Process for treating wood
United States Patent 3900615

Lumber and other wood products are impregnated with one or more additives such as preservatives, fire retardants, dyes, and the like, using a halogenated hydrocarbon solvent as the sole heat transfer medium and carrier by heating the wood with hot solvent vapor, soaking the hot wood in a pressurized solvent solution of the additive, and releasing the pressure after removing the solvent solution from contact with the wood, thereby causing most of the solvent in the wood to flash off. The process is preferably run in essentially continuous manner by operating three or more interconnected treating chambers simultaneously in offset cycle sequence using a common source of boiling treating solution under moderate superatmospheric pressure.

Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
8/402, 34/339
International Classes:
B27K3/02; B27K3/08; B27K3/40; (IPC1-7): B27K3/08; B27K3/40
Field of Search:
21/7 117
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US Patent References:
3200003Process for impregnating wood with pentachlorophenol and composition therefor1965-08-10Bescher
2931737Impregnation process1960-04-05Thornton
2860070Method of drying and impregnating wood1958-11-11McDonald

Primary Examiner:
Trenor, William R.
Attorney, Agent or Firm:
Baker, Glwynn R.
I claim

1. A process for treating wood which consists essentially of the steps:

2. The process of claim 1 wherein the solvent vaporized from the treated wood is condensed and recycled to the process.

3. The process of claim 2 wherein the solvent is methylene chloride.

4. The process of claim 3 wherein the wood treating agent is pentachlorophenol.

5. The process of claim 1 wherein the heated wood is immersed in a boiling wood treating solution under superatmospheric pressure and the hot solvent vapors from the boiling solution are used at least in part, also under superatmospheric pressure, to contact and preheat a second batch of wood in the treating process, said second batch of wood after thus having been preheated then being immersed for treating under superatmospheric pressure in the boiling solution removed from the first treated wood, thereby obtaining an essentially continuous wood treating process.


This invention relates to a process whereby wood is impregnated with modifying additives such as preservatives, fire retardants, dyes, and the like. It relates particularly to an essentially continuous process utilizing methylene chloride or a similar halogenated solvent as the carrier and heat transfer agent for impregnating wood with pentachlorophenol or other preservatives, dyes, fire retardants, and the like.

A number of processes are known in which halogenated hydrocarbon solvents such as perchloroethylene, trichloroethylene, and methylene chloride have been used as the principal carriers for wood treating agents. Representative of these processes are those described by Hudson, U.S. Pat. No. 2,650,885, Tanner Canadian Pat. No. 519,730; McDonald, U.S. Pat. No. 2,860,070; and Dunn, Canadian Pat. No. 863,885. All of these processes, by the nature of their steps and the time required to complete a single cycle of operation, i.e., treatment of a single charge of wood, are either unsuited to large scale general use or applicable for commercial treatment of only a few kinds of wood. For example, Hudson and McDonald use the relatively high boiling perchloroethylene and trichloroethylene which are not readily removable from the interior of the treated wood even when vacuum treatment is applied. Tanner describes a solvent impregnation of wood apparently carried out at ambient temperature with solvent removal carried out by any of various means with no mention of solvent recovery, evidently a slow process which would be wasteful of solvent. Dunn discloses a commercially operable process using methylene chloride as the treating medium, but one which would apparently have inefficient utilization of steam and cooling water requirements.

The present process is a modification of the latter procedure whereby rapid and essentially continuous treatment of wood is accomplished with highly efficient use of both energy and treating materials. It is applicable to both the more permeable woods such as southern yellow pine and ponderosa pine and the denser hard woods.


It has now been found that the above desirable objectives are obtained by a process which comprises contacting wood at about 50°C. to about 125°C. with a chlorinated aliphatic hydrocarbon solvent vapor under superatmospheric pressure for a time sufficient to heat the wood to approximately the temperature of the solvent vapor, immersing the heated wood in a boiling solution of at least one wood treating agent in said chlorinated solvent at said temperature range and under superatmospheric pressure for a time sufficient to cause substantial impregnation of the wood by the solution, removing the excess solution from contact with the impregnated wood, and reducing the pressure on the impregnated wood at least to about atmospheric pressure, thereby causing a flash vaporization of at least a substantial part of the solvent contained in the wood.

The advantages of this process are most efficiently applied when it is operated in an essentially continuous manner using a plurality of pressure treating vessels appropriately interconnected and coordinated so as to operate in staggered cycles using a common source of pressurized boiling treating solution, using pressurized solvent vapor from a vessel in the wood soaking part of the treating cycle to preheat new wood in another vessel, and having common means for condensing solvent vapor and recycling the condensate in the process, and a common solution make-up tank.

In such a mode of operation, energy requirements per treating vessel are minimized and a substantially continuous process is obtained wherein hand labor is reduced essentially to loading new wood into the treating vessels and unloading the treated wood.


The drawing is a diagrammatic representation of a preferred wood treating apparatus of the invention having three treating chambers in the form of horizontally disposed cylindrical presure vessels, each having one closed end and an open end fitted with a door for loading and unloading wood, the door being capable of being closed and sealed to withstand moderate superatmospheric pressures. The three cylinders have a common source of hot treating solution (the reboiler), a common solution make-up tank, common condensing means for solvent vapor at about atmospheric pressure and another such means for condensing solvent vapor at superatmospheric pressure, and appropriate circulating pumps for treating solution, all of the above with suitable interconnecting lines and valves, temperature and pressure regulators, vents and drains as required.


The following description of apparatus suitable for carrying out the present invention has particular reference to the drawing. Three pressure cylinders 10, 11 and 12 are provided, each with a door or end, not shown, through which the wood to be treated can be loaded or unloaded into the cylinder, and which door will seal the cylinder in a manner to prevent escape of liquid or vapors from the cylinder under superatmospheric pressure. Each cylinder 10, 11 and 12 is provided with a surge drum 13, 14 and 15, respectively. A high pressure water cooled condenser 16 is connected through piping 24 to each surge drum 13, 14 and 15. A temperature responsive control valve 16A maintains the pressure within the condenser 16 through temperature sensing element 16B. The condensate from condenser 16 passes to pump 16D through line 16C to reboiler 17. Reboiler 17 is connected through piping 21 to the three cylinders 10, 11 and 12. Each cylinder 10, 11 and 12 is connected to a low pressure condenser 18, having a temperature responsive control 18A associated with temperature sensing element 18B. The condensate from the low pressure condenser 18 is directed to fresh solvent and penta mix storage tank 19. Penta make-up and fresh solvent make-up are introduced into tank 19 through lines 19A and 19B, respectively. Pump 20 withdraws solvent from tank 19 on demand and delivers it to reboiler 17. Various other piping is provided in an interconnecting manner to transfer fluids from one cylinder to another. Valves are provided to control the flow of fluids. These valves are piping are fully identified and their operative cycles are set forth in the following description and tables.

The present invention is illustrated hereafter with regard to treating yellow pine dimension lumber. Other woods require different treating cycles. However, it can generally be determined that the average treating cylinder, for example, requires about 30 minutes to load and an equal period to unload; that a normal cylinder takes about 75 minutes to purge of air and heat the lumber to the treating temperature; that the equipment (pumps, piping, etc.) limits the rate of filling the cylinders with liquid and the build-up of pressure such that filling requires about 45 minutes. It is known that yellow pine lumber bundles require about 105 minutes to impregnate the bundle. Draining, solvent recovery and let-down to atmosphere are about equal in total time requirement to filling, soaking and purging. An equation to enable the operator to adjust his cycles to a multiple cylinder operation would take the form of:

2L + P + 2FD + S + R/no. cyl.= total cycle time


L = time to load or unload

P = time to purge and heat with vapors

Fd = time to fill or drain

S = time to soak

R = time to recover solvent

Substituting the times in minutes we set forth above, we find the time for a single cycle in a single cylinder equals

2(30) + 75 + 2(45) + 105 + 120 = 450 minutes (or 7h 30m) cycle for one cylinder.

Knowing this and deciding to use three cylinders, we can divide the cycle into three equal periods of 150 minutes (2 hour 30 minutes) and thus know the second cylinder should be ready to load and start a cycle 2h 30m after the first cylinder was opened for loading and the third cylinder should be ready to load 5h following the first cylinder zero time. This permits utilization of the cylinders to exchange vapors, treating liquid, etc., conserving energy while accomplishing adequate treating.

An offset cycle sequence is described in detail in Tables I and II for the three cylinder apparatus of the drawing. The individual operating steps are allotted certain numbers of time units as an indication of the approximate relative lengths of the steps. This would vary depending upon the dimensions of the wood pieces and the kind of wood particularly. For a representative wood treating procedure, each time unit might be 15 minutes, for example.

TABLE I ______________________________________ Step 1, Time Units 1-2 Cylinder 1 Cylinder 2 Cylinder 3 ______________________________________ Wood is loaded in and cylinder door is closed Wood is soaked in pressurized boiling solution admitted from Cylinder 3 Solution has been drained from soaked wood into Cylinder 3, then pressure is reduced to atmospheric through low pressure condenser to flash off solvent in the wood Step 2, Time Units 3-7 Purge air from cylinder with hot solvent vapor from Cylinder Wood remains soaking Solvent is flashed off as above Step 3, Time Units 8-10 Fill with pressurized boiling solution from 2 Drain solution to 1 Admit air, open door and remove wood Step 4, Time Units 11-17 Wood soaks Reduce to atmospheric pressure, flash off solvent through low pressure condenser Load in new wood, purge air and heat wood with hot solvent vapor from 1 Step 5, Time Units 18-20 Drain solution to 3 Admit air, open to remove wood Admit solution from 1, wood soaks Step 6, Time Unit 21 Flash off solvent from wood through low pressure condenser and reduce to atmospheric pressure Load in wood Wood soaks Step 7, Time Units 22-26 Flashing off as in Time Unit 21 Purge air and heat wood with hot pressurized solvent vapor from 3 Wood soaking Step 8, Time Units 27-28 Reduce to atmospheric pressure, admit air Heat wood with vapor from 3, admit hot solution from Wood soaking, then drain solution to 2 Step 9, Time Units 29-30 Unload wood Wood soaks Solution drained to 2 ______________________________________

Table II lists the valving sequence during the treating procedure of Table I. For each valve, an "O" marked under a particular time unit indicates that the valve is open. A valve not so marked is closed.

TABLE II __________________________________________________________________________ Time Unit 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Valve No. A 0 0 0 0 0 21 B 0 0 0 0 0 0 0 0 0 0 C A 0 0 0 0 0 22 B 0 0 0 0 0 0 0 0 0 0 C A 0 0 0 23 B 0 0 0 C A 0 0 0 0 0 0 0 0 0 0 24 B 0 0 C 0 0 0 A 0 0 0 B 0 0 0 0 0 25 C D 0 0 0 0 0 E F 0 0 0 A 26 B C 0 0 A 27 B 0 0 0 0 0 C 0 0 0 0 0 0 0 0 Time Unit 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 Valve No. A 0 0 0 0 0 21 B C 0 0 0 0 0 0 0 0 0 0 A 0 0 0 0 0 22 B C 0 0 0 0 0 0 0 0 0 0 A 0 0 0 23 B 0 0 0 C 0 0 0 0 0 0 A 24 B 0 0 0 0 0 0 0 0 C 0 0 0 0 0 0 0 A 0 0 B 25 C 0 0 0 0 0 D E 0 0 0 0 0 F 0 0 A 0 0 26 B 0 0 C A 0 0 0 0 0 0 0 0 0 0 27 B 0 0 0 C __________________________________________________________________________

In a treating procedure as outlined in Tables I and II using a boiling methylene chloride solution of pentachlorophenol as the treating solution and operating at a solvent vapor temperature of about 78°C. and a gauge pressure of about 64 lbs./sq. in. in the wood impregnation step, typical operating pressures in lbs./sq. in. gauge in Cylinder 1, in the reboiler, and in the high pressure condenser are about as follows in the various steps.

TABLE III ______________________________________ Step Pressure, psig No. Cylinder 1 Reboiler H.P. Condenser ______________________________________ 1 0 64 64 2 initially 0, 62 30 then to 30 3 30-64 63 64 4 64 64 to 62 64 to 30 5 64-50 63 64 6 50-0 64 64 7 0 64 to 62 64 to 30 8 0 62-63 30 to 64 9 0 63 64 ______________________________________

The preliminary heating step wherein the wood is contacted with hot pressurized solvent vapor is preferably carried out at about 50°-100°C. and a pressure from about 1.1 to about 5 atmospheres. This step requires from about 0.5 hour to about 4 hours depending upon the temperature employed, the kind of wood, the dimensions of the pieces, and the amount of moisture in the wood.

The same preferred temperature range applies to the soaking step in which the hot wood is immersed in superheated treating solution. Somewhat higher pressures are preferred for this step, for example, up to about 15 atmospheres. Soaking time varies according to the degree of penetration wanted, the dimensions of the pieces being treated, and the moisture content and density of the wood. Soaking times as long as 10 hours can be employed, but preferably the soaking time is about 0.5-3 hours.

Chlorinated solvents suitable for use in this process are those having boiling points between about 35°C. and about 125°C. and these include the polychloromethanes such as methylene chloride, chloroform, and carbon tetrachloride; the polychloroethanes such as ethylene dichloride, methylchloroform, and 1,1,2-trichloroethane; and the polychloroethylenes such as 1,2-dichloroethylene, trichloroethylene, and perchloroethylene. Other chlorinated aliphatic solvents which can be used are propylene dichloride and compounds as described above where some of the chlorine atoms have been replaced by fluorine, for example, 1,2-dichloro-1,1,2,2-tetrafluoroethane, dichlorofluoromethane and trifluoromethane. Since an essential feature of this process is the use of solvent vapor and treating solution at temperatures significantly above the normal boiling point of the solvent, a solvent having a relatively low boiling point is preferred, for example, in the range of about 35°-80°C. Methylene chloride is particularly preferred.

Pentachlorophenol or a mixture of pentachlorophenol with a minor proportion of tetrachlorophenol is the preferred wood preservative used in the process. Preferably, the treating solution contains about 2-10 percent by weight of pentachlorophenol. Other wood preserving chemicals can be used, either alone or in combination with pentachlorophenol in the treating solution. Additionally, the treating solution may contain components such as fire retarding chemicals, dyes, or other such additives to modify the properties of the treated wood.

The process is most advantageously used for impregnating seasoned wood with preservatives, fire retardant chemicals, dyes, or other wood treating compositions or mixtures thereof. However, it can also be applied to green or partially seasoned wood in which case it becomes a combined seasoning and impregnating process. Somewhat longer heating and soaking steps may then be necessary.


The following example describes the procedure used and the results obtained from a single cycle of operation adaptable to multiple chamber operation where the individual cycles are staggered and coordinated such that the treating solution filling time for one chamber coincides with the solution removal time of another.

The treating chamber was a horizontally disposed steel cylinder about 12 feet long and 2 feet in diameter with one end permanently closed and a sealable door at the other end. It contained a steam coil along the bottom, a tram for moving lumber into and out of the cylinder, and it was equipped with appropriate piping connections for admission and removal of liquid and vapor, including connections to a water-cooled condenser and liquid storage. The whole system was adapted to operation under moderate superatmospheric pressure.

Ten seasoned yellow pine two-by-fours eight feet long were fastened in place on the tram within the cylinder, the cylinder door was closed, and enough 5% pentachlorophenol in methylene chloride was introduced to cover the steam coil. Steam was passed into the coil to heat the solution to boiling and the air in the cylinder was vented as it was displaced by methylene chloride vapor. After the air had been vented from the system, heating was continued so as to maintain the treating cylinder at about 78°C. and 30 p.s.i.g. After 3.75 hours under these conditions when the lumber was heated through, additional treating solution was pumped into the cylinder to cover the wood. Steam pressure in the coil was then increased to obtain a final temperature of 91°C. and a pressure of 64 p.s.i.g. within the cylinder. The wood was subjected to the action of the solution under these conditions for 45 minutes. The steam was then shut off and the solution was drained from the cylinder into a storage tank. The cylinder was then vented through a water-cooled condenser, whereupon most of the solvent in the hot lumber vaporized to be condensed in the condenser and collected.

The cooled lumber was later removed from the cylinder and was found to have gained 5 pounds in weight, about three pounds of this being pentachlorophenol and the rest methylene chloride. 7 days' air drying removed the residual solvent. The dried two-by-fours contained about one pound of pentachlorophenol per cubic foot and showed good penetration. The treatment had also reduced the water content of the lumber from 15.2% to 13.4%. The treated wood showed no checks or cracks and there was little or no discoloration.