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

First Water-Soluble, then Water-Repellent

Gypsum plasterboard used in damp interiors is easily identified by its typical green coloring.

Efficient Hydrophobization

In order to obtain solid and free-flowing additives from available liquid agents, silicone manufacturers and formulators so far make use of a trick: they take the active agent, which is itself a liquid, and either encapsulate it within a powder-form material or adsorb it onto a powdery carrier material, thereby creating a vehicle for introducing and blending the agent into the dry-mix mortar formulation. In practice, silicone manufacturers do not even actually use the active agent itself (an alkyl silicic acid), but rather a liquid precursor of the active agent, such as an alkyl silicic ester, a polyalkyl silicic ester or mixtures of these. They do this because the actual active agent is highly reactive, ruling out the option of isolating it and using it directly.

When gypsum dry-mix mortars that include this kind of carrier-based or encapsulated active agent precursor are mixed with water, the liquid substance leaves its “packaging” and is converted into the actual active agent (an alkyl or polyalkyl silicic acid) via hydrolysis. There are numerous disadvantages associated with the use of these traditional powder-form water repellents, however.

  • The powdery additive contains very little active substance, because the carrier and/or encapsulating material can only accommodate 30 percent of the active ingredient. Loading these materials with larger amounts of liquid yields a sticky powder that is no longer free flowing. Because the carrier and/or encapsulating material itself has no effect whatsoever,up to 70 percent of the bulk of the additive contributes nothing toward making the mortar water repellent. Conventional powdery hydrophobizing agents are therefore inefficient.
  • Mixing the gypsum dry-mix mortar with water results in hydrolysis, and, if this reaction is to proceed at a practical rate, the mortar has to be rendered highly alkaline. If the pH of the blended compound is too low, the gypsum will set faster than the precursor can hydrolyze and be converted to the active agent itself, thus rendering the additive largely ineffective. Yet, even if the pH is sufficiently high, the chemical reaction does take a while to proceed. Water repellency, in other words, would not take effect in the set gypsum plaster material for some time.

Furthermore, traditional powdery water repellents cause mixing problems. Once packaged, the liquid organosilicon compounds can migrate from carrier materials and/or from the encapsulation material onto the surrounding bulk solid and the calcium sulfate hemihydrate. This can even occur while the materials are still in storage, thus making the binders and fillers water repellent before the mixing water is added. Poor wetting properties result from this undesirably premature hydrophobization: mixing therefore takes a long time and is dusty, because the mixing water cannot wet the water-repellent plaster dust.