Rates of photooxidation induced crosslinking and chain scission in thermoset polymer coatings - II. Effect of hindered amine light stabilizer and ultraviolet light absorber additives

The chemical stress relaxation behavior of two acrylic/melamine clearcoats has been investigated to determine the effect of hindered amine light stabi lizers (HALS) and ultraviolet light absorbers (UVA) on the rates of photoox idation induced crosslink/chain scission and crosslink formation. The addit ion of a HALS to one of the clearcoats, Mel'-A, slows the rate of crosslink /chain scission and in high enough concentrations stops the formation of cr osslinks. In the other clearcoat, Mel'-N, the addition of small amounts of either HALS or UVA slows the rate of photooxidation. The overall changes in crosslink density are the same for Mel'-A containing small amounts of HALS and for Mel'-A containing moderate amounts of both HALS and UVA, indicatin g that the same changes in crosslink density can be achieved by different d egradation pathways. These findings indicate that small changes in clearcoa t stabilization packages can have dramatic effects on long-term weathering performance of acrylic/melamine coatings. (C) 2000 Elsevier Science Ltd. Al l rights reserved.