Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2001-01-31
2002-08-13
Lipman, Bernard (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S387000
Reexamination Certificate
active
06433103
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to the production of polyethylene resins, and particularly, to the production of polyethylene resins for use in blow molding applications.
BACKGROUND
Polyethylene has been used in the production of various blow molded products, such as bottles. Examples of such products include household industrial containers, such as bleach bottles, detergent bottles and the like. Blow molding is accomplished by extruding molten polyethylene resin as a parison or hollow tube into a mold cavity while simultaneously forcing air into the parison so that the parison expands, taking on the shape of the mold. The molten polyethylene cools within the mold until it solidifies to produce the desired molded product.
During blow molding, the polyethylene will typically expand or swell upon exiting the die of the extruder. Although some swelling of the extruded resin may be acceptable and even desirable, excessive swelling can result in defective products and production difficulties. Excessive parison swell can result in molded parts having flash in non-pinchoff areas and/or neck/tail flash which is difficult to trim. In general, more swell makes it more difficult to control the wall thickness of the parison for optimal wall thickness distribution of the products made. Too little swell can be detrimental as well, making it particularly important to control the degree of swell. When a resin exhibits too little swell, it can become difficult or impossible to properly fill out the extremities of the mold, such as the handle of a blow molded bottle. It is, therefore, desirable to control swelling in blow molding operations to within acceptable limits.
U.S. Pat. No. 5,486,575 discloses improving certain properties of chromium produced polyethylene blow molding resins through the addition of organic peroxides. The improvements were directed towards increasing the stress crack resistance of blow molded polyethylene bottles. The improved results were seen in polyethylene resins having a broad molecular weight distribution, i.e. M
w
/M
n
being greater than 10. Although acceptable swell characteristics were achieved with respect to those polyethylene resins having the broad molecular weight distributions described, poor swell characteristics were observed for polyethylenes having more narrow molecular weight distributions.
Polyethylene resins produced with Ziegler-Natta catalysts have also been modified through the use of free radical initiators, such as oxygen, to improve the polymer properties. U.S. Pat. Nos. 5,728,335 and 5,962,598 discuss the modification of polyethylene resins used in films through such means. Ziegler-Natta produced polyethylene resins used in blow molding resins are typically bimodal resins wherein a low molecular weight polymer and a high molecular weight polymer are combined to provide a broad molecular weight distribution to improve the melt properties of the resin.
It is desirable to improve the swell characteristics of polyethylene blow molded resins having more narrow molecular weight distributions than has heretofore been achieved. It is also desirable to improve other properties of polyethylene resins and to improve upon the processability of such polyethylene resins, particularly those used in connection with blow molded products.
SUMMARY
A method of processing polyethylene, and the polyethylene produced by such method, for use as an extruded blow molding resin to improve the properties and processability thereof is provided. The method comprises modifying a polyethylene resin having a molecular weight distribution (Mw/Mn) of less than about 10 by the addition of a free radical initiator to the resin during extrusion of the resin to increase the rheological breadth of the polyethylene from about 10 to about 60% compared to unmodified polyethylene resin extruded under similar conditions.
In certain embodiments, the polyethylene resin may have a melt flow index of at least 1 or greater prior to extrusion. The modified polyethylene produced by such method may also have a YI value reduced by at least about 10% compared to unmodified polyethylene resin extruded under similar conditions.
The free radical initiator may be any source capable of generating a free radical. Examples include air, oxygen or peroxide, which may be added in the amount of from 5 to 100 ppm. The polyethylene resin may be a chromium-catalyst produced polyethylene. The polyethylene resin may also include polyethylene and ethylene copolymers of C
3
to C
10
alpha-olefins.
In another embodiment of the invention, a method of processing polyethylene resin having desired properties for use as a blow molding resin is provided. The method comprises selecting a first unextruded polyethylene resin having a first initial melt index that provides a desired final melt index after extrusion of the selected polyethylene resin. The first polyethylene resin is modified during extrusion with a selected amount of a free radical initiator so that the first polyethylene resin is modified to provide a modified melt index of the first polyethylene resin that is different from the desired final melt index after extrusion. A second unextruded polyethylene resin is selected having a desired initial melt index based upon the differences between the modified melt index and the desired final melt index to provide a second desired final melt index upon modification of the second polyethylene resin with the selected amount of the free radical initiator after extrusion. This second polyethylene is modified with the free radical initiator while the second polyethylene resin is in a molten state and extruded.
In certain embodiments, the first and second desired final melt indices may be substantially equal, and may be modified with the selected amount of free radical initiator. The desired initial melt index may be greater than the initial melt index of the first polyethylene resin, and the modified second polyethylene resin may have a greater rheological breadth compared to the first polyethylene resin upon extrusion.
Additionally, the unmodified first polyethylene resin and the modified second polyethylene resin may have final melt indices after extrusion that are substantially the same, and wherein extruder pressures for the modified polyethylene resin are less than those for the unmodified polyethylene resin during extrusion.
In another aspect of the invention, a method of processing polyethylene for use as an extruded blow molding resin to improve the color properties thereof is provided. The method comprises modifying a polyethylene resin by the addition of a free radical initiator to the resin during extrusion of the resin to obtain a polyethylene resin having a YI value reduced by at least about 10% compared to unmodified polyethylene resin extruded under similar conditions.
DETAILED DESCRIPTION
It has been found that by the addition of long-chain branching of polyethylene resins for use in blow molding applications by the utilization of a free radical initiator with the polyethylene prior to extrusion, improvements in the properties and processability of the resins can be achieved. The invention has particular application to those polyethylene resins having narrow molecular weight distributions. Improvements in the way of increased rheological breadth of the ethylene polymers are achieved. In particular, the rheological breadth of the polyethylene can be increased from about 10% to about 60% compared to unmodified polyethylene resin extruded under similar conditions. This directly bears on and results in improvements in the processability, as well as improvements in swell characteristics and other properties of the blow molded resin. Improvements in the color of the extruded polyethylene resin have also been observed by the addition of the free radical initiator.
The polyethylene used for blow molding applications may be polyethylene homopolymers or copolymers of ethylene and C
3
-C
10
alpha-olefin monomers. When copolymers are used, the ethylene content will typically range from about 90 t
Clark Curt D.
Coffy Tim J.
Guenther Gerhard K.
Hicks Ben W.
Nairn Lea Ann
Bergen Grady K.
Fina Technology, Inc.
Lipman Bernard
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