Silane coupling agents find applications as adhesion promoters for composites and coatings. Silanes have the ability to form covalent bonds with inorganic substrates and epoxy resins. Most commonly, epoxycyclohexyl- and glycidoxy-functional silanes are used to pretreat fillers or are blended with the epoxy resin. Amine-functional silanes can likewise be used to pretreat the filler or blended with the hardener component of two-part systems. Treatment of fillers in epoxy adhesives improves dispersibility, increases mechanical properties, and improves humidity resistance.
Single-component liquid cure epoxy adhesives and coatings employ dimethylbutylidene-blocked aminosilanes. These materials show excellent storage stability in resin systems, but are activated by moisture provided by water adsorbed on substrate surfaces or from humidity. Deblocking begins in minutes and is generally complete within two hours in sections with a diffusional thickness of < 1 mm.
Primer coatings for metal substrates utilize dipodal silanes to improve wet adhesion. Comparative results for the addition of a non-functional dipodal silane (SIB1817.0 bis(triethoxysilyl)ethane) in an EVA system are shown below. Epoxy systems use non-functional dipodal silanes in conjunction with epoxysilanes. Functional dipodal silanes such as SIB1833.0 bis(triethoxysilylpropyl)amine are used with aminosilanes.
Effect of Dipodal Silane on Bond Strength
|Primer on metal
(10% in isopropanol)
|Wet adhesion to titanium (N/cm)
|Wet adhesion to cold-rolled steel(N/cm)
|methacryloxypropylsilane + 10% dipodal
|28.0 (cohesive failure)
P. Pape et al, in Silanes and Other Coupling Agents, ed. K. Mittal, 1992, VSP, p105
In low moisture conditions, stable mixtures of cyclic azasilanes and compounds or polymers containing epoxy groups can be formed. When exposed to moisture, there is a high speed ring opening of the cyclic azasilane, which deprotects amine functionality. The amine functionality can react with epoxy functionality, forming hybrid siloxane/silsesquioxane-epoxy materials. Examples include a moisture-cure epoxy system with bisphenol A and N-n-butyl-aza-2,2-dimethoxysilacyclopentane (SIB1932.4). Moisture-initiated chain extension was demonstrated with a difunctional epoxy siloxane (DMS-E09) and N-methyl-aza-2,2,4-trimethylsilacyclopentane (SIM6501.4).
Silane Coupling Agents for Epoxy Resins Selection Chart
|Coupling agent class
|Suggestions for primary screening
|Epoxy (linear aliphatic or bisphenol A)
|Epoxy, UV cure (cycloaliphatic)