Contact-angle development of polymer melts

In powder coatings, the quality of the final film is often associated with its smoothness. This is affected by the wetting and levelling process which occurs during film formation and is mainly determined by surface tension (driving force) and viscosity (resistance).

Especially for the surface tension the requirements are complex. On the one hand a lower surface tension facilitates the substrate wetting process, but if it is too low levelling is poor, resulting in wavy surfaces (“orange peel”). On the other hand, a higher surface tension promotes levelling, but if it is too high the wetting is poor, resulting in crater defects. Therefore, the surface tension of a powder coating has to be optimised carefully. It can be adjusted by addition of “levelling” or “surface flow” compounds to a formulation as well as changing the composition of the resin. Most of the current knowledge however is based on trial and error and a more systematic approach should provide better fundamental insight into the interrelation between the different parameters.

Since there is no generally accepted method of measuring wetting and levelling, a new method has been devised by which the actual melting and flow process can be monitored. The method provides information about the wetting and flow process of polymers by measuring the contact-angle of polymer melts as a function of temperature and gives significant and consistent results while data analysis provides quantitative information.

In this presentation, the method will be explained and examples will be given, showing the effects of structural parameters of coating binders on their wetting and flow behaviour.