Page 92 - Handbook of Adhesives and Sealants

60 Chapter Two

Figure 2.6 An illustration of good and poor wetting by an
adhesive spreading over a surface. 9

The term is the interfacial tension of the solid material in equilib-
SV
rium with a fluid vapor; is the surface tension of the fluid material
LV
in equilibrium with its vapor; and is the interfacial tension be-
SL
tween the solid and liquid materials. Complete, spontaneous wetting
occurs when 0 , or the material spreads uniformly over a substrate
to form a thin film. A contact angle of 0 occurs with a pure water
droplet on a clean, glass slide. Therefore, complete spontaneous wet-
ting occurs when cosine 1.0 or when:

SV SL .
LV
Wetting is favored when the substrate surface tension, ,orits
SV
critical surface energy, , is high, and the surface tension of the wet-
C
ting liquid, , is low (i.e., C substrate adhesive ). Low energy polymers,
LV
therefore easily wet high energy substrates such as metals. Con-
versely, polymeric substrates having low surface energies will not be
readily wet by other materials and are useful for applications requir-
ing nonstick, passive surfaces.
Thus, most common adhesive liquids readily wet clean metal sur-
faces, ceramic surfaces, and many high energy polymeric surfaces.
However, common adhesives do not wet low energy surfaces. This ex-
plains why organic adhesives, such as epoxies, have excellent adhesion
to metals, but offer weak adhesion on many untreated polymeric sub-
strates, such as polyethylene, polypropylene, and the fluorocarbons.

For good wetting: adhesive C substrate