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Heat exposure and Acrylic/Polycarbonate degradation in skylights

August 5, 2020

Skylights can be made from a number of hard, transparent materials, typically some type of plastic. The two general classes of plastic most commonly used include acrylic glass and polycarbonates. These materials can become yellow over time in response to ultraviolet (UV) from the sun, indicating a weakening of their mechanical strength in addition to the loss of transparency. The warranties of some skylights protect the buyer from excessive yellowing, but they may also require regular cleaning.


Polymethyl methacrylate (PMMA), commonly known as acrylic glass, also has several trade names such as Acrylite, Lucite, Perspex, and Plexiglas. It’s a transparent thermoplastic material often used as a shatter-resistant alternative to normal glass, although it can also be a casting resin in inks and other coatings. Acrylic glass has greater impact resistance than standard glass and a density of 1.2 grams per cubic centimeter (g/cm3), which is less than half that of standard glass.

Polycarbonates (PC) are a group of thermoplastic polymers that contain the carbonate chemical group. This structure provides these materials with greater strength than acrylic glass, and some types are also transparent. Polycarbonate also has a greater tolerance for temperature extremes, which is a particularly desirable property for skylights. Furthermore, polycarbonate is highly transparent to visible light, more so than many types of glass.

Polycarbonate is easy to work and mold into a particular shape such as a skylight dome. However, it’s also easy to scratch. Manufacturers must, therefore, protect polycarbonate skylights with a hard coating.


Scientists can measure the yellowing of materials with various yellow indices (YI)s that allow them to compare a material’s current color with that of its original state. This difference in color is also known as its delta YI since the Greek letter delta is often used in science to indicate a change in value. This process generally involves taking a series of measurements under controlled conditions to obtain values for parameters. A formula then uses these parameters to calculate a single value for that particular YI.

Industries currently use two YIs to assess the yellowing of materials, which include ASTM D1925-70 and ASTM E313-73. Both formulas use the X, Y, and Z tristimulus values defined by the CIE 1931 XYZ color space. The specific definition of these values is complex, but X, Y, and Z generally correspond to the primary colors red, green, and blue, respectively.

ASTM D1925-70, also known as the Standard Test Method for Yellowness Index of Plastics, was discontinued in 1995, although some manufacturers still use it. It uses the formula YI = 100 (1.274641506X – 1.057434092Z)/Y. ASTM E313-73 is also known as the Standard Test Method for Indexes of Whiteness and Yellowness. ASTM reapproved this standard in 1993, which uses the following formula YI = 1 – (0.847Z)/Y.

Clear air has a YI=0.303, while YI=2 is the lowest amount of yellowing that’s generally noticeable. YI=10 is moderate yellowing, and YI=20 and higher indicate extreme yellowing. Skylight warranties with yellowing provisions often guarantee YI=2 or less for up to ten years.


A skylight typically requires cleaning once or twice each year to delay yellowing as long as possible and keep the skylight within its warranty. The following procedure will accomplish this without further damaging the skylight.

Rinse the skylight with cold water to remove the majority of the debris. Prepare a mixture of 80 percent lukewarm water and 20 percent mild household detergent, and use it to wash the skylight with a soft, clean cloth or cellulose sponge. Rinse the skylight with pressurized cold water. Dry it with a chamois cloth to prevent water spots. Homeowners may also want to finish by waxing the skylight with plastic polish.

It’s essential to avoid cleaning plastic skylights with anything other than mild detergent, as other materials can damage them. Abrasive powder cleaners can scratch the surface, giving the skylight a hazy appearance. Chemicals like acetone, benzene, and gasoline can permanently discolor plastic skylights.

Don’t use squeegees or similar applicators, as they can trap grit that can cause scratches. Clean skylights only when required, as excessive cleaning also increases the risk of scratches. It’s also important to avoid cleaning skylights during very hot weather, which can warp the skylight when the water suddenly cools it.


Vtech’s patented reaction injection molding (RIM) process involves chemically bonding the light directly to the frame, which makes our skylights the best choice for the money. We use only glass in our skylights, thus preventing yellowing, scratching, cracking, and leaking. All Vtech solid-state skylights are covered by our 20-year transferable warranty, which doesn’t require maintenance on the skylights.

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