Method for preparing biodegradable films from plant-based raw materials
FIELD OF THE INVENTION 
     The invention relates to a method for preparing biodegradable films from plant-based raw materials. Such methods are gaining more and more importance, for example, in the production of environmentally compatible packing material. The object is to produce films having sufficient strength that can easily be disintegrated or dissolved into their original natural components. 
     BACKGROUND OF THE INVENTION 
     Methods for preparing thermoplastic granules by using starches have been disclosed previously (U.S. Pat. Nos. 4,133,784, 4,021,388, DE-A-23 22 440). Such granules are processed to films in a second procedural step. In so doing, the starch is used as filler material in synthetic polymers. Furthermore, methods for preparing and modifying destructured thermoplastic starch have been disclosed (EP-A-378 646, EP-A-397 819). 
     WO 90/05161 A1 shows thermoplastically processable starch as well as a method for preparing it. In the process aggregates and, if desired, various further additives are added to raw starch material to reduce the melting temperature of the starch to a level below its decomposition temperature. This is to improve the structure of the starch melt so as to facilitate handling in preparing and processing thermoplastic starch. 
     EP-A1-0 474 095 discloses a process of preparing biodegradable articles, such as dishes, cups, cardboard boxes, etc. using plant-based raw material, such as starch, as starting material. This material is supplemented with additives, plastified and converted in a first extruder to an intermediate to form a billet or granules. Then the intermediate is molded in a second extruder into the desired article. 
     Similarly, conventional film preparation from plant-based raw materials proceeds in two completely separate steps. The biopolymer melt destructured and plastified in the extrusion process is cooled at the end of the first step, forced through a die under high pressure and granulated. In order to avoid biopolymer foaming, it is required to cool the melt. The resulting high shear stress causes damage to the molecular structure of the starch while various molecule fragments are split off. The polymer chains of the starch are shortened and the molecular weight is reduced. The obtained starch granules are then processed to films in a further extrusion process subject to similar conditions. In the process the biopolymer is again decomposed as the molecular weight is reduced and the chains are shortened. The two-fold damage to the molecular structure, especially the shortening of the amylose chains, causes immediate deterioration of the properties (tensile strength, elasticity, etc.) of the thus obtained films. What is more, the two-fold extrusion is time-, energy- and cost-consuming. 
     By contrast, the object underlying the present invention is to provide a cost-efficient method for preparing biodegradable films from plant-based raw materials while avoiding the drawbacks of the prior art, improving the quality of the obtained films and achieving, for instance, high tensile strength and elasticity. 
     This object is achieved by providing the features indicated in the claims. 
     SUMMARY OF THE INVENTION 
     In achieving the object, the invention is based on the concept of optionally disintegrating and modifying plant-based raw materials, e.g., in the form of carbohydrates, in a continuous and one-step process without involving intermediates and without interruption, of plastifying as well as of compressing and molding them into films. A suitable configuration of a screw-type extruder to transport the film materials and suitable temperature conditions ensure careful processing of the biopolymers. 
     The plant-based raw materials can include carbohydrates, e.g., flour or native starch in natural or hybrid form-derived, for example, from potatoes, manioc, peas, beans, corn, wax corn, corn with high amylose content, grain such as wheat and fractions prepared therefrom, barley or sorghum, starch derivatives consisting of physically and/or chemically modified starch, cellulose derivatives, plant rubber (carbohydrate polymer), hemicellulose, polysaccharides, hydrocolloids or mixtures of one or more of those raw materials. 
     The invention has the following advantages. 
     The method of the invention reduces the shear strength, fragmentation and molecular weight loss of the biopolymers. As a result, the preparation process is considerably facilitated because the reaction extruder and the film preparation form a process unit. The storage of hygroscopic pellets in an aluminum compound under vacuum is no longer necessary. Compounding and film preparation occur in one step. The production is more cost-efficient as no packing and transportation of the pellets are required and as less personnel and equipment costs are incurred. In addition, energy for transporting and melting the pellets can be saved. The obtained films exhibit improved properties in comparison to the prior art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is further illustrated below by means of drawings. 
     FIG. 1a shows a lateral view of a production line of the invention for the production of blown films. 
     FIG. 1b shows a top view of a production line of the invention as shown in FIG. 1a. 
     FIG. 2a shows a lateral view of a production line of the invention for the production of flat films. 
     FIG. 2b shows a top view of a production line of the invention as shown in FIG. 2a. 
     FIG. 3a shows an embodiment of the invention featuring the end of a screw-type extruder comprising a double tubular die. 
     FIG. 3b shows an embodiment of the invention featuring the end of a screw-type extruder comprising a tubular die. 
     FIG. 3c shows an embodiment of the invention featuring the end of a screw-type extruder comprising a flat die. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1a and 1b show a production line of the invention for the production of blown films in two views. Depicted are individual production sections 1 to 6 and a die 7 interconnected by a screw-type extruder (not shown) that transports and mixes or kneads the film materials. A mixture of solid plant-based raw materials and, if desired, additives is measured with a metering device 17 (only shown in FIG. 1a) and added via the first inlet 11 of the first section 1. At the same time, liquid additives can be added via the second inlet 12. Examples of additives include emulsifiers, plastifiers, preservatives and water. In section 2 the mixture is heated and kneaded. Liquid additives can be added via a third inlet 13. In a third section 3 the temperature is further increased and the plant-based raw materials are allowed to react with the additives, thus forming a melt. In a fourth section 4 the melt is evacuated in a vacuum apparatus 14 by allowing water to evaporate from the melt and thus causing the melt to cool down. The melt is further cooled in a fifth section 5. In a sixth section 6 the melt is compressed so as to cause a pressure built-up in the die and then blown through a tubular die 7 to form a film 15. A similar production line is depicted in FIGS. 2a and 2b (metering device 17 is only shown in FIG. 2a). The difference between FIGS. 2a, 2b and FIGS. 1a, 1b lies in the fact that a flat film 16 is molded by a flat die 9. 
     FIG. 3a shows an embodiment of the invention wherein a blown film 15 is produced by means of a double tubular die 8. Two one-lead screws 19 are provided behind a double-lead screw 10 to transport the melt to the die 8. 
     In FIG. 3b a blown film 15 is produced by means of a tubular die 7. The die is supplied with the melt by a gear pump 18 provided behind the double-lead screw 10. 
     In an embodiment comprising a flat die 9 for the production of a flat film 16 as shown in FIG. 3c the double-lead screw 10 feeds the melt directly to the die 9. 
     The screw-type extruder 10 is preferably shaped like a double-lead screw and preferably has the following configurations in each of sections 1 to 6: right-handed in sections 1 and 2; right- and left-handed in section 3 while optionally alternating several times; steeply right-handed in sections 4 and 5; and flatly right-handed in section 6. These differing configurations ensure controlled and differing transport rates, kneading and mixing in each of the sections and thus the desired careful handling of the biopolymers. 
     Sections 1 to 6 and dies 7, 8 or 9 exhibit a controlled temperature distribution. Preferred temperature conditions include: 50° C. in section 1; 80°-100° C. in section 2; 120°-160° C. in section 3; 120°-160° C. in section 4; 80° to 120° C. in section 5; 40°-60° C. in section 6; and 50° to 70° C. at dies 7, 8 or 9. 
     The embodiments of the method of the invention are shown below. A two-shaft screw-type extruder (ZSK 40) manufactured by Werner und Pfleiderer, Germany, was used. 
     EXAMPLE 1 
     A solids mixture comprising potato starch and an emulsifier (mono-diglyceride) as well as the liquid components water and glycerol were continuously added to the two-shaft screw-type extruder. 
     
         ______________________________________                                    
Solids:                                                                   
Potato starch          99%                                                
Emulsifier             1%                                                 
                       100%                                               
Liquid additives:                                                         
Water                  14%                                                
Glycerol               86%                                                
                       100%                                               
Mixing ratio during the process:                                          
Solids                 75%                                                
Liquids                25%                                                
Temperature profile:                                                      
Section 1:             50°                                         
                               C.                                         
Section 2:             80°                                         
                               C.                                         
Section 3:             120°                                        
                               C.                                         
Section 4:             120°                                        
                               C.                                         
Section 5:             80°                                         
                               C.                                         
Section 6:             60°                                         
                               C.                                         
Die:                   60°                                         
                               C.                                         
RPM:                   170     min.sup.-1                                 
Torque:                30%                                                
Pressure:              80      bar                                        
Blow ratio:            1:4                                                
______________________________________                                    
 
    
     A transparent film having the following properties was obtained: 
     
         ______________________________________                                    
Film thickness:        35     μm                                       
Tensile strength along length:                                            
                       12.1   N/mm.sup.2                                  
Tensile strength along width:                                             
                       11.8   N/mm.sup.2                                  
Duration of dissolution of 10 g of film                                   
                       15     minutes                                     
in 100 g of water at 20° C.:                                       
______________________________________                                    
 
    
     EXAMPLE 2 
     A solids mixture comprising potato starch and an emulsifier (mono-diglyceride) as well as the liquid components water and glycerol were continuously added to the two-shaft screw-type extruder. 
     
         ______________________________________                                    
Solids:                                                                   
Potato starch          99%                                                
Emulsifier             1%                                                 
                       100%                                               
Liquid additives:                                                         
Water                  20%                                                
Glycerol               80%                                                
                       100%                                               
Mixing ratio during the process:                                          
Solids                 75%                                                
Liquids                25%                                                
Temperature profile:                                                      
Section 1:             50°                                         
                               C.                                         
Section 2:             80°                                         
                               C.                                         
Section 3:             140°                                        
                               C.                                         
Section 4:             140°                                        
                               C.                                         
Section 5:             100°                                        
                               C.                                         
Section 6:             60°                                         
                               C.                                         
Die:                   60°                                         
                               C.                                         
RPM:                   170     min.sup.-1                                 
Torque:                20%                                                
Pressure:              60      bar                                        
Blow ratio:            1:4                                                
______________________________________                                    
 
    
     A transparent film having the following properties was obtained: 
     
         ______________________________________                                    
Film thickness:        30     μm                                       
Tensile strength along length:                                            
                       10.2   N/mm.sup.2                                  
Tensile strength along width:                                             
                       9.4    N/mm.sup.2                                  
Duration of dissolution of 10 g of film                                   
                       11     minutes                                     
in 100 g of water at 20° C.:                                       
______________________________________                                    
 
    
     EXAMPLE 3 
     A solids mixture comprising potato starch and an emulsifier (mono-diglyceride) as well as the liquid components water and glycerol were continuously added to the two-shaft screw-type extruder. 
     
         ______________________________________                                    
Solids:                                                                   
Potato starch          99%                                                
Emulsifier             1%                                                 
                       100%                                               
Liquid additives:                                                         
Water                  10%                                                
Glycerol               90%                                                
                       100%                                               
Mixing ratio during the process:                                          
Solids                 75%                                                
Liquids                25%                                                
Temperature profile:                                                      
Section 1:             50°                                         
                               C.                                         
Section 2:             80°                                         
                               C.                                         
Section 3:             160°                                        
                               C.                                         
Section 4:             160°                                        
                               C.                                         
Section 5:             120°                                        
                               C.                                         
Section 6:             60°                                         
                               C.                                         
Die:                   60°                                         
                               C.                                         
RPM:                   230     min.sup.-1                                 
Torque:                15%                                                
Pressure:              52      bar                                        
Blow ratio:            1:4                                                
______________________________________                                    
 
    
     A transparent film having the following properties was obtained: 
     
         ______________________________________                                    
Film thickness:        33     μm                                       
Tensile strength along length:                                            
                       9.4    N/mm.sup.2                                  
Tensile strength along width:                                             
                       8.2    N/mm.sup.2                                  
Duration of dissolution of 10 g of film                                   
                       8      minutes                                     
in 100 g of water at 20° C.:                                       
______________________________________                                    
 
    
     EXAMPLE 4 
     A solids mixture comprising corn starch and an emulsifier (mono-diglyceride) as well as the liquid components water and glycerol were continuously added to the two-shaft screw-type extruder. 
     
         ______________________________________                                    
Solids:                                                                   
Corn starch            98%                                                
Emulsifier             2%                                                 
                       100%                                               
Liquid additives:                                                         
Water                  14%                                                
Glycerol               86%                                                
                       100%                                               
Mixing ratio during the process:                                          
Solids                 70%                                                
Liquids                30%                                                
Temperature profile:                                                      
Section 1:             50°                                         
                               C.                                         
Section 2:             80°                                         
                               C.                                         
Section 3:             140°                                        
                               C.                                         
Section 4:             140°                                        
                               C.                                         
Section 5:             100°                                        
                               C.                                         
Section 6:             60°                                         
                               C.                                         
Die:                   60°                                         
                               C.                                         
RPM:                   200     min.sup.-1                                 
Torque:                18%                                                
Pressure:              45      bar                                        
Blow ratio:            1:3                                                
______________________________________                                    
 
    
     An opaque film having the following properties was obtained: 
     
         ______________________________________                                    
Film thickness:        42     μm                                       
Tensile strength along length:                                            
                       8.2    N/mm.sup.2                                  
Tensile strength along width:                                             
                       6.8    N/mm.sup.2                                  
Duration of dissolution of 10 g of film                                   
                       24     minutes                                     
in 100 g of water at 20° C.:                                       
______________________________________                                    
 
    
     EXAMPLE 5 
     A solids mixture comprising wheat flour and an emulsifier (mono-diglyceride) as well as the liquid components water and glycerol were continuously added to the two-shaft screw-type extruder. 
     
         ______________________________________                                    
Solids:                                                                   
Type 405 wheat flour   98%                                                
Emulsifier             2%                                                 
                       100%                                               
Liquid additives:                                                         
Water                  25%                                                
Glycerol               75%                                                
                       100%                                               
Mixing ratio during the process:                                          
Solids                 70%                                                
Liquids                30%                                                
Temperature profile:                                                      
Section 1:             50°                                         
                               C.                                         
Section 2:             80°                                         
                               C.                                         
Section 3:             140°                                        
                               C.                                         
Section 4:             140°                                        
                               C.                                         
Section 5:             100°                                        
                               C.                                         
Section 6:             60°                                         
                               C.                                         
Die:                   60°                                         
                               C.                                         
RPM:                   200     min.sup.-1                                 
Torque:                20%                                                
Pressure:              55      bar                                        
______________________________________                                    
 
    
     The material was molded by a flat slot die and drawn out to a film. 
     An opaque film having the following properties was obtained: 
     
         ______________________________________                                    
Film thickness:        94     μm                                       
Tensile strength along length:                                            
                       9.6    N/mm.sup.2                                  
Tensile strength along width:                                             
                       9.0    N/mm.sup.2                                  
Duration of dissolution of 10 g of film                                   
                       64     minutes                                     
in 100 g of water at 20° C.:                                       
______________________________________                                    
 
    
     EXAMPLE 6 
     A solids mixture comprising potato starch and an emulsifier (mono-diglyceride) as well as the liquid components water and glycerol were continuously added to the two-shaft screw-type extruder. At the end of the two-shaft section the material was fed to a tubular die via a gear pump. 
     
         ______________________________________                                    
Solids:                                                                   
Potato starch          99%                                                
Emulsifier             1%                                                 
                       100%                                               
Liquid additives:                                                         
Water                  14%                                                
Glycerol               86%                                                
                       100%                                               
Mixing ratio during the process:                                          
Solids                 80%                                                
Liquids                20%                                                
Temperature profile:                                                      
Section 1:             50°                                         
                               C.                                         
Section 2:             100°                                        
                               C.                                         
Section 3:             140°                                        
                               C.                                         
Section 4:             140°                                        
                               C.                                         
Section 5:             80°                                         
                               C.                                         
Section 6:             40°                                         
                               C.                                         
Die:                   50°                                         
                               C.                                         
RPM:                   200     min.sup.-1                                 
Torque:                62%                                                
Pressure at pump inlet:                                                   
                       80      bar                                        
Pressure at pump outlet:                                                  
                       210     bar                                        
Blow ratio:            1:4                                                
______________________________________                                    
 
    
     A transparent film having the following properties was obtained: 
     
         ______________________________________                                    
Film thickness:        29     μm                                       
Tensile strength along length:                                            
                       14.1   N/mm.sup.2                                  
Tensile strength along width:                                             
                       12.6   N/mm.sup.2                                  
Duration of dissolution of 10 g of film                                   
                       10     minutes                                     
in 100 g of water at 20° C.:                                       
______________________________________                                    
 
    
     EXAMPLE 7 
     A solids mixture comprising potato starch and an emulsifier (mono-diglyceride) as well as the liquid components water and glycerol were continuously added to the two-shaft screw-type extruder. At the end of the two-shaft section the material was passed on to two one-lead screws and from there on fed to a double tubular die 8. In the screws two concentric tubes were formed from the two product flows, the thus obtained tubes were immediately laminated one on top of the other and together blown to a film. 
     
         ______________________________________                                    
Solids:                                                                   
Potato starch          99%                                                
Emulsifier             1%                                                 
                       100%                                               
Liquid additives:                                                         
Water                  20%                                                
Glycerol               80%                                                
                       100%                                               
Mixing ratio during the process:                                          
Solids                 75%                                                
Liquids                25%                                                
Temperature profile:                                                      
Section 1:             50°                                         
                               C.                                         
Section 2:             100°                                        
                               C.                                         
Section 3:             120°                                        
                               C.                                         
Section 4:             120°                                        
                               C.                                         
Section 5:             80°                                         
                               C.                                         
Section 6:             60°                                         
                               C.                                         
Die:                   70°                                         
                               C.                                         
RPM:                   160     min.sup.-1                                 
Torque:                44%                                                
Pressure:              50      bar                                        
Blow ratio:            1:4                                                
______________________________________                                    
 
    
     A transparent film having the following properties was obtained: 
     
         ______________________________________                                    
Film thickness:        38     μm                                       
Tensile strength along length:                                            
                       11.4   N/mm.sup.2                                  
Tensile strength along width:                                             
                       9.4    N/mm.sup.2                                  
Duration of dissolution of 10 g of film                                   
                       31     minutes                                     
in 100 g of water at 20° C.:                                       
______________________________________