WO1997006951A1 - Films comprising metallocene catalyzed polyethylene - Google Patents

Abstract

A self supporting film having one or more layers wherein at least one layer has a percent haze of less than 17.8 and the polymer of that layer consists essentially of a polyethylene having a density of at least 0.925 grams per cc, a molecular weight distribution of no more than 4, optionally containing a fluoroelastomer, and methods for making such film are disclosed.

Description

FILMS COMPRISING METALLOCENE CATALYZED POLYETHYLENE

Field of the Invention

This apphcation is a Continuation-in-Part of copending U.S. Patent

Apphcation Serial No. 08/515,498 filed August 15, 1995, the disclosure of which

is incorporated herein by reference. This invention relates to film of polymers

produced from a monomer consisting essentially of ethylene. In another aspect,

the present invention relates to polyethylene film having a good balance of

physical, processing, and optical properties.

Background of the Invention

In its broadest sense, the term "film" as used herein refers to

self-supporting materials having a wide range of thicknesses. Examples would

include thicknesses in the range of 0.05 to about 40 mils, more typically about

0.25 to about 5 mils (1 mil equals 1/1000 of an inch). Films can be made using

a variety of techniques such as casting, blowing, and extrusion.

Good clarity in polyethylene blown film as indicated by low Haze

and high Gloss has been noted in the past to be dependent upon several factors. Typically the Haze increases (and the Gloss decreases) as the polymer density and

molecular weight distribution increases. Also, it has been noted that typically the

surface roughness increases as the molecular weight distribution and density

increases. Film stiffness on the other hand, which is often a desired property of

the blown film dependent upon the actual application, has been noted to increase

as density increases. Therefore, there has usually been a trade-off between film

clarity and stiffness in polyethylene blown film.

Often in forrning multi-layered films, a base layer of high molecular

weight high density polyethylene or medium molecular weight high density weight

polyethylene has been employed to provide strength and a low density

polyethylene or linear low density polyethylene layer has been provided to provide

other properties. Often, however, it has been noted that the low density

polyethylene and linear low density polyethylene layers are tacky and sticky

unless antiblock agents are included. Such antiblock agents, however, generally

also have an adverse effect upon the clarity and physical properties.

An object of the present invention is to provide a method for

producing films of ethylene polymers having a density of at least about 0.925 g/cc

which have a good balance of processing, physical, and optical properties.

Other aspects, objects, and advantages of the present invention will

be apparent from the following comments. Summary of the Invention

In accordance with the present invention, there is provided a

unusually clear self-supporting film comprising at least one layer having a percent

haze of less than 17.8 wherein the polymer of said layer consists essentially of

polyethylene having a density of at least about 0.925 g/cc and a molecular weight

distribution of no more than 4. The narrow molecular weight polyethylene having

a density of at least about 0.925 g/cc is preferably selected from polyethylenes

which can be formed into a 1 mil blown film having a percent haze of less than

17.8, or most preferably no more than 10.

In one preferred embodiment, the film has only one layer of polymer

consisting essentially of polyethylene having a density in the range of 0.93 to

about 0.945 g/cc and a molecular weight distribution in the range of about 1.5 to

about 4, or more preferably about 1.5 to about 3.5. In another preferred

embodiment the film is multilayered and at least one layer has a percent haze of

less than 17.8, more preferably a percent haze of less than 10, and comprises

polyethylene having a density of at least 0.925 g/cc and a molecular weight

distribution no more than 4.

Detailed Description of the Invention

The polyethylene useful for producing the inventive films can be

produced using a suitable metallocene-containing polymerization catalyst system. In a particularly preferred embodiment the polyethylene is produced in a slurry,

i.e. particle form, type process wherein the polymer is formed under conditions

such that the polymer is produced in the form of solid particles that can be readily

separated from the liquid polymerization diluent. In such particle form

polymerizations it is preferable that the metallocene-containing catalyst system be

employed in a form that is substantially insoluble in the polymerization diluent

during the polymerization process. Various techniques are known for producing

such relatively insoluble catalyst systems. Some examples are shown in U.S.

5,354,721; 5,411,925; and 5,414, 180.

One particularly preferred type of relatively insoluble solid

metallocene catalyst system can be produced by prepolymerizing a mixture of a

metallocene, preferably a metallocene having olefinically unsaturated substituents,

and a suitable cocatalyst in the presence of an olefin, generally containing 2 to 8

carbon atoms. In particularly preferred embodiment the solid catalyst system is

obtained by polymerizing ethylene in the presence of an alkane liquid diluent

under slurry polymerization conditions using a special type of metallocene-based

catalyst system. The catalyst system is a solid catalyst prepared by (a) combining

5-(9-fluorenyl)-5-(cyclopentadienyl)-hexene-l zirconium dichloride and

methylaluminoxane in a liquid, (b) prepolymerizing ethylene in the resulting

hquid, and (c) separating the resulting solid prepolymerized catalyst system from

the hquid. It is preferred that the liquid employed in step (a) be an organic liquid in which the methylaluminoxane is at least partially soluble. Preferably some

aromatic solvent is employed in step (a). Examples of aromatic solvents include

benzene, toluene, ethylbenzene, diethylbenzene, and the like. Preferably the

amount ofthe hquid should be such as to dissolve the product of reaction between

the metallocene and the aluminoxane, provide desirable polymerization viscosity

for the polymerization, and to permit good mixing. During the mixing, the

temperature would preferably be kept below that which would cause the

metallocene to decompose. Typically the temperature would be in the range of

about -50°C to about 150°C. Preferably, the metallocene, the aluminoxane, and

the liquid diluent are combined at room temperature, i.e. around 10°C to 30°C.

The reaction between the aluminoxane and the metallocene is relatively rapid.

The reaction rate can vary over a wide range, however, it is generally desired that

they be contacted for an amount of time in the range of about 1 minute to about

1 hour.

It is also within the scope of the invention to carry out the step (a)

in the presence of a particulate solid. Any number of particulate solids can be

employed. Typically this solid would be any inorganic solid that does not

interfere with the desired end results. Examples include porous supports such as

talc, inorganic oxides, resins to support material such as particulate polyolefins.

Examples of inorganic oxide materials include metal oxides of Groups II-V, such as silica, alumina, silica-alumina, and mixtures thereof. Other examples of

inorganic oxides are magnesia, titania, zirconia, and the like.

If a solid is employed, it is generally desirable for the solid to be

thoroughly dehydrated prior to use. Preferably it is dehydrated so as to contain

less than 1 percent loss on ignition. Thermal dehydration may be carried out in

a vacuum or while purging with a dry inert gas such as nitrogen at a temperature

of about 20°C to about 1000°C and preferably from about 300°C to about 870°C.

Pressure considerations are not viewed as critical. The duration of the thermal

treatment can be from about 1 to about 24 hours as needed.

Dehydration can also be accomplished by subjecting the solid to a

chemical treatment in order to remove water and reduce the concentration of

surface hydroxyl groups. Chemical treatment is generally capable of converting

all water hydroxyl groups in the oxide surface to relatively inert species. Useful

chemical agents are for example, carbon monoxide, carbonyl sulfide,

trimethylaluminum, ethyl magnesium chloride, chloro silanes such as SiCl₄,

disilazane, trimethylchlorosilane, dimethylamino trimethylsilane, and the like.

The amount of aluminoxane and metallocene used in forrning a

liquid catalyst system for the prepolymerization can vary over a wide range.

Typically, however, the molar ratio of the aluminum in the aluminoxane to the

transition metal ofthe metallocene is in the range of about 1 : 1 to about 20,000: 1,

more preferably a molar ratio of about 50: 1 to about 2,000: 1 is used. If a particulate solid, i.e. silica, is used, generally it is used in an amount such that the

weight ratio of the metallocene to the particulate solid is in the range of about

0.00001/1 to 1/1, more preferably 0.0005/1 to 0.2/1.

The prepolymerization is conducted in the liquid catalyst system,

which can be a solution, a slurry, or gel in a liquid. A wide range of olefins can

be used for the polymerization. Typically, however, the prepolymerization will

be conducted using an olefin, preferably selected from ethylene and non-aromatic

alpha olefins, such as propylene. It is within the scope of the invention to use a

mixture of olefins, for example, ethylene and a higher alpha olefin can be used for

the prepolymerization. The use of a higher alpha olefin, such as 1 -butene, with

ethylene, is believed to increase the amount of copolymerization occurring

between the olefin monomer and the olefinically unsaturated portion of the

metallocene.

The prepolymerization can be conducted under relatively mild

conditions. Typically this would involve using low pressures of the olefin and

relatively low temperatures designed to prevent site decomposition resulting from

high concentrations of localized heat. The prepolymerization typically occurs at

temperatures in the range of about -15 °C to about +150°C, more typically in the

range of about 0°C to about +30°C. The amount of prepolymer can be varied but

typically would be in the range of from about 1 to about 95 weight percent of the

resulting prepolymerized solid catalyst system, still more preferably about 5 to about 80 weight percent. It is generally desirable to carry out the

prepolymerization to at least a point where substantially all of the metallocene is

in the solid rather than in the liquid, since that maximizes the use of the

metallocene.

After the prepolymerization, the resulting solid prepolymerized

catalyst is separated from the liquid reaction mixture. Various techniques known

in the art can be used for carrying out this step. For example, the material could

be separated by filtration, decantation, or vacuum evaporation. It is currently

preferred, however, not to rely upon vacuum evaporation since it is considered

desirable to remove substantially all of the soluble components in the liquid

reaction product ofthe prepolymerization from the resulting solid prepolymerized

catalyst before it is stored or used for subsequent polymerization. After separating

the solid from a hquid, the resulting solid is preferably washed with a hydrocarbon

and dried using a high vacuum to remove substantially all the liquids or other

volatile components that might still be associated with the solid. The vacuum

drying is preferably carried out under relatively mild conditions, i.e. temperatures

below 100°C. More typically the prepolymerized solid is dried by subjection to

a high vacuum at a temperature of about 30 °C until a substantially constant weight

is achieved. A preferred technique employs at least one initial wash with an

aromatic hydrocarbon, such as toluene, followed by a wash with a paraffinic

hydrocarbon, such as hexane, and then the vacuum drying. It is also within the scope of the present invention to add a

particulate solid to the liquid catalyst system after it has been formed and then to

carry out the prepolymerization in the presence of that solid. Another option is to

add a particulate solid ofthe type aforementioned after the prepolymerization or

after the solid prepolymerized catalyst system has been separated from the liquid.

This resulting solid prepolymerized catalyst system is capable of

preparing polymers of ethylene having a fairly wide range of densities. Typically,

in preparing the lower density versions, the ethylene is polymerized in

combination with a smaller amount, generally less than 20 mole percent, of at least

one other alpha olefin, generally containing about 3 to about 10 carbon atoms,

examples of which include aliphatic hydrocarbons such as butene- 1, pentene- 1,

hexene- 1, 4-methylpentene-l, octene- 1, and the like. The solid prepolymerized

catalyst system can be employed using slurry polymerization conditions.

Typically the polymerization temperature would be selected so as to provide slurry

polymerization conditions in the particular liquid diluent selected. Typically the

temperature would be in the range of about 20°C to about 130°C. With isobutane

as the hquid diluent, temperatures in the range of about 60 °C to about 110°C have

been found desirable. For producing polymers for film apphcations, it is generally

desirable to produce a polymer having a melt index of less than 5. This can be

accomplished by adjusting the molar ratio of hydrogen to ethylene in the polymerization process, changing the reactor temperature, and/or changing the

ethylene concentration.

When the polymerization is carried out in a continuous loop slurry

process, it is generally desirable to include in the reaction mixture a small amount

of an antistatic agent. An example of such as antistatic agent is the material sold

by DuPont Chemical Co. under the trade name Stadis 450.

In a particle form type polymerization the above described type of

catalyst system is capable of producing polyethylene homopolymers and

copolymers having densities of 0.925 g/cm or higher with molecular weight

distributions of no more than 4 that are useful for making films having percent

haze of less than 17.8, especially preferred polyethylenes having densities in the

range of 0.925 to 0.95g/cc. The polymers produced in that manner have low flow

activation energies, i.e. below about 25 kJ/mole, anda critical shear stress at the

onset of melt fracture of less than 4 x IO⁶ dyne/cm². This is considered to indicate

that the polymers are substantially linear polymers substantially free of long chain

branching. The number of long chain branches in such polymers is considered to

be less than 0.01/1000 carbon atoms. The term "long chain branching" as used

herein refers to branches having a chain length of at least 6 carbon atoms. A

method of deterrnining long chain branching is disclosed in Randal, Rev.

Macromol. Chem. Phys., C29 (243), 285-297. The ethylene polymers produced in a particle form process with that

catalyst system are also beheved to have a very uniform distribution of short chain

branches both at the intramolecular level (monomer sequence distributions along

the chain) and at the intermolecular level (monomer distribution between polymer

chains of different molecular weights). Homopolymers and ethylene-hexene

copolymers produced with such catalysts are particularly unusual in that they

contain ethylene branches even though no butene comonomer was employed in the

polymerization. It is theorized that butene is formed insitu in the polymerization

and that such results in a very uniform distribution of the ethylene branches. The

shear stress response of such polymers is essentially independent of the molecular

weight distribution.

It is typically desirable to add stabilizers to the polymer recovered

from the polymerization process. A number of suitable stabilization packages are

known in the art. Stabilizers can be incorporated into the polymer during a

pelletization step or by reextrusion of previously produced pellets. One example

of a stabilizer would be Irganox® 1010 antioxidant which is believed to be a

hindered polyphenol stabilizer containing tetrakis [methylene 3-(3,5-di

tertbutyl-4-hydroxy-phenylpropionate)] methane produced by Ciba-Geigy

Coφoration. Another example is the PEP-Q® additive which is a product of

Sandoz Chemical, the primary ingredient of which is believed to be

tetrakis-(2,4-di-tertbutyl-phenyl)-4,4' biphenyl phosphonite. Other common stabilizer additives include calcium stearate or zinc stearate. Still other stabilizers

commonly used include Ultranox 626 antioxidant which is a product of GE, the

primary ingredient of which is believed to be bis(2,4-di-t-butylphenyl)

pentaerythritol diphosphite, and Ultranox 627A antioxidant which is believed to

be Ultranox 626 containing about 7 weight percent of a magnesium aluminum

hydrocarbonate. Such stabilizer additives can be employed in generally any

suitable amount. The amounts used are generally the same as have been used for

other polyethylene polymers. Often the amounts for each additive is less than 0.2

weight percent based upon the weight ofthe polymer.

The molecular weight ofthe polyethylene used to make the inventive

film can vary over a wide range. Typically for forrning films by blowing it is

desirable for the polymer to have a melt index in the range of about 0.1- lOdg/min,

more preferably about 0.2-5dg/min. Generally ifthe melt index ofthe polymer is

less than about 1, it is often desirable to incoφorate a processing enhancing

amount of a fluoroelastomer processing aid. One example is the fluoroelastomer

sold under the trade name Viton by E. I. DuPont de Nemours & Co. Another

example is the fluoropolymer sold under the trade name Dynamar FX-9613 by 3M

Company. The amount of fluoropolymer employed can vary over a wide range

depending upon the particular results desired. Typically it would be employed in

an amount in the range of about 0.01 to about 1 weight percent based upon the

weight of the polyethylene. In some cases the fluoroelastomer is employed in form of a masterbatch in which the fluoroelastomer is dispersed in a polymer such

as LLDPE copolymer of butene and ethylene. One example of such a material is

Ampacet 10919 processing aid masterbatch available from AMPACET Coφ.

In some applications it may be desirable to include in the polymer

of one or more of the layers a slip/anti-block agent, particularly for layers

produced from polymers having a density of less than 0.925g/cc. Generally such

materials are inorganic compounds. Some examples include mica, talc, silica,

calcium carbonate, and the like. A typical example would be Ampacet 10430

slip/antiblock concentrate available from AMPACET Coφ.

It is also within the scope of the present invention for the

polyethylene used to produce the inventive films to contain various other additives

normally included in polyethylenes, such as heat stabilizers, weather stabilizers,

lubricants, etc, in amounts that do not impact unduly on the objects of the present

invention. It is also within the scope ofthe present invention to blend the required

narrow molecular weight polyethylene having a density of at least about 0.925

with other polymers so long as the amount ofthe other polymers does not unduly

detract from the beneficial properties of the required polyethylene, i.e. low haze

and good handling properties. Generally the required polyethylene is greater than

about 50 weight percent ofthe polymer, more typically at least 90 weight percent

ofthe polymer, and still more preferably at least about 99.5 weight percent of the

polymer. It is within the scope ofthe present invention to prepare single layer

films having a haze of less than 17.8 using polyethylene having a density of at

least 0.925 and a molecular weight distribution of no more than about 4. It is

considered that such films can be produced by casting, blowing, or extrusion.

It is also within the scope ofthe present invention to use such a layer

of film to form a multilayered film. The polymers employed in the other layers

can be selected from generally any of the polymeric materials generally used in

producing films. Thus the other layers need not be limited to polymers of ethylene

but could contain other polymers such as propylene-butene copolymer,

poly(butene-l), styrene-acrylonitrile resin, acrylonitrile-butadiene-styrene resin,

polypropylene, ethylene vinyl acetate resin, polyvinylchloride resin,

poly(4-methyl-l -pentene), and the like. Multilayers can be formed using

techniques generally known in the art, such as, for example co-extrusion.

One particularly preferred example of a multilayered film includes

one layer having a percent haze of less than 17.8 comprising a polyethylene having

a density in the range of about 0.925 to about 0.945 g/cc and a molecular weight

distribution of no more than 4 and another layer comprising a second polyethylene

having a molecular weight distribution greater than 4, more preferably greater than

6, and still more preferably greater than 10, such as polyethylenes produced using

Phillips chromium catalysts or Ziegler-Natta type catalysts. For some applications it is also desirable for the polyethylene with

the broader molecular weight distribution to have a higher density than the

polyethylene having the narrower molecular weight distribution, for example a

density of at least about 0.945 g/cc. In a preferred embodiment of this type there

are at least three layers and the outer layers have a haze of less than 17.8 percent

and comprise a polyethylene having a density in the range of about 0.925 to about

0.945 g/cc and a molecular weight distribution of no more than 4, and and the

inner layer comprises a polyethylene having a density of at least about 0.945 g/cc.

In another preferred embodiment there are at least three layers and

the outer layers have a haze of less than 17.8 percent and consists essentially of

polyethylene having a density in the range of about 0.925 to about 0.945 g/cc and

a molecular weight distribution of no more than 4, and and the inner layer

comprises polyethylene having a molecular weight distribution of at least 10 and

a density of less than 0.93 g/cc, most preferably a density in the range of 0.91 to

0.929 g/cc with a HLMI in the range of about 12 to about 24 dg/min.

The most preferred multilayered films are those in which the

multilayered film itself has a percent haze of less than 17.8, even more preferably

a percent haze of less than 10. In the currently preferred three layer film the outer

layers each have a thickness in the range of about 5 to about 25 percent ofthe total

thickness of the three layered film. A particularly preferred inner layer is one

having a thickness equal to about 50 to about 90 percent of the total thickness of the three layered film, with the polymer of that inner layer being a low density

linear copolymer of ethylene and 1 -hexene produced using a Phillips Cr catalyst

in a particle form polymerization process, particularly a copolymer having a

density in range of about 0.91 to about 0.929 g/cc, an HLMI in the range of about

12 to 24 dg/min. and a molecular weight distribution greater than 10.

It is also within the scope ofthe present inventive mutilayered films

to have a layer of polyethylene having a broader molecular weight distribution and

a lower density than the polyethylene in the layer having a percent haze of less

than 17.8 , for example one layer could have a percent haze of less than 17.8 and

be composed of a polyethylene having a density at least 0.925 g/cc and molecular

weight distribution of at least 4 and a second layer could be composed of a

polyethylene having a density of less than 0.925g/cc, such as for example a low

density polyethylene produced by a high pressure process.

It is also within the scope of the present invention to have a

multilayered film in which one layer has a percent haze of less than 17.8 wherein

the polymer consists essentially of a polyethylene having a density of at least

0.925 g/cc and a molecular weight distribution of less than 4 and another layer

composed of a low density polyethylene having a narrow molecular weight

distribution and good clarity. In that case the inventive layer of polyethylene

provides stiffiiess that may not be provided by the lower density polyethylene

without detracting from the clarity of the lower density polyethylene as much as would a similar density polymer produced by a Phillips chromium catatalyst or a

Ziegler-Natta type titamum-containing coordination catalyst.

A layer having a percent haze of less 17.8 made of a polyethylene

having a density of less than 0.935 g/cc typically has a much lower melting point

than polymers ofthe same density and molecular weight produced by conventional

transition metal coordination catalysts or Phillips chromium catalysts. If a lower

melt temperature layer is desired it may therefor be advantageous to use the

polyethylenes having a density in the range of 0.925 to 0.935 g/cc and a molecular

weight distribution of less than 4 to form the layer having the haze of less than

17.8.

In a particularly preferred embodiment all the polyethylene layers

are polyethylenes produced using metallocene catalysts which have molecular

weight distributions of less than 4.

A further understanding ofthe present invention and its objects and

advantages will be provided by the following examples.

EXAMPLES

Example I

A large batch of solid particulate metallocene-based catalyst was

prepared. The preparation involves reacting the metallocene (but-3-enyl)

(cyclopentadienyl) (fluorenyl) (methyl) methane zirconium dichloride which is

also known as (5-cyclopentadenyl) (5-fluorenyl) hex-l-ene zirconium dichloride with a 10 weight percent solution of methylaluminoxane in toluene to give a

soluble olefin polymerization catalyst system. Davison 948 silica, dried thermally

and treated with trimethylaluminum, was added to the liquid catalyst system. To

heterogenize this system the terminal unsaturated group of the metallocene was

copolymerized with ethylene by adding ethylene to maintain a pressure in the

reaction vessel at 3 to 4 psig and stirring while the temperature was maintained at

about 20 °C. After about two hours, the ethylene addition was stopped and the

slurry was filtered. The solid was washed with toluene and then with hexane and

dried overnight using a membrane pump until no more solvent appeared on the

condenser. The resulting pink powder was dried an additional 5 hours in a high

vacuum. The solid was sieved through a 60 mesh screen and combined with

Cabosil HS-5, a fumed silica which had been dried thermally and treated with

trimethylaluminum.

The resulting solid metallocene-based catalyst system was then

employed in a pilot plant scale continuous loop reactor under slurry type

polymerization conditions. The feedstocks to the reactor were passed through

alumina drier beds prior to being sent to the reactor. The reactor was a stainless

steel pipe loop reactor. Circulation was achieved by a propeller within the reactor.

Reactant concentrations were monitored by flash gas analysis using two on-line

gas chromatographs. The polymerizations were conducted in isobutane as a liquid diluent

using varying amounts of ethylene and hexene- 1 comonomer to obtain a number

of lots of polyethylene fluff. Copolymers of ethylene and hexene- 1 having

densities varying from 0.9179 to 0.9402 g/cc were produced using the solid

metallocene based catalyst system. The polyethylene copolymers of various

densities were compounded with a typical stabilization package comprising

0.06 weight percent Irganox 1010, 0.12 weight percent PEP-Q, and 0.05 weight

percent zinc stearate based upon the weight ofthe polymer.

The resulting polymers were then evaluated for various physical

properties and were employed in the production of films using a 4 inch Sano

blown film line having a 1.5 inch single screw extruder. The film die is a spiral

mandrel die with four entry ports and is 4 inches in diameter. The die had a dual

lip air ring mounted on it which was used to cool and stabilize the extruded

bubble. Film blowing parameters were employed that are typical of linear-low

density polyethylene type processing conditions, including a 0.06 inch die gap,

190°C extruder barrel and film die set temperatures, 2.5: 1 blowup ratio, no stalk,

i.e. "in-pocket extrusion" in 1 mil film thickness. The screw rotation was adjusted

to keep the extrusion rate between 55 and 60 pounds per hour, so that the film

properties so obtained would scale directly (i.e., be the same as or at least very

similar) with those obtained from larger, commercial scale equipment. For some of the polyethylene copolymers runs were also made

where the copolymer had been compounded with 0.07 weight percent of FX-9613

fluoropolymer. As controls films were also produced using the commercially

available Dow 2045A copolymer, which is believed to be a linear low density

polyethylene copolymer produced using a non-metallocene titanium-based catalyst

system. Also, films were made using a copolymer produced by a Phillips chrome

resin.

Various characteristics ofthe polymer and the polymerization were

characterized. Examples of characteristics determined in various cases include

Haze (ASTM D-l 003 using an XL-211 Hazeguard System from Garder/Neotec

Instruments Division); density in grams/mL (ASTM D 1505-68); High Load Melt

Index (HLMI) in grams of polymer/10 minutes 190°C (ASTM D1238-86,

Condition 190/21.6); Melt Index (MI) in grams of polymer/10 minutes 190°C

(ASTM D1238-86, Condition 190/2.16); Shear Stress Response (SR) determined

by dividing HLMI by MI; Molecular weights by size exclusion chromatography,

i.e. weight averge molecular weight referred to herein as M_w and number average

molecular weight referred to herein as M„; and Heterogenity index (HI) or

molecular weight distribution (MWD) being determined by dividing M_w by M„.

The (SEC) size exclusion chromatography was conducted using a linear column

capable of resolving the wide range of molecular weights generally observed in

polyolefins, such as polyethylene. The property referred to herein as flow-activation energy, also

sometimes referred to as energy of activation, i.e. Ea, reflects the sensitivity of a

polymer melt viscosity to temperature. This is generally viewed as a function of

the linear vs network character of the polymer. The molecular weight and the

molecular weight distribution are also generally viewed as factors affecting the

flow activation energy. The Ea in terms of kJ/mol can be readily determined from

data obtained from a dynamic rheometer such as Rheometrics Inc. (RMS 800)

dynamic rheometer. A standard prescription for summarizing the

viscosity-temperature dependence of polymer melts has long been available in the

scheme known as the Williams-Landel-Ferry (WLF) supeφosition which is

described in the classic text entitled "Viscoelastic Properties of Polymers", 3rd

Edition (John Wiley & Sons, New York, 1980) by John D. Ferry. Data needed for

establishing the temperature dependence of dynamic viscosity versus frequency,

or viscosity vs shear rate, are not difficult to obtain at various temperatures in a

range between melting and the onset of chemical degradation. In order to ensure

that the Ea values are most accurate, it is desirable to optimize the data to produce

optimally smooth isothermal master curves according to the WLF time-

temperature supeφosition but using a least squares closeness-of-fit criterion based

on Carreau-Yasuda model parameters that have been shown previously to

give highly precise fits to single temperature polyethylene data . This can be done

in various ways. The currently preferred technique involves subjecting the dynamic viscosity frequency curves obtained from a Rheometrics, Inc. dynamic

viscometer to a proprietary computer program entitled "Rheology Analysis

Program CY" covered by Phillips Petroleum Company unpublished copyright

which was filed for registration on January 31, 1995. This proprietary computer

program is available for use by others under a licensing program.

Discussions of the Carreau-Yasuada model can be found in

Dynamics of Polymeric Liquids. Second ed. (John Wiley & Sons, New York,

1987) by R. Byron Bird, Robert C. Armstrong, and Ole Hassager; as well in

C. A. Hieber and H. H. Chiang, "Some correlations involving the shear viscosity

of polystyrene melts," Rheol. Acta. 28, 321-332 (1989) and CA. Hieber and

H. H. Chiang, Shear-rate-dependence modeling of polymer melt viscosity," Polym.

Ene. Sci. 32. 031-938 f 1992V

The copolymers produced using the metallocene-based catalyst

system have some distinct differences from the Dow 2045A polymer and the

polymer produced using a Phillips chromium catalyst. Specifically, the polymers

produced using a metallocene-based catalyst had molecular weight distributions

in a range of 2.17 to 2.31 and unusually low melting points for their density. The

Dow polymer had a broader molecular weight distribution. The polymer produced

using a Phillips chromium catalyst a molecular weight distribution that was even

broader than that of the Dow polymer. In addition, the SR or HLMI/MI for the

polymers produced using the metallocene-based catalyst were in the range of 17 to 18 whereas the Dow resin was 30. From rheological data and Carreau-Yasuda

parameters at 190°C, the flow activation energies ofthe polymers were compared.

The polymers produced from the metallocene-based system had flow activation

energies in the range of 20.48, to 23.71 kJ/mol. The Dow 2045A polymer in

contrast had a flow activation energy, Ea, of 25.47 kJ/mol. The metallocene-based

polymers were also evaluated to determine the concentration of terminal vinyl

groups. The percent of chains with a tei inal vinyl were in the range of 30 to

about 42.9 percent, a value of which is somewhat lower than that normally

observed for copolymers produced using chromium type catalysts. Carbon ¹³NMR

analysis also indicated that the metallocene-based polymers showed the evidence

of trace amounts of ethyl and butyl short chain branches which may have come

from in-situ generated one olefin oligomers. As determined by FTIR

spectroscopy, the total branching of the metallocene produced resins varied from

about 0.4 to about 2.1 mole percent. The number of vinyl groups per 1000 carbon

atoms for the metallocene based resins as determined by FTIR was in the range of

0.087 to 0.145.

A summary of the polyethylene properties and the properties of

selected films is shown in the following table. Polyethylene Properties Film Properties

Film Density MI MWD Dart, MD TD Tear, g Haze, % Gloss, g/cc g Tear, g %

IA 0.9179 1.06 2.17 388 200 398 4.06 119.7

IB 0.9179 1.06 2.17 708 299 429 3.73 134.3

2A 0.9216 1.36 2.24 169 237 411 5.9 111.5

3A 0.9222 1.89 2.21 256 253 429 - —

3B 0.9222 1.89 2.21 145 174 453 5.66 118.2

4A 0.9256 0.98 2.31 153 170 422 — -

4B 0.9256 0.98 2.31 152 222 355 ~ -

5A 0.9402 0.87 2.31 30 19 147 - -

5B 0.9402 0.87 2.31 <30 24 168 5.74 121.4

Dow 0.9200 1.00 4.17 216 461 755 17.8 ~

2045

Cr 0.9230 - 24.0 - - - 27.08 30 Resin

In the above table if there is an A after the film number, it refers to

a film prepared without any fluoroelastomer, whereas if there is a B after the

number, it refers to a film produced using a polymer containing 0.07 weight

percent fluoroelastomer. No fluoroelastomer was used in the control runs where

films were produced from the Dow resin and the Phillips chromium resin.

The table demonstrates that in some cases the addition of

fluoropolymer improved the dart impact strength. It is important to note that the metallocene based resin was much clearer and smoother than the film ofthe resin

with lower density that was produced with a Phillips chromium catalyst. While

the metallocene resin having a density of 0.9402 g/cc had somewhat lower values

for dart impact and tear resistance, the fact still remains that the copolymer

produced using the metallocene is capable of producing very clear films at

densities much higher than that normally employed in making films. In addition

films made from the higher density resins have the additional property of greater

stiffness than the films made from lower density polymer, a definite advantage in

some applications.

It was further noticed that the films produced from the lower density

metallocene based resins, i.e. those having a density of less than 0.925 g/cc

exhibited significant friction in the wooden take-up slats. In addition, the

tackiness and blocking decreased as resin density increased. Accordingly, for the

best balance of processing and clarity properties, the metallocene produced resins

having a density of at least about 0.925 g/cc were preferable. Additional runs

were made that demonstrated that it was possible to produce 0.5 mil films using

the special polyethylene copolymers having a density of at least about 0.925 g/cc

and a narrow molecular weight distribution. Example II

A coextruded blown film having three layers was produced using

a medium density metallocene prepared using the same type of catalyst system

described in Example I and a low density linear polyethylene produced using a

Phillips chromium catalyst process. Both ethylenes were copolymers of ethylene

and 1 -hexene. The medium density metallocene-produced polymer had a density

of 0.9309 g/cc and a melt index of 0.87 dg/min. The low density linear

polyethylene produced with the Phillips chromium catalyst process had a density

in the range of 0.919 to 0.923 and a HLMI in the range of 15 to 21 dg/min. If one

produced a 1 mil film using the chromium low density linear polyethylene, it is

possible to obtain good physical properties, however, the optical properties are

less than would be desirable for clear film applications, i.e. the percent haze is

greater than 17.8 in such a film. A 1 mil film produced using the metallocene

catalyst system had lower tear resistance than the low density linear polyethylene

produced using the chromium catalyst. The 1.5 mil coextruded film was extruded

using a Sano coextrusion dye. Processing parameters included 3.0: 1 blow up

ratio, 0.060 inch die gap at 200 lb/hour rate. The bubble configuration was

"pocket". The process was caπied out to produce a product in which 60 percent

ofthe thickness was the low density linear polyethylene and the two outer layers

each were 20 percent ofthe thickness, the two outer layers being the metallocene

polyethylene. The metallocene polyethylene was compounded with 1 weight percent of Ampacet 10919, which is believed to be a butene-ethylene linear low

density polyethylene containing about 3 weight percent of the fluoroelastomer

processing aid. A comparison of various properties of approximately 1 mil fihns

of each of the two resins and of the 1.58 mil coextruded film are set forth in the

following table.

Comparison of Films

Metallocene

Property Tested Coextruded Cr Polymer Polymer

Gauge mil 1.58 1.01 1.08

E. Tear MD g 101 103 58

E. Tear TD g 685 323 272

T.E.D.D. ft-lbs 1.23 1.45 0.886

Dart g 96 216 110

Ten. @ Yield Md psi 1800 -- 2150

Ten. @ Yield TD psi 1850 — 2300

Ten. @ Break MD psi 4450 — 3750

Ten. @ Break TD psi 4350 — 4150

Elongation MD % 517 — 506

Elongation TD % 723 — 630

Haze 7.4 >17.8 4.4

Gloss 115.6 — 129

The data shows that the coextruded film has improved optical

properties as compared to the low density linear chromium based polyethylene and

improved properties in toughness as compared to the films made only from the metallocene polymer. Of particular note is the fact that the haze ofthe coextruded

film is significantly lower than that of the polymer of the inner layer.

Claims

THAT WHICH IS CLAIMED:

1. A self-supporting film having one or more layers wherein at

least one layer is a low haze layer having a percent haze of less than 17.8 wherein

the polymer consists essentially of polyethylene selected from homopolymers of

ethylene and copolymers of ethylene and one or more other olefins selected from

the group consisting of alpha-olefins having 3 to 10 carbon atoms, said

polyethylene having a density of at least 0.925 g/cc, a molecular weight

distribution of no more than 4, and optionally containing a fluoroelastomer.

2. A film according to claim 1 wherein said film consists of a

single layer.

3. A film according to claim 2 wherein said film has a percent

haze of less than 10.

4. A film according to claim 3 wherein the polyethylene used to

produce said film has a densityof at least about 0.93 g/cc.

5. A film according to claim 4 which is a blown film.

6. A film according to claim 5 having a thickness in the range of

about 0.25 to about 5 mils.

7. A film according to claim 6 wherein the polyethylene is a

copolymer of ethylene and 1 -hexene which contains ethyl branches.

8. A film according to claim 7 wherein the polyethylene has a

density in the range of about 0.93 to about 0.945 g/cc.

9. A film according to claim wherein 8 the polyethylene has a

molecular weight distribution of no more than 3 and the film has a percent haze

of no more than 6.

10. A film according to claim 9 wherein the polyethylene has a melt

index in the range of about 0.2 to about 5 dg/min.

11. A film according to claim 10 wherein the polyethylene has a

shear stress response value in the range of about 16 to about 20.

12. A film according to claim 11 wherein the polyethylene has less

than 0.1 long chain branches per 1000 carbon atoms.

13. A film according to claim 12 wherein at least about 20 percent

of the polymer chains ofthe polyethylene contain terminal vinyl groups.

14. A film according to claim 13 wherein the polyethylene is

prepared by polymerizing ethylene and hexene in the presence of an alkane liquid

diluent under slurry polymerization conditions using a catalyst consisting

essentially of the solid catalyst prepared by (a) combining 5-(9-fluorenyl)-5-

(cyclopentadienyl)-hexene-l zirconium dichloride and methylaluminoxane in a

hquid, (b) prepolymerizing ethylene in the resulting liquid, and (c) separating the

resulting solid prepolymerized catalyst system form the liquid.

15. A film according to claim 6 wherein the polyethylene is

produced by the homopolymerization of ethylene in the presence of an alkane

liquid diluent under slurry polymerization conditions using a catalyst consisting essentially of the solid catalyst prepared by (a) combining 5-(9-fluorenyl)-5-

(cyclopentadienyl)-hexene-l zirconium dichloride and methylaluminoxane in a

hquid, (b) prepolymerizing ethylene in the resulting liquid, and (c) separating the

resulting solid prepolymerized catalyst system form the liquid.

16. A film according to claim 1 containing three layers wherein the

polymer of each outer layer consists essentially of a polyethylene which can be the

same or different polyethylene selected from homopolymers of ethylene and

copolymers of ethylene and one or more other olefins selected from the group

consisting of alpha-olefins having 3 to 10 carbon atoms, said polyethylene having

a density of at least 0.925 g/cc and a molecular weight distribution of no more than

4, and optionally containing a fluoroelastomer.

17. A film according to claim 16 wherein the polyethylene of the

inner layer has a density less than that ofthe two outer layers.

18. A film according to claim 17 wherein the polyethylene of the

inner layer has a molecular weight distribution of at least 10.

19. A film according to claim 18 wherein the polyethylene ofthe

two outer layers is selected from polyethylenes having a density in the range of

about 0.93 to about 0.945 g/cc.

20. A film according to claim 19 wherein the polyethylene of the

inner layer has a density in the range of about 0.91 to about 0.929 g/cc and an

HLMI in the range of about 12 to 24 dg/min. 21. A film according to claim 20 wherein the polyethylene of the

two outer layers is selected from the same or different polyethylene selected from

polyethylenes having a melt index in the range of 0.2 to 5 dg/rnin.

22. A film according to claim 21 which is a coextruded blown film

having a percent haze of less than 10 percent and a thickness in the range of 0.25

to 5 mil.

23. A film according to claim 22 wherein at least one ofthe outer

layers contains polyethylene having a melt index of less than about 2 dg/min and

said polyethylene contains about 0.01 to about 1 weight percent fluoroelastomer.

24. A film according to claim 1 having at least a second layer

wherein the polymer consists essentially of polyethylene having a density of at

least about 0.945 and a molecular weight distribution greater than 6.

25. A film according to claim 1 having at least a first and second

layer wherein the polymer ofthe second layer consists essentially of polyethylene

having a density of less than 0.925 g/cc and a molecular weight distribution of less

than 4 and the first layer has a percent haze of less than 17.8, wherein the polymer

of the first layer consists essentially of a polyethylene selected from

homopolymers of ethylene and copolymers of ethylene and one or more other

olefins selected from the group consisting of alpha-olefins having 3 to 10 carbon

atoms, said polyethylene having a density of at least 0.925 g/cc and a molecular weight distribution of no more than 4 and a melt index in the range of about 0.2

to about 10 dg/min, optionally containing a fluoroelastomer.

26. A film according to claim 1 wherein the polymer employed in

making the low haze layer has a melt index of at least about 2 dg/min and does

not contain any fluoroelastomer.

27. A film according to claim 1 wherein the polyethylene used in

forming at least one of the at least one low haze layers has a melt index of less

than 2 dg/min and contains a processing enhancing amount of fluoroelastomer.

28. A film according to claim 1 wherein the polyethylene used in

forming at least one of said at least one low haze layer has a melt index of less

than 2 dg/min and contains about 0.01 to about 1 weight percent fluoroelastomer

based on the weight ofthe polyethylene in said layer.

29. A coextruded blown film having a percent haze of less than

17.8 consisting of three layers wherein the polymer of the each outer layers is the

same or different polyethylene selected from polyethylenes having a density of at

least 0.925 g/cc, less than 0.1 long chain branches per 1000 carbon atoms, and a

molecular weight distribution of less than 4 and the polymer of the inner layer

consists essentially of a copolymer of ethylene and 1 -hexene having a density in

range of about 0.91 to about 0.929 g/cc, an HLMI in the range of about 12 to 24

dg/min. and a molecular weight distribution greater than 10 produced using a

chromium oxide containing catalyst in a particle form polymerization process. 30. A film according to claim 29 having a haze of no more than 10

wherein the polymer ofthe outer layers is selected from copolymers of ethylene

and 1-hexene having a density of at least about 0.93 g/cc, wherein the thickness

of each outer layer is in the range of about 5 to about 25 percent of the total

thickness of said film, and wherein ifthe polymer of each ofthe outer layers was

formed into a one mil film the films would each have a lower haze than a one mil

film produced from the polymer ofthe inner layer under the same conditions.