Electronic Thesis and Dissertation Repository


Master of Engineering Science


Chemical and Biochemical Engineering


Jesse Zhu


The major challenge in fine powder coating technology is to improve the flowability of powders, so that the fine particles can be fluidized, pneumatically transported and then sprayed onto substrates homogenously. Although this can be achieved by adding commercial inorganic nanoparticles as flow additives, it also causes problems including film defects (seeds and fish eyes), because the inorganic additives are not fully compatible with the organic fine powder coating materials during curing. In this study, the concept of modified additives were exploited to improve the film quality and to enhance the performance of the modified additives on improving the flowability of the fine coating powders. The modified additives were produced by encapsulating the commercial nano-additives with three organic materials, epoxy, polyester and hybrid with a wet encapsulation method. The encapsulated additives were evaluated by TEM to make sure they are still in nano-scale.

Functionality tests of the modified additives were conducted through the flowability measurements of fine polyurethane coating powder samples with modified additives. Sixty-six fine powder samples were tested for their angle of repose (AOR, a semi-static flowability) and avalanche angle (AVA, a dynamic flowability test). The results showed that both AOR and AVA were significantly affected by the encapsulating materials and their weight percentage, as well as the total loading radios of additives used in fine powders.

An adhesion force model was proposed to reveal the mechanism on how the additive improving the flowability of fine powder. Based on this model, the optimum loading ratio of the additive for fine particles were predicted, which agreed well with the experimental results. Thus, this model can be used to predict the optimum loading ratio of the additive for different host particle sizes.