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Powder coating processes

Powder coating is a relatively new procedure which has a number of advantages compared to classic coating procedures with liquid materials. Achieving a high-quality powder coating greatly depends on the behavior of the powder during processing and application. It is therefore important to monitor and understand this behavior in order to ensure efficient processes and the required end result.

The advantages of powder coating

Powder coating is a technology which is growing in popularity and has numerous applications. Benefits of this approach are that it requires no solvent to keep the binder and filler parts in a liquid suspension and no finisher, allowing for environmentally friendly processing. Powder coatings can also be thicker and tougher than traditional coatings.In addition, the powder coatings are more durable and dry faster in most cases than wet coatings.

Application of powder coatings

A powder coating is a free-flowing dry powder which is deposited on a conductive base material (also called the substrate). The success of the powder coating process depends on a number of factors, including:

⦁ how it reacts to pneumatic conveyance

⦁ how it behaves during and after spraying

⦁ how it sticks to the substrate after heating

All these behaviors can be predicted and described using an oscillatory rheometer as part of production monitoring and quality control. Evaluating the behavior and quality of the sample and controlling the process requires a rheometer with powder capabilities and a disposable parallel-plate geometry with Peltier-heated hood.

Powder reaction to pneumatic conveyance

To ensure both good transport capability and correct application to the substrate the powder used needs to have high air retention capability and be fluidizable. These properties are both typically checked by bed collapse experiments. The bed collapse technique is one of the standard techniques used for studying the fluidization properties of powders and can be carried out in a powder cell on a rheometer. The fluidization constants (both incipient as well as full fluidization) need to be known in order to ensure optimal calibration of the pneumatic conveying equipment which transports the powder to be sprayed. The apparent viscosity of the powder is a further parameter which can be assessed in rheological measurements to reveal the influence of different formulations and flow additives, for example.

Powder behavior during and after spraying

To convey the powder in the process or move it from the container for application to the substrate requires pneumatic transport. Thereby the powder is fluidized, i.e. mixed with air and set in motion. From this fluidized bed, the powder is transported through the process to the processing site. To achieve optimal transport, it is necessary to keep a continuously dense flow. The transport process and the processing of the powder coating require the exact knowledge of the fluidization and the flow properties. These are influenced by a variety of factors, such as particle size and shape, chemical structure, humidity, temperature, and static charge. Furthermore, additives can be used to obtain better fluidizability, although these may cause a poor surface finish if the viscosity is increased by the additive. The influence of additives on the viscosity and therefore the surface finish can be investigated on a rheometer.

After the powder has been fluidized and transported, it passes to a nozzle for application to the substrate. The nozzle represents the last barrier for the powder. Here the powder is exposed to a high shear load. The flow behavior of the powder changes under shear load, similar to what happens with liquids. This behavior can be simulated by applying a “shear rate ramp” in a Powder Cell and determining the changes in the powder properties.

Powder after heating

After application, the coating is submitted to a heat treatment in which polymerization takes place and the coating is expected to both bond to the substrate and form a continuous, smooth surface. In this step, the powder needs to have high enough cohesion (inter-particulate adhesion) so that the coating does not “crumble off” before or during the heat treatment stage (curing). While the cohesion can be studied in a powder cell, the curing itself can be measured with classical rheology: Measurements determine and help keep constant the curing point at which the applied powder bonds and produces an (ideally) smooth surface. After application, the quality of the coating can be analyzed with tribology measurements on the same rheometer used previously.

Conclusion

A rheometer with implemented Powder Cell allows the investigation of many relevant properties of powder coatings in order to optimize their transport and application.