Page 100 - Handbook of Adhesives and Sealants

68 Chapter Two

the radius and half the thickness of a thin, circular layer of liquid in
contact with the solid; and LV is the surface tension of the liquid. If
R is greater than R , then p will be greater than p . Hence, the two
1
2
1
2
plates will be forced together because of the pressure difference p
2
p . From this equation it follows that many common liquids that spon-
1
taneously wet two such solid surfaces will hold them together.
In reality, however, the assumptions stated above are never realized.
Extremely close fitting, perfectly smooth, solid adherends would be
very expensive to prepare. The absence of dust or other particles on
the surface could be critically important in forming a strong joint. The
resistance of the joint to stress is determined solely by the viscosity
of the liquid film; hence, only if the viscosity is very great could the
joint withstand practical loading pressures.
Since real surfaces are not smooth or perfectly flat, it is necessary
to understand the effects of surface roughness on joint strength. A
viscous liquid can appear to spread over a solid surface and yet have
many gas pockets or voids in small surface pores and crevices. Even
if the liquid does spread spontaneously over the solid, there is no cer-
tainty that it will have sufficient time to fill in all the voids and dis-
place the air. The gap-filling mechanism is generally competing with
the setting mechanism of the liquid.
Problems occur when the liquid solidifies rapidly after being ap-
plied. Two examples are fast curing epoxies and fast gelling hot melt
adhesives. Very fast reacting epoxy adhesive systems generally do not
have the high adhesion strength that slower curing epoxy systems
have. One reason for this (there are others primarily related to the
chemistry of these fast acting systems) is that the curing reaction does
not provide sufficient time for the adhesive to fill the crevices on the
substrate surface. Cyanoacrylate adhesives, on the other hand, are
also very fast curing adhesives that provide exceptionally high bond
strengths on many substrates. Although cyanoacrylate resins set rap-
idly, their viscosity and wetting characteristics are such that they
quickly permeate the crevices and valleys on a substrate surface.
When a hot melt adhesive is applied in melt form to a cold metal
surface, the adhesion is much weaker than if the hot melt was applied
to a preheated metal surface and then allowed to cool at a slower rate
along with the substrate. When the hot melt makes contact with the
cold surface the adhesive gels immediately, and there is no time for
the adhesive to fill-in the cavities on the surface.
In certain cases, roughening of the surface may be undesirable—
actually decreasing the resulting joint strength. There is a noticeable
difference in measuring the contact angle of a liquid spreading over a
clean dry surface, , and the contact angle measured when the liquid
A
recedes from a previously wet surface, . When a difference is ob-
R