Silicon elastomer, reinforcement with silica

The graphical representation of o/VjAT(e - 1/e ) vs. leads to a straight line in the case of silicone elastomers reinforced with silica. The experimental results, reported by A.M. Bueche, are really located on a straight line, in the limits of performed measurements, i.e. 0 < v,< 0.25 and 1 < e < 1.5, respectively. Figure... 

A new approach to combating iodine deficiency through controlled release of iodide by a silicone elastomer has been reported (Fisch et ai, 1993). Silicone elastomers or silicone rubbers made from cross linked polymers are reinforced with silica. Silicones, being chemically inert, are safe and stable over a wide range of temperatures and have a high permeability to small molecules. Sodium iodide, being a small molecule, can be intercalated into the silicone matrix from where iodine is released in a controlled manner when water is passed through it. 

Compared to dimethyl silicone elastomers, vinyl silicone elastomers have much reduced compression set. A further point is that although silicone elastomers are mostly reinforced with silica, special purpose electrically conducting elastomers are filled with carbon black. Carbon black does not inhibit cure by less active peroxides and may be used with vinyl silicone elastomers. This filler does, however, inhibit vulcanization by benzoyl peroxide (and its derivatives) and therefore cannot be added to dimethyl silicone gums. 

Deng, Q., Hahn, J.R., Stasser, J., Preston, J.D. and Bums, G.T., Reinforcement of silicone elastomers with treated silica xerogels silica-silicone IPNs. Rubber Chem. Technol., 73(4), 647-665 (2000). 

The main chemical routes for vulcanisation of silicone elastomers are 1) Elevated temperatures cures and 2) Room temperature vulcanisation mechanisms. Organic peroxide cures are used in elevated temperature cures. Since the organic peroxides are inhibited by most carbon blacks, non black reinforcing fillers such as precipitated silicas, titanium dioxide and zinc oxide are used. Room temperature vulcanisation is normally used with low consistency silicone elastomers. 

Many of the silicone elastomers that are used in biomedical applications are produced by Dow Chemical Corp., under the trade name SILASTIC . For example, a typical medical-grade silicone (like SILASTIC MDX4-4210 Medical grade elastomer) contains, after curing, cross-linked drmethylsiloxane polymer and silica for reinforcement. Silcones are also reinforced with PET (Dacron) fiber meshes for certain biomedical applications. For implantable medical devices, it is important to realize that the cured polymer contains residual catalysts and silicone cross-linkers, which are necessary for the polymerization. 

High-consistency thermosetting medical grade silicone elastomer compounds are prepared from high molecular weight polydior-ganosiloxanes compounded with high-surface fumed silica (approximately 400 mVg). Silica is the only material known that adequately reinforces silicone elastomer. 

Dynamic mechanical thermal analysis of several of the norbornene functional organic resins and the silicone resins gave relatively unremarkable results. The maximum for tan 5 peaks were in good agreement with Tg detamiined by DSC. The silicone elastomer (with 35% fumed silica as reinforcing filler) exhibited a Tg of ca. -90°C and a Tm at ca. -30 C which is typical for this type of polymer. 

For example, in heat-cured mbber systems a common loading with synthetic silica is around 30%. For economic reasons, precipitated silicas are used in increasing volumes in silicone elastomers such as high-consistency mbber (HCR), liquid silicone mbber (LSR), and two-component room temperature vulcanized elastomers (RTV2). The reinforcement provided by precipitated silicas is only slightly lower than that provided by fumed silicas. However, fumed silicas are necessary in silicone sealants and other high-end applications because of their desirable properties like low moisture content. 

The traditional TPS for launcher fairings and re-entry capsules consists of an external ablative insulation, fixed or bonded onto a metallic primary structure. Ablative materials are based on thermosets (phenolic and epoxy resins) or elastomers (ethylene-propylene and silicone rubbers) usually filled and reinforced with cork, cotton, glass, silica, quartz, carbon, silicon carbide, nylon and aramid in the form of powders, fibres, fabrics and felt (Table 2). 

Synonyms Dimethyl silicones and siloxanes, reaction prods, with silica Uses Rheology control agent, reinforcing agent, free-flow agent for adhesives. coatings, powd. coatings, defoamers, elastomers, inks, lubricants. liq. resins, sealants, silicones, toners thixotrope for vinyl ester resins... 

Evidenced has been obtained previously that the reinforcing effect of different grades of fumed silicas on silicone elastomers is influenced by the surface fractality [1] and that the surface roughness increases with the specific surface energy. The aim of the present work is to demonstrate variations by calling on NMR and infrared spectroscopic methods, which are applied to fumed silica samples that have been carefully characterized through adsorption methods including IGC analysis. 

Deng, Q. Q. Hahn, J. R. Stasser, J. Preston, J. D. Burns, G. T., Reinforcement of Silicone Elastomers with Treated Silica Xerogels Silica-Silicone IPNs. 

Fumed silica has an ultimate particle size as small as 5 nm, which is finer than hydrated precipitated silica. With a conventional Banbury mixer, a filler this fine would be very difficult to disperse in a general-purpose elastomer compound. Fumed silica can be dispersed using a dough mixer in a gum silicone to achieve the ultimate reinforcement improvement. Fumed silica costs more to produce than hydrated precipitated silica. 

The work previously reported on the reinforcement of PDMS by in situ generated silica is essentially related to silicone elastomers formed by the use of functionally-terminated PDMS fluids that undergo catalyzed cross-linking reactions. Typically, silica precipitation can be carried out before or after or alternatively during curing [42]. In the third procedure, hydroxyl-terminated PDMS chains are blended with enough TEOS that will simultaneously end-link the precursor chains and provide the silica particles. 

Properties of Silicone Rubbers. Gum vulcanizates have essentially no tensile strength. Fillers are therefore essential. Reinforcing silicas are frequently used. Even then, the tensile properties and abrasion resistance of silicone rubber vulcanizates are poor in comparison with of other types of elastomer. However, the properties change very little with increasing temperatures. These polymers excel in high-temperature applications. 

Amorphous silica lacks a definite shape and an orderly atomic latticework. Types of amorphous silica include glass, vitreous glass fiber, large flocculated silica gel, diatomaceous earth, and opaline phase-fused silica in granular and powder form. These products provide high purity, reinforcement, great surface area, flatting properties, and thixotropy. Fused silica has an extremely low coefficient of linear thermal expansion and has become quite popular, especially in electronics applications. The amorphous silicas are compatible with thermosets, thermoplastics, silicone rubber, and other elastomers. 

Microamorphous silica is similar to amorphous silica except that particles measure in the submicrometer range. Examples of microamorphous silica are fumed silica and precipitated silica. These are used when high surface area, high purity, high gloss, and superior reinforcement properties are needed. They are compatible with thermosets, thermoplastics, silicone rubber, and other elastomers. Fumed silica is often used to provide thixotropy in resin mixtures. 

As reported by Zelikin et al. (571), hydrophobic pyrogenic silicas were used as reinforcing Tillers in silicone rubber giving a tensile strenjgth of 71 kg cm and 460% elongation at break. However, the reinforcement of this type of elastomer is now probably considerably advanced by new techniques of introducing particulate silica with optimum characteristics. 

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