MDO MULTILAYER FILM

Abstract

A multilayer film having Machine Direction Orientation (MDO) is prepared by first co-extruding a multilayer film, then stretching the multilayer film in the machine direction at a temperature lower than the melting point of the polyethylene resin that is used to prepare the film. At least one layer of the film is a first polyethylene composition having a density of from about 0.94 to about 0.97 g/cc and at least one second layer is prepared from a polyethylene composition having a lower density than the first polyethylene composition. This disclosure enables the manufacture of films having outstanding barrier properties (low Water Vapor Transmission Rate, WVTR, and low Oxygen Transmission Rate, OTR) and good physical properties.

Claims

1. A method for producing an oriented multilayer film comprising A) co-extruding a multilayer film comprising 1) a first layer prepared from a first high density polyethylene composition having a melt index, I.sub.2, of from about 0.2 to about 10 grams per 10 minutes, a density of from about 0.94 to about 0.97 g/cc and from about 100 to about 9000 parts per million of a nucleating agent; and 2) a second layer prepared from a second polyethylene composition having a melt index, I.sub.2, of from about 0.2 to about 10 grams per 10 minutes and a density which is lower than the density of said first high density polyethylene composition by an amount of from about 0.010 to about 0.060 g/cc; and B) stretching said multilayer film in the machine direction at a stretch ratio of from about 1:2 to about 1:12; wherein melt index is measured according to ASTM D1238 at 190° C. using a 2.16 kg weight and density is measured according to ASTM D792.

2. The method of claim 1 wherein said stretching is done at a temperature below about 120° C.

3. The method of claim 1 wherein said multilayer film is a three layer film having at least one skin layer prepared from said second polyethylene composition and a core layer prepared from said first high density polyethylene composition.

4. The method of claim 1 wherein said multilayer film comprises at least five layers and wherein said multilayer film has two skin layers prepared from said second polyethylene composition and at least one core layer prepared from said first high density polyethylene composition.

5. The method of claim 1 wherein said multilayer film has a thickness of from about 0.5 to about 3 mils after said stretching.

6. A film made by the method of claim 1.

7. A package prepared from a film according to claim 6.

8. A method for producing an oriented multilayer film comprising: A) co-extruding a multilayer film comprising 1) a first layer prepared from a first high density polyethylene composition having a melt index, I.sub.2, of from about 0.2 to about 10 grams per 10 minutes; a density of from about 0.95 to about 0.97 g/cc and a Mw/Mn of from about 5 to about 12, with the proviso that said first polyethylene composition is a blend of at least two blend components comprising a first blend component having a Mw/Mn of from about 2 to about 4 and a second blend component having a Mw/Mn of from about 2 to about 4; and 2) a second layer prepared from a second polyethylene composition having a melt index, I.sub.2, of from about 0.2 to about 10 grams per 10 minutes and a density which is lower than the density of said first polyethylene composition by an amount of from about 0.010 to about 0.060 g/cc; and B) stretching said multilayer film in the machine direction at a stretch ratio of from about 1:2 to about 1:12. wherein melt index is measured according to ASTM D1238 at 190° C. using a 2.16 kg weight and density is measured according to ASTM D792.

9. The method of claim 8 wherein said first blend component has a melt index I.sub.2 and said second blend component has a melt index I.sub.2′; wherein the ratio (I.sub.2/I.sub.2′) is greater than about 10.

10. The method of claim 8 wherein said stretching is done at a temperature below about 120° C.

11. The method of claim 8 wherein said multilayer film is a three layer film having two skin layers prepared from said second polyethylene composition and a core layer prepared from said first high density polyethylene composition.

12. The method of claim 8 wherein said multilayer film comprises at least five layers and wherein said multilayer film has two skin layers prepared from said second polyethylene composition and at least one core layer prepared from said first high density polyethylene composition.

13. The method of claim 8 wherein said multilayer film has a thickness of from about 0.5 to about 3 mils after said stretching.

14. The method of claim 8 wherein said first high density polyethylene composition further contains from about 100 to about 2000 parts per million of a nucleating agent.

15. A film made by the method of claim 8.

16. A package prepared from a film according to claim 8.

17. The method of claim 1 wherein said nucleating agent comprises the calcium salt of hexahydrophthalic acid.

Description

EXAMPLE

[0061] The following test methods were used.

[0062] Melt Index: “I.sub.2”, was determined according to ASTM D1238. [Note: I.sub.2 measurements are made with a 2.16 kg weight at 190° C.] Test results are reported in units of grams/10 minutes.

[0063] Number average molecular weight (Mn), weight average molecular weight (Mw) and MWD (calculated by Mw/Mn) were determined by high temperature Gel Permeation Chromatography “GPC” with differential refractive index “DRI” detection using universal calibration.

[0064] Secant Modulus (MD/TD) was determined according to ASTM D882.

[0065] Density was determined using the displacement method according to ASTM D792.

[0066] Gloss was determined by ASTM D2457.

[0067] Haze was determined by ASTM D1003.

[0068] Water Vapor Transmission Rate (“WVTR”, expressed as grams of water vapor transmitted per 100 square inches of film per day at a specified film thickness (mils), or g/100 in.sup.2/day) was measured in accordance with ASTM F1249-90 with a MOCON permatron developed by Modern Controls Inc. at conditions of 100° F. (37.8° C.) and 100% relative humidity.

[0069] The following polyethylene compositions were used in this example.

[0070] sHDPE-1 is an ethylene homopolymer composition that is prepared in a dual reactor solution polymerization process using a single site catalyst, using procedures in substantial accordance with those described in U.S. Pat. No. 7,737,220. A high molecular weight homopolymer (having an Mw/Mn of about 2) is prepared in the first reactor and a low molecular weight homopolymer is prepared in the second reactor. The I.sub.2 of the low molecular weight homopolymer from reactor 2 is more than ten times higher than the I.sub.2 of the high molecular weight homopolymer from reactor 1. The density of the overall blend composition is about 0.967 g/cc, the melt index (I.sub.2) is about 1.2 g/10 minutes and the Mw/Mn is about 8.

[0071] sHDPE-1 also contains about 1200 ppm of a nucleating agent (sold under the trade name HYPERFORM® HPN20E by Milliken Chemicals).

[0072] HDPE-2 is an ethylene homopolymer made with a Ziegler Natta catalyst (containing Ti). It has a density of about 0.96 g/cc and a melt index of about 1.2 [19G].

[0073] HDPE-3 is an ethylene homopolymer made with a Ziegler Natta catalyst (containing Ti). It has a density of about 0.96 g/cc and a melt index of about 1 (19C).

[0074] sLLDPE-1 is an ethylene-octene copolymer made with a single site catalyst (containing Ti). It has a density of about 0.916 g/cc, a melt index of about 0.7 g/cc, a CDBI of greater than 70%, an Mw/Mn of about 2.8 and was prepared in a dual reactor solution polymerization process.

[0075] For convenience, some of the above described properties are provided in Table 1.

TABLE-US-00001 TABLE 1 Polyethylene Composition Type Melt Index, dg/min Density, g/cc sHDPE-1 1.2 0.967 HDPE-2 1.2 0.96 HDPE-3 1 0.96 sLLDPE 0.7 0.916

[0076] Three layer films were then prepared on a blown film line. For convenience, the three layers may be referred to as A/B/C—with layers A and C being the external layers (often referred to as “skin” layers) and layer B being the core layer. Table 2 shows the composition and thickness of the films.

[0077] These thick films were then subjected to a Machine Direction Orientation (MDO) process. The MDO was done at temperatures between about 98 to about 121° C. at the stretch ratios shown in Table 3. By way of example, (and for clarity), the precursor film 1 from Table 2 was used to prepare 3 MDO films (stretch ratios 1:6; 1:8; and 1:9) as shown in Table 3.

[0078] Table 3 shows that various thinner films were prepared by stretching the thick film by increasing amounts.

[0079] The thickness and the Water Vapor Transmission rate of these films are reported in Table 3. As shown in Table 3, comparative monolayer films prepared from only the nucleated HDPE composition (sHDPE-1) have excellent barrier properties (as indicated by low WVTR values) and comparative films prepared from only the sLLDPE composition have comparatively poor barrier properties.

[0080] Precursor films comprising two skin layers of sLLDPE and a core layer of sHDPE had intermediate barrier properties. However, when these precursor films were subjected to the MDO process at stretch ratios of at least 1:6, a very surprising and unexpected observation was made—the absolute WVTR values of these films is better than the WVTR value of the film prepared from sHDPE-1 alone.

TABLE-US-00002 TABLE 2 Film Layer A (wt %) Layer B (wt %) Layer C (wt %) 1 sHDPE-1 sHDPE-1 sHDPE-1 (25) (50) (25) 2 HDPE-2 HDPE-2 HDPE-2 (25) (50) (25) 3 sLLLDPE-1 sHDPE-1 sLLDPE-1 (25) (50) (25) 4 sLLDPE-1 sHDPE-1 sLLDPE-1 (15) (70) (15) 5 sLLDPE-1 HDPE-3 sLLDPE-1 (25) (50) (25)

TABLE-US-00003 TABLE 3 % change Film WVTR against non- Gauge G/100 MDO film Film Stretch Ratio mil In.sup.2 .Math. mil/D (1:1) 1 Stretch Ratio 1:1 7.2 0.1728 0.0 1.6 Stretch Ratio 1:6 1.7 0.2329 34.8 1.8 Stretch Ratio 1:8 1.1 0.20493 18.6 1.9 Stretch Ratio 1:9 0.9 0.15822 −8.4 2 Stretch Ratio 1:1 6.7 0.37989 0.0 2.6 Stretch Ratio 1:6 1.4 0.36274 −4.5 2.8 Stretch Ratio 1:8 0.9 0.25929 −31.7 3 Stretch Ratio 1:1 6.3 0.28287 0.0 3.4 Stretch Ratio 1:4 1.8 0.30618 8.2 3.6 Stretch Ratio 1:6 1.4 0.2559 −9.5 3.7 Stretch Ratio 1:7 1.1 0.1804 −36.2 3.8 Stretch Ratio 1:8 0.8 0.12136 −57.1 4 Stretch Ratio 1:1 7 0.2184 0.0 4.4 Stretch Ratio 1:4 1.7 0.19261 −11.8 4.6 Stretch Ratio 1:6 1.4 0.20104 −7.9 4.7 Stretch Ratio 1:7 1.1 0.1298 −40.6 4.8 Stretch Ratio 1:8 1 0.1231 −43.6 5 Stretch Ratio 1:1 7.3 0.53874 0.0 5.5 Stretch Ratio 1:5 1.6 0.29392 −45.4 Note: The stretch ratio numbers reflect an aim point; the film thickness values were measured.

INDUSTRIAL APPLICABILITY

[0081] The films of this invention are suitable for use in a wide variety of packaging applications.