Siloxane-silica fillers

In Situ Formed Siloxane-Silica Filler for Rubbery Organic Networks... 

Mark and his co-workers reported the reinforcement of poly(dimethylsiloxane) networks by silica gel particles [1-6]. For example, bis(silanol)-terminated poly-(dimethylsiloxane) was reacted with tetraethoxysilane in the presence of acid-catalyst to produce the reinforced siloxane networks. The reaction proceeded homogeneously. The content of the silica filler can be controlled by the feed ratio of polysiloxane and tetraethoxysilane. 

Both one-component and two-component silicone sealants contain polydimethyl siloxane as the base polymer along with fillers such as calcium carbonate and/or fumed silica fillers, plasticisers (silicone oil) and a variety of cross-linking agents and adhesion promoters. Two-component sealants utilise a catalyst such as dibutyl tin dilaurate, alkyl silicate esters and metallic salts (Maslow, 1982). 

Experiments were performed on DC745U and M97 silica-filled silicone polymers as described elsewhere [1-5]. The gum stocks for all formulations were co-block polymers of dimethylsiloxane, diphenylsiloxane, methylphenylsiloxane, and/or methylvinyl siloxane. The gum stock was reinforced with high surface area silica filler and crosslinked with peroxide curing agents. These materials were tested in both new as well as service return conditions. 

During the cure (or vulcanization) process, Si-O-Sl bonds are formed, and these are the same stable bonds present in the silica filler and polymer backbone. The result is a 3-dlmensional network of polymer and silica consisting exclusively of siloxane bonds. 

Studies of uniaxial extension on noncrystallizable elastomer, poly(phenyl methyl siloxane) showed results which are consistent and comparable with those obtained for PDMS, suggesting that the crystallization is not important for this type of reinforcement [20]. Other examples for reinforcement effects achieved with the addition of silica fillers include polyisobutylene [24], poly(ethyl acrylate) [3], poly (tetra methylene oxide) [29,30], and some high-temperature polymers such as aromatic polyamides [14,33,34], polyi-mides [15,38,39], polybenzoxazoles [16,17], and polyben-zobisthiazoles [16,17]. Results indicated that the modulus increases with increase in silica content while the tensile... 

Physical forms. The molding compound will consist of 20 to 25% resin (phenyl and methyl siloxanes), 75% filler (glass fiber and fused silica mix), a lead-based catalyst pigment, and lubricants. The compounds are free-flowing granular in form and are available in opaque colors (mostly red). They are readily moldable in compression, transfer, and injection molding processes. 

With many synthetic elastomeric polymers, the strength properties obtained from a non-reinforced crosslinked polymer are very low and generally unsuitable for industrial applications. Silicones are no exception and although carbon black can be used for reinforcement, fine particle size fume silica is the usual choice for property enhancement. The incorporation of these highly surface-active silicas into silicone gums is a difficult process due to the rapid interaction between polymer and filler resulting in a pseudo-vulcanised mass. For this reason a variety of siloxane based filler treatments are generally used to control viscosity and other properties. 

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]. 

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. 

On the other hand, the alkoxide system presented several problems in formulation. The system first chosen as a model consisted of a trimethoxymethyl silane crosslinker, 8000 centistoke HEB siloxane, and a catalyst. A number of catalysts were used and each exhibited different cure rates and electrical properties. DuPont tetraalkoxytitante-Tyzor appears to he one of the better catalysts used in this type of curing system. Fillers are usually incorporated into the silicone formulation to improve mechanical properties, promote adhesion, and to serve as light screening and pigment agents. Cab-o-sil, a form of fumed silica, carbon-black, titanium dioxide and calcium carbonate are then used as RTV fillers. 

Abstract Plasma polymerization is a technique for modifying the surface characteristics of fillers and curatives for rubber from essentially polar to nonpolar. Acetylene, thiophene, and pyrrole are employed to modify silica and carbon black reinforcing fillers. Silica is easy to modify because its surface contains siloxane and silanol species. On carbon black, only a limited amount of plasma deposition takes place, due to its nonreactive nature. Oxidized gas blacks, with larger oxygen functionality, and particularly carbon black left over from fullerene production, show substantial plasma deposition. Also, carbon/silica dual-phase fillers react well because the silica content is reactive. Elemental sulfur, the well-known vulcanization agent for rubbers, can also be modified reasonably well. 

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. 

Compression set is an important property of elastomers which is affected by the choice of filler. Studies were conducted on silica in silicon rubber vulcanizates. Figure 8.60 shows the relationship between the surface area of silica and compression set. As the surface area increases compression set increases. The increase surface area contributes to an increase in the number of functional groups on the surface of silica. These groups can potentially react with siloxane. When they do, there is a good interaction of filler with matrix which contributes to reduction of compression set (Figure 8.61). ... 

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