Headlight and trim aftercare

Why do plastic headlights turn yellow and cloudy?

Most headlights are made of plastic. AAA's December 11, 2018 release says sunlight breaks down the plastic coating, causing discoloration that diminishes the headlight's ability to provide adequate light.

In AAA's laboratory tests of headlights from two sedans about 11 years old, the low beam produced just 22 percent of the light of a new headlight. AAA says headlights can begin showing signs of deterioration as early as three to five years, depending on where and how the vehicle is used. In Consumer Reports' experience, some makes and models seem to be more susceptible to clouding than others, depending on the lens's size, angle and composition and on whether the car is garaged or spends its life parked on the street.

Can anything permanently stop headlights from yellowing, and how long did restoration last in testing?

AAA's Via article says nothing can permanently stop headlights from yellowing or clouding.

Consumer Reports tested restoration kits in 2016 and found that many of the tested lenses had begun to haze over again a year later.

Why does black plastic trim fade?

Dr. Beasley's, a car-care maker, says most exterior plastic trim is made from plastics such as polypropylene and polyethylene. It explains that UV energy can break the chemical bonds in these polymer chains, a process called photo-oxidation, and the black or gray color begins to fade.

The same car-care source describes the white, zebra-like streaks that can show up on trim. They follow stress bands in the molded plastic, which weaken as the part expands and contracts with temperature, and the carbon black that colors the plastic is pushed out where they run.

What signs of headlight deterioration does AAA describe?

AAA's December 2018 release notes that most drivers are not in the habit of routinely inspecting their headlights. It suggests checking them for changes in appearance such as yellowing or clouding, and says that if the bulb is difficult to see, it is time to have the lenses replaced or restored as soon as possible.