Applications of polysiloxanes

It is quite difficult to enumerate all the possible applications of polysiloxanes. The reader is directed to other sources that are devoted to the technology and applications of polysiloxanes [1-2, 4-5]. The following serves to acquaint the reader with the large range of applications that these polymers possess. 

Conventional utility of silicone oils has been commented upon already. Thus, these possess a low VTC and this enables their utility as hydraulic fluids for machinery that operate at very low temperatures. Silicone oils are used routinely as heat-transfer fluids and dielectric insulating materials. 

The low surface tension of silicone oils have allowed their use as antifoam agents in various applications such as textile dyeing, fermentation, fruit juice processing, antacid tablets and so on. 

Silicone elastomers have exceptional stability to both high as well as low temperatures. Thus, silicone rubbers are quite flexible at temperatures as low as -60 to -70 °C. They do not seem to suffer much degradation up to 300 °C. Silicone resins find electrical and non electrical applications. Thus, these are used in the insulation of electrical equipment and for laminating printed circuit boards. 

The biocompatibility and low toxicity of polysiloxanes have allowed their utility in artificial body organs, artificial skin, soft-contact lenses etc. 

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One of the most impressive applications of polysiloxanes, particularly in the arts, is their use in making molds of intricate surfaces. This is illustrated in Figure 4.18. In this case, the surface to be copied was vertical and non-movable, so a non-slumping end-linkable paste had to be used rather than a liquid. After the surface was coated, it was cured to give a remarkably faithful reproduction that was easily removable from the original surface. 

Knowledge of the polymerization and copolymerization of cyclosiloxanes and the structure of the polymers so formed is fundamental to understanding the structure-property relationships of polysiloxanes. Such knowledge in the areas of polymer stability, reactivity and surface activity is of prime importance in the industrial application of polysiloxanes. 

Several of the papers in this volume focus not only on the synthesis of new materials but also on properties and applications. The applications of polysiloxanes as photoresists (van de Grampel, Chapter 8), polyphosphazenes for microencapsulation of biologically active species (Allcock, Chapter 17), and inorganic-organic hybrid oxopolymers with optical applications (Schrnidt, Chapter 15) are examples. New information is also presented on the photochemistry and photophysics (Hoyle, Chapter 25), the oxygen permeability (Kajiwara, Chapter 21), and a variety of optical properties of polyphosphazenes (Allcock). 

Hydrosilation reactions have been one of the earlier techniques utilized in the preparation of siloxane containing block copolymers 22,23). A major application of this method has been in the synthesis of polysiloxane-poly(alkylene oxide) block copolymers 23), which find extensive applications as emulsifiers and stabilizers, especially in the urethane foam formulations 23-43). These types of reactions are conducted between silane (Si H) terminated siloxane oligomers and olefinically terminated poly-(alkylene oxide) oligomers. Consequently the resulting system contains (Si—C) linkages between different segments. Earlier developments in the field have been reviewed 22, 23,43> Recently hydrosilation reactions have been used effectively by Ringsdorf 255) and Finkelmann 256) for the synthesis of various novel thermoplastic liquid crystalline copolymers where siloxanes have been utilized as flexible spacers. Introduction of flexible siloxanes also improved the processibility of these materials. 

Great Lakes has reported that functionalisation with graftable moieties results in a product which can be chemically bound to a polysiloxane backbone, e.g. Silanox MD. Functionalisation of polysiloxanes with HALS (polymer-bound HALS, P-HALS) and phenolic antioxidants has been described [22]. Functionalised polysiloxanes (Figure 3.23) exhibit high stabilisation activity in critical applications such as PP fibres and PE cables [58]. 

The TT-electron system-substituted organodisilanes such as aryl-, alkenyl-, and alkynyldisilanes are photoactive under ultraviolet irradiation, and their photochemical behavior has been extensively studied (1). However, much less interest has been shown in the photochemistry of polymers bearing TT-electron substituted disilanyl units (2-4). In this paper, we report the synthesis and photochemical behavior of polysiloxanes involving phenyl(trimethylsilyl)-siloxy units and silicon polymers in which the alternate arrangement of a disilanylene unit and a phenylene group is found regularly in the polymer backbone. We also describe lithographic applications of a double-layer system of the latter polymers. 

Although fabrics made from microfibres generally have a softer handle and better drape than those from conventional fibres, these properties can be further improved to a significant extent by the application of silicone softeners, the best results being obtained with aminofunctional polysiloxanes [491]. 

The unique surface characteristics of polysiloxanes mean that they are extensively used as surfactants. Silicone surfactants have been thoroughly studied and described in numerous articles. For an extensive, in-depth discussion of this subject, a recent chapter by Hill,476 and his introductory chapter in the monograph he later edited,477 are excellent references. In the latter monograph, many aspects of silicone surfactants are described in 12 chapters. In the introduction, Hill discusses the chemistry of silicone surfactants, surface activity, aggregation behavior of silicone surfactants in various media, and their key applications in polyurethane foam manufacture, in textile and fiber industry, in personal care, and in paint and coating industries. All this information (with 200 cited references) provides a broad background for the discussion of more specific issues covered in other chapters. Thus, surfactants based on silicone polyether co-polymers are surveyed.478 Novel siloxane surfactant structures,479 surface activity and aggregation phenomena,480 silicone surfactants application in the formation of polyurethane foam,481 foam control and... 

Research of biologically active silicone materials continues. The synthesis and characterization of polysiloxanes having bioactive pendant groups,556 557 and the preparation of bioactive porous organic-inorganic hybrids for medical applications,558 have been reported. 

Dvornic, P. R. Thermal Properties of Polysiloxanes. In Silicon-Containing Polymers. The Science and Technology of Their Synthesis and Applications, Jones, R. G., Ando, W., Chojnowski, J., Eds. Kluwer Dordrecht, 2000 pp 185-212. 

The applications of chiral polysiloxanes in enantiomer analysis in various fields of chemistry have been extensively reviewed6,124,128. Noteworthy is the use of Chirasil-Val (in both enantiomeric forms) for trace enantiomer analysis in the realm of EPC and enzymatic transformations31109. Enantiomeric impurities down to levels as low as <0,005% have been determined31185 (Figures 24a and 24b). 

Spectroscopic techniques are extremely useful for the characterization of filler surfaces treated with surfactants or coupling agents in order to modify interactions in composites. Such an analysis makes possible the study of the chemical composition of the interlayer, the determination of surface coverage and possible coupling of the filler and the polymer. This is especially important in the case of reactive coupling, since, for example, the application of organofunctional silanes may lead to a complicated polysiloxane interlayer of chemically and physically bonded molecules [65]. The description of the principles of the techniques can be found elsewhere [15,66-68], only their application possibilities are discussed here. 

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