The Truth About Perspex® UV Resistance – and Why It’s Important

We are frequently asked whether Acrylic sheet blocks UV rays, or whether Acrylic sheet is UV stable. A quick search on your favourite search engine will yield lots of results - unfortunately, many of those results provide incorrect, misleading or inadequate information. This blog post cuts through the myths and misinformation about Perspex® and UV resistance.

What is Acrylic sheet?

Acrylic sheet, scientifically known as polymethyl methacrylate (PMMA) with the chemical formula (C₅O₂H₈)ₙ, is a thermoplastic renowned for its exceptional optical clarity, durability, and versatility. Often marketed under trade names such as Perspex®, Plexiglas®, Lucite®, and Acrylite®, it is produced through the polymerisation of methyl methacrylate monomers into long, stable chains that form an amorphous, non-crystalline structure.

This gives the material its remarkable ability to transmit up to 92% of visible light, making it even clearer than glass of the same thickness, while weighing only about half as much.

It is more impact-resistant than glass, though less so than polycarbonate, and is easily fabricated by cutting, drilling, machining, and thermoforming. Its combination of light weight, optical performance, and durability makes it a popular choice for a wide range of applications, including glazing and windows, retail and exhibition displays, illuminated signage, aquariums, protective barriers, lighting diffusers, and decorative furniture elements.

What are UV rays?

Ultraviolet (UV) rays are part of the electromagnetic light spectrum, sitting just beyond the violet end of visible light with shorter wavelengths (about 10–400 nanometres).

The light spectrum is simply the range of all the different colours (or wavelengths) of light, from the shortest to the longest.

At one end, you have shorter wavelengths (measured in nanometres), like ultraviolet (UV), which, despite being invisible to the naked eye, travel at the same speed as visible light but can carry more energy. Then comes visible light, which is the small part of the spectrum our eyes can detect — from violet (the shortest visible wavelength) through blue, green, yellow, orange, to red (longest visible wavelength). Beyond red is infrared (IR), which we also can’t see, but we can feel as heat. UV rays are broken down into three main types:

1. UV-A (315–400 nm) – Closest to visible light, least energetic, penetrates deeper into skin.
2. UV-B (280–315 nm) – More energetic, causes sunburn and DNA damage.
3. UV-C (100–280 nm) – Highly energetic, germicidal, mostly absorbed by Earth’s atmosphere.

It is therefore accurate to state that the lower the wavelength, the higher the energy, and the higher the potential for causing damage.

Why do UV rays matter?

UV rays matter to almost all living species. UV rays can be harmful because they carry enough energy to damage fundamental biological molecules like DNS, proteins and lipids.

In humans, whilst small amounts of UV-B can help the skin to produce Vitamin D, which is essential for bone health and immune function, it can also cause damage such as:

• DNA Damage: UV-B (280–315 nm) penetrates the skin’s outer layers and directly damages DNA in skin cells by forming thymine dimers. This can lead to mutations that cause skin cancer (basal cell carcinoma, squamous cell carcinoma, melanoma).
• Sunburn: Inflammation caused by cellular injury from UV exposure — the redness, pain, and swelling are part of the body’s healing response.
• Photoaging: UV-A (315–400 nm) penetrates deeper into the dermis, breaking down collagen and elastin. Over time, this leads to wrinkles, sagging, and pigmentation changes.

In plants, the effects can be quite similar to those effects experienced by humans. High UV exposure can stunt growth and reduce crop yields, lower the plants ability to absorb visible light for photosynthesis, and impair reproduction.

Both humans and plants can benefit from low levels of UV for short periods, however, overexposure can have disastrous, even deadly, consequences. Given that Perspex® Acrylic sheet is often used for various types of glazing, behind which will frequently be human beings or plant life, it’s clear to see why it’s important to reduce the amount of UV being transmitted through acrylic sheets. Whilst blocking UV light is vitally important to protect the objects behind the acrylic sheet, it is also important to ensure that the UV rays will not cause the acrylic sheet to degrade or discolour over time. Materials that are able to withstand UV rays without significant degradation or discolouration are known as UV stable materials.

UV Stable vs UV Blocking

The difference comes down to what the UV protection is for — the material itself, or what’s behind it.

UV Stable materials are designed to withstand sunlight without degrading. They resist yellowing, cracking, and weakening when exposed to UV rays over long periods, ensuring the sheet retains its durability and appearance.

UV Blocking materials, on the other hand, stop ultraviolet rays from passing through. This protects whatever is behind the sheet — people, plants, or valuable objects — from UV damage. However, unless the sheet is also UV stable, the material itself may still degrade over time.

Is Perspex® Acrylic a natural UV blocker and UV stable?

Now that we have explained what UV rays are, why UV rays are important, and stated the difference between UV Blocking and UV Stable materials, let’s move on to providing an answer for the main topic of this blog post. Most of the online search results provide very short answers as to whether Acrylic sheet resists and/or blocks UV rays, normally, simply stating that it does. This isn’t strictly true.

"PMMA (polymethyl methacrylate) is naturally UV stable, but only partially a UV blocker."

PMMA (polymethyl methacrylate) is naturally UV stable because its molecular structure is built from strong carbon–carbon and carbon–oxygen bonds that resist breaking under UV exposure, and it lacks vulnerable chemical groups like aromatic rings or halogens that make other plastics degrade quickly in sunlight. It can absorb and harmlessly dissipate much of the UV energy it encounters, so it resists yellowing, brittleness, and loss of strength over many years outdoors.

However, PMMA is only a partial UV blocker because, while it effectively absorbs most UV-B and a portion of UV-A, it still allows some longer-wave UV-A (closer to visible light) to pass through. Without added UV-absorbing additives, it won’t block the entire UV range — meaning it protects itself well but doesn’t fully shield objects behind it from all UV damage.

The transmission curve below was measured using 3mm and 8mm clear Perspex® cast acrylic sheets manufactured without UV-absorbing additives (known as a UV Transmitting Acrylic Sheet). The graph highlights two key observations:

• UV transmission decreases as sheet thickness increases — thicker acrylic allows less UV light to pass through
• At the most harmful end of the spectrum (UV-C, 100–280 nm), acrylic sheet naturally blocks nearly all ultraviolet radiation

This is why it is accurate to state that Acrylic that has not been treated with UV-absorbing additives is only a partial UV blocker.

Most Acrylic manufacturers, including Perspex®, add UV-absorbers into the material during the manufacturing process. UV absorbers work by soaking up harmful ultraviolet light before it can damage a material, then safely releasing that energy in a harmless form — usually as heat. There are two major advantages of adding UV-absorbers to a material:

• They protect materials by acting like a sponge for UV light — they catch the energy before it can snap chemical bonds in the underlying material, resulting in a more UV-stable material which can withstand the effects of UV rays for longer periods
• They lower the UV transmission rate because they intercept and absorb UV photons before those photons can pass through the material

Once UV-absorbers have been added to Acrylic Sheet, it can still only be called a partial UV blocker because it does not block 100% of the UV rays, however, the UV transmission is further reduced. The transmission curve shown below was measured against a 3mm clear Perspex® Acrylic Sheet manufactured with UV-absorbers.

Compared to the UV Transmitting Acrylic Sheet, standard Perspex® acrylic sheet manufactured with UV-absorbers, blocks almost all UV light up to around 360 nm. In contrast, the unmodified UV Transmitting Acrylic sheet begins transmitting UV at much shorter, higher-energy wavelengths (around 245 nm), and by the time the wavelength reaches 360 nm, it is allowing about 90% of the UV light to pass through. This demonstrates how the addition of UV-absorbers lowers the amount of UV transmission, making it safer for human and plant life to be behind Perspex® Acrylic Sheets, whilst also increasing the materials ability to block UV light before it can disrupt the material’s molecular structure and cause photodegradation, in-turn increasing the UV stability of the material and increasing its lifespan.

Can the UV resistance of Acrylic Sheets be further increased?

Perspex®, being one the World’s leading manufacturers of Acrylic Sheet, has also produced a range of materials that even further limit the UV transmission – VE and VA Grade Acrylic Sheet. The need for clear Acrylic’s that have even lower UV transmission rate stems from the need in certain industries to be able to show priceless artworks or historical artefacts whilst minimising the risk of the object behind the Perspex® degrading over time from UV exposure – think of museums, high-end art galleries, biological laboratories and in retail and showroom displays.

The transmission curve below shows the UV transmission rates for standard Perspex Clear Cast Acrylic, Clear Cast VE Grade and Clear Cast VA Grade.

Summary

• Perspex® Acrylic Sheet is naturally UV Stable and a partial UV Blocker
• UV-Absorbing additives are added to Perspex® Cast Acrylic to increase the material’s operational lifespan and lower the UV transmission rate
• Standard Perspex® Clear Cast Acrylic will block almost all of the UV light in the range of 200-360nm
• The thickness of an Acrylic Sheet will affect the UV transmission rate – the thicker the material, the lower the transmission rate
• Whilst not covered in this article, Coloured Cast Acrylic will absorb more UV than Clear Cast Acrylic, due to the colour pigments, which have good UV stability
• Perspex® have a range of materials, Clear Cast VE Grade and Clear Cast VA Grade, which offer even lower UV transmission rates

The Perspex Guarantee

Clear Cast Acrylic Sheet

Perspex® clear cast acrylic sheet will not yellow and will retain high light transmission for 30 years.

• Light Transmission
  The light transmission for clear Perspex® will not fall below 85% (test method: ASTM D 1003 when measured using a cleaned and polished flat sample of 1mm thickness taken from the exposed sheet).
• Yellowness Index
  The yellowness index of clear cast Perspex® will not be more than 10 after a period of 30 years, dependent on the application and surrounding environment. This value is measured using a cleaned and polished flat sample in accordance with DIN 5036.

Download the Clear Cell Cast Acrylic Sheet Guarantee

Coloured Cast Acrylic Sheet

Perspex® cast acrylic Standard Colours sheet will comply in service with the performance specifications for a period of ten years.

• Elastic Modulus
  The rigidity of Perspex® acrylic will remain substantially unchanged in service. This means that the elastic modulus of a sample cut from a section of Perspex® acrylic sheet will not be less than 2500 MPa as delivered and for ten years after delivery (test method: ISO 178 measured on specimens removed from the exposed Perspex® acrylic installation).
• Tensile Strength
  Perspex® will suffer no unacceptable change in brittleness. This means that the tensile strength as delivered and for ten years after delivery of a sample cut from a section of Perspex® will not be less than 50 MPa. (test method: ISO R 527 measured on specimens, removed from the exposed Perspex® ).
• Colourfastness
  During a ten year outdoor exposure period all coloured surfaces can be expected to exhibit at least some degree of change. Lucite International guarantees that any changes in colour of the Perspex® sheet will be so small as to pass undetected by the casual observer at street level to the exposed Perspex® acrylic.

Download the Cell Cast Acrylic Sheet Colours Guarantee