Abstract:
The mechanical properties of wood allow it to be used for numerous purposes. For
most purposes, drying of the wood material from the green state, sawn from the log, is
first required. This drying step significantly improves the strength properties of wood.
It is therefore clear that moisture in wood plays an important role in determining the
bulk mechanical properties. Over the last century, many studies have been carried out
to investigate the way in which the water content wood affects the bulk mechanical
properties. More recent studies have focused to the individual chemical components
that make up wood to understand the observed changes in bulk mechanical properties.
Models of the nanostructure of wood contained; cellulose, hemicellulose, and lignin,
and the arrangement and location of these components in terms of their mechanical
properties was interpreted through what was described as the 'slip-stick' mechanism,
by which wood, in its green state, maintained its molecular and mechanical properties
under external stresses. This model, while insightful, failed to account for the presence
and the role of water in the nanostructure of wood.
In this work, synchrotron based X-ray diffraction and NMR studies, have been used to
develop a new model, in which water plays a vital role in the determination of the
mechanical properties of wood in its green, part-dried, and rewet states. X-ray
diffraction showed that changes occur to the molecular packing of cellulose crystallites
with change in moisture content, and that these changes begin to occur under mild
drying conditions, i.e. drying in air at ambient temperatures. These changes depend on
the severity of drying, whether ambient or forced oven drying, and are to some extent
reversible. A spin-diffusion model was constructed using dimensions obtained from Xray
diffraction, comparisons between predictions and experimental data from an NMR
study showed that the location of water was dependent on the moisture history of
wood. In the green state, at least some of the water in the wood cell wall forms a layer,
between the cellulose crystals and the hemicellulose and lignin matrix. If dried and
then rewet, this water associated with the cellulose crystals was not present to the
same degree as in the green state, allowing a closer association of the hemicellulose
with the cellulose. The effect of this change in water distribution in the wood cell wall
on the bulk mechanical wood properties was shown in mechanical testing. The
nanostructure of the wood cell wall therefore should be considered to contain cellulose,
hemicellulose, lignin and water, where each component contributes, according to its
molecular properties, dynamic mechanical properties which are reflected in the bulk
material properties.
