Photo-stabilisation and photo-degradation of organic coatings containing a hindered amine light stabiliser: Part III—Kinetics of stabilisation during free radical oxidation

Abstract

The kinetics of nitroxide formation and decay and hindered amine light stabiliser consumption in organic coatings doped with bis (2,2,6,6-tetramethyl-4-piperidinyl) sebacate (HALS I) have been interpreted using a scheme based on conventional free radical oxidation. Rate constants have been determined by fitting experimental nitroxide kinetic data from coatings doped variously with HALS I- or a HALS-based nitroxide. Three rate constants are important to the understanding of the stabilisation kinetics of HALS I: the rate constant for the conversion of HALS I to nitroxide, the rate constant for the scavenging of free radicals by nitroxide to form aminoethers and the rate constant for the regeneration of nitroxide from aminoethers. These rate constants have been determined for both an acrylic/urethane and an acrylic/melamine coating as a function of exposure conditions. The rate constant for radical scavenging by nitroxide is independent of coating type. It is found that aminoethers are more easily converted to nitroxide than is HALS I. It is also found that the rate constants for the generation of nitroxide from HALS I are independent of humidity for the acrylic urethane coating but increase with increasing humidity in the acrylic/melamine coating. It is suggested that this is due to the influence of formaldehyde, released during hydrolysis of the acrylic/melamine coating, on the free radical chemistry.