Production of multimodal polyethylene

Details Production of multimodal polyethylene Production of multimodal polyethylene

1999-09-24

2001-04-17

Nutter, Nathan M. (Department: 1711)

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

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C525S240000

Reexamination Certificate

active

06218472

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a process for the preparation of polyethylenes, having a multimodal molecular weight distribution, more particularly a bimodal or trimodal molecular weight distribution and exhibiting outstanding mechanical properties for blow-molding, film and pipe applications.
DESCRIPTION OF THE PRIOR ART
Polyolefins such as polyethylenes which have high molecular weight generally have improved mechanical properties over their lower molecular weight counterparts. However, high molecular weight polyolefins can be difficult to process and can be costly to produce. Polyolefins having a bimodal molecular weight distribution are desirable because they can combine the advantageous mechanical properties of high molecular weight fraction with the improved processing properties of the low molecular weight fraction.
For many HDPE applications, polyethylene with enhanced toughness, strength and environmental stress cracking resistance (ESCR) is important. These enhanced properties are more readily attainable with high molecular weight polyethylene. However, as the molecular weight of the polymer increases, the processibility of the resin decreases. By providing a polymer with a broad or bimodal MWD, the desired properties that are characteristic of high molecular weight resin are retained while processability, particularly extrudibility, is improved.
There are several methods for the production of bimodal or broad molecular weight distribution resins: melt blending, reactor in series configuration, or single reactor with dual site catalysts.
Use of a dual site catalyst for the production of a bimodal resin in a single reactor is also known.
Chromium catalysts for use in polyolefin production tend to broaden the molecular weight distribution and can in some cases produce bimodal molecular weight distribution but usually the low molecular part of these resins contains a substantial amount of the comonomer. Whilst a broadened molecular weight distribution provides acceptable processing properties, a bimodal molecular weight distribution can provide excellent properties. In some cases it is even possible to regulate the amount of high and low molecular weight fraction and thereby regulate the mechanical properties.
Ziegler Natta catalysts are known to be capable of producing bimodal polyethylene using two reactors in series. Typically, in a first reactor, a low molecular weight homopolymer is formed by reaction between hydrogen and ethylene in the presence of the Ziegler Natta catalyst. It is essential that excess hydrogen be used in this process and, as a result, it is necessary to remove all the hydrogen from the first reactor before the products are passed to the second reactor. In the second reactor, a copolymer of ethylene and hexene is made so as to produce a high molecular weight polyethylene.
Metallocene catalysts are also known in the production of polyolefins. For example, EP-A-0619325 describes a process for preparing polyolefins such as polyethylenes having a multimodal or at least bimodal molecular weight distribution. In this process, a catalyst system which includes at least two metallocenes is employed. The metallocenes used are, for example, a bis (cyclopentadienyl) zirconium dichloride and an ethylene-bis (indenyl) zirconium dichloride. By using the two different metallocene catalysts in the same reactor, a molecular weight distribution is obtained which is at least bimodal.
Polyethylene resins are known for the production of pipes. Pipe resins require high resistance against slow crack growth as well as resistance to rapid crack propagation yielding impact toughness. There is a need to improve in the performance of currently available pipe resins.
EP-A-0571987 discloses a process for producing an ethylenic polymer composition using multistage polymerisation. The catalyst comprises, as principal components, a transition metal compound, a compound capable of reacting with the transmission metal compound to form an ionic complex and an organoaluminium compound.
EP-A-0600482 discloses the production of a resin composition of laminates which includes two polyethylene components, one of the components being prepared using a metallocene catalyst comprising ethylene-bis (4,5,6,7-tetrahydroindenyl) zirconium dichloride.
EP-A-0575123 discloses an ethylene polymer composition which may be produced using a metallocene catalyst.
EP-A-0605952 discloses an ethylene/alpha-olefin comonomer composition and a polymerisation process therefor which employs at least two kinds of specific metallocene compounds.
EP-A-0735090 dicloses a polyethlyene resin composition which is produced by physical blending of three polyethylene components.
EP-A-0791627 discloses a polyethylene resin composition which is produced using a metallocene catalyst.
WO-A-95/26990 discloses a process for producing polymers of multi-modal molecular weight distributions using metallocene catalysts.
SUMMARY OF THE INVENTION
The present invention aims to overcome the disadvantages of the prior art.
The present invention provides a process for the preparation of polyethylene resins having a multimodal molecular weight distribution which comprises:
(i) contacting ethylene monomer and a comonomer comprising an alpha-olefin having from 3 to 10 carbon atoms with a first catalyst system in a first reactor under first polymerisation conditions in a slurry process to produce a first polyethylene having a first molecular weight, an HLMI of not more than 0.5 g/10 min and a first density of not more than 0.925 g/ml and the first catalyst system comprising (a) a first metallocene catalyst of general formula R″(CpR
m
) (Cp′R′
n
)MQ
2
, wherein Cp is a cyclopentadienyl moiety, Cp′ is a substituted or unsubstituted fluorenyl ring; each R is independently hydrogen or hydrocarbyl having 1 to 20 carbon atoms in which 0≦m≦4; each R′ is independently hydrocarbyl having 1 to 20 carbon atoms in which 0≦n≦8, R″ is a bridge which comprises a C
1
-C
20
alkylene radical, a dialkyl germanium or silicon or siloxane, or an alkyl phosphine or amine radical, which bridge is substituted or unsubstituted, M is a Group IVB transition metal or vanadium and each Q is hydrocarbyl having 1 to 20 carbon atoms or halogen, the metallocene catalyst a centroid-M-centroid angle in the range 105° to 125°; and (b) a cocatalyst which activates the catalyst component;
(ii) providing a second polyethylene having a second lower molecular weight and higher density than the first polyethylene; and
(iii) mixing together the first and second polyethylenes to form a polyethylene resin having a multimodal molecular weight distribution.
In this specification, the HLMI is measured by the procedures of ASTM D 1238 using a load of 21.6 kg at a temperature of 190° C.
The first polyethylene may be monomodal or bimodal, the second polyethylene may have a monomodal molecular weight distribution and may have been produced using a metallocene catalyst, a Ziegler-Natta catalyst or a chromium-oxide based catalyst. Alternatively, the second polyethylene may have a bimodal molecular weight distribution and has been produced using one or two of those different catalyst systems. The first and second polyethylenes may be mixed together with a third polyethylene to provide a trimodal molecular weight distribution in the resultant polyethylene resin. The third polyethylene may be produced using a metallocene catalyst, a Ziegler-Natta catalyst or a chromium-oxide based catalyst.
The first and second polyethylenes may be mixed by chemical blending or physical blending. For chemical blending, the first and second polyethylenes are made in two serially connected reactors using a common Cp fluorenyl metallocene catalyst (a), or three serially connected reactors for making a polyethylene resin having a trimodal molecular weight distribution, in which a third polyethylene is chemically blended with the first and second polyethylenes. In an alternative arrangement, the first and second polyethylenes may be chemica

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