The species and density of functional groups of sol

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The species and density of functional groups of sol particles affect anticorrosion efficiency of hybrid coatings. Duran et al. [ 23 ] has reviewed the sol composition, syn- thesis and processing conditions, deposition method, curing and structure–property-processing relationships of sol–gel coatings. Hybrid coatings were prepared by acidic hydro- lysis of GPS and titanium organic compounds in 2-propa- nol solution, and diketones used as complexing agents for controllable hydrolysis of titanium organics. The best anticorrosive protection of AA2024 in chloride solutions is provided by the films prepared with titanium tetrapropox- ide and acetylacetone as starting materials [ 24 ]. 3 Influence of organic components The introduction of organic groups or polymer into inor- ganic sol–gel coatings eliminates the drawbacks of inor- ganic coatings. The major drawbacks of inorganic sol–gel coatings from the standpoint of corrosion resistant layers are: (1) thick coatings ( [ 1 l m) are difficult to achieve without cracking [ 25 ]; (2) brittle inorganic films have high crack-forming potential [ 25 ]; (3) pure inorganic films have apertures [ 26 ]. These disadvantages make inorganic sol–gel films poor physical barriers that cannot provide adequate corrosion protection. There are two methods to reduce permeability of the inorganic films by organic components: one is defect healing action; another is introduction of organic compo- nents into inorganic sol–gel films. The defect healing action that inorganic coatings are covered by suitable organic alksilane reduces the perme- ability of inorganic coating and is often used for hybrid coatings with little organic component or inorganic films to eliminate defects. The oxygen transmission rate of the hybrid coating on poly(ethylene terephthalate) (PET) films substrate which replicates perception of permeability of the coating can demonstrate if the coating is dense and defect-free. The oxygen transmission rate of 48 nm thick SiO 2 coatings on a PET film is reduced by a factor of three after applying an 8 nm thick layer of oligomerized APS by reducing macro- defect and nano-defect of the film. The defect-healing effect of the amino-silane on the SiO 2 barrier coating leads to a twofold reduction of oxygen permeability [ 27 ]. 1 wt% aminosilane solution is applied on SiO 2 -coated PET films. The permeability of the silane–silica hybrid coating on PET is reduced twofold compared to that of the SiO 2 /PET film [ 28 ]. Densification of inorganic coating is also a kind of defect healing action. Sol–gel films treated by UV radiation at room temperature densificating the coatings show cor- rosion resistance of about 2.5 times higher than conven- tional films dried at 300 ° C [ 29 ]. The presence of organic components in sols forms dense films and reduces coating porosity, which makes the gel network more flexible and less prone to cracking during 176 J Sol-Gel Sci Technol (2010) 54:174–187 123
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further drying process [ 30 ]. Moreover, organic groups have low surface tension compared with inorganic oxides.
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