The synthesis reaction involved usually results from

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The synthesis reaction involved usually results from the conjugation of two mechanisms, namely 12 , 13 , 16 : hydrolysis of metal alkoxides (step 1, Fig. 5 ) to produce hydroxyl groups in the presence of stoichiom- etric water (generally in the presence of acid or base catalyst) followed by polycondensation of the resulting hydroxyl groups and residual alkoxyl groups to form a three-dimensional network (step 2, Fig. 5 ). The first step is the formation of a covalent bond between the organic and inorganic components giving rise to the ‘‘seed’’ precursor molecule. The conversion of the precursors into OIH materials proceeds via the formation of siloxane (Si–O–Si) bonds. This process takes place by hydrolyzing monomeric tetrafunctional alkoxide precursors employing a mineral acid (e.g., HCl) or base (e.g., NH 3 ) as a catalyst. Figure 6 shows a schematic of a practical example including the generic steps in the preparation of a particular example of class II OIH material involving Fig. 3: Interactions established between the organic and inorganic components for OIHs class I. (a) organic molecules immobilized in an inorganic network ; (b) organic molecules embedded in an inorganic network followed by polymerization forming a semi-interpene- trating network; (c) simultaneous formation of two networks from organic molecules and inorganic precursors forming an interpenetrating network (adapted from Prado et al. 21 ) H 3 C H 3 C H 3 C R1 R1 M O O OH M O O OH M O OH Organic phase Inorganic phase (M represents a network–forming element such as Si, Ti, Zr, Al, B, etc. R1 is typically an alkyl group) Fig. 4: Organic–inorganic interactions for OIHs class II (adapted from Prado et al. 21 ) J. Coat. Technol. Res.
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the incorporation of a polyether oligomer within the gel matrix. 22 Class III (Fig. 7 ) includes OIHs that are based on the combination of both types of interactions assigned to classes I and II. An example of this kind of hybrid is the material obtained by an organic polymer contain- ing hydrolyzable alkoxylanes (SiOR) 3 and hydrogen acceptor groups such as carbonyls, amines, and imides. 21 Sol–gel coatings for metals Corrosion protection of metallic substrates has long been one of the key roles performed by organic coatings, one of the most cost-effective means of providing practical protection from corrosion to easily corrodible metallic structures and items. The produc- tion of eco-friendly sol–gel coatings to prevent corro- sion on metallic substrates was one of the emerging areas of application, competing with conventional chromate and phosphate CCCs. Corrosion protective coatings are not just a barrier layer between the object and its environment. They should act to: protect small and local areas of exposed substrate and stop the spread of damage; limit the passage of current on the substrate; slow/inhibit oxygen (and other oxidants’) mobility toward the metal surface; minimize water and electrolyte penetration and release embedded inhibitor species that contribute to substrate passivation or block corrosion reac- tions.
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  • Fall '19
  • 1984, oIh, OIH coatings

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