Process for producing polyethylene

Details Process for producing polyethylene Process for producing polyethylene

2001-07-24

2003-03-18

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...

C526S087000, C526S123100, C526S124300, C526S160000, C526S161000, C526S348500, C526S348600, C502S104000, C502S152000, C502S155000

Reexamination Certificate

active

06534613

ABSTRACT:

FIELD OF INVENTION
The present invention relates to a polymerization process for the production of a polyethylene. Preferably the polyethylene has a low level of extractables. Films produced from the polyethylene are characterized by having high strength properties.
BACKGROUND OF INVENTION
Polyethylene polymers are well known and are useful in many applications. In particular, linear polyethylene polymers possess properties which distinguish them from other polyethylene polymers, such as branched ethylene homopolymers commonly referred to as LDPE (low density polyethylene). Certain of these properties are described by Anderson et al. U.S. Pat. No. 4,076,698.
A particularly useful polymerization medium for producing polyethylene polymers is a gas phase process. Examples of such are given in U.S. Pat. Nos. 3,709,853; 4,003,712; 4,011,382; 4,302,566; 4,543,399; 4,882,400; 5,352,749 and 5,541,270 and Canadian Patent No. 991,798 and Belgian Patent No. 839,380.
Ziegler-Natta type catalyst systems for the polymerization of olefins are well known in the art and have been known at least since the issuance of U.S. Pat. No. 3,113,115. Thereafter, many patents have been issued relating to new or improved Ziegler-Natta type catalysts. Exemplary of such patents are U.S. Pat. Nos. 3,594,330; 3,676,415; 3,644,318; 3,917,575; 4,105,847; 4,148,754; 4,256,866; 4,298,713; 4,311,752; 4,363,904; 4,481,301 and Reissue 33,683.
These patents disclose Ziegler-Natta type catalysts that are well known as typically consisting of a transition metal component and a co-catalyst that is typically an organoaluminum compound. Optionally used with the catalyst are activators such as halogenated hydrocarbons and activity modifiers such as electron donors.
The use of halogenated hydrocarbons with Ziegler-Natta type polymerization catalysts in the production of polyethylene is disclosed in U.S. Pat. No. 3,354,139 and European Patent Nos. EP 0 529 977 B1 and EP 0 703 246 A1. As disclosed, the halogenated hydrocarbons may reduce the rate of ethane formation, improve a catalyst efficiency, or provide other effects. Typical of such halogenated hydrocarbons are monohalogen and polyhalogen substituted saturated or unsaturated aliphatic, alicyclic, or aromatic hydrocarbons having 1 to 12 carbon atoms. Exemplary aliphatic compounds include methyl chloride, methyl bromide, methyl iodide, methylene chloride, methylene bromide, methylene iodide, chloroform, bromoform, iodoform, carbon tetrachloride, carbon tetrabromide, carbon tetraiodide, ethyl chloride, ethyl bromide, 1,2-dichloroethane, 1,2-dibromoethane, methylchloroform, perchloroethylene and the like. Exemplary alicyclic compounds include chlorocyclopropane, tetrachlorocyclopentane and the like. Exemplary aromatic compounds include chlorobenzene, hexabromobenzene, benzotrichloride and the like. These compounds may be used individually or as mixtures thereof.
It is also well known, in the polymerization of olefins, particularly where Ziegler-Natta type catalysts are employed, to utilize, optionally, electron donors. Such electron donors often aid in increasing the efficiency of the catalyst and/or in controlling the stereospecificity of the polymer when an olefin, other than ethylene, is polymerized. Electron donors, typically known as Lewis Bases, when employed during the catalyst preparation step are referred to as internal electron donors. Electron donors when utilized other than during the catalyst preparation step are referred to as external electron donors. For example, the external electron donor may be added to the preformed catalyst, to the prepolymer, and/or to the polymerization medium.
The use of electron donors in the field of propylene polymerization is well known and is primarily used to reduce the atactic form of the polymer and increase the production of the isotactic polymers. The use of electron donors generally improves the productivity of the catalyst in the production of isotactic polypropylene. This is shown generally in U.S. Pat. No. 4,981,930.
In the field of ethylene polymerization, where ethylene constitutes at least about 70% by weight of the total monomers present in the polymer, electron donors are utilized to control the molecular weight distribution (MWD) of the polymer and the activity of the catalyst in the polymerization medium. Exemplary patents describing the use of internal electron donors in producing linear polyethylene are U.S. Pat. Nos. 3,917,575; 4,187,385; 4,256,866; 4,293,673; 4,296,223; Reissue 33,683; U.S. Pat. Nos. 4,302,565; 4,302,566; and 5,470,812. The use of an external monoether electron donor, such as tetrahydrofuran (THF), to control molecular weight distribution is shown in U.S. Pat. No. 5,055,535; and the use of external electron donors to control the reactivity of catalyst particles is described in U.S. Pat. No. 5,410,002.
Illustrative examples of electron donors include carboxylic acids, carboxylic acid esters, alcohols, ethers, ketones, amines, amides, nitriles, aldehydes, thioethers, thioesters, carbonic esters, organosilicon compounds containing oxygen atoms, and phosphorus, arsenic or antimony compounds connected to an organic group through a carbon or oxygen atom.
SUMMARY OF THE INVENTION
The polymerization process of the present invention comprises the introduction into a polymerization medium containing ethylene and optionally other olefin(s), a Ziegler-Natta type polymerization catalyst, at least one halogenated hydrocarbon, at least one compound containing at least one carbon-oxygen-carbon linkage (C—O—C) of the formula R
1
—O(—R
2
—O)
n
—R
3
where n ranges from 0 to 30, and R
1
, R
2
and R
3
independently contain from 1 to 30 carbon atoms and from 0 to 30 heteroatoms of an element, or mixtures thereof, selected from Groups 13, 14, 15, 16 and 17 of the Periodic Table of Elements, and further wherein R
1
, R
2
and/or R
3
can be linked and form part of a cyclic or polycyclic structure, as an external electron donor, and as a co-catalyst at least one compound of the formula,
X
n
ER
3-n
,
wherein,
X is hydrogen, halogen, or mixtures of halogens, selected from fluorine, chlorine, bromine and iodine;
n ranges from 0 to 2;
E is an element from Group 13 of the Periodic Table of Elements such as boron, aluminum and gallium; and
R is a hydrocarbon group, containing from 1 to 100 carbon atoms and from 0 to 10 oxygen atoms, connected to the Group 13 element by a carbon or oxygen bond.
The external electron donor as defined herein, the co-catalyst defined herein, and the halogenated hydrocarbon may be added to the polymerization medium in any manner. The external electron donor as defined herein, the halogenated hydrocarbon, and/or the co-catalyst defined herein may be added to the catalyst just prior to addition to the polymerization medium, or added separately from the catalyst to the polymerization medium in any manner known in the art. For example, the external electron donor as defined herein may optionally be premixed with the co-catalyst prior to addition to the polymerization medium.
If a gas phase fluidized bed process is utilized for polymerization of the ethylene, it may be advantageous to add the external electron donor as defined herein prior to the heat removal means, e.g., the heat exchanger, to slow the rate of fouling of said heat removal means.
All mention herein to elements of Groups of the Periodic Table are made in reference to the Periodic Table of the Elements, as published in “Chemical and Engineering News”, 63(5), 27, 1985. In this format, the Groups are numbered 1 to 18.
DETAILED DESCRIPTION OF THE INVENTION
The present inventors have unexpectedly discovered that a particular combination of a Ziegler-Natta catalyst, at least one halogenated hydrocarbon, as a co-catalyst at least one compound of the formula,
X
n
ER
3-n
,
wherein,
X is hydrogen, halogen, or mixtures of halogens, selected from fluorine, chlorine, bromine and iodine;
n ranges from 0 to 2;
E is an element from Group 13 of the Periodic Table of Elements such as boron, aluminum and gallium; and
R is a hydrocarbon group, conta

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