Thermal insulation: filled energy saving bar

They are even better hidden than screws, of which you can at least still see the heads, but they are just as ubiquitous: insulating webs are stuck inside aluminum and metal profiles for windows and facade glazing - as thermal separators between the outside and inside, because otherwise cold or heat would flow unchecked through the metal. An inconspicuous component, therefore, that is becoming even more important with global warming and the need to reduce CO2 emissions.

But although it has been established for over four decades, the insulating web has potential for improvement. An Empa team led by Michel Barbezat and Giovanni Terrasi from the Mechanical Systems Engineering department is working on a new type of product - together with experts from the metal construction company Hochuli in Wigoltingen, which has founded its own sister company "hochuli advanced" for this purpose.

The highlight of the "Alpet" insulating web: Inside the glass-fiber-reinforced plastic is a foam strip made of polyethylene terephthalate - PET, in other words, from recycled bottles. The many air pores in this layer insulate efficiently: The thermal conductivity of the prototypes is on average, depending on the web width, around 0.1 W/mK - far less than a standard insulating web made of the plastic polyamide (around 0.25 W/mK) and also significantly lower than high-end products available today.

Fine tuning of many details

A simple approach, as it seems at first glance - but in order to turn the idea into a product, it was necessary within the framework of a Innosuisse-project required a lot of concept work. For example, the production method: After testing various processes, the experts decided on extrusion, in which heated, molten plastic is pressed through a slot as a soft dough and shaped in this way - around the strip of PET. But this in turn raised questions, for example about the proportion of air voids in this "filling.

"As many as possible" is the answer in order to achieve a high insulating effect. But "too many" would have jeopardized the stability of the prepared PET strand, because its sheathing with the black plastic takes place at temperatures of up to 300 degrees and high pressure. The researchers also adapted the thickness of this "shell" to the necessities: as narrow as possible for a good insulating effect - but thick enough so that the web later also withstands the mechanical loads well; including the post-treatment of the finished overall profile, such as powder coating or anodizing.

From numerous samples, the developers distilled seven variants for testing - a total of around 1,000 linear meters of insulating web - and from this finally the final prototype as the basis for finished products. An important step in the production process was also achieved along the way: The web welds itself virtually by itself due to the heat - without the need for another "seam" to be glued. "This is already a big advantage," says metal construction engineer Frank Hochuli from the industrial partner. "There are no local weak points where material could come off. And the fewer work steps, the cheaper the product."

A lengthy process that was worth it in the end, as Empa researcher Barbezat also finds. "Technically, we certainly have very good chances," he says. The measured values and the product, which can be easily recycled despite the use of two materials, are already convincing. And the experts are also confident about the necessary long-term stability over many years, which his team tried to estimate with experiments and the innovative "stepped isostress method".

Independent audits in Germany

Of course, Barbezat sees a challenge in convincing a large system house that manufactures and sells profiles for window manufacturers to cooperate with him. "After all, compared to large companies, we are 'nobodies,'" he says. So in order to win customers, you also need an independent blessing from above - and so the partners sent their "Alpet" insulation bar to the ift testing institute in Rosenheim, Bavaria, which has been a reference in the industry for decades.

The experts there not only repeated Swiss tests, but also subjected the prototypes to fire tests, fracture tests and other stresses - for example, to invisible microcracks after 1000 hours of storage in oil or light acid, or to strong tension in the transverse direction. According to Frank Hochuli, official certifications for fire behavior and static load capacity have now been issued; only the certificate for thermal insulation is still outstanding - due to minor differences in individual measured values, which, of course, would not change the positive results, according to the engineer.

More environmentally friendly with little effort

What effect would the new insulating bar have on the overall "window" system, including glass panes, aluminum profiles, seals and all other details? Compared with today's high-end designs, Hochuli estimates that the thermal insulation in a new office building, for example, could be improved by up to a fifth. And because the web with its "dovetail" as a mounting connection is compatible with all common systems, existing solutions can be upgraded relatively easily - for example, for high requirements according to passive house standards. "With aluminum profiles, you can only do that with great difficulty these days," he says, "and our system would certainly make it easier.

Detailed tests would then also be carried out by the manufacturers themselves, in their own laboratories with their own profile systems. Talks on this are already taking place, according to the developer: "I'm definitely optimistic!" he says. "Otherwise, I would hardly have founded a company to bring our idea to market." Nevertheless, the further path will not be a solo effort: the Empa experts will continue to support the insulating web with their know-how after years of support.

The Stepped Isostress Method

How does a plastic component deform over a period of 30 or more years - under continuous load and large temperature differences? Common "creep tests" in the laboratory over 1000 hours under combined thermal-mechanical loads do not allow any serious statements to be made. In order to be able to make long-term predictions, however, new methods have been developed and refined in recent years, including the "Stepped Isostress Method" (SSM), which Empa experts applied to the new insulating web. In simple terms, the deformations from numerous individual tests with increasing loads are added up to form a "master curve" that provides information about larger periods of time. This procedure can also be carried out with a single measurement run with gradually increasing loads - and makes it easier to investigate critical temperature ranges at the same time.