Reinforced flexible container
BACKGROUND OF THE INVENTION 
     The present invention relates to a reinforced flexible container for transportation and storage of bulk material. The container comprises a conventional flexible container which can be made from woven textile fibers, laminated plastics or textile coated with plastic or the like. It comprises a bottom section, a central section and an upper section with lifting loops, and it can also be equipped with an inner sack of impervious material. The lifting loops can be integral parts of the container itself, but may also comprise separate loops or lifting straps secured to the container. 
     Several types of such containers for transportation of bulk material are used today. Some of these known containers are used only once, while others can be used several times. Among such known flexible containers are those described in British Pat. No. 1,431,582, published Apr. 7, 1976; and in British Pat. No. 1,475,019, published June 1, 1977, U.S. Pat. No. 4,136,713. 
     The container according to the above U.S. patent is preferably equipped with an inner sack of impervious material, and preferably the upper section of the container comprises integrated lifting loops and a central filling opening. The special feature of this container is that its bottom section consists of at least two pairs of equally wide flaps which are direct extensions of the container walls, and that such flaps are joined at their lower edges such that the joints thereby formed cross each other at one point. This bottom construction has by dynamic testing been found to endure greater strains than the bottom construction according to the above mentioned British Pat. No. 1,475,019. 
     For certain applications however, conventional flexible containers do not meet the requirements regarding container strength. For transportation of dangerous material like ammonium nitrate for instance, it is required that the container shall endure a specified drop against a plain floor, etc. without rupturing. By example, Verkehr der Bundesrepublik Deutschland, i.e. the authorities of the German Federal Republic, require that flexible containers having a volume of 250-1250 liters shall endure a drop of 1.2 meter when they are filled with pulverent, dangerous materials. References made to &#34;Amtsblatt der Bundesministers f ,uml/u/ r Verkehr der Bundesrepublik Deutschland&#34;, No. 6, page 254-259, of Mar. 31, 1976 volume 30, published in Bonn, Germany. In this application dangerous material means that classified as such in packaging group 3 by the Intergovernmental Maritime Consultative Organization (IMCO). 
     The disadvantage of the known flexible containers is that they burst just above the bottom when dropped, in filled condition, against a floor etc. Tests have shown that most of them burst already when the drop height is only 0.3 meter. 
     SUMMARY OF THE INVENTION 
     The main object of the present invention is to provide flexible containers which may be employed for transportation and storage of dangerous material. Another object is to provide such container by just reinforcing known flexible containers without substantially changing their basic construction or method of manufacture. 
     Reinforcing known containers by increasing the weaving density or the thickness of the fabric has proved to be expensive for making containers that could endure a drop height of 1.2 meter. In addition to increasing weaving density and/or thickness of the fibres, this method also requires that the seams be reinforced. 
     The inventors have developed a simple method of obtaining sufficiently strong flexible containers without altering the basic construction of known containers. 
     The special feature of the invention is that a flexible container is equipped with at least one belt around its central section and that the belt or belts have a total width which maybe as much as the height of the bulk material in the filled container. Preferably such belt width corresponds to 1/3-2/3 of the height of the bulk material of the filled container, and the belt is than placed around the lower part of the container. 
     The belt or belts can be made from the same material as the container itself, and preferably the belt is formed of round woven textile material. The belt can, however, also be made by joining together a piece of woven textile material. 
     The belt can also consist of a wide strap with tightening means such that the strap is secured around the container subsequent to the filling of the container. 
     The invention also includes a flexible container equipped with several narrow belts. The same reinforcement is then obtained with a total width of the belts somewhat less than the width necessary with a single belt. The application of several belts is, however, in most cases more expensive and labour consuming. 
     The most preferred performance of the invention is to place a round woven belt around the container before filling. During the filling operation the container walls will be pressed against the belt and keep it in the right position. Optionally, the belt can be secured flexibly to the container by means of, for instance, tape or threads through the belt and the container wall. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will be apparent from the following detailed description, taken with the accompanying drawings, wherein: 
     FIG. 1 is a schematic perspective view of a reinforced flexible container unit according to one embodiment of the present invention; and 
     FIG. 2 is a schematic perspective view of a tubular woven reinforcing belt employable in the unit of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One example of a reinforced flexible container according to the invention is schematicly shown in FIG. 1. Around the container 1 there is a belt 2. Threads 3 for securing the belt 2 are shown, even though they are not necessary. FIG. 2 shows a round of tubular woven belt 2. 
     The most essential advantage of the invention is that it makes it possible in a very simple way to reinforce conventional flexible containers when that is necessary. This means that flexible containers can be manufactured for different purposes by the same method and equipment and only those containers that must endure drop strain have to be equipped with a belt and this is preferably done immediately before filling the container. Accordingly, a reinforced flexible container is attained without altering the basic construction of the container itself. 
     Another advantage of the invention is that it can be applied to several types of flexible containers. A further advantage of the invention is that the belt can be used for marking purposes, as informative label, etc. 
     EXAMPLE 
     Flexible containers according to U.S. Pat. No. 4,136,723 were filled with 800 kilos of bulk material (NPK-fertilizer prills) and exposed to varying drop tests. Three such containers were not equipped with belts, while three such containers were equipped with belts of woven polypropylene having the same weaving density and fibre thickness as the container itself. A circular belt was made from a piece of woven polypropylene joined together by zigzag seams. Such belts are somewhat weaker than round woven belts made from the same material. The width of the belts were 2/3 of the height of the bulk material in the filled containers. The belts were thread around the containers immediately before filling. 
     The following table shows the test results: 
     
                       TABLE                                                       
______________________________________                                    
        Container                                                         
        without Container                                                 
         belt   with belt                                                 
Drop height                                                               
          1     2     3   4   5     6                                     
______________________________________                                    
0.10 m                                                                    
0.20      ok    ok    ok                                                  
0.30      R     R     R                                                   
0.40                                                                      
0.50                      ok        ok                                    
0.60                                                                      
0.70                                                                      
0.80                      ok  ok    ok                                    
0.90                                                                      
1.00                                                                      
1.10                                                                      
1.20                      ok  ok,ok.sup.x                                 
                                    ok  German Limit                      
1.30                                                                      
1.40                                                                      
1.50                      ok  ok    ok                                    
1.60                                R                                     
______________________________________                                    
 
    
     Each container without a belt burst at the second drop which was 0.3 meter high. Two of the reinforced containers were exposed to four drop tests and were still completely intact, marked ok in the table. One of the reinforced containers was dropped five times and burst at the fifth drop which was 1.6 meter high, marked R in the table. 
     It was proved during the tests that by equipping a conventional flexible container with a belt, it was reinforced such that it could endure for instance several drops of more than 1.2 meter, contrary to a conventional container without the best which burst at a drop of 0.3 meter. The containers first burst at the side seams just above the bottom of the containers. If stronger belts than those in the above example are used, the reinforced containers will endure drops of even more than 1.6 meter.