Page 82 - Handbook of Adhesives and Sealants

50 Chapter Two

good adhesion, but it is a poor adhesive or sealant. Its failure is usu-
ally cohesive. Cohesive strength of an adhesive or sealant is at least
as important as its adhesive strength. Like a weak link in a chain,
the bond will fail at the place where the intermolecular forces are the
weakest.
Adhesive forces hold two materials together at their surfaces. Co-
hesive forces hold adjacent molecules of a single material together.
Adhesive or sealant joints may fail either adhesively or cohesively.
Adhesive failure is failure at the interface between adherend and the
adhesive. An example would be the peeling of cellophane tape from a
glass surface if the adhesive film separates cleanly from the glass.
Cohesive failure is failure within the adhesive or one of the adherends.
Cohesive failure would result if two metal substrates held together
with grease were pulled apart. The grease would be found on the two
substrates after the joint failed. The grease would have failed cohe-
sively. Another example of cohesive failure is if two wooden panels
were bonded together with an epoxy adhesive and then pulled apart.
Most likely, the resulting failure would show that particles of wood
fiber were left embedded in the adhesive. In this case, the wood or
adherend failed cohesively.
Both adhesive and cohesive forces are the result of forces existing
between atoms or molecules. These forces are the result of unlike
charge attractions between molecules. The positive portion of one mol-
ecule attracts the negative portion of adjacent molecules. The more
positive or negative the charged sites and the closer together the mol-
ecules, the greater will be the forces of attraction.
Adhesive or cohesive forces can be attributed to either short or long
range molecular interactions. These are also referred to as primary or
secondary bonds. Table 2.1 characterizes these forces. The exact types
of forces that could be operating at the interface are generally thought
to be the following:

van der Waals forces (physical adsorption)
Hydrogen bonding (strong polar attraction)
Ionic, covalent, or co-ordination bonds (chemisorption)

Short-range molecular interactions include covalent, ionic, and me-
tallic forces. Covalent forces result from chemical reactions such as
provided by some surface treatments on glass fiber. Welding, soldering,
or brazing processes form metallic bonds. However, these forces gen-
erally are not at work in the more common, everyday adhesive appli-
cations. The most important forces relative to adhesion are the sec-