Abstract
The migration and segregation of a minor silicone containing additive in a multilayer, organic coating system has been investigated by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The silicone containing additive employed was the most compatible thermally stable, polyester modified poly(dimethyl siloxane) (PDMS) flow agent. A polyester/polyurethane (PU) based primer and a poly(vinylidene difluoride) (PVdF) based topcoat on an aluminium substrate were used as a model, multilayer, organic coating system. XPS and SIMS characterisation of the PU primer formulation (with and without addition of the PDMS based flow agent), confirmed that the PDMS based flow agent segregated to the PU primers air/coating surface. Characterisation of the PVdF topcoats air/coating surface, after application and curing over the PU primers, revealed the presence of the PDMS based flow agent at the PVdF air/coating surface when the topcoat was applied to the PU primer containing the PDMS based flow agent. Ultra-low-angle microtomy (ULAM) was employed to produce an ultra-low-angle taper that passes through the entire thickness of the PVdF topcoat (~20 µm). XPS linescan analysis along the ULAM taper indicated that the PDMS based flow agent had migrated from the PU primer surface into the bulk of the PVdF topcoat. Analysis of the shape of the silicon concentration profile revealed the existence of a silicon concentration gradient and indicated that the PDMS based flow agent was segregating towards the PVdF topcoats air/coating surface. Such migration and segregation phenomena have major implications for formulators in the coatings/paint industries.