Wood Protection with Beading Effect - Wacker Chemie AG

Wood Protection with Beading Effect

This outdoor weathering wall at the Burghausen plant was subjected to wind and the elements for over one and a half decades. After seventeen years’ exposure to the elements, water still beads up on the boards that had been impregnated with a silicone resin emulsion (fourth board from left). By contrast, the surface of the untreated board (left) is now so porous, that water penetrates it immediately. The treated board also has far fewer cracks than the untreated board.

Effective Protection Retained

After application and drying, wood preservatives afford a good level of protection. However, this effect wears off significantly as the years pass. Impregnations leach out, wood stains bleach and paint peels off. This means that wood impregnations and coatings must be repeatedly re-applied. “To stop the new coating immediately flaking off, and to avoid unsightly edges or flatness imperfections, previously treated wood usually needs to be sanded, dusted and cleaned before the new coating is applied – this is a time-consuming and expensive process,” says Sebastian Hock, marketing manager for silicone building-protection agents at WACKER.

Manufacturers of wood preservatives have long sought better and longer-lasting ways of preserving this natural material. As well as waxes, oils and resins, organosilicon compounds are also used in wood preservatives. These compounds are naturally water repellent and exhibit a high degree of water-vapor permeability. This explains why silicone resins have proved so successful in building protection over the decades. By virtue of its SILRES® product portfolio, WACKER is Europe’s largest producer of silicone-based building protection agents.

Unlike silicone fluids and silicone rubber, which chiefly consist of linear polydimethylsiloxane chains, silicone resins are fully crosslinked polymethylsiloxanes. Their high degree of curing usually renders these materials solid and brittle. They adhere very well to mineral building materials, forming a long-lasting water-repellent silicone-resin network. Therefore on mineral building materials, silicone resins make the perfect hydrophobic active substances . On wood, however, they often become far less effective after just a few months’ weathering. The reason is that, although the silicone drastically reduces the wood's moisture content, it never totally eradicates it. Consequently, wood that is impregnated with silicone resin emulsions continues to swell and shrink, and so the silicone resin is gradually “forced off.”

However, this problem can be resolved by replacing some of the crosslinking trifunctional methylsiloxane units with linear dimethylsiloxane units. The outcome is a somewhat more flexible, more elastic silicone resin, which adheres well to substrates that have a tendency to swell – it is therefore also suitable for wood.

“Following intensive research, our applications engineers have now succeeded in synthesizing a functional silicone resin that is ideal for formulating wood preservatives,” says Dr. Rudolf Hager, head of the Construction Chemicals business team at WACKER SILICONES. In line with the current trend of using waterborne products in wood protection – as in all other areas of building protection – the product is designed to be a waterborne, 50 percent emulsion. SILRES® WH, as it is called, is usually diluted 1:4 to 1:9 with water for direct application to wood. The product can also be admixed in undiluted form to waterborne stains in amounts of up to three percent.

“Over the decades, silicones have proved hugely successful at protecting and restoring mineral building materials, and now water-repellent silicone resins are able to preserve wood too,” explains Martin Sebald, the applications engineer, who, along with Albert Hausberger, was responsible for developing SILRES® WH. Following treatment with the silicone resin emulsion – whether in the form of a colorless water-repellent agent or incorporated as an additive into colored stains – wood outdoors would not need to be painted and renovated as often.