Patent Description:
As a film for packaging foods and the like, a laminate film is widely used in the related art, the laminate film being obtained by laminating a substrate film and a film formed of an ethylene-based polymer by melt-extruding a resin composition containing an ethylene-based polymer on the substrate film. In a case where such a film formed of an ethylene-based polymer is used on a surface of the laminate film, the film is suitably used as a sealant layer, and in a case where the film is used inside the laminate film, the film is suitably used as an adhesive layer. Therefore, it is required for the film to have excellent transparency and adhesive strength.

As a material for the ethylene-based polymer used in the laminate film, for example, <CIT> discloses a polyethylene-based resin material for lamination, the polyethylene-based resin material being obtained by mixing <NUM> to <NUM> part by weight of a radical initiator with <NUM> parts by weight of a polyethylene-based resin composition containing <NUM> to <NUM> wt% of a high-pressure radical polymerization polyethylene-based resin and <NUM> to <NUM> wt% of a high-pressure radical polymerization polyethylene-based resin other than the above polyethylene-based resin, and modifying the mixture.

In <CIT> is disclosed the preparation of a fireproof composite polyolefin material by successively adding individual components to the polyolefin and extruding them at different temperatures to form pellets.

In order to improve transparency and adhesive strength of the film formed of an ethylene-based polymer, it is required to reduce thickness unevenness of the film and thus to improve flatness. However, the film formed of an ethylene-based polymer of <CIT> has a problem in that thickness unevenness is large.

The present invention has been made in view of such a problem, and an object of the present invention is to provide an ethylene-based polymer capable of obtaining a film in which thickness unevenness is reduced and a film containing the ethylene-based polymer.

According to the present invention, there is provided a high-pressure low-density polyethylene produced by a method comprising:.

As specified above, the high-pressure low-density polyethylene is produced by a method comprising: a step (A) of melt-kneading a mixture containing a high-pressure and a radical initiator at a temperature TA(°C); a step (B) of melt-kneading the melt-kneaded product obtained in the step (A) at a temperature TB(°C); and a step (C) of melt-kneading the melt-kneaded product obtained in the step (B) at a temperature Tc(°C), in which the following Expression (<NUM>) is satisfied: <MAT>.

According to the present invention, a film contains the high-pressure low-density polyethylene.

According to the present invention, it is possible to provide a high-pressure low-density polyethylene as an ethylene-based polymer capable of obtaining a film in which thickness unevenness is reduced and a film containing the high-pressure low-density polyethylene.

Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments.

In a high-pressure low-density polyethylene according to the present embodiment, the following Expressions (<NUM>) and (<NUM>) are satisfied: <MAT> wherein,.

Here, the LAOS method is a method of applying a large and fast shear strain to a sample and observing and analyzing a response stress of the sample.

ηL means a value obtained by applying a sinusoidal shear strain (deformation) to a sample and plotting a deformation rate and a response stress to create a Lissajous curve and dividing a response stress at the highest deformation rate by a deformation rate at that time, that is, a viscosity. When ηL is measured while changing a magnitude of the shear strain (deformation), ηL varies depending on a structure of the high-pressure low-density polyethylene. Specifically, when the shear strain (deformation) is large, the high-pressure low-density polyethylene is oriented in a shear direction (deformation direction). Therefore, ηL is decreased. A decrease level of ηL varies depending on ease of the orientation of the high-pressure low-density polyethylene in the shear direction (deformation direction). When the high-pressure low-density polyethylene is difficult to be oriented in the shear direction (deformation direction), it means that the high-pressure low-density polyethylene is entangled and difficult to move. Therefore, it can be said that ηL<NUM>,<NUM>%/ηL<NUM>% is a parameter that reflects the amount of entangled branch structure.

In general, in the measurement of ηL<NUM>%, a sinusoidal shear strain of <NUM> is applied to a sample for <NUM> cycles, and an average value of values obtained in the latter <NUM> cycles is a measured value. In general, a time required for the measurement of ηL<NUM>% is <NUM> seconds.

In general, in the measurement of ηL<NUM>,<NUM>%, a sinusoidal shear strain of <NUM> is applied to a sample for <NUM> cycles, and an average value of values obtained in the latter <NUM> cycles is a measured value. In general, a time required for the measurement of ηL<NUM>,<NUM>% is <NUM> seconds.

In general, the measurement of ηL<NUM>% is performed first, and ηL is measured while increasing a strain of γ<NUM> stepwise. Specifically, ηL is measured while increasing a strain of γ<NUM> = <NUM>% to <NUM>% (that is, γ<NUM> = <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, or <NUM>%) in <NUM> logarithmically evenly spaced steps. Similarly, ηL is measured while increasing a strain of γ<NUM> = <NUM>% to <NUM>,<NUM>% in <NUM> logarithmically evenly spaced steps, and ηL is measured while increasing γ<NUM> = <NUM>,<NUM>% to <NUM>,<NUM>% in <NUM> logarithmically evenly spaced steps. As described above, in general, a time required for the measurement of one ηL is <NUM> seconds.

In a method of producing the high-pressure low-density polyethylene to be described below, the value of ηL<NUM>,<NUM>%/ηL<NUM>% can be increased by increasing the amount of radical initiator mixed to more than <NUM>% by mass, and the value of ηL<NUM>,<NUM>%/ηL<NUM>% can be decreased by reducing the amount of radical initiator mixed to less than <NUM>% by mass.

A ratio of the intensity of the fifth harmonic to the intensity of the first harmonic (I5/I1) varies depending on the structure of the high-pressure low-density polyethylene. Specifically, the larger the distribution (bias) of the entangled branch structure of the high-pressure low-density polyethylene is, the larger the I5/I1 tends to be. Therefore, it is considered that I5<NUM>,<NUM>%/I1<NUM>,<NUM>% is a parameter that reflects the distribution (bias) of the entangled branch structure.

In general, in the measurement of the response stress at a strain of γ<NUM> = <NUM>,<NUM>%, a sinusoidal shear strain of <NUM> is applied to a sample for <NUM> cycles, and an average value of values obtained in the latter <NUM> cycles is a measured value. In general, a time required for the measurement of the response stress at the strain of γ<NUM> = <NUM>,<NUM>% is <NUM> seconds.

In the same manner as described above, in general, the measurement of the response stress at a strain of γ<NUM> = <NUM>% is performed first, and the measurement of the response stress is performed while increasing the strain of γ<NUM> stepwise. Specifically, the response stress is measured while increasing each of a strain of γ<NUM> = <NUM>% to <NUM>%, a strain of γ<NUM> = <NUM>% to <NUM>,<NUM>%, and a strain of γ<NUM> = <NUM>,<NUM> to <NUM>,<NUM>% in <NUM> logarithmically evenly spaced steps. As described above, in general, a time required for the measurement of one response stress is <NUM> seconds.

In a method of producing the high-pressure low-density polyethylene to be described below, the value of I5<NUM>,<NUM>%/I1<NUM>,<NUM>% can be increased by raising a temperature Tc to higher than <NUM>, and the value of I5<NUM>,<NUM>%/I1<NUM>,<NUM>% can be decreased by lowering the temperature Tc to lower than <NUM>.

In the high-pressure low-density polyethylene according to the present embodiment, the following Expressions (<NUM>') and (<NUM>') are preferably satisfied, and the following Expressions (<NUM>") and (<NUM>") are more preferably satisfied, from the viewpoint of further reducing thickness unevenness of a film. <MAT> <MAT> <MAT> <MAT>.

The high-pressure low-density polyethylene according to the present embodiment preferably has a melt flow rate (MFR) of <NUM>/<NUM> or more and <NUM>/<NUM> or less, and more preferably has a melt flow rate (MFR) of <NUM>/<NUM> or more and <NUM>/<NUM> or less, the MFR being measured at a temperature of <NUM> and a load of <NUM>, from the viewpoint of processing stability. Note that the MFR is measured at a temperature of <NUM> and a load of <NUM> according to the method A defined in JIS K7210-<NUM>.

The high-pressure low-density polyethylene according to the present embodiment preferably has a molecular weight distribution of <NUM> or more and <NUM> or less, and more preferably has <NUM> or more and <NUM> or less, from the viewpoint of processing stability. Note that the molecular weight distribution is a ratio of a weight average molecular weight Mw in terms of polystyrene to a number average molecular weight Mn in terms of polystyrene (Mw/Mn), Mw and Mn being measured by a gel permeation chromatography (GPC) method.

The GPC measurement is performed under the following conditions, and a peak is specified by defining a baseline on a chromatogram based on the description of ISO <NUM>-<NUM>.

Mobile phase: Mobile phase is used by adding <NUM>. 1w/V of BHT to orthodichlorobenzene (manufactured by Wako Pure Chemical Industries, Ltd. , special grade).

Standard substance for GPC column calibration: Standard polystyrenes manufactured by Tosoh Corporation were weighed in combinations as shown in Table <NUM>, <NUM> of orthodichlorobenzene (the same composition as that of the mobile phase) was added to each of the combinations, and the mixture was dissolved in room temperature, thereby preparing the standard substance.

The high-pressure low-density polyethylene according to the present embodiment preferably has a crosslinked structure, from the viewpoint of reducing thickness unevenness of a film.

As the ethylene-based polymer is a high-pressure low-density polyethylene, the thickness unevenness of a film is further reduced.

The high-pressure low-density polyethylene is a low-density polyethylene produced by a high-pressure radical polymerization method. In general, a high-pressure low-density polyethylene is produced by continuously polymerizing ethylene monomers at a pressure of <NUM>,<NUM> to <NUM>,<NUM> atm and <NUM> to <NUM> in the presence of oxygen or an organic peroxide as a polymerization initiator in a pressure-resistant polymerization reactor.

An MFR of the high-pressure low-density polyethylene is preferably <NUM>/<NUM> or more and <NUM>/<NUM> or less, more preferably <NUM>/<NUM> or more and <NUM>/<NUM> or less, and still more preferably <NUM>/<NUM> or more and <NUM>/<NUM> or less, from the viewpoint of reducing an extrusion load during formation of a film.

A density of the high-pressure low-density polyethylene is preferably <NUM>/m<NUM> or more and <NUM>/m<NUM> or less, more preferably <NUM>/m<NUM> or more and <NUM>/m<NUM> or less, and still more preferably <NUM>/m<NUM> or more and <NUM>/m<NUM> or less. Note that the density is measured according to the method A specified in JIS K7112-<NUM> after performing annealing described in JIS K6760-<NUM>.

A molecular weight distribution of the high-pressure low-density polyethylene is preferably <NUM> or more and <NUM> or less, and more preferably <NUM> or more and <NUM> or less.

A melt flow rate ratio (MFRR) of the high-pressure low-density polyethylene is preferably <NUM> or more and less than <NUM>, and more preferably <NUM> or more and <NUM> or less. Note that the MFRR refers to a ratio of an H-MFR to the MFR. The H-MFR is measured at a temperature of <NUM> and a load of <NUM> according to the method A defined in JIS K7210-<NUM>.

The high-pressure low-density polyethylene according to the present embodiment may contain a thermoplastic resin and a thermoplastic elastomer different from the high-pressure low-density polyethylene.

Examples of the thermoplastic resin and the thermoplastic elastomer different from the high-pressure low-density polyethylene can include linear low-density polyethylene, ultra-low-density polyethylene, an ethylene-α-olefin copolymer, an ethylene-(meth)acrylate copolymer, a metal salt of an ethylene-(meth)acrylate copolymer, an ethylene-methyl methacrylate copolymer, and ethylenepropylene copolymer rubber.

Contents of the thermoplastic resin and the thermoplastic elastomer different from the high-pressure low-density polyethylene is preferably <NUM>% by mass or less, and more preferably <NUM>% by mass or less, with respect to a total mass of <NUM>% by mass of resin components contained in the high-pressure low-density polyethylene according to the present embodiment.

The high-pressure low-density polyethylene according to the present embodiment can include additives such as an antioxidant, a slipping agent, an antistatic agent, a processability improver, an anti-blocking agent, a weather-resistant stabilizer, a release agent, a flame retardant, a metallic soap, wax, an antifungal agent, an antibacterial agent, a filler, and a foaming agent, if necessary.

Examples of the antioxidant can include a phenolic stabilizer such as <NUM>,<NUM>-di-t-butyl-p-cresol (BHT), tetrakis[methylene-<NUM>-(<NUM>,<NUM>-di-t-butyl-<NUM>-hydroxyphenyl)propionate]methane (manufactured by Ciba Specialty Chemicals Inc. , trade name: IRGANOX (registered trademark) <NUM>), or n-octadecyl-<NUM>-(<NUM>'-hydroxy-<NUM>,<NUM>'-di-t-butylphenyl)propionate (manufactured by Ciba Specialty Chemicals Inc. , trade name: IRGANOX (registered trademark) <NUM>), a phosphite stabilizer such as bis(<NUM>,<NUM>-di-t-butylphenyl)pentaerythritol diphosphite or tris(<NUM>,<NUM>-di-t-butylphenyl)phosphite, and a phenol phosphite bifunctional stabilizer such as <NUM>-[<NUM>-(<NUM>-t-butyl-<NUM>-hydroxy-<NUM>-methylphenyl)propoxy]-<NUM>,<NUM>,<NUM>,<NUM>-tetra-t-butyldibenzo[d,f][<NUM>,<NUM>,<NUM>]dioxaphosphepin (manufactured by Sumitomo Chemical Co. , trade name: SUMILIZER (registered trademark) GP). A content of the antioxidant is preferably <NUM>% by mass or more and <NUM>% by mass or less, and more preferably <NUM>% by mass or more and <NUM>% by mass or less, with respect to a total mass of <NUM>% by mass of the high-pressure low-density polyethylene.

Examples of the slipping agent can include erucamide, a higher fatty acid amide, and a higher fatty acid ester. A content of the slipping agent is preferably <NUM>% by mass or more and <NUM>% by mass or less, and more preferably <NUM>% by mass or more and <NUM>% by mass or less, with respect to the total mass of <NUM>% by mass of the high-pressure low-density polyethylene.

Examples of the antistatic agent can include a glycerin ester of a fatty acid having <NUM> to <NUM> carbon atoms, sorbitan acid ester, and polyethylene glycol ester. A content of the antistatic agent is preferably <NUM>% by mass or more and <NUM>% by mass or less, and more preferably <NUM>% by mass or more and <NUM>% by mass or less, with respect to the total mass of <NUM>% by mass of the high-pressure low-density polyethylene.

An example of the processability improver can include a fatty acid metal salt such as calcium stearate. A content of the processability improver is preferably <NUM>% by mass or more and <NUM>% by mass or less, and more preferably <NUM>% by mass or more and <NUM>% by mass or less, with respect to the total mass of <NUM>% by mass of the high-pressure low-density polyethylene.

Examples of the anti-blocking agent can include silica, diatomaceous earth, calcium carbonate, and talc. A content of the anti-blocking agent is preferably <NUM>% by mass or more and <NUM>% by mass or less, and more preferably <NUM>% by mass or more and <NUM>% by mass or less, with respect to the total mass of <NUM>% by mass of the high-pressure low-density polyethylene.

Theses additives may be added to the high-pressure low-density polyethylene, and a masterbatch obtained by adding additives to an high-pressure low-density polyethylene may be mixed with the high-pressure low-density polyethylene. When two or more high-pressure low-density polyethylenes are contained, the additives may be added after the two or more high-pressure low-density polyethylenes are blended in advance, may be added to one high-pressure low-density polyethylene, or may be added to each of the high-pressure low-density polyethylenes.

A method of producing the high-pressure low-density polyethylene according to the present embodiment includes: a step (A) of melt-kneading a mixture containing an high-pressure low-density polyethylene and a radical initiator at a temperature TA(°C); a step (B) of melt-kneading the melt-kneaded product obtained in the step (A) at a temperature TB(°C); and a step (C) of melt-kneading the melt-kneaded product obtained in the step (B) at a temperature Tc(°C), in which the following Expression (<NUM>) is satisfied: <MAT>.

As the radical initiator contained in the mixture, peroxide is preferred, and a cyclic organic peroxide represented by the following Formula (I) is more preferred. <CHM>
<CHM>.

Here, R<NUM> to R<NUM> each independently represent an alkyl group having <NUM> to <NUM> carbon atoms, a phenyl group, or an alkyl-substituted phenyl group. Among them, it is preferable that R<NUM> to R<NUM> each independently represent an alkyl group having <NUM> to <NUM> carbon atoms. In addition, among R<NUM> to R<NUM>, it is more preferable that R<NUM> to R<NUM> are alkyl groups having the same structures and R<NUM> to R<NUM> are alkyl groups having the same structures, and it is still more preferable that R<NUM> to R<NUM> are methyl groups and R<NUM> to R<NUM> are ethyl groups.

The radical initiator may be an organic peroxide other than the cyclic organic peroxide represented by Formula (I). Examples of the organic peroxide other than the cyclic organic peroxide represented by Formula (I) can include dicumyl peroxide, di-t-butyl peroxide, <NUM>,<NUM>-dimethyl-<NUM>,<NUM>-di-(t-butylperoxy)hexane, <NUM>,<NUM>-dimethyl-<NUM>,<NUM>-di(t-butylperoxy)hexyne-<NUM>, <NUM>,<NUM>-bis(t-butylperoxyisopropyl)benzene, <NUM>,<NUM>-bis(t-butylperoxy)-<NUM>,<NUM>,<NUM>-trimethylcyclohexane, n-butyl-<NUM>,<NUM>-bis(t-butylperoxy)valerate, benzoyl peroxide, p-chlorobenzoyl peroxide, <NUM>,<NUM>-dichlorobenzoyl peroxide, t-butylperoxy benzoate, t-butylperoxy isopropyl carbonate, diacetyl peroxide, lauroyl peroxide, and t-butyl cumyl peroxide. These organic peroxides may be used alone or in combination of two or more thereof. Among them, <NUM>,<NUM>-dimethyl-<NUM>,<NUM>-di-(t-butylperoxy)hexane is preferred from the viewpoint of ease of handling.

The amount of radical initiator mixed is preferably <NUM>% by mass or more, more preferably <NUM>% by mass or more, and still more preferably <NUM>% by mass or more, with respect to the total mass of <NUM>% by mass of the high-pressure low-density polyethylene contained in the mixture, from the viewpoint of strength of a molded body. The amount of radical initiator mixed is preferably <NUM>% by mass or less, more preferably <NUM>% by mass or less, and still more preferably <NUM>% by mass or less, with respect to the total mass of <NUM>% by mass of the high-pressure low-density polyethylene contained in the mixture, from the viewpoint of fluidity.

The temperature TA is preferably <NUM> or higher and <NUM> or lower, more preferably <NUM> or higher and <NUM> or lower, and still more preferably <NUM> or higher and <NUM> or lower. The temperature TB is preferably a temperature at which a half-life of the radical initiator is longer than <NUM> minute. Specifically, the temperature TB is preferably <NUM> or higher and <NUM> or lower, more preferably <NUM> or higher and <NUM> or lower, and still more preferably <NUM> or higher and <NUM> or lower. The temperature TC is preferably a temperature at which a half-life of the radical initiator is <NUM> minute or shorter. Specifically, the temperature Tc is preferably <NUM> or higher and <NUM> or lower, more preferably <NUM> or higher and <NUM> or lower, and still more preferably <NUM> or higher and <NUM> or lower, from the viewpoint of productivity.

The time for melt-kneading at the temperature TA(°C) is preferably <NUM> minutes or longer and more preferably <NUM> minutes or longer, from the viewpoint of uniform dispersibility, and is preferably <NUM> minutes or shorter and more preferably <NUM> minutes or shorter, from the viewpoint of productivity. The time for melt-kneading at the temperature TB(°C) is preferably <NUM> minutes or longer and more preferably <NUM> minutes or longer, from the viewpoint of uniform dispersibility, and is preferably <NUM> minutes or shorter and more preferably <NUM> minutes or shorter, from the viewpoint of productivity. The time for melt-kneading at the temperature Tc(°C) is generally a time equal to or longer than a half-life of the organic peroxide. Specifically, the time for melt-kneading at the temperature Tc(°C) is preferably <NUM> minutes or longer and more preferably <NUM> minutes or longer, from the viewpoint of strength of a molded body, and is preferably <NUM> minutes or shorter and more preferably <NUM> minutes or shorter, from the viewpoint of fluidity.

The melt-kneaded product obtained in each of the steps is preferably pellets.

As a melt-kneading apparatus, various known mixers such as a single-screw extruder, a twin-screw extruder, an open-type mixing roll, a closed-type Banbury mixer, a heat roll, and a kneader can be used. In the melt-kneading, all components to be kneaded may be collectively melt-kneaded or some components may be kneaded, and then, unselected components may be added and melt-kneaded.

In the method of producing the high-pressure low-density polyethylene, it is preferable that the step (A) is a step of performing melt-kneading using a melt-kneading extruder (a), the step (B) is a step of performing melt-kneading using a melt-kneading extruder (b), the step (C) is a step of performing melt-kneading using a melt-kneading extruder (c), and the melt-kneading extruder (a), the melt-kneading extruder (b), and the melt-kneading extruder (c) are different from each other. With such a configuration, workability can be improved.

Note that the same melt-kneading extruders may be used in any two steps of the step (A), the step (B), and the step (C), and the same melt-kneading extruders may be used in all the steps. In a case where the same melt-kneading extruders are used, a plurality of steps can be performed by changing the melt-kneading temperature stepwise in the melt-kneading extruders.

A film according to the present embodiment contains the high-pressure low-density polyethylene.

The film according to the present embodiment is a multi-layer film including a substrate film and a film formed of the high-pressure low-density polyethylene. The substrate film may be a substrate film having one or two or more layers.

An example of the substrate film can include a film formed of a polyamide resin such as nylon <NUM> or nylon <NUM>, a polyester resin such as polyethylene terephthalate or polybutylene terephthalate, cellophane, a paper, a paperboard, a fabric, an aluminum foil, stretched polypropylene, or polyethylene. The substrate film may include an anchor coat layer. The substrate film having two or more layers is obtained by dry-laminating or extrusion coating the respective layers.

An example of a method of producing the multi-layer film can include a method of melt-extruding a resin composition containing the high-pressure low-density polyethylene on a substrate film and laminating layers. By the extrusion coating process, it is possible to form a multi-layer film without molding defects such as edge break and film cracking. Therefore, the film according to the present embodiment is excellent in film formability. Note that the edge break is a phenomenon in which a molten film formed of an ethylene-based polymer is broken during the extrusion coating process. The film cracking is a phenomenon in which a long hole is generated at a part of a molten film formed of an ethylene-based polymer in a machine direction (MD direction) and a non-laminated part is thus generated.

In a case where the film formed of the high-pressure low-density polyethylene in the multi-layer film is used on a surface of the multi-layer film, the film is used as a sealant layer, and in a case where the film is used inside the multi-layer film, the film is used as an adhesive layer. In addition, in a case where the resin composition containing an high-pressure low-density polyethylene is laminated on a substrate film after being subjected to extrusion coating, the resin composition may be applied onto an anchor coat layer of the substrate film.

A thickness of the film according to the present embodiment is preferably <NUM> or more and <NUM> or less, and more preferably <NUM> or more and <NUM> or less.

Note that the high-pressure low-density polyethylene and the film according to the present embodiment are not limited to the above embodiment, and various modifications can be made without departing from the gist of the present invention. In addition, configurations, methods, or the like of embodiments other than the above embodiments may be optionally employed and combined, and configurations, methods, or the like according to one embodiment may be applied to configurations, methods, or the like according to another embodiment.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples. However, the present invention is not limited to the following Examples.

A melt flow rate was measured at a temperature of <NUM> and a load of <NUM> according to the method A according to the method defined in JIS K7210-<NUM>.

<NUM> parts by mass of a radical initiator (manufactured by Kayaku Nouryon Corporation, Trigonox <NUM>) was immersed in <NUM> parts by mass of a pulverized powder of a high-pressure low-density polyethylene as an ethylene-based polymer (manufactured by Sumitomo Seika Chemicals Company, Ltd. , FLO-THENE FG801NN). Masterbatch pellets (hereinafter, referred to as MB pellets) were prepared by melt-kneading the obtained immersed powder at <NUM> for <NUM> minute using a melt-kneading extruder (a) having a screw diameter of <NUM> (manufactured by UNIPLAS Corporation).

The obtained MB pellets were added to <NUM> parts by mass of a high-pressure low-density polyethylene as an ethylene-based polymer (manufactured by Sumitomo Chemical Co. , SUMIKATHENE CE4506) so that a concentration thereof was <NUM>,<NUM> ppm, and the mixture was melt-kneaded at <NUM> for <NUM> minute using a melt-kneading extruder (b) having a screw diameter of <NUM> (manufactured by TANABE PLASTICS MACHINERY CO. ), thereby obtaining pellets. The obtained pellets were melt-kneaded at <NUM> for <NUM> minute using another melt-kneading extruder (c) having a screw diameter of <NUM> (manufactured by TANABE PLASTICS MACHINERY CO. ), thereby obtaining pellets.

The obtained pellets were left to stand at <NUM> for <NUM> minutes (pre-heat step), were pressurized at <NUM> and <NUM> MPa for <NUM> minutes (press step), and were gradually cooled at <NUM> for <NUM> minutes (gradual cooling step), thereby obtaining a press sheet having a thickness of <NUM>. A circular sheet having a diameter of <NUM> was punched out from the obtained press sheet to prepare a measurement sample. The obtained measurement sample was measured according to a LAOS method using a dynamic viscoelasticity measuring device (manufactured by TA Instruments, Inc. , ARES-G2).

A viscosity ηL<NUM>% at the highest shear rate of the measurement sample at a strain of γ<NUM> = <NUM>% and a viscosity ηL<NUM>,<NUM>% at the highest shear rate of the measurement sample at a strain of γ<NUM> = <NUM>,<NUM>% were measured at a temperature of <NUM> and a frequency of <NUM>. In each of the measurement of ηL<NUM>% and the measurement of ηL<NUM>,<NUM>%, a sinusoidal shear strain of <NUM> was applied to the measurement sample for <NUM> cycles, and an average value of values obtained in the latter <NUM> cycles was a measured value. In addition, the measurement of ηL<NUM>% was performed first, and the measurement of ηL<NUM>,<NUM>% was performed while increasing each of a strain of γ<NUM> = <NUM>% to <NUM>%, a strain of γ<NUM> = <NUM>% to <NUM>,<NUM>%, and a strain of γ<NUM> = <NUM>,<NUM>% to <NUM>,<NUM>% in <NUM> logarithmically evenly spaced steps. The ratio of ηL<NUM>,<NUM>%/ηL<NUM>% was <NUM> and Expression (<NUM>) was satisfied.

In addition, an intensity I1<NUM>,<NUM>% of a first harmonic and an intensity of I5<NUM>,<NUM>% of a fifth harmonic were measured at a temperature of <NUM> and a frequency of <NUM>, the intensities being obtained by Fourier-transforming (manufactured by TA Instruments, Inc. , software name: TRIOS ver5. <NUM>) a response stress of the measurement sample at a strain of γ<NUM> = <NUM>,<NUM>%. In each of the measurement of I1<NUM>,<NUM>% and the measurement of I5<NUM>,<NUM>%, a sinusoidal shear strain of <NUM> was applied to the measurement sample for <NUM> cycles, and an average value of values obtained in the latter <NUM> cycles was a measured value. In addition, the measurement of the response stress at a strain of γ<NUM> = <NUM>% was performed first, and the measurement of the response stress was performed while increasing each of a strain of γ<NUM> = <NUM>% to <NUM>%, a strain of γ<NUM> = <NUM>% to <NUM>,<NUM>%, and a strain of γ<NUM> = <NUM>,<NUM>% to <NUM>,<NUM>% in <NUM> logarithmically evenly spaced steps. The ratio of I5<NUM>,<NUM>%/I1<NUM>,<NUM>% was <NUM> and Expression (<NUM>) was satisfied. The results are shown in Table <NUM>.

The obtained pellets were extruded, laminated, and molded on a PET substrate having a thickness of <NUM> under conditions of a T-die inner width of <NUM>, an air gap of <NUM>, an high-pressure low-density polyethylene laminate thickness of <NUM>, a temperature directly below a T-die of <NUM>, and a lamination speed of <NUM>/min using a coextrusion coating machine provided with a T-die having a width of <NUM> at a distal end of an extruder having a screw diameter of <NUM> (manufactured by Sumitomo Heavy Industries Modern, Ltd. In the obtained laminate sample, a thickness of the high-pressure low-density polyethylene was measured in a width direction with a width of <NUM> at the central portion of the sample using a desk-top offline thickness gauge (manufactured by Yamabun Electronics Co. , TOF-5R01). A standard deviation of the obtained measured value was <NUM>. The results are shown in Table <NUM>.

MB pellets were prepared in the same manner as that of Example <NUM>. The obtained MB pellets were added to <NUM> parts by mass of a high-pressure low-density polyethylene as an ethylene-based polymer (manufactured by Sumitomo Chemical Co. , SUMIKATHENE CE4506) so that a concentration thereof was <NUM>,<NUM> ppm, and the mixture was melt-kneaded at <NUM> using a melt-kneading extruder (b) having a screw diameter of <NUM> (manufactured by TANABE PLASTICS MACHINERY CO. ), thereby obtaining pellets. The obtained pellets were melt-kneaded at <NUM> using another melt-kneading extruder (c) having a screw diameter of <NUM> (manufactured by TANABE PLASTICS MACHINERY CO. ), thereby obtaining pellets.

As a result of measuring ηL<NUM>,<NUM>%/ηL<NUM>% using the obtained pellets according to the LAOS method in the same manner as that of Example <NUM>, ηL<NUM>,<NUM>%/ηL<NUM>% was <NUM> and Expression (<NUM>) was satisfied. In addition, as a result of measuring I5<NUM>,<NUM>%/I1<NUM>,<NUM>%, I5<NUM>,<NUM>%/I1<NUM>,<NUM>% was <NUM> and Expression (<NUM>) was satisfied. The results are shown in Table <NUM>.

An extruded and laminated sample was prepared using the obtained pellets in the same manner as that of Example <NUM>, and a thickness of the high-pressure low-density polyethylene laminate layer having the central portion with a width of <NUM> was measured in a width direction. A standard deviation of the obtained measured value was <NUM>.

MB pellets were prepared in the same manner as that of Example <NUM>. The obtained MB pellets were added to <NUM> parts by mass of a high-pressure low-density polyethylene as an ethylene-based polymer (manufactured by Sumitomo Chemical Co. , SUMIKATHENE CE3049) so that a concentration thereof was <NUM> ppm, and the mixture was melt-kneaded at <NUM> using a melt-kneading extruder (b) having a screw diameter of <NUM> (manufactured by TANABE PLASTICS MACHINERY CO. ), thereby obtaining pellets. The obtained pellets were melt-kneaded at <NUM> using another melt-kneading extruder (c) having a screw diameter of <NUM> (manufactured by TANABE PLASTICS MACHINERY CO. ), thereby obtaining pellets.

An immersed powder was prepared in the same manner as that of Example <NUM>. The obtained immersed powder was added to <NUM> parts by mass of a high-pressure low-density polyethylene as an ethylene-based polymer (manufactured by Sumitomo Chemical Co. , SUMIKATHENE CE4506) so that a concentration thereof was <NUM>,<NUM> ppm, the mixture was kneaded in advance, and the mixture was melt-kneaded at <NUM> using a melt-kneading extruder (c) having a screw diameter of <NUM> (manufactured by TANABE PLASTICS MACHINERY CO. ), thereby obtaining pellets.

As a result of measuring ηL<NUM>,<NUM>%/ηL<NUM>% using the obtained pellets according to the LAOS method in the same manner as that of Example <NUM>, ηL<NUM>,<NUM>%/ηL<NUM>% was <NUM> and Expression (<NUM>) was satisfied. On the other hand, as a result of measuring I5<NUM>,<NUM>%/I1<NUM>,<NUM>%, I5<NUM>,<NUM>%/I1<NUM>,<NUM>% was <NUM> and did not satisfy Expression (<NUM>). The results are shown in Table <NUM>.

An extruded and laminated sample was prepared using the obtained pellets in the same manner as that of Example <NUM>, and a thickness of the high-pressure low-density polyethylene layer having the central portion with a width of <NUM> was measured in a width direction. A standard deviation of the obtained measured value was <NUM>, and thickness unevenness was generated.

As a result of measuring ηL<NUM>,<NUM>%/ηL<NUM>% using pellets of a high-pressure low-density polyethylene as an ethylene-based polymer (manufactured by Sumitomo Chemical Co. , SUMIKATHENE L420) according to the LAOS method in the same manner as that of Example <NUM>, ηL<NUM>,<NUM>%/ηL<NUM>% was <NUM> and did not satisfy Expression (<NUM>). In addition, as a result of measuring I5<NUM>,<NUM>%/I1<NUM>,<NUM>%, I5<NUM>,<NUM>%/I1<NUM>,<NUM>% was <NUM> and did not satisfy Expression (<NUM>). The results are shown in Table <NUM>.

An extruded and laminated sample was prepared using the pellets in the same manner as that of Example <NUM>, and a thickness of the high-pressure low-density polyethylene layer having the central portion with a width of <NUM> was measured in a width direction. A standard deviation of the obtained measured value was <NUM>, and thickness unevenness was generated.

Claim 1:
A high-pressure low-density polyethylene produced by a method comprising:
a step (A) of melt-kneading a mixture containing a high-pressure low-density polyethylene and a radical initiator at a temperature TA(°C);
a step (B) of melt-kneading the melt-kneaded product obtained in the step (A) at a temperature TB(°C); and
a step (C) of melt-kneading the melt-kneaded product obtained in the step (B) at a temperature TC(°C),
wherein the following Expression (<NUM>) is satisfied: <MAT>
the high-pressure low-density polyethylene satisfying the following Expressions (<NUM>) and (<NUM>): <MAT>
wherein,
ηL<NUM>% represents a viscosity (Pa·sec) at the highest shear rate of the high-pressure low-density polyethylene at a strain of γ<NUM> = <NUM>% of the high-pressure low-density polyethylene measured by a large amplitude oscillatory shear (LAOS) method at <NUM> and <NUM>, and
ηL<NUM>,<NUM>% represents a viscosity (Pa·sec) at the highest shear rate of the high-pressure low-density polyethylene at a strain of γ<NUM> = <NUM>,<NUM>% of the high-pressure low-density polyethylene measured by the LAOS method at <NUM> and <NUM>, and <MAT>
wherein,
I1<NUM>,<NUM>% represents an intensity of a first harmonic wave obtained by Fourier-transforming a response stress of the high-pressure low-density polyethylene at a strain of γ<NUM> = <NUM>,<NUM>% of the high-pressure low-density polyethylene measured by the LAOS method at <NUM> and <NUM>, and
I5<NUM>,<NUM>% represents an intensity of a fifth harmonic obtained by Fourier-transforming a response stress of the high-pressure low-density polyethylene at the strain of γ<NUM> = <NUM>,<NUM>% of the high-pressure low-density polyethylene measured by the LAOS method at <NUM> and <NUM>.