Silicone rubber swelling

The methyl silicone rubbers swell substantially in hydrocarbon oils and solvents. The introduction of cyanoethyl groups is beneficial in improving the resistance to swelling in these materials but the polymers have not achieved technical significance. Of greater importance are the fluorosilicones such as the rubbers containing the structure ... 

Silicone rubber-hydrogel composite is a two-phase system that is capable of swelling in water. The hydrogels prepared have different chemical compositions, size and shape of particles, and correspondingly different specific surfaces. It was found that the mechanical properties of silicone rubber-hydrogel composites depend mostly on the magnitude of the contact surface of both phases. ... 

FIG. 8 Evolution of the contact angle, 0(f7), of TCP on silicone rubber (RTV 630, General Electric Co.), (a) preswollen by TCP, (b) silicone rubber, and (c) on silicone rubber after allowing for swelling effects. 

Equation (2.79) expresses the driving force in pervaporation in terms of the vapor pressure. The driving force could equally well have been expressed in terms of concentration differences, as in Equation (2.83). However, in practice, the vapor pressure expression provides much more useful results and clearly shows the connection between pervaporation and gas separation, Equation (2.60). Also, the gas phase coefficient, is much less dependent on temperature than P L. The reliability of Equation (2.79) has been amply demonstrated experimentally [17,18], Figure 2.13, for example, shows data for the pervaporation of water as a function of permeate pressure. As the permeate pressure (p,e) increases, the water flux falls, reaching zero flux when the permeate pressure is equal to the feed-liquid vapor pressure (pIsal) at the temperature of the experiment. The straight lines in Figure 2.13 indicate that the permeability coefficient d f ) of water in silicone rubber is constant, as expected in this and similar systems in which the membrane material is a rubbery polymer and the permeant swells the polymer only moderately. 

Helix-electrodes contact peripheral nerves on a small area due to their open structure. They consist of a silicone rubber substrate with one or two embedded platinum wire electrodes (Fig. 1). They were developed at the Huntington Medical Research Institute (HMRI) in a monopolar and a bipolar version [17]. The latest versions have been applied in paralyzed people at the sacral spinal roots for micturition control and incontinence. Their advantage can be seen in the self-sizing properties due to the spiral structures when the nerve is affected by edematous swelling after the implantation trauma. 

Fig. 23. Changes of crosslinking density n,. in netformation from a low-molecular-weight silicone rubber at 20 °C, calculated from the elastic modulus (1) and equilibrium swelling in toluene (2) [46]...

Similarly, McGinnity has shown that incorporation of hygroscopic carriers such as sodium alginate into silicone rubber based matrix devices greatly inaeased the rate of morphine sulfate release and changed its temporal character to approximate more nearly a zero-order pattern. It was noted that inclusion of a carrier caused the device to swell considerably because of water imbibition and no doubt a complex series of events account for the effect on release rate. 

Silicones are resistant to water between 0°C and 100°C. Steam, at higher temperatures in prolonged attack, destroys the system. Normally, silicone rubbers are resistant to salt solution and to dilute solutions of acids and bases. However, they are not resistant to organic solvents. In this case a reversible swelling takes place. 

Fig. 7 Swelling of rubber samples in organic solvents measured in the Diamond TMA. NR, natural rubber NBR, nitrile butyl rubber SiR, silicon rubber HR, isobutyl rubber. (Courtesy of the PerkinElmer Instruments, Norwalk, CT.)...

Container integrity can be a problem with certain samples. The Teflon surface of the septum on the 40-mL VOA vial serves as a barrier between hydrocarbons and the soft silicone rubber of the septum. Just a few punctures of this barrier by a syringe needle may allow the vapor to swell the septum. Both light crude oil and condensate samples can swell a septum to a point where it splits and releases material. These problems of container integrity and the volatility of both the matrix and the analyte make it a challenge to find a suitable Statistical Quality Control (SQC) sample for this test. 

Ethylene-acrylic elastomers exhibit very good resistance to hot oils and hydrocarbon- or glycol-based proprietary lubricants and to transmission and power steering fluids. The swelling characteristics of E A will be retained better than those of the silicone rubbers after oil immersion. 

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