It is critical to use the right type of adhesive to correctly and securely bond mirrors to their backings, because of the multi-layer structure of the mirror. The reality is that most bonding occurs on a mirror’s back coating, which means the adhesive has to bond the structure to that back coating. Only a handful of adhesives can perform well on this kind of multi-layer structure. To prevent mirror delamination the manufacturer must select the appropriate adhesive – otherwise it’s only a matter of time before the product fails.
Organic Solvents and the Smell Test
Most strong organic solvents, including solvents such as toluene, xylene, MEK and acetone generally have a strong chemical odor due to their tendency to evaporate rapidly (think model airplane adhesive or Liquid Nails™). This property which makes them smell also makes them very useful in common paints and adhesives but unfortunately can weaken bonds in multi-layer structures such as mirrors, causing the entire structure to delaminate. For those who aren’t experts with chemical data sheets, there is a rudimentary, yet surprisingly effective test to rule out some candidate adhesives. Solvent-based adhesives give off a powerful, unmistakable smell. If the adhesive has a strong organic chemical odor, it probably won’t work well on a mirror.
Urethane and Silicone: Built for Bonding with Mirrors
Two types of adhesives that get the best results with mirrors are urethanes and silicones, neither of which are solvent-based adhesive systems.
Urethane adhesives are usually as sold as two-part adhesives. When part A is mixed with part B, this initiates a chemical reaction that polymerizes the constituents and creates an amazing adhesive bond. Since there are little or no solvents to evaporate, there is no strong odor and there is no organic solvent to extract that can cause the mirror to delaminate. Depending on the application, urethane adhesives can be formulated to cure rapidly or to give the assembler some work time. In either case, once the adhesive is cured, it creates a permanent and irreversible bond. Urethane adhesives are fairly rigid once cured, which is good in some applications and bad in others. Unfortunately some small percentage of people are hypersensitive to the constituents of urethane adhesives, so read the MSDS sheets and follow appropriate safety and hygiene guidelines.
Silicones can also be formulated as either two-part or single part adhesives. Single-part silicones typically generate acetic acid as a byproduct of the chemical reaction (which gives off the smell of vinegar). Acetic acid isn’t a very strong solvent and doesn’t generally contribute to the delamination of mirrors. Two-part silicones are completely free of acetic acid, and therefore give off no vinegar odor. Much like the description above for urethanes, mixing part A and part B initiates the chemical reaction that polymerizes the constituents and creates the bond. Once fully cured, silicones are permanent (often with an expected lifetime of over 30 years in all weather), can tolerate a wide temperature range, are waterproof, and importantly – for some applications – are extremely pliable, tolerating a huge amount of strain prior to failure.
Common Adhesives That Won’t Work on Mirrors
Many people experiment with common adhesives without understanding that these products will probably delaminate the mirror. These products include:
Liquid Nails ™, or other construction adhesives suitable for wood
Methacrylate adhesives (such as WeldOn™ or Acryfix ™)
Any cyanoacrylate adhesive such as Super Glue™
Silicone VS Urethane: Which One is Superior?
Both products have their advantages and disadvantages. Silicone may be the superior product – it’s not uncommon for it to last up to 30 years, and it is much more resistant to UV light. The drawback is that silicone is messy in small applications and requires a greater degree of skill. Silicone is often used in industrial applications and is available in five-gallon pails for dispensing with automated equipment. It is also available in smaller quantities, usually in the one-part variety, for dispensing from a caulking gun. So, for large industrial applications with an experienced person, silicone is likely the most appropriate choice.
Urethanes work extremely well, too, and are usually more user-friendly for small jobs in limited quantities because they are often sold with convenient application tools. One special type of urethane, polyurethane foam, is an excellent adhesive, but it is not at all easy to dispense. This is an exception to the statement above.
Choosing the Right Adhesive To Prevent Delamination
Often, manufacturers experiment with different adhesives that they’ve picked up from the local hardware store, without realizing that due to the multi-layer structure of mirrors, adhesives loaded with organic solvents will cause the structure to delaminate entirely. To prevent failure, start your experiments with a silicone or urethane adhesive – these options will more likely create a strong bond that will stay secure far longer than the other common adhesives out there.