Patent Publication Number: US-2005140058-A1

Title: Apparatus and method for continuous formation of composites having filler and thermoactive materials, and products made by the method

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application is a continuation-in-part application of copending U.S. patent application Ser. No. 08/988,680, filed on Dec. 11, 1997, which is incorporated herein by reference. 
    
    
     FIELD  
      This invention concerns an apparatus and method for making composite products, including board products, that includes applying a hot, dry gas to filler and thermoactive materials, particularly cellulosic and thermoplastic materials, in the continuous production of the composites.  
     BACKGROUND  
      Products that combine wood materials with thermoplastic materials are known. These products generally are made using batch processes, such as processes that employ heated platens to apply heat and a compression force to the substrate, instead of continuous processes.  
      Recently, products comprising waste plastics and waste cellulosic materials have been developed, most of which are made by extrusion or injection-die methods. Examples of patented inventions concerning wood/plastic composite products include: 
          (a) Smith&#39;s U.S. Pat. No. 3,995,980, which describes forming mixtures of materials using three separate delivery systems, and thereafter extruding products comprising the mixture;     (b) Goforth et al.&#39;s U.S. Pat. No. 5,088,910, which describes an extrusion process for making synthetic wood products from recycled materials, such as low or high density polyethylene;     (c) Wold&#39;s U.S. Pat. No. 5,435,954, which discusses a method for forming wood-plastic composites comprising placing mixtures of such materials in molds and subjecting the mixture to sufficient temperatures to cause the material to occupy the mold and assume its shape; and     (d) Reetz&#39; U.S. Pat. Nos. 5,155,146 and 5,356,278, incorporated herein by reference, which describe extrusion apparatuses and processes for processing charges that include expanded thermoplastic materials, such as polystyrene.        

      There are several disadvantages associated with the inventions discussed above. A principal problem associated with extrusion and injection methods is that the particle size of the materials used to form the composite must be fairly small. Otherwise, the particle size of the composite mixture is too high to be extruded or injection molded efficiently. Moreover, extrusion and injection processes are further limited by the ratio of filler materials, such as wood, to the thermoactive materials that can be used in the charge (i.e., the mixture of filler material and thermoactive material used to form the final product). This puts undesirable constraints on the products that can be produced.  
      Another problem associated with these prior processes and apparatuses involving heated platens is that they produce products batchwise, instead of continuously. This substantially reduces product throughput. For example, heated platens take too long to heat composites completely throughout their cross section. If the temperature of the platens is increased too much in an effort to speed production, the composite product may burn or scorch, particularly at temperatures above about 400° F. Moreover, many processes that use platen presses require that the platen not only be heated but also cooled during each production cycle. This decreases product throughput and is expensive in view of the energy required to complete the serial heating and cooling steps.  
      Steam injection processes also can be used to produce composites. However, the initial steam heating stage is followed by continued heating to remove all of the water applied to the composite during the steam injection process. The combination of heating the composite to form products, followed by continued heating to remove water, requires a longer period of time and is more expensive than is desirable in a commercial process.  
      German Patent No. 14 53 374 (the &#39;374 patent) describes a continuous process for forming composites comprising waste plastic and waste wood. A mixture of waste plastic and waste wood is pressed in the nip between two rollers and hot air is applied to the substrate as it travels around the rollers. The structural features of the apparatus described in the &#39;374 patent are limiting. For example, the &#39;374 patent teaches applying hot gas to only one of the two major opposed surfaces of a substrate at a time. As the substrate passes over one roller gas is applied to one surface, then as the substrate passes over a second roller, hot gas is applied to the opposite surface. There is considerable energy loss, and therefore added expense, as a result of heated gas being vented to the atmosphere after passing through the composite. This also may present a health problem in that vented gas may include volatile organic compounds (VOCs) that present a health risk.  
      Furthermore, by following the radius of the rollers, the outside surface of the mat is stretched relative to the inside surface. This causes fissures on the outside surface. Such fissures are undesirable when manufacturing a thick product in combination with an outside surface comprised of a fines material (i.e., an outside surface comprised of small particles).  
      Despite the inventions discussed above, there still is a need for an effective and efficient apparatus and method for continuously forming composite products.  
     SUMMARY  
      The present invention overcomes the difficulties of the prior art by providing an effective and efficient composite consolidation apparatus and method for continuously forming composite products comprising filler materials and thermoactive materials. The apparatus and method are particularly suited for forming composites comprising waste cellulosic materials and waste thermoplastics.  
      One embodiment of the consolidation apparatus includes a hot-gas distribution system having at least one pair of gas cells, more typically plural paired gas cells, such as rollers or hoods, for applying hot air to the charge. A first cell of each pair applies gas to the charge, and generally is referred to as an application roller. The second cell of each pair, referred to as a suction roller, operates at a pressure less than the application roller, i.e., a pressure differential exists between the application roller and the suction roller. Certain embodiments of the apparatus include at least one set of baffles positioned adjacent a cell, at least one shroud positioned about a cell, or at least one set of baffles positioned adjacent a first cell and at least one shroud positioned about a second cell to eliminate or substantially reduce the amount of gas that is vented to the surrounding atmosphere.  
      The consolidation apparatus can be used in combination with other apparatuses to form a system. One embodiment of the system comprises: (1) a mixer, such as a cyclone, for continuous or batchwise formation of mixtures of filler material and thermoactive material; (2) optionally a prepress for optional densification of the mixture prior to subsequent treatment; (3) a consolidation apparatus having a thermal consolidation zone, and perhaps a densifying zone, for continuously applying hot-gas to a moving charge, the zone having at least one pair of and perhaps plural paired gas cells wherein a first cell of each pair applies gas to the moving charge and wherein a second cell of each pair operates at a pressure less than in the first cell; and (4) a mechanical densifying apparatus for applying a densifying pressure to the charge downstream of the consolidation zone. The system may further include a mat-forming apparatus downstream of the mixer and upstream of the consolidation zone.  
      The invention further comprises a method for continuously forming composites. A mixture is formed comprising a waste thermoactive material and a waste filler material. The mixture is then continuously consolidated by applying a hot, dry noncondensable gas to the mixture. The apparatus described above may be used to continuously apply the gas to the mixture, and the mixture may move continuously through a zone where the consolidating gas is applied. Generally, but not necessarily, the filler material comprises cellulosic material, and the thermoactive material is a thermoplastic material. The mixture may further include materials selected from the group consisting of biocides, fungicides, fire retardants, conductive materials, pigments, water retardants, wax-like materials, coupling agents, crosslinking agents, and combinations thereof.  
      Still another embodiment of the present invention comprises a method for forming a product from cellulosic material and thermoactive material. The method comprises first providing a mixture comprising a predetermined amount of cellulosic material and a predetermined amount of thermoplastic material. A mat is formed from the mixture, and the mat is then consolidated. The consolidated mat is pressed to a predetermined final product thickness. At least a portion of a surface of the mat or product is then surface modified. Consolidating the mixture into a mat can include forming a first face layer of the face mixture, forming a core layer of the core layer mixture on the first face layer, and forming a second face layer of the face layer mixture on the core layer. Forming the core layer may include depositing mixtures of wood flakes and plastic flakes according to mixture flake size in successive layers such that the core layer has larger flakes adjacent the first face layer and adjacent the second face layer, and smaller flakes between the larger flakes in a middle portion of the core layer. The method also can include fusing a sheet comprising a plastic material to least one major planar surface of the mat. The plastic material can then be surface modified, such as by e-beam or flame treating the material. Grafting chemicals can be applied to surface modified mats, plastic material or products.  
      A method for forming a product having a first face layer and a core layer also is described. The method comprises preforming a first face layer to form a preformed a first face layer. A core layer is deposited on an upper surface of the preformed first face layer.  
      A method for forming a face layer for a composite product also is described. The method comprises making a face layer mixture comprising predetermined amount of thermoplastic material and cellulosic material. The mixture is deposited on a forming surface, and the mixture is then heated to an activation temperature. The method can also include applying heat energy to the mixture to form a melted face layer.  
      A composite product also is described. The composite comprises a first portion comprising a first core formed from a mixture comprising a cellulosic material and a thermoplastic material, and at least one face layer attached to at least one major planar surface of the first core. The at least one face layer is formed from a mixture comprising a cellulosic material and a thermoplastic material. The product also comprises a second portion bonded to the first portion. The portion comprises a second core formed from a mixture comprising a cellulosic material and a thermoplastic material, and at least one face layer attached to at least one major planar surface of the second core, the at least one face layer is formed from a mixture comprising a cellulosic material and a thermoplastic material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a flow chart illustrating certain process steps used to form composites that include filler materials and thermoactive materials in accordance with the invention.  
       FIG. 2  is a schematic, side elevational view illustrating a cyclone mixer for mixing filler and thermoactive material in accordance with the invention.  
       FIG. 3  is a schematic, longitudinal sectional view of an embodiment of a continuous consolidation and densifying apparatus in accordance with the invention.  
       FIG. 4  is a partial schematic longitudinal sectional view showing a portion of a continuous consolidation apparatus in accordance with a second embodiment of the invention.  
       FIG. 5  is a schematic longitudinal sectional view showing a third embodiment of a continuous consolidation apparatus in accordance with the invention, including a continuous foraminous conveying belt.  
       FIG. 6  is a schematic longitudinal sectional view showing a fourth embodiment of a continuous consolidation apparatus in accordance with the invention having plural hoods for applying hot gas to a charge and removing the gas after it passes through the charge.  
       FIGS. 7, 8  and  9  are front, side and plan views, respectively, of a specific embodiment of an alternative consolidation apparatus.  
       FIG. 10  is a slightly enlarged side view of the apparatus of  FIG. 8  with some detail removed to shown the path of the conveyers through the apparatus.  
       FIG. 11  is an enlarged view of the delivery end of the apparatus of  FIG. 8 .  
       FIG. 12  is a sectional view of the apparatus of  FIGS. 7, 8  and  9  taken along line  12 - 12  in  FIG. 11 .  
       FIG. 13  is a sectional view of the apparatus of  FIGS. 7, 8  and  9 , except without the extensions rollers and the auxiliary roller, taken along line  13 - 13  in  FIG. 11 .  
       FIG. 14  is an enlarged view of a take-up roll.  
       FIG. 15  is a schematic view of a process line showing apparatus for blending, conveying, face forming, consolidating, laminating, pressing and surface treating.  
       FIG. 16  is a flow chart illustrating process steps used to prepare plastic material for blending.  
       FIG. 17  is a flow chart illustrating process steps used to prepare wood material for blending.  
       FIG. 18  is a flow chart illustrating process steps for a specific implementation of forming a board product.  
       FIG. 19  is a partial perspective view of a board product with a core and two faces.  
       FIG. 20  is a magnified cross-sectional view of the core showing the size distribution and orientation of the flakes.  
       FIG. 21  is a schematic side view of a specific implementation of a face forming apparatus.  
       FIG. 22  is a schematic side view of another specific implementation of a face forming apparatus.  
       FIG. 23  is a schematic perspective view of a lamination apparatus showing laminate being applied to both major surfaces of a board product.  
       FIG. 24  is a schematic perspective view of a surface treatment apparatus showing both major surfaces of a board product being subjected to surface treatment.  
       FIG. 25  is a partial perspective view of a board product made by joining two previously formed board products together.  
       FIG. 26  is a partial perspective view of a board product made by joining two previously formed board products together at their core sides.  
       FIG. 27  is a schematic side view of a screening apparatus.  
       FIG. 28  is a schematic top view of a first stage of the screening apparatus of  FIG. 26 . 
    
    
     DETAILED DESCRIPTION  
      The flow chart of  FIG. 1  illustrates certain process steps used to form composite products that include filler materials and thermoactive materials. The first steps in the process require selecting appropriate filler material, selecting appropriate thermoactive material, and thereafter forming a mixture comprising such materials. The mixture may be used as a charge for the continuous consolidation apparatuses illustrated in  FIGS. 3-6 . Alternatively, the mixture may be processed before being consolidated by the apparatuses of  FIGS. 3-6 , such as by using a preliminary preheating and/or pressing stages to provide an intermediate substrate. One example of an intermediate substrate suitable as a charge for the illustrated continuous consolidation apparatuses is a mat of the composite material. Mats can be formed using conventional apparatuses known in the art.  
      The apparatuses illustrated in  FIGS. 3-6  continuously consolidate charges in a consolidation stage by applying hot gas thereto using the illustrated hot-gas distribution systems. As used herein, “consolidates” or “consolidation,” means that the mixture of filler and thermoactive material is processed from a first initial density to a second, greater density of from about 2.5 to about 60 pounds per cubic foot (pcf), and more typically from about 3 pcf to about 50 pcf, preferably about 25 pcf upon exit from the consolidation apparatus. The second, greater density results, for example, as the thickness dimension of the charge decreases upon application of the hot gas (i.e., thermal consolidation), and perhaps a simultaneous densifying force (mechanical consolidation), thereto. It also should be appreciated that the density of the charge may be serially increased by thermal and/or mechanical consolidation as the charge moves through the consolidation zone.  
      As indicated by  FIG. 1 , the consolidated product may then be further compressed to an even greater density in a densifying stage, such as by using a conventional press to a density of about 50 to about 60 pcf, preferably about 50 to about 52 pcf. However, the apparatuses of  FIGS. 3-5  may be designed to both compress the charge and consolidate the charge to a greater density than could be achieved by hot gas consolidation alone. And, each pair of cells forming the apparatus may increase the force applied to the charge moving through a consolidation zone. Alternatively, the apparatuses may include (1) a first consolidation stage wherein the density of the charge generally increases by application of the hot gas, and (2) a second densifying stage wherein greater compression forces, and perhaps cooler temperatures than in the heating stage, are applied to the composite product to achieve the product&#39;s final desired density, as shown in  FIG. 3 .  
      The preferred materials, without limitation, for preparing the composite products comprise waste cellulosic materials and waste thermoactive materials, such as waste plastics. Each of these materials is described below, followed by a discussion of the apparatuses illustrated in the drawings.  
     I. Materials for Forming Composites  
     A. Filler Materials  
      Without limitation, a partial list of filler materials includes all natural and synthetic fibers, examples of which include cellulosic materials, carbon-based materials such as carbon fibers, glass fibers, and mixtures of these materials. A currently preferred filler material is cellulosic material.  
      The cellulosic material may be virgin wood materials, i.e., materials that have not been used previously to form products, such as wood chips, sawdust, cotton, hemp, straw, or combinations of such materials. Alternatively, the cellulosic material may comprise waste products, such as used paper, peanut shells, used cotton, used railroad ties, fibers derived from paper mill sludge, fibers derived from recycling mill sludge, and combinations of such materials. Moreover, the cellulosic material may comprise virgin materials mixed with waste materials.  
      Single-layer products made in accordance with the present invention typically include both cellulosic materials and plastic materials where the average particle size that ranges anywhere from about {fraction (3/16)} inch in length to about {fraction (3/4)} inch in length. The strength of the product may be affected by the size of the particles used to form the board product, but cellulosic and plastic materials having particle sizes that range anywhere from about {fraction (3/16)} inch in length to about 3 inches in length (preferably about 1½ inches) have been found suitable for making single-layer products, or the core portion of multilayered board products. Multilayered products made in accordance with the present invention often have one or more layers that include “fines,” i.e., materials having an average particle size of less than about {fraction (3/16)} inch, and more typically having a particle size so that approximately 80% of the particles pass through a 14 mesh size screen.  
      In a first generation without orientation, the aspect ratio of the flakes ranged from about 2:1 to about 8:1, and preferably from about 2:1 to about 6:1. With orientation, the aspect ratio of the largest flakes is about 16:1 to about 20:1.  
     B. Thermoactive Materials  
      The filler material is mixed with a thermoactive material. “Thermoactive” refers to both thermoset and thermoplastic materials. Thermoplastic materials generally are preferred materials because waste thermoplastics can be remelted, allowing the melted thermoplastic material to wick along and to flow around the filler materials. The thermoactive materials act as binders for the filler particles once the thermoactive materials are heated to a temperature sufficient to make them flow, in the case of thermoplastics, or heated to the cure temperature in the case of thermoset materials.  
      As with the filler material, the thermoactive material may be any material now known or hereafter discovered that is useful for forming composite products. Moreover, the thermoactive material may be virgin, i.e., materials that have not been used previously for any purpose. Alternatively, the thermoactive material can be a waste material, particularly waste thermoplastic materials.  
      Examples of suitable thermoactive materials include, but are not limited to: polyamides and copolymers thereof; polyolefins and copolymers of polyolefins, with particular polyolefin examples including polyethylene, polypropylene, polybutene, polyvinyl chloride, acrylate derivatives, acetate derivatives, etc; polystyrene and copolymers of polystyrene; polycarbonates; polysulfones; polyesters; polyvinyl chloride; polyvinylidene chloride; copolymers of vinyl chloride and vinylidene chloride; and mixtures of these materials.  
      This list should not be considered an exhaustive list of thermoactive materials that can be used to form composites. Any readily available, relatively nontoxic thermoactive material which (1) can be made to flow to coat filler fibers or particles, or which can be heated to a curing temperature, and (2) which materials act as suitable binders for the fibrous material, can be used.  
     C. Additional Materials  
      The composites that are produced according to the present invention are not limited to having only filler materials and thermoactive materials. A partial list of additional materials that can be used to form such composites includes preservatives, biocides, fungicides, fire retardants, conductive materials such as carbon black, pigments, water retardants, wax-like materials, coupling agents (which are used to enhance the interaction between the filler material and the thermoactive material), crosslinking agents, and combinations thereof.  
      Crosslinking agents have been found to decrease the creep observed with composite products made in accordance with the present invention. “Crosslinking” refers to reactions that occur with thermoactive materials, either intermolecularly or intramolecularly, most typically intramolecularly, and is distinguished from coupling agents which form bonds between thermoactive materials and the cellulose. See the examples provided below for more detail concerning crosslinking the thermoactive materials and creep. A number of crosslinking agents can be used to practice the method of the present invention. For example and without limitation, suitable crosslinking agents can be selected from the group consisting of organic peroxides, such as dicumyl peroxide, t-butyl peroxide, benzoyl or dibenzoyl peroxide, t-butyl peroxybenzoate, butyl 4,4-di-(t-butylperoxy)valerate, t-butyl cumyl peroxide, di-(2-t-butylperoxyisopropyl)benzene, di-2,4-dichlorobenzoylperoxide, 1,1-di-(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne, azonitriles, such as 2,2′-azobisisobutyronitrile, azo-type derivatives, such as 2,2-azoisobutene and triazobenzene, and other free-radical generators, such as benzenesulfonyl azide and 1,4-dimethyl-1,4-diphenyltetrazene, and any combination of these crosslinking agents. Particularly suitable crosslinking agents are selected from the group consisting of dicumyl peroxide, t-butyl peroxide, benzoyl or dibenzoyl peroxide, t-butyl peroxybenzoate, and combinations thereof, with dicumyl peroxide being a currently preferred crosslinking agent for use in making cellulose/thermoactive composites according to method of the present invention.  
      Generally, the crosslinking agents are mixed with the thermoactive component or components prior to forming mixtures comprising the thermoactive component/crosslinking materials and cellulose. This can be accomplished in a batch process by forming a solution, typically an organic solution, comprising a crosslinking agent or agents, and then applying the solution to the thermoactive material. Alternatively, the thermoactive material may be immersed in the solution comprising the crosslinking agent. In a continuous commercial process, the crosslinking agent likely will be applied to the thermoactive material by atomizing liquid crosslinking agent, or a solution comprising the crosslinking agent, and spraying the atomized material onto the thermoactive material.  
     II. Mixing Filler and Thermoactive Materials  
      Once the desired materials are selected as described above, the materials are then combined to form a mixture. The materials may be mixed by hand or by using a hand actuated mixer. However, for commercial production it is preferred to mix the materials using a large-capacity, continuous or batch blending apparatus that tumbles, oscillates, shakes, or otherwise thoroughly mixes the materials. Such apparatuses are referred to herein as mixers.  
      The filler material and the thermoactive material may be mixed using a cyclone mixing and/or heating apparatus  10  illustrated in  FIG. 2 . Cyclone  10  also can be used solely as a heating chamber for preheating a previously formed mixture of filler material and thermoactive material prior to the mixture being consolidated in one of the apparatuses of  FIGS. 3-6 . Cyclone  10  includes a top  12 , walls  14 , and a bottom outlet  16 . Cyclone  10  also includes a gas supply conduit  18  which passes through wall  14 . Gas conduit  18  is coupled to a gas heater  20  and conveys hot, pressurized gas from a gas source (not illustrated) to interior region or chamber  22  adjacent top  12  of cyclone  10 . The heater heats the gas to a temperature of from about 250EF to about 600EF. Gas conduit  18  is coupled to wall  14  so as to substantially prevent the hot gas from being vented to the atmosphere.  
      Cyclone  10  also includes at least one additional supply conduit  24  that passes through wall  14  and into the interior region  22 . If the cyclone  10  is used solely to preheat the filler material and thermoactive material, then the conduit  24  transports a preformed mixture of these materials to the interior  22  of the cyclone  10 . Alternatively, if cyclone  10  is being used as both a mixing and heating chamber, then the cyclone  10  may include a third supply conduit  26 . One of the conduits  24  and  26  transports comminuted filler material from a filler material storage unit (not illustrated) to interior region  22 . The other of the conduits  24  or  26  transports comminuted thermoactive material from a thermoactive material storage unit (also not illustrated) to interior region  22 .  
      The cyclone  10  is capable of performing several functions, including forming mixtures, heating premixes of suitable mixtures, and simultaneously heating and forming mixtures. The mixing and/or heating functions occur in interior chamber  22 . Filler material and thermoactive material naturally descend in a cyclonic flow path  23  towards, and eventually through, outlet  16  and onto a conveyor  28 . Conveyor  28  conveys the filler-thermoactive material composition to the consolidation apparatuses illustrated in  FIGS. 3-6 .  
      From the foregoing, it will be apparent that cyclone  10 , when continuously supplied with filler and thermoactive materials, either separately or in a premix, provides a continuous mixer, and perhaps heater, for the materials. As a result, a mixture or hot mixture may be supplied in a continuous stream, or charge, to the conveyor  28 .  
       FIG. 2  also shows that cyclone  10  may include a hot gas exhaust and recycling conduit  30 . This conduit is used to recycle gas from the interior region  22  back to gas heater  20 . Alternatively, recycling conduit  30  may be used to supply hot gas to the hot gas distribution systems illustrated in  FIGS. 3-6 .  
      Plural cyclones similar to cyclone  10  also may be used. For example, two or more cyclones  10  can be arranged adjacent each other to deliver mixtures onto a conveyor to positions adjacent each other across the width of a conveyor. This arrangement of plural cyclones  10  can be used to form mats and other charges.  
      Once formed and deposited on conveyor  28 , the mixture should be sufficiently permeable to a hot, dry noncondensable gas (discussed in more detail below) so as to allow the hot gas to circulate throughout the composite. The gas circulation can be affected by the ratio of the filler material to the thermoactive material. This ratio is best determined by reference to the attributes desired in the final product. In general, mixtures comprising a 9:1 ratio, by weight, of filler-to-thermoactive materials to 9:1 ratio, by weight, of filler-to-thermoactive materials can be used. Working embodiments of the invention have made mixtures comprising roughly a 1:1 ratio, by weight, of filler particles and thermoactive materials, and currently it is believed that the best results are obtained when the filler materials comprise about 60 weight percent or less of the mixture. Dispensing materials by volume would also be possible, particularly if the bulk density of the materials can be adequately controlled.  
      The filler particles and plastic particles may be of different sizes and shapes; however, it has been found that the best results, in terms of obtaining a thoroughly mixed material, are obtained when the filler particles or fibers and the plastic particles or fibers are of roughly the same size and shape. Moreover, the larger the particle size, the more time it takes to melt solid thermoactive materials, and the less thoroughly covered are the filler materials by the thermoactive materials. Thus, powdered filler material and thermoactive materials may be used. The particles also generally are mixed at ambient temperatures and under relatively dry conditions, i.e., no added water is used during the formation of the mixture. Additional materials, as discussed above, may be mixed with the filler and thermoactive materials in the mixer.  
     III. Continuous Consolidation  
     A. Background  
      One primary advantage of the present invention is that it allows for the continuous, thermal consolidation, and if desired, mechanical densification, of mixtures continuously supplied as described above. Steam can be used to form the composites by thermal consolidation. However, dry, noncondensable gases, particularly air, are best used for the hot-gas consolidation process. “Dry” refers to a gas in which water is not a major component, although “dry” does include materials that have some water or water vapor. For example, air generally includes some water, the amount depending upon the location. “Dry” does not include gases wherein a major fraction is water, and preferably does not include materials wherein the amount of water exceeds the saturation point of the gas at room temperature.  
      “Noncondensable” refers to materials that remain in a gaseous state at ambient conditions. One benefit of using a noncondensable gas is that the pressure and temperature of the gas can be independently controlled. This generally is not true for condensable gases, such as steam. When steam is used as the medium for applying heat to the composite, relatively high pressures must be used in order to maintain the gas at the desired temperature.  
      There a number of gases that satisfy the stated criteria for a dry, noncondensable gas. Such gases include, without limitation, air, nitrogen, carbon dioxide, and combinations of these and other gases.  
      The temperature of the gas also is an important consideration. For thermoactive materials, the temperature generally must be high enough to “activate” the material. With reference to thermoplastic materials, this generally means that the temperature is sufficiently high to allow the thermoplastic material to become more flowable, i.e., less viscous in nature, so that the material can flow over and around the filler materials. For thermoset materials, there generally is no precise temperature at which the material cures. Generally, the cure rate for thermoset materials depends upon the temperature, i.e., there is a direct correlation between temperature and cure rate.  
      Some guidance can be provided for selecting an appropriate activation temperature for a given thermoplastic or thermoset material. However, it also should be appreciated that the precise activation temperature depends on a number of factors. A partial list of such factors would include the particular materials being used to form the composite, the thickness of the composite, the ability of the materials forming the composite to absorb heat, and the heat capacity or insulating properties associated with the apparatus used to thermally consolidate, and perhaps mechanically densify, the composite while being heated or heated and densified.  
      Thermoplastic materials generally have an activation temperature in the range of from about 250EF to about 600EF, and more typically from about 400EF to about 600EF. For thermoset materials, curing may begin at temperatures of as low as about 100EF, although higher temperatures also may be used. The cure rate of thermoset materials also may be enhanced, and the curing temperature lowered, by using catalysts.  
     B. Consolidation System  
       FIG. 3  illustrates an apparatus  40  for thermally consolidating and, if desired, mechanically densifying, a filler-thermoactive material charge. Gas-permeable conveyor  28  delivers to apparatus  40  continuously a charge  42  comprising a mixture of thermoactive material and filler, as supplied, for example, from cyclone  10 . Charge  42  may be a loose mixture of thermoactive material and filler, known in the art as a fluff, or may be in the form of a partially consolidated mat formed in a pre-consolidation step, which is not shown.  
      Charge  42  is moved into an enclosed consolidation and heating zone  44  by conveyor  28  through inlet  46 . Zone  44  substantially reduces or prevents exposure of people adjacent the apparatus to volatile organic compounds (VOCs) by acting as a containment hood to remove fumes, fines and VOCs that may be emitted during the consolidation process. The enclosed consolidation zone also helps minimize heat loss from the hot gas to the surroundings.  
      Consolidation zone  44  houses a plurality of hot-air distribution cells, one embodiment of which comprises perforated or otherwise gas-permeable rollers  50   a - 50   h  arranged in pairs on opposite sides of a charge  42 , for applying hot gas to and drawing hot gas at least partially into and perhaps through charge  42 . The actual number of rollers  50  used in a particular embodiment is not critical, and is more likely defined by processing times, production rate, nature and size of the filler and thermoactive materials, and characteristics desired in the final product.  FIG. 3  illustrates eight rollers  50   a - 50   h  arranged in pairs to engage the major opposed surfaces of charge  42 . For example, roller  50   a  is paired with roller  50   b.    
      Apparatus  40  also includes at least one additional paired set of rollers  52   a ,  52   b  located in a region exterior to zone  44  in a densifying stage of the apparatus downstream from the described consolidation stage. In the illustrated embodiment, hot-gas distribution rollers  50   a - 50   h  consolidate charge  42  from a first density, i.e., the density of charge  42  prior to entering zone  44 , to a second density. This is illustrated in  FIG. 3  as a decrease in the thickness of charge  42  from a first thickness to a second thickness in zone  44 . Rollers  52   a ,  52   b  apply positive pressure to the charge  42  to densify the charge from the second density and thickness to a third density and a thickness. The third density and thickness may be those of the final product, or there may be an additional densifying stage (not illustrated) subsequent to the densification stage represented by rollers  52   a ,  52   b.    
      Apparatus  40  includes a hot gas distribution system for applying hot gas to, and into, charge  42 . The flow of gas through the system can be either counter to the direction the charge  42  moves, or it can be in the same direction the mat moves through the apparatus. Currently, the preferred flow of gas through the system is indicated by arrows  54 , which show that the hot gas flows in a direction counter to the movement of charge  42  through apparatus  40 . Hot pressurized gas from source  56  flows through checkpoint  58  in the direction of arrow  54 . Gas checkpoint  58  may include both pressure and temperature sensors to monitor the pressure and temperature of the gas as it flows through checkpoint  58  and into first densifying roller drum  52   a.    
      Each pair of rollers is coupled so that one is a hot gas application roller and the other of the pair is a suction or evacuation (if a vacuum pump is used) roller. In other words, a pressure differential is created across the pair of rollers. The gas application roller applies gas to one major surface of the charge  42  while the evacuated roller helps draw gas through the charge  42  and into the evacuated roller. For example, with the arrow  54  indicating flow direction, roller  52   a  operates as a hot gas application roller and roller  52   b  operates as an evacuated roller, thus creating a pressure differential across the charge to help the hot gas penetrate the charge and thus perform its consolidation function.  
      Each roller  50   a - 50   h  and  52   a ,  52   b  is substantially identical and includes a stationary central region  60  for receiving hot gas from or directing the gas to charge  42 , depending upon the function of the roller as either an application or suction or evacuation roller. As an application roller, hot gas feeds into roller  52   a  by a hot gas conduit (not illustrated) and into central portion  60 . Central portion  60  is fluidly coupled to a hot-gas distribution region  62  which rotates on central portion  60 . External surface portion  64  of the roller is perforated, or is otherwise rendered gas permeable, so as to allow hot gas to flow from hot-gas distribution region  62  through surface  64  and into the charge under a pressure greater, but perhaps only slightly greater, than ambient. In the case of a suction or evacuation roller, gas flow is in the opposite direction, and central portion  60  is maintained under a negative pressure through connection to a suction fan or vacuum pump (not shown).  
      The rotation of the rollers  50   a - 50   h  and  52   a , 52   b  is synchronized. As a result, hot gas application region  62  of roller  52   a  allows hot gas to flow to charge  42  and hot gas evacuation region  66  of roller  52   b  receives gas after it flows through charge  42 . In this manner, the application of hot gas to charge  42  through roller  52   a  is coupled to the gas drawing capability of roller  52   b . Alternatively, the rollers may include an internal, stationary baffle (not shown) that allows hot air to be expelled through perforate rollers.  
      Gas exiting from roller  52   b  is routed into zone  44  as indicated by the gas flow arrow  54 . Prior to entering zone  44 , hot gas may flow through sensor  68 , which may include a temperature sensor, a pressure sensor, or both a pressure and a temperature sensor. The temperature and pressure of the hot gas can be continuously monitored at sensor  68  prior to the introduction of the hot gas through a second gas checkpoint  70 . Gas checkpoint  70  houses a compressor and heater (not illustrated) to (1) increase or decrease the gas flow rate, (2) increase or decrease the gas temperature or (3) increase the temperature and decrease the flow rate, or (4) increase the flow rate and decrease the temperature, or (5) increase or decrease both the temperature and pressure of the gas as it enters rollers  50   h . Alternatively, a charge sensor (not shown) can be positioned between pairs of rollers to directly measure the temperature of the charge. The sensor could provide temperature information to pairs of cells so that the temperature, and perhaps flow rate of air through each pair of cells, can be adjusted.  
      Whereas roller  52   b  is an evacuated roller in the illustrated embodiment, roller  50   h  is a gas application roller. Roller  50   g , the roller coupled to roller  50   h , is an evacuation roller. Thus, the arrangement of rollers  50   g  and  50   h , with respect to the application of hot air to the opposed major surfaces of charge  42 , is opposite the combination of rollers  52   a  and  52   b . In this manner, the application of hot air can be “pulsed” or “reversed” relative to a particular point on the moving charge, i.e., hot gas is applied to one major surface of charge  42  at a first position along apparatus  40  and the charge  42  and to the second major surface of charge  42  at a second position along apparatus  40  and the charge  42 . This arrangement currently is believed to ensure sufficient hot gas penetration through the cross section of charge  42  to melt or cure the thermoactive material throughout the entire cross section, and to equalize the temperature gradient throughout the cross section of the charge  42 .  
      Air passing through charge  42  and into evacuation roller  50   g  then feeds through a third gas checkpoint  72  prior to flowing through roller  50   e . Again, at gas checkpoint  72 , the pressure and temperature of the gas can be monitored to determine whether either of these variables must be adjusted. Gas flowing from checkpoint  72  then enters gas application roller  50   e , which is coupled to a evacuated roller  50   f . The gas drawn through charge  42  by roller  50   f  is then fed through a third gas checkpoint  74 . Gas flows through the remaining rollers  50   a - 50   d  and through a final checkpoint  78  prior to either being (1) vented to the atmosphere, or (2) recycled into an upstream portion of the gas distribution system.  
       FIG. 3  also illustrates that apparatus  40  may include baffles  80 . Baffles  80  generally are arranged adjacent each of the gas rollers  50   a - 50   h  and  52   a ,  52   b . Baffles  80  are positioned to help prevent loss of gas as it enters or exits through surface  64  of each of the rollers  50   a - 50   h , and  52   a ,  52   b.    
       FIG. 4  illustrates an alternative embodiment of a baffle system that may be used instead of or in combination with the rollers  50   a - 50   h  and  52   a ,  52   b . The embodiment illustrated in  FIG. 4  shows only four rollers being housed in consolidation zone  44 . It will be understood that the number of rollers in either of the embodiments of  FIGS. 3 and 4  may vary. The purpose of shrouds  82  is the same as that of baffles  80 , i.e., to prevent or reduce the amount of gas escaping from the system as the gas is applied to the charge  42 .  FIG. 4  illustrates that each of the rollers includes a shroud  82  designed to substantially completely encase the roller therein. It also is possible to use a combination of baffles  80  and shrouds  82 .  
       FIG. 5  illustrates still another embodiment of a continuous consolidation apparatus  100 . Again, the number of rollers illustrated may vary according to the particular application desired. Furthermore, structures illustrated in  FIG. 5  that are similar to those illustrated in  FIG. 3  or  4  will be identified by like reference numbers.  
      A primary feature illustrated in  FIG. 5  is the use of continuous foraminous belts  102 ,  104 . Foraminous belt  102  is trained around belt feed rollers  106   a - 106   d . Continuous foraminous belt  104  is trained around belt feed rollers  108   a - 108   d . The foraminous belts  102  and  104  are positioned between charge  42  and the rollers  50   a - 50   h  and  52   a , 52   b . Belts  102  and  104  have two primary functions. First, these belts act as conveyors to convey charge  42  through zone  44 . Second, belts  102  and  104  eliminate or reduce the introduction of fines from charge  42  into the components of apparatus  100 .  
       FIG. 6  illustrates still another alternative embodiment of a gas distribution system for applying a hot gas to a charge  42  in zone  44 . Again, like reference numbers will be used to designate structures in  FIG. 6  that are similar to those illustrated in  FIGS. 3-5 .  
      A primary feature illustrated in  FIG. 6  is the use of an alternative gas distribution system for distributing hot gas to charge  42 . With reference to  FIGS. 3-5 , the hot-gas distribution system comprises a series of coupled rollers for both applying gas to and drawing gas through charge  42 .  FIG. 6  illustrates paired gas distribution hoods  110   a - 110   h  being arranged in paired fashion on opposite sides of charge  42 . Hot-gas distribution conduit  112  feeds hot gas through gas checkpoint  70  and into hood  100   h . Hood  110   h  therefore is an application hood. Hood  110   g  is an evacuated hood for drawing hot gas through charge  42 . As with the previous embodiment, hot gas flowing through the charge  42  is then fed through a gas checkpoint  72  and thereafter through conduit  112  into hood  110   e . As a result, hood  110   e  is a gas application hood, whereas coupled hood  110   f  is an evacuated hood for drawing hot gas through the charge  42 .  
     Iv. Operation  
      The operation of the apparatus will now be described with reference to using thermoplastics as the thermoactive material. The filler material and the thermoplastic material are comminuted, shredded or otherwise reduced to sizes suitable for producing composites. A room-temperature or preheated mixture of the filler material and thermoactive material is formed, such as by using cyclone or cyclones  10 . The mixture is then deposited onto conveyor belt  28  as a charge, which leads to the consolidation apparatuses.  
      The exact pressure to which the gas is pressurized before application to charge  42  in zone  44  depends on a number of factors, such as the materials being used, the speed at which the production line operates, the flow rate, the size of the particles used to form the composite, the thickness of the composite, etc. In general, the pressure of the hot gas as applied to the charge  42  ranges from about 1 psi to about 50 psi. Surprisingly, it has been determined that the melting of thermoactive material does not prevent hot air from passing through the mat. As a result, the pressure of the gas generally varies from slightly above atmospheric, such as about 0.01 psig to at least about 10 psig above atmospheric pressure, with about 0.01 to about 2 psig being typical, and about 1 psig or less being preferred.  
      As hot gas is applied to composite  42 , the volume of the composite decreases if the thermoactive material is a thermoplastic. This is because the thermoplastic material melts and apparently wicks along and flows around the filler material. The mixture thereafter appears to collapse under its own weight to occupy less volume than the mixture comprising solid thermoplastic material, which is referred to herein as thermal consolidation. This is particularly true if thermoplastics are used as the thermoactive material because such materials melt upon application of hot gas. The consolidation apparatuses of  FIGS. 3-6  may be designed solely to thermally consolidate (as opposed to a densifying) charge  42 , and therefore not compress the composite  42  to a final product density, if the cells do not exert a compression force on the charge. Alternatively, the consolidation apparatuses may exert a compression force to the composite  42 . The force applied by the final press typically ranges from about 100 psi to about 1,000 psi, with about 500 psi being typical.  
      Once the charge  42  exits outlet  48 , it may be further processed to provide an aesthetically pleasing commercial product. For example, charge  42  may be (1) sanded to provide a smooth surface, (2) embossed with desired patterns, (3) coated with an exterior coating so as to provide a water-impermeable exterior, (4) covered with a paper-based exterior coating as is known in the art of oriented strand board, (5) laminated with veneer facings, (6) painted, or (7) any combination of 1-6.  
      Certain of the thermoactive/cellulosic composites made in accordance with the present invention have been surface modified in order to be painted or otherwise surface decorated. Methods for modifying certain thermoactive materials are disclosed in AU 9514510 and 9515286, which are incorporated herein by reference. These methods apparently concern modifying polymeric materials, particularly polyethylene, such as by corona discharge and/or flame treatment oxidation. Flame treatment oxidation is a currently preferred method for oxidizing the surface of the composite product. Oxidation alone is generally not sufficient for promoting durable adhesion to conventional oil or water based paints, especially when wet (see Example 5). Typically, grafting chemicals are thereafter attached to the oxidized polymeric material for coupling other materials, such as paint or veneers, to the oxidized thermoactive material.  
      But, there are other methods for oxidizing the surface of composite products made in accordance with the present invention for coupling grafting chemicals to the product&#39;s surface. Currently, the three most likely approaches for modifying the surface of composite products are as follows: (1) flame and/or corona discharge oxidation, as discussed above; (2) photoreactions, particularly ultraviolet irradiation in the presence of azido compounds, including but not limited to perfluorophenyl azides; and (3) E-beam treatment of the composite product, perhaps simultaneously with the application of grafting chemicals. One possible approach will be to both crosslink the thermoactive material of the composite product by E-beam (see Example 7) while simultaneously applying surface grafting chemicals to the surface of the product.  
     V. Other Specific Implementations  
      Following are descriptions of specific implementations of products, processes and apparatus for forming composite materials, including, e.g., board products. In the description that follows, the thermoactive material is a thermoplastic material (“plastic”) and the filler material is a cellulosic material (e.g., wood), although other materials may be used in accordance with the guidelines described above. Except as specifically indicated, the following description of products, processes and apparatus is consistent with the description above.  
      A. Products  
      Consistent with the examples discussed below, a description of specific implementations of products follows. In particular, various board products are described, but other products are also possible. “Board products,” as used herein, refers to materials made for use in construction or repair applications, typically but not necessarily in substitution for any wood piece, e.g., in lumber, sheet stock, siding, posts, panels, decking (i.e., having a higher load-bearing capacity as required by building codes), tongue-and-groove material, finishing layer material, underlayment material, trim and fascia stock, molded products (such as containers, packaging, etc.), etc., for structural, aesthetic and reliability purposes.  
      Board products formed by the processes and apparatus described below can be configured in any desired dimensions. In a specific implementation, the board product is formed as a board, i.e., a rectilinear solid having two generally parallel major surfaces (sides), two generally parallel minor surfaces (edges) and two ends. Customarily, boards are produced as relatively long members having a width much less than their length and a thickness less than or equal to their width. In other implementations, the board product may be a 4 feet by 8 feet sheet with a specified thickness, i.e., similar in size to typical plywood and other sheet materials.  
      As one example, the finished board may be {fraction (7/16)} inch thick. The length and width of the board are variable, but according to one implementation, the board formed has a length of 17 feet and a width of 30 inches. For aesthetic purposes, the board may include a finishing face to cover the core. The face also provides weather resistance.  
      The thickness of the face can vary dependent on the thickness of the finished board product. The face thickness can be from about {fraction (1/48)} inch to about {fraction (3/4)} inch, and typically is in the range of about {fraction (1/36)} inch to about {fraction (1/4)} inch. For a specific implementation of a {fraction (7/16)} inch board, the face may be about {fraction (1/24)} in thick. The face is preferably sufficiently thick to shade and cover flakes in the core that “telegraph.” Also, the face is preferably formed on at least both major surfaces of the board to provide a balanced construction free from stress concentration on only one major surface. The ends and side edges of the board also can be formed with a face, if desired.  
      The core material and the finishing face material are each predominately a mixture of plastic material and wood material. The ratio by weight of wood to plastic in the core mixture may be from about 10%/90% to about 90%/10%, and preferably from about 45%/55% to about 55%/45%. A desirable ratio is about 50%/50%. The ratio by weight of wood to plastic in the face may be from about 30%/70% to about 0%/100% (i.e., the face may be formed entirely of plastic), and preferably about 30%/70%. The product as formed has a density of about 40 to about 65 pcf, and preferably about 50 to about 52 pcf.  
      B. Process  
      Referring to  FIGS. 16-18 , according to one implementation of the process, the plastic is collected and sorted (step  202 ). The collection and sorting may take place at a remote site. The sorting may include sorting the plastic by type, e.g., distinguishing high density plastic material and low density plastic material, discarding undesired material, etc. The plastic material to be used may include any fraction of waste or non-virgin plastic.  
      The sorted plastic is then granulated. According to a specific implementation, the plastic is granulated in a U.S. Granulator 2436 5KN granulator. The plastic was granulated into two sizes to make working embodiments: (1) flakes, for use in the core of the product (step  204 ); and (2) fines, for use in the faces of the product (step  206 ). In one implementation, the sorted plastic material is granulated once to obtain the coarse flakes, and a portion of the coarse flakes are granulated a second time to produce the fines. In particular, the plastic may be ground into {fraction (1/2)} inch flakes by using a {fraction (1/2)} inch screen plate in the granulator (i.e., a plate with an aperture allowing pieces of up to about {fraction (1/2)} inch to pass). Some of the {fraction (1/2)} inch flakes can be granulated into fines with a U.S. Granulator radial granulator having a {fraction (1/8)} inch screen plate. Other flake sizes could be made, including from {fraction (3/16)} inch to {fraction (3/8)} inch. Commercial embodiments will likely use {fraction (7/16)} inch flakes.  
      After the plastic is granulated, it is sorted by weight (step  208 ). The granulated plastic may be conveyed, e.g., to a storage area or a subsequent process step, with forced air. The sorted plastic may be stored for later use or used directly as part of a continuous process.  
      The raw wood material also is collected and sorted (step  212 ). The raw wood material may include virgin wood, recovered wood and stumps. In a specific implementation, the raw wood material was reduced in size and then sorted by passing it over a first plate with a 4 inch diameter hole positioned above a second plate with 1 inch diameter hole. The wood pieces that pass through the first plate but not the second plate, i.e., wood pieces between about {fraction (3/4)} inch and about 7 inches (subject to screen efficiency), are segregated into (1) large wood pieces (typically from about 1½ inches to 7 inches) and (2) small wood pieces (typically from about {fraction (3/4)} inch to about 4 inches). The wood pieces that pass through both the first and the second plate (i.e., wood pieces smaller than about 1 inch) are screened off and used to form the fines.  
      Multiple flakers will likely be used. In commercial embodiments, sorting as described herein may not be required.  
      Optionally, the large and small wood pieces may be washed to remove excess debris (step  214 ). Grinding moist wood also reduces the amount of dusting that occurs as a result of the process. In a specific implementation, the large wood pieces and the small wood pieces are each soaked for about two hours and then washed in a conventional chip washer. In commercial processes, the wood pieces will be soaked from 30 seconds up to about 2 hours, depending on the cleanliness required.  
      The large wood pieces and the small wood pieces are flaked to desired sizes. In a specific implementation, the large wood pieces and the small wood pieces were separately flaked to desired sizes (step  216 ). After flaking, the flakes from the large wood pieces (about 1½ inch to about 7 inches long) have a mean length of about 3 inches, and range from about 1 inch to about 5 inches (typically about 2 inches to about 4 inches). The flakes from the small wood pieces (about {fraction (3/4)} inch to about 4 inches) have a mean length of about 1¼ inches, and range from about {fraction (3/8)} inch to about 3 inches (typically about {fraction (3/4)} inch to about 1½ inches).  
      If necessary, the flakes may be dried to a desired moisture content according to any suitable known method (step  218 ). If the moisture content is higher than about 5%, the throughput of subsequent processing is lessened due to the required drying time. In one specific implementation, the flakes from the large pieces were dried separately from the flakes from the smaller pieces.  
      The flakes can be dried using a conventional gas conveyor dryer. One suitable dryer is the Proctor Schwartz gas conveyer dryer, although other suitable dryers and drying methodologies would be known to those of ordinary skill in the art. Preferably, the flakes can be dried to a moisture content of about 2%.  
      The flakes can be further screened to segregate pieces larger than and smaller than a desired size or sizes. Such oversized and undersized material can be collected and ground to form the wood fines (step  220 ). In one specific implementation, the flakes from the large wood pieces were screened with a 1¼ inch diameter punch plate and a {fraction (3/16)} inch diameter punch plate. Separately, the flakes from the small wood pieces were screened with only the {fraction (3/16)} inch punch plate. After screening, the “accepts,” i.e., the remaining large and small flakes, occur in the core mixture in ratios from about 20% large flakes/80% small flakes to about 60% large flakes/40% small flakes.  
      The smaller material that passes through the {fraction (3/16)} inch diameter plate can be double refined to form the wood fines using a conventional refiner. One suitable refiner is a Bauer single or double disk refiner (24 inch diameter). The resulting refined material was screened through a U.S. No. 20 mesh screen and a U.S. No. 30 screen (in series). Accepts represent 80% of the original material. The material that passes through both screens is discarded as waste.  
      By segregating the wood into large pieces and small pieces, and flaking the large pieces separately from the small pieces, a greater portion of longer flakes is preserved than if the large pieces and the small pieces are ground together.  
      The ground wood flakes and fines are sorted by weight (step  222 ). The ground wood flakes and fines may be stored for later use or used directly as part of a continuous process.  
      The moisture content of the plastic material, the wood material and/or both materials in combination may be periodically monitored throughout the process as necessary, as is known to those of ordinary skill in the art.  
      To form the core, the plastic material (i.e., the plastic flakes) and the wood flakes are blended together in a suitable core mixture (steps  226 ,  228 ), as discussed above. In a specific implementation, the plastic material and the wood material were blended together in a conventional rotary drum blender. Any other suitable type of blending that does not break or split the fibers in the wood also may be used.  
      To form the faces, the plastic fines and the wood fines were blended together in a face mixture (steps  230 ,  232 ). In a specific implementation, the plastic fines and the wood fines are blended together using a conventional twin screw ribbon blender. The ratio by weight of wood to plastic in the face mixture may be from about 70%/30% to about 0%/100%, and preferably about 50%/50% to about 30%/70%. Because the fines used to form the faces are relatively small, they may be lost before being processed into the completed faces. For example, fines may be lost at gaps in a moving belt as part of a continuous process or at transitions between processing steps.  
      According to one implementation, the faces are pre-consolidated (i.e., consolidated before the subsequent consolidation of the board product having both the faces and the core). In a preferred embodiment, an amount of the face material that has been deposited on a lower surface having a stick-resistant coating (e.g., PTFE) is heated to the activation temperature of the face material while being subjected to pressure from an upper stick-free surface. As described in more detail below, the lower and upper stick-free surfaces may be moving belts coated with PTFE such that the faces can be formed in a continuous process. In another implementation, the face material is heated to its activation temperature while it travels through a long tunnel. In another implementation, infrared (IR) radiation, e.g., from lamps, is used to heat the face material to the activation temperature. In the heated tunnel and IR radiation implementations, the face is mechanically consolidated downstream of the heating area, e.g., by using a roller at a temperature below the temperature of the face material.  
      The upper melting point of polyethylene is about 254° F. Thermal degradation of wood begins at about 360° F. Because the pre-consolidation process is time-temperature dependent, different temperatures and dwell times can be used. For example, if higher temperatures are used, dwell times at the higher temperatures can be decreased to lessen cellulose degradation.  
      The pre-consolidated faces can be stored for future use (i.e., by rolling) or can be used continuously in a consolidation step for forming the board product from the faces and a core.  
      According to one implementation, a first face is formed, the core is formed on an upper major surface of the first face, and then a second face is placed over the upper major surface of the core.  FIG. 19  is a partial perspective view of the three-layer construction, showing a first face F 1 , a core C and a second face F 2 . Either or both of the face forming operation and core consolidation operation (described below), or a combination thereof, may be configured as a continuous process.  
      Either or both of the first face and the second face may be formed previously. For example, the second face may be formed previously, and added to the upper surface of the core at an appropriate time. Alternatively, the first face may be formed previously, and the second face may be formed in parallel with the core and then added to the core. It also is possible to form the core separately from the faces, and then subsequently add the faces.  
      Preforming at least one of the faces, i.e., before the core material is deposited on that face, has several benefits. By forming the faces on a solid belt, thin, uniform faces can be formed with consistency. The belt provides a flat, regular reference surface, and the face formed on the belt is thus flat, regular and suitable as a reference surface for the core. With a preformed face as a bottom surface of the mat, the mat can be easily transferred from one conveyor to another (e.g., across gaps) without excess dusting and/or loss of material. With a preformed face, the wood and plastic fines are entrapped and not subject to being blown away, e.g., in the consolidation apparatus to be described below. Decreased dusting results in a lower fire risk. Further, the premelted face prevents the mat from sticking to the conveyor and other surfaces within the consolidation apparatus.  
      According to the implementation described above, the board product before consolidation was formed as a mat by depositing the core mixture in a predetermined fashion on the first face as the first face moves on a moving belt (step  234 ). In particular, the wood flakes are aligned and graduated. In one specific implementation, the wood flakes were aligned, graduated and deposited with the plastic flakes, which were also graduated and deposited, using two Acrowood disk screeners arranged in series and opposed in the movement direction.  
      In particular, the screeners were used to deposit the core mixture such that the bottommost flakes were largest, progressively decreasing in size to about the middle of the core, and then progressively increasing in size from the middle of the core to its upper surface, with the uppermost flakes being about the same size as the bottommost flakes, as shown in  FIG. 20 .  
      After the core material is deposited, the second face is placed onto the upper surface of the core to complete the mat.  
      The mat formed of the first face, the core and the second face is subjected to consolidation, generally by applying both heat and pressure (step  236 ), although not necessarily simultaneously. The mat for a {fraction (7/16)} inch thick board product is about 7½ inches thick before consolidation. According to one implementation, the mat is moved through a consolidation apparatus on a moving belt while being subjected to heat and pressure. The application of heat and pressure is regulated to control the properties of the product, including degradation of the cellulose, loss of material (e.g., through the sides), etc.  
      One implementation of the consolidation apparatus uses hot air to raise the plastic material of the mat to its activation temperature. In this implementation, the moving belt is perforated such that the hot air can be circulated through the moving belt and mat as they travel through the consolidation apparatus. The air temperature within the consolidation apparatus may be from about 100° F. to about 600° F., and preferably is about 250° F. to about 450° F. In one working implementation, the air temperature was about 400° F. The air can be circulated at any pressure from 0.2 to 1.2 psi, and typically from about 0.2 to 0.6 psi (preferably 0.4 psi). After the mat is consolidated, it can be cooled to facilitate its release from the moving belt. Cooling the mat causes it to shrink slightly.  
      The consolidated mat can then be laminated (step  238 ). It also is possible to laminate the mat prior to consolidation.  
      Laminating a thin layer of plastic to one or both major surfaces of the consolidated mat provides a consistent quality surface that will more readily receive a surface treatment as described below than a non-laminated surface. Laminating a surface as described also ensures complete plastic coverage of that surface and provides a water resistant surface. The laminate is applied on the major surfaces and then subjected to heat (about 250° F.) and pressure to cause it to couple or fuse to the faces, thereby ensuring a non-discrete laminate. Laminating both major surfaces ensures a balanced construction, which helps prevent warping.  
      In a specific implementation, the laminate was a 2 millimeters thick sheet of polyethylene. Preferably, the polyethylene has known, consistent properties. If polyethylene is used as the laminate material, it flows and bonds with the polyethylene material in the faces without resulting in any discernible interface. Other polymeric materials (such as polyolefins) also may be used as a laminate.  
      Following lamination, the composite is pressed to the final thickness of the board product using a conventional pressing apparatus (step  240 ). A suitable pressing apparatus is an IECO C-frame press. In specific implementations, the board product also was embossed with a pattern (e.g., woodgrain) and/or cooled as it was pressed.  
      The pressing action applied on the major surfaces of the board product to form it to its final dimensions requires only about 100-400 psi, and more typically 200-300 psi, depending upon pressing rate, product thickness and density requirements. In a specific implementation, the applied pressure in the pressing action is about 250 psi. The upper limits of the pressure applied during the pressing action are determined to avoid loss of material through the side edges of the board product. Under certain conditions, it may be desirable to first cool the edges before and/or during pressing of the major surfaces. Such cooling will cause the edges to harden and prevent material loss through the edges. The press can be stationary, in which each board product is inserted individually, or moving, in which case the board products travel through the press in a continuous process.  
      Following pressing, the board product can be cut or trimmed to final dimensions using conventional equipment and methods. Thereafter, the board product is subjected to surface treatment (step  242 ). Alternatively, the product may be surface treated before being trimmed to size.  
      In a specific implementation, the board product was surface oxidized by flame treatment, and a grafting chemical was applied. After drying using conventional drying technology, which allows solvent to escape, the board product was ready for painting using known apparatus and methods.  
      C. Apparatus  
       FIG. 21  shows a specific implementation of a face forming apparatus  300 . The face forming apparatus has a housing  302 , an entrance end  304  and a delivery end  306 . The housing  302  preferably is insulated to minimize heat loss.  
      An upper moving belt  308  and a lower moving belt  310 , which are driven at the same speed by a belt driving unit (not shown), are rotatably supported by upper moving belt rollers  312  and by lower moving belt rollers  314 , respectively. A portion of the upper moving belt  308  is heated as it circulates through the housing  302  by contact with an upper platen  316 . Similarly, a portion of the lower moving belt  310  is heated as it circulates through the housing  302  by contact with a lower platen  318 .  
      The lower moving belt  310  extends beyond the entrance end  304  to provide a surface upon which to receive a desired face material mixture for forming the faces. As illustrated, a forming head  315  dispenses the face material onto the lower moving belt  310  as the lower moving belt approaches the entrance end  304 .  
      The distance separating the upper moving belt roller  312  from the lower moving belt roller  314  at the entrance end  304  may be greater or less than a starting thickness  320  of the face being formed. The distance separating the upper moving belt roller  312  from the lower moving belt roll  314  at the delivery end  306  is equivalent to a formed thickness of the face. Thus, the upper moving belt  308  slopes slightly downward with respect to the lower moving belt  310 , which is generally horizontal, in the direction from the entrance end  304  to the delivery end  306 . In a specific implementation where the face mat was {fraction (3/8)} inch thick, the starting thickness  320  was about two and one-half times greater than the formed thickness  322 .  
      The upper moving belt  308  and the lower moving belt  310  are preferably solid to minimize loss of face material through the surface of the belts, particularly the lower moving belt  310 . Each of the upper moving belt  308  and lower moving belt  310  was formed of a synthetic conductive material. In a specific implementation, the belts were coated with a stick resistant material to facilitate release of the formed face from the belts. An example of a suitable stick resistant material is polytetrafluoroethylene (PTFE). Other similar materials also could be used.  
      As stated, each of the upper moving belt  308  and the lower moving belt  310  is heated by conduction through contact with the heated upper platen  316  and the heated lower platen  318 , respectively. In a specific implementation, the belts were heated to a temperature of about 300° F. to about 600° F., and preferably, to between about 400° F. and 500° F., which is above the activation temperature point of the plastic material.  
      The length of the face-forming apparatus  300 , belt operating temperatures and belt speeds may be varied according to desired processing requirements, including, e.g., face thickness, power requirements and throughputs. In a specific implementation for making a {fraction (1/20)} inch face, the time required to form one face using 450° F. upper and lower platens was about 15 seconds to about 1 minute, and preferably less than 30 seconds.  
      If necessary, the face forming apparatus  300  can include an optional cooling station  324 . In a specific implementation, the cooling station  324  included an upper cooling roll  326  and a lower cooling roll  328  (or corresponding cooling platens). As illustrated, the upper moving belt  308  was extended to circulate around the upper cooling roll  326 , and the lower moving belt  310  was extended to circulate around the lower cooling belt  328 . The cooling rolls  326 ,  328  are at a lower temperature than the portions of the respective belts  308 ,  310  exiting the delivery end  306 . Thus, these portions of the belts  308 ,  310  are cooled by contact with the respective cooling rolls  326 ,  328 . In turn, the formed face is cooled through contact with the cooled belt portions, and the cooled face shrinks slightly, thus facilitating its release from the belts  308 ,  310 .  
      As illustrated, the upper cooling roll  326  and the lower cooling roll  328  are separated from each other by a distance about equal to the formed thickness  322  of the face. The upper cooling roll  326  and the lower cooling roll  328  are maintained at a temperature, e.g., by cooling with a circulating fluid, below the melting temperature of the plastic material, e.g., at about ambient temperature. Cooler temperatures promote result in faster cooling. Other suitable cooling methods also can be used.  
      Other face forming methodologies also may be used. For example, the faces may be formed using IR radiation. In a specific implementation, the face material was deposited on a solid surface, and then subjected to IR radiation from above to melt the plastic in the face material. One specific implementation of a face forming apparatus  400  is illustrated in  FIG. 22 .  
      As illustrated, face material is dispensed from a forming head  402  onto a moving belt  404  to provide the initial form of a face F. The moving belt is rotatably supported on moving belt rolls  406 . The moving belt  404  is circulated beneath one or more IR units  414  that are positioned to direct IR energy onto an upper surface of the face as the face moves through the housing  310  on the moving belt  404  over a surface  415 . The IR units heat the plastic material in the face, preferably to at least its activation temperature. After the face is heated, the face passes beneath a press roll  416  that presses the face against a flat surface, such as a surface  417 .  
      Depending upon the intensity of the IR radiation and the desired thickness of the face, the surface of the face that travels on the moving belt  404  must also be subjected to direct IR treatment. In one specific implementation, the face is turned over by a face turning apparatus  418 . The untreated surface of the face is then fed on the moving belt  404  and subjected to IR treatment.  
      In the illustrated embodiment, the face turning apparatus  418  has a moving belt  419  rotatably supported by rollers  423 . The path of the moving belt  419  is configured to bend around a turning roller  421 . As illustrated, a face F that has been surface treated on one major surface (shown in dashed lines) travels on the moving belt  419 , enters the nip formed by the moving belt  419  and the turning roller  421 , follows the circumference of the turning roller  421  in the counterclockwise direction and is turned over near the 1 o&#39;clock position of the turning roller  421 . The face is then released from between the turning roller  421  and the moving belt  419 , and allowed to slide down a guide plate  425 .  
      The illustrated face forming apparatus  400  is configured to operate in a forward direction  420 . Alternatively, the face forming apparatus may be configured to operate in a reverse direction  422  (e.g., in a second pass to process the second of two surfaces), as well as the forward direction  420 .  
      In yet another implementation of the face forming apparatus, a long, insulated housing  429  (shown in dashed lines), is provided in place of the IR units  414 . The face is heated as it travels the length of the housing, and reaches the activation temperature near the end of the housing.  
       FIGS. 27 and 28  show a specific implementation of a screening apparatus  250 . As shown in  FIG. 27 , the screening apparatus  250  is arranged above a face F of a mat M being developed through deposition of core material. The screening apparatus  250  has a first stage  252  and a second stage  254 , which are arranged in series in the process direction  105  of the moving mat M. The first stage  252  and the second stage  254  are positioned as shown to be inclined upwardly from horizontal relative to the process direction  105  at an angle θ. In a specific implementation, the angle theta is about 10°.  
      The first stage  252  has multiple sorting zones, including, e.g., zones  256 ,  258  and  260  as illustrated. Similarly, the second stage  254  has multiple sorting zones, including, e.g., the zones  262 ,  264  and  266  as illustrated. Each of the multiple sorting zones is sized to allow a different flake size to pass through and be deposited upon the mat M. As illustrated, the first stage  252  and the second stage  254  are opposed to each other, and the sizes of the zones  262 ,  264  and  266  in the second stage are the same as the sizes of the zones  260 ,  258  and  256  of the first zone  252 , respectively.  
      Referring to  FIG. 28 , which shows the configuration of the first stage  252  (the second stage  254  being similar, but reversed), each of the zones  256 ,  258  and  260  includes a plurality of sprockets  272 ,  274  and  276 , respectively, that are rotatably supported on axles  278  extending through a generally rectangular frame  279 . The axles  278  are drivingly coupled together. The sprockets on adjacent axles  278  are interspersed with each other. Within the zone  256 , the axles  278  are spaced by an interradial distance  280  (i.e., in the process direction  105 ), e.g., {fraction (1/4)} inch.  
      Within the zone  258 , the axles  278  are spaced by an interradial distance  282 , e.g., {fraction (1/8)} inch. Within the zone  260 , the axles  278  are spaced by an interradial distance  284 , e.g., {fraction (1/16)} inch. Because the zones  262 ,  264  and  266  of the second stage  254  have the same sizes as the zones  260 ,  258  and  256  for the illustrated first stage  252 , respectively, the interradial distances for the zones  262 ,  264  and  266  are the same as the interradial distances  284 ,  282  and  280 , respectively.  
      In continuous operation after initialization, core material is dispensed from a first stage dispenser or chute  268  onto the first stage  252 , and from a second stage dispenser or chute  270  onto the second stage  254 , while the mat M is moving in the process direction  105 . The first stage  252  is driven in a first stage direction  253  from left to right in  FIG. 27  by a conventional chain drive mechanism (not shown). Thus, the dispensed core material is progressively graduated and aligned as it travels from the zone  260  through the zone  258  to the zone  256 . In particular, the sprockets  276 ,  274  and  272  in the respective zones contact the flakes and tend to align them lengthwise in the first stage direction  253  as the flakes are moved by the action of the sprockets through one or more of the zones  260 ,  258  and  262 .  
      At the same time, flakes in the core material dispensed onto the first stage  252  begin passing through the first stage  252  and being deposited on the face F according to their size. The smallest flakes tend to be deposited first, i.e., through the zone  260 . Slightly larger flakes, which may have been aligned in the zone  260 , begin passing through the first stage  252  in the zone  258 . Finally, the largest flakes, which may have been aligned in the zones  260  and  258 , begin passing through the first stage  252  in the zone  256 . Some flakes pass through the first stage  252  directly, whereas others pass through to a limited extent until they contact the upper surface of the developing mat M and are pulled slightly by the movement of the mat M in the process direction.  
       FIG. 27  shows the development of the mat M during a continuous phase after an initialization period has elapsed. In particular,  FIG. 27  shows the development of one point on the mat, a leading edge LE, from the beginning of the continuous phase (at LE 0 ) as the mat M moves from right to left in the process direction  105 . During this initialization period, which is not illustrated, the dispenser  268  begins operating, and the first flakes that pass through the first stage  252  and contact the face F are the smallest flakes, i.e., the flakes that can pass through the zone  260 . Similarly, slightly larger flakes begin passing through the zone  258 . As described above, however, the desired graduation of the core material begins with the largest flakes being deposited as the bottommost layer onto the face F. Therefore, after the first stage  252  begins operating, the initialization period continues until a point on the face F begins receiving the largest flakes first. This point becomes the leading edge LE of the mat M, and is designated LE 0 . The material deposited during initialization ahead of the leading edge LE is recycled for later use during the continuous phase.  
      During continuous operation, as the leading edge LE moves forward in the process direction  105  below the zone  258 , slightly smaller flakes are deposited on the largest flakes (beginning at LE 1 ). Similarly, still smaller flakes are deposited on the slightly smaller flakes as the leading edge moves forward below the zone  260  (beginning at LE 2 ).  
      Still during continuous operation, the progressive development of the mat M continues as the leading edge LE moves forward beyond the first stage  252  to below the zone  262  of the second stage  254 . From the zone  262 , flakes approximately the same in size as those deposited from the zone  260  continue to be deposited (beginning at LE 3 ). As the leading edge LE moves forward below the zone  264 , larger flakes are deposited on the smaller flakes (beginning at LE 4 ). Finally, as the leading edge LE moves forward below the zone  266 , the largest flakes are deposited (beginning at LE 5 ), thereby forming the uppermost layer and completing the core (at LE 6 ).  
      The second stage  254  is driven similar to the first stage  252  by a conventional chain drive (not shown), but in a second stage direction  255  opposite the first stage direction  253 .  
      A specific implementation of the consolidation apparatus is shown in  FIGS. 7-14 . As shown in  FIGS. 8-10 , a consolidation apparatus  100  has a feed end  102  through which a mat M is fed, a main body portion  103  in which the mat M is consolidated, and a delivery end  104  through which the consolidated mat M is received. A lower conveyor  106  and an upper conveyor  108 , which are rotated at the same speed in a feed direction  105 , contact and feed the mat M in a feed direction from the feed end  102  to the delivery end  104 . The consolidation apparatus  100  also includes a hot gas distribution system  10  ( FIG. 9 ), which is described below in greater detail.  
      The portion of the lower conveyor  106  that transports the mat M is positioned approximately parallel to the surface on which the consolidation apparatus  100  rests, e.g., a floor. The portion of the upper conveyor  108  that contacts and transports the mat M slopes downwardly from the feed end  102  to the delivery end  104 . Thus, as the mat M is transported in the feed direction, the mat M is mechanically consolidated, i.e., pressed to a smaller thickness. In a specific implementation, the mat M had a thickness of about 7½ inches at the feed end  102  and a thickness of about ½ to about 4 inches (preferably {fraction (3/4)} inch to 1½ inches) at the delivery end  104 . The nip at the delivery end  104  should be cooler than the mat M.  
      Near the feed end  102 , an extended throat section having a length L 1  is provided to ensure sufficient back pressure for the operation of the hot gas distribution system  110  (i.e., to ensure that air is guided through the mat M rather than escaping outward through the feed end  102 ).  
      The path of the lower conveyor  106  and the upper conveyor  108  is most clearly seen in  FIG. 10 . The lower conveyor  106  and the upper conveyor  108  are each endless belts having a perforated surface that allows hot air from the hot gas distribution system  110  to impinge on the upper and/or lower surfaces of and to pass through the mat M. The lower conveyor  106  and the upper conveyor  108  each has a conveyor guidance system  112 , a feed roller  114 , idler rollers  116 , and, near the delivery end  104 , an idler brush  120 , a rotary brush  124 , a drive roller  126  and a take-up roller and pneumatic cylinder  130 . The upper conveyer  108  also has a feed roller  114  positioned near the feed end  102 , whereas one of the idler rollers  116  is positioned at a corresponding location for the lower conveyor  106 .  
      At the delivery end  104 , the lower conveyor  106  has a lower nip roller  118 , and the upper conveyor  108  has an upper nip roller  119 . The space between the outer peripheries of the lower nip roller  118  and the upper nip roller  119  is adjustable depending upon the desired thickness of the consolidated mat M at the delivery end  104 . The lower nip roller  118  and the upper nip roller  119  are each driven and temperature controlled. As illustrated in  FIG. 12 , the lower nip roller has peripheral flanges  131  at its outer edges to prevent the core material from flowing outward in a direction perpendicular to the movement direction of the conveyors.  
      Further, at the delivery end  104 , each of the lower conveyor  106  and the upper conveyor  108  has an extension roller  121  of a smaller diameter than the lower nip roller  118  and the upper nip roller  119 , respectively. An auxiliary roller  123  is also provided adjacent the upper extension roller  121 . The extension rollers  121  allow the lower conveyor  106  and the upper conveyor  108  to reverse direction at a lesser angle with respect to the process direction  105  than if only the lower nip roller  118  and the upper nip roller  119  were present. As a result of this lesser angle, the mat M is more readily released from the lower conveyor  106  and the upper conveyor  108  because the force to separate the conveyors from the mat M is directed more nearly parallel with the process direction  105 .  
      Each rotary brush  124  has an element that contacts the surface of and cleans an adjacent one of the drive rollers  126 . Similarly, each idler brush  120  has an element that contacts the surface of and cleans an adjacent one of the idler rollers  116 . The take-up roll and pneumatic cylinder arrangement  130  allows the tension of each respective conveyor to be adjusted.  
      The lower conveyor  106  and the upper conveyor  108  include belt supports  128 , one being shown for the lower conveyor  106 . The lower conveyor  106  and the upper conveyor  108  are driven at the same speed by variable frequency drive assemblies (not shown) coupled to each of the drive rollers  126 . The inclination of the upper conveyor  108 , which is supported by an adjustable frame  158 , can be adjusted using a lift/lower system  136 .  
      At the delivery end  104 , an exit tunnel having a length L 2  is formed in the space provided by the converging lower conveyor  106  and upper conveyor  108 . Similar to the throat at the feed end  102 , the exit tunnel provides sufficient back pressure to ensure operation of the hot gas distribution system  110 .  
      The hot gas distribution system  110  includes one or more distribution zones. As illustrated in  FIG. 9 , the hot gas distribution system  110  is divided into a first distribution zone  138  and a second distribution zone  140  in the feed direction  105 . As shown, a partition  142  separates and isolates the first distribution zone  138  from the second distribution zone  140 , thereby preventing short-circuiting. Thus, the first distribution zone  138  and the second distribution zone  140  are not fluidly connected. Each of the first distribution zone  138  and the second distribution zone  140  has a fan  144  that is driven by a fan drive  146 . Each fan  144  draws hot air or other suitable gas from a duct  148 . The hot air drawn from the duct  148  is first heated by a heat exchanger  150 . Each heat exchanger  150  has a hot-side heat exchange with a hot fluid (e.g., hot oil) conveyed by an inlet pipe  152  through the heat exchanger  150  and out through an outlet pipe  154 . Each fan  144  conveys the hot air through a plenum  156  into the main body  103  of the consolidation apparatus  100 .  
      As illustrated best in  FIG. 9 , each of the first distribution zone  138  and the second distribution  140  includes at least a pair of sliding covers for changing the airflow path in the respective zone. The pair of sliding covers for each zone are aligned in the vertical direction and located on the fan side of the conveyors at the junction of a fan chamber  153  (i.e., essentially the airflow path between the consolidation apparatus  100  and the fan  144 /duct  148 ) and a plenum chamber  155  (i.e., essentially the airflow path between the consolidation apparatus  100  and the plenum  156 ).  
       FIG. 9  shows upper sliding covers  165 ,  167 , the lower sliding covers being similar. In particular,  FIG. 9  shows an upper sliding cover  165  for the first distribution zone  138 , and an upper sliding cover  167  for the second distribution zone  140 .  FIG. 7  shows the upper sliding cover  167  in vertical alignment with a lower sliding cover  177  for the second distribution zone  140 .  
      As illustrated in  FIG. 9 , the upper sliding covers  165 ,  167  can be positioned to cover one of a pair of openings  169  or  171  and  173  or  175 , respectively, in the wall of the consolidation apparatus  100 . The lower sliding covers are positioned to cover similar openings. For example, for the upper sliding cover  167 , the opening  169  connects the fan chamber  153  and the interior of the consolidation apparatus  100 , whereas the opening  171  connects the plenum chamber  155  and the interior of the consolidation apparatus  100 . Typically, the upper and lower sliding covers for either zone are configured such that one of the covers is positioned to cover the opening in the respective fan chamber  153 , and the other positioned to cover the opening in the respective plenum chamber  155 . As a result, an air flow path through both conveyors and the mat between the conveyors is created.  
      According to one specific implementation, the first distribution zone  138  was configured such that air flowed from the top down through the upper conveyor  108 , the mat M and the lower conveyor  106 . Conversely, the second distribution zone  140  was configured such that the air flowed upwardly through the lower conveyor  106 , the mat M, and the upper conveyor  108 , i.e., in a direction reversed from the first distribution zone  138 . Arrows A in  FIG. 7  show the air flow direction in the second distribution zone  140 , the air flow direction in the first distribution zone  138  being opposite the arrows A.  
      In this specific implementation, the upper sliding cover  165  for the first distribution zone  148  was positioned to cover the opening  171 , and the lower cover for the same zone was positioned to cover the opening from the fan chamber  153  into the consolidation apparatus. As a result, an air flow path is established in the first distribution zone from the fan chamber  153  through the opening  169 , into the consolidation apparatus  100  downwardly through the upper conveyor  108 , mat M and lower conveyor  106  and out the plenum chamber  155  (i.e., through the opening not covered by the lower sliding cover). Conversely, the upper sliding cover  167  for the second distribution zone is slid to cover the opening  173 , and the lower sliding cover  177  is slid to cover the opening from the plenum chamber  155  into the consolidation apparatus  100 . As a result, an air flow path is established in the second distribution zone  150  from the fan chamber  153  through the opening into the consolidation apparatus  100  (i.e., through the opening not covered by the sliding cover  177 ) upwardly through the lower conveyor  106 , mat M and upper conveyor  108  and out onto the plenum chamber  155  (i.e., through the opening  171 ).  
      At an air temperature of about 400° F., the time for consolidation of a {fraction (7/16)} inch board product using the illustrated air flow configuration is about 60 seconds. The air flow rate in each of the first distribution zone  138  and the second distribution zone  140  is set to create a pressure differential of about 4 in H 2 O to about 12 in H 2 O, and preferably about 8 in H 2 O to about 10 in H 2 O. The first distribution zone  138  and the second distribution zone  140  are independent of each other, and thus the zones can be configured to convey air in the same direction or in opposite directions.  
      Each fan  144  includes a spark detector  164  ( FIG. 9 ) that detects sparks, e.g., caused by excessively high temperatures and/or combustible gases in the air flow, to shut down the hot has distribution system  110 .  
      Each of the first distribution zone  138  and the second distribution zone  140  includes an exhaust duct  149  ( FIG. 7 ) for allowing a portion of the air in each zone to be exhausted, in which case make-up air (e.g., fresh air) is added as a replacement. In general, a major portion of the air in each zone is reheated and recycled, which makes the process more efficient. To minimize fire risk within the consolidation apparatus, however, a portion of the used air may be exhausted, which removes some VOCs, dust and fines via continuous dilution. In a specific implementation, the quantity of fresh make-up air is about 30% by volume.  
       FIG. 23  shows a schematic representation of a specific implementation lamination apparatus  600 . The lamination apparatus  600  has a pair of supply rollers  602 ,  604  positioned to supply laminate material L for laminating major surfaces of the product prior to pressing. After the laminate material (e.g., polyethylene) has been applied, the product travels through a pair of heated lamination rollers  606 ,  608  that position the laminate L on the product and nip the product to its substantial caliper. In one implementation, a conventional Dubois 2-sided dry-film laminator operating at a roller temperature above about 250° F. was used as the lamination apparatus  600 . According to the methods used in lamination, there is no discrete boundary between the applied laminate and the underlying surface.  
      Following final densification in a suitable pressing apparatus, the product is subjected to surface treatment.  FIG. 24  shows a schematic representation of a surface treatment apparatus  800 . As illustrated, the product travels on a belt to a flame treatment apparatus  802 . At the flame treatment apparatus  802 , the product transitions from the belt to small rollers  803  positioned at the entrance and exit of the flame treatment apparatus  802 . Within the flame treatment apparatus  802 , at least both major surfaces of the product are surface oxidized by flame treatment in a single pass by flame treatment elements  804 . One suitable flame treatment apparatus is the Wise Corporation twin burner flame treater. Thereafter, the product enters a spraying station  806 . Within the spraying station  806 , which is conventional, at least both major surfaces of the product are sprayed with a grafting chemical by spraying elements  808 .  
       FIG. 15  shows a schematic production line layout  500  of the various apparatus used to produce board products according to the invention. In particular, the illustrated production line layout illustrates (1) continuous face forming in parallel with (2) continuous consolidation, lamination and pressing. As described above, flaked wood and granulated plastic are sorted and prepared described above in a material preparation and supply area  400 , and then supplied to the face forming apparatus  300 . Faces are formed with the face forming apparatus  300 . In a parallel arrangement, board products are formed in the consolidation apparatus  100 , laminated in a lamination apparatus  600  and then pressed to final dimensions in a press  700 . Although not shown, an appropriate surface treatment apparatus, such as the surface treatment apparatus  800  described above, may be positioned downstream of the press  700  to continuously treat the pressed board product.  
     Vi. EXAMPLES  
      The following examples are provided solely to illustrate certain particular features of the present invention, but the invention should not be limited to the particular features described.  
     Example 1  
      This example describes the formation of a {fraction (7/16)}-inch-thick composite product having a density of about 50 pounds/ft 3  and comprising about 50% waste polyethylene. Waste thermoplastic material, primarily polyethylene, but perhaps containing minor fractions of other thermoplastic materials, and wood were comminuted into flakes. A mixture was then formed by hand comprising about 115 grams of comminuted thermoplastic material and about 126 grams of wood flakes having a moisture content of about 9.8%. This mixture was then placed in a containment bin for thermal consolidation in a batch hot-air consolidation apparatus that uses the principles of the apparatuses illustrated in  FIGS. 3-6 , the batch apparatus having only one cell for applying hot air to the entire area of one surface of the mixture in the containment bin. Hot air at a temperature of about 400° F. was applied to the mixture generally at a pressure of less than about 1-2 psig for a period of about 1 minute. The thermally consolidated mixture was removed from the consolidation apparatus and pressed to its final density in a conventional platen press at a pressure of about 550 psig.  
     Example 2  
      Composite products made in accordance with the present invention may advantageously be overlaid with a paper sheet or material, a plastic sheet or material, or both. For example, portions of the cellulosic material may extend upwardly from the surface of the board product, which is referred to herein as telegraphing. Overlaying the board product with a paper sheet or material, a plastic sheet or material, or both, solves problems associated with telegraphing. The present example describes the formation of a board product having an overlying layer of a thermoplastic material.  
      A board product was made as substantially described in Example 1. A 2 millimeter-thick sheet of low density polyethylene was then placed on each major opposing surface of a warm composite product after thermal consolidation. The overlaid product was then pressed for a period of about 2 minutes at about 550 psig in a conventional heated platen press heated to a temperature of about 275°.  
     Example 3  
      This example describes the formation of a {fraction (7/16)}-inch-thick three-layer board product having a core between two outer layers comprising filler and thermoplastic fines. A first mixture was made comprising 17 grams of thermoplastic material fines, primarily polyethylene, and 18 grams wood fines having a moisture content of about 11.1%. This mixture was formed into a mat in a containment bin. A second mixture for the product&#39;s core was then made comprising about 82 grams thermoplastic material and 102 grams cellulosic wood flakes having a moisture content of about 12.42%. This mixture was formed into a mat on top of the mat situated in the containment bin. Finally, a third layer substantially identical to the first layer was placed on top of the core layer in the containment bin.  
      Air at a temperature of about 400° F. was applied to the mixture at a pressure of about 1-2 psig for a period of about 1 minute. The thermally consolidated mixture was removed from the consolidation apparatus and pressed to its final density at a pressure of about 550 psig using a conventional platen press.  
     Example 4  
      This example describes the formation of a {fraction (7/16)}-inch-thick three-layer board product having a core between two outer layers comprising fines, the board product being overlaid with a plastic layer. A three-layer board product was made substantially as described above in Example 3. A 0.002-inch-thick sheet of low density polyethylene was then placed on each major opposing surface of the board product after thermal consolidation. The overlaid product was then pressed in a conventional platen press at a pressure of about 550 psig and a temperature of about 275° for a period of about 2 minutes.  
     Example 5  
      This example describes the formation of a {fraction (7/16)} inch board having a density of about 50 pounds/ft 3  and comprising about 50% polyethylene, the board product being surface modified and painted. A board product was made substantially as described above in Example 1. The surface of the product was subjected to corona discharge to oxidize the surface of the product (products also have been made where the surface of the product was oxidized by flame treatment). A solution, such as an aqueous solution, an organic solution, particularly alcoholic solutions, and most typically an aqueous/organic solution (e.g., water and alcohol) of surface-modifying agents, such as silanes, ketonates, zirconates, amines, chromium compounds, etc., was applied to the product. The surface-modified composite product was then painted and allowed to dry.  
      The adhesion of the paint to the composite product was then tested using an Elcometer according to ASTM D4541-89 and compared to products that had not been surface modified. These tests showed a significant improvement in the wet and dry adhesion of the surface modified and painted product as shown in Table 1. Wet adhesion involved exposure to condensed humidity conditions for one week and one month at 60° C.  
               TABLE 1                          Dry and Wet Adhesion on a Polyethylene Surface       (Pull-off Strength in MPa)                             Treatment   Dry   Wet (1 week)   Wet (1 month)               Untreated   0   0   0       Corona only   1.9 ± 0.1   2.1 ± 0.8   1.8 ± 0.8       Corona plus graft   3.1 ± 0.4   2.3 ± 0.6   2.3 ± 0.3                  
 
      Further tests showed that for flame treated material, the non-surface-modified and painted product fails at the paint-product interface. Surface-modified products exhibited a large percentage of cohesive failure of the product itself, not at the paint-product interface. See Table 2.  
               TABLE 2                          Surface Treatment and Graft Chemical Application       on Cohesive Failure (% CFS)                                 Treatment   Pull-off Strength (MPa)   % CFS                       Flame only   2.1 ± 0.5   18           Flame plus graft   2.7 ± 0.2   68                      
 
     Example 6  
      This example discusses the production of composite products having crosslinked thermoactive materials. Waste thermoplastic material, primarily polyethylene, and wood were comminuted into flakes. A solution (0.5 g/ml in hexanes) comprising various percents of peroxide crosslinking agents, in this example dicumyl peroxide, by weight of the thermoplastic material as indicated below in Table 1 was sprayed onto the thermoplastic material. A mixture was then formed by hand comprising about 115 grams of the comminuted thermoplastic material (after soaking in the crosslinking agent solution) and about 126 grams of wood flakes having a moisture content of about 9.8%. This mixture was then placed in a containment bin for thermal consolidation. Hot air was applied to the mixture at a pressure of about 1-2 psig and a temperature of about 400EF in the consolidation apparatus for a period of about 1 minute. The thermally consolidated mixture was removed from the consolidation apparatus and pressed to its final density at a pressure of about 550 psig using a conventional platen press.  
      The creep rate (displacement/time) of the products made according to this example was then determined with respect to the gel fraction of the product, which indicates the percent crosslinking that occurred with the thermoactive material. The gel fraction was determined according to ASTM D2765-95 modified to account for the wood in the composite, where the wood was treated as a filler in the method. For purposes of comparison, the creep rate for a product made without crosslinking the thermoactive material was measured as being 4.76×10 −4  mm/minute at a load of 50 Newtons. Loads for normal use of the product are expected to be about 0.1 to about 5 Newtons. Composite products made according to the method of the present invention and having crosslinked thermoactive material had substantially reduced creep rates as shown by Table 3.  
                       TABLE 3                           Gel Fraction           Peroxide Addition   (% of plastic)   Creep Improvement (%)                  0   0   —       2   33 ± 3   84       6   30 ± 4   78                  
 
     Example 7  
      This example further discusses the production of composite products having crosslinked thermoactive materials. Waste thermoplastic material, primarily polyethylene, and wood were comminuted into flakes. A mixture was then formed by hand comprising about 115 grams of comminuted thermoplastic material and about 126 grams of wood flakes having a moisture content of about 9.8%. This mixture was then placed in a containment bin for thermal consolidation. Hot air was applied to the mixture at a pressure of about 1-2 psig and a temperature of about 400EF in the consolidation apparatus for a period of about 1 minute. The thermally consolidated mixture was removed from the consolidation apparatus and pressed to its final density at a pressure of about 550 psig using a conventional platen press.  
      The composite product was then subjected to electron-beam (E-beam) treatment to crosslink the thermoplastic material. The E-beam crosslinking was done by E-beam Services of Cranberry, N.J., but also could be done by other entities, such as the Atomic Energy Commission Laboratory, Whiteshell, Manitoba, Canada. The product can be subjected to E-beam treatment at any time following thermal consolidation, but typically is best accomplished while the product is still warm. Various E-beam doses in Mrads were tried. The creep rate (displacement/time) of the products made according to this example was then determined with respect to the gel fraction of the product. The gel fraction again was determined according to ASTM D2765-95 modified to account for the wood in the composite, where the wood was treated as a filler in the method.  
      The percent decrease in creep relative to a non-crosslinked composite product was determined, as summarized below in Table 4. These results are substantially similar to the results presented for chemically crosslinked substrates. E-beam likely will be a preferred process for commercial production because it can be implemented less expensively than can chemical crosslinking.  
                       TABLE 4                       E-Beam Dose   Gel Fraction           (Mrads)   (% of plastic)   Creep Improvement (%)                                            0   0   —       6   40 ± 4   85       16   60 ± 4   86                  
 
     Example 8  
      As shown in  FIG. 25 , a first board product having a first face F 1 , a core C A  and a second face F 2  can be joined using a conventional adhesive L A  to a second board product having a first face F 3 , a core C B  and a second face F 4 . Depending upon the particular adhesive used, the surfaces to be adhered may have to be flame treated and sprayed with a grafting chemical. With conventional epoxy, normally only flame treatment would be required. Alternatively, the faces can be joined with a molten layer of polyethylene that would bond the two board products together upon cooling.  
      The first board product and the second board product may have the same or different dimensions. As one example, if the illustrated first and second board products each have a thickness of {fraction (7/16)} inch, then a {fraction (7/8)} inch board product can be formed by joining the first and second board product together at one of their respective major surfaces.  
      In this particular example, the first board product and the second board product were surface modified using flame treatment, and the modified surfaces were joined using a Durobond two-part epoxy applied by hand. The assembly was clamped and allowed to cure overnight. The assembly was then exposed to water for 2 days and outdoor weathering for one week. No delamination occurred.  
     Example 9  
      As shown in  FIG. 26 , a first board product having a first face F 1  and a core C A  was joined using a conventional adhesive L A  to a second board product having a core C B  and a second face F 2 , at the intersection of the core C A  and the core C B , consistent with the description in Example 8.  
      The present invention has been described in accordance with preferred embodiments. However, it will be understood that certain substitutions and alterations may be made thereto without departing from the spirit and scope of the invention.