Silanes, reaction + silica

The silanization reaction has been used for some time to alter the wetting characteristics of glass, metal oxides, and metals [44]. While it is known that trichlorosilanes polymerize in solution, only very recent work has elucidated the mechanism for surface reaction. A novel FTIR approach allowed Tripp and Hair to prove that octadecyl trichlorosilane (OTS) does not react with dry silica. 

Network Formation with Silica The Silanization Reaction.802... 

ZnO influences the characteristics of a compound twofold Primarily, ZnO influences the kinetics of the silica-silane reaction. Secondly, ZnO enhances the scorching behavior of a sulfur-containing coupling agent. This effect is clearly visible when looking at the G at 100% strain and the 300% modulus. 

Silica compounds are generally processed in conventional internal mixers, preferably with intermeshing rotors. These mixers are designed and optimized for carbon black-fiUed compounds in which mixing is based only on physical processes. When a silica-silane reinforcing system is used, additionally a chemical reaction, the sUanization, occurs. One of the main influencing factors of the silanization reaction is the concentration of ethanol in the compound as well as in the mixer [25,26]. As the silanization finally reaches an equilibrium, low concentrations of ethanol in the compound are expected to enhance the reaction rate. 

High operation temperature (limited by scorch phenomena) in order to raise the speed of the silane reaction with silica and the diffusion coefficient of ethanol... 

The efficiency of the silanization reaction is increased by aU measures enhancing devolatilization of ethanol from the silica compound in the mixer. One possible measure is the reduction of the ethanol concentration in the void space of the mixer, thus increasing the driving force for mass... 

W. Dierkes, Economic mixing of silica-mbber compounds Interaction between the chemistry of the silica-silane reaction and the physics of mixing, Ph.D. thesis, University of Twente, The Netherlands, 22 April 2005, ISBN 90-365-2185-8. 

Reaction of Chlorodimethyl(2,3,4,5-tetramethyl-2,4-cyclopentadien-l-yl)silane with Silica Materials. Chlorodimethyl-(2,3,4,5-tetramethyl-2,4-cyclopentadien-l-yl)silane (excess) was added to a mixture of amine-functionalized silica with hexanes in a drybox. 2,6-Di-tert-butylpyridine (excess) was added as a proton sponge. The mixture was allowed to react while stirring for 24 h. The solid was filtered and washed with hexanes and THF in the drybox. The solid was then contacted with another aliquot of chlorodimethyl-(2,3,4,5-tetramethyl-2,4-cyclopentadien-l-yl)silane and the procedure was repeated. 

In practice, the common method for silanization of silica is to mix a chlorosilane with a hydrated silica in a suitable organic solvent. A common mechanism reported for this reaction is that the chlorosilane is first hydrolyzed by the water and this is followed by the condensation with the surface hydroxyl groups to form a strong Si—O—Si surface bond.72... 

An alternative mechanism for the base-promoted reaction of silanes with silica has been described by Blitz et al.83 In this mechanism, the base attacks directly to the surface silanols. The bonded amine renders the silanol more nucleophilic which then attacks the silicon atom of an approaching silane, giving rise to a pentacoordinate intermediate. 

Thus, in a typical base-promoted silanization on silica, it is more likely that both polymerization and surface reaction occur to some extent. Both mechanisms can account for polymerization. The intermediate formed by attachment of the amine to the chlorosilane could react with nucleophiles (i.e., molecular water) other than the surface silanols. In the mechanism described by Blitz et al. the chlorosilane (either attached or in solution) could be hydrolyzed to the trisilanol by molecular water and the trisilanol offers an additional source of silanols for base attachment and subsequent polymerization. Polymerization often results in a thick silane layer on the surface that in many cases is undesirable. 

FIGURE 6-2. (a) Types of silanols on the surface of silica, (b) Reaction of various chlorosubstituted silanes with silica gel. 

Surface Modification with Chlorosilanes. Chlorosilanes are volatile and very sensitive to the presence of trace amounts of water. Vapor-phase silanization reactions with rigorous control of the absence of water show that Cl SiMe4-n-type chlorosilanes react with aerosil (nonporous microparticulate silica) as... 

A typical RPLC column is packed with porous silica beads. The pore sizes have a length scale in the range 50-100 A. The silica substrate is derivatized by a silanization reaction which attaches alkanes to hydroxyl groups on the surface. Chains of length 8-18 carbon atoms are utilized, with the Cjg case being common. Surface densities up to about 50% of close packing are typical. The mobile phase most often contains a cosolvent, such as methanol or acetonitrile, in addition to water. 

The latter procedure has so far been applied successfully (/) to a large number of silicas suitable for column chromatography (60-200 mesh). Attempts to extend these techniques to thin-layer silicas have not been successful. In the case of silicas with calcium sulfate binder, experimental values of 5,. and St appear quite variable and do not agree with values that can be inferred from surface area, pore diameter, and chromatographic activity data. This suggests that calcium sulfate in some way interferes with the silica-silane reaction. For thin-layer silicas without binder, erroneously low values of S jSf are obtained. This is believed to be due to... 

Compared to morphology, filler chemistry has been only slightly studied, partly because of the difficulty of such characterizations and more probably because since the 1970s reinforcement is broadly considered as a physical interaction between elastomer and filler. So carbon black chemical characterizations mainly date from the 1960s, and few new technical methods have been applied to carbon black surface characterization since then. The situation is somewhat different for silicas, because silica reinforcement is the consequence of a chemical reaction of silane with silica surface. Few studies have been published in the elastomer reinforcement area, probably because silica surface was already well characterized for other applications. 

As noted earlier, three types of functional silanol groups exist on the silica surface isolated hydroxyl groups, geminal groups (two -OH groups on one Si atom), and vicinal groups (see Figure 9.6). The silanization reaction is illustrated in Eq. (9.5). 

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