Extensibility silicone rubber

Hydrophobicity loss and recovery in silicone rubbers has been extensively studied and reviewed in a few recent publications with numerous references.581,582 The ability to recover hydrophobicity after oxidation or contamination is considered as one of the important features determining the applicability of silicones in electrical and other outdoor insulation. The methods of studying of what is known as aging and recovery mechanisms are discussed. 

Sequence distribution studies on several types of rubber by 13C-NMR technique have been reported. Some of the more recent reports include silicone rubbers [28-30], SBR [31], acrylonitrile-butadiene rubber (NBR) [32,33], polyurethane [34,35], polyepichlorohydrin [36], ethylene-norbonene [37] and ethylene-propylene rubber [4, 16, 25, 38-44]. The NMR studies on EPDM have been carried out extensively, because it is one of the important parameters, which control the physical properties of the elastomer. For example, ethylene sequence can influence the crystallisation kinetic and melting behaviour of the rubber [38]. 

Table 1. Energy for first (A,) and second (A2) deformation to the extension ratio X = 2, and their difference (AA) as a function of temperature for cured, unfilled and filled silicon rubbers [43,46, 56] value for the defoimation energy is given in kJ mol" ...

Fig. 5. shows the technology progress of the extension fatigue durability of silicone rubber. At present, silicone rubber achieves a fatigue durability of about 50 million cycles by using improved dispersion uniformity of the reinforcing silica filler [12]... 

Silicone rubbers play a huge role in automotive applications due to their low temperature characteristics, thermal and chemical resistance, and general purpose use. From Table 8.1, we can see that the useful temperature range goes from -50 to 250°C. For automotive applications, this range covers all the operating system temperatures that a vehicle will see. Table 8.2 shows service life at continuous use for silicone rubbers. The time durations shown here are extensive for an elastomer and represent a great improvement over unsaturated carbon-based elastomers. 

With this determination of NkT for uniaxial compression, the obtained agreement between model and experiments over a very wide range of relative extension is shown in Fig. 6.7. Arruda and Boyce (1993) demonstrated equally good agreement for their other experiments on silicone rubber and gum rubber. 

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. ... 

Having mentioned the factors influencing the bond between the silicone rubber and the substrate and bearing in mind the vast number of possible material combinations the necessity of preliminary extensive tests for adhesion is obvious. 

Usually, sealants and adhesive materials for construction applications are evaluated by looking at the engineering side, butnotthe chemistry of the material. As a result, only tests that measure the mechanical properties are used. Most of the studies on the viscoelastic properties use traditional tests such as tensile testing to obtain data, which can be used in complicated mathematical equations to obtain information on the viscoelastic properties of a material. For example, Tock and co-workers studied the viscoelastic properties of stmctural silicone rubber sealants. According to the author, the behavior of silicone mbber materials subjected to uniaxial stress fields carmotbe predicted by classical mechanical theory which is based on linear stress-strain relationship. Nor do theories based on ideal elastomers concepts work well when extensions exceed... 

They presented a theoretical approach to predict the behavior of silicone rubber under uniaxial stress. The model is based on the concept of the classical Maxwell treatment of viscoelasticity and stress relaxation behavior, and the Hookean spring component was replaced by an ideal elastomer component. From the test data, the substitution permits the new model estimation of the cross-link density of the silicone elastomer and allows a stress level to be predicted as a complex function of extension, cross-link density, absolute temperature, and relaxation time. Tock and co-workersh" ] found quite good agreementbetweenthe experimental behavior based on the new viscoelastic model. By using dynamic mechanical analysis (DMA), the authors would have been able to obtain similar information on the silicone elastomer. 

In liquid mixtures of type (2), the solutions of primary interest are azeotropic and other mixtures containing variable amounts of water in organics dehydration of organic solvents containing very small amounts of water. Removal of water from azeotropic mixtures of ethanol-water, isopropanol-water, etc., is extensively practiced using polymeric membranes (of crosslinked polyvinyl alcohol) that are highly polar and selective for water. On the other hand, the membranes that are used to remove VOCs selectively from aqueous solutions are usually highly nonpolar rubbery polymeric membranes, e.g. dimethyl siloxane (silicone rubber). 

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