Photo-discoloration of UV-curable acrylic coatings and the underlying wood

Wood specimens coated with four different UV-curable acrylic clear coatings were exposed to UV light. All samples turned yellow after the exposure. Among these specimens, the one coated with aromatic urethane modified acrylic coating showed the deepest yellowing. Uncoated wood specimens covered with free films of the coatings under study were used to simulate coated wood. After accelerated lightfastness tests, it was observed that the color and chemical changes of silicone and aliphatic urethane modified acrylic free films were very slight. However, the discoloration of the wood underneath these two films was quite distinct. This observation makes it obvious that the color change of wood coated with so-called durable coatings was primarily caused by the discoloration of wood underneath, not the coatings. The degree of discoloration of aromatic urethane and epoxy modified acrylic films increases with irradiation time. An increase in discoloration is also evident on the wood specimens underneath these two films. These facts show that in addition to inhibiting the discoloration of coatings, it is also necessary to reduce the intensity of ultraviolet light reaching the wood underneath the coatings. (C) 1999 Elsevier Science Ltd. All rights reserved.