Photodegradation of thermally-modified Scots pine and Norway spruce investigated on thin micro-veneers

The exposure to ultraviolet light results in surface photodegradation of uncoated wood during exterior application. When using thermally-modified wood in exterior applications, chemical changes that occur during the modification process might affect a subsequent photodegradation. This study investigates the impact of artificial weathering on the photodegradation of thin micro-veneers of thermally-modified Scots pine and Norway spruce by means of FT-IR spectroscopy and micro-tensile strength testing. FT-IR spectra revealed photodegradation reactions of lignin that resulted in the disappearance of the peak at 1508 cm⁻¹ after 144 h, irrespective of the thermal modification process. Loss in micro-tensile strength by photodegradation was higher for finite- than for zero-span micro tensile strength as a result of the loss of amorphous carbohydrates. Although strength loss of unweathered veneers was evident for thermally modified wood, the rate at which strength loss occurs during artificial weathering was considerably decreased by the thermal modification process. It was concluded that the amorphous carbohydrates were less degraded during artificial weathering as they were already pre-degraded during the thermal modification process.