CHEM-E0120: An Introduction to Wood
Properties and Wood Products
Wood-water relationships
Mark Hughes
29th September 2016
There are just 3 basic rules when it
comes to wood and water:
1. Keep it dry!
2. keep it dry!!
3.keep it dry!!!
How does water affect wood?
• Dimensional changes:
– Initial shrinkage (from the ‘green’ state to ‘air dry’)
– Small changes in response to fluctuations in ambient
relative humidity (diurnal and seasonal changes)
– Warping and other unwanted distortions
• Changes in both short- and long-term mechanical
properties
– Strength, stiffness and toughness
– ‘Mechanosorptive’ properties
• Susceptibility to biodeterioration
– Decreased risk of attack with lower moisture content
Water in wood
• When wood is freshly felled, it contains an abundance of
water
• This water is found in two states:
• As liquid water in the void spaces (lumen) in the middle of the
cells. This is known as ‘free’ water because it is not chemically
or physically bound to the wood structure (this is the ‘sap’
seen coming from fresh wood – it contains dissolved
substances)
• And as ‘bound’ water, which is found in the cell wall and is
bound to the cell wall polymers though hydrogen bonds (the
same type of bonding that holds water molecules together,
which results in it being a liquid at room temperature)
• When wood dries from ‘green’, the free water is lost first,
followed by bound water
Free water & bound water
A. Cell wall fully saturated,
lumen filled with water
(‘green’)
B. Thin film of free water
remaining on lumen
surface
C. No remaining free water,
but the cell wall is fully
saturated
D. Water in cell wall below
the fully saturated point
(A)
(C)
(B)
(D)
Wood & water – the basics
• The amount of water in wood is known as the moisture
content (MC) and is generally expressed, on a weight basis,
as the percentage of the (oven) dry mass of wood: this is
often described as the ‘oven dry method’
• The MC in wood finds an equilibrium with the ambient
relative humidity, termed the equilibrium moisture content
or, simply, EMC
• ‘Green’ (i.e. freshly felled and never dried) wood has a MC
often exceeding 100%. The MC of ‘air dried’ wood is
somewhere in the region of 8-18% depending on the
environmental conditions
Moisture content
• The total amount of water in
wood is known as the moisture
content
• Remember: this tells us only the
total amount of water and does
not differentiate between
‘bound’ and ‘free’ water
• ‘Oven’ dry means dried at >100
oC until constant weight (usually
at 103oC)
• Can also be expressed on the
basis of the mass of green wood.
This is known as the ‘green’ MC
M init  M od
MC % 
100%
M od
MC is the moisture content expressed as a
percentage
Minit is the initial mass of the sample
Mod is the “oven dry” mass of the sample
Equilibrium moisture content
• Equilibrium moisture content or EMC, is the
moisture content that wood reaches when it
is placed in certain conditions of temperature
and relative humidity. In other word the wood
reaches equilibrium with its surroundings
Most wood properties are highly dependent upon moisture
content. It is therefore very important to measure the properties
under ‘standard conditions’ of relative humidity (RH) and
temperature (generally 65% RH and 20 oC temperature). This is
defined in various Standards
Relative humidity
• Relative humidity (RH) is the term used to
describe the amount of water vapour that
exists in a gaseous mixture of air and water
vapour
• It is the ratio of the partial pressure of water
vapour in the mixture to the saturated vapour
pressure of water at a given temperature
• It is dependent upon temperature
Wood and water
• Wood is a ‘hygroscopic’ material, i.e. it will
attract moisture from the surroundings
• When dry wood gets wet, it swells, leading to
‘sticking’ doors, drawers and peeling
paintwork etc. Conversely, when wood dries,
it shrinks, distorts and cracks
• But it is only loss/gain of bound water that
affects the dimensions of wood
Wood-based products such as particleboard, medium density
fibreboard (MDF) and plywood also respond to moisture, but the
effect is also complicated by their own structure
Importance of drying
• Wood is usually dried to bring the MC close to
the final MC that it will equilibrate to whilst in
service and so will undergo smaller
dimensional changes
• e.g. at 65% RH, 20 oC, MC is around 12%
• To reduce the MC below a level at which
biological attack will be favoured (generally
regarded to be around 20%)
Note: drying does not entirely eliminate
dimensional changes!
Measurement of MC in practice
• Gravimetric methods (by weighing). This is the
most accurate and reliable method, but is not
always practicable
• Moisture meters – electrical devices that measure
the conductivity
• This is proportional to the amount of (bound)
water in wood
• Works well within a certain range (typically 5-20%)
but at the extremes the measurements become
inaccurate
MC of ‘green’ wood
(Source: Dinwoodie, 2000)
Fibre saturation point
Free water, bound water and fibre
saturation point (f.s.p.)
A. Cell wall fully saturated,
lumen filled with water
(‘green’)
B. Thin film of free water
remaining on lumen
surface
C. No remaining free water,
but the cell wall is fully
saturated
D. Water in cell wall below
the fully saturated point
(A)
(C)
(B)
(D)
Fibre saturation point (‘f.s.p.’)
• The point at which the bound water is at a maximum
and no free water remains
• Fibre saturation point is a concept (as it is impossible
to ‘see’ or measure directly the point at which there
is no free water and only bound water exists)
• Nevertheless it is an important concept as below
f.s.p. many of the material properties of wood
change
• The moisture content at f.s.p. is generally thought to
be in the region of 30% (very difficult to measure
experimentally), though new evidence suggests that
it may be closer to 40%
• Theoretically the MC at 100% relative humidity
f.s.p. and mechanical properties
f.s.p.
bound
free
(Source: Dinwoodie, 2000)
Why is wood hydrophilic?
Bound water: why does wood attract
water?
• Water is a ‘polar’ molecule. Because of its polarity, it is
attracted to the polar hydroxyl (–OH) groups in the cell wall
polymers (mainly in the amorphous regions) of wood and
forms ‘hydrogen’ bonds
• The hydrogen bonds ‘bind’ the water to the wood – hence the
term ‘bound’ water
• Hydrogen bonds are relatively strong (but only a fraction of
that of the covalent bonds that bind the glucose units of the
cellulose chain together) and so the association between
wood and water is relatively strong
Remember that the extensive inter- and intra-molecular hydrogen bonding in
the wood cell wall polymers also accounts for the crystal structure of cellulose
and helps control the structure of wood
Hydrogen bonding
• Hydrogen bonds form when polar molecules
or moieties (parts of the molecule) are in
close proximity
• The polarity in the case of –OH (hydroxyl)
groups (moieties) arise because the
hydrogen atom is attached to an oxygen
atom which is slightly electronegative,
causing a dipole
• Opposites attract!
• Most common example is water
• –OH groups are present in abundance in the
cell wall polymers of wood
• Hydrogen ‘bonding’ should not to be
confused with the strong chemical (covalent)
bonding that holds the atoms in the
polymers together
Hydrogen bond
Bond energies, e.g.:
H-bond: 21 kJ/mol
C-O bond: 358 kJ/mol
Inter- and intra- molecular hydrogen
bonding
Cellulose
Hemicelluloses
Lignin
Some further thoughts…
• Water molecules are small! But cell wall
accessibility is an issue
• Hydroxyl groups in crystalline cellulose are mainly
involved in inter- and intra- molecular bonding and
are therefore not able to interact with water
• As hemicelluloses are branched molecules, they are
only slightly crystalline. They are more accessible
and therefore more hydroscopic
• Lignin is rather more hydrophobic
Water molecules are small!
~3.2Å (3.2 x 10-10 m), 0.32 nm!
Finding equilibrium
RH & EMC
(Source: Dinwoodie, 2000)
Relative humidity and EMC
• RH changes with
temperature and
other climatic factors
and can range widely
from <30% to >90%
even in house
interiors
• Wood will try to reach
equilibrium with those
surroundings
(Source: Desch & Dinwoodie, 1981)
MC and dimensional change
• As MC (below f.s.p.) changes, so too do the
dimensions of wood. This leads to the
phenomenon of ‘movement’
• Movement can be problematic in many ways:
– Doors, windows, etc
– Irreversible swelling in wood-based panels
References and further reading
• Dinwoodie, J.M. (2000). Timber: Its nature and
behaviour
• Desch, H.E. and Dinwoodie, J.M. (1981). Timber: Its
Structure, Properties, and Utilisation, Sixth edition.
Macmillan, London; New York