Introduction to Glass Bonding

Posted on 3/11/2013 8:14:03 AM By Sandy Niks

There are several areas of glass bonding – such as windshield, rear window, side windows, headlamps, and bonding to glass (stickers/labels, rearview mirror button, etc.). This article will concentrate on bonding to body openings.

Early applications first used butyl tapes, then polysulfide rubber materials. These were lower strength materials that held the windshield and backlight (rear window) in place, using the large bond area of the two substrates. These materials also functioned as a sealant, to prevent water intrusion. A decorative reveal molding was also used to cover the interface of the exposed bonded joint edge and the painted metal body opening.

New structural requirements took advantage of the large bond area and the strength of the glass to not just hold the glass in place, but to make the joint load-bearing and thus part of the load bearing structure to pass roof crush tests. For this, a stronger adhesive/sealant was needed, and urethanes became the material usually chosen for the application. They had the strength, the flexibility for bonding two materials with very different expansion coefficients, and could seal against moisture. Using structurally bonded glass has allowed for new design possibilities, such as eliminating the reveal molding to have a flush glass to roof line.

Windshield bonding has several steps. One substrate is the painted metal of the body opening. Testing includes all of the colors, to insure there isn’t some incompatibility with the chemistry of a particular color. In the assembly plant, the painted opening may have an additional cleaning step, then the [glass] body primer. On the glass side, there is a cleaner/primer, followed by a blackout primer (to protect the urethane adhesive from light), then the adhesive bead is applied, and the windshield is placed in the body opening.

Over the years, some of these steps could be eliminated. For example, the electrodeposition (ELPO) primer applied to the vehicle body makes an excellent substrate for bonding. The [glass] body primer may be eliminated. For this to work, the body opening must be masked in some manner (masking tape, reusable rubber part that covers and seals the opening, etc.) so the subsequent paint layers are not applied to the bond area. The mask must also be tight enough that there is no paint overspray, as this tends to a weak substrate to adhere to. Furthermore, the mask itself must not leave any residue that would interfere with the bond. Another way to eliminate the [glass] body primer is with the use of the base coat/clear coat paint system. Since the bond would be to a consistent clear coat over the base (color) coat, the variability due color chemistry may not be relevant. In modern automotive assembly plants, the cleaning step(s) may be eliminated. The conveyor belt system has been largely replaced by mobile platforms in final assembly, resulting in a much cleaner (and quieter) environment. Air conditioning and filtering have also greatly reduced potential cleanliness issues. On the glass side, the blackout primer has usually been replaced by a black glass frit that has been applied by the glass manufacturer.

The adhesive itself can be a one- or two-part urethane, with the second part more of an accelerator since the adhesive will eventually cure on its own in the presence of moisture. Other adhesives have also been investigated, such as “warm melts” which could be used in conjunction with the urethane, providing a rapid bond to hold the glass in place while the urethane cures.

Only one company was located in the search for suppliers used by the vehicle manufacturers. This doesn’t mean there aren’t others. This is not an endorsement of any company.


Dow Automotive

Other companies located were for aftermarket repair. This will be discussed at a future date.


Handbook of Adhesive Technology, edited by A. Pizzi, K. L. Mittal, Marcel Dekker, Inc., 1994, ISBN:  0-8247-8947-1, Chapter 40, “Adhesives in the Automotive Industry”, by Eckhard H. Cordes.

Handbook of Adhesives, Third Edition, edited by Irving Skeist, Ph.D., Van Nostrand Reinhold, 1990, ISBN:  0-442-28013-0, Chapter 10, “Butyl Rubber and Polyisobutylene”, J. J. Higgins, F. C. Jagisch, and N. E. Stucker; Chapter 16, “Polysulfide Sealants and Adhesives”, Julian R. Panek; Chapter 20, “Polyurethane- and Isocyanate-Based Adhesives”, C.S. Schollenberger;  Chapter 45, “Adhesives in the Automobile Industry”, by G. L. Schneberger P. E.

Engineered Materials Handbook®, Volume 3, Adhesives and Sealants, Hal F. Brinson, Technical Chairman ASM International Handbook Committee, ASM International, 1990, ISBN: 0-87170-281-9, pp 108-112, “Urethanes”, by Cynthia L. Kreider; pp203-207, “Urethanes”, by John F. Regan; pp 551-557, “Automotive Applications for Adhesives”,  by Kieran Drain and Sarat Chandrasekharan; pp 604-612, “Applications for Sealants”, by Kent Adams.

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