pu-schaum.center · Practical tips

Practical tips

Be it for installing windows or doors, attaching thermal insulation composite systems or simply filling openings in walls: construction foam has proven its practical value many times and is appreciated as a practical tool by tradesmen and handymen alike. To ensure that these all-round helpers function correctly, the PU Foam Information Centre has put together some practical tips for handling assembly foam.

Basics about using construction foam

Anyone using a construction foam can for the first time is unfamiliar with a few things that is second nature to a professional. For instance, the construction foam can should be shaken well before use. 1-component PU foam hardens better, if the surface is moistened beforehand. The assembly foam can is always held with the valve facing downwards. The PU Foam Information Centre has compiled an information sheet with basic information about handling construction foam cans.

Use the construction foam upside down

PU foam professionals know that assembly foam cans need to be held so that the valve faces downwards. This is the only way to completely empty the can. But why is this so? In contrast to classic aerosol cans, assembly 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 construction foam 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 ensuring that there is always a sufficient amount of PU foam at the valve opening and that the foam can be applied evenly without interruption.

Removing construction foam

Anyone who has worked with assembly foam knows the problem: however careful one is, some foam always splashes over and lands where it is not wanted, for example on the window frame. If this is noticed in time, the construction foam can be simply wiped off with a cloth. But once it has hardened, things get problematic.

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Ensure that the processing temperature is right

The conventional 1-component PU foam should be processed at a temperature between 5° and 30° Celsius so that the hardened PU foam has the required quality. At temperatures below 5 degrees the hardening process is slower which impacts on the cell structure. The hardened foam can become brittle. If the PU foam becomes too warm during processing, it may lose structural stability. This is similar to how cream behaves on a hot waffle. Therefore, ensure that the environment and also the material onto which the foam is applied do not reach a temperature above 30° Celsius. 

These temperature specifications only apply to foam that has not yet fully reacted. Once hardened, PU foam is temperature resistant from -40° C to +80°C.

Tip: Warm up cold foam and cool down warm foam carefully

You can warm up cool PU foam cans in lukewarm water. Never place the can into hot water or expose it to extreme heat, for instance on the radiator or using a Bunsen burner, because the propellant in the can could expand too quickly causing the can to burst.

Foam that is too warm can be brought down to the right temperature in a cool water bath.


Tip: Use special winter foam

If the outside temperature lies permanently below the optimum processing temperature, manufacturers recommend using so-called winter foams that have been specially developed for frosty days. Depending on their formula, they produce high quality insulation and foaming results at temperatures up to minus 10°C.

Moisten the 1-K-PU-foam

Most 1-component PU foams need a certain amount of moisture from the environment to harden. Therefore, the surfaces and the freshly applied PU foam should be moistened. A general rule is that a PU foam can with 750 millilitres (ml) of foam requires about 60 ml of water for correct hardening.

But, be careful! There are now PU foam on the market that do not need moisture to harden. Therefore, always check the instructions by the manufacturer on the packaging.

What happens, if there is not enough moisture.

If a PU foam cannot find enough moisture, it hardens much more slowly and does not expand as much. The 1C PU foam hardens from the outside (where moisture impacts first) inwards. If there is not enough moisture in the environment, the outer layer of the foam develops a hard skin, whilst the active PU foam underneath remains unhardened. If the humidity increases at a later date, this foam can expand weeks later and deform the parts.

This is how PU foam adheres correctly

PU foam adheres to almost all surfaces that are strong, clean and free of dust and grease. Only silicon, oils, greases, polyethylene, Teflon or similar surfaces are not suitable for PU foam. One special case are surfaces, such as plaster, that can actively absorb moisture. This must be dampened enough to ensure that the PU foam finds adequate moisture for its hardening process. Otherwise, it could impair the adhesion and edge adhesion. We recommend a prior coat of primer or bonding agent in this case.

Protect PU foam against direct sunlight

PU foam is decay and rot-proof; however, it is not resistant to sunlight or any other UV radiation. 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.

Shake the PU foam can well before and during use

Just before they are used, PU foam cans must be shaken thoroughly, at least 25 times, to ensure that the viscous PU foam and the propellant in the can get mixed. This creates a homogenous mass and the propellant can fulfill its three important functions.

  • It triggers the viscous PU fluid.
  • It transports the PU from the can.
  • It acts as an expanding material and cell creator for the PU foam that exits the can, thereby creating the required fine-pore structure that insulates so efficiently.

If the can is used for a longer time, re-shake now and again.

What happens if these instructions are ignored?

If the PU foam and the propellant do not form a homogeneous masse, partial mixing gaps can be created and the PU foam only expands slightly.

Store PU foam cans upright

If the PU foam cans are not used immediately, they should be stored in an upright position so that the valves do not become clogged.

Returning spent PU foam cans free-of-charge in Germany

Spent assembly foam cans should not be placed in the household waste, yellow sack, mixed construction or tin plate container. Your residual contents are too valuable for this! Construction foam cans can now be recycled to 95%; 80% of this in materials. In Germany, PDR recycles these practical cans in an environmentally-friendly way for free. PDR collects the spent construction foam cans in boxes from the companies free-of-charge. Anyone who only uses PU foam cans occasionally can return them to the DIY centre or the municipal waste depot free-of-charge. Please visit www.pdr.de and use the online search tool to find out where there is a return station close to you. For all other information about PDR's free collection system, please visit:

Working with PU foam

There are many types of PU foam. For instance, 1-component PU foam differs from 2-component PU foam in terms of its hardening behaviour. 1-component PU foam needs contact with ambient moisture to harden. In contrast, 2-component PU foam requires another reaction partner to harden, i.e. one that is in the packaging and is mixed in with the prepolymer just before the application. Another difference is the way it is applied. If a PU foam gun is used, we speak of gun foam. Alternatively, an adapter tube can also be used to apply the foam.

1-component PU foam cans with an adapter and safety valve

1-component PU foams applied with an adapter tube are a practical solution for people that only use construction foam occasionally. The adapter tube is simply screwed to the valve of the PU foam can just before use. 

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1-component gun PU foam

People who use 1-component PU foam on a regular basis usually prefer working with a separate PU foam gun that is screwed onto the construction foam can. The benefits lie in the easier dosage of the applied construction foam.

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2-component PU foam is very strong and hardens very quickly and evenly. This helps to control the hardening process. 2-component assembly foam is typically used to fit doors.

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