Abstract:
A wood particle mixture is described especially made from raw wood for an extruded or injection-molded wood-plastic composite material, especially manufactured from wood-plastic compound, which significantly improve the strength properties in a wood plastic composite material. Each wood particle has certain dimensions and certain shaping.

Description:
INTRODUCTION AND BACKGROUND  
       [0001]     The present invention relates to a wood particle mixture and also to a method for producing a wood particle mixture, especially from raw wood, for an extruded or injection-molded wood-plastic composite material, especially wood-plastic compound. In addition, the invention relates to a device for performing the method according to the invention.  
         [0002]     It is known to include such wood particle mixtures in various plastics, especially in polypropylene, wherein each individual wood particle is encased by plastic. Such wood-plastic composite materials are known from WO 96/34045. Because their strength properties are largely dependent on the homogeneity of the compound, the individual wood particles of the wood particle mixture lie in a range of sizes featuring pre-defined limits. However, the shape of the individual particles and their alignment within the wood-plastic composite material also have a strong influence on the strength properties.  
       SUMMARY OF THE INVENTION  
       [0003]     A wood particle mixture, a method, and a device of the class described in the introduction, which significantly improve the strength properties in a wood-plastic composite material are described herein.  
         [0004]     According to the invention, there is described a wood particle mixture, especially made from raw wood for an extruded or injection-molded wood-plastic composite material, especially manufactured from wood-plastic compound, characterized in that each wood particle has two cut surfaces which define its thickness and which run approximately in the direction of its grain, and each wood particle has cut edges and/or fracture edges defining its length in the direction of the grain and also its width perpendicular to the direction of the grain. In a further aspect of the invention, there is described a method for producing a wood particle mixture, especially made from raw wood, for an extruded or injection-molded wood-plastic composite material, e.g., manufactured from wood-plastic compounds, especially for producing the herein described wood particle mixture, characterized in that flat chips are generated from the raw wood by means of at least one blade, the flat chips are cut up, the cut-up flat chips are broken up into flat splinters, and the flat splinters are filtered.  
         [0005]     The wood particle mixture is distinguished in that each of its wood particles has two cut surfaces that define its thickness and run approximately in the direction of its grain, and in that each wood particle has cut and/or fracture edges that define its length in the grain direction and define its width perpendicular to the grain direction. Wood particles shaped in this way thus feature fewer, but elongated, fibers relative to its body volume. The use of such wood particle mixtures in a wood-plastic composite material has the advantage that the connections between the individual wood particles and the plastic run essentially along the grain, so that the individual fibers effectively increase the tensile strength and flexural strength of the wood-plastic composite material.  
         [0006]     According to one refinement of the invention, at least 90 wt % of the wood particles have a thickness between 0.2 mm and 2.0 mm, preferably a thickness between 0.4 mm and 0.5 mm, and a length and width up to 8.0 mm. These details on size ranges are to be understood as preferred ranges, with which sufficient homogeneity in the wood composite material can be achieved. However, especially preferred is that at least 90 wt % of the wood particles have a thickness between 0.2 mm and 1.4 mm and a length and width up to 2.5 mm.  
         [0007]     According to a next refinement of the invention, the cut surfaces running in the grain direction are arranged approximately parallel to each other. Thus, the individual wood particles each receive the shape of a flattened fiber strand section, which further improves the strength properties of the wood-plastic composite material.  
         [0008]     It is proposed that each wood particle consist of spruce wood. Spruce wood satisfies the requirements for the wood composite material and is available in sufficient quantities particularly economically in Europe. It is further proposed that each wood particle consist of loblolly pine wood, which is available in the USA in sufficient quantities at economical prices. However, it is also conceivable to use other types of wood, e.g., if these can be obtained especially economically in one region or part of the world or if these exhibit a special suitability in the particular use for wood-plastic composite materials. Mixtures of different woods can also be used. Any suitable polymer, including thermoplastics and thermosetting resins can be used to make the extruded or injection molded articles.  
         [0009]     It is further proposed that the wood particle mixture have a moisture content of less than 15% relative to the absolute dry mass. The low moisture content is primarily important to guarantee uniform operating conditions for the extrusion of wood plastic compounds. The extrusion process generates high temperatures, which lead to controlled evaporation of the moisture bound in the wood particle mixture. The same also applies for injection molded parts. Furthermore, with a moisture content of less than 15%, fungal attack is for the most part prevented, so that the final product made from the wood-plastic composite material is fit for storage.  
         [0010]     According to another refinement of the invention, the wood particle mixture has an extract content of less than 5.5% relative to the absolute dry mass. This specification is based on measurements according to the ethanol-cyclohexane method or the hot-water method. A low extract content, especially a low resin content, is primarily important, so that the top surfaces of the extrusion molds do not stick and to be able to guarantee on the extrusion molds constant friction resistance that is not too high.  
         [0011]     According to the method of the invention it is proposed that flat chips be generated from the raw wood by means of at least one blade, that the flat chips be cut up, that the cut-up flat chips be broken up into flat splinters, and that the flat splinters be sifted. An especially large-volume chip pocket is advantageously allocated to each blade, so that the flat chips feature an especially low curvature.  
         [0012]     According to a refinement of the invention, the flat chips are generated by blades arranged parallel to the grain of the raw wood. This has the advantage that the flat chips in the chip pocket are bent only around a line running in the direction of the grain. The individual fibers are thus not bent.  
         [0013]     In the following, the individual method steps are described in more detail:  
         [0014]     The flat chips generated from the raw wood are led into a wet bunker, which represents intermediate storage for continuous removal of the flat chips. A stirring machine integrated in the wet bunker prevents sticking or bridging of the flat chips. Then the flat chips are supplied from the wet bunker via a vibrating flat channel at least to a cutting mill with a filter passage. The cutting mill advantageously features a cutting roll with V-shaped blades and also a counter blade corresponding to the blades. By means of air, the cut-up flat chips are suctioned from the cutting mill through the filter passage, led into a wet cyclone, and there separated from the air stream. Then the cut-up flat chips are led from the wet cyclone into a dosing bunker. From the dosing bunker, the cut-up flat chips are led by means of air into a dryer. The dosing bunker is used as intermediate storage for the dryer. This has a pressure generator and a cyclone, wherein the flat chips are led through the dryer with the air stream generated by the pressure generator and are separated again from the air stream in the cyclone. The cut-up and dried flat chips are then led from the dryer into a dryer bunker, which is used in turn as intermediate storage, and from which the cut-up and dried flat chips are supplied via a worm to a hammer mill, which also features a filter passage. Here, the cut-up and dried flat chips are broken up into flat splinters with beating tools and suctioned through the filter passage from the hammer mill. Then the flat splinters are led into a drying cyclone, where they are again separated from the air stream. From the drying cyclone, the flat chips are then led into a filtering device, in which the flat splinters lying within a certain range of sizes for the wood particle mixture are filtered out and flat splinters lying above the predetermined range of sizes are fed back into the drying bunker. The returned flat splinters are thus reduced in size again in the hammer mill.  
         [0015]     In the filtering device, the flat splinters are sorted according to the size of their cut surfaces defining their thickness. The filtering device advantageously has a damping plate, on which the flat splinters are laid flat. By means of vibrating motion, the flat splinters then slide over the inclined damping plate onto the first flat sieve under which further flat sieves are arranged for filtering out predetermined ranges of sizes of the flat splinters.  
         [0016]     According to an especially advantageous refinement of the invention, it is preferred that the flat splinters filtered from the filtering device and lying within a predetermined range of sizes be led to a wobble sieve, which separates out flat splinters lying below the predetermined range of sizes for the wood particle mixture. In this way, additional fine material, which could not be separated by the filtering device, is subsequently separated in an especially efficient way.  
         [0017]     According to another refinement of the invention, the flat chips are cut up in undefined directions. This is realized especially in that the flat chips are supplied via the vibrating flat channel in an undefined alignment to the V-shaped blades of the cutting mill.  
         [0018]     According to an especially advantageous refinement of the invention, extracts, especially resins, are washed out from the raw wood and/or the flat chips and/or the cut-up flat chips and/or the flat splinters and/or the wood particle mixture. This is realized, e.g., with the ethanol-cyclohexane method or the hot-water method.  
         [0019]     The device according to the invention is used for performing the method according to the invention and is distinguished in that the filtering device has at least two flat sieves arranged one after the other with different mesh widths. The flat sieves are used to filter out the flat splinters lying in a predetermined range of sizes for the wood particle mixture. To be able to realize the size ratios of the individual wood particles mentioned for the wood particle mixture according to the invention, the first flat sieve has an open mesh width between 2 mm and 4 mm. In contrast, the second flat sieve has an open mesh width between 0.5 mm and 1.0 mm. The meshes of the flat sieves are shaped like squares.  
         [0020]     In addition, it is provided that the cutting mill have at least one filter passage on the output side, which is formed as an elongated filter, whose elongated holes have a length between 40 mm and 70 mm and a width between 3 mm and 8 mm.  
         [0021]     The hammer mill has at least two different alternating filter passages on the output side, which are formed as elongated filters, wherein the difference between the filter passages consists essentially in that the elongated holes of one of the filter passages has approximately twice the width of the elongated holes of the other filter passage. Such a configuration advantageously prevents the flat splinters from clogging or stopping up the filter passages. The elongated holes of the filter passages arranged in the hammer mill have a length between 10 mm and 50 mm and a width between 0.5 mm and 2.5 mm.  
         [0022]     According to a refinement of the invention, the elongated holes of the filter passages arranged in the hammer mill and also the filter passages arranged in the cutting mill have a rotational spacing or a bridge width between 0.5 mm and 5.0 mm to each other.  
         [0023]     The blade for generating the flat chips has a length between 15 mm and 25 mm. The blades are advantageously assembled from a plurality of cutting teeth, which are in turn distributed over the circumference of a cutting roll. In addition, it is proposed that the blade be set in the device such that the ratio of the cutting speed divided by the advancing speed is between 550 and 750, preferably 650. A cutting speed of 39 m/s thus produces a preferred advancing speed of approximately 0.06 m/s. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The invention will be further understood with reference to the accompanying drawings, wherein:  
         [0025]      FIG. 1  shows a plan view of a layout for a device for performing the method according to the invention;  
         [0026]      FIG. 2  shows a perspective view of a flat chip for producing a wood particle mixture according to the invention at an enlarged scale;  
         [0027]      FIG. 3  shows a perspective view of reduced size flat chips from the flat chip shown in  FIG. 2 ; and  
         [0028]      FIG. 4  shows a perspective view of flat splinters from the cut-up flat chips shown in  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0029]      FIG. 1  shows a plan view of a layout for a device, for which the method according to the invention for producing a wood particle mixture is to be described in the following:  
         [0030]     Tree trunks are stripped by a bark peeling device  1 . The debarked tree trunks are then fed to a cutter shaft chipper  2  or to a cutter ring chipper, in which the stripped tree trunks are machined into flat chips. From the cutter shaft chipper  2 , the flat chips are transported via a worm  3  and an elevator  4  into a wet bunker  5 . The wet bunker  5  is configured as intermediate storage for continuous removal of the flat chips. From here, the flat chips are transported by means of two worms  6 ,  6 ′ and two vibrating flat channels  7 ,  7 ′ into two cutting mills  8 ,  8 ′, in which the flat chips are cut up. By means of air, the cut-up flat chips are then suctioned from the cutting mills  8 ,  8 ′ and led into a wet cyclone  9 , which separates the cut-up flat chips from the air. From the wet cyclone  9 , the cut-up flat chips are led with a pressure chain conveyor  10  into a dosing bunker  11 . The dosing bunker  11  is formed as intermediate storage for a dryer  12  and is connected to the dryer  12  by means of a pressure chain conveyor  13 . In this dryer, the cut-up flat chips are dried, which is why the previously described system components are referred to with the term “wet side” and the system components described in the following are referred to with the term “dry side.” 
         [0031]     The cut-up flat chips are suctioned through the dryer  12  and led on the output side into a dryer bunker  15  via a pressure chain conveyor  14 . The dryer bunker  15  is formed as intermediate storage for two hammer mills  16 ,  16 ′, which are each connected via a vibrating flat channel  17 ,  17 ′ and a common worm  18  to the dryer bunker  15 . In the hammer mills  16 ,  16 ′, the cut-up and dried flat chips are broken up into flat splinters. The flat splinters are suctioned by means of air from the hammer mills  16 ,  16 ′ and led into a drying cyclone  19 . In this cyclone the flat splinters are separated from the air stream. From the drying cyclone  19 , the flat splinters are led by means of an inclined damping plate  20  into a filtering device  21 . In the filtering device, the flat splinters lying in a predetermined range of sizes for the wood particle mixture according to the invention are filtered out by means of flat sieves  22  arranged one above the other. Flat splinters lying above the predetermined range of sizes are fed back from the filtering device  21  via a pressure chain conveyor  23  into the dryer bunker  15 . The flat splinters lying in the predetermined range of sizes are led from the filtering device  21  via an elevator (not shown) into a finished goods silo (not shown).  
         [0032]      FIG. 2  shows a flat chip for producing a wood particle mixture according to the invention. The flat chip has two cut surfaces  25 ,  26 , which define its thickness and which run in the direction of its grain  24 . The width of the flat chip in the direction of its grain  24  is defined by cut edges  27 ,  27 ′. The length of the flat chip perpendicular to the direction of its grain  24  is defined by fracture edges  28 ,  28 ′.  
         [0033]      FIG. 3  shows reduced size flat chips from the flat chip shown in  FIG. 2 . The reduced size flat chips have cut edges  29 , which run in undefined directions relative to the grain  24 . Equivalent surfaces and edges are provided with the same reference numbers.  
         [0034]      FIG. 4  shows flat splinters from the cut-up flat chips shown in  FIG. 3 . The flat splinters have fracture edges  30 , which run approximately in the direction of the grain  24 . Equivalent surfaces and edges are provided with the same reference numbers.  
         [0035]     Further variations and modifications of the foregoing will be apparent to those skilled in the art and are intended to be encompassed by the claims appended hereto.  
         [0036]     German prior application 103 27 848.6 of Jun. 18, 2003, is relied on and incorporated herein by reference.