MXPA01000148A - Polyethylene compositions having improved optical and mechanical properties and improved processability in the melted state - Google Patents

Abstract

Polyethylene composition comprising (percentages by weight):A) from 60 to 95%of an LLDPE copolymer having a density from 0.905 to 0.935 g/cm3, Mw/Mn values less than 4 and F/E ratio values greater than 20;B) from 5 to 40%of one or more crystalline copolymers of propylene selected from among defined copolymers of propylene with ethylene and/or higher alpha-olefins.

Description

POLYETHYLENE COMPOSITIONS THAT HAVE IMPROVED OPTICAL AND MECHANICAL PROPERTIES AND CAPACITY OF IMPROVED PROCESSING IN CASTING STATE DESCRIPTIVE MEMORY The present invention relates to polyethylene compositions containing a linear copolymer of low density of ethylene (LLDPE, for its acronym in English) having a narrow molecular weight distribution, and a crystalline copolymer of propylene. The films obtained from said compositions possess an excellent balance of mechanical and optical properties. In addition, the aforementioned compositions are easily processable in their molten state, since they do not require large energy costs in the machines used for their processing and do not cause high pressures in the heads of the machines. The ethylene copolymer used for the compositions of the present invention possesses a molecular weight distribution, in terms of the ratio between the weight average molecular weight (PMP) and the number average molecular weight (PMn), which is in terms of PMp / PMn, which is particularly narrow (corresponding to the values of PMp / PMn less than 4) and therefore, typical of the polyethylenes obtained with metallocene catalysts. »^^. ^^^^^^^^^^^^^^^^^^^^ 11 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ In this regard, the compositions of the present invention differ from the compositions described in published patent applications WO 93/03078 and WO 95/20009, in which the LLDPE copolymer (which is mixed with a crystalline copolymer of propylene) is prepared with Ziegler-Natta catalysts and therefore has PMp / PMn values typically greater than or equal to 4. In accordance with the US patent 4,871, 813, it is possible to prepare copolymers of LLDPE having PMp / PMn values of less than 4 (from 2.5 to 6) even when the catalyst used is of the Ziegler-Natta type, however in the examples only one LLDPE copolymer is used which it has a PMp / PMn value of 4. Also, in the US patent cited above, the LLDPE copolymer is mixed with a propylene copolymer; however, the crystallinity of this propylene polymer is somewhat low, as indicated by the low values of heat of fusion (less than or equal to 75 J / g) and, in particular, the degree of crystallinity (less than 35%) . The aforementioned documents show that by means of the addition of the propylene copolymer to the LLDPE copolymer, polyethylene compositions are obtained which have an improved processing capacity in their molten state, in the aforementioned sense. In accordance with the patent of E.U.A. 4,871, 813, this effect is achieved without substantially changing the optical and mechanical properties of the film, compared with those of a film obtained from the pure LLDPE copolymer. In accordance with the published patent application WO 95/20009, the addition of the crystalline propylene copolymer is capable of improving the processing capacity in the molten state, as well as the resistance of the polyethylene film to impact and tearing. However, the optical properties of the compositions possessed by an LLDPE copolymer obtained with Ziegler-Natta catalysts and a propylene copolymer are lower, in particular as regards the values of turbidity and brightness, with respect to those typical of a LLDPE copolymer obtained with metallocene catalysts and having relatively high F / E ratio values, ie greater than 20. The aforementioned LLDPE copolymers obtained with metallocene catalysts generally show turbidity values less than about 20% and brightness values greater than about 30% (measured in blown film of a thickness of 25 μm by the method described in the examples). Corresponding to said high values of turbidity and gloss, said LLDPE polymers also possess satisfactory values of impact resistance (Dart test) and tear resistance (Elmef).

However, the LLDPE copolymers obtained with metallocene catalysts show an unsatisfactory processing capacity in the molten state. This is why it is particularly convenient to obtain polyolefin compositions having the aforementioned optical properties, with the best possible balance of impact resistance and tear strength and good processability in their molten state. In the patent of E.U.A. No. 5,674,945, there are disclosed polyethylene compositions 10 containing a copolymer of LLDPE obtained with metallocene catalysts and a propylene copolymer having a density greater than or equal to 0.900 g / m2. In particular, a copolymer containing 0.2 mol% of butene and a copolymer containing 3.4 mol% of ethylene and 1.6 mol% of butene is used in the examples. In both cases, the relative amount of propylene copolymer in the polyethylene compositions is 10% by weight, and the transparency of the films obtained from said compositions turns out to be substantially free of change compared to the transparency of the films obtained from the corresponding LLDPE copolymers in their pure state. In addition, the films obtained from the aforementioned compositions show high values of tensile modulus of elasticity, '? lÉíi_r •• - "-'" - '-' •. * .. »• > - -J- *** ** _? _ Dl_Hi_riíM _? ___ áÉ ?? __ ^ rii_M ___ MÉ_tt _ ^ __ M higher than those of the films obtained from the corresponding LLDPE copolymers in their pure state, and high breaking load values. The technical problem of obtaining an excellent balance of optical properties, resistance to impact and resistance to tearing is not considered. Now, polyethylene compositions have been created that completely satisfy the aforementioned requirements, thanks to an unusual and particularly favorable balance of mechanical and optical properties and processing capacity in the molten state. Accordingly, the object of the present invention is constituted by polyethylene compositions compng (percentages by weight): A) from 60 to 95%, preferably from 60 to 90%, most preferably from 70 to 88%, of a copolymer of ethylene with an alpha-olefin CH2 = CHR, in which R is an alkyl radical containing from 1 to 18 carbon atoms carbon (LLDPE copolymer), said copolymer having a density of 0.905 to 0.935 g / cm3, preferably 0.910 to 0.930, most preferably 0.915 to 0.925 g / cm3 (measured in accordance with ASTM D 4883), PMp / PMn values less than 4, preferably from 1.5 to 3.5, most preferably from 1.5 to 3 (measured by GPC, ie gel permeation chromatography) and F / E ratio values greater than 20, preferably from 25 to 70, most preferably from 25 to 50 (measured in accordance with ASTM D 1238); B) from 5 to 40%, preferably from 10 to 35%, most preferably from 12 to 30%, of one or more crystalline copolymers of propylene selected from (i) copolymers of propylene with ethylene containing from 3 to 8%, preferably from 4 to 6%, of ethylene; (ii) copolymers of propylene with one or more alpha-olefins CH2 = CHR ', wherein R1 is an alkyl radical having from 2 to 8 carbon atoms or an aryl radical, containing from 5 to 25%, preferably from 8 to 20%, of alpha-olefins CH2 = CHR '; (Ii) copolymers of propylene with ethylene and one or more alpha-olefins CH2 = CHR ', wherein R1 has the aforementioned meaning, and contains from 0.1 to 8%, preferably from 0.5 to 5%, most preferably from 1 to 4%, ethylene, and from 0.1 to 20%, preferably from 1 to 15%, most preferably from 2.5 to 15%, in Particularly from 2.5 to 10%, of alpha-olefins CH2 = CHR ', with the proviso that the total content of ethylene and alpha-olefins CH2 = CHR' in the copolymers (ii) is greater than or equal to 5%. %. Optionally, in order to improve the optical properties, the compositions of the present invention may contain, in addition to the components A) and B), from 0.5 to 10%, preferably from 1 to 6%, by weight of a LDPE polyethylene (component C), in relation to the total weight of A) + B) + C). As seems obvious from the above description, polymers containing two or more types of comonomers are also include in the definition of copolymers. The aforementioned compositions are generally characterized by turbidity values of less than or equal to 25%, preferably less than or equal to 20%, in particular between 20 and 5%, and gloss values greater than about 30%, in particular between 30 to 60% (measured in blown film of a thickness of 25 μm by the method described in the examples). In addition, the compositions of the present invention are generally characterized by dart test values greater than or Equal to 150 g, preferably greater than or equal to 200 g, in particular between 150 and 400 g, preferably between 200 and 400 g (measured in blown film of a thickness of 25 μm by the method described in the examples). In addition, the compositions of the present invention show particularly high tear strength values (Elmendorf), as such and in consideration with the dart test values. Generally, said values are greater than or equal to 400 g (measured in blown film of a thickness of 25 μm by the method described in the examples) in the transverse direction (DT), in particular between 400 and 800 g, and greater than or Equal to 150 g in the machine direction (DM); preferably greater than or equal to 200 g, in particular between 150 and 350 g, preferably between 200 and 350 g. Generally, component A) of the compositions of the present invention have an ethylene content greater than or equal to 60% by weight, in particular from 60 to 99%, preferably greater than or equal to 70% by weight, in particular from 70 to 99%, most preferably greater than or equal to 80% by weight, in particular from 80 to 99%.

Some examples of alpha-olefins CH2 = CHR present in component A) of the compositions of the present invention are propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl l-1-pentene, 1-ketene, 1 -decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene and 1-eicosene. Some preferred examples are 1-butane, 1-hexane and 1-octane. Generally, component A) possesses E values of molten material flow rate (MFR E in accordance with ASTM D 1238) from 0.1 to 100 g / 10 min. In addition, said component A) preferably has a fraction content soluble in xylene at 25 ° C less than or equal to 5% by weight. The trace of the DSC (Differential Scanning Calorimetry) of said component A) preferably shows a single melting peak (typical of a crystalline phase); this peak is generally located at a temperature greater than or equal to 100 ° C. LLDPE copolymers having the aforementioned characteristics for component A) are known in the art and can be obtained by conventional polymerization processes (in gas phase, in suspension or in solution) using catalysts comprising a compound of an element of transition, preferably Ti, Zr or Hf, or of the lanthanide series, wherein this element is combined with at least one cyclopentadienyl group, and a cocatalyst, in particular an alumoxane or a compound capable of forming an alkyl cation. • riRTTÉIf1 ^ - - "- - - • ***» ^ * "'*' *» > ^ * > ~ * "* < '»> ~ * - ~ * ™ - -.- w «« * ^ * Some examples of the aforementioned catalysts and polymerization processes are described in published patent application WO 93/08221. Some examples of alpha-olefins CH2 = CHR 'present in component B) of the compositions of the present invention are 1-butane, 1-pentene, 4-methylpentene-1, 1-hexene and 1-ketene. 1 -butene is preferred. Generally, component B) has L values of melt flow rate (MFR L in accordance with ASTM D 1238) of 0.1 to 500 g / 10 min., preferably from 1 to 50, most preferably from 6 to 25 10 g / 10min. Preferably said component B) has density values (measured in accordance with ASTM D 4883) of less than 0.9 g / cm3, in particular of 0.890 to 0.899, most preferably 0.892 to 0.899. In addition, said component B) preferably has the following characteristics: content of fraction insoluble in xylene at 25 ° C greater than 70% by weight, very preferably greater than or equal to 75%, in particular greater than or equal to 85% in weight; heat of fusion (measured in accordance with ASTM D 3418-82) greater than 50 J / g, most preferably greater than or equal to 60 J / g, in particular greater than or equal to 70 J / g, for example from 75 to 95 J / g; melting point (measured in accordance with STM DD 3418-82) less than 140 ° C, most preferably 120 to 140 ° C; __________ • -. - 3-*-- . .-. . ___..___ ftfflff-i ***** - > - «» > * «• -» »- • - • ---. ^. ^ M ^^ Mi.

PMp / PMn values greater than 3.5, in particular from 3.5 to 15. The crystalline propylene copolymers having the above-mentioned characteristics for component B) are known in the art, and can be obtained by conventional polymerization processes using Ziegler-Natta catalysts stereospecific supported by magnesium halides. Said catalysts contain, as an essential component, a solid catalyst component comprising a titanium compound having at least one titanium-halogen bond and an electron donor compound, both supported by a magnesium halide. As cocatalysts a alalkyl compound and an electron donor compound are generally used. Catalysts having the aforementioned characteristics are described, for example, in the patent of E.U.A. 4,399,054 and European Patent 45977. The LDPE (low density polyethylene) polymer constituting component C) of the compositions of the present invention is an ethylene homopolymer or an ethylene copolymer containing minor amounts of comonomers, such as acrylate butyl, prepared by high pressure polymerization using free radical initiators. The density of said LDPE polymer generally ranges from 0.910 to 0.925 g / cm 3 (measured in accordance with ASTM D 4883).

* * ¿?? **? I. . ^ •• -! •! , ^ T * A ^ __ te_f_____ ^ _ ^.-_ > ___ ¿_...---; . ... _ Mfl Afc_ «, The MFR E values of said LDPE polymer are generally in the range of 0.1 to 50 g / 10 min, preferably 0.3 to 20 g / 10 min. LDPE polymers having the above characteristics mentioned for component C) are known in the art. Some specific examples are commercially available polymers with the trade names of Escorene, and Lupolen (BASF). In addition to the aforementioned components, the compositions of the present invention may contain other polymeric components, such as olefin elastomers, or ethylene / propylene (EPR) or ethylene / propylene / diene (EPDM) elastomers, and additives commonly used in the process. technical, as stabilizers (in particular phenolic antioxidants and process stabilizers such as organic phosphites), pigments, fillers, nucleating agents, releasing agents, lubricating and antistatic agents, flame retardants and plasticizers. The compositions of the present invention can be prepared by means of polymerization processes in two or more consecutive steps, using at least one step the catalysts described above for the preparation of component A) and at least one different step in the catalysts. of Ziegler-Natta described above for the preparation of component B), and optionally adding component C) by mixing in the molten state.

Naturally, it is also possible to prepare the compositions of the present invention by mixing components A), B), and optionally C), in the molten state. The melt mixing processes conveniently used are of the conventional type and are based on the use of mixing devices known in the art, such as single and double-screw extruders. In view of its easy processing capacity in the molten state and its excellent optical and mechanical properties, the compositions of the present invention are particularly suitable for the preparation of molded articles in general, and in particular of film, single layer or multiple layers , either fused or mono or biaxially oriented, including blown films, in which at least one layer comprises the aforementioned compositions. The processes for the preparation of blown film are well known in the art and comprise an extrusion step through a head with an annular opening. The product of this stage is a tubular extrudate that is inflated with air, to obtain a tubular bubble that cools and collapses to obtain the film. The following examples are given for the purpose of illustrating but not limiting the present invention. The following materials are used for these examples: ^ teA .. > . *. - * «- * - tJB__i ___« u ^^^ 1 ^^ M? »*.

A) Copolymer of LLDPE ethylene / 1-ketene copolymer, marketed by DOW CHEMICAL COMPANY under the name "Affinity-1570", containing 10.8% by weight of 1-ketene (deteed by 13C NMR) and having the following 5 characteristics: Density (ASTM D 4883): 0.9130 g / cm3 PMpPMn (GPC): approx. 2.5 MFR E (ASTM D 1238): 1.0 g / 10 min F / E (ASTM D 1238): 41 10 Soluble in xylene at 25 ° C (% by weight): 3.2 B) Crystalline propylene copolymer A propylene copolymer having the following characteristics is used: Butene content (% by weight) 5.3 Ethylene content (% by weight): 2.2 Density (g / cm3): 0.895 MFR L (g / 10 min): 6 Soluble in xylene at 25 ° C (% by weight) 10 Note: the aforementioned contents of butene and ethylene are measured by IR spectroscopy, the density by ASTM D 4883, and the insoluble content in xylene (and hence the soluble content) is deteed by the following method: __? i_i__g? _l_M ____ M_É kiiíii? á? l l_ft_p_rlimi? i - "* • - * • '" - .. - ^ «-___-_ * __ a_ 2.5 g of copolymer together with 250 cm3 of oxylene are placed in a conical glass flask, equipped with condenser and magnetic stirrer. The temperature rises to the boiling point of the solvent in 30 minutes. The clear solution thus formed is allowed to reflux with stirring for another 30 minutes. The closed flask is then placed in a water and ice bath for 30 minutes and then in a water bath under a thermostat at 25 ° C for 30 minutes. The solid formed is filtered on paper at a high filtration rate. 100 cm3 of the liquid obtained from the filtration are emptied into an aluminum container, previously weighed, and everything is placed in a heating plate to evaporate the liquid in a stream of nitrogen. The container is then placed in an oven at 80 ° C and kept under vacuum until a constant weight is reached. The aforementioned propylene copolymers are prepared using high performance Ziegler-Natta catalysts and stereospecificity, supported by magnesium chloride, in the polymerization.

EXAMPLE 1 The aforementioned components A) and B) are mixed in their molten state in a single-screw extruder (Flag TR-60) under the following conditions: Temperature profile: 185, 195, 200, 205, 210, 215, 235, melted at 230 ° C; Propeller revolutions: 70 rpm; Performance: 67 kg / hr The relative amounts of the aforementioned components are equal to 80% by weight of A) and 20% by weight of B), relative to the total weight of the composition. From the composition obtained in this way, a blown film of 25 μm thickness is prepared using a COLLIN-25 machine under the following conditions: Temperature profile: 155, 165, 175, 185, 190, 190, 190, 190, melted 200 ° C; Propeller revolutions: 90 rpm; Performance: 67 kg / hr Blowing rate 2.5.

The properties shown in table 1 are measured in the film thus prepared. For comparison purposes, Table 1 also shows the properties of a blown film obtained and tested under the same conditions as in Example 1, but using component A) in its pure state (reference example 1).

TABLE 1 With respect to table 1, the head pressure corresponds to the pressure measured in the extruder head while the motor input refers to the extruder motor. In addition, the properties of the films shown in Table 1 are measured by the following standard ASTM methods: Turbidity: ASTM D 1003 Brightness: ASTM D 2457 Dart test: ASTM D 1709 10 Elmendorf: ASTM D 1922.

Claims (7)

NOVELTY OF THE INVENTION CLAIMS

1. - A polyethylene composition comprising (percentages by weight): A) from 60 to 95% of a copolymer of ethylene with an alpha-olefin CH2 = CHR, wherein R is an alkyl radical containing from 1 to 18 carbon atoms , said copolymer having a density of 0.905 to 0.935 g / cm3, PMp / PMn values less than 4, and F / E ratio values greater than 20; B) from 5 to 40% of one or more crystalline propylene copolymers selected from (i) copolymers of propylene with ethylene containing from 3 to 8% ethylene; (ii) copolymers of propylene with one or more alpha-olefins CH2 = CHR ', wherein R1 is an alkyl radical having from 2 to 8 carbon atoms or an aryl radical, containing from 6 to 25% alpha-olefin CH2 = CHR '; (iii) copolymers of propylene with ethylene and one or more alpha-olefin CH2 = CHR ', wherein R1 has the aforementioned meaning, containing 0.1 to 8% ethylene and 0.1 to 20% alpha-olefin CH2 = CHR ', with the proviso that the total content of ethylene and alpha-olefin CH2 = CHR' in the copolymers (ii) is greater than or equal to 5%.

2. The polyethylene composition according to claim 1, further characterized in that it contains: C) from 0.5 to 10% by weight, based on the total weight of A) + B) + C), of LDPE polyethylene.

3. - The polyethylene composition according to claim 1, further characterized in that the component B) has a density less than 0.900 g / cm3.

4. The polyethylene composition according to claim 1, further characterized in that the component B) has a fraction insoluble in xylene at 25 ° C greater than 70% by weight.

5. The polyethylene composition according to claim 1, further characterized in that it has turbidity values, measured in blown film of a thickness of 25 μm in accordance with ASTM D 1003, less than or equal to 25%.

6. A film of a layer or multiple layers, characterized in that at least one layer comprises the polyethylene composition according to claim 1.

7. A blown film according to claim 6.