Acrylics
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Toughened
acrylics are fast curing and offer high
strength and toughness. Both one and two
part systems are available. In two part
systems, no mixing is required because
the adhesive is applied to one substrate,
the activator to the second substrate,
and the substrates joined. They tolerate
minimal surface preparation and bond well
to a wide range of materials.
Process
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Structural
acrylic adhesives cure by mixing two separate
parts—a resin and an activator. Once
the two components are mixed, a room-temperature
chemical reaction occurs, delivering a
very strong bond to metals, plastics and
composites. Structural acrylic adhesives
require little surface preparation and
can be formulated to deliver application-specific
open times from minutes to hours.
The mix ratio
for the adhesives is forgiving and allows
some margin for error, although assemblers
should be aware that, once mixed, the
adhesives generate heat during the exothermic
curing process. Luckily, manufacturers
can minimize the effects of this heat
by controlling the amount of adhesive
dispensed, the size of the assembly and
the substrates used. For example, a large
metal assembly will dissipate heat faster
than a small metal, plastic or composite
part.
Note that
acrylic adhesives will not bond well to
wood and rubber. Also, because most structural
acrylics cannot resist temperatures above
250 F, processing that involves elevated
temperatures, such as a paint bake cycle,
may present problems. However, a few structural
acrylics can withstand temperatures up
to 400 F for short periods of time, allowing
for use in paint bake cycles without a
significant loss in bond strength.
Ultimately,
a structural acrylic adhesive will require
a cure time of up to 24 hours to achieve
full strength. However, many formulations
allow handling of assemblies in just minutes
(fixture time). To keep production lines
moving, mechanical fasteners are sometimes
used to temporarily hold the assembly
in place while the adhesive cures. These
small fasteners are used only sparingly
and do not require through-holes.
High-performance
adhesive formulations are available for
bonding specific substrates such as aluminum,
stainless steel, carbon steel or galvanized
materials. Adhesives are also being formulated
with “toughening” agents to
improve impact and peel resistance. For
example, rubber-toughened structural acrylic
formulations can be used in those applications
requiring excellent cold impact resistance,
long-term fatigue resistance and durability.
Uses
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Acrylics
will develop a tough, durable bond to
a wide variety of substrates, including
metals, plastics and ceramics, thermoplastics
like polypropylene and ABS, and thermoset
like fiberglass, sheet molded compounds
(SMC) and fiber reinforced plastics (FRP)
but they will not bond well to wood and
rubber.
