WO1995009721A1

WO1995009721A1 - Wood preservation method

Info

Publication number: WO1995009721A1
Application number: PCT/FI1994/000440
Authority: WO
Inventors: Liisa Viikari; Johanna Buchert; Anne-Christine Ritschkoff
Original assignee: Valtion Teknillinen Tutkimuskeskus
Priority date: 1993-10-01
Filing date: 1994-09-30
Publication date: 1995-04-13
Also published as: AU7700994A; FI94323B; FI94323C; FI934338A0

Abstract

A method is provided for protecting wood against undesirable reactions caused by microorganisms. According to the present invention wood is washed with an acid in order to remove at least a part of those transition metals contained in the wood which are essential for the mechanism of decay and other metabolic reactions of rot fungi. Preferably, wood is washed with a mineral acid or a solution thereof having a pH in the range of 0.5 to 2, whereby at least about 30 % of the iron and the manganese is removed. In order to prevent contamination caused by metals from the soil and coming with rain water, monovalent cations can be attached to the wood instead of the removed transition metals. By means of the present invention it becomes possible to protect wood against decay, blue-staining and moulding in a simple, non-toxic way.

Description

WOOD PRESERVATION METHOD

The present invention relates to a wood preservation method in accordance with the preamble of claim 1.

According to such a method, wood is protected against undesirable reactions caused by microorganisms by preventing the growth and spread of the microorganisms in the wood. Th present invention also deals with timber according to claim 13, which is protected against undesirable reactions of microorganisms.

Wood decay fungi and a number of other microorganisms can metabolically utilize the structural components of wood. Brown-rot fungi remove cellulose and hemicellulose from wood, while white-rot and soft-rot fungi are also capable of utilizing the lignin components o wood. Brown- and soft-rot decay is characterized by rapid deterioration of the strength properties of wood already in the initial stage of decay. Brown- and soft-rot fungi are among the worst culprits in boreal climate zones for causing damage in timber and wood constructions and they account for annual losses of several billions of Finnmarks through damages caused to wood constructions.

Wood can be protected chemically against damages caused by decay fungi by various preservation methods employing preservatives of varying efficacy. Wood preservatives presently used can be coarsely classified as follows: water-borne preservatives (CCA and CC preservatives), oil-borne preservatives (tribytyl tin naphthenate, tribytyl tin oxide, dichloro fluanide, iodopropynyl butynyl carbamate and mixtures of penta- and tetrachlorophenols) and creosote oil (the fraction of coal tar distilling above 200 °C).

The known preservatives have considerable drawbacks. Thus, they contain toxic compounds, and therefore their use has be to approved by authorities. The efficiency of the common preservatives is based on the general toxicity of these compounds which is generally directed towards the basic (vital) metabolic reactions common to all living organisms, such as cell respiration or production of the key energy metabolite, ATP. Because these preservatives are non-specific toxins, serious health (e.g., carcinogenicity) and environmental (soil and waterway contamination) risks are involved with the use of conventional wood preservatives. Health risks are imposed on all eucaryotic organisms including plants, animals and man. If t content of copper, arsenic and chromium in a CCA preservative are decreased, however, problems in fixing the preservative into wood result, with a significant reduction of the preservative's efficacy paralleling the reduction of heavy metal concentrations.

It is an object of the present invention to overcome the drawbacks of the prior art and to provide an entirely novel method for protecting wood against decay and similar undesirable reactions caused by microorganisms which impair the strength properties of wood or otherwise decrease the economic value of the material.

The present invention is based on the idea that at least a part of the metals (Fe, Mn, Cu, Zn or other transition metals) which are normally present in the wood and which are essential f the wood degradation mechanism or metabolism of the rotting fungi is removed from the wood by using an acid wash. In addition to the removal of these metals, a suitable cation, such as monovalent metal ion, or another compound of small molecular size, enhancing the prevention of decay fungi growth, can be added instead of the metals.

More specifically, the method according to the present invention is characterized by what is stated in the characterizing part of claim 1.

The lumber according to the present invention is, again, characterized by what is stated in th characterizing part of claim 13.

In the context of the present invention, the term "undesirable reactions" of microorganisms is used for referring to wood degradation and decay caused principally by fungi and moulds. The greatest damage to wood is caused by the above-mentioned rot fungi, such as the brown rot and the white-rot fungi, but also the damage caused by blue-stain fungi and by mould is of economical importance.

As examples of the brown-rot fungi, the following species should be mentioned: dry-rot . ingus (βerpula lacrymans), cellar fungus (Coniophora puteanά), white-pore fungus (Poria placenta) and sauna fungus (Gloeophyllum trabeum). Rot fungi decompose structural components of wood, that is, cellulose and hemicellulose, by virtue of reactions ending in hydrolytic and oxidizing radical reactions. Conventionally, decay of wood is characterized by the weight loss of the wood.

Damage to wood (that is, colour defects) is caused by blue-stain and mould fungi. Of the fungi causing mould damages, strains worthπmentioning are those belonging to the Cladosporium, Alternaria, Helminthosporium, Penicillium, Aspergillus, Epicoccus and Rhizo- pus families. Mould fungi belonging particularly to the Penicillium and Aspergillus families cause extensive damage in indoor spaces and structures. Blue-stain fungi most frequently found in wood include strains of the Ambrosiella, Aureobasidium, Ceratocystis, Cladosporium and Phialophora families. Most common blue-stain strains attacking sawn pine

belong to the Aureobasidium pullulans and Ceratocystis families, e.g., C. pilifera. Besides these strains, blue-stain in spruce wood is caused by, e.g., Ceratocystis piceae and C. coerulescens.

In addition to moulds belonging to the above-mentioned strains, strains of the Sclerophoma family occur in sawn pine wood such as Sclerophoma entoxylina.

The present invention can be utilized to preserve wood against undesirable reactions of all above-mentioned microorganisms. The present invention is preferably used for protecting felled timber, such as sawn wood and pilars, but it can also be employed for treatment of wood material in the form of saw dust and chips and products formed therefrom.

It is known in the art that the metals contained in wood can be removed or bound into a form, which is no more available for microorganisms, by using chelating agents. We here refer to the solution presented in the Finnish Patent Application No. 915166. In the method described herein, the removal of metals refers to the detachment or washing out of metals bound to the wood by ionic bonds. Especially important is the removal of transition metals which are present in native wood. These metals are of central importance in the degradation mechanism of wood and other metabolic reactions of the rot fungi.

During wood decay transition metals, such as Fe, Mn, Cu and Zn, interact in wood with, e.g., hydrogen peroxide while producing radicals, or through the metabolism of the micro¬ organisms. In the latter case, the above mentioned microorganisms abstract metal ions from the surroundings and use them in the active centres of enzymes, taking part in the degradatio of wood. In connection with the present invention, it has been observed that the removal of transition metals prevents the action of these metals during production of the radicals required by wood decay (degradation of lignin and carbohydrates).

According to the present invention, it is preferred to change the transition and other harmful metals present in the wood-containing material for another counter ion, such as a monovalent metal (e.g., Na, Li, K etc.), which does not act as a transition metal. In this way, it is possibl to prevent contamination caused by transition metals coming from the soil or rain water. Simultaneously, the additional advantage is achieved, that certain hydrolytic enzymes (such as xylanases), participating in the degradation of wood, are inhibited. This is illustrated by Example 5.

In connection with the present invention, the term "acid wash" refers to a procedure, in which the wood material is treated with a solution having a pH of about 3 or lower. The purpose of the acid wash is to remove a substantial part of the metals in the native wood. Typically, at least 30 % of the transition metals bound to the wood and playing a key role in the wood degradation mechanisms and in fungal metabolism are removed. (The amount mentioned is calculated separately for each of the transition metals considered). In particular, one aims at removing at least 30 % of the transition metals Fe and Mn. Therefore, it is typical for the wood material treated by the method of invention, that the content of transition metals in considerably lower than in native wood. Because the metal concentration of the wood depend on whether heartwood or sapwood is considered, the corresponding type of native wood is taken as a reference.

The metals are removed by using an acid treatment, wherein the wood is washed by an acid having a pH in the range of 0.5 to 2, preferably about 1. Suitable acids include mineral and organic acids, and mixtures of these. Of the mineral acids e.g. hydrochloric, sulphuric and phosphoric acids and of the organic acids formic and acetic acids can be mentioned. According to one preferred embodiment hydrochloric acid is used in a concentration of, e.g., 0.1 M. Also mixtures of strong and weak acids can be used in the present invention. As an example the mixture of hydrochloric and boric acid should be mentioned. By using boric acid the fire resistance of wood material can be improved.

According to the present inveniion, acid washing of the wood material is carried out by impregnating the wood with an acid solution and by allowing the solution act on the woods for such a long period of time that at least a part of the metals is transferred into the liquid phase, by splitting the ionic bonds. The impregnation treatment preferably aims at thoroughly removing the metals, at least from the sapwood. This treatment can be carried out under or without pressure. Before treating the wood with acid, it is preferred to remove at least a part of the air contained in the wood, in order to ensure that the acid solution penetrates as deep a possible within the wood material. If it is desired to treat only the surface of the wood material, e.g. for preserving the surface against fungi, the acid treatment can be carried out without removal of air.

According to a first embodiment of the present invention, the wood material is impregnated under pressure using an acid solution. After the impregnation, the acid and the metals are withdrawn in vacuum from the wood and the material is further impregnated with water, possibly supplemented with buffering compounds, to increase the pH again. Any hydrolysis o the wood, caused by residual acid, is thereby avoided.

According to a second embodiment of the present invention, metals are removed by impregnation of wood with acid without pressure. In this case, however, the treatment time is longer, and the destruction of wood components by the acid more significant.

In order to determine the most suitable treatment time, the removal of metals in both cases can be monitored by using ϋnic adsorption spectroscopy. In particular the amounts of iron and manganese are decisive for a successful result of this method. The acid treatment can be repeated, if necessary.

The wood, treated by the acid, can be used as such. In that case, it is advantageous to wash the acid-treated wood with water (supplemented with buffering compounds, if necessary) to increase the pH, before usage of the wood material.

The vacant space left by the removed metals can, however, preferably be filled with a monovalent cation or by a cation with a small molecular size. Suitable compounds of these types are, for instance, different kinds of non-transition metals, such as lithium, sodium, beryllium, magnesium or calcium. Also other, non-metallic compounds, such as organic compounds, with a small molecular size, are suitable. This stage of the treatment is preferably carried out by impregnating the acid-washed wood material under pressure with a solution, containing the required compound at a suitable molar ratio or weight percentage (typically 0.1 - 10 %). According to a preferred embodiment of the present invention the binding of the metal can be enhanced by adjusting the pH-value using the corresponding metal hydroxide, in which case the impregnation solution contains the corresponding metal hydroxide in addition to the metal chloride. At the same time the pH is adjusted to a suitable value. The adsorption of the metals can be monitored by atomic adsorption spectroscopy and the amount needed for the prevention of decay can be controlled.

Considerable advantages are obtained by the present invention. Using the method the transition metals needed by the microorganisms can be removed. The effective compounds, added to replace the removed metals, prevent contamination of wood by metals originating from external sources, such as rain water or soil. No non-specific toxic compounds are used, neither are any additional investments needed for the application of this method at saw mills using wood preservation. The wood products treated with this method are safe and the destruction of products and residues, e.g. by combustion, does not cause any risks to the environment. The natural light colour of wood is retained in the treated wood material. The adsorption of monovalent cations or other compounds of small molecular size to the wood makes possible to introduce coloured pigments instead of the removed metals, if coloured wood products are desired.

In the following, the invention will be described in more detail with the aid of some working examples. Example 1

Removal of metals from sawdust and replacement of metals by monovalent cations

Sapwood dust from pine (40 g/1) was washed with a 0.1 M HCl-solution, whose pH was about 0.9 to 1. The acid wash was carried out by impregnation under pressure, and then the sawdust was washed with water, .11 the pH was 4. The sawdust was dried at 60 °C overnight. The acid wash was repeated three times. The removal of metals from was followe by determining the concentrations of Fe and Mn in the wood material. The metal analyses were carried out by atomic adsorption spectroscopy.

The acid-washed wood powder was impregnated under pressure with 0.1 M LiCl. The pH of the sawdust was increased to pH 6, pH 8 ja pH 10 using LiOH as follows:

pH 6 32 g sawdust / 400 ml 0.1 M LiCl. The pH was adjusted with 1400 μl of 1 M LiOH.

pH 8 32 g sawdust / 400 ml 0.1 M LiCl. The pH was adjusted with 3900 μl of 1 M

LiOH.

pH 10 32 g sawdust / 420 ml 0.1 M LiCl. The pH was adjusted with 7500 μl of 1 M

LiOH.

The sawdust was dried at 60 °C and the removal of metals with respect to the Fe, Mn and Li concentrations was followed by using an atom absorbtion spectrometer. Based on the results obtained it appeared that all the manganese and most of the iron (about 70 %) could be removed by an acid wash. The higher the binding pH the better the Li-ions were bonded to the unoccupied sites in the wood. The results are presented in Table 1. Table 1. Metal analysis results. Metal concentrations indicated as μg/g.

Wood Fe (μg/g) Mn (μg/g) Li (μg/g)

Control 6.3 72.1 < 1

Acid-washed 2.0 0.14 < 1

Li-added, pH 6 1.1 0.03 500

Li-added, pH 8 0.7 0.06 610

Li-added, pH 10 0.9 0.02 690

Example 2

Growth test on wood powder

Sawdust treated with Li-ions was sterilized in an autoclave (1 arm, 120 °C) for 30 min. The sawdust was distributed on agar plates (3 g / plate, 90 x 90 mm), the dust was moistured with distilled water, containing 1 % agar. Sauna fungus Gloeophyllum trabeum and cellar fungus

Coniophora puteana were chosen as test organisms for experiments on wood substrates treate with lithium.

During the tests the ability of the fungi to grow on the substrates was followed by measuring the diameter of the fungal growth and by comparing these with those of the control plates.

The control plates contained untreated sawdust. The fungi were grown in growth chambers at 22 °C and 75 % relative humidity until the reference plates were fully grown. The results of the growth experiments are presented in Tables 2a, 2b, 2c and 2d. Table 2a. The growth of the cellar fungus on lithium-containing plates. The results are expressed as diameters (mm) of fungal growth. The markings +, +/- refer to the viability of the fungal mycelium (inspected microscopically). Thus in some case the fungus was alive, but was not able to spread on the plate.

Growth time Control Li-added, pH 6 Li-added, pH 8 Li-added, pH 10 (days) (mm) (mm) (mm) (mm)

0 0 0 0 0

7 12.7 + 0 0

12 47.2 + 0 0

24 85.0 +/- 0 0

Table 2b. The growth of the sauna fungus on lithium-containing plates. The results are expressed as diameters (mm) of fungal growth. The markings +, +/- refer to the viability of the fungal mycelium (inspected microscopically). Thus in some case the fungus was alive, but was not able to spread on the plate.

Growth time Control Li-wood, pH 6 Li-wood, pH 8 Li-wood, pH 10 (days) (mm) (mm) (mm) (mm)

0 0 0 0 0

7 33.0 0 0 0

12 58.5 0 0 0

21 85.0 0 0 0 Example 3

Washing-out of metals and adsorption of lithium on sapwood samples of pine

The samples selected for this decay testing were washed with a diluted acid solution in order to remove the metals contained in the wood. Thus, samples of pine sapwood (30 x 15 x 5 mm) were subjected to acid wash with 0.1 M HC1 by pressure impregnation. After the wash the acid was removed under pressure and the test specimens were impregnated with water. The samples were washed with water until their pH had been increased to about 3 to 4. After the wash the samples were cautiously dried in a heating chamber at 60 °C. The acid wash wa repeated three times.

The metal-free parts of the wood samples were filled with lithium. The impregnation solution were prepared based on the results obtained in connection with the growth test. The pH of th solutions (0.1 M LiCl) were adjusted with LiOH as follows:

pH 6 500 ml 0.1 M LiCl + 1800 μl 1 M LiOH pH 8 500 ml 0.1 M LiCl + 4950 μl 1 M LiOH pH 10 500 ml 0.1 M LiCl + 9200 μl 1 M LiOH

The removal of the metals contained in the wood and the bonding of lithium thereto was determined by atomic absorption spectroscopy by analyzing the wood for the concentrations of Fe, Mn and Li. The metal contents of the wood samples after the different treatment stage are depicted in Table 3. As will appear from said table, the concentration of iron decreased b more than 45 % and that of manganese by more than 90 %.

Table 3. Results of metal analysis of wood samples. Metal contents indicated as μg/g.

Wood Fe (μg/g) Mn (μg/g) Li (μg/g)

Control 4.2 47.1 < 1

Acid-washed 2.3 4.3 < 1

Li-wood, pH 6 2.6 1.6 1020

Li-wood, pH 8 2.7 7.9 1360

Li-wood, pH 10 1.3 3.4 1570

Example 4

Decay testing of acid-washed and lithium-treated wood

The decay test was carried out according to standard EN 113, suitably modified, in kolle dishes which were filled with water agar on the bottom (1 %). The substrates were sterilized in an autoclave (20 min, 120 °C). The rot fungi selected for this test were two brown-rot fungi, viz. cellar fungus (Coniophora puteana) and sauna fungus (Gloeophyllun trabeum). Th duration of the test was 10 weeks and it was performed in a growth chamber at about 22 °C and at a relative humidity of 75 %.

The pine sapwood pieces (30 x 15 x 5 mm) chosen for the test were placed after sterilization by autoclaving (30 min, 120 °C) in kolle dishes upon a sterile rubber ring. The test fungi were inoculated upon each sample. Both a treated (acid-washed or Li-treated) sample and a control sample were placed in each kolle dish. Three parallel test samples were used in the test. The efficacy of the treatment was characterized by the weight loss (in percents) caused by the fungus. The weight loss was calculated on basis of the initial dry weight of the wood piece and the dry weight of the wood piece after the decay test. The results are indicated in Table 4. Table 4. Results of decay testing. The numerical value indicate the weight losses (in per cents) caused by the fungi.

Fungus Control Acid washed Li wood Li wood Li wood wood pH 6 pH 8 pH 10

Cellar fungus 19.6 4.2 3.3 3.4 4.0

Sauna fungus 22.7 5.1 2.6 2.8 3.8

Example 5

Enzymatic hydrolysis of a lignocellulosic substrate

A lignocellulosic substrate was first subjected to an acid wash, and then the substrate was converted into the Li-form, as described above. It was subjected to hydrolytic enzymes produced by Trichoderma reesei. The degree of hydrolysis was determined on basis of the amount of reducing sugars. The results are given in Table 5, which shows that the Li-form material was hydrolyzed only to an extent of 16 % compared with the control.

Table 5 Hydrolysis of untreated and Li-form substrate

Relative degree of hydrolysis

Control 100 %

Substrate in lithium form 16 %

Claims

CLAIMS:

1. A method for protecting wood against undesirable reactions caused by microorganisms, c h a r a c t e r i z e d in that the wood to be protected is subjected to an acid wash in order to remove transition metals natively contained in the wood.

2. The method according to claim 1, which comprises removing at least 30 % of those transition metals, which are bound to the wood by means of ionic bonds and which are of significance as regards the decay mechanism of the fungi and the other metabolic reactions.

3. The method according to claim 1, wherein iron, manganese, copper or zinc is removed from the wood by means of the acid wash.

4. The method according to any one of claims 1 to 3, wherein the wood is washed with a mineral acid or a solution thereof, having a pH of about 0.5 to 2.

5. The method according to claim 1, wherein the metals are removed by

- removing at least a part of the air contained in the wood, - impregnating the wood with an acid solution,

- allowing the acid solution to act on the wood for such a long period of time that a desired amount of metals are transferred to liquid phase and

- withdrawing the metal-containing acid solution from the wood.

6^ The method according to claim 5, wherein the wood is pressure impregnated with an acid solution.

7. The method according to claim 5 or 6, wherein the wood material is impregnated with water after the acid wash in order to raise the pH.

8. The method according to any one of claims 5 to 7, wherein said method is repeated at least once in order more effectively to remove the metals.

9. The method according to claim 5, wherein the acid-washed wood is treated with a solution containing small-sized metal ions or compounds of a small molecular size, which can be adsorbed to the wood instead of the removed transition metals.

10. The method according to claim 9, wherein a solution is used which contains said ions or compounds in a concentration of 0.1 - 10 %.

11. The method according to claim 9, wherein a solution is used which contains lithium, sodium, calcium, beryllium and/or magnesium ions.

12. The method according to claim 9, wherein the adsorption of the metal ions is enhanced by adjusting the pH with the aid of the corresponding metal hydroxide.

13. Sawn wood which is protected against undesirable reactions caused by microorganisms, c h a r a c t e r i z e d in that the concentration of transition metals therein is essentially lower than the corresponding concentration of native wood.

14. Sawn wood according to claim 13, wherein the concentration of both iron and manganese is at least 30 % smaller than that of the corresponding native wood.