ADHESIVES LIGHTWEIGHTING
ADHESIVE ADVANTAGES
As the automotive industry uses
more and more mixed materials,
adhesives will continue to grow in
importance, and the increasing popularity
of polymer composites exemplifies their
advantage.
It is, of course, possible to join polymer
composites to other materials using
traditional mechanical attachments, such
as clips and screws. However, it requires
an extra step to mould or create features
for the attachment, which can create
stress concentrations, leading to cracking
and premature failure. Drilling holes in
composite materials will reduce strength
by introducing discontinuities into the
matrix and reinforcing fibres.
To add to this, any mechanical
attachment method will add weight to the
structure. Non-mechanical techniques,
such as heat and friction welding, can be
used for some composites, but these are
energy- and tooling-intensive, and have
limitations in the possible geometries and
substrate combinations.
Just as there are many different types
of polymer, so there is a wide variety of
adhesives, such as epoxies, acrylics and
urethanes, that allow design engineers
to create products that meet structural
integrity requirements. These structural
adhesives work with multiple substrates,
including plastics, metals and composites,
without losing performance properties.
Even low-surface energy plastics, such as
polypropylene and polyethylene, which are
difficult to join, can be bonded with the
latest generation of speciality structural
adhesives.
As well as forming strong bonds,
structural adhesives can lower overall
costs, increase product durability and
make a significant contribution to
lightweighting. Durability is improved
because adhesives distribute stress
across the bonded area, while fasteners,
rivets and spot welding can create
stress concentrations – leading to weak
points across the substrates.
Adhesives also seal the
entire bonding area,
while providing a
high strength joint.
As well as
forming strong
bonds, structural
HOW IT’S
DONE
A few examples
of where
structural
adhesives have
successfully
improved design
and manufacture in the
automotive sector show the
range of potential applications. In one
instance, a manufacturer used adhesive
bonding to attach Acrylonitrile Butadiene
Styrene (ABS) inserts to a glass-filled
polypropylene bumper. It was the only
way to provide good
adhesion and a
permanent structural
bond to both surfaces
adhesives can lower overall
costs, increase product
durability and made a
significant contribution
without pre-treatment that
to lightweighting
also maintained the required surface
appearance. Similarly, adhesive bonding
is routinely used to attach interior
trim panels, such as bonding a large
thermoplastic bulk head to an external
aluminium frame. And it’s not just for
mass-produced cars. Speciality vehicles,
such as buses and coaches, can also
benefit. In one case, a polyurethane
adhesive sealant was used to bond
a glass-reinforced plastic side panel
to a coated steel frame. The adhesive
withstood the loads applied to the panel
during use, and allowed for the necessary
movement created by the vibrations and
differential thermal expansion.
“Many designers may feel that a line
of adhesive can never be as effective as
a line of rivets,” adds Pagliuca. “However,
many tests and practical examples show
that adhesive bonding can outperform
mechanical fastening techniques. As well
as cutting part count and component
weight, adhesives can seal gaps, reduce
internal vehicle noise and act as the
best interface between a wide range of
dissimilar materials.”
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