MXPA96005478A - Medicum module film and fabricac method - Google Patents

Abstract

Medium-modulus polyethylene films and bags are made with improved tear strength performance for heavy duty packaging applications and hot fill applications. The film compositions contain linear high molecular weight polyethylene and a linear ethylene / α-olefin interpolymer. The film is at least about 31 microns thick, and is characterized by having a calculated film density in the range of 0.923 to 0.95 grams / cubic centimeter (g / cc), and typically has a tear resistance to impact when less 30 percent larger than the current industrial polyethylene film. The novel film has a dimensional stability and superior resistance properties that allow to lower the caliber in a significant way in relation to the industrial films used as overwraps, shirts and work bags weighs

Description

MEDIUM MODULE FILM AND MANUFACTURING METHOD This invention pertato a medium modulus polyethylene film and to a method for the preparation of this film. The novel film can be prepared by variable stem blow extrusion. The film has properties of high resistance to tearing and impact. The film can be used in heavy-duty packaging and in boat applications, and also in hot-fill packaging applications. Polyethylene films with tear and impact resistance properties are needed to pack and ship heavy items, such as building materials, meadows and gardens, salt, polymeric granules. Heavy duty films and bags must also have good rigidity (modulus). Good film strength properties are required to prevent bag ruptures and product losses during distribution, while stiffness provides good dimensional stability. Dimensional stability is important during manufacturing and packaging operations, because it assists in maintaining the proper placement of the film or bag as it is transported through the different stations of the equipment during the manufacturing operational steps of the bag and filling of product. Dimensional stability at elevated temperatures is also required in some ances during the product filling step, when the product (eg, salt) is hot packed, such as, for example, in some form-fill-seal packaging operations. The heavy duty packaging currently involves single layer and multilayer polyethylene films having a calculated density of the film as low as about 0.920 grams / cubic centimeter. Typical polyethylene film compositions for heavy duty packaging include: (a) blends of linear low density polyethylene (LLDPE) with low density polyethylene (LDPE), (b) high density polyethylene (HDPE) modified by addition rubber and other elastomers (eg, polybutylene) to impart impact resistance, (c) linear low density polyethylene mixed with a low molecular weight high density polyethylene (LMW-HDPE), (d) linear low polyethylene density mixed with a high speed high melt flow polyethylene, or (e) linear low density polyethylene mixed with partially isotactic polymers. See, for example, U.S. Patent No. 5,041,401 to Shirodkar et al., U.S. Patent No. 5,102,955 to Calabro et al., And U.S. Patent No. 4,828,906 to Nishimura et al. . The polyethylene composition described by Thiersault et al. Is also known from U.S. Patent No. 4,786,688, which conta80 to 98 weight percent high density polyethylene, and 2 to 20 weight percent linear low density polyethylene, which is claimed to be useful for thin film (20 microns), and for blow molding applications. Additionally, ternary polymer blends have been used in this packaging application. For example, in U.S. Patent No. 4,824,912, Su et al describe linear low density polyethylene mixed with minor amounts of a low molecular weight high density polyethylene (LMW-HDPE), and a high density polyethylene. of high molecular weight (HMW-HDPE), for the processability and improvements in the properties of the film on the linear low density polyethylene used alone. The prior art shows that the linear ethylene polymers currently used in the manufacture of polyethylene films provide greater tear resistance as the density increases to approximately 0.920 grams / cubic centimeter, and then show substantially lower tear strengths as the density decreases above about 0.920 grams / cubic centimeter. Attempts to improve tear resistance by increasing the thickness of the film have only been marginally effective. When the thickness of the film is increased to improve the strength properties, the rigidity of the polyethylene films of the current art is increased disproportionately with the properties of impact and tear resistance, and therefore, the films thicker ones offer practitioners little or no additional benefit. Accordingly, although a variety of polyethylene films and film compositions are known, polyethylene films of the prior art are not completely satisfactory for use in heavy duty packaging applications, because they do not offer the desired balance of high strength to tear and impact with the stiffness or modulus of film required, and / or do not have the desired dimensional stability. Accordingly, it is an object of the present invention to provide a polyethylene film with improved tear strength and impact resistance, and with good dimensional stability, as well as a method for manufacturing it, that can be used in heavy duty packaging applications. and shipping, and for use in hot fill packaging applications. Applicants have discovered a novel medium modulus polyethylene blown film having good impact and tear resistance, and a method for preparing this film. The novel film comprises: (A) from 60 to 95 weight percent, based on the combined weights of components (A) and (B), of at least one linear high molecular weight ethylene polymer having a density in the scale from 0.92 to 0.96 grams / cubic centimeter, and a melt index I5 on the scale of 0.1 to 3 grams / 10 minutes, and (B) from 5 to 40 percent by weight, based on the combined weights of the components (A) and (B), of at least one linear ethylene / α-olefin interpolymer, characterized in that it contains at least one α-olefin monomer, and because it has a density on the scale of 0.85 to 0.92 grams / cubic centimeter, and an I2 melt index on the scale of 0.3 to 3 grams / 10 minutes. The novel method for producing this medium modulus polyethylene film is a variable stem extrusion process, which comprises the steps of: (1) providing an extrudable thermoplastic composition containing (A) from 60 to 95 weight percent, based on the combined weights of the components (A) and (B), of at least one linear high molecular weight ethylene polymer having a density on the scale of 0.92 to 0.96 grams / cubic centimeter, and an I5 melt index on the scale of 0.1 to 3 grams / 10 minutes, and (B) of 5 to 40 weight percent, based on the combined weights of components (A) and (B), of at least one ethylene interpolymer / - linear olefin, characterized in that it contains at least one α-olefin monomer, and because it has a density in the scale of 0.85 to 0.92 grams / cubic centimeter, and a melt index I2 in the range of 0.3 to 3 grams / 10 minutes. (2) introducing the composition of step (1) into a heated film extrusion apparatus equipped with an annular die, (3) extruding that composition through the annular die to form a molten or semi-molten thermoplastic tube of said composition, which is subsequently blows and stretches down through the tightening and unwinding rollers, to form a flat film with a thickness greater than about 31 microns, and (4) to transport the film formed in step (3) for subsequent use by lowering by the line of the film extruding apparatus of step (2), or collecting the film formed in step (3) for subsequent off-line use. The film of the present invention has better performance of tear and impact resistance and excellent dimensional stability which is not ordinarily expected for medium modulus polyethylene films. The new movie has at least 30 percent, and preferably a 50 percent improvement in impact and tear strength properties relative to prior art polyethylene films having approximately the same film density, melt index and film thickness. These improvements allow Practitioners to meet the specified requirements of heavy duty films at substantially lower costs by down calibration and / or by using higher loads of diluent and recycled material. Figure 1 graphs the data describing the relationship between Mw / Mn and IIQ I2 For three different types of polymer: substantially linear polyethylene, heterogeneous linear polyethylene, and linear homogeneous polyethylene. Figures 2 to 8 are used to summarize graphically the data presented in the Examples. Figure 2 graphs the relationship between tear strength and film thickness for an Invention Film prepared from Film Composition A, and for Comparative Films prepared from Film Compositions B, C and D Figure 3 graphs the relationship between tear strength and film thickness for an Invention Film prepared from the Film Composition, and for the Comparison Polylulas prepared from the Film Compositions B. , E and F.

Figure 4 graphs the relationship between tear strength and film thickness for the Invention Films prepared from the Compositions of Film A, H and I, and for the Comparative Films prepared from the Film Composition G. Figure 5 graphs the relationship between the tear strength and the film thickness for the Films of the Invention prepared from the Compositions of Film A, J and K, and for Comparative Films prepared from Film Compositions B and C. Figure 6 graphs the relationship between tear strength and film thickness for Invention Films prepared from of the Compositions of Film A, L and M, and for the Comparative Films prepared from the Film Composition B. Figure 7 graphs the relationship between the tear strength and film thickness for the Invention Films prepared from the Film Compositions A, H, I, J, K, L and M, and for the Comparative Films prepared from the Film Compositions D, C, D, E, F and G, and includes the linear regression equation for each composition. Figure 8 graphs the relationship between tear strength and film density for Invention Films prepared from Film Compositions A, H, I, J, K, L and M, and Comparative Films prepared from Film Compositions B, C, G and O, as well as the calculated or predicted tear strength of the blend compositions, based on a high density, high molecular weight polyethylene of 0.942 grams / cubic centimeter, and an ultra-low density polyethylene of 0.905 grams / cubic centimeter, in different proportions. Figure 9 graphs the relationship between tear strength and film density for Invention Films prepared from Film Compositions A, H, K, L and M, and Comparative Films prepared from the Film Compositions B, C, G and O, as well as the calculated or predicted tear strength of the blend compositions, based on a high density, high molecular weight polyethylene of 0.942 grams / cubic centimeter, and an ultra high molecular weight polyethylene. -Low density of 0.905 grams / cubic centimeter, in different proportions. The terms "ultra-low density polyethylene" (ULDPE), "very low density polyethylene" (VLDPE) and "very low density linear polyethylene" (LVLDPE), have been used interchangeably in the polyethylene technique to designate the polymeric subset of linear low density polyethylenes having a density less than, or equal to, about 0.915 grams / cubic centimeter. Then the term "linear low density polyethylene" (LLDPE) is applied to those linear polyethylenes having a density greater than about 0.915 grams / cubic centimeter. The terms "heterogeneous" and "heterogeneously branched" are used herein in the conventional sense with reference to a linear ethylene / α-olefin polymer having a comparatively low short chain branching distribution index. The short chain branching distribution index (SCBDI) is defined as the weight percentage of the polymer molecules having a comonomer content within 50 percent of the average total molar comonomer content. The short chain branching distribution index of the polyolefins can be determined by well-known elution fractionation techniques with elevation of temperature, such as those described by Wild et al., Journal of Polymer Science, Poly. Phvs. Ed., Volume 20, page 441 (1982), L.D. Cady, "The Role of Comono er Type and Distribution in LLDPE Product Peformance", SPE Regional Technical Conference, Quaker Square Hilton, Akron, Ohis, October 1-2, pages 107-119 (1985), or the Patent of the States United States of America Number 4,798,081. The heterogeneous linear ethylene / α-olefin polymers typically have a short chain branching distribution index of less than about 30 percent.

The terms "homogeneous" and "homogeneously branched" are used herein in the conventional sense with reference to an ethylene / α-olefin polymer having a comparatively high short chain branching distribution index (SCBDI), determined by techniques fractionation by elution with well-known temperature rise. The homogeneous ethylene / α-olefin polymers typically have a short chain branching distribution index greater than or equal to about 30 percent. The term "medium modulus" is used herein with reference to the novel film, to mean that the calculated film density is in the range of 0.923 to 0.95 grams / cubic centimeter. The term "calculated film density" is used herein to mean the density of the film when calculated from known weight fractions, and the measured hardened densities of the polymers or component layers. The term "coarse" is used herein with reference to the novel film to mean a film thickness greater than about 31 microns. The term "variable stem extrusion" is a new term in the art, used herein to express the distance between the annular film die and the height of the stem or the bubble expansion point, which can be varied from O centimeters up to more than 366 centimeters during the manufacture of the film by blowing. The term includes both the extrusion of blown film in a well-known pouch and the extrusion of blown film in the stem. The term "high stem extrusion" is used herein in the conventional sense to mean a distance between the annular film die and the air ring that is greater than, or equal to, 76 centimeters. The term "low stem extrusion" is used in the present in the conventional sense to mean a distance in the scale of 12.7 centimeters to 76 centimeters. The term "hot fill" refers herein to a product packing or filling operation, wherein the temperature of the product is greater than 45 ° C. The term "heavy work" is generally referred to industrial articles packaged in bulk, or having a weight of a single package greater than 4.5 kilograms. The density of the polymers used to make the medium modulus film of the present invention is measured according to ASTM D-792, and reported as grams / cubic centimeter (g / cc). The measurements reported in the Examples below are determined after the polymer samples have been annealed for 24 hours at ambient conditions. Melt index measurements are made in accordance with ASTM D-1238, Condition 190 ° C / 2.16 kilograms (kg), and Condition 190 ° C / 5 kilograms, and are known as I2 and I5, respectively. The melt index is inversely proportional to the molecular weight of the polymer. Therefore, the higher the molecular weight, the lower the melting index, although the relationship is not linear. The melt index is reported as grams / 10 minutes. For the purposes of this invention, in the calculation of certain values of the Examples, the values I5 and I2 are related to each other by a factor of about 5.1; for example, a merger of index 1.0 I2 is equivalent to approximately a merger index of 5.1 I5. Melt index determinations can also be performed with even higher weights, such as according to ASTM D-1238, Condition 190 ° C / 10 kilograms, and Condition 190 ° C / 21.6 kilograms, and are known as I10 and I2 ? .6 'respectively. The term "melt flow rate" is defined herein in the conventional sense as the ratio of a higher melt index determination to a lower weight determination. For the measured melt index values I10 and I2, the melt flow ratio is conveniently designated as I10 / I2. For the values I 1 6 and I10, the ratio is referred to as l2i.6 Il? * Occasionally other melt flow ratios are used with respect to polyethylene compositions, such as, for example, I5 / I2, based on measurements of melt index I5 and I2 . In general, the determinations of l2i.6 I? Oe and Is / I2 provide similar melt flow values, and the IIQ I2 values are usually greater than the values by a factor of approximately 4.4, and this factor is used for the purposes of the present invention in the calculation of certain values of the Examples. The tear strength of the film of the present invention is measured according to ASTM D1922, and reported in grams. The tear strength is measured in both the machine direction (MD) and the transverse direction (CD). The term "tear strength" is used in the present to represent the average between tear strength values - in the machine direction and in the transverse direction, and in the same way, is reported in grams. The impact resistance of the film of the present invention is measured according to ASTM D1709. Where indicated and according to the ratio of higher thicknesses produce increased performance values, the tear and impact results are normalized to exactly 50 microns by proportional increases or proportional decreases, based on the actual measured film thickness (microns). ); however, these normalization calculations are only made and reported where the thickness variations are less than 10 percent, that is, where the measured thickness is in the range of 45 to 56 microns. The medium modulus polyethylene film of the present invention has a film density calculated in the scale from 0.923 grams / cubic centimeter to 0.95 grams / cubic centimeter, especially from 0.926 grams / cubic centimeter to 0.948 grams / cubic centimeter, and more especially from 0.93 grams / cubic centimeter to 0.945. The thickness of the film is generally greater than about 31 microns, especially in the scale of 37 microns to 217 microns, and more especially in the scale of 50 microns to 198 microns. These films have a tear strength or, alternatively, an impact resistance, at least 30 percent greater than, and more preferably at least 50 percent greater than, the tear strength or impact resistance of a film. of comparative polyethylene of the prior art having approximately the same film density, melt index and film thickness. The tear resistance of the novel film can be determined by the following equation: tear strength (grams) = Ax + Bx2 + C where A, B and C are numerical values, and x is the thickness of the film (thousandths); when A is less than, or equal to, about 150, B is greater than, or equal to, about 12.5, preferably greater than, or equal to, about 13.5, and more preferably greater than, or equal to, about 14.5; and when A is greater than about 150, B is on the scale of -80 to 40, preferably -70 to 20, and more preferably, -60 to 0. For example, the expression 96.621x + 16.186X2 + 59.767 it is believed that it represents the tear resistance of the film of the present invention, while the expression 138.22x + 4.8116x2 - 19.364 does not. These representative expressions or equations are specific to the film composition. Figure 7 provides other example expressions based on the thickness of the film, which are representative of the film of the present invention. The tear resistance of the novel film can be determined by the following equation: tear strength (grams) = (2.065 x 106) (Z) 2"3.8983 x 106) (Z) + 1.84015 x 106 where Z is the density calculated from the film in grams / cubic centimeter.This novel film can be conveniently formed into bags, and is useful in heavy-duty packaging and shipping applications, as well as in hot-fill packaging applications, where films with good balance of properties, that is, high strength and medium modulus with good resistance to tearing, impact and dimensional stability High molecular weight linear ethylene polymers, Component (A), to be used in the preparation of polyethylene film of the present invention are a known class of compounds that can be produced by any well-known particulate polymerization process, such as polymerization in paste and polymerization in gas phase. Preferably, linear high molecular weight ethylene polymers are produced using well known Phillips or Ziegler type coordination catalysts, although metallocene catalyst systems can also be used. Although preferred, with conventional Ziegler-type catalysts, pulp polymerization processes are generally limited to polymer densities greater than about 0.940 grams / cubic centimeter, and are especially limited to polymer densities greater than about 0.935 grams / cubic centimeter , that is, approximately 0.935 grams / cubic centimeter is the practical lower trade limit for pulp polymerization. The high molecular weight linear ethylene polymer can be an ethylene homopolymer or a copolymer of ethylene with at least one α-olefin of 3 to 20 carbon atoms, however, preferably, the high molecular weight linear polymer is a copolymer with at least one α-olefin of 3 to 20 carbon atoms, such as 1-propylene, 1-butene, 1-isobutylene, 4-methyl- l-pentene, 1-hexene, 1-heptene and 1-octene. More preferably, the linear high molecular weight ethylene polymer is an ethylene / 1-butene copolymer prepared by a low pressure pulp polymerization process. The novel film comprises 60 to 95 weight percent linear high molecular weight ethylene polymer, preferably 65 to 90 weight percent, and more preferably 70 to 85 weight percent. Component (A) can also be a mixture of linear ethylene polymers. These mixtures can be prepared on site (for example, by making a mixture of catalysts in a single polymerization reactor, or by using different catalysts in separate reactors connected in parallel or in series), or by physically mixing the polymers. The linear high molecular weight ethylene polymer has a melt index I5 in the range of 0.1 grams / 10 minutes to 3 grams / 10 minutes, preferably 0.1 grams / 10 minutes to 2 grams / 10 minutes, and more preferably, from 0.15 grams / 10 minutes to 1 gram / 10 minutes. Additionally, the linear polymer preferably has a bimodal molecular weight distribution (MWD), and an I2i.6 ratio on the scale of 1 to 12, preferably on a scale of 3.5 to 10, more preferably on the scale from 4 to 8, and most preferably in the range from 4.5 to 6. The linear high molecular weight ethylene polymer, which includes, but is not limited to, linear low density polyethylene, linear polyethylene of medium density and polyethylene of high density, and mixtures thereof, preferably have a density in the scale of 0.92 grams / cubic centimeter to 0.96 grams / cubic centimeter, more preferably in the range of 0.93 grams / cubic centimeter to 0.96 grams / cubic centimeter, and very preferably on the scale of 0.935 grams / cubic centimeter to 0.958 grams / cubic centimeter. The linear ethylene / α-olefin interpolymers useful in this invention are a known class of compounds, which includes both heterogeneously branched linear, ethylene / α-olefin interpolymers, catalysed with Ziegler, conventional, as well as ethylene / α-olefin interpolymers. homogeneously branched linear. Ultra-low density polyethylene and heterogeneously branched linear low density polyethylene are well known materials and are commercially available. These are typically prepared using Ziegler-Natta catalysts in solution or gas phase polymerization processes, Anderson et al., United States Patent Number 4,076,698, is illustrative. These traditional Ziegler type linear polyethylenes are not homogeneously branched, and have no long chain branching. The ultra-low density polyethylene and the typical heterogeneously branched linear low density polyethylene have molecular weight distributions Rw / Mn, on the scale of 3.5 to 4.1. Ultra-low density polyethylene and homogeneously branched linear low density polyethylene are also well known. The Elston description in U.S. Patent Number 3,645,992 is illustrative. Ultra-low density polyethylene and homogeneously branched linear low density polyethylene can be prepared in conventional polymerization processes using Ziegler-type catalysts, such as, for example, zirconium and vanadium catalyst systems, as well as using metallocene catalyst systems , such as, for example, those based on hafnium. The description of Ewen et al. In U.S. Patent Number 4,937,299 and the description of Tsutsui et al. In U.S. Patent Number 5, 218,071 are illustrative. This second class of linear polyethylenes are homogeneously branched polymers, and like the traditional heterogeneous linear polyethylenes of Ziegler type, they do not have long chain branching. The ultra-low density polyethylene and the homogeneously branched linear low density polyethylene typically have molecular weight distributions, / Mn of about 2. Commercial examples of the homogeneously branched linear polyethylenes include those sold by Mitsui Petrochemical Industries under the designation "T? FMER", and by Exxon Chemical Company under the designation "EXACT". The linear ethylene / α-olefin interpolymers used in this invention are not in the same class of compounds as the unique class of substantially linear ethylene polymers defined in U.S. Patent Nos. 5,272,236 and 5,278,272, Lai et al. . The linear ethylene / α-olefin interpolymers used to make the novel film of the present invention are distinguished from the unique polymers described by Lai et al., In which the substantially linear ethylene / α-olefin interpolymers have excellent processability, yet when they have relatively narrow molecular weight distributions (ie, the Mw / Mn ratio is typically about 2). Still more surprisingly, as described in U.S. Patent No. 5,278,272 by Lai et al., The melt flow rate (I10 / I2) of the substantially linear ethylene polymers can vary essentially independently of the polydispersity index (ie, the molecular weight distribution, Mw / Mn). As illustrated in Figure 1, the rheological behavior of substantially linear ethylene / α-olefin polymers, and represents a dramatic counterdistinction to the homogeneous linear ethylene / α-olefin polymer described by Elston, and heterogeneous linear polyethylene polymerized with conventional Ziegler. , done, for example, according to the description of Anderson et al., in US Pat. No. 4,076,698, in which both heterogeneous linear and homogeneous linear ethylene / α-olefin polymers have rheological properties such that, As the polydispersity index increases, the I10 / I2 value also increases. The linear ethylene / α-olefin interpolymer, Component (B), for use in the preparation of the medium-modulus thick polyethylene film of the present invention, contains at least one α-olefin monomer. The interpolymer can be produced by polymerization processes in solution and gas phase. However, when they are produced by a gas phase process, and the interpolymer is a copolymer containing only one α-olefin, the α-olefin must be greater than 6 carbon atoms. When produced by the preferred solution process, the interpolymer can contain at least one α-olefin of 3 to 20 carbon atoms, such as 1-propylene, 1-butene, 1-isobutylene, 1-hexene, 4-methyl- 1-pentene, 1-heptene and 1-octene, as well as other types of monomer, such as styrene, styrenes substituted by halogen or alkyl, tetrafluoroethylene, benzocyclobutane vinyl, 1,4-hexadiene, 1,7-octadiene, and cycloalkenes, for example, cyclopentene, cyclohexene and cyclooctene. Although the interpolymer can be a terpolymer wherein at least two α-olefin monomers are polymerized with ethylene, preferably the interpolymer is a copolymer with an α-olefin monomer copolymerized with ethylene, and more preferably, the ethylene interpolymer / α - Linear olefin, Component (B), is a copolymer of ethylene and 1-octene. The novel film is prepared using 5 to 40 weight percent linear ethylene / α-olefin interpolymer, preferably 10 to 35 weight percent, and more preferably 15 to 30 weight percent. The linear ethylene / α-olefin interpolymer used to prepare the film of the present invention has a melt index I2 on the scale of 0.3 grams / 10 minutes, at 3 grams / 10 minutes, preferably 0.3 grams / 10 minutes to 2.5 grams / 10 minutes, and more preferably from 0.4 grams / 10 minutes to 2 grams / 10 minutes. The linear ethylene / α-olefin interpolymer has a density of less than about 0.92 grams / cubic centimeter, more preferably in the range of 0.85 grams / cubic centimeter to 0.916 grams / cubic centimeter, and most preferably in the 0.86 grams / centimeter scale cubic to 0.91 grams / cubic centimeter. The proportion I? Or I2 °? E ° s linear ethylene / α-olefin interpolymers is in the range of 5.63 to 30, preferably less than about 20, especially less than about 15, and more especially less than about 10. The Preparation of polyethylene film by blown film extrusion is well known. See, for example, U.S. Patent Number 4,632,801 by Dowd, which discloses a typical blown film extrusion process. In the typical process, a polyethylene composition is introduced into a screw extruder, where it is melted, and the composition is forced through an annular film die to form a molten tube. Air is then supplied through the annular die to inflate the tube in a "bubble" with the desired diameter. The air is contained inside the bubble by the annular die and the tightening rollers downstream of the die, where subsequently the bubble collapses into a flat film. The final thickness of the film is controlled by the extrusion speed, the bubble diameter and the tightening speed, which can be controlled by variables, such as the screw speed, the hauling speed, and the speed of the winding machine The increase in the speed of extrusion with a constant diameter of the bubble and constant speed of tightening, will increase the final thickness of the film. The typical blown extrusion process can be broadly classified as "stem" or "bag" extrusion. In stem extrusion, inflation and bubble expansion are controlled or present at a significant distance above the annular die. The air ring, normally of a single-lip construction, provides an air flow external to the tube and parallel to the machine direction, such that the molten tube maintains the approximate diameter of the die of the annular film until it is inflated to a height of at least 12.7 centimeters above the die cancel. Internal bubble cooling can also be used, as well as an internal bubble stabilizer to ensure optimal bubble stability during manufacturing. It is known that the stem extrusion allows to have a better molecular relaxation, and as such, mitigates excessive orientation in one direction, and therefore, allows to have balanced physical properties of the film. The increase in stem height or expansion, generally provides higher properties in the transverse direction (CD) and, therefore, higher average properties of the film. The stem extrusion, and particularly the high stem extrusion, is very useful for the preparation of blown films from high molecular weight polyethylene compositions, such as, for example, high molecular weight high density polyethylene (HMW). HDPE) and high molecular weight low density polyethylene (HMW-LDPE) that have sufficient melt strength to ensure adequate stability of the bubble. In bag extrusion, air is supplied by an air ring disposed immediately adjacent the annular die, to cause the bubble to exit the die to inflate and expand immediately. The air ring is typically a type of double lip to ensure greater stability of the bubble. Bag extrusion is used more widely than stem extrusion, and is generally preferred for the lower melt strength, lower melt strength polyethylene compositions, such as, for example, linear low density polyethylene ( LLDPE) and ultra-low density polyethylene (ULDPE). Films can be prepared both in a single layer and in multiple layers by the extrusion of stem and bag, and the films of the present invention can be single-layer or multi-layer structures. Multilayer polyethylene films can be prepared by any technique known in the art, including, for example, co-extrusion, lamination or combinations of both. However, the preferred medium modulus thick polyethylene film of the present invention is a single layer film structure. Although the film of the present invention can be prepared by variable stem extrusion, bag extrusion, and low stem extrusion, it is preferred, wherein the linear high molecular weight ethylene polymer, Component (A) has a low molecular weight index. I5 melt greater than about 0.5 grams / 10 minutes, particularly greater than about 0.6 grams / 10 minutes, and most particularly greater than about 0.7 grams / 10 minutes. High-stem extrusion is preferred, where the distance between the die and the bubble expansion presentation is normally from 76 to 107 centimeters, that is, from 6 to 10 diameters of the die, for the preparation of the film. the present invention, wherein the linear high molecular weight ethylene polymer, Component (A) has a melt index I5 of less than or equal to about 0.5 grams / 10 minutes, particularly less than about 0.4 grams / 10 minutes. , and more particularly less than about 0.3 grams / 10 minutes. Components (A) and (B), used to prepare the film of the present invention, can be mixed individually (i.e., where a component itself is a polymer blend of two or more sub-component polymers), or can be mixed yes by any suitable element known in this field. Suitable elements are believed to include dry mixing by tumbling the components together before loading the blown film extruder, feeding by weight of the components directly into the blown film extruder, melt mixing of the components by means of extrusion of compound or side arm before the introduction into the blown film extruder, multiple reactor polymerization of the components with reactors in series or in parallel, and optimally with different types of catalyst and / or monomer in each reactor, or the like, as well as combinations thereof. In addition to the above equations with respect to the performance of tear resistance and impact of the film of the present invention, fractionation by elution with temperature rise (TREF) can also be used to have the "footprint" or identify the novel film of this invention, as well as the film compositions used to make the novel film. Also additives may be included, such as antioxidants (for example, hindered phenolics, such as Irganox® 1010 or Irganox® 1076 supplied by Ciba Geigy), phosphites (for example, Irgafos® 168 also supplied by Ciba Geigy), filler additives ( for example, PIB), Standostab PEPQMR (supplied by Sandoz), pigments, dyes, and fillers, in the film of the present invention, or in the polymeric compositions used to make it, to the extent that these additives or ingredients do not interfere csn the best performance of tear resistance and impact discovered by the Requesters. Although not generally required, the film of the present invention may also contain additives to improve anti-blocking and coefficient of friction characteristics, including, but not limited to, untreated and treated silicon dioxide, talc, calcium carbonate. , and clay, as well as primary, secondary, and substituted fatty acid amides, release agents, silicone coatings, etc. Other additives, such as quaternary ammonium compounds alone or in combination with ethylene-acrylic acid copolymers (EAA), or other functional polymers, can also be added to improve the antistatic characteristics of the film of the present invention, and to allow , for example, a heavy-duty package of electronically sensitive articles. Conveniently, due to the improved strength properties of the novel film, recycled and waste materials, as well as polymeric diluents, can be incorporated or mixed in the film compositions used to make the novel film at higher loads than which is typically possible with the polyethylene film compositions of the prior art, and still provide or maintain the desired performance properties for successful use in heavy duty packaging and shipping applications. Suitable diluent materials include, for example, elastomers, rubbers and polyethylenes modified with anhydride (for example, linear low density polyethylene and high density polyethylene grafted with polybutylene and maleic anhydride), as well as with high pressure polyethylenes, such as, for example, low density polyethylene (LDPE), ethylene / acrylic acid interpolymers (EAA), ethylene / vinyl acetate interpolymer (EVA), and ethylene / methacrylate interpolymers (EMA), and combinations thereof.

EXAMPLES The following examples illustrate some of the particular embodiments of the present invention, but the following should not be construed to mean that the invention is limited only to the particular embodiments shown. Table 1 lists different types of resin to be used in the investigation of the requirements for improved medium module films.

Table 1 Types of Resin and Properties * HMW-HDPE and MDPE resins are supplied by The Dow Chemical Company. ULDPE resins are supplied by The Dow Chemical Company and Union Carbide Corporation.

Tables 2 to 8 summarize the different resins of components and film compositions to be used in the studies to determine the requirements for thick films of medium modulus with better strength properties. With the exceptions of the Comparative Films prepared from Compositions B, N and O, all the Invention Films and Comparative Films comprising polymer blends, as well as the Comparative Films prepared from Composition G (a composition of a single polymer, unmixed, high molecular weight), were prepared using a Kiefel high stem blown film line of seven zones, equipped with a grooved barrel extruder 70 millimeters in diameter, a decompression screw, an annular die 113 millimeters in diameter, a die gap of 1.2 millimeters, and no internal bubble cooling. During these fabrications, the blowing rate remained at 3.3 / 1, the height of the neck remained at 104 centimeters, and the output remained at 100 kilograms per hour, for thicknesses greater than 12.7 microns and at 77 kilograms per hour for smaller thicknesses that, or equal to 12.7 microns, to provide a flat film of 58.4 centimeters. The Comparative Films prepared from Compositions B, N and 0 (medium molecular weight unmixed single polymer compositions), used a bag blown film line equipped with a 24: 1 L / D extruder, 64 millimeters in diameter, a barrier screw, an annular die of 15.2 centimeters in diameter, and a die gap of 1.78 millimeters. The blowing rate remained at 2.5 / 1, and the output remained at 64 kilograms per hour, or 8.7 kilograms / hour / centimeter of the die circumference). With the exception of the Comparative Films prepared from Composition B, which was prepared with an inclined profile of the extruder, all the film preparations used a reverse temperature profile. The melting temperature of all film preparations was maintained at 213-221 ° C. The physical properties of the Films of the Invention and the resulting Comparative Films from the Compositions A-0 as a function of the thickness are also summarized in Tables 2 to 8. The Tables report the calculated density of the film. As the determinations of calculated film density, the I5 values of the composition reported in the Tables were also derived from the calculations of the weight fraction. For the purposes of the present invention and for the component polymers, all reported I2 values less than 0.5 grams / 10 minutes, and I5 values greater than 1.0 grams / 10 minutes, are values calculated based on the following relationship; 1.0 02 = 5.1 I5. Additionally, for the component polymers, the reported I2i.β values of less than 4.0, and the I10 / I2 values greater than 15, are also values calculated based on the following relationship: 4.4 I10 / I2 = 1.0 I21.6 / I10 . For the purposes of the present invention, and as an example, the following calculation is the calculation of the weight fraction to determine the calculated film density of Example of Invention 1, which comprises 80 weight percent of a polyethylene of high density that has a density of 0.942 grams / cubic centimeter, and 20 weight percent of an ultra-low density polyethylene having a density of 0.905 grams / cubic centimeter. calculated film density (in g / cc) = (0.8) (0.942 g / cc) + (0.2) (0.905 g / cc) = 0.935 g / cc. The following calculation example is the calculation of the weight fraction to determine the calculated I5 of the composition of Example of the Invention 1, which comprises 80 weight percent of a high density polyethylene having an I5 of 0.75 grams / 10 minutes, and 20 weight percent of an ultra-low density polyethylene having an I2 of 1.0 grams / 10 minutes: Ir calculated from the composition (g / 10 min.) = (0.8) (0.26 I5) + (0.2) (1.0 I2) (5.1 I5 / 1.0 I2) = 0.71 Ig. The following calculation example is the factor-based calculation to determine the I5 melt index of ultra-low density polyethylene having an I2 of 0.8 grams / 10 minutes, which was used to prepare the Composition A: I5 cal culated of the component polymer (in g / 10 min.) = (0. 8 I2) (5 .1 I5 / 1 .0 I2) = 4. 08 I $.

The following calculation example is the factor-based calculation to determine the ratio I2i.6 I? O ° ^ the ultra-low density polyethylene that has a proportion I10 / I2 of 8.7, which was used to prepare Composition A: I -i? pQ calculated from the component polymer = (8. 7 I10 / I2) (1. 0 I21.6 / I10 + 4. 4 I1Q / I2) = 1.98 I21.6 / I10. The following calculation example is the normalization calculation to determine the tear resistance of the Example of Invention 1, in 50 microns, where the tear strength is 516 grams in 54 microns: tear strength at 2 thousandths (in grams) = (516 g) (2.0 mils / 2.12 mils) = 487 grams.

Table 2 Film Compositions and Physical Properties of the Film (Continuation Table 2) Denotes the Comparative Examples only; that is, the examples are not examples of the present invention. Film Density Cal. Denotes the calculated density of the film.

Table 3 Film Compositions and Physical Properties of the Film (Continuation Table 3) Denotes the Comparative Examples only; that is, the examples are not examples of the present invention. Film Density Cal. Denotes the calculated density of the film.

Table 4 Film Compositions and Physical Properties of the Film (Continuation Table 4) Denotes the Comparative Examples only; that is, the examples are not examples of the present invention. Film Density Cal. denotes the calculated density of the film.

Table 5 Film Compositions and Physical Properties of the Film (Continuation Table 5) Denotes the Comparative Examples only; that is, the examples are not examples of the present invention. Film Density Cal. Denotes the calculated density of the film.

Table 6 Film Compositions and Physical Properties of the Film (Continuation Table 6) Denotes the Comparative Examples only; that is, the examples are not examples of the present invention. Film Density Cal. Denotes the calculated density of the film.

\ Table 7 Film Compositions and Physical Properties of the Film (Continuation Table 7) Denotes the Comparative Examples only; that is, the examples are not examples of the present invention. Film Density Cal. Denotes the calculated density of the film.

Table 8 Film Compositions and Physical Properties of the Film (Continuation Table 8) Denotes the Comparative Examples only; that is, the examples are not examples of the present invention. Film Density Cal. Denotes the calculated density of the film.

The physical property data of Tables 2 to 8 and Figures 2 to 7 show that the films prepared in accordance with the present invention exhibit substantially improved tear strengths, as compared to other films prepared with and without admixture, having the same film density and film thickness, and a similar melt index. Figure 2 specifically illustrates that the Invention Films prepared from Composition A exhibit superior tear strength at a film thickness greater than 31 microns, particularly in the scale from 37 microns to 217 microns, and especially in the scale from 50 microns to 198 microns, compared to the Comparative Films prepared from Compositions B, C and D, in equivalent densities. Figures 2 and 3 show that the process in solution and / or 1-octene is preferred over the gas and / or 1-butene and 1-hexene phase process, to produce an ethylene / α-olefin interpolymer of higher density. suitable low, Component B, to be mixed with a suitable high molecular weight linear ethylene interpolymer, Component A, to prepare the novel film of the present invention. The figures show that in a mix combination of 80/20 by weight, with a suitable Component A, a lower density ethylene / octene copolymer translated by a solution process (Films of the Invention prepared from Composition A, ie, Examples of Invention 1, 3 and 4) produces significantly better tear strengths on the scale from 37 microns to 217 microns (than Comparative Films based on a lower density ethylene / α-olefin interpolymer produced by a gas process, either with 1-butene or 1-hexene as the comonomer) (Comparative Films prepared from Compositions D, E and F, ie, Comparative Examples 13 to 24). In direct comparisons, at approximately 152 microns, the Invention Films prepared from Composition A, exhibit a tear strength as high as approximately 60 percent higher than Comparative Films based on the interpolymers produced in the gas. However, the Applicants believe that this difference is not due to the type of process itself, but reflects a higher requirement of α-olefin when a gas phase process is used to produce a suitable Component B. Figure 4 shows the Invention Films prepared from Compositions A, H and I, which are significantly and not obviously superior to Comparative Films prepared from Composition G, a high molecular weight linear ethylene interpolymer that It has a density of approximately 0.942 grams / cubic centimeter and an I5 of approximately 0.26. The superior performance of the Invention Films prepared from Composition H (Examples of Invention 26-28) is particularly not obvious, since their film densities are equivalent to the film densities of the Comparative Films prepared from of Composition G (Comparative 29 to 32). The superior performance of the Invention Films prepared from Compositions H and I, demonstrate that Component A can have a density in the range of 0.935 grams / cubic centimeter to 0.95 grams / cubic centimeter, although not necessarily limited to same Figure 5 illustrates the superior tear resistance performance of the Invention Films prepared from Compositions A, J and K, as compared to Comparative Films prepared from Compositions B and C. Figure 6 also shows that Component B can have a density on the scale of 0.901 grams / cubic centimeter to 0.912 grams / cubic centimeter, and an I2 melt index on the scale of 0.8 grams / 10 minutes to 1.0 grams / 10 minutes, although it does not need to be limited each one to it. Figure 6 shows the novel operation of the Invention Films prepared from Compositions A, L and M. Figure 6 also shows that superior performance of the novel film of the present invention can be obtained with blend combinations of Components A and B at 70 percent / 30 percent, 80 percent / 20 percent, and 85 percent / 15 percent by weight, respectively. Figure 7 shows the relationship between film thickness and tear resistance for the Invention Films prepared from Compositions A, H, I, J, K, L and M, and for Comparative Films prepared at from Compositions B, C, D, E, F and G. Figure 7 also shows the respective equations resulting from the regression analysis for each composition. A comparison of the equations belonging to the Films of the Invention and to the Comparative Films indicates the tear resistance of the novel film of the present invention, which corresponds to the following expression: tear strength (grams) = Ax + Bx ^ + C where A, B and C are numerical values, and X is the thickness of the film in thousandths; when A is less than, or equal to 150, B is greater than, or equal to approximately 12.5; and when A is greater than about 150, B is on the scale of -80 to 40. Figure 8 shows the relationship between film density and tear strength at 50 microns for the Invention Films prepared from Compositions A, H, I, J, K, L and M, as well as for the Comparative Films prepared from Compositions B, C, G and O. Figure 8 also shows the predicted average tear strength (calculated) for compositions based on a high density, high molecular weight polyethylene resin of 0.942 grams / cubic centimeter, and an ultra-low density polyethylene resin of 0.905 grams / cubic centimeter, in 100 percent mixing ratios / 0 percent, 70 percent / 30 percent, 80 percent / 20 percent, and 0 percent / 100 percent, respectively. Figure 8 indicates that although the calculated tear strengths of the high density polyethylene and high molecular weight / ultra-low density polyethylene blends are similar to the actual tear strengths of linear resins at equivalent densities, the The invention exhibits synergistically superior tear strengths at equivalent densities. Figure 9 shows the relationship between film density and tear strength for the most preferred medium modulus thick film of the present invention (ie, wherein Component A has a density greater than about 0.935 grams / cubic centimeter , and Component B has a density greater than about 0.901 grams / cubic centimeter), and the same Comparative Films and calculated films of Figure 8. Figure 9 shows the most preferred Invention Film (the films prepared from the Compositions A, H, J, L and M), which is not just synergistically superior, but is also increasingly and exponentially higher at equivalent densities of less than about 0.935 grams / cubic centimeter. Figure 9 also indicates that the novel film can be further characterized by the following equation: tear strength (grams) = (2.065 x 106) (Z) 2 - (3.8983 x 106) (Z) + 1.84015 x 106 where Z is the calculated density of the film in grams / cubic centimeter. In another evaluation, Invention Film Examples 1, 27, 28 and 34 are compared to commercial heavy-duty medium-duty films used as padded carpet wrappers, insulation shirts, hot-filled bags of salt, and bags for Meadows and gardens, Tables 9 to 11 show that these Invention Films surprisingly exhibited superior property balances at higher film densities and reduced film thicknesses, where lower densities and increased thicknesses are ordinarily required to improve properties. As an example of the significant dimensional stability and down calibration improvements, and the cost savings of the novel film of the present invention in relation to the commercial cushioned carpet wrap film (Comparative Film 56), the 9 shows the Invention Film Example 27 which exhibited superior tensile performance, ultimate tensile strength, and tear strength, with equivalent impact strength in a film thickness approximately 33 percent lower .

Table 9 Movie Properties of Invention Films and Commercial Films * Comparative Film 55 is a commercial single layer film consisting of a mixture of 60/40 by weight of linear low density polyethylene and a low density polyethylene (LDPE) sold in the bag applications segment. Heavy work for meadows and gardens. The Comparative Film 56 is a commercial single layer film consisting of low density polyethylene (LDPE) sold in the heavy-duty, cushioned carpet wrapping application segment.

Table 10 Movie Properties of Invention Films and Commercial Films * Comparative Film 57 is a commercial single layer film consisting of a blend of 20/30 by weight linear 100% low density polyethylene, and 100% low density polyethylene (LDPE) sold in the applications of heavy duty bags to pack polymer resins. The Comparative Film 58 is a commercial multilayer film consisting of a coextrusion of 25/75 by weight of a high density polyethylene modified with rubber coextruded with a mixture of 50/50 by weight of linear low density polyethylene and polyethylene of Low density (LDPE), and sold in the application segment of hot filled salt bags application.

Table 11 Movie Properties of Invention Films and Commercial Films * Comparative Films 59 and 60 are commercial multilayer films consisting of a 50/50 coextrusion of polybutylene and high density polyethylene, and are sold in the heavy duty insulation jacket application segment.

Claims (11)

1. A medium modulus polyethylene film, characterized in that it has a film thickness greater than 31 microns, and a tear strength at least 30 percent greater than the tear strength of a comparative linear ethylene polymer film having essentially the same same density, thickness, and melt index, which comprises: (A) from 60 to 95 weight percent, based on the combined weight of Components (A) and (B), of at least one linear ethylene polymer of high molecular weight having a density on the scale of 0.92 to 0.96 grams / cubic centimeter, and a melt index I5 on the scale of 0.1 to 3 grams / 10 minutes, and (B) of 5 to 40 weight percent , based on the combined weight of Components (A) and (B), of at least one linear ethylene / α-olefin interpolymer which contains at least one α-olefin monomer having more than 6 carbon atoms, and has a density on the scale of

0. 85 to 0.92 grams / cubic centimeter, and an I2 melt index on the scale of 0.3 to 3 grams / 10 minutes.

2. The film of claim 1, wherein the film thickness is in the range of 37 microns to 217 microns.

3. The film of claim 1, wherein the calculated density of the film is in the range of 0.923 grams / cubic centimeter to 0.95 grams / cubic centimeter.

The film of claim 1, wherein the linear high molecular weight ethylene polymer is an interpolymer of ethylene and at least one α-olefin selected from the group consisting of 1-propylene, 1-butene, 1- hexene, 4-methyl-1-pentene and 1-octene.

5. The film of claim 5, wherein the linear high molecular weight ethylene polymer is a copolymer of ethylene and 1-butene.

6. The film of claim 1, wherein the high molecular weight linear ethylene polymer is prepared by a particulate polymerization process.

The film of claim 1, wherein the linear ethylene / α-olefin interpolymer is a copolymer of ethylene and 1-octene.

The film of claim 1, wherein the polymer of the linear ethylene / α-olefin interpolymer is prepared by a solution polymerization process.

9. A method for the preparation of a medium modulus polyethylene film, characterized in that it has a tear strength at least 30 percent greater than the tear strength of a comparative polyethylene film having essentially the same density, thickness and thickness. melt index, which comprises the steps of: (1) providing an extrudable thermoplastic composition containing: (A) from 60 to 95 weight percent, based on the combined weight of Components (A) and (B), from at least one linear high molecular weight ethylene polymer having a density on the scale of 0.92 to 0.96 grams / cubic centimeter, and a melt index I5 on the scale of 0.1 to 3 grams / 10 minutes, and (B) from 5 to 40 weight percent, based on the combined weight of Components (A) and (B), of at least one linear ethylene / α-olefin interpolymer containing at least one α-olefin having more than 6 carbon atoms, and that it has a density in the scale of 0.85 to 0.92 grams / cubic centimeter, and a melt index I2 in the scale of 0.3 to 3 grams / 10 minutes, (2) introducing the composition of step (1) in an extrusion apparatus of heated film equipped with an annular die, (3) extruding the composition through the annular die to form a molten or semi-molten thermoplastic tube of said composition, which is subsequently blown beyond the diameter of the die, and stretched through the rollers of tightening and unwinding, to form a flat film with a thickness greater than about 31 microns, and (4) to transport the film formed in step (3) for subsequent use by going down the line of the blown film extrusion apparatus of step (2), or collecting the film formed in step (3) for subsequent off-line use.

The method of claim 9, wherein the extruding apparatus is a line of variable stem extrusion film,

11. The film produced by the method of claim 9.