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Filler Types Susceptible to Silane Treatment
Because they rely on reaction with filler surface hydroxyls to produce a stable layer, the organo-silanes are most effective on fillers with high concentrations of such sites. This includes silicas, silicates (including glass), oxides, and even hydroxides, although problems can arise if the surface has a high pH, when exposure to moisture may cause the bond to break down.
Silanes have little effect on carbon blacks. They are also not generally effective on surfaces such as sulfates and carbonates.
As can be judged from the description of their chemistry, organo-silanes are complex species, which often have the potential of reacting with themselves as well as with filler surfaces. Careful control of the coating process is thus important.
The noncoupling silane versions are generally quite volatile, and filler coating is usually carried out in the gas phase by exposing the filler to their vapors. This ensures maximum reaction with the surface and minimum self-condensation.
On the other hand, the coupling versions are less volatile and are liquids at ambient temperatures. The filler can therefore be coated using the neat liquid or a solution of the latter in a suitable solvent. Silane coupling agents have been most used for coating glass fibers. In this specific application, prehydrolysis and partial self-condensation have been found to aid the coating process, and this is the usual procedure employed in much of the scientific work on silane treatment of particulate fillers. Unfortunately, this approach is not often used in real applications as it is too impractical. In many instances, the filler coating is actually carried out “in situ” during the compounding process, essentially utilizing the polymer matrix as the solvent and the mixer as the reactor.
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