CHEM-E2105
Wood and Wood Products
Wood-water relationships I
Mark Hughes
31st January 2017
How does water affect wood?
• Dimensional changes:
– Initial shrinkage from “green” conditions
– Warping and other unwanted distortions
– Small changes in response to fluctuations in relative humidity
(“movement”)
• 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
– Suitable conditions for some decay/fungus: 25-30°C, MC 3550%
– <20% thought to be a “safe” threshold MC
Today’s topics
1. States of water in
wood
2. Moisture content
3. Fibre Saturation Point
4. Equilibrium Moisture
Content
5. Sorption
Silicone nanofilaments coating on wood surface creating water repellent surface.
Image: http://thefutureofthings.com/3907-water-repellent-fabric/
1. States of water in wood
Water in wood
• “Green” wood (i.e. wood that is in the native state, having never
been dried) contains both “free” water and “bound” water
• Free water is liquid water present in the void spaces within
wood, i.e. the cell lumen. Free water is not chemically
associated with the cell wall polymers and therefore does not
influence mechanical properties
• Bound water is “intimately associated” with the cell wall
polymers through hydrogen bonding with accessible hydroxyl (OH) groups on the cell wall polymers (amorphous cellulose,
hemicelluloses and lignin)
– Has been further categorized as “freezing” and “non-freezing” bound water.
Recently it has been considered that only non-freezing bound water and free
water are present in solid wood
Image: https://nationalvetcontent.edu.au/alfresco/d/d/workspace/SpacesStore/b2f0fcee-47cb-4650-b248f533d73d5428/13_05/toolbox13_05/unit9_selecting_timber/section4_seasoning/lesson3_the_drying_process.htm
Free water & bound water
1.
1.
2.
3.
4.
2.
3.
4.
Cell-wall fully saturated, lumen filled with water (“green”)
Above 25-30% moisture content cell cavities contain some
water -> free water on the cell wall surface.
At the “fibre saturation point” (see later slides), no remaining
free water, but cell-wall remains fully saturated (i.e. only bound
water)
Water in cell-wall below the fully saturated point, cell-walls lose
bound water and shrink.
(Image: http://www.trada.co.uk/images/onlinebooks/A01A024F-B667-4A6D-8443-E92A295E68B4/)
2. Moisture content
Wood & water
• The amount of water in wood is known simply as
the moisture content (MC)
• Moisture content does itself does not describe the
state of the water in wood, though certain
inferences can be made depending upon the MC
• In the hydroscopic range, where only bound water is
present in wood, the MC at equilibrium with the
ambient relative humidity (RH) is termed the
equilibrium moisture content (or EMC)
Moisture content
• The total amount of water
in wood is known as the
moisture content . This can
be “bound” water or a
combination of “free” water
and “bound” water
• Moisture content is
generally expressed in
terms of the oven dry mass
of wood
• “Oven dry” means dried at
>100 oC until constant
weight (usually at 103oC)
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
MC of “green” wood
Green 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
(Source: Dinwoodie, 2000)
Measurement of MC in practice
• Various methods, e.g.:
– Gravimetric methods (by
weighing)
– Moisture meters – electrical
devices that measure the
conductivity of wood or
capacitance (proportional to the
amount of water)
– Works well within a certain range
(typically 5-20%) but at the
extremes the measurements
become inaccurate
(Upper image: http://www.exotek-instruments.com/Moisture-meters/Wood-buildingmaterials/MC-380XCA.htm)
(Lower image: http://www.woodsa.co.za/2014/November/MoistureMeters.php)
3. Fibre saturation point
Free water, bound water and fibre
saturation point
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)
• The moisture content at f.s.p. was generally thought to be in
the region of 30% (very difficult to measure experimentally),
corresponding with a change in properties (e.g. mechanical)
• Solute exclusion or measurement of EMC at RH approaching
100% (>99.9%) gives f.s.p. values around 10% higher, i.e. 40%
• The “last” 10% probably has little effect on mechanical
properties, accounting for the differences
MC and mechanical properties
bound
free
(Source: Dinwoodie, 2000)
FSP and mechanical properties
FSP
bound
free
(Source: Dinwoodie, 2000)
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
• For examples, at 65% RH, 20 oC, the MC is around
12%
• To reduce the MC below a level at which biological
attack will be favoured (generally regarded to be
around 20% MC)
Note: drying does not entirely eliminate
dimensional changes!
4. Equilibrium moisture content
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
• EMC is not only dependent on the RH but also
sorption history (see later slide)
Most wood properties are dependent upon moisture content. It is
therefore very important to measure the properties under “standard
conditions” of relative humidity 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
• and is dependent upon temperature
5. Sorption
Relative humidity & EMC
•
Example water vapour sorption measurement of Scots pine at 20°C
(Image: Kristiina Laine)
Influence of moisture history
– Warm air can hold much more water than cold air
Source: Dinwoodie, 2000
Relationship between
RH, MC and temperature
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 arise because the hydrogen atom is
attached to an oxygen atom which is slightly
electronegative, causing a dipole
• –OH groups are present in abundance in the
cell wall polymers of wood: hemicelluloses
possess the most, followed by cellulose and
lignin. Known as “sorption sites”
• Estimates that 1-2 H2O molecules “attach” to
each sorption site at f.s.p.
• Hydrogen “bonding” should not to be confused
with the strong chemical (covalent) bonding
that hold the atoms in the polymers together
Hydrogen bond
Bond energies, e.g.:
H-bond:
21 kJ/mol
C-O bond: 358 kJ/mol
(Image: http://www.differencebetween.net/)
Inter- and intra- molecular hydrogen
Intra-molecular Hbonding
bonding within one
cellulose chain
Inter-molecular Hbonding between
neighboring
cellulose chains
Cellulose
Hemicelluloses
Lignin
Where do the water molecules go?
• Water is attracted to accessible hydroxyl groups in
the amorphous regions of wood:
• At the microfibrillar level this means the disordered
regions of cellulose and hemicellulose surrounding the
crystalline cellulose core
Example visualisation of the crystalline parts of cellulose microfibrils
Disordered / amorphous
Ordered / crystalline
Some further thoughts…
• Water molecules are small! So 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 hydroscopic;
• Lignin is rather more hydrophobic
Water molecules are small!
~3.2Å (3.2 x 10-10 m), 0.32 nm!
Moisture content (%)
20
Total
Data
15
10
Multilayer
5
Monolayer
0
0
20
40
60
80
100
Relative humidity (%)
(Source: C.A.S. Hill)
(Image: Laine, 2014: based on Salmén 1990)
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)
Rate of sorption
• Moisture transport
results from:
– Vapor diffusion in a
porous system
– Sorption
– Diffusion of bound water
• Many factors affect the
rate….
(Figure from: Engelund et al, 2013)
References and further reading
• Desch, H.E. and Dinwoodie, J.M. (1981). Timber: Its Structure,
Properties, and Utilisation, Sixth edition. Macmillan, London;
New York
• Dinwoodie, J.M. (2000). Timber: Its nature and behaviour
• Engelund, E.T., Thygesen, L. G., Svensson, S. and Hill, C.A.S.
(2013). A critical discussion of the physics of wood-water
interactions. Wood Sci Technol 47: 141-161