“Again, this test demonstrates the durability of SILRES® WH,” says WACKER applications engineer Albert Hausberger. Even after 4,000 hours of irradiation with UV-B light – the equivalent of several years’ open-air exposure – the silicone resin emulsion still affords outstanding protection. For a 1:4 dilution, water absorption is at most 15 percent. 1:9 dilutions perform only marginally worse. By way of comparison, the untreated wood samples that were artificially weathered for 4,000 hours had a water-absorption rate of 50 percent, or half their own weight (see graph on right).
Wood treated with SILRES® WH can, in principle, also be painted if some weathering of the impregnated substrate is allowed first. However, one thing needs to be remembered: smooth wooden surfaces will absorb only some of the silicone – the rest remains on the surface. As a result, and especially in the case of highly concentrated wood preservatives, paint adhesion may be restricted and leveling impaired. For this reason, SILRES® WH should not be employed as a primer.
Wood stains to some degree mark the transition from colorless impregnating agents to film-forming wood varnishes. To an extent depending on the pigment and binder content, they vary from being translucent to transparent. The most commonly employed binders are water-soluble acrylic and alkyd resins. The solids content determines whether the wood stains are low-build or high-build. Low-build types have a solids content of up to 30 percent, whereas, for their high-solids counterparts, it’s up to 40 percent.
Wood stains contain much more binder than conventional exterior paints. Accordingly, they produce a water-repellent film when they dry. A good wood stain, i.e. one with a high binder content, is characterized by low water absorption. Since the wood cannot swell or shrink as much, stains also boost dimensional stability. Unfortunately, their weatherability is often unsatisfactory. They show signs of cracking or even start to flake off after just one year’s outdoor weathering. When that happens, the wood absorbs a great deal more water.
“This provides a great opportunity for the silicone additive,” explains Sebastian Hock from WACKER. “Even a small admixture is enough to influence the film properties and ensure that the paint or stain is permanently water repellent.”
These tests also reveal something else: the lower the binder content, the more pronounced is the hydrophobic effect of the silicone additive. “The water repellency of low-binder wood stains can be impressively enhanced by admixing SILRES® WH,” stresses Martin Sebald from the WACKER Applications Technology department. “Often, an addition of just one percent in the formulation is enough.” Good water repellency, though, is always accompanied by improved durability and appearance of the wood, he adds.