New Silicone Resin Raises Technology Standard for Exterior Facade Paints and Plasters

For years, exterior facade paints have had two important functions: they enhance the appearance of the facade and protect it against adverse weather conditions. For this reason, top-tier exterior facade paint manufacturers have optimized these two properties considerably. Silicone resin paints are currently the state-of-the-art technology for exterior facades. These paints contain emulsified silicone resin in addition to the main binder, which is usually styrene acrylic dispersion. This makes it possible to produce paints with excellent resistance to adverse weather conditions and a long service life. However, customer expectations have increased in recent years. There is a need for more color tone options, slower soiling of the facade compared to the past, and all of this should be achieved in an environmentally friendly manner as possible. Researchers at Evonik have developed a new silicone resin with TEGO® Phobe 1659 that meets these rising demands. Moreover, this resin can be used at much lower concentrations compared to commonly used alternatives in formulations, thus providing cost savings.

High Water Vapor Diffusion, Low Water Absorption

Exterior facade paints must provide good water vapor diffusion so that moisture from the inside does not accumulate within the wall. Under certain conditions, this moisture can create pressure that may cause non-vapor-permeable coatings to burst. Due to the low binder content, not all gaps between filler and pigment particles can be filled, thus creating a micropore network. Therefore, high water vapor diffusion is achieved at a critically high PVC (pigment volume concentration). However, these pores present surfaces that can be affected by water penetrating, for example, due to rain. Hydrophilic pores absorb water intensively; the intensity can be seen when a vertical cylindrical capillary pore example is taken. The height at which water rises (h) is described by the capillary equation as follows: Here σ is the surface tension of water and ρ is its density, θ is the contact angle of water on the capillary surface, g is the gravitational constant, and r is the radius of the pore. For a hydrophilic pore with a contact angle of 0°C and a radius of 100 nm, the height at which water rises is 157 meters, which is even higher than a water column from Cologne Cathedral!

This effect must be minimized, which can be done most effectively through the surface chemistry of the capillary. When the contact angle θ > 90°C, water stops rising and begins to fall: under the same conditions mentioned above, with a contact angle of 140°C, water falls by 100 meters.

Therefore, for water vapor diffusion, the pore surfaces must be made hydrophobic without significantly interfering with the microporous structure. In practice, this is achieved by adding silicone resin emulsions. Researchers at Evonik were able to measure the hydrophobicizing effect of the new silicone resin in experiments. They applied a paint containing the silicone resin completely onto mineral-based surfaces and kept these surfaces in water for 24 hours. During this period, the amount of water absorbed was found to be much lower compared to paint formulations containing similar products commonly used in the market. The W24 water absorption coefficient remained below 0.1 kg/(m²*h½) and was 60–80% lower. Even with a concentration of only 2.0%, this new silicone resin achieved these very good values, which had not been achieved before. (Chart 1) Normally, a coating's W24 value decreases each time it is exposed to water, because surfactants that allow water to penetrate the pore network are washed away and separated from the coating. In practice, the advantage is that the W24 value remains as low as possible even after being exposed to water only a few times. This is called early water resistance. TEGO® Phobe 1659 reaches nearly optimal W24 values when exposed to water for only a second time. With similar products, the first two stages proceed much more poorly. From the user's perspective, this means that paints formulated with TEGO® Phobe 1659 have high water resistance immediately after application and drying, not after several heavy rains.

Transmission Electron Microscopy

Researchers obtained electron microscopy images to further investigate the distribution of silicone resin within the paint. They also determined the local distribution of elements using spatially resolved, energy-dispersive X-ray analysis (EDX). To enable classification of the silicone resin, only calcium carbonate and titanium dioxide were used instead of fillers or pigments containing silicon such as quartz or silica. A commonly used styrene acrylic dispersion was used as the primary binder. Paint layers containing TEGO® Phobe 1659 and a standard silicone resin commonly used in the market at rates of 2.0% and 4.8% were separated from the surface and embedded in epoxy resin under vacuum. Researchers then prepared thin sections 180 nm thick and examined them. A representative result is shown in Figure 1. (Figure 1)

Figure 1. Transmission electron image of a silicone resin paint containing 2.0% TEGO® Phobe 1659. Magnification rate: 2000x. Three areas marked with I, II, and III in the figure can be distinguished from each other.

The paint contains 2.0% TEGO® Phobe 1659 and has a porous, non-homogeneous structure as expected. The image of paint layers containing a different silicone resin at a higher ratio looks the same at first glance. Three areas marked with I, II, and III in the figure can be distinguished from each other. Area I is light-colored, amorphous, and homogeneous. In this section, EDX shows almost only carbon (C) and oxygen (O). This looks like pure binder. Area II is dark-colored, similarly homogeneous structures with high electron density. EDX here shows titanium (Ti) and calcium (Ca); therefore, this area must be pigment or filler. Finally, there is again area III with high electron density and non-homogeneous structure; EDX here shows silicon (Si) in addition to carbon (C) and oxygen (O). This looks like silicone resin. How the silicone resin is distributed within the film is decisive in terms of effectiveness. Upon detailed examination, there are marked differences between TEGO® Phobe 1659 and the market standard product in this regard. Figures 2A and B show high-resolution images where the silicone resin can be seen in addition to the primary binder (Figures 2A/B).

Figure 2A. Transmission electron image of a paint containing a standard silicone resin commonly used in the market at a rate of 4.8%. Magnification rate: 4000x. Part of the silicone resin (dark-colored) is isolated by the primary binder.

Figure 2B. Transmission electron image of a silicone resin paint containing 4.8% TEGO® Phobe 1659. Magnification rate: 4000x. The primary binder (light-colored) and silicone resin (dark-colored) are located separately from each other.

Figure A shows that large portions of the silicone resin commonly used in the market are encapsulated by the primary binder; this undoubtedly negatively affects effectiveness. Figure B shows that when TEGO® Phobe 1659 is used, the silicone resin and binder are almost completely separated. This allows the silicone resin to fully demonstrate its effectiveness. The cause of this microscopic phase separation is probably on the one hand the molecular structure of the silicone resin and on the other hand the emulsifiers used. A second difference between these two silicone resins also emerges in the EDX spectra of the pigments. (Chart 2)

Chart 2. EDX of pigment particles. When TEGO® Phobe 1659 is used, silicon (Si) is found alongside titanium (Ti).

The EDX spectrum of the standard product commonly used in the market contains some titanium and calcium, but no silicon. In contrast, the EDX spectrum of TEGO® Phobe 1659 shows both titanium and silicon. This means that pigment particles are coated with a silicone resin layer. In addition to the phase separation mentioned above, the high affinity of TEGO® Phobe 1659 for pigments and fillers also explains the product's special effectiveness at low concentrations.

Low Dirt Pickup

In an accelerated soiling test conducted in the laboratory that reflects the trends of exterior facade air effects, researchers proved that exterior facades with TEGO® Phobe 1659-based formulations pick up dirt twice as slowly as market standard products. (Figure 3)

Figure 3. Soiling tendency. TEGO® Phobe 1659-based formulations show L values up to 30% lower in the soiling test.

Evonik used a special machine for this test. Tar, soot, and dirt known as Japanese standard dirt were added to water. Test panels painted with different silicone resin paints were subjected to dirty water in the machine. The machine then heats the panels to 50°C and subsequently cleans them with clean water. This cycle is continuously repeated for a defined period. After the soiling simulation, the L values indicating the degree of soiling reached by the formulation with the new silicone resin structure were up to 30% lower compared to formulations containing other silicone resins. (Chart 3)

Formulation with Organic Pigments

The new silicone resin's high pigment affinity is also important for pigment concentration pull. Tests have shown that color pastes containing silicone resin have better performance than pigment concentrations drawn without silicone resin. Thus, pigment concentrate can be saved in the preparation of the formulation while still achieving the same color intensity. Another advantage of TEGO® Phobe 1659 is its versatility: it can also be used with organic pigment-based pigment concentrates, thus providing a much wider color range than has been possible with most silicone resins to date. (Chart 4)

Chart 4. Rub-out values after adding 5% pigment concentrate

 

Process and Storage Advantages

TEGO® Phobe 1659 is also suitable for liquid plasters in addition to exterior facade paints. Here, additional advantages emerge in terms of workability, as the current flow limit in the market is now much higher. Especially after storage, tests with plasters formulated with the new silicone resin showed that the flow limit was only about one-third that of plasters containing similar products, independent of the storage duration. Formulations containing the new silicone resin structure have a very convenient rheological profile that facilitates application for those processing them. Additionally, due to the homogeneous distribution of silicone resin, an evenly proportioned structure emerges within the plaster. (Figure 4)

Figure 4. Surface structure of plasters. TEGO® Phobe 1659-based formulations have a more evenly proportioned surface structure.

This not only enhances visual appeal, but also prevents the formation of large cracks that would absorb more water. Even when the paint is exposed to 50°C for a long period, its viscosity remains much lower compared to other products on the market. This makes application easier after storage. Even after 256 days of storage in the test, no effect on viscosity was observed; in similar products and paints without additives, this is five times (!) higher. G' dynamic coefficient analyses also support this observation: even after one week of storage, this value was much lower compared to a similar product and formulation without hydrophobicizing agent. After 256 days of storage, the G' dynamic coefficient doubled in TEGO® Phobe 1659, while it increased eightfold in the standard product and formulation without additives. (Chart 5)

Chart 5. G' storage modulus as a function of time. With TEGO® Phobe 1659, G' remains one-third lower from the outset and increases only marginally over time. Without silicone resin or with market standard products, G' increases significantly as time progresses.

Due to TEGO® Phobe 1659's high affinity for pigments and fillers, it stabilizes the formulation, helping to obtain more stable products. Finally, this means that formulations containing TEGO® Phobe 1659 have a much longer service life.

Environmentally Friendly to Access a Broad Market

Since all these functions can only be achieved with thin paint layers, the material requirements of the applicator are reduced. Furthermore, it is environmentally beneficial because researchers selected raw materials from the outset that will meet the high requirements of finished product certifications and thus smoothly overcome regulatory barriers in the relevant countries. Possible certifications include Blue Angel, the Nordic Ecolabel, and the EU Ecolabel.

Technology Standard Elevated

In conclusion, with TEGO® Phobe 1659, a silicone paint with better technical properties can be obtained even at low concentrations. From the end-user perspective, it offers more color options and reduces maintenance costs. All of this is achieved in conjunction with high environmental properties, thus introducing a new technological standard to the market for hydrophobicizing agents for exterior facade paints and plasters. Markus Vogel Senior Technical Manager for Construction Paints Evonik Resource Efficiency GmbH     Roger Reinartz Senior Technical Manager for Leather and Paper Coatings Evonik Resource Efficiency GmbH   Daniel Brünink Global Marketing Director for Decorative Paints Evonik Resource Efficiency GmbH