Analysis of lignin degradation on wood surfaces to create a UV-protecting cellulose rich layer.

Wood is sensitive to photochemical degradation. The development of a wood surface protected against sun radiation remains a major challenge. Specimens from different species have been exposed to the outdoor environment and analysed at different depths and time steps by Fourier transform infrared spectroscopy. After 2 months of weathering, the peak representing the C=C aromatic bond disappeared on the surface. The radiation penetration depth in the latewood of softwoods is less than that in earlywood. The degradation of beech and oak is more gradual with depth and comparable to spruce and larch earlywood. The destruction of the anatomical structure begins with pits cracking. Then, the cracks proceed into the S2 layer, the middle lamella is degraded, leading to cells delamination and reduction of the surface mechanical strength. Treatment of the specimens with pressurised water vapour creates a cellulose layer on the surface, more obvious for hardwood than for softwood.