Assembly foam

from Wikipedia, the free encyclopedia

OCF , often foam insulation , insulation foam , filling foam , insulating foam or PU foam mentioned, is an in-situ foam , which in the building industry for sealing is used. One then speaks of “foaming”.

Assembly foam is usually based on polyurethane , hence the frequently used names PU foam and PUR foam . It sticks very well to many materials. It is available as a one-component foam (1K) or as a two-component version (2K).

One-component assembly foam

Spray cans with one-component assembly foam

This assembly foam (1-component PU foam) is stored in aerosol cans , and in large quantities in pressure containers. The foam is applied directly from the can. The processing takes place with a hand dispenser or a dosing device. In 1-component cans, isocyanate and polyol are mixed in the can during manufacture, which is called a prepolymer . These compounds are made from higher molecular weight polyols with a stoichiometric excess of isocyanate. In this way, there are compounds which already have urethane bonds, but which on the other hand still have reactive isocyanate groups (so-called isocyanatopolyurethanes) which are accessible to reaction with moisture. The cans must be shaken before use to mix the components well. A propellant causes the material to escape at high speed and to expand into foam. Contact with the surrounding air sets the chemical reaction in motion that causes the foam to harden. In order to be able to set, one-component foams need water, which they extract from the air. Normally, the moisture contained in the air (at least 40% relative humidity) is sufficient; it is only advisable to moisten the affected surfaces on warm and dry days. The ambient temperature, relative humidity and the humidity of the contact materials are decisive for the foam quality and the course of the reaction. At an ambient temperature of 20 ° C, the 1K PU foam is cured after about five to eight hours. The in- situ polyurethane foams (frothing foams) used in construction expand spontaneously as soon as the dissolved and / or emulsified propellants evaporate immediately after being removed from the pressure vessel. The primary amines formed during the hardening reaction react directly with other isocyanate groups to form polyureas. The formation of carbon dioxide through the decomposition of the unstable carbamic acid derivatives can be critical in this reaction . Depending on the formulation, the carbon dioxide formed can cause bubbles to form in the cured polymer layer. This undesirable property is favored by a high content of isocyanate groups, by a low viscosity, fast curing and also with a porous substrate.

In order to speed up this process and achieve better quality, it is advisable to spray the points (the connection surfaces) with water before and after foaming. The foams harden from the outside, where the moisture acts first, inwards. Most foams are tack-free after approx. 10 minutes and can also be cut after approx. 45–60 minutes. After three to five hours, they can be fully loaded. 1-component cans make up the majority of the market in Germany at 80 percent.

Two-component assembly foam

2-component foams have a further reaction partner: the so-called crosslinker or hardener. This is either located in the can as an additional packaging unit or is supplied separately. Moistening to harden is not necessary here. The user must activate the hardener as described on the can. Evenly colored foam shows the success of the mixing process. 2-component foams, in which the substances in the can react, must be processed within the time specified on the label. The 2-component foam achieves higher strengths than 1-component foam and cures very quickly and evenly. This simplifies the control of the hardening.

In the case of different systems, the two components are brought together and the reaction is triggered either immediately after exiting the can or by mixing within the cartridge. When the reaction is triggered within the cartridge, the processing time begins, which with this system can only take a few minutes (usually 5 to 10 minutes). With two-component foams, the substrate must be absolutely dry. The foam hardens completely within 20 to 30 minutes, regardless of the humidity of the air or material. There is no risk of post-reaction as with one-component foam.

2-component foams consist of a low molecular weight polyisocyanate and an equally relatively low molecular weight polyol in a stoichiometric ratio. Bifunctional monomers essentially produce linear, thermoplastic products that are also used as hot melt adhesives. Trifunctional and multifunctional monomers lead to highly crosslinked, thermoset adhesive layers. Systems with different pot lives can be set depending on the reactivity of the starting monomers . Too long reaction times can be shortened by adding accelerators (e.g. triethylamine , lead oleate ). By carefully selecting the monomers, the properties that determine the adhesive layer, such as strength, elasticity, mechanical deformation behavior and resistance to chemical influences, can be controlled.

More PUR foams

Isocyanate-containing foams

Polyurethanes (PUR) are a particularly versatile group of plastics for use in a wide variety of areas of daily life. PUR can be found as rigid or flexible foam in effective heat and cold insulation, furniture upholstery, car seats or mattresses. The versatile material is also used as insulation for electrical or electronic components and can be found in modern high-performance paints, but also in the most modern sports shoes and floor coverings.

PUR foam variants

An isocyanate-reduced foam is a foam in which the content of methylenediphenyl isocyanate (MDI) has been reduced to a value between 0.1 - <1%. Other foams are reduced in MDI content to such an extent that they have a value of <0.1% of reactive chains in the MDI. In addition, there are assembly foams that also contain the polyurethane framework, but no longer contain any free isocyanate groups, as these were chemically converted in a preliminary stage to form a neutral urethane framework. The curing of the foams is based on reactive silane groups which, after foaming from the can, react with the ambient moisture to cross-link and thus to cure the applied foam. Since no carbon dioxide is formed as a blowing agent, they are foamed with propane or butane . When the silane groups are crosslinked, the reaction product is methanol .


Mounting foam serves as an insulating material and prevents air exchange through otherwise leaky areas (" thermal bridges ").

Diagram of thermal conductivity PU foam.jpg

It applies for example in the window assembly and the door frame nmontage. When cured, assembly foams are resistant to water, oil, petrol, alkalis and various solvents , but not to UV radiation . The foam is rot-proof and rot-resistant. PU foam is very often used in the assembly of door frames or the installation of windows. The space between the wall and the door frame is punctually filled with assembly foam (only for doors). A problem with PU foam is that it can swell very much after it has been applied, but this can be prevented by appropriate processing or processing equipment. PU foam is used for insulating, filling, gluing and insulating and is available from building materials retailers in cans with a few hundred milliliters (300–750 ml). 1K foam requires moisture to harden. This moisture comes on the one hand from the surrounding materials (e.g. bricks) and on the other hand from the air. In order to achieve the best possible result, additional pre-moistening of the joint is necessary, except in the case of frost (ice formation), and this is absolutely required by all manufacturers of 1K foams. Most of the shortcomings in the manufacture of one-component PU foam joints are due to insufficient moisture. 2K foam hardens in itself without the effect of moisture, as the components required for hardening are already in the can and are mixed when activated. 1K foams with silane groups are processed in the same way as conventional 1K PU foams and also require humidity for curing.

By using special assembly foam known as frame foam, better sound insulation values ​​can generally be achieved when installing windows and doors. In most cases, these foams also swell less.

Well foam is intended for use in civil engineering in permanently damp locations, e.g. B. for sealing manhole rings.

Fresh foam that has not yet dried can be removed with acetone or agents containing acetone. Cured foam can only be removed mechanically.

In addition to its insulating effect, PU foam is particularly valued for its simple and quick processing.

Working hours insulation materials in comparison.jpg


In Germany, used PUR foam cans are classified as hazardous waste ( special waste ) for recycling. This classification is necessary because empty PUR foam cans still contain liquid residues. For this reason, they may only be disposed of using defined return systems. Local waste disposal companies accept individual cans free of charge. Many hardware stores also take back used assembly foam cans. Consumers of larger quantities can also call the respective licensor (usually using a free call number), register their quantities and make them available for free collection. The respective licensor can be recognized by the logo on the PUR foam can.

The return takes place on the basis of § 25 Recycling Management Act (KrWG) in connection with § 8 of the packaging ordinance . This means that manufacturers comply with Section 23 (product responsibility) from the KrWG.

safety instructions

Isocyanates as the main components of assembly foams have long been considered hazardous substances because they are B. May cause eye, skin and respiratory tract irritation. In more recent studies, these substances are even suspected of causing cancer. All construction foams that contain more than 1% MDI must therefore be accompanied by the note H351 - Attention! Suspected of causing cancer (previously R40 - suspected carcinogenic effect ) can be labeled. According to the German Chemicals Prohibition Ordinance , the products labeled in this way were only allowed to be sold to private customers by competent staff for a few years and were not freely accessible, but had to be locked in a showcase, for example. Construction foams that contain less than 1% free isocyanate, e.g. B. isocyanate-reduced foams or those that crosslink via silanes. With the latter, the free isocyanates are completely chemically reacted with special alpha-silanes during the production of the prepolymer. If the foam accidentally gets on the skin, it can simply be rubbed off without further ado and does not leave behind the black stains typical of PU foams.

With the entry into force of the updated Chemicals Prohibition Ordinance on January 26, 2017, the self-service ban for the sale of PU foam cans has been lifted. Products containing MDI are no longer subject to the regulation provided that they carry the hazard statement H351. For the trade, this means that construction foam cans can be freely accessible again on the sales shelf.

If 1K construction foam is processed in closed rooms without ventilation, the propellant gas (propane-butane mixture) may be sufficient to form an explosive gas-air mixture together with the room air. Sparks from light switches and brush fires from drills, vacuum cleaners, etc. Ä. Sufficient for an ignition of the gas-air mixture. The gas from 750 ml construction foam (1 standard can) is sufficient for approx. 6 m³ of an explosive gas-air mixture.

Individual evidence

  1. a b c Gerd habenicht: Gluing basics, technologies, applications . Springer-Verlag, 2008, ISBN 978-3-540-85266-7 , pp. 97 ( limited preview in Google Book search).
  2. a b Ulf Hestermann, Ludwig Rongen: Frick / Knöll building construction theory 2 . Springer-Verlag, 2012, ISBN 978-3-8348-2140-9 , pp. 557 ( limited preview in Google Book search).
  3. ↑ When it comes to disposal, it is important to ensure that the take-back systems also comply with Section 8, Paragraph 3, for the prescribed recycling of residual materials (Section 3, Paragraph 7, No. 3).
  4. ChemVerbotsV - unofficial table of contents. Retrieved April 8, 2019 .

Web links

Wiktionary: construction foam  - explanations of meanings, word origins, synonyms, translations