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Simplifying adhesive decisions for your application
The following questions will help you narrow adhesive choices to two or three possibilities
for evaluation.
Q: What materials will be bonded?
A: Structural adhesives work by adhering to the top
surface of the bonded parts, so it’s important to know the
exact material and condition on those surfaces. For metals,
will the adhesive be applied to bare metal, or will there be a
paint or coating on the surface? For plastics, exactly which
base resin? Could there be residual release agents on the
surfaces used for mold release?
Q: What is the preferred cure speed?
A: The chosen structural adhesive must have enough
work life (open time, pot life) to allow proper mixing and
application of the adhesive and assembling of the bonded
parts. Smaller assemblies or shorter cycle time production
processes may be able to use a faster curing adhesive
with a work life of only five minutes or less, while larger
assemblies that require alignment and clamping will
probably need a work life of 20 minutes or more.
Q: What surface preparation will be required?
A: Structural adhesives generally prefer clean, rough, dry
surfaces for highest bond strength. This typically means
either light abrasion and solvent cleaning of the surface, or
solvent cleaning followed by chemical etching or applying a
primer. Adhesion tests should be performed to determine
the adequate surface preparation for a specific application.
Q: What types of joints are best for
structural adhesives?
A: Joint designs that put the adhesive bond under shear,
tension, or compression forces will provide the highest
strength. Designs that tend to apply peel or cleavage
forces to the adhesive, where the applied stresses are
not distributed over the entire bond area, will have lower
bond strength, but the bond may still be sufficient for the
needs of the application. In addition, optimum bond line
thickness typically ranges from 0.005" to 0.020". The
adhesive qualification process should always include testing
of prototype assemblies to ensure the adhesive will provide
enough performance.
Q: How are structural adhesives used
and applied?
A: Structural adhesives come in many forms, including
low viscosity liquids and non-sag pastes, one- and two-
component formulations, short and long work lives, and
various package sizes and shapes. Most two-part structural
adhesives are available in both bulk containers and
convenient, easy-to-use dispense cartridge mixing systems.
Q: What are the general characteristics of
the different types of structural adhesives?
A: All structural adhesives provide at least 1,000 psi
of overlap shear strength to aluminum, but the different
adhesive chemistries have various properties:
• Epoxy adhesives come in both two-part adhesives (that
cure upon mixing the two components) and one-part
adhesives (that cure with temperature). They generally
have the highest strength and overall performance. They
also provide the best resistance to high temperatures,
solvents and outdoor weathering. They adhere well to
metals, woods and concrete, and the flexible epoxy
adhesives also bond to some plastics and rubbers. Epoxy
adhesives usually require clean, abraded surfaces to
obtain maximum bond strength.
• Acrylic adhesives are two-part adhesives that provide
excellent bond strength and durability, although slightly
lower than epoxy adhesives. However, they have
several features that make them easier to use in many
applications and manufacturing processes. These
benefits include a much faster cure speed, higher
tolerance for oily or unprepared bonding surfaces and the
ability to bond a wide variety of materials, including nearly
all plastics. Newer acrylic adhesive formulations are room
temperature stable with a long shelf life, and some have
much lower odor than regular acrylic adhesives.
• Urethane adhesives are two-part adhesives that are
relatively flexible when cured, and therefore tend to
have excellent impact resistance and good adhesion to
most plastics. They also bond well to woods, concrete
and rubbers, but they tend to have reduced resistance
to solvents and high temperatures. Uncured adhesive
components are sensitive to moisture.
• Cyanoacrylate adhesives (instant adhesives) are one-
component, lower viscosity liquids that cure extremely quickly
with just contact pressure and surface moisture. They adhere
well, with thin bond lines, to plastics, metals and rubbers.
With the use of primers, they can also adhere to low surface
energy plastics and elastomers. They tend to have low
flexibility, peel strength and impact resistance compared
to other structural adhesives. They are generally used for
applications such as gasket bonding and smaller assemblies.
• Anaerobic adhesives are one-part adhesives that cure
on active metal surfaces when oxygen gets excluded
from the bond line. These products keep your factory
running efficiently, reducing maintenance and leakage.
They do not bond well to glass, plastics or rubbers, and
are primarily used for applications such as locking threads
and sealing pipe connections.
Tensile is pull exerted equally over the
entire joint. Pull direction is straight and
away from the adhesive bond.
Shear is pull directed across the
adhesive, forcing the substrates to
slide over each other.
Cleavage is pull concentrated at one
edge of the joint, exerting a prying force
on the bond. The other edge of the joint
is theoretically under zero stress.
Peel is concentrated along a thin line
at the edge of the bond where one
substrate is flexible.