Accelerated Weathering of Coated and Uncoated Beech Wood Modified with Citric Acid

Chemical modifi cation of wood can minimize wood drawbacks in exterior application, such as moisture absorption, swelling and shrinkage, susceptibility to photodegradation and microbial attack. We modifi ed a beech wood with 7.0 % water solution of citric acid and 6.5 % sodium-hypophosphite monohydrate (SHP) as a catalyst and investigated the color stability and weathering performance of coated and uncoated wood under accelerated weathering conditions. The modifi ed and unmodifi ed beech wood samples were coated with transparent water-borne stain and transparent solvent-borne stain, and with reference stain. The accelerated outdoor exposure was conducted in a QUV weathering tester (Q-Panel Company) equipped with UVA-340 fl orescent lamps for 56 days (1344 hours). The surface of samples was examined for color and gloss changes, adhesion and appearance of fl aking, cracking, blistering and chalking. The overall color change (E*) of coated beech wood samples at the end of exposure was smaller in unmodifi ed wood samples. The trend of gloss changes of modifi ed and unmodifi ed wood samples was very similar during the exposure. At the end of exposure unmodifi ed beech wood samples exhibited better weathering performance than modifi ed beech wood samples. In order to achieve the optimal protection of wood modifi ed with citric acid the new type of fi nishes should be developed.