Filler silicone rubber

TGA traces for silicone rubber and its composites with 8.1 vol.% of filler (---) silicone rubber without filler (-) silicone rubber/organo-MMT... 

Room temperature vulcanising silicone rubbers (r. t. v. rubbers) have proved of considerable value where elaborate processing equipment is not available. These rubbers are low molecular weight silicones with reactive end-groups and loaded with reinforcing fillers. The RTV silicone rubbers may be classified into two types ... 

Calcium silicate, sodium aluminium silicate and treated varieties of aluminium silicate used as reinforcing fillers in rubber compounding. Silicone Rubber... 

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. 

The degree of moisture present affects the properties of the silicone rubber vulcanisate. Moisture levels also determine the ease with which the filler is incorporated into the silicone rubber. Low moisture levels improve the final physical properties but definitely detract from the incorporation speed of the silica filler. 

Zinc oxide (ZnO) is widely used as an active filler in rubber and as a weatherability improver in polyolefins and polyesters. Titanium dioxide (TiOj) is widely used as a white pigment and as a weatherability improver in many polymers. Ground barites (BaS04) yield x-ray-opaque plastics with controlled densities. The addition of finely divided calcined alumina or silicon carbide produces abrasive composites. Zirconia, zirconium silicate, and iron oxide, which have specific gravities greater than 4.5, are used to produce plastics with controlled high densities. 

Fig. 9. Dependence of the energy contribution on the filler (filled rubbers) or hard phase (thermo elastoplastics) content. 1 — filled silicone rubber1221 Sil-51 (A), Sil-4600 ( ) multiblock copolymer polyarylate-PDMS (O) us) graft copolymer of PDMS and AN ( x) 128>. 2 — Butyl rubber with high abrasion furnace black125). 3 — Butyl rubber with medium thermal black 125). 4 — SBR-filled rubber 126). 5 — aerosil //j Si-filled silicon rubber138). 6 — EPR-filled rubber 129,130). 7 — plastisized PVC filled with aerosil131132). 8 — SBS block copolymers 134)...

Graftcopolymerization onto silicone rubber is rather difficult to achieve and is often accompanied by unwanted changes in physico-mechanical properties of the polymer caused by initiating agents. To overcome the problem, silica was introduced into the rubber matrix as an active filler capable of binding cationic compounds such a cationic compound being y-aminopropyltriethoxysilane. Schematically the pathway for heparinization of the latter may be presented as follows ... 

Carbon microspheres yield syntactic foams with resistivities that are astonishingly low for these materials. Novolac syntactic foams with carbon microspheres have resistivities of 0.02-0.5 Ohm m (depending on the filler concentration)77 this is ten orders of magnitude lower than for glass microspheres in the same binder For materials made from carbon microspheres and silicone rubbers, the resistivity depends exponentially on the temperature, viz. 0.08 Ohm m at 20 °C, 0.2 Ohm m at 60 °C, and 200 Ohm m at 95 °C 1). Consequently, carbon microspheres make it possible to produce syntactic foams with electric properties appropriate for semiconductors. 

A low-resolution proton NMR method is one of the few techniques that have so far proved to be suitable for studying elastomer-filler interactions in carbon-black-filled conventional rubbers and silica-filled silicon rubbers [20, 62, 79]. It was pointed out by McBrierty and Kenny that Many of the basic characteristics of filled elastomers are revealed by low resolution spectra while the more sophisticated techniques and site specific information refine interpretations and clarify motional dynamics [79]. 

The rubber compounds prepared in this way are analysed and packed into polyethylene bags. Then they are loaded into metal cylinders or rubberised bags and sent to the consumer. Silicone rubber compounds can be stored for 3 to 6 months depending on the type of fillers and stabilisers. 

Fillers such as silica In silicone rubber have the same effect as crystallinity, reducing polymer motion by physical crosslinking and increasing the tortuosity of the diffusion path (14,15). 

Lopour P et al. (1993) Silicone rubber-hydrogel composites as polymeric biomaterials. IV Silicone matrix-hydrogel filler interaction and mechanical properties. Biomaterials 14(14) 1051—1055... 

Figure 12.2. MWNTs dispersions at 0.01 wt% in various solvents one week after their preparation. [Reproduced by permission of Kautschuk Gummi Kunststoffe from L. Bokobza and M. Rahmani "Carbon nanotubes Exceptional reinforcing fillers for silicone rubbers", KGK, 62,112,2009].

Methyl silicone rubber also shares the excellent electrical properties of the resins and oil. A molded sample with silica filler had a dielectric constant of 3.0 at room temperature over a range of 60 to 1010 cycles. The loss factor remains at 0.004 from 60 to 107 cycles and then rises rapidly to 0.037 at 109 cycles and 0.055 at 1010 cycles. At 102° C. the values remain the same except for a small decrease in dielectric constant (caused by a decrease in density) and a slight indication of enhanced d-c conductivity. The rubber does not seem to be affected by ozone. 

The surface of PDMS is hydrophobic which results in poor wettability with aqueous solvents and promotes non-specific protein adsorption. It is also relatively inert to chemical modification [25]. The liquid silicon rubber chosen for fabrication of the reaction plate contained pyrogenic silicic acid as a filler. Aside from its effect on elastomer properties the silicic acid can be expected to provide additional silanol... 

The high mechanical strength of natural and organic rubbers as used in tires is due to the incorporation of pyrogenic carbon blacks as active fillers. Elastomers of a more polar polymer backbone, such as polyacrylates, polyurethanes or polysulphides, require fillers of higher polarity. In particular the performance of polydimethylsiloxane elastomers (silicone rubber) is basically related to the addition of fumed silica. 

Stress-strain properties for unfilled and filled silicon rubbers are studied in the temperature range 150-473 K. In this range, the increase of the modulus with temperature is significantly lower than predicted by the simple statistical theory of rubber elasticity. A moderate increase of the modulus with increasing temperature can be explained by the decrease of the number of adsorption junctions in the elastomer matrix as well as by the decrease of the ability of filler particles to share deformation caused by a weakening of PDMS-Aerosil interactions at higher temperatures. 

Molecular mechanisms for stress-softening are also discussed. It is shown that this phenomenon is not related to the chain slippage or to a conversion of a "hard" adsorbed phase to a soft one. The obtained results assume that the stress-softening in silicon rubbers is caused by two possible reasons changes in the positions of filler particles relative to the direction of stretching at the first deformation and by a re-distribution of the topological hindrances. It is shown that the tensile strength at break as a fiinction of temperature is closely related to the chain dynamics at the elastomer-filler interface. 

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