Filler silica particle

The surface of silica is covered by a layer of acidic silanol and siloxane groups. This highly polar and hydrophilic character of the filler surface results in a low compatibihty with the rather apolar polymer. Besides, highly attractive forces between silica particles result in strong agglomeration forces. The formation of a hydrophobic shell around the silica particle by the sUica-sUane reaction prevents the formation of a filler-filler network by reduction of the specific surface energy [3]. 

These fillers are cage-like silicon-oxygen structures, and have been called the smallest possible silica particles. The most common structure has eight... 

Silica used as a filler for rubbers is silicon dioxide, with particle sizes in the range of 10-40 nm. The silica has a chemically bound water content of 25% with an additional level of 4-6% of adsorbed water. The surface of silica is strongly polar in nature, centring around the hydroxyl groups bound to the surface of the silica particles. In a similar fashion, other chemical groups can be adsorbed onto the filler surface. This adsorption strongly influences silica s behaviour within rubber compounds. The groups found on the surface of silicas are principally siloxanes, silanol and reaction products of the latter with various hydrous oxides. It is possible to modify the surface of the silica to improve its compatibility with a variety of rubbers. 

Other additives such as silanes, titanates and zirconates are also used to overcome the processing characteristics of silica fillers. Silanes not only give improved processability of silica-filled compounds, but also provide improved crosslinks between the silica particle surface and the rubber molecular chains giving increased physical properties. The use of silane coupling agents at a... 

There is a variety of miscellaneous fillers that are of interest for reinforcing elastomers such as PDMS. Examples are ground-up silica xerogels,119 carbon-coated silica,120 and functionalized silica particles.121-123... 

This paper represents an overview of investigations carried out in carbon nanotube / elastomeric composites with an emphasis on the factors that control their properties. Carbon nanotubes have clearly demonstrated their capability as electrical conductive fillers in nanocomposites and this property has already been commercially exploited in the fabrication of electronic devices. The filler network provides electrical conduction pathways above the percolation threshold. The percolation threshold is reduced when a good dispersion is achieved. Significant increases in stiffness are observed. The enhancement of mechanical properties is much more significant than that imparted by spherical carbon black or silica particles present in the same matrix at a same filler loading, thus highlighting the effect of the high aspect ratio of the nanotubes. 

The tensile strength of cross-linked polysiloxane elastomers is low, but can be markedly improved by reinforcement with a filler. The material of choice is fumed silica with high surface area, which can increase the strength by a factor of 20. It is thought that the silica particles agglomerate to form a three-dimensional network within the siloxane, greatly reinforcing the stmcture. 

Silica particles do not induce any modification of the stress state in the material, and so no extended plasticity in the matrix. Naiiosized silica particles can be considered as a modifier of polymer chain displacement, and not as a reinforcement filler. There is adsorption of PP on silica surface and consequent reduction of molecular mobility with a large increase of elastic modulus. We do not observe any process zone, and mechanisms as particle/matrix decohesion as well as crack pinning or blunting are not effective. 

F . 10. Relationsh between filler content in epoxy resin composition containing 70 pm silica particles (42) and alumina trihydrate particles of differing sizes (51)... 

It is well known that the reinforcing potential of fillers can only be realized if a good dispersion of the filler is achieved. Traditionally, various carbon blacks have been used as fillers, more recently (modified) silica particles are being increasingly utilized. As shown below, the filler distribution can be directly assessed by AFM. 

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. 

Examples of size distribution functions are shown ill Figs. 1.4 and 1.5. Figure 1.4 shows number distributions of commercially produced silica particles in terms of the fraction of particles in the,size range around dp, dN/N d dp) = na(,dp)fNxs where is the total particle concentration. The total particle surface area corresponding to each size distribution is shown. Commercial silica manufactured by the oxidation of SiCU is used as a filler (additive) in rubber. Both coordinate axes in Fig. 1.4 are linear, and the area under each curve should be normalized to unity. A bimodal volume distribution with a minimum near a particle size of 1 is shown in Fig. 1.5. Distributions of this type are often observed for atmospheric aerosols (Chapter 13) the volume of aerosol material per unit volume of gas above and below a micron is about the same as shown by the area under the curve. Bimodal distributions are also often observed in aerosols from industrial sources as discus.sed below. 

Watanabe et al (1996) describe the non-linear dynamic rheology of a concentrated spherical silica-particle-filled ethylene glycol/glycerol system. The strain dependence was well described by the BBCZ-type constitutive equation, until the shear-thickening regime. The shear thickening was qualitatively described in relation to the structure of the suspended filler particles. 

Petti (1994) showed the importance of silica-particle shape and maximum packing volume fractions on the chemoviscosity and spiral flow of highly filled epoxy-resin moulding compounds. Ultrahigh concentrations of silica could be used using the correct filler. 

A noteworthy finding has been that all the materials show two distinct relaxation dynamics, a fast and a slow relaxation [60]. The fast mode corresponds to relaxation of bulky polymer molecules, while the slow mode is related to relaxation of the filler structure with much longer time scales. As silica particles are physically connected with adsorbed polymer molecules, the formed polymer-particle network is a temporary physical network. On a long time scale, relaxation of this network occurs when immobilized polymer molecules connecting silica particles become free, via dissociation from silica particles or disentanglement from other immobilized polymer molecules. 

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