US2618813A

US2618813A - Method for making cellulosic board

Info

Publication number: US2618813A
Application number: US184888A
Authority: US
Inventors: Edward A Patton; Forrest F Beil
Original assignee: Curtis Companies Inc
Current assignee: Curtis Companies Inc
Priority date: 1950-09-14
Filing date: 1950-09-14
Publication date: 1952-11-25
Anticipated expiration: 1969-11-25

Description

zmwm

Nov. 25,, 1952 E. A. PATWM mm.

'METHOD FOR MAKING CELLULOSIC BOARD 4 Sheds-Shem 1 Filed Sept. 14, 1.950

Nov. 25, 1952 Filed Sept. 14, 1950 E. A. PAWUN ET AL METHOD FOR MAKING CELLULOSIC BOARD zmwm Sheets-Sheet 2 Nov. 25, "21952 E. A. PATTON ETAL Zfimfiw METHOD FOR MAKING CELLULOSIC BOARD Filed Sept. 14, 1950 4: Sheets-Sheet 3 Nov. 25, 1952 E. A. PATTON mm.

ma ma METI-IOD FOR MAKING CELLULOSIC BOARD Filed Sept. 14, 1950 4 Sheets-Sheet 4 UN [TED d'iATES Patented Nov. 25, 952

iowa, assignors to Curtis Companies incorporated, Clinton, liowa, a corporation of Iowa Application September 14, 1950, Serial No. 184,888

(Cl. ldti.5)

3 Claims. i This invention relates to a method for manufacturing a compressed wood board from granulated wood such as sawdust, finely subdivided wood waste of the type obtained in mill work plants, and the like.

Reference is made to the copending applica-' tions of Edward A. Patton, Merle W. Baker, Forrest F. Beil and Charles F. Curtis. I1, Serial No. 28,158 (filed May 20, 1948, entitled Board of Compressed Cellulose Material and Method and Apparatus for Manufacturing the Same now forfeited) and of Edward A. Patton and Forrest F. Beil Serial No. 100,004 (filed June 18, 1949, and entitled Cellulosic Board and Method of Treating the Same). These two applications show methods for manufacturing a compressed cellulosic board characterized by high strength, freedom from warping and many other desirable characteristics. Reference is also made to the copending applications of Merle W. Baker, Forrest F. Bell, Charles F. Curtis, H, and Edward A. Patton Serial No. 59,903 (filed November 13, 1948, and entitled Apparatus for Manufacturing Boards of Compressed Cellulose Material and the Like now U. S. Patent No. 2,583,249), Serial No. 59,902 (filed November 13,1948 and entitled Pan Filling Machine) and Serial No. 117,634 (filed September 24, 1949 and entitled Pan Filling Machine). These three applications show apparatus for practicing the methods of said first mentioned two applications.

The apparatus and method of said five copending applications involve compression molding of a mixture of granulated wood and a small amount of resinous binder in relatively shallow generally fiat pan or tray-like molds. More particularly, the copending applications disclose the molding of mixtures of granulated wood with a resinous binder. These mixtures may contain as little as 4% resin and are characterized by a moisture content of at least 5%. The resinous binder is preferably, but not necessarily, thermosetting and is characterized by a capacity for flowing under the temperature and pressure conditlons maintained during the molding operation for an appreciable period of time before the resin is set or cured or otherwise brought into the condition in which the binder is present in the hi1- ished board. In the molding operation, a pressure of at least 150 lbs. per square inch, but less than 500 lbs. per square inch is maintained, at least in the initial stage. Further, the exact pressure employed is correlated with the moisture content of the molding mixture as disclosed in detail herelnbelow. The temperature is maintained for a sufliclent time to cure or set the resin. or otherwise bring the resin into the condition characteristic of the finished board. Further, the margins of the layer of mixed resin and wood being molded are compressed to from 40% to of the thickness of the remaining portions of the compressed layer. Finally, the pressure is released slowly (within a time of several seconds or minutes), rather than all at once. The compressed edges may be trimmed on to leave a panel or board of uniform thickness.

The significance oi the above disclosed steps is explained as follows:

Since the margins of the layer being molded are compressed very much more than the remaining portions of the layer, the moisture content of the mixed wood and resin is maintained practically constant and uniform throughout the molding operation. In other words, the compressed margins or edges act as a seal to prevent the escape of moisture and the moisture content is kept uniform throughout the layer being molded. There is, therefore, no tendency to warping or curling after the molding operation has been completed due to uneven moisture loss with consequent greater shrinkage of areas of relatively great moisture loss. Further, at a temperature of at least 280 F., a pressure of at least pounds per square inch and a moisture content of at least 5%, and when the pressure has been correlated with the moisture content as described hereinbelow, the wood particles are rendered plastic and flow so as to form a board characterized by low porosity. h i strength and resistance against chipping, in spite of the relatively small amounts of resinous binder present in the board. In this connection, it should be mentioned that since the resinous binder flows under the temperature and pressure conditions maintained during theirdtia'lstage of the molding operation, the resinous binder is distributed over the wood particles in a manner that utilizes more fully the binding properties of the resin. Finally, when the moisture content has been correlated with the pressure as disclosed hereinbelow, there is little or no tendency to blister when the pressure is released slowly. Pressure must be maintained at least long enough to cause both the wood particles and the resin to flow into the positions characteristic of the final board but the pressure may be reduced slowly and water vapor released, if desired, before the resin is completely cured as long as the moisture is retained for a suincient period to plasticize the wood particles for flow into their final positions. The pressure is strength,

kept below 500 lbs. per square inch, since otherwise the wood particles will be at least partly hydrolyzed or changed chemically, as evidenced by darkening, lessened ability to absorb oil stains, and other undesirable changes. Further, at pressures of 500 lbs. per square inch or higher, the boards tend to blister on release of pressure, even when such release is carried out slowly.

The board or sheet material prepared according to said method is characterized by high cohesiveness no tendency toward chipping or to the breaking off of small particles, Particularly at edges), uniform physical characteristics (strength, rigidity, and the like) from the center of the board or sheet all the way to the edge, freedom from warping, a tendency to swell at humidities higher than normal, it at all, principally in a direction normal to the plane of the board, a hygroscopicity no higher than ordinary wood, resistance against bending, ability to take paint and other finishes in the same manner as ordinary wood and a capacity for being sawed, nailed, screwed or planed.

When the boards prepared as disclosed hereinabove are removed from the pans or molds or trays in which they have been formed, the boards are hot (about 212 F.) and have a moisture content of about 2%. After cooling, and on standing exposed to the atmosphere, the boards absorb moisture, finally reaching a moisture content that remains more or less constant. In other words, the moisture content of the boards reaches an equilibrium level which depends to some extent on the moisture content of the surrounding atmosphere. At a, relative humidity of the atmosphere ranging from 60 to 70%, the moisture content of the boards becomes constant at the level ranging from 6 to 7% board moisture content.

Several days may be required for the compressed boards to reach their equilibrium with respect to moisture content. During this time the boards undergo dimensional changes, for absorption of moisture is accompanied by expansion of the boards. In other words, the boards reach a dimensional equilibrium at the same time as the board moisture content reaches an equilibrium level. As a result, the boards, immediately after cooling on removal from the pans or molds, tend to warp or to be otherwise distorted if then and there sawed into panels which at once are incorporated with a cabinet or door or like structure having sufficient rigidity to prevent free expansion or other dimensional changes in the panel made part of said structure. Of course, once the boards have reached their moisture and dimensional equilibria, the boards may be incorporated with doors, cabinets or like structures in the form of panels having any desired dimensions, and will then not warp or be otherwise distorted.

The boards in question reach their equilibria with respect to moisture content and dimensions most rapidly if individually set on edge with their broad faces exposed to the atmosphere. If stacked with their broad faces contacting, the boards require a much longer time to reach their equilibria. But setting each board on edge even for a matter of hours or days requires a considerable amount of space, equipment and labor. On the other hand, prolonged storage of stacks of boards requires a sometimes inconvenient amount of space together with the necessity of inspections for determining the condition of the stacked boards.

It is therefore an important object of the present invention to provide a method for mak ing compressed boards from comminuted wood characterized by stability with respect to moisture content and dimensions so that said boards can immediately be incorporated with structures such as doors and cabinet in which dimensional changes are prevented, the boards thereafter not being subject to warping or distortion due to dimensional changes therein.

Another object of this invention is to provide a method for making compressed boards from comminuted wood in which the wood particles are not changed or decomposed chemically so that the finished boards have all the physical and chemical properties to be expected in merely compressed wood particles including uniform light color and ability to absorb oil stains uniformly.

Other and further objects and features will become apparent from the following description and appended claims as illustrated by the accompanying drawings showing, diagrammatically and by way of examples, apparatus and products according to this invention. More particularly:

Figure 1 is a diagrammatical side view of apparatus utilized for effecting a complete process in the manufacture of boards from granulated wood;

Figure 2 is a. diagrammatic view of the hydraulic system of a hot press forming part of the apparatus of Figure 1;

Figure 3 is a perspective view of a pan or tray used as a mold in the apparatus of Figure 1;

Figure 4 is an enlarged sectional view, with parts broken away, of the pan of Figure 3 with the material to becompressed therein, as it appears prior to compression, with a cover disposed on top of the material;

Figure 5 is a view similar to Figure 4 but showing the parts as they appear after the material has been compressed;

Figure 6 is an enlarged sectional view, with parts broken away, of the compressed material after it has been removed from the pan;

Figure 7 is a view similar to Figure 6 but showing the finished board with the compressed or densified margins cut off, the cut off material being shown in dotted lines;

Figure 8 is an enlarged perspective view of a water spray device forming part of the apparatus of Fig. l

Figure 9 is a. cross sectional view taken along the line 9-9 of Figure 8;

Figure 10 is a cross sectional view taken along the line Ill-l0 of Figure 9;

Figure 11 is a cross sectional view taken along the line II-H of Figure 9; and

Figure 12 is an enlarged fragmentary cross sectional view taken along the line |2-|2 of Figure 9.

Referring specifically to Figure 1 for a description of apparatus for practicing the present invention, numeral H designates a conduit which conveys, for instance, wood waste from a dust collector system in a wood making plant. The wood waste may contain a high percentage of knot sections, say, 50% knot sections and 50% machine waste.

The refuse or waste is delivered by the conduit H to a cyclone I2, which is preferably equipped with a magnetic separator (not shown) to remove any metal therefrom, which may cause sparks and possibly a fire. From the cyclone l2, the material is delivered through a conduit 13 to aeraera ii an ordinary commercial hammer-mill it, which pulverizes or granulates the waste material and is equipped with a suitable screen not shown) to deliver pulverized waste directly into a storage bin 45. This part of the process is continuous, the remainder being accomplished by a batch method. The storage bin iii may be provided with an automatic shut-off device (not shown), which shuts off the delivery of waste through the conduit ll, when a predetermined level has been reached in the bin l5. I

The pulverized material is fed through an outlet IG from the bin ill to a belt conveyor H. A screw conveyor i not shown) is provided in the outlet conduit It, and the belt conveyor ill and screw conveyor are synchronized electrically by any suitable means to introduce a predetermined amount of pulverized material into a waste measure I8. An automatic water valve (not shown) delivers a predetermined amount of water to each measured batch of pulverized material that is delivered to a muller or mixer iii. A predetermined amount of powdered or liquid resin or other binder is added to each measured batch of pulverized material. After mulling, this mixture is then dumped back onto a belt conveyor 24 through an outlet conduit 23, and is delivered to a hopper 25 of the pan filling machine.

From the hopper 25, the pulverized and mixed material is delivered to a belt system, which is generally indicated by the numeral 25. The entire pan filling machine (which may be the machine shown in said copending application Serial No. 117,634) is supported on a table it, and the pans which are filled by the machine are shown generally at 28. A continuous belt 29, and a second continuous belt 33, are provided for conveying the pulverized mixed material to the pans, and for conveying the pans to a loading rack 32, respectively. It will be noted that the loading rack is provided with a number of shelves or supports 32a for the pans 28. From the loading rack 32, the pans are delivered either by manual or mechanical means to a hot press generally indicated at 33. The hot press itself is of modified standard design, and pressure is applied to the material in the pans, and at the same time the material is heated. When the compression step is completed, the pans with the compressed ma{ terial therein are delivered to the unloading rack 34, which likewise has a number of shelves 36a for the reception of the pans 28. The pans are then removed from the rack fi l, either manually or mechanically, and the compressed material is taken out of the pans by inverting them. The inverted pans 28 are then placed on a gravity roller conveyor 35, which terminates adjacent the pan loading mechanism.

The compressed boards indicated at it; are placed on edge in a cooling raclr ti. After having been cooled, the boards are passed through a spray device generally indicated at 32 in which each board or panel is moved between opposed sets of spray nozzles. After the boards or panels have been sprayed with water, they are piled on a support to form a stack M in which the boards are left for a few hours. During this time the moisture distributes itself evenly through each of the boards or panels which are then ready for trimming and sanding.

Referring now to Figures 3 through 7 for a detailed description or the pans 28 and the boards formed therein, the pan 28 is formed, preferably, of aluminum, because of its lightness and heat conductivity. Furthermore, the aluminum is of d fairly thick gauge and does not have too much tendency to warp under heat. Also. there is very little tendency for the compressed material to stick to the aluminum stu'face. Obviousjy, however, other metals may be used for the pans, such as brass or iron.

The pan 28, preferably, comprises a flat base member bu which has an angle shaped flange 5i secured thereto, as by rivets 52. The horizontal portion lid of the angle 5! overlies the base plate 5G, for a purpose herein described.

The mixed granulated material to be compressed is shown in Figure 4 by the numeral 55. A cover 369 is suitably superimposed over the material 55 and has its edges spaced from the upstanding portion of the angle 5 6.

When the compressed board lid (shown in Figures 6 and '7) has been removed from the pan 28, the board d ll includes a thin flange t5 extending completely around the main portion of the board, being formed between the horizontal angle portion til and the opposed margins of the cover it. As shown in Figure 7, the thin flange or border 65 together with a slightly upturned edge 36 (formed between the edge of the cover 38 and the upstanding portion of the angle El) are removed to leave a board of uniform thickness. The portions tlti and d6 thus removed, for instance, by sawing, may then be returned to the waste or refuse material and repulverized.

Referring specifically to Figure 2, a hydraulic system for operating a hot press is there shown. The hydraulic system is constructed along conventional lines except for the inclusion of a needle throttling valve 52 for a purpose to be described.

The hydraulic system comprises an oil supply tank 63; to which is connected a high pressure pump 6d and a. high volume pump 65. A pipe line 66 is connected with the outlet of the high pressure pump and has two normally closed valves 67, 68 therein. A normally closed check valve 69 is provided in an outlet pipe H from the high volume pump ed. A pipe line 72 is connected between the valves El and 58 to a cylinder drain valve chamber i3. Likewise, pipe H5 is connected to a pipe l5 extending between the valves '58 and 89 and pipe it is also connected to the cylinder drain valve chamber F3. The pipe is then connects the cylinder drain valve chamber it with a hydraulic piston H which provides the required pressure for the hot press 33.

The press 33 and the hydraulic system are provided with a standard electrical timer (not shown), which maintains a high pressure on the hydraulic system until the pressing is completed. At this time, a solenoid operated valve 18 is partially opened. The needle valve 62 functions as a pilot valve for the valve it and is adjusted so as to provide a very small opening. The high pressure hydraulic fluid from the hydraulic piston ll slowly passes back through the pipe 76, through one branch of the pipe l2, through the needle valve 62, through the partially open solenoid operated valve l8 and back to the oil supply tank through a pipe l e. Since the valve 72 is so adjusted as to permit the fluid to flow slowly into the main valve, a number of seconds Will elapse while the initial pressure is being reduced. As soon as the pilot valve 62 is full of hydraulic fluid, the main valve '58 is opened completely and the press opens rapidly. A pipe 8% having a. hand valve therein connects the cylinder drain valve chamber ill with the oil supply tank 53 for the aeraera obvious purpose of draining the cylinder of the hydraulic piston 'I'i when desired.

Since there is considerable internal steam pressure or superheated water in the panel during the pressing operation, the panel 40 is liable to blister or explode if the press is permitted to open instantaneously. With the needle valve 62 installed as shown and adjusted to a very small opening, the high pressure oil is forced to pass through the small opening when the solenoid valve I8 trips or opens. Therefore, several seconds are required for the pressure to be reduced on the press 33, thus causing a gradual release of pressure in the board and eliminating blistering and possible exploding of the pane1 40.

The operation of the apparatus and process has already been described up to the time that the mixed pulverized material is delivered to the hopper 25. The pan filling device preferably includes means for maintaining the granulated material in loose form. The pan filling device fills the pans 28 with a layer or granulated material of uniform thickness, regardless of any warping or twisting of the pan. After the pans have been filled with the material to be compressed, the pans are carried into the hot press 33, the covers having been placed over the pans so that they may be forced downwardly into each pan when proceeding according to the specific method illustrated in Figures 4 through 7.

Since the metal leg 53 cannot be compressed, the portion of the panel immediately above it is compressed considerably more than the remainder of the panel, producing an extremely

dense edge

45, 40 around the panel, which prevents the escape of an excessive amount of steam and moisture during the pressing process. A seal results around the edge of the panel, and in addition to preventing the escape of moisture, also maintains uniform moisture distribution throughout the panel during pressing. Uniform moisture distribution brings about uniform physical characteristics in the finished panel and, in particular, minimizes internal strains in the panel, resulting in a flat panel with a minimum tendency to bow or curl due to internal strains resulting from unequal distribution of moisture during pressing.

Similar results are obtained when proceeding according to the methods illustrated in Figures through 17 in said copending application Serial No. 59,903.

Another difiiculty encountered in retaining the steam in the panel during the pressing operation is the tendency of the panel to explode, due to internal steam pressure, when the pressure on the hot press 33 is released. This has been overcome by installing a needle valve 82 in the hydraulic system of the hot press to permit very gradual release of pressure on the panel, as explained above.

After the boards have been formed by compression in the hot press 33 and thereafter removed from the pans 28, as by inversion of the pans, the still hot boards are placed on edge in the cooling rack M and there exposed to the atmosphere. Ordinarily an exposure of from 20 to minutes (or not more than minutes) suffices to cool the boards down to approximately 100 F. or at least to a temperature, say, of 120 F., where moisture not in excess or" 7% would not evaporate off rapidly. In other words, the panels or boards are cooled down to a temperature at which the final mixture equilibrium content (0 to 7%) can be maintained throughout the boards or panels. If the panels are sprayed while at 212 F. (the temperature at which the panels are removed from the hot press), the panels warp. Such warping is apparently due to rapid evaporation of the applied moisture from the panel surfaces at this elevated temperature so that in fiat piling one panel surface is exposed to the air for a few seconds longer than the other surface and therefore has a lower moisture content than the other surface. Such uneven distribution of moisture as between the two sides of the panel causes severe cupping of the panel.

The spray device 42 for applying moisture to the panels is shown in detail in Figures 8 through 12. The device is supported by legs and includes a relatively shallow longitudinally extending trough 94 provided with a discharge conduit 00. Two sprocket wheels 88 rotatably disposed in said trough near the ends thereof have a chain I00 trained therearound. The upper run of this chain moves over the bottom of a channel I02 extending above and parallel with the trough 94, having its side walls extending above the chain I00. One of the wheels 98 is rotated by force, transmitted from a motor I04 through a chain I06 or by any other suitable means.

A rod I08 is supported by standards IIO above, slightly to one side of and in parallelism with the channel I02. Thus, the edge of a panel 40 may be placed on the chain I00 in the channel I02 and the upper margin of the panel will then be supported by the rod I08 as the panel is moved longitudinally by the chain I00. At its discharge end, the rod i08 is bent horizontally, say, by 45, as indicated at 108a. to cause the boards to tip over as they are discharged from the spray device.

The trough 04 includes a central upwardly divergent portion 94a forming the bottom of a spray chamber having side walls I II and a roof H2. The side walls III are each formed with a vertical recess IIIa accommodating a vertically extending water pipe II4 supplied with water through a conduit I I6. A plurality of nozzles I I8 extend transversely into the spray chamber from each of the pipes H4. These nozzles are spaced progressively further apart toward the bottom of the spray chamber. If desired, each of the pipes H4 may be branched at the level of the highest nozzle IIS, as shown at Illa, and this branch may terminate in a nozzle II8a. Such a nozzle arrangement provides for the application of more water at the upper margin of each panel 40, to compensate for the run down of water over the sides of the panel.

The spray chamber is also provided with end walls I20 each formed with a vertical inclined slot I22 through which the panels 40 enter and leave the spray chamber. To prevent splashing of water through these slots, resilient flaps I24 may be aihxed to the outside of the walls I22 to project over the slots I22, for instance, into wiping relationship with the panels 40.

As explained hereinabove, the panels 40, after cooling in the racl: 4|, are passed through the spray device 42 and there moistened with water. The moistened panels are thereafter piled fiat as a stack 44 and left there until moisture and dimensional equilibrium has been reached, which will ordinarily be a matter of a few hours. The panels may then have their

borders

45, 46 trimmed off and, after sanding, are then in finished condition.

The nozzles H8 form fan-shaped sprays directed onto both sides of the vertically disposed boards. The above described nozzle arrangement and the fact that the chain I00 moves the boards slowly and at a constant slow speed through the aeiacis it spray chamber cause the boards to be moistened uniformly. Any slight excess of water (above the amount required to raise the board moisture content to 6 or 7%) sprayed into the boards is not absorbed but runs 017: the boards, when the boards are sprayed at about 100 F.

In this connection, it may be pointed out that the compressed boards, when removed hot from the press, contain approximately 2% moisture. The rest of the original moisture content is lost as steam when the press is opened. By adding about 4 to 5% moisture, the panels or boards are rendered perfectly stable dimensionally at arelative humidity of 60 to 70% and dimensional changes at other relative humidities are reduced to such small values that they can be disregarded.

The following materials have been found to provide a very satisfactory panel formed of compressed wood or other cellulose type material. Disintegrated wood of any species of tree may be used. Very satisfactory results have been obtained with pine wood. Preferably at least 50% of the wood is disintegrated to a 16 to 40 mesh particle size.

The resin employed is preferably one having a how point not higher than 125 C. lhe resin may be a thermosetting resin capable of flowing for an appreciable period of time before it is cured or set in the press and capable of acting as a bonding agent for the wood particles. We prefer to use a resin having a curing time of from 40 to 100 seconds at 150 C. Resins of various chemical compositions share these characteristics. We can use, for instance, resins oi the phenolformaldehyde type or the urea-formaldehyde type, or iurfural resins and the like. Obviously, resins characterized by excessive tendency to absorb water or by insufidcient resistance to weathering agents or having other undesirable characteristics should not be employed.

We have successfully used, inter alia, three phenolformaldehyde resins characterized by the following flow points and cure times:

It is understood that the thermosetting resins herein referred to are capable of curing or'setting under the conditions of the molding operation. In other words, the binding agents employed may or may not be resinous when initially incorporated with the granulated wood but are definitely present as resins in the finished panels. We may therefore employ binding compositions made up of resin-forming materials in any resinforming stage short of the final or cured or set stage. The resinous binding agent may be employed in wet or dry condition. We prefer to use a solid finely pulverized resin-forming composition, since such products are most easily and most uniformly blended or mixed with the wood particles. Nevertheless, we can also employ moist or dissolved or dispersed resin-forming compositions, due regard then being bad for the moisture content of the resin-forming composition when making up the mixture to be molded.

The amount of resin employed may range upwardly from 4% to 5 of the mixture being molded. We prefer to employ from. 5% to 8% resin. When a dry powdered phenolfornialdehyde resin is used, very satisfactory results have been obtained at a resin content of from 6% to '7 Blistering occurs at resin contents of about 14% or higher, for such high resin contents apparently prevent the free escape of steam from the resulting dense boards when the press is opened. We prefer to keep the resin content at from 5% to 8%, to keep the cost at a minimum. Obviously, the exact amount of resin to be used will vary somewhat according to the specific nature of the particular resin being used. In general, more resin is used when the wood is more finely disintegrated.

The water content of the molding mixture is maintained at from 5% to 25%, depending on the pressure employed in the molding operation. At lower moisture contents, the panels obtained are characterized by excessive thickness, structural weakness, excessive porosity, the presence of voids in the interior of the panel and by pitted surfaces, even when relatively high pressures are used. At moisture contents in excess of 25% there is a tendency for the panels to stick or adhere to the mold walls and to the formation of blisters or even to explosive disintegration of the panel on release of the pressure, whether or not such release is accomplished slowly, if sufiicient pressure has been used to form a firm panel. The correlation between the moisture content and the pressure is discussed hereinbelow. Wood waste accumulated from millwork operations commonly contains about 6 to 8% moisture. This moisture content is taken into account when the total moisture content of the pressing mixture is calculated.

It should be understood that besides the above enumerated ingredients, other materials may also be incorporated with the molding mixture. Such added material may include pigments such as titanium dioxide, iron oxides and the like, inert fillers such as chalk or barium sulfate, materials commonly used as fillers or extenders for resins, finely divided carbon and many other materials.

The above disclosed ingredients of the molding mixture are mixed with each other at a temperature below the flow point of the resin.

The pressure applied during the hot molding operation ranges from 150 to 400 pounds per square inch or higher but does not exceed 500 pounds per square inch. The pressure is correlated with the moisture content of the molding mixture according to the following table:

I P i B d i cloisture rcssurc in cum s per roa ontent Square inch Range Preferred hmmmc Range Percent Percent 22-25 L. 17-23 20 12-18 15 7-l3 in 5-8 7 l Pressure in Pounds For I 7 Square Inch Moisture Content ill 1 or (cut Minimum Maximum 150 200 Hill) 300 400 400 500 The exact pressures and moisture contents to be employed will vary, within the tabulated limits, according to a number of factors such as the thickness, strength and density required or desired in the finished panel. Obviously, these characteristics vary according to the end use of the finished panel. Further, moisture contents and pressures will vary somewhat, within the tabulated limits, according to the nature and prior preparation of the wood, the nature and amount of specific resin employed, and like factors. In making panels suitable for most, if not all purposes, on a large scale, we prefer to use a molding mixture containing from 10 to 15% moisture, and to press this mixture at from 300 to 400 pounds per square inch, using a powdered phenol-formaldehyde resin as binding agent in an amount ranging from to 8%. Thus, a batch of material to be molded may have the following composition: 86.3% by weight pulverized mill waste 7.7% by weight water 6.0% by weight powdered phenol-formaldehyde resin having a how point of li0-125 C. and a cure time of 80-100 seconds at 150 C.

As explained hereinabove, the pressure is at least sufficient, at the prevailing moisture content and temperature, to cause the wood to be plasticized and at the same time not great enough to cause blistering when the pressure is released slowly.

The temperature of molding is at least 280 or 300 F. A temperature Of 338 F. insures very satisfactory results with the above disclosed specific mixture. In general, the temperature must be sufficient to bring about curing or setting of any thermosetting resin employed. The time of molding should be sufiicient to bring about curing or setting at the prevailing temperature. Ordinarily, from about 3 /2 to minutes molding time is sufficient. With the above disclosed specific mixture, a molding time of 5 minutes has been found satisfactory. The full pressure should be applied at the beginning of the molding operation, to insure fiow of resin before the resin is cured or set. When longer molding times and higher temperatures are employed, the resulting panel material will be more stable dimensionally under varying humidity conditions. i. e. the material is less hygroscopic.

The pressure is applied for a period of time to compress the layer of molding mixture to its final dimensions. If desired, the full pressure can be applied throughout the whole molding operation, although very good results have also been obtained by slowly reducing the pressure to a lower value as soon as complete compression has been effected.

The molds may be coated with magnesium stearate to prevent adherence. For the same purpose, the molds may be preheated, say. to 150 to 175 F. before the press mixture is introduced.

In the molding operation, the margins on the layer being compressed are compressed to about 40% to 60% of the thickness of the middle portions of the finished panel. Some warping tendency is evident if the margins are compressed to less than 60% of the thickness of the remainder of the board. Wood cannot be compressed to lem than about one-third of its original thickness. Hence. when the edges or margins have been compressed to about one-third of the thickness of the remaining portions of the panel, these margins act as stops preventing further compressing of the middle of the panel.

Preferably, the margins are compressed to about 45% to 55% of the thickness of the middle portions of the panel. In the case of the above disclosed specific mixture, very satisfactory results have been obtained by compressing the margins to one-half of the thickness of the remaining portions of the panel. In the case of a panel 4 ft. square, compressed margins 1 wide function very satisfactorily to seal the moisture content of the pressing mixture.

To prevent darkening, either of the whole panel (excluding the densified sealing edge or margin) or parts thereof, and to permit uniform absorption of oil stains and the like, the pressing operation is conducted so that no significant decomposition or other chemical changes are efiected in the wood particles during the pressing step. For this purpose, the molding pressure is kept below 500 pounds per square inch and the full molding pressure is applied for less than 10 minutes, at least when the pressure ranges between 400 and 500 pounds per square inch. At pressures below 400 pounds per square inch, the pressure may be applied for longer periods than 10 minutes. Finally, the temperature is also kept below levels causing discoloration of the wood and reduced stain absorption. More particularly, temperatures up to about 360 F. are safe at pressures below 500 pounds per square inch applied for less than 10 minutes. At pressures less than 400 pounds per square inch, temperatures higher than 360 F. may be used, say, up to 400 F. However, as long as the flow point of the resin-forming binder is exceeded by about to F., no particular advantage is gained by further raising the temperature.

To show the effects of various pressures applied for various times, the following experiments are described. All these experiments were carried out in the above described apparatus, the margins of the layer of material being molded being densified as described hereinabove.

A molding mixture consisting of 93% sawdust (dry basis) and 7% water was compressed for 10 minutes at 345 F. and at a pressure of 500 pounds per square inch and yielded a board having a strength barely sufiicient to hold the board together.

A second molding mixture consisting of 86% sawdust (dry basis), 7% water and 7% of a resin binder was compressed for 10 minutes at 345 F. and at a pressure of 500 pounds per square inch and yielded a blistered board of dark color.

A third molding mixture consisting of 87% sawdust (dry basis) 6% water and 7% of a resin binder was compressed for 4 /2 minutes at 320 F. and at a pressure of 480 pounds per square inch and yielded a board of uniformly light color devoid of blisters.

A fourth molding mixture consisting of 86% sawdust (dry basis), 7% water and 7% resin binder was compressed for '10 minutes at 345 F. and at a pressure of 480 pounds per square inch and yielded a board having a dark central portion and a light colored marginal portion. The central portion absorbed less oil stain than the lighter marginal portion.

It should be noted that the above disclosed restrictions as to pressure and time apply only to the conditions under which the present method is carried out, i. e., where the margin of the layer of granulated wood-resin mixture is densified to form a seal retaining moisture within the molding mixture inside said densified margin. In this case, the margin is densified to almost or about its!) the limit of its compressibility, while this is not true of the material inside said margin. The latter material is not densified as much as the margin. The densified margin (which is subsequently trimmed off) is usually darkened and characterized by reduced capacity for absorbing oil stains or the like, as compared with the material within the margin. Thus, the step of compressing the margins more than the rest of the layer being molded serves not only to form a seal against the escape of moisture but the heavily densified margin material also serves as a stop preventing similar heavy compression of the material within the margin which otherwise would be darkened and have its ability to absorb oil stains reduced. In other words, the heavy densification of the marginal material permits the applications of relatively high over-all compressing forces which would cause over-all darkening and other undesirable changes in the compressed board in the absence of such marginal densification.

The edge sealing discussed in the preceding paragraph further contributes to uniformity in strength throughout the compressed boards, as illustrated by the following experiment. A number of souare boards or 2 foot width were prepared using a sawdust mixture containing about 7% resin binder, as tabulated:

Boards Nos. 1 and 2 were about equally strong. Both these boards showed a gradual slight increase in strength inwardly from the edge for a distance of or 6 inches and were then fairly uniform in strength all the way to the center.

Boards Nos. 3 and 4 were quite weak over a marginal area 2 inches wide. At a line extending 2 inches from the edge, there was an abrupt increase in strength. From this line inwardly, the strength increased gradually to a very high value at the center of the board.

From the foregoing it will be apparent that we have provided an improved method and apparatus for forming dense board from granulated or pulverized wood or other cellulose material.

The panels or boards prepared as disclosed hereinabove are made up of wood that has not been modified chemically to a significant extent and of resin in an amount of, say, from 6 to 7%. The boards will have about the same hygroscopic characteristics (tendency to absorb water) as the wood from which the boards have been prepared. The color of the boards is approximately the same as the wood contained therein. It should be noted, in this connection, that the color of the boards is uniform and does not vary locally, as contrasted to the different colors of the sap wood and heart wood oi pine and to the local color variations in plywood. The boards prepared from a pressing mixture containing from 10% to moisture at from 300 to 400 pounds pressure are characterized by moduli of rupture in static bending of from 4000 to 5000 pounds per square inch in all directions. With respect to strength in static bending, these boards are one-half as strong as solid wood. one-half as strong as 3-ply plywood with surface plies running in the long direction, and twice as strong as B-ply plywood with surface plies running cross-wise. As to impact resistance these boards compare favorably with inch pine plywood or fir wood of equal thickness or with solid wood of equal thickness, and greatly exceed many conventional building boards. For comparison with the latter, our boards are distinguished by firmer edges that will not splinter like plywood nor dent as easily as plywood or solid lumber when the edges are treated roughly. The surface of our boards resist denting many times better than pine plywood or solid lumber. Our boards shrink or swell but little. For instance, a panel of 3-foot width will swell or shrink only about inch with a moisture change of 6%, while a pine plywood panel will swell or shrink inch and pine lumber panel will shrink or swell l;- inch. Our boards are superior to plywood in resistance against warping and not as liabale to damage on subjection to elevated temperatures. Our boards have surfaces-excellently adapted to receive a finish, such as paint, being more absorptive so that the paint will be more firmly bonded thereto, and the paint coats do not show the hair line checks typical of painted veneers, and due to al ternate transverse swelling and contraction of oriented cellulosic fibers. The initial paint coat applied to our boards yields a finish similar to painted metal. The surface of our boards accepts readily any color stain and the stain will not bring out any local color variations, as is the case with lumber or plywood. The boards may be treated to simulate natural wood grain and the staining characteristics of natural wood by the methods disclosed in said copending application Serial No. 100,004. Our boards are easily machined, with any wood-working machinery, and can consistently be produced with any desired hardness, color, size or other characteristics.

Changes in composition and procedure may be made without departing from the real spirit and purpose of our invention, and it is our intention to cover by our claims any modified forms which may be reasonably included within their scope without sacrificing any of the advantages thereof.

We claim as our invention:

1. In a method of preparing a cellulosic board which comprises providing a mixture comprising mechanically disintegrated wood and a resinforming binder in an amount ranging from about 4% to about 14% by weight of said mixture, said mixture having a moisture content ranging from about 5% to about 25%, subjecting a layer of said mixture at an elevated temperature ranging from about 280 to 400 F. to a pressure ranging from about to about 500 pounds per square inch for at least 3 but less than 10 minutes, said pressure being correlated within said range with lthte moisture content of said mixture as tabua ed:

' Pressure in Pounds Per said elevated temperature being maintained below 360 F. whenever said pressure amounts to at least 400 pounds per square inch, the margins of said layer being compressed more than the remainder of said layer to 'seal said remainder against moisture loss during the pressing operation, and thereafter slowly releasing said pressure to prevent blistering of the compressed board, the board thus prepared having a moisture content of less than 6%, the improvement comprising exposing the compressed board to the atmosphere for cooling the board at least down to 120 F. and thereafter moistening the board with water in an amount sufficient to raise the moisture content of said board at least to 6%.

2. In a method of preparing a cellulosic board which comprises providing a mixture comprising mechanically disintegrated wood and a resinforming binder in an amount ranging from about 4% to about 14% by weight of said mixture, said mixture having a moisture content ranging from about 10% to about 25%, subjecting a layer of said mixture at an elevated temperature ranging from about 280'" to 400 F. to a pressure ranging from about 150 to about 400 pounds per square inch for at least 3 but less than 10 minutes, said pressure being correlated within said range with the moisture content of said mixture as tabulated:

Pressure in Pounds Per Square Inch Minimum Maximum Moisture Content in il-rccm 150 a lo-QO "i 200 i 300 10-15 300 i 400 id to F. and thereafter moistening the board with water in an amount sufficient to raise the moisture content of said board at least to 6%.

3. In a method of preparing a cellulosic board which comprises providing a mixture comprising mechanically disintegrated wood and a resinforming binder in an amount ranging from about 4% to about 14% by weight of said mixture, said mixture having a moisture content ranging from about 10% to about 15%, subjecting a layer of said mixture to a temperature of from about 280 to 400 F. and a pressure of about 300 to 400 pounds per square inch for at least 3 /2 but less than 10 minutes, the margins of said layer being compressed more than the remainder of said layer to seal said remainder against moisture loss during the pressing operation, and thereafter slowly releasing said pressure to prevent blistering of the compressed board, the board thus prepared having a moisture content of less than 6%, the improvement comprising exposing the compressed board to the atmosphere for cooling the board at least down to 120 F. and thereafter moistem'ng the board with water in an amount suflicient to raise the moisture content of said board at least to 6%.

EDWARD A. PA'I'ION.

FORREST F. BEIL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 667,385 Bright Feb. 5, 1901 1,900,166 Dix Mar. 7, 1933 2,033,411 Carson Mar. 10, 1936 2,044,213 Irvine June 16, 1936 2,214,641 Massey et al Sept. 10, 1940 2,314,797 Morris et a1 Mar. 23, 1943 2,348,081 Linzell May 2, 1944 2,402,554 Irvine et al June 25, 1946 2,437,492 Allen Mar. 9, 1948 2,480,851 Goss Sept. 6, 1949

Claims

1. IN A METHOD OF PREPARING A CELLULOSIC BOARD WHICH COMPRISES PROVIDING A MIXTURE COMPRISING MECHANICALLY DISINTEGRATED WOOD AND A RESINFORMING BINDER IN AN AMOUNT RANGING FROM ABOUT 4% TO ABOUT 14% BY WEIGHT OF SAID MIXTURE, SAID MIXTURE HAVING A MOISTURE CONTENT RANGING FROM ABOUT 5% TO ABOUT 25%, SUBJECTING A LAYER OF SAID MIXTURE AT AN ELEVATED TEMPERATURE RANGING FROM ABOUT 280* TO 400* F. TO A PRESSURE RANGING FROM ABOUT 150 TO ABOUT 500 POUNDS PER SQUARE INCH FOR AT LEAST 3 1/2 BUT LESS THAN 10 MINUTES, SAID PRESSURE BEING CORRELATED WITHIN SAID RANGE WITH THE MOISTURE CONTENT OF SAID MIXTURE AS TABULATED: