Search

Wood-water interactions are central to the utilization of wood in our society since water affects many important characteristics of wood. This topic has been investigated for more than a century, but new knowledge continues to be generated as a result of improved experimental and computational methods. This review summarizes our current understanding of the fundamentals of water in wood and highlights significant knowledge gaps. Thus, the focus is not only on what is currently known but equally important, what is yet unknown. The review covers locations of water in wood; phase changes and equilibrium states of water in wood; thermodynamics of sorption; terminology including cell wall water (bound water), capillary water (free water), fiber saturation point, and maximum cell wall moisture content; shrinkage and swelling; sorption hysteresis; transport of water in wood; and kinetics of water vapor sorption in the cell wall., Wood-water interactions are central to the utilization of wood in our society since water affects many important characteristics of wood. This topic has been investigated for more than a century, but new knowledge continues to be generated as a result of improved experimental and computational methods. This review summarizes our current understanding of the fundamentals of water in wood and highlights significant knowledge gaps. Thus, the focus is not only on what is currently known but equally important, what is yet unknown. The review covers locations of water in wood; phase changes and equilibrium states of water in wood; thermodynamics of sorption; terminology including cell wall water (bound water), capillary water (free water), fiber saturation point, and maximum cell wall moisture content; shrinkage and swelling; sorption hysteresis; transport of water in wood; and kinetics of water vapor sorption in the cell wall.

Water vapor sorption is an important characteristic of wood as an engineering material. In addition to affecting engineering properties such as the heat capacity, thermal conductivity, strength, stiffness, and dimensions, moisture is fundamental to many wood degradation mechanisms including fungal decay. Water vapor sorption isotherms have been gaining increasing attention in the wood literature as more laboratories adopt automated sorption balances, frequently called dynamic vapor sorption (DVS) analyzers. With DVS, the collection of sorption isotherms can be automated, completely transforming what was once a series of tedious manual measurements. The adoption of DVS, however, preceded careful studies of the precision and accuracy of the measurement; as a result, many of the previously published studies have been cast under scrutiny in recent years as the technique has been further studied. This paper summarizes the advancements in understanding of the relationship between experimental method and experimental errors with DVS along with best practices that should be used when collecting DVS data in future studies.

Sorption isotherm models are frequently used to understand the interactions of water with cellulosic materials. Many commonly used models, such as the Guggenheim-Anderson-de Boer, Hailwood-Horrobin, and Dent isotherm models, are based on idealized physical systems. In the scientific literature, these models are often used to predict properties of wood such as the monolayer capacity, which is a representation of the number of available sorption sites in the material. Reporting such properties assumes that an idealized system adequately represents actual wood-water interactions. These models can be reduced to the same form, where the ratio of water activity, aw, to equilibrium moisture content (EMC), is a second-order polynomial function of aw. Here we review twelve models that have a parabolic form, fit these models to high quality water vapor sorption isotherm data for wood at multiple temperatures, and calculate physical properties from the model parameters. Moreover, variability in model predictions arising from measurement uncertainties in aw and EMC values is propagated using a Monte Carlo method. Predicted physical properties are tested against independently measured values of hydroxyl accessibility, relative amounts of primary and secondary water, and enthalpy of sorption. None of the models accurately predicts any of the physical properties, even when the reported measurement uncertainties are increased by a factor of three. Therefore, it must be concluded that parabolic sorption isotherm models are not valid for water in wood cell walls., Sorption isotherm models are frequently used to understand the interactions of water with cellulosic materials. Many commonly used models, such as the Guggenheim-Anderson-de Boer, Hailwood-Horrobin, and Dent isotherm models, are based on idealized physical systems. In the scientific literature, these models are often used to predict properties of wood such as the monolayer capacity, which is a representation of the number of available sorption sites in the material. Reporting such properties assumes that an idealized system adequately represents actual wood-water interactions. These models can be reduced to the same form, where the ratio of water activity, aw, to equilibrium moisture content (EMC), is a second-order polynomial function of aw. Here we review twelve models that have a parabolic form, fit these models to high quality water vapor sorption isotherm data for wood at multiple temperatures, and calculate physical properties from the model parameters. Moreover, variability in model predictions arising from measurement uncertainties in aw and EMC values is propagated using a Monte Carlo method. Predicted physical properties are tested against independently measured values of hydroxyl accessibility, relative amounts of primary and secondary water, and enthalpy of sorption. None of the models accurately predicts any of the physical properties, even when the reported measurement uncertainties are increased by a factor of three. Therefore, it must be concluded that parabolic sorption isotherm models are not valid for water in wood cell walls.

The performance of cellulosic materials is intimately linked to their moisture content. Over the last century, many studies have been devoted to examining the moisture uptake in equilibrium with surrounding environmental conditions. However, with the adoption of automated sorption balances in laboratories worldwide, it has become increasingly popular to study the approach to equilibrium, i.e., sorption kinetics. Recording the moisture content change between two equilibrium states produces a wealth of data, which necessitates a robust methodology for analysis. In this study, we employ exponential decay analysis on sorption kinetic data obtained in a previous study for loblolly pine, holocellulose, office paper and microcrystalline cellulose. Unlike other methods that are constrained by a fixed number of fitting parameters or similar constraints, exponential decay analysis is a flexible, data-driven methodology for analyzing sorption kinetics. No physical meaning is assigned to the output from the analysis. However, the methodology yields robust results in relation to various measurement uncertainties. As a result, exponential decay analysis can provide a reliable description of the dominant time scales of sorption kinetics. Hereby, the methodology offers a new way to evaluate the suitability of theoretical models for describing sorption kinetics.

Sorption of water vapor in cellulosic materials has been studied for more than 100 years. However, the last 2 decades have seen a surge in studies of sorption kinetics due to the adoption of automated sorption balances in laboratories worldwide. Sorption kinetic data is commonly fitted with the parallel exponential kinetics (PEK) model, which is the sum of two exponential functions. The PEK model parameters are often interpreted as representing physical quantities or material properties, but their exact definition remains undetermined. Nonetheless, PEK fitting of sorption data has been widely used to identify various physical properties. Recent work has called into question the protocols used to collect sorption data with automated sorption balances and the methods used to analyze the data. Typically, data acquisition is interrupted before equilibrium when the rate of mass change fulfills a user-specified criterion, and this has been shown to severely mischaracterize both the equilibrium moisture content and the full kinetic behavior. The objective of this work is to examine whether interrupting data acquisition before equilibrium affects the PEK model parameters and further explore whether PEK model interpretations have physical significance as claimed in the literature. The analysis shows that the PEK model cannot capture the actual form of the sorption curves for loblolly pine and microcrystalline cellulose across a range of relative humidity (RH) steps in both absorption and desorption. PEK model parameters depend not only on the mass stability criterion that controls the measurement hold time, but also on the RH step making even qualitative comparisons of a single sample across different RH steps meaningless. The PEK model cannot be used to derive physically meaningful properties from water vapor sorption measurements and its use as a tool for exploring the properties of cellulosic materials interacting with water is not recommended.

Water vapor sorption is the most fundamental aspect of wood-moisture relations. It is directly or indirectly related to the physical properties of wood and the onset of wood-damage mechanisms. While sorption properties of cellulosic materials have been utilized since antiquity, the time-dependent transition from one moisture content to another (i.e., sorption kinetics) has received much less attention. In this critical review, we present the state-of-the-art of water vapor sorption kinetics in wood. We first examine different experimental methods that have been used to measure sorption kinetics, from the quartz helix vacuum balance beginning in earnest in the 1930s, to automated sorption balances used recently. We then give an overview of experimental observations and describe the physical phenomena that occur during the sorption process, which potentially govern the following kinetics: boundary layer mass transfer resistance, heat of sorption, cell wall diffusion, swelling, and polymer mobility. Finally, we evaluate theoretical models that have been proposed for describing sorption kinetics, considering both experimental data and the physical processes described in the previous section. It is clear that no previously developed model can phenomenologically describe the sorption process. Instead, new models are needed. We conclude that the development of new models will require more than simple gravimetric measurements. In addition to mass changes, complementary techniques are needed to probe other important physical quantities on multiple length scales.

It has been known for over 35 years that commonly used sorption isotherm models fail to correctly predict wood–water properties such as heat of sorption. Despite this, their use to determine thermodynamic quantities and monolayer moisture contents persists and in fact is increasing in frequency. In this paper, we recommend the use of the “ABC isotherm,” which is mathematically equivalent to sorption isotherm models commonly used for wood but has the added benefits of simplicity, avoidance of a conceptual image of water sorption that is contradicted by measurements, and avoidance of any additional step of identifying quantities that are physically incorrect.