The tendency of many Plastic materials to yellow is due to degradation by UV or visible light.
Optical brighteners (OBs) or fluorescent whitening agents are chemicals that manufacturers add to their products, such as papers, plastics, and textiles, to give them a brighter and cleaner surface, and reduce the rate of natural yellowing that occurs over time. OBs trick our eyes into seeing a brighter white.
These chemical are excited by wavelengths of light in the near-ultraviolet (UV) range (360 to 365 nm) and then emit light in the blue range (400 to 440 nm).