MXPA06010220A - Machine-direction oriented multilayer films - Google Patents

Abstract

A method for making films is disclosed. The method comprises orienting in the machine direction a multilayer film at a draw-down ratio effective to give the film a dart-drop strength that increases with increasing draw-down ratio. The multilayer film comprises at least one layer of a linear low density polyethylene and at least one layer of a high density polyethylene or a medium density polyethylene.

Description

MOVIES OF MULTIPLE LAYERS ORIENTED IN ADDRESS OF THE MACHINE FIELD OF THE INVENTION The invention relates to polyethylene films. More particularly, the invention relates to multilayer films oriented in the machine direction.

BACKGROUND OF THE INVENTION Polyethylene is divided into high density polyethylene (HDPE, density 0.941 g / cm3 or greater), medium density (MDPE, density from 0.926 to 0.940 g / cm3), low density (LDPE, density from 0.910 to 0.925). g / cm3) and low linear density (LLDPE, density 0.910 to 0.925 g / cm3). See ASTM D4976-98: Standard specification for polyethylene plastic molding and extrusion materials. Polyethylene can also be divided by molecular weight. For example, ultra high molecular weight polyethylene denotes those having a weight average molecular weight (Mw) greater than 3,000,000. See US patent no. 6,265,504. High molecular weight polyethylene usually denotes those that have an Mw from 130,000 to 1,000,000. One of the main uses of polyethylene (HDPE, MDPE, LLDPE and LDPE) is in film applications, such as sacks of groceries, coatings of institutional and consumer cans, merchandise bags, shipping sacks, food packaging films, multi-walled bag liners, bags of agricultural products, delicatessen wrappers, stretch wrappers and shrink wraps. The key physical properties of polyethylene film include tear strength, impact force, tensile strength, rigidity and transparency. The film stiffness can be measured by module. The module is the resistance of the film to deformation under tension. The direction in the machine direction (MDO) is known for the polyolefin industry. When a polymer is forced under uniaxial tension, the orientation becomes aligned in the direction of pulling. For example, the US patent no. 6,391, 41 1 shows the MDO of high molecular weight HDPE films (both Mn and Mw greater than 1,000,000). However, MDO of high molecular weight HDPE films are limited because these films are difficult to stretch at a high drag rate. Current polyethylene films typically compromise various properties, such as modulus, yield strength and breaking force, to meet the packing requirements for dart fall impact force. Polymer films that do not compromise such properties are desirable to improve the performance of the bags, as well as the economy associated with the production and filling of the bisas. For example, by increasing the module and the yield strength of the film, larger bags can be produced, which could allow packing larger quantities of products while retaining their shape after being handled by the consumer. Bags with higher modules would also allow filling lines to run faster, improving the overall economy of the filling process. By increasing the yield strength of the film, it would be less likely that the bags would stretch under tension and therefore retain the original shape and dimensions. This would reduce the amount of breaks, which are a result of film performance and weight loss under load. In addition, the printed surface of the bag would not be distorted, maintaining the aesthetic quality of the packaging and intensifying brand recognition by the consumer. In addition, films that do not compromise the aforementioned properties could allow the reduction in film thickness, thus improving the economy associated with the products. Such innovations are desirable for everyone in the heavy-duty shipping bag industry to create new products that provide both performance and economy benefits.

SHORT DESCRIPTION OF THE INVENTION The method of the invention comprises orienting a multi-layered film in the machine direction (MD) at an effective downward drag ratio to give the film a dart fall force that increases with the proportion of drag down growing. The multilayer film comprises at least one layer of a linear low density polyethylene (LLDPE) and at least one layer of a high density polyethylene (HDPE) or a medium density polyethylene (MDPE). When a movie is stretched, its dart fall impact force is usually reduced as the film becomes thinner. Surprisingly I have found that when a multilayer film is oriented in the machine direction beyond a certain drag-down ratio, the dart-falling force of the film increases with the increasing drag-down ratio and the oriented film may optionally have a higher dart drop value than that of the original film. In this way, the invention provides a new method for producing a multilayer film oriented in the machine direction (MDO), which has a combination of high modulus, high tension and high dart fall impact force.

DETAILED DESCRIPTION OF THE INVENTION The method of the invention comprises orienting a multi-layered film in the machine direction (MD) at a drag-down rate to give the film a dart-dropping force that increases with drag ratio downward increasing. The multilayer film comprises at least one layer of a linear low density polyethylene (LLDPE) and at least one layer of a high density polyethylene (HDPE) or a medium density polyethylene (MDPE). Suitable LLDPE preferably is ethylene copolymers with 5 wt% to 15 wt% of a long chain α-olefin, such as 1-butene, 1-hexene and 1-ocene. Suitable LLDPE includes those having a density in the range of about 0.910 g / cm3 to about 0.925 g / cm3. Suitable LLDPE also includes so-called very low density polyethylene (VLDPE). The suitable VLDPE has a density within the range of 0.865 g / cm3 to 0.910 g / cm3. The suitable MDPE preferably has a density in the range of about 0.926 g / cm3 to about 0.940 g / cm3. More preferably, the density is within the range of about 0.930 g / cm3 to about 0.940 g / cm3. The preferred MDPE is a copolymer comprising from about 85% by weight to about 98% by weight of recurring units of ethylene and from about 2% by weight to about 15% by weight of recurring units of an α-olefin of C3 to C 0. Suitable C3 to C10 α-olefins include propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene and 1-octene, and the like, and mixtures thereof. Preferably, the MDPE has a bimodal or multimodal molecular weight distribution. The method for making bimodal or multimodal MDPE is known. For example, the US patent no. 6,486,270 shows the preparation of MDPE by a multi-zone process. The suitable HDPE preferably has a density in the range of about 0.941 g / cm3 to about 0.970 g / cm3. More preferably, the density is within the range of about 0.945 g / cm3 to about 0.965 g / cm3. Most preferably, the density is within the range of 0.958 g / cm3 to 0.962 g / cm3. Preferably, LLDPE, MDPE and HDPE have an Ml2 from about 0.01 to about 1.5 dg / min, and more preferably from about 0.01 to about 1.0 dg / min. Preferably, LLDPE, MDPE and HDPE have an MFR of from about 50 to about 300. The melt index (Ml2) is usually used to measure the polymer molecular weight and the melt flow rate (MFR) is used to measure the molecular weight distribution. A higher Ml2 indicates a lower molecular weight. A higher MFR indicates a wider molecular weight distribution. The MFR is the ratio of the high load melting index (HLMI) to Ml2. The Ml2 and HLMI can be measured in accordance with ASTM D-1238. The Ml2 is measured at 190 ° C under 2.16 kg of pressure. The HLMI is measured at 190 ° C under 21.6 kg of pressure. Preferably, the LLDPE, MDPE and HDPE have a number average molecular weight (Mn) within the range of from about 10,000 to about 500,000, more preferably from about 1,100 to about 50,000, and most preferably from about 1 1,000. up to approximately 35,000. Preferably, the LLDPE, MDPE and HDPE have a weight average molecular weight (Mw) in the range of about 120,000 to about 1,000,000, more preferably from about 135,000 to about 500,000 and most preferably from about 140,000 to about 250,000. Preferably, LLDPE, MDPE and HDPE have a molecular weight distribution (Mw / Mn) in the range of about 3 to about 20, more preferably from about 4 to about 18, and most preferably from about 5 to about 17.

The Mw, Mn and Mw / Mn are obtained by gel permeation chromatography (GPC) in a Waters GPC2000CV high temperature instrument equipped with a mixed-bed GPC column (Polymer Labs B-LS mixed) and 1, 2.4 -trichlorobenzene (TCB) as the mobile phase. The mobile phase is used at a nominal flow rate of 1.0 ml / min and a temperature of 145 ° C. No antioxidant is added to the mobile phase, but 800 ppm BHT is added to the solvent used for sample solution. The polymer samples are heated at 175 ° C for two hours with gentle agitation every 30 minutes. The injection volume is 100 microliters. The Mw and Mn are calculated using the% cumulative equalization calibration procedure employed by the Waters Millennium 4.0 computation program. This involves first generating a calibration curve using narrow polystyrene standards (PSS, Waters Corporation products), then developing a polyethylene calibration using the universal calibration procedure. Suitable LLDPE, MDPE and HDPE can be produced by Ziegler, single-site catalysts or any other olefin polymerization catalyst. Zielger catalysts are well known. Examples of suitable Ziegler catalysts include titanium halides, titanium alkoxides, vanadium halides and mixtures thereof. Ziegler catalysts are used with cocatalysts, such as alkyl aluminum compounds. The simple site catalysts can be divided into metallocene and not metallocene. The simple metallocene site catalysts are transition metal compounds containing cyclopentadienyl (Cp) ligands or Cp derivative. For example, the US patent no. 4,542, 199 shows metallocene catalysts. The non-metallocene simple site catalysts contain ligands other than Cp, but have the same catalytic characteristics as the metallocenes. Simple metallocene site catalysts may contain heteroaromatic ligands, for example, boraaryl, pyrrolyl, azaborolinyl or quinolinyl. For example, US patents nos. 6,034,027, 5,539, 124, 5,756.61 1 and 5,637,660 show non-metallocene catalysts. Optionally, the multilayer film comprises other layers such as gas barrier, adhesive, medical, flame retardant and the like layers. Suitable materials for the optional layers include poly (vinylidene) chloride, poly (vinyl) alcohol, polyamide (Nylon), polyacrylonitrile, ethylene-vinyl acetate (EVA) copolymers, ethylene-methyl acrylate (EMA) copolymers, ethylene-acrylic acid copolymers (EAA), ionomers, polyolefins grafted with maleic anhydride, K-resins (styrene / butadiene block copolymers), and poly (ethylene) terephthalate (PE), the like and mixtures thereof. Multilayer films can be made by coextrusion, coating and any other lamination process. They can be done by processes of cast or blown film. The blown film process includes high stacking and pocket processes. The difference between the high stacking process and the pocket process is that in the high stacking process, the extruded tube is inflated a distance (ie, the stacking length) from the extrusion die, while the extruded tube is The pocket process is inflated as the tube exits the extrusion die. The multilayer film is stretched uniaxially in the machine direction (or processing). This is called MDO. During the MDO, the film from the blown film line or other film process is heated to an orientation temperature. Preferably, the orientation temperature is between 60% of the difference between the glass transition temperature (Tg) and the melting point ™ and the melting temperature ™. For example, if the mixture has a Tg of 25 ° C and a Tm of 125 ° C, the orientation temperature is preferably within the range of approximately 60 ° C to approximately 125 ° C. The heating is preferably carried out using multiple heating rollers. Next, the heated film is fed to a slow feed roller with a pinch roller, which has the same roller speed as the heating rollers. The film then enters a fast drag roller. The fast drag roller has a speed that is 2 to 10 times faster than the slow drag roller, which effectively stretches the film on a continuous basis. The stretched film then enters the thermal quenching rollers, which allows stress relaxation by holding the film at an elevated temperature for a period. The tempering temperature is preferably within the range of about 100 ° C to about 125 ° C and the tempering time is within the range of about 1 to about 2 seconds. Finally, the film is cooled through cooling rollers at room temperature. The ratio of the film thickness before and after the orientation is called the "drag-down ratio". For example, when a 0.01524 cm (6 mil) film is stretched to 0.001524 cm (0.6 mil) the drag ratio down is 10: 1. According to the method of the invention, the drag-down ratio is sufficiently high at which the dart force of the film increases with the drag-down ratio. As expected, when the multilayer film is MD-oriented, its dart drop value decreases with increasing downward drag ratio. However, I found surprisingly that when the film is oriented beyond a certain point, the dart drop value increases with the drag ratio down. As the orientation continues, the oriented film may have a higher final dart drop value than that of the unoriented film. The critical point beyond which the dart drop value increases with the drag ratio down depends on many factors, including the properties of the layers, the film process conditions and the MDO conditions. Preferably, the drag-down ratio is greater than 6: 1. More preferably, the drag-down ratio is greater than 8: 1. Most preferably, the rate of drag down is greater than 10: 1. Preferably, the multilayer film is oriented to a degree that the layers of the film begin to delaminate and form a multi-walled film. The invention includes the oriented MD film made by the method of the invention. The invention also includes the multi-walled film made by the method of the invention. The film of the invention not only has a high modulus and high tensile strength, but also has high impact force of dart fall. The film of the invention is particularly useful for making heavy duty bags due to its combination of high modulus, high tensile strength and high impact force. Preferably, the film of the invention has a modulus of 1% MD and TD drier (cross direction) greater than 150,000 psi, more preferably greater than 14,000 kg / cm 2 (200,000 psi) and most preferably greater than 17,575 kg / cm2 (250,000 psi). The module is tested in accordance with ASTM E-1 1 1-97. Preferably, the film has a tensile strength MD at yield and rupture greater than 2.109 kg / cm2 (30,000 psi), more preferably greater than 2.460.5 kg / cm2 (35,000 psi) and most preferably greater than 2.812 kg / cm2 ( 40,000 psi). The tensile force is tested in accordance with ASTM D-882. Preferably, the film has a haze less than 30%, and more preferably less than 50%. The mist is tested in accordance with ASTM D1003-92: Standard test method for haze and light transmittance of transparent plastics, Oct. 1992. Preferably, the film has a gloss greater than 20 and more preferably greater than 30. The gloss is tested in accordance with ASTM D2457-90: Standard test method for specular gloss of plastic films and solid plastics. The following examples merely illustrate the invention. Those skilled in the art will recognize that there are many variations within the spirit of the invention and scope of the claims.

EXAMPLES 1-6 Machine direction orientation of three layer films of LLDPE / MDPE / LLDPE Medium density polyethylene (XL3805, product of Equistar Chemicals, LP, Ml2: 0.057 dg / min, density: 0.938 g / cm3, Mn: 18,000, Mw: 209,000) is coextruded with a linear low density polyethylene (GS707, product of Equistar Chemicals, LP, density: 0.915 g / cm3, Ml2: 0.700 dg / min, Mn: 30,000, Mw: 120,000) and converted into a three layer film of equal layers (LLDPE / MDPE / LLDPE ) with a thickness of 0.0355 cm (14.0 mil) in a die of 1000 mm with a die opening of 2.5 mm. The films are produced in pocket and blowing proportions (BUR) of 2.1. The films are then stretched in thinner films in the machine direction with downstream ratios of 4, 5, 6, 7, 8, and 9.3: 1 in Examples 1-6, respectively. The drag ratio down to 9.3: 1 is the maximum downward drag ratio limited by the targeting equipment and not the polymer film. The film properties are listed in Table 1. It is shown that at lower drag ratios, the dart drop values decrease with increasing downward drag ratios as expected. After a particular drag ratio, the dart drop values begin to increase and significantly exceed that dart drop value of the initial film.

TABLE 1 Properties vs. Proportion of dragging down of multilayer films COMPARATIVE EXAMPLES 7-1 1 Orientation in the direction of the HDPE monolayer film machine Examples 1-6 were repeated, but the films were made as a monolayer HDPE structure (L5005, product of Equistar Chemicals, LP, density: 0.949 g / cm3, Ml2: 0.057 dg / min, Mn: 12,600, Mw: 212,000). The film properties are listed in Table 2, which shows that the dart drop values decrease significantly with increasing downward drag ratio and the drastic somersault in the dart drop values seen with the multilayer films in the Examples 1-6 are not observed. The drag ratio down to 7.9: 1 is the maximum drag-to-bottom ratio limited by the targeting equipment and not by the polymer film.

TABLE 2 Properties vs. drag ratio down of monolayer films COMPARATIVE EXAMPLES 12-19 Machine direction orientation of MDPE-LLDPE monolayer film machines Examples 1-6 are repeated, but the films are made as a monolayer of the MDPE mixture (XL3805, product of Equistar Chemicals, LP, Ml2: 0.057 dg / min, density: 0.38 g / cm3, Mn: 18,000, Mw: 209,000) and LLDPE (GS707, product of Equistar Chemicals, LP, density: 0.915 g / cm3, Ml2: 0.700 dg / min, Mn: 30,000 Mw: 120,000). The components in the mixture have proportions such that the percentage of each material present in the overall film is the same as that of the multilayer films shown in Examples 1-6. The film properties are listed in Table 3, which shows that the dart drop values decrease significantly with increasing downward drag ratio and the drastic somersault seen with the multilayer films in Examples 1-6 is not observed . The drag ratio below 10.6: 1 is the maximum downward drag ratio limited by the orientation equipment and not by the polymer film.

TABLE 3 Properties vs. drag ratio down of monolayer MDPE-LLDPE blend films

Claims (14)

1. CLAIMS 1 . A method comprising orienting a multilayer film in the machine direction to an effective drag ratio to give the film a dart fall force that increases with the increasing downward drag ratio, wherein the film comprises at least a layer of a linear low density polyethylene (LLDPE) and at least one layer of a high density polyethylene (HDPE) or a medium density polyethylene (MDPE).
2. 2. The method of claim 1, wherein the HDPE has a density in the range of 0.941 g / cm3 to 0.970 g / cm3.
3. 3. The method of claim 1, wherein the MDPE has a density in the range of 0.926 g / cm3 to 0.940 g / cm3.
4. 4. The method of claim 1, wherein the LLDPE has a density within the range of 0.865 to 0.925 g / cm3.
5. The method of claim 1, wherein the film is oriented at an effective drag-down rate to cause the film to delaminate.
6. The method of claim 1, wherein the film is oriented at a downward drag ratio to give the film a greater dart fall force than that of the original film.
7. 7. The method of claim 1, wherein the LLDPE, HDPE and MDPE each have a weight average molecular weight (Mw) in the range of 120,000 to 1,000,000.
8. The method of claim 7, wherein the Mw is within the range of 135,000 to 500,000.
9. 9. The method of claim 7, wherein the Mw is within the range of 140,000 to 250,000.
10. The method of claim 1, wherein the LLDPE, HDPE and MDPE each have an average number-average molecular weight (Mn) in the range of 10,000 to 500,000. eleven .
11. The method of claim 10, wherein the Mn is within the range of 1 1,000 to 50,000.
12. 12. The method of claim 10, wherein the Mn is within the range 1 1, 000 to 35,000.
13. 13. A oriented film made by the method of claim 1.
14. 14. A multi-walled film by the method of claim 5.