Scratch response of high-performance thermoset and thermoplastic powders deposited by the electrostatic spray and ‘hot dipping’ fluidised bed coating methods: The role of the contact condition

Abstract

The scratch performance of powder-coated substrates depends on many factors, including the coating material and thickness, the deposition techniques, and the characteristics of the baking processes. In the present investigation, two thermoplastic (PPA571, an alloy of acid-modified polyolefins) and thermoset (TGIC-free, transparent, bronze-pigmented polyester) organic powders were deposited onto flat aluminium substrates by the ‘hot dipping’ fluidised bed and electrostatic spray coating technologies. In the former technology, the metal substrates were pre-heated to 300 °C before being dipped in the fluidised bed and powder coated. In the latter technology, after electrostatic painting, the coated substrates were submitted to a curing process for 20 min at 170 °C to allow the formation of continuous films. The resulting coatings differed in morphology, thickness and adhesion strength. The scratch responses of the coatings were evaluated by progressive-mode scratch tests. Different contact conditions between the scratching geometry and the coating surface were analysed using three conical (Rockwell C) diamond indenters with tip radii of 100, 200 and 800 μm. Electrostatically sprayed thermoset coatings showed a better overall scratch resistance with minor deformation events and damage of a lesser extent compared to the thermoplastic coatings. On the contrary, the thermoplastic coatings displayed very singular scratch properties regardless of the deposition process, having the largest deformation events and most massive interfacial rupture.