CA1122484A - Wood preservation - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Wood is treated by a novel impregnation process wherein water-borne wood preservative treatment materials, such as CCA salts or a sodium chlorophenate, are forced under pressure into the wood and the solution is held within the wood by maintaining the treatment pressure until the impregnants become affixed to the wood by precipitation or chemical depo-sition. A significant"kickout" of depleted treatment liquid occurs when the pressure within the pressure vessel is reduced following the impregnation period, to a pressure at least equal to the internal wood pressure prior to treatment, and preferably to a pressure below the initial wood pressure. The process is preferably carried out at temperatures elevated to 65°C or higher and it is often desirable to heat the treatment liquid, or alternatively to drain the treatment liquid and use steam or a second heated treatment liquid to heat the wood. In any case, care must be taken to maintain a sufficient pressure within the treatment vessel to prevent premature "kickout" of the treatment liquid before 90% or more of the treatment materials have become deposited within the wood.

Description

This invention relates to the pressure impregnation of wood with water-borne wood treatment materials to improve the properties of the wood. More particularly, the invention relates to a process for the impregnation of wood in which water-borne wood treatment materials are deposited within the wood, as by precipitation or chemical affixation, in a manner that combines the effectiveness and other advantages of the full-cell process, with low residual liquids in the wood ohtainable with empty-cell processes.

BACKGROUND OF THE INVENTION

The present invention con-templates the use of what basic-ally may be considered to be an "empty-cell" impregnation process, as distinguished from a "full-cell" process. The essential difference between these two processes lies in the fact that in the full-cell process, the liquid forced into the wood is retained by the wood after impregnation, while in the empty-cell process most of the treatment fluid is expelled from the wood after impregnation. The terms "full" and "empty" derive from the fact that the cells of the wood are substantially filled with impreg-nants in the full-cell process, but tend only to be coated with the impregnant in the empty-cell process.

The full-cell process makes use o a vacuum/pressure impregnation c~cle in which the wood is Eirst placed under vacuum and then, without admitting air, the treatment vessel is filled with the treatment liquid. Ater the wood is fully immersed in the liquid, the pressure is increased to perhaps ten atmospheres or so and the liquid is Eorced into the wood. After the wood has been treated to the point of refusal, or until a predetermined gross absorption of the treatmeht li~uid has been achieved, the pressure is relieved and the treatment fluid is drained from the vessel. Usually a short vacuum cycle follows to remove excess 8~

fluid from the surface o~ the wood.
q'he distinctive feature of the empty-cell process is that at the end of the process, the pressure is reduced to, and preferably below, the initial pressure within the wood prior to impregnation so that a kickout of treatment liquid will result.
The most common way to achieve this is to place the wood under initial pressure prior to the time that it is contacted with the treatment liquid. This initial pressure is maintained during the time that the treatment vessel is filled with the treatment l.iquid and then the pressure is increased to a second, higher pressure, forcing the treatment liquid into the wood against the air pressure initially established within the wood. As a result when the pressure is relieved, the air compressed within the wood expands and expels much of the liquid that was forced into the wood. Equivalent results can be obtained by starting the impregnation while the wood is at or slightly below atmoshpheric pressure and then pulling a vacuum at the end of the treatment cycle. In any event, it is common to establish a vacuum in the treatment vessel after the impregnation pressure has been relieved to increase the recovery of the treatment liquid and shorten the period of time in which liquid will exude from the surface of the wood. The liquid expelled from the wood by the difference between the internal and external pressures is referred to as '~kickout"
liquid which term is used the meaning so defined throughout the following description.
The empty-cell process is especially advantageous as compared with the full-cell process for treating wood with water-borne materials which may be toxic since the wood, after treatment, is ready for economical shipment, for further treatment, or for immediate use since there is no need to dry the wood to remove the treatment liquid from the interior surfaces of the wood.

Despite the inherent advantage of ending up with "dry"

2 -wood, the empty-cell process has not, for some purposes, been of practical utility in impregnating wood for a number of different reasons. For example, when treating wood with water-soluble pres-ervatives - for example chromium/copper/arsenic salts known as "CCA" salts - the kickout that is inherent in empty-cell processes cannot economically be reused since the kickout fluid will include water-soluble reducing substances leached from the wood, such as hemicelluloses, which, it is generally believed, react with the CCA salts to precipitate a dense sludge that fouls the equipment.
This makes it impossible to recycle without periodically removing insolubles, and further, the kickout cannot be discharged as waste because of its toxicity and the danger it poses to the env-ironment. For these and other reasons, such as the preerential rate at which CCA salts are e~tracted from solutlon, it has become standard practice to avoid problems attendant with recycling or otherwise disposing of depleted CCA solutions by using full-cell techniques and simply letting the treatment liquid remain in the wood, until it evaporates, which may take up to several months of drying in air.
In addition to CCA salts, another effective and commonly used wood preservative that is difficult to use in ernpty-cell processes is chlorinated phenol, most commonly the tetra- and penta-chlorophenols (collectively referred to as "PCP"). PCP is not soluble in neutral or acid solutions and, in order to orm an impregnating solution, it is conventional to dissolve PCP in hiyh-boiling petrochemicals, Stoddard's solvents, kerosene, and the like. Empty-cell impreynation methods are sornetimes used to coat the cell walls with the preservative solution since the hydrocarbon solvents must be recovered to make the process econ-omical and to prevent bleedlng of the oil-based solution which would make it impossible to apply conventional ~inishes to the wood. However, even though most of the treatment solution is ~L~2~

removed from the wood in empty-cell processes, the amount of solvent consumed represent.s the major portion of the cost of materials in impregnating the wood with PCP.
To reduce the cost of the oil-based solvents, other proc-esses have been replacing the empty-cell methods in which the solvent is a low-boiling liquid that may be recovered and reused.
Typically, in these processes, low-boiling aliphatic hydrocarbon solvents for PCP are volatilized after impregnation, leaving a water-insoluble residue of PCP within the wood. These processes are not without attendant disadvantages which include the increa-sed cost of treatment apparatus, the need to recover the gasified solvents, both for purposes of economy and environment protection and, as has been suggested in the literature, the protection afforded the wood is less lasting when volatile solvents are used.
A further disadvantage in the use of low-boiling hydro-carbon solvents is that the protection afEorded the wood at its outer surfaces is sometimes less than satisfactory. It has been suggested that the reason for this lies in the fact that during volatilization, some of the PCP adjacent the surface of the wood is carried away and the amount of PCP left at this region is inadequate to protect the wood.
SUMMARY STATEMENT OF THE INVENTION
The present inventlon is directed to providing an improved process Eor pressure-impregnation oE wood with a water-borne preservative treatment material, wherein an aqueous solution of treatment liquid is forced into the wood at a pressure greater than the ini-tial internal pressure of the wood, the treatmen-t liquid is retained within the wood under an elevated pressure and a temperature at least 65C for a time of at ]east one hour and sufficient to allow a major weight proportion of the preservative treatment material to become affixed to or deposited within the wood, and then the pressure is reduced to allow the spent ~,Zff~J~

treatment liquid to be expelled or exude, and then be collected.
It is contemplated that the inventive process as outlined will be practiced with aqueous solutions of wood-preserving subst-ances, which substances may comprise salts of copper, chromium or arsenic or a mixture thereof, or a sodium chlorophenate, or an alkaline solution of sodium pentachlorophenate.
It is further contemplated that the inventive process as above recited be performed with heating of the wood to a tempera-ture from about 65C to about 120C, which heating may be effected variously by heating the treatment fluid, or by steam, or by the application of a second heated treatment liquid while maintaining the elevated pressure.
In a further aspect the invention is to be understood to include use of a second treatment liquid which may contain wood treatment agents selected from the group consisting of coloring agents, wood-softening agents, antichecking agents, film formers, flame retardants, antistatic agents, polymerizable mixtures and additional wood preservatives~
It is also contemplated that the process according to-the invention include the step of pretreatment of the wood with an acid, which acid may be acetic acid, or with metal ions to precipitate sugars within the wood.
From yet another aspect the invention may be seen to include the provision of the step of irnpregnating woocl with a solution of pentachlorophenol and a low-boiling hydrocarbon or chlorinated solvent for the pentachlorophenol before the aqueous treatment liquid is introduced into the vessel, such aqueous treatment fluid containing CCA salts.
In yet another aspect, the invention is to be understood to provide the further step of impregnating the wood following the steps of impreynation and pressure reduction, with creosote.

By the practice of the invention as above outlined, the amounts of treatment materials retained within the wood are maxi-mized and the retention of liquids within the wood following the process is reduced to a minimum.
In the case of treatments with CCA salts, the process of this invention is of particular utility since the kickout liquid is substantially depleted o~ metal ions hence recovery or disposal problems are largely avoided. Further, the treated wood is dis-charged from the treatment vessel in substantially dry state, thereby permitting of immediate use, shipment, or even subsequent treatment such as impregnation with creosote ~o meet requirements for marine applications.
Recognition of the fact that the treatment materials can be reacted in situ during pressure impregnation makes it possible to protect wood, using PCP preservatives, without the use of hydrocarbon solvents. Rather, in the practice of this invention, sodium penta- or tetra-chlorophenate is dissolved in an alkaline solution, the wood is impregnated with the solution, and the solution is held within the wood without permitting kickout of spent liquid to occur until the natural acidity of the wood causes precipitation of water-insoluble PCP within the wood. If the acidity of the wood is not sufficient to precipitate PCP at a useful rate, the wood may be first treated in an empty-cell proc-ess with an acid, such as acetic acid, prior to impregnation with the sodium penta-chlorophenate solution. It is believed that by acidifyi.ng the penta-chlorophenate to form the insoluble PCP
precipitate in situ in the wood, the retention of the PCP and protection o:E -the wood is made more lasting than can be achieved when using either volatile or nonvolatile petroleum-based solvents.
~s would be expected, the reaction between the treatment materials and the wood is greatly accelerated when the wood is heated during the time the impregnating solution is held within q7 .". . ~:

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the wood. Suitable times and temperatures will vary substantially depending upon the species of wood being treated and the nature of the treatment liquid, but can easily be determined by simple experimentation.

DESCRIPTION OF PREFERRED_EM~ODIMENTS OF THE INVENTION

In Examples I to IX detailed process steps are recounted exemplifying practical modes in which the invention may be put into effect, and tabular data is presented showing retention of preservative materials in the treated wood~.`

As the examples will show, the treatment of Southern pine with CCA salts at varying temperatures of the wood has been found to be most efficient in the range from about 65C to 121C. At temperatures lower than these, the reaction rates are inconveniently slow, and at higher temperatures, damage may be done to the wood.
A preferred temperature range for treating Southern~Pine with CCA salts is from about 81C to 116C and, more preferably, from about 93C to 110C.
Treatment times will vary considerably and, depending on the temperature used in the treatment cycle, satisfactory results have been obtained using treatment times of from about one to seven hours. Longer times can, o~ course, be used, but in the interest of productivity of the process, no purpose is served in prolonging the treatment time after the desired reactions have neared or reached completion..

EXAMPLE

A charge o kiln-dry Southern pine tomato stakes having 15~ moisture content, and of yrape stakes of the same wood and moisture content, was ~iven a modified empty-cell treatment with a 2.0~ (oxide basis) solution of CCA as follows.
I'he charge comprised thirty tomato stakes measuring 2,5 cm by 2,5 cm by 183 cm (0.035 Cubic meters) and 15 grape stakes ' ' ~ , measuring ~,1 cm by 4,1 cm by 183 cm (0.046 cubic meters).

An ini-tial pressure of 0,7 kg/cm of air above one atmos-phere was introduced into the treatment vessel and held for five minutes. The vessel was then filled with the CCA solution without relieving the initial pressure, and the pressure was i.ncreased to 7 kg/cm in a forty-~ive minute period by increasing the pressure 0,7 kg/cm at five minute intervals.
Excess preservative liquid was removed from the vessel while maintain ng cylinder pressure at 7 kg/cm .
The gross absorption of the CCA solution during the press-urization period was 575 Kg/M .
The charge was steamed 1.5 hours at 7 kg/cm pressure with the maximum temperature of 116C being reached in one hour, and then held at this temperature for 0.5 hours A kickback sample taken at the end of the steami.ng cycle had a pH of 3.1.
The charge was exposed to 660 mm Hg vacuum for one hour before being removed and weighed to determine net solution reten-tion. The net solution retention obtained during treatment was 164 ~g/M .

Analysis of the original treating solution and kickbackresulting from the treatment Oe this charge gave the ol:Lowing result:

Sample pH CuO Cr~3 ~S~05 % reduction of:
CuO CrO3 AS25 Treatment 1.5 0.345 0.9580.718 - - -solution Kickback 3.1 0.021 0:0160.01993.9 98.3 97.4 _ E~LE II

Precipitation of Cu and Cr from acid copper chromate in .2æ~s~

in kiln-dry Southern pine (20% moisture content) by steaming before permitting the kickout to occur.
A piece of kiln-dry Southern pine of diameter 16,5 cm by 45 cm long was impregnated as follows:
A. Preservative Solution:
The solution of acid copper chromate was prepared in 2.38%
strength (oxid~ basis) from a commercial con~entrate "CELCUR~"
providing 3.84% copper sulfate (anhydrous), 5.01% sodium dichr~mate (anhydrous), and 0.20~ chromic acid (anhydrous) by dilution with water. The pH of this fresh solution was 3.4 B. Impregnation Cycle:
A pressure of 2,8 kg/cm2 of air atmosphere was initially held for five minutes, after which the treatment vessel was filled with the preservative solution at this pressure, and the pressure was then increased to 9,8 kg/cm and held for two hours.
Excess preservative solution was drained from the vessel while maintaining the c~linder pressure at 9,8 kg/cm2.
A~kickout sample was taken at the end of the pressure period but before starting tha steaming, by momentarily reducing the pressure slightly. The pH of this kickout sample was 3.85.

The gross absorption of preservative during the pressure period was 469,4 Kg/M3.
The sample was steamecl at 100C Eor three hours while maintaining 9,8 kg/cm vessel pressure.
A kickout sample taken at the end oE the steaming cycle had a pH oE 5.45.
The sample was exposed to 660 mm Hg vacuum ~or one hour before being removed and ~eighed to determine net solution retention.
A sample of the drip obtained during the Einal vacuum had g _ a pH of 5.50.

The net solution retention obtained during treatment was 145.7 Kg/M so that the gross retention was reduced by over 320,5 Kg/M .

A disc obtained from the middle of this sample after treatment was dried i.n an oven to determine its average moisture contentO Its moisture content expressed as a percentage of oven-dry weight was 40.3%.

Analysis of the samples obtained during the treatment of this sample for Cu and Cr yave the results indicated in the table below~ In this table, Sample I was the original treatment solution;
Sample 2 was the kickout after impregnation but before steaming;
Sample 3 was the kickout after impregnation and steamin~; and Sample 4 was the dr;p from the wood during the final vacuum.

Sample pH %Cu %reduction % CrO3 % reduction of of CuO Cr~3 _ 1 3.~0 0.717 ~ 1.661 2 3.85 0.628 12.41 1.527 8.07

3 5.45 0.014 98.06 0.027 98.37

4 5.50 0.025 96.58 0.009 99.46 _ .
The disc obtained from this sample after treatment and steaming indicated complete penetration by the preservative.

E AM LE III
Wood was given an empty-cell pre-treatment with a 0.5~
Barium hydroxide solution prior to treatment with CCA -to determine i.f the sugars could be precipitated in the wood so that they would not conta.minate the kickout.
Analysis of the kickout after the CCA impregnation cycle showed that the reduction oE the trea.tment solution in CuO was 100.00% and 99.56% in CrO3 and As2O5.
Enough CCA concentrate was added to a porkion of the kickout to bring its concentratinn up to 2.0~ (oxide basis). No precipitate occurred in this sample after two weeks' storage in the laboratory, indicating that the wood sugars were precipitated in the wood and did not contaminate the kickout -to any observable extent.
EXAMPLES IV, V AND VI
Samples of Southern pine wood were tre~ted with CCA salts under varying process conditions t as shown in the following table.
Column headed "Initial Pressure" in the table denotes the pressure to which the wood was exposed prior to impregnation. Column headed "Impregnation pressure" indicates the pressure that was established within the treatment vessel after the vessel was filled with the treatment liquid. "Steaming Temperature" denotes the ultimate ~-hlgh temperature reached, over a period of about one hour, after ~-the impregnation pressure was imposed. "Holding Time" denotes the period of time during which impregnation pressure and steaming temperature was maintained within the vessel.
The other columns indicate the percentage of the active ingredien-ts in the initial treatment solution, the kickout, and the percentage of the active ingredients that were retained within the wood. Note that in EXAMPLES IV and V where the temperature oE the treatment vessel was not increased above ambient, the deposition of the CCA salts did not begin to approach completion even aEter a holding time of as long as six hours. In contrast to thisl EXAMPLE VI shows th~t at a steaminy temperature of 98C
well over 95% of the CCA salts were deposited in the wood after a holding time of onl~ two hours.

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EXAMPLE VII

A piece of kiln-dry Southern pine 14,6 cm in diameter and 45 cm long was impregnated as follows:
A. Preservative Solution: 1.50% technical grade ~ sodium penta-chlorophenate in water; original p~I = 10.3.

B. Impregnation Cycle:
The wood was placed in a pressure treatment vessel wh~ch was then pressurized with air at 2.1 kg/cm2. Without relieving this pressure, the vessel was filled with the preservative solution and the air pressure was increased to 10.5 kg/cm2 which was held for two hours. ~ .:

The preservative was drained away from the wood while the cylinder pressure was maintained at 10.5 kg~cm . Gross absGrp-tion of preservative during the pressure period was 523 Kg./~3.

The wood was then steamed for three hours at 100 C while maintaining 10.5 kg/cm pressure in the cylinder. A kickout sample taken by momentarily reducing the pressure at the end of the steaming period had a pH of 5Ø The wood was placed under a vacuum of 660 mm Hg for one hour and was then removed from the cylinder and weighed. The net solution retention by gain in weight during treatment was 122 Kg/M3 which means that the amount of kickout expressed as a percentage of total gross absorption was 76.6%.

A sample of the drip recovered during the final vacuum had a pH of 5.15.

C. Analysis of Borings ~or PCP:

Three increment cores were taken near midlength of the sample at 120 intervals around the circumference. The cores were zoned .into 1,~5 cm segments and assayed for PCP by the lime ignition method. The results of the analyses are tabulated on the next page.

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Depth from Surface, cm. PCP Content, Kg/M

0.0 - 1.25 6.33 1,25 -2.54 2.47 2,54 - 3,8 3,12 3,8 - 5,0 2,47 D. Analysis of Original Treating Solution and Kickout After Steaming for PCP:
Samples of the original treating solution and kickout were analyzed by the lime igni-tion method Eor PCP with results as follows:
Original Treating Soluti~n.......... 1O143% PCP content Kickout after steaming.............. 0.026~ PCP content.
The percentage reduction in PCP (i.e., retained in the wood) was 97.7%, E. Test For Fixation of PCP to Wood:

A disc of axial length 1,90 cm in length was cut from the midlength of the treated pole section. A strip 1,90 cm wide was sawn from this disc so -that the pith was in -the center of the strip. The outer 7,6 cm on opposite sides of the pitch in this strip was sectioned into 1,25 cm incremen-ts for the acid test and into 2,54 cm increments for the pH determination.

To test for acid, each block w~s splintered into 3,1 mm square pieces and covered with 50 ml of boiling disLilled water.
The mixture was shaken for -thirty minu-tes and fil-tered through Whatm~n 1~541 Eilter paper. 3 ml of the filtrate was placed in a test tube and dilute HCl was added to determine if any PCP precip-itated upon addition of the MCl. The sensitivity o~ the method related is 100 ppm.
The results of the acid test are shown by table in the following page.

Sample Distance From Wood Surface Precipitate measured in cm Occurrence ::
Present Absent - , 0.0 to 1,25 very slight 1,5 to 2,54 yes 2,54 to 3,8 yes 3,8 to 5,0 yes 5,0 to 6,3 yes

6,3 to 7,6 yes F. Moisture Content After Treatment:
A disc 1,90 cm thick was sawed from the middle of the treated pole section and oven-dried to determine the average moisture content immediately after treatment. The moisture content of this disc was 38.6%.

EXAMP~E VIII
The acidity of the wood being treated was relied on solely in this Examp~e to reduce the pH of the treatment solution to precipitate PCP within the wood. The wood was first treated with acetic acid in a modified empty~cell process prior to impreynation with the pentachlorophenate solution to determine if the added acidity would make the process more efficient.
A sample of Southern pine wood was placed in a treatment vessel and placed under 0,7 kg/cm2 air pressure above atmosphere~
W:ithout relieviny the pressure, an 0.8 molar solution o.E ~lacial acetic acid was introduced :into the vessel and the ai:r pressure was lnc~reasecl to 8,4 ]cg/cm and held Eor two hours. The acetic acid was drained away :Erom the wood while maintaininy 8,4 ky/cm2 air pressùre and the wood was then steamed for three hours at 99C. The pressure was then relieved and a vacuum pulled for one hour.
Immediately following the above acid treatment of the wood, 4~

the identical process operations were repeated as recited in Example VII using a 4% alkaline solution of sodium penta-chloro-phenate as the impregnate.

~ fter treatment with PCP r the kickout was found to contain less than 0,008% PCP thus indicating -that over 99.8% of the PCP
in the treatment solution was deposited in the wood. A test for leachable PCP as described in Example 1 above was run, and the results were negative, indicating that the PCP precipitated in the wood could not be leached in observable quantities (i.e., greater than 100 ppm) by boiling the treated wood in water.

Borings were made in the -treated wood and the amount of PCP depoeited in the wood was determined at various depths by lime ignition:

_ Sample PCP, Kg/M

0,0 to l,25 cm 17,75 l,25 to 2,54 13,56 2,54 to 3,8 12,68 3,8 to 5,00 12,28 ; . n The preceding Examples VII and VIII demonstrate that the chemically precip~tated PCP is tightly held and is not leachable, if at all, except in tract amounts. It is at once obvious that b~ achieving this fixation o~ -the PCP within the wood, the e~ectiveness oE the wood preservative will be maint-ained over prolonyed periods of -time.

~ secon~ar~ bene~it whi.ch is yained in afEixiny the PCP
within the wood lies in the Eact that the potential danyer of PCP
to the environment is greatly reduced when the PCP is tightly held within the wood. In contrast to the results obtained through the practice of this invention, when conventional high-boiling petrochemical solvents are used and PCP is not precipitated in the wood but i5 held in solution by organic solvents, the PCP may leach from the wood over a period of time, depending on such factors as the type of soil and other yround conditions to which the wood is exposed, changes in temperature, barometric pressure, humidity, and other ambient conditions.

EXAMPI,E IX
_ A charge of air-dried pine fence posts with 25~ moisture content was placed in a pressure vessel . The posts were of dimensions lO cm diameter by about 260 cm length. An initial air pressure of 1,4 kg/cm2 above atmosphere was introduced into the treatment vessel and held for about five minutes.

The vessel was then filled with a CCA solution of 2%
strength (oxide basis) as in EXAMPLE I without relieving the initial vessel pressure. The pressure was increased to over 9,8 kg/cm over a period of about 55 minutes. The temperature of the CCA impregnating solution was about 32C and the gross absorption achieved was about 503 Kg/M .
When the re~uired gross absorption had been obtained, steam was admitted to the coils in the cylinder and the CCA solution in the cylinder was heated to about 88C within 30 minutes and this temperature was maintained for 60 minutes. No precipitating of CCA salts from the treatment solution was noted during this heating cycle.
At the end of the heating period, the CCA solution was removed from the cylinder wi-thout permitting the pressure on the system to chanye, following which the pressure on the system was released to collect the kickout separate e rom the CCA solution.
The temperature Oe the kickout solution was 82C. A final vacuum of 60 minutes at 655 mm Hy followed after releasing the pressure on the system.
The wood was removed from the cylinder and weiyhed. It was determined tha-t the net preservative solution retention was 4~

112 Kg/M .

Results of analyses performecl on the CCA be:fore treatment, after -treatment and heating, and on a sample of the kickout solution after heating, are contained in the table below:

SOLUrION p~:[ METAL CO2~T~, % OXIDE BASIS
CuO CrO3 AS205 rrotal Salt CCA before -treatment 1.92 0.304 0.870 0.643 1.817 CCA after treatment 2.03 suhstantially the same as ahove and heating Kickout 4.18 0.008 0.017 0.022 0.047 An increment core was taken from midlength of each post for analysis for Cu, Cr and As retention after treatmentO The cores were cut into 1,25 cm segments and similar segments were combined for analysis. Results for these analyses are contained in the table below:

Depth From Surface, cm. Retention of Metals Kg/M
~Oxide Basis) CuO CrO3 As2O5 Total Salt 0.o to 1,25 2.58 5.64 4.02 12.26 1,25 ~o 2.54 1,97 3,91 0.30 6.17 2.54 to 3,8 1,18 2,84 2,24 ~,24 rrhe modi:Eied empty-cell process of this invention has been descrihed primarily w:i.th regard to -the impreynation of wood w:ith sodium chlo:r:i.natecl phenates and CC~ salts, but it can readily be unclerstood hy those of ordinary skill in the wood treatment arts that the invention is of utility in treating wood with other wood conditioners. For example, the water-borne materials may include such things as wood-softening agents, antl-checking agents, film-formers, coloriny ayents, flame retardants, 30 antistatic agents, dimensional stabilizers, and other wood-treating : :

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agents. The process of this invention may also be used to leach materials, such as sugars, from wood or conversely to precipitate sugars in the wood prior to a subsequent preservation step. The pH of the treatment liquid can be adjusted to maximize solubiliz-ation of the sugars and, if it is the object to fix the sugars in the wood, cations such as barium or copper may be included in the treatment liquid to form insoluble products with the sugars.

The modified empty-cell process of this invention makes it possible to conduct a plurality of sequential impregnations without the intervening drying or curing steps required in the full-cell process. For example, substantially immediately after wood is treated with CCA salts in accordance with this invention, it may be impregnated with an oil-based preservative. Another example would be to follow the CCA treatment with a PCP treatment.
This increases the degree of protection for the wood and is advantageous if arsenic salts must be excluded from the treatment liquid because of environmental hazards.

In a further modification of this invention, CCA salts can be de~osited at the surface regions of wood while the wood is being treated with a solution of PCP and liquefied hydrocarbon.
When the pressure is relieved on the system, volatilization of the hydrocarbon will cause kickout of the CCA treatment liquid.

In the above examples, the treatment solution was removed Erom contact with the wood, while maintaining impregnation pressures, prior to the time that the wood was heated. This is not essential and, if desired, the temperature of the wood may be raised by heatiny the impregnating solution, as by steam coils, prior to the time it is drained from the vessel. Ln another variation of the practice of this invention, the treatment solution may, in some instances, be heated prior to the time it is l~Z~

introduced into the vessel. Also, the treatment liquid may be drained from the vessel while maintaining sufficient pressure to avoid kickout, and s-team or a second liquid used to heat the woodO

In this latter regard, a liquid may be intxoduced;' into the vessel while maintaining sufficient pressure to prevent kickout. If a hot water bath is used for this final treatment, it may prove advantageous to include water-borne wood treatment materials such as the above-mentioned wood-softening agents, anti-checking agents, film-formers, coloring agents, flame retardants, antistatic agents, dimensional stabilizers, and other wood-treating agents. Also, if the first treatment was with PCP, a dilute solution of CCA salts may advantageously be included in the water to provide additional protection to the surface of the wood. Since the CCA salts will tightly adhere to the wood, this additional protection may prove to be of value to protect against any possible loss or migration of the PCP
from the surface areas of the wood. Thus, it can be understood that this final treatment step may serve not only to aid in precipitating or affixing the preservative to -the wood, but also to improve at least the surface properties of the wood.

Claims (18)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for preservative treatment of wood comprising:
- placing the wood in a pressure treatment vessel at an initial pressure;
- introducing an aqueous treatment liquid carrying preservative treatment materials into the vessel;
- increasing the pressure within the vessel to above the initial pressure to impregnate the wood with the treatment liquid;
- heating the treatment liquid to a temperature of at least 65°C;
- maintaining a pressure within the vessel sufficient to retain the treatment liquid in the wood for at least one hour and until at least 90% of the treatment materials carried in the treatment liquid become affixed to or deposited within the wood;
- reducing the pressure within the vessel to or below the initial pressure to permit expulsion of spent treatment liquid from the wood; and - collecting the expelled spent treatment liquid.

2. A method according to Claim 1 wherein the preservative treatment materials contain salts of copper, chromium or arsenic.

3. A method according to Claim 1 wherein the preservative treatment materials contain a sodium chlorophenate.

4. A method according to Claim 3 wherein the aqueous treat-ment liquid is an alkaline solution of sodium pentachlorophenate.

5. A method according to Claim 1 wherein the wood is heated while the treatment liquid is retained within the wood.

6. A method according to Claim 5 wherein the wood is heated to a temperature from about 65°C to about 120°C.

7. A method according to Claim 5 wherein the wood is heated by heating the treatment liquid.

8. A method according to Claim 5 wherein the wood is heated by steam.

9. A method according to Claim 5 wherein the wood is heated with a second treatment liquid.

10. A method according to Claim 9 wherein the second treatment liquid contains wood treatment agents selected from the group consisting of coloring agents, wood-softening agents, anti-checking agents, film formers, flame retardants, anti-static agents, polymerizable mixtures and additional wood preservatives.

11. A method according to Claim 3 wherein the wood is pre-treated with an acid.

12. A method according to Claim 11 wherein the acid is acetic.

13. A method according to Claim 1 or Claim 6 or Claim 11 wherein the wood is pretreated using the method of Claim 1 with hot water or steam as the aqueous treatment liquid.

14. A method according to Claim 1 or Claim 6 or Claim 11 wherein the wood is pretreated with metal ions to precipitate sugars within the wood.

15. A method according to Claim 1 wherein the wood is impregnated with a solution of pentachlorophenol and a low-boiling hydrocarbon or chlorinated solvent for the pentachloro-phenol before the aqueous treatment liquid is introduced into the vessel and wherein the treatment liquid contains CCA salts.

16. A method according to Claim 1 or Claim 11 or Claim 15 wherein the treated wood is further impregnated with creosote.

17. A method for the preservative treatment of wood comprising:
- placing the wood in a pressure treatment vessel at an initial gas pressure above one atmosphere;
- introducing an aqueous treatment liquid carrying a solution of CCA salts into the vessel;
- elevating the pressure within the vessel over a period of time at least 45 minutes above the initial pressure to impreg-nate the wood with the treatment liquid;
- removing excess treatment liquid while maintaining said elevated pressure;
- heating the wood to attain a temperature between 65°C and 116°C over a time interval of about one hour at said elevated pressure and maintaining said temperature for about 0.5 hours;
- reducing the pressure within the vessel to below one atmosphere;
and - collecting spent treatment liquid expelled from the wood.

18. The method according to Claim 17 wherein the wood is heated by steam.