Biaxially Oriented MDPE Film

Abstract

The biaxially oriented polyethylene (BOPE) process uses selected polyethylenes with moderate density and very broad molecular weight distributions that allow for easier orientation in the BOPE process compared to previously used polyethylene resins with higher densities and / or narrower molecular weight distributions.

Description

[Technical field]

[0001] Biaxially oriented polyethylene (BOPE) films are made from medium density polyethylene (MDPE) which has a very broad molecular weight distribution. [Background technology]

[0002] It is well known that polymer films can be oriented by stretching the film in two directions. The film may be stretched sequentially, first in the "machine" direction (MD) and then in the "transverse" direction (TD), or simultaneously (with stretching forces applied in both directions simultaneously). One common stretching process is known as the "tenter frame" process. The resulting film is generally called "biaxially oriented" or "bi-oriented". The tenter frame process is commonly used with films made from polyamides, polyethylene terephthalate (PET), and especially polypropylene (PP). However, the tenter frame process has been less successful with polyethylene (PE), as PE is relatively difficult to stretch. Some technical success has been achieved using high density polyethylene (HDPE) in the tenter frame process (described in U.S. Patent No. 6,946,203 (Lockhart et al.)).

[0003] The use of linear low density polyethylene (LLDPE) to prepare BOPE has also been proposed (see US Pat. No. 6,469,137 (Joyner et al.) and US Pat. No. 10,363,700 (Yun et al.)).

[0004] The inventors have 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 the Invention

[0005] In one embodiment, the present invention provides a biaxially oriented polyethylene film comprising at least 60% by weight of a medium density polyethylene, said medium density polyethylene comprising: 1) 0.94~0.95g / cm 3 density; 2) a melt index I2 of 0.2 to 5 g / 10 min (measured by ASTM D1238 at 190°C using a load of 2.16 kg); and 3) Molecular weight distribution Mw / Mn of 10 to 50 has.

[0006] In another embodiment, the present invention provides a method for preparing a biaxially oriented polyethylene film comprising the steps of: A) providing a base structure comprising at least 60% by weight of a medium density polyethylene, said medium density polyethylene being 1) 0.94~0.95g / cm 3 density; 2) a melt index I2 of 0.2 to 5 g / 10 min (measured by ASTM D1238 at 190°C using a load of 2.16 kg); and 3) Molecular weight distribution Mw / Mn of 10 to 50 The process comprising: B) stretching the base structure in the machine direction to an extent of about 3:1 to about 10:1; and C) stretching the base structure laterally to a ratio of about 3:1 to about 10:1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] The tenter frame process is commonly used to prepare biaxially oriented films and is suitable for the present invention. The tenter frame process is well known to those skilled in the art of film manufacturing, so only a brief description is given herein. The process begins with an extruder 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 is quenched with a chill roll, MD stretching or MD orientation (MDO) is achieved by pulling the base structure with several rolls rotating at gradually increasing surface speeds. Following MD stretching, clips (attached to chains) grab the edges of the moving web and carry it to an oven. In the oven, the edges of the base structure are pulled apart to widen the sheet, thereby providing TD orientation (TDO). This orientation / stretching results in a thinning of the film in proportion to the orientation or stretch ratio. For example, to prepare a 1 mil finished film with a 5:1 stretch ratio in MD and 8:1 stretch ratio in TD, the process must begin with a 40 mil sheet. Further details can be found in the textbook "Film Processing Advances" (Kanai et al., 2014, Hanser Publishers).

[0008] Biaxial orientation can improve the toughness, barrier properties, optics, heat resistance, and stiffness of the film, but conventional PE is considered less suitable for tenter frame processes due to its poor stretchability (compared to polypropylene and PET).

[0009] The BOPE films prepared according to the present disclosure are suitable for use in a wide variety of packaging applications. In one embodiment, the BOPE films can be used in laminate structures, for example, as print webs when laminated to a sealant web made from lower density polyethylene. This type of laminate structure can be recycled more easily compared to conventional laminate structures that include a layer of polyester or polypropylene laminated to a layer of polyethylene.

[0010] <mdpe> The medium density polyethylene (MDPE) used in this disclosure has a density of 0.94 to 0.95 g / cm 3 (g / cc) as measured by ASTM D972.

[0011] The MDPE also has a polydispersity index Mw / Mn of 10 to 50 (particularly 10 to 30).

[0012] In one embodiment, the MDPE has a Mz of 550,000 to 1,500,000, in particular 600,000 to 1,500,000.

[0013] In one embodiment, the BOPE film of the present disclosure is made with 60-100% by weight MDPE having all of the above properties. In one embodiment, the BOPE film contains 70-90% of this MDPE. In one embodiment, the BOPE film contains 80-95% of this MDPE, with the remaining polymer used to prepare the BOPE film also being polyethylene (using only polyethylene to prepare the BOPE film allows the film to be more easily recycled compared to films made with a blend of polymers).

[0014] In one embodiment, the MDPE is made using a chromium catalyst in a gas phase polymerization. The resulting MDPE may contain long chain branching (LCB). In another embodiment, the MDPE is made using a Ziegler-Natta catalyst, and the resulting MDPE contains little or no LCB.

[0015] <Blends with other polymers> The BOPE films of the present disclosure are prepared from a polymer composition comprising at least 60% by weight of MDPE as defined above. The use of blends of polymers to prepare BOPE films is known in the art and is also contemplated by the present disclosure. Examples of polymers suitable for use in blends according to the present disclosure include: 1) Linear low density polyethylene (LLDPE). In one embodiment, the LLDPE has a melt index (I2) of 0.1 to 10 (particularly 0.9 to 2.3) g / 10 min and a melt index (I2) of 0.89 to 0.935 g / cm 3 has a density of; 2) High density polyethylene (HDPE), especially with a melt index (I2) of 0.1 to 10 (especially 0.4 to 0.9) g / 10 min and a density of at least 0.95 g / cm 3 HDPE; 3) High pressure low density polyethylene (HPLD), which is prepared by free radical polymerization of ethylene, in particular, HPLD having a melt index (I2) of 0.1 to 10 g / 10 min and a density of 0.92 to 0.94 g / cm 3 HPLD.

[0016] For clarity, the term LLDPE as used herein is meant to include "plastomers," which is a subset of the LLDPE family having relatively low densities of 0.89 to 0.91 g / cc.

[0017] <Multi-layer base structure> In the art of preparing BOPE films, it is known to use multilayer films as starting (unstretched) films. These starting films are relatively thick before being stretched and are often referred to as "sheets" rather than films. For convenience, such unstretched multilayer sheets can be referred to as "base structures". A suitable base structure according to the present disclosure comprises at least 60% by weight of MDPE as defined above, based on the total weight of the base structure. In one embodiment, the MDPE forms the "core" layer (i.e. the inner layer of the multilayer base structure).

[0018] Examples of polymers that can be used to prepare the other layers include the LLDPE, HDPE, and HPLD mentioned above.

[0019] In one embodiment, the multi-layer base structure includes at least three layers, including two skin layers (i.e., layers on the outer surface of each of the base structures) and one or more core layers. In one embodiment, as disclosed in US Patent Application Publication No. 2016 / 0031191 (hereinafter referred to as "Paulino'191"), one skin layer can be made of HDPE and the other skin layer is a sealing layer. As disclosed in Paulino'191, the sealing layer may include LLDPE (especially when polymerized with a metallocene catalyst), "plastomers", elastomers and blends thereof. Ethylene-octene plastomers (as well as blends of these elastomers with other LLDPEs, HDPEs and / or HPLDs) can also be used in the sealing layer. Furthermore, the use of plastomers in the skin layers may improve the optical properties of the BOPE film, so it is also contemplated to use these plastomers (or blends containing plastomers) in both skin layers. In one embodiment, the core layer comprises MDPE as defined in claim 1 and both skin layers comprise ethylene-octene plastomer. In another embodiment, a multi-layer structure comprising at least five layers may comprise two outer skin layers made from a plastomer and two "skin adjacent" layers made from a blend of plastomer and higher density polyethylene.

[0020] Additionally, it is 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 polyamides.

[0021] The polymers used in this disclosure typically contain antioxidants in conventional amounts, such as hindered phenols, phosphites, or blends of both, as is well known to those skilled in the art. Other optional additives include antiblocks, slip agents, and nucleating agents, such as those disclosed in Paulino '191. Additionally, zinc glycerate may be used as an (optional) nucleating agent, such as is commercially available under the trademark IRGASTAB® 287. EXAMPLES

[0022] The invention is further illustrated by the following non-limiting examples.

[0023] <Test Method> Melt index was determined according to ASTM D1238 (at 190° C., using a load of 2.16 kg, “Condition I2,” reported in g / 10 min).

[0024] Density was determined according to ASTM D972 and is reported as g / cc.

[0025] Mn, Mw, and Mz (g / mole) were determined by high temperature gel permeation chromatography (GPC) with differential refractive index (DRI) detection using universal calibration (e.g., ASTM-D6474-99). GPC data were obtained using an instrument sold under the trade name "Waters 150c" with 1,2,4-trichlorobenzene as the mobile phase at 140°C. Samples were prepared by dissolving the polymer in this solvent and tested without filtration. Molecular weights are expressed as polyethylene equivalents with a relative standard deviation of 2.9% for number average molecular weight ("Mn") and 5.0% for weight average molecular weight ("Mw"). 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 (1-2 mg / mL) were prepared by heating the polymer in 1,2,4-trichlorobenzene (TCB) and rotating on a wheel in an oven at 150 °C for 4 h. The antioxidant 2,6-di-tert-butyl-4-methylphenol (BHT) was added to the mixture to stabilize the polymer against oxidative degradation. The BHT concentration was 250 ppm. The sample solutions were chromatographed at 140 °C using a high-temperature chromatography unit PL220 equipped with four Shodex columns (HT803, HT804, HT805, and HT806) with TCB as the mobile phase at a flow rate of 1.0 mL / min and differential refractive index (DRI) as the concentration detector. To protect the column from oxidative degradation, BHT was added to the mobile phase at a concentration of 250 ppm. The sample injection volume was 200 mL. Raw data was processed with CIRRUS® GPC software. The column was calibrated with narrow distribution polystyrene standards. The molecular weight of polystyrene was converted to the molecular weight of polyethylene using the Mark-Houwink equation as described in ASTM Standard Test Method D6474.

[0026] Other test methods that can be used to characterize the BOPE films are summarized in Table 1.

[0027]

Table 1

[0028] <Part B: Preparation of BOPE Film> The biaxially oriented polyethylene (BOPE) film was prepared by a tenter frame process using the conditions described below.

[0029] <A. Preparation of the Undrawn Film (or "Base Structure")> The multilayer (3 - layer) sheet was co - extruded from three single - screw extruders through a 12 - inch casting die. The melt streams were combined within a multi - cavity die prior to extrusion. The multilayer sheet extruded from the die was cooled and quenched in a two - roll horizontal design equipped with an air knife. For convenience, this undrawn multilayer sheet may be referred to herein as the "base structure". The weight of the polymer used in each of the three layers is indicated in the form of A / B / C. For example, a base structure having two outer layers (or skin layers) each containing 5 wt% of the total polymer and a core layer containing 90% is described as 5 / 90 / 5.

[0030] <B. Biax Process> In this example, a sequential stretching process was used. First, stretching / orientation in the machine direction was performed. Then, the "oriented" sheet was stretched in the transverse direction. In some (comparative) examples, it was not possible to stretch the film in both the MD and TD directions.

[0031] Machine direction orientation (MDO) can be generated using one - or two - stage compression roll drawing (CRD) at a maximum temperature of 275°F and a maximum draw ratio of 7.5:1.

[0032] Transverse direction orientation (TDO) was produced in multiple zones (preheat, stretch zone, annealing, and one cooling zone). The temperatures in the stretch zones were up to 280° F. and the stretch ratios were up to 12:1.

[0033] MDO is achieved by preheating the base structure and stretching the sheet between two rolls rotating at different speeds. The difference in the speed of the rolls determines the stretch ratio. Stretching can be done with a single set of stretch rolls or over a series of stretch rolls. Stretching is generally done below the crystalline melting temperature (Tm) of the film.

[0034] The MDO film is fed into a tenter frame oven by clips on chains attached to rails and preheated. The film is stretched as the rails diverge from one another, and the edges of the film are pulled to stretch the web. The film is stretched and the edges of the film are pulled to stretch the web. 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 temperature equivalent to or slightly higher than MDO.

[0035] A summary of the process conditions is given below. TIFF0007624984000003.tif66164

[0036] [Example 1] Comparative BOPE films were made using the HDPE polymers shown in Table 2.

[0037] [Table 2]

[0038] The qualitative properties of the stretched films made from HDPE are shown in Table 3. These BOPE films are for comparison. The use of HDPE to prepare BOPE in a tenter frame process is disclosed in US Patent No. 6,946,203. This patent clearly indicates that SCLAIR 19C is suitable for preparing BOPE (however, this patent also teaches that the core layer of the BOPE film needs to be loaded with particulates to reduce density).

[0039] [Table 3]

[0040] As previously described, the base structure was prepared using three extruders to provide an A / B / C film structure (A and C for the outer or "skin" layers, and B for the core layer). The "MD Stretch" and "TD Stretch" values ​​in Table 3 indicate the machine direction (longitudinal) and transverse direction stretch ratios, respectively, with NA indicating that the base structure could not be adequately stretched.

[0041] The final column indicates a pass or fail ranking (pass indicates that the film could be stretched / oriented to the extent indicated in Table 3). The "ranking" is a qualitative assessment of the film's appearance, with higher numbers indicating better quality.

[0042] [(Comparison) Example 2-MDPE] The properties of the medium density polyethylene (MDPE) used in this example are shown in Table 4. Of note, MDPE has a density of 0.945 g / cc (which is desirable for use in the films of the present disclosure) and a polydispersity index (Mw / Mn) of 4.52 (which is not broad enough for use in the films of the present disclosure).

[0043] This MDPE was used to prepare a base structure using the procedure described in Part B above (this MDPE was used in each of the A, B, and C layers to produce an A / B / C structure). Attempts to prepare a BOPE film (using the procedure above) were unsuccessful and were given a rating of "Fail".

[0044] [Table 4]

[0045] [(Invention) Example 3-MDPE] The properties of the MDPE used in this example are shown in Table 5.

[0046] The MDPE used in this example was made by copolymerization of ethylene and hexene in a gas phase process using a chromium catalyst.

[0047] This MDPE was used to prepare a base structure using the procedure described in Part B above (this MDPE was used in each of the A, B, and C layers to produce an A / B / C structure).

[0048] Biaxially oriented polyethylene (BOPE) films were prepared from this base structure using the procedure described in Part B above.

[0049] The BOPE film made 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., this BOPE film was clearly superior to the comparative BOPE film made with HDPE (Example 1 above).

[0050] [Table 5]

[0051] The properties of BOPE films made from the inventive MDPE of Table 5 are shown in Table 6.

[0052] [Table 6]

[0053] [(Invention) Example 4-MDPE] The MDPE used in this example was prepared by copolymerization of ethylene and butene in a solution polymerization process using a Ziegler-Natta catalyst. The properties of this particular MDPE are shown in Table 7. In a more general embodiment, MDPE made by this method having an Mn of 11,000-15,000, an Mz of 600,000-800,000, and an Mw / Mn of 10-12 is suitable for use in this disclosure.

[0054] [Table 7]

[0055] This MDPE was used to prepare a base structure using the procedure described in Part B above (this MDPE was used in each of the A, B, and C layers to produce an A / B / C structure).

[0056] Biaxially oriented polyethylene (BOPE) films were prepared from this base structure using the procedure described in Part B above.

[0057] The BOPE films made with an MD stretch ratio of 6.5 and a TD stretch ratio of 9 were assigned a qualitative appearance rating of 8. The properties of the BOPE films made from the inventive MDPE of Table 7 are shown in Table 8.

[0058] [Table 8] [Industrial Applicability]

[0059] Biaxially oriented polyethylene (BOPE) films are made from medium density polyethylene with a very broad molecular weight distribution. These BOPE films are suitable for the preparation of packages that can be used to package a wide variety of consumer goods. The contents of the invention according to the claims as originally filed in this application are as follows: [Section 1] A biaxially oriented polyethylene film comprising at least 60% by weight of a medium density polyethylene, the medium density polyethylene comprising: 1) 0.94~0.95g / cm 3 density; 2) Melt index I: 0.2-5g / 10min 2 (measured by ASTM D1238 at 190°C with a load of 2.16 kg); and 3) Molecular weight distribution Mw / Mn of 10 to 50 The above film having the formula: [Section 2] Item 2. The film according to item 1, further characterized in that the medium density polyethylene has an Mz of 550,000 to 1,500,000. [Section 3] 3. The film according to item 1 or 2, characterized in that it is stretched in the machine direction to a ratio of about 3:1 to about 10:1. [Section 4] Item 4. The film according to item 2 or 3, characterized in that it is stretched in the transverse direction to a ratio of about 3:1 to about 10:1. [Section 5] Item 1. The film according to item 1, wherein when prepared from a base structure having at least three layers, at least one core layer of the base structure comprises the medium density polyethylene. [Section 6] 1. A method for preparing a biaxially oriented polyethylene film, comprising the steps of: A) providing a base structure comprising at least 60% by weight of a medium density polyethylene, said medium density polyethylene being 1) 0.94~0.95g / cm 3 density; 2) Melt index I: 0.2-5g / 10min 2 (measured by ASTM D1238 at 190°C with a load of 2.16 kg); and 3) Molecular weight distribution Mw / Mn of 10 to 50 The process comprising: B) stretching the base structure in the machine direction to an extent of about 3:1 to about 10:1; and C) stretching the base structure laterally to a ratio of about 3:1 to about 10:1. [Section 7] Item 7. The method according to item 6, further characterized in that the medium density polyethylene has an Mz of 550,000 to 1,500,000. [Section 8] 8. The method of claim 6 or 7, wherein the base structure is at least three layers, and at least one core layer of the base structure comprises the medium density polyethylene. [Section 9] Item 2. The method according to item 1, wherein the MDPE has an Mn of 11,000 to 15,000, an Mz of 600,000 to 800,000, and an Mw / Mn of 10 to 12. [Section 10] Item 10. The method of item 9, wherein the MDPE is made by a solution polymerization process in the presence of a Ziegler-Natta catalyst. [Section 11] 9. The method of claim 8, wherein at least one skin layer of the base structure comprises an ethylene-octene plastomer. [Section 12] Item 12. The method of item 11, wherein both skin layers of the base structure comprise ethylene-octene plastomer. < / mdpe>

Claims

1. A method for preparing a biaxially oriented polyethylene film, comprising the steps of: A) providing a base structure comprising at least 60% by weight of a medium density polyethylene, said medium density polyethylene being 1) 0.94-0.95g / cm ３ density of; 2) Melt index I of 0.2 to 5 g / 10 min ２ (measured by ASTM D1238 at 190° C. with a load of 2.16 kg); and 3) Molecular weight distribution Mw / Mn of 10 to 50 The process comprising: B) stretching the base structure in the machine direction to an extent of about 3:1 to about 10:1; and C) stretching the base structure in the transverse direction to an extent of about 3:1 to about 10:

1. The method includes: the base structure is at least three layers, at least one core layer of the base structure comprises the medium density polyethylene; The medium density polyethylene is further characterized by having a Mz of 550,000 to 1,500,000; The medium density polyethylene (MDPE) is made by copolymerization of ethylene and butene in a solution polymerization process using a Ziegler-Natta catalyst, or by copolymerization of ethylene and hexene in a gas phase process using a chromium catalyst. The above method.

2. 2. The method of claim 1, wherein the MDPE has a Mn of 11,000-15,000, a Mz of 600,000-800,000, and a Mw / Mn of 10-12.

3. 3. The process of claim 1 or 2, wherein the MDPE is made in a solution polymerization process in the presence of a Ziegler-Natta catalyst.

4. The method of any one of claims 1 to 3, wherein at least one skin layer of the base structure comprises an ethylene-octene plastomer.

5. The method of any one of claims 1 to 4, wherein both skin layers of the base structure comprise ethylene-octene plastomer.

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