Page 105 - Handbook of Adhesion Promoters

98 Polymer Modification to Improve Adhesion

its adhesion to quartz, primarily through lower residual stress in the samples and increased
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energy dissipation on debonding.
The higher concentration of hydroxyl groups in the polymer does not prevent the
adhesion loss under wet conditions but makes it easier for the polymer to restore adhesion
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during drying, as it provides more anchoring sites to the metal surface.
The interactions at the cured epoxy resin/oxidized aluminum interface were acid-
base interactions formed between amino groups in 4,4'-diaminodiphenylmethane (used as
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curative of the epoxy system) and acidic sites on the oxidized aluminum.
Steel was covalently modified by graphene oxide nanosheets reacted with 3-amino-
propyltriethoxysilane to improve the adhesion and corrosion protection properties of an
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epoxy coating. The graphene oxide film inserted −NH groups onto the surface of
2
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modified steel (Figure 6.3). The epoxide groups formed strong covalent bonds with the
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−NH groups that exist on the surface of the modified steel. The high ionic resistance of
2
the coating restricts the access of chloride ions to the steel surface and sodium cations to
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cathodic regions. The deposition of graphene oxide film on steel surface improved the
adhesion strength and corrosion protection and reduced the cathodic delamination rate of
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the epoxy coating.
Current literature on epoxy coating indicates that epoxy resins are not resistant to
moisture and, therefore, interfaces require reinforcement with adhesion promoters using
one of the systems included in the above discussion.
6.5 SILANE GRAFTING
High-density polyethylene surface modified by radio-frequency discharge plasma was
subsequently grafted by alkoxysilane to form a new surface containing polar functional
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groups. The reaction of the polar groups with vinyl silanes significantly improved
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hydrophilicity of the polymer and its adhesion.
Isotactic copolymers of propene and dienes were synthesized through Ziegler-Natta
catalysis and functionalized with silanes through Speier catalysis, either at one end of lin-
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ear chains or at the end of short ramifications. Silane functions can create chemical
bonds with glass fibers and the structure of the dangling chains allows cocrystallization
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with the polypropylene matrix. The increase of surface energy with the silane content
shows that only part of the silane functions is grafted on glass, the others are oriented
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toward the free surface.
The ethylene-octene copolymer matrix was functionalized via silane grafting, using
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monofunctional vinyltriethylsilane or bifunctional vinyltriethoxysilane agents. Two
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types of silica were used, unmodified and modified with octylsilane. The formation of
covalent bonds between the vinyltriethoxysilane functionality and the hydroxyl groups
present on the surface of the particles, generated strong polymer/filler interactions, result-
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ing in improved filler dispersion.
Ultra-thin layers of 7-octenyltrimethoxysilane and its mixtures with n-octylt-
rimethoxysilane have been grafted as adhesion primers for silicone coatings on the
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mechanically polished aluminum alloy. The 7-octenyltrimethoxysilane improved adhe-
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sion of polymer films but its mixtures did not. This is in relation to the number of C=C
functions ready to enter into a cooperative hydrosilylation reaction involving the primer
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and the forming silicone network.