Rheology Solutions on rheological analysis of powder coatings:
Powder coatings are a cost effective and an environmentally friendly alternative to liquid coatings.
Traditional coating systems form networks via a thermally activated process. This usually involves placing a powder coated surface into an oven at temperatures between 160°C and 200°C.
The powder melts at around 80°C to 120°C and becomes an even film. At temperatures between 110°C and 140°C the cross linking reaction starts.
The curing time typically lasts between 15 and 30 minutes, depending on the temperature and reactivity of the system.
UV curing powder coating systems have been established, where the curing process takes place in a shorter amount of time, even at temperatures below 100°C. A further advantage of the UV induced network formation is that it works virtually independently of temperature. This opens up new application possibilities for coating of heat sensitive surfaces like wood or synthetics.
The melting and curing relationship of powder coatings can be rheologically analysed via oscillatory 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 monitored without disturbing the process i.e. at very small oscillation amplitudes. The rheological measurements that are usually of interest are complex viscosity ), complex modulus G, the loss factor tan as well as the phase displacement angle.
As many powder coatings become very rigid after the complete curing process, parallel measurements using a plate/plate geometry PP20 with 20 mm plate diameter are recommended for these rheological tests.
Rheological analysis of powder coatings take place at temperatures of somewhere between 80°C and 250°C. Below are experimental results which can be measured with an air-bearing rheometer such as the Haake Mars.
The temperature range can be easily covered using an electric temperature control unit consisting of a lower and an upper heating system.