Process for producing multimodal polyolefin composition in extruder apparatus

Abstract

A continuous process for preparing a multimodal polyolefin composition comprising one or more antioxidants, the process comprising: preparing a multimodal polyolefin in the form of a polyolefin powder having a mass median diameter D50 of polyolefin particles in the range from 300 [mu] m to 2500 [mu] m in a series of polymerization reactors; mixing the polyolefin powder and the organic peroxide without melting the polyolefin powder; melting and homogenizing the mixture in an extruder device; adding one or more antioxidants to the molten polyolefin composition; homogenizing the combination of the molten polyolefin composition and the antioxidant to form an antioxidant-equipped molten polyolefin composition; and granulating the molten polyolefin composition provided with the antioxidant agent.

Description

Technical Field

[0001] The present disclosure provides a continuous process for preparing a multimodal polyolefin composition comprising one or more antioxidants. The present disclosure particularly provides a continuous process for preparing a multimodal polyolefin composition comprising one or more antioxidants from a multimodal polyolefin in the form of polyolefin powder using an organic peroxide. Background Art

[0002] Polyolefins are widely used commercial polymers. In order to achieve and maintain the desired properties, polymers are equipped with additional substances. These so-called plastic additives are auxiliary compounds that significantly influence the properties of the polymer and make the polymer commercially useful, although the additives are added only in small quantities. The additives are usually combined with the polyolefin directly after the polymerization during the pelletizing step. A widely used class of additives is antioxidants. These polymer additives inhibit the degradation of the polymer caused by reaction with atmospheric oxygen. Antioxidants are effective at all stages of the polymer life cycle. Antioxidants allow plastic articles to be produced faster and with fewer defects, extend the service life of the articles, and facilitate recycling.

[0003] A particular class of additives that may be added after the polymerization present in the polyolefin pelletizing step is a free radical generator, such as a peroxide. For example, a free radical generator may be employed for modifying the molecular structure of the polyolefin.

[0004] An important application of polyolefins is the preparation of pipes, and in particular the preparation of pressure pipes, i.e. pipes designed to transport fluids under conditions where the pressure inside the pipe is higher than the pressure outside the pipe. Suitable materials for producing polyolefin pipes having excellent mechanical properties (including good long-term pressure resistance and good chemical resistance) are polyolefin pipes having a density of from 0.940 g / cm 3 Up to 0.968g / cm 3 Bimodal or multimodal polyethylene.

[0005] In order to be suitable for the manufacture of tubes, polyolefins need to have good processability in tube extrusion. Particularly for the production of large diameter tubes, one aspect of this processability is the high resistance of the resulting tube to "sag". "Sag" is the effect during melt extrusion of a tube that a portion of the melt can flow from the upper part of the tube to the lower part of the tube, resulting in unacceptable tube wall thickness variations around the circumference of the tube.

[0006] In order to improve the sag behavior of pipes produced from bimodal or multimodal polyethylene, peroxides can be added in the pelletizing of bimodal or multimodal polyethylene. For example, WO 2013 / 101767A2 discloses the production of a bimodal high density polyethylene polymer composition by feeding a bimodal high density polyethylene base resin and a peroxide into an extrusion device and mixing the polymer composition and the peroxide in the extrusion device until substantially homogeneous.

[0007] WO 2016 / 005044A1 relates to a multimodal polyethylene composition that can be made into a pipe. The multimodal polyethylene composition comprises a high-density multimodal ethylene polymer component and an ultra-high molecular weight ethylene polymer component. The high-density multimodal polyethylene composition is preferably prepared in a compounding step prior to the pipe extrusion step. It is also disclosed that a peroxide masterbatch is added during compounding to induce LCB / crosslinking reactions.

[0008] WO 2016 / 064984A1 relates to crosslinked metallocene-catalyzed polyethylene copolymers, and articles (e.g., pipes) made therefrom. The crosslinked metallocene-catalyzed polyethylene copolymers can be obtained by contacting a polyethylene copolymer base resin with a peroxide modifier. The peroxide modifier can be used as a powder, as a masterbatch, in a carrier (e.g., polypropylene, calcium carbonate, etc.), or a combination thereof.

[0009] WO 2017 / 112642A1 discloses a polyethylene formulation comprising a partially crosslinked multimodal polyethylene composition, which can be extruded into various shaped articles, such as, for example, a pipe. The polyethylene formulation is obtained by combining a multimodal polyethylene resin with a peroxide masterbatch.

[0010] WO 2018 / 022885 A1 discloses a pipe comprising a bimodal high molecular weight high density polyethylene extruded in the presence of one or more organic peroxides. In the method for preparing the pipe, a porous polypropylene random copolymer is used as a carrier for the organic peroxide.

[0011] EP 2 966 1223 A1 discloses a multimodal polyethylene composition which can be manufactured into pipes showing improved pressure resistance, these pipes exhibit improved pressure resistance and creep resistance. Optionally, a free radical generator (such as a peroxide) can be added to produce a modified polymer composition, thereby increasing η 0.05 value.

[0012] EP 3 450 127 A1 discloses a process for preparing a polyolefin composition comprising supplying a bimodal or multimodal polyolefin in the form of a polyolefin powder having a mass median diameter D50 in the range from 400 μm to 2500 μm and one or more additives to a mixing device, mixing the polyolefin powder and the additives, and transferring the mixture to an extruder device to melt and homogenize the mixture.

[0013] Nevertheless, there is still a need to provide polyethylene compositions comprising multimodal polyethylene having high sag resistance and having reduced gel levels. Summary of the invention

[0014] The present disclosure provides a continuous process for preparing a multimodal polyolefin composition comprising one or more antioxidants, the process comprising the following steps:

[0015] a) preparing a multimodal polyolefin in the form of a polyolefin powder having a mass median diameter D50 of the polyolefin particles in the range from 300 μm to 2500 μm, the multimodal polyolefin being a multimodal polyethylene, by polymerizing one or more 1-olefins in the presence of an olefin polymerization catalyst at a pressure of from 0.5 MPa to 10 MPa and at a temperature of from 30°C to 160°C in a series of two or more polymerization reactors;

[0016] b) providing an organic peroxide in an amount of from 20 ppm to less than 100 ppm, preferably from 25 ppm to 95 ppm, more preferably from 28 ppm to 80 ppm, relative to the polyolefin powder;

[0017] c) mixing the polyolefin powder prepared in step a) and the organic peroxide provided in step b) at a temperature ranging from 10° C. to 100° C. without melting the polyolefin powder to form a polyolefin peroxide mixture;

[0018] d) melting and homogenizing the polyolefin peroxide mixture obtained in step c) in an extruder device, wherein at least 90% by weight of the organic peroxide decomposes and forms a molten polyolefin composition;

[0019] e) adding one or more antioxidants to the molten polyolefin composition obtained in step d) to form a combination of a molten polyolefin composition and an antioxidant;

[0020] f) homogenizing the combination of the molten polyolefin composition and the antioxidant to form a molten polyolefin composition provided with the antioxidant; and

[0021] g) pelletizing the molten polyolefin composition provided with the antioxidant.

[0022] In some embodiments, the multimodal polyolefin in the form of polyolefin powder is produced in one or more gas phase polymerization reactors and at least one gas phase polymerization reactor is a multizone circulation reactor in which one polymerization zone is a riser in which growing polymer particles flow upwards under fast fluidization or transport conditions and the other polymerization zones are subzones of a downcomer in which growing polymer particles flow downwards in a densified form, wherein the riser and the downcomer are interconnected and the polymer particles leaving the riser enter the downcomer and the polymer particles leaving the downcomer enter the riser, thereby establishing a circulation of polymer particles through the riser and the downcomer.

[0023] In some embodiments, the series of polymerization reactors includes a fluidized bed reactor upstream of a multi-zone circulation reactor.

[0024] In some embodiments, the olefin polymerization catalyst is a Ziegler or Ziegler-Natta catalyst comprising the reaction product of an aluminum alkyl and a titanium compound supported on a magnesium halide.

[0025] In some embodiments, the extruder device is a combination of two co-rotating twin-screw extruders, and the polyolefin powder and the organic peroxide are transferred to the first twin-screw extruder, step d) is carried out in the first twin-screw extruder, and the stream of the molten polyolefin composition formed in step d) is provided to the second twin-screw extruder and step f) is carried out, or the extruder device is a counter-rotating continuous mixer equipped with a gear pump or a co-rotating twin-screw extruder equipped with a gear pump.

[0026] In some embodiments, the molten polyolefin composition provided with an antioxidant is passed through a melt filter prior to pelletizing.

[0027] In some embodiments, the multimodal polyolefin in the form of polyolefin powder produced in step a) and the organic peroxide provided in step b) are supplied to a mixing device, and the mixing of step c) is performed in the mixing device.

[0028] In some embodiments the multimodal polyolefin in the form of polyolefin powder produced in step a) is directly supplied to an extruder device or the multimodal polyolefin composition in the form of polyolefin powder produced in step a) is supplied to a mixing device, mixed with one or more additives different from antioxidant and organic peroxide in the mixing device and the mixture comprising polyolefin powder and additives different from antioxidant and organic peroxide is transferred from the mixing device to an extruder device and the organic peroxide provided in step b) is supplied to the extruder device at a position where the polyolefin powder or the mixture comprising polyolefin powder and additives different from antioxidant and organic peroxide has not yet melted.

[0029] In some embodiments, the mixing of step c) is performed in an atmosphere of reduced oxygen content, and the oxygen content in the gas phase within the mixing device is less than 5% by volume.

[0030] In some embodiments, in step b), the organic peroxide is provided as a liquid or as a component of a liquid, or the organic peroxide is provided in the form of a mixture of the organic peroxide and the polyolefin powder.

[0031] In some embodiments, in step b), the organic peroxide is provided as a liquid or as a component of a liquid, and the liquid comprising the organic peroxide is added to the polyethylene powder by passing the liquid through a spring injector comprising a preloaded spring at an injection pressure of 0.5 to 4 MPa.

[0032] In some embodiments, the organic peroxide is provided as a peroxide solution having an active oxygen content ranging from 0.1 wt % to 10 wt %.

[0033] In some embodiments, the multimodal polyolefin produced in step a) has an M from 15 to 40. w / M n .

[0034] In some embodiments, the multimodal polyolefin produced in step a) is a multimodal polyethylene, preferably having a molecular weight ranging from 0.940 g / cm 3 Up to 0.968g / cm 3 Multimodal polyethylene of density.

[0035] In some embodiments, the present disclosure provides a process for preparing a tube, the process comprising the following steps:

[0036] a) preparing a multimodal polyolefin composition by a process as claimed in any one of claims 1 to 14; and

[0037] b) forming the multimodal polyolefin composition into a pipe. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 The reactor set-up for producing the bimodal or multimodal polyethylene employed in the process of the present disclosure is schematically shown.

[0039] Figure 2 A spring-loaded syringe for adding a liquid including an organic peroxide to polyethylene powder is schematically shown.

[0040] Figure 3 The set-up for preparing the polyolefin composition according to the present disclosure is schematically shown, wherein a liquid comprising an organic peroxide is added to a polyolefin powder in a mixing device, and a masterbatch of one or more antioxidants is added to the molten and homogenized mixture of the polyolefin powder and the peroxide solution.

[0041] Figure 4Schematically illustrated is an assembly for preparing a polyolefin composition according to the present disclosure, wherein a liquid comprising an organic peroxide is added to a polyolefin powder in an extruder device, and a masterbatch of one or more antioxidants is added to the molten and homogenized mixture of the polyolefin powder and the peroxide solution.

[0042] Figure 5 A comparative set-up for preparing a polyolefin composition is schematically shown, wherein a liquid comprising an organic peroxide (as a mixture with additives comprising one or more antioxidants) is added to a polyolefin powder, again in a mixing device. DETAILED DESCRIPTION

[0043] The present disclosure provides a continuous process for preparing a multimodal polyolefin composition comprising one or more antioxidants. The process comprises as step a) the step of preparing a multimodal polyolefin in the form of a polyolefin powder.

[0044] Step a) of the process of the present disclosure involves the preparation of multimodal polyethylene by homopolymerization or copolymerization. Preferred comonomers in the polymerization of ethylene are up to 40 wt.% C 3 -C 8 -1-olefins, preferably 1-butene, 1-pentene, 1-hexene, 1-octene or mixtures thereof. Particularly preferred are those wherein ethylene is mixed with from 0.01 to 20% by weight, more preferably from 0.05 to 12% by weight, of one or more C 3 -C 8 - Process for 1-olefin copolymerization. Particularly preferred comonomers for ethylene copolymerization are 1-hexene and / or 1-butene.

[0045] The polyolefin in the form of polyolefin powder according to the present disclosure is a multimodal polyolefin. The term "multimodal" can refer to the mode of molecular weight distribution. Such multimodal polyolefins can be obtained by polymerizing or copolymerizing one or more olefins under different reaction conditions in a series of two or more polymerization reactors, or by adopting a mixed catalyst system including two or more types of polymerization catalysts, which produce polyolefins of different molecular weights under the same polymerization conditions. Therefore, "mode" indicates how many different polymerization conditions are used in the preparation of polyolefins, independently indicating whether the mode of the molecular weight distribution can be considered as the maximum value separated in a gel permeation chromatography (GPC) curve, or how many different types of polymerization catalysts are included in a mixed polymerization catalyst system. The term multimodal, which is often used in the art and is also used herein, can include bimodal. In addition to molecular weight distribution, polyolefin polymers can also have a comonomer distribution, wherein preferably, the average comonomer content of the polymer chain with a higher molecular weight is higher than the average comonomer content of the polymer chain with a lower molecular weight. However, the same or very similar reaction conditions can also be adopted in all polymerization reactors in a series of polymerization reactors, and thereby narrow molecular weight polyolefins can be prepared. However, a complication in producing multimodal polyolefins in a series of polymerization reactors operating under different reaction conditions is that the composition of the individual polyolefin particles of the polyolefin powder may vary drastically due to the different residence times of the individual polyolefin particles in the different reactors.

[0046] All industrially known polymerization processes can be used for the preparation of multimodal polyolefins. These polymerization processes include solution processes, suspension processes and gas phase processes. Processes of this type are generally known to the person skilled in the art. Among the polymerization processes mentioned, gas phase polymerization, in particular in a gas phase fluidized bed reactor or a multizone circulation reactor, and suspension polymerization, in particular in a loop reactor or a stirred tank reactor, are preferred.

[0047] The polymerization for preparing multimodal polyolefin in step a) of the process of the present disclosure is operated at a pressure of from 0.5 MPa to 10 MPa, preferably from 1.0 MPa to 8 MPa, in particular from 1.5 MPa to 4 MPa, wherein these pressures, like all pressures given in the present disclosure, have to be understood as absolute pressures, i.e. pressures with MPa dimensions (abs). The polymerization is carried out at a temperature of from 30°C to 160°C, and preferably from 65°C to 125°C, wherein temperatures in the upper part of the range are preferred for preparing relatively high density ethylene polymers, and temperatures in the lower part of the range are preferred for preparing lower density ethylene copolymers.

[0048] The multimodal polyolefin in the form of a polyolefin powder prepared in step a) of the process of the present disclosure is prepared by polymerizing one or more 1-olefins in a series of two or more polymerization reactors. Preferably, at least one polymerization reactor is a gas phase polymerization reactor. In a preferred embodiment, the multimodal polyolefin is prepared in a series of two or more gas phase polymerization reactors, preferably in a series of two or more fluidized bed reactors, or in a series comprising a multizone circulation reactor and one or more different gas phase polymerization reactors such as a fluidized bed reactor; or the multimodal polyolefin is prepared in a series of first one or more suspension polymerization reactors such as a loop reactor, and subsequently in one or more gas phase polymerization reactors such as a fluidized bed reactor.

[0049] In a preferred embodiment of the present disclosure, at least one polymerization reactor in a series of polymerization reactors is a gas phase polymerization reactor, and the gas phase polymerization reactor comprises a riser unit and a downcomer, wherein the growing polymer particles flow upward under fluidization, fast fluidization or conveying conditions, and wherein the downcomer, wherein the growing polymer particles flow downward in a densified form.

[0050] The riser unit in which the growing polymer particles flow upward may include a fluidized bed of growing polymer particles. The riser unit is then operated as a fully mixed gas phase reactor such as a fluidized bed reactor. A fluidized bed reactor is a reactor in which polymerization is carried out in a bed of polymer particles which is maintained in a fluidized state by feeding a reaction gas mixture at the lower end of the reactor, usually below a gas distribution grid having a function of distributing the gas flow, and removing the gas again at its upper end. The reaction gas mixture is then returned to the lower end of the reactor via a recirculation line equipped with a compressor and a heat exchanger for removing the heat of polymerization. The velocity of the reaction gas mixture must be high enough to firstly fluidize the mixed bed of finely divided polymers present in the tubes serving as the polymerization zone, and secondly to effectively remove the heat of polymerization.

[0051] When a polymerization unit comprising a fluidized bed of growing polymer particles is used as the riser unit, the downcomer may be located within, around or adjacent to the gas phase reactor. It is also possible to use two or more separate polymerization units as downcomers, wherein the growing polymer particles flow downward in a densified form.

[0052] In a particularly preferred embodiment of the present disclosure, the riser unit is a riser in which the upward movement of the growing polymer particles occurs under fast fluidization or transport conditions. Fast fluidization conditions in the riser are established by feeding the reaction gas mixture at a speed higher than the transport speed of the polymer particles. The speed of the reaction gas mixture is generally included between 0.5m / s and 15m / s, preferably between 0.8m / s and 5m / s. The terms "transport speed" and "fast fluidization conditions" are well known in the art. For the definition of these terms, see, for example, "D. Geldart, "Gas Fluidization Technology", page 155 and below, J. Wiley & Sons Ltd., 1986".

[0053] The part of the polymerization reactor where the growing polymer particles flow downward in a densified form can be called a "downcomer", but it can also be designated as a "moving bed" or "settled bed" unit or reactor. The term "densified form" is understood to mean that the ratio between the mass of polymer and the reactor volume is higher than 80% of the "cast bulk density" of the resulting polymer. Thus, for example, at a polymer bulk density equal to 420 kg / m 3 In the case of "densified form" it is meant that the polymer mass / reactor volume ratio is at least 336 kg / m 3 The "poured bulk density" of a polymer is a parameter well known to the person skilled in the art: it can be measured in accordance with DIN EN ISO 60: 1999. The density of the solids in the reactor is defined as the mass of polymer per volume of the reactor occupied by the polymer.

[0054] Typically, a downcomer is a portion of a polymerization reactor containing a bed of growing polymer particles that moves downwardly in a substantially plug flow mode. "Plug flow mode" means that there is little or preferably no backmixing of the polymer particles.

[0055] In order to replace the reacted olefins and control the gas flow in the downcomer, a gas or liquid feed stream can be introduced into the downcomer at one or more positions. The feed stream preferably includes ethylene and can also include one or more comonomers, inert components such as propane or hydrogen. According to the amount of the gas or liquid feed stream added to the downcomer and the pressure conditions in the downcomer, the gas medium surrounding the polymer particles can be designed to move downwards simultaneously with the polymer particles or move upwards countercurrently with the polymer particles. When the liquid stream is fed to the downcomer, these liquid streams are preferably evaporated in the downcomer, which contributes to the composition of the reaction gas mixture in the downcomer. When the downcomer is operated with more than one feed stream, the feed point for introducing the feed stream into the downcomer is preferably evenly distributed on the height of the downcomer.

[0056] In the most preferred embodiment of the present disclosure, one of the polymerization reactors is a multi-zone circulation reactor. Such reactors are described, for example, in WO 97 / 04015A1 and WO 00 / 02929A1 and have two interconnected polymerization zones: a riser and a downcomer, in which the growing polymer particles flow upward under fast fluidization or conveying conditions, and in which the growing polymer particles flow in a densified form under gravity. The polymer particles leaving the riser enter the downcomer, and the polymer particles leaving the downcomer are reintroduced into the riser, thereby establishing a circulation of the polymer between the two polymerization zones, and the polymer alternately passes through the two zones multiple times. In such polymerization reactors, a solid / gas separator is arranged above the downcomer to separate the polyolefin and reaction gas mixture from the riser. The growing polyolefin particles enter the downcomer, and the separated reaction gas mixture of the riser is continuously recycled to one or more points reintroduced into the polymerization reactor through a gas recirculation line. Preferably, most of the recirculated gas is recycled to the bottom of the riser. The recirculation line is preferably equipped with a compressor and a heat exchanger for removing polymerization heat. Preferably, the line for catalyst feed is arranged on the riser and the polymer discharge system is located in the bottom part of the downcomer.The introduction of make-up monomer, comonomer, hydrogen and / or inert components may occur at different points along the riser and downcomer.

[0057] According to a preferred embodiment of the present disclosure, the reaction gas mixture leaving the riser unit is partially or completely prevented from entering the downcomer to establish different polymerization conditions between the riser unit and at least a portion of the downcomer. This can be achieved, for example, by feeding a spacer fluid in the form of a gas and / or liquid mixture into the downcomer, preferably in the upper part of the downcomer. The spacer fluid should have a suitable composition different from the gas mixture present in the riser unit. The amount of spacer fluid added can be adjusted in such a way that an upward gas flow countercurrent to the flow of the polymer particles is generated, in particular at the top thereof, to act as a barrier to the gas mixture entrained in the particles from the riser unit.

[0058] The spacer fluid preferably comes from the recycle gas stream and is more preferably obtained by partial condensation of this stream. Thus, in addition to the monomers to be polymerized, the spacer fluid may also contain inert compounds used as polymerization diluents, such as nitrogen or alkanes having from 1 to 10 carbon atoms, hydrogen or other components of the reaction gas mixture.

[0059] The polymerization in a gas phase polymerization reactor of a series of polymerization reactors can also be carried out in condensing or supercondensing mode, in which part of the circulating reaction gas mixture is cooled below the dew point and returned to the reactor separately as liquid phase and gas phase or together as a two-phase mixture, in order to additionally utilize the enthalpy of evaporation for cooling the reaction gases.

[0060] In a preferred embodiment of the present disclosure, wherein a series of polymerization reactors comprises a gas phase polymerization reactor comprising a riser unit and a downcomer, the other polymerization reactors in the series of polymerization reactors may be any type of low pressure polymerization reactor, such as a gas phase reactor or a suspension reactor. If the polymerization process of a series of polymerization reactors comprises suspension polymerization, the suspension polymerization is preferably carried out upstream of the gas phase polymerization. Suitable reactors for carrying out such suspension polymerization are, for example, loop reactors or stirred tank reactors. Suitable suspension media are in particular inert hydrocarbons such as isobutane or a mixture of hydrocarbons or the monomers themselves. Such additional polymerization stages carried out in suspension may also include a prepolymerization stage. If the multi-stage polymerization of olefins comprises an additional polymerization stage carried out in the gas phase, the additional gas phase polymerization reactor may be any type of gas phase reactor, such as a horizontally or vertically stirred gas phase reactor, a fluidized bed reactor or a multi-zone circulation reactor. Such additional gas phase polymerization reactors may be arranged downstream or upstream of the gas phase polymerization reactor. In a particularly preferred embodiment of the present disclosure, the gas phase polymerization reactor comprising a riser unit and a downcomer is part of a series of polymerization reactors, wherein the fluidized bed polymerization reactor is arranged upstream of the gas phase polymerization reactor.

[0061] Figure 1 The set-up of a series of polymerisation reactors comprising a fluidised bed reactor and a multizone circulation reactor for producing a multimodal polyolefin according to step a) of the process of the present disclosure is schematically shown.

[0062] The first gas phase reactor, a fluidized bed reactor (1), comprises a fluidized bed (2) of polyolefin particles, a gas distribution grid (3) and a speed reduction zone (4). The speed reduction zone (4) generally has an increased diameter compared to the diameter of the fluidized bed portion of the reactor. The polyolefin bed is kept in a fluidized state by an upwardly flowing gas which is fed through a gas distribution grid (3) placed at the bottom portion of the reactor (1). The gas stream of the reaction gas mixture leaving the top of the speed reduction zone (4) via a recycle line (5) is compressed by a compressor (6), transferred to a heat exchanger (7) where it is cooled and then recirculated to the bottom of the fluidized bed reactor (1) at a point below the gas distribution grid (3) at position (8). If appropriate, the recycle gas can be cooled in the heat exchanger to below the dew point of one or more of the recycle gas components in order to operate the reactor with condensed material, i.e. in condensing mode. In addition to unreacted monomer, the recycle gas may include inert condensable gases, such as alkanes, and inert non-condensable gases, such as nitrogen. Make-up monomer, hydrogen and optional inert gases or process additives may be fed into the reactor (1) at various locations, for example via line (9) upstream of the compressor (6). Typically, the catalyst is fed into the reactor (1) via line (10) which is preferably placed in the lower part of the fluidized bed (2).

[0063] The polyolefin particles obtained in the fluidized bed reactor (1) are discontinuously discharged via pipeline (11) and fed to a solid / gas separator (12) in order to avoid the gas mixture from the fluidized bed reactor (1) entering the second gas phase reactor. The gas leaving the solid / gas separator (12) leaves the reactor as waste gas via pipeline (13), while the separated polyolefin particles are fed to the second gas phase reactor via pipeline (14).

[0064] The second gas phase reactor is a multi-zone circulation reactor (21) comprising a riser (22) and a downcomer (23) through which the polyolefin particles repeatedly pass. In the riser (22), the polyolefin particles flow upward in the direction of arrow (24) under fast fluidization conditions. In the downcomer (23), the polyolefin particles flow downward in the direction of arrow (25) under the action of gravity. The riser (22) and the downcomer (23) are suitably interconnected by interconnecting bends (26) and (27).

[0065] After flowing through the riser (22), the polyolefin particles and the reaction gas mixture leave the riser (22) and are transported to a solid / gas separation zone (28). This solid / gas separation can be achieved by using conventional separation devices such as centrifugal separators such as cyclone separators. From the separation zone (28), the polyolefin particles enter the downcomer (23).

[0066] The reaction gas mixture leaving the separation zone (28) is recycled to the riser (22) via a recycle line (29) equipped with a compressor (30) and a heat exchanger (31). Between the compressor (30) and the heat exchanger (31), the recycle line (29) splits and the gas mixture is divided into two separate streams: the line (32) conveys a part of the recycle gas into the interconnecting bend (27), while the line (33) conveys another part of the recycle gas to the bottom of the riser (22) in order to establish fast fluidization conditions therein.

[0067] The polyolefin particles coming from the first gas phase reactor via the pipeline (14) enter the multi-zone circulation reactor (21) at the interconnection bend (27) at the position (34). The polyolefin particles obtained in the multi-zone circulation reactor (21) are continuously discharged from the bottom part of the downcomer (23) via the discharge pipeline (35).

[0068] A portion of the gas mixture leaving the separation zone (28) leaves the recirculation line (29) after having passed through the compressor (30) and is sent via the line (36) to the heat exchanger (37) in which a portion of the gas mixture is cooled to a temperature at which the monomers and optionally the inert gases are partially condensed. A separation vessel (38) is located downstream of the heat exchanger (37). The separated liquid is withdrawn from the separation vessel (38) via the line (39) and fed to the downcomer (23) via the lines (40), (41), (42) and (43) by means of a pump (44), wherein the feed stream introduced via the line (40) is supplied to generate a barrier for preventing the reaction gas mixture of the riser (22) from entering the downcomer (23). Make-up monomer, make-up comonomer and optionally inert gas and / or process additives can be introduced into lines (41), (42) and (43) via lines (45), (46) and (47) and then fed into downcomer (23) at monomer feed points (48), (49) and (50). Make-up monomer, make-up comonomer and optionally inert gas and / or process additives can also be introduced into recycle line (29) via line (51). The gas mixture obtained as a gas phase in separation vessel (38) is recycled to recycle line (29) via line (52).

[0069] The bottom of the downcomer (23) is provided with a control valve (53) having an adjustable opening for regulating the flow of polyolefin particles from the downcomer (23) through the interconnecting bend (27) into the riser (22). Above the control valve (53), an amount of a recycle gas mixture from the recycle line (29) via the line (54) is introduced into the downcomer (23) to promote the flow of polyolefin particles through the control valve (53).

[0070] The polymerization for preparing multimodal polyolefins can be carried out using all conventional olefin polymerization catalysts. This means that the polymerization can be carried out, for example, using a Phillips catalyst based on chromium oxide, using a Ziegler or Ziegler-Natta catalyst based on titanium, using a single active site catalyst or using a mixture of such catalysts. The preparation and use of these catalysts for olefin polymerization are generally known. In a preferred embodiment of the present disclosure, the polymerization catalyst is a Ziegler or Ziegler-Natta catalyst comprising the reaction product of an aluminum alkyl and a titanium compound supported on a magnesium halide.

[0071] The multimodal polyolefin produced in step a) of the process of the present disclosure is obtained in the form of a polyolefin powder, which means in the form of relatively small particles. The mass median diameter D50 of these polyolefin particles is in the range of from 300 μm to 2500 μm, preferably from 400 μm to 2300 μm, and in particular from 800 μm to 2100 μm, determined by dry sieve analysis according to DIN 53477 (November 1992).

[0072] The multimodal polyolefin produced in step a) of the process of the present disclosure preferably has a molecular weight of from 0.940 to 0.968 g / cm 3 Preferably, the density is between 0.945 and 0.965 g / cm 3 In the range of 0.945 to 0.955 g / cm 3 Density must be understood as the density determined in accordance with DIN EN ISO 1183-1:2004, method A (immersion) with 2 mm thick compression mouldings which were pressed at 180° C., 20 MPa for 8 minutes and subsequently crystallized in boiling water for 30 minutes.

[0073] The preferred polyolefin produced in step a) of the process of the present disclosure is a multimodal polyethylene having a melt flow rate MFR of from 0.5 to 300 g / 10 min, more preferably from 1 to 100 g / 10 min, even more preferably from 1.2 to 100 g / 10 min, and in particular from 1.5 to 50 g / 10 min, measured according to DIN EN ISO 1133:2005 condition G at 190°C under a load of 21.6 kg. 21.6 Preferred multimodal polyethylenes have an M of from 15 to 40, especially from 20 to 35. w / M n , where M w is the weight average molecular weight, and M n is the number average molecular weight, measured by GPC (gel permeation chromatography). GPC is preferably a high temperature gel permeation chromatography performed as described in ISO 16014-1, -2, -4, 2003, wherein 1,2,4-trichlorobenzene (TCB) is used as solvent, the temperature of the apparatus and the solution is 135°C, and an infrared detector capable of being used with TCB is used as a concentration detector.

[0074] In step b) of the process of the present disclosure, an organic peroxide is provided in an amount of from 20 ppm to less than 100 ppm of the organic peroxide relative to the polyolefin powder. Examples of suitable organic peroxides are dicumyl peroxide, di-tert-butyl peroxide, tert-butyl peroxybenzoate, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, 3,6,9-triethyl-3,6,9-trimethyl-1,2,4,5,7,8-hexoxylanthanide, representative 3,6,9-trimethyl-3,6,9-tri(alkyl)-1,2,4,5,7,8-hexoxylanthanide wherein the alkyl group is propyl or ethyl, tert-butyl peroxyneodecanoate, tert-amyl peroxypivalate, 1,3-bis(tert-butylperoxyisopropyl)benzene, and the like. Preferably, the organic peroxide is 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, 3,6,9-triethyl-3,6,9-trimethyl-1,2,4,5,7,8-hexanone or a representative 3,6,9-trimethyl-3,6,9-tri(alkyl)-1,2,4,5,7,8-hexanone wherein the alkyl radical is propyl or ethyl.

[0075] In a preferred embodiment of the present disclosure, the organic peroxide is employed in the form of a polyolefin mixture, which is preferably prepared by adding the organic peroxide in pure form or as a solution of the organic peroxide in a diluent such as a hydrocarbon to a polyolefin powder. In the polyolefin mixture, the preferred polyolefin mixture has an organic peroxide content in the range of from 0.5 to 25 wt%, preferably from 1 to 20 wt%, and more preferably from 2 to 10 wt%.

[0076] In a particularly preferred embodiment of the present disclosure, a liquid comprising an organic peroxide is provided in step b) of the process of the present disclosure. The liquid comprising an organic peroxide may be a liquid peroxide, or the liquid comprising an organic peroxide may be a solution of a peroxide. Preferably, the liquid comprising an organic peroxide is a solution of a peroxide. The liquid comprising an organic peroxide is provided in an amount of an organic peroxide from 20ppm to less than 100ppm, preferably from 25ppm to 95ppm, more preferably from 28ppm to 80ppm or from 40ppm to 95ppm, and particularly from 45ppm to 95ppm relative to polyolefin powder. In other words, the liquid comprising an organic peroxide is added in an amount corresponding to an addition amount of an organic peroxide from 20ppm to less than 100ppm, preferably from 25ppm to 95ppm, more preferably from 28ppm to 80ppm or from 40ppm to 95ppm; and particularly from 45ppm to 95ppm. Preferably, the organic peroxide is adopted in the form of a solution in a diluent such as a hydrocarbon. Preferred peroxide solutions have an active oxygen content in the range from 0.1 to 10 wt %, preferably from 0.2 to 5 wt %, more preferably from 0.3 to 2 wt %, and in particular from 0.5 to 1 wt %.

[0077] Some steps of preparing the polyolefin composition including one or more antioxidants of the present disclosure are carried out in an extruder device. Suitable extruder devices for the process of the present disclosure are extruders or continuous mixers. These extruders or mixers can be single-stage or two-stage machines, which melt and homogenize the polyethylene composition. Examples of extruders are pin extruders, planetary extruders or co-rotating disc processors. Other possibilities are the combination of a mixer with a discharge screw and / or a gear pump. Preferred extruders are screw extruders, and in particular extruders that are constructed to be twin-screw machines. Twin-screw extruders and continuous mixers with discharge elements are particularly preferred, and in particular continuous mixers with counter-rotating twin rotors, or the extruder device comprises at least one co-rotating twin-screw extruder. Machines of this type are conventional in the plastics industry and are manufactured, for example, by Coperion GmbH, Stuttgart, Germany; KraussMaffei Berstorff GmbH, Hannover, Germany; The Japan Steel Works LTD., Tokyo, Japan; Farrel Corporation, Ansonia, USA; or Kobe Steel, Ltd., Kobe, Japan. Suitable extruder devices are usually also equipped with a unit for pelletizing the melt, such as an underwater pelletizer.

[0078] In a particularly preferred embodiment of the present disclosure, the extruder device is a combination of two co-rotating twin-screw extruders, wherein the mixture comprising polyolefin powder and organic peroxide is transferred to the first twin-screw extruder, the molten polymer stream from the first twin-screw extruder is provided to the second twin-screw extruder, and the second twin-screw extruder completes the homogenization process. In other particularly preferred embodiments of the present disclosure, the polyethylene composition is prepared in a counter-rotating continuous mixer equipped with a gear pump or in a co-rotating twin-screw extruder equipped with a gear pump.

[0079] Step c) of the process of the present disclosure provides mixing the polyolefin powder prepared in step a) and the organic peroxide in step b) at a temperature ranging from 10° C. to 100° C. without melting the polyethylene powder. Preferably, the mixing of step c) is carried out at a temperature ranging from 20° C. to 90° C., more preferably from 60° C. to 80° C.

[0080] Preferably, the mixing of step c) is carried out in an atmosphere with a reduced oxygen content, which is preferably achieved by injecting nitrogen into the mixing device. Preferably, the oxygen content in the gas phase in the mixing device is less than 5% by volume, more preferably less than 1% by volume, and in particular less than 0.2% by volume.

[0081] In preferred embodiments of the present disclosure, the mixing of step c) is carried out in a mixing device. In these embodiments, the multimodal polyolefin in the form of a polyolefin powder prepared in step a) and the organic peroxide provided in step b) are supplied to the mixing device for mixing. The mixing device can be any device that allows dry mixing of particles. The mixing device can be operated continuously or discontinuously. Preferably, the mixing device is operated continuously.

[0082] A preferred dry mixing device is a paddle mixer, which comprises one or two horizontally oriented rotating shafts, more preferably two horizontally oriented counter-rotating shafts. The shafts are equipped with paddles of appropriate geometry. The rotating shafts move the composition of polyethylene powder and additives horizontally along the axis of the shafts and simultaneously intensively mix the composition. Such paddle mixers are commercially available, for example, from Adenau, Germany. Co., Ltd. or J. Engelsmann AG, Ludwigshafen, Germany. Preferred mixing devices are other vertical batch mixers, such as the Henschel- The preferred dry blending device is also a single screw conveyor equipped with a blending element. Preferably, such blending elements are modifiable devices, such as adjustable paddles or grooved screw flights, to allow control of the blending level.

[0083] In other preferred embodiments of the present disclosure, the mixing of step c) is carried out in an extruder device. In these embodiments, the multimodal polyolefin in the form of a polyolefin powder prepared in step a) and the organic peroxide provided in step b) are supplied to the extruder device for mixing. Preferably, the polyolefin powder is transferred to the hopper of the extruder device and then introduced into the extruder device from the hopper. Preferably, the polyolefin powder is transferred to the extruder device by gravity. Preferably, the introduction of the polyolefin powder into the extruder device occurs in an atmosphere with a reduced oxygen content, which is preferably achieved by injecting nitrogen into the hopper. Preferably, the oxygen content in the gas phase in the hopper is less than 5% by volume, more preferably less than 1% by volume, and in particular less than 0.2% by volume.

[0084] In a preferred embodiment of the present disclosure, the organic peroxide is provided in liquid form, and the liquid comprising the organic peroxide is added to the polyethylene powder by passing the liquid through a spring injector comprising a preloaded spring at an injection pressure of from 0.5 to 4 MPa, more preferably from 1 to 2 MPa, and feeding the liquid into a mixing device or an extruder device.

[0085] Figure 2 Schematically illustrated is a spring injector for adding a liquid comprising an organic peroxide to polyethylene powder. The spring injector (61) consists of a body (62) comprising an injection hole (63), an internal needle (64), a sealing device (65), a compression spring (66) and a compression adjustment knob (67) for setting a spring preload, and contains an injection chamber (68). The liquid comprising an organic peroxide is continuously fed into the injection chamber (68) through a feed tube (69). The compression spring (66) pushes the needle (64) against the injection hole (63) according to the preload of the compression spring (66). Once the pressure in the injection chamber (68) overcomes the preload on the compression spring (66), the peroxide solution is finely sprayed on the polyethylene powder. Additionally, the preload of the compression spring (66) prevents powdery materials such as polyethylene fines or additives from entering the injection chamber (68), thereby causing potential blockage.

[0086] In an extruder device, in step d), a mixture comprising polyolefin powder and organic peroxide is melted and homogenized to form a molten polyethylene composition. This preferably occurs by applying heat and mechanical energy to the polyolefin peroxide mixture. The conditions in the extruder device and the chemical structure of the organic peroxide are selected in such a way that at least 90% by weight, more preferably at least 95% by weight, and in particular from 98% to 100% by weight of the organic peroxide decompose.

[0087] Subsequently, in step e) of the process of the present disclosure, one or more antioxidants are added to the molten polyolefin composition obtained in step d) to form a combination of a molten polyolefin composition and an antioxidant.

[0088] The subsequent step f) of the process of the present disclosure is a step of homogenizing the combination of the molten polyolefin composition and the antioxidant to form a molten polyolefin composition provided with the antioxidant. Step f) results in further homogenization of the polyolefin melt itself and also provides a uniform distribution of the one or more antioxidants in the polyethylene melt.

[0089] In a preferred embodiment of the present disclosure, the extruder device is a combination of two co-rotating twin-screw extruders, and the polyolefin powder and the organic peroxide are transferred to the first twin-screw extruder, step d) is carried out in the first twin-screw extruder, and the molten polyolefin composition stream formed in step d) in the first twin-screw extruder is supplied to the second twin-screw extruder and step f) is carried out. In other preferred embodiments of the present disclosure, the extruder device is a counter-rotating continuous mixer equipped with a gear pump or a co-rotating twin-screw extruder equipped with a gear pump.

[0090] Preferably, the specific energy applied in step d) and step f), i.e. the sum of thermal and mechanical energy per amount of material treated, is from 120 kWh / t to 350 kWh / t, more preferably from 160 kWh / t to 300 kWh / t, and in particular from 200 kWh / t to 260 kWh / t.

[0091] In a preferred embodiment of the present disclosure, the molten polyolefin composition equipped with an antioxidant passes through a melt filter before pelletizing. Suitable melt filters include one or more active screens with mesh openings of 50 μm to 400 μm. Particularly preferred melt filters include melt filters including one or more active screens with mesh openings of 75 μm to 200 μm or melt filters including at least two screens with mesh openings of 205 μm to 350 μm. The combination of such screens is described, for example, in WO 2012 / 152775 A1.

[0092] The subsequent step g) of the process of the present disclosure provides for pelletizing the antioxidant-provided molten polyethylene composition comprising the multimodal polyolefin and one or more antioxidants. Thus, step g) converts the molten polyethylene composition into pellets.

[0093] In addition to adding antioxidants and organic peroxides, standard practice in the preparation of polyolefin compositions is to also add other additives. The additive of the suitable additional type for preparing polyolefin compositions is for example melt stabilizer, light stabilizer, acid scavenger, lubricant, processing aid, antiblocking agent, smoothing agent, antistatic agent, antifogging agent, pigment or dye, nucleator, flame retardant or filler. Usually in polyethylene composition, add several additives. Multiple additives can be different types of additives. However, several additives representing a type can also be added in a polyethylene composition. All these types of additives are normally commercially available, and are described in, for example, Hans Zweifel, Plastics Additives Handbook, the 5th edition, Munich, 2001.

[0094] In a preferred embodiment of the process of the present disclosure, further additives other than antioxidants and organic peroxides are added in step e) together with one or more antioxidants to the molten polyolefin composition obtained in step d).

[0095] In other preferred embodiments of the process of the present disclosure, the multimodal polyolefin in the form of polyolefin powder prepared in step a) and other additives other than antioxidants and organic peroxides are supplied to a mixing device and mixed therein to form a polyolefin powder mixture without melting the polyolefin powder. In these embodiments, the organic peroxide provided in step b) can also be supplied to the mixing device. This means that the mixing in the mixing device represents step c) of the process of the present disclosure, and other additives other than antioxidants and organic peroxides are added to the polyolefin powder together with the organic peroxide. However, it is also possible to supply the organic peroxide to the extruder device and not to supply the organic peroxide to the mixing device, and to mix other additives other than antioxidants and organic peroxides with the polyolefin powder in the mixing device to obtain a polyolefin powder additive mixture, which is transferred from the mixing device to the extruder device. This means that, according to this possibility of the process of the present disclosure, step c) of the process of the present disclosure is to mix the organic peroxide provided in step b) with the polyolefin powder additive mixture obtained in the mixing device in the extruder device at a position where the polyolefin powder additive mixture has not yet melted.

[0096] Figure 3 The set-up for preparing the polyolefin composition according to the present disclosure is schematically shown, wherein a liquid comprising an organic peroxide is added to a polyolefin powder in a mixing device, and a masterbatch of one or more antioxidants is added to the molten and homogenized mixture of the polyolefin powder and the peroxide solution.

[0097] The polyolefin powder is provided to a hopper (101) of a dosing device (102) operated by a motor (103). The dosing device (102) feeds the polyolefin powder to a hopper (106) of a screw conveyor (107) operated by a motor (108). The peroxide solution is transferred from a storage container (111) to the screw conveyor (107) by a pump (110) and injected into the screw conveyor (107) at a position (113) downstream of the hopper (109) by a syringe (112).

[0098] The screw conveyor (107) feeds the mixture of polyolefin powder and peroxide solution into the hopper (114) of the first extruder (115) operated by the motor (116). In the extruder (115), the mixture of polyolefin powder and peroxide solution is melted and homogenized, and the peroxide is decomposed.

[0099] The extruder (115) feeds the molten mixture of polyolefin powder after decomposition of organic peroxide into a second extruder (118) operated by a motor (119) via a pipeline (117). A masterbatch prepared from one or more antioxidants and polyolefin powder is provided to a hopper (131) of a dosing device (132) operated by a motor (133), and the dosing device (132) feeds the masterbatch into a hopper (134) of the second extruder (118). In the extruder (118), the masterbatch fed into the hopper (134) is melted, and a mixture of the molten masterbatch and the melt provided by the extruder (115) is homogenized. The homogenized melt passes through a starter valve (120) and a melt filter (121) and is transferred to an underwater pelletizer (122) operated by a motor (123) to form pellets.

[0100] Figure 4 Schematically illustrated is an assembly for preparing a polyolefin composition according to the present disclosure, wherein a liquid comprising an organic peroxide is added to a polyolefin powder in an extruder device, and a masterbatch of one or more antioxidants is added to the molten and homogenized mixture of the polyolefin powder and the peroxide solution.

[0101] In addition, the peroxide solution is transferred from the storage container (111) to the first extruder (115) by the pump (110) and injected into the extruder (115) by the injector (125) at the position (126) downstream of the hopper (114) where the polyolefin powder has not yet melted. Figure 4 The assembly and Figure 3The assembly for preparing the polyolefin composition shown is the same. In addition, a mixture of additives that are neither organic peroxides nor antioxidants is provided to a hopper (141) of another dosing device (142) operated by a motor (143). The dosing device (142) feeds the mixture of additives that are neither organic peroxides nor antioxidants into a hopper (109) of a screw conveyor (107) located downstream of the hopper (106).

[0102] Figure 5 A comparative set-up for preparing a polyolefin composition is schematically shown, wherein a liquid comprising an organic peroxide (as a mixture with additives comprising one or more antioxidants) is added to a polyolefin powder, again in a mixing device.

[0103] The process of the present disclosure allows the preparation of polyolefin compositions including multimodal polyolefins from polyolefin powders having a mass median diameter D50 of polyolefin particles in the range from 400 μm to 2500 μm, these polyolefin compositions having an excellent combination of properties including good sag behavior and low gel content of pipes prepared from these polyolefin compositions. Furthermore, the process can be carried out continuously in an economical manner.

[0104] In another aspect, the present disclosure provides a process for preparing a pipe, the method comprising the steps of: a) preparing a polyolefin composition by a process according to the present disclosure, and b) forming the polyolefin composition into a pipe. The pipe is preferably a pipe having a SAG value of from 10 to 17.

[0105] Examples

[0106] Melt flow rate MFR was determined according to DIN EN ISO 1133-1:2012-03 at a temperature of 190° C. and a load of 5 kg. 5 .

[0107] The SAG value is determined in a rotating parallel plate rheometer with a shear stress of 300 Pa for 5200 seconds at 230°C with a plate opening of 1 mm. The SAG value is defined as 1 / 10 of the percent creep strain measured at the end of the test. Lower SAG values ​​correspond to more uniform tube thickness.

[0108] The gel count and gel area were determined by preparing cast films, analyzing film defects with the aid of an optical scanning device, and classifying and counting film defects according to their size (circular diameter). The films were prepared by an extruder (Collin 25 model) equipped with a cooling roll and winder (model collin cooling roll 144 / 230) and analyzed by an optical film surface analyzer, model FSA100 (manufactured by OCS Optical Control Systems GmbH, Witten, Germany) with a flash camera system. The device has the following features

[0109] -Screw diameter: 25mm;

[0110] - Screw length: 25D;

[0111] -Compression ratio: 3.5:1;

[0112] - Screw arrangement 27D: 9D for feeding, 7D for compression, 9D for metering;

[0113] - Die width (slit die): 150mm;

[0114] - Resolution: 25μm×25μm;

[0115] And operate under the following conditions

[0116] -T 1 200℃;

[0117] -T2 210℃;

[0118] -T 3 220℃;

[0119] -T 4, 5 (adapter) 230℃;

[0120] -T 6, 7, 8 (mold) 240℃;

[0121] - Take-out speed 3.0m / min;

[0122] - Adjust the screw speed to a film thickness of 50 μm;

[0123] -Throughput 1.0 to 1.5 kg / h (target 1.15 kg / h);

[0124] - Air shower room open,

[0125] - Cooling roller temperature 60°C;

[0126] - Camera Thresholds Threshold 1: 50% to Threshold 2: 45%.

[0127] To start the measurement, the extruder and take-off unit are set to specific conditions and start with a material with a known gel level. When the extruder shows stable conditions for temperature and melt pressure, start the film inspection software. After operating the extruder for at least half an hour with the starting material or after the gel count reaches a known gel level, feed the first sample to be measured into the extruder. After 45 minutes of reaching a stable gel level, start the counting process until the camera has checked at least 3m 2 After this, the next sample was fed into the extruder and the counting process for the next sample was started 45 minutes after a stable gel count was reached again. The camera was used to check at least 3 m 2 The counting process was set for all samples in the form of the area of ​​the film and the number of defects measured in each size category was normalized to 1 m 2 membrane.

[0128] The determination of the particle size distribution of the produced polyolefin powder was carried out by dry sieve analysis using an AS200 controlled vibrating sieve vibrator (Retsch GmbH, Haan, Germany) with a set of 7 calibrated sieves (125 μm, 250 μm, 500 μm, 710 μm, 1000 μm, 1400 μm and 2000 μm) according to DIN 53477 (November 1992). After weighting the fractions on each sieve, the mass median diameter D50 of the polyolefin particles was obtained by calculating the particle size distribution using the software Easy Sieve 4.0 of the device.

[0129] Example 1

[0130] In such Figure 1 Polyethylene is produced in a series of fluidized bed reactors and a multi-zone circulating reactor (MZCR) having two interconnected reaction zones as shown.

[0131] 12 g / h of the Ziegler-Natta catalyst prepared according to Example 6 of WO2018 / 114453A1 was fed into a first stirred pre-contact vessel using 0.7 kg / h of liquid propane, and triisobutylaluminum (TIBA) and diethylaluminum chloride (DEAC) were also added thereto. The weight ratio of TIBA to DEAC was 7:1. The weight ratio of alkylaluminum to catalyst solid was 5:1. The first pre-contact vessel was maintained at 50°C with a residence time of 30 minutes. The catalyst suspension of the first pre-contact vessel was continuously transferred to a second stirred pre-contact vessel, which was operated with a residence time of 30 minutes and maintained at 50°C. The catalyst suspension was then continuously transferred to a fluidized bed reactor (1) via pipeline (10).

[0132] In a fluidized bed reactor (1), ethylene is polymerized in the presence of propane as an inert diluent using hydrogen as a molecular weight regulator. 50 kg / h of ethylene and 230 g / h of hydrogen are fed to the fluidized bed reactor (1) via line (9). No comonomer is added. The polymerization is carried out at a temperature of 80°C and a pressure of 2.9 MPa.

[0133] The polyethylene obtained in the fluidized bed reactor (1) has an MFR of 80 g / 10 min. 2.16 and 0.967 g / cm 3 density.

[0134] The polyethylene obtained in the fluidized bed reactor (1) is continuously transferred to a multizone circulation reactor (21) operating at a pressure of 2.5 MPa and a temperature of 80°C, measured at the beginning of the line (29) where the reaction gas mixture has just left the separation zone (28). The riser (22) has an internal diameter of 200 mm and a length of 19 m. The downcomer (23) has a total length of 18 m, divided into an upper part of 5 m with an internal diameter of 300 mm, a lower part of 13 m with an internal diameter of 150 mm, and a conical part with a length of 0.43 m between the upper and lower parts. The final polymer is discharged discontinuously via the line (35).

[0135] A liquid stream of 50 kg / h is fed as a spacer fluid to the upper part of the downcomer (23) via the pipeline (40). The liquid for generating the spacer originates from a recycle gas mixture partially condensed in a heat exchanger (37) under operating conditions of 60° C. and 2.5 MPa and the liquid and gas components are separated in a separation vessel (38). The composition of the liquid spacer fluid is such that the gas generated by evaporating the spacer fluid has 4.2% by volume of ethylene, 0.02% by volume of hydrogen, 0.6% by volume of 1-hexene and 95.18% by volume of propane.

[0136] Additional monomers were fed to the downcomer at three monomer feed points below the spacer. The combined amount of fresh monomers fed to the downcomer (23) was 20 kg / h of ethylene and 1.7 kg / h of 1-hexene. Additionally, 5 kg / h of propane, 25.5 kg / h of ethylene and 1.3 g / h of hydrogen were fed to the recycle line (29) via line (51).

[0137] Of the polyethylene finally obtained in the series of fluidized bed reactor (1) and multizone circulation reactor (21), 51% by weight is produced in the fluidized bed reactor (1) and 49% by weight is produced in the multizone circulation reactor (21).

[0138] The final polyethylene obtained has an MFR of 0.21 g / 10 min. 5and 0.948 g / cm 3 The mass median diameter D50 of the polyethylene powder is 1624 μm.

[0139] Example 2

[0140] like Figure 3 As shown, the polyethylene powder prepared in Example 1 was metered into a screw conveyor provided by SomefOfficina MeccanicaE Fonderia of Occhiobello, Italy at a feed rate of 180.93 kg / h. In addition, a liquid metering pump provided by Lewa GmbH, Leonberg, Germany was used, such as provided by Coperion Group of Stuttgart, Germany. Figure 2 The liquid syringe shown, at an injection pressure of 1 MPa, a homogeneous solution of 8 wt% 2,5-dimethyl-2,5-di-(tert-butylperoxy)-hexane (Peroxan HX, supplied by PERGAN GmbH, Bocholt, Germany) and 92 wt% paraffin oil OB 22AT (supplied by Conqord Oil SRL, Castellar Guidobono, Italy) was metered into the screw conveyor to obtain a solution containing 0.808 wt% active oxygen. The amount of peroxide solution added was 500 wt ppm (corresponding to 40 wt ppm of 2,5-dimethyl-2,5-di-(tert-butylperoxy)-hexane) relative to the final polyethylene composition. In order to prevent the presence of oxygen, nitrogen was added at 0.5 m 3 The feed rate of / h is injected into the screw conveyor.

[0141] The homogeneous mixture of polyethylene powder and peroxide produced by the screw conveyor mixer is then fed into the feed hopper of a twin screw extruder ZSK 50 of a ZSKNT 50 / 58 extrusion line (Coperion Group, Stuttgart, Germany), which includes a first twin screw extruder ZSK 50 and a second twin screw extruder ZSK 58. In the ZSK 50 extruder, the mixture of polyethylene powder and peroxide is melted and homogenized.

[0142] To provide a polyethylene powder with additives, Songnox 1010 (supplied by SONGWON Industries Ltd., Ulsan, South Korea), Irgafos 168 (supplied by BASF SE, Ludwigshafen, Germany) and calcium stearate "M" (supplied by So.GIS Industria Chimica Spa, Sospiro, Italy) were extruded through a Berstorff ZE25 twin-screw extruder (KraussMaffei Group GmbH, Munich, Germany) to form a homogeneous masterbatch consisting of 80% of the powder prepared in Example 1 and 20% of the additives. Figure 3 As shown, the obtained additive masterbatch is directly fed to a ZSK 58 twin screw extruder, the second stage of a ZSKNT 50 / 58 extrusion line at a feed rate of 4.07 kg / h. The amount of the additive added corresponds to an additive content of 1600 ppm (by weight of Songnox 1010), 1600 ppm (by weight of Irgafos 168), and 1200 ppm (by weight of calcium stearate M) in the final polyethylene composition.

[0143] The mixture of masterbatch and polyethylene melt supplied by the ZSK 50 extruder was then subjected to further homogenization and melt filtration steps in an extruder ZSK 58 and finally pelletized using a Filtec UW50 underwater pelletizing system (Filtec srl, Badia Polesine, Italy). For melt filtration, the melt was passed through a screen bank consisting of three 60 mesh metal screens and a 20 mesh metal screen used as mechanical support.

[0144] The applied operating conditions and characterization results of the obtained pellet samples are shown in Table 1.

[0145] Comparative Example A

[0146] Except that a masterbatch of the additive is not first prepared and the masterbatch is fed into the Figure 3 The ZSK 58 twin screw extruder shown here is based on Figure 5The process of Example 2 was repeated under the same conditions except that the obtained additive powder premix, polyethylene powder and peroxide solution were metered into a screw conveyor at two different feeding points and mixed thoroughly therein. The homogeneous mixture of polyethylene powder, additive and peroxide produced by the screw conveyor mixer was then fed into the feed hopper of the ZSK 50 extruder of the ZSKNT 50 / 58 extrusion line. The content of additives in the final polyethylene composition was the same as in Example 2, and the addition amount of 2,5-dimethyl-2,5-di-(tert-butylperoxy)-hexane was also 40 ppm by weight in this case.

[0147] The applied operating conditions and characterization results of the obtained pellet samples are shown in Table 1.

[0148] Comparative Example B

[0149] The process of Comparative Example A was repeated under the same conditions except that the addition amount of 2,5-dimethyl-2,5-di-(tert-butylperoxy)-hexane was 80 ppm by weight (corresponding to 1000 ppm by weight of peroxide solution) relative to the final polyethylene composition.

[0150] The applied operating conditions and characterization results of the obtained pellet samples are shown in Table 1.

[0151] Table 1

[0152]

[0153]

[0154] Comparison between Example 2 and Comparative Example A shows that the process according to the present disclosure produces a multimodal polyethylene composition by adding the antioxidants Songnox 1010 and Irgafos 168 to a molten polyethylene composition obtained by melting and homogenizing a mixture of a multimodal polyethylene in the form of a polyethylene powder and an organic peroxide to produce a multimodal polyethylene composition which has a very low level of macrogels and allows the preparation of a pipe with high sag resistance. In contrast, when the same multimodal polyethylene powder and the same amount of peroxide and antioxidant are used but the antioxidant is added to the polyethylene powder together with the peroxide, the sag behavior deteriorates sharply and the amount of macrogels increases. Comparison between Comparative Example A and Comparative Example B further shows that increasing the amount of peroxide under the same process conditions improves the sag resistance, but at the expense of an even increased level of macrogels.

Claims

1. A continuous process for preparing a multimodal polyolefin composition comprising one or more antioxidants, said process The following steps are involved: a) preparing a multimodal polyolefin in the form of a polyolefin powder having a mass median diameter D50 of the polyolefin particles in the range from 300 μm to 2500 μm by polymerizing one or more 1-olefins in the presence of an olefin polymerization catalyst at a pressure of from 0.5 MPa to 10 MPa and a temperature of from 30°C to 160°C in a series of two or more polymerization reactors, said multimodal polyolefin prepared in step a) being a multimodal polyethylene; b) providing an organic peroxide in an amount of from 20 ppm to less than 100 ppm of the organic peroxide relative to the polyolefin powder; c) mixing the polyolefin powder prepared in step a) and the organic peroxide provided in step b) at a temperature ranging from 10° C. to 100° C. without melting the polyolefin powder to form a polyolefin peroxide mixture; d) melting and homogenizing the polyolefin peroxide mixture obtained in step c) in an extruder device, wherein at least 90% by weight of the organic peroxide decomposes and forms a molten polyolefin composition; e) adding one or more antioxidants to the molten polyolefin composition obtained in step d) to form a combination of a molten polyolefin composition and an antioxidant; f) homogenizing the combination of the molten polyolefin composition and the antioxidant to form a molten polyolefin composition provided with an antioxidant; as well as g) pelletizing the molten polyolefin composition provided with the antioxidant.

2. The process according to claim 1, wherein the multimodal polyolefin in the form of a polyolefin powder is produced in one or more gas phase polymerization reactors and at least one of the gas phase polymerization reactors is a multizone circulation reactor in which one polymerization zone is a riser in which growing polymer particles flow upwards under fast fluidization or transport conditions and the other polymerization zones are subzones of downcomers in which the growing polymer particles flow downwards in a densified form, wherein the riser and the downcomer are interconnected and polymer particles leaving the riser enter the downcomer and polymer particles leaving the downcomer enter the riser, thereby establishing a circulation of polymer particles through the riser and the downcomer.

3. The process of claim 2, wherein the series of polymerization reactors comprises a fluidized bed reactor upstream of the multi-zone circulation reactor.

4. The process according to any one of claims 1 to 3, wherein the olefin polymerization catalyst is a Ziegler or Ziegler-Natta catalyst comprising the reaction product of an aluminum alkyl and a titanium compound supported on a magnesium halide.

5. The process according to any one of claims 1 to 4, wherein the extruder device is a combination of two co-rotating twin-screw extruders, and the polyolefin powder and the organic peroxide are transferred to a first twin-screw extruder, step d) is carried out in the first twin-screw extruder, and the stream of the molten polyolefin composition formed in step d) is provided to a second twin-screw extruder and step f) is carried out, or the extruder device is a counter-rotating continuous mixer equipped with a gear pump or a co-rotating twin-screw extruder equipped with a gear pump.

6. The process according to any one of claims 1 to 5, wherein the molten polyolefin composition provided with an antioxidant is passed through a melt filter before pelletizing.

7. The process according to any one of claims 1 to 6, wherein the multimodal polyolefin in the form of polyolefin powder produced in step a) and the organic peroxide provided in step b) are supplied to a mixing device and the mixing of step c) is carried out in the mixing device.

8. Process according to any one of claims 1 to 6, wherein the multimodal polyolefin in the form of a polyolefin powder prepared in step a) is supplied directly to the extruder device or the multimodal polyolefin composition in the form of a polyolefin powder prepared in step a) is supplied to a mixing device, mixed with one or more additives different from antioxidants and organic peroxides in said mixing device and the mixture comprising said polyolefin powder and said additives different from antioxidants and organic peroxides is transferred from said mixing device to said extruder device and the organic peroxide provided in step b) is supplied to the extruder device at a position where said polyolefin powder or said mixture comprising said polyolefin powder and said additives different from antioxidants and organic peroxides has not yet melted.

9. The process according to any one of claims 1 to 8, wherein the mixing of step c) is carried out in an atmosphere of reduced oxygen content and the oxygen content in the gas phase within the mixing device is less than 5% by volume.

10. The process according to any one of claims 1 to 9, wherein in step b), the organic peroxide is provided as a liquid or as a component of a liquid, or the organic peroxide is provided in the form of a mixture of the organic peroxide and a polyolefin powder.

11. The process according to any one of claims 1 to 10, wherein in step b), the organic peroxide is provided as a liquid or as a component of a liquid and the liquid comprising the organic peroxide is added to the polyethylene powder by passing the liquid through a spring injector comprising a preloaded spring at an injection pressure of 0.5 to 4 MPa.

12. The process according to claim 11, wherein the organic peroxide is provided as a peroxide solution having an active oxygen content in the range from 0.1 wt% to 10 wt%.

13. The process according to any one of the preceding claims, wherein the multimodal polyolefin produced in step a) has an M of from 15 to 40. w / M n .

14. The process according to any one of the preceding claims, wherein the multimodal polyolefin produced in step a) is a multimodal polyethylene having a molecular weight of from 0.940 g / cm2, measured at 23°C according to DIN EN ISO 1183-1:2004. 3 Up to 0.968g / cm 3 density.

15. A process for preparing a tube, The following steps are involved: a) preparing a multimodal polyolefin composition by a process as claimed in any one of claims 1 to 14; as well as b) forming the multimodal polyolefin composition into a pipe.

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