Multilayer films and methods of making the same

Abstract

Disclosed are multilayer films which can provide balanced improvement in film mechanical performance, where the film comprises a propylene-based polymer and an elastic ethylene copolymer.

Claims

1. A multilayer film, comprising: (a) two outer layers; (b) a core layer between the two outer layers, the core layer comprising about 50 wt % to about 100 wt % of a propylene-based polymer, based on total weight of polymer in the core layer; and (c) two inner layers each between the core layer and each outer layer, wherein at least one of the inner layers comprises about 50 wt % to about 100 wt % of an ethylene copolymer, based on total weight of polymer in the inner layer, wherein the ethylene copolymer comprises a low crystalline polymer comprising greater than or equal to about 70 wt % units derived from ethylene, less than or equal to about 30 wt % units derived from propylene, and less than about 5 wt % of units derived from C.sub.4-C.sub.20 -olefins, based on total weight of the polymer, and having the following properties: (i) crystallinity derived from ethylene; (ii) a heat of fusion of about 20 to about 85 J/g; (iii) a polydispersity index (M.sub.w/M.sub.n) of less than about 2.5; (iv) a reactivity ratio of about 0.5 to about 1.5; (v) a proportion of inversely inserted propylene units based on 2, 1 insertion of propylene monomer in all propylene insertions, as measured by .sup.13C NMR of less than 0.5 wt %; and (vi) a branching index greater than about 0.5; wherein the low crystalline polymer is prepared in a single reactor; wherein the multilayer film has: (i) a 1% Secant Modulus of at least about 20%, 50%, or 100% higher in Machine Direction (MD) and of at least about 20%, 50%, or 100% higher in Transverse Direction (TD); and (ii) an Elmendorf tear strength of at least about 20% higher in MD and of at least about 20% higher in TD, compared to that of a film free of the propylene-based polymer in the core layer and the ethylene copolymer in the inner layer, but is otherwise identical in terms of film structure, layers' compositions, and the film's overall thickness.

2. The multilayer film of claim 1, wherein the core layer consists of about 100 wt % of a propylene-based polymer, based on the total weight of polymer in the core layer.

3. The multilayer film of claim 1, wherein at least one of the two inner layers consists of about 100 wt % of the ethylene copolymer, based on total weight of polymer in the inner layer.

4. The multilayer film of claim 1, wherein the propylene-based polymer comprises one or more of (i) a polypropylene homopolymer; (ii) a copolymer derived from propylene and one or more C.sub.2 and/or C.sub.4 to C.sub.10 -olefin comonomers, wherein the copolymer has at least about 60 wt % propylene-derived units, based on total weight of the copolymer, and (iii) an impact copolymer having between 75 and 95 wt % homopolypropylene and between 5 and 30 wt % of ethylene-propylene rubber, wherein ethylene propylene rubber comprises less than about 50 wt % ethylene, and blends thereof.

5. The multilayer film of claim 1, wherein the propylene-based polymer is a polypropylene homopolymer.

6. The multilayer film of claim 1, wherein the ethylene copolymer further comprises a low crystalline polymer blend composition, comprising: (i) from 65 wt % to 90 wt % based on the total weight of the blend of an ethylene -olefin elastomer having either no crystallinity or crystallinity derived from ethylene, having 70 wt % or more units derived from ethylene; and (ii) from 10 wt % to 35 wt % based on the total weight of the blend of a propylene polymer having 40 wt % or more units derived from propylene, including isotactically arranged propylene derived sequences; wherein the ethylene -olefin elastomer and the propylene polymer are prepared in separate reactors arranged in parallel configuration.

7. The multilayer film of claim 1, wherein at least one of the outer layers comprises at least about 80 wt % of a polyethylene derived from ethylene and one or more C.sub.3 to C.sub.20 -olefin comonomers, based on total weight of polymer in the outer layer, wherein the polyethylene has a density of from about 0.900 to about 0.945 g/cm.sup.3, an MI, I.sub.2.16, of from about 0.1 to about 15 g/10 min, an MWD of from about 1.5 to about 5.5, and an MIR, I.sub.21.6/I.sub.2.16, of from about 10 to about 100.

8. The multilayer film of claim 7, wherein at least one of the outer layers further comprises a low density polyethylene (LDPE).

9. The multilayer film of claim 1, wherein the two outer layers have a total thickness of at most about two thirds of the total thickness of the multilayer film.

10. The multilayer film of claim 1, wherein the two outer layers are identical.

11. The multilayer film of claim 1, wherein the two inner layers have a total thickness of at most about 50% of the total thickness of the multilayer film.

12. The multilayer film of claim 1, wherein the core layer has a total thickness of at most about 75% of the total thickness of the multilayer film.

13. The multilayer film of claim 1, wherein the two inner layers are identical.

14. A multilayer film, comprising: (a) two outer layers, each comprising: (i) at least about 80 wt % of a polyethylene derived from ethylene and one or more C.sub.3 to C.sub.20 -olefin comonomers, based on total weight of polymer in the outer layer, wherein the polyethylene has a density of from about 0.900 to about 0.945 g/cm.sup.3, an MI, I.sub.2.16, of from about 0.1 to about 15 g/10 min, an MWD of from about 1.5 to about 5.5, and an MIR, I.sub.21.6/I.sub.2.16, of from about 10 to about 100; and (ii) an LDPE; (b) a core layer between the two outer layers, the core layer comprising about 100 wt % of a polypropylene homopolymer, based on total weight of polymer in the core layer; and (a) two inner layers each between the core layer and each outer layer, wherein each of the inner layers comprises about 100 wt % of an ethylene copolymer, based on total weight of polymer in the inner layer, wherein the ethylene copolymer is a low crystalline polymer comprising greater than or equal to about 70 wt % units derived from ethylene, less than or equal to about 30 wt % units derived from propylene, and less than about 5 wt % of units derived from C.sub.4-C.sub.20 -olefins, based on total weight of the polymer, and having the following properties: (i) crystallinity derived from ethylene; (ii) a heat of fusion of about 20 to about 85 J/g; (iii) a polydispersity index (M.sub.w/M.sub.n) of less than about 2.5; (iv) a reactivity ratio of about 0.5 to about 1.5; (v) a proportion of inversely inserted propylene units based on 2, 1 insertion of propylene monomer in all propylene insertions, as measured by .sup.13C NMR of less than 0.5 wt %; and (vi) a branching index greater than about 0.5; wherein the polymer is prepared in a single reactor, wherein the multilayer film has: (i) a 1% Secant Modulus of at least about 20%, 50%, or 100% higher in Machine Direction (MD) and of at least about 20%, 50%, or 100% higher in Transverse Direction (TD); and (ii) an Elmendorf tear strength of at least about 20% higher in MD and of at least about 20% higher in TD, compared to that of a film free of the propylene-based polymer in the core layer and the ethylene copolymer in the inner layer, but is otherwise identical in terms of film structure, layers' compositions, and the film's overall thickness.

15. The multilayer film of claim 14, wherein the thickness ratio between each of the outer layers, each of the inner layers, and the core layer is about 2:1:2.

16. A method for making a multilayer film, comprising the steps of: (a) preparing two outer layers; (b) preparing a core layer between the two outer layers, the core layer comprising about 100 wt % of a propylene-based polymer, based on total weight of polymer in the core layer; (c) preparing two inner layers each between the core layer and each outer layer, wherein at least one of the inner layers comprises about 100 wt % of an ethylene copolymer, based on total weight of polymer in the inner layer, wherein the ethylene copolymer is a low crystalline polymer comprising greater than or equal to about 70 wt % units derived from ethylene, less than or equal to about 30 wt % units derived from propylene, and less than about 5 wt % of units derived from C.sub.4-C.sub.20 -olefins, based on total weight of the polymer, and having the following properties: (i) crystallinity derived from ethylene; (ii) a heat of fusion of about 20 to about 85 J/g; (iii) a polydispersity index (M.sub.w/M.sub.n) of less than about 2.5; (iv) a reactivity ratio of about 0.5 to about 1.5; (v) a proportion of inversely inserted propylene units based on 2, 1 insertion of propylene monomer in all propylene insertions, as measured by .sup.13C NMR of less than 0.5 wt %; and (vi) a branching index greater than about 0.5; wherein the polymer is prepared in a single reactor; and (d) forming a film comprising the layers in steps (a) to (c), wherein the multilayer film has: (i) a 1% Secant Modulus of at least about 20%, 50%, or 100% higher in Machine Direction (MD) and of at least about 20%, 50%, or 100% higher in Transverse Direction (TD); and (ii) an Elmendorf tear strength of at least about 20% higher in MD and of at least about 20% higher in TD, compared to that of a film free of the propylene-based polymer in the core layer and the ethylene copolymer in the inner layer, but is otherwise identical in terms of film structure, layers' compositions, and the film's overall thickness.

17. The method of claim 16, wherein the multilayer film in step (d) is formed by blown extrusion, cast extrusion, coextrusion, blow molding, casting, or extrusion blow molding.

Description

EXAMPLES

(1) The present invention, while not meant to be limited, may be better understood by reference to the following example and tables.

(2) The example illustrates stiffness and tear resistance demonstrated by inventive samples (Samples 1-6) of five layers comprising 100 wt % (based on total weight of polymer in the core layer) of a polypropylene homopolymer in the core layer and 100 wt % (based on total weight of polymer in the inner layer) of an elastic ethylene copolymer in each inner layer between the core layer and each outer layer, in comparison with comparative samples (Samples A-D) using other polymers in place of the elastic ethylene copolymer alone or in place of both the polypropylene homopolymer and the elastic ethylene copolymer, but otherwise identical in terms of film structure, layers' compositions, and the film's overall thickness.

(3) Polymer products used in the samples include: Moplen HP456J polypropylene homopolymer resin (MFR (230 C./2.16 kg): 3.4 g/10 min) (LyondellBasell Industries N.V., Netherlands); low crystalline ethylene-propylene polymers EP1 (ethylene content: 78 wt %, density: 0.880 g/cm.sup.3), EP2 (ethylene content: 74 wt %, density: 0.873 g/cm.sup.3), EP3 (ethylene content: 85 wt %, density: 0.892 g/cm.sup.3), and EP 4 (ethylene content: 86 wt %, density: 0.892 g/cm.sup.3) (ExxonMobil Chemical Company, Houston, Tex., USA); low crystalline ethylene-propylene blend compositions EP5 (ethylene content: 74 wt %, density: 0.879 g/cm.sup.3) and EP6 (ethylene content: 73 wt %, density: 0.873 g/cm.sup.3) (ExxonMobil Chemical Company, Houston, Tex., USA); VISTAMAXX 6102FL performance polymer (ethylene content: 16 wt %, density: 0.862 g/cm.sup.3, MFR (230 C./2.16 kg): 3 g/10 min) and VISTAMAXX 3020FL performance polymer (ethylene content: 11 wt %, density: 0.874 g/cm.sup.3, MFR (230 C./2.16 kg): 3 g/10 min) (ExxonMobil Chemical Company, Houston, Tex., USA); EXCEED 1018KB mPE resin (density: 0.918 g/cm.sup.3, MI (190 C./2.16 kg): 1.0 g/10 min) and EXCEED 1018HA mPE resin (density: 0.918 g/cm.sup.3, MI (190 C./2.16 kg): 1.0 g/10 min) (ExxonMobil Chemical Company, Houston, Tex., USA); and EXXONMOBIL LDPE LD 150BW LDPE resin (density: 0.923 g/cm.sup.3, MI: 0.75 g/10 min) (ExxonMobil Chemical Company, Houston, Tex., USA). All samples were prepared with a thickness of 50 m at a layer thickness ratio of 2:1:2:1:2 on a W&H coextrusion blown film line with a BUR of 2.5. Samples were conditioned at 23 C.2 C. and 50%10% relative humidity for at least 40 hours prior to determination of all properties. Structure-wise formulations of the film samples and test results thereof are respectively depicted in Tables 1 and 2.

(4) The films of the examples (Samples 1-6 and A-C) are S layer blown films using a dual lip internal bubble cooling system. The die diameter was 280 mm, die gap was 1.4 mm, film thickness was 50 m, blow up ratio was 2.5, total output was between about 225-230 kg/hr for Samples 1-6 and about 200 kg/hr for Sample A, and about 120 kg/hr for Samples B-D, the frost line height was about 1000 mm for Samples 1-6 and Sample A, and about 800 mm for Samples B-D.

(5) 1% Secant modulus of the films were measured in both MD and TD by a method based on ASTM D882 with static weighing and a constant rate of grip separation using a Zwick 1445 tensile tester with a 200N. Since rectangular shaped test samples were used, no additional extensometer was used to measure extension. The nominal width of the tested film sample is 15 mm and the initial distance between the grips is 50 mm. A pre-load of 0.1N was used to compensate for the so called TOE region at the origin of the stress-strain curve. The constant rate of separation of the grips is 5 mm/min upon reaching the pre-load and 5 mm/min to measure 1% Secant modulus (up to 1% strain). 1% Secant modulus is calculated by drawing a tangent through two well defined points on the stress-strain curve. The reported value corresponds to the stress at 1% strain (with x correction). The result is expressed as load per unit area (N/mm.sup.2). The value is an indication of the film stiffness in tension. The 1% Secant Modulus is used for thin film and sheets as no clear proportionality of stress to strain exists in the initial part of the curve.

(6) Elmendorf tear strength was measured in both MD and TD based on ASTM D1922-06a using the Tear Tester 83-11-01 from TMI Group of Companies and measures the energy required to continue a pre-cut tear in the test sample, presented as tearing force in gram. Samples were cut across the web using the constant radius tear die and were free of any visible defects (e.g., die lines, gels, etc.).

(7) TABLE-US-00001 TABLE 1 Structure-wise formulations (wt %) for all film samples Sample No. Outer Inner Core 1 EXCEED 1018KB EP1 (100) Moplen HP456J (95) (100) EXXONMOBIL LDPE LD 150BW (5) 2 EXCEED 1018KB EP2 (100) Moplen HP456J (95) (100) EXXONMOBIL LDPE LD 150BW (5) 3 EXCEED 1018KB EP3 (100) Moplen HP456J (95) (100) EXXONMOBIL LDPE LD 150BW (5) 4 EXCEED 1018KB EP4 (100) Moplen HP456J (95) (100) EXXONMOBIL LDPE LD 150BW (5) 5 EXCEED 1018KB EP5 (100) Moplen HP456J (95) (100) EXXONMOBIL LDPE LD 150BW (5) 6 EXCEED 1018KB EP6 (100) Moplen HP456J (95) (100) EXXONMOBIL LDPE LD 150BW (5) A EXCEED 1018KB EXCEED 1018KB EXCEED 1018KB (95) (95) (95) EXXONMOBIL EXXONMOBIL EXXONMOBIL LDPE LD 150BW (5) LDPE LD 150BW (5) LDPE LD 150BW (5) B EXCEED 1018KB EXCEED 1018HA Moplen HP456J (95) (100) (100) EXXONMOBIL LDPE LD 150BW (5) C EXCEED 1018KB VISTAMAXX Moplen HP456J (95) 6102FL (sub-skin next to (100) EXXONMOBIL outside bubble skin)/ LDPE LD 150BW (5) VISTAMAXX 3020FL (sub-skin next to inside bubble skin) (100) D EXCEED 1018KB VISTAMAXX Moplen HP456J (95) 3020FL (100) (100) EXXONMOBIL LDPE LD 150BW (5)

(8) TABLE-US-00002 TABLE 2 Mechanical properties for all film samples 1% Secant 1% Secant Modulus Modulus MD TD Elmendorf Elmendorf Sample No. (N/mm.sup.2) (N/mm.sup.2) Tear MD (g) Tear MD (g) 1 485 451 949 1473 2 482 456 990 1526 3 478 462 819 1420 4 482 468 848 1372 5 508 473 1078 1628 6 483 461 1084 1496 A 199 196 516 934 B 571 495 477 912 C 495 477 661 753 D 502 509 589 889

(9) It can be observed from the above test results in Table 2 that, whether the elastic ethylene copolymer described herein is present in the inner layers (Samples 1-6) or not (Samples B-D), the film samples (all samples except for Sample A) including the polypropylene homopolymer in the core layer exhibited enhanced stiffness, as demonstrated by a higher 1% Secant Modulus both in MD and TD than that of the sample with a core layer otherwise formulated (Sample A). However, tear resistance remained unchanged without the elastic ethylene copolymer (Samples B-D), unless the film samples were equipped with such elastic ethylene copolymer in the inner layers in addition to the polypropylene homopolymer in the core layer (Samples 1-6). As a result of the combined effect imposed by the polypropylene homopolymer and the elastic ethylene copolymer, the inventive samples outperformed the comparative samples, excelling simultaneously in stiffness and tear resistance, as represented by improvement in both 1% Secant Modulus and Elmendorf tear strength.

(10) Without being bound by theory, it is believed that the inventive structure-wise formulation design described herein can serve as a desired alternative to the current solutions available for film performance optimization over a broad range of end-uses.

(11) All documents described herein are incorporated by reference herein, including any priority documents and/or testing procedures. When numerical lower limits and numerical upper limits are listed herein, ranges from any lower limit to any upper limit are contemplated. As is apparent from the foregoing general description and the specific embodiments, while forms of the invention have been illustrated and described, various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited thereby.