Chemisorption and physical adsorption of light stabilizers on pigment and ultrafine particles in coatings

Hindered amine light stabilizers (HALS) and UV absorbers (UVA) added to pig mented coatings adsorb on certain pigment and ultrafine oxide particles. An alysis of adsorption isotherms taken in xylene solutions and spectroscopic data revealed that chemisorption is the dominant interaction mechanism betw een the light stabilizers and the pigment particles. In contrast, the stabi lizer interactions with ultrafine oxide particles are based on van der Waal s forces, i.e., physical adsorption. In thermoset coatings, the chemisorbed stabilizers remain bound to the pigment surface on cure. The physically ad sorbed stabilizers desorb on cure and migrate within the coatings. The chem istries involved in chemisorption were investigated using Fourier transform infrared spectroscopy and time-of-flight secondary ion mass spectroscopy. Stabilizers containing ester linkages hydrolyze to varying degrees into car boxylate salts and alcohols. Acetyl groups also hydrolyze. Some of the chem isorbed light stabilizers lose the ability to protect coatings from photode gradation.