A hermetic seal is any type of sealing that makes a given object airtight (preventing the passage of air, oxygen, or other gases). The term originally applied to airtight glass containers, but as technology advanced it applied to a larger category of materials, including rubber and plastics. Hermetic seals are essential to the correct and safe functionality of many electronic and healthcare products. Used technically, it is stated in conjunction with a specific
test method
A test method is a method for a test in science or engineering, such as a physical test, chemical test, or statistical test. It is a definitive procedure that produces a test result. In order to ensure accurate and relevant test results, a test met ...
and conditions of use.
Uses
![FeAp 92-1a - keyboad and display PCB - Eternacell Lithium battery G03-8634](https://upload.wikimedia.org/wikipedia/commons/d/d6/FeAp_92-1a_-_keyboad_and_display_PCB_-_Eternacell_Lithium_battery_G03-8634.jpg)
Some kinds of
packaging
Packaging is the science, art and technology of enclosing or protecting products for distribution, storage, sale, and use. Packaging also refers to the process of designing, evaluating, and producing packages. Packaging can be described as a co ...
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
Food preservation includes processes that make food more resistant to microorganism growth and slow the oxidation of fats. This slows down the decomposition and rancidification process. Food preservation may also include processes that inhibit ...
practices, such as
vacuum packing
Vacuum packing is a method of packaging that removes air from the package prior to sealing. This method involves placing items in a plastic film package, removing air from inside and sealing the package. Shrink film is sometimes used to have a tig ...
and
canning
Canning is a method of food preservation in which food is processed and sealed in an airtight container (jars like Mason jars, and steel and tin cans). Canning provides a shelf life that typically ranges from one to five years, although u ...
.
Packaging
Packaging is the science, art and technology of enclosing or protecting products for distribution, storage, sale, and use. Packaging also refers to the process of designing, evaluating, and producing packages. Packaging can be described as a co ...
materials include
glass
Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...
,
aluminum can
An Aluminum can (British English: Tin can) is a single-use container for packaging made primarily of aluminum.
It is commonly used for food and beverages such as milk and soup but also for products such as oil, chemicals, and other liquids. Globa ...
s,
metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
foils, and gas impermeable
plastic
Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...
s.
Some buildings designed with
sustainable architecture
Sustainable architecture is architecture that seeks to minimize the negative environmental impact of buildings through improved efficiency and moderation in the use of materials, energy, development space and the ecosystem at large. Sustainable ...
principles may use airtight technologies to
conserve energy. Under some
low energy building
Low-energy buildings, which include zero-energy buildings, passive houses and green buildings, may use any of a large number of techniques to lower energy use.
The following are some of the techniques used to achieve low-energy buildings, which e ...
,
passive house
"Passive house" (german: Passivhaus) is a voluntary standard for energy efficiency in a building, which reduces the building's ecological footprint. It results in ultra-low energy buildings that require little energy for space heating or coo ...
,
low-energy house
A low-energy house is characterized by an energy-efficient design and technical features which enable it to provide high living standards and comfort with low energy consumption and carbon emissions. Traditional heating and active cooling systems ...
,
self-sufficient homes
An autonomous building is a building designed to be operated independently from infrastructural support services such as the electric power grid, gas grid, municipal water systems, sewage treatment systems, storm drains, communication services, ...
,
zero energy building
A Zero Energy Building (ZEB), also known as a Net Zero Energy (NZE) building, is a building with net zero energy consumption, meaning the total amount of energy used by the building on an annual basis is equal to the amount of renewable energy c ...
, and
superinsulation
Superinsulation is an approach to building design, construction, and retrofitting that dramatically reduces heat loss (and gain) by using much higher levels of insulation and airtightness than normal. Superinsulation is one of the ancestors of t ...
standards, structures must be more air-tight than other lesser standards. Air barriers are not effective if construction joints or service penetrations (holes for pipes, etc.) are not sealed.
Airtightness is a measure of the amount of warm (or cool) air that can pass through a structure. Mechanical ventilation system can recover heat before discharging air externally.
Green building
Green building (also known as green construction or sustainable building) refers to both a structure and the application of processes that are environmentally responsible and resource-efficient throughout a building's life-cycle: from planni ...
s may include windows that combine triple-pane insulated glazing with
argon
Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
or
krypton
Krypton (from grc, κρυπτός, translit=kryptos 'the hidden one') is a chemical element with the symbol Kr and atomic number 36. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere and is often ...
gas to reduce
thermal conductivity
The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa.
Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
and increase
efficiency
Efficiency is the often measurable ability to avoid wasting materials, energy, efforts, money, and time in doing something or in producing a desired result. In a more general sense, it is the ability to do things well, successfully, and without ...
. In
landscape
A landscape is the visible features of an area of land, its landforms, and how they integrate with natural or man-made features, often considered in terms of their aesthetic appeal.''New Oxford American Dictionary''. A landscape includes the ...
and exterior construction projects, airtight seals may be used to protect general services and
landscape lighting
Landscape lighting or garden lighting refers to the use of outdoor illumination of private gardens and public landscapes; for the enhancement and purposes of safety, nighttime aesthetics, accessibility, security, recreation and sports, and socia ...
electrical connections/splices. Airtight implies both waterproof and vapor-proof.
Applications for hermetic sealing include
semiconductor electronics
A semiconductor device is an electronic component that relies on the electronic properties of a semiconductor material (primarily silicon, germanium, and gallium arsenide, as well as organic semiconductors) for its function. Its conductivity li ...
,
thermostat
A thermostat is a regulating device component which senses the temperature of a physical system and performs actions so that the system's temperature is maintained near a desired setpoint.
Thermostats are used in any device or system tha ...
s,
optical device
Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviole ...
s,
MEMS
Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems constitute the technology of microscopic devices, ...
, and
switch
In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
es. Electrical or electronic parts may be hermetic sealed to secure against
water vapor
(99.9839 °C)
, -
, Boiling point
,
, -
, specific gas constant
, 461.5 J/( kg·K)
, -
, Heat of vaporization
, 2.27 MJ/kg
, -
, Heat capacity
, 1.864 kJ/(kg·K)
Water vapor, water vapour or aqueous vapor is the gaseous pha ...
and foreign bodies to maintain proper functioning and reliability.
Hermetic sealing for airtight conditions is used in
archiving
An archive is an accumulation of historical records or materials – in any medium – or the physical facility in which they are located.
Archives contain primary source documents that have accumulated over the course of an individual or ...
significant historical items. In 1951, The
U.S. Constitution
The Constitution of the United States is the supreme law of the United States of America. It superseded the Articles of Confederation, the nation's first constitution, in 1789. Originally comprising seven articles, it delineates the natio ...
,
U.S. Declaration of Independence
The United States Declaration of Independence, formally The unanimous Declaration of the thirteen States of America, is the pronouncement and founding document adopted by the Second Continental Congress meeting at Pennsylvania State House (l ...
, and
U.S. Bill of Rights
The United States Bill of Rights comprises the first ten amendments to the United States Constitution. Proposed following the often bitter 1787–88 debate over the ratification of the Constitution and written to address the objections rais ...
were hermetically sealed with
helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
gas in glass cases housed in the
U.S. National Archives
The National Archives and Records Administration (NARA) is an " independent federal agency of the United States government within the executive branch", charged with the preservation and documentation of government and historical records. It i ...
in
Washington, D.C.
)
, image_skyline =
, image_caption = Clockwise from top left: the Washington Monument and Lincoln Memorial on the National Mall, United States Capitol, Logan Circle, Jefferson Memorial, White House, Adams Morgan, ...
In 2003, they were moved to new glass cases hermetically sealed with argon.
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
epoxy resin
Epoxy is the family of basic components or cured end products of epoxy resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide functional group is also coll ...
s and their processes can create a hermetic bond to copper, brass, stainless steel, specialty alloys, plastic, or epoxy itself with similar
coefficients of thermal expansion, and are used in the manufacture of hermetic electrical and fiber optic hermetic seals. Epoxy-based seals can increase signal density within a feedthrough design compared to other technologies with minimal spacing requirements between electrical conductors. Epoxy hermetic seal designs can be used in hermetic seal applications for low or high vacuum or pressures, effectively sealing gases or fluids including helium gas to very low helium gas leak rates similar to glass or ceramic. Hermetic epoxy seals also offer the design flexibility of sealing either copper alloy wires or pins instead of the much less electrically conductive
Kovar
Kovar (trademark of CRS Holdings, inc., Delaware) is a nickel–cobalt ferrous alloy compositionally identical to Fernico 1, designed to have substantially the same thermal expansion characteristics as borosilicate glass (~5 × 10−6 /K between ...
pin materials required in glass or ceramic hermetic seals. With a typical operating temperature range of −70 °C to +125 °C or 150 °C, epoxy hermetic seals are more limited in comparison to glass or ceramic seals, although some hermetic epoxy designs are capable of withstanding 200 °C.
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 the weaker of the two types and is generally used for low-intensity applications such as in light bulb bases.
![Glass-to-metal compression seals](https://upload.wikimedia.org/wikipedia/commons/6/61/Glass-to-metal_compression_seals.JPG)
"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.
Ceramic-to-metal hermetic seals
Co-fired ceramic
Co-fired ceramic devices are monolithic, ceramic microelectronic devices where the entire ceramic support structure and any conductive, resistive, and dielectric materials are fired in a kiln at the same time. Typical devices include capacitors, ...
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
PTFE
Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Chemour ...
sealing rings (high vacuum tight, air leakage rate 10
−6 mBar × L/sec and below),
o-rings
An O-ring, also known as a packing or a toric joint, is a mechanical gasket in the shape of a torus; it is a loop of elastomer with a round cross-section, designed to be seated in a groove and compressed during assembly between two or more par ...
(optionally encapsulated o-rings), or PTFE sleeves, sometimes used instead of
grease that can dissolve into contamination.
PTFE tape
Thread seal tape (also known as PTFE tape, Teflon tape, or plumber's tape) is a polytetrafluoroethylene (PTFE) film tape commonly used in plumbing for sealing pipe threads. The tape is sold cut to specific widths and wound on a spool, making it ...
, PTFE resin string, and wax are other alternatives that are finding widespread use, but require a little care when winding onto the joint to ensure a good seal is produced.
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
Laboratory glassware refers to a variety of equipment used in scientific work, and traditionally made of glass. Glass can be blown, bent, cut, molded, and formed into many sizes and shapes, and is therefore common in chemistry, biology, and anal ...
for a long time in high-temperature applications (such as for
continuous distillation
Continuous distillation, a form of distillation, is an ongoing separation in which a mixture is continuously (without interruption) fed into the process and separated fractions are removed continuously as output streams. Distillation is the sep ...
), the grease may eventually contaminate the chemicals.
Also,
reagent
In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...
s may react with the grease,
especially under
vacuum
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...
. 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
Vaseline ()Also pronounced with the main stress on the last syllable . is an American brand of petroleum jelly-based products owned by transnational company Unilever. Products include plain petroleum jelly and a selection of skin creams, soa ...
(a common substitute) and are more chemically inert than other substitutes.
Cleaning
Ground glass joints are translucent when physically free of debris and clean. Solvents, reaction mixtures, and old grease show up as transparent spots. Grease can be removed by wiping with an appropriate solvent;
ether
In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again be c ...
s,
methylene chloride
Dichloromethane (DCM or methylene chloride, methylene bichloride) is an organochlorine compound with the formula . This colorless, volatile liquid with a chloroform-like, sweet odour is widely used as a solvent. Although it is not miscible with ...
,
ethyl acetate
Ethyl acetate ( systematically ethyl ethanoate, commonly abbreviated EtOAc, ETAC or EA) is the organic compound with the formula , simplified to . This colorless liquid has a characteristic sweet smell (similar to pear drops) and is used in glues ...
, or
hexane
Hexane () is an organic compound, a straight-chain alkane with six carbon atoms and has the molecular formula C6H14.
It is a colorless liquid, odorless when pure, and with boiling points approximately . It is widely used as a cheap, relatively ...
s work well for
silicone
A silicone or polysiloxane is a polymer made up of siloxane (−R2Si−O−SiR2−, where R = organic group). They are typically colorless oils or rubber-like substances. Silicones are used in sealants, adhesives, lubricants, medicine, cooking ...
- and
hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...
-based greases.
Fluoroether-based greases are quite impervious to organic solvents. Most chemists simply wipe them off as much as possible. However, some fluorinated solvents can remove fluoroether greases, but are more expensive than laboratory solvents.
Testing
Standard test methods are available for measuring the
moisture vapor transmission rate
Moisture vapor transmission rate (MVTR), also water vapor transmission rate (WVTR), is a measure of the passage of water vapor through a substance. It is a measure of the permeability for vapor barriers.
There are many industries where moisture c ...
,
oxygen transmission rate Oxygen transmission rate (OTR) is the measurement of the amount of oxygen gas that passes through a substance over a given period. It is mostly carried out on non-porous materials, where the mode of transport is diffusion, but there are a growing nu ...
, 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
A glass bottle is a bottle made from glass. Glass bottles can vary in size considerably, but are most commonly found in sizes ranging between about 200 millilitres and 1.5 litres. Common uses for glass bottles include food condiments, soda, liq ...
may have an effective total barrier but the
screw cap A screw cap or closure is a common type of closure for bottles, jars, and tubes.
Usage
A screw closure is a mechanical device which is screwed on and off of a "finish" on a container. Either continuous threads or lugs are used. It must be engi ...
closure and the closure liner might not.
See also
*
Building insulation
Building insulation is any object in a building used as insulation for thermal management. While the majority of insulation in buildings is for thermal purposes, the term also applies to acoustic insulation, fire insulation, and impact insul ...
*
Glass-to-metal seal
Glass-to-metal seals are a very important element of the construction of vacuum tubes, electric discharge tubes, incandescent light bulbs, glass encapsulated semiconductor diodes, reed switches, pressure tight glass windows in metal cases, Vacuum I ...
*
Heat sealer
A heat sealer is a machine used to seal products, packaging, and other thermoplastic materials using heat. This can be with uniform thermoplastic monolayers or with materials having several layers, at least one being thermoplastic. Heat sealing ...
*
Permeation
In physics and engineering, permeation (also called imbuing) is the penetration of a permeate (a fluid such as a liquid, gas, or vapor) through a solid. It is directly related to the concentration gradient of the permeate, a material's intrinsic ...
*
Moisture vapor transmission rate
Moisture vapor transmission rate (MVTR), also water vapor transmission rate (WVTR), is a measure of the passage of water vapor through a substance. It is a measure of the permeability for vapor barriers.
There are many industries where moisture c ...
Notes
{{packaging
Seals (mechanical)