Page 408 - Handbook of Adhesives and Sealants

Adhesives Families 355

Elastomeric and thermoplastic adhesives are generally not used in
joints requiring continuous loading because of their tendency to creep
under stress. They are also degraded by many common service en-
vironments. These adhesives find greatest use in low-strength ap-
plications. Elastomeric adhesives represent a separate classifica-
tion because they exhibit unique rheological properties. Elastomeric
adhesives are generally thermosetting, although there are also ther-
moplastic elastomers that are used in adhesive formulations. Ther-
moplastic adhesives are generally used as hot melts or in solvent so-
lutions, emulsions, or dispersions. They are generally noted by their
convenience and quick setting characteristics.

10.3 Structural Adhesives
Structural adhesives consist of adhesive compositions that are nor-
mally used in high strength, permanent applications. Often they pro-
vide the primary means of attachment in structural applications. They
are generally formulated from thermosetting resins that require
chemical crosslinking either with the addition of a curing agent and/
or heat. The types of polymeric resin families that are formulated into
structural adhesives are usually classified as thermosets or modified
hybrids. Some high strength thermosetting elastomers, such as poly-
urethane, can also be classified as a structural adhesive. Common
polymeric resin families that are used to formulate structural adhe-
sives are described in the following sections.

10.3.1 Epoxies
Epoxy adhesives were introduced commercially in 1946 and have a
wide application of use in the automotive, industrial, and aerospace
markets. Epoxies are probably the most versatile family of adhesives
because they bond well to many substrates and can be easily modified
to achieve widely varying properties. This modification usually takes
the form of:

1. Simple additions of organic or inorganic fillers and components
2. Selection of the appropriate epoxy resin or combination of resins of
which many are available
3. Modification of the resulting resin through the choice of curing
agent and reaction mechanism

Therefore, it is incorrect to define ‘‘epoxy’’ adhesives in a generic man-
ner as if all these adhesives had similar properties. Depending on the
type of resin and curing agent used and on the specific formulation,