POLYMER BLEND FOR THE PRODUCTION OF A BIORIENTED POLYMER FILM

FIELD OF THE INVENTION

[0001] The present disclosure relates to a polymer blend, a use thereof, a method of production of a bioriented polymer film and a bioriented polymer film.

BACKGROUND OF THE INVENTION

[0002] Polymeric films are widely used, both in industrial manufacturing processes and in the nonindustrial sector for the wholesale and retail delivery of goods to the consumer market.

[0003] Currently, films composed of ethylene based thermoplastic polymers dominate certain of these market applications - such as the market for household disposables, trash bags and liners; overwrap films and bags for laundry and dry cleaning goods; and shipping and carryout bags for retail merchandising of non-perishable goods. In other aspects of the consumer goods delivery market, ethylene based polymer films only weakly compete, if at all, with other more expensive polymer films such as plasticized polyvinyl chloride films and/or polypropylene films - such as in the heat-shrink wrap film market for the taut-contour fit wrapping of various items, particularly perishables such as cuts of meat, poultry, and fish. Yet for other applications, such as for packaging of produce, package constructions for cereals, dry foods, and snack foods ethylene based polymer films compete somewhat in certain circumstances of these applications.

[0004] Due to their valuable mechanical and optical properties, bi-oriented polymeric films are increasingly requested for packaging applications.

[0005] However, to achieve an optimal profile of properties, bioriented polymeric films generally have a multiple layer structure, with layers of different polymeric materials, like for instance polypropylene, polyethylene, polyethylene terephthalate, polyamides, ethylene polyvinyl alcohol.

[0006] Such complex structures require complex processing in the film preparation and are hardly compliant with the present sustainability and recyclability requirements. [0007] An attractive candidate for the preparation of bioriented films with reduced complexity, in terms of composition and structure, is polyethylene, in particular the high density polyethylene (HDPE).

[0008] In fact, HDPE can potentially achieve high mechanical properties, as it can be oriented up to relatively high stretch ratios.

[0009] Moreover, HDPE can achieve valuable optical properties, when properly treated.

[0010] However, the use of HDPE in the preparation of bioriented films has been strongly limited by processing difficulties, resulting into often insufficient final properties.

[0011] The object of the present disclosure is to provide a polymer blend, a use thereof, a method of production of a bioriented polymer film and a bioriented polymer film that allow the drawbacks of the known art to be at least partially overcome, and which are, at the same time, simple and inexpensive to implement.

SUMMARY OF THE INVENTION

[0012] Thus the present disclosure provides a polymer blend comprising:

A) from 65 to 98 wt.% of a first polyethylene component, which has a density from 0.948 to 0.960 g/cm ³ , a Melt Index MIF from 30 to 100 g/lOmin and Melt Flow Ratio MIF/MIP from 15 to 30 (in particular, up to 25); and

B) from 2 to 35 wt.% of a second polyethylene component having a density from 0.910 to 0.945 g/cm ³ and a Melt Index MIE from 0.1 to 3 g/lOmin.

[0013] The above wt.% of the first polyethylene component and of second polyethylene component are with respect to the overall weight (i.e. the sum of weights) of the first polyethylene component and of second polyethylene component. In some non-limiting cases, the above wt.% of the first polyethylene component and of second polyethylene component are with respect to the overall weight of the polymer blend.

[0014] It has been experimentally observed that the present polymer blend permits to obtain bioriented polymer films with surprisingly good mechanical (in particular, the tensile modulus and the strength at break) and esthetical (in particular, haze and gloss) properties.

DETAILED DESCRIPTION OF THE INVENTION [0015] In the present text: MIF indicates the Melt Index measured with 21.6 kg at 190°C; MIP indicates the Melt Index measured with 5 kg at 190°C; and MIE indicates the Melt Index with 2.16 kg at 190°C.

[0016] The first polyethylene component A) and the second polyethylene component B) can be selected from ethylene homopolymers and ethylene copolymers containing alpha-olefin monomer units (preferably in amounts up to 10% by weight) and their mixtures. Examples of the said alpha-olefin monomer units are those having from 3 to 8 carbon atoms, in particular propylene, 1 -butene, 1 -pentene, 1 -hexene, 1 -octene and 4-methyl-l -pentene. 1 -butene and 1- hexene are preferred.

[0017] Said homopolymers and copolymers can be obtained by way of polymerization processes in the presence of coordination catalysts. Said processes and the homopolymers and copolymers obtained from them are widely described in the art.

[0018] In particular it is possible to carry out the polymerization process in the presence of a Ziegler-Natta catalyst or single site catalyst.

[0019] As is well known, a Ziegler-Natta catalyst comprises the product of the reaction of an organometallic compound of group 1, 2 or 13 of the Periodic Table of elements with a transition metal compound of groups 4 to 10 of the Periodic Table of Elements (new notation). In particular, the transition metal compound can be selected among compounds of Ti, V, Zr, Cr and Hf and is preferably supported on MgCE.

[0020] Particularly preferred catalysts comprise the product of the reaction of said organometallic compound of group 1, 2 or 13 of the Periodic Table of elements, with a solid catalyst component comprising a Ti compound supported on MgCh.

[0021] Preferred organometallic compounds are the organo-Al compounds.

[0022] The single site catalysts are known in the art and are generally selected from metallocene and non-metallocene single site catalysts.

[0023] Examples of metallocene single site catalysts are zirconocenes and hafnocenes, for instance cyclopentadienyl or indenyl complexes of zirconium or hafnium, like bis (cyclopentadienyl) zirconium dichloride; bis (indenyl) zirconium dichloride or bis (indenyl) hafnium di chloride.

[0024] Examples of non-metallocene single site catalysts are iron complex compounds preferably having a tridentate ligand. [0025] Particularly suited tridentate ligands are 2,6-Bis[l-(phenylimino)ethyl] pyridine and preferably the corresponding compounds wherein both the two phenyl groups are substituted in the ortho-position with a halogen or tert, alkyl substituent.

[0026] Specific examples are 2,6-Bis[l-(2-tert.butylphenylimino)ethyl]pyridine iron(II) di chloride; 2,6-Bis[l-(2-tert.butyl-6-chlorophenylimino)ethyl]pyridine iron(II) di chloride or 2,6- Bis[l-(2,4-dichlorophenylimino)ethyl]pyridine iron(II) di chloride.

[0027] Said metallocene and non-metallocene single site catalysts can also be used in combination.

[0028] Preferably, the single site catalysts are reacted with activating compounds (cocatalysts), preferred examples of which are aluminoxanes, such as mono-methylaluminoxane (MAO), for instance.

[0029] The polymerization, which can be continuous or batch, is carried out, in the presence of said catalysts, following known techniques and operating in liquid phase, in the presence or not of inert diluent, or in gas phase, or by mixed liquid-gas techniques.

[0030] Reaction time, pressure and temperature relative to the polymerization steps are not critical, however it is best if the temperature is from 50 to 100°C. The pressure can be atmospheric or higher.

[0031] The regulation of the molecular weight is carried out by using known regulators, hydrogen in particular.

[0032] The second polyethylene component B) can also consist of or comprise a low density polyethylene (LDPE) selected from ethylene homopolymers or copolymers produced in a high pressure free radical polymerization.

[0033] Examples of LDPE copolymers include ethylene-vinyl acetate copolymers, ethylenevinyl alcohol copolymers, ethylene-acrylate copolymers, ethylene-methacrylate copolymers, ethylene copolymers containing alpha-olefin monomer units and mixtures thereof.

[0034] Suitable examples of alpha-olefin monomer units in the LDPE copolymers are the same as previously described.

[0035] There are two basic high pressure polymerization processes for the manufacture of LDPE: autoclave and tubular. [0036] The LDPE made by the autoclave reactor process has a high concentration of long chain branches, resulting into high values of elongational hardening, and a relatively broad molecular weight distribution that make it easy to process.

[0037] The autoclave polymerization is generally carried out in the presence of radical initiating agents selected from organic peroxides.

[0038] The tubular reactor process does not necessarily require the use of organic peroxides. It can be carried out by using oxygen alone as the radical initiating agent, thus allowing to prepare a LDPE which is free from the products of chemical degradation of organic peroxides.

[0039] The said LDPE can also be prepared with a mixed process combining both autoclave and tubular reactors.

[0040] Process operating conditions can include, but are not limited to, a pressure in the range of from 70 MPa to 700 MPa and a temperature in the range of from 150°C to 500°C.

[0041] The polymerization can be carried out in the presence of one or more chain transfer agents known in the art, such as propylene, propane and propionic aldehyde.

[0042] Such chain transfer agents are used to regulate the molecular weights.

[0043] The said processes and the resulting LDPE product are well known in the art. For instance, US patent. No. 3,691,145 and US patent application No. 2010/0076160 teach producing LDPE in a tubular reactor process.

[0044] In general, the term “copolymer” is meant to include also polymers containing more than one kind of comonomers, such as terpolymers.

[0045] All the said ethylene homopolymers and copolymers are available on the market. Specific commercial polymers suited for producing the present polymer blend are described in the examples.

[0046] Advantageously but not necessarily, the first polyethylene component has a tensile modulus of at least 650 MPa (in particular, at least 800 MPa; more in particular, at least 850 MPa). According to some non-limiting embodiments, the first polyethylene component has a tensile modulus of up to 1300 MPa (in particular, up to 1200 MPa; more in particular, up to 1100 MPa).

[0047] Advantageously but not necessarily, the second polyethylene component has tensile modulus up to 1000 MPa (in particular, up to 800 MPa). According to particularly preferred embodiments, the second polyethylene component has tensile modulus up to 400 MPa (more particularly, up to 300 MPa). [0048] According to some non-limiting embodiments, the second polyethylene component has tensile modulus of at least 100 MPa (in particular, at least 200 MPa; more in particular, at least 250 MPa).

[0049] Advantageously but not necessarily, the second polyethylene component has a mass-average molar mass Mw lower than 170000 g/mol (in particular, lower than 160000 g/mol). [0050] According to particularly preferred but not limiting embodiments, the second polyethylene component has a Mw lower than 130000 g/mol (in particular, lower than 120000 g/mol).

[0051] In some non-limiting cases, the second polyethylene component has a Mw higher than 90000 g/mol (in particular, higher than 100000 g/mol).

[0052] Advantageously but not necessarily, the second polyethylene component has a dispersity Mw/Mn lower than 22.0 (in particular, lower than 17.0).

[0053] According to particularly preferred but not limiting embodiments, the second polyethylene component has a Mw/Mn lower than 10.0 (in particular, lower than 8.0).

[0054] In some non-limiting cases, the second polyethylene component has a Mw/Mn higher than 5.0 (in particular, higher than 6.0).

[0055] According to some non-limiting embodiments, the second polyethylene component has a number-average molar mass Mn higher than 9000 g/mol (in particular, higher than 13000 g/mol); in particular, lower than 20000 g/mol (more in particular, lower than 17000 g/mol).

[0056] According to some non-limiting embodiments, the second polyethylene component has a number-average molar mass Mz lower than 800000 g/mol (in particular, lower than 400000 g/mol); in particular, higher than 200000 g/mol (more in particular, higher than 250000 g/mol).

[0057] Advantageously but not necessarily, the first polyethylene component has a massaverage molar mass Mw higher than 175000 g/mol (in particular, higher than 185000 g/mol).

[0058] In some non-limiting cases, the first polyethylene component has a Mw lower than 250000 g/mol (in particular, lower than 210000 g/mol).

[0059] Advantageously but not necessarily, the first polyethylene component has a dispersity Mw/Mn higher than 22.0 (in particular, higher than 25.0; more in particular, higher than 26.0).

[0060] In some non-limiting cases, the first polyethylene component has a Mw/Mn lower than 34 (in particular, lower than 30). [0061] According to some non-limiting embodiments, the first polyethylene component has a number-average molar mass Mz higher than 800000 g/mol (in particular, higher than 900000 g/mol); in particular, lower than 150000 g/mol (more in particular, lower than 120000 g/mol).

[0062] According to some non-limiting embodiments, the first polyethylene component has a number-average molar mass Mn lower than 9000 g/mol (in particular, lower than 8000 g/mol); in particular, higher than 4000 g/mol (more in particular, higher than 6000 g/mol).

[0063] It has been experimentally observed that with a second polyethylene component having a relatively low tensile modulus and/or a low Mw (and/or Mz) and/or Mw/Mn (see above) it is possible to obtain bioriented polymer films with particularly good mechanical and esthetical properties.

[0064] Advantageously but not necessarily, the first polyethylene component has a density of at least 0.950 g/cm ³ . Alternatively or additionally, the second polyethylene component has a density up to 0.940 g/cm ³ (in particular, up to 0.930 g/cm ³ ). In some non-limiting cases, the second polyethylene component has a density of at least 0.915 g/cm ³ .

[0065] Advantageously but not necessarily, the first polyethylene component has a Melt Index MIF from 45 to 80 g/lOmin. Alternatively or additionally, said second polyethylene component having a Melt Index MIE from 0.6 to 2 g/lOmin.

[0066] Advantageously but not necessarily, the polymer blend comprises from 67 to 95 wt.%, with respect to the overall weight (i.e. the sum of weights) of the first polyethylene component and of second polyethylene component, of said first polyethylene component. In addition or alternatively, the polymer blend comprises up to 33 (in particular, up to 25; more in particular, up to 15) wt.%, with respect to the overall weight (i.e. the sum of weights) of the first polyethylene component and of second polyethylene component, of said second polyethylene component. In some instances, the polymer blend comprises at least 5 (in particular, at least 10) wt.%, with respect to the overall weight (i.e. the sum of weights) of the first polyethylene component and of second polyethylene component, of said second polyethylene component.

[0067] Advantageously but not necessarily, the first polyethylene component has a Melt Index MIP from 0.5 (in particular, from 1.0) to 15 (in particular, to 10) g/lOmin, in particular from 1.0 to 10 g/lOmin.

[0068] According to some non-limiting embodiments, the polymer blend comprises: - (from 1; in particular, from 2) up to 33 (in particular, up to 20) wt.% of at least one third polyethylene component having a density from 0.920 to 0.950 g/cm ³ , a Melt Index MIE from 0.1 to 3 g/lOmin and a tensile modulus of at least 400 MPa (in particular, at least 500 MPa);

- from 65 to 98 wt.%, with respect to the overall weight (i.e. the sum of weights) of the first, the second and the third polyethylene component, of the first polyethylene component; and

- from 2 to 33 wt.%, with respect to the overall weight (i.e. the sum of weights) of the first, the second and the third polyethylene component, of the second polyethylene component, which has a tensile modulus of up to 350 MPa (in particular, up to 300 MPa); the wt.% of the third polyethylene component being with respect to the overall weight (i.e. the sum of weights) of the first polyethylene component, of second polyethylene component and of third polyethylene component.

[0069] According to preferred but non-limiting embodiments, the sum of the weights of the second polyethylene component and of the third polyethylene component is from 3 (in particular from 5) to 35 (in particular, to 33) wt.%, with respect to the overall weight (i.e. the sum of weights) of the first, the second and the third polyethylene component.

[0070] Additionally or alternatively, the third polyethylene component has a density from 0.920 to 0.950 g/cm ³ , a Melt Index MIE of 0.1 to 3 g/lOmin, and a Mw lower than 170000 g/mol (in particular, lower than 160000) and higher than 135000 (in particular, higher than 145000; more in particular, higher than 150000). In some non-limiting cases, the third polyethylene component has a Mw/Mn lower than 25 (in particular lower than 22) and higher than 11 (in particular, higher than 12).

[0071] According to some non-limiting embodiments, the second polyethylene component has a Mw lower than 130000 g/mol (in particular, lower than 120000 g/mol).

[0072] It has been experimentally observed that the presence of both the second and the third polyethylene components permits to obtain bioriented polymer films with surprisingly good esthetical properties (in particular, haze).

[0073] Advantageously but not necessarily, the wt.% of the second polyethylene component is equal or higher than the wt.% of the third polyethylene component (the wt.% of the third polyethylene component and of the second polyethylene component being with respect to the overall weight - i.e the sum of weights - of the first polyethylene component, of second polyethylene component and of third polyethylene component). [0074] Advantageously but not necessarily, the polymer blend comprises from 2 to 25 (in particular, to 20; more in particular, to 15) wt.%, with respect to the overall weight (i.e. the sum of weights) of the first, the second and the third polyethylene component, of the second polyethylene component.

[0075] Advantageously but not necessarily, the polymer blend comprises from 2 to 25 (in particular, to 20; more in particular, to 15) wt.%, with respect to the overall weight (i.e. the sum of weights) of the first, the second and the third polyethylene component, of the third polyethylene component.

[0076] According to non-limiting embodiments, the polymer blend consists of the first polyethylene component and the second polyethylene component (and, optionally, the third polyethylene component).

[0077] The present polymer blend can also contain conventional additives.

[0078] Examples of these additives are heat stabilizers, antioxidants, UV absorbers, light stabilizers, metal deactivators, compounds which destroy peroxide, and basic costabilizers, typically in amounts of from 0.01 to 10 % by weight, preferably from 0.1 to 5 % by weight, with respect to the total weight of the polymer blend.

[0079] In another embodiment, it is herein provided a process for the production of the polymer blend described above. The process comprises a combination step, during which the first polyethylene component and the second polyethylene component (and, optionally, the third polyethylene component) are combined by melting and mixing the components, and the mixing is effected in a mixing apparatus at temperatures generally of from 160 to 250°C.

[0080] Any known apparatus and technology can be used for this purpose.

[0081] Useful melt-mixing apparatus in this context are in particular extruders or kneaders, and particular preference is given to twin-screw extruders. It is also possible to premix the components at room temperature in a mixing apparatus.

[0082] In another embodiment, it is herein provided a use of a polymer blend as described above for producing a bioriented polymer film.

[0083] According to some embodiments, the use comprises a stretching step, during which a film of the polymer blend is stretched in a first and a second direction crosswise (in particular, perpendicular) to each other. [0084] In particular, the film of the polymer blend is stretched in the first direction with a stretch ratio from 3 : 1 to 9: 1. In addition or alternatively, the film of the polymer blend is stretched in the second direction with a stretch ratio from 3 : 1 to 7: 1.

[0085] In some non-limiting cases, the primary film before stretching has a thickness of at least 0.3 mm (in particular, at least 0.5mm), and the bioriented polymer film has a thickness of less than 250 pm (in particular, less than 100 pm; more in particular, less than 50 pm).

[0086] In another embodiment, it is herein provided a method of production of a bioriented polymer film, comprising the stretching step as disclosed above. In particular, also the polymer blend and the bioriented polymer film are as disclosed above.

[0087] More precisely but not necessarily, mono or multilayer bioriented films can be prepared with known processes.

[0088] In particular they can be prepared using the tenter frame process. In the tenter frame process, the polymer is extruded as a film directly onto a chilled roller and the film is then passed through a stretching unit by rollers moving faster than the rate at which the polymer is extruded. This orients the film in the machine direction (MD).

[0089] The film extrusion is carried out with known techniques, preferably operating at temperatures from 180 to 300°C.

[0090] In an orientation stage, carried out in the stretching unit, the main operative conditions are, preferably:

- Pre-heating temperature: 120 - 130°C;

- Pre-heating time: 60 - 100 sec.;

- Stretch rate: 40 - 80%/sec.;

- Stretch ratio: 3: 1 - 9: 1.

[0091] The film is then fed into a tenter frame for transverse direction orientation. In the tenter frame the film is maintained at the pre-heating temperature and is gripped along each edge by clamps that are attached to moving chains. These move outwards to stretch the film in the transverse direction (TD). After stretching, the film is heat-set to hold the orientation and then reeled up.

[0092] At this stage, the main operative conditions are, preferably:

- Stretch rate: 30 - 60%/sec.;

- Stretch ratio: 3: 1 - 7: 1. [0093] The bioriented films can also be conveniently produced using the twin-bubble method. This method involves the production of a primary tubular film with concentric layers (when the film is multilayer) by extrusion of the polymer components constituting the various layers through an annular slot. The primary film is calibrated and rapidly cooled and then heated and oriented in the machine and transverse direction by blowing with compressed air (TD) and increasing the speed of the take-up roll (MD). The bioriented film is then rapidly cooled to stabilize the molecular orientation of the film.

[0094] In all the said processes, heating can be carried out by using, for instance, IR lamps or hot air or other heating elements, like electrical resistance heaters.

[0095] The biorientation provides balanced mechanical characteristics. Biaxial film orientation greatly improves film's tensile strength, flexibility, and toughness. Orientation also enables the films to be used for heat-shrinking applications.

[0096] Patent application WO97/22470 also discloses methods for making oriented films. [0097] In another embodiment, it is herein provided a bioriented polymer film consisting of a polymer blend as disclosed above.

[0098] According to some non-limiting embodiments, the bioriented polymer film has a thickness of less than 250 pm (in particular, less than 100 pm; more in particular, less than 50 pm). [0099] In particular, the bioriented polymer film is obtained with the previously described method.

[0100] Preferably, the bioriented polymer film has one or more of the following properties: Haze from 1.5 to 20%, more preferably from 1.5 to 6.5;

Gloss on film (at 45°C): from 45 to 100 GU;

Tensile Modulus MD: from 700 to 1600 MPa;

Tensile Modulus TD: from 800 tol800 MPa;

Strength at break MD: from 60 to 200 MPa;

Strength at break TD: from 130 to 250 MPa;

Elongation at break MD: from 40 to 200%;

Elongation at break TD: from 40 to 200%.

EXAMPLES [0101] The practice and advantages of the various embodiments, compositions and methods as provided herein are disclosed below in the following examples. These examples are illustrative only, and are not intended to limit the scope of the appended claims in any manner whatsoever.

[0102] The following analytical methods are used to characterize the polymer compositions.

[0103] Melt flow index

[0104] Determined according to to ISO 1133-1 2012-03 at 190°C with the specified load.

[0105] Density

[0106] Determined according to ISO 1183-1 :2012 at 23°C, immersion method.

[0107] Tensile modulus

[0108] Determined according to ASTM D882-18.

[0109] Gloss

[0110] Determined according to ASTM D-2457-13.

[0111] Haze

[0112] Determined according to ASTM D- 1003-13.

[0113] Molecular Weight Distribution Determination

[0114] The determination of the means Mw, Mn and Mz and of Mw/Mn derived therefrom was carried out by high-temperature gel permeation chromatography using a method described in ISO 16014-1, -2, -4, issue of 2003. The specifics according to the mentioned ISO standards are as follows: Solvent 1, 2, 4-tri chlorobenzene (TCB), temperature of apparatus and solutions 145°C and as concentration detector a PolymerChar (Valencia, Patema 46980, Spain) IR-4 infrared detector, capable for use with TCB. A WATERS Alliance 2000 equipped with the following pre-column SHODEX UT-G and separation columns SHODEX UT 806 M (3x) and SHODEX UT 807 (Showa Denko Europe GmbH, Konrad-Zuse-Platz 4, 81829 Muenchen, Germany) connected in series was used.

[0115] The solvent was vacuum distilled under Nitrogen and was stabilized with 0.025% by weight of 2,6-di-tert-butyl-4-methylphenol. The flowrate used was 1 ml/min, the injection was 500pl and polymer concentration was in the range of 0.01% < cone. < 0.05% w/w. The molecular weight calibration was established by using monodisperse polystyrene (PS) standards from Polymer Laboratories (now Agilent Technologies, Herrenberger Str. 130, 71034 Boeblingen, Germany) in the range from 580g/mol up to 11600000g/mol and additionally with Hexadecane.

[0116] The calibration curve was then adapted to Polyethylene (PE) by means of the Universal Calibration method (Benoit H., Rempp P. and Grubisic Z., & in J. Polymer Sci., Phys. Ed., 5, 753(1967)). The Mark-Houwing parameters used herefore were for PS: kps= 0.000121 dl/g, aps=0.706 and for PE kp£= 0.000406 dl/g, apE=0.725, valid in TCB at 135°C. Data recording, calibration and calculation was carried out using NTGPC_Control_V6.02.03 and NTGPC V6.4.24 (hs GmbH, HauptstraBe 36, D-55437 Ober-Hilbersheim, Germany) respectively.

[0117] Comonomer content

[0118] The comonomer content was determined by means of IR in accordance with ASTM D 6248 98, using an FT-IR spectrometer Tensor 27 from Bruker, calibrated with a chemometric model for determining ethyl- side-chains in PE for butene- 1 as comonomer and butyl- side-chains in PE for hexene- 1 as comonomer.

[0119] Example 1

[0120] This example discloses the production of samples of bioriented polymer films and the characteristics of the obtained films.

[0121] The following commercially available starting materials, sold by LyondellBasell Industries, have been used.

Hostalen GD 9555 (GD9555)

- MIP: 3.0 g/10 min;

MIF: 63 g/10 min;

Density: 0.953 g/cm ³ ;

Tensile modulus: 1050 MPa;

Tensile Stress at Yield: 25 MPa;

Tensile Strain at Yield: 10%.

Luflexen hyPE 35P FA (hyPE)

- MIE: 0.75 g/10 min;

Density: 0.936 g/cm ³ ;

Tensile modulus: 600 MPa;

Tensile Stress at Yield: 16 MPa.

Lupolen 2420F (LP 2420F) - MIE: 0.75 g/10 min;

Density: 0.923 g/cm ³ ;

Tensile modulus: 260 MPa;

Tensile Stress at Yield: 11 MPa.

[0122] The molecular weights are reported in Table 1 below.

Table 1

[0123] Samples of bioriented polymer films with the following polymer blends have been produced (the percentages are by weight with respect overall weight of the polymer blend).

- A: 100% GD9555

- B: 90% GD9555 + 10% hyPE

- C: 90% GD9555 + 10% LP 2420F

- D: 80% GD9555 + 10% hyPE + 10% LP 2420F

- E: 70% GD9555 + 15% hyPE + 15% LP 2420F

[0124] In order to obtain the bioriented polymer films the following procedure has been used.

[0125] A primary film having a thickness of 1 mm was prepared using a Leonard line with the following features and under the following conditions:

Extruder diameter: 40 mm, L/D 27;

- Dosing gear pump;

- Flat die, with lip width 200 mm, die lip gap of 1 mm;

- Melt temperature: 240°C;

3 chill rolls having a diameter of 160 mm, with roll temperature of 45°C;

- Film cutting unit.

[0126] From the primary film, 93 x 93 mm specimens were cut. The specimens were then oriented using a Brueckner KARO IV stretching unit, under the following conditions: - Pre-heating temperature: 123°C; heating time: 80 sec; stretching speed: 45 - 60%/sec; stretching area: 70 x 70 mm (out of clamps); stretch ratio: MD 6: 1; TD 5:1; final thickness: 23 pm.

[0127] The bioriented polymer films produced had the characteristics indicated in Table 2 below.

Table 2

[0128] MD means in the “machine direction”. In other words, it means that the measurement is carried out the direction of the extrusion.

[0129] TD means in the “Transversal direction”. In other words, it means that the measurement is carried out in a direction substantially perpendicular to the direction of the extrusion.

[0130] Before having carried these tests, the skilled person would have believed that the addition of a polymer having low density to a base polymer having high density would have provided bioriented polymer films with sensibly lower mechanical properties (in particular, tensile modulus and strength at break).

[0131] The obtained results are, thus, definitely surprising: contrary to what expected in most instances, it has been obtained an improvement; or at least only a slight worsening.