US2668779A

US2668779A - Pressure process for impregnating wood

Info

Publication number: US2668779A
Application number: US65663A
Authority: US
Inventors: Mervin J Herman
Original assignee: Koppers Co Inc
Current assignee: Beazer East Inc
Priority date: 1948-12-16
Filing date: 1948-12-16
Publication date: 1954-02-09
Anticipated expiration: 1971-02-09

Description

Feb. 9, 1954 M. J. HERMAN PRESSURE PROCESS FOR IMPREGNATING WOOD 2 Sheets-Sheet 1 Filed Deo. 16, 1948 I FJ Woe/f TANK TREAT/NG CyL/None Mensa/ef ,0a/np ADJUJTABLE naw @Nr/@OL TIME //V MINUTES Mae w/v u'. Alge/wmv Feb. 9, 1954 M. J. HERMAN PRESSURE PROCESS FOR IMPREGNATING WOOD Filed Dec' le' 1948 2 sheets-sheenw 2 N Q s u) "I un I y.. w

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marre/2Min Patented Feb. 9, `1954 Mervin J. Herman,` rrville, Ohio,` assigner to Koppers. Company Inc., Pittsburgh,- Pa., a corporation of. Delaware Application December 16, 1948, Serial No. 65,663

7 Glaims. (Cl. 117-59) This invention relates to wood preservation, and? particularly toA an improved pressure process or method for impregnating wood with preservative material.

In order to prevent decay of wood or timbers, and thereby to increase their life, itis common practice to impregnate the wood or timbers with a preservative, such as creosote or other material. The protection afforded by the application of preservative material is dependent upon deep and reasonably uniform penetration of the Wood or timber by the preservative material. Itis customary to treat wood or timbers with preservativefin closed cylinders under pressure so the preservative material is forced into the body of the wood.

However, with known methods of impregnation, it has been found that it is very difficult, if not impossible, when treating some types of green or incompletely seasoned wood, such as soft pine or other conifers, to secure adequate and uniform penetration of the preservative. This is particularly true when the Wood which is being treated is green or is not fully seasoned, and is of a type having a louT specific gravity, such as less than 0.45.

In the processes now employed wood is subjected to preservative under relatively high pressure, such as 150 to 200 pounds to the square inch, for a substantial period of time, such as 4 or 5 hours, and at a relatively high temperature. such as 180 F'. to 210 F. In some cases, before the wood is impregnated, it is subjected to a preliminary treatment with steam under W pressure, such as 2O pounds to the square inch. Al-

though increases in pressure. tend to increase the `amount of preservative absorbed by the Wood, it may cause the penetration to be erratic or uneven, that is, to cause the depth to which the preservative penetrates to be much less in some areas than in others. It appears that the application of pressure causes compression of the outer layers of the Wood, particularly after the Wood is weakened and softened by steaming. The compression of the outer layers of the Wood causes `collapse of the cells of the wood. This collapse of the Wood cells occurs Where high pressure is applied While the cells are empty and before they have become lled with preservative, This co1- lapse. of the wood cells is especially likely to occur the wood cells in an area, there is formed a relativelygmpenetrablelayer which restricts, or even 2 the interior of the Wood, and thus makes it impossible to` secure the uniform, deep penetration of the preservativeV which is essential to satisfactory preservation of the Wood.

An object of this invention is to provide an improved method or process of pressure-treating wood.

A further object of the invention is to provide an improved process or method of the type described which makes it possible to successfully treat wood or timbers which cannot 'be satisfactorily treated by the methods heretofore known.

Another object of the invention is to provide an improved process or method of pressure-treating Wood in which the preservative material is supplied to the pressure vessel at a rate governed by the abilityy of the wood being treated to absorb the preservative.

A further object of the invention is to provide an improved process or method of pressure-treating wood which insures that uniform deep penetration of the Wood being treated will always be secured.

Another object of the invention is to provide an improved process or method of pressure-treating Wood which operates to control the pressure to which the wood is subjected to values which Will insure rapid absorption of the preservative, but will not` cause collapse of the cells of the Wood.

A further object of the invention is to provide an improved process or method of the type described Which automaticallyv operates to restrict the pressure to which the Wood is subjected to relatively low values until the preservative has penetrated the Wood to a substantial depth so that the cells in the outer layers of the Wood become filled with preservative and do not collapse When the pressure is increased.

Another object of the invention is to provide an improved process or method of pressure-treating wood which may be employed regardless or whether the wood is being treated by the fullcell process, the Lowry process, or the Rueping process.

A further object of the invention is to provide an improved process or method of the type described` Which can be carried out automatically and` which requires a minimum` of attention.

Another object of the invention is to provide an improved wood preserving process which is arranged so,- that. the pressure to which the wood is subjected is gradually increased in accordance with increases inl thcpressure gradient from the suriace of thewood. tothe untreated areas within the wood as the depth of penetration of the preservative liquid increases.

Other objects of the invention and features of novelty will be apparent from the following description taken in connection with the accompanying drawings.

ln the drawings, Fig. 1 is a iiow sheet illustrating diagrammatically apparatus suitable for carrying out my improved process; and

Figs. 2 and 3 are diagrams showing the pressure conditions and preservative absorption throughout a treating cycle under my improved process, and also showing the absorption of preservative liquid by the wood being treated.

Referring to the drawings, Figure. 1 is a diagram of a wood preserving plant arranged to supply of liquid exceeds this rate, the pressure in the treating cylinder will build up too rapidly and will collapse the cells of the wood, thereby `interfering with penetration of the preservative into the wood. If the rate of supply of preservative liquid is less than the rate at which the employ my improved process. As shown, there is a pressure or treating cylinder in which the wood being treated is subjected to preservative liquid under pressure,l al work tank adapted to hold a quantity of preservative liquid, and a pressure pump for supplying liquid under pressure from the Work tank to the treating cylinder through an adjustable flow control means for controlling the flow of preservative into the treating cylinder.

The operation of the equipment during treat- -ment of wood by the Rueping process will be described, but as explained above, the process provided by this invention is equally applicable to the treatment of wood by the full-cell process, or by the Lowry process.

A quantity of Wood to be treated is placed in the treating cylinder, but before being placed in the treating cylinder the wood is measured so .that the number of cubic feet of wood in the cylinder is accurately known. In addition, the species of the wood being treated is known. After the wood has been placed in the treating cylinder, the treating cylinder doors are sealed, and the wood may be then subjected to any desired preliminary treatment, such as steaming. Thereafter, a vacuum is drawn on the charge in the treating cylinder and is maintained for a suitable period, such as from one and a half to three hours. Green or partly seasoned wood is thus treated to drive out moisture and to open the pores of the wood.

Air is now injected into the cylinder until the desired pressure is obtained, and for purposes of illustration it will be assumed that the pressure in the cylinder is increased to pounds to the square inch Which is the standard specication for the Rueping process. The cylinder is now filled with hot preservative oil in such away that the injected air is trapped in the wood. This may be accomplished by forcing'the preservative oil into the bottom of the cylinder and allowing air to escape from the top of the cylinder just rapidly enough to maintain a constant pressure.

After the treating cylinder has been filled with preservative, the further supply of liquid to the treating cylinder is regulated by the control equipment shown in the drawings. This equipment is arranged to supply to the treating cylinder a substantially constant volume of liquid per minute irrespective of pressure conditions in the treating cylinder.` As hereinafter pointed out, the supply of liquid to -the'treating cylinder at a uniform rate causes the pressure in the treating cylinder to gradually inerease. The rate 0f supply of liquid to the treating cylinder is selected inaccordance with' the species of the wood being treated and'in a'c'cordanc'sewith'jthe total volume of wood inthe treating'r 'cylinder s'th'at preservative can be absorbed by the wood, the time required for the treatment of a charge will be unnecessarily prolonged and the expense of the treatment will be unnecessarily increased.

As the volume of the wood in the charge, ,the species of the Wood, and the rate at which 'liquid can be absorbed by this species of wood are known, the rate of supply of liquid to the treating cylinder to produce uniform, deep penetration of the preservative in the minimum time may be readily calculated.

As previously explained, the rate of supply of liquid to the treating cylinder is selected in accordance with the ability of the wood being treated to absorb the liquid, and is substantially as rapid as possible Without causing collapse of the cells of the Wood. This results in a gradual increase in the pressure in the treating cylinder during the initial part of the treating'period, while this is followed by a rapid increase in the pressure in the treating cylinder during the latter part of the treating period.

When the wood is first subjected to the pressure of the preservative liquid in the treating cylinder, the cells of the Wood are empty, except for the air under pressure present therein,l and if excessive pressure is applied to the wood, the cells will collapse and will interfere with subsequent penetration of liquid into the wood. The liquid gradually penetrates the cells of the wood and lls them with liquid, displacing or compressing the air present therein, so that the pressure of the liquid in the cells builds up substantially to the pressure of the liquid in the cylinder. After a cell has become lled with liquid, or with liquid together with air under pressure, it will not be collapsed as a result of an increase in the pressure of the liquid in the treating cylinder. This is particularly true as the pressure of the liquid and air within the cells builds up so that the pressure on opposite sides of the walls of the cells is substantially equal.

The cells in the outer layers of the wood be come filled with liquid or with liquid and air under pressure first, and thereafter the liquid penetrates inwardly to the inner layers of woodso that there is a gradual increase in the thickness of the layer in which the wood cells are filled with liquid or with liquid and air under pressure. The layers of wood present some resistance to the flow of liquid so the effective pressure of the liquid after it reaches the inner layers of the wood is materially less than the pressure of the liquid in the treating cylinder at the surface of the wood. As-the layer of cells filled with liquid increases in thickness, there is a corresponding increase in the amount of reduction in the eiectivefpres; sure ofth'e liquid applied tothe empty. orpartly `ber in the charge.

:aecomo empty cells, located inwardly of vthe filled cells. Hence, even though the pressure of the liquid in the treating cylinder builds up to a .relatively high `value during the latter part of the treating period, the effective pressure applied to `the `Wood cells which have not already become 4filled with liquid, that is, to the cells located a substantial `distance below the surface of the wood, is vrelatively lovv and is below the value which will lcause collapse of these cells. The cells which are located nearer the surface of the wood, and therefore are subjected to higher effective pressures, become filled or substantially lilled with liquid before the high effective pressures are developed `so these cells are not collapsed by these high pres- STATES.. i

As the rate of supply yci. liquid to the treatin cylinder is selected in accordance with the 4ability of the wood to absorb the liquid, the pressure in the treating cylinder increases only at Aa rate sufficient to cause the liquid to penetrate the Wood and be absorbed, that is to lill the cells of the wood, and does not increase so rapidly as to cause` collapse of the cells of the Wood. As collapse of the wood cells does not occur, the preservative liquid can readily penetrate all areas of the wood so uniform deep penertration of the Wood by the preservative is secured. Furthermore, as the preservative liquid is supplied at a rate selected in accordance with the ability of the Wood to absorb the liquid, the processing of a charge of wood is carried out in the minimum time, thus limiting the expense of the treatment and making possible the treatment of a maximum amount of Wood with a set of equipment.

After the charge has been in the cylinder for a period `of time long enough to obtain the desired absorption, the further supply ofliquid to the treating cylinder is cut off, the pressure in the cylinder is released, the preservative "is drained from the cylinder, and the charge is subjected to a high final vacuum for a period of thirty minutes or more. As soon as the pressure is released, the compressed air in the Wood expands and forces out a considerable amount of the preservative that was injected. The iinal vacuum hastens the recovery of preservative and shortens the period during which the preservative will continue to drip from the timber. After the final vacuum has been discontinued, the cylinder door is opened and the charge is removed.

Figs. 2 and 3 are diagrams showing the pressure conditions in the cylinder during the treatment of a charge of 1200 cubic feet of Norway or red pine by the Rueping process. Fig. 3 plots the pressure curves of Fig. 2 on a logarithmic scale in which the logarithm of the pressure is given as a function of tlietime. An initial air pressure of 20 pounds per square inch was established on the charge and this pressure was maintained while the cylinder was lled with preservative liquid. Thereafter, preservative liquid was supplied to the treating cylinder at a rate such as to cause 20,000 pounds to be supplied in 6 hours. This is 1,6% pounds for each cubic foot of timber in the charge, and is at the rate of 2% pounds o preservative per hour for each cubicfoot of tim- The pressure treatment was terminated after about ilve hours and forty minutes, whereupon the pressure was released and the charge was subjected to a Iinal vacuum.

The broken line Illil in Fig. 2 indicates the quantity of preservative supplied to the cylinder and it will be seen that between.l9,000 and 20,000 pounds of preservative had been supplied to the 6 treating'` cylinder at the time the pressure was released. `'lhis liquid is in addition to that required to initially ll the treating cylinder. As a result of release of the .pressure and thev establishment `of a vacuum in the treating cylinder, considerable liquid was expelled from the timber forming the charge and the net retained liquid totalled 9,000 pounds, giving ahet retention of '7l/2 pounds per cubic foot of timber in the charge.`

The solid line loi in Figs. 2 and 3 indicates the pressure in the treating cylinder throughout the period of 'the pressure treatment. The initial pressure required to offset the preliminary air inch so the line itil starts at this pressure. The pressureincreased `very little in the first hour, and had risen only to about 26 pounds, an increase of only 6 pounds in one hour, even though approximately `3,333Y pounds of preservative liquid had been pumped into the treating cylinder in this period. A-s the increase n pressure o'vi the entire period of one hour Was only 6 pounds to the square inch, relatively little force was exerted on the timber in 'the charge to compress the timber or vcollapse the 'wood cells during this period. Hence, it is clear that the 'preservative liquid supplied to 'the treating cylinder during this period penetrated into the timber and the slight increase in pressure which did occur was that which was necessary to overcome the 'rsistance to now of liquid through the wood as the depth of penetration of the liquid increased;

ln like manner there `vvas a further gradual increase in pressure during the second hour and at the `end of the'se'cond hour the pressure was about 33 pounds, 0r an increase of only 7 pounds over the pressure present at the end of the first hour. The increase in pressure during this periool was so limited in amount, and was distributed over so long a period, that it did not cause collapse of the Wood cells of the timber in the charge, but was sufficient only to overcome resistance of the vvo'od to penetration yof the preservative.

There was va further gradual increase in the pressure during the third hour and at the end of the third hour the pressure was approximately 41 pounds, or about 8 pounds higher than at the end of the second hour. l

similarly, the pressure gradually increased during the fourth and fifth hours, the pressure at the end of the fourth hour being about 5o pounds, or approximately 9 pounds higher than at the end of the third hour. At the end of the fth hour the pressure was about 61 pounds, or about 1l pounds higher than at the end of the fourth hour.

After the fifth hourk the pressure increased rapidly so that at the end of the pressure treating period. the pressure was about 87 pounds, or an increase of about 26 poundsl in less than an hour. The rapid increase in 'the pressure in the treating cylinder near the end of' the pressure period indicates the increasing resistance to the penetration of the preservative into the timber in the cylinder. Ill addition, it shows that the pelletration of preservative into the timber was appreaching the point of refusal, and that further increases in pressure, or continued exposure of the timber to preservative under pressure, would not materially increase the amount of preservative absorbed bythe timber.

The slow Vgradual increase in pressure which the disgramsshow'to have occurred in the treatina cylinder showsthat the liquid supplied to the treating cylinder'penetrat'ed into the'timber as fast as the liquid was supplied to the treating cylinder.

The diagram also shows that the rise in pressure in the treating cylinder was so gradual that the cells in the timber had ample opportunity to become filled or substantially filled with preservative liquid so that the pressure within the cells was built up'rapidly enough to enable the cells to withstand the increased external pressure with the result that collapse of the cells does not occur. The absence of cell collapse in timbers treated by this process is confirmed by microscopic inspection of pieces of wood treated by this process. On the other hand, when timbers of the same type treated by the conventional pressure treating processes are examined microscopically, the cell walls are shown to be collapsed.

It will be observed in Fig. 3 that the slope of the pressure curve is substantially constant over the major portion of the treating period; inl other words, that the logarithm of the pressure is essentially a linear function of the time over the major portion of the treating period. Thus over the first five hours, 88 per cent of the treating period, the curve is essentially linear. Thereafter as the wood in the treating cylinder nears saturation the system becomes nearer and nearer a pure hydraulic system. The pressure, therefore, begins to increase rapidly thus indicating that the wood has taken up about all the preservative it will hold. l

It will be observed further that the pressure curve falls below the straight line defined by points Piti and P2152. As t1 equals zero, this line may be represented by the equation:

If the preservative is fed to the cylinder ata rate faster than that at which the preservative goes into the wood, the system will be shifted toward a purely hydraulic system..` Theslope of the log P curve, therefore, increases as afunction of the time as shown between'the fthand sixth hours on the curve. v,

The absence of collapse of the cells in timbers -treated in accordance with this invention results in uniform deep penetration'of the preservative liquid. Pieces of wood treated in accordance with this invention and then sawed in two at a point some distance from the ends of the piece show that the preservative has thoroughly and uniformly penetrated the sap wood portion of the timber. Where the pieces Which have been treated are round poles, ,the penetration of the Apreservative liquid is to a substantially uniform depth throughout the circumference of the piece.

On the other hand, similar inspection of timbers of the same type treated yby the conventional pressure treating processes showsv that the penetration of the preservativeisextremely uneven and non-uniform, beingrelatively, deepbin some areas,and quite shallow in others. The uniform deep penetration of the preservative secured by this process provides the maximum protection for the treated timbers with a minimum expenditure for preservative.

Furthermore, this process does not increase the length of the treating process even though in this process the pressure in the treating cylinder is much lower throughout a substantial part of the treating cycle than in conventional processes.

Although this improved process has been illustrated and described in connection with apparatus which causes preservative liquid to be supplied to the treating cylinder at a constant rate, and thus produces a controlled gradual increase in the pressure in the treating cylinder, it is contemplated that the supply of liquid to the treating cylinder may be regulated in other ways to provide substantially the same gradual increase in the pressure in the treating cylinder throughout the treating period.

Similarly, although the process has been illustrated and described in connection with the treatment of one species of green wood, the process is not limited to use in the treatment of this species of wood, but is equally useful in the treatment of other species of soft woods of low density. Likewise, the process is not limited to use in the treatment of green woods, but is advantageous in the treatment of partially seasoned woods which are too green to be treated successfully by conventional processes intended for treatment of fully seasoned woods. Accordingly, the term "pine lumber is intended to include all types of soft or coniferous timber and is not limited` to species commonly known as pines.

The specic gravity of the wood is determined by the methods commonly employed in the industry. A piece of wood is dried in a kiln or oven at a temperature of C. until it reaches a weight that does not vary. The weight of the piece after drying is then compared with the weight of a volume of water equal to the volume of the piece of wood in its-green state, that is, prior to drying of the piece.

It has been found that when treating green lumber the maximum rate of supply of liquid to the treating cylinder which can be safely employed is about 3 pounds per cubic foot of lumber `per hour. As the lumber seasons, it is progressively easier to treat, and on partially seasoned lumber the rate of supply of liquid to the treating cylinder may be increased to about 4.5 pounds vper cubic foot per hour.`

Although I have illustrated and described one form of improved process for impregnatingwood with preservative, it -is to be understood that the process is not limited to the specific details described and that numerous changes and modiications may be made without departing from the spirit and scope of the accompanying claims.

The preferred form of the invention having been thus described, what is claimed is:

l. In a process for impregnating unseasoned wood in which the wood is first dried by heating with steam followed by drawing a vacuum on the thus treated wood and then impregnated with a liquid impregnant the method .whereby unseasoned pine having a specic gravity less than 0.45 may be effectively impregnated which comprises; in the impregnating step confining the dried, unseasoned wood and impregnating liquid under iluid pressure ina closed pressure Avessel and pumping additional impregnating :liquid .inist-,he sorlosedivessel at a A substantially constant rate over substantially the entire period of the impregnation, said rate being less than 3-4.5 pounds of impregnating liquid per cubic foot of wood per hour whereby each increment of impregnating liquid pumped into the lled treating vessel necessarily is forced into the wood and the pressure is a function of the resistance the wood offers to the flow of impregnating liquid into its interior.

2. The process of claim 1 in which the treating vessel is completely lled with impregnating liquid and the system is purely a hydraulic one except for the wood in the treating vessel whereby each increment of impregnating liquid pumped into the lled treating vessel necessarily is forced into the wood and the pressure is a function of the resistance the wood oers to the flow of impregnating liquid into its interior.

3. The process of claim 2 in which the pressure does not increase at a rate of more than 9 pounds per square inch per hour initially.

4. The process of claim 3, in which the rate of change of pressure over the rst hour is in the order of 6 pounds per square inch per hour and in which the rate of change of pressure does not double itself until after about the fourth hour.

5. The process of claim 4 in which the wood treated is green Norway pine and in which the rate specified is about 3 pounds of impregnating liquid per cubic foot of Wood per hour.

6. A process for impregnating unseasoned pine having a specific gravity less than 0.45

10 which comprises charging the wood into a closed pressure vessel, steaming the wood therein, drawing a vacuum on the steamed wood, placing the wood under a pressure of air in order to charge the Wood with air, displacing the air with impregnating liquid without changing the pressure, and then pumping additional impregnating liquid into the vessel at a substantially constant rate not greater than 3-4.5 pounds of impregnating liquid per cubic foot of wood per hour and such that the pressure continually ncreases over the entire period of the impregnation and then releasing the" pressure on the system and withdrawing the impregnating liquid.

7. The process of claim 6 in which the wood treated is green Norway pine and in which the rate specied is about 3 pounds of impregnating liquid per cubic foot of wood per hour.

MERVIN J. HERMAN.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

1. IN A PROCESS FOR IMPREGNATING UNSEASONED WOOD IN WHICH THE WOOD IS FIRST DRIED BY HEATING WITH STREAM FOLLOWED BY DRAWING A VACUUM ON THE THUS TREATED WOOD AND THEN IMPREGNATED WITH A LIQUID IMPREGNANT THE METHOD WHEREBY UNSEASONED PINE HAVING A SPECIFIC GRAVITY LESS THAN 0.45 MAY BE EFFECTIVELY IMPREGNATED WHICH COMPRISES; IN THE IMPREGNATING STEP CONFINING THE DRIED, UNSEASONED WOOD AND IMPREGNATING LIQUID UNDER FLUID PRESSURE IN A CLOSED PRESSURE VESSEL AND PUMPING ADDITIONAL IMPREGNATING LIQUID INTO THE SO-CLOSED VESSEL AT A SUBSTANTIALLY CONSTANT RATE OVER SUBSTANTIALLY THE ENTIRE PERIOD OF THE IMPREGNATION, SAID RATE BEING LESS THAN 3-4.5 POUNDS OF IMPREGNATING LIQUID PER CUBIC FOOT OF WOOD PER HOUR WHEREBY EACH INCREMENT OF IMPREGNATING LIQUID PUMPED INTO THE FILLED TREATING VESSEL NECESSARILY IS FORCED INTO THE WOOD AND THE PRESSURE IS A FUNCTION OF THE RESISTANCE THE WOOD OFFERS TO THE FLOW OF IMPREGNATING LIQUID INTO ITS INTERIOR.