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Comparative study of photodegradation of six wood species after short-time UV exposure

Abstract

The effect of UV irradiation on different wood species represents an open subject of research. Six species, half softwoods and half hardwoods, all important for both historic and modern furniture, were subjected to the action of artificial UV radiation at a constant temperature of 40 °C. The species were: fir (Abies alba), pine (Pinus sylvestris), larch (Larix decidua), ash (Fraxinus excelsior), cherry (Prunus/Cerasus avium) and walnut (Juglans regia). Previous research showed that important and rapid changes occur within the first 24 h of exposure, and therefore, a short exposure time was adopted in this research, where the specimens were subjected to the action of UV-rich artificial light (UVB–UVA–Vis, 295–600 nm, UVA-Spot 400T source), at a constant temperature of 40 °C, in a climatic chamber. The effect of exposure was measured after 6, 12, 18 and 24 h. The test samples were covered with black cardboard on half of their surface to block the access of UV radiation and allow the investigation of temperature effect with none or limited UV penetration. The experiments proved that UV radiation caused rapidly colour changes (after 6 h), as darkening, monitored by direct observations and colour measurements in the CIE Lab system. The overall colour difference ΔE evolved in time, reaching, after 24 h UV exposure at 40 °C, a maximum for pine (15.93) and minimum for ash (7.05). FTIR–ATR analyses were performed to determine surface chemical structure changes associated with the measured colour changes. UV radiation caused photodegradation phenomena as lignin degradation and photooxidation processes leading to the formation of carbonyl containing chromophores, even after short-time exposure periods (6 h), which evolved in time following patterns influenced by the wood species. The moderate temperature had only a limited effect on the covered specimens, on both colour and surface chemistry changes, compared to the effect combined with UV irradiation, but particularities were observed in the behaviour of the wood species studied. The combined analysis of FTIR–ATR results with CIE Lab measurements showed only a slight trend in case of yellowness, which was associated with lignin degradation, but none for redness, associated mostly with wood extractives. The different behaviour of the wood species studied may be related to the differences in their chemical composition, especially lignin and extractives content.

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Acknowledgments

We hereby acknowledge the structural founds project PRO-DD (POS-CCE, O.2.2.1., ID 123, SMIS 2637, ctr. No 11/2009) for providing the infrastructure used in this work. The authors are also grateful to Mrs. Mihaela Pamfile Lazar for her contribution to colour measurements.

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Correspondence to Maria Cristina Timar.

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Timar, M.C., Varodi, A.M. & Gurău, L. Comparative study of photodegradation of six wood species after short-time UV exposure. Wood Sci Technol 50, 135–163 (2016). https://doi.org/10.1007/s00226-015-0771-3

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Keywords

  • Lignin
  • Wood Species
  • Lignin Degradation
  • Juglans Regia
  • Black Cardboard