NL2027813A - Polyethylene powder and molded article - Google Patents

Abstract

Provided is a polyethylene powder that is used as a starting material to produce various molded articles not only having impact resistance and abrasion resistance compatible with each other but also having excellent transparency and excellent ease of finding stains. The polyethylene powder comprises, as a constituent unit, an ethylene unit and/or an ethylene unit and a unit of an @— olefin having 3 or more and 8 or less carbon atoms, and has a viscosity—average molecular weight of 1,000,000 or more and 10,000,000 or less, wherein a yellowness YI, a whiteness WI, and a density p of a sheet molded article under the compression molding conditions described in JIS K6936—2 satisfy the following Expression (1): 0.503(— YI)XWI/p<2.0 .…

Description

P129908NL00 TITLE: POLYETHYLENE POWDER AND MOLDED ARTICLE

BACKGROUND OF THE INVENTION Field of the Invention

[0001] The present invention relates to a polyethylene powder and a molded article. Description of the Related Art

[0002] It has been conventionally known that since a polyethylene powder, particularly an ultra-high-molecular weight polyethylene powder, has a high molecular weight as compared with general-purpose polyethylene, it is excellent in stretching processability, has high strength, has high chemical stability and is excellent in long-term reliability. From these reasons, the polyethylene powder, particularly an ultra-high-molecular weight polyethylene powder, is used as a starting material for molded articles, such as microporous membranes for separators of secondary batteries typified by a lead storage battery and a lithium-ion battery, and fibers.

[0003] The polyethylene powder, particularly an ultra-high- molecular weight polyethylene powder, is excellent in various characteristics, such as impact resistance, abrasion resistance, sliding properties, low-temperature properties and chemical resistance, as compared with general-purpose polyethylene. On that account, the polyethylene powder, particularly an ultra-high-molecular weight polyethylene powder, is also used as a starting material for not only lining materials for hoppers, chutes and the like, bearings, gears and roller guide rails but also molded articles such as bone substitutes, bone conductive materials and osteoinductive materials.

[0004] On the other hand, in the case of the ultra-high- molecular weight polyethylene powder, extrusion molding of a resin alone is difficult because the molecular weight is high, and therefore, compression molding (press molding) or molding with a special extruder such as a ram extruder is often carried out.

In any of such various molded articles as described above, it is important to make impact resistance and abrasion resistance compatible with each other.

From such a viewpoint, techniques for making impact resistance and abrasion resistance compatible with each other in molded articles using a polyethylene powder are disclosed in Japanese Patent Laid-Open No. 2007-23171, Japanese Patent No. 4173444, and Japanese Patent Laid- Open No. 2015-157905.

[0005]

SUMMARY OF THE INVENTION Technical Problem

[0006] In recent years, in various molded articles using, as a starting material, such a polyethylene powder as described above, demands for not only possession of excellent impact resistance and abrasion resistance but also improvement in transparency of the molded articles and capability of easily finding stains of those various molded articles (hereinafter, sometimes referred to as ease of finding stains of molded articles) are increasing.

However, regarding the molded articles described in the aforesaid Japanese Patent Laid-Open No. 2007-23171, Japanese Patent No. 4173444, and Japanese Patent Laid- Open No. 2015-157905, verification about ease of finding stains of molded articles has not been done at all, and they have a problem that there is room for improvement in the ease of finding stains of molded articles.

[0007] Then, in the light of the problem of the prior art, it is an object of the present invention to provide a polyethylene powder which makes impact resistance and abrasion resistance of various molded articles using the polyethylene powder as a starting material compatible with each other and from which molded articles excellent in transparency and excellent in ease of finding stains can be obtained.

Solution to Problem

[0008] In order to solve the above problem, the present inventor (s) has earnestly studied, and as a result, has found that a polyethylene powder which has a viscosity- average molecular weight in the prescribed range and in which a yellowness YI, a whiteness WI and a density p of a sheet molded article under the compression molding conditions described in JIS K6936-2 satisfy the prescribed relationship can solve the above problem, and the inventor(s) has completed the present invention. That is to say, the present invention is as follows.

[0009]

[1] A polyethylene powder comprising, as a constituent unit, an ethylene unit and/or an ethylene unit and a unit of an a-olefin having 3 or more and 8 or less carbon atoms, and having a viscosity-average molecular weight of 1,000,000 or more and 10,000,000 or less, wherein a yellowness YI, a whiteness WI, and a density p of a sheet molded article under the compression molding conditions described in JIS K6936-2 satisfy the following Expression (1):

0.50 {(-YI) xWI/p<2.0 == Expression (1)

[2] The polyethylene powder according to the above [1], wherein the a-olefin is 1-propene or l-butene, and the content of the o-olefin is 1.0 mol or less.

[3] The polyethylene powder according to the above [1] or [2], wherein the polyethylene powder has a bulk density of 0.30 g/mL or more and less than 0.60 g/mL.

5 [4] The polyethylene powder according to any one of the above [1] to [3], wherein the total of magnesium, titanium and aluminum element contents as measured by an inductively coupled plasma mass spectrometer (ICP/MS) is 1 ppm or more and 50 ppm or less.

[5] The polyethylene powder according to any one of the above [1] to [4], wherein in a melting integral curve by differential scanning calorimetry (DSC), a temperature at the time of 30% melting is 120°C or higher and 140°C or lower, a temperature at the time of 50% melting is 125°C or higher and 145°C or lower, and a temperature at the time of 70% melting is 130°C or higher and 150°C or lower.

[6] The polyethylene powder according to the above [5], wherein in the melting integral curve by differential scanning calorimetry (DSC), the temperature at the time of 50% melting is 130°C or higher and 140°C or lower.

[7] A molded article of the polyethylene powder according to any one of the above [1] to [6].

[8]

The molded article according to the above [7], wherein the molded article is a compression molded article.

[9] The molded article according to the above [7], wherein the molded article is an extrusion molded article.

[10] The molded article according to the above [7], wherein the molded article is a stretch molded article.

[11] The molded article according to the above [7], wherein the molded article is a microporous membrane.

[12] The molded article according to the above [7], being a fiber.

Advantageous Effects of Invention

[0010] According to the present invention, there can be provided a polyethylene powder which makes impact resistance and abrasion resistance of various molded articles using the polyethylene powder as a starting material compatible with each other and from which molded articles excellent in transparency and excellent in ease of finding stains are obtained.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, embodiments for carrying out the present invention (referred to as "the present embodiments" hereinafter) will be described in detail.

The following embodiments are examples to describe the present invention, and the present invention is not limited to the following contents and can be variously changed and carried out without departing from the scope of the present invention.

[0012] [Polyethylene powder] The polyethylene powder of the present embodiment comprises, as a constituent unit, an ethylene unit and/or an ethylene unit and a unit of an oa-olefin having 3 or more and 8 or less carbon atoms, and has a viscosity- average molecular weight of 1,000,000 or more and 10,000,000 or less, and a yellowness YI, a whiteness WI, and a density p of a sheet molded article of the polyethylene powder under the compression molding conditions described in JIS K6836-2 satisfy the following Expression (1).

0.50<(=YI)xWI/p<2.0 ... Expression (1) According to the polyethylene powder of the present embodiment, impact resistance and abrasion resistance of various molded articles are made compatible with each other, and the molded articles are excellent in transparency and are excellent in ease of finding stains.

[0013]

(Constituent unit) The polyethylene powder of the present embodiment comprises, as a constituent unit, an ethylene unit and/or an ethylene unit and a unit of an a-olefin having 3 or more and 8 or less carbon atoms.

The o-olefin having 3 or more and 8 or less carbon atoms is copolymerizable with ethylene, and the a-olefin having 3 or more and 8 or less carbon atoms is not particularly limited, and is, for example, at least one a-olefin selected from the group consisting of a linear, branched or cyclic a-olefin, a compound represented by the formula: CH:=CHR} (here, R! is an aryl group optionally substituted with a group having 1 to 6 carbon atoms), and a linear, branched or cyclic diene having 4 to 7 carbon atoms.

Among these, 1-propene and/or l-butene is preferable as the a-olefin from the viewpoints of abrasion resistance, heat resistance, impact resistance and strength of a molded article, and the constituent amount thereof in all the monomers is preferably 1.0 mol or less, more preferably 0.9 mol% or less, and still more preferably 0.8 mol% or less.

From the viewpoints of excellent transparency and ease of finding stains, the constituent amount thereof in all the monomers is preferably 0.01 mol? or more, more preferably 0.04 mol% or more, and still more preferably

0.1 mol% or more.

[0014]

In the present specification, naming of each monomer unit constituting a polymer follows naming of a monomer from which the monomer unit is derived. For example, an "ethylene unit” means a constituent unit of a polymer produced as a result of polymerization of ethylene that is a monomer, and its structure is a molecular structure in which two carbons of ethylene become a polymer main chain. An "o-clefin unit” means a constituent unit of a polymer produced as a result of polymerization of an o- olefin that is a monomer, and its structure is a molecular structure in which two carbons of an olefin derived from the a-olefin become a polymer main chain.

[0015] (Viscosity-average molecular weight) The polyethylene powder of the present embodiment has a viscosity-average molecular weight of 1,000,000 or more and 10,000,000 or less, preferably 1,200,000 or more and 9,500,000 or less, and more preferably 1,500,000 or more and 9,000,000 or less. Since the viscosity-average molecular weight is in the above range, abrasion resistance, heat resistance, impact resistance, strength, and molding processability can be made compatible with one another in a molded article using the polyethylene powder of the present embodiment as a starting material.

[0016] The viscosity-average molecular weight (Mv) of the polyethylene powder of the present embodiment can be calculated from the following Expression (2) using an intrinsic viscosity [nq] (dl/g) determined by extrapolating reduced viscosities to a concentration 0, the reduced viscosities being determined using a plurality of solutions obtained by dissolving the polyethylene powder in decahydronaphthalene solutions in different concentrations under the temperature conditions of 135°C. Specifically, the viscosity-average molecular weight can be determined by the method described in the examples.

Mv = (5.34x104%)x[n]t-42 … Expression (2)

[0017] The viscosity-average molecular weight of the polyethylene powder of the present embodiment can be controlled to be in the above numerical value range by adopting a method of allowing hydrogen to exist in the polymerization system or a method of adjusting the polymerization temperature in the polymerization step for the polyethylene powder.

[0018] In a sheet molded article obtained from the polyethylene powder of the present embodiment under the compression molding conditions described in JIS K6936-2, a yellowness YI, a whiteness WI, and a density p satisfy the following Expression (1).

0.50<(=YI)xWI/p<2.0 es Expression (1) (-YI)xWI/p is preferably 0.80 or more and less than

1.9, and more preferably 1.0 or more and less than 1.8.

The polyethylene powder of the present embodiment satisfies a relationship of the above Expression (1), and therefore, in various molded articles using the polyethylene powder of the present embodiment as a starting material, impact resistance and abrasion resistance are made compatible, an effect of improving transparency is obtained, and ease of finding stains becomes excellent. In order to control the polyethylene powder of the present embodiment to satisfy the above range, it is effective to adopt, for example, a method of adding an aliphatic saturated alcohol in a small amount of 100 ppb or more and less than 1 ppm, a method of setting a solid catalyst component concentration based on the inert hydrocarbon medium described later to 5 mg/L or more and 15 mg/L or less, or a method of setting an oxygen concentration in a dryer in the drying described later to 100 ppm or less, after the polymerization is completed. The aliphatic saturated alcohol is not particularly limited, and methanol or ethanol is more preferable. The yellowness YI, the whiteness WI, and the density p can be measured by the method described in the examples.

[0019] (Bulk density) The bulk density of the polyethylene powder of the present embodiment is preferably 0.30 g/mL or more and

0.60 g/mL or less, more preferably 0.33 g/mL or more and

0.57 g/mL or less, and still more preferably 0.35 g/mL or more and 0.55 g/mL or less.

Since the bulk density of the polyethylene powder of the present embodiment is in the above range, handling properties of the polyethylene powder are enhanced.

In general, the bulk density varies depending on a catalyst used in the polymerization step, so that by appropriately selecting the catalyst, the bulk density can be controlled to be in the above numerical value range, or by adjusting the polymerization temperature or the slurry concentration in the polymerizer, the bulk density can be controlled, and moreover, the productivity of the polyethylene powder per unit catalyst can be controlled.

For example, it is possible to control the bulk density of the polyethylene powder by adjusting the polymerization temperature during the polymerization for the polyethylene powder, and by increasing the polymerization temperature, the bulk density can be decreased. It is also possible to control the bulk density of the polyethylene powder by adjusting the slurry concentration in the polymerizer, and by increasing the slurry concentration, the bulk density can be increased.

As described above, by adjusting the polymerization temperature or the slurry concentration, activity of the catalyst can be adjusted, and the productivity of the polyethylene powder per unit catalyst can be controlled.

The bulk density of the polyethylene powder can be measured by the method described in the examples.

[0020] (Total of magnesium, titanium and aluminum element contents) In the polyethylene powder of the present embodiment, the total of magnesium, titanium and aluminum element contents is preferably 1 ppm or more and 50 ppm or less, more preferably 2 ppm or more and 40 ppm or less, and still more preferably 3 ppm or more and 30 ppm or less.

Since the total of magnesium, titanium and aluminum element contents in the polyethylene powder of the present embodiment is in the above range, appearances of various molded articles using the polyethylene powder as a starting material are improved, and a waste ratio of molded articles arising from appearances is decreased, so that the productivity can be enhanced.

It is possible to control the total of magnesium, titanium and aluminum element contents in the polyethylene powder of the present embodiment by, for example, adjusting the productivity of the polyethylene powder per unit catalyst, and by increasing the productivity, the total thereof can be controlled to be in the above numerical value range.

The total of magnesium, titanium and aluminum element contents in the polyethylene powder of the present embodiment is regarded as that measured by an inductively coupled plasma mass spectrometer (ICP/MS), and specifically, the total thereof can be measured by the method described in the examples.

[0021] (Melting integral curve by DSC) In a melting integral curve of the polyethylene powder of the present embodiment by DSC (differential scanning calorimetry), it is preferable that a temperature at the time of 30% melting be 120°C or higher and 140°C or lower, a temperature at the time of 50% melting be 125°C or higher and 145°C or lower, and a temperature at the time of 70% melting be 130°C or higher and 150°C or lower. It is more preferable that a temperature at the time of 30% melting be 122°C or higher and 138°C or lower, a temperature at the time of 50% melting be 127°C or higher and 143°C or lower, and a temperature at the time of 70% melting be 132°C or higher and 148°C or lower. It is still more preferable that a temperature at the time of 30% melting be 125°C or higher and 135°C or lower, a temperature at the time of 50% melting be 130°C or higher and 140°C or lower, and a temperature at the time of 70% melting be 135°C or higher and 145°C or lower.

Since the temperatures are in the above ranges, impact resistance and abrasion resistance are made compatible, excellent transparency is obtained, and ease of finding stains becomes excellent, in various molded articles using the polyethylene powder as a starting material.

A method to allow the temperatures at the time of melting of the polyethylene powder to satisfy the above ranges is, for example, a method of adding an aliphatic saturated alcohol in a small amount of less than 1 ppm after the completion of the polymerization.

The aliphatic saturated alcohol is not particularly limited, and, for example, methanol or ethanol is a preferred one.

The measurement by DSC can be carried out by the method described in the examples.

[0022] (Other components) The polyethylene powder of the present embodiment may be used in combination with various known additives, as needed. Examples of the additives include a heat stabilizer, a lubricant and a hydrogen chloride absorbent.

Examples of the heat stabilizers include, but are not limited to, heat-resistant stabilizers, such as tetrakis[methylene (3, 5-di-t-butyl-4- hydrxy)hydrocynnamate] methane and distearyl thiodipropionate; and weathering stabilizers, such as bis{2,;2',6,6'-tetamethyl-4-piperidine) sebacate and 2-{2- hydroxy-t-butyl-5-methylphenyl)}-5-chlorobenzotriazole.

Examples of the lubricants and the hydrogen chloride absorbents include, but are not limited to, fatty acid metal salts, such as calcium stearate, magnesium stearate, zinc stearate, calcium palmitate, magnesium palmitate and zinc palmitate.

[0023] [Molded article] The molded article of the present embodiment is obtained by molding the above-mentioned polyethylene powder of the present embodiment.

The polyethylene powder may be molded, as it is, by various molding machines, or may be molded by various molding machines after the polyethylene powder is mixed with an organic peroxide.

[0024] (Organic peroxide) The organic peroxide that is used when the polyethylene powder is molded has a function of an organic peroxide crosslinking agent.

The organic peroxide is not particularly limited as long as it is an organic substance contributing to crosslinking of an ethylene-based polymer and having an atomic group -0-0- in a molecule, and examples thereof include organic peroxides, such as dialkyl peroxide, diacyl peroxide, hydroperoxide and ketone peroxide; organic peresters, such as alkyl perester; and peroxydicarbonate.

Specific examples of the organic peroxides include, but are not limited to, dicumyl peroxide, di-tert-butyl peroxide, 2,5-dimethyl-2,5-di{tert-butylperoxy)hexane,

2, 5-dimethyl-2, 5-di (tert-butylperoxy)hexyne-3, 1,3- bis (tert-butylperoxyisopropyl)benzene, 1,1-bis(tert- butylperoxy)-3,3,5-trimethylcyclohexane, n-butyl-4,4- bis (tert-butylperoxy) valerate, benzoyl peroxide, p- chlorobenzoyl peroxide, 2,4-dichlorobenzoyl peroxide, tert-butyl peroxybenzoate, tert-butyl perbenzoate, tert- butyl peroxyisopropyl carbonate, diacetyl peroxide, lauroyl peroxide, tert-butyl cumyl peroxide, o,a'- di (tert-butylperoxy)diiscpropylbenzene.

Among these, 2,5-dimethyl-2,5-bis(t- butylperoxy) hexane (trade name "PERHEXA 25B" manufactured by NOF Corporation), 2,5-dimethyl-2,5-bis(t- butyloxy)hexyne-3 (trade name "PERHEXYNE Z25B" manufactured by NOF Corporation), dicumyl peroxide, and 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclchexane are preferable.

[0025] (Method for molding polyethylene powder, various molded articles) Examples of methods for molding the polyethylene powder of the present embodiment include, but are not limited to, compression molding (press molding), extrusion molding and stretch molding.

The compression molding is a method in which the starting material powder is homogeneously spread in a mold and heated and pressurized to mold the powder, and then the resulting molded article is cooled and taken out. Through this method, a compression molded article is obtained. A plate-like molded article may be used as a product, as it is, or a block may be prepared and then subjected to cutting or the like to prepare a final product.

In the extrusion molding, a screw extruder, or a ram extruder for extruding a molded article by moving a piston back and forth is used. Through this method, an extrusion molded article is obtained. By changing a shape of an outlet of the extruder, molded articles of various shapes such as sheet, flat plate, profile and pipe are obtained.

The stretch molding is, for example, a method of stretching a sheet to perform film processing or a method of stretching a strand to perform fiber processing.

Through this method, a stretch molded article is obtained.

[0026] [Method for producing polyethylene powder] A method for producing the polyethylene powder of the present embodiment is not particularly limited, and is, for example, a method for producing the polyethylene powder using common Ziegler-Natta catalysts or metallocene catalysts. Particularly a method for producing the polyethylene powder using Ziegler-Natta catalysts is preferable. As the Ziegler-Natta catalysts, those disclosed in [0032] to [0068] of the aforesaid Japanese Patent Laid-Open No. 2015-157905 can be used.

When a solid catalyst component and an organometallic compound component (sometimes both being together referred to as a "catalyst" hereinafter) are added to the polymerization system for an ethylene-based polymer, they may be separately added to the polymerization system or may be added to the polymerization system after they are mixed in advance. When the solid catalyst component and the organometallic compound component are used in combination, the ratio between them is not particularly limited, and the amount of the organometallic compound component based on 1 g of the solid catalyst component is preferably 0.01 mmol or more and 1,000 mmol or less, more preferably 0.1 mmol or more and 500 mmol or less, and still more preferably 1 mmol or more and 100 mmol or less. By mixing them, electrostatic adhesion to a storage tank, piping or the like can be prevented.

[0027] The method for polymerizing a monomer in the production process for the polyethylene powder is, for example, a method in which ethylene or monomers containing ethylene and an Qg-olefin having 3 or more and 8 or less carbon atoms are polymerized by suspension polymerization. The suspension polymerization is preferable from the viewpoint of capability of efficiently removing heat of polymerization. In the suspension polymerization, an inert hydrocarbon medium can be used as a medium, and an olefin itself can also be used as a solvent.

Examples of the inert hydrocarbon media include, but are not limited to, aliphatic hydrocarbons, such as propane, butane, isobutane, pentane, isopentane, hexane, heptane, octane, decane, dodecane and kerosine; 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 of these.

When the polyethylene powder of the present embodiment is produced, the catalyst concentration based on the inert hydrocarbon medium is preferably 5 mg/L or more and 15 mg/L or less.

[0028] In usual, the polymerization temperature in the production process for the polyethylene powder of the present embodiment is preferably 20°C or higher and 100°C or lower, more preferably 30°C or higher and 95°C or lower, and still more preferably 40°C or higher and 920°C or lower. Since the polymerization temperature is 20°C or higher, industrially efficient production is feasible. On the other hand, since the polymerization temperature is 100°C or lower, continuously stable operations are feasible.

[0029]

In usual, the polymerization pressure in the production process for the polyethylene powder of the present embodiment is preferably normal pressure or higher and 15 MPa or lower, more preferably 0.1 MPa or higher and 14 MPa or lower, and still more preferably 0.2 MPa or higher and 13 MPa or lower. Since the polymerization pressure is normal pressure or higher, a polyethylene powder in which the total amount of metals and the total amount of chlorine are large tends to be obtained, and since the polymerization pressure is 13 MPa or lower, a polyethylene powder in which the total amount of metals and the total amount of chlorine are small tends to be stably produced. That is to say, by adjusting the polymerization pressure to be in the above numerical value range, the total amount of metals and the total amount of chlorine can be controlled to be in appropriate ranges.

[0030] It is also possible to carry out the polymerization step for the polyethylene powder of the present embodiment in two or more stages different in reaction conditions. The viscosity-average molecular weight of the resulting polyethylene powder can be controlled by adopting a method of allowing hydrogen to exist in the polymerization system or a method of changing the polymerization temperature, as described in West German Patent Application Publication No. 3127133. By adding hydrogen to the polymerization system as a chain transfer agent, it becomes possible to control the viscosity- average molecular weight in an appropriate range. When hydrogen is added to the polymerization system, the molar fraction of hydrogen is preferably 0.01 mol% or more and 30 mol% or less, more preferably 0.01 mol&% or more and 25 mol? or less, and still more preferably 0.01 mol3 or more and 20 mol3 or less. In the present embodiment, other known components useful for producing the polyethylene powder can be contained in addition to such components as described above.

[0031] In the polymerization for the polyethylene powder of the present embodiment, it is preferable to use an antistatic agent such as STATSAFE 3000 manufactured by Innospec Inc. in order to inhibit electrostatic adhesion of a polymer to the polymerization reactor. As the STATSAFE 3000, dilute one obtained by diluting it with an inert hydrocarbon medium can be added to the polymerization reactor by a pump or the like. In this case, the addition amount is preferably 0.1 ppm or more and 50 ppm or less, and more preferably 20 ppm or more and 50 ppm or less, based on the production of the polyethylene powder per unit time.

[0032] As a method for drying the polyethylene powder of the present embodiment after the polymerization, a drying method without applying high heat is preferable from the viewpoint of prevention of deformation of a powder or fusion bonding of powder particles. The type of the dryer is preferably rotary kiln type, paddle type, a fluidized bed dryer or the like. The drying temperature is preferably 50°C or higher and 150°C or lower, and more preferably 70°C or higher and 100°C or lower. It is also effective to introduce an inert gas such as nitrogen to the dryer to accelerate drying, and specifically, the oxygen concentration in the dryer is preferably set to less than 100 ppm.

[0033] [Use application of molded article] The polyethylene powder of the present embodiment can be processed by various processing methods. A molded article of the polyethylene powder of the present embodiment can be applied to various uses. Examples of main uses to which the molded article is preferably applied include microporous membranes, e€.d., separators for secondary batteries such as lithium-ion secondary batteries and lead storage batteries, fibers, lining materials for hoppers, chutes, etc. because of non-tackiness and low friction coefficient, and bearings, gears, roller guide rails, bone substitutes, bone conductive materials or osteoinductive materials, which require self-lubricating properties, low friction coefficient and abrasion resistance. Examples

[0034] Hereinafter, the present embodiment will be described in more detail using concrete examples and comparative examples, but the present embodiment is in no way limited to the following examples and comparative examples.

Methods for measuring various characteristics and physical properties are shown below.

[0035] [Measuring methods and conditions] {1) Viscosity-average molecular weight (Mv) A viscosity-average molecular weight of a polyethylene powder was determined by the method shown below in accordance with ISO 1628-3 (2010).

First, 20 mg of a polyethylene powder was weighed into a melting tube, the melting tube was purged with nitrogen, then 20 mL of decahydronaphthalene {to which 1 g/L of 2,6-di-t-buty1-4-methylphenol had been added) was added, and they were stirred at 150°C for 2 hours to dissolve the polyethylene powder, thereby obtaining a solution.

In a constant temperature bath at 135°C, a fall time (ts) of the solution between marked lines was measured using a Cannon-Fenske viscometer (product of SIBATA SCIENTIFIC TECHNOLOGY LTD., Product No. 100). Also, regarding each of samples in which the amount of the polyethylene powder had been changed to 10 mg, 5 mg and

2.5 mg, a fall time (ts) between marked lines was measured similarly to the above.

A fall time (tb) of only decahydronaphthalene containing no polyethylene powder, as a blank, was measured.

Reduced viscosities (nsp/C) of the polyethylene powders determined in accordance with the following Expression (3) were each plotted to derive a liner expression of the concentration (C) (unit: g/dL) and the reduced viscosity (nsp/C) of the polyethylene powder, and an intrinsic viscosity ([n]) given by extrapolation to the concentration 0 was determined.

nsp/C = (ts/tb-1)/0.1 (unit: dL/g) ... Expression (3) Next, using the following Expression (4) and using the above intrinsic viscosity [Mm], a viscosity-average molecular weight {Mv) was calculated.

Mv = (5.34x104) x[n]+:*3 == Expression (4)

[0036] {2) Content of a-olefin unit Measurement of a content (mol%) of polymerization units derived from an a-olefin in a polyethylene powder was carried out in accordance with the method disclosed in G. J. Ray, et al. Macromolecules, 10, 773 (1977), and using a signal of methylene carbon observed in a +3C-NMR spectrum, the content was calculated from its integrated intensity.

Measuring device: ECS-400 manufactured by JECL Ltd.

Observation nucleus: +°C Observation frequency: 100.53 MHz Pulse width: 45° (7.5 usec) Pulse program: single pulse dec PD: 5 sec Measuring temperature: 130°C Number of integration times: 30,000 or more Reference: PE (-eee-}) signal, 29.9 ppm Solvent: orthodichlorobenzene-d4 Sample concentration: 5 to 10 mass? Dissolving temperature: 130 to 140°C

[0037] (3) Yellowness YI, whiteness WI Using a polyethylene powder, a compression molded sheet was prepared in accordance with the compression molding conditions for a specimen described in JIS K6936- 2, and a yellowness YI and a whiteness WI of the sheet were measured using a CR-20 Color Reader (manufactured by KONICA MINOLTA, INC.).

[0038] (4) Bulk density A bulk density of a polyethylene powder was measured in accordance with JIS K-6721:1997.

[0039] (5) Density (p) A density was measured in accordance with ASTM D

1505.

A compression molded sheet was prepared in accordance with the compression molding conditions for a specimen described in JIS K6936-2, then a section cut out from the sheet was annealed at 120°C for 1 hour and then cooled at 25°C for 1 hour, and the resulting section was used as a specimen.

[0040] {6) Total of magnesium, titanium and aluminum element contents The total of magnesium, titanium and aluminum element contents in a polyethylene powder was determined by pressure decomposing the polyethylene powder using a microwave decomposition device (model: ETHOS TC, manufactured by Milestone General K.K.), measuring element concentrations of magnesium, titanium and aluminum as metals in the polyethylene powder by an internal standard method using ICP-MS (inductively coupled plasma mass spectrometer, model: X Series X7, manufactured by Thermo Fisher Scientific), and calculating the sum of them.

[0041] (7) Temperature at the time of melting of prescribed amount in melting integral curve by DSC Measurement of a melting integral curve of a polyethylene powder by DSC was carried out using DSC (manufactured by PerkinElmer, trade name: DSC 8000). In an aluminum pan, 8 to 10 mg of a polyethylene powder was introduced, then the aluminum pan was set in the DSC, thereafter, a melting curve obtained by holding the polyethylene powder at 50°C for 1 minute and then heating it up to 190°C at a temperature increasing rate of 10°C/min was integrated, and with the proviso that the percentage before the beginning of melting was 0% and the percentage at the time of completion of melting was 100%, temperatures at the time of 30% melting, at the time of 50% melting and at the time of 70% melting were determined.

[0042] (8) Evaluation of ease of finding stains of molded article In a mold of 1 m square and a height of 3 cm in a hot press molding machine, a mixture obtained by adding 10 ppm of carbon black (manufactured by YONEYAMA YAKUHIN KOGYO Co., LTD.) to 28 kg of a polyethylene powder was introduced in a state of free fall, the mixture was compression molded at a preset temperature of 210°C and a gauge pressure of 10 MPa for 12 hours, and then the mixture was subjected to cooling process for terminating heating while keeping the pressure, thereby obtaining a molded article. The molded article was cut into products of 20 cm square, the resulting 25 cut products of 20 cm square were visually evaluated, and presence or absence of stains caused by carbon black was confirmed with the naked eye. The judgement criteria are as follows.

OD: The number of molded articles in which stains were confirmed is 8 or more. A: The number of molded articles in which stains were confirmed is 4 or more and 7 or less. Xx: The number of molded articles in which stains were confirmed is 3 or less.

[0043] {9) Evaluation of impact resistance Using a polyethylene powder, a compression molded sheet was prepared in accordance with the compression molding conditions for a specimen described in JIS K6936-

2. The average cooling rate was changed to 1°C/min. The resulting compression molded sheet was cut into a specimen described in JIS K7111-1/1fA, and the specimen was subjected to Charpy impact test by the method described in JIS K7111-1. The judgement criteria of the test results are as follows. (0: The Charpy impact strength is 80 kJ/m? or more. A: The Charpy impact strength is 60 kJ/m? or more and less than 80 kJ/mZ. Xx: The Charpy impact strength is less than 60 kJ/m.

[0044] (10) Evaluation of abrasion resistance A compression molded sheet was prepared in the same manner as in the above (9). The average cooling rate was changed to 1°C/min.

A specimen obtained by cutting the compression molded sheet into a size of 60 mm x 35 mm x thickness 4 mm was attached to a rotary shaft, introduced into a sand slurry of No. 4 silica sand 2 kg/water 2 L, and subjected to abrasion test at a rotational speed of 1,750 rpm for a testing time of 24 hours.

From the mass measurement results before and after the test, an abrasion loss ratio was determined by the following Expression (5}.

The judgement criteria of the evaluation were as follows.

(0: The abrasion loss ratio is 5% or less.

A: The abrasion loss ratio is more than 5% and 82 or less.

x: The abrasion loss ratio is more than 8%.

Abrasion loss ratio (%) = (W1-W2)/W1x100 Expression (5) Wl: mass before abrasion test, W2: mass after abrasion test

[0045] [Catalyst Synthesis Example 1: Preparation of solid catalyst component [A]] (1) Synthesis of starting material (a-1) In an 8 L stainless steel autoclave having been thoroughly purged with nitrogen, 2,000 mL (equivalent to 2,000 mmol in terms of magnesium and aluminum) of a hexane solution of Mge(CsHs)12Al (C2Hs)z of 1 mol/L was introduced, and while stirring at 50°C, 146 mL of a n-

butanol hexane solution of 5.47 mol/L was dropwise added over a period of 3 hours. After the completion of the addition, the line was washed with 300 mL of hexane. Further, stirring was continued at 50°C over a period of 2 hours. After the reaction was completed, the reaction product having been cooled down to ordinary temperature was used as a starting material (a-1). The concentration of magnesium in the starting material (a-1) was 0.704 mol/L.

[0046] (2) Synthesis of starting material (a-2) In an 8 L stainless steel autoclave having been thoroughly purged with nitrogen, 2,000 mL (equivalent to 2,000 mmol in terms of magnesium and aluminum) of a hexane solution of Mgs{(C4Hs)12Al1 (C2Hs)3 of 1 mol/L was introduced, and while stirring at 80°C, 240 mL of a hexane solution of methyl hydrogen polysiloxane (manufactured by Shin-Etsu Chemical Co., Ltd.) of 8.33 mol/L was pressure fed, and further, stirring was continued at 80°C over a period of 2 hours. After the reaction was completed, the reaction product having been cooled down to ordinary temperature was used as a starting material (a-2). The total concentration of magnesium and aluminum in the starting material (a-2) was 0.786 mol/L.

[0047] {3) Synthesis of carrier (A-1)

In an 8 L stainless steel autoclave having been thoroughly purged with nitrogen, 1,000 mL of a hexane solution of hydroxy trichlorosilane of 1 mol/L was introduced, and 1,340 mL (equivalent to 0.943 mol in terms of magnesium) of a hexane solution of an organomagnesium compound that was the starting material (a-1) was dropwise added at 65°C over a period of 3 hours, and further, the reaction was continued at 65°C for 1 hour while stirring.

After the reaction was completed, a supernatant was removed, and the remainder was washed with 1,800 mL of hexane four times, thereby obtaining a carrier (A-1). As a result of analysis of this carrier (A-1), the amount of magnesium contained per 1 g of the solid was 7.5 mmol.

[0048] (4) Preparation of solid catalyst component [A] While stirring 1,970 mL of a hexane slurry containing 110 g of the carrier (A-1) at 10°C, 103 mL of a hexane solution of titanium tetrachloride of 1 mol/L and 131 mL of the starting material {(a-2) were simultaneously added to the hexane slurry over a period of 3 hours.

After the addition, the reaction was continued at 10°C for 1 hour.

After the reaction was completed, a supernatant was removed, and the remainder was washed with hexane four times to remove an unreacted starting material component, thereby preparing a solid catalyst component [A].

[0049] [Production of polyethylene powder] To a Bessel type 300 L polymerization reactor equipped with a stirrer, hexane, ethylene, an o-olefin, hydrogen, the solid catalyst component [A], a cocatalyst component, and STATSAFE 3000 {manufactured by The Associated Octel Company Limited) were continuously fed under the conditions shown in the following Table 1 and Table 2 and the conditions shown in the following examples and comparative examples, thereby producing polyethylene powders as below.

[0050] (Example 1: PE-1) The polymerization temperature was kept at 58°C by jacket cooling.

Hexane was fed at 55 L/hour.

As the catalysts, a mixture of triisobutylaluminum and diisobutylaluminum hydride that were cocatalyst components, and the solid catalyst component [A] were used.

The solid catalyst component [A] was added to the polymerizer at a rate of 0.7 g/hour, and the mixture of triisobutylaluminum and diisobutylaluminum hydride was added to the polymerizer at a rate of 3 mmol/hour.

The solid catalyst component [A] and the mixture of triisobutylaluminum and diisobutylaluminum hydride were added in equal amounts so that the rate might become 5 L/hour.

Likewise, STATSAFE 3000 was added in such a manner that the concentration thereof based on the polyethylene powder became 15 ppm. As the a-olefin, l-butene was continuously added in such a manner that the 1-butene concentration became 6.6 mol% based on the gas phase ethylene concentration. The polymerization pressure was kept at 0.3 MPa by continuously feeding ethylene. Under these conditions, stirring was sufficiently carried out so that the contents in the polymerization reactor might become homogenous. The production rate of the polyethylene powder was 10 kg/hour.

The catalytic activity was 30,000 g-PE/g-solid catalyst component [A].

The polymer slurry of polyethylene was continuously drawn out into a flash drum having a pressure of 0.05 MPa so that the level in the polymerization reactor might be kept constant, and unreacted ethylene was separated. Thereafter, the slurry was successively subjected to a solvent separation step, and thereafter, it was sent to a drying step. The dryer was a drum type one, and in a stream of nitrogen, the temperature of the jacket was set to 80°C, and the oxygen concentration in the dryer was adjusted to 80 ppm.

Continuous operations were able to be stably carried out without existence of massive polymers and without clogging of the slurry drawing piping. Methanol was added as the aliphatic saturated alcohol while adjusting the methanol to 150 ppb based on the resulting polyethylene powder, then they were homogenously mixed by a Henschel mixer, and using a sieve having an opening of 425 um, particles which had not passed through the sieve were removed.

The polyethylene powder (PE-1) obtained as above was evaluated by the aforesaid methods.

The evaluation results are set forth in Table 1.

[0051] (Example 2) In the homogenous mixing by a Henschel mixer after the drying step, calcium stearate (manufactured by DAINICHI CHEMICAL INDUSTRY CO., LTD., C60) was mixed in such a manner that the concentration became 1,000 ppm based on the polyethylene powder. Regarding other conditions, the same operations as in Example 1 were carried out.

The polyethylene powder (PE-2) obtained as above was evaluated by the aforesaid methods.

The evaluation results are set forth in Table 1.

[0052] (Example 3) As the a-olefin, l-propene was used. Regarding other conditions, the same operations as in Example 1 were carried out.

The polyethylene powder (PE-3) obtained as above was evaluated by the aforesaid methods.

The evaluation results are set forth in Table 1.

[0053] (Example 4) As the aliphatic saturated alcohol, ethanol was used. Regarding other conditions, the same operations as in Example 1 were carried out. The polyethylene powder (PE-4) obtained as above was evaluated by the aforesaid methods. The evaluation results are set forth in Table 1.

[0054] (Example 5) The polymerization temperature was kept at 77°C. As the a-olefin, 1l-butene was continuously added in such a manner that the 1-butene concentration became 6.4 mol? based on the gas phase ethylene concentration. Hydrogen was continuously added in such a manner that the hydrogen concentration became 0.9 mol& based on the gas phase ethylene concentration. Regarding other conditions, the same operations as in Example 1 were carried out. The polyethylene powder (PE-5) obtained as above was evaluated by the aforesaid methods. The evaluation results are set forth in Table 1.

[0055] (Example 6) The polymerization temperature was kept at 45°C. As the a-olefin, 1-butene was continuously added in such a manner that the 1-butene concentration became 7.0 mol& based on the gas phase ethylene concentration. The polymerization pressure was kept at 0.5 MPa. Regarding other conditions, the same operations as in Example 1 were carried out.

The polyethylene powder (PE-6) obtained as above was evaluated by the aforesaid methods.

The evaluation results are set forth in Table 1.

[0056] (Example 7) The polymerization temperature was kept at 73°C. As the a-clefin, l-butene was continuously added in such a manner that the 1-butene concentration became 0.3 mols based on the gas phase ethylene concentration. The polymerization pressure was kept at 0.15 MPa. Regarding other conditions, the same operations as in Example 1 were carried out.

The polyethylene powder (PE-7) obtained as above was evaluated by the aforesaid methods.

The evaluation results are set forth in Table 1.

[0057] (Example 8) The polymerization temperature was kept at 57°C. Regarding other conditions, the same operations as in Example 7 were carried out.

The polyethylene powder (PE-8) obtained as above was evaluated by the aforesaid methods.

The evaluation results are set forth in Table 1.

[0058] (Example 9)

The polymerization temperature was kept at 58°C, and as the a-olefin, 1-butene was continuously added in such a manner that the l-butene concentration became 2.0 mol&® based on the gas phase ethylene concentration, and by allowing nitrogen to flow in the dryer, the oxygen concentration in the dryer was adjusted to 150 ppm. Regarding other conditions, the same operations as in Example 1 were carried out. The polyethylene powder (PE- 9) obtained as above was evaluated by the aforesaid methods. The evaluation results are set forth in Table 1.

[0059] (Example 10) By allowing nitrogen to flow in the dryer, the oxygen concentration in the dryer was adjusted to 80 ppm, and feeding of hexane was adjusted in such a manner that the feed rate became 40 L/hour. Regarding other conditions, the same operations as in Example 9 were carried out. The polyethylene powder (PE-10) obtained as above was evaluated by the aforesaid methods. The evaluation results are set forth in Table 1.

[0060] (Example 11) By allowing nitrogen to flow in the dryer, the oxygen concentration in the dryer was adjusted to 80 ppm, and the resulting polyethylene powder was homogeneously mixed by a Henschel mixer without adding methanol. Regarding other conditions, the same operations as in

Example 9 were carried out. The polyethylene powder (PE- 11) obtained as above was evaluated by the aforesaid methods.

The evaluation results are set forth in Table 1.

[0061] (Comparative Example 1) The polymerization temperature was kept at 83°C. As the a-olefin, l-butene was continuously added in such a manner that the 1-butene concentration became 6.2 mol? based on the gas phase ethylene concentration. Hydrogen was continuously added in such a manner that the hydrogen concentration became 0.2 mol? based on the gas phase ethylene concentration. Further, feeding of hexane was adjusted in such a manner that the feed rate became 40 L/hour. Furthermore, by allowing nitrogen to flow in the dryer, the oxygen concentration in the dryer was adjusted to 150 ppm. Regarding other conditions, the same operations as in Example 1 were carried out.

The polyethylene powder (PE-12) obtained as above was evaluated by the aforesaid methods.

The evaluation results are set forth in Table 2.

[0062] (Comparative Example 2) Feeding of hexane was adjusted in such a manner that the feed rate became 55 L/hour, and the resulting polyethylene powder was homogeneously mixed by a Henschel mixer without adding methanol. Regarding other conditions, the same operations as in Comparative Example 1 were carried out. The polyethylene powder (PE-13) obtained as above was evaluated by the aforesaid methods. The evaluation results are set forth in Table 2.

[0063] (Comparative Example 3) The oxygen concentration in the dryer was adjusted to 80 ppm, and the resulting polyethylene powder was homogeneously mixed by a Henschel mixer without adding methanol. Regarding other conditions, the same operations as in Comparative Example 1 were carried out. The polyethylene powder (PE-14) obtained as above was evaluated by the aforesaid methods. The evaluation results are set forth in Table 2.

[0064] (Comparative Example A4) The resulting polyethylene powder was homogeneously mixed by a Henschel mixer without adding methanol. Regarding other conditions, the same operations as in Comparative Example 1 were carried out. The polyethylene powder (PE-15) obtained as above was evaluated by the aforesaid methods. The evaluation results are set forth in Table 2.

[0065] (Comparative Example 5) The polymerization temperature was kept at 75°C. The a-olefin was not added. Addition of the aliphatic saturated alcohol was not carried out. Regarding other conditions, the same operations as in Example 1 were carried out. The polyethylene powder (PE-16) obtained as above was evaluated by the aforesaid methods. The evaluation results are set forth in Table 2.

[0066] (Comparative Example 6) The polymerization temperature was kept at 73°C. As the a-olefin, 1-butene was continuously added in such a manner that the l-butene concentration became 0.3 mol? based on the gas phase ethylene concentration. Addition of the aliphatic saturated alcohol was not carried out. In the homogenous mixing by a Henschel mixer after the drying step, calcium stearate (manufactured by DAINICHI CHEMICAL INDUSTRY CO., LTD., (C60) was mixed in such a manner that the concentration became 1,000 ppm based on the polyethylene powder. Regarding other conditions, the same operations as in Example 1 were carried out. The polyethylene powder (PE-17) obtained as above was evaluated by the aforesaid methods. The evaluation results are set forth in Table 2.

[0067] (Comparative Example 7) The polymerization temperature was kept at 47°C. The a-olefin was not added. The polymerization pressure was kept at 0.4 MPa. Addition of the aliphatic saturated alcohol was not carried out. Regarding other conditions, the same operations as in Example 1 were carried out. The polyethylene powder (PE-18) obtained as above was evaluated by the aforesaid methods. The evaluation results are set forth in Table 2.

[0068] (Comparative Example 8) The polymerization temperature was kept at 80°C. As the a-clefin, l-butene was continuously added in such a manner that the 1-butene concentration became 0.15 mol& based on the gas phase ethylene concentration. Hydrogen was continuously added in such a manner that the hydrogen concentration became 0.2 mols based on the gas phase ethylene concentration. The polymerization pressure was kept at 0.15 MPa. Addition of the aliphatic saturated alcohol was not carried out. In the homogenous mixing by a Henschel mixer after the drying step, calcium stearate (manufactured by DAINICHI CHEMICAL INDUSTRY CO., LTD., C60) was mixed in such a manner that the concentration became 500 ppm based on the polyethylene powder. Regarding other conditions, the same operations as in Example 1 were carried out. The polyethylene powder (PE-19) obtained as above was evaluated by the aforesaid methods. The evaluation results are set forth in Table 2.

[0069] [Table 1]

TT 58 Thurs There 66 | 66 | 66 | 66 Hyogen gas phase concentration (mol | 0 | 0 | 0 0 | 0s | 0 | o | 0 | o | 0 | 0 ä 5 5 07 | Poymerzatonpresare (ua) | 03 03 03 | 03 | 05 | 046 | of | 05 | 03 | 03 | E 506 | Action amount of cam stearaelppm) | 0 1909 eee Ooenconcenratnnderem) | 80 | ®0 | w | & | @ | w | ®@ | a | 0 | a | | methanol ethanol | methanol | methanol | methanol methanol IE EE | 0 | 50 | 0 | 0 | 0 | 50 | 0 | 0 | fe | m0 | 0 wesw | es | 721 | 72 | 02 9 | ov 1-butene 1-propene 1-butene | 1-butene | 1-butene 1-butene

0.42 0.40 0.37 0.14 1 129 130 ne 131 129 134 Temperature at the time of DSC 70% melting (°C} 137 137 138 138 139 135 141 143 136 137 137

Ee © | © | © | © | © | © | 2 | 2] 2 | = [2 Semimeamses | © | 0 | 0 | 0 | 0 | 0 | 2 | 4 0 | oo] | Fomine | 0 | 0 | 0 | 0 0 | 0] 0] oo] ol of

[0070] [Table 2] Example 1 | Example 2 | Example 3 | Example 4 | Example 5 | Example 6 | Example 7 | Example 8 | eR at | Polymerizaiontemperawerc) | 8 | 8 | 8 | 8 | 7 | 1 | 4&4 | 8 HOI hie Inte | he DE en et 0.3 0.15 | Hydrogen gas phase concentration {mol%) | 02 | 02 | 02 | 02 | o | o | 0.2 Catalyst concentration (mg/L) 12.7 12.7 co 0 0 0 | 0 | 1 0 | 50 | | Oxwgenconeentrationindyer(pem | 10 | 150 | 8 | 150 | 8& [| & WN SRNR 5% NA IAS RA SAS: I 0 0 | 0 | 0 0 | 0 | | Yelownessvi | 82 | 86 | 80 | 78 | 273 | 81 93 63 | WhienessWl | 543 | 51 | 540 | 538 | 939 | ®@5 | ®0 | 655 929 923 939

Qs 0.49 0.49 0.47 0.46 28 28 3.1 0.44 a-Olefin species | tbutens | T-butene | 1-butene | T-butene | - | 1-butene : Content of a-olefin unit (mol%) | 042 | 041 | 043 | 042 | 0 | 003 | 0 | 001 | Bulk density (g/mL) 0.48 0,39 | Amount of Mg + Ti + Al (ppm) [18 19 | 2 + 2 | 14 | 10 20 48 | Temperature at the time of DSC 30% melting (°C) 134 137 Temperature at the time of DSC 50% melting (°C) | Temperature at the time of DSC 70% melting (°C) | 138 | 139 | 138 | 138 | 143 | 142 | 141 | 146 |

- 46 -

[0071] It has been found that according to the polyethylene powders of Examples 1 to 11, molded articles not only having excellent impact resistance and abrasion resistance but also having excellent transparency and excellent ease of finding stains were obtained. Industrial Applicability

[0072] The polyethylene powder of the present invention has industrial applicability as a material for lining materials for hoppers, chutes, and the like because of non-tackiness and low friction coefficient, bearings, gears, roller guide rails, bone substitutes, bone conductive materials and osteoinductive materials, which require self-lubricating properties, low friction coefficient and abrasion resistance, separators for secondary batteries such as lithium-ion secondary batteries and lead storage batteries, and fibers.

Claims (1)

Conclusions A polyethylene powder comprising, as a constituent unit, an ethylene unit and/or an ethylene unit and an -olefin unit having 3 or more and 8 or less carbon atoms, and having a viscosity average molecular weight of 1,000,000 or more and 10,000,000 or less, wherein a yellowness YI, a whiteness WI, and a density p of a sheet-formed article under the compression molding conditions described in JIS K6936-2 satisfy the following Expression (1): 0.50 <(- YDxWI/p<2.0... Expression (1) The polyethylene powder according to claim 1, wherein the -olefin is 1-propylene or 1-butene, and the content of the a-olefin is 1.0 mol. % or less IS. The polyethylene powder according to claim 1 or 2, wherein the polyethylene powder has a bulk density of 0.30 g/ml or more and less than 0.60 g/ml. The polyethylene powder according to any one of claims 1 to 3, wherein the total of magnesium, titanium and aluminum element contents as measured by an inductively coupled mass spectrometer (ICP/MS) is 1 ppm or more and 50 ppm or less. The polyethylene powder according to any one of claims 1 to 4, wherein in an integral melting curve by differential scanning calorimetry (DSC), a temperature at the time of 30% melting is 120°C or more and 140°C or less, a temperature at the time of 50 % melting is 125 °C or higher and 145 °C or lower, and a temperature at the time of 70 % melting is 130 °C or higher or 150 °C or lower. The polyethylene powder according to claim 5, wherein in the integral melting curve by differential scanning calorimetry (DSC), the temperature at the time of 50% melting is 130°C or more or 140°C or less. A molded article of the polyethylene powder according to any one of claims 1 to 6. The molded article of claim 7, wherein the molded article is a compression molded article. The molded article of claim 7, wherein the molded article is an extrusion molded article. The molded article of claim 7, wherein the molded article is a stretch molded article 1s. The molded article of claim 7, wherein the molded article is a microporous membrane. The molded article of claim 7, wherein the molded article is a fiber 1s.