DEGRADATION OF WOOD SURFACES DURING NATURAL WEATHERING - EFFECTS ON LIGNIN AND CELLULOSE AND ON THE ADHESION OF ACRYLIC LATEX PRIMERS

Radiata pine veneers and blocks were exposed to natural weathering und er Australian summer conditions over a period of 30 days. Infrared spe ctroscopy revealed that there was perceptible surface delignification after 4 hours exposure, substantial surface delignification after 3 da ys exposure and almost complete surface delignification after 6 days. Rapid lignin degradation was also suggested by measurements of the aci d insoluble lignin content of weathered veneers. Viscometry determinat ions on holocellulose samples from weathered veneers and unweathered c ontrols indicated significant depolymerisation of cellulose after 4 da ys exposure. A tape test was used to assess the adhesion of acrylic la tex primers to weathered blocks. The adhesion of exterior acrylic prim ers decreased on weathered wood surfaces and was significantly lower o n specimens that were weathered for 5 to 10 days. An oil-modified acry lic primer showed greater adhesion to weathered wood surfaces. Primer adhesion was lower on weathered radial surfaces than on similarly expo sed tangential surfaces. The practical implications of these findings for the coating of exterior wood with acrylic latex primers are discus sed briefly.