. The potential-area cthanpes with molecular packing were unusual. The electric nionient per monomer has been obtained, and its significance is brieflj discussed.
pol?-iiiethylsiloxalles, those under 5 centistokes (cs.) in viscosity apparmtly being distillation cuts. The samples of this series having low-er viscosities than 5 cs. comprised a small range of niolecular w i g h t s approaching pure compounds a t the loffest viscosities (0.65) 1.0, and 1.5 cs.). This is evidenced by the close agreement of the surface tensions and densities with those measured for the linear dimer, trimer, and tetramer, The polyethylsiloxane.< and polymethl-lphenylsiloxanes used were producers’ samples of mixed homologous compounds. The type classifications of these fluids given in Table I are based on the analyses of this laboratory. Almost all of the measurements n-ere made in 8 constant temperature room held at 20” += 0.2O C. The rrlative
