Polymerization catalyst and process

Details Polymerization catalyst and process Polymerization catalyst and process

1999-11-12

2002-10-15

Wu, David W. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S105000, C526S106000, C526S348400, C526S348000

Reexamination Certificate

active

06465586

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to modified supported chromium oxide olefin polymerization catalyst systems.
It further relates to a method of making ethylene polymers and copolymers.
Supported chromium oxide catalyst systems have been used for many years in the polymerization of olefins. Ethylene can be polymerized by contacting the monomer with a silica-supported chromium oxide catalyst systems, the reaction being carried out in an inert liquid at temperatures below 110° C. for producing solid polymer suspended in the liquid or at temperatures above 110° C. for solution polymerization. The properties of the resulting polymer depend upon a number of factors, including the type of catalyst system employed and its activation temperature, the reaction pressure, and the reaction temperature. It is generally known that titanium can be added to the supported chromium oxide catalyst systems to produce a polymer having an increased melt index and a broader molecular weight distribution. It is also generally known that certain substances called promoters or adjuvants can be used in combination with chromium oxide catalyst systems to modify the properties of the polymer.
The use of chromium catalyst systems with certain organoboron promoters generally is known to broaden the polymer molecular weight distribution. Use of titanium in the catalyst system support or boron adjuvants in the reactor can result in improvements in the environmental stress crack resistance (ESCR) of the resultant polymers, as well as increased catalyst system productivity. Unfortunately, these modifications can increase the amount of low molecular weight polymer and oligomers that are formed. Low molecular weight polymers and oligomers can contribute smoke and odor during the processing of the polymer, or resin, as it is molded into bottles and other articles of manufacture.
Another important characteristic of a blow molding resin is the swell properties of the resin. Two kinds of swell are critical during blow molding. These are “weight swell” and “diameter swell”; the later also is referred to herein as “die swell”. As polymer, or resin, is extruded under pressure through a die opening and into a mold, a polymer has a tendency to swell as it exits the die. This is known as weight swell and can be determinative of the thickness of bottle wall, as well as the overall weight of the resultant blow molded product. For example, a resin which is extruded through a 0.02 inch die gap might yield a bottle wall thickness of 0.06 inches, in which case the weight swell is said to be 300%. A resin that swells too much can produce a bottle with too thick of a wall. To compensate, the die opening, or gap, can be narrowed by manual adjustment. However, any decrease in die gap can increase the resistance to the flow of the resin through the die. Narrower die gaps can result in higher shear rates during extrusion which also can increase melt fracture leading to a rough bottle surface. Thus, a resin which can be described as easily processable must exhibit low weight swell, which allows a wide die gap.
Diameter, or die, swell refers to how much a parison flares out as it is extruded from the die. For example, a resin extruded through a circular die of one (1) inch diameter can yield a parison tube of 1.5 inches in diameter; the die swell is said to be 50%. Die swell is significant because molds usually are designed for a certain amount of flare; too much die swell can interfere with molding of a bottle handle. A high degree of weight swell often causes high die swell because of the narrow die gap. Unfortunately, a narrow gap also increases the resistance to polymer flow. Thus, as used herein, a polymer which is considered easily processable also should exhibit low die swell.
Attempts have been made to obtain ethylene polymers having a broader molecular weight distribution (MWD) and consequent improved environmental stress crack resistance imparted by organoboron promoters and titania-containing catalyst systems. One of the most prevalent problems associated with such attempts is an increase of the amount of swell exhibited by the resin as it exits the die. Swell can be decreased by raising the activation temperature of the catalyst system, however, this also can reduce the polymer ESCR. Swell also can be decreased by lowering the melt index (MI), but this generally makes the resin more difficult to process, as measured by output rate and melt fracture. Swell also can be decreased by adding more chromium to the catalyst system, but this tends to impart a dirty color to the final polymer product. Thus, it has been difficult to produce a resin that maintains good swell and processing characteristics, high ESCR, and a low volatiles content.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a process by which high density ethylene polymers and copolymers can be manufactured having a high stress crack resistance are prepared in high yield.
It is a further object of the invention to minimize the production of oligomers and low molecular weight polymer during the polymerization process.
It is a further object of this invention to minimize the amount of die swell and weight swell exhibited by the resin during molding.
It is a further object of this invention to provide a blow molding polymer which processes well in the blow molding machine.
In accordance with one embodiment of the invention, a polymerization catalyst system is provided which comprises chromium oxide on a silica-titania, wherein said support comprises less than about 3 weight percent titanium, in combination with an organoboron promoter. Further according to the invention, an ethylene polymer or copolymer is produced by contacting an ethylene monomer, and an optional comonomer, with an activated catalyst system comprising chromium oxide on a silica-titania, wherein said support comprises less than about 3 weight percent titanium, in the presence of an organoboron promoter. The resulting polymer is produced in high yield and has a combination of improved properties, including high shear response, good environmental stress crack resistance, and high density.
DETAILED DESCRIPTION OF THE INVENTION
The silica containing substrates, or supports, used in the invention catalyst systems are silica or silica-alumina gels. Such gels conventionally are prepared by mixing an acid such as sulfuric acid with an aqueous solution of an alkali metal silicate such as sodium silicate to produce an aqueous gel, or hydrogel. The silicate preferably is added to the acid, and the reaction mixture is strongly agitated. The mixing temperature can range from about 1° C. to about 43° C. The resulting hydrogel is approximately 3 to about 12 weight percent SiO
2
and has a pH in a range of about 3 to about 9. The hydrogel is aged at a temperature of about 18° C. to 98° C. for a suitable time, generally more than one hour. Silica gels often have a minor portion, generally not exceeding 20 weight percent, of alumina or other metal oxides, an the support of the invention includes composite silica gels comprising silica and alumina, thoria, zirconia and like substances.
As used in this disclosure, the term “support” refers to a carrier for another catalytic component. However, by no means, is a support necessarily an inert material; it is possible that a support can contribute to catalytic activity and selectivity.
The hydrogel then is washed with water and either an ammonium salt solution or a dilute acid to reduce the alkali metal content of the hydrogel to less than about 0.1 weight percent. The ammonium salt solution is preferably one such as ammonium nitrate or an ammonium salt of an organic acid which volatizes upon subsequent calcination.
Water in the hydrogel can be removed by any conventional method, such as, foe example, by spray drying, vacuum oven drying, or air oven drying at temperatures above 100° C. If the hydrogel is dried by heating, it is not necessary to add an agent to the gel to prevent shrinkage of the pores.
The support must comprise titanium.

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