Page 71 - Handbook of Adhesives and Sealants

An Introduction to Adhesives and Sealants 39

life, then the resulting performance characteristics of the joint may
also change.

1.6.1.5 Joint design. The adhesive or sealant joint should be designed
to take advantage of the high shear and tensile strength properties of
most materials and to spread the resulting load over as great an area
as possible. Such design considerations will be discussed in the follow-
ing chapters. Although adequate adhesive-bonded assemblies have
been made from joints designed for mechanical fastening, the maxi-
mum benefits of the adhesive can be obtained only in assemblies spe-
cifically designed for adhesive bonding.

1.6.1.6 Selection and control of materials and manufacturing pro-
cesses. When determining which adhesives are suitable candidates
for an application, a number of important considerations must be
taken into account. The factors most likely to influence adhesive se-
lection were listed in Table 1.1. With regard to these controlling fac-
tors, the many adhesives available can usually be narrowed to a few
candidates that are most likely to be successful.
The appropriate manufacturing processes must then be chosen to
provide consistent, high strength joints within the allotted time and
cost. The exact manufacturing process will depend on many factors
including the choice of adhesive or sealant and the availability of
equipment. However, once chosen, the manufacturing process must be
rigidly controlled with regard to the incoming adhesive or sealant ma-
terials and with regard to the incoming substrate materials. A change
in processing parameters could change the degree of stress in the joint,
or even the chemical nature of the interphase regions.
Should the user decide to change substrate suppliers, he or she
should re-verify completely the entire bonding processes. An example
is the case of a vulcanized elastomeric substrate such as neoprene.
There are many formulations that an elastomer supplier can use to
meet a material specification. However, the formulations may contain
compounds (e.g., low molecular weight extenders, plasticizers, etc.)
that drastically reduce the adhesion of any material to the surface.

1.6.2 Mechanism of bond failure
As there are general similarities regarding the development of suc-
cessful adhesive and sealant joints, there are similarities regarding
the nature of adhesive or sealant failure. Joints may fail in adhesion
or cohesion or by some combination of the two. Adhesive failure is an
interfacial bond failure between the adhesive and adherend. Cohesive