Silica-filled silicon rubbers

The frequency of the chain motions of one of the two types of chain units, inferred from the minimum Tj around 195 K (-78 °C), is similar to that observed in unfilled PDMS. The mobility of the chain units of the other type is greatly hindered, resulting in a T1 minimum at higher temperature (at about 7 °C). This minimum becomes more pronounced in proportion to the total surface of filler particles in the mixture. It was suggested that the observed chain immobilisation is caused mainly by physical adsorption of PDMS chain units adjacent to the silica surface and to a lesser extent by entropy constraints at the silica surface [113]. 

The estimated thickness of the PDMS-silica interface is about 1 nm [105, 109, 110], which is comparable with that of carbon-black-filled conventional rubbers. The adsorption 

The relative number of immobilised adsorbed chain units in a single type of silica is proportional to Mn 0 5 [117]. The fraction of immobilised chain units is larger in hydroxyl-terminated PDMS [117]. This suggests that the majority of hydroxyl chain ends are linked to the silica surface due to the formation of double hydrogen bonds. 

When the temperature increases, both the fraction of less mobile, adsorbed chain units and the lifetime of the chain units in the adsorbed state decrease. The lifetime of PDMS chain units in the adsorbed state approaches zero at approximately -73 °C and 227 °C at the surface of hydrophobic and silylated silicas, respectively [108, 113]. 

The frequency of PDMS chain mobility in the interfacial layer is largely dependent on the type of silica surface. Water adsorption by the hydrophilic silicas causes the degree of chain immobilisation in the interface to decrease [107]. Sililation of the silica surface results in a significant decrease in the strength of adsorption interactions at the silica 

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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]. 

Shim, S. E. Isayev, A. I., Ultrasonic Devulcanization of Precipitated Silica-Filled Silicone Rubber. Rubber Chem. Technol. 2001,74,303-316. 

Currently, breast cancer is the most frequently diagnosed cancer in both white and African-American women. According to latest reports, of the 662,870 cancer cases reported for women in 2005, 32% were breast cancer. The incidence of breast cancer increased from 1 in 20 in 1960 to 1 in 8 today. More than 100,000 women per year require mastectomy for treatment, and every year 75% decide to have reconstmction. About half the number of these women select prostheses made of silica-reinforced silicone-rubber shell filled with silicone gel, while the other half have the same shell filled with physiological saline. Reportedly, gel-fiUed prostheses feel more natural, but are associated with true or perceived health problems and remain highly controversial. 

As in carbon-black-filled EPDM and NR rubbers, the physical network in silica-filled PDMS has a bimodal structure [61]. A loosely bound PDMS fraction has a high density of adsorption junctions and topological constraints. Extractable or free rubber does virtually not interact with the silica particles. It was found that the density of adsorption junctions and the strength of the adsorption interaction, which depends largely on the temperature and the type of silica surface, largely determine the modulus of elasticity and ultimate stress-strain properties of filled silicon rubbers [113]. 

Takala M, Ranta H, Nevalainen P, Pakonen P, Pelto J, Karttunen M, Virtanen S, Koivu V, Pettersson M, Sonerud B, Kannus K (2010) Dielectric properties and partial discharge endurance of polypropylene-silica nanocomposite. IEEE Trans Diel Electr Insul 17 1259-1267 Tanaka T, Kozako M, Fuse N, Ohki Y (2005) Proposal of a multi-core model for polymer nanocomposite dielectrics. IEEE Trans Diel Electr Insul 12 669-681 Vaughan AS, Swingler SG, Zhang Y (2006) Polyethylene nanodielectrics the influence of nanoclays on structure formation and dielectric breakdown. Trans lEE Jpn 126 1057-1063 Venkatesulu B, Thomas MJ (2010) Erosion resistance of alumina-filled silicone rubber nanocomposites. IEEE Trans Diel Electr fiisul 17 615-624 Weibull W (1951) A statistical distribution function of wide applicability. J Appl Mech Trans ASME 18 293-297... 

Silicone elastomers ( silicone rubber ) filled with silica have outstandir properties, and many commonly used products are made of these materials. Silicone rubbers filled with silica exhibit high mechanical strength and high chemical stability, and can be used at temperatures ranging fixnn -50 °Cto+300°C[l]. 

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). 

Organofunctional polysiloxanes with silanol groups can be cold cured with methyl triacetoxysilane, tetrabutyl titanate, etc. On the other hand, organofunctional polysiloxanes with about 0.2% vinyl groups are hot cured with peroxides. All these silicon rubbers are filled with highly dispersed silica since the unfilled rubber is practically useless as an elastomer. 

Commercial 50 Durometer Red Silicon Rubber (SI) filled 50% by volume with silica particles of irregular shape, typically 10 /xm in diameter, and very fine Fe203 powder. 

Natural mbber crystallizes on elongation—a phenomenon called strain-induced crystallization—what enhances mechanical properties. However, a filler in the form of carbon black is typically added to natural rubber to additionally modify the mechanical properties. Elastomers which cannot undergo strain-induced crystallization contain even more fillers. Carbon black is used in such cases also, but silicone rubbers are filled with silica. 

Thongsang et al. examined the properties of fly ash/precipitated silica-filled NR vulcanizates. NR used is grade STR20. The silica hybrid filler in this work consisted of mixtures of fly ash silica (FASi) and precipitated silica (PSi). The major component of FASi was silicon dioxide, at 33.5%. The sihca content of FASi added to the rubber compounds based on the fly ash chemical composition. Fly ash with the particle sizes of 25 mm and 45-74 mm were studied. The precipitated silica used was Hi-Sil 233-S which had a pH of 6.8, a bulk density of 0.235 g/cm, and a surface area of 128 m /g. The total sihca contents filled into the NR compounds were 10 and 40 phr for low and high total sihca loadings, respectively. The FASi PSi ratios used were 100 0, 75 25, 50 50, 25 75 and 0 100. 

Fig. 32 Silica cell for performing liquidjliquid electrochemical EPR experiments (a) interface between two immiscible liquids, (b), platinum wire counter electrode 1, (c) silver wire reference electrode 1, (d) platinum wire counter electrode 2, (e) silver wire reference electrode 2, (f) capillary to fill lower portion of cell, (g) thin-layer portion of cell, (h) Teflon /silicone rubber sleeves surrounding lower portions of reference electrodes,...

It has been shown that the viscoelastic losses of OH-terminated poly(dimethyl-siloxane), crosslinked with tetra-functional silicates, decreases with increased crosslink density. Furthermore, identical results were obtained when the polymer was crosslinked with y-irradiation, in bulk and in solution this indicates that there is no significant change in the number of interchain entanglements, and these are responsible for the observed losses. Vulcanization studies of poly(di-methylsiloxane)s, y-irradiated up to 500 Mrad, have shown linear correlation of the crosslink density with swelling, indentation and extension behaviour up to 160 Mrad, and exponentially for higher doses. Basic principles for the thermal stabilization of silicone rubbers, filled with carbon blacks and silica, have been discussed and a tentative stabilization mechanism put forward. ... 

Aranguren MI, Mora E, Macosko CW, Saam J (1994) Rheological and mechanical properties of filled rubber silica-silicone. Rubber Chem Technol 67 820... 

Figure 11.21 NMR images of a silicone rubber filled with silica obtained through two differing spatially resolved H echo techniques (a) is a spin-echo image and (b) is a gradient echo-derived image. Using both methods, agglomerations of silica filler particles (darker areas) are clearly visible. Reproduced with permission from [16] Copyright (1997) American Chemical Society.

Silicone rubber and, in general polar polymers, are by nature materials of choice for preparing silica filled systems however limited to niche applications, with respect to the range of properties that such specialty polymers may offer. In order to develop optimum reinforcing performance with more common diene elastomers, silica must be chemically treated as we will see below, because contrary to carbon blacks, silica particles do not develop spontaneous strong interactions with nonpolar polymers. It is nevertheless interesting to see that, even with comparable size and structure, pure silica does not affect the mechanical properties of vulcanized rubber compounds in the same manner as carbon black. 

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