Static & Dynamic Surface Tension of Coatings

Carole Moules, Camtel Ltd

INTRODUCTION

More companies are becoming aware of dynamic measurements for surface tension and when faced with a limited budget for a Quality Control instrument should they choose dynamic or static tensiometer? Static surface is obtained by measuring the maximum force at a liquid/gas interface on a sample where the interface is static i.e. there is no creation of new interfaces during the test. It is obtained using a force measuring device with a Du Noüy ring that is an ASTM standard test and historically it has been the basis of many companies compulsory method for determining surface tension. However when liquids reach a certain viscosity there is no doubt that rheological flow under gravity can have an effect on the ring technique, perhaps causing the lamella to break before maximum force has been reached. Elastic materials may enhance the stretching ability of the liquid and increased force may result. As such some companies now use the Wilhelmy plate with the tensiometer, which has the benefit of sitting at the surface rather than being pulled above it, thus allowing rheological effects to play out.

Regardless of whether the ring or plate is used the value obtained is a static one. When the industrial process or application involves the creation of new surfaces such as in spraying, then what relevance is the static value? The good news is that it is important, as it is the value that helps to predict wetting. The bad news is that in a dynamic process the static value may not be reached by the time the liquid hits the solid, which will then not be wetted. The following sections describe the principles behind both static and dynamic surface tension measurements with an example from a company faced with a choice between the two types of instrument for QC purposes. Would they be better opting for the static or the dynamic system? Specific examples will be made using the Camtel CIT-100 Surface & Interfacial Tensiometer for static measurements and Sita t60 Bubble Tensiometer for dynamic measurements