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


First Water-Soluble, then Water-Repellent

The biggest disadvantage of gypsum-bound construction materials is their low moisture resistance. Thanks to SILRES® BS Powder S, a highly efficient, powdered water repellent is now available for the first time for treating gypsum-based dry-mix mortars.

Dr. Daniel Schildbach, in charge of an applications lab run by the Construction Chemicals business team, with lab employee Theresia Genzinger.

Gypsum is a general-purpose material, with applications well beyond drywall construction. Construction elements and construction materials based on gypsum are environmentally friendly to manufacture and can be rapidly and inexpensively processed. They create a pleasant indoor climate, regulate interior humidity and always feel pleasantly warm and dry to the touch.

When set, gypsum has two weaknesses, however: it has relatively poor hardness, and is slightly – yet noticeably – soluble in water. The solubility of calcium sulfate dihydrate is roughly 2.6 g/L at 20 °C. Compared to silicate-based building materials, which are practically insoluble in water, this is quite high. Furthermore, gypsum-based construction materials lose their compressive strength when saturated with moisture. Plus, prolonged exposure to water irreversibly damages the material structure.

Drywall Construction in Damp Locations

Gypsum-based construction elements and construction materials should therefore only be used in damp rooms if they have been made effectively hydrophobic, i.e. water-repellent. However, all the materials currently available for rendering gypsum construction materials water repellent have the disadvantage that they are only available in liquid form. Organosilicon water repellents, based either on what are known as H-siloxanes (polymethylhydrosiloxanes) or on alkali alkyl siliconates, have proven highly effective for gypsum building panels – such as gypsum plasterboard, fiberboard or block – and established themselves on the market accordingly. The former are used as oils or aqueous emulsions, while the latter are in aqueous solution – they are therefore not suitable for use as additives for formulating dry-mix mortars.

“WACKER has now designed a completely new gypsum hydrophobizing agent, SILRES® BS Powder S, which can be used in the dry-mixing process,” says Dr. Rudolf Hager, head of the Construction Chemicals business team at WACKER. “This additive allows manufacturers to produce gypsum-based wall plasters, joint fillers, top coat materials and adhesives that can be used throughout the interior of a building.”

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.

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.”

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
Fast and dust-free: the SILRES® BS Powder S hydrophobizing agent is used in the in-plant dry-mixing process before mixing water is added on the building site.

The aim of the WACKER developers was to reduce the water absorption of the set dry mixtures to less than five weight percent, as has been specified for damp-room gypsum board for several years by the standard EN 520:2004. According to the experts, below this limit, joint fillers can be used generally in all interior rooms, including damp rooms. The EN 13963 standard, which was updated and revised in 2014, confirms this. Accordingly, only Type H1 jointing compounds are approved for challenging damp-room applications. These materials absorb no more than five percent by weight of water, even after immersion in water for two hours.

“The tests on different gypsum test specimens showed that our new hydrophobizing agent suppresses the penetration of water extremely effectively,” explains product developer Dr. Daniel Schildbach from WACKER’s Technical Service in Burghausen. “After two hours’ immersion in water, the water absorption remains below five percent – assuming that the dosage rate is correct.

Requirements to Be Met by Water Repellents for Gypsum-Based Dry-Mix Mortars

  • Water absorption < 5%
  • Quantity used < 0.5%
  • Mixing characteristics (when mixing the dry-mix mortar with water): excellent
  • Dust formation (when mixing the dry-mix mortar with water): none

The starting point for Dr. Schildbach’s study was a commercially available gypsum joint filler to which SILRES® BS Powder S had been added at various concentrations. According to Dr. Schildbach, “Unlike traditional powder-form water repellents, our new additive can be used in gypsum construction materials to keep water absorption very low over long periods of time. SILRES® BS Powder S additionally imparts a pronounced beading effect to the construction material surface.

Outdoor weathering tests conducted near WACKER’s Burghausen site also confirm that SILRES® BS Powder S provides long-lasting protection against water. Large, uncoated gypsum-plaster test objects still displayed pronounced beading after two years of weathering. When they were again immersed in water, they absorbed 70 percent less water than the test objects without water repellent. “However, the driving rain typical of Western Europe had eroded the soft surface of the gypsum plaster considerably,” reports Dr. Schildbach. He therefore does not recommend using gypsum for outdoor renders in such climatic regions. In regions that experience little rainfall, such as Southern Europe or South Asia, on the other hand, SILRES® BS Powder S can increase the durability of existing outdoor gypsum plasters.