First Water-Soluble, then Water-Repellent - Wacker Chemie AG


First Water-Soluble, then Water-Repellent

Between 0.1 and 0.2 weight percent SILRES® BS Powder S is added to a commercially available gypsum joint filler (left-hand bar). The test specimens obtained were then immersed in water for two hours. The chart shows how much water by weight percent was absorbed by the test specimens.

“Manufacturers of gypsum construction materials need water repellents that are suitable for use in dry-mix processes, are exceptionally efficient and pose no problems when mixed with water,” explains Dr. Rudolf Hager, who heads the Construction Chemicals business team. “SILRES® BS Powder S is not subject to any of the problems associated with conventional powder-form gypsum hydrophobizing agents. It consists only of the pure active ingredient, without any carriers or encapsulation material, and is therefore extremely efficient.”

Patented SILRES® BS Powder S consists of potassium methylsiliconate, a material that has been used in building protection since the 1950s, for example for surface hydrophobizing of coarse ceramic clay products, or for integral water-repellent treatment of prefabricated gypsum elements. Up to now, however, alkali alkyl siliconates – as indicated earlier – have only been produced and used in the form of aqueous solutions.

“With the development of alkali alkylsiliconates, which can be obtained as free-flowing solids on an industrial scale, we have made a major breakthrough,” explains Rudolf Hager. WACKER has developed a variety of drying processes for making powders with different properties.

“We were also able to adjust the potassium-silicon ratio so as to optimize the balance between stability, reactivity and pH in gypsum-based construction materials.”

  • Gypsum test specimens being made in WACKER’s applications lab in Burghausen.

  • SILRES® BS Powder S is added to one specimen, but not to the other.

  • The specimens are then immersed in water,

  • Before being weighed on a set of scales to determine which one absorbed more water.

Because potassium methyl siliconates already contain the silica groups necessary for the interaction with gypsum, there is no need for a preliminary chemical reaction, namely hydrolysis, to generate these groups. The siliconate itself is already in its effective form, which means that, unlike traditional water repellents, the new product eliminates the need for elevating the pH of the material formulation in order to induce alkaline activation. The active substance itself is surprisingly hydrophilic and water soluble, and the gypsum dry-mix mortar treated with it can therefore be mixed rapidly and without additional dust formation. When the calcium sulfate hemihydrate is blended with the mixing water, the water-soluble potassium methylsiliconate goes into solution very quickly. While the plaster sets, the siliconate selectively interacts with the growing gypsum crystals, using its hydrophilic silica groups to anchor itself onto the gypsum crystal surfaces. Its hydrophobic methyl groups, however, point away from the crystal surfaces of the calcium sulfate dihydrate being formed. This orientation causes the siliconate to lose its original hydrophilic properties and form a shield that prevents water from further penetrating the gypsum crystals in the set material, while the outstanding permeability of the gypsum construction materials to water vapor remains unaffected.

“Our innovative water repellent is an effective way to stop the penetration of water.”

Dr. Daniel Schildbach Technical Marketing, WACKER SILICONES