Polycarbonate is an amorphous engineering thermoplastic renowned for its extraordinary impact resistance. Developed independently by Dr. Hermann Schnell at Bayer AG in Germany and by Daniel Fox at General Electric in the United States in 1953, polycarbonate was commercialised by GE under the Lexan brand and by Bayer under Makrolon. Both brands remain the global standards, though polycarbonate is now manufactured by SABIC, Teijin, Mitsubishi Chemical, and many others.
Impact Strength: 200x Glass, 10x Acrylic
The defining property of polycarbonate is impact strength. Under standardised Izod impact testing, polycarbonate registers approximately 600 to 850 joules per metre, compared to roughly 20 J/m for cast acrylic. In practical terms, a polycarbonate sheet of 3 millimetres thickness can withstand repeated blows from a hammer without cracking, while glass of the same thickness shatters on first impact. Polycarbonate is rated at approximately 200 times the impact resistance of glass and 10 times that of acrylic of equal thickness. This is why polycarbonate is specified for riot shields, aircraft windows, bullet-resistant glazing, machine guards, and safety spectacles.
The impact resistance is a direct consequence of polycarbonate's molecular structure. The polymer backbone contains bulky bisphenol-A units connected by flexible carbonate linkages. This structure allows large-scale molecular motion under stress, absorbing impact energy through viscoelastic deformation rather than brittle fracture. Unlike acrylic, which fractures cleanly along stress lines, polycarbonate yields and stretches before breaking, forming a ductile hinge rather than sharp shards.
Multiwall Sheets for Roofing and Greenhouses
Polycarbonate multiwall sheets, also called twin-wall or triple-wall, are extruded profiles with parallel internal ribs that create air channels between two or more solid faces. The most common configuration is twin-wall, typically 6 to 10 millimetres thick, with a single row of vertical ribs. Triple-wall sheets, at 16 millimetres thickness, incorporate two layers of internal ribs and three solid faces, achieving an R-value of approximately 1.45 in imperial units or a U-value of around 3.0 W/m2K.
These sheets are the dominant glazing material for commercial greenhouses in Europe and North America. They provide diffuse light transmission of roughly 75 to 80% for clear grades, while opal or bronze-tinted grades reduce light and solar heat gain. The cellular structure provides thermal insulation that single-pane glass cannot match, which is why Dutch and Canadian greenhouse operators have standardised on 16mm triple-wall polycarbonate for year-round vegetable production.
UV Coextruded Cap Layer and Weathering
Unprotected polycarbonate degrades rapidly under ultraviolet radiation. The UV energy breaks carbonate bonds in the polymer backbone, causing yellowing, embrittlement, and loss of impact strength within a few years of outdoor exposure. To solve this, polycarbonate sheet manufacturers coextrude a thin UV-absorbing cap layer, typically 50 to 80 microns thick, on the weather-facing surface during sheet production. This layer contains UV absorbers that convert UV radiation to harmless heat before it reaches the bulk polycarbonate. High-quality multiwall sheets carry 10-year warranties against yellowing and impact loss, and well-made sheets from manufacturers like Palram (Suntuf) and Brett Martin (Marlon) routinely last 15 to 20 years in temperate climates.
BPA Content and Scratching Susceptibility
Polycarbonate is manufactured from bisphenol-A and phosgene, and BPA remains present in trace amounts within the finished polymer. The concern over BPA leaching, particularly from polycarbonate food and beverage containers, led to the phase-out of polycarbonate baby bottles in Canada in 2008, followed by the EU in 2011 and the US FDA ban in 2012. For construction applications, where the material is not in prolonged contact with food or drink, BPA migration is not a relevant concern.
The major practical disadvantage of polycarbonate is its soft surface. Polycarbonate is significantly softer than glass and acrylic, scratching readily from abrasive cleaners, dust, and even paper towels. Hard-coated polycarbonate, with a silicone or polysiloxane coating applied by dip or flow coating, improves scratch resistance substantially. Products like Lexan Margard offer abrasion resistance approaching that of glass, though at a cost premium of roughly 30 to 50% over standard sheet.