SILRES® Wood Hydrophobizing Agent for Long-Lasting Protection

The best way to protect wood from moisture outdoors is to apply a water-repellent impregnation. However, the effectiveness of such products decreases over time. Protective coatings need to be renewed after a few years.

Wacker has developed a resin emulsion that provides significantly longer-lasting moisture protection compared to conventional wood paints. In addition to their water-repellent and breathable properties, the new silicone resins, which are sufficiently flexible, enable wood to withstand swelling and shrinkage without problems. Wood is frequently used as an outdoor building material: it is durable and robust, easy to work with, and versatile. Another important property is its ecological sustainability as a renewable raw material. However, wood has a major enemy: Water. Rain and moisture not only damage the appearance of wood. They can also lead to infestation by fungi, mold, and biological pests, resulting in irreparable damage to the wood. For this reason, when wood is used outdoors, it must be protected with an appropriate coating. Chemical wood preservatives are broadly divided into two classes: impregnation agents, which contain water-repellent components such as resins or oils, as well as fungicides and insecticides. These penetrate the wood and, through their impregnating effects, provide protection from the inside out, so to speak. In contrast, film-forming coatings such as wood varnishes, paints, or polishes form a closed water-repellent protective film when they dry. These types of systems also contain biocides in their composition that, in addition to binders and protective agents, often provide protection against insect infestation and mold. Additionally, UV light-absorbing pigments prevent graying that occurs in wood when exposed to direct sunlight.

New Solution in Wood Protection: SILRES^® WH

Wood preservatives provide good protection after application and drying. However, this protection decreases significantly over the years. For this reason, impregnation and painting of wood must be repeatedly applied. To prevent new paint from peeling and to avoid unsightly edges and rough surfaces during painting, coated wood typically needs to be sanded, dusted, and cleaned before repainting. This is a time-consuming and expensive process. For this reason, wood preservative manufacturers have long been researching ways to achieve better and longer-lasting protection of natural wood. In addition to waxes, oils, and resins, organosilicon compounds are also used in wood preservatives. Such compounds are naturally water-repellent and highly vapor-permeable. Because of this, silicone resins have proven themselves for years in the field of building protection. With its SILRES® product portfolio, Wacker is the largest producer of silicone-containing building protection products across Europe. Silicone resins are highly crosslinked polymethylsiloxanes and are generally hard and brittle due to their high degree of crosslinking. They adhere very well to mineral-based building materials, creating a permanent water-repellent silicone resin network on these surfaces. For this reason, silicone resins are ideal hydrophobic active agents on mineral-based building materials. However, this problem can be solved by using linear di-methylsiloxanes instead of some of the crosslinked trifunctional methylsiloxane units. The silicone resin created in this way is somewhat more flexible and elastic and can adhere well to expanding substrates, making it suitable for wood. Through intensive research, Wacker synthesized the optimal silicone: SILRES^® WH. This is a functional silicone resin specially developed for wood preservative formulations. As in other areas of building protection, aqueous products are in demand for wood protection, so it is designed as a 50% water-based emulsion. Silres WH can be applied to wood diluted with water in a 1:4 to 1:9 ratio, or it can be added to water-based wood varnish at up to 3%.

Fewer Fungicides, Better Weathering Properties

Wood impregnation agents must protect wood especially against decay, fungi, and biological pest infestation. The risk of such damage can be reduced by lowering the moisture content of the wood. In other words, by using components with permanent water repellency in impregnation agents, the use of biocides, particularly fungicides, can be partially reduced. This is important not only from an ecological and toxicological perspective for manufacturers and users, but also from an economic one. The most important criterion in evaluating wood hydrophobicizing agents is their weathering properties. Many impregnation agents such as waxes provide impressive hydrophobicity before weathering. After exposure to outdoor weather conditions for one to two years or to rapid abrasion in a laboratory environment for 1,000 to 2,000 hours, a different picture emerges: During this period, organic waxes break down completely, conventional silicone resins "fragment and disintegrate," and linear silicones, also known as silicone oils, stand out due to excessive soiling. In contrast, SILRES^® WH has permanent water-repellent effect. A spruce wood sample was immersed for three minutes in an impregnation solution consisting of one part SILRES^® WH and four parts water in a laboratory test. The test specimen was then subjected to artificial weathering in a Q-UV test chamber for 2,000 hours alongside an untreated wood panel. As shown in Figure 1, both wood panels showed graying due to lignin degradation. However, unlike the untreated wood that absorbs water like a sponge, the impregnated test specimen retains strong water repellency: water runs off the surface.

Long-Term Tests

There are differing views on the relationship between artificial UV weathering in laboratory conditions and weathering under open-air conditions. For this reason, Wacker conducted various outdoor tests at its facilities in Burghausen several years ago. The results are impressive: wood panels impregnated with silicone resin retain their water impermeability even after seventeen years of weathering in the open air. Figure 2 clearly shows that cracking in hydrophobicized wood is significantly less than in untreated wood. Wood exposed to unprotected sunlight shows graying. Hydrophobicized silicones cannot prevent this. However, it is possible to prevent such graying in various ways, such as by adding UV absorbers or using colored pigments that deflect UV light. Tests show that wood panels coated with a pigmented coating modified with SILRES^® WH are very well protected against the effects of direct sunlight. Wood treated with SILRES^® WH can in principle be repainted once the paint on its surface weathers. However, there is a point to consider. On smooth surfaces, only part of the silicone penetrates into the wood. The rest remains on the surface. In this way, especially when wood preservatives are used at high concentrations, the adhesion and spreading of paint may be insufficient. For this reason, SILRES® WH should not be used as a primer. Figure 2. Weathering test: These wood panels were exposed to weathering in outdoor conditions for 17 years. The surface of the untreated board (left) became so open-pored during this time that water penetrated immediately. On the adjacent wood, water ran off the surface without adhering. Wood treated with SILRES^® WH continues to be protected by the water-repellent effect of the silicone resin.

Additives Used in Wood Varnishes

Wood varnishes, to some extent, represent the transition from colorless impregnation agents to film-forming wood polishes. Depending on their pigment and binder content, they are more or less transparent or semi-transparent. The most preferred binders are water-soluble acrylic and alkyd resins. Depending on their solid content varying from 30 to 40 percent, they are divided into thin-film-forming and thick-film-forming wood varnishes. Wood varnishes essentially contain significantly higher amounts of binder compared to classical facade paints. In this way, they form a water-repellent film when they dry. A good wood varnish with a rich binder content has low water absorption. Since wood swells and shrinks to a lesser extent, these varnishes also provide higher dimensional stability. However, their weathering resistance is not satisfactory. Even after one year of weathering in the open air, cracks may appear or the varnish may begin to peel. In such cases, water absorption by the wood increases significantly. This is precisely where the advantage of silicone additive comes into play. Even at low concentrations, it affects film properties, enabling the paint or varnish to exhibit permanent water-repellent effect. Figure 3 gives us an idea of how much the durability of a qualitatively high-value wood varnish can be improved with the addition of SILRES^® WH. The spruce wood in the photograph was coated with an aqueous varnish. An alkyd resin dispersion was used as the binder. The SILRES^® WH dosage is 3%. After 1,000 hours of UV-B weathering, distinct water-repellent effect is visible on the wood treated with SILRES^® WH on the right. The tests carried out also show the following: as binder content decreases, the hydrophobicizing effect of the silicone additive becomes more pronounced. The water-repellent properties of wood varnishes with low binder content are dramatically improved with the addition of SILRES^® WH. Generally, an addition of 1% to the formulation is sufficient. However, good hydrophobicity is not always accompanied by an improvement in the durability and appearance of the wood.

Conclusion

SILRES^® WH provides a silicone resin emulsion that is flexible and elastic enough to adhere tightly to wood and thus provide protection against water and moisture for many years. In this way, outdoor wood will require repainting and repair less frequently. This makes it possible to avoid losses and expensive repairs. Wood impregnated with SILRES^® WH remains dry for many years and is therefore not subject to infestation by biological pests or fungi. In this way, the use of biocides and other environmentally harmful components in wood preservatives can be reduced. SILRES^® WH provides permanent hydrophobicity both as a colorless hydrophobicizing agent and as an additive in wood varnishes. Water balance is kept at a very low level, even after years of weathering in the open air. Another point that is at least as important as the above is that every user is assured, thanks to the generally recognized superior water-repellent effect of silicones, that the best is being done for valuable natural wood protection. Burcu Atmaca Gölbaşı / Sales Manager - Wacker Silicones