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LUCiANO C. SCALA and WILLIAM M. HICKAM Westinghouse Research Laboratories, Pittsburgh 35, Pa.
Thermal and Oxidative Degradation of Silicones Combination of a closed gascirculating system and a mass spectrometer is a practical way for following quantitatively evolution of gaseous degradation products from solid and liquid polymers in various atmospheres. Degradation of silicone polymers involves loss of volatile materials, oxidation of polymer, and degradation and volatilization of the more unstable oxidized structures. Without crosslinking initiators, phenyl-substituted organosilicon compounds offer greater resistance to thermal and oxidative degradation than methyl- or vinyl-substituted silicones.
HIGH
temperatures at which modern electrical equipment is operated require thermally stable insulation. Silicones, have been used extensively, but they gradually deteriorate mechanically and dielectrically when exposed to the combined action of atmospheric oxygen and heat evolved. Degradation reactions of some silicone insulating materials were investigated. Materials
(DM),, prepared by hydrolysis of equimolar proportions of dimethyldiethoxysilane and methyltriethoxysilane, was a hard, infusible, colorless solid with a methyl-silicon ratio of 1.5. I t was ground in a Wiley mill to 40-mesh and used without further treatment. DC200 fluid (dimethylpolysiloxane, Dow Corning Corp.) had a viscosity of 100 cs. a t 25' C., a molecular weight of 6000, and a methyl-silicon ratio slightly higher than 2. Low boiling components were distilled off at 240' C. in vacuo for 4 hours. Its structure is: (CH3)aSi-0-
E(: >. Si-0
-Si(CH3)3
MePhViSil, an organosilicon compound prepared by D. W. Lewis of these laboratories, is a phenyl- and vinylcontaining linear silicone fluid, endstopped with trimethylsilyl groups, with a viscosity of 48 cs. at 25' C. and low molecular weight. After heating under a pressure of 0.09 mm. of mercury to 137' C. for 4 hours, its viscosity increased to 82.4 cs. at 25' C. The ma-
terial of higher viscosity was used. Quantity of dissolved air was negligible. OctaDhenvlcvclotetrasiloxane (melting p o h t 208-21 1' C.) has the foimula [(CeHdzSiOl4. PhSil is the Droduct of hvdrolvsis and high temperatAre alkaline conddnsation of diphenyldichlorosilane. After stripping low boiling materials, the polymer is a waxy material, softening a t about 100' C., and soluble in common organic solvents; . carbon, 70.43%; hydrogen, 5.0370; silicon, 15.52y0. The number of substituent groups per silicon atom is 1.77, which suggests loss of phenyl groups during preparation. Apparatus .and Procedure
The apparatus consists basically of a closed system in which a known volume of air or oxygen is circulated in one direction through the sample by a magnetic pump (Figure 1). The pump is a Teflon-covered iron piston, free to move u p and down inside a glass cylinder surrounded by four solenoids. Current flows in the coils successively when contact is made with microswitches mounted on a rotating wheel. The oscillating motion of the piston, induced by the ascending and descending magnetic field, causes a continuous, unidirectional flow of gas through the sample by means of four check valves. The speed of the pump, which can be varied, is about 0.2 liter per minute. This pump is essentially that described by Von Fuchs and Diamonds ( 5 ) , with some modifications by H. E. Mahncke. The sample holder is of a simple bubbler type for liquid materials; solid samples rest on a sintered-glass filter disk. A weighed sample is introduced into the system, which is tested for leaks and filled with the desired gas in a state of high purity. The apparatus is connected to a Consolidated Electrodynamics Corp. Model 21-103-C mass spectrometer by a three-way, straight-through metal needle valve, the traps are immersed in dry ice-trichloroethylene mixture, and the pump is started. After a few minutes a sample of gas from the system, usually 0.1 to 0.2 cc., is admitted to the mass spectrometer to determine the gas background. Heat is then applied to the sample. Samples of the circulating gas, about 0.05 cc. a t atmospheric pressure, are removed periodically and analyzed ; the precision of the results is within 3Y0. Pressure inside the system is recorded. At the conclusion of each run, weight losses and weights of condensed degrada-
tion products are ascertained. Materials that condense in the dry ice traps and residues are analyzed by infrared and by elemental determination. The evolution of gaseous oxidation products can be followed without substantially altering experimental conditions. Results and Discussion
Experimental results are summarized in Table I.
Degradation in Air. METHYLSILICONES. Reaction with atmospheric oxy-
gen did not take place at an observable rate below 200' C. Oxygen uptake a t 300' C. for (DM)z was about 1.6 times that at 250' C. for approximately the same reaction time; however, the amount of carbon dioxide evolved a t 300' ,C. was larger than a t 250' C. by a factor of 13.4. This may mean that a t 250' C. the unoxidized resin picked up oxygen faster than it released it as carbon dioxide; a t 300' C. the oxidized portions of the silicone became more unstable and tended to be released at a faster rate than oxygen was absorbed. DC200 gelled to a solid state in 3 hours at 250' C., showing cross linking through active points brought about by oxidation of methyl groups. The higher reactivity of DC200 over (DM)z may be associated with the greater relative number of reactive methyl groups in the former. Other products of degradation of both (DM)=and DC200 were a low molecular weight silicone of the same structure as the starting material, and paraformaldehyde. The former probably derived from cleavage of the original structure; the latter by the following schemes (7-4) :
S
I Electric Coils
2 Teflon Covered Iron Piston 3 Valve System 4. Sample Holder 5 Cold Trap
6 7 8 9
Cold Trap Needle Valve Gas Reservoir Flowmeter
Figure 1. Air or oxygen is circulated in a closed system VOL. 50, NO. 10
OCTOBER 1958
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1 1
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120
I
I
360
240
480
Time, Min.
Figure 2: Rates of oxygen absorption by (DM),, MePhViSil, and PhSil in air Initiation RCHS
+
0
2 +
RCHz.
+
(1)
RCHzOzH + RCHz0. f H O . (4) CHzO RCHz0. -+ R . (5)
+
Formaldehyde polymerized to paraformaldehyde when it came into contact with the cold traps. The presence of carbon dioxide and hydrogen among the products of degradation of methyl silicones may be due to oxidation : CHzO
+ '/zO2
HCOOH
4
+ HI
HCOOH
(6)
+ COa
(7)
VINYL-CONTAIMNG SILICONES.The vinyl-containing silicone exhibited a much higher susceptibility to oxidation than the methyl silicones, as shown by oxygen absorption and gas evolution at comparable temperatures. MePhViSil gelled in 3.3 hours at 200' C. (compared to 3 hours for DC200 at 250' C.). The viscosity of both MePhViSil and DC200 increased with time on heating; this could alter the rate of oxygen uptake and
Table I.
( D M k air
DC200, air
MePhViSil, air
c.
200 250 250 300 150 200 250 100 125 150 200
ylated
Organosilicon
Compounds.
When octaphenylcyclotetrasiloxane was heated in a stream of air at 300' C.. large amounts of a white, solid material escaped from the reaction vessel. While most of the condensate was octaphenylcyclotetrasiloxane, an appreciable
Reaction Time,
Moles Gas/G. Sample, X lo5
Hr.
Hz
6.0 6.3 11.0 6.0 4.3 6.5 3.0 6.6 6.9 7.0 3.3
0
0.59 0.55 13.44 0 0 4.45
coz
co
0 1.56 1.83 21.0 Traces Traces 2.10
0
0
0
0.45 0.98 0.39
2.75 3.72 2.42
0 0
0 0 0 0 0 0
11.6 9.7 0
0 PhSil Air
02
1584
300 400
6.5 6.2 3.2
0 0
400
3.2
02 0 88.0 93.7 139.0 0 0 120.0 0 37.9 78.3 54.4 0 0
0
0 7.1 6.2
4.6 2.6
0 27.5 22.8
9.0
8.7
17.0
240.0
INDUSTRIAL AND ENGINEERING CHEMISTRY
fraction melted sharply at 188' C. This is thought to be the trimer hexaphenylcyclotrisiloxane (melting point 190' C.) probably generated by breaking of Si-0-Si bonls of the siloxane tetramer, followed by rearrangement to the lower molecular weight trimer. Neither evolution of gases nor oxygen absorption was reported by the mass spectrographic analyses. Heating the: tetramer a t 215' C. in a stream of oxygen showed no measurable results. When PhSil, a completely phenylated polymer, was heated at 300" C. in air, no gases were evolved nor was oxygen absorbed. However, about 10% of the weight of the original sample was found in the cold traps as a pale yellow solid, soluble in acetone. Infrared analyses showed that the light red residue and condensed distillates had retained the basic structure of the original materia!. O n heating PhSil a t 400' in air, appreciable evolution of carbon monoxide and dioxide and absorption of oxygen were observed. Infrared spectra of the condensed portions and the glassy, red residue were indistinguishable from those of the starting material. PhSil heated in air evolved no hydrogen but both methyl silicones and MePhViSil did; PhSil liberated carbon monoxide, whereas methyl silicones did not. When PhSil was exposed to a current of oxygen at 400' C., considerably more gas was evolved than in air at the same temperature. The infrared spectra of the glassy residue, condensed distillates. and starting material were identical. A relatively large amount of hydrogen was evolved from PhSi! in oxygen a t 400" C., with none from PhSil in air at 400'. Figure 2 shows rates of oxygen absorption by methyl-, vinyl-, and phenylcontaining silicones at three significant temperatures. Gas evolution curves are similar in shape to those of Figure 2. Acknowledgment
Gas Evolution and Oxygen Absorption
Temp., Material
gas evolution. For this reason the observed rates at the higher stage of oxidation may not be completely valid, although they are fairly reproducible. The vinyl content of MePhViSil decreased with increasing temperature, until at 200' C. about half the unsaturation had disappeared a t the end of the run. However, the carbon, hydrogen, and silicon in the residues from reactions at 150" and 200' C. did not differ appreciably from the starting material. These facts suggest that gelation of MePhViSil a t 200' is brought about predominantly through reaction of the vinyl groups and the substituent groups do not separate from the polymeric chain to a great extent under these conditions. Infrared analysis showed the condensates to be silicones of low molecular weight possessing the same basic structure as the starting material, except for carbonyl groups found in the residue too. These groups probably account for the evolution of carbon monoxide. The absence of paraformaldehyde from the reaction products may be due to the ease of oxidation of the vinyl groups, which inhibited the formaldehyde-forming reaction. That atmospheric oxygen is the principal cause of the deterioration of MePhViSil was shown by the fact that when MePhViSil was heated in vacuo at 150' C. no change could be detected in chemical or physical properties. The absence of benzene, polyphenyls, and phenols indicates that under these conditions the phenyl groups in MePhViSil are not attacked by oxygen. Degradation of Completely Phen-
0
The authors thank R. N. Wenzel, C. W. Lewis, L. W. Frost, and D. W. Lewis for discussion and suggestions and J. H. Lady for help in interpretation of infrared spectra. Literature Cited
(1) Atkins, D. C., Murphy, C. M., Saunders, C. E., IND.ENG.CHEM.39, 1395 (1947). (2) Farmer, E. H., Trans. Faraday Sot. 38, 340 (1942). (3) Grassie, N., "Chemistry of High Polymer Degradation," Chap. 4, Interscience, New York, 1956. (4) Taylor, H. S., Tobolski, A. V., J. Am. Chem. SOC. 67, 2063 (1945). (5) Von Fuchs, G. H., Diamonds, H., IND.END.CHEM.34, 927 (1942). RECEIVED for review February 24, 1958 ACCEPTED June 26, 1958
