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.