WO2008132054A1

WO2008132054A1 - Polyethylene compositions

Info

Publication number: WO2008132054A1
Application number: PCT/EP2008/054624
Authority: WO
Inventors: Lene Karlberg Bjorndal; Arne Peder Jonassen; Erling Lofgren; Ragnhild Olsson
Original assignee: NORFOLIER AS
Current assignee: NORFOLIER AS
Priority date: 2007-04-26
Filing date: 2008-04-17
Publication date: 2008-11-06
Anticipated expiration: 2009-10-26

Abstract

The present invention is directed to polyethylene compositions and high performance shrink films based thereon combining excellent mechanical properties such as stiffness and toughness with good processability and good optical properties. These polyethylene compositions have densities of 0,918 to 0,927 g/cm3 and a Ml (meltindex) of 0,2-2,5 g/10 min along with bimodal LLDPE with densities of 0,920 to 0,930 g/cm3 and a Ml of 0,2-0,6 g/10 min. The compositions can be used for film applications requiring this unique combination of properties, such as, but not exclusively, packaging.

Description

POLYETHYLENE COMPOSITIONS

FIELD OF THE INVENTION The present invention relates to polyethylene compositions and high performance shrink films based thereon combining excellent mechanical properties such as stiffness and toughness with good processability and good optical properties. These polyethylene compositions can therefore be used for film applications requiring this unique combination of properties, such as, but not exclusively, packaging.

BACKGROUND OF THE INVENTION

Shrink film has been used for years in the packaging industry to wrap articles. It is well known to use linear low density polyethylene (LLDPE) in admixture with low density polyethylene (LDPE) in shrink film compositions. Compositions comprising of from 20 to 40% by weight of LLDPE with 80 to 60% by weight of LDPE are commonly used. Indeed the addition of LLDPE to LDPE in shrink film compositions is well known in order to avoid the formation of holes that could occur during the retraction of shrink film made from pure LDPE.

Nevertheless the currently available polyethylene resins suffer from major drawbacks. The low density polyethylene (LDPE) resins exhibit excellent optical and processing properties but they have poor mechanical properties and poor rigidity. Linear low density polyethylene (LLDPE) resins have excellent mechanical properties but have medium optical properties and poor processability. Indeed LLDPE leads to bubble instability and its extrusion is difficult. If mixed with LDPE they have improved processability properties but their mechanical properties are reduced.

Metallocene-catalysed linear low density polyethylene (mLLDPE) resins have excellent mechanical properties but poor optical properties and processability requiring extrusion equipment specially designed for mLLDPE with wide die gap. If mixed with LDPE they have very good optical and good sealing properties, but the mechanical properties are reduced.

Wherever high rigidity is needed, LDPE and LLDPE compositions will require overly thick structures. Especially for LLDPE, where excellent impact and tear i properties render its down-gauging capability useful, the lack of rigidity is a main drawback because high rigidity is a requirement for product packaging.

It is an object of the present invention to provide polyethylene compositions for mono or multilayers films, that achieve good balanced shrink properties in so machine direction (MD) and transverse direction (TD) with fast shrink speed and high cohesion force at room temperature while keeping a good rigidity, excellent optical properties and an easy processing in film blowing process.

SUMMARY OF THE INVENTION

The present inventors have surprisingly found that these objects are particularly well met, particularly for relatively thin shrink films, with homogeneous compositions of polyethylene (PE) with a density of 0,918 to 0,927 g/cm3 and a Ml (meltindex) of 0,2-2,5 g/10 min along with bimodal LLDPE with a density of 0,920 to 0,930 g/cm3 and a Ml of 0,2-0,6 g/10 min. Moreover the inventors have found that metallocene LLDPE with a density of 0,918 to 0,930 g/cm3 and a Ml of 0,7-1 ,5 g/10 min is preferably included in the composition; alternatively EBA (ethylene butyl acrylate) with BA content of 2 to 10%, a density of 0,922-0,925 g/cm3 and a Ml of 0,2- 0,4 g/10 min is to be used instead of metallocene LLDPE. Other polymers compatible with said blend can be added to the blend to a total to amount not to exceed 33% by weight based on the total weight of the polymers.

Accordingly the present invention, in one aspect, provides a homogeneous blend comprising: • 20-60% (w/w) Polyethylene with a density of 0.918 to 0.927 g/cm3 and a Ml (meltindex) of 0.2-2.5 g/10 min;

• 10-40% (w/w) Bimodal LLDPE with a density of 0.920 to 0.930 g/cm3 and a Ml of 0.2-0.6 g/10 min; and

• 10-40% (w/w) Metallocene LLDPE with a density of 0.918 to 0.930 g/cm3 and a Ml of 0.7-1 .5 g/10 min.

In another aspect the present invention provides a homogeneous blend comprising

• 20-60% (w/w) Polyethylene with a density of 0.918 to 0.927 g/cm3 and a Ml (meltindex) of 0.2-2.5 g/10 min; • 10-40% (w/w) Bimodal LLDPE with a density of 0.920 to 0.930 g/cm3 and a Ml of 0.2-0.6 g/10 min; and • 10-40% (w/w) EBA (ethylene butyl acrylate) with BA content of 2 to 10% (w/w), a density of 0.922-0.925 g/cm3 and a Ml of 0.2- 0.4 g/10 min.

It has been found by the inventors that such blends or compositions exhibit superior properties when extruded as thin films, both mono-extruded and co-extruded. Concerning the latter the present invention specifically provides a blown film comprising: A core layer of :

• 40-60% Polyethylene with a density of 0.918 to 0.927 g/cm3 and a Ml (meltindex) of 0.2-2.5 g/10 min; and • 40-60% Bimodal LLDPE with a density of 0.920 to 0.930 g/cm3 and a Ml of 0.2-0.6 g/10 min

Outer layers with:

• 50-80% Metallocene LLDPE with a density of 0.918 to 0.930 g/cm3 and a Ml of 0.7- 1 .5 g/10 min; and

• 20-50% Polyethylene with a density of 0.918 to 0.927 g/cm3 and a Ml (meltindex) of 0.2-2.5 g/10 min.

Alternatively the outer layers may be prepared with 0-50% EBA (ethylene butyl acrylate) with BA content of 2 to 10%, a density of 0,922-0,925 g/cm3 and a Ml of 0,2- 0,4 g/10 min instead of metallocene LLDPE.

The compositions of the present invention are particularly suited for manufacturing shrink wrap, wrap, hoods, sacks, carrier bags, cover films and building films.

DETAILED DESCRIPTION OF THE INVENTION

The manufacture of the low density polyethylenes used in the present invention is known in the art and is described for example in "Encyclopedia of Polymer Science and Engineering", second edition, Volume 6, on pages 436 to 444 (LLDPE).

The catalyst system used to produce the polyethylene required by the present invention comprises a metallocene component. The metallocene component can be any metallocene component known in the art. Among the preferred metallocenes used one can cite among others bis tetrahydro-indenyl compounds and bis indenyl compounds as disclosed for example in WO 96/35729. The most preferred metallocene catalyst is ethylene bis (4,5,6,7-tetrahydro-1-indenyl) zirconium dichloride.

The metallocene may be supported according to any method known in the art. In the event it is supported, the support used in the present invention can be any organic or inorganic solids, particularly porous supports such as talc, inorganic oxides, and resinous support material such as polyolefin.

In the present invention, the LLDPE can be monomodal, bimodal or multimodal, preferably bimodal. The metallocene catalyst utilized to produce the polyethylene required by the present invention can be used in gas, solution or slurry polymerizations. Preferably the polymerization process is conducted under slurry phase polymerization conditions. It is preferred that the slurry phase polymerization conditions comprise a temperature of from 20 to 125O, preferably from 60 to 1 1 O⁰C and a pressure of from 0.1 to 8 MPa, preferably from 2 to 5 MPa for a time between 10 minutes and 4 hours, preferably between 0.4 and 2.5 hours. High pressure ranges like 100 to 2000 bars can be used for polymerization in high pressure tubular or autoclave reactors. It is preferred that the polymerization reaction be run in a diluent at a temperature at which the polymer remains as a suspended solid in the diluent. Diluents include, for examples, propane, isobutane, n- hexane, n- heptane, methylcyclohexane, n-pentane, e-butane, n-decane, cyclohexane and the like as well as mixtures thereof. The preferred diluent is isobutane. The diluent can be under liquid or super critical state.

The polymerization of the LLDPE used in the present invention can be conducted in a continuous reactor. The continuous reactor is preferably a loop reactor. During the polymerization process, at least one monomer, the catalytic system and a diluent are flowed in admixture through the reactor.

Alternatively for a bimodal production of LLDPE, two reactors in series can be used. In the present invention average molecular weights can be further controlled by the introduction of some amount of hydrogen or by changing the temperature during polymerization.

Standard additives such as antioxidants may be used for both long term and processing stabilization and if desired, one or more pigments and/or dyes and/or processing aids like fluoro elastomers can also be added. Antistatic, antifog, antiblocking or slip additives may also be added. According to embodiments of the present invention, compositions of LLDPE and PE are obtained either by preliminary dry blend or extrusion or by direct blend in the hopper or via the extruder.

The present invention in particularly preferred embodiments provides the use of the homogeneous blend according to the present invention to produce a monolayer blown film or one or more layers of a multilayer blown film wherein said layers are arranged in any order.

As a consequence of a higher shrinkage in transverse direction and a higher cohesion force of the film produced according to the invention it will be possible to make thinner shrink film leading to a significant cost reduction.

The blend as described here above may also be used in the production of lamination films, barrier films and reticulation applications. These films may also be metallized, corona treated, printed and laminated.

EXAMPLE

A shrink film of 150 microns thickness was prepared by blown film extrusion as a monolayer film using a conventional film extruder. The extruder was equipped with a die of diameter 200 mm and die gap 1 mm. Film blowing took place at an extrusion temperature of 200⁰C; a blow up ratio of 1 :3 and a frost line height of 900 mm. The composition undergoing this blow film extrusion was obtained by forming a blend comprising: 20-60% (w/w) Polyethylene with a density of 0.918 to 0.927 g/cm3 and a Ml (meltindex) of 0.2-2.5 g/10 min; 10-40% (w/w) Bimodal LLDPE with a density of 0.920 to 0.930 g/cm3 and a Ml of 0.2-0.6 g/10 min; and 10-40% (w/w) Metallocene LLDPE with a density of 0.918 to 0.930 g/cm3 and a Ml of 0.7-1 .5 g/10 min.

Claims

1 . A homogeneous blend comprising:

• 20-60% (w/w) Polyethylene with a density of 0.918 to 0.927 g/cm3 and a Ml (meltindex) of 0.2-2.5 g/10 min;

2. A homogeneous blend comprising

• 10-40% (w/w) EBA (ethylene butyl acrylate) with BA content of 2 to 10% (w/w), a density of 0.922-0.925 g/cm3 and a Ml of 0.2- 0.4 g/10 min.

3. A homogeneous blend according to claim 1 or 2, wherein the Polyethylene has a density of 0.920 to 0.925 g/cm3.

4. A homogeneous blend according to any one of claims 1 -3, wherein the Bimodal LLDPE has a density of 0.923 to 0.927 g/cm3

5. A homogeneous blend according to claim 1 , wherein the metallocene LLDPE has a density of from 0.922 g/cm3 to less than 0.927 g/cm3.

6. A homogeneous blend according to claim 2, wherein the EBA has a density of 0.924g/cm3

7. Use of the homogeneous blend of any one of claims 1 to 6 to produce a monolayer blown film or one or more layers of a multilayer blown film.

8. Films prepared with the homogeneous blend according to any one of claims 1 to 6.

9. A blown film comprising: A core layer of :

Outer layers with:

10. A blown film comprising:

A core layer of :

• 40-60% Polyethylene with a density of 0.918 to 0.927 g/cm3 and a Ml (meltindex) of 0.2-2.5 g/10 min; and

• 40-60% Bimodal LLDPE with a density of 0.920 to 0.930 g/cm3 and a Ml of 0.2-0.6 g/10 min

Outer layers with:

• 50-80% EBA (ethylene butyl acrylate) with BA content of 2 to 10%, a density of 0,922-0,925 g/cm3 and a Ml of 0,2- 0,4 g/10 min; and • 20-50% Polyethylene with a density of 0.918 to 0.927 g/cm3 and a Ml (meltindex) of 0.2-2.5 g/10 min.