Understanding Polymer Backbone Composition Variables for Corrosion Performance and Failure

Diglycidyl ether of bisphenol-A (DGEBA) based polyepoxides are widely used in corrosion resistant coatings. Three different molecular weight DGEBA-based building blocks (Epon^® 828, 1001 and 1007) were reacted with stoichiometric proportions of Polylink 4200 [4,4’-methylenebis (N-sec-butyl aniline)] to result in a series of high molecular weight thermoplastic coating polyepoxides with varying nitrogen content.  The thermoplastic materials allow for molecular weight versus exposure to be characterized regularly unlike the traditional thermoset systems.  In order to establish a baseline of knowledge the model polymers were applied over Al-2024 panels and subjected to accelerated weathering methods, i.e., salt spray test (ASTM B 117), UVB radiation following ASTM D 4587 and accelerated thermal degradation at a mild yet controllable 105°C.  The unexposed and exposed polyepoxides were characterized via FTIR, GPC and DSC.  The thermoplastic material characterization confirm several key gains in understating, 1) both UV and thermal exposure protocols resulted in quantifiable differences in rates of change that correlate directly with the varying level of nitrogen in the polymer backbone.  However, corrosion specific exposure conditions (ASTM B 117), did not result in molecular weight changes regardless of backbone composition.  The results provide strong support that polyepoxides (albeit thermoplastic) exhibit no detectable chain scission events during corrosion exposure conditions contrary to frequent literature citations suggesting bond breakage during corrosion induced failure.