Chapter 22
Chemical Modification of Wood
R. M. Rowell
22.1. Introduction
After millions of years of evolution, wood was designed to perform in a wet
environment, and nature is programmed to recycle it, in a timely way. back
to the basic building blocks of carbon dioxide and water through biological,
thermal, aqueous, photochemical, chemical, and mechanical degradation. The
properties of wood are, for the most part, a result of the chemistry of its cell
wall components. The polysaccharides (cellulose and hemicelluloses) are mainly
responsible for (i) moisture uptake and release in changing environments that
result in changes in wood volume (dimensional instability), (ii) recognition
mechanism for biological degradation (attack by fungi, termites, etc.), (iii)
thermal instability (pyrolysis and burning), (iv) chemical degradation (by acids
and bases), and, to some degree, (v) degradation due to ultraviolet radiation
(weathering, lignin degradation).
22.1.1. Chemical modification of wood
If the chemistry of the wood cell wall polymers is changed, the polymer
properties change as does the performance of the modified wood. The chemical
modification of wood can be defined as a chemical reaction between some
reactive part of wood and a simple single chemical reagent, with or without
catalyst, to form a covalent bond between the two. This excludes chemical impregnations (monomer impregnations that polymerize in situ, but do not bond
with the cell wall), polymer inclusions, coatings. and heat treatments.
Chemical modification of wood has been used (i) to isolate various cell wall
polymers, (ii) to study differences in properties as a result of changing the
chemistry. and (iii) to improve the performance properties of wood.
R. M. Rowell
674
Many chemical reaction systems have been published for the modification
of wood and the systems have been reviewed in the literature [1-7]. The chemicals include anhydrides (such as acetic, butyric, phthalic, succinic, maleic,
propionic, and butyric anhydride), acid chlorides, ketene carboxylic acids,
isocyanates, formaldehyde, acetaldehyde, difunctional aldehydes, chloral, phthalaldehydic acid, dimethyl sulfate, alkyl chlorides, 3-propiolactone, acrylonitrile,
and epoxides (such as ethylene, propylene, and butylene oxides, and difunctional
epoxides).
Of all the studies of the chemical modification of wood and other lignocellulosic resources, none has gone past the research stage. During the past
few years, many laboratories around the world have concentrated their efforts
in this field on acetylation, using liquid acetic anhydride.
22.1.2.
Acetylation
of
wood
The reaction of acetic anhydride with wood results in esterification of the
accessible hydroxyl groups in the cell wall, with the formation of by-product
acetic acid [8]. This by-product must he removed. as the human nose is quite
sensitive to its odor. While this is easily done in the case of wood particles
and fibers, it is somewhat difficult to do in solid wood. Acetylation is a singleaddition reaction, which means that one acetyl group is on one hydroxyl group
with no polymerization:
wood–OH
+
(CH3–CO)2O
Acetic anhydride
wood–OOC–CH3 + CH3–COOH
Acetylated wood
Acetic acid
Thus, all the weight gain in acetyl can be directly converted into units of
blocked hydroxyl groups. This is not true for a reaction where polymer chains
are formed (e.g., epoxides and isocyanates). In these cases, the weight gain
cannot be converted into units of blocked hydroxyl groups.
Acetylation has also been done using ketene gas [9-13]. In this case, the
wall hydroxyl groups are esterified, but no by-product acetic acid is formed.
While this is an interesting chemistry and eliminates a by-product, it has been
shown that the reaction rate is determined by the rate of diffusion of the vapor
into the wood [1]. Since the rate of diffusion into a porous solid varies inversely
with the square of its thickness, the reaction of wood with ketene has been
restricted to a maximum wood thickness of about 3 mm if it is to he carried
out within a reasonable time length.
22.2. History and process of acetylation
The acetylation of wood was first performed in Germany in 1928 by Fuchs,
using acetic anhydride and sulfuric acid as a catalyst [14]. Fuchs found an acetyl
weight gain of over 40%, which meant that he decrystallized the cellulose in
the process. He used the reaction to isolate lignin from pinewood. In the same
year. Horn [15] and Suida and Titsch [16] acetylated beech wood to remove
Chemical Modification of Wood
675
hemicelluloses in a similar lignin isolation procedure. A year later, Suida and
Titsch [17] acetylated powdered beech and pine using pyridine or dimethylaniline as a catalyst to yield an acetyl weight gain of 30% to 35% after 15 to
35 days at 100°C. In 1945, Tarkow first demonstrated that acetylated balsa
was resistant to decay [10]. In 1946, Tarkow, Stamm and Erickson [18,19] first
described the use of wood acetylation to stabilize wood from swelling in water.
Since the 1940s, many laboratories around the world have looked at the acetylation of many different types of wood and agricultural resources.
Through the years, many catalysts have been tried for acetylation, both
with liquid and vapor systems. These include zinc chloride [20], urea-ammonium
sulfate [21], dimethylformamide [21-23], sodium acetate [18], magnesium persulfate [24-26],trifluoroacetic acid [24], boron trifluoride [22], and γ-rays [27].
The newest acetylation technology, developed at the Forest Products
Laboratory and Chalmers University of Technology in Sweden in the early 1980s.
uses a controlled and limited amount of acetic anhydride, without a catalyst
or co-solvent, and a small amount of acetic acid at a reaction temperature of
between 120 and 130°C for solid wood and 120 to 165°C for fibers and particles.
For solid wood, vacuum is applied before the anhydride is introduced, the
mixture is heated by microwaves, and the chemicals are recovered in a heated
vacuum step. For flakes, particles, or fiber, no vacuum or pressure is applied
and the reaction is carried out in a screw reactor. the chemical recovery being
done using steam. These processes are described in detail later.
The rate-controlling step in the chemical modification of solid wood is the
penetration of the reagent into the cell wall [8]. In the reaction of liquid acetic
anhydride with wood, at an acetyl weight percent gain (WPG) of about 4.
there is more bonded acetyl in the S2 layer than in the middle lamella. At a
WPG of about 10, acetyl is equally distributed throughout the S2 layer and
middle lamella. At a WPG over 20, there is a slightly higher concentration of
acetyl in the middle lamella than in the rest of the cell wall. These results were
found using chloroacetic anhydride and following the fate of the chlorine by
energy-dispersive X-ray analysis [8].
Questions have been raised about the long-term stability of the acetate
group in wood. Table 22.1 shows the stability of acetyl groups in pine and aspen
flakes to cyclic exposure to 30 and 90% relative humidity (RH) (3 months at
30% RH, followed by 3 months at 90% RH) [28,29]. Within the experimental
error, no loss of acetyl occurred over 41 cycles. This experiment has been
Table 22.1. Stability of acetyl groups in pine and aspen flakes after cyclic exposure to 90
and 30% RH
Acetyl content (%) by cycle number
Cycles
0
13
21
33
41
Pine
18.6
18.2
16.2
18.0
16.5
Aspen
17.9
18.1
17.1
17.8
17.1
676
R. M. Rowell
ongoing for more than 20 years; the results continue to show no loss of acetyl
from humidity cycling.
The mass balance in the acetylation reaction shows that all the acetic
anhydride consumed in the acetylation of hardwood and softwood could be
accounted for as increased acetyl content in the wood, acetic acid resulting
from hydrolysis by moisture in the wood or as unreacted acetic anhydride [30].
The consumption of acetic anhydride can be calculated stoichiometrically, based
on the degree of acetylation and the moisture content of the wood. This is
true for all wood acetylated to date.
The weight gain caused by acetylation and the acetyl content found by
chemical analysis are shown in Table 22.2. At lower WPG levels, the acetyl
content is higher, compared to the WPG values. This may be caused by the
removal of extractives and some cell wall polymers into the acetic anhydride
solution, resulting in an initial specimen weight loss. At WPG values higher
than about 15, the acetyl content and the WPG values are almost the same.
Table 22.2. Weight percent gain (WPG) and acetyl analysis of pine and aspen
Species
Pine
Aspen
WPG (%)
Acetyl content (%)
0
1.4
6.0
7.0
14.8
15.1
21.1
20.1
0
3.9
7.3
10.1
14.2
16.9
17.9
19.1
22.3. Properties of acetylated wood
22.3.1. Moisture and water sorption
The replacement of some hydroxyl groups of the cell wall polymers by bonded
acetyl groups reduces the hygroscopicity of wood. Table 22.3 shows the fiber
saturation point for acetylated pine and aspen [4]. As the level of acetylation
increases, the fiber saturation point decreases, in both softwood and hardwood.
Table 22.4 shows the equilibrium moisture content (EMC) of control and acetylated pine and aspen at several levels of acetylation and three levels of relative
humidity. In all cases, as the level of chemical weight gain increases, EMC is
reduced in the reacted wood [31].
Sorption-desorption isotherms for acetylated spruce fibers are shown in
Figure 22.1 [32]. The 10-min acetylation curve represents a WPG of 13.2 and
Chemical Modification of Wood
677
Table 22.3. Fiber saturation point for acetylated pine and aspen
WPG (%)
Pine (%)
Aspen (%)
0
45
46
6
24
—
8.7
—
29
10.4
16
—
13.0
—
20
17.6
—
15
18.4
14
—
21.1
10
—
Table 22.4. Equilibrium moisture content of acetylated pine and aspen
Equilibrium moisture content at 27°C
Species
Pine
Aspen
WPG (%)
30% RH
65% RH
90% RH
0
5.8
12.0
21.7
6.0
4.1
9.2
17.5
10.4
3.3
7.3
14.4
14.8
2.8
6.0
11.6
18.4
2.3
5.0
9.2
20.4
2.4
4.3
8.4
0
4.9
11.1
21.5
7.3
3.2
7.8
15.0
11.5
2.7
6.9
12.9
14.2
2.3
5.9
11.4
17.9
1.6
4.8
9.4
the 4-h curve a WPG of 19.2. Untreated spruce reached an adsorptiondesorption maximum at about 35% moisture content, while acetylated spruce
at 19.2 WPG reached a maximum of about 10%. Moisture is presumed to be
sorbed either as primary or secondary water. Primary water is the water sorbed
to primary sites with high binding energy, such as the hydroxyl groups.
Secondary water is the water sorbed to sites with less binding energy, i.e., the
water molecules are sorbed on top of the primary layer. Since some hydroxyl
sites are esterified with acetyl groups, there are fewer primary sites to which
water sorbs. And since the fiber is more hydrophobic as a result of acetylation,
there may also be fewer secondary binding sites.
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R. M. Rowell
Figure 22.1. Sorption-desorption isotherms for acetylated spruce fibers [32].
Changes in dimension are a great problem in wood composites, as compared
to solid wood. Composites undergo not only normal bulk wood swelling (reversible swelling), but also swelling caused by the release of residual compressive
stresses imparted to the board during the composite pressing process (irreversble swelling). Water sorption causes both reversible and irreversible swelling:
some reversible shrinkage occurs when the board dries.
The dimensional stability (measured as anti-shrink efficiency, ASE) of acetylated solid pine is 71% at a WPG of 19.7 after a 5-day water soaking test.
The rate and extent of thickness swelling in pine fiberboard (5% phenolic resin
content) at the same level of acetylation is greatly reduced as a result of acetylation. At the end of 5 days of water soaking, control boards swelled 36% whereas
boards made from acetylated fiber swelled less than 4%, giving an ASE of 93%.
After drying at the end of the test, control boards exhibited a greater degree
of irreversible swelling, compared to boards made from acetylated fiber [33].
The dimensional stability resulting from acetylation is caused by the bulking
of the bonded acetyl groups in the cell wall polymer hydroxyl groups. Because
the volume of the cell wall is swollen to near the original green volume, little
swelling can occur when water enters the wood. Acetylated wood can sorb water
through capillary action and, to some extent, in the cell wall. Since the water
molecule is smaller than the acetyl group, some swelling can occur in "completely acetylated wood" , but swelling does not exceed the elastic limit of the
cell wall.
Chemical Modification of Wood
679
22.3.2. Resistance to biological attack
Fungi. Various types of solid wood, particleboards, and flakeboards made from
acetylated wood have been tested for resistance to different types of organisms
[34-40].
Acetylated wood has been tested with several types of decay fungi in an
ASTM standard 12-week soil block test, using the brown-rot fungus Gloeophyllum trabeum or the white-rot fungus Trametes versicolor. Table 22.5 shows the
resistance of pine acetylated to several levels of chemical modification to attack
by brown- and white-rot fungi [35,41]. As the level of acetylation rises, the
resistance to fungal attack increases.
Table 22.5. Resistance of acetylated pine against brown- and white-rot fungi
WPG (%)
Weight loss after 12 weeks (%)
Brown-rot
White-rot
0
61.3
7.8
6.0
34.6
4.2
10.4
6.7
2.6
14.8
3.4
<2
17.8
<2
<2
The weight loss resulting from fungal attack is the method most frequently
used to determine the effectiveness of a preservative treatment to protect wood
from decay. In some cases, especially for brown-rot fungal attack, the strength
loss may be a more important measure of attack since large strength losses
are known to occur in solid wood at very low wood weight loss [42]. A dynamic
bending creep test has been developed to determine the strength loss when
wood composites are exposed to a brown- or white-rot fungus [43–45].
In a bending creep test of aspen flakeboards, control boards made with
phenol-formaldehyde adhesive failed in an average of 71 days when exposed
to the brown-rot fungus Tyromyces palustris and in 212 days when exposed
to the white-rot fungus Traetes versicolor [41,46]. At failure, the weight loss
averaged 7.8% for T. palustris and 31.6% for T. versicolor. Isocyanate-bonded
control flakeboards failed in an average of 20 days with T. palustris and 118
days with T. versicolor, with an average weight loss at failure of 5.5% and
34.4%, respectively [41]. Very little or no weight loss occurred with both fungi
in flakeboards made using either phenol-formaldehyde or isocyanate adhesive
with acetylated flakes. None of these specimens failed during the 300-day test
period. Mycelium fully covered the surfaces of isocyanate-bonded control
flakeboards within 1 week, but the mycelial development was significantly slower
in control flakeboards bonded with phenol-formaldehyde. Both isocyanate- and
phenol-formaldehyde-bonded acetylated flakeboards showed surface mycelium
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R. M. Rowell
colonization during the test. but since the fungus did not attack the acetylated
flakes, little strength was lost.
In similar bending creep tests, both control and acetylated pine particleboards made using a melamine-urea-formaldehyde adhesive failed because T.
palustris attacked the adhesive in the glue line [46]. Mycelium invaded the inner
part of all boards, colonizing in both the wood and glue line in control boards,
but only in the glue line in acetylated boards. These results show that the glue
line is also important in protecting composites from biological attack.
After 16-week exposure to T. palustris, the internal bond strength (IBS)
of control aspen flakeboards made using a phenol-formaldehyde resin was
reduced by more than 90%; the IBS of flake boards made using an isocyanate
resin was reduced by 85% [34]. After a 6-month exposure in moist non-sterile
soil, the same control flakeboards made using a phenol-formaldehyde resin lost
65% IBS and those made using an isocyanate resin lost 64% IBS. Failure was
due mainly to strength reduction in the wood. resulting from fungal attack.
Acetylated aspen flakeboards lost much less IBS during the 16-week exposure
to T. palustris or 6-month soil exposure.
The resistance to fungal attack by chemical modification is probably related
to low moisture sorption, below that needed for biological attack. The mechanism may also be blocking of specific enzymatic reactions as a result of changes
in configuration and conformation of the polymers in the cell wall of the
modified wood. In the case of brown-rot fungal attack. it has been suggested
that the reduced moisture content of acetylated wood prevents the fungus from
initiating the breakdown of hemicelluloses as an energy source [47-49]. This
mechanism is consistent with the data from soil block weight loss tests and
strength loss tests.
Another test to determine the fungal and bacterial resistance of acetylated
composites is a fungal cellar containing brown-, white-, and soft-rot fungi and
tunneling bacteria. Control blocks were destroyed in less than 6 months, while
flakeboards made from acetylated furnish above 16 WPG showed no attack after
1 year [41,50] (Table 22.6). These data show that no attack occurs until swelling of the wood takes place [41]. This fungal cellar test was continued for some
additional 12 years, with no attack at 17.9 WPG, thus providing additional
evidence that the moisture content of the cell wall is critical to fungal attack
[51,52].
Acetylated solid wood and flakeboards have been subjected to in-ground
tests [37,53,54] throughout the world. In the United States, Sweden, and New
Zealand, specimens showed little or no attack after 10 years of exposure. In
Indonesia, specimens failed in less than 3 years [53]. mainly as a result of termite
attack. In Sweden, acetylated pine at a WPG of 21.2 has been outperforming
wood treated with chromated copper arsenate at 10.3 kg/m3 after 8 years of
exposure [54].
Termites. Table 22.7 shows the results of a 2-week termite test using
Reticulitermes flavipes (subterranean termites) on several types of chemically
Chemical Modification of Wood
681
Table 22.6. Fungal cellar tests of aspen flakeboards made from control and acetylated flakesa,b
Rating at intervals (months)c
WPG
2
3
4
5
6
12
24
36
72
144
—
—
—
—
—
0
S/2
S/3
S/3
S/3
S/4
7.3
S/0
S/1
S/1
S/2
S/3
S/4
—
—
—
—
11.5
0
0
S/0
S/1
S/2
S/3
S/4
—
—
—
13.6
0
0
0
0
S/0
S/1
S/2
S/4
—
—
16.3
0
0
0
0
0
0
0
0
0
0
17.9
0
0
0
0
0
0
0
0
0
0
a Non-sterile
soil containing brown-, white, and soft-rot fungi and tunneling bacteria
bonded with 5% phenol-formaldehyde adhesive
c Rating system: 0 = no attack; 1 = slight attack; 2 = moderate attack; 3 = heavy attack;
4 = destroyed; S = swollen
bFlakeboards
Table 22.7. Resistance of acetylated pine to Reticulitermes flavipes
WPG (%)
0
10.4
17.8
21.6
Weight loss (%)
31
9
6
5
modified pine [41]. The lack of complete resistance to attack may be attributed
to the severity of the test. However, termites can live on acetic acid and
decompose cellulose to mainly acetate. The termite survival was quite high at
the end of the tests, which indicates that the modified wood was not toxic to
these insects.
Marine organisms. Acetylated wood is resistant to attack by marine organisms.
In Florida, control specimens were destroyed within 6 months to 1 year, mainly
because of attack by Limnoria tripunctata, while acetylated wood showed good
resistance [55] (Table 22.8). In similar tests in Sweden, acetylated wood failed
after 2 years of exposure and control specimens failed in less than 1 year [54].
Table 22.8. Resistance of acetylated pine to marine organisms a
WPG (%)
Exposure (years)
0
1
22.0
3
a Teredinid
Mean rating of attack by various organism b
Limnoriid and terebransc
Shaeromad
2-4
8
borers
b Rating system: 10 = no attack; 9 = slight attack; 7 = some attack; 4 = heavy attack;
0 = destroyed
c Installed in Key West, FL
d Installed in Tarpon Springs, FL
3.4
8.8
682
R. M. Rowell
For both control and acetylated specimens, failure was due to attacks by
crustaceans adn molluscs.
22.3.3.
Thermal
properties
Table 22.9 shows the results of thermogravimetric and evolved gas analysis of
acetylated pine. Both unreacted and acetylated samples showed two peaks in
the thermogravimetric runs. The lower temperature peak represents the
hemicellulose fraction and the higher peak the cellulose in the fiber. Acetylated
pine fibers pyrolyze at about the same temperature and rate [56]. The heat of
combustion and rate of oxygen comsumption are approximately the same for
control and acetylated fibers, which means that the added acetyl groups have
approximately the same carbhon hydrogen, and oxygen content as that of the
cell wall polymers.
Table 22.9. Thermal properties of acetylated pine
Temperature at
Heat of
Rate of O2
max weight lossa
combustion
combustion
(%)
(°C)
(kcal/g)
(mm/g s)
0
335/375
2.9
0.06/0.13
338/375
3.1
0.08/0.14
WPG
21.1
a
Lower temperature peak represents hemicellulose fraction and higher peak cellulose
in the fiber
22.3.4.
Weathering
The reaction of wood with acetic anhydride has also been shown to improve
the ultraviolet resistance of wood [57]. After 700 h of accelerated weathering,
controls eroded at a rate of about 0.12 µm/h or about 0.2%/h (Table 22.10).
Acetylation reduced the surface erosion by 50%. The depth of penetration
Table 22.10. Weight loss and erosion of acetylated aspen after 700 h accelerated weathering
WPG
(%)
Weight loss
(%/h)
Erosion rate
Reduction in erosion
Penetration depth
(µm/h)
(%)
(µm)
0
0.019
0.121
—
199-210
21.2
0.010
0.059
51
85-105
caused by weathering was about 200 µm for unmodified boards and half that
for acetylated boards. In outdoor tests, the color of acetylated pine flakeboards
remained light yellow after 1 year, while that of control boards turned from
dark orange to light gray [57]. The acetylated pine retained its bright color
for 10 years.
Chemical Modification of Wood
683
22.4. Mechanical properties
Wood composites made from acetylated furnish show a slight increase in
strength as a result of acetylation. Acetylation increases the modulus of rupture
(MOR) and the modulus of elasticity (MOE), but does not change the internal
bond strength (IBS) [58,59] (Table 22.11). The MOR value is above the
minimum standard as given by the American Hardboard Association [60].
Acetylation has been shown to have little effect on the strength properties of
thin flakes [61]. The slight decrease in some strength properties resulting from
acetylation may be attributed to the hydrophobic nature of the acetylated
furnish, which may not allow the water-soluble phenolic or isocyanate resins
to penetrate the flakes.
Table 22.11. Mechanical properties of fiberboards made from control and acetylated
pine fibera
WPG (%)
MOR (MPa)
MOE (GPa)
IBS (MPa)
0
53
3.7
2.3
19.6
61
4.1
2.3
31
—
—
ANSI Standard
a Fiberboards made with 10% phenolic resin. MOR, modulus of rupture: MOE,
modulus of elasticity; IBS, internal bond strength
The strength properties of wood are very dependent on the moisture content
of the cell wall. The mechanical properties of fiber stress at proportional limit,
work to proportional limit, and maximum crushing strength are most affected
by changing moisture content by only ±1% below FSP [54,62]. The fact that
the EMC and FSP of the acetylated fiber are much lower than those of
unmodified fiber alone accounts for their difference in strength properties.
Recent research has shown that bonded acetylated wood, when wet, retains
more than 70% of its dry shear strength [63].
22.5. Commercialization of acetylated wood
Despite the vast amount of research on the chemical modification of wood and,
specifically, on the acetylation of wood, commercialization has not come easily.
The first patent on wood acetylation was filed by Suida in Austria in 1930
[64]. Nearly two decades later, Stamm and Tarkow [65] filed a patent on the
acetylation of wood and boards using pyridine as a catalyst. In 1961, the
Koppers Company [66] published a technical bulletin on the acetylation of wood
using no catalysis but an organic co-solvent [67,68]. Later, Otlesnov and Nikitina
[69] came close to commercializing acetylation, but the process was discontinued,
presumably because it was not cost-effective. In the late 1980s in Japan, Daiken
started the commercial production of acetylated wood for flooring, called alphawood that is still in operation today.
684
R. M. Rowell
Many companies are currently involved in the commercialization of wood
acetylation. Those known to the author are A-Cell in Sweden. TitanWood in
Holland, and Weyerhaeuser in the United States, as well as commercial developments in North Wales, Malaysia, Sweden, Norway, Germany, The United
Kingdom, and New Zealand.
Today, the preferred method of acetylating wood is to use a Limited amount
of liquid acetic anhydride without a catalyst or co-solvent [70,71]. Variations
of this procedure have been used to modify fibers. particles, flakes, chips,
veneers, and wood of various sizes. The fact that only a limited quantity of
acetic anhydride is used means that, less chemical has to be heated during the
reaction and less chemical has to he cleaned up after the reaction. A small
amount of acetic acid seems to be needed in the reaction mixture to swell the
cell wall.
Two new processes are presently under way in Sweden to commercialize
the acetylation of wood. One is a fiber process and the other is a process to
acetylate wood of large dimensions, using a microwave technology. The fiber
process [72-76] uses a limited amount of anhydride and utilizes a screw reactor
and two stripper units to remove the anhydride and acid (Figure 22.2). The
solid wood acetylation process is based on microwave heating of the anhydride
Figure 22.2. Fiber acetylation pilot plant in Sweden
Chemical Modification of Wood
685
Figure 22.3. Microwave reactor for solid wood in Sweden
and wood [77-79] (Figure 22.3). The absorption of the microwave energy in
acetic anhydride impregnated wood is preferred to other methods of heating
since it heats only specific parts of the wood, provides some self-regulation of
the overall temperature rise, and promotes a more uniform heating pattern.
Acetic anhydride is supplied to the reactor, under vacuum, then pressure is
applied for a short time, and the excess anhydride is removed by another
vacuum step. Microwave energy is then applied to heat the anhydride-soaked
wood. The penetration depth of the microwaves at 2450 MHz is approximately
10 cm, ehcih means thatthe technology can be used to acetylate large wood
members. the variation in acetyl content, both within and betweeen samples,
is less than 2%. Microwave energy can also be used to remove the excess acetic
anhydride and by-product acetic acid after acetylation.
One concern about the acetylation of wood, using acetic anhydride as the
reagent, has been the acetic acid by-product. Many past attempts have been
made for the "complete removal" of the acid to eliminate its odor. A complete
removal of acetic acid has not been achieved in both the fiber process and
the solid wood microwave process.
22.6. Applications and economics of wood acetylation
Because of the cost of acetylation, acetylated wood will find applications in
value-added products. Since acetylated wood has greatly improved properties,
as compared to unmodified wood, codes and standards will need to be developed. Consumer acceptance will require the education of the architects, designers,
and purchasing agents, as well as the general public. General applications in-
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R. M. Rowell
clude transportation, sports equipment, military, and construction, where higher
tolerances for stability and durability are required. In 2005, The ICC Evaluation
Service drafted a document on the acceptance criteria for acetylated wood preservative systems, for the use of acetylated wood in adverse environments [80].
The first suggested application of acetylated solid wood is for the residential
decking market. For many years, the main product for residential decking was
wood, pressure treated with chromated copper arsenate (CCA), until it was
withdrawn from the market at the end of 2003. CCA-treated wood represented
80% of the total market up to that point. Wood/plastic lumber has also entered
the residential decking market; as of 2005, it had achieved a market share of
16 to 20%. Acetylated wood could compete in this market, but it would be
limited by the amount of anhydride production. Projections indicate that if
the entire global output of acetic anhydride were to be used to acetylate wood
for residential decking, it would only represent about a 20% market share.
For a unit size of 5/4 by 6 inches (1 inch = 2.54 cm) by 8 feet (1 foot =
30.5 cm), the cost of acetylated lumber, compared to that of standard treated
wood and plastic lumber, is as follows: standard treated lumber is $0.50 to
$0.65, plastic lumber is $2.75 to $5.90, and a very early estimate for acetylated
lumber is $3.50 to $4.50. The range of prices depends on the seller, and the
price for acetylated wood is the best estimate at this time. Based on these
data, acetylated wood could be cost-effective for the residential decking market.
In the construction sector, acetylated wood has also been considered for exterior
doors and windows, for wet rooms, for use under roofs and siding composite
boards, and for reusable construction plywood.
Acetylated fiber will also find applications in value-added composites and
has already been used to produce experimental exterior-profiled door skins,
window components, lightweight sports equipment, automotive parts, and
exterior composite furniture. It has also been studied as a component in fiberthermoplastic composites. Up to 70% acetylated fiber has been used to make
a conical extruded polypropylene thermoplastic composite.
The cost of acetylated fiber depends on the size of the plant: the larger
the plant, the lower the fiber cost. In 1992, Sheen from British Petroleum.
projected that for a fiber acetylation plant that produces 10 000 tons/year,
the cost of fiber would be US$0.31/lb (0.454 kg). assuming a ketene step is
used in the process [81]. Under the same assumption, the DanACell company
in Denmark/Sweden predicts costs of US$0.32/lb for a 8 000 tons/year plant,
US$0.27/lb for a 20 000 tons/year plant, and US$0.20/lb for a 100 000 tons/year
plant. All of these cost projections depend on the price of acetic anhydride
and the costs related to chemical recovery, equipment. and processes.
22.7. Conclusions
The acetylation of wood went from an analytical technique to isolate cell wall
polymers in the 1920s to a commercial reality in Japan in the late 1980s. It
may be a commercial reality in several other countries in the near future. Its
Chemical Modification of Wood
687
first application in North America may be acetylated solid wood for decking
material, replacing the CCA-treated wood.
Acetylation of wood reduces the hygroscopocity and increases the dimensional stability and resistance to biological attack. It has found and will continue
to find value-added markets where these two properties are desired, or required.
Since acetic anhydride is produced in many places in the world, acetylation of
wood fits into a global infrastructure of acetic anhydride production and acetate
recovery.
It will be interesting to see how acetylation will be marketed around the
world. Will it he sold as a preservative treatment for wood used in the decking
market? Will acetylated wood be considered as a new wood species because
of its higher content of naturally occurring acetyl groups? Will its increased
dimensional stability and biological resistance open up new markets for wood?
It will also be interesting to see how acetylated wood is accepted in the marketplace by consumers, builders, and craftsmen. Codes and standards will need
to be developed, as well as guarantees of service life. Acetylated wood certainly
will have a high life cycle-to-benefit ratio.
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Stoyko Fakirov
Debes Bhattacharyya
Handbook of
Engineering
Biopolymers
Homopolymers, Blends and Composites
HANSER
Hanser Publishers, Munich • Hanser Gardner Publications, Cincinnati
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Prof. Dr. Stoyko Fakirov, The University of Auckland, Department of Mechanical Engineering.
Private Bag 92019, Auckland, New Zealand
Prof. Dr. Debes Bhattacharya, The University of Auckland, Department of Mechanical Engineering,
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