Process for obtaining polyethylene with reduced gel content

Abstract

A process for obtaining a polyethylene having a reduced gel content comprising subjecting a polyethylene (I) having an ER value of from 1 to 8 and a MIE value of from 0.1 to 5 g / 10 min to thermal visbreaking.

Description

Technical Field

[0001] The present invention provides a process for obtaining polyethylene with a reduced gel content, which comprises a thermal visbreaking step. Background Art

[0002] Polyethylene is generally considered to be the most widely used thermoplastic polymer material.

[0003] In fact, it is used to prepare a very wide variety of extruded, injection-molded and blow-molded articles, such as from packaging articles (especially films and sheets) to shopping bags, bottles, containers, tubes and automotive parts.

[0004] However, in order to obtain optimal mechanical and optical properties in such applications, polymer uniformity plays a decisive role.

[0005] In fact, polyethylene generally has a broad molecular weight distribution and can thus be regarded as a blend of fractions with different molecular weights. It can also contain fractions with different compositions, especially different types and amounts of comonomers.

[0006] This complexity of molecular weight and composition can easily impair the polymer uniformity, due to the formation of separate domains commonly referred to as "gels", thereby deteriorating the mechanical and optical properties.

[0007] Gels, also known as "specks" or "white spots", are mainly high molecular weight, high viscosity particles in a low viscosity matrix.

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

[0009] Such polyethylene recyclates obtained by separation from waste streams have a rather inhomogeneous and variable composition, resulting in a particularly high amount of gels.

[0010] Therefore, there has been a continuing need for an effective method capable of providing polyethylene with a reduced gel content, especially a large gel content.

[0011] According to WO2017001384A1 and WO2018096016A1, the gel content is reduced by melt-processing polyethylene in the presence of an organic peroxide.

[0012] According to US2935502, the gel content is reduced by heating polyethylene to a temperature above about 100 °C, preferably from about 100 °C to about 300 °C, at a pressure above about 7,500 p.s.i. and extruding the molten polyethylene through a fine channel or capillary.

[0013] It has now been found that polyethylene with a reduced gel content can be effectively obtained by thermo-mechanical degradation of polyethylene having specific properties, without using peroxides and without the need to operate at high pressure. SUMMARY OF THE INVENTION

[0014] The present invention provides a process for obtaining polyethylene with a reduced gel content, said process comprising subjecting polyethylene (I) having an ER value from 1 to 8, preferably from 2 to 6 and a MIE value from 0.1 to 5 g / 10 min to thermo-mechanical degradation, thereby obtaining processed polyethylene (II) having a MIE / ER ratio from 0.2 to 2.44, preferably from 0.2 to 2.40;

[0015] wherein ER is calculated by the following formula:

[0016] at a value of G" = 0.5 kPa (5,000 dyn / cm 2 ),

[0017] ER = (1.781*10 -3 ) * G';

[0018] where:

[0019] G' = storage modulus;

[0020] G" = loss modulus;

[0021] both G' and G" are measured by dynamic oscillatory shear in a plate-plate rotational rheometer at a temperature of 190 °C;

[0022] MIE is the melt flow index measured according to ISO 1133-2:2011 at 190 °C and a load of 2.16 kg.

[0023] The processed polyethylene (II) thus obtained has valuable mechanical and optical properties, as well as high melt strength, as shown by the high value of the force F(max) required to tear the strand in the Rheotens test. DETAILED DESCRIPTION

[0024] The expression "polyethylene" is used herein to include, as alternatives, single ethylene polymers and polyethylene compositions, i.e. compositions comprising two or more ethylene polymers.

[0025] 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 polyethylene (I) having the ER value as defined above to thermo-mechanical degradation, the MIE / ER ratio from 0.2 to 2.44, preferably from 0.2 to 2.40, is obtained.

[0026] Particularly preferred MIE / ER ratios are:

[0027] - from 0.3 to 2.44; or

[0028] - from 0.3 to 2.40; or

[0029] - from 0.2 to 2.35; or

[0030] - from 0.3 to 2.35.

[0031] Preferably, the ratio MIE(II) / MIE(I) between the MIE of the processed polyethylene (II) and the MIE of the polyethylene (I) is equal to or higher than 1.2, preferably equal to or higher than 1.5, in particular from 1.2 to 15 or from 1.2 to 10, or from 1.5 to 15, or from 1.5 to 10.

[0032] Particularly preferred is a ratio MIE(II) / MIE(I) of from 1.2 to 6.0, in particular from 1.5 to 5.5.

[0033] The polyethylene (I) used in the present process may consist of virgin polyethylene or contain virgin polyethylene, or may consist of recycled polyethylene or contain recycled polyethylene.

[0034] The polyethylene (I) may also consist of a blend of virgin polyethylene and recycled polyethylene or contain a blend of virgin polyethylene and recycled polyethylene.

[0035] The expression "virgin polyethylene" means polyethylene which has not yet undergone any process for the production of finished or semi-finished products, such as, for example, packaging films, tubes, bottles, containers or semi-finished products such as fibres or sheets for thermoforming.

[0036] Thus, apart from possible pelletization (which is still considered part of the polymer production process), virgin polyethylene has not undergone post-treatment.

[0037] As used herein, the expression "recycled polyethylene" means post-consumer recycled ("PCR") polyethylene and / or post-industrial recycled ("PIR") polyethylene. PCR polyethylene 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 polyethylene 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.

[0038] Thus, generally speaking, recycled polyethylene is a material derived from article manufacturing processes.

[0039] Typically, two main polyolefin fractions are obtained by a step of separating from other polymers such as PVC, PET or PS, namely recycled polyethylene (including HDPE, MDPE, LDPE and LLDPE) and recycled polypropylene (including homopolymers, random copolymers and multiphase copolymers).

[0040] However, recycled polyethylene can still contain significant amounts, even high amounts, of other polymers such as those specified above, and also including polypropylene.

[0041] Other polymers and other common materials such as fillers may also be present.

[0042] Preferably, polyethylene (I) contains 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 unprocessed polyethylene (I).

[0043] A particularly preferred ethylene polymer is LDPE.

[0044] As used herein, "LDPE" refers to ethylene homopolymers and ethylene copolymers produced in free radical polymerization.

[0045] The polymerization is typically carried out at high pressure, as will be explained in detail below.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] Typically, the term "copolymer" is also intended to include polymers containing more than one comonomer, such as terpolymers.

[0050] Preferably, the density of LDPE measured at 23 °C according to ISO 1183-1:2012 is from 0.910 to 0.940 g / cm 3 , more preferably from 0.915 to 0.935 g / cm 3 .

[0051] There are two basic high-pressure polymerization processes for manufacturing LDPE: autoclave and tubular.

[0052] LDPE prepared by the autoclave reactor process (“autoclave LDPE”) has a high concentration of long-chain branches, resulting in a high tensile hardening value and a relatively broad molecular weight distribution that makes it easy to process.

[0053] Autoclave polymerization is typically carried out in the presence of a free radical initiator selected from organic peroxides.

[0054] 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 can be prepared that does not contain chemical degradation products of organic peroxides.

[0055] The LDPE can also be prepared by a hybrid process that combines both an autoclave and a tubular reactor.

[0056] 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.

[0057] The polymerization gas can optionally contain one or more chain transfer agents known in the art, such as propylene, propane, and propionaldehyde.

[0058] Such chain transfer agents are used to regulate the molecular weight.

[0059] The processes and the resulting LDPE products 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.

[0060] Relative to the total weight of polyethylene (I), LDPE can be present in polyethylene (I) in an amount equal to or higher than 35% by weight, preferably equal to or higher than 45% by weight. The upper limit of the amount of LDPE in polyethylene (I) can be 70% by weight, or 80% by weight, or 90% by weight, or 100% by weight in all cases relative to the total weight of polyethylene (I).

[0061] Virgin LDPE polymers having the properties cited above are known in the art. Specific examples are polymers commercially available under the trade names Lupolen (LyondellBasell) and Petrothene (Equistar).

[0062] Recycled polyethylene materials containing LDPE with the above-cited properties are also known in the art. A specific example is a polyethylene composition commercially available under the trade name Nextfilm (Suez).

[0063] Polyethylene (I) can also consist of one or more ethylene polymers different from LDPE or contain one or more ethylene polymers different from LDPE as described above. The ethylene polymers 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.

[0064] They are ethylene homopolymers and ethylene copolymers containing α-olefin monomer units (preferably in an amount of at most 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.

[0065] 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.

[0066] In particular, the polymerization process can be carried out in the presence of a Ziegler-Natta catalyst or a single-site catalyst.

[0067] 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.

[0068] Preferred organometallic compounds are organo-Al compounds.

[0069] 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.

[0070] 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.

[0071] Examples of non-metallocene single-site catalysts are iron complex compounds preferably having tridentate ligands.

[0072] An example of high-density recycled polyethylene having the above characteristics is 5603 grey (LyondellBasell).

[0073] Preferably, the polyethylene (I) has one or more of the following characteristics:

[0074] - a density from 0.910 to 0.960 g / cm 3 、more preferably from 0.910 to 0.950 g / cm 3 、especially from 0.910 to 0.940 g / cm 3 、or from 0.915 to 0.935 g / cm 3 ;

[0075] - a MIP from 1 to 10 g / 10 min, where MIP is the melt flow index measured according to ISO 1133-2:2011 at 190 °C and 5 kg load;

[0076] - a MIP / MIE ratio from 2 to 10;

[0077] - a Mw from 80,000 to 350,000 g / mol;

[0078] - a Mw / Mn ratio from 3 to 20 or from 4 to 15;

[0079] where Mw is the weight-average molecular weight and Mn is the number-average molecular weight, both measured by GPC (gel permeation chromatography).

[0080] Preferably, the processed polyethylene (II) has one or more of the following characteristics:

[0081] - a density from 0.910 to 0.960 g / cm 3 、more preferably from 0.910 to 0.950 g / cm 3 、especially from 0.910 to 0.940 g / cm 3 、or from 0.915 to 0.935 g / cm 3 ;

[0082] - a MIP from 5 to 20 g / 10 min;

[0083] - MIE from 1 to 10 g / 10 min;

[0084] - MIP / MIE ratio from 1.5 to 8;

[0085] - Mw from 50,000 to 250,000 g / mol;

[0086] - Mw / Mn ratio from 3 to 20 or from 4 to 15;

[0087] - ER from 0.8 to 6, preferably from 1 to 5;

[0088] - Total gel amount / m less than 150,000, more preferably less than 100,000, most preferably less than 80,000 2 ;

[0089] - Gel amount / m of gels with a diameter higher than 600 μm less than 20 2 quantity.

[0090] Details of the test method are given in the examples.

[0091] Thermal viscosity reduction cracking involves treating polyethylene (I) at a temperature and / or mechanical shear energy sufficient to break polymer chains, mainly polymer chain branching or crosslinking.

[0092] In some embodiments, thermal viscosity reduction cracking is carried out by heating the 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, and most preferably at a temperature equal to or higher than 310 °C, with the upper limit preferably being 500 °C in all cases.

[0093] In particular, thermal viscosity reduction cracking can be carried out at the following temperatures:

[0094] - From 280 °C to 500 °C; or

[0095] - From 290 °C to 500 °C; or

[0096] - From 300 °C to 500 °C; or

[0097] - From 310 °C to 500 °C; or

[0098] - From 280 °C to 480 °C; or

[0099] - From 290 °C to 480 °C; or

[0100] - From 300 °C to 480 °C; or

[0101] - From 310 °C to 480 °C; or

[0102] - From 280 °C to 460 °C; or

[0103] - from 290 °C to 460 °C; or

[0104] - from 300 °C to 460 °C; or

[0105] - from 310 °C to 460 °C.

[0106] 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.

[0107] It is known that thermal visbreaking can be carried out in conventional mixing devices commonly used for processing polymers in a molten state.

[0108] In particular, the processed polyethylene (II) of the present invention can be prepared by processing 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 polyethylene. Examples of extruders are pin extruders, planetary extruders, or co-rotating disk processors. Other possibilities are combinations of mixers with discharge screws and / or gear pumps. Preferred extruders are screw extruders, and in particular extruders configured as twin-screw machines. Particularly preferred are twin-screw extruders with discharge elements and continuous mixers, especially 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 generally also equipped with units for pelletizing the melt, such as underwater pelletizers.

[0109] As is known to those skilled in the art, the degree of visbreaking, and thus the increase in melt flow index in the thermal visbreaking step, is mainly affected by temperature and specific energy input.

[0110] In an extruder apparatus, the specific energy input (SEI) refers to the energy input mechanically applied to the melt by the rotation of the screw and is related to the power consumption of the motor. 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 degradation cracking step will be.

[0111] Indicatively, the SEI value can be from 0.15 to 0.4 kWh / kg, preferably from 0.20 to 0.35 kWh / kg.

[0112] Furthermore, one or more additives can be fed into the processed polyethylene (II). The feeding of these additives can be carried out before, during or after the thermal degradation cracking.

[0113] 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 of different types. However, several representatives of one type of additive can also be added to the low-density polyethylene. All these types of additives are generally commercially available and are described, for example, in Hans Zweifel, Plastics Additives Handbook, 5th Edition, Munich, 2001.

[0114] In some embodiments, in the case where antioxidant addition is used in combination with thermal degradation cracking, the antioxidant is added after most of the degradation cracking reaction has occurred.

[0115] The processed polyethylene (II) of the present invention is particularly suitable for applications where good melt strength and uniformity are required or desired, such as for the preparation of films, especially cast films or blown films.

[0116] The film is prepared by methods known in the art, especially by an extrusion process.

[0117] 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.

[0118] To prepare a blown film by an extrusion process, the molten polymeric material is extruded through a circular die. The extruded stretched material is tubular, which is inflated by air to form a tubular bubble. The bubble is cooled and then flattened and wound up.

[0119] Examples

[0120] The various embodiments, 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.

[0121] The following analytical methods were used to characterize the polymer compositions.

[0122] Melt flow index

[0123] Determined according to ISO 1133-1:2012-03 at 190 °C and a specified load.

[0124] Density

[0125] Determined according to ISO 1183-1:2012 at 23 °C.

[0126] Cast film measurement

[0127] The film measurement of the gel was carried out on an OCS extruder model ME 202008-V3, which has a screw diameter of 20 mm, a screw length of 25D, and a slit die width of 150 mm. The casting line is equipped with a cooling roll and a winder (model OCS CR-9). The optical equipment consists of an OSC film surface analyzer camera with a resolution of 26 μm × 26 μm, model FTA-100 (flash camera system). After purging the resin for 1 hour to stabilize the extrusion conditions, the inspection and value recording were carried out after 30 minutes. The resin was extruded at 220 °C, and the take-up speed was ca. 2.7 m / min to produce a film with a thickness of 50 μm. The temperature of the cooling roll was 70 °C. The inspection with the surface analyzer camera provided the total content of the gel and the content of gels with a diameter higher than 600 μm, as reported in Table 1.

[0128] Molecular weight distribution determination

[0129] 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 published 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 with TCB as the 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.

[0130] The solvent is vacuum distilled under nitrogen and stabilized with 0.025% by weight of 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 11,600,000 g / mol and additionally hexadecane.

[0131] 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-Houwink parameters used here for PS: 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, Hauptstraße 36, D-55437 Ober-Hilbersheim, Germany) respectively.

[0132] Complex shear viscosity η 0.02 (eta(0.02)) and ER

[0133] Measured as follows at an angular frequency of 0.02 rad / s and 190 °C.

[0134] The sample was melt-pressed at 200 °C and 200 bar for 4 minutes into a 1 mm thick plate. A disk specimen with a diameter of 25 mm was punched and inserted into a rheometer preheated at 190 °C. Any commercially available rotational rheometer can be used for the measurement. Here, an Anton Paar MCR 300 with a plate-plate geometry was used. At T = 190 °C, a so-called frequency sweep was performed 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. The standardized basic software was used to calculate the rheological properties, namely the storage modulus G', the loss modulus G", the phase lag δ (= arctangent (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 .

[0135] 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:

[0136] At the value of G" = 5,000 dyn / cm 2 value

[0137] ER = (1.781 * 10 -3 ) * G'.

[0138] As will be appreciated 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 degree of non-linearity in the log G' vs. log G" plot. 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 。

[0139] Comonomer content

[0140] The comonomer content was determined by IR using an FT-IR spectrometer Tensor 27 from Bruker according to ASTM D 624898.

[0141] Melt strength

[0142] The melt strength of a 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 a polymer melt by pulling a vertical melt strand at a constant draw rate or at a linearly accelerating rate in a Rheotens spinning machine located below a capillary die under a constant force.

[0143] 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 melt time of 10 min, the polymer was extruded at a shear rate of 50 1 / s. The polymer strand exits the capillary die and the die exit velocity v0 was recorded. At a strand length of 74 mm, the two upper wheels pull the melt strand downward with an acceleration of 2.4 mm / s 2 and the velocity v was recorded (the two lower wheels are 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.

[0144] SEI

[0145] 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 directly measured, 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.

[0146] Example 1

[0147] Use commodity-grade T 800 sold by Suez as polyethylene (I).

[0148] is recycled LDPE, which is made essentially of 50% LDPE and 50% LLDPE by weight, having the properties reported in Table 1, where it is identified as T 800.

[0149] The processed polyethylene (II) of the example is obtained by extruding polyethylene (I) in an extruder Leistritz ZSE 27MAXX. The machine parameters are:

[0150] - Rotational speed: 700 rpm;

[0151] - Output: 30 kg / h;

[0152] - Temperature:

[0153] Zone 1: 300 °C, Zone 2: 320 °C, Zones 3 - 10: 340 °C, Die: 300 °C;

[0154] - SEI: 0.30 kWh / kg.

[0155] Thermal degradation cracking occurs under the said conditions. The properties of the thus obtained polyethylene (II) are reported in Table 1.

[0156] Table 1

[0157] T 800 Example 1* MIP [g / 10min] 4.27 10.2 MIE [g / 10min] 1.35 3.81 <![CDATA[Density [g / cm 3 > 0.925 0.919 Mw [g / mol] 127710 83966 Mw / Mn 5.9 7.0 MIP / MIE 3.2 2.7 ER 2.5 1.7 MIE / ER 0.54 2.2 <![CDATA[Total gel / m 2 and]]> 199548 58010 <![CDATA[gel / m 2 >600 μm and]]> 47 1 F(max) [N] 0.078 0.062

[0158] * Polyethylene (II)

[0159] The data reported in Table 1 show that the present process allows to provide polyethylene with a significantly reduced gel content while maintaining a high melt strength, as evidenced by the high F(max) value.

[0160] Example 2

[0161] Use commercial recycled polyethylene 5603 grey as polyethylene (I) and perform thermal degradation cracking under the same conditions as reported in Example 1.

[0162] The properties of the said polyethylene (I) together with the properties of the polyethylene (II) of the thus obtained example are reported in Table 2, where it is identified as CR 5603.

[0163] Table 2

[0164] CR 5603 Example 2* MIE [g / 10min] 0.4 1.98 MIP [g / 10min] 1.88 7.22 <![CDATA[Density [g / cm 3 > 0.956 0.9588 ER 4.91 2.82 MIE / ER 0.08 0.7 <![CDATA[Total gel / m 2 and]]> 217300 136581 <![CDATA[gel / m 2 301 - 700 μm and]]> 4517 2124 <![CDATA[gel / m 2 701 - 1500 μm and]]> 132 10 <![CDATA[Gel / m 2 >1500 μm and]]> 0 0

[0165] * Polyethylene (II)

Claims

1. A process for obtaining polyethylene with a reduced gel content, the process comprising subjecting polyethylene (I) having an ER value from 1 to 8, preferably from 2 to 6, and a MIE value from 0.1 to 5 g / 10 min to thermo-oxidative degradation cracking, thereby obtaining processed polyethylene (II) having a MIE / ER ratio from 0.2 to 2.44, preferably from 0.2 to 2.40; 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'; where: 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; MIE is the melt flow index determined according to ISO 1133-2:2011 at 190 °C and a load of 2.16 kg.

2. The process according to claim 1, wherein the polyethylene (I) consists of recycled polyethylene or contains recycled polyethylene.

3. The process according to claim 1 or 2, wherein the ratio MIE(II) / MIE(I) between the MIE of the processed polyethylene (II) and the MIE of the polyethylene (I) is equal to or higher than 1.2, preferably equal to or higher than 1.

5.

4. The process according to claim 1 or 2, wherein the thermo-oxidative degradation cracking is carried out by heating the 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.

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

6. The process according to claim 1 or 2, wherein the polyethylene (I) has at least one of the following additional characteristics: - From 0.910 to 0.960 g / cm 3 , more preferably from 0.910 to 0.950 g / cm 3 , especially from 0.910 to 0.940 g / cm 3 , or from 0.915 to 0.935 g / cm 3 of density; - a MIP from 1 to 10 g / 10 min, where MIP is the melt flow index determined according to ISO 1133-2:2011 at 190 °C and a load of 5 kg; - a MIP / MIE ratio from 2 to 10; - a Mw from 80,000 to 350,000 g / mol; - a Mw / Mn ratio from 3 to 20 or from 4 to 15; where Mw is the weight-average molecular weight and Mn is the number-average molecular weight, both measured by GPC (gel permeation chromatography).

7. The process according to claim 1 or 2, wherein the polyethylene (I) contains LDPE.

8. The process according to claim 1 or 2, wherein the polyethylene (I) contains one or more polyethylene components selected from HDPE, MDPE, LLDPE, and mixtures thereof.

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

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

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