Abstract:
A method for making in a single step a multiple-ply panel comprising the steps of forming and sandwiching a raw mat assembly of resinated furnish between two sheets of green incised wood veneer, applying a layer of thermosetting high-moisture tolerant adhesive at interfaces between the green veneer and core mat, and applying pressure and heat to cure the consolidated panel assembly for a duration sufficient to secure good internal bond within the substrate panel.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to the manufacture of plywood. More particularly, this invention relates to the manufacture of plywood having a resinated mat core such as a waferboard core.  
         BACKGROUND OF THE INVENTION  
         [0002]    Hardwood plywood is defined in the ANSI/HPVA HP-1-2000standard as, “A panel composed of an assembly of layers or plies of veneer or veneers in combination with a lumber core, particleboard core, MDF core, hardboard core, or of special core material joined with an adhesive. Except for special constructions, the grain of alternate plies is at right angles and the face veneer is a hardwood species.” 
           [0003]    Traditionally hardwood plywood was made by “peeling” tree stems on a large rotary lathe to yield wet veneers of uniform thickness. The wet veneers were next clipped to uniform width and stacked in separate bins by grade. Then they were dried in roller dryers to remove excessive moisture as required for gluing and pressing, sorted again into different grades, repaired for natural defects and conditioned for uniform moisture content. Conditioned veneers were next coated with appropriate glue between adjacent plies, laid up in a veneer assemblies having face and back veneer as outer faces with respective grain angles along a “major axis”, centres and intermediate plies with respective grain angles perpendicular to the face and back veneers, and a core which is an inner ply with a respective grain angle in the same direction as that of the outer plies.  
           [0004]    The face and back veneers usually consist of thin decorative veneers from, such wood species as oak, cherry, birch, maple, etc. The inner plies were usually made from aspen, spruce and other lower cost species. The glued veneer assemblies were pre-pressed as a single stack for moisture equalization and tack build-up, and pressed in a multi-opening hot press into plywood panels. The final thickness of hardwood panels varied according to the number of plies. Each pressed plywood panel was held together by the cured resin bond. After conditioning in hot stacks, these plywood panels were trimmed to size (usually 4ft ×8 ft or 1.2 m ×2.4 m in nominal size), patched for external defects, and sanded to meet the product specification requirements.  
           [0005]    More recently a process has been developed to substitute the veneer inner plies for a homogenous wafercore (a term for waferboard used as core) to simplify and improve the consistency of the core material. By adding intermediate plies of veneer on each side, the resulting wafercore substrate provides a uniform and stable surface for thin decorative veneers.  
           [0006]    “Waferboard” is defined in CSA 0437.0-93 as, “A panel containing layers of only randomly placed wafers.” “Wafers” in turn are defined as, “a specific type of wood flake produced as a primary function of specialized equipment (i.e. a waferizer) and having a controlled length of at least 30 mm (1¼ inch) along the grain direction, a controlled thickness, and a variable or controlled width. Each wafer is essentially flat and has the grain of the wood running predominantly in the plane of the wafer. In overall character, wafers resemble small pieces of thin veneer. Wafers purposely produced with a narrow width to facilitate alignment are called strands.” 
           [0007]    The more recent process yields a balanced panel of stressed-skin construction for industrial application with superior strength, good dimensional stability, good machinability and decorative advantage with less thickness variation and lower formaldehyde emissions than more traditional plywood panels. Either process has similar drawbacks in requiring the drying of green veneer which requires labour and time to monitor and control the drying conditions, plus costly natural gas as a purchased fuel to provide heat for the dryers.  
           [0008]    Either process involves a significant amount of handling in the drying of the green veneer. The purchase and operation of a dryer represents capital and maintenance costs.  
           [0009]    It is an object of the present invention to provide a faster and more economical process for the manufacture of composite plywood.  
         SUMMARY OF THE INVENTION  
         [0010]    A method for making a multiple-ply wood composite panel including the steps of:  
           [0011]    (i) forming a panel substrate by sandwiching a mat of resinated furnish between two sheets of green incised wood veneer;  
           [0012]    (ii) applying a thermosetting and moisture compatible interface adhesive between the wet veneer sheets and resinated furnish to form an unconsolidated panel;  
           [0013]    (iii) applying pressure and heat to the unconsolidated panel in an amount and for a duration sufficient to cure the resin and the adhesive to form said composite panel.  
           [0014]    The resinated furnish may be formed from wood wafers, wood strands, wood particles, wood fibres, wood veneer and lumber. In one embodiment the resinated finish is of random wafers.  
           [0015]    Preferably, the sheets of green veneer are selected to be within a similar range of moisture content through a moisture sorting system in the ranges of 30% to 49%, 50% to 80%, 81% to 100% and over 100% based on an oven-dry method.  
           [0016]    The interface adhesive may be an MDI resin system or equivalent in moisture resistance to be applied at a rate from 35 to 70 grams per square meter depending on wood species, surface roughness and moisture content of veneer. The resin may be cured at a temperature of from 130° C. to 190° C. and a pressure of from 24 to 28 kg/cm 2  for a duration of 4 to 6 minutes.  
           [0017]    Heat may be applied from opposed hot press platens having a surface temperature of from 200° C. to 220° C. Resin curing time may be from 14 to 18 seconds/mm thickness depending on the board thickness, veneer moisture content and the type of adhesive system used.  
           [0018]    Powder or liquid phenolic adhesives may be used for the resinated finish and may be applied in amounts from 1.75% to 2.5% by weight (dry basis) for a composite core material depending on the grade requirement. Alternatively, regular or fortified low-fume resins may be used for interior grade panels.  
           [0019]    The depth of incision on green veneer sheets may be all the way through their respective thicknesses.  
           [0020]    The composite panels formed as described above may be further sanded on one or both sides after pressing for lamination with thin decorative veneers as final face and/or back coverings. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0021]    Preferred embodiments of the present invention are described below with reference to the accompanying illustrations in which:  
         [0022]    [0022]FIG. 1 is a schematic diagram illustrating a multiple-ply panel pressing process according to the present invention. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0023]    [0023]FIG. 1 illustrates a multiple-ply panel  10  between opposed platens  20  of a typical hot press. The balance of the hot press is not illustrated but would typically be steam or thermal-oil heated and of known construction. Representative of a suitable hydraulic hot press is a MULTI-DAYLIGHT® steam heated press with simultaneous arm closing and an electronic press controller. Alternatively, a continuous hot press with proper set-up can be used to achieve a similar result.  
         [0024]    The panel  10  includes a mat of resinated furnish hereinafter referred to as “a resinated mat”  12  sandwiched between two sheets of green incised wood veneer  14 . “Resinated furnish” is a term used to refer to wafers in combination with phenolic resins prior to consolidation. Between the veneer sheets  14  and the resinated mat  12  is a layer of thermosetting moisture-tolerant adhesive  16 .  
         [0025]    The panel  10  may initially be formed as an unconsolidated panel with two veneer sheets  14  coated with adhesive  16  applied to an inner face  22  of the sheets  14  or alternatively to the resinated mat  12  before overlaying the veneer sheets  14  over the resinated mat  12 .  
         [0026]    The unconsolidated panel I then pressed between the hot press platens  20  to cure the resinated furnish and set the interface adhesive and form the consolidated panel  10 . The surface temperature of the platens  20 , the type of resin and adhesive, the amount of resin and adhesive  16 , the pressure and duration of heating and pressing must be selected to suit the dimensions and the grade of the panel being pressed. Using the pressing parameters set out below as a guideline, a skilled press operator can optimize the pressing strategy for a specific product.  
         [0027]    The incising of veneer is becoming a common practice in the plywood industry. Basically, it involves using a proprietary cutter to make incisions in the veneer. The incisions may extend partially or completely through the veneer. The purpose of the incising is to provide a conduit for steam formed during hot pressing to escape easily through the veneer and avoid undesirable delamination due to blows and blisters at the end of the pressing cycle. Preferably the green wood veneer for the present process is incised completely through the veneer thickness, and the incisions are regularly spaced throughout the entire veneer sheet  14 .  
         [0028]    As the adhesive  15 , an MDI resin (isocyanate resin) or an equivalent resin system can be applied at the rate from 35 to 70 grams per square meter depending on wood species, surface roughness and moisture content of veneer.  
         [0029]    As a wafercore binder, a combination of “slack wax” and powdered or liquid phenolic resin may be used. Slack wax or emulsified wax would first be applied to the wood furnish at the rate from 0.75% to 1.0% by weight. Phenolic resin is subsequently applied at the rate from 1.75 to 2.5% by weight (oven-dry basis). Alternatively other resin systems such as high-moisture resistant low-fume fortified urea-formaldehyde resin can be applied as well.  
         [0030]    The hot press platens would typically have a surface temperature from 190° C. to 220° C. depending on the resin system and moisture content of the mat assembly. Typical pressing time 14-18 seconds/mm of board thickness.  
         [0031]    To ensure a balanced panel, both sheets  14  of incised green wood veneer within the unconsolidated panel should be selected to be within a similar range of moisture content. In practice, it has been found that the green veneer can be sorted into moisture groups having 30% to 49%, 50% to 80%, 81% to 100% and greater than b 100% moisture based on oven-dry weight of the veneer.  
         [0032]    Hot pressing typically generates steam from water inside the green veneer which in turn “injects” itself inwardly through the incisions. This drives the heat faster into the core layer and assists in the wood plasticization and resin curing process. The process requires a high-moisture tolerant adhesive that cannot be washed out by steam.  
         [0033]    Once resins inside the unconsolidated panel  10  have been cured, the resultant consolidated panel  10  may be sanded and hardwood veneer applied to one or both sides in a normal veneering process similar to hardwood plywood.  
         [0034]    Thus the above process produces a multiple-ply panel in one step and eliminates the need to dry, sort and otherwise process the wet wood veneer. It also eliminates the need for a veneer dryer or the use of natural gas as a fuel source. The platens  20  may be steam heated and the fuel for generating the steam may be wood waste hence converting scrap into fuel rather than requiring natural gas.  
         [0035]    The above description is intended in an illustrative rather than a restrictive sense. Variations to the exact process parameters set out may be apparent to those skilled in the relevant art without departing from the scope of the invention as defined by the claims set out below.