Polymer films

ES 2 229 675 T3

DESCRIPTION

Polymeric films.

Background of the invention

Invention sector

The present invention relates to polymeric films. In particular, the present invention relates to medium density polyethylene compositions and polymeric films blown from said polyethylene compositions and having improved impact resistance and excellent mechanical properties and good processability.

Description of Related Art

There is an increasing demand for thinner films (size reduction) due to environmental aspects and cost reduction. Because the performance and strength of films in packaging lines should be maintained despite size reduction, stiffer films with high mechanical properties are required to meet future demands.

Polyolefin materials used for blown film applications can be divided into the following five material groups:

1) Low Density Polyethylene (LDPE) with a wide molecular weight distribution (MWD) and produced by radical polymerization in a reactor;

2) Linear Low Density Polyethylene (LLDPE) with a narrow MWD and produced by polymerization in the presence of Ziegler catalysts in a reactor;

3) High Density / Medium Density Polyethylene (HD / MDPE) with a wide MWD and produced by polymerization in the presence of Cr catalysts in a reactor;

4) High Density Polyethylene (HDPE) with a bimodal (broad) MWD and produced by polymerization in the presence of Ziegler catalysts in two reactors in series (cascade);

5) Linear Low Density Polyethylene (LLDPE) with a bimodal (broad) MWD and produced by polymerization in the presence of Ziegler catalysts in two cascade reactors.

The market for polyethylene (PE) films is continually developing better solutions for packaging requirements based on the above types of materials.

Unimodal Linear Low Density (LLD) materials (item 2 above) used in film applications, typically having densities less than 926 kg / m³, show good mechanical properties (such as resistance to tearing or dart drop). There are applications, however, in which its stiffness is not high enough. Higher density materials are used in such applications. When the density has been increased to increase stiffness, the mechanical properties have been affected by compromise.

Bimodal LLD film materials (item 5 above) have a superior balance between mechanical properties and stiffness as well as superior processability compared to unimodal LLD. This type of material has been described, for example, in a Finnish Patent Application No. 945926. Other materials of the bimodal LLDPE type are disclosed in European Patent Application Nos. 0 492 656, 0 691 367, 0 773 257 and 0 773 258.

However, there are applications where even higher stiffness is required. There are bimodal high density film materials available (point 4 above) with high stiffness. This type of material is discussed, for example, in European Patent Application No. 0 691 353. Unfortunately, a reduction in mechanical properties is then observed.

A problem associated with bimodal (or multimodal) polyethylenes is the inherent heterogeneity of the material, due to the fact that it consists of components that have molecular weights (or melt flow rates) and comonomer contents (or densities, or degrees branching) very different. The problem associated with differences in molecular weight is discussed, for example, in an article by Bohm, Enderle and Fleissner (Industrial Synthesis of Bimodal Polyethylene Grades with Improved Properties), where it is stated (page 355) that problems should be expected when polymers with a proportion of their molecular weights greater than 10 are mechanically mixed. The problems associated with the different degree of branching (or different density) are studied by Morgan, Hill, Barham and Frye in a recent article (Liquid-Liquid Phase Separation in Ternary Blends of Linear Polyethylene with Two Ethylene Butylene Copolymers, Polymer, Vol. 38 , No. 8, pp. 1903-1909, 1997). The authors show that when a linear PE is mixed with an ethylene-butene copolymer having a relatively low degree of branching, phase separation can occur (eg, Figure 1 and Figure 4). In an article by Ala2

ES 2 229 675 T3 mo, Kim, Mandelkern, Galante, Wignall, Londono and Stehling (The Cocrystallization of Mixtures of Homopolymers and Copolymers: Polyolefins, Polym. Mater. Sci. Eng., Vol. 78, pp. 213-214, 1998 , page 213) it is also stated that when a linear PE is mixed with a branched PE having a branching concentration greater than 8 mol%, phase separation takes place in the melt.

The consumption of MDPE for blown films is increasing and the market is routinely supplied by Cr-based unimodal products. These materials are widely used in coextruded films to improve stiffness, but make a relatively small contribution to other physical properties, such as resistance to corrosion. impact required for packaging. The processability in a blown film line, as well as the physical properties of the final film are highly dependent on the polymer structure, especially MWD and SCBD.

Thus, as is apparent from the foregoing, available film blowing materials provide limited alternatives in terms of balance between stiffness and other mechanical properties.

Summary of the invention

It is an object of the present invention to eliminate the problems of the prior art and to provide novel blown polymeric films.

This and other objects, together with the advantages of these over known processes and products, which will be apparent from the following specification, are achieved by the present invention as described and claimed below.

As mentioned above, the general belief in the art has been that if one seeks to increase stiffness, then some of the mechanical properties must be sacrificed to increase the density of the material. However, contrary to this belief, it has now been surprisingly found that if the material with the highest density is bimodal and the increase in density is carried out in a specific way, the mechanical properties are not affected as it is applied. increases stiffness. The result is a material with mechanical properties equal to LLD but with improved stiffness.

In particular, the present material comprises a bimodal medium density polyethylene with

i. 30 to 50% by weight, preferably 35-45%, and in particular 37-43%, of a low molecular weight part with a density of at least 960 kg / m³ and a MFR₂ > 100 g / 10 min and a low comonomer content and ii. 70 to 50% by weight, preferably 65 to 55%, and in particular 63-57%, of a high molecular weight part with a density calculated to be in the range of 890 to 920 kg / m³ and a high comonomer content, and the present polyethylene has a density of about 920-945 kg / m³, in particular 925-940 kg / m³, and melt flow velocity in the range of MFR_2Y 5 to 50 g / 10 min. The comonomer content of the high molecular weight part is at least about two times higher, preferably at least about 3 times higher than in the low molecular weight part.

The present bimodal MDPE material has an elongation viscosity that increases with increasing elongation rate. Therefore, it is particularly well suited to blown film production. The films according to the present invention show, depending on the density and the MFR of the composition and the blowing conditions of the film, a tensile strength in the transverse direction of 15 MPa or more, and a secant modulus of 1% in the machine direction of at least 300 MPa and in the cross direction of 400 MPa or more.

The present compositions are preferably produced by polymerizing or copolymerizing ethylene in a cascade reactor consisting of at least two reactors operating with different amounts of hydrogen and comonomers to produce a high molecular weight portion in one of the reactors and a low molecular weight part in the other.

More specifically, the present polyethylene is characterized by what is stated in the characterization part of claim 1.

Considerable advantages are obtained by means of the present invention. Thus, the present invention provides a product with improved tear resistance, as well as excellent film homogeneity, compared to conventional Cr-based materials at the same stiffness levels.

In addition, the product has an improved stiffness / impact resistance balance and higher stiffness, allowing for size reduction and increased subsequent conversion speeds.

The novel MPDE material is particularly suitable for blown film, with a processability

Enhanced ES 2 229 675 T3 that allows it to be processed on a variety of film lines (LD, LLD and HD lines) and film coextrusion lines.

In the following, the present invention will be examined in more detail with the aid of the following detailed description and with reference to the attached figures.

Brief description of the drawings

Figure 1 shows the relationship between stiffness and blown film mechanical properties of unimodal and bimodal polymer compositions comprising 1-butene and 1-hexene, respectively, as comonomers;

Figure 2 shows the elongation viscosity, according to the Cogswell method, plotted against the equivalent shear rate in the matrix for two samples prepared from the present polyethylene composition compared to a reference sample; and Figure 3 shows the elongation viscosity, according to the Cogswell method, versus the rate of elongation for a sample prepared from the present polyethylene composition compared to a reference sample.

Detailed description of the invention

Definitions

For the purpose of the present invention, "emulsion reactor" designates any reactor, such as a continuous or batch stirred tank reactor or a tubular reactor, which operates in emulsion and in which the polymer is formed in the form of particles. According to a preferred embodiment, the emulsion reactor comprises a tubular reactor.

By "gas phase reactor" is meant any mechanically mixed or fluidized bed reactor. Preferably, the gas phase reactor comprises a mechanically stirred fluidized bed reactor with gas velocities of at least 0.2 m / s.

By "Melt Flow Rate" or the abbreviated form "MFR" is meant the weight of a polymer extruded through a standard cylindrical die at a standard temperature in a laboratory rheometer carrying a standard piston and load. MFR is a measure of the melt viscosity of a polymer and therefore also of its molar mass. The abbreviation "MFR" is generally provided with a numerical subscript indicating the piston load in the test. Thus, for example, MFR₂ designates a load of 2.16 kg and MFR_2Y a load of 21.6 kg. MFR can be determined using, for example, one of the following tests: ISO 1133 C4, ASTM D 1238, and DIN 53735.

By "Flow Rate Ratio" or the abbreviation FRR is meant a ratio between two MFR values ​​measured from the same polymer using different fillers. The abbreviation FRR is generally provided with a subscript indicating which charges have been used in the determination of FFR. Thus, FRR₂i_/5has been obtained as a ratio of MFR_2Y to MFR₅. The FFR is a measure of the width of MWD. A high FRR corresponds to a wide MWD.

Linear polyethylene is blown to form a film on two types of machines. Typically, linear low-density PE is processed in machines operating under so-called "low neck" conditions, which means that a relatively low blowing rate and cooling line height have been used during the blowing of the film. relatively low. Polymers that are commonly processed with a low neck have a relatively low molecular weight (with an MFR2 of about 1). Relaxation of the polymer takes place in the zone of the molten polymer between the die and the cooling line.

On the other hand, high-density PE is usually processed in machines under so-called "high neck" conditions, in which a relatively high blowing rate and relatively high cooling line height are used. Polymers that are commonly processed with a high neck have a relatively high molecular weight (with an MFR_2Y of about 7). Some relaxation occurs in the zone of the molten polymer between the die and the quench line, but especially the biaxial orientation of the polymer occurs. Usually, the orientation increases the strength of the polymer in the direction of the orientation. This means that under high neck conditions a mechanically stronger film is obtained, and for example, the impact from dart drop is increased.

Polyethylenes known in the art (and listed on page 1 above) have been specifically designated for one of these applications. A relevant characteristic of the present invention is its flexibility, which allows it to be used in these two applications.

Table 2 (in the Examples section) of the present application shows an example of these methods of operation. There, a machine made by Windmoller and Holscher, with a 60mm extruder and a 200mm die with a 1.2mm die opening, ran with a 3: 1 blow ratio and a line height of

ES 2 229 675 T3 650 mm cooling. This is an example of "low neck" conditions. On the other hand, a machine made by Alpine, with a 65mm extruder and a 160mm die and a 1.5mm die opening ran with a blow ratio of 4: 1 and a cooling line height of 1280 mm. This is an example of "high neck" conditions.

The polymer composition

The present invention relates to a medium density polyethylene composition having a bimodal molar mass distribution comprising a high molar mass part and a medium or low molar mass part. The high molar mass portion contains comonomers that improve the mechanical properties of the polymer. Surprisingly, this improvement is not linear. Figure 1 shows the relationship between stiffness and mechanical properties of four different MDPE-type polymer compositions, two have a unimodal MWD and two have a bimodal MWD.

As is evident from figure 1, the use of a higher olefin (1-hexene vs. 1-butene) improves the mechanical properties, so that these remain at an acceptable level even if the stiffness increases somewhat, for example , by increasing the density of the polymer. As is known in the art, comonomers typically accumulate in the low molar mass portion of the polymer molecular weight distribution of a unimodal product. In contrast, for bimodal polymers, comonomers are concentrated in the high molar mass portion. As a result, as is also evident from Figure 1, at the same level of stiffness, comonomers will not only improve mechanical properties compared to the corresponding unimodal polymer, but will also have a suspension effect in lowering mechanical properties when stiffness increases. Thus, over a wide range of high stiffness, the mechanical properties of bimodal medium density polyethylenes are significantly superior to corresponding unimodal polymers. Bimodal polyethylene, having 1-butene as a comonomer, usually has superior mechanical properties compared to a unimodal material having 1-hexene as a comonomer.

Generally, a polymer composition with an amount of comonomers from 0.1 to 10% is preferred, in particular, the amount is from 2 to 8% by weight.

According to an embodiment of the present invention, the polyethylene composition comprises (i) a low molecular weight part with a density of more than 960 kg / m³ and a MFR₂ greater than 100 g / 10 min (ii) one part of high molecular weight, and the polyethylene composition has a density of about 925-940 kg / m³ and a melt flow rate in the range of MFR₂₁ = 5 to MFR₂= 15 g / 10 min.

Preferably, the above polyethylene composition has a density of 925-940 kg / m³, approximately, and a melt flow velocity in the range of MFR₂₁ 5 to 50 g / 10 min.

Furthermore, the polyethylene composition described above preferably comprises

- 30 to 50% by weight, preferably 35-45%, and in particular 37-43%, of a low molecular weight part with a density of at least 960 kg / m³ and a MFR₂ > 100 g / 10 min and a low comonomer content; Y

- 70 to 50% by weight, preferably 65 to 55%, and in particular 63-57%, of a high molecular weight part with a density calculated to be in the range of 890 to 920 kg / m³ and a high comonomer content.

The medium density polyethylene according to the present invention comprises 70 to 50% of a high molar mass part, and 30 to 50% of a low molar mass part and a Melt Flow Rate of polymer in the MFR range₂₁ 5 to MFR₂ About 15 g / 10 min, preferably in the MFR range₂₁5 to 40 g / 10 min approximately. The composition contains about 2 to 8% comonomers, and the low molar mass portion contains less than 1% comonomers. The density of the polymeric product is between 920 and 945 kg / m³, in particular 925-940 kg / m³ approximately. Charpy impact strength at -20 ° C is generally greater than 90 kJ / m², and the Ratio of Flow Rates FRR₂i_/5 is over 20.

As specific examples of preferred embodiments, the following bimodal ZN-based MDPE polymers for blowing films can be mentioned:

an MDPE with an MFR₂₁ 20 g / 10 min and a density of 931 kg / m³, an MPDE with an MFR₂₁ 13 g / 10 min and a density of 937 kg / m³.

ES 2 229 675 T3

Surprisingly, it has been found that a composition according to the present invention shows strain hardening, that is, its elongation viscosity increases with increasing elongation rate. This characteristic opposes the behavior of unimodal materials prepared using chromium catalysts, which show fluidization by deformation, in which the elongation viscosity decreases with increasing elongation speed. This is depicted in Figures 2 and 3, which show the elongation viscosities of the material according to the present invention and a prior art material as a function of shear rate and elongation rate, respectively.

This difference has a significant consequence, for example, in blowing films. In practice, it is not possible to maintain exactly constant operating conditions and material feed on a film line. If the material feed to the die decreases, the elongation rate will increase. If the material has a deformation fluidization behavior, its elongation viscosity decreases. This leads to the material having a lower tensile strength and consequently variations in the thickness of the film occur. However, if the material shows strain hardening, its elongation viscosity increases with increasing elongation rate. The increased viscosity resists elongation and thus an even greater film thickness is obtained.

Due to excellent mechanical properties in combination with sufficient rigidity and excellent processability, the present polyethylene compositions are remarkably suitable for blowing films. The films will have a homogeneous appearance and, in general, will show the following characteristics:

A. If the composition has a density between 929-934 and a MFR₂₁ between 12-30 g / 10 min, the film is generally blown using "low neck" conditions and the resulting film has the following properties:

- tensile strength in the transverse direction of at least 15 MPa;

- 1% secant modulus in the machine direction of at least 300 MPa and in the cross direction of at least 400 MPa; Y

- when processed to 35-45 μιη film thickness, the film has a dart drop of at least 5 g / μιη.

B. On the other hand, if the composition has a density between 935-939 and a MFR₂₁ between 7-20 g / 10 min, the film can be blown using "low neck" conditions and then the film has the following properties:

- tensile strength in the transverse direction of at least 20 MPa;

- 1% secant modulus in the machine direction of at least 400 MPa and in the cross direction of at least 500 MPa; Y

- when processed to a film with a thickness of 35-45 nm, the film has a dart drop of at least 5 g / μιη.

C. The film can also be blown using high neck conditions and then the resulting film has the following properties:

- tensile strength in the transverse direction of at least 20 MPa;

- 1% secant modulus in the machine direction of at least 400 MPa and in the cross direction of at least 500 MPa; Y

- when processed to a film with a thickness of 20-30 nm, the film has a dart drop of at least 8 g / μιη.

Polymerization process

To produce the polymeric compositions, ethylene is polymerized in the presence of a suitable catalyst, preferably a Ziegler-Natta catalyst (see below), at an elevated temperature and pressure. The polymerization is carried out in a series of polymerization reactors selected from the group of emulsion reactors and gas phase reactors. A tubular reactor is a particularly preferred embodiment of polymerization in emulsion reactors. The high molar mass part and the medium or low molar mass part of the product can be prepared in any order in the reactors.

As previously disclosed, a Ziegler-Natta catalyst is preferably used to produce the composition. Said catalyst comprises an active compound, which is generally composed of a transition metal from groups 4-5 of the Periodic Table of the Elements (IUPAC, 1990). Usually the active compound is a Ti, V or Zr compound containing a halogen. Often the catalyst also contains compounds of other metals, such as halogen-containing Mg and / or Al compounds.

ES 2 229 675 T3

The catalyst can be unsupported or supported on any particulate support known in the art. Typically, the support material is an oxide of an inorganic element, such as silicon, aluminum, titanium, or aluminum. It can also be a mixed oxide of the elements mentioned above. However, organic supports, which are usually organic polymers, can also be used. Thus, the support can be silica, alumina, zirconia, silica-alumina, silica-titania, silica-zirconia, polystyrene, etc.

An example of a suitable unsupported catalyst is shown, for example, in EP-B-491566. Some examples of suitable supported catalysts are indicated, for example, in EP-B-688794 or EP-B-604850. A particularly suitable catalyst is the one prepared according to patent EP-B-688794, since this catalyst is capable of producing ethylene (co) polymers in high yield both at high and low hydrogen concentration. This is a beneficial feature when producing a composition according to the present invention, in which a polymerization step is carried out at a high hydrogen concentration and a polymerization step is carried out at a low hydrogen concentration.

The catalyst is used in combination with a cocatalyst, which is generally a compound of a Group 2 or 13 metal, containing an alkyl, usually an aluminum alkyl, such as triethylaluminum.

The compositions according to the present invention can advantageously be produced in a multistage polymerization process comprising two or more reactors in cascade. Although it may be possible to use a process comprising only cascade emulsion reactors, such a process is not recommended, due to the problems that can occur when a component having a low density dissolves in the reaction diluent. Therefore, it is recommended to use a process that comprises at least one gas phase reactor. A process comprising a tubular reactor and a gas phase reactor, according to the specification of European Patent EP-B-517868, has been found to be especially beneficial in producing the composition. The process enables stable continuous operation, leading to a homogeneous and uniform product.

It should also be noted that the composition according to the present invention can also be produced in a single reactor using a mixed catalyst, in which one catalyst particle contains different types of active species. In this case, both active species can be metallocene species, or one or both of them can be Ziegler species.

Next, the reactor system will be described with particular reference to a system similar to that disclosed in European Patent specification No. 0 517 868 and comprising a tubular reactor (referred to as "the first reactor") and a gas phase reactor (referred to as "the second reactor"), in this order. However, it should be understood that the reactor system may comprise any number of reactors and in any other order.

In each polymerization stage it is also possible to use comonomers selected from the group of olefins C_3-18, preferably C olefins_4-10such as 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-heptene, 1-octene, 1-nonene and 1-decene, as well as mixtures thereof. The use of comonomers in the preparation of the high molar mass portion is particularly preferred.

In addition to the actual polymerization reactors used to produce the bimodal ethylene homo- or copolymer, the polymerization reactor system may also include various additional reactors, such as prereactors. Prereactors include any reactor to prepolymerize the catalyst and to modify the olefin supply, if necessary. All reactors of the reactor system are preferably arranged in series (in a cascade).

According to the present invention, polymerization comprises the steps of

- subjecting ethylene, optionally, hydrogen and / or comonomers, to a first polymerization reaction in a first reactor or reaction zone,

- recovering the product of the first polymerization from the first reaction zone,

- introduce the product of the first polymerization into a second reactor or reaction zone,

- introducing additional ethylene and optionally hydrogen and / or comonomers into the second reaction zone,

subjecting the additional ethylene and optional hydrogen and / or comonomers to a second polymerization reaction in the presence of the product of the first polymerization to produce a product of the second polymerization, and

- recovering the product of the second polymerization from the second reaction zone.

Thus, in the first stage of the process, ethylene with the optional comonomer (s), together with the catalyst, is introduced into the first polymerization reactor. Along with these components, hydrogen is introduced into the reactor as a molar mass regulator in the amount required to achieve the desired molar mass of the polymer. Alternatively, the feed to the first reactor may consist of the reaction mixture from a reactor

ES 2 229 675 T3 above, if any, together with the new monomer added, the hydrogen and / or optional comonomers and the cocatalyst. In the presence of the catalyst, ethylene and the optional comonomer will polymerize and form a particulate product, that is, polymer particles, which are suspended in the fluid circulated in the reactor.

The polymerization medium usually comprises the monomer (ie ethylene) and / or a hydrocarbon, and the fluid is liquid or gaseous. In the case of an emulsion reactor, in particular a tubular reactor, the fluid is a liquid and the polymer suspension is continuously circulated through the emulsion reactor, whereby more suspension of polymer in particulate form will be produced in a hydrocarbon or monomer medium.

The emulsion reactor conditions are selected such that at least 20% by weight, preferably at least 35% by weight of the total production is polymerized in the emulsion reactor (s). The temperature is in the range of 40 to 110 ° C, preferably in the range of 70 to 100 ° C. The reaction pressure is in the range of 25 to 100 bar, preferably 35 to 80 bar. In order to produce a polyethylene having a density in excess of 960 kg / m³, the polymerization is preferably carried out under supercritical conditions at temperatures above 90 ° C.

In emulsion polymerization, more than one reactor can be used in series. In this case, the polymer suspension in an inert hydrocarbon produced in the emulsion reactor is introduced, without separation of inert components and monomers, periodically or continuously into the next emulsion reactor, which operates at a lower pressure than the emulsion reactor. previous.

The heat of polymerization is removed by cooling the reactor through a cooling jacket. The residence time in the emulsion reactor should be at least 10 minutes, preferably 20-100 minutes, to obtain a sufficient degree of polymerization.

According to one of the embodiments of the present invention, light inert hydrocarbons are introduced into the reactor. Some examples of such hydrocarbons are propane, isobutane, n-butane, and isopentane. Preferably, propane is used as the light inert hydrocarbon.

As discussed above, if the desired product is a low molar mass polyethylene, hydrogen is introduced into the reactor. Hydrogen can be added to the reactor in a proportion of at least 100 mol H₂/ kmol ethylene, preferably 300-600 mol H₂/ kmol ethylene.

The pressure of the product of the first polymerization, including the reaction medium, is lowered after the first reaction zone to evaporate volatile compounds from the product, for example, in a flash vessel. As a result of the flash expansion, the polyethylene-containing product stream does not contain hydrogen and can be subjected to a second polymerization in the presence of additional ethylene to produce a high molar mass polymer.

To produce an ethylene composition having a tensile strength in cross direction creep of at least 15 MPa, a 1% secant modulus in the machine direction of at least 300 MPa and in the machine direction cross section of at least 400 MPa; and a dart drop of at least 5 g / pm, ethylene, hydrogen, and optional comonomers are reacted in the first stage to produce a polymer having a MFR₂ 100 g / 10 min or more.

The second reactor is preferably a gas phase reactor, in which ethylene and preferably comonomers are polymerized in a gaseous reaction medium.

The gas phase reactor can be a conventional fluidized bed reactor, although other types of gas phase reactors can be used. In a fluidized bed reactor, the bed consists of the formed and growing polymer particles as well as the still active catalyst together with the polymer fraction. The bed is kept in a fluidized state by introducing gaseous components, for example monomers, at a flow rate such that it will cause the particles to act like a fluid. The fluidizing gas can also contain inert carrier gases, such as nitrogen and propane, and also hydrogen as a modifier. The fluidized gas phase reactor can be equipped with a mechanical mixer.

The gas phase reactor used can operate in a temperature range of 50 to 115 ° C, preferably between 60 and 110 ° C and at a reaction pressure between 10 and 40 bars and a monomer partial pressure between 1 and 20 bars.

In the second polymerization stage, a second polymerization product is preferably produced having an MFR₂₁ less than 50 g / 10 min.

The pressure of the product of the second polymerization, including the gaseous reaction medium, can then be released, after the second reactor, to optionally separate part of the gaseous components and possible volatile components of the product, for example, in a container of expansion. The overhead stream or part of it is recirculated to the second reactor.

The division of production between the high molar mass polymerization reactor and the polymerization reactor8

ES 2 229 675 T3 medium or low molar mass ratio is 50-70: 50-30. Preferably, 35 to 45%, in particular 37 to 43%, of the ethylene copolymer or homopolymer is produced under conditions that provide a polymer having an MFR2 of 100 g / 10 min or more and constituting the molar mass part of the polymer, and 65 to 55%, particularly 63 to 57%, of the ethylene homopolymer or, preferably, copolymer, is produced under conditions that provide a polymer having a MFR₂₁ less than 50 g / 10 min, in particular 5 to 50 g / 10 min and constituting the high molar mass part of the polymer. The density of the low molar mass part is preferably above 960 kg / m³ and the density of the final polymer is preferably from 925 to 940 kg / m³.

The present ethylene polymers and copolymers can be blended and optionally compounded with additives and adjuvants conventionally used in the art. Thus, suitable additives include antistatic agents, flame retardants, light and heat stabilizers, pigments, processing aids, and carbon black. Fillers such as chalk, talcum powder, and mica can also be used.

The following examples illustrate the present invention.

Description of analytical methods

Charpy Impact Resistance

Charpy impact strengths are measured according to ISO 179 method. The test specimen is supported as a single horizontal bar and is broken by a single swing of a pendulum with the impact line midway between the supports and the opposite notch ( eventual).

Dart drop

Dart drop is measured using the ISO 7765-1 method. A dart with a 38 mm diameter hemispherical head is dropped from a height of 0.66 m onto a film held above a cavity. If the sample fails, the weight of the dart decreases and if it does not fail, the weight increases. A minimum of 20 samples need to be tested. A weight resulting from the failure of 50% of the samples is calculated.

Tensile strength in creep

The tensile strength in yield is obtained from a tensile experiment. The experiment is carried out according to the ISO 1184 method. The sample is spread along its major axis at a constant speed.

Secant modulus

The secant modulus at 1% elongation is also obtained from a tensile test. The value is the ratio of stress to strain at 1% strain on the stress-strain curve.

Tear resistance

Tear resistance is measured using the ISO 6383 method. The force required to propagate the tear transversely to a film sample is measured using a pendulum device. The pendulum swings by gravity through an arc, tearing the sample from a pre-cut slit. The sample is held on one side by the pendulum and on the other by a stationary member. Tear strength is the force required to tear the sample.

Example 1

A tubular reactor of a production scale plant operated at a temperature of 95 ° C and a pressure of 60 bar. Ethylene, hydrogen, diluent propane and a polymerization catalyst prepared according to Patent EP 688794 were added to the reactor, so that 5.0 tons / h of polyethylene were formed with an MFR₂ 410 g / 10 min and a density of 970 kg / m³. The polymer containing the active catalyst was separated from the reaction medium and transferred to a gas phase reactor, in which additional ethylene, hydrogen and 1-butene comonomer were added, so that 12.5 tons / h of polyethylene with an MFR₂₁ 20 g / 10 min and a density of 930.5 kg / m³. The material was mixed with 2000 ppm by weight of Irganox B225 additive polymer and 1500 ppm of calcium stearate and pelletized. The fraction of the high MFR material (or low molecular weight material) in the total polymer was 40%. The 1-butene content of the polymer was analyzed and found to be 6.4% by weight, or 3.3% mole. Since the material made in the tubular reactor did not contain any comonomers, the comonomer content of the fraction obtained in the gas phase reactor had to be 11% by weight or 5.8% molar. The corresponding density was estimated to be 904 kg / m³.

Example 2

A gas phase reactor operated at a temperature of 83 ° C and a pressure of 25 bar. Ethylene, hydrogen, 1-hexene comonomers and a polymerization catalyst prepared according to Patent EP 688794 were added to the reactor so that 8 kg / h of a polyethylene with an MFR₂₁ of 1 g / 10 min approximately and a

ES 2 229 675 T3 density of 917 kg / m³. The polymer containing the active catalyst was separated from the reaction medium and transferred to another gas phase reactor in which additional ethylene, hydrogen and 1-hexene comonomers were added, so that a total of 13 kg / h was obtained. polyethylene with an MFR₂₁ 13 g / 10 min and a density of 937 kg / m³. The material was mixed with 2000 ppm by weight of Irganox B225 additive polymer and 1500 ppm of calcium stearate and pelletized. The fraction of the high MFR material (or low molecular weight material) in the total polymer was 39%. The density of the low molecular weight fraction was therefore estimated to be 968 kg / m³.

Example 3

The material prepared according to Example 1 was blown into film on a Windmoller and Holscher film line with a 60mm extruder, a 200mm die, a 1.2mm die aperture and a "Blow-up ratio" (BUR) of 3 and a Cooling Line Height (FLH) equal to 3 times the diameter of the die (3 DD). The properties of the material and the film can be seen in the third column of Table 1.

Comparative Example 1

A commercially available unimodal material was blown onto film. The film was blown into a 200mm die, 1.4mm die aperture, a BUR of 2, and a low FLH. (Data is taken from a presentation by JC Dewart: Extended Medium Density Polyethylene Application Range with Metallocene Catalysed MDPE. The presentation was made at the Eurofilm 97 conference in Frankfurt, June 3-5, 1997, organized by Applied Market Information, Bristol , UK). The properties of the material and the film can be seen in the second column of Table 1.

Comparative Example 2

Another commercially available unimodal material was blown into film, using the conditions presented in Example 3. The material and film properties can be seen in the first column of Table 1 and also in the first column of Table 2.

TABLE 1

<td>MDPE type</td><td>Example Comparative 2</td><td>Example Comparative 1</td><td>Bimodal</td>

<td>MFR₂i [g / lOmin]</td><td> 13</td><td> 14,5</td><td> 20</td>

<td>Density [kg / m^J]</td><td> 939</td><td> 934</td><td> 931</td>

<td>Comonomer</td><td>C₆</td><td>EC</td><td>c,</td>

<td>Catalyst</td><td>Cr</td><td>Cr</td><td>ZN</td>

<td>Dart Impact [g]</td><td> 72</td><td> 105</td><td> 201</td>

<td>Tear resistance MD [N]</td><td> 0,3</td><td> 0,4</td><td> 1,7</td>

<td>Creep resistance TD [MPa]</td><td> 25</td><td></td><td> 17</td>

<td>MD / TD secant module</td><td> 500/640</td><td></td><td> 320/450</td>

<td>Film thickness (pm)</td><td> 40</td><td> 40</td><td> 40</td>

Example 4

The material prepared according to Example 2 was blown into a film using the conditions presented in Example 3. The material and film properties are shown in the third column of Table 2.

Example 5

The material prepared according to Example 2 was blown into film on an Alpine film line with a 65mm extruder, a 160mm die, a 1.5mm die aperture and a "Blow-Up Ratio" (BUR ) of 4 and a Cooling Line Height (FLH) equal to 8 DD. The properties of the material and the film can be seen in the fourth column of Table 2.

Example 6

The elongation viscosity of the samples prepared in Example 1 and Example 2, as well as the sample used in Comparative Example 2, was measured using a capillary rheometer and calculating the elongation viscosity from the pressure drop data using the method Cogswell. The method is described in more detail in Cogswell: Polymer melt rheology, a guide for industrial practice, Woodhead Publishing Ltd. 1997, ISBN 1 85573 198 3. The

ES 2 229 675 T3 elongation viscosity is shown as a function of shear rate and elongation rate in accompanying Figure 2 and Figure 3, respectively. The figures show that the material according to the present invention shows strain hardening (elongation viscosity increases with increasing elongation rate or shear rate) while conventional materials show strain thinning (elongation viscosity decreases with increasing shear rate). elongation or shear rate).

Comparative example 3

The material used in Comparative Example 2 was blown into a film as described in Example 5. The material and film properties are shown in the second column of Table 2.

TABLE 2

<td>MDPE type</td><td>Example Comparative 2</td><td>Example Comparative 3</td><td>Example 4</td><td>Example 5</td>

<td>MFR21 [g / lOmin]</td><td> 13</td><td> 13</td><td> 13</td><td> 13</td>

<td>Density [kg / m^J]</td><td> 939</td><td> 939</td><td> 937</td><td> 937</td>

<td>Comonomer</td><td>c₆</td><td>C₆</td><td>c₆</td><td>c₆</td>

<td>Catalyst</td><td>Cr</td><td>Cr</td><td>ZN</td><td>ZN</td>

<td>Indicted*</td><td>TO</td><td>B</td><td>TO</td><td>B</td>

<td>Film thickness Εμπι]</td><td> 40</td><td> 25</td><td> 40</td><td> 25</td>

<td>Gels [n / A4]</td><td> 30-50</td><td> 30-50</td><td> 5</td><td> 5</td>

<td>Appearance of the movie</td><td>It is truc tured / nuanced</td><td>Structured / nuanced</td><td>Homogeneous</td><td>Homogeneous</td>

<td>Dart Impact [g]</td><td> 72</td><td> 180</td><td> 185</td><td> 250</td>

<td>MD tear [N]</td><td> 0,3</td><td> 0,3</td><td> 1,1</td><td> 0,69</td>

<td>Resistance in the creep TD [MPa]</td><td> 25</td><td> 27</td><td> 23</td><td> 24</td>

<td>MD / TD secant module</td><td> 500/640</td><td> 460/670</td><td> 460/590</td><td> 480/630</td>

* Processing conditions

A = 60mm Windmoller and Holscher extruder with 200mm die / 1.2mm die opening, BUR = 3: 1, FLH = 650mm

B = Alpine 65mm extruder with 160mm die / 1.5mm die opening, BUR = 4: 1, FLH = 1280mm

The main advantages of this product are improved impact resistance and tear resistance, as well as excellent film homogeneity and processability, compared to conventional Cr-based materials at the same stiffness levels. This type of product is particularly suitable for HD blown film processing and coextrusion lines.