Polycarbonate (PC) is a strong, transparent thermoplastic polymer valued for its high impact resistance and lightweight properties. While used extensively in applications like safety glazing, automotive parts, and roofing, its inherent UV resistance is nuanced. Untreated polycarbonate is inherently susceptible to damage from ultraviolet radiation, making it unsuitable for prolonged outdoor exposure. However, nearly all commercial polycarbonate intended for exterior use is specially manufactured with advanced treatments to provide long-lasting UV protection.
Inherent Susceptibility to Photodegradation
The fundamental vulnerability of polycarbonate lies within its molecular structure, specifically the carbonate linkages and aromatic rings. When high-energy ultraviolet photons from sunlight strike the material, these weak points readily absorb the energy. This absorption initiates a destructive process known as photodegradation, which includes photo-oxidation and photo-Fries rearrangement reactions.
These chemical reactions cause the covalent bonds within the polymer chain to rupture, leading to a decrease in the material’s molecular weight. As the chains break, various degradation products are formed. These new chemical groups absorb visible light in the yellow spectrum, which is the primary cause of the material’s noticeable discoloration. This chemical breakdown is concentrated near the surface, typically penetrating only about 25 micrometers deep into the material.
Engineering Methods for UV Protection
Manufacturers employ sophisticated techniques to fortify polycarbonate against solar radiation for outdoor use. The most common and robust method is co-extrusion, which permanently bonds a protective layer to the base polymer during the manufacturing process. A thin cap layer containing a high concentration of UV-absorbing additives is melted and extruded simultaneously with the bulk polycarbonate sheet.
This co-extruded layer is typically only a fraction of a millimeter thick but contains the necessary UV stabilizers. It is seamlessly joined to the underlying PC, which eliminates the risk of peeling or delamination. These stabilizers work by preferentially absorbing the harmful UV radiation and dissipating it as heat energy.
In addition to surface protection, chemical compounds can be incorporated directly into the polymer bulk as internal additives. These stabilizers are mixed into the resin before molding or extrusion to provide a degree of protection throughout the material. They are often used in conjunction with co-extrusion for applications requiring maximum longevity and clarity. This combination ensures the polycarbonate can absorb up to 99.5% of harmful UV wavelengths, allowing it to maintain performance for over a decade in harsh outdoor environments.
Physical and Mechanical Effects of Failure
When a polycarbonate material’s UV protection fails, or if an untreated sheet is exposed, degradation manifests in distinct, observable ways. The most immediate effect is optical deterioration, commonly seen as yellowing or discoloration, which reduces transparency and light transmission capabilities.
The chemical breakdown also leads to significant changes in physical integrity. The surface develops fine cracks, known as crazing, which scatters light and reduces clarity. Mechanically, the polymer suffers a substantial loss of toughness, transitioning from a ductile material to one that is brittle and susceptible to fracture.