Process for modifying melt flow index of low density polyethylene

Abstract

A process for producing a low density polyethylene having an improved melt flow index comprising subjecting a precursor polyethylene (I) comprising 35% or more by weight of LDPE to thermal visbreaking.

Description

TECHNICAL FIELD

[0001] The present invention provides a process for producing low density polyethylene having an improved melt flow index, which comprises a thermal degradation cracking step.

[0002] The product thus obtained is particularly suitable for foamed articles and films. BACKGROUND OF THE INVENTION

[0003] Low density polyethylene is a well-known thermoplastic with a variety of uses.

[0004] In fact, low density polyethylene obtained by free radical polymerization of ethylene, especially LDPE, is commonly used in many applications, including films for packaging, agricultural films, shopping bags, heavy duty transport bags, foamed articles, lids and closures, pipes, tubes, automotive parts, household utensils, medical applications, liners and toys.

[0005] Depending on the specific application, different polymer property distributions are required.

[0006] In particular, for foamed articles (such as foamed articles for flexible packaging) and films, a precise balance of molecular weight, and thus melt flow index and melt strength, is required.

[0007] However, the virgin LDPE grades available on the market (obtained directly from polymerization) do not necessarily have this property balance.

[0008] In addition, a particularly advantageous source of LDPE material currently represents LDPE recyclates, which mainly come from the differential recycling of post-consumer and / or industrial plastic waste.

[0009] Such LDPE recyclate materials, which are obtained by separation from waste streams and usually contain significant amounts of other polyethylene components, especially linear low density polyethylene (LLDPE), have rather variable properties and usually require processing, such as treatment with free radical initiators and / or blending with virgin LDPE, to make them suitable for the desired uses.

[0010] Therefore, considering the end uses of low density polyethylene materials, there has been a continuing need for effective processing methods capable of adjusting the main and most determining polymer properties of low density polyethylene materials.

[0011] Thermal degradation cracking of ethylene polymers is generally known, as reported for example in WO0136495.

[0012] It has now been found that due to the optimal balance of melt flow index and melt strength, thermal degradation cracking of low density polyethylene consisting of or containing LDPE allows the obtaining of end products that are particularly suitable for foamed articles and films.

[0013] Such a result is achieved by appropriately selecting the low-density polyethylene material and the degree of thermo-oxidative degradation, which is represented by the ratio of specific polymer properties before and after the thermo-oxidative degradation. SUMMARY OF THE INVENTION

[0014] The present invention provides a process for producing low-density polyethylene having an improved melt flow index, which comprises subjecting a precursor polyethylene (I) to thermo-oxidative degradation, the precursor polyethylene (I) comprising 35% by weight or more, preferably 40% by weight or more, in particular from 35% to 100%, or from 40% to 100% of LDPE, based on the total weight of the precursor polyethylene (I), the precursor polyethylene (I) having:

[0015] 1 I ) a density of from 0.910 to 0.940 g / cm 3 , preferably from 0.915 to 0.935 g / cm 3 , measured at 23 °C according to ISO 1183-1:2012;

[0016] 2 I ) a MIP value of from 0.3 to 7 g / 10 min, preferably from 0.5 to 6 g / 10 min;

[0017] 3 I ) a ratio MIP / MIE of from 2 to 7, preferably from 2.5 to 6, where MIP is the melt flow index at 190 °C and a load of 5 kg, and MIE is the melt flow index at 190 °C and a load of 2.16 kg, both measured according to ISO 1133-2:2011;

[0018] 4 I ) an ER value of from 1.8 to 8, preferably from 2 to 6;

[0019] Thereby obtaining a low-density polyethylene product having a ratio MIE / ER of from 0.2 to 2.44, preferably from 0.2 to 2.40, and:

[0020] 1) a density of from 0.910 to 0.940 g / cm 3 , preferably from 0.915 to 0.935 g / cm 3 ;

[0021] 2) a MIP value of from 3 to 20 g / 10 min, preferably from 4 to 15 g / 10 min, where MIP ratio 2) / 2 I ) is equal to or greater than 1.5, preferably equal to or greater than 1.8, in particular from 1.5 to 15, or from 1.5 to 10, or from 1.8 to 15, or from 1.8 to 10;

[0022] wherein ER is calculated by the following formula:

[0023] At a value of G" = 0.5 kPa (5,000 dyn / cm 2 ),

[0024] ER = (1.781 * 10 -3 ) * G';

[0025] where:

[0026] G' = storage modulus;

[0027] G” = loss modulus;

[0028] Both G' and G" are measured by dynamic oscillatory shear in a plate - plate rotational rheometer at a temperature of 190 °C.

[0029] The low - density polyethylene product thus obtained not only has a relatively high melt flow index value but also has a high melt strength, as indicated by the high value of the force F(max) required to tear the strand in the Rheotens test. Detailed Embodiments

[0030] The expression "low - density polyethylene" is used herein to include, as alternatives, single ethylene polymers and polyethylene compositions, i.e., compositions comprising two or more ethylene polymers.

[0031] As shown in the examples, thermo - mechanical degradation has the effect of reducing the ER value while increasing the melt flow index value, such that by subjecting the precursor polyethylene (I) having an ER value as defined above 4 I ) to thermo - mechanical degradation, the MIE / ER ratio from 0.2 to 2.44 is obtained.

[0032] Particularly preferred MIE / ER ratio values are:

[0033] - from 0.3 to 2.44; or

[0034] - from 0.3 to 2.40; or

[0035] - from 0.2 to 2.35; or

[0036] - from 0.3 to 2.35.

[0037] Preferably, the resulting MIE value of the low - density polyethylene product of the present invention is 1 g / 10 min or higher, particularly from 1 to 10 or from 1 to 8 g / 10 min.

[0038] Preferably, the precursor polyethylene (I) comprises in total (i.e., including the previously mentioned LDPE) at least 70% by weight of ethylene polymer, more preferably at least 80% by weight of ethylene polymer, and most preferably at least 90% by weight of ethylene polymer. In all cases, the upper limit is preferably 100% by weight of ethylene polymer. The amount refers to the total weight of the precursor polyethylene (I).

[0039] As previously mentioned, the precursor polyethylene (I) may consist of virgin LDPE or contain virgin LDPE, or may consist of LDPE recyclate or contain LDPE recyclate.

[0040] The precursor polyethylene (I) may also consist of a blend of virgin LDPE and LDPE recyclate or contain a blend of virgin LDPE and LDPE recyclate.

[0041] The expression "virgin LDPE" refers to a polymer that has not undergone any process for producing a finished or semi-finished product, such as a packaging film, tube, bottle, container, or semi-finished products such as fibers or sheets for thermoforming.

[0042] Thus, except for possible pelletization (which is still considered part of the polymer production process), virgin LDPE has not undergone post-production processing.

[0043] As used herein, "LDPE recyclate" refers to post-consumer recycled ("PCR") LDPE and / or post-industrial recycled ("PIR") LDPE. PCR LDPE recyclate is derived from end products that have completed their life cycle as consumer goods and would otherwise be disposed of as waste (e.g., polyethylene water bottles). PIR LDPE recyclate is derived from plastic waste generated as waste in industrial processes. PCR polyolefins include polyolefins that have been collected in commercial and residential recycling programs, including flexible packaging (cast film, blown film, and BOPP film), rigid packaging, blown bottles, and injection-molded containers.

[0044] Thus, generally speaking, LDPE recyclate is a material derived from the manufacturing process of products.

[0045] Typically, through a step of separating from other polymers such as PVC, PET, or PS, two main polyolefin fractions are obtained, namely polyethylene recyclate (including HDPE, MDPE, LDPE, and LLDPE) and polypropylene recyclate (including homopolymers, random copolymers, and multiphase copolymers). The polyethylene recyclate can be further separated to recover a fraction containing a significant amount, especially 35% or more by weight relative to the total weight, of LDPE.

[0046] Preferably, the precursor polyethylene (I) has one or more of the following additional characteristics:

[0047] - MIE from 0.1 to 3 g / 10 min;

[0048] - Mw from 100,000 to 350,000 g / mol;

[0049] - Mw / Mn ratio from 3 to 20 or from 4 to 15;

[0050] wherein Mw is the weight-average molecular weight and Mn is the number-average molecular weight, both measured by GPC (gel permeation chromatography) as explained in the examples.

[0051] As used herein, "LDPE" refers to ethylene homopolymers and ethylene copolymers produced in free radical polymerization.

[0052] The polymerization is usually carried out under high pressure, as explained in detail below.

[0053] Examples of LDPE copolymers include ethylene-vinyl acetate copolymers, ethylene-vinyl alcohol copolymers, ethylene-acrylate copolymers, ethylene-methacrylate copolymers, ethylene-α-olefin copolymers, and mixtures thereof.

[0054] Suitable examples of α-olefin comonomers in LDPE copolymers include C3-C 10 α-olefins such as propylene, 1-butene, 1-hexene, 1-octene, and mixtures thereof.

[0055] When present, the comonomer can be present in an amount of up to 15% by weight, 10% by weight, or 5% by weight, relative to the total weight of the copolymer.

[0056] Generally, the term "copolymer" is intended to also include polymers containing more than one comonomer, such as terpolymers.

[0057] There are two basic high-pressure polymerization processes for manufacturing LDPE: autoclave and tubular.

[0058] LDPE prepared by the autoclave reactor process ("autoclave LDPE") has a high concentration of long-chain branches, resulting in high tensile hardening values and a relatively broad molecular weight distribution that makes it easy to process.

[0059] Autoclave polymerization is usually carried out in the presence of a free radical initiator selected from organic peroxides.

[0060] The tubular reactor process does not necessarily require the use of organic peroxides. It can be carried out by using only oxygen as a free radical initiator, and thus LDPE free of chemical degradation products containing organic peroxides can be prepared.

[0061] The LDPE can also be prepared by a hybrid process combining both a high-pressure autoclave and a tubular reactor.

[0062] The process operating conditions can include but are not limited to a pressure of 70 Mpa to 700 Mpa, preferably 140 to 190 MPa, and a temperature of 150 °C to 500 °C, preferably 150 °C to 320 °C.

[0063] The polymerization gas can optionally contain one or more chain transfer agents known in the art, such as propylene, propane, and propionaldehyde.

[0064] Such chain transfer agents are used to adjust the molecular weight.

[0065] The process and the resulting LDPE product are well-known in the art. For example, U.S. Patent No. 3,691,145 and U.S. Patent Application No. 2010 / 0076160 teach the production of LDPE in a tubular reactor process.

[0066] Virgin LDPE polymers having the properties cited above for the precursor polyethylene (I) are known in the art. Specific examples are polymers commercially available under the trade names Lupolen (LyondellBasell) and Petrothene (Equistar).

[0067] LDPE recycle compositions having the characteristics cited above for the precursor polyethylene (I) are also known in the art. Specific examples are polyethylene compositions commercially available under the trade name Nextfilm (Suez).

[0068] The precursor polyethylene (I) and thus the low-density polyethylene product of the present invention obtained by thermo-oxidative degradation can contain one or more additional polyethylene components, which are particularly selected from HDPE (high-density polyethylene, typically having a density of 0.940 to 0.965 g / cm 3 ), MDPE (medium-density polyethylene, typically having a density of 0.926 to 0.940 g / cm 3 ), LLDPE (linear low-density polyethylene, typically having a density of 0.900 to 0.939 g / cm 3 ), and mixtures thereof.

[0069] One or more of these additional components can be present, for example, in the LDPE recycle.

[0070] They are ethylene homopolymers and ethylene copolymers containing α-olefin monomer units (preferably in an amount of up to 10% by weight) and mixtures thereof. Examples of the α-olefin monomer units are those having 3 to 8 carbon atoms, especially propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, and 4-methyl-1-pentene. 1-butene and 1-hexene are preferred.

[0071] The homopolymers and copolymers can be obtained by a polymerization process in the presence of a coordination catalyst. The process and the homopolymers and copolymers obtained therefrom are widely described in the art.

[0072] In particular, the polymerization process can be carried out in the presence of a Ziegler-Natta catalyst or a single-site catalyst.

[0073] As is well known, the Ziegler-Natta catalyst comprises the reaction product of an organometallic compound of Group 1, 2, or 13 of the Periodic Table with a transition metal compound of Groups 4 to 10 (new numbering system) of the Periodic Table. In particular, the transition metal compound can be selected from compounds of Ti, V, Zr, Cr, and Hf, and is preferably supported on MgCl2. A particularly preferred catalyst comprises the reaction product of the organometallic compound of Group 1, 2, or 13 of the Periodic Table with a solid catalyst component comprising a Ti compound supported on MgCl2.

[0074] Preferred organometallic compounds are organo-Al compounds.

[0075] The single-site catalyst is well known in the art and is generally selected from metallocene single-site catalysts and non-metallocene single-site catalysts.

[0076] Examples of metallocene single-site catalysts are zirconocenes and hafnocenes, such as cyclopentadienyl or indenyl complexes of zirconium or hafnium, such as bis(cyclopentadienyl)zirconium dichloride; bis(indenyl)zirconium dichloride or bis(indenyl)hafnium dichloride.

[0077] Examples of non-metallocene single-site catalysts are iron complex compounds preferably having a tridentate ligand.

[0078] Thermal degradation cracking involves treating a precursor polyethylene (I) at a temperature and / or mechanical shear energy sufficient to break polymer chains, mainly polymer chain branching or crosslinking.

[0079] In some embodiments, thermal degradation cracking is carried out by heating the precursor polyethylene (I) at a temperature equal to or greater than 280 °C, preferably at a temperature equal to or greater than 290 °C, more preferably at a temperature equal to or greater than 300 °C, and most preferably at a temperature equal to or greater than 310 °C. The preferred upper limit is preferably 500 °C in all cases.

[0080] In particular, the thermal visbreaking can be carried out at the following temperatures:

[0081] - from 280 °C to 500 °C; or

[0082] - from 290 °C to 500 °C; or

[0083] - from 300 °C to 500 °C; or

[0084] - from 310 °C to 500 °C; or

[0085] - from 280 °C to 480 °C; or

[0086] - from 290 °C to 480 °C; or

[0087] - from 300 °C to 480 °C; or

[0088] - from 310 °C to 480 °C; or

[0089] - from 280 °C to 460 °C; or

[0090] - from 290 °C to 460 °C; or

[0091] - from 300 °C to 460 °C; or

[0092] - from 310 °C to 460 °C.

[0093] In some embodiments, the thermal visbreaking is carried out in the absence or substantially absence of oxygen, where substantially absence of oxygen means less than or equal to 1.0% by weight, less than or equal to 0.10% by weight, or less than or equal to 0.01% by weight based on the total weight of the polymer in the thermal visbreaking zone.

[0094] It is known that the thermal visbreaking can be carried out in a conventional mixing device commonly used for processing polymers in a molten state.

[0095] In particular, the low-density polyethylene of the present invention can be prepared by processing a precursor polyethylene (I) in an extruder device. Suitable extruder devices are extruders or continuous mixers. These extruders or mixers can be single-stage or two-stage machines that melt and homogenize the low-density polyethylene. Examples of extruders are pin-type extruders, planetary extruders or co-rotating disk processors. Other possibilities are combinations of mixers with discharge screws and / or gear pumps. The preferred extruder is a screw extruder, and in particular an extruder configured as a twin-screw machine. Particularly preferred are twin-screw extruders with discharge elements and continuous mixers, in particular continuous mixers with counter-rotating twin rotors, or the extruder device comprises at least one co-rotating twin-screw extruder. Machines of this type are conventional in the plastics industry and are manufactured, for example, by Leistritz Extrusionstechnik GmbH, Nuremberg, Germany; Coperion GmbH, Stuttgart, Germany; KraussMaffei Berstorff GmbH, Hannover, Germany; The Japan Steel Works LTD., Tokyo, Japan; Farrel Corporation, Ansonia, USA; or Kobe Steel, Ltd., Kobe, Japan. Suitable extruder devices are usually also equipped with units for pelletizing the melt, such as underwater pelletizers.

[0096] As is known to the skilled person, the degree of visbreaking, and thus the increase in the melt flow index in the thermal visbreaking step, is mainly influenced by the temperature and the specific energy input.

[0097] In the extruder device, the specific energy input (SEI) refers to the energy input from the motor and measured by a torque sensor. It can be expressed in kWh / kg. The higher the temperature and / or the SEI value, the higher the melt flow index value produced by the thermal visbreaking step will be.

[0098] Indicatively, the SEI value can be from 0.15 to 0.4 kWh / kg, preferably from 0.20 to 0.35 kWh / kg.

[0099] In addition, one or more additives can be fed into the low-density polyethylene. The feeding of these additives can be carried out before, during or after the thermal visbreaking.

[0100] Such additives are common in the art. Suitable types of additives for preparing polyethylene compositions are, for example, antioxidants, melt stabilizers, light stabilizers, acid scavengers, lubricants, processing aids, anti-blocking agents, slip agents, antistatic agents, anti-fogging agents, pigments or dyes, nucleating agents, flame retardants or fillers. Usually several additives are added. The various additives can be different types of additives. However, several representatives of one type of additive can also be added to the low-density polyethylene. All these types of additives are usually commercially available and are described, for example, in Hans Zweifel, Plastics Additives Handbook, 5th Edition, Munich, 2001.

[0101] In some embodiments, when the antioxidant addition is used in combination with thermal visbreaking, the antioxidant is added after most of the visbreaking reaction has occurred.

[0102] As previously mentioned, the low-density polyethylene products of the present invention are particularly suitable for applications where significant melt strength is required or desired, such as especially for the preparation of foamed products (such as foamed products for flexible packaging) or films, in particular cast films or blown films. In fact, it is preferably characterized by an F(max) value of 0.04 N or higher, especially from 0.04 to 2 N, measured at 190 °C with a Rheotens device at an acceleration of 2.4 mm / s 2 of the acceleration measurement.

[0103] Independently or in combination with the F(max) value, the low-density polyethylene product of the present invention preferably has at least one of the following additional characteristics:

[0104] 3) A MIP / MIE ratio from 1.8 to 6, preferably from 2 to 5, where the MIP / MIE ratio 3 I / 3) is equal to or greater than 1.05, preferably equal to or greater than 1.08, especially from 1.05 to 1.4, or from 1.05 to 1.3, or from 1.08 to 1.4, or from 1.08 to 1.3;

[0105] 4) An ER value from 1 to 4, preferably from 1.2 to 4, more preferably from 1.2 to 3.5, where the ER ratio 4 I / 4) is equal to or greater than 1.2, preferably equal to or greater than 1.3, especially from 1.2 to 3, or from 1.2 to 2.5, or from 1.3 to 3, or from 1.3 to 2.5;

[0106] - A Mw from 60,00 to 180,000 g / mol;

[0107] - A Mw / Mn ratio from 3 to 18 or from 4 to 13.

[0108] Foamed articles can be produced by a chemical foaming process or by a physical foaming process. Physically foamed polyolefin foams are typically produced with blowing agents such as isobutane, pentane, and cyclopentane. Generally, physically foamed polyolefin foams have the advantage that they produce higher expansion and thus lower density compared to chemically foamed polyolefin foams. The foam can be uncrosslinked or crosslinked.

[0109] In some embodiments, the foam comprising the low-density polyethylene of the present invention has a density in the range of 12 kg / m 3 to 60 kg / m 3 Such foams can be used in protective packaging for electronic devices, furniture, fruits, glass articles, toys, etc., or in any other article that requires cushioning protection against shock and / or vibration. The foam can also be used in protective packaging for articles where thermal insulation is desired.

[0110] The film is prepared by methods well known in the art, particularly by an extrusion process.

[0111] To prepare a cast film by an extrusion process, the molten polymeric material is forced through a long, thin, rectangular die. The extrudate is in the form of a film. The film is cooled and then wound up.

[0112] To prepare a blown film by an extrusion process, the molten polymeric material is extruded through a circular die. The stretched extrudate is tubular, which is inflated with air to form a tubular bubble. The bubble is cooled and then flattened and wound up.

[0113] Examples

[0114] The various examples, compositions, and methods provided herein are disclosed in the following examples. These examples are merely illustrative and are not intended to limit the scope of the appended claims in any way.

[0115] The following analytical methods are used to characterize the polymer compositions.

[0116] Melt flow index

[0117] Determined according to ISO 1133-1 2012-03 at 190 °C and a specified load.

[0118] Density

[0119] Determined according to ISO 1183-1:2012 at 23 °C.

[0120] Molecular weight distribution measurement

[0121] The average Mw and Mn and the derived Mw / Mn are determined by high-temperature gel permeation chromatography using the methods described in ISO 16014-1, -2, -4 issued in 2003. The details according to the said ISO standard are as follows: solvent 1,2,4-trichlorobenzene (TCB), temperature of the apparatus and solution 135 °C, and PolymerChar (Valencia, Paterna 46980, Spain) IR-4 infrared detector which can be used together with TCB as a concentration detector. Use a WATERS Alliance 2000 equipped with the following pre-columns SHODEX UT-G and separation columns SHODEX UT 806M(3x) and SHODEX UT 807 (Showa Denko Europe GmbH, Konrad-Zuse-Platz 4, 81829 Munich, Germany) connected in series.

[0122] The solvent is vacuum distilled under nitrogen and stabilized with 0.025 wt% 2,6-di-tert-butyl-4-methylphenol. The flow rate used is 1 ml / min, the injection volume is 500 μl, and the polymer concentration is in the range of 0.01% < concentration < 0.05% w / w. Molecular weight calibration is established by using monodisperse polystyrene (PS) standards from Polymer Laboratories (now Agilent Technologies, Herrenberger Str. 130, 71034 Boeblingen, Germany) in the range from 580 g / mol to 11600000 g / mol and additionally hexadecane.

[0123] Then the calibration curve is adjusted to polyethylene (PE) by the universal calibration method (Benoit H., Rempp P. and Grubisic Z., & in J. Polymer Sci., Phys. Ed., 5, 753 (1967)). The Mark-Houwing parameters for PS used here: k PS = 0.000121 dl / g, α PS = 0.706, and for PE, k PE = 0.000406 dl / g, α PE = 0.725, valid in TCB at 135 °C. Data recording, calibration and calculation are carried out using NTGPC_Control_V6.02.03 and NTGPC_V6.4.24 (hsGmbH, Hauptstraβe 36, D-55437 Ober-Hilbersheim, Germany) respectively.

[0124] Complex shear viscosity η 0.02 (eta(0.02)) and ER

[0125] Measured as follows at an angular frequency of 0.02 rad / s and 190 °C.

[0126] The sample was melt-pressed at 200 °C and 200 bar for 4 minutes into a 1 mm thick plate. A disc specimen with a diameter of 25 mm was punched and inserted into a rheometer preheated at 190 °C. Measurement can be carried out using any commercially available rotational rheometer. Here, an Anton Paar MCR 300 with plate-plate geometry was used. At T = 190 °C, a so-called frequency sweep was carried out at a constant strain amplitude of 5% (after annealing the sample at the measurement temperature for 4 minutes), and the stress response of the material in the range of excitation frequencies ω from 628 to 0.02 rad / s was measured and analyzed. Standardized basic software was used to calculate the rheological properties, namely the storage modulus G', the loss modulus G", the phase lag δ (= arctan(G" / G')) and the complex viscosity η*, as a function of the applied frequency, i.e., η*(ω) = [G'(ω) 2 + G"(ω) 2 1 / 2 / ω. The value of the latter at an applied frequency ω of 0.02 rad / s is η 0.02 .

[0127] ER was determined by the following method: R. Shroff and H. Mavridis, "New Measures of Polydispersity from Rheological Data on Polymer Melts", Journal of Applied Polymer Science, 57 (1995) 1605 (see also column 10, lines 20 to 30 of US Patent No. 5,534,472). Calculated from:

[0128] At the value of G" = 5,000 dyn / cm 2

[0129] ER = (1.781 * 10 -3 ) * G'.

[0130] As will be recognized by those skilled in the art, when the lowest G" value is greater than 5,000 dyn / cm 2 , the determination of ER involves extrapolation. The calculated ER value will then depend on the non-linearity in log G' versus log G". The temperature, plate diameter and frequency range were chosen such that within the resolution of the rheometer, the lowest G" value is close to or less than 5,000 dyn / cm​​2 。

[0131] Comonomer content

[0132] The comonomer content was determined by IR using an FT-IR spectrometer Tensor 27 from Bruker according to ASTM D 624898.

[0133] Melt strength

[0134] The melt strength of the polymer is an important parameter in melt processing operations where stretching or drawing is involved in one or more stages of the process. It depends on molecular parameters such as molecular weight, molecular weight distribution, and / or polymer branching. The test apparatus measures the extensional properties of the polymer melt by pulling a vertical melt strand at a constant draw speed or at a linearly accelerating speed in a Rheotens spinning machine located below a capillary die under a constant force.

[0135] Melt strength analysis was carried out at 190 °C on a Rheotester 1000 (barrel diameter 12 mm, capillary die L / D = 20 / 2) equipped with a RHEOTENS 71.97 device. The RHEOTENS consists of two upper and two lower driven counter-rotating wheels connected to a very sensitive balance system. The vertical gap between the wheels is 0.3 mm. After a 10 min melt time, the polymer was extruded at a shear rate of 50 1 / s. The polymer strand left the capillary die and the die exit velocity v0 was recorded. At a strand length of 74 mm, the two upper wheels pulled the melt strand downwards with an acceleration of 2.4 mm / s 2 and the velocity v was recorded (the two lower wheels were only used to additionally stabilize the strand during the downward pull). The draw ratio at break λ = v / v0, the break velocity, and the break force (F(max) = melt strength) of the melt strand were recorded.

[0136] SEI

[0137] As will be known to those skilled in the art, there are different methods to calculate the specific energy input SEI. The SEI can be calculated by dividing the motor power by the material flow rate. The motor power is equal to the product of the torque and the angular velocity. The equation for SEI is as follows: Here, 2*π*n represents the angular velocity ω, M D represents the torque, represents the material flow. When the torque of the motor cannot be measured directly, the maximum current and the actual current of the motor can also be used to approximately calculate the SEI according to the following equation. Here, n represents the screw speed, I represents the current, and η 传动is the efficiency of the drive.

[0138] Example 1

[0139] Use the commercial grade Lupolen sold by LyodellBasell Industries 2420D as the precursor polyethylene (I).

[0140] Lupolen 2420D is virgin LDPE with the properties reported in Table 1, where it is identified as LP 2420D.

[0141] The low-density polyethylene product of Example 1 was obtained by extruding the precursor polyethylene (I) in an extruder Leistritz ZSE 27MAXX. The machine parameters were:

[0142] - Rotation speed: 500 rpm;

[0143] - Output: 30 kg / h;

[0144] - Temperature:

[0145] Zone 1: 300 °C, Zone 2: 320 °C, Zones 3 - 10: 340 °C, Die: 300 °C;

[0146] - SEI: 0.25 kWh / kg.

[0147] The properties of the low-density polyethylene thus obtained are reported in Table 1.

[0148] Example 2

[0149] Use the commercial grade Nextfilm sold by Suez T 800 as the precursor polyethylene (I).

[0150] Nextfilm is recycled LDPE, which is essentially made of 50% LDPE and 50% LLDPE by weight and has the properties reported in Table 1, where it is identified as T 800.

[0151] The low-density polyethylene product of Example 2 was obtained by extruding the precursor polyethylene (I) in an extruder Leistritz ZSE 27MAXX. The machine parameters were:

[0152] - Rotation speed: 700 rpm;

[0153] - Output: 30 kg / h;

[0154] - Temperature:

[0155] Zone 1: 300 °C, Zone 2: 320 °C, Zones 3 - 10: 340 °C, Die: 300 °C;

[0156] - SEI: 0.30 kWh / kg.

[0157] Table 1

[0158] LP 2420D Example 1 LP 2420H MIP [g / 10min] 1.12 6.84 7.74 MIE [g / 10min] 0.25 1.80 1.93 <![CDATA[Density [g / cm 3 > 0,922 0.923 0.922 Mw [g / mol] 153196 123721 109464 Mw / Mn 13.6 10.7 7.1 MIP / MIE 4.5 3.8 4.0 ER 4.0 2.3 1.9 MIE / ER 0.06 0.78 1.02 F(max) [N] 0.138 0.094 0.088

[0159] Continued Table 1

[0160] T 800 Example 2 LP 2420K MIP [g / 10min] 4.27 10.2 13.7 MIE [g / 10min] 1.35 3.81 4.15 <![CDATA[Density [g / cm 3 > 0.925 0.919 0.922 Mw [g / mol] 127710 83966 100445 Mw / Mn 5.9 7.0 7.9 MIP / MIE 3.2 2.7 3.3 ER 2.5 1.7 1.5 MIE / ER 0.54 2.2 2.77 F(max) [N] 0.078 0.062 0.042

[0161] For reference purposes, Table 1 also reports the properties of the commercial LDPE grades Lupolen 2420H (identified as LP 2420H) and Lupolen 2420K (identified as LP 2420K).

[0162] These are the target grades of the low - density polyethylene products of Example 1 and Example 2, respectively.

[0163] The data reported in Table 1 show that the process of the present invention allows to obtain melt flow index values that are substantially the same as the respective target grades, with an even improved melt strength, as evidenced by the higher F(max) values.

Claims

1. A process for producing low density polyethylene having an improved melt flow index, which comprises subjecting a precursor polyethylene (I) to thermo-oxidative degradation, said precursor polyethylene (I) comprising 35% by weight or more, preferably 40% by weight or more, especially from 35% to 100%, or from 40% to 100% of LDPE, based on the total weight of the precursor polyethylene (I), said precursor polyethylene (I) having: 1 I )The density measured at 23 °C according to ISO 1183-1:2012 ranges from 0.910 to 0.940 g / cm 3 , preferably from 0.915 to 0.935 g / cm 3 ; 2 I ) an MIP value ranging from 0.3 to 7 g / 10 min, preferably from 0.5 to 6 g / 10 min; 3 I ) A ratio MIP / MIE from 2 to 7, preferably from 2.5 to 6, where MIP is the melt flow index at 190 °C and a load of 5 kg, and MIE is the melt flow index at 190 °C and a load of 2.16 kg, both determined according to ISO 1133-2:2011; 4 I ) an ER value from 1.8 to 8, preferably from 2 to 6; Thereby obtaining a low density polyethylene product having a specific MIE / ER of from 0.2 to 2.44, preferably from 0.2 to 2.40, and: 1) A density from 0.910 to 0.940 g / cm 3 、 preferably from 0.915 to 0.935 g / cm 3 ; 2) An MIP value from 3 to 20 g / 10 min, preferably from 4 to 15 g / 10 min, where the MIP ratio 2) / 2 I ) is equal to or greater than 1.5, preferably equal to or greater than 1.8, especially from 1.5 to 15, or from 1.5 to 10, or from 1.8 to 15, or from 1.8 to 10; wherein ER is calculated by the following formula: At a value of G" = 0.5 kPa (5,000 dyn / cm 2 ), ER = (1.781 * 10 -3 ) * G'; wherein: G' = storage modulus; G" = loss modulus; Both G' and G" are measured by dynamic oscillatory shear in a plate-plate rotational rheometer at a temperature of 190 °C.

2. The process according to claim 1, wherein said thermo-oxidative degradation is carried out by heating said precursor polyethylene (I) at a temperature equal to or higher than 280 °C, preferably at a temperature equal to or higher than 290 °C, more preferably at a temperature equal to or higher than 300 °C, most preferably at a temperature equal to or higher than 310 °C.

3. The process according to claim 1 or 2, wherein said thermo-oxidative degradation is carried out in an extruder device.

4. The process according to claim 1 or 2, wherein said precursor polyethylene (I) has at least one of the following additional characteristics: - an MIE of from 0.1 to 3 g / 10 min; - an Mw of from 100,000 to 350,000 g / mol; - an Mw / Mn ratio of from 3 to 20 or from 4 to 15; wherein Mw is the weight average molecular weight and Mn is the number average molecular weight, both measured by GPC (gel permeation chromatography).

5. The process according to claim 1 or 2, wherein said precursor polyethylene (I) comprises virgin LDPE, or LDPE recyclate, or a blend of virgin LDPE and LDPE recyclate.

6. The process according to claim 1 or 2, wherein said precursor polyethylene (I) comprises one or more additional polyethylene components selected from HDPE, MDPE, LLDPE and mixtures thereof.

7. The process according to claim 1 or 2, wherein said low density polyethylene product has an MIE of 1 g / 10 min or higher, especially from 1 to 10 or from 1 to 8 g / 10 min.

8. The process according to claim 1 or 2, wherein said low density polyethylene product has at least one of the following additional characteristics: 3) A ratio MIP / MIE from 1.8 to 6, preferably from 2 to 5, where the ratio MIP / MIE I ) / 3) is equal to or greater than 1.05, preferably equal to or greater than 1.08; 4) An ER value from 1 to 4, preferably from 1.2 to 4, more preferably from 1.2 to 3.5, where the ER ratio to 4 I ) / 4) is equal to or greater than 1.2, preferably equal to or greater than 1.3; - an Mw of from 60,00 to 180,000 g / mol; - an Mw / Mn ratio of from 3 to 18 or from 4 to 13; - Measured at an acceleration of 2.4 mm / s at 190 °C using a Rheotens device 2 F(max) values of 0.04 N or higher, in particular from 0.04 to 2 N.

9. Low density polyethylene obtainable by the process according to claim 1 or 2.

10. An article comprising the low density polyethylene according to claim 9.

11. The article according to claim 10, which is in the form of a foamed article or a film.