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Heterogeneous/homogeneous copolymerRelated Patent Categories: Synthetic Resins Or Natural Rubbers -- Part Of The Class 520 Series, Natural Rubber Compositions Having Nonreactive Materials (dnrm) Other Than: Carbon, Silicon Dioxide, Glass Titanium Dioxide, Water, Hydrocarbon, Halohydrocarbon, Ethylenically Unsaturated Reactant Admixed With A Preformed Reaction Product Derived From: (a) At Least One Polycarboxylic Acid, Ester, Or Anhydride; (b) At Least One Polyhydroxy Compound; And (c) At Least One Fatty Acid Glycerol Ester, Or A Fatty Acid Or Salt Derived From A Naturally Occurring Glyceride, Tall Oil, Or A Tall Oil Fatty Acid, At Least One Solid Polymer Derived From Ethylenic Reactants Only, Polymer Mixture Of Two Or More Solid Polymers Derived From Ethylenically Unsaturated Reactants Only; Or Mixtures Of Said Polymer Mixture With A Chemical Treating Agent; Or Products Or Processes Of Preparing Any Of The Above Mixtures, Solid Polymer Derived From Ethylene Or PropyleneHeterogeneous/homogeneous copolymer description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060041074, Heterogeneous/homogeneous copolymer. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0001] This invention relates to polyethylene compositions having a unique balance of properties. Preferred polyethylene compositions may be easily "processed" to produce plastic films having very good tear strength, impact strength and optical properties. BACKGROUND OF THE INVENTION [0002] Linear low density polyethylene ("LLDPE") copolymers that are prepared by the copolymerization of ethylene with a higher alpha olefin using a conventional Ziegler-Natta type catalyst system are known to contain three different polymer fractions (or "modes"), namely 1) a low molecular weight fraction which contains a high comonomer content; 2) a copolymer fraction of intermediate molecular weight and intermediate comonomer content; and 3) a high molecular weight fraction which contains little or no comonomer. The low molecular weight fraction is sometimes also described as being "highly branched" (due to the high comonomer content) and/or "grease" (due to the low molecular weight). The high molecular weight content is also sometimes described as "homopolymer". The "grease" fraction often causes organoleptic problems and may even limit or restrict uses in which the polymer comes in contact with food. The "homopolymer" fraction generally imparts a stiffness to the resin and melts at a higher temperature than the other fractions. In general, the non-uniformity of the molecular weight and the comonomer content is a distinguishing characteristic of conventional Ziegler resins. It is now common to refer to these resins as "heterogeneous". [0003] In contrast, "homogeneous" polyethylene copolymers are generally characterized by having a narrow molecular weight distribution and a narrow composition distribution. The term "homogeneous" was proposed by one of us to describe such polymers in United States Patent (USP) U.S. Pat. No. 3,645,992 (Elston), the disclosure of which is incorporated herein by reference. [0004] As noted in Elston '992, homogeneous polymers have a distinct melting point due to the uniform polymer architecture. The homogeneous polymers disclosed in the Elston '992 patent were prepared with a vanadium catalyst system which is insufficiently active to permit widespread commercial use. [0005] Advances in catalyst technology now permit the production of homogeneous ethylene copolymers at commercially viable rates. For example, the metallocene/aluminoxane catalysts disclosed by Kaminsky (U.S. Pat. No. 4,542,199) and improved by Welborn (U.S. Pat. No. 5,324,800); the monocyclopentadienyl catalysts disclosed by Stevens et al. (U.S. Pat. No. 5,064,802) and Canich (U.S. Pat. No. 5,055,438); the ketimine catalysts disclosed by McMeeking et al. (U.S. Pat. No. 6,114,481); and the phosphinimine catalysts disclosed by Stephan et al (U.S. Pat. No. 6,063,879) are all highly active for the preparation of homogeneous copolymers. [0006] A particularly important end use of LLDPE is the manufacture of films. Films prepared from homogeneous LLDPE generally have good optical properties, good organoleptic properties and excellent impact strength. [0007] However, films prepared from homogeneous LLDPE generally have poor tear strength, particularly in the so-called "machine direction". In addition, homogeneous resins are difficult to "process" (i.e. to convert to films). This poor processability is manifested by high energy demands required to extrude the resin (e.g. large current draws on the electric motors used to drive the extrudes) and/or poor melt strength. [0008] Attempts have been made to blend heterogeneous (Ziegler-Natta, or "Z/N") resins with homogeneous resins in order to produce a resin blend which is easier to convert to film and/or to produce film having higher impact strength and good tear properties. U.S. Pat. No. 5,530,065 (Farley, to Exxon) teaches that a trivial blend of a conventional heterogeneous Z/N resin and a metallocene resin has a balance of properties which are suitable for some film properties. Similarly, U.S. Pat. Nos. 5,844,045 and 5,869,575 (Kolthamer, to Dow) also disclose that simple blends of a conventional heterogeneous Z/N resin and homogeneous resin prepared with a monocyclopentadienyl catalyst are also suitable for preparing films. [0009] However, it will also be appreciated that the simple blends of the above '065, '045 and '575 patents all contain the low molecular weight "grease" due to the use of the Z/N catalyst to prepare some of the blend composition. Moreover, the disclosures of the '065, '045 and '575 patents are silent with respect to the hexane extractables contents of the blends. [0010] Thus, films prepared from conventional heterogeneous resins have comparatively poor impact strength, optical properties and organoleptic properties--but do have very good tear strength. Conversely, films prepared from homogeneous resins have excellent impact strength, optical properties and organoleptic properties--but poor tear strength. Previous attempts to utilize resin blends to eliminate this problem have not been completely successful. Simple blends of heterogeneous resins with homogeneous resins provide films with sub-optimal organoleptic properties (presumably because of the "grease" fraction in the conventional heterogeneous resin). [0011] Another attempt to solve this problem is by preparing blends of more than one homogeneous resin is disclosed in U.S. Pat. Nos. 5,382,630 and 5,382,631 (Stehling et al., to Exxon). Stehling et al. '631 teach blends which are characterized by the substantial absence of blend components having a higher molecular weight and a lower comonomer content than other blend components (e.g. the high molecular weight homopolymer of conventional heterogeneous resins). These blends are shown to be useful for the preparation of structures having improved tear properties. However, the disclosure of this patent is silent on the subject of impact properties and we have observed that films made from such blends may suffer a very large loss of dart impact strength when the amount of lower molecular weight, higher density component is sufficient to enhance tear strength. One of us disclosed a dual reactor solution polymerization process to prepare a homogeneous copolymer composition which is useful for the preparation of films (Brown, U.S. Pat. No. 6,372,864). None of the inventive copolymer compositions disclosed in Brown '864 contained the high molecular weight, high density (very low comonomer) fraction which is an essential element of the compositions of this invention. [0012] Thus, films prepared from conventional heterogeneous resins have comparatively poor impact strength, optical properties and organoleptic properties--but do have very good tear strength. Conversely, films prepared from homogeneous resins have excellent impact strength, optical properties and organoleptic properties--but poor tear strength. Previous attempts to utilize resin blends to eliminate this problem have not been completely successful. Simple blends of heterogeneous resins with homogeneous resins provide films with sub-optimal organoleptic properties and optical properties (presumably because of the "grease" fraction in the conventional heterogeneous resin). SUMMARY OF THE INVENTION [0013] We have now discovered a heterogenized/homogeneous polymer composition which may be used to prepare films having an improved balance of impact strength, tear strength and organoleptic properties. [0014] In addition, the heterogenized/homogeneous polymer compositions of this invention are surprisingly easy to "process" in machinery used to convert the compositions into films (in comparison to homogeneous resins). [0015] The heterogenized/homogeneous polymer compositions must be prepared using a "homogeneous catalyst"--i.e. a catalyst system that will produce homogeneous polymers (having a narrow molecular weight distribution and a narrow composition distribution) in a conventional polymerization reactor. In addition, the heterogenized/homogeneous polymer compositions of this invention must contain at least one first copolymer fraction and a second high molecular weight/high density fraction. This second fraction is somewhat analogous to the "homopolymer" fraction of heterogeneous resins. In this sense, the compositions may be referred to as being "heterogenized". Thus, the polymer compositions of this invention are made with "homogenous" components prepared with a "homogeneous" catalyst system but they must also contain a high molecular weight, high density component which is analogous to a "heterogeneous" resin. [0016] Thus, in one embodiment, the present invention provides a heterogenized/homogeneous polymer composition prepared with a homogeneous catalyst system, said composition comprising: [0017] A) a first polymer fraction having a density of from 0.880 to 0.945 grams per cubic centimeter as measured by ASTM D792; a melt index, I.sub.2, of from 0.1 to 200 grams per 10 minutes as determined by ASTM D1238; less then 2 weight % hexane extractables; and a substantial absence of homopolymer wherein said first polymer fraction comprises at least one homogeneous copolymer of ethylene and at least one C.sub.4to10 alpha olefin, and wherein each of said at least one homogeneous copolymer is characterized by having a molecular weight distribution, Mw/Mn, of less then three; and [0018] B) a second polymer fraction having a higher molecular weight then said first fraction; a higher density then said first fraction; and a lower alpha olefin content then said first fraction, wherein said second polymer fraction comprises at least one second homogeneous polymer of ethylene, optionally with at least one C.sub.4to10 alpha olefin comonomer, and wherein each of said at least one second homogeneous polymer of ethylene is characterized by having a molecular weight distribution, Mw/Mn, of less than 3. [0019] As noted above, the second fraction must have both of a higher molecular weight and a lower comonomer content than the first fraction. [0020] It will be appreciated by those skilled in the art that it is extremely difficult to prepare such a polymer composition by a simple mechanical blend of the two polymer fractions. Accordingly, it is highly preferred to prepare the present compositions by solution blending--especially via a solution polymerization process in which the fractions are blended in situ. It is particularly preferred to use two continuously stirred tank reactors (CSTR's) to prepare the compositions--although a single tube reactor (plug flow reactor) or a combination of a tube reactor and a CSTR may also be suitably employed. [0021] The compositions of this invention may be used to prepare a wide variety of goods including injection molded parts, rotomolded parts and film. Preferred compositions which are described in more detail below are especially suitable for the preparation of films. The film may be prepared by conventional "cast" or "blown bubble" techniques. Monolayer films or multilayer films (prepared by coextrusion of multiple layers of laminates) are possible. The resulting films may be used to package foods and consumer goods in sealed packages, including sealed packages for liquids. The films are also suitable for preparing trash bags, "heavy duty packages" (for such goods as peat moss--and other gardening items including bark, fertilizer and decorative gravel--that are exposed to the outdoors in gardening centers); shrink films (which may be used in high performance packaging for poultry or cuts of meat); pallet wraps (to protect goods on pallets during shipping and/or outdoor storage); and stretch films. [0022] The plastic parts and films prepared from the polymer compositions of this invention may include conventional additives such as antioxidants (e.g. hindered phenols and phosphates); UV stabilizers such as hindered amines; antiblocks (e.g. talc and silica); antistatic agents (e.g. low molecular weight polyethylene glycol); processing aids (e.g. fluoropolymers and polyethylene glycols having a molecular weight of from 2,000 to 8,000); pigments and the like. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Continue reading about Heterogeneous/homogeneous copolymer... Full patent description for Heterogeneous/homogeneous copolymer Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Heterogeneous/homogeneous copolymer patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. 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