Page 33 - Handbook of Adhesives and Sealants

An Introduction to Adhesives and Sealants 3

comparable processing requirements and failure mechanisms, and the
fundamentals of how they work are similar. Therefore, adhesives and
sealants are often considered together, as they are in this Handbook.
However, different specifications and test methods apply to adhesives
and sealants, and most often they are designed to perform different
functions. Their definitions hint at these differing functions.
Adhesive—a substance capable of holding at least two surfaces to-
gether in a strong and permanent manner.
Sealant—a substance capable of attaching to at least two surfaces,
thereby, filling the space between them to provide a barrier or pro-
tective coating.
Adhesives and sealants are often considered together because they
both adhere and seal; both must be resistant to their operating envi-
ronments; and their properties are highly dependent on how they are
applied and processed. Adhesives and sealants also share several com-
mon characteristics.

They must behave as a liquid, at some time in the course of bond
formation, in order to flow over and wet (make intimate contact
with) the adherends.
They form surface attachment through adhesion (the development
of intermolecular forces).
They must harden to carry sometimes continuous, sometimes vari-
able load throughout their lives.
They transfer and distribute load among the components in an as-
sembly.
They must fill gaps, cavities, and spaces.
They must work with other components of the assembly to provide
a durable product.

Adhesives are chosen for their holding and bonding power. They are
generally materials having high shear and tensile strength. Structural
adhesive is a term generally used to define an adhesive whose strength
is critical to the success of the assembly. This term is usually reserved
to describe adhesives with high shear strength (in excess of 1,000
pounds per square inch or psi) and good environmental resistance.
Examples of structural adhesives are epoxy, thermosetting acrylic, and
urethane systems. Structural adhesives are usually expected to last
the life of the product to which they are applied.
Non-structural adhesives are adhesives with much lower strength
and permanence. They are generally used for temporary fastening or