All home improvement enthusiasts have used silicone at some time in their lives. Whenever joints have to be sealed in the bathroom or the kitchen, this viscous mass is a necessity; the paste is easy to apply and cures quickly at room temperature. But what about painting over the silicone seams or using the product to lay a new parquet floor in the living room? Preferably not. On such occasions, the professional will opt for polyurethane, which has better paint adhesion as a sealant and much better mechanical properties as an adhesive. Researchers from Bayer Polymers have now developed a range of adhesives and sealants that combine the advantages of polyurethanes with those of silicones, thereby minimizing their disadvantages at the same time: silane-terminated polyurethanes (STP).
Normally, isocyanates (highly reactive chemical groups containing nitrogen, carbon and oxygen atoms) are positioned at each end of the polyurethane molecule. They react with the water in moist air, resulting in pairs of thread ends that link up and release carbon dioxide (CO2).
Bayer owes the fact that it can make the polyethers in its new adhesives so long - molecular weights of over 10,000 g/mol are possible - to its "Impact" technology. "This makes it possible to produce defined chain lengths with exactly the desired number of end-groups," says Simon. "And that means we are close to large-scale realization of the ideal molecule." STP binders can now be made as soft or as hard as required. "The longer the chains, the softer the binder," Simon explains.
They will probably only really be able to play out their strengths to the full under extreme conditions, says Dr. Mathias Matner, who looks after applications development in the adhesives and sealants team. He is thinking here of the "facades of high-rise buildings, which are subjected to extreme temperature fluctuations." At high temperatures, the facade panels of Eternit or plastic expand; when it is cold, they shrink. "The sealing joints must be able to balance out these movements, which may be as much as a few centimeters," he says. But even in the traditional domains of polyurethane adhesives (for example, when laying the wooden planks on the deck of a sailing boat), STPs could be especially useful. The first products are already on the market under the names Desmoseal® and Baycoll®.
For more information on the basic principles of bonding in simple form, visit http://www.Bayer-ls.com/ls/lswebcms.nsf/id/arch_e .