US3900615A - Process for treating wood - Google Patents

Abstract

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.

Description

United States Patent Dunn, Jr. 1 Aug. 19, 1975 1 1 PROCESS FOR TREATING WOOD [75] Inventor: James L. Dunn, Jr. Lake Jackson Primary bmnlmerw1ll1am R. Trenor Attorney, Agent, or Fzrm--G1wynn R. Baker [73] Assignee: The Dow Chemical Company, 57 ABSTRACT Midland, Mich. 1 Lumber and other wood products are impregnated 122] Flled: 1972 with one or more additives such as preservatives, fire [21 APPL Nu: 297.464 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 va- [52] U.S. C1. 427/317; 8/65; 21/7; por Soaking the hot wood i a pressurized Solvent 34/95; 34/13-8; 34/16-5 lution of the additive, and releasing the pressure after 51 11 C1 B27k 3/08; B271 3/40 removing the Solvent solution from Contact with the [58] Fleld of Search 21/7; 1 17/59, 147, 149, wood thereby Causing most of the sowem in the wood 1 8/65; 34/95 to flash off. The process is preferably run in essentially continuous manner by operating three or more inter- 1561 References C'ted connected treating chambers simultaneously in offset UNITED STATES PATENTS cycle sequence using a common source of boiling 2,860,070 11/1958 McDonald 117/147 treating Solution under moderate superatmospheric 2,931,737 4/1960 Thornton 117/147 pressure. 3,200,003 8/1965 Bescher 117/59 1 3,685,959 11 1972 Dunn 21 7 5 Clams l Drawmg F'gure I f WO/E/Ou/ -g s Wmer in 2 "5102. 1'

7'0 619/: pressure 24 condenser 24 1 24 V V 3 b 24b 0 26a 2 255g OVEf/Ieoo vapor C rans er V r\ 2 v urye V 1 Sur e drum dru /n 1 5 25 Cy/ina er 2 Cy/fnaer 2 (y/b70616 1/0 or 1 a 25) V frr nsper 25 *M"! 25a 2 250 5 J Q Z7 4/ u/a Low ress. 9 vraZzS/sr cono nser 27c Reba/Yer g l'n/ef 22b V 27 0 22a Q 21 Reboi/er calf/e); 315 216 21a 1 7/175 17} V 5/ eam #1817 18a //'7 Low ressure 1,

' COHZZ'QS {e Corifienser 18 n O 0a Pen/a Mak up 186 V E/ven/ 19b 20 I W fresh fso/vep/ 20a f ue/7 am/x PROCESS FOR TREATING WOOD BACKGROUND or THE INVENTION This invention'relates to' a, process wherebyi wo od is impregnated with modifying additives suchas 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.' 7 v 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, US. Pat. No. 2,650,885, Tanner Canadian Pat. No. 519,730; McDonald, US. Pat. No. 2,860,070; and Dunn, Canadian Pat. No. 863,885. All of these processes, by thenature 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 interiorof the treated wood even when vacuum treatment isapplied. 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 disclosesa commercially operable process using methylene chloride as the treating medium, but one which would apparently have inefficient utilization of steam and cooling water requirements. I

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. lt is applicable to both the more permeable woods such as southern yellow pine and ponderosa pine and the denser hard woods. I

SUMMARY OF INVENTION It has now been found that the above desirable objectives are obtained by a process which comprises contacting wood at about 50C. to about 125C. 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,

2 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 vaporvand 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.

BRIEF DESCRIPTION OF THE DRAWING 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 endfitted with a door for loading and unloading wood, the doorsbeing capable of being closed and sealed to withstand moderate superatmosphericpressures. 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, allof the above with suitable interconnecting lines and valves,gtemperature and pressure regulators, ventsand drains as required.

DETAILED DESCRIPTION Thefollowing description of apparatus suitable for carryingout the present invention has particular reference to the drawing. Three pressure cylinders '10, I1 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 l0, l1 and 12 is provided-with a surge drum l3, l4 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 tempera-v ture sensing element 168. 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 193, 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 3 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 TABLE I Step I, Time Units 1-2 Cylinder 2 Cylinder 3 Cylinder l liquid and the build-up of pressure such that filling rewmd fi i' has and cylinder door is pressurized boiling drained from soaked qulres about mml 1te5' It known'that y q p closed solution admitted wood into Cylinder 3 lumber bundles require about 105 minutes to impregfrom Cylmdcr 3 prcssurqls reduced I to atmosphenc through natethe bundle. Draining, solvent recovery and letlow pressure condenser down to atmosphere are about equal in total time reto flash ofl'solvent in the quirement to filling, soaking and purging. An equation to enable the operator to adjust his cycles to a multiple Step 2, Time Units 3-7 li d Operation ld t k th f f; Purge air from Wood remains Solvent is flashed off as cylinder with hot soaking above v solvent vapor from 2L P 2FD .S R/no. cyl.- total cycle time Cylinder 2 where Step 3, Time Units 8-10 L time IO load 01 unload Fill with pressurized Drain solution to l Admit air, open door P time to purge and heat i vapors lzooiling solution from and remove wood FD time to fill or drain s ti t k Step 4, Time Units ll-l7 Wood soaks Reduce to Load in new wood, R time (0 recover solvent atmospheric purge air and heat wood Substituting the times in minutes we set forth above, we prtlzssurehflashhoff twith hot solvent vapor so vent t roug low mm 1 find the time for a single cycle in a single cylinder pressure condenser equa s Step 5 Time Units 18-20 2( 2(45) 450 minutes (or 7" 25 Drain solution to 3 Admit air. open to Admit solution from I,

cycle for one cylinder. remove wood wood soaks K th' d d idi t u e three 0 linders we stcp 'Timc 2] f g an ng O S Flash ofi solvent Load in wood Wood soaks can divide the cycle into three equal periods of from wood through minutes (2 hour 30 minutes) and thus know the second d cylinder should be ready to load and start a cycle 2' 3O reduce to 30'" after the first cylinder was opened for loading and mmosphcflc Pressure h Step 7, Time Units 22-26 the third cylinder should be ready to load 5 following Flushing off as in Purge air and hem 7 wood Soaking the first cylinder zero time. This permits utilization of Time Un t 21 wood Withdholl the cylinders to exchange vapors treating liquid, etc., 35 3 conserving energy while accomplishing adequate treat- Step 8. Time Units 27-28 ing Reduce to Heat wood with Wood soaking then atmospheric vapor from 3, admit drain solution to 2 An offset cycle sequence is described in detail in Tapressure admit hm 80min f 3 bles l and ll for the three cylinder apparatus of the U d )d svtspojfTingc Units 29-30l d d 7 drawing. The individual operating steps are allotted 5 0 certain numbers of time units as an indication of the ap- 40 roximate relative len ths of the ste s. This would var p I p y Table II lists the valving sequence during the treating depending upon the dimensions of the wood pieces and n v procedure of Table I. For each valve, an O marked the kind of wood particularly. For a representative under a particular time unit indicates that the valve is wood treating procedure, each time unit might be 15 minutes for example open. A valve not so marked is closed.

1 TABLE ll 'TimeUnit l 2 3 4 5 6 7 s 9 10 ll l2 l3 l4 15 Valve Not A 0 0 0 0 0 21 B 0 0 0 0 0 o 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 o 0 0 0 0 C u 0 0 0 0 o 0 0 TABLE ll-continued f 1 Time Unit Vulvc No. v i

C v 0 0 i1 0 0 0 0 .0 A 0 0 u 0 1' 22 B Y 11 C 0 0.. o .0 0 0 (1, 0 A o 1) 0 23 B 0 0 0 C .0 0. 0 0 0 -0 A v 24 B '0 (1 0 0 0 0 0 0 A 0 0 B 25 C 0 0. 0 0 0 26, k B 0 0 g C i 4 v 1 A I o 0 0 0 0 0 0 0 0 0 In a treating procedure as outlined in Tables I and II carbon tetrachloride; the :polychloroethanes, as using aboiling methylene chloride solution of pentaethylene dichloride, methylchlorof0rm ;.;and-l,l',2=trichlorophenol as the treating solution and operating ata' chloroethane; and the polychloroethylenessuch as 1, 2 solvent vaportemperatu're' of about 78 C.and a gauge 'dichloroethylene, trichloro ethylene, andperchlorqethpr ssu of u 4 q iniin th WOO'd i p g ylene. Other chlorinated aliphatic solventsjwh-ieh can n t p. typical operating pressur s in l/ q: n. 30 beused are propylene dichlorideand compounds-as deg ug in yl n r i the oiler, and in the high scribed above where some of-the chlorineatomshave pressure condenser are about as followsin the various been'replaced by fluorine-{for example, l, 2 d ichloro"- steps. l 1,1,2,2-tetrafluoroethane, dichlorofluoromethane{and TABL "I trifluoromethane. Since an essential feature Qfthispicess is the use of solvent vapor and treating solu t1on at I *Stcp' Prcssure,psig r t Y. I I No; Cylinder] A Rcboilcr H.P.Condenscr temperatures Slgnlficantlyv'abqve P p p point of the solvent, a solvent-having a relat1yely..-,low l .1 64 boiling point-is'preferred,.for example, in the rangeof- 2 initially (l, 62 30 I Q then to 30 about 35 -80C. Methylene chloride is particularly-pre- 3 30-64 ferredi 1 4 t 30 f: 1 64 62 6 2 Pentachlorophenol or a mixture of pentachlorophe- 6 5041 64 64 nol with a minor proportion of tetrachlorophenol is the Z, 23 2 28 3 preferred wood preservative used in the process. Pref- 9 0 63 64 erably, the treating solution contains about 2-10 percent by weight of pentachlorophenol. Other wood preserving chernicals can be used, either alone or in com- The preliminary heating step wherein the wood is bination with pentachlorophenol in the treating solucontacted with hot pressurized solvent vapor is preferation. Additionally, the treating solution may contain bly carried out at about 100C. and a pressure components such as fire retarding chemicals, dyes, or from about 1.] to about 5 atmospheres. This step re- '50 other such additives to modify the properties of the quires from about 0.5 hour to about 4 hours depending treated wood. upon the temperature employed, the kind of wood, the The process is most advantageously used for impregdimensions of the pieces, and the amount of moisture nating seasoned wood with preservatives, fire retardant in the wood. chemicals, dyes, or other wood treating compositions The same preferred temperature range applies to the or mixtures thereof. However, it can also be applied to soaking step in which the hot wood is immersed in sugreen or partially seasoned wood in which case it beperheated treating solution. Somewhat higher prescomes a combined seasoning and impregnating prosures are preferred for this step, for example, up to cess. Somewhat longer heating and soaking steps may about l5 atmospheres. Soaking time varies according then be necessary. to the degree of penetration wanted, the dimensions of EXAMPLE the pieces being treated, and the moisture content and density of the wood. Soaking times as long as 10 hours The following example describes the procedure used can be employed, but preferably the soaking time is and the results obtained from a single-cycle of operaabout 0.5-3 hours. tion adaptable to multiple chamber operation where Chlorinated solvents suitable for use in this process are those having boiling points between about 35C. and about C. and these include the polychloromethanes such as methylene chloride, chloroform, and

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 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 78C. 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 91C. and a pressure of 64 p.s.i.g. within the-cylinder. Thewood 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 conderiser,-'whereupon' most of the solvent in the hot lumber vapo rized to be condensed in the condenser and collected;

"moved 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 8 from 15.2% to 13.4%. The treated wood showed no checks or cracks and there was little or no discoloration.

I claim:

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

a. contacting wood at about 50C. to about C. with a vaporized chlorinated aliphatic solvent under superatmospheric pressure for a time sufficient to preheat the wood to approximately the temperature of the vaporized solvent,

b. immersing the heated wood in a solution of at least one wood treating agent in said chlorinated solvent at about said temperature and under superatmospheric pressure for about 01-10 hours,

c. removing the excess solution from contact with the thereby impregnated wood, and

d. reducing the pressure on the impregnated wood at least to atmospheric pressure, thereby causing vaporization of at least a substantial part of the solvent contained in the wood.

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. i

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.

Claims (5)

1. A PROCESS FOR TREATING WOOD WHICH CONSISTS ESSENTIALLY OF THE STEPS: A. CONTRACTING WOOD AT ABOUT 50*C. TO ABOUT 125*C, WITH A VAPORIZED CHLORINATED ALIPHATIC SOLVENT UNDER SUPERATMOSPHERIC PRESSURE FOR A TIME SUFFICIENT TO PREHEAT THE WOOD TO APPROXIMATELY THE TEMPERATURE OF THE VAPORIZED SOLVENT. B. IMMERSING THE HEATED WOOD IN A SOLUTION OF AT LEAST ONE WOOD TREATING AGENT IN SAID CHLORINATED SOLVENT AT ABOUT SAID TEMPERATURE AND UNDER SUPERATMOSPHERIC PRESSURE FOR ABOUT 0.1-10 HOURS, C. REMOVING THE EXCESS SOLUTION FROM CONTACT WITH THE THEREBY IMPREGNATED WOOD, AND D. REDUCING THE PRESSURE ON THE IMPREGNATED WOOD AT LEAST TO ATMOSPHERIC PRESSURE, THEREBY CAUSING VAPORIZATION OF AT LEAST A SUBSTANTIAL PART OF THE SOLVENT CONTAINED IN THE WOOD.

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.

Images