Photostabilization and photodegradation in organic coatings containing a hindered amine light stabilizer. Part VII. HALS effectiveness in acrylic melamine coatings having different free radical formation rates

Abstract

The ability of hindered amine light stabilizer (HALS) bis-(2,2,6,6-tetramethyl-4-piperidinyl) sebacate to reduce the photooxidation rate in two different acrylic melamine polymer coatings has been studied using both infrared and electron spin resonance spectroscopies. These coatings differ primarily in the rate at which free radicals are formed during photolysis. Under controlled photolysis conditions similar to sunlight (near-ambient), the photoinitiation rate (PIR) of free radicals in one coating is some five times higher than that of the other coating, resulting in a photooxidation rate that is some 2·5 times greater than that for the coating with the lower PIR value. When harsh, short wavelength ultraviolet light is used, the PIR values and photooxidation rates of both coatings are higher. The addition of hindered amine (2% by weight) to the coating with the higher PIR value reduces its photooxidation rate by roughly a factor of 2·6 for both exposures. The addition of the hindered amine to the coating with the lower PIR value reduces its photooxidation rate by a factor of 1·9 for the harsh exposure, but does not reduce its photooxidation rate under near-ambient exposure conditions. These results point out the risk of using harsh, accelerated tests to evaluate stabilizer performance and also suggest that there is a limit to HALS inhibition in acrylic melamine coatings.