Photostabilization of wood using low molecular weight phenol formaldehyde resin and hindered amine light stabilizer

Abstract

Phenol formaldehyde (PF) resin containing low molecular weight hydroxymethyl phenol species and a water soluble hindered amine light stabilizer (HALS) was synthesized and used to treat wood veneers. Treated veneers were exposed to natural weathering for 25, 35 and 50 days and the effectiveness of the modified PF resins as photoprotective treatments for wood was assessed. We hypothesized that the photostability of veneers would be positively correlated with concentration of PF resin (10, 20 and 30% w/v) and HALS (1 or 2% w/w) in solutions used to treat veneers. There was an inverse relationship between resin concentration in treatment solutions and tensile strength losses of treated veneers exposed to natural weathering. Mass losses of veneers treated with solutions containing 20 or 30% PF resin were lower than those of veneers treated with 10% PF resin. The addition of 2% HALS to solutions containing 30% PF resin had a significant (p < 0.05) effect at restricting mass and tensile strength losses of treated veneers exposed to the weather for 50 days. A PF resin treatment containing 30% resin and 2% HALS was as effective as chromic acid (a noted photostabilizer for wood) at restricting mass and tensile strength losses of veneers during natural weathering. Treated veneer in a plywood-type composite became darker and redder when exposed outdoors, but the addition of HALS to the resin restricted color changes of veneer treated with 10% PF resin. A 30% PF resin treatment on its own or containing 2% HALS protected wood's cellular structure from destruction during accelerated weathering and restricted, but did not prevent, delignification of wood. We conclude that the effectiveness of low molecular weight PF resin as a photoprotective treatment for wood can be improved by increasing the concentration of PF resin and by combining it with a water soluble HALS.