Page 128 - Construction Waterproofing Handbook

ABOVE-GRADE WATERPROOFING 3.17
Silanes
Silanes contain the smallest molecular structures of all silicone-based materials. The small
molecular structure of the silane allows the deepest penetration into substrates. Silanes,
like siloxanes, must have silica present in substrates for the chemical action to take place
that provides water repellency. These materials cannot be used on substrates such as wood,
metal, or limestone that have no silica present for chemical reaction.
Of all the silicone-based materials, silanes require the most difficult application proce-
dures. Substrates must have sufficient alkalinity in addition to the presence of moisture to
produce the required chemical reaction to form silicone resins. Silanes have high volatility
that causes much of the silane material to evaporate before the chemical reaction forms the
silicon resins. This evaporation causes a high silane concentration, as much as 40 percent,
to be lost through evaporation.
Should a substrate become wet too quickly after application, the silane is washed out
from the substrate-prohibiting proper water-repellency capabilities. If used during extremely
dry weather, after application substrates are wetted to promote the chemical reaction nec-
essary. The wetting must be done before all the silane evaporates.
As with other silicone-based products, silanes applied properly form a chemical bond
with a substrate. Silanes have a high repellency rating when tested in accordance with
ASTM C-67, with some products achieving repellency over 99 percent. As with urethane
sealers, their high cost limits their usage. (See Table 3.8.)

Siloxanes
Siloxanes are produced from the CL-silane material, as are other silicone masonry water
repellents. Siloxanes are used more frequently than other clear silicones, especially for
horizontal applications. Siloxanes are manufactured in two types, oligomerous (short chain
of molecular structure) and polymeric (longer chain of molecular structure) alky-
lalkoxysiloxanes.
Most siloxanes produced now are oligomerous. Polymeric products tend to remain wet
or tacky on the surface, attracting dirt and pollutants. Also, polymeric siloxanes have poor
alkali resistance, and alkalis are common in masonry products for which they are intended.
Oligomerous siloxanes are highly resistant to alkaline attack, and therefore can be used
successfully on high alkaline substrates such as cement-rich mortar.
Siloxanes react with moisture, as do silanes, to form the silicone resin that acts as
the water-repellent substance. Upon penetration of a siloxane into a substrate it forms

TABLE 3.8 Silane Water-Repellent Properties

Advantages Disadvantages
Deepest penetration capabilities High evaporation rate during application
of all silicone-based products
Forms chemical bond with Dry substrates must be wetted to ensure
substrate with good chemical reactions before evaporation
permeability rating
Good weathering characteristics High cost of material