Ultraviolet light (UV radiation) is a form of electromagnetic energy extending just beyond the violet end of the visible light spectrum. Whether UV radiation passes through plastic depends entirely on the material’s specific chemical composition, thickness, and specialized protective additives. Some common plastics naturally block nearly all UV radiation, while others are almost transparent to it, allowing the high-energy light to pass through easily.
Understanding the UV Spectrum
The ultraviolet spectrum is divided into three distinct bands based on wavelength. UVC radiation (100 to 280 nanometers, or nm) is the highest-energy and shortest-wavelength band, making it the most destructive to organic materials. Fortunately, all solar UVC is absorbed by the Earth’s atmosphere and does not reach the surface.
UVB radiation (280 to 315 nm) occupies the medium wavelength range and is responsible for causing sunburn and skin cancer. The longest wavelength band is UVA (315 to 400 nm), which accounts for approximately 95% of the UV radiation reaching the Earth’s surface. Understanding these three bands is essential because a plastic might block the high-energy UVB and UVC while allowing the longer-wavelength UVA to pass through.
Molecular Structure Determines Blocking
A plastic’s ability to block UV light is rooted in the chemical bonds and molecular architecture of its polymer chains. When UV radiation strikes a plastic, the energy must be absorbed to be blocked, a process often initiated by molecules called chromophores. These chromophores are chemical groups within the polymer structure or present as impurities that can absorb UV photons, causing them to enter an excited state.
Once the UV energy is absorbed, the chromophore dissipates it, often by converting it into harmless heat, a process that prevents the light from passing through the material. If the energy is not safely dissipated, it can generate free radicals, leading to a chain reaction known as photodegradation that breaks the polymer bonds. Manufacturers combat this by including additives, such as organic UV absorbers, which are designed to absorb UV light and protect the plastic itself from structural damage.
How Common Plastics Handle UV
The UV transmission properties of common plastics vary significantly. Polyethylene Terephthalate (PET), widely used in beverage bottles, inherently blocks most UVB and UVC radiation, but clear PET typically transmits a significant amount of UVA light above 340 nm. While this protects the plastic from rapid deterioration, UV-blocking additives are often included to protect contents sensitive to UVA.
Polycarbonate (PC), an extremely impact-resistant plastic used for safety glasses and skylights, naturally absorbs UV radiation across the relevant spectrum, including UVA and UVB. This absorption protects people underneath the material, but it causes the polycarbonate itself to yellow and degrade over time unless a protective layer or UV stabilizer is applied.
Acrylic (also known as Plexiglass or PMMA) naturally blocks nearly all UV light below 400 nm, allowing very little UVA or UVB transmission, making it a common choice for applications requiring inherent UV protection. Polyvinyl Chloride (PVC), a durable material used in piping and window frames, is highly susceptible to UV degradation, leading to yellowing and embrittlement, and therefore requires the addition of pigments or UV stabilizers to scatter and absorb the light.
Real World Effects and Applications
The selective transmission and blocking capabilities of plastics have consequences for their real-world use and longevity. The destructive effect of UV light on polymers is demonstrated through weathering, where prolonged exposure causes chalking, discoloration, and a loss of mechanical strength. This degradation occurs predominantly in the surface layer, typically less than 0.5 millimeters deep, as the damaged layer absorbs light and shields the plastic beneath.
In practical applications, clear polycarbonate is used for car headlamps and safety lenses because it provides high impact resistance and blocks both UVA and UVB rays for personal protection. Conversely, the high-energy UVC band, which is completely blocked by the atmosphere, is effectively used in germicidal lamps to sterilize surfaces. This radiation is potent, meaning plastic components used near these lamps must be highly specialized, or a thin glass shield must be used to prevent rapid degradation of the plastic.