STEAM PRE-HEATING IN ORIENTED STRAND BOARD PRODUCTION

Abstract: Oriented strand board (10) is produced using a continuous belt press (22) and using an isocyanate binder with steam exposure of the mat (18) of wood material prior to entry into the press. The steam exposure provides moisture which softens wood fibers and enhances lignin flow in the mat. In addition, the moisture reacts with isocyanate binder to produce polyureas from the isocyanate binder (e.g. the preferred binder is pMDI); thereby reducing the cure conditions (i.e. temperature, pressure and time) required for the press to fuse the mat and binder together. The isocyanate binder also reacts with hydroxyl groups in the cellulose and hemicellulose strands of the wood material to allow both chemical and physical bonds to be created with the wood material. The oriented strand board exiting the press has a higher moisture content and more resistant thickness swell under humid conditions.

Description: DESCRIPTION STEAM PRE-HEATING IN ORIENTED STRAND BOARD PRODUCTION Related U.S. Application Data Continuation-in-part of Serial No. 08/968,946, November 12, 1997. Technical Field The invention is generally related to oriented strand board and, more particularly, to improving the production efficiency and product quality in oriented strand board manufacturing. Background Art Oriented strand board is commercially available from a number of companies including J. M. Huber Corporation, Georgia-Pacific corporation, Louisiana-Pacific, and a number of other sources. This material has multiple layers of wood "flakes" or "strands" bonded together by a binding material such as phenol formaldehyde resin or isocyanate resin together with sizing materials such as paraffinic waxes. The flakes are made by cutting thin slices with a knife edge parallel to the length of a debarked log. The flakes are typically 0.01 to 0.5 inches thick, although thinner and thicker flakes can be used in some applications, and are typically, less than one inch to several inches long and less than one inch to a few inches wide. The flakes typically are longer than they are wide. In the fabrication of oriented strand board, the flakes are first dried to remove water, and are then coated with a thin layer of binder and sizing material. The coated flakes are then spread on a conveyor belt in a series of alternating layers, where one layer will have the flakes oriented generally in line with the conveyor belt, and the succeeding layer of flakes oriented generally perpendicular to the conveyor belt, such that alternating layers have flakes oriented in generally perpendicular to one another. The word "strand" is used to signify the cellulosic fibers which make up the wood, and, because the grain of the wood runs the length of the wood flake, the "strands" in the oriented strand board are oriented generally perpendicular to each other in alternating layers. The layers of oriented "strands" or "flakes" are finally subjected to heat and pressure to fuse the strands and binder together. The resulting product is then cut to size and shipped. Typically, the resin and sizing comprise less than 10% by weight of the oriented strand board product. The fabrication of oriented strand boards is described in U.S. Patent 5,525,394 to Clarke et al., and that patent is herein incorporated by reference. Oriented strand board has been used in sheathing walls, wooden I-beam structural supports, and in roofs and floors where strength, light weight, ease of nailing and dimensional stability under varying moisture conditions are the most important attributes. Oriented strand board is sold at a substantial discount compared to structural grades of soft plywood. Several other patents describe the production of oriented strand board. U.S. Patent 5,635,248 to Hsu et al. describes a process for producing a smooth hard finish on products such as oriented strand board. In Hsu, a foamed polymerized latex emulsion is applied to the surface and dried. After drying, the emulsion is crushed, and then cured to form the coating, with post-cure heat treatments being found to improve the hardness of the coating. U.S. Patent 5,554,429 to Iwata et al. discloses an oriented strand board flooring material which is indicated to have significant moisture resistance. In Iwata, the oriented strand board is fabricated with the surface layers having strands with longer average length values and wider average width values than the centrally located layers. Iwata uses a foaming urethane resin and a non-foaming aqueous emulsion-type phenol resin in combination to join the wood strips together. Iwata also contemplates attaching a decorative sheet of material (e.g., oak sheets) to the oriented strand board surface using an aqueous polymeric isocyanate adhesive, and subsequently overcoating the decorative sheet with a polyurethane, thus producing high gloss, decorative, wood flooring which has the appearance of oak. The complete contents of each of these patents is herein incorporated by reference. Recently, efforts have been made to produce oriented strand board on a continuous basis. Prior production systems had utilized a batch press operation to fuse the mat of flakes together. These systems required the use of a saw and separation procedures to isolate individual lengths of matted flakes. Advances were made in presses and shuttling sub-systems which allowed multiple lengths of matted flakes to be simultaneously pressed and fused together. However, despite these advances, it was envisioned that the use of continuous presses would be advantageous in oriented strand board production since their use would eliminate cutting and separating procedures prior to pressing, thereby increasing production capacity. Examples of continuous belt presses for making oriented strand board can be found in U.S. Patent 5,520,530 to Siempelkamp and U.S. Patent 5,596,924 to Gerhardt, and an example of a continuous production process to form particleboard or fiberboard is described in U.S. Patent 5,538,676 to Biefeldt, and the complete contents of each of these patents is herein incorporated by reference. In oriented strand board production processes that utilize continuous belts to move materials through the pressing step, a conveyor moves the mat through two opposing, closely spaced belts which press the flakes together. A pair of heated plates or a heated moveable ram and an opposing table are positioned behind the opposing, closely spaced belts, and provide heat and additional pressing forces. Thus, as the mat is moved through the opposing, closely spaced belts, the wood fibers are pressed together both by the belts and by the plates or ram, and the binder and filler are heated to a point where the individual wood flakes or "strands" are fused to form a continuous flake board or "strand" board product. Disclosure of the Invention According to the invention, isocyanate-based binder materials, such as methylene diphenyl diisocyanate (MDI) and polymeric methylene diphenyl diisocyanate (pMDI), are used in a continuous oriented strand board or "flake board" manufacturing process. A mat of wood flakes or "strands" is produced which includes alternating layers of strands oriented generally perpendicular to each other. The strands are coated with an isocyanate binder, which is preferably pMDI, as well as filler material. The typical filler material is a paraffin based wax; however, wood fines, dyes, flours, and the like may also be included. The mat is carried on a conveyor to a continuous press. Just prior to entering the continuous press, the mat is subjected to a steam treatment which both raises the temperature and the moisture content of the mat. Steam treatment softens the wood fibers and lowers the Tg of lignin to start lignin flow. Due to the use of isocyanate binder materials, the moisture provided by the steam as well as the moisture emanating from the wood material itself chemically reacts with the isocyanate to produce polyureas. In addition, the isocyanate binder materials chemically react with hydroxyl moieties on the cellulose and hemicellulose constituents of the wood material to produce urethane bonds. Heretofore, isocyanate and, in particular, pMDI was not used as the resin or binder material in methods of producing oriented strand board which included the step of pre-heating wood material to obtain desirable moisture levels. This was due to the belief that the presence of pMDI during the pre-heating step would create a condition known as pre-curing whereby the resin would set before the wood material was compressed. However, in the process disclosed herein such pre-curing does not take place. Further, the reactivity of the isocyanates with water and hydroxyl moities allows curing of the binder and fusion with the wood material to be achieved under relatively mild press operating conditions (e.g., temperature, pressure, and time of exposure) as well as at a faster production rate. This, in turn, reduces the amount of volatile organic compound (VOC) emissions produced. Furthermore, the oriented strand board emanating from the continuous press has a relatively high moisture content which allows it to withstand thickness swelling under humid conditions. Brief Description of the Drawings The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of the preferred embodiments of the invention with reference to the drawings, in which: Figure 1 is a cut-away isometric view of an oriented strand board showing the general orientation of wood flakes in the pressed, composite product; Figure 2 is a schematic drawing showing the reaction chemistry of pMDI and phenol-formaldehyde (PF) resin systems; Figures 3a and 3b are a schematic drawings showing the chemical and mechanical bonding of pMDI resin to wood material in oriented strand board, and the mechanical bonding of PF resin to wood material in oriented strand board, respectively; and Figure 4 is a schematic side view of a continuous oriented strand board manufacturing system and method according to the present invention. Best Mode of Carrying Out Invention With reference to Figure 1, it can be seen that oriented strand board 10 is comprised of multiple layers 12, 14, and 16, of wood "flakes" or "strands" which are preferably oriented generally peφendicular to each other in adjacent layers. However, it should be noted that the layers of strands may be oriented parallel to one another or oriented in a variety of other ways. The size of the strands can vary and the number of layers in the oriented strand board can vary to meet a wide range of design requirements. In addition, the size of strands in different layers may also vary. As discussed above, the strands are held together by a binding material, and the oriented strand board typically includes a wax material for sizing. The oriented strand board 10 can be produced by a variety of techniques; however, common to all fabrication processes is a step of subjecting layers 12, 14, and 16 to high temperature and pressure to fuse and bind them together using the binding material. In the practice of this invention the binding material is an isocyanate, and most preferably a polyisocyanate such as pMDI. Figure 2 compares the cure reaction of pMDI to the PF resin systems described in U.S. Patent 5,538,676 to Biefeldt. It is noted that with pMDI, urethane bonds are formed between hydroxyl moieties on the wood surface, such as those which occur along cellulose and hemicellulose chains. In addition, in the presence of moisture, urea bonds are created between pMDI monomers. In sharp contrast, in phenol-formaldehyde resins, the monomer sub-units react via a condensation reaction and release water as a reaction byproduct. Thus, the presence of excessive moisture inhibits curing of PF resin systems. Figure 3a shows that pMDI is chemically and mechanically bonded to wood material, while PF resin systems are most likely mechanically bonded to wood material in oriented strand board production. During fabrication of an oriented strand board, a substantial amount of energy is used to drive off water molecules prior to and during curing of the resin. When an isocyanate resin such as pMDI is used, chemical reactions proceed with the water molecules and hydroxy moieties because they are thermodynamically favored. The net result is that an isocyanate resin system such as pMDI will achieve a stronger fusion with the wood material under reduced temperature and exposure time conditions compared to PF resin systems. Figure 4 illustrates a continuous oriented strand board forming process according to the present invention. A mat 18 of wood flakes progresses from left to right on a conveyor 20 through a continuous press 22 to produce a continuous sheet of oriented strand board 24. The conveyor is preferably coated with a release agent to facilitate the releasing of the board from the press without delamination or blistering. Typical release agents are wax-based release agents such as Blackhawk Specialty Chemical's EX-24 or soap-based release agents such as Houghton International's #8315. As discussed in conjunction with Figure 1, the wood flakes or "strands" are positioned on the conveyor 20 as alternating layers where the "strands" in adjacent layers are oriented generally perpendicular to one another. The number of layers will vary depending on the application and desired thickness of oriented strand board to be produced. Typically, the mat 18 will be 1 to 20 inches thick. The individual strands in the mat will be pre-coated with isocyanate binder, sizing, such as paraffin wax, and/or other materials such as dyes, etc., using conventional processes. The preferred isocyanate binder is pMDI, and it is commercially available from the ICI Polyurethanes Group of New Jersey (as Rubinate pMDI), and from other commercial sources. Preferably, the isocyanate binder comprises about 1.5 to about 8% by weight of the mat, and the sizing materials comprise about 0.5% to about 4% by weight of the mat. Prior to depositing the wood flakes on the conveyor, they are dried. The moisture content of the mat 18 is preferably 2% to 20% by weight. Since pMDI and other isocyanates beneficially react with water, the moisture content need not be as strictly controlled or be as low as that which is employed when PF binder resins are used. Prior to entering the continuous press, the mat 18 of wood material is exposed to a steam treatment by steam sources 26. The steam sources may be positioned on opposite sides of the mat 18. In the preferred embodiment, the conveyor 20 will be made of porous wire material such that steam can penetrate through to the bottom of the mat 18. The steam functions to soften the wood fibers. Steam also lowers the glass transition temperature (Tg) of lignin, and thereby enhances lignin flow in the wood material in the mat 18. As explained above in conjunction with Figures 2 and 3, water enhances the cure reaction of isocyanates thereby allowing relatively lower temperatures and reduced press times to be used in the press than if phenol-formaldehyde resin systems were utilized. In addition, an oriented strand board product 24 having higher moisture content has improved resistance to thickness swelling caused by exposure to humidity. The amount of steam can vary depending on the thickness of the mat, or the desired characteristics of the end product. It is expected that in most applications the steam will raise the temperature in the mat 18 from about 50 to about 95 °C and the moisture content from about 6 to about 24%. Because isocyanates such as pMDI react with water molecules and hydroxyl moities at relatively low temperatures, the temperature and pressure and time of exposure conditions in the continuous press 22 can be reduced compared to when PF resins are used in order to achieve curing. Thus, no additional pre-heat stations are required prior to pressing as is the case in U.S. Patent 5,538,676 to Biefeldt. However, also because of the reaction, it is important that the mat 18 proceed directly into the press 26 so that the binder can fuse with the wood material. In prior art systems, pMDI has been used as a binder for oriented strand board production, but it has not been used in combination with steam pre- treatment. These prior systems employed a batch/shuttle mechanism in combination with a multi-port press such that multiple oriented strand boards would be simultaneously produced in a single pressing; however, the separated strand board sections would need to be stacked and shuttled into position prior to pressing. If steam pre-treatment were employed in these systems, the reaction of the pMDI with water may proceed too rapidly prior to pressing, thus limiting chemical reactions with the wood material and possibly hindering mechanical joining of the isocyanate to the wood (as shown in Figure 3). Hence, prior to this invention, it was believed by those of skill in the art that steam pre- treatment could not be employed in oriented strand board manufacturing which utilized an isocyanate resin such as pMDI. However, because continuous presses 22 can receive and process steam treated mat 18 on a continuous basis, it has been discovered that isocyanates can be used as the binder and that steam pre-treatment can be advantageously employed to achieve benefits in cure/press conditions and benefits in the physical properties of the oriented strand board 24 produced in oriented strand board manufacturing. The continuous press 22 can be similar to those described in U.S. Patents 5,520,530, 5,538,676, and 5,596,924; however, a wide variety of continuous presses can be used in the practice of this invention. The chief requirement for the continuous press 22 is that it be able to continuously take in mat 18 and press and heat the mat 18 to fuse the isocyanate binder to the wood material, and continuously output oriented strand board 24. A continuous press 22 will typically have a pair of closely spaced, opposing conveyors 28, and internal, heated press plates 30 which can be progressively and repetitively moved toward each other. Instead of heated press plates 30, one moveable plate or "ram" and one stationary plate can be used. The heated press plates are responsible for exerting a pressure on the mat material at a temperature which both cures the resin binder and fuses the wood and binder together. The press plates 30 will typically move closer together than the gap between the opposing conveyors 28, and the distance between the press plates 30 can be varied to accommodate the production of oriented strand board 24 of differing thicknesses. The temperature employed in the press 22 can vary depending on the application and properties of the oriented strand board to be produced, as well as the time period to traverse the press 22. In most applications employing isocyanate resin binders, the temperature of the belts in the press 22 will range from about 120 to about 260°C. When pMDI is used as the binder, the preferred temperature for the belts in press 22 ranges from about 175 to about 227°C. It should be apparent to those skilled in the art that the temperature can be varied to achieve similar end product results by varying the pressure and/or residence time in the press 22. The pressure exerted by the press plates 30 can be varied in a similar manner to the temperature. In most applications in the practice of this invention the maximum pressure will range from about 300 to about 900 psi. Likewise, the residence time in the press 22 can be varied and is dependent on the length of the press 22, the speed of the conveyor 20, and the thickness of the panel. In most applications in the practice of this invention the residence time will range from 0.5 to 10 minutes. The residence time in the press and product properties may also be modified through the addition of catalysts or polyols to the isocyanate or pMDI binder, or to the wood strands. It is preferred that the temperature, pressure, and time in the press 22 be selected to allow for complete curing of the isocyanate resin and fusion with the wood material. These operational specifications for the press, as well as the moisture content of the mat 18 as adjusted by the steam generators 26 can be varied to achieve the continuous production of oriented strand board 24 of desired moisture content. Experiments have shown that higher moisture content oriented strand board 24 is more resistant to thickness expansion and linear expansion resulting from exposure to humidity. In the preferred embodiment, the parameters used in the press and the steam generator will produce oriented strand board 24 having a moisture content ranging from about 4 to about 12% by weight. While the invention has been described in terms of its preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims. By way of example, and not limitation, the steam pre-heating process described above may be utilized in the production of other engineered wood products such as wood composite lumber, rim board, webstock, particleboard and fiberboard.

Claims: CLAIMS 1. A method for producing oriented strand board, comprising the steps of: providing a quantity of wood material present in the form of strands; coating said wood material with an isocyanate-based binder; forming a mat from said wood material wherein said wood material is layered with alternating layers and wherein said strands in adjacent layers are oriented generally peφendicularly to each other; exposing said mat of wood material to steam; and pressing said mat of wood material with a continuous belt press to fuse said isocyanate-based binder and said wood material together to form an oriented strand board. 2. The method of claim 1 wherein said step of exposing raises a temperature in said mat from about 50 to about 95°C. 3. The method of claim 1 wherein said step of exposing raises a moisture content in said mat from about 6 to about 24% by weight. 4. The method of claim 1 wherein said step of pressing is performed at a temperature of from about 120 to about 260°C, at a pressure of from about 300 to about 900 psi, and for a time period of from about 0.5 to about 10 minutes. 5. The method of claim 1 wherein said isocyanate-based binder is pMDI. 6. The method of claim 5 wherein said binder includes an agent selected from the group consisting of catalysts and polyols. 7. The method of claim 4 wherein said oriented strand board has a moisture content of from about 4 to about 12% by weight. 8. The method of claim 1 wherein said isocyanate binder is pMDI, and said pMDI comprises of from about 1.5 to about 8 wt % of said mat of wood material. 9. The method of claim 1 wherein said pressing step is performed within about 0.5 to about 10 minutes after said step of exposing. 10. A method for producing an engineered wood product, comprising the steps of: providing a quantity of wood material present in the form of strands; coating said wood material with an isocyanate-based binder; forming a mat from said wood material wherein said wood material is layered; exposing said mat of wood material to steam; and pressing said mat of wood material with a continuous belt press to fuse said isocyanate-based binder and said wood material together to form an engineered wood product. 11. The method of claim 10 wherein said wood material is layered with alternating layers having strands oriented generally peφendicularly to each other. 12. The method of claim 10 wherein said step of exposing raises a temperature in said mat from about 50 to about 95°C. 13. The method of claim 10 wherein said step of exposing raises a moisture content in said mat from about 6 to about 24% by weight. 14. The method of claim 10 wherein said step of pressing is performed at a temperature of from about 120 to about 260°C, at a pressure of from about 300 to about 900 psi, and for a time period ranging from about 0.5 to about 10 minutes. 15. The method of claim 10 wherein said isocyanate-based binder is pMDI. 16. The method of claim 15 wherein said engineered wood product has a moisture content ranging from about 4 to about 12% by weight. 17. The method of claim 10 wherein said isocyanate-based binder is pMDI, and said pMDI comprises about 1.5 to about 8 wt % of said mat of wood material. 18. The method of claim 10 wherein said pressing step is performed within about 0.5 to about 10 minutes after said step of exposing. 19. A method for producing oriented strand board, comprising the steps of: forming a mat of wood material present in the form of strands wherein said wood material is layered, and wherein said wood material is coated with an isocyanate-based binder; exposing said mat of wood material to steam; and pressing said mat of wood material with a continuous belt press to fuse said isocyanate-based binder and said wood material together. 20. The method of claim 19 wherein said step of exposing raises a moisture content in said mat from about 6 to about 24% by weight. 21. The method of claim 19 wherein said isocyanate-based binder is pMDI. 22. The method of claim 21 wherein said binder includes an agent selected from the group consisting of catalysts and polyols. 23. An engineered wood product having a moisture content of from about 4% to about 12% produced by providing a quantity of wood material present in the form of strands, coating said wood material with an isocyanate-based binder, forming a mat from said wood material wherein said wood material is layered, exposing said mat of wood material to steam, and pressing said mat of wood material with a continuous belt press at a temperature of from about 120 to about 260°C and a pressure of from about 300 to about 900 psi to fuse said isocyanate-based binder and said wood material and form the engineered wood product.

Inventors: CARLSON BRIAN S, PALARDY ROBERT D, CARLSON, BRIAN, S., PALARDY, ROBERT, D.
Applicants: CARLSON BRIAN S, HUBER CORP J M, PALARDY ROBERT D, CARLSON, BRIAN, S., J. M. HUBER CORPORATION, PALARDY, ROBERT, D.
Application year: 1998
Main classification: B29C43
IPC codes: B27N3, B32B21