Doctor of Philosophy
Chemical and Biochemical Engineering
Powder coating, which utilizes polymer resin based powder, is a relatively young technology for decorative and protective purposes. It was first developed in the early 1950s and gradually grew into automotive finishes. Compared to conventional liquid coating, powder coating is an environmentally friendly technology, given that there is no emission of harmful solvent throughout the entire operating process. It is also much more efficient, economical and energy saving as a result of the capability to reuse all the powder. However, there are still some problems in the powder coating process limiting its application. This thesis focused on studying some key aspects of the manufacturing and application processes of both coarse powder and fine powder. Several new techniques were developed during the process.
The study started with an investigation of powder coating manufacturing processes. Most of the production is aiming at maximizing the productivity, while the produced powder has a wide particle size distribution, leading to a poor surface condition. In order to narrow the particle size distribution, a new design of classifying cyclone for an air classifying mill (ACM) with a reversed air inlet was studied. Experimental results indicated that this novel cyclone design could effectively reduce the span of the particle size distribution, without deteriorating the collection efficiency compared to conventional cyclones.
In addition, the spraying properties of powder coatings were investigated during electrostatic spraying. In order to overcome the Faraday Cage effect, a modified corona spray gun with a multi-electrode design supplied by an alternating charging pattern was invented. Experimental results demonstrated that the Faraday Cage effect could be significantly mitigated by using the new spray gun at various gun voltages. Besides, the new design of spray gun could provide higher transfer efficiency compared to its conventional counterpart.
Moreover, this study also improved the powder coating processing technique involving metallic pigments. To achieve better bonding between metallic pigments and powder coating particles, a liquid bonding agent (bonder) was introduced into the blending process. It was found that the concentration of metallic pigments changed minutely from the pre-sprayed and post-sprayed powder, indicating that the recycled powder could be reused. Therefore, this new bonding method of utilizing liquid bonder could provide a strong affinity between the powder coating and metallic pigments so as to prevent separation of the two materials during spraying.
At the end, a pre-heating method of applying powder coating onto thermoplastic substrates, which helps powder deposition and adhesion onto plastic substrates, was studied. Three widely used powder coatings have been tested for this method. The coating films were evaluated by both visual inspection and instrumental measurements. It was found that this new processing method could provide a smooth surface as well as a strong adhesion to the difficult substrate. Furthermore, for the purpose of protecting the thermoplastic substrate from deformation during the curing process, a UWO low-cure method was applied to these three powder coatings. Compared with commercial low-cure powder, the UWO low-cure coatings perform better in a few aspects.
The discoveries and analysis in this thesis work are contributing to powder coating development. Several original techniques have been invented in this study and they could provide a good guideline for future work of modern powder coating technology.
Gao, Shan, "Study on Developments of Processes for Powder Coatings" (2016). Electronic Thesis and Dissertation Repository. 3984.