TECHNICAL FIELD

Biaxially oriented polyethylene (BOPE) films are made from medium density polyethylene (MDPE) having a very broad molecular weight distribution.

BACKGROUND ART

It is well known that polymeric films may be oriented by stretching the films in two directions. The films may stretch sequentially - first in the “machine” direction (MD) and then in the “transverse” direction (TD), or simultaneously (with stretching forces being applied in both directions at the same time). One common stretching process is known as the “tenter frame” process. The resulting films are generally referred to as being “biaxially oriented” or “bi-oriented”. The tenter frame process is commonly used with films made from polyamide, polyethylene terephthalate (PET) and especially polypropylene (PP). However, the tenter frame process has been less successful with polyethylene (PE) because PE is comparatively difficult to stretch. Some technical success was achieved using high density polyethylene (HDPE) in the tenter frame process - as described in United States Patent No. 6,946,203 (Lockhart et al.).

The use of linear low density (LLDPE) has also been proposed to prepare BOPE: See U.S. Patent Nos. 6,469,137 (Joyner et al.) and 10,363,700 (Yun et al.).

We have now discovered that a different type of polyethylene - namely medium density polyethylene (MDPE) -- can be used to prepare BOPE films, provided that the MDPE has a very broad molecular weight distribution.

SUMMARY OF INVENTION

In an embodiment, the invention provides a biaxially oriented polyethylene film comprising at least 60 weight percent of a medium density polyethylene having:

1 ) a density of from 0.94 to 0.95 grams per cubic centimeter.

2) a melt index, I2, as measured by ASTM D1238 at 190°C using a 2.16 kg load of from 0.2 to 5 grams/10 minutes; and

3) a molecular weight distribution, Mw/Mn, of from 10 to 50.

In another embodiment, the invention provides a process to prepare a biaxially oriented polyethylene film comprising:

A) providing a base structure comprising at least 60 weight % of a medium density polyethylene having: 1 ) a density of from 0.94 to 0.95 grams per cubic centimeter.

2) a melt index, h, as measured by ASTM D1238 at 190°C using a 2.16 kg load of from 0.2 to 5 grams/10 minutes; and

3) a molecular weight distribution, Mw/Mn, of from 10 to 50;

B) stretching said base structure to a degree of from about 3:1 to about 10:1 in the machine direction; and

C) stretching said base structure to a degree of from about 3:1 to about 10:1 in the transverse direction.

DESCRIPTION OF EMBODIMENTS

The tenter frame process is commonly used to prepare biaxially oriented film and is suitable for the present invention. The tenter frame process is well known to persons skilled in the art of film making, so only a brief description is provided herein. The process begins with an extruder that is equipped with a slot die to form a sheet. For convenience, this extruded sheet is referred to herein as the base structure. Once the base structure has been quenched on chill rolls, MD stretching or MD orientation (MDO) is accomplished by pulling the base structure using several rolls that rotate with progressively increasing surface speeds. Following MD stretching, clips (that are attached to chains) grip the edges of the moving web and carry it into an oven. In the oven, the edges of the base structure are drawn apart making the sheet wider, thus providing TD orientation (TDO). The orientation/stretching causes the film to become thinner, proportionally to the orientation or stretch ratios. For example, to prepare a 1-mil finished film with 5:1 stretch ratio in MD and 8:1 stretch ratio in the TD, the process must start out with a 40-mil sheet. Further details are provided by Kanai et al. in the textbook “Film Processing Advances” (2014); Hanser Publishers.

Biaxial orientation may improve toughness, barrier, optics, heat resistance, and stiffness of films. Flowever, conventional PE is not considered to be well suited to the tenter frame process as it has poor stretchability (compared to polypropylene or PET).

The BOPE films prepared according to this disclosure are suitable for use in a wide variety of packaging applications. In an embodiment, the BOPE film may be used in a laminated structure - for example, the BOPE film may be used as the print web when laminated to a sealant web made from a lower density polyethylene. This type of laminated structure may be more easily recycled in comparison to conventional laminated structures that contain a layer of polyester or polypropylene that is laminated to a layer of polyethylene.

MDPE

The medium density polyethylene (MDPE) used in this disclosure has a density of from 0.94 to 0.95 grams per cubic centimeter (g/cc) as measured by ASTM D972.

The MDPE also has a polydispersity index, Mw/Mn, of 10 to 50 (especially from 10 to 30).

In an embodiment, the MDPE has a Mz of from 550,000 to 1 ,500,000, especially from 600,000 to 1 ,500,000.

In an embodiment, the BOPE films of this disclosure are made with from 60 to 100 weight % of MDPE having all of the characteristics described above. In an embodiment, the BOPE films comprise from 70 to 90% of this MDPE. In an embodiment, the BOPE films comprise from 80 to 95% of this MDPE and the remaining polymer(s) used to prepare the BOPE film is also a polyethylene (because the use of only polyethylene to prepare the BOPE film allows the film to be more easily recycled in comparison to a film made with a mixture of polymers).

In an embodiment, the MDPE is made with a chromium catalyst in a gas phase polymerization. The resulting MDPE may contain some long chain branching (LCB). In another embodiment, the MDPE is made with a Ziegler Natta catalyst and the resulting MDPE contains little or no LCB.

Blends with Other Polymers

The BOPE films of this disclosure are prepared from a polymer composition comprising at least 60 weight % of the above defined MDPE. It is known in the art of preparing BOPE films using blends of polymers and this is also contemplated by the present disclosure. Examples of polymers that are suitable for use in blends according to this disclosure include:

1) Linear Low Density Polyethylene (LLDPE). In an embodiment, the LLDPE has a melt index (I2) of from 0.1 to 10 (especially from 0.9 to 2.3) grams per 10 minutes and a density of from 0.89 to 0.935 grams per cubic centimeter; 2) High Density Polyethylene (HDPE), especially HDPE having a melt index(l2) of from 0.1 to 10 (especially from 0.4. to 0.9) grams per 10 minutes and a density of at least 0.95 grams per cubic centimeter; and

3) High Pressure Low Density Polyethylene (HPLD) which is prepared by the free radical polymerization of ethylene, especially HDLD having a melt index (I2) of from 0.1 to 10 grams per minutes and a density of from 0.92 to 0.94 grams per cubic centimeter.

For clarity, the term LLDPE as used herein is meant to include “plastomers”, where the term plastomer is a subset of the LLDPE family having a relatively low density of from 0.89 to 0.91 g/cc.

Multi -Laver Base Structures

It is known in the art of preparing BOPE films to use a multi-layer film as the (unstretched) starting film. These starting films are comparatively thick before being stretched and are often referred to as a “sheet” instead of a film. For convenience, such an unstretched, multi-layer sheet may be referred to as a “base structure”. Suitable base structures according to this disclosure contain at least 60 weight % of the above defined MDPE, based on the total weight of the base structure. In an embodiment, the MDPE forms a “core” layer (i.e. an interior layer of a multi-layer base structure).

Examples of polymers that may be used to prepare other layers include the LLDPE; HDPE and HPLD described above.

In an embodiment, the multi-layer base structure contains at least three layers including two skin layers (i.e. the layers on each exterior surface of the base structure) and one or more core layers. In an embodiment, one skin layer may be made from HDPE and the other skin layer is a seal layer, as disclosed in published U.S. Patent Application No. 2016/000031191 (referred to hereafter as “Paulino ‘191 ”). As disclosed in Paulino Ί 91 , the seal layer may comprise a LLDPE (particularly when polymerized with a metallocene catalyst); “plastomers”; elastomers and blends thereof. The use of ethylene-octene plastomers (and blends of those elastomers with another LLDPE; HDPE and/or HPLD) may also be used in a seal layer. Furthermore, the use of plastomers in skin layers may improve the optical properties of the BOPE film so it is also contemplated to use these plastomers (or blends that contain plastomers) in both skin layers. In an embodiment, the core layer comprises the MDPE defined in claim 1 and both skin layers comprise an ethylene-octene plastomer. In another embodiment, a multilayer structure that contains at least 5 layers may have two external skin layers made from a plastomer and two “adjacent-to-skin” layers that are made from a blend of a plastomer and a polyethylene having a higher density.

In addition, it is also known to use a layer of a “barrier resin” to improve the barrier properties of BOPE films. Examples of suitable barrier resins include Ethylene-Vinyl Alcohol (EVOH) and polyamide.

The polymers used in the present disclosure will typically contain an antioxidant (such as a hindered phenol; a phosphite or a blend of both) in conventional amounts, as is well known to those skilled in the art. Other, optional additives include antiblocks; slip agents and nucleating agents (such as those that are disclosed in Paulino ‘191. Furthermore, the use of zinc glycerolate as an (optional) nucleating agent is also contemplated (this nucleating agent is commercially available, for example, under the trademark IRGASTAB ^® 287). EXAMPLES

The invention is further illustrated by the following non limiting examples.

Test Methods

Melt index was determined according to ASTM D1238 (at 190°C, using a 2.16 kg load, “condition I2” and is reported in grams/10 minutes).

Density was determined according to ASTM D972 and is reported as g/cc.

Mn, Mw and Mz (g/mol) were determined by high temperature Gel Permeation Chromatography (GPC) with differential refractive index (DFtl) detection using universal calibration (e.g. ASTM-D6474-99). GPC data was obtained using an instrument sold under the trade name “Waters 150c”, with 1 ,2,4- trichlorobenzene as the mobile phase at 140°C. The samples were prepared by dissolving the polymer in this solvent and were run without filtration. Molecular weights are expressed as polyethylene equivalents with a relative standard deviation of 2.9% for the number average molecular weight (“Mn”) and 5.0% for the weight average molecular weight (“Mw”). The molecular weight distribution (MWD) is the weight average molecular weight divided by the number average molecular weight, Mw/Mn. The term “polydispersity index” also refers to Mw/Mn. The z- average molecular weight distribution is Mz/Mn. Polymer sample solutions (1 to 2 mg/mL) were prepared by heating the polymer in 1 ,2,4-trichlorobenzene (TCB) and rotating on a wheel for 4 hours at 150°C in an oven. The antioxidant 2,6-di-tert- butyl-4-methylphenol (BHT) was added to the mixture in order to stabilize the polymer against oxidative degradation. The BHT concentration was 250 ppm. Sample solutions were chromatographed at 140°C on a PL 220 high-temperature chromatography unit equipped with four Shodex columns (HT803, HT804, HT805 and HT806) using TCB as the mobile phase with a flow rate of 1 .0 mL/rminute, with a differential refractive index (DRI) as the concentration detector. BHT was added to the mobile phase at a concentration of 250 ppm to protect the columns from oxidative degradation. The sample injection volume was 200 mL. The raw data were processed with CIRRUS ^® GPC software. The columns were calibrated with narrow distribution polystyrene standards. The polystyrene molecular weights were converted to polyethylene molecular weights using the Mark-Houwink equation, as described in the ASTM standard test method D6474.

Other test methods that may be used to characterize BOPE films are summarized in Table 1. TABLE 1

Part A: Test Methods

Part B: Preparation of BOPE Films

Biaxially oriented polyethylene (BOPE) films were prepared in a tenter frame process using conditions that are described below.

A. Preparation of Unstretched Film (or “Base Structure”)

A multi-layer (three layer) sheet, is co-extruded from three single screw extruders through a 12 inch casting die where the melt streams are combined in the multicavity die prior to extrusion. After extrusion from the die, the multi-layer sheet is chilled and quenched on a two-roll horizontal design with air knife. For convenience, this unstretched multilayer sheet is sometimes referred to herein as a “base structure”. The weight of polymer used in each of the three layers is indicated by an A/B/C format. For example, a base structure having two exterior layers (or skin layers) that each contain 5 weight % of the total polymer and a core layer containing 90% is described as 5/90/5.

B. Biax Process

A sequential stretching process was used in this example. The stretching/orientation in the machine direction was performed first. The “oriented” sheet was then stretched in the transverse direction — in some (comparative) examples, it was not possible to stretch the films in both the MD and TD directions.

Machine direction orientation (MDO) can be produced using single stage, or two-stage, Compression Roll Drawing (CRD), at temperatures up to 275°F and draw ratios up to 7.5:1 .

The transverse direction orientation (TDO) was produced in multiple zones: preheat, stretch zone, and annealing, plus one cooling zone. Stretch zone temperatures were up to 280°F, draw ratio up to 12:1 .

MDO is achieved by pre-heating the base structure and stretching the sheet between two rolls that are turning at different speeds. The difference in the speeds of the rolls determines the stretch ratio. Stretching can be performed in one set of draw rolls or performed over a series of draw rolls. The stretching is generally performed at temperatures below the crystalline melting temperatures of the film (Tm).

The MDO film is fed into the tenter frame oven using clips on chains attached to rails, and pre-heated. The film is stretched as the rails diverge from one another which pull the edges of the film causing the web to stretch. The width of the film is set by the distance between the rails and can be adjusted to achieve the desired stretch ratio. TDO is performed at a similar or slightly higher temperatures than MDO.

A summary of process conditions is provided below: Example 1

Comparative BOPE films were made with the HDPE polymers shown in

Table 2.

TABLE 2

Comparative HD PE Note: HDPE-4 is sold under the trademark name SCLAIR ^® 19C by NOVA Chemicals

Qualitative properties of the stretched films made from HDPE are shown in Table 3. These BOPE films are comparative. The use of HDPE to prepare BOPE in the tenter frame process is disclosed in U.S. Patent No. 6,946,203. This patent specifically identifies SCLAIR 19C as being suitable for the preparation of BOPE (although this patent also teaches that the core layer of the BOPE film should be filled with a particulate in order to reduce density). TABLE 3

Comparative BOPE Films From HDPE

As previously noted, the base structures were prepared using three extruders to provide an A/B/C film structure (with the exterior or “skin” layers being A and C and the core layer B). “MD stretch” and “TD stretch” values in Table 3 show the stretch ratios in the machine direction and the transverse direction (respectively); NA indicates a failure to stretch the base structure adequately.

The final column provides a pass or fail ranking (with pass indicating that it was possible to stretch/orient the films to the degree indicated in Table 3); the “ranking” is a qualitative evaluation of the film appearance, with higher numbers indicating better quality.

(Comparative) Example 2 - MDPE

The properties of the medium density polyethylene (MDPE) used in this example are shown in Table 4. Of note, the MDPE has a density of 0.945 g/cc (which is desirable for use in the films of this disclosure and a polydispersity index (Mw/Mn) of 4.52 (which is not broad enough for use in the films of this disclosure).

A base structure was prepared with this MDPE using the procedures described in Part B above (to produce an A/B/C structure with this MDPE used in each of the A, B, and C layers). Attempts to prepare a BOPE film (using the procedures described above) were not successful and were given a “fail” rating. TABLE 4

MDPE (Comparative)

(Inventive) Example 3 - MDPE Properties of the MDPE used in this example are shown in Table 5.

The MDPE used in tis example was made by the copolymerization of ethylene with hexene in a gas phase process using a chromium catalyst.

A base structure was prepared with this MDPE using the procedures described in Part B above (to produce an A/B/C structure, with this MDPE used in each of the A, B and C layers).

A biaxially oriented polyethylene (BOPE) film was prepared from this base structure using the procedures described in Part B above.

A BOPE film prepared with an MD stretch ratio of 4.75 and a TD stretch ratio of 8 was assigned a qualitative appearance rating of 10 - i.e. it was clearly superior to the comparative BOPE films made with HDPE (Example 1 above).

TABLE 5 MDPE (Inventive) Properties of BOPE film made from the inventive MDPE of Table 5 are shown in Table 6.

TABLE 6 (Inventive) Example 4 -MDPE

The MDPE used in this example was prepared by the copolymerization of ethylene with butene in a solution polymerization process using a Ziegler Natta catalyst. Properties of this specific MDPE are shown in Table 7. In a more general embodiment, an MDPE made in this manner having an Mn of from 11 ,000 to 15,000, an Mz of from 600,000 to 800,000 and an Mw/Mn of from 10 to 12 is suitable for use in the present disclosure. TABLE 7

MDPE (Inventive)

A base structure was prepared with this MDPE using the procedures described in Part B above (to produce an A/B/C structure, with this MDPE used in each of the A, B and C layers).

A biaxially oriented polyethylene (BOPE) film was prepared from this base structure using the procedures described in Part B above.

A BOPE film prepared with an MD stretch ratio of 6.5 and a TD stretch ratio of 9 was assigned a qualitative appearance rating of 8.

Properties of BOPE film made from the inventive MDPE of Table 7 are shown in Table 8.

TABLE 8

INDUSTRIAL APPLICABILITY

Biaxially oriented polyethylene (BOPE) films are made from a medium density polyethylene having a very broad molecular weight distribution. These BOPE films are suitable for the preparation of packages that may be used to package a wide variety of consumer goods.