The water vapour adsorptionedesorption behaviour of naturally aged Tilia cordata Mill. wood Carmen-Mihaela Popescu a, * , Callum A.S. Hill b a “Petru Poni” Institute of Macromolecular Chemistry of Romanian Academy, 700487 Iasi, Romania b Forest Products Research Institute, Edinburgh Research Partnership Joint Research Institute in Civil and Environmental Engineering, Edinburgh Napier University, Edinburgh EH10 5DT, UK article info Article history: Received 5 February 2013 Received in revised form 20 April 2013 Accepted 17 May 2013 Available online 31 May 2013 Keywords: Dynamic water vapour sorption Tilia cordata wood Parallel exponential kinetic (PEK) model Sorption isotherm abstract The effect caused by the natural ageing of wood on the sorption properties during two consecutive sorption cycles of historical Tilia cordata Mill. wood has been investigated and compared with a reference sample. Differences were found in the sorption isotherm between the first and second sorption cycles for most of the samples and also between the reference and historical wood. All samples exhibited sorption hysteresis, but with important differences in behaviour observed with the historic wood samples. It was interesting to note that the oldest historic wood sample did not display any discernible difference between the first and second sorption cycle throughout much of the hygro- scopic range and also exhibited absolute hysteresis values essentially identical with the reference sample. These results indicate that the sorption behaviour of wood is dependent upon the previous exposure of the wood to atmospheric relative humidity. The sorption kinetics was also analysed in terms of the parallel exponential kinetics (PEK) model, and excellent fits to the data were obtained. The PEK model describes the dynamic sorption behaviour in terms of a fast and slow kinetic process and important differences in behaviour for these two processes were found. It is thought that the fast process is associated with a physical diffusion phenomenon, while the slow kinetics process is considered to be associated with cell wall matrix polymer relaxation processes. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Wooden Cultural Heritage is extremely rich and diverse, but, over time, is subjected to complex transformations influenced by numerous stress factors [1]. Apart from the environmental pa- rameters that contribute significantly to the degradation of wood, the rate of degradation may also be affected by the chemical structure of wood which interacts with environmental factors such as oxygen, moisture, pH, temperature, exposure to visible and ul- traviolet light, micro-organisms and contaminants [2]. Wood is a natural composite consisting mainly of cellulose, hemicelluloses and lignin which are strongly enmeshed and linked by hydrogen bonds and covalent cross-linkages. In addition to these polymeric components, wood may contain varying quantities of ex- tractives, including sugars, flavonoids, tannins, terpenes, fats and/or waxes [3,4]. It is well known that wood is a hygroscopic material; it can adsorb and/or desorb water from the surrounding environment, tending to reach an equilibrium condition when the atmospheric relative humidity (RH) is stable. The relationship between the equilibrium moisture content (EMC) and RH at constant tempera- ture is known as a sorption isotherm. The sorption isotherm is characteristic of the material, but is influenced by the temperature and previous sorption history of the sample under examination. Under unstable environmental relative humidity conditions, the wood moisture is changing continuously and equilibrium is rarely reached. Wood is also a highly anisotropic material, a significant factor which influences different properties, such as: hygroscopic- ity, shrinkage, swelling, rheology or mechano-sorption [5]. Under- standing the sorption behaviour is important for understanding of the degradation mechanisms influencing the natural ageing of wooden structures. Characterising historic wood is a challenging task due to the complex structure and its extent of degradation. Although different analytical techniques have been used to study historic wood in order to determine the species and the conservation state, the study of the water sorption behaviour of historic wood, is relatively * Corresponding author. Tel.: þ40 232217454; fax: þ40 232211299. E-mail address: mihapop@icmpp.ro (C.-M. Popescu). Contents lists available at SciVerse ScienceDirect Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab 0141-3910/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2013.05.021 Polymer Degradation and Stability 98 (2013) 1804e1813