Robust, energy-efficient windows with polyurethane composites

The efficient thermal insulation of buildings not only reduces energy consumption, it also spares the wallets of the owners and tenants. Furthermore, it makes a real contribution towards conserving resources and protecting the climate. Windows and facades, in particular, are a common source of heat loss. To reduce this heat loss, it is important not only to use multi-pane glazing but also to ensure that the window frames are well insulated.
Large windows that flood the room with light are highly fashionable at the moment, particularly in public buildings. The frames are therefore becoming smaller, yet they must still offer correspondingly high rigidity and dimensional stability. To comply with all these requirements, Bayer MaterialScience has developed an efficient, robust and durable material solution based on fiber composites with a tailored polyurethane matrix.

Bayer_Foto_2013-0545“These composites are noted for their very good thermal insulation,” explains Dr. Andreas Hoffmann, head of the project in the Europe, Middle East and Africa region. It has a low thermal conductivity similar to that of wood and polyvinyl chloride (PVC). “In favorable cases, the window frames satisfy the requirements of energy-saving regulations and passive house standards,” says Hoffmann.

The material’s outstanding mechanical properties are attributable primarily to the high glass fiber content of around 80 percent by weight. This makes it possible to manufacture profiles with a very narrow visible height and low installation depth, for example for the glazing for roofs or elements where space is limited, like sliding doors for balconies and terraces. A heat-insulating lift-and-slide door of Internorm International GmbH is a current example. Thanks to a ground sill with a height of only four centimeters and a small framework construction the door has a huge glass facade and floods the room with light.

Alternatively, large profiles can also be produced for particularly large glass facade windows. The good mechanical properties also help to reduce overall weight. This facilitates assembly and later opening of the window elements.

The composite materials are protected from the weather by a suitable coating or by the application of a laminating film. This eliminates any risk of rotting as in the case of a wooden frame or of embrittlement as with PVC. Thanks to the material’s good dimensional stability, only minor stresses occur at the joint between the glass and the window frame when subjected to marked changes of temperature. The joint remains permanently air-tight.

The fiber-reinforced profiles are produced by a continuous process known as pultrusion. This involves pulling rovings of e.g. glass or carbon fibers from rolls through a closed injection box, where they are impregnated by the liquid polyurethane matrix. Under the influence of heat, the fiber composite is formed and fully cured in the die – in this case, a window profile. It is then cut to size by a saw.

Finally, the parts – possibly after further machining by grinding, milling or drilling – are integrated into the window or facade elements. Working with polyurethane systems generally enables higher production speeds than with alternative matrix materials such as polyester, vinyl esters or epoxy resins.

The possibilities for polyurethane pultrudates do not end with their use in windows and facades, however. Highly promising applications also exist in the transport sector, where, compared with metal, they exhibit very good mechanical properties and lower weight. Other examples include infrastructure projects such as bridge construction and utility poles. Because of their high breaking resistance and wear resistance, U-shaped sections made from pultrudate are used for the construction of industrial facilities.