Process for modifying melt flow index of low density polyethylene

Abstract

A process for the production of low density polyethylene having an improved melt flow index comprising subjecting a precursor polyethylene (I) comprising 35 wt% or more LDPE to thermal visbreaking and blending the intermediate polyethylene product (II) so obtained with additional polyethylene (III).

Description

Technical Field

[0001] The present disclosure provides a method for producing low-density polyethylene having an improved melt flow index, the method comprising a thermo-oxidative degradation step.

[0002] The product thus obtained is particularly suitable for foamed articles and films. Background Art

[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, in particular LDPE, is commonly used in many applications, including films for packaging, agricultural films, shopping bags, heavy-duty transport bags, foamed articles, lids and closures / tubes, pipes, 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 is currently represented by 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 generally 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 major and most determining polymer properties of low-density polyethylene materials.

[0011] Thermo-oxidative degradation 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, thermo-oxidative degradation of low-density polyethylene consisting of or comprising LDPE allows the obtaining of end products particularly suitable for foamed articles and films.

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

[0014] The present disclosure provides a method for producing low - density polyethylene having an improved melt flow index, the method comprising:

[0015] (a) subjecting a precursor polyethylene (I) to thermo - visbreaking, the precursor polyethylene (I) comprising 35 wt% or more, preferably 40 wt% or more, especially 35 wt% to 100 wt%, or 40 wt% to 100 wt% of LDPE, based on the total weight of the precursor polyethylene (I), the precursor polyethylene (I) having:

[0016] 1 I ) a density of 0.910 to 0.940 g / cm 3 , preferably 0.915 to 0.935 g / cm 3 , measured at 23 °C according to ISO 1183 - 1:2012;

[0017] 2 I ) a MIP value of 0.3 to 7 g / 10 min, preferably 0.5 to 6 g / 10 min;

[0018] 3 I ) a ratio MIP / MIE of 2 to 9, preferably 2.5 to 7, 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;

[0019] 4 I ) an ER value of 1.8 to 8, preferably 2 to 6;

[0020] Thereby obtaining an intermediate polyethylene product (II) having a MIP 2 I ) value higher than MIP 2 II ), where the ratio MIP 2 II ) / 2 I ) is 1.5 or greater, preferably 3 or greater, more preferably 10 or greater, and preferably having an upper limit of 25 in all cases; and

[0021] (b) blending the intermediate polyethylene product (II) obtained in the thermo - visbreaking step (a) with a further polyethylene (III) having a density of 0.910 to 0.940 g / cm 3, preferably 0.915 to 0.935 g / cm 3 with a density of 1 III ) and different from MIP 2 II ) of MIP 2 III ) value, where MIP 2 III ) / 2 II ) ratio is 0.03 to 3, preferably 0.05 to 2, and MIP 2 III ) - MIP 2 II ) the absolute value of the difference is at least 5, preferably at least 8, and in all cases the upper limit is preferably 20, more preferably 15;

[0022] Thus, a low - density polyethylene product (IV) with a ratio of MIE / ER of 0.2 to 2.8, preferably 0.3 to 2.5, is obtained, and:

[0023] 1) 0.910 to 0.940 g / cm 3 , preferably 0.915 to 0.935 g / cm 3 density;

[0024] 2) MIP value of 3 to 20 g / 10 min, preferably 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 1.5 to 15, or 1.5 to 10, or 1.8 to 15, or 1.8 to 10;

[0025] where ER is calculated by:

[0026] ER = (1.781 * 10 -3 ) * G'

[0027] at a value of G” = 0.5 kPa (5,000 dyn / cm 2 );

[0028] where:

[0029] G' = storage modulus;

[0030] G” = loss modulus;

[0031] Both G' and G” are measured using dynamic oscillatory shear in a plate - plate rotational rheometer at a temperature of 190 °C;

[0032] 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 shown by the high value of the force F(max) required to tear the strand in the Rheotens test. Detailed Description

[0033] The expression "low density polyethylene" is used herein to include, as alternatives, a single ethylene polymer and polyethylene compositions, i.e., compositions comprising two or more ethylene polymers.

[0034] As shown in the examples, the thermo-oxidative 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 (4 I ) value as defined above to thermo-oxidative degradation, the MIE / ER ratio of 0.2 to 2.8, preferably 0.3 to 2.5, is obtained.

[0035] Particularly preferred MIE / ER ratio values are:

[0036] - 0.2 to 2.44; or

[0037] - 0.3 to 2.44; or

[0038] - 0.2 to 2.40; or

[0039] - 0.3 to 2.40; or

[0040] - 0.2 to 2.35; or

[0041] - 0.3 to 2.35.

[0042] Preferably, the obtained MIE value of the low density polyethylene product (IV) of the present invention is 1 g / 10 min or higher, particularly 1 to 10 or 8 g / 10 min.

[0043] Preferably, the precursor polyethylene (I) comprises in total (i.e., including the previously described LDPE) at least 70 wt% of ethylene polymers, more preferably at least 80 wt% of ethylene polymers, most preferably at least 90 wt% of ethylene polymers, with the preferred upper limit in all cases being 100 wt% of ethylene polymers. The amount refers to the total weight of the precursor polyethylene (I).

[0044] As previously mentioned, the precursor polyethylene (I) may consist of or include virgin LDPE, or may consist of or include recycled LDPE.

[0045] The precursor polyethylene (I) may also consist of or include a blend of virgin LDPE and recycled LDPE.

[0046] Preferably, the additional polyethylene (III) consists of or includes virgin LDPE.

[0047] The expression "virgin LDPE" means a polymer that has not undergone any method for producing a finished product, such as a packaging film, tube, bottle, container, or semi-finished product, such as fibers or sheets for thermoforming.

[0048] Therefore, unused LDPE has not been post-treated, except for possible pelletization, and it is still considered part of the polymer production method.

[0049] As used herein, "LDPE recyclate" means 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.

[0050] Thus, generally speaking, LDPE recyclate is a material derived from the method of manufacturing articles.

[0051] 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 wt% or more relative to the total weight, of LDPE.

[0052] Preferably, the precursor polyethylene (I) has one or more of the following additional characteristics:

[0053] - An MIE of 0.1 to 3 g / 10 min;

[0054] - An Mw of 100,000 to 350,000 g / mol;

[0055] - An Mw / Mn ratio of 3 to 20 or 4 to 15;

[0056] where 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.

[0057] Preferably, the intermediate polyethylene product (II) has at least one of the following characteristics:

[0058] 2 II ) An MIP value of 5 to 30;

[0059] 3 II ) A MIP / MIE ratio of 2.5 to 8, where the MIP / MIE ratio is 3 I ) / II ) Equal to or greater than 1.05, especially 1.05 to 2;

[0060] 4 II ) An ER value of 1 to 5, where the ER ratio is 4 I ) / 4 II ) Equal to or greater than 1.2, especially 1.2 to 4;

[0061] - MIE of -1 to 10 g / 10 min;

[0062] - Mw of -80,000 to 200,000 g / mol;

[0063] - Mw / Mn ratio of -3 to 20, preferably 4 to 15.

[0064] Preferably, the additional polyethylene (III) has at least one of the following characteristics:

[0065] 2 III ) A MIP value of 0.5 to 15;

[0066] 3 III ) A ratio of MIP / MIE of 1 to 8;

[0067] 4 III ) An ER value of 1 to 6;

[0068] - MIE of -0.1 to 5 g / 10 min;

[0069] - Mw of -80,000 to 250,000 g / mol;

[0070] - Mw / Mn ratio of -3 to 20 or 4 to 15.

[0071] As used herein, "LDPE" refers to ethylene homopolymers and ethylene copolymers produced in free radical polymerization.

[0072] The polymerization is typically carried out at high pressure, as will be explained in detail below.

[0073] 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.

[0074] 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.

[0075] When present, the comonomer can be present in an amount of up to 15 wt%, 10 wt% or 5 wt% relative to the total weight of the copolymer.

[0076] Generally, the term "copolymer" is intended to also include polymers containing more than one comonomer, such as terpolymers.

[0077] There are two basic high-pressure polymerization methods for making LDPE: autoclave and tubular reactor.

[0078] LDPE prepared by the autoclave reactor method ("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.

[0079] Autoclave polymerization is usually carried out in the presence of a free radical initiator selected from organic peroxides.

[0080] The tubular reactor method does not necessarily require the use of organic peroxides. It can be carried out by using oxygen alone as a free radical initiator, thus allowing the preparation of LDPE that does not contain chemical degradation products of organic peroxides.

[0081] The LDPE can also be prepared by a hybrid method combining both autoclave and tubular reactors.

[0082] The method 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.

[0083] The polymerization gas can optionally contain one or more chain transfer agents known in the art, such as propylene, propane, and propionaldehyde.

[0084] Such chain transfer agents are used to regulate the molecular weight.

[0085] The method 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 method.

[0086] The original LDPE polymers having the properties cited above for the precursor polyethylene (I) and for the additional polyethylene (III) are known in the art. Specific examples are polymers commercially available under the trade names Lupolen (LyondellBasell) and Petrothene (Equistar).

[0087] 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).

[0088] The precursor polyethylene (I) and thus the low-density polyethylene product (IV) of the present invention may include 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.

[0089] One or more of these additional components may be present, for example, in the LDPE recycle.

[0090] Thus, when the additional polyethylene (III) consists of or contains the original LDPE, the method of the present invention allows a significant amount of recycled material to be introduced into the original LDPE.

[0091] The additional components are ethylene homopolymers and ethylene copolymers containing α-olefin monomer units (preferably in an amount of up to 10 wt%) and mixtures thereof. Examples of the α-olefin monomer units are those having 3 to 8 carbon atoms, particularly propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, and 4-methyl-1-pentene. 1-butene and 1-hexene are preferred.

[0092] The homopolymers and copolymers can be obtained by polymerization methods in the presence of coordination catalysts. The methods and the homopolymers and copolymers obtained therefrom are widely described in the art.

[0093] In particular, the polymerization process can be carried out in the presence of Ziegler-Natta catalysts or single-site catalysts.

[0094] It is well known that Ziegler-Natta catalysts include the reaction products (new notation) of organometallic compounds of Groups 1, 2, or 13 of the Periodic Table with transition metal compounds of Groups 4 to 10 of the Periodic Table. In particular, the transition metal compounds can be selected from compounds of Ti, V, Zr, Cr, and Hf, and are preferably supported on MgCl2. Particularly preferred catalysts include the reaction products of the organometallic compounds of Groups 1, 2, or 13 of the Periodic Table with solid catalyst components including Ti compounds supported on MgCl2.

[0095] The preferred organometallic compound is an organo-Al compound.

[0096] Single-site catalysts are known in the art and are generally selected from metallocene and non-metallocene single-site catalysts.

[0097] Examples of metallocene single-site catalysts are zirconocene and hafnocene, 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.

[0098] Examples of non-metallocene single-site catalysts are preferably iron complex compounds having a tridentate ligand.

[0099] Thermal visbreaking involves treating the precursor polyethylene (I) at a temperature and / or mechanical shear energy sufficient to break polymer chains, mainly polymer chain branching or crosslinking.

[0100] In some embodiments, thermal visbreaking 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.

[0101] In particular, thermal visbreaking can be carried out at the following temperatures:

[0102] - 280 °C to 500 °C; or

[0103] - 290 °C to 500 °C; or

[0104] - 300 °C to 500 °C; or

[0105] - 310 °C to 500 °C; or

[0106] - 280 °C to 480 °C; or

[0107] - 290 °C to 480 °C; or

[0108] - 300 °C to 480 °C; or

[0109] - 310 °C to 480 °C; or

[0110] - 280 °C to 460 °C; or

[0111] - 290 °C to 460 °C; or

[0112] - 300 °C to 460 °C; or

[0113] - 310 °C to 460 °C.

[0114] 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 wt%, less than or equal to 0.10 wt%, or less than or equal to 0.01 wt%, based on the total weight of the polymer in the thermal visbreaking zone.

[0115] It is known that thermal visbreaking can be carried out in conventional mixing equipment commonly used for processing polymers in the molten state.

[0116] In particular, the intermediate polyethylene product (II) of the present disclosure can be prepared by processing the 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 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. Preferred extruders are screw extruders, especially 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 includes at least one co-rotating twin-screw extruder. This type of machine is conventional in the plastics industry and is 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.

[0117] As 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.

[0118] In an extruder device, 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 SEI value, the higher the melt flow index value produced by the thermal visbreaking step.

[0119] Indicatively, the SEI value can be from 0.15 to 0.4 kWh / kg, preferably from 0.20 to 0.35 kWh / kg.

[0120] The blending step (b) can be carried out in an extruder device of the same type as the aforementioned thermal visbreaking cracking step (a).

[0121] The temperature at which the blending step (b) is carried out is preferably low enough to avoid further thermal visbreaking cracking, in particular equal to or lower than 250 °C, more preferably equal to or lower than 240 °C.

[0122] The lower limit of the temperature at which the blending step (b) is carried out is generally equal to or higher than the melting point of the polymeric material used in this step.

[0123] Preferably, it is 190 °C, more preferably 210 °C.

[0124] The relative amounts of the intermediate polyethylene product (II) and the additional polyethylene (III) are determined by the respective melt flow index values of the low-density polyethylene product (IV) and the desired melt flow index value.

[0125] Indicatively, the following relative amounts can be selected:

[0126] - 10 wt% to 90 wt% of (II) and 90 wt% to 10 wt% of (III); or

[0127] - 15 wt% to 85 wt% of (II) and 85 wt% to 15 wt% of (III); or

[0128] - 10 wt% to 40 wt% of (II) and 90 wt% to 60 wt% of (III); or

[0129] - 90 wt% to 60 wt% of (II) and 10 wt% to 40 wt% of (III);

[0130] All the said amounts refer to the total weight of the intermediate polyethylene product (II) and the additional polyethylene (III).

[0131] Furthermore, 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 cracking.

[0132] 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.

[0133] 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.

[0134] As previously mentioned, the low-density polyethylene product (IV) of the present application is particularly suitable for applications where significant melt strength is required or desired, such as particularly for the preparation of foamed articles (e.g., foamed articles for flexible packaging) or films, especially cast films or blown films. In fact, it is preferably characterized by an F(max) value of 0.04 N or higher, particularly 0.04 to 2 N, measured at an acceleration of 2.4 mm / s with a Rheotens device at 190 °C. 2

[0135] Independently or in combination with the F(max) value, the low-density polyethylene product (IV) of the present invention preferably has at least one of the following additional characteristics:

[0136] 3) An MIP / MIE ratio of 1.8 to 8, preferably 2 to 6, where the MIP / MIE ratio 3 I ) / 3) is equal to or greater than 1.05, preferably equal to or greater than 1.08, particularly 1.05 to 1.4, or 1.05 to 1.3, or 1.08 to 1.4, or 1.08 to 1.3;

[0137] 4) An ER value of 1 to 4, preferably 1.2 to 4, more preferably 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, particularly 1.2 to 3, or 1.2 to 2.5, or 1.3 to 3, or 1.3 to 2.5;

[0138] - A Mw of -60,000 to 180,000 g / mol;

[0139] - A Mw / Mn ratio of -3 to 18 or 4 to 13.

[0140] Foamed products can be produced by chemical foaming methods or by physical foaming methods. 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, compared to chemically foamed polyolefin foams, they produce higher expansion and thus lower density. The foam can be uncrosslinked or crosslinked.

[0141] In some embodiments, foams including the low-density polyethylene of the present invention have a density 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 articles that require cushioning protection against impact and / or vibration. The foam can also be used in protective packaging for articles that require thermal insulation.

[0142] The film is prepared by methods well known in the art, particularly by extrusion methods.

[0143] To prepare a cast film by an extrusion method, 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.

[0144] To prepare a blown film by an extrusion method, 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.

[0145] Examples

[0146] The various embodiments, compositions, and methods provided herein are disclosed in the following examples. These examples are illustrative only and are not intended to limit the scope of the appended claims in any way.

[0147] The following analytical methods are used to characterize the polymer compositions.

[0148] Melt flow index

[0149] Determined at 190 °C and a specified load according to ISO 1133-1 2012-03.

[0150] Density

[0151] Determined at 23 °C according to ISO 1183-1:2012.

[0152] Molecular weight distribution determination

[0153] The determination of the average Mw and Mn and the Mw / Mn derived therefrom is carried out by high-temperature gel permeation chromatography using the methods described in ISO 16014-1, -2, -4 in 2003. The details according to the said ISO standard are as follows: Solvent 1,2,4-trichlorobenzene (TCB), equipment temperature and solution 135 °C and a PolymerChar (Valencia, Paterna 46980, Spain) IR-4 infrared detector capable of being used with TCB as the concentration detector. WATERS Alliance 2000 was used, which is equipped with the following pre-column SHODEX UT-G and separation columns SHODEX UT 806M(3x) and SHODEX UT 807 (Showa Denko Europe GmbH, Konrad-Zuse-Platz 4, 81829 München, Germany) connected in series.

[0154] The solvent was vacuum distilled under nitrogen and stabilized with 0.025 wt% of 2,6-di-tert-butyl-4-methylphenol. The flow rate used was 1 ml / min, the injection was 500 μl and the polymer concentration was 0.01% < concentration < 0.05% w / w. Molecular weight calibration was established by using monodisperse polystyrene (PS) standards in the range of 580 g / mol to 11,600,000 g / mol from Polymer Laboratories (now Agilent Technologies, Herrenberger Str. 130, 71034 Böblingen, Germany) and additional hexadecane.

[0155] Then the calibration curve was adapted 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 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, correction and calculation were carried out using NTGPC_control_V6.02.03 and NTGPC_V6.4.24 (hs GmbH, Hauptstraße 36, D-55437 Ober-Hilbersheim, Germany) respectively.

[0156] Complex shear viscosity η 0.02 (eta(0.02)) and ER

[0157] Measurements were made at an angular frequency of 0.02 rad / s and 190 °C as follows.

[0158] The sample was melt-pressed at 200 °C and 200 bar for 4 minutes to form a 1 mm thick plate. A disk specimen with a diameter of 25 mm was punched and inserted into a rheometer preheated to 190 °C. This measurement can be carried out using any commercially available rotational rheometer. Here, an Anton Paar MCR300 with a plate-plate geometry was used. At T = 190 °C, at a constant strain amplitude of 5%, a so-called frequency sweep was carried out (after annealing the sample at the measurement temperature for 4 minutes), and the stress response of the material was measured and analyzed in the excitation frequency ω range of 628 - 0.02 rad / s. Rheological properties, namely storage modulus G', loss modulus G", phase lag δ (= arctan(G" / G')) and complex viscosity η* as a function of the applied frequency, i.e., η*(ω) = [G'(ω) 2 + G"(ω) 2 1 / 2 / ω. At the applied frequency ω of 0.02 rad / s, the latter value is η 0.02 .

[0159] ER was determined by the method of R. Shroff and H. Mavridis, "New Measures of Polydispersity from Rheological Data on Polymer Melts," J. Applied Polymer Science 57 (1995) 1605 (see U.S. Patent No. 5,534,472, column 10, lines 20 - 30). It is calculated from:

[0160] ER = (1.781 * 10 -3 ) * G'

[0161] at a value of G" = 5,000 dyn / cm 2 .

[0162] 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 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 .​

[0163] Comonomer content

[0164] The comonomer content was determined by IR using an FT-IR spectrometer Tensor 27 from Bruker according to ASTM D 6248-98.

[0165] Melt strength

[0166] The melt strength of the polymer is an important parameter in melt processing operations where elongation or stretching 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 tensile properties of the 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.

[0167] At 190 °C on a Rheotester 1000 (12 mm barrel diameter, 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 10 minutes of melt time, the polymer is extruded at a shear rate of 50 l / s. The polymer strand exits the capillary die and the die exit velocity v0 is 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 is 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 are recorded.

[0168] SEI

[0169] 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, and represents the material flow rate. 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 传输 is the transmission efficiency.

[0170] Example 1

[0171] Use the commercial grade 2420D sold by LyondellBasell Industries as the precursor polyethylene (I).

[0172] Use the commercial grade 2420H sold by LyondellBasell Industries as the additional polyethylene (III).

[0173] 2420D and 2420H are unused LDPEs with the properties reported in Table 1, where they are identified as "LP 2420D" and "LP 2420H", respectively.

[0174] In method step (a), the low-density polyethylene product (IV) of Example 1 is obtained by first extruding the precursor polyethylene (I) in an extruder Leistritz ZSE 27MAXX.

[0175] The machine parameters are:

[0176] - Rotation speed: 750 rpm;

[0177] - Throughput: 20 kg / h;

[0178] - Temperature:

[0179] Zone 1: 300 °C, Zone 2: 320 °C, Zones 3 - 10: 340 °C, die: 300 °C;

[0180] - SEI: 0.32 kWh / kg.

[0181] The properties of the intermediate polyethylene product (II) thus obtained are reported in Table 1, where it is identified as "Int.(II)". Then, in method step (b), the intermediate polyethylene product (II) is blended with the additional polyethylene (III). The relative amounts in the blend are 20 wt% of the intermediate polyethylene product (II) and 80 wt% of the additional polyethylene (III).

[0182] Method step (b) is carried out in the same extruder Leistritz ZSE 27MAXX as step (a).

[0183] The machine parameters are:

[0184] - Rotation speed: 200 rpm;

[0185] - Throughput: 30 kg / h;

[0186] - Temperature:

[0187] Zone 1: 200 °C, Zone 2: 220 °C, Zone 3 - 10: 240 °C, Die: 220 °C;

[0188] SEI: 0.08 kWh / kg.

[0189] The properties of the low-density polyethylene product (IV) thus obtained are reported in Table 1, where it is identified as "Prod.(IV)".

[0190] Table 1

[0191] LP 2420D LP 2420H Int.(II) Prod.(IV) MIP[g / 10min] 1.12 7.74 19.9 8.39 MIE[g / 10min] 0.25 1.93 5.41 2.09 <![CDATA[Density [g / cm 3 > 0.922 0.922 0.924 0.922 Mw[g / mol] 153196 109464 103233 112930 Mw / Mn 13.6 7.1 8.4 10.0 MIP / MIE 4.5 4.0 3.7 4.0 ER 4.0 1.9 2.2 1.9 MIE / ER 0.06 1.02 2.48 1.09 F (maximum)[N] 0.138 0.088 0.043 0.086

[0192] The data reported in Table 1 show that the process of the present invention allows the introduction of a significant amount of visbreaking LP2420D (Int.(II)) into LP 2420H, while substantially not changing the properties of LP 2420H.

[0193] Therefore, it is possible to add a viscous recycle to LP 2420H, replacing the alternative intermediate polyethylene product (II) obtained by visbreaking LP 2420D, while still maintaining satisfactory properties.

[0194] Example 2

[0195] The commercial grade 2420D sold by LyodellBasell Industries is used both as the precursor polyethylene (I) and as the additional polyethylene (III).

[0196] In process step (a), the low-density polyethylene product (IV) of Example 2 was obtained by first extruding the precursor polyethylene (I) in an extruder Leistritz ZSE 27MAXX.

[0197] The machine parameters were:

[0198] - Rotational speed: 700 rpm;

[0199] - Throughput: 30 kg / h;

[0200] - Temperature:

[0201] Zone 1: 300 °C, Zone 2: 320 °C, Zone 3 - 10: 340 °C, Die: 300 °C;

[0202] - SEI: 0.27 kWh / kg.

[0203] The properties of the intermediate polyethylene product (II) thus obtained are reported in Table 2, where it is identified as "Int.(II)". The intermediate polyethylene product (II) is then blended with additional polyethylene (III) in method step (b). The relative amounts in the blend are 85 wt% of the intermediate polyethylene product (II) and 15 wt% of the additional polyethylene (III).

[0204] Method step (b) is carried out in the same extruder Leistritz ZSE 27MAXX as step (a).

[0205] The machine parameters are as follows:

[0206] - Rotational speed: 200 rpm;

[0207] - Throughput: 30 kg / h;

[0208] - Temperature:

[0209] Zone 1: 200 °C, Zone 2: 220 °C, Zones 3 - 10: 240 °C, Die: 220 °C;

[0210] SEI: 0.08 kWh / kg.

[0211] The properties of the low - density polyethylene product (IV) thus obtained are reported in Table 2, where it is identified as "Prod.(IV)".

[0212] Table 2

[0213] Int.(II) Prod.(IV) MIP[g / 10min] 11.5 6.57 MIE[g / 10min] 3.06 1.65 <![CDATA[Density [g / cm 3 > 0.922 0.923 Mw[g / mol] 117280 133918 Mw / Mn 9.2 8.9 MIP / MIE 3.76 3.98 ER 2.41 2.63 MIE / ER 1.27 0.63 F (maximum)[N] 0.081 0.1516

[0214] The data reported in Table 2 indicate that the present method allows obtaining a melt flow index value substantially the same as that of LP 2420H, with an even improved melt strength, as evidenced by a higher F(max) value.

Claims

1. A method for producing low density polyethylene with an improved melt flow index, comprising: (a) subjecting a precursor polyethylene (I) to thermo-oxidative degradation, said precursor polyethylene (I) comprising 35 wt% or more, preferably 40 wt% or more, particularly 35 wt% to 100 wt%, or 40 wt% to 100 wt% of LDPE, relative to 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 is 0.910 to 0.940 g / cm 3 , preferably 0.915 to 0.935 g / cm 3 ; 2 I ) An MIP value of 0.3 to 7 g / 10 min, preferably 0.5 to 6 g / 10 min; 3 I ) A ratio of MIP / MIE of 2 to 9, preferably 2.5 to 7, where MIP is the melt mass-flow rate at 190 °C and a load of 5 kg, and MIE is the melt mass-flow rate at 190 °C and a load of 2.16 kg, both determined in accordance with ISO 1133-2:2011; 4 I ) An ER value of 1.8 to 8, preferably 2 to 6; Thereby obtaining an intermediate polyethylene product (II) having an MIP 2 I ) value higher than that of MIP 2 II ), wherein the MIP ratio 2 II ) / 2 I ) is 1.5 or greater, preferably 3 or greater, more preferably 10 or greater, and preferably 25 or less in all cases; and (b) Blend the intermediate polyethylene product (II) obtained in the thermal visbreaking cracking step (a) with additional polyethylene (III), said additional polyethylene (III) having a density of from 0.910 to 0.940 g / cm 3 , preferably from 0.915 to 0.935 g / cm 3 3 III ) and an MIP 2 II ) value different from MIP 2 III ), wherein the ratio of MIP 2 III / 2 II ) is from 0.03 to 3, preferably from 0.05 to 2, and the absolute value of the difference between MIP 2 III ) and MIP 2 II ) is at least 5, preferably at least 8, and in all cases preferably has an upper limit of 20, more preferably 15; Thereby obtaining a low density polyethylene product (IV) having a specific MIE / ER of 0.2 to 2.8, preferably 0.3 to 2.5, and: 1) A density of 0.910 to 0.940 g / cm 3 , preferably 0.915 to 0.935 g / cm 3 ; 2) An MIP value of 3 to 20 g / 10 min, preferably 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 1.5 to 15, or 1.5 to 10, or 1.8 to 15, or 1.8 to 10; Wherein ER is calculated by: ER = (1.781 * 10 -3 ) * G' At a value of G” = 0.5 kPa (5,000 dyn / cm 2 ); Where: G' = storage modulus; G'' = loss modulus; Both G' and G'' are measured using dynamic oscillatory shear in a plate-plate rotational rheometer at a temperature of 190 °C.

2. The method according to claim 1, wherein said thermo-oxidative degradation step (a) is carried out by heating said 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, most preferably at a temperature equal to or greater than 310 °C.

3. The method according to claim 1 or 2, wherein said thermo-oxidative degradation step (a) is carried out in an extruder device.

4. The method according to claim 1 or 2, wherein said precursor polyethylene (I) has at least one of the following additional characteristics: - MIE of 0.1 to 3 g / 10 min; - Mw of 100,000 to 350,000 g / mol; - Mw / Mn ratio of 3 to 20 or 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).

5. The method 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 method 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 method according to claim 1 or 2, wherein said additional polyethylene (III) comprises virgin LDPE.

8. The method according to claim 1 or 2, wherein the relative amounts of the intermediate polyethylene product (II) and the additional polyethylene (III) are: - 10 wt% to 90 wt% of (II) and 90 wt% to 10 wt% of (III); or - 15 wt% to 85 wt% of (II) and 85 wt% to 15 wt% of (III); or - 10 wt% to 40 wt% of (II) and 90 wt% to 60 wt% of (III); or - 90 wt% to 60 wt% of (II) and 10 wt% to 40 wt% of (III); All said amounts refer to the total weight of the intermediate polyethylene product (II) and the additional polyethylene (III).

9. The method according to claim 1 or 2, wherein the low density polyethylene product (IV) has an MIE of 1 g / 10 min or higher, in particular 1 to 10 or 1 to 8 g / 10 min.

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

11. Low density polyethylene obtainable by the method according to claim 1 or 2.

12. A manufactured article comprising the low density polyethylene according to claim 11.

13. The manufactured article according to claim 12, which is in the form of a foamed article or a film.