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Polyurethane foam on the front line of construction

09.07.2020

Text: Maria Ševeljova, Product Managing Department

In the insulation and sealing work of modern buildings, you cannot manage without polyurethane foam. Although foams have been on the market for quite a long time, manufacturers are constantly being asked to advise on how the foam works and what the criteria are to make the right choice for the particular field of application.

Synthetic polymer polyurethane was discovered in Germany in the second half of the 1930s, and a dozen years later, a rigid foam material for insulation was introduced in the USA. As it is a polymer that is resistant to chemicals, weather conditions and mechanical wear, the production method of which is relatively simple and which has plenty of application areas, no better alternative has been found to polyurethane in construction. High quality polyurethane foam is characterised by fine porosity, uniform structure, good joint stability, very good adhesion to the substrate and rapid hardening.

Put simply, single-component polyurethane foams are divided into basic and special foams. Special foams are foam adhesives and fire rated foams as well as elastic foams. Basic foams are meant for year-round or winter use, and they include foams with high yield or low post-expansion, filler foams, spray or low isocyanate content foams, etc.

The general rule is that straw foams are characterised by significant post-expansion and gun foams by small post-expansion. The difference comes from physics – if in the case of a straw foam pressure is released when foam extrudes from can, then in the case of a gun foam it is released at the gun´s end, which ensures a uniform fluffy structure, minimum post-expansion and maximum yield. Between these two there are foams with a thin applicator in which case the foam is compressed in the trigger. Compared to the straw foam, the yield is better and the post-expansion is less significant. As a result of product and chemical development, a number of foams with a specific composition have been developed which have been adjusted in such a way that the use in a particular work stage is as convenient as possible. For example, an elastic straw foam the expansion of which is less than average or a spray foam with a spray nozzle, which expands 100%. The characteristics of a high-quality foam adhesive is always low expansion pressure.

Polyurethane foams are primarily intended for insulation, sealing and filling. Foams cannot perform a bearing function and are not suitable for fixing. In different countries the construction practices are different, but mechanical fastenings are almost always strictly recommended, especially when installing windows. Once again, the exception is foam adhesive, which can also be used for fixing. In the development of foam adhesives, much attention has been paid to their good adhesive properties. Although conventional foams also have good adhesive properties, foam should not be used as an adhesive. There are specific requirements for adhesion, and using foam instead of adhesive in critical places may not work.

Polyurethane foam requires air humidity to expand, and the previously moistened surface ensures a better structure for the foam. Moistening is recommended in summer and for porous and dry surfaces. In the case of winter foams (up to -18°C), the air humidity may be lower, which has already been taken into account in the composition of the foams used at minus temperatures. However, it must be ensured that the substrates are free of ice and frost.

The joint or surface on which the product is to be installed must be moistened and not the foam itself. Water damages the structure of the foam. It is the best to use a sprayer where the liquid comes out in a gentle mist.  Water droplets must not remain on the surface of the joint. If necessary, the droplets must dry before using the foam. If the foam is applied in two layers, it is also necessary to moisten the first layer of the foam. In the light of the above, polyurethane foams cannot be applied in a closed room or opening because there is not sufficient humidity required for the foam to harden. 

It is necessary to shake the foam can 20 times before use in order to mix the ingredients and ensure good foam structure. Then, the can must be turned upside down and screwed on the gun. If the can is screwed on the gun without shaking it first, some gas will come out. Gas has an impact on the yield of the product and some of the foam may remain in the can.

Gas is the most hazardous ingredient in the polyurethane foam (explosion risk), and thus foam cans must be stored and transported in an upright position. As the can is under pressure and flammable, the foam must be protected against overheating and kept away from sources of ignition.  Considering the safety of aerosol, the product must not be left in the sun on the construction site or in the car. The suitable storage temperature is +5°C to +30°C. It is important that the can is intact and free of any dents. After use, the cans must be emptied and taken for waste collection.

Application of polyurethane foam does not present any risks when using personal protective equipment, such as gloves and protective goggles, and the room is ventilated. Installed and hardened foam does not emit harmful compounds.

The life expectancy of polyurethane foam protected against UV radiation is very long. It does not rot or mould, it does not collapse or become brittle, and it withstands extensive temperature and humidity fluctuations.

Important to know:

–        When choosing the foam, the specific work requirements and application temperature of the product must be taken into account. The main uses of polyurethane foams are insulation, filling and sealing gaps, and installing doors and windows. For special work, specific foams such as foam adhesive or fire rated foam are available. The more sensitive the materials that are used on a site, the more important it is to pay attention to the expansion pressure of the product.

–        Traditionally, foams are applied from bottom to the top, in the form of a zigzag. If the application direction of the foam is not so important in a narrow joint, then for wider joints the joint must be built up. When applying the foam from top to bottom, it may simply fall down. Foam adhesive may be released straight and, in the case of spray foam, the application direction is not very important.

–        The joint must be 2/3 filled with gun foam and half with a straw foam so that there is sufficient expansion room for the foam.

–        Polyurethane foam is a good insulation material that helps to save energy. When installing windows, foam sealing helps to reduce air leakages and heat losses and save on heating costs.