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Anti-Flutter Adhesives

Posted on 12/10/2012 10:53:29 AM By Sandy Niks
  

We recently reviewed hem flange bonding around the perimeter of metal closure panels. Now let’s discuss anti-flutter adhesives, also used in bonding the inner to outer panels, although more commonly used on horizontal closure panels such as hoods and decklids with less application on vertical panels like doors or liftgates. The function of an anti-flutter adhesive, as the name implies, is to reduce or eliminate any “fluttering” or vibration of the outer and inner panels relative to each other.

The challenges of automotive metal bonding described previously (see the November 4 blog) are still present. One difference between the two applications is bondline thickness. A hem flange maintains a very thin bondline, around 013 mm (0.005 in), while an anti-flutter bondline thickness is generally around 20 mm (0.75 – 0.8 in). The shape of the applied adhesive is also very different. Instead of a bead, the adhesive is applied in the shape of a gumdrop or chocolate drop. It is strategically spaced across the inside surface of the outer panel, aligning with the shape of the inner panel. The placement also provides support for the outer panel to pass compression tests (developed to survive someone sitting on the hood without causing a dent). After application of both adhesives, the inner panel is placed and the perimeter of the outer panel folded over to form the hem flange.

The anti-flutter adhesive used has a low modulus and lower strength than the hem flange adhesive, thus it is also sometimes referred to as an anti-flutter adhesive/sealant. It is also thixotropic, so it doesn’t move during the subsequent processing until final cure in the paint ovens.

Because this application uses thick bondlines, the single overlap shear test that is commonly used when evaluating metal bonding adhesives isn’t the primary test when comparing candidates for the anti-flutter application. The cross peel test (similar to SAE J1553) has been found to be more representative of the application. The two metal panels to be bonded are placed perpendicular to each other, with the overlap (and the adhesive) in the middle, forming a “cross”. Spacers or a fixture may be necessary to maintain the bondline during cure. In testing, the sample is pulled apart in tension until the bondline is destroyed. Release of the adhesive to the substrate is unacceptable. The strength levels at which the adhesive has cohesively separated are compared. Anti-flutter adhesives often expand, so another test (found in SAE J250) is to evaluate how much, 10%, 50%, etc. Other testing used in the evaluation and characterization of anti-flutter adhesives is similar to other adhesives. Please view a prior post for more details.

The materials used in this application include vinyl plastisols, elastomeric or rubber-based, and warm-applied adhesives. A few example suppliers include:

Henkel

Uniseal

Bostik

Hutchinson Worldwide


Reminder – this is not an endorsement of any material.


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Figure 1: What a typical anti-flutter adhesive “gumdrop” looks like.

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Figure 2: Another formation used instead of “gumdrop.”




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