Lynne Wong's PhD thesis

6.9 FIBRE SATURATION POINT
Berry and Roderick (2005) in their review on plant-water relations introduce the idea of
fibre saturation point (FSP), well known and in routine practical use by engineers and
material scientists for at least 50 years. Fibre saturation point is routinely used to estimate
the volume fractions of solid, liquid and gas phases in bulk timber, it is based on the
concept that a certain (and repeatable) amount of water is chemically bound to cellulose
and other substances in wood. This water, also called bound water, exists in an integrated
mixture of cell wall material and bound water, is recognized as a distinct phase called
‘solid solution’ by Stamm (1964), a separate phase from the adjacent water in either a pure
liquid phase or a vapour phase.
The following illustrates well the concept of fibre saturation point. When a small volume
of liquid water is poured onto oven-dried timber, observations show that when equilibrium
is attained, the added water is not visible in the voids, but it must be located inside the cell
wall matrix; which will swell as a result and the strength of the timber will progressively
decline as the moisture content increases. If the addition of water is continued, the
moisture content increases further, and the system will reach a new equilibrium state when
liquid water begins to accumulate in the voids. The moisture content at which this occurs
is called the fibre saturation point, which has been described as the moisture content at
which the cell walls are fully saturated with liquid moisture but the cell cavities contain no
water.
Water exists in timber in three phases: as vapour in the gas-filled voids, as ‘free’ or ‘bulk’
liquid water in the voids and as bound water in the cell wall matrix (Stamm, 1967a). The
volume fraction of the three phases in timber can be estimated given its fresh volume, fresh
mass, dry mass and the fibre saturation point. At the fibre saturation point, all of the liquid
water is bound water, as it represents the maximum amount of water that can be taken up
from the vapour phase by a unit mass of timber at a given temperature (Browning, 1963).
If the fibre saturation point of timber is measured before and after pulverization to a larger
surface area keeping the total mass the same, the results do not differ showing that there is
a distinct number of binding sites for water in the timber, and the number of the binding
sites is independent of the surface area of the sample (Stamm, 1964). The forces that hold
the water preferentially in the cell walls are chemical bonding and/or capillary (i.e. surface)
forces. The support of this statement comes from the fact that heat of wetting is evolved
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