Plexiglass, a trade name for the transparent plastic polymethyl methacrylate (PMMA), is a common material used in applications from skylights to picture frames. Its exceptional clarity and durability often lead people to question its effectiveness at protecting against the sun’s invisible, energetic radiation. The answer to whether this acrylic glass blocks ultraviolet (UV) light depends entirely on the specific type of UV ray and the material’s formulation. Understanding the inherent properties of standard PMMA and its modifications provides the full picture of its sun-shielding capabilities.
Understanding UV Radiation
Ultraviolet radiation from the sun is categorized into three main types based on wavelength, which dictates their energy and ability to penetrate materials and the skin. The longest wavelength is UVA, which ranges from 320 to 400 nanometers (nm), and penetrates deep into the skin, contributing significantly to photoaging and tanning. The medium wavelength, UVB, spans 280 to 320 nm and is the primary cause of sunburn and most skin cancers. The shortest and most energetic form is UVC (100 to 280 nm), though almost all of it is naturally filtered out by the Earth’s ozone layer.
Standard Acrylic and UV Transmission
The inherent chemical structure of standard, unmodified acrylic glass acts as a highly effective filter for the shorter, high-energy UV wavelengths. Due to the molecular composition of PMMA, it naturally absorbs nearly 100% of UVC and a large majority of UVB radiation, the rays responsible for sunburn. This built-in protection is why standard acrylic sheets resist degradation and yellowing better than many other plastics.
However, standard acrylic performs quite differently regarding the longer-wavelength UVA radiation. These rays, which span up to 400 nm, are largely transmitted through the material, often passing through at rates between 70% and 90%. The high transmission rate of UVA means that while standard acrylic will prevent a sunburn, it offers incomplete protection against the full spectrum of solar damage.
Specialized UV Filtering Acrylics
Manufacturers have engineered specialized versions of PMMA to provide comprehensive protection across the entire UV spectrum. These materials are created by incorporating specific chemical compounds, known as UV absorbers, into the acrylic polymer during the manufacturing process. These additives, such as benzotriazole or benzophenone compounds, function by absorbing UV photons and converting their energy into harmless, low-level heat.
This modification dramatically increases the material’s blocking power, resulting in UV-filtering acrylic that can block up to 98% of all UV radiation below 400 nm. These specialty sheets are often used in applications where complete UV exclusion is necessary, such as museum display cases, high-end picture framing, and architectural glazing. Their effectiveness depends on the concentration and type of additive, not the material’s thickness, allowing even thin sheets to provide superior protection.
Practical Implications of UV Transmission
The difference between blocking UVB and transmitting UVA has significant consequences for both human health and the longevity of materials behind the acrylic. For human health, using standard acrylic in applications like car windows or patio enclosures means that while sunburn is largely prevented, long-term skin damage continues. The high transmission of UVA rays allows for photoaging, which includes wrinkling, leathery texture, and the breakdown of collagen and elastin in the skin. Therefore, standard acrylic should not be relied upon as a full shield against sun damage.
Furthermore, the incomplete blocking of UV light impacts the acrylic material itself and any items placed behind it. When UV light penetrates the polymer structure, it triggers a process called photodegradation, which causes the material to yellow, haze, and become structurally weaker over time. Using UV-filtering acrylic mitigates this degradation, ensuring that the sheet maintains its optical clarity and strength for a much longer period. This engineered protection is why UV-stable acrylics are guaranteed to retain their light transmission for decades, making them necessary for preserving valuable art and historical artifacts.