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Polyurethane Adhesives at Home in Wood Constructions

The use of wood in the construction industry is currently experiencing a renaissance. New joining methods and construction principles, not to mention the discovery of the classic material for modern architectural solutions, have opened up entirely new possibilities for building with wood. No longer relegated to home construction, wood is now used for office and administrative buildings, schools, multi-family dwellings, bridges, theaters and even transmission towers. This trend is being fostered by the evolution of the lumber industry from a pure handcraft to the industrial production of semi-finished goods that can be joined quickly and precisely on site to form larger elements, making the construction of large and architecturally interesting structures fast and economical.

The current success of load-bearing wood construction would be unthinkable without the use of high-performance adhesives, however. Modern one-component polyurethane systems offer a number of decisive advantages for this application. They are applied directly from the drum; there is no need to mix components prior to applying the adhesive, nor are there any pot lives to be considered. Odorless and free of formaldehyde and solvents, the adhesives are also highly environmentally compatible. The production process is very economical, requiring less adhesive than other adhesive systems and allowing parts to be stably and permanently joined after a short press time. Purbond AG of Sempach Station, Switzerland, is the market and technology leader for one-component polyurethane (PU) adhesives for the manufacture of load-bearing wood elements. Bayer MaterialScience is the leading supplier of the isocyanate prepolymers used as raw materials for this application. Before they are introduced to the market, adhesives for engineered wood must be tested and certified. In Germany, the Materials Testing Institute (MPA) of the University of Stuttgart is responsible for testing and assessment. The MPA uses test methods derived from previous work with phenol-resorcinol and melamine-urea resins to determine such things as the adhesive strength, application properties and long-term durability of adhesives. The first one-component polyurethane adhesive certified in Germany for the manufacture of load-bearing wood components received general regulatory approval in 1994. In the eyes of the MPA, Purbond and Bayer MaterialScience, the methods underpinning current standards do not give sufficient consideration to the outstanding properties of polyurethane adhesives.

To obtain even more reliable test results and ensure the continued market success of polyurethane systems, the MPA and Bayer MaterialScience plan to develop test methods with the support of Purbond to improve predictions of long-term behavior. This work will incorporate the latest scientific findings, with the results of the fundamental and application-oriented projects to be used to improve existing standards.

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