To determine the "leveling" performance of varnish by measuring the boundary angle, it can be carried out by measuring the surface tension of the contact surface between the liquid phase and the solid phase, but the general method can only measure the interface between the liquid and the liquid. Surface tension at the interface between liquid and gas. (Related instrument: leveling meter)
Because it is difficult to measure the surface tension at the interface between the liquid and the solid, the surface tension can only be indirectly measured by the degree of wetting the solid by the liquid.
When several drops of liquid (1) are filled on the surface of the solid body (3), the cutting angle cut at the interface between the air (2) and the liquid drop forms a certain province at the intersection point of the three surfaces and the solid surface. This negative is called Boundary angle θ.
The relationship between the boundary angle value and the surface loading force on the front phase interface can be shown by the following formula: Selective wet-wet performance of varnish.
Liebendier explained that the physical meaning of the above formula is: the smaller the surface tension and the smaller the surface free energy at the interface with the air (the smaller the polarity difference between the liquid and the solid at the interface), that is The closer the liquid and the solid are in terms of molecular properties, the better (more perfect) the body is wetted by the liquid. Therefore, if the cohesion between the particles of the solid and the liquid is greater than or equal to the cohesion between the molecules of the body itself, The wetting effect can be carried out better. It can be seen that the better the fluidity of the varnish, the stronger the adhesion of its film to the surface of the body. (Related instruments: adhesion Tester)
In order to determine the wettability ("flow rate") of varnishes on any kind of surface, it seems that the most satisfactory method is to measure the wettability of such varnishes by measuring the boundary negative.
To this end, drop a drop of the liquid to be tested on the solid surface, and use the parallel light group to project the drop on the light-shielding plate; then draw a tangent line on the liquid-filled outline on the light-selecting plate and measure the boundary angle.
In order to ensure the accuracy of the test results, the solid surface reflected on the shading plate needs to be clearly and distinctly reflected by a thin line and a clear outline of the droplet is drawn.
