Many people assume that plastic is plastic, but saying that all plastic is the same is much like saying that all wood is the same or that all metal is the same. Replacement Domes primarily uses two different types of transparent polymers (clear plastics). Each polymer has different properties, different benefits and drawbacks, and are generally used for different applications. There are several other transparent polymers available in the industry.;

The two polymers primarily used by Replacement Domes are represented by complicated chemical names that few can remember or spell, so the end result is an alphabet soup of acronyms:

  • Polymethylmethacrylate acrylic is just called “acrylic” or “PMMA;”
  • Polycarbonate often becomes “PC;”

Just as balsa wood and bamboo and mahogany are totally different, these two classes of polymers are radically different from one another. Furthermore, in the case of acrylic there is a big difference between extruded acrylic versus cell cast acrylic. Both are forms of PMMA, but are made by totally different manufacturing processes that result in some important property differences, leading to different end uses.

The following chart offers a glimpse into the differences, uses and applications of these materials.

Some other polymers that are transparent include PETG (glycol modified polyester), PS (polystyrene) and some grades of PVC (polyvinylchloride).

PETG is sometimes used in mirror but is not UV stable whatsoever, and this greatly reduces the application range for this polymer. Polystyrene (PS) is quite brittle and therefore cracks too easily for most durable goods. PS is more commonly used in disposable applications where long-life is not required. PVC is very tough and durable but is a bit too cloudy to make a high quality mirror.

Hence the decision generally boils down to finding the best balance of properties between extruded acrylic, cell cast acrylic, or polycarbonate. The requirements of the end use will dictate which polymer is the best choice.

The following chart offers a glimpse into the differences, uses and applications of these materials.

Extruded Acrylic (PMMA) Cell Cast Acrylic (PMMA) Polycarbonate (PC)
Typical End Uses Convex mirrors, dome mirrors, skylight domes. Camera domes. Playground domes and some skylights.
Beneficial Material Features Very clear, UV stable, great optics, uniform thickness, scratch resistant versus PC due to hardness. Low cost. Hard and clear. Has unique forming properties suitable for camera domes. Highest resistance to breakage due to high impact strength. Highest maximum service temperature and best fire rating.
Negative Features or Disadvantages Hardness results in brittleness compared to PC. Poor thickness uniformity & more expensive than extruded acrylic. Needs protection for UV stability. Softness renders PC very susceptible to scratching unless coated. Highest cost.
Impact Resistance Much better than glass, but inferior to PC. Does not stretch much before breakage. Similar to extruded acrylic in this respect. Highest impact strength, often considered “unbreakable” in most applications.
UV Resistance Excellent: typically durable 10 – 20 years in outdoor applications under full sun, or longer if given extra UV protection. Depends highly on the manufacturer, grade, and UV protection. Can be good or bad for outdoor uses. Not as naturally UV stable as extruded acrylic. Typically lasts 3-5 years outdoors unprotected, 10 years with a UV protection layer.
Fire Performance Burns thoroughly and cleanly with minimal smoke. Adds fuel to any fire. Same as extruded acrylic. Somewhat fire retardant; more so with special additives. If burned, smoke is very toxic.
Indoor/Outdoor Usages Our most commonly used polymer. Its optics, UV stability, and scratch resistance make it excellent for use in mirrors, both indoors and outdoors. Very safe compared to glass. Also very lightweight compared to glass. Typically limited to camera domes at Replex due to higher cost, and poor thickness uniformity compared to extruded acrylic. Used anywhere that safety is a prime consideration, such as playgrounds, eyeglasses, safety goggles and hockey visors. High impact strength makes it good for vandalism-prone mirrors, such as those found in prisons. Used for skylights in hurricane prone areas.
Known Brands Optix™, Plexiglas™ N/A Lexan™
Cost Lowest cost among these three material types, both to purchase and to manufacture into domes. Manufacturing process makes this more expensive than extruded acrylic. Most expensive of the three material types, and more expensive to process as well. Must be dried before forming.