Development of smooth finishes in electrostatic fluidized bed (EFB) coating process of high-performance thermoplastic powders (PPA 571 H)

Abstract

This paper deals with the analysis of the evolution of the surface morphology of metal substrates coated with high-performance thermoplastic powders, namely PPA 571 H, by using electrostatic fluidized bed (EFB) process. Attention has been particularly focused on the relationship between baking time and temperature of EFB coated substrates and the morphological characteristics of the resulting polymeric films.First, thermal behaviour of PPA 571 H polymeric powders was characterized by using standard calorimetric techniques. Accordingly, PPA 571 H melting kinetic was experimentally deduced. Based upon experimental findings, predictive analytical model was also developed and employed to trace 'iso-conversion' curves out.Second, metal substrates, made from low carbon steel (AISI 1040), were EFB coated and baked at several baking time and temperatures. Combined analyses of scanning electron and confocal microscopes were led to measure the evolution of the films surface morphology under different baking conditions. Accordingly, a relationship between film morphologies and melting degree was sought. Consistent trends of roughness parameters versus baking parameters were found, with smoother finishes of the polymeric films being achieved for higher degrees of melting, that is, for higher baking temperature and time. Full maps and related analytical models of the finishing levels according to baking parameters were also built up, hence providing first useful indications to powder coaters on how to best deal with their settings.

Combined use of scratch tests and CLA profilometry to characterize polyester powder coatings

“…R a consistently decreases with the baking time and temperature up to values as low as 0.18 µm and 1.7 µm, respectively. These values are typical of smooth finish powder coating systems and agree with data reported in the scientific and technical literature [11,15,28,29].…”

“…Polyester powder onto the large-sized metal substrate does not have enough time to heat up to the prescribed curing temperature (150 and 175°C, respectively) in such a short curing program (just 5 min), part of the heat being absorbed by the large-sized metal substrate. In addition to this, it is well know that viscosity vs. temperature trends of a powder coatings are characterized by a first decreasing branch (which can even last several minutes [27]) followed by a rapid increase (the rate at which viscosity increases depends on curing temperature [11,22,29]) after which the viscosity approaches its minimum (which is lower when higher temperatures are considered [11,22,27,29]). Therefore, being of a short duration, the baking program chosen (150 and 175°C for 5 min) results in too short a permanence time for the polyester coating to be at viscosity values low enough to permit the full leveling of the powders and the formation of smooth films.…”

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

“…When the polyphthalamide is used, the preheated substrate releases its heat to the surrounding powder, causing it to melt, coalesce and level up to form a continuous film. The polyphthalamide is a semi-crystalline thermoplastic material, and it only needs to be melted until a smooth film is formed from the loose powder [20]. When the polyester is used, the preheated substrate continues to release its heat to convert the powder to a continuous film.…”

Wood-Reinforced Polyphthalamide Resins: MultiFunctional Composite Coating for Metal Substrates

“…The higher the baking temperature would be, the faster the initial decrease in resin viscosity and therefore its capability to melt and level over a flat substrate would be. However, the development of smooth finishes after postheating of polyphthalamidebased coatings deposited by electrostatic FB was detailed by Barletta et al [15] in an earlier study, as well. Differently, the wooden particles dispersed inside the polymeric material in the coating systems A-D do not melt neither when they come in touch with the hot metal substrate nor during the postheating.…”