Technology: Wacker, University of Munich advance precious metal-free silicone curing

Munich, Germany – A research team from the Technical University of Munich (TUM) and the Munich-based Wacker Group has developed a curing process for silicones that does not require precious metals.

Developed at Wacker's Institute of Silicon Chemistry in Garching, near Munich, the process uses silicone building blocks with silirane units to cure silicone rubber without the need to use precious-metal catalysts, TUM said in a release 1 Dec.

According to TUM, siliranes are three-membered rings with a high degree of ring strain, which makes them reactive. 

The rings comprise one silicon atom and two carbon atoms, and can either react directly, upon ring-opening, with suitable functional groups – without releasing any byproducts – or generate what are known as silylenes through thermal or photochemical activation.

The highly reactive compounds can, in turn, react with any functional groups or synthetic building blocks typically used in the production of silicone rubber. 

Depending on the type of activation process and the choice of starting compounds, TUM said the method opens up "a variety of pathways" for curing silicones with silirane-containing crosslinkers.

Silicones are synthetic polymers consisting of an inorganic silicon-oxygen backbone modified with organic side groups 

Before use, the material must be converted to a rubber-elastic state through chemical crosslinking. 

A key methods used in the industry is addition-curing, since this crosslinking process does not release any volatile byproducts and results in particularly high-quality silicone elastomers.

However, the catalysts required for crosslinking contain precious metals such as platinum, which make the manufacturing process relatively expensive. These metals also remain in the silicone permanently.

According to TUM, the silicone elastomers produced through the new crosslinking process have “exceptional purity”, as they contain neither volatile substances nor traces of precious metals. 

“That’s especially true for elastomers, which are crosslinked via a ring-opening reaction. Silicones like these are particularly suitable for medical applications or as encapsulants for the electronics industry,” said Bernhard Rieger, who holds the Wacker chair of macromolecular chemistry and led the research team.

The process, according to TUM, needs further fine-tuning to “overcome a few more hurdles” to demonstrate practical viability in industrial use.

“But we can already clearly see the advantages of the method”, Rieger notes. “Given the global growth in demand for precious metals, which are only available in limited quantities, alternative concepts like the use of siliranes to crosslink silicone without precious metals could play an important role in protecting important resources.”