On Wood–Water Interactions in the Over-Hygroscopic Moisture Range—Mechanisms, Methods, and Influence of Wood Modification

Maria Fredriksson

doi:10.3390/f10090779

Abstract

Wood is a hygroscopic material that absorbs and desorbs water to equilibrate to the ambient climate. Within material science, the moisture range from 0 to about 95–98% relative humidity is generally called the hygroscopic moisture range, while the exceeding moisture range is called the over-hygroscopic moisture range. For wood, the dominating mechanisms of moisture sorption are different in these two moisture ranges; in the hygroscopic range, water is primarily bound by hydrogen bonding in cell walls, and, in the over-hygroscopic range, water uptake mainly occurs via capillary condensation outside cell walls in macro voids such as cell lumina and pit chambers. Since large volumes of water can be taken up here, the moisture content in the over-hygroscopic range increases extensively in a very narrow relative humidity range. The over-hygroscopic range is particularly relevant for durability applications since fungal degradation occurs primarily in this moisture range. This review describes the mechanisms behind moisture sorption in the over-hygroscopic moisture range, methods that can be used to study the interactions between wood and water at these high humidity levels, and the current state of knowledge on interactions between modified wood and water. A lack of studies on interactions between modified wood and water in the over-hygroscopic range was identified, and the possibility of combining different methods to acquire information on amount, state, and location of water in modified wood at several well-defined high moisture states was pointed out. Since water potential is an important parameter for fungal growth, such studies could possibly give important clues concerning the mechanisms behind the increased resistance to degradation obtained by wood modification.

Highlights

In the living tree, water and nutrients are transported from the roots to the transpiring leaves in the canopy through the xylem in the wood stem, a transport mechanism which requires that a continuous path of water [1]
The objectives of this review are to describe the mechanisms behind moisture sorption in the over-hygroscopic moisture range, the methods that can be used to study the interactions between wood and water at these high humidity levels, and the current state of knowledge on interactions between modified wood and water
Despite the importance of the over-hygroscopic moisture range for fungal degradation, and the importance of the change in interactions between modified wood and water in decay protection, few studies focused on the influence of wood modification on the interaction between wood and water in this moisture range

Summary

Introduction

Water and nutrients are transported from the roots to the transpiring leaves in the canopy through the xylem in the wood stem, a transport mechanism which requires that a continuous path of water [1]. The structure of the xylem is, specialized in conducting and holding water, something that needs to be considered when using wood as a construction material. The moisture content of the xylem in the living tree is high, in the range of 100–200% (mass of water in relation to dry mass) in sapwood depending on wood species [2]. After a tree is felled, the wood, needs to be dried before it can be used as a construction material. If the wood is kept in a relatively dry environment, e.g., indoors, it will not return to as high moisture levels as in the living tree

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