Stabilization of the Oxide Interface

Effect of silane coupling agents on the durability of steel joints immersed in water 

Kinloch further points out that, although silanes may increase the stability of the interface and enhance durability, the polysiloxane layer formed then becomes the weakest part of the joint. Water will presumably concentrate in this layer and the joint durability will depend on the hydrolytic stability of the various —Si—O—Si— and —Si—O—M— bonds present.  

The use of silane coupling agents has also been reported to improve the durability of 6A1-4V-Ti alloy joints bonded with an epoxy adhesive.  

As already mentioned, the weakest component of an aluminum joint is the oxide itself rather than the adhesive/oxide interface. The use of coupling agents would not be expected to provide an enhancement in durability under these circumstances. However, Brockmann has found that priming aluminum with 2,4,5,7-tetrahydroxyflavonene can substantially improve the durability of joints prepared with phenolic or nitrile-epoxy adhesives. This compound may be acting as a hydration inhibitor rather than as a coupling agent. 

DeNicola and Bell have reported attempts to improve durability of epoxy-bonded steel joints using coupling agents containing )8-diketone and polyfunctional mercaptoester chelating groups. The success of this approach again depends on the hydrolytic stability of the chemical bonds formed with the oxide surface. 

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