[Expert advice] PU foam sealant. Key concepts

How PU foam sealant came about

PU foam sealant in the form it is known in today came into wide use in the 1980s. But polyurethane foam, of which PU foam sealant is one kind, was invented much earlier, back in the 1940s, by the Swiss chemist Otto Bayer, who ran a laboratory at the Bayer chemical company. Incidentally, Otto himself was no relation to Friedrich Bayer, one of the founders of the company — just a namesake.

One-component, one-and-a-half-component and two-component PU foam sealant

PU foam sealant comes as one-component and two-component. In a one-component foam the can holds a premixed prepolymer and a blowing gas, also called a propellant. As it leaves the can the prepolymer foams up, starts to react with the moisture in the air and cures. If there is not enough moisture, curing is impeded and large voids can be left inside the body of the foam.

One-and-a-half-component foam, often called two-component in everyday speech, is stored in a can made up of two parts. One part holds a prepolymer, practically the same as in a one-component foam, and the other a catalyst that speeds up curing. The products from the two parts of the can are mixed immediately before use. Compared with one-component foam, one-and-a-half-component foam has a higher density, less post-expansion and a lower yield. In return it cures very quickly. Foam like this is used to fix window and door frames in openings quickly, in place of mechanical fasteners. One-and-a-half-component foam is used fairly rarely: it costs more, has a lower foam yield and has to be applied within 15 minutes of activation, otherwise it sets inside the can. In the overwhelming majority of cases a one-component foam makes more economic sense.

Two-component foam is produced at the point of use by mixing two different components with dedicated equipment. A great many products are made by this technology: from mattresses and car seats to thermal insulation, shoe soles and wood substitutes.

Where PU foam sealant is used

Thanks to properties such as low air permeability, low thermal conductivity and ease of use, PU foam sealant has found its use in sealing gaps when windows and doors are installed, filling cracks, insulating openings for pipe and cable runs, and insulating balconies and other building structures. More than 2,000 fields of application of PU foam sealant are known today, from construction to art. It has to be clearly understood that standard PU foam sealant is not recommended for waterproofing, because it absorbs moisture. Only special types of PU foam sealant may be used for waterproofing, and only in some cases. PU foam sealant is also degraded by ultraviolet light, so it always has to be protected from sunlight.

The excellent adhesion of foamed polyurethane to most surfaces has also found use in construction. Dedicated products appeared, such as foam adhesive based on polyurethane foam. They differ from standard PU foam sealant in having relatively low initial expansion and post-expansion but higher bonding strength. These products are used to bond thermal insulation boards to walls and as a bonding compound for construction blocks, wood-based materials, gypsum board and metal roof tiles.

Foam yield of PU foam sealant

Probably the first specification end users look at. And it does matter: the more foam comes out of the can, the more work can be done with it. That is a direct saving of both time and money. So what does the foam yield depend on?

First of all on the amount of active substance filled into the can. The mass of the can can serve as a measure of this. It is often found that cans from different manufacturers that look identical and declare the same foam yield differ greatly in mass. All other things being equal, a heavier can should give more foam than a lighter one.

The yield, however, does not depend on the fill of the can alone. Cured foam from different manufacturers can have different properties, density for example. A heavier can does not always give a higher yield than a lighter one. In the same way, the foam that gives the larger volume does not always turn out better on the other properties. It may, for instance, have a lower density and therefore poorer thermal insulation.

People who decide to check for themselves whether the foam yield matches the manufacturer's figure often find the volume smaller than expected and hurry to accuse the manufacturer of bad faith. But the reason frequently lies not in “short-filling” the buyer but in the test conditions. Foam yield is quoted for standard conditions, taken to be a temperature of +23°C and a humidity of 50%. The maximum foam yield can only be obtained in a laboratory, following the manufacturer's test procedure in full. In dry weather or in frost, for example, the foam yield can be one and a half or even two times lower. As for comparing the yield of different cans, such a comparison is only valid if the samples are tested under the same conditions, by the same person, from the same gun and, best of all, at the same time.

Initial expansion of PU foam sealant

Initial expansion is the increase in volume of the liquid foam immediately after it leaves the nozzle. The mechanism is as follows. The gases and the prepolymer sit in the can under a pressure of about six atmospheres. The can is shaken before use, the gases mix with the prepolymer and partly dissolve in it. On leaving the can the mixture goes through a sharp drop in pressure, and the gas bubbles compressed inside expand rapidly and form foam. The process is the same as a carbonated drink foaming up when a sealed bottle is opened. That is why it matters to shake the can thoroughly before use: without it the output will not be a good foam with the declared yield.

Naturally, the amount of initial expansion depends heavily on external conditions: air temperature, the way the foam is applied and the skill of the operator.

Post-expansion of PU foam sealant

Post-expansion is the increase in the volume of the foam after initial expansion has finished and before it is fully cured. It is quoted as a percentage. Post-expansion happens as the prepolymer reacts with moisture. This reaction releases carbon dioxide, the structure is formed and the foam cures. The amount of post-expansion depends on the formulation used and, for different manufacturers and different types of foam, can range from 15% to 60% for professional foam and from 200% to 300% for DIY foam. Post-expansion is a very important figure that directly affects the quality of most work done with foam. Before working with a foam that is new to you, it is therefore advisable to run a trial, establish how much it post-expands and allow for this parameter during the job.

Curing pressure of PU foam sealant

As it expands, the foam presses on the structure. How strong that pressure is depends not only on the degree of post-expansion but on other properties of the foam as well. Foams with a high degree of post-expansion do not always press hard on the structure. This can only be established by trial and then, of course, allowed for when working with a particular brand of foam. When switching to another foam, keep in mind that its curing pressure may be higher and that it may distort the structure more.

Cutting time of PU foam sealant

This term means the time after which the foam has hardened enough to be worked mechanically: to trim the excess and to prepare it for painting or filling. Manufacturers give this figure on the can; as a rule it is a few tens of minutes. Keep in mind, though, that the figure is quoted for ideal conditions. In practice it is best to make a test cut before any mechanical work and make sure the foam has hardened enough.

Full cure time of PU foam sealant

Full cure time is the time in which all the chemical reactions in the foam come to an end and the foam takes on its final structure. Cure time depends on several parameters: on the quality of the foam itself, on the width of the joint, on the amount of moisture available and on the temperature. The faster moisture penetrates the foam, the faster and the better the curing goes. That is exactly why it is advisable to mist the surfaces the foam will go onto before application and to mist the filled joint once more afterwards. Excessive wetting should be avoided, though — the surface must be damp, not wet. Temperature works as in any chemical reaction: the warmer it is, the faster the reaction goes. Under standard conditions the cure time of PU foam sealant is about 12 hours, but in frost or in dry weather curing goes much more slowly and can stretch over several days. As for the width of the joint, numerous tests by various manufacturers show that moisture can penetrate curing foam to a depth of no more than 3 cm. Moisture reaches layers lying deeper than 3 cm from the edge with difficulty, so the diameter of a foam bead applied in one pass must not exceed 6 cm. If it is thicker, there is a high risk that the middle of the bead will never cure and a void will form there. A joint like this will have poorer sound and thermal insulation and can break down easily. That is exactly why large openings have to be filled with foam layer by layer. The second layer can be applied no earlier than a skin has formed on the first. And the surface the second layer will go onto must always be misted.

“Shrinkage” of PU foam sealant

During curing the carbon dioxide formed in the foam, which creates excess pressure inside, gradually leaves the cells and is replaced by air. Depending on how fast these processes run, the foam may shrink or expand. International practice regards dimensional changes of ±10% as acceptable for installing PVC windows and doors.

Storage conditions and shelf life of PU foam sealant

Cans of PU foam sealant must always be stored upright, valve up, at a temperature between +5°C and +25°C. Only under these conditions does the manufacturer guarantee that the foam will keep its properties for the whole shelf life stated on the packaging. The temperature limits for storing the foam may differ from the limits at which it can be applied. Winter foam, for example, can be used with the can down to -10°C, but if it is stored in freezing conditions it will become unusable long before the date given on the can. Freezing the foam is permitted, but the cans then have to be thawed correctly if the foam is to keep its working properties. They must be thawed slowly, without sudden heating.

Application conditions for PU foam sealant

Application conditions differ between types of PU foam sealant and are usually given on the can. For summer foams the air temperature is normally between +5°C and +35°C; the best winter foams, KUDO^® ARKTIKA NORD for example, can be used at air temperatures down to -25°C.

The outside air temperature at which PU foam sealant may be applied has to be told apart from the temperature of the can itself. The winter foam KUDO^® ARKTIKA, for example, can be used at temperatures from -18°C to +35°C, while the temperature of the can must not fall below -10°C. This is considered a very good figure, because KUDO^® foams use the AFC (Advanced Freeze Control) technology, which allows work with a chilled can. For foams without technologies of this kind the permitted can temperature is usually above 0°C. If the can has cooled below the critical temperature, it has to be warmed by putting it in warm water for a while. Never heat a can with an open flame or a heat gun — overheating can make the can burst. One more important point: the difference between the temperature of the foam and the outside air temperature must not be too large, otherwise the foam may simply run in the opening after application. A dedicated chart can be used to choose the optimum temperature for KUDO^® foam.

Sufficient humidity is a no less important condition for correct application of PU foam sealant; as a rule it must be at least 50%. The foam cures by reacting with moisture, so for a sound joint it is advisable always to mist the surface the foam will go onto before starting work and to mist the filled joint again after application. If the foam is applied in several layers, every layer has to be misted.

Fire-rated PU foam sealant

Fire-rated PU foam sealant is used where fire safety requirements are higher than usual. As a rule fire-rated foam is pink or red, occasionally gray. This makes it easy to check which foam was used in a structure — fire-rated or standard.

Fire resistance and flammability must be told apart. Flammability means the ability of a material to support combustion, while fire resistance means the ability of a material to keep its integrity (E) and its insulation performance (I). Fire resistance ratings are tested on joints 100 and 200 mm deep and 10 to 40 mm wide. What is measured is the time in minutes for which the material was able to keep its integrity and insulation performance under an open flame.

Fire resistance ratings of KUDO^® PU foam sealant

When studying the fire resistance figures of different brands of foam, keep in mind that tests can be run on different types of joint: a uniform joint of foam alone and a combined joint of foam and basalt wool. If the test was run on a combined joint, this is always stated in the specifications. Joints like these almost always have higher fire resistance figures, but that does not mean that the foam in them has a higher fire resistance of its own. Only figures for joints of the same type can be compared correctly.

Rules for working with PU foam sealant

Since PU foam sealant sticks to hands very well and comes off them very badly, protective gloves should always be worn when working with it.

The can must always be shaken before use so that the components inside mix thoroughly. Without this, the output will not be a good foam.

Since the foam cures in the presence of moisture, the surface being worked on has to be misted before the foam is applied. At sub-zero temperatures the moisture can freeze on the surface. Small areas of the surface should therefore be misted and foamed straight away, giving the moisture no time to freeze.

Vertical joints are best foamed from the bottom up — it is easier and more convenient that way.

When applying the foam, always allow for its post-expansion and try to apply it so that no trimming is needed once it has cured. The reason is that a fairly dense skin forms on the surface of the foam and reduces how readily the foam takes up moisture. Cut that skin away and the foam will absorb more moisture.

After the foam is applied, the joint should be misted once more for faster and better curing.

PU foam sealant is degraded by ultraviolet light, so once cured the joint must always be protected with plaster or in some other way.