Polyethylene film

Details Polyethylene film Polyethylene film

2000-10-31

2002-03-12

Henderson, Christopher (Department: 1713)

Stock material or miscellaneous articles

Composite

Of addition polymer from unsaturated monomers

Reexamination Certificate

active

06355359

ABSTRACT:

FIELD OF THE INVENTION
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 {fraction (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 forming 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. Pat. Nos. 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-1 zirconium dichloride and methylaluminoxane in a liquid, (b) prepolymerizing ethylene in the resulting liquid, and (c) separating the resulting solid prepolymerized catalyst system from the liquid. 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 of the liquid 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 SiCl
4
, disilazane, trimethylchlorosilane, dimethylamino trimethylsilane, and the like.
The amount of aluminoxane and metallocene used in forming a liquid catalyst system for the prepolymerization can vary over a wide rang

Affiliated with

Also associated with

Rating

Polyethylene film does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Polyethylene film, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Polyethylene film will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2860232