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Rheological analysis of powder coatings

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Powder coatings are a cost effective and environmentally friendly alternative to liquid coatings. Traditional coatings systems network via a thermally activated process. This usually involves placing a powder coated surface into an oven between 160°C to 200°C in temperature.

The powder melts at around 80°C to 120°C and becomes an even film. At temperatures of between 110°C to 140°C the network reaction begins. The curing time typically lasts between 15 and 30 minutes, depending on the temperature and reactivity of the system. New to the market place are U.V. curing powder coating systems, where the curing process takes place in a shorter amount of time, even at temperatures of below 100°C.

A further advantage of the U.V. induced networking is that it works virtually independently of temperature. This opens up new application possibilities for coating heat sensitive surfaces like wood or synthetics. When powder coating different substrates, it is not only smooth, tough surfaces that are desired, but also adhesive strength, corrosion protection and durability.

The melting and curing relationship of powder coatings can be rheologically analysed using oscillation tests with an air bearing rheometer. The advantage of this method is that the structural change of the coating as a function of time or temperature can be demonstrated without disturbing the process, that is, at very small oscillation amplitudes.

The rheological measurements that are usually of interest are complex viscosity h, complex modulus G, the loss factor tan d as well as the phase displacement angle d. As many powder coatings become very hard after the complete curing process, parallel measurements using plate geometry PP20 with 20 mm plate diameter are recommended for the rheological tests.

Rheological analysis of powder coatings takes place at temperatures of somewhere between 80°C and 250°C. This range can easily be covered using the electric temperature unit TC501 with cone heating. The following outlines some recommendations of practical examples and typical measurement definitions.

All measurements have been carried out using the air bearing rheometer Haake RheoStress, with the high temperature measuring unit TC501 with cone heating and the parallel plate geometry PP20 (20mm plate diameter). As a standard measurement the gap was set at 0.5 mm.

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