Wood Protection with Beading Effect - Wacker Chemie AG


Wood Protection with Beading Effect

Outside, the best way to protect wood against moisture is to treat it with a water-repellent impregnating agent or a coating. However, conventional protective coatings usually need to be renewed every few years. In SILRES® Wood Hydrophobization, WACKER has now developed a breathable silicone resin emulsion that protects wood against moisture for significantly longer than conventional treatments.

16 percent of all single-family and duplex houses in Germany are constructed from wood. Now, this environmentally friendly material is even being used to build apartment houses and skyscrapers.

Wood is one of the oldest building materials known to humans – and is currently enjoying a renaissance. Take, for example, the Cedar House project in Stockholm, Sweden. With its 13 stories rising 43 meters into the sky, it will be the tallest building in the world to be made entirely from wood. Its facade of Canadian cedar makes a striking impression in Hagastaden's new high-tech quarter, yet blends seamlessly with the surrounding concrete and glass edifices.

Rediscovering a Construction Material

Theoretically, wood is the perfect building material: lighter than steel, but with the same load-bearing capacity, it is easy to process, has myriad uses and lends itself to industrial pre-fabrication. What is more, wood releases far fewer CO2 emissions than any other building material. Timber designs now account for some 16 percent of all detached and duplex houses in Germany, and they are also becoming increasingly popular for apartment complexes.

“There is nothing exotic about wooden constructions anymore – everything’s high-tech these days,” says German architect Tom Kaden, who has erected a seven-story apartment block with a timber frame and wooden walls in the trendy district of Prenzlauer Berg in the heart of Berlin.

However, the renewable raw material has one arch-enemy: water. “Not only do rain and moisture spoil its appearance, they also promote blue stain fungus, mold and insect attack – which can lead to irreparable damage,” explains Albert Hausberger, jointly responsible for the development of building-protection agents in WACKER SILICONES’ Applied Technology department.

Therefore anybody who uses wood outside should protect it with a suitable treatment. Chemical wood preservatives can be roughly divided into two classes: impregnating agents and film-forming coatings. Impregnating agents contain water-repellent components, such as waxes, resins and oils, as well as, occasionally, fungicides and insecticides. They penetrate into the wood, and due to their impregnating effect, protect it, as it were, from the inside out. Film-forming coatings work differently – they dry to form an intact water-repellent protective film. As well as binders and preservatives, these types of systems also frequently contain biocides, which protect against insect attack and mold. In addition, UV-absorbing pigments ensure that the wood does not turn gray in direct sunlight. Film-forming coatings typically include wood stains, paints and varnishes.

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.

The spruce boards shown here were coated with a water-based impregnation (top image) or coated with a waterborne stain (bottom image) and were then exposed to UV-B light for 1,000 hours. SILRES® WH was added on the right-hand board in each case. The result is a markedly hydrophobic effect.

Wood Protection Possible without Biocides

Wood-impregnating agents are especially designed to protect wood against attack by rot, fungi and insects. The risk of such damage can be mitigated by reducing the moisture content in the wood. Moreover, water-repellent agents go a long way to preserving the appearance of the wood, even though by their nature they cannot prevent UV radiation from turning it gray. The pronounced beading of water-repellent agents acts as a clear sign of effective protection.

Weatherability is the most important criterion by which the quality of water repellents for wood can be judged. Many impregnating agents, such as waxes, exhibit impressive water repellency prior to weathering. But things look very different after one or two years’ open-air weathering or 1,000 to 2,000 hours’ accelerated weathering in the laboratory. By then, organic waxes have been completely degraded, conventional silicone resins have become “chapped” and linear silicones, i.e. silicone fluids, have become extensively soiled. Not so with SILRES® WH, which bestows long-lasting water repellency – as preliminary laboratory tests show. These consisted of immersing a spruce board for three minutes in an impregnating solution composed of one part SILRES® WH and four parts water. The test sample and an untreated board were then artificially weathered in a QUV tester for 2,000 hours. Both boards turn gray due to degradation of the lignin – not even water-repellent silicones can prevent that. However, unlike the untreated board, which sucks up the water like a sponge, the impregnated test sample is still highly water-repellent – the water rolls off the surface in beads.

The water repellency of low-binder wood stains can be impressively enhanced by admixing SILRES® WH.

Martin Sebald Technical Marketing, WACKER SILICONES

Effective Even in Low Concentrations

But there are ways to prevent such graying, e.g. by adding UV absorbers or by using pigments that keep the UV light at bay. Tests show that wood treated with a pigmented coating which has been modified with SILRES® WH enjoys very good protection against the consequences of direct sunlight.

Capillary water absorption is usually determined in the floating test. Adapted from ASTM D 5401-03, this consists in placing each side of an untreated sample and of impregnated wooden boards, which have been exposed to UV light for different lengths of time, in water for 15 minutes at a time and then weighing them. The weight gain indicates the amount of water absorbed by the substrate.

As the floating test shows, solar UV radiation promotes the absorption of water by untreated boards, which can cause irreparable damage. This can be largely prevented with SILRES® WH. The silicone resin emulsion is so effective that even highly dilute solutions offer adequate protection.

“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.