OLEFIN POLYMER COMPOSITIONS CONTAINING METAL CARBOXYLATE CROSS-LINKING RETARDERS

Abstract: Polymer composition comprising a linear low density ethylene copolymer containing about 1-30 mol % of alpha-olefin comonomer and having improved resistance to cross-linking during thermoforming are provided. The ethylene copolymer has a narrow compositional distribution of chain branching as reflected by compositional distribution breadth index of at least 50 % and is stabilized to inhibit cross-linking during thermoforming by the addition of an effective amount of a metal carboxylate of C1-C22 carboxylic acid.

Description: OLEFIN POLYMER COMPOSITIONS CONTAINING METAL CARBOXYLATE CROSS TNKTNG RRTARDERS BACKGROUND OF THE INVENTION Field of the Invention The invention relates to linear low density ethylene polymer compositions stabilized to inhibit crosslinking during melt processing. Description of the Related Art Various types of polyethylene are known in the art. Low density polyethylene ("LDPE") is generally prepared at high pressure using free radical initiators and typically has a density in the range of 0.915-0.940 g/cm 3 . LDPE is also known as "branched" polyethylene because of the relatively large number of long chain branches extending from the main polymer backbone. High density polyethylene ("HDPE") usually has a density in the range of greater than 0.940 to 0.960 g/cm 3 . HDPE is prepared using a coordination catalyst e.g., Ziegler-Natta type catalysts, at low, moderate or high pressures. HDPE is generally linear without any substantial side chain branching, and is a substantially a crystalline polymer. Linear low density polyethylene ("LLDPE") is generally prepared in the same manner as HDPE, but incorporates a relatively minor amount of alpha-olefin comonomer such as butene, hexene or octene to introduce enough short chain branches into the otherwise linear polymer to reduce the density of the resultant polymer into the range of that of LDPE. Introducing larger concentrations of comonomer can also reduce the density of the ethylene copolymers into the 0.900 to 0.915 g/cm range of very low density polyethylene (VLDPE) and in the "plastomer range" (i.e. 0.88-0.90 g/cm 3 ). The Ziegler/Natta coordination catalysts used to copolymerize ethylene and the alpha-olefin generally produce an LLDPE with a relatively broad weight molecular weight distribution, i.e., Mw/Mn greater than about 3. Such LLDPE's also have relatively broad compositions in that the proportion of alpha-olefin comonomer molecules incorporated into the polymer molecules varies. Generally, the lower molecular weight polymer molecules contain a relatively higher proportion of the alpha-olefin comonomer than the higher molecular weight polymer molecules. A polyethylene such as LLDPE having a broad molecular weight distribution is undesirable in many respects, depending on the desired end use application. For example, LLDPE resins known in the prior art containing relatively high molecular weight molecules are subject to orientation which results in anisotropic properties in the machine versus transverse direction of a fabrication process. On the other hand, LLDPE resins containing relatively lower molecular weight molecules, in which the comonomer is invariably concentrated, tend to exhibit high block and tackiness in fabricated films. These lower molecular weight, highly branched molecules interfere with the proper function of certain additives compounded in the resin, increase the percentage of extractable polymer, and increase fouling in the polymerization plant. The relatively high alpha-olefin comonomer content of these low molecular weight polymer molecules causes such polymer molecules to be generally amorphous and to exude to the surface of fabricated parts, thereby producing an undesirable sticky surface. Prior art polyethylenes such as LLDPE also generally tend to have a very broad, non-uniform distribution of comonomer content, i.e., some polymer molecules have a relatively high alpha-olefin comonomer content while others have a relatively low content. Generally, the polymer molecules of low comonomer content are relatively more crystalline and have a high melting temperature, whereas the high comonomer content polymer molecules are more amorphous and melt at a lower temperature. The presence of a higher melting component is disadvantageous in many applications, for example where softness or clarity is desired. On the other hand, the presence of a lower melting component frequently results in a high quantity of extractables, which limit food contact applications. Recently, a new class of LLDPE polymers has been developed based upon copolymers of ethylene and a minor content of at least one alpha-olefin comonomer. These polymers are prepared preferably using a metallocene transition metal catalyst and exhibit an average molecular weight distribution (Mw/Mn) of <_3 and a compositional distribution breadth index (CDBI) of at least 50%. These copolymers and their method of preparation are more particularly disclosed in US Patent 5,382,631, the complete disclosure of which is incorporated herein by reference. One of the main distinctions which differentiates the newer LLDPE materials from the LLDPE of the prior art is that the branching alpha-olefin comonomer tends to be more uniformly and randomly distributed along the polymer chain rather than concentrated in the lower molecular weight fractions of chain molecules as is the case with prior art LLDPE described above. Because of this more uniform comonomer distribution and a narrow molecular weight distribution, the newer LLDPE materials avoid many of the disadvantages of conventional LLDPE materials as described above, particularly when used to prepare films for packaging applications. It has been observed, however, that the metallocene-polymerized LLDPE polymers tend to have a higher susceptibility towards molecular crosslinking when subjected to thermoform shearing forces, e.g. extrusion, than the conventional LLDPE materials such as prepared using Ziegler/Natta transition metal catalyst systems. This crosslinking phenomena is reflected by gels present in extruded film and by a decrease in the melt index of the polymer after extrusion. It is believed that this phenomena is caused by the organometallic structures of the metallocene catalyst residues and their silica supports present in the LLDPE polymer, from which free radicals can be generated under the high heat conditions of extrusion. Whereas thermal degradation processes in conventional LLDPE tends to produce more low molecular weight species which serve to plasticize the polymer, the thermal degradation processes in metallocene polymerized LLDPE tends to favor increased crosslinking of the molecular chains, likely because the short chain branches are more randomly spaced along the polymer chains and thus less susceptible to scission. Accordingly, it is a primary object of this invention to provide linear low density ethylene polymer compositions which are stabilized to inhibit crosslinking of the molecular chains when subjected to shear conditions encountered during thermoforming. SUMMARY OF THE INVENTION The invention provides a composition comprising a mixture of: (a) a linear low density ethylene polymer containing from about 1-30 mol% of at least one alpha- olefin comonomer and having an average molecular weight distribution Mw/Mn of < 3 and a compositional distribution breadth index of at least 50%; and (b) at least one metal carboxylate of a Ci to C 22 saturated or unsaturated carboxylic acid, said metal carboxylate present in said composition in an amount sufficient to inhibit crosslinking of said composition when said composition is heated under conditions of shear at a temperature above the melting point of said ethylene polymer. The invention also provides a process for melt processing a polymer composition comprising: (a) forming a composition comprising a mixture of a linear low density ethylene polymer containing from about 1-30 mol% of at least one alpha-olefin comonomer and having an average molecular weight distribution Mw/Mn of < 3 and a compositional distribution breadth index of at least 50%, and at least about 0.005 wt.%, based on the weight of said ethylene polymer, of a metal carboxylate of a C r C^ carboxylic acid; (b) thermoforming said composition to form a shaped article at a temperature above the melting point of said ethylene polymer under mixing conditions of shear sufficient to cause scission of at least some of the polymer chains of said ethylene polymer; and (c) recovering said shaped article. Shaped articles, e.g. films, prepared by mixing and shaping the polymer composition of this invention exhibit a lower degree of crosslinking as evidenced by less gel formation in the extruded film and a higher polymer melt index after thermoforming than an otherwise identical composition which is free of the metal carboxylate stabilizer. DETAILED DESCRIPTION OF THE INVENTION Prior to a detailed discussion of this invention, it should be pointed out that it is known in the art to include minor quantities of various metal carboxylates into polyethylene compositions to enhance various physical properties of the composition or films prepared therefrom. For example, US Patent 4,454,281 discloses low density ethylene copolymer compositions containing a combination of metal salt of a fatty acid and an antiblocking agent, present to improve the optical properties of extruded film. A similar composition containing a combination of a diben2ylidene sorbitol and a metal stearate is disclosed in EPA 0 091 612. In addition, molded articles prepared from polyethylene and exhibiting improved surface gloss and low haze and containing minor amounts of a mixture of a metal salt of a C 7 -C 22 fatty acid and a metal salt of an aromatic carboxylic acid are disclosed in UK Patent 1,338,142. However, none of these or other prior art publications disclose the use of metal carboxylates in combination with the LLDPE polymers of this invention to inhibit crosslinking of the polymer when heated under conditions of temperature and shear such as encountered during extrusion of the polymer composition. The linear low density ethylene (LLDPE) polymer component of the present invention is a copolymer (interpolymer) of from about 70-99 mol% of ethylene and from about 1-30 mol% of one or more alpha-olefin comonomers, said polymer having a density in the range of from about 0.9 to about 0.94 g/cm . The preferred alpha-olefin content of the ethylene polymer lies in the range of from about 2-15 mol%. The molecular weight of the LLDPE component may range from 1,000 to 1,000,000 or more depending on the particular end use, preferably 10 4 -10 5 , and especially 2x10 -5x10 . As used herein, the terms "average molecular weight" and "molecular weight" refer to weight average molecular weight unless otherwise indicated. The linear polyethylene component preferably has a narrow molecular weight distribution (MWD). By "narrow MWD" is meant that the ratio of the weight average molecular weight (M w ) to the number average molecular weight (M„) is less than or equal to 3.0. Particularly preferred are the linear polyethylene components having a very narrow MWD, i.e. M^/M,, less than or equal to 2.5, and especially less than or equal to 2.0. Molecular weight distributions of ethylene interpolymers are readily determined by techniques known in the art, such as, for example, size exclusion or gel permeation chromatography. The linear polyethylene component preferably has a composition distribution (CD) such that the composition distribution breadth index (CDBI) is at least 50%, more preferably greater than about 60% and most preferably greater than about 70%. CDBI is a measure of composition distribution, and is defined as the weight percent of the copolymer molecules having a comonomer content within 50% (that is, 25% on each side) of the median total molar comonomer content. The CDBI of a copolymer is readily determined utilizing well known techniques for isolating individual fractions of a sample of the copolymer. One such technique is Temperature Rising Elution Fraction (TREF), as described in Wild et al., J. Poly. Sci., Poly. Phys. Ed., vol. 20, p. 441 (1982), which is incorporated herein by reference. To determine CDBI, a solubility distribution curve is first generated for the copolymer. This may be accomplished using data acquired from TREF techniques described above. This solubility distribution curve is a plot of the weight fraction of the copolymer that is solubilized as a function of temperature. This is converted to a weight fraction versus composition distribution curve. For the purpose of simplifying the correlation of composition with elution temperature, the weight fractions less than 15,000 are ignored. These low weight fractions generally represent a trivial portion of the polymer. The remainder of this description and the appended claims maintain this convention of ignoring weight fractions below 15,000 in the CDBI measurements. From the weight fraction versus composition distribution curve the CDBI is determined by establishing what weight percent of the sample has comonomer content within 25% each side of the median comonomer content. Further details of determining CDBI of a copolymer are known to those skilled in the art, see, for example, PCT Patent Application WO 93/03093, published February 18, 1993. Unless otherwise indicated, terms such as "comonomer content", "average comonomer content" and the like refer to the bulk comonomer content of the indicated copolymer on a molar basis. The linear polyethylene of the invention may be prepared by the use of activated catalyst systems of the metallocene type known to provide narrow CD/MWD resins. Cyclopentadienylide catalyst systems using a metallocene complex in conjunction with an alumoxane cocatalyst or reaction product thereof are suitable for preparing the polymer components utilized in the invention. Similarly, catalyst systems with ionizing cocatalysts capable of providing non-coordinating anions will also be suitable in this regard. The metallocene catalyst may be represented by the general formula (C ^ nj MR^R' p wherein CY is a substituted or unsubstituted cyclopentadienyl ring; M is a Group IVB, or VB transition metal; R and R' are independently selected from halogen, hydrocarbyl groups, or hydrocarboxyl groups having 1-20 carbon atoms; m = 1-3, n = 0-3, p = 0-3, and the sum of m+n+p equals the oxidation state of M. Various forms of the catalyst system of the metallocene type may be used for polymerization to prepare the polymer components of the present invention including those of the homogenous or the heterogenous, supported catalyst type wherein the catalyst and alumoxane cocatalyst are together supported or reacted together onto an inert support for polymerization by gas-phase, high pressure, slurry, or solution polymerization. A more complete description of these metallocene catalysts and their method of preparation may be found in the above referenced US Patent 5,382,631, and also in US Patents 5,408,017, 5,470,927, 5,483,014 and WO 96/04319. Metal carboxylates which are suitable as crosslinking retarders in this invention include metal salts of carboxylic acids having from 1 to about 22 carbon atoms, more preferably from 2 to about 18 carbon atoms. The number of carbon atoms includes the carboxylic acid group. Typical acids are monocarboxylic saturated or unsaturated acids such as formic, acetic, heptylic, caprylic, capric, lauric, palmitic, stearic and behenic acids as well as their unsaturated analogs such as oleic and ricinoleic acids. The carboxylic acid may also include aromatic acids such as benzoic or naphthenic acid and their derivatives. Suitable salt-forming cations include zinc, calcium, copper, cadmium, aluminum, sodium, potassium, nickel, magnesium, barium, lead and iron, most preferably cations of Group I to Group III metals of the Periodic Table. The most preferred carboxylates include zinc acetate and zinc stearate. The quantity of metal carboxylate added to the ethylene polymer composition to hinder polymer crosslinking may generally range from about 0.005 up to about 1 wt.%, more preferably from about 0.01 up to about 0.5 wt.%, and most preferably from about 0.01 up to about 0.25 wt.%, based on the weight of the ethylene polymer present in the composition. The composition of the invention may also include a blend of the LLDPE of the invention with up to about 50 wt.%, based on total polymer content, of one or more different olefin polymers such as other low, medium or high density polyethylenes, polypropylene, copolymers of ethylene and propylene and like thermoplastics. In a preferred embodiment of the invention, the composition also contains one or more antioxidant (stabilizer) materials, such as phenolic, phosphite or phosphonite antioxidants. Suitable phenolic antioxidants which may be used include polyalkyl-substituted phenols such as 2,6-di-t-butyl-p-cresol, octadecyl-3-(3',5'-di-t-butyl-4'- hydroxyphenyl) propio-nate, octadecyl-(3,5-di-tert-butyl-4-hydroxyhydro- cinnamate), 2,6-di-tert-butyl-4-me-thylphenol, tetrakis (methylene-3(3', 5'-di-t-butyl- 4-hydroxyphenyl)propionate) meth-ane, 1 ,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4- hydroxybenzyl) benzene, l,3,5-tris(3,5-di-t-butyl-4-hydroxybenzyl) isocyanurate, and n-octadecyl-8-(4'-hydroxy-3',5'-di-t-butylphenyl) propionate. These phenolic antioxidants may be used either individually or in combination of two or more thereof. Suitable phosphite and phosphonite stabilizers which can be used include alkyl and aryl phospites such as tri-n-octyl, tri-n-decyl and tri (mixed mono and dinonyl phenyl) phosphite, distearyl pentaerythritol diphosphite; tetrakis (2,4-t-butylphenyl)- 4,4' biphenylene diphosphite; bis(2,4-di-t-butylphenyl)-pentaerythritol diphosphite; tris (2,4-di-t-butylphenyl) phosphite; and like materials. The most preferred stabilizes are those having the formula: wherein R is C 2 -C lg alkyl or alkyl-substituted phenyl. A particularly preferred phosphite is of the above formula wherein R is -Cι 8 H 37 , marketed by Borg Warner under the tradename WESTON™619, or WESTON™399. Mixtures of phenolic and phosphite antioxidants may also be used. The antioxidant is preferably added to the composition at a level of from about 100 to about 5,000 parts per million, more preferably 150 to 2500 parts per million, based on the polymer content of the composition. The composition may also contain one or more adjuvant materials which are commonly employed in ethylene polymer-based extrudable compositions, including plasticizers, fillers, pigments, lubricants, slip agents, processing aids, dyes, pigments and like materials. The composition may also contain additional decomposition inhibitors or free radical scavengers such as zinc or magnesium oxide, polyakylene glycol anti- gelation agents such as polyethylene glycol or polypropylene glycol. These components or combinations thereof may be generally present in the composition at levels in the range of from about 0.01 to about 1 wt%, based on the weight of the polymer component of the composition. These and other adjuvants, as well as the metal carboxylate, may be incorporated into the composition either at the time of polymer composition is pelletized or by the user as a separate additive package prior to the thermoforming of the composition to form shaped articles. The polymer composition of this invention may be thermoformed to form shaped articles such as films, containers and molded three dimensional articles by well known techniques such as blown film or cast film extrusion, uni- or biaxial orientation, blow molding, injection molding or rotomolding. In these methods, the polymer composition is first mixed under conditions of shear in a suitable mixing device such as a screw extruder, Banbury mixer or Brabender plasticorder and heated to a temperature above the melting point of the polymer components of the composition, generally in the range of about 140 °C up to about 350 °C, more preferably from about 150 °C to about 300 °C. Tubular film may be prepared using an extruder/mixer by passing the extrudate through an annular die in an upward or downward direction and the resulting tubular film expanded to the desired extent using a pressurized gas, cooled and flattened, followed by slitting to form a film. Other shaped articles such as bottles, lids and other molded shapes may be prepared by subjecting extrudate or molten polymer to well known injection molding, blow molding or rotomolding techniques. The following examples are illustrative of the invention. Materials identified in the examples are as follows: ECD 103 - a metallocene/alumoxane polymerized copolymer of ethylene and about 3 mol% hexene - MI of 1.04 dg./min, density of 0.9169 g/cm , and an ash of 274 ppm. ECD 103' - a metallocene/alumoxane polymerized copolymer of ethylene and about 3 mol% hexene - MI of 1.13 dg./min, density of 0.9161 g/cm , and an ash of 544 ppm. LLDPE 3001 - Ziegler/Natta polymerized copolymer of ethylene and about 3 mol% hexene - MI of 0.88 dg/min, density of 0.9187 g/cm 3 , and an ash of 367 ppm. IRGANOX™ 1076 - octadecyl - 3-(3*,5'-ditert-butyl-4 * - hyroxyphenyl) propionate. W-399 - phosphite PEG - polyethylene glycol (anti gelation agent) PPA - fluoroelastomer (processing agent) EXAMPLE 1 A series of 12 different formulations as described in Table 1 were prepared by mixing the indicated ingredients in a small scale Brabender plasticorder at 250 °C for the times indicated in Table 1. The resulting blends were evaluated for Melt Index (MI - ASTM 1238 - Cond. E), Flow Index (HMJ. - ASTM 1238 - Cond F), Melt Index Ratio (MIR - Flow Index/Melt Index) and Swell Ratio (SR), which is a measure of the diameter of the polymer strand extruded in the MI measurement and is inversely proportional to the melt index. Table 1 Formulations 1-2, 5-6 and 9-10 contain zinc oxide (ZnO) which is a known additive for minimizing the formation of free radical groups, while formulations 3-4, 7-8 and 11-12 contain zinc acetate, an additive within the scope of the invention. Comparison of the met index and melt flow properties show that the compositions based on the metallocene polymerized ethylene polymer containing the zinc acetate additive exhibit an increase in melt index and melt flow properties which is indicative of a reduced crosslinking propensity under the conditions of shear mixing. This effect is less predominant with respect to samples 11 and 12 as compared with samples 9 and 10 which each contain a Ziegler/Natta polymerized ethylene polymer. EXAMPLE 2 A series of 19 different formulations each containing 50 g of ECD- 103 polyethylene and other components in the quantities listed in Table 2 were processed in a Brandender™ plasticorder at 250 °C for the times indicated in Table 2. Sample 13 reflects the properties of unprocessed virgin ECD- 103, whereas samples 14 and 15 are processed controls containing no additives. The Irganox 1076, W-399 and indicated zinc salts were added as an additive package in the quantities listed in the table prior to mixing. Samples 16 and 17 also contained 200 ppm of PEG and 800 ppm of PPA. The data showed a synergistic effect with respect to the use of the additive package components Irganox™ 1076 and W 399 used in combination with zinc acetate or zinc stearate as compared with formulations which did not contain this combination of ingredients or which contained ZnO, as reflected by a comparison of the melt index values of samples 18, 19, 28 and 29 with samples 16 and 17, as well as a comparison of the melt index values of samples 22, 23, 30 and 31 with samples 26- 27. I A rS I f r, z note : All blends contained 50 g of ECD- 103 copolymer and were processed at 250 °C.

Claims: CLAIMS 1. A polymer composition comprising a mixture of: a) a linear low density ethylene polymer containing from about 1-30 mol% of at least one alpha-olefin comonomer and having an average molecular weight distribution Mw/Mn of < 3 and a compositional distribution breadth index of at least 50%; and b) at least one metal carboxylate of a C r C 22 saturated or unsaturated carboxylic acid, said metal carboxylate present in said composition in an amount sufficient to inhibit crosslinking of said composition when said composition is heated under conditions of shear at a temperature above the melting point of said ethylene polymer. 2. The composition of claim 1 wherein said ethylene polymer has a compositional distribution breadth index of greater than about 60%. 3. The composition of claims 1 or 2 wherein said ethylene polymer has a compositional distribution breadth index of greater than about 70%. 4. The composition of claims 1-3 wherein said comonomer comprises at least one C 3 -C 12 alpha-olefin. 5. The composition of claims 1-4 wherein said ethylene polymer has an Mw/Mn of less than 3.0. 6. The composition of claims 1-5 wherein said ethylene polymer is produced by copolymerizing a mixture of ethylene and said at least one alpha-olefin comonomer in the presence of a metallocene transition metal catalyst having the formula (CY) m MR l' p wherein CY is a substituted or unsubstituted cyclopentadienyl ring; M is a Group IVB or VB transition metal; R and R are independently selected from halogen, hydrocarbyl or hydrocarboxyl groups having 1-20 carbon atoms; m = 1-3, n = 0-3, p = 0-3 and the sum of m+n+p equals the oxidation state of m. 7. The composition of claims 1-6 wherein said metal carboxylate contains 2-18 carbon atoms. 8. The composition of claims 1-7 wherein said metal is a Group I to Group III metal of the Periodic Table. 9. The composition of claim 8 wherein said metal is zinc. 10. The composition of claim 9 wherein said metal carboxylate comprises zinc acetate. 11. The composition of claim 9 wherein said metal carboxylate comprises zinc stearate. 12. The composition of claims 1-11 wherein said metal carboxylate is present in said composition at a level of from about 0.005 up to about 1 wt.%, based on the weight of said ethylene polymer. 13. The composition of claim 12 wherein said metal carboxylate is present in said composition at a level of from about 0.01 up to about 0.5 wt.%. 14. The composition of claims 1-13 wherein said ethylene polymer has a density in the range of from about 0.9 to about 0.94 g/cm 3 . 15. The composition of claims 1-14 further containing an effective amount of an antioxidant selected from the group consisting of phenolics, phosphites, phosphonites and mixtures thereof. 16. The composition of claim 15 wherein said antioxidant comprises a hindered phenol. 17. The composition of claim 15 wherein said antioxidant comprises a phosphite. 18. A process for thermoforming a polymer composition comprising: a) forming a composition comprising a mixture of a linear low density ethylene polymer containing from about 1-30 mol% of at least one alpha-olefin comonomer and having an average molecular weight distribution Mw/Mn of < 3 and a compositional distribution breadth index of at least 50%, and at least about 0.005 wt.%, based on the weight of said ethylene polymer, of a metal carboxylate of a C r C 22 carboxylic acid; b) thermoforming said composition to form a shaped article at a temperature above the melting point of said ethylene polymer under mixing conditions of shear sufficient to cause scission of at least some of the polymer chains of said ethylene polymer; and c) recovering said shaped article. 19. The process of claim 15 wherein said composition is thermoformed at a temperature in the range of from about 140 °C to about 350 °C. 20. The process of claim 15 wherein said composition has a higher melt index after said thermoforming step than the melt index of an otherwise identical composition which is free of said metal carboxylate.

Inventors: ARJUNAN PALANISAMY, ARJUNAN, PALANISAMY
Applicants: EXXON CHEMICAL PATENTS INC, EXXON CHEMICAL PATENTS INC.
Application year: 1998
Main classification: C08K5
IPC codes: B29C51, C08F210, C08K5, C08L23