Polyethylene powder and molded article

Abstract

To provide polyethylene powder capable of obtaining powder excellent in stability of a molecular weight, and excellent in whiteness degree and cleanness; and to provide a compact therefrom.SOLUTION: There is provided polyethylene powder in which an intrinsic viscosity is 3.0 dl / g or more and below 30.0 dl / g, and heat flux of 60 minutes after switching oxygen is 0.03 mW or more and below 0.25 mW, in oxidation induction time measurement (150°C, under oxygen, mass of sample is 5 mg), based on ISO11357-6(2018).SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to polyethylene powder and a molded article.

Background Art

[0002] Polyethylene is easy to melt-process, and molded articles obtained using polyethylene have high mechanical strength, excellent chemical resistance, rigidity, etc., and have thus been conventionally used in a wide variety of applications such as films, sheets, microporous membranes, fibers, foams, and sintered bodies. In particular, ultra-high molecular weight polyethylene has higher mechanical strength and excellent slidability and abrasion resistance, and thus has high practical applicability.

[0003] However, ultra-high molecular weight polyethylene has low fluidity even when melted at a temperature above its melting point, so it is difficult to pelletize and is used in powder form. As a method for molding the ultra-high molecular weight polyethylene powder, there are known a compression molding method in which the ultra-high molecular weight polyethylene powder is compression-molded under heating and then cut into the desired shape, and a molding method in which the powder is dissolved in a solvent such as liquid paraffin, stretched, and then the solvent is removed to form a sheet or filament (see, for example, Patent Documents 1 and 2).

[0004] On the other hand, in recent years, the requirements for cleanliness with respect to chemical liquids, cleaning water, etc. used in the manufacturing processes of displays, electronic components, and various media devices have been increasing more and more. For example, when using ultra-high molecular weight polyethylene containing additives, impurities, oligomers generated by deterioration, etc. as containers for storing high-purity chemicals, ultrapure water, etc. (hereinafter also referred to as objects to be contacted) or filters for filtration, there is a problem that these substances may elute and contaminate or alter the objects to be contacted. In view of such problems, regarding high-purity polyethylene resin materials, for example, a resin composition of ultra-high molecular weight polyethylene with an intrinsic viscosity of 10 dl / g or more and 80 dl / g or less and polyethylene with a molecular weight distribution (Mw / Mn) of 4 or less is a polyethylene resin material with a low content of low-molecular-weight components and excellent cleanliness, and a technology to this effect has been disclosed (see, for example, Patent Document 3).

[0005] Also, for example, a technology has been disclosed that ultra-high molecular weight polyethylene with an intrinsic viscosity of 10 dl / g or more and 60 dl / g or less has a low chlorine content, suppresses corrosion of the molding machine, and does not require the addition of metal soaps or the like as a neutralizing agent, and thus has excellent cleanliness (see, for example, Patent Document 4).

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0007] Regarding a compression molded article of ultra-high molecular weight polyethylene powder, in order to exhibit excellent physical properties, it is necessary to completely fuse the powders together and perform compression processing at a high temperature for a long time. In addition, regarding the above compression molded article, in recent years, there has been an increasing number of applications that require a higher strength and excellent dimensional accuracy, and compression molding has been performed at a high temperature for a longer time than in the past. In order to relieve residual strain after molding, it has also become necessary to perform annealing at a temperature below the melting point for a long time. Furthermore, even when using ultra-high molecular weight polyethylene powder for applications such as separators and high-strength fibers, the management of undissolved powder has become strict. For this reason, in order to completely dissolve ultra-high molecular weight polyethylene powder in a solvent, it has been kneaded at a high temperature for a longer time than in the past. In various applications described above, ultra-high molecular weight polyethylene with excellent cleanliness and whiteness is required.

[0008] However, in the technologies disclosed in Patent Document 1 and Patent Document 2, in order to suppress the deterioration of ultra-high molecular weight polyethylene powder, after adding a large amount of antioxidants and neutralizing agents such as calcium stearate, heating processing is performed to produce a molded article and high-strength fibers. The molded article has a problem that additives elute during use, resulting in poor cleanliness. In addition, Patent Document 3 describes suppressing discoloration (yellowing) and oxidative deterioration caused by titanium, and having few low-molecular weight components and being clean, but there is no description regarding deterioration caused by other metal components and the polymer structure of ultra-high molecular weight polyethylene powder. It has a problem that a decrease in molecular weight and gelation occur in the compression molding process that is heated for a long time. Furthermore, Patent Document 4 does not describe reducing the chlorine content in ultra-high molecular weight polyethylene powder, but has many impurities such as catalyst residues and no description for suppressing deterioration. It has a problem that its use is restricted for applications that require extremely strict cleanliness.

[0009] Therefore, in the present invention, in view of the problems of the above-described prior art, an object is to provide a polyethylene powder and a molded article thereof that can obtain a molded article excellent in molecular weight stability, whiteness, and cleanliness.

Means for Solving the Problems

[0010] As a result of intensive studies to solve the above problems, the inventors of the present invention have found that a polyethylene powder having a predetermined intrinsic viscosity and a heat flow in a predetermined oxidation induction time measurement can solve the above problems, and have completed the present invention. That is, the present invention is as follows.

[0011] [1] The intrinsic viscosity is 3.0 dl / g or more and less than 30.0 dl / g, In the oxidation induction time measurement (150 ° C, under oxygen, sample mass 5 mg) in accordance with ISO 11357-6 (2018), the heat flow after 60 minutes from the oxygen switching is 0.03 mW or more and less than 0.25 mW, a polyethylene powder. [2] The polyethylene powder according to [1] above, wherein the Mg content is less than 10.0 ppm. [3] The polyethylene powder according to [1] or [2] above, wherein the Ti content is less than 3.0 ppm. [4] The polyethylene powder according to any one of [1] to [3] above, wherein the Al content is less than 8.0 ppm. [5] The polyethylene powder according to any one of [1] to [4] above, wherein the α-olefin content is less than 0.10 mol%. [6] When heated at 200 ° C under air for 1 hour Oxidation The polyethylene powder according to any one of [1] to [5] above, wherein the maximum weight increase rate is less than 2.0%. [7] The polyethylene powder according to any one of the above [1] to [6], wherein the total mass ratio of particles having a particle diameter of 106 μm or more and less than 212 μm is 50% by mass or more and less than 75% by mass. [8] The compacted bulk density is 0.50 g / cm 3 or more and less than 0.65 g / cm 3 and The value obtained by dividing the compacted bulk density by the loose bulk density (compacted bulk density / loose bulk density) is 1.05 or more and less than 1.30. The polyethylene powder according to any one of the above [1] to [7]. [9] The polyethylene powder according to any one of the above [1] to [8], wherein the Ca content is less than 10 ppm.

[10] A molded article of the polyethylene powder according to any one of the above [1] to [9]. [Advantages of the Invention]

[0012] According to the present invention, it is possible to provide a polyethylene powder and a molded article thereof that can obtain a molded article excellent in molecular weight stability, whiteness, and cleanliness. [Embodiments for Carrying Out the Invention]

[0013] Hereinafter, embodiments for carrying out the present invention (hereinafter also referred to as "the present embodiment") will be described in detail. Note that the following present embodiment is an exemplification for explaining the present invention, and is not intended to limit the present invention to the following contents. The present invention can be variously modified and implemented within the scope of its gist.

[0014] [Polyethylene Powder] The polyethylene powder of the present embodiment has an intrinsic viscosity of 3.0 dl / g or more and less than 30.0 dl / g, and in the oxidation induction time measurement (150 °C, under oxygen, sample mass 5 mg), the heat flow after 60 minutes from the oxygen switching is 0.03 mW or more and less than 0.25 mW. Since the polyethylene powder of the present embodiment has the above configuration, a molded article excellent in molecular weight stability, whiteness, and cleanliness can be obtained. The polyethylene powder of the present embodiment is a so-called ultra-high molecular weight polyethylene powder having an intrinsic viscosity of 3.0 dl / g or more.

[0015] The polyethylene constituting the polyethylene powder of the present embodiment is not limited to the following, and for example, an ethylene homopolymer or a copolymer of ethylene and another comonomer is preferably mentioned. The other comonomer is not particularly limited, and examples thereof include α-olefins and vinyl compounds. The α-olefin is not limited to the following, and for example, α-olefins having 3 to 20 carbon atoms can be mentioned. Specifically, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-nonene, 1-decene, 1-undecene, 1-dodecene, 1-tridecene, 1-tetradecene and the like can be mentioned. The vinyl compound is not limited to the following, and for example, vinylcyclohexane, styrene and its derivatives can be mentioned. Further, if necessary, non-conjugated polyenes such as 1,5-hexadiene and 1,7-octadiene can be used as other comonomers. When the polyethylene powder of the present embodiment is a copolymer, the copolymer may be a ternary random polymer. The other comonomer may be used alone or in combination of two or more. When the amount of the ethylene copolymer is 100 mol%, the amount of the other comonomer is preferably 0.1 mol% or less, more preferably 0.08 mol% or less, and still more preferably 0.06 mol% or less. By adjusting the amount of the other comonomer to 0.1 mol% or less, the generation of tertiary carbon that is easily deteriorated can be reduced, and the generation of oligomers also tends to be suppressed. In addition, the polymerization reaction tends to have high catalytic activity, and the amount of the metal component of the catalyst residue tends to be easily suppressed. The amount of comonomer in polyethylene can be confirmed by infrared analysis, NMR method, etc.

[0016] (Intrinsic viscosity [η]) The intrinsic viscosity [η] of the polyethylene powder of this embodiment is 3.0 dL / g or more and less than 30.0 dL / g, preferably 3.5 dL / g or more and less than 28.0 dL / g, more preferably 4.0 dL / g or more and less than 26.0 dL / g. The intrinsic viscosity can be measured in decahydronaphthalene at 135°C. When the intrinsic viscosity [η] is 3.0 dL / g or more, there are few low molecular weight components with poor oxidation resistance deterioration, and chain scission and crosslinking due to deterioration during molding processing are suppressed, and the molded body tends to have excellent deterioration durability. In addition, the molded body obtained from the polyethylene powder of this embodiment tends to have sufficient mechanical strength and abrasion resistance. On the other hand, when the intrinsic viscosity [η] is less than 30.0 dL / g, the polymerization reaction tends to have high catalytic activity, and the amount of metal components of the catalyst residue tends to be easily suppressed. In addition, since the moldability is excellent, the fusion adhesion between polyethylene powders is improved, resulting in excellent mechanical strength, and the solubility of the polyethylene powder also tends to increase. The intrinsic viscosity [η] of the polyethylene powder can be controlled within the above numerical range by appropriately adjusting the polymerization conditions and the like described later. Specifically, the intrinsic viscosity [η] can be controlled by making hydrogen present as a chain transfer agent in the polymerization system or by changing the polymerization temperature. The intrinsic viscosity [η] of the polyethylene powder of this embodiment can be obtained by preparing a solution in which the polyethylene powder is dissolved at different concentrations in decahydronaphthalene, measuring the solution viscosity of the solution at 135°C, and extrapolating the reduced viscosity calculated from the measured solution viscosity to a concentration of 0. Specifically, it can be obtained by the method described in the examples below.

[0017] (Heat flow 60 minutes after oxygen switching in oxidation induction time measurement) In the polyethylene powder of the present embodiment, the heat flow 60 minutes after oxygen switching in the measurement of oxidation induction time (150 °C, under oxygen, sample mass 5 mg) in accordance with ISO11357-6 (2018) is 0.03 mW or more and less than 0.25 mW, preferably 0.04 mW or more and less than 0.24 mW, more preferably 0.05 mW or more and less than 0.23 mW. When the heat flow 60 minutes after oxygen switching in the oxidation induction time measurement is 0.03 mW or more, a high-purity molded body tends to be obtained. On the other hand, when the heat flow 60 minutes after oxygen switching in the oxidation induction time measurement is less than 0.25 mW, the polyethylene powder is less likely to be oxidized and deteriorated, the cleavage and cross-linking of molecular chains due to deterioration during molding processing are suppressed, and when it is made into a molded body, it has excellent deterioration durability and a molded body with high whiteness tends to be obtained. The heat flow in the oxidation induction time measurement is measured in accordance with ISO11357-6 (2018), and the heat generation due to the oxidation reaction of polyethylene can be measured using a differential scanning calorimeter (DSC) or the like. As a measurement method, after heating a test sample in a nitrogen atmosphere and reaching 150 °C, the atmosphere is switched to an oxygen atmosphere, and the heat flow 60 minutes after reaching a constant temperature of 150 °C is measured. The heat flow 60 minutes after oxygen switching in the oxidation induction time measurement at 150 °C will be described in detail in the examples below.

[0018] As a method for controlling the heat flow 60 minutes after oxygen switching in the oxidation induction time measurement to be 0.03 mW or more and less than 0.25 mW, a method of preparing polyethylene having a structure with few impurities that promote deterioration as a polyethylene structure that is difficult to deteriorate without blending additives such as antioxidants can be mentioned. Specifically, reducing the generation of tertiary carbon, which is easily deteriorated, to a predetermined amount or less, reducing the double bonds at the ends of polymer chains generated by abnormal reactions or thermal deterioration during polymerization to a predetermined amount or less, reducing metal components such as catalyst residues that promote oxidation deterioration and oligomer components generated by abnormal reactions to a predetermined amount or less, etc. can be mentioned. As a more specific method, ethylene gas, a solvent, a catalyst, etc. are continuously supplied into the polymerization system, and unreacted ethylene gas and the solvent are continuously discharged together with the produced polyethylene powder, and a continuous polymerization with a uniform system is carried out; the main catalyst and the cocatalyst are alternately introduced from the same line to suppress local rapid polymerization; as a highly active polymerization reaction, the amount of the metal component of the catalyst residue is suppressed; for the cooling of the polymerization reaction, jacket cooling and a condenser for cooling the vaporized solvent, etc. are used in combination to prevent the abnormally reacted polyethylene powder entrained in the vaporized solvent from being recharged into the polymerization reactor; ethanol is introduced into the buffer tank before separating the polyethylene powder and the solvent so that the amount of ethanol becomes 10% by mass of the solvent amount to remove the metal component and the oligomer component as much as possible; for the polyethylene powder after polymerization, steam of water and isopropyl alcohol (70 / 30% by mass) is sprayed, and while blowing nitrogen gas at 100°C or higher and 110°C or lower, hydrochloric acid gas generated from the volatilized solvent and the catalyst is removed, etc. Usually, in the production process of polyethylene powder, due to local abnormal reactions during the polymerization reaction, insufficient purification, deterioration during drying, etc., it becomes polyethylene powder that is easily oxidized and deteriorated. However, the polyethylene powder of this embodiment has a great feature in that it has a configuration that is difficult to be oxidized and deteriorated.

[0019] (Mg content) The Mg content of the polyethylene powder of this embodiment is preferably less than 10.0 ppm, more preferably less than 8.0 ppm, and still more preferably less than 6.0 ppm. When the Mg content is less than 10.0 ppm, the oxidative degradation of polyethylene can be effectively suppressed, and the molecular weight reduction and gelation due to the crosslinking reaction can be further effectively suppressed. In addition, when it is made into a molded body, it has excellent deterioration durability, and there is a tendency to suppress the coloring and embrittlement of the molded body.

[0020] The Mg content can be controlled within the above numerical range by increasing the productivity of polyethylene per unit catalyst. The productivity of polyethylene per unit catalyst can be controlled by adjusting the polymerization temperature, polymerization pressure, and slurry concentration in the reactor when producing polyethylene. That is, to increase the productivity of polyethylene per unit catalyst of the polyethylene powder of the present embodiment, the polymerization temperature can be increased, the polymerization pressure can be increased, and / or the slurry concentration can be increased. The catalyst to be used is not particularly limited, and common Ziegler-Natta catalysts, metallocene catalysts, etc. are preferably mentioned. As a method for increasing the productivity of polyethylene per unit catalyst, further, the main catalyst and the cocatalyst are alternately introduced from the same line to suppress local rapid polymerization, the cooling of the polymerization reaction uses a combination of jacket cooling and a condenser for cooling the vaporized solvent, etc., to prevent the abnormally reacted polyethylene powder entrained in the vaporized solvent from being recharged into the polymerization reactor, ethanol is introduced into the buffer tank before separating the polyethylene powder and the solvent so that it becomes 10% by mass of the solvent amount to remove metal components and oligomer components as much as possible, the polyethylene powder and the solvent are separated by a centrifugal separation method, and the amount of solvent contained in the polyethylene powder before drying is made 70% by mass or less based on the mass of the polyethylene powder. The Mg content of the polyethylene powder can be measured by the method described in the examples below.

[0021] (Ti content) The Ti content of the polyethylene powder of the present embodiment is preferably less than 3.0 ppm, more preferably less than 2.5 ppm, and even more preferably less than 2.0 ppm. When the Ti content is less than 3.0 ppm, the oxidative degradation of polyethylene can be effectively suppressed, and the molecular weight reduction and gelation due to cross-linking reactions can be more effectively suppressed. Also, when it is made into a molded body, it has excellent deterioration durability, and there is a tendency to suppress the coloring and embrittlement of the molded body. As a method for making the Ti content less than 3.0 ppm, the same method as the method for controlling the Mg content described above can be applied.

[0022] (Al content) The Al content of the polyethylene powder of the present embodiment is preferably less than 8.0 ppm, more preferably less than 7.0 ppm, and still more preferably less than 6.0 ppm. When the Al content is less than 8.0 ppm, oxidative degradation of polyethylene can be effectively suppressed, and molecular weight reduction and gelation due to crosslinking reactions can be more effectively suppressed. Further, when formed into a molded article, it has excellent deterioration durability, and coloring and embrittlement of the molded article tend to be suppressed. As a method for making the Al content less than 8.0 ppm, the same method as the method for controlling the Mg content described above can be applied.

[0023] (Content of α-olefin) When the ethylene copolymer is 100 mol%, the content of α-olefin in the polyethylene powder of the present embodiment is preferably less than 0.10 mol%, more preferably 0.08 mol% or less, and still more preferably 0.06 mol% or less. By making the content of α-olefin less than 0.10 mol%, the generation of tertiary carbon that is easily deteriorated can be reduced, and the generation of oligomers also tends to be suppressed. Further, the polymerization reaction tends to have high catalytic activity, and the amount of metal components in the catalyst residue tends to be suppressed. The content of α-olefin in the polyethylene powder can be measured by infrared analysis or NMR method. The content of α-olefin in the polyethylene powder of the present embodiment can be controlled within the above numerical range by adjusting the addition amount of α-olefin to ethylene added in the polymerization reactor.

[0024] (Maximum oxidation weight increase rate) When heated at 200 °C in air, the maximum oxidation weight increase rate of the polyethylene powder of the present embodiment is preferably less than 2.0%, more preferably less than 1.9%, and still more preferably less than 1.8%. When the maximum oxidation weight increase rate of the polyethylene powder when heated at 200°C in air is less than 2.0%, the oxidation of the initial polyethylene is suppressed, and subsequent chain reaction decomposition is less likely to occur. Also, the storage stability of the polyethylene powder tends to improve.

[0025] When polyethylene is heated in air, weight gain due to oxidation, which is the starting reaction of deterioration, occurs. The smaller this weight gain rate, the more likely it is to suppress subsequent molecular weight reduction due to molecular chain scission and gelation due to cross-linking reactions. Also, when polyethylene powder is stored for a long time in summer, it slightly deteriorates during storage, but polyethylene powder with a low likelihood of initial oxidation reaction also tends to have improved storage stability, so the smaller the maximum oxidation weight increase rate, the more preferable. As a method for controlling the maximum oxidation weight increase rate of the polyethylene powder when heated at 200°C in air to less than 2.0%, continuously supply ethylene gas, solvent, catalyst, etc. into the polymerization system, and continuously discharge unreacted ethylene gas and solvent together with the produced polyethylene powder, perform continuous polymerization with a uniform system inside, alternately introduce the main catalyst and the co-catalyst from the same line to suppress local rapid polymerization, suppress the amount of metal components of the catalyst residue as a highly active polymerization reaction, use a jacket cooler and a condenser for cooling the vaporized solvent, etc. for the cooling of the polymerization reaction, and prevent abnormally reacted polyethylene powder entrained in the vaporized solvent from being re-introduced into the polymerization reactor, introduce ethanol into the buffer tank before separating the polyethylene powder and the solvent so that it becomes 10% by mass of the solvent amount to remove metal components and oligomer components as much as possible, spray steam of water and isopropyl alcohol (70 / 30 mass%) onto the polyethylene powder after polymerization, and while blowing nitrogen gas at 100°C or higher and 110°C or lower, completely remove hydrochloric acid gas generated from the volatilized solvent and catalyst, etc.

[0026] (Total mass ratio of particle size 106 μm or more and less than 212 μm) The polyethylene powder of this embodiment preferably has a total mass ratio of particles with a particle size of 106 μm or more and less than 212 μm of 50% by mass or more and less than 75% by mass, more preferably 52% by mass or more and less than 73% by mass, and even more preferably 54% by mass or more and less than 71% by mass. When the total mass ratio of particles with a particle size of 106 μm or more and less than 212 μm is 50% by mass or more, there is less fine powder and coarse powder, so the powder has excellent fluidity. Also, when kneading a solvent such as liquid paraffin with the polyethylene powder, generation of lumps due to fine powder and occurrence of undissolved residues due to coarse powder that are difficult to dissolve can be suppressed. On the other hand, when the total mass ratio of particles with a particle size of 106 μm or more and less than 212 μm is less than 75% by mass, during compression molding using a press or ram extruder, it is easier to achieve close packing in the mold, and there is a tendency for less residual molding strain in the molded body.

[0027] In the polyethylene powder of this embodiment, as a method for controlling the total mass ratio of particles with a particle size of 106 μm or more and less than 212 μm to 50% by mass or more and less than 75% by mass, a method of using a catalyst with a small particle size distribution as the catalyst used for the polymerization of polyethylene can be mentioned. Also, by adjusting the conditions during the polymerization of polyethylene, it is possible to control the mass ratio of the polyethylene powder at the above-mentioned particle size. For example, the total mass ratio of particles with a particle size of 106 μm or more and less than 212 μm can be controlled by lowering the polymerization pressure or shortening the residence time in the reactor. The total mass ratio of particles with a particle size of 106 μm or more and less than 212 μm can be calculated by dividing the total mass of the powder above the 106 μm and 150 μm sieves by the total powder mass when classifying by sieves with mesh openings of 300 μm, 212 μm, 150 μm, 106 μm, 75 μm, and 53 μm.

[0028] (Average particle size) Also, from the viewpoints of processability and handleability, the preferred average particle diameter of the polyethylene powder of the present embodiment is 500 μm or less, more preferably 400 μm or less, and even more preferably 300 μm or less. In the integral curve obtained by integrating the mass of the polyethylene powder remaining on each sieve obtained above from the side with a smaller mesh size, the particle diameter at which the mass becomes 50% is the average particle diameter.

[0029] (Consolidated bulk density) The polyethylene powder of the present embodiment preferably has a consolidated bulk density of 0.50 g / cm 3 or more and less than 0.65 g / cm 3 More preferably, it is 0.52 g / cm 3 or more and less than 0.63 g / cm 3 Even more preferably, it is 0.54 g / cm 3 or more and less than 0.61 g / cm 3 and less. When the consolidated bulk density is 0.50 g / cm 3 or more, there are few aggregates of polyethylene powder and polyethylene powder with irregular shapes, and it is close to a spherical shape. Therefore, the powder fluidity is excellent, and when compression molding is performed using a press or a ram extruder, it is easy to fill the mold most densely, and it is difficult for molding strain to remain in the molded body. On the other hand, when the consolidated bulk density is less than 0.65 g / cm 3 , the powder solubility is excellent, and it becomes possible to process at a lower temperature and in a shorter time, and there is a tendency to obtain a higher purity polyethylene powder.

[0030] As a method for controlling the consolidated bulk density of the polyethylene powder of the present embodiment to be 0.50 g / cm 3 or more and less than 0.65 g / cm 3 , methods of synthesis using a general Ziegler-Natta catalyst or a metallocene catalyst can be mentioned. In particular, a method of synthesis using the catalyst described later is preferred. Also, by suppressing the heat generation amount due to the rapid polymerization reaction that occurs when producing the polyethylene powder, the consolidated bulk density of the polyethylene powder of the present embodiment can also be controlled within the above range. Specific methods include continuously supplying ethylene gas, a solvent, a catalyst, etc. into the polymerization system, and continuously discharging unreacted ethylene gas and the solvent together with the produced polyethylene powder, performing a continuous polymerization with a uniform system inside; introducing the main catalyst and the cocatalyst alternately from the same line to suppress local rapid polymerization; using a condenser that cools the jacket cooling and the vaporized solvent, etc. for the cooling of the polymerization reaction, and preventing the abnormally reacted polyethylene powder entrained in the vaporized solvent from being recharged into the polymerization reactor, and the like.

[0031] (Bulk density when consolidated / Bulk density when loose) For the polyethylene powder of this embodiment, the value obtained by dividing the bulk density when consolidated by the bulk density when loose (bulk density when consolidated / bulk density when loose) is preferably 1.05 or more and less than 1.30, more preferably 1.07 or more and 1.27 or less, and even more preferably 1.09 or more and 1.24 or less. When the value obtained by dividing the bulk density when consolidated by the bulk density when loose is within the range of 1.05 or more and less than 1.30, the polyethylene powder has excellent fluidity, can further suppress the generation of voids during compression molding, and can further enhance the mechanical properties of the molded body. The bulk density when loose is the density measured in a state where the powder is loosely filled (not tapped) in a container of a predetermined volume. The bulk density when consolidated is the density measured in a state where a container of a predetermined volume filled with powder is repeatedly dropped (tapped) from a certain height at a certain speed until the bulk density of the powder in the container becomes substantially constant. Specifically, the bulk density when consolidated and the bulk density when loose can be measured by the methods described in the examples below. The polyethylene powder with the value obtained by dividing the bulk density when consolidated by the bulk density when loose within the above range can be obtained by controlling the particle size distribution, particle shape, etc.

[0032] (Ca content) For the polyethylene powder of this embodiment, the Ca content is preferably less than 10.0 ppm, more preferably less than 8.0 ppm, and even more preferably less than 6.0 ppm. Since the Ca content is less than 10.0 ppm, less Ca elution occurs, resulting in a higher purity polyethylene powder, which can be suitably used for containers and the like that dislike elution of impurities. As a method for controlling the Ca content of the polyethylene powder of the present embodiment to less than 10.0 ppm, there are a method of using a catalyst that does not contain Ca, a method of not blending a neutralizing agent such as calcium stearate or an additive containing Ca, and the like. The Ca content of the polyethylene powder can be measured by the method described in the examples below.

[0033] 〔Manufacturing method of polyethylene powder〕 The polyethylene powder of the present embodiment can be manufactured by a conventionally known polymerization method. The polymerization method is not limited to the following, but for example, there is a method of (co)polymerizing ethylene or a monomer containing ethylene by a slurry polymerization method, a gas phase polymerization method, or a solution polymerization method. In particular, a slurry polymerization method capable of efficiently removing the heat of polymerization is preferable. In the slurry polymerization method, an inert hydrocarbon medium can be used as the medium. The inert hydrocarbon medium is not particularly limited, and examples thereof include aliphatic hydrocarbons such as propane, butane, isobutane, pentane, isopentane, hexane, heptane, octane, decane, dodecane, and kerosene; alicyclic hydrocarbons such as cyclopentane, cyclohexane, and methylcyclopentane; aromatic hydrocarbons such as benzene, toluene, and xylene; halogenated hydrocarbons such as ethyl chloride, chlorobenzene, and dichloromethane; and mixtures thereof. In the polymerization step of the polyethylene powder, it is preferable to use an inert hydrocarbon medium having 6 to 10 carbon atoms. If the number of carbon atoms is 6 or more, low molecular weight components generated by side reactions during ethylene polymerization or deterioration of polyethylene are relatively easily soluble, and can be easily removed in the step of separating polyethylene and the polymerization medium. On the other hand, if the number of carbon atoms is 10 or less, adhesion of the polyethylene powder to the reaction tank and the like is suppressed, and industrial stable operation tends to be possible.

[0034] In the production process of the polyethylene powder of this embodiment, the polymerization temperature is preferably 30°C or higher and 100°C or lower, more preferably 35°C or higher and 95°C or lower, and even more preferably 40°C or higher and 90°C or lower. If the polymerization temperature is 30°C or higher, the activity of the catalyst tends to increase, the metal components, which are catalyst residues remaining in the polyethylene, can be reduced, and furthermore, industrial efficient production tends to be achievable. On the other hand, if the polymerization temperature is 100°C or lower, the adhesion to the reaction tank can be suppressed, and the generation of abnormally reacted polyethylene powder entrained in the vaporized solvent can be suppressed, and stable operation tends to be achievable. In the production process of the polyethylene powder, the polymerization pressure is preferably normal pressure or higher and 2.0 MPa or lower, more preferably 0.1 MPa or higher and 1.5 MPa or lower, and even more preferably 0.1 MPa or higher and 1.0 MPa or lower.

[0035] The polymerization reaction can be carried out by any of the batch, semi-continuous, and continuous methods. In particular, continuous polymerization is preferred. By continuously supplying ethylene gas, solvent, catalyst, etc. into the polymerization system and continuously discharging ethylene gas, solvent, catalyst, etc. together with the produced polyethylene powder, it becomes possible to suppress the partial high-temperature state due to the rapid reaction of ethylene, suppress the deactivation of the catalyst and side reactions, and the polymerization system tends to be more stabilized. Also, it is possible to carry out the polymerization in two or more stages with different reaction conditions.

[0036] In the production process of the polyethylene powder of this embodiment, polyethylene is produced using a catalyst component. Examples of the catalyst component include, but are not limited to, Ziegler-Natta catalysts, metallocene catalysts, Phillips catalysts, etc. As the Ziegler-Natta catalyst, for example, those described in Japanese Patent No. 5767202 can be preferably used. As the metallocene catalyst, although not limited to the following, for example, those described in JP-A-2006-273977 and Japanese Patent No. 4868853 can be preferably used. Further, the catalyst component used in the production process of the polyethylene powder of the present embodiment may contain a cocatalyst such as triisobutylaluminum and Tebbe reagent. The average particle diameter of the above-described catalyst is preferably 0.1 μm or more and 20 μm or less, more preferably 0.2 μm or more and 16 μm or less, and still more preferably 0.5 μm or more and 12 μm or less. If the average particle diameter of the catalyst is 0.1 μm or more, problems such as scattering and adhesion of the obtained polyethylene powder can be prevented. Further, when the average particle diameter of the catalyst is 20 μm or less, problems such as the polyethylene powder becoming too large and sedimenting in the polymerization system or causing blockage of the line in the post-treatment process of the polyethylene powder can be prevented. The particle size distribution of the catalyst is preferably narrow, and fine powder and coarse powder can also be removed by a sieve, centrifugation, or cyclone.

[0037] In the production process of the polyethylene powder of the present embodiment, it is preferable to alternately add the main catalyst and the cocatalyst from the bottom of the polymerization reactor using the same introduction line. As the addition method, although not limited, after continuously adding the solid catalyst component as the main catalyst for 1 minute, stopping for 1 minute, and then preferably repeating the operation of continuously adding a cocatalyst such as triisobutylaluminum for 1 minute and stopping for 1 minute. The catalytic reaction starts when the main catalyst and the cocatalyst come into contact with each other. By alternately adding from the same introduction line, the contact between the high-concentration main catalyst and the cocatalyst immediately after addition is reduced, and polyethylene can be produced without causing a rapid reaction. Therefore, as a method for producing the polyethylene of the present embodiment, which is excellent in oxidation degradation resistance, it is most preferable.

[0038] In the process for producing the polyethylene powder of this embodiment, ethylene gas is preferably introduced from the bottom of the polymerization reactor. Further, for example, as described in German Patent Application Publication No. 3127133, the intrinsic viscosity of the obtained polyethylene powder can be controlled by causing hydrogen to be present in the polymerization system or by changing the polymerization temperature. Also, by adding hydrogen as a chain transfer agent into the polymerization system, the intrinsic viscosity can be controlled within an appropriate range. When hydrogen is added into the polymerization system, the molar fraction of hydrogen is preferably 0 mol% or more and 30 mol% or less, more preferably 0 mol% or more and 25 mol% or less, and even more preferably 0 mol% or more and 20 mol% or less. In addition, in this embodiment, other known components useful for the production of polyethylene can be included in addition to the above-described components.

[0039] In the process for producing the polyethylene powder of this embodiment, the deactivation of the catalyst system used is not particularly limited, but it is preferably carried out in a buffer tank before the stretch separation for separating the polyethylene powder and the solvent. Examples of the agent for deactivating the catalyst system include, but are not limited to, oxygen, water, alcohols, glycols, phenols, carbon monoxide, carbon dioxide, ethers, carbonyl compounds, alkynes, and the like. The deactivation of the catalyst system is preferably carried out by introducing alcohols into the buffer tank so that the amount of the alcohols becomes 5% by mass to 20% by mass of the solvent amount. By adding a predetermined amount of alcohols to an inactive hydrocarbon medium having 6 to 10 carbon atoms and stirring, metal components that are catalyst residues, oligomer components generated by side reactions, and the like can be efficiently extracted.

[0040] In the process for producing the polyethylene powder of this embodiment, after the polymerization reaction, the solvent and the polyethylene powder are separated. Examples of the solvent separation method include a decantation method, a centrifugation method, a filter filtration method, and the like. From the viewpoint of high separation efficiency between the polyethylene powder and the solvent, the centrifugation method is preferred. The amount of the solvent contained in the polyethylene powder after solvent separation is not particularly limited, but is preferably 70% by mass or less, more preferably 60% by mass or less, and still more preferably 50% by mass or less based on the total mass of the polyethylene powder. By setting the solvent contained in the polyethylene powder to 70% by mass or less, metal components, oligomer components, etc. contained in the solvent tend to hardly remain in the polyethylene powder.

[0041] In the manufacturing process of the polyethylene powder of this embodiment, after separating the solvent, it is preferable to perform a drying treatment. The drying temperature is preferably 80°C or higher and 150°C or lower, more preferably 90°C or higher and 140°C or lower, and still more preferably 100°C or higher and 130°C or lower. If the drying temperature is 80°C or higher, efficient drying is possible. On the other hand, if the drying temperature is 150°C or lower, it is possible to dry while suppressing thermal degradation of the polyethylene powder. Also, in this drying process, a mixed liquid of water and alcohols (20 / 80 mass% to 80 / 20 mass%) is sprayed onto the polyethylene powder after polymerization, and an inert gas is blown at a flow rate of 10 m 3 / hour or more, and it is preferable to dry while removing the volatilized solvent. By spraying an aqueous solution containing more volatile alcohols and drying while efficiently removing the volatiles, it is possible to reduce low molecular weight components generated by side reactions and hydrochloric acid generated by deactivation of the catalyst, which is preferable.

[0042] (Additive) The polyethylene powder of this embodiment can contain other known components useful for the production of polyethylene powder in addition to the above components. The polyethylene powder of this embodiment may further contain additives such as a neutralizing agent, for example. The neutralizing agent is used as a catcher for chlorine contained in the polyethylene powder or as a molding processing aid, etc. The neutralizing agent is not particularly limited, and examples thereof include stearates of alkaline earth metals such as calcium, magnesium, and barium. The content of the neutralizing agent is not particularly limited, but is preferably 1,000 ppm or less, more preferably 600 ppm or less, and even more preferably 200 ppm or less based on the total amount of the polyethylene powder. In the polyethylene powder of the present embodiment, when using the polyethylene powder and a molded body made of the polyethylene powder, it is preferably not used because additives may elute.

[0043] The content of the additive contained in the polyethylene of the present embodiment can be determined by subjecting the additive in the polyethylene powder to Soxhlet extraction with tetrahydrofuran (THF) for 6 hours and separating and quantifying the extract by liquid chromatography. The polyethylene powder of the present embodiment can also blend polyethylenes having different intrinsic viscosities, molecular weight distributions, etc., and can also blend other resins such as low-density polyethylene, linear low-density polyethylene, polypropylene, and polystyrene.

[0044] 〔Molded body〕 The polyethylene powder of the present embodiment can be applied to various uses by various processing methods. The molded body of the present embodiment can be produced by molding the polyethylene powder of the present embodiment. Since the molded body is highly pure and excellent in heat resistance, it can be suitably used as a microporous membrane, fiber, sheet-like and block-like molded body. Examples of such molded bodies include separators for secondary batteries, particularly separators for lithium-ion secondary batteries, lead-acid battery separators, high-strength fibers, and the like. Moreover, by taking advantage of the characteristics of ultra-high molecular weight polyethylene, such as excellent abrasion resistance, high slidability, high strength, and high impact resistance, through solid forming methods such as extrusion molding, press molding, and cutting processing, it can be used for gears, rolls, curtain rails, pachinko ball rails, the inner lining sheets of storage silos for grains, etc., sliding-imparting coatings for rubber products, ski boards and ski soles, lining materials for heavy machinery such as trucks and shovel cars, etc. In addition, the molded body obtained by sintering the polyethylene powder of the present embodiment can be used, for example, for filters, dust trap materials, suction conveyance sheets, etc. Furthermore, it can be suitably used for applications that require cleanliness, such as containers for storing high-purity chemicals, ultrapure water, etc. (hereinafter also referred to as the objects to be contacted), and filters for filtration.

Examples

[0045] Hereinafter, the present embodiment will be described in detail with specific examples and comparative examples, but the present embodiment is not limited in any way by the following examples and comparative examples. The measurement methods for various physical properties and characteristics are shown below.

[0046] 〔Measurement methods and evaluation methods for various physical properties and characteristics〕 ((1) Measurement of intrinsic viscosity [η]) 10 mg of polyethylene powder was put into 20 mL of decahydronaphthalene (containing 1 g / L of 2,6-di-t-butyl-4-methylphenol), and stirred at 150 °C for 2 hours to dissolve the polymer. The solution was placed in a constant temperature bath at 135 °C, and the dropping time (ts) between the calibration marks was measured using a Cannon-Fenske viscometer. Similarly, three solutions were prepared by changing the mass of the polyethylene powder, and the dropping times were measured. The dropping time (tb) of only decahydronaphthalene without polyethylene powder was measured as a blank. The reduced viscosities (ηsp / C) of the polymers obtained according to the following formula were plotted respectively to derive a linear equation of the concentration (C) (unit: g / dL) and the reduced viscosity (ηsp / C) of the polymer, and the intrinsic viscosity [η] extrapolated to a concentration of 0 was obtained. (ηsp / C) = (ts / tb - 1) / C

[0047] ((2) Heat flow 60 minutes after oxygen switching in the measurement of oxidation induction time) The heat flow 60 minutes after oxygen switching in the measurement of oxidation induction time was measured by an oxidation induction time test conforming to ISO11357 - 6 (2018). Measurement was carried out using a differential scanning calorimeter (manufactured by Shimadzu Corporation, product name: DSC - 60Plus). First, 5 mg of aluminum oxide was put into an aluminum pan for DSC measurement as a reference. This was set on the left side of the furnace of DSC - 60Plus, and an aluminum pan containing 5 mg (accurately weighed) of polyethylene powder was set on the right side of the furnace. While flowing nitrogen gas into the furnace at 25 mL / min, the temperature was raised from 40°C to 150°C at a heating rate of 25°C / min, and after standing for 5 minutes in that state, nitrogen was switched to oxygen and the measurement was started. The sampling interval during measurement was set to 0.5 seconds. The value obtained by subtracting the heat flow at the time of switching nitrogen to oxygen from the heat flow after 60 minutes was taken as the heat flow 60 minutes after in the measurement of oxidation induction time.

[0048] ((3) Mg, Ti, Al, Ca contents) The polyethylene powder was decomposed under pressure using a microwave decomposition apparatus (model ETHOS TC, manufactured by Milestone General), and the elemental concentrations of magnesium, titanium, aluminum, and calcium were measured as the metal contents in the polyethylene powder by ICP - MS (inductively coupled plasma mass spectrometer, model X series X7, manufactured by Thermo Fisher Scientific) using the internal standard method.

[0049] ((4) α - olefin content) The measurement of the α - olefin content in the polyethylene powder was carried out according to the method disclosed by G.J. Ray et al. in Macromolecules, 10, 773 (1977). 13 It was calculated from the area intensity using the signal of methylene carbon observed by C - NMR spectrum. Measuring device: ECS-500 manufactured by JEOL Ltd. Observed nucleus: 13 C Observed frequency: 100.53 MHz Pulse width: 45° (7.5 μsec) Pulse program: single pulse dec PD: 5 sec Measuring temperature: 130 °C Number of integrations: 30,000 times or more Reference: PE(-eee-) signal, 29.9 ppm Solvent: ortho-dichlorobenzene-d4 Sample concentration: 5 - 10 wt% Dissolution temperature: 130 - 140 °C

[0050] ((5) Maximum oxidation weight increase rate) Using a thermogravimetric analyzer (manufactured by Perkin Elmer, trade name "Pyris1 TGA"), with a sample weight of 5 mg, an oxygen flow rate of 10 mL / min, and a heating rate of 30 °C / min, the temperature was raised from room temperature to 200 °C, and the weight increase rate was measured while holding at that temperature for 1 hour. It was confirmed that the weight of the polyethylene powder increased most due to oxidation within about 30 minutes from the start of the measurement, and then the weight decreased. The ratio at which the weight increased most was defined as the maximum oxidation weight increase rate.

[0051] ((6) Total mass ratio and average particle size of particles with a particle size of 106 μm or more and less than 212 μm) The total mass ratio of particles with a particle size of 106 μm or more and less than 212 μm was measured as follows. 100 g of polyethylene powder was weighed into a 200 mL polycup, 1 g of carbon black was added, and the mixture was thoroughly stirred with a medicine spoon. The stirred polyethylene powder was classified by sieving through sieves with mesh openings of 300 μm, 212 μm, 150 μm, 106 μm, 75 μm, and 53 μm in accordance with JIS Z 8801 standard. The ratio obtained by dividing the total mass of the powder on the sieves of 106 μm and 150 μm by the total powder mass was defined as the total mass ratio (mass%) of particles with a particle size of 106 μm or more and less than 212 μm. Also, in the integral curve obtained by integrating the mass of the polyethylene powder remaining in each sieve from the side with the smaller aperture size, the particle size at 50% by mass was defined as the average particle size.

[0052] ((7) Packed bulk density / Loose bulk density) Using a powder tester PT-X type (manufactured by Hosokawa Micron), the packed bulk density and the loose bulk density were measured. 100 cm made of stainless steel 3 In a cylindrical container made of stainless steel, the sample supply device was vibrated until the polyethylene powder piled up in the container, and the polyethylene powder was allowed to flow down. The density measured by scraping off the excess polyethylene powder on the container using a blade was defined as the loose bulk density (g / cm 3 ). Also, for a 100 cm 3 cylindrical container made of stainless steel, a cap was put on, the sample supply device was vibrated to allow the polyethylene powder to flow down, and tapping was performed at a stroke length (tapping height) of 18 mm, a tapping speed of 60 times / minute, and a tapping number of 180 times. Then, the density measured by scraping off the excess polyethylene powder on the container using a blade was defined as the packed bulk density (g / cm 3 ). The measured value of the packed bulk density measured as described above was divided by the measured value of the loose bulk density to obtain the value of packed bulk density / loose bulk density.

[0053] ((8) Whiteness of the molded body) The molded body was preheated at 220 °C and 5 MPa for 5 minutes in accordance with ISO 11542-2, and then heated and compressed at 220 °C and 10 MPa for 45 minutes. Then, annealing was performed at 100 °C for 24 hours. The whiteness of the molded body was measured and evaluated using a color difference meter CR-20 (manufactured by Konica Minolta) in accordance with the measurement standard CIE / ASTM E 313-96. The evaluation criteria are as follows. 〇 ··· The whiteness of the molded body is 95 or more. △ ··· The whiteness of the molded body is 90 or more and less than 95. × ··· The whiteness of the molded body is less than 90. The polyethylene powder of the example had a high whiteness, excellent oxidation resistance even after long-term heat compression, and did not discolor, resulting in excellent whiteness.

[0054] ((9) Molecular weight reduction rate during kneading) When the total of polyethylene powder and liquid paraffin was 100 parts by mass, 30 parts by mass of polyethylene powder and 70 parts by mass of liquid paraffin were blended to prepare a slurry-like liquid. The obtained slurry-like liquid was fed into a twin-screw extruder through a feeder and kneaded at 230 °C for 30 minutes to obtain a polyethylene gel. The obtained polyethylene gel was extruded from a T-die installed at the tip of the extruder and immediately cooled and solidified with a casting roll cooled to 25 °C to form a gel-like sheet. This gel-like sheet was stretched 7×7 times at 120 °C using a simultaneous biaxial stretching machine to obtain a stretched film. Then, this stretched film was immersed in hexane to completely extract and remove the liquid paraffin, and then vacuum dried at 50 °C for 12 hours. The molecular weight reduction rate was calculated by multiplying by 100 the value obtained by subtracting the intrinsic viscosity of the stretched film from the intrinsic viscosity of the raw material polyethylene powder according to the following formula and dividing by the intrinsic viscosity of the raw material polyethylene powder. [((Intrinsic viscosity of polyethylene powder) - (Intrinsic viscosity of stretched film)) / (Intrinsic viscosity of polyethylene powder)] × 100 (%) The molecular weight reduction rate was evaluated according to the following criteria. The evaluation criteria are as follows. 〇 ··· The molecular weight reduction rate is 90% or more. △ ··· The molecular weight reduction rate is 85% or more and less than 90%. × ··· The molecular weight reduction rate is less than 85%.

[0055] ((10) Cleanliness) 500 mL of ultrapure water (purified using Trepure LV-10T (manufactured by Toray Industries, Inc.) (registered trademark)) was put into a 1 L round glass container, covered, shaken and washed for 30 seconds, and drained. This shaking and washing was repeated 5 times. This container was refilled with 500 mL of ultrapure water, and a molded body with a width of 2 cm, a length of 15 cm, and a thickness of 4 mm, prepared in accordance with ISO 11542-2, was introduced into it. The container was then capped and shaken for 30 seconds for washing, and the water was drained. This shaking wash was repeated 5 times. Next, 500 mL of ultrapure water was put into a 1 L round glass container containing the molded body, and it was shaken at 70 °C for 4 weeks. After 4 weeks, 5 mL was collected from this filled water, and the number of particles of 0.1 μm or more leached into it was measured using a particle counter (KL-22, manufactured by Rion Co., Ltd.). The number of particles contained in the water was determined as follows and taken as the cleanliness. The calculation formula and evaluation criteria are shown below. Cleanliness: Number of particles in water (particles / mL) = {(count number (particles)) × (amount of ultrapure water 100 (mL))} / {sampling amount 5 (mL) × container capacity 200 (mL)} 〇 ··· Less than 40 particles / mL. △ ··· 40 particles / mL or more and less than 70. × ··· 70 particles / mL or more.

[0056] [(Production Example) Synthesis of Catalyst] (Preparation of Solid Catalyst Component [A]) 1,600 mL of hexane was added to an 8 L stainless steel autoclave sufficiently purged with nitrogen. While stirring at 10 °C, 800 mL of a 1 mol / L titanium tetrachloride hexane solution and 800 mL of a hexane solution of an organomagnesium compound represented by 5 (C 4 H 9 ) 11 (OSiH) 2 were simultaneously added over 4 hours. After the addition, the temperature was slowly raised, and the reaction was continued at 10 °C for 1 hour. After completion of the reaction, 1,600 mL of the supernatant was removed, and the solid catalyst component [A] was prepared by washing 5 times with 1,600 mL of hexane. The amount of titanium contained in 1 g of this solid catalyst component [A] was 3.05 mmol.

[0057] (Preparation of Solid Catalyst Component [B]) <Synthesis of raw material (b-1)> Charge 2,000 mL of a hexane solution of 1 mol / L Mg 6 (C 4 H 9 ) 12 Al(C 2 H 5 ) 3 (equivalent to 2,000 mmol of magnesium and aluminum) into an 8 L stainless steel autoclave sufficiently purged with nitrogen. While stirring at 50 °C, dropwise add 146 mL of a 5.47 mol / L n-butanol hexane solution over 3 hours. After completion, wash the line with 300 mL of hexane. Further, continue stirring at 50 °C for 2 hours. After completion of the reaction, cool to room temperature and use it as raw material (b-1). The concentration of magnesium in raw material (b-1) was 0.704 mol / L. <Synthesis of raw material (b-2)> Charge 2,000 mL of a hexane solution of 1 mol / L Mg 6 (C 4 H 9 ) 12 Al(C 2 H 5 ) 3 (equivalent to 2,000 mmol of magnesium and aluminum) into an 8 L stainless steel autoclave sufficiently purged with nitrogen. While stirring at 80 °C, dropwise add 240 mL of an 8.33 mol / L hexane solution of methylhydrogenpolysiloxane (manufactured by Shin-Etsu Chemical Co., Ltd.) over 3 hours. After completion, wash the line with 300 mL of hexane. Further, continue stirring at 80 °C for 2 hours. After completion of the reaction, cool to room temperature and use it as raw material (b-2). The total concentration of magnesium and aluminum in raw material (b-2) was 0.786 mol / L. <Synthesis of (B-1) carrier> 1,000 mL of a hexane solution of 1 mol / L hydroxytrichlorosilane was charged into an 8 L stainless steel autoclave fully substituted with nitrogen, and 1,340 mL of a hexane solution of an organomagnesium compound of raw material (b-1) (equivalent to 943 mmol of magnesium) was added dropwise thereto at 65°C over 3 hours, and the reaction was continued with stirring at 65°C for 1 hour. After completion of the reaction, the supernatant was removed, and the residue was washed 4 times with 1,800 mL of hexane to obtain a support (B-1). As a result of analyzing this support, the magnesium content per 1 g of the solid was 7.5 mmol. <(4) Preparation of solid catalyst component [B]> To 1,970 mL of a hexane slurry containing 110 g of the support (B-1), 103 mL of a hexane solution of 1 mol / L titanium tetrachloride and 131 mL of raw material (b-2) were simultaneously added dropwise at 10°C over 3 hours with stirring. After the addition, the reaction was continued at 10°C for 1 hour. After completion of the reaction, the supernatant was removed, and the residue was washed 4 times with hexane to remove unreacted raw material components, thereby preparing a solid catalyst component [B].

[0058] [Example 1] (Production of polyethylene) Hexane, ethylene, 1-butene, hydrogen, and a catalyst were continuously supplied to a 300 L vessel-type polymerization reactor equipped with a stirrer. Hexane was supplied from the bottom of the polymerization reactor at 80 L / Hr. Ethylene gas was supplied from the bottom of the polymerization reactor to maintain the polymerization pressure at 0.3 MPa. 1-Butene was supplied to the gas layer portion at the upper part of the polymerization reactor so that the concentration in the gas phase was 0.25 mol% with respect to the gas phase concentration of ethylene, and hydrogen was supplied so that the concentration in the gas phase was 11.2 mol% with respect to the gas phase concentrations of ethylene and 1-butene. As a catalyst, a solid catalyst component [A] and triisobutylaluminum as a promoter were used. The solid catalyst component [A] was added alternately from the bottom of the polymerization reactor at a rate of 0.2 g / Hr, and triisobutylaluminum was added at a rate of 10 mmol / Hr using the same introduction line. As the addition method, after continuously adding the solid catalyst component [A] for 1 minute, it was stopped for 1 minute, and then the operation of continuously adding triisobutylaluminum for 1 minute and stopping for 1 minute was repeated. The polymerization temperature was maintained at 78 °C by using jacket cooling and a condenser for cooling the vaporized solvent and the like in combination. A filter with a mesh size of 75 μm was installed in the pipe through which the solvent cooled by the condenser returned to the polymerization reactor to prevent polyethylene powder mixed in the solvent from being re-introduced into the polymerization reactor. The catalyst activity was 12,000 g-PE / g-solid catalyst component [A], and the slurry concentration was 27%. The obtained polymerization slurry was continuously withdrawn into a flash tank equipped with a stirrer so that the level of the polymerization reactor was kept constant, and unreacted ethylene and hydrogen were separated. Next, the polymerization slurry was continuously withdrawn into a buffer tank equipped with a stirrer so that the level of the flash tank was kept constant. Ethanol was introduced into the buffer tank to be 10% by mass of the solvent amount, and it was stirred for 1.5 hours. Subsequently, it was continuously fed into a centrifuge so that the level of the buffer tank was kept constant to separate polyethylene from other solvents and the like, and polyethylene powder was obtained. The content of solvents and the like with respect to polyethylene at that time was 55%. The separated polyethylene powder was dried at 105 °C for 2 hours. This drying process was carried out by spraying steam of water and isopropyl alcohol (70 / 30 mass%) onto the polyethylene powder after polymerization, and blowing nitrogen gas at a flow rate of 15 m 3 / hour to remove the volatilized solvent. The obtained polyethylene powder was passed through a sieve with a mesh size of 425 μm, and those that did not pass through the sieve were removed to obtain the polyethylene powder of Example 1. The physical properties of the obtained polyethylene powder are shown in Table 1.

[0059] [Example 2] In the polymerization step, without introducing 1-butene, hydrogen was introduced so that the gas-phase concentration of ethylene was 10.6 mol%, and polyethylene powder of Example 2 was obtained by the same operation as in Example 1 except that solid catalyst component [B] was used instead of solid catalyst component [A]. The catalyst activity was 12,000 g-PE / g-solid catalyst component [B], and the slurry concentration was 29%.

[0060] [Example 3] In the polymerization step, the polymerization temperature was 76 °C, without introducing 1-butene, and hydrogen was introduced so that the gas-phase concentration of ethylene was 0.18 mol%. Polyethylene powder of Example 3 was obtained by the same operation as in Example 1. The catalyst activity was 16,000 g-PE / g-solid catalyst component [A], and the slurry concentration was 28%.

[0061] [Example 4] In the polymerization step, the polymerization temperature was 71 °C, the polymerization pressure was 0.35 MPa, and hydrogen was introduced so that the gas-phase concentration of ethylene was 0.20 mol%. Polyethylene powder of Example 4 was obtained by the same operation as in Example 1 except that solid catalyst component [B] was used instead of solid catalyst component [A]. The catalyst activity was 20,000 g-PE / g-solid catalyst component [B], and the slurry concentration was 25%.

[0062] [Example 5] In the polymerization step, polyethylene powder of Example 5 was obtained by the same operation as in Example 4 except that calcium stearate was introduced at 150 ppm with respect to the polyethylene powder. The catalyst activity was 20,000 g-PE / g-solid catalyst component [B], and the slurry concentration was 25%.

[0063] [Example 6] In the polymerization process, the polymerization temperature was 65°C, the polymerization pressure was 0.35 MPa, 1-butene was 0.35 mol% based on the gas-phase concentration of ethylene, and hydrogen was introduced to be 0.01 mol% based on the gas-phase concentrations of ethylene and 1-butene. A polyethylene powder of Example 6 was obtained by the same operation as in Example 1, except that solid catalyst component [B] was used instead of solid catalyst component [A], and ethanol was not introduced into the buffer tank. The catalyst activity was 10,000 g-PE / g-solid catalyst component [B], and the slurry concentration was 25%.

[0064] 〔Example 7〕 In the polymerization process, a polyethylene powder of Example 7 was obtained by the same operation as in Example 1, except that the polymerization temperature was 57°C, the polymerization pressure was 0.36 MPa, 1-butene and hydrogen were not introduced, and solid catalyst component [B] was used instead of solid catalyst component [A]. The catalyst activity was 8,000 g-PE / g-solid catalyst component [B], and the slurry concentration was 20%.

[0065] 〔Comparative Example 1〕 In the polymerization process, a polyethylene powder of Comparative Example 1 was obtained by the same operation as in Example 1, except that the polymerization temperature was 85°C, the polymerization pressure was 0.90 MPa, 1-butene was 3.7 mol% based on the gas-phase concentration of ethylene, and hydrogen was introduced to be 24.2 mol% based on the gas-phase concentrations of ethylene and 1-butene. The catalyst activity was 50,000 g-PE / g-solid catalyst component [A], and the slurry concentration was 27%.

[0066] 〔Comparative Example 2〕 In the polymerization process, a polyethylene powder of Comparative Example 2 was obtained by the same operation as in Comparative Example 1, except that the polymerization temperature was 85°C, the polymerization pressure was 0.85 MPa, 1-butene was not introduced, and hydrogen was introduced to be 33.2 mol% based on the gas-phase concentration of ethylene. The catalyst activity was 51,000 g-PE / g-solid catalyst component [A], and the slurry concentration was 26%.

[0067] 〔Comparative Example 3〕 In the polymerization process, except that the polymerization temperature was 61 °C, the polymerization pressure was 0.32 MPa, 1-butene was 6.5 mol% with respect to the gas-phase concentration of ethylene, hydrogen was not introduced, and solid catalyst component [B] was used instead of solid catalyst component [A], polyethylene powder of Comparative Example 3 was obtained by the same operation as in Example 1. The catalyst activity was 10,000 g-PE / g-solid catalyst component [B], and the slurry concentration was 25%.

[0068] 〔Comparative Example 4〕 Polyethylene powder of Comparative Example 4 was obtained by the same operation as in Example 1, except that 150 ppm of pentaerythrityl-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] was added as an antioxidant.

[0069] 〔Comparative Example 5〕 In the polymerization process, the polymerization temperature was 45 °C, the polymerization pressure was 0.28 MPa, 1-butene was 0.35 mol% with respect to the gas-phase concentration of ethylene, hydrogen was introduced so as to be 0.23 mol% with respect to the gas-phase concentrations of ethylene and 1-butene, solid catalyst component [B] was used instead of solid catalyst component [A], solid catalyst component [B] was introduced at a rate of 0.3 g / Hr, triisobutylaluminum was introduced at a rate of 15 mmol / Hr, and ethanol was not introduced into the buffer tank. Polyethylene powder of Comparative Example 5 was obtained by the same operation as in Example 1. The catalyst activity was 4,500 g-PE / g-solid catalyst component [B], and the slurry concentration was 35%.

[0070] 〔Comparative Example 6〕 In the polymerization process, polyethylene powder of Comparative Example 6 was obtained by the same operation as in Example 6, except that solid catalyst component [B] and triisobutylaluminum as a cocatalyst were continuously added from separate introduction lines and ethanol was not introduced into the buffer tank. The catalyst activity was 10,000 g-PE / g-solid catalyst component [B], and the slurry concentration was 25%.

[0071] 〔Comparative Example 7〕 In the polymerization process, the solid catalyst component [A] and triisobutylaluminum as a cocatalyst were continuously added from separate introduction lines, the temperature was adjusted only by jacket cooling without using a condenser, ethanol was not introduced into the buffer tank, only water was sprayed, and nitrogen gas was blown at 3 m 3 / hour. Except for this, polyethylene powder of Comparative Example 7 was obtained by the same operation as in Example 3. The catalyst activity was 14,500 g-PE / g-solid catalyst component [B], and the slurry concentration was 28%.

[0072]

Table 1

[0073]

Table 2

Industrial Applicability

[0074] The polyethylene powder of the present invention is excellent in molecular weight stability even under more severe processing conditions, and the molded article containing the polyethylene powder becomes a molded article excellent in whiteness and cleanliness. Therefore, as materials for various films, sheets, microporous membranes, fibers, foams, pipes, etc. that require high purity, it has high industrial applicability.

Claims

1. The limiting viscosity is 3.0 dl / g or more and less than 30.0 dl / g, in the measurement of the oxidation induction time (150 °C, under oxygen, sample mass 5 mg) in accordance with ISO 11357-6 (2018), the heat flow 60 minutes after oxygen switching is 0.03 mW or more and less than 0.25 mW and, a polyethylene powder.

2. The content of Mg is less than 10.0 ppm, the polyethylene powder according to Claim 1.

3. The content of Ti is less than 3.0 ppm, the polyethylene powder according to Claim 1 or 2.

4. The content of Al is less than 8.0 ppm, the polyethylene powder according to any one of Claims 1 to 3.

5. The content of α-olefin is less than 0.10 mol%, the polyethylene powder according to any one of Claims 1 to 4.

6. The maximum oxidation weight increase rate when heated at 200 °C under air for 1 hour is less than 2.0%, the polyethylene powder according to any one of Claims 1 to 5.

7. The total mass ratio of particles with a particle size of 106 μm or more and less than 212 μm is 50% by mass or more and less than 75% by mass, the polyethylene powder according to any one of Claims 1 to 6.

8. The solid bulk density is 0.50 g / cm 3 or more and less than 0.65 g / cm 3 and is The value obtained by dividing the compacted bulk density by the loose bulk density (compacted bulk density / loose bulk density) is 1.05 or more and less than 1.30, the polyethylene powder according to any one of Claims 1 to 7.

9. The content of Ca is less than 10 ppm, the polyethylene powder according to any one of Claims 1 to 8.

10. A molded article of the polyethylene powder according to any one of Claims 1 to 9.

Images