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Adhesion of organic coatings II

Adhesion of organic coatings II

Impurities at the surface of cleaned metallic substrates are composed of organic molecules or water.

A good chemical bond between a coating and the substrate can only result if the functional groups of the polymer can build a close network of bonds with the OH groups at the surface of the substrate.

Because the position taken by the impurities is the same as the functional groups of the polymer the bonding strength diminishes with increasing impurity density at the surface. Part of the positions will be taken by the functional groups by displacement, but a greater part remains well anchored.  

In this figure (from Ph.D Thesis J. van den Brand, TU Delft 2004) it can be seen that the lowering of the bondingscapacity of an aluminium substrate for a carboxylic acid as a function of the “ageing” is more severe in a contaminated atmosphere. 

The conclusion is obvious: after surface treament coating should take pace immediately. Only then an optimal adhesion can be obtained.