Page 101 - Handbook of Adhesives and Sealants

Theories of Adhesion 69

served between these two angles, the most common cause is the en-
trapment of liquid in the valleys and pores of the surface as the liquid
advances over it. When the liquid recedes, the surface uncovered usu-
ally includes wet areas. For this reason is smaller than . However,
R
A
when extreme care is used in preparing clean, sufficiently smooth sur-
faces, no differences are found. Wenzel 15 noticed that the apparent or
measured contact angle ( ) between the liquid and the surface of the
solid and the true contact angle ( ) between the liquid and the surface
follow the relationship:

cos
r
cos

where r is a measure of roughness (r 1.0 for smooth surface and
generally r 1.5 to 3.0 for machined and ground metal surfaces). Here
can be thought of as the contact angle between the liquid and its
envelope to the solid surface, and is the contact angle between the
liquid and the surface at the air-liquid-solid contact boundary. Since
most organic liquids exhibit contact angles of less than 90 degrees on
clean polished metals, the effect of roughening the surface is to make
. Thus, wetting is improved, and the resulting joint strength is
positively affected by the roughening. However, when the surface is
wet by a liquid with a contact angle greater than 90 degrees, then the
effect of roughening is to make and to reduce wetting.
This effect is evident when trying to bond a low energy surface, such
as polyethylene, with an epoxy adhesive. Without surface preparation,
the contact angle that the liquid epoxy makes on the polyethylene is
greater than 90 degrees. Therefore, epoxy adhesive joints made by
roughening the surface of polyethylene will be weaker than joint
strengths made with a non-roughened surface. This is sometimes dis-
concerting to those accustomed to bonding metal surfaces. With metal
surfaces, most adhesives make contact angles less than 90 degrees,
and surface roughening is usually, and correctly, looked upon as a pos-
sible way of improving joint strength. However, with untreated poly-
ethylene, the effect is just the opposite—exactly what is not wanted.

2.4.2 Setting or solidification
When a liquid adhesive or sealant solidifies, the loss in joint strength
is much greater than simply the loss of contact area due to the con-
siderations noted above. On setting or curing, there can be possible
reductions in adhesive strength due to:

1. Localized stress concentration points at the interface