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Glass Bonding – Testing

Posted on 3/25/2013 10:50:23 AM By Sandy Niks
  

Continuing with the last blog’s topic on bonding glass to body openings, this time we’ll look at testing for these bonding applications - windshield and backlight.

Many of the general requirements (MSDS, etc.), in-coming material requirements (density, VOC, etc,), shipping requirements (settling, etc.), and processing requirements (open time, etc.) are similar to other pumpable materials used in manufacturing. See the blog on “Materials Specifications” posted in December 2012 for this information. Depending on whether the material is a one-part or two-part, other requirements and testing may include a pressure flowmeter “press-flow” viscosity (SAE J1524), color (black for mixed adhesive or one-part), solids content (ASTM D2834), and slump resistance (SAE J243). Other bulk cured properties may include hardness (Shore A, ASTM D2240) and elongation (ASTM D412).

The adhesive performance requirements use tests developed to better represent the windshield bonding application. The single overlap shear test typically used in other structural bonding applications has been modified to include thicker bondlines, as well as some of the concerns with testing glass. SAE J1529, “Overlap Shear Test for Automotive Type Sealant for Stationary Glass Bonding”, is the resulting test procedure. It is currently being reviewed and updated by the SAE Adhesives and Sealants Committee. The dimensions of the painted metal substrate, glass substrate, and adhesive/sealer bead are being converted to metric – and not just the metric equivalent to the inch – to be more acceptable globally. In order to maintain the alignment for shear testing, special fixturing for preparing and testing the test specimens is recommended. The test fixture also has the glass edge supported by the fixture, not clamped as is done with the metal coupon, minimizing the potential for glass breakage due to clamp pressure.

Also in development are two additional standards. Once approved and published, these two will replace the similar company test procedures.  The first test, “Quick Knife Adhesion Test for Stationary Glass Bonding”, is a qualitative screening test that evaluates compatibility of the adhesive/sealer to the substrate. Metal or plastic panels are sent through the production paint system to receive all paint layers – such as primer, base (color) coat, and clear coat. Glass panels will have the black ceramic frit. Cleaning and/or adhesive priming steps are executed per the adhesive/sealer manufacturer’s recommendations in the lab as part of the test preparation.  The test panels are 100 mm x 300 mm. About 250 mm of adhesive/sealer is applied as a bead and allowed to cure. The bead is grasped and, after an initial starting cut parallel to the substrate, cuts are made perpendicular to the surface, keeping a steady pull on the bead. (Please see the test method, when published, for complete details.) What is left of the bead on the panel is evaluated for separation mode – did the adhesive/sealer release cleanly from the paint; did layers of the paint separate; did the adhesive/sealer leave a layer on the panel.

The second test, “Glass Bonding Primer Link-up Rate Test via Tape Method”, is more of a test for processing requirements and uses substrate panels as in the previous test. After the adhesive primer is applied, an “X” pattern is cut into the panel. A tape is applied down the center. It is then pulled off at 15 minute intervals, revealing another “X” each time. The exposed primer and tape is examined each time. The time is recorded when no primer is transferred to the tape.

Note: To participate in the review and development of these glass bonding test procedures, please send me a comment.

Most adhesive joints are hidden from view, and also hidden from significant light exposure. This is not always true for glass bonding. Therefore, environmental and durability testing for glass bonding adhesive/sealants includes exposure to light, whether in a lab or at an outdoor facility. The test specimens described in the three tests above may be used for this evaluation.

In addition to these tests, there are vehicle tests. As mentioned in the previous blog, there are federally [U.S.A] mandated roof crush tests.  Here is an example of this test being executed provided.

Standards may be obtained through the standards developing organization  - ASTM International, SAE International  - or through a distributor, such as IHS.



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