Page 409 - Handbook of Adhesives and Sealants

356 Chapter Ten

‘‘epoxy’’ adhesives can offer the user an almost infinite assortment of
end-properties as well as a wide diversity of application and curing
characteristics.
Because of their good wetting characteristics, epoxy adhesives offer
a high degree of adhesion to all substrates except some low surface
energy, untreated plastics and elastomers. Cured epoxies have ther-
mosetting molecular structures. They exhibit excellent tensile-shear
strength but poor peel strength unless modified with a more resilient
polymer. Epoxy adhesives offer excellent resistance to oil, moisture,
and many solvents. Low shrinkage on curing and high resistance to
creep under prolonged stress are characteristics of many high quality
epoxy adhesives. Epoxy resins have no evolution of volatiles during
cure and are useful in gap-filling applications.
Commercial epoxy adhesives are composed primarily of an epoxy
resin and a curing agent. The curing agent may be incorporated into
the resin to provide a single component adhesive, or else it may be
provided in a separate container to be mixed into the resin prior to
application. Epoxy adhesives are commercially available as liquids,
pastes, films, and solids. Epoxy adhesives are generally supplied as
100% solids, but some are available as sprayable solvent systems. A
list of selected epoxy adhesives and their properties are presented in
Table 10.6.
Most commercially available epoxy adhesives are either single com-
ponent, heat curing adhesives or multiple component adhesives that
cure at either room or elevated temperature. Generally, epoxy sys-
tems that cure at elevated temperatures have a higher cross-
linking density and glass transition temperature than systems that
are formulated to cure at room temperature. This provides the ele-
vated temperature curing epoxies with better shear strength, espe-
cially at elevated temperatures, and better environmental resistance.
However, they usually have poor toughness and peel strength.
Depending on the epoxy resin and curing agent used, room-
temperature-curing adhesive formulations can harden in as little as
several minutes at room temperature, but most systems require from
18 to 72 hrs to reach full strength. The room temperature curing epoxy
adhesives can also be cured at elevated temperatures, if faster curing
times are required. The curing time is greatly temperature-dependent,
as shown in Fig. 10.1.
Once the curing agent is added to the epoxy resin the adhesive must
be used in a time period that is dependent on the type of resin and
curing agent and on the ambient conditions. With most room temper-
ature systems, the time period is short. The maximum time from mix-
ing to application is considered the pot-life of the system. For most
single component chemistries, the allowable time from mixing the