Abstract:
A method of producing a composition which is composed of from 5 to 50 wt. % of caoutchouc, preferably natural caoutchouc; from 4 to 25 wt. % of starch powder; from 10 to 50 wt. % of one of polybetahydroxybutyrate or cellulose; and from 0 to 15 wt. % of auxiliary agent, and which is biodegradable includes providing caoutchouc in one of flaked or granular form; providing initial materials selected from the group consisting of starch, polybetahydroxybutyrate, cellulose, and auxiliary agent, in ground form as one of a powder or granulated; homogeneously mixing the initial materials into the caoutchouc to provide a homogeneous mixture: plasticizing the homogeneous mixture by heating in one of a screw-type extruder or injection molding machine to provide a plasticized mass; and ejecting the plasticized mass from the extruder or injection-molding machine and allowing the mass to cool. Preferably, plasticizing is carried out of a temperature ranging from 170° C. to 180° C.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a biodegradable material and a method of producing it. Biodegradable materials are those that decompose under the influence of environmental conditions and the action of destructive agents. Biodegradable materials are already known in which peas are used as the base material. It is also known to incorporate starch into conventional polymers to achieve partial degradability. In these starch-filled plastics, however, only the inserted starch component degrades, making the plastic matrix break down into numerous small pieces. 
     It is an object of the invention to propose a material that is completely biodegradable and is suited for incorporation therein round or fibrous particles to form composites. 
     SUMMARY OF THE INVENTION 
     This object is accomplished by providing a biodegradable material characterized by the following composition (wt. %): 
     
         ______________________________________caoutchouc            5 to 50%starch powder         4 to 25%polybetahydroxybutyrate                10 to 50%auxiliary agents      0 to 15%.______________________________________ 
    
     The biodegradable material may alternatively be characterized by the following composition (wt. %): 
     
         ______________________________________caoutchouc            5 to 50%starch powder         4 to 25%cellulose powder     10 to 50%auxiliary agents      0 to 15%.______________________________________ 
    
     The biodegradable material may have a thermoplastic matrix and be inserted, granulated or fibrous fillers comprising regenerative raw materials, characterized in that the matrix comprises 
     
         ______________________________________caoutchouc            5 to 50 wt. %starch powder         4 to 25 wt. %polybetahydroxybutyrate                10 to 50 wt. %.______________________________________ 
    
     The biodegradable composite may have a thermoplastic matrix and be inserted, granulated or fibrous fillers comprising regenerative raw materials, characterized in that the matrix comprises 
     
         ______________________________________caoutchouc            5 to 50 wt. %starch powder         4 to 25 wt. %cellulose powder     10 to 50 wt. %.______________________________________ 
    
     The biodegradable composite may have a thermoplastic matrix and be inserted, granulated or fibrous fillers comprising regenerative raw materials, characterized in that the matrix comprises 
     
         ______________________________________caoutchouc            5 to 50 wt. %starch powder         4 to 25 wt. %polybetahydroxybutyrate/cellulose                10 to 50 wt. %.--;______________________________________ 
    
     All of these basic composition contain 5 to 50 wt. % of caoutchouc and 4 to 25 wt. % of starch. A third component is added to these base components of caoutchouc and starch in a proportion of 10 to 50%. This component is either polybetahydroxybutyrate or cellulose powder. The former is obtained through polymerization of beta-hydroxybutyric acid. It is also commercially available under the trade name &#34;Biopol&#34; (by Zeneca). 
     Caoutchouc is an essential component of the materials of the invention. Natural caoutchouc is preferred because it is completely biodegradable. However, it is also possible to use artificial caoutchoucs that are at least partially biodegradable, or that degrade after lengthy storage under conditions that encourage degradation, depending on the type of additives used. The starch powder and the third component, which is also essentially in powder form, are dispersed homogeneously in the base caoutchouc mass, and are no longer visible, at least to the naked eye. The material of the invention has more or less elasticity depending on the caoutchouc component. It is therefore better suited for the production of molded bodies in which elastic properties take priority over solidity and stability. 
     A crucial feature of composites is that sufficient adhesion exist between the inserted particles, for example round or fibrous particles, and the matrix material. The mechanical properties are basically determined by the quality of the boundary surfaces between the particles and the matrix. It has been seen that the basic composition according to invention can take in relatively large quantities of fillers or reinforcing agents in the manner of a GRP (glass fiber-reinforced plastic). According to the invention, granulated or fibrous materials comprising regenerative raw materials are used as reinforcing insertions. These raw materials can be, for example, ground grains or plant fibers. The materials obtained in this way are distinguished by a greater hardness and solidity in comparison to the basic material. They are therefore suited for the production of larger and, in particular, sheet molded bodies, for example, for parts for vehicle interior trim. Auxiliary agents can be added to the materials of the invention in proportions of up to about 15%. These agents include substances that modify the materials without significantly influencing their mechanical properties. Examples of such substances are solvents (e.g., stearin) for improving the plasticization of the initial materials in injection-molding machines or extruders (see below). They may also be dyes or animal-deterring agents. 
     A further object of the invention is to propose a method of producing the materials of the invention. Caoutchouc (in uncured form) is known to be a relatively tough, tacky, &#34;rubberlike&#34; mass into which it is difficult to incorporate powdered or granulated materials. This is the case for both artificial and natural caoutchouc. The latter is typically present in the form of so-called sheets that have already been obtained, in the country of origin, from the latex of the caoutchouc plant through evaporation of the water component. Working a powder, such as starch or cellulose or more coarse-grained particles, into this type of caoutchouc mass would require heavy-duty cutting and kneading machines that use a tremendous amount of energy. The invention proposes using the caoutchouc in the form of flakes or grains. In this form it can easily be mixed homogeneously with the other components in simple mixers. Caoutchouc is commercially available in flakes or grains. In the next method step, the initial mixture obtained in this way is plasticized by heating in a conventional screw-type extruder or a conventional injection-molding machine. During this treatment, the caoutchouc flakes or grains are heated to a softening temperature. The individual components of the initial mixture are mixed thoroughly by the action of the screw and an extruder, as well as that of an injection-molding machine. The tough, tacky caoutchouc typically tends to solidify on the screw and the inside walls of the screw chamber of the above-mentioned machines. This effect is contrary to homogeneous, thorough mixing of the components. It also leads to increased friction and thus to an increase in the temperature of the mass to be processed. The increase in temperature results in undesired changes, particularly hardening of the caoutchouc. If, however, caoutchouc flakes or grains are processed, in accordance with the invention, in a mixture with the above-cited components in the extruder or injection-molding machine, this effect is avoided. The caoutchouc is effectively prevented from sticking to the screw and the inside machine walls by the fact that the individual caoutchouc particles are surrounded, at least in the initial stage of plasticization, by a powdered casing in the screw chamber. During the further course of processing, the individual caoutchouc flakes particles combine to form a homogeneous base mass in which the additives are uniformly distributed. The result of processing in the screw chamber of the above-cited machines is therefore a plasticized mass that can be extruded and injected. As the next method step, this mass is either extruded in the form of a strand or injected into a mold. In the former case, the extruded material strand is ground, that is, granulated. The material of the invention is then present in granular form, and can be kept in intermediate storage for a practically arbitrary length of time and used to produce injection-molded parts. In the latter case, the material of the invention is injected directly into a mold, and the desired molded part is obtained in a direct manner. 
     It has proven advantageous to maintain a temperature of 170° C. to 180° C. during plasticization of the initial mass in the screw chamber. At temperatures below 170° C., the caoutchouc cannot be sufficiently softened, so the additives cannot be satisfactorily worked in. However, other temperature effects could also have an impact. In particular, at the dominant temperatures, the added natural substances experience a significant change and conversion that could have a significant impact on the material properties. For this reason, the temperature cannot be increased greatly above 180° C. The natural substances, comprising primarily starch, cellulose and proteins, are changed so much at the higher temperatures that the obtained material no longer possesses the desired properties. In particular, carbon-containing components such as cellulose or starch can burn or carbonize. The material of the invention is able to take in a maximum filler component of 65%. Consequently, a broad spectrum of composites can be produced that possess widely-varying graduations of solidity and hardness. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Injection-molded parts such as cups, plates, bowls and parts for vehicle interior trim, were produced with the composites of the invention. The produced bodies were inspected closely and tested with regard to their suitability. Test pieces were also produced for collecting characteristic data, as explained below. All of these experiments revealed that materials having particularly advantageous properties can be obtained if the following composition is maintained (wt. %): 
     
         ______________________________________caoutchouc           10 to 30%potato starch         5 to 15%polybetahydroxybutyrate/cellulose                20 to 30%fillers              20 to 60%auxiliary agents     max. 10%______________________________________ 
    
     Granular materials and fibers of a wide variety of plants can be used as fillers for composites. In particular, granular materials from grain seeds such as corn, rye and wheat are considered. The fibers of Chinese grass or the capsular fruits of the kapok tree, for example, are considered as fibrous materials. Wood fibers or cotton fibers can also be used. 
     If the proportion of fillers exceeds 60%, the composite becomes noticeably brittle. The base matrix comprising caoutchouc, potato starch and polybetahydroxybutyrate (hereinafter referred to as PBHB) or cellulose, respectively, is no longer capable of holding together the inserted particles; its absorptive capability is partly exceeded. The material also becomes brittle if the caoutchouc proportion drops below 10%. With caoutchouc contents of more than 30%, the material becomes increasingly elastic, and loses more and more of the properties of a composite, namely hardness and solidity. With a filler content of more than about 60%, the mechanical properties worsen noticeably. In particular, the tensile strength decreases. The regenerative raw material additives are hydrophilic by nature, that is, they absorb water. If the proportion of inserted regenerative raw materials is more than 60%, the swelling property of the composite reaches an intolerable degree. Moreover, high particle proportions impede the uniform dispersement during plasticization. Auxiliary agents such as citric acid, used to protect against animal damage (deters rodents like mice and rats), or stearin as a solvent should be limited to a total proportion of 10% to essentially preclude a negative influence on the mechanical properties. 
     The advantage of using popped grains such as popcorn is that composites of lower density can be obtained. The fact that these materials contain air also increases the thermal insulating capability of the composites. 
    
    
     The invention is described by way of examples: 
     EXAMPLE 1 
     
         ______________________________________natural caoutchouc flakes              26%potato starch       6%popcorn            13%rye                26%PBHB               26%citric acid         4%______________________________________ 
    
     EXAMPLE 2 
     
         ______________________________________natural caoutchouc flakes              19%potato starch      12%oats               19%popcorn            12%dammar resin       12%PBHB               19%citric acid         3%stearin             2%gelatin powder      2%______________________________________ 
    
     EXAMPLE 3 
     
         ______________________________________natural caoutchouc flakes              13%potato starch      13%oats               13%rye                26%PBHB               20%stearin             3%citric acid         3%gelatin powder      2%popcorn             7%______________________________________ 
    
     Stearin was added to the compositions of Examples 1 through 3 as a solvent. The &#34;base matrix&#34; of these compositions comprises caoutchouc, potato starch and PBHB. In the above examples, as well as in examples discussed below, potato starch was used as a base component of the base matrix, particularly for cost reasons. However, other types of starch, such as rice, corn or grain starches, can also be used. 
     A composite according to the invention was produced as follows: 
     The flaked or granular caoutchouc was mixed homogeneously with the remaining components in a conventional mixer. At least the potato starch was present as a powder or flour, while the fillers, that is, popcorn, rye and oats, were present in granular form. After mixing, the latex flakes were uniformly distributed in the other components, or vice versa. The initial mixture prepared in this way was plasticized in a 75-ton injection-molding machine. It was poured into the filling hopper of the machine, through which it traveled into the screw chamber. The machine was heated such that the mass located in the screw chamber had a temperature of 170° C. to 180° C. Caoutchouc softens in this temperature range. The fillers and auxiliary agents were worked into the caoutchouc matrix by the rotation of the screw. At the dominant temperature, the added natural substances also changed. Experiments to explore this have not yet been undertaken. It is assumed, however, that the temperature effect on the natural substances in the screw chamber is not insignificant for the properties of the later material. The caoutchouc flakes, which are by nature sticky, at least in the heated state, were prevented from solidifying on the screw or the inside walls of the screw chamber by the encasing of the caoutchouc flakes with starch powder or other powdered components. The mass was fully plasticized in the screw chamber after 1 minute at the latest. 
     Different molded bodies, such as cups, bowls and test pieces, were injected with the plasticized mass. In the tests described below, the test pieces were used to determine characteristic mechanical data. The compositions of Examples through 3 are hereinafter referred to as Z1, Z2 and Z3. 
     Bending Test in Accordance with DIN 53452 
     
                       TABLE 1______________________________________Flexural strength (N/mm.sup.2)Test No.   Z1           Z2     Z3______________________________________1          23.38        22.22  40.422          25.09        21.17  43.173          23.39        21.55  37.364          23.86        19.43  41.295          20.91        19.54  38.62average    23.33        20.78  40.17flexuralstrength______________________________________ 
    
     
                       TABLE 2______________________________________Expansion at Maximum Force (%)Test No.   Z1           Z2     Z3______________________________________1          2.35         3.14   3.212          3.00         2.36   3.143          2.47         2.57   2.864          2.23         2.71   3.085          1.68         2.28   3.05average    2.35         2.61   3.07expansionatmaximumforce______________________________________ 
    
     Test pieces having a rectangular, average cross section of 4.16 mm×9.89 mm were used in the bending tests according to Tables 1 and 2. The test speed was 2 mm/min. 
     Determination of the Modulus of Elasticity in the Bending Test in Accordance with DIN 53457 
     Test pieces having an average thickness of 4.1 mm and an average width of 9.9 mm were tested. 
     
                       TABLE 3______________________________________Modulus of Elasticity (N/mm.sup.2)Test No.   Z1           Z2      Z3______________________________________1          1821.2       1270.1  1749.91          1821.2       1270.1  1749.92          1707.3       1472.5  1844.73          1725.0       1177.3  1881.4average    1751.2       1306.6  1825.3modulusofelasticity______________________________________ 
    
     Determination of the Length Expansion Coefficient 
     Test pieces having dimensions of 15×10.5×117 mm were injected from compositions Z1 and Z2. The length expansion was determined at a heating speed of 120 K/h. All three tests reveal a relatively constant course of expansion up to about 80° C. The expansion coefficient α of this constant range is shown in Table 4. 
     
                       TABLE 4______________________________________Expansion Coefficient  10-5K-1!:Z1              Z2     Z3______________________________________12.5            13.9   12______________________________________ 
    
     Determination of the Melt-Flow Index in Accordance with DIN 53735 
     This test was only performed with material having the Example 1 (Z1) composition. The test temperature was 190° C. Small pieces of a test mass were placed in a test cylinder and heated to determine the heat-flow index. On its underside, the test cylinder has a nozzle from which the softened mass is pressed out as a strand by a pressing stamp (load 2.61 kp) inserted into the test cylinder. As a result, an MFI (Melt Flow Index) of 2.59 g/10 min. could be ascertained. 
     Determination of Burning Behavior in Accordance with DIN 75200 
     The determination of burning behavior is required particularly for materials for vehicle interior appointments. Plates were produced in accordance with the injection-molding method and stored over a period of 48 hours at standard climate (23° C., 50% relative air humidity). The plates possessed dimensions of 139 mm×79 mm×3 mm. The plates were ignited at one edge, and the path and time of burning were determined. Five samples of each composition (Z1 through Z3) were tested. The following table summarizes the average burning speed and the maximum burning speed of the respective 5 samples. 
     
                       TABLE 5______________________________________Burning Behavior (Average and Max. Burning speeds inmm/min)     Z1         Z2     Z3______________________________________average     33.4         18.2   44.4burning speedmaximum     39.8         19.9   49.0burning speed______________________________________ 
    
     Determination of Density 
     To determine density, injection-molded plates measuring 2.9 mm×79 mm×139 mm were weighed with precision of 0.001 g. The plates were measured to determine their volume, and density was calculated from the values for volume and the masses. 
     
                       TABLE 6______________________________________Density (g/cm.sup.3)     Z1         Z2     Z3______________________________________average     1.24         1.23   1.20density______________________________________ 
    
     Determination of Absolute Moisture Content in Accordance with DIN 52351 
     To determine the relative moisture, plates of compositions Z1, Z2 and Z3 were stored for 48 hours under standard climate conditions (23° C., 50% relative air humidity) and, afterward, their weight gain with respect to the dry state after storage in moist air and in water was determined. The results are summarized in Table 7. 
     
                       TABLE 7______________________________________Average Absolute Moisture Content (%):     Z1         Z2     Z3______________________________________avg. absolute       0.21         0.81   0.43moisturecontent______________________________________ 
    
     Determination of Water Absorption and Swelling Based on DIN 52351 
     To determine water absorption and swelling, plates made from materials of compositions Z1, Z2 and Z3 were first stored for 48 at a standard climate (23° C., 50% relative air humidity). A portion of the samples were stored in moist air (95% relative air humidity, 55° C.), and the average changes in thickness and mass were determined as a percentage value after 24, 48, 72 and 96 hours. 
     
                       TABLE 8______________________________________Average Thicknesses and Average Change in Mass(change in thickness Δd and change in mass Δm in %) duringstorage in moist air and in waterΔ      Z1         Z2       Z3______________________________________Δd/Δm     24 h    0.00/0.61  1.54/1.25                              0.96/0.86after     48 h    0.27/0.68  1.54/1.25                              0.96/0.94storage in     72 h    0.83/0.68  1.54/1.25                              0.96/0.94air       96 h   --         --     --Δd/Δm     24 h   11.24/2.14 15.85/3.76                              2.25/0.96after     48 h   13.61/2.83 15.85/5.12                              3.46/1.52storage in     72 h   14.87/3.56 16.69/6.35                              3.46/1.86water     96 h   14.87/4.19 16.69/7.33                              3.46/2.19______________________________________ 
    
     Determination of Tension and Expansion in the Tensile Test in Accordance with DIN 53455 
     Test rods having an average thickness of 4 mm and an average width of 10 mm (standard rod No. 3) were produced from compositions Z1-Z3. Tensile strength and expansion were determined once at a test speed of 5 mm/min and another time at 50 mm/min. Expansion was determined--in deviation from the DIN standard--from the change in spacing of the clamping jaws. The test results are summarized in Tables 9 and 10. 
     
                                           TABLE 9__________________________________________________________________________Tensile Strength (N/mm.sup.2)Z1               Z2         Z3Test No. 5 mm/min      50 mm/min            5 mm/min                 50 mm/min                       5 mm/min                            50 mm/min__________________________________________________________________________1     5.92 7.94  14.11                 14.69 12.10                            14.012     6.79 6.65  13.46                 10.98 11.68                            16.863     6.64 7.04  10.13                 13.35 11.73                            19.484     6.83 7.84  11.18                 13.01 11.43                            13.985     6.57 12.05 14.05                 16.49 11.10                            13.42avg.  6.55 8.30  12.57                 13.70 11.61                            15.55tensilestrength__________________________________________________________________________ 
    
     
                                           TABLE 10__________________________________________________________________________Expansion at Maximum Force (%)Z1               Z2         Z3Test No. 5 mm/min      50 mm/min            5 mm/min                 50 mm/min                       5 mm/min                            50 mm/min__________________________________________________________________________1     6.54 8.54  9.22 9.16  11.84                            13.462     6.86 7.30  7.86 8.76  11.92                            14.043     7.16 8.86  8.00 6.80  12.00                            15.844     7.92 8.66  8.78 7.60  11.32                            12.625     7.18 8.66  8.76 10.10 12.56                            10.84avg.  7.13 8.44  8.52 8.48  11.93                            13.36expansionat maximumforce__________________________________________________________________________ 
    
     In addition to the above-described Examples 1-3, other materials having widely-varying compositions were produced and processed in accordance with the injection-molding method. Bowls, cups and plate-shaped injection-molded parts were produced. The components and content ranges of these examples are summarized in Table 11. 
     
                                           TABLE 11__________________________________________________________________________Examples 4-19 (Contents in % by weight)Examples4       5 6  7 8 9 10                11                  12 13 14 15 16                                17                                  18 19__________________________________________________________________________Base matrix, 50   70     33.4        60          80            80              80                75                  63.4                     65.8                        68.7                           82.5                              60                                85                                  66.6                                     53.5totalFlake 10   20     6.3        10          30            40              50                25                  6.7                     9.9                        21.5                           8  6 25                                  8.3                                     4.1caoutchoucPotato starch 10   10     6.3        --          10            20              20                --                  10 6.2                        4.3                           5.3                              4 10                                  8.3                                     4.1&#34;PBHB&#34; 30   20     20.8        10          20            --              10                25                  46.7                     -- -- 53.2                              26                                50                                  25 41.2Celluose --   20     -- 40          20            20              --                25                  -- 49.7                        42.9                           16 24                                --                                  25 4.1powderFillers, total 40   30     56.3        40          20            20              20                25                  36.8                     31 21.5                           5.3                              38                                15                                  33.4                                     34.1Popcorn --   --     -- --          --            --              --                --                  6.7                     9.3                        4.3                           -- --                                --                                  -- --Rye   --   --     -- --          --            --              --                --                  10 3.1                        -- -- 18                                --                                  -- --Oats  --   --     -- --          --            --              --                --                  6.7                     -- -- -- --                                --                                  -- --Corn  40   30     56.3        40          20            20              20                25                  6.7                     12.4                        4.3                           5.3                              16                                15                                  16.7                                     21.8Soy   --   --     -- --          --            --              --                --                  6.7                     6.2                        -- -- 4 --                                  16.7                                     8.2Slate powder --   --     -- --          --            --              --                --                  -- -- 12.9                           -- --                                --                                  -- 4.1Auxiliary 10   --     10.5        --          --            --              --                --                  -- 3.1                        9.9                           12.2                              2 --                                  -- 12.3agents, totalLemon juice --   --     -- --          --            --              --                --                  -- 3.1                        1.3                           1.6                              --                                --                                  -- 4.1Dammar 10   --     6.3        --          --            --              --                --                  -- -- 4.3                           5.3                              1 --                                  -- 4.1resinStearin --   --     -- --          --            --              --                --                  -- -- 4.3                           5.3                              --                                --                                  -- 4.1Gelatine --   --     4.2        --          --            --              --                --                  -- -- -- -- 1 --   --powder__________________________________________________________________________