Clear plastic, once pristine and transparent, often develops a yellow tint over time. This is not merely a sign of aging or dirt, but indicates complex chemical changes within the plastic material itself. Understanding these processes explains why it happens and how its progression might be managed.
The Primary Chemical Reaction
The main reason clear plastics turn yellow is photodegradation, primarily influenced by ultraviolet (UV) light. Plastics are long chains of molecules called polymers. When exposed to UV radiation, chemical bonds within their structure break down, forming highly reactive free radicals.
These free radicals react with oxygen in the air, a process called photo-oxidation. This incorporates oxygen into the polymer, creating new chemical structures. Among these are “chromophores,” molecular groups that absorb visible light. Their absorption of certain light wavelengths makes the plastic reflect yellow light, causing discoloration.
Common clear plastics like polycarbonate (PC) and polyvinyl chloride (PVC) are susceptible. Polycarbonate yellows from UV exposure and oxidation. PVC can yellow from photo and thermal degradation. The overall process is autocatalytic, meaning the initial degradation products can accelerate further reactions, leading to more free radicals and reactive oxygen species.
Other Contributing Factors
While UV light and oxidation are primary drivers, other environmental factors also contribute to plastic yellowing. Elevated temperatures accelerate the degradation of the polymer structure, causing it to break down faster and promoting increased oxidation. This causes plastics in hot environments, like direct sunlight or industrial settings, to yellow more quickly.
Certain chemicals or oils can also cause discoloration. Substances in cleaning products, solvents, or human skin oils can react with plastic components, leading to degradation or color changes. Atmospheric pollutants, such as nitrogen oxides (NOx), can react with plastic additives, contributing to a yellowish or pinkish tint, a phenomenon sometimes called “gas fading.”
Preventing Yellowing
Preventing clear plastic from yellowing involves minimizing exposure to degradation factors. Keeping plastic items out of direct sunlight reduces UV light exposure. Storing plastics in cooler environments slows chemical reactions. Regular cleaning with mild, non-abrasive cleaners removes surface oils and residues that contribute to degradation.
During manufacturing, specific additives enhance a plastic’s resistance to yellowing. UV stabilizers, like UV absorbers and Hindered Amine Light Stabilizers (HALS), protect the polymer. UV absorbers absorb harmful UV radiation, while HALS neutralize free radicals. Plastics with these protections or UV-resistant coatings maintain clarity longer.
Restoring Yellowed Plastic
For plastic that has already yellowed, methods exist to reverse discoloration, though results vary. A common approach, “Retrobrighting,” uses hydrogen peroxide, often with UV light. Hydrogen peroxide acts as an oxidizing agent; when activated by UV light, it forms reactive hydroxyl radicals. These radicals break down the chromophores causing the yellow appearance, essentially bleaching the plastic.
To do this, submerge the yellowed plastic in a hydrogen peroxide solution and expose it to direct sunlight or an artificial UV light source for hours or days. The yellowing indicates that the plastic’s molecular structure has already been altered. While the color may be lessened, the underlying material integrity might remain compromised. Caution is advised with strong chemicals like concentrated hydrogen peroxide, and the effect can be temporary, with yellowing potentially returning.