Silicone rubbers developments

Silicone rubber, developed by Dow Corning company, is one of the few polymers developed for medical use. The repeating unit is dimethyl sUoxane which is polymerized by a condensation polymerization. Low molecular weight polymers have low viscosity and can be cross-linked to make a higher molecular weight, rubber-like material. Medical grade silicone rubbers contain stannous octate as a catalyst and can be mixed with a base polymer at the time of implant fabrication. 

Fig. 1.32 Insulating material evolution -from silicone rubber developed in 1958 to the latest copolymer used for the first time in 2006...

The early 1980s saw considerable interest in a new form of silicone materials, namely the liquid silicone mbbers. These may be considered as a development from the addition-cured RTV silicone rubbers but with a better pot life and improved physical properties, including heat stability similar to that of conventional peroxide-cured elastomers. The ability to process such liquid raw materials leads to a number of economic benefits such as lower production costs, increased ouput and reduced capital investment compared with more conventional rubbers. Liquid silicone rubbers are low-viscosity materials which range from a flow consistency to a paste consistency. They are usually supplied as a two-pack system which requires simple blending before use. The materials cure rapidly above 110°C and when injection moulded at high temperatures (200-250°C) cure times as low as a few seconds are possible for small parts. Because of the rapid mould filling, scorch is rarely a problem and, furthermore, post-curing is usually unnecessary. 

One of the important breakthroughs in the silicone PSA industry has been the development of new release liners based on fluorosilicones [116]. These new materials significantly improve the release characteristics of the silicone PSAs, something that has been difficult to achieve because of the high adhesion silicone PSAs show to low energy substrates, including silicone rubbers and liners. 

Silicone rubber as a shaft seal and backing material has a number of special applications. It can be used over a temperature range of —60°C to 260°C (—76°F to 500°F) in air or suitable fluids. Its abrasion resistance is good with hard shafts having a 0.000254 mm RMS surface finish. Commercial grades of silicone rubber are compatible with most industrial chemicals up to 260°C (500°F). In lubricating oils, the limiting temperature is 120°C (250°F), but special types have been developed for use up to 200°C (392°F). 

Development work by the US Navy has included attempts to use expls other than RDX and HMX viz, HNS and TACOT (Tetranitro-1,2,5,6-tetraazadibenzocyclooctatetrene) together with binders such as RTV silicon rubbers. As reported by Stott (Ref 43) such work was not fruitful because of the low deton vel and d of the developed compns... 

Many other heterogeneous electrodes have been developed based on, e.g., calcium oxalate or stearate in paraffin, barium sulphate in paraffin or silicone-rubber, bismuth phosphate or iron(III) phosphate in silicone-rubber, caesium dodecamolybdophosphate in silicone-rubber and amminenickel nitrate in phenol-formaldehyde resin39 these permit the determination, respectively, of Ca and oxalate, Ba and sulphate, Bi or Fe(HI) and phosphate, Cs, Ni and nitrate, etc. 

The first move in this direction was to improve the weatherability of impact-resistant polystyrene. Because polybutadiene, the most widely used rubber in impact-resistant polystyrene, is unsaturated, it is sensitive to photooxidation, and impact-resistant polystyrene is therefore not suitable for outdoor applications. A saturated rubber might be able to help here. In the ABS sector this has been successfully tried out with acrylate rubber (77) and EPDM (78, 79), and the latter has also been used in impact-resistant polystyrene (80, 81) This development has elicited satisfactory responses only in certain areas and more work still has to be done. For instance, attempts have been made to improve resistance to weathering by using silicone rubber (82 ). This approach is effective, but economic factors still stand in its way. Further impetus may also be expected from stabilizer research. Hindered secondary amines (83), to which considerable attention has recently been paid, are a first step in this direction. 

Schmid et al. used the same principle to develop sensors to be incorporated into FI systems for the determination of ascorbic acid in fruit juices [38] and that of lactic acid in dairy products [39]. The membrane used in both applications consisted of decacyclene dissolved in silicone rubber that was treated similarly as the membrane in glucose sensors (Fig. 3.4.B). The oxygen optrode was coated with a sheet of carbon black as optical insulation in order to protect it from ambient light or intrinsic sample fluorescence. Ascorbic acid oxidase or lactic acid oxidase was immobilized by adsorbing it onto carbon black and cross-linking it with glutaraldehyde. The FI system automatically buffered and diluted the food samples, thereby protecting the biosensor from a low pH and interferents. 

EnviroSep, Inc. (EnviroSep), has developed a thick-film absorption technology for the removal of volatile organic compounds (VOCs) from water. The technology uses a proprietary form of silicone rubber to absorb contaminants. The vendor claims the technology is effective for VOCs with less than 2% solubility in water and a boiling point of less than 200°C for the pure compound and is most efficient for use at sites with contaminant concentrations between 10 parts per million (ppm) and 2000 ppm. The technology is intended for aqueous waste streams. 

Recently developed blood oxygenators are disposable, used only once, and can be presterilized and coated with anticoagulant (e.g., heparin) when they are constructed. Normally, membranes with high gas permeabilities, such as silicone rubber membranes, are used. In the case of microporous membranes, which are also used widely, the membrane materials themselves are not gas permeable, but gas-liquid interfaces are formed in the pores of the membrane. The blood does not leak from the pores for at least several hours, due to its surface tension. Composite membranes consisting of microporous polypropylene and silicone rubber have also been developed. 

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