CA1247308A - Process for acetylating a wood material - Google Patents

Abstract

Abstract:
The invention provides a process for acetylating a wood material. The process comprises impregnating the wood material with an aqueous solution of an alkali metal acetate, drying the wood material containing the alkali metal acetate solution, and treating the dried wood material with acetic anhydride optionally diluted with an organic solvent at temperature between 100°C and 150°C. The treated wood material resists decay and dimensional changes due to moisture absorption and release.

Description

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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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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 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 ~2~7~

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 ~l2~7~

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 - 8 - ~ % ~73~

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 ~2~73~3 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-- 10 - ~2~7~

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 ~2~73~

(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 ~, L73~3 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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Claims (12)

Claims:

1. A process for acetylating a wood material which comprises:
(a) impregnating the wood material with an aqueous solution of an alkali metal acetate until the alkali metal acetate penetrating 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 acetyl-ation of the wood material, said acetic anhydride being at a temperature of 100°C to 150°C.

2. The process according to claim 1, wherein said impreg-nation step (a) is accomplished under pressure conditions effective to deaerate said wood material and impregnation of said aqueous alkali metals solution accomplished under pressure conditions effective to accelerate impregnation.

3. The process according to claim 1, wherein the impregna-tion of step (c) is accomplished with an admixture of acetic anhydride and alkali metal acetate.

4. The process according to claim 1, wherein the impregna-tion of step (c) is accomplished with an admixture of acetic anhydride and an organic solvent.

5. The process according to claim 4, wherein the organic solvent is benzene, toluene, xylenes or mixtures thereof.

6. The process according to claim 5, wherein the weight ratio of acetic anhydride and the organic solvent ranges from 20:80 to 70:30.

7. The process according to claim 1, comprising an additional step of pre-impregnating the dried wood material with acetic anhydride, said additional step being carried out subsequent to drying step (b) and prior to impregnating step (c).

8. An acetylated wood material obtained by the steps of (a) impregnating the wood material with an aqueous solution of an alkali metal acetate until the alkali metal acetate penetrating 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 acetyl-ation of the wood material, said acetic anhydride being at a temperature of 100°C to 150°C.

9. The process according to claim 1, wherein the alkali metal acetate is sodium acetate or potassium acetate.

10. The process according to claim 1, wherein impregnation step (a) is accomplished by elevating the temperature of said aqueous alkali metal solution to about 40°C to about 80°C.

11. The process according to claim 1, wherein said impregna-tion step (a) is accomplished under pressure conditions effective to accelerate the impregnation of said aqueous alkali metal acetate solution.

12. The process according to claim 1, wherein said impregna-tion step (a) is accomplished under pressure conditions effective to deaerate said wood material and impregnation of said aqueous alkali metal acetate solution being accomplished at atmospheric pressure.