Inorganic sols not only strengthen adhesion on metal

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Inorganic sols not only strengthen adhesion on metal substrate, but also improve comprehensive performances of polymer. The film prepared from neat phenolic resole is not suitable for being coating due to its high crackforming J Sol-Gel Sci Technol (2010) 54:174–187 175 123
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potential and poor mechanic strength. Phenolic resole hybrid coatings prepared from silica sol derived from TEOS, with up to 45 wt% silica-contents, are crack-free. Moreover, hardness and resistance to fire of the hybrid coatings are remarkably improved [ 16 ]. It was shown in our previous work that the change of functional groups of silica/phenolic resole hybrid coatings by FTIR is investigated, the hybrid coatings and neat phenolic resole coatings are cured in the same conditions (120 ° 2 h), there are more hydroxyl groups, ether bonds in the hybrid material than that in phenolic resin [ 17 ]. These functional groups in the hybrid coatings provide excellent mechanic strength and good adhesion between metals and hybrid coatings. As undercoat, the hybrid coatings have enough functional groups to react with those of top coat so that the multiple coatings cure without interface between undercoat and topcoat. The top coat prepared from di- glycidyl ether of bisphenol A crosslinked with N-(2-ami- noethyl)-3-aminopropyltrimethoxysilane is applied on the undercoat based on 2-hydroxy-ethylmethacrylate, 3-methacryloxypropyltrimethoxysilane, and tetraethoxysi- lane. The top layer provided silanol groups to react with the methoxysilane groups and methacrylate groups of the undercoat layer to produce covalent bonds during the polymerization [ 18 ]. The introduction of inorganic sol into other polymers to improve the hybrid coatings is discussed here briefly. Hybrid coatings consisting of PMMA and SiO 2 sol are synthesized by using 3-(Trimethoxysilyl)propyl methacry- late as a coupling agent. The coatings with low silica loading (e.g., 5 wt%) on cold-rolled steel (CRS) coupons are found to be much superior in anticorrosion efficiency over those of neat PMMA, which is due to the increase of adhesion strength of hybrid sol–gel coatings on CRS cou- pons relative to neat PMMA [ 19 ]. Sol–gel/epoxy resin hybrid coatings also show enhanced mechanical strength such as hardness and abrasion resis- tance. The epoxy hybrid coatings exhibit good performance as the undercoat and have good anti-corrosion of metals even without corrosion inhibitors such as hexavalent chromates [ 20 ]. Hybrid coatings are prepared, using alkyds with sol–gel precursors, and SiO 2 /alkyd resins form Si–O–C bonds. The heat resistant property of the coatings is improved [ 21 ]. However, the sol–gel precursor was found to interact with the driers to hinder cure of the alkyd resin [ 22 ]. Therefore, when sol–gel precursors are introduced into other oxidizing coatings, the cure rate of the hybrid coatings is supposed to be slower.
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