Page 85 - Handbook of Adhesives and Sealants

Theories of Adhesion 53

surface energy and surface tension are not the same. Still it is often
convenient to refer to indiscriminately as either surface energy or
surface tension, but it is inaccurate because the ‘‘tension’’ in the sur-
face of the solid is greater than the surface energy. It is an easy matter
to measure the surface tension of a liquid in equilibrium with its va-
por, , but not to measure the surface energy of a solid. Measure-
LV
ments on high energy solids are mostly made near the melting point;
whereas, it is the room temperature properties that mainly concern
adhesive studies. Surface free energies of low surface energy solids
(i.e., polymeric materials) have been indirectly estimated through con-
tact angle measurement methods as explained below.
In a contact angle measurement, a drop of liquid is placed upon the
surface of a solid. It is assumed that the liquid does not react with the
solid and that the solid surface is perfectly smooth and rigid. The drop
is allowed to flow and equilibrate with the surface. The measurement
of the contact angle, (theta), is usually done with a goniometer that
is simply a protractor mounted inside a telescope. The angle that the
drop makes with the surface is measured carefully. A diagram of the
contact angle measurement is shown in Fig. 2.2.
A force balance between the liquid and the solid can be written as:
cos
LV SV SL
where LV is the liquid-vapor interfacial tension, is the contact angle,
SV is the solid-vapor interfacial tension, and SL is the solid-liquid
interfacial tension. This is known as the Young equation after the
4
scientist who originated the analysis. The SV is the solid-vapor in-
terfacial energy and not the true surface energy of the solid. The sur-
face energy is related to SV through the following relationship:

SV e

where is the true surface energy of the solid and is a quantity
e
known as the equilibrium spreading pressure. It is a measure of the
energy released through adsorption of the vapor onto the surface of
the solid, thus lowering the surface energy.

Figure 2.2 Schematic diagram of the contact angle and its
surface free energy (tension) components.