Page 319 - Handbook of Adhesives and Sealants

Adhesive Classifications 281

cals. However, at times these materials could possibly be used in long
term applications, depending on the severity of the application.
Nonstructural adhesives are sometimes used with other types of fas-
teners including mechanical fasteners. In these applications, the ad-
hesive bond is considered a secondary fastener. The use of a nonstruc-
tural adhesive in concert with mechanical fasteners may allow one to
reduce the number of mechanical fasteners that would normally be
used and also provide additional value in the assembly such as vibra-
tion damping, sealing, or insulation.

8.2.2 Chemical composition
The classification of adhesives by chemical composition describes ad-
hesives in the broadest sense as being either thermosetting, thermo-
plastic, elastomeric, or alloys (hybrids) of these. These classifications
are described in Table 8.1. Usually, the chemical composition is di-
vided further into major chemical types or families within each group,
such as epoxy, urethane, neoprene, and cyanoacrylate. The polymeric
families of resins that are useful in developing adhesive formulations
are described in detail in the Chapter 10.

8.2.2.1 Thermosetting adhesives. Thermosetting adhesives are mate-
rials that cannot be heated and softened repeatedly after their initial
cure. Once cured and crosslinked, the bond can be softened somewhat
by heat, but it cannot be remelted or restored to the flowable state
that existed before curing. Thermoset materials form infusible and
insoluble materials. These adhesives generally degrade and weaken
upon heating at high enough temperatures because of oxidation or
molecular chain session.
Thermosetting adhesive systems cure by an irreversible chemical
reaction at room or elevated temperatures, depending on the type of
adhesive. This chemical reaction is often referred to as crosslinking.
The crosslinking that occurs in the curing reaction is brought about
by the linking of two linear polymers, resulting in a three dimensional
rigidized chemical structure. A model crosslinked structure is shown
in Fig. 8.1. Crosslinking usually occurs by chemical reaction. Some
reactions require heat for initiation and completion; others can be
completed at room temperature. With certain ‘‘room temperature’’ cur-
ing adhesives, it is the internal heat of reaction generated by the cur-
ing mechanism, called the exotherm, which actually provides the en-
ergy required to completely cure the polymeric material.
Substantial pressure may also be required with some thermosetting
adhesives, yet others are capable of providing strong bonds with only