by Bob Beranek
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Every now and then, I get on the glassBYTEs.com™/AGRR™ magazine forum and read the comments and solutions given to technicians concerning the everyday problems they encounter. I must say that many on the forum are unselfish, competent professionals. I would love to meet some of you over a beer to discuss the hints and tricks of automotive glass installation because I am confident that I would learn a lot.

BobThursday06252015This weekend I read a request from an Australian reader wanting to know what we here in America use for a mirror pad adhesive. One contributor responded that he uses the ever popular Loctite mirror adhesive. The poster added that he couldn’t remember the last time he had to reattach a mirror pad. Pad technology has made strides in the way mirrors are attached to the glass surface. Mirrors rarely fall off any more, mainly due to the introduction of this new type of adhesive.

The older mirror pads were attached to the windshield with a small amount of the poly-vinyl butyral (PVB), which is used for the windshield’s lamination. The problem is that with the introduction of heat from the sun, the glass would expand and contract so often that the PVB would eventually separate from the glass. The use of Loctite mirror adhesive works great to permanently attach the pad back to the glass surface because it is an anaerobic adhesive. It will not separate due to contraction and expansion.

However, about the same time as the new adhesive for mirror pads was introduced, so were mirror pad frits. This black paint dot applied to the number four surface of the glass posed another problem many do not realize. If an anaerobic adhesive is used to attach a mirror pad to a mirror frit, it can cause a stress break emanating from the center of the mirror pad. This is caused by the restriction of free expansion and contraction of the glass. Both the glass and the adhesive must be allowed to breathe or a stress break can occur.BobThursday206252015

Glass breathes when there is a change in temperature. Transparent glass has less movement than glass with a frit applied to it. Black paint absorbs more of the heat energy found in light. The glass breathes more easily on the edges and at the mirror pad where the black frit is applied. The adhesive you use must be allowed to move with it. If it is restricted, the result is a stress break.

Loctite mirror glue or an anaerobic adhesive works well when the glass is transparent and permanent adhesion is the goal. However, I would suggest that when the need arises to adhere a mirror pad to a mirror frit, a more flexible adhesive be used to allow for the glass to breathe. I have used an epoxy, channel bond adhesive, or another flexible fast curing adhesive to attach the pad to the frit. Any others could cause a fracture when the temperature changes drastically.

Comments (9)

  1. […] TODAY’S BLOG: Mirror Pad Adhesion […]

    • David said on 26-06-2015

      Thank you very much for the info Bob. I too would like to sit with you over a beer, I could learn some new stuff too, as I just did. I will adjust my technique accordingly.

  2. DAVE said on 25-06-2015

    We found the hard way years ago Loctite will break the glass on frit area, thanks for the tips

  3. stanley said on 26-06-2015

    PVB is the best for providing the room for expansion or extraction, however it requires autoclave process, it won’t works at fitting shop.

    Loctite glue is normally used at fitting shop, it often fall off or cause break, not reliable way.

    3M special tape is better, the only problem is the tape is black, it will be visible when there is not a ceramic print at the place.

  4. Mike said on 28-06-2015

    I see a TON of stress breaks around some of the older Hyundai mirror brackets on SGC brand glass. Like, every one! That window was made to fail.

  5. Glasseye said on 29-06-2015

    When I was fitting, I used Loctite 317 with Loctite 734 activator which worked well, used correctly. I also made sure to lubricate the pivot joint on the mirror, prior to refitting to ensure smooth adjustment without exerting too much pressure on the mirror pad

  6. Daniel Dinu said on 10-07-2015

    All VW vehicles, in particulary the ones provided with light+rain sensors have urethane bonded mirror pads. It is mandatory to use the safe method to set the new pad.
    I lately encounter more and more cheap windshiels that come without mirror pads and I believe Loctite
    adhesive sets may create problems as the assembly ages.
    Bottom line: I would rather use urethane rather than Loctite.

  7. Chris Courtney said on 29-03-2016

    Concerning the mirror pad adhesion, I am wondering if it matters if the frit is painted in between the 2-3 or directly on the inside\4 surface. In which scenario is Loctite safe to use or should it just not be used when painted on the 4 surface. Some of the DOT 177 windshields have the frit in between the glass and it looks like they used Loctite from the factory, but the DOT 15 appear attached with a pad and no Loctite. Also, the frit is painted directly on the 4 surface, inside. Sounds confusing, but not sure when to use Loctite or 3M Channel Bonding Adhesive to reattach a mirror bracket. Any help with this would be greatly appreciated.

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