Fillers silanes, reactivity

In its simplest terms, the titanate function (1) mechanism may be classed as proton-reactive through solvolysis (monoalkoxy) or coordination (neoalkoxy) without the need of water of condensation, while the silane function (1) mechanism may be classed as hydroxyl-reactive through a silanol-sUoxane mechanism requiring water of condensation. The silane s silanol-siloxane water of condensation mechanism limits its reactions to temperatures below 100 °C, thereby reducing the possibility of in situ reaction in the thermoplastic or elastomer melt above 100 °C as is possible with titanates. In addition, a variety of particulate fillers such as carbonates, sulfates, nitrides, nitrates, carbon, boron and metal powders used in thermoplastics, thermosets, and cross-linked elastomers do not have surface silane-reactive hydroxyl groups, while almost all three-dimensional particulates and species have surface protons, thereby apparently making titanates universally more reactive. 

It can be seen that modification of filler snrfaces both to aid processing and improve composite properties is an important and active area of research. While there are a considerable number of treatments proposed, they all follow the principle of a filler surface reactive group linked to an organic backbone, which may carry further functionality. The main variation is in the group used to achieve surface reaction. As we have seen this may be an acid or acid precursor, an aluminate, borate, phosphate, silane, titanate or zirconate. 

While organo-silane treatments are extensively used in both thermoset and elastomer applications, their use in thermoplastics has so far been somewhat restricted. This is because they do not react with the surface of calcium carbonate, one of the principal fillers used in this type of polymer and because of the lack of a suitable reactive functionality for most of the thermoplastic polymers. Today they are principally used in conjunction with glass fibres, calcined clays, aluminium and magnesium hydroxides, micas and wollastonite. The main thermo-... 

Spectroscopic techniques are extremely useful for the characterization of filler surfaces treated with surfactants or coupling agents in order to modify interactions in composites. Such an analysis makes possible the study of the chemical composition of the interlayer, the determination of surface coverage and possible coupling of the filler and the polymer. This is especially important in the case of reactive coupling, since, for example, the application of organofunctional silanes may lead to a complicated polysiloxane interlayer of chemically and physically bonded molecules [65]. The description of the principles of the techniques can be found elsewhere [15,66-68], only their application possibilities are discussed here. 

Hydrosilylation is also a very useful chemical modification which leads to silane modified polymers with special properties [60-62]. Silane modified polymers have improved adhesion to fillers and better heat resistance. It also acts as a reactive substrate for grafting or moisture catalysed room temperature vulcanisation. Guo and co-workers [61] carried out catalytic hydrosilylation of BR using RhCl(PPh3)3 as the catalyst. Hydrosilylation reactions followed anti-Markovnikov rule as shown in the Scheme 4.4. 

All of these mechanisms which affect crosslink density were confirmed by experimental studies. The classic case of a reactive particle filler is silica filled polysiloxane (Figure 6.25). Silica particles have numerous OH groups which react with the crosslinking component of polysiloxane. Modification of silica by silanes reduces reinforcement. 

Dutta and Ryan (1979) examined the effects of fillers (carbon black and silane-surface-treated silica) on the cure of DGEBA/MPDA epoxy-amine systems. They found that the rate constants of the cure reaction are affected by the presence of the fillers in an unusual fashion (a function of temperature and concentration) with respect to concentrations up to 10%. This was postulated to be due to the reactive surface groups on the fillers. The reaction order, however, is not affected. 

To formulate a successful composite material, and in particnlar to ensnre that there is adequate stress transfer from matrix to filler phase, a conpling agent is deployed at the matrix-filler interface. The type of silane nsed for conventional dental composite resins effectively forms a mono-molecnlar hydrophobic layer on the snrface of the inorganic filler particles. In silanating the reactive ionomer glass in this way, the chemical reactivity of the glass is affected. It is no longer quite so hydrophilic, and hence is less susceptible to acid attack in the presence of moisture. 

Chem. Descrip. Phenyl trimethoxy silane CAS 2996-92-1 EINECS/ELINCS 221-066-9 Uses Surf, treatment, pigment/filler treatment in primers, water repellents, paints, inks, and adhesives reactive intermediate tor silicone resin synthesis and org. resin modification adhesion promoter Properties Low-vise, liq. sp.gr. 1.05 vise. 1.7 cSt flash pt. (CC) 29 C 100% act. 

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