Polyethylene resin multi-layer foam sheet and its manufacturing method

Abstract

To provide a polyethylene-based resin multilayer foam sheet that is electrically conductive and highly reduces contamination of an object to be packaged, and a method for manufacturing the same.SOLUTION: A multilayer foam sheet 1 comprises: a polyethylene-based resin foam layer 2; a surface layer 4 laminated on at least one side of the foam layer 2; and an electrically conductive layer 3 provided between the surface layer 4 and the foam layer 2. The electrically conductive layer 3 includes an ethylene-based copolymer containing a structural unit derived from a monomer having a polar group; a polyethylene-based resin different from the ethylene-based copolymer; and conductive carbon. A content of conductive carbon in the conductive layer 3 is 5 to 15 mass%. The surface layer 4 is composed of one or more kinds of linear polyethylene selected from the group consisting of linear low-density polyethylene and high-density polyethylene, or a mixed resin containing linear polyethylene and low-density polyethylene, in which a blending amount of linear polyethylene is 8 mass% or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Polyethylene resin foam sheets using polyethylene resins as a base resin have high flexibility and excellent shock absorption properties and are therefore used for applications such as cushioning materials and packaging materials. Among these, cushioning materials and packaging materials used for packaging electronic devices, electronic components, etc. may be required to be electrically conductive in addition to protecting the packaged items.

[0003] As an example of a conductive foamed sheet, Patent Document 1 describes a conductive polyethylene-based resin foamed sheet obtained by mixing a masterbatch containing conductive carbon with a polyethylene-based resin and extruding and foaming the mixture.

[0004] Patent Document 2 describes an electrically conductive polyethylene-based resin multi-layer foamed sheet obtained by heating and foaming an expandable multi-layer thermoplastic resin sheet consisting of at least two layers: a non-conductive polyethylene-based resin containing a thermal decomposition type foaming agent and a conductive thermoplastic resin layer containing conductive carbon. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 61-31440 [Patent Document 2] Japanese Patent Application Publication No. 62-231728 Summary of the Invention [Problem to be solved by the invention]

[0006] The foam sheet of Patent Document 1 needs to be blended with a relatively large amount of conductive carbon to impart conductivity to the foam sheet. However, if the amount of conductive carbon blended into the foam sheet is too large, the foaming properties may be hindered, and properties such as cushioning properties necessary for use as a cushioning material or packaging material may be impaired.

[0007] Similarly, the foamed sheet of Patent Document 2 also has a surface resistivity of 1×10 7 In order to achieve a resistance of Ω or less, it is necessary to blend a relatively large amount of conductive carbon into the conductive thermoplastic resin layer.

[0008] Furthermore, the foam sheets of Patent Documents 1 and 2 have a risk that the conductive carbon may fall off the foam sheet and contaminate the surrounding area. In particular, when the amount of conductive carbon in the foam sheet increases, the conductive carbon may easily fall off the foam sheet, and the surrounding area of ​​the foam sheet may easily become contaminated by the conductive carbon. Furthermore, when the foam sheet is used as a packaging material, the conductive carbon that falls off the foam sheet may migrate to the packaged item, contaminating the packaged item.

[0009] In recent years, depending on the application of electronic components and the like, a highly clean state may be required, and packaging materials used for such applications are required to further reduce the contamination of the packaged items so that the packaging materials themselves do not become a source of contamination.

[0010] The present invention has been made in view of the above background, and aims to provide a polyethylene-based resin multi-layer foamed sheet that is electrically conductive and can significantly reduce contamination of packaged items, and a method for producing the same. [Means for solving the problem]

[0011] One aspect of the present invention is a polyethylene resin multi-layer foamed sheet according to the following items [1] to

[10] .

[0012] [1] A polyethylene resin foam layer, a surface layer provided on at least one side of the polyethylene-based resin foam layer; a conductive layer provided between the surface layer and the foam layer, the conductive layer comprises an ethylene-based copolymer having a structural unit derived from ethylene and a structural unit derived from a monomer having a polar group, a polyethylene-based resin different from the ethylene-based copolymer, and conductive carbon; the conductive layer contains the conductive carbon in an amount of 5% by mass or more and 15% by mass or less; the surface layer is made of a mixed resin containing one or more linear polyethylenes selected from the group consisting of linear low-density polyethylenes and high-density polyethylenes, or the linear polyethylenes and low-density polyethylenes, with the linear polyethylene content being 8% by mass or more; The surface resistivity of the surface of the multilayer foamed sheet on which the surface layer is provided is 1×10 8 A polyethylene resin multi-layer foam sheet having a modulus of elasticity of less than Ω.

[0013] [2] The polyethylene-based resin multi-layer foamed sheet according to [1], wherein the foamed layer contains low-density polyethylene. [3] The polyethylene-based resin multi-layer foamed sheet according to [1] or [2], wherein the surface layer contains at least a linear low-density polyethylene as the linear polyethylene. [4] The polyethylene-based resin multi-layer foam sheet according to any one of [1] to [3], wherein the linear polyethylene contained in the surface layer has a melt flow rate of 12 g / 10 min or more at a temperature of 190°C and a load of 2.16 kg.

[0014] [5] The polyethylene-based resin multi-layer foam sheet according to any one of [1] to [4], wherein the linear polyethylene contained in the surface layer has a tensile strength of 3.0 MPa or more at a temperature of 95°C. [6] The polyethylene-based resin multi-layer foam sheet according to any one of [1] to [5], wherein the linear polyethylene contained in the surface layer has a tensile strength of 15 MPa or more at a temperature of 23°C. [7] The polyethylene-based resin multi-layer foam sheet according to any one of [1] to [6], wherein the crystallization temperature of the linear polyethylene contained in the surface layer is 100°C or higher and 115°C or lower.

[0015] [8] The melting point Tm of the linear polyethylene contained in the surface layer S and the melting point Tm of the polyethylene resin used in the foam layer C Difference from Tm S -Tm C The polyethylene resin multi-layer foam sheet according to any one of [1] to [7], wherein the temperature is 0°C or higher and 15°C or lower. [9] The basis weight of the surface layer of the polyethylene-based resin multi-layer foamed sheet is 1 g / m 2 More than 10g / m 2 The polyethylene resin multi-layer foam sheet according to any one of [1] to [8] below.

[10] The apparent density of the polyethylene-based resin multi-layer foamed sheet is 30 kg / m 3 More than 150kg / m 3 The polyethylene resin multi-layer foam sheet according to any one of [1] to [9] below.

[0016] Another aspect of the present invention is a method for producing a multi-layer polyethylene resin foam sheet according to the following item

[11] .

[0017]

[11] A method for producing a multi-layer polyethylene resin foamed sheet, comprising co-extruding a foam layer-forming melt for forming a polyethylene resin foamed layer, a conductive layer-forming melt for forming a conductive layer, and a surface layer-forming melt for forming a surface layer, to produce a polyethylene resin foamed layer having the polyethylene resin foamed layer, the surface layer provided on at least one side of the polyethylene resin foamed layer, and the conductive layer provided between the surface layer and the foamed layer, the foam layer-forming melt is obtained by kneading a polyethylene resin and a physical foaming agent, the conductive layer-forming melt is obtained by kneading an ethylene-based copolymer having a structural unit derived from ethylene and a structural unit derived from a monomer having a polar group, a polyethylene-based resin different from the ethylene-based copolymer, and conductive carbon; the conductive carbon is blended in an amount of 5% by mass or more and 15% by mass or less in the conductive layer-forming melt, a method for producing a polyethylene-based resin multi-layer foamed sheet, wherein the surface layer-forming molten material is obtained by kneading one or more linear polyethylenes selected from the group consisting of linear low-density polyethylenes and high-density polyethylenes, or 8 mass% or more of the linear polyethylene and low-density polyethylene relative to the mass of the surface layer-forming molten material. [Effects of the Invention]

[0018] According to the above-described aspect, it is possible to provide a polyethylene-based resin multi-layer foamed sheet (hereinafter referred to as "multi-layer foamed sheet") that is electrically conductive and can significantly reduce contamination of packaged items, and a method for producing the same. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is a cross-sectional view of a polyethylene-based resin multi-layer foamed sheet in an example. DETAILED DESCRIPTION OF THE INVENTION

[0020] (Polyethylene resin multi-layer foam sheet) The multilayer foam sheet has a multilayer structure including three layers: a polyethylene-based resin foam layer (hereinafter referred to as "foam layer"), a surface layer provided on at least one side of the foam layer, and a conductive layer provided between the surface layer and the foam layer. The surface layer is provided on the outermost surface of the multilayer foam sheet. Each layer included in the multilayer foam sheet is laminated and bonded to an adjacent layer. The surface layer and the conductive layer in the multilayer foam sheet are preferably laminated by coextrusion, and all layers in the multilayer foam sheet are preferably laminated and bonded to an adjacent layer by coextrusion.

[0021] For example, the multilayer foam sheet may be composed of three layers: a foam layer, a conductive layer laminated on one side of the foam layer, and a surface layer laminated on the conductive layer. Alternatively, the multilayer foam sheet may be composed of five layers: a foam layer, conductive layers laminated on both sides of the foam layer, and a surface layer laminated on each conductive layer. Furthermore, the multilayer foam sheet may have a layer between the foam layer and the conductive layer that has a different composition from the foam layer, the conductive layer, and the surface layer.

[0022] In the multilayer foam sheet, a conductive layer composed of a resin composition containing the specific resin and conductive carbon is provided on at least one side of the foam layer. Furthermore, a surface layer composed of the specific resin is provided on the surface of the multilayer foam sheet. By providing the conductive layer from the specific resin composition in this way, conductivity can be increased without excessively increasing the amount of conductive carbon blended. Furthermore, by providing a surface layer composed of the specific resin on the surface of the multilayer foam sheet, the multilayer foam sheet can significantly reduce contamination of packaged items while maintaining high conductivity.

[0023] From the viewpoints of more reliably preventing static damage to packaged items such as electronic devices and electronic components and further reducing the contamination of the multi-layer foam sheet, it is preferable that the conductive layer and the surface layer are provided on both sides of the polyethylene-based resin foam layer. That is, the multi-layer foam sheet preferably has a five-layer structure including a foam layer, surface layers provided on both sides of the foam layer and positioned on the outermost surfaces of the multi-layer foam sheet, and conductive layers positioned between each surface layer and the foam layer.

[0024] <Total thickness> The total thickness of the multi-layer foam sheet is preferably 0.05 mm or more and 3.0 mm or less. By making the total thickness of the multi-layer foam sheet 0.05 mm or more, more preferably 0.1 mm or more, and even more preferably 0.2 mm or more, the cushioning properties of the multi-layer foam sheet can be further improved. Furthermore, by making the total thickness of the multi-layer foam sheet 3.0 mm or less, more preferably 2.0 mm or less, even more preferably 1.5 mm or less, and particularly preferably 1.2 mm or less, the handleability of the multi-layer foam sheet can be further improved, making it easier to package the packaged items.

[0025] The method for measuring the overall thickness of a multi-layer foam sheet is as follows. First, the multi-layer foam sheet is cut along a plane perpendicular to the extrusion direction. Ten measurement positions are set on this cut surface so as to divide the length of the cross section in the width direction (i.e., the direction perpendicular to both the extrusion direction and the thickness direction) into 11 equal parts. The thickness at each measurement position is measured by, for example, observing the measurement positions using a microscope. The arithmetic mean value of these thicknesses is then taken as the overall thickness of the multi-layer foam sheet.

[0026] <Apparent density> The apparent density of the multilayer foam sheet is 30 kg / m 3 More than 150kg / m 3 The apparent density of the multi-layer foam sheet is preferably 30 kg / m or less. 3 More preferably, 35 kg / m 3 More preferably, 40 kg / m 3 By setting the apparent density of the multilayer foam sheet at 150 kg / m or more, sufficient strength for use as a cushioning material or packaging material can be easily ensured. 3 Less than or equal to 120 kg / m 3 or less, more preferably 100 kg / m 3 By setting the content below, the multi-layer foamed sheet can be sufficiently secured in terms of lightness and flexibility, and the cushioning properties of the multi-layer foamed sheet can be further improved.

[0027] The method for measuring the apparent density of a multilayer foam sheet is as follows. First, the multilayer foam sheet is cut in the width direction (i.e., the direction perpendicular to both the extrusion direction and the thickness direction) to obtain a test piece. The shape of the test piece can be, for example, a rectangle whose longitudinal dimension is the same as the overall width of the multilayer foam sheet and whose transverse dimension is 10 cm. The mass (unit: g) of this test piece is divided by the area of ​​the test piece, and then converted into units to obtain the basis weight of the multilayer foam sheet, i.e., the mass per 1 m of the multilayer foam sheet. 2 Mass per unit (unit: g / m 2 The apparent density (unit: kg / m) of the multi-layer foam sheet is calculated by dividing the basis weight of the multi-layer foam sheet by the total thickness of the multi-layer foam sheet obtained by the above-mentioned method and then converting the result into units. 3 ) can be calculated.

[0028] <Surface resistivity> The surface resistivity of the surface of the multilayer foamed sheet having the surface layer is 1×10 8 The multilayer foamed sheet having a surface resistivity in this range is suitable as a cushioning material or packaging material for electronic components, electronic devices, etc., since it can easily remove static electricity charged on the packaged items. From the same viewpoint, the surface resistivity of the surface of the multilayer foamed sheet on the side having the surface layer is 5×10 7 Ω or less is preferable, and 1×10 7 The surface resistivity of the surface of the multilayer foamed sheet having the surface layer is preferably less than 1×10 3 It is preferably Ω or more.

[0029] The surface resistivity of the multilayer foam sheet is measured by a measurement method conforming to JIS K6271-1:2015. Specifically, a square test piece with sides of 100 mm is first taken from the multilayer foam sheet. Electrodes are attached to the surface of the test piece that has the surface 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 foam sheet.

[0030] [Polyethylene resin foam layer] The polyethylene-based resin foam layer is mainly composed of a polyethylene-based resin.

[0031] <Polyethylene resin> In this specification, the term "polyethylene resin" refers to a resin containing 50 mol% or more of structural units derived from ethylene. Examples of polyethylene resins include polyethylenes such as low-density polyethylene (PE-LD), linear low-density polyethylene (PE-LLD), and high-density polyethylene (PE-HD), as well as ethylene copolymers having structural units derived from ethylene and structural units derived from monomers having polar groups, such as ethylene-vinyl acetate copolymer (EVA), ethylene-methyl acrylate copolymer (EMA), and ethylene-methyl methacrylate copolymer (EMMA).

[0032] The low-density polyethylene mentioned above has a long-chain branched structure, and its density is usually 910 kg / m 3 More than 930kg / m 3 Linear low-density polyethylene is a copolymer of ethylene and an α-olefin having 4 to 8 carbon atoms, has a substantially linear molecular chain, and its density is usually less than 910 kg / m 3 More than 942kg / m 3 High density polyethylene is an ethylene homopolymer or a copolymer of ethylene and an α-olefin having 4 to 8 carbon atoms, and its density is usually 942 kg / m 3 Higher than.

[0033] The foam layer may contain one type of polyethylene resin or two or more types of polyethylene resins. From the viewpoint of further improving the flexibility, cushioning properties, and foamability of the multi-layer foam sheet, the polyethylene resin constituting the foam layer is preferably either low-density polyethylene or a mixed resin of low-density polyethylene and linear low-density polyethylene, and more preferably low-density polyethylene.

[0034] The melting point of the polyethylene resin contained in the foam layer is preferably 100°C or higher and 135°C or lower. In this case, a foam layer having excellent extrusion foamability and excellent shock-absorbing properties can be stably formed. From this viewpoint, the melting point of the polyethylene resin contained in the foam layer 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 of polyethylene resins can be measured using the plastic transition temperature measurement method specified in JIS K7121:2012. First, a test specimen is conditioned by setting the heating and cooling rates to 10°C / min according to the "measurement of melting temperature after a certain heat treatment." Then, heat flux DSC (i.e., differential scanning calorimetry) is performed at a heating rate of 10°C / min to obtain a DSC curve. The melting point is determined as the apex temperature of the endothermic peak in the resulting DSC curve. If multiple endothermic peaks appear in the DSC curve, the apex temperature of the melting peak with the largest area is determined as the melting point, relative to the higher-temperature baseline.

[0036] The melt flow rate (MFR) of the polyethylene resin contained in the foam layer is preferably 0.5 g / 10 min to 15 g / 10 min, more preferably 1 g / 10 min to 8 g / 10 min, and even more preferably 1.5 g / 10 min to 5 g / 10 min, in order to achieve excellent extrusion foamability. The MFR of the polyethylene resin in this specification is a value measured in accordance with JIS K7210-1:2014 at a test temperature of 190°C and a load of 2.16 kg.

[0037] <Other polymers> The foam layer may contain a polymer other than the polyethylene-based resin, provided that the above-described effects are not impaired. Examples of the polymer other than the polyethylene-based resin include thermoplastic resins other than polyethylene-based resins, such as polystyrene-based resins, and elastomers, such as ethylene-propylene rubber and styrene-butadiene-styrene block copolymers. The content of the polymer other than the polyethylene-based resin in the foam layer is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably 5 parts by mass or less, per 100 parts by mass of the polyethylene-based resin. It is particularly preferred that the content be 0 parts by mass, i.e., the polymer component constituting the foam layer is composed solely of the polyethylene-based resin.

[0038] <Additives> The foam layer may contain additives such as a cell regulator, an antioxidant, a heat stabilizer, a weathering agent, an ultraviolet absorber, a flame retardant, a filler, an antibacterial agent, etc. The amount of the additive in the foam layer 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, per 100 parts by mass of the polyethylene resin.

[0039] [Conductive layer] The conductive layer is provided between the polyethylene-based resin foam layer and the surface layer. The conductive layer is mainly composed of a resin composition containing an ethylene-based copolymer having structural units derived from ethylene and structural units derived from a monomer having a polar group, a polyethylene-based resin different from the ethylene-based copolymer, and conductive carbon. The conductive layer contains 5% to 15% by mass of conductive carbon. The conductive layer is preferably in a non-foamed state from the viewpoint of improving the conductivity, handleability, and appearance of the multi-layer foamed sheet. The non-foamed state includes a state in which the layer is not foamed during production and does not contain any bubbles, and a state in which the layer is slightly foamed during production and the bubbles subsequently disappear, meaning that there is almost no bubble structure within the layer. However, the conductive layer may contain a small number of very small bubbles.

[0040] When the conductive layer is made of the specific resin composition, the multilayer foamed sheet can be made conductive with a relatively small amount of conductive carbon. The reason for this is not entirely clear at present, but the following reasons are thought to be possible.

[0041] 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.

[0042] Because the ethylene-based copolymer and the polyethylene-based resin in the resin composition are incompatible with each other, a phase mainly composed of the polyethylene-based resin and a phase mainly composed of the ethylene-based copolymer are formed in the conductive layer. When such a morphology is formed in the conductive layer, it is believed that the conductive carbon is unevenly distributed in either the phase composed of the polyethylene-based resin or the phase composed of the ethylene-based copolymer. The uneven distribution of the conductive carbon in either phase facilitates the formation of a conductive network of conductive carbon particles. As a result, it is believed that conductivity is easily exhibited even when the amount of conductive carbon blended is small.

[0043] <Ethylene copolymer> The ethylene copolymer used in the conductive layer has at least a structural unit derived from ethylene and a structural unit derived from a monomer having a polar group. The ethylene copolymer may be, for example, a copolymer of ethylene and a monomer having a polar group, or a copolymer of ethylene, a monomer having a polar group, and a monomer other than these monomers. The amount of structural units derived from the other monomers contained in the ethylene copolymer is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 3% by mass or less, and most preferably 0% by mass, i.e., the ethylene copolymer is a copolymer of ethylene and a monomer having a polar group.

[0044] Examples of ethylene copolymers used in the conductive layer include ethylene-vinyl acetate copolymer (i.e., EVA), ethylene-methyl methacrylate copolymer (i.e., EMMA), ethylene-methyl acrylate copolymer (i.e., EMA), ethylene-methacrylic acid copolymer (i.e., EMAA), ethylene-acrylic acid copolymer (i.e., EAA), ethylene-ethyl methacrylate copolymer (i.e., EEMA), ethylene-ethyl acrylate copolymer (i.e., EEA), and ethylene-butyl acrylate copolymer (i.e., EBA). From the viewpoint of further increasing the conductivity of the multilayer foam sheet, the conductive layer preferably contains one or more ethylene copolymers selected from the group consisting of ethylene-vinyl acetate copolymer, ethylene-methyl methacrylate copolymer, ethylene-methyl acrylate copolymer, and ethylene-methacrylic acid copolymer, and more preferably contains ethylene-vinyl acetate copolymer.

[0045] The content of structural units derived from monomers having polar groups in the ethylene copolymer used in the conductive layer is preferably 30% by mass or more and 50% by mass or less. By making the content of structural units derived from monomers having polar groups in the ethylene copolymer 30% by mass or more, the conductivity of the multi-layer foamed sheet can be further improved. From the viewpoint of further enhancing this effect, the content of structural units derived from monomers having polar groups in the ethylene copolymer 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.

[0046] Furthermore, by setting the content of structural units derived from monomers having a polar group in the ethylene copolymer to 50% by mass or less, the handleability of the multi-layer foamed sheet can be further improved, and production stability when laminating the conductive layer to the foamed layer can be further improved. From the viewpoint of further enhancing these effects, the content of structural units derived from monomers having a polar group in the ethylene copolymer is more preferably 48% by mass or less, and even more preferably 45% by mass or less.

[0047] The melt flow rate of the ethylene copolymer used in the conductive layer at a temperature of 190°C and a load of 2.16 kg is preferably 20 g / 10 min or more and 100 g / 10 min or less, and more preferably 40 g / 10 min or more and 80 g / 10 min or less. This can further improve the adhesion between the conductive layer and the foam layer. Furthermore, this can more stably develop the conductivity of the multilayer foam sheet. The MFR of the ethylene copolymer in this specification is a value measured in accordance with JIS K7210-1:2014 at a test temperature of 190°C and a load of 2.16 kg.

[0048] The melting point of the ethylene copolymer used in the conductive layer 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 of the ethylene copolymer used in the conductive layer within the above-mentioned specific range, the conductivity of the multi-layer foam sheet can be further improved, and production stability can be further improved when laminating the conductive layer to the foam layer. The melting point of the ethylene copolymer can be measured by the same method as that of the polyethylene resin used in the foam layer.

[0049] <Polyethylene resin> The conductive layer contains a polyethylene-based resin other than the ethylene-based copolymer. Examples of polyethylene-based resins used in the conductive layer include polyethylenes such as low-density polyethylene (PE-LD), linear low-density polyethylene (PE-LLD), and high-density polyethylene (PE-HD). The polyethylene-based resin other than the ethylene-based copolymer used in the conductive layer is preferably polyethylene, and more preferably one or more polyethylenes selected from the group consisting of low-density polyethylene and linear low-density polyethylene. In this case, conductivity can be more reliably imparted to the multilayer foam sheet even when the amount of conductive carbon blended is relatively small.

[0050] The melting point of the polyethylene resin other than the ethylene copolymer used in the conductive layer is preferably 100°C or higher and 120°C or lower, more preferably 102°C or higher and 115°C or lower. In this case, even when the multi-layer foamed sheet is produced by coextrusion, the conductive layer can be stably laminated and bonded to the foamed layer. The melting point of the polyethylene resin used in the conductive layer can be measured by the same method as the melting point of the polyethylene resin used in the foamed layer described above.

[0051] The difference Tm1-Tm2 between the melting point Tm1 of the polyethylene resin other than the ethylene copolymer used in the conductive layer and the melting point Tm2 of the ethylene copolymer is preferably 30°C or more and 80°C or less.

[0052] By setting the melting point difference (Tm1-Tm2) to 30°C or more, sufficient electrical conductivity can be more easily imparted to the multi-layer foam sheet. Furthermore, by setting the melting point difference (Tm1-Tm2) to 80°C or less, adhesion of the ethylene copolymer to a sizing device or the like during the production process of the multi-layer foam sheet can be easily suppressed, making it easier to obtain a good multi-layer foam sheet. This also makes it easier to avoid problems such as softening of the ethylene copolymer during use or fusion of multi-layer foam sheets when stored in a stacked state, which would impair handleability. To ensure these effects, the melting point difference (Tm1-Tm2) is more preferably 40°C or more and 70°C or less.

[0053] The polyethylene resin used in the conductive layer preferably has a melt flow rate of 5 g / 10 min to 80 g / 10 min at a temperature of 190°C and 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. This can further improve the adhesion between the conductive layer and the foam layer. This can also more reliably impart conductivity to the multilayer foam sheet.

[0054] <Blending ratio> The blending ratio of the ethylene copolymer to the polyethylene resin other than the ethylene copolymer in the conductive layer is preferably 20:80 to 80:20 by mass, more preferably 50:50 to 75:25, and even more preferably 55:45 to 70:30, in which case the conductivity of the multi-layer foamed sheet can be further improved.

[0055] <Conductive carbon> The resin composition constituting the conductive layer contains conductive carbon, i.e., a substance that is mainly composed of carbon atoms and has conductivity. Specific examples of conductive carbon include conductive carbon black such as furnace black, acetylene black, thermal black, and Ketjen Black (registered trademark). The conductive layer may contain two or more types of conductive carbon. From the viewpoint of further reducing the amount of conductive carbon blended while ensuring the conductivity of the multilayer foam sheet, it is preferable that the conductive layer contains highly conductive carbon black such as Ketjen Black as the conductive carbon.

[0056] 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 conductivity of the multi-layer foamed sheet can be further increased. From the viewpoint of further increasing the conductivity 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 D2414-79.

[0057] The BET specific surface area of ​​the conductive carbon is 600m 2 / g or more 2000m 2 / g or less. In this case, the conductivity of the multilayer foamed sheet can be further increased. From the viewpoint of further increasing the conductivity of the multilayer foamed sheet, the BET specific surface area of ​​the conductive carbon is preferably 700 m 2 / g or more 1600m 2 In the multilayer foam sheet of the present invention, the conductive layer containing the ethylene-based copolymer, the polyethylene-based resin other than the ethylene-based copolymer, and the conductive carbon is formed as a layer separate from the foam layer, and therefore the conductive carbon having a large specific surface area can be blended into the foam layer without inhibiting the foamability of the foam layer.

[0058] The amount of conductive carbon in the conductive layer is 5% by mass or more and 15% by mass or less. By setting the amount of conductive carbon to 5% by mass or more, the multilayer foamed sheet can be made conductive. From the viewpoint of further increasing the conductivity of the multilayer foamed sheet, the amount of conductive carbon in the conductive layer is preferably 6% by mass or more, and more preferably 7% by mass or more. If the amount of conductive carbon in the conductive layer is too small, it becomes difficult to form a conductive network of conductive carbon particles in the conductive layer, which may result in a decrease in the conductivity of the multilayer foamed sheet. The amount of conductive carbon is approximately equal to the amount of conductive carbon in the conductive layer of the multilayer foamed sheet.

[0059] Furthermore, by setting the blending amount of conductive carbon to 15% by mass or less, it is possible to reduce the shedding of conductive carbon from the multi-layer foamed sheet. From the viewpoint of further reducing the shedding of conductive carbon from the multi-layer foamed sheet, the blending amount of conductive carbon is preferably 12% by mass or less, more preferably 10% by mass or less, and even more preferably less than 10% by mass. If the blending amount of conductive carbon in the conductive layer is too high, the conductive carbon will be more likely to fall off from the multi-layer foamed sheet, which may cause contamination around the multi-layer foamed sheet.

[0060] <Other polymers> The conductive layer may contain other polymers besides the ethylene copolymer and polyethylene resin, provided that the aforementioned effects are not impaired. Examples of other polymers besides the ethylene copolymer and polyethylene resin include thermoplastic resins other than polyethylene resins, such as polystyrene resins, and elastomers, such as ethylene-propylene rubber and styrene-butadiene-styrene block copolymers. It is preferable that the conductive layer does not contain a polymer with a melting point higher than that of the polyethylene resin contained in the foam layer. This can further improve manufacturing stability when producing a multilayer foam sheet having a conductive layer by the co-extrusion method described below. The content of other polymers than the ethylene copolymer and polyethylene resin in the conductive layer 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.

[0061] <Additives> The conductive layer may contain additives such as antioxidants, heat stabilizers, weathering agents, ultraviolet absorbers, flame retardants, fillers, antibacterial agents, etc. The amount of additives in the conductive layer 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, per 100 parts by mass of the total of the ethylene copolymer and the polyethylene resin other than the ethylene copolymer.

[0062] <Average thickness> The average thickness of the conductive layer is preferably 1 μm or more and 20 μm or less. By making the average thickness of the conductive layer 1 μm or more, more preferably 3 μm or more, the multi-layer foamed sheet can be more reliably imparted with electrical conductivity. Furthermore, by making the average thickness of the conductive layer 20 μm or less, more preferably 18 μm or less, even more preferably 15 μm or less, and particularly preferably 10 μm or less, the contamination of the multi-layer foamed sheet can be more reliably reduced.

[0063] The method for measuring the average thickness of the conductive layer is as follows. First, the multilayer foam sheet is cut in a plane perpendicular to the extrusion direction. Ten measurement positions are set on this cut surface so as to divide the length of the cut surface in the longitudinal direction (i.e., the direction perpendicular to both the extrusion direction and the thickness direction) into 11 equal parts. The cross section of the multilayer foam sheet at these measurement positions is observed using a microscope, and the thickness of the conductive layer at each measurement position is measured. The arithmetic mean value of these thicknesses is taken as the average thickness (unit: μm) of the conductive layer.

[0064] <Basic weight> The basis weight of the conductive layer is 1 g / m 2 More than 50g / m 2 The basis weight of the conductive layer is preferably 1 g / m or less. 2 More preferably, 2 g / m 2 More preferably, 3 g / m 2 More than 5 g / m 2 By setting the basis weight of the conductive layer to 30 g / m or more, the conductivity of the multilayer foamed sheet can be more reliably imparted. 2 Less than 20 g / m, more preferably 2 or less, more preferably 15 g / m 2 Below 10 g / m, particularly preferably 2 By setting the following, it is possible to more reliably reduce the contamination of the multi-layer foam sheet. When the conductive layer is laminated on both sides of the polyethylene-based resin foam layer, the basis weight of the conductive layer means the basis weight per side.

[0065] The method for measuring the basis weight of the conductive layer per side is as follows. First, the average thickness of the conductive layer is calculated using the method described above. After converting the unit of this average thickness, the density of the conductive layer (unit: g / m 3 ) to obtain the basis weight of the conductive layer (unit: g / m 2 ) can be obtained. The density of the conductive layer includes the density of conductive carbon and other additives contained in the conductive layer.

[0066] When the multi-layer foam sheet is produced by co-extrusion, the basis weight of the conductive layer per side can also be calculated using the discharge rate X1 (unit: g / hour) of the conductive layer per side, the width W (unit: m) of the multi-layer foam sheet, and the take-up speed L (unit: m / hour) of the multi-layer foam sheet according to the following formula (1): Conductive layer basis weight = [X1 / (L × W)] (1)

[0067] When a multi-layer foamed sheet is produced by co-extruding a surface layer-forming melt, a conductive layer-forming melt, and a foam layer-forming melt, a conductive layer having a small basis weight and a small thickness that cannot be formed by thermal lamination or the like can be formed, and electrical conductivity can be stably exhibited.

[0068] [Surface layer] The surface layer is located on the outermost surface of the multi-layer foam sheet and is substantially composed of one of the following resins (1) and (2): (1) One or more linear polyethylenes selected from the group consisting of linear low-density polyethylenes and high-density polyethylenes. (2) A mixed resin containing linear polyethylene and low-density polyethylene, the linear polyethylene content being 8% by mass or more.

[0069] The surface layer is preferably in a non-foamed state from the viewpoints of improving the electrical conductivity, handling properties, and appearance of the multi-layer foamed sheet and reducing contamination, although a small amount of very small bubbles may be contained in the surface layer.

[0070] The multilayer foam sheet has a surface layer made of the specific resin, which provides high conductivity and reliably reduces the shedding of conductive carbon from the conductive layer, thereby significantly reducing contamination of the packaged items. This is thought to be because the use of the specific resin as the resin constituting the surface layer can suppress the formation of pinholes when the surface layer is laminated on the conductive layer. In this specification, pinholes refer to defects such as small holes formed on the surface of the multilayer foam sheet.

[0071] If the multilayer foam sheet does not have a surface layer, the conductive carbon in the conductive layer is exposed on the outermost surface of the multilayer foam sheet, which may result in insufficient reduction of contamination of the packaged goods. Furthermore, if the surface layer does not contain the linear polyethylene, for example, even if a surface layer made of low-density polyethylene is laminated on a conductive layer containing conductive carbon, the contamination of the packaged goods may not be sufficiently reduced. This is thought to be because, in a multilayer foam sheet having a conductive layer containing conductive carbon as a filler, if the surface layer does not contain linear polyethylene, the formation of pinholes on the surface of the multilayer foam sheet cannot be sufficiently prevented.

[0072] The surface layer contains at least linear polyethylene, i.e., one or more polyethylenes selected from the group consisting of linear low-density polyethylene and high-density polyethylene. The surface layer preferably contains at least linear low-density polyethylene as the linear polyethylene. This allows the surface layer to be more stably laminated and bonded to the conductive layer, and further reduces local variations in surface resistivity on the surface of the multilayer foam sheet.

[0073] <Linear polyethylene> The linear polyethylene used for the surface layer preferably has a melt flow rate of 12 g / 10 min or more at a temperature of 190°C and a load of 2.16 kg, and more preferably 15 g / 10 min or more. In this case, the surface layer can be more stably laminated and bonded to the conductive layer, and local variations in surface resistivity on the surface of the multi-layer foamed sheet can be further reduced. From the viewpoint of extrusion stability, the upper limit of the melt flow rate of the linear polyethylene used for the surface 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.

[0074] The linear polyethylene used for the surface layer preferably has a tensile strength at 95°C of 3.0 MPa or more, more preferably 3.5 MPa or more, even more preferably 4.0 MPa or more, and particularly preferably 4.5 MPa or more. In this case, the shedding of conductive carbon from the multi-layer foamed sheet can be further reduced, thereby further reducing contamination of the packaged items. This may be because, for example, the use of linear polyethylene having the specified tensile strength makes the surface layer tougher during co-extrusion production of the multi-layer foamed sheet, reducing the likelihood of pinholes forming in the surface layer. The upper limit of the tensile strength at 95°C of the linear polyethylene used for the surface layer may be, for example, 10 MPa, more preferably 8.0 MPa, from the viewpoint of further improving the extrusion stability and take-up stability of the multi-layer foamed sheet.

[0075] From the same viewpoint, the tensile strength of the linear polyethylene used for the surface layer at 23° C. is preferably 15 MPa or more, more preferably 16 MPa or more, even more preferably 17 MPa or more, and particularly preferably 18 MPa or more. The upper limit of the tensile strength of the linear polyethylene used for the surface layer at 23° C. is preferably 30 MPa, from the viewpoint of imparting flexibility to the surface of the multilayer foamed sheet and further suppressing damage to the packaged items.

[0076] The tensile strength of the linear polyethylene at each of the temperatures described above is measured in accordance with JIS K7161-1: 2014. Taking into consideration the extrusion foaming temperature of the polyethylene-based resin multi-layer foamed sheet described below, the measurement temperature was set to 95°C, assuming the temperature of the resin when the surface layer is formed by coextrusion.

[0077] The crystallization temperature of the linear polyethylene used for the surface layer is preferably 100°C or higher, more preferably 102°C or higher, and even more preferably 105°C or higher. In this case, contamination of the packaged items can be further reduced, and the surface layer can be laminated and adhered to the conductive layer more stably. One possible reason for this is that, by using a linear polyethylene having the specific crystallization temperature, the surface layer is more likely to solidify immediately after extrusion when producing a multi-layer foamed sheet by coextrusion, making it less likely that pinholes will form in the surface layer.

[0078] The crystallization temperature of the linear polyethylene used in the surface layer is measured using a heat flux differential scanning calorimeter in accordance with JIS K7121: 2012. When multiple crystallization peaks appear in the DSC curve, the peak temperature of the crystallization peak with the highest peak height is taken as the crystallization temperature.

[0079] Melting point Tm of linear polyethylene used in the surface layer S is preferably 110°C or higher and 135°C or lower, more preferably 115°C or higher and 130°C or lower. In this case, the surface layer can be stably formed even when the multi-layer foamed sheet is produced by co-extrusion. In addition, from the viewpoint of further reducing the contamination of the packaged goods by the multi-layer foamed sheet, the melting point Tm of the linear polyethylene used for the surface layer is S and the melting point Tm of the polyethylene resin used in the foam layer C Difference from Tm S -Tm C is preferably -5°C or higher, more preferably 0°C or higher, and even more preferably 5°C or higher. From the viewpoint of further reducing the variation in the surface resistivity of the multi-layer foamed sheet, the melting point Tm of the linear polyethylene used in the surface layer is S and the melting point Tm of the polyethylene resin used in the foam layer C Difference from Tm S -Tm CIt is preferable that the melting point is 15° C. or less. The method for measuring the melting point of the linear polyethylene used in the surface layer is the same as the method for measuring the melting point of the polyethylene resin used in the foam layer described above.

[0080] <Low-density polyethylene> The surface layer may contain low-density polyethylene. For example, the surface layer may be made of the same low-density polyethylene as that used in the foam layer. When the surface layer contains low-density polyethylene, the cushioning properties for the packaged items can be further improved. Furthermore, when the surface layer contains linear polyethylene and low-density polyethylene, the amount of linear polyethylene in the surface layer is set to 8% by mass or more in order to reliably obtain the above-mentioned effects of the linear polyethylene. From the same viewpoint, the amount of linear polyethylene in the surface layer is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more.

[0081] <Other polymers> The surface layer may contain a polymer other than linear polyethylene and low-density polyethylene, provided that the above-described effects are not impaired. Examples of polymers other than linear polyethylene and low-density polyethylene include thermoplastic resins other than polyethylene-based resins, such as polystyrene-based resins, and elastomers, such as ethylene-propylene rubber and styrene-butadiene-styrene block copolymers. From the viewpoint of production stability when producing a multi-layer foam sheet having a surface layer by the co-extrusion method described below, the melting point of the polymer contained in the surface layer is preferably equal to or lower than the melting point of the polyethylene-based resin contained in the foam layer. The content of the polymer other than linear polyethylene and low-density polyethylene in the surface layer 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.

[0082] <Additives> The surface layer may contain additives such as antioxidants, heat stabilizers, weathering agents, ultraviolet absorbers, flame retardants, fillers, and antibacterial agents. On the other hand, the surface layer preferably does not contain conductive carbon, which may contaminate the surrounding area of ​​the multilayer foamed sheet, or antistatic agents such as low-molecular-weight antistatic agents and high-molecular-weight antistatic agents. The amount of additives in the surface layer 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, per 100 parts by mass of the linear polyethylene and low-density polyethylene combined.

[0083] <Average thickness> The average thickness of the surface layer is preferably 1 μm or more and 20 μm or less. By making the average thickness of the surface layer 1 μm or more, more preferably 2 μm or more, and even more preferably 3 μm or more, it is possible to further reduce the shedding of conductive carbon. Furthermore, by making the average thickness of the surface layer 20 μm or less, more preferably 18 μm or less, even more preferably 15 μm or less, and particularly preferably 10 μm or less, it is possible to more stably exhibit conductivity in the multilayer foamed sheet.

[0084] The method for measuring the average thickness of the surface layer is as follows. First, the multilayer foam sheet is cut in a plane perpendicular to the extrusion direction. On this cut surface, 10 measurement positions are set so as to divide the length of the cut surface in the longitudinal direction (i.e., the direction perpendicular to both the extrusion direction and the thickness direction) into 11 equal parts. The cross section of the multilayer foam sheet at these measurement positions is observed using a microscope, and the thickness of the surface layer at each measurement position is measured. The arithmetic mean value of these thicknesses is taken as the average thickness of the surface layer (unit: μm).

[0085] <Basic weight> The surface layer has a basis weight of 1 g / m 2 More than 20g / m 2 The surface layer preferably has a basis weight of 1 g / m or less. 2 More preferably, 2 g / m 2 More preferably, 3 g / m 2By doing so, the formation of pinholes in the surface layer can be more effectively suppressed, and the contamination of the multilayer foamed sheet can be further reduced. In addition, when the basis weight of the surface layer is 20 g / m 2 or less, more preferably 10 g / m 2 or less, still more preferably 8 g / m 2 or less, particularly preferably 4.5 g / m 2 or less, the local variation in conductivity on the surface of the multilayer foamed sheet can be further reduced, and conductivity can be more reliably imparted to the multilayer foamed sheet. When the surface layer is provided on both sides of the multilayer foamed sheet, the basis weight of the surface layer means the basis weight per side.

[0086] Also, from the viewpoint of more reliably imparting conductivity to the multilayer foamed sheet and more reliably suppressing the variation in surface resistivity, the basis weight of the surface layer is preferably smaller than the basis weight of the conductive layer, and more preferably less than 0.85 times the basis weight of the conductive layer. That is, when the basis weight of the surface layer is represented by R2 (unit: g / m 2 ) and the basis weight of the conductive layer is represented by R1 (unit: g / m 2 ), the relationship between R1 and R2 is preferably R2 < R1, and more preferably R2 < R1 × 0.85.

[0087] The method for measuring the basis weight of the surface layer per side is as follows. First, the average thickness of the surface layer is calculated by the method described above. After converting the unit of this average thickness, the basis weight (unit: g / m 3 ) of the surface layer can be obtained by multiplying the density (unit: g / m 2 ) of the surface layer.

[0088] When the multilayer foamed sheet is manufactured by coextrusion, the basis weight of the surface layer per side can also be obtained by the following formula (2) using the discharge amount X2 (unit: g / hour) of the surface layer per side, the width W (unit: m) of the multilayer foamed sheet, and the take-up speed L (unit: m / hour) of the multilayer foamed sheet. Basis weight of surface layer = [X2 / (L × W)] ··· (2)

[0089] <Pinhole area ratio> The multilayer foam sheet has a surface layer made of the specific resin, which can prevent pinholes from forming when the surface layer is laminated to a conductive layer. The pinhole area ratio of the multilayer foam sheet is preferably 5% or less, more preferably 3.5% or less, and even more preferably 2% or less. In this case, the shedding of conductive carbon from the conductive layer can be more reliably reduced, and contamination of the packaged items can be further reduced. The lower limit of the pinhole area ratio of the multilayer foam sheet is 0%.

[0090] The pinhole area ratio of the multilayer foamed sheet described above was measured as follows. The surface layer of the multilayer foamed sheet was observed at an arbitrary position using a scanning electron microscope, and an electron microscope image of the measurement position was obtained. In this electron microscope image, a square measurement area with a side length of 1000 μm was set. In the measurement area, a pinhole area ratio of 70 μm formed on the surface layer was measured. 2 The above holes are regarded as pinholes, and the ratio of the area occupied by pinholes to the area of ​​the measurement region is calculated. The same procedure is carried out for 15 or more positions on the multi-layer foamed sheet, and the arithmetic mean value of the ratios of the area occupied by pinholes in these measurement regions is taken as the pinhole area ratio of the multi-layer foamed sheet.

[0091] (Method of manufacturing multi-layer foam sheet) The multilayer foam sheet can be produced, for example, by a coextrusion foaming method. Specifically, in the method for producing a polyethylene-based resin multilayer foam sheet, a foam layer-forming melt for forming a polyethylene-based resin foam layer, a conductive layer-forming melt for forming a conductive layer, and a surface layer-forming melt for forming a surface layer are coextruded to produce a polyethylene-based resin multilayer foam sheet comprising a polyethylene-based resin foam layer, a surface layer provided on at least one side of the polyethylene-based resin foam layer, and a conductive layer provided between the surface layer and the foam layer. The foam layer-forming melt contains a polyethylene-based resin and a physical foaming agent. The conductive layer-forming melt contains an ethylene-based copolymer having structural units derived from ethylene and structural units derived from a monomer having a polar group, a polyethylene-based resin other than the ethylene-based copolymer, and conductive carbon. The conductive layer-forming melt contains conductive carbon in an amount of 3% by mass or more and 15% by mass or less. The surface layer-forming melt contains linear polyethylene and, optionally, low-density polyethylene. The surface layer-forming melt contains linear polyethylene and, optionally, low-density polyethylene. The surface layer-forming melt contains linear polyethylene in an amount of 8% by mass or more.

[0092] In carrying out this method, a known co-extrusion apparatus used in the field of extrusion foaming can be used. More specifically, for example, the multi-layer foamed sheet can be produced using a co-extrusion apparatus including a foam layer-forming extruder configured to be able to extrude a foam layer-forming melt, a conductive layer-forming extruder configured to be able to extrude a conductive layer-forming melt, and a surface layer-forming extruder configured to be able to extrude a surface layer-forming melt, and a co-extrusion die to which the discharge ports of these extruders are connected.

[0093] [Melted material for forming foam layer] The melt for forming a foam layer contains at least a polyethylene resin and a physical foaming agent. The melt for forming a foam layer can be produced, for example, by the following method. First, the polyethylene resin and additives added as needed are fed into an extruder for forming a foam layer and melt-kneaded. Next, a physical foaming agent is fed under pressure to the melt containing the polyethylene resin melted in the extruder and further kneaded, thereby obtaining a melt for forming a foam layer.

[0094] The physical foaming agent may be an organic or inorganic physical foaming agent. Examples of organic physical foaming agents include aliphatic hydrocarbons such as propane, normal butane, isobutane, normal pentane, isopentane, normal hexane, and isohexane; alicyclic hydrocarbons such as cyclopentane and cyclohexane; chlorinated hydrocarbons such as methyl chloride and ethyl chloride; and fluorinated hydrocarbons such as 1,1,1,2-tetrafluoroethane and 1,1-difluoroethane. Examples of inorganic physical foaming agents include nitrogen, carbon dioxide, air, and water. The foam layer-forming melt may contain one type of physical foaming agent or two or more types of physical foaming agents.

[0095] From the viewpoint of compatibility with polyethylene-based resins and foamability, the foam layer-forming melt preferably contains an organic physical foaming agent as the physical foaming agent, and more preferably contains an organic physical foaming agent containing normal butane, isobutane, or a mixture thereof as the main component.

[0096] The amount of the physical foaming agent can be appropriately set depending on the type of foaming agent, the desired apparent density, etc. For example, when a mixed butane consisting of 30% by mass of isobutane and 70% by mass of normal butane is used as the physical foaming agent, 3 parts by mass to 30 parts by mass, preferably 4 parts by mass to 20 parts by mass, more preferably 10 parts by mass to 20 parts by mass, of the mixed butane may be added to 100 parts by mass of the polyethylene resin.

[0097] It is preferable to add a cell control agent to the foam layer-forming melt. The cell control agent may be an inorganic or organic cell control agent. Examples of inorganic cell control agents include metal borates such as zinc borate, magnesium borate, and borax, as well as sodium chloride, aluminum hydroxide, talc, zeolite, silica, calcium carbonate, and sodium bicarbonate. Examples of organic cell control agents include sodium 2,2-methylenebis(4,6-tert-butylphenyl)phosphate, sodium benzoate, aluminum benzoate, and sodium stearate. Furthermore, a mixture of citric acid and sodium bicarbonate, or a mixture of an alkali citrate and sodium bicarbonate, may also be used as the cell control agent. The foam layer-forming melt may contain one or more cell control agents. The amount of the cell control agent in the foam layer-forming melt may be appropriately determined depending on the type of physical foaming agent, the desired apparent density, the cell diameter, and the like.

[0098] [Conductive layer forming melt] The conductive layer-forming melt contains at least an ethylene-based copolymer, a polyethylene-based resin other than the ethylene-based copolymer, and conductive carbon. When preparing the conductive layer-forming melt, for example, the ethylene-based copolymer, the polyethylene-based resin other than the ethylene-based copolymer, the conductive carbon, and additives added as needed are supplied to an extruder for conductive layer formation. Then, these are melt-kneaded in the extruder to obtain the conductive layer-forming melt.

[0099] The conductive layer-forming melt may contain a volatile plasticizer as an additive. The volatile plasticizer has the effect of reducing the melt viscosity of the conductive layer-forming melt and is configured to volatilize from the conductive layer after co-extrusion. The volatile plasticizer can bring the extrusion temperature of the conductive layer-forming melt closer to the extrusion foaming temperature of the foam layer-forming melt during co-extrusion. The volatile plasticizer can also improve the melt elongation of the softened conductive layer. As a result, adding a volatile plasticizer to the conductive layer-forming melt makes it less likely that the bubbles of the polyethylene-based resin foam layer will be destroyed by the heat of the conductive layer during foaming of the foam layer-forming melt, and further, the conductive layer will be more likely to elongate in response to the expansion of the polyethylene-based resin foam layer during foaming.

[0100] Examples of the volatile plasticizer 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 conductive layer-forming melt may contain one type of volatile plasticizer, or two or more types of volatile plasticizers.

[0101] The boiling point of the volatile plasticizer is preferably 120°C or lower, and more preferably 80°C or lower. Volatile plasticizers having a boiling point within this range naturally volatilize and are removed from the conductive layer after coextrusion. The lower limit of the boiling point of the volatile plasticizer is approximately -50°C.

[0102] The amount of the volatile plasticizer can be appropriately set depending on the compositions of the conductive layer and the polyethylene-based resin foam layer. For example, the amount of the volatile plasticizer can be 5 parts by mass or more and 50 parts by mass or less per 100 parts by mass of the ethylene-based copolymer and the polyethylene-based resin other than the ethylene-based copolymer contained in the conductive layer. From the viewpoint of further improving the conformability of the conductive layer-forming melt and reducing variation in the thickness of the conductive layer, the amount of the volatile plasticizer is preferably 5 parts by mass or more, more preferably 7 parts by mass or more, and even more preferably 10 parts by mass or more per 100 parts by mass of the ethylene-based copolymer and the polyethylene-based resin other than the ethylene-based copolymer.

[0103] On the other hand, from the viewpoint of stably laminating the conductive layer to the foam layer, the amount of the volatile plasticizer is preferably 50 parts by mass or less, more preferably 45 parts by mass or less, and even more preferably 40 parts by mass or less, per 100 parts by mass of the total of the ethylene-based copolymer and the polyethylene-based resin other than the ethylene-based copolymer.

[0104] [Surface layer forming melt] The melt for forming the surface layer contains at least linear polyethylene. When preparing the melt for forming the surface layer, for example, linear polyethylene and, if necessary, low-density polyethylene and additives are supplied to a surface layer forming extruder. Then, these are melt-kneaded in the extruder to obtain the melt for forming the surface layer. Furthermore, a volatile plasticizer can be blended into the melt for forming the surface layer, if necessary. The volatile plasticizer used in the melt for forming the surface layer is the same as the volatile plasticizer used in the melt for forming the conductive layer.

[0105] [Co-extrusion] In coextrusion, the foam layer-forming melt, the conductive layer-forming melt, and the surface layer-forming melt formed in each extruder are introduced into a coextrusion die and extruded in layers from the extrusion opening of the coextrusion die. The coextrusion die may be, for example, a flat die with a linear extrusion opening. In this case, a laminate of the foam layer-forming melt, the conductive layer-forming melt, and the surface layer-forming melt is extruded into a sheet from the extrusion opening of the flat die. When the laminate is extruded into the atmosphere from the extrusion opening, the foam layer-forming melt expands while foaming. As a result, the conductive layer-forming melt and the surface layer-forming melt are stretched. The sheet-like laminated foam extruded from the extrusion opening is then drawn along a widening device and cooled, solidifying the foam layer-forming melt, the conductive layer-forming melt, and the surface layer-forming melt. This fixes the cell structure formed by foaming and stabilizes the dimensions. This allows a multilayer foamed sheet to be obtained.

[0106] The coextrusion die may also be, for example, an annular die equipped with an annular extrusion port. In this case, a laminate of a foam layer-forming melt, a conductive layer-forming melt, and a surface layer-forming melt is extruded into a cylindrical shape from the extrusion port of the annular die. When the laminate is extruded into the atmosphere from the extrusion port, the foam layer-forming melt expands while foaming. Accordingly, the conductive layer-forming melt and the surface layer-forming melt are stretched. The tubular laminated foam extruded from the extrusion port is then widened from the inside with compressed air or the like, while the inside is drawn along a widening device such as a mandrel and cooled, thereby solidifying the foam layer-forming melt, the conductive layer-forming melt, and the surface layer-forming melt. This fixes the cell structure formed by foaming and stabilizes the dimensions. Finally, the tubular laminated foam is cut open on the widening device to obtain a multilayer foam sheet. When an annular die is used as the coextrusion die, it is easy to produce wide multilayer foam sheets, for example, those with a width of 1000 mm or more. Furthermore, it is easy to produce a thin multilayer foamed sheet, for example, having an overall thickness of 3 mm or less.

[0107] Conventionally, in a polyethylene resin multilayer foam sheet produced by coextrusion, by providing a conductive layer containing conductive carbon, 8 To achieve a surface resistivity of less than Ω, it was necessary to incorporate a relatively large amount of conductive carbon into the conductive layer. The reason for this is thought to be as follows.

[0108] As described above, when a multi-layer foamed sheet is produced by coextrusion, the foam layer-forming melt extruded from the coextrusion die rapidly expands due to foaming, and the conductive layer-forming melt is strongly stretched in response to the expansion of the foam layer-forming melt. During this stretching, the conductive carbon particles in the conductive layer are separated from each other, and if the amount of conductive carbon blended is insufficient, it becomes difficult to maintain the conductive network.

[0109] Furthermore, because the laminated foam extruded from the coextrusion die is rapidly cooled, the base resin of the conductive layer is likely to solidify in a state in which the conductive carbon particles are separated by stretching. Furthermore, the extrusion foaming temperature of the foam layer-forming melt is set to a relatively low temperature, in the range of 100 to 130°C, which is near the melting point of the polyethylene-based resin that constitutes the polyethylene-based resin foam layer. Accordingly, the base resin of the conductive layer is more likely to solidify under the extrusion temperature conditions of the conductive layer-forming melt, which are set to a relatively low temperature. For these reasons, it is believed that it has been difficult to stably form a conductive network in the conductive layer in conventional multilayer foam sheets produced by coextrusion.

[0110] In contrast, as described above, the conductive layer of the multi-layer foamed sheet contains two mutually incompatible resins: an ethylene-based copolymer having structural units derived from ethylene and structural units derived from a monomer having a polar group, and a polyethylene-based resin other than the ethylene-based copolymer. Therefore, it is believed that the conductive carbon is unevenly distributed in one of the phases in the multi-layer foamed sheet. As a result, it is believed that a conductive network of conductive carbon particles is easily formed and maintained. Furthermore, when the difference Tm1 - Tm2 between the melting point Tm1 of the polyethylene-based resin other than the ethylene-based copolymer used in the conductive layer and the melting point Tm2 of the ethylene-based copolymer is within the above-mentioned range, it is believed that during the cooling process after co-extrusion, stretching is alleviated before the conductive layer solidifies, and the conductive network is more easily reconstructed.

[0111] Furthermore, as described above, the multilayer foam sheet contains at least linear polyethylene in the surface layer. By laminating a surface layer containing linear polyethylene on a conductive layer containing an ethylene copolymer, a polyethylene resin other than an ethylene copolymer, and conductive carbon, it is believed that pinhole formation in the surface layer can be suppressed. As a result, it is believed that contamination of packaged items can be further reduced.

[0112] Therefore, according to the above-mentioned production method, it is possible to produce by co-extrusion a multi-layer foamed sheet that has high conductivity and can significantly reduce contamination of packaged items. [Example]

[0113] Examples of the multi-layer foam sheet and the method for producing the same are described below.

[0114] The resins used in this example are shown in Table 1. The symbols shown in the "Type of Resin" column in Table 1 have the following meanings:

[0115] PE-LD: Low density polyethylene iC6-PE-LLD: Linear low-density polyethylene containing 4-methyl-1-pentene as a copolymer component C6-PE-LLD: Linear low-density polyethylene containing 1-hexene as a copolymer component C4-PE-LLD: Linear low-density polyethylene containing 1-butene as a copolymerization component PE-HD: High density polyethylene EVA: Ethylene-vinyl acetate copolymer

[0116] [Table 1]

[0117] The "MFR" column in Table 1 lists the melt flow rate of each resin measured at a temperature of 190°C and a load of 2.16 kg according to the method specified in JIS K7210-1 (2014). The melting point and crystallization temperature of each resin in Table 1 are values ​​measured using the method described above, i.e., the plastic transition temperature measurement method specified in JIS K7121:2012.

[0118] The "23°C tensile strength," "95°C tensile strength," and "95°C breaking elongation" columns in Table 1 show the tensile strength of each resin at 23°C, the tensile strength of each resin at 95°C, and the elongation at break of each resin at 95°C, measured in accordance with JIS K7161-1:2014.

[0119] The tensile strength of each resin at 23°C was measured using a method in accordance with JIS K7161-1:2014. Specifically, a resin film 0.85±0.03 mm thick was first prepared, and then test specimens having the dumbbell-shaped No. 1 shape specified in JIS K6251, Section 4.1 (shape and dimensions of test specimens) were punched out of the film to prepare test specimens. Using a universal testing machine "Tensilon RTC-1250A" manufactured by A&D Co., Ltd., a tensile test was performed on the test specimens at a test speed of 500 mm / min in an atmosphere of 23°C, and the tensile strength of the test specimens was measured.

[0120] The above measurement was carried out five times for each resin, and the arithmetic mean value of the obtained tensile strengths was taken as the tensile strength of the resin.

[0121] The tensile strength and elongation at break of each resin at 95°C were measured in the same manner as described above, except that the universal testing machine thermostatic chamber "TLF-III-40-B" was attached to the universal testing machine, the ambient temperature of the space where the tensile test was performed was adjusted to 95°C ± 2°C, and the test specimen was left in that atmosphere for 30 seconds before the tensile test. Note that the elongation at break was calculated based on the gauge length of a No. 1 dumbbell (specifically, 40 mm). If the test specimen did not break even after reaching the maximum possible displacement in the tensile test (specifically, 300 mm), the "95°C elongation at break" column in Table 1 was entered as ">750."

[0122] The mass fraction (unit: mass%) of structural units derived from monomers having polar groups in ethylene-vinyl acetate copolymer is shown in the "Modification rate" column of Table 1. Note that low-density polyethylene, linear low-density polyethylene, and high-density polyethylene do not contain structural units derived from monomers having polar groups, and therefore the symbol "-" is shown in the "Modification rate" column of Table 1 for these resins.

[0123] The conductive carbon used in this example is highly conductive carbon black ("Ketjenblack EC300J" manufactured by Lion Specialty Chemicals Co., Ltd.). The conductive carbon in this example has a porosity of 60%, a primary particle size of 40 nm, a dibutyl phthalate (DBP) supply rate of 365 mL / 100 g, and a BET specific surface area of ​​800 m. 2 / g. In the "conductivity imparting agent" columns of Tables 2 to 7, the above-mentioned highly conductive carbon black is described as "CB."

[0124] The porosity of the conductive carbon described above is a value determined by dividing the bulk density of the conductive carbon by the true density of the conductive carbon. The primary particle diameter of the conductive carbon is a value determined by observation with a transmission electron microscope. The DBP oil absorption of the conductive carbon is a value measured in accordance with ASTM D 2414-79. The BET specific surface area is a value determined in accordance with ASTM D 2414.

[0125] Example 1 As shown in FIG. 1, the multilayer foam sheet 1 of Example 1 has a five-layer structure consisting of a foam layer 2, conductive layers 3 laminated on both sides of the foam layer 2, and surface layers 4 laminated on each conductive layer 3. The foam layer 2 is composed of a low-density polyethylene as shown in Table 2. The conductive layer 3 is composed of a resin composition containing an ethylene copolymer, low-density polyethylene as a polyethylene resin, and conductive carbon as a conductivity imparting agent. The specific composition of the resin composition constituting the conductive layer 3 is as shown in Table 2. The surface layer 4 is composed of a mixed resin of linear low-density polyethylene as a linear polyethylene and low-density polyethylene. The specific composition of the mixed resin constituting the surface layer 4 is as shown in Table 2.

[0126] The specific method for producing the multi-layer foamed sheet of Example 1 is as follows. First, a co-extrusion apparatus is prepared, which includes an extruder for forming a foamed layer, an extruder for forming a conductive layer, an extruder for forming a surface layer, and a co-extrusion die connected to the discharge ports of these extruders. The co-extrusion die in this example is an annular die having an annular extrusion port.

[0127] To prepare the molten material for forming the foam layer, the polyethylene resin shown in Table 2 and 1 part by mass of a cell control agent per 100 parts by mass of the polyethylene resin were fed into an extruder for forming the foam layer, and these were melt-kneaded in the extruder. The cell control agent used was a mixture of citric acid and sodium bicarbonate ("Fine Cell Master PO217K" manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.). "Fine Cell Master" is a registered trademark of Dainichiseika Color & Chemicals Mfg. Co., Ltd.

[0128] A physical foaming agent was added under pressure to a mixture of a molten polyethylene resin and a cell control agent in an extruder, and the mixture was further mixed in the extruder to obtain a molten material for forming a foam layer. A mixed butane consisting of 65% by mass of normal butane and 35% by mass of isobutane was used as the physical foaming agent. The amount of the physical foaming agent was 8 parts by mass per 100 parts by mass of the polyethylene resin.

[0129] To prepare the conductive layer-forming melt, the types and amounts of ethylene copolymer, low-density polyethylene, and conductive carbon shown in Table 2 were fed into the conductive layer-forming extruder, and 25 parts by mass of a volatile plasticizer was fed per 100 parts by mass of the ethylene copolymer and low-density polyethylene combined. These were then kneaded in the extruder to obtain the conductive layer-forming melt. A butane mixture consisting of 65% by mass of normal butane and 35% by mass of isobutane was used as the volatile plasticizer.

[0130] To prepare the surface layer-forming melt, low-density polyethylene and linear low-density polyethylene of the types and amounts shown in Table 2 were fed to the surface layer-forming extruder, and 17 parts by mass of a volatile plasticizer was fed per 100 parts by mass of the low-density polyethylene and linear polyethylene combined. These were then kneaded in the extruder to obtain the surface layer-forming melt. A butane mixture consisting of 65% by mass of normal butane and 35% by mass of isobutane was used as the volatile plasticizer.

[0131] The melts thus produced in each extruder were simultaneously fed into a co-extrusion die, where they merged so that the conductive layer-forming melt was laminated on both sides of the foam layer-forming melt, and the surface layer-forming melt was laminated on the conductive layer-forming melt. These melts were then co-extruded through the extrusion opening of the co-extrusion die and foamed to produce a cylindrical laminated foam in which conductive layers were laminated on both sides of the polyethylene-based resin foam layer and a surface layer was laminated on each conductive layer. A mandrel with a diameter of 360 mm was inserted inside the laminated foam, and the laminated foam was cut open while being drawn along the mandrel, yielding the multi-layer foam sheet of Example 1. The conductive layer and surface layer of the multi-layer foam sheet thus obtained were both in an unfoamed state.

[0132] (Examples 2 to 4 and Example 10) The multi-layer foamed sheets of these Examples have the same structure as that of Example 1, except that the type of linear low-density polyethylene used in the surface layer was changed as shown in Tables 2 and 4. The methods for producing the multi-layer foamed sheets of Examples 2 to 4 and Example 10 were the same as that of Example 1, except that the type of linear low-density polyethylene in the molten material for forming the surface layer was changed as shown in Tables 2 and 4.

[0133] (Examples 5, 12 and Comparative Example 6) The multi-layer foamed sheets of these Examples and Comparative Example 6 had the same structure as the multi-layer foamed sheet of Example 1, except that the blending ratio of the linear low-density polyethylene to the low-density polyethylene in the surface layer was changed as shown in Tables 3, 4, and 7. The manufacturing methods of the multi-layer foamed sheets of Examples 5, 12, and Comparative Example 6 were the same as the manufacturing method of the multi-layer foamed sheet of Example 1, except that the blending ratio of the linear low-density polyethylene to the low-density polyethylene in the molten material for forming the surface layer was changed as shown in Tables 3, 4, and 7.

[0134] (Examples 6 and 11) The multi-layer foam sheets of these Examples have the same structure as that of Example 1, except that the surface layers are composed only of linear low-density polyethylenes shown in Tables 3 and 4. The method for producing the multi-layer foam sheet of Example 6 is the same as that for producing the multi-layer foam sheet of Example 1, except that the surface layer-forming molten material does not contain low-density polyethylene, but instead uses linear low-density polyethylenes shown in Tables 3 and 4.

[0135] Example 7 The multi-layer foam sheet of Example 7 has the same structure as the multi-layer foam sheet of Example 1, except that high-density polyethylene was used instead of linear low-density polyethylene as the linear polyethylene used in the surface layer, and the blending ratio of the high-density polyethylene to the low-density polyethylene was changed as shown in Table 3. The production method of the multi-layer foam sheet of Example 7 is the same as the production method of the multi-layer foam sheet of Example 1, except that high-density polyethylene was used in place of linear low-density polyethylene in the molten material for forming the surface layer, and the blending ratio of the high-density polyethylene to the low-density polyethylene was changed as shown in Table 3.

[0136] (Examples 8 to 9) The multi-layer foam sheets of these Examples have the same structure as the multi-layer foam sheet of Example 1, except that the blending ratios of the low-density polyethylene, the ethylene-based copolymer, and the conductive carbon contained in the conductive layer were changed as shown in Tables 3 and 4. The manufacturing methods of the multi-layer foam sheets of Examples 8 and 9 were the same as the manufacturing method of the multi-layer foam sheet of Example 1, except that the blending ratios of the low-density polyethylene, the ethylene-based copolymer, and the conductive carbon contained in the conductive layer-forming molten material were changed as shown in Tables 3 and 4.

[0137] (Examples 13 to 16) The multi-layer foamed sheets of these Examples have the same structure as the multi-layer foamed sheet of Example 1, except that the basis weight of the conductive layer and / or surface layer was changed as shown in Table 5. The manufacturing method of the multi-layer foamed sheets of Examples 13 to 16 was the same as the manufacturing method of the multi-layer foamed sheet of Example 1, except that the discharge speed during manufacturing was changed so that the basis weight of the conductive layer and / or surface layer of the multi-layer foamed sheet would be the value shown in Table 5.

[0138] (Comparative Example 1) The multi-layer foamed sheet of Comparative Example 1 has the same structure as the multi-layer foamed sheet of Example 1, except that it does not have a surface layer, as shown in Table 6. The manufacturing method of the multi-layer foamed sheet of Comparative Example 1 is the same as that of Example 1, except that no surface layer is laminated on the conductive layer.

[0139] (Comparative Example 2) The multi-layer foamed sheet of Comparative Example 2 has the same structure as the multi-layer foamed sheet of Example 1, except that the surface layer does not contain linear polyethylene and is composed only of low-density polyethylene, as shown in Table 6. The method for producing the multi-layer foamed sheet of Comparative Example 2 is the same as the method for producing the multi-layer foamed sheet of Example 1, except that the linear polyethylene is not blended into the molten material for forming the surface layer.

[0140] (Comparative Example 3) The multi-layer foamed sheet of Comparative Example 3 has the same structure as the multi-layer foamed sheet of Example 1, except that the amount of conductive carbon in the conductive layer and the type of linear low-density polyethylene used in the surface layer were changed as shown in Table 6. The manufacturing method of the multi-layer foamed sheet of Comparative Example 3 was the same as that of Example 1, except that the amount of conductive carbon in the molten material for forming the conductive layer and the type of linear low-density polyethylene used in the molten material for forming the surface layer were changed as shown in Table 6.

[0141] (Comparative Examples 4 to 5) The multi-layer foam sheets of these comparative examples have the same structure as the multi-layer foam sheet of Example 1, except that the conductive layer does not contain an ethylene-based copolymer and the amount of conductive carbon and the type of linear low-density polyethylene used in the surface layer are changed as shown in Tables 6 and 7. The manufacturing methods of the multi-layer foam sheets of Comparative Examples 4 and 5 are the same as those of Example 1, except that the ethylene-based copolymer is not blended in the conductive layer-forming molten material and the amount of conductive carbon and the type of linear low-density polyethylene blended in the surface layer-forming molten material are changed as shown in Tables 6 and 7.

[0142] (Reference example) The multilayer foam sheet of the Reference Example has the same structure as the multilayer foam sheet of Example 1, except that the conductive layer does not contain an ethylene copolymer, a polymeric antistatic agent is blended in place of conductive carbon as a conductivity-imparting agent, and the surface layer is composed solely of low-density polyethylene as shown in Table 7. The manufacturing method of the multilayer foam sheet of the Reference Example is the same as that of Example 1, except that the conductive layer-forming melt does not contain an ethylene copolymer, the polymeric antistatic agent is blended in place of conductive carbon in the amount shown in Table 7, and the surface layer-forming melt is composed solely of low-density polyethylene as shown in Table 7. The polymeric antistatic agent used in this example is specifically "Plectron (registered trademark) LMP" manufactured by Sanyo Chemical Industries, Ltd. In Table 7, the polymeric antistatic agent is abbreviated as "ASP."

[0143] (evaluation) The evaluation methods for the various properties shown in Tables 2 to 7 are as follows.

[0144] [Melt viscosity of resin composition constituting conductive layer] The melt viscosity of the conductive layer-forming melt in the Examples, Comparative Examples, and Reference Examples was measured using a capillary rheometer ("Capillograph 1D" manufactured by Toyo Seiki Co., Ltd.) with an orifice diameter of 1 mm, an orifice length of 10 mm, a measurement temperature of 190°C, and a shear rate of 100 sec -1The melt viscosities of the conductive layer-forming melts of the Examples, Comparative Examples and Reference Examples are as shown in Tables 2 to 7.

[0145] [Total thickness of multi-layer foam sheet] First, the multi-layer foam sheet was cut in a plane perpendicular to the extrusion direction. Ten measurement positions were set on this cut surface at equal intervals in the longitudinal direction of the cut surface (i.e., in a direction perpendicular to both the extrusion direction and the thickness direction). These measurement positions were observed using a microscope, and the thickness at each measurement position was measured. The arithmetic mean value of these thicknesses was taken as the total thickness of the multi-layer foam sheet, and is shown in Tables 2 to 7.

[0146] [Basis weight of multi-layer foam sheet] A square test piece with a side length of 25 mm was taken from the multi-layer foam sheet, and the mass (unit: g) of the test piece was measured. The mass of this test piece was converted into a unit to determine the basis weight of the multi-layer foam sheet, i.e., the weight per 1 m of the multi-layer foam sheet. 2 Mass per unit (unit: g / m 2 The basis weights of the multi-layer foamed sheets of the Examples, Comparative Examples and Reference Examples are as shown in Tables 2 to 7.

[0147] [Expansion ratio of multi-layer foam sheet] The basis weight of the multi-layer foam sheet obtained by the above-mentioned method was divided by the total thickness of the multi-layer foam sheet, and the apparent density (unit: kg / m) of the multi-layer foam sheet was calculated by converting the unit. 3 ) was calculated. The expansion ratio was calculated by dividing the density of the polyethylene resin constituting the foam layer of the multi-layer foam sheet by the apparent density of the multi-layer foam sheet. The expansion ratios of the multi-layer foam sheets of Examples, Comparative Examples, and Reference Examples are shown in Tables 2 to 7.

[0148] [Basis weight of conductive layer and surface layer] The basis weight of the conductive layer and the basis weight of the surface layer per side (unit: g / m) were calculated based on the following formulas (1) and (2) using the discharge amount X1 (unit: g / h) of the conductive layer per side, the discharge amount X2 (unit: g / h) of the surface layer per side during co-extrusion of the multi-layer foam sheet, the width W (unit: m) of the multi-layer foam sheet, and the take-up speed L (unit: m / h) of the multi-layer foam sheet. 2 ) was calculated. Conductive layer basis weight = [X1 / (L × W)] (1) Surface layer basis weight = [X2 / (L×W)] (2)

[0149] [Surface resistivity] The surface resistivity of the multilayer foam sheet was measured using a measurement method in accordance with JIS K6271-1:2015. Specifically, a square test piece with sides of 100 mm was taken from the center of the multilayer foam sheet in the width direction. Electrodes were attached to the surface of the surface layer of the test piece, and a voltage of 1 V was 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 was recorded as the surface resistivity of the multilayer foam sheet. The surface resistivity was measured using a "Hiresta UX MCP-HT800" manufactured by Nitto Seiko Analytech Co., Ltd., and a double ring electrode "MCP-JB-04" manufactured by Nitto Seiko Analytech Co., Ltd. The surface resistivities of the multilayer foam sheets of the Examples, Comparative Examples, and Reference Examples are shown in Tables 2 to 7.

[0150] [Surface resistivity variation] The surface resistivity was measured at multiple positions on the surface layer of the multilayer foam sheet using a measurement method in accordance with JIS K7194:1994. Specifically, the surface resistivity was measured at 21 measurement positions at the center of the width direction of the multilayer foam sheet, shifted by 25 mm in the longitudinal direction of the multilayer foam sheet (i.e., the extrusion direction). The surface resistivity was measured using a "Loresta GP MCP-T610" manufactured by Nitto Seiko Analytech Co., Ltd., and the electrode was a four-probe electrode "MCP-TPLSP" manufactured by Nitto Seiko Analytech Co., Ltd.

[0151] Similar measurements were performed at a position 300 mm to the left and 300 mm to the right of the center in the width direction, for a total of 63 measurement positions, to measure the surface resistivity. If the ratio of the maximum value to the minimum value of the surface resistivity thus obtained was less than 10, the symbol "A" was entered in the "Variation in Surface Resistivity" column of Tables 2 to 7; if it was more than 10 but less than 100, the symbol "B" was entered in the same column; and if it was more than 100, the symbol "C" was entered in the same column. The method for measuring the surface resistivity at each measurement position was the same as the method described above.

[0152] [Pinhole area ratio] At the center of the width direction of the multilayer foamed sheet, the surface layer was observed using a scanning electron microscope while shifting the position in the longitudinal direction (i.e., the extrusion direction) of the multilayer foamed sheet, and electron microscope images were obtained at six measurement positions. The observation magnification was 100x. The same operation was performed at positions 300 mm to the left and 300 mm to the right of the center in the width direction, and a total of 18 electron microscope images were obtained.

[0153] Next, in each of the 18 electron microscope images thus obtained, a square measurement area with a side length of 1000 μm was set by image analysis. 2 These holes were regarded as pinholes, and the ratio of the area occupied by pinholes to the area of ​​the measurement region was calculated.

[0154] The arithmetic mean value of the ratio of the area occupied by pinholes in the 18 measurement regions obtained by the above procedure was defined as the pinhole area ratio of the multi-layer foamed sheet. The pinhole area ratios of the multi-layer foamed sheets of Examples, Comparative Examples, and Reference Examples are shown in Tables 2 to 7.

[0155] [Amount of dust generated] A test piece was taken from the multilayer foam sheet, and foreign matter adhering to the surface of the test piece was roughly removed using a clean roller. Next, a bag was made from the test piece with the surface cleaned by the clean roller facing inward. Ultrapure water was poured into the bag to rinse the inner surface of the bag, and foreign matter adhering to the inner surface was removed. The bag was then placed in a glove box.

[0156] In a glove box, 200 mL of ultrapure water was poured into the bag, and then ultrasonic cleaning was performed at a frequency of 100 kHz for 1 minute. After ultrasonic cleaning was completed, the number of foreign particles with a diameter of 2 μm or more contained in the ultrapure water in the bag was counted using a particle counter. The number of foreign particles thus measured was divided by the area of ​​the inner surface of the bag to determine the amount of dust generated per unit area (unit: particles / cm). 2 ) was calculated. The amounts of dust generated by the multilayer foam sheets of Examples, Comparative Examples, and Reference Examples are shown in Tables 2 to 7. In the above-mentioned method, the foreign matter mixed into the ultrapure water in the bag specifically includes conductive carbon that has fallen off the conductive layer and polyethylene resin pieces that have fallen off from the edges of the pinholes, etc. The smaller the amount of dust generated, the less likely it is that the packaged item will be contaminated.

[0157] [exterior] The appearance of the multilayer foam sheet was visually observed to evaluate the lamination state of each layer. In the "Appearance" column of Tables 2 to 7, a uniform color tone of the multilayer foam sheet was indicated by the symbol "A," a slightly non-uniform color tone was indicated by the symbol "B," and a significantly non-uniform color tone was indicated by the symbol "C."

[0158] [Table 2]

[0159] [Table 3]

[0160] [Table 4]

[0161] [Table 5]

[0162] [Table 6]

[0163] [Table 7]

[0164] As shown in Tables 2 to 5, the multi-layer foamed sheets of Examples 1 to 16 each have a foamed layer, conductive layers made of the resin composition having the specific composition and laminated on both sides of the foamed layer, and surface layers made of the specific resin and laminated on the conductive layers. Therefore, the multi-layer foamed sheets of Examples 1 to 16 have a 1×10 8 The multilayer foamed sheet has a surface resistivity of less than Ω and can reduce the amount of dust generated from the multilayer foamed sheet. Such a multilayer foamed sheet is electrically conductive and can significantly reduce contamination of the packaged items. In addition, in Examples 1 to 16, the foamed layer, conductive layer, and surface layer having such a configuration are laminated and bonded to adjacent layers by coextrusion. Therefore, the productivity is superior to that of a method in which a functional layer such as a conductive layer is laminated by thermal lamination or the like.

[0165] Among these Examples, Examples 5 and 7 in Table 3 have the same configuration except for the type of linear polyethylene contained in the surface layer. Example 5 is superior to Example 7 in terms of the variation in surface resistivity and appearance. Therefore, a comparison between Examples 5 and 7 shows that by using linear low-density polyethylene as the linear polyethylene contained in the surface layer, the surface layer can be stably laminated and bonded to the conductive layer, making it easier to obtain a multilayer foamed sheet with good appearance and further reducing the variation in surface resistivity.

[0166] Furthermore, Examples 1 to 4 in Table 2 and Example 10 in Table 4 have the same configuration except for the type of linear polyethylene contained in the surface layer. Examples 1 to 3 suppress the occurrence of pinholes and further reduce the amount of dust generated compared to Examples 4 and 10. Therefore, from a comparison of Examples 1 to 4 and Example 10, it can be seen that by using a resin with a higher tensile strength at a predetermined temperature and a higher crystallization temperature as the linear polyethylene contained in the surface layer, it is possible to further reduce contamination of packaged items.

[0167] Furthermore, Example 6 in Table 3 and Example 11 in Table 4 have the same configuration except for the type of linear polyethylene contained in the surface layer. Example 6 is superior in surface resistivity variation and appearance compared to Example 11. Therefore, from a comparison between Example 6 and Example 11, it can be seen that by using a linear polyethylene resin with a higher melt flow rate contained in the surface layer, the surface layer can be stably laminated and bonded to the conductive layer, making it easier to obtain a multilayer foamed sheet with good appearance and further reducing the variation in surface resistivity.

[0168] On the other hand, the multi-layer foamed sheet of Comparative Example 1 shown in Table 6 did not have a surface layer, and therefore generated more dust than the examples.

[0169] The multi-layer foamed sheet of Comparative Example 2 had a surface layer, but the surface layer did not contain linear polyethylene, and therefore generated more dust than the examples.

[0170] The multi-layer foamed sheet of Comparative Example 3 contained too little conductive carbon, so that the multi-layer foamed sheet could not be made conductive.

[0171] The multi-layer foamed sheet of Comparative Example 4 did not contain an ethylene-based copolymer in the conductive layer, and therefore, when the amount of conductive carbon was set within the above-mentioned specific range, the multi-layer foamed sheet could not be made conductive.

[0172] As shown in Table 7, the amount of conductive carbon blended in the multi-layer foamed sheet of Comparative Example 5 was greater than that in Comparative Example 4 in order to impart conductivity to the multi-layer foamed sheet. However, the amount of conductive carbon blended was too high, resulting in increased dust generation.

[0173] The multi-layer foamed sheet of Comparative Example 6 generated a large amount of dust because the amount of linear polyethylene blended in the surface layer was too small.

[0174] In the reference example, instead of the conductive layer containing conductive carbon, an antistatic layer containing a polymeric antistatic agent is provided between the foam layer and the surface layer. As shown in the reference example, when an antistatic layer containing a polymeric antistatic agent is provided between the foam layer and the surface layer, it is possible to reduce the amount of dust generation even if the surface layer does not contain linear polyethylene.

[0175]

[0049] While specific embodiments of the multi-layer foamed sheet and the method for producing the same according to the present invention have been described above based on the examples, the specific embodiments of the multi-layer foamed sheet and the method for producing the same according to the present invention are not limited to the examples, and the configurations can be appropriately changed within the scope of the gist of the present invention. [Explanation of symbols]

[0176] 1 Polyethylene resin multi-layer foam sheet 2 Polyethylene resin foam layer 3 Conductive layer 4 Surface layer

Claims

1. a polyethylene resin foam layer; a surface layer provided on at least one side of the polyethylene-based resin foam layer; a conductive layer provided between the surface layer and the foam layer, the conductive layer comprises an ethylene-based copolymer having a structural unit derived from ethylene and a structural unit derived from a monomer having a polar group, a polyethylene-based resin different from the ethylene-based copolymer, and conductive carbon; the conductive layer contains the conductive carbon in an amount of 5% by mass or more and 15% by mass or less; the polyethylene-based resin contained in the conductive layer is polyethylene, the surface layer is made of a mixed resin containing one or more linear polyethylenes selected from the group consisting of linear low-density polyethylenes and high-density polyethylenes, or the linear polyethylenes and low-density polyethylenes, and the blending amount of the linear polyethylene is 8% by mass or more; The surface resistivity of the surface of the multilayer foamed sheet on which the surface layer is provided is 1×10 ８ A polyethylene resin multi-layer foam sheet having a modulus of elasticity of less than Ω.

2. 2. The polyethylene-based resin multi-layer foamed sheet according to claim 1, wherein the ethylene-based copolymer contained in the conductive layer has a melting point of 30°C or higher and 80°C or lower, and the polyethylene-based resin contained in the conductive layer has a melting point of 100°C or higher.

3. 2. The polyethylene-based resin multi-layer foamed sheet according to claim 1, wherein the ethylene-based copolymer contained in the conductive layer has a content of structural units derived from monomers having a polar group of 30% by mass or more and 50% by mass or less.

4. a polyethylene resin foam layer; a surface layer provided on at least one side of the polyethylene-based resin foam layer; a conductive layer provided between the surface layer and the foam layer, the conductive layer comprises an ethylene-based copolymer having a structural unit derived from ethylene and a structural unit derived from a monomer having a polar group, a polyethylene-based resin different from the ethylene-based copolymer, and conductive carbon; the conductive layer contains the conductive carbon in an amount of 5% by mass or more and 15% by mass or less; the ethylene-based copolymer and the polyethylene-based resin contained in the conductive layer are incompatible with each other, the surface layer is made of a mixed resin containing one or more linear polyethylenes selected from the group consisting of linear low-density polyethylenes and high-density polyethylenes, or the linear polyethylenes and low-density polyethylenes, and the blending amount of the linear polyethylene is 8% by mass or more; The surface resistivity of the surface of the multilayer foamed sheet on which the surface layer is provided is 1×10 ８ A polyethylene resin multi-layer foam sheet having a modulus of elasticity of less than Ω.

5. a polyethylene resin foam layer; a surface layer provided on at least one side of the polyethylene-based resin foam layer; a conductive layer provided between the surface layer and the foam layer, the conductive layer comprises an ethylene-based copolymer having a structural unit derived from ethylene and a structural unit derived from a monomer having a polar group, a polyethylene-based resin different from the ethylene-based copolymer, and conductive carbon; the conductive layer contains the conductive carbon in an amount of 5% by mass or more and 15% by mass or less; the polyethylene resin contained in the conductive layer does not contain a structural unit derived from a monomer having a polar group, the surface layer is made of a mixed resin containing one or more linear polyethylenes selected from the group consisting of linear low-density polyethylenes and high-density polyethylenes, or the linear polyethylenes and low-density polyethylenes, and the blending amount of the linear polyethylene is 8% by mass or more; The surface resistivity of the surface of the multilayer foamed sheet on which the surface layer is provided is 1×10 ８ A polyethylene resin multi-layer foam sheet having a modulus of elasticity of less than Ω.

6. The polyethylene-based resin multi-layer foamed sheet according to any one of claims 1 to 5, wherein the foam layer comprises a low-density polyethylene.

7. The polyethylene-based resin multi-layer foamed sheet according to any one of claims 1 to 5, wherein the surface layer contains at least a linear low-density polyethylene as the linear polyethylene.

8. The polyethylene-based resin multi-layer foamed sheet according to any one of claims 1 to 5, wherein the linear polyethylene contained in the surface layer has a melt flow rate of 12 g / 10 min or more at a temperature of 190°C under a load of 2.16 kg.

9. The polyethylene-based resin multi-layer foamed sheet according to any one of claims 1 to 5, wherein the linear polyethylene contained in the surface layer has a tensile strength of 3.0 MPa or more at a temperature of 95°C.

10. The polyethylene-based resin multi-layer foamed sheet according to any one of claims 1 to 5, wherein the linear polyethylene contained in the surface layer has a tensile strength of 15 MPa or more at a temperature of 23°C.

11. The polyethylene-based resin multi-layer foamed sheet according to any one of claims 1 to 5, wherein the crystallization temperature of the linear polyethylene contained in the surface layer is 100°C or higher and 115°C or lower.

12. The melting point Tm of the linear polyethylene contained in the surface layer Ｓ and the melting point Tm of the polyethylene resin used in the foam layer Ｃ The difference Tm Ｓ -Tm Ｃ The polyethylene resin multi-layer foamed sheet according to any one of claims 1 to 5, wherein the temperature is 0°C or higher and 15°C or lower.

13. The surface layer of the polyethylene-based resin multi-layer foamed sheet has a basis weight of 1 g / m ２ 10g / m or more ２ The polyethylene-based resin multi-layer foamed sheet according to any one of claims 1 to 5, wherein:

14. The apparent density of the polyethylene resin multi-layer foamed sheet is 30 kg / m ３ More than 150kg / m ３ The polyethylene-based resin multi-layer foamed sheet according to any one of claims 1 to 5, wherein:

15. A method for producing a polyethylene-based resin multi-layer foamed sheet, comprising co-extruding a foam layer-forming melt for forming a polyethylene-based resin foam layer, a conductive layer-forming melt for forming a conductive layer, and a surface layer-forming melt for forming a surface layer, to produce a polyethylene-based resin multi-layer foamed sheet having the polyethylene-based resin foam layer, the surface layer provided on at least one side of the polyethylene-based resin foam layer, and the conductive layer provided between the surface layer and the foam layer, the foam layer-forming melt is obtained by kneading a polyethylene resin and a physical foaming agent, the conductive layer-forming melt is obtained by kneading an ethylene-based copolymer having a structural unit derived from ethylene and a structural unit derived from a monomer having a polar group, a polyethylene-based resin different from the ethylene-based copolymer, and conductive carbon; the conductive carbon is blended in an amount of 5% by mass or more and 15% by mass or less in the conductive layer-forming melt, the polyethylene-based resin contained in the conductive layer-forming melt is polyethylene, the surface layer-forming melt is made by kneading one or more linear polyethylenes selected from the group consisting of linear low-density polyethylenes and high-density polyethylenes, or the linear polyethylene and low-density polyethylene in an amount of 8% by mass or more relative to the mass of the surface layer-forming melt, The surface resistivity of the surface of the multilayer foamed sheet on which the surface layer is provided is 1×10 ８ A method for producing a polyethylene-based resin multi-layer foam sheet having a hardness of less than Ω.

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