Polycarbonate (Lexan)

Polycarbonate is the toughest clear plastic in common use. It was first synthesised in 1953 by Hermann Schnell at Bayer in Germany and independently by Daniel Fox at General Electric in the United States. Bayer sold it as Makrolon and GE as Lexan, and both names are still widely used in industry today. Most polycarbonate sold in Australia comes from Sabic (the former GE Plastics business), Covestro (the former Bayer business) and a number of Asian manufacturers.

Chemically, polycarbonate is built from a backbone of carbonate groups linked by bisphenol A. That backbone gives the polymer two unusual properties for a clear plastic: extreme impact resistance and a high glass transition temperature. The result is a material that you can hit with a hammer, drop from a height or shoot at with a small calibre projectile and it simply will not break. It deforms, it dents, but it almost never shatters. That is why polycarbonate is the material of choice for police riot shields, machine guards, hockey rink surrounds, bank teller screens and bullet resistant glazing.

The mechanical numbers tell the story. Polycarbonate is roughly 250 times more impact resistant than glass of the same thickness and about 30 times more impact resistant than acrylic. It maintains that toughness across an unusually wide temperature range, from around minus 40 C at the cold end to about 120 C at the hot end. Acrylic, by comparison, becomes brittle below 0 C and softens above 80 C. That temperature range is why polycarbonate dominates outdoor applications in industrial Australia, from pergola roofs in Brisbane summers to greenhouses in Tasmanian winters.

There is one significant trade off. Polycarbonate is soft. Standard polycarbonate scratches more easily than acrylic, glass or almost any other clear material. Run a key across an unprotected polycarbonate panel and you will see a visible mark. For this reason, most polycarbonate used in glazing and signage is sold with one or both surfaces coated with a hard coat. The hard coat is a thin layer of cured silicone or acrylic that brings the surface hardness up close to acrylic levels and gives the panel a useful service life. UV protection is typically built into the same coating, so a UV protected, hard coat polycarbonate panel can sit outside for 10 to 15 years with negligible yellowing.

Polycarbonate is also chemically sensitive. Ammonia, methylated spirits, acetone, and many petroleum based cleaners will craze the surface, the same way they do on acrylic but often faster. Cleaning is done with warm soapy water and a soft cloth. Avoid paper towel, which contains wood fibres hard enough to scratch the surface. For sealants and adhesives in service, neutral cure silicone is the safe choice. Many acetic acid silicones will craze polycarbonate.

In terms of cost, polycarbonate sits well above acrylic. Expect to pay roughly 1.5 to 2.5 times more for clear solid sheet in equivalent thicknesses, and more again for tinted, hard coated or twin wall configurations. Multiwall polycarbonate, the hollow panel product widely used for pergola and greenhouse roofing, is much cheaper per square metre but is not optically clear and is only suitable for translucent applications. For a one to one replacement of glass with similar transparency, solid sheet polycarbonate is the product.

Polycarbonate is fully recyclable as a thermoplastic, and increasing volumes of clean offcuts and end of life parts are being processed back into pellets for reuse in less demanding applications. From a sustainability perspective, polycarbonate is best chosen where its toughness genuinely justifies the higher embodied cost. Using polycarbonate where acrylic would do the job adds unnecessary expense and impact.

When not to use polycarbonate: where appearance is the highest priority and there is no real impact risk, acrylic is cheaper, harder to scratch and slightly clearer. Where chemical resistance is the issue, polypropylene or HDPE will outlast polycarbonate by a long margin. And where extreme heat is present, polycarbonate will warp before glass does. For most other clear glazing problems, polycarbonate is the safe, durable choice.