Degradation of wood surfaces during natural weathering. Effects on lignin and cellulose and on the adhesion of acrylic latex primers

Abstract

Radiata pine veneers and blocks were exposed to natural weathering under Australian summer conditions over a period of 30 days. Infrared spectroscopy revealed that there was perceptible surface delignification after 4 hours exposure, substantial surface delignification after 3 days exposure and almost complete surface delignification after 6 days. Rapid lignin degradation was also suggested by measurements of the acid insoluble lignin content of weathered veneers. Viscometry determinations on holocellulose samples from weathered veneers and unweathered controls indicated significant depolymerisation of cellulose after 4 days exposure. A tape test was used to assess the adhesion of acrylic latex primers to weathered blocks. The adhesion of exterior acrylic primers decreased on weathered wood surfaces and was significantly lower on specimens that were weathered for 5 to 10 days. An oil-modified acrylic primer showed greater adhesion to weathered wood surfaces. Primer adhesion was lower on weathered radial surfaces than on similarly exposed tangential surfaces. The practical implications of these findings for the coating of exterior wood with acrylic latex primers are discussed briefly.