Foamed polysiloxanes

Figure 3 3D computerized tomography (CT) image of a foamed polysiloxane incorporating a small admixture of carborane cages. 

Aging Complexities Induced by the Organotin Catalyst in Foamed Polysiloxane Elastomers... 

An important route to the production of foamed polysiloxane rubbers relies on the use of a low molecular weight silicone gum and on a series of tin catalysed cure and gas formation reactions. Normally, a dihydroxy terminated poly dimethylsiloxane containing a small amount of silica filler, together with a polyfunctional crosslinking agent (e.g. tetrapropylsilicate), is used as the starting material. 

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. 

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... 

Anti-foaming Agent 900 Dimethyl polysiloxane 1.5 25th p 18 648, p 31, 1980... 

Not all copolymers which are produced by step-growth processes are random in nature. Block copolymers are also of major interest. An example of the synthesis of elastic polyurethane fibers was given in Section 1.5.4. Block and graft copolymers of polysiloxane-poly(alkylenc ethers) with segments like5-9 are used as surfactants in the production of polyurethane foams with uniform cell sizes. 

The surfactants employed for polyurethane foams can be also used for preparing foamed composites. The surfactants include silicone surfactants, which consist of polysiloxane-polyoxyalkylene block-copolymers. 

The same reaction (11.4) is currently used to obtain silicon emulsifier for flexible and rigid PU foams, by the reaction of polydimethylsiloxane of relatively high MW (3000-5000 daltons or more) having several -Si-H groups in the main polysiloxanic chain and a propylene oxide (PO) - ethylene oxide (EO) copolymer, block or preferably random copolymers, having minimum 50% EO units (reaction 11.6). 

Weighed amounts of bio-polyols, catalyst (Stannous 2-ethyl hexanoate), surfaetant (polyether-modified polysiloxane), and blowing reagent (distilled water) were mixed well in a paper cup. A prescribed amoimt of MDI was added to the mixture, which was then stirred with a high-speed stirrer (5,000 rpm) for 15 s. After stirring, the mixture was poured immediately into another paper cup, and the foam was allowed to rise and set at ambient conditions (22 °C). Finally, the foam samples were cured at room temperature for 24 h before any analysis can be conducted. The formulations of the prepared foams are shown in Table 2. 

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