Starch filled coextruded degradable polyethylene film

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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 "polyethylene" 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 "starch" 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.sup.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.sup.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 (.mu.m) 50 50 34 45 GLOSS (%) 70 25.7 67 25 TENSILES: Elong. MD (%) 400 160 615 585 Elong. TD (%) 650 615 705 660 Stress MD (kPa) 24115 16363 30660 17225 Stress TD (kPa) 22392 11747 26660 16536 C.O.F. 0.60 0.39 1.0 0.53 TEAR MD (g/mm) 3740 2559 6693 5315 TD (g/mm) 10236 9449 13779 13385 IMPACT (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.sup.3 ; DOWLEX 2027A--a LLDPE/octene copolymer having a melt index of 4.0 and a density of 0.941 g/cm.sup.3 ; Rexene 1031S--a low density polyethylene homopolymer having a melt index of 0.80 and a density of 0.924 g/cm.sup.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.sup.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 __________________________________________________________________________ ADM Dowlex Dowlex Rexene Rexene Ampacet 012406/ CM Example 2047A 2027A 1031S 1068 20835 012401 80707 __________________________________________________________________________ core- 40% 53% 7% exterior 65% 35% 3, 4 core- 40% 45.5% 7.5% 7% exterior 65% 35% 5, 6 core- 40% 45.5% 7.5% 7% exterior 60% 30% 8%/2% 7, 8 core- 40% 30.5% 22.5% 7% exterior 65% 35% 9, 10 core- 40% 30.5% 22.5% 7% exterior 60% 30% 8%/2% 11, 12 core- 40% 38% 15% 7% exterior 62.5% 32.5% 4%/1% 13 core- 40% 38% 15% 7% exterior 65% 35% 14 core- 40% 45.5% 7.5% 7% exterior 62.5% 32.5% 4%/1% 15 core- 40% 38% 15% 7% exterior 60% 30% 8%/2% 16 core- 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.degree. 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 0.965 Gauge (mils) 1.24 1.35 1.30 1.30 1.34 1.39 1.36 1.31 Stress at: D-882 5% Elongation MD 290 299 320 299 286 293 317 339 (gms) TD 271 273 287 302 264 294 296 295 10% Elongation MD 589 612 638 612 581 572 637 659 (gms) TD 509 566 568 584 522 561 554 540 25% Elongation MD 796 824 850 824 789 747 846 858 (gms) TD 586 663 666 688 614 651 641 629 40% Elongation MD 1037 1023 1073 1023 1000 902 1029 1031 (gms) TD 617 692 691 719 644 685 656 640 Ultimate MD D-882 2180 2006 1955 1898 1868 1461 1727 1461 Stress (gms) TD 1644 1387 1472 1379 1327 1443 1327 1269 Ultimate MD D-882 350 308 271 306 279 315 335 315 Elongation (%) TD 755 603 657 607 634 666 660 619 Tear MD D-1922 26 20 29 21 20 23 27 20 Strength (gms) TD 202 265 279 270 283 322 263 232 Impact MD D-1709 64 70 53 65 <45 <45 <45 <45 Coefficient of (1) 1.08 0.96 0.96 1.02 1.23 0.83 0.83 0.86 Friction (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 0.959 Gauge (mils) 1.36 1.30 1.15 1.29 1.29 1.31 1.32 Stress at: D-882 5% Elongation MD 317 330 236 298 269 313 342 (gms) TD 277 309 227 273 305 309 342 10% Elongation MD 637 633 531 595 531 618 660 (gms) TD 555 574 435 538 589 632 637 25% Elongation MD 846 830 739 788 700 805 859 (gms) TD 650 666 511 628 687 746 749 40% Elongation MD 1029 1015 938 969 874 770 1033 (gms) TD 667 692 534 649 689 770 776 Ultimate MD D-882 1479 1666 2180 1716 1734 1716 1730 Stress (gms) TD 1447 1110 1139 1110 1505 1681 1423 Ultimate MD D-882 317 300 350 335 341 335 308 Elongation (%) TD 681 531 648 557 638 691 622 Tear MD D-1922 53 25 19 21 29 41 28 Strength (gms) TD 281 279 311 285 219 318 271 Impact MD D-1709 53 50 <45 55 69 56 <45 Coefficient of (1) 1.14 0.89 1.06 0.93 0.86 1.01 0.84 Friction.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 427 431 471 420 405 472 461 581 10% Elongation MD 589 764 756 688 700 724 704 922 (gms) TD 694 706 803 689 667 752 747 890 25% Elongation MD 736 994 990 909 917 903 894 1050 (gms) TD 753 775 866 760 720 793 795 853 40% Elongation MD 1037 1215 1220 1112 1143 1047 1041 1156 (gms) TD 763 779 891 768 715 757 759 806 Ultimate MD D-882 2180 1859 1824 1707 1724 1488 1460 1421 Stress (gms) TD 1639 1200 1699 1443 1520 1363 1481 1116 Ultimate MD D-882 350 234 205 254 229 270 270 244 Elongation (%) TD 720 553 662 649 692 622 655 516 Tear MD D-1922 26 28 33 16 12 23 24 45 Strength (gms) TD 275 273 295 390 283 330 357 383 Impact MD D-1709 -- -- -- -- -- -- -- -- __________________________________________________________________________ Example ASTM 10 11 12 13 14 15 16 __________________________________________________________________________ 5% Elongation MD 579 408 308 414 395 520 506 (gms) TD 528 426 323 465 461 454 527 10% Elongation MD 886 698 550 702 690 830 895 (gms) TD 822 688 547 742 775 732 815 25% Elongation MD 1011 889 744 899 889 997 1071 (gms) TD 823 747 597 780 859 773 814 40% Elongation MD 1119 1058 936 1059 1076 1156 1207 (gms) TD 797 745 589 748 862 745 758 Ultimate MD D-882 1341 1550 1352 1549 1701 1561 1583 Stress (gms) TD 881 1274 1110 1274 1300 1332 1443 Ultimate MD D-882 202 260 204 278 263 240 257 Elongation (%) TD 417 573 627 573 548 643 642 Tear MD D-1922 12 25 14 37 16 20 24 Strength (gms) TD 342 336 413 302 289 338 359 Impact 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 --