Mechanism of ultraviolet stabilization of polymers by aromatic salicylates

Many derivatives of phenyl salicylate turn yellow when they are exposed, in solution, to ultraviolet light. This limits their usefulness as ultraviolet stabilizers for plastics. Evidence is presented to support the postulation that the yellowing is due to the formation, by a photochemical ortho rearrangement, of a derivative of 2,2′‐dihydroxybenzo‐phenone. Aromatic salicylates, such as 2,6‐dialkylphenyl salicylate, in which the ortho rearrangement is blocked, are found to be nonyellowing stabilizers, although less effective than the better yellowing varieties, such as p‐octylphenyl salicylate. Loss of stabilizer by volatilization, either in compounding or during subsequent exposure, is shown to be an important factor in limiting the effective performance of the stabilizer.