Polyethylene composition suitable for the preparation of films and process for preparing the same

The present invention relates to a novel polyethylene composition, to a process for the preparation thereof, as well as to a film comprising such a polyethylene composition.

In the field of preparation of polyethylene films, particularly in the field of medium density (MDPE) and high density (HDPE) films, there is a long-felt need of providing films having, at the same time, a number of mechanical and physical properties, and in particular adequate mechanical strength, processability and transparency, which are normally conflicting with each other.

In the present description and in the following claims, the expression “medium density film” is used to indicate a film having a density ranging from above 0.930 to 0.940 g/cm³, while the expression “high density film” is used to indicate a film having a density above 0.940 g/cm³.

In polyethylene film applications, a possible way to evaluate the above-mentioned properties may be made through the following parameters which, in the present description and in the following claims, are defined and determined as specified hereinbelow.

The mechanical strength of a polyethylene film may be effectively evaluated, for example, by means of the dart drop impact, which gives a measure of the puncture resistance of a film under shock loading. In the present description and in the following claims, the dart drop will be referred to as determined by ASTM D 1709, Method A.

The processability of the composition on which the polyethylene film is based may be determined in terms of MFR according to standard ISO 1133, condition G, corresponding to a measurement performed at a temperature of 190° C. and under a weight of 21.6 kg.

The transparency of a polyethylene film may be expressed in terms of haze, gloss and/or of clarity. In the present description and in the following claims, the haze will be referred to as determined by ASTM D 1003-00 on a BYK Gardener Haze Guard Plus Device on at least 5 pieces of 10×10 cm film, while the gloss will be referred to as determined by ISO 2813 and the clarity will be referred to as determined by ASTM D 1746-03 on a BYK Gardener Haze Guard Plus Device, calibrated with calibration cell 77.5, on at least 5 pieces of film 10×10 cm.

In the field of films, the above-mentioned mechanical and optical parameters should range in ranges meeting the requirements set by the packaging industry in the production, for example, of hygiene films and laminating films for food packaging, where transparency should be as high as possible.

Several polyethylene films are known whose properties essentially depend, in addition to on the nature of the composition on which the films are based, also on the way in which the film is prepared and, in particular, on the kind of process used to prepare the same. Among the different steps used to carry out the process, a key role is played by the catalyst system selected in the (co)polymerization step(s) which are carried out to obtain the polyethylene starting from ethylene and, optionally, one comonomer or more comonomers.

Accordingly, in the present description and in the following claims, the term “polymer” is used to indicate both a homopolymer, i.e. a polymer comprising repeating monomeric units derived from equal species of monomers, and a copolymer, i.e. a polymer comprising repeating monomeric units derived from at least two different species of monomers, in which case reference will be made to a binary copolymer, to a terpolymer, etc. depending on the number of different species of monomers used.

Among the prior art medium density polyethylene films, films prepared by means of chromium catalysts are known. Although substantially suitable for the purpose, the polyethylene films based on chromium catalysts suffer from an insufficient mechanical strength and a very poor transparency. By way of illustrative example, the known polyethylene films prepared by means of a chromium catalyst have a dart drop impact ranging from 150 to 200 g, a MFR (190/21.6) ranging from 10 to 15 g/10 min, a haze ranging from 70 to 80%, and a clarity ranging from 8 to 15%, such values being essentially a function of the film thickness.

Such values of mechanical strength and transparency are considered unacceptable, particularly in food packaging applications. In the attempt of improving the transparency, low density polyethylene (LDPE) prepared by high-pressure polymerization, which is known for being transparent, has been added to the medium density polyethylene prepared by means of chromium catalysts. In the present description and in the following claims, the term LDPE is used to indicate a polyethylene having a density from 0.910 to 0.930 g/cm³.

For example, an LDPE film having a density of 0.930 g/cm³ and a MFR (190/2.16) of 1 g/10 min may have a clarity of above 99% at a thickness of 50 μm.

Although the compositions made of MDPE and LDPE show an increased transparency, for example in terms of a certain increase of clarity up from an initial value of about 13% (MDPE alone) to a final value 56% (MDPE added with LDPE) at a thickness of 50 μm, a first disadvantage of these compositions is that such increase of transparency is still insufficient for film applications in food industry. A second disadvantage is that such a relative increase of transparency is obtained at the expenses of the mechanical strength. In particular, for example, MDPE films having a dart drop impact of 180 g, when added with LDPE, may have a dart drop impact in the range of 130-165 g depending on the amount of LDPE added to MDPE. Such worsening of the mechanical properties of the mixture is deemed to depend on the intrinsic poor mechanical dart drop impact of the LDPE.

Thus, no significant improvement in transparency has been attained by adding a LDPE to a MDPE prepared by means of a chromium catalyst and the relative improvement of the transparency inevitably results in an unacceptable worsening of the mechanical properties of the film.

It is also known to use a blend of a metallocene-catalyzed medium density polyethylene (mMDPE) with low density polyethylene (LDPE) and/or a linear low density polyethylene (LLDPE), to produce blown films, as for example described by patent U.S. Pat. No. 6,114,456. Compositions of such kind have sufficient processability and are used to make blown films which have to some extent the good optical properties of LDPE and the good mechanical properties of mMDPE. However, such compositions have the main disadvantage in that the dart drop impact sensibly decreases as the density increases.

Patent application WO 01/62847 discloses a bimodal extrusion composition of polyethylene which is prepared by (co)polymerizing ethylene in a multistage polymerization sequence of successive polymerization stages in the presence of a single site catalyst. According to WO 01/62847, the bimodal composition of polyethylene can be extruded with addition of small amounts, namely 10 wt-% or less, of high pressure LPDE by blending or by coextrusion. The addition of LDPE to such a bimodal composition, however, does not allow to obtain a film product having adequate optical properties.

A polyethylene film made from a composition of a high-density polyethylene (HDPE) and a low-density polyethylene (LDPE) prepared by high-pressure polymerization process is also known and has been hitherto used as a packaging material utilizing its transparency. However, the mechanical strength of the polyethylene film is yet insufficient. Therefore, there have been attempts to improve the impact resistance thereof. In order to improve the impact resistance, U.S. Pat. No. 6,426,384 for example teaches to prepare a polyethylene film for packaging starting from a polyethylene resin composition comprising a linear low-density polyethylene prepared using a metallocene-based catalyst and a high-density polyethylene prepared using a Ziegler type catalyst. However, the increase of the impact resistance is still insufficient.

EP-A1-1 470 185 describes a blend from about 20% by weight to about 80% by weight of a high-molecular weight, medium density polyethylene having a multimodal molecular weight distribution and about 20% by weight to about 80% by weight of a linear low density polyethylene. The medium density polyethylene is prepared by using Ziegler catalysts. The blend may optionally contain a third polymer, such as for example low density polyethylene, in an amount preferably less than 50% by weight of the total blend. However, the dart drop impact and the tear strength of the films prepared starting from such blend are inadequate.

In view of the above, the Applicant has perceived the need of providing a polyethylene composition, as well as a process for the preparation thereof and a film comprising such a polyethylene composition which, in sharp contrast to the prior art, although having a density which may range in the medium-high density range, has a high dart drop impact and a high transparency, while maintaining a good degree of processability so as to permit to use low working temperatures.