Biaxially oriented MDPE film

Abstract

The biaxially oriented polyethylene (BOPE) process uses a selected polyethylene having a medium density and a very broad molecular weight distribution. The use of this selected polyethylene facilitates stretching in the BOPE process compared to previously used polyethylene resins having higher density and / or narrower molecular weight distribution.

Description

Technical Field

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

[0002] It is well known that polymer films can be oriented by stretching the film in two directions. The film can be stretched sequentially - first in the "machine" direction (MD), and then in the "transverse" direction (TD), or simultaneously (applying stretching forces in both directions at the same time). A common stretching method is called the "tenter" method. The resulting film is usually called "bidirectional orientation" or "bi-oriented". The tenter method is usually used with films made of polyamide, polyethylene terephthalate (PET) and especially polypropylene (PP). However, in the case of polyethylene (PE), the tenter method is less successful because PE is relatively difficult to stretch. As described in U.S. Patent No. 6,946,203 (Lockhart et al.), some technical success has been achieved using high-density polyethylene (HDPE) in the tenter method.

[0003] The use of linear low density (LLDPE) to prepare BOPE has also been proposed: see US Pat. Nos. 6,469,137 (Joyner et al.) and 10,363,700 (Yun et al.).

[0004] We have now found that a different type of polyethylene, namely medium density polyethylene (MDPE), can be used to prepare BOPE films if 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 wt% of a medium density polyethylene having:

[0006] 1) Density is 0.94 to 0.95 g / cm3.

[0007] 2) a melt index I2 of 0.2 to 5 g / 10 min as measured by ASTM D1238 at 190°C using a 2.16 kg load; and

[0008] 3) The molecular weight distribution Mw / Mn is 10 to 50.

[0009] In another embodiment, the present invention provides a method for preparing a biaxially oriented polyethylene film, comprising:

[0010] A) providing a base structure comprising at least 60 wt.% of a medium density polyethylene having:

[0011] 1) Density is 0.94 to 0.95 g / cm3.

[0012] 2) a melt index I2 of 0.2 to 5 g / 10 min as measured by ASTM D1238 at 190°C using a 2.16 kg load; and

[0013] 3) a molecular weight distribution Mw / Mn of 10 to 50;

[0014] B) stretching the chassis in the longitudinal direction to a degree of from about 3:1 to about 10:1; and

[0015] C) stretching the chassis in the transverse direction to a ratio of about 3:1 to about 10:1.

[0016] Detailed Description of the Embodiments

[0017] The tenter method is commonly used to prepare biaxially oriented films and is suitable for use in the present invention. The tenter method is well known to those skilled in the art of film making, so only a brief description is provided herein. The method starts with an extruder equipped with a slot die to form a sheet. For convenience, the extruded sheet is referred to herein as a base structure. Once the base structure has been quenched on a cooling roll, MD stretching or MD orientation (MDO) is completed by dragging the base structure with several rollers rotating at gradually increasing surface speeds. After MD stretching, a clip (which is connected to a chain) clamps the edge of the moving web and transports it to an oven. In the oven, the edges of the base structure are pulled apart to make the sheet wider, thereby providing TD orientation (TDO). Orientation / stretching causes the film to become thinner, which is proportional to the orientation or stretch ratio. For example, to prepare a 1 mil finished film with a longitudinal stretch ratio of 5:1 and a transverse stretch ratio of 8:1, the method must start with a 40 mil sheet. Further details are provided by Kanai et al. in “Film Processing Advances” (2014); Hanser Publishers.

[0018] Biaxial orientation can improve the film's toughness, barrier properties, optics, heat resistance, and stiffness. However, conventional PE is considered less suitable for the tenter frame method because of its poor stretchability (compared to polypropylene or PET).

[0019] BOPE films prepared according to the present disclosure are suitable for use in a wide variety of packaging applications. In one embodiment, the BOPE film can be used in a laminated structure - for example, when laminated to a sealant web made of low-density polyethylene, the BOPE film can be used as a printing web. This type of laminated structure can be more easily recycled than a traditional laminated structure containing a polyester layer or a polypropylene layer laminated to a polyethylene layer.

[0020] MDPE

[0021] The medium density polyethylene (MDPE) used in the present disclosure has a density of 0.94 to 0.95 grams per cubic centimeter (g / cc) as measured by ASTM D972.

[0022] MDPE also has a polydispersity index Mw / Mn of 10 to 50, especially 10 to 30.

[0023] In one embodiment, the MDPE has an Mz of 550,000 to 1,500,000, especially 600,000 to 1,500,000.

[0024] In one embodiment, the BOPE film of the present disclosure is made of 60 to 100 wt% of MDPE having all of the above properties. In one embodiment, the BOPE film comprises 70 to 90% of such MDPE. In one embodiment, the BOPE film comprises 80 to 95% of such MDPE, and the remaining (one or more) polymers used to make the BOPE film are also polyethylene (because using only polyethylene to make the BOPE film allows the film to be more easily recycled than films made from polymer blends).

[0025] In one embodiment, the MDPE is made using a chromium catalyst in a gas phase polymerization. The resulting MDPE may contain some long chain branches (LCB). In another embodiment, the MDPE is made using a Ziegler Natta catalyst and the resulting MDPE contains little or no LCB.

[0026] Blends with other polymers

[0027] The BOPE film of the present disclosure is prepared from a polymer composition comprising at least 60% by weight of the above defined MDPE. The use of a blend 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:

[0028] 1) Linear low density polyethylene (LLDPE). In one embodiment, the melt index (I2) of LLDPE is

[0029] 0.1 to 10 (especially 0.9 to 2.3) g / 10 min and a density of 0.89 to 0.935 g / cm3;

[0030] 2) high density polyethylene (HDPE), especially HDPE having a melt index (I2) of 0.1 to 10 (especially 0.4-0.9) g / 10 min and a density of at least 0.95 g / cm3; and

[0031] 3) High pressure low density polyethylene (HPLD) produced by free radical polymerization of ethylene, especially HDLD having a melt index (I2) of 0.1 to 10 g / min and a density of 0.92 to 0.94 g / cm3.

[0032] For clarity, the term LLDPE as used herein is intended to include "plastomers," wherein the term plastomer is a subset of the LLDPE family having a relatively low density of 0.89 to 0.91 g / cc.

[0033] Multi-layer infrastructure

[0034] It is known in the art of preparing BOPE films to use multilayer films as (unstretched) starting 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 may be referred to as "base structures". Suitable base structures according to the present disclosure contain 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).

[0035] Examples of polymers that can be used to prepare the other layers include the above-mentioned LLDPE; HDPE and HPLD.

[0036] In one embodiment, the multilayer base structure contains at least three layers, including two skin layers (i.e., layers on each outer surface of the base structure) and one or more core layers. In one embodiment, one skin layer may be made of HDPE, while the other skin layer is a sealant layer, as disclosed in published U.S. Patent Application No. 2016 / 000031191 (hereinafter referred to as "Paulino '191"). As disclosed in Paulino '191, the sealant layer may include LLDPE (especially when polymerized with a metallocene catalyst); "plastomers"; elastomers and blends thereof. The use of ethylene-octene plastomers (and blends of these elastomers with another LLDPE; HDPE and / or HPLD) can also be used in the sealant layer. In addition, the use of plastomers in the skin layers can 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 above and both skin layers comprise ethylene-octene plastomers. In another embodiment, a multilayer structure containing at least 5 layers may have two outer skin layers made of a plastomer, and two "skin-adjacent" layers made of a blend of a plastomer and a polyethylene having a higher density.

[0037] Furthermore, it is also known to use a "barrier resin" layer to enhance the barrier properties of BOPE films. Examples of suitable barrier resins include ethylene vinyl alcohol (EVOH) and polyamides.

[0038] As is well known to those skilled in the art, the polymers used in the present disclosure will generally contain conventional amounts of antioxidants (e.g., hindered phenols; phosphites, or blends of the two). In addition, optional additives include antiblocks; slip agents and nucleating agents (e.g., those disclosed in Paulino '191. In addition, the use of glycerolated zinc as an (optional) nucleating agent is also contemplated (such nucleating agents are commercially available, for example, under the trademark 287). Example

[0039] The present invention is further illustrated by the following non-limiting examples.

[0040] Test Method

[0041] Melt index is determined according to ASTM D1238 (at 190°C using a 2.16 kg load, "Condition 12" and reported in grams / 10 minutes).

[0042] Density was determined according to ASTM D972 and is reported in g / cc.

[0043] Mn, Mw and Mz (g / mol) are determined by high temperature gel permeation chromatography (GPC) using differential refractive index (DRI) detection using a universal calibration (e.g., ASTM-D6474-99). GPC data are obtained using an instrument sold under the trade name "Waters 150c" at 140°C using 1,2,4-trichlorobenzene as the mobile phase. The samples are prepared by dissolving the polymer in the solvent and 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"). 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. The polymer sample solution (1 to 2 mg / mL) was prepared by heating the polymer in 1,2,4-trichlorobenzene (TCB) and rotating on a hub in an oven at 150°C for 4 hours. 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 solution was 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, a flow rate of 1.0 mL / min, and a differential refractive index (DRI) as a concentration detector. BHT was added to the mobile phase at a concentration of 250 ppm to protect the column from oxidative degradation. The injection volume was 200 mL. The raw data was processed by GPC software. The column was calibrated using narrow distribution polystyrene standards. Polystyrene molecular weights were converted to polyethylene molecular weights using the Mark-Houwink equation as described in ASTM Standard Test Method D6474.

[0044] Table 1 summarizes other test methods that can be used to characterize BOPE films.

[0045] Table 1

[0046] Part A: Test Methods

[0047]

[0048]

[0049] Part B: Preparation of BOPE film

[0050] Biaxially oriented polyethylene (BOPE) films were prepared in a tenter frame process using the following conditions.

[0051] A. Preparation of the Unstretched Film (or "Base Structure")

[0052] The multilayer (three-layer) sheet is coextruded by three single screw extruders through a 12-inch casting die, where the melt streams are combined in a multi-cavity die before extrusion. After extrusion from the die, the multilayer sheet is cooled and quenched on a two-roll horizontal design with an air knife. For convenience, this unstretched multilayer sheet is sometimes referred to as a "base structure" in this article. The weight of the polymer used in each of the three layers is expressed in an A / B / C format. For example, a base structure with two outer layers (or skin layers) each containing 5% by weight of the total polymer and a core layer containing 90% is described as 5 / 90 / 5.

[0053] B. Two-way approach

[0054] 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 (control) examples, the film could not be stretched in both the MD and TD directions.

[0055] Machine direction orientation (MDO) can be produced using single or two stage compression roll stretching (CRD) at temperatures up to 275°F and stretch ratios up to 7.5:1.

[0056] Transverse direction orientation (TDO) occurs in multiple zones: preheat, stretch, and anneal, plus a cooling zone. The stretch zone temperature is up to 280°F, and the stretch ratio is up to 12:1.

[0057] MDO is achieved by preheating the base structure and stretching the sheet between two rollers rotating at different speeds. The difference in roller speed determines the stretch ratio. Stretching can be performed in a set of stretching rollers or on a series of stretching rollers. Stretching is usually performed at a temperature below the crystalline melting temperature (Tm) of the film.

[0058] The MDO film is fed into a tenter oven using clips on chains attached to rails and preheated. The film is stretched as the rails move away from each other, pulling on the edges of the film causing the web to stretch. The width of the film is set to the distance between the rails and can be adjusted to achieve the desired stretch ratio. TDO is performed at similar or slightly higher temperatures than MDO.

[0059] An overview of the method conditions is provided below:

[0060] Target melt temperature in the extruder 465-475°F Die Width (inches) 12 Layer ratio 5 / 90 / 5 Casting roll temperature (°F) 120-150°F Casting roll speed (ft / min) 15 MDO draw roll temperature (°F) 190(250°F) MDO stretch ratio 4.75：1-6.5：1 Tenter Frame Stretch Zone Temperature (°F) 230-255°F TDO 7∶1-9：1 .

[0061] Example 1

[0062] The control BOPE films were made from the HDPE polymers shown in Table 2.

[0063] Table 2

[0064] Comparison with HDPE

[0065]

[0066] Note: HDPE-4 is manufactured by NOVA Chemicals under the trade name 19C for sale.

[0067] Qualitative properties of stretched films made from HDPE are shown in Table 3. These BOPE films are controls. The use of HDPE in a tenter frame process to make BOPE is disclosed in U.S. Pat. No. 6,946,203. The patent specifically states that SCLAIR 19C is suitable for making BOPE (although the patent also teaches that the core layer of the BOPE film should be filled with particles to reduce density).

[0068] Table 3.

[0069] Comparative BOPE film of HDPE

[0070]

[0071] As previously described, the chassis was prepared using three extruders to provide an A / B / C film structure (where the outer or "skin" layers are A and C, and the core layer is B). The "MD Stretch" and "TD Stretch" values ​​in Table 3 show the stretch ratio in the machine direction and the transverse direction (respectively); NA indicates that the chassis failed after sufficient stretching.

[0072] The last column provides a pass or fail grade (where pass means the film could be stretched / oriented to the extent shown in Table 3); "Grade" is a qualitative assessment of the film's appearance, where higher numbers indicate better quality.

[0073] (Comparative) Example 2-MDPE

[0074] The properties of the medium density polyethylene (MDPE) used in this example are shown in Table 4. Notably, the MDPE had 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.

[0075] This MDPE was used to prepare a base structure (to produce an A / B / C structure, where this MDPE was used in each of the A, B, and C layers) using the procedure described above in Section B. Attempts to prepare a BOPE film (using the above procedure) were unsuccessful and were given a "failed" rating.

[0076] Table 4

[0077] MDPE(control)

[0078] <![CDATA[Density (g / cm 3 )]]> 0.945 <![CDATA[Melt Index I2 (g / 10 min) at 190 °C]]> 1.7 Stress Index 1.25 <![CDATA[Melt Index I2 (g / 10 min) at 230 °C]]> 2 Branch frequency / 1000C 2.7 Comonomer ID Octene <![CDATA[M n ]]> 19440 <![CDATA[M w ]]> 87892 <![CDATA[M z ]]> 224656 <![CDATA[Polydispersity index (M w / M n )]]> 4.52 .

[0079] (Present invention) Example 3-MDPE

[0080] The properties of the MDPE used in this example are shown in Table 5.

[0081] The MDPE used in this example is prepared by copolymerization of ethylene and hexene using a chromium catalyst in a gas phase process.

[0082] A base structure was prepared with this MDPE using the procedure described in Section B above (to produce an A / B / C structure in which this MDPE was used in each of the A, B, and C layers).

[0083] A biaxially oriented polyethylene (BOPE) film was prepared from this base structure using the procedure described in Section B above.

[0084] The 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 - ie, it was significantly better than the control BOPE film made with HDPE (Example 1 above).

[0085] Table 5

[0086] MDPE (present invention)

[0087]

[0088]

[0089] The properties of BOPE films made from the inventive MDPE of Table 5 are shown in Table 6.

[0090] Table 6

[0091] XOtD 4.75X8 Membrane physical properties Thickness Profile Ave 0.81 Thickness Profile (Min-Max) 0.79-0.84 Film toughness Dart impact (g / mil) 696 Low friction puncture-(J / mm) Puncture(J / mm) Dynatup membrane maximum load (1b) 25.4 Total energy (Ftlb) 0.46 Maximum load energy (Ftlb) 0.43 Film tear resistance Tear-MD(g / mil) 14 Tear-TD(g / mil) 6 Membrane stiffness 1% second modulus-MD(MPa) 1338 1% Second modulus-TD(MPa) 2225 Membrane tensile strength Tensile strength at break-MD(MPa) 129 Tensile strength at break-TD(MPa) 298 Elongation at break-MD(%) 129 Elongation at break-TD(%) 28 Tensile yield strength-MD(MPa) 129 Tensile yield strength-TD(MPa) 298 Tensile energy (J) MD 3.12 Tensile energy (J) TD 1.36 Film Optics 45° glossiness 59 Haze(%) 8.2 <![CDATA[Water vapor transmission rate (g / 100in 2 / 24hr)]]> 0.3655 <![CDATA[Oxygen Transmission Rate (cc / 100in 2 / 24hr)]]> 138.74 .

[0092] (Present invention) Example 4-MDPE

[0093] The MDPE used in this example was prepared by copolymerizing ethylene with butene using a Ziegler Natta catalyst in a solution polymerization process. The properties of this particular MDPE are shown in Table 7. In a more general embodiment, an MDPE made in this manner having an Mn of 11,000 to 15,000, an Mz of 600,000 to 800,000, and an Mw / Mn of 10 to 12 is suitable for use in the present disclosure.

[0094] Table 7

[0095] MDPE (present invention)

[0096] <![CDATA[Density (g / cm 3 )]]> 0.95 <![CDATA[Melt Index I2 (g / 10 min) at 190 °C]]> 0.85 Comonomer ID Butene <![CDATA[M n ]]> 11,800 <![CDATA[M w ]]> 118,000 <![CDATA[M z ]]> 612,000 <![CDATA[Polydispersity index (M w / M n )]]> 10.0 .

[0097] A base structure was prepared with this MDPE using the procedure described in Section B above (to produce an A / B / C structure in which this MDPE was used in each of the A, B, and C layers).

[0098] A biaxially oriented polyethylene (BOPE) film was prepared from this base structure using the procedure described in Section B above.

[0099] The BOPE film produced with an MD stretch ratio of 6.5 and a TD stretch ratio of 9 was assigned a qualitative appearance rating of 8.

[0100] The properties of BOPE films made from the inventive MDPE of Table 7 are shown in Table 8.

[0101] Table 8

[0102]

[0103]

[0104] Industrial Applicability

[0105] Biaxially oriented polyethylene (BOPE) films are made from medium density polyethylene having a very broad molecular weight distribution. These BOPE films are suitable for making packaging that can be used to package a wide variety of consumer products.

Claims

1. A biaxially oriented polyethylene film comprising at least 60% by weight of a medium density polyethylene having: 1) Density is 0.94 to 0.95 g / cm3; 2) a melt index, I2, of 0.2 to 5 g / 10 min as measured by ASTM D1238 at 190°C using a 2.16 kg load; and 3) Molecular weight distribution Mw / Mn is 10 to 50; wherein the medium density polyethylene is further characterized by having an Mz of 550,000 to 1,500,000; and wherein the medium density polyethylene is prepared by copolymerizing ethylene and butene using a Ziegler Natta catalyst in a solution polymerization process or by copolymerizing ethylene and hexene using a chromium catalyst in a gas phase process; and 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, The film is stretched to a degree of 3:1 to 10:1 in the longitudinal direction and to a degree of 3:1 to 10:1 in the transverse direction.

2. A method for preparing a biaxially oriented polyethylene film, comprising: A) providing a base structure comprising at least 60 wt.% of a medium density polyethylene having: 1) Density is 0.94 to 0.95 g / cm3; 2) a melt index, I2, of 0.2 to 5 g / 10 min as measured by ASTM D1238 at 190°C using a 2.16 kg load; and 3) Molecular weight distribution Mw / Mn is 10 to 50; B) stretching the chassis in the longitudinal direction to a ratio of 3:1 to 10:1; and C) stretching the chassis in the transverse direction to a ratio of 3:1 to 10:1; wherein the base structure has at least three layers, and wherein at least one core layer of the base structure comprises the medium density polyethylene; wherein the medium density polyethylene is further characterized by having an Mz of 550,000 to 1,500,000; and The medium density polyethylene is prepared by copolymerizing ethylene and butene using a Ziegler Natta catalyst in a solution polymerization process or by copolymerizing ethylene and hexene using a chromium catalyst in a gas phase process.

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

4. The method of claim 2, wherein at least one skin layer of the base structure comprises ethylene-octene plastomer.

5. The method of claim 4, wherein both skin layers of the base structure comprise ethylene-octene plastomer.

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