JP2024000635A - Polyethylene-based resin multilayer foam sheet and method for manufacturing the same - Google Patents

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

The present invention relates to a polyethylene resin multilayer foam sheet and a method for manufacturing the same.

BACKGROUND ART Polyethylene resin foam sheets, which use polyethylene resin 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, for example, cushioning materials and packaging materials used for packaging electronic devices, electronic parts, etc. are sometimes required to have electrical conductivity in addition to protection for the packaged object.

As an example of a conductive foam sheet, for example, Patent Document 1 describes a conductive polyethylene resin foam sheet obtained by mixing a masterbatch containing conductive carbon with a polyethylene resin and extruding and foaming the mixture. Are listed.

Further, Patent Document 2 discloses a foamable multilayer thermoplastic resin sheet consisting of at least two layers: a non-conductive polyethylene resin containing a pyrolytic foaming agent and a conductive thermoplastic resin layer containing conductive carbon. A conductive polyethylene resin multilayer foam sheet obtained by heating and foaming is described.

Japanese Unexamined Patent Publication No. 61-31440 Japanese Unexamined Patent Publication No. 62-231728

The foamed sheet of Patent Document 1 requires a relatively large amount of conductive carbon to be blended into the foamed sheet in order to impart electrical conductivity. However, if the amount of conductive carbon added to the foam sheet increases, the foamability will be inhibited, and there is a risk that properties such as cushioning properties necessary for use as a cushioning material or packaging material will be impaired.

Similarly, in the foamed sheet of Patent Document 2, in order to make the surface resistivity 1×10 ⁷ Ω or less, it is necessary to incorporate a relatively large amount of conductive carbon into the conductive thermoplastic resin layer.

Further, in the foam sheets of Patent Document 1 and Patent Document 2, the conductive carbon may fall off from the foam sheet and contaminate the area around the foam sheet. In particular, when the amount of conductive carbon in the foam sheet increases, the conductive carbon tends to fall off from the foam sheet, and the area around the foam sheet may be easily contaminated by the conductive carbon. Furthermore, when a foam sheet is used as a packaging material, there is a risk that conductive carbon that has fallen off from the foam sheet may transfer to the packaged item, thereby contaminating the packaged item.

In recent years, depending on the use of electronic parts, etc., a highly clean state is sometimes required, and the packaging materials used for such uses have been designed to further reduce the contamination potential of the packaged items so that the packaging material itself does not become a source of contamination. There were times when I was asked to reduce the amount.

The present invention has been made in view of this background, and aims to provide a polyethylene resin multilayer foam sheet that is electrically conductive and capable of highly reducing contamination of packaged items, and a method for manufacturing the same. It is something.

One aspect of the present invention resides in polyethylene resin multilayer foam sheets according to [1] to [10] below.

[1] Polyethylene resin foam layer,
a surface layer provided on at least one side of the polyethylene resin foam layer;
A multilayer foam sheet comprising a conductive layer provided between the surface layer and the foam layer,
The conductive layer comprises an ethylene copolymer having a structural unit derived from ethylene and a structural unit derived from a monomer having a polar group, a polyethylene resin different from the ethylene copolymer, and conductive carbon. including;
The amount of the conductive carbon in the conductive layer is 5% by mass or more and 15% by mass or less,
The surface layer contains one or more types of linear polyethylene selected from the group consisting of linear low-density polyethylene and high-density polyethylene, or the linear polyethylene and low-density polyethylene, and the linear polyethylene It is composed of a mixed resin with a blending amount of 8% by mass or more,
A polyethylene resin multilayer foam sheet, wherein the surface resistivity of the surface on the side where the surface layer is provided among the surfaces of the multilayer foam sheet is less than 1×10 ⁸ Ω.

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

[5] The polyethylene resin multilayer foam 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. sheet.
[6] The polyethylene resin multilayer 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 resin multilayer foam sheet according to any one of [1] to [6], wherein the linear polyethylene contained in the surface layer has a crystallization temperature of 100° C. or more and 115° C. or less.

[8] The difference Tm S - Tm _C between the melting point Tm _S of the linear polyethylene contained in the surface layer and the melting point Tm _C of the polyethylene resin used in the foam layer is 0° _C or more and 15°C or less. , the polyethylene resin multilayer foam sheet according to any one of [1] to [7].
[9] The polyethylene resin multilayer according to any one of [1] to [8], wherein the surface layer of the polyethylene resin multilayer foam sheet has a basis weight of 1 g/m ² or more and 10 g/m ² or less. foam sheet.
[10] The polyethylene resin multilayer foam sheet according to any one of [1] to [9], wherein the polyethylene resin multilayer foam sheet has an apparent density of 30 kg/m ³ or more and 150 kg/m ³ or less.

Another aspect of the present invention is a method for producing a polyethylene resin multilayer foam sheet according to [11] below.

[11] A foam layer forming melt for forming a polyethylene resin foam layer, a conductive layer forming melt for forming a conductive layer, and a surface layer forming melt for forming a surface layer. By co-extruding the polyethylene resin foam layer, the surface layer provided on at least one side of the polyethylene resin foam layer, and the conductive layer provided between the surface layer and the foam layer. A method for producing a polyethylene resin multilayer foam sheet, the method comprising: producing a polyethylene resin multilayer foam sheet having a layer,
The foam layer forming melt is obtained by kneading a polyethylene resin and a physical foaming agent,
The conductive layer forming melt is an ethylene copolymer comprising a structural unit derived from ethylene and a structural unit derived from a monomer having a polar group, and a polyethylene resin different from the ethylene copolymer; Made by kneading conductive carbon,
The amount of the conductive carbon in the conductive layer forming melt is 5% by mass or more and 15% by mass or less,
The surface layer forming melt is one or more types of linear polyethylene selected from the group consisting of linear low density polyethylene and high density polyethylene, or 8 mass relative to the mass of the surface layer forming melt. % or more of the linear polyethylene and low-density polyethylene are kneaded.

According to the above aspect, there is provided a polyethylene resin multilayer foam sheet (hereinafter referred to as a "multilayer foam sheet" as appropriate) that is electrically conductive and can highly reduce contamination of packaged items, and a method for manufacturing the same. can be provided.

FIG. 1 is a cross-sectional view of a polyethylene resin multilayer foam sheet in an example.

(polyethylene resin multilayer foam sheet)
The multilayer foam sheet includes a polyethylene resin foam layer (hereinafter referred to as "foam layer"), a surface layer provided on at least one side of the foam layer, and a polyethylene resin foam layer provided between the surface layer and the foam layer. It has a multilayer structure including three layers including a conductive 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. It is preferable that the surface layer and the conductive layer in the multilayer foam sheet are laminated by coextrusion, and it is preferable that all the layers in the multilayer foam sheet are laminated and bonded to adjacent layers by coextrusion.

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. Further, the multilayer foam sheet may be composed of five layers: a foam layer, a conductive layer laminated on both sides of the foam layer, and a surface layer laminated on each conductive layer. Furthermore, a layer having a composition different from that of the foam layer, the conductive layer, and the surface layer may be provided between the foam layer and the conductive layer in the multilayer foam sheet.

In the multilayer foam sheet, a conductive layer made 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 made of the specific resin is provided on the surface of the multilayer foam sheet. In this way, by forming the conductive layer from the above-mentioned specific resin composition, conductivity can be increased without excessively increasing the amount of conductive carbon. Furthermore, by providing a surface layer made of the specific resin on the surface of the multilayer foam sheet, the multilayer foam sheet can highly reduce contamination of the packaged items while maintaining high conductivity. can.

In order to more reliably suppress damage to packaged items such as electronic devices and electronic parts due to static electricity, and to further reduce contamination of the multilayer foam sheet, the conductive layer and surface layer should be made of a polyethylene resin foam layer. Preferably, they are provided on both sides. That is, the multilayer foam sheet has a foam layer, a surface layer provided on both sides of the foam layer and located on the outermost surface of the multilayer foam sheet, and a conductive layer located between each surface layer and the foam layer. It is preferable to have a five-layer structure.

<Overall thickness>
The overall thickness of the multilayer foam sheet is preferably 0.05 mm or more and 3.0 mm or less. By setting the overall thickness of the multilayer foam sheet to 0.05 mm or more, more preferably 0.1 mm or more, and still more preferably 0.2 mm or more, the cushioning properties of the multilayer foam sheet can be further improved. Furthermore, by setting the overall thickness of the multilayer foam sheet to 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 multilayer foam sheet can be improved. This makes it easier to pack items.

The method for measuring the total thickness of the multilayer foam sheet is as follows. First, the multilayer foam sheet is cut along a plane perpendicular to the extrusion direction. On this cut surface, 10 measurement positions are set so that the length of the cut surface in the width direction (that is, the direction perpendicular to both the extrusion direction and the thickness direction) is divided into 11 equal parts. The thickness at each measurement position is measured by a method such as observing these measurement positions using a microscope. Then, the arithmetic mean value of these thicknesses is taken as the total thickness of the multilayer foam sheet.

<Apparent density>
The multilayer foam sheet preferably has an apparent density of 30 kg/m ³ or more and 150 kg/m ³ or less. By setting the apparent density of the multilayer foam sheet to 30 kg/m ³ or more, more preferably 35 kg/m ³ or more, even more preferably 40 kg/m ³ or more, it is possible to easily ensure sufficient strength as a cushioning material or packaging material. I can do it. Furthermore, by setting the apparent density of the multilayer foam sheet to 150 kg/m ³ or less, more preferably 120 kg/m ³ or less, and even more preferably 100 kg/m ³ or less, the lightness and flexibility of the multilayer foam sheet can be sufficiently ensured. However, the cushioning properties of the multilayer foam sheet can be further improved.

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 (that is, the direction perpendicular to both the extrusion direction and the thickness direction), and a test piece is taken. The shape of the test piece can be, for example, a rectangle whose longitudinal dimension is the same as the full width of the multilayer foam sheet and whose transverse dimension is 10 cm. After dividing the mass of this test piece (unit: g) by the area of the test piece and converting it into units, the basis weight of the multilayer foam sheet, that is, the mass per 1 m ² of the multilayer foam sheet (unit: g/m ² ) is calculated. The apparent density (unit: kg/m ³ ) of the multilayer foam sheet can be calculated by dividing the basis weight of the multilayer foam sheet by the overall thickness of the multilayer foam sheet obtained by the method described above, and then converting into units. .

<Surface resistivity>
The surface resistivity of the surface of the multilayer foam sheet on the side having the surface layer is less than 1×10 ⁸ Ω. A multilayer foam sheet having a surface resistivity within this range can easily remove static electricity from objects to be packaged, etc., and is therefore suitable as a cushioning material or packaging material for electronic parts, electronic equipment, etc. From the same viewpoint, the surface resistivity of the surface of the multilayer foam sheet on the side having the surface layer is preferably 5×10 ⁷ Ω or less, more preferably less than 1×10 ⁷ Ω. . Moreover, it is preferable that the surface resistivity of the surface of the multilayer foam sheet on the side having the surface layer is 1×10 ³ Ω or more.

The surface resistivity of the multilayer foam sheet is measured by a measuring method based on JIS K6271-1:2015. Specifically, first, a square test piece with a side of 100 mm is taken from the multilayer foam sheet. After attaching an electrode to the surface of this test piece on the side having the surface layer, a voltage of 1 V is applied between the electrodes in an atmosphere of a temperature of 23° C. and a relative humidity of 50%. Then, the surface resistivity (unit: Ω) after one minute has passed after the voltage is applied is defined as the surface resistivity of the multilayer foam sheet.

[Polyethylene resin foam layer]
The polyethylene resin foam layer is mainly composed of polyethylene resin.

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

The aforementioned low-density polyethylene has a long chain branched structure, and its density is usually 910 kg/m ³ or more and less than 930 kg/m ³ . Linear low-density polyethylene is a copolymer of ethylene and α-olefin having 4 to 8 carbon atoms, and has a substantially linear molecular chain, and its density is usually 910 kg/m ³ It is not less than 942 kg/m ³ or less. 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 higher than 942 kg/m ³ .

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

The melting point of the polyethylene resin contained in the foam layer is preferably 100°C or more and 135°C or less. In this case, it is possible to stably form a foam layer that has excellent extrusion foaming properties and excellent cushioning properties. From this viewpoint, the melting point of the polyethylene resin contained in the foam layer is preferably 100°C or more and 130°C or less, more preferably 105°C or more and 120°C or less, and 108°C or more and 115°C or less. is even more preferable.

The melting point of the polyethylene resin can be measured by the plastic transition temperature measuring method specified in JIS K7121:2012. First, according to "Measuring the melting temperature after a certain heat treatment", the test piece is conditioned by setting the heating rate and cooling rate to 10° C./min. Thereafter, heat flux DSC (that is, differential scanning calorimetry) is performed with the heating rate set to 10° C./min to obtain a DSC curve. The apex temperature of the endothermic peak in the obtained DSC curve is defined as the melting point. In addition, when a plurality of endothermic peaks appear in the DSC curve, the apex temperature of the melting peak with the largest area is set as the melting point, with the baseline on the high temperature side as a reference.

The melt flow rate (MFR) of the polyethylene resin contained in the foam layer is preferably 0.5 g/10 minutes or more and 15 g/10 minutes or less, and 1 g/10 minutes or more and 8 g/10 minutes or more, since it has excellent extrusion foaming properties. It is more preferably 10 minutes or less, and even more preferably 1.5 g/10 minutes or more and 5 g/10 minutes or less. The MFR of the polyethylene resin in this specification is a value measured at a test temperature of 190° C. and a load of 2.16 kg based on JIS K7210-1:2014.

<Other polymers>
The foamed layer may contain other polymers than the polyethylene resin as long as the above-mentioned effects are not impaired. Examples of polymers other than polyethylene resins include thermoplastic resins other than polyethylene resins such as polystyrene resins, and elastomers such as ethylene propylene rubber and styrene-butadiene-styrene block copolymers. The content of other polymers other than polyethylene resin in the foam layer is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and 5 parts by mass or less based on 100 parts by mass of polyethylene resin. Parts by mass or less are more preferable, and it is particularly preferable that the polymer component constituting the foamed layer is 0 parts by mass, that is, the polymer component constituting the foamed layer consists only of polyethylene resin.

<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, and an antibacterial agent. The blending 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 3 parts by mass or less based on 100 parts by mass of the polyethylene resin. is even more preferable.

[Conductive layer]
The conductive layer is provided between the polyethylene resin foam layer and the surface layer. The conductive layer is mainly composed of an ethylene copolymer having a structural unit derived from ethylene and a structural unit derived from a monomer having a polar group, a polyethylene resin different from the ethylene copolymer, and a conductive It is made of a resin composition containing carbon. Further, the content of conductive carbon in the conductive layer is 5% by mass or more and 15% by mass or less. The conductive layer is preferably in a non-foamed state from the viewpoint of improving the conductivity, handleability, and appearance of the multilayer foam sheet. Note that the non-foamed state includes a state in which no foaming occurs during manufacturing and no bubbles are included, and a state in which foaming occurs slightly during manufacturing and then the bubbles disappear, and means that there is almost no cell structure in the layer. . However, a small amount of very small bubbles may be included in the conductive layer.

By forming the conductive layer from the above-mentioned specific resin composition, conductivity can be imparted to the multilayer foam sheet using a relatively small amount of conductive carbon. Although the reason for this is not necessarily clear at present, the following reasons may be considered, for example.

In general, when conductive carbon is dispersed in thermoplastic resin such as polyethylene resin, adjacent conductive carbon particles are located close to each other within a certain distance, resulting in a conductive network formed by the conductive carbon particles. is formed, and conductivity is developed.

Since the ethylene copolymer and polyethylene resin in the resin composition are incompatible with each other, the conductive layer contains a phase mainly composed of polyethylene resin and a phase mainly composed of ethylene copolymer. A phase is formed. When such a morphology is formed in the conductive layer, it is thought that conductive carbon is unevenly distributed in one of the phases made of polyethylene resin and the phase made of ethylene copolymer. When conductive carbon is unevenly distributed in either phase, a conductive network of conductive carbon particles is likely to be formed. As a result, it is thought that conductivity is likely to be developed even if the amount of conductive carbon blended is small.

<Ethylene copolymer>
The ethylene copolymer used for 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 other monomers other than these monomers. You can. The amount of structural units derived from the other monomer contained in the ethylene copolymer is preferably 10% by mass or less, more preferably 5% by mass or less, and 3% by mass or less. is more preferable, and most preferably 0% by mass, that is, the ethylene copolymer is a copolymer of ethylene and a monomer having a polar group.

Examples of the ethylene copolymer used in the conductive layer include ethylene-vinyl acetate copolymer (i.e., EVA), ethylene-methyl methacrylate copolymer (i.e., EMMA), and 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-acrylic acid Examples include ethyl copolymer (ie, EEA), ethylene-butyl acrylate copolymer (ie, EBA), and the like. From the viewpoint of further increasing the conductivity of the multilayer foam sheet, the conductive layer contains ethylene-vinyl acetate copolymer, ethylene-methyl methacrylate copolymer, ethylene-methyl acrylate copolymer, and ethylene-methacrylic acid copolymer. It is preferable that one or more ethylene copolymers selected from the group consisting of polymers are contained, and it is more preferable that an ethylene-vinyl acetate copolymer is contained.

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 setting the content of the structural unit derived from a monomer having a polar group in the ethylene copolymer to 30% by mass or more, the conductivity of the multilayer foam sheet can be further improved. From the viewpoint of further enhancing such effects, the content of structural units derived from monomers having polar groups in the ethylene copolymer is preferably more than 30% by mass, and preferably 35% by mass or more. More preferably, the content is particularly preferably 40% by mass or more, and most preferably greater than 40% by mass.

In addition, by controlling the content of structural units derived from monomers with polar groups in the ethylene copolymer to 50% by mass or less, the handleability of the multilayer foam sheet is further improved, and the conductive layer is laminated to the foam layer. It is possible to further improve manufacturing stability when manufacturing. From the viewpoint of further enhancing these effects, the content of structural units derived from monomers having polar groups in the ethylene copolymer is more preferably 48% by mass or less, and preferably 45% by mass or less. is even more preferable.

The melt flow rate of the ethylene copolymer used for the conductive layer at a temperature of 190° C. and a load of 2.16 kg is preferably 20 g/10 minutes or more and 100 g/10 minutes or less, and 40 g/10 minutes or more and 80 g/10 minutes or less. It is more preferable that In this case, the adhesiveness between the conductive layer and the foam layer can be further improved. Moreover, in this case, the conductivity of the multilayer foam sheet can be more stably developed. The MFR of the ethylene copolymer in this specification is a value measured at a test temperature of 190° C. and a load of 2.16 kg based on JIS K7210-1:2014.

The melting point of the ethylene copolymer used in the conductive layer is preferably 30°C or more and 80°C or less, more preferably 32°C or more and 75°C or less, and even more preferably 35°C or more and 70°C or less. preferable. By setting the melting point of the ethylene copolymer used for the conductive layer within the above specific range, the conductivity of the multilayer foam sheet is further improved, and the manufacturing stability when laminating the conductive layer on the foam layer is further improved. can be done. The method for measuring the melting point of the ethylene copolymer is the same as the method for measuring the melting point of the polyethylene resin used for the foam layer.

<Polyethylene resin>
The conductive layer contains a polyethylene resin other than the ethylene copolymer. Examples of the polyethylene resin 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 resin other than the ethylene copolymer used in the conductive layer is preferably polyethylene, and preferably one or more polyethylenes selected from the group consisting of low density polyethylene and linear low density polyethylene. More preferred. In this case, even when the amount of conductive carbon blended is relatively small, conductivity can be more reliably imparted to the multilayer foam sheet.

The melting point of the polyethylene resin other than the ethylene copolymer used in the conductive layer is preferably 100°C or more and 120°C or less, more preferably 102°C or more and 115°C or less. In this case, even when producing a multilayer foam sheet by coextrusion, the conductive layer can be stably laminated and bonded to the foam layer. Note that the method for measuring the melting point of the polyethylene resin used for the conductive layer is the same as the method for measuring the melting point of the polyethylene resin used for the foamed layer described above.

The difference Tm 1 - _{Tm 2} between the melting point Tm ₁ of the polyethylene resin other than the ethylene copolymer used in the conductive layer and the melting point Tm ₂ of the ethylene copolymer is preferably 30° C. or more and 80° C. or less. .

By setting the melting point difference Tm ₁ -Tm ₂ to 30° C. or more, sufficient electrical conductivity can be more easily imparted to the multilayer foam sheet. In addition, by setting the melting point difference Tm ₁ - Tm ₂ to 80°C or less, it is possible to easily prevent the ethylene copolymer from adhering to the sizing equipment, etc. during the manufacturing process of the multilayer foam sheet, and to produce a good multilayer foam sheet. can be obtained more easily. In addition, in this case, the ethylene copolymer may soften during use, or the multilayer foam sheets may fuse together when stored in a stacked state, impairing handleability. This allows you to more easily avoid getting caught. From the viewpoint of more reliably obtaining these effects, the melting point difference Tm ₁ -Tm ₂ is more preferably 40°C or more and 70°C or less.

The melt flow rate of the polyethylene resin used for the conductive layer at a temperature of 190° C. and a load of 2.16 kg is preferably 5 g/10 minutes or more and 80 g/10 minutes or less, and 10 g/10 minutes or more and 65 g/10 minutes or less. More preferably, it is 12 g/10 minutes or more and 50 g/10 minutes or less. In this case, the adhesiveness between the conductive layer and the foam layer can be further improved. Moreover, in this case, conductivity can be more reliably imparted to the multilayer foam sheet.

<Blending ratio>
The blending ratio of the ethylene copolymer and the polyethylene resin other than the ethylene copolymer in the conductive layer may be ethylene copolymer: polyethylene resin = 20:80 to 80:20 in terms of mass ratio. Preferably, the ratio of ethylene copolymer to polyethylene resin is 50:50 to 75:25, and even more preferably the ratio of ethylene copolymer to polyethylene resin is 55:45 to 70:30. In this case, the conductivity of the multilayer foam sheet can be further improved.

<Conductive carbon>
The resin composition constituting the conductive layer contains conductive carbon, that is, a substance that is mainly composed of carbon atoms and has conductivity. Preferred examples of the conductive carbon include conductive carbon blacks such as furnace black, acetylene black, thermal black, and Ketjenblack (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 mixed while ensuring the conductivity of the multilayer foam sheet, the conductive layer contains highly conductive carbon black such as Ketjen black as conductive carbon. It is preferable.

The dibutyl phthalate (DBP) oil absorption amount 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 multilayer foam sheet can be further improved. From the viewpoint of further increasing the conductivity of the multilayer foam 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. . Note that the above-mentioned dibutyl phthalate (DBP) oil absorption is a value measured according to ASTM D2414-79.

Further, the BET specific surface area of the 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 foam sheet can be further improved. From the viewpoint of further increasing the conductivity of the multilayer foam 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 foam sheet of the present invention, the conductive layer containing an ethylene copolymer, a polyethylene resin other than the ethylene copolymer, and conductive carbon is formed as a separate layer from the foam layer. , conductive carbon having a large specific surface area can be blended without inhibiting the foamability of the foam layer.

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 be 5% by mass or more, conductivity can be imparted to the multilayer foam sheet. From the viewpoint of further increasing the conductivity of the multilayer foam sheet, the content of conductive carbon in the conductive layer is preferably 6% by mass or more, 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 lead to a decrease in the conductivity of the multilayer foam sheet. Note that the amount of conductive carbon blended is approximately equal to the content of conductive carbon in the conductive layer in the multilayer foam sheet.

Further, by setting the amount of conductive carbon to be 15% by mass or less, it is possible to reduce the amount of conductive carbon falling off from the multilayer foam sheet. From the viewpoint of further reducing the shedding of conductive carbon from the multilayer foam sheet, the blending amount of conductive carbon is preferably 12% by mass or less, more preferably 10% by mass or less, and 10% by mass. More preferably, it is less than When the amount of conductive carbon in the conductive layer is too large, the conductive carbon tends to fall off from the multilayer foam sheet, which may cause contamination of the surroundings of the multilayer foam sheet.

<Other polymers>
The conductive layer may contain other polymers than the ethylene copolymer and polyethylene resin as long as the above-mentioned effects are not impaired. Examples of polymers other than ethylene copolymers and polyethylene resins include thermoplastic resins other than polyethylene resins such as polystyrene resins, ethylene propylene rubber, and styrene-butadiene-styrene block copolymers. Examples include elastomers. It is preferable that the conductive layer does not contain a polymer having a higher melting point than the polyethylene resin contained in the foam layer. In this case, production stability can be further improved when producing a multilayer foam sheet including a conductive layer by the coextrusion method described below. The content of other polymers other 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, and 5% by mass or less. It is more preferable that the amount is at least 3% by mass, particularly preferably 3% by mass or less.

<Additives>
The conductive layer may contain additives such as antioxidants, heat stabilizers, weathering agents, ultraviolet absorbers, flame retardants, fillers, and antibacterial agents. The amount of the additive in the conductive layer is preferably 10 parts by mass or less, for example, 10 parts by mass or less, and 5 parts by mass based on the total of 100 parts by mass of the ethylene copolymer and the polyethylene resin other than the ethylene copolymer. It is more preferably at most 3 parts by mass, and even more preferably at most 3 parts by mass.

<Average thickness>
The average thickness of the conductive layer is preferably 1 μm or more and 20 μm or less. By setting the average thickness of the conductive layer to 1 μm or more, more preferably 3 μm or more, conductivity can be more reliably imparted to the multilayer foam sheet. Further, by setting the average thickness of the conductive layer to 20 μm or less, more preferably 18 μm or less, still more preferably 15 μm or less, particularly preferably 10 μm or less, the contamination of the multilayer foam sheet can be more reliably reduced.

The method for measuring the average thickness of the conductive layer is as follows. First, the multilayer foam sheet is cut along a plane perpendicular to the extrusion direction. On this cut surface, 10 measurement positions are set so that the length of the cut surface in the longitudinal direction (that is, the direction perpendicular to both the extrusion direction and the thickness direction) is divided 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 defined as the average thickness (unit: μm) of the conductive layer.

<Basic weight>
The basis weight of the conductive layer is preferably 1 g/m ² or more and 50 g/m ² or less. By setting the basis weight of the conductive layer to 1 g/m ² or more, more preferably 2 g/m ² or more, still more preferably 3 g/m ² or more, particularly preferably 5 g/m ^{2 or} more, conductivity can be imparted to the multilayer foam sheet. This can be applied more reliably. In addition, by setting the basis weight of the conductive layer to 30 g/m ² or less, more preferably 20 g/m ² or less, even more preferably 15 g/m ² or less, particularly preferably 10 g/m ² or less, the multilayer foam sheet can be contaminated. It is possible to more reliably reduce the In addition, when a conductive layer is laminated|stacked on both surfaces of a polyethylene resin foam layer, the basis weight of the said conductive layer means the basis weight per one side.

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 by the method described above. After converting the unit of this average thickness, the basis weight (unit: g/m ² ) of the conductive layer can be obtained by multiplying the density (unit: g/m ³ ) of the conductive layer. Note that the density of the conductive layer is a density including conductive carbon and other additives contained in the conductive layer.

When producing a multilayer foam sheet by coextrusion, the discharge amount of the conductive layer per side X1 (unit: g/hour), the width W of the multilayer foam sheet (unit: m), the take-up speed L of the multilayer foam sheet ( The basis weight of the conductive layer per one side can also be determined by the following formula (1) using (unit: m/hour).
Basis weight of conductive layer=[X1/(L×W)]...(1)

When producing a multilayer foam sheet by coextruding a surface layer forming melt, a conductive layer forming melt, and a foam layer forming melt, it is necessary to co-extrude a melt for forming a surface layer, a melt for forming a conductive layer, and a melt for forming a foam layer. It is possible to form a conductive layer with a small basis weight and a small thickness, and to stably exhibit conductivity.

[Surface layer]
The surface layer is located on the outermost surface of the multilayer foam sheet. The surface layer is substantially composed of any one of the resins shown in (1) and (2) below.
(1) One or more types of linear polyethylene selected from the group consisting of linear low density polyethylene and high density polyethylene.
(2) A mixed resin containing linear polyethylene and low-density polyethylene, in which the blending amount of linear polyethylene is 8% by mass or more.

The surface layer is preferably in a non-foamed state from the viewpoint of improving the conductivity, ease of handling, appearance, and reducing contamination of the multilayer foam sheet. However, a small amount of very small air bubbles may be included in the surface layer.

Since the multilayer foam sheet has a surface layer made of the specific resin, it has high electrical conductivity, reliably reduces the shedding of conductive carbon from the conductive layer, and reduces contamination of the packaged items. can be highly reduced. This is thought to be because by using the above-mentioned specific resin as the resin constituting the surface layer, formation of pinholes can be suppressed when the surface layer is laminated on the conductive layer. Note that in this specification, a pinhole refers to a defect such as a small hole formed on the surface of a multilayer foam sheet.

When 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, so there is a possibility that the contamination of the packaged object cannot be sufficiently reduced. Furthermore, even when the surface layer does not contain the linear polyethylene, for example, when a surface layer made of low-density polyethylene is laminated on a conductive layer containing conductive carbon, the contamination of the packaged items can be sufficiently reduced. There is a possibility that you may not be able to do so. This sufficiently suppresses the formation of pinholes on the surface of a multilayer foam sheet with a conductive layer containing conductive carbon as a filler if the surface layer does not contain linear polyethylene. This is probably due to the inability to do so.

The surface layer contains at least one or more types of polyethylene selected from the group consisting of linear polyethylene, that is, linear low density polyethylene and high density polyethylene. It is preferable that the surface layer contains at least linear low density polyethylene as linear polyethylene. In this case, the surface layer can be laminated and adhered to the conductive layer more stably, and local variations in surface resistivity on the surface of the multilayer foam sheet can be further reduced.

<Linear polyethylene>
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 12 g/10 minutes or more, more preferably 15 g/10 minutes or more. In this case, the surface layer can be laminated and adhered to the conductive layer more stably, and local variations in surface resistivity on the surface of the multilayer foam sheet can be further reduced. Note that 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 minutes, for example, from the viewpoint of extrusion stability, and 50 g/10 minutes. More preferably, the amount is 30 g/10 minutes.

The tensile strength at a temperature of 95° C. of the linear polyethylene used for the surface layer is preferably 3.0 MPa or more, more preferably 3.5 MPa or more, even more preferably 4.0 MPa or more, It is particularly preferable that the pressure is 4.5 MPa or more. In this case, it is possible to further reduce the shedding of the conductive carbon from the multilayer foam sheet, and the contamination of the packaged object can be further reduced. The reason for this is, for example, that by using linear polyethylene having the above-mentioned specific tensile strength, the surface layer becomes tougher when a multilayer foam sheet is manufactured by coextrusion, and pinholes are formed in the surface layer. It is possible that it becomes difficult to Note that the upper limit of the tensile strength of the linear polyethylene used for the surface layer at a temperature of 95° C. may be, for example, 10 MPa, from the viewpoint of further increasing the extrusion stability and take-up stability of the multilayer foam sheet. More preferably, it is .0 MPa.

From the same viewpoint, the tensile strength at a temperature of 23°C of the linear polyethylene used for the surface layer is preferably 15 MPa or more, more preferably 16 MPa or more, even more preferably 17 MPa or more, and 18 MPa or more. It is particularly preferable that it is above. In addition, the upper limit of the tensile strength of the linear polyethylene used for the surface layer at a temperature of 23°C is 30 MPa from the viewpoint of imparting flexibility to the surface of the multilayer foam sheet and further suppressing damage to the packaged items. It is preferable that there be.

The tensile strength of linear polyethylene at each temperature mentioned above is measured based on JIS K7161-1:2014. In addition, in consideration of the extrusion foaming temperature of the polyethylene resin multilayer foam sheet described later, 95° C. was adopted as the measurement temperature assuming the temperature of the resin when the surface layer is formed by coextrusion.

The crystallization temperature of the linear polyethylene used in 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, the contamination of the packaged object can be further reduced, and the surface layer can be laminated and bonded to the conductive layer more stably. The reason for this is, for example, that by using linear polyethylene having the above-mentioned specific crystallization temperature, when producing a multilayer foam sheet by coextrusion, the surface layer is more likely to solidify quickly immediately after extrusion, and the surface layer It is thought that pinholes are less likely to form.

Note that the crystallization temperature of the linear polyethylene used for the surface layer is measured using a heat flux differential scanning calorimeter based on JIS K7121:2012. When a plurality of 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.

The melting point _TmS of the linear polyethylene used for the surface layer is preferably 110°C or more and 135°C or less, more preferably 115°C or more and 130°C or less. In this case, even when producing a multilayer foam sheet by coextrusion, the surface layer can be stably formed. In addition, from the viewpoint of further reducing the contamination of the packaged objects of the multilayer foam sheet, the difference Tm between the melting point Tm _S of the linear polyethylene used for the surface layer and the melting point Tm _C of the polyethylene resin used for the foam layer is _S -Tm _C is preferably -5°C or higher, more preferably 0°C or higher, and even more preferably 5°C or higher. In addition, from the viewpoint of reducing variations in the surface resistivity of the multilayer foam sheet, the difference Tm S between the melting point Tm _S of the linear polyethylene used for the surface layer and the melting point Tm _C of the polyethylene resin used for the foam _layer is -Tm _C is preferably 15°C or less. The method for measuring the melting point of the linear polyethylene used for the surface layer is the same as the method for measuring the melting point of the polyethylene resin used for the foamed layer described above.

<Low density polyethylene>
The surface layer may contain low density polyethylene. For the surface layer, for example, low-density polyethylene similar to the low-density polyethylene used for the foamed layer described above can be used. When the surface layer contains low-density polyethylene, the cushioning properties for the packaged object can be further improved. In addition, when the surface layer contains linear polyethylene and low-density polyethylene, the blending amount of linear polyethylene in the surface layer is 8% by mass in order to ensure the above-mentioned effects of linear polyethylene. The above shall apply. From the same viewpoint, the blending 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.

<Other polymers>
The surface layer may contain polymers other than linear polyethylene and low-density polyethylene to the extent that the above-mentioned effects are not impaired. Examples of polymers other than linear polyethylene and low-density polyethylene include thermoplastic resins other than polyethylene resins such as polystyrene resins, and elastomers such as ethylene propylene rubber and styrene-butadiene-styrene block copolymers. etc. From the viewpoint of production stability when producing a multilayer foam sheet with a surface layer by the coextrusion method described below, the melting point of the polymer contained in the surface layer should be lower than the melting point of the polyethylene resin contained in the foam layer. It is preferable that there be. The content of other polymers 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, and 5% by mass or less. It is more preferable that the amount is 3% by mass or less, and particularly preferably 3% by mass or less.

<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, it is preferable that the surface layer does not contain conductive carbon or antistatic agents such as low molecular weight antistatic agents and polymeric antistatic agents that cause contamination of the surroundings of the multilayer foam sheet. . The blending amount of the additive in the surface layer is, for example, preferably 10 parts by mass or less, more preferably 5 parts by mass or less, based on 100 parts by mass of the linear polyethylene and low-density polyethylene in total. More preferably, it is 3 parts by mass or less.

<Average thickness>
The average thickness of the surface layer is preferably 1 μm or more and 20 μm or less. By setting the average thickness of the surface layer to 1 μm or more, more preferably 2 μm or more, still more preferably 3 μm or more, it is possible to further reduce shedding of the conductive carbon. Further, by setting the average thickness of the surface layer to 20 μm or less, more preferably 18 μm or less, still more preferably 15 μm or less, particularly preferably 10 μm or less, the multilayer foam sheet can more stably exhibit conductivity.

The method for measuring the average thickness of the surface layer is as follows. First, the multilayer foam sheet is cut along a plane perpendicular to the extrusion direction. On this cut surface, 10 measurement positions are set so that the length of the cut surface in the longitudinal direction (that is, the direction perpendicular to both the extrusion direction and the thickness direction) is divided 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 defined as the average thickness (unit: μm) of the surface layer.

<Basic weight>
The basis weight of the surface layer is preferably 1 g/m ² or more and 20 g/m ² or less. By setting the basis weight of the surface layer to 1 g/m ² or more, more preferably 2 g/m ² or more, even more preferably 3 g/m ² or more, the formation of pinholes in the surface layer can be more effectively suppressed, and the multilayer The contamination of the foam sheet can be further reduced. In addition, by setting the basis weight of the surface layer to 20 g/m ² or less, more preferably 10 g/m ² or less, even more preferably 8 g/m ² or less, particularly preferably 4.5 g/m ² or less, the multilayer foam sheet It is possible to further reduce local variations in conductivity on the surface of the multilayer foam sheet and more reliably impart conductivity to the multilayer foam sheet. In addition, when the surface layer is provided on both sides of a multilayer foam sheet, the basis weight of the said surface layer means the basis weight per one side.

In addition, from the viewpoint of more reliably imparting conductivity to the multilayer foam sheet and more reliably suppressing variations in surface resistivity, the basis weight of the surface layer is preferably smaller than the basis weight of the conductive layer, More preferably, it is less than 0.85 times the basis weight of the conductive layer. That is, when the basis weight of the surface layer is expressed as R2 (unit: g/m ² ) and the basis weight of the conductive layer is expressed as R1 (unit: g/m ² ), the relationship between R1 and R2 is R2<R1 It is preferable that R2<R1×0.85.

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 ² ) of the surface layer can be obtained by multiplying the density (unit: g/m ³ ) of the surface layer.

When producing a multilayer foam sheet by coextrusion, the discharge amount of the surface layer per side X2 (unit: g/hour), the width W of the multilayer foam sheet (unit: m), the take-up speed L of the multilayer foam sheet ( The basis weight of the surface layer per one side can also be determined by the following formula (2) using (unit: m/hour).
Basis weight of surface layer=[X2/(L×W)]...(2)

<Pinhole area ratio>
Since the multilayer foam sheet has a surface layer made of the above-mentioned specific resin, it is possible to suppress the formation of pinholes when the surface layer is laminated on the 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, falling off of the conductive carbon from the conductive layer can be more reliably reduced, and contamination of the packaged object can be further reduced. Note that the lower limit of the pinhole area ratio of the multilayer foam sheet is 0%.

The method for measuring the pinhole area ratio of the multilayer foam sheet described above is as follows. At any position of the multilayer foam sheet, the surface layer is observed using a scanning electron microscope, and an electron microscope image of the measurement position is obtained. In this electron microscope image, a square measurement area with a side length of 1000 μm is set. In the measurement region, a hole formed in the surface layer with an area of 70 μm ² or more is regarded as a pinhole, and the ratio of the area occupied by the pinhole to the area of the measurement region is calculated. Similar operations are performed at 15 or more positions on the multilayer foam sheet, and the arithmetic mean value of the area ratios occupied by pinholes in these measurement regions is taken as the pinhole area ratio of the multilayer foam sheet.

(Method for manufacturing multilayer foam sheet)
The multilayer foam sheet can be manufactured, for example, by a coextrusion foaming method. That is, in the method for manufacturing a polyethylene resin multilayer foam sheet, a foam layer forming melt for forming a polyethylene resin foam layer, a conductive layer forming melt for forming a conductive layer, and a surface layer are used. By co-extruding the surface layer forming melt for forming the polyethylene resin foam layer, the surface layer provided on at least one side of the polyethylene resin foam layer, and the space between the surface layer and the foam layer. A polyethylene resin multilayer foam sheet having a conductive layer provided thereon is produced. The foam layer-forming melt contains a polyethylene resin and a physical foaming agent. The melt for forming a conductive layer consists of an ethylene copolymer having a structural unit derived from ethylene and a structural unit derived from a monomer having a polar group, a polyethylene resin other than the ethylene copolymer, and conductive carbon. Contains. The content of conductive carbon in the melt for forming a conductive layer is 3% by mass or more and 15% by mass or less. The surface layer forming melt contains linear polyethylene and low density polyethylene added as necessary. The blending amount of linear polyethylene in the melt for forming the surface layer is 8% by mass or more.

In carrying out such a method, a known coextrusion device used in the field of extrusion foaming can be used. More specifically, for example, an extruder for forming a foam layer configured to be able to extrude a melt for forming a foam layer, an extruder for forming a conductive layer configured to be able to extrude a melt for forming a conductive layer, The multilayer foam sheet is produced using a coextrusion device equipped with a surface layer forming extruder configured to be able to extrude a surface layer forming melt and a coextrusion die to which discharge ports of these extruders are connected. It can be made.

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

As the physical foaming agent, an organic physical foaming agent or an inorganic physical foaming agent can be used. Examples of organic physical blowing agents include aliphatic hydrocarbons such as propane, normal butane, isobutane, normal pentane, isopentane, normal hexane, and isohexane, alicyclic hydrocarbons such as cyclopentane and cyclohexane, methyl chloride, ethyl chloride, etc. and fluorinated hydrocarbons such as 1,1,1,2-tetrafluoroethane and 1,1-difluoroethane. Examples of the inorganic physical blowing agent 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.

From the viewpoint of compatibility with the polyethylene resin and foamability, it is preferable that the melt for forming a foam layer contains an organic physical foaming agent as a physical foaming agent, such as normal butane, isobutane or these. It is more preferable that the mixture contains an organic physical blowing agent as a main component.

The blending amount of the physical blowing agent can be appropriately set depending on the type of blowing agent, desired apparent density, and the like. For example, when using a mixed butane consisting of 30% by mass of isobutane and 70% by mass of normal butane as a physical blowing agent, 3 parts by mass or more and 30 parts by mass or less, preferably 4 parts by mass, based on 100 parts by mass of polyethylene resin. The mixed butane may be added in an amount of at least 10 parts by mass and at most 20 parts by mass, more preferably at least 10 parts by mass and at most 20 parts by mass.

It is preferable to add a cell regulator to the foam layer-forming melt. As the foam regulator, an inorganic foam regulator or an organic foam regulator can be used. Examples of the inorganic bubble control agent include boric acid metal salts such as zinc borate, magnesium borate, and borax, sodium chloride, aluminum hydroxide, talc, zeolite, silica, calcium carbonate, and sodium bicarbonate. Examples of organic foam regulators include sodium phosphate-2,2-methylenebis(4,6-tert-butylphenyl), sodium benzoate, aluminum benzoate, and sodium stearate. Furthermore, a mixture of citric acid and sodium bicarbonate, a mixture of an alkali citric acid salt and sodium bicarbonate, etc. can also be used as a foam regulator. The foam layer-forming melt may contain one type of cell regulator, or may contain two or more types of cell regulators. The amount of the cell regulator in the foam layer-forming melt may be appropriately set depending on the type of physical foaming agent, desired apparent density, cell diameter, and the like.

[Melt for forming conductive layer]
The conductive layer forming melt contains at least an ethylene copolymer, a polyethylene resin other than the ethylene copolymer, and conductive carbon. When producing a melt for forming a conductive layer, for example, an ethylene copolymer, a polyethylene resin other than the ethylene copolymer, conductive carbon, and additives added as necessary are added to an extruder for forming a conductive layer. supply the agent. Then, by melting and kneading these in an extruder, a melt for forming a conductive layer can be obtained.

The melt for forming a conductive layer may contain a volatile plasticizer as an additive. The volatile plasticizer has the effect of lowering the melt viscosity of the melt for forming the conductive layer, and is configured to volatilize from the conductive layer after coextrusion. The volatile plasticizer can bring the extrusion temperature of the conductive layer-forming melt close to the extrusion foaming temperature of the foam layer-forming melt during coextrusion. Further, the volatile plasticizer can improve the melt elongation of the conductive layer in a softened state. As a result, by adding a volatile plasticizer to the melt for forming a conductive layer, the bubbles in the polyethylene resin foam layer are less likely to be destroyed by the heat of the conductive layer during foaming of the melt for forming the foam layer. Further, the conductive layer easily expands following the expansion of the polyethylene resin foam layer during foaming.

Examples of volatile plasticizers include aliphatic hydrocarbons having 3 to 7 carbon atoms, alicyclic hydrocarbons having 3 to 7 carbon atoms, aliphatic alcohols having 1 to 4 carbon atoms, and 2 to 8 carbon atoms. Examples include the following aliphatic ethers. The conductive layer forming melt may contain one type of volatile plasticizer, or may contain two or more types of volatile plasticizer.

The boiling point of the volatile plasticizer is preferably 120°C or lower, more preferably 80°C or lower. A volatile plasticizer having a boiling point in such a range naturally volatilizes and is removed from the conductive layer after coextrusion. Note that the lower limit of the boiling point of the volatile plasticizer is approximately -50°C.

The blending amount of the volatile plasticizer can be appropriately set depending on the composition of the conductive layer and the polyethylene resin foam layer. For example, the blending amount of the volatile plasticizer is 5 parts by mass or more and 50 parts by mass or less based on 100 parts by mass of the ethylene copolymer and the polyethylene resin other than the ethylene copolymer contained in the conductive layer. be able to. From the viewpoint of further improving the followability of the melt for forming the conductive layer and reducing the variation in the thickness of the conductive layer, the amount of volatile plasticizer blended should be It 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, based on a total of 100 parts by mass of the resin.

On the other hand, from the viewpoint of stably laminating the conductive layer on the foam layer, the blending amount of the volatile plasticizer is 50 parts by mass per 100 parts by mass of the ethylene copolymer and the polyethylene resin other than the ethylene copolymer. It is preferably at most parts by mass, more preferably at most 45 parts by mass, even more preferably at most 40 parts by mass.

[Melted material for forming surface layer]
The surface layer forming melt contains at least linear polyethylene. In producing the surface layer-forming melt, for example, linear polyethylene, low-density polyethylene and additives added as necessary are supplied to the surface layer-forming extruder. Then, by melting and kneading these in an extruder, a melt for forming a surface layer can be obtained. Furthermore, a volatile plasticizer may be added to the surface layer forming melt as 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.

[Coextrusion]
To perform coextrusion, as described above, the foam layer forming melt, the conductive layer forming melt, and the surface layer forming melt formed in each extruder are led to the coextrusion die, and the extrusion of the coextrusion die is carried out. Extrude from the outlet in layers. The coextrusion die may be, for example, a flat die with a straight 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 port of the flat die. When the laminate is extruded into the atmosphere from the extrusion port, the foam layer-forming melt expands while foaming. Along with this, the conductive layer forming melt and the surface layer forming melt are stretched. Then, the sheet-like laminated foam extruded from the extrusion port is cooled while being taken along the width expanding device, thereby solidifying the foam layer forming melt, the conductive layer forming melt, and the surface layer forming melt. let This fixes the cell structure formed by foaming and stabilizes the dimensions. Through the above steps, a multilayer foam sheet can be obtained.

Further, the coextrusion die may be, for example, an annular die equipped with an annular extrusion port. 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 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. Along with this, the conductive layer forming melt and the surface layer forming melt are stretched. Then, the cylindrical laminated foam extruded from the extrusion port is expanded from the inside with compressed air, etc., and the inside is taken along a width-expanding device such as a mandrel and cooled, thereby creating a melt for forming a foam layer. , solidify the conductive layer forming melt and the surface layer forming melt. This fixes the cell structure formed by foaming and stabilizes the dimensions. Finally, a multilayer foam sheet can be obtained by cutting open the cylindrical laminated foam on a widening device. When an annular die is used as the coextrusion die, it is easy to produce a wide multilayer foam sheet, for example, having a width of 1000 mm or more. Further, it is easy to manufacture a thin multilayer foam sheet having an overall thickness of 3 mm or less, for example.

Conventionally, when trying to achieve a surface resistivity of less than 1×10 ⁸ Ω by providing a conductive layer containing conductive carbon in a polyethylene resin multilayer foam sheet produced by coextrusion, It was necessary to incorporate a relatively large amount of conductive carbon. The reason is thought to be as follows.

As mentioned above, when attempting to produce a multilayer foam sheet by coextrusion, the melt for forming the foam layer extruded from the coextrusion die rapidly expands due to foaming, so the melt for forming the conductive layer is Following the expansion of the forming melt, it is strongly stretched. During this stretching, the conductive carbon particles in the conductive layer are separated from each other, so if the amount of conductive carbon is insufficient, it tends to be difficult to maintain the conductive network.

Further, since the laminated foam extruded from the coextrusion die is rapidly cooled, the base resin of the conductive layer tends to solidify while the conductive carbon particles are separated from each other 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 around the melting point of the polyethylene resin constituting the polyethylene resin foam layer. Correspondingly, under extrusion temperature conditions of the conductive layer forming melt set to a relatively low temperature, the base resin of the conductive layer is more likely to solidify. For these reasons, it is thought that in conventional multilayer foam sheets produced by coextrusion, it has been difficult to stably form a conductive network in the conductive layer.

On the other hand, in the multilayer foam sheet, as described above, the conductive layer contains an ethylene copolymer and an ethylene copolymer having a structural unit derived from ethylene and a structural unit derived from a monomer having a polar group. It contains two types of resins that are mutually incompatible with a polyethylene resin other than the polymer. Therefore, in the multilayer foam sheet, it is considered that conductive carbon is unevenly distributed in one of the phases. As a result, it is thought that a conductive network of conductive carbon particles is likely to be formed and the conductive network is likely to be maintained. Further, when the difference Tm ₁ - _{Tm 2} between the melting point Tm ₁ of the polyethylene resin other than the ethylene copolymer used in the conductive layer and the melting point Tm ₂ of the ethylene copolymer is within the above range, It is thought that in the cooling process after coextrusion, the stretching is relaxed and the conductive network is more likely to be rebuilt before the conductive layer solidifies.

Furthermore, in the multilayer foam sheet, as described above, the surface layer contains at least linear polyethylene. In this way, 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, pinholes in the surface layer can be eliminated. It is thought that the formation of can be suppressed. As a result, it is considered that the contamination of the packaged items can be further reduced.

Therefore, according to the manufacturing method, it is possible to manufacture a multilayer foam sheet by coextrusion, which has high conductivity and can highly reduce contamination of the packaged items.

Examples of the multilayer foam sheet and its manufacturing method will be described below.

Table 1 shows the resins used in this example. The meanings of the symbols shown in the "Type of resin" column of Table 1 are as follows.

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

The "MFR" column of Table 1 lists the melt flow rate of each resin measured at a temperature of 190° C. and a load of 2.16 kg based on the method specified in JIS K7210-1 (2014). Further, the melting point and crystallization temperature of each resin in Table 1 are values measured by the method described above, that is, the plastic transition temperature measuring method specified in JIS K7121:2012.

The "23℃ tensile strength" column, the "95℃ tensile strength" column, and the "95℃ elongation at break" column in Table 1 contain the values of each resin at a temperature of 23℃ measured based on JIS K7161-1:2014. The tensile strength, the tensile strength of each resin at a temperature of 95°C, and the elongation at break of each resin at a temperature of 95°C are listed.

The tensile strength of each resin at a temperature of 23°C was measured by a method based on JIS K7161-1:2014. Specifically, first, a resin film with a thickness of 0.85 ± 0.03 mm was created, and a dumbbell-shaped No. 1 shape specified in JIS K6251 4.1 (shape and dimensions of test piece) was made from the film. A test piece was created by punching out a test piece having the following properties. Using a universal material testing machine "Tensilon RTC-1250A" manufactured by A&D Co., Ltd., a tensile test was performed on the test piece at a test speed of 500 mm/min in an atmosphere of 23 ° C, and the tensile strength of the test piece was measured. did.

The above measurements were performed five times for each resin, and the arithmetic mean value of the obtained tensile strengths was taken as the tensile strength of the resin.

The tensile strength and elongation at break of each resin at a temperature of 95°C were measured by attaching a constant temperature bath for universal testing machine "TLF-III-40-B" to the above-mentioned universal material testing machine, and using the atmosphere in the space where the above-mentioned tensile test was performed. The same method was used except that the temperature was adjusted to 95° C.±2° C., and the tensile test was conducted after the test piece was left standing in the atmosphere for 30 seconds. In addition, the elongation at break is a value calculated based on the distance between the gauge lines (specifically, 40 mm) in dumbbell type 1, and the maximum displacement that can be performed in the tensile test (specifically, 300 mm) is reached. If the test piece did not break even after the test, ">750" was written in the "95°C elongation at break" column in Table 1.

In addition, in the "Modification rate" column of Table 1, the mass fraction (unit: mass %) of the structural unit derived from the monomer having a polar group in the ethylene-vinyl acetate copolymer is listed. Note that low-density polyethylene, linear low-density polyethylene, and high-density polyethylene do not contain structural units derived from monomers having polar groups. The symbol "-" is written.

The conductive carbon used in this example is highly conductive carbon black ("Ketjenblack EC300J" manufactured by Lion Specialty Chemicals Co., Ltd.). The porosity of the conductive carbon in this example is 60%, the primary particle diameter is 40 nm, the amount of dibutyl phthalate (that is, DBP) supplied is 365 mL/100 g, and the BET specific surface area is 800 m ² /g. In addition, in the "conductivity imparting agent" column of Tables 2 to 7, the above-mentioned highly conductive carbon black was described as "CB".

The porosity of the conductive carbon described above 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 of conductive carbon is a value determined by observation using a transmission electron microscope. The DBP oil absorption of conductive carbon is a value measured according to ASTM D 2414-79. The BET specific surface area is a value determined in accordance with ASTM D 2414.

(Example 1)
As shown in FIG. 1, the multilayer foam sheet 1 of Example 1 consists of a foam layer 2, conductive layers 3 laminated on both sides of the foam layer 2, and a surface layer 4 laminated on each conductive layer 3. It has a 5-layer structure. The foam layer 2 is made of low density polyethylene shown in Table 2. The conductive layer 3 is made 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 made of a mixed resin of linear low density polyethylene as linear polyethylene and low density polyethylene. The specific composition of the mixed resin constituting the surface layer 4 is as shown in Table 2.

The method for manufacturing the multilayer foam sheet of Example 1 is specifically as follows. First, a coextrusion device is prepared that includes an extruder for forming a foam layer, an extruder for forming a conductive layer, an extruder for forming a surface layer, and a coextrusion die to which discharge ports of these extruders are connected. . The coextrusion die in this example is an annular die having an annular extrusion opening.

In producing the foam layer-forming melt, the polyethylene resin shown in Table 2 and 1 part by mass of a cell regulator per 100 parts by mass of the polyethylene resin were supplied to the foam layer-forming extruder. These were melt-kneaded in an extruder. A mixture of citric acid and sodium bicarbonate ("Fine Cell Master PO217K" manufactured by Dainichiseika Kagyo Co., Ltd.) was used as the bubble control agent. "Fine Cell Master" is a registered trademark of Dainichiseika Kagyo Co., Ltd.

A physical foaming agent was supplied under pressure to a mixture of a polyethylene resin and a cell regulator melted in an extruder, and the mixture was further kneaded in the extruder to obtain a melt for forming a foam layer. As the physical foaming agent, a mixed butane consisting of 65% by mass of normal butane and 35% by mass of isobutane was used. The blending amount of the physical foaming agent was 8 parts by mass with respect to 100 parts by mass of the polyethylene resin.

In producing the conductive layer-forming melt, the ethylene copolymer, low-density polyethylene, and conductive carbon of the type and amount shown in Table 2 are supplied to the conductive layer-forming extruder, and the ethylene copolymer 25 parts by mass of volatile plasticizer was supplied for a total of 100 parts by mass of the combined material and low density polyethylene. Then, by kneading these in an extruder, a melt for forming a conductive layer was obtained. As the volatile plasticizer, a mixed butane consisting of 65% by mass of normal butane and 35% by mass of isobutane was used.

In producing the surface layer-forming melt, low-density polyethylene and linear low-density polyethylene of the types and amounts shown in Table 2 are supplied to the surface-layer-forming extruder, and the low-density polyethylene and linear low-density polyethylene are 17 parts by mass of volatile plasticizer was supplied for a total of 100 parts by mass with polyethylene. Then, by kneading these in an extruder, a melt for forming a surface layer was obtained. As the volatile plasticizer, a mixed butane consisting of 65% by mass of normal butane and 35% by mass of isobutane was used.

The melts produced in each extruder in this way are simultaneously supplied to the coextrusion die, and in the coextrusion die, the conductive layer forming melt is laminated on both sides of the foam layer forming melt, and the conductive layer is laminated on both sides of the foam layer forming melt. The melt for surface layer formation was laminated on top of the melt for formation. Then, by coextruding and foaming these melts from the extrusion port of a coextrusion die, conductive layers are laminated on both sides of the polyethylene resin foam layer, and a cylindrical shape is formed in which a surface layer is laminated on each conductive layer. A laminated foam was produced. A mandrel having a diameter of 360 mm was inserted inside the laminated foam, and the laminated foam was cut open while being taken along the mandrel to obtain the multilayer foam sheet of Example 1. The conductive layer and surface layer of the multilayer foam sheet thus obtained were both in a non-foamed state.

(Examples 2 to 4 and Example 10)
The multilayer foam sheets of these Examples had the same structure as the multilayer foam sheet 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. have. The methods for producing multilayer foam sheets of Examples 2 to 4 and Example 10 were as follows, except that the type of linear low-density polyethylene in the melt for forming the surface layer was changed as shown in Tables 2 and 4. The method for manufacturing the multilayer foam sheet in Example 1 is the same.

(Example 5, Example 12 and Comparative Example 6)
The multilayer foam sheets of these Examples and Comparative Example 6 had the following differences, except that the blending ratio of linear low-density polyethylene and low-density polyethylene in the surface layer was changed as shown in Tables 3, 4, and 7. It has the same structure as the multilayer foam sheet of Example 1. The method for manufacturing the multilayer foam sheets of Example 5, Example 12, and Comparative Example 6 was as follows: Table 3, Table 4, and Table 4 The method for manufacturing the multilayer foam sheet of Example 1 was the same as that of Example 1 except for the changes shown in 7.

(Example 6 and Example 11)
The multilayer foam sheets of these Examples had the same structure as the multilayer foam sheets of Example 1, except that the surface layer was composed only of linear low-density polyethylene shown in Tables 3 and 4. ing. The method for manufacturing the multilayer foam sheet of Example 6 was the same as that of Example 1, except that low-density polyethylene was not blended into the melt for forming the surface layer, and linear low-density polyethylene shown in Tables 3 and 4 was used. This method is similar to the method for producing a multilayer foam sheet.

(Example 7)
The multilayer foam sheet of Example 7 had the following features: as the linear polyethylene used in the surface layer, high density polyethylene was used instead of linear low density polyethylene, and the blending ratio of high density polyethylene and low density polyethylene was changed. It has the same structure as the multilayer foam sheet of Example 1 except for the changes shown in Table 3. The method for manufacturing the multilayer foam sheet of Example 7 included the addition of high-density polyethylene instead of linear low-density polyethylene to the melt for forming the surface layer, and the mixing ratio of high-density polyethylene and low-density polyethylene. The method for producing the multilayer foam sheet of Example 1 was the same as that of Example 1 except for the changes shown in Table 3.

(Examples 8-9)
The multilayer foam sheets of these Examples had the following points, except that the blending ratios of low-density polyethylene, ethylene copolymer, and conductive carbon contained in the conductive layer were changed as shown in Tables 3 and 4. It has the same structure as the multilayer foam sheet of Example 1. The method for manufacturing the multilayer foam sheets of Examples 8 and 9 was as follows: The blending ratios of low density polyethylene, ethylene copolymer, and conductive carbon in the conductive layer forming melt were as shown in Tables 3 and 4. The manufacturing method of the multilayer foam sheet of Example 1 is the same as that of Example 1 except that the method was changed to .

(Examples 13 to 16)
The multilayer foam sheets of these Examples have the same structure as the multilayer foam sheet of Example 1, except that the basis weights of the conductive layer and/or surface layer were changed as shown in Table 5. The methods for producing multilayer foam sheets of Examples 13 to 16 were as follows, except that the discharge speed during production was changed so that the basis weight of the conductive layer and/or surface layer of the multilayer foam sheet became the value shown in Table 5. The method for manufacturing the multilayer foam sheet in Example 1 is the same.

(Comparative example 1)
As shown in Table 6, the multilayer foam sheet of Comparative Example 1 has the same structure as the multilayer foam sheet of Example 1, except that it does not have a surface layer. The method for manufacturing the multilayer foam sheet of Comparative Example 1 is the same as the method for manufacturing the multilayer foam sheet of Example 1, except that the surface layer is not laminated on the conductive layer.

(Comparative example 2)
As shown in Table 6, the multilayer foam sheet of Comparative Example 2 is the same as the multilayer foam sheet of Example 1, except that the surface layer does not contain linear polyethylene and is composed only of low density polyethylene. They have similar configurations. The method for manufacturing the multilayer foam sheet of Comparative Example 2 is the same as the method for manufacturing the multilayer foam sheet of Example 1, except that linear polyethylene is not blended into the melt for forming the surface layer.

(Comparative example 3)
The multilayer foam sheet of Comparative Example 3 is the same as that 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. It has the same structure as a foam sheet. The method for manufacturing the multilayer foam sheet of Comparative Example 3 was as follows: The method for manufacturing the multilayer foam sheet of Example 1 is the same as that of Example 1 except for the changes.

(Comparative Examples 4-5)
The multilayer foam sheets of these comparative examples do not contain an ethylene copolymer in the conductive layer, and the amount of conductive carbon and the type of linear low-density polyethylene used in the surface layer are as shown in Table 6. It has the same structure as the multilayer foam sheet of Example 1 except for the changes shown in Table 7. In the method for producing multilayer foam sheets of Comparative Examples 4 and 5, the ethylene copolymer was not blended into the melt for forming a conductive layer, but the amount of conductive carbon was adjusted and the ethylene copolymer was blended into the melt for forming the surface layer. The method for producing the multilayer foam sheet of Example 1 was the same except that the type of linear low-density polyethylene was changed as shown in Tables 6 and 7.

(Reference example)
The multilayer foam sheet of the reference example has the following features: no ethylene copolymer is included in the conductive layer, a polymer antistatic agent is added instead of conductive carbon as a conductivity imparting agent, and the surface layer is It has the same structure as the multilayer foam sheet of Example 1 except that it is made of only low density polyethylene shown in Table 7. The manufacturing method of the multilayer foam sheet of the reference example does not include an ethylene copolymer in the melt for forming the conductive layer, but instead of the conductive carbon, a polymer type antistatic agent is added in the amount shown in Table 7. The method for producing the multilayer foam sheet of Example 1 was the same except that the melt for forming the surface layer was composed only of the low-density polyethylene shown in Table 7. The polymer type antistatic agent used in this example is specifically "Pellectron (registered trademark) LMP" manufactured by Sanyo Chemical Industries, Ltd. In Table 7, the polymer type antistatic agent is described as "ASP".

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

[Melt viscosity of resin composition constituting the conductive layer]
Using a capillary rheometer (“Capillograph 1D” manufactured by Toyo Seiki Co., Ltd.), the melt viscosity of the conductive layer forming melts in Examples, Comparative Examples, and Reference Examples was measured. Note that the orifice diameter of the capillary rheometer was 1 mm, the orifice length was 10 mm, and the measurement was performed at a measurement temperature of 190° C. and a shear rate of 100 sec ⁻¹ . The melt viscosities of the conductive layer forming melts of Examples, Comparative Examples, and Reference Examples were as shown in Tables 2 to 7.

[Overall thickness of multilayer foam sheet]
First, the multilayer foam sheet was cut along a plane perpendicular to the extrusion direction. On this cut surface, ten measurement positions were set so that the intervals in the longitudinal direction of the cut surface (that is, the direction perpendicular to both the extrusion direction and the thickness direction) were equal. 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 overall thickness of the multilayer foam sheet and is listed in Tables 2 to 7.

[Basic weight of multilayer foam sheet]
A square test piece with a side of 25 mm was taken from the multilayer foam sheet, and the mass (unit: g) of the test piece was measured. By converting the mass of this test piece into units, the basis weight of the multilayer foam sheet, that is, the mass per 1 m ² of the multilayer foam sheet (unit: g/m ² ) was calculated. The basis weights of the multilayer foam sheets of Examples, Comparative Examples, and Reference Examples were as shown in Tables 2 to 7.

[Foaming ratio of multilayer foam sheet]
After dividing the basis weight of the multilayer foam sheet obtained by the method described above by the total thickness of the multilayer foam sheet, the apparent density (unit: kg/m ³ ) of the multilayer foam sheet was calculated by converting the unit. Then, the foaming ratio was calculated by dividing the density of the polyethylene resin constituting the foam layer of the multilayer foam sheet by the apparent density of the multilayer foam sheet. The expansion ratios of the multilayer foam sheets of Examples, Comparative Examples, and Reference Examples were as shown in Tables 2 to 7.

[Basic weight of conductive layer and surface layer]
Discharge amount of the conductive layer per side during coextrusion of the multilayer foam sheet X1 (unit: g/hour), discharge amount of the surface layer X2 (unit: g/hour), width W of the multilayer foam sheet (unit: m) Using the take-up speed L (unit: m/hour) of the multilayer foam sheet, the basis weight of the conductive layer and the basis weight of the surface layer (unit: g/hour) per one side are calculated based on the following formulas (1) and (2). m ² ) was calculated.
Basis weight of conductive layer=[X1/(L×W)]...(1)
Basis weight of surface layer=[X2/(L×W)]...(2)

[Surface resistivity]
The surface resistivity of the multilayer foam sheet was measured using a measurement method based on JIS K6271-1:2015. Specifically, a test piece having a square shape of 100 mm on a side was taken from the center in the width direction of the multilayer foam sheet. After attaching electrodes to the surface of the surface layer of this 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 (unit: Ω) after 1 minute had passed since the voltage was applied was defined as the surface resistivity of the multilayer foam sheet. To measure the surface resistivity, "Hiresta UX MCP-HT800" manufactured by Nitto Seiko Analytech Co., Ltd. was used, and the double ring electrode "MCP-JB-04" manufactured by Nitto Seiko Analytech Co., Ltd. was used as the electrode. Using. The surface resistivities of the multilayer foam sheets of Examples, Comparative Examples, and Reference Examples were as shown in Tables 2 to 7.

[Variations in surface resistivity]
The surface resistivity was measured at a plurality of positions on the surface layer of the multilayer foam sheet by a measuring method based on JIS K7194:1994. Specifically, the surface resistivity was measured at 21 measurement positions while shifting the positions by 25 mm in the longitudinal direction (that is, the extrusion direction) of the multilayer foam sheet in the widthwise center of the multilayer foam sheet. In addition, "Loresta GP MCP-T610" manufactured by Nitto Seiko Analytech Co., Ltd. was used to measure the surface resistivity, and a four-probe method electrode "МCP-TPLSP" manufactured by Nitto Seiko Analytech Co., Ltd. was used as the electrode. there was.

Similar measurements were performed at positions 300 mm to the left and 300 mm to the right from the center in the width direction, and the surface resistivity was measured at a total of 63 measurement positions. If the ratio of the maximum value to the minimum value of the surface resistivity obtained in this way is less than 10, mark ``A'' in the ``Surface resistivity variation'' column of Tables 2 to 7, and 10. If it exceeds 100, then the symbol "B" is written in the same column, and if it exceeds 100, the symbol "C" is written in the same column. Note that the method for measuring the surface resistivity at each measurement position is the same as the method described above.

[Pinhole area ratio]
At the center of the multilayer foam sheet in the width direction, the surface layer was observed using a scanning electron microscope while shifting the position in the longitudinal direction of the multilayer foam sheet (in other words, in the extrusion direction). I got the statue. Note that the observation magnification was 100 times. Similar operations were performed at positions 300 mm to the left and 300 mm to the right from the center in the width direction, and a total of 18 electron microscope images were measured.

Next, by image analysis, a square measurement area with a side length of 1000 μm was set in each of the 18 electron microscope images thus obtained. Then, in each measurement region, holes formed in the surface layer with an area of 70 μm ² or more were regarded as pinholes, and the ratio of the area occupied by the pinholes to the area of the measurement region was calculated.

The arithmetic mean value of the ratio of the area occupied by pinholes in the 18 measurement regions obtained by the above operation was defined as the pinhole area ratio of the multilayer foam sheet. The pinhole area ratios of the multilayer foam sheets of Examples, Comparative Examples, and Reference Examples were as shown in Tables 2 to 7.

[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 inside. After filling the bag with ultrapure water and rinsing the inner surface of the bag to remove foreign matter adhering to the inner surface, the bag was placed in a glove box.

After pouring 200 mL of ultrapure water into the bag in the glove box, ultrasonic cleaning at a frequency of 100 kHz was performed for 1 minute. After the ultrasonic cleaning was completed, the number of foreign substances with a diameter of 2 μm or more contained in the ultrapure water in the bag was counted using a particle counter. By dividing the number of foreign particles thus measured by the area of the inner surface of the bag, the amount of dust generated per unit area (unit: pieces/cm ² ) was calculated. The amounts of dust generated by the multilayer foam sheets of Examples, Comparative Examples, and Reference Examples were as shown in Tables 2 to 7. In the above method, the foreign matter mixed into the ultrapure water in the bag is specifically conductive carbon that has fallen off from the conductive layer, pieces of polyethylene resin that have fallen off from the edges of the pinholes, etc. This means that the smaller the amount of dust generated, the lower the contamination of the packaged items.

[exterior]
The appearance of the multilayer foam sheet was visually observed, and the lamination state of each layer was evaluated. In the "Appearance" column of Tables 2 to 7, the symbol "A" indicates that the color tone of the multilayer foam sheet is uniform, the symbol "B" indicates that the color tone is slightly uneven, and the symbol "B" indicates that the color tone is significantly uneven. If so, the symbol "C" is written.

As shown in Tables 2 to 5, the multilayer foam sheets of Examples 1 to 16 were composed of a foam layer and a resin composition having the above-mentioned specific composition, and conductive layers laminated on both sides of the foam layer. and a surface layer made of the specific resin and laminated on the conductive layer. Therefore, the multilayer foam sheets of Examples 1 to 16 have a surface resistivity of less than 1×10 ⁸ Ω, and can reduce the amount of dust generated from the multilayer foam sheets. Such a multilayer foam sheet has electrical conductivity and can highly reduce contamination of the packaged items. Further, in Examples 1 to 16, the foam layer, conductive layer, and surface layer having such a structure are laminated and bonded to adjacent layers by coextrusion. Therefore, it is superior in productivity compared to a method in which functional layers such as conductive layers are laminated by thermal lamination or the like.

Among these Examples, Example 5 and Example 7 in Table 3 have the same configuration except for the type of linear polyethylene contained in the surface layer. In addition, Example 5 is superior to Example 7 in terms of variation in surface resistivity and appearance. Therefore, from a comparison between Example 5 and Example 7, 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, and a good It can be seen that it is possible to more easily obtain a multilayer foamed sheet having a good appearance, and to further reduce variations in surface resistivity.

Further, 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. In Examples 1 to 3, the generation of pinholes is suppressed and the amount of dust generated is further reduced compared to Examples 4 and 10. Therefore, from a comparison of Examples 1 to 4 and Example 10, by using a resin that has 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 It can be seen that the contamination of packages can be further reduced.

Further, 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. In addition, Example 6 is superior to Example 11 in terms of variation in surface resistivity and appearance. Therefore, from a comparison between Example 6 and Example 11, by using a resin with a higher melt flow rate of the linear polyethylene contained in the surface layer, the surface layer can be stably laminated and bonded to the conductive layer, resulting in good results. It can be understood that it is possible to more easily obtain a multilayer foamed sheet having a beautiful appearance, and to further reduce variations in surface resistivity.

On the other hand, since the multilayer foam sheet of Comparative Example 1 shown in Table 6 did not have a surface layer, the amount of dust generated was greater than that of the example.

Although the multilayer foam sheet of Comparative Example 2 had a surface layer, since the surface layer did not contain linear polyethylene, the amount of dust generated was greater than that of the example.

In the multilayer foam sheet of Comparative Example 3, the amount of conductive carbon blended was too small, so it was not possible to impart conductivity to the multilayer foam sheet.

Since the multilayer foam sheet of Comparative Example 4 does not contain an ethylene copolymer in the conductive layer, when the amount of conductive carbon is within the above specified range, conductivity is imparted to the multilayer foam sheet. I couldn't.

As shown in Table 7, in the multilayer foam sheet of Comparative Example 5, the amount of conductive carbon blended was larger than that of Comparative Example 4 in order to impart conductivity to the multilayer foam sheet. However, as the amount of conductive carbon blended was too large, the amount of dust generated increased.

In the multilayer foam sheet of Comparative Example 6, the amount of linear polyethylene blended in the surface layer was too small, resulting in a large amount of dust generation.

The reference example is an example in which an antistatic layer containing a polymer type antistatic agent was provided between the foam layer and the surface layer in place of the conductive layer containing conductive carbon. As shown in the reference example, when an antistatic layer containing a polymer type antistatic agent is provided between the foam layer and the surface layer, dust generation may occur even if the surface layer does not contain linear polyethylene. It is possible to reduce the amount.

As above, specific aspects of the multilayer foam sheet and the manufacturing method thereof according to the present invention have been explained based on Examples. The present invention is not limited to this, and the configuration can be changed as appropriate without departing from the spirit of the present invention.

1 polyethylene resin multilayer foam sheet 2 polyethylene resin foam layer 3 conductive layer 4 surface layer

Claims (11)

A polyethylene resin foam layer,
a surface layer provided on at least one side of the polyethylene resin foam layer;
A multilayer foam sheet comprising a conductive layer provided between the surface layer and the foam layer,
The conductive layer comprises an ethylene copolymer having a structural unit derived from ethylene and a structural unit derived from a monomer having a polar group, a polyethylene resin different from the ethylene copolymer, and conductive carbon. including;
The amount of the conductive carbon in the conductive layer is 5% by mass or more and 15% by mass or less,
The surface layer contains one or more types of linear polyethylene selected from the group consisting of linear low-density polyethylene and high-density polyethylene, or the linear polyethylene and low-density polyethylene, and the linear polyethylene It is composed of a mixed resin with a blending amount of 8% by mass or more,
A polyethylene resin multilayer foam sheet, wherein the surface resistivity of the surface on the side where the surface layer is provided among the surfaces of the multilayer foam sheet is less than 1×10 ⁸ Ω. The polyethylene resin multilayer foam sheet according to claim 1, wherein the foam layer contains low density polyethylene. The polyethylene resin multilayer foam sheet according to claim 1 or 2, wherein the surface layer contains at least linear low density polyethylene as the linear polyethylene. The polyethylene resin multilayer foam sheet according to claim 1 or 2, wherein the linear polyethylene contained in the surface layer has a melt flow rate of 12 g/10 minutes or more at a temperature of 190° C. and a load of 2.16 kg. The polyethylene resin multilayer foam sheet according to claim 1 or 2, 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. The polyethylene resin multilayer foam sheet according to claim 1 or 2, wherein the linear polyethylene contained in the surface layer has a tensile strength of 15 MPa or more at a temperature of 23°C. The polyethylene resin multilayer foam sheet according to claim 1 or 2, wherein the linear polyethylene contained in the surface layer has a crystallization temperature of 100°C or more and 115°C or less. A difference Tm S - Tm _C between the melting point Tm _S of the linear polyethylene contained in the surface layer and the melting point _{Tm C} of the polyethylene resin used in the foam layer is 0°C or more and 15°C or less. The polyethylene resin multilayer foam sheet according to 1 or 2. The polyethylene resin multilayer foam sheet according to claim 1 or 2, wherein the surface layer of the polyethylene resin multilayer foam sheet has a basis weight of 1 g/m ² or more and 10 g/m ² or less. The polyethylene resin multilayer foam sheet according to claim 1 or 2, wherein the polyethylene resin multilayer foam sheet has an apparent density of 30 kg/m ³ or more and 150 kg/m ³ or less. Co-extrusion of a foam layer forming melt to form a polyethylene resin foam layer, a conductive layer forming melt to form a conductive layer, and a surface layer forming melt to form a surface layer. By doing so, the polyethylene resin foam layer, the surface layer provided on at least one side of the polyethylene resin foam layer, and the conductive layer provided between the surface layer and the foam layer, A method for producing a polyethylene resin multilayer foam sheet, the method comprising:
The foam layer forming melt is obtained by kneading a polyethylene resin and a physical foaming agent,
The conductive layer forming melt is an ethylene copolymer comprising a structural unit derived from ethylene and a structural unit derived from a monomer having a polar group, and a polyethylene resin different from the ethylene copolymer; Made by kneading conductive carbon,
The amount of the conductive carbon in the conductive layer forming melt is 5% by mass or more and 15% by mass or less,
The surface layer forming melt is one or more types of linear polyethylene selected from the group consisting of linear low density polyethylene and high density polyethylene, or 8 mass relative to the mass of the surface layer forming melt. % or more of the linear polyethylene and low-density polyethylene are kneaded.

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