Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2001-10-01
2003-10-14
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
06632884
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a process for the production of polymer compositions. In addition, the present invention concerns films prepared of bimodal polymer compositions obtained by the present process. In particular, the present invention relates to bimodal films having an improved balance between the optical and mechanical properties and a good processability.
2. Description of Related Art
The processability on a film blowing line as well as the physical properties of the final film depend largely on the polymer structure, especially on the molecular weight distribution (MWD). If the polymer is bimodal, i.e., if the MWD is broad, the polymer can be expected to exhibit a good processability. Other important properties, which naturally depend on the application the polymer material is used in, comprise optical properties (i.e., the film should be clear and glossy) and mechanical properties.
Conventionally, linear low density polyethylene (PE-LLD) having a bimodal molecular weight distribution is produced by polymerization in the presence of Ziegler catalysts in two cascaded reactors. Similarly, high density polyethylene (PE-HD) having a bimodal MWD has been produced by polymerization in the presence of Ziegler catalysts in two reactors in series.
Medium density polyethylene (PE-MD) for blown film is typically supplied by unimodal Cr-based products. These materials are extensively used in coextruded films as a stiffness-improvement, but give relatively less contribution to other physical properties like impact required by packaging.
The use of a metallocene catalyst in a two-stage polymerization process is known from EP-A-447035 and EP-A-881237. Bimodal polyethylene for film is known from e.g. EP-A-605952, EP-A-691353. EP-A-691367 and WO-A-9618662.
EP-A-447035 discloses an ethylene polymer composition having a density of 860-940 kg/m
3
and an intrinsic viscosity of 1-6 dl/g, which would mean with a rough calculation a MFR
2
in a range of approximately 0,04-60 g/10 min. The composition has been produced using a catalyst comprising a ligand that has a cycloalkadienyl skeleton and an organoaluminum oxy-compound. The publication refers to the reduced fraction of polymer soluble in n-decane in the resins produced according to the invention. It further states that when the fraction of such polymer is low the polymer has excellent anti-blocking properties.
In addition, the publication states that the target has been to produce materials having the clarity of the unimodal metallocene-based resin but a superior processability. The publication does not, however, disclose whether or not the good clarity and improved processability actually were achieved. The melt flow rate region disclosed in the publication is considerably large, which seems to indicate that it was not clear what combination of density and intrinsic viscosity would result in best processability and best clarity. Comparative examples shall show that many materials satisfying the definitions of EP-A447035 are not appropriate for producing films with a good combination of optical and mechanical properties.
EP-A-605952 discloses a polymer composition comprising two different ethylene polymers, which have been obtained by using a catalyst comprising at least two different metallocene compounds. This type of catalyst is sometimes referred to as dual site catalyst. The publication discloses that the two ethylene polymers may be polymerized separately and blended in an extruder, or the polymers may be dissolved and then combined. The polymers may also be produced in a two stage polymerization. The composition can be used to prepare films.
The examples show that the compositions where a dual site catalyst was used to prepare the polymer components produced films with good optical properties, high impact strength and good processability (or mouldability). Comparative examples 1 and 2 further disclose that compositions where a single component catalyst was used to prepare the polymer components, produced films with inferior optical and mechanical properties and poor processability.
EP-A-881237 discloses a two-stage process to produce ethylene polymers, wherein a metallocene catalyst based on a tetrahydroindenyl compound was used in a two-stage polymerization process. The document further discloses that the density of the polymer may range from 900 to 970 kg/m
3
and the high load melt index (MFR
21
) from 0.1 to 45000 g/10 min. The examples disclose that the polymer was produced in loop and CSTR reactors. The materials disclosed in the examples had a density between 938 and 955 kg/M
3
and a melt index MFR
2
between 0.18 and 1.2 g/10 min. The use of the polymer was not disclosed and no practical examples concerning the use of the polymer were given.
EP-A-691367 discloses a film extruded from an in-situ blend of ethylene polymers prepared using Ziegler-Natta catalysts. The publication states that the resulting resins have a high mechanical strength. It is also stated in the publication that the film has a good processability and a low blocking tendency. The optical properties or gel level are not referred to. It is, however, known in the art that films made of such blends tend to be hazy.
EP-A-691353 discloses a process for preparing an in-situ blend giving a low gel level film. The process comprises polymerizing ethylene (with comonomer) in a cascade of gas phase reactors using a Ziegler-Natta catalyst. The publication further discloses that the resulting material has a good processability in blown film line.
WO-A-9618662 discloses a process for producing both high density and linear low density film material. The process comprises a cascade of a loop and a gas phase reactor. In the process, also a prepolymerizer is included. The publication also states that metallocene catalysts may be used in the process. However, it does not reveal the purpose for using a metallocene catalyst nor the advantages of it.
Thus, as apparent from the above, the available materials for films give limited alternatives in terms of a balance between clarity and mechanical properties.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved process for producing suitable polyethylene materials for the production of films.
It is another object of the present invention to provide novel polymer compositions for film-making.
It is a further object of the present invention to eliminate the problems of the prior art and to provide novel polymer films.
These and other objects, together with the advantages thereof over known processes and products, which shall become apparent from the specification which follows, are accomplished by the invention as hereinafter described and claimed.
The present invention is based on the provision of bimodal polyethylene compositions comprising
a first (low molecular weight) component with MFR
2
at least 10 g/10 min and a density higher than the density of the composition,
at least one other component,
said composition having a melt flow rate in the range MFR
2
=0.1-5.0 g/10 min and a density of 905-960 kg/m
3
.
According to one embodiment, the present invention provides a bimodal polyethylene compositions comprising
a first (low molecular weight) component with MFR
2
at least 10 g/10 min and a density higher than the density of the composition,
at least one other component,
said composition having a melt flow rate in the range MFR
2
=0.1-5.0 g/10 min and a density of 915-960 kg/m
3
.
The composition is further characterized by a shear thinning index (SHI) of 3-20, viscosity of 5000-25000 Pas and storage modulus G′
5kPa
of 800-2500 Pa. It can be used for manufacturing polyethyiene films. The films according to the invention exhibit excellent balance between optical and mechanical properties.
The composition for polyethylene films can be produced by polymerizing or copolymerizing ethylene in a reactor cascade formed by at least two reactors in the presence of a metallocene catalyst capable of producing a high molec
Hokkanen Harri
Kallio Kalle
Kinnunen Esa
Knuuttila Hilkka
Lappi Eija
Borealis Technology Oy
Nutter Nathan M.
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