M.T.I. Bhuiyan, H. Zhang, J. Zhu
Keywords: ultra-fine, powder coating, XDLVO, nanoadditive, fluidization, compatibility, Geldart’s Group-C powders, surface tension
Summary:The goal of this research is to develop environmentally friendly ultra-fine powder coatings with quality finish. Despite friendlier technology compared to conventional wet paints in economic, environmental and major film-quality aspects the use of powder coatings is still confined within 1/5th of coating industry. One of the existing major concerns in conventional powder coating technology is its lack of freedom of controlling film thickness with smooth finish. Due to excessive cohesiveness of Group - C particles powder coating industry mainly uses Group - A powders which constrains the film thickness mostly around 80 um whereas around 40 um is most desired. To minimize this limitation Zhu and Zhang pioneered a nanoadditive technology for the fluidization of ultra-fine powder coatings. [1-3] However, finding the right balance of the surface energy of nanoparticles (e.g. silica, alumina, titania, carbon black, and hydrophobic metal oxide nanoparticles) as per host resin matrix (polyesters, epoxies, polyacrylics, polyurethanes, and their hybrids) has been a challenge to avoid film defects like craters, seeds and pin-holes. The size of the primary nanoparticles and their clusters also play role in film quality. A theoretical analysis by XDLVO method was done in the selection of primary particles to study the effect of surface energy and size of typical nanomaterials on the fluidization behavior of powders during powder spraying and compatibility behavior of resin matrix during curing. A series of data of inter-particle adhesiveness and cohesiveness in terms of free energy (∆G) were obtained by using the surface tension components (γLW, γ+, γ-, γAB). Our theoretical results in consistence with experimental observation imply that lower is the surface energy of guest nanoparticles irrespective of host resin particles, better is the powder fluidization. The specific inter-particle contact area and surface energy at the contacts govern the flow ability of particles in a fluidized bed. The film appearance is linked to the interaction of nanoparticles in resin melt during curing. Poor compatibility causes film defects like craters, seed, orange peel, and reduction in gloss. We have found that closer is the surface energy between host and guest particles, better is the film appearance. The results also imply that creating a separation distance of at least 10 nm by using guest nanoparticles nullifies the cohesive interaction between two host Group-C powder particles. This study on nanoadditives for ultra-fine powder coatings shows the promise of new insights how creating nano-roughness on fine particle and modifying the surface energy makes Geldart’s Group-C powders behave like Group-A in a gas-solid fluidized bed.