The plastic used in mirrors (typically acrylic or polycarbonate) is a thermal insulator. This means that when there is a change in temperature, one side of a piece of plastic may be warmer than the other, and it takes some time for the heat to soak through and equalize the temperature throughout. In the meantime, when there is a temperature gradient in the plastic, the plastic bends to accommodate the temperature inconsistency within.
How Does Temperature Impact Flat Mirrors?
When you start out with something flat, even the slightest amount of curvature produces visible and noticeable image changes due to the variation in the radius of the curvature, either convex or concave. Replex Plastics maintains that it’s better to know one’s limitations than to raise false expectations of performance, and the fact is that it’s very difficult to keep flat plastic mirrors perfectly flat!
How Does Temperature Impact Acrylic Mirrors?
Acrylic mirrors endure length changes as well, but the pre-existing curve makes such changes generally imperceptible to the human eye without the aid of a measuring apparatus. Acrylic is good to -40°F without distorting the mirror. Much lower than that the Acrylic becomes brittle, and shrinks a lot and depending on the shrinkage of the backer, the mirror may look distorted or crack due to stress caused by the shrinkage.
How Does Temperature Impact Polycarbonate Mirrors?
Polycarbonate is more heat resistant that Acrylic, in that it can handle hot temperatures without significant material degradation. Polycarbonate can also withstand temperatures down in the -40°F without becoming brittle or cracking. Use of Polycarbonate is idle for many mirrors especially ones used in outdoor applications, however it is recommended to have the mirror hard coated as Polycarbonate is softer than Acrylic and can scratch easily. Polycarbonate material can also come in UV Resistant grades making them great for all types of outdoor applications. Some UV resistant grades warranty 20+ years in the field.
How Does Moisture Impact Acrylic Mirrors?
Moisture also contributes to changes in length — and therefore curvature — in acrylic mirrors. Both absorption and desorption of water in the acrylic can result in curvature changes similar in magnitude to that which would result from temperature shifts.
In mirrors with aluminum on one side, all moisture is absorbed from the front (not the metallized) side, and it can take weeks or even months to reach equilibrium within. In the meantime, the moisture inconsistency results in changes in curvature. As with temperature-induced changes, these changes induced by moisture are most noticeable in flat mirrors, and much less so with convex mirrors.
Choosing the Right Housing
It is also critical to choose the right housing for the mirror. Since plastics can grow and shrink at four to six times the rate of most metals, an acrylic mirror is better served by a plastic housing, as opposed to a steel housing — unless it has enough edge clearance to allow for thermal expansion and contraction of the acrylic.
Contact Us for Additional Questions
We find that Replex’s clients make the best design decisions when they know the strengths and limitations of the materials with which they’re working. Replex is here to help our clients succeed and can provide more detail as needed for your application. Click to Contact Us.