Page 220 - Handbook of Adhesion Promoters
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            trigger the osmotic process.  These phenomena are hindered or even prevented by the use
            of coatings having high adhesion to the substrate and low water transport kinetics; funda-
                                                 2
            mental is the cleaning and surface preparation.
                Elevated pressure (35 atm) changed the electrochemical behavior and deteriorated
            coating protectiveness by accelerating water absorption, increasing diffusion coefficient
            and changing diffusion type. The loss of wet adhesion, induced by blisters and corrosion
            products on the interface, became the main reason for coating failure and thus the key fac-
                                     3
            tor controlling coating lifetime.  Figure 3.2 shows the mechanism of failure of coatings
                            3
            under high pressure.
                Adhesion promoters, such as silanes are frequently used in coatings. In addition to
            the promoting adhesion, the films of γ-glycidoxypropyltrimethoxysilane formed on the
            galvanized steel diminished the corrosion current of the metal, but they did not have bar-
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            rier effect in the protection of substrate (polymer coating needed).
                The nanoPANI in the epoxy coating works as an adhesion promoter and corrosion
                   5
            inhibitor.  The PANI coatings prevented corrosion by two possible mechanisms, operating
            simultaneously; (a) improvement in barrier properties and (b) formation of a passive oxide
                                 5
            layer on the metal surface.
                The corrosion of steel protected with glass flake epoxy coatings with and without an
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            adhesion  promoter  was  examined  by  electrochemical  impedance  spectroscopy.  An
            improvement in the protective behavior of the coating due to the use of adhesion promoter
            is related to a large increase in the resistance in the pores of the coating because of an
            improvement in the initial, wet, and recovered adhesion between glass flakes and epoxy
                 6
            binder.
                In combination coating by titanate and silane, the existence of Ti–O–Si, Fe–O–Ti
                                               7
            and Fe–O–Si covalent linkages was found.  The three-dimensional network obtained by
            polycondensation adheres to the surface, forming stable covalent bonds during the densifi-
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            cation process but the coating is pervious to diffusion of water/ions.
                The coupling layer of phosphoric acid monoalkyl ester has been formed in the epoxy
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            coating and it has promoted the formation of a durable interphase.  The higher crosslink
            density in the epoxy network was indicated by the increased glass transition temperature
                                                                      8
            and the increased corrosion performance of the coated aluminum sample.
                Aminopropylphosphonic acid formed of densely packed monolayers on the alumin-
                       9
            ium substrate.  Formation of ionic bonds with the aluminum oxyhydroxide surface via
            acid/base interactions in bi-dentate conformation caused a strong inhibition of anodic de-
                          9
            adhesion process.
                The surfactants bearing a phosphonic acid head-group and polymer-reactive group
                                                                10
            were able to adsorb spontaneously onto the aluminium surface.  They formed oriented
                                  10
            layers on the metal surface.  The effects of the bifunctional surfactants were equivalent to
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            the chromated panels due to the corrosion inhibition and improved adhesive strength.
                The 4-amino-butyl-phosphonic acid used as the surface modifier showed a signifi-
                                                               11
            cant influence of pH on its efficiency as an adhesion promoter.  When the amino group
            was protonated at a pH=5.3, the molecule was attached to carbon steel at both ends with
                                                 11
            no  significant  improvement  in  performance.   At  pH=8,  the  molecule  had  a  greatly
            improved surface packing density with the amino group outwards from the surface in the
                             11
            preferred orientation.  In this condition, an epoxy coating demonstrated substantial resis-
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