Page 336 - Handbook of Adhesives and Sealants

294 Chapter Eight

working time that will depend on the type of adhesive, the chemical
reaction involved, and the conditions present in the bonding environ-
ment, mainly temperature. The working life may vary from several
minutes to several hours. Thus, working life could be a significant
parameter in the scheduling of production, and it could be a significant
cost item if significant quantities of adhesive have to be scrapped be-
cause its working life has expired.
Epoxies, polyurethanes, acrylics, phenolics, and silicones are ex-
amples of common adhesive systems that are available in multiple
parts. Most of these adhesives will cure at room temperature and heat
can be used to accelerate the curing process. Certain multiple part
adhesive formulations must be cured at elevated temperatures.
Certain thermosetting acrylic adhesives are unique in that they are
multiple part systems that do not require mixing. Multiple part ‘‘no-
mix’’ adhesives are systems that do not require careful metering and
no mixing is involved. Part ‘‘A’’ (e.g., resin portion) of the adhesive can
be applied to one substrate and Part ‘‘B’’ (e.g., catalyst portion) applied
to the other substrate. Cure then occurs at room temperature when
the coated substrates are mated. Cure can also be achieved by coating
the substrates with an accelerator/primer. Once coated, the substrates
can be stored for a significant period of time. When bonding is re-
quired, the substrates are removed from storage and only the single
resin component needs to be applied. Once the resin is applied to the
primed substrates, free radical polymerization begins. So in essence,
we have a single component adhesive that requires a primer to cure.
Figure 8.3 shows the ‘‘accelerator cure’’ concept and the ‘‘honeymoon’’
cure concept where Parts A and B are applied to different substrates.
The primary advantages of these thermosetting acrylic adhesives is
their ease of application and fast setting characteristics relative to
epoxy or urethane. The cure occurs at room temperature and depend-
ing on the nature of the specific adhesive formulation, full cure can
be completed in minutes to several hours. Cure can also be accelerated
by heat.

8.2.3.1.2 Single component systems that cure via catalyst or hardener. Most
single component systems consist of two or more premixed compo-
nents. The curing agent or hardener is incorporated with the resin
base and all the other fillers and additives. This eliminates end-user
metering and mixing processes that are required of two part systems.
Consistency is also usually better with the single component systems.
However, refrigerated storage conditions may be required, and these
adhesives have a limited shelf life.
Single component heat curing adhesives incorporate a latent curing
agent or hardener in the formulation. This hardener is activated with