Preparation of polyethylene

Details Preparation of polyethylene Preparation of polyethylene

2001-12-28

2004-04-27

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...

C526S172000, C526S352000, C526S348000, C526S131000, C526S134000

Reexamination Certificate

active

06727331

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to polyethylene. More particularly, the invention relates to polyethylene that has improved rheological properties and thermal processability.
BACKGROUND OF THE INVENTION
Single-site catalysts often produce olefin polymers having relatively low molecular weight and narrow molecular weight distributions. The uniformity of molecular weight distribution, although improving tensile strength and other physical properties of polymer products, makes the thermal processing more difficult. Olefin polymers having low molecular weights also cause problems in thermal processing because they have low viscosity and low melt strength at low shear rates.
In contrast, polyethylenes made with Ziegler catalysts have relatively high molecular weight and broad molecular weight distribution. They have high melt strength but low melt index. It is desirable to produce a polyethylene having a combination of the properties of single-site polyethylene and Ziegler polyethylene. U.S. Pat. No. 6,127,484, for example, teaches a multiple-zone, multiple-catalyst process for making polyethylene. A single-site catalyst is used in a first reaction zone to produce a polymer having a relatively low molecular weight and a Ziegler catalyst is used in a second zone to make a polymer having a relatively high molecular weight. The two polymers are mixed to give a polymer having a broad molecular weight distribution and improved processability.
It is also known that increasing long-chain branching can improve processability of polyethylene made with single-site catalysts (see WO 93/08221). The existence of long-chain branching in polyethylene is particularly important for blown film extrusion and blow molding processes. However, achieving long-chain branching often requires the use of specific catalysts. For example, WO 93/08221 teaches how to increase the concentration of long-chain branches in polyethylene by using constrained-geometry single-site catalysts.
New polyethylene is needed. Ideally, the polyethylene would have both high melt indexes like single-site polyethylene and high melt strength like Ziegler polyethylene.
SUMMARY OF THE INVENTION
The invention is a polyethylene having improved rheological properties. The polyethylene has a melt index (MI
2
) from about 0.01 to about 50 dg/min and a melt strength (&eegr;) that satisfies MI
2
×&eegr;≧3.5. The polyethylene can be made by using an azaborolinyl-containing single-site catalyst in the presence of hydrogen, where the hydrogen consumption is controlled to be less than about 30 wt %.
DETAILED DESCRIPTION OF THE INVENTION
The polyethylene of the invention has unique rheological properties. It has a melt index MI
2
from about 0.01 dg/min to about 50 dg/min. Preferably, the MI
2
is from about 0.05 dg/min to about 10 dg/min. MI
2
is measured at 190° C. under 2.16 kilograms of pressure, according to ASTM D-1238.
The polyethylene of the invention has a melt strength &eegr; that satisfies MI
2
×&eegr;≧3.5. Preferably, &eegr; satisfies MI
2
×&eegr;≧4.0. More preferably, &eegr; satisfies MI
2
×&eegr;≧5.0.
Melt strength &eegr; is the ability of a polymer melt, in an extensional-type deformation, to withstand disturbances that tend to destabilize the process. For example, melt strength in a blown film process usually refers to the bubble stability. Melt strength is commonly characterized by low shear viscosity. See M. H. Naitove and J. H. Schut,
Plastics Technology
, (October 1993) 41-44. Low shear viscosity can be conveniently obtained by dynamic viscoelastic measurements. In this test, an oscillatory shear deformation with a frequency &ohgr; and a strain &ggr; is imposed on the polymer melt and the resultant stress response is measured. The ratio of the stress to the strain is the complex modulus, G*, from which the complex viscosity, &eegr;*, is obtained:
&eegr;*=G*/&ohgr;
See J. M. Dealy and K. F. Wissbrun,
Melt Rheology and It Role in Plastics Processing
, Van Nostrand Reinhold, New York (1990). Melt strength is defined as:
&eegr;=&eegr;*×10
−5
The &eegr; is measured at 190° C. and at G*=5000 dyn/cm
2
.
We have found that the known polyethylenes have a value of MI
2
×&eegr; significantly lower than that of polyethylenes of the invention. For instance, Comparative Example 5 shows that a polyethylene made with a borabenzene-based single-site catalyst has MI
2
×&eegr; of 3.1. Moreover, Comparative Examples 6-9 show that polyethylenes made with Ziegler catalysts have MI
2
×&eegr; values that are even lower.
Preferably, the polyethylene of the invention comprises less than about 15 wt % of C
3
to C
10
&agr;-olefin recurring units. Examples of suitable C
3
to C
10
&agr;-olefins are propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 4-methyl-1-pentene, and the like, and mixtures thereof. 1-Hexene and 1-butene are preferred. Incorporating long-chain &agr;-olefins reduces the density of polyethylene. The density of the polyethylene is preferably from about 0.89 g/cm
3
to about 0.96 g/cm
3
. More preferably, the density is within the range of about 0.90 g/cm
3
to about 0.94 g/cm
3
. Most preferably, the density is from about 0.90 g/cm
3
to about 0.93 g/cm
3
.
The polyethylene preferably has a weight average molecular weight (Mw) from about 10,000 to about 1,500,000 and a molecular weight distribution (Mw/Mn) less than about 3.5. More preferably, the Mw is from about 30,000 to about 1,000,000. Most preferably, the Mw is from about 30,000 to about 800,000. The Mw/Mn is more preferably less than about 3.0.
The invention includes a process for making the polyethylene. The process uses an azaborolinyl-containing single-site catalyst. Preferably, the single-site catalyst is a Group 3-10 transition metal compound that contains at least one azaborolinyl ligand. Group 4 transition metal compounds are preferred. Azaborolinyl-containing single-site catalysts are known. For instance, U.S. Pat. No. 5,902,866, the teachings of which are incorporated herein by reference, teaches the preparation of azaborolinyl-containing single-site catalysts and the polymerization of an olefin by using the catalysts.
The catalyst contains other ligands. The total number of ligands satisfies the valence of the transition metal. Other suitable ligands include substituted or unsubstituted cyclopentadienyls, indenyls, fluorenyls, halides, C
1
-C
10
alkyls, C
6
-C
15
aryls, C
7
-C
20
aralkyls, dialkylamino, thioether, siloxy, alkoxy, and the like, and mixtures thereof. Halides, cyclopentadienyls, and indenyls are preferred.
Examples of suitable single-site catalysts are (azaborolinyl)(indenyl)titanium dichloride, (azaborolinyl)(indenyl)zirconium dichloride, (azaborolinyl)(cyclopentadienyl)titanium dichloride, (azaborolinyl)(cyclopentadienyl)zirconium dichloride, and the like, and mixtures thereof. (Azaborolinyl)(cyclopentadienyl)zirconium dichloride is preferred.
Optionally, the catalyst is immobilized on a support. The support is preferably a porous material such as inorganic oxides and chlorides, and organic polymer resins. Preferred inorganic oxides include oxides of Group 2, 3, 4, 5, 13, or 14 elements. Preferred supports include silica, alumina, silica-aluminas, magnesias, titanias, zirconias, magnesium chloride, and crosslinked polystyrene. Silica is most preferred.
Preferably, the support has a surface area in the range of about 10 to about 900 m
2
/g, a pore volume in the range of about 0.1 to about 4.0 mL/g, an average particle size in the range of about 10 to about 500 &mgr;m, and an average pore diameter in the range of about 10 to about 1000 Å. Supports are preferably modified by heat treatment, chemical modification, or both. For heat treatment, the support is preferably heated at a temperature from about 50° C. to about 800° C. More preferably, the temperature is from about 50° C. to about 300° C.
Suitable chemical modifiers include organoaluminum, organosilicon, organomagnesium, and organoboron compounds. Organosilicon and organoboron compound

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