I/EC CONDENSATES Vapor Pressures of Vacuum Cements at 23.0" h 2.0" C. (R.M. Zabel, N.I.T., January 1933)
Epoxy Resin a s Sealant for High Vacuum Systems
IN
years the increased use of high vacuum systems, in both research and technology, has magnified the need for new and better sealing materials of both temporary and permanent varieties. Many types of waxes, greases, and cements of the temporary variety have been used to provide air-tight bonds between similar and dissimilar materials. This article deals specifically with the investigation of a resin-type cement, of the permanent variety, to give an air-tight bond from room to elevated temperatures between dissimilar materials, more specifically glass to brass, and in liquid nitrogen between polystyrene and brass, and between polystyrene and epoxy. A series of experiments was conducted with several commercial cements in an attempt to bond glass to brass where the evacuated system was to be subjected to elevated temperatures, and plastic to metal for operation in liquid nitrogen. At elevated temperatures two major problems limit the use of certain types of cements. The dissimilar rates of expansion of the unlike adherends create strains within the bond, resulting in mechanical weakening of the joint and loss of air-tight seal. On the other hand, as the temperature rises the increase of vapor pressure of the sealant increases the pressure in the system. A vacuum sealant for use at elevated temperatures should have an extremely low vapor pressure at the operating temperature and a balanced coefficient of expansion to cope with the expansion characteristics of the adherends. Epoxy resins were studied as potential sealants and were compared with existing sealing compounds. The epoxy sealant, RECENT
pa. Ion Current per M a .
consisting of Araldite CN 502 (Ciba Co.) and 8 to 10% triethylenetetramine, was allowed to set for 24 hours at room temperature. The epoxy seal was found to have many physical properties which made it applicable to high vacuum. Observation. At operating temperatures of 25' to 140' C. no separation or breakdown of the bond was observed. Even after several successive hearings the resin bond showed no signs of brittleness, cracking, or separation. The resin partially softens and flows to compensate for expansions of the unlike materials. The actual bond strength was not determined. However, the resin when cured for optimum strength can withstand a force equivalent to 8000 pounds per square inch in pure tension, 18,000 in compression a t room temperature and slightly less a t elevated temperatures. In shear, as an adhesive, an epoxy resin can withstand 2000 to 5000 pounds per square inch, depending on test conditions, methods of preparation, and surface conditions. Probably the most important feature of a high vacuum cement is a low vapor pressure throughout the working temperature range. Although it is difficult to obtain an absolute value for the vapor pressure of a material of this type, the method of R. M. Zabel, with some modification, gave a value for comparison with available data on vacuum cement? : 1.7 to 1.8 pa. of ion current per ma. of electron current, at 25' C. The amount of vaporization of the epoxy sealant as a function of temperature was determined. Vacuum seal was maintained for as long as 0.5 hour in liquid nitrogen with no pressure change in the vacuum sys-
Amount of vaporization of epoxy sealant as function of temperature
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Substance Blank Borosilicate glass Brass Iron Iron heavily coated with rust Wax Picein (melting point 105O C.) Picein (melting point 80° C.) Dennison's Royal Scarlet Beeswax 1 part, rosin 1 part Glycol-phthalic anhydride resin Extrafine Sealing (express) Universal Beeswax 4 parts, Venice turpentine 1 part Dennison's No. 2 de Khotinsky (hard) de Khotinsky (medium) de Khotinsky (soft) Glyptal Lacquer (baked 4 hours at 2000 C.) Lacquer (dried 14 days) Green (baked 4 hours at ZOOo
C.)
Black (baked 4 hours at 200' C.) Red (baked 4 hours at ZOOo C .) Stopcock grease Central Scientific Co. Lubriseal (improved formula) Gum chicle and Duco N-Dibutyl phthalate (25' C.) Epoxy resin (25'-28' C., determined by V. L. Denniger)
Electron Current
0.7 1.0 3.5 250.0 3.5 4.0
4.5 5.5 8.5 10.0 10.5 12.0 12.0 12.0 15.0 21.5 25.0 2.0 5.0 3.5 5.5 7.0 7.0
7.5 50.0 5.4
X -7-1
e
8
tem. The same vacuum seal showed no visible breakdown following five separate immersions in liquid nitrogen from room temperature-Le., thermal shock did not cause the seal to fail. The excellent dielectric properties, iow vapor pressure, and excellent bonding qualities of epoxy resins make them applicable for bringing wires, electrodes, etc., into vacuum systems. Solvents such as acetone, often used in cleaning vacuum systems, have no effect on the sealant. The data recorded and plotted do not represent an attempt to relate vapor pressure of the resin to temperature, but rather serve to show the contribution of the resin to the destruction of the high vacuum as the temperature is increased. S. S. STIVALA V. L. DENNlGERl Stevens institute of Technology, Hoboken, N. J. Local section, ACS, Newark, N. J., January 1955. From a thesis submitted by V. L. Denniger in partial fulfillment of requirements for the degree o f , M.E. with high honor. 1 Present address, U, S. Army.
