Class ii includes oihs where the inorganic and

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Class II includes OIHs where the inorganic and organic components are connected by covalent bonds (as shown in Fig. 4 ). The support network is formed by condensation of chemical structural units (called gel precursors) where organic and inorganic components coexist. During the gel process, the addition of the precursor’s structure replicates a stable macromolecu- Commonly used variatons of the sol–gel processing Aqueous sol–gel chemistry Non–aqueous sol–gel chemistry Hydrolysis and condensation of metal alkoxides Organic–inorganic hybrids Polymer pyrolysis Colloidal dispersion Method 2 and 3 Method 1 Method 3 Method 1 and 3 Method 2 Method 1 Fig. 2: Different routes of the sol–gel process and their relation with the three systematic approaches proposed by Hench and West 14 Precursors Sol Gel Drying <100°C Xerogel Aerogel Monolith Fibers Films Fig. 1: Main steps to obtain materials by sol–gel process- ing (temperatures <100 ° C) (adapted from Hench and West 14 ) J. Coat. Technol. Res.
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lar network where these two basic components are chemically bonded. The sol–gel synthesis of OIHs class II based on siloxanes can be easily synthesized because Si–C sp 3 bonds are rather covalent and therefore quite stable toward attack by nucleophilic species such as water, alcohols, and hydroxylated ligands, among others. The precursors of these compounds are organo-substituted silicic acid esters of general formula R ¢ n Si(OR) 4 À n , bridged precursors of silesquioxanes X 3 Si–R ¢ –SiX 3 (X = Cl, Br, OR) in which R ¢ can be any organofunc- tional group and n is generally 1 or 2. Organic groups R ¢ may bind to an inorganic network with two distinct purposes, namely as network modifiers or as network formers . If R ¢ is a simple nonhydrolyzable group, it will have a network modifying effect. On the other hand, if R ¢ bears any reactive group that can, for instance, polymerize or co-polymerize (e.g., methacryloyl, epoxy, or styryl groups) or undergo hydrolysis-condensation (trialkoxysilyl groups), it will act as a network former. 16 The synthesis of this class of OIHs usually involves polycondensation reactions between di-, tri-, or poly- functional organosiloxanes and metal alkoxides (e.g., Si(OR) 4 , Ti(OR) 4 , Zr(OR) 4 , etc.). Polyfunctional alk- oxysilanes and metal alkoxides are effective crosslink- ers allowing for the production of OIHs materials with mechanical and thermal properties between those of polymers and glasses. The results of hydrolysis and polycondensation of these precursors are organo-poly- siloxanes that have properties such as hydrophobicity, corrosion protection, low dielectric constants, or good scratch resistance. 21 Due to the chemical and physical properties of the OIHs class II that are suitable to prevent corrosion/oxidation of the metallic substrates these are the main focus of this review.
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  • Fall '19
  • 1984, oIh, OIH coatings

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