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Are Polycarbonate Mirrors Really "Unbreakable?"

Polycarbonate "Impact Strength"

The material property most related to cracking is called "Impact Strength". Calling that property "impact strength" is a bit of a misnomer because the unit of measure is actually a measure of energy per unit of material thickness. This property measures how much energy must be imparted to a given thickness of material in order to break it. This is not at all the same thing as how much force is applied. The "impact strength" of polycarbonate is roughly 20 times the impact strength of standard acrylic, which itself is already much higher impact strength and therefore unbreakable compared to glass. As you know, glass is actually very fragile, making it difficult to ship. Furthermore once glass is broken, the glass fragments are sharp and dangerous. To summarize, acrylic is much more impact resistant than glass, and polycarbonate is much more impact resistant than acrylic. Neither acrylic nor polycarbonate produce dangerous fragments when broken.

Idea of Being "Unbreakable":

The term "unbreakable" is rather unscientific as well. If you hit polycarbonate or any material with enough energy, it will break. Nothing on this planet is actually unbreakable except in science fiction movies. But the practical reality is that under all normal circumstances and even in many abnormal circumstances (such as in prisons), polycarbonate is very difficult to break. When you hit polycarbonate with a blunt object, the material will flex but not easily snap. It is a bit like the difference between a hard and rigid piece of wood which can snap versus a softer but pliable branch which bends but does not break. While bending it absorbs considerable energy. Think of the energy absorbed while bending a wooden bow, which then shoots the arrow when released. Polycarbonate is like the flexible branch which flexes but does not break. Acrylic is more like the harder and more rigid wood which is strong but will snap if flexed too far. As a result of this, polycarbonate is much softer than acrylic and therefore polycarbonate is much more easily scratched than acrylic. Of course you can add scratch resistant coatings to acrylic or polycarbonate, but for most applications that process is far too expensive to be practical.

Another Option to Add Rigidity to Polycarbonate for Special Applications:

The idea of adding rigid foam behind acrylic or polycarbonate is a good one. It can be done on either acrylic or polycarbonate mirror with no problem. If a polycarbonate dome is backed up with polyurethane foam, it will be able to withstand much higher mechanical forces with less flexing. We have the capability of backing up lenses and domes with foam if you are interested. In a prison application where you need maximum strength, you will want to fully foam the entire cavity, which will add considerable cost. If you just want to stiffen the item a little bit, we can sparingly spray apply the foam on a more limited basis at a lower cost as compared to filling the entire cavity.

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