CHEM-E2115
Wood products: application and
performance
Wood modification
28th April 2016
Why modify wood?
• Increased demand for high quality wood
• Wood absorbs / desorbs water
Dimensional instability
Changes in mechanical properties
• Wood decay:
Fungal, Bacterial, Insects, UV light
• Mechanical abrasion
• Wood burns
• Aesthetic reasons
‘Active’ and ‘passive’ modification types
Active modification:
Chemical nature of the wood is changed
Passive modification:
Wood properties are affected, but without any alteration of the
chemistry of the material. Either the cell wall is filled and/or
lumens (but with some provisos!).
What is wood “modification”?
“Wood modification involves the action of a chemical, biological
or physical agent upon the material, resulting in a desired
property enhancement during the service life of the modified
wood. The modified wood should itself be non toxic under service
conditions and, furthermore, there should be no release of any
toxic substances during service, or at end of life following
disposal or recycling of the modified wood. If the modification is
intended for improved resistance to biological attack, then the
mode of action should be non biocidal”
(Hill, 2006)
Wood modification in brief
Frequently the aim is to control moisture in the cell wall, in
which case the structure or chemistry of the CELL WALL is
altered in some fundamental way
(Emil Engelund Thybring, http://www.ifb.ethz.ch/woodmaterialsscience/people/emilt)
Prof. Lauri Rautkari
28.4.2016
6
Modification types
Chemical modification
• “The reaction of a chemical reagent with the wood
polymeric constituents, resulting in the formation of
a covalent bond between the reagent and the wood
structure”
(C.A.S. Hill)
• The reaction of wood with a reagent. This can result
in the formation of a bond between a wood –OH
group, or cross-linking between two or more –OH
groups.
• Leads to improved DS and resistance to biological
decay
Example: acetylation
Thermal modification
Probably oldest and most commercialised wood modification
technology
Reconstruction of stone age hut (6000 BC)
“thermally modified” poles
Principles of thermal modification
There are several ways in which the modification can be
undertaken and this leads to a variety of process variables that
need to be considered. These include:
–
–
–
–
–
–
–
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The temperature of treatment
The duration of treatment
The treatment atmosphere
Whether is it a “closed” or “open” system
Wood species
Wet or dry
Sample dimensions
The use of catalysts
ThermoWood® process
• Modification
temperature 180-210°C
• Modification time 2-4 h
• Normal pressure under
superheated steam
ThermoWood® properties
Improves:
• Durability
• Dimensional stability
-70%
Reduces:
• MOE (5-20%)
• MOR (10-20%)
• Hardness (0-10%)
• Impact strength (3080%)
Impregnation modification
“Any method that results in the filling of the wood
substance with an inert material (impregnant) in
order to bring about a desired performance change”
• Impregnate the cell wall with a monomer which is
subsequently polymerised in situ. Acts by bulking
the cell wall
• Only lumens can be impregnated (but this will not
necessarily affect dimensional stability or
durability)
Prof. Lauri Rautkari
28.4.2016
14
Example: polyethylene glycol (PEG)
impregnation
NB: PEG is not chemically bound to the cell wall, nor polymerized in situ,
so can leach out of the wood
Surface modification
• “The application of a chemical, physical or biological
agent to the wood surface in order to effect a desired
performance improvement”
(Hill, 2006)
• Chemical, biological (using enzymes) or physical
processes (e.g. plasma) to selectively alter the surface
characteristics of wood
• Affects the surface (rather than bulk) of the material to
improve bonding with another material or increase
weathering resistance
Example: plasma modification
Video clip
http://www.plasmatreat.com/plasma-treatment/plasmapretreatment/plasma-activation_surface-activation.html
• Plasma is created by heating certain gases in electromagnetic field or
microwave generator
• Plasma is used to break surface layer molecular bonds leading to an
altered surface chemistry, depending on the process gas
Prof. Lauri Rautkari
28.4.2016
17
Other types of modification
Mechanical modification
• Cross-lamination
• Compression modification (“densification”)
Other material/process combinations
• When does modified wood cease to be wood at all?
• E.g. delignify wood, impregnate with a polymer solution, hot
press: result = cellulose nano-composite
Preservation
• This is not strictly modification, but as the properties of wood
are altered by preservative treatment
Hybrid modification-preservation
• Oil heat treatment (with e.g. a boron compounds)
Impregnation
modification
Impregnation modification
Impregnation modification involves impregnating “the
cell wall of wood with a chemical or a combination of
chemicals, that then react to form a material that is
‘locked’ into the cell wall”
(Hill, 2006)
It is a “passive” rather than an “active” modification
technique
Impregnation modification
• The wood must be in its swollen state – so the
impregnant can penetrate the cell wall
• The impregnant molecules must be small enough
to enter the cell wall (pore diameter approximately
2-4 nm)
• Two main mechanisms:
• Monomer or oligomer (few monomers), with
subsequent polymerisation
• Diffusion of soluble material into the cell wall, with
subsequent treatment to make it insoluble
Wood: dry and wet state?
Green (never dried)
Dried
Dried → Soaked
Department of Forest Products Technology
28.4.2016
22
Principle
• When wood swells, the void volume (“micro-pores”) in the
cell wall increases
• The voids are then filled with a substance, initially in the
liquid state, that then becomes solid (“cures” or solidifies) by
some mechanism, maintaining the “artificially” swollen state
• The solid filling the voids may be chemically bonded or there
may be an interaction, but as the void spaces have an irregular
structure the solidified impregnant becomes “locked” in place
• Movement is reduced because the wood remains in the
artificially swollen state so the limits of movement are
reduced
• As the mass of wood is increased, there is a relationship
between WPG and dimensional stability
Conditions - process
• Pressure treatment can help introduce the modifying material
into the gross (macro) structure of the wood (i.e. lumena and
other macroscopic voids)
• Species will be important, as will whether it is sapwood or
heartwood being impregnated (extractives, border pits)
• Also, the size of the molecules that can penetrate the cell wall
are dependent upon the size of the voids. Typically 2-4 nm is
quoted, but may be smaller.
Department of Forest Products Technology
4/28/2016
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Conditions - process
• Resins transported into the cell wall in a suitable solvent,
ideally one which also swells the wood - preferably this will be
water!
• If modification throughout the wood is required, then the
impregnant must also be distributed throughout the bulk
wood
• Does the modification material form a “coating” on the
exposed surfaces of the wood (i.e. lumena), or does it enter
the wood cell wall?
• Forming a coating could be effective provided it is not
breached (broken), in which case moisture will be able
penetrate the cell wall. The rate of moisture sorption can be
altered but not the final EMC
Properties of impregnated wood
• Ideally, the impregnant enters wood material and
occupies space. I.e. it “bulks” the cell wall
• This makes the wood swollen, giving dimensional
stability
• Reduces the space for water molecules so, reduces
EMC
• Improves decay resistance by blocking micropores in
the cell wall, preventing access to enzymes
Impregnation approaches
• Impregnation with liquid (aqueous-based) resins,
such as:
• Phenol formaldehyde – PF
• Melamine formaldehyde – MF
• Urea formaldehyde – UF
• Furfurylation
• Furfuryl alcohol with in situ polymerisation
• Low molecular weight furan polymers in solvent (water)
• Dimethyloldihydroxyethyleneurea (DMDHEU)
• Impregnation with vegetable oils, PEG (polyethylene
glycol), etc.
Resin impregnation
Research started in the 1930’s (Stamm and co-workers at FPL
Madison)
Initial work with PF resins:
• Impregnated with PF resins to up to 100% resin addition (WPG)
• Improved dimensional stability (ASE up to 58%)
• Improved resistance to biodeterioration
Examples of PF resin modified wood
• “Impreg”: Resin impregnated veneers dried at low
temperature, to low MC (about 10%), then hot-pressed to
compress the wood and cure the resin
• “Insulam”: Densified, PF resin-impregnated wood produced by
C-K Composites, USA. Manufactured from veneers and is used
for its high mechanical strength, good dielectric properties
and dimensional stability
• “Permawood®”: Also known as Lignostone®, Permawood is
formed from beech veneers, which are consolidated under
heat and pressure together with thermosetting synthetic
resins. Claimed properties include; low specific weight, good
electrical insulation, withstands high mechanical loading
Permawood®
• Permawood®:
http://www.permali.co
m/permawood.html
• Lignostone®:
http://www.lignostone.c
om/
Furfurylation
• First developed in the 1940’s
• Modification with furfuryl alcohol, derived from corn
cobs or sugar cane residues.
• Conventional impregnation plants used, with curing
(catalysed process) in a conventional kiln
• Gives good dimensional stability and decay resistance
Furfurylated wood
Commercial production by Kebony ASA, Norway
The furfurylation process gives a dark, tropical
hardwood, appearance to the wood
Chemical modification
Background
• Chemical modification can be regarded as an active
modification because it results in a chemical change
in the cell wall macromolecules
• Active modification can also be brought about by
thermal or biological processes:
• e.g. heat or thermal treatment brings about a change
in the cell wall chemistry
• Enzymatic processes
Cell wall chemistry
Three main polymer groups forming the structure of
the cell wall:
• Cellulose
• Hemicelluloses
• Lignin
All these polymer groups contain hydroxyl groups (–
OH). Hydroxyl groups can be considered to be the
main chemical groups that undergo reaction
Types of chemical modification reaction
Acetylation (esterification with acetic anhydride#). The most
widely studied chemical modification and the one that
presently shows the most promise of commercialisation
Other methods include reactions with:
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•
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Other linear anhydrides
Cyclic anhydrides
Acetylation with ketene gas
Carboxylic acids
Acid chlorides
Isocyanates
Epoxide modification
Alkyl halide
Aldehyde modification (formaldehyde)
# Anhydride: “without water”
Size of molecule
Acetic anhydride (C4H6O3)
Propionic anhydride (C6H10O3)
Butyric anhydride (C8H14O3)
Reaction kinetics
May need to enhance this, by e.g.:
Raising the temperature – up to a certain point! Too high a
reaction temperature will lead to unacceptable degradation of
the cell wall polymers
Reaction time – kinetic process, the longer the reaction
proceeds, the greater the degree of reaction
Effect of reaction time on the acetylation
process
• Degree of reaction measured
as WPG (“weight percent
gain”), i.e. a measure of the
weight of chemical adduct
• As the reaction proceeds,
WPG increases, but not
linearly
WPG(%)  M MOD  M OOD  / M OOD 100
Where:
MMOD is the modified oven dry weight
MOOD is the original oven dry weight (i.e. before modification)
Effect of reaction temperature
• As the reaction temperature increases, the rate of
reaction increases
• If the reaction carried out at too high a temperature,
then degradation will result
• 120oC regarded as the upper limit
Other considerations
• Unlikely to consume 100% of reagent that enters the
wood, therefore must remove un-reacted chemical
(may be e.g. toxic, corrosive or harmful!)
• By-product of reaction must also be removed (if
there is one)
• Organic solvent used to deliver reagent into the
wood. Must be removed at the end
Acetylation
Reaction of wood hydroxyls with acetic anhydride to form a
acetyl adduct, linked by an ester bond. Acetic acid (in dilute form
this is vinegar) is produced as a by product of the reaction
Reaction scheme for acetylation:
Acetylation
Video: http://www.youtube.com/watch?v=aWtLcYYRM8c
Department of Forest Products Technology
28.4.2016
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Properties
• WPGs of > 25% achieved. Too high, then degradation
of wood structure can occur
• Improved dimensional stability
• Change in mechanical properties
• Improved resistance to biodeterioration
Mechanical properties
Reduced EMC leads to increased tensile strength, MOE
and MOR, but:
Degradation can occur from:
• Temperature of reaction
• Liberated acid in the structure
Therefore competing effects
Biodeterioration
Acetylation increases wood’s decay resistance
Around 20% WPG gives good protection against brown
rot fungi
Mechanism of protection not fully understood:
• Threshold EMC to support decay ~20%
• Acetylation lowers EMC, so if below around 20% this will
reduce decay
• Also blocking of micro-pores?
Sneek bridge, the Netherlands
Department of Forest Products Technology
4/28/2016
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Department of Forest Products Technology
4/28/2016
49
Further reading
Hill, C.A.S. Wood Modification – Chemical, Thermal and Other
Processes. John Wiley and Sons Ltd., Chichester, UK, 2006
• There are a few copies here and in main library
• E-book can be found in MyiLibrary (via Nelli portal)
Navi P., Sandberg D. Thermo-hydro-mechanical processing of wood
engineering sciences. EPFL Press, Lausanne, 2012