Page 414 - Handbook of Adhesives and Sealants

Adhesives Families 359

Figure 10.1 Characteristics of a
particular epoxy adhesive under
different cure time and temper-
ature relationships. 1

hardener into the resin (done by the adhesive manufacturer) to ap-
plication is relatively long and dependent on the storage conditions.
This time period is defined as the adhesive’s shelf life.
An exothermic reaction occurs on curing of epoxy adhesive systems.
Exotherm is the heat that is self-generated by the curing reaction. If
the exotherm is not controlled, it can lead to drastically reduced pot
life and even to a dangerous situation where the exotherm can be
great enough to cause combustion. The exotherm can be minimized by
lowering the temperature of the mixed components and by limiting
the mixing batch size.
Epoxy adhesives are often given a post-cure. Room temperature cur-
ing and elevated temperature curing epoxy adhesives are cured to a
state in which the resin is hard and the joint is capable of movement
(i.e., handling strength). This provides the opportunity of releasing the
fixturing used to hold the parts in place and moving the bonded com-
ponent to the next manufacturing stage. Once handling strength has
been reached, the joint is post-cured without fixturing by placing it in
an oven or other heat source until it achieves full strength. Often post
curing can be combined with some other post-bonding process such as
paint curing or component drying. The epoxy adhesive manufacturer
should be consulted to determine if post-curing is a viable option.
Many epoxy adhesives are capable of being B-staged. A B-stage
resin is one in which a limited reaction between the resin and hard-
ener has taken place so that the product is in a semi-cured but solid
state. In the B-staged state, the polymeric adhesive is still fusible and
soluble. On additional heating, the adhesive will progress from the B-
stage to a completely cured state. This will usually be accompanied
by moderate flow. The advantage of B-stage resins is that they permit
the formulation of one component adhesives such as films, powders,
and preforms. The user can then purchase an adhesive product that
does not require metering or mixing. There is very little waste asso-
ciated with B-staged products, and they generally provide better con-