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continuous increase in the sedimentation coefficient.
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- THE ELECTRICAL
CONDUCTIVITY MOLTEN SILICA
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BY M. B. PANISH Research and Advanced Deueloprnert Dzvisron, AVCO Corp., Wilmangton, M a s s . Received December 16, 1968
In connection with studies of the properties of materials a t high temperatures we have determined
Yol. 63
NOTES
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(1) J. Cohen, J. A p p l . Phus., 28, 795 (1957). ( 2 ) R. Sosman, "Properties of Silica," American Chemical Society Monograph, 1927, p. 528. (3) H. E. Seeman, Phya. Rev., 81, 119 (1928).
THE ROLE OF CARBON DIOXIDE I N CATALYZED SILOXANE CLEAVAGE BY L. E. ST. PIERRE AND A. M. BUECHE General Electric Cornpanu, Researrh Laboratory, Schenectady, N . Y . Received December 16. 1958
Osthoff, Bueche aiid Grubb' have reported the
16 12 T I M E (HOURS).
4
20
24
Fig. 1.-Stress relaxation of polydimethylsiloxane gum containing 0.01yopotassium hydroxide and benzoyl peroxide decomposition fragments in dried carbon dioxide and nitrogen at 130'.
OC.
tained by previous w o r k e r ~ . l - ~Sosman's averaged data for several earlier workers has been plotted along with the smoothed curve from Seeman's data and several points obtained by Cohen. The higher conductivity obtained by Cohen easily could be due to the approximately 0.2% impurity in the silica used in his work. During the degassing procedures, it was noticed t,hat the conductivity often decreased in spite of the fact that t'he melt was coming into increasingly better contact with the electrodes. It thus appears that adsorbed water (as OH-) contributes markedly to the conductivity of the solid, and it is possible that most of the water was removed by degassing before the final measurements on molten silica were made. Inspection of the cells after cooling showed that the silica had wet the tungsten, a,nd that once this had occurred, there was no observable reaction between tungsten and silica. The tungsten electrodes remained, in fact, mirror bright under the silica surface. It was, therefore, assumed that the contact resistance was negligible in comparison with the resistance of the silica. Above the surface of the melt, the tungsten was eroded, this obviously being due to the reaction of tungsten with oxygen deriving from the decomposition of silica to gaseous silicon monoxide and oxygen. Acknowledgment.-The author appreciates the assistance of Mr. F. Bourgelas, Mr. R. Levy and Mr. A. Kocsi in constructing and assembling the apparatus. This work was supported by the U. S. Air Force under Contract No. AF-04(645)-30.
I 8
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I DRY
Nz
T 0 0 4
t TIME (HOURS).
Fig. 2.-Stress relaxation of polydimethylsiloxane gum containing 0.01% potassium hydroxide (cross linked with high energy electrons) in dried carbon dioxide and nitrogen a t 130'.
effects of COz and water on the rate of chemical stress relaxation of polydimethylsiloxane elastomers. The effect of water is clear if it is assumed that it reacts with the catalytic agents, benzoic acid and potassium hydroxide, which are present. However, the acceleratory effect of COz was inexplicable on the basis of known siloxane chemistry. It was therefore decided to reinvestigate the effect of COZ,taking special precautions to purify it. The apparatus used and polymer preparations are those described by Osthoff, Bueche and Grubb.1 Dew point measurements were made by observing the temperature a t which a gas stream exhibited condensation on a glass mirror. Matheson "Bone-Dry" carbon dioxide was used and three methods of drying were tested: (i) passage through three 15 cm. tubes packed wit,h anhydrous. MgSO1; (ii) pasFage through three 1 meter tubes packed with activated alumina; (iii) passage through two 20 em. traps packed with brass turnings and cooled in Dry Ice-acetone baths. The standard of comparison for the stress relaxation runs were thoae made in laboratory line dry nitrogen. Therefore it was desirable that the COSused be of comparable dryness to offset the known effects of water. The line nitrogen had a dew point of -60'F. which indicated a water content of 0.0056 volume %. Carbon dioxide dried by means of MgS04, activated alumina and packed cooled traps contained 0.180, 0.0092 and 0.005% water, respectively. (1) R. C . Osthoff, A. M. Bueche and W. T. Grubb, J. A n . Chem. Soc., 76, 4659 (1954).
