Hermetic seal
A hermetic seal is any type of
Etymology
The word hermetic comes from the Greek god Hermes. A hermetic seal comes from alchemy in the tradition of Hermeticism. The legendary Hermes Trismegistus supposedly invented the process of making a glass tube airtight using a secret seal.[1]
Uses
Some kinds of packaging must maintain a seal against the flow of gases, for example, packaging for some foods, pharmaceuticals, chemicals and consumer goods. The term can describe the result of some food preservation practices, such as vacuum packing and canning. Packaging materials include glass, aluminum cans, metal foils, and gas impermeable plastics.
Some buildings designed with
Applications for hermetic sealing include
Hermetic sealing for airtight conditions is used in
In the funeral industry, some caskets and burial vaults are hermetically sealed by a rubber seal and being locked.
Types of epoxy hermetic seals
Typical epoxy resins have pendant hydroxyl (-OH) groups along their chain that can form bonds or strong polar attractions to oxide or hydroxyl surfaces. Most inorganic surfaces—i.e., metals, minerals, glasses, ceramics—have polarity so they have high surface energy. The important factor in determining good adhesive strength is whether the surface energy of the substrate is close to or higher than the surface energy of the cured adhesive.
Certain
Types of glass-to-metal hermetic seals
When the glass and the metal being hermetically sealed have the same coefficient of thermal expansion, a "matched seal" derives its strength from bond between the glass and the metal's oxide. This type of glass-to-metal hermetic seal is generally used for low-intensity applications such as in light bulb bases.[4]
"Compression seals" occur when the glass and the metal have different coefficients of thermal expansion such that the metal compresses around the solidified glass as it cools. Compression seals can withstand very high pressure and are used in a variety of industrial applications.
Compared to epoxy hermetic seals, glass-to-metal seals can be operated up to much higher temperatures (250 °C for compression seals, 450 °C for matched seals). The material selection is however more limited due to thermal expansion constraints. The sealing process is performed at roughly 1000 °C in an inert or reducing atmosphere to prevent discoloration of the parts.[5]
Ceramic-to-metal hermetic seals
Co-fired ceramic seals are an alternative to glass. Ceramic seals exceed the design barriers of glass to metal seals due to superior hermetic performance in high stress environments requiring a robust seal. Choosing between glass versus ceramic depends on the application, weight, thermal solution and material requirements.
Glassware sealing
Sealing solids
Glass taper joints can be sealed hermetically with
Grease
A thin layer of grease made for this application can be applied to the ground glass surfaces to be connected, and the inner joint is inserted into the outer joint such that the ground glass surfaces of each are next to each other to make the connection. In addition to making a leak-tight connection, the grease lets two joints be later separated more easily. A potential drawback of such grease is that if used on laboratory glassware for a long time in high-temperature applications (such as for continuous distillation), the grease may eventually contaminate the chemicals.[8] Also, reagents may react with the grease,[9][10] especially under vacuum. For these reasons, it is advisable to apply a light ring of grease at the fat end of the taper and not its tip, to keep it from going inside the glassware. If the grease smears over the entire taper surface on mating, too much was used. Using greases specifically designed for this purpose is also a good idea, as these are often better at sealing under vacuum, thicker and so less likely to flow out of the taper, become fluidic at higher temperatures than Vaseline (a common substitute) and are more chemically inert than other substitutes.
Cleaning
Ground glass joints are translucent when clean and free of debris. Solvents, reaction mixtures and old grease appear as transparent spots. Grease can be removed by wiping with an appropriate solvent;
Testing
Standard test methods are available for measuring the moisture vapor transmission rate, oxygen transmission rate, etc. of packaging materials. Completed packages, however, involve heat seals, joints, and closures that often reduce the effective barrier of the package. For example, the glass of a glass bottle may have an effective total barrier but the screw cap closure and the closure liner might not.
See also
Notes
- ^ "Online Etymology Dictionary". www.etymonline.com.
- ^ "Origins of the Charters of Freedom Project". 2001-06-25. Retrieved 2015-11-07.
- ^ "Hermetic Feedthrough Whitepaper | Douglas Electrical Components". Douglas Electrical Corp. Retrieved 2021-12-23.
- ^ "Hermetic Seal | Glass-to-Metal Seal | Elan Technology in USA". Elan Technology. Retrieved 2015-12-03.
- ^ "Glass-to-Metal Sealing Technology | Dietze Group". Dietze Group. Retrieved 2019-07-01.
- ^ Glindemann, D., Glindemann, U. (2001). “Greaseless Taper Jointed Glassware and Containers hermetic tight with new PTFE Sealing Ring”. Fusion (ASGS) 48 (2): 29–33.
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- ^ Rob Toreki (2006-12-30). "Glassware Joints". Interactive Learning Paradigms Inc.
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