Polyethylene homopolymer composition having balanced properties

Abstract

A polyethylene homopolymer composition having a weight average molecular weight Mw of 75,000 to 95,000, an Mz of 200,000 to 325,000, a molecular weight distribution Mw / Mn of 6 to 12, and a melt index I2 of 1.5 to 2.8 g / 10 min can be used to prepare a film having a good balance of optical properties and moisture resistance.

Description

[Technical field]

[0001] The polyethylene homopolymer composition can be used to prepare films that have a good balance of optical and barrier properties. [Background technology]

[0002] Polyethylene homopolymers prepared with peroxide initiators in a high pressure polymerization process are widely available commercial products commonly referred to as high pressure / low density polyethylene (or "HPLD"). Such HPLD generally contain significant amounts of long chain branching (or "LCB"). The presence of LCB reduces the density of the homopolymer polyethylene. In addition, the presence of LCB modifies the melt rheology of the polyethylene in a manner that is desirable for many manufacturing processes, particularly the production of blown films. Films made from HPLD typically exhibit very good optical properties (high gloss and low haze), but have poor "barrier" properties (i.e., these films are relatively moisture permeable). It is also generally accepted that the presence of LCB can reduce some physical properties of the polyethylene compared to "linear" polyethylene homopolymers with the same melt index.

[0003] It is also known to produce "linear" ethylene homopolymers by homopolymerization of ethylene with coordination catalysts (such as Ziegler-Natta or "Z / N" catalysts). The resulting ethylene homopolymers are essentially free of LCBs when prepared with Z / N catalysts. These linear ethylene homopolymers have sharp melting points, making them suitable for some injection molding applications. In addition, plastic films prepared from linear ethylene homopolymers have high moisture vapor resistance (or, in other words, low water vapor transmission rate or low "WVTR"). Summary of the Invention

[0004] In one embodiment, there is provided a polyethylene composition comprising at least two ethylene homopolymer blend components, said composition comprising: (i) a density of 0.96 to 0.97 g / cc; (ii) Mw between 75,000 and 90,000; (iii) Mn between 7,000 and 12,000; (iv) Mz between 200,000 and 325,000; (v) a molecular weight distribution Mw / Mn of 6 to 12; (vii) Melt index I of 1.5 to 2.8 g / 10 min 2 (Determined by ASTM D1238 at 190°C using a load of 2.16 kg), and (viii) 25 to 55% by weight of the composition has a molecular weight of 20,000 or less; the composition comprises a first blend component comprising 45-60% by weight of the composition and a second blend component comprising 55-40% by weight of the composition; The first blend component comprises: A1) Mw / Mn of 1.8 to 2.5, and A2) Melt index I: 3,000-25,000g / 10min 2 (Determined by ASTM D1238 at 190°C using a load of 2.16 kg), having The second blend component comprises: B1) Mw / Mn of 1.8 to 2.5, and B2) Melt index I: 0.05-0.5g / 10min 2 (Determined by ASTM D1238 at 190°C using a load of 2.16 kg), has.

[0005] In one embodiment, there is provided a polyethylene composition comprising at least two ethylene homopolymer blend components, said composition comprising: (i) a density of 0.96 to 0.97 g / cc; (ii) Mw between 75,000 and 90,000; (iii) Mn between 7,000 and 12,000; (iv) Mz between 200,000 and 325,000; (v) a molecular weight distribution Mw / Mn of 6 to 12; (vii) Melt index I of 1.5 to 2.8 g / 10 min 2 (Determined by ASTM D1238 at 190°C using a load of 2.16 kg), and (viii) 25 to 55% by weight of the composition has a molecular weight of 20,000 or less; the composition comprises a first blend component comprising 45-60% by weight of the composition and a second blend component comprising 55-40% by weight of the composition; The first blend component comprises: A1) Mw / Mn of 1.8 to 2.5, and A2) Melt index I: 3,000-8,500g / 10min 2 (Determined by ASTM D1238 at 190°C using a load of 2.16 kg), having The second blend component comprises: B1) Mw / Mn of 1.8 to 2.5, and B2) Melt index I: 0.05-0.5g / 10min 2 (Determined by ASTM D1238 at 190°C using a load of 2.16 kg), has.

[0006] In one embodiment, the polyethylene composition has an Mz less than, or equal to, 300,000.

[0007] In one embodiment, the polyethylene composition has an Mn of less than or equal to 10,000.

[0008] In one embodiment, the polyethylene composition comprises a nucleating agent.

[0009] In one embodiment, the polyethylene composition comprises a nucleating agent comprising a calcium salt of hexahydrophthalic acid.

[0010] In one embodiment, films prepared from the polyethylene composition offer a good balance of optical properties (good gloss and low haze) and barrier properties (low water vapor transmission rate, WVTR).

[0011] One embodiment is a film made from the polyethylene composition having a haze of less than 30%.

[0012] In one embodiment, there is provided a method for preparing a polyethylene composition comprising at least two ethylene homopolymer blend components, the method comprising the steps of contacting at least one single-site polymerization catalyst system with ethylene under polymerization conditions for the ethylene in at least two polymerization reactors comprising a first polymerization reactor and a second polymerization reactor, and blending together the polyethylene produced in each polymerization reactor, wherein hydrogen is added to the first polymerization reactor to provide a concentration of from 0.5 to 2 ppm in the first polymerization reactor and hydrogen is added to the second polymerization reactor to provide a concentration of from 35 to 55 ppm in the second polymerization reactor. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The polyethylene homopolymer compositions of the present disclosure are made from at least two ethylene "homopolymer" blend components. As used herein, the term "homopolymer" is meant to convey its conventional meaning, i.e., that the polymer is prepared from substantially only ethylene monomer (although those skilled in the art will recognize that very small amounts (less than 1%) of higher alpha olefins may be present in conventional "homopolymers" as a result of contamination of the ethylene stream and / or polymerization medium).

[0014] In one embodiment, the composition has a very high melt index ("I 2 The blend comprises at least one homopolymer blend component having a copolymerizable functional group (also referred to as a "copolymer copolymer").

[0015] In one embodiment, the composition has a very high melt index ("I 2 The blend comprises at least one homopolymer blend component having a copolymerizable functional group (also referred to as a "copolymer copolymer").

[0016] In one embodiment, the overall polyethylene composition comprises: (i) a density of 0.96 to 0.97 g / cc; (ii) Mw between 75,000 and 95,000; (iii) an Mn of 12,000 or less (or in another embodiment, 10,000 or less); (iv) Mz of 200,000 to 325,000 (or in another embodiment, 300,000 or less); (v) Mw / Mn between 6 and 12; (vi) 1.5-2.8 g / 10 min I 2 (determined by ASTM D1238 at 190°C using a load of 2.16 kg), and (vii) a molecular weight distribution such that 25-55% by weight (or in other embodiments, greater than 30% by weight) of said composition has a molecular weight less than 20,000 (i.e., if the molecular weight distribution of a homopolymer composition were plotted as a conventional "molecular weight versus mass fraction" plot, the cumulative weight percentage of the compositions of the present invention would be such that at least 25% by weight and up to 55% by weight have a molecular weight less than 20,000); has.

[0017] In one embodiment, the overall polyethylene composition comprises: (i) a density of 0.96 to 0.97 g / cc; (ii) Mw between 75,000 and 95,000; (iii) an Mn of 12,000 or less (or in another embodiment, 10,000 or less); (iv) Mz of 200,000 to 325,000 (or in another embodiment, 300,000 or less); (v) Mw / Mn between 6 and 12; (vi) 1.5-2.8 g / 10 min I 2(determined by ASTM D1238 at 190°C using a load of 2.16 kg), and (vii) a molecular weight distribution such that 25-45% by weight (or in other embodiments, greater than 30% by weight) of said composition has a molecular weight less than 20,000 (i.e., if the molecular weight distribution of a homopolymer composition were plotted as a conventional "molecular weight versus mass fraction" plot, the cumulative weight percentage of the compositions of the present invention would be such that at least 25% by weight and up to 45% by weight have a molecular weight less than 20,000); has.

[0018] In one embodiment, the composition is essentially free of polyethylene having a molecular weight greater than 900,000 (more preferably, less than 1% by weight has a molecular weight greater than 900,000).

[0019] In one embodiment, the polyethylene composition has a number average molecular weight Mn of 7,000 to 10,000.

[0020] In one embodiment, the polyethylene composition has a Z-average molecular weight Mw from 200,000 to 300,000.

[0021] In one embodiment, at least 25% by weight, or at least 30% by weight, or at least 35% by weight, or at least 40% by weight of the polyethylene composition has a molecular weight less than 20,000.

[0022] In one embodiment, at least 99% by weight of the polyethylene composition has a molecular weight less than 900,000.

[0023] Both homopolymer blend components often also have narrow molecular weight distributions (Mw / Mn less than or equal to 2.5).

[0024] In one embodiment of the present disclosure, the polyethylene composition comprises at least two ethylene homopolymer blend components: a first blend component comprising 45-60 wt.% of the polyethylene composition, and a second blend component comprising 55-40 wt.% of the polyethylene composition.

[0025] In one embodiment, the first blend component has a lower weight average molecular weight, Mw, than the second blend component.

[0026] In one embodiment, the first blend component has a higher melt index I than the second blend component. 2 (As determined by ASTM D1238 at 190°C using a load of 2.16 kg).

[0027] In one embodiment of the present disclosure, the first blend component has a Mw / Mn of 1.8 to 2.5.

[0028] In one embodiment of the present disclosure, the first blend component has a melt index I of 3,000 to 25,000 g / 10 min, or 3,000 to 20,000 g / 10 min, or 3,000 to 15,000 g / 10 min, or 3,000 to 10,000 g / 10 min, or 3,000 to 8500 g / 10 min. 2 (As determined by ASTM D1238 at 190°C using a load of 2.16 kg).

[0029] In one embodiment of the present disclosure, the second blend component has a Mw / Mn of 1.8 to 2.5.

[0030] In one embodiment of the present disclosure, the second blend component has a melt index I of 0.05 to 0.5 g / 10 min. 2 (As determined by ASTM D1238 at 190°C using a load of 2.16 kg).

[0031] As illustrated in the examples, the compositions can be used to prepare films that have a good balance of optical and barrier properties.

[0032] Without wishing to be bound by theory, it is believed that: 1) The good optical properties of films made from these compositions are a result of Mz being controlled to values ​​of 325,000 or less (especially 300,000 or less). 2) The excellent barrier properties of the film (low WVTR) are the result of a combination of: (i) a low molecular weight of the first blend component (i.e., at least 25 wt.% of the blend has an absolute molecular weight less than 20,000); a low Mn of the composition (≦12,000, especially ≦10,000); (ii) both blend components have a molecular weight distribution (Mw / Mn) of ≦2.5; and (iii) a large difference in the molecular weights of the blend components, resulting in a molecular weight distribution of the composition between 6 and 12.

[0033] The homopolymer compositions of the present disclosure can generally be used to prepare a wide variety of molded or extruded articles. However, they are often used in films, particularly blown films. Certain plastic films produced with the present compositions have a desirable balance of optical and barrier properties (or, in other words, low WVTR). Such films may be monolayer or multilayer. In multilayer films, the compositions may be suitably used in at least one "skin layer" or in the "core layer" of a three-, five-, or seven-layer film. In one embodiment, the composition is used in the core layer of a multilayer film, and a conventional HDPE resin (such as one prepared with a Ziegler-Natta catalyst or a chromium catalyst) is used in the skin layer of the multilayer film. In one embodiment, the multilayer film may also include a core layer made of an ethylene-vinyl alcohol (EVOH) copolymer to further improve the barrier properties.

[0034] The composition can be prepared by any conventional blending technique or by homopolymerization of ethylene in a multi-reactor polymerization system. When the composition is produced by a polymerization process, a catalyst capable of producing polyethylene having a molecular weight distribution Mw / Mn of 2.5 or less is used in all polymerization reactors. The use of a dual reactor solution polymerization process using a "single-site catalyst" is suitable and is illustrated in the examples.

[0035] Multi-reactor polymerization systems are well known to those skilled in the art. A description of a dual reactor solution polymerization system that can be used in the present disclosure is given in U.S. Patent No. 6,372,864 (Brown).

[0036] The term "single-site catalyst" is also well known to those skilled in the art and is used herein to convey its conventional meaning. In general, single-site catalysts copolymerize ethylene with higher alpha olefins (such as butene, hexene, or octene) in a well-mixed polymerization reactor in a manner that produces ethylene homopolymers having narrow molecular weight distributions (i.e., Mw / Mn of 2.5 or less) and a regular distribution of the comonomer within the copolymer.

[0037] Examples of single-site catalyst systems include the following catalysts: metallocenes, constrained geometry catalysts, or phosphinimine catalysts when used in combination with methylaluminoxane ("MAO") cocatalysts or boron activators (such as trityl or anilinium salts of tetrakis(pentafluorophenyl)boron).

[0038] Further description of single-site catalysts is provided in US Pat. No. 6,689,847 (including references therein).

[0039] Additional examples of "linear" ethylene homopolymers produced with single-site catalysts (such as "metallocene" or "constrained geometry" catalysts) are suggested in U.S. Pat. No. 6,419,966 (Davis). Similarly, a review of linear ethylene homopolymers prepared with "constrained geometry" catalysts is suggested in 2003 / 0088021A1 (Van Dun '03). Additionally, the Van Dun disclosure provides a review of resin blends that include ethylene homopolymer blend components having a molecular weight distribution greater than 2.5 (i.e., blend components not prepared with a single-site catalyst). Van Dun specifically teaches that typical single-site catalyst-derived homopolymers suffer from two shortcomings: (a) they cannot achieve as high a density for a given molecular weight as comparable Ziegler products, and (b) they exhibit narrow Mw / Mn over the entire molecular weight range.

[0040] It is noted that catalysts that do not readily produce long chain branching ("LCB") are generally used in the present disclosure because the presence of LCB can reduce the density and / or stiffness (as evidenced by a lower elastic modulus) of films made from the polyethylene homopolymer compositions of the present disclosure.

[0041] LCB is 13 C Nuclear Magnetic Resonance (NMR) and quantified using the method defined by Randall (Rev. Macromol. Chem. Phys. C29(2 and 3) pp. 285-297). In one embodiment, the compositions of the present disclosure contain less than 0.3 long chain branches per 100 carbon atoms, and more particularly less than 0.1 long chain branches per 1,000 carbon atoms.

[0042] The use of a single-site phosphinimine catalyst system in a dual reactor polymerization process to prepare compositions according to the present disclosure is described in the following non-limiting examples.

[0043] <Additives> HDPE is often sold with additive packages that include primary antioxidants (Part 1 below) and secondary antioxidants (Part 3 below). Primary antioxidants can be used in amounts between 200 and 2,000 ppm. Similarly, secondary antioxidants can be used in amounts between 200 and 2,000 ppm. Other (optional) additives are also described below.

[0044] 1. <Primary antioxidant> 1.1 <Alkylated monophenols> For example, 2,6-di-tert-butyl-4-methylphenol; 2-tert-butyl-4,6-dimethylphenol; 2,6-di-tert-butyl-4-ethylphenol; 2,6-di-tert-butyl-4-n-butylphenol; 2,6-di-tert-butyl-4-isobutylphenol; 2,6-dicyclopentyl-4-methylphenol; 2-(α-methylcyclohexyl)-4,6-dimethylphenol; 2,6-di-octadecyl-4-methylphenol; 2,4,6-tricyclohexylphenol; and 2,6-di-tert-butyl-4-methoxymethylphenol.

[0045] 1.2 <Alkylhydroquinone> For example, 2,6-di-tert-butyl-4-methoxyphenol; 2,5-di-tert-butylhydroquinone; 2,5-di-tert-amyl-hydroquinone; and 2,6-diphenyl-4-octadecyloxyphenol.

[0046] 1.3 <Hydroxylated thiodiphenyl ether> For example, 2,2'-thio-bis-(6-tert-butyl-4-methylphenol); 2,2'-thio-bis-(4-octylphenol); 4,4'-thio-bis-(6-tert-butyl-3-methylphenol); and 4,4'-thio-bis-(6-tert-butyl-2-methylphenol).

[0047] 1.4 <Alkylidene bisphenol> For example, 2,2'-methylene-bis-(6-tert-butyl-4-methylphenol); 2,2'-methylene-bis-(6-tert-butyl-4-ethylphenol); 2,2'-methylene-bis-(4-methyl-6-(α-methylcyclohexyl)phenol); 2,2'-methylene-bis-(4-methyl-6-cyclohexylphenol); 2,2'-methylene-bis-(6-nonyl-4-methylphenol); 2,2'-methylene-bis-(6-nonyl-4-methylphenol) ;2,2'-methylene-bis-(6-(α-methylbenzyl)-4-nonylphenol);2,2'-methylene-bis-(6-(α,α-dimethylbenzyl)-4-nonyl-phenol);2,2'-methylene-bis-(4,6-di-tert-butylphenol);2,2'-ethylidene-bis-(6-tert-butyl-4-isobutylphenol);4,4'-methylene-bis-(2,6-di-tert-butylphenol);4,4'-methylene-bis-(6-tert-butyl- 2-Methylphenol;1,1-bis-(5-tert-butyl-4-hydroxy-2-methylphenol)butane;2,6-di-(3-tert-butyl-5-methyl-2-hydroxybenzyl)-4-methylphenol;1,1,3-tris-(5-tert-butyl-4-hydroxy-2-methylphenyl)butane;1,1-bis-(5-tert-butyl-4-hydroxy-2-methylphenyl)-3-dodecyl-mercaptobutane;Ethylene glycol-bis-(3,3,-bi di-(2-(3'-tert-butyl-2'hydroxy-5'methylbenzyl)-6-tert-butyl-4-methylphenyl)-terephthalate; and other phenols such as monoacrylate esters of bisphenols such as ethylidenebis-2,4-di-t-butylphenol monoacrylate.

[0048] 2. <UV absorbers and light stabilizers> 2.1 <2-(2'-hydroxyphenyl)-benzotriazole> For example, 5'-methyl-, 3', 5'-di-tert-butyl-, 5'-tert-butyl-, 5'(1,1,3,3-tetramethylbutyl)-, 5-chloro-3', 5'-di-tert-butyl-, 5-chloro-3'-tert-butyl-5'-methyl-3'-sec-butyl-5'-tert-butyl-, 4'-octoxy, 3', 5'-di-tert-amyl-3', 5'-bis-(α,α-dimethylbenzyl)-derivatives.

[0049] 2.2 <2-Hydroxy-benzophenone> For example, 4-hydroxy-4-methoxy-, 4-octoxy-, 4-decyloxy-, 4-dodecyloxy-, 4-benzyloxy, 4,2',4'-trihydroxy- and 2'-hydroxy-4,4'-dimethoxy derivatives.

[0050] 2.3 <Sterically hindered amines> For example, bis(2,2,6,6-tetramethylpiperidyl)-sebacate; bis-5(1,2,2,6,6-pentamethylpiperidyl)-sebacate; n-butyl-3,5-di-tert-butyl-4-hydroxybenzylmalonic acid bis(1,2,2,6,6-pentamethylpiperidyl)ester; condensation product of 1-hydroxyethyl-2,2,6,6-tetramethyl-4-hydroxy-piperidine with succinic acid; N,N'-(2,2,6,6-tetramethylpiperidyl)-sebacate; tetramethylpiperidyl)-hexamethylenediamine and condensation product of 4-tert-octylamino-2,6-dichloro-1,3,5-s-triazine; tris-(2,2,6,6-tetramethylpiperidyl)-nitrilotriacetate, tetrakis-(2,2,6,6-tetramethyl-4-piperidyl)-1,2,3,4-butane-tetracarbonic acid; and 1,1'(1,2-ethanediyl)-bis-(3,3,5,5-tetramethylpiperazinone). These amines are typically referred to as HALS (Hindered Amines Light Stabilizing) and include butanetetracarboxylic acid 2,2,6,6-tetramethylpiperidinol ester. Such amines include hydroxylamines derived from hindered amines. For example, di(1-hydroxy-2,2,6,6-tetramethylpiperidin-4-yl)sebacate; 1-hydroxy-2,2,6,6-tetramethyl-4-benzoxypiperidine; 1-hydroxy-2,2,6,6-tetramethyl-4-(3,5-di-tert-butyl-4-hydroxyhydrocinnamoyloxy)-piperidine; and N-(1-hydroxy-2,2,6,6-tetramethyl-piperidin-4-yl)-ε-caprolactam.

[0051] 3. <Secondary antioxidant> 3.1 <Phosphites and phosphonites> For example, triphenyl phosphite; diphenyl alkyl phosphate; phenyl dialkyl phosphate; tris(nonylphenyl) phosphite; trilauryl phosphite; trioctadecyl phosphite; distearyl pentaerythritol diphosphite; tris(2,4-di-tert-butylphenyl) phosphite; diisodecyl pentaerythritol diphosphite; 2,4,6-tri-tert-butylphenyl-2-butyl-2-ethyl-1,3-propanediol phosphite; bis(2,4-di-tert-butylphenyl) pentaerythritol diphosphite tristearyl sorbitol triphosphite; and tetrakis(2,4-di-tert-butylphenyl) 4,4'-biphenylene diphosphonite.

[0052] 3.2 <Hydroxylamine and amine oxide> For example, N,N-dibenzylhydroxylamine, N,N-diethylhydroxylamine, N,N-dioctylhydroxylamine, N,N-dilaurylhydroxylamine, N,N-ditetradecylhydroxylamine, N,N-dihexadecylhydroxylamine, N,N-dioctadecylhydroxylamine, N-hexadecyl-N-octadecylhydroxylamine, N-heptadecyl-N-octadecylhydroxylamine, and N,N-dialkylhydroxylamines derived from hydrogenated tallow amine. Similar amine oxides are also suitable.

[0053] 4. Slip agents For example, oleamide; erucamide; stearamide; behenamide.

[0054] 5. <Fillers, antiblocks, and reinforcing agents> For example, calcium carbonate; diatomaceous earth; natural and synthetic silica; silicates; glass fibers; asbestos; talc; kaolin; mica; barium sulfate; metal oxides and hydroxides; carbon black; and graphite.

[0055] 6. <Other additives> For example, plasticizers; epoxidized vegetable oils such as epoxidized soybean oil; lubricants; emulsifiers; pigments; optical brighteners; flame retardants; antistatic agents; antifog agents; foaming agents; and thiosynergists such as dilauryl thiodipropionate or distearyl thiodipropionate.

[0056] 7. <Nucleating Agent> In one embodiment, the composition of the present disclosure is used to prepare a film that includes a nucleating agent.Examples of suitable nucleating agents include: the cyclic organic structure (and its salt, such as disodium bicyclo[2.2.1]heptene dicarboxylate) disclosed in US Patent No. 5,981,636; the saturated version of the structure disclosed in US Patent No. 5,981,636 (disclosed in US Patent No. 6,465,551 (Zhao et al., to Milliken)); the salt of certain cyclic dicarboxylic acids with hexahydrophthalic acid structure (or "HHPA" structure) disclosed in US Patent No. 6,599,971 (Dotson et al., to Milliken); zinc glycerolate; and phosphate esters, such as those disclosed in US Patent No. 5,342,868 and sold by Asahi Denka Kogyo under the trade names NA-11 and NA-21. In one embodiment, the nucleating agent is a cyclic dicarboxylate or salt thereof, particularly a divalent metal or metalloid salt (particularly a calcium salt) of the HHPA structure disclosed in U.S. Pat. No. 6,599,971. For clarity, the HHPA structure generally includes a ring structure having six carbon atoms in the ring and two carboxylic acid groups that are substituents on adjacent atoms of the ring structure. The other four carbon atoms in the ring may be substituted, as disclosed in U.S. Pat. No. 6,599,971. In one embodiment, the nucleating agent is 1,2-cyclohexanedicarboxylic acid, calcium salt (CAS Registry No. 491589-22-1), and in one embodiment, the nucleating agent is added in an amount of 200 to 2,000 ppm (particularly 500 to 1,500 ppm) based on the weight of the HDPE.

[0057] In the examples, the physical properties of the polymers were measured using the methods described below.

[0058] Melt Index I 2 The measurement was performed according to ASTM D-1238 ("I 2 " was performed using a load of 2.16 kg at 190°C, and "I 21 " used a load of 21 kg at 190°C).

[0059] Density (g / cc) was measured according to ASTM D792. Film density was measured according to ASTM D1505.

[0060] The haze of the films was measured according to ASTM D1003.

[0061] The gloss of the films was measured according to ASTM D2457.

[0062] Mn, Mw and Mz (g / mol) were determined by gel permeation chromatography and measured according to ASTM D6474-99.

[0063] If the blend components are made as separate polymers, the above methods can be used to directly measure the properties of the blend components and the overall composition, such as melt index, density, Mn, Mw, and Mw / Mn. However, if the composition is made in a multi-reactor process that produces an in-situ blend, curve fitting techniques can be used to deconvolute / estimate the properties and amounts of the blend components in the overall composition. Generally, this type of deconvolution is done by starting with a GPC chromatogram / curve of the whole resin and then using multiple Flory distributions to derive a "best fit" of the chromatogram / curve. This deconvolution technique is well known to those skilled in the art.

[0064] Techniques for estimating the melt index of low molecular weight blend components are described below. Those skilled in the art will appreciate that melt index (I 2 ) is inversely proportional to the molecular weight of the polyethylene resin. 2A homopolymer HDPE resin with a narrow molecular weight distribution (<3) was identified by constructing a plot of log(weight average molecular weight, Mw) versus log(melt index, I 2 The homopolymer HDPE resins had narrow molecular weight distributions (less than 3) but different Mw's ranging from about 30,000 to 150,000. (Those skilled in the art will appreciate that reproducible I values ​​may be obtained for polyethylene resins having molecular weights outside this range.) 2 values ​​are difficult to obtain).

[0065] These I 2 Using a log / log plot of I vs. Mw, the I of such homopolymer HDPE resins was 2 The following relationship between and Mw was calculated: I 2 =(1.774×10 19 )×(Mw -3.86 ).

[0066] Using extrapolation (based on the above relationships), the I of the low molecular weight fraction of the compositions of the invention shown in the examples 2 The Mw value was used to estimate the I 2 The values ​​were estimated.

[0067] Water Vapor Transmission Rate ("WVT"), grams of water vapor per 100 square inches of film per day for a given film thickness (in mils), or g / 100in 2 / day) was measured at 100°F (37.8°C) and 100% relative humidity in accordance with ASTM F1249-90 using a MOCON permatron developed by Modern Controls Inc. Oxygen Transmission Rate (OTR, cubic centimeters of oxygen per 100 square inches of film per day at a specified film thickness (mils), or cc / 100in 2 / day) was measured using an instrument sold under the brand name MOCON OXTRA System, model 2 / 21T.

[0068] The polymerization conditions and polymer properties are shown in Table 1. EXAMPLES

[0069] <Polymerization> In these examples, a dual reactor solution polymerization process was used in which the contents of the first reactor flowed into the second reactor. The use of two reactors results in an "in-situ" polymer blend. Both reactors were well stirred to provide well-mixed conditions. The volume of the first reactor was 12 liters and the volume of the second reactor was 24 liters. The first reactor was pressurized at about 13,000 kPa (about 2.0×10 3 The first reactor was operated at a reactor pressure of 100 psi. The second reactor was at a low enough pressure to facilitate continuous flow from the first reactor to the second reactor. The solvent used was methylpentane. The process is continuous in all feed streams.

[0070] The catalyst used in all experiments was a titanium(IV) complex with one cyclopentadienyl ("Cp") ligand, two chloride ligands, and one tri(tert-butyl)phosphinimine ligand, i.e., CpTi(NP( t Bu) 3 )Cl 2 The catalyst concentrations added to each reactor, expressed as parts per million (ppm) of Ti (based on the total weight of the reactor contents), are shown in Table 1.

[0071] A boron cocatalyst (i.e., triphenylcarbenium or "trityl" salt of tetrakispentafluorophenyl boron) was used in approximately stoichiometric amounts (based on titanium content in the catalyst). Commercially available methylaluminoxane ("MMAO7", Akzo Nobel) was also included at an Al / Ti ratio of about 40 / 1. 2,6-di-tert-butylhydroxy-4-ethylbenzene was added to the MAO to scavenge free trimethylaluminum (TMA) in the MAO (at an Al / OH ratio of about 0.5 to 1, based on Al content in the TMA).

[0072] Hydrogen is added to the reactors as a chain transfer agent, in one embodiment, 0.5 to 2 parts per million (ppm) by weight (based on the total weight of the reactor contents) of hydrogen is added to the first reactor and 35 to 55 ppm of hydrogen is added to the second reactor.

[0073] The term "ethylene split" refers to the percentage of total ethylene that is directed to the reactor.

[0074] Other polymerization conditions and some properties of the resulting high density polyethylene ("HDPE") composition are shown in Table 1.

[0075] [Table 1]

[0076] <Part C.1: Blown Film> Films were prepared on a blown film line (Gloucester Engineering Corporation, Gloucester, Massachusetts) using the HDPE composition shown in Table 1. The composition contained 500 parts per million (ppm) of a primary antioxidant (a hindered phenol sold under the trademark IRGANOX® 1076), 500 ppm of a secondary antioxidant (a phosphite sold under the trademark IRGAFHOS® 168), and a nucleating agent. The nucleating agent was purchased from Milliken (reported to be a combination of a) a calcium salt of HHPA and b) zinc stearate in a 2 / 1 weight ratio) and was added in an amount of 1200 ppm. The blown film line was fitted with a single screw extruder with a 2.5 inch (6.35 cm) diameter screw, a length / diameter screw ratio of 24:1, and a 4 inch (10.16 cm) diameter annular die. The die gap was set at 35 mils. A conventional temperature profile was used for extrusion of HDPE. Dual air rings were used for film cooling. Films were prepared using a film thickness of 1.5 mils and a blow-up ratio (BUR) of 2:1.

[0077] The film properties are shown in the table.

[0078] [Table 2]

[0079] The WVTR values ​​shown in Table 2 were measured on a 1.5 mil thick film (i.e., the values ​​are not "normalized" to what would be expected for a 1 mil thick film). HDPE-1 MI is a comparative example - this HDPE has a melt index of 1.2 g / 10 min, which is too low for the compositions of this disclosure. HDPE-1 MI is a commercially successful product in the preparation of barrier films used to package dry foods.

[0080] [Table 3]

[0081] <Optical properties considerations> Again, the values ​​obtained are reported "as measured" (i.e., not "normalized") for a 1.5 mil thick film, and HDPE-1 is a comparative example made with a commercially available HDPE having a melt index of 1.2 g / 10 min, which is too low for the compositions of the present disclosure. Inventive Example Films 1-5 were all shown to exhibit better "barrier" properties than films made with HDPE-1 (see Table 2). The data in Table 3 show that Inventive Example Films 1-4 exhibit better optical properties than the comparative film made from HDPE-1. Inventive Example Films 1-4 also exhibit better optical properties than the two comparative films shown at the bottom of Table 3, which is interesting because Comparatives 1-2 were also made with HDPE resin having a melt index of about 2 (as required for the compositions of the present disclosure). Inventive Example Films 1-3 have particularly good optical properties. Example compositions 1-3 all have an Mn of less than 10,000; an Mz of less than 300,000 and an Mw of less than 90,000.

[0082] <Preparation of multilayer films> In this example, a multilayer film with three layers (A / B / C structure) was prepared. The skin layers (A and C) constituted 25 wt% (each) of the total structure and were made of conventional HDPE (prepared with a Ziegler-Natta catalyst in a conventional solution polymerization reactor, with a melt index I of approximately 1 g / 10 min). 2 and a density of about 0.95 g / cc. The core layer (50% by weight of the total structure) was made with Inventive Example Composition 1 or 2 from Example 1. The barrier properties of the films are shown in Table 4.

[0083] [Table 4]

[0084] [Table 5]

[0085] <Considerations on modulus of elasticity> It is desirable for HDPE films to have relatively high stiffness (as evidenced by having relatively high modulus values). The compositions of the present disclosure have a melt index higher than that of HDPE-1, and a higher melt index typically results in a lower stiffness / modulus. However, the data in Table 5 show that the compositions of the present disclosure can also be used to prepare films with desirable stiffness / modulus. Secant modulus was generally measured according to ASTM D882.

[0086] Non-limiting embodiments of the present disclosure include the following.

[0087] Embodiment A. A polyethylene composition comprising at least two ethylene homopolymer blend components, the composition comprising: (i) a density of 0.96 to 0.97 g / cc; (ii) Mw between 75,000 and 95,000; (iii) Mn between 7,000 and 12,000; (iv) Mz between 200,000 and 325,000; (v) a molecular weight distribution Mw / Mn of 6 to 12; (vii) Melt index I of 1.5 to 2.8 g / 10 min 2 (Determined by ASTM D1238 at 190°C using a load of 2.16 kg), and (viii) 25 to 55% by weight of the composition has a molecular weight of 20,000 or less; the composition comprises a first blend component comprising 45-60% by weight of the composition and a second blend component comprising 55-40% by weight of the composition; The first blend component comprises: A1) Mw / Mn of 1.8 to 2.5, and A2) Melt index I: 3,000-25,000g / 10min 2 (Determined by ASTM D1238 at 190°C using a load of 2.16 kg), having The second blend component comprises: B1) Mw / Mn of 1.8 to 2.5, and B2) Melt index I: 0.05-0.5g / 10min 2 (Determined by ASTM D1238 at 190°C using a load of 2.16 kg), having The polyethylene composition.

[0088] Embodiment B. The polyethylene composition according to embodiment A, wherein the Mn is 7,000 to 10,000.

[0089] Embodiment C. The polyethylene composition according to embodiment A or B, wherein the Mz is 200,000 to 300,000.

[0090] Embodiment D. The first blend component has a melt index I of 3,000 to 8,500 g / 10 min. 2 The polyethylene composition of embodiment A, B or C, having a thermal expansion coefficient of 1:1 (determined by ASTM D1238 at 190° C. using a load of 2.16 kg).

[0091] Embodiment E. The polyethylene composition of embodiment A, B, C or D, wherein at least 40% by weight of the composition has a molecular weight less than 20,000.

[0092] Embodiment F. The polyethylene composition of embodiment A, B, C, D or E, wherein at least 99% by weight of the composition has a molecular weight of 900,000 or less.

[0093] Embodiment G. The polyethylene composition of embodiment A, B, C, D, E or F comprising a nucleating agent.

[0094] Embodiment H. The polyethylene composition of embodiment G, wherein the nucleating agent comprises a salt of a dicarboxylic acid.

[0095] Embodiment I. The polyethylene composition of embodiment G, wherein the nucleating agent comprises a calcium salt of hexahydrophthalic acid.

[0096] Embodiment J. A film made from the polyethylene composition of embodiment A, B, C, D, E, F, G, H or I.

[0097] Embodiment K. A film made from the polyethylene composition of embodiment A, B, C, D, E, F, G, H or I having a haze value of less than 30%.

[0098] Embodiment L. A method for preparing the polyethylene composition of embodiment A, comprising: contacting at least one single-site polymerization catalyst system with ethylene under polymerization conditions for said ethylene in at least two polymerization reactors, the polymerization reactor comprising a first polymerization reactor and a second polymerization reactor; blending together the polyethylene produced in each polymerization reactor; The above process, wherein hydrogen is added to the first polymerization reactor to provide a concentration of 0.5 to 2 ppm in the first polymerization reactor, and hydrogen is added to the second polymerization reactor to provide a concentration of 35 to 55 ppm in the second polymerization reactor.

[0099] Embodiment M. The method of embodiment L, wherein the polymerization conditions are solution polymerization conditions.

[0100] Embodiment N. The method of embodiment M, wherein the at least two polymerization reactors are operated such that there is a temperature difference of at least 30° C. between the at least two polymerization reactors. [Industrial Applicability]

[0101] Provided is a polyethylene homopolymer composition that can be used to prepare polymeric films having a good balance of optical properties and moisture resistance.

Claims

1. 1. A polyethylene composition comprising at least two ethylene homopolymer blend components, said composition comprising: (i) a density of 0.96 to 0.97 g / cc; (ii) Mw from 75,000 to 95,000; (iii) Mn of 7,000 to 12,000; (iv) Mz of 200,000 to 325,000; (v) a molecular weight distribution Mw / Mn of 6 to 12; (vii) a melt index I of 1.5 to 2.8 g / 10 min. ２ (Determined by ASTM D1238 at 190° C. using a load of 2.16 kg); and (viii) 25 to 55 weight percent of said composition is characterized by having an absolute molecular weight of less than or equal to 20,000 as determined by gel permeation chromatography and measured in accordance with ASTM D6474-99; the composition comprises a first blend component comprising 45-60% by weight of the composition, and a second blend component comprising 55-40% by weight of the composition; The first blend component comprises: A1) Mw / Mn of 1.8 to 2.5, and A2) Melt index I of 3,000 to 25,000 g / 10 min ２ (Determined by ASTM D1238 at 190° C. using a load of 2.16 kg); having The second blend component comprises: B1) Mw / Mn of 1.8 to 2.5, and B2) Melt index I of 0.05 to 0.5 g / 10 min ２ (Determined by ASTM D1238 at 190° C. using a load of 2.16 kg); having The polyethylene composition.

2. The polyethylene composition according to claim 1, wherein the Mn is from 7,000 to 10,000.

3. The polyethylene composition according to claim 2, wherein the Mz is from 200,000 to 300,000.

4. The first blend component has a melt index I of 3,000 to 8,500 g / 10 min. ２ 2. The polyethylene composition of claim 1, having a modulus of elasticity (determined by ASTM D1238 at 190° C. using a load of 2.16 kg).

5. 2. The polyethylene composition of claim 1, wherein at least 40% by weight of the composition has an absolute molecular weight of less than 20,000.

6. 2. The polyethylene composition of claim 1, wherein at least 99 weight percent of the composition has an absolute molecular weight less than or equal to 900,000.

7. The polyethylene composition according to any one of claims 1 to 6, comprising a nucleating agent.

8. The polyethylene composition of claim 7, wherein the nucleating agent comprises a salt of a dicarboxylic acid.

9. 8. The polyethylene composition of claim 7, wherein the nucleating agent comprises a calcium salt of hexahydrophthalic acid.

10. A film made from the polyethylene composition according to any one of claims 1 to 9.

11. 2. A process for preparing the polyethylene composition of claim 1, comprising the steps of: The method includes the steps of contacting at least one single-site polymerization catalyst system with ethylene under polymerization conditions for the ethylene in at least two polymerization reactors, including a first polymerization reactor and a second polymerization reactor; blending together the polyethylene produced in each polymerization reactor; Hydrogen is added to the first polymerization reactor to provide a concentration of 0.5 to 2 ppm in the first polymerization reactor, and hydrogen is added to the second polymerization reactor to provide a concentration of 35 to 55 ppm in the second polymerization reactor. The above method.

12. The method of claim 11 , wherein the polymerization conditions are solution polymerization conditions.

13. 13. The method of claim 12, wherein the at least two polymerization reactors are operated such that there is a temperature difference of at least 30°C between the at least two polymerization reactors.

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