Note: Descriptions are shown in the official language in which they were submitted.
~2~173~
Improved process for acetylating a wood material
The present invention relates to a process for
improving the quality of a wood material and, more
particularly, to an improved process for the acetylation
S of wood material to impart, for example, an increased
resistance both to decay and to deformation.
It is to be noted that the term "wood material"
empLoyed for the purpose o~ the description o the present
invention is to be understood as including lumber, boards,
strips, chips, fibers and any other material made of wood
Wood material~ have long been used as materials
for buildin~s, furniture, fixtures~ etc. ~lthough wo~d
material generally has numerous desirable properties, it
also has some disadvantageous properties as a material for
many structural purposes. One of the disadvantages is that
wood material is susceptible to decay by the action of a
great number of fungi and/or insects such as termites. In
recent years, the problems associated with the decay of
wood material have been increasing in modernized buildings
2Q which have an air-tight structure and which are continu
ously air-conditioned.
Another disadvantageous property inherent to wood
material is that it is susceptib]e to deformation, such as
warpage, twisting, expansion and contraction, due to re-
peated absorption and release of moisture according to thesurrounding climatic conditions.
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73~3
It is already know that the acetylation of wood
material is very helpful in solving the above problems.
For instance, Irving S. Goldstein et al. describe a process
for acetylating wood material in the absence of a catalyst
in U.S. Patent Specification No. 3,094,431 and Foeest
Products Journal, 1961 (8), 363-370. However, Goldstein's
process requires a great deal of time for completion of
the acetylation. In addition, the following disadvantages
inherent in the acetylation reaction have not been solved
by this method.
1) Acetic acid formed during the acetylation
reaction accumulates in the acetylating agent, e.g. acetic
anhydride, and lowers the acetylation eEficiency of the
agent. In addition, the acetic acid itself causes swelling
and deterioration of the wood material.
2) In general, the wood material to be acetylated
contains a considerable amount oE water, which reacts with
acetic anhydride penetrating into the wood material to Eorm
acetic acid. Thus, the water contained in the wood mate-
2Q rial reduces th~ acetylation eEficiency oE the acetylatingagellt.
On the otller hand, a process Eor acetylating
cellulose per se in the presence of a catalyst is also
known. The ~.ypical method Eor the acetylation of cellu-
lose is the so~called "sul~uric acid-catalization method",
and i9 disclosed in Plastic Materials, No. 17, "Fiber
Resin", 55-66, published by Nilc~an Kogyo Shimbunsha in
Japan (1970). This method comprises treating cellulose
with a mixture of acetic acid and a catalytic amount of
sulfuric acid and subsequently treating the cellulose with
an acetylating agent comprising acetic anhydride. When
this method is applied to a wood material, the above
disadvantages can be reduced. However, this method causes
the following different disadvantages:
1) Surfuric acid causes carbonization, deterio-
ration or discoloration of the wood material;
,.,
_ 3 _ ~ 3
(
2) ~ter completion of the acet~lation, it is
very difficult to remove the sulfuric acid remaining in
the wood material, which has the indesirable action as
stated above.
Another typical method for the acetylation of
cellulose previously known is the "perchloric acid-
catalization method" which is also described in the
literature mentioned above. However, this method r
when applied to a wood material, also exhibits similar
disadvantages to those seen in the sulfuric acid-
catalization method and, therefore, is not satisfactory.
As stated above, none of the previously known
methods is particularly suited for the acetylation of
wood materials and, therefore, an improved method for
lS acetylating wood materials, which permits easy and
economical production of wood material having improved
properties such as a high resistance to decay, a high
resistance to changes in dimension and a high durability,
has long been desired.
In acco~dance with an aRpect of t~e invention there
~ provided a p~oce~s ~or ace~ylating a wood material which
aomprise~: ~a) impregnating the wood material with an aqueous
solution of an alkali metal acetate until the alkali metal
acetate penetsating into the wood material comprises 2 to 20%
by weight of the dry weight of the wood material, (b) drying
the wood material containing the alkali metal acetate solution,
~c) impregnating the dried wood material with acetic anhydride
for a period of time effective to cause the acetylation of the
wood material, said acetic anhydride being at a temperature of
100C to 150C.
It is to be noted that the phrases "acetylation of a
wood material" and "a wood material is acetylated" used in this
specification are intended to mean the acetylation of the con-
stituents of wood material, particularly the hydroxyl groups of
cellulose~ which is the major component of the wood material.
The improved process for acetylating a wood material
according to the invention is described in more detail below.
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_ 4 _ ~ ~7~
The ~iest step of the process of the invention
consists of impregnating the wood material with an a~ueous
solution of an alkali metal acetate which, for the purpose
of the present invention, means an alkali metal salt of
acetic acid and, in particular, sodium acetate and potas-
sium acetate. The concentration of the alkali metal
acetate in the solution is not critical, but it usually
ranges between 2 and 30~ by weight. The impregnation
can be accomplished by simply dipping the wood material
in the aqueous solution of the alkali metal acetate.
However, the use of an aqueous solution of an alkali metal
acetate having an elevated temperature between 40C and
80C is advantageous in accelerating the impregnation.
In addition, a suitable impregnation can be attained in a
short time by the use of a pressure impregnation technique
wheren the impregnation is carried out under pressure, a
vaccum impregnation technique wherein a wood material is
first placed uncler reduced pressure for the purpose of
deaeration and subsequently the impregna~ion is carried
out at atmospheric pressure, or a vaccum-pressure impreg-
nation technique wherein the wood material is first sub-
jected to deaeration in the sam~ manner as in the vaccum
impregnation technique and, thereafter, the impregnation
is carried out under pressure as in the pressure impreg-
nation technique, using a reactor resistant to bothpositive and negative pressures.
The amount of the alkali metal acetate to be
penetrated into the wood material is generally 2 to 20% by
weight, preferably 5 to 18~ by weight, based on the dry
weight of the wood material to be treated.
The second step of the process of the invention
is drying the wood material resulting from the first step.
This step is carried out by using a conventional dryer for
the purpose of removing most of the water contained in the
wood material. The drying is preferably conducted so that
the water content of the wood material may decrease to
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3[3~3
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less than 5~ by weight of the wood material, whereby an
efficient acetylation in the subsequent step can be
accomplished.
The dried wood material thus obtained is then
subjected to acetylation, the last step of the process of
the invention. The acetylation can be carried out by the
use of either acetic anhydride or acetic anhydride diluted
with an organic solvent.
(A) Acetylation with acetic anhydride
Acetylation is carried out, for example, by adding
acetic anhydride to the dried wood material present in the
reactor and heating to a temperature not lower than 100C,
preferably between 100C and 140C. The reaction time
required for the acetylation varies depending on the re-
action conditions. However, the reaction is generally con-
ducted to the extent that the weight of the wood material
increases about 15~ with respect to the staring weight. It
shouLd be noted that acetic anhydride already heated to an
elevated temperature can be added to the reactor in order
2Q to shorten the reaction time.
~ urill~ the ahove treatment, the precipitates of
the alkali metal ac~tate which are uniormly dispersed
within the wood material catalyze the reaction between
acetic anhydride and the hydroxyl groups of the cellulose
of the wood material, and an efficient acetylation of the
material i9 accomplished. Since the wood material has
been dried in the second step and contains only a small
amount of water reactable with acetic anhydride, undesir-
able decomposition of the latter into acetic acid can be
remarkably reduced as compared with non-dried wood
material.
The wood material acetylated in the manner stated
above warps and twists very little, and has an improved
resistance to deformation, an improved resistance to decay
and an improved durability. In addition, the alkali metal
acetate, a catalyst in the acetylation reaction, imparts
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no disadvantage to the wood material even if it remains
within the wood material after washing of the material
with water. Accordingly, no deterioration of the wood
material arises due to the remaining catalysts, such as
occurs in the case of the sulfuric acid-catalization
method and the perchloric acid-catalization method.
The above process can overcome most of the afore-
mentioned drawbacks inherent in the prior art. However,
the formation of acetic acid, which is a by-product of the
acetylation reaction, cannot be avoided. Since it is usual
that acetic anhydride is recycled for the acetylation
reaction, the acetic acid formed as a by-product accumu-
lates in the acetic anhydride and lowers the acetylation
efficiency. The inventors have now found that this in-
convenience caused by the acetic acid accumulated in aceticanhydride can be avoided by incorporating an alkali metal
acetate into the acetic anhydride employed as the acety
latin~ agent, The alkali metal acetate to be added to
the acetic anhydride can be identical to or dieEerent from
the alkali metal acetate penetrated into the wood material
durillg the eirst step. By this treatment, the acetic acid
which i5 Eormecl during the acetylation reaction and removed
from the wood material reacts with the alkali metaL acetate
to form a complex in accordance with the ollowing reaction
scheme, whereby the apparent concentration oE Eree acetic
acid in the acetic anhydride solution can be maintained
within a limited level and, accordingly, a decrease of the
acetylating rate of the solution due to the free acetic
acid can be avoided.
2CH3COOH t CH3COOM~ -~CH3cOOM 2CH3COOH
(M: sodium or potassium~
The complex dissolved in the acetic anhydride
solution precipitates upon lowering of the temperature of
the acetic anhydride solution, for example, down to about
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-- 7
20 to 40C. The precipitate can be separated by filtration
from the acetic anhydride and the latter can be recycled
for another acetylation reaction. The alkali metal acetate
can be recovered from the separated complex by treating
the latter with an alkali metal hydroxide solution, for
example, sodium or patassium hydroxide solution.
As will be understood from the above description,
one of the preferred embodiments of the present invention
comprises impregnating a wood material with an aqueous
solution of an alkali metal acetate, drying the wood
material containing the alkali metal acetate, and treating
the dried wood material at temperature between 100C and
1~0C with acetic anhydride admixed with an alkali metal
acetate, the first mentioned and second mentioned alkali
metal acetates being identical to or different from each
other.
(B) ~cetylation with acetic anhydride diluted
with an or~3anic solvent
The acetylation can also be conducted according
to the general procedure desceibed in (A) employing acetic
anhydride diluted with an organic solverlt instead oE acetic
anhydridc alone and carrying out the acetylatioll reaction
at a tempe~ature between 100C and 150C.
Be~ore the description proceeds, it is to be noted
that the organic solvent useable in the practice of the
present invention to dilute the acetic anhydride includes
all kind of organic solvent, except (1) those capable of
reacting with acetic anhydride, such as alcohols; (~)
those which make wood material swell to a great extent,
such as pyridine and formaldehyde; (3) those which are
more expensive than acetic anhydride; and (4) those having
an extremely low boiling point. The preferred organic
solvents are aromatic hydrocarbons such as benzene,
toluene and xylenes, and mixtures thereof.
In contrast to the aorementioned process (A)
wherein an expensive acetic anhydride is used above, this
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modified process employs a mixture of a less expensive
organic solvent and acetic anhydride. Accordingly, acety-
lation of the wood material according to the modified
process can be accomplished more economically than the
process (A). Elowever, another and important advantage
of the modified process involving the dilution of acetic
anhydride with an organic solvent resides in the following
point. Acetic anhydride has a strong tendency to extract
various components of wood material. Therefore, the acetic
anhydride used for the acetylation of the wood material
will become contaminated by the extracted components.
Such extraction of the components causes on the one hand
a decrease of the acetylating efficiency of the acetic
anhydride and on the other hand causes a loss in strength
of the wood material.
It has been ound that this disadvantage can be
remarkably diminished by using acetic anhydride diluted
with an or~anic solvent and that the decrease of the
acetylating eEficiency due to the dilution Oe the acetic
allllydride is tlegll~ibl~.
Ace~ic anhydrlde ha~ a strongly irritating and
stimulating ordor and, thereÇore, the use o large
quatltitie~ Oe acetic anhydride, especially in the hot
state, is unÇavorable to workers from the viewpoint of
environment hygiene. This disadvantage can also be
minimized by the use oÇ a mixture of acetic anhydride
and an organic solvent.
The concentration of acetic anhydride in the
mixture may range between 20 and 70% by weight. The
reaction rate snay fall if the concentration is below the
lower limit, and the aforementioned advantage may decrease
if the concentration is above the upper limit.
Thus, as will be readily understood from the
above description, another preferred embodiment of the
present invention comprises impregnating the wood material
with an aqueous solution of an alkali metal acetate, drying
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g
the wood material containing the alkali metal acetate, and
treating the dried wood material at temperature between
100C and 150C with a mixture of acetic anhydride and an
organic solvent, the concentration of the acetic anhydride
in the mixture ranging between 20 and 70~ by weight.
The drawbacks of the prior art can be substan-
tially overcome by employing the preferred method described
above. However, the most preferred embodiment of the pre-
sent invention is one which corresponds to the process just
mentioned above and further comprises an additional step
consisting of pre-impregnating the wood material with ace-
tic anhydride optionally diluted with an organic solvent
after the second step and before the last acetylation step.
Since wood material has a complex tissue structure
which hampers a uniform penetration of the acetylating
agent comprising acetic anhydride, the last acetylation
step sometimes ~ails to accomplish a rapid and complete
acetylation ve the wood material. ~his disadvantage can
be overcom~ by the use o~ a pre-impregnation step using
the acetylatill~ a~ent prior to the ~inal actylation step.
Th~ pre-inlpregnati~rl o~ the dried wood material
with the ~cetylatin~ ~gent can he carried out simply by
dipping the wood material into the agent. ~lowever, in
order to obviate uneven impregnation of the agent due to
the air present within the wood material, it is preferred
to conduct the dipping under pressure. In addition, it is
most preferred to conduct deaeration of the wood material
under reduced pressure prior to the dipping.
The final step, i.e., the acetylation of the wood
material pre-treated as above can be conducted e~actly in
the same manner as previously stated.
The acetylated wood materials obtained in
accordance with any one of the processes described above
can be processed in the same manner as non-acetylated wood
materials to form various semi-finished or finished wood
products. For instance, strips, chips and fibers acety-
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lated by the process of the invention can be processed to
form plywood or laminated veneer lumber, particle board
and Eiber board, respectively.
The following detailed Examples are presented by
way of illustration of certain specific embodiments of the
invention.
Example 1
A batch of ten spruce veneers of 3mm x 60mm x 55mm
in size was impregnated with a 5% aqueous sodium acetate
solution by means of a vaccum-pressure impregnation
technique and was then dried absolutely. This yielded
dried veneers containing 13~ by weight of sodium acetate.
The veneers were dipped in a mixture consisting of m-xylene
and acetic anhydride (60 : 40 by weight) heated to about
125C and were kept in the mixture for 30 minutes, whereby
acetylation of the wood material took place. After the
completion of the reaction, the veneers were placed under
reclucecl pressure to remove the mixture let within the
boards, washed with hot water and dried. Acetylated
veneers were~ thus obtained. The avecage value of the
wei~llt gaitls Oe the veneets, i.e., the apparent acetylation
rates o~ the veneers, was 23 percent by wei~ht.
The above process was repeated ten times under
the same conditions using the same mixture supplemented
with a ~resh mixture required for compensating for the
consumed acetic anhydride, while eresh spruce veneers were
employed each time. The average value of the acetylation
rates was not less than 20 percent by weight in each trial.
No deterioration such as cracks was observed in all of the
resultant acetylated boards.
Exam~ 2
A batch of ten hemlock veneers of 3mm x 60mm x
60mm in size (about 409) were impregnated with a 5% aqueous
sodium acetate solution and dried in the same manner as in
Example 1.
Sodium acetate (lOg) was added to acetic anhydride
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(500g), and the mixture was heated to 120C. The dried
veneers obtained above were dipped in the hot mixture,
and acetylation was allowed to proceed for 30 minutes.
The above process was repeated ten times using
the same mixture supplemented with fresh acetic anhydride
required for compensating ~or the consumed anhydride, while
a fresh batch of ten hemlock veneers was employed each
time. The average acetylation rate of the last batch was
24 percent by weight, which nearly equaled the average
acetylation rate of the first batch. Thus, a substantial
decrease of the acetylation efficiency was not observed in
the reactant mixture which had been reused no less than
ten times.
A halE of the liquid portion of the mixture was
15 removed from the reactor and cooled to 20C, whereby a
complex, consisting oE acetic acid formed during acety-
lation and sodium acetate from the mixture, precipitated.
The complex was separated Erom the li~uid portion of the
mi~ture, and the latter was returned to the reactor. On
20 the other han~, the complex, aEter havin~ been dissolved
in water, was d~composed to sodium acetate and acetic acid
by the addition of a 10~ a~ueous sodium hydroxide solution.
The recovered sodium acetate was also returned to the
reactor.
A ~:resh batch o hemlock veneers is treated in
the same manner as described above, in a reactor charged
with the recovered mixture to form the acetylated veneers
having an acetylation rate of 24 percent by weight and
high strength characteristics.
Example 3
A batch of ten spruce veneers of 3mm x 60mm x 55mm
in size were impregnated with a 5% aqueous sodium acetate
solution and then completely dried in the same manner as
in Example 1. This gave veneers containing 15% by weight
35 of sodium acetate. The veneers were then subjected to
deaeration under reduced pressure over ten minutes and
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impregnated with a mixture consisting of m-xylene and
acetic anhydride (60 : 40 by weight) under pressure.
The resultant veneers which contained sodium
acetate and the mixture were dipped in the same kind of a
mixture heated to about 125C and kept in the mixture for
20 minutes at the same temperature. After the completion
of the reaction, the veneers were placed under reduced
pressure to remove the mixture left therein, washed with
hot water and dried. Veneers having an average acetylation
rate of about 25 percent were thus obtained.
The above process was repeated ten times under the
same conditions using the same mixture supplemented with a
fresh mixture required for compensating for the consumed
acetic anhydride, while a fresh batch of ten spruce veneers
was employed each time. The average value of the acety-
lation ~ates of the resultant veneers was not less than 20
percent by weic~ht in each trial. No deterioration such as
cracks were observed in all of the resultant acetylated
veneers.
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impregnated with a mixture consisting of m-xylene and
acetic anhydride (60 : 40 by weight) under pressure.
The resultant veneers which contained sodium
acetate and the mixture were dipped in the same kind of a
mixture heated to about 125C and kept in the mixture for
20 minutes at the same temperature. After the completion
of the reaction, the veneers were placed under reduced
pressure to remove the mixture left therein, washed with
hot water and dried. Veneers having an average acetylation
rate of about 25 percent were thus obtained.
The above process was repeated ten times under the
same conditions using the same mixture supplemented with a
fresh mixture required for compensating for the consumed
acetic anhydride, while a fresh batch of ten spruce veneers
was employed each time. The average value of the acety-
lation ~ates of the resultant veneers was not less than 20
percent by weight in each trial. No deterioration such as
cracks were observed in all of the resultant acetylated
veneers.
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