pu-schaum.center · FAQ PU foam

Questions and answers about PU foam

Construction foam, filling foam, window foam, insulating foam, frame foam, PU in-situ foam, PUR foam or polyurethane foam: there are almost as many names for PU foam as there are applications. But regardless of the name, some questions are repeatedly asked when working with the products. The most frequent ones have been listed here.

Ecology and recycling

You can either return single cans to the local municipal depot or to DIY or construction materials stores. You can find the next free return option nearby using the search machine at der Homepage von PDR: https://www.pdr.de/index.php?id=112

The specialist recycling company, PDR Recycling GmbH, will pick up larger quantities of spent PU foam cans. For more information at www.pdr.de

As a precaution, the European Legislation has classified MDI (diphenyl methane-4,4´-diisocyanate) as Category 3 and carcinogenic; it also bears the symbols XN and the R40 statement (Limited evidence of a carcinogenic effect). This is the basis for the German classification in the Chemicals Prohibition Ordinance and the Ordinance of the Protection of Hazardous Substances.

On the contrary!! PU foams from pressurised packaging are primarily used for thermal insulation. The excellent insulating properties achieve outstanding energy-savings and therefore reduce CO2 emissions. This has been proven by tests conducted by the ift Rosenheim.

Expert report by the ift Rosenheim
Expert report by the ift Rosenheim Annex
Table: Thermal conductivity of insulation materials

Benefits of PU foam

Depending on the number of installed windows, an annual sum of approx. 115 million euros is saved.

PU foams in pressurised cans are easy to use and very mobile. Joints can be insulated, sealed and adhered in one work step, e.g. after windows have been attached mechanically. The result is a draught-free and thermally insulated joint. Thanks to its environmentally-friendly properties, it also ensures a positive climate in rooms.

Also, PU foam is permanently resistant to mould, damp, bacteria and many alkalis and acids.

Yes, because they have the best insulating properties. Mineral construction materials such as plaster, mortar or concrete have almost no thermal insulation effect. Glass-wool fillings decompose gradually, thereby loosing their thermal insulation effect. And of course insulation materials such as sheep's wool or spray cork only have half the insulation value. Also, natural insulation materials are often preserved with chemicals. 

PU foams ensure that joints are permanently sealed. The applied foam also gets into the smallest of gaps and hollow spaces.

Table: Thermal conductivity of insulation materials

Cost comparison of thermal insulation for typical window installations.

What average costs are generated when insulating a window joint with PU foam, and what are the costs of using mineral wool? This was the central issue addressed in the expert report by Prof. Klaus Layer, expert at the Mannheim Chamber of Commerce and Industry for the glazing industry.

The study based its result on the assumption that the window joint was created correctly both from a building and also a technical point of view. It uses a typical calculation that looks at both the factor material and the factor work at realistic prices. Also, the various installation situations for new buildings and renovation work were addressed.

The results:

The costs were determined per running metre and show that using mineral wool is 2.5 – 5.1 times more expensive than using PU foam. The use of PUR foam saves public and private builders in Germany an annual sum of 115 million euros (as per 2005) in construction projects. The precise calculations and coefficients are stated in the expert report.

Table: Thermal conductivity of insulation materials
Expert report about the cost situation for insulating window joints

The decisive λ-value is 0.030-0.040 W/mK and is achieved throughout the foamed area if the foam is applied correctly in compliance with the manufacturer's instructions. For instance, the value for mineral wool is highly dependent on the skill of the tradesman filling the gaps.

Table: Thermal conductivity of insulation materials

Questions about the product

There are so-called MDI-free PU foams (MDI is the abbreviation for isocyanate). These are assembly foams that have a polyurethane structure but no more free isocyanate groups because they were turned into a non-reactive urethane structure in a previous chemical process. The foam hardening process is based on reaction-capable silane groups that, once the foam has left the can, react with the ambient moisture leading to the interconnection and therefore the hardening of the applied PU foam.

For more information, please go to https://www.pu-schaum.center/en/pu-foam/pu-foam-types.html

Manufacturers supply PU foams in various colours. Among these foams, there are some that are white. However the colour of the decorative projects are not decisive in the long run because PU foam is not light fast. This means: is changes colour over time. If the hardening foam should have a different colour at a later date, paint the foam to protect it from the effect of light. 

The first cans with construction foam were launched in the 70s. For more background information go to: https://www.pu-schaum.center/en/polyurethane-pu/history.html

In the 1990s the Association for the Control of Emissions in Products for Flooring Installation, Adhesives and Construction Products (GEV) introduced the EMICODE to offer assurance to consumers about how to handle adhesives, sealing agents and insulating materials. This is a competition-neutral classification system that assesses the emission behaviour of products based on a strict testing procedure. PU foams that were tested were classified as very low-emission and received the best possible category EC1plus. 

You can simply wipe off PU foam using a dry cloth. Due to its good adhesion properties, hardened construction foam can only be removed mechanically.  Larger excesses can be simply cut off with a sharp knife. For all other residues, use a PU foam remover (not to be confused with acetone-based PU cleaners) that most manufacturers of assembly foams stock. The PU remover is simply applied to the hardened PU foam residues using a cloth after the time stated on the packaging. However the PU foam should be tried first at an invisible place because it could corrode the surfaces. 

Well stocked DIY stores and specialist stores sell construction foams.

PU foam can be used in many places in the caravan, for instance as insulation and for affixing the shower tub. Due to the fact that a caravan on the road is exposed to many vibrations, a flexible PU foam should be selected that moves with the movements of the vehicle without cracking. The name of the foam usually indicates whether or not the foam is flexible. Usually, the product names contain a term that indicates the foam properties such as 'flexible', or 'elastic', otherwise simply ask the vendor or read the product data sheet. The flexibility of the foam is stated via the compression force (compression around 10 % of the layer thickness). The lower the value, the higher the flexibility.

However, it is not recommended applying PU foam to fill out hollow spaces in vehicle bodies. Moisture deposits in the wall structures are likely. If the hollow space above the moisture is sealed with PU foam, this will encourage corrosion. 

No. PU foam is not suitable for this.

The winter foams sold in Germany can usually be used up to -10°C. PU foams that work at lower temperatures are sold in other countries that have much longer and colder winters. You should ask the manufacturers. There is a list of leading European manufacturers of PU foam at https://www.pdr.de/en/divisions/recycling-of-pu-foam-cans/system-partners/

Fitting doors is one of the most common areas of use of foam in Germany. However, special door frame foams should be used; the normal insulating and filling foams are not suitable.

No. PU foam does not corrode other materials, for instance cable jackets.

1-component PU foam needs to come into contact with normal humidity in the air to harden. 1-component assembly foam hardens from the outside, where the moisture takes effect first, to the inside. In contrast, 2-component PU foams do not react with ambient moisture, instead they need another reaction partner. This is usually found as an extra packaging element in the can. It needs to be activated separately before starting. This process is described on the can. 2-component PU foam hardens from inside out. It is much stronger than 1-component construction foam and hardens very quickly and evenly. This helps to control the hardening process. 2-component assembly foam is typically used to fit doors. A classic application for the 1-component construction foam is insulating windows.

No. PU foams do not contain formaldehyde, neither can they create formaldehyde. Note: There is a urea-formaldehyde foam that is occasionally used in the construction field. This should not be confused with PU foams.

The so-called MAK value represents the maximum workplace concentration and refers to the level of diphenyl methane-4.4-diisocyanate. This value is 0.01 ppm and is not reached when the room is adequately ventilated, even if PU foams are used permanently. 

The Chemical Professional Association took measurements of room air in the surrounding area (10 cm away from hardened foam) and found that the isocyanate values in the air lay below the detection limit. Despite this, the safety advice on every pressurised packaging regarding the use of the foam should be observed.

The main constituents of PU foam are polyalcohols (polyols), isocyanates (diphenyl methane-4,4´-diisocyanate), propellants and, depending on the application area or purpose, stabilisers, accelerators and flame retardants.

No, once the foam is hard, it has a neutral impact on the environment. Please refer to the results of the EMICODE examinations. PU foams that were tested were classified as very low-emission with the label EC1plus (= very low emission). Click here for more details about the Emicode.

No. However, for technical reasons the foam is always covered whenever PU foam is used. For instance, when PU foam is used in its main application field, i.e. ensuring that window joints are air-tight, window insulation is stipulated to prevent draughts and heat loss.

Also, purely for appearance, the PU foam should always be covered. If assembly foams are exposed to light permanently, the hardened foam will turn light to dark brown. This not only does not look nice, it makes the foam brittle. Therefore the hardened foam should be protected from UV light by a coat of paint or plaster, or by applying a sealing agent.

It is a question of time. If, for instance, the windows of a building are insulated with PU foam and the foam is not immediately protected from direct sunlight by sealing tape, the foam will turn yellow. If over a period of about three to four months the sealing is replaced, this is unproblematic because the foam below the surface still fulfils its function and is undamaged. However, the discoloured layer should be removed before the joint is finally sealed. If the foam has been exposed to UV radiation for a longer time, it will become brittle and lose its insulating function gradually.

No, and it doesn’t need to be! PU foam is primarily used for insulating window joints. This is why it needs excellent thermal insulating values; these are achieved by the high air inclusions in the hardened construction foam. A further important quality feature of PU foam is its good flexibility that allows the filled openings to move with the window frame as it is exposed to temperature and wind loads. Concrete cannot do both, concrete is only stable and hard.

Yes, there are special foams that mice do not like. While mouseholes can be temporarily plugged with normal construction foam, rodents can gnaw through the foam again, if they are so inclined. Rodent-resistant construction foams are available in well-stocked specialist retailer.

Processing PU foam

It is usually very easy to paint or plaster over PU foam. It is usually compatible with the common pants, lacquers and plasters.

Well foam was developed for filling larger hollow spaces that will be placed underwater at some point. It can also be used when moving shaft rings, drain shafts and domestic sewage plants. It is distinguished by its resistance to diluted chemicals, petrol and its rotting-resistance (resistance to bacteria from the earth). The hardened foam is therefore also resistant below water. Always observe the processing information, for instance the surfaces that are to be adhered should be adhered within the open time of the foam (usually approx. 5 minutes). However, PU foam cannot be applied directly under water.

PU foam is ideally suited for thermal insulation of the window joints. Its adhesive effect also helps when fitting the window and, to a certain extent, PU foam is also airtight. However further steps are necessary to ensure that the window is airtight and is fitted correctly in line with the recognised rules of engineering. For instance, mechanical attachment of the window frame is mandatory.

There are a large number of technical books that address this highly complex trade. The most well-known is the RAL guide 'Window Fitting'. For more information, please go to: https://www.pu-schaum.center/en/service/quality-and-legal-considerations.html

Filling openings in brick walls with PU foam is one of the most common application fields for this material in the can.

It is generally possible to re-use partially spent 1-K-PU foam cans. To do this, either the adapter tube or the spray gun need to be cleaned directly after processing using a PU foam cleaner, and all foam residues that could harden need to be removed. However, the opened construction foam can should still be used up as soon as possible. All cans, partially used or new, should be stored in an upright position. Partially used cans in particular tend to have clogged valves unless special clog-free valves are used (e.g. so-called safety valves).

2-K cans must be used in one go. They cannot be used later because the hardening component was activated and hardens the foam still inside the can.

The most common reason for this is an insufficient amount of moisture. PU prepolymer needs moisture to completely react. When making model moulds, we recommend slightly wetting the walls of the mould and later the foam itself. Alternatively, you can use a 2-K-PU foam instead of a 1-K-PU foam.

No, because hardened PU foam is environmentally neutral.

No, PU foam is not sensitive to moisture. This has been proven in test certificates issued by specialist institutions such as the ift Rosenheim.

ift expert report

In contrast to classic aerosol cans, PU foam cans do not have a so-called riser tube that pumps the fluid content outwards when the spray head is pressed. Because PU foam is viscous, a transport tube of this kind would become clogged. Therefore, the can needs to be turned upside down to empty it after the pre-polymer has been mixed with the propellant by means of shaking. This causes the viscous mass to sink downwards to the valve and ensures that there is always a sufficient amount of PU foam at the valve opening and the foam can be applied evenly without interruption.

In the case of 1-component foams in aerosol cans, the pre-polymer in the can is mixed in an ideal ratio so that the user only needs to apply the foam. The foam hardens within an hour after coming into contract with ambient moisture. After this, PU foam is resistant to mould and rotting, it has no odour and is physiologically safe.

2-component foams require an additional reaction partner to harden. This is located as an extra packaging element, either in the can or supplied separately. To ensure that the foam has the right mixing ratio, the hardener needs to be activated just before it is used and the can needs to be shaken thoroughly. The precise instructions for the product are stated on the can and must be strictly observed to ensure that foam develops optimum quality properties. The benefit of this method is that the material becomes very strong very quickly and expands less. If no hardener is added, 2-component foams react just like their 1-component counterparts, however with a slight delay.

The aerosol cans containing 2-component foams should not be confused with the 2-component spray foams that are used in some European countries for insulating large wall, ceiling, floor and roof surfaces. Here, the hardener and pre-polymer are supplied in large canisters and mixed directly at the building site using special machines. This insulation method should only be carried out by trained specialists to guarantee the correct mixing ratio and safe application.

Fresh PU foam that has not yet hardened can be simply wiped off with a cleaning cloth. Once the assembly foam is hard, removal is much more difficult. Due to its good adhesion properties, hardened construction foam is difficult to remove mechanically. Larger excesses can be simply cut off with a knife. For all other residues, use a PU foam remover that most manufacturers of assembly foams stock. The PU cleaning agent is simply applied to the hardened PU foam residues as stated in the instructions. Within about an hour, it has softened the hard construction foam allowing it to be simply wiped off with a damp cloth. Caution: the cleaning agent can corrode and dull painted or coated surfaces. Therefore, always check compatibility on an inconspicuous place before using. It is also advisable checking the technical information sheet of the product. It is generally not a good idea to allow the product to come in contact with the skin. You can only try to rub off the foam with plenty of water, detergent and patience. It is better to wear protective clothing when using PU foams, i.e. gloves and long sleeves so that no drops land on the skin.

The hardening process of 1-component assembly foam can be accelerated if water is sprayed slightly onto the surface. The reason: 1-component PU foam needs moisture to harden. It hardens from outside in. If there is no moisture, because the air is to dry, this process is delayed.

Anyone working with construction foam should wear gloves and goggles. One of the greatest advantages of PU foam is its excellent adhesion properties. The effect you want on your building is something you need to avoid on your skin! Even very fresh foam is very sticky and can only be removed from the skin mechanically or with chemicals. Therefore, appropriate protective clothing should be worn to avoid accidental contact with the eyes or skin.

No, PU foam can absorb large quantities of water and is therefore not suitable for sealing walls.

In this case we do not recommend using a classic PU foam because pumice absorbs too much moisture and this could prevent the PU foam from hardening.

PU foam is not suitable for use under water or for contact with water.

PU foam is compatible with all common paints, lacquers and plasters. Simply use a sharp knife to cut away any excess PU foam to create a smooth surface.

Even cans held with the valve facing upwards can work, if a few simple details are observed. Here are six tips:

1. Use a PU foam can that is not yet opened.

2. If possible, use fresh foam. You can recognise this by shaking the can. The more fluid the content, the fresher the PU foam!

3. The PU foam can should be warm. This means the foam should be about 25 °C. This makes the content more fluid and it can exit the can more easily!

4. Shake the can thoroughly before use, so that the content is mixed well. This improves the quality of the exiting PU foam.

5. Open the valve fully.

6. Use a small construction can if possible. The smallest 1-component PU foams are supplied in 150 ml cans. The next largest can is approx. 300 ml in size. 400 and 500 ml are OK. The 750 - 850 ml foams are too large for this purpose.

Anyone who observes these tips will be able to work without problems. However, a little more residual content than usual will be left in the PU foam can.

That depends! The basic rule used to be that the more the foam expands the better; nowadays, expansion in excess of 50% is no longer desirable. Even experienced craftsmen and women cannot estimate how much foam needs to be applied, if it expands by more than half again. This is why so-called low-expansion PU foam is now available that reaches its final volume just a few seconds after application. These kinds of low-expansion foams do not necessarily provide a lower foam volume than others, it is simply that the chemical reaction takes place earlier. Normally, a one-litre can yield between 20 and 30 litres.