US2350135A - Wood impregnation - Google Patents

Description

y 0, 1944. A. J. STAMM 2,350,135

WOOD IMPREGNATION Filed Nov. 16, 1942 4 Sheets-Sheet 2 5 INVENTOR A.J.$TAMM 67 2 .gm BY W ATTORN EYS May 30, 1944. A. J. STAMM WOOD IMPREGNATION 4 Sheets-Sheet 4 Filed Nov. 16, 1942 MN nn mm mm INVENTOR I A.J.STAMM ATTOR N EYS Patented May 30, 1944 WOOD IMPREGNATION Alfred J. Stamm, Madison, Wis., assignor to Claude R. Wickard, as Secretary of Agriculture of the UnltedStates of America, and his suc- Application November 16, 1942, Serial No. 465,732

Claims.

(Cl. l17116) (Granted under the act of March 3, 1883, as

amended April 30, 1928; 3'10 This application is made under the act of March 3, 1883, as amended by the act of April 30, 1928, and the invention herein described and claimed, if patented, may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment to me of any royalty thereon.

This invention relates to a process and apparatus for impregnating wood in th form of either a thin lumber or veneer. The wood may be either dried or in a green, water-soaked condition. and this invention concerns the impregnation of such wood with water-soluble solutes, such as, by way of illustration, stabilizing resins, firer'etardant chemicals, and toxic preservative chemicals. With respect to the dry woods, impregnation may be accomplished with various chemicals dissolved in organic solvents.

The principal object of this invention is to impregnate wood of the type mentioned by a process which is simple and quick as compared to those processes of the prior art, and which is adaptable to continuous operations as distinguished from batch operations.

According to the prior art, green veneer, and sometimes dry or partially dry veneer, is treated with water-soluble solutes by merely soaking the green or dry veneer in an aqueous solution of the solute. Although this process is simple, it is very time-consuming. For example, fi-inch thick Douglas-fir veneer containing an amount of water equal to the dry weight of the wood (100 percent moisture content) will require about 14 hours soaking at room temperature in a 50 percent aqueous solution of phenol-formaldehyde resin-forming mix to take up 30 percent of resinforming solids on the basis of the dry weight of the untreated wood by diilusion into the wood structure.

When dry or partially dry wood is soaked in an aqueous solution, capillary absorption accompanies diffusion, but the combined processes are even slower than diflusion alone into veneer containing a large amount of water, as the eflective liquid cross section through which diflusion takes place is materially reduced. Forexample, .fi-inch thick dry Douglas-fir veneer (6 percent moisture content) will require over '75 hours soaking to take up the same amount of resini'orming solids as are taken up by the aforementioned water-soaked Douglas-fir veneer in 14 hours. '1

It is desirable, therefore, from the standpoint of the rate of takeup of solute, to apply this method to water-saturated rather than dry veneer. In this case, however, water must diffuse out from the wood as the solute diffuses in. When fresh solute is added to the treating bath to maintain the necessary outside concentration, the volume of the bath solution increases. It is hence necessary to remove part of the solution from the bath at intervals. The solute can be recovered from the removed solution in some cases by evaporating the water. In the case of the dimension-stabilizing, resin-forming systems, evaporation cannot be resorted to, as it causes premature curing of the resin. Treating watersaturated veneer by the difiusion method with such resin-forming mixes is hence wasteful of resin.

Both the rates of difiusion into wood and capillary absorption by wood can be increased by raising the temperature. The diffusion at C. would, theoretically, be about four times that at 250 C., because of the change of temperature and the accompanying change in viscosity of the water. The capillary absorption would be increased about 2.6 times for the same temperature change because of the decreased surface tension and decreased viscosity of the solution. In cases where heating is not detrimental to the solution, it is found advantageous to heat the bath, but even then the takeup of solute is slow.

immersing heated dry Or partially dry veneer in the solution at room temperature aids capillary absorption because the air in th capillary structure which expanded on heating again contracts in the cooler solution, thus setting up a partial vacuum which aids capillary rise. A vacuum amounting to only A; of an atmosphere can be obtained by heating the wood to 100 C. This will aid in taking up only a volume of liquid equal to one-fourth of the void volume of the wood.

Veneer can also be treated by dipping in solution, brushing a solution on the surface, or applying solution with a glue spreader. Sufficient solution for most purposes is taken up by the surface in one application only when the veener is li o-inch thick or less, or when the solution is quite viscous. When thicker veneer and low-viscosity solutions are used, it is necessary to dry the veneer and reapply the solution to the surface a number of times. This involves costly handllng. When solutions of very high concentration are used to avoid multiple applications, diffusion is greatly retarded and hence the solvent tends to evaporate before the solute has an opportunity to diffuse completely into the structure.

The most rapid and efficient treating method now in use comprises increasing the rate and amount of capillary absorption by a vacuum, pressure, or combined vacuum-pressure method in a treating cylinder. Veneer of different species in thicknesses up to /a-inch can be treated with an amount ofsolution equal to the weight of the wood in from. minutes to 4 hours by applying air pressures ranging from to 75 pounds per square inch to the immersed veneer. This method of treating, although fairly rapid, requires expensive equipment, a great deal of handling of the material, and can only be operated on a batch basis.

impregnating veneer by the method of the present invention overcome all the aforementioned deficiencies of the prior art. The method comprises passing veneer, which preferably has been preheated to make it plastic, between pressure rollers beneath the surface of the liquid with which it is to be impregnated. From the standpoint of preventing degradation of the veneer, it is preferable that the veneer sheets pass between the rollers at right angles to the axis of the rollers. This avoids injury to the wood and prevents curling of the sheets. It is desirable to compress the veneer to at least one-half the original thickness when an appreciable takeup of solute is desired. A large part of the air or moisture contained in the coarse-capillary structure is thus squeezed out. The veneer recovers very rapidly from the compression as the roller pressure is released, soaking in a volume of solution equal to the volume of air and water displaced. Besides the takeup of solution due to the mechanical treatment, there is an additional takeup resulting from the contraction of the air caused by the cooling of the hot veneer in the bath. To avoid having the temperature in the bath increase appreciably above room temperature, or any other desired point, by the hot veneer, cooling coils can be immersed in the bath near the compression rollers.

Green veneer, that is, veneer containing moisture above the fiber-saturation point (about percent moisture content), which contains free water will tend to lose water while being compressed, and dilute the treating solution. To avoid this, veneer which contains free water should be compressed to about the same degree between another pair of rollers before entering the compression rollers of the treating bath so that the expressed moisture will not enter the bath. Toaid in carrying away the expressed water and thus avoid reabsorption by the wood, a blast of hot air may be directed across each face of the veneer parallel to the axis of the rollers and as near the surface of the rollers as possible. In the case of dry, or partially dry veneer, containing moisture below the fiber-saturation point, it is unnecessary to subject the veneer to a predrying compression, so that only the treating bath rollers are necessary.

About half the void volume of fi-inch thick air-dry veneer of different species can be filled with an aqueous solution in one minute or less by the method of this invention when the veneer is compressed to half its original thickness between the treating rollers, even when fairly viscous solutions are used.

Green, water-soaked veneer, as it comes from the cutter knives, can also be impregnated by the method of my invention by first passing the veneer through a pair of hot drying rollers to squeeze out a quantity of the free water, prior to 16 passing through the rollers of the treating batl". I have found that there is a tendency for the expressed water to be reabsorbed by the veneer, on the outlet side of the rollers, unless some means for removing the water from the surface of the wood is provided. This surface water can be adequately removed by blowing air, preferably hot air, over both surfaces of the wood parallel to the axis of the rollers with the nozzle placed as near as possible to the line of emergence of the veneer from the rollers.

When appreciable amounts of free water remain in the veneer as it enters the impregnating solution, both the amount and rate of solution takeup are reduced. Green veneer should, therefore, be reduced to as near the fiber-saturation point as possible in the preliminary heating and compressing steps,

Although some species of wood will take up a sufiicient amount of solution for most purposes by cold compression, there is a definite advantage in compressing the veneer while hot to avoid fiber rupture, to insure complete springback of the wood, and to aid penetration of the liquid by the contraction of the hot air in the wood structure as itcools in the solution, Compressing the wood while hot also reduces the work necessary to effect compression. Woods which tend to take up too much solution when compressed to half their original thickness should, when hot, be compressed to a lesser extent.

In the case of the more difficultly treated woods, it may be desirable to keep theem immersed in the solution, after they pass through the rollers, for more than one minute, because the springback or recovery from the compression is somewhat slower. It is generally of no advantage to leave the veneer immersed in the solution for more than ten minutes.

Birch wood has a specific gravity which is about optimum for compression to half the original thickness. Denser woods cannot be compressed to this extent. Any attempt to do so would be disastrous to the equipment.

Woods which are readily compressed to half of their original thicknesses can be fed between rollers set to cause such compression. Light softwood veneers -inch thick can be easily fed between rollers set to compress then to e-inch thick.

Most of the air removed from the veneer in the process of compression beneath the treating solution moves in the fiber direction through the veneer sheets and comes out at the ends of the sheets. Because of this, solution takeup occurs throughout the thickness of the veneer rather than just a fractional distance into the veneer from the faces. This is of value in reducing the subsequent storage time of the wet treated veneer to get a uniform distribution of the solute throughout the structure prior to drying. For example, veneers E-inch thick impregnated with Bakelite resinoid BR15,100 ordinarily require from 3 to 20 hours of aging, depending upon the veneers, under nondrying conditions (absence of heat and circulation) to permit them to come to practically complete swelling equilibrium.

In the accompanying drawings, there are illustrated suitable means for carrying out the methods described above, which, together with the following description, will more fully disclose this invention.

In the drawings:

Figure 1 is a plan view of an apparatus which may be used for impregnating the wood.

Figure 2 is a section along the line 2-2 of Figure 1.

Figure 3 is a section along the line 22 of Figure 1.

Figure 4 is a modified form of gearing for the compression rollers.

Figure 5 is a. plan view of an apparatus for removing free water from green, water-soaked wood prior to impregnation.

Figure 6 is a section along the line 8- of Figure 5.

Figure 7 is a section along the line 1-1 of Figure 5.

Figure 8 is a section along the line 82 of Figure 5. 7

Referring with more particularity to the drawings, in which like numerals designate like parts, the apparatus illustrated in Figures 1, 2, and 3 comprises a tank I for holding the solution with which the wood is to be impregnated. The shape of this tank is preferably as illustrated, in the general form of a V, to economize on space, materials, and volume of treating bath. The lower end is provided with a drainage valve |2. The tank is supported by suitable frame members II. One end of the tank is provided with a pair of compression rollers l4 and I5, between which the wood is compressed. The rollers are disposed so that the space between them is below the liquid level of the tank and so as to direct downward into the tank veneer passing therebetween.

The rollers l4 and I5 are preferably geared together to avoid slippage therebetween. In order to adjust the clearance between the rollers to permit the use of the apparatus with veneers of difierent standard thicknesses, the following means are provided: The bottom roller It is fixed to a shaft l6 which is rotatably mounted in bearings I! and I8 on the sides l9 and 20 of the tank. The upper roller I4 is fixed to a shaft 2| which is rotatably mounted in bearings 22 and 23 movable in slots 24 and 25 in the sides I! and 20 of the tank. The slots 24 and 25 are disposed above the liquid level and the bearings 22 and 23 are each provided with a pair of flanges 26 and 21 on either side of the slot, which can be drawn together by bolts 28 to secure the bearings in selected positions along the slots. The shafts i5 and 2| extend outward on one side of the tank and are provided with gearing units 29 and 20, respectively. The gearing unit 29 comprises a plurality of gears 3|, 32, 33, 34, and 35 of different sizes, secured together and slidably mounted on and rotatable with the shaft 2| by means of a sliding key arrangement 35A or by any other suitable means. A collar 36, secured to the unit, is provided with a set screw 31 to secure the unit in different positions along the shaft. The gearing unit 30 is provided with the same number and sizes of gears 38, 33, 40, 4|, and 42, respectively, but these gears are arranged in.reverse order to those of the first unit. The unit 30 is attached to the shaft I6 for rotation therewith, substantially as shown. The shaft [6 is also provided with a pulley 43 for the transmission of power thereto by a belt 44 from a source of power, such as an electric motor 45. It is important to have the gears of the gearing units properly arranged in order to prevent locking. In gearing unit 29, the smallest gear 3| should be on one side and the next largest gear 35 on the other side. The next size gear 32 should be adjacent the smallest gear, and the next larger size gear 34 should be adjacent the gear 35, and so forth. In other words,

cording to their sizes. This staggered arrangement permits gears of equal sizes to be meshed with each other without any of the other gears meshing. By these means the rollers l4 and II can be geared to each other in a 1 to 1 relation 311th different amounts of clearance between Another type of gearing is illustrated in Figure 4, and it embraces beveled gears 40 and 41, secured to the shafts l4 and 2|, respectively. These beveled gears mesh with other beveled gears 4! and 42 mounted for rotation with a shaft 80 at right angles to the shafts l8 and 2|. The shaft 54 is fixed to the frame of the machine by means of brackets II and 52. The gear 43 is fixed to the shaft 50, but the gear 4| is slidable thereon so as to mesh with the gear 41 in different positions of the roller IS. The movable gear 43 may be adjustably secured to the shaft 54 by means of a set screw 53 through a collar 54, integral with the gear. The shaft 52 is driven by power, delivered to a pulley I5 fixed thereto.

The ends of the shafts l4 and 2| on the extended sides are rotatably disposed in a fixed bearing 56 and a movable bearing 51, respectively, supported by a rigid bracket 58 secured to the tank. The bearing 51 is movable in a slot 53 of the bracket and is provided with flanges ill and ii and bolts 32, similar to the bearings 22 and 22.

Adjacent the fixed roller N, there are disposed cooling units 33 in the tank for the purpose of carrying 01! heat from the liquid, imparted to it by the heated veneers. These coils are connected to a suitable source of cooling medium (not shown).

As the wood veneers are passed between the rollers l4 and I5, they descend into the liquid bath in the tank. Guide rollers 64 and 25 on the outside of the tank permit easier handling of the veneers and other guide rollers 46 and i1 placed within the tank keep the veneers in the solution for a sumcient period of time and direct them the gears should be staggered on either side ac- 7 through the liquid bath so that one end comes up before the other. This makes it easier to withdraw the veneer from the bath.

When green, water-soaked veneer is used, it is necessary to give it a preliminary treatment for v the purpose of expressing a portion of the free water therein. This may be accomplished by the apparatus illustrated in Figures 5, 6, 7, and 8, which comprises a suitable frame structure that carries a pair of hollow compression rollers I and 69. These rollers are adjustable with respect to each other, like the rollers 4 and I5, and are provided with similar means for this purpose. The various elements of such means are designated by numerals corresponding to like parts in Figures 1, 2, and 3.

Plane surfaces 10 and H are provided on either side of the rollers to support the veneers.

Shafts 12 and 13 and 14, 15, which support the rollers 68 and 69, respectively, are hollow to carry hot air through these rollers. Hollow shafts l2 and I4, on one side of the machine, are connected tothe outlet pipe 16 of a hot-air generator II through a control valve II by means of sealed couplings I9 and N, which permit rotation of the shafts 12 and I4. An extension joint II is provided in the pipe I between the shafts I2 and 14 to permit displacement of the shafts when the distance between the rollers is changed. The hollow shaft I3 and II, on the other side of the machine, are connected to the inlet 42 of the generator 11 in a similar manner by means of scaled couplings 82 and I4 and extension joint 85.

On one side of the rollers, air nozzles 86 and 81 are disposed through the side of the machine so as to direct blasts of hot air across the upper and lower faces of the veneers as they emerge from the rollers. These nozzles are connected to the hot-air generator 'II by means of a pipe 88.

The followim examples will further illustrate this invention and the manner in which it may be practiced:

Example 1.-Air-dry veneer or thin lumber containing any amount of moisture up to the fibersaturation point (about 30 percent moisture content) is heated in a drier to a temperature oi 200 F. to 300 F. under such conditions that no appreciable drying is effected. This generally requires circulation of air having a relative humidity of 75 percent or more. These veneers are then removed from the drier and directly fed into the apparatus, such as illustrated in Figures 1, 2, and 3, between the compression rollers thereof, with the clearance between the rollers set at aboutone-half the thickness of the veneer. As the veneers pass between the rollers, they are immersed in the treating bath, which may, by way of illustration, consist of a 50 percent aqueous solution of a phenol-formaldehyde resin-forming mix. After the veneer has been held submerged in the solution [or about one minute, it recovers substantially completely from the compression and soaks in the solution. The veneer may then be removed from the treating bath.

Example 2.-Green, water-soaked veneer as it comes from the cutter knives or thin, green lumber prior to seasoning having a moisture content above the fiber-saturation point is first passed through a veneer drier, preferably a continuous drier to heat the veneer to about 200 F. without preventing any drying which may normally take place. The veneers are then fed directly from the drier to a device such as that illustrated in Figures 5 to 8, inclusive, in which the pressure rollers are set to compress the veneer to half its original thickness, the while permitting hot air to blow across the faces of the veneer through the nozzles 86 and 81 to remove any expressed water from the surface of the veneers. Promptly, upon removal from this operation, the veneers are fed. to the apparatus, illustrated in Figures 1, 2, and 3, in a manner similar to that explained above in Example 1.

Instead of using phenol-formaldehyde resinforming mix, other types of aqueous solutions may be used. Examples of other resin-forming mixes are urea-formaldehyde, melamine-formaldehyde, and phenol-furfural. Also, inorganic fireretardant salts may be used, such as monoammonium acid phosphate, diammonium acid phosphate, and other phosphates, borates, and so forth. A number of toxic salts may also be used, such as zinc chloride, sodium fluoride, sodium arsenate, and other commercial mix salts and organic compounds. These examples may be used in :any desired concentration.

A number of difierent types of preheating devices to plasticize the wood can be used. The heating can be done with steam, hot air, or with hot rollers or hot plates. When steam is used, it should be dry steam to avoid undue capillary condensation in the wood. Preheating above the boiling point of water is feasible only for veneer containing water below the fiber-saturation point. Preheating above 300 F. is not desirable because of the imbrittling effect upon the wood.

In the drying of green veneer, it may be advantageous, if the moisture content is relatively high, to put the veneer through the drying rollers before heating so as to avoid the expense of heating a large excess of free water.

The extent to which the wood is compressed in either the drying or treating rollers may vary, depending upon the amount of solution which it is desired to take up, and is limited only by the extent of the void volume of the wood. The fraction of the original thickness to which dry veneer can be compressed can be calculated by dividing the oven-dry specific gravity of the wood by 1.46.

r The fraction of the original thickness'to which veneer at the fiber-saturation point can be compressed can be calculated by dividing the green specific gravity by 1.12.

The length of time the compressed veneer should remain immersed in the solution after having been compressed will vary with the species and the amount of solution that it is desired to take up. Spruce and cottonwood take up an adequate amount of solution for most purposes in about 10 seconds. Birch may require immersion for as long as 10 minutes.

Having thus described my invention, I claim:

1. Th process comprising plasticizing with heat a water-soaked wood veneer, expressing and removing free water from the coarse capillary structure of the veneer to establish a void volume, thence simultaneously submerging the plasticized, dewatered veneer in an aqueous solution and passing it between compression rollers, thence holding the veneer in the solution to permit it to take up the solution as it recovers from the compression.

2. The process comprising plasticizing with heat at a temperature of 200 F. a water-soaked wood veneer, expressing and removing free water from the coarse capillary structure of the veneer to establish a void volume, thence simultaneously submerging the plasticized, dewatered veneer in an aqueous solution and passing it between compression rollers, thence holding the veneer in the solution to permit it to take up the solution as it recovers from the compression.

3. The process for impregnating water-soaked wood veneer with an aqueous solution comprising plasticizing it with heat, expressing and removing free water from the coarse capillary structure in an amount substantially comparable to the quantity of the solution with which it is desired to impregnate the veneer, thence simultaneously submerging the plasticized, dewatered veneer in an aqueous solution and passing it between compression rollers, thence holding the veneer in the solution to permit it to take up the solution as it recovers from the compression.

4. Apparatus comprising a tank, co-acting compression rollers mounted on the tank, means for adjusting the clearance between said rollers, means for rotatably driving said rollers, the clearance between said rollers being disposed below the maximum liquid level of the tank, and means for cooling the liquid in the tank, said cooling means being disposed adjacent the rollers.

5. The process comprising plasticizing with heat a wood veneer having a void volume, thence simultaneously submerging the plasticized veneer in an aqueous solution cooler than the veneer and passing it between compression rollers, thence holding the veneer in the solution to permit it to take up the solution as it recovers from the compression.

ALFRED J. STAMM.

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