Siloxane process

PEI, poly(carbonate-b-siloxane), processability and impact strength Giles White, 1983... 

There is also the siloxane process, which simply mixes the various additives (silane chemicals, cross-linking agents, antioxidants, catalyst, colorants, etc.) along with the polyethylene base resin using a high-intensity mixer prior to feeding the mixture into the pipe extruder. The siloxane process requires a special type of extrusion screw. 

The acid monolayers adsorb via physical forces [30] however, the interactions between the head group and the surface are very strong [29]. While chemisorption controls the SAMs created from alkylthiols or silanes, it is often preceded by a physical adsorption step [42]. This has been shown quantitatively by FTIR for siloxane polymers chemisorbing to alumina illustrated in Fig. XI-2. The fact that irreversible chemisorption is preceded by physical adsorption explains the utility of equilibrium adsorption models for these processes. 

Figure 12 shows an optional step at the end of the process ia which the siUca is chemically reacted with a siUcone oil, typically polydimethyl siloxane, to render the product hydrophobic. Other aftertreatments are also commonly employed, such as wax coatings that enhance the performance of precipitated siUcas used as flatting agents ia paints (78). 

The behavior of oligosiloxanediols in the presence of strong bases is different. The contribution to the overall process of the disproportionation reaction, involving a migration of the ultimate siloxane unit between siloxane molecules, is much greater and may even completely dominate the polycondensation reaction (80). The reactivity enhancement of the siloxane bond adjacent to the sHanolate anion can be understood in terms of n(0) (7 (SiO) conjugation. 

The mechanism of anionic polymerization of cyclosiloxanes has been the subject of several studies (96,97). The first kinetic analysis in this area was carried out in the early 1950s (98). In the general scheme of this process, the propagation/depropagation step involves the nucleophilic attack of the silanolate anion on the sUicon, which results in the cleavage of the siloxane bond and formation of the new silanolate active center (eq. 17). 

SiHcone mbber has a three-dimensional network stmcture caused by cross-linking of polydimethyl siloxane chains. Three reaction types are predominantiy employed for the formation of siHcone networks (155) peroxide-induced free-radical processes, hydrosdylation addition cure, and condensation cure. SiHcones have also been cross-linked using radiation to produce free radicals or to induce photoinitiated reactions. 

Silicones, an important item of commerce, are widely available commercially (9,494). The principal manufacturers of silicone operate direct-process reactors to produce dimethyl dichi orosilane and, ultimately, polydimethyl siloxane. Typical plants produce more than 450 t per year. The siUcone industry is a global enterprise in the 1990s, with principal producers in the United States (Dow Coming, GE, and OSi), Europe (Wacker Chemie, Hbls, Rhc ne-Poulenc, and Bayer), and Southeast Asia (Shin-Etsu, Toshiba SiUcones, and Dow Coming, Japan). Table 15 Hsts the approximate sales of the principal producers for 1991. 

The third process for cross-linking is the Sioplas process developed by Dow. The first stage of this involves the grafting of an easily hydrolysable trialkoxyvinylsilane onto the polyethylene chain, the site activation having been achieved with the aid of a small amount of peroxide. The compound is then extruded onto the wire, which is collected on a drum. The drum is then exposed to hot water, or, more commonly, low-pressure steam. The water hydrolyses the alkoxy groups, which then condense to form a siloxane cross-link. ... 

Surface active agents are important components of foam formulations. They decrease the surface tension of the system and facilitate the dispersion of water in the hydrophobic resin. In addition they can aid nucleation, stabilise the foam and control cell structure. A wide range of such agents, both ionic and non-ionic, has been used at various times but the success of the one-shot process has been due in no small measure to the development of the water-soluble polyether siloxanes. These are either block or graft copolymers of a polydimethylsiloxane with a polyalkylene oxide (the latter usually an ethylene oxide-propylene oxide copolymer). Since these materials are susceptible to hydrolysis they should be used within a few days of mixing with water. 

The competition at 200°C between an aliphatic and an aromatic amine toward the formation of an imide is a very selective reaction (Fig. 5.1) for the formation of an aliphatic imide.141 This reaction suggests that the reactive processing in extruder, for example, could be used to transform a melt-processable polyimide with an oligomer end capped with an aliphatic amine. In order to get a perfecdy alternate block polyimide-block siloxane, Rogers et al. used low-temperature transimidization.142 An oligomeric aromatic imide was end capped... 

Development of several new siloxane-imide copolymers for commercial applications have also been reported by Lee 181) and Berger58). Although no information was given in terms of the chemical compositions of these materials, most of these polymers were reported to be processable by solution or melt processing techniques, most probably due to their high siloxane contents. However, due to the presence of low (—20 to —120 °C) and high (>230 °C) temperature Tg s, it was clear that multiphase copolymers have been synthesized. Molecular weights and thermal stabilities, etc, were not reported. 

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. 

---