Doctor of Philosophy
Chemical and Biochemical Engineering
The metallic effect powder coatings (MEPC), composed of regular powder coating materials and metallic pigments, share an increasing market because they provide an aesthetic metallic effect and also enhance protection. Mainstream industrial production employs a thermal bonding technology to incorporate and immobilize the metallic pigments to coating particles. However, a series of inherent shortcomings of the thermal technology have affected the promotion and deployment of MEPC, such as mis-bonding, metallic flake deformation, inability for high-temperature bonding, etc. To address these issues, cold, single-component-heating (SCH) and microwave bonding methods have been proposed and tested in this thesis work.
The cold and SCH bonding methods both were shown to have enhanced bonding quality and small Al-content relative difference of less than 5%, but could not compete with the microwave bonding method that has high heating rates and easy real-time control, and is more readily for industrialization. A thorough study on the microwave bonding method from lab to industrial-scale experiments provided a comprehensive understanding of the process and mechanism of microwave bonding including the bonding quality and color stability. Microwaves exhibited significant advancements in heating rate (10-23°C/min), which enabled a much higher production efficiency and a lower energy consumption than the current bonding technology. In the microwave bonding, the temperature of particles is easy to control. Furthermore, the bonding quality increased by 100% and the color is much more stable compared to the current commercial method. Based on solid experiments and reliable results, the microwave bonding devices have been scaled up from lab-scale (0.05 kg/batch) to pilot-scale (10 kg/batch) and finally to industrial-scale (100 kg/batch). The industrial microwave bonding machine successfully overcame the shortcomings of the current commercial bonding machine, and achieved better bonding quality and more stable color with higher productivity and less energy consumption.
In conclusion, the microwave bonding method shows obvious superiority over the current bonding technology, as well as the cold and SCH bonding methods studied here. It avoids the current shortcomings of bonding, provides better bonding quality and color stability, and achieves higher production efficiency. The industrial-scale microwave bonding machines are expected to replace the current bonding machines in the very near future.
Summary for Lay Audience
Powder coating is an environmentally friendly painting process that has 100% solid ingredients of resin, pigment, filler, and additives. The excluding of solvent avoids atmosphere pollution. Metallic effect powder coating (MEPC) provides a finish with high protection and sparkling (metal) effect that has been heavily applied in the fields of domestic appliances, building materials, automobile parts, electronic instrumentation and other industrial products.
The most important production procedure for MEPC is the bonding treatment that aims to bond the coating particle with metallic pigment to ensure the color stability of the final films. The current commercial bonding technology employs high-speed stirring and water/oil jackets to heat and bond the two components. However, there are several inherent shortcomings, such as mis-bonding, bending and grinding of metallic pigments, high-temperature bonding, etc.
In this work, to overcome the above shortcomings and improve bonding quality, several bonding methods have been proposed and tested. Cold bonding is using water-soluble binders to bond the coating particles and metallic pigments at room temperature. In the single-component-heating bonding, the metallic pigments are heated alone and then immediately dispersed into the fluidized coating particles to bond them. At last, the microwave bonding method has been established and developed, which selectively heats the two materials and bonds them. The microwave bonding device has been scaled up from lab-scale (0.05 kg/batch) to pilot-scale (10 kg/batch) and finally to industrial-scale (100 kg/batch).
The bonding quality and color stability of the bonded samples were analyzed by a series of characterizations, such as ash test, gas-volumetric analysis, FTIR, optical microscope, colorimeter, SEM, and EDS. The results show that the microwave bonding method obtains better bonding quality and color stability than the cold, SCH and current commercial bonding methods. More specifically, the bonding rate increased from 35% to 72% and the color difference decreased from 2.9 to 0.3. In addition, microwave bonding device provides higher productivity with lower energy consumption compared to the commercial technology.
In summary, the microwave bonding method gains better bonding quality and higher color stability than the current commercial bonding technology. The industrial microwave bonding machine is expected to replace the current bonding machine and create enormous commercial value in the near future.
Liu, Wei, "Development of New Bonding Technologies for Powder Coatings" (2021). Electronic Thesis and Dissertation Repository. 7933.
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Available for download on Thursday, June 22, 2023