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Typical silicone sealant . If the silica content is lower , this substance can also be made liquid.
Structural steel in the breakthrough in a wall with a 90-minute fire resistance period . A fire protection mortar and a silicone , which is attached underneath the trapezoidal sheet metal to seal off a smoke-tight room, serve as a partition or sealant .

Sealants are materials for sealing joints , gaps, openings and the like, which in practice can be exposed to movements. In contrast to a seal made of solids, a sealant is required to adhere to the joint flanks in order to fulfill its function. For this reason, all sealants can be assigned to adhesives according to their definition according to DIN EN 923 . Other names for sealant are joint compound, sealant, joint sealant, soft seal or liquid seal .

The term sealant can be viewed as a generic term for sealants and other types of sealing. In construction, materials for sealing larger areas are also known as sealing agents .


Sealing joints and gaps is an activity that humans and animals have mastered for a long time. While it was necessary in prehistoric times to seal cracks in pile dwellings against wind and weather, for example with grass and clay, in antiquity vessels and boats were sealed with naturally occurring products such as beeswax , asphalt (pitch) or tree resin . One of these original types of sealing has survived to this day in a modified way in the construction of wooden boats: Here the gaps in the hull are sealed by caulking with tow and tar . In the animal world, too, was and is being sealed, for example by beavers when building dams or by bees .

The history of modern sealants begins with the development of window putty around 1700 . After the advent of polymer chemistry in the 1930s , a rapid development of synthetic sealants ( butyl rubber , polysulfide , silicone , polyurethane , SMP sealants) began in the 1950s and 1960s . The further development of sealants will not stop in the future either: New polymers with new properties are already being tested, for example to improve the environmental friendliness of manufacture, use and disposal or to optimize the price-performance ratio: requirements that meet future sealant developments.

Mode of action and classification of sealants

A sealant must meet two basic requirements: It must have adhesion to the components to be sealed so that the medium, against which it is to be sealed, cannot penetrate between component and sealant and thus make the joint permeable. Adhesion can either take place via chemical bonds, as is the case with chemically reactive sealants, or via physical interactions, for example electrostatic forces. It is assumed that a chemical bond to the substrate is more permanent than a physical bond; however, in many cases the physical forces are completely sufficient for an effective seal. The joint flanks must be prepared accordingly so that the adhesive forces - regardless of their type - can develop properly: All loose components such as rust, dust, etc. must be removed mechanically, oils, greases and other separating agents must be removed with suitable solvents. As with adhesives , the preparation of the substrate is a major factor in the success or failure of the sealing operation. If a joint starts to leak, it is usually not due to adhesive failure, but usually due to inadequate joint preparation.

The cohesion of the sealant is its internal strength and describes the resistance to tearing of the sealant. The cohesion has to be so high that a pending medium, even under pressure, cannot find a way through the sealant itself.

The sealants can be classified according to many criteria:

  • According to reactivity
  • According to the mechanical behavior
    • elastic
    • plastic
  • According to the underlying basic chemistry
  • After application

In the case of a chemically reactive system , chemical reactions take place after being ejected from the cartridge with the ingress of atmospheric moisture, which lead to crosslinking / solidification of the injected sealant mass. The 1-component systems discussed here are condensation reactions in which - depending on the basic chemistry - different small molecules ( carbon dioxide (CO 2 ), water , alcohols , acetic acid etc.) are split off. The rate of reaction depends on the temperature of the environment and the amount of water vapor in the air. The higher the ambient temperature, the faster a chemical reaction takes place. In most cases, there is also a higher air humidity at higher temperatures, which leads to a further acceleration of the hardening reaction, which begins with skin formation. In cool dry weather, hardening and skin formation take significantly longer than, for example, in Central European summer. This must be taken into account during sealing operations, i.e. in summer the sealant must be smoothed off relatively quickly before the onset of skin formation, otherwise the surface will not be smooth. In winter you can take more time with the tooling; however, the skin is then sticky longer.

Physically reactive sealants only change physically, for example through cooling after melting, through loss of solvent or loss of water. There are no chemical reactions whatsoever within the sealant or between the sealant and the substrate.

Non-reactive sealants are identical in the delivery and application state. There is no reaction.

The usual classification of the most important sealant technologies is shown in the following overview:

Classification of the most important sealants according to reactivity (curing behavior) and basic chemistry

Chemically reactive sealants

Reactive sealant with intumescence in the Grand Coulee Dam . Improper sealing in a concrete wall with a 2-hour fire resistance period using a liquid sealant (CP25). In the event of fire , the sealant containing sodium silicate foams up.


The best-known silicone sealants, all of which harden in the air, can be divided into the acetate systems, amine / amineoxy systems, oxime systems, benzamide systems, ester systems, enoxy systems and alkoxy systems.

The best known is the acetate system , which splits off acetic acid when it hardens (acetic cross-linking) . It is characterized by very high stability (heat, UV radiation, weathering) and good adhesion to substrates such as glass , enamel , porcelain and also anodized aluminum . For cement-based substrates (e.g. concrete, mortar, screed), sensitive metals (e.g. copper, brass, lead) and plastics, acetate systems are generally unsuitable or only suitable to a limited extent. The acetate systems are used extensively in the interior fitting of houses in the sanitary area and are probably the epitome of a silicone for the layperson.

The amine / amineoxy systems have a characteristic, fish-like odor when hardened . As long as this rather unpleasant odor occurs, it is a sign that the sealant has not yet fully cured and must therefore not be exposed to any strain. Although amine or amineoxy systems lead to extremely stable products that harden significantly even in cold weather, they can only be found on the market for special applications.

Benzamide systems also play a subordinate role in the market . These also split off condensation products with a characteristic odor when it hardens. Today they are only used to a very limited extent in window construction and for special applications.

Oxime systems are neutrally curing (neutrally crosslinking) silicone sealants . During curing, they split off relatively inert, characteristically smelling ketoximes that do not attack sensitive substrates such as those in the electronics industry. Wherever the more chemically aggressive splitting-off molecules such as acetic acid and amine are not desired, oxime systems have a good chance of ensuring a functioning seal. Due to the toxicity of the cleavage products (2-butanone oxime (MEKO), 2-propanone oxime (DMKO) and / or 2-pentanone oxime (MPKO)), the use of oxime silicones is now to be viewed as critical. The construction trade association recommends using other silicone systems or other sealant technologies and avoiding the use of oxime silicones.

The ester systems are one of the more recent developments in the field of neutrally curing (neutrally crosslinking) silicone sealants. In contrast to oxime silicones, ester silicones do not release any toxic by-products; they are suitable for sensitive substrates and show a wide range of adhesion on many materials. They are now used in many applications (e.g. as a sanitary sealant, as a glazing sealant, for facade joints, for floor joints, etc.). During the hardening process, they split off ester molecules, which, like other silicone systems, have a characteristic odor. After curing, however, they are completely odorless.

The alkoxy systems are also neutrally curing (neutrally crosslinking) silicone sealants and have been on the market for many years. Alkoxysilicones split off lower alcohols during curing (methanol and / or ethanol) and therefore have a barely perceptible odor during curing. They are suitable for sensitive substrates, show a wide range of adhesion on many materials and are used in many applications.

Enoxy systems are also neutrally curing (neutrally crosslinking) silicone sealants . Enoxysilicones split off acetone during curing. They have a rather subordinate importance in the market and are used in special applications in the industrial sector.

Polyurethane sealants

Sealants based on polyurethane (PU) harden in humid air, releasing little carbon dioxide. They are used extensively in sealing operations in the transport industry (car repair, caravan repair and in metal construction) and for connecting joints on windows and doors. In addition to their sealing effect, they can also be used as elastic adhesives, for example to glue solar modules to caravan roofs. Soft polyurethane sealants are used in large quantities to seal structural joints (according to DIN 18540). In contrast to silicones, yellowing can occur on light formulations when exposed to sunlight.

Sealants based on silane-modified polymers (SMP)

Silane-modified polymers as the basic material of these sealant systems are neutrally hardening polymers, which split off alcohol when exposed to moisture. This means that UV-stable sealants and elastic adhesives that adhere to most substrates without primer can be formulated. What is interesting about this technology is the fact that the primer (primer = adhesion promoter for difficult substrates), which normally has to be applied separately, is built into the sealant. The application of sealant actually includes two operations, namely the application of the primer and the application of the sealant itself. The universal applicability and the advantages described are the reason for the growing importance of this class of sealant in the transport industry, in repair and now also in the first do-it-yourself applications. Sealants based on SMP are available on the market under different names depending on the exact raw material basis (e.g. MS polymer, hybrid polymer, PUSI, SPUR and others).

Polysulfide sealants

Polysulfide technology is the oldest reactive sealant technology. 2-component polysulphide sealants ( polysulphides ), which are very resistant to media, were and are used in the glazing of wooden windows. Since they smell quite intensely of sulfur compounds, they could hardly establish themselves in indoor applications. In the case of polysulphide sealants, on the other hand, there are noteworthy applications, especially in the 2-component area, where polysulphides are used on a large scale as so-called secondary seals to seal joints in the petrol station area and to manufacture insulating glass . Due to their excellent fuel resistance and very good flexibility at low temperatures, the polysulphide sealants are widely used in aircraft construction. The connecting elements of the fuel tanks in the wings of an aircraft as well as the fuselage structure are sealed with interlayer and caterpillar sealing compounds.

Chemically non-reactive sealants

Butyl sealants

They are based on butyl rubber , are more or less permanently sticky and are mainly sold in the form of tapes, cords or diecuts. These products are often used in metal construction (sheet metal construction, ventilation construction or heating construction). Since the butyl sealants cannot transmit forces because they are permanently plastic, the individual substrates must be mechanically connected to one another. In the automotive industry, butyl sealants from barrel melting systems are processed and used to seal between two metal sheets, which are connected to one another by spot welding, for example. Butyl sealants are very non-polar and adhere to most polar and non-polar substrates without pre-treatment.

Solvent-based sealants

If you add a certain amount of solvent to butyl sealants (and also those based on other polymers), you get products that are very easy to process. When spraying, they flow onto the substrate without great pressure, wet it and usually build up good adhesion, even to non-polar substrates. The loss of solvent during drying can result in slightly concave surfaces.

Acrylate sealants

On the basis of acrylate dispersions , water-based sealants can be formulated, which are also referred to as acrylic sealants or dispersion sealants. Compared to reactive systems such as silicone sealants, SMP sealants and polyurethane sealants, the acrylate sealants usually have a significantly lower elasticity, which means that there are restrictions on their use in joints subject to movement. The adhesion properties on many materials are also comparatively limited. In the construction industry, they are often used as joint sealants on absorbent, mineral substrates and painted wooden components, they are often also suitable on some metals and plastics. Typical applications are the sealing of connection joints in the interior (e.g. in walls / ceilings, on door frames and window frames, in drywall) and the sealing of cracks in facades. During the drying process, the water contained evaporates, which results in a visible loss of volume. Due to the water content, acrylate sealants are usually sensitive to both frost and rain before and during drying. The drying speed is temperature and weather dependent. Acrylate sealants have a comparatively low resistance to chemicals and cleaning agents and are therefore not suitable for joints in the sanitary area or for floor joints.


Sealants can contain pollutants that evaporate mainly during processing and curing, and sometimes for a long time afterwards. In a study, the Stiftung Warentest found pollutant loads in 14 of 50 products tested that did not comply with the official health protection requirements, particularly in the case of sealants and synthetic resin ready-to-use plasters. The Stiftung Warentest points out that there are also low-pollutant products for almost all applications that are labeled with an eco-label .

Until around the mid-1970s, polychlorinated biphenyls (PCB) were used as plasticizers in joint sealing compounds.

See also


  • Manfred Pröbster: Compact dictionary of sealants and joints . Fraunhofer IRB Verlag, Stuttgart 2010, ISBN 978-3816781110 .
  • P. Birkholz among others: repairs at home . 3rd edition Stiftung Warentest, Berlin 2010, ISBN 978-3-86851-012-6 .
  • M. Pröbster: Building sealants . Vieweg + Teubner Verlag, Wiesbaden 2008, ISBN 3-8348-0290-5 .
  • M. Pröbster: Modern industrial sealants . Vulkan Verlag, Essen 2006.
  • E. Baust, W. Fuchs: Practical handbook sealants . 5th edition IVD Industrieverband Dichtstoffe (Ed.), HS Public Relations Verlag, Düsseldorf o. J.

Web links

  • - Information page of the IVD Industrieverband Dichtstoffe e. V.

Individual evidence

  1. DIN EN 923: 2016-03 Adhesives - Terms and Definitions , DIN German Institute for Standardization e. V., Beuth Verlag GmbH, Berlin
  2. Archived copy ( memento of the original from October 20, 2018 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot /
  3. Archived copy ( memento of the original from October 20, 2018 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot /
  5. Stiftung Warentest : Thick Air After Renovation , test 05/2007, accessed on June 11, 2012.
  6. ^ Hansjörg Kieper, Heinz-Dieter Neumann, Rita Rachor-Ebbinghaus: Polychlorinated biphenyls in building construction. In: Hazardous substances - cleanliness. Air . 71, No. 1/2, 2011, ISSN  0949-8036 , pp. 10-14.