Polyethylene compositions for injection molding

FIELD OF THE INVENTION

0001This invention relates to thermoplastic compositions of polyethylene polymers suitable for fabrication into useful products by injection molding.

BACKGROUND

0002Injection molding is the most important process for producing moldings from thermoplastics. This significance is due to the ability of injection molding to manufacture complex molding geometries in a single stage with high levels of reproducibility. Plastics finishing is largely unnecessary and a high degree of automation is possible. All manner of consumer goods and commodity articles are manufactured using injection molding of polyethylene thermoplastics.

0003To injection mold a part, polyethylene thermoplastic pellets, granules or powders are melted and injected under pressure into the cavity of a mold where the melted resin is solidified by cooling for subsequent removal. More detailed discussion of injection molding may be found in <nplcit id="ncit0001" npl-type="b"><text>Ullman's Encyclopedia of Industrial Chemistry, vol. A20, Plastics Processing, pages 688-696 (VCH Publishers, 1992</text></nplcit>).

0004Blends of polyethylene resins have been proposed to improve physical properties, including impact strength, environmental stress crack resistance (ESCR), and chemical resistance.

0005<patcit id="pcit0001" dnum="US4438238A"><text>U.S. Patent No. 4,438,238</text></patcit> describes blends for extrusion processing, injection molding and films, where a combination of two ethylene-α-olefin copolymers with different densities, intrinsic viscosities and number of short chain branching per 1000 carbon atoms is attributed with such physical properties.

0006<patcit id="pcit0002" dnum="US4461873A"><text>U.S. Patent No. 4,461,873</text></patcit> describes ethylene polymer blends of a high molecular weight ethylene polymer, preferably a copolymer, and a low molecular weight ethylene polymer, preferably an ethylene homopolymer, for improved film properties and ESCR, useful in the manufacture of film, in blow molding techniques, or in the production of pipes and wire coating.

0007<patcit id="pcit0003" dnum="EP0423962A"><text>EP 0 423 962</text></patcit> describes ethylene polymer compositions particularly suitable for gas pipes, said to have improved ESCR, comprising two or more kinds of ethylene polymers different in average molecular weight, at least one of which is a high molecular weight ethylene polymer having an intrinsic viscosity of 4.5 to 10.0 dl/g in decalin at 135 °C and a density of 0.910 to 0.930 g/cm³, and another of which is a low molecular weight ethylene polymer having an intrinsic viscosity of 0.5 to 2.0 dl/g, as determined for the first polymer, and a density of 0.938 to 0.970 g/cm³.

0008<patcit id="pcit0004" dnum="US5082902A"><text>U.S. Patent No. 5,082,902</text></patcit> describes blends of linear polyethylenes for injection and rotational molding said to have reduced crystallization times with improved impact strength and ESCR. The blends comprise: (a) a first polymer having a density of from 0.85 to 0.95 g/cm³ and a melt index (MI) of 1 to 200 g/10min; and (b) a second polymer having a density of 0.015 to 0.15 g/cm³ greater that the density of the first polymer and an MI differing by no more that 50% from the MI of the first polymer.

0009<patcit id="pcit0005" dnum="US5306775A"><text>U.S. Patent No. 5,306,775</text></patcit> describes polyethylene blends said to have a balance of properties for processing by any of the known thermoplastic processes, specifically including improved ESCR. These compositions have: (a) low molecular weight ethylene resins made using a chromium oxide-based catalyst and having a density at least 0.955 g/cm³ and MI) between 25 and 400 g/10min; and (b) high molecular weight ethylene copolymer resins with a density not higher than 0.955 g/cm³ and a high load melt index (HLMI) between 0.1 and 50 g/10min.

0010<patcit id="pcit0006" dnum="US5382631A"><text>U.S. Patent No. 5,382,631</text></patcit> describes linear interpolymer polyethylene blends having molecular weight distribution (M_w/M_n) ≤ 3 and composition distribution (CDBI) ≤ 50%, where the blends are generally free of fractions having higher molecular weight and lower average comonomer contents than other blend components. Improved properties for films, fibers, coatings, and molded articles are attributed to these blends. In one example, a first component is an ethylene-butene copolymer with a density of 0.9042 g/cm³, M_w/M_n of 2.3, and an MI of 4.0 dg/min and a second component is a high density polyethylene (HDPE) with a density of 0.9552 g/cm³, M_w/M_n of 2.8, and an MI of 5.0 dg/min. The blend is ascribed with improved tear strength characteristics.

0011<patcit id="pcit0007" dnum="US6329054B"><text>US6329054</text></patcit> discloses a cable sheating composition from polyethylene components having different densities and melt index. The overall melt index is less than 3.0 g/10min. <patcit id="pcit0008" dnum="US5082902A"><text>US5082902</text></patcit> discloses a molding composition having two components of different density but with melt indices differing by less than 50%. <patcit id="pcit0009" dnum="US3183283A"><text>US3183283</text></patcit> contains components of different melt index but the difference in density is not quantified.

0012There is a continuing need for polyethylene-based compositions having improved environmental stress cracking resistance, particularly those suitable for injection molding applications.

4.

SUMMARY OF THE INVENTION

0013In accordance with the present invention, polyolefin-based blend compositions suitable for injection molding, injection molded articles, and processes for injection molding articles are provided and more particularly a polyethylene composition comprising: <ol id="ol0001" compact="compact"><li>(a) a first polyethylene having a melt index of 0.1 to 3.0 g/10 min and a density of from 0.905 to 0.938 g/cm³; and</li><li>(b) a second polyethylene having a melt index of 10 to 500 g/10 min and a density of 0.945 to 0.975 g/cm3,</li></ol> wherein the composition has a density of from 0.920 to 0.973 g/cm³ and a melt index of 4 to 30 g/10 min, the melt indices being measured according to ASTM D1238, condition 190 °C, 2.13 kg, and wherein the density of the second polyethylene is from 0.037 to 0.062 g/cm³ greater than the density of the first polyethylene.

0014The polyethylene composition may include a first polyethylene having a melt index (I_2.16) of 0.3 to 3.0 g/10 min. The first polyethylene may be a metallocene-catalyzed polyethylene. Both the first and the second polyethylenes may be metallocene-catalyzed polyethylenes or at least one of the first and second polyethylenes is a metallocene-catalyzed polyethylene.

0015In another aspect, the invention provides an injection molded article formed from or including a polyethylene composition, the polyethylene composition including a first polyethylene having a melt index of 0.3 to 3.0 g/10 min and a density of from 0.905 to 0.938 g/cm³; and a second polyethylene having a melt index of 10 to 500 g/10 min and a density of 0.945 to 0.975 g/cm³, wherein the composition has a density of from 0.920 to 0.973 g/cm³ and a melt index of 4 to 30 g/10 min, and wherein the density of the second polyethylene is from 0.037 to 0.062 g/cm³ greater than the density of the first polyethylene. The first polyethylene may be a metallocene-catalyzed polyethylene or, both the first and the second polyethylenes are metallocene-catalyzed polyethylenes.

0016In another aspect the invention provides a process for forming an injection molded article, the process carried out by: (a) providing a polyethylene composition, the polyethylene composition including a first polyethylene having a melt index of 0.3 to 3.0 g/10 min and a density of from 0.905 to 0.938 g/cm³; and a second polyethylene having a melt index of 10 to 500 g/10 min and a density of 0.945 to 0.975 g/cm³, wherein the composition has a density of from 0.920 to 0.973 g/cm³ and a melt index of 4 to 30 g/10 min, and wherein the density of the second polyethylene is from 0.037 to 0.062 g/cm³ greater than the density of the first polyethylene; and (b) injection molding the composition to form an injection molded article. Again the first polyethylene may be a metallocene-catalyzed polyethylene or both the first and the second polyethylenes are metallocene-catalyzed polyethylenes.

0017The invention also provides provides a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, except that the metallocene catalyzed polyethylene has an Mw/Mn ratio of from 1.4 to 4.0.

0018The invention also may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, except that the metallocene catalyzed polyethylene has an Mw/Mn ratio of from 1.8 to 3.5.

0019The invention may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, except that the first polyethylene has a density of from 0.910 to 0.935 g/cm³.

0020The invention may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, except that the first polyethylene has a melt index of 0.1 to 2.0 g/10 min.

0021The invention may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, except that the first polyethylene has a melt index of 0.1 to 1.0 g/10 min.

0022The invention may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, except that the first polyethylene has a melt index of 0.1 to 3.0 g/10 min.

0023The invention may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, except that the first polyethylene has a melt index of 0.3 to 1.0 g/10 min.

0024The invention may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, except that the second polyethylene has a density of from 0.950 to 0.972 g/cm³.

0025The invention may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, except that the second polyethylene has a density of from 0.955 to 0.970 g/cm³.

0026The invention may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, except that the second polyethylene has a density of from 0.960 to 0.968 g/cm³.

0027The invention may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, except that the second polyethylene has a melt index of 10 to 300 g/10 min.

0028The invention may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, except that the second polyethylene has a melt index of 30 to 200 g/10 min.

0029The invention may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, except that the second polyethylene has a melt index of 50 to 100 g/10 min.

0030The invention may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, except that the composition has a density of from 0.930 to 0.970 g/cm³.

0031The invention may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, except that the composition has a density of from 0.940 to 0.965 g/cm³.

0032The invention may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, except that the composition has a density of from 0.950 to 0.960 g/cm³.

0033The invention may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, except that the density of the second polyethylene is from 0.038 to 0.062 g/cm³ greater than the density of the first polyethylene.

0034The invention may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, except that the density of the second polyethylene is from 0.040 to 0.060 g/cm³ greater than the density of the first polyethylene.

0035The invention may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, except that the composition has a melt index I_2.16 of from 4 to 10 g/10 min.

0036The invention may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, wherein the blend includes 80% to 20% by weight of the first polyethylene and 20% to 80% by weight of the second polyethylene, based on the total weight of the first and second polyethylenes.

0037The invention may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, wherein the blend includes 70% to 30% by weight of the first polyethylene and 30% to 70% by weight of the second polyethylene, based on the total weight of the first and second polyethylenes.

0038The invention may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, wherein the blend includes 60% to 40% by weight of the first polyethylene and 40% to 60% by weight of the second polyethylene, based on the total weight of the first and second polyethylenes.

0039The invention may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments, wherein at least one of the first and second polyethylenes is a blend of two or more polyethylene resins.

0040The invention may provide a polyethylene composition, an injection molded article, or a process of forming an injection molded article, in accordance with any of the preceding embodiments except the immediately preceding embodiment, wherein the composition includes only the first and second polyethylenes, except that minor amounts of conventional additives can also be present.

DETAILED DESCRIPTFON

0041The inventive compositions surprisingly and advantageously provide improved ESCR for polyethylene injection molding applications, relative to compositions having the same melt index and density.

0042By preparing several samples of proposed blend polyethylene components and then subjecting blends prepared from them to analytical testing, it was determined that peak values of ESCR are obtained when the melt index (I_2.16) and the difference in density and of the blend components were within specific ranges, as described herein. At smaller density differences for the two components, ESCR was improved over single component compositions, but was significantly deficient to those within the range for the inventive compositions. Increasing the width of the density range between the components within the invention range increased the ESCR improvement until a peak was reached in which ESCR no longer improved and began to diminish. Examining the melting peaks of the sample blends with a differential scanning calorimeter (DSC) helps illustrate the region in which ESCR improvements are no longer realized by increasing the difference in densities between the two components. This is shows by the point where by further increasing the width of the density range, the two components no longer completely cocrystallize, as evidenced by the presence of a secondary lower melting peak in the DSC scan. When the density range was wider than that described above, evidence of loss of cocrystallizability became apparent as a second melting peak or shoulder began to appear in the scans. The blends exhibiting even minimal incidence of a second shoulder had diminished ESCR improvements.

0043The first polyethylene of the polymer blends of the invention is a polyethylene copolymer derived from the coordination polymerization of principally ethylene with a minor amount of one or more copolymerizable monomers. Particularly improved end-product properties are obtained using such copolymers having a narrow molecular weight distribution (Mw/Mn, or "MWD"), e.g., Mw/Mn of from a lower limit of 1.4 or 1.6 or 1.8 or 2.0 to an upper limit of 4.0 or 3.8 or 3.5 or 3.0, with ranges from any lower limit to any upper limit being contemplated. Suitable comonomers include C₃-C₂₀ alpha-olefins, preferably C₃ C₈, C₅-C₂₀ cyclic olefins, preferably C₇-C₁₂ cyclic olefins, C₇-C₂₀ vinyl aromatic monomers, preferably styrene, and C₄-C₂₀ geminally disubstituted olefins, preferably isobutylene. The most preferred comonomers include propylene, 1-butene, 1-hexene, 4-methyl-1-pentene and 1-octene. The density of the copolymer is determined largely by comonomer content and typically ranges from 0.905 or 0.910 g/cm³ to 0.938 or 0.935 g/cm³, with ranges from any lower limit to any upper limit being contemplated. Some amount of long-chain branching may be present, but the density limitations are largely due to the presence of comonomer. These ethylene copolymers are of higher molecular weight than the second polyethylene of the blends, as shown by a melt index I_2.16 as measured according to ASTM D1238, condition 190 °C. 2.16 kg (formerly condition "E"), of from 0.1 or 0.3 to 3.0 or 2.0 or 1.0 g/10 min, with ranges from any lower limit to any upper limit being contemplated.

0044The second polyethylene of the polymer blends of the invention has a higher density and a lower molecular weight than the first polyethylene. The second polyethylene can be derived from ethylene and, optionally, minor amounts of any of the comonomers listed above for the first polyethylene. The density can be from a lower limit of 0.945 or 0.950 or 0.955 or 0.960 g/cm³ to an upper limit of 0.975 or 0.972 or 0.970 or 0.968 g/cm³, with ranges from any lower limit to any upper limit being contemplated. It should be appreciated that, the specific choice of densities must be consistent with the density differences described herein. The melt index I_2.16 of the second polyethylene, as measured according to ASTM D1238, condition 190 °C, 2.16 kg, can be from a lower limit of 10 or 30 or 50 to an upper limit of 500 or 300 or 200 or 100 g/10 min, with ranges from any lower limit to any upper limit being contemplated. The second polyethylene can be any conventional polyethylene having the properties described herein, and can have a broad or narrow molecular weight distribution. In a particular embodiment, the second polyethylene has a value of Mw/Mn of from a lower limit of 1.4 or 1.6 or 1.8 or 2.0 to an upper limit of 4.0 or 3.8 or 3.5 or 3.0, with ranges from any lower limit to any upper limit being contemplated.

0045Industrial methods of producing the polyethylene components of the invention are well known in the art as is exemplified in the references cited above. Any such method capable of producing polyethylene polymer components according to the invention will be suitable. Such methods include gas phase, liquid phase (or solution), and slurry phase polymerization processes, either alone or in combination. By alone, reference is made to series or serial production in a single reactor or in more than one reactor. Reactor blends will also be suitable, such as by the use of mixed catalysts or mixed polymerization conditions in a single reactor. Gas phase processes are particularly suited in view of economic advantages. Such processes use supported catalysts and are conducted in polymerization reactors under gas phase conditions suitable for linear low density ethylene copolymers prepared by coordination polymerization. Illustrative examples may be found in <patcit id="pcit0010" dnum="US4543399A"><text>U.S. Patent Nos. 4,543,399</text></patcit>, <patcit id="pcit0011" dnum="US4588790A"><text>4,588,790</text></patcit>, <patcit id="pcit0012" dnum="US5028670A"><text>5,028,670</text></patcit>, <patcit id="pcit0013" dnum="US5352749A"><text>5,352,749</text></patcit>, <patcit id="pcit0014" dnum="US5382638A"><text>5,382,638</text></patcit>, <patcit id="pcit0015" dnum="US5405922A"><text>5,405,922</text></patcit>, <patcit id="pcit0016" dnum="US5422999A"><text>5,422,999</text></patcit>, <patcit id="pcit0017" dnum="US5436304A"><text>5,436,304</text></patcit>, <patcit id="pcit0018" dnum="US5453471A"><text>5,453,471</text></patcit>, <patcit id="pcit0019" dnum="US5462999A"><text>5,462,999 </text></patcit>and <patcit id="pcit0020" dnum="US5463999A"><text>5,463,999</text></patcit>, and International applications <patcit id="pcit0021" dnum="WO9428032A"><text>WO 94/28032</text></patcit>, <patcit id="pcit0022" dnum="WO9507942A"><text>WO 95/07942</text></patcit> and <patcit id="pcit0023" dnum="WO9600245A"><text>WO 96/00245</text></patcit>. These processes use either traditional Ziegler-Natta catalysts or later organometallic catalysts characterized as having essentially single polymerization sites due to the arrangement of ancillary ligands on or about the metal center. Metallocene catalysts are representative "single site catalysts" and are preferred in this invention in embodiments having narrow molecular weight distribution polyolefins. Typically, the processes are conducted at temperatures of from -100 °C to 150 °C, more typically from 40°C to 120 °C, at pressures up to about 7000 kPa, typically from 690 kPa to 2415 kPa. Continuous processes using fluidized beds and recycle streams as the fluidizing medium are preferred.

0046Slurry polymerization processes are suitable for both components and particularly suited for the high density components of the invention. These processes are typically described as those in which the polymerization medium can be either a liquid monomer, like propylene, or a hydrocarbon solvent or diluent, advantageously aliphatic paraffin such as propane, isobutane, hexane, heptane, cyclohexane, or an aromatic one such as toluene. Slurry solids typically include the forming polymer and inert camer-supported catalysts. Catalysts are typically Ziegler-Natta, and/ or one or more single site catalysts, such as metallocenes. The polymerization temperatures may be those considered low, e.g., less than 50 °C, typically 0 °C-30 °C, or may be in a higher range, such as up to about 150 °C, typically from 50 °C up to about 80 °C, or at any ranges between the end points indicated. Pressures can vary from about 100 to about 700 psia (0.76-4.8 MPa). Additional description is given in <patcit id="pcit0024" dnum="US4182810A"><text>U.S. Patent Nos. 4,182,810</text></patcit>,<patcit id="pcit0025" dnum="US5274056A"><text> 5,274,056</text></patcit>, <patcit id="pcit0026" dnum="US6319997B"><text>6,319,997</text></patcit>, <patcit id="pcit0027" dnum="US6380325B"><text>6,380,325</text></patcit>, <patcit id="pcit0028" dnum="US6420497B"><text>6,420,497</text></patcit>, <patcit id="pcit0029" dnum="WO9421962A"><text>WO 94/21962</text></patcit> and <patcit id="pcit0030" dnum="WO9932531A"><text>WO 99/32531</text></patcit>.

0047The polyethylene blend compositions in accordance with the present invention can include the first polyethylene in an amount of from a lower limit of 20 or 30 or 40 wt% to an upper limit of 80 or 70 or 60 wt%, based on the total weight of the first and second polyethylenes, with ranges from any lower limit to any upper limit being contemplated. Similarly, the polyethylene blend compositions in accordance with the present invention can include the second polyethylene in an amount of from a lower limit of 20 or 30 or 40 wt% to an upper limit of 80 or 70 or 60 wt%, based on the total weight of the first and second polyethylenes, with ranges from any lower limit to any upper limit being contemplated.

0048Additionally, either or both of the first polyethylene and the second polyethylene can be a sub-blend of two or more polyethylenes so long as the sub-blend has the properties described herein.

0049Although the description herein focuses on first and second polyethylenes, in some embodiments, the polyethylene blend composition can further include additional polymeric components, including additional polyethylenes, provided that the overall blend composition has the recited properties.

0050The weight percentages recited herein for the first and second polyethylene components are based on the total weight (100%) of the first and second polyethylene components.

0051blend can have a density of from a lower limit of 0.920 or 0.930 or 0.940 or 0.950 g/cm³ to an upper limit of 0.973 or 0.970 or 0.965 or 0.960 g/cm³, with ranges from any lower limit to any upper limit being contemplated.

0052The blend can have a difference in the density of the first and second polyethylenes, with the density of the second polyethylene being greater, of from a lower limit of 0.037 or 0.038 or 0.040 g/cm³ to an upper limit of 0.062 or 0.060 g/cm³, with ranges from any lower limit to any upper limit being contemplated.

0053The melt index I_2.16 of the blend can be from a lower limit of 2 or 3 or 4 g/10 min to an upper limit of 200 or 100 or 50 or 30 or 10 /10 min.

0054The first and second polyethylenes have weight average molecular weights Mw₁ and Mw₂, respectively, conforming to the relationship <maths id="math0001"><math display="block"><mfrac><msub><mi mathvariant="italic">Mw</mi><mn>1</mn></msub><msub><mi mathvariant="italic">Mw</mi><mn>2</mn></msub></mfrac><mo>></mo><mn>1.</mn></math><img file="EP1546252B1_D0001.tif" /></maths>

0055The densities of the first and second polyethylenes, ρ₁ and ρ₂, respectively, conform to the relationship <maths id="math0002"><math display="block"><mfrac><msub><mi mathvariant="italic">ρ</mi><mn>1</mn></msub><msub><mi mathvariant="italic">ρ</mi><mn>2</mn></msub></mfrac><mo>&lt;</mo><mn>1.</mn></math><img file="EP1546252B1_D0002.tif" /></maths>

0056It is well-known in the art that, all other factors being equal, ESCR is inversely proportional to density, and inversely proportional to melt index. It has been surprisingly found that polyethylene blend compositions of the invention show ESCR values greater than those of conventional compositions having the same density and melt index, but not having the inventive combination of properties described herein, such as melt indexes, densities, and density differences.

0057Additives may be used as needed. Typical additives include one or more of antioxidants, anti-static agents, UV stabilizers, foaming agents, processing aids, nucleating agents, nanocomposites, fiber reinforcements and pigments. Illustrative pigments or colorants include titanium dioxide, carbon black, cobalt aluminum oxides such as cobalt blue, and chromium oxides such as chromium oxide green. Pigments such as ultramarine blue, which is a silicate. Phthalocyanine blue and iron oxide red will also be suitable. Such are typically used an amounts from 0 wt% to not more than 15 wt%, based on the total weight of the first and second polyethylene components.

EXAMPLES

0058Mz, Mw and Mn can be measured using gel permeation chromatography (GPC), also known as size exclusion chromatography (SEC). This technique utilizes an instrument containing columns packed with porous beads, an elution solvent, and detector in order to separate polymer molecules of different sizes. In a typical measurement, the GPC instrument used is a Waters chromatograph equipped with ultrastyro gel columns operated at 145 °C. The elution solvent used is trichlorobenzene. The columns are calibrated using sixteen polystyrene standards of precisely known molecular weights. A correlation of polystyrene retention volume obtained from the standards, to the retention volume of the polymer tested yields the polymer molecular weight.

0059Average molecular weights M can be computed from the expression: <maths id="math0003"><math display="block"><mi mathvariant="normal">m</mi><mo>=</mo><mfrac><mrow><munderover><mo mathvariant="italic">∑</mo><mi mathvariant="italic">i</mi><mspace width="1em" /></munderover><msub><mi mathvariant="italic">N</mi><mi mathvariant="italic">i</mi></msub><mo>⁢</mo><msubsup><mi mathvariant="italic">M</mi><mi mathvariant="italic">i</mi><mrow><mi mathvariant="italic">n</mi><mo mathvariant="italic">+</mo><mn>1</mn></mrow></msubsup></mrow><mrow><munderover><mo mathvariant="italic">∑</mo><mi mathvariant="italic">i</mi><mspace width="1em" /></munderover><msub><mi mathvariant="italic">N</mi><mi mathvariant="italic">i</mi></msub><mo>⁢</mo><msubsup><mi mathvariant="italic">M</mi><mi mathvariant="italic">i</mi><mi mathvariant="italic">n</mi></msubsup></mrow></mfrac></math><img file="EP1546252B1_D0003.tif" /></maths>

0060where N_i is the number of molecules having a molecular weight M_i. When n = 0, M is the number average molecular weight Mn. When n = 1, M is the weight average molecular weight Mw. When n = 2, M is the Z-average molecular weight Mz. The desired MWD function (e.g., Mw/Mn or Mz/Mw) is the ratio of the corresponding M values. Measurement of M and MWD is well known in the art and is discussed in more detail in, for example, <nplcit id="ncit0002" npl-type="b"><text>Slade, P. E. Ed., Polymer Molecular Weights Part II, Marcel Dekker, Inc., NY, (1975) 287-368</text></nplcit>; <nplcit id="ncit0003" npl-type="b"><text>Rodriguez, F., Principles of Polymer Systems 3rd ed., Hemisphere Pub. Corp., NY, (1989) 155-160</text></nplcit>; <patcit id="pcit0031" dnum="US4540753A"><text>U.S. Patent No. 4,540,753</text></patcit>; <nplcit id="ncit0004" npl-type="s"><text>Verstrate et al., Macromolecules, vol. 21, (1988) 3360</text></nplcit>; and references cited therein.

0061Environmental Stress Crack Resistance (ESCR) (bent strip) is determined in accordance with ASTM D 1693, condition B, 10% IGEPAL^™ IGEPAL^™ is a nonylphenoxy poly(ethylenoxy)ethanol surfactant available from Rhone Polenc, Cranbury, NJ. All ESCR values cited herein are ASTM D 1693 condition B, 10% IGEPAL^™ F50 values, and are given in units of hours.

0062Polymer density (g/cm³) is determined using a compression molded sample, cooled at 15 °C per hour and conditioned for 40 hours at room temperature according to ASTM D1505-68 and ASTM D1928, procedure C.

0063Polymer melt flow rates can be determined at 190 °C according to ASTM D-1238. I_21.6 is the "flow index" or melt flow rate of the polymer measured according to ASTM D-1238, condition 190 °C, 21.6 kg, and I_2.16 is the "melt index" or melt flow rate of the polymer measured according to ASTM D-1238, condition 190 °C, 2.16 kg. The ratio of I_21.6 to I_2.16 is the "melt flow ratio" or "MFR". The melt flow rate I_21.6 is also sometimes termed the "high load melt index" or HLMI. Melt flow rates are reported in units of grams per 10 minutes (g/10 min) or equivalently decigrams per minute (dg/min).

Examples 1-8, Comparative Examples 1-2

0064Table 1 illustrates the invention in examples 1a-b through 8a-3b, with comparative examples Comp 1 and Comp 2a-c. Each "a" row illustrates a first polyethylene component and each "b" row illustrates a second polyethylene component. In Comp 2, the "c" row indicates a third polyethylene component. The column "Δ density" provides the difference in density of the two components for each illustrated blend. In Comp 2, the difference in density is the difference between components 2a and 2c. Comp 1 illustrates a comparative single polyethylene component within the density and melt index range typical for injection molding compositions. Comp 2 illustrates a comparative blend where the density difference is less than 0.037 g/cm³ but the blend melt index and density are the same as Example 1.

0065The polyethylene resins in Table 1 were prepared generally in accordance with the examples in <patcit id="pcit0032" dnum="US5382631A"><text>U.S. Patent No. 5,382,631</text></patcit>, except where noted. A zirconocene activated with alumoxane on a silica support, 12 wt% methylalumoxane and 3.5 wt% zirconium, was used as polymerization catalyst in a gas phase reactor operated at (185 °F) 85 °C, with a gas phase consisting of 70 vol% ethylene, 0.5 - 2.0 vol% hexene, 200-800 parts per million hydrogen, with remainder being nitrogen. From (50 to 75 pounds) 22.6 to 33.9 kg per hour were produced in each polymerization run.

0066ESCR values in Table 1 given as ranges indicate that the sample failure occurred at an undetermined time between the times shown. The ESCR value for blend 6a/6b indicates that the sample was intact when testing was stopped at 605 hours.

0067Comparative Examples 1 and 2 have the same density as Example 1, and the same or comparable melt index, but show poor ESCR performance (4.5 hours versus 78.5-143 hours). <tables id="tabl0001" num="0001"><table frame="topbot"><title>Table 1</title><tgroup cols="7" colsep="0"><colspec colnum="1" colname="col1" colwidth="27mm" /><colspec colnum="2" colname="col2" colwidth="12mm" /><colspec colnum="3" colname="col3" colwidth="26mm" /><colspec colnum="4" colname="col4" colwidth="16mm" /><colspec colnum="5" colname="col5" colwidth="16mm" /><colspec colnum="6" colname="col6" colwidth="18mm" /><colspec colnum="7" colname="col7" colwidth="20mm" /><thead><row rowsep="0"><entry valign="top">Example</entry><entry align="center" valign="top">Wt%</entry><entry align="center" valign="top">Melt Index I_2.16</entry><entry align="center" valign="top">Density</entry><entry align="center" valign="top">Mw/Mn</entry><entry align="center" valign="top">Δ density</entry><entry align="center" valign="top">ESCR, F₅₀</entry></row><row><entry valign="top" /><entry align="center" valign="top" /><entry align="center" valign="top">(g/10 min)</entry><entry align="center" valign="top">(g/cm³)</entry><entry align="center" valign="top" /><entry align="center" valign="top">(g/cm³)</entry><entry align="center" valign="top">(hr)</entry></row></thead><tbody><row rowsep="0"><entry>1a</entry><entry align="center">30.6</entry><entry align="center">0.46</entry><entry align="char" char="." charoff="12">0.911</entry><entry align="center">2.50</entry><entry align="char" char="." charoff="11" /><entry align="center" /></row><row rowsep="0"><entry>1b</entry><entry align="center">69.4</entry><entry align="center">56.6</entry><entry align="char" char="." charoff="12">0.970</entry><entry align="center">3.8</entry><entry align="char" char="." charoff="11" /><entry align="center" /></row><row><entry>1a/1b Blend</entry><entry align="center">100</entry><entry align="center">6.8</entry><entry align="char" char="." charoff="12">0.952</entry><entry align="center" /><entry align="char" char="." charoff="11">0.059</entry><entry align="center">78.8-143</entry></row><row rowsep="0"><entry>2a</entry><entry align="center">27.2</entry><entry align="center">0.46</entry><entry align="char" char="." charoff="12">0.911</entry><entry align="center">2.50</entry><entry align="char" char="." charoff="11" /><entry align="center" /></row><row rowsep="0"><entry>2b</entry><entry align="center">72.8</entry><entry align="center">56.6</entry><entry align="char" char="." charoff="12">0.970</entry><entry align="center">3.8</entry><entry align="char" char="." charoff="11" /><entry align="center" /></row><row><entry>2a/2b Blend</entry><entry align="center">100</entry><entry align="center">7.5</entry><entry align="char" char="." charoff="12">0.954</entry><entry align="center" /><entry align="char" char="." charoff="11">0.059</entry><entry align="center">69.5</entry></row><row rowsep="0"><entry>3a</entry><entry align="center">25.5</entry><entry align="center">0.46</entry><entry align="char" char="." charoff="12">0.911</entry><entry align="center">2.50</entry><entry align="char" char="." charoff="11" /><entry align="center" /></row><row rowsep="0"><entry>3b</entry><entry align="center">74.5</entry><entry align="center">56.6</entry><entry align="char" char="." charoff="12">0.970</entry><entry align="center">3.8</entry><entry align="char" char="." charoff="11" /><entry align="center" /></row><row><entry>3a/3b Blend</entry><entry align="center">100</entry><entry align="center">8.6</entry><entry align="char" char="." charoff="12">0.955</entry><entry align="center" /><entry align="char" char="." charoff="11">0.059</entry><entry align="center">6.5-23.5</entry></row><row rowsep="0"><entry>4a</entry><entry align="center">23.8</entry><entry align="center">0.46</entry><entry align="char" char="." charoff="12">0.911</entry><entry align="center">2.50</entry><entry align="char" char="." charoff="11" /><entry align="center" /></row><row rowsep="0"><entry>4b</entry><entry align="center">76.2</entry><entry align="center">56.6</entry><entry align="char" char="." charoff="12">0.970</entry><entry align="center">3.8</entry><entry align="char" char="." charoff="11" /><entry align="center" /></row><row><entry>4a/4b Blend</entry><entry align="center">100</entry><entry align="center">9.3</entry><entry align="char" char="." charoff="12">0.956</entry><entry align="center" /><entry align="char" char="." charoff="11">0.059</entry><entry align="center">6.5-23.5</entry></row><row rowsep="0"><entry>5a</entry><entry align="center">22.2</entry><entry align="center">0.46</entry><entry align="char" char="." charoff="12">0.911</entry><entry align="center">2.50</entry><entry align="char" char="." charoff="11" /><entry align="center" /></row><row rowsep="0"><entry>5b</entry><entry align="center">77.8</entry><entry align="center">56.6</entry><entry align="char" char="." charoff="12">0.970</entry><entry align="center">3.8</entry><entry align="char" char="." charoff="11" /><entry align="center" /></row><row><entry>5a/5b Blend</entry><entry align="center">100</entry><entry align="center">10.0</entry><entry align="char" char="." charoff="12">0.957</entry><entry align="center" /><entry align="char" char="." charoff="11">0.059</entry><entry align="center">5-6.5</entry></row><row rowsep="0"><entry>6a</entry><entry align="center">30</entry><entry align="center">0.45</entry><entry align="char" char="." charoff="12">0.919</entry><entry align="center">2.59</entry><entry align="char" char="." charoff="11" /><entry align="center" /></row><row rowsep="0"><entry>6b</entry><entry align="center">70</entry><entry align="center">56.6</entry><entry align="char" char="." charoff="12">0.970</entry><entry align="center">3.8</entry><entry align="char" char="." charoff="11" /><entry align="center" /></row><row><entry>6a/6b Blend</entry><entry align="center">100</entry><entry align="center">4.8</entry><entry align="char" char="." charoff="12">0.955</entry><entry align="center" /><entry align="char" char="." charoff="11">0.051</entry><entry align="center">> 605</entry></row><row rowsep="0"><entry>7a</entry><entry align="center">24</entry><entry align="center">0.45</entry><entry align="char" char="." charoff="12">0.919</entry><entry align="center">2.59</entry><entry align="char" char="." charoff="11" /><entry align="center" /></row><row rowsep="0"><entry>7b</entry><entry align="center">76</entry><entry align="center">56.6</entry><entry align="char" char="." charoff="12">0.970</entry><entry align="center">3.8</entry><entry align="char" char="." charoff="11" /><entry align="center" /></row><row><entry>7a/7b Blend</entry><entry align="center">100</entry><entry align="center">5.4</entry><entry align="char" char="." charoff="12">0.958</entry><entry align="center" /><entry align="char" char="." charoff="11">0.051</entry><entry align="center">60-78</entry></row><row rowsep="0"><entry>8a</entry><entry align="center">35.5</entry><entry align="center">0.86</entry><entry align="char" char="." charoff="12">0.919</entry><entry align="center">2.43</entry><entry align="char" char="." charoff="11" /><entry align="center" /></row><row rowsep="0"><entry>8b</entry><entry align="center">64.5</entry><entry align="center">56.6</entry><entry align="char" char="." charoff="12">0.970</entry><entry align="center">3.8</entry><entry align="char" char="." charoff="11" /><entry align="center" /></row><row><entry>8a/8b Blend</entry><entry align="center">100</entry><entry align="center">7.0</entry><entry align="char" char="." charoff="12">0.952</entry><entry align="center" /><entry align="char" char="." charoff="11">0.051</entry><entry align="center">21-39</entry></row><row><entry>Comp 1*</entry><entry align="center">100</entry><entry align="center">6.5</entry><entry align="char" char="." charoff="12">0.952</entry><entry align="center">3.6</entry><entry align="char" char="." charoff="11" /><entry align="center">4.5</entry></row><row rowsep="0"><entry>Comp 2a</entry><entry align="center">29</entry><entry align="center">3.0</entry><entry align="char" char="." charoff="12">0.935</entry><entry align="center">2.82</entry><entry align="char" char="." charoff="11" /><entry align="center" /></row><row rowsep="0"><entry>Comp 2b</entry><entry align="center">34</entry><entry align="center">3.0</entry><entry align="char" char="." charoff="12">0.947</entry><entry align="center">2.87</entry><entry align="char" char="." charoff="11" /><entry align="center" /></row><row rowsep="0"><entry>Comp 2c</entry><entry align="center">37</entry><entry align="center">56.6</entry><entry align="char" char="." charoff="12">0.970</entry><entry align="center">3.8</entry><entry align="char" char="." charoff="11" /><entry align="center" /></row><row rowsep="0"><entry>Comp 2a/2b/2c</entry><entry align="center">100</entry><entry align="center">6.8</entry><entry align="char" char="." charoff="12">0.952</entry><entry align="center" /><entry align="char" char="." charoff="11">0.035</entry><entry align="center">4.5</entry></row><row><entry>Blend</entry><entry align="center" /><entry align="center" /><entry align="char" char="." charoff="12" /><entry align="center" /><entry align="char" char="." charoff="11" /><entry align="center" /></row></tbody></tgroup><tgroup cols="7" rowsep="0"><colspec colnum="1" colname="col1" colwidth="27mm" /><colspec colnum="2" colname="col2" colwidth="12mm" /><colspec colnum="3" colname="col3" colwidth="26mm" /><colspec colnum="4" colname="col4" colwidth="16mm" /><colspec colnum="5" colname="col5" colwidth="16mm" /><colspec colnum="6" colname="col6" colwidth="18mm" /><colspec colnum="7" colname="col7" colwidth="20mm" /><tbody><row><entry namest="col1" nameend="col7" align="justify">*Commercial HDPE for injection molding (HD6706, ExxonMobil Chemical)</entry></row></tbody></tgroup></table></tables>

0068Various tradenames used herein are indicated by a ™ symbol, indicating that the names may be protected by certain trademark rights. Some such names may also be registered trademarks in various jurisdictions.