Abstract:
A starch filled, coextruded degradable polyethylene film includes a starch filled inner layer positioned between two outer layers that contain a prodegradant. The outer layers of the film, which contain no starch, enable the film to retain desirable physical properties similar to conventional polyethylene film. The prodegradant causes the outer layers of the film to degrade under appropriate conditions exposing the starch filled inner layer. The starch in the inner layer then serves as a source of nutrients for microorganisms which consume starch, leaving a porous structure that is vulnerable to oxidation, hydrolysis, direct enzyme action or combinations of these processes.

Description:
TECHNICAL FIELD 
     This invention relates to a starch filled coextruded degradable film. 
     BACKGROUND OF THE INVENTION 
     Polyethylene films are used in a wide variety of applications including stretch/cling (pallet wraps etc.), grocery bags, heavy duty shipping sacks, disposable diapers, food wraps and agricultural films. Large volumes of polyethylene film are used in these applications on a daily basis. Decomposition of conventional polyethylene films, however, takes long periods of time under most conditions. Thus, a need has arisen for a degradable polyethylene film suitable for use in various products. 
     A degradable composition comprising a synthetic resin, a degradable granular filler such as natural starch granules and a substance autoxidizable to yield a peroxide is described in U.S. Pat. No. 4,016,117 issued Apr. 5, 1977 to Griffin. Purportedly, articles formed from the composition described in this patent degrade as the starch granules exposed at or adjacent the surface of the article are degraded and leached away followed successively by degradation of particles at the interior to produce a cellular structure which is more readily attacked by the processes of oxidation, hydrolysis, direct enzyme action or combinations of these processes. 
     The use of starch as a filler material in the production of thin polyethylene films, however, causes major problems in the manufacturing process. Starch, a hydrophilic material is incompatible with polyethylene, a hydrophobic material. Due to the relative incompatibility of starch with polyethylene and the difference in the surface energies of the respective materials, starch migrates to the surface of the meltstream during the extrusion process and collects on the die lips where the shear rates are significant. The starch deposited on the die lips oxidizes and intermittently picks off into the passing film material causing holes and defects in the film product. 
     The use of starch as filler material in polyethylene film products also has a significant impact on the physical properties of the film product. Major reductions in gloss, elongation, toughness, tear strength, impact and coefficient of friction result from the use of starch as a filler material. Although the magnitude of the changes in physical properties varies with different types of polyethylene, e.g., low density polyethylene (LDPE) vs. linear low density polyethylene (LLDPE), the changes are nonetheless significantly deleterious. 
     Thus, there is a need for a polyethylene film that is degradable and which simultaneously substantially retains the desirable properties of conventional polyethylene film. 
     SUMMARY OF THE INVENTION 
     The present invention provides a starch filled coextruded multilayer degradable polyethylene film. A starch filled inner layer is positioned between two outer layers that contain a prodegradant. The starch filled inner layer provides a source of nutrients for microorganisms. The outer layers of the film, which contain no starch, enable the film to retain the desired physical properties, similar to conventional polyethylene film. When the film is disposed of the prodegradant causes the outer layers of the film to degrade exposing the starch filled inner layer. The starch in the inner layer then may act as a source of nutrients for microorganisms which consume the starch leaving a porous structure that is vulnerable to oxidation, hydrolysis, direct enzyme action or combinations of these processes. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The film of the present invention is a multilayer polyethylene film produced with conventional coextrusion processes. The term &#34;polyethylene&#34; as used herein refers to low, medium and high density polyethylenes, and mixtures thereof, including low density polyethylene (LDPE), linear low density polyethylene (LLDPE), ultra low density polyethylene (ULDPE), ethylene copolymers and mixtures of polyethylene and other polymers suitable for the manufacture of films and sheets. 
     The inner layer of the film of the present invention contains from about 3% to about 40% starch. All percentages used herein are by weight unless otherwise indicated. Starches are polysaccharide compounds which are converted to sugar upon hydrolysis. The term &#34;starch&#34; as used herein refers to any starch obtained from cereal grains or root crops such as corn, maize, wheat, rice and potatoes, other starches, starch components, modified starch products and mixtures thereof. Starch is a hydrophilic material, having a strong tendency to bind or absorb water. Polyethylene, on the other hand, is a hydrophobic material, basically antagonistic to water. Consequently, polyethylene and starch are basically incompatible and the inclusion of starch granules in a polyethylene film results in a film having less desirable properties than conventional polyethylene film. In order to compensate for the change in physical properties, the present invention provides two starch free outer layers, each comprising from about 5% to about 20% of the total film thickness. Preferably, each outer layer comprises about 10% of the total film thickness. 
     Since the starch-free outer layers of the film prevent the exposure of the starch contained in the inner layer, the film would not normally degrade through attack on the starch. The present invention, however, provides a prodegradant in the outer layers to facilitate the degradation of the outer layers resulting in the exposure of the starch filled inner layer. The functional components of the prodegradant are: (1) a transition metal such as manganese or iron and salts thereof; and, (2) a quantity of an unsaturated polymer such as vegetable oils, fats, fatty acids, styrene-butadiene-styrene block copolymer or other unsaturated polymers of a wide variety. Although the specific reaction kinetics and dynamics are not well understood, it appears that the transition metal or transition metal salt reacts initially with the unsaturated polymer or fatty acid source at the surface of the film to produce peroxides and hydroperoxides in the initial phase of the reaction. The peroxides and hydroperoxides then initiate free radical chain reactions and ultimate oxidation of the polyethylene film. Thus, the outer layers of the film of the present invention degrade through oxidation and expose the starch filled inner layer. The starch filled inner layer may also contain prodegradant in order to increase the rate of degradation. 
     Preferably the outer layers of the film of the present invention contain from about 5% to about 15% of a commercially available prodegradant system sold by Archer Daniels Midland Co., Decatur, Ill. 62525 under the trade designations ADM 012401 and ADM 012406. ADM 012401 is a metal catalyst concentrate containing approximately 7200 ppm manganese in a linear low density polyethylene base having a density of approximately 0.918 gm/cm 3  and a melt index of about 2.0. ADM 012406 is 50% styrene-butadiene-styrene unsaturated block copolymer and 50% linear low density polyethylene having a density of about 0.924 and a melt index of about 20.0. The styrene-butadiene-styrene copolymer contains about 70% butadiene. The ratio of ADM 012406 to ADM 12401 is preferably about 4:1. More preferably, the outer layers of the film contain about 10% of the prodegradant system. 
     The films of the present invention may be produced in thickness from about 0.75 mils to about 7.0 mils using conventional cast and blown film coextrusion techniques. 
     Preferably, the film is produced in thickness from between about 1.0 mil to about 2.0 mils. 
     The invention will be further described with respect to the following examples; however, the scope of the invention is not to be limited thereby. 
     EXAMPLE 1 
     In order to illustrate the effect of incorporating starch in a polyethylene film, films were produced from a low density polyethylene resin and from a linear low density polyethylene resin in a conventional blown film extrusion process. Each resin was used to produce a conventional and a starch filled film. A commercially available starch master batch sold by Ampacet Corp., 250 S. Terrace Ave., Mount Vernon, NY 10550 under the trade designation Ampacet 20835 was added to the resins used to produce the starch filled films at a rate resulting in a starch concentration in the films of 6% by weight. Ampacet 20835 is a linear low density polyethylene having a density of about 0.924 gm/cm 3  and a melt index of about 20.0 that contains 40% starch, 14% unsaturated styrene-butadiene-styrene block copolymer and about 500 ppm manganese. The films were tested and the results are set forth in Table 1 below. 
     
                       TABLE 1______________________________________Effect of Starch on Film Properties               LDPE             LLDPE               +6%              +6%PROPERTY    LDPE    STARCH    LLDPE  STARCH______________________________________AV. GAUGE (μm)         50     50         34     45GLOSS (%)     70    25.7        67     25TENSILES:Elong. MD (%)        400     160       615    585Elong. TD (%)        650     615       705    660Stress MD (kPa)       24115   16363     30660  17225Stress TD (kPa)       22392   11747     26660  16536C.O.F.      0.60    0.39      1.0    0.53TEARMD (g/mm)    3740   2559       6693   5315TD (g/mm)   10236   9449      13779  13385IMPACT (g/mm)        3740   1260       7480   5905______________________________________ 
    
     The foregoing example illustrates the deleterious effects on the physical properties of films resulting from the addition of starch. 
     EXAMPLES 2-16 
     Coextruded polyethylene films were produced using conventional cast film techniques. The films were cast with a starch filled center layer and exterior layers containing varying concentrations of prodegradant. The following commercially available materials were used to produce the films: DOWLEX 2047A--LLDPE/octene copolymer having a melt index of about 2.3 and a density of about 0.917 g/cm 3  ; DOWLEX 2027A--a LLDPE/octene copolymer having a melt index of 4.0 and a density of 0.941 g/cm 3  ; Rexene 1031S--a low density polyethylene homopolymer having a melt index of 0.80 and a density of 0.924 g/cm 3  ; Rexene 1068--a low density polyethylene homopolymer having a melt index of about 5.5 and a density of about 0.922 g/cm 3  ; Quantum CM80707--a white color concentrate containing about 50% titanium dioxide in a low density polyethylene base; Ampacet 20835--a starch filled polyethylene aster batch; and, ADM 012401 with ADM 012406--a transition metal salt/unsaturated polymer prodegradant system. The compositions of the core and exterior layers of the film are set forth in Table 2 below. 
     
                                           TABLE 2__________________________________________________________________________                       ADMDowlex     Dowlex          Rexene              Rexene                  Ampacet                       012406/                            CMExample2047A     2027A          1031S              1068                  20835                       012401                            80707__________________________________________________________________________core-40%       53%               7%exterior  65%      35%3, 4core-40%       45.5%    7.5%     7%exterior  65%      35%5, 6core-40%       45.5%    7.5%     7%exterior  60%      30%      8%/2%7, 8core-40%       30.5%   22.5%     7%exterior  65%      35%9, 10core-40%       30.5%   22.5%     7%exterior  60%      30%      8%/2%11, 12core-40%       38%     15%       7%exterior  62.5%    32.5%    4%/1%13core-40%       38%     15%       7%exterior  65%      35%14core-40%       45.5%    7.5%     7%exterior  62.5%    32.5%    4%/1%15core-40%       38%     15%       7%exterior  60%      30%      8%/2%16core-40%       30.5%   22.5%     7%exterior  62.5%    32.5%    4%/1%__________________________________________________________________________ 
    
     In order to test the degradability of the films of Examples 2-16, the films were aged at approximately 160° F. For 24 days at a relative humidity of approximately 50%. The physical properties of the films were tested prior to aging and at 8-day intervals during the aging process. The results of the tests are set forth in Tables 3-6 below. 
     
                                           TABLE 3__________________________________________________________________________Film Properties Prior to Aging__________________________________________________________________________Example      ASTM            2   3   4   5   6   7   8   9__________________________________________________________________________Density (gms/cm.sup.3)             0.935                 0.937                     0.935                         0.940                             0.935                                 0.965                                     0.936                                         0.965Gauge (mils)     1.24                1.35                    1.30                        1.30                            1.34                                1.39                                    1.36                                        1.31Stress at:   D-8825% Elongation     MD     290 299 320 299 286 293 317 339(gms)     TD     271 273 287 302 264 294 296 29510% Elongation     MD     589 612 638 612 581 572 637 659(gms)     TD     509 566 568 584 522 561 554 54025% Elongation     MD     796 824 850 824 789 747 846 858(gms)     TD     586 663 666 688 614 651 641 62940% Elongation     MD     1037                1023                    1073                        1023                            1000                                902 1029                                        1031(gms)     TD     617 692 691 719 644 685 656 640Ultimate  MD D-882            2180                2006                    1955                        1898                            1868                                1461                                    1727                                        1461Stress (gms)     TD     1644                1387                    1472                        1379                            1327                                1443                                    1327                                        1269Ultimate  MD D-882            350 308 271 306 279 315 335 315Elongation (%)     TD     755 603 657 607 634 666 660 619Tear      MD D-1922             26  20  29  21  20  23  27  20Strength (gms)     TD     202 265 279 270 283 322 263 232Impact    MD D-1709             64  70  53  65 &lt;45 &lt;45 &lt;45 &lt;45Coefficient of (1)            1.08                0.96                    0.96                        1.02                            1.23                                0.83                                    0.83                                        0.86Friction (2)     0.57                0.59                    0.53                        0.64                            0.62                                0.50                                    0.53                                        0.50__________________________________________________________________________Example          ASTM                10  11  12  13  14  15  16__________________________________________________________________________Density (gms/cm.sup.3)                 0.936                     0.960                         0.961                             0.952                                 0.940                                     0.956                                         0.959Gauge (mils)         1.36                    1.30                        1.15                            1.29                                1.29                                    1.31                                        1.32Stress at:       D-8825% Elongation MD     317 330 236 298 269 313 342(gms)         TD     277 309 227 273 305 309 34210% Elongation         MD     637 633 531 595 531 618 660(gms)         TD     555 574 435 538 589 632 63725% Elongation         MD     846 830 739 788 700 805 859(gms)         TD     650 666 511 628 687 746 74940% Elongation         MD     1029                    1015                        938 969 874 770 1033(gms)         TD     667 692 534 649 689 770 776Ultimate      MD D-882                1479                    1666                        2180                            1716                                1734                                    1716                                        1730Stress (gms)  TD     1447                    1110                        1139                            1110                                1505                                    1681                                        1423Ultimate      MD D-882                317 300 350 335 341 335 308Elongation (%)         TD     681 531 648 557 638 691 622Tear          MD D-1922                 53  25  19  21  29  41  28Strength (gms)         TD     281 279 311 285 219 318 271Impact        MD D-1709                 53  50 &lt;45  55  69  56 &lt;45Coefficient of (1)   1.14                    0.89                        1.06                            0.93                                0.86                                    1.01                                        0.84Friction.sup.1 (2)   0.55                    0.54                        0.55                            0.56                                0.62                                    0.57                                        0.50__________________________________________________________________________ .sup.1 Subsequent to extrusion, the film was passed between a polished chrome roller and a rubberfaced roller. (1) refers to the side corresponding to the polished chrome roller and (2) refers to the side corresponding to the rubberfaced roller. 
    
     
                                           TABLE 4__________________________________________________________________________Film Properties After Eight Days__________________________________________________________________________Example    ASTM          2   3   4   5   6   7   8   9__________________________________________________________________________5% Elongation   MD     290 437 429 393 407 426 399 605(gms)   TD     427 431 471 420 405 472 461 58110% Elongation   MD     589 764 756 688 700 724 704 922(gms)   TD     694 706 803 689 667 752 747 89025% Elongation   MD     736 994 990 909 917 903 894 1050(gms)   TD     753 775 866 760 720 793 795 85340% Elongation   MD     1037              1215                  1220                      1112                          1143                              1047                                  1041                                      1156(gms)   TD     763 779 891 768 715 757 759 806Ultimate   MD D-882          2180              1859                  1824                      1707                          1724                              1488                                  1460                                      1421Stress (gms)   TD     1639              1200                  1699                      1443                          1520                              1363                                  1481                                      1116Ultimate   MD D-882          350 234 205 254 229 270 270 244Elongation (%)   TD     720 553 662 649 692 622 655 516Tear    MD D-1922            26               28  33  16  12  23  24  45Strength (gms)   TD     275 273 295 390 283 330 357 383Impact  MD D-1709          --  --  --  --  --  --  --  --__________________________________________________________________________Example        ASTM              10  11  12  13  14  15  16__________________________________________________________________________5% Elongation       MD     579 408 308 414 395 520 506(gms)       TD     528 426 323 465 461 454 52710% Elongation       MD     886 698 550 702 690 830 895(gms)       TD     822 688 547 742 775 732 81525% Elongation       MD     1011                  889 744 899 889 997 1071(gms)       TD     823 747 597 780 859 773 81440% Elongation       MD     1119                  1058                      936 1059                              1076                                  1156                                      1207(gms)       TD     797 745 589 748 862 745 758Ultimate    MD D-882              1341                  1550                      1352                          1549                              1701                                  1561                                      1583Stress (gms)       TD     881 1274                      1110                          1274                              1300                                  1332                                      1443Ultimate    MD D-882              202 260 204 278 263 240 257Elongation (%)       TD     417 573 627 573 548 643 642Tear        MD D-1922               12  25  14  37  16  20  24Strength (gms)       TD     342 336 413 302 289 338 359Impact      MD D-1709              --  --  --  --  --  --  --__________________________________________________________________________ 
    
     
                                           TABLE 5__________________________________________________________________________Film Properties After Sixteen Days__________________________________________________________________________Example    ASTM          2   3   4   5   6   7   8   9__________________________________________________________________________5% Elongation   MD     419 458 423 715 635 516 496 --(gms)   TD     446 438 470 618 618 595 592 --10% Elongation   MD     746 782 707 1073                          980 814 829 --(gms)   TD     722 728 737 875 935 905 910 --25% Elongation   MD     989 1018                  922 1215                          1137                              977 1000                                      --(gms)   TD     795 796 792 963 --  939 943 --40% Elongation   MD     1216              1228                  1124                      1333                          1295                              1119                                  1123                                      --(gms)   TD     799 793 789 --  --  891 881 --Ultimate   MD D-882          2154              1879                  1711                      1609                          1629                              1494                                  1534                                      798Stress (gms)   TD     1707              1463                  1357                      907 972 1322                                  1478                                      575Ultimate   MD D-882          327 235 221  97 117 275 282  3Elongation (%)   TD     744 660 627  24  20 572 630  4Tear    MD D-1922           22  47  2   8   12  54  47 &lt;16Strength (gms)   TD     305 300 291  74  60 364 384 &lt;16Impact.sup.2   MD D-1709           50 &lt;45 &lt;45 &lt;45 &lt;45 &lt;45 &lt;45 &lt;45__________________________________________________________________________Example        ASTM              10  11  12  13  14  15  16__________________________________________________________________________5% Elongation       MD     831 408 492 414 458 806 732(gms)       TD     --  573 545 437 504 583 64210% Elongation       MD     --  698 755 702 779 1146                                      1061(gms)       TD     --  846 790 682 803 --  74725% Elongation       MD     --  889 888 899 987 --  1148(gms)       TD     --  847 733 737 861 --  --40% Elongation       MD     --  1058                      1000                          1059                              1175                                  --  1221(gms)       TD     --  711 --  729 851 --  --Ultimate    MD D-882              855 1550                      1222                          1549                              1727                                  1198                                      1172Stress (gms)       TD     488 889 822 1087                              1252                                  733 962Ultimate    MD D-882               5  260 103 278 236  18  41Elongation (%)       TD      3   53  37 550 559  5   11Tear        MD D-1922              &lt;16  12  5   21  17 &lt;16 &lt;16Strength (gms)       TD     &lt;16 216  37 314 294 &lt;16  40Impact.sup.3       MD D-1709              &lt;45 &lt;45 &lt;45 &lt;45 48  &lt;45 &lt;45__________________________________________________________________________ .sup.2 Tested at 12 days. .sup.3 Tested at 12 days. 
    
     
                                           TABLE 6__________________________________________________________________________Film Properties After 24 Days__________________________________________________________________________Example    ASTM          2   3   4   5   6   7   8   9__________________________________________________________________________5% Elongation   MD     256 418 377 .sup. --.sup.4                          651 516 458 .sup. --.sup.4(gms)   TD     390 377 402 --  602 437 502 --10% Elongation   MD     685 794 709 --  1073                              915 850 --(gms)   TD     716 705 732 --  864 749 831 --25% Elongation   MD     935 1041                  938 --  1232                              1103                                  1042                                      --(gms)   TD     810 788 808 --  --  791 867 --40% Elongation   MD     1158              1248                  1133                      --  --  1236                                  1169                                      --(gms)   TD     809 788 800 --  --  748 831 --Ultimate   MD D-882          1890              1933                  1733                      --  1189                              1614                                  1493                                      --Stress (gms)   TD     1882              1309                  1453                      --  914 1221                                  1040                                      --Ultimate   MD D-882          273 257 225 --   22 214 206 --Elongation (%)   TD     785 595 653 --   8  618 410 --Tear    MD D-1922           30  55  33 --  &lt;16  38  31 &lt;16Strength (gms)   TD     374 363 324 --  &lt;16 434 436 &lt;16Impact  MD D-1709           45  50 &lt;45 --  &lt;45 &lt;45 &lt;45 --__________________________________________________________________________Example        ASTM              10  11  12  13  14  15  16__________________________________________________________________________5% Elongation       MD     .sup. --.sup.4                  642 562 398 519 --  918(gms)       TD     --  697 550 420 533 334 16110% Elongation       MD     --  1018                      903 776 907 --  --(gms)       TD     --  554 --  733 892 --  --25% Elongation       MD     --  --  --  1005                              1085                                  --  --(gms)       TD     --  --  --  798 923 --  --40% Elongation       MD     --  --  --  1170                              1216                                  --  --(gms)       TD     --  --  --  780 823 --  --Ultimate    MD D-882              --  1069                      980 1724                              1582                                  406 952Stress (gms)       TD     --  987 720 1115                              959 535 987Ultimate    MD D-882              --   14  18 285 167  3   5Elongation (%)       TD     --   8   6  547 105  6   8Tear        MD D-1922              --   8  &lt;16  17  45 &lt;16 &lt;16Strength (gms)       TD     --   12 &lt;16 417 353 &lt;16 &lt;16Impact      MD D-1709              --  &lt;45 &lt;45 &lt;45 &lt;45 &lt;45 &lt;45__________________________________________________________________________ .sup.4 Film crumbled at touch. 
    
     As can be seen from the foregoing examples, the films of the present invention provide a combination of initially satisfactory physical properties with subsequent degradability. 
     While the particular embodiments of the present invention have been described, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. The following claims are intended to cover all such modifications that are within the scope of this invention.