Silicones for Construction

Silicones have a very wide range of uses in building and construction work. These range from joint seals to water-repellent treatment of breeze-blocks and additives for paints through to effective thermal insulation and resultant energy savings. They are also used when damaged structures need to be protected against further damage. This will be illustrated in the following example:

Silicones in Masonry Protection

A section of the Fürstenland bridge in St. Gallen, Switzerland is shown in the following picture. Environmental influences, such as damp and the use of deicing salt, caused the steel skeleton of the bridge to start corroding, as can be seen from the rust deposits on the concrete surface. Some of the concrete even broke off, and that jeopardized the long-term safety of the bridge.

To counteract the problem of weathering, the entire concrete surface was treated with SILRES^® BS. Its active agent is iso-octyltriethoxysilane (content: 80 %). The other 20 % is mainly water to ensure that the active agent penetrates far into the concrete. The water is also needed for forming the water-repellent silicone resin network. The silicone resin provides adequate protection for the pores and capillaries in the concrete. This impregnating process requires just one working step, namely application of the SILRES^® BS .

The following three pictures clearly show how the SILRES^® BS works:

Fig. 1. The cream is applied to the surface of the concrete object. Because it is white, it stands out from the untreated concrete and this allows it to be applied uniformly.

Fig. 2. After a short time, it has already soaked so far into the concrete that it can hardly be seen.

Fig. 3. About two hours later, nothing more can be seen of the white cream and the concrete is almost back to its typical appearance – only now it is optimally protected against corrosion. Just 200 g of the cream is enough to cut water absorption by the concrete by about 80 % at a depth of 3 mm. Aside from being used to treat damaged concrete structures, this method can of course also be employed as a preventive measure for new buildings.
- See also the experiments “Hydrophobic properties of silicone fluids", " Silicones in masonry protection", the video (as wmv or as mov) and flash-animation.

Application Areas of Silicones in House Building

The next diagram gives some idea of where silicones are used in buildings:

As the diagram and the explanations show, silicones are mainly used for impregnating, structural glazing and sealing in buildings. The corresponding components are protected optimally against damp and related weathering effects.

Joints and Seals

Silicones have a wide range of bonding, sealing and jointing applications. In modern buildings, silicone rubber compensates movement between glass panes and aluminum frames that are caused by wind and temperature fluctuations. Masonry, too, is constantly subjected to stresses and movements due to temperature fluctuations, the effects of moisture, shrinkage of construction materials and mechanical shocks. When selecting jointing material, therefore, it is essential to assess its mechanical deformation properties.

Transparent insulating films of GENIOMER® are used to soundproof laminated glass in buildings. As GENIOMER® conducts light, it is also possible to produce indirect lighting effects in windows.

The above diagram illustrates the deformation properties of elastic and plastic sealants. The application profile of a joint depends on whether the jointing material is more plastic or more elastic in nature. If the material is too plastic, cohesive failure ensues on account of the chewing-gum effect. If the material is too elastic, adhesive failure may occur. In this case, a strong tensile force is generated at the joint flanks. To avoid joint failures of this kind, joint sealants with intermediate properties are used, i.e. sealants that show partly elastic and partly plastic behavior. Silicones, i.e. polysiloxanes, are ideal for this purpose. The polysiloxanes used are cured by means of moisture in the air or addition of a curing agent. A further advantage of silicones is their good resistance to weathering influences, chemicals and radiation. These materials are moreover safe to handle and easy to apply.

Silicones and Thermal Insulation

Silicones play two different roles in thermal insulation.

First, the heat transfer resistance of materials decreases as they absorb moisture. Water uptake by construction materials is reduced considerably if they are treated with silicones such as SILRES^® BS, which was described above, to make them water-repellent. The heat transfer resistance then hardly decreases at all, ensuring efficient insulation. Second, silicone foams are excellent heat insulators and therefore have many applications in this field. One of the main uses of these foams is as insulation for district-heating pipes. Brief mention may also be made here of flame-retarding silicone rubber foams. These are used for safety reasons wherever necessary.