Dimensional stability of wood in presence of water
Conference Paper
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abstract
Wood is our most important raw material. It is important not only because it is used for literally
hundreds of products, but also because it is a renewable natural resource. Through a carefully
and planned use, forests could provide a perpetual supply of wood. All wood in growing trees
contains a considerable amount of water as part of the photosynthesis and the growing
processes. This water is commonly called sap. The main goal of this work is to study the water
movement in wood: first of all, the drying process, which occurs before the manufacture and
use as finished wood products, and secondly the gain and loss of water in response to changes
in environmental conditions that surround the wood. The moisture content relationship has an
important influence on wood properties and performance.
Wood is dimensionally stable when moisture content is greater than the fibre saturation point
(MCfs). Below MCfs wood dimensional changes and it gains moisture (swells) or loses
moisture in the form of bound water. The level of MCfs depends on the relative humidity and
temperature of the surrounding air. Shrinkage and swelling are the cause of many of the
problems that occur in wood during drying and in use, therefore, an understanding of them will
help minimize such problems. Splitting, warping, and open joints are examples of problems
that occur due to uneven shrinkage.
An experimental program was defined with the aim to evaluate the dimensional stability of
hardwood and softwood species. The Pine softwood and the Ash hardwood of the North-east
region of Portugal will be analysed. A group of thirty specimens were made for each specimens
of wood. The assumed geometry for the specimens is 40×40×10 mm, based on NP EN 614
recommendations. Before running the tests, one half of the specimens of each species were
dried in an oven at 103ºC ± 2ºC, while the other one half was saturated in a water tank until a
constant mass is attained. The tests were carried out in a climatic chamber with a constant
internal environment of 20ºC and 60% (RH), during a period time equal to 24 hours or more,
until stabilization of dried and saturated specimens. Mass is considered constant when the
difference between two consecutive weight measurements, delayed 2 hours, is less than 0.5%.
