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 pigmented coatings adsorb on certain pigment and ultrafine oxide particles. Analysis of adsorption isotherms taken in xylene solutions and spectroscopic data revealed that chemisorption is the dominant interaction mechanism between the light stabilizers and the pigment particles. In contrast, the stabilizer interactions with ultrafine oxide particles are based on van der Waals forces, i.e., physical adsorption. In thermoset coatings, the chemisorbed stabilizers remain bound to the pigment surface on cure. The physically adsorbed stabilizers desorb on cure and migrate within the coatings. The chemistries 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 carboxylate salts and alcohols. Acetyl groups also hydrolyze. Some of the chemisorbed light stabilizers lose the ability to protect coatings from photodegradation.