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Will Bonding Endure a Crash

The use of “structural” adhesives to attach outer door skins, hoods, deck lids and body panels is nothing new. In fact, the vehicle manufacturers have been doing it for years.

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Adhesives are used with both plastic and steel panels
in a variety of passenger-car and truck applications, so, consequently,
they must often be used during the collision repair process if
an OEM component that was glued in place needs to be replaced.

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What’s new is the use of high-strength adhesives
as an alternative to spot welding or MIG welding for attaching
exterior steel body panels during collision repairs.

Why use an adhesive instead of welding? According
to the suppliers of adhesives, bonding offers numerous advantages:

  • It saves labor time by reducing the amount of finish work
    needed after installing a panel;

  • It provides a better environmental seal, especially in critical
    rust-prone areas, such as the joint between the fender and inner
    wheel housing;

  • It won’t weaken heat-treated steel or alter the metallurgy
    of the metal in any way;

  • It won’t affect the corrosion protection offered by E-coated,
    epoxy-primed or galvanized steel;

  • It’s as strong or stronger than spot welding. Many adhesives
    have shear strengths in the range of 3,000 to 4,000 pounds per
    square inch (psi);
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  • Bonding distributes stresses more evenly, unlike welding,
    which concentrates stress at specific points;

  • The use of adhesives reduces noise and vibration by creating
    quieter, stiffer seams. This, in turn, reduces metal fatigue and
    the risk of cracking;

  • Bonding costs no more than welding when all the costs of finishing
    and materials are factored in;

  • Bonding allows dissimilar materials, such as aluminum and
    steel, that cannot be welded together to be joined without the
    use of special fasteners. The adhesive also creates a barrier
    between the two metals, which helps to prevent electrolytic corrosion;
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  • Bonding allows plastic panels to be attached to steel or other
    types of plastic without the use of fasteners; and

  • Adhesives make it possible for vehicle manufacturers to use
    thinner-gauge sheet metal because bonding doesn’t create the panel
    distortion or warpage that can result from welding.

    Crash-Test Results

    In March 1996, several suppliers of structural adhesives participated
    in a series of crash tests to find out if their adhesives could
    meet the requirements for the Federal Motor Vehicle Safety Standard
    (FMVSS) 301. The tests were conducted by MGA Research Corp., an
    internationally recognized, independent test facility in Burlington,
    Wis.

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    The FMVSS 301 standard covers the integrity of the fuel system.
    To pass the test, a vehicle’s fuel tank must remain intact and
    not leak more than a few ounces of fuel following a 30-mph rear-end
    collision. The test is conducted by ramming a stationary vehicle
    from behind with a 4,000-pound mobile barrier.

    For each of the participating adhesive manufacturers, three identical
    vehicles were crash tested (different vehicle makes were used
    by the various adhesive manufacturers). One car was stock. The
    driver-side rear quarter panel and rear body panel were removed
    from the other two test cars, then one car’s panels were reattached
    by MIG welding, while the remaining car’s panels were glued in
    place with adhesive only.

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    All vehicles, including the ones with the panels held by adhesive
    alone, successfully passed the FMVSS 301 crash tests. The panel
    seams that had been glued in place held up just as well as the
    ones that had been MIG welded. The test also revealed that the
    use of adhesive didn’t reduce the quarter panel’s ability to deform
    and dissipate the impact forces created by the crash.

    The FMVSS 301 crash tests proved that adhesives do have the strength
    necessary for automotive metal-bonding applications and can be
    safely and successfully used as a repair alternative to welding
    in many applications.

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    In January 1997, further proof of the strength of adhesives was
    obtained by Lord Corporation when a vehicle repaired with its
    adhesive successfully passed an FMVSS 208 test, which tests the
    integrity of the roof in a rollover crash. For this test, the
    vehicle (a Nissan Sentra with a bonded roof panel) was mounted
    at a 23-degree angle with the passenger side toward the ground.
    The vehicle was then towed down a test track at 32 mph and catapulted
    over to simulate a rollover. The test showed that the adhesive-bonded
    roof maintained its integrity, and it confirmed the effectiveness
    of the roof-skin bonding technique that Lord had developed.

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    But don’t sell your welding equipment just yet. Though structural
    adhesives have proven their ability to hold various kinds of panels
    in place during FMVSS crash tests, they’re still limited to certain
    kinds of applications and aren’t a substitute for all kinds of
    welding. No adhesive manufacturer currently recommends using a
    structural adhesive on structural body components, such as frame
    rails, subframes or rocker panels.

    Define Structural

    "We don’t like to refer to these products as structural adhesives
    because we feel the term is misleading," says Tom Bray, of
    3M Automotive Trades Division. "When you’re talking about
    structural components in automotive applications, you’re talking
    frame rails, posts and rocker panels. We do not recommend using
    outer-panel adhesive on such components."

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    Bray says the term "structural" applies to the strength
    properties of the adhesive only. "One definition of structural
    means at least 1,000 pounds per square inch of shear overlap strength,"
    he says. "A lot of adhesives have three to four times that
    much. But another definition of structural is anything that’s
    strong enough to cause substrate failure. Using this definition,
    Elmer’s glue on wood is a structural adhesive because wood will
    break before the glue pops away.

    "If you bond an SMC panel onto a Chevrolet front-wheel-drive
    minivan, you only need about 400 psi of shear strength before
    the SMC breaks. So a structural adhesive that’s designed for this
    type of application would not be the same as a structural adhesive
    formulated for a higher strength metal-bonding application."

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    It’s important that body shop technicians don’t get confused about
    structural adhesives, says Bray. Techs should not use them to
    bond structural components on a vehicle because the adhesives
    are not approved for this type of use; they’re strictly for exterior
    sheet-metal panels only.

    Bray says that although General Motors says its plastic quarter
    panels on its first generation FWD minivans are structural, they
    only add structural rigidity to the vehicle. They’re not load-bearing
    components like the frame rails and rockers. The same applies
    to exterior sheet-metal panels on most unibodies. The truly structural
    members are clearly identified in collision repair manuals.

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    "The best advice we can give is this: If a technician doesn’t
    know if a component is structural or not, we recommend welding
    it rather than trying to bond it," he says.

    Bray also says 3M is the only adhesive manufacturer that recommends
    welding the seam between the rear body panel and quarter panel
    rather than bonding it. "Our concern is not strength, but
    that seam separation might occur in this area in certain kinds
    of rear collisions," he says. "If the damage is not
    noticed when the vehicle is repaired, there’s a possibility the
    parts might peel loose from one another.

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    "Another recommendation we make is that when doing a beltline
    cut on a quarter [not replacing the full quarter], the panels
    should be welded rather than bonded. This will prevent the line
    from showing and will give you

    a better cosmetic repair."

    Types Available

    Metal-bonding adhesives are available from various suppliers and
    come in three basic categories: acrylics, epoxies or urethanes.

    Acrylics – Lord Corporation’s Fusor Metal Bonding Adhesive
    is a two-part acrylic. This type of adhesive has a working time
    of about 45 minutes and sets up in one-and-a-half to two hours.
    At that point, the adhesive is hard enough so the clamps can be
    removed and the technician can continue finishing. Full cure is
    in eight hours. Heat is not recommended with this type of adhesive
    to speed curing.

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    Epoxies – Dominion Sure Seal’s Epoxy Door Skin Adhesive,
    Kent’s Uniweld Door Skin Adhesive, SEM Product’s Nonsag Door Skin
    Adhesive and 3M’s Panel Bonding Adhesive are all two-part epoxies.
    Working times and curing times vary with the product. Kent and
    SEM have working times of about 30 minutes, with set up in one
    hour and full cure in 24 hours at 70 degrees F. 3M also has a
    working time of about 30 minutes but recommends that clamps be
    left in place for four hours before proceeding with work. Full
    cure is also in 24 hours. Dominion’s working time is about two
    hours, with a six-hour setup time and 24 hours for full cure.
    The curing time with all these products can be greatly decreased
    by applying heat. Heat lamps at 120 to 140 degrees can reduce
    curing time to about an hour.

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    Urethanes – Polymer Engineering Corporation’s Duramix
    Medium Metal Panel Door Skin Adhesive is a two-part urethane.
    Duramix has a working time of 45 minutes, with a six-hour setup
    time and 24 hours for full cure. Heat can also be used with this
    product to reduce cure times. For every 20-degree increase in
    temperature over 75 degrees F, the cure time will be cut in half.

    It’s important to note that all adhesives must be kept warm for
    curing to occur. When ambient temperatures drop below about 50
    degrees, curing stops. For this reason, vehicles should be kept
    indoors in a heated area until the adhesive has time to cure.

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    Technicians need to be aware of the differences in adhesives because
    each type of product is unique. What’s more, application instructions
    vary from one adhesive manufacturer to another, so you should
    always follow the specific application and usage recommendations
    by the product’s manufacturer.

    For example, Tom Oliver of SEM says its Nonsag Door Skin Adhesive
    should be used to bond door skins only. The product has a shear
    strength of 2400 psi, which is fine for door skins but not for
    other metal-bonding applications. Oliver says SEM is currently
    developing a new adhesive for quarter panels and roofs.

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    Metal Bonding

    Jim Perritt of Lord Corporation says the nice thing about using
    adhesives to bond metal is that it’s hard to screw up. "Unlike
    welding, it takes no special skills, " says Perritt. "But
    you do have to use the product correctly."

    Perritt says surface preparation is the key to getting a strong,
    long-lasting bond. "You do essentially the same kind of surface
    preparation that you do for welding, except that you should also
    grind the primer off."

    Most other adhesive manufacturers agree on this point. Though
    the E-coating on new sheet metal provides corrosion protection,
    the quality of E-coats can vary (some may not adhere as well as
    others, say the adhesive manufacturers). What’s more, E-coatings
    can sometimes separate from a panel at 20 to 30 degrees F below
    zero, which may cause a bonded seam to pull loose. That’s why
    the adhesive manufacturers recommend removing all paint, primer
    and E-coat prior to bonding.

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    What about corrosion? Adhesive manufacturers say that as long
    as all of the exposed metal is covered with adhesive, there should
    be no concerns about corrosion. A squeegee can be used to spread
    the adhesive over the exposed metal on both mating surfaces to
    assure complete coverage prior to bonding, and a small bead of
    adhesive can then be applied to the panel to glue it in place.

    When replacing a quarter panel, the bulk of the damaged panel
    can be cut away with an air saw or chisel. The mating flanges
    should not be cut; instead, they should be removed by drilling
    out the spot welds with a spot weld cutter. The remaining edges
    can then be chiseled away.

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    Once the panel has been removed, any damage along the mating edges
    should be straightened using a hammer and dolly. The mating edges
    on the body, along with the entire outer edge of the new panel,
    should be ground down to bare metal so the adhesive will bond
    to the metal itself and not to the paint or primer (which might
    peel loose).

    After trial fitting the parts to make sure they align properly,
    a 1/4- to 3/8-inch bead of adhesive can be applied along the prepared
    edge of the new panel. A small amount of adhesive should be squirted
    out of the applicator before applying the sealer to the panel
    to make sure the adhesive is mixing properly. Once the adhesive
    is applied, the panel can be positioned on the vehicle and clamped
    in place.

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    If a panel needs to be readjusted, it should never be pulled loose
    – doing so can introduce air bubbles that might weaken the bond.
    If the panel needs to be adjusted, it should be slid to maintain
    contact.

    Panels should be clamped evenly all the way around to provide
    proper support; but don’t clamp the panel too tightly because
    doing so can squeeze the adhesive too thin and prevent proper
    curing and bonding. Jim Bidwell of Polymer Engineering recommends
    a minimum thickness of 5 to 20 mils with its urethane-based product,
    while Mike Medrek of Kent recommends a minimum thickness of 1/16
    inch with its epoxy-based product.

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    If you’re skeptical about using an adhesive to install a quarter
    panel, one adhesive manufacturer suggests using adhesive along
    the wheel-well area and dog leg where it comes down to meet the
    back door, then welding the rest of the panel in place. Using
    the adhesive here will give a better seal in an area that’s hard
    to seal and minimize the risk of corrosion.

    Replacing Door Skins and Roofs

    Door skins can be removed by grinding away the edges, then removing
    the outer panels. Damaged areas on the mating flange should then
    be straightened with a hammer and dolly. The entire outer edge
    of the door frame should be ground down to bare metal to remove
    all paint and primer, and the inner edges of the new door skin
    should be scuffed before prefitting the parts to check alignment.
    A 1/8-inch bead of adhesive can then be applied to the door skin.
    The door skin should be lightly clamped in place while rolling
    the hem flanges. Allow one-and-a-half to two hours for the adhesive
    to set, and then scuff the edges of the door skin with a scratch
    pad and prime per the paint manufacturer’s instructions.

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    When replacing a roof panel, the E-coating should be removed in
    the bond area (a red scratch pad is recommended). A 1/8-inch bead
    of adhesive can then be applied to the new panel and clamped in
    place.

    Removing the Adhesive

    Making collision repairs on a vehicle that has bonded panels requires
    the use of a heat gun to loosen the adhesive. Epoxies and acrylics
    soften and lose their grip when heated to 400 degrees F (250 degrees
    for urethane). A chisel can then be used to pry apart the mating
    surfaces to peel the panel loose.

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    Adhesives are strong in the shear mode but weak in the peel mode,
    so pulling panels loose isn’t that difficult once the material
    is heated and starts to loosen.

    Another technique that works is to use an air chisel, saw, cutoff
    wheel or shears to cut away most of the damaged panel, leaving
    about a one- or two-inch strip around the edge that has the glue
    on it. The remaining strip can be pried loose and peeled off.

    Once the panel has been removed, all traces of the old adhesive
    must be removed by scraping or grinding. The mating surface then
    needs to be straightened, ground down to bare metal and cleaned
    prior to installing the replacement panel.

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    A Gripping Subject

    Like other changes and advances made to the collision repair process,
    high-strength adhesives are a subject worth investigating. The
    use of these adhesives is a relatively new procedure – a procedure
    that might prove beneficial to your business if you take the time
    to learn how to use them properly.

    Writer Larry Carley is a contributing editor to BodyShop Business.

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