Patent Publication Number: US-6656399-B1

Title: Method for producing shaped bodies

Description:
This application claims the benefit under 35 U.S.C. 371 of PCT/EP 99/03155 filed May 7, 1999. 
    
    
     The present invention relates to a method of manufacturing shaped bodies, in particular plates, from particles having a surface with a wax layer, in particular straw, hemp stalks, maize stalks, grapevine stalks or the like, which are scattered to form a mat and subsequently pressed to form the shaped body. 
     A method of this kind is known from DE-OS 3 021 455. In this method, the particles are first comminuted by chopping in a first process step, whereupon they are split up into chip fibers in a second, subsequent step. 
     In a further subsequent process step, the chip fibers are dried, whereupon the dried chip fibers are admixed with a binder and a paraffin emulsion. 
     The particles which have been coated with glue in this way are further comminuted in a further process step, with the further comminution of the glue-coated particles being important to the method described in this publication. It is namely specified that the number of those particles surfaces which have an unfavorable effect on the later bonding during the pressing of the mat are substantially reduced by said further comminution of the glue-coated particles. At the same time, the binder is distributed even better to all now present surfaces of the particles, and thus further homogenized, during the further comminution due to the smearing effect known from chip gluing. 
     While boards with good strength properties are generated with the method described in DE-OS 3 021 455, a relatively high binder content and the addition of a paraffin emulsion is required. Furthermore, a relatively large number of separate process steps are required, with each process step increasing the costs of the process. 
     It is the object of the invention to develop a method of the kind first mentioned such that the required proportion of binder can be reduced and the number of required process steps reduced. 
     This object is satisfied in accordance with the invention starting from a method of the kind first mentioned by the particles first being comminuted in a single process step, being heated in a following process step and further comminuted in a subsequent process step and a binder begin finally admixed with the further comminuted particles. 
     It has surprisingly been found that, on the one hand, a separate process step to split up the comminuted particles subsequent to the first comminution process can be dispensed with if, in accordance with the invention, the comminuted particles are first immediately heated, then further comminuted and only then admixed with a binder. On the other hand, with this procedure, the quantity of binder required can be reduced with respect to the known methods without impairing the strength properties of the plates generated. 
    
    
     The method in accordance with the invention is thus simpler and of lower cost than the method of the kind first mentioned, since both a separate process step can be saved and the required proportion of binder reduced. Whereas the required dosage of the binder in the method of the kind first mentioned is given as 6% solid resin/atro straw, in the method in accordance with the invention a dosage of approximately 4.5% is sufficient. 
     In accordance with an advantageous embodiment of the invention, the comminuted particles are roasted, that is heated for so long and/or to such a temperature that they attain a certain brittle porosity, in particular at their surfaces, in particular so that cracks occur on the surfaces of the particles. The wax layer present on the surface of the particles splits off during the further comminution process following the heating due to the brittle porosity of the particles created by the heating in accordance with the invention. This achieves two positive effects. On the one hand, the addition of a separate paraffin emulsion can be omitted since the split off wax particles take on its function. On the other hand, the binder added after the further comminution is absorbed better by the surface of the particles now freed of the wax layer so that less binder needs to be added in total. 
     In the following, typical manufacturing and quality characteristics are given by way of a preferred embodiment of the manufacturing method in accordance with the invention: 
     The starting particles are poured into a chopper with an initial moisture content, for example, of approximately 15% and are there comminuted in one single process step. The comminuted particles are subsequently heated and thereby dried until they have a moisture content of approximately 2-3% by weight. 
     A roasting of the comminuted particles is effected by the heating, whereby in particular the surface of the particles attains a brittle porosity which presents itself, for example, as irregularly arranged cracks on the surfaces of the particles. 
     In a next process step, the roasted particles are further comminuted, whereby the wax layer present on the surface splits off and the split-off particles are mixed with the further comminuted particles. The further comminuted, non-fractionated particles are admixed with binder to an amount of approximately 4.5%/atro straw so that a glued moisture content of approximately 4% by weight is achieved. 
     The glue-coated particles are scattered to form a mat using suitable scattering methods and pressed to form the desired shaped bodies, for example, at 200° C. under a compression pressure of max. 3.5 N/mm 2  for a compression time factor of, for example, 11.5 seconds per millimeter of thickness of the mat. With a mat thickness of approximately 19 mm, a compression time of approximately 3.6 minutes thus results. 
     Typical quality characteristics of a board manufactured in accordance with the method in accordance with the invention are: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Board thickness 
                 19 mm 
               
               
                   
                 Bulk density 
                 640 kg/m 3   
               
               
                   
                 Bending strength 
                 16 N/mm 2   
               
               
                   
                 Transverse tensile strength 
                 0.57 N/mm 2   
               
               
                   
                 2 hours of swelling 
                 3.2% 
               
               
                   
                 24 hours of swelling 
                 16.5%