Epoxy-siloxanes

Adherent films would not necessarily require formation of covalent bonds at the Interface, since localized lntermolecul u dispersion forces that u e operative In the adsorption of coatings (with good wetting properties) should provide stable interfacial bond conditions. Among candidate materials which could fulfill the requirements of good adhesion and substrate protection from moisture are epoxy-modified polyurethanes and epoxy-siloxane polymers. 

The epoxy/siloxane/PACM-20 mixture was poured into a hot (120 °C) RTV-silicone mold of the precise shapes to be used for solid-state testing. The mixture was cured at 160 °C for 2.5 hours. The curing time and temperature chosen were considered to provide enough mobility for network formation. This conclusion was partially based on earlier studies which found a glass transition temperature of 150 °C for Epon 828/PACM-20 3S). 

Sun et al. reported a significant accelerating effect by free-radicals on the polymerization of cycloaliphatic epoxy siloxanes in the presence of diaryliodonium salts. The optimum ratio of a free-radical initiator to the iodonium salt being 1 2. Some commercially available onium slats are listed in Table 2.10. 

Hou, S. S. Chung, Y. P. Chan, C. K. Kup, C. P. Function and performance of silicone copolymer. Part IV. Curing behavior and characterization of epoxy-siloxane copolymers blended with diglycidyl ether of bisphenol-A. Polymer, 2000, 41(9), 3263. Arends, C. B. Polymer Toughening, New-York, Marcel Dekker 1996. 

H. S. Barud, J. M. A. Caiut, J. Dexpert-Ghys, Y. Messaddeq, and S. J. L. Ribeiro, Transparent bacterial cellulose-boehmite-epoxi-siloxane nanocomposites. Composites A 43, 973-977 (2012). 

Matrix system Epoxy/ siloxane composition curatives Moisture absorption (%) Heat distortion temperature C Glass transition temperature C LOI... 

Silane coupling agents may contribute hydrophilic properties to the interface, especially when amino functional silanes, such as epoxies and urethane silanes, are used as primers for reactive polymers. The primer may supply much more amine functionality than can possibly react with the resin at the interphase. Those amines that could not react are hydrophilic and, therefore, responsible for the poor water resistance of bonds. An effective way to use hydrophilic silanes is to blend them with hydrophobic silanes such as phenyltrimethoxysilane. Mixed siloxane primers also have an improved thermal stability, which is typical for aromatic silicones [42]. 

Figure 7.38 Epoxy structures (1) bisphenol-A-based epoxy, (2) brominated bisphenol-A-based epoxy, and (3) siloxane epoxy.

Poly(unsaturated ester)-siloxane segmented copolymers have been prepared by the polycondensation of epoxy-terminated polydimethylsiloxanes and carboxy-terminated poly(ethylene adipate-co-maleate) oligomers 243). Reactions have been conducted in cellosolve solvent, at 140-150 °C, in the presence of 2% by weight potassium hydroxide catalyst. The molecular weights reported were fairly low. The same group has also prepared poly(hexamethylene adipate)-polydimethylsiloxane copolymers con-... 

In addition to its higher reactivity towards epoxy groups, another advantage provided by the reaction of ethylpiperazine terminated siloxanes with Epon Resin 828 was... 

Schematic Representation of Siloxane Modification of Epoxy Resins 69n5)...

After these preliminary studies, McGrath and co-workers have used ethylpiperazine terminated siloxane oligomers with varying molecular weights and backbone compositions throughout their studies 69 114,11S). Modifications of epoxy resins with siloxane oligomers were performed in two steps as depicted in Reaction Scheme XXIII. 

Siloxane containing interpenetrating networks (IPN) have also been synthesized and some properties were reported 59,354 356>. However, they have not received much attention. Preparation and characterization of IPNs based on PDMS-polystyrene 354), PDMS-poly(methyl methacrylate) 354), polysiloxane-epoxy systems 355) and PDMS-polyurethane 356) were described. These materials all displayed two-phase morphologies, but only minor improvements were obtained over the physical and mechanical properties of the parent materials. This may be due to the difficulties encountered in controlling the structure and morphology of these IPN systems. Siloxane modified polyamide, polyester, polyolefin and various polyurethane based IPN materials are commercially available 59). Incorporation of siloxanes into these systems was reported to increase the hydrolytic stability, surface release, electrical properties of the base polymers and also to reduce the surface wear and friction due to the lubricating action of PDMS chains 59). 

Other reports on the morphology and mechanical behavior of organosiloxane containing copolymeric systems include polyurethanes 201 202), aliphatic 185, 86) and aromatic117,195> polyesters, polycarbonates 233 236>, polyhydroxyethers69,311, siloxane zwitterionomers 294 295) and epoxy networks 115>. All of these systems display two phase morphologies and composition dependent mechanical properties, as expected. 

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