Chapter 8: Physical Properties of
Renewable Composites
Moisture is Everything…Nearly
• Natural fibers are used by plants for two purposes: provide
structure and transport water.
– We seek these fibers for the first purpose, but its affinity for water
governs most of its use and properties.
• Moisture content in a tree can range from about 30% to more than
200%.
– Water is contained in basically two locations: within the pore structure
of the wood (lumens) or absorbed by the wood polymers via hydrogen
bonding. The absorbed water is usually about 30%.
– Absorbed moisture is termed bound water, and the water in the pores
is termed free water.
– When drying natural materials, bound water requires much more
energy to remove than free water. Additionally, there may be a small
adsorbed moisture layer on the natural fiber surface that requires
more energy to remove than free water but less than bound water.
Relationship Between Temperature
and Moisture
Source: Wood Handbook
Moisture Property Relationships
•
•
•
Natural materials, notably wood,
have anisotropic physical properties
along with mechanical properties.
Anisotropic shrinking and swelling is
a large problem to control in service
and when drying for wood and other
natural materials.
Swelling and shrinking are controlled
in composites and wood by
– Barriers and coatings that limit moisture
sorption such as paint and laminates
– Adding more resin to composites to
resist swelling
– Adding waxes to coat fibers and make
them more hydrophobic
– Chemical modification of wood
polymers, such as acetylation, to
replace hydrophilic hydroxyl groups
with more hydrophobic chemical
groups.
Source: Wood Handbook
Composite Properties are Affected by
Moisture in the Panel
In general, the denser the product the more
it will be affected by moisture.
Moisture absorbed impacts the performance
of panel products. In these figures, MBL (wet
process medium density fiber board), MDF (dry
processed medium density fiber board), and
HB (hard board) were tested.
Source: Bekhta and Niemz. 2009. European
Journal of Wood Products. 67: 339-342.
Relationship Between Temperature
and Moisture in Composites
• Moisture relationships
can be easily modeled
in WPCs
• Fickian behavior
F  Dm
 m /  t   F /  x
• Water diffusion
(Arrhenius)
D  D 0e
EA
RT
Density
• Density more than any other characteristic will influence
other properties such as moisture sorption, thickness swell,
thermal properties, and mechanical properties.
• Wood’s density mostly varies between 320 and 720 kg/m3
but can be 160 kg/m3 for balsa to 1,040 kg/m3 for some
tropical species. Density may vary more than 10% within a
species.
• Agriculture fibers density
– Straw has a low bulk density of approximately 134 kg/m3.
– Cellulose fiber by itself may have a density of 1,500 kg/m3
– Handling and dispersing fibers evenly in a matrix and
transporting them becomes an economic problem.
• Acoustic properties are related to density.
Friction
• Friction is an important property for many
construction applications such as decking,
flooring, stairs, etc. It is another property that
is heavily dependent on moisture content. A
fresh, dry, smooth wood surface coefficient of
friction will be 0.3 to 0.9 for a surface near the
fiber saturation point. Other coefficients of
friction are 1 for a tire, 0.8 for steel, and 0.2
for polyethylene.
Thermal Properties
Material
a1
(10-6 m/m)/K
a2
(10-6 m/m)/K
Thermal
conductivity
(W/mK)
Specific Heat
(kJ/kg K)
Graphite/epo
xy
0.88
31.0
NA
NA
E-glass/epoxy
6.3
20
NA
NA
epoxy
55
55
0.35
NA
1020 steel
12
12
43
0.49
Concrete
14.5
14.5
1.7
0.75
Wood (oak)
4.9
5.4
0.17
2
Polyethylene
200
200
0.5
1.67
Source: http://www.engineeringtoolbox.com/
The coefficients of thermal expansion (a) in two principle material directions. This
relates to how much the material will deform if the temperature raises 1K.
Thermal Properties
Thermal stability of Switchgrass
•
•
Natural fibers start to thermally
degrade above 100°C, but undergoes
significant degradation above 200°C.
The amorphous polysaccharide
degrade first. This property has
significant implications on processing
conditions and end use of materials.
Thermal degradation is a kinetic
process, meaning that it depends on
time and temperature.
Combustion properties have
tremendous importance for fire
protection and bioenergy.
– Autoignition temperature for wood and
coal are 300 and 400°C respectively.
– The combustion energy contained in
wood (hardwood), switchgrass, and coal
(anthrocite) are 8500, 7990, and 14,500
Btu/lb.
Electrical Properties
• The resistivity of steel is 10-7
W m.
• Wood is a very good
electrical insulating
material, but insulating
polymers are on the order
of 1016 W m.
• Cellulose crystals are weakly
piezoelectric, which means
that they will deform when
subjected to an electric
current. Quartz is an
example of a piezoelectric
material.
Electrical resistance varies with moisture
content and is how many hand-held
moisture meters work.
Source: Wood Handbook
Electromagnetic Spectrum
•
•
Electromagnetic radiation interacts with natural materials in different ways and may serve different
purposes.
Radio and microwaves (non-ionizing radiation)
–
–
•
Interact with water and other polar molecules in a rotating electromagnetic field to cause dielectric heating
Useful in curing adhesives (e.g. Parallam) and in drying wood
Ionizing radiation (gamma, x-ray, electron beam, UV)
–
–
–
–
Interacts with oxygen, unsaturated carbon bonds, and aromatic rings
Can penetrate into materials
Generates free radicals that can degrade natural materials
UV exposure although not high enough energy to penetrate deep into natural materials will degrade the
surface causing oxidation and degradation of the material surface. Coatings, paint, and other additives can
be used to preserve the surface.
Source: http://mc2.gulf-pixels.com/wp-content/uploads/2009/07/Electromagnetic-Spectrum2.jpg
Radiation and Natural Materials
• Ionizing radiation interacts with matter
• Intensity interacts with a material according to
the relationship
– I=I0exp(-mx) where I is the intensity in the material as a
function of and I0 is the incident, and m is the linear
absorption co-efficient of the material, which depends
on the material and the type of radiation. Values for of
m for wood depend on density, such as 0.065 to 0.11
cm-1 for poplar over its density range for g radiation
and around 3.0 cm-1 for b radiation (Wood
Handbook).
Summary
• Natural materials have a number of appealing
physical properties that make it very desirable
for use in composites, such as low density to
strength, insulating, reactive surface, etc.
• Care still is needed when using natural
materials so that they do not degrade. This
requires protection from moisture, thermal
degradation, and UV degradation.