Surface qualification of weathered wood

It is expected that surface delignification of wood would enhance the durability of a transparent coating. To assert suitability of outdoor natural weathering as a wood surface delignification process, wood samples of four species (Picea abies, Larix deciduas, Fagus sylvatica, Quercus robur) have been exposed to outdoor environment for one year. Regularly, their roughness, colour and water contact angle have been measured. Besides, samples of the same species have been exposed to UV irradiation at 50 W/m2 for 100 hours. In addition to the same measurements as for the naturally weathered samples, FTIR measurements have been done on those samples. Four steps during the first year of natural weathering have been clearly identified. First, contact angle, roughness and darkness increase. In a second phase they decrease. In a third phase, darkness and roughness increase again, while contact angle stay low. Finally, beech gets lighter again, while contact angle on oak increases. These four phases respectively correspond to lignin degradation, leaching of the degraded lignin, fungal colonization, and steady erosion. From these results, it can be expected that a coating applied after the second phase (1 month), or after a forced leaching of the lignin after the first phase (2 weeks), will have an enhanced durability.