Polymerization process

Details Polymerization process Polymerization process

2001-01-31

2003-12-02

Choi, Ling-Siu (Department: 1713)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S348000, C526S352000, C526S106000, C526S129000, C526S131000

Reexamination Certificate

active

06657023

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to the polymerization and copolymerization of a mono-1-olefin monomer, such as ethylene, with a higher alpha-olefin comonomer.
Supported chromium catalysts long have been a dominant factor in the production of high density olefin polymers, such as polyethylene. As originally commercialized, these catalyst systems were used in solution polymerization processes. However, it became evident early that a slurry process was a more economical route to many commercial grades of olefin polymers, that is, a polymerization process carried out at a temperature low enough that the resulting polymer is largely insoluble in the diluent.
It is well known that mono-1-olefins, such as ethylene, can be polymerized with catalyst systems employing vanadium, chromium or other metals on a support, such as alumina, silica, aluminum phosphate, titania, zirconium, magnesium and other refractory metal supports. Initially, such catalyst systems primarily were used to form homopolymers of ethylene. Soon copolymers were developed wherein comonomers such as propylene, 1-butene, 1-hexene or other higher mono-1-olefins were copolymerized with ethylene to provide resins tailored to specific end uses.
Often, high density and/or high molecular weight copolymers can be used for blow molding applications because the blow molding process enables rapid processing into a desired molded product. Theoretically, any type of resin can be made to flow more easily by merely lowering the molecular weight, (i.e., by raising the melt index.) However, this is rarely practical because of other penalties that occur because of a higher melt index (MI). A higher melt index can result in a decrease in melt strength, which can cause a parison to tear or sag during extrusion because the parison is unable to resist its own weight. As used in this disclosure, a parison is an extruded cylinder of molten polymer before it is blown by air pressure to fill a mold. Additionally, a higher MI can cause bottle properties such as environmental stress crack resistance (ESCR) and impact strength to decrease. One of the most prevalent problems associated with raising the MI is an increase of the amount of swell exhibited by the resin as it exits the die.
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 is 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 in 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 the 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 gap that accompanies it. Unfortunately, increasing the melt index of a resin usually increases both weight swell and die swell of the polymer. Thus, as used herein, a resin which is considered easily processable also should exhibit low die swell.
SUMMARY OF THE INVENTION
Therefore, it is an object of this invention to provide an improved olefin polymerization process.
It is another object of this invention to provide a process to produce copolymers of ethylene and mono-1-olefins that can be processed at increased production rates and have a decreased weight swell.
It is still another object of this invention to provide a process to produce copolymers of ethylene and mono-1-olefins that can be processed at increased production rates and have a decreased die swell.
In accordance with this invention, herein is provided a polymerization process comprising contacting under slurry polymerization conditions at a temperature within a range of about 200° F. to about 226° F. (about 93° C. to about 108° C.) in an isobutane diluent:
a) ethylene monomer;
b) at least 1 mono-1-olefin comonomer having about three to eight carbon atoms per molecule;
c) a catalyst system comprising chromium supported on a silica-titania support, wherein said support comprises from about 1 to about 10 weight percent titanium, based on the weight on the support, wherein said catalyst system has a pore volume within a range of about 0.5 to about 1.3 ml/g, a surface area within a range about 150 to 400 m
2
/g, and said catalyst system has been activated at a temperature within a range of about 800° F. to about 1300° F. (about 427° C. to about 704° C.);
d) a trialkylboron compound; and
e) recovering an ethylene/mono-1-olefin copolymer.
In accordance with another embodiment of this invention, a copolymer comprising ethylene and a mono-1-olefin having from about 3 to about 8 carbon atoms carbon atoms per molecule is provided, wherein said copolymer has a high load melt index (HLMI) within a range of about 10 to about 80 g/10 minutes, a density within a range of about 0.95 to 0.96 g/cc, a weight swell lower than about 380%, and a die swell lower than about 43%. An environmental stress crack resistance (ESCR) of greater than about 200 hours, a M
w
/M
n
of greater than about 12 and the onset of melt fracture of greater than about 2000 sec
−1
.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Catalyst
As used in the description herein, the terms “cogel” and “cogel hydrogel” are arbitrarily used to describe cogelled silica and titania. The term “tergel” is used to describe the product resulting from gelation together of silica, titania, and chromia. References to “silica” mean a silica-containing material generally comprised of 80 to 100 weight percent silica, the remainder, if any, being selected from alumina, boria, magnesia, thoria, zirconia, or mixtures thereof. Other ingredients which do not adversely affect the catalyst or which are present to produce some unrelated results also can be present.
The support for the catalyst of this invention must be a cogel of silica and a titanium compound. Such a cogel hydrogel can be produced by contacting an alkali metal silicate such as sodium silicate with such as an acid, carbon dioxide, or an acidic salt. The preferred procedure is to utilize sodium silicate and an acid such as sulfuric acid, hydrochloric acid, or acetic acid, with sulfuric acid being the most preferred due to less corrosivity and greater acid strength. The titanium component must be coprecipitated with silica and thus most conveniently the titanium compound will be dissolved in the acid or alkali metal silicate solution.
The titanium compound preferably is incorporated with the acid. The titanium compound can be incorporated in the acid in any form in which it will be subsequently incorporated in the silica gel formed on combination of the silicate and the acid (preferably by means of adding the silicate to the acid) and from which form it is subsequently convertible to titanium oxide on calcination. Suitable titanium compounds include, but are not limited to, halides such as TiCl
3
and TiCl
4
, nitrates, sulfates, oxalates and alkyl titanates. In instances where carbon dioxid

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