Polyethylene-based resin multilayer foamed sheet and method for manufacturing the same

Abstract

The polyethylene-based resin multilayer foamed sheet has: a polyethylene-based resin foamed layer including a polyethylene-based resin (A) as a base resin; and a conductive layer laminated to at least one side of the foamed layer. The conductive layer includes: a mixed resin of polyethylene (B) selected from the group consisting of low-density polyethylene and linear low-density polyethylene, and an ethylene-based copolymer (C) having a structural unit derived from ethylene and a structural unit derived from a monomer having a polar group; and a conductive carbon. The content of the conductive carbon in the conductive layer is 3% by mass or more and 15% by mass or less. The difference Tm B between the melting point Tm C of the polyethylene (B) and the melting point Tm B of the ethylene-based copolymer (C) is 30°C or more and 80°C or less. C ​

Description

TECHNICAL FIELD

[0001] The present application relates to a polyethylene-based resin multilayer foamed sheet and a method for producing the same. BACKGROUND

[0002] A polyethylene-based resin foamed sheet using a polyethylene-based resin as a base resin is used for cushioning materials, packaging materials, and the like, because it has high softness and excellent impact absorption properties. Among them, for example, cushioning materials and packaging materials used for packaging electronic devices, electronic parts, and the like, are required to have conductivity in addition to the protection of the packaged objects.

[0003] As an example of a foamed sheet having conductivity, a conductive polyethylene-based resin foamed sheet obtained by mixing a master batch containing conductive carbon and a polyethylene-based resin and extrusion foaming is described in Patent Literature 1.

[0004] In addition, a conductive polyethylene-based resin multilayer foamed sheet obtained by heat foaming a foamed multilayer thermoplastic resin sheet composed of at least two layers of a non-conductive polyethylene-based resin containing a thermal decomposition type foaming agent and a conductive thermoplastic resin layer containing conductive carbon is described in Patent Literature 2.

[0005] PRIOR ART DOCUMENTS

[0006] PATENT LITERATURE

[0007] Patent Literature 1: Japanese Patent Application Laid-Open No. 61-31440

[0008] Patent Literature 2: Japanese Patent Application Laid-Open No. 62-231728 SUMMARY

[0009] PROBLEMS TO BE SOLVED BY THE INVENTION

[0010] The foamed sheet of Patent Literature 1 needs to incorporate a relatively large amount of conductive carbon in the foamed sheet in order to impart conductivity. However, if the amount of conductive carbon incorporated in the foamed sheet is increased, there is a risk that the foaming property is hindered and the cushioning properties and the like required for use as a cushioning material or a packaging material are impaired.

[0011] Similarly, the foamed sheet of Patent Literature 2 needs to incorporate a relatively large amount of conductive carbon in the conductive thermoplastic resin layer in order to have a surface resistivity of 1 x 10 7 Ω or less.

[0012] Furthermore, in Patent Documents 1 and 2, conductive carbon detaches from the foam sheet, posing a risk of contaminating the surrounding area. In particular, if the amount of conductive carbon in the foam sheet increases, there is a risk that the conductive carbon will easily detach from the foam sheet, causing contamination of the surrounding area. Additionally, when the foam sheet is used as packaging material, there is a risk that the conductive carbon detached from the foam sheet may transfer to the packaged item and contaminate it.

[0013] The present invention was made in view of the above background, and its object is to provide a polyethylene resin multilayer foam sheet that is conductive and can reduce the shedding of conductive carbon from the foam sheet, and a method for manufacturing the same.

[0014] means for solving problems

[0015] One aspect of the present invention relates to a multilayer foamed sheet based on polyethylene resin, wherein,

[0016] The polyethylene resin multilayer foam sheet comprises:

[0017] A polyethylene resin foam layer comprising a polyethylene resin (A) as a base resin; and

[0018] A conductive layer is laminated on at least one side of the polyethylene resin foam layer.

[0019] The conductive layer comprises:

[0020] A mixed resin of one or more polyethylenes (B) selected from the group consisting of low-density polyethylene and linear low-density polyethylene, and an ethylene copolymer (C) having structural units derived from ethylene and structural units derived from monomers having polar groups; and

[0021] The conductive carbon in the mixed resin,

[0022] The amount of conductive carbon in the conductive layer is 3% by mass or more and 15% by mass or less.

[0023] The polyethylene (B) contained in the conductive layer has a melting point Tm. B The melting point Tm of the ethylene copolymer (C) C The difference Tm B -Tm C The temperature is above 30℃ and below 80℃.

[0024] Another aspect of the present invention relates to a method for manufacturing a polyethylene-based resin multilayer foamed sheet, which involves co-extruding a foaming layer forming melt for forming a polyethylene-based resin foam layer and a conductive layer forming melt for forming a conductive layer to produce a polyethylene-based resin multilayer foamed sheet having the polyethylene-based resin foam layer and the conductive layer laminated on at least one side of the polyethylene-based resin foam layer.

[0025] The foaming layer forming melt is prepared by mixing polyethylene resin (A) as the base resin and a physical foaming agent.

[0026] The conductive layer forming melt is a compound made by mixing one or more polyethylenes (B) selected from the group consisting of low-density polyethylene and linear low-density polyethylene, an ethylene copolymer (C) having structural units derived from ethylene and structural units derived from monomers having polar groups, and conductive carbon.

[0027] The amount of conductive carbon in the melt used to form the conductive layer is 3% by mass or more and 15% by mass or less.

[0028] The melting point Tm of the polyethylene (B) B The melting point Tm of the ethylene copolymer (C) C The difference Tm B -Tm C The temperature is above 30℃ and below 80℃.

[0029] Invention Effects

[0030] The aforementioned polyethylene resin multilayer foam sheet (hereinafter, appropriately referred to as "multilayer foam sheet") can impart conductivity to the multilayer foam sheet even with a small amount of conductive carbon by configuring the conductive layer in the specific manner described above. Furthermore, the conductive layer having the specific composition described above can suppress the shedding of conductive carbon.

[0031] In the manufacturing method described above, a multilayer foamed sheet is obtained by co-extruding a melt for forming a foamed layer having the specific composition described above and a melt for forming a conductive layer. By making the composition of the melt for forming the conductive layer specific as described above, the manufacturing method can impart conductivity to the multilayer foamed sheet even with a small amount of conductive carbon. Furthermore, according to the manufacturing method described above, a multilayer foamed sheet that reduces the shedding of conductive carbon can be obtained.

[0032] As described above, a polyethylene resin multilayer foam sheet with conductivity and a method for manufacturing the same can be provided. Detailed Implementation

[0033] (Polyethylene resin multilayer foamed sheet)

[0034] The above multilayer foamed sheet has two or more layers including a polyethylene resin foamed layer and a conductive layer laminated on at least one side of the polyethylene resin foamed layer. The layers included in the multilayer foamed sheet are laminated and bonded to the adjacent layers. The layers in the multilayer foamed sheet are preferably laminated and bonded by co-extrusion with the adjacent layers.

[0035] For example, the above multilayer foamed sheet can be composed of two layers of a polyethylene resin foamed layer and a conductive layer laminated on one side of the polyethylene resin foamed layer. Alternatively, the above multilayer foamed sheet can be composed of three layers of a polyethylene resin foamed layer and conductive layers laminated on both sides of the polyethylene resin foamed layer. Further, a layer having a different composition from the polyethylene resin foamed layer and the conductive layer can be provided between the polyethylene resin foamed layer and the conductive layer in the above multilayer foamed sheet, or on the outermost surface of the multilayer foamed sheet. From the viewpoint of more reliably preventing damage to the electronic device, electronic component, or the like due to static electricity, it is preferable that the conductive layer be laminated on both sides of the polyethylene resin foamed layer, and more preferable that the conductive layer be on the surface of the multilayer foamed sheet.

[0036] <Overall thickness>

[0037] The overall thickness of the above multilayer foamed sheet is preferably 0.05 mm or more and 3.0 mm or less. By making the overall thickness of the multilayer foamed sheet 0.05 mm or more, more preferably 0.1 mm or more, and further preferably 0.2 mm or more, the cushioning property of the multilayer foamed sheet can be further improved. In addition, by making the overall thickness of the multilayer foamed sheet 3.0 mm or less, more preferably 2.0 mm or less, further preferably 1.5 mm or less, and particularly preferably 1.2 mm or less, the handleability of the multilayer foamed sheet can be further improved, and the baling of the packaged article can be more easily performed.

[0038] The method for measuring the overall thickness of the multilayer foamed sheet is as follows. First, the multilayer foamed sheet is cut along a surface perpendicular to the extrusion direction. In the cut surface, ten measurement positions are set in such a manner that the length of the cut surface in the width direction (i.e., the direction that is at right angles to both the extrusion direction and the thickness direction) is divided into eleven equal parts. The thickness of each measurement position is measured by a method such as observation using a microscope. Then, the arithmetic mean of the above thicknesses is taken as the overall thickness of the multilayer foamed sheet.

[0039] <Apparent density>

[0040] The apparent density of the above multilayer foamed sheet is preferably 30 kg / m 3 or more and 150 kg / m 3 or less. By making the apparent density of the multilayer foamed sheet 30 kg / m 3The apparent density of the multilayer foamed sheet is preferably 35 kg / m 3 The apparent density of the multilayer foamed sheet is further preferably 40 kg / m 3 The apparent density of the multilayer foamed sheet is preferably 35 kg / m 3 The apparent density of the multilayer foamed sheet is more preferably 120 kg / m 3 The apparent density of the multilayer foamed sheet is further preferably 100 kg / m 3 The apparent density of the multilayer foamed sheet is more preferably 120 kg / m

[0041] The apparent density of the multilayer foamed sheet is measured as follows. First, the multilayer foamed sheet is cut in the width direction, and a test piece is collected. The test piece can be, for example, a rectangular shape having the same length as the total width of the multilayer foamed sheet in the longitudinal direction and a width of 10 cm in the lateral direction. The mass of the test piece (unit: g) is divided by the area of the test piece, and the basis weight of the multilayer foamed sheet, that is, the mass per 1 m 2 The mass of the multilayer foamed sheet (unit: g / m 2 ) is measured. The basis weight of the multilayer foamed sheet is divided by the overall thickness of the multilayer foamed sheet obtained by the above method, and then unit conversion is performed, whereby the apparent density of the multilayer foamed sheet (unit: kg / m 3 ) can be calculated.

[0042] <surface resistivity>

[0043] The surface resistivity of the surface of the side having the conductive layer in the above multilayer foamed sheet is preferably 1 x 10 3 Ω or more and 1 x 10 7 Ω or less. The multilayer foamed sheet having the surface resistivity in this range can easily remove static electricity carried by a packaged article or the like, and thus is suitable as a cushioning material, a packaging material, and the like for electronic components, electronic devices, and the like. From the same viewpoint, the surface resistivity of the above multilayer foamed sheet is more preferably 5 x 10 6 Ω or less.

[0044] The surface resistivity of the multilayer foamed sheet is measured according to the measurement method of JIS K6271-1:2015. Specifically, first, a test piece having a square shape of 100 mm on a side is collected from the multilayer foamed sheet. After an electrode is attached to the surface of the side having the conductive layer of the test piece, a voltage of 1 V is applied between the electrodes in an atmosphere having a temperature of 23°C and a relative humidity of 50%. Then, the surface resistivity (unit: Ω) at the time point of 1 minute after the voltage is applied is taken as the surface resistivity of the multilayer foamed sheet.

[0045] [Polyethylene-based resin foamed layer]

[0046] Polyethylene resin foam is a foam made of polyethylene resin (A) as the base resin.

[0047] <Polyethylene-based resin (A)>

[0048] In this specification, polyethylene resin refers to a resin containing 50 mol% or more of structural units derived from ethylene. Examples of polyethylene resins (A) include, for instance, high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and ethylene-vinyl acetate copolymers (EVA) containing 50 mol% or more of structural units derived from ethylene.

[0049] Furthermore, the low-density polyethylene exemplified as the aforementioned polyethylene-based resin (A) refers to polyethylene with a long-chain branched structure and a density of 910 kg / m³. 3 Above and below 930 kg / m 3 Linear low-density polyethylene (LLDPE) refers to a copolymer of ethylene and α-olefins with 4 to 8 carbon atoms, with essentially linear molecular chains and a density of 910 kg / m³. 3 Above and below 930 kg / m 3 High-density polyethylene (HDPE) refers to ethylene homopolymer or copolymer of ethylene and α-olefins with 4 to 8 carbon atoms, with a density of 930 kg / m³. 3 The above refers to polyethylene.

[0050] The polyethylene resin foam layer may contain one polyethylene resin (A) selected from the group consisting of ethylene homopolymers and ethylene copolymers, or it may contain two or more polyethylene resins (A). Preferably, the polyethylene resin (A) contains 60 mol% or more of ethylene-derived structural units, more preferably 70 mol% or more.

[0051] From the viewpoint of further improving the softness, cushioning and foaming properties of the above-mentioned multilayer foam sheet, in the polyethylene resin foam layer, the polyethylene resin (A) preferably contains more than 50% by mass of low-density polyethylene, more preferably more than 80% by mass, even more preferably more than 90% by mass, and particularly preferably 100% by mass, that is, the base resin of the polyethylene resin foam layer is composed only of low-density polyethylene.

[0052] Melting point Tm of polyethylene resin (A) A Preferably, the temperature is above 100°C and below 135°C. This is achieved by adjusting the melting point Tm of the polyethylene resin (A). A Within the aforementioned range, a foamed layer with excellent extrusion foaming properties and excellent cushioning properties can be stably formed. From this perspective, the melting point Tm of the polyethylene-based resin (A) is... AIt is preferably 100°C or higher and 130°C or lower, more preferably 105°C or higher and 120°C or lower, further preferably 108°C or higher and 115°C or lower.

[0053] The melting point Tm of the polyethylene-based resin (A) A It can be measured by the transition temperature measurement method for plastics prescribed in JIS K7121:2012. First, in accordance with "the case where the melting temperature is measured after a certain heat treatment", the heating rate and the cooling rate are set to 10°C / minute, and the state adjustment of the test piece is performed. Thereafter, the heating rate is set to 10°C / minute, and the heat flux DSC (i.e., differential scanning calorimetry) is performed, and the DSC curve is obtained. The peak temperature of the endothermic peak in the obtained DSC curve is taken as the melting point. In addition, in the case where a plurality of endothermic peaks appear in the DSC curve, the peak temperature of the melting peak having the largest area is taken as the melting point with reference to the baseline on the high temperature side.

[0054] From the aspect that the extrusion foaming property is excellent, the melt flow rate (MFR) of the polyethylene-based resin (A) is preferably 0.5 g / 10 minutes or more and 15 g / 10 minutes or less, more preferably 1 g / 10 minutes or more and 8 g / 10 minutes or less, further preferably 1.5 g / 10 minutes or more and 5 g / 10 minutes or less. In addition, the MFR of the polyethylene-based resin in the present specification is a value measured under the conditions where the test temperature is 190°C and the load is 2.16 kg based on JIS K7210-1:2014.

[0055] <Other polymers>

[0056] Within a range that does not impair the above-described effects, other polymers than the polyethylene-based resin (A) can be contained in the polyethylene-based resin foamed layer as a base resin. As the other polymers than the polyethylene-based resin (A), for example, thermoplastic resins such as polystyrene-based resins, ethylene-propylene rubber, elastomers such as styrene-butadiene-styrene block copolymers, and the like can be exemplified. The content of the other polymers than the polyethylene-based resin (A) in the polyethylene-based resin foamed layer is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, further preferably 5 parts by mass or less, particularly preferably 0 parts by mass, that is, the polyethylene-based resin foamed layer contains only the polyethylene-based resin (A) as the polymer component constituting the foamed layer, with respect to 100 parts by mass of the polyethylene-based resin (A).

[0057] <Additives>

[0058] The polyethylene-based resin foamed layer can contain a bubble adjusting agent, an antioxidant, a heat stabilizer, a weathering agent, a UV absorber, a flame retardant, a filler, an antibacterial agent, and the like. The amount of the additive contained in the polyethylene-based resin foamed layer is, for example, preferably 10 parts by mass or less, more preferably 5 parts by mass or less, and further preferably 3 parts by mass or less, relative to 100 parts by mass of the polyethylene-based resin (A).

[0059] [Conductive layer]

[0060] The conductive layer is laminated on one or both sides of the polyethylene-based resin foamed layer. The base resin of the conductive layer is a mixed resin of the polyethylene (B) and the ethylene-based copolymer (C). From the viewpoint of improving the conductivity, handleability, and appearance of the multilayer foamed sheet, the conductive layer is preferably in a non-foamed state. However, a small amount of extremely fine bubbles can be contained in the conductive layer as long as the handleability and the like of the multilayer foamed sheet are not impaired.

[0061] [Polyethylene (B)]

[0062] The conductive layer contains the polyethylene (B) selected from one or two or more kinds of polyethylene from the group consisting of a low-density polyethylene and a linear low-density polyethylene. The low-density polyethylene exemplified as the polyethylene (B) refers to a polyethylene having a long-chain branched structure, a density of 910 kg / m 3 or more and less than 930 kg / m 3 , and the linear low-density polyethylene refers to a copolymer of ethylene and an α-olefin having 4 to 8 carbon atoms, a substantially linear molecular chain, a density of 910 kg / m 3 or more and less than 930 kg / m 3 . Among them, the polyethylene (B) is preferably a low-density polyethylene. The melting point Tm B of the polyethylene (B) is preferably 100°C or higher and 120°C or lower, and more preferably 102°C or higher and 115°C or lower. The melting point Tm B of the polyethylene (B) is within the above range, the conductive layer can be stably laminated and bonded to the polyethylene-based resin foamed layer even in the case where the multilayer foamed sheet is manufactured by co-extrusion. Further, the measurement method of the melting point Tm B of the polyethylene (B) is the same as the measurement method of the melting point Tm A of the polyethylene-based resin (A) described above.

[0063] The melt flow rate of the polyethylene (B) at a temperature of 190°C under a load of 2.16 kg is preferably 5 g / 10 minutes or more and 80 g / 10 minutes or less, more preferably 10 g / 10 minutes or more and 65 g / 10 minutes or less, and further preferably 12 g / 10 minutes or more and 50 g / 10 minutes or less. By making the melt flow rate of the polyethylene (B) be within the above specific range, the adhesion between the conductive layer and the polyethylene-based resin foamed layer can be further improved. In this case, the conductivity of the multilayer foamed sheet can be more stably exhibited.

[0064] <ethylene-based copolymer (C)>

[0065] The ethylene-based copolymer (C) 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-based copolymer (C) can 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 another monomer other than these monomers. The amount of the structural unit derived from the above another monomer contained in the ethylene-based copolymer (C) is preferably 10% by mass or less, more preferably 5% by mass or less, further preferably 3% by mass or less, and most preferably 0% by mass, that is, the ethylene-based copolymer (C) is a copolymer of ethylene and a monomer having a polar group.

[0066] The above multilayer foamed sheet has the conductivity by setting the base resin of the conductive layer to a mixed resin of the polyethylene (B) and the ethylene-based copolymer (C), and can reduce the detachment of the conductive carbon from the multilayer foamed sheet. The reason is not necessarily clear at present, but is considered as follows.

[0067] Generally, in the case where the conductive carbon is dispersed in a thermoplastic resin such as a polyethylene-based resin, adjacent conductive carbon particles exist in close proximity to each other at a distance or less, thereby forming a conductive network formed of the conductive carbon particles and exhibiting the conductivity.

[0068] In the above multilayer foamed sheet, since the above polyethylene (B) and the above ethylene-based copolymer (C) are incompatible with each other, a phase mainly composed of the polyethylene (B) and a phase mainly composed of the ethylene-based copolymer (C) are formed in the conductive layer. It is considered that, if such a morphology is formed in the conductive layer, the conductive carbon is biased to either one of the phase composed of the polyethylene (B) and the phase composed of the ethylene-based copolymer (C). Also, by biasing the conductive carbon to either one of the phases, it is easy to form a conductive network formed of the conductive carbon particles. As a result, it is considered that the conductivity is easily exhibited even if the amount of the conductive carbon to be compounded is small.

[0069] Further, in the conductive layer, if the electrically conductive carbon is biased to either of the phase composed of the polyethylene (B) and the phase composed of the ethylene-based copolymer (C), the amount of the electrically conductive carbon exposed on the surface of the conductive layer can be reduced. Thus, it is considered that the electrically conductive carbon can be inhibited from peeling off from the multilayer foamed sheet. As a result, the electrically conductive carbon peeled off from the multilayer foamed sheet can be inhibited from being transferred to the packaged article to contaminate the packaged article.

[0070] As the ethylene-based copolymer (C), for example, 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), ethylene-butyl acrylate copolymer (i.e., EBA), and the like can be exemplified. From the viewpoint of further improving the electric conductivity of the multilayer foamed sheet and further reducing the peeling of the electrically conductive carbon, it is preferable that the conductive layer contain one or two or more ethylene-based copolymers (C) selected from the group consisting of ethylene-vinyl acetate copolymer and ethylene-methyl methacrylate copolymer, and it is more preferable that the conductive layer contain ethylene-vinyl acetate copolymer.

[0071] The content of the structural unit derived from the monomer having a polar group in the ethylene-based copolymer (C) is preferably 30% by mass or more and 50% by mass or less. By making the content of the structural unit derived from the monomer having a polar group in the ethylene-based copolymer (C) 30% by mass or more, the electric conductivity of the multilayer foamed sheet can be further improved, and the peeling of the electrically conductive carbon from the multilayer foamed sheet can be more effectively inhibited. From the viewpoint of further improving the above-mentioned effects, the content of the structural unit derived from the monomer having a polar group in the ethylene-based copolymer (C) is more preferably more than 30% by mass, further preferably 35% by mass or more, particularly preferably 40% by mass or more, and most preferably more than 40% by mass.

[0072] Further, by making the content of the structural unit derived from the monomer having a polar group in the ethylene-based copolymer (C) 50% by mass or less, the handleability of the multilayer foamed sheet can be further improved, and the manufacturing stability at the time of laminating the conductive layer to the polyethylene-based resin foamed layer can be further improved. From the viewpoint of further improving the above-mentioned effects, the content of the structural unit derived from the monomer having a polar group in the ethylene-based copolymer (C) is more preferably 48% by mass or less, and further preferably 45% by mass or less.

[0073] The melting point Tm of the ethylene-based copolymer (C) C30°C or higher and 80°C or lower, more preferably 32°C or higher and 75°C or lower, and further preferably 35°C or higher and 70°C or lower. By making the melting point Tm C of the ethylene-based copolymer (C) within the above specific range, the electrical conductivity of the multilayer foamed sheet can be further improved, and the manufacturing stability at the time of laminating the conductive layer to the polyethylene-based resin foamed layer can be further improved. Further, the melting point Tm C of the ethylene-based copolymer (C) is measured by the same method as the melting point Tm A of the polyethylene-based resin (A) described above.

[0074] The melting point Tm B of the polyethylene (B) and the melting point Tm C of the ethylene-based copolymer (C) are different from each other by Tm B -Tm C is 30°C or higher and 80°C or lower. By using the melting point difference Tm B -Tm C of the polyethylene (B) and the ethylene-based copolymer (C), the electrical conductivity can be imparted to the multilayer foamed sheet.

[0075] In the case where the melting point difference Tm B -Tm C is less than 30°C, there is a risk that the electrical conductivity cannot be sufficiently imparted to the multilayer foamed sheet. In addition, in the case where the melting point difference Tm B -Tm C exceeds 80°C, the ethylene-based copolymer (C) easily adheres to a sizing device or the like during the manufacturing process of the multilayer foamed sheet, and there is a risk that a good multilayer foamed sheet cannot be obtained. In addition, in this case, there is a risk that the operability is impaired due to the softening of the ethylene-based copolymer (C) during use, or the fusion of the multilayer foamed sheets to each other during the storage of the multilayer foamed sheets in a stacked state.

[0076] The melt flow rate of the ethylene-based copolymer (C) at a temperature of 190°C and a load of 2.16 kg is preferably 10 g / 10 minutes or higher and 120 g / 10 minutes or lower, and more preferably 20 g / 10 minutes or higher and 80 g / 10 minutes or lower. In this case, the electrical conductivity of the multilayer foamed sheet can be further improved. In addition, by making the melt flow rate of the ethylene-based copolymer (C) within the above specific range, the adhesion strength between the conductive layer and the layer adjacent to the conductive layer can be further improved. For example, in the case where the conductive layer and the polyethylene-based resin foamed layer are laminated and adhered to each other, the adhesion strength between the two can be further improved.

[0077] <Blending ratio>

[0078] The mixing ratio of the polyethylene (B) to the ethylene-based copolymer (C) in the conductive layer is preferably 80:20 to 20:80 by mass of the polyethylene (B) : ethylene-based copolymer (C), more preferably 50:50 to 25:75, and even more preferably 45:55 to 30:70. In this case, the conductivity of the multilayer foamed sheet can be further improved.

[0079] <Conductive carbon>

[0080] The conductive carbon is a substance mainly composed of carbon atoms and having conductivity. As the conductive carbon, specifically, conductive carbon black such as furnace black, acetylene black, thermal-cracking carbon black, and Ketjen black is preferably exemplified. Two or more kinds of conductive carbon can be contained in the conductive layer. From the viewpoint of further reducing the mixing amount of the conductive carbon while ensuring the conductivity of the multilayer foamed sheet, it is preferable to contain high-conductivity carbon black such as Ketjen black as the conductive carbon in the conductive layer.

[0081] The dibutyl phthalate (DBP) oil absorption amount of the above-mentioned conductive carbon is preferably 150 mL / 100 g or more and 700 mL / 100 g or less. In this case, the conductivity of the multilayer foamed sheet can be further improved. From the viewpoint of further improving the conductivity of the multilayer foamed sheet, the DBP oil absorption amount 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. Furthermore, the above-mentioned dibutyl phthalate (DBP) oil absorption amount is a value measured in accordance with ASTM D2414-79.

[0082] In addition, the BET specific surface area of the above-mentioned conductive carbon is preferably 600 m 2 / g or more and 2000 m 2 / g or less. In this case, the conductivity of the multilayer foamed sheet can be further improved. From the viewpoint of further improving the conductivity of the multilayer foamed sheet, the BET specific surface area of the conductive carbon is more preferably 700 m 2 / g or more and 1600 m 2 / g or less. In the multilayer foamed sheet of the present application, the conductive layer containing the above-mentioned specific polyethylene (B), ethylene-based copolymer (C), and conductive carbon is formed as a layer different from the foamed layer, and thus a high-conductivity carbon having a large specific surface area can be mixed without hindering the foamability of the polyethylene-based resin foamed layer.

[0083] The content of the electrically conductive carbon in the electrically conductive layer is 3% by mass or more and 15% by mass or less. By making the content of the electrically conductive carbon 3% by mass or more, the electrically conductive layer can be imparted with electric conductivity. From the viewpoint of further improving the electric conductivity of the multilayer foamed sheet, the content of the electrically conductive carbon in the electrically conductive layer is preferably 4% by mass or more, and more preferably 5% by mass or more. In the case where the content of the electrically conductive carbon in the electrically conductive layer is too small, it is difficult to form an electrically conductive network formed of the electrically conductive carbon particles within the electrically conductive layer, and there is a risk of causing a decrease in the electric conductivity of the multilayer foamed sheet. Furthermore, the content of the electrically conductive carbon in the electrically conductive layer is approximately equal to the content of the electrically conductive carbon in the multilayer foamed sheet.

[0084] In addition, by making the content of the electrically conductive carbon 15% by mass or less, the electrically conductive carbon can be reduced from the multilayer foamed sheet. From the viewpoint of further reducing the electrically conductive carbon from the multilayer foamed sheet, the content of the electrically conductive carbon is preferably 12% by mass or less, more preferably 10% by mass or less, and further preferably less than 10% by mass. In the case where the content of the electrically conductive carbon in the electrically conductive layer is too large, there is a risk that the electrically conductive carbon easily falls off from the multilayer foamed sheet, and causes contamination around the multilayer foamed sheet.

[0085] <Other polymers>

[0086] Within a range that does not impair the above-described effects, the electrically conductive layer can contain other polymers than the polyethylene (B) and the ethylene-based copolymer (C). As the other polymers than the polyethylene (B) and the ethylene-based copolymer (C), for example, thermoplastic resins such as polyethylene-based resins other than the polyethylene (B) and polystyrene-based resins, elastomers such as ethylene-propylene rubber and styrene-butadiene-styrene block copolymer, and the like can be exemplified. From the viewpoint of manufacturing stability when the electrically conductive layer is stacked on the polyethylene-based foamed layer, it is preferable that the electrically conductive layer does not contain a polymer having a higher melting point than the polyethylene-based resin (A) contained in the polyethylene-based foamed layer. The content of the other polymers than the polyethylene (B) and the ethylene-based copolymer (C) in the electrically conductive layer is preferably 20% by mass or less, more preferably 10% by mass or less, further preferably 5% by mass or less, and particularly preferably 3% by mass or less.

[0087] <Additives>

[0088] The electrically conductive layer can contain an antioxidant, a heat stabilizer, a weather resistant agent, an ultraviolet absorber, a flame retardant, a filler material, an antibacterial agent, and the like. The content of the additive in the electrically conductive layer is, for example, preferably 10 parts by mass or less, more preferably 5 parts by mass or less, and further preferably 3 parts by mass or less, with respect to 100 parts by mass of the total of the polyethylene (B) and the ethylene-based copolymer (C).

[0089] <Average thickness>

[0090] 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, and further preferably 5 μm or more, the conductivity of the multilayer foamed sheet can be more stably exhibited. In addition, by making the average thickness of the conductive layer 20 μm or less, more preferably 18 μm or less, further preferably 15 μm or less, and particularly preferably 10 μm or less, the peeling of the conductive carbon from the multilayer foamed sheet can be more effectively suppressed.

[0091] The average thickness of the conductive layer is measured as follows. First, the multilayer foamed sheet is cut along a plane perpendicular to the extrusion direction. In the cut surface, ten measurement positions are set in such a manner that the length of the cut surface in the direction perpendicular to both the extrusion direction and the thickness direction is equally divided into eleven. The cross section of the multilayer foamed sheet at the measurement positions is observed using a microscope, and the thickness of the conductive layer at each measurement position is measured. The arithmetic mean of the thicknesses is taken as the average thickness of the conductive layer (unit: μm).

[0092] <Basic Weight>

[0093] The basic weight of the conductive layer is preferably 1 g / m 2 or more and 20 g / m 2 or less. By making the basic weight of the conductive layer 1 g / m 2 or more, more preferably 5 g / m 2 or more, and further preferably 7 g / m 2 or more, the conductivity of the multilayer foamed sheet can be more stably exhibited. In addition, by making the basic weight of the conductive layer 20 g / m 2 or less, more preferably 18 g / m 2 or less, further preferably 15 g / m 2 or less, and particularly preferably 10 g / m 2 or less, the peeling of the conductive carbon from the multilayer foamed sheet can be more effectively suppressed. Furthermore, in the case where the conductive layer is laminated on both surfaces of the polyethylene-based resin foamed layer, the basic weight of the conductive layer refers to the basic weight of each surface.

[0094] The basic weight of the conductive layer of each surface is measured as follows. First, the average thickness of the conductive layer is calculated by the above-described method. After the unit of the average thickness is converted, the density of the conductive layer (unit: g / m 3 ) is multiplied, whereby the basic weight of the conductive layer (unit: g / m 2 ) can be obtained. Furthermore, the density of the conductive layer is the density including the conductive carbon, other additives, and the like contained in the conductive layer.

[0095] In the case of manufacturing the multilayer foamed sheet by co-extrusion, the basis weight of the conductive layer of each side can be calculated by the following (1) using the discharge amount X [g / hour] of the conductive layer of each side, the width W [m] of the multilayer foamed sheet, and the pulling speed L [m / hour] of the multilayer foamed sheet.

[0096] The basis weight [g / m 2 ] of the conductive layer = (X / (L x W))... (1)

[0097] In the case of manufacturing the multilayer foamed sheet by co-extruding the melt for forming the conductive layer and the melt for forming the foamed layer, a conductive layer having a small basis weight and a thin thickness that cannot be formed by heat lamination or the like can be formed, and the electrical conductivity can be stably exhibited.

[0098] (Method for manufacturing multilayer foamed sheet)

[0099] The above multilayer foamed sheet can be manufactured, for example, by a co-extrusion foaming method. That is, in a method for manufacturing a polyethylene-based resin multilayer foamed sheet, a polyethylene-based resin multilayer foamed sheet having a polyethylene-based resin foamed layer and a conductive layer laminated to at least one side of the polyethylene-based resin foamed layer is manufactured by co-extruding a melt for forming the polyethylene-based resin foamed layer and a melt for forming the conductive layer. The melt for forming the foamed layer contains a polyethylene-based resin (A) and a physical foaming agent. The melt for forming the conductive layer contains a polyethylene (B), an ethylene-based copolymer (C) having a structural unit derived from ethylene and a structural unit derived from a monomer having a polar group, and a conductive carbon. The content of the conductive carbon in the melt for forming the conductive layer is 3% by mass or more and 15% by mass or less. Furthermore, the melting point Tm B of the polyethylene (B) is 30°C or more and 80°C or less from the melting point Tm C of the ethylene-based copolymer (C). B -Tm C .

[0100] In implementing the method, a publicly known co-extrusion device used in the field of extrusion foaming can be used. More specifically, for example, the above multilayer foamed sheet can be manufactured using a co-extrusion device having a foamed layer forming extruder configured to be able to extrude the melt for forming the foamed layer, a conductive layer forming extruder configured to be able to extrude the melt for forming the conductive layer, and a co-extrusion die connected to the discharge ports of the above extruders.

[0101] [Melt for forming foamed layer]

[0102] The melt for forming a foamed layer contains at least the polyethylene-based resin (A) and a physical blowing agent. The melt for forming a foamed layer can be produced, for example, by the following method. First, the polyethylene-based resin (A) and additives added as necessary are supplied to an extruder for forming a foamed layer, and melt-kneaded. Next, the physical blowing agent is supplied to the melt containing the molten polyethylene-based resin (A) in the extruder while being pressurized, and further kneaded, whereby the melt for forming a foamed layer can be obtained.

[0103] As the physical blowing agent, an organic physical blowing agent, an inorganic physical blowing agent, or the like can be used. As the organic physical blowing agent, for example, aliphatic hydrocarbons such as propane, n-butane, isobutane, n-pentane, isopentane, n-hexane, isohexane, alicyclic hydrocarbons such as cyclopentane, cyclohexane, chlorinated hydrocarbons such as chloromethane, chloroethane, fluorinated hydrocarbons such as 1,1,1,2-tetrafluoroethane, 1,1-difluoroethane, and the like can be exemplified. As the inorganic physical blowing agent, for example, nitrogen, carbon dioxide, air, water, and the like can be exemplified. In the melt for forming a foamed layer, one kind of physical blowing agent can be contained, or two or more kinds of physical blowing agents can be contained.

[0104] From the viewpoints of compatibility with the polyethylene-based resin (A) and foaming property, in the melt for forming a foamed layer, as the physical blowing agent, an organic physical blowing agent is preferably contained, and an organic physical blowing agent in which n-butane, isobutane, or a mixture thereof is a main component is more preferably contained.

[0105] The blending amount of the physical blowing agent can be appropriately set depending on the kind of the blowing agent, the desired apparent density, and the like. For example, in the case where a mixed butane composed of isobutane 30% by mass and n-butane 70% by mass is used as the physical blowing agent, 3 parts by mass or more and 30 parts by mass or less, preferably 4 parts by mass or more and 20 parts by mass or less, and more preferably 10 parts by mass or more and 20 parts by mass or less of the mixed butane can be added with respect to 100 parts by mass of the polyethylene-based resin (A).

[0106] It is preferable to add a bubble adjusting agent to the melt for forming the foamed layer. As the bubble adjusting agent, an inorganic bubble adjusting agent, an organic bubble adjusting agent can be used. As the inorganic bubble adjusting agent, metal borate such as zinc borate, magnesium borate, borax, sodium chloride, aluminum hydroxide, talc, zeolite, silica, calcium carbonate, sodium bicarbonate, etc. can be listed. As the organic bubble adjusting agent, sodium phosphate-2,2-methylenebis(4,6-tert-butylphenyl), sodium benzoate, aluminum benzoate, sodium stearate, etc. can be listed. Further, a mixture of citric acid and sodium bicarbonate, a mixture of an alkali salt of citric acid and sodium bicarbonate, etc. can also be used as the bubble adjusting agent. In the melt for forming the foamed layer, one bubble adjusting agent can be contained, or two or more kinds of bubble adjusting agents can be contained. The amount of the bubble adjusting agent to be added to the melt for forming the foamed layer is appropriately set according to the kind of the physical foaming agent, the desired apparent density, the bubble diameter, etc.

[0107] [Conductive layer forming melt]

[0108] The melt for forming the conductive layer contains at least the polyethylene (B), the ethylene-based copolymer (C), and the electrically conductive carbon. When the melt for forming the conductive layer is produced, for example, the above-described polyethylene (B), the ethylene-based copolymer (C), the electrically conductive carbon, and an additive added as necessary are supplied to an extruder for forming the conductive layer. Then, by melt-kneading them in the extruder, the melt for forming the conductive layer can be obtained.

[0109] In the melt for forming the conductive layer, a volatile plasticizer can be contained as an additive. The volatile plasticizer has an effect of reducing the melt viscosity of the melt for forming the conductive layer, and is configured to be volatilized from the conductive layer after co-extrusion. The volatile plasticizer enables the extrusion temperature of the melt for forming the conductive layer to be close to the extrusion temperature of the melt for forming the foamed layer at the time of co-extrusion. In addition, the volatile plasticizer can increase the melt elongation of the conductive layer in a softened state. As a result, by adding the volatile plasticizer to the melt for forming the conductive layer, the bubbles of the polyethylene-based resin foamed layer are less likely to be broken by the heat of the conductive layer during the foaming of the melt for forming the foamed layer, and further, the conductive layer is easily elongated following the expansion of the polyethylene-based resin foamed layer during the foaming.

[0110] As the volatile plasticizer, for example, an aliphatic hydrocarbon having 3 or more and 7 or less carbon atoms, an alicyclic hydrocarbon having 3 or more and 7 or less carbon atoms, an aliphatic alcohol having 1 or more and 4 or less carbon atoms, an aliphatic ether having 2 or more and 8 or less carbon atoms, etc. can be listed. In the melt for forming the conductive layer, one volatile plasticizer can be contained, or two or more kinds of volatile plasticizers can be contained.

[0111] The volatile plasticizer preferably has a boiling point of 120°C or lower, more preferably 80°C or lower. A volatile plasticizer having a boiling point in this range is naturally dispersed from the conductive layer after co-extrusion and removed from the conductive layer. In addition, the lower limit of the boiling point of the volatile plasticizer is approximately -50°C.

[0112] The blending amount of the volatile plasticizer can be appropriately set in accordance with the composition of the conductive layer and the polyethylene-based resin foamed layer, and the like. For example, the blending amount of the volatile plasticizer can be set to 5 parts by mass or more and 50 parts by mass or less with respect to 100 parts by mass of the total of the polyethylene (B) and the ethylene-based copolymer (C). From the viewpoint of further improving the followability of the melt for forming the conductive layer and reducing the deviation in the thickness of the conductive layer, the blending amount of the volatile plasticizer is preferably 5 parts by mass or more, more preferably 7 parts by mass or more, and further preferably 10 parts by mass or more with respect to 100 parts by mass of the total of the polyethylene (B) and the ethylene-based copolymer (C).

[0113] On the other hand, from the viewpoint of stably laminating the conductive layer, the blending amount of the volatile plasticizer is preferably set to 50 parts by mass or less, more preferably 45 parts by mass or less, and further preferably 40 parts by mass or less with respect to 100 parts by mass of the total of the polyethylene (B) and the ethylene-based copolymer (C).

[0114] [Co-extrusion]

[0115] When co-extrusion is performed, the melt for forming the foamed layer and the melt for forming the conductive layer formed in each extruder as described above are introduced into a co-extrusion die, and are extruded in a layered manner from the extrusion port of the co-extrusion die. The co-extrusion die can be, for example, a flat die having a linear extrusion port. In this case, the laminate of the melt for forming the foamed layer and the melt for forming the conductive layer is extruded in a sheet shape from the extrusion port of the flat die. When the laminate is extruded from the extrusion port into the atmosphere, the melt for forming the foamed layer expands while foaming. Along with this, the melt for forming the conductive layer is stretched. Then, the sheet-shaped laminate foam extruded from the extrusion port is drawn while being cooled along a tentering device, whereby the melt for forming the foamed layer and the melt for forming the conductive layer are solidified. In this way, the bubble structure formed by foaming is fixed, and the size is stabilized. By the above operation, a multilayer foamed sheet can be obtained.

[0116] Further, the co-extrusion die can also be a ring die having a ring-shaped extrusion opening. In this case, the laminate of the foaming layer-forming melt and the conductive layer-forming melt is extruded from the extrusion opening of the ring die into a cylindrical shape. When the laminate is extruded from the extrusion opening into the atmosphere, the foaming layer-forming melt expands while foaming. Along with this, the conductive layer-forming melt is stretched. Then, the cylindrical laminate foam extruded from the extrusion opening is expanded by compressed air or the like from the inside, and is cooled while being pulled from the inside along a mandrel or the like expansion device, whereby the foaming layer-forming melt and the conductive layer-forming melt are solidified. Thus, the bubble structure formed by foaming is fixed, and the dimensional stability is achieved. Finally, by cutting the cylindrical laminate foam on the expansion device, a multilayer foamed sheet can be obtained. In the case where the ring die is used as the co-extrusion die, it is easy to manufacture a multilayer foamed sheet having a width of, for example, 1000 mm or more. Further, it is easy to manufacture a foamed sheet having a thickness of, for example, 3 mm or less.

[0117] In the past, in a polyethylene-based resin multilayer foamed sheet produced by co-extrusion, when it is intended to achieve a surface resistivity of, for example, 1 x 10 7 Ω or less by providing a conductive layer containing conductive carbon, it is necessary to incorporate a relatively large amount of conductive carbon in the conductive layer. The reason for this is considered as follows.

[0118] As described above, in the case where a multilayer foamed sheet is intended to be produced by co-extrusion, the foaming layer-forming melt extruded from the co-extrusion die rapidly expands by foaming, and thus the conductive layer-forming melt is strongly stretched in pursuit of the expansion of the foaming layer-forming melt. At this time, the conductive carbon particles in the conductive layer are separated from each other, and thus if the amount of the conductive carbon incorporated is insufficient, it is easy to fail to maintain the conductive network.

[0119] Further, since the laminate foam extruded from the co-extrusion die is rapidly cooled, the base resin of the conductive layer is easily solidified in a state where the conductive carbon particles are separated from each other by stretching. Further, the extrusion foaming temperature of the foaming 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 (A) constituting the polyethylene-based resin foaming layer. Correspondingly, under the extrusion temperature condition of the conductive layer-forming melt set to a relatively low temperature, the base resin of the conductive layer is more easily solidified. For these reasons, it is considered that in the past multilayer foamed sheet produced by co-extrusion, it is difficult to stably form a conductive network in the conductive layer.

[0120] On the contrary, in the multilayer foamed sheet described above, as described above, the mutually incompatible two kinds of resins of the polyethylene (B) and the ethylene-based copolymer (C) having the structural unit derived from ethylene and the structural unit derived from the monomer having a polar group are contained in the conductive layer. Therefore, it is considered that in the multilayer foamed sheet described above, the electrically conductive carbon is present in either phase. It is considered as a result thereof that the electrically conductive network formed of the electrically conductive carbon particles is easily formed, and the electrically conductive network is easily maintained.

[0121] Further, it is considered that since the ethylene-based copolymer (C) having a melting point difference Tm B -Tm C the resin having a melting point in the above-described specific range, extension is able to be eased during the period until the conductive layer is solidified in the cooling process after the co-extrusion. It is considered as a result thereof that the electrically conductive network is easily restructured.

[0122] Thus, according to the manufacturing method described above, even if the blending amount of the electrically conductive carbon is small, it is possible to manufacture the multilayer foamed sheet having electric conductivity by the co-extrusion. In addition, according to the manufacturing method described above, it is possible to easily obtain the multilayer foamed sheet which is thin in thickness, excellent in cushioning property, and further low in contamination property to the packaged article.

[0123] Examples

[0124] Examples of the multilayer foamed sheet and the manufacturing method thereof described above will be described below. Furthermore, the specific modes of the multilayer foamed sheet and the manufacturing method thereof according to the present application are not limited to the modes of the examples shown below, and the constitution can be appropriately changed within the scope not impairing the gist of the present application.

[0125] First, the resins and the electrically conductive carbon used in the present example are shown in Table 1 and Table 2.

[0126] Table 1

[0127]

[0128] The mass fraction (unit: mass %) of the structural unit derived from the monomer having a polar group in the ethylene-based copolymer is described in the column of "Modification rate" in Table 1. Furthermore, the structural unit derived from the monomer having a polar group is not contained in the resins other than the ethylene-based copolymer, and therefore the notation "-" is described in the column of "Modification rate" in Table 1 for these resins. In addition, the resin notation "mLLDPE" means a linear low-density polyethylene polymerized using a metallocene catalyst.

[0129] Further, the melting point of each resin in Table 1 is a value determined by the above-described method, that is, the transition temperature measurement method for plastics prescribed in JIS K7121:2012. The melt flow rate determined based on the method prescribed in JIS K7210-1 (2014) under the conditions of a temperature of 190°C and a load of 2.16 kg is described in the column of "MFR" in Table 1.

[0130] Table 2

[0131]

[0132] The physical properties of the conductive carbon are shown in Table 2. In Table 2, the void fraction is a value obtained by dividing the bulk density of the conductive carbon by the true density of the conductive carbon. The primary particle diameter is a value obtained based on observation by a transmission electron microscope. The DBP oil absorption is a value determined in accordance with ASTM D 2414-79. The BET specific surface area is a value obtained in accordance with ASTM D 2414.

[0133] Further, CB1 in Table 2 specifically is "KETEN BLACK EC300J" manufactured by Lion Specialty Chemicals Co., Ltd.

[0134] (Examples 1 to 8)

[0135] The multilayer foamed sheet of Examples 1 to 8 is a non-crosslinked multilayer foamed sheet having a three-layer structure composed of a polyethylene-based resin foamed layer containing a polyethylene-based resin (A) and a conductive layer laminated on both faces of the polyethylene-based resin foamed layer. The conductive layer contains the polyethylene (B), the ethylene-based copolymer (C), and the conductive carbon in the kinds and amounts shown in Table 3.

[0136] The production method of the multilayer foamed sheet of Examples 1 to 8 is specifically described as follows. First, a co-extrusion device provided with an extruder for forming a foamed layer, an extruder for forming a conductive layer, and a co-extrusion die connected to the discharge port of the above-described extruders is prepared. The co-extrusion die in this example is a ring die having a ring-shaped discharge port.

[0137] In producing the melt for forming a foamed layer, the polyethylene-based resin (A) in the kind shown in Table 3 and a bubble adjusting agent at 1 part by mass with respect to 100 parts by mass of the polyethylene-based resin (A) are supplied to the extruder for forming a foamed layer, and they are melt-kneaded in the extruder. Further, as the bubble adjusting agent, a mixture of citric acid and sodium bicarbonate ("FINECELL MASTER PO217K" manufactured by Ouchi Shinko Kaisha, Ltd.) is used. "FINECELL MASTER" is a registered trademark of Ouchi Shinko Kaisha, Ltd.

[0138] The mixture of the polyethylene-based resin (A) and the bubble adjuster melted in the extruder was supplied with a physical blowing agent while being pressurized, and further kneading was performed in the extruder. By the above operation, a melt for forming a foamed layer having an extrusion temperature shown in Table 3 was obtained. Further, as the physical blowing agent, mixed butane composed of n-butane 65 mass% and isobutane 35 mass% was used. The amount of the physical blowing agent was 8 parts by mass with respect to 100 parts by mass of the polyethylene-based resin (A).

[0139] In the production of the melt for forming a conductive layer, the polyethylene (B), the ethylene-based copolymer (C), and the conductive carbon shown in Table 3 and in the amounts were supplied to the extruder for forming a conductive layer, and 25 parts by mass of the volatile plasticizer with respect to 100 parts by mass of the total of the polyethylene (B) and the ethylene-based copolymer (C) was supplied. Then, they were kneaded in the extruder, whereby a melt for forming a conductive layer having an extrusion temperature shown in Table 3 was obtained. Further, as the volatile plasticizer, mixed butane composed of n-butane 65 mass% and isobutane 35 mass% was used.

[0140] The melts thus produced in the respective extruders were supplied to a co-extrusion die while maintaining the extrusion temperatures shown in Table 3, and they were merged in the co-extrusion die to laminate the melt for forming a conductive layer on both surfaces of the melt for forming a foamed layer. Then, these melts were co-extruded from the extrusion port of the co-extrusion die at the discharge amounts shown in Table 3 to be foamed, whereby a laminated foamed body of a cylindrical shape in which a conductive layer was laminated on both surfaces of a polyethylene-based resin foamed layer was produced. Further, the discharge amount of the conductive layer shown in Table 3 was the discharge amount per surface. The laminated foamed body was drawn along a mandrel having a diameter of 360 mm at the drawing speed shown in Table 3 while being cut, whereby the multilayer foamed sheet of Example 1 to Example 8 was obtained. The width, the overall thickness, the basis weight, and the apparent density of the multilayer foamed sheet of Example 1 to Example 8 were the values shown in Table 3. In addition, the conductive layer of each of these multilayer foamed sheets was in a non-foamed state.

[0141] (Comparative Example 1)

[0142] The multilayer foamed sheet of Comparative Example 1 did not contain the ethylene-based copolymer (C) in the conductive layer and was the same as the multilayer foamed sheet of Example 1 except for this. In the production method of the multilayer foamed sheet of Comparative Example 1, the ethylene-based copolymer (C) was not compounded in the melt for forming a conductive layer, and the manufacturing conditions and the like were changed as shown in Table 4, and the production method of the multilayer foamed sheet of Example 1 was the same except for this.

[0143] (Comparative Example 2, Comparative Example 4)

[0144] In the multilayer foamed sheet of Comparative Example 2 and the multilayer foamed sheet of Comparative Example 4, the polyethylene (B) in the conductive layer was changed to a polyethylene having a melting point difference Tm B -Tm C The multilayer foamed sheet of Comparative Example 3 had the same configuration as the multilayer foamed sheet of Example 1, except that the composition of the melt for forming the conductive layer and the manufacturing conditions were changed as shown in Table 4. Other than this, the method for producing the multilayer foamed sheet of Comparative Example 3 was the same as the method for producing the multilayer foamed sheet of Example 1.

[0145] (Comparative Example 3)

[0146] The multilayer foamed sheet of Comparative Example 3 had the same configuration as the multilayer foamed sheet of Example 1, except that the composition of the melt for forming the conductive layer and the manufacturing conditions were changed as shown in Table 4. Other than this, the method for producing the multilayer foamed sheet of Comparative Example 3 was the same as the method for producing the multilayer foamed sheet of Example 1.

[0147] (Comparative Example 5)

[0148] The multilayer foamed sheet of Comparative Example 5 had the same configuration as the multilayer foamed sheet of Comparative Example 1, except that the amount of the electrically conductive carbon in the conductive layer was increased. In the method for producing the multilayer foamed sheet of Comparative Example 5, the amount of the electrically conductive carbon in the melt for forming the conductive layer and the manufacturing conditions were changed to the values shown in Table 4. Other than this, the method for producing the multilayer foamed sheet of Comparative Example 5 was the same as the method for producing the multilayer foamed sheet of Comparative Example 1.

[0149] (Evaluation)

[0150] The melt viscosity of the conductive layer, the overall thickness and the basis weight of the multilayer foamed sheet, the average thickness and the basis weight of the conductive layer of the multilayer foamed sheet, the apparent density, the foaming ratio, the contamination property, and the electrical conductivity of the multilayer foamed sheets of the examples and the comparative examples were evaluated by the following methods.

[0151] [Melt viscosity of the conductive layer]

[0152] The melt viscosities of the melts for forming the conductive layers of Examples 1 to 8 and Comparative Examples 1 to 5 were measured using a capillary rheometer ("Rheovis 2100" manufactured by CEAST). In addition, the orifice diameter of the capillary rheometer was 1 mm, the orifice length was 10 mm, the measurement temperature was 190°C, and the shear rate was 100 sec -1 The melt viscosities of the melts for forming the conductive layers of the examples and the comparative examples are shown in Table 3 or Table 4.

[0153] [Overall thickness of the multilayer foamed sheet]

[0154] First, the multilayer foamed sheet was cut along a face perpendicular to the extrusion direction. In the cut surface, ten measurement positions were set at equal intervals in the length direction of the cut surface (i.e., the direction at right angles to both the extrusion direction and the thickness direction). The thickness of each measurement position was measured using a microscope. The arithmetic mean of the thickness was taken as the overall thickness of the multilayer foamed sheet and is shown in Tables 3 and 4.

[0155] [Basic Weight of Multilayer Foamed Sheet]

[0156] A test piece of square shape with one side of 25 mm was cut from the multilayer foamed sheet, and the mass (unit: g) of the test piece was measured. The basic weight of the multilayer foamed sheet, i.e., the mass per 1 m 2 The basic weight of the multilayer foamed sheet (unit: g / m 2 ) was calculated by unit conversion of the mass of the test piece. The basic weight of the multilayer foamed sheet of the examples and comparative examples is shown in Tables 3 or 4.

[0157] [Average Thickness of Conductive Layer]

[0158] The multilayer foamed sheet was cut along a face perpendicular to the extrusion direction. In the cut surface, ten measurement positions were set at equal intervals in the length direction of the cut surface (i.e., the direction at right angles to both the extrusion direction and the thickness direction). The cross section of the multilayer foamed sheet at the measurement positions was observed using a microscope, and the thickness of the conductive layer at each measurement position was measured. The arithmetic mean of the thickness was taken as the average thickness of the conductive layer (unit: pm). The average thickness of the conductive layer in the multilayer foamed sheet of the examples and comparative examples is shown in Tables 3 or 4.

[0159] [Basic Weight of Conductive Layer]

[0160] The basic weight of the conductive layer of each face was calculated based on the following (1) formula using the discharge amount X [g / hour] of the conductive layer of each face at the time of co-extrusion of the multilayer foamed sheet, the width W [m] of the multilayer foamed sheet, and the pulling speed L [m / hour] of the multilayer foamed sheet.

[0161] Basic weight of conductive layer [g / m 2 ] = (X / (L x W))...(1)

[0162] [Apparent Density of Multilayer Foamed Sheet]

[0163] The basic weight of the multilayer foamed sheet (unit: g / m 2 ) obtained by the above method was divided by the overall thickness of the multilayer foamed sheet, and unit conversion was performed, whereby the apparent density of the multilayer foamed sheet (unit: kg / m 3 ) was calculated.

[0164] [Pollution]

[0165] After overlapping the multilayer foamed sheets of the examples and comparative examples with cleaning paper, at a density of 150 g / cm³, 2 The load presses the multi-layer foam sheet against the cleaning paper. In this state, the multi-layer foam sheet is moved back and forth relative to the cleaning paper, causing it to slide against the paper. Furthermore, the amplitude of the reciprocating movement is 5 mm, and the period is 1 second.

[0166] At a time point 100 seconds after the start of the sliding motion, the multilayer foam sheet was removed from the cleaning paper. Then, the hue of the cleaning paper was measured using a spectrophotometer (SE-2000, manufactured by Nippon Denshoku Kogyo Co., Ltd.), obtaining the L* value in the CIE 1976 L*a*b* color space for the portion that slid with the multilayer foam sheet. Then, based on the previously measured L* value of the cleaning paper before sliding with the multilayer foam sheet... * Value and L of the cleaning paper after sliding * The difference in values ​​is used to evaluate the contamination potential of multilayer foam sheets.

[0167] The ΔL* value is recorded in the "Staining Potential" column of Tables 3 and 4. This ΔL* value is obtained by subtracting the L* value of the cleaning paper after sliding from the L* value of the cleaning paper before sliding. The L* value represents lightness; a larger value means a brighter hue. When the multilayer foam sheet slides against the cleaning paper, conductive carbon that detaches from the multilayer foam sheet adheres to the cleaning paper. Moreover, if the amount of conductive carbon adhering to the cleaning paper increases, the hue of the cleaning paper darkens. Therefore, the smaller the ΔL* value shown in the "Staining Potential" column of Tables 3 and 4 for the multilayer foam sheet, the less likely the conductive carbon is to detach from the multilayer foam sheet, and the less likely it is to stain the surrounding area.

[0168] [Electrical Conductivity]

[0169] The surface resistivity of the conductive layer was measured according to the method described in JIS K6271-1:2015. Specifically, a square test piece with one side measuring 100 mm was collected from a multilayer foam sheet. After electrodes were mounted on the surface of the conductive layer in the test piece, a voltage of 1 V was applied between the electrodes in an atmosphere with a temperature of 23°C and a relative humidity of 50%. The surface resistivity (in Ω) at the point 1 minute after the voltage was applied was then taken as the surface resistivity of the multilayer foam sheet. Furthermore, a HirestaUXMCP-HT800 manufactured by Nitto Seiko Analytech Co., Ltd. was used for the surface resistivity measurement. The surface resistivity of the multilayer foam sheets of the examples and comparative examples are shown in Table 3 or Table 4.

[0170]

[0171]

[0172] As shown in Table 3, the conductive layer in the multilayer foamed sheet of Examples 1 to 8 contains polyethylene (B), an ethylene-based copolymer (C) having a structural unit derived from ethylene and a structural unit derived from a monomer having a polar group, and electrically conductive carbon. In addition, in the conductive layer of the multilayer foamed sheet of Examples 1 to 8, the blending amount of the electrically conductive carbon and the melting point difference Tm of the polyethylene (B) and the ethylene-based copolymer (C) are in the above specific ranges. Furthermore, in Examples 1 to 8, the conductive layer having such a configuration is laminated and bonded with a polyethylene-based resin foamed layer by co-extrusion. The multilayer foamed sheet of Examples 1 to 8 has a surface resistivity of 1 x 10 B -Tm C Ω or less. Furthermore, the multilayer foamed sheet of Examples 1 to 8 has a surface resistivity of 1 x 10 7 Ω or less. Furthermore, the multilayer foamed sheet of Examples 1 to 8 has a surface resistivity of 1 x 10

[0173] As can be understood from the comparison of Table 3 and Table 4, the multilayer foamed sheet having a conductive layer having the above specific composition shows excellent electrical conductivity even if the electrically conductive carbon in the conductive layer is blended in a small amount of 10 mass% or less. In addition, for example, as can be understood from the comparison of Example 1 and Comparative Example 1, and Example 4 and Comparative Example 4 in which the blending amount of the electrically conductive carbon in the conductive layer is the same degree, the multilayer foamed sheet having a conductive layer having the above specific composition has a small ΔL* value and can reduce the contamination of the surroundings.

Claims

1. A polyethylene resin multilayer foam sheet, wherein, The polyethylene resin multilayer foam sheet comprises: A polyethylene resin foam layer comprising polyethylene resin A as a base resin; and A conductive layer is laminated on at least one side of the polyethylene resin foam layer. The conductive layer comprises: A mixed resin selected from one or more polyethylene Bs in the group consisting of low-density polyethylene with long-chain branched structure and linear low-density polyethylene, and an ethylene copolymer C having structural units derived from ethylene and structural units derived from monomers with polar groups. as well as The conductive carbon in the mixed resin, The amount of conductive carbon in the conductive layer is 3% by mass or more and 15% by mass or less. The melting point Tm of the polyethylene B contained in the conductive layer B The melting point Tm of the ethylene copolymer C C The difference Tm B -Tm C Temperatures must be between 30°C and 80°C. The mass ratio of polyethylene B to ethylene copolymer C in the conductive layer is 80:20 to 20:

80.

2. The polyethylene resin multilayer foam sheet according to claim 1, wherein, The mass ratio of polyethylene B to ethylene copolymer C in the conductive layer is 50:50 to 25:

75.

3. The polyethylene resin multilayer foam sheet according to claim 1 or 2, wherein, The content of structural units derived from monomers with polar groups in the ethylene copolymer C is 30% by mass or more and 50% by mass or less.

4. The polyethylene resin multilayer foam sheet according to claim 1 or 2, wherein, The conductive layer comprises one or two ethylene copolymers C selected from the group consisting of ethylene-vinyl acetate copolymers and ethylene-methyl methacrylate copolymers.

5. The polyethylene resin multilayer foam sheet according to claim 1 or 2, wherein, The ethylene copolymer C has a melt flow rate of more than 20 g / 10 minutes and less than 100 g / 10 minutes at a temperature of 190°C and a load of 2.16 kg.

6. The polyethylene resin multilayer foam sheet according to claim 1 or 2, wherein, The basis weight of the conductive layer is 1 g / m³. 2 Above and 20g / m 2 the following.

7. The polyethylene resin multilayer foam sheet according to claim 1 or 2, wherein, The apparent density of the polyethylene resin multilayer foam sheet is 30 kg / m³. 3 Above and 150kg / m 3 the following.

8. The polyethylene resin multilayer foam sheet according to claim 1 or 2, wherein, The overall thickness of the polyethylene resin multilayer foam sheet is 0.05 mm or more and 3.0 mm or less.

9. The polyethylene resin multilayer foam sheet according to claim 1 or 2, wherein, The conductive carbon has an oil absorption capacity of more than 200 mL / 100 g and less than 600 mL / 100 g for dibutyl phthalate.

10. The polyethylene resin multilayer foam sheet according to claim 1 or 2, wherein, The surface resistivity of the side of the polyethylene resin multilayer foam sheet having the conductive layer is 1×10⁻⁶. 3 Ω or higher and 1×10 7 Below Ω.

11. A method for manufacturing a polyethylene-based resin multilayer foam sheet, comprising co-extruding a foam layer forming melt for forming a polyethylene-based resin foam layer and a conductive layer forming melt for forming a conductive layer, thereby producing a polyethylene-based resin multilayer foam sheet having the polyethylene-based resin foam layer and the conductive layer laminated on at least one side of the polyethylene-based resin foam layer, wherein... The foaming layer forming melt is formed by mixing polyethylene resin A, which serves as the base resin, with a physical foaming agent. The conductive layer forming melt is made by compounding one or more polyethylene B selected from the group consisting of low-density polyethylene with long-chain branched structure and linear low-density polyethylene, an ethylene copolymer C having structural units derived from ethylene and structural units derived from monomers with polar groups, and conductive carbon. The amount of conductive carbon in the melt used to form the conductive layer is 3% by mass or more and 15% by mass or less. The melting point Tm of the polyethylene B B The melting point Tm of the ethylene copolymer C C The difference Tm B -Tm C Temperatures must be between 30°C and 80°C. The mass ratio of polyethylene B to ethylene copolymer C in the conductive layer is 80:20 to 20:80.

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