US3560251A - Wood treatment method - Google Patents

Abstract

AFTER WOOD HAS BEEN IMPREGNATED WITH AN AQUEOUS SOLUTION OF A WOOD PRESERVATIVE IT IS DRIED BY HEATING IT IN A HEATED BATH OF HIGH-BOILING OIL IN A CHAMBER, UNDER VACUUM, THE TEMPERATURE AND SUBATMOSPHERIC PRESSURE BEING ADJUSTED TO REMOVE THE SOLVENT AT A TEMPERATURE BELOW THE NORMAL BOILING POINT OF THE SOLVENT.

Description

United States Patent 3,560,251 WOOD TREATMENT METHOD Bror Olof Hager, Djursholm, Sweden, assignor to Hager Aktiebolag, Stockholm, Sweden No Drawing. Filed July 11, 1966, Ser. No. 563,979 Claims priority, application Germany, July 19, 1965, P 14 92 511.2 Int. Cl. Bc 11/10; B27k 3/02 US. Cl. 117102 5 Claims ABSTRACT OF THE DISCLOSURE After Wood has been impregnated with an aqueous solution of a wood preservative it is dried by heating it in a heated bath of high-boiling oil in a chamber, under vacuum, the temperature and subatmospheric pressure being adjusted to remove the solvent at a temperature below the normal boiling point of the solvent.

The present invention relates to a method of treating wood (and similar organic substances) to combat decay and damage by insects. According to the complete method, the so-treated material is dried and substances may be, or are, added thereto which give it different valuable properties. The method is carried out in the following way.

The wood is treated with a preserving solution, preferably under pressure, according to usual standard processes. In order to facilitate the subsequent drying it is desirable to introduce no more solvents than necessary into the wood. Preferably the Lowry or Ruping treatment is to be observed (as opposed to full cell or open tank treatment). The preserving treatment is followed by drying the treated woodpreferably, under vacuun1in a warm high-boiling liquid which is immiscible with the preserving solution.

The preference for the Lowry or Ruping procedure stems from the following facts.

By the full cell treatment, the wood is exposed to a vacuum before the solution is introduced under pressure into the wood. This treatment gives the highest possible retention of solution in the wood, in that not only the walls of the cells in the wood are saturated with solution but also the cells are filled with solution. As an example, it can be noted that in Swedish pine treated by full cell treatment there is usually introduced about 300 liters of solution per cubic meter of wood. The Lowry treatment, on the other hand, starts without initial vacuum, the solution being directly introduced into the wood by pressure. Then the solution in the pressure cylinder (or, equivalent pressure vessel) which has not been actually introduced into the wood is removed from the cylinder by a pump, and a vacuum is applied to the interior of the vessel. Part of the solution introduced into the wood is recovered from the wood by means of the compressed air introduced into the wood during the pressure period. By treating Swedish pine according to the Lowry process usually about 150 liters of the preserving solution are introduced into, and well distributed in, the wood.

The Ruping treatment is still further developed in order to effect a low net retention. It is begun with an air pressure of some kilograms on the wood before the solution is introduced. This means that an increased air quantity is compressed within the wood when the solution thereafter is introduced under an elevated pressure. In the subsequent vacuum period more solution is recovered than in the Lowry treatment. When treating Swedish pine according to the Ruping treatment usually at the most 100 liters of solution per cubic meter are introduced into the wood. In this respect, the Ruping treatment has some technical advantages. The apparatus for its use is, how- "Ice ever, so expensive that in spite of the higher amount of solution introduced, the Lowry treatment generally is to be preferred for the process according to the present invention.

By the different impregnating treatments different quantities of solution are thus introduced into the wood. Therefore also different concentrations of the solutions are used, corresponding to the amount of solution introduced so that the wood always obtains the correct (e.g., the same) amount of preservative per unit volume.

According to the invention, it is the intention in the first line to use aqueous solutions of preservatives which, after the treatment, are fixed in the wood. In some instances, the preserving substance is dissolved in water by means of ammonia and ammoniacal salts. Solutions of this kind are, for example, described in US. Pats. 2,908,- 607 and 3,089,810. They have the important property of being stable enough to be used in the Lowry and Ruping treatments.

After the impregnating treatment has been carried out and the preservative solution has been introduced and distributed in the wood and excess solution has been pumped away from the pressure cylinder a, high boiling liquid is introduced. By high boiling liquid is here meant a liquidfor instance, an oilboiling over 100 C., preferably, over 250300 C. The wood is dried by the use of this oil. This is done by keeping the oil at an elevated temperature, and keeping the cylinder (with the wood in it) under a high vacuum. In order to obtain a mild drying, and to avoid inconveniences, e.g., foaming, from arising, the different moments of the drying process can be increased successively to full effect. These moments are the introduction of the oil into the cylinder, its heating and the raising of the vacuum. Antifoaming means can also be used. The final temperature of the oil ought to be about C., but higher and lower final temperatures can be tolerated depending upon the resistance of the wood. The normal variation in the final temperature can be said to be between 60 and C. In this connection it is noted that high temperatures cause risks of damage in the wood in the form of warps, cracks, tensions within the wood, etc. Such risks are normally not present when drying at 80 C. When treating easily damaged woods, the final temperature has to be somewhat decreased, with some consequent prolongation of the driving time.

Steam coming from the wood gives the oil an effective stirring and, therefore, a good and even heat transfer is obtained. Accordingly, special arrangements in the cylinder for circulating the oil are not needed. When the drying begins, the wood contains much preserving solution and is, therefore, relatively very heavy, and hence it sinks in the light medium which consists of the oil highly mixed with steam bubbles. Gradually, the wood dries and grows lighter: at the same time the oil becomes less and less mixed with steam bubbles and hence the mixture of oil and steam bubbles grows heavier. The wood eventually begins to float. During a prolonged drying procedure the wood and the oil-steam mixture thus have about the same weight per volume and this greatly favors the contact between oil and wood, and likewise favors the effective execution of the drying process. The pressure between the different wood pieces will therefore not be so heavy, and laying crosspieces between the different wood pieces is, as a rule, not necessary. All these circumstances promote the rate of heating of the Wood. By the additional vacuum, the drying proceeds both ex tremely fast and mild, since the water evaporates from the wood by boiling under reduced pressure.

A fast drying above the boiling point, without the aid of vacuum, is out of question, because the high temperatures which are needed in such a case tend to cause damages in the wood.

After the wood has been dried, the oil is pumped out of the cylinder. This has to be done with maintained vacuum, unless an exceptionally high amount of oil in the wood is wanted. If the vacuum is broken with the oil left in the cylinder the consequence will be that the air pressure will force oil into the wood, the cells of which in this moment will be evacuated. They have been filled with steam at a low pressure, and when the vacuum is broken the steam condenses. After the oil has been removed from the cylinder the wood is taken out. The treatment is then completed. The so-treated wood has protection against wood destroyers: it has been dried andowing to the oil in the outer layer of the wood-has acquired water repellency which means that the wood swells and shrinks less than does wood which has not undergone this treatment.

The oil in which the wood is dried can be parafiin oil or other high boiling oil; however, one had better use different drying oils, such as linseed oil or linseed oil mixtures. Several oils of drying character in the petrol industry are suitable either as they occur or in different mixtures. If paraffin oil or other nondrying oil is used the surface of the wood will often be a little smeary, which condition will not happen when drying oils are being used. Of course it takes some days before the oils dry so that all smear disappears.

Different substances can be introduced into the oil in which the wood is heated, in order to give the wood different wanted properties; or, the oil can be replaced by some substances of this kind. The drying can thus be carried out in solutions or in melts of waxes and/ or resins in order to obtain an increased water-repellency or a hard surface. As further example chloroparaflin can be mentioned: this gives the wood certain fire-protecting properties which is or may be desirable. Chloroparaffin can be used alone or dissolved in a high-boiling oil. If the proportion of chlorine in the chloroparafiin is high it is difficult to use it without mixing it with oil. Other organic substances containing chlorine or phosphorus have fireprotecting properties and can be used in a similar way. An illustrative commercial product of this kind containing phosphorus is called Phosgard. Good results have been obtained with tricresylphosphate and similar substances. They can with advantage be used together with drying oils. For example, tricresylphosphate and linseed oil in equal parts is a suitable composition. The so-obtained fire protection has been very apparent and it has also had the very important property to be resistant against leaching.

Furthermore, suitable coloring substances can be introduced into the oil in order to color the wood in the course of the drying. Different kinds of color substances have been investigated: best permanence has been obtained with pigment colors, which have proven to be superior to colors dissolved in the oil. The pigments have to be very finely subdivided; moveover, they should emulsify readily. Otherwise, they are not easy to use.

To promote ease in carrying out the process in practice, some details ought to be observed. Thus, it is important to remove all solution from the cylinder after the treatment of the wood. No pockets where solution can remain should occur in the tapping system. In such a case the retained solution will be evaporated to dryness during the following drying of the wood in the oil. The preserving agents present in the residual solution form solid products under such circumstances, which solid products have to be removed separately or they will form coverings on wood and on apparatus details. In order to ensure against this risk, one can introduce into the oils (in which the wood is dried) some water-insoluble substances which are solvents for the preserving agents and solid products formed thereof, Products with such dissolving properties are, for instance, tall oil and naphthenic acid.

Another practical detail is to remove the oil from the cylinder after the drying period so that no residual oil will be left. Such residue would be mixed with the treating solution in continuous operation. The warm oil is comparatively easy to remove as it flows readily in this state. An especially effective way to remove oil residues is to steam the cylinder with the wood after the removal of the oil. By such a treatment both apparatus and wood will be cleaned very fast from oil residues on their surfaces.

As above stated, the process is chiefly adapted for drying wood that has been treated with aqueous treating solutions. On the other hand, it is operable in cases in which the preservative is dissolved in an organic solvent. Wood treated in this way can be dried in a high boiling liquid in which the organic solvent is insoluble. The solvent which evaporates during the drying period can be condensed and recovered. Such organic solvents insoluble in paraffin oil are, for instance, alcohols and organic acids.

EXAMPLE As an example of how the process can be carried out the following may be given:

Pine wood of one and two inches thickness was treated with an aqueous ammoniacal solution containing 0.6 percent copper, 0.3 percent chlorophenol, 1.2 percent am monia and 2.5 percent carbon dioxide, percent here meaning percent by weight. The treating solution was forced into the wood under 8 kilograms per square centimeter pressure during 90 minutes, whereafter the solution was removed from the impregnating cylinder and a high vacuum was applied during 30 minutes. A considerable amount of solution was evacuated from the wood and was removed from the cylinder by a pump. With maintained high vacuum, a mixture of oils having a temperature of 40 C. was successively introduced into the cylinder. The oil mixture contained equal parts of linseed oil and a slowly drying, high-boiling oil and in the oil mixture a brown color pigment was dispersed. The pigmented oil mixture was heated by steam in pipelines in the bottom of the treating cylinder in a scant hour up to C.

At this final temperature, and under a high vacuum, the drying of the wood proceeded during 4 hours. The oil thereafter was removed from the cylinder in which the vacuum was maintained until all free oil was eX pelled. Steaming was then carried out during 10 minutes in order to remove oil residues. The wood was then taken out from the cylinder.

The product had been treated in respect to conservatron, to drying, to water-repelling properties and to coloring. The waterwhich in the form of solution had been introduced into the wood during the treatment-had been removed for the most part. The amount of oil absorbed by the Wood was less than 40 kilograms per cubic meter, and was to be found in the outermost layers of the wood.

An interesting variation of the treating method of the present invention is the treatment of moist (green) wood. The introduction of the solution into the wood can hereby be carried out in the form of a diffusion treatment and this can be used for the treatment of practically all kinds of wood, including varieties which in the dry state are difficult to treat under pressure. Also, in these cases it is an advantage to use ammonia and/or ammoniacal salts as dissolving agents. In this way preservatives are obtained which can both penetrate into the moist wood by diffusion and also be fixed in the Wood after drying. A suitable solution for treatment of green wood according to the diffusion method is an aqueous solution containing 1.2 percent copper, 0.6 percent arsenic pentoxide, 4 percent ammonia and 5 percent carbon dioxide. The carbon dioxide stabilizes the solution and makes it more resistant to evaporation and hydrolysis. It can be replaced by other acids such as acetic or formic acid.

In carrying out the process just mentioned, the moist wood is kept in the preservative solution a certain time during which the preservative penerates by diffusion into the wood. The time for this treatment varies with the dimensions and the kind of wood. As a general rule it may be said that one-inch-thick wood usually is kept in the solution a week while poles are treated for a month. After a sutficient amount of preservative has penerated into the wood the preservative solution is removed. Preservative has now entered into the outer layers of the wood. It has to be distributed further into the wood: accordingly, the wood should not be dried but rather be kept in moist state for some time for purposes of equalization of distribution. This distribution time should be considerable: it ought to be at least as long-and may amount to as much as three times as long-as was the time interval during which the wood has been kept in the preserving solution. The drying may be avoided by covering the wood, keeping it enclosed or surrounded by oil during the distribution period. If the oil is kept warm, i.e., at 4050 C., the distribution of the preservative proceeds faster and this period can be shortened. Thereafter, the above-described drying in oil follows and then the treatment of the wood is finished.

When treating moist wood, pressure can be used with advantage-a full cell treatment is in this case to be preferred-or an open tank treatment can be used instead of keeping the wood a long time in the solution to obtain the penetrating diffusion. Such treatments are specially useful when the wood has been dried to some extent and thus is not quite moist or green. By these treatments more preservative (solution) is usually introduced into the wood than by diffusion treatments. Therefore it is desirable to use a weaker solution-for instance, a solution containing half the amount of the ingredients used for the diffusion solution, the said half amount being the same as is used for the Lowry treatment. With such a solution moist pine wood can be treated with remarkably good results. Of important interest is the fact that even spruce and other kinds of wood which are difiicult to impregnate by ordinary methods can be successfully treated.

As long as the wood is moist it is permeable for the impregnating solution. A high vacuum is applied to the wood in the treating cylinder, whereafter the solution is introduced and is forced into the wood under a pressure of, say, 8 kilograms per square centimeter. The free solution is then removed from the cylinder, and a suitable oil is introduced, which oil at 80 C. dries the wood under a high vacuum as earlier described. It is here to be observed htat a longer drying time is necessary because the wood contains much more water than after a Lowry treatment.

By treating moist wood it is desirable to use methods which give the wood the highest possible amount of preservative solution. Depending on the water content the air space in the wood cells is definitely smaller than in dry wood, and for this reason also the possibility of introducing preserving solution is much less. As a rule, it is most suitable to fill this cell space completely with solution. In moist wood the air space in the cells is too small to be used as intended when carrying out the Lowry or Ruping treatment. When treating moist wood, the diflFerent pressure methods therefore give similar results. By full cell treatment of moist wood, however, often so good a penetration and distribution can be obtained that the drying in oil can start at once thereafter: a distribution period is thus not necessary. When the demands on the treatment are not so high this way can be used with advantage. It is to be observed that during the drying period an apparent distribution of the preservatives occurs. Here we thus have the very favorable situation that undried wood can be preserved, dried, colored, etc., and ready for delivery in one day. The only question left after the treatment is that the drying oils, if used, need some days for being solidified.

As said above, it is a considerable advantage to use the Lowry treatment for handling the drying of the wood in the best way. Depending on the differences in apparatus and in the conditions of the wood it is difficult to give exact figures for the drying times. As a rule, 4 to 6 hours drying time is used for sawn Wood when the Lowry process is applied. During this time usually more than percent of the water introduced into the wood will have been evaporated. For larger dimensions and poles, the drying times often are considerably longer. In order to obtain effective operation, the process can thereby be full-filled during a night shift. Drying times of for example 16 to 20 hours can then be used without disadvantage and without noticeable capacity decrease.

When full cell treatment of moist wood is used, the drying process is prolonged and it is also more difiicult to obtain even drying. It is here impossible to give general rules. Very often, however, the drying time will be two or three times longer by full cell treatment than is called for by Lowry treatment.

I claim:

1. Method of preserving wood which comprises treating the same, in a closed space and at a pressure at least as high as atmospheric pressure, with an aqueous solution of a wood preservative, which after the treatment is fixed in the wood in nonleachable condition, treating the so-impregnated wood, in said enclosed space, in a heated liquid bath of a high-boiling medium, with said space being maintained under subatmospheric pressure and the temperature of the bath and the sub-atmospheric pressure being adjusted to remove the water present in the impregnated wood at a temperature below the normal boiling point of water and within the range of from about 60 to about C. and withdrawing said high-boiling medium from the so-treated wood in said space whilst said space is maintained at subatmospheric pressure, whereby to minimize retention of said medium by the so-treated wood.

2. The improved method defined in claim 1, in which the high-boiling medium is selected from the group consisting of high-boiling nondrying oils, drying oils and mixtures thereof, and in which the high-boiling medium boils over 250-300 C.

3. Method according to claim 1, in which the aqueous preserving solution contains copper compounds dissolved in ammonia compounds giving difficulty soluble preserving copper compounds in the wood after the treatment and the evaporation of the ammonia compounds.

4. Method according to claim 1, in which fire retardants are dissolved in the high boiling liquid.

5. Method according to claim 1, in which fine granular colour pigments are dispersed in the high boiling liquid.

References Cited UNITED STATES PATENTS 247,602 9/1881 Boulton 117-116 360,947 4/1887 Boulton 117-116 109,874 12/1870 Cresson 117-102 990,246 4/ 1911 Fetterman 34-95 1,756,797 4/1930 Rawson 117-102 2,892,261 6/1959 Hutchinson 34-95 3,061,508 10/1962 Morriss et al 21-7X 3,200,003 8/ 1965 Bescher 21-65X FOREIGN PATENTS 159,784 11/1954 Australia 117-119.8

OTHER REFERENCES Special Methods of Seasoning Wood-Boiling in Oil, Forest Products Laboratory Report No. R1665, Madison, Wis., 1947.

Hunt, George M., and Garratt, George A.-: Wood Preservation, McGraw-Hill, New York, 1938 (pp. 127- 129 and 409-410 relied on).

MORRIS O. WOLK, Primary Examiner B. S. RICHMAN, Assistant Examiner US. Cl. X.R.