Abstract:
There is provided a method for preparing composite materials from renewable raw materials wherein natural raw materials, especially in fiber form, comprising starch as binder are molded by adding water in accordance with a predetermined temperature and time profile to form boards and molded articles. The advantages of this method reside in said materials being fully biodegradable and free from harmful substances.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method for preparing biodegradable natural fiber materials (wood, textile fibers, paper, etc.) comprising binders from renewable raw materials. Such methods are gaining more and more importance, for example, in the production of environmentally compatible and formaldehyde-free particle boards. 
     BACKGROUND OF THE INVENTION 
     The majority of composite materials made of wood chips or wood fibers is used in the form of flat boards in the furniture or construction business. Unlike in the paper or textile industry, starch paste is not used at all in the wood industry or merely to serve as extender for synthetic resin glue. 
     So far, organic binders from basic petrochemical materials have mainly been employed in the production of board-like wood products. Even if only small portions of such synthetic binders are replaced by binders made of renewable raw materials, substantial ecological and macroeconomic advantages will result. 
     In conventional processes for the production of board-like materials, for example, wood fibers have been prepared by hot-pressing using synthetic binders. The synthetic binders are crosslinked or cured by chemical reactions (polymerization, polyaddition, etc.). During those chemical reactions the wood particles are caused by external pressure to exhibit a degree of compression higher than that corresponding to their bulk density. By curing the synthetic binders, the elastic recovery of the wood particles is reduced. The pressing process is completed when the restoring forces have been fully absorbed by the binders. All conventional methods of preparation have in common that the wood particles are cured prior to the application of glue. Depending on the portion of synthetic binder, a wood chip humidity of 10 to 15% will result after the glue has been applied. Due to the vapor pressure resulting during hot pressing, higher wood chip humidities in conventional preparatory methods will cause tension in the material in excess of the tensile strength in the transverse direction of the board. After the press is opened, the board will crack. 
     SUMMARY OF THE INVENTION 
     The object underlying the present invention is to provide an environmentally compatible and cost-efficient method for preparing fully biodegradable natural fiber materials free from harmful substances. 
     This object is achieved by providing the features indicated in the claims. 
     In achieving the object, the invention is based on the concept of utilizing starch-based binders in the method for preparing fully biodegradable natural fiber materials, coordinating and controlling the procedural steps and selecting a recipe for the amount of binder, water and, if desired, further additives, based on the amount of dry wood chips or wood fibers, thus optimizing the residence time during hot pressing and shaping the mechanical properties of the composite materials prepared in accordance with the desired usage. The humidity of the wood chips used as fibrous material is used as solvent and swelling agent in the production of the natural fiber mixture. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Starch and/or dextrin gums can be economically useful for the preparation of composite materials made of wood chips and wood fibers especially if the necessary procedural requirements are converted to industrial manufacture to enable the preparation of substitutable products in amounts comparable to raw materials for use in synthetic resin binders. 
     Native starch obtainable from various plants is insoluble in cold water and therefore cannot be used as binder in such state. A chemical and/or physical process is required to impart binding properties to the starch. Irrespective of the process used, the starch is meant to be converted to a colloidal system. Evaporation of the solvent (water) yields a solid matrix that may bond more or less strongly to other substances by adhesion. 
     Advantageously, the materials, boards, molded articles and profiles based on natural fibers and prepared in accordance with the present invention are made entirely from renewable raw materials. They are biodegradable, completely free from harmful substances (formaldehyde-free) and can also be used for thermal applications. 
     In the method of the invention the technological parameters during pressing include in particular the press temperature (e.g., 160° C.-220° C.), the mold pressure, the press closing rate, the press time, the nonwoven fabric temperature and the mold phases (supercompression, release, heating period, calibration period). 
     The technological parameters are optimized relative to the recipe parameters and the properties of the raw materials. 
     In addition to the press-associated parameters, the amount of water required for the starch to swell is a recipe factor relevant for the press time. Therefore, suitable additives should be added to the binders, causing the water-binder ratio to be reduced, so that it is possible to activate the humidity of the wood as solvent or swelling agent. 
     The mixture of raw materials is prepared in a suitable mixer, preferably a plough blade mixer with a knife head. All ingredients such as, for example, wood chips, water and starch flour are placed in the mixer as prescribed. After sufficient mixing, the mixture is placed in a mold, compressed for a short time, covered and placed in a heatable press. The cake is then molded using a special method at a predetermined temperature and released from the mold to form the finished board. Thus, fiber composite boards having various mechanical properties can be produced. 
     Additionally, plywood or decorated boards can be placed in the mold and pressed onto the fiber composite boards. 
     Also, the mixture of raw materials can be molded by extruders comprising suitable dies and, if desired, pressed onto the aforementioned boards by extrusion. 
     If binders from renewable raw materials (e.g., starch) are used, the binding process will not be the result of a chemical reaction. The curing of the natural binders is a physical process initiated by evaporation of water. Curing increases as drying proceeds. Such characteristic difference between natural binders and reactive, synthetic binders requires a completely different method that deviates substantially from conventional preparatory processes. 
     Embodiments of the method of the invention are shown below. 
     EXAMPLE 1 
     1,760 g of medium-fine wood chips were placed in a 130 liter plough blade mixer with a knife head and sprayed with 520 g of water. Then 520 g of potato starch were added and mixed for one more minute. 
     Of this mixture, 50% was removed and uniformly sprinkled on a metal sheet in a 420×420 mm (inner size) frame. The bulk was compressed at a force of 700N for a short time. The thus obtained cake was covered with a further metal sheet and placed in a heatable press. 
     The cake was compressed at a press temperature of 170° C. for two seconds to a level of 10 mm. The mold pressure was maintained at 0.1N/mm 2  for 20 minutes. The distance between the pressing surfaces was adjusted to 10 mm for one minute. Then the mold was released and the finished board removed. 
     The laboratory experiments led to the following results: 
     
         ______________________________________Bulk density         530    g/lFlexural strength    18     N/mm.sup.2______________________________________ 
    
     EXAMPLE 2 
     1,760 g of medium-fine wood chips were placed in a 130 liter plough blade mixer with a knife head and sprayed with 520 g of water. Then 520 g of corn starch were added and mixed for one more minute. 
     Of this mixture, 50% was removed and uniformly sprinkled on a metal sheet in a 420×420 mm (inner size) frame. The bulk was compressed at a force of 700N for a short time. The thus obtained cake was covered with a further metal sheet and placed in a heatable press. 
     The cake was compressed at a press temperature of 170° C. for two seconds to a level of 10 mm. The mold pressure was maintained at 0.1N/mm 2  for 20 min. The distance between the pressing surfaces was adjusted to 10 mm for one minute. Then the mold was released and the finished board removed. 
     The laboratory experiments led to the following results: 
     
         ______________________________________Bulk density         546    g/lFlexural strength    14     N/mm.sup.2______________________________________ 
    
     EXAMPLE 3 
     1,760 g of medium-fine wood chips were placed in a 130 liter plough blade mixer with a knife head and sprayed with 520 g of water. Then 520 g of cationic wheat starch were added and mixed for one more minute. 
     Of this mixture, 50% was removed and uniformly sprinkled on a metal sheet in a 420×420 mm (inner size) frame. The bulk was compressed at a force of 700N for a short time. The thus obtained cake was covered with a further metal sheet and placed in a heatable press. 
     The cake was compressed at a press temperature of 170° C. for two seconds to a level of 10 mm. The mold pressure was maintained at 0.1N/mm 2  for 20 minutes. The distance between the pressing surfaces was adjusted to 10 mm for one minute. Then the mold was released and the finished board removed. 
     The laboratory experiments led to the following results: 
     
         ______________________________________Bulk density         528    g/lFlexural strength    21     N/mm.sup.2______________________________________ 
    
     EXAMPLE 4 
     1,760 g of fine wood chips were placed in a 130 liter plough blade mixer with a knife head and sprayed with 520 g of water. Then 520 g of potato starch were added and mixed for one more minute. 1,760 g of coarse wood chips were placed in a 130 liter plough blade mixer with a knife head and sprayed with 520 g of water. Then 520 g of potato starch were added and mixed for one more minute. 
     Of each mixture, 25% was removed and sprinkled on a metal sheet in a 420×420 mm (inner size) frame as follows: at first, one half of the fine wood chips, then the coarse wood chips and finally the other half of the fine wood chips. The bulk was compressed at a force of 700N for a short time. The thus obtained cake was covered with a further metal sheet and placed in a heatable press. 
     The cake was compressed at a press temperature of 170° C. for two seconds to a level of 11 mm. The mold pressure was maintained at 0.1N/mm 2  for 14 minutes. The distance between the pressing surfaces was adjusted to 10 mm for one minute. Then the mold was released and the finished board removed. 
     The laboratory experiments led to the following results: 
     
         ______________________________________Bulk density         524    g/lFlexural strength    19     N/mm.sup.2______________________________________ 
    
     EXAMPLE 5 
     1,760 g of medium-fine wood chips were placed in a 130 liter plough blade mixer with a knife head and sprayed with 520 g of water. Then 520 g of wheat starch were added and mixed for one more minute. 
     Of this mixture, 50% was removed and uniformly sprinkled on a metal sheet in a 420×420 mm (inner size) frame as follows: 
     At first, a 3 mm base layer, then 100 g of water, next the remaining mixture and finally 100 g of water were sprayed onto the sheet. The bulk was compressed at a force of 700N for a short time. The thus obtained cake was covered with a further metal sheet and placed in a heatable press. 
     The cake was compressed at a press temperature of 190° C. for 20 seconds to a level of 9.5 mm. The distance between the pressing surfaces was maintained at 12.5 mm for 12 minutes. Next, the distance between the pressing surfaces was adjusted to 10 mm for three minutes. Then the mold was released and the finished board removed. 
     The laboratory experiments led to the following results: 
     
         ______________________________________Bulk density         551    g/lFlexural strength    17     N/mm.sup.2______________________________________