Method for manufacturing polyethylene resin multi-layer foam sheet, and polyethylene resin multi-layer foam sheet

Abstract

To provide a method for manufacturing a polyethylene-based resin foam sheet that exhibits excellent antistatic properties and suppresses the adhesion of metal ions to contacted objects, and a polyethylene-based resin foam sheet provided thereby.SOLUTION: A method for manufacturing a polyethylene-based resin multi-layer foamed sheet, wherein the amount of conductive carbon in the conductive carbon-containing resin layer is 0.01 g / m2 or more and 0.5 g / m2 or less, the physical blowing agent comprises one or more organic physical blowing agents selected from hydrocarbons having 3 to 5 carbon atoms and dialkyl ether having alkyl groups with 1 to 3 carbon atoms, and nitrogen, the sum (A+B) of the amount A of the organic physical blowing agent added and the amount B of nitrogen added is 0.5 mol or more but not more than 5 mol per 1 kg of the resin component constituting the foaming resin melt for forming the foamed layer, and the amount B of nitrogen added is 0.1 mol or more but not more than 0.4 mol per 1 kg of the resin component constituting the foaming resin melt for forming the foamed layer.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a multi-layer polyethylene resin foam sheet, and a multi-layer polyethylene resin foam sheet. [Background technology]

[0002] Polyethylene-based resin foam sheets are used in a variety of applications. Examples of applications of polyethylene-based resin foam sheets include cushioning materials such as packaging materials and packing materials. When polyethylene-based resin foam sheets are used for applications such as packaging materials, the polyethylene-based resin foam sheets generally come into contact with the packaged items. In this specification, polyethylene-based resin foam sheets are also simply referred to as foam sheets. Furthermore, items that come into contact with the foam sheets, such as packaged items packaged in the foam sheets, are also referred to as contacted items.

[0003] Patent Document 1 discloses the development of a multilayer foamed sheet having a layer structure imparted with antistatic properties as a polyethylene-based resin foamed sheet. The foamed sheet disclosed in Patent Document 1 can provide effects such as preventing dust caused by static electricity from adhering to objects that come into contact with it. Specific examples of applications of foamed sheets with antistatic properties include packaging materials for precision instruments and slip sheets for liquid crystal glass substrates. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2020-138357 Summary of the Invention [Problem to be solved by the invention]

[0005] Generally, to impart antistatic properties to a foam sheet, an antistatic agent is incorporated into the foam sheet. Polymeric antistatic agents are widely used as the antistatic agents incorporated into foam sheets. A metal salt is typically added to the polymeric antistatic agent to impart antistatic properties. That is, foam sheets imparted with antistatic properties may contain metal ions derived from the metal salt added to the polymeric antistatic agent. Therefore, foam sheets imparted with antistatic properties may transfer metal ions to objects that come into contact with the foam sheet (i.e., metal ions may adhere to the objects). For this reason, there is room for improvement in terms of suppressing the transfer of metal ions to objects that come into contact with the foam sheet, depending on applications where metal ion adhesion is a concern, such as semiconductors and glass substrates used in precision electronic devices.

[0006] One object of the present invention is to provide a method for producing a polyethylene-based resin multi-layer foamed sheet that has good antistatic properties and can suppress the transfer of metal ions to objects that come into contact with it, and to provide a polyethylene-based resin multi-layer foamed sheet that can be produced by the method. [Means for solving the problem]

[0007] The following techniques [1] to [9] are presented regarding aspects of the present invention. [1] A method for producing a polyethylene-based resin multi-layer foamed sheet comprising a polyethylene-based resin foamed layer and a conductive carbon-containing resin layer by co-extruding a foamable resin melt for forming a foamed layer, which is obtained by kneading a polyethylene-based resin with a physical foaming agent, and a conductive carbon-containing resin melt for forming a conductive carbon-containing resin layer, which is obtained by kneading a polyethylene-based resin with conductive carbon, The amount of conductive carbon in the conductive carbon-containing resin layer is 0.01 g / m 2 More than 0.5g / m 2 is as follows: the physical blowing agent contains one or more organic physical blowing agents selected from hydrocarbons having 3 to 5 carbon atoms and dialkyl ethers having an alkyl group with 1 to 3 carbon atoms, and nitrogen; the sum (A+B) of the amount A of the organic physical foaming agent added and the amount B of the nitrogen added is 0.5 mol or more and 5 mol or less per kg of the resin component constituting the foam layer-forming foamable resin melt, A method for producing a polyethylene-based resin multi-layer foamed sheet, wherein the amount B of nitrogen added is 0.1 mol or more and 0.4 mol or less per kg of the resin component constituting the foamable resin melt for forming the foam layer. [2] The method for producing a polyethylene-based resin multi-layer foamed sheet according to the above [1], wherein the ratio (A / B) of the amount A of the organic physical foaming agent to the amount B of the nitrogen is 2 or more and 18 or less. [3] The method for producing a polyethylene-based resin multi-layer foamed sheet according to [1] or [2] above, wherein no powdery inorganic substance (but not including a chemical foaming agent) and no chemical foaming agent are added to the foam layer-forming molten foamable resin, or wherein a powdery inorganic substance (but not including a chemical foaming agent) and / or a chemical foaming agent are added to the foam layer-forming molten foamable resin, and the total amount of the inorganic substance and the chemical foaming agent added is less than 0.1 parts by mass per 100 parts by mass of the resin components constituting the foam layer-forming molten foamable resin. [4] The density of the polyethylene resin multi-layer foam sheet is 20 kg / m 3 More than 200kg / m 3 A method for producing a polyethylene-based resin multi-layer foamed sheet according to any one of the above [1] to [3], which is as follows: [5] The method for producing a polyethylene-based resin multi-layer foamed sheet according to any one of [1] to [4] above, wherein the polyethylene-based resin multi-layer foamed sheet has an average cell diameter of 100 μm or more and 1000 μm or less. [6] The method for producing a polyethylene-based resin multi-layer foamed sheet according to any one of [1] to [5] above, wherein the polyethylene-based resin multi-layer foamed sheet comprises the polyethylene-based resin foamed layer, the conductive carbon-containing resin layer laminated on at least one side of the polyethylene-based resin foamed layer, and a resin surface layer laminated on the conductive carbon-containing resin layer. [7] A polyethylene-based resin foam layer and a conductive carbon-containing resin layer containing a polyethylene-based resin and conductive carbon, having a density of 20 kg / m 3 More than 200kg / m 3 A polyethylene-based resin multi-layer foamed sheet as follows: The amount of conductive carbon in the conductive carbon-containing resin layer is 0.01 g / m 2 More than 0.5g / m 2 is as follows: The total amount of sodium ions, potassium ions and lithium ions extracted from the polyethylene-based resin multi-layer foamed sheet is 0.10 ng / cm 2 A polyethylene-based resin multi-layer foam sheet characterized by having the following (including 0): [8] The polyethylene-based resin multi-layer foamed sheet according to [7] above, wherein the polyethylene-based resin multi-layer foamed sheet has an average cell diameter of 100 μm or more and 1000 μm or less. [9] The polyethylene-based resin multi-layer foamed sheet according to [7] or [8], wherein the ash content of the polyethylene-based resin multi-layer foamed sheet is 0.1% by mass or less (including 0). [Effects of the Invention]

[0008] According to the present invention, there are provided a method for producing a polyethylene-based resin multi-layer foamed sheet that has good antistatic properties and can suppress adhesion of metal ions to objects that come into contact with the sheet, and a polyethylene-based resin multi-layer foamed sheet that can be produced by the method. [Brief explanation of the drawings]

[0009] [Figure 1] 1A to 1D are cross-sectional views each showing a schematic example of a polyethylene-based resin multi-layer foamed sheet according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] An example of an embodiment of the present invention will be described in detail below, but the present invention is not limited to the embodiment described below.

[0011] This specification discloses a method for producing a polyethylene-based resin multi-layer foamed sheet, and a polyethylene-based resin multi-layer foamed sheet. In this specification, an example of an embodiment of the present invention will be described in the order of the method for producing a polyethylene-based resin multi-layer foamed sheet and the polyethylene-based resin multi-layer foamed sheet. When describing that the method for producing a polyethylene-based resin multi-layer foamed sheet, the polyethylene-based resin multi-layer foamed sheet, application examples, and examples are applied to the entire specification, the term "in this specification" may be used.

[0012] In this specification, when multiple numerical ranges are defined for "items defined using numerical values ​​(referred to as numerically defined items (NM))" such as a numerical value corresponding to a specific attribute or an amount of addition, for each numerically defined item (NM), the upper and lower limit values ​​in the combinations defining each numerical range may be independently combined in any combination. In this specification, a combination defining a numerical range refers to a combination of an upper limit value and a lower limit value. Note that a numerical range defined by any combination of an upper limit value and a lower limit value includes the upper limit value, the lower limit value, and values ​​between the upper limit value and the lower limit value (i.e., the numerical range is equal to or greater than the lower limit value and equal to or less than the upper limit value). For example, if the numerical value for a target numerical requirement (NM) (such as the density or amount of a specific compound) is between MA1 and MB1, preferably between MA2 and MB2, and more preferably between MA3 and MB3, the lower limit of the target numerical requirement (NM) may be any value (selected lower limit) selected from the group consisting of MA1, MA2, and MA3, and the upper limit of the target numerical requirement (NM) may be any value (selected upper limit) selected from the group consisting of MB1, MB2, and MB3. Furthermore, the numerical range of the target numerical requirement (NM) may be a numerical range (selected numerical range) that is between the "selected lower limit" and the "selected upper limit." Note that MA1, MA2, MA3, MB1, MB2, and MB3 represent numerical values.

[0013] Furthermore, unless otherwise specified, even when different numerical requirements (NM) are combined, the upper limit of each numerical requirement (NM) may be individually considered to be the "selected upper limit." This also applies to the lower limit and numerical range of the numerical requirements (NM).

[0014] In addition, in this specification, the expression "MA1 to MB1", which expresses a numerical range using the symbol "to", is synonymous with "not less than MA1 and not more than MB1", and represents a numerical range that includes MA1 and MB1, which are the endpoints of the numerical range.

[0015] In this specification, a polyethylene-based resin multi-layer foam sheet may be abbreviated as a multi-layer foam sheet. In addition, in this specification, a polyethylene-based resin foam layer may be abbreviated as a foam layer. In this specification, a conductive carbon-containing resin layer may be referred to as a carbon-containing layer. In this specification, low-density polyethylene may be abbreviated as PE-LD, as necessary. In addition, in this specification, linear low-density polyethylene may be abbreviated as PE-LLD.

[0016] [Method of manufacturing polyethylene resin multi-layer foam sheet] The method for producing a polyethylene-based resin multi-layer foamed sheet (method for producing a multi-layer foamed sheet) according to the present invention is a method for producing a polyethylene-based resin multi-layer foamed sheet comprising a polyethylene-based resin foam layer and a conductive carbon-containing resin layer. The method for producing a multi-layer foamed sheet according to the present invention is a method for producing a multi-layer foamed sheet by co-extruding a foam layer-forming foamable resin melt and a conductive carbon-containing resin layer-forming resin melt. The foam layer-forming foamable resin melt can form a polyethylene-based resin foam layer, and the conductive carbon-containing resin layer-forming resin melt can form a conductive carbon-containing resin layer.

[0017] In the method for producing a multi-layer foamed sheet according to the present invention, the polyethylene-based resin multi-layer foamed sheet produced has a conductive carbon content of 0.01 g / m in the carbon-containing layer. 2 More than 0.5g / m 2In the method for producing a multi-layer foamed sheet according to the present invention, the physical foaming agent contained in the foam layer-forming molten resin contains an organic physical foaming agent and nitrogen, and the organic physical foaming agent and nitrogen satisfy predetermined conditions, as described below.

[0018] In this specification, the foamable resin melt for forming a foam layer may be abbreviated as "foamable resin melt." The resin melt for forming a conductive carbon-containing resin layer may be abbreviated as "carbon-containing layer forming resin melt." When there is no need to distinguish between the foamable resin melt for forming a foam layer and the resin melt for forming a conductive carbon-containing resin layer, they will be collectively referred to simply as "melt." The same applies to the case where a resin melt for forming a resin surface layer, which will be described later, is used. That is, when there is no need to distinguish between the foamable resin melt for forming a foam layer, the resin melt for forming a conductive carbon-containing resin layer, and the resin melt for forming a resin surface layer, they will be collectively referred to simply as "melt."

[0019] In this specification, a method of forming a structure having a laminated structure by co-extruding molten materials is referred to as a co-extrusion method.

[0020] The method for producing a multilayer foam sheet according to the present invention produces a multilayer foam sheet having a structure in which a carbon-containing layer is laminated on at least one side (one surface) of a foam layer. Examples of the structure of the multilayer foam sheet include a structure in which a carbon-containing layer is laminated on both sides of a foam layer (first laminate structure) and a structure in which a carbon-containing layer is laminated on one side of a foam layer (second laminate structure), as described below. Examples of the structure of the multilayer foam sheet include a structure in which a carbon-containing layer and a resin surface layer are laminated on both sides of a foam layer (third laminate structure), and a structure in which a carbon-containing layer and a resin surface layer are laminated on one side of a foam layer (fourth laminate structure), as described below. In the third and fourth laminate structures, the resin surface layer is located on the outermost surface of the multilayer foam sheet, and the carbon-containing layer is sandwiched between the foam layer and the resin surface layer.

[0021] In the following description of the method for producing a multilayer foam sheet, a basic example of the production method and an additional example of the production method will be described as examples of the production method. In the basic example of the production method, an example of a method for forming a first laminate structure will be described. In the explanation accompanying the basic example, an example of a method for forming a second laminate structure will be described. In the additional example of the production method, an example of a method for forming a third laminate structure will be described. In the explanation accompanying the additional example, an example of a method for forming a fourth laminate structure will be described. Note that, as will be described later, detailed explanations of the additional examples that are common to the basic example (steps, conditions, etc.) will be omitted.

[0022] [1 Basic example of manufacturing method] In the basic example of the manufacturing method, a method for manufacturing a multi-layer foamed sheet having a first laminate structure in which a carbon-containing layer is laminated on both sides of a foamed layer will be described. Note that the co-extrusion method described here is only an example and does not limit the manufacturing method of the multi-layer foamed sheet according to the present invention.

[0023] (co-extrusion method) A co-extrusion method for producing a multi-layer foamed sheet will now be described. The apparatus used to carry out the co-extrusion method is not particularly limited. As an apparatus for forming a multi-layer foamed sheet having a first laminate structure (a laminate structure formed of two types of layers, i.e., three layers) in which carbon-containing layers are laminated on both sides of the foamed layer as described above, for example, the following co-extrusion extruder (co-extrusion apparatus) can be used. The co-extrusion apparatus includes an extruder for forming a foamed layer, an extruder for forming a carbon-containing layer, and a co-extrusion die. The outlet of the extruder for forming a foamed layer and the outlet of the extruder for forming a carbon-containing layer are connected to the co-extrusion die.

[0024] In the example of the co-extrusion device shown here, the extruder for forming a foam layer is a tandem extruder having a first extruder and a second extruder connected in series, and the extruder for forming a carbon-containing layer is a third extruder connected to a co-extrusion die attached downstream of the tandem extruder that serves as the extruder for forming a foam layer. The co-extrusion die has a melt path formed so that the resin melt for forming a carbon-containing layer is laminated on both sides of the foamable resin melt for forming a foam layer. When the extruder for forming a foam layer and the extruder for forming a carbon-containing layer are not distinguished, they are simply referred to as "extruders." This also applies to the extruder for forming a resin surface layer shown in the additional example of the manufacturing method described below. When the foamable resin melt for forming a foam layer and the resin melt for forming a carbon-containing layer are not distinguished, they are simply referred to as "melt." This also applies to the resin melt for forming a resin surface layer shown in the additional example of the manufacturing method described below.

[0025] In the example using the coextrusion device described above, the coextrusion method can be realized by introducing the foamable resin melt for forming the foam layer (foamable resin melt) and the resin melt for forming the carbon-containing layer, which are formed in the respective extruders, into a coextrusion die and simultaneously extruding them from the extrusion opening of the coextrusion die. As the coextrusion die, for example, a circular die equipped with a circular extrusion opening is used. However, a flat die equipped with a linear extrusion opening may also be used as the coextrusion die.

[0026] In the example of the co-extrusion device shown above, a laminate of a foamable resin melt and a carbon-containing layer-forming resin melt is extruded into a cylindrical shape from the extrusion port of the co-extrusion die. When the foamable resin melt is extruded into the atmosphere from the extrusion port of the co-extrusion die, the foamable resin melt foams and expands to form a foam layer, thereby forming a tubular laminated foam. The tubular laminated foam has a structure in which a foam layer and a layer composed of a carbon-containing layer-forming resin melt are laminated. The layers composed of the carbon-containing layer-forming resin melt are formed on both sides of the foam layer. Furthermore, as the layer composed of the foamable resin melt expands, the layer composed of the carbon layer-forming resin melt is stretched. The tubular laminated foam extruded from the extrusion port is widened from the inside with compressed air or the like, and the inside of the tubular laminated foam is cooled while being taken up by a take-up machine such as a roller along a widening device such as a mandrel, thereby solidifying the foam layer and the layer composed of the carbon-containing layer-forming resin melt. This largely fixes the cell structure formed in the foam layer. The layer composed of the carbon-containing layer-forming resin melt becomes a carbon-containing resin layer. Then, the cylindrical laminated foam is cut open with a cutter or the like on the width-expanding device. In this way, a multi-layer foam sheet is obtained having a laminated structure (a three-layer laminated structure) including a foam layer and carbon-containing resin layers laminated on both sides of the foam layer (laminated in a state of being directly adhered to the foam layer). When an annular die is used as the co-extrusion die, it is easy to produce a wide multi-layer foamed sheet, for example, having a width of 1000 mm or more. Furthermore, when an annular die is used as the co-extrusion die, it is easy to produce a thin multi-layer foamed sheet, for example, having an overall thickness of 3 mm or less. From this viewpoint, it is preferable to use an annular die as the co-extrusion die.

[0027] From the viewpoint of stably producing a multi-layer foamed sheet by co-extrusion and obtaining a multi-layer foamed sheet that is easily recycled, it is preferable that neither of the melts contains a crosslinking agent.

[0028] (Co-extrusion method when the multi-layer foam sheet has a second laminate structure) The above explanation of the co-extrusion method has been given as an example of a production method in which the multi-layer foam sheet has a first laminate structure. In the case where the multi-layer foam sheet has a second laminate structure, the following co-extrusion die is preferably employed. That is, a co-extrusion die is used in which a melt path is formed so that the carbon-containing layer-forming resin melt is laminated on one side of the foam layer-forming foamable resin melt. In this case, the resulting tubular laminated foam has a structure in which a foam layer and a layer composed of the carbon-containing layer-forming resin melt are laminated, and the layer composed of the carbon-containing layer-forming resin melt is laminated on one side of the foam layer. The tubular laminated foam is then cut open with a cutter or the like to obtain a multi-layer foam sheet having the second laminate structure.

[0029] (Foamable resin melt for forming foam layer) The foamable resin melt for forming a foam layer (foamable resin melt) contains a polyethylene-based resin and a physical foaming agent. The foamable resin melt is a mixture of the polyethylene-based resin and the physical foaming agent. The polyethylene-based resin contained in the foamable resin melt is in a molten state. The foamable resin melt can be produced, for example, by the following method. For example, a polyethylene-based resin is supplied to a first extruder of the foam layer-forming extruder described above, and the polyethylene-based resin is melt-kneaded in the first extruder. Thereafter, a physical foaming agent is supplied into the extruder downstream of the first extruder, and the polyethylene-based resin and the physical foaming agent supplied in the first extruder are kneaded together. This produces a foamable resin melt. For convenience of explanation, the polyethylene-based resin contained in the foamable resin melt may be referred to as a polyethylene-based resin (F) in this specification. The polyethylene-based resin contained in the carbon-containing layer-forming resin melt described later may be referred to as a polyethylene-based resin (A). Furthermore, the polyethylene resin used to form the resin melt for forming the resin surface layer may be referred to as polyethylene resin (B).

[0030] (Polyethylene resin) In this specification, polyethylene-based resins (polyethylene-based resin (F), polyethylene-based resin (A), and polyethylene-based resin (B)) refer to resins containing 40% by mass or more of structural units derived from ethylene. The polyethylene-based resin (polyethylene-based resin (F)) used to form the foamed layer is preferably a resin containing 50% by mass or more of structural units derived from ethylene, more preferably a resin containing 70% by mass or more of structural units derived from ethylene, and even more preferably a resin containing 80% by mass or more of structural units derived from ethylene. Examples of polyethylene-based resins used when preparing a foamable resin melt include polyethylenes such as high-density polyethylene, low-density polyethylene, and linear low-density polyethylene.

[0031] In this specification, low-density polyethylene has a long-chain branched structure and a density of 910 kg / m 3 More than 930kg / m 3 This indicates polyethylene of less than 100%. Linear low-density polyethylene is a copolymer of ethylene and an α-olefin, and refers to polyethylene whose molecular chain is substantially linear. The density of linear low-density polyethylene is 910 kg / m 3 More than 940kg / m 3 It is preferable that: High density polyethylene has a density of 941 kg / m 3 The above polyethylene is shown. In addition, in JIS K 6899-1:2015 "Plastics - Symbols and abbreviations - Part 1: Basic polymers and their properties," low-density polyethylene is abbreviated as "PE-LD," linear low-density polyethylene is abbreviated as "PE-LLD," and high-density polyethylene is abbreviated as "PE-HD."

[0032] The foamable resin melt may contain one type of polyethylene resin, or may contain two or more types of polyethylene resins.

[0033] From the viewpoint of improving the flexibility, cushioning properties, and foamability of the multilayer foamed sheet obtained by applying the production method of the present invention, it is preferable to use a polyethylene resin containing low-density polyethylene as a main component as the polyethylene resin (F). Specifically, the foamable resin melt contains preferably 50% by mass or more of low-density polyethylene (where the ratio of the total mass of the resin components contained in the foamable resin melt is taken as 100% by mass), more preferably 60% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more.

[0034] The melting point TmA of the polyethylene resin (polyethylene resin (F)) contained in the foamable resin melt is preferably 100°C or higher and 135°C or lower. By setting the melting point TmA of the polyethylene resin within the above range, it is possible to obtain the effect of improving the foamability of the foamable resin melt and further the effect of improving the cushioning properties of the multi-layer foamed sheet. From the viewpoint of improving these effects, the melting point TmA of the polyethylene resin is preferably 100°C or higher and 130°C or lower, more preferably 105°C or higher and 120°C or lower, and even more preferably 108°C or higher and 115°C or lower.

[0035] The melting point TmA of the polyethylene resin (F) can be measured by the plastic transition temperature measurement method specified in JIS K7121:2012. Specifically, a polyethylene resin is used as a test specimen. The test specimen is then conditioned with heating and cooling rates of 10°C / min according to the "measurement of melting temperature after a certain heat treatment" procedure. The heating temperature is then set to 10°C / min, and heat flux DSC (i.e., differential scanning calorimetry) is performed using the test specimen. A DSC curve is obtained by using the temperature at the apex of the endothermic peak in the resulting DSC curve as the melting point. Note that if multiple endothermic peaks appear in the DSC curve, the temperature at the apex of the melting peak with the largest area, relative to the baseline, is used as the melting point.

[0036] From the viewpoint of improving the foamability of the foamable resin melt, the melt flow rate (MFR) of the polyethylene resin (F) is preferably 0.5 g / 10 min or more and 15 g / 10 min or less, more preferably 1 g / 10 min or more and 8 g / 10 min or less, and even more preferably 1.5 g / 10 min or more and 5 g / 10 min or less. Note that the MFR (g / 10 min) of the polyethylene resin in this specification is a value measured in accordance with JIS K7210-1:2014 under conditions of a test temperature of 190°C and a load of 2.16 kg.

[0037] (physical foaming agent) The foamable resin melt contains a physical foaming agent. The physical foaming agent contained in the foamable resin melt contains an organic physical foaming agent and nitrogen. That is, the foamable resin melt uses an organic physical foaming agent and nitrogen in combination as physical foaming agents. The organic physical foaming agent and nitrogen may be supplied to the extruder at the same time or at different times.

[0038] (organic physical foaming agent) The organic physical blowing agent constituting the physical blowing agent is one or more organic compounds (one type of organic compound or a "combination of multiple types of organic compounds") selected from the group of organic compounds consisting of hydrocarbons having 3 to 5 carbon atoms and dialkyl ethers having alkyl groups with 1 to 3 carbon atoms. Examples of the "hydrocarbons having 3 to 5 carbon atoms" include butane, pentane, propane, and hydrofluoroolefins such as 1-chloro-3,3,3-trifluoropropene (HCFO-1233zd). Examples of the "dialkyl ethers having alkyl groups with 1 to 3 carbon atoms" include dimethyl ether. The organic physical blowing agent is preferably composed primarily of butane and / or dimethyl ether. Specifically, when the molar ratio of the total organic compounds constituting the organic physical blowing agent is taken as 100 mol%, the sum of the proportion of butane and the proportion of dimethyl ether in the organic physical blowing agent is preferably 50 mol% or more, more preferably 60 mol% or more, even more preferably 80 mol% or more, and particularly preferably 90 mol% or more. Furthermore, when the molar ratio of all organic compounds constituting the organic physical blowing agent is taken as 100 mol %, the proportion of butane in the organic physical blowing agent is preferably 50 mol % or more, more preferably 80 mol % or more, and even more preferably 90 mol % or more.

[0039] (nitrogen) Nitrogen, which constitutes the physical foaming agent, is used as a type of inorganic physical foaming agent.

[0040] In the present invention, the physical foaming agent added to the foamable resin melt contains one or more organic physical foaming agents selected from hydrocarbons having 3 to 5 carbon atoms and dialkyl ethers having an alkyl group with 1 to 3 carbon atoms, and nitrogen, and the amount A of the organic physical foaming agent added and the amount B of the nitrogen added satisfy the following predetermined conditions:

[0041] (Amount of organic physical foaming agent and nitrogen added) The amount A of the organic physical blowing agent added and the amount B of nitrogen added are the amounts of the organic physical blowing agent added to the foamable resin melt, and are also the amounts of the agents (molar amounts) per kg of the resin components constituting the foamable resin melt.

[0042] (Total of the amount of organic physical foaming agent added (A) and the amount of nitrogen added (B) (A+B)) In the foamable resin melt, the sum of the amount A of the organic physical foaming agent added and the amount B of nitrogen added (referred to as the total (A+B)) is adjusted to a value in the numerical range of 0.5 mol to 5 mol per kg of the resin components constituting the foamable resin melt. When the value of the total (A+B) falls within this numerical range, the foamability of the foamable resin melt is appropriate, and a multi-layer foamed sheet having a good foaming state can be obtained. From the viewpoint of improving the foamability of the foamable resin melt, the value of the total (A+B) is preferably in the range of 0.8 mol to 4 mol, more preferably in the range of 1 mol to 3 mol, per kg of the resin components.

[0043] (Amount of added nitrogen B) In the foamable resin melt, the amount of nitrogen added (B) is adjusted to a value in the range of 0.1 mol to 0.4 mol per kg of the resin components constituting the foamable resin melt, provided that the above-mentioned condition for the total amount of physical foaming agents (A + B) is satisfied. By adding the amount of nitrogen added (B) in the foamable resin melt of 0.1 mol or more per kg of the resin components, the foamability of the foamable resin melt can be improved, and a multi-layer foamed sheet with a low density can be obtained.

[0044] From the viewpoint of further enhancing the effect of improving the foamability of the foamable resin melt, the amount B of nitrogen added to the foamable resin melt is preferably 0.12 mol or more, and more preferably 0.16 mol or more, per kg of the resin component.

[0045] From the viewpoint of preventing excessive miniaturization of the average size (average bubble diameter) of bubbles formed in the foamed layers of the resulting multi-layer foamed sheet and enabling stable withdrawal of the multi-layer foamed sheet during extrusion, the amount B of nitrogen added to the foamable resin melt is preferably 0.4 mol or less per kg of the resin component. From the viewpoint of more reliably obtaining the effect of suppressing miniaturization of bubbles, the amount B of nitrogen added to the foamable resin melt is more preferably 0.38 mol or less per kg of the resin component.

[0046] When the foamable resin melt contains an organic physical foaming agent and nitrogen and satisfies the above-mentioned condition of the numerical range of (A+B) and the above-mentioned condition of the numerical range of the added amount B, the foamability of the foamable resin melt can be improved even when the addition of a cell control agent to the foamable resin melt is omitted or the amount of the cell control agent added to the foamable resin melt is limited to a small amount. The improved foamability of the foamable resin melt allows the production of a multilayer foamed sheet with a good foaming state and a low density.

[0047] (Amount of organic physical foaming agent added A) The amount A of the organic physical foaming agent may be determined depending on the desired density of the foamed sheet, etc. However, the determined value of the amount A is within a range (numerical range of the amount A) in which the amount B can be determined so as to satisfy the above-mentioned condition of the numerical range of (A+B) and the above-mentioned condition of the numerical range of the amount B.

[0048] For example, the amount A of the organic physical foaming agent may be adjusted to 0.4 mol or more and 4.6 mol or less per kg of the resin components constituting the foamable resin melt. From the viewpoint of easily realizing stable production of the multi-layer foamed sheet, the amount A of the organic physical foaming agent is preferably 0.5 mol or more and 4.0 mol or less, and more preferably 0.8 mol or more and 3.5 mol or less, per kg of the resin components.

[0049] When the physical foaming agent contained in the foamable resin melt contains two or more organic physical foaming agents, the total amount of the two or more organic physical foaming agents is determined by setting the total amount of each organic physical foaming agent added to the foamable resin melt (total amount of the two or more organic physical foaming agents) as the value of the above-mentioned addition amount A. Then, the addition amount of each organic physical foaming agent is determined according to the value of the total amount of the two or more organic physical foaming agents.

[0050] The physical foaming agent contained in the foamable resin melt may be used in combination with the organic physical foaming agents and physical foaming agents other than nitrogen, as long as the object and effect of the present invention are not impaired.

[0051] (Ratio of the amount of organic physical blowing agent added (A) to the amount of nitrogen added (B) (A / B)) From the viewpoint of enabling stable withdrawal of the multilayer foamed sheet during extrusion and stable production of a multilayer foamed sheet with a good foaming state, it is preferable that the ratio (A / B) of the amount of added organic physical foaming agent A to the amount of added nitrogen B per kg of the resin components constituting the foamable resin melt be 2 or more and 18 or less. From the viewpoint of enhancing the effect of suppressing excessive micronization of bubbles when foaming the foamable resin melt and from the viewpoint of facilitating the production of a multi-layer foamed sheet having a lower density, it is preferable that the value of the ratio (A / B) is 3 or more.

[0052] From the viewpoint of easily obtaining a thin multi-layer foamed sheet having a relatively small cell diameter and suitable for packaging, the ratio (A / B) is preferably 16 or less, more preferably 12 or less, and even more preferably 8 or less.

[0053] (About foam adjusters) When conventional manufacturing methods are used to produce polyethylene-based resin multilayer foamed sheets, a cell control agent is often added to a foamable resin melt. Cell control agents are generally considered to have the effect of forming cell nuclei in the resin components when foaming the foamable resin melt. Cell nuclei are considered to have a structure that promotes good separation of the organic physical foaming agent from the resin components that constitute the foamable resin melt. Good separation of the organic physical foaming agent from the resin components that constitute the foamable resin melt promotes efficient foaming of the foamable resin melt.

[0054] Examples of foam control agents include powdered inorganic substances (inorganic foam control agents) and chemical foaming agents (chemical foam control agents). However, chemical foaming agents are excluded from inorganic substances. Examples of inorganic substances include metal borates such as zinc borate, magnesium borate, and borax, talc, sodium chloride, aluminum hydroxide, zeolite, silica, and calcium carbonate. It is considered preferable to use inorganic materials that do not generate gas upon thermal decomposition. Examples of chemical foaming agents include sodium bicarbonate-citric acid-based chemical foaming agents such as mixtures of sodium bicarbonate and citric acid or mixtures of sodium bicarbonate and alkali metal citrates such as sodium citrate, azodicarbonamide, hydrazodicarbonamide, azobisisobutyronitrile, N,N'-dinitrosopentamethylenetetramine, P,P'-oxybisbenzenesulfonylhydrazide, and ammonium carbonate. Chemical foaming agents have the effect of forming bubble nuclei within the foamable resin melt by generating gas through chemical reactions or thermal decomposition when foaming the foamable resin melt. In addition, in the case of sodium bicarbonate-citric acid-based chemical foaming agents, the reaction product (residue) is also thought to act as bubble nuclei.

[0055] When conventional manufacturing methods are used to produce polyethylene-based resin multi-layer foamed sheets, inorganic substances such as talc (excluding chemical foaming agents) and / or sodium bicarbonate-citric acid-based chemical foaming agents tend to be used as cell control agents. The sodium bicarbonate-citric acid-based chemical foaming agent is a mixture of sodium bicarbonate and citric acid and / or sodium citrate. Examples of sodium citrate include monosodium citrate, disodium citrate, and trisodium citrate. These cell control agents are solid at room temperature and generally in powder form. In the present invention and this specification, a powdery inorganic substance (excluding chemical foaming agents) refers to a powdery substance composed of an inorganic compound different from chemical foaming agents, with a median diameter (d50) of approximately 1 μm to 100 μm as measured by laser diffraction / scattering particle size distribution analysis. A powdery chemical foaming agent refers to a chemical foaming agent with a median diameter (d50) of approximately 1 μm to 100 μm as measured by laser diffraction / scattering particle size distribution analysis. Note that room temperature refers to 20°C. Also, being solid at room temperature means being solid under the conditions at 20°C specified in "8.1 Conditions of the analysis location" of JIS K0050:2019 (General rules for chemical analysis methods).

[0056] Based on the above-described description of the cell regulator, when the multilayer foam sheet is used as a packaging sheet (e.g., a sheet material for packaging semiconductors), from the viewpoint of reducing contamination (adhesion of metal ions or particles) of objects to be contacted, such as packaged goods, it is preferable not to add a powdery inorganic substance (excluding chemical foaming agents) or a chemical foaming agent to the foamable resin melt. This does not prohibit the addition of a powdery inorganic substance (excluding chemical foaming agents) and / or a chemical foaming agent to the foamable resin melt. However, from the viewpoint of easily ensuring the effect of reducing contamination of objects to be contacted, when a powdery inorganic substance (excluding chemical foaming agents) and / or a chemical foaming agent is added to the foamable resin melt, it is preferable that the total amount of the inorganic substance and the chemical foaming agent added be less than 0.1 parts by mass per 100 parts by mass of the resin components constituting the foamable resin melt. From the viewpoint of ensuring the effect of reducing contamination of contacted objects, it is more preferable that the total amount of the inorganic substance and the chemical foaming agent added is 0.05 parts by mass or less, and even more preferable that the total amount is 0.02 parts by mass or less, relative to 100 parts by mass of the resin components constituting the foamable resin melt.

[0057] By reducing the amount of inorganic matter (excluding chemical foaming agents) added, the amount of inorganic matter that can be desorbed from the multi-layer foamed sheet can be reduced, resulting in a multi-layer foamed sheet that is less susceptible to desorption of components derived from the inorganic matter. By reducing the amount of inorganic matter that desorbs from the multi-layer foamed sheet, the possibility of inorganic matter particles adhering to the contacted object can be reduced. Furthermore, by reducing the amount of chemical foaming agent added, it is possible to prevent unreacted or undecomposed components of the chemical foaming agent, or residues after chemical reaction or thermal decomposition, from remaining in the multi-layer foamed sheet. Furthermore, since unreacted or undecomposed components or residues after reaction may contain metal salts such as sodium salts, the prevention of unreacted or undecomposed components of the chemical foaming agent or residues after reaction can prevent metal ions derived from the metal salts from adhering to the contacted object. The multi-layer foamed sheet is capable of suppressing adhesion of the above-mentioned components derived from inorganic substances, components derived from chemical foaming agents, and components derived from metal ions to objects that come into contact with the sheet, and therefore the multi-layer foamed sheet can be suitably used in applications such as electronic device-related products, such as semiconductors and glass substrates, which require a high level of cleanliness to prevent deterioration of product performance.

[0058] In the present invention, nitrogen is dissolved in a resin melt together with an organic physical blowing agent at a predetermined ratio as a partial or complete replacement for the cell control agent used together with the organic physical blowing agent. While the reason why good foamability can be achieved in this manner is unclear, it is believed that the solubility of nitrogen in the molten polyethylene resin in the extruder is moderately low compared to the solubility of organic physical blowing agents such as butane. Therefore, when the physical blowing agent separates from the resin components contained in the foamable resin melt as the pressure decreases during extrusion, it is likely that nitrogen separates before the organic physical blowing agent. This is thought to result in the nitrogen separating from the resin melt during extrusion foaming, generating tiny bubbles. These bubbles then act as bubble nuclei to promote the separation of the organic physical blowing agent from the resin melt, thereby growing the bubbles, resulting in good foaming.

[0059] (Melted resin for forming carbon-containing layer) The carbon-containing layer-forming resin melt contains a polyethylene-based resin (polyethylene-based resin (A)) as a resin component and conductive carbon. In addition to the polyethylene-based resin (A) and conductive carbon, various additives may be added to the carbon-containing layer-forming resin melt as needed.

[0060] The molten resin for forming a carbon-containing layer is a mixture of a polyethylene-based resin (polyethylene-based resin (A)) and conductive carbon. The polyethylene-based resin contained in the molten resin for forming a carbon-containing layer is in a molten state. The molten resin for forming a carbon-containing layer can be produced, for example, by the following method. For example, the polyethylene-based resin (A) and conductive carbon are supplied to the above-mentioned extruder for forming a carbon-containing layer, and the polyethylene-based resin (A) and conductive carbon are melt-kneaded. At this time, an additive such as a volatile plasticizer may be supplied to the extruder for forming a carbon-containing layer, as needed. This allows the molten resin for forming a carbon-containing layer to be obtained.

[0061] (Polyethylene (A1)) The polyethylene-based resin (polyethylene-based resin (A)) contained in the molten resin for forming the carbon-containing layer preferably contains polyethylene (A1). As the polyethylene (A1) contained in the molten resin for forming the carbon-containing layer, it is preferable to use one or two polyethylenes selected from the group consisting of low-density polyethylene and linear low-density polyethylene. The low-density polyethylene exemplified as polyethylene (A1) is a polyethylene having a long-chain branched structure and a density of 910 kg / m 3 More than 930kg / m 3 Linear low-density polyethylene refers to polyethylene that is a copolymer of ethylene and an α-olefin and has a substantially linear molecular chain. Linear low-density polyethylene is a polyethylene with a density of 910 kg / m 3 More than 940kg / m 3 It is preferable that: In addition, in JIS K 6899-1:2015 "Plastics - Symbols and abbreviations - Part 1: Basic polymers and their properties," low-density polyethylene is abbreviated as "PE-LD" and linear low-density polyethylene is abbreviated as "PE-LLD." The polyethylene (A1) preferably contains low-density polyethylene as a main component. Specifically, the proportion of low-density polyethylene in the polyethylene (A1) is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more.

[0062] The melting point TmA1 of the polyethylene (A1) is preferably 100°C or higher and 120°C or lower, and more preferably 102°C or higher and 115°C or lower. When the melting point TmA1 of the polyethylene (A1) is within the above range, the carbon-containing layer can be stably laminated and bonded to the foamed layer when a multi-layer foamed sheet is produced by co-extrusion. The melting point TmA1 of the polyethylene (A1) can be measured by the same method as the method for measuring the melting point TmA of the polyethylene-based resin constituting the foamable resin melt described above.

[0063] The polyethylene (A1) preferably has a melt flow rate of 5 g / 10 min to 80 g / 10 min at a temperature of 190°C under a load of 2.16 kg, more preferably 10 g / 10 min to 65 g / 10 min, and even more preferably 12 g / 10 min to 50 g / 10 min. By setting the melt flow rate of the polyethylene (A1) within the above-mentioned specific range, the adhesion between the carbon-containing layer and the foam layer can be further improved. Furthermore, the antistatic properties of the multi-layer foam sheet can be more stably exhibited.

[0064] The polyethylene resin contained in the molten resin for forming the carbon-containing layer preferably contains an ethylene copolymer (A2) in addition to the polyethylene (A1).

[0065] (Ethylene-based copolymer (A2)) The ethylene-based copolymer (A2) preferably has at least a structural unit derived from ethylene and a structural unit derived from a monomer having a polar group. The ethylene-based copolymer (A2) may be, for example, a copolymer of ethylene and a monomer having a polar group. The ethylene-based copolymer (A2) may also be, for example, a copolymer of ethylene, a monomer having a polar group, and a monomer other than these monomers. In this case, the amount of the structural unit derived from the other monomer is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less, of the ethylene-based copolymer (A2).

[0066] Examples of the ethylene copolymer (A2) include ethylene-vinyl acetate copolymer (EVA), ethylene-methyl methacrylate copolymer (EMMA), ethylene-methyl acrylate copolymer (EMA), ethylene-methacrylic acid copolymer (EMAA), ethylene-acrylic acid copolymer (EAA), ethylene-ethyl methacrylate copolymer (EEMA), ethylene-ethyl acrylate copolymer (EEA), ethylene-butyl acrylate copolymer (EBA), etc. From the viewpoint of stably imparting antistatic properties to the multi-layer foamed sheet and further reducing the shedding of conductive carbon from the multi-layer foamed sheet, it is preferred that the carbon-containing layer-forming resin melt contain, as a resin component, one or more ethylene copolymers (A2) selected from the group consisting of ethylene-vinyl acetate copolymer and ethylene-methyl methacrylate copolymer.

[0067] The content of structural units derived from monomers having a polar group in the ethylene copolymer (A2) is preferably 30% by mass or more and 50% by mass or less. By ensuring that the content of structural units derived from monomers having a polar group in the ethylene copolymer (A2) is 30% by mass or more, the antistatic performance of the multi-layer foamed sheet obtained by the production method of the present invention can be further improved, and the detachment of conductive carbon from the multi-layer foamed sheet can be more effectively suppressed. From the viewpoint of further enhancing these effects, the content of structural units derived from monomers having a polar group in the ethylene copolymer (A2) is more preferably more than 30% by mass, even more preferably 35% by mass or more, particularly preferably 40% by mass or more, and most preferably more than 40% by mass.

[0068] Furthermore, by setting the content of structural units derived from monomers having a polar group in the ethylene copolymer (A2) to 50% by mass or less, the effect of further improving the production stability of the multi-layer foamed sheet can be obtained. From the viewpoint of further enhancing this effect, the content of structural units derived from monomers having a polar group in the ethylene copolymer (A2) is more preferably 48% by mass or less, and even more preferably 45% by mass or less.

[0069] The melting point TmA2 of the ethylene copolymer (A2) is preferably 30°C or higher and 80°C or lower, more preferably 32°C or higher and 75°C or lower, and even more preferably 35°C or higher and 70°C or lower. By setting the melting point TmA2 of the ethylene copolymer (A2) within the above-mentioned specific range, the antistatic properties of the multi-layer foamed sheet can be stably exhibited and the stability of the laminate structure in which the carbon-containing layer is laminated on the foamed layer can be further improved. The melting point TmA2 of the ethylene copolymer (A2) can be measured by the same method as the method for measuring the melting point TmA of the polyethylene resin constituting the foamable resin melt described above.

[0070] The melt flow rate of the ethylene copolymer (A2) at a temperature of 190°C under a load of 2.16 kg is preferably 10 g / 10 min or more and 120 g / 10 min or less. In this case, the multi-layer foamed sheet obtained by the production method of the present invention can stably exhibit antistatic properties. Furthermore, by setting the melt flow rate of the ethylene copolymer (A2) within the above-mentioned specific range, the adhesive strength between the carbon-containing layer and the foamed layer can be further increased. Note that, when the obtained multi-layer foamed sheet has a structure in which a resin surface layer and a carbon-containing layer, which will be described later, are laminated, the adhesive strength between the carbon-containing layer and the foamed layer and the adhesive strength between the resin surface layer and the carbon-containing layer can be increased.

[0071] When polyethylene (A1) and ethylene copolymer (A2) are used as the polyethylene resin for forming the carbon-containing layer-forming resin melt, the blending ratio of the polyethylene (A1) to the ethylene copolymer (A2) is preferably 80:20 to 20:80, more preferably 50:50 to 30:70, by mass. The blending ratio shown here is a value when the sum of the mass of the polyethylene (A1) and the mass of the ethylene copolymer (A2) is taken as 100. The mass ratio shown here is the mass ratio of (mass of polyethylene (A1)):(mass of the ethylene copolymer (A2)). When the blending ratio of the polyethylene (A1) to the ethylene copolymer (A2) is within the above range, the multilayer foamed sheet obtained by the production method of the present invention can more stably exhibit antistatic properties. The resin component contained in the carbon-containing layer-forming resin molten material preferably contains a mixture of polyethylene (A1) and an ethylene copolymer (A2) as a main component. Specifically, the carbon-containing layer-forming resin molten material preferably contains the polyethylene (A1) and the ethylene copolymer (A2) in a total content of 50% by mass or more (where the proportion of the total mass of the resin components contained in the carbon-containing layer-forming resin molten material is 100% by mass), more preferably 60% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more.

[0072] The polyethylene resin contained in the carbon-containing layer-forming resin melt contains polyethylene (A1) and ethylene copolymer (A2), which enhances the effect of stably imparting antistatic properties to the resulting multi-layer foamed sheet and the effect of reducing the shedding of conductive carbon from the multi-layer foamed sheet. The reason for these effects is not clear, but the following is thought to be the reason.

[0073] Generally, when conductive carbon is dispersed in a thermoplastic resin such as a polyethylene-based resin, adjacent conductive carbon particles are present in close proximity to each other within a certain distance, forming a conductive network of the conductive carbon particles and exhibiting conductivity.

[0074] When the polyethylene-based resin contained in the carbon-containing layer-forming resin melt contains polyethylene (A1) and an ethylene-based copolymer (A2), the polyethylene (A1) and the ethylene-based copolymer (A2) are incompatible with each other. Therefore, during the production of a multi-layer foamed sheet, a phase consisting primarily of polyethylene (A1) and a phase consisting primarily of ethylene-based copolymer (A2) are formed in the carbon-containing layer. When such a morphology is formed in the carbon-containing layer, the conductive carbon is thought to be unevenly distributed in either the phase consisting of polyethylene (A1) or the phase consisting of ethylene-based copolymer (A2). The uneven distribution of the conductive carbon in either phase is thought to efficiently form a conductive network of conductive carbon particles. As a result, the multi-layer foamed sheet is likely to exhibit antistatic properties even when the amount of conductive carbon blended is small.

[0075] Furthermore, it is believed that the amount of conductive carbon exposed on the surface of the carbon-containing layer can be reduced by distributing the conductive carbon unevenly in either the phase composed of polyethylene (A1) or the phase composed of ethylene copolymer (A2) in the carbon-containing layer, which is believed to prevent the conductive carbon from falling off from the multi-layer foamed sheet.

[0076] (Conductive carbon) The carbon-containing layer-forming resin melt contains conductive carbon. Conductive carbon refers to a substance primarily composed of carbon atoms and having electrical conductivity. Examples of conductive carbon include conductive carbon blacks such as furnace black, acetylene black, thermal black, and ketjen black. The carbon-containing layer-forming resin melt may contain two or more types of conductive carbon. From the viewpoint of further reducing the amount of conductive carbon used while ensuring the antistatic properties of the multi-layer foamed sheet, it is preferable that the carbon-containing layer-forming resin melt contains at least one type of conductive carbon black selected from furnace black and ketjen black as the conductive carbon. By laminating a carbon-containing layer on a foamed layer using a carbon-containing layer-forming resin melt containing conductive carbon, antistatic properties can be imparted to the multi-layer foamed sheet.

[0077] The dibutyl phthalate (DBP) oil absorption of the conductive carbon is preferably 150 mL / 100 g or more and 700 mL / 100 g or less. In this case, the antistatic properties of the multi-layer foamed sheet can be further improved. From the viewpoint of further improving the antistatic properties of the multi-layer foamed sheet, the DBP oil absorption of the conductive carbon is more preferably 200 mL / 100 g or more and 600 mL / 100 g or less, and even more preferably 300 mL / 100 g or more and 600 mL / 100 g or less. The dibutyl phthalate (DBP) oil absorption is a value measured in accordance with ASTM D 2414-79.

[0078] The BET specific surface area of ​​conductive carbon is 600m 2 / g or more 2000m 2 / g or less. In this case, the antistatic property of the multilayer foamed sheet can be further improved. From the viewpoint of further improving the antistatic property of the multilayer foamed sheet, the BET specific surface area of ​​the conductive carbon is preferably 700 m 2 / g or more 1600m 2 It is more preferable that the saturation coefficient is 1 / g or less.

[0079] (Amount of conductive carbon added) The amount of conductive carbon added to the molten resin for forming the carbon-containing layer is 0.01 g / m 2 More than 0.5g / m 2 The following value is set. When the amount of conductive carbon added is such a value, the multilayer foamed sheet exhibits antistatic properties while suppressing the occurrence of pinholes during production. Furthermore, suppressing the occurrence of pinholes can prevent the resin forming the multilayer foamed sheet from falling off near the pinhole-forming portion of the multilayer foamed sheet. This can prevent the fallen resin from adhering to objects that come into contact with the multilayer foamed sheet or scattering into the surrounding environment. In this specification, pinholes refer to defects such as small holes formed on the surface of the multilayer foamed sheet. It has been found that, when a multi-layer foamed sheet is produced using a nitrogen-containing physical foaming agent as in the present invention, an excessively large amount of conductive carbon per area in the carbon-containing layer is likely to cause pinholes and the multi-layer foamed sheet to generate dust. By setting the amount of conductive carbon in the carbon-containing layer to a specific value or less, it is possible to suppress the generation of pinholes during the production of the multi-layer foamed sheet, even when a nitrogen-containing physical foaming agent is used, and a multi-layer foamed sheet that is less likely to generate dust can be obtained. From the viewpoint of suppressing the occurrence of pinholes, the blending amount of conductive carbon in the carbon-containing layer is 0.3 g / m 2 Preferably, it is 0.2 g / m or less. 2 It is preferable that:

[0080] Regarding the amount of conductive carbon added to the resin melt for forming the carbon-containing layer, from the viewpoint of more reliably imparting antistatic properties to the multi-layer foamed sheet, the amount of conductive carbon blended in the carbon-containing layer is 0.02 g / m 2 It is preferable that the content is 0.05 g / m or more. 2 More preferably, it is equal to or greater than this.

[0081] In the present invention, the amount of conductive carbon contained in the carbon-containing layer is determined based on the area (m 2 The amount of conductive carbon contained in the carbon-containing layer is expressed as, for example, the basis weight (BW1) (g / m 2 The basis weight of the carbon-containing layer can be calculated by multiplying the "weight ratio (mass ratio (mass%)) (MRC) of the conductive carbon contained in the carbon-containing layer" by the product ((BW1) × (MRC) / 100). For example, if the basis weight of the carbon-containing layer is 1 g / m², 2 When the blending ratio (mass ratio) of the conductive carbon contained in the carbon-containing layer is 8 mass %, the blending amount of the conductive carbon is 0.08 (g / m 2 )

[0082] From the viewpoint of being able to stably obtain a multi-layer foamed sheet in which the blending amount of conductive carbon contained in the carbon-containing layer is within the above-mentioned range, the blending ratio of conductive carbon in the carbon-containing layer is preferably 2% by mass or more and 15% by mass or less, more preferably 3% by mass or more and 12% by mass or less, and even more preferably 4% by mass or more and 10% by mass or less.

[0083] The amount of conductive carbon added to the total of the resin component and conductive carbon contained in the molten resin for forming the carbon-containing layer is a value that roughly corresponds to the blending amount of conductive carbon in the carbon-containing layer.

[0084] (Other polymers) The carbon-containing layer-forming resin melt may contain a polymer other than the polyethylene-based resin (polyethylene-based resin (A)) described above. Examples of the other polymer include thermoplastic resins such as polystyrene-based resins, and elastomers such as ethylene propylene rubber and styrene-butadiene-styrene block copolymers. From the viewpoint of stably forming a laminate structure of the carbon-containing layer and the foamed layer, it is preferable that the carbon-containing layer does not contain a polymer having a melting point higher than that of the polyethylene-based resin (polyethylene-based resin (F)) contained in the foamed layer. Furthermore, the content of the other polymer is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, and particularly preferably 3% by mass or less, when the total amount of the polyethylene-based resin components contained in the carbon-containing layer-forming resin melt is taken as 100% by mass.

[0085] (additives) The carbon-containing layer-forming resin melt may contain additives such as antioxidants, heat stabilizers, weathering agents, ultraviolet absorbers, flame retardants, fillers, and antibacterial agents. However, it is preferable that the additives are not compounds containing metals. The amount of additives in the carbon-containing layer-forming resin melt is, for example, preferably 10 parts by mass or less, more preferably 5 parts by mass or less, and even more preferably 3 parts by mass or less, relative to 100 parts by mass of the total polyethylene resin components of the carbon-containing layer-forming resin melt.

[0086] A volatile plasticizer may be added as an additive to the carbon-containing layer-forming resin melt. Examples of volatile plasticizers include aliphatic hydrocarbons having from 3 to 7 carbon atoms, alicyclic hydrocarbons having from 3 to 7 carbon atoms, aliphatic alcohols having from 1 to 4 carbon atoms, and aliphatic ethers having from 2 to 8 carbon atoms. The carbon-containing layer-forming resin melt may contain one type of volatile plasticizer or two or more types of volatile plasticizers. From the viewpoints of ease of dissipation from the multi-layer foamed sheet after extrusion and efficient plasticization of the melt, it is preferable to use butane as the volatile plasticizer.

[0087] The multilayer foam sheet obtained by the production method described above in [1. Basic Example of Production Method] has a laminated structure in which a carbon-containing layer is formed on at least one surface (at least one side) of the foam layer, as described above. The production method for a multilayer foam sheet according to the present invention may be configured to further form a resin surface layer on the surface of the multilayer foam sheet obtained by the production method described in [1. Basic Example of Production Method]. That is, the production method for a multilayer foam sheet according to the present invention may be configured to obtain a multilayer foam sheet having a laminated structure in which a carbon-containing layer and a resin surface layer are formed in this order on at least one surface (one side) of the foam layer. Such a production method for a multilayer foam sheet according to the present invention is referred to as an [Additional Example of Production Method], as described above. Next, the additional example of the production method will be described.

[0088] [2 Additional examples of manufacturing methods] The additional example of the manufacturing method describes a method for manufacturing a multilayer foam sheet having a third laminate structure in which a carbon-containing layer and a resin surface layer are laminated on both sides of a foam layer. Examples of methods for laminating the resin surface layer include a co-extrusion method (co-extrusion method) to form a structure in which a carbon-containing layer and a resin surface layer are laminated on both sides of a foam layer, and a thermal lamination method (thermal lamination method) to thermally laminate the resin surface layer onto the surface of the carbon-containing layer. Note that the method described here is merely an example and does not limit the manufacturing method of the multilayer foam sheet according to the present invention. All of the content described in [1 Basic Example of Manufacturing Method] can also be applied to [2 Additional Example of Manufacturing Method]. The multi-layer foamed sheet has a resin surface layer laminated and bonded to the carbon-containing layer, which makes it easier to suppress detachment of resin containing conductive carbon, etc., originating from the carbon-containing layer, from the multi-layer foamed sheet, and also makes it easier to smooth the surface of the multi-layer foamed sheet, thereby making it easier to improve the appearance of the multi-layer foamed sheet.

[0089] (co-extrusion method) This section describes a method for forming a structure in which a carbon-containing layer and a resin surface layer are laminated on both sides of a foam layer by a co-extrusion method. The apparatus used to perform the co-extrusion method in this additional example of the manufacturing method is not particularly limited. As an apparatus for forming a multi-layer foam sheet having a third laminate structure (a laminate structure formed of three types of layers, i.e., five layers) in which a carbon-containing layer and a resin surface layer are laminated on both sides of the foam layer as described above, for example, a co-extrusion extruder (co-extrusion apparatus) such as the one shown below can be used. The co-extrusion apparatus includes an extruder for forming a foam layer, an extruder for forming a carbon-containing layer, an extruder for forming a resin surface layer, and a co-extrusion die. The co-extrusion die is connected to the outlet of the extruder for forming the foam layer, the outlet of the extruder for forming the carbon-containing layer, and the outlet of the extruder for forming the resin surface layer. The co-extrusion die has a melt path formed so that the resin melt for forming the carbon-containing layer and the resin melt for forming the resin surface layer are laminated in this order on both sides of the foamable resin melt for forming the foam layer.

[0090] The co-extrusion method can be achieved by introducing the foamable resin melt for forming the foam layer (foamable resin melt), the resin melt for forming the carbon-containing layer, and the resin melt for forming the resin surface layer, which are formed inside each extruder, into a co-extrusion die and simultaneously extruding them from the extrusion opening of the co-extrusion die. In an additional example of the manufacturing method, a laminate (five-layer laminate) formed by stacking the foamable resin melt, the resin melt for forming the carbon-containing layer, and the resin melt for forming the resin surface layer is extruded into a cylindrical shape from the extrusion opening of the co-extrusion die. Of this laminate, the layer composed of the resin melt for forming the resin surface layer becomes the resin surface layer. Note that in this additional example of the manufacturing method, the other aspects of forming such a laminate may be the same as the co-extrusion method described above in [1 Basic Example of Manufacturing Method].

[0091] (Co-extrusion method when the multi-layer foam sheet has a fourth layer structure) The above description of the co-extrusion method has been given as an example of a production method for a multi-layer foam sheet having a third layer structure. When the multi-layer foam sheet has a fourth layer structure, the following co-extrusion die is preferably used. That is, a co-extrusion die is used that has a melt path formed so that the carbon-containing layer-forming resin melt and the resin surface layer-forming resin melt are laminated on one side of the foam layer-forming foamable resin melt. In this case, the resulting tubular laminated foam has a laminated structure of a foam layer, a layer composed of the carbon-containing layer-forming resin melt, and a layer composed of the resin surface layer-forming resin melt, and the layer composed of the carbon-containing layer-forming resin melt and the resin surface layer-forming resin melt are laminated on one side of the foam layer. The tubular laminated foam is then cut open with a cutter or the like to obtain a multi-layer foam sheet having a fourth layer structure.

[0092] (Foamable resin melt and resin melt for forming carbon-containing layer) When comparing the additional example of the manufacturing method with the basic example of the manufacturing method described above, the foamable resin melt and the resin melt for forming the carbon-containing layer used may be the same (same conditions such as composition and material), and therefore detailed explanations of the foamable resin melt and the resin melt for forming the carbon-containing layer will be omitted in the explanation of the additional example of the manufacturing method.

[0093] (Melted resin for forming resin surface layer) The resin melt for forming the resin surface layer preferably contains a polyethylene-based resin (polyethylene-based resin (B)) as a resin component. The resin melt for forming the resin surface layer can be produced, for example, by applying the following method. A polyethylene-based resin and, if necessary, additives are supplied to an extruder for forming the resin surface layer. Then, these are melt-kneaded in the extruder to obtain a melt for forming the surface layer. Note that, for the polyethylene-based resin (polyethylene-based resin (B)), reference can be made to the details of the polyethylene-based resin (F) contained in the foamable resin melt described in [1 Basic Example of the Production Method]. The polyethylene-based resin (B) and the polyethylene-based resin (F) may be composed of the same type of resin, or may have different compositions. The same applies to the relationship between the polyethylene-based resin (B) and the polyethylene-based resin (A). The same applies to the relationship between the polyethylene-based resin (A) and the polyethylene-based resin (F).

[0094] (linear polyethylene) The polyethylene resin (polyethylene resin (B)) used to form the resin melt for forming the resin surface layer preferably contains linear polyethylene. The linear polyethylene refers to one or more polyethylenes selected from the group consisting of linear low-density polyethylene and high-density polyethylene. The resin melt for forming the resin surface layer preferably contains at least linear low-density polyethylene as the linear polyethylene, which allows the resin surface layer to be more stably laminated and bonded to the carbon-containing layer and also prevents local variations in surface resistivity on the surface of the multi-layer foamed sheet. From this viewpoint, the resin component contained in the resin molten material for forming the resin surface layer preferably contains linear polyethylene as a main component. Specifically, the resin molten material for forming the resin surface layer preferably contains linear polyethylene in an amount of 50% by mass or more (where the ratio of the total mass of the resin components contained in the resin molten material for forming the resin surface layer is taken as 100% by mass), more preferably 60% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more.

[0095] (Melt flow rate of linear polyethylene) The linear polyethylene used for the resin molten material for forming the resin surface layer preferably has a melt flow rate of 12 g / 10 min or more, more preferably 15 g / 10 min or more, at a temperature of 190°C and a load of 2.16 kg. This allows the resin surface layer to be stably bonded to the carbon-containing layer. Furthermore, the resulting multilayer foamed sheet is less likely to have local variations in surface resistivity. From the viewpoint of extrusion stability, the upper limit of the melt flow rate of the linear polyethylene used for the surface resin layer at a temperature of 190°C and a load of 2.16 kg is preferably 100 g / 10 min, more preferably 50 g / 10 min, and even more preferably 30 g / 10 min.

[0096] (melting point of linear polyethylene) The melting point TmS of the linear polyethylene used in the resin melt for forming the resin surface layer is preferably 110°C or higher and 135°C or lower, more preferably 115°C or higher and 130°C or lower. When the melting point TmS falls within this range, the resin surface layer can be stably formed even when a multi-layer foamed sheet is produced by co-extrusion. The method for measuring the melting point TmS of the linear polyethylene is the same as the method for measuring the melting point TmA of the polyethylene resin constituting the foamable resin melt described in [1 Basic Example of Production Method].

[0097] Regarding the polyethylene resin used to form the resin melt for forming the resin surface layer, the definitions of linear low-density polyethylene and high-density polyethylene are the same as those explained in [1 Basic example of manufacturing method].

[0098] A volatile plasticizer may be added as an additive to the resin melt for forming the resin surface layer. Examples of volatile plasticizers include aliphatic hydrocarbons having 3 to 7 carbon atoms, alicyclic hydrocarbons having 3 to 7 carbon atoms, aliphatic alcohols having 1 to 4 carbon atoms, and aliphatic ethers having 2 to 8 carbon atoms. The resin melt for forming the carbon-containing layer may contain one type of volatile plasticizer or two or more types of volatile plasticizers. From the viewpoints of ease of dissipation from the multi-layer foamed sheet after extrusion and efficient plasticization of the melt, it is preferable to use butane as the volatile plasticizer.

[0099] In the above explanation, a case where a structure in which a carbon-containing layer and a resin surface layer are laminated on both sides of a foam layer is formed by a co-extrusion method is used as an example. A case where a method of laminating a resin surface layer on the surface of a carbon-containing layer (thermal lamination method) is applied will be further explained.

[0100] (Thermal lamination method) The following method can be exemplified as a method for producing a multilayer foamed sheet using the thermal lamination method. Using the method described in [1. Basic Example of Production Method], a laminate (laminate C) is formed by co-extrusion, in which a carbon-containing layer is laminated on both sides of a foamed layer. A sheet (sheet S) containing a resin component constituting the resin melt for forming a resin surface layer is produced. Then, sheet S is laminated onto the surface of the carbon-containing layer of laminate C under heating conditions. This results in a multilayer foamed sheet having a structure in which a carbon-containing layer and a resin surface layer are laminated on both sides of a foamed layer.

[0101] From the viewpoint of ensuring that the antistatic properties of the multi-layer foamed sheet can be stably exhibited even when the thickness of the multi-layer foamed sheet is reduced, it is preferable to form a structure in which a carbon-containing layer and a resin surface layer are laminated on both sides of the foamed layer by a co-extrusion method.

[0102] The above describes examples of the production method according to the present invention in [1. Basic Example of Production Method] and [2. Additional Example of Production Method]. Such a production method according to the present invention is preferably a method for producing a multi-layer foamed sheet as shown below.

[0103] (Density of multi-layer foam sheet) The method for producing a multi-layer foamed sheet according to the present invention is preferably a method for producing a multi-layer foamed sheet having a density that satisfies the following numerical ranges, although this does not prohibit the density of the multi-layer foamed sheet from being outside the specified numerical ranges.

[0104] The density of the multi-layer foamed sheet obtained by the method for producing a multi-layer foamed sheet according to the present invention is 20 kg / m 3 More than 200kg / m 3 It is preferable that the saturation is 30 kg / m or less. 3 More than 100kg / m 3 It is more preferable that the density is not more than 100%. When the density of the multilayer foamed sheet is a value satisfying the above-mentioned numerical range, the multilayer foamed sheet can be suitably used for various applications, such as packaging materials for fragile items, packaging materials for glass panels for image display devices such as liquid crystal displays and glass plates for cover glasses of mobile phones and the like, and packaging materials for electronic devices. The density of the multilayer foamed sheet is the density of the multilayer foamed sheet as a whole. Therefore, for example, when the multilayer foamed sheet includes a foam layer and a carbon-containing layer, the density is the density of the entire multilayer foamed sheet including the foam layer and the carbon-containing layer. When the multilayer foamed sheet includes a foam layer, a carbon-containing layer, and a resin surface layer, the density is the density of the entire multilayer foamed sheet including the foam layer, the carbon-containing layer, and the resin surface layer.

[0105] The density of the multi-layer foam sheet is calculated by dividing the basis weight of the multi-layer foam sheet by the average thickness of the multi-layer foam sheet and converting the result into units. The basis weight and average thickness of the multi-layer foam sheet can be determined, for example, by the method described below.

[0106] (Basis weight of multi-layer foam sheet) The basis weight (total basis weight) of the multi-layer foamed sheet obtained by the method for producing a multi-layer foamed sheet according to the present invention is not particularly limited. However, from the viewpoint of suitability for use as a cushioning material such as a packing material, wrapping material, or slip sheets, a basis weight of 10 g / m is preferred. 2 More than 200g / m 2 Preferably, it is 12 g / m or less. 2 More than 150g / m 2 More preferably, it is 15 g / m or less. 2 More than 100g / m 2 It is even more preferable that:

[0107] The basis weight of the multi-layer foam sheet is determined by dividing the multi-layer foam sheet into a predetermined area (m 2 A sample having a thickness of 1 / 2 mm was cut out from the multilayer foam sheet, and the mass (g) of the sample was measured. 2 This can be obtained by converting it into

[0108] (Average thickness of multi-layer foam sheet) The average thickness (total thickness) of the multilayer foamed sheet obtained by the method for producing a multilayer foamed sheet according to the present invention is not particularly limited, but from the viewpoint of further enhancing the effect of enabling use as a sheet material having a thickness suitable for use as a packaging material, wrapping material, or interleaf, the average thickness of the multilayer foamed sheet is preferably 0.05 mm to 10 mm, more preferably 0.1 mm to 8 mm, and even more preferably 0.1 mm to 6 mm. Furthermore, from the viewpoint of further enhancing the effect of enabling use as a sheet material having a thickness suitable for use as an interleaf, the average thickness of the multilayer foamed sheet is preferably 0.05 mm to 3 mm, and more preferably 0.1 mm to 2 mm.

[0109] The average thickness of a multi-layer foam sheet is measured by the following method. A cross section (transverse direction (TD) cross section) perpendicular to the machine direction (MD) (the extrusion direction of the resin when coextrusion is performed) of the multi-layer foam sheet is cut out, and five or more sections for observation (selected sections) are randomly selected from the cross section in the transverse direction (TD) of the multi-layer foam sheet. Next, each selected section is magnified approximately 50 times using a microscope or the like to obtain an enlarged image of each selected section. Next, the thickness of the multi-layer foam sheet is measured at 10 random locations on each enlarged image. The arithmetic mean of all the measured thicknesses (mm) is determined to be the average thickness (mm) of the multi-layer foam sheet. The thickness of the multi-layer foam sheet is measured using a multi-layer foam sheet that has been conditioned for at least 24 hours at a temperature of 23±5°C and a relative humidity of 50%.

[0110] (Average bubble diameter of multi-layer foam sheet) The method for producing a multi-layer foamed sheet according to the present invention is preferably a method for producing a multi-layer foamed sheet having an average cell diameter (D) that satisfies the following numerical ranges, although this does not prohibit the multi-layer foamed sheet from having an average cell diameter (D) outside the specified numerical ranges.

[0111] The average cell diameter (D) of the multi-layer foamed sheet obtained by the method for producing a multi-layer foamed sheet according to the present invention is preferably in the range of 100 μm or more and 1000 μm or less, more preferably 200 μm or more and 1000 μm or less, even more preferably 300 μm or more and 900 μm or less, and even more preferably 400 μm or more and 850 μm or less.

[0112] The average cell diameter (D) of the multilayer foam sheet is determined as follows. First, a sample having a cross section (extrusion direction cross section) along the extrusion direction (MD) at the center of the width direction of the multilayer foam sheet and a sample having a cross section (transverse direction (TD) cross section) perpendicular to the extrusion direction of the multilayer foam sheet are cut out. Next, three observation areas are selected near the center of the width direction and near both ends of the width direction of the sample having the width direction cross section, and three observation areas are randomly selected from the extrusion direction cross section. The selected observation areas are magnified approximately 50 times using a microscope or the like to obtain enlarged images (observation images) of each cross section so that they include the entire thickness of the multilayer foam sheet. Note that each cross section is formed to be a cross section along the thickness direction. Next, 10 bubbles are randomly selected from each of the three width direction cross sections, and the maximum length of each selected bubble in the thickness direction and the maximum length of the bubble in the width direction are measured. In addition, ten bubbles are randomly selected from each of the three cross-sections in the extrusion direction, and the maximum length of each bubble in the extrusion direction is measured. The arithmetic mean of all the measured bubble diameters in each direction is determined as the average bubble diameter (D) of the multi-layer foamed sheet.

[0113] (Average number of bubbles in the thickness direction of the multi-layer foam sheet) The average number of bubbles in the thickness direction of the multilayer foam sheet is preferably 0.5 bubbles / mm or more and 5 bubbles / mm or less, more preferably 0.6 bubbles / mm or more and 4 bubbles / mm or less, even more preferably 0.7 bubbles / mm or more and 3 bubbles / mm or less, and even more preferably 0.8 bubbles / mm or more and 2.8 bubbles / mm or less, from the viewpoint that the multilayer foam sheet has a desired expansion ratio (density), a good appearance, and good cushioning properties and can be suitably used as a cushioning material for packaging materials, packing materials, etc.

[0114] The average number of cells in the thickness direction of the multi-layer foamed sheet is determined as follows. The multilayer foam sheet is cut in a direction perpendicular to the extrusion direction of the multilayer foam sheet to form a cut surface (widthwise cross section), and five or more observation points are randomly selected in the widthwise direction of the foam sheet. The selected observation points are then observed at a magnification of approximately 50x to obtain enlarged images of each point. Ten lines are then randomly drawn on the cut surface of each enlarged image along the thickness direction of the multilayer foam sheet, and the number of bubbles intersecting these lines is counted. The number of counted bubbles is then divided by the length of each line and converted to a unit to determine the number of bubbles (number / mm) per thickness of the multilayer foam sheet at each measurement point. The arithmetic mean value of the number of bubbles per thickness of the multilayer foam sheet measured in this manner at a total of 50 or more points is calculated, and this is the average number of bubbles in the thickness direction of the multilayer foam sheet.

[0115] [3 Actions and Effects] Generally, to impart antistatic properties to a polyethylene-based resin foam sheet, a polymeric antistatic agent is incorporated into the polyethylene-based resin foam sheet. Here, a polyethylene-based resin foam sheet containing a polymeric antistatic agent is referred to as a conventional foam sheet. Conventional foam sheets are prone to contain metal ions. When such conventional foam sheets are used as packaging sheets for semiconductors, liquid crystal panels, electronic devices, and the like, there is a high possibility that the metal ions contained in the conventional foam sheets will migrate to the objects they come into contact with, such as the semiconductors, liquid crystal panels, electronic devices, and other packaged items.

[0116] According to the method for producing a multi-layer foamed sheet of the present invention, a multi-layer foamed sheet can be obtained by co-extrusion of the foam layer-forming foamable resin melt and the carbon-containing layer-forming resin melt. The multi-layer foamed sheet obtained by the method of the present invention achieves antistatic properties due to the conductive carbon contained in the carbon-containing layer. Therefore, compared with conventional foamed sheets that are imparted with antistatic properties using a polymeric antistatic agent, the multi-layer foamed sheet obtained by the method of the present invention has a reduced content of metal ions derived from the polymeric antistatic agent.

[0117] Furthermore, when producing a foamed sheet having a structure in which a conductive carbon-containing layer is laminated on the surface of a foamed layer, pinholes may occur on the surface of the foamed sheet depending on the composition of the resin melt used to form the conductive carbon-containing layer. Excessive pinholes may deteriorate the appearance of the foamed sheet, and the resin forming the conductive carbon-containing layer may be detached from the foamed sheet due to friction or other factors. This tendency is more likely to occur when producing a multi-layer foamed sheet using a nitrogen-containing physical foaming agent. According to the method for producing a multi-layer foamed sheet of the present invention, the amount of conductive carbon added to the resin melt for forming the carbon-containing layer is set within a predetermined range, thereby reducing the occurrence of pinholes. Therefore, according to the method for producing a multi-layer foamed sheet of the present invention, deterioration in the appearance of the multi-layer foamed sheet can be suppressed. Furthermore, according to the method for producing a multi-layer foamed sheet of the present invention, the possibility of detachment of the resin containing conductive carbon from the multi-layer foamed sheet can be reduced.

[0118] When conventional foam sheets are manufactured, a cell regulator has generally been used. When a powdery inorganic substance is used as the cell regulator, there is a high possibility that the inorganic substance released from the conventional foam sheet will migrate to objects that come into contact with the sheet, such as semiconductors, liquid crystal panels, and electronic devices. Furthermore, when a chemical foaming agent is used as the cell regulator, there is a high possibility that metal ions and reaction residues derived from metal salts contained in the chemical foaming agent will migrate to objects that come into contact with the sheet, such as semiconductors, liquid crystal panels, and electronic devices.

[0119] In the method for producing a multi-layer foamed sheet according to the present invention, nitrogen is contained as a physical foaming agent in the foam layer-forming foamable resin melt in a manner that satisfies a predetermined condition. It can be considered that nitrogen also functions as a cell control agent. Therefore, the method for producing a multi-layer foamed sheet according to the present invention can produce a multi-layer foamed sheet without adding a cell control agent that is solid at room temperature. Therefore, the method for producing a multi-layer foamed sheet according to the present invention can reduce contamination of contacted objects.

[0120] Furthermore, the method for producing a multi-layer foamed sheet according to the present invention can provide a multi-layer foamed sheet of the present invention, which will be described later. The multi-layer foamed sheet of the present invention has a concentration of alkali metal ions extracted from the multi-layer foamed sheet of the present invention of 0.10 ng / cm. 2 The following (including 0): From this, it can be seen that the method for producing a multi-layer foamed sheet according to the present invention can reduce contamination of objects with metal ions (particularly alkali metal ions).

[0121] [Polyethylene resin multi-layer foam sheet] The polyethylene-based resin multi-layer foamed sheet according to the present invention will now be described. As described above, the polyethylene-based resin multi-layer foamed sheet is simply referred to as a multi-layer foamed sheet. As shown in FIGS. 1A to 1D, the multi-layer foamed sheet according to the present invention comprises a polyethylene-based resin foamed layer (hereinafter referred to as a foamed layer) and a conductive carbon-containing resin layer (carbon-containing layer) containing a polyethylene-based resin and conductive carbon. FIGS. 1A to 1D are cross-sectional views showing an example of the multi-layer foamed sheet according to the present invention.

[0122] In FIG. 1A, the multilayer foam sheet 10A has a structure (a laminated and bonded structure) (first laminate structure) in which carbon-containing layers 12 are laminated on both sides of the foam layer 11. In FIG. 1B, the multilayer foam sheet 10B has a structure (a second laminate structure) in which the carbon-containing layer 12 is laminated on one side of the foam layer 11. In FIG. 1C, the multilayer foam sheet 10C has a structure (a third laminate structure) in which the carbon-containing layer 12 and the resin surface layer 13 are laminated in this order on both sides of the foam layer 11. In FIG. 1D, the multilayer foam sheet 10D has a structure (a fourth laminate structure) in which the carbon-containing layer 12 and the resin surface layer 13 are laminated in this order on one side of the foam layer 11. In the third and fourth laminate structures, the resin surface layer 13 is located on the outermost side. In the first and third laminate structures, the carbon-containing layers 12 formed on both sides of the foam layer 11 may be made of the same material or different materials. In the third laminate structure, the resin surface layer 13 laminated on the carbon-containing layer 12 on one side of the foam layer 11 and the resin surface layer 13 laminated on the carbon-containing layer 12 on the other side of the foam layer 11 may be made of the same material or different materials. In the first to fourth laminate structures, the foam layer and the carbon-containing layer are laminated in a state of being directly bonded to each other (the foam layer and the carbon-containing layer are laminated and bonded). In the third and fourth laminate structures, the resin surface layer and the carbon-containing layer are laminated in a state of being directly bonded to each other (the carbon-containing layer and the resin surface layer are laminated and bonded).

[0123] The multi-layer foamed sheet of the present invention can be obtained, for example, by carrying out the "method for producing a polyethylene-based resin multi-layer foamed sheet" of the present invention, as described above.

[0124] [Density of multi-layer foam sheet] The multilayer foamed sheet of the present invention has a density (total density) of 20 kg / m 3 More than 200kg / m 3 The density of the multi-layer foamed sheet can be determined by the method described in the above-mentioned "Method for producing a multi-layer polyethylene resin foamed sheet."

[0125] [Foam layer] The foam layer has a structure obtained by foaming the above-mentioned foamable resin melt. The foam layer contains a polyethylene-based resin as a resin component. The polyethylene-based resin constituting the foam layer may be the same resin as the polyethylene-based resin (polyethylene-based resin (F)) contained in the foamable resin melt, and the above-mentioned description of the polyethylene-based resin contained in the foamable resin melt may be applied.

[0126] [Carbon-containing layer] As described above, the carbon-containing layer contains a polyethylene-based resin and conductive carbon. The carbon-containing layer can be formed of a layer composed of the above-described carbon-containing layer-forming resin melt. The carbon-containing layer may be a layer containing a resin component that constitutes the carbon-containing layer-forming resin melt. The resin that constitutes the carbon-containing layer may be the same resin as the resin contained in the carbon-containing layer-forming resin melt, and the above-described resin contained in the carbon-containing layer-forming resin melt may be applied. Therefore, the polyethylene-based resin that constitutes the carbon-containing layer may have the same resin composition as the polyethylene-based resin (A).

[0127] (Basis weight of carbon-containing layer) The basis weight of the carbon-containing layer is 0.1 g / m 2 More than 20g / m 2 The basis weight of the carbon-containing layer is preferably 0.5 g / m or less. 2 More preferably, 1 g / m 2 By setting the carbon-containing layer at the above level, antistatic properties can be more reliably imparted to the multi-layer foamed sheet. In addition, from the viewpoint of further suppressing the generation of dust from the multi-layer foamed sheet due to friction or the like, the basis weight of the carbon-containing layer is preferably 10 g / m 2 More preferably, it is 5 g / m or less. 2 More preferably, it is 3 g / m or less. 2 When the carbon-containing layer is laminated on both sides of the polyethylene-based resin foam layer, the basis weight of the carbon-containing layer means the basis weight per side.

[0128] The method for measuring the basis weight of the carbon-containing layer per side is as follows. First, the average thickness of the carbon-containing layer is calculated. After converting the unit of this average thickness, the density of the carbon-containing layer (unit: g / m 3 ) to obtain the basis weight (unit: g / m 2 ) can be obtained. The density of the carbon-containing layer includes the density of the conductive carbon and other additives contained in the carbon-containing layer. The average thickness of the carbon-containing layer is measured by the following method. A cross section (transverse direction (TD) cross section) perpendicular to the machine direction (MD) (the extrusion direction of the resin when coextrusion is performed) of the multilayer foam sheet is cut out, and five or more observation sections (selected sections) are randomly selected from the cross section containing the carbon-containing layer in the transverse direction (TD) of the multilayer foam sheet. Next, each selected section is enlarged using a microscope or the like to obtain an enlarged image of each selected section. Next, the thickness of the carbon-containing layer is measured at 10 random locations on each enlarged image. The arithmetic mean of all the measured thicknesses is defined as the average thickness of the carbon-containing layer. The thickness of the carbon-containing layer is measured using a multilayer foam sheet that has been conditioned for at least 24 hours under conditions of a temperature of 23±5°C and a relative humidity of 50%.

[0129] When a multi-layer foamed sheet is produced by co-extrusion as in the production method of the present invention, the basis weight of the carbon-containing layer per side can also be calculated using the following formula (1).

[0130] Basis weight of carbon-containing layer = [X1 / (L × W)] (1)

[0131] In the above formula (1), X1 is the discharge amount of the carbon-containing layer per side (the total discharge amount of the resin components and conductive carbon constituting the resin melt for forming the carbon-containing layer per side) (unit: g / hour), W is the width of the multi-layer foamed sheet (unit: m), and L is the take-up speed of the multi-layer foamed sheet (unit: m / hour).

[0132] In the multi-layer foamed sheet according to the present invention, the blending amount of conductive carbon in the carbon-containing layer is 0.01 g / m2 More than 0.5g / m 2 and the amount of alkali metal ions extracted from the polyethylene resin multi-layer foamed sheet is 0.10 ng / cm or less. 2 The following (including 0):

[0133] (Amount of conductive carbon added) The amount of conductive carbon in the carbon-containing layer is 0.01 g / m 2 More than 0.5g / m 2 The fact that the conductive carbon content is below the above-mentioned range is the same as that in the description of the amount of conductive carbon added to the molten resin for forming a carbon-containing layer. Furthermore, the definition of the amount of conductive carbon added, the preferred numerical value (numerical range) of the amount of conductive carbon added, and the effects are all the same as those in the description of the amount of conductive carbon added to the molten resin for forming a carbon-containing layer, and therefore detailed description thereof will be omitted.

[0134] (amount of alkali metal ions) In the present invention, the alkali metal ions extracted from the polyethylene-based resin multi-layer foamed sheet refer to sodium ions, potassium ions, and lithium ions. The amount of alkali metal ions refers to the amount of eluted alkali metal ions obtained by the elution test described below. Therefore, in the multi-layer foamed sheet of the present invention, the total amount of sodium ions, potassium ions, and lithium ions extracted from the polyethylene-based resin multi-layer foamed sheet is 0.10 ng / cm. 2 The multi-layer foamed sheet has a total content of sodium ions, potassium ions and lithium ions of 0.10 ng / cm 2 or less (including 0), an effect can be obtained in that even when the multilayer foamed sheet is used as a sheet for packaging semiconductors, liquid crystal panels, electronic devices, etc., the possibility of contamination of contacted objects such as semiconductors, liquid crystal panels, electronic devices, etc. by alkali metal ions can be reduced. From the viewpoint of enhancing this effect, it is preferable that the total amount of sodium ions, potassium ions, and lithium ions is 0.05 ng / cm or less. 2It is preferable that the concentration is 0.01 ng / cm or less (including 0). 2 It is more preferable that the number is equal to or less than (including 0).

[0135] The amount of alkali metal ions eluted from the multilayer foam sheet of the present invention can be measured by the elution test described below. In the elution test, a test piece having a thickness of 0.5±0.1 g is cut out from the multilayer foam sheet. The test piece is immersed in 10 ml of ion-exchanged water. While the test piece is immersed, the ion-exchanged water is heated at 95°C for 30 minutes. This heating causes ions to be extracted from the test piece. The heated ion-exchanged water is referred to as the test solution. The amount of alkali metal ions contained in the test solution is then measured by applying ion chromatography to the test solution. That is, the amount (ng / ml) of alkali metal ions eluted from the test piece into the ion-exchanged water is measured. Then, the amount of alkali metal ions eluted per unit area of ​​the multilayer foam sheet (ng / cm) is calculated based on the mass of the test piece and the basis weight of the test piece (multilayer foam sheet). 2 ) is calculated, and this calculated value is defined as the amount of eluted alkali metal ions. The amount of alkali metal ions measured using ion chromatography can be performed in accordance with JIS K0127:2013. Ion chromatography can be performed using an apparatus such as the ion chromatograph "INTEGRION" manufactured by Thermo Fisher Scientific. The conditions for the elution test may be those based on the "Test Method for Measurement of Ionic Contamination on Semiconductor Leadframes" specified in "SEMI G52-90."

[0136] (Ash content of multi-layer foam sheet) The ash content of the multi-layer foam sheet according to the present invention is preferably 0.1% by mass or less (including 0). Measuring the ash content of the multi-layer foam sheet allows the amount of components derived from the cell control agent (inorganic and chemical foaming agents) remaining in the multi-layer foam sheet to be confirmed. For example, a conventional multi-layer foam sheet manufactured using talc as the inorganic substance may have a high ash content. Furthermore, a conventional multi-layer foam sheet manufactured using a citric acid-sodium bicarbonate-based chemical foaming agent may contain sodium components contained in the chemical foaming agent in the ash of the foam sheet. These inorganic substances and sodium components that may be contained in the ash are both components that may migrate to contacted materials, etc. Therefore, keeping the ash content below a predetermined value may enhance the effect of reducing migration to contacted materials, etc. From this viewpoint, the ash content of the multilayer foamed sheet is preferably 0.05% by mass or less (including 0), more preferably 0.02% by mass or less (including 0), and even more preferably 0.01% by mass or less (including 0).

[0137] The ash content of the multilayer foam sheet can be measured by a method conforming to JIS K7250-1:2006. Specifically, a sample is cut from the multilayer foam sheet. After measuring the mass of the sample, the sample is placed in a crucible and the crucible is heated. After heating, the mass of the combustion residue (ash) of the sample in the crucible is measured. The ratio (M1 / M2 × 100) (%) of the measured ash mass (M1) to the sample mass (M2) is calculated, and the calculated ratio (M1 / M2 × 100) is defined as the ash content (%) of the foam sheet.

[0138] (Surface resistivity) From the viewpoint of enabling the multilayer foamed sheet to stably exhibit antistatic properties, the surface resistivity of the multilayer foamed sheet according to the present invention on the side where the carbon-containing layer is provided is set to 1×10 13 Ω or less is preferable, and 5×10 12 Ω or less is more preferable, and 1×10 12The lower limit of the surface resistivity of the multi-layer foamed sheet on the side where the carbon-containing layer is provided is not limited as long as the intended object of the present invention can be achieved, but it is preferably about 1×10 3 In addition, from the viewpoint of improving the suitability of the multilayer foam sheet when it is used as a cushioning material for electronic components and electronic devices, the lower limit of the surface resistivity is 1 × 10 7 Ω, preferably 1×10 8 Ω, preferably 1×10 9 When the surface resistivity of the multi-layer foamed sheet is within the above-mentioned range, the multi-layer foamed sheet can be evaluated as having the function of preventing static electricity.

[0139] The surface resistivity of the carbon-containing layer side of the multi-layer foamed sheet according to the present invention is the surface resistivity of surface 10A1 and / or surface 10A2 in multi-layer foamed sheet 10A shown in FIG. 1A. The surface resistivity of the carbon-containing layer side of multi-layer foamed sheet 10B shown in FIG. 1B is the surface resistivity of surface 10B1. In multi-layer foamed sheet 10C shown in FIG. 1C, the surface resistivity of the carbon-containing layer side is the surface resistivity of surface 10C1 and / or surface 10C2. In multi-layer foamed sheet 10D shown in FIG. 1D, the surface resistivity of the carbon-containing layer side is the surface resistivity of surface 10D1. When a carbon-containing layer is provided on both sides of the multi-layer foamed sheet, it is preferable that the surface resistivity of each of the two sides of the multi-layer foamed sheet both satisfy the above-mentioned numerical range.

[0140] The surface resistivity of the multilayer foamed sheet can be measured using a method conforming to JIS K6271-1:2015. Specifically, a square test piece measuring 100 mm on a side is taken from the multilayer foamed sheet. Electrodes are attached to the surface of the test piece having the carbon-containing layer, and a voltage of 1 V is applied between the electrodes in an atmosphere of 23°C and 50% relative humidity. The surface resistivity (unit: Ω) measured 1 minute after the voltage application is taken as the surface resistivity of the multilayer foamed sheet.

[0141] The polyethylene-based resin multi-layer foamed sheet of the present invention can be obtained by carrying out the "method for producing a polyethylene-based resin multi-layer foamed sheet" of the present invention, as described above. The preferred values ​​(and numerical ranges), definitions, and specifying methods for the density, basis weight, average thickness, average cell diameter, and average number of cells in the thickness direction of the multi-layer foamed sheet of the present invention are the same as the preferred values ​​(and numerical ranges), definitions, and specifying methods for the density, basis weight, average thickness, and average cell diameter of the multi-layer foamed sheet described above in the "method for producing a polyethylene-based resin multi-layer foamed sheet."

[0142] Therefore, the average cell diameter (D) of the polyethylene-based resin multi-layer foamed sheet according to the present invention is preferably 100 μm to 1,000 μm, more preferably 200 μm to 1,000 μm, even more preferably 300 μm to 900 μm, and even more preferably 400 μm to 850 μm. When the average cell diameter (D) of the multi-layer foamed sheet satisfies the above-mentioned range, the multi-layer foamed sheet is likely to have good appearance and cushioning properties, making it suitable for use as a cushioning material for packaging and other purposes.

[0143] [Resin surface layer] When the multilayer foam sheet has a resin surface layer, the resin surface layer may be formed from a resin melt for forming the resin surface layer. The resin surface layer contains the resin components constituting the resin melt for forming the resin surface layer. The resin constituting the resin surface layer may be the same resin as the resin contained in the resin melt for forming the resin surface layer, and the above-mentioned description of the resin contained in the resin melt for forming the resin surface layer may apply. Therefore, the resin surface layer may be a layer containing a polyethylene-based resin, and the polyethylene-based resin contained in the resin surface layer may have the same resin composition as the polyethylene-based resin (B). Note that the resin surface layer is preferably in a non-foamed state from the viewpoints of improving the antistatic properties, handleability, and appearance of the multilayer foam sheet and reducing dust generation. However, a small amount of very small air bubbles may be contained in the resin surface layer.

[0144] (Basis weight of resin surface layer) The basis weight of the resin surface layer is 0.5 g / m 2 More than 20g / m 2 The resin surface layer preferably has a basis weight of 0.5 g / m or less. 2 More preferably, 1 g / m 2 By setting the basis weight of the resin surface layer to the above value, it is possible to more effectively suppress the formation of pinholes in the resin surface layer, and to further reduce the migration of metal ions from the multi-layer foamed sheet and the scattering of dust originating from the carbon-containing layer. 2 Less than 5g / m, more preferably 2 or less, more preferably 4 g / m 2 Below 3 g / m, particularly preferably 2 By setting the thickness of the resin surface layer to the thickness of the multi-layer foamed sheet, it is possible to more reliably impart antistatic properties to the multi-layer foamed sheet by suppressing local variations in the antistatic properties on the surface of the multi-layer foamed sheet. When the resin surface layer is provided on both sides of the multi-layer foamed sheet, the basis weight of the resin surface layer means the basis weight per side.

[0145] The method for measuring the basis weight of the resin surface layer per side is as follows: First, the average thickness of the resin surface layer is calculated. After converting the unit of this average thickness, the density of the resin surface layer (unit: g / m3 ) to obtain the basis weight of the resin surface layer (unit: g / m 2 ) can be obtained. The average thickness of the resin surface layer is measured by the following method. A cross section (transverse direction (TD) cross section) perpendicular to the machine direction (MD) (the resin extrusion direction when coextrusion is performed) of the multilayer foam sheet is cut out, and five or more observation sections (selected sections) are randomly selected from the cross section containing the resin surface layer in the transverse direction (TD) of the multilayer foam sheet. Next, each selected section is enlarged using a microscope or the like to obtain an enlarged image of each selected section. Next, the thickness of the resin surface layer is measured at 10 random locations on each enlarged image. The arithmetic mean of all the measured thicknesses is defined as the average thickness of the resin surface layer. The thickness of the resin surface layer is measured using a multilayer foam sheet that has been conditioned for at least 24 hours at a temperature of 23±5°C and a relative humidity of 50%.

[0146] When the multi-layer foamed sheet is produced by co-extrusion, the basis weight of the resin surface layer per side can also be calculated using the following formula (2).

[0147] Basis weight of resin surface layer = [X2 / (L × W)] (2)

[0148] In the above formula (2), X2 is the discharge amount of the resin surface layer per side (the discharge amount of the resin components constituting the resin molten material for forming the resin surface layer per side) (unit: g / hour), W is the width of the multi-layer foamed sheet (unit: m), and L is the take-up speed of the multi-layer foamed sheet (unit: m / hour).

[0149] [Actions and Effects of the Multilayer Foam Sheet of the Present Invention] The multi-layer foamed sheet of the present invention has a carbon-containing layer containing conductive carbon, and the amount of conductive carbon in the carbon-containing layer is set within a predetermined range, and the amount of alkali metal ions eluted is set within a predetermined range. As a result, the multi-layer foamed sheet of the present invention has antistatic properties, is able to reduce contamination of objects that come into contact with the carbon-containing layer by metal ions, and is able to suppress the generation of resin dust and other particles originating from the carbon-containing layer.

[0150] [Application example] The multilayer foam sheet of the present invention can be suitably used as a cushioning material such as packaging materials for semiconductors and electronic devices, and interleaving sheets for glass substrates, particularly as a cushioning material for logistics.

[0151] The present invention will now be described in more detail with reference to examples, although the present invention is not limited to these examples. [Example]

[0152] In order to carry out Examples 1 to 8 and Comparative Examples 1 to 5, the following resins, physical foaming agents, antistatic agents, cell regulators, and devices were prepared.

[0153] (resin) Four types of resins were prepared as shown in Table 1. The four types of resins are abbreviated as PE-LD1, PE-LD2, PE-LLD, and EVA, respectively. PE-LD1 and PE-LD2 are low-density polyethylenes. PE-LLD is linear low-density polyethylene, specifically a copolymer of ethylene and an α-olefin with six carbon atoms, and EVA is an ethylene-vinyl acetate copolymer. Table 1 also shows the density (g / cm) of the resins. 3 ), MFR (g / 10 min), and melting point (°C). The content (mass%) of structural units derived from vinyl acetate (a monomer with a polar group) in the EVA is 41%.

[0154] [Table 1]

[0155] The values ​​listed under "Melting Point" in Table 1 are values ​​measured using the plastic transition temperature measurement method specified in JIS K7121:2012. The values ​​listed under the "MFR" column in Table 1 are melt flow rate values ​​measured at a temperature of 190°C and a load of 2.16 kg using the method specified in JIS K7210-1 (2014).

[0156] (physical foaming agent) As the physical blowing agent, mixed butane (a mixture of 70% by weight of normal butane and 30% by weight of isobutane) and nitrogen were prepared. Although mixed butane is an organic physical blowing agent, it is also used as a volatile plasticizer, as described below.

[0157] (Conductive carbon) As the conductive carbon, furnace black (brand name "VULCAN XC max22" (manufactured by CABOT CORPORATION) (oil absorption capacity 320 ml / 100 g)) was prepared. The oil absorption capacity refers to the above-mentioned "dibutyl phthalate (DBP) oil absorption capacity of conductive carbon." In Tables 2 and 3, conductive carbon is abbreviated as CB. In the explanations of the examples and comparative examples, conductive carbon is sometimes abbreviated as CB. CB was prepared in advance as a 17% concentration masterbatch (referred to as carbon masterbatch) based on PE-LD2, and when CB was used, the carbon masterbatch was used.

[0158] In addition, two types of polymeric antistatic agents were prepared as follows: The two types of polymeric antistatic agents are abbreviated as ASP-1 and ASP-2.

[0159] ASP-1 is a brand name of "Pelectron LMP" (manufactured by Sanyo Chemical Industries, Ltd.) ASP-1 is a polyether polymer to which lithium salt is added.

[0160] ASP-2 is the brand name "Entira SD-100" (manufactured by Mitsui Dow Polychemicals). ASP-1 is an ionomer and contains potassium.

[0161] (Foam adjuster) The foam control agents prepared were a chemical foaming agent and an inorganic substance. The chemical foaming agent was the product name "Fine Cell Master PO217K" (manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.). This chemical foaming agent was a mixture of sodium bicarbonate and monosodium citrate. The inorganic substance was talc. For the talc, a masterbatch (talc-added masterbatch) was prepared in advance by filling low-density polyethylene with talc at a concentration of 20%, and when talc was used, the "talc-added masterbatch" was used.

[0162] (Device) The following first and second co-extrusion devices were used.

[0163] (First co-extrusion device) The first co-extrusion device includes an extruder for forming a foam layer, an extruder for forming a carbon-containing layer, and a co-extrusion die. The extruder for forming a foam layer is a tandem extruder in which two extruders, a first extruder with a diameter of 90 mm and a second extruder with a diameter of 120 mm, are connected in series. The extruder for forming a carbon-containing layer is an extruder with a diameter of 40 mm. In the first co-extrusion device, the outlet of the second extruder and the outlet of the extruder for forming a carbon-containing layer are connected to the co-extrusion die. The device is configured so that a foamable resin melt for forming a foam layer can be introduced from the outlet of the second extruder toward the co-extrusion die. The device is configured so that a carbon-containing layer-forming resin melt can be introduced from the outlet of the carbon-containing layer-forming extruder toward the co-extrusion die. The co-extrusion die is an annular die and has a melt flow path formed so that the carbon-containing layer-forming resin melt can be laminated on both sides of the foamable resin melt for forming a foam layer. The co-extrusion die has a lip diameter of 108 mm at its die outlet.

[0164] (Second co-extrusion device) The second co-extrusion device includes an extruder for forming a foam layer, an extruder for forming a carbon-containing layer, an extruder for forming a resin surface layer, and a co-extrusion die. The extruder for forming a foam layer and the extruder for forming a carbon-containing layer are configured similarly to the first co-extrusion device. The extruder for forming a resin surface layer is an extruder with a diameter of 40 mm. In the second co-extrusion device, the outlet of the second extruder, the outlet of the extruder for forming a carbon-containing layer, and the outlet of the extruder for forming a resin surface layer are connected to the co-extrusion die. The co-extrusion die is configured so that the resin melt for forming the resin surface layer can be introduced from the outlet of the extruder for forming the resin surface layer toward the co-extrusion die. The co-extrusion die is an annular die, and forms a melt flow path so that the resin melt for forming the carbon-containing layer and the resin melt for forming the resin surface layer can be layered in this order on both sides of the foamable resin melt for forming the foam layer. The co-extrusion die has a lip diameter of 108 mm at its die outlet.

[0165] Examples 1 and 2 (Foamable resin melt for forming foam layer) Using a first co-extrusion device, the resin shown in the "Foam Layer" column of Table 2 (PE-LD1 in Examples 1 and 2) was supplied to the first extruder and heated and kneaded at approximately 200°C to obtain a first resin melt. Next, a physical foaming agent was injected into the first resin melt. An organic physical foaming agent (mixed butane) and nitrogen were used as the physical foaming agent. The amounts of the organic physical foaming agent and nitrogen added are as shown in Table 2. In Table 2, the amount A of the organic physical foaming agent and the amount B of nitrogen added per kg of the resin components constituting the foam layer-forming foamable resin melt are expressed as "mol / kg." The first resin melt into which the physical foaming agent was injected was supplied to a second extruder, and the temperature of the first resin melt was adjusted to approximately 110°C to obtain a foam layer-forming foamable resin melt.

[0166] The "Foam Layer" column in Table 2 also lists the total amount (A+B) (mol / kg) of the amount of organic physical blowing agent (A) and the amount of nitrogen (B) added, and the ratio (A / B) of the amount of organic physical blowing agent (A) to the amount of nitrogen added (B) (the value of [amount of organic physical blowing agent added] / [amount of nitrogen added]). This also applies to Examples 3 to 8 in Table 2 and Comparative Examples 1 to 5 in Table 3.

[0167] (Melted resin for forming carbon-containing layer) The resin shown in the "Carbon-containing layer" column of Table 2 (a mixed resin of PE-LD2 and EVA in Examples 1 and 2) and conductive carbon were supplied to the carbon-containing layer-forming extruder, and the resin and conductive carbon were heated in the carbon-containing layer-forming extruder to obtain a second resin melt adjusted to approximately 200°C. A volatile plasticizer was injected into the second resin melt, and the resin temperature was adjusted to 125°C to obtain a carbon-containing layer-forming resin melt. The conductive carbon was supplied by feeding a carbon masterbatch into the carbon-containing layer-forming extruder. Mixed butane was used as the volatile plasticizer injected into the second resin melt, and the amount of mixed butane added to the second resin melt was 17 parts by mass per 100 parts by mass of the total resin components and conductive carbon constituting the resin melt. In Table 2, the mixed resin of PE-LD2 and EVA is referred to as PE-LD2 / EVA. The term PE-LD2 / EVA is also used in Table 3. The blending ratio (mass ratio) of PE-LD2 / EVA / conductive carbon in the carbon-containing layer was PE-LD2 / EVA / conductive carbon = 33.0 / 61.5 / 5.5 in Example 1, and PE-LD2 / EVA / conductive carbon = 32.0 / 60.0 / 8.0 in Example 2.

[0168] The total output rate of the resin components constituting the carbon-containing layer-forming resin melt and the conductive carbon was 5 kg / h in Example 1 (2.5 kg / h per side) and 6 kg / h in Example 2 (3 kg / h per side). The output rate of the resin components constituting the foam layer-forming foamable resin melt was 60 kg / h (for both Examples 1 and 2). The carbon-containing layer-forming resin melt and the foam layer-forming foamable resin melt were introduced into a co-extrusion die, and the carbon-containing layer-forming resin melt merged on both sides of the foam layer-forming foamable resin melt in the co-extrusion die. The carbon-containing layer-forming resin melt and the foam layer-forming foamable resin melt were extruded from the co-extrusion die in a laminated state. At this time, a cylindrical multilayer foam was formed. The cylindrical multilayer foamed body has a cylindrical foam layer formed by foaming a foam layer-forming foamable resin melt, and has a structure in which layers of a carbon-containing layer-forming resin melt are laminated (laminated and directly bonded) on both the inner and outer surfaces of the cylindrical foam layer. The extruded cylindrical laminated foamed body was expanded using a mandrel with a diameter of 350 mm, and then taken up by a take-up machine (roll) while being slit open to produce a polyethylene-based resin multilayer foamed sheet. Note that when the cylindrical laminated foamed body was taken up by the take-up machine (roll), the take-up speed was 25 m / min in Example 1, and 19 m / min in Example 2.

[0169] The resulting polyethylene resin multi-layer foamed sheet was measured for total basis weight (g / m 2 ), total thickness (average thickness) (mm), density (kg / m 3 ), average bubble diameter (μm), and average number of bubbles in the thickness direction (pcs / mm) were measured. 2 ), total thickness (average thickness) (mm), density (kg / m 3 The average cell size (μm), average cell diameter (μm), and average number of cells in the thickness direction (cells / mm) were determined using the method described in the above "Method for producing a polyethylene-based resin multi-layer foamed sheet." The results are shown in Table 2.

[0170] In addition, for the obtained polyethylene resin multi-layer foamed sheet, the basis weight (g / m2 ), the amount of conductive carbon in the carbon-containing layer (amount of carbon per area) (g / m 2 ) was measured using the method described in the above description of the "Polyethylene-based resin multi-layer foamed sheet." The basis weight (g / m 2 ) was calculated from the extrusion conditions using the above formula (1). The results are shown in Table 2.

[0171] Furthermore, the surface resistivity (Ω) and ash content (mass%) of the polyethylene-based resin multi-layer foamed sheet were measured. The results are shown in Table 2.

[0172] (Surface resistivity) The surface resistivity of the multilayer foam sheet was determined as the average of the surface resistivity values ​​at five equally spaced measurement points across the width of the sheet. The surface resistivity at each measurement point was determined using a Nitto Seiko Analytech "Hiresta UX MCP-HT800" instrument, manufactured by Nitto Seiko Analytech Co., Ltd., by measuring the surface resistivity one minute after applying a voltage of 50 V according to the method of JIS K6271 (2001). The average of the surface resistivity values ​​obtained at each measurement point was calculated, and this calculated value was determined as the surface resistivity of the multilayer foam sheet. Note that the surface resistivity was measured on the surface that faced the mandrel during the production of the multilayer foam sheet. The surface resistivity of the multilayer foam sheet is 1×10 13 (Ω) or less, the multilayer foamed sheet can be evaluated as having antistatic properties.

[0173] (ash content) The ash content of the multilayer foam sheet was measured in accordance with JIS K7250-1:2006 as follows. A sample weighing approximately 10 g was cut from the multilayer foam sheet. After measuring the mass of the sample, the sample was placed in a crucible and heated for 1 hour in an electric furnace with an ambient temperature set to 600°C. After heating, the mass of the combustion residue (ash) of the sample was measured. The ratio (M1 / M2 × 100) (%) of the measured ash mass (M1) to the sample mass (M2) was calculated, and the calculated ratio (M1 / M2 × 100) was determined as the ash content (%) of the multilayer foam sheet.

[0174] For polyethylene resin multi-layer foam sheets, the amount of ion elution (ng / cm 2 ), the amount of ions transferred to the surface of the glass material (ng / cm 2 ), pinhole occurrence rate (%), and number of particles (particles / m 3 ) was measured according to the following method. The results are shown in Table 2.

[0175] (Amount of ion elution from multi-layer foam sheet) The multilayer foam sheet was cut into a piece weighing 0.5±0.1 g, and the cut multilayer foam sheet was used as a measurement sample. The measurement sample was immersed in 10 ml of ion-exchanged water. The ion-exchanged water was heated at 95°C for 30 minutes. This heating caused ions to be extracted from the test piece into the ion-exchanged water. The heated ion-exchanged water is referred to as the test solution. The test solution was then used for ion chromatography analysis to measure the amount of alkali metal ions eluted (ng / ml) (the total amount of eluted sodium ions, eluted potassium ions, and eluted lithium ions). The elution of alkali metal ions was measured in accordance with JIS K0127:2013. Specifically, the amount of alkali metal ions eluted (ng / ml) extracted into the ion-exchanged water constituting the test solution was measured using an ion chromatograph "INTEGRION" manufactured by Thermo Fisher Scientific. Based on the measured amount of elution, the weight of the measurement sample, and the basis weight of the measurement sample (multi-layer foam sheet), the amount of ions eluted from the multi-layer foam sheet per unit area of ​​the multi-layer foam sheet (ng / cm 2 ) was calculated, and the calculated value was defined as the total amount of alkali metal ions extracted from the multi-layer foam sheet.

[0176] (Amount of ions transferred to the surface of the glass material) The multilayer foamed sheet was cut into four pieces measuring 60 mm in the width direction and 110 mm in the extrusion direction. Next, a laminated structure was produced by alternately stacking the four cut-out multilayer foamed sheets and three sheets of glass material (glass plate material) (alkali-free glass, product name "Eagle XG" (manufactured by Corning)) measuring 50 mm × 100 mm. A 20 g / cm 2 coating was applied to the top surface of the laminated structure. 2 Weight (base area of ​​weight: 126cm 2 The laminated structure with the weight placed on it was then placed in a thermo-hygrostat chamber at a temperature of 60°C and a humidity of 80%, and allowed to stand in the chamber for 24 hours to promote the migration of components from the multilayer foamed sheet to the glass surface.

[0177] After leaving it for 24 hours, the glass material located in the middle of the three glass materials constituting the laminated structure (the glass material located second from the top) was used as the measurement sample. The measurement sample was immersed in 10 ml of ion-exchanged water. The ion-exchanged water was heated at a temperature of 95°C for 30 minutes. This heating caused ions to be extracted from the test piece into the ion-exchanged water. The heated ion-exchanged water is referred to as the test liquid. Thereafter, the amount of eluted alkali metal ions (ng / ml) was measured by ion chromatography analysis using this test liquid. The ion chromatography analysis was performed using the same method as that used to measure the "amount of eluted ions from the multilayer foam sheet" described above. The measured amount of elution was calculated based on the area (100 cm) of the measurement sample. 2 ) to determine the amount of alkali metal ions transferred from the multilayer foam sheet to the surface of the glass material (ion transfer amount, unit: ng / cm 2 ) was calculated. From the viewpoint of suppressing the transfer of metal ions to the contacted object, the amount of ion transfer is 0.002 ng / cm 2 It is preferable that the concentration is 0 or less (including 0), and 0.001 ng / cm 2 It is more preferable that the number is equal to or less than (including 0).

[0178] (Pinhole occurrence rate) As described above, pinholes refer to defects such as small holes formed on the outermost surface of a multi-layer foam sheet. The pinhole occurrence rate (%) is defined as the percentage (%) of the total area of ​​the pinhole-formed region on the outermost surface of the multi-layer foam sheet (area occupancy rate). The pinhole occurrence rate was measured as follows: Three locations were randomly selected in the width direction of the multi-layer foam sheet, and rectangular sheet pieces measuring 5 mm square were cut out from the multi-layer foam sheet at each of the three locations. These sheet pieces were used as test pieces. Using the test pieces, images of the test piece surfaces (i.e., images of the outermost surface of the multi-layer foam sheet) were taken with an electron microscope. The observation magnification of the electron microscope was set to 100x. For each test piece, images of the test piece surface were taken in six fields of view. A total of 18 observation images were obtained as images of the test piece surfaces. For each observation image, the areas of pinholes present in each observation field (image area) were tallied, and the total area was multiplied by the total area of ​​the observation fields (34.5 mm 2 The pinhole occurrence rate (%) was calculated by dividing the measured value by the measured value. From the viewpoint of stably suppressing detachment of resin and the like originating from the carbon-containing layer from the multi-layer foamed sheet due to friction or the like, the pinhole occurrence rate is preferably 3% or less (including 0), more preferably 2% or less (including 0), and even more preferably 1% or less (including 0).

[0179] (number of particles) The dust count indicates the amount of particles generated from resin fragments and the like generated from the multi-layer foam sheet. The value indicating the dust count is the number of particles (particles / m) measured based on the airborne dust generation test in accordance with SEMI-G67-0996 shown below. 3) was adopted. A class 1 clean bench was prepared, and an airborne dust generation test was conducted using the space within the class 1 clean bench. The clean bench was equipped with an airflow generator to generate a clean air flow in advance. The airborne dust generation test was conducted as follows. After pre-cleaning the multilayer foam sheet within the clean bench, the multilayer foam sheet was torn approximately every 5 seconds on the upstream side of the air flow direction within the clean bench to promote dust generation from the multilayer foam sheet (to promote particle generation). The generated particles were contained in the clean air flowing through the space within the clean bench. An airborne particle analyzer (model number "KC2B", manufactured by Rion Co., Ltd.) was installed downstream of the air flow direction, and particles contained in the clean air were sucked into the airborne particle analyzer. The number of particles with an equivalent volume sphere diameter of 0.08 μm or more that were sucked in was counted by the airborne particle analyzer, and the measured value was used as the dust number (particles / m 3 ) was established. From the viewpoint of maintaining the cleanliness of the usage environment and the objects that come into contact with the polyethylene resin multi-layer foam sheet, the dust emission count measured by the air dust emission test is 20,000 particles / m 3 It is preferable that the number of particles is 15,000 or less (including 0) 3 It is more preferable that the number of particles is 10,000 or less (including 0), and 10,000 particles / m 3 It is more preferable that the number is equal to or less than (including 0).

[0180] Examples 3 to 8 In Examples 3 to 8, a foamable resin melt for forming a foam layer and a resin melt for forming a carbon-containing layer were obtained by applying the same method as in Example 1 using a second co-extrusion device and the resins and physical foaming agents shown in Table 2, respectively. In Table 2, the compounding ratio (mass ratio) of PE-LD2 / EVA / conductive carbon in the carbon-containing layer was PE-LD2 / EVA / conductive carbon = 33.0 / 61.5 / 5.5 in Examples 3 and 5 to 8, and PE-LD2 / EVA / conductive carbon = 32.0 / 60.0 / 8.0 in Example 4.

[0181] (Melted resin for forming resin surface layer) The resin shown in the "Resin surface layer" column in Table 2 (PE-LLD in Examples 3 to 8) was supplied to an extruder for forming a resin surface layer, and the resin was heated in the extruder for forming a resin surface layer, thereby obtaining a resin melt for forming a resin surface layer.

[0182] The total output rate of the resin components constituting the carbon-containing layer-forming resin melt and the conductive carbon was 4 kg / h (2 kg / h per side), the output rate of the resin components constituting the resin surface layer-forming resin melt was 6 kg / h (3 kg / h per side), and the output rate of the resin components constituting the foamable resin melt for forming a foam layer was 60 kg / h. The carbon-containing layer-forming resin melt, the resin surface layer-forming resin melt, and the foamable resin melt for forming a foam layer were introduced into a co-extrusion die, and the carbon-containing layer-forming resin melt and the resin surface layer-forming resin melt were merged on both sides of the foamable resin melt for forming a foam layer in the co-extrusion die. A structure in which the carbon-containing layer-forming resin melt and the resin surface layer-forming resin melt were laminated on both sides of the foamable resin melt for forming a foam layer in this order was formed, and the resulting mixture was extruded from the co-extrusion die. At this time, a cylindrical multilayer foam was formed. The cylindrical multilayer foamed body has a cylindrical foam layer formed by foaming a foam layer-forming foamable resin melt, and has a structure in which a layer of a carbon-containing layer-forming resin melt and a layer of a resin surface layer-forming resin melt are laminated (laminated and directly bonded) on both the inner and outer surfaces of the cylindrical foamed layer. The extruded cylindrical laminated foamed body was expanded on a mandrel with a diameter of 350 mm, and cut open while being taken up by a take-up machine (roll), to produce a polyethylene-based resin multilayer foamed sheet. In the carbon-containing layer-forming resin molten material, 17 parts by mass of mixed butane as a volatile plasticizer was added relative to 100 parts by mass of the total of the resin components and conductive carbon constituting the carbon-containing layer-forming resin molten material. Also, in the resin surface layer-forming resin molten material, 17 parts by mass of mixed butane as a volatile plasticizer was added relative to 100 parts by mass of the resin components constituting the resin surface layer-forming resin molten material. When the cylindrical laminated foam body was taken up by a take-up machine (roll), the take-up speed was 20 m / min in Examples 3 and 5 to 8, and 30 m / min in Example 4.

[0183] The resulting polyethylene resin multi-layer foamed sheet was measured for total basis weight (g / m 2 ), total thickness (average thickness) (mm), density (kg / m 3 ), average bubble diameter (μm) and average number of bubbles in the thickness direction (pcs / mm) were measured. 2 ), total thickness (average thickness) (mm), density (kg / m 3 The average cell diameter (μm) and the average number of cells in the thickness direction (cells / mm) were determined using the method described in the above "Method for producing a polyethylene-based resin multi-layer foamed sheet." The results are shown in Table 2.

[0184] In addition, for the obtained polyethylene resin multi-layer foamed sheet, the basis weight (g / m 2 ), the amount of conductive carbon in the carbon-containing layer (g / m 2 ), basis weight of resin surface layer (g / m 2 ) was measured using the method described in the above description of the "Polyethylene-based resin multi-layer foamed sheet." The basis weight (g / m 2 ) is calculated by the above formula (1) using the basis weight (g / m 2 ) was calculated using the above formula (2) under the conditions at the time of extrusion. Furthermore, the surface resistivity (Ω), ash content (mass%), and ion elution amount (ng / cm) of the polyethylene-based resin multi-layer foamed sheet were measured using the method described in Example 1. 2 ), the amount of ions transferred to the surface of the glass material (ng / cm 2 ), pinhole occurrence rate (%), and number of particles (particles / m 3 The results are shown in Table 2.

[0185] Comparative Examples 1 and 2 In Comparative Examples 1 and 2, the same method as in Example 4 was carried out, except that the amount of nitrogen contained in the physical foaming agent was set to the amount shown in Table 3. The blending ratio (mass ratio) of PE-LD2 / EVA / conductive carbon in the carbon-containing layer was also the same as in Example 4. In Comparative Examples 1 and 2, poor foaming occurred in the molten foamable resin for forming the foam layer. In Comparative Example 1, many areas where foaming was not observed occurred in the structure obtained by coextrusion, resulting in a low-density multi-layer foamed sheet (specifically, a multi-layer foamed sheet with a density of 500 kg / m 3 In Comparative Example 2, the structure obtained by coextrusion frequently broke during take-up by rolls, making it difficult to produce a multilayer foam sheet. Thus, in Comparative Examples 1 and 2, a good multilayer foam sheet could not be obtained. For this reason, measurements corresponding to the items listed in the "Physical properties of multilayer foam sheet" and "Evaluation results" columns of Table 3 were not performed for Comparative Examples 1 and 2.

[0186] Comparative Example 3 In Comparative Example 3, the addition of nitrogen to the foamable resin melt was omitted, a bubble adjusting agent was added to the foamable resin melt, a chemical foaming agent was used as the bubble adjusting agent, and the blending amount of conductive carbon (g / m) contained in the carbon-containing layer was 2 ) was used in the amounts shown in Table 3. The same method as in Example 4 was carried out. The blending ratio (mass ratio) of PE-LD2 / EVA / conductive carbon in the carbon-containing layer was also the same as in Example 4. The amount of the chemical foaming agent used as a cell adjusting agent was 1.0 part by mass with respect to 100 parts by mass of the resin component constituting the foamable resin melt.

[0187] The total basis weight (g / m) of each polyethylene-based resin multi-layer foamed sheet obtained in Comparative Example 3 was 2 ), total thickness (average thickness) (mm), density (kg / m 3 ), average bubble diameter (μm) and average number of bubbles in the thickness direction (pcs / mm) were measured. 2 ), total thickness (average thickness) (mm), density (kg / m 3The average cell size (μm), average cell diameter (μm), and average number of cells in the thickness direction (cells / mm) were determined using the method described in the above "Method for producing a polyethylene-based resin multi-layer foamed sheet." The results are shown in Table 3.

[0188] For each of the polyethylene-based resin multi-layer foamed sheets obtained in Comparative Example 3, the basis weight (g / m 2 ), the amount of conductive carbon in the carbon-containing layer (g / m 2 ), basis weight of resin surface layer (g / m 2 ) was measured using the method described in the above description of the "Polyethylene-based resin multi-layer foamed sheet." The basis weight (g / m 2 ) is calculated by the above formula (1) using the basis weight (g / m 2 ) was calculated using the above formula (2) under the conditions at the time of extrusion. Furthermore, the surface resistivity (Ω), ash content (mass%), and ion elution amount (ng / cm) of the polyethylene-based resin multi-layer foamed sheet were measured using the method described in Example 1. 2 ), the amount of ions transferred to the surface of the glass material (ng / cm 2 ), pinhole occurrence rate (%), and number of particles (particles / m 3 ) was measured. The results are shown in Table 3.

[0189] Comparative Examples 4 and 5 In Comparative Examples 4 and 5, unlike Example 1, the addition of nitrogen to the foamable resin melt was omitted, and instead a bubble control agent was added to the foamable resin melt, with an inorganic bubble control agent (talc in Comparative Examples 4 and 5). In Comparative Examples 4 and 5, unlike Example 1, instead of adding conductive carbon to the carbon-containing layer, a polymer-type antistatic agent (ASP-1 in Comparative Example 4, ASP-2 in Comparative Example 5) was added in the type and amount shown in Table 3. The resins used in the carbon-containing layer (resins constituting the layer to which the polymer-type antistatic agent was added) were the types shown in the "Resin" column of Table 3. Except for these points, Comparative Examples 4 and 5 were carried out in the same manner as Example 1. In Table 3, for the items related to the carbon-containing layer (the "Conductive Carbon" column and the "Basis Weight of Carbon-Containing Layer" column), the parts where a polymer-type antistatic agent was used are written in parentheses. The "Conductive Carbon" column lists the type and amount of polymeric antistatic agent, and the "Basis Weight of Carbon-Containing Layer" column lists the basis weight of the layer to which the polymeric antistatic agent was added. In Comparative Example 4, the blending ratio (mass ratio) of PE-LD1 / polymeric antistatic agent (ASP-1) was 85 / 15. In Comparative Example 5, the blending ratio (mass ratio) of PE-LD1 / polymeric antistatic agent (ASP-2) was 85 / 15. In Comparative Examples 4 and 5, the amount of inorganic substance (talc) used as a cell adjusting agent was 3.0 parts by mass relative to 100 parts by mass of the resin component constituting the foamable resin melt.

[0190] The polyethylene-based resin multi-layer foamed sheets obtained in Comparative Examples 4 and 5 were measured for their total basis weight (g / m 2 ), total thickness (average thickness) (mm), density (kg / m 3 ), average bubble diameter (μm) and average number of bubbles in the thickness direction (pcs / mm) were measured. 2 ), total thickness (average thickness) (mm), density (kg / m 3 The average cell size (μm), average cell diameter (μm), and average number of cells in the thickness direction (cells / mm) were determined using the method described in the above "Method for producing a polyethylene-based resin multi-layer foamed sheet." The results are shown in Table 3.

[0191] For the polyethylene-based resin multi-layer foamed sheets obtained in Comparative Examples 4 and 5, the basis weight (g / m) of the layer to which the polymeric antistatic agent was added (corresponding to the carbon-containing layer in the Examples) was 2 ) was measured. 2 ) was calculated from the extrusion conditions using the formula (1) for the carbon-containing layer shown in the explanation of the [Polyethylene-based resin multi-layer foamed sheet] above. In the formula (1), X1 was applied as the extrusion rate (unit: g / hour) of the "layer containing a polymer-type antistatic agent" per side. Furthermore, using the method described in Example 1, the surface resistivity (Ω), ash content (mass%), and ion elution amount (ng / cm) of the polyethylene-based resin multi-layer foamed sheet were measured. 2 ), the amount of ions transferred to the surface of the glass material (ng / cm 2 ), pinhole occurrence rate (%), and number of particles (particles / m 3 ) was measured. The results are shown in Table 3.

[0192] [Table 2]

[0193] [Table 3]

[0194] Based on Examples 1 to 8 and Comparative Examples 1 to 5, it was confirmed that a multi-layer foamed sheet having a desirable density, good antistatic properties, and reduced adhesion of metal ions (alkali metal ions) to objects contacted therewith can be obtained by providing a conductive carbon-containing resin layer containing a conductive carbon content within a predetermined range, and by providing a conductive carbon-containing resin layer containing a conductive carbon content within a predetermined range. [Explanation of symbols]

[0195] 10A, 10B, 10C, 10D Multilayer foam sheet 11 Foam layer 12 Carbon-containing layer 13 Resin surface layer

Claims

1. A method for producing a polyethylene-based resin multi-layer foamed sheet comprising a polyethylene-based resin foamed layer and a conductive carbon-containing resin layer by co-extruding a foamable resin melt for forming a foamed layer, the foamed resin melt being obtained by kneading a polyethylene-based resin with a physical foaming agent, and a conductive carbon-containing resin layer forming melt being obtained by kneading a polyethylene-based resin with conductive carbon, the method comprising: The amount of conductive carbon in the conductive carbon-containing resin layer is 0.01 g / m ２ 0.5g / m or more ２ is as follows: the physical blowing agent comprises one or more organic physical blowing agents selected from hydrocarbons having 3 to 5 carbon atoms and dialkyl ethers having an alkyl group having 1 to 3 carbon atoms, and nitrogen; the sum (A+B) of the amount A of the organic physical foaming agent added and the amount B of the nitrogen added is 0.5 mol or more and 5 mol or less per kg of the resin component constituting the foamable resin molten material for forming a foam layer, The method for producing a polyethylene-based resin multi-layer foamed sheet, wherein the amount B of nitrogen added is 0.1 mol or more and 0.4 mol or less per kg of the resin component constituting the foamable resin melt for forming the foam layer.

2. 2. The method for producing a polyethylene-based resin multi-layer foamed sheet according to claim 1, wherein a ratio (A / B) of an amount A of the organic physical foaming agent to an amount B of the nitrogen is 2 or more and 18 or less.

3. 3. The method for producing a polyethylene-based resin multi-layer foamed sheet according to claim 1 or 2, wherein neither a powdery inorganic substance (but not including a chemical foaming agent) nor a chemical foaming agent is added to the foam layer-forming molten foamable resin, or wherein a powdery inorganic substance (but not including a chemical foaming agent) and / or a chemical foaming agent is added to the foam layer-forming molten foamable resin, and the total amount of the inorganic substance and the chemical foaming agent added is less than 0.1 parts by mass per 100 parts by mass of the resin components constituting the foam layer-forming molten foamable resin.

4. The density of the polyethylene resin multi-layer foam sheet is 20 kg / m ３ More than 200kg / m ３ The method for producing a polyethylene-based resin multi-layer foamed sheet according to claim 1 or 2, wherein the method is as follows:

5. 3. The method for producing a polyethylene-based resin multi-layer foamed sheet according to claim 1, wherein the polyethylene-based resin multi-layer foamed sheet has an average cell diameter of 100 μm or more and 1000 μm or less.

6. 3. The method for producing a polyethylene-based resin multi-layer foamed sheet according to claim 1, wherein the polyethylene-based resin multi-layer foamed sheet comprises the polyethylene-based resin foam layer, the conductive carbon-containing resin layer laminated on at least one side of the polyethylene-based resin foam layer, and a resin surface layer laminated on the conductive carbon-containing resin layer.

7. A polyethylene-based resin foam layer and a conductive carbon-containing resin layer containing a polyethylene-based resin and conductive carbon, ３ More than 200kg / m ３ A polyethylene-based resin multi-layer foamed sheet as follows: The amount of conductive carbon in the conductive carbon-containing resin layer is 0.01 g / m ２ 0.5g / m or more ２ is as follows: The total amount of sodium ions, potassium ions and lithium ions extracted from the polyethylene-based resin multi-layer foamed sheet is 0.10 ng / cm ２ A polyethylene-based resin multi-layer foamed sheet, characterized in that:

8. 8. The polyethylene-based resin multi-layer foamed sheet according to claim 7, wherein the polyethylene-based resin multi-layer foamed sheet has an average cell diameter of 100 μm or more and 1000 μm or less.

9. 9. The polyethylene-based resin multi-layer foamed sheet according to claim 7, wherein the ash content of the polyethylene-based resin multi-layer foamed sheet is 0.1% by mass or less (including 0).

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