Page 98 - Handbook of Adhesives and Sealants

66 Chapter Two

During the third period in an adhesive’s life (service exposure),
weak boundary layers could occur by environmental moisture diffus-
ing through either the adhesive or the adherend and locating at the
interface. Plasticizers, solvents, or other low molecular weight sub-
stances also may migrate out of the adherend or adhesive and deposit
at the interface. The bonding of plasticized polyvinyl chloride, for ex-
ample, can be difficult because the plasticizers will migrate to the in-
terphase with time. Notice the way some plasticized binder covers will
stick together on your bookshelf? This is due to the highly mobile
nature of the plasticizer. Another weak boundary layer that can form
during aging occurs by the adherend continuing to cure or chemically
react with its surroundings. Corrosion of an aluminum adherend could
occur at the interface and weaken the joint strength. On exposure to
moderately elevated temperatures, under-cured phenolic substrates,
for example, continue to liberate moisture as a by-product of the cur-
ing reaction, thereby degrading the joint strength.

2.4 Stages in an Adhesive’s or a Sealant’s
Life
As was indicated in the previous section, the life of an adhesive bond
has several important stages no matter whether it is in an adhesive
or a sealant joint. These stages are:

(1) Application and wetting
(2) Setting or solidification
(3) Operating in service
The degree of interfacial adhesion is greatly determined by stages 1
and 2. Stages 2 and 3 determine the degree of cohesive strength. All
stages markedly influence the overall joint performance. From a prac-
tical standpoint, the last stage is where the ultimate performance of
the adhesive or sealant is measured. It includes aging of the joint
materials in the service environment as well as exposure to any
stresses in the application.
There are processes going on in all three stages that ultimately will
affect the adhesion and the performance of the adhesive or sealant. It
is important to understand these processes and the effects that they
will have on the quality of the joint.

2.4.1 Application and Wetting
As explained under the adsorption theory of adhesion, an adhesive
must first wet the substrate and come into intimate contact with it.