Plastic exhibits a wide range of behaviors when exposed to ultraviolet (UV) radiation. The ability of a plastic to block UV light depends entirely on the specific chemical structure of the polymer used. Some plastic types inherently absorb and block most UV wavelengths, providing effective protection for their contents or underlying surfaces. Other common plastics, however, are naturally transparent to UV light, meaning they transmit the radiation freely unless specialized additives are incorporated during manufacturing.
Understanding Ultraviolet Radiation
Ultraviolet radiation, an invisible form of electromagnetic energy, is categorized into three main types based on wavelength. The longest-wavelength UV, known as UVA, ranges from 315 to 400 nanometers (nm) and accounts for the majority of solar UV that reaches the Earth’s surface. This lower-energy radiation penetrates deeply and is primarily associated with long-term material degradation and skin aging.
The mid-range UVB radiation spans from 280 to 315 nm and is more energetic than UVA. Although much of the sun’s UVB is filtered by the ozone layer, the remaining portion is responsible for sunburn and significant polymer damage, such as embrittlement and discoloration. The shortest and most energetic type is UVC, covering 100 to 280 nm, which is completely blocked by the atmosphere but is used in artificial germicidal lamps.
How Plastic Materials Interact with UV
A plastic’s interaction with UV is determined by its molecular architecture, specifically whether its polymer chains contain groups capable of absorbing high-energy photons. Blocking occurs through an absorption mechanism, where UV energy is captured by chemical structures within the material and converted into harmless heat. Polymers that feature aromatic rings are naturally effective at absorbing shorter UV wavelengths.
In contrast, polymers composed of simple, repeating carbon-hydrogen chains tend to be highly transparent to UV light, especially in the UVA and UVB ranges. When UV energy is absorbed by a susceptible plastic, it can trigger a process called photo-oxidation, breaking the polymer’s molecular bonds and generating free radicals. This chemical degradation is what causes plastics to crack, become brittle, or turn yellow over time.
Performance of Common Plastic Types
Polyethylene (PE) and Polypropylene (PP)
Polyethylene (PE) and Polypropylene (PP), two of the most common polyolefins used in packaging and outdoor items, are poor inherent UV blockers. Their simple chemical structure means they transmit most UVA and UVB light. Without additives, they quickly degrade when exposed to sunlight, leading to rapid loss of mechanical strength.
Polyethylene Terephthalate (PET) and Polycarbonate (PC)
Polyethylene Terephthalate (PET), widely used for beverage bottles, contains aromatic rings that enable it to inherently absorb most UV light up to approximately 320 nm, effectively blocking UVB but allowing some UVA transmission. Polycarbonate (PC), a high-impact plastic used for safety glasses and skylights, also has an inherent ability to absorb UV, typically blocking wavelengths shorter than about 290 nm. However, PC requires specialized coatings or stabilizers to prevent yellowing and maintain clarity during prolonged outdoor use.
Acrylic (PMMA)
Acrylic (Polymethyl Methacrylate or PMMA) is known for its glass-like clarity and is naturally resistant to UV degradation because its molecular structure does not readily absorb the harmful wavelengths that cause photo-oxidation. Standard grades of acrylic transmit a significant amount of UV light. Specialized UV-filtering acrylics, often used for museum display cases, are manufactured to block up to 98% of all UV radiation below 400 nm.
Modifying Plastics for UV Resistance and Protection
Manufacturers modify base resins to achieve the desired level of UV protection, either to prevent the plastic itself from degrading or to shield the contents. This is accomplished by incorporating UV stabilizers, which protect the polymer chain, or UV absorbers, which act as a screen. UV absorbers capture the UV radiation and harmlessly dissipate the energy as low-level heat, preventing it from reaching the underlying material.
Another common modification is the addition of Hindered Amine Light Stabilizers (HALS), which function by scavenging the free radicals created when UV light initiates degradation. For maximum UV blocking, an opaque pigment like carbon black is often added, especially to polyolefins like PE and PP. Carbon black acts as an effective physical screen, preventing UV penetration and significantly extending the service life of the plastic component. The thickness of the material is also a factor, as a thicker section of plastic will naturally absorb or scatter more UV light, confining the inevitable degradation to a thin surface layer.