Physical Properties of Wood

1. 1
Chapter 9 The physical nature of wood
Basically all the physical properties of wood are
determined by the factors inherent in its structural
organization. These may be summarized under five headings:
• The amount of cell wall substance present in a given volume
of wood
• The amount of water present in the cell wall
• The proportionate composition of the primary chemical
components of the cell wall and the quantity as well as the
nature of the extraneous substances present
• The arrangement and orientation of the wall materials in the
cells and in the different tissues
• The kind, size, proportions, and arrangement of the cells
making up the woody tissue

2. 2
The first of these factors is measured by the specific
gravity of the wood and furnishes the most useful index to the
predicted physical behavior of wood.
The second factor profoundly affects the total physical
behavior of wood, not only because the addition of water to
the cell wall changes its density and dimensions, but because
of its effect on plasticity and transfer of energy within a piece
of wood.
The third factor is related to many of the special
properties of certain kinds of wood as well as to the
deviations from expected quantitative behavior.
The last two are the cause for the large difference, which
are found in the physical responses of wood with respect to
the grain direction, i.e., for the anisotropic behavior of woods.

3. 3
Topical highlights:
Ⅰ. Wood and water
Ⅱ. Specific gravity and density
Ⅲ. Heat in relation to wood
Ⅳ. Electrical properties of wood

4. 4
Ⅰ. Wood and water
Water is a natural constituent of all parts of a living tree.
In the xylem portion, water (moisture) commonly makes up
over half the total weight.
Since almost all properties of wood and wood products
are affected by water, it is important to understand the
nature of water in wood and how it is associated with the
microstructure. This paragraph is devoted to this subject
and in addition covers the proper use of wood products to
assure satisfactory performance under a variety of service
conditions.

5. 5
1. Location of water in wood
1-1 Location
• In the cell walls
Water can stays in the cell walls at any moisture content level.
• In the cell lumens
Water stays in the cell lumens only when the wood is saturated,
in other words, when the moisture content of wood is above the
fiber saturation point (FSP).

6. 6
1-2 Fiber saturation point
• Definition
The point at which all the liquid water in the lumen has
been removed but the cell wall is still saturated is termed
the fiber saturation point (FSP).
• Implication
This is a critical point, since below this point almost all
properties of wood are altered by changes in moisture
content.
• Ranges
MC = 28-35%, usually take it as 30%.

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2. Nature of water in wood
2-1 Free water
• stayed in the cell lumens
• held by the force of surface tension
• easy to remove, just like the water in a pool
2-2 Bound water
• stayed in the cell walls
• held by adsorption force through H-bond
• difficult to remove
• H-bond force increases as MC decreases
2-3 Conception of absorption and adsorption
• Absorption: results from surface tension forces.
• Adsorption: involves the attraction of water molecules
to hydrogen bonding sites.

8. 8
3. Determination of moisture content
3-1 Definition
The moisture content (MC) is defined as the weight of the
waterexpressed as a percentage of oven dry weight of the
wood.
Thus the absolute moisture content of wood :

9. 9
The relative moisture content of wood:
3-2 Determination
• In laboratory:
• In factory: Using a Electricity Resistance MC Meter
3-3 MC of green wood
• Heartwood: 33-98%
• Sapwood : 44-250%
• Some species, e.g., Eucalyptus goes to 300% .
weight of water
MC % = × 100
Weight of wood with water
weight with water - OD wood weight
MC %= × 100
OD wood weight

10. 10
3-4 Equilibrium MC
• Definition
This is the moisture content of wood that corresponds to
a certain weather condition.
• Ranges : 10-20%, usually take it as 15%.
• Affecting factors: H, T, The drying condition of wood
3-5 Hysteresis phenomenon
EMC of wood under adsorption process is always lower
than that of wood under desorption process.

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4. The principle of movement of water in wood
above and below fiber saturation point
Any movement of water in wood involves the permeability of
its microscopic and submicroscopic structure.
4-1 Above fiber-saturation point
The coarser capillaries contain free liquids. The molecules of
water adjacent to the capillary walls are not free but bound by
chemosorption. The movement of liquid water above the fiber
saturation point is caused by capillary forces.
4-2 Below fiber saturation point
Bound water moves through the cell walls due to moisture
gradients set up across the cell walls. This movement is a diffusion
phenomenon.

12. 12
5. Relation of moisture content to the
environment
Wood exposed to an atmosphere containing moisture in the form of
water vapor will come, in time, to a steady moisture-content condition
called the equilibrium moisture content (E.M.C.). This steady-moisture
state depends on the relative humidity, the temperature of the
surrounding air, and the drying conditions to which the wood has
previously been exposed; it fluctuates with changes in one or both of
these atmospheric conditions.
Products manufactured from wood tend in most cases to have
slightly lower EMCs than the raw wood from which they are produced.
This is partially because of the heat-treatment effect mentioned above
but also because of the addition of resins, coatings, and sizing material,
which in themselves are usually less hygroscopic than wood. Plywood
and laminated wood products have EMC characteristics very similar to
wood or lumber. Fiber and particle products, however, many exhibit
considerably different characteristics.

13. 13
6. Moisture movement in
wood
In order to arrive at an equilibrium condition the moisture
must move within the piece of wood. The movement
internally is dependent on time and the direction of
movement with respect to the major axes of the wood.
6-1 Difference in direction
• Water in wood moves 12 to 15 times faster along the grain as
it does across it.
• But for thin boards, especially veneers, most water lost or
gained through the sides instead of from the ends.

14. 14
6-2 Time influence
It is apparent that at the 1st stage the moisture gradient is steep
and the loss of water is high. As the wood becomes drier, the
moisture gradient decreases and approaches zero as the final
moisture content equilibrium for the lumber is reached.

15. 15
7. Moisture content of green wood
7-1 Definition
MC of wood in fleshly felled condition
7-2 Significance
• affect the weight logs and green lumber
• affecting shipment cost
• affecting purchase cost on weight basis
7-3 Ranges
• heartwood: 33-98%
• sapwood : 44-250%
7-4 Influence factors
• tree species and age
• location in a tree
• the felling season

16. 16
8. Shrinking and swelling
As wood loses moisture below the FSP, i.e., loses bound water, it
shrinks. Conversely, as water enters the cell wall structure, the wood
will swell.
• Phenomenon
Wood floor swells in spring and summer and shrinks in fall and
winter.
• Definition
change in dimension from swollen size
shrinkage (% ) = × 100
swollen dimension of wood
change in dimension from dry size
swelling ( %) = × 100
dry dimension of wood

17. 17
• Not completely reversible in swelling and shrinkage
— due to the two different reference bases
— due to the hysteresis effect in the adsorption process
• Mechanisms of shrinking and swelling of wood
— Shrinking: the collective effect of cellulose chains moving
closer when losing water.
— Swelling: the collective effect of cellulose chains moving
apart when gaining water.

18. 18
8-1 Anisotropic dimensional changes in wood
• Definition
The observed dimensional changes in wood are unequal along
the three structural directions (St > Sr > Sz). This is called wood
anisotropy in dimensional changes.
• General ranges of shrinkage
St: 6-12%; Sr: 3-6%; Sz: 0.1-0.3%
• Differential shrinkage
D = St / Sr
• Means to assess stability of wood species
The smaller the value of St, Sr and D, the better.

19. 19
8-2 Basic causes for anisotropic dimensional changes
(1) Sz < Sr and St
The difference between longitudinal and transverse directions
can be traced to cell and microfibril orientation.
— Shrinkage results from the closing action of cellulose chains in the
cell walls, so that it occurs mainly in the direction transverse to the
cellulose chains;
— In the cell walls, S2-layer is the main part, amounting about 70%,
and the microfibril angle on the S2-layer is very small (10-30o
). That is
to say that on the cell walls, most of the cellulose chains are nearly
parallel to the cell axes. So for a single cell, shrinkage mainly occurs in
the direction transverse to the cell axes.
— In a piece of wood, most of the cells are arranged parallel to the
length of the log, so when millions of cells shrink transverse to their cell
axes, the collective effect is transverse shrinking to the log length.

20. 20
(2) Sr < St
The main causes for difference in radial and tangential
directions are as follows:
— Restraint action of the radially oriented wood rays
The low radial shrinkage of ray tissues restrains the
relatively weak early wood and reduce its radial
dimensional change.
— Earlywood and latewood arrangement
Tangential shrinkage and swelling are largely controlled
by the changes in the late wood, since this part of the
growth increment is strong enough to force the early
wood to comply with it. The radial dimensional
changes, on the other hand, are a summation of the
weighted contributions of each part of the annual
increment. It is smaller than that in the tangential
direction because of the presence of the low-shrinking
earlywood component in the total.

21. 21
— The numerous and large pits on radial walls
Usually the radial walls are heavily pitted with large pits, and the
tangential walls are sparely pitted with small ones. Pits are actually
holes on the cell wall, no shrinkage can occur to it. Therefore radial
walls will shrink less because of the heavily pitted large pits.
— Other factors
* The restraint action of the radially oriented knots
* The higher lignin content in radial walls than in tangential walls
* The smaller microfbril angle in radial walls than in the
tangential walls
* Tangential shrinkage begins at higher moisture content than
radial shrinkage.

22. 22
8-3 Dimensional changes of wood in service
It follows from the previous discussion that wood used where
the humidity fluctuates will continually change moisture content
and therefore dimension.
For interior uses such as furniture and millwork, it is much
more critical for satisfactory performance to use lumber at the
proper moisture content.

23. 23
8-4 Modifying factors for dimensional changes
• Wood density
Wood density can be used as a rough indicator of the dimensional
changes in wood. Generally speaking, the denser the wood, the
larger the dimensional change of the wood when suffers humidity
fluctuation.
• Content of extractives
Wood extractives tend to restrict shrinkage by their bulking action
on the cell wall.
• Content of hemicelluloses
Wood with low content of hydrolysable hemicelluloses tends to be
more stable dimensionally than those woods with normal or high
contents of these types of polysaccharides

24. 24
8-5 Means of reducing moisture-induced
dimensional change in wood products
• Preventing moisture sorption by coating the product, such as painting.
• Preventing dimensional change by restraint that makes movement
difficult orimpossible, such as plywood production.
• Treating wood with material that replaces the bound waterin the cell
wall, such as with polyethylene glycol.
• Treating wood to produce mutual cross-linking of the hydroxyl groups in
the cell wall, such as with formaldehyde.
• Impregnation wood with plastic monomers, such as methyl methacrylate,
e.g., WPC production.

25. 25
Ⅱ. Specific gravity and density
wood density is its single most important physical
characteristic. Most mechanical properties of wood are
closely correlated to it. The physic-mechanical properties of
wood are determined by three characteristics:
• the porosity or proportion of void volume, which can be
estimated by measuring the density;
• the organization of the cell structure, which includes the
microstructure of the cell walls and the variety and
proportion of cell types ;
• the moisture content.

26. 26
1. Definition of wood density and specific gravity
• Density: D = mass / volume
• Specific gravity: sg = weight / volume
= weight of wood / weight of water of same volume at 4o
C
The two are equal in numerical value, so they are
interchangeable in practice.
2. Importance of D
D ↑, wood substance ↑, wood strength ↑
3. Determination of wood density
• Weighting: a method according to the definition
• X-ray density meter
• Pylodyn wood density tester

27. 27
4. Density of wood substance
Density of wood substance, which constitutes the cell
wall, has been determined to be about 1.5 grams per cubic
centimeter in the ovendry state. Little variation exists in
this value for different kinds of wood, as long as the same
experimental procedures are employed.
5. All kinds of terms on wood density
• Basic density= Wmin / Vmax
• Oven-dry density= Wmin / Vmin
• Air-dry density: density of air dried wood
• Green wood density: density of green wood

28. 28
Ⅲ. Heat in relation to wood
1. Thermal properties of wood
1-1 Thermal conductivity of wood (K)
• Conception
Thermal conductivity is a measure of the rate of heat flow
through materials subjected to a temperature gradient. In the
English system thermal conductivity of wood (K) is measured
as the amount of heat, in British thermal units (Btu), that will
flow in 1 hour through a homogeneous material 1 inch thick
and 1 foot square, when 1 temperature difference is℉
maintained between the surfaces.
• Affecting factors
— direction of heat flow with respect to the grain, Kz > Kt, Kr
— moisture content of wood, MC ↑, K↑
— specific gravity of wood, sg ↑, K ↑

29. 29
• Wood possesses good heat insulating properties

30. 30
• The thermal insulating value (R)
The thermal insulating value (R) of wood is the reciprocal of
the thermal conductivity; i.e., R=1/K. From the previous
discussion it is apparent that the insulating value for wood is
inversely proportional to the specific gravity and moisture
content. This relationship explains the use of low-density, dry
balsa wood (Ochroma lagopus Sw.) for insulating purposes.
•Evaluation of thermal conductivity of wood
The transverse thermal conductivity (K) of wood can be evaluated:
K = G (1.39+C (MC) ) + 0.165
G: specific gravity
MC: moisture content in percent
C: a constant depending on the moisture content, with a value
of 0.028 below 40 percent MC, and 0.038 above

31. 31
1-2 Thermal diffusivity of wood(ρ)
• Conception
Thermal diffusivity is a measure of the speed with which a
material can absorb heat from its surroundings. It is defined as
the ratio of thermal conductivity to the product of density and
specific heat.
ρ= K / (D*c)
• Ranges of ρ
Wood has a low thermal diffusivity because of the low
thermal conductivity and moderate values forboth density
and specific heat. Wood is quoted as having a thermal
diffusivity of 0.00025 square inch persecond in comparison
with 0.02 square inch persecond forsteel and 0.001 square
inch persecond formineral wool. The low thermal-diffusivity
values associated with wood account forthe comfortably
warm feeling of wood furniture in contrast with metals and
plastic when these are used forfurniture.

32. 32
1-3 Thermal expansion of wood ( )α
• Conception
The measurement of the dimensional changes of wood
caused by temperature differences is called the coefficient of
thermal expansion
• Ranges of α
The coefficient of thermal expansion forthe longitudinal
direction (αL) in the temperature range from –50 to +50℃ ℃
averages 3.39×10-6
perdegree Celsius, regardless of kind of
wood and its specific gravity.
In the transverse direction the expansion is about 10 times
that in the longitudal direction. Foran average specific
gravity of 0.46, the coefficient of radial expansion (αR) is
25.7×10-6
perdegree Celsius, while that fortangential
expansion (αT) is 34.8×10-6
perdegree Celsius.

33. 33
• Wood possesses good thermal stability
Longitudinal thermal expansion in wood is small in comparison
with that of other common solid materials. However, the
transverse thermal expansion in wood is great than it is for any of
the metals and other common materials.
For example, the coefficient of thermal expansion per degree
Celsius for steel is 10×10-6
, for aluminum 24×10-6
,and for flint glass
7.9×10-6
.
The reason that the thermal changes are not more commonly
recognized is that wood is usually used within a narrow range of
temperatures and the dimensional changes caused by moisture
fluctuations are large enough and usually in an opposite sense so
that the thermal effects are masked.

34. 34
2. Combustion of wood
2-1 Ignition of wood
The ignition temperature of wood is usually given as about
275℃ and is actually the temperature at which wood begins to
decompose exothermically.
2-2 Fuel value of wood
Fuel value of wood is primarily determined by the density of
the wood and its moisture content. It is modified by variations in
lignin content and to a much greaterextent by the presence of
extractives such as resins and tannins.
The heat of combustion (H), i.e., the heat in Btu produced by
burning 1 pound of ovendry wood, averages about 8500 Btu for
hardwoods and 9000 Btu forconifers. These values forheat of
combustion bearlittle relationship to a particularkind of wood
and vary only from 5 to 8 percent at a maximum.

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Ⅳ. Electrical properties of wood
1. Direct-current electrical properties of wood
1-1 Conception
The electrical properties of wood are measured by its
resistivity, orspecific resistance, orby its reciprocal,
conductivity.
1-2 Ranges of r
Air-dry wood is an excellent electrical insulator, with dc
resistivity in the order of 3×1017
to 3×1018
ohm-centimeters at
room temperature.

36. 36
1-3 Affecting factors
• Moisture content of wood
The resistivity decreases rapidly by an approximate factor of
three for each percentage moisture content increase up to the
fiber saturation point. at the fiber saturation point it becomes
approximately that of water alone, i.e., 105 to 106 ohm-
centimeters.
• Temperature of wood
Temperature is also an inseparably related influence causing a
decrease in the resistivity with increasing temperatures.
• Grain direction
In general resistivity across the grain is from 2.3 to 4.5 times
greater than that along the grain for conifers and from 2.5 to
8.0 times greater for hardwoods.

37. 37
2. Alternating-current characteristics of
wood
2-1 Conception
One measure of the insulating capacity of a material
underalternating current is its dielectric constant ( ).ε
This is expressed as the ratio of the charge held by a
condenser in which the electrodes are separated by a
dielectric material such as wood, to the charge held by the
condenserwith the electrodes separated by a vacuum at a
given voltage.
2-2 Ranges of ε
ovendry wood substance is a nonconductor, with a
dielectric constant of about 2.
Above the fibersaturation point, the dielectric constant of
fully saturated wood approaches that of water, which has a
value of 81.

38. 38
2-3 Affecting factors
• Wood density
D↑, ε↑
• Moisture content of wood
MC↑, ε↑
• Frequency of the alternating current
f ↑, ε↑
• Grain direction of wood
In dry wood the dielectric constant is from 1.3 to 1.5
times greater in the longitudinal direction than it is in
the transverse direction.

39. 39
Reflection and practice:
1. Definition and implication of fiber saturation point,
and its general range?
2. What is called free water and bound water?
3. Definition of moisture content?
4. Conception of equilibrium moisture content of wood
and its general range?
5. mechanisms of wood shrinking and swelling?
6. Anisotropy of wood in dimensional shrinkage?
7. Definition of differential shrinkage?
8. Basic causes for anisotropy in dimensional shrinkage?
9. Definition of basic density of wood?
10.Affecting factors of thermal conductivity of wood?
11.Influence of moisture content and temperature on
electrical resistivity?
12.Concept of dielectric constant?