Classifications

• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
  • C08K5/00—Use of organic ingredients
  • C08K5/04—Oxygen-containing compounds
  • C08K5/09—Carboxylic acids; Metal salts thereof; Anhydrides thereof
  • C08K5/098—Metal salts of carboxylic acids
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
  • C08L23/00—Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers
  • C08L23/02—Compositions of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Compositions of derivatives of such polymers not modified by chemical after-treatment
  • C08L23/04—Homopolymers or copolymers of ethene
  • C08L23/08—Copolymers of ethene
  • C08L23/0807—Copolymers of ethene with unsaturated hydrocarbons only containing more than three carbon atoms
  • C08L23/0815—Copolymers of ethene with aliphatic 1-olefins
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
  • C08L2314/00—Polymer mixtures characterised by way of preparation
  • C08L2314/06—Metallocene or single site catalysts

Definitions

• the invention relates to linear low density ethylene polymer compositions stabilized to inhibit crosslinking during melt processing.
• LDPE Low density polyethylene
• free radical initiators 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 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.,
• HDPE 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 ).
• VLDPE very low density polyethylene
• 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.
• 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.
• 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.
• 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.
• 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.
• the presence of a lower melting component frequently results in a high quantity of extractables, which limit food contact applications.
• LLDPE polymers 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.
• 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.
• 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.
• thermal degradation processes in conventional LLDPE tends to produce more low molecular weight species which serve to plasticize the polymer
• 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.
• the invention provides a composition
• 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
• 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.
• 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 .
• 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).
• MWD narrow molecular weight distribution
• MWD narrow MWD
• MWD is meant that the ratio of the weight average molecular weight (M w ) to the number average molecular weight (M Thread) 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.
• 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.
• TREF Temperature Rising Elution Fraction
• 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.
• 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.
• catalyst systems with ionizing cocatalysts capable of providing non-coordinating anions will also be suitable in this regard.
• 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.
• 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.
• 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.
• the composition also contains one or more antioxidant (stabilizer) materials, such as phenolic, phosphite or phosphonite antioxidants.
• antioxidant stabilizer
• 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) isocyan
• 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:
• R is C 2 -C lg alkyl or alkyl-substituted phenyl.
• a particularly preferred phosphite is of the above formula wherein R is -C ⁇ 8 H 37 , marketed by Borg Warner under the tradename WESTONTM619, or WESTONTM399. Mixtures of phenolic and phosphite antioxidants may also be used.
• 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.
• 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.
• compositions 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.
• 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.
• 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.
• 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.
• 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.
• MI Melt Index
• HMJ. - ASTM 1238 - Cond F Flow Index
• MIR - Flow Index/Melt Index Melt Index Ratio
• Swell Ratio Swell Ratio
• 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.
• ZnO zinc oxide
• 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.

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• Chemical Kinetics & Catalysis (AREA)
• Medicinal Chemistry (AREA)
• Polymers & Plastics (AREA)
• Organic Chemistry (AREA)
• Compositions Of Macromolecular Compounds (AREA)
• Manufacture Of Macromolecular Shaped Articles (AREA)
• Blow-Moulding Or Thermoforming Of Plastics Or The Like (AREA)
• Addition Polymer Or Copolymer, Post-Treatments, Or Chemical Modifications (AREA)

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• 1998
  • 1998-02-23 EP EP98911399A patent/EP0970143A1/de not_active Withdrawn
  • 1998-02-23 CA CA002283482A patent/CA2283482A1/en not_active Abandoned
  • 1998-02-23 WO PCT/US1998/003504 patent/WO1998042777A1/en not_active Application Discontinuation
  • 1998-02-23 JP JP54570298A patent/JP2001518136A/ja active Pending

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