Polycarbonate is a durable, thermoplastic polymer valued for its high impact resistance and optical clarity. Standard polycarbonate naturally blocks nearly 100% of ultraviolet (UV) radiation up to approximately 380 nanometers (nm). This inherent UV-blocking capability makes the material a popular choice for everything from construction glazing to protective eyewear lenses.
The Science Behind Polycarbonate’s Inherent UV Absorption
The ability of polycarbonate to absorb UV radiation stems from the presence of specific chemical groups within its polymer chain. Polycarbonate is a polymer derived from bisphenol A, and this structure contains aromatic rings, which are six-carbon rings with delocalized electrons. These aromatic rings function as chromophores, meaning they are the part of the molecule that absorbs light energy.
When high-energy UV photons, particularly those in the UVB (290–320 nm) and UVC (100–290 nm) ranges, strike the polycarbonate, the aromatic rings absorb this energy. This absorption process prevents the radiation from passing through the material. This natural absorption acts as a built-in protective mechanism, giving the raw material a UV cutoff point close to the visible light spectrum.
While this absorption protects what is behind the polycarbonate, the absorbed energy initiates a process called photodegradation in the material itself. Chemical reactions, such as photo-Fries rearrangement and photo-oxidation, occur on the surface of the plastic. These reactions cause the material to yellow and haze over time when exposed to sunlight.
Commercial Variations and Protective Coatings
The natural UV absorption of polycarbonate is a strength, but its susceptibility to photodegradation necessitates commercial treatments for long-term outdoor use. Manufacturers often add UV stabilizers directly into the bulk material or apply specialized coatings to the surface. These modifications serve two primary purposes: protecting the polycarbonate from itself and enhancing the level of protection offered to the user.
To prevent the yellowing caused by sun exposure, a thin layer of UV-absorbing material is frequently co-extruded onto the surface of the sheet. This co-extruded layer contains powerful UV absorbers that preferentially absorb the radiation, shielding the underlying polycarbonate and extending the material’s lifespan to ten years or more. This surface treatment ensures that the polymer maintains its clarity and structural integrity in exterior applications like architectural glazing and roofing.
For specialized applications, particularly in eyewear, coatings are used to extend the protection slightly past the material’s natural 380 nm cutoff, up to 400 nm. The “UV400” standard signifies that the lens blocks virtually all UV radiation, encompassing the entire UVA and UVB range. This provides complete protection for the eyes.
Comparing UV Performance to Other Transparent Materials
Polycarbonate’s inherent UV performance often makes it a superior default choice compared to other common transparent materials. Standard soda-lime window glass, for instance, effectively blocks UVB and UVC radiation. However, common window glass transmits a significant portion of UVA radiation (320–400 nm), which is the longest-wavelength UV light.
In contrast, polycarbonate naturally blocks nearly all UVA and UVB rays, making it a better shield against the sun’s full spectrum of UV light without any special treatment. Acrylic, or Polymethyl Methacrylate (PMMA), exhibits high UV resistance and clarity but generally transmits more UV light than polycarbonate in its standard form. While some acrylics incorporate UV-blocking additives, the raw material requires modification to match the performance that polycarbonate achieves naturally.