WO2012119956A1

WO2012119956A1 - Method of preparing pelleted polyethylene products

Info

Publication number: WO2012119956A1
Application number: PCT/EP2012/053661
Authority: WO
Inventors: Peter Stevens; Daan Dewachter; Etienne Laurent
Original assignee: Total Research & Technology Feluy
Priority date: 2011-03-04
Filing date: 2012-03-02
Publication date: 2012-09-13

Abstract

The present invention relates to a method of preparing a pelleted polyethylene product by: (i) producing a first polyethylene resin in the presence of a metallocene catalyst, said first polyethylene resin having a density of from 0.940 to 0.970 g/cm3 as measured with ASTM D-1505 standardized test at a temperature of 23°C; (ii) extruding the first polyethylene resin with at least one additive selected from pigments, UV-stabilizers, anti-oxidants, fillers, anti-static agents, dispersive aid agents, processing aids, anti-acid agents, and mixtures thereof, into a pelleted first polyethylene resin; (iii) separately producing a second polyethylene resin in the presence of a Ziegler- Natta and/or chromium catalyst; and (iv) physically blending 1 % to 35% by weight of the pelleted first polyethylene resin with the polyethylene of step (iii) to produce the pelleted polyethylene product, with % by weight being based on the total weight of the pelleted polyethylene product.

Description

Method of preparing pelleted polyethylene products Technical field of the invention

The present invention relates to a method for preparing a pelleted polyethylene product and to the use of pelleted polyethylene. The invention can advantageously be used in chemical manufacturing, specifically in the polymerization of olefins.

Background of the invention

Polyolefins, such as polyethylene (PE), are synthesized by polymerizing monomers, such as ethylene (CH₂=CH₂). Because it is cheap, safe, stable to most environments and easy to be processed polyolefins are useful in many applications. Polyethylene can be classified into several types, such as but not limited to LDPE (Low Density Polyethylene), LLDPE (Linear Low Density Polyethylene), and HDPE (High Density Polyethylene) as well as High Molecular Weight (HMW), Medium Molecular Weight (MMW) and Low Molecular Weight (LMW). Each type of polyethylene has different properties and characteristics.

Olefin (such as ethylene) polymerizations are frequently carried out in a loop reactor using monomer (such as ethylene), diluent and catalyst, optionally an activating agent, optionally one or more co-monomer(s), and optionally hydrogen..

Polymerization in a loop reactor is usually performed under slurry conditions, with the produced polymer usually in a form of solid particles suspended in diluent. The slurry is circulated continuously in the reactor with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent. Polymer slurry is discharged from the loop reactor by means of settling legs, which operate on a batch principle to recover the slurry. Settling in the legs is used to increase the solid concentration of the slurry finally recovered as product slurry. The product slurry is further discharged through heated flash lines to a flash tank, where most of the diluent and unreacted monomers are flashed off and recycled.

After the polymer product is collected from the reactor and the hydrocarbon residues are removed, the polymer product is dried resulting in a polymer resin. Additives can be added and finally the polymer may be mixed and pelletized resulting in polymer product.

During the mixing step, polymer resin and optional additives are mixed intimately in order to obtain a polymer product as homogeneous as possible. Preferably, mixing is done in an extruder wherein the ingredients are mixed together and the polymer product and optionally some of the additives are melted so that intimate mixing can occur. The melt is then extruded into a rod, cooled and granulated, e.g. to form pellets. In this form the resulting compound can then be used for the manufacturing of different objects. Optionally, two or more different polyethylene resins are produced separately and subsequently mixed, representing a physical blending process.

Complications may also occur during physical blending of different polyolefin resins into a polyolefin product. Even though such blends could be advantageous in usage, the complications related to physical blending may require complicated mixing machines and/or extensive mixing processes or even lead to non-homogenous polymer mixtures that are not optimal for application in end-products. Consequently, there remains a need in the art for homogeneous polymer product while ensuring low production costs and high- quality end-products.

Summary of the invention

Surprisingly, the present inventors have found a way to improve polyolefin preparation processes and overcome the above and other problems of the prior art. Accordingly, the present invention relates to a method of preparing a pelleted polyethylene product by:

(i) producing a first polyethylene resin in the presence of a metallocene catalyst, wherein said first polyethylene resin has a density of from 0.940 to 0.970 g/cm³;

(ii) extruding the first polyethylene resin with at least one additive selected from pigments, UV-stabilizers, anti-oxidants, fillers, anti-static agents, dispersive aid agents, processing aids, anti-acid agents and mixtures thereof, into a pelleted first polyethylene resin;

(iii) separately producing a second polyethylene resin in the presence of a Ziegler- Natta and/or chromium catalyst; and

(iv) physically blending 1 % to 35% by weight of the pelleted first polyethylene resin with the polyethylene of step (iii) to produce the pelleted polyethylene product, preferably physically blending 3 to 20% by weight, more preferably 3.5 to 10% by weight of the pelleted first polyethylene resin with the polyethylene of step (iii) to produce the pelleted polyethylene product.

Preferably, the present invention relates to a method of preparing a pelleted polyethylene product by:

(i) producing a first polyethylene resin in the presence of a metallocene catalyst, said first polyethylene resin having a density of from 0.940 to 0.970 g/cm³ as measured with ASTM D-1505 standardized test at a temperature of 23°C; (ii) extruding the first polyethylene resin with at least one additive selected from pigments, UV-stabilizers, anti-oxidants, fillers, anti-static agents, dispersive aid agents, processing aids, anti-acid agents, and mixtures thereof, into a pelleted first polyethylene resin;

(iv) physically blending 1 % to 35% by weight of the pelleted first polyethylene resin with the polyethylene of step (iii) to produce the pelleted polyethylene product, with % by weight being based on the total weight of the pelleted polyethylene product.

Surprisingly, the pelleted first polyethylene resin can be used for convenient adjustment of the properties of second polyethylene resin, allowing for flexible yet precise processing conditions, while producing homogeneous and high-quality end-products.

The present invention also relates to the use of pelleted polyethylene resin comprising a first polyethylene resin prepared in the presence of a metallocene catalyst, having a density of from 0.940 to 0.970 g/cm³, and at least one additive selected from pigments, UV-stabilizers, anti-oxidants, fillers, anti-static agents, for preparing a physical blend comprising from 1 % to 35% by weight of said pelleted polyethylene resin and a second polyethylene resin prepared in the presence of a Ziegler-Natta or Chromium catalyst. The present invention will now be further described. In the following passages, different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

Detailed description of invention

Before the present method used in the invention is described, it is to be understood that this invention is not limited to particular methods, components, or devices described, as such methods, components, and devices may, of course, vary. It is also to be understood that the terminology used herein is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.

As used herein, the singular forms "a", "an", and "the" include both singular and plural referents unless the context clearly dictates otherwise. The terms "comprising", "comprises" and "comprised of" as used herein are synonymous with "including", "includes" or "containing", "contains", and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps. The terms "comprising", "comprises" and "comprised of" also include the term "consisting of". The recitation of numerical ranges by endpoints includes all numbers and fractions subsumed within the respective ranges, as well as the recited endpoints.

The term "about" as used herein when referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, is meant to encompass variations of +/-10% or less, preferably +1-5% or less, more preferably +/-1 % or less, and still more preferably +/-0.1 % or less of and from the specified value, insofar such variations are appropriate to perform in the disclosed invention. It is to be understood that the value to which the modifier "about" refers is itself also specifically, and preferably, disclosed.

Unless otherwise defined, all terms used in disclosing the invention, including technical and scientific terms, have the meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. By means of further guidance, definitions for the terms used in the description are included to better appreciate the teaching of the present invention.

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments. Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

In one aspect, the present invention relates to a method of preparing a pelleted polyethylene product by physical blending (a) from 1 % to 35% by weight of a pelleted first polyethylene resin based on the total weight of the pelleted polyethylene product, with (b) a second polyethylene resin produced in the presence of a Ziegler-Natta and/or chromium catalyst, wherein said pelleted first polyethylene resin comprises a first polyethylene resin prepared in the presence of a metallocene catalyst, having a density of from 0.940 to 0.970 g/cm³, and at least one additive selected from pigments, UV-stabilizers, anti- oxidants, fillers, anti-static agents.

Surprisingly, the present inventors have found that using from 1 % to 35% by weight of the first polyethylene resin in pelleted form may advantageously improve the properties of a second polyethylene resin produced in the presence of a Ziegler-Natta and/or chromium catalyst. For instance, the pelleted first polyethylene resin may be chosen as a function of a fraction of the second polyethylene that is judged to be missing. Thus, if the second polyethylene has insufficient LMW fractions, a pelleted first polyethylene resin with LMW is preferably added. If the second polyethylene has insufficient HMW, then a pelleted first polyethylene resin with HMW is preferably added. Alternatively, the pelleted first polyethylene resin may be chosen as a function of the density of one or more fractions of the second polyethylene.

If the second polyethylene has a bimodal molecular weight distribution, the pelleted first polyethylene resin may for instance be chosen as a function of the difference in the average MFI and/or the average density between the HMW and the LMW fractions of the second polyethylene.

The process preferably comprises producing a first polyethylene resin having a density of from 0.940 to 0.970 g/cm³ in the presence of a metallocene catalyst in step (i). Preferably said first metallocene produced polyethylene resin has a density of 0.950 to 0.965 g/cm³.

For the purpose of this invention, "resin" is defined as the polymer material that is produced in the loop reactor with the hard catalyst particle at the core of each grain of the powder and which is also sometimes referred to as "fluff". For the purpose of the invention, "polyethylene product" or "polyethylene pellet" is defined as ethylene polymer material that is produced through compounding and homogenizing of the resin, for instance with mixing and/or extruder equipment.

Preferably, the first polyethylene resin of step (i) is subsequently extruded in step (ii) with at least one additive into a pelleted first polyethylene resin. Use of a pelleted form of the first polyethylene resin is preferred as it allows for easy storage, for convenient adjustment of the properties of second polyethylene resin, for flexible yet precise processing conditions, and for production of homogeneous and high-quality end-products. Surprisingly, the present inventors have found that incorporating higher levels of additives in the first polyethylene that is in a pelleted form is particularly useful when adjusting the properties of the pelleted polyethylene product obtained in step (iv) of the invention. For instance, it may not be required anymore to add these additives in step (vi), providing additional processing flexibility, particularly in the case of pigment which might for instance otherwise stain processing equipment.

Preferably, the additive is selected from pigments, UV-stabilizers, anti-oxidants, fillers, anti-static agents, dispersive aid agents, processing aids, anti-acid agents, and mixtures thereof. Preferably, the pelleted first polyethylene resin is extruded with a pigment.

Preferred levels of additives are from 2 to 60% by weight of the pelleted first polyethylene resin. Preferably, the pelleted first polyethylene resin comprises pigment at a level of at least 2,5% by weight, more preferably at least 2,6 % by weight and preferably at most 55% by weight, preferably at most 50% by weight, preferably at most 46 % by weight, based on the weight of the pelleted first polyethylene resin.

Preferably, the polyethylene resin of step (iii) (second polyethylene resin) is separately produced in the presence of a Ziegler-Natta and/or chromium catalyst. More preferably, the polyethylene resin of step (iii) (second polyethylene resin) is separately produced in the presence of a Ziegler-Natta catalyst. Preferably, step (iii) produces a resin of the second polyethylene that is used in extrusion step (iv) of the invention. According to an embodiment of the invention, in some cases, the resin of the second polyethylene is preferably first extruded into a pelleted second polyethylene product before the extrusion step (iv) with the pelleted first polyethylene resin.

Preferably, the pelleted polyethylene product is prepared in step (iv) by physical blending and melting 1 % to 35% by weight of the pelleted first polyethylene resin of step (ii) with the second polyethylene resin of step (iii). In an embodiment, step (ii) and/or step (iv) are performed in device for continuously melting and blending. Preferably, physical blending takes place in a device for continuously melting and blending said resins selected from a mixer, an extruder or combinations thereof. For example, the device can be an extruder and/or a mixer. Preferably, the device is an extruder. A preferred extruder is a co-rotating twin screw. A preferred mixer is a counter-rotating twin screw.

Preferably, the final polyethylene product comprises from 1 to 35% by weight of the pelleted first polyethylene resin, more preferably from 2% to 30% by weight, most preferably from 2% to 28% by weight and preferably from 3% to 25% by weight, more preferably from 3% to 20% by weight and most preferably from 3.5% to 15% by weight of the pelleted first polyethylene resin. Preferably, the polyethylene product comprises at most 99% by weight of the second polyethylene resin, more preferably at most 98% by weight, more preferably at most 97.5% by weight, and preferably at most 97% by weight, and preferably at most 95% by weight, more preferably at most 90% by weight.

Preferably, the polyethylene product of the invention has a multimodal molecular weight distribution. Preferably, the pelleted first polyethylene resin of step ii) has a monomodal molecular weight distribution. The present inventors have surprisingly found that physically blending a monomodal pelleted first polyethylene resin with the polyethylene of step (iii) leads to a homogeneous polyethylene product with optimal characteristics under acceptable processing conditions. Preferably, the second polyethylene resin has a monomodal or bimodal molecular weight distribution. More preferably, the second polyethylene resin has a bimodal molecular weight distribution.

By the term "monomodal polymers" or "polymers with a monomodal molecular weight distribution" it is meant, polymers having one maxima in their molecular weight distribution curve defined also as unimodal distribution curve. By the term "polymers with a bimodal molecular weight distribution" or "bimodal polymers" it is meant, polymers having a distribution curve being the sum of two unimodal molecular weight distribution curves. By the term "polymers with a multimodal molecular weight distribution" or "multimodal" polymers it is meant polymers with a distribution curve being the sum of at least two, preferably more than two unimodal distribution curves. By the term "monomodal polyethylene" or "polyethylene with a monomodal molecular weight distribution" it is meant, polyethylene having one maxima in their molecular weight distribution curve defined also as unimodal distribution curve. By the term "polyethylene with a multimodal molecular weight distribution" or "multimodal" polyethylene product it is meant polyethylene with a distribution curve being the sum of at least two, preferably more than two unimodal distribution curves.

Preferably, the pelleted polyethylene product has a High Load Melt Index (HLMI) of from 2 to 50 g/10min, preferably from 5 to 20g/10min. Preferably, the pelleted polyethylene product has a density of from 0.930 to 0.965g/cm³ as measured with the ASTM D-1505 standardized test at a temperature of 23°C.

According to the invention, the HLMI is determined with the ASTM D-1238 standardized test which uses a temperature of 190°C and a load of 21.6 kg. The density is determined with the ASTM D-1505 standardized test at a temperature of 23°C.

The pelleted polyethylene product of the invention is especially useful in pipe applications. In another aspect, the present invention relates to the use of pelleted polyethylene produced in the presence of metallocene and comprising a first polyethylene resin prepared in the presence of a metallocene catalyst, having a density of from 0.940 to 0.970 g/cm³, and at least one additive selected from pigments, UV-stabilizers, anti- oxidants, fillers, anti-static agents, for preparing a physical blend comprising from 1 % to 35% by weight of said pelleted polyethylene and a second polyethylene resin prepared in the presence of a Ziegler-Natta or Chromium catalyst. Preferably, the pelleted polyethylene and the polyethylene resin are subsequently physically blended, more preferably extruded, to produce a pelleted final polyethylene product.

In a preferred embodiment, the present invention relates to the use of pelleted polyethylene resin comprising a first polyethylene resin prepared in the presence of a metallocene catalyst, having a density of from 0.940 to 0.970 g/cm³ as measured with ASTM D-1505 standardized test at a temperature of 23°C, and at least one additive selected from pigments, UV-stabilizers, anti-oxidants, fillers, anti-static agents, for preparing a physical blend comprising from 1 % to 35% by weight of said pelleted polyethylene resin and a second polyethylene resin prepared in the presence of a Ziegler- Natta or Chromium catalyst, with % by weight being based on the total weight of the pelleted polyethylene product.

Polyolefin, and in particular polyethylene, resins are preferably prepared in a loop reactor that preferably comprises interconnected pipes, defining a reactor path, and wherein a slurry is preferably pumped through said loop reactor. Preferably, each of the polyethylene resins of the invention is separately produced.

As used herein, the term "polymerization slurry" or "polymer slurry" or "slurry" means substantially a multi-phase composition including at least polymer solids and a liquid phase, the liquid phase being the continuous phase. The solids include catalyst and a polymerized olefin, such as polyethylene. The liquids include an inert diluent, such as isobutane, dissolved monomer such as ethylene, co-monomer, molecular weight control agents, such as hydrogen, antistatic agents, antifouling agents, scavengers, and other process additives.

As used herein, the term "catalyst" refers to a substance that causes a change in the rate of a polymerization reaction without itself being consumed in the reaction. In the present invention, it is especially applicable to catalysts suitable for the polymerization of ethylene to polyethylene. These catalysts will be referred to as ethylene polymerization catalysts or polymerization catalysts. The invention is suitable for supported heterogeneous catalysts. In the present invention it is especially applicable to ethylene polymerization catalysts such as metallocene catalysts, Ziegler-Natta catalysts and chromium catalysts.

The term "metallocene catalyst" is used herein to describe any transition metal complexes consisting of metal atoms bonded to one or more ligands. The metallocene catalysts are compounds of Group IV transition metals of the Periodic Table such as titanium, zirconium, hafnium, etc., and have a coordinated structure with a metal compound and ligands composed of one or two groups of cyclopentadienyl, indenyl, fluorenyl or their derivatives. Use of metallocene catalysts in the polymerization of olefins has various advantages. Metallocene catalysts have high activities and are capable of preparing polymers with enhanced physical properties. The key to metallocenes is the structure of the complex. The structure and geometry of the metallocene can be varied to adapt to the specific need of the producer depending on the desired polymer. Metallocenes comprise a single metal site, which allows for more control of branching and molecular weight distribution of the polymer. Monomers are inserted between the metal and the growing chain of polymer.

In a preferred embodiment, the metallocene catalyst has a general formula (I) or (II):

(Ar)₂MQ₂ (I); or

R"(Ar)₂MQ₂ (II)

wherein the metallocenes according to formula (I) are non-bridged metallocenes and the metallocenes according to formula (II) are bridged metallocenes;

wherein said metallocene according to formula (I) or (II) has two Ar bound to M which can be the same or different from each other;

wherein Ar is an aromatic ring, group or moiety and wherein each Ar is independently selected from the group consisting of cyclopentadienyl, indenyl (IND), tetrahydroindenyl (THI) or fluorenyl, wherein each of said groups may be optionally substituted with one or more substituents each independently selected from the group consisting of halogen, a hydrosilyl, a SiR₃ group wherein R is a hydrocarbyl having 1 to 20 carbon atoms, and a hydrocarbyl having 1 to 20 carbon atoms, and wherein said hydrocarbyl optionally contains one or more atoms selected from the group comprising B, Si, S, O, F, CI and P; wherein M is a transition metal selected from the group consisting of titanium, zirconium, hafnium and vanadium; and preferably is zirconium;

wherein each Q is independently selected from the group consisting of halogen; a hydrocarboxy having 1 to 20 carbon atoms; and a hydrocarbyl having 1 to 20 carbon atoms and wherein said hydrocarbyl optionally contains one or more atoms selected from the group comprising B, Si, S, O, F, CI and P; and

wherein R" is a divalent group or moiety bridging the two Ar groups and selected from the group consisting of a C C₂₀ alkylene, a germanium, a silicon, a siloxane, an alkylphosphine and an amine, and wherein said R" is optionally substituted with one or more substituents each independently selected from the group consisting of halogen, a hydrosilyl, a SiR₃ group wherein R is a hydrocarbyl having 1 to 20 carbon atoms, and a hydrocarbyl having 1 to 20 carbon atoms and wherein said hydrocarbyl optionally contains one or more atoms selected from the group comprising B, Si, S, O, F, CI and P.

The term "hydrocarbyl having 1 to 20 carbon atoms" as used herein is intended to refer to a moiety selected from the group comprising a linear or branched C C₂₀ alkyl; C₃-C₂o cycloalkyl; C₆-C₂o aryl; C₇-C₂₀ alkylaryl and C₇-C₂₀ arylalkyl, or any combinations thereof. Exemplary hydrocarbyl groups are methyl, ethyl, propyl, butyl, amyl, isoamyl, hexyl, isobutyl, heptyl, octyl, nonyl, decyl, cetyl, 2-ethylhexyl, and phenyl. Exemplary halogen atoms include chlorine, bromine, fluorine and iodine and of these halogen atoms, fluorine and chlorine are preferred. Exemplary of the alkylene groups is methylidene, ethylidene and propylidene.

Illustrative examples of metallocene catalysts comprise but are not limited to bis(cyclopentadienyl) zirconium dichloride (Cp₂ZrCI₂), bis(cyclopentadienyl) titanium dichloride (Cp₂TiCI₂), bis(cyclopentadienyl) hafnium dichloride (Cp₂HfCI₂); bis(tetrahydroindenyl) zirconium dichloride, bis(indenyl) zirconium dichloride, and bis(n- butyl-cyclopentadienyl) zirconium dichloride; ethylenebis(4,5,6,7-tetrahydro-1-indenyl) zirconium dichloride, ethylenebis(l-indenyl) zirconium dichloride, dimethylsilylene bis(2- methyl-4-phenyl-inden-1-yl) zirconium dichloride, diphenylmethylene (cyclopentadienyl)(fluoren-9-yl) zirconium dichloride, and dimethylmethylene [1-(4-tert- butyl-2-methyl-cyclopentadienyl)](fluoren-9-yl) zirconium dichloride.

The metallocene catalysts generally are provided on a solid support. The support should be an inert solid, which is chemically unreactive with any of the components of the conventional metallocene catalyst. The support is preferably a silica compound. In a preferred embodiment, the metallocene catalyst is provided on a solid support, preferably a silica support.

The term "Ziegler-Natta catalyst" refers to catalysts preferably of the a general formula M¹X_V, wherein M¹ is a transition metal compound selected from group IV to VII, wherein X is a halogen, and wherein v is the valence of the metal. Preferably, M¹ is a group IV, group V or group VI metal, more preferably titanium, chromium or vanadium and most preferably titanium. Preferably, X is chlorine or bromine, and most preferably, chlorine. Illustrative examples of the transition metal compounds comprise but are not limited to TiCI₃, TiCI₄. Suitable ZN catalysts for use in the invention are described in US6930071 and US6864207, which are incorporated herein by reference. Preferably, the first polyethylene resin is produced in the presence of a Ziegler-Natta catalyst with an average particle size (D50) of at most 50 μηι, preferably, at most 30 μηι, more preferably at most 15 μηι, more preferably at most 13 μηι, most preferably at most 10 μηι, most preferably at most 8 μηι, for example at most 5 μηι and preferably at least 3 μηι.

The D50 is defined as the particle size for which fifty percent by volume of the particles has a size lower than the D50. The measurement of the average particle size (D50) can be made according to the International Standard ISO 13320:2009 ("Particle size analysis - Laser diffraction methods"). For example, Malvern Instruments' laser diffraction systems can advantageously be used. The D50 can be measured by laser diffraction analysis on a Malvern type analyzer after having put the supported catalyst in suspension in cyclohexane. Suitable Malvern systems include the Malvern 2000, Malvern MasterSizer (such as Mastersizer S), Malvern 2600 and Malvern 3600 series. Such instruments together with their operating manual meets or even exceeds the requirements set-out within the ISO 13320 Standard. The Malvern MasterSizer (such as Mastersizer S) may also be useful as it can more accurately measure the D50 towards the lower end of the range e.g. for average particle sizes of less 8 μηι, by applying the theory of Mie, using appropriate optical means.

Suitable Ziegler Natta catalysts of average particle size (D50) of at most 15 μηι are commercially available from W. R. Grace and Company, such as SYLOPOL®5910 which has an average particle size of 10 μηι, or from Lyondellbasell.

The term "chromium catalysts" refers to catalysts obtained by deposition of chromium oxide on a support, e.g. a silica or aluminium support. Illustrative examples of chromium catalysts comprise but are not limited to CrSi0₂ or CrAI₂0₃.

The catalyst is preferably added to the loop reactor as a catalyst slurry. As used herein, the term "catalyst slurry" refers to a composition comprising catalyst solid particles and a diluent. The solid particles can be suspended in the diluent, either spontaneously or by homogenization techniques, such as mixing. The solid particles can be non- homogeneously distributed in a diluent and form a sediment or deposit. Optionally, activating agent is used in processes according to the invention. The term "activating agent" refers to materials that can be used in conjunction with a catalyst in order to improve the activity of the catalyst during the polymerization reaction. In the present invention, it particularly refers to an organo-aluminium compound, being optionally halogenated, having general formula AIR¹¹ R¹²R¹³ or AIR¹¹ R¹²Y, wherein R¹¹ , R¹², R¹³ is an alkyl having from 1 to 6 carbon atoms and R¹¹ , R¹², R¹³ may be the same or different and wherein Y is hydrogen or a halogen, as disclosed in US6930071 and US6864207, which are incorporated herein by reference. Preferred activating agents are Tri-Ethyl Aluminum (TEAI), Tri-lso-Butyl Aluminum (TIBAI), Tri-Methyl Aluminum (TMA), and Methyl-Methyl- Ethyl Aluminum (MMEAI). TEAI is particularly preferred. In an embodiment, the activating agent is added to the loop reactor in an activating agent slurry at a concentration of less than 90% by weight of the activating agent slurry composition, more preferably from 10 to 50% by weight, for instance around 20% by weight. Preferably, the concentration of the activating agent in the loop reactor is lower than 200ppm, more preferably from 10 to 100 parts per million, most preferably from 20-70ppm and for instance around 50ppm.

As used herein, the term "monomer" refers to olefin compound that is to be polymerized. Examples of olefin monomers are ethylene and propylene. Preferably, the invention is directed to ethylene.

As used herein, the term "diluent" refers to diluents in a liquid state, liquid at room temperature and preferably liquid under the pressure conditions in the loop reactor. Diluents which are suitable for being used in accordance with the present invention may comprise but are not limited to hydrocarbon diluents such as aliphatic, cycloaliphatic and aromatic hydrocarbon solvents, or halogenated versions of such solvents. The preferred solvents are C12 or lower, straight chain or branched chain, saturated hydrocarbons, C5 to C9 saturated alicyclic or aromatic hydrocarbons or C2 to C6 halogenated hydrocarbons. Non-limiting illustrative examples of solvents are butane, isobutane, pentane, hexane, heptane, cyclopentane, cyclohexane, cycloheptane, methyl cyclopentane, methyl cyclohexane, isooctane, benzene, toluene, xylene, chloroform, chlorobenzenes, tetrachloroethylene, dichloroethane and trichloroethane. In a preferred embodiment of the present invention, said diluent is isobutane. However, it should be clear from the present invention that other diluents may as well be applied according to the present invention.

Suitable ethylene polymerization includes but is not limited to homopolymerization of ethylene, copolymerization of ethylene and a higher 1 -olefin co-monomer. As used herein, the term "co-monomer" refers to olefin co-monomers which are suitable for being polymerized with ethylene monomers. Co-monomers may comprise but are not limited to aliphatic C3-C20 alpha-olefins. Examples of suitable aliphatic C3-C20 alpha- olefins include propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene and 1-eicosene. The term "co-polymer" refers to a polymer, which is made by linking two different types of in the same polymer chain. The term "homo-polymer" refers to a polymer which is made by linking ethylene monomers, in the absence of co-monomers. In an embodiment of the present invention, said co-monomer is 1-hexene.

In a preferred embodiment, reactants comprising the monomer ethylene, isobutane as hydrocarbon diluent, a catalyst, the co-monomer 1-hexene are used.

The polymerization can be performed over a wide temperature range. Preferably, the temperature is within the range of about 0°C to about 1 10°C. A more preferred range is from about 60°C to about 100°C, more preferably from about 80 to 1 10°C,

The reactor pressure is preferably held between 20 and 100 bar, 30 to 50 bar, more preferably at pressure of 37 to 45 bar. In an embodiment, the slurry flow can be set between 5 and 15 m/s.

The pelleted polyethylene product of the invention can be easily produced under flexible processing conditions by using the pelleted first polyethylene resin, while leading to homogeneous pelleted polyethylene product. The method provides advantages such as easy mixing; and easy of processing. The present invention also provides improved master batches, by improving the average MFI and/or the average density. The invention allows preparing polyethylene products with tailor made properties.

The following non-limiting example illustrates the invention.

Example

A first polyethylene resin is produced in a single loop in the presence of a metallocene catalyst. Ethylene was injected with 1-hexene together with the catalyst. The metallocene catalyst used was an activated supported ethylene bis (tetrahydro-indenyl) zirconium dichloride. Isobutane was used as diluent. The polymerization conditions are indicated in Table 1. The produced first polyethylene resin has a density of 0.960 g/cm³ and a Ml₂ of 8g/10 min with the Melt Index Ml₂ being measured by the procedure of ASTM D-1238 using a temperature of 190°C and a load of 2.16 kg. Table 1

TIBAL = triisobutylaluminium

The first polyethylene resin is extruded with 40% by weight of carbon black pigment, and 4.5% by weight of anti-oxidant, 2.7% by weight of anti-acid, into a pelleted first polyethylene resin.

A second polyethylene fluff is produced in a double loop reactor in the presence of a Ziegler-Natta. A Ziegler-Natta catalyst slurry (0.7% by weight; D₅₀ of 5μηι) was fed under 200-240 kg/h isobutane flushing into a double loop reactor. TEAI activating agent (concentration of 20% by weight; at 50 ppm in the reactor) was fed with 100kg/h isobutane. Ethylene monomer was fed at 5-10°C. The reactor temperature was kept at 90°C.

The Ziegler-Natta catalyst of D₅₀ of 5 μηι was prepared as described herein:

Step! BEM/TEAI (1 :0.03) + 2-ethylhexanol (2-EHOH) →Mg(2-EHO)₂; wherein BEM= MgRR' with RH and R'H are butane and ethane, respectively;

Step 2. Mg(2-EHO)₂ + CITi(OiPr)₃→ product A

Step 3. Product A + 2TiCI₄/titanium (IV) butoxide (TNBT)→product B

Step 4. Product B + TiCI₄→product C

Step 5. Product C + TiCI₄→ Product D

Step 6. Product D + TEAI→catalyst 5μηι The second polyethylene fluff produced has a density of 0.958 g/cm³ and HLMI of 8 g/10 min.

5% by weight of the pelleted first polyethylene resin is physically blended with the second polyethylene fluff to produce a pelleted polyethylene product.

The pelleted final product displayed good mechanical properties in particular for pipe application.

Claims

1. Method of preparing a pelleted polyethylene product by:

(i) producing a first polyethylene resin in the presence of a metallocene catalyst, said first polyethylene resin having a density of from 0.940 to 0.970 g/cm³ as measured with ASTM D-1505 standardized test at a temperature of 23°C;

(ii) extruding the first polyethylene resin with at least one additive selected from pigments, UV-stabilizers, anti-oxidants, fillers, anti-static agents, dispersive aid agents, processing aids, anti-acid agents, and mixtures thereof, into a pelleted first polyethylene resin;

2. Method according to claim 1 , wherein said pelleted first polyethylene resin of step ii) has a monomodal molecular weight distribution.

3. Method according to any of claims 1 to 2, wherein the first polyethylene resin is extruded with a pigment in step (ii).

4. Method according to any of claims 1 to 3, wherein said second polyethylene resin of step iii) has a bimodal molecular weight distribution.

5. Method according to any of claims 1 to 4, wherein the pelleted polyethylene product has a High Load Melt Index (HLMI) of from between 2 and 50 g/10 min, with the HLMI being measured by the procedure of ASTM D-1238 using a temperature of 190°C and a load of 21.6 kg.

6. Method according to any of claims 1 to 5, wherein the pelleted polyethylene product has a density of from 0.930 to 0.965 g/cm³ as measured with ASTM D-1505 standardized test at a temperature of 23°C.

7. Method according to any of claims 1 to 6, wherein step (ii) and/or step (iv) are performed in device for continuously melting and blending.

8. Method according to claim 7, wherein said device is an extruder and/or a mixer. Use of pelleted polyethylene resin comprising a first polyethylene resin prepared in the presence of a metallocene catalyst, having a density of from 0.940 to 0.970 g/cm³ as measured with ASTM D-1505 standardized test at a temperature of 23°C, and at least one additive selected from pigments, UV-stabilizers, anti-oxidants, fillers, anti-static agents, for preparing a physical blend comprising from 1 % to 35% by weight of said pelleted polyethylene resin and a second polyethylene resin prepared in the presence of a Ziegler-Natta or Chromium catalyst, with % by weight being based on the total weight of the pelleted polyethylene product.