SREP^® explained:
Expert insights into long-lasting facade protection

Lenine De Sousa: SREP^® (Silicone resin emulsion paints) is a patented innovation by WACKER that combines the benefits of mineral-based and synthetic-resin-bound coatings. The ideal SREP^® formulation includes both silicone resins and silicone oils. Although these two chemistries are often mistakenly considered the same, they differ significantly in both structure and performance. Let’s hear from the experts to clarify the distinction.

Hello Christophe, it’s a pleasure to have you here today. With your extensive background in chemistry and coatings, and your current role at WACKER, you bring a wealth of expertise to this discussion. Thank you for joining me.

Christophe Cheikh: Hello Lenine. Thank you for having me. It’s a privilege to discuss such an important topic with you and to bring it to the coatings’ community.

Lenine: That’s fascinating. Silicone chemistry is clearly at the heart of this innovation. However, there seems to be some confusion in the market about the differences between silicone resins and silicone additives. Could you clarify this for us?

Christophe: Certainly. Silicone chemistry is indeed diverse, and understanding the differences between silicone resins and silicone oils is crucial. Silicone oils, often referred to as silicone additives, are primarily composed of difunctional silicone units. These provide a linear structure, which imparts flexibility and a softening effect. Silicone oils tend to migrate to the surface of the coating, enhancing properties like hydrophobicity and open time. However, when overdosed, they can also increase surface tackiness, which may lead to higher dirt pick-up over time.

If you look at the standard chemical structure of a silicone oil, we have a small amount of trifunctional building blocks and a large proportion of difunctional ones.

On the other hand, silicone resins are predominantly composed of trifunctional silicone units, which create a crosslinked, three-dimensional network within the coating. This structure provides long-lasting protection, reduced water uptake, and a harder surface, which minimizes dirt pick-up. In SREP^®, the silicone resin binder is deeply integrated into the coating, forming a stable and durable network.

Lenine: That’s a clear distinction. So, if I understand correctly, silicone oils are more surface-active, while silicone resins form a robust internal network. How does this difference impact the performance of a facade coating?

Christophe: Exactly. Silicone oils strongly improve water beading at the coating interface thanks to their very low surface tension, but they can lead to higher tackiness, which attracts dirt over time. In contrast, silicone resins are distributed through the entire coating matrix, forming a durable, UV-stable network. This ensures long-term protection against water ingress and UV radiation while maintaining a cleaner surface over time.

Lenine: That’s an important point. As mentioned earlier, SREP^® combines both silicone resins and oils. Why is this combination so effective?

Christophe: The key is to formulate SREP^® above the CPVC (critical pigment volume concentration), thus creating a porous system that allows for breathability and water vapor to easily diffuse through the coating. Left untreated, the water uptake would increase via capillary uptake. The solution consists in treating these pores with hydrophobic silicone. To do so, the combination of silicone oil and resin is advised to leverage the strength of both chemistries. This balance is crucial for achieving the unique property of SREP^®. Initially the silicone oil provides surface hydrophobicity. However, as weathering progresses and alters the surface composition, the silicone resin takes over, providing a long-term protection through the entire coating. As previously explained, an excessive amount of silicone oil can lead to increased surface tackiness, which in turn promotes dirt pick-up.

Lenine: I understand there’s a French standard, FD T30-808, that defines how siloxane paints should be formulated. Could you elaborate on its significance?

Christophe: Yes, the FD T30-808 standard is very important. In the absence of an international or European definition for siloxane paint, the standards define such coatings as containing at least one-part silicone resin to one-part organic binder. When a “real” silicone resin is used – composed entirely of trifunctional units – 80% residual weight of such a one-to-one mixture will be found after calcination. Calcinating a mixture of an organic binder with silicone resin should yield 40% residual matter.

By partially replacing the organic binder with silicone resin, the coating is formulated above its critical pigment volume concentration (CPVC), as described by SREP^® technology.

It is important to note that the standard does not differentiate between siloxane technologies, which may lead formulators to mistakenly use silicone oils instead.

Lenine: If I understand well, the standard does not consider the difference between silicone resins and oils. Why do you specifically mention silicone resins?

Christophe: If silicone oils were used instead of silicone resins, the facade would suffer from issues like increased dirt pick-up and reduced long-term protection. This standard helps ensure that SREP^® formulations meet the highest quality and performance benchmarks.

Lenine: That’s a critical insight. Let’s talk about testing. How do you demonstrate the differences between silicone additives and silicone resins in the lab?

Christophe: We can differentiate both materials in different ways. In the analytical lab, these structures can be easily verified via an infra-red spectrum to determine whether the silicone material is a resin or a silicone oil. And in the applications lab, we simulate the aging of a facade using QUV-B radiation, which mimics weathering conditions. We then evaluate the surface of the coating using scanning electron microscopy and measure water contact angles. These tests reveal how the composition of the coating surface changes over time. For example, non-functionalized silicone oils can be washed out together with the natural degradation of the organic binder, exposing fillers and pigments to the environment. In contrast, silicone resins maintain their structure and hydrophobicity in the coating’s matrix, providing better long-term stability.

Lenine: That’s a rigorous approach. But how do these lab results translate to real-world performance?

Christophe: We have extensive outdoor weathering data to validate our lab findings. For instance, we’ve tested facade paints at our outdoor weathering station in Burghausen, Germany. After two years of exposure at a 60° angle to the southwest, we observed significant differences. Paints containing only silicone oils showed higher dirt pick-up, while SREP^® formulations optimized with both silicone oil and resin remained much cleaner. These results align perfectly with our expectations and demonstrate the superior performance of SREP^® in real-world conditions.

Lenine: Those results are compelling. Before we wrap up, is there a key takeaway you’d like to leave with us?

Christophe: Certainly. The key takeaway is that not all silicones are created equal. Silicone oils and silicone resins serve different purposes and have distinct impacts on coating performance. SREP^® technology, with its unique combination of both, offers the best of both worlds: initial surface benefits from silicone oils and long-term durability from silicone resins.

By adhering to standards like FD T30-808 and leveraging the right silicone chemistry, we can ensure facades remain protected and aesthetically pleasing for years to come. While formulating a coating without silicone resins might provide the right level of water protection, it will be detrimental for the aesthetic aspects in the middle term.

At this stage, I would like to repeat that silicone oil and silicone resin present different structures, which impart different properties to the coatings. These structures can be easily verified via an infra-red spectrum to determine whether the silicone is a resin or an additive. Such a spectrum can be easily provided by any silicone supplier.

Lenine: Thank you, Dr. Cheikh, for sharing your expertise and shedding light on the science behind SREP^®. It’s been an enlightening discussion.

Christophe: Thank you, Lenine. I hope this helps to better understand the value of SREP^® technology and the critical role of silicone chemistry in coatings.