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.
This 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.
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.
