This paper reviews recent findings on wood–water interaction and puts them into context of established knowledge in the field. Several new findings challenge prevalent theories and are critically discussed in an attempt to advance current knowledge and highlight gaps. The focus of this review is put on water in the broadest concept of wood products, that is, the living tree is not considered. Moreover, the review covers the basic wood–water relation, states and transitions. Secondary effects such as the ability of water to alter physical properties of wood are only discussed in cases where there is an influence on state and/or transition.

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A critical discussion of the physics of wood–water interactions

Recent insights into the physical biology of plant cell walls are reviewed, summarizing the essential differences between primary and secondary cell walls and identifying crucial gaps in our knowledge of their structure and biomechanics. Unexpected parallels are identified between the mechanism of expansion of primary cell walls during growth and the mechanisms by which hydrated wood deforms under external tension. There is a particular need to revise current “cartoons” of plant cell walls to be more consistent with data from diverse approaches and to go beyond summarizing limited aspects of cell walls, serving instead as guides for future experiments and for the application of new techniques.

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Comparative structure and biomechanics of plant primary and secondary cell walls.

Summarizes information on wood as an engineering material. Presents properties of wood and wood-based products of particular concern to the architect and engineer. Includes discussion of designing with wood and wood-based products along with some pertinent uses.

https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190/chapter_04.pdf

Moisture relations and physical properties of wood

Low-field permanent magnets for industrial process and quality control.

Wood Chemistry, Fundamentals and Applications, Second Edition, examines the basic principles of wood chemistry and its potential applications to pulping and papermaking, wood and wood waste utilization, pulping by-products for production of chemicals and energy, and biomass conversion.

Wood chemistry: fundamentals and applications.

Abstract Desorption isotherms at 20°C for untreated, acetylated, and furfurylated Norway spruce [Picea abies (L.) Karst.] sapwood were established in the 91.9–99.9% relative humidity (RH) range. Three methods were employed to secure various constant RH levels: saturated salt solutions, climate chambers, and the pressure plate technique. The curve form for the untreated samples did not show an upward bend, except perhaps above 99.5% RH, indicating that – contrary to what has hitherto been assumed – capillary condensation does not play a significant role for water sorption in wood below fiber saturation. Three additional results corroborate this conclusion: (1) calculation of the theoretical contribution of capillary condensation to the moisture content (MC) in wood based on idealized microstructural geometries by means of the Kelvin and Laplace equations resulted in very small contributions to the equilibrium moisture content (EMC), i.e., below 0.35% moisture at 99.9% RH. (2) The ratio between the EMC of acetylated and untreated samples did not show an increasing trend for increasing RH, as would have been the case if capillary condensation had taken place in both untreated and acetylated wood. (3) Low field time domain nuclear magnetic resonance results showed that only the relaxation curves from the furfurylated samples were affected systematically by freezing, indicating that neither untreated nor acetylated wood contained significant amounts of capillary condensed water.

Water sorption in wood and modified wood at high values of relative humidity: Part I: Results for untreated, acetylated, and furfurylated Norway spruce

It is a commonly accepted notion that the equilibrium moisture content (EMC) of wood at a given relative humidity (RH) is highest during initial desorption of green wood due to an irreversible loss of hygroscopicity during the 1 st desorption. The basis for this notion is investigated by assessing how drying and saturation procedures influence the differences between the 1 st and the 2 nd desorption curves for Norway spruce (Picea abies (L.) Karst.) sapwood. The study establishes 1 st and 2 nd desorption isotherms for a variety of initial conditions and it covers the RH range from 60.1% to 99.9%. The state of the water is not affected by oven-drying and rewetting as demonstrated by time domain low field NMR relaxometry. The results challenge the conclusions of earlier studies and indicate that in these studies the 2 nd desorption was initiated at much too low EMC and therefore fails to describe a boundary desorption isotherm. Instead, it becomes an intermediate desorption isotherm starting at the adsorption boundary curve and crossing over to eventually meet the desorption boundary curve. The results also show that vacuum drying at room temperature only gives a modest loss of hygroscopicity compared to the green state. Conversely, oven-drying at 1038C results in a more significant loss of hygroscopicity, except for RH above 96% where an increase in EMC surprisingly is seen.

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Equilibrium moisture content (EMC) in Norway spruce during the first and second desorptions

A theoretical study of the amount of moisture held in wood as capillary condensed water in the relative humidity (RH) range of 90–99.9% is carried out. The study is based on idealized geometries of the softwood structure related to micrographs. It is confined to structural elements such as bordered pits and the pointed ends of tracheids. The theoretical amount of water in these elements is found by employing the Kelvin equation. An equal amount of earlywood and latewood cells with different geometries and with different amounts of pits is assumed. The effect of pit aspiration is considered, and different degrees of pit aspiration are assigned to earlywood and latewood. We suggest based on the results that capillary condensation makes only a very small contribution to the equilibrium moisture content. At 99.9% RH the contribution amounts to less than 0.0035 kg water per kg dry wood. This is in line with the experimental results presented in Part 1 of this study.

Water sorption in wood and modified wood at high values of relative humidity. Part 2: Appendix. Theoretical assessment of the amount of capillary water in wood microvoids

The time-dependent mechanical behaviour (TDMB) of wood is important when using the material for structural purposes. Recently, a new method for predicting the TDMB by numerical modelling was established based on the assumption that TDMB is caused by the sliding of the microfi brils past each other. In this study, the TDMB is examined via creep experiments on small specimens of Norway spruce latewood. The results of these are compared with results from numerical modelling. The experiments include results at two levels of moisture content and three levels of temperature, enabling an investigation of these two climatic factors on TDMB of wood. It was found that the mechanical response of wood tissue is the sum of responses from both tracheids and middle lamella, with only the previous being reversible. The effect of moisture and temperature differed in that the latter affected the elastic and time-dependent responses equally. Moisture, on the other hand, reduced both the elastic properties and the activation energy barrier for sliding of the microfi brils, but furthermore changed the microfi bril angle of the sample as a result of swelling. Hereby, moisture had a larger effect on the time-dependent response than the elastic. All of these effects were predicted by numerical modelling.

Emil Tang Engelund

Papers

A critical discussion of the physics of wood–water interactions

Water sorption in wood and modified wood at high values of relative humidity: Part I: Results for untreated, acetylated, and furfurylated Norway spruce

Equilibrium moisture content (EMC) in Norway spruce during the first and second desorptions

Water sorption in wood and modified wood at high values of relative humidity. Part 2: Appendix. Theoretical assessment of the amount of capillary water in wood microvoids

Tensile creep and recovery of Norway spruce influenced by temperature and moisture