Page 89 - Handbook of Adhesives and Sealants

Theories of Adhesion 57

W W + W
Experimental Expected Additional

Interfacial Viscoelastic

Bonding Deformation
Figure 2.4 The measured work of adhesion is made up of thermodynamic and
mechanical components.

adhesive, interphase, and adherend the practical work of adhesion is
equal to the theoretical work of adhesion.
It should be realized that the above discussion on forces and work
of adhesion is very simplistic and summarizes a great deal to a fault.
There are also significant debates over the applicability and direct
usefulness of these relationships. However, the following conclusions
can be derived and are of significant assistance to the user of adhe-
sives and sealants:

1. The work of adhesion is at a maximum when the contact angle, ,
equals 0 degrees, that is when the liquid spreads completely on the
surface of the solid. This condition implies that there are stronger
forces between the molecules of the liquid and the substrate than
between the liquid molecules themselves.
2. Adhesion will tend to go to zero as the contact angle increases
above 90 degrees.
3. Under conditions of perfect wetting of a surface by a liquid, W
A
2 . Hence W W .
LV A C
These conclusions will be discussed further in the following sections.

2.2.5 Surface attachment theory of joint
strength
The preceding discussion looks at adhesive failure and cohesive failure
as separate modes. However, in practice they result from the same
joint specimen. One can generalize on the influence of the degree of
interfacial surface attachment on the adhesive joint strength and on
the mode of failure. The degree of interfacial surface attachment may
vary due to wetting, boundary layer effects, or other phenomena that
influence the degree of adhesion at the interface. The different inter-
facial states of adhesion are summarized in Fig. 2.5.