Preparation of ultra-high-molecular-weight polyethylene

Details Preparation of ultra-high-molecular-weight polyethylene Preparation of ultra-high-molecular-weight polyethylene

1999-09-22

2001-07-24

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...

C526S127000, C526S132000, C526S160000, C526S172000, C526S352000, C526S943000, C502S117000, C502S152000, C502S200000, C502S202000

Reexamination Certificate

active

06265504

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to an ultra-high-molecular-weight polyethylene (UHMWPE) compositions that have exceptionally narrow molecular weight distributions. The invention also relates to a process for preparing the UHMWPE compositions with a single-site catalyst having a heteroatomic ligand.
BACKGROUND OF THE INVENTION
Ultra-high-molecular-weight polyethylene (UHMWPE) has a molecular weight that is 10 to 20 times greater than high-density polyethylene (HDPE). It has been defined by ASTM as having a weight average molecular weight (Mw) greater than 3,000,000.
In addition to the chemical resistance, lubricity, and excellent electrical properties of conventional HDPE, UHMWPE offers major advantages in toughness, abrasion resistance, and freedom from stress-cracking.
UHMWPE is produced by Ziegler polymerization. U.S. Pat. No. 5,756,600 teaches how to make an ultra-high-molecular-weight polyethylene with Ziegler catalyst. The process requires exceptionally pure ethylene and other raw materials. An &agr;-olefin comonomer, such as 1-butene, may be incorporated into UHMWPE according to U.S. Pat. No. 5,756,600. Like conventional HDPE, UHMWPE made by Ziegler polymerization has a broad molecular weight distribution, and usually its polydispersity Mw/Mn (Mw: weight average molecular weight, Mn: number average molecular weight) is within the range of 5 to 20.
Newly developed metallocene and single-site catalysts advantageously provide polyethylene and other polyolefins with very narrow molecular weight distribution (Mw/Mn from 1 to 5). The narrow molecular weight distribution results in reduced low molecular weight species. These new catalysts also significantly enhance incorporation of long-chain &agr;-olefin comonomers into polyethylene, and therefore reduce its density. Unfortunately, however, these catalysts produce polyethylene having a lower molecular weight than that made with Ziegler catalyst. It is extremely difficult to produce UHMWPE with metallocene and single-site catalysts. For example, U.S. Pat. No. 5,444,145 teaches preparation of polyethylene having a Mw up to 1,000,000 with a cyclopentadienyl metallocene catalyst. However, its molecular weight is significantly lower than the required for UHMWPE.
UHMWPE that has a narrow molecular weight distribution is unknown and is needed. A feasible process for preparing it would be valuable.
SUMMARY OF THE INVENTION
The invention is an ultra-high-molecular-weight polyethylene (UHMWPE) composition that has a narrow molecular weight distribution. The UHMWPE has a weight average molecular weight (Mw) greater than about 3,000,000 and a molecular weight distribution (Mw/Mn) less than about 5.
The invention includes a process for preparing UHMWPE. The process comprises polymerizing ethylene with a single-site and a non-alumoxane activator at temperature within the range of about 40° C. to about 110° C. The single-site catalyst comprises a Group 3-10 transition or lanthanide metal and a heteroatomic ligand. The process is performed in the absence of hydrogen, &agr;-olefin comonomers, and aromatic solvents.
The invention also includes film, sheet, pipe and other articles made from the UHMWPE of the invention.
We surprisingly found that when a heteroatomic ligand containing catalyst is used, ultra-high-molecular-weight polyethylene can be obtained only in the presence of a non-alumoxane activator, only in the absence of an &agr;-olefin comonomer, aromatic solvent, and hydrogen, and only at a relatively low polymerization temperature.
DETAILED DESCRIPTION OF THE INVENTION
The invention is an ultra-high-molecular-weight polyethylene (UHMWPE) that has a narrow molecular weight distribution. It has a weight average molecular weight (Mw) greater than about 3,000,000, and a molecular weight distribution (Mw/Mn) less than about 5. Preferably, its Mw is greater than about 4,500,000, and its Mw/Mn less than about 3.
The UHMWPE of the invention incorporates essentially no &agr;-olefin comonomer. It preferably has a density within the range of about 0.94 g/cm
3
to about 0.98 g/cm
3
.
The UHMWPE of the invention has significant advantages over that prepared by Ziegler polymerization. Because it contains reduced level of low molecular weight species, the UHMWPE of the invention has improved moisture-barrier properties, chemical resistance, and mechanical strength.
The UHMWPE of the invention also possesses significant advantages over known polyethylenes prepared with metallocene and single-site catalysts. Although known polyethylenes have narrow molecular weight distributions, they do not have ultra-high molecular weights, and they lack optimal properties for many applications. UHMWPE of the invention uniquely provides excellent environmental stress-crack resistance, chemical resistance, impact resistance, abrasion resistance, high tensile strength, and high moisture-barrier properties.
The UHMWPE of the invention has a variety of uses. In particular, it can be advantageously used to make film, pressure pipe, large-part blow molding, extruded sheet, and many other articles. It can be used alone or blended with other resins. Techniques for making these articles are well known in the polyolefin industry.
The invention includes a process for preparing the UHMWPE. The polymerization of ethylene is conducted with a “single-site” catalyst. By “single-site,” we mean catalysts that are distinct reactive species rather than mixtures of different species. The single-site catalyst is an organometallic compound having a heteroatomic ligand. Suitable metals are Group 3-10 transition or lanthanide metals. Preferably, the metal is titanium, zirconium, or hafnium.
The single-site catalyst contains at least one heteroatomic ligand. Preferably, the heteroatomic ligand is a substituted or unsubstituted boraaryl, azaborolinyl, pyridinyl, pyrrolyl, indolyl, carbazolyl, or quinolinyl group.
In addition to a heteroatomic ligand, other ligands are used. 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
6
-C
20
arylkyls, dialkylamino, siloxy, alkoxy, and the like, and mixtures thereof.
The catalyst is used with a non-alumoxane activator. Alumoxane compounds, such as methyl alumoxane or ethyl alumoxane, are not suitable activators for the process of the invention. When an alumoxane activator is used with the single-site catalyst, the UHMWPE cannot be made.
Suitable non-alumoxane activators include alkyl aluminums, alkyl aluminum halides, anionic compounds of boron or aluminum, trialkylboron and triarylboron compounds, and the like. Examples are triethylaluminum, trimethylaluminum, diethylaluminum chloride, lithium tetrakis(pentafluorophenyl) borate, triphenylcarbenium tetrakis(pentafluorophenyl) borate, lithium tetrakis(pentafluorophenyl) aluminate, tris (pentafluorophenyl) boron, tris(pentabromophenyl) boron, and the like. Other suitable activators are known, for example, in U.S. Pat. Nos. 5,756,611, 5,064,802, and 5,599,761, and their teachings are incorporated herein by reference.
Activators are generally used in an amount within the range of about 0.01 to about 100,000, preferably from about 0.1 to about 1,000, and most preferably from about 0.5 to about 50, moles per mole of the single-site catalyst.
The polymerization is conducted at a temperature within the range about 40° C. to 110° C. preferably about 50° C. to 80° C. A high polymerization temperature results in a low molecular weight of polyethylene. If the temperature is too high, UHMWPE cannot be obtained.
The polymerization is preferably conducted under pressure. The reactor pressure is preferably in the range of about 150 to about 5,000 psi, more preferably from about 300 to about 3,000 psi, and most preferably from about 500 to about 2,000 psi. Generally, the higher the pressure, the more productive the process.
The process of the invention includes solution, slurry, and gas phase polymerizations. Solution polymerization is

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