Polymer compositions

Details Polymer compositions Polymer compositions

2000-09-25

2002-12-03

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...

C526S160000, C526S943000, C526S123100, C526S124100, C526S124300, C526S154000, C526S158000

Reexamination Certificate

active

06489427

ABSTRACT:

The present invention relates to novel polymer compositions-and in particular to polymer compositions having improved physical properties as well as improved processability.
In recent years there have been many advances in the production of polyolefin copolymers due to the introduction of metallocene catalysts. Metallocene catalysts offer the advantage of generally higher activity than traditional Ziegler catalysts and are usually described as catalysts which are single-site in nature. Because of their single-site nature the polyolefin copolymers produced by metallocene catalysts often are quite uniform in their molecular structure. For example, in comparison to traditional Ziegler produced materials, they have relatively narrow molecular weight distributions (MWD) and narrow Short Chain Branching Distribution (SCBD). By narrow SCBD, it is meant that the frequency of short chain branches, formed where comonomers incorporate into the polyolefin chain, is relatively independent of molecular weight. Although certain properties of metallocene products are enhanced by narrow MWD, difficulties are often encountered in the processing of these materials into useful articles and films relative to Ziegler produced materials. In addition, the uniform nature of the SCBD of metallocene produced materials does not readily permit certain structures to be obtained.
One approach to improving processability has been the inclusion of long chain branching (LCB), which is particularly desirable from the viewpoint of improving processability without damaging advantageous properties. U.S. Pat. Nos. 5,272,236; 5,278,272; 5,380,810; and EP 659,773, EP 676,421, relate to the production of polyolefins with long chain branching.
Another approach is the addition of the polymer processing aids to the polymer prior to fabrication into films or articles. This requires extra processing and is expensive.
A different approach to the problem has been to make compositions which are blends or mixtures of individual polymeric materials with the aim being to maximise the beneficial properties of given components while minimising its processing problems. This also requires extra processing which increases the cost of materials produced. U.S. Pat. Nos. 4,598,128; 4,547,551; 5,408,004; 5,382,630; 5,583,631; and 5,326,602; and WO 94/22948 and WO 95/25141 relate to typical blends.
Another way to provide a solution to the processability problems and to vary SCBD has been the development of various cascade processes, where the material is produced by a series of polymerisations under different reactor conditions, such as in a series of reactors. Essentially, a material similar in some ways to a blend is produced, with a modality greater than one for various physical properties, such as the molecular weight distribution. While polyolefin compositions with superior processability characteristics can be produced this way, these methods are expensive and complicated relative to the use of a single reactor. Processes of interest are disclosed in U.S. Pat. No. 5,442,018, WO 95/26990, WO 95/07942 and WO 95/10548.
Another potentially feasible approach to improving processability and varying SCBD has been to use a multicomponent catalyst. In some cases, a catalyst which has a metallocene catalyst component and a conventional Ziegler-Natta catalyst component are used on the same support to produce a multimodal material. In other cases two metallocene catalysts have been used in polyolefin polymerisations. Components of different molecular weights and compositions are produced in a single reactor operating under a single set of polymerisation conditions. This approach however is difficult from the point of view of process control and catalyst preparation. Catalyst systems of interest are disclosed in WO 95/11264. The polymer produced in this disclosure are blends of polymers having a bimodal molecular weight distribution. They are also prepared specifically in a gas phase fluidised bed reactor.
We have now surprisingly found novel polyethylene compositions which exhibit both improved physical properties exemplified by their initial fracture toughness and improved processability as exemplified by their dynamic viscosity. The novel polyethylenes also show a high density and high molecular weight.
Thus according to the present invention there is provided a polyethylene having an annealed density in the range 900 to 980 kg/m
3
and a molecular weight greater than 200,000 characterised in that the polyethylene has
(a) an initiation fracture toughness (G
p
) at −40° C. of greater than 20 kJ/m
2
,
(b) a dynamic viscosity of <3000 Pa.s at 100 rad/sec (190° C.) and <3000 kpa.s at 0.01 rad/sec (190° C.); and
(c) a mass flow rate (MFR) under 21.6 kg load (190° C.) of 0.01-100 g/10 min.
Preferably the polyethylene according to the present invention has a fracture toughness greater than 25kJ/m
2
, a dynamic viscosity <2500 Pa.s at 100 rad/sec (190° C.) and <1500 kPa.s at 0.01 rad/sec (190° C.) and a MFR of 0.2-10 g/10 min (21.6 kg load).
The preferred density for the polyethylenes of the present invention is in the range 930 to 970 kg/m
3
and a molecular weight preferably greater than 400,000.
The polyethylenes of the present invention may also be characterised by having a die swell in the range 10-80%.
Thus according to another aspect of the present invention there is provided a polyethylene having an annealed density in the range 900-980 Kg/m
3
and a molecular weight greater than 200,000 characterised in that the polyethylene has
(a) an initiation fracture toughness (Gp) at −40° C. of greater than 20 kJ/m
2
(b) a dynamic viscosity of <3000 Pa.s at 100 rad/sec (190° C.) and <3000 Kpa.s at 0.01 rad/sec (190° C.), and
(c) a die swell in the range 10-80%.
The polyethylenes of the present invention preferably have a die swell in the range 20-40%.
The novel polyethylenes of the present invention may suitably be prepared by use of a multisite catalyst having two catalytic components. For example a Ziegler component and a component based on a metallocene complex. Alternatively the multisite catalyst may comprise two metallocene components.
Examples of suitable metallocene components are complexes which may be represented by the general formula:
(Cp)
m
M R
x
R
1
y
wherein Cp is a substituted or unsubstituted cyclopentadienyl nucleus; M is a Group IVA, VA or VIA transition metal in particular Zr, Ti, Hf, R and R
1
are independently hydrocarbyl having 1-20 carbon atoms, halogen or other suitable monovalent ligand; m=1−3, x=0−3 and y=0−3 wherein the sum of m, x and y equal the oxidation state of M.
Preferred metallocenes are those wherein M is Zr, Hf or Ti, R and R
1
are alkyl or halogen and m=2.
Preferred metallocene complexes are those described in EP 129368, EP 206794 and EP 586167.
Particularly suitable are complexes in which the cyclopentadienyl nucleus is substituted by alkyl groups for example butyl groups.
A particularly suitable complex is bis(2-butyl) cyclopentadienyl zirconium dichloride.
The Ziegler component of the multisite catalyst may typically be a traditional Ziegler polymerisation catalyst well known to those skilled in the art. Preferably the Ziegler component of the multisite catalyst is one comprising atoms of titanium and halogen usually chloride and also preferably magnesium. Such catalysts and their preparation will be well known to those skilled in the art.
The multisite catalyst is preferably supported and is most preferably supported on an inorganic support for example silica, alumina or magnesium chloride. The preferred support is silica.
The multisite catalyst may be supported by any traditional method of support. For example the support may preferably be initially impregnated by the Ziegler component prior to impregnation with the metallocene.
Preferably the silica support has been precalcined by heating before impregnation.
The multisite catalyst may also comprise an activator. The preferred activator is aluminoxane, most preferably methyl a

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